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Home , Gadolinium, Thorium dioxide

FACULTY OF SCIENCE

Synthesis of dioxide doped with via a wet chemical route

Peter Zsabka

Promotor: Prof. Dr. Koen Binnemans Molecular Design and Synthesis,Department of Thesis presented in Chemistry, KU Leuven fulfilment of the requirements

Co-promotor: for the degree of Master of Science Prof. Dr. Thomas Cardinaels in Chemistry Radiochemical Analysis and Processes, Institute for Nuclear Materials Science, SCK•CEN Academic year 2014-2015 © Copyright by KU Leuven

Without writtenpermission of the promotors and the authors it is forbidden to reproduce or adapt in any form or by any means any part of this thesis. Requests for obtaining the right to reproduce or utilize parts of this publication should be addressed to KU Leuven, Faculteit Wetenschappen, Geel Huis, Kasteelpark Arenberg 11 bus 2100, 3001 Leuven (Heverlee), Tel.: +32 16 32 14 01.

A written permission of the promotors is also required to use the methods, products, schematics and programs described in this work for industrial or commercial use, and for submitting this publication in scientific contests.

I Abstract (English)

Thorium gained interest as a source of 233U, an alternative to 235U-based nuclear fuel, since the onset of the commercial utilization of nuclear power. Nowadays enormous reserves of thorium(IV) nitrate and thoria (ThO2) have been built up as a side product of extraction of rare-earth elements from ore. Without valorisation such reserves only count as highly radioactive waste that needs to be securely buried and monitored. From a technological point of view, the most straightforward utilization of thorium is its use as ThO2 ceramic pellets in pressurized water reactors. Fabrication of high ThO2 pellets is more difficult compared to currently used UO2 pellets due to the very high of thoria. Until know, pilot projects in which thoria pellets were used usually applied extensive mechanical treatment (milling or grinding) to improve the sinterability of pellet feed material. Wet chemical synthetic routes are generally considered as more safe compared to dry mechanical treatment, as hazardous dust formation can be avoided and powder properties can be engineered to some extent. Among others, the optimal process parameters of the precipitation route has been intensively investigated in the past as it is a cheap and simple wet chemical method and a candidate for industrial upscaling.

Wet chemical synthesis routes were applied to investigate the feasibility of fabricating high- density thoria pellets without having recourse to any mechanical treatment of precursor powder feed material. Three synthetic routes were studied. On the basis of literature data, nano-sized thorium(IV) oxalate hexahydrate and derived thoria powder was obtained reproducibly under carefully selected reaction and heat-treatment parameters. As the powders proved to be highly prone to form agglomerates, the sintered pellets shown sign of heterogeneous , and as a consequence reproducibility in terms of both pellet density and pellet integrity was very low. By adding trace amount of alumina sintering aid to the thoria powder, significant improvement in density has been attained. Further studies are needed to draw a definite conclusion whether the addition of sintering aid (either alumina or niobia) could ameliorate the abovementioned shortcomings of oxalate-route derived powder.

The second method was homogenous precipitation route, in which diethyl oxalate was thermally decomposed to generate in situ oxalic . The latter type of thoria powder was fluffy and free flowing.Pellets produced via homogeneous precipitation route sintered to low , being unfavourable with respect to economy during breeding, but more favourable for reprocessing due to ease in dissolution of irradiated pellets. In the third route tetrabutylammonium oxalate was used as precipitating reagent. In the latter case the precipitate could not be thermally decomposed and further processing of the material was abandoned.

The second goal of the thesis was to study the effect of gadolinium on lattice parameter of thoria powder and sintered pellet samples.The Gd- serves as a consumable neutron absorber, the use of which is promoted by the aim of obtaining higher neutron economy during the irradiation cycle.For the purpose of studying the effect of doping, the well-established oxalate precipitation route was applied.

Lattice parameters of sintered Gd-doped thoria pellets fabricated from wet chemical route- derived powder are reported here for the first time. Evidence is given on the solid solution

II formation during the sintering process. Lattice parameter values of powder samples were determined after calcination at 700 °C and 1000 °C for 1 h. The X-ray diffractogram of these samples showed the presence of two cubic phases. Lattice parameter values were subsequently determined on pellets that were sintered at 1750 °C for 8 h. Sintering resulted in one single cubic for all applied dopant concentration indicating solid solution formation. Besides the decrease of lattice parameter, gadolinium doping on thoria also caused a progressive decrease in the sintered pellet densities with higher dopant concentration.

Although wet chemical routes are considered to provide highly homogeneous distribution of constituting elements, evidence was found, that due to crystallization processes and thermal decomposition patterns during calcination of precursor powders leave back inclusions with composition different from that of the bulk matrix. The amount of inhomogeneity is nonetheless lower than the limit of detection attainable with XRD method, and was only possible to capture by EDS.

III Abstract (Nederlands)

Thorium wekte belangstelling als een bron van 233U, een alternatief van 235U-gebaseerde nucleaire brandstof, sinds het begin van commercieel gebruik van kernenergie. In deze tijd zijn enorme reserves thorium(IV) nitraat en thorium dixoxide (ThO2) ontstaan als een bijproduct van extractie van zeldzame aardelementen uit monaziet erts. Zonder valorisatie worden zodanige reserves alleen als hoogradioactief afval geacht dat veilig moet begraven en gecontroleerd worden. Ten opzichte van technologie is de eenvoudigste benuttiging van thorium als ThO2 keramische pellets in drukwaterreactors worden gebruikt. Fabricage van

ThO2 pellets met hoge dichtheid is meer ingewikkeld in vergelijking met de tegenwoordig gebruikte UO2 pellets vanwege het heel hoge smeltpunt van . Tot nu toe hebben piloot projecten waarbij thorium dioxide pellets werden gebruikt gewoonlijk extensieve mechanische behandeling (malen of vermalen) toegepast om de sinterabiliteit van pellet voedingsmateriaal te verbeteren. Natte chemische synthetische route is in het algemeen meer veilig geacht dan droge mechanische behandeling, want gevaarlijke stofvorming kan vermeden worden en poedereigenschappen kunnen tot op zekere hoogte gepland worden. Onder anderen werden de optimale procesparameters van oxalaat precipitatie weg in het verleden intensief onderzocht, omdat deze een goedkope en eenvoudige natte scheikundige methode is en een kandidaat voor industriele uitbreiding is.

Natte chemische synthetische route werden aangewend om de uitvoerbaarheid van productie van thorium dixoxide pellets met hoge dichtheid te onderzoeken zonder toepassing van enige mechanische behandeling van precursor poeder voedingsmateriaal. Drie synthetische routes werden bestudeerd. Op grond van vaste literatuurgegevens werden thorium(IV) oxalaat hexahydraat en afkomstig thorium dixoxide poeder in nano-maat reproduceerbaar met zorgvuldig geselecteerde reactie en warmtebehandeling parameters verkregen. Aangezien dat gebleken is dat de poeders zeer vatbaar voor de vorming van agglomeraten zijn, tonen de gesinteerde pellets het teken van heterogene sinteren, en als gevolg de reproducibiliteit ten aanzien van zowel pellet dichtheid als pellet integriteit zeer laag was. Door toevoeging van spoor hoeveelheid aluminiumoxide sinterhulpmiddel aan thorium dixoxide poeder, werd aanzienlijke verbetering betreffende dichtheid bereikt. Verdere studies zijn nodig voor het trekken van definitieve conclusie of de toevoeging van sinterhulpmiddel (hetzij aluminiumoxide hetzij ) de bovenvermelde tekortkomingen van uit oxalaat-route komende poeder zou kunnen verbeteren.

De tweede methode was de homogene precipitatie route, waarbij diethyl oxalaat thermisch ontbonden was om in situ oxaalzuur voort te brengen. Het laatstgenoemde type van thorium dioxide poeder was donzig en vrij vloeibaar. Pellets geproduceerd via homogene precipitatie route werd tot lage dichtheid gesinteerd, welk ongunstig aangaande neutron economie gedurende voortbrenging is, maar gunstiger is voor reprocessing ten gevolge van vergemakkelijking in het oplossen van bestraalde pellets. Bij de derde route werd tetrabutylammonium oxalaat als precipitatie reagens gebruikt. In voormeld geval kon het precipitaat thermisch niet ontbonden worden en verdere verwerking van het materiaal was gestopt.

De tweede doelstelling van het thesis (proefschrift) was om de invloed van de doping van gadolinium op rooster parameter van thorium dioxide poeder en gesinteerde pellet monsters

IV te bestuderen. Gd-dopant (Gd-doping agens) dient als een verbruikbare neutron absorber. Het gebruik ervan wordt met de bedoeling bijgestaan om hogere neutron economie tijdens de bestralingscyclus te krijgen. Om de invloed van doping te bestuderen, werd de goed gegronde oxalaat precipitatie route aangewend.

Rooster parameters van gesinteerde Gd-gedoopt thorium dioxide pellets geproduceerd uit poeder afkomstig van chemische route zijn hier allereerst gepubliceerd. Bewijs wordt geleverd met betrekking tot vorming van solide oplossing tijdens sinterproces. Rooster parameterwaarden van poeder monsters werden na calcinatie bij een temperatuur van 700 °C en 1000 °C voor 1 h bepaald. Het röntgendiffractogram van deze monsters toonde de aanwezigheid van twee kubieke fasen aan. Rooster parameter waarden werden vervolgens betreffende pellets bepaald die bij een temperatuur van 1750 °C voor 8 h werden gesinteerd. Het sinteren leidde tot een enkel kubieke fase voor alle gebruikte dopant concentratie die op vorming van solide oplossing wijst. Behalve de vermindering van rooster parameter, veroorzaakte gadolinium doping bij thorium dioxide ook een progressieve vermindering in dichtheid van gesinteerde pellets met hoger doping agens concentratie.

Hoewel natte chemische routes worden geacht om zeer homogene distributie van uitmakende elementen te verlenen, werd bewijs gevonden dat dankzij kristallisatie processen en thermische ontbinding patroon gedurende calcinatie van precursor poeders inclusies achterlaten met samenstelling verschillend van deze van bulk matrix. De hoveelheid van inhomogeniciteit is tevens lager dan de detectiegrens die door middel van XRD methode kan bereikt worden en was alleen mogelijk om door EDS gevangen te worden.

V List of Abbreviations

BET Brunauer–Emmett–Tellertheory

CSGP Complex sol-gel process

CTAB cetyl-trimethyl ammonium bromide

DTA Differential thermal analysis

EDS Energy-dispersive X-ray spectroscopy

EGA-MS Evolving-gas analysis coupled mass spectroscopy

HMTA Hexamethyleneammine

ICDD International Centre for Diffraction Data

OM Optical Microscopy

ORNL Oak Ridge National Laboratory

PGE poly(ethylene-glycol)

PVA Polyvinyl-alcohol

SCK•CEN StudiecentrumvoorKernenergie – Centre d’Etude de l’EnergieNucléaire – BelgianNuclear Research Centre

SEM: Scanning microscope

SGMP Sol-gel microsphere pelletization process

TD Theoretical density

TG Thermogravimetry

XRD: X-ray diffraction

VI Table of Contents Abstract(English)...... II Abstract(Nederlans)...... IV

List of abbreviations...... VI

Acknowledgement...... 3 Preface ...... 4 1. Introduction ...... 6 2. Scope and objectives of thesis work...... 7 3. Literature...... 8 3.1. Oxalate precipitation route ...... 8 3.1.1. Synthesis...... 8 3.1.2 Studies on the thermal decomposition of thorium(IV) oxalate...... 11 3.2. Direct denitration method ...... 15 3.3. Gel routes...... 15 3.3.1. Sol-Gel Processes...... 16 ORNL process...... 16 External gelation process ...... 17 Internal gelation techniques: ...... 17 The Sol-gel Microsphere Pelletization Process ...... 19 Gel supported precipitation ...... 19 3.3.2. Pechini method ...... 19 3.3.3. Citrate-gel method...... 20 3.3.4. Combustion processes ...... 20 3.4. Powder production methods via synthesis of ...... 21 3.5. Precipitation of thorium(IV) oxalate via homogeneous precipitation route ...... 24 3.6. Precipitation of thorium(IV) oxalate in ionic liquid medium...... 24

3.7. Synthesis routes for doped ThO2 ...... 25

3.8. Influence of contraction on lattice parameter of Th1-xLnxO2-x/2...... 25 3.9. The effect of using of sintering aids ...... 26 4. Experimental techniques ...... 28 4.1. Synthetic techniques ...... 28 4.1.1. Synthesis of thorium(IV) oxalate hexahydrate via oxalate precipitation route: ...... 28 4.1.2. Synthesis of thorium(IV) oxalate via homogeneous precipitation route...... 29 1 4.1.3. Synthesis of thorium(IV) oxalate via precipitation in ionic liquid media...... 30 4.1.4 Coprecipitation ...... 31 4.1.5 Calcination ...... 31 4.2. Characterization techniques...... 32 BET –surface area measurement ...... 32 Dilatometry...... 32 TG ...... 32 EGA ...... 32 XRD ...... 33 SEM ...... 34 OM ...... 35 Sintering ...... 35 Density measurements ...... 35 5. Results and discussion...... 36 5.1. Results of powder sample synthesis...... 36 5.2. BET surface area measurement results...... 36 5.3. Thermogravimetry analysis and evolved gas analysis...... 40 5.4. X-ray diffraction results ...... 45 5.5. Scanning-electron microscopy...... 53 5.6. Dilatometry measurements ...... 61 5.7. Optical microscopy of sintered and polished pellets ...... 64 5.8. Density measurements on sintered pellets...... 67 6. Conclusion...... 73 7. Health, Safety and Environment ...... 74 8. References...... 75 Annex I...... 79

Annex II...... 87

Annex III...... 89

2 Acknowledgement First and foremost I’d like to express my gratitude to Prof. Dr. Koen Binnemans, Prof.Dr. Thomas Cardinaels and Dr. Marc Verwerft, who made it possible to pursue an exciting research in SCK-CEN whereby I could gain experience on state-of-the-art analytical instruments and methods used in nuclear fuel research. Moreover, their guidance, enthusiastic encouragement and useful critiques of this research contributed to the finalization of the thesis to the largest extent.

I would also like to thank for all the help of Angela Baena for whom I'm extremely thankful and indebted to her for sharing expertise, and for her sincere and valuable guidance and encouragement extended to me.

I would like to express my gratitude for Gregory Leinders who thought me about the ins and outs of XRD, providing guidance with sample preparation techniques for both XRD and SEM and for the numerous measurements on STA.

I wish to express my sincere thanks for the help provided by Peter Dries and Koen Vanaken in numerous instances with dilatometry measurements, with sintering campaigns and even with unhappy nuclear incidents I run into during the thesis work.

For valuable help with the instructions on the use of scanning electron microscope and with sample manipulation inside of the hot cell I’d like to thank the members of the NMA , Ann Lenaerts, Jelle Van Eyken, Ben Vos and Wouter Van Renterghem.

I would like to grab the opportunity to express my thanks to all the staff members ofSCK•CENfor helping me with handling analytical instruments or providing solutions for any problems that emerged.

Last but not least, I’d like to thank for all the support of my family.

3 Preface Thorium’s main source is the monazite ore. The chemical processes are well established for the extraction and purification of the compound in the form of thorium dioxide

1,2 . Thorium dioxide (ThO2, thoria) crystallizes in a face centered cubic structure (or 5 -1 Oh space group) with a lattice constant of 0.56 nm. As it has low phonon energy (~ 450 cm ) radiative losses are low, making it a promising host material in optical applications3. The of the Th4+- is 0.104 nm, which makes substitution of every rare-earth ion possible.

ThO2 can accomodate large concentrations of and the equivalent amount of vacancies4.

Figure 1. Face-centered cubic of thoria – image prepared using PANalitical’s Highscore X'Pert code on the basis of X-ray diffractogram of the sample ThR1002-S36-B.

The idea of using thorium as a nuclear fuel has been investigated during decades by many research groups, although the intensity of support for the research was not constant. Most of the projects were abandoned at the end of the 1980’s (a valuable historical overview is given by Lung and Gremm5), with India left alone to work on its own three stage nuclear program (a short overview is given by Banerjee et al.6).

Thoria fuel properties can be compared to those of the conventional urania fuels, as collected by Lombardi et al.7 a) higher (by almost 50% over a significant range of temperature1 according to Kasten8), b) a much higher melting

1 ThO2: 9.5 kW/m and UO2: 6.5 kW/m, respectively.

4 temperature (by approx. 400-500 °C), c) a lower coefficient of , d) a lower value for the diffusion coefficient of fission gases and e) significantly less (by 9.7%) theoretical density. The last property combined with the lesser heat capacity of thoria fuel compared to urania fuel, results in a lower stored thermal energy per unit of volume in ThO2 fuel.

As of the beginning of the new millenium, few of the former stakeholders and also new players (China and Norway for instance) expressed a vivid interest to (re-)start their own or joined efforts to put thorium on the map of the energy production on an industrial scale. Financial risks associated with the development of a new nuclear power plant design serve as a very strong incentive to conduct research in a direction, where a new type of nuclear fuel could be applied as a simple replacement of the well-established urania-based fuels.

5 1. Introduction The use of thorium as a nuclear fuel in the most prevailing type of nuclear reactors (pressurized water reactors) require the attainment of high density oxide ceramic pellets. Engineering the properties (size, size distribution, shape, and state of agglomeration) of starting oxide powder material are decisive on the powder consolidation step and the microstructure of the fired body – as summarized by Rahaman.9

Particle size has its most profound effect on sintering (the densification of the material is in inverse relationship with the particle size), normally, a particle size less than 1 μm allows one to obtain high density sintered body within reasonable sintering times. Fine powders on the other hand are very prone to agglomeration, in the dry state. Formation of agglomerates in the powder starting material should be avoided, as it causes a heterogenous packing in the green body and to differential sintering during the firing stage (ie.: different regions of the material shrink at different rates, inducing mechanical tension that to the formation of large pores and crackings). Agglomerates can be hard agglomerates in which particles are held together by chemical bonds, or soft agglomerates which only consist of particles held together by van der Waals interactions. Fine and monodisperse powder can be only obtained, if the formation of any kind of agglomerates is avoided. In practice, that is almost impossible. However, if only soft agglomerates are formed, they can be deaggregated either mechanically (pressing or milling) or by dispersing in liquid.

Wide distribution of particle size is favourable for obtaining of high density in the green body, but during the sintering the larger particles coarsen while smaller ones’ size shall decrease. In practice the use of a narrow size distribution (or monodisperse) powder (preferably with spherical or equiaxial shape) is more beneficial for obtaining a uniformly packed green body and a high density sintered body.

Presence of surface impurities can lead to formation of liquid phase at the sintering temperature which causes formation of grains which grow faster than others (leading to non- uniform grains).

6 2. Scope and objectives of thesis work The thesis work aimed at the design and synthesis of thoria and gadolinium-doped thoria powders via wet chemical routes allowing the production of high density sintered pellets, and the determination of the effect of doping on the unit cell parameter of a series of Th1-xGdxO2- x/2. Preceding extensive studies of Horlait et al. were to be furthered by investigation of the unit cell parameters on sintered pellets.10

Wet chemical routes in principle should directly yield powders, that have such morphology not requiring further mechanical treatment (grinding or milling especially, which create hazardous dust). A major drawback of mechanical comminution is that it introduces contamination from the materials used (mainly zirconia or SiO2 particles in case of ball-mill). Such contamination can furthermore adhere to surface of particles and during sintering form liquid phases, resulting in heterogeneous sintering.

Powders of mixed derived from wet chemical routes should show a higher degree of homogeneity than oxide powders comminuted by only physical means.

Various synthetic routes were selected as candidates, promising production of highly sinterable thoria powders. Since the most extensively studied method of thoria powder synthesis is via the oxalate precipitation route, this method served as a reference or standard method, with which the usefulness of other methods were compared.

7 3. Literature

3.1. Oxalate precipitation route

3.1.1. Synthesis In order to obtain monodisperse particles, it is necessary to maintain reaction conditions where only homogeneous precipitation takes place. This requires the complete exclusion of contaminants in the solution or on the vessel’s walls moreover surfaces with which the solution is in contact should be smooth. According to the LaMer model of nucleation, we can obtain rather uniform size distribution in a precipitate, if we can induce the nucleation to take place all of a sudden, i.e.: all nuclei are created within a very short of time interval.11 To reach this, the concentration of the reactants should be brought substantially above the saturation concentration of the precipitate at the temperature of the reaction. Supersaturation can only be attained, if the reactants are free of impurities, especially solid particles, which would facilitate heterogeneous nucleation. Moreover, to reduce the time interval within which the nucleation step takes place, it is necessary to work with rather low concentrations of the reactants.

Monodispersity needs to be maintained in later stages of the precipitation process, which means that the further growth of the primary particles need to occur uniformly, in a diffusion controlled manner. The latter requires a slow release/introduction of additional reagents.

Particle growth following the quick nucleation step can be described by the Ostwaldripening, which provides an explanation for the process, whereby smaller particles dissolve and larger particles grow on the expense of the former ones.12 The system proceeds towards the thermodynamic equilibrium, where the whole amount of the solid phase forms a single crystal.

