Monoclinic ZrO2 Phase Research Articles

Comparative study of aerogels nanostructured catalysts: Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2−

A series of Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2− catalysts were prepared in one step by the sol–gel method and dried in hypercritical conditions of the solvent. The characteristic properties of those solids were investigated using many techniques: the XRD, the physisorption of N2, the IR, the UV–visible, and the X-ray photoelectron spectroscopy (XPS). Textural analysis reveals the mesoporosity of all the aerogel catalysts. Moreover, the addition of alumina to nickel sulfated zirconia at high calcination temperature increased twice the specific surface area, from 72 to 158 m2/g. XRD patterns show that nickel-promoted sulfated zirconia calcined at different temperature develops the tetragonal and the monoclinic ZrO2 phase, whereas the nickel sulfated zirconia alumina exhibits only the ZrO2 tetragonal phase. The addition of aluminum to nickel sulfated zirconia induces significant changes in symmetry of nickel by the migration of Ni ions from octahedral to tetrahedral coordination. XPS spectroscopy shows that the nickel in Ni/ZrO2–SO4 2− catalysts is more reducible than those in Ni/ZrO2–Al2O3–SO4 2−. Nickel sulfated zirconia catalyst exhibits higher activity than nickel sulfated zirconia alumina, in the n-hexane isomerization reaction.

Thermal Expansion of HfO2 and ZrO2

The thermal expansion of a low symmetry crystal can be much more interesting than the lattice parameter expansion would suggest. Here, the complete thermal expansion tensors for monoclinic and tetragonal phases of ZrO2 and HfO2 have been measured in air, by high‐resolution, high‐temperature X‐ray diffraction. These results reveal the highly anisotropic nature of thermal expansion in the monoclinic phase as well as a cooperative movement of ions and the existence of a zero thermal expansion plane.

Fabrication of Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Micro/Nano Composites Using Powder Alkoxide Mixtures

Al2O3/ZrO2 composites have been investigated as the bio-inert material. In the present work Al2O3/ZrO2 Micro/Nano composites were prepared by colloidal method. The effects of the concentration of zirconium propoxide and the sintering temperature were studied on the distribution of ZrO2 nano-grainsin Al2O3 matrix and mechanical properties of composites. XRD patterns for all compositions showed a single phase of a-Al2O3, and the tetragonal and monoclinic ZrO2 phases. Samples sintered at 1650°C demonstrated the high bulk density with value of 86-92% of theoretical density. ZrO2 nano-grains were dispersed within Al2O3 grain and the grain boundary. The sample of the highest flexural strength of 710 MPa was synthesized using 100% Zr propoxide solution and sintered at 1650o C. The flexural strength was found to increase with increasing the concentration of Zr propoxide and sintering temperature.

Preparation of m-ZrO2 compacts by microwave sintering

Dense un-doped monoclinic ZrO2 (m-ZrO2) compacts were prepared successfully by microwave sintering with a TE666 resonant mode and temperature ranges from 1350°C to 1550°C. XRD and SEM techniques were used to characterize the samples. A dense and homogeneous microstructure was observed with the grain size of 1μm, which is smaller than that of conventional sintered ones. A monoclinic ZrO2 phase without detectable tetragonal ZrO2 (t-ZrO2) was found for both microwave sintered and conventional sintered samples. Density of samples was measured according to the Archimedes principle. The results showed a relative density of 99.0% for microwave sintered samples which is higher than that of conventional sintered ones. The microhardness was investigated by indentation test with the highest value of 4.0GPa. Meanwhile, microcracks were observed on the edges of the indentation but not at the tips for microwave sintered samples which indicated good toughness. Optimized microwave sintering temperature of m-ZrO2 compacts was 1450°C for 30min with the microhardness of 4.0GPa and relative density of 99.0%.

Formation and electrochemical characterization of anodic ZrO2–WO3 mixed oxide nanotubular arrays

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85mV and −230mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03mF/cm2 at a scan rate of 10mV/s and the lowest was 1.93 mF/cm2 at 1V/s in 1M sulfuric acid solution.

