Fine Zirconia Powders Research Articles

Electrochemical Reduction of Zirconia in Melts Based on Mixture of Calcium Chloride and Calcium Oxide

The results of studies of the electrochemical reduction of zirconium dioxide on liquid gallium cathode in a molten mixture of CaO-CaCl2-MCl (M = Li, Na) are presented. The fine zirconia powder were used for reduction. As a result of a complex electrochemical and chemical reactions during electrolysis (Fray-Farthing-Chen (FFC) -Cambridge-Process), zirconia is reduced on liquid gallium cathode to zirconium forming wherein a fine powder that does not contain impurities both of oxygen and gallium. Oxygen ions associated with zirconium in zirconia, are moved into the melt under the influence of the electric field, are transported to the anode and discharged thereon. The components of the melt and the gallium are practically not spent on the recovery of zirconia. One of the necessary conditions of the realization of this process is the presence of a suitable melt capable of dissolving a sufficient amount of metal oxides (Ca, Mg, etc.). These oxides are necessary for the electrochemical reduction of ZrO2 and for discharge of oxygen anions, which are formed herewith. The mixture of CaO(5,0) - CaCl2(39,9) - LiCl(55,1 mol.%) tmp. = 4950C, and CaO(5,0) -CaCl2(48,0) -NaCl(, 0 mol.%) tmp. = 5000C were investigated as such melts. The powder of zirconium, which is forming during electrolysis, is easily removed from the cell due to the difference in the specific weights and crystallization temperatures of the molten oxide-chloride mixture and the gallium. The extraction degree of zirconium from zirconia depends on the cation composition of the melt and is the higher, the lower the its acid strength is. For example, the extraction degree of zirconium from zirconia is not lower than 76% and essentially independent of temperature in the temperature range 650-8500C in the melt CaO-CaCl2-NaCl, which acid strength is less than that of melt CaO-CaCl2-LiCl. At lower temperatures, the degree of extraction of zirconium in this melt decreases. The intensive reduction of zirconia begins at lower temperatures (580-6000C) in the molten CaO-CaCl2-LiCl mixture. The extraction degree of zirconium from zirconia also substantially independent of temperature in this melt and is not lower than 65% in the temperature range of 600-8000C. The extraction degree of zirconium from zirconia reaches a maximum after reaching of certain values of the current density and subsequently is practically independent from it. This may be due to the fact that reducing agent of the zirconia in this range of current density become preferably intermetallic compounds of calcium with gallium. The maximum extraction degree of zirconium in the melt CaO-CaCl2-NaCl is achieved at current densities exceeding 0.3 A/cm2, and in the melt CaO-CaCl2-LiCl - at current densities above 0.7 A/cm2. The molten mixture of CaO-CaCl2-NaCl provides more favorable conditions for the recovery of zirconia. The average size of the particles of obtained zirconium powder is of 0.001-0.003 microns, specific surface area and the average of micropore size decreases with increasing current density.

Preparation of Mullite‐ and Zircon‐Based Ceramics Using Kaolinite and Zirconium Oxide: A Sintering Study

The interactions between zirconia (ZrO2) powder and three Algerian hydrated kaolinitic clays were studied at high temperatures. The analysis by X‐ray diffraction of the prepared products allowed to follow the different phase developments during heat treatment and to identify the parameters controlling the zirconia conversion into zircon (ZrSiO4). It was found that ZrSiO4 formation, occurring at temperatures above 1150°C, is enhanced by the presence, in the clays, of fusing impurities such as K, Fe, Ca, and Mn, and by a decrease in zirconia particle size. A reactional mechanism, involving zirconia, a flux, and cristobalite is proposed. Moreover, the effect of zirconia additions on sintering was studied. It was also found that the increase in the porosity ratio of the final products for zirconia levels above 20 wt% was governed by a decrease in the flux amount, due to its lower clay content. Finally, it was shown that ceramics obtained by sintering at 1400°C for 2 h of a mixture of 38 wt% of fine zirconia powder and 62 wt% of the more reactive clay were mainly constituted of zircon and mullite.