The abovementioned conditions for the nucleation and subsequent growth of monodisperse particles can be met by i) hydrolysis of -organic compounds (metal alkoxides);13,14 ii) hydrolysis of aqueous solutions of metal salts,15 or iii) other methods, whereby the anions or eventually the cations from a reactant can be released slowly.16 Modification of the speed of diffusion of reactants in the reaction media can also be a technique to exert some control over the growth processes.

The oxalate route is the most extensively studied and documented method for the production of thoria powders. A typical process flow diagram for the fabrication of ThO2 pellets is shown below in Figure 3.1.

Th(NO3)4(aq.)

(COOH)2(aq.)

Th(C2O4)2(s.) precipitation

Calcination

ThO2 powder 8 Micronization

Granulation

Cold compaction

CO2 pre-treatment

Sintering under H2

Figure 3.1. A process flow diagram for the fabrication of ThO2 pellets via oxalate powder route, reproduced on the basis of work by Clayton.17

Numerous papers were published which discuss optimization of the precipitation conditions with the aim of obtaining a thoria powder that could be sintered to a higher density. The advantage of using thorium oxalate as an intermediate material for synthesizing thoria is that thorium(IV)oxalate is pseudomorphic with thoria consequently the properties of the oxide product can be controlled to some extent by the preparation parameters of the oxalate intermediate.18

Earliest attempts to determine optimal conditions on the precipitation of thorium oxalate were made by Allred et al.19 They studied the effect of temperature on the thorium(IV)oxalate precipitation (and subsequent calcination) obtained from an aqueous thorium(IV)nitrate solution by drop wise addition of aqueous oxalic acid solution (this order of addition of reactants is also called as “direct strike”).

Thoria powder made from thorium(IV)oxalate precipitated at 10 °C was composed of cubic particles with an „edge-to-thickness ratio” of approximately 3:2, while samples made from oxalate precipitated at 100 °C were composed of platelets, „edge-to-thickness ratio” of approximately 6:1.

The largest specific surface area was obtained for samples made from oxalate precipitated at 40 °C and calcined between 600 °C to 900 °C temperatures. On the basis of the measurement on their samples the authors proposed a relationship between the surface area (S) and the crystallite size (D) of the powder:

(3.1.) whereρ is the density, and 1/F is the packing factor (this refers to the relative crystallite surface area unavailable for adsorption in the B.E.T. surface area method).

One of the most extensive studies published on the effect of experimental parameters is that of Pope and Radford whoused both oxalate route and direct denitration

9 process.20Thorium(IV)oxalate was prepared by starting from thorium(IV)nitrate aqueous solution (pH ≈ 0) via direct strike. Product was dried at 100 °C for 12 hours in an oven.

Calcination of the thorium(IV)oxalate and nitrate salts was investigated at the following temperatures: 450 °C, 600 °C, 750 °C, 900 °C, 1000 °C, 1250 °C and 1450 °C for 2 h in air atmosphere. According to X-ray and TGA analysis, approximately 500 °C is required for the completion of the conversion of thorium(IV)oxalate to thoria.

Contrary to the relatively moderate changes in the particle size distributions at higher calcination temperatures, the BET surface area of all the investigated types of powders have decreased. For instance, the oxalate derived-oxide calcined at 450 °C had a surface area of approximately 26.7 m2/g, while if calcined at 1450 °C, the surface area was 1.6 m2/g.

As far as the morphology is concerned, the oxalate derived thoria particles displayed a high degree of regularity. The particles were of a nearly perfect tetragonal symmetry and particle rounding was seen after calcination. The particles formed agglomerates of a 1 – 1.5 magnitude higher than the constituting particles. On the contrary, the nitrate derived thoria powders showed a very high degree of agglomeration, and both large and very small particles of irregular shapes and surface were found.

The authors performed a systematic investigation of the dependence of sintered body density on the green body densities at various temperatures of calcination. Duplicates of the samples were sintered at 1650 °C both in air and in H2 atmospheres for a period of 3 h. In general, the samples sintered in air gave 1-5 % higher densities. The powders calcined at 600 °C reached their maximum density (being 88 – 92 %TD) during 3 hours according to the experiments.

It is noticeable, that on the one hand the oxalate-derived thoria gave higher densities, on the other hand the higher the calcination temperature, the lower the sintered density became. The reason for these observations was explained with the following considerations. The compact made from oxalate-derived thoria powder contains individual crystallites which are in direct contact with each other. The porosity therefore is eliminated during the sintering step, and the sintered body shall exert a fairly homogenous/even distribution of round shaped pores. In contrast, the nitrate-derived thoria powder showed a better sinterability within the aggregates and not between them. As a result a lot of porosity was trapped inside yielding low sintered densities for high surface area powder.

Gradual increase of entrapped porosity in oxalate derived thoria at higher calcination temperatures is the reason for the gradual sintered body densities (simply more and more strong the aggregates become during the calcination, and the same pressure applied shall be insufficient to break them down).

At the Pacific Northwest Laboratory of US DoE, White et al. worked on the optimization of precipitation conditions for thorium(IV)oxalate.21 Most sinterable thoria powders were obtained via precipitation at 10 °C using mechanical stirring for 15 min. From the process parameters, temperature has the largest effect on particle morphology, the surface area crystallite size.

The conclusion of the study is, that at lower precipitation temperature (i.e. T = 10 °C) the specific surface area is higher and the average particle size is smaller, than for samples

10 precipitated at 70 °C. The grain sizes of the samples were between ca. 10- 20 μm and were distributed uniformly. BET surface area of thoria powders was 12.4 m2/g in case of products obtained by continuous precipitation and 8.7 m2/g for thoria derived from direct strike.

The product was dried at 115 °C, decomposed at 350 °C (in order to avoid the melting of

Th(C2O4)2·2H2O) and calcined at 900 °C - each step lasted for 24 h in air. Particles were sieved at a 40 mesh screen. Compaction into green pellets was done with 276 MPa pressure. The thoria green pellets (with densities between 56-59 % T.D.) obtained by the White et al. could be sintered up to 96.0 - 96.6 % T.D. without the need to apply milling before the sintering process.

Ananthasivan et al.investigated the sintering properties of thoria obtained from thorium(IV)oxalate precipitated in water and in non-aqueous solvents.22 In order to do away with agglomerates formed from fine particles, a de-agglomeration step was performed on the oxalate-route derived products by ultrasonication both in aqueous and non-aqueous media (methanol, ethanol, propan-2-ol and propan-1-one). (Thoria derived from the non-aqueous route yielded lower crystallite size and higher bulk densities, while the aqueous route yielded bigger crystallite sizes and lower bulk densities).

Thorium(IV)oxalate was precipitated from nitrate solution by direct strike at 25 °C. The precipitate was de-agglomerated twice in 250 ml of the medium by agitating for 15 min. in an ultrasonic agitator. Dried product was calcined in air at 700 °C for 4 h and compacted at 120 MPa into pellets. The pellets were sintered in air for 4 h at 1200 °C and 1600 °C by using a heating and cooling rate of 300 K/h.

It was found, that thorium(IV)oxalate powders obtained from the aqueous method and de- agglomerated in alcohols gave cuboidal morphology. If the de-agglomeration was performed in propan-2-one, then the oxalate platelets ’flocked’ together resulting in spheroidal particles. The authors suggest that the presence of hydroxyl groups in the solvents facilitates the dispersion of the oxide and , while with a ketone medium, the inter-particle cohesion is stronger.

Thoria derived by de-agglomeration procedure from aqueous precipitation could be sintered to 97 % T.D., while starting from non-aqueous precipitated thorium(IV)oxalate, nanocrystalline thoria (diameter = 2-5 nm) was obtained, that could not be sintered to such high densities.

3.1.2 Studies on the thermal decomposition of thorium(IV)oxalate To conserve native powder morphology it is inevitable that the thermal treatment during calcination have to be designed in a way to avoid i) melting of the thorium(IV)oxalate monohydrate, since it would result in hard agglomerates, that impair homogeneous sintering; ii) occurrence of pre-mature sintering. There is no consensus on the exact mechanism of decomposition according to an overview of literature. A proper understanding of the thermal decomposition mechanism of thorium(IV)oxalate hexahydrate would be important in order to choose the least vigorous parameters to calcine precursor materials. Too long heating times or too high applied temperatures – although might be beneficial to completely remove even traces of impurities – are detrimental to the morphology of the powder, as it is quite common, that premature sintering takes place during calcination.

11 One of the earliest studies on thorium(IV) oxalate thermal decomposition was conducted by Beckett and Winfield.23 The chemical composition of thermally decomposed thorium(IV)oxalate hexahydrate was determined on the basis of changes in the thermograms recorded during decomposition. Decomposition of thorium(IV)oxalate was found to start around 200 °C (the highest decomposition rate at 295 °C) via the formation of

Th(CO3)2intermediate.

A similar decomposition scheme was described by D’Eye and Sellman.24 The effect of applied heating rate was studied on the thermal decomposition of thorium(IV)oxalate dihydrate in air atmosphere. According to their findings, the decomposition of thorium(IV)oxalate occurs through a carbonate intermediate above 270 °C:

Th(C2O4)2 → Th(CO3)2 + 2 CO (3.2.)

Th(CO3)2 → ThO2 + 2 CO2 (3.3)

The total amount of CO and CO2 formed exceeded theory and the authors explained this by stating, that CO disproportionated to C and CO2 and subsequent oxidation of elemental carbon with oxygen from air.

Wendlandt et al. analyzed and interpreted the differential thermal analysis curves for oxalate hydrate compounds of Th, Sc, U, Y and a few other rare-earth elements.25 Starting from

Th(C2O4)2.6H2O that was obtained from homogeneous precipitation route using methyl oxalate as a source of precipitating agent.

An endothermic peak was found at 145 °C and identified as a transition:

Th(C2O4)2.6H2O → Th(C2O4)2·2H2O + 4 H2O (3.4.)

A second endothermic peak was found at 270 °C and assigned as:

Th(C2O4)2·2H2O → Th(C2O4)2 + 2 H2O (3.5.)

A third endothermic peak at 385 °C and a broad exothermic peak centered at around 560 °C are both assigned as:

Th(C2O4)2 → ThO2 + 2 CO + 2 CO2 (3.6.)

The DTA curves of all rare-earth oxalates had the following features in common: all of the endothermic dehydration peaks occurred below 350 °C, while the decomposition of the anhydrous metal oxalates which were manifested in both an endo- and an exothermic peaks, took place from 350 °C to 900 °C. On the basis of trends observed, rare-earth oxalates could be grouped into three groups:

Group I contain Ln, Pr, Nd-oxalates, of which all exhibit decomposition from decahydrate to anhydrous oxalate without formation of an intermediate hydrate. The DTA curves showed two peaks centered around 185 °C and 260 °C respectively.

Group II contain Sm, Eu, Gd, Tb-oxalates all showed three endothermic peaks in the 50 °C– 350 °C temperature range, centered about 160 °C, 200 °C and 285 °C. Assignment of the peaks was not possible to intermediate hydrates because the dehydration reactions are complex and overlapping.

12 Group III contain the oxalates of Dy, Ho, Er, Y, Tm and Lu, of which all exhibited a broad endothermic peak in the DTA curves centered about 125 °C- 160 °C. According to weight- loss measurements, these peaks could be assigned to the formation of metal-oxalate dihydrate. Interestingly, the anhydrous oxalate was not formed, but the dihydrate decomposed directly to metal oxides (manifested as an endothermic peak centered about 415 °C - 460 °C).

Termogravimetry, differential thermal analysis and specific surface area measurement techniques were used by Aybers to study the kinetics of the thermal decomposition of thorium(IV)oxalate dihydrate. X-ray diffractogram was only obtained from the starting material, but not from intermediate species.26 Based on the interpretation of a constructed surface area-temperature curve; the decomposition was found to occur between 100 °C and 580 °C as per reactions (3.7)-(3.11) below.

Th(C2O4)2·2H2O → Th(C2O4)·H2O + H2O (3.7.)

Th(C2O4)·H2O → Th(C2O4) + H2O (3.8.)

Th(C2O4) → Th(CO3)2 + 2 CO (3.9.)

Th(CO3)2 → ThOCO3 + CO2 (3.10.)

ThOCO3 → ThO2 + CO2 (3.11.)

In a more recent paper Oktay and Yayli investigated the morphology, size and size distribution of thorium(IV)oxalate particles obtained via direct strike using either mechanical or ultrasonic agitation.27 The thermal decomposition process was investigated by thermogravimetric (TGA) and differential thermal analysis (DTA) techniques.

Based on the thermogravimetric analysis, the dried samples were in the form of

Th(C2O4)2·2H2O, and the thermal decomposition proceeded in three consecutive stages as per equations (3.12.) – (3.14.).

Stage 1 (200 °C - 260 °C): Th(C2O4)2·2H2O → Th(C2O4)2·H2O + H2O (3.12.)

Stage 2 (260 °C - 373 °C): Th(C2O4)2·H2O → Th(C2O4)2 + H2O (3.13.)

Stage 3 (373 °C – 498 °C): Th(C2O4)2 → ThO2 + 2 CO + 2 CO2 (3.14.)

One of the most detailed analysis available in this field is that of Joseph et al. who compared five former studies on the thermal decomposition of thorium(IV)oxalate hexahydrate and pointed out crucial differences on either the applied heating rates, the used starting material’s composition or even the absence of XRD analysis of starting materials and intermediates that were assumed to be present.28

To clarify the ongoing processes during the thermal decomposition,TGA-DTA studies were conducted. These studies were repeated in different heating rates (1, 2, 5 and 10 °C/min.) and with two applied sample mass: 15 mg and 85 mg respectively (to provide similar circumstances as those applied by former authors on the topic). To identify the chemical composition of intermediates, samples were taken from eight selected temperatures: 75 °C, 200 °C, 240 °C, 310 °C, 355 °C, 370 °C, 388 °C and 600 °C (calcination was interrupted and sample cooled down at a rate of 60 °C / min. to avoid unwanted changes). Sample handling

13 was performed in a manner to prevent moisture uptake from air during manipulation and during XRD-measurement.

According to their findings, the decomposition of thorium(IV)oxalate occurs in four stages (taking the example of a sample heated at 1 °C /min. rate. No evidence was found to the existence of anhydrous form of the oxalate salt. A thorium(IV)oxycarbonate (ThOCO3) intermediate was identified and XRD-pattern confirmed, that this compound is by nature amorphous.

Th(C2O4)2·6H2O → Th(C2O4)2·2H2O + 4 H2O above 70 °C (3.15.)

Th(C2O4)2·2H2O → Th(C2O4)2·H2O + H2O above 180 °C (3.16.)

Th(C2O4)2·H2O → ThOCO3 + H2O + 2 CO + CO2 above 297 °C (3.17.)

ThOCO3 → ThO2 + CO2 above 388 °C (3.18.)

Amount of free carbon impurities present in samples heated above 550 °C was found to be 60 ppm.

In a more recent study Dash et al.conducted an experiment, where the results of TGA and evolved gas analysis-mass spectrometry (EGA-MS) were correlated during a temperature programmed decomposition of thorium(IV)oxalate.29 By virtue of the combination of the two methods, and a deconvolution code applied on the EGA-MS spectra, it was possible to substantiate the subsequent phases during the thermal decomposition. According to the results, the total decomposition - marked by the upper temperature limit of CO2 - evolution only occurred at 626 °C (being in contrast to former results). The decomposition was claimed to proceed through the formation of two intermediates, Th(CO3)2 and ThO(CO3).

The release span of CO2 attributed to the decomposition of the Th(CO3)2 was investigated separately in a study by Dash et al.30 On the basis of TGA results, the thermal decomposition takes place without any induction period, starts already at 27 °C and it is only completed at 627 °C. The loss of 2 equivalents of CO2 molecules occurs in two partially separated steps. According to the deconvoluted EGA-MS spectra recorded during the TGA measurements, the release of the first equivalent of CO2 is characterized by a broad peak centered around 230 °C, while the release of the second equivalent is characterized by a sharper peak centered around 380 °C.

Adding to the confusion concerning the decomposition reactions, Raje and Reddy in a later publication concluded that all former publications stating the formation of thorium(IV) oxalate monohydrate and anhydrous thorium(IV) oxalate misinterpreted a multistep process, whereby the hexahydrate form de-hydrates to dihydrate, which in turn polymerizes to form a polynuclear thorium(IV) oxalate complex.31

The evolution of the particle morphology with temperature of nano-grained thoria produced 32 by thermal decomposition of Th(C2O4)2·2H2O was investigated by Tyrpekl et al. Oxalate precursors were prepared by reverse strike. Contrary to all former reports, the authors stated, that the oxalate product obtained was the monoclinic C2/c dihydrate form of thorium(IV)oxalate, and not hexahydrate form. The experimental precautions to avoid re- hydration of the dihydrate via uptake of water from air were not elucidated.

14 High temperature XRD allowed for the determination of change in lattice parameter, grain size and strain with the temperature in the range of 20 °C - 700 °C. According to the diffractograms recorded during the decomposition of the oxalate starting material, the oxalate structures vanish around 300 °C to give birth to amorphous intermediate, which in turn starts to crystallize into ThO2 from 400 °C and above. Unit cell parameter of thoria nanoparticles was found to be 5.600(6) Å.

The conclusion that should be drawn from these results is that literature results are often difficult to compare due to different experimental procedures or process parameters. They nonetheless provided a useful guidance for the start of the thermal decomposition studies presented below. Calcination was conducted with an insertion of an intermediate step: 5 h dwell time at 350 °C based on the findings of Wendlandt et al. The highest temperature of the calcination program had to be determined on the basis of TG analysis coupled to EGA- MS, since the absence of a consensus on this value in the literature.

3.2. Direct denitration method

Probably the most simple route for the production of thoria powders consists of simple heating of aqueous thorium(IV)nitrate solutions until complete removal of water followed by the thermal decomposition of nitrate . As Belle and Berman concluded, products obtained by this simple scheme are unfortunately composed of coarse particles that cannot be worked up into acceptable quality ceramics without excessive ball milling.33Similar conclusion was brought by the authors of IAEA TECDOC 1450, which stated, that the direct denitration route was abandoned because it yields less sinterable thoria powder. The main reason for lower sinterability is the low specific surface area and large crystallite sizes.

The surface areas (as determined by nitrogen B.E.T. method34) of thoria powders obtained via thermal decomposition (at 600 °C, 700 °C and 900 °C) of thorium(IV)nitrate were measured by Veron.35 (The respective values: 55 m2/g, 29 m2/g and 10 m2/g indicate the initial sintering of the powders).

The behavior of wet ground thoria-based powders produced by hydrothermal, , and „pot” denitration were compared by Palmer et al.36 The powders obtained from thermal denitration consisted large, hard particles. The heating was performed on hydrated thorium(IV)nitrate crystals in the presence of steam in a rotary furnace at 500 °C. Heat treatment on the powders didn’t change the morphology, but resulted in growth of them. Powders were wet ground resulting in colloidal slurries. Subsequent drying gave cakes which were crushed. The green and sintered densities were 64.1 % and 96.2 % T.D., respectively.

3.3. Gel routes

A common feature of the various methods collected below, is that all of them involve the production of a semi-rigid gel or a highly viscous resin intermediate, which are processed into ceramics making them subject to high temperatures. An advantage of these methods is good chemical homogeneity, while a disadvantage is that after the calcination the decomposition product is often not actually a powder but a mass consisting of charred lumps-necessitating milling.

15 3.3.1. Sol-Gel Processes Originally, these methods were developed for making thin films, powders and fibres, and in some cases for making monolithic ceramic bodies. Gel derived powders often have an amorphous structure and high surface area, making it relatively easy to obtain high sintered densities at lower sintering temperatures. As of the 1960’s sol-gel techniques were adapted for production of nuclear fuel because they have the advantage avoid the need to handle powders with a high risk of hazardous radioactive dust formation.37 Working with only fluidic compounds has an advantage of reduction of process steps and the ease in upscaling and facilitated remote handling during the operation. Vaidya categorized the sol-gel processes developed for nuclear fuel synthesis into three groups: a) ORNL process, b) external gelation process and c) internal gelation process.38

Sol-gel routes most frequently use precursors in the form of metal alkoxides. In the case of thorium, the alkoxides can be obtained from its anhydrous chloride form by adding the dry alcohol and as per (3.19.).

ThCl4 + 4 ROH + NH3 → Th(OR)4 + 4 NH4Cl (s) (3.19.)

This reaction however is especially dependent on the extent by which the water can be excluded from the reaction vessel (these processes actually require the use of glove box and dehydrated solvents).

Another method used for the preparation of metal alkoxydes is:

MClz + 4 NaOR → M(OR)z + NaCl(s) (3.20.)

The hydrolysis-reaction of the metal-alkoxides can be formulated generally as per equation (III.21):

M(OR)z + z/2 H2O → MOz/2 + z ROH (3.21.)

ORNL process:

The ORNL sol-gel process (as summarized by Haas39 and Wymer40) was based on the 41 hydrothermal denitration reaction starting from Th(NO3)4 aqueous solution. Thoria powder which was used for the production of the sol was prepared by forcing superheated steam through a thin layer of thorium(IV) nitrate solution at gradually increasing temperatures (from 200 °C till reaching 485 °C at the highest point). The obtained powder is subsequently stirred - vigorously in diluted (with an NO3 / ThO2 ratio 0.11) at 80 °C to yield a stabilized sol.

