Synthesis of a Metal Oxide by Forming Solvate Eutectic Mixtures and Study of Their Synthetic Performance Under Hyper- and Hypo-Eutectic Conditions

crystals

Article Synthesis of a Metal Oxide by Forming Solvate Eutectic Mixtures and Study of Their Synthetic Performance under Hyper- and Hypo-Eutectic Conditions

Omar Gómez Rojas 1 , Simon R. Hall 2,* and Tadachika Nakayama 1,*

1 Nagaoka University of Technology, 1603-1, Kamitomioka Nagaoka, Niigata Prefecture 940-2188, Japan; [email protected] 2 Complex Functional Materials Group, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK * Correspondence: [email protected] (S.R.H.); [email protected] (T.N.)

Received: 29 April 2020; Accepted: 18 May 2020; Published: 21 May 2020

Abstract: The synthesis of YBa2Cu3O7 x (YBCO or 123) superconductor was carried out under − hyper- and hypo-eutectic conditions with different ammonium compounds, i.e., ammonium nitrate, formate, acetate, carbonate, bicarbonate, and tetramethylammonium nitrate. The aim was to find more affordable synthetic pathways using highly available and cheaper compounds, as well as to study the crystal formation under no-carbon conditions when ammonium nitrate was employed. Best results were obtained when eutectic conditions were achieved, namely by ammonium nitrate and YBaCu nitrates in a 5:1 molar ratio (81% of the superconductor). Ammonium formate, acetate, carbonate, and bicarbonate did not produce eutectic mixes. Temperature analysis of the reaction carried out by ammonium nitrate/YBaCu nitrates indicated the formation of barium carbonate, despite no carbon source being used in this reaction. This phenomenon is further discussed in this work. Consequently, tetramethylammonium nitrate, as a chelator and carbon source, was used, providing >96% of the superconductor.

Keywords: metal-containing eutectic mixtures; high-temperature superconductor; metal oxide; ammonium compounds

1. Introduction The synthesis of metal oxides has been an intensive research field due to the plethora of properties that these materials can provide [1]. For this reason, synthetic procedures have been constantly evolving to create new, faster, less time- and energy-consuming routes. Different approaches have been found to be successful such as wet routes, either surfactant-free [2] or assisted by a liquid like ethylene glycol [3], hydrothermal [4], ionothermal [5], sol–gel [6], and microwave-assisted [7] syntheses, just to mention a few. Every synthetic procedure has advantages and disadvantages, but in order to reduce the latter, more complex approaches have been taken, for example, the combination of sol–gel and hydrothermal methods which have achieved interesting results [8,9], or the use of ionic liquids as solvents and chelators and an organic source as the coagulant agent as well as non-specific chelating compound [10–13]. Despite the success that the ionic liquids have shown in different areas via ionogels [14], as electrolytes [15], catalysts [16], or as solvents for metal extraction [17], the use of such for the synthesis of complex metal oxides has not seen great diversification, partly due to the uncertainty around their role in the synthesis. A recent study, however, has provided insight into their behavior at early stages of the synthetic protocol [12]. Another matter to be solved is the elevated price that ionic liquids tend to have, for example, 1-Ethyl-3-methylimidazolium acetate ( 95.0%) sold by Sigma ≥

Crystals 2020, 10, 414; doi:10.3390/cryst10050414 www.mdpi.com/journal/crystals Crystals 2020, 10, 414 2 of 11

Aldrich costs the equivalent of ¥54,500/100 g. As a result of their expense, variations of these solvents have been developed, named deep eutectic solvents. With similar behavior and chemical interactions, these solvents provide cheaper and easier to synthesize alternatives and have already found success in the synthesis of metal oxides [18,19]. In this work, instead of solubilizing metal compounds in a deep eutectic solvent, we used the metal compounds themselves as eutectic forming agents, significantly reducing the expenses related to reactants (tetramethylammonium nitrate ¥29,600/100 g, tetramethylammonium formate ¥37,357/100 g, ammonium nitrate ¥1664/100 g, ammonium acetate ¥1828/100 g, ammonium carbonate ¥6120/100 g, ammonium bicarbonate ¥3640/100 g, and ammonium formate ¥12,200/100 g; all the prices shown are from Sigma Aldrich). This is analogous to the solvation of metal compounds in ionic liquids [20]. Here, the synthesis of the high-temperature superconductor YBa2Cu3O7 x (YBCO) oxide is investigated with a variety of different compounds such as ammonium − nitrate, ammonium formate, ammonium acetate, ammonium carbonate, ammonium bicarbonate, and tetramethylammonium nitrate. Furthermore, the synthesis is also investigated under hypo- and hyper-eutectic conditions in order to examine the effect of this variation in the pursuit of high yields of YBCO.

