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.     

Enthalpies of formation in the scandia‐zirconia system

The scandia‐zirconia (ScZ) solid solutions have been attracting attention from the communities interested in solid‐oxide fuel cells because they possess the highest ionic conductivity among zirconia‐based materials. However, this system shows a relatively large number of polymorphs with lack of thermodynamic data to enable comprehensive phase control for property optimization. In this work, the enthalpy of formation of the ScZ system within the range 0–20 mol% Sc₂O₃ is determined by combining the surface energy values with enthalpy of drop solution data obtained from high‐temperature oxide melt solution calorimetry. The heats of formation, a key data for understanding phase stability, for five polymorphs: monoclinic (m), tetragonal (t), cubic (c), and rhombohedral (β and γ) are reported for the first time.     

The Effects of pH and Alkaline Earth Ions on the Formation of Nanosized Zirconia Phases Under Hydrothermal Conditions

The phase composition and crystallite size of zirconia formed under hydrothermal conditions is strongly dependent on crystallization conditions, in particular, pH and a mineralizer. Monoclinic ZrO₂ formed easily in the strong acid and basic media. When pH<1 or >14, monoclinic ZrO₂ was exclusively obtained and its crystallite size increased with increasing pH. In the range of pH 1–14, products of hydrothermal reaction are a mixture of monoclinic and tetragonal ZrO₂. The effects of Mg²⁺, Ca²⁺ and Sr²⁺ ions on the formation of zirconia under hydrothermal conditions were investigated. The presence of these bivalent M²⁺ ions was in favour of the formation of tetragonal (or cubic) ZrO₂. The nanosized ZrO₂ crystallites of cubic and tetragonal symmetry were obtained at pH 10, 220°C for 2 h and in the case of Ca²⁺ and Sr²⁺ as mineralizer, respectively.     

Energy Crossovers in Nanocrystalline Zirconia

The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse-grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high-temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO₂ was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m². The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m², for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/surface area gave surface enthalpies of 4.2 J/m² for the monoclinic form and 0.9 J/m² for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO₂ was estimated to be 10±1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol.     

Synthesis of cubic and monoclinic hafnia nanoparticles by pulsed plasma in liquid method

Hafnia (HfO₂, hafnium dioxide) is a wide band gap and high-κ material, and the metastable cubic hafnia has a much higher permittivity compared with the normal monoclinic hafnia. Here, we employ a one-step process, the pulsed plasma in liquid (PPL) method to synthesize two types of hafnia nanoparticles (NPs): one which is mainly in cubic phase (cubic: 81.7 at%, monoclinic: 18.3 at%) and the other which is in monoclinic phase. High-resolution transmission electron microscopy images showed that the particles were small (particle size ~3 nm). X-ray absorption fine structure analysis showed no chemical shifts, indicating that the synthetic hafnia NPs contained no oxygen vacancy. The synthetic hafnia NPs mainly in cubic phase showed a much higher relative permittivity than that of the commercial hafnia (monoclinic), and have a larger band gap than the synthetic monoclinic hafnia NPs.     

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO₂ on the metallic properties of Pt, when supported on WO₃–ZrO₂, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO₂,^2- deposited on amorphous Zr(OH)₄ before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Formation and reactivity of nitrides. IV. Titanium and zirconium nitrides

The reactivities of the interstitial titanium and zirconium nitrides have been compared. Samples of these nitrides have been converted to oxides by being calcined in air. Changes in phase composition, surface area, crystallite and aggregate sizes have been correlated with oxidation time and temperature. Crystallites of rutile, TiO₂, split off from the remaining titanium nitride before they sinter, and inhibit further oxidation. Zirconium nitride oxidation is complicated by formation of tetragonal ZrO₂ at higher temperatures, particularly over 1200°, and monoclinic ZrO₂ at lower temperatures. The nitride initially forms the so-called ‘amorphous’ cubic ZrO₂, notably between 400–600°, which may be stabilised somewhat by the remaining cubic ZrN. Subsequently, there is a further fractional volume increase while formation of monoclinic ZrO₂ is being completed.     

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.     

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.     

