ZrO2 Crystals Research Articles

The aging effect on the low temperature mechanical strength of 3.2YSZ single crystals manufactured by the Skull melting method

Single crystals of ZrO2 containing 3.2mol% of Y2O3 were manufactured by the Skull melting method. The single crystals were composed of the t′-ZrO2 phase. For a comparison of the mechanical strength, 3mol% of Y2O3 added tetragonal zirconia polycrystal (3Y-TZP) (Tosoh TZ-3Y®) samples were also prepared. The biaxial flexure strengths of the single crystal and 3Y-TZP ceramics were 1328±537MPa and 1227±312MPa, respectively. After autoclave treatment at 200°C for 24h, the strength of the single crystals and polycrystalline ceramics changed to 1345±251MPa and 281±31MPa, respectively. This result shows that the t′-ZrO2 single crystals have excellent resistance in moisture-enhanced mechanical degradation. Since the martensitic transformation of the tetragonal (t) and monoclinic (m) has crystallographic correspondences between the lattices, the relationship will lead to low interface energy at the t–m interface and high binding energy between the t and m phases. As far as the crystallographic relationship is maintained in the t′-ZrO2 single crystals, different from polycrystalline 3Y-TZP, the partial coherency seems to retard low temperature degradation – monoclinic phase pull out or crack generation – of single crystals.

Study of the tribological performance of zirconia-based materials in pair with VT-9 titanium alloy

The paper presents a study of the possibility of using composite nanostructured ceramic materials and crystals based on zirconia as bearing materials for dry operation at temperatures of up to 150°C. The results of experimental study of the dry friction of the materials in pair with alpha-titanium alloy VT-9 are presented. It is shown that at a sliding velocity of 0.004 m/s, contact pressure of 3.3 MPa, and temperature of 150°C, their antifriction properties do not differ significantly. ZrO2 crystals have a slight superiority in wear resistance because of their higher microhardness. The wear resistances of the alpha-alloy in pair with the ceramics and the crystal are practically the same. A power pattern of the dependence of the wear rate on the pressure for the ZrO2 crystal and the VT-9 (α) alloy is confirmed.

Hot corrosion control in plasma sprayed YSZ coating by alumina layer with evaluation of microstrcuture and nanoindentation data (H, E)

Thermal barrier coatings (TBCs), which contain yttria stabilized zirconia (YSZ) are used as thermal insulation coatings on hot section components in gas turbine in order to increase operation temperature for higher efficiency. Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which comes as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of two types of plasma sprayed TBCs was evaluated: a) usual YSZ (yttria stabilized zirconia) and b) layer composite of (YSZ/Al2O3) in which Al2O3 was as a top coat on YSZ layer. Hot corrosion tests were carried out on the surface of coatings in molten salt (Na2SO4 + V2O5) at 1050 °C for 40 h. Spallation was observed in YSZ coating due to formation of monoclinic ZrO2 and YVO4 crystals as hot corrosion products. Alumina coating as outer layer in YSZ/Al2O3 reduced the penetration of molten salt into YSZ and resulted in the further resistance of TBC against hot corrosion. Also nanoindentation tests demonstrated that the hardness (H) and elastic modulus (E) of the usual TBC increased substantially after hot corrosion whereas these properties exhibited the less variations in YSZ/Al2O3.

Preparation and characterization of sol–gel derived zirconia coated carbon fiber

Zirconia coated carbon fiber for thermal and oxidation protection was prepared through a sol–gel technique. A zirconia sol was synthesized with the zirconium oxychloride, ammonia and polyethylene glycol, each of them acting as the zirconium resource, precipitant and surfactant. The morphology, phase composition and thermal properties of the zirconia coated carbon fiber were evaluated by SEM, EDS, XRD and TG-DTA. The increase of dipping cycles can raise the thickness of the coating, while attaching more ZrO2 crystals on the coating surface. Mass variations of the 3mol% YSZ and 2mol% YSZ coated carbon fibers at 800°C are 20% and 40%, less than the 70% of the uncoated carbon fiber.