2. Materials and Methods

2.1. Materials Tetramethylammonium nitrate (425257), tetramethylammonium formate (438375), ammonium nitrate (A9642), ammonium acetate (A1542), ammonium carbonate (207861), ammonium bicarbonate (09830), ammonium formate (516961), barium nitrate (217581), and copper(II) nitrate trihydrate (V800130) were purchased from Sigma-Aldrich Japan, and yttrium(III) nitrate hydrous (150316) was purchased from High Purity Chemicals Japan. Barium nitrate (021-09035, 99.9%) was purchased from 1 Wako Japan. Deionized water was obtained using a MilliQ PureLab Ultra (18.2 M Ω cm− ). None of the materials required further puriﬁcation and were used as received.

2.2. Generation of Metal-Containing Deep Eutectic Solvents For the generation of metal-containing deep eutectic solvents, every ammonium compound, namely ammonium nitrate, formate, acetate, carbonate, bicarbonate, and tetramethylammonium nitrate, and formate was mixed with the metal nitrates, viz. yttrium nitrate, barium nitrate, and copper nitrate, in a range of 20:1 to 1:20 ammonium compound/metal nitrate. Initially, the aqueous solutions were mixed correspondingly, and left at 70 ◦C for a period of 24 h to ensure full dehydration. After that, solid products were obtained in all the mixes of ammonium compounds with every single metal nitrate, in all the range of molar ratios. The eutectic point was then noted by raising the temperature, using a Fisher Scientiﬁc Isotemp Advanced stirring hotplate, until the solid became entirely liquid. However, from all the ammonium compounds used, eutectic mixtures could only be produced using ammonium nitrate and tetramethyl ammonium nitrate in interaction with yttrium nitrate, copper nitrate, and barium nitrate. When ammonium nitrate was employed, copper nitrate and yttrium nitrate gave eutectic mixtures at the same molar ratio, i.e., 5:1 ammonium nitrate/copper–yttrium nitrate. However, barium nitrate exhibited eutectic behavior on the molar ratio of 20:1 ammonium nitrate/barium nitrate. The use of tetramethyl ammonium nitrate provided the same results as observed for ammonium nitrate.

2.3. Generation of Aqueous Precursors Yttrium nitrate (0.05 M), barium nitrate (0.1 M), and copper nitrate (0.15 M) were individually mixed in vials with deionized water under stirring until all salts were dissolved. Similarly, ammonium compounds, i.e., ammonium nitrate (0.5 M), formate, acetate (0.5 M), carbonate (0.5 M), bicarbonate (0.5 M), and tetramethylammonium nitrate (0.5 M), and formate (0.5 M), were also dissolved under constant stirring with DI water until all the solid material was fully dissolved. CrystalsCrystals 20202020,, 1010,, x 414 33 of of 11 11

2.4. Heating Protocols 2.4. Heating Protocols All the mixes of ammonium compounds with metal nitrates were calcined following the same calcinationAll the route: mixes max of ammonium temperature compounds 920 °C, at a withramp metal rate of nitrates 5 °C/min were with calcined a dwell following time of 2 theh. same calcination route: max temperature 920 ◦C, at a ramp rate of 5 ◦C/min with a dwell time of 2 h. 2.5. Characterization 2.5. Characterization For the characterization of the samples FE-SEM JEOL JSM 67000F, powder X-ray diffraction For the characterization of the samples FE-SEM JEOL JSM 67000F, powder X-ray diffraction (pXRD) was carried out on a Rigaku RINT2000 diffractometer (CuKα 1 radiation at λ = 1.5418 Å). (pXRD) was carried out on a Rigaku RINT2000 diffractometer (CuKα 1 radiation at λ = 1.5418 Å). Rietvield analysis was done via Profex 3.12.1 Software [21]. Diffraction patterns were analyzed using Rietvield analysis was done via Profex 3.12.1 Software [21]. Diffraction patterns were analyzed using the Inorganic Crystal Structure Database (ICSD) reference numbers for phase identification (Table the Inorganic Crystal Structure Database (ICSD) reference numbers for phase identification (Table S3). S3). Size distribution was done via ImageJ software [22]. Thermogravimetric analysis was performed Size distribution was done via ImageJ software [22]. Thermogravimetric analysis was performed on a on a Bruker TG-DTA2000SA. Bruker TG-DTA2000SA.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Eutectic Eutectic Mixtures Mixtures