The hydrothermal reaction is formulated as per equation (3.22.):

Th(NO3)4 + 2 H2O → ThO2 + 4 HNO3 (3.22.)

In order to obtain highly dispersible thoria powder it proved to be crucial, that no local overheating2 and consequent nitrate decomposition took place. Inappropriate heat transfer leads to a sticky/gummy product, which is impossible to disperse further on. The dispersion of thoria powder produced by the hydrothermal process was made by mixing the powder with

2 Fluidized bed and rotating drum arrangements were both used in ORNL for this purpose. 16 an aqueous solution of nitric acid (at temperature between 60 °C - 80 °C), in which the nitrate ions serve as a peptizing ion.

The aqueous thoria sol is then dispersed into droplets in an organic liquid (2-ethyl-1-hexanol) in the presence of surfactants (Ethomeen S/15 to prevent coalescence and Span 80 to prevent clustering of partially gelledspheres), followed by the removal of water, resulting in solid gel particles. The surfactants help to avoid the agglomeration of the particles. The gel particles usually have spherical shape upon passing the orifice applied for the droplet- formation, although the amount and nature of surfactants can heavily modify the morphology. Uniformity of droplet and derived gel particle sizes could be improved by applying vibration on the nozzles.

After separation from the organic liquid, these rather narrow size distribution particles are subject to drying, compaction and further heat treatment.

Laboratory-scale studies demonstrated the feasibility to prepare homogeneous powder products containing oxides of Th/U, Pu and lanthanides via sol-gel process without the need to apply hydrothermal denitration. According to McBride,42 the process to synthesize mixed thorium- oxide sols was based on solvent extraction using a long-chain aliphatic amine (Amberlite LA-2: n-lauryltrialkylmethyl amine) dissolved in an inert diluent (n-paraffin). When the aqueous solutions of the nitrates were brought into contact at 50-60 °C with the organic phase containing the long-chain aliphatic amine extractant, a partial denitration of the metal ions occurred. Thereafter the phases were separated, and the aqueous phase heated to 95-100 °C for 10 min. whereby the yellow solution turned into a red-colored sol, containing 10 - 40 Å sized crystallites. A second contact with the organic phase removed nitrates further.

The sol was subsequently further dried by vacuum evaporation of water (avoidance of heating was necessary to maintain the sol's stability). The spheres were formed by free-fall mechanism or by using vibrated orifices.

External gelation process:

The external gelation of thorium was developed in Kern Forchung Analage (KFA) by Ringel and Zimmer in Germany.43 According to this process, the sol is prepared by adding ammonia gas to the pre-neutralized solution of Th(NO3)4 or (Th/U)(NO3)4or (Th/Pu)(NO3)4 mixtures in a controlled manner. The gas is introduced to a jacketed and water-heated vessel through a hollow, rotating disperser shaft. Subsequently urea and ammonium nitrate was added to the solution. Droplets are formed from this solution and gelled in gaseous ammonia. The gelled spheres are washed with an aqueous ammonia solution at 60 °C and dried on a special belt drier in the temperature range of 150-500 °C, finally sintered at 1200 °C.

Internal gelation techniques: The internal gelation method was developed in KEMA, Netherlands and investigated in many other research groups, including ORNL. Collins et al. provided an overview of the chemistry behind the internal gelation method.44

In internal gelation processes concentrated, cold (typically 0 °C) metal nitrate feed solutions are mixed with cold and concentrated hexamethyleneammine (HMTA) and urea solutions.

17 The mixture is amenable to form droplets, if pushed through an orifice, which, upon entering into hot silicone oil (kept at 90 °C) undergoes a gelatinous precipitation.

Collins et al. in their experiment used uranyl-nitrate solution that was partially pre-neutralized - with NH4OH to give an NO3 /U mol ratio of 1.50 to 2.00. The uranium concentration of each solution was chosen as 1.84 mol/dm3 and the HMTA and urea feed solution concentrations were both 3.1 mol/dm3. Urea forms a stable complex at low temperatures (i.e. 0 °C) with the metal ions present, but this complex becomes less stable with raise of temperature. The complexation reaction is:

2+ 2+ UO2 + 2 CO(NH2)2 UO2[CO(NH2)2]2 (3.23.)

At 0 °C, a uranyl-to-urea molar ratio⇌ of ~ 1.0 was sufficient to get a stable broth. Urea is also supposed to accelerate the decomposition of HMTA, although the mechanism is not clear. (It was found, that in the presence of excess acid, protonated HMTA will also decompose, but not as fast as in the presence of urea).

These experiments identified four reactions taking place during the course of the process:

i) decomplexation:

2+ 2+ UO2[CO(NH2)2]2 2 CO(NH2)2 + UO2 (3.24.)

ii) hydrolysis: ⇌

2+ + UO2 + 2 H2O UO2(OH)2 + 2 H (3.25.)

iii) HMTA protonation:⇌

+ + (CH2)6N4 + H [(CH2)6N4H] (3.26.)

iv) HMTA decomposition⇌

+ + - + - (CH2)6N4.H + 3 H + 4 NO3 + 6 H2O 4 NH4 + 4 NO3 + 6 CH2O (3.27.)

Temperature has an overwhelming importance⇌ in the process, since both the decomplexation reaction and the hydrolysis reaction (both being an initiation step in the course of gelatinous precipitation) depend on it.

Matthews and Hart investigated the optimal sphere properties, calcination, pressing and sintering conditions for gel-derived spheres UO2 obtained from internal gelation method, as 45 well as ThO2 and ThO2 - 25 wt.% UO2mixtures obtained from external gelation method. According to the investigations; the calcination temperature had the largest effect on the pressing behavior of UO2 spheres. Quite similarly to the findings of Pope and Radford higher calcination temperatures gave lower sintered body densities in all cases.

Internally gelated UO2 spheres had a coarser structure and could be deformed more easily than the externally gelated samples.

2 The samples of ThO2 powders (spheres with a specific surface area of 45 g/m and a crystalline size of 7 nm) derived from external gelation were calcined, compacted and sintered. The morphology of the sphere surfaces was similar to the externally gelled UO2- spheres i.e.: they had a high surface area and small crystallite size. The authors could not

18 manage to obtain pellets which were free of cracks. Changing of calcination temperature, sphere size, pressing parameters and addition of sintering aid or the application of milling were of no use in ameliorating the integrity of the product.

A complex formation step was included into the synthesis-scheme applied by Deptula et al. who have developed a so-called complex sol-gel process (CSGP) for the production of U and Th-oxides.46 The addition under reflux conditions of ascorbic acid to the metal salt solution created ascorbate-complexes. The ascorbate-thorium sol upon dropwise introduction into 2- ethylhexanol-1 quickly formed an emulsion (due to water extraction), making it is easy to prepare gelled microspheres by adding ammonia solution.

The major drawback of this process is that during the heating, at 70 °C a violent, exothermic reaction takes place between ascorbic acid and nitrate ions. In the case of uranium, the similar step led sometimes even to explosion.

The Sol-gel Microsphere Pelletization Process Apart from the three abovementioned group of processes, alternative sol-gel based methods were developed (for example: the microsphere impregnation technique by Rajesh et al.).47 The Sol-Gel Microsphere Pelletization process (SGMP) combines the advantages of the sol- gel process and the pellet fabrication which is still the standard form of nuclear fuel worldwide as opposed to sphere-pac type of fuel pins. In a series of publications, Yamagishi and Takahashi reported fabrication of thoria pellets through microsphere pelletization without additives.48

A sol was prepared by partially neutralizing a hot thorium(IV)nitrate aqueous solution by + - introducing gaseous ammonia. (Pn = [NH4 ]/[NO3 ] = 0.50).

In the SGMP the gelation takes place by forcing ThO2 sol through a two-fluid nozzle that is placed on the top of the gelation column. The drops are formed, upon entering into ammoniac hexone (methyl isobutyl ketone) and concentrated ammonia solution. The gel particles are collected at the bottom of the column into reservoirs. The humidification of the sintering atmosphere proved to be a beneficial measure to obtain high density pellets (up to 99.9 % T.D.) from high- containing starting materials.49,50

Gel supported precipitation A method developed at Harwell, UK consisted of addition and mixing of a water-soluble polymer (dextran) with the aqueous solution of thorium(IV) nitrate to obtain a homogeneous solution51. Droplets were formed from this mixture through a 1 mm diameter jet and the Th(IV)-ion content was precipitated in the form of hydroxide by the addition of ammonium- hydroxide or hydroxide. Upon precipitation, 3-4 mm diameter spheres were obtained, that were subsequently washed and dried. The role of the sacrificial polymer is to help the droplet formation. To avoid carbon impurities the produced microspheres were heated to 250 °C under vacuum where the polymer was decomposed.

3.3.2. Pechini method This method is an adaptation of the synthesis route elaborated originally to produce lead and alkaline-earth titanates for the purpose of capacitors by Pechini.52

According to the original patent, one equivalent of the selected metal ions is brought into solution in their alkoxide, hydrated oxide or α-hydroxycarboxylate form. To this solution 2 to 8

19 equivalent of citric acid and an excess of a polyhydroxy-alcohol, like ethylene-glycol is added and stirred until a clear single liquid phase is obtained. The dopant metal in its oxide, hydroxide, carbonate or alkoxide (i.e. in basic) form is added before the removal of excess solvent by heating. The obtained intermediate is a transparent resin, which contains the selected metal ions in homogeneously mixed state. It was found, that by simple mixing of metal ion containing solutions in the same chemical form would result in non-uniform composition crystal growth and usually with a wide size distribution. Also nitrates, chlorides and acetates should be avoided, because they can be insoluble in the solid resin form, and in that way crystal formation would take place before the thermal decomposition of the resin intermediate yielding powder that usually requires milling or grinding.

The formed resin is decomposed at moderate temperatures: 540 °C - 650 °C which was found to be beneficial in keeping the resulting particle sizes small.

The method was successfully adapted for thoria powder synthesis and -oxide doped thoria powder synthesis by Ganesan et al. and resulted in fine powders that could be processed into pellets with sintered to densities up to 99 % of theoretical density.53Thorium(IV) nitrate and yttrium(III)nitrate solutions were mixed in the required ratio and to the solution citric acid was added in a total metal ion-to-citric acid ratio of 1.0. Ethylene glycol was added in the same molar quantity as citric acid. The homogeneous solution was subsequently heated up to 90-95 °C where gel formation took place during 4-5 hours. Formed gel was dried in oven at 140-150 °C for 15 hours followed by calcination, pelletization and sintering at 1500 °C for 2 h with a 10 °C/min. heating and cooling rate.

3.3.3. Citrate-gel method This method was used for the synthesis of high temperature superconducting ceramic, YBCO by Schafer et al.54The use of citric acid has the advantage over other fuels in the ignition reaction that it allows for less violent reaction conditions, and usual laboratory glassware can be used.

The process consisted of the addition of nitrate solutions of the three metal ions to a citric acid solution kept at pH ~ 6. The heating of the solution at 75 °C gave a viscous liquid, containing polybasic chelates. Heating this material further at 85 °C gave an amorphous solid that was subjected to pyrolysis at 900 °C. The product was a crystalline powder.

Cosentino and Muccillo followed a citrate technique, to synthesize ThO2 – n mol% Y2O3 powders.55 To create a given composition of mixed oxide system, a given amount of thorium(IV)nitrate and yttrium(III)nitrate was mixed with citric acid and ethylene glycol (the latter two compound were added in a ratio of 60:40 wt.%) in a vessel kept at a 60 °C bath.

After mixing, the temperature was raised to 120 °C to eliminate NO2 while a brownish resin was formed in the vessel. This resin was subsequently calcined –by first heating to 400 °C for 6 h in air (residual carbon content remained high, 7-22 wt.%) followed by heating to 800 °C for 24 h under flowing oxygen (residual carbon content decreased to 0.3 wt. %).

3.3.4. Combustion processes These methods involve the mixing of a metal nitrate solution and glycine or another complexing compound, followed by the evaporation of the solvent by heating. The result is a highly viscous mass, containing metal ions in a complexed form, preventing precipitation. At this stage the material usually shows a good chemical homogeneity. The next step is ignition of the material, where glycine or other complexing agent serves as the fuel for the oxidization 20 by nitrate ions. Although the ignition gives a very fine powder, it can be extremely violent, leading to explosions therefore it can only conducted safely on small quantities.

Nanocrystalline thoria powders were obtained via glycine-nitrate combustion by Purohit et al.56The choice for glycine was due to the fact that as a zwitterion, is effectively complexing a lot of metal ions and prevents their precipitation hence maintaining a compositional homogeneity in the solution.

The experiment was conducted by mixing an aqueous solution ofthorium(IV)nitrate and glycine in two selected molar ratio. The clear solution was then dehydrated on a hot plate at a temperature of (80 ± 5) °C yielding a viscous liquid. In the next step, the hot plate was heated up to 200 °C, where auto-ignition occurred. The ignition resulted in the development of voluminous gases.

A similar method was applied by Purohit et al. to synthesize ultrafine ceria powders via the glycine-nitrate combustion.57 With the application of fuel deficient glycine-to nitrate ratio (0.3 in this case), they obtained fine powders with a surface area of 75 m2, while with stoichiometric or fuel rich compositions gave lower specific surface area powders (71 and 38 m2/g respectively).

The feasibility of citrate gel combustion synthesis of thoria was investigated by Chandramouli et al.58 Combustion synthesis makes use of the exothermic chemical reaction between a fuel (like urea or citric acid or other additive) and an oxidant (it can be oxygen from air) when an aqueous solution of citric acid and thorium(IV) nitrate salt was heated by using microwave energy (2450 MHz and an output of 600-700 W) or a hotplate.

Synthesis of thoria was also attempted by using urea or polyvinyl-alcohol (PVA) as a fuel.59 In general, the combustion reaction proved to be more violent compared to the citrate route, often resulting in fire.

3.4. Powder production methods via synthesis of nanoparticles

In the quest searching for methods to produce highly sinterable thoria powders, the synthetic routes developed by nanochemistry should not be overlooked. Monodisperse spherical metal oxide nanoparticles could be synthesized by using a reverse micelle-synthesis, whereby the reaction is literally confined to the volume of single micelles, which are fairly uniform in size and shape. An archetype of large number of variants on this theme was developed by Boutonnet et al. who applied a basic concept of restricting the chemical reaction to take place in small confinements constituted of small water pools separated from one other by an apolar solvent.60 The assumption is that each of such water pool (stabilized by surfactant molecules) hosts one single nucleus and protects it from aggregation and especially Ostwald ripening until the completion of the reaction. This method therefore provides a control over nucleation and growth, the separation of which is usually very difficult in any other manner. One of the reactants is introduced in one microemulsion, the other in an another microemulsion, using the same surfactant molecule, these two microemulsions are then poured into an apolar solvent which basically serves the purpose of a medium where the reverse micelles can perform free Brownian motion. Every now and then the micelles containing reactants collide and occasionally exchange the content of their water pool. Since the collision and opening-up of the surfactant layer requires energy, the transport of the

21 reactants is a slow process, while the reaction inside the water pool is comparatively fast. For 3 6 a [water] / [AOT ] in heptane microemulsion, the rate constant of comminution is kcom.≈ 10 - 107 dm3 * mol-1* s-1 as Fischer et al. determined.61 On this basis Capek estimated that only one out of a thousand of collisions between reversed micelles can lead to the exchange of their aqueous-phase content.62

Since the purpose of these techniques is to engineer the final morphology of the particles created, there is a need to study the structure of the reversed micelles themselves, serving as template. A small-angle neutron scattering study was conducted by Li et al. on (IV)-containing [water] / [AOT] microemulsion system (toluene was used as the apolar media).63 Experimental results show a slight contraction of the micelle sizes from 23 Å to 19 Å when the ZrOCl2 concentration is raised from 0.1 M to 1.0 M in the aqueous phase for a given water-to-oil ratio (Wo). Moreover, evidences for the existence of “two water populations in the pool” were found; an outer layer of water molecules, more strongly bound to the surfactant head groups, and a non-bound (bulk) region in centre of the water pool.)

As Holmberg has pointed out, there are many examples, where there is no or just poor correlation between the structure and size of the micelle and the particles’ shape created within.64 The control on the morphology is a function of other factors as well (the type of counter ion of the metal ion can have a profound impact on the crystallization). In the case of

CuO/ZrO2 nanocomposite synthesized by Vahidshad et al., using AOT as surfactant and cyclohexane as “oil” phase, the particles proved to be of spherical shape and had an average diameter 10-35 nm.65 Such a size range is at least ten times smaller than the average size of thorium(IV)oxalate particles prepared by the oxalate precipitation route. The successful synthesis of nanoparticles using zirconium(IV) suggests, that there is a room for study the feasibility in the case of thorium(IV) ions.

The homogeneous precipitation route combined with microemulsion-mediated method as applied by Elen et al. proved to be successful for the synthesis of spherical ZnO nanoparticles.66 The goal was to overcome the tendency of ZnO to grow in elongated shapes that are not beneficial for optical applications. To have a stable emulsion, the anionic head group of Na(AOT) (dioctyl sulfosuccinate sodium salt) was exchanged with Zn2+-ions. This way the modified surfactant acted as a metal source and also as a microemulsion stabilizer.

In a non-aqueous solvent based surfactant-assisted synthesis Hudry et al. reported the successful production of thorium and uranium oxide nanocrystals with sizes of 4.5 or 10.7 nm in diameter.67 Through modifying the synthesis circumstances, the authors managed to control the shape of nanoparticles (quantum dots, nanorods and branched nanocrystals).

Two different organic media were tested for the preparation of urania and thoria nanoparticles i) mixture of benzyl ether (solvent) and oleic acid and oleylamine (co- surfactants)4; ii) a quaternary system in which the oleylamine was replaced with an equimolar amount of trioctylamine and trioctylphosphine oxide5. It was also observed, that the presence of oleylamine prevents the formation of thoria even at temperatures as high as 350 °C.

3 AOT refers to the dioctyl sulfosuccinate anion, a commonly used surfactant compound. 4 The starting oleic acid-to-actinide and oleic acid-to-oleylamine ratios were fixed at 10 and 1, respectively. 5 In the second case, the starting oleic acid-to-actinide ratio was reduced to 2 and the trioctylamine and tryoctylphosphine oxide-to-actinide ratios were fixed at ten. 22 A cationic surfactant assisted reverse micellar route was used by Gupta et al. to produce 3+ 68 ThO2 nanorods activated by Sm -ions (a warm light emitting product). Although the ionic radius of Th4+ -ions (0.104 nm) is close to those of the trivalent rare-earth ions, substitution is not unlimited in composition ranges, because the charge imbalance causes oxygen vacancies, which further on causes steric constraints in the lattice. Former investigations (by Horlait et al.) on tetravalent hafnia, zirconia and ceria suggested the existence of a mechanism that involves the creation of oxygen vacancies (Vo..) to counterbalance the lack of some positive charges due to the incorporation of the trivalent lanthanides.69

A reverse micellar route using a cationic surfactant cetyl trimethyl ammonium bromide (CTAB) allowed Vaidya et al. to obtain thorium(IV)oxalate nanoparticles with an average diameter 20 ± 0.3 nm.70 The microemulsion used for the synthesis contained 16.76 % CTAB, 13.9 % n-butanol (co-surfactant), 59.29 % isooctane and 10.05 % aqueous phase. For the synthesis of the thorium(IV)oxalate precursor, two solutions, one containing thorium(IV)nitrate and another one containing ammonium oxalate in aqueous solution was mixed slowly and stirred for 15 hours. When doped product was prepared, the required amount of Sm3+-ions were added to the microemulsion of thorium nitrate. After stirring, the product was separated by centrifugation and washed with a 1:1 mixture of CHCl3 and methanol and dried at room temperature. Thorium(IV)oxalate was thermally decomposed at 500 °C for 6 h (decomposition temperature chosen on the basis of the DTA analysis).

Monodisperse uranium oxide particles (identified as UO2·nH2O) with well-controlled shapes and hierarchical structures of were produced by Bouala et al.71 who adapted a precipitation method elaborated for ceria mesoparticles by Wang et al.72They synthesized shape- controlled ceria nanoparticles by adding 1 mmol of Ce(NO3)3·6H2O to a 50 ml aqueous solution containing 2 mmol/dm3 poly(ethylene-glycol) (PEG6) (Ms = 20,000). The mixture was stirred for 30 minutes and 25 mmol urea (acting as a precipitating agent) was added, the solution transferred to a three-necked round-bottom flask to reflux at about 100 °C for 3 h.

The formation of CeOHCO3 product is supposed to take place via the following steps:

+ - (1) CO(NH2)2 + 3H2O → 2NH4 + 2OH + CO2 (3.28.)

2- + (2) CO2 + H2O → CO3 + 2 H (3.29.)

3+ - 2- (3) Ce + OH + CO3 → CeOHCO3 (3.30.)

This process adapted by Bouala et al. to the synthesis of U(IV) oxide particles resulted in spherical shapes of 120 nm in diameter and a narrow size distribution. Bouala et al. have applied a higher amount of urea in the reaction: 5 g of urea for 1 mmol of U4+.

According to the PXRD measurements, the samples were identified as UO2·nH2O. The primary building blocks of these spheres were found to be crystallites with an average diameter of 3 nm. It is supposed, that the secondary structures (or "superparticles" as the authors call them) are the results of the self-organization of the nano-crystallites. If the heating was conducted for periods exceeding 1.5 h, the spheres retained their original size but started to agglomerate into a third level of hierarchy particles larger than 1 µm.

