Stabilized nanoparticles of metastable ZrO2 with Cr3+/Cr4+ cations: preparation from a polymer precursor and the study of the thermal and structural properties

Nanoparticles of stabilized ZrO₂ in a single cubic phase by 5–20 at.% (of total metal cations) Cr³⁺/Cr⁴⁺ addition are obtained through a chemical method using a polymer matrix made of sucrose and polyvinyl alcohol. On heating at 250–900 °C in air, the polymer network decomposes and burns out leaving behind a dispersed microstructure in 10–25 nm diameter particles of cubic ZrO₂ in spherical shape. A modified microstructure comprises of 10–14 nm crystallites of dispersed tetragonal phase, or both tetragonal and monoclinic phases, in cubic phase appear on a prolong (2 h or larger) heating of precursor at 900–950 °C. Particles in tetragonal ZrO₂ are in acicular shape, while the monoclinic phase is in the shape of platelets. The Cr³⁺/Cr⁴⁺ additive facilitates formation of cubic phase in small particles on a controlled decomposition and combustion of precursor. It stabilizes small particles by inhibiting their growth by forming a thin amorphous surface layer over them.     

Eight mole percent yttria stabilized zirconia powders by organic precursor route

An organic precursor-mixing route has been developed for preparation of 8 mol% yttria stabilized zirconia (8YSZ) ceramics. Polymeric salt of succinic acid with yttrium and zirconium has been prepared separately by treating sodium succinate with yttrium chloride and zirconyl chloride followed by washing with water and drying at 120 °C. Thorough mixing of the two salts in stoichiometric proportions by planetary ball milling followed by calcination at 850 °C resulted in a precursor powder containing nanocrystalline (∼40 nm) monoclinic zirconia, tetragonal YSZ, cubic YSZ and yttria. Compacts prepared after deagglomeration of powder by planetary ball milling produce 8YSZ ceramics having density 99.3% TD on sintering at 1550 °C for 2 h.     

Electrospinning of Y2O3- and MgO-stabilized zirconia nanofibers and characterization of the evolving phase composition and morphology during thermal treatment

The current paper focuses on the fabrication of yttria and magnesia stabilized zirconia nanofibers via electrospinning from zirconyl chloride octahydrate and polyvinylpyrrolidone precursors with minor additions of yttrium nitrate hexahydrate (3 mol.%) or magnesium nitrate hexahydrate (10 mol.%). The precursor materials were dissolved in an ethanol-water mixture in a ratio of 75:25. After successful fiber preparation, the thermal decomposition behavior of the starting materials and the subsequent phase evolution at elevated temperatures were studied. Pure tetragonal zirconia nanofibers were obtained for the composition stabilized with 3 mol.% yttria when the thermal treatment was conducted with a heating rate of 10 K/min up to 1100 °C. In future research work, these tetragonal zirconia nanofibers will be used as reinforcing material in metal matrix composites based on metastable austenitic steel. The combination of the TRIP/TWIP-effect in the steel matrix with the stress-assisted tetragonal to monoclinic phase transformation in the tetragonal stabilized zirconia will lead to a composite material with outstanding mechanical properties.     

Phase transformation kinetics of 3 mol% yttria partially stabilized zirconia (3Y-PSZ) nanopowders prepared by a non-isothermal process

A crystalline nanopowder of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) has been synthesized using ZrOCl₂ and Y(NO₃)₃ as raw materials throughout a co-precipitation process in an alcohol-water solution. The phase transformation kinetics of the 3Y-PSZ freeze dried precursor powders have been investigated by nonisothermal methods. Differential thermal and thermogravimetric analyses (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been utilized to characterize the 3Y-PSZ nanocrystallites. When the 3Y-PSZ freeze dried powders are calcined in the range of 703-1073 K for 2 h, the crystal structure is composed of tetragonal and monoclinic ZrO₂. The BET specific surface area of the 3Y-PSZ freeze dried precursor powders calcined at 703 K for 2 h is 118.42 m²/g, which is equivalent to a crystallite size of 8.14 nm. The activation energy from tetragonal ZrO₂ converted to monoclinic ZrO₂ in the 3Y-PSZ freeze dried precursor powders was determined as 401.89 kJ/mol. The tetragonal (T) and monoclinic (M) ZrO₂ phases coexist with a spherical morphology, and based on TEM examination have a size distribution between 10 and 20 nm. When sintering green compacts of the 3Y-PSZ, a significant linear shrinkage of 8% is observed at about 1283 K. On sintering the densification cycle is complete at approximately 1623 K when a total shrinkage of 32% is observed and a final density above 99% of theoretical was achieved.     