3.1.1.3.1.1. Ammonium Ammonium Nitrate Nitrate WithWith thethe aimaim of of reducing reducing cost cost expenses expenses of chemical of chemical compounds compounds and to analyzeand to analyze the crystal the growth crystal in growththe absence in the of carbonabsence sources, of carbon ammonium sources, nitrateammoni wasum chosen. nitrate Eutectic was chosen. mixtures Eutectic were formedmixtures between were formedmetal nitrates between and metal ammonium nitrates and nitrate. ammonium Individual nitrat metale. Individual nitrates metal were combinednitrates were with combined the molecule, with theand molecule, their eutectic and their points eutectic noted, points 105 and noted, 118 ◦105C for and copper 118 °C nitrate for copper and yttrium nitrate and nitrate, yttrium respectively, nitrate, respectively,both in a molar both ratio in ofa 5:1molar ammonium ratio of nitrate5:1 ammonium/copper-yttrium nitrate/copper-yttrium nitrates (Figure1). Eutecticnitrates mixes(Figure were 1). Eutecticformed withmixes barium were formed nitrate inwith a molar barium ratio nitrate of 20:1 in ammonium a molar ratio nitrate of 20:1/barium ammonium nitrate. Combinationnitrate/barium of nitrate.YBaCu Combination nitrate mixture of andYBaCu ammonium nitrate mixture nitrate and at 5:1 ammonium molar ratio nitrate resulted at 5:1 in amolar deep ratio melting resulted point in at a100 deep◦C, melting which waspoint ﬁve at degrees100 °C, which lower was than five the degre copperesnitrate lower than and ammoniumthe copper nitrate nitrate and eutectic ammonium mixture. nitrateDespite eutectic barium mixture. nitrate not Despite forming barium a eutectic nitrate solvent not forming at that molar a eutectic ratio, solvent no precipitates at that molar were observedratio, no precipitates(Figure1). were observed (Figure 1).

Figure 1. Phase diagram showing the eutectic behavior of the metal nitrates when mixed with Figure 1. Phase diagram showing the eutectic behavior of the metal nitrates when mixed with ammonium nitrate in diﬀerent molar ratios. ammonium nitrate in different molar ratios. 3.1.2. Ammonium Carbonate, Ammonium Bicarbonate, Ammonium Acetate, and 3.1.2.Ammonium Ammonium Formate Carbonate, Ammonium Bicarbonate, Ammonium Acetate, and Ammonium FormateAmmonium carbonate, ammonium bicarbonate, ammonium acetate, and ammonium formate were also employed; however, eutectic mixtures could not be formed. In those cases, depending on Ammonium carbonate, ammonium bicarbonate, ammonium acetate, and ammonium formate were also employed; however, eutectic mixtures could not be formed. In those cases, depending on

Crystals 2020, 10, x 4 of 11 Crystals 2020, 10, 414 4 of 11 the molar ratio between the metal cation and the ammonium compound, two different interactions werethe molar observed. ratio betweenWhen the the molar metal ra cationtio of ammonium and the ammonium nitrate was compound, higher than two the di ﬀmetalerent nitrates, interactions the ammoniumwere observed. was able When to thecoordinate molar ratiothe metal. of ammonium On the other nitrate hand, was when higher mola thanr ratios the of metal ammonium nitrates, compoundsthe ammonium were was lower able toin coordinatecomparison the with metal. the On metal the othernitrates, hand, the when carboxylic molar ratiosgroup, of i.e., ammonium acetate, carbonate,compounds bicarbonate, were lower and in formate, comparison coordinated with the th metale metal nitrates, cations. the This carboxylic was observed group, by i.e., changes acetate, in color,carbonate, from bicarbonate, dark blue andto dark formate, green. coordinated The appe thearance metal cations.of color Thisbased was on observed cation bycoordination changes in environmentcolor, from dark agrees blue with to dark previous green. Thestudies appearance [23,24]. ofA colortable based summarizing on cation the coordination data can environmentbe found in Figureagrees 2. with previous studies [23,24]. A table summarizing the data can be found in Figure2.

(a)

(b)

Figure 2. 2. (a)(a) Tabulated Tabulated data data summarizing summarizing the the eutectic eutectic be behaviorhavior of of each each mix mix of of ammonium ammonium compounds compounds and metal nitrates (white), and maximum percenta percentagege of the superconductive phase and the molar ratioratio atat which which it wasit was obtained obtained (gray); (gray); (b) summary (b) summary of the maximum of the percentage maximum of thepercentage superconductive of the superconductivephase obtainedvia phase diﬀ erentobtained ammonium-containing via different ammonium-containing compounds in dicompoundsﬀerent molar in different ratios with molar the ratiosmetal with nitrates. the metal nitrates.

3.2. Synthesis Synthesis of of YBCO YBCO Superconductor Superconductor The synthesis of YBCO (123) superconductor was attempted with the mixes of ammonium nitrate, ammonium carbonate, ammonium bicarbonate, ammonium acetate, and ammonium ammonium formate formate

