Zirconia Crystals Research Articles

Microstructural development during crystallization firing of a dental-grade nanostructured lithia-zirconia glass-ceramic

The microstructural development during crystallization firing of a commercially-available dental-grade nanostructured lithia-zirconia glass-ceramic (Vita Suprinity® PC) was unraveled using a wide battery of ex-situ and in-situ characterization techniques. It was found that the milling blocks are slightly crystallized glass-ceramics, with a complex chemical composition and consisting of partially de-polymerized glass plus lithium silicate (Li2SiO3) nanocrystals. It was also found that during crystallization firing the glassy matrix first reacts with part of the Li2SiO3 to form lithium disilicate (Li2Si2O5) at ∼810−820 °C, and then lithium orthophosphate (Li3PO4) precipitates from the glass. This results in glass-ceramics with abundant nanocrystals embedded in a sparse zirconosilicate glass matrix (containing many other cations subsumed) that, due to its high viscosity, inhibited crystal growth. Therefore, these dental glass-ceramics are not reinforced with zirconia (ZrO2) crystals unless over-fired above ∼890 °C and at the expense of its singular nanostructure. Finally, this study opens doors for optimizing the clinical performance of these dental glass-ceramics via microstructural tailoring.

Application of statistical moment method to investigate mechanical properties of yttria-stabilized zirconia

The mechanical properties of yttria-stabilized zirconia (YSZ) crystal under extreme conditions are theoretically investigated using the analytic free energy formulas derived from the statistical moment method. We calculate the elastic moduli and constants under temperature up to 2100 K and pressure up to 20 GPa using Buckingham potential. The dopant concentration, temperature, and pressure dependences of the elastic moduli and constants of YSZ crystal are studied taking into account the anharmonic effects of the lattice vibrations. Our results are in reasonable agreement with previous experiments and simulations. This approach can then be developed extensively to investigate the thermodynamic and mechanical properties of the other ceramic materials used in solid oxide fuel cells.

Effect of heat treatment on the structure and mechanical properties of partially gadolinia-stabilized zirconia crystals

ABSTRACT (ZrO2)1-x (Gd2O3) x (x= 0.028–0.04) is heat-treated at 1600°C in air or in vacuum. Partially gadolinia-stabilized zirconia crystals are grown by the skull melting technique. The effect of annealing on the phase compositions, structures, and mechanical properties of the crystals is studied using X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and indentation. A comparison with previously obtained data on the structure and properties of crystals after growth shows that annealing the crystals leads to the redistribution of gadolinia between the tetragonal phases: the t phase is depleted of gadolinia while the t’ phase becomes enriched. The gadolinia-enriched t’ phase undergoes multistage twinning. Thus, the sizes of the twins in the t’ phase are far smaller than those in the t phase. Annealing the crystals increases their fracture toughness, and this increase is larger for all crystals after air annealing. The increase in the fracture toughness of the crystals is caused by a decrease in the gadolinia concentration in the transformable t phase, which enhances the effect of the transformation hardening mechanism. The fracture toughness of the crystals is further increased by the ferroelastic hardening mechanism, the effect of which is enhanced with an increase in the quantity of the t’ phase in the crystals.

Microstructural responses of Zirconia materials to in-situ SEM nanoindentation

Development of optimal shaping processes for pre-sintered and sintered zirconia materials requires a fundamental understanding of damage and deformation mechanisms at small-scale contacts with diamond tools. This paper reports on responses of zirconia materials with distinct microstructures to nanoindentation associated with diamond machining using a Berkovich diamond indenter. In-situ nanoindentation was performed in a scanning electron microscope (SEM) and in-process filmed to record small contact events. Indentation morphology was SEM-mapped at high-magnifications. Although both pre-sintered porous and sintered dense zirconia materials mechanically revealed the quasi-plastic behavior in indentation, there were distinct responses of the two materials to quasi-plasticity at the microstructural level. For pre-sintered porous zirconia, the quasi-plasticity was attributed to shear faults resulting from breaking pore networks as microstructurally discrete interfaces, to lead to compression, fragmentation, pulverization and microcracking of zirconia crystals in indentation imprints. In contrast, sintered dense zirconia had shear band-induced quasi-plastic deformation, accompanied with localized tensile microfracture. A material index associated with the mechanical properties ranked the lower quasi-plasticity for pre-sintered porous zirconia than its sintered dense state, predicting more machining-induced damage in the former than the latter. Significantly higher indentation imprint volumes induced in indented pre-sintered porous zirconia than sintered dense state previses higher machining efficiency for the former than the latter. The microstructure-dependent indentation mechanisms provide the fundamental knowledge into micromechanics of abrasive machining of zirconia materials and may lead to a new microstructural design for zirconia materials to achieve a balanced machining efficiency and damage control.

