Thermal shock resistance of Mg-PSZ

The thermal shock resistance (TSR) of Mg-PSZ (a partially stabilized ZrO₂) containing either tetragonal t- or monoclinic m-ZrO₂ precipitates was studied. The samples containing t-ZrO₂ had superior TSR, particularly in having a higher retained strength after thermal fracture had been initiated. The t-ZrO₂ in the surface regions of shocked samples transformed to a new polymorph with orthorhombic (o) symmetry; the stress-induced t o transformation may be responsible in part for the improved TSR in samples containing t-ZrO₂ compared to those in which all the particles have m symmetry.     

Preparation of monodispersed Y-doped ZrO2 powders

3 mol% Y-doped ZrO₂ powders prepared by the controlled hydrolysis of metal alkoxides were monodispersed and grown to 0.5m after 5 h ageing. The as-prepared powder was amorphous and hydrate but transformed into a tetragonal single phase by heating. Furthermore, the Y₂O₃ concentration of each particle was almost the same in all particles. The synthesis conditions such as ageing time, ageing temperature and water concentration greatly affected the particle morphology. The refluxing of the alkoxide solution was particularly necessary to prepare the monodispersed particles. On the basis of the variation of size distribution with ageing time, the mechanism of particle growth was discussed.     

Preparation of composite Al2O3-TiO2 particles from organometallic precursors and transformations during heating

Equimolar Al₂O₃-TiO₂ composite powders were prepared via controlled hydrolysis of organometallic precursors, sometimes in the presence of submicrometre commercial-Al₂O₃ or anatase-TiO₂ particles. Variations in the chemical procedures were used aimed at different submicrostructures within the resulting powders. Heating such powders in air shows that structural behaviour is influenced by the micromorphology of the composite particle. Transformation temperatures of the titania phases seem to depend upon some size parameter which would represent their morphology within the powders. Studies performed on a series of non-equimolar Al₂O₃-TiO₂ composite powders showed that the temperature of -Al₂O₃ formation may be decreased by 210° C possibly due to a seeding effect of rutile. Pseudobrookite Al₂TiO₅ was never detected at 1300° C in air.     

Preparation of ZrO2 coated graphite powders

Graphite powders were coated with ZrO₂ by the controlled hydrolysis of a zirconium oxychloride aqueous solution. Thehydrolysis process was carried out with temperature control because of the low wettability of ZrO₂ to the surface of thegraphite. PVA was added to the solution for the enhancement of Zr ion adsorption. The surface of the graphite particles coatedwith ZrO₂ was observed by TEM. There are two types of ZrO₂ particles; (a) primary particles a few nm in size and (b) secondary particles with 0.1 m size were obtained. The data of oxidation weight loss and surface potentialshow that the graphite surface was successfully modified by the forced hydrolysis of the zirconium chloride aqueous solution.     

Strength improvement in beta″ alumina by incorporation of zirconia

Beta alumina ceramic electrolytes for use in Na/S batteries are inherently weaker than most engineering ceramics due to the presence of weakly-bonded conduction planes in the crystal structure and to difficulties in controlling grain growth during firing. Substantial improvement in microstructural control is obtained by incorporation of monoclinic zirconia (m-ZrO₂) or partially stabilized zirconia (PSZ) resulting in increases in strength and fracture toughness to around 350 MPa and 4 MPam^1/2, respectively. PSZ may adversely influence the electrical resistivity of the ceramic owing to the presence of impurities. With most zirconia powders a high level of retention of tetragonal zirconia (t-ZrO₂) is obtained at levels of addition up to 15% by weight ZrO₂. At these levels ZrO₂/-Al₂O₃ ceramics show low resistivity and stable resistance in Na/S cells.     

