Pure Monoclinic Zirconia Research Articles

Study of the alumina sintering process with a low zirconia content

One of the strategies for improving alumina ceramics' strength and optical properties is to eliminate grain growth during sintering. In this work, a casting process from water suspension was applied to prepare bulk alumina specimens doped with zirconia inclusions in the amount up to 0.5 wt.%. The effect of pure monoclinic (m-ZrO 2 ) or cubic zirconia (ZrO 2 stabilised with 8 mol.% Y 2 O 3 ) on the sintering and alumina grain growth up to 1600 °C was studied. The results demonstrate that even a negligible addition of zirconia particles (0.2 wt.%) reduces grain growth after sintering up to 1400 °C. At the sintering temperature of 1600 °C, significant grain growth was observed for both pure and doped alumina ceramics. However, the careful analysis of the grain size distribution revealed narrower distribution for zirconia-doped alumina compared to pure alumina ceramic. The Vickers hardness decreases with the increasing sintering temperature due to grains' growth independently on the load applied. The indentation fracture toughness of samples sintered at 1400 °C does not significantly depend on the presence of inclusions added in amounts up to 0.5 wt.%.

Upshot of Concentration of Zirconium (IV) Oxynitrate Hexa Hydrate on Preparation and Analyses of Zirconium Oxide (ZrO₂) Nanoparticles by Modified Co-Precipitation Method.

In the present work, pure nanocrystalline monoclinic Zirconia (ZrO₂) has been successfully synthesized and optimized by the modified co-precipitation method. The concentration of raw material has been optimized with the fixed amount of precipitation agent (Potassium hydroxide KOH). The thermal history of the precursor has been examined through TG/DTA analysis. All the samples are subjected to study the structure, fingerprints of the molecular vibrations, and morphology analyses. The representative sample has been analyzed through Transmission Electron Microscope (TEM) and X-ray Photo Electron Spectroscopy (XPS) analyses. The as-prepared sample exhibits the better crystallinity and surface morphology with lesser particle size (190 nm) when the raw material concentration is 0.2 M. The as-prepared ZrO₂ filler (0, 3, 6, 9, and 12 wt.%) is spread through the enhanced polymer electrolyte P(S-MMA) (27 Wt.%)-LiClO₄ (8 wt.%)-EC + PC (1;1 of 65 wt.%) complex system via solution casting method. The as-synthesized electrolyte films are examined via complex impedance analysis. P(S-MMA) (27 wt.%)-LiCIO₄ (8 wt.%)-EC + PC (1 ;1 of 65 wt.%)-6 wt.% of ZrO₂ shows the high ionic conductivity 2.35 × 10-3 Scm-1. Temperature-dependent ionic conductivity studies obey the non-linear behavior. The enhanced ZrO₂ has been expected to enhance the other electrochemical properties of the lithium secondary battery.

Hydrothermal Synthesis, Characterization and Dielectric Properties of Zirconia Nanoparticles

Simple hydrothermal method has been used to synthesize zirconia (ZrO2) nanoparticles in alkaline medium. The powder X-ray diffraction technique (PXRD), electron microscopic studies (SEM and TEM) and BET surface area studies were done to characterize the as-synthesized nanoparticles. The crystalline nature and phase analysis of the ZrO2 nanoparticles was determined by X-ray diffraction studies which revealed that the as-synthesized powder is pure monoclinic zirconia (m-ZrO2). The particle size was estimated by TEM and HRTEM studies, which comes out to be about 25nm. The surface area of as-prepared ZrO2 nanoparticles was found to be 186m2/g with DA pore radius of 11.9A by using BET surface area studies. The dielectric constant and dielectric loss of the nanoparticles were found to be 7.5 and 0.0094 for as-synthesized nanoparticles at 500 kHz frequency at room temperature.

Metastable Nanocrystalline Zirconia in Light of the Nucleation Theory

It is demonstrated that the theoretical foundation for the existence of metastable nonmonoclinic zirconia due to nanocrystallinity contradicts the fundamental aspects of the classical nucleation theory. As a consequence, the stabilization of the thermodynamically less stable zirconia modifications cannot be related to metastability. The experimental findings suggest that nonmonoclinic zirconia at room temperature results from chemical reactions, the products of which slightly deviate from pure monoclinic zirconia in terms of composition and, hence, in terms of structure.