The effect of sulfate on the crystal structure of zirconia

Zirconia can be prepared to produce either tetragonal phase or predominantly monoclinic phase upon calcination at 500 °C. The precursors for each phase of zirconias was treated with 1N H₂SO₄ to produce a sulfated material. The results reveal that sulfation causes the tetragonal phase to be formed for both types of zirconia contrary to the data before sulfation, and sulfation increases the crystallization exotherm by 150 °C.     

Specific Features in the Behavior of Amorphous Zirconium Hydroxide: I. Sol–Gel Processes in the Synthesis of Zirconia

Finely crystalline zirconia powder with a crystal size of less than 100 nm is synthesized. It is found that, upon decomposition of zirconium hydroxide, the amorphous phase is formed as an intermediate product. This phase is structurally disordered and involves impurities (OH^– and H₂O). It is demonstrated that the crystallization of amorphous zirconium hydroxide under different conditions provides a way of preparing low-temperature tetragonal and cubic or monoclinic zirconia modifications.     

Surface Enthalpy, Enthalpy of Water Adsorption, and Phase Stability in Nanocrystalline Monoclinic Zirconia

A fundamental issue that remains to be solved when approaching the nanoscale is how the size induces transformation among different polymorphic structures. Understanding the size-induced transformation among the different polymorphic structures is essential for widespread use of nanostructured materials in technological applications. Herein, we report water adsorption and high-temperature solution calorimetry experiments on a set of samples of single-phase monoclinic zirconia with different surface areas. Essential to the success of the study has been the use of a new ternary water-in-oil/water liquid solvothermal method that allows the preparation of monoclinic zirconia nanoparticles with a broad range of (BET) Brunauer–Emmett–Teller surface area values. Thus, the surface enthalpy for anhydrous monoclinic zirconia is reported for the first time, while that for the hydrous surface is a significant improvement over the previously reported value. Combining these data with previously published surface enthalpy for nanocrystalline tetragonal zirconia, we have calculated the stability crossovers between monoclinic and tetragonal phases to take place at a particle size of 28 ± 6 nm for hydrous zirconia and 34 ± 5 nm for anhydrous zirconia. Below these particle sizes, tetragonal hydrous and anhydrous phases of zirconia become thermodynamically stable. These results are within the margin of the theoretical estimation and confirm the importance of the presence of water vapor on the transformation of nanostructured materials.     

The effect of zirconia substitution on the high-temperature transformation of the monoclinic-prime phase in yttrium tantalate

Amorphous yttrium tantalate, as well as solid solutions containing zirconia, transform on heating to a monoclinic-prime phase and then, with further heating, to a crystalline tetragonal (T) solid solution phase at ～1450?°C. On subsequent cooling the tetragonal phase converts by a second-order displacive transformation to a different monoclinic phase not to the monoclinic-prime phase. On subsequent reheating and cooling, the phase transformation occurs between the monoclinic (M) and tetragonal phases, and the monoclinic-prime phase cannot be recovered. The limit of zirconia solubility in both the monoclinic-prime and monoclinic phases lies between 25 and 28?m/o ZrO₂, consistent with previous first-principles calculations. The monoclinic-prime phase is stable up to at least 1400?°C for 100?h for zirconia concentrations from 0 to ～60?m/o ZrO₂. This temperature exceeds the temperature of the equilibrium M-T phase transformation suggesting that the monoclinic-prime phase transforms directly to the tetragonal phase by a reconstructive transformation and is unaffected by the zirconia in solid solution.     