Dispersion of ZrO2 particles in aqueous suspensions by ammonium polyacrylate

The effects of poly anionic-electrolyte (ammonium polyacrylate, PAA) as a dispersant on two kinds of ZrO₂ (monoclinic and yttria-doped tetragonal zirconia) aqueous suspensions were examined by the measurements of-potential and viscosity, the sedimentation test and the determination of the wet point and flow point of the powders. Additions above 2.5 wt% PAA to zirconia gave a negative high-potential above –30 mV, and then –45 and –30 mV were obtained for monoclinic and tetragonal zirconia above 5 wt% PAA, respectively. A high negative-potential above –30 mV was retained with 5 wt% PAA for a change in pH over a wider range (pH 6 to 10 for monoclinic ZrO₂, 7 to 9 for tetragonal ZrO₂) in comparison to that of ZrO₂ without dispersant. The increase of the-potential resulted in a decrease in the viscosity. The evaluation of dispersion by the sedimentation test was correlated well with the value of-potential and the viscosity of the suspensions. The presence of native positive charge of monoclinic and tetragonal zirconia powders required an excess amount of PAA to attain dispersion of the suspension. There was a small difference in the least amount of PAA required to attain good dispersion between monoclinic and tetragonal ZrO₂. The difference was also indicated by changes of the flow point on PAA addition. Addition of 0.1% PAA to monoclinic ZrO₂ and 0.25 wt% to tetragonal ZrO₂ gave a maximum value of the flow point, whereas the positive-potential fell to zero. Measurement of the flow point was a simple and useful technique for rapid evaluation of a required amount of dispersant for ZrO₂ suspensions.     

Estimation of formation mechanism of spherical fine ZrO2-SiO2 particles by ultrasonic spray pyrolysis

Spherical fine ZrO₂-SiO₂ (11) particles were synthesized from a hydrolysed mix-solution of Si(OC₂H₅)₄ and ZrOCI₂ · 8H₂O by ultrasonic spray pyrolysis, and the formation mechanism of the particles discussed. All of the resultant particles identified as t-ZrO₂ and amorphous SiO₂ with an atomic order dispersion were spherical, and mainly of diameter 0.8 to 1.0m. It was estimated that the three-dimensional ladder siloxane chains containing an equimolar Zr⁴⁺ homogeneously dispersed were formed by hydrolysis, and an atomized droplet itself converted into an isolated ZrO₂-SiO₂ (11) particle without aggregation.     

Effect of NiO dissolution on the microstructural development of t′-ZrO2 with 5 mol% Y2O3

The ageing (1300 °C for 1–300 h) dependence of the microstructure of the metastable tetragonal (t-) ZrO₂ (plasma-sprayed Y-PSZ with 5 mol% Y₂O₃) in the sintered (1300 °C, 5 h) t-ZrO₂/NiO (10 mol%) composite was studied by transmission electron microscopy. Short-term (10 h) ageing caused incomplete decomposition of t-ZrO₂ into tweed assemblages containing t-ZrO₂ relics and tetragonal (t-) ZrO₂ precipitates in the cubic (c-) ZrO₂ matrix. The t-ZrO₂ relics retained anti-phase domain boundary (APB) and primary deformation twins, but no secondary deformation twins. Further ageing (e.g. up to 100 or 300 h) caused growth of the twin variants of t-ZrO₂ with {101} habit plane. The large t-ZrO₂ variants transformed directly into m-ZrO₂ twins, but not into t-ZrO₂ colonies. In addition to the relics in the aged samples, t-ZrO₂ with APB may also form locally from a solute-redistributed c-ZrO₂ by slow cooling.     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Formation and sintering of 75 mol% alumina/25 mol% zirconia (2–3.5 mol% yttria) composite powder prepared by the hydrazine method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 25 mol% ZrO₂ have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO (OH) gel solid solutions and amorphous ZrO₂. The formation process leading to -Al₂O₃/t-ZrO₂ composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 °C under 196 MPa. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed.     

Transformation toughening and grain size control inβ″-Al2O3/ZrO2 composites

In order to fabricate transformation-toughened-Al₂O₃ and optimize its mechanical and electrical properties it was found to be necessary to carefully control the particle size distribution of the starting powders and their mixing. The ionic resistivity of the composites depended primarily on the volume fraction of ZrO₂. Additions between 10 and 20 vol% produced materials with ionic resistivities (300° C) between 6 and 10 cm and eliminated exaggerated grain growth of the-Al₂O₃. Comparison of-Al₂O₃ composites containing either tetragonal (t-) ZrO₂ or cubic (c-) ZrO₂ with the single phase material showed that the major strengthening mechanism is the reduction in critical flaw size. This occurred by the elimination of the flaw population associated with abnormally large grains. For maximum increases in fracture toughness and strength, however, the use of t-ZrO₂ (transformation toughening) as a second phase is preferred.     