6Flexible PEG chains contain a large number of oxygen atoms which tend to coordinate to the metal ions and modify the direction of further growth process. 23 Calcination of the uranium hydroxy carbonate powders at 1000 °C resulted in significant changes in the morphology, due to the visible start of (premature-) sintering, while at 700 °C the spheres retained their original shape (pseudomorphism). Consequently a 650 °C calcination temperature could be applied.

3.5. Precipitation of thorium(IV)oxalate via homogeneous precipitation route

The modification of precipitate morphology can be attempted also by using a different source of precipitant. Homogeneous precipitation method has been applied to treat liquid actinide- contaminated waste(by Germain and Pasquiou73, Gompper74), and to produce Pu(IV)oxalate precipitate, using diethyl oxalate reagent by Rao et al.75 and dimethyl oxalate by Abrahamson76. In the latter process 0.5 M (IV)nitrate aqueous solution was used with addition of to adjust the (+IV) of the plutonium ions. Solid dimethyl oxalate dissolved in water was added to the solution after 1 h of peroxide digestion of the plutonium(IV)nitratesolution. The reaction vessel was kept at 60 ± 0.5 °C for 1 h and mixed by bubbling Ar gas into it. Quantitative precipitation was reached after 1 h.

3.6. Precipitation of thorium(IV)oxalate in ionic liquid medium

Although the ultimate candidates for a highly sinterable thoria powder should be monodisperse and submicron-sized spherical particles that can be obtained via surfactant assisted methods, the throughput of such techniques is often niche. Other alternatives are also sought for to find better throughput reaction schemes.

An ionic liquid with a different anion, [(P4444 )2][oxalate] has been recently characterized by 3+ Quiroz-Guzman et al. who tested complexation properties of [P66614][HOx] IL with Nd - ions, and found that a blue solid precipitate is formed.77 The authors did not perform a characterization of the morphology of the powders. The presence of alkyl chains might alter the surface energies of forming crystallites resulting in largely different morphologies from oxalate precipitation conducted in aqueous media. Moreover, the relatively higher viscosity of the ionic liquid compared to aqueous solutions might also prevent the rapid growth of crystals, especially in the initial stages of mixing. Use of other solvents than water might also allow for avoiding thixotropic stages in the precipitation (which is the case with the oxalate precipitation route), which pose a real problem when effective stirring needs to be maintained throughout the reaction.78

Thus use of ionic liquid serve two purposes: i) as a source of oxalate ions, it shall be a precipitating agent; ii) as a viscous medium it is expected that it shall slow down the transport of ions towards nuclei, which might allow some control over the growth properties of thorium oxalate precipitate. Since the pseudomorphism between thorium(IV)oxalate hexahydrate and

ThO2derived from it is well-known, it is expected, that the sinterability of the thoria powder can be to some extent affected by the circumstances of the reaction between the anion of the ionic liquid and the Th4+-ions.

24 3.7. Synthesis routes for doped ThO2

In most wet chemical synthetic routes lanthanide-doped thoria were produced by the simple addition of solution of the selected lanthanide salt in the required molar proportion to an aqueous thorium(IV)nitrate-solution.

One of the most comprehensive knowledge of the different phases and phase- 79 transformations in ThO2- Ln2O3 systems was provided by Sibieude and Foex. According to the described ThO2- Gd2O3 phase diagram, at 1750 °C (applied for sintering of pellets in the present thesis work) the limit of Gd2O3 in ThO2 is around 30 mol%. Similar results were obtained by Keller et al.80

Exceptionally high density yttrium-doped thoria disks were prepared by Antony et al., who applied a modified version of the combustion techniques.81 Th(IV) and Y(III) ions were mixed in nitric acid aqueous solutions, added to a hot mixture of ethylene glycol and citric acid (the latter two compounds formed an ester in situ). Heating for a period of 1-2 h at 90 °C yielded a „syrupy liquid”. Further heating at 350 °C for 3-4 h turned the solution into a brown-colored, but transparent gel that was decomposed at 500 °C in air. The sample, at this stage had a black color from the carbon impurities, but densified to 99 % of TD, upon pressing a 180 MPa and heating to 1350 °C under Ar atmosphere for 20 h.

Yildiz et al.82 have investigated the compactibility and sintering behavior of the nanocrystalline (Th1-xCex)O2 powders prepared via co-precipitation reaction according to a previous process published by Yildiz83. Thoria and Ce-doped thoria powders were obtained from calcination of Th(OH)4 precipitated from aqueous solutions using ammonia gas as precipitating agent.

3.8. Influence of on lattice parameter of Th1-xLnxO2-x/2

A wet chemical route - oxalate (co)-precipitation - was used for the synthesis of various lanthanide-ion (Nd, Sm, Gd, Dy, Er, Yb) doped thoria powders by Horlait et al.84 The lattice parameters were determined using XRD. Factors affecting the unit cell size are best demonstrated in the case of Gd(III)-doping. Gd(III)-ions have a very similar ionic radii to that of Th(IV), (r =1.053 Å and 1.050 Å respectively), still at every composition the lattice unit shrinked. This phenomenon was explained as being due to the creation of oxygen vacancies and decrease in coordination number from 8 to 6. For a given doping rate the heavier lanthanides gave smaller lattice parameters. Expansion was only noticed in the case of Th1- xNdxO2-2/x and Th1-xSmxO2-2/x solid solutions.

Morphological evolution of oxalate samples during calcination was followed in situ by using a furnace inserted inside the SEM chamber. Thorium(IV)oxalate co-precipitated with the oxalates of Sm(III), Dy(III) and Gd(III) all were crystallized in monoclinic C2/m and appeared as squared platelets.

Lattice parameters of various lanthanide-ion doped thoria sintered pellets synthesized via a dry route were determined by Muta et al.85 Pellets were prepared by mixing and ball-milling the respective oxides, followed by cold pressing, sintering at 1600 °C during 72 h under 4%

H2-Ar gas mixture. After the first sintering, the pellets were crushed and re-sintered using Spark-plasma sintering. The following equation was found to provide a good approximation

25 of the lattice parameter as a function of additive concentration, oxygen defects and ionic radii:

∆r x 0.018969 y 0.55974 nm (3.31) wherea a1is.9445 the lattice parameter (in nm unit), x and y are the molar fraction of the dopant ion and oxygen defect, respectively, Δr expresses the difference in ionic radii of Th(IV) and the dopant ion (in nm units).

3.9. The effect of using of sintering aids

The use of sintering aids to allow for higher sintered densities at lower temperatures were known for a long time (Matzke).86 The mechanism by which a sintering aid additive enhance the sintering was found to be two-fold; i) speeding up diffusion (in the case of TiO2 doping caused a 250 times faster diffusion of uranium compared to undoped urania) ii) inhibition of grain growth.

To obtain high density thoria pellets at low temperatures Balakrishna et al. compared the efficiency of traditionally used magnesia (MgO) and the newly introduced application of 87 niobia (Nb2O5). MgO additive can be effectively used to enhance the sintering in both reducing and oxidizing (air) atmosphere at high temperatures like 1700 °C. Niobia additive proved to be effective in the case of sintering in air atmosphere at low temperatures – even at 1150 °C 96-97 % of TD were achieved. The explanation of the mechanism of effect of additives was formulated as the following. Sintering involves the diffusion of ions that have to maintain the charge balance, since macroscopic separation of electric charges would require energies that are not available at the sintering temperature. Therefore the rate controlling step is the diffusion of the slowest diffusing species (according to Matzke, in the case of thoria this is the Th(IV) ion).88 Ion mobility is enhanced by the presence of imperfections in the lattice, like point defects. A higher concentration of point defects can be created by heating, as the native point defect concentration evolves in accordance with equation (3.32).

⁄ (3.32) whereN stands for the total concentration of different defect types (vacant oxygen, vacant metal sites, oxygen and metal interstitials). Activated sintering did not take place in the case of the same heat treatment of similar samples under hydrogen atmosphere as reported by Balakrishna et al.89 In this later study the minimum required amount of niobia for an effective sintering at 1150 °C in air was found to be 0.25 mol%. When 0.125 mol% was applied, the densification went into completion only at higher temperature, at 1200 °C.

Apart from heating, the concentration of defects can also be modified by changing the oxygen partial pressure of the sintering atmosphere since point defects result also from exchange reactions between solid and the gas phases. Doping can be a third way of increasing the defect concentration in oxides. Pentavalent niobium(V) ions give rise to thorium vacancies or oxygen interstitials. Divalent , like Mg2+-ions in the presence of a reducing atmosphere on the contrary give rise to oxygen vacancies and thorium interstitials.

26 In a more recent study, Kutty et al. used dilatometry tests to compare the shrinkage behaviour of pure and 0.25 mol% niobia-doped thoria green compacts.90 The results showed a drastic, five-fold increase in the strain rate in the temperature region between 1300 °C and 1350 °C for the doped samples compared to the pure thoria pellets.

Ananthasivan et al. investigated the sinterability of oxalate-derived thoria doped with calcia or magnesia.91 The thoria samples were subjected to ultrasonication-enhanced de- agglomeration. The concentration of sintering aids was chosen to correspond to a final composition of 0.1, 0.25 and 0.5 mol% with respect to CaO or MgO. Calcined thoria powder was pressed at 200 MPa. The sintering temperature was 1200 °C, 1400 °C and 1600 °C for different samples. The sintered bodies had a density between 95.5 and 97 % T.D. In general calcia-doped thoria sintered better than magnesia doped thoria. The densities obtained at as low as 1200 °C sintering were above 95 % T.D. in the case of all samples.

27 4. Experimental techniques

Table IV.1.List of chemicals and their suppliers used for the synthesis.

Compound Supplier Compound Supplier

Th(NO ) 1.9 M aqueous 3 4 Solvay PEG 20000 VWR solution

Tetrabutylammonium Oxalic acid (anhydrous) VWR Sigma- hydroxide 40 m/m % Aldrich aqueous solution

Gd2(NO3)3 (99.9 % purity) Alfa Aesar NaOH VWR

Diethyl oxalate VWR Urea VWR

All the chemicals were used as obtained from the commercial suppliers, without further purification.

4.1. Synthetic techniques

4.1.1. Synthesis of thorium(IV)oxalate hexahydrate via oxalate precipitation route:

Thorium(IV)nitrate aqueous solution was used at a concentration of c ≈ 1 M, obtained from dilution of a weighted amount of liquid from the original batch. Typically 100 ml 1 M Th(NO3)4 stock solutions were used via the measurement of the mass of the liquid from the original batch from the supplier.

3 3 mLOT = 88.394 g, VLOT = m/ρ = 88.394 g / 1.755 g/cm = 50.4 cm

3 3 nTh(NO3)4 = c*V = 1.959 mol/dm * 0.050 dm = 0.099 mol

This amount of thorium(IV) nitrate solution was diluted to 100 ml in a calibrated volumetric flask. The liquid showed volume contraction upon mixing therefore the volume was adjusted after thorough mixing. Actual concentration of the stock solution was 0.987 mol/dm3.

79.997 g of the Th(NO3)4 stock solution was measured and poured into a 500 ml Erlenmeyer flask, which was immersed into a cooling bath kept at 10 °C with the use of an external chiller (Julabo). The solution was stirred with a 40 mm long Teflon coated stirring bar at high speed (it was constantly adapted during the course of the reaction to avoid it is bumping around.

The neck of the flask was covered with a slice of Parafilm to avoid contamination of the experimental set up. This precaution always must be kept in mind, as the solution was splashing quite intensively due to the stirring, especially, when the precipitation is already progressing.

The precipitating reagent oxalic acid was used in the form of a 0.75 M solution. The stock solution was prepared by dissolving 16.881 g oxalic acid in 200 ml distilled water, and further

28 diluting to 250 ml after transferring to a calibrated volumetric flask. Calculated volume (containing 1.05 –fold equivalent of the Th(IV) ions) of the oxalic acid was added to the cooled thorium(IV)nitrate solution (i.e.: direct strike) using a glass burette with a very narrow outlet. The reaction vessel used was a 500 ml Erlenmeyer flask. The oxalic acid was added to the stirred thorium(IV)nitrate solution drop wise (at an approx. 1 droplet/second rate). After the addition of the last drop, stirring was continued for an additional 15 min. period.

4+ + Th (aq.) + 2 (COOH)2(aq.) + 10 H2O→ Th[(COO)2]2·6H2O (s.) + 4 H3O (4.1.)

The aged precipitate was transferred onto a Buchner funnel mounted with a Whatman GF/F 1825-90 type glass fiber filter paper (with a pore size of 0.7 µm) and rinsed three times with 150 ml water, with a simultaneous mixing of the paste-like product with a spatula. Care was taken to avoid hurting the filter paper (very prone to form holes when wet). The last rinsing is followed by the collection of the paste from the Buchner funnel into 250 ml glass beaker containing 150 ml distilled water. The beaker was subsequently put into ultrasonic bath for 5 min. ultrasonic agitation. The suspension was filtered again after the ultrasonic treatment and rinsed with 150 ml distilled water at the end. Bulk water was removed from the product by pulling air through it using the vacuum pump for 30 min. giving a cake-like brittle material. Collected product was further dried in an oven at 70 °C overnight. Dried product, in most cases were composed of rather hard agglomerates that could be pulverized by crushing using a mortar and a pestle before calcination. Overall product yield varied between 85-90 %. The sticking of the powder to the reaction vessel walls and into the filtering paper accounted for the most important part of loss.

Since the study of the effect of calcination temperature/time, the effect of sintering circumstances as well as dilatometryrequired a lot of samples, multiple batches had to be produced. The precipitation was conducted at (10±2) °C using ice-bath (in the case of the first batch) or temperature-controlled chiller (in the case of subsequent batches).

4.1.2. Synthesis of thorium(IV)oxalate via homogeneous precipitation route

Under this method, thorium(IV)oxalate hexahydrate was precipitated using homogeneous precipitation, whereby the precipitating reagent is generated in situ, by acid-catalyzed thermal decomposition of diethyl-oxalate as per equation (4.2). The idea is that this way a low oxalic acid concentration can be maintained during the course of the precipitation reaction and the distribution of the reagent should be more homogeneous compared to mechanical mixing of the reagents, promising a finely divided and mono-disperse thorium(IV)oxalate powder as a product.

+ + CH3CH2OOC-COO-CH2-CH3 + 2 H3O → (COOH)2 + 2 CH3CH2OH + 2 H (4.2.)

Diethyl oxalate is only partially miscible with water at room temperature. However, the heating of the mixture leads to a single phase, between 75 °C – 80 °C the two phases become mutually soluble (water-diethyl oxalate has an upper critical solubility temperature).

60 ml (0.442 mol) diethyl oxalate was measured into a 500 ml round-bottom flask, and the 1

M Th(NO3)4 stock solution containing 0.056 mol Th(IV) was added to it at room temperature. To reduce the amount of Fe3+ and Mn2+- ion impurities (both identified as catalysts for nitric oxidation of oxalate ions) MilliQ water was used for the preparation of stock solution.

29 The reaction vessel was immersed into a heating bath that was progressively heated up to 60 °C. An Allihn-type condenser connected to the hoses of a closed-circuit Julabo chiller was put on top of the flask. The solution was stirred vigorously throughout the precipitation reaction. The mixture was progressively heated up to 61- 65 °C where it became a suspension with an opaque appearance. At 76 °C brownish color of evolving NO gas could be observed. This is the product of a by-reaction: nitrate ions and oxalic acid in strongly acidic medium enter into -reaction as per equations (4.3. - 4.4.).

- 2 NO3 + 3 (COOH)2 + 2 H3O + = 2 NO + 6 CO2 + 6 H2O (4.3.)

NO + 1/2 O2 → NO2 (4.4.)

The solution at high temperatures was easy to stir in the initial stages of the precipitation reaction. However, at later stages, when the reaction vessel was let to cool down and stirring was stopped, it was very difficult to re-start stirring, as the product sedimented into a dense, gummy material. Due to the formation of such dense and gummy material filtering could be performed only on the still hot suspension, using a P5 glass filter. Removal of adsorbed ions was effected by a thorough rinsing with 3* 100 ml distilled water.

The thorium(IV)oxalate hexahydrate product had a markedly different behavior from the product obtained via the oxalate precipitation route. The white, fluffy precipitate was easy to stir with spatula and did not form hard agglomerates. Moreover, the dry powder is rather free- flowing and less prone to withhold significant amounts of adsorbed water. Heat treatment of the precursor product was conducted according to the scheme followed under the oxalate precipitation route. Overall product yield was 88 % and 94 % in the case of two batches prepared.

4.1.3. Synthesis of thorium(IV) oxalate via precipitation in ionic liquid media

Another way to control the powder morphology besides keeping temperature low is to use a liquid medium, where the mass transport of reactants toward nucleation seeds is slowed down due to higher viscosity than in aqueous-media based reactions. To synthesize tetrabutylammonium-oxalate, 55 ml ethanolic solution of 6.391 g anhydrous oxalic acid and 100 g 40 m/m% aqueous solution of tetrabutylammonium-hydroxide was mixed in 1:2 molar ratio (equation 4.5.) and stirred for 2 h.

2 [P4444][OH] + (COOH)2 → [(P4444)2][Ox] + 2 H2O (4.5.)

Excess water and ethanol content of the viscous yellowish colored liquid was removed using Rotavap. Since the ionic liquid is sensitive to low pH (oxalic acid precipitates out below pH = 2), 30.223 g thorium(IV)nitrate stock solution(ρ = 1.755 g/cm3, c = 1.959 mol/dm3) was partially neutralized to pH ≈ 3 – 3.5 before addition to ionic liquid.

The thorium(IV)nitrate solution was added drop wise to the vigorously stirred and cooled ionic liquid. The temperature of the reaction vessel was kept at 10 °C. Precipitation was instantaneous as per equation (4.6)

4+ - + 2 [(N4444)2][(COO)2] + Th (aq.) + 4 NO3 + 6 H2O→ Th[(COO)2]2(s.).6H2O + 4 [N4444] (aq.) + 4 - NO3 (aq.) (4.6)

30 According to the expectations, the precipitate was finely dispersed in the ionic liquid matrix. In order to assure the second role of the ionic liquid to be effective (slowing down the mass transport towards the surface of the formed nuclei), ionic liquid was used in excess with respect to the quantity of oxalate consumed by the precipitation reaction (4.6).

The precipitate was collected after 30 min. digestion time by pouring the suspension onto a glass frit and by washing it with ethanol and distilled water, to remove excess ionic liquid, and tetrabutylammonium cations and nitrate anions. Product yield was only 57 % due to clogging of significant amount of the powder into the glass frit of the P5 glass filter.

4.1.4Coprecipitation

Gd-doped thoria powders were synthesized according to the oxalate precipitation route, as described under section 4.1.1. The said procedure was the only one which provided high density sintered thoria pellets, thus for the purposes of doped samples, the other methods were not used. The synthetic procedure was modified by adding calculated amount of 1 M stock solution of gadolinium(III) nitrate to the thorium(IV) nitrate solution before the precipitation reaction.

4.1.5 Calcination

Powder samples obtained from oxalate route and homogeneous precipitation route were calcined in a muffle furnace under air atmosphere using alumina crucibles. The following temperature program was applied for pure thorium(IV) oxalate, as well as Gd-doped thorium(IV) oxalate samples. First the temperature was raised from 20 °C during 6 h to 350 °C where the dwell time was 12 h (this isothermal stage was inserted to fully remove crystalline water from the thorium(IV)oxalate). Thereby the melting of thorium(IV)oxalate monohydrate could be avoided which is crucial for the preserving the powder morphology as emphasized by White et al.92. The temperature was further raised to 700 °C during 5 h and kept at that temperature for a dwell time of 1 h. The furnace was cooled back to room temperature during 2 h. After the decomposition, a snow-white powder was obtained. The calcined product in most cases contained hard agglomerates that had to be crushed mechanically by a mortar and a pestle before further processing into green pellets. In the case of the sample that contained the highest dopant concentration, the EGA-MS coupled to the TG- analysis indicated, that the decomposition was incomplete at 700 °C, thus an additional calcination at 850 °C for 1 h was applied to remove remaining traces of carbon impurities.

The powder obtained from precipitation in tetrabutylammonium oxalate could not be completely decomposed due to the formation of heat resistant product, thus the further processing of that batch was not attempted.

31 4.2. Characterization techniques

BET –surface area measurement Specific surface area of thoria powders obtained from thorium(IV)oxalate starting materials produced via wet chemical methods were measured using the BET nitrogen adsorption - method with a Micromeritics Tristar 3020 Surface area analyzer. Powder samples were degassed for 2-3 h at 180 °C under vacuum before the measurement.

Dilatometry Green pellets’ shrinkage behaviour was monitored by using a dilatometer (Netzsch Gerätebau GmbH, Germany). The dilatometry measurements were run in an alumina tube (see Figure 4.1.), using zirconia spacers to separate the pellet from the alumina push-rod and the furnace tube walls.

Figure 4.1. The “heart” of the experimental set-up used for the dilatometry measurements. The green pellet with the zirconia spacer disc, the push rod and the thermocouple can be seen inside the alumina tube.