Graded structures for damage resistant and aesthetic all-ceramic restorations

Objectives Clinical studies revealed several performance deficiencies with alumina- and zirconia-based all-ceramic restorations: fracture; poor aesthetic properties of ceramic cores (particularly zirconia cores); and difficulty in achieving a strong ceramic–resin-based cement bond. We aim to address these issues by developing a functionally graded glass/zirconia/glass (G/Z/G) structure with improved damage resistance, aesthetics, and cementation properties. Methods Using a glass powder composition developed in our laboratory and a commercial fine zirconia powder, we have successfully fabricated functionally graded G/Z/G structures. The microstructures of G/Z/G were examined utilizing a scanning electron microscopy (SEM). The crystalline phases present in G/Z/G were identified by X-ray diffraction (XRD). Young's modulus and hardness of G/Z/G were derived from nanoindentations. Critical loads for cementation radial fracture in G/Z/G plates (20 mm × 20 mm, 1.5 or 0.4 mm thick) bonded to polycarbonate substrates were determined by loading with a spherical indenter. Parallel studies were conducted on homogeneous yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) controls. Results The G/Z/G structure consists of an outer surface aesthetic glass layer, a graded glass–Y-TZP layer, and a dense Y-TZP interior. The Young's modulus and hardness increase from surface to interior following power-law relations. For G/Z/G plates of 1.5 and 0.4 mm thick, critical loads for cementation radial fracture were 1990 ± 107 N (mean ± S.D., n = 6) and 227 ± 20 N (mean ± S.D., n = 6), respectively, which were ∼30 and 50% higher than those for their monolithic Y-TZP counterparts (1388 ± 90 N for 1.5 mm and 113 ± 10 N for 0.4 mm thick; mean ± S.D., n = 6 for each thickness). A 1-sample t-test revealed significant difference ( p < 0.001) in critical loads for radial fracture of G/Z/G and homogeneous Y-TZP for both specimen thicknesses. Significance Our results indicate that functionally graded G/Z/G structures exhibit improved damage resistance, aesthetics, and potentially cementation properties compared to homogeneous Y-TZP.

Sintering Kinetics at Constant Rates of Heating: Mechanism of Silica‐Enhanced Sintering of Fine Zirconia Powder

The sintering behavior of 3 mol% Y2O3‐doped ZrO2 powders with and without a small amount of SiO2 was investigated to clarify the effect of SiO2 addition on the initial sintering stage. The shrinkage behavior of a powder compact was measured under constant rates of heating (CRH). The sintering rate increased remarkably with the addition of a small amount of SiO2. The apparent activation energy (nQ) and apparent frequency factor , where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying the sintering‐rate equation to the CRH data. The diffusion mechanism changed from grain‐boundary diffusion (GBD) to volume diffusions (VD) on SiO2 addition, and both nQ and increased with the GBD→VD change. It is, therefore, concluded that the sintering rate increases by SiO2 addition because the increase in rather than nQ is predominant.

Mechanism of Alumina‐Enhanced Sintering of Fine Zirconia Powder: Influence of Alumina Concentration on the Initial Stage Sintering

The isothermal shrinkage behavior of 2.9 mol% Y2O3‐doped ZrO2 powders with 0–1 mass% Al2O3 was investigated to clarify the effect of Al2O3 concentration on the initial sintering stage. The shrinkage of the powder compact was measured at constant temperatures in the range of 950°–1050°C. The Al2O3 addition increased the densification rate with increasing temperature. The values of apparent activation energy (nQ) and apparent frequency‐factor term (β0n), where n is the order depending on the diffusion mechanism, were estimated at the initial sintering stage by applying a sintering‐rate equation to the isothermal shrinkage data. The diffusion mechanism changed from grain‐boundary diffusion (GBD) to volume diffusion (VD) by Al2O3 addition and both nQ and β0n increased with increasing Al2O3 concentration. The kinetic analysis of the sintering mechanism suggested that the increase of densification rate by Al2O3 addition largely depends on the increase of β0n, that is, the increases of n with GBD→VD change and β0 with an increase in Al2O3 content, although the nQ also increases with Al2O3 addition. This enhanced sintering mechanism is reasonably interpreted by the segregated dissolution of Al2O3 at ZrO2 grain boundaries.

Sintering mechanism of fine zirconia powders with alumina added by powder mixing and chemical processes

The present study examined the sintering behavior of fine zirconia powders (containing 2.9 mol% Y2O3) with and without small amounts of Al2O3 added by different ways: powder mixing (PM), homogeneous precipitation (HP), and hydrolysis of alkoxide (HA). The initial sintering behavior was examined by both measurement methods of a constant rate of heating and isothermal shrinkage. In the PM process, Al2O3 particles pinned the shrinkage of zirconia powder compact at the initial stage and diffuse toward zirconia surface to enhance the sintering. This initial sintering mechanism was explained by the grain-boundary diffusion for the Al2O3-free powder and the volume diffusion for Al2O3-added powder. When Al2O3 was added to zirconia powder by HP and HA processes, the densification rate was more stimulated compared to the PM process. The initial sintering mechanism did not change by the way for Al2O3 addition. The Al2O3 addition by chemical processes of HP and HA tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al2O3 by those processes.