Texturing of 3Y-TZP zirconia by slip casting in a high magnetic field of 17.4 T

Ceramics can be textured in a strong magnetic field, by aligning the suspended particles during colloidal processing and retaining this alignment in the green body. Attempts to align tetragonal zirconia particles however were not successful, not even in a high magnetic field of 17.4T. In the current work, monoclinic zirconia was successfully aligned with its (100) plane perpendicular to the magnetic field direction by slip casting in a 17.4T field. Textured tetragonal zirconia was reactively synthesized during sintering of undoped pure monoclinic zirconia and co-precipitated 8mol% yttria-stabilized zirconia. The sintered tetragonal zirconia inherited the alignment of the monoclinic zirconia particle precursor with a (100)m//(001)t orientation relationship. In other words, the t-ZrO2 grains were aligned with their (001) plane perpendicular to the magnetic field direction. The indentation toughness of the ceramic cast in the magnetic field is anisotropic and has an exceptionally high value of 14.6MPam0.5 along the [001] direction. A 3-pt flexural strength of 968±77MPa (load applied parallel to the magnetic field) was obtained for the ceramic cast in the strong magnetic field, which was 26% higher than for the ceramic cast without magnetic field.

Fast and tunable synthesis of ZrO2 nanocrystals: mechanistic insights into precursor dependence.

In this work, ZrO2 nanocrystals (NCs) are synthesized via a solvothermal treatment in benzyl alcohol, which is an established method for the synthesis of many metal oxide nanocrystals. We found that the use of microwave heating allows for a reduction in reaction time from 2 days in the autoclave to merely 4 h in the microwave. Furthermore, we were able to tune the crystallographic phase from pure cubic to pure monoclinic zirconia by changing the reaction mechanism through the use of a different zirconium precursor. Via GC-MS measurements, we found that the release of a strong acid during synthesis controls the key mechanism behind the control over crystal phase formation. The as-synthesized ZrO2 NCs (cubic or monoclinic) are small in size (3-10 nm), yet aggregated. However, aggregate-free NCs are generated through a surface-functionalization with carboxylic acid ligands, providing stabilization in apolar solvents via steric hindrance. Solution (1)H NMR was used to study the details of this post-modification step and the surface chemistry of the resulting aggregate-free NCs. This led to the conclusion that not only a different crystal structure but also a different surface chemistry is obtained, depending on the precursor composition.

Texturing of 3Y-TZP zirconia by electrophoretic deposition in a high magnetic field of 17.4T

Abstract Ceramics can be textured during colloidal processing by aligning the suspended particles in a strong magnetic field and retaining this alignment in the green body. Attempts to align tetragonal zirconia particles however were not successful, not even in 12 T magnetic fields. In the current work, monoclinic zirconia was successfully aligned with its (1 0 0) plane perpendicular to the magnetic field direction by electrophoretic deposition (EPD) in a 17.4 T field. Moreover, textured tetragonal zirconia was developed by reactive sintering of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia. The sintered tetragonal zirconia inherited the alignment of the monoclinic zirconia particle precursor and aligned with its (0 0 1) plane perpendicular to the magnetic field direction. The toughness of the (0 0 1)-oriented 3Y-TZP along the [0 0 1] direction of the textured zirconia was 65% higher than normal to it and 48% higher than the randomly oriented material.

Synthesis and optical properties of pure monoclinic zirconia nanosheets by a new precursor

Pure monoclinic zirconia nanosheets were successfully prepared by the polymeric complex method. In this approach, zirconium acetylacetonato, Zr(acac)4, citric acid (CA) and ethylene glycol (EG) were used as the source of Zr4+ and the chelating and polymerization agent, respectively. The effects of heat treatment and EG:CA ratios on the structure, morphology and zirconia particle size were investigated. UV–vis spectra illustrated that ZrO2 nanosheets did not show any extrinsic states aroused from surface trap states, defect states, or impurities.