Stabilized formation of tetragonal ZrO2 thin film with high permittivity

We have investigated the effect of the film thickness and lattice spacing of Zr oxides on the crystalline structures of Zr oxide films formed by atomic layer deposition (ALD) and sputtering methods to achieve higher-k tetragonal ZrO2 (t-ZrO2) films. In the ALD case, the t-ZrO2 crystalline structure stably formed over a wide range of film thicknesses (8 to 130nm). The Zr oxide films with a t-ZrO2 structure prepared by ALD and post-deposition annealing at 350°C achieved a relative permittivity of 35±2. On the other hand, the ratio of the t-ZrO2 phase to the monoclinic (m-) ZrO2 phase decreases with increasing thickness of the Zr oxide film formed by sputtering, although it can be increased by Ge incorporation. Focusing on the relationship between the lattice spacing and the crystalline phase of Zr oxide, the t-ZrO2 crystalline structures are stably formed with decreasing lattice spacing of the t-ZrO2 crystalline structure. The effect of the lattice spacing on the t-ZrO2 crystalline structure formation is suitably explained in terms of the bulk (volume) energy of the t-ZrO2 phase.

Structural, down- and phase selective up-conversion emission properties of mixed valent Pr doped into oxides with tetravalent cations

We report on structure-property relationships in Pr-doped CeO2 and ZrO2 using X-ray diffraction (XRD), Raman, UV to Vis Diffuse Reflectance (DR-UV/Vis), X-ray Photoelectron (XPS), and luminescence (PL) spectroscopies. Both 3+ and 4+ valence states of Pr are evidenced, irrespective of the host and calcination temperature, T (T = 500 and 1000 °C) with consequences on absorption, surface, vibrational and luminescence properties. Only zirconia represents a suitable host for Pr(3+) luminescence. The distinct trivalent Pr centers and their excitation mechanism are identified in relation to the tetragonal and monoclinic phases of ZrO2. A near-infrared to visible up-conversion (UPC) emission of Pr(3+) is observed upon excitation at 959 nm which occurs, most probably, via a two-photon excited state process. By using a multi-wavelength, time-gated excitation, the UPC process is established as phase selective, i.e. only Pr(3+) located in the monoclinic sites of the mixed phase, monoclinic and tetragonal ZrO2 (T = 1000 °C) contribute to the UPC emission. We believe that, besides the local symmetry, a key role in phase selective UPC is played by the presence of Pr(3+) low-lying 4f 5d levels. To the best of our knowledge, this is the first report of phase selective up-conversion emission in a lanthanide doped multi-phase host.

Sintering and Performance of MgO from Bischofite with ZrO<sub>2</sub> Additive

Effect of ZrO2 on sintering properties of MgO prepared from natural brine from Qarhan Salt Lake, crystalline bischofite and MgCl2·6H2O(AR) was studied. The results showed that ZrO2 of addition exceeded 1 wt% had promotive effect on sintering of magnesia prepared from MgCl2·6H2O(AR). While 1.5 wt% ZrO2 was added, the bulk density of sample was 3.37 g/cm3, and relative density was 94.1%. With 1 wt% ZrO2, the bulk densities of magnesia prepared from crystalline bischofite and brine were 3.10 and 3.27 g/cm3, and the relative densities increased by 10.0% and 19.6%, respectively. The main reasons for promoting MgO sintering with ZrO2 were that monoclinic phase ZrO2 converted to cubic phase Zr2O and excess ZrO2 reacted with MgO to generate Mg0.13Zr0.87O1.87, which activated lattice and promoted the diffusion of ions during sintering process. In addition, liquid phase sintering was also one of the main reasons for promoting the sintering of brine magnesia.

