metals

Communication Synthesis and Structural Analysis of -

Alessandro Figini Albisetti 1, Carlo Alberto Biffi 2,* and Ausonio Tuissi 2

1 EDISON S.p.A.-Research and Development Division, Via Foro Buonaparte 31, Milano 20121, Italy; alessandro.fi[email protected] 2 CNR-IENI—Institute for Energetic and Interphases, C.so Promessi Sposi 29, Lecco 23900, Italy; [email protected] * Correspondence: carloalberto.biffi@cnr.it; Tel.: +39-0341-499181

Academic Editor: Sherif D. El Wakil Received: 2 March 2016; Accepted: 16 August 2016; Published: 23 August 2016

Abstract: A new copper–zirconium oxide was synthesized at ambient pressure in air during a thermal treatment. Its was analyzed using X-ray Powder Diffraction, and the atomic ratio between copper and zirconium was found performing a Rietveld analysis. An accurate analysis, also comparing this new compound with others present in the literature and which present a similar structure, enables us to characterize the new mixed oxide well. Scanning electron microscopy and thermogravimetric analyses were also performed in order to completely characterize this new material, which is interesting both from an academic point of view for its crystal structure and from an industrial one due to the formation of copper–zirconium-based shape memory alloys during thermal treatment.

Keywords: X-ray Powder Diffraction; differential thermal analysis; Rietveld refinement; shape memory alloys

1. Introduction Zirconium dioxide has been extensively studied, and it is still under analysis for its very important and useful chemical and physical properties [1–4]. A large number of attempts were made mixing this oxide with other elements, especially from the transition periods: [5,6], [7], [8], [9], etc. Until now, only a few mixed metal containing both zirconium and copper were reported in the literature, though none of them were provided along with a complete structural characterization [10,11]—specifically the cubic and tetragonal phases, which are more interesting from an application point of view and which could have been analyzed by a better method than the monoclinic analysis. During a thermodiffractometric study of a copper– [12–15], we observed the formation of an oxide on the surface of the Cu10Zr7 sample. The experimental conditions in which the oxide was synthesized were the following: starting from room temperature (RT), the temperature was increased in 50 K steps until 1373 K (in air). After each step, an X-ray measurement—taken 10 min from the stabilization of the temperature—was performed. A change in the composition of the sample was observed starting at 923 K. Analyzing the last diffractogram (recorded at 1373 K) we noticed that a mixed metal oxide containing copper and zirconium, isomorphous to the crystal structure of monoclinic zirconia, was probably present. Copper(I) oxide was not present, except in a small amount, which we took into account during the structural analysis; however, we could not separate it in the Energy Dispersive Spectroscopy EDS analysis. We were interested in a thorough characterization of this new monoclinic zirconia phase because of its possible presence in copper-zirconium shape memory alloys (SMAs) after heat treatments.

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Understanding this phase means understanding its contribution to the physical properties of that class of alloys. For this reason, we decided to combine the X-ray data with the data obtained by Metals 2016, 6, 195 2 of 6 Scanning Electron Microscopy (SEM) and Thermogravimetry coupled with Differential Thermal Analysistreatments. (TG/DTA). Understanding this phase means understanding its contribution to the physical properties of that class of alloys. For this reason, we decided to combine the X‐ray data with the data 2. Materialsobtained andby Scanning Methods Electron Microscopy (SEM) and Thermogravimetry coupled with Differential