Plasma Dissociated Zircon (PDZ) Processing; Influence of the Zr:Si Ratio in the Composition, Microstructure and Thermal Re-Crystallization

Plasma dissociated zircon (PDZ) has attractive applications in ceramic colouring, as well as the preparation of fine ZrO2 crystals, coatings and refractory materials. Generally the PDZ is obtained from different natural or synthetic zircon powders, if the cooling rate is high enough a complete dissociation is achieved, with rounded (dendritic) zirconia grains imbibed in a fused silica (non crystalline) matrix. The principal objective of this work is to establish the influence of the incorporation of both products of the zircon dissociation (fused silica and crystalline zirconia) to the initial powders mixture utilized for the processing of PDZ in an indigenous thermal plasma argon furnace. Both structural and microstructural characterization was carried out. A thermal annealing at 1500¼C with different soaking time (1-15hours) was completed in order to establish the thermal stability to the thermodynamically favoured recombination, showing that after this time a recombination was achieved, and this behaviour was not influenced by the Zr:Si ratio. The addition of fused silica resulted in an important decrease of the zirconia grains size, while the zirconia addition had the opposite result, showing that within this range (1-10μm) the final zirconia grains size can be selected and controlled with this processing variable. The present processing route showed an alternative way for obtaining different size of monoclinic zirconia powder.

Steam reforming of ethanol over carburized alkali-doped nickel on zirconia and various supports for hydrogen production

The steam reforming of ethanol over carburized alkali-, La- and Ce-doped nickels on zirconia and various supports was studied to determine the support effects, alkali addition and carburization. The 823 K-carburized Ni/ZrO 2 and Ni/V 2O 5 catalysts exhibited higher activities than the Ni catalysts supported on CeO 2, Al 2O 3, SiO 2–Al 2O 3 and MgO and a reduced catalyst. The cesium-doped Ni/ZrO 2 promoted more hydrogen formation than the Li-, K-, La- and Ce-doped catalysts. From the XRD measurement, crystals of ZrO 2, Ni carbide and Ni metal were formed during the carburization of the Ni/ZrO 2 catalyst at 823 K. No Ni carbide was observed on the MgO, CeO 2 Al 2O 3, and V 2O 5 carburized catalysts. The Cs 0.2Ni/ZrO 2 and Cs 0.5Ni 0.5/Zr exhibited no carbon deposition at 7 h. The carburized and cesium-doped Ni/ZrO 2 was more effective in decreasing the carbon deposition. From the IR study, the formed carbonated species were attributed to the formation of CO 3 − from the formed CO 2 (CO 2 + O 2 − → CO 3 −). In the pulse experiment of H 2 18O, Cs 0.2Ni/ZrO 2 helped to facilitate the dissociation of H 2O to 2H and O and the decomposition of CH 3CHO to hydrogen and CO 2 through the CH 3CO species. The TPD after ethanol adsorption on Ni/ZrO 2 with and without Cs-doping showed that the Cs-doped Ni/ZrO 2 catalyst promoted the C–H breaking through the CH 3CO species more than the Ni/ZrO 2 catalyst.

Influence of lattice parameters on the dielectric constant of tetragonal ZrO2 and La-doped ZrO2 crystals in thin films deposited by atomic layer deposition on Ge(001)

In ZrO2 crystals, the highest dielectric constant (k) is ascribed to the tetragonal phase. By the use of density functional theory and synchrotron radiation x-ray diffraction, we show how the a and c lattice parameters of the tetragonal phase influence the resulting k. Highest k values are obtained at increasing both a and c, while k is reduced for compressive strained cells. The determination of a and c on La-doped ZrO2 and ZrO2 thin films deposited by atomic layer deposition on Ge (001) allowed us to elucidate the influence of La doping and Ge diffusion on the k value.

Effect of Atmospheric Water Vapor on the Thermally Induced Crystallization in Zirconia Gel

Effect of water vapor on the thermally induced crystallization of tetragonal ZrO2 in hydrous gel precursor was investigated using TG‐DTA under controlled water vapor pressures, p(H2O). Existence of the temperature region for preliminary nucleation in the region lower than that of the crystallization exothermic peak was identified. The rate of preliminary nucleation was largely enhanced by the atmospheric water vapor, resulting in the increase of number of preexisting nuclei for the subsequent growth and the systematic shift of DTA exothermic peak of crystallization to the lower temperatures with increasing p(H2O). From a systematic shift of the DTA exothermic peak depending on the annealing time at a constant temperature, the rate behavior of preliminary nucleation under high p(H2O) was characterized as the constant rate nucleation. By the preannealing treatment in the temperature region of preliminary nucleation under high p(H2O), the number of preexisting nuclei is nearly saturated without any partial crystallization. From a series of DTA exothermic peaks of crystallization recorded by heating the preannealed samples at different heating rates, the apparent activation energy, Ea, for the crystal growth of tetragonal ZrO2 from the preexisting nuclei was estimated to be Ea = 187.3 ± 3.8 kJ mol−1.