Novel fabrication of high-quality zirconia ceramic thin films by polyglycol-assisted spreading deposition

A polyglycol-assisted spreading deposition (PASD) method has been used to prepare the high-quality ZrO2 thin film on single crystalline Si (111) substrate. The structure and composition of the films were studied by SEM, XRD and UV Raman techniques. Characterization results indicated that the ZrO2 films were composed of nano-sized small particles and have a crack-free, dense and homogeneous microstructure. After a heat treatment of 10min at 700–900°C, the films showed a mixture of monoclinic and tetragonal ZrO2 phases. And the grain sizes of monoclinic and tetragonal phases in the ZrO2 films were 5–16nm and 10–18nm, respectively. In addition, the thin films of Y2O3-stabilized ZrO2 (YSZ), La2Zr2O7, and the multilayer thin films of La2Zr2O7/YSZ were also fabricated by the PASD method.

Deposition of ZrNxOy films by filtered cathodic vacuum arc

Zirconium oxynitride (ZrNxOy) thin films were prepared by filtered cathodic vacuum arc technique using N2/O2 mixtures. The effect of N2/O2 ratios on the microstructure, optical and mechanical properties of films had been investigated. X-ray diffraction results showed that o-Zr3N4(O) phase was observed at low oxygen flowrate, while tetragonal and monoclinic phases of ZrO2 were detected at high one. Optical test revealed that the films changed from semitransparent to transparent, and the absorption edge shifted to lower wavelength with the increase in oxygen flowrate. The refractive indexes of films ranged from 2·24 to 3·47, and the optical band gap increased from 2·06 to 4 eV with the increasing oxygen content, which implied that the films had potentials in optoelectronic applications.

Optical properties of monoclinic HfO2 studied by first-principles local density approximation + U approach

The band structures and optical properties of monoclinic HfO2 are investigated by the local density approximation + U approach. With the on-site Coulomb interaction being introduced to 5d orbitals of Hf atom and 2p orbitals of O atom, the experimental band gap is reproduced. The imaginary part of the complex dielectric function shows a small shoulder at the edge of the band gap, coinciding with the experiments. This intrinsic property of crystallized monoclinic HfO2, which is absent in both the tetragonal phase and cubic phase, can be understood as a consequence of the reconstruction of the electronic states near the band edge following the adjustment of the crystal structure. The existence of a similar shoulder-like-structure in the monoclinic phase of ZrO2 is predicted.

Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion

The effect of amorphous (am-), monoclinic (m-), and tetragonal (t-) ZrO2 phase on the physicochemical and catalytic properties of supported Cu catalysts for ethanol conversion was studied. The electronic parameters of Cu/ZrO2 were determined by in situ XAS, and the surface properties of Cu/ZrO2 were defined by XPS and DRIFTS of CO-adsorbed. The results demonstrated that the kind of ZrO2 phase plays a key role in the determination of structure and catalytic properties of Cu/ZrO2 catalysts predetermined by the interface at Cu/ZrO2. The electron transfer between support and Cu surface, caused by the oxygen vacancies at m-ZrO2 and am-ZrO2, is responsible for the active sites for acetaldehyde and ethyl acetate formation. The highest selectivity to ethyl acetate for Cu/m-ZrO2 catalyst up to 513K was caused by the optimal ratio of Cu0/Cu+ species and the high density of basic sites (O2−) associated with the oxygen mobility from the bulk m-ZrO2.

Interface Phenomena Responsible for Bonding between TRIP Steel and Partially Stabilised Zirconia as Revealed by TEM

In this study, we demonstrate results of the transmission electron microscopy investigations of phenomena at the interface between partially stabilized zirconia (PSZ) and TRIP steel (steel with the Transformation Induced Plasticity effect). The interface phenomena were studied in thin film samples, which were prepared as a model system by the deposition of PSZ on TRIP steel using magnetron sputtering, and in a bulk composite containing TRIP steel and PSZ powder. The bulk composite was produced by Spark Plasma Sintering. The phase composition and the orientation relationship of several crystallites of PSZ and TRIP steel in the vicinity of their mutual interface were analysed using high resolution transmission electron microscopy (HRTEM). The phase identification was facilitated by using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS) in scanning transmission electron microscope. In particular, the analysis of the zirconium N2,3 edge in the electron energy loss spectra allowed the cubic, tetragonal and monoclinic ZrO2 phases to be distinguished from each other. Thus mainly EELS complemented the HRTEM analysis of the orientation relationships. The same experimental methods were utilized to follow the local phase transformations near the PSZ/TRIP steel interfaces after a mechanical deformation of the samples.

