Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the properties of nanocrystalline ZrO<Subscript>2</Subscript> + 3 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powder

We have studied the properties of nanocrystalline ZrO₂〈3 mol % Y₂O₃〉 and 90 wt % ZrO₂〈3 mol % Y₂O₃〉-10 wt % Al₂O₃ powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al₂O₃ doping raises the phase transition temperatures of the metastable low-temperature ZrO₂ polymorphs and that the structural transformations of the ZrO₂ and Al₂O₃ in the doped material inhibit each other.     

A novel spray co-precipitation method to prepare nanocrystalline Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powders for transparent ceramics

A novel spray co-precipitation method was adopted to synthesize well dispersed nanocrystalline Y₂O₃ powders for transparent ceramics. Several analytic techniques such as XRD, SEM, BET and UV–Vis–NIR spectrophotometer were used to determine the properties of coprecipitated powders, and the microstructure and optical properties of as-fabricated ceramics. The influences of the aging time on powders and ceramics were systematically investigated. Precursors were completely reached to yield the Y₂O₃ phase after being calcined at 1250 °C in air. The calcined Y₂O₃ powders exhibited an approximately spherical morphology with narrow size distribution and weak agglomeration, with mean particle size of ~140 nm. The co-precipitated nanopowders with an aging time of 12 h exhibited the best sintering activity due to the low agglomeration, and the in-line transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h in vacuum reached to 77.2% at 1064 nm (1 mm thickness).     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> co-stabilized ZrO<Subscript>2</Subscript>-WC composites

Y₂O₃ + Nd₂O₃ co-stabilized ZrO₂-based composites with 40 vol% WC were fully densified by pulsed electric current sintering (PECS) at 1350 °C and 1450 °C. The influence of the PECS temperature and Nd₂O₃ co-stabilizer content on the densification, hardness, fracture toughness and bending strength of the composites was investigated. The best combination of properties was obtained for a 1 mol% Y₂O₃ and 0.75 mol% Nd₂O₃ co-stabilized composite densified for 2 min at 1450 °C under a pressure of 62 MPa, resulting in a hardness of 15.5 ± 0.2 GPa, an excellent toughness of 9.6 ± 0.4 MPa.m^0.5 and an impressive 3-point bending strength of 2.04 ± 0.08 GPa. The hydrothermal stability of the 1 mol% Y₂O₃ + 1 mol% Nd₂O₃ co-stabilized ZrO₂-WC (60/40) composites was compared with that of the equivalent 2 mol% Y₂O₃ stabilized ceramic. The double stabilized composite did not degrade in 1.5 MPa steam at 200 °C after 4000 min, whereas the yttria stabilized composite degraded after less than 2000 min. Moreover, the (1Y,1Nd) ZrO₂-WC composites have a substantially higher toughness (~9 MPa.m^0.5) than their 2Y stabilized equivalents (~7 MPa.m^0.5).     

Self-propagating high-temperature synthesis of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript> powders for optical ceramics

A technique has been developed for the self-propagating high-temperature synthesis of lutetium oxide (Lu₂O₃) powders using citric acid, glycine, and lutetium acetate as fuels. We have carried out thermodynamic analysis of synthesis conditions and examined the effect of the nature of the fuel on the properties of the resultant powders. Using vacuum sintering at a temperature of 1780°C and powders prepared with glycine as a fuel and containing 25 mol % yttrium oxide and 5 mol % lanthanum oxide as sintering aids, we have obtained transparent lutetium oxide-based ceramics.     