Characterization of Hafnium-Zirconium-Oxide-Nitride films grown by ion beam assisted deposition

Hafnium-Zirconium-Oxide-Nitride (Hf1−xZrxO1−yNy) films are prepared by ion beam assisted deposition on p–Si and quartz substrates with a composite target of sintered high-purity HfO2 and ZrO2. The thermal stability and microstructure characteristics for Hf1−xZrxO1−yNy films have been investigated. EDS results confirmed that nitrogen was successfully incorporated into the Hf1−xZrxO2 films. XRD and Raman analyses showed that the Hf1−xZrxO1−yNy films remain amorphous after 1100 °C under vacuum ambient, and monoclinic HfO2 and ZrO2 crystals separate from Hf1−xZrxO1−yNy films with a increase of the annealing temperature up to 1300 °C. Meanwhile, the Hf1−xZrxO1−yNy films can also effectively suppress oxygen diffusion during high temperature annealing. AFM measurements demonstrated that surface roughness of the Hf1−xZrxO1−yNy films increase slightly. Then the optical properties of the samples were observed in the ultraviolet–visible range at room temperature. The variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature has also been discussed briefly.

Oxidation Behavior of SiC Platelet‐Reinforced ZrB2 Ceramic Matrix Composites

Zirconium diboride (ZrB2) ceramic matrix composites reinforced by silicon carbide platelets (SiCpl) were prepared by hot pressing. Five groups of specimens were prepared, in which the contents of SiC platelets were 0, 5, 10, 15 and 20 vol%, respectively. The oxidation behaviors of specimens at different temperatures from 800° to 1600°C for 1 h and at 1200°C for different times from 0.5 to 4 h were investigated. With the increase of SiCpl content, the mass gain exhibited a decreasing trend. Oxidation thermodynamics and kinetics of the SiCpl/ZrB2 composites were discussed. A characteristic structure of hollow quadrangular prismy ZrO2 crystal emerged on the sample surface after oxidation was found and the possible formation mechanism was also presented. The surface element analysis indicated that the main products of oxidation were m‐ZrO2, SiO2, and borosilicate (aluminosilicate).

Hydrothermal Synthesis and Characterization of Phosphor Doped Tetragonal Zirconia Nanocrystallites

Using zirconium oxychloride hydrate ( ZrOCl2•8H2O) and ammonia water (NH3•H2O) as raw materials, and ammonium dihydrogen phosphate (NH4H2PO4) as additives, tetragonal zirconia (t-ZrO2) with size range of 8–12 nm were prepared by coprecipitation method under hydrothermal conditions. The influence factors on phase transformation and the particle size such as phosphor loading, hydrothermal temperature and calcination temperature were studied by X-ray diffraction (XRD), Fourier transform Roman spectra (FT-Roman), the Brunauer-Emmett-Teller (BET) method and X-ray photoelectron spectroscopy (XPS) techniques etc. Research results show that a small amount of phosphor has been incorporated into the framework of ZrO2 crystals, producing a certain amount of oxygen vacancies. Phosphor can effectively restrain crystal particles growth and improve the thermal stability of metastable t-ZrO2. The phosphor doped t-ZrO2 had a high surface area (244.2 m2/g). In contrast to the pure ZrO2 particles readily aggregating, the phosphor species deposited on the framework of ZrO2 crystals prevented the agglomeration of the primary particles during calcinations.

Macroscopic and microscopic electrical characterizations of high-k ZrO2 and ZrO2/Al2O3/ZrO2 metal-insulator-metal structures

In order for sub-10 nm thin films of ZrO2 to have a dielectric constant larger than 30 they need to be crystalline. This is done by either depositing the layer at higher temperatures or by a postdeposition annealing step. Both methods induce high leakage currents in ZrO2 based dielectrics. In order to understand the leakage a thickness series of ultrathin ZrO2 and nanolaminate ZrO2/Al2O3/ZrO2 (ZAZ) films, deposited by atomic layer deposition, was investigated. After deposition these films were subjected to different rapid thermal annealing (RTA) processes. Grazing incidence x-ray diffraction and transmission electron microscopy yield that the crystallization of ZrO2 during deposition is dependent on film thickness and on the presence of an Al2O3 sublayer. Moreover, the incorporation of Al2O3 prevents crystallites from spanning across the entire film during RTA. C-V and I-V spectroscopies show that after a 650 °C RTA in N2 the capacitance equivalent oxide thickness of 10 nm ZAZ films is reduced to 1.0 nm while maintaining low leakage currents of 3.2×10−8 A/cm2 at 1 V. Conductive atomic force microscopy studies yield that currents are not associated with significant morphological features in amorphous layers. However, after crystallization, the currents at crystallite grain boundaries are increased in ZrO2 and ZAZ films.