Relating cumene hydrocracking activity to the acidic center of CrO3–ZrO2

The properties of CrO3–ZrO2 and Pt/CrO3–ZrO2 were studied by IR and ESR spectroscopy to elucidate the active sites in n-heptane isomerization and cumene cracking. Surface analyses showed the presence of cubic, monoclinic and tetragonal phases of ZrO2 with a BET specific surface area of about 150m2/g for both catalysts. 2,6-Lutidine adsorbed IR confirmed the presence of treble doublet bands at 1675+1660, 1650+1625 and 1640+1630cm−1 corresponding to Brønsted acid sites and quartet doublet bands at 1604+1580, 1595+1580, 1590+1580 and 1570+1560cm−1 corresponding to Lewis acid sites. The presence of Pt enhanced the activity of CrO3–ZrO2 in cumene hydrocracking, in which Pt facilitated the formation of protonic acid sites at 1675+1660 and 1650+1625cm−1 through a hydrogen spillover mechanism. The IR study verified that the Lewis acidic center at 1570+1560cm−1 was involved in the formation of protons by trapping electrons. Meanwhile, neither CrO3–ZrO2 nor Pt/CrO3–ZrO2 was active in n-heptane hydroisomerization.

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

Abstract The effects of Mn3O4 addition and reductive atmosphere (N2:H2 = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn3O4 added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO2 was obtained in 1 mol% Mn3O4 added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO2 phases were observed for 1 mol% Mn3O4 added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO2 phase existed stably as Mn3O4 added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.

Structural and spectroscopic properties of high temperature prepared ZrO2–TiO2 mixed oxides

ZrO₂-TiO₂ mixed oxides of various composition, with the molar fraction of TiO₂ ranging from 0.1% to 15%, have been prepared via sol-gel synthesis and then calcined at 1273 K to check both their thermal stability and physicochemical properties. These solids are usually employed in photocatalytic processes and as active phase supports in heterogeneous catalysis. As indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO₂ matrix are formed in the whole range of Ti molar fraction examined. Materials with low Ti loading (0.1%–1%) are basically constituted by the monoclinic phase of ZrO₂ while the tetragonal phase becomes prevalent at 15% of TiO₂ molar fraction. The presence of Ti ions modify the electronic structure of the solid as revealed by investigation of the optical properties. The typical band gap transition of ZrO₂ undergoes, in fact, a red shift roughly proportional to the Ti loading which reach the remarkable value of 1.6 eV for the sample with 10% of molar Ti concentration. Comparing chemical analysis of the solids with XPS data it has been put into evidence that the titanium ions distribution into the solid is not uniform and the concentration of Ti⁴⁺ tend tomore » be higher in subsurface layers than in the crystal bulk. The introduction of titanium ions in the structure increases the reducibility of the solid. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti³⁺ reduced centres which are characterized by a typical EPR signal. Ti³⁺ defects forms, as also forecast by theoretical modelling of the solid, as their energy is lower than that of other possible reduced defective centers. The reduced solids are able to transfer electrons to adsorbed oxygen molecules in mild condition resulting in the formation of surface superoxide anions (O₂⁻) which are stabilized on surface Zr⁴⁺ or, alternatively, on Ti⁴⁺ ions according to the sample composition. - Graphical abstract: The presence of Ti in ZrO₂ modifies UV–vis and electronic properties of mixed oxides. Highlights: • ZrO₂–TiO₂ mixed oxides have been prepared via sol–gel synthesis. • Solids with low Ti loading are constituted by the monoclinic phase of ZrO₂. • The presence of Ti ions modify the electronic structure of the solid. • The introduction of titanium ions increases the reducibility of the solid.« less

Effect of gadolinium doping on phase transformation and microstructure of Gd2O3-Y2O3-ZrO2 composite coatings prepared by electrophoretic deposition

