metals Communication Synthesis and Structural Analysis of Copper-Zirconium Oxide 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 crystal structure 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: cerium [5,6], scandium [7], vanadium [8], hafnium [9], etc. Until now, only a few mixed metal oxides 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–zirconium alloy [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. Metals 2016, 6, 195; doi:10.3390/met6090195 www.mdpi.com/journal/metals Metals 2016, 6, 195 2 of 6 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 crucible 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 tungsten 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 oxygen, 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 Metals 2016, 6, 195 3 of 6 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 uspredominantly 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.