The effect of atomic polarization on the diffusion coefficient of oxygen ions in cubic zirconia

We study single crystal zirconia using molecular dynamics method. We calculate the diffusion coefficient of oxygen ions in the temperature range from the occurrence of diffusion at our calculation parameters to the loss of stability of the crystal. In the potential of interatomic interactions, in addition to conventional terms, we introduce into our model the interaction due to the polarization of atoms. These interactions turned out to be crucial for the correspondence between the calculations and available experimental data.

Physico-mechanical properties and microstructure of multi-phase ceramic composites based on zircon and dolomite mixtures

Four composites containing zircon and dolomite were designed by adding dolomite from 5wt% to 35wt% at the expense of zircon content. Densification parameters in terms of bulk density, apparent porosity and linear change were determined at different firing temperatures (1200°C–1400°C). Cold crushing strength of sintered composites, phase composition and microstructure were investigated. Samples contain 35wt% of dolomite and fired at 1200°C for 2 hours exhibited higher porosity (AP ∼ 51.25%) than other samples and can be used as porous ceramics. This is due to CO2 evaporation through the thermal decomposition of dolomite. Dense ceramics can be obtained by adding 5wt% of dolomite and fired at 1400°C for 2 hours (bulk density ∼3.67 g/cm3 and apparent porosity ∼4.5%). Only zirconia and diopside crystalline phases were detected at composite containing 35wt% of dolomite and fired at 1400°C. Due to the liquid phase sintering process, the densification parameters of the sintered samples have been enhanced by increasing the temperature. The mechanical properties of the sintered samples were improved due to the transformation toughening mechanism of tetragonal zirconia. Microstructure and EDAX analysis of the sintered composites show the presence of sub-prismatic zircon and rounded zirconia crystals as well as irregularly dark diopside crystals.

Influence of alumina addition on structural and catalytic properties of sulphated zirconia in isomerization of n-hexane

New binary catalytic systems based on sulphated zirconia-alumina were synthesized by controlled hydrolysis of alkoxides using relative molar ratios of constituents 1:1, 1:2 and 1:3 in favour of zirconia. The obtained differences in the final catalytic material?s properties may be related to organic precursor memory effect, the applied calcinations temperatures and used alumina contents. The addition of alumina to zirconia affected the structural and surface properties stabilizing bare zirconia by means of slower phase transformation of tetragonal zirconia crystal phase into monoclinic one, further resulting in smaller crystallites sizes and higher surface density of acidic function (sulphates). The best alumina impact on physico-chemical properties was achieved when the highest amount of alumina (e.g. 50%) and lower calcination temperature (500?C) were used, causing relatively high steady-state activity in isomerization of n-hexane at reaction temperature 250 ?C. On the other hand, higher calcination temperature (600 ?C) played a role in the genesis of greater sulphate density. Catalytic activity and selectivity are expressed as a complex synergistic function of relative density of acid sites together with positive status of other optimized physico-chemical properties of the catalytic material (by activation and calcination temperatures).

Preparation of zirconium coating on zirconium-niobium alloy substrate by pulsed current electrodeposition from fluoride molten salt

ABSTRACT Zirconium coating on Zirconium-Niobium alloy substrate was obtained by pulsed current electrodeposition from FLINAK-K2ZrF6 molten salt at 1023 K in argon atmosphere. The microstructure and morphology of the coating were investigated. The adhesion between coating and substrate, porosity of the coating surface was tested. Metallic zirconium crystal of zirconium coating is a hexagonal crystal structure with the preferential growth orientation of (002). The porosity of the coating surface is 1.7%. The adhesion between coating and substrate is good in the scratch test, and the adhesive force of the zirconium coating is 10.5 N. Furthermore, on the inner and outer walls of a 10 mm diameter tube, the same quality zirconium coating can be obtained.