4 mol.% Y2O3 stabilized ZrO2 (YSZ) doped with various quantities of Gd2O3 (G-YSZ) ceramic coatings were synthesized by electrophoretic deposition method, and followed by vacuum sintering and isothermally annealing at 1000 °C for different durations. X-ray diffraction (XRD) was used to investigate their phase composition. Scanning electron microscopy (SEM) was employed to examine their microstructure, while energy dispersive X-ray spectrometer (EDS) was used to assess composition of the composite coatings. The results indicated that YSZ coating was composed of tetragonal and monoclinic phase after vacuum sintering at 1000 °C for 2 h under vacuum (<10−3 Pa). G-YSZ composite coatings were composed of tetragonal, monoclinic phase and a small amount of Gd2Zr2O7 phase after vacuum sintering at 1000 °C for 2 h, whereas the content of monoclinic phase in G-YSZ composite coatings increased with increasing Gd2O3 concentration. It was found that G-YSZ composite coatings were composed of tetragonal ZrO2 phase, monoclinic ZrO2 phase and cubic phase, whereas Gd2Zr2O7 phase disappeared, after G-YSZ composite coatings were isothermally annealed at 1000 °C in air for 100 h. A detailed description of the results and their discussion was presented in the paper.

Effect of oxide nanofillers on fabrication, structure, and properties of zirconia-based composites

Abstract We have studied the effect of NiO on the sintering of yttria-stabilized zirconia at temperatures ranging from 1300 °C to 1500 °C in air and argon environments. It was found that the addition of NiO stabilized the cubic phase of ZrO2 independently from the sintering atmosphere. The monoclinic phase of ZrO2 formed only during sintering within the air environment at temperatures higher than 1450 °C. The transformation of NiO to Ni by reversible decomposition depends on the sintering atmosphere, and this can lead to variations in the nature of inclusions and in changes of the structure and properties of nanocomposite materials in the system ZrO2–NiO(Ni). NiO and Ni inclusions can increase the indentation fracture toughness of zirconia–nickel oxide composite material more than 50%, which can be compared with zirconia ceramics during sintering in a neutral atmosphere alone.

Effect of voltage on properties of microarc oxidation films prepared in phosphate electrolyte on Zr–1Nb alloy

A thin protective film on Zr–1Nb alloy was fabricated by AC microarc oxidation (MAO) in phosphate electrolyte. The morphology, microstructure and phase constituents of MAO films under different discharge voltages were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Their surface roughness and microhardness were measured. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate corrosion behaviors of films. The equivalent circuits of EIS plots for bare Zr–1Nb alloy and MAO films were proposed. It was found that all of films at different discharge voltages consisted of the monoclinic ZrO2 and tetragonal ZrO2 phases, but the content of monoclinic ZrO2 was much higher than that of tetragonal ZrO2 phase. Microhardness of MAO films is over 570HV, which is higher than that of Zr–1Nb alloy substrate with 245HV. The film fabricated in higher voltage shows lower corrosion current density and higher impedance, which displays better corrosion resistance. The EIS results identify that the inner compact layer of MAO films plays a key role in improving the corrosion resistance of Zr–1Nb alloy.

Preparation of NASICON Disk by Tape Casting and its CO&lt;sub&gt;2&lt;/sub&gt; Sensing Properties

In this work, NASICON-type disks with the formula, Na3Zr2Si2PO12 were prepared by non-aqueous tape casting method. The effect of the dispersant on the slurry viscosity was investigated, triethanolamine was found to be an effective dispersant for NASICON slurry. The correlation between the overall conductivity and the sintering conditions (temperature and time) for the NASICON disk was also studied. Green tapes were calcined at 900°C, 1000°C, 1100°C for 6h and 12h, respectively. Results revealed that the overall conductivity increased with the increasing of the sintering temperature and decreased with the increasing of the sintering time. The segregation of resistive monoclinic ZrO2 phase was examined to have a negative effect on the overall conductivity. The CO2 sensor using NASICON disk and Li2CO3-BaCO3 complex thick film was fabricated and evaluated, the sensitivity was about 82.9 mV/decade at 450°C.