Preparation of YSZ/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite coatings via electrophoretic deposition of nanopowders

Using electrophoretic deposition (EPD), we have produced YSZ individual ceramic coatings and YSZ/Al₂O₃ composite coatings for a wide range of applications in modern materials research. YSZ and Al₂O₃ nanopowders were prepared by high-energy physical dispersion techniques, namely, by a laser evaporation–condensation process and electroexplosion of wire, respectively. Stable nonaqueous suspensions for the EPD process have been prepared using YSZ and Al₂O₃ nanopowders with an average particle size of 11 and 22 nm, respectively. The YSZ/Al₂O₃ composite coating produced by sintering at 1200°C has been shown to have higher density in comparison with the YSZ individual coating produced at the same temperature. X-ray diffraction characterization showed that the YSZ/Al₂O₃ composite coating consisted of two crystalline phases: α-Al₂O₃ (corundum) (42 wt %) and cubic ZrO₂〈Y₂O₃〉 (58 wt %). Quantitative analysis of electron micrographs of the surface of the films showed that the YSZ individual coating produced by sintering at 1200°C had a loose structure and contained pores (9%), as distinct from the composite coating, which had a dense, porefree grain structure.     

Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO<Subscript>2</Subscript>, MgO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript>

Biologically derived hydroxyapatite from calcinated (at 850 °C) bovine bones (BHA) was doped with 5 wt% and 10 wt% of SiO₂, MgO, Al₂O₃ and ZrO₂ (stabilized with 8% Y₂O₃). The aim was to improve the sintering ability and the mechanical properties (compression strength and hardness) of the resultant BHA-composites. Cylindrical samples were sintered at several temperatures between 1,000 and 1,300 °C for 4 h in air. The experimental results showed that sintering generally occurs at 1,200 °C. The BHA–MgO composites showed the best sintering performance. In the BHA–SiO₂ composites, extended formation of glassy phase occurred at 1,300 °C, resulting in structural degradation of the resultant samples. No sound reinforcement was achieved in the case of doping with Al₂O₃ and zirconia probably due to the big gap between the optimum sintering temperatures of BHA and these two oxides.     

Preparation and properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> partially stabilized ZrO<Subscript>2</Subscript> crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y₂O₃ partially stabilized ZrO₂ crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y₂O₃ content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Synthesis of Ba<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript>−Sr<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript> solid solution and their dielectric properties

Oxides of the type, Ba_3-xSr_xZnNb₂O₉ (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.     

Preparation of apatite-type La<Subscript>9.33</Subscript>(SiO<Subscript>4</Subscript>)<Subscript>6</Subscript>O<Subscript>2</Subscript> electrolyte by urea-nitrates combustion

Apatite-type La_9.33(SiO₄)₆O₂ powders have been prepared by urea-nitrates combustion at low temperature. Process parameters of combustion and characteristics of electrolyte were studied and optimized. Gelation time of precursor has been shortened distinctly by introducing an appropriate solvent system. Molar ratio of nitric acid to lanthanum oxide dependence of the nature of the phases has first been characterized. Well-crystallized La_9.33(SiO₄)₆O₂ powders with an average size of 30.5 nm were obtained at a calcining temperature as low as 800°C for 12h. Dense ceramic with a relative density of 96% was prepared by sintering the green compact of these nanopowders at 1400°C for 3 h. The sintering body exhibited a high ionic conductivity of 4.38 × 10⁻³ S/cm at 700°C.     

Formation behavior of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> from CaTiO<Subscript>3</Subscript>, CuO and TiO<Subscript>2</Subscript>

The formation behavior of CaCu₃Ti₄O₁₂ (CCTO) had been investigated via solid state reaction from CaTiO₃, CuO and TiO₂ powders. In the temperature range from 750 to 1,200 °C, the reaction sequence was traced by XRD, and the microstructure evolution of calcined powders was also investigated by SEM. CCTO began to form owing to the reaction between CaTiO₃, CuO and TiO₂ at around 850 °C, and became the major phase at 1,000 °C. Finally, the single phase CCTO was obtained at 1,150 °C. However, CCTO was decomposed at CaTiO₃, CuO and TiO₂ when the temperature increased to 1,200 °C. In addition, no other intermediate phases occurred in the synthesized process. The formation behaviors indicated that CaTiO₃ prevented the formation and growth of CCTO.     