High-purityThermal Analysis Zr and (TG/DTA). Cu metals were melted in a vacuum arc furnace with a non-consumable tungsten2. Materials electrode. and Methods The nominal atomic composition of the chosen alloy was Cu58.8Zr41.2 (at %). The furnace was equipped with a water-cooled copper in order to avoid the contamination of the liquidHigh pool.‐purity The Zr material and Cu metals was melted were melted in an inertin a vacuum atmosphere arc furnace (argon with at 400 a non mbar)‐consumable to minimize electrode. The nominal atomic composition of the chosen alloy was Cu58.8Zr41.2 (at %). The the oxidation, and the obtained button ingot was re-melted six times, being flipped over after each furnace was equipped with a water‐cooled copper crucible in order to avoid the contamination of meltingthe stepliquid in pool. order The to material ensure homogeneitywas melted in an of inert the chemical atmosphere composition. (argon at 400 The mbar) sample to minimize was subjected the to grindingoxidation, in and an agate the obtained mortar button in order ingot to producewas re‐melted the powder; six times, later, being the flipped powder over was after heated each up ◦ to 1100meltingC (1373 step in K). order The to morphology ensure homogeneity of the of powder the chemical was analyzedcomposition. utilizing The sample a LEO was 1430 subjected scanning electronto grinding microscope in an (SEM)agate mortar from Oxfordin order Instrumentsto produce the (Abingdon-on-Thames, powder; later, the powder UK). was In heated addition, up to high temperature1100 °C differential(1373 K). The thermal morphology analysis of the (HTDTA) powder was analyzed also performed utilizing using a LEO a 1430 Q600TA scanning TG-DTA systemelectron from TAmicroscope Instruments (SEM) (New from Castle,Oxford DE,Instruments USA), on (Abingdon a small sample‐on‐Thames, (about UK). 10 mg)In addition, in a temperature high rangetemperature of 200–1100 differential◦C (473–1373 thermal K) analysis at a 20 ◦(HTDTA)C (K)/min was heating also performed rate in orderusing toa Q600TA check the TG‐ oxidation.DTA Diffractionsystem data from (CuK TA αInstruments, k = 1.5418 (New Å) were Castle, collected DE, USA.), using on a θa:θ smallvertical sample scan (about Panalitycal 10 mg) X’Pert in a PRO temperature range of 200–1100 °C (473–1373 K) at a 20 °C (K)/min heating rate in order to check the diffractometer (Almelo, The Netherlands) equipped with parallel (Soller) slits (0.04 rad) and a RTMS oxidation. Diffraction data (CuKα, k = 1.5418 Å) were collected using a θ:θ vertical scan Panalitycal (RealX’Pert Time PRO Multiple diffractometer Strip) detector. (Almelo, The Netherlands) generator was equipped operated with at parallel 40 kV (Soller) and 30 slits mA. (0.04 Slits rad) used: ◦ ◦ ◦ divergenceand a RTMS 0.5 ; the(Real scan Time was Multiple performed Strip) in detector. the 5 –50 The generator2θ range. was operated at 40 kV and 30 mA. Slits used: divergence 0.5°; the scan was performed in the 5–50° 2θ range. 3. Results and Discussion First,3. Results the productand Discussion was observed utilizing X-ray Powder Diffraction (XRD) for microstructural characterization,First, the and product SEM was for observed a morphological utilizing X and‐ray compositional Powder Diffraction characterization. (XRD) for microstructural Figure1 shows the XRDcharacterization, patterns of theand initial SEM for Cu a10 morphologicalZr7 phase at roomand compositional temperature, characterization. and of the product Figure obtained 1 shows after heatingthe upXRD to patterns 1373 K: of it the is evident initial Cu that10Zr the7 phase heating at room produces temperature, a compound and of the having product characteristic obtained after peaks, whichheating are the up subject to 1373 of K: investigation it is evident that in the this heating work. produces a compound having characteristic peaks, which are the subject of investigation in this work.

Figure 1. X‐ray powder diffraction (XRD) diffractogram of the Cu10Zr7 sample before and after Figure 1. X-ray powder diffraction (XRD) diffractogram of the Cu10Zr7 sample before and after heating up toheating 1373 K. up to 1373 K.

The Secondary Electrons (SE) mode enabled us to observe how the surface of each single grain Thewas Secondaryattacked by , Electrons forming (SE) mode an almost enabled 10 μm us‐thick to observe layer of howoxide the (estimated surface from of each lateral single view grain was attacked by oxygen, forming an almost 10 µm-thick layer of oxide (estimated from lateral view of

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SEMMetals observation). 2016, 6, 195 It is clearly visible how the grain tends to flake, separating the oxide layer from3 theof 6 bulk grain (see Figure2). of SEM observation). It is clearly visible how the grain tends to flake, separating the oxide layer from the bulk grain (see Figure 2).