Effect of Heat Treatment on the Thermal Conductivity of Single Crystals of ZrO2-Based Solid Solutions Stabilized with Scandium and Yttrium Oxides

The effect of heat treatment at 1000°C for 400 h on the thermal conductivity of zirconium dioxide crystals stabilized with scandium oxide (ZrO2)1 – x(Sc2O3)x (x = 0.08–0.10) and simultaneously with scandium and yttrium oxides (ZrO2)1 – x – y(Sc2O3)x(Y2O3)y (x = 0.003–0.20, y = 0.02–0.025) has been studied. In the crystals of zirconium dioxide stabilized with scandium oxide, the most noticeable changes in the thermal conductivity are observed in the 9ScSZ crystals, in which the phase composition is changed and a marked content of the rhombohedral phase appears. These changes are less noticeable in the 8ScSZ crystals and they are mainly due to the ordering of oxygen vacancies and the changes in the microstructures of the samples, and there are no changes in the 10ScSZ crystals. The 10ScSZ crystals have the minimum electrical conductivity before and also after annealing, which is determined by the highest content of scandium oxide in the solid solution. Insignificant changes in the thermal conductivity are observed in crystals of partially stabilized zirconium dioxide co-alloyed with scandium and yttrium oxides. The cubic 8Sc2YSZ and 10Sc2YSZ crystals demonstrate only slight changes in the thermal conductivity, the character of the temperature dependence of the thermal conductivity, and the phase compositions. The alloying of the zirconia–based solid solutions with yttrium oxide simultaneously with scandium oxide enables one to increase the stability of their phase compositions and structurally dependent thermal and electrical physical characteristics.

Improving the quality of surgical scalpels with ceramic blades

The article discusses the issues of increasing resistance to brittle fracture of surgical scalpels with ceramic blades from nanostructured crystals of partially stabilized zirconium dioxide. The statement is substantiated that the strength properties of crystals can be increased by increasing the crack resistance of crystals by alloying with rare-earth elements. The kinetic microindentation method is used to study the characteristics of mechanical properties and crack resistance of samples of alloyed crystals. An assumption about the efficiency of alloying crystals with cobalt is substantiated. The bending strength test of promising crystal compositions is carried out. Based on the study, recommendations are formulated on the use of zirconium dioxide crystals for surgical scalpel blade manufacture.

Zirconia crystals for tribotechnical applications doped by rare earth elements

The article discusses the impact of rare-earth microadditives on the mechanical properties of nanostructured zirconia crystals stabilized with yttrium oxide as applied to the friction units of devices. The mechanical properties of the surface layers were studied by the nondestructive method of kinetic microindentation. Compressive strength was determined by the standard destructive compression test method. Comparative tribological tests were carried out with dry sliding friction of the test samples on steel according to the disk-finger scheme. The effect of the ionic radius of the alloying element on the compressive strength is investigated. It was established that the doping of zirconia crystals is ambiguous, because it can improve and worsen the tribological properties of crystals. On the basis of the study, a comparison is made of the mechanical properties of crystals and ceramics used in the supports of the axis of precision instruments and areas that are promising for the use of zirconia crystals in instrumentation are identified.

Tribological properties of a plain bearing with a sleeve made of a nanostructured zirconia crystal

The article deals with the issues of the working capacity of a plain bearing with a sleeve made of nanostructured partially stabilized zirconium dioxide. The tribological properties of the shaft-sleeve interface under boundary friction conditions are investigated. An X-ray phase analysis of samples of zirconium dioxide crystals with various additives of yttrium oxide stabilizer has been performed. The optimal composition for the manufacture of bearing bushings has been selected. A procedure for tribological testing of a sliding bearing has been developed. Special attention is paid to the influence of the clearance between the shaft and the bushing on the tribotechnical characteristics of the interface. The calculation of the wear rate of the sliding bearing is carried out taking into account the clearance. A method for calculating the service life of a bearing is proposed. The prospect of using nanostructured crystals of zirconium dioxide for the manufacture of sleeve bearing bushings for precision mechanisms and devices has been experimentally confirmed

Reduction of thermal-stress-induced birefringence in a Faraday rotator based on a crystal with a negative optical anisotropy parameter

Thermally induced polarization distortions in a Faraday rotator based on a crystal with a negative value of the optical anisotropy parameter was investigated in experiments. A terbium-containing cubic zirconia crystal was taken as a magneto-active medium. A crystal was cut in the orientation in which the axes of thermal-stress-induced birefringence are oriented in one direction. It was demonstrated that the distortions in such a Faraday rotator were reduced by an order of magnitude compared to the minimum possible depolarization arising with the use of a magneto-optical element with the standard [001] orientation, but were still higher than the calculated values. It was shown that for successful development of Faraday isolators based on crystals with a negative optical anisotropy parameter, the theoretical model describing thermal-stress-induced birefringence needs improvement.