The role of crystalline phase of zirconia in catalytic conversion of ethanol to propylene

Zirconia catalysts can selectively convert ethanol to propylene and exhibit excellent catalytic stability. However, the effects of crystalline phase of ZrO₂ on the catalyst active sites and catalytic performance have not been fully recognized. In this work, when Y or La was doped into ZrO₂, the monoclinic to tetragonal phase transition occurred, and the propylene yields were improved to 44.0% and 42.3%, respectively. The effects of different crystalline phases of ZrO₂ on the ethanol to propylene reaction were analyzed by density functional theory. Comparing the monoclinic, tetragonal and cubic phases of ZrO₂, the tetragonal phase ZrO₂ has the lowest oxygen vacancy formation energy and is likely to form oxygen vacancies to convert ethanol to propylene. Moreover, the adsorption energy of ethanol on tetragonal ZrO₂ is moderate, which is not only beneficial to ethanol conversion, but also reduces catalyst deactivation caused by excessive adsorption. Therefore, tetragonal ZrO₂ shows practical significance for catalyzing ethanol to propylene.     

Thermodynamic calculations of t to m martenstic transformation of ZrO2–CaO binary system

The phase transformation of different polymorphs in zirconia is very important for the processing and mechanical properties of zirconia ceramics. This paper adopts thermodynamic model which is suitable for ceramic system to calculate the Gibbs free energy change of tetragonal and monoclinic phases in ZrO₂–CaO binary system. The difference of the Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CaO as a function of composition and temperature, namely t → m phase transformation driving force, is thermodynamically calculated from the binary systems. Furthermore, in 8.0 mol% CaO–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, was obtained as 1270.3 K, and martensitic transformation starting temperature (M_s) for t → m transformation of this ceramic with a mean grain size of 2.0 mm was calculated as 805.9 K, which is good agreement with experiment one of 793 K with 12.9 K residual.     

Microstructural study and hot corrosion behavior of bimodal thermal barrier coatings under laser heat treatment

In this study, nanostructured YSZ powders were deposited on the Hastalloy X Superalloy substrate coated with a metallic bond coat by plasma spraying to produce a nanostructured thermal barrier coating with bimodal microstructure. After that, the coated samples were heat-treated using a Nd:YAG laser. Then, the microstructures of the conventional and nanostructured TBCs before and after the laser glazing process were examined using a scanning electron microscope (SEM). The coating phases were studied by X-ray diffractometry (XRD). The high-temperature corrosion behavior of the nanostructured plasma sprayed coating in the presence of Vanadium pentoxide and Sodium sulfate molten salt was compared with that of the conventional coatings before and after laser treatment at 1050 °C. The hot corrosion results showed that the coatings had a similar degradation mechanism based on which the corrosive molten salt reacted with the stabilizer of YSZ, producing hot corrosion products such as YVO₄. It led to an unwanted phase transformation from tetragonal (t) to monoclinic (m) Zirconia and the final degradation of the TBC system. However, reducing molten salt penetration, decreasing surface roughness and, reduction of the specific surface area are three important mechanisms that improved hot corrosion resistance, finally extending the lifetime of the glazed samples. The results also showed that the nanostructured TBC had higher hot corrosion resistance in comparison with other samples.     

Preparation of zirconia powders by sol-gel route of sodium glycozirconate complex

Zirconia powders were prepared by a sol-gel method, using sodium glycozirconate complex as precursor synthesized via the Oxide One Pot Synthesis (OOPS) process. Gelation of this precursor was achieved through the variation of the hydrolysis ratio without the use of the dopants. The gel samples were also calcined at different temperatures. The resulting zirconia was characterized using X-ray diffraction (XRD), SEM and nitrogen adsorption/desorption. The solid materials obtained after heat treatment at 500 °C by varying the hydrolysis ratio have large surface areas of 154-220 m² g⁻¹ and a narrow pore size distribution in the mesopore region. By variation of the heat treatment, the zirconia xerogels existed in either an amorphous, tetragonal, or monoclinic form at room temperature. Based on XRD data the first identifiable crystalline structure developed from the amorphous phase was the tetragonal polymorph, which was formed between 500 and 800 °C. When the temperature was raised to 1000 °C, zirconia powder with a monoclinic structure was obtained. Surface areas about 280 m² g⁻¹ was obtained after calcination at 400 °C, which drop to ca. 70 m² g⁻¹ following treatment at 1000 °C.     