Figure 2. Scanning electron electron microscopy microscopy (SEM) (SEM) image image of of powder powder grains grains of of different different size size are are shown. shown. Each of them present the Cu10Zr7 compound, showing the characterizing micrometric oxide layer. EachFigure of 2. them Scanning present electron the Cu 10microscopyZr7 compound, (SEM) showing image of the powder characterizing grains of micrometric different size oxide are layer. shown. Each of them present the Cu10Zr7 compound, showing the characterizing micrometric oxide layer. On the other hand, the Back‐Scattered Electrons (BSE) mode enabled us to determine a sort of On the other hand, the Back-Scattered Electrons (BSE) mode enabled us to determine a sort of confirmationOn the other of the hand, Cu/Zr the ratio: Back the‐Scattered chemical Electrons composition (BSE) of modethe oxide enabled was predominantlyus to determine Zr a ‐sortbased of confirmation of the Cu/Zr ratio: the chemical composition of the oxide was predominantly Zr-based (asconfirmation expected), of because the Cu/Zr of theratio: higher the chemical reactivity composition of Zr with of O the than oxide Cu. was The predominantly chemical composition, Zr‐based (as expected), because of the higher reactivity of Zr with O than Cu. The chemical composition, calculated(as expected), on the because basis of of EDS the analysis,higher reactivity is the following: of Zr with Cu11.6 OZr than27.5O 60.9Cu. (at The %). chemicalFor the high composition, reactivity calculated on the basis of EDS analysis, is the following: Cu11.6Zr27.5O60.9 (at %). For the high reactivity ofcalculated Zr with O, on TG/DTA the basis analysis of EDS analysis,was performed is the following: on small sample Cu11.6Zr of27.5 CuO60.910Zr (at7 intermetallic %). For the high (about reactivity 10 mg of Zr with O, TG/DTA analysis was performed on small sample of Cu10Zr7 intermetallic (about 10 mg inof weight)Zr with O,till TG/DTA1373 K (the analysis same wastemperature performed reached on small during sample the of X ‐Curay10 Zranalysis).7 intermetallic Figure (about 3 shows 10 themg in weight) till 1373 K (the same temperature reached during the X-ray analysis). Figure3 shows the weightin weight) gained, till 1373 which K (thecan samebe divided temperature into two reached parts, duringas a function the X‐ rayof the analysis). temperature: Figure the 3 shows first one the weight gained, which can be divided into two parts, as a function of the temperature: the first one withweight a gain gained, rate whichof about can 8.2 be × divided10−4 %/K intoin the two temperature parts, as a rangefunction [473; of 1023the temperature: K] and the second the first one one of with a gain rate of about 8.2 × 10−4 %/K in the temperature range [473; 1023 K] and the second one of aboutwith a 4.6 gain × 10 rate−3 %/K of about in the 8.2 temperature × 10−4 %/K rangein the [1023;temperature 1373 K]. range [473; 1023 K] and the second one of about 4.6 × 10−3 %/K in the temperature range [1023; 1373 K]. about 4.6 × 10−3 %/K in the temperature range [1023; 1373 K].

Figure 3. TG/DTA (thermogravimetry coupled with differential thermal analysis) scan of Cu10Zr7

