Unusual structural phase transition in nanocrystalline zirconia

The present work is the first example demonstrating that a hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The X-ray diffraction pattern of the hydrous zirconia heated to 310 °C shows that diffraction peaks begin to emerge and reveals a just crystallized mixture of predominantly monoclinic zirconia (70%) with some tetragonal zirconia(30%). In other words, the hydrous zirconia formed in the present work yields the predominantly monoclinic structure coexisting with the tetragonal one as soon as crystallization starts at low temperature (310 °C). This is an important exception to the general principle that amorphous zirconia precursors first convert to the tetragonal structure of zirconia with increasing calcination temperature and then transform to the monoclinic one at a higher temperature (∼600 °C). At the crystallization temperature (310 °C), the monoclinic crystallite size is about 17 nm and the tetragonal one 28 nm. The monoclinic crystallite is much smaller than the tetragonal one with which it co-exists. This result is also not consistent with the traditional view that a critical particle size effect is responsible for the stability of the tetragonal and monoclinic structures. When the temperature (310 °C) is slightly raised to 320 °C, the XRD pattern shows a nearly pure monoclinic zirconia. The crystallite size of the monoclinic zirconia is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. The unusual structural phase transition has been investigated by several complementary experimental tools: X-raydiffraction and surface analyses, and infrared and Raman spectroscopies. PACS 81.07.-b; 64.70.Nd; 82.80.-d; 78.67.-n; 81.05.Je     

The structure and phase composition of stabilized zirconia-based nanosystems exposed to shock-wave treatment

The special features of the structure and phase composition of nanocrystalline zirconia-based powders subjected to shock-wave treatment are studied. The investigations show that zirconia with small amounts of yttria and/or alumina is in nanocrystalline and quasi-amorphous states representing a nonequilibrium solid solution of ZrO2 (Y, Al) and that an increase in the monoclinic phase abundance is associated with a reduction in the critical size of tetragonal crystallites due to an accumulation of lattice microdistortions. The monoclinic phase in powders with yttria and alumina additions is not formed even with shock compression at pressures up to 20 GPa. This is attributed to the fact that the resultant lattice microdistortion level is inadequate to destabilize the nanocrystalline tetragonal phase. Relaxation of microdistortions on annealing causes the critical size of tetragonal crystallites to increase. As this takes place, the monoclinic phase is converted into the tetragonal one.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 61–70, September, 2004.     

Investigations on composition and morphology of electrochemical alumina and alumina–yttria stabilised zirconia deposits

Conversion coatings modified by deposits of electrolytic alumina added or not with yttria and/or zirconia, have been studied which are well known for their resistance to chemical attack and high temperature. Conversion coating, characterised by a particular morphology and strong interfacial adhesion with the substrate, facilitate the electrochemical deposition of ceramic layers and enhance their adhesion to the substrate. Zirconia–alumina coating behaviour at 1000°C is similar to that of alumina coating; from 800°C, the chromium diffuses from the stainless steel through the electrolytic refractory coating up to the external interface, provokes discontinuities and can modify its protective character. Yttrium stabilises the cubic and the tetragonal form of the zirconia; so, during cooling, the phase transformation near 1000°C of tetragonal zirconia to monoclinic form cannot take place.     

Synthesis, structure, microstructure and mechanical characteristics of MOCVD deposited zirconia films

Zirconia (ZrO₂) thin films were deposited by metal organic chemical vapor deposition (MOCVD) on (1 0 0) Si over temperature and pressure ranges from 700 to 900 °C and 100 to 2000 Pa, respectively. The oxide films were characterized by field emission microscopy and X-ray diffraction so that microstructure and ratios of monoclinic and tetragonal phases could be estimated according to the process conditions. The mechanical behaviour of the substrate-film systems was investigated using Vickers micro-indentation and Berkovitch nano-indentation tests. The characteristics of silicon are not modified by the presence of a thin film of silicon oxide (10 nm), formed in the reactor during heating. Young's modulus and the hardness of tetragonal zirconia phase, 220 and 15 GPa, respectively, are greater than values obtained for monoclinic phase, 160 and 7 GPa, respectively. The zirconia films are well adherent and the toughness of tetragonal zirconia phase is greater than that of monoclinic phase.     