Sonochemical synthesis and characterization of nano-sized zirconium(IV) complex: new precursor for the preparation of pure monoclinic and tetragonal zirconia nanoparticles

In this study, synthesis and characterization of two polymorphs of a new nano-sized zirconium(IV) complex, [ZrO(dmph)I2] (1), {dmph = 2,9-dimethyl-1,10-phenanthroline (neocuproine)}, have been investigated in two different solvents. The reaction between zirconyl nitrate pentahydrate and potassium iodide with dmph as a ligand under ultrasonic irradiation in methanol and mono-ethylene glycol (MEG) leads to the formation of the nano-sized Zr(IV) complex. Characterization of the Zr(IV) complex has been performed using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and elemental analysis. The thermal stability of the compound 1 has been studied by thermal gravimetric and differential thermal analyses. Particle sizes of the compound 1 have been decreased after thermal treatments in an autoclave. Pure monoclinic (m) and tetragonal (t) zirconia (ZrO2) nanoparticles were readily synthesized from thermal decomposition of the Zr(IV) complex as a new precursor in presence of methanol and MEG as solvents, respectively. Zirconium oxide was characterized by FT-IR, XRD, and SEM to depict the phase and morphology. The results showed that, pure zirconia was produced with particle size about 59 nm and crystal system was monoclinic when methanol was used as a solvent during complexation process. On the other hand, particle sizes of zirconia with tetragonal structure were significantly reduced to about 39 nm, when MEG was used as solvent.

Influence of thermal treatment on the formation of zirconia nanostructured powder by thermal decomposition of different precursors

The paper presents some results concerning the preparation of zirconia powders starting from ZrOCl2·8H2O by using two synthesis methods: (a) precipitation with NH3, at 90°C, and (b) thermal decomposition of carboxylate precursors, obtained in the reaction of zirconium nitrate and two different alcohols, 1,3-propanediol (PD) and poly(vinyl alcohol) (PVA), at 150°C. The precursors obtained at different temperatures have been characterized by thermal analysis (TG, DTA) and FT-IR spectroscopy. DTA analysis evidenced very clearly the transition temperatures between zirconia crystalline phases. The precursors have been annealed at different temperatures in order to obtain zirconia powders and the as obtained powders have been characterized by means of X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM). In case of precipitation method the presence of the tetragonal phase was observed at 400°C, while the monoclinic phase appears at temperatures higher than 400°C, becoming major crystalline phase starting with 700°C. In case of the powders prepared by thermal decomposition of carboxylate precursors, the tetragonal phase was formed at temperatures below 700°C, when the monoclinic phase begin to crystallize as secondary phase, in a higher proportion for the samples synthesized with 1,3-propanediol. All powders annealed at 1200°C are pure monoclinic zirconia. SEM images have evidenced for the zirconia powders annealed at 1000°C particles with diameters up to 150nm, agglomerated in micrometer-sized aggregates, more individualized and homogenous than that obtained in the case of zirconia powder synthesized with poly(vinyl alcohol).

Phase composition controllable preparation of zirconia nanocrystals via solvothermal method

Zirconia nanopowders are synthesized via solvothermal method at a temperature as low as 180°C without any additives. The pure tetragonal zirconia transforms into nearly pure monoclinic zirconia gradually with the progressing of solvothermal reaction, and the morphology of as-prepared nanoparticles grows up from spherical to short rodlike. The results of crystallite size estimated from XRD patterns using Scherrer equation show that the size of monoclinic zirconia ranges from 10nm to 50nm, while the size of tetragonal zirconia is below 5nm.

Magnetic field oriented tetragonal zirconia with anisotropic toughness

(0 0 1)-oriented 3 mol% yttria stabilized tetragonal zirconia (3Y-TZP) has been developed by reactive synthesis of undoped pure monoclinic zirconia and co-precipitated 8 mol% yttria-stabilized zirconia (8Y-ZrO 2). The dispersed pure monoclinic ZrO 2 powder, having magnetic anisotropy, was first aligned in a strong magnetic field and co-sintered in a randomly distributed cubic 8Y-ZrO 2 fine matrix powder. The reactive sintering resulted in a 3Y-TZP ceramic with a (0 0 1) orientation. The (0 0 1)-oriented 3Y-TZP showed a substantial toughness anisotropy, i.e. the toughness along the [0 0 1] direction is 54% higher than that of its perpendicular direction. Moreover, the toughness along the [0 0 1] direction is 49% higher than that of a non-textured isotropic reactively synthesized 3Y-TZP and 110% higher than that of an isotropic co-precipitated powder based 3Y-TZP. The substantially enhanced toughness was interpreted in terms of the tetragonal to monoclinic martensitic phase transformability.