Crystals 2020, 10, x 5 of 11 inCrystals combination2020, 10, 414 with YBaCu nitrates, as well as in different molar ratios. The results can be seen5 of in 11 Figure 2. In none of the cases was the phase percentage of the superconductive crystalline phase abovein combination 80%, based with on Rietveld YBaCu nitrates,refinement; as wellfor comparison, as in different a control molar reaction ratios. The with results only YBaCu can be nitrates seen in wasFigure performed,2. In none ofgenerating the cases was69% the of phasethe total percentage mass of of thethe superconductivesuperconductive crystalline phase (Figure phase above2a,b). Tabulated80%, based data on Rietveldof every refinement;molar ratio for can comparison, be found in a Table control S1. reaction In regard with to only the YBaCusyntheses nitrates with wasthe mixesperformed, of ammonium generating nitrate, 69% of ammonium the total mass carbonate, of the superconductive ammonium bicarbonate, phase (Figure ammonium2a,b). Tabulated acetate, dataand ammoniumof every molar formate ratio canwith be YBaCu found nitrates, in Table the S1. highes In regardt percentage to the syntheses of the superconductor with the mixes ofwas ammonium obtained withnitrate, ammonium ammonium nitrate carbonate, and in ammonium the respective bicarbonate, eutectic point ammonium previously acetate, found, and namely ammonium 80% formateat a 5:1 molarwith YBaCu ratio of nitrates, ammonium the highest nitrate/YBaCu percentage nitrates of the (F superconductorigure 2a,b). This was highlights obtained the with importance ammonium of havingnitrate anda highly in the mobile respective system eutectic which point allows previously fast mass found, transport, namely and 80% better at a 5:1 metal molar to ratio metal of interactions.ammonium nitrate/YBaCu nitrates (Figure2a,b). This highlights the importance of having a highly mobileRegardless system which of this, allows the target fast mass superconductive transport, and phase better is metalstill a torelatively metal interactions. low amount. Complete tabulatedRegardless data can of be this, found the targetin Table superconductive S1. phase is still a relatively low amount. Complete tabulated data can be found in Table S1. 3.3. Temperature Analysis 3.3. Temperature Analysis 3.3.1. Via Rietveld Refinement of 5:1 Ammonium Nitrate/YBaCu Nitrates 3.3.1. Via Rietveld Refinement of 5:1 Ammonium Nitrate/YBaCu Nitrates Previous studies have shown that during the calcination, in every case, metal carbonates were formedPrevious and those studies crystal have compositions shown that were during identified the calcination, as promoters in every of high case, yi metalelds carbonatesof single crystal were phasesformed [25]. and The those metal crystal carbonates compositions are formed were via identified the interaction as promoters of the ofmetal high cation yields with of single a carboxylic crystal coordinatingphases [25]. Thesphere metal [12]; carbonates therefore, are with formed the us viae of the compounds interaction ofsuch the as metal ammonium cation with nitrate, a carboxylic YBaCu nitrates,coordinating and no sphere carbon [12 ];source therefore, added with to the the reacti use ofon, compounds the formation such of as such ammonium crystals nitrate,should YBaCunot be expected.nitrates, andThe nothermal carbon route source is addedshown toin theFigure reaction, 3. At thethe formationbeginning, of ammonium such crystals nitrate should is almost not be solelyexpected. observed The thermalat 120 °C, route present is shown as 88% in of Figure the material.3. At the However, beginning, due ammonium to the lower nitrate boiling is almost point, i.e.,solely 180 observed °C [26], itat will 120 immediately◦C, present evaporate, as 88% of theand material. at 220 °C However,83% barium due nitrate to the and lower copper boiling oxide point, are observed.i.e., 180 ◦C[ Barium26], it willnitrate immediately will gradually evaporate, decompose and at from 220 ◦220C 83% to 620 barium °C, and nitrate copper and copperoxide sees oxide a gradualare observed. gain in Barium total phase nitrate percentage will gradually from 220 decompose to 720 °C fromcomposing 220 to 44% 620 ◦ofC, the and reactive copper material oxide sees at thea gradual last temperature. gain in total However, phase percentage contrary fromto ever 220y toexpectation, 720 ◦C composing barium 44%carbonate of the is reactive also observed material duringat the last the temperature.reaction process, However, highligh contraryted with to an every asterisk expectation, (*) in Figure barium 3, first carbonate seen at 420 is also °C with observed only 8%during but with the reaction a positive process, tendency highlighted on the next with 200 an asterisk°C, covering (*) in 55% Figure and3, first59% seen of the at total 420 ◦massC with at only520 and8% but620 with°C, respectively. a positive tendency It is important on the next to 200rememb◦C, coveringer that no 55% sources and 59% of carbon of the total were mass added at 520 to andthe reaction.620 ◦C, respectively. It is important to remember that no sources of carbon were added to the reaction.

Figure 3. Graphical representation via qualitative analysis of pXRD patterns from the temperature Figure 3. Graphical representation via qualitative analysis of pXRD patterns from the temperature analysis of the synthesis of YBa2Cu3O7 x (YBCO) (123) superconductor in the 5:1 molar ratio of − analysisammonium of the nitrate synthesis/YBaCu of nitrates. YBa2Cu3O7-x (YBCO) (123) superconductor in the 5:1 molar ratio of ammonium nitrate/YBaCu nitrates.