Nanocrystallization and Phase Transformation in Monodispersed Ultrafine Zirconia Particles from Various Homogeneous Precipitation Methods

Monodispersed ultrafine (nano- to micrometer) zirconia precursor powders were synthesized by three different physicochemical methods: (I) forced hydrolysis, (II) homogeneous precipitation in inorganic salt solutions, and (III) hydrolysis/condensation of alkoxide. The forced hydrolysis method produced monoclinic nanocrystalline particles (cube shaped) of nanometer scale, which depended largely on the initial salt concentration. Methods II and III, both involving the use of alcohol as a solvent, exhibited a faster particle formation rate and generated amorphous ultrafine (submicrometer) monodispersed microspheres, indicating that the presence of alcohol may have stimulated particle nucleation due to its low dielectric property (and, thus, the low solubility of nucleus species in mixed water-alcohol solutions). Nucleation and growth of the particles in solutions are discussed based on the measurements obtained by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). High-temperature X-ray diffraction (HTXRD) and TGA/DTA studies elucidated the differences in phase transformation for different types of powders. The most interesting finding was the nonconventional monoclinic nanocrystal nucleation and growth that occurred prior to transformation to the tetragonal phase (at 1200°C) during the heat treatment of the nanocrystalline powders produced by the forced hydrolysis.     

Raman study of yttria-stabilized zirconia interfacial coatings on Nicalon™ fiber

The undoped, 3Y- and 9Y-stabilized ZrO₂ interfacial coatings on SiC-based fiber type Nicalon™ were fabricated by sol–gel approach and studied using Raman spectroscopy. Raman spectroscopy proved to be a very successful method for revealing beyond question the monoclinic, tetragonal and cubic modification in the as-prepared and exposed to air ZrO₂-coated Nicalon™ fibers. The quantitative phase analysis in the tetragonal or tetragonal/monoclinic two-phase interfacial zirconia coatings was done using an accurate calibration curve directly determined from the Raman spectra of standard mixtures with known monoclinic and tetragonal phase ratios. It was found that the undoped ZrO₂ coating on Nicalon™ fiber was composed of monoclinic together with tetragonal modification in approximately equal fractions whereas after exposition to air the t → m phase transformation occurred in full extent. The 3YSZ coating also underwent the t → m transformation, with the extent of this transformation being different for various areas of the same filament and for various filaments.A monitoring of the t → m phase transformation within ZrO₂ coating on Nicalon™ fiber using micro-Raman spectroscopy makes it possible quantitatively to evaluate an ability of ZrO₂ as oxidation resistance and readily deformable weak interfacial coating for CMC's.     

Structural Characterization of High-Temperature Zirconia Ceramics by Perturbed Angular Correlation Spectroscopy

Perturbed angular correlation spectra of γ-rays emitted following the decay of dilute ¹⁸¹Hf in several zirconia ceramics are reported. Spectra for monoclinic and tetragonal zirconia, a tetragonal zirconia/yttria alloy, a cubic zirconia/yttria alloy, and two mixed tetragonal/cubic-phase zirconia/yttria alloys were measured as a function of temperature to 1470°C. The spectrum observed for each phase has a unique signature, and the spectrum of mixed-phase materials can be used to determine the relative amounts of the different phases. These data give a cubic/(cubic + tetragonal) phase boundary that is somewhat lower in temperature than indicated by current phase diagrams.     

Phase transformation behavior of 3 mol% yttria partially-stabilized ZrO2 (3Y–PSZ) precursor powder by an isothermal method

The phase transformation behavior of freeze-dried 3 mol% yttria–partially-stabilized zirconia (3Y–PSZ) precursor powder has been studied. When the freeze-dried 3Y–PSZ precursor powder was calcined at 773–1073 K for 2 h, the crystalline structure was composed of tetragonal and monoclinic ZrO₂ as primary and secondary phases, respectively. The freeze-dried 3Y–PSZ precursor powder after calcination at 773 K, the monoclinic ZrO₂ content abruptly increased from 8.00% to 31.51% and the tetragonal ZrO₂ content suddenly decreased from 92.00% to 68.49%, with the duration increasing from 0.5 to 1 min. The activation energy of the isothermal transformation from tetragonal to monoclinic was 7.02 kJ/mol. The kinetics equation for the phase transformation from tetragonal to monoclinic in the freeze-dried 3Y–PSZ precursor powder between 773 K and 1273 K for various durations is described as ln(1/1−α)=1/2.61[t^2.61(1.50×10⁻³)^2.61exp(−7.02×10³/RT)]$\ln (1/1-\alpha )=1/2.61[t^{2.61}{(1.50\times {10}^{-3})}^{2.61}\exp (-7.02\times {10}^3/RT)]$; whereas, the HRTEM image shows a typical monoclinic ZrO₂ domain because of the stress-induced tetragonal to monoclinic ZrO₂ martensitic transformation that has occurred.     