Facile Synthesis of Pure Monoclinic and Tetragonal Zirconia Nanoparticles and Their Phase Effects on the Behavior of Supported Molybdena Catalysts for Methanol-Selective Oxidation

Pure monoclinic (m) and tetragonal (t) zirconia nanoparticles were readily synthesized from the reaction of inorganic zirconium salts (e.g., hydrated zirconyl nitrate) and urea in water and methanol, respectively, via a facile solvothermal method. The role of the solvents was crucial in the formation of the pure ZrO(2) phases, whereas their purity was essentially insensitive to other variables, including reaction temperature, reactant concentration, pH, and zirconium salts. Water as the solvent led to the transformation of hydrous ZrO(2) precipitates initially formed with tetragonal structures to thermodynamically more stable m-ZrO(2) via the dissolution-precipitation process, whereas methanol favored the removal of water molecules from the precursors via their reaction with urea, consequently maintaining the tetragonal structures. The obtained tetragonal samples were found to possess superior hydrothermal stability compared to those reported previously, which provides the possibility for systematically studying the effects of ZrO(2) phases on many catalytic reactions involving water as a reactant or product. Using these pure m- and t-ZrO(2) phases as supports, dispersed MoO(x) catalysts were synthesized at MoO(x) surface densities of approximately 5.0 Mo/nm(2), which is close to one monolayer of coverage. Characterization by X-ray diffraction and Raman spectroscopy confirmed that the pure ZrO(2) phases remained unchanged in the presence of the MoO(x) domains and the MoO(x) domains existed preferentially as 2D polymolybdate structures. The catalysts were subsequently examined for selective methanol oxidation as a test reaction. m-ZrO(2) support led to 2-fold greater oxidation rates than for t-ZrO(2) support, reflecting the higher intrinsic reactivity of the MoO(x) domains on m-ZrO(2). This is consistent with their higher reducibility probed by temperature-programmed reduction with H(2) (H(2) TPR). These observed effects of the ZrO(2) phases provide the basis for designing catalysts with tunable redox properties and reactivity.

The effect of zirconia polymorphs on the structure and catalytic properties of V 2O 5/ZrO 2 catalysts

The effect of zirconia polymorphs on the activity and selectivity of V 2O 5/ZrO 2 for the alkylation of phenol with methanol has been investigated. Relatively pure monoclinic zirconia (m-ZrO 2) and tetragonal zirconia (t-ZrO 2) were prepared. A series of vanadium oxide supported on m- and t-ZrO 2 with varying vanadium oxide content from 2.5 to 15 wt% were prepared by wet impregnation method. The catalysts were characterized by X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (UV-DRS), temperature-programmed reduction of H 2 (TPR), temperature-programmed desorption (TPD) of NH 3, specific surface area and pore size distribution measurements. The XRD results suggest that highly dispersed or amorphous state of vanadium oxide is present at lower loadings. X-ray diffraction patterns also indicate the presence of crystalline V 2O 5 phase from 7.5 wt% of V 2O 5 on m-ZrO 2 support and there is no indication for the formation of V 2O 5 crystalline phase up to 15 wt% of V 2O 5 on t-ZrO 2 support. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V 5+ species at lower loadings in all catalysts. The temperature-programmed reduction of H 2 shows a single reduction peak corresponding to V 5+ to V 3+ in all the samples. TPD of ammonia suggests that surface acid density of m-ZrO 2 phase is found to be more than t-ZrO 2 phase. Acidity of the supported catalysts is increased with increase in V 2O 5 loading up to monolayer formation and decreased with the appearance of crystalline phase of V 2O 5. At monolayer coverage, V 2O 5/m-ZrO 2 is fourfold more active than V 2O 5/t-ZrO 2 for alkylation of phenol at 673 K. Selectively C-alkylated products were formed on all the catalysts. A linear correlation is observed between catalytic activity and surface acidity of catalysts.