Large-Bulk Solidified Al<sub>2</sub>O<sub>3</sub>-ZrO<sub>2</sub>(Y<sub>2</sub>O<sub>3</sub>) Prepared by Self-Pressure Assisting Combustion Synthesis under High Gravity

By introducing self-pressure processing into combustion synthesis under high gravity, Al2O3-ZrO2(4Y) eutectic composite ceramics without shrinkage cavities and holes were achieved, and the self-pressure processing also made the macro-crack be controlled and the ceramic densification be promoted evidently. XRD results showed the eutectic ceramics were composed of α-Al2O3, t-ZrO2 and a few m-ZrO2 phases, and the volume fraction of the transformable t-ZrO2 increased with self-pressure force increasing. SEM images showed that with increasing self-pressure force, the rod-shaped eutectic colonies were refined and the volume fraction of the colonies increased. Meanwhile, the irregular ZrO2 crystals around eutectic colonies transformed to be fine t-ZrO2 spherical crystals gradually, and thickness of eutectic colonies decreased. With the changes in microstructures of the ceramics, the mechanical properties of the ceramics were improved greatly.

Embedding and atomic orbitals hybridization

AbstractIn calculation, the electronic structure of crystals, especially those containing point defects, the embedding approach is proved to be useful and convenient. In this approach, a finite part of the crystal, referred to as cluster, is considered instead of infinite crystal and the influence of the rest of the crystal is simulated by the embedding potential. The key problems of this approach are the cluster selection and the embedding potential generation. To select a cluster, the Wigner‐Seitz unit cell is used in the present approach and every border atom, situated at the unit cell face, edge, or vertex is symmetrically “divided” among adjacent unit cells sharing this atom. The atomic hybrid orbitals are used for the border atoms partition between neighboring clusters. It is shown that contrary to the conventional hybridization scheme, the nonorthogonal and even linearly dependent atomic hybrid orbitals can be used to construct the border atom density matrix. This density matrix can be made to satisfy the proper point symmetry and to match the number of equivalent hybrid orbitals and the number of nearest neighbors. Two different types (one‐center and multicenters) of the embedding potential corresponding to the border atom are considered in the article. As a particular example of the border atom the oxygen ion in ZrO2, MgO, and TiO2 rutile crystals is considered. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Persistent mechanoluminescence induced by elastic deformation of ZrO2:Ti phosphors

ZrO2:Ti phosphors show such a strong mechanoluminescence (ML) that it can be seen in day light with naked eye. When a pellet of ZrO2:Ti phosphor mixed in epoxy resin is deformed in the elastic region at a fixed strain rate using a testing machine, ML intensity increases linearly with time, and when the deformation is stopped, ML intensity decreases exponentially with time. For a given strain rate, ML intensity increases linearly with pressure, and for a given pressure, ML intensity increases linearly with the strain rate. The total ML intensity, in the deformation region, increases quadratically with pressure; however, the total ML intensity in the post-deformation region increases linearly with pressure. ML intensity decreases with successive number of pressings, whereby the reduced ML intensity can be recovered by UV-irradiation of the sample. ML intensity increases linearly with density of filled electron traps and it is optimum for a particular concentration of Ti in ZrO2. ML intensity should change with increasing temperature of the phosphors. Although ZrO2 is non-piezoelectric as a whole, it seems that the local structures near the Ti ions in ZrO2 crystals are in the piezoelectric phase. The elastico ML in ZrO2 phosphors can be understood on the basis of the localized piezoelectrification-induced detrapping model. According to this model, the localized piezoelectric field near Ti ions causes detrapping of electrons and subsequently the detrapped electrons moving in the conduction band are captured by the energy state of excited Ti4+ ions, whereby excited Ti4+ ions are produced and consequently the decay of excited Ti4+ ions gives rise to the light emission. The expressions derived on the basis of this model are able to explain satisfactorily the characteristics of ML. The relaxation time of localized piezoelectric charges and the threshold pressure for the ML emission can be determined from ML measurements. The long decay of elastico ML indicates the possibility of exploring persistent elastico ML, which may be useful for the fabrication of dim light sources capable of operating without any external power.