Crystals 2020, 10, 414 6 of 11 Crystals 2020, 10, x 6 of 11

To avoid any possible formation due to impurities on the crucible, syntheses with new crucibles To avoid any possible formation due to impurities on the crucible, syntheses with new crucibles were carried out with no changes on the outcome whatsoever. The reactants employed were of high were carried out with no changes on the outcome whatsoever. The reactants employed were of purity (>99%) and with the best quality standards; therefore, to uncover the reason for barium high purity (>99%) and with the best quality standards; therefore, to uncover the reason for barium carbonate formation, several experiments were performed. First, individual reactants, namely, carbonate formation, several experiments were performed. First, individual reactants, namely, barium barium nitrate, copper nitrate, and yttrium nitrate were placed in separate furnaces and new nitrate, copper nitrate, and yttrium nitrate were placed in separate furnaces and new crucibles, calcined crucibles, calcined at a maximum temperature of 620 °C for 2 h, and a ramp rate of 1 °C/min. As at a maximum temperature of 620 C for 2 h, and a ramp rate of 1 C/min. As expected, copper nitrate expected, copper nitrate and yttrium◦ nitrate provided pure phase◦ copper oxide and yttrium oxide and yttrium nitrate provided pure phase copper oxide and yttrium oxide (Figure4a,b). However, (Figure 4a,b). However, results with barium nitrate provided a mix of barium carbonate and barium results with barium nitrate provided a mix of barium carbonate and barium silicate (B2SiO4), silicon silicate (B2SiO4), silicon being provided by the surface of the crucible. To ensure that these results being provided by the surface of the crucible. To ensure that these results were reproducible, crucibles were reproducible, crucibles from a different provider were used, as well as the use of barium nitrate from a diﬀerent provider were used, as well as the use of barium nitrate provided by Sigma-Aldrich provided by Sigma-Aldrich Japan (217581, > 99%). The product once again exhibited a mix of barium Japancarbonate (217581, and> barium99%). The silicate product (Figur oncee 4c). again The exhibited observed a results mix of bariumare due carbonateto the highly and reactive barium silicatenature (Figureof barium4c). The[27]; observedadditionally, results simi arelar duebehavior to the of highly barium reactive nitrates nature being of replaced barium by [ 27 carbonates]; additionally, has similaralso been behavior reported of barium[28,29]. Subsequently, nitrates being the replaced reaction by protocol carbonates from has 520 also to 820 been °C reportedsaw the barium [28,29]. Subsequently,nitrate decomposition the reaction at 620 protocol °C, the formation from 520 toof y 820ttrium◦C sawoxide the with barium 8% of nitratethe total decomposition mass percentage at 620at 720◦C, °C, the and formation the presence of yttrium of barium oxide copper with oxide 8% of with the 35% total of mass the total percentage mass percentage at 720 ◦C, at and820 °C. the presenceThose crystal of barium compositions, copper oxide copper with oxide, 35% of bari theum total copper mass percentageoxide, yttrium at 820 oxide,◦C. Thoseand barium crystal compositions,carbonate, reacted copper to oxide, form bariumthe superconductive copper oxide, yttriumphase with oxide, 32% and and barium 81% carbonate,at 820 and reacted 920 °C, to formrespectively. the superconductive phase with 32% and 81% at 820 and 920 ◦C, respectively.

(a)

(b) (c)

FigureFigure 4. 4.Powder Powder X-rayX-ray didiffractionﬀraction patternspatterns ofof ((aa)) yttriumyttrium oxide,oxide, ((bb)) coppercopper oxide,oxide, and (c) barium • carbonatecarbonate and and barium barium silicate silicate ( ). ( All). All the reactionsthe reactions were were carried carried out using out theirusing respective their respective metal nitrates, metal • calcinednitrates, in calcined new crucibles, in new crucibles, and a maximum and a maximum temperature temperature of 620 ◦C. of 620 °C.

BasedBased on on this this result, result, thethe formationformation ofof metalmetal carbonatescarbonates isis essentialessential due to the fact that those crystallinecrystalline compositions compositions thrive thrive even even under under non-carbonnon-carbon conditions,conditions, asas elementselements like barium can react with carbon that the atmosphere provides. Therefore, adding chemical compounds that supply carbon to the reaction media provides faster thermal synthetic routes and higher purity [12,25].