TG Powders obtained via wet-chemical routes were subject to thermogravimetry analysis using an STA 449 F3 Jupiter Simultaneous TG-DSC instrument (Netzsch Gerätebau GmbH, Germany). Typical sample amount used was between 400-500 mg, which was loaded into an alumina crucible. A blank experiment was run under the same conditions as the main experiment to correct for the buoyancy effect.

EGA The chemical composition of volatile decomposition products evolved during the run of TG experiments were analysed using a QMS 403-C Aëolos (Netzsch Gerätebau GmbH, Germany) mass-spectrometer coupled to the furnace of the STA instrument. The evolved gas analysis curves were deconvoluted using Multiple peak fit (for datasets containing flat baselines) or Peak analyser (for datasets including non-flat baseline) option of Origin code.

Three species were selected to follow the decomposition process, H2O, CO and CO2. The EGA-MS results together with the TG results allowed selection of optimal calcination

32 temperatures (the mildest possible heat treatment reduces the risk of premature sintering to take place).

XRD X-ray diffractograms of powder samples and sintered pellets were taken using a Phillips X’Pert (Model: NUA 40049) X-ray diffractometer. The applied X-ray tube was “Empyrean CuLFF”-type and the current and tension used were 45 mA and 40 kV respectively. The used wavelength was 0.154059 nm, corresponding to the CuKα1-line. The beam was line-focused. The incident and diffracted beams were collimated by passing through Soller slits with 0.02 rad opening. Measurements were run by scanning a range between 2θ = 20° - 140° with a 2.122° step unit and a 62.2 s counting time per steps.

The detector module used was an X'Celerator scanning line detector used with a Ni-β-filter.

Thoria and thorium(IV)oxalate hexahydrate powder samples were mounted on a steel sample holder, using back-loading technique or dispersed in isopropanol to mount on a zero- background Si disc sample holder (in cases where the amount of sample occasionally was insufficient to sue the back-loading technique). The measurements were performed using Bragg-Brentano focusing geometry which allows for a higher speed of recording powder X- ray diffractograms, but provide slightly asymmetric line shapes for low-angle reflections.

The obtained X-ray diffractograms were evaluated by comparing with reported reference patterns available from ICDD database; for gadolinium(III)oxalate hydrate pattern No. 00-

020-0411, for thorium(IV) oxalate hexahydrate pattern No. 00-014-0816, for ThO2 pattern No.

00-042-1462 and for Gd2O3 reference pattern No. 00-042-1465 were used.

Crystallite sizes of the powder samples were determined using the Scherrer93 formula as interpreted by Langford and Wilson94 (equation 4.5).

(4.5)

Where d refers to the mean size of the crystallite domains present in the sample. K is a dimensionless shape factor, close to unity (in the present work a value of 0.9 was used),λ is the wavelength of the used monochromatic X-ray beam, β is the peak width, and θ is the Bragg-angle.

The peak width is calculated for each peak as the total peak area over the height of the peak. In order to correct for the peak broadening resulting from instrumental factors, peak width from a diffractogram recorded under the same instrument settings from a reference material

(LaB6) was subtracted from the peak width values of the samples. For the purpose of the crystallite size calculations the average of calculated crystallite sizes from peaks from 2θ = 27° to 75° were used.

33 Figure IV. 2. Back-loaded sample holder filled with powder sample (left side) and zero- background sample holder (used for samples, where only a small amount of sample was available (right side).

SEM Large-magnification images were taken using a Field Emission Scanning Electron Microscope, model: JEOL JSM-7100 F. The instrument is installed inside of a hot cell, requiring remote manipulation of the samples for insertion into the vacuum-chamber.

Powder samples were mounted on Al-cylinders by preparing a suspension in isopropanol and using 2-3 droplets of such suspension.

Sintered pellets were mechanically polished followed by a thermal etching (which basically consists of heating for 4 h at 1300 °C in a tube furnace to reveal grain etches). After cooling down, the pellets were coated with 5 nm Pt using a Polaron SC7640 sputter coater. Pellets were fixed onto the Al-stubs with paste that further helped in conducting the charges built up due to the electron bombardment.

(a) (b) Figure 4.3. The hot cell with the sample-loading rail of the electron microscope (4.3.(a)) the vacuum chamber with powder samples loaded into the chamber (4.3.(b)).

34 Figure 4.4.Polished pellet samples mounted on Al-stubs with silver paste before Pt-sputter coating.

OM Sintered pellets were embedded in a resin for the purpose of mechanical polishing and subsequently their microstructure was analysed with a Leica MEF4M optical microscope (Leica Microsystems GmbH, Austria). Images were taken from the edge, the centre and from a third point located at the half of the radius of the pellet with 5, 10, 20, 50 and 100-fold magnifications.

Sintering Sintering of green pellets was performed in either a Linn HT furnace (Linn Electronic Co. Germany) using a reducing atmosphere (necessitated by the use of Mo-heating elements), or a tube furnace (Carbolite) with an oxidizing atmosphere. Heating and cooling of the samples were conducted at a 2.5 °C/min. rate to minimize the thermal stress in the fragile green pellets (knowing the very low thermal conductivity of thoria). Pellets were heated to 1750 °C and kept at this temperature for 8 h. The temperature-distribution was not absolutely homogeneous, as the deposition of Mo-vapours (see images in Annex III. of as-sintered pellets), was always more emphasized on the parts more distant from the crucible’s plate.

Density measurements Densities of green and sintered pellets were determined by using a digital calliper to measure geometrical parameters. In the case of pellets where the geometrical density was found to be above 90 % TD (i.e. no open porosity is supposed to be left), the density was also determined using the Archimedes principle.

35 5. Results and discussion

5.1. Results of powder sample synthesis

The process parameters of the oxalate precipitation route were chosen on the basis of literature data (mechanical stirring, reaction vessel kept at 10 °C during precipitation, slow addition of 0.75 M oxalic acid solution to 1 M Th(NO3)4 stock solution). It was found, that the efficiency of stirring is impaired in later stages of the precipitation reaction, especially in the case of batches aiming at thoria powder more than 15 g. An upscaling of this process would necessitate the use of more powerful stirring devices than a conventional magnetic stirrer. Insufficient stirring results in local excesses of the reagent and also contributes to the agglomeration of precipitated particles. The need for slow addition of oxalic acid aqueous solution elongated the reaction time up to several hours.

Powder product obtained via the oxalate precipitation route is prone to form hard agglomerates, and would be difficult to use in remotely processed facilities as it is sticking to walls.

Homogeneous precipitation route provided free-flowing white fluffy powder that was easy to filter and remove from surfaces. Green pellets pressed from this powder were not as fragile as those pressed from the oxalate precipitation route derived powders. Moreover, quantitative precipitation reaction went into completion rather fast, within one hour.

The third tested route, using tetrabutylammonium oxalate ionic liquid as precipitating reagent yielded a product, that was prone to form agglomerates while wet, and highly charging while dry. The latter property made it difficult to manipulate the material, as it easily contaminated any plastic or glass tools brought in contact with it.

5.2. BET surface area measurement results

Results of the surface area measurements performed on powder samples calcined under air atmosphere are listed in Table 5.1. – 5.3.

Table 5.1. Specific surface area of oxalate precipitation route derived thoria powders as determined by the BET method.

36 Milling Process and time and Calcination temperature and Surface area Composition Sample ID precipitation frequency time (m2/g) parameters (h/Hz)

6.73 ± 0.02 Oxalate 350 °C / 12 h + ThR1003- precipitation route ThO2 - 6.63 ± 0.01 p3 700 °C / 8 h Tprecip. = 10 °C 6.06 ± 0.01

Oxalate 13.82 ± 0.03 350 °C / 12 h + ThR1004- precipitation route ThO2 - p1 700 °C / 1 h 13.35 ± 0.03 Tprecip. = 10 °C

Oxalate 350 °C / 12 h + ThR1010- precipitation route ThO2 - 8.78 ± 0.03 p3 700 °C / 1 h Tprecip. = 10 °C

Oxalate 350 °C / 12 h + ThR1012- precipitation route 1 h / 20 ThO2 7.93 ± 0.02 p3 Hz 700 °C / 1 h Tprecip. = 10 °C

Oxalate ThR1012- 350 °C / 12 h + ThO precipitation route - 7.64 ± 0.02 2 p4 Tprecip. = 10 °C 700 °C / 1 h

ThO2 ThR1014-p Oxalate - 350 °C / 12 h + 8.20 ± 0.03

37 precipitation route 700 °C / 1 h

Tprecip. = 10 °C 7.20 ± 0.03

Oxalate 44.88 ± 0.20 precipitation route 5 h / 30 350 °C / 12 h + ThO2 ThR1014-p Tprecip. = 10 °C Hz 700 °C / 1 h 45.33 ± 0.20

38 Table 5.2.Specific surface area of homogeneous-precipitation route thoria powders as determined by the BET method.

Milling Calcination Surface Precipitation time and Composition Sample ID temperature area temperature frequency and time (m2/g) (h/Hz)

- 14.00 ± 350 °C / 12 h + 0.03 Tprecip.= (65- ThO2 ThR1011-p3 75) °C 700 °C / 1 h 14.29 ± 0.03

- 10.60 ± 350 °C / 12 h + 0.03 ThO2 ThR1013-p3 Tprecip.= 65 °C 700 °C / 1 h 10.36 ± 0.04

Table 5.3.Specific surface area of oxalate precipitation-route Gd-doped thoria powders as determined by the BET method.

Calcination Gd O 2 3 Surface area Composition Sample ID temperature and concentration (m2/g) time (mol %)

350 °C / 12 h + ThO2 / Gd2O3 ThGd5R1005-p3 2.56 13.94 ± 0.03 700 °C / 1 h

350 °C / 12 h + ThO2 / Gd2O3 ThGd10R1006-p4 5.29 13.57 ± 0.04 700 °C / 1 h

350 °C / 12 h + ThO2 / Gd2O3 ThGd30R1007-p3 17.23 23.17 ± 0.08 700 °C / 1 h

350 °C / 12 h + 13.38 ± 0.03

ThO2 / Gd2O3 ThGd30R1007-p3 700 °C / 1 h + 17.23 9.25 ± 0.10 850 °C / 1 h

Remark: The specific surface area of sample ThGd30R1007-p3 containing the highest amount of Gd measured after calcination at 700 °C for 1 h proved to be significantly higher compared to other samples. The TG-analysis of the sample however confirmed, that the

39 decomposition was incomplete and a repeated calcination at 850 °C for 1 h was necessary, providing similar specific surface areas as the other samples.

Main observations that can be discerned on the basis of the results are the following. The specific surface area of the powder samples obtained from oxalate precipitation route fall in the region of 6.06 m2/g and 13.82 m2/g. The large difference shows the dependence of the powder properties on the efficiency of stirring during precipitation.Multiple batches were prepared under similar conditions using the oxalate precipitation route, the only difference between them being the batch sizes and the calcination time. The efficiency of stirring varied significantly depending on the size of batch because of the progressive densification of the reaction mixture.The used experimental set-up (magnetic stirrer and a 500 ml flat-bottom flask) was suitable for preparing thorium(IV) oxalate hexahydrate up to 30 g (0.059 mol). Larger batch sizes caused the stirring bar to bump around, and stirring was poor, thus the homogeneity of oxalic acid concentration could not be ascertained and agglomeration of the thorium(IV) oxalate particles took place.

Powder samples derived from thorium(IV)oxalate precipitated using the homogeneous precipitation route gave powders with specific surface areas between 10.36 m2/g and 14.29 m2/g. Although these values are close to those found for powders obtained using the oxalate precipitation route, the morphologies and size distributions thereof were substantially different as proven by SEM. Homogeneous precipitation route derived powders were composed of significantly larger particles (several μm in diameter) and formed agglomerates of several dozens of μm in diameter. The comparison suggests that these measured specific surface areas are also due to the porosity of the particles.

The specific surface area of Gd(III)–doped thoria powders obtained using the oxalate precipitation route seem to fall in a narrow region, within 13.37 m2/g and 13.94 m2/g. From the SEM observations, it was found, that in the case of higher dopant concentrations the particles in general have progressively larger average diameter. Similar specific areas of significantly different morphologies suggest again, that the specific surface areas measured are to a large extent due to an inherent porosity of these powders and not only to outer surfaces of the individual particles (Figure 5. 22. showing the porous surface of particles serves as another evidence for this finding).

5.3. Thermogravimetry analysis and evolved gas analysis

Thorium(IV)oxalate-hexahydrate and co-precipitated thorium(IV)oxalate-Gd(III)oxalate- hydrate powders were subject to thermogravimetry analysis. The samples were inserted in an alumina crucible and heated at a constant 2.5 °C/min. rate until 1000 °C with 1 h dwell time. The slow heating rate allowed the identification of several stages during the decomposition.

The gases evolved during the TGA runs were analysed using a mass spectrometer coupled to the chamber of the STA. The signals of three species, water, CO and CO2 were plotted as a function of the temperature. The results of the deconvoluted CO signals are to be found in Annex I.

40 TG curve of thorium(IV) oxalate obtained from the H2O signal in EGA of thorium(IV) oxalate obtained from the oxalate precipitation route oxalate precipitation route Th C O . 5.3 H O T = 45 °C ( 2 4)2 2 100 0,00020 T = 133 °C Heating rate: 2.5 °C/min. 90 Th(C O ) . 1.6 H O T = 160 °C 2 4 2 2 0,00015 Protective Ar flow: 20 ml/min. Th C O . 1.2 H O T = 247 °C ( 2 4)2 2 Purge: 88 ml/min. 80 0,00010 T = 233 °C 70 T = 321 °C 0,00005

Mass change / % 60 Ionic current / A

Heating rate: 2.5 °C/min. T = 700 °C ThO2 0,00000 50 Protective Ar flow: 20 ml/min. Purge: 88 ml/min. 0 200 400 600 800 1000 0 200 400 600 800 1000 T / °C T / °C

CO signal in EGA of thorium(IV) oxalate obtained from the 2 CO signal in EGA of thorium(IV) oxalate obtained from the oxalate precipitation route oxalate precipitation route Heating rate: 2.5 °C/min. 0,00006 T = 137 °C Protective Ar flow: 20 ml/min. 0,00125 Purge: 88 ml/min. T = 216 °C 0,00005 T = 369 °C T = 319 °C 0,00120 0,00004 Heating rate: 2.5 °C/min. Protective Ar flow: 20 ml/min. 0,00115 0,00003 Purge: 88 ml/min.

0,00002 T = 400 °C T = 318 °C 0,00110 T = 522 °C Ionic current / A

Ionic current / A 0,00001 T = 552 °C 0,00105

0,00000

0,00100 0 200 400 600 800 1000 0 200 400 600 800 1000 T / °C T / °C

Figure 5.1. TG-curve and deconvoluted water, CO and CO2 signal of thorium(IV)oxalate hydrate powder sample(ID ThR1010-p1) obtained from oxalate precipitation route

TGA curve of thorium(IV) oxalate hexahydrate powder obtained via the homogeneous precipitation route

T = 42 °C Th(C2O4) . 6 H O 100 2 2

Th C O . 2 H O T = 143 °C ( 2 4)2 2 Th C O . H O T = 243 °C ( 2 4)2 2 80

60 Mass change / %

T = 700 °C ThO2 Heating rate: 2.5 °C / min. Gas flow: Ar purge: 65 ml/min.

40 Ar protective flow: 56 ml/min. 0 200 400 600 800 1000 T / °C

41 H O signal in EGA of thorium(IV) oxalate obtained from the CO signal in EGA of thorium oxalate obtained from the 2 2 homogeneous precipitation route 0,00014 homogeneous precipitation route Heating rate: 2.5 °C/min. 0,000035 Heating rate: 2.5 °C/min. Protective Ar flow: 20 ml/min. Protective Ar flow: 20 ml/min. 0,00012 T = 115 °C Purge: 88 ml/min. 0,000030 Purge: 88 ml/min. T = 373 °C

0,00010 0,000025

0,00008 0,000020 T = 347 °C T = 220 °C 0,000015 0,00006 0,000010 T = 334 °C Ionic current / A 0,00004 T = 419 °C Ionic current / A 0,000005 T = 537 °C 0,00002 0,000000

0,00000 -0,000005 0 200 400 600 800 1000 0 100 200 300 400 500 600 700 800 T / °C T / °C

Figure 5.2. TG-curve and deconvoluted water and CO2 signal of thorium(IV)oxalate hydrate powder sample (ID: ThR1011-p1) obtained via homogeneous precipitation route

TG curve of 30 mol% Gd(III)-doped thorium oxalate hydrate powder 110

T = 42 °C Th Gd (C O ) .5.9 H O 100 0.95 0.05 2 4 1.975 2

90 Th Gd C O . 1.9 H O T = 142 °C 0.95 0.05( 2 4)1.975 2 Th Gd C O . 0.7 H O T = 231 °C 0.95 0.05( 2 4)1.975 2 80

70

Mass change / % 60 T = 700 °C 50 Heating rate: 2.5 °C/min. Th Gd O Protective Ar flow: 20 ml/min. 0.95 0.05 1.975 Purge: 88 ml/min. 40 0 200 400 600 800 1000 1200 T / °C

H2O signal in EGA of 5 mol% Gd(III)-doped CO2 signal in EGA of 5 mol% Gd(III)-doped thorium(IV) oxalate hydrate powder 0,00010 thorium(IV) oxalate hydrate powder CO2 signal in EGA of 5 mol% Gd(III)-doped Th(IV)oxalate hydrate powder Ion current Peak1 Peak2 Heating rate: 2.5 °C/min. 0,000016 Peak3 T = 375 °C Peak4 Peak5 PeakSum T = 111 °C Protective Ar flow: 20 ml/min. Purge: 88 ml/min. 0,000014 0,00008 0,000012 Heating rate: 2.5 °C/min. 0,000010 Protective Ar flow: 20 ml/min. Purge: 88 ml/min. 0,00006 0,000008 T = 338 °C T = 398 °C 0,000006 T = 216 °C Ion current / A Ion current / A 0,00004 0,000004 T = 301 °C T = 300 °C T = 549 °C 0,000002 T = 214 °C T = 380 °C 0,00002 0,000000 0 200 400 600 800 1000 0 100 200 300 400 500 600 700 800 T / °C T / °C

Figure 5.3. TG-curve and deconvoluted water and CO2 signal of 5 mol% Gd(III)-doped thorium(IV)oxalate hydrate powder sample (ID: ThGd5R1005-p1).

42 TG curve of 10 mol% Gd(III)-doped thorium(IV)oxalate Water signal in EGA of 10 mol% Gd(III)-doped thorium(IV) oxalate hydrate powder powder hydrate 105 Water signal in EGA of 10 mol% Gd(III)-doped Th(IV)oxalate hydrat Th Gd (C O ) . 6.11 H O e T = 40 °C 0.9 0.1 2 4 1.95 2 Peak1 100 0,00014 T = 123 °C Peak2 Peak3 Peak4 95 T = 115 °C PeakSum 0,00012 Heating rate: 2.5 °C/min. 90 T = 185 °C Th Gd (C O ) . 2.4 H O 0.9 0.1 2 4 1.95 2 Protective Ar flow: 20 ml/min. 85 Th Gd (C2O4) . 0.3 H O T = 247 °C 0.9 0.1 1.95 2 0,00010 Purge: 88 ml/min. 80

75 0,00008 T = 225 °C 70 Mass change / %

65 Ion current / A 0,00006 T = 306 °C 60 Heating rate: 2.5 °C/min. 0,00004 55 T = 700 °C T = 387 °C Protective Ar flow: 20 ml/min. 52.2 % 50 Th0.9Gd0.1O1.95 Purge: 88 ml/min. 0,00002 100 200 300 400 500 600 700 800 900 1000 0 200 400 600 800 1000 T / °C T / °C

Figure 5.4. TG-curve and deconvoluted water signal of 10 mol% Gd(III)-doped thorium(IV)oxalate hydrate powder sample (ID: ThGd10R1006-p1).

TGA curve of 30 mol% Gd(III)-doped thorium(IV) oxalate hydrate powder

Th Gd (C2O4) .5.8 H O 100 T = 42 °C 0.70 0.30 1.85 2

90 Th Gd C O .2.2 H O T = 148 °C 0.70 0.30( 2 4)1.85 2

T = 241 °C Th Gd (C2O4) .0.5 H O 80 0.70 0.30 1.85 2

70 Mass change / % 60 Heating rate: 2.5 °C/min. T = 800 °C 50 Protective Ar flow: 20 ml/min. Th Gd O Purge: 88 ml/min. 0.70 0.30 1.85 0 200 400 600 800 1000 T / °C

Water signal in EGA of 30 mol%Gd (III)-doped CO2 signal in EGA of 30 mol%Gd(III)-doped thorium(IV)oxalate hydrate sample thorium(IV) oxalate hydrate sample 2,20E-009 T = 116 °C Heating rate: 2.5 °C/min. 3,50E-010 2,00E-009 Heating rate: 2.5 °C/min. Protective Ar flow: 20 ml/min. Protective Ar flow: 20 ml/min. T = 376 °C 1,80E-009 Purge: 88 ml/min. 3,00E-010 Purge: 88 ml/min. 1,60E-009 2,50E-010 1,40E-009 T = 335 °C T = 187 °C 2,00E-010 1,20E-009 T = 415 °C 1,00E-009 T = 230 °C 1,50E-010 T = 302 °C 8,00E-010 1,00E-010 T = 245 °C Ion current / A T = 536 °C 6,00E-010 T = 294 °C Ion current / A 5,00E-011 4,00E-010 T = 371 °C T = 670 °C

2,00E-010 0,00E+000

0,00E+000 -5,00E-011 0 200 400 600 800 1000 0 200 400 600 800 1000 T / °C T / °C

Figure 5.5. TG-curve and deconvoluted water and CO2 signal of 30 mol% Gd(III)-doped thorium(IV)oxalate hydrate powder sample (ID: ThGd30R1007-p1).