Transparent and Scratch-Resistant C:ZrOx Coatings on Polymer and Glass by Plasma-Enhanced Chemical Vapor Deposition

Transparent and scratch-resistant zirconium oxide thin films were deposited in radio-frequency plasma-enhanced chemical vapor deposition process on glass and polycarbonate substrates using zirconium-tetra-tert-butoxide, [Zr(OtBu)4], as the precursor. Investigations on film morphology (AFM), thickness (cross-sectional SEM), phase structure (X-ray diffraction), chemical composition (X-ray photoelectron spectroscopy), and optical properties (UV/Vis, ellipsometry) revealed the interplay of process parameters (plasma power, gas composition, deposition time, and precursor flux) on the composition and properties of the coatings. High-quality transparent coatings (>90%) with a tunable refractive index (n=1.7–2.1) and abrasion-resistant properties were deposited on both polymer and glass substrates. Despite low deposition temperatures (<100°C), the coatings showed good adherence to the substrate and extraordinary barrier properties that were further improved by depositing intermediary SiOx-layers. Phase analysis of the predominantly amorphous films showed the incipient crystallization of ZrO2.

Crystallization of hydrous zirconia and hafnia during hydrothermal treatment

The products of microwave-hydrothermal (MW-HT) treatment of amorphous hydrous zirconia and hafnia xerogels were studied by powder X-ray diffraction and thermal analysis. Decreasing gel precipitation pH leads to an increase in ZrO2 and HfO2 crystallization rates and enhances the formation of stable monoclinic ZrO2.

Effects of Hydrothermal Conditions of ZrO2 on Catalyst Properties and Catalytic Performances of Ni/ZrO2 in the Partial Oxidation of Methane

ZrO2 was prepared via the hydrothermal method and used as the support of nickel catalysts in the partial oxidation of methane to syngas. The hydrothermal conditions (such as hydrothermal temperature, hydrothermal time, and the concentration of NaOH used) necessary for ZrO2 preparation were investigated and optimized. The physicochemical properties of ZrO2 supports and Ni/ZrO2 catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption, temperature-programmed reduction (TPR), and thermogravimetric analysis (TGA). The results showed that low hydrothermal temperatures, short hydrothermal time, and low concentrations of NaOH benefited the formation of tetragonal ZrO2. Elevating the hydrothermal temperature (from 160 °C to 200 °C), prolonging the hydrothermal time (from 6 h to 48 h), or increasing the concentration of NaOH (from 5 wt % to 40 wt %) would lead to the disappearance of tetragonal ZrO2 and an increase in the degree of monoclinic ZrO2 crystallization. The Ni/ZrO2 catalysts with ZrO2 prepared with high NaOH concentration or with ZrO2 prepared at long hydrothermal times possessed excellent catalytic activities and resistance to carbon formation. Whereas the Ni/ZrO2 catalysts with ZrO2 prepared with low NaOH concentration or with ZrO2 prepared at short hydrothermal time deactivated soon because of carbon deposition.

Changes in Microstructures and Properties of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/ZrO&lt;sub&gt;2&lt;/sub&gt; (Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) with Different Content of ZrO&lt;sub&gt;2&lt;/sub&gt;

Based on preparation of Al2O3/ZrO2 (4Y) by combustion synthesis in high-gravity field, the microstructure transformation and properties of the materials are investigated through adjusting the ZrO2 (4Y) content in the composites. As the content of ZrO2 changed from 37% to 40%, the microstructures of the ceramics transformed the sphere-like tetragonal ZrO2 crystals from the rod-shaped colonies with nanocrystalline structures. Al2O3/33%ZrO2 (4Y) had the maximum relative density, hardness and flexural strength due to the low solidification temperature, the highest volume fraction of the colonies, small-size defect and high fracture toughness, whereas Al2O3/44%ZrO2 (4Y) was somewhat weakened in strength in despite of its highest fracture toughness.

Preparation and Characterization of Unique Foamy Zirconia Single Crystals via Sol-Gel-Hydrothermal Method

Porous ZrO2 single crystals were prepared through a sol-gel-hydrothermal method and characterized by XRD, HR-TEM, SEM and FT-IR. The results show that the dried sol gel was converted into monoclinic zirconia crystals after calcined above 550 °C. The zirconia single crystal has a porous structure with the crystal sizes of 20-70 nm. The origin of the pores was the gaseous hydrolysates of the superstoichiometric urotropine. The gel structure and the hydrothermal treatment are the required conditions resulting in the pores in crystals. After sintered at 1400 °C, the disc of the obtained ZrO2 crystals was dense. However, Y2O3 should be added to avoid fracture due to phase change if the disc was used as heat insulating layer.