Y2O3–Nd2O3 double stabilized ZrO2–TiCN nanocomposites

Yttria-neodymia double stabilized ZrO₂-based nanocomposites with 40 vol% electrical conductive TiCN were fully densified by means of pulsed electric current sintering (PECS) in the 1400–1500 °C range. The Y₂O₃ stabilizer content was fixed at 1 mol% whereas the Nd₂O₃ co-stabilizer content was varied between 0.75 and 2 mol% in order to optimise the mechanical properties. The mechanical (Vickers hardness, fracture toughness and bending strength), electrical (electrical resistivity) and microstructural properties were investigated and the hydrothermal stability in steam at 200 °C was assessed.The nanocomposites with 1–1.75 mol% Nd₂O₃, PECS at 1400 or 1450 °C, have an excellent fracture toughness of 8 MPa m^1/2, although the grain size of both ZrO₂ and TiCN phases after densification is in the 100 ± 30 nm range. Moreover, the composites combine a hardness of about 13 GPa, a bending strength of 1.1–1.3 GPa with a low electrical resistivity (1.6–2.2 × 10^?5 Ω m) allowing electrical discharge machining. The hydrothermal stability of the double stabilizer nanocomposites was higher than for yttria-stabilized ZrO₂-based composites with the same overall stabilizer content.     

Preparation of Y2O3 stabilised ZrO2 ceramic nanopowders by surface doping

Abstract

Weakly agglomerated nanocrystalline Y₂O₃–ZrO₂ powder was prepared by dispersion Y₂O₃ on the surface of ZrO₂ nanopowder (7·3 nm) that was derived from gas phase synthesis. The utmost dispersion capacity of Y₂O₃ on the surface of ZrO₂ was determined to be 0·16 gY₂O₃ /gZrO₂ (or 8·7 mol.-%Y₂O₃–ZrO₂ ) which suggests that 3 mol.-%Y₂O₃ would be homogeneously dispersed on ZrO₂ and no phase segregation would occur during surface doping. The results show that the tetragonal phase content in surface doped ZrO₂ increased with grain growth or heating temperatures, unlike the undoped and the bulk doped ZrO₂ . The stabilisation of the tetragonal phase resulted from the incorporation of Y ³⁺ cations from the surface into the grains of ZrO₂ . This conclusion is supported by the X-ray photoelectron spectroscopy and X-ray diffraction evidence. Surface doped powders have a strong tendency to improve the anticoarsening ability and suppress grain growth, especially at higher doping levels and at lower heating temperatures.     

Preparation and characterization of Y2O3 hollow spheres

Y₂O₃ hollow spheres are prepared by coating the anionic poly (styrene-methyl acrylic acid) (PSA-A) latex with an amorphous yttrium layer and subsequent calcining at elevated temperatures in air. The aging temperature, aging time, the concentrations of Y(III) and urea are systematically varied to establish the optimum conditions for the formation of the coating shell. Zeta potential measurement, transmission electron microscopy, scanning electron microscopy, X-ray diffraction and infrared measurements are used to characterize the PSA-A latex, core-shell spheres and Y₂O₃ hollow spheres. A mechanism for the formation of the PSA-A/yttrium compounds core-shell spheres is suggested. The key factor to prepare smooth-coated spheres is obtained.     

纳米Al2O3/ZrO2复合粉体的制备及表征

高性能的复合粉体是制备纳米复相陶瓷材料的关键.采用醇-水溶液加热法结合共沉淀过程制备纳米Al2O3/ZrO2复合粉体,研究了不同沉淀剂对粉体团聚的影响,利用透射电镜、X射线衍射、热重-差热分析、比表面积测定等技术对获得的纳米复合粉体进行了表征.结果表明:采用NH4HCO3作为沉淀剂可以得到几乎无团聚的碱式碳酸盐前驱物,该前驱物在煅烧过程中的物相变化显示四方相氧化锆(t-ZrO2)的形成温度大幅度地提高,同时在较低温度下生成了α-Al2O3,在1 100℃转变为t-ZrO2相和α-Al2O3相;粉体中两相颗粒分散良好、粒径一致、无硬团聚,其平均粒径为15～20 nm,比表面积为69.5 m2·g-1.     