Ethylbenzene to styrene in the presence of carbon dioxide over zirconia

ZrO₂ itself was found be active for the dehydrogenation of ethylbenzene, especially in the presence of CO₂, which was aimed to be utilized as an oxidant. This positive effect of CO₂ was highly dependent on the crystalline phases of zirconia. The higher the tetragonal phase contained in ZrO₂, the higher the ethylbenzene conversion and styrene selectivity that were obtained. Highly tetragonal ZrO₂ was more active in oxidative dehydrogenation than monoclinic ZrO₂. The differences of catalytic activities could be ascribed to the differences of the surface area and CO₂ affinity related with surface basicity.     

Synthesis and phase stability of precursor derived HfO2/Si-C-N-O nanocomposites

Hafnium alkoxide modified polysilazane was synthesized by the drop-wise addition of hafnium tetra(n-butoxide) to polysilazane. The solid state thermolysis (SST) temperature and the ceramic yield for both the polysilazane and modified polysilazane were determined by performing thermogravimetry. Fourier transform infrared spectroscopy was performed to understand the polymer to ceramic conversion as well as the bonding characteristics of the ceramics. The modified polymer after crosslinking was subjected to SST at 800 °C at a constant heating rate of 5 °C/min for a dwell time of 2 h in atmospheric ambience. From the X-ray diffractograms, the as-thermolysed ceramics were observed to remain X-ray amorphous and on heat-treatment resulted in the crystallization of tetragonal hafnia. However, on heat-treatment at 1500 °C, reverse phase transformation from tetragonal to monoclinic hafnia was observed. Raman spectroscopy and transmission electron microscopy were employed to further understand the phase evolution. The thermal stability and the influence of amorphous matrix on the coarsening of HfO₂ were also evaluated.     

Phase stability in scandia‐zirconia nanocrystals

Scandia‐zirconia system has great technological interest as it has the highest ionic conductivity among doped zirconia ceramics. However, polymorphism is the most important limiting factor for application of this material. Considering that there is a strong grain size dependence on phase transitions in this class of materials, mapping out the stable polymorph as a function of grain size and composition may lead to more efficient compositional design. In this article, the phase stability of zirconia‐scandia nanocrystals was investigated based on experimental thermodynamic data. Exploiting recent advances in microcalorimetry, reliable surface energy data for five polymorphs of scandia‐zirconia system: monoclinic, tetragonal, cubic, rhombohedral β and γ are reported for the first time. Combining surface energy values with bulk enthalpy data obtained from oxide melt drop solution calorimetry allowed us to create a predictive phase stability diagram that shows the stable zirconia polymorph as a function of composition and grain size of the specimen within the range of 0‐20 mol% scandia.     

The role of microstructure on high-temperature oxidation behavior of hafnium carbide

Microstructure development of the products formed upon oxidation of hafnium carbide (HfC_x, x = 0.65, 0.81, or 0.94) at 1300°C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO₂C_x that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase-separated region vary from cubic to monoclinic as grains coarsen from ca. 2–20 nm, respectively. Immediately adjacent to the phase-separated region is carbon-free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high-temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO₂C_x product.     

Mechanical properties and thermal conductivities of 3YSZ-toughened fully stabilized HfO2 ceramics

In this work, the 3 mol% yttria stabilized zirconia (3YSZ) composed of tetragonal phase has been introduced into the 10 mol% Er₂O₃ stabilized cubic hafnia (10ErSH) matrix to improve its fracture toughness. The effects of the addition of 3YSZ on the phase composition, microstructure, mechanical properties and thermal conductivities of the 10ErSH have been investigated. The results showed that all the 3YSZ-toughened 10ErSH samples were composed of cubic phase and a little (<10 mol%) monoclinic phase. The introduced tetragonal phase of 3YSZ fully disappeared even when the volume fraction of 3YSZ reached 50%, indicating that the phase transformation occurred during 1500 °C. The fracture toughness for the sample with 50% 3YSZ was improved by 60% compared with the pure 10ErSH ceramics owing to the sub-mico/micro hybrid structure, which changed the crack propagation mode and consumed part of the crack extension energy. Additionally, the thermal conductivity slightly decreased due to the mass and radius misfits induced by substitution atoms (Zr⁴⁺, Er³⁺ and Y³⁺). Considering the improved mechanical and thermal properties, the 3YSZ-toughened 10ErSH ceramics may be alternative TBC materials.     