intermetallic,FigureFigure 3.3. TG/DTATG/DTA showing (thermogravimetry the rate of oxide formation coupled withwith upon differentialdifferential heating. thermalthermal analysis)analysis) scanscan of of Cu Cu1010ZrZr77 intermetallic,intermetallic, showingshowing thethe raterate ofof oxideoxide formationformation uponupon heating.heating. All the XRD diffractograms (recorded during the thermal analysis) were analyzed, and the one recordedAllAll thethe at XRDXRD1373 diffractogramsdiffractogramsK was chosen (recorded(recorded as the most duringduring significant. thethe thermalthermal After analysis)analysis) a quick werewere qualitative analyzed,analyzed, analysis, andand thethe oneonethe presencerecordedrecorded at ofat 1373some 1373 K peaks wasK was chosen belonging chosen as the toas most secondarythe significant.most unknownsignificant. After phases a quickAfter was qualitativea quick shown, qualitative analysis, but their the presenceanalysis, presence canthe of besomepresence neglected peaks of belongingsome in order peaks to to obtainbelonging secondary info unknownrmationto secondary on phases the unknown new was oxide shown, phases found. but was their The shown, sample presence but was can their cooled be presence neglected to room can in temperature,orderbe neglected to obtain inand information order a new to obtain diffractogram on the information new oxide was on found.recorded the new The inoxide sample order found. wasto confirm cooled The sample to the room stability was temperature, cooled of the to roomnew and compound.atemperature, new diffractogram Performing and a wasnew recorded adiffractogram LeBail inrefinement order was to confirmrecorded (where the inthe stability order extra to ofpeaks confirm the new are the compound.taken stability in account Performingof the newbut neglectedacompound. LeBail refinement in Performingthe real (where refinement), a theLeBail extra itrefinement was peaks observed are taken(where that in accountthethe compoundextra but peaks neglected had are the intaken sa theme realin cell account refinement), and space but groupitneglected was of observed the in themonoclinic thatreal therefinement), zirconia compound [P2it was had1/c; aobserved the = 5.185(1); same cellthat b = andthe 5.216(2); compound space c group = 5.375(2); had of thethe β monoclinicsame= 98.86(3)]; cell and zirconia after, space a Rietveld[P2group1/c ;ofa refinement= the 5.185(1); monoclinicb was= 5.216(2); carried zirconia cout= [P2 5.375(2); to1 /assessc; a =β 5.185(1);the= 98.86(3)]; real copper/zirconium b = after,5.216(2); a Rietveld c = 5.375(2); ratio refinement and β confirm= 98.86(3)]; was carriedthe after,crystal out a structure.Rietveld refinement Performing was the carried Rietveld out torefinement assess the real(with copper/zirconium the TOPAS [16] ratio software), and confirm imposing the crystal the monoclinicstructure. PerformingZrO2 structure the withoutRietveld copperrefinement substitution, (with the we TOPAS obtained [16] a highsoftware), value ofimposing agreement the coefficientmonoclinic (RZrOwp 2= structure0.287), which without means copper a badsubstitution, correlation we between obtained the a highcalculated value ofand agreement the real coefficient (Rwp = 0.287), which means a bad correlation between the calculated and the real

Metals 2016, 6, 195 4 of 6 to assess the real copper/zirconium ratio and confirm the crystal structure. Performing the Rietveld refinement (with the TOPAS [16] software), imposing the monoclinic ZrO2 structure without copper substitution, we obtained a high value of agreement coefficient (Rwp = 0.287), which means a bad correlation between the calculated and the real structure. We repeated the refinement substituting a part of Zr atoms with Cu atoms. Checking the agreement coefficient (Rwp = 0.082) and analyzing the output data from the refinement, it was possible to determine the Zr/Cu ratio (8.09), which means that the mixed-metal oxide has the formula Cu0.11Zr0.89O2. The atomic positions and cell parameters were refined, obtaining the output data summarized in Tables1 and2. In Table1, a brief comparison with the crystal structure of the standard ZrO2 is made in order to highlight the differences in cell parameters and . Figure4 indicates the agreement between experimental and calculated data after the Rietveld refinement. The copper content measured by EDS is overestimated with respect to the one obtained by XRD. The reason for this disagreement depends on the different penetration depths of the two techniques: X-rays can penetrate only the surface layer, while EDS can also observe the underlyingMetals 2016, 6, 195 matrix. 5 of 6

FigureFigure 4. 4.Rietveld Rietveld refinement, refinement, shownshown in the the range range 5°–50° 5◦–50 2◦θ2. θ.