91Zr NMR characterisation of phases in transformation toughened zirconia.

A detailed 91Zr NMR investigation is presented of the five component (cubic, monoclinic, tetragonal, orthorhombic and delta) phase mixture in the transformation toughened engineering ceramic, magnesia-partially-stabilised zirconia (MgPSZ). The phases are distinguished on the basis of the quadrupole interaction displayed in the powder pattern of the 91Zr(1/2, -1/2) transition. This paper reports: (i) the first characterisation of the magnesia-fully-stabilised cubic phase at the eutectic composition (13.5 mol% MgO); (ii) the observation of a poorly ordered tetragonal phase on fast cooling ZrO2 (9.3 mol% MgO) from the cubic phase field at 1720 degrees C, and the subsequent growth and ordering of the tetragonal phase precipitates due to further annealing; (iii) the observation of the (partial) transformation of the cubic phase to the ordered delta-phase (Mg2Zr5O12) on annealing MgPSZ at 1100 degrees C for 8 h; and (iv) the observation of the transformation of the tetragonal phase into the orthorhombic phase after cooling in liquid nitrogen, and the reverse transformation after heating to 600 degrees C.     

Stress driven phase transformation in ZrO2 film

Zirconia thin layers (250 nm) were deposited on stainless steel substrates using organo-metallic injection chemical vapour deposition (MOCVD) process with zirconium beta-diketonate as precursor at low oxygen pressure and 900 °C. Low roughness zirconia films were made up of a mixture of tetragonal and monoclinic phases depending on the process conditions. As the zirconia tetragonal phase is known to be stabilized by small grain size and/or internal compressive stresses, tensile and/or compressive external stress fields were applied at room temperature using a bending test device. Then, XRD measurements were used to determine tetragonal/monoclinic phase ratio and also residual stresses in the films before and after the tests. The film surface was observed at the various stages of the experiments by field electron gun-scanning electron microscopy (FEG-SEM).Under these stress fields, phase transformation occurs in the film, from tetragonal structure to a monoclinic one. Some preferential tetragonal planes give rise to monoclinic ones. The external stress field is also likely to redistribute the internal stresses within the films.     

二氧化锆相结构的漫反射光谱表征

利用紫外漫反射光谱考察了二氧化锆基催化剂的表面相组成 ,结合X射线粉末衍射、差热分析和X射线光电子能谱研究了铂化以及Al2 O3 的引入对样品中ZrO2 体相和表面相结构的影响。结果表明 :通氢还原铂化过程使部分的ZrO2 由四方相向单斜相转化 ,而且在催化剂表面这一效应更为显著。随着基体中引入Al2 O3 量的增加 ,二氧化锆的相变温度不同程度地向高温转移 ,添加Al2 O3 使ZrO2 得到较好的分散并抑制单斜相二氧化锆的生成。     

Monoclinic-tetragonal phase transition in zirconium and hafnium dioxides: A high-temperature raman scattering investigation

Zirconium dioxide ZrO₂ and hafnium dioxide HfO₂ are investigated using high-temperature Raman spectroscopy in the temperature range 300–2080 K, including the regions of the monoclinic-tetragonal phase transitions revealed in these materials. An analysis is made of the specific features observed in the evolution of the high-temperature Raman spectra of both the monoclinic (m) and tetragonal (t) modifications of ZrO₂ and HfO₂ with variations in the temperature. The polarized Raman spectra of the metastable tetragonal phases in solid solutions based on zirconia and hafnia are used to identify the symmetry of vibrations in the spectra of the tetragonal modifications of pure zirconium and hafnium dioxides, which exist at high temperatures.     