Crystals 2020, 10, 414 7 of 11

Crystals 2020, 10, x 7 of 11 with carbon that the atmosphere provides. Therefore, adding chemical compounds that supply carbon to theTo reaction prove the media formation provides of carbonates faster thermal due syntheticto the interaction routes and of barium higher puritywith the [12 carbon,25]. contained in theTo atmosphere, prove the formation a reaction ofunder carbonates argon atmosp due tohere the interaction was done. ofThe barium outcome with showed the carbon no carbonates contained inin the atmosphere,least, although a reaction only weak under noise/signal argon atmosphere ratio diffractions was done. The were outcome observed, showed namely, no carbonates yttrium bariumin the least, copper although oxide with only weakthe chemical noise/signal composition ratio diff Yractions0.5Ba3Cu were1.5O5.5 observed,, and yttrium namely, oxide yttrium (Figure barium S1). copper oxide with the chemical composition Y0.5Ba3Cu1.5O5.5, and yttrium oxide (Figure S1). 3.3.2. Via Rietveld Refinement of 5:1 Tetramethylammonium Nitrate/YBaCu Nitrates 3.3.2. Via Rietveld Refinement of 5:1 Tetramethylammonium Nitrate/YBaCu Nitrates As a proof of concept, and due to the fact that the synthesis could not be done by a different syntheticAs a pathway, proof of concept,namely, andby avoiding due to the the fact synt thathesis the of synthesis metal carbonates could not using be done only by ammonium a different nitratesynthetic and pathway, YBaCu namely, nitrates by avoidingin the reaction, the synthesis tetramethylammonium of metal carbonates usingnitrate only was ammonium used. By nitrate the incorporationand YBaCu nitrates of carbon in the sources reaction, in tetramethylammoniumthe reaction, compared nitrate to the was synthesis used. By carried the incorporation out by using of ammoniumcarbon sources nitrate/YBaCu in the reaction, nitrates, compared higher to the total synthesis mass percentage carried out of by the using superconductive ammonium nitrate phase/YBaCu was expectednitrates, higherto be formed. total mass percentage of the superconductive phase was expected to be formed. TheThe synthesis synthesis with with tetramethylammonium tetramethylammonium nitrate nitrate goes as followed (Figure 55):):

Figure 5. Graphical representation via qualitative analysis of pXRD patterns from the temperature Figureanalysis 5. of Graphical the synthesis representation of YBCO (123) via superconductorqualitative anal inysis the of 5:1 pXRD molar patterns ratio of tetramethylfrom the temperature ammonium analysisnitrate/metal of the nitrates. synthesis of YBCO (123) superconductor in the 5:1 molar ratio of tetramethyl ammonium nitrate/metal nitrates. At the beginning of the temperature analysis, from 120 to 220 ◦C, tetramethyl ammonium nitrate wasAt the the majority beginning phase of the with temperature>80% of theanalysis, total fr phaseom 120 percentage to 220 °C, tetramethyl across this temperatureammonium nitrate range. wasThis the phase majority is consumed phase with on further >80% of heating the total with phas thee majoritypercentage of theacross phase this percentage temperature barium range. nitrate This phase(81%), is and consumed a small percentageon further heating of copper with oxide the (19%).majority From of the 420 phase to 520 percentage◦C, barium barium nitrate nitrate continuously (81%), anddecomposes, a small percentage from 37% toof 0%,copper and oxide hence, (19%). barium From carbonate 420 to 520 starts °C, to barium form, presentnitrate ascontinuously 31% of the decomposes,material at 420 from◦C and37% 68%to 0%, at 520and◦ hence,C. From barium that point carbonate onwards, starts barium to form, carbonate present will as 31% only of decay the materialin total phaseat 420 percentage,°C and 68% at and 520 as °C. a consequence,From that point barium onwards, copper barium oxide carbonate will appear will firstonly at decay 520 ◦inC. totalAt this phase point, percentage, all the necessary and as a precursor consequence, phases barium for the copper formation oxide of will YBCO appear are first present at 520 in the°C. system,At this point,namely, all copperthe necessary oxide, precursor barium copper phases oxide,for the andformation barium of carbonate.YBCO are present YBCO in is the first system, seen at namely, 820 ◦C copperand subsequently oxide, barium is present copper as oxide, 96% atand 920 barium◦C. Powder carbonate. diffraction YBCO patterns is first of seen the synthesisat 820 °C of and the subsequentlysuperconductive is present phase via as 5:1 96% molar at ratio920 °C. of ammoniumPowder diffraction and tetramethyl patterns ammonium of the synthesis nitrate canof alsothe superconductivebe found in Figure phase S2. Additionally,via 5:1 molar tabulatedratio of ammonium data comparing and tetramethyl the total mass ammonium percentage nitrate obtained can also by beammonium found in Figure nitrate /S2.YBaCu Additionally, nitrates and tabulated tetramethylammonium data comparing the nitrate total/YBaCu mass percentage nitrates can obtained be found by in ammoniumTable S2. nitrate/YBaCu nitrates and tetramethylammonium nitrate/YBaCu nitrates can be found in Table S2.