Effect of the energy deposition modes on the structural stability of pure zirconia

One of the most important goals in materials science is to be able to design materials with specific properties. Irradiation seems to be a powerful tool for the design of advanced materials because of its ability to modify over different scales the microstructure of solids. Nowadays, it is clearly proved that irradiation induces order–disorder phase transitions in metallic alloys and in some ceramics. Recent investigations on pure monoclinic zirconia have clearly shown that a displacive phase transition can be induced by irradiation. In this work, the impact of the energy deposition modes on the structural stability of pure monoclinic ZrO2 is discussed in detail. Based on experimental evidences, a microscopic model is proposed to explain the displacive phase transition observed in this material after irradiation by low and high energy ions within the Landau theory framework. Even if defects generated by low and high energy ions are quite different, these defects are able to quench the same tetragonal phase in pure zirconia.

Synthesis of zirconia nanopowders from various zirconium salts via polyacrylamide gel method

Pure monoclinic zirconia nanopowders were synthesized via a simple, fast and low cost method; polyacrylamide gel method. Also, the effect of initial salt precursor on thermal behavior of gel network and structure of the synthesized nanoparticles was studied with thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The XRD results showed that the presence of nitrate ions not only retarded the crystallization, but also delayed the tetragonal to monoclinic phase transformation of zirconia nanoparticles which resulted in smaller particle sizes in comparison with the chloride samples. However, TG-DTA analysis confirmed accelerator role of nitrate ions on degradation of polymeric network. Thus, it was expected that zirconia nanopowders synthesized by nitrate samples have bigger sizes than the chloride base powders. Therefore, the presence of nitrate ions affects the synthesized nanoparticle size via two different mechanisms: the retarded crystallization and polymeric network degradation. But, TEM images revealed that the controlling mechanism is the former one.

Evidence of extended defects in pure zirconia irradiated by swift heavy ions

X-ray diffraction, transmission electron microscopy, and optical spectroscopy were used to investigate the microstructure of polycrystalline samples of pure monoclinic zirconia irradiated by high energy ions. These techniques point out the existence of extended defects and they allow to monitor the tetragonal to monoclinic phase transition as a function of the temperature during isochronal annealings. The Landau theory approach explains the phase transition mechanism via the presence of an important strain field induced by dislocations. Though high and low energy ions can effectively stabilize the same tetragonal phase in the irradiated layer, only point defects are produced at low energy whereas extended defects are also observed at high energy, showing the strong influence of the energy deposition modes on the observed microstructures.

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.

Impedance spectroscopy of reduced monoclinic zirconia

Zirconia doped with low-valent cations (e.g. Y3+ or Ca2+) exhibits an exceptionally high ionic conductivity, making them ideal candidates for various electrochemical applications including solid oxide fuel cells (SOFC) and oxygen sensors. It is nevertheless important to study the undoped, monoclinic ZrO2 as a model system to construct a comprehensive picture of the electrical behaviour. In pure zirconia a residual number of anion vacancies remains because of contaminants in the material as well as the thermodynamic disorder equilibrium, but electronic conduction may also contribute to the observed conductivity. Reduction of zirconia in hydrogen leads to the adsorption of hydrogen and to the formation of oxygen vacancies, with their concentration affected by various parameters (e.g. reduction temperature and time, surface area, and water vapour pressure). However, there is still little known about the reactivities of defect species and their effect on the ionic and electronic conduction. Thus, we applied electrochemical impedance spectroscopy to investigate the electric performance of pure monoclinic zirconia with different surface areas in both oxidizing and reducing atmospheres. A novel equivalent circuit model including parallel ionic and electronic conduction has previously been developed for titania and is used herein to decouple the conduction processes. The concentration of defects and their formation energies were measured using volumetric oxygen titration and temperature programmed oxidation/desorption.

A nearly pure monoclinic nanocrystalline zirconia

Generally, monoclinic zirconia is considered to be much more difficult to prepare at low temperatures and particularly in a pure state. The present work is the first example that shows that the hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The crystallite size of the monoclinic zirconia produced in the present work is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. Such a small monoclinic crystallite arises from some of the chemical and physical factors built into the solution-gelation-xerogel process such as acidic preparation-pH, rapid precipitation, and moderate aging time and drying temperature, which result in a structure different from those of the existing zirconium hydroxides. In addition, the hydrous zirconia exhibits a unique thermal behavior in two respects: first, a sudden weight drop in the region of exothermic peak of the thermogravimetric curve is seen, suggesting that the main decomposition of the hydrous zirconia occurs in this region; second, there is an endothermic peak at high temperature in the differential thermal analysis curve, indicating the presence of coordinated water in the hydrous zirconia.