Sintering Mechanism of Fine Zirconia Powders with Alumina Added by Various Ways

Small amounts of Al2O3 were added to fine zirconia powder by different ways: powder mixing, hydrolysis of alkoxide, and homogeneous precipitation. During a constant rate heating process, the Al2O3 addition slightly raised the starting temperature of densification of powder compact, and the densification was remarkably stimulated by Al2O3 at temperatures above about 1100oC. According to an isothermal analysis of densification, the densification rate was retarded by Al2O3 addition just after the start of sintering and then the densification rate increased significantly during sintering compared to Al2O3-free powder. These results mean that Al2O3 particles pinned the shrinkage of zirconia powder compact at the initial stage, and diffuse toward zirconia surface to enhance the sintering. The sintering mechanism was explained by the grain-boundary diffusion for the Al2O3-free powder and the volume diffusion for Al2O3-added powder. When the Al2O3 was added to zirconia powder by homogeneous precipitation and alkoxide methods, the densification rate was more stimulated compared to powder mixing method. The sintering mechanism did not change by the way for Al2O3 addition. The Al2O3 addition by the chemical process tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al2O3 by these chemical processes.

Sintering Kinetics at Isothermal Shrinkage: II, Effect of Y2O3 Concentration on the Initial Sintering Stage of Fine Zirconia Powder

The shrinkage behavior of fine zirconia powders containing 2.9 and 7.8 mol% Y2O3 was investigated to clarify the effect of Y2O3 concentration on the initial sintering stage. The shrinkage of powder compact was measured under both conditions of constant rates of heating (CRH) and constant temperatures. CRH measurements revealed that when the Y2O3 concentration of fine zirconia powder increased, the starting temperature of shrinkage shifted to a high temperature. Isothermal shrinkage measurements revealed that the increase in Y2O3 concentration causes the shrinkage rate to decrease. The values of activation energy (Q) and frequency‐factor term (β0) of diffusion at initial sintering were estimated by applying the sintering‐rate equation to the isothermal shrinkage data. When the Y2O3 concentration increases, both Q and β0 of diffusion increase. It is, therefore, concluded that the increase in Y2O3 concentration of fine zirconia powder decreases the shrinkage rate because of increasing Q of diffusion at the initial stage of sintering.

Sintering Kinetics at Isothermal Shrinkage: Effect of Specific Surface Area on the Initial Sintering Stage of Fine Zirconia Powder

The isothermal shrinkage behaviors of fine zirconia powders (containing 2.8–2.9 mol% Y2O3) with specific surface areas of about 6 and 16 m2/g were investigated to clarify the effect of specific surface area on the initial sintering stage. The shrinkage of powder compact was measured under constant temperatures in the range of 1000°–1100°C. The increase in specific surface area enhanced the densification rate with increasing temperature. The values of activation energy (Q) and frequency‐factor term (β0) of diffusion at initial sintering were estimated by applying the sintering‐rate equation to the isothermal shrinkage data. The Q of diffusion changes little but the β0 increases with the increase in specific surface area. It is therefore concluded that the increase in the specific surface area of fine zirconia powder enhances the shrinkage rate because of an increase in the β0 at the initial stage of sintering.

Sintering Kinetics at Constant Rates of Heating: Effect of Al2O3 on the Initial Sintering Stage of Fine Zirconia Powder

The sinterabilities of fine zirconia powders including 5 mass% Y2O3 were investigated, with emphasis on the effect of Al2O3 at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al2O3 increased the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact including Al2O3 was lower than that of a powder compact without Al2O3. The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al2O3 included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al2O3 enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage.