Crystallite and Grain-Size-Dependent Phase Transformations in Yttria-Doped Zirconia

In pure zirconia, ultrafine powders are often observed to take on the high-temperature tetragonal phase instead of the "equilibrium" monoclinic phase. The present experiments and analysis show that this observation is one manifestation of a much more general phenomenon in which phase transformation temperatures shift with crystallite/grain size. In the present study, the effect of crystallite (for powders) and grain (for solids) size on the tetragonal → monoclinic phase transformation is examined more broadly across the yttria–zirconia system. Using dilatometry and high-temperature differential scanning calorimetry on zirconia samples with varying crystallite/grain sizes and yttria content, we are able to show that the tetragonal → monoclinic phase transformation temperature varies linearly with inverse crystallite/grain size. This experimental behavior is consistent with thermodynamic predictions that incorporate a surface energy difference term in the calculation of free-energy equilibrium between two phases.     

Perturbed-Angular-Correlation Study of Zirconias Produced by the Sol-Gel Method

Zirconia powders, prepared by a sol-gel method using zirconium n -propoxide (ZNP) and different H₂O/ZNP and alcohol/ZNP ratios, were investigated by the perturbed-angular-correlation method, along with X-ray diffraction (XRD), Raman spectroscopy, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) techniques. The hyperfine interaction was measured after annealing the samples at increasing temperatures up to 1080°C. Two disordered structures, giving tetragonal patterns in XRD and Raman analysis, were identified in as-produced powders, which consisted of chains of fully hydrolyzed species and chains containing residual organic groups. As the annealing temperature was increased, crystallization occurred, producing the tetragonal and monoclinic phases of zirconia. All powders were essentially monoclinic at the highest temperatures. Those representing low H₂O/ZNP ratios retained a large amount of tetragonal phase over a wide temperature range and transformed to monoclinic with a higher activation energy.     

The low-temperature conversion of methyl formate on zirconium oxide

Zirconium oxide samples prepared by a variety of methods were found to have surface areas from 7 to 226 m²/g and relative amounts of tetragonal to (tetragonal + monoclinic) phases from 0 to 100%. Methyl formate is converted to carbon monoxide and methanol on these samples at temperatures of 220–265 °C. All catalyst samples examined are stable at these temperatures for times-on-stream up to 3 h. Although the conversion varies with the relative amounts of tetragonal phase present and decreases with increase in the calcination temperature, a clear dependence on surface area is also observed. The conversion of methyl formate is found to be first order with activation energies of 80 (±10) kJ/mol. The data can be fitted to a Langmuir-Hinshelwood equation.     

Structural origin of size effect on piezoelectric performance of Pb(Zr,Ti)O3

Grain size effect plays a vital role in piezoelectric performance from both scientific and technological view. However, the underlying structural mechanism related to grain size is still unclear. In the present study, the structural mechanism of grain size effect on piezoelectric performance has been revealed in the prototype Pb(Zr,Ti)O₃ system by using in-situ synchrotron X-ray diffraction. The miniaturization of grain size tends to favor the appearance of higher symmetric tetragonal phase, while a single monoclinic phase is determined in the coarse-grained ceramics. The direct structural evidence reveals that both tetragonal and monoclinic phases in the fine-grained ceramics are less sensitive to the electric field, corresponding to the inferior piezoelectric performance, while the single monoclinic phase in the coarse-grained ceramics is more active to be driven by the electric field, generating good piezoelectric behavior. Both domain switching ability and lattice strain are suppressed with decreasing grain size, which directly leads to the deterioration in piezoelectric performance. The current results will benefit the structural understanding of the size effect of piezoelectric and other related systems.