The stoichiometry indicated on the right side of the TG-curves are only for indication of calculated elemental composition, and not referring to solid solutions. According to the XRD 43 measurements (see results later), the Gd-doped oxalate-derived oxide samples contain at least two phases.

Comparison of TG-curves of thorium(IV) oxalate and Gd(III)-doped thorium(IV) oxalate hexahydrate powders obtained via different routes

30 mol% Gd-doped Th[(COO) ] 100 2 2 10 mol% Gd-doped Th[(COO)2]2

5 mol% Gd-doped Th[(COO)2]2 Th[(COO) ] from oxalate precipitation route 90 2 2

Th[(COO)2]2 from IL route

80 77.8%

70 Mass change / % 60 Heating rate: 2.5 °C/min. 50 Protective Ar flow: 20 ml/min. Purge: 88 ml/min. 50.5% 0 200 400 600 800 1000 T / °C

Figure 5. 6. The comparison of the TG-curves of samples obtained from various synthesis routes.

Thermogravimetric analysis coupled with evolved-gas mass-spectrometry was chosen to select the lowest possible calcination temperature. The deconvolution of the evolved-gas signals allowed identification more accurately the individual steps of water loss, CO and CO2 evolution.

XRD-patterns of the precipitated material showed, that the raw product obtained via the oxalate route is thorium(IV)oxalate hexahydrate (See Figure A.II.1. in Annex II.). Slow heating rate allowed identification of several stages in the decomposition process (see Figures 5.1 and 5.2). Decomposition in the case of samples obtained via both oxalate- precipitation route and homogeneous precipitation route occurred in three consecutive steps. From the thorium(IV)oxalate hexahydrate starting compound, first the dihydrate form of the salt was obtained when reaching 160 °C and 143 °C respectively. In the second step, the monohydrate form of the salt was obtained around 247 °C and 243 °C respectively. The anhydrous form of the salt was in fact not formed, as the decomposition of the organic anion already started at temperatures, where crystalline water was still present in the sample. These dehydration and subsequent decomposition processes are need to be separated in time as much as possible. Although the anhydrous form is not formed as an intermediate, the melting of the material at slightly elevated temperatures was reported to be a real risk (being detrimental to the particle morphology), unless water content is completely removed. On the basis of plotted water and CO EGA-MS signals, 350 °C with 5 h dwell time was chosen with the intention to minimize the chance of melting before the decomposition of oxalate anions take place.

Results in the case of Gd(III)-doped samples indicate a very important difference from pure thorium(IV)oxalate hexahydrate samples, pointing out, that separation of dehydration and oxalate decomposition processes overlap more and more with a higher dopant concentration. The result is markedly different particle morphology. Evidence for this

44 conclusion can be found on Figures 5.3. - 5.5. showing that the complete release of water only took place at significantly higher temperatures (reaching even 371-380 °C for Gd-doped samples), and at the same time, the decomposition of organic anions occurs at progressively lower temperatures (even as low as 245 °C) compared to the case of undoped sample. Taking these findings into account, the higher degree of agglomeration and the visible widening of size distribution in case of calcined Gd(III)-doped powder samples (see SEM Figures V. 19-21.) can be explained. The consequence of the overlap of dehydration and decomposition processes is that a partial melting of the doped samples during calcination inevitably takes place, and the efficiency of the 350 °C / 5h step that helped to separate these stages in the case of pure thoria sample is reduced if not niche. If a phase transition affecting only one of the components co-precipitated takes place during the calcination, the homogeneity of elemental distribution of the sample is certainly affected as well.

As the TGA-curves and the coupled EGA-mass spectroscopy results both confirmed, the thermal decomposition of pure thorium(IV)oxalate samples went into completion below 600 °C, a safe 700 °C was chosen for the calcination of powder samples dedicated for subsequent pellet fabrication. The sufficiently higher temperature ascertained that carbon impurity (which affects sinterability of thoria adversely) is only left back in trace amounts.

This dehydration scheme is in accordance with the results of Joseph et al.27 Due to the difficulties in ensuring water-free conditions between TGA and XRD measurements, the X- ray diffractograms of the intermediate products were not recorded. Thus, the formation of thorium(IV) carbonate or thorium(IV)oxycarbonate (as claimed by Joseph et al.27and Dash et al.28) cannot be verified from these results.

As far as the decomposition process in the case of Gd(III)-doped samples is concerned, a marked difference found was the progressive raising with increasing Gd-dopant concentration of the temperature where the decomposition terminated. This result was expected on the basis of the results of Wendlandt et al.25 In the case of sample containing 30 mol% Gd-dopant, this temperature limit raised even up to 800 °C, accordingly the part of the powder sample used for pellet feed material was re-calcined at 850 °C for 1 h.

Thermal decomposition of the product precipitated in tetrabutylammonium oxalate ionic liquid didn’t go into completion according to the TGA curve (see Figure 5.6). The presence of a significant amount of tetrabutylammonium cations as impurity resulted in a heat-resistant intermediate material rendering this synthesis route inappropriate for the purpose of obtaining highly sinterable thoria powder.

5.4. X-ray diffraction results

X-ray diffraction technique was used for the identification of the crystal structure, the determination of Scherrer crystallite size and lattice parameter of powder samples as well as sintered pellets. The obtained X-ray diffractograms were evaluated by comparing with reported reference patterns available from ICDD database; for gadolinium(III)oxalate hydrate pattern No. 00-020-0411, for thorium(IV) oxalate hexahydrate pattern No. 00-014-0816, for

ThO2 pattern No. 00-042-1462 and for Gd2O3 reference pattern No. 00-042-1465 were used.

Besides the as-precipitated oxalate products, two series of pure thoria and Gd-doped thoria powder samples (calcined at two different temperatures) and sintered pellet samples

45 weresubject to measurements. The thorium(IV)oxalate precipitates corresponded with the ICDD reference pattern of Th(IV)oxalate hexahydrate-form (Card No. 014-0816).

Table 5.4.Lattice parameters of thoria powder samples.

Scherrer Synthetic Lattice Sample ID Composition crystallite size route parameter (Å) (nm)

oxalate

ThR1002-p1 ThO2 precipitation 5.597(1) 41 route

oxalate

ThR1004-p1 ThO2 precipitation 5.601(1) 85 route

oxalate

ThR1010-p1 ThO2 precipitation 5.5976(2) 163 route

oxalate

ThR1010-p3 ThO2 precipitation 5.5992(7) 59 route

homogeneous

ThR1011-p1 ThO2 precipitation 5.5979(2) 443 route

homogeneous

ThR1011-p3 ThO2 precipitation 5.6027(7) 71 route

homogeneous

ThR1011-p4 ThO2 precipitation 5.6016(9) 74 route

oxalate

ThR1012-p3 ThO2 precipitation 5.5973(3) 747 route

oxalate

ThR1012-p4 ThO2 precipitation 5.596(1) 54 route

oxalate

ThR1014-p3 ThO2 precipitation 5.598(1) 49 route

46 Table 5.5. La+ttice parameters and Scherrer-crystallite sizes of Gd(III)-doped thoria powder samples subject to calcination at 700 °C (and 850 °C in the case of 30 mol% Gd(III)-doped thorium(IV)oxalate).

Scherrer Number of Lattice Calcination Sample ID Composition crystallite phases parameter (Å) T/t (°C/h) size (nm) present

ThGd5R1005-p3 Th0.95Gd0.05O1.975 5.598(1) 84 1 700/1

ThGd5R1005-p4 Th0.95Gd0.05O1.975 5.597(1) 43 1 700/1

ThGd10R1006-p4 Th0.90Gd0.10O1.95 5.591(1) 84 2 700/1

700/1 ThGd30R1007-p3 Th0.90Gd0.10O1.95 5.5882(6) 62 2 + 850/1

Table 5.6. Lattice parameters of Gd(III)-doped thoria powder samples heated to 1000 °C during the TG-analysis.

Scherrer Number of Lattice Calcination Sample ID Composition crystallite phases parameter (Å) T/t (°C/h) size (nm) present

ThGd5R1005-p1 Th0.95Gd0.05O1.975 5.5975(2) 303 1 1000/1

ThGd10R1006-p1 Th0.90Gd0.10O1.95 5.5955(2) 219 2 1000/1

ThGd30R1007-p1 Th0.90Gd0.10O1.95 5.5965(5) 87 2 1000/1

In the case of samples where gadolinium(III)oxalate was co-precipitated with thorium(IV)oxalate, a second phase was observable for all dopant concentrations (see Figures A.II.2-4 in Annex II.), which corresponds with monoclinic gadolinium(III) oxalate hydrate (ICDD Reference pattern No. 020-0411). This finding indicates that co-precipitation does notnecessarily yield a product that is chemically homogeneous on a microscopic level, but instead, crystallites, rich in one of the compounds might coexist even in the moment of nucleation. In literature, there are evidences for such a phenomenon; oxyhydroxide nanoparticles synthesized via coprecipitation with lead showed the presence of inhomogeneity in the form of adsorbed layers of tracer on primarily precipitated carrier.95In tracer studies, it has been already observed earlier that coprecipitation occurs only in case the tracer ion and its counter ion are isomorphus to the carrier compound’s precipitate.96 This is probably not the case, as Gd(III) oxalate hydrate crystallizes in monoclinic form, while the crystal structure of Th(IV) oxalate hexahydrate is not described in in any of the available reference patterns (only the dihydrate form is specified as being orthorhombic, see ICDD card No. 00-018-1365).

47 The proper understanding of the processes underlying the exact mechanism of coprecipitation would require extended studies.

Powder samples calcined at 700 °C for 1 h (and at 850 °C for 1 h in the case of 30 mol% Gd- doped samples) were measured using backloading technique, while samples calcined at 1000 °C for 1 h were mounted on a zero-background sample holder Si disc because the available sample amount was insufficient for backloading. Pellets sintered at 1750 °C for 8 h were polished before recording the X-ray diffractograms to obtain a smooth surface.

XRD-patterns of ThO and Th Gd O powders calcined at 700 °C 2 1-x x 2-x/2 6000 ThO 2 Th Gd O 0.95 0.05 1.975 Th Gd O 0.90 0.10 1.95 220 Th Gd O 0.70 0.30 1.85 4000 ThO2 311

ThO2

Counts Gd O 2 3 2000 222 Gd O Gd O 2 3 2 3 ThO2

0 30 35 40 45 50 55 60 65 2

Figure 5.7. Comparison of XRD patterns of pure and Gd(III)-doped thoria powder samples calcined at 700 °C.

XRD-patterns of ThO and Th Gd O powders calcined at 1000 °C 2 1-x x 2-x/2 ThO 2 Th Gd O 0.95 0.05 1.975 Th Gd O 0.90 0.10 1.95 220 Th Gd O ThO 4000 0.70 0.30 1.85 2 311

ThO2

Counts 2000 Gd O Gd O 2 3 2 3 Gd O 2 3 222

ThO2

0

25 30 35 40 45 50 55 60 65 2

48 Figure 5.8. Comparison of XRD patterns of pure and Gd(III)-doped thoria powder samples calcined at 1000 °C.

XRD-patterns of ThO and Th Gd O pellets sintered at 1750 °C 2 1-x x 2-2/x

ThO 5000 2 222 Th Gd O ThO 0.95 0.05 1.975 2 Th Gd O 4000 0.90 0.10 1.95 Th Gd O 0.70 0.30 1.85

3000 Counts 2000

1000

0 56 57 58 59 60 61 62 2

Figure 5.9. Comparison of XRD patterns of pure and Gd(III)-doped thoria pellet samples sintered at 1750 °C.

Counts 4007_ThGd5R1005p1

ThO2 100,00 2 4 % 2 2 4 300 5 1 1 1 5

200

100

0 75 80 85 90 95 Position [°2Theta]

Counts

4000

2000

0 30 40 50 60 70 80 90 100 110 120 130

Figure 5.10. Th0.95Gd0.05O1.975 powdersample: Enlargement of the region in the proximity of peak 422, indicating, that a second phase (if present) is below the limit of detection.

49 Counts 4004_ThGd10R1006-p1 ThO2 100,0 % 111; Th O2 3000

2000 200; Th O2

1000 Gd2O3 Gd2O3 Gd2O3

0 30 35 Position [°2Theta]

Counts

3000 2000 1000 0 30 40 50 60 70 80 90 100 110 120 130

Figure 5.11. Th0.90Gd0.10O1.95 powdersample: Enlargement of the region in the proximity of peak 200, indicating, that a second phase is present.

Calcination at 700 °C for 1 h as well as at 1000 °C for 1 h were insufficient to obtain a solid solution in the case of Gd(III)-doped thoria samples containing 10 and 30 mol% dopant (see Figures 5.7, 5.8, 5.11.). In the case of powder samples containing 5 mol% Gd(III) as dopant, the second, gadolinia-rich phase is below the detection limit (Figure 5.10.). An important effect of the calcination temperature on the crystallite sizes can be discerned from Tables 5.4- 5.6. Higher calcination temperatures (1000 °C) resulted in a significant increase of crystallite sizes in the case of doped, and in the case of most pure thoria powders.

Table 5.7.Lattice parameters of thoria pellet samples sintered at under various circumstances.

Lattice Number of Sintering T/t Sample ID Composition parameter (Å) phases present (°C/h)

ThR1001-S36-A ThO2 5.59755(4) 1 1750/8

ThR1002-S32-A ThO2 5.59801(4) 1 1750/8 (under air)

ThR1002-S36-B ThO2 5.59782(2) 1 1750/8

ThR1003-S36-A ThO2 5.59797(2) 1 1750/8

ThR1010-D-A ThO2 5.59776(6) 1 1625/1

ThR1012-S36-B ThO2 5.59788(3) 1 1750/8

Comment: Samples were sintered under reducing atmosphere, with the exception of ThR1002-S32-A pellet.

50 Table 5.8. Lattice parameters of Gd-doped thoria pellet samples sintered at 1750 °C under reducing atmosphere in Linn-furnace during 8 h dwell time.

Lattice Number of Sintering Sample ID Composition parameter (Å) phases present T/t (°C/h) ThGd5R1005- Th Gd O 5.59408(3) 1 1750/8 S36-B 0.95 0.05 1.975 ThGd10R1006-C Th0.90Gd0.10O1.95 5.58822(3) 1 1750/8 ThGd30R1007-A Th0.70Gd0.30O1.85 5.56299(4) 1 1750/8

Counts 4003_1 ThO2 100,0 %

6000 3 3 1 3 3 4 2 2 2 4 4000 0 2 4 5 1 1 1 5

2000

0 75 80 85 90 95 Position [°2Theta]

Counts 40000 30000 20000 10000 0 30 40 50 60 70 80 90 100 110 120 130

Figure 5.12.Th0.95Gd0.05O1.975pellet sample: Enlargement of the region in the proximity of peak 422, indicating, that only a single cubic phase is present.

Counts 4049_ThGd30R1007-A 6000 ThO2 100,0 % 3 3 1 3 3

4000 4 2 0 2 4 4 2 2 2 4 5 1 1 1 5 4 0 0 0 4

2000

0 70 80 90 100 Position [°2Theta]

Counts 40000 30000 20000 10000 0 30 40 50 60 70 80 90 100 110 120 130

Figure 5.13.Th0.70Gd0.30O1.85pelletsample: Enlargement of the region in the proximity of peak 422, indicating, that only a single cubic phase is present.

51 Lattice parameter vs. Gd-dopant concentration in thoria powder samples

5,60

o 5,59

Å

5,58

5,57

Lattice parameter / A Powder samples calcined in air (present study) 1000 °C/1 h Powder samples calcined in air (present study) 700 °C/1 h 5,56 Horlait et al. powder samples calcined in air 1000 °C/1 h Muta et al. (Pellets presintered 1600 °C/72 h + SPS 1600 °C/40 min. Pellets sintered at 1750 °C/8h in red. atm. (present study) 0 5 10 15 20 25 30 Gd dopant concentration x in Th Gd O 1-x x 2-x/2

Figure 5.14. Plots of lattice parameter vs. Gd-dopant concentration of powder and pellet samples compared with literature data (Horlait et al.69 and Muta et al.85).

In the case of sintered pellets, for all the dopant concentrations the solid solution formation is ascertained by the presence of one single cubic phase (Figures 5.12-13.). All these results are in accordance with the thoria-gadolinia phase diagrams constructed by Keller et al.80The progressive solid solution formation is also supported by the comparison of lattice parameter vs. Gd-dopant concentration plots (Figure 5.14.). The fact, that lattice parameters determined for samples calcined at 700 °C are lower than those determined for samples calcined at 1000 °C is in contradiction with expectations. It can nontheless be explained by the uncertainity of peak positions caused by the peak broadening effect of small particle size. Lattice parameters of Gd-doped powder samples clearly indicate, that the solid solution formation cannot go into completion at 1000 °C for 10 and 30 mol% dopant concentrations, as claimed by Horlait et al.69 who used similar synthesis route and calcination program as in the present study. The lattice parameters of the sintered pellets in the case of 5 and 10 mol% Gd-dopant concentration are similar to the values reported by Muta et al.85 and Horlait et al.69 but in the case of 30 mol % Gd-dopant concentration, the lattice parameter found to be significantly lower, than that determined by Horlait et al.69

The product obtained via precipitation in ionic liquid media proved to be amorphous, as witnessed by the very broad peaks on the recorded diffractogram (see Figure A.II.5). The total absence of the thorium(IV)oxalate hexahydrate pattern indicates, that the precipitate is not simply containing impurities of the ionic liquid, but instead, the organic cations are constituent part of the precipitated material, the exact composition of which is unknown. As the powder could not be purified and calcined, further processing of it into green compacts was not attempted.

52 5.5. Scanning-electron microscopy

Sharpest images on both pure and Gd-doped thoria and thorium(IV)oxalate powder samples could be taken while using low accelerating voltages (typically 2 – 7 kV) and low probe currents and the smallest aperture of the instrument. Below there is a collection of high- resolution SEM images of pure thoria powder sample.

Figure 5.15.Upper row: thorium(IV)oxalate hexahydrate particles obtained via the oxalate precipitation route (ThR1003-p2). Lower row: ThO2 particles obtained via oxalate precipitation route and calcined at 700 °C / 1 h (ThR1003-p3)

53 Figure 5.16.Upper row: thorium(IV)oxalate hexahydrate powder sample (ThR1014-p2),

Middle row: ThO2 obtained from oxalate precipitation route, and calcined at 700 °C / 1h

(ThR1014-p4), Lower row: Milled thoria powder (5 h at 30 Hz, 10 mm ZrO2 milling ball) obtained from oxalate precipitation route (ID: ThR1014-p3).

Thoria particles obtained via the oxalate precipitation route have squared-platelet morphology. The platelets themselves are not fully densified and they retain a high degree of porosity. The squared platelets are stacked on top of each other to form cubic secondary particles with a typical size of 200-400 nm in diameter. These secondary, cubic particles in turn are highly prone to form larger agglomerates, with diameters ranging from 1 μm to several dozens of μm.

The comparison of the morphology of precursor thorium(IV)oxalate-hexahydrate and the thoria powder obtained through calcination shows the well-known pseudomorphism of the oxalate form.

A SEM study on the effect of milling on powder morphology of thoria obtained via the oxalate route revealed that the applied milling (30 Hz for 5 h using 10 mm ZrO2 milling balls) of the porous platelet-structures result in a baked material instead of a simple breaking of the platelets. The originally existing squared-platelet morphology is completely lost, and fragments aggregated into large particles of several dozens of μm in diameter.

SEM study on the particle morphology of the thoria powder obtained via homogeneous precipitation route

54 Figure 5.17.Upper row: thorium(IV)oxalate hexahydrate powder obtained via homogeneous precipitation route (ThR1011-p2). Middle row: thorium(IV)oxalate hexahydrate powder obtained via homogeneous precipitation route (ThR1011-p2).Lower row: ThO2 powder particles derived from homogeneous precipitation route (ThR1013-p3).

SEM images of Gd-doped thoria and thorium(IV)oxalate powder samples

55 Figure 5.18.Upper row: 5 mol% Gd-doped thorium(IV)oxalate hexahydrate powder sample (ThGd5R1005-p2). Lower row: 5 mol% Gd(III)-doped ThO2 obtained from oxalate precipitation route, and calcined at 700 °C / 1 h (ThGd5R1005-p4).

One of the marked effect of the presence of Gd(III), as a dopant is that the secondary particles composed of platelets have rounded edges, as opposed to the rectangular features of pure thoria powders. Moreover, the particle sizes are on average 3-4 times larger than those of pure thoria powder particles prepared under similar conditions via the oxalate route.

56 Figure 5.19.Upper row: 10 mol% Gd(III)-doped thorium(IV)oxalate hexahydrate powder sample (ThGd10R1005-p2). Lower row: 10 mol% Gd-doped ThO2 obtained from oxalate precipitation route, and calcined at 700 °C / 1h (ThGd10R1006-p4).