The influence of the support composition on the physicochemical and catalytic properties of Cu catalysts supported on Zirconia-Alumina for methanol steam reforming

ZrAl materials with different compositions have been synthesized by the hydrothermal method and used as supports of Cu catalysts for hydrogen production from methanol steam reforming (MSR). The effect of the support composition, Zr/Al ratio, on the physicochemical and catalytic properties of CuZrAl catalysts was studied. The characterization results show that there is an optimal support composition (Zr/Al ratio of 0.4), allowing to achieve the highest methanol conversion due to enhanced dispersion and reducibility of copper. Zr-rich catalysts displayed low MSR activity, which is likely due to the segregation and crystallization of zirconia, resulting in the formation of large copper particles with lowered dispersion and reducibility. The catalyst with the optimal Zr/Al ratio shows the highest methanol conversion, enhanced selectivity and good long term stability, rendering this catalyst a promising material for the production of high purity H2 from MSR.

Tough, strong, hard, and chemically durable enstatite‐zirconia glass‐ceramic

AbstractStrong glass‐ceramics (GCs) have been envisaged and widely researched for various applications, including large architectural panels, ballistic impact protection, bioactive medical implants, and odontological prostheses. Here, we report on the development and characterization of a novel hard, strong and tough enstatite‐zirconia (MgSiO3‐ZrO2) glass‐ceramic derived from a 51SiO2–35MgO–6Na2O–4ZrO2–4TiO2 (mol%) glass. The best GC was developed by treating glass samples for nucleation at 700°C for 12 hours, followed by crystal growth at 1090°C for 3 minutes. It was characterized by X‐ray fluorescence (XRF), differential scanning calorimetry (DSC), X‐ray diffraction (XRD), scanning electron microscopy (SEM), and high‐resolution transmission electron microscopy (HR‐TEM), and contained plate‐like enstatite, zirconia, and Ti‐containing crystals. We investigated the nucleating ability of ZrO2 and TiO2 in inducing internal nucleation. In the early stage of crystallization, enstatite spherulites were observed, which were precipitated by heterogeneous nucleation on previously nucleated ZrO2 nano‐crystals. At more advanced stages, at high temperatures, they transformed into plate‐like crystals. The ball‐on‐three‐balls strength, elastic modulus, and Vickers micro‐hardness of the GC are 323 ± 26 MPa, 146 ± 13 GPa, and 6.9 ± 0.1 GPa (load = 5N), respectively. The indentation (KC), single‐edge notched beam bending (KIC), and crack tip (Ktip) fracture toughness are 2.8 ± 0.6 MP.m0.5, 2.2 ± 0.3 MP.m0.5, 1.9 ± 0.3 MP.m0.5, respectively. The crack propagation profile after a controlled Vickers indentation was quite intricate. The enstatite and zirconia crystals enhanced crack deflection, bridging and branching, hindering crack propagation. According to the ISO 6872 for dental materials, the chemical solubility of our GC is 80 ± 5 μg/cm2. Due to this positive combination of high strength, toughness, hardness, and chemical durability, this new glass‐ceramic is envisioned as a candidate for several applications and could be further developed for memory disc substrates, architectural cladding and tiles, ceramic glazes, and dental materials.

Nano-Structured Crystals of Zirconium Dioxide for Extreme Applications

The paper discusses application possibilities of nanostructured crystals of partly stabilized zirconium dioxide in greaseless running of plain bearings in order to support normal operation of friction joints at high temperatures and in aggressive conditions. This issue is a burning one in different industries, e.g. circulating pumps in nuclear power engineering, mixing units of crystal synthesizing lines in chemical engineering; motors in space-based targets, rotors of guide blades in axial compressors of airplane engines, supports of high-speed gas centrifuges. Micro-hardness was investigated, and friction surfaces were explored using methods of X-ray, electron-microscopic and element analysis to find a wear mechanism for crystals ZrO2- Y2O3. The paper reports on tribological properties of partly stabilized zirconium dioxide crystals and prospects of their industrial usage.

Preliminary results on the role of the deposition of small amounts of ZrO2 on Al2O3 support on the partial oxidation of methane and ethane over Rh and Ni supported catalysts