Stability of tetragonal ZrO2 phase in Al2O3 prepared from Zr-Al organometallic compounds

Zr-Al organometallic compounds have been spray-dried and heated at temperatures 600 to 1400°C to prepare ZrO₂-Al₂O₃ composite powders. The powders consist of balloon-like particles 0.5 to 2 m in diameter with homogeneously dispersed tetragonal ZrO₂ grains 0.1 to 0.2 m in diameter. The tetragonal fraction of ZrO₂ in the composite powders is higher than that in the powders prepared from sols of Zr(OBuⁿ)₄ and Al[OCH(CH₃)₂]₃. The fraction is affected by the organofunctional group in the Zr-Al compounds.Zr(OBuⁿ)₄ = Zr(OC₄Hgⁿ)₄; Al[OCH(CH₃)₂]₃ = Al(OPrⁱ)₃.     

X-ray diffraction study of compositionally homogeneous, nanocrystalline yttria-doped zirconia powders

The crystal structure of compositionally homogeneous, nanocrystalline ZrO₂ - X mol% Y₂O₃ (X = 2.8, 4, 5, 6, 7, 8, 9, 10, 11, and 12) powders synthesized by a nitrate-citrate gel-combustion process has been studied by X-ray diffraction. By applying the Rietveld method, it was found that all the powders presented the tetragonal phase (P4 ₂/nmc space group). The axial ratio c/a _f decreased with increasing Y₂O₃ content and became almost unity at 9 mol% Y₂O₃. However, powders in the compositional range of ZrO₂ - 9 to 12 mol% Y₂O₃ exhibited the oxygen atoms displaced from their ideal sites of the cubic phase along the c axis, which is known as the t-form of the tetragonal phase. A conjunct analysis of crystallite size and microstrain of all the powders is also presented.     

Precipitation of ultrafine powders of zirconia polymorphs and their conversion to MZrO3 (M = Ba,Sr, Ca) by the hydrothermal method

Ultrafine powders of ZrO₂ with a high degree of crystallinity and chemical purity are hydrothermally precipitated from aqueous solutions of impure zirconyl oxychloride or the acid extract of zircon (ZrSiO₄)-frit at 2 to 8 MPa and 180 to 230 ° C. Monoclinic ZrO₂ is produced from aqueous hydrochloric acid, whereas tetragonal ZrO₂ is formed from the same medium when sulphate ions are present with [SO ₄ ^2– ]/[Cl–] 0.08. If cation impurities such as Y³⁺ or Ce³⁺ are incorporated, the stability range of the tetragonal phase is extended to higher temperatures. Ultrafine powders are characterized by X-ray broadening methods, TEM and thermally as well as mechanically induced transformation characteristics. The tetragonal ZrO₂ powder is constituted of polydomain crystallites with higher hydroxyl ion content than the monoclinic phase. Both series of powders convert to BaZrO₃, SrZrO₃ or CaZrO₃ perovskites when suspended in the corresponding hydroxide solution at 190 to 480 °C and 2 to 100 M Pa.     

Thermal shock of ß-alumina with zirconia additions

Sodium-alumina electrolyte tubes were prepared from spray dried precursor powders with additions of 2, 5, 10 and 15 wt % ZrO₂ made by the sodium zirconate route. The thermal shock behaviour of ring segments cut from the tubes was examined by quenching into water at 0° C. At the 2% ZrO₂ level the dispersed ZrO₂ particle size was low, <0.5m, and the tetragonal phase was retained. For higher weight fractions, particle coarsening during the-alumina sintering schedule was extensive and the large particles transformed to monoclinic on cooling. At the 15% ZrO₂ level the-alumina had a slightly reduced strength and high resistance to thermal shock and to thermal shock damage, quenching into water from 800° C effecting only a 43% reduction in strength. These observations are consistent with the effects of microcracking in the vicinity of second phase ZrO₂ particles.     