CrystalsCrystals 20202020,, 1010,, x 414 88 of of 11 11

3.4. Thermogravimetry Analysis (TGA) 3.4. Thermogravimetry Analysis (TGA) ThermogravimetricThermogravimetric analysis (TGA)(TGA) (Figure(Figure6 a)6a) and and di differentialfferential thermal thermal analysis analysis (DTA) (DTA) (Figure (Figure6b) 6b)of the of reactionsthe reactions using using 5:1 molar 5:1 ratiomolar eutectic ratio eutectic mixtures mixtures of ammonium of ammonium nitrate and nitrate tetramethyl and tetramethyl ammonium ammoniumnitrate in combination nitrate in combination with YBaCu with nitrates YBaCu were nitrates also performed. were also performed. For the analysis, For the neutral analysis, atmosphere neutral atmospherein the form ofin argonthe form gas of was argon used. gas For was a 5:1used. molar For ratioa 5:1 ofmolar ammonium ratio of nitrateammonium to metal nitrate nitrate, to metal there nitrate,was a loss there of 8%was of a total loss massof 8% percentage, of total mass which percentage, was due towhich the early was evaporation due to the early of ammonium evaporation nitrate of ammonium nitrate and possible moisture adsorbed by the material. At 173 °C (Figure 6a I*), there and possible moisture adsorbed by the material. At 173 ◦C (Figure6a I*), there was a significant loss in wasmass a percentagesignificant from loss 93%in mass to 40% percentage due to the from complete 93% evaporationto 40% due of to ammonium the complete nitrate. evaporation Consistently, of ammonium nitrate. Consistently, in this period, DTA analysis showed exothermic behavior, viz. from in this period, DTA analysis showed exothermic behavior, viz. from 240 to 280 ◦C, highlighted in 240 to 280 °C, highlighted in Figure 6b blue*. From 280 °C (Figure 6a II*) to 500 °C (Figure 6a III*) Figure6b blue*. From 280 ◦C (Figure6a II*) to 500 ◦C (Figure6a III*) there was a gradual decay in mass there was a gradual decay in mass percentage of only 5%. However, soon after there was a second percentage of only 5%. However, soon after there was a second sharp decay, from 35% at 500 ◦C to 18% sharp decay, from 35% at 500 °C to 18% at 622 °C, from the barium nitrate decomposition. In this at 622 ◦C, from the barium nitrate decomposition. In this range of temperatures, from 500 to 622 ◦C, rangeDTA analysisof temperatures, showed from endothermic 500 to 622 behavior °C, DTA until analysis the veryshowed end endothermic of the range behavior of temperatures, until the very from end of the range of temperatures, from 610 to 622 °C, where exothermic behavior was again observed. 610 to 622 ◦C, where exothermic behavior was again observed. From that point onwards, the TGA Fromtotal massthat point percentage onwards, remained the TGA constant, total mass as well percentage as a continual remained endothermic constant, as behavior well as observeda continual in endothermicDTA analysis. behavior observed in DTA analysis. Conversely,Conversely, TGATGA onon tetramethyl tetramethyl ammonium ammonium nitrate nitrate samples samples in combination in combination with YBaCu with nitrates,YBaCu nitrates,with the with same the 5:1 same molar 5:1 ratio, molar showed ratio, no showed differences no differences on the mass on percentage the mass untilpercentage 250 C (Figureuntil 2506a °C I ) ◦ • (Figurewas reached. 6a I•) Shortlywas reached. after, there Shortly was after, a considerable there was weighta considerable loss of 69%, weight from loss 99% of to 69%, 30%, from in the 99% range to 30%, in the range of temperatures between 250 and 380 °C (Figure 6a II •). DTA analysis in this range of temperatures between 250 and 380 ◦C (Figure6a II ). DTA analysis in this range of temperatures, of temperatures, from 250 to 380 °C, exhibited exothermic• behavior from 270 to 375 °C, highlighted from 250 to 380 ◦C, exhibited exothermic behavior from 270 to 375 ◦C, highlighted in Figure6B *red, in Figure 6B *red, specifically represented in two peaks at 290 and 328 °C. Such weight loss and specifically represented in two peaks at 290 and 328 ◦C. Such weight loss and exothermic behavior can exothermicbe attributed behavior mainly tocan tetramethyl be attributed ammonium mainly nitrateto tetramethyl evaporating ammonium and nitrate nitrate from evaporating YBaCu nitrates and as nitratewell. From from thatYBaCu point nitrates onwards, as well. there From was that a constant point onwards, tendency there to lose was weight, a constant dropping tendency 7%more to lose of weight, dropping 7% more of total mass from 380 to 660 °C (Figure 6a III •), and an unvarying total mass from 380 to 660 ◦C (Figure6a III ), and an unvarying endothermic behavior observed by endothermic behavior observed by the DTA• analysis. Subsequently, the decomposition of barium the DTA analysis. Subsequently, the decomposition of barium carbonate to release CO2 afforded a carbonatefurther mass to release loss of CO 2%.2 afforded a further mass loss of 2%.

(a) (b)

FigureFigure 6. 6. (a(a) )TGA TGA analysis, analysis, and and (b (b) )differential diﬀerential thermal thermal analysis analysis (DTA) (DTA) analysis analysis of of the the synthesis synthesis of of YBCOYBCO (123) (123) superconductor superconductor via via 5:1 5:1 molar molar ratio ratio of of ammonium ammonium nitrate nitrate and and tetramethyl tetramethyl ammonium ammonium nitrate/YBaCunitrate/YBaCu nitrates. nitrates.