Interdependence between green compact property and powder agglomeration and their relation to the sintering behaviour of zirconia powder

Interdependence of green density and corresponding powder agglomeration and their influence on the sintering behaviour of commercial fine zirconia powders under a constant rate of heating (non-isothermal sintering) were investigated. Agglomeration of the powder was controlled by different time periods of ball-milled processing and was defined as the size ratio of sedimentationally-measured particle size to the size of primary particles which were microscopically-determined (hereinafter termed agglomeration parameter or AP). Green compact density shows to be approximately linearly related to powder agglomeration under identical consolidation technique, which is decreased with increasing degree of agglomeration. Both the green density and powder agglomeration affect sintering behaviour over entire sintering schedule. For a given AP the shrinkage rate reduces with increasing green compact density and vice versa, which is consistent with the literatured reports. The experimental results also showed that compacts with identical starting density showed a lower shrinkage rate when the compacts contained less agglomeration (i.e. low AP) than does for high-AP compacts. However, a higher end-point density can be obtained for low-AP compacts, suggesting a better packing structure of the powders. The use of agglomeration parameter defined currently, which is taken as an indication of the level of powder agglomeration in commercial fine ceramic powders, is likely to provide some useful understanding in characterising the sintering behaviour and possibly potential evolution of sintered microstructure on sintering.

Effect of pH and Concentration on the Synthesis of Monodispersed Spherical Fine Zirconia Powders Using Gas–Liquid Phase Reaction

ABSTRACTAmmonia gas was blown into the solution of zirconium ion to induce homogeneous precipitation of supersaturated zirconium ion at gas-liquid interface with increase in pH. The precipitates formed using interface of gas-liquid phase were decomposed into fine spherical zirconia powder of high purity. As Concentration increase, mean diameter of particles increases to 140, 180, 240, 290 and 630nm. At pH of 4.5, maximum yield of 98.7% was obtained. From the above pH of 4.5, yield has been kept constant. Above pH of 5.0, large aggregates of precipitate consisting of primary particles were formed, and this may have been caused due to the existence of isoelectric point. Below pH of 4.5, almost aggregate-free fine spherical powders with particle size of below 100nm were produced.

Pressure filtration and sintering of fine zirconia powder

Pressure filtration is applied to the consolidation of 3 mol% yttria-doped zirconia fine powder with average particle size of 60 nm over an applied pressure range of 2.5 to 10.0 MPa. Two kinds of aqueous suspension with different rheological behavior, well-dispersed and insufficiently dispersed, respectively, are prepared by changing the amount of dispersant. The kinetics and mechanics of the consolidation of particles are discussed from the dehydration and consolidation rates of the suspensions based on Darcy's law and the Kozeny-Carman equation. The consolidated layer prepared from the well-dispersed suspension shows a good sinterability. Cold isostatic pressing at 400 MPa increases the sintered density of the consolidated layer prepared from the insufficiently dispersed suspension.

Effects of synthesis parameters on electrical conductivity and microstructural development of fine zirconia powders

Effects of synthesis parameters on electrical conductivity and microstructural development of fine zirconia powders

Preparation of Tetragonal Zirconia Ceramics in ZrO&lt;sub&gt;2&lt;/sub&gt;-Sc&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; System by Hydrolysis and Homogeneous Precipitation and Their Phase Stability

Fine zirconia powders doped with 3 to 6mol% Sc2O3 have been prepared by the urea-based homogeneous precipitation method using monoclinic zirconia sols previously formed by the hydrolysis of ZrOCl2 solution. Highly dense Sc2O3-doped tetragonal zirconia ceramics, Sc-TZP (>99%) with a homogeneous microstructure were obtained by sintering these powders at 1300°C or 1400°C for 1h and their phase stability was investigated. The average grain size and the crystal phase of ZrO2(+Sc2O3) materials were observed to depend on the Sc2O3 contents. Addition of 3.5mol% Sc2O3 was necessary to fabricate zirconia bodies consisting of fully tetragonal phase by sintering at 1300°C. This value corresponds to nealy 1.5 times the content of Y2O3 in the ZrO2-Y2O3 system. The average grain size of the 4mol%Sc-TZP sintered at 1400°C was <0.5μm. Although the grain size is depending on the sintering temperature, it increased gradually with increases in Sc2O3 content from 3 to 6mol% and in the content of the cubic phase. The monoclinic zirconia contents transformed from the tetragonal phase in the surface of the 4 to 6mol% Sc-TZP under hydrothermal conditions at 180°C for 50h were about 93 to 70%.