Figure 5.20.Upper row: 30 mol% Gd-doped thorium(IV)oxalate hexahydrate powder sample

(ThGd30R1007-p2). Lower row: 30 mol% Gd-doped ThO2 obtained from oxalate precipitation route, and calcined at 700 °C / 1h followed by a second calcination at 850 °C / 1h (ThGd30R1007-p3)

The particles of the Gd-doped thoria powder samples are composed of platelets, which are stacked on top of one other. If we compare the Gd-doped thoria particles’ morphology, a clear trend can be found. The higher the Gd-dopant concentration, the wider is the size distribution, the higher the degree of agglomeration and the larger the average particle size.

The image in the right bottom corner of Figure 5.20shows the presence of pores on the surface of the platelet. An enlargement of the porous feature can be seen on the high magnification Figure 5.21.

57 Figure 5.21. Porous surface of the 30 mol% Gd-doped thoria particles obtained via oxalate precipitation route.

Powder samples obtained via precipitation using ionic liquid media:

Figure 5.22.Upper row: thorium(IV)oxalate-hexahydrate powder sample (ThR1015-p2) obtained via precipitation in tetrabutylammonium oxalate ionic liquid. Lower row: amorphous material, containing ThO2 and heat-resistant residual impurity from the ionic liquid. The sample was calcined at 1000 °C / 1h (ThR1015-p1).

The particles in the upper row images show a narrow size distribution, and have a rather rounded shape (tempting to state that they are spherical, but not all of the images ascertain, that we really see spheres and not just rather flat circles), quite different from the typical squared platelet morphology. If thermal decomposition would go into completion such morphology would be ideal for obtaining high sintered densities at moderate sintering temperatures. The presence of impurities from the ionic liquid is ascertained by the incomplete decomposition, as witnessed in the TG-analysis.

58 Microstructure characterization with SEM

Characterization of the powder product obtained via the oxalate precipitation route proved, that the selected reaction circumstances (direct strike, 10 °C reaction temperature, slow addition of oxalic acid solution) resulted in a fine thorium(IV)oxalate hexahydrate powder which is composed of squared platelets (Figure 5.15.). The squared platelets have a fairly narrow size distribution, falling between 300-500 nm in diameter. The morphological similarity of oxalate precursor and derived oxide products (pseudomorphism) was found back in the case of every sample produced via oxalate precipitation route as well as those produced via homogeneous precipitation route (see Figures 5.15- 5.20.).

The Gd-doped powder samples show a progressive growth of average particle size with higher dopant concentrations as well as a higher degree of agglomeration and loss of regularity in shapes. While pure thoria and its precursor oxalate are composed of squared platelets stacked on each other to form cubic-shaped secondary particles. Powder samples containing 5 mol% Gd-dopant are composed of cylinders with a diameter of 1 μm on average. The cylinders in turn are again composed of circular platelets stacked on each other, with visible void places left between them. Powders containing 10 and 30 mol% Gd as dopant are made of even larger cylinders that aggregate into tertiary, irregular structures of several μm in diameter. The cylinders in these cases are also composed of platelet structures, which are themselves porous (see Figure 5.21.) and contain significant amount of void spaces.

The progressive growth of agglomerate sizes and the widening of particle size distribution are factors usually considered as unfavourable for the sinterability of the powder material, and therefore it should be avoided as much as possible. In the present case, as discussed above, this is an inevitable result of the thermal decomposition behaviour as modified by the presence of gadolinium(III) oxalate hydrate in the thorium(IV)oxalate hexahydrate matrix.

As the presence of agglomerates in green pellets results in heterogeneous sintering, the effect of milling was investigated on pure thoria powder with the aim of crushing only the agglomerates (but leaving the platelets intact). The applied method (5 h 30 Hz with a 10 mm

ZrO2 milling ball) resulted however in even larger particles than the original agglomerates (see the comparison in Figure 5.16.). According to the BET surface area measurement, the milled powder had the highest specific surface area of all samples analysed. This finding can only be explained if the large particles retained a very high degree of internal porosity.

Powder samples obtained from the homogeneous precipitation route behaved macroscopically as a very free-flowing material, basically leaving no traces of contamination on glass or paper surfaces if poured off. Free-flowing property of materials intended in remote-handling fabrication facilities is usually a necessity. Surprisingly, this free-flowing product is composed of rather large squared platelets grown into each other forming hard agglomerates of several dozens of μm in diameter (see Figure 5.17. upper and lower rows). Besides the rectangular forms, several ill-defined, highly porous particles (Figure 5.17. middle row) were also noticed, but the reason for the presence of two largely distinct morphologies within the same batch is unknown.

This free-flowing macroscopic behaviour of rectangular shaped-particles could be explained in the light of the particle size; the particles are probably larger than the average roughness of the surfaces, and therefore they simply cannot get stuck into them. 59 Powder samples precipitated using ionic liquid media were composed of spherical/rounded particles with an average diameter of 1 μm (Figure 5.22. upper row). The particles formed soft agglomerates, of several dozens of μm in diameter. The as-precipitated particles’ morphology would be ideal for the purposes of sintering, as it is very easy to obtain highly packed green bodies of spherical particles as opposed to rectangular ones. Unfortunately, calcination even up to 1000 °C was insufficient to fully decompose the tetrabutylammonium cations retained by the precursor. Moreover, as visible in Figure 5.23, (lower row), the original morphology was completely lost during the heat treatment, and large, shapeless particles were left back.

Energy dispersive spectrometry

Figure 5.23.SEM image and atomic mapping by EDS analysis on pellet with a composition

Th0.9Gd0.1O1.95.

Energy dispersive spectrometry using a Bruker Quantax XFlash 6/100 detector was coupled to SEM (JEOL 7100) to analyse the chemical composition of sintered pellet samples. On the basis of the X-ray diffractogram recorded from pellets it is expected, that a mapping of the sample should show a high degree of homogeneity. In fact, the higher sensitivity of this analytical method compared to XRD allowed to show, that Gd2O3-rich regions can be found in a fairly homogeneous matrix, even in the case of only 10 mol% Gd-dopant concentration (see bright green spots in bottom-left image on Figure 5.23.). This finding (together with X- ray patterns of calcined powder samples) again supports, that simple co-precipitation does not directly allow for obtaining a powder sample that is homogeneous on an atomic scale, for multiple reasons. One of these being the different crystal structure of precursor oxalates of Th(IV) and Gd(III), leading to nucleation process whereby crystallites of non-uniform

60 composition are formed from the supersaturated solutions. Another reason is that described in the thermogravimetry section above.

5.6. Dilatometry measurements

Seven green pellet samples were subject to dilatometry measurements. Among these were green pellets that pressed at 340 MPa using Specac Atlas 8T hydraulic uniaxial press from feed powder obtained from oxalate precipitation route. Sintering behaviour of green pellets pressed from (i) pure thoria, (ii) 5 mol% Gd-doped thoria powder (a duplicate of samples), (iii) thoria powder containing 0.65 mol% Al2O3 sintering aid, (iv) thoria powder dried at 200 °C for 8 h followed by immediate pressing and sintering, (v) thoria powder obtained from homogeneous precipitation route and (vi) from milled (20 Hz, 30 min.) oxalate-route derived powder were followed. The samples were heated at a constant 5 °C/min. rate until 1625 °C with 1 h dwell time under air atmosphere in the case of the first five samples and under reducing atmosphere in the case of the last two samples. A blank experiment was run under the same parameters to account for the heat expansion of the push-rod and the zirconia spacers.

Dilatometry curves of pure thoria and Gd- doped thoria pellets

0,00

-0,02

-0,04

) -0,06 o -0,08 Heating rate: 5 °C/min. L/L (

d -0,10 ThO2 pellet from oxalate route 5 mol% Gd-doped ThO2 pellet from oxalate route -0,12 ThO2 pellet from oxalate route with 0.65 mol% Al2O3 5 mol% Gd-doped ThO2 pellet from oxalate route with 0.65 mol% Al2O3 ThO pellet from predried oxalate route-derived powder -0,14 2 ThO2 pellet from homogeneous prec. route ThO pellet from milled, oxalate route-derived powder -0,16 2 200 400 600 800 1000 1200 1400 1600 T / °C

Figure 5.24. Comparison of dilatometry curves of green pellets pressed from pure thoria, 5 mol% Gd-doped thoria and thoria containing Al2O3 sintering aid. The arrow in red indicates the onset of the solid solution formation in the case of Gd-doped thoria pellets.

61 Dilatometry curves of pure thoria and Gd- doped thoria pellets )

o 0,00

L/L Heating rate: 5 °C/min. ( d

ThO2 pellet from oxalate route

5 mol% Gd-doped ThO2 pellet from oxalate route

ThO2 pellet from oxalate route with 0.65 mol% Al2O3

5 mol% Gd-doped ThO2 pellet from oxalate routewith 0.65 mol% Al2O3

ThO2 pellet from predried oxalate route-derived powder

ThO2 pellet from homogeneous prec. route

ThO2 pellet from milled, oxalate route-derived powder

100 150 200 250 300 T / °C

Figure 5.25. Comparison of dilatometry curves of green pellets pressed from pure thoria, 5 mol% Gd-doped thoria and thoria containing Al2O3 sintering aid. Zoom into the temperature range of 50 °C - 350 °C. The arrow in red indicates the sudden expansion of a pellet caused by release of entrapped moisture (due to phase transition at 100 °C). The arrow in black indicates a different type of expansion, occurring above 200 °C.

Dilatometry curves of pure thoria and Gd- doped thoria pellets

0,00 ) o

L/L Heating rate: 5 °C/min. ( d

-0,02 ThO2 pellet from oxalate route

5 mol% Gd-doped ThO2 pellet from oxalate route

ThO2 pellet from oxalate route with 0.65 mol% Al2O3

5 mol% Gd-doped ThO2 pellet from oxalate routewith 0.65 mol% Al2O3

ThO2 pellet from predried oxalate route-derived powder

ThO2 pellet from homogeneous prec. route ThO pellet from milled, oxalate route-derived powder -0,04 2 550 600 650 700 750 800 850 900 T / °C

Figure 5.26. Comparison of dilatometry curves of green pellets pressed from pure thoria, 5 mol% Gd-doped thoria and thoria containing Al2O3 sintering aid. Zoom into the temperature range of 600 °C - 950 °C.

62 Table 5.9. Density values of the pellets sintered in the dilatometer (heating scheme: 5 °C/min. heating rate till 1625 °C with 1 h dwell time).

Geometr Geometrical Metal T.D. ical Geometrical Gd O density Sample ID 2 3 Gd fraction x - density density (sintered) Synthetic (g cm (sintered) route (mol in (Th 3 (green) -3 (wt%) 1- ) (g cm ) %) Gd )O -3 (%T.D.) x x 2-x/2 (g cm ) ThGd5R1005-A oxalate route 3.49 3.03 0.05 9.84 5.70 8.34 84.70 oxalate route + mechanical ThGd5Al0.65R100 3.49 3.03 0.05 9.84 5.47 8.86 89.96 mixing with 5-C sintering aid (Al2O3)

ThR1010-A 0.00 0.00 0.00 10.00 5.62 8.51 85.11 oxalate route oxalate route + mechanical ThAl0.65R1010-B 0.00 0.00 0.00 10.00 5.51 8.34 83.36 mixing with sintering aid (Al2O3) oxalate route pellet pressed ThR1010-C 0.00 0.00 0.00 10.00 5.19 7.91 79.12 from predried powder homogeneous ThR1011-A 0.00 0.00 0.00 10.00 5.50 7.79 77.89 precipitation route oxalate route + 0.00 0.00 0.00 10.00 85.06 ThR1012-A 5.75 8.51 milling

63 All the examined samples showed incomplete densification, which can be attributed to the short dwell time and the moderate temperature applied.

Shrinkage behaviour of green pellets compacted using a uniaxial hydraulic press at pressure of 350 MPa was studied in a dilatometer containing an alumina tube. Heating of samples were conducted until 1625 °C at a constant 5 °C/ min. rate and with 1 h dwell time.

Pellets composed of pure ThO2 followed rather similar densification behaviour (sintering started already at approximately 725 °C), while in the case of Gd-doped samples a marked change in the shrinkage is apparent at tempaeratures above 1100 °C indicating the solid solution formation (Figure 5.24.).

Pure thoria pellet pressed from powder obtained from oxalate precipitation route showed a sudden change around 100 °C (note, that the temperature values shown by the thermocouple are less accurate in such low temperatures). Since the pellet shown a major after the sintering, the abrupt change in the expansion (Figure 5.25.) is attributed to the crack caused by the sudden release of entrapped moisture inside the green pellet. This observation shows, that although thoria itself is not reported to be hygroscopic, the large specific surface is prone to host a significant amount of adsorbed water, thus green pellets should be pressed from pre-dried feed material. To test this, one of the pellets (see the green dotted line in Figure 5.25.) was pressed from a powder that was heated to 200 °C for 8 h before pressing. Sintering was started immediately after the pressing of that pellet. Apparently the avoidance of moisture did help to reduce the cracking caused by entrapped moisture inside of the green pellets.

The pellet containing 0.65 mol% Al2O3 sintering aid also have shown a rather unusual behaviour, but in this case the cracking was caused most probably due to internal stress built up in the green pellet during the uniaxial pressing (the cracking seems to have started only above 150 °C). The very low thermal conductivity of thoria contributes to the aggravation of any defects already present in the green body before sintering.

The sintering of the pellets started in the region of 725-750 °C, with the exception of the pellet that has been pressed from pure thoria powder obtained from the oxalate precipitation route and used without any further treatment (see Figure 5.26.). This early start of sintering also supported the appropriateness of chosen upper temperature (700 °C) for calcination of pure thoria and Gd-doped thoria powders (with the exception of 30 mol% dopant concentration). At higher calcination temperatures, premature sintering would have taken place resulting in the formation of hard agglomerates lowering the sinterability of green pellets. Although the sintering started at unusually low temperatures, from the collected density values in Table 5.9. it is visible, that 1625 °C with only 1 h dwell time is insufficient to obtain high density pellets.

5.7. Optical microscopy of sintered and polished pellets

Pure thoria pellets sintered in Linn-furnace under reducing atmosphere at 1750 °C for 8 h resulted in high density sintered bodies, however all of them showed the presence of major cracks as witnessed by the overview images on Figure 5.27.

64 Overview and 5x-magnification (of the middle region) of thoria pellet obtained from oxalate precipitation-route derived powder (ID: ThR1001-S36-A)

Overview and 5x-magnification (of the middle region) of thoria pellet obtained from oxalate precipitation-route derived powder (ID: ThR1002-S36-B)

Overview and 5x-magnification (of the edge region) of thoria pellet obtained from oxalate precipitation-route derived powder (ID: ThR1003-S36-A)

Optical microscopy image (5x-magnification) of thoria pellet obtained from powder synthesized via oxalate precipitation route (pellet ID: ThR1010-S36A)

65 Figure 5.27.Selected images of polished pellets representing the overview and one 5-fold magnification of a given sample.

Overview and 5x-magnification (of the centre region) of thoria pellet Figure 5.28.Images of polished pelletobtained from thoria powder synthesized via homogeneous precipitation route (pellet ID: ThR1011-S36-B)

Overview and 5x-magnification (of the edge region) of thoria pellet Figure 5.29.Images of polished pelletobtained from milled powder synthesized via oxalate precipitation route (pellet ID: ThR1014-B).

Overview and 5x-magnification (of the midway region) of 5 mol% Gd-doped thoria pellet obtained from powder synthesized via oxalate precipitation route (pellet ID: ThGd5R1005—S36-B)

Overview and 5x-magnification (of the centre region) of 10

66 mol% Gd-doped thoria pellet obtained from powder synthesized via oxalate precipitation route (pellet ID: ThGd10R1006—S36-C)

Overview and 5x-magnification (of the centre region) of 30 mol% Gd-doped thoria pellet obtained from powder synthesized via oxalate precipitation route (pellet ID: ThGd30R1007-A) Figure 5.30.Images ofpolished Gd-doped thoria pelletsobtained from powder synthesized via oxalate precipitation route.

As one of the purposes of the present study was to test the viability of fabricating high- density sintered pellets as a model material for nuclear fuel via wet chemical route and without using mechanical treatment of the powder precursor, the pellets’ overall integrity also had to be investigated. A nuclear fuel pellet has to be free of open pores and cracks, as the ceramic matrix of the pellet is the most important line of defence against the release of volatile and water-soluble products. On Figures 5.27 – 5.30 a collection of overview and 5-fold magnifications show, that in most of the cases the pellets themselves suffer from major defects (deep cracks, porous and fragile surfaces). The cracks are most probably formed due to two reasons, one of them being the release of entrapped moisture during the heating of the green pellet – as proven by the dilatometry studies. Another reason is the internal mechanical stress built up in the green body during the pellet pressing. The latter phenomenon can be seen in the cracks formed in concentric circles on pellets ThR1003-S36-A,ThR1014-B and ThGd5R1005—S36-B. The internal stress is possibly caused by the uniaxial pressing, and the release process from the dye (the sheer forces might be substantial, when the lubrication of the dye walls are insufficient). Pellets pressed from 10 and 30 mol% Gd-doped thoria powder were free of major defects, similarly to the case of pellet pressed from homogeneous-precipitation route derived thoria powder. The major drawback in the case of these latter pellets was, that their density was significantly inferior to those of the pure thoria pellets derived from oxalate precipitation route.

5.8. Density measurements on sintered pellets

Besides an overall integrity, nuclear fuel pellets also need to be densified to a very high degree, in order to exclude the possibility of a further shrinkage caused by continued sintering during the period of irradiation. Significant volumetric changes might lead to failure of a fuel element, which should be avoided for safety and economic reasons. These considerations require that any fuel ceramic pellet obtain at least 95 % TD during fabrication. In this respect pure thoria pellets fabricated from oxalate-precipitation route derived powder are more promising, than those obtained from homogeneous precipitation route derived powders. As thermal decomposition could not be effected on precursors obtained from other

67 routes, only these two types of powders were used for pellet fabrication. On the basis of the dilatometry measurement results, higher sintering temperature and longer dwell time was chosen (1750 °C and 8 h respectively).

Pellet densities were determined by weighting and measuring their geometrical parameters. In those instances, where the geometrical densities were found to be above 90-92 % TD, immersion densities were also determined using the Archimedes’ principle. Results of pellet density measurements on pure thoria pellets sintered at 1750 °C for 8 h in the Linn-furnace under reducing atmosphere (O2 potential in the furnace was - 420 kJ/mol)are listed in Table 5.10.

68 Table 5.10. Density values of sintered thoria pellets.

Applied Geometri T.D. Green Immersio Immersio Geometric pressure cal Sintering density n density n density al density Synthetic route / Sample ID - temperature (g *cm (MPa) density Macroscopic aspect 3) (g*cm-3) (g*cm-3) (%T.D.) (%T.D.) (°C) / time (h) (g*cm-3)

oxalate route / failed ThR1001-A 10.00 340 5.55 9.89 98.95 9.25 92.51 1750 /8 (major cracks)

oxalate route / failed ThR1002-B 10.00 340 5.64 9.82 98.16 9.08 90.75 1750 /8 (major cracks)

oxalate route / chips ThAl0.12-1002-A 10.00 340 5.78 9.86 98.67 9.67 96.72 1750 /8 missing

oxalate route /failed ThAl0.12-1002-B 10.00 340 5.69 9.90 99.03 9.49 94.92 1750 /8 (edge broken down)

oxalate route /no visual ThAl0.12-1002-C 10.00 340 5.82 9.89 98.89 9.73 97.24 1750 /8 defect

oxalate route /failed ThR1003-A 10.00 340 5.94 9.79 97.97 9.66 96.56 1750 /8 (major cracks)

homog. precip. route / ThR1011-A 10.00 340 5.81 9.11 91.11 8.82 88.16 1750 /8 no visual defect but porous surface

homog. precip. route / ThR1011-B 10.00 453 5.19 8.92 89.21 8.69 86.85 1750 /8 no visual defect but porous surface

69 oxalate route, milled ThR1014-A 10.00 453 6.03 8.91 89.10 9.29 92.88 1750 /8 powder

oxalate route, milled powder / failed (major ThR1014-B 10.00 453 6.13 9.77 97.66 9.43 94.28 1750 /8 circular cracks and missing chips)

Remark: The samples in blue colour were pressed from pure thoria powders obtained from the oxalate route and the feed powder was mixed with 0.12 mol% alumina sintering aid.

Table 5.11. Density values of sintered Gd-doped thoria pellets obtained via oxalate precipitation route.

As-fabricated content Applie T.D. Metal d Green Immersio Immersion Geometrical Geometrical - Macroscopi Gd O fraction pressu density n density density density density Sample ID 2 3 Gd (g cm c aspect x in (Th 3 re -3 -3 -3 (mol 1- ) (g cm ) (g cm ) (%T.D.) (g cm ) (%T.D.) (wt%) Gd )O (MPa) %) x x 2- x/2 failed/circular 3.49 3.03 0.05 9.86 94.42 ThGd5R1005-B 340 5.59 9.59 97.26 9.31 cracks large piece ThGd10R1006-B 7.09 6.15 0.10 9.73 560 5.76 - - 8.13 83.56 missing / broken only shallow 7.09 6.15 0.10 9.73 95.58 ThGd10R1006-C 560 5.89 9.62 98.86 9.30 cracks no visual ThGd30R1007-A 22.73 19.72 0.30 9.23 453 5.48 8.06 87.32 7.86 85.16 defects no visual ThGd30R1007-B 22.73 19.72 0.30 9.23 453 5.48 8.03 86.99 7.88 85.37 defects

70 Remark: The samples containing identical amount of dopant concentration (i.e.: ThGd10R1006-B and C, as well as ThGd30R1007-A and B) were prepared under similar conditions thus being duplicates.