Production and Characterization of Zirconia (ZrO2) Ceramic Nanofibers by Using Electrospun Poly(Vinyl Alcohol)/Zirconium Acetate Nanofibers as a Precursor

Zirconia (ZrO₂) inorganic ceramic nanofibers were produced using electrospinning of the poly(vinyl alcohol)/zirconium acetate as a precursor followed by calcinating and sintering to decompose the polymer and turn the metal salt (zirconium acetate) into the metal oxide. Characterization of the nanofibers, including polymer thermal decomposition, chemical and crystal structure, phase transformations, and fiber morphology were investigated by simultaneous thermal analysis (STA), thermomechanical analysis (TMA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). The results showed that the polymer decomposition started at 250°C and zirconia nanofibers with different phases (tetragonal and monoclinic) were obtained by the calcination of the precursor nanofibers at various temperatures between 500°C and 1100°C. The initially crystallized zirconia phase, which formed at 500°C, was tetragonal and with increasing calcination temperature, zirconia nanofibers with increasing amount of monoclinic phase were formed. Consequently, at 1100°C, the tetragonal phase disappeared and was transformed to the monoclinic phase of the zirconia completely. Increasing the calcination temperature caused the fiber average diameter decrease and grain growth took place due to the removal of the polymer and organic groups; neighboring grains sintered to each other and formed fibers with a high aspect ratio. At 1100°C the grains size was about the same as the fiber diameter.     

On the nature of the KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> high-temperature transformation

For over two decades, the high-temperature phase transition (HTPT) at around T _p = 180 °C on KH₂PO₄ (KDP), which involves an ionic conductivity increase, constitutes a controversial subject; while most authors ratify a physical transformation (tetragonal → monoclinic phase transition), others defend the chemical transformation. A careful high-temperature phase behavior examination of this acid salt by means of modulated and conventional differential scanning calorimetry, thermogravimetric analysis, simultaneous thermogravimetric and differential scanning calorimetry, impedance spectroscopy, and temperature evolution of X-ray diffraction was performed to provide a possible solution to this long-standing issue. We found that the structural phase transition does not take place. Instead, a chemical transformation occurs at T _p. When KDP is heated through this temperature, the sample initially corresponding to a single phase (tetragonal) transforms to a sample composed of two solid phases: tetragonal KDP, located at its bulk, and monoclinic potassium metaphosphate (KPO₃), located at its surface. Most of the water produced evaporates, but a small portion of liquid water bonds to KPO₃. Because this is of polymeric nature, it takes the role of a host matrix that contains liquid water regions. Consequently, given that part of the water dissolves a portion of surface salt (providing protons), the surface sample system behaves in a similar manner to a polymer electrolyte membrane where the proton transport mechanism includes the vehicle type, using hydronium (H₃O⁺) as a charge carrier. On further heating, the bulk tetragonal KDP phase reduced to its total decomposition. The metastability of the high-temperature phase below T _p is also explained.     

基于热膨胀性质的ZrO2 固体电解质性能与相关系模型

ZrO₂固体电解质室温下各相的相对含量、高温可相变量是决定材料抗热震性和导电性能的关键因素.固体电解质抗热震性与导电性能的匹配,对低氧含量下的钢水定氧起着重要作用.以此为前提,建立了电解质材料体系的抗热震性与室温相比例之间的线性模型,提出了其高温电导率与相变之间的演化模型.为制备用于测低氧活度的高精度冶金氧传感器提供参考依据.     

Characterization of laser sealed coatings of yttria partially stabilized zirconia

The present paper reports an experimental investigation based on X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) analyses of phases formed after laser sealing of plasma sprayed coatings of 8.5 wt% yttria partially stabilized zirconia (YPSZ). X-ray diffraction and X-ray step-scanning analyses showed that the plasma sprayed and sealed coatings consisted mainly of t′ phase with a very small amount of monoclinic phase (m phase) in the plasma sprayed coatings. It was also found that the small amounts of m and cubic phases (c phase) present in the sealed coatings were dependent on laser processing specific energies (specific energy is equal to laser power/traverse speed x beam diameter). It was also found that rhombohedral (r) phase formed after laser sealing of coatings at higher specific energies. A direct relationship between c/a ratio of transformable tetragonal phase (t phase) produced by thermal treatment of sealed coatings and nontransformable tetragonal phase (t′ phase) and the amount of Yttria was obtained. A new equation was derived, which describes the relationship between Yttria concentration and the c/a ratio of tetragonal phases (t or t′).     