Hydrothermal Synthesis and Sintering of Fine Powders in CeO&lt;sub&gt;2&lt;/sub&gt;-ZrO&lt;sub&gt;2&lt;/sub&gt; System

Fine powders in CeO2-ZrO2 system have been prepared from the precipitate mixtures of cerium (III) nitrate solution, zirconium oxychloride solution and an excess amount of ammonia under hydrothermal conditions at 180°C, and their changes in crystal phase and surface area by calcination, sinterability, the crystal phase and the microstructure of the sintered bodies have been investigated. Twelve mol% CeO2-doped fine zirconia powders hydrothermally synthesized showed metastable cubic zirconia phase without monoclinic phase and they gradually transformed into tetragonal phase by calcining at above 600°C. The samples except for 12mol% CeO2-ZrO2 consisted of two phase mixtures of zirconia and ceria. The crystallite size of 12mol% CeO2-doped zirconia particles and CeO2 particles in the 70mol% CeO2-ZrO2 powder was about 6nm and 20nm, respectively. Dense CeO2-ZrO2 ceramics were obtained by sintering these powders at above 1400°C for 1h. The sinterability of the powders increased with increasing CeO2 concentration. The calcined 80mol% CeO2-ZrO2 powders pressed at 98MPa reached a maximum density with single cubic CeO2 solid solution at 1300°C for 1h. The average grain size of 12mol% CeO2-doped tetragonal zirconia ceramics sintered at 1500°C was 1.5μm. Although the grain size of the sintered body in the two-phase region (tetragonal zirconia and cubic CeO2 solid solution) was below 1.0μm, it increased with increasing CeO2 concentration within the single phase region of cubic CeO2 solid solution.

Influence of some variables of the precipitation process on the structural characteristics of fine zirconia powders

Zirconia gels precipitated at pH 6, 10.4 and 13.5 from zirconyl chloride solutions have been investigated by thermogravitry and differential thermal analysis techniques. The total mass loss is 20% but increases for the lower pH studied. Oxide crystallization occurred at 447°C, and no clear relation appears between processing conditions and thermal effects. The calcined powders were investigated by X-ray diffraction and nitrogen adsorption techniques. X-ray powder patterns only exhibited the tetragonal reflections after precipitation performed at pH 13.5 at room temperature. However, for a slight increase in the precipitant temperature the diffraction profile shows a mixture of monoclinic and tetragonal phases. The order of addition of solutions during the precipitation process produces some differences concerning crystallite sizes. Specific surface area values are found to be independent of the pH of precipitation, but strongly affected by the washing media.

Formation of Coating Film on Milling Balls for Mechanical Alloying

A homogeneous coating film can be obtained on the mechanical alloying (MA) ball surface by ball milling of fine zirconia powder and Ti-34 mass%Al powder. X-ray diffraction pattern of MA powder containing 80 mass% zirconia is broad by MA for 360 ks under 35 kPa argon atmosphere. Fine zirconia powders combine a coating film with a ball and alloy Zr or O with Ti or Al in a coating film. The coating film of approximately 8 μm in thickness, which consists of Ti, Al, Zr and O, is provided on the ball surface. Furthermore, a metallic coating film of approximately 10 μm in thickness can be obtained on the ball surface by ball milling with zirconia balls and metal powder, such as Ti-Al, Ti-Si, Fe-Al and Fe-Si.

Influence of Zirconia Addition on the Sintering Behavior of Bimodal Size Distributed Alumina Powder Mixtures

Fine zirconia powder was added to the bimodal size distributed alumina powder mixtures and the influence of zirconia addition on the sintering behavior of the powder mixtures was investigated. At low firing temperature, that is, in the initial stage of sintering, zirconia retarded the densification of the powder mixtures and also enlarged the pore size of the compacts. At high firing temperature, the influence of zirconia addition was different depending on the packing structure of the green compact. When the coarse alumina particles were dispersed in the matrix of the fine particles, zirconia promoted the densification of the mixtures. This result was explained by the promotion of the densification between the fine and the coarse alumina particles in the intermediate stage of sintering and by the migration of zirconia and the hindrance effect of zirconia on the grain growth of alumina in the final stage of sintering. When the coarse alumina particles formed a skeletal structure, zirconia did not promote the densification of the mixtures. This result was explained by the dependence of the densification of the compacts only on the densification among the coarse alumina particles.

Use of coupling agents as a route to improvement of the compressibility of fine zirconia and alumina powders

In order to improve the compressibility of fine zirconia and alumina powders, powders were surface-treated with aluminate, silane and titanate coupling agents. The surface modification reduced both powder/powder friction and powder/die-wall friction, which increased the density of the compacts. At 2% additions, the effectiveness of the coupling agents on density increase was in the order, silane > titanate > aluminate. In zirconia systems with different titanate concentrations it was found the optimum amount of coupling agent could be approximated by the amount required for monolayer coverage on the powder surface.