Alumina supported samples have been prepared by impregnation of the support with a solution of zirconium isopropoxide in n-propanol to obtain after calcination a theoretical monolayer of zirconia (1ZrAl). The formation of a layer of tetragonal zirconia crystals over alumina was observed. Catalysts were evaluated in the partial oxidation of methane (CPOM) and ethane(CPOE) and characterized by elemental chemical analysis, BET specific surface area, X-ray diffraction, X-ray photoelectron spectroscopy and hydrogen chemisorption.The impregnation with Ni and Rh maintains this layer of tetragonal ZrO2, although microcrystals with a local distorted tetragonal environment around the Zr have probably been formed as inferred by the broadness of the XRD zirconia peaks.Different Zr environments and different charge densities on the Zr sites are detected after the test in CPOM and CPOE in Ni/1ZrAl and Rh/1ZrAl catalysts. After the tests in CPOM and CPOE, no ZrO2 peaks are detected in Ni/1ZrAl, which indicates that the ZrO2 layer is either amorphous or formed by ZrO2 nanocrystals. On the other hand, a slight interaction between zirconia and alumina is observed, which leads to an increase in the positive charge density on the Zr. In the case of Rh/1ZrAl after both tests, a layer of distorted tetragonal zirconia microcrystals and no interaction between zirconia and alumina are detected. Catalytic tests in CPOM and CPOE make it possible to determine that the modified support improves significantly the performances of the catalysts. The variation in the local environment of the Zr sites and in its charge density favor the adsorption of intermediates/reaction products at high temperatures on the Zr sites, particularly in the amorphous state, which leads to an enhancement in hydrogen production through STRM and Dry reforming of methane.

Phase field modeling of crack propagation in shape memory ceramics – Application to zirconia

Shape memory ceramics (SMCs) are promising candidates for actuators in extreme environments such as high temperature and corrosive applications. Despite outstanding energy dissipation, compared to metallic shape memory materials, SMCs suffer from a sudden brittle fracture. While the interaction of crack propagation and phase transformation in SMCs have been subject of several experimental and theoretical studies, mainly at the macroscale, the fundamental understanding of the dynamic interaction of crack propagation and martensitic transformation is poorly understood. In this work, we use the phase field framework to fully couple the martensitic transformation to the variational formulation of brittle fracture. The model is parameterized for single crystal zirconia which experiences tetragonal to monoclinic transformation during crack propagation. For the mode I of fracture, the opening mode, crack shows an unusual propagation path that is in good agreement with the experiments and indicates the significant role of phase transformation on the crack propagation path. The investigation on the effect of lattice orientation on crack propagation shows that the lattice orientation has a significant influence not only on the crack propagation path but also on the magnitude of the transformation toughening. In a constrained crystal the maximum (minimum) toughening, under mode I loading, occurs when the crystal lattice orientation makes the angle of 50 (90) degrees with the crack surface.

Phase compositions, structures and properties of scandia-stabilized zirconia solid solution crystals co-doped with yttria or ytterbia and grown by directional melt crystallization

The effect of co-doping an impurity on the transport parameters and cubic phase stabilization of ZrO2–Sc2O3-based solid solutions has been compared for Y2O3 and Yb2O3 dopants. Solid solution crystals of (ZrO2)1-x-y(Sc2O3)x(Yb2O3)y and (ZrO2)1-x-y(Sc2O3)x(Y2O3)y (x = 0.08–0.10, y = 0.01, 0.02) were grown by directional melt crystallization in a cold crucible. Co-doping of the (ZrO2)1-x(Sc2O3)x crystals (x = 0.08–0.10) with 2 mol% Y2O3 or Yb2O3 for all experimental compositions produced transparent homogeneous single crystals of the t” phase or the cubic phase. For co-doping of the scandia-stabilized zirconia crystals with 1 mol% Y2O3 or Yb2O3, the stabilization of the high-temperature phases depended on the Sc2O3 content in the solid solutions. For co-doping of the solid solutions with 1 mol% Y2O3 or Yb2O3, the conductivity of the crystals increased with the Sc2O3 concentration. For co-doping of the crystals with 2 mol% Y2O3 or Yb2O3, the conductivity decreased with the Sc2O3 concentration. At scandia concentrations of 9–10 mol%, the high-temperature conductivity of the crystals co-doped with ytterbia were higher compared to those with yttria co-doping. The (ZrO2)0.9(Sc2O3)0.09(Yb2O3)0.01 crystals had the highest conductivity over the entire experimental temperature range.

Analysis of surface structure of zirconia crystals in case of friction against steel

The article considers the formation of the third body during dry friction of the nanostructured zirconia crystals partially stabilized with yttria against steel. The assumption is substantiated that the tribological properties of the studied friction pair are determined by the properties of the films formed on the surface of the crystals. Friction tests under sliding conditions were performed according to the “disk-finger” scheme. The results of electron microscopic examination of the friction surfaces of crystals are presented. The elemental composition was determined, and the phase composition of the transfer films of various sections of the crystal friction surface was calculated. At high magnifications, it was found that the friction surface of samples with 2-4 mol.% of Y2O3 has the sufficiently homogeneous structure of the films with traces of boundaries of smaller particles of the transferred material. Destruction of the friction surface of a sample with a Y2O3 content of 8 mol.% occurs at a deeper level and affects not only the layer of secondary structures, but also the underlying layers of the base material.