Phase evolution and chemical durability of Zr1‐xCexO2 (0 ≤ x ≤ 1) ceramics

Ce‐doped zirconia ceramics with general stoichiometry of Zr_1‐xCe_xO₂ (0 ≤ x ≤ 1) have been obtained by substitution of Ce⁴⁺ for Zr⁴⁺ in ZrO₂. The phase and microstructure evolutions of the ceramics were investigated, and the effects of composition, temperature, and pH value on the chemical durability of the ceramics were also studied. The results show that the phase transformation from monoclinic to tetragonal takes place at about x = 0.2, and from tetragonal to cubic at about x = 0.6. It is found that the increase in Ce content and/or sinter temperature promote the phase transformation. The leaching studies show that the normalized leaching rates of Ce (LR_Ce) increase with increasing Ce content. Moreover, LR_Ce in acid solution are higher than those in neutral and alkaline solution. After 42 days, LR_Ce is 10^?5 ~ 10^?7 g m^?2 d^?1 under all different leaching conditions, exhibiting their excellent chemical durability.     

Growth of nanolaminate structure of tetragonal zirconia by pulsed laser deposition

Alumina/zirconia (Al₂O₃/ZrO₂) multilayer thin films were deposited on Si (100) substrates at an optimized oxygen partial pressure of 3 Pa at room temperature by pulsed laser deposition. The Al₂O₃/ZrO₂ multilayers of 10:10, 5:10, 5:5, and 4:4 nm with 40 bilayers were deposited alternately in order to stabilize a high-temperature phase of zirconia at room temperature. All these films were characterized by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM), and atomic force microscopy. The XRD studies of all the multilayer films showed only a tetragonal structure of zirconia and amorphous alumina. The high-temperature XRD studies of a typical 5:5-nm film indicated the formation of tetragonal zirconia at room temperature and high thermal stability. It was found that the critical layer thickness of zirconia is ≤10 nm, below which tetragonal zirconia is formed at room temperature. The XTEM studies on the as-deposited (Al₂O₃/ZrO₂) 5:10-nm multilayer film showed distinct formation of multilayers with sharp interface and consists of mainly tetragonal phase and amorphous alumina, whereas the annealed film (5:10 nm) showed the inter-diffusion of layers at the interface.     

X-ray diffraction study of phase transformation in hydrolyzed zirconia nanoparticles

The occurrence of metastable tetragonal (t′) phase and its transformation into monoclinic (m) in nanocrystalline ZrO₂ were systematically studied by using X-ray diffraction with respect to the grain size and processing conditions. The zirconia particles in the average size range 7–20 nm were prepared by hydrolyzis of zirconium oxy-chloride solution for different hydrolyzing periods. Drastic changes in grain size and the phase formation with the hydrolyzing time were observed. The amorphous ZrO₂ crystallizes in the t′ phase around 718 to 753 K on heating. However, if the crystallization occurs during the hydrolyzis itself, the monoclinic phase is formed even the grain size is less than 10 nm. The microstructural defects seem to dominate the grain size effect causing the reduction in crystal symmetry in the nanocrystalline zirconia obtained after 72 h of hydrolyzis. ©     

Monoclinic phase-free,low temperature spark plasma sintering of CeO2-doped ZrO2 ceramics and its associated benefits on mechanical properties

Consolidating a CeO₂-doped ZrO₂ ceramics, free from monoclinic phase using spark plasma sintering (SPS) is a major challenge faced by previous researchers; Ce⁺⁴ → Ce⁺³ conversion under reducing environments was assigned as the prime factor. We report dense (> 95 % of theoretical density) 20 mol. % CeO₂-doped ZrO₂ ceramics, free from monoclinic phase and any of micro/ macro-cracks via SPS. The sintering temperature (1175 ℃) used for the present work was the lowest compared to previous reports on the same system. Phase analysis revealed a mixture of tetragonal (major phase) and cubic phase (minor). No depletion of cerium (Ce) from the ZrO₂ matrix and no additional/impurity phases were noted after SPS; a common issue that has been observed in most of the previous works. Sintered ceramics showed appreciably high hardness (>11 GPa); the obtained toughness was in-between of tetragonal and cubic CeO₂-ZrO₂ ceramics.