Sintering temperature depression in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> by mechanical milling

High-energy milling of Al₂O₃ with hardened steel milling media has confirmed that nanocrystalline powders are readily formed. At a ball to charge mass-ratio of 20:1, the crystallite size falls below 30 nm in just 2 h and below 15 nm in 4 h. The as-milled powders are contaminated with Fe which increases linearly with increased milling time, reaching ∼10 wt% after 16 h. The HCl leaching process of Karagedov and Lyakhov [Karagedov and Lyakhov (1999) Nanostruct Mater 11(5):559] was found to remove a large proportion of the Fe, but residual Fe was found with XRF analysis. Milled and leached samples show significant sintering temperature depression to approximately 1100 °C and produce sintered densities greater than 94% without the application of pressure. Milling induced lattice expansion of the Al₂O₃ is observed which we posit to be due to defect formation rather than Fe absorption. The respective roles of small crystallite size and lattice defects in reducing the sintering temperature are discussed.     

Ionic conductivity in the Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system

Nanocrystalline Lu₂O₃-TiO₂ (33.3–44 mol % Lu₂O₃) materials with a partially disordered pyrochlore structure, prepared via heat treatment in the range 1400–1750°C, are found to possess high oxygen ionic conductivity. Their 740°C conductivity is 10^-3 to 10^-2 S/cm, depending on the heat-treatment temperature and composition, which is comparable to that of the well-known fluorite solid electrolyte ZrO₂-9 mol % Y₂O₃.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 324–331.Original Russian Text Copyright © 2005 by Shlyakhtina, Mosunov, Stefanovich, Knotko, Karyagina, Shcherbakova.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

The ZrO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> phase diagram

The ZrO₂-TiO₂ phase diagram was determined experimentally between 800 and 1200°C, 1 atm, extending our knowledge of this system to temperatures previously inaccessible for equilibrium experiments due to sluggish kinetics. The crystallization of the ordered (Zr,Ti)₂O₄ phase from the oxides was facilitated by the addition of flux (CuO or Li₂MoO₄/MoO₃), and seeds. Two ordered (Zr,Ti)₂O₄ phases with different compositions were identified, and their phase relationships with TiO₂ and ZrO₂ solid solutions investigated. Structure data, superstructure reflections and composition were used to locate the ordering phase transition of (Zr,Ti)₂O₄ in equilibrium with ZrO₂ and TiO₂. At the onset of ordering between 1130 and 1080°C, (Zr,Ti)₂O₄ is of composition X_Ti = 0.495 ± 0.02, and displays a dramatic change in b-dimension. At 1060°C and below, the composition of (Zr,Ti)₂O₄ is significantly more Ti-rich and dependent on temperature, ranging from X_Ti = 0.576 at 1060°C to 0.658 at 800°C. This variability in composition of the ordered phase contrasts with previous studies that suggested the composition to be constant at either X_Ti = 0.667 [ZrTi₂O₆] or 0.583 [Zr₅Ti₇O₂₄]. When grown at low temperatures and with lithium molybdate, the crystals of ordered (Zr,Ti)₂O₄ are acicular to needle shape, and develop distinct square cross-sections and end facets.     

A novel method for producing open-cell Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-ZrO<Subscript>2</Subscript> ceramic foams with controlled cell structure

A novel method was introduced to prepare open-cell Al₂O₃–ZrO₂ ceramic foams with controlled cell structure. This method used epispastic polystyrene (EPS) spheres to array ordered templates and centrifugal slip casting in the interstitial spaces of the EPS template to obtain cell struts with high packing density. Aqueous Al₂O₃–ZrO₂ slurries with up to 50 vol.% solid contents were prepared and centrifuged at acceleration of 2,860g. The effect of the solid contents of slurries on segregation phenomena of different particles and green compact uniformity were investigated. In multiphase system, the settling velocities of Al₂O₃ and ZrO₂ particles were calculated. Theory analysis and calculated results both indicated segregation phenomenon was hindered for slurries with 50 vol.% solid content. The cell struts of sintered products had high green density (61.5%TD), sintered density (99.1%TD) and homogeneous microstructures after sintered at 1,550 °C for 2 h. The cell size and porosity of Al₂O₃–ZrO₂ ceramic foams can be adjusted by changing the size of EPS spheres and the load applied on them during packing, respectively. When the porosity increased from 75.3% to 83.1%, the compressive strength decreases from 3.82 to 2.07 MPa.     