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO₃ powders were precipitated by reacting fine TiO₂ particles with a strongly alkaline solution of Ba(OH)₂ under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO₃ increased from 50 nm at 90°C to 100 nm at 240°C. At synthesis temperatures below 150°C, the BaTiO₃ particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO₃ powder is discussed.     

Formation of monodispersed Ta2O5 powders

The Ta₂O₅ powders synthesized by the hydrolysis of tantalum pentaethoxide, Ta(OC₂H₅)₅ in alcoholic solution were monodispersed fine oxide particles, which were a uniform, spherical shape, non-agglomerate, and had a narrow size distribution. They grew to 1.2m after ageing for 1 h after hydrolysis. Powder X-ray diffraction and differential thermal analysisthermogravimetric analysis showed the particles were amorphous and hydrated. These particles lost the water at 290° C and gave well-crystalline Ta₂O at 740° C. Throughout these thermal processes, the particle morphology was kept almost the same.     

Preparation of Ultrafine Tetragonal ZrO2–CeO2Solid Solutions

Nanometer-sized tetragonal Zr_{1 – x} Ce _x O_{2 –} powders were prepared by hydroxide precipitation and sol–gel processing. The effects of gel-aging time and solution concentration on the phase composition and particle size of the powders were studied. The ceramics prepared by sintering ultrafine powders at 1500°C had a density of 6.0 g/cm³, open porosity of 4%, and closed porosity below 1%.     

A study of the oxidation products of some LiRF4 phases

The effects of oxidation in both normal and moist air upon powdered single crystals of the scheelite-type compounds LiYF₄, LiErF₄, -YLF (LiY_0.434 Er_0.5 Tm_0.055 Ho_0.011 F₄) and -LuLF (LiLu_0.434 Er_0.5 Tm_0.05 Ho_0.011 F₄) have been studied by X-ray powder diffraction and differential thermal analysis. For all the materials the oxidation rate is enhanced in the presence of moisture. The oxidation process results in the formation of rare-earth oxyfluoride phases, LiYF₄, forming yttrium oxyfluoride (with a rhombohedral phase predominating) and LiErF₄ forming erbium oxyfluoride (with a tetragonal phase predominating). The LiYF₄ compound oxidizes more rapidly than LiErF₄ both in normal and moist air. The more complex -YLF compound oxidizes to form a mixture containing significant quantities of both the rhombohedral and tetragonal rare-earth oxyfluoride phases. The -LuLF compound oxidizes to form the tetragonal rare-earth oxyfluoride phase together with a very small amount of the rhombohedral variety. The melting reactions of oxidized samples are very broad thermal events which are coupled with the presence of a strong LiF/LiRF₄-type eutectic component. These observations are rationalized in terms of the appearance of excess LiF due to selective oxidation of the trifluoride component.     

Powders of Pb(ZrxTi1−x)O3 by sol-gel coating of PbO

Powders of lead zirconate titanate (PZT) are prepared by coating a PbO powder with very fine particles containing titanium and zirconium derived from an alkoxide sol. The size of the particles in the coating increases from 10 nm to 100 nm with the increase in the pH during preparation from 3 to 10.5. The powders calcine at low temperature ( 600 °C) to single-phase PZT without the formation of any intermediate phase. The tetragonal and rhombohedral phases are found to coexist betweenx = 0.535 and 0.545. Careful calcination of the powders is needed to ensure removal of residual organics; otherwise the residual carbon reduces PbO to metallic lead leading to high currents during poling. Loss of lead on sintering is significantly lower (1.4 wt%) as compared to samples from conventional powders (6.4 wt%). Successful poling of the sintered samples is achieved when the powders are prepared without using sol stabilizers and are calcined without pelletizing. Washing of the powders is not helpful, perhaps because it leads to change in stoichiometry due to loss of material. A dielectric constant of 950 andd ₃₃ > 190 pCN^–1 are easily achieved.