4. Structural Characterization Scanning electron microscopy was performed for samples collected when the reaction was carried out using tetramethylammonium nitrate/metal nitrates, in molar ratios varying from 20:1 to 1:20

Crystals 2020, 10, x 9 of 11

4. Structural Characterization

CrystalsScanning2020, 10, 414electron microscopy was performed for samples collected when the reaction 9was of 11 carried out using tetramethylammonium nitrate/metal nitrates, in molar ratios varying from 20:1 to 1:20 (Figure S3), and at a maximum calcination temperature of 920 °C. From this, it was clear that there(Figure was S3), a and tendency at a maximum for the calcinationsize of crystallites temperature to be of 920reduced◦C. From while this, the it wasconcentration clear that there of both was compoundsa tendency for was the in size the of range crystallites of 1:1 to≤5 be (Figure reduced 7b,c). while Specifically, the concentration 1:1 was of bothshown compounds to be the wasmost in uniform,the range whereas of 1:1 5 (Figure5:1, and7 b,c).3:1 also Speciﬁcally, exhibited 1:1 simila was shownr sizes toof be crystallites, the most uniform, with the whereas majority 5:1, of andthe ≤ crystallites3:1 also exhibited being around similar sizes0.5 to of1 crystallites,µm, and with with only the slight majority amounts of the of crystallites bigger crystals being aroundwith various 0.5 to sizes1 µm, from and withabove only 1 to slight 7 µm. amounts It was also of bigger noted crystals that when with the various metal sizes nitrates from were above in 1 a to higher 7 µm. molar It was ratioalso notedthan tetramethyl that when theammonium metal nitrates nitrate, were the incrys a highertallite sizes molar were ratio increased than tetramethyl considerably ammonium (Figure 7a).nitrate, the crystallite sizes were increased considerably (Figure7a).

FigureFigure 7. HistogramsHistograms obtained obtained via via image image analysis analysis showing showing the the particle particle size size distribution distribution obtained obtained via via ((aa)) 5:1, ( b) 1:1, and (c) 1:5 molarmolar ratiosratios ofof tetramethyltetramethyl ammonium nitratenitrate/YBaCu/YBaCu nitrates. ( (aa––cc)) SEM SEM imagesimages of of their respective syntheticsynthetic conditions. The The reactions reactions were were carried carried out out at at a a maximum temperaturetemperature of of 920 920 °C.◦C. 5. Conclusions 5. Conclusions During this study, it has been proven that finding the eutectic point gives better crystalline results in termsDuring of puritythis study, of the it sample. has been This proven can bethat attributed finding tothe the eutectic fast mass point transport gives better that thecrystalline system resultsprovides in whenterms aof eutectic purity mixtureof the sample. is formed. This Furthermore, can be attributed highly to reactive the fast elements, mass transport such as that barium, the system provides when a eutectic mixture is formed. Furthermore, highly reactive elements, such as in the absence of carbonaceous material will react with the atmospheric CO2, forming barium carbonate, barium,and with in Si, the provided absence byof carbonaceous the crucible, to material form barium will react silicates, with inthe order atmospheric to form aCO stable2, forming configuration. barium carbonate,Therefore, and adding with a carbonSi, provided source by to the the crucible, reaction to will form improve barium the silicates, overall resultsin order by to facilitating form a stable the configuration.formation of barium Therefore, carbonate adding and a carbon restraining source barium to the from reaction reacting will with improve undesired the overall elements results such by as facilitatingsilicon contained the formation in the crucible. of barium carbonate and restraining barium from reacting with undesired elements such as silicon contained in the crucible. These results not only provide a better understanding of the use of ionic liquids/deep eutectic These results not only provide a better understanding of the use of ionic liquids/deep eutectic mixtures for the synthesis of metal oxides but have also provided greater clarity in the factors important mixtures for the synthesis of metal oxides but have also provided greater clarity in the factors to consider when searching for molecules to act as cations. Furthermore, the study here has supplied important to consider when searching for molecules to act as cations. Furthermore, the study here clarification on the relevance of metal carbonates during the syntheses of complex metal oxides has supplied clarification on the relevance of metal carbonates during the syntheses of complex metal produced via calcination in air. oxides produced via calcination in air. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/5/414/s1.

Author Contributions: O.G.R. carried out experiments, data collection and writing of the article. T.N. and S.R.H. Supplementaryprovided valuable Materials: guidance The and following important are discussions available which online made at www.mdpi.com/xxx/s1. the writing of this article possible. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding. Crystals 2020, 10, 414 10 of 11

Acknowledgments: O.G. would like to thank Thi Mai Dung for valuable discussions and Suematsu Hisayuki for helping with TGA analysis. O.G. would also like to thank the WISE program for funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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