Effect of the precipitation sequence on phase formation in the ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied phase formation processes during heat treatment of precipitates in the ZrO₂-Al₂O₃ and ZrO₂-CeO₂-Al₂O₃ systems. During heat treatment of powders prepared by coprecipitation of precursors to ZrO₂, CeO₂, and Al₂O₃, α-Al₂O₃ is formed at higher temperatures, which is due to the formation and decomposition of T-ZrO₂ and metastable Al₂O₃ phases. The precipitation sequence in the ZrO₂-CeO₂-Al₂O₃ system influences the lattice parameters of the forming T-ZrO₂-based solid solutions because of the different degrees of Ce⁴⁺ and Al³⁺ substitutions for Zr⁴⁺.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> stabilizer content and annealing on the structural transformations of ZrO<Subscript>2</Subscript>

The structure of partially stabilized zirconia crystals has been studied by transmission electron microscopy before and after annealing. Structural characterization of Y₂O₃-doped (2.8 to 4 mol %) zirconia before annealing showed that all of the samples consisted of twin domains whose size was dependent on the stabilizer content. Annealing at 2100°C increased the domain size in the composition range 2.8–3.7 mol % Y₂O₃ and reduced it at 4 mol % Y₂O₃. These structural changes allowed us to determine the position of the representative point relative to the phase boundary in the equilibrium phase diagram of the system.     

Varistor properties and aging behavior of ZnO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> ceramics with sintering process

The microstructure, electrical properties, and DC-accelerated aging behavior of the ZnO-V₂O₅-Mn₃O₄ ceramics were investigated at different sintering temperatures of 850–925°C. The microstructure of the ZnO-V₂O₅-Mn₃O₄ ceramics consisted of ZnO grain as a primary phase, and Zn₃(VO₄)₂ which acts as a liquid-phase sintering aid, in addition to Mn-rich phase as secondary phases. The maximum value (3,172 V/cm) and minimum value (977 V/cm) of breakdown field were obtained at sintering temperature of 850 and 900°C, respectively. The nonlinear coefficient exhibited the highest value, reaching 30 at 925°C and the lowest value, reaching 4 at 850°C. The optimum sintering temperature was 900°C, which exhibited not only high nonlinearity with 24 in nonlinear coefficient, but also the high stability, with %ΔE_1mA = −0.9% and %∆α = −12.5% for DC-accelerated aging stress of 0.85 E_1mA/85°C/24 h.     

Preparation of fine-grained ceramics by hot-pressing of Ce<Subscript>0.09</Subscript>Zr<Subscript>0.91</Subscript>O<Subscript>2</Subscript>/MgO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder

We have studied the effect of hot-pressing conditions (different pressure rise rates and isothermal holding temperatures in the range 1450–1550°C) on the microstructure of ceramics produced from nanopowder with the composition Ce_0.09Zr_0.91O₂/MgAl₆O₁₀/γ-Al₂O₃ (20.6, 37.4, and 42.0 wt %, respectively). Firing at 1450°C for 1 h made it possible to obtain fine-grained ceramics with less than 3 μm in grain size. The compaction pressure rise rate was shown to be a key parameter under such thermal conditions (20 + 10°C/min → 1450°C). Grain growth was prevented most effectively when the maximum load, 30 MPa, was reached at a temperature of 1000°C. Under such conditions, the grain size was 0.4–0.8 μm and the relative density reached 98.8%.