Influence of environment and grinding on the crystallisation mechanism of ZrO2 gel

The influence of grinding and surrounding atmosphere on the thermal transformations of zirconia gel has been studied. The XRD analysis of the products obtained by thermal decomposition of zirconia gel has shown that pure tetragonal phase is obtained if the gel decomposition is carried out under high vacuum or dry inert atmosphere, while monoclinic zirconia results from the decomposition of the zirconia gel under air or inert gas saturated with water vapour. A mechanism for the thermal crystallisation of zirconia gel has been proposed from the study of the variation of the crystal size of the monoclinic and tetragonal zirconia phases formed as a function of the temperature and the surrounding atmosphere.The thermal decomposition of ground zirconia leads to the formation of ZrO₂ with a percentage of tetragonal phase closed to 90% irrespectively of the surrounding atmosphere. The stabilisation of the tetragonal phase by grinding seems to be connected with the formation of tetragonal zirconia nuclei that cannot be observed by XRD. The crystallisation enthalpy measurements carried out by DSC support this conclusion.     

Structural phase transitions and elastic properties of zirconia

The structural phase transformations in various phases (monoclinic, ortho I, ortho II, and tetragonal) of zirconia (ZrO₂) have been investigated using an effective interionic interaction potential. The cohesive energy, the equation of state, and the elastic properties of these phases have also been studied and found to reproduce well the experimentally observed data for almost all the phases of zirconia ceramics.     

Neutron diffraction study of the size-induced tetragonal to monoclinic phase transition in zirconia nanocrystals

Accurate neutron powder diffraction experiments at several temperatures allow one to monitor the reconstructive tetragonal to monoclinic phase transition as a function of the size of zirconia nanoparticles. The structure of the tetragonal phase observed in the nanocrystals is identical to that observed in micrometric zirconia above 1400 K. A uniaxial strain depending on grain size is observed. The phase transition occurs above a threshold crystal size. These results are analyzed within the Landau theory and can be understood as a mechanism of size-dependent phase transition where the primary order parameter is altered by the nanoparticle size.     

Surfactant-assisted synthesis of defective zirconia mesophases and Pd/ZrO2: Crystalline structure and catalytic properties

Mesoporous zirconia nanophases with structural defects were synthesized by using a surfactant-templated method. Physicochemical properties and crystalline structures of the zirconia nanophases were studied by means of thermogravimetric analysis (TGA), N₂ physosorption isotherm and in situ Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The resultant materials show typical mesoporous features which vary with calcination temperature. The cationic surfactant in the network of the solids induces structural deformation and defect creation. The zirconia consists of monoclinic and tetragonal nanophases which contains many structural defects, and its crystalline structure shows microstrain. Both, concentration of lattice defects and degree of the crystal microstrain, decrease as the calcination temperature is increased. When CO is adsorbed on the surface of Pd/ZrO₂, linear bonds of CO–Pd⁰, CO–Pd^δ+ and CO–Zr⁴⁺ are formed, accompanying with CO₂ production. Catalytic evaluation shows that the Pd/ZrO₂ catalyst is very active for CO oxidation and NO reduction. In the case of oxygen absence from reaction mixture, high selectivity to N₂ is achieved without any NO₂ formation. In the oxygen rich condition, CO conversion is enhanced but less than 19% NO₂ is produced. N₂O is formed only in the reducing condition and its selectivity is sensitive to reaction temperature. The possible mechanisms of NO + CO and NO + CO + O₂ reactions over Pd/ZrO₂ catalyst related to reactant dissociation on the Pd metals and to defective structure of the nanozirconia support are discussed.     

AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles

The incorporation of monoclinic zirconia nanoparticles and their subsequent transformation is examined for coatings formed on magnesium by plasma electrolytic oxidation under AC conditions in silicate electrolyte. The coatings are shown to comprise two main layers, with nanoparticles entering the coating at the coating surface and through short-circuit paths to the region of the interface between the inner and outer coating layers. Under local heating of microdischarges, the zirconia reacts with magnesium species to form Mg₂Zr₅O₁₂ in the outer coating layer. Relatively little zirconium is present in the inner coating layer. In contrast, silicon species are present in both coating layers, with reduced amounts in the inner layer.     

Internal stresses and stability of the tetragonal phase in zirconia thin layers deposited by OMCVD

Zirconia thin films were deposited by OMCVD (organo-metallic chemical vapour deposition) at various temperatures and oxygen partial pressures on a AISI 301 stainless steel substrate with Zr(thd)₄ as precursor. The as deposited 250 nm thin zirconia films presented a structure consisting of two phases: the expected monoclinic one and also an unexpected tetragonal phase. According to the literature, the stabilization of the tetragonal phase (metastable in massive zirconia) can be related to the crystallite size and/or to the generated internal compressive stresses.To analyze the effect of internal and external stresses on the thin film behaviour, in-situ tensile experiments were performed at room temperature and at high temperature (500 °C).Depending on the process parameters, phase transformations and damage evolution of the films were observed. Our results, associated to XRD (X-ray diffraction) analyses, used to determine phase ratios and residual stresses within the films, before and after the mechanical experiments, are discussed with respect to their microstructural changes.     

Effect of Hydrolysis Conditions on the Direct Formation of Nanoparticles of Ceria–Zirconia Solid Solutions from Acidic Aqueous Solutions

The effect of the cation concentration, hydrolysis temperature, and composition in the CeO₂–ZrO₂ system on the direct precipitation of ceria–zirconia solid solutions and the structure of the precipitates from acidic aqueous solutions of (NH₄)₂Ce(NO₃)₆ and ZrOCl₂ by hydrolysis under hydrothermal conditions were investigated. Nanometer-sized (8–10 nm) ceria–zirconia solid solution particles in a composition range of 0 to 60 mol% ZrO₂ were directly precipitated from the solutions with total metal cation concentration less than 0.2 mol/dm³ by simultaneous thermal hydrolysis at 150–240°C. The crystalline phase of the precipitates gradually changed from cubic and/or tetragonal to monoclinic with increasing the cation concentration of the solution from 0.2 to 0.8 mol/dm³ at the starting composition of 50 mol% ZrO₂ under hydrolysis condition of 150°C for 48 h, which was attributed to decrease in the supply of hydrolyzed Ce component caused by decrease in the hydrolysis ratio of (NH₄)₂Ce(NO₃)₆. Ceria–zirconia solid solutions containing large amount of ZrO₂ maintained high specific surface area and small-sized crystallite after heat-treatment at 900–1000°C for 1 h.     

The control of very low oxygen partial pressures with stabilized zirconia cell

Control of very low oxygen partial pressures was performed in the range of 10⁻²¹-10⁻²⁴ Pa at 750°C by pumping oxygen into the purified hydrogen stream with a stabilized zirconia cell. The oxygen partial pressures were monitored by a stabilized zirconia sensor. The stabilized zirconia oxygen sensor was calibrated by H₂-CO₂ gas buffer mixture in the range of oxygen pressure from 10⁻¹⁷ to 10⁻²¹ Pa, and oxygen partial pressures below 10⁻²¹ Pa were measured by extrapolating the calibration line to very low oxygen partial pressures. The lowest oxygen partial pressure controlled was 10⁻²⁴ Pa at 750°C, which was limited by gas leaks in the system and also by the reduction of the ionic transference number in solid electrolyte used as the oxygen pump.