5.Table Conclusions 1. Crystal data and data analysis parameters for the Cu/Zr oxide and a comparison with the ZrO2Fromstructure the Rietveld (Refinement refinement, agreement a good indices structural and 2 characterizationθ range are the minimumof this new and stable the mixed maximum metal oxide,values fromCu0.11 differentZr0.89O2, certifiedwas performed. laboratories). It is of particular interest in studying the compositional modification in copper‐zirconium alloys during heating treatment. In this work, we perform a thermal analysis startingFormula from Cu10Zr7, a minor Cu0.11 Zrphase0.89O present2 in CuZrZrO alloys.2 [17] We are interested in carrying out furtherfw investigations(g·mol−1) on other copper120.18‐zirconium phases present - in copper‐zirconium SMAs, in order to fullySystem characterize that class Monoclinic of compounds. Further Monoclinic work may require the study of the secondary unknownSpace phases Group in order to evaluate P21/c the diffusion process P21/c of copper atoms in the matrix, thus discoveringa (Å) the formation5.1843 mechanism (6) of the CuZr 5.1460 oxides. b (Å) 5.2164 (9) 5.2116 Acknowledgments: The authors thank Giordano Carcano (CNR‐IENI Unit of Lecco) for his important c (Å) 5.3734 (9) 5.3130 contribution in SEM analyses. Mean error ±0.0009 - Author Contributions: Figiniβ (◦ )Albisetti A. and Biffi 98.842 C.A. (4) conceived and designed 99.222 the experiments; Biffi C.A. prepared the sample andV performed(Å3) the SEM and143.59 HTDTA (2) analyses; Figini Albisetti 143.6 A. performed the X‐ray experiment; all the authors analyzedZ the data and wrote4 the paper. 4 d (g·cm−3) 4.86 5.82 Conflicts of Interest: The authors declare no conflict of interest. 2θ range (◦) 5–120 7–162 Rp 0.061 0.031–0.136 References Rwp 0.082 0.039–0.187 1. Steiner, S.A.; Baumann,RBragg T.F.; Bayer, B.C.; Blume,4.79 R.; Worsley, M.A.; 2.0–16.1 Moberlychan, W.J.; Shaw, E.L.; Schoegl, R.; Hart, A.J.; Hofmann S.; et al. Nanoscale Zirconia as a Nonmetallic Catalyst for Graphitization of and Growth of Single‐ and Multiwall Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 12144. 2. Liu, J.; Meng, X.; Banis, M.N.; Cai, M.; Li, R.; Sun, X. Crystallinity‐Controlled Synthesis of Zirconium Oxide Thin Films on ‐Doped Carbon Nanotubes by Atomic Layer Deposition. J. Phys. Chem. 2012, C116, 14656. 3. Piticescu, R.M.; Piticescu, R.R.; Taloi, D.; Badilita, V. Hydrothermal synthesis of ceramic nanomaterials for functional applications. Nanotechnology 2003, 14, 312. 4. Chen, Y.Y.; Wei, W.J. Processing and characterization of ultra‐thin yttria‐stabilized zirconia (YSZ) electrolytic films for SOFC. State Ionics 2006, 177, 351.

5. Varez, A.; Garcia‐Gonzalez, E.; Jolly, J.; Sanz, J. Structural characterization of Ce1−xZrxO2 (0 ≤ x ≤ 1) samples prepared at 1650 °C by solid state reaction: A combined TEM and XRD study. J. Eur. Ceram. Soc. 2007, 27, 3677. 6. Yashima, M.; Hirose, T.; Katano, S.; Suzuki, Y. Structural changes of ZrO2‐CeO2 solid solutions around the monoclinic‐tetragonal phase boundary. Phys. Rev. B Condens. Matter Mater. Phys. 1995, 51, 8018.

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Table 2. Fractional atomic coordinates and occupancies.

Atom XYZ Occ. Cu 0.2899 0.0428 0.2187 0.11 Zr 0.2899 0.0428 0.2187 0.89 O1 0.0552 0.3123 0.3615 1 O2 0.4078 0.7448 0.5408 1

4. Conclusions From the Rietveld refinement, a good structural characterization of this new stable mixed metal oxide, Cu0.11Zr0.89O2, was performed. It is of particular interest in studying the compositional modification in copper-zirconium alloys during heating treatment. In this work, we perform a thermal analysis starting from Cu10Zr7, a minor phase present in CuZr alloys. We are interested in carrying out further investigations on other copper-zirconium phases present in copper-zirconium SMAs, in order to fully characterize that class of compounds. Further work may require the study of the secondary unknown phases in order to evaluate the diffusion process of copper atoms in the ceramic matrix, thus discovering the formation mechanism of the CuZr oxides.

Acknowledgments: The authors thank Giordano Carcano (CNR-IENI Unit of Lecco) for his important contribution in SEM analyses. Author Contributions: Figini Albisetti A. and Biffi C.A. conceived and designed the experiments; Biffi C.A. prepared the sample and performed the SEM and HTDTA analyses; Figini Albisetti A. performed the X-ray experiment; all the authors analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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