Table 5.10.lists both geometrical and immersion densities of pellets, former providing a lower (90.51 % TD – 96.56 % TD), while the latter a higher bound (97.55 % TD – 98.95 % TD) for these values. Immersion density values written in italics refer to unreliable measurements due to the presence of open porosity or visible cracks on the pellet rendering immersion density measurements inaccurate.

In the case of three samples, the effect of the addition of 0.12 mol% Al2O3 powder to the thoria pellet feed powder was tested. The powders were commingled using mechanical mixing. As reproduced in the highlighted boxes, all the three derived pellets sintered to very high densities (94.92 – 97.24 % TD geometrical and 98.67-99.03% TD in case of immersion density measurements). Moreover, these pellets were also free of major defects (see images in Annex III on figure A.III.1.). The grey colour visible on one side of the pellets is due to the deposition of Mo metal from the Mo-heating elements of the Linn-furnace used for the sintering. OM and SEM images of these samples are not available, but would be also necessary for a fully supported conclusion on the effect of the sintering aid.

Since the small amount of additive both helped to obtain high density and apparently good quality pellets, while defect-free pellet could not be fabricated from pure thoria powder, it is tempting to conclude, that the very high melting point of thoria necessitates the use of sintering aids/additives even in the case of nano-sized powders obtained from oxalate precipitation route.

The oxalate-route derived thoria pellets pressed from previously milled (30 Hz for 5 h using 10 mm zirconia milling ball) powder sintered to densities similar to those from non-milled powders (89.10 – 97.60 % TD geometrical and 92.88–94.28 % TD in case of immersion density measurements).

Pellets fabricated from homogeneous precipitation route derived powder could not densify to sufficiently high densities (86.85 – 88.16 % TD in the case of geometrical and 89.21 – 91.11 % TD in the case of immersion densities). Significant amount of porosities were left, probably because of the poor packing of very large sized particles constituting the thoria powder used for the fabrication. Although both the precursor thorium(IV)oxalate hexahydrate powder as well as the derived oxide are extremely free-flowing, and thus ideal for remote processing, high density pellets could not be fabricated from it. Further experiments on the effect of milling on this specific type of powder would be necessary to investigate whether sinterability could be improved.

A marked effect of the Gd-doping on oxalate-route derived thoria pellets is a sharp decrease of sintered densities with higher Gd-dopant concentrations compared to pure thoria samples. Sample containing 5 mol% Gd as dopant sintered to 94.56 % TD according to geometrical and 97.41 % TD according to immersion density measurement.

In the case of 10 mol% Gd-dopant concentration, two pellets from the same batch already showed a significant difference in the densities, one densified to 96.00 % TD another only to 83.90 % TD although the same pressure was applied during the green pellet fabrication. Most probably the packing of the powder inside the dye before the pressing was significantly

71 better in the case of the second sample. With 30 mol% Gd dopant concentration both of the samples sintered to densities inferior to 87 % TD and 89 % TD in case of geometrical and immersion density measurements. These values are too low for a pellet to be suitable for nuclear fuel usage, even though their overall integrity was better than most of the pure thoria pellets.

72 6. Conclusion

Wet chemical synthesis routes were applied to investigate the feasibility of fabricating high- density thoria pellets without having recourse to any mechanical treatment of precursor powder feed material. On the basis of solid literature data, nano-sized thorium(IV)oxalate hexahydrate and derived thoria powder was obtained reproducibly under carefully selected reaction and heat-treatment parameters. As the powders proved to be highly prone to form agglomerates, the sintered pellets shown sign of heterogeneous sintering, and as a consequence reproducibility in terms of both pellet density and pellet integrity was very low. By adding trace amount of alumina sintering aid to the thoria powder, significant improvement has been attained as those pellets sintered to very high densities. Further studies are needed to draw a definite conclusion whether the addition of sintering aid (either alumina or niobia) could ameliorate the shortcomings of oxalate-route derived powder.

Pellets produced via homogeneous precipitation route sintered to low densities, being unfavourable with respect to neutron economy during breeding, but more favourable for reprocessing due to ease in dissolution of irradiated pellets. The optimal choice for pellet quality, density and integrity has to be assessed taking into account the entire nuclear fuel cycle, the necessity of adaptation of synthesis methods to remote handling and upscaling.

Further work is necessary to investigate the sol-gel techniques developed decades ago, as well as more recent reverse-micelle based synthetic routes or other techniques elaborated in nano-chemistry should be used to obtain spherical particles that are more promising in obtaining high densities. Alternative routes might circumvent the morphological shortcomings of the oxalate-based methods, which pose the main obstacle in reaching high density sintered bodies. Such exotic synthesis routes are nonetheless certainly more expensive and produce larger amount of contaminated waste.

Gd-dopant serves as a consumable neutron absorber in advanced nuclear fuel. With the use of Gd-doped nuclear fuel, highly enriched fuel can be applied, which contributes to a more economic operation without impairing safety. The effects of doping on thoria are manifold and needs to be studied thoroughly for technical purposes.

Lattice parameters of sintered Gd-doped thoria pellets fabricated from wet chemical route- derived powder are reported here for the first time. Evidence is given on the solid solution formation during the sintering process. For the purpose of the present study the well- established oxalate precipitation route was applied besides the less known homogeneous precipitation route to convert thorium(IV)nitrate solution into solid ThO2.

Although wet chemical routes are considered to provide highly homogeneous distribution of constituting elements, evidence was found, that due to crystallization processes and thermal decomposition patterns during calcination of precursor powders leave back inclusions with composition different from that of the bulk matrix. The amount of inhomogeneity is nonetheless lower than the limit of detection attainable with XRD method, and was only possible to capture by EDS.

73 7. Health, Safety and Environment

Synthesis and characterization of thorium(IV)-containing materials were regarded as work conducted with open radioactive sources until the material was processed into dense sintered bodies. Manipulation of open radioactive sources was only conducted in a protected environment: a fume hood, a glove box or hot-cell (collectively named as working units). Transportation between working units of samples or tools get in contact with radioactive material or suspected of contamination was performed after repeated wipe-tests for the exclusion of contamination of areas outside the working units. The largest hazard with α- emitter 232Th nuclide is the incorporation into human body either by digestion or inhalation of fine powder/dust. Since the purpose of the present work was to create finest achievable powder, this risk was even elevated. Samples in powder or green compact state were therefore only removed from a working unit in a perfectly sealable transport container, the outer surface of which was repeatedly checked for contamination.

Manipulation inside a working unit was only performed above a leakage tray with high walls and covered with multiple adsorbing papers to allow for an easier clean-up in case of contamination from splashing of liquid or dropping of powdery material.

One of the difficulty of 232Th-detection is, that it basically has to rely on α-detection of a rather low intensity emitter, which, if present only in small amounts can pass a single wipe test unnoticed. To overcome the uncertainty in measurement, multiple wipe tests were applied on every item that were to be removed from a working unit.In addition, the γ-activity of the daughter elements was also measured. Activity of dust particles in air particles were monitored on a daily basis by using aerosol-collector filtering paper through which a constant air-flow was applied. Hand and feet monitors were also used before leaving the controlled area every occasion to prevent contamination being carried outside of the zone.

The mother solution of thorium(IV)nitrate had an additional risk due to its high acidity (4 M of

HNO3) which further imposed precautions to take during manipulations. Pipetting was performed inside a fume hood using a pipetting ball. Care was taken to avoid the droplets of liquid to contaminate surroundings.

Since the thesis project foresaw the wet chemical synthesis of thorium dioxide, the liquid wastes had to be dealt with appropriately. One of the most important aspects of chosen methods was how it is possible to solidify completely the solute content of produced liquid waste. In this respect the oxalate precipitation route and homogeneous precipitation routes were the most straightforward, mother liquors could be let completely dry after neutralization with NaOH solution. Solidified waste then could be disposed as solid 232Th-contaminated waste. Methods involving the use of organic liquids were restricted to low amounts and wastes were also solidified by allowing drying of the liquid in air.

74 8. References

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Annex I. EGA analysis results

a) Pure thorium(IV)oxalate hexahydrate sample EGA-MS analysis (Sample ID: ThR1010-p1) obtained from the oxalate precipitation route

Table A.I.1 Peak deconvolution results of the EGA signal of H2O in the case of thorium(IV)oxalate hexahydrate powder sample (ID: ThR1010-p1).

Area (arbitrary Center (T /°C) units) 1 0.01206 133 2 0.00173 233 3 0.00262 321

Table A.I.2. Peak deconvolution results of the EGA signal of CO in the case of thorium(IV)oxalate hexahydrate powder sample (ID: ThR1010-p1).

Area (arbitrary Center (T /°C) units) 1 0.02384 137 2 0.01113 236 3 0.01144 318 4 0.02554 522

Table A.I.3. Peak deconvolution results of the EGA signal of CO2 in the case of thorium(IV)oxalate hexahydrate powder sample (ID: ThR1010-p1).

79 Area (arbitrary Center (T /°C) units) 1 0.00116 318 2 0.00172 369 3 0.00222 400 4 1.37767E-4 552

a. ) Pure thorium(IV)oxalate hexahydrate sample EGA-MS analysis (Sample ID: ThR1011-p1) obtained from the homogeneous precipitation route

Table A.I.4. Peak deconvolution results of the EGA signal of H2O in the case of thorium(IV)oxalate hexahydrate powder sample (ID: ThR1011-p1).

Area (arbitrary Center (T /°C) units) 1 0.00717 115 2 0.00142 220 3 0.00208 334 Table A.I.5 Peak deconvolution results of the EGA signal of CO2 in the case of thorium(IV)oxalate hexahydrate powder sample (ID: ThR1011-p1).

Area (arbitrary Center (T /°C) units) 1 0.00214 347 2 3.13384E-4 373 3 4.16069E-4 419 4 1.01371E-4 537

80 Peak Analysis

Data Set:% ([Book2]FitPeaks1,@WL,Input.IDTR1.IDTC2) Date:2015.04.20 Baseline:ExpDec1 Chi^2=-- Adj. R-Square=-- # of Data Points=% ([Book2]FitPeaks1,@WL,RegStats.C1.N) SS=-- Degree of Freedom=% ([Book2]FitPeaks1,@WL,RegStats.C1.DOF)

8,0x10-4

7,0x10-4 Ionic current (A) current Ionic

6,0x10-4

5,0x10-4 0 500 1000 T (°C) Fitting Results

Peak Index Peak Type Area Intg FWHM Max Height Center Grvty Area IntgP 1. Gaussian 0,00995 81,96944 1,1E-4 122,74159 43,04558 2. Gaussian 0,00628 62,80035 9E-5 223,21191 27,17991 3. Gaussian 0,00625 76,14936 8E-5 322,7098 27,04661 4. Gaussian 6,3E-4 133,60576 0 554,76209 2,7279

Figure A.I.1. Deconvolution of EGA CO-signal and the calculated areas of peaks in the case of thorium(IV) oxalate hexahydrate powder sample obtained via the homogeneous precipitation route (ID: ThR1011-p1).

81 Gd-doped samples’ EGA-MS analysis b) 5 mol% Gd(III) doped thorium(IV)oxalate hexahydrate sample

Table A.I.6. Peak deconvolution results of the EGA signal of H2O in the case of a 5 mol% Gd(III)-doped thorium(IV)oxalate hexahydrate powder sample (ID: ThGd5R1005-p1).

Area (arbitrary Center (T /°C) units) 1 0.00343 111 2 0.00112 216 3 6.6475E-4 300 4 1.83653E-4 380

Table A.I.7. Peak deconvolution results of the EGA signal of CO2 in the case of a 5 mol% Gd(III)-doped thorium(IV)oxalate hexahydrate powder sample (ID: ThGd5R1005-p1).

Area (arbitrary Center (T /°C) units) 1 -2.36E-4 214 2 1.91E-4 301 3 0.00101 338 4 4.09E-4 375 5 0.00113 398

82 Peak Analysis

Data Set:% ([Book3]FitPeaks1,@WL,Input.IDTR1.IDTC2) Date:2015.04.19 Baseline:ExpDec1 Chi^2=-- Adj. R-Square=-- # of Data Points=% ([Book3]FitPeaks1,@WL,RegStats.C1.N) SS=-- Degree of Freedom=% ([Book3]FitPeaks1,@WL,RegStats.C1.DOF)

1,0x10-3

9,0x10-4 Ion current (A) current Ion

8,0x10-4

0 500 1000 T (°C) Fitting Results

Peak Index Peak Type Area Intg FWHM Max Height Center Grvty Area IntgP 1. Gaussian 0,0138 87,56235 1,5E-4 118,84702 41,52183 2. Gaussian 0,00581 58,54577 9E-5 214,35601 17,47455 3. Gaussian 0,01066 111,68612 9E-5 298,37132 32,06788 4. Gaussian 0,00297 425,52103 1E-5 437,0941 8,93574

Figure A.I.2. Peak deconvolution results of the EGA signal of CO in the case of a 5 mol% Gd(III)-doped thorium(IV)oxalate hexahydrate powder sample (ID: ThGd5R1005-p1).

Table A.I.7. Peak deconvolution results of the EGA signal of CO in the case of a 5 mol% Gd(III)-doped thorium(IV)oxalate hexahydrate powder sample.

Area (arbitrary Center (T /°C) units) 1 0.01391 118 2 0.00581 214 3 0.01066 298 4 0.00301 437

c) 10 mol% Gd(III)-doped thorium(IV)oxalate hydrate sample EGA-MS analysis results:

83 Table A.I.8. EGA water signal peak deconvolution results of thorium(IV)oxalate sample containing 10 mol% Gd as dopant

Area (arbitrary Center (T /°C) units) 1 0.00625 123 2 0.00219 225 3 0.00145 306 4 4.59E-4 387

Peak Analysis

Data Set:% ([Book1]FitPeaks1,@WL,Input.IDTR1.IDTC2) Date:2015.04.19 Baseline:ExpDec1 Chi^2=-- Adj. R-Square=-- # of Data Points=% ([Book1]FitPeaks1,@WL,RegStats.C1.N) SS=-- Degree of Freedom=% ([Book1]FitPeaks1,@WL,RegStats.C1.DOF)

1,1x10-3

1,0x10-3 Sum (A) Sum

9,0x10-4

8,0x10-4 0 500 1000 T (°C)

Fitting Results Peak Index Peak Type Area Intg FWHM Max Height Center Grvty Area IntgP 1. Gaussian 0,02149 118,38607 1,7E-4 129,21603 45,38339 2. Gaussian 0,00883 70,84408 1,2E-4 230,88495 18,65217 3. Gaussian 0,01111 86,48831 1,2E-4 312,6122 23,46648 4. Gaussian 0,00592 436,60049 1E-5 415,67119 12,49796

Figure A.I.3. Peak deconvolution results of the EGA signal of CO in the case of a 10 mol% Gd(III)-doped thorium(IV)oxalate hydrate powder sample (ID: ThGd10R1006-p1).

In the case of CO-signals the line was usually not found to be flat, consequently a different peak deconvolution algorithm had to be used. In all cases of CO signals in the evolved gas-analysis by mass spectrometer an exponential base line was determined before the peak fitting step.

84 d) 30 mol% Gd(III)-doped thorium(IV)oxalate hydrate sample EGA-MS analysis results:

Table A.I.9. EGA water signal peak deconvolution results of thorium(IV)oxalate sample containing 30 mol% Gd as dopant

Area (arbitrary Center (T /°C) units) 1 1.12683E-7 116 2 1.03341E-8 187 3 3.3848E-8 230 4 1.52599E-8 294 5 6.43154E-9 371

Table A.I.10. EGA CO2 signal peak deconvolution results of thorium(IV)oxalate sample containing 30 mol% Gd(III) as dopant

Area (arbitrary Center (T /°C) units) 1 3.82E-9 245 2 6.32E-9 302 3 3.08E-9 335 4 7.09E-9 376 5 2.19E-8 415 6 1.37E-9 536 7 7.54E-9 536

85 Peak Analysis

Data Set:% ([COEGA]FitPeaks1,@WL,Input.IDTR1.IDTC2) Date:2015.04.19 Baseline:ExpDec1 Chi^2=-- Adj. R-Square=-- # of Data Points=% ([COEGA]FitPeaks1,@WL,RegStats.C1.N) SS=-- Degree of Freedom=% ([COEGA]FitPeaks1,@WL,RegStats.C1.DOF)

1,3x10-8

1,2x10-8 Ion current Ion

1,1x10-8

1,0x10-8 0 500 1000 T Fitting Results

Peak Index Peak Type Area Intg FWHM Max Height Center Grvty Area IntgP 1. Gaussian 0 53,02335 0 148,54217 17,8886 2. Gaussian 0 28,64025 0 187,35875 3,6315 3. Gaussian 0 59,86518 0 216,58478 18,45477 4. Gaussian 0 40,24302 0 250,28074 3,82218 5. Gaussian 0 81,87442 0 297,46126 25,94301 6. Gaussian 0 205,43229 0 305,5911 -3,44308 7. Gaussian 0 106,38446 0 576,42644 4,55198 8. Gaussian 0 155,22649 0 928,14628 4,73393 9. Gaussian 0 104,34025 0 383,48867 5,68993 10. Gaussian 0 70,39283 0 93,50762 18,72719

86 Figure A.I.4. Peak deconvolution results of the EGA signal of CO in the case of a 30 mol% Gd(III)-doped thorium(IV)oxalate hydrate powder sample (ID: ThGd30R1007-p1).

Annex II. XRD-patterns of thorium(IV) oxalate hexahydrate precursor materials

Counts 3917_ThR1010-p2

3000 C4 O8 Th !6 H2 O O H2 H2 !6 !6 Th Th O8 O8 C4 C4 O H2 !6 Th O8 O C4 H2 !6 Th O8 C4 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

2000 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

1000 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

0 25 30 35 40 Position [°2Theta]

Counts 4000 3000 2000 1000 0 30 40 50 60 70 80 90 100 110 120 130 Figure A.II.1. X-ray diffractogram of thorium(IV) oxalate hexahydrate precursor obtained via the oxalate precipitation route

Counts 3816ThGd5R1005-p2

4000 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 O H2 !6 Th O8 C4

3000 O22 Gd2 H20 C6 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O; C6 H20 Gd2 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C6 H20 Gd2 O22 Gd2 H20 C6 2000 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O; C6 H20 Gd2 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4

1000

0 30 40 50 Position [°2Theta]

Peak List

Accepted Patterns

Figure A.II.2. X-ray diffractogram of thorium(IV) oxalate hexahydrate containing 5 mol% Gd as dopant obtained via the oxalate precipitation route

87 Counts 3867_1

4000 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

3000 C6 H20 Gd2 O22 Gd2 H20 C6 C4 O8 Th !6 H2 O; C6 H20 Gd2 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

2000 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 C6 H20 Gd2 O22 Gd2 H20 C6 C6 H20 Gd2 O22 Gd2 H20 C6

1000

0 25 30 35 40 45 50 Position [°2Theta]

Counts 4000 3000 2000 1000 0 30 40 50 60 70 80 90 100 110 120 130 Figure A.II.3. X-ray diffractogram of thorium(IV) oxalate hexahydrate containing 10 mol% Gd as dopant obtained via the oxalate precipitation route

Counts 4000 ThGd30R1007-p2_3918_1 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 O H2 !6 Th O8 C4 3000 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O; C6 H20 Gd2 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4 C4 O8 Th !6 H2 O; C6 H20 Gd2 O22 Gd2 H20 C6 O; H2 !6 Th O8 C4

2000 O22 Gd2 H20 C6 C4 O8 Th !6 H2 O H2 !6 Th O8 C4 C4 O8 Th !6 H2 O H2 !6 Th O8 C4

1000

0 30 40 50 Position [°2Theta]

Counts 4000 3000 2000 1000 0 30 40 50 60 70 80 90 100 110 120 130 Figure A.II.4. X-ray diffractogram of thorium(IV) oxalate hexahydrate containing 30 mol% Gd as dopant obtained via the oxalate precipitation route

88 Counts 4070_ThR1015-p2

1000

500

0 30 40 50 60 70 80 90 100 110 120 130 Position [°2Theta]

Counts

1000

500

0 30 40 50 60 70 80 90 100 110 120 130 Figure A.II.5. X-ray diffractogram of thorium(IV) oxalate hexahydrate obtained via the precipitation in ionic liquid

Annex III.

Images of sintered thoria pellets with 0.12 mol% alumina sintering aid.

ThAl0.12R1002-A ThAl0.12R1002-B ThAl0.12R1002-C Figure A.III.1. Images of thoria pellets (containing 0.12 mol% Al2O3 sintering aid) sintered at 1750 °C/8h in Linn furnace under reducing atmosphere. Note the gradient in the deposited amount of Mo vapours on the right-side image, evidencing the presence of a thermal gradient during the sintering process.

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