Preparation and characterization of dispersed Rh2O3 on tetragonal ZrO2

Samples of well-dispersed hexagonal Rh₂O₃ on tetragonal ZrO₂ have been prepared by the code composition of the nitrates at 900°C. A comparison of the stability towards reduction of the bulk and dispersed Rh₂O₃ products demonstrates the influence of an interaction between the dispersed metal oxide and the support.     

溶胶凝胶-微波加热技术制备Y2O3稳定ZrO2纳米粉体的研究

以硬脂酸为分散荆。采用溶胶凝胶-微波加热技术研究了制备Y2O3稳定ZrO2(YSZ)纳米粉体的条件与方法。分别用红外光谱(IR)、X射线粉末衍射(XRD)、透射电镜(TEM)、比表面积(BET)测定及电测量技术对该纳米粉体的结构与性能进行了表征。结果表明：在700℃以上微波处理得到立方相YSZ纳米粉体，其粒子分散性好，形貌为椭球体，平均粒径约为37nm，在480～980℃温度范围内呈现出良好的电导率，并且有较高的稳定性。     

An experimental technique to evaluate the effective thermal conductivity of Y2O3 stabilized ZrO2 coatings

An experimental device was set up to determine thermal resistance and conductivity of 8% yttria-stabilized zirconia deposited by plasma spray method on cylindrical specimen. In this experimental setup, coated surface of the sample was exposed to a high temperature environment and inner metal surface was cooled by flowing air, simulating actual gas turbine applications. Overall heat resistance at the outside surface of thermal barrier coating was adopted to assess thermal advantage due to the thermal barrier coating deposited on air-cooled cylindrical specimen. 28% less heat was extracted at 1000°C by applying 1.2 mm thick thermal barrier coating. Temperatures of the outside surface of the coated samples increased with increasing coating thickness with respect to the same furnace temperature since the sample with thicker coating was less thermally conductive and retarded heat transfer. The overall heat resistances of samples between the outside surface of sample and the flowing air inside the sample assembly were estimated. Then, the thermal conductivity of coating could be determined from the difference of overall thermal resistances of two selected samples with varying coating thickness.     

Effect of sintering temperature on properties of Al2O3 whisker reinforced 3 mol% Y2O3 stabilized tetragonal ZrO2 (TZ-3Y) nanocomposites

In present study, effect of sintering temperature on density and hardness of 3 mol% yttria-stabilized tetragonal zirconia (referred to as TZ-3Y) composite reinforced with alumina whiskers (5, 10, 15 and 20 wt.%) has been studied. Initially, Ammonium Aluminum Carbonate Hydroxide (AACH) whiskers were added in TZ-3Y composite and transformed into alumina during sintering performed at different temperatures i.e. 1400, 1500 and 1650 °C. Results revealed that for all sintering temperatures, with increase in whisker concentration, sintered density decreased and hardness increased conversely. Maximum hardness of 14.47 GPa was achieved with 10 wt.% whiskers addition when sintered at 1500 °C. However, with addition of CTAB (1 wt.%) as deflocculating agent the hardness was further improved to 15.11 GPa. While sintering at 1650 °C a decrease in hardness was observed. It was mainly due to high temperature morphological change of whiskers i.e. transformation of whiskers into alumina rich grains.     

Preparation and properties of Y2O3-doped ZrO2 thin films by the sol–gel process

Y₂O₃-doped ZrO₂ (YZ) thin films were prepared on alumina substrates by the dip-coating process. The dip-coating solution consisted of a homogeneous sol, and was prepared by using the respective chlorides as raw materials, with ethylene glycol, 2-butanol and distilled water as solvents. The thin films containing 0–20 mol % Y₂O₃ were successfully produced by thermal treatment above 600 °C. The characterization for the film preparation was performed by means of thermogravimetric–differential thermal analysis for the thermal analysis, and scanning electron microscopy for the morphological analysis and thickness measurements. The properties of the films were characterized in terms of a study of the crystalline phase, the crystallite size, the microstructure and the electrical conductivity by using X-ray diffraction, scanning electron microscopy and the complex impedance techniques. In all YZ thin films, the tetragonal phase was stable at low temperatures as a result of the crystallite size effect. However, at higher temperatures, the tetragonal phase was transformed into either the monoclinic phase or the cubic phase, depending on the doping concentration. The YZ thin film of 8 mol% Y₂O₃ content was stabilized to almost cubic phase at 1000 °C. Resonable conductivity behaviour at YZ was observed for the YZ thin films. The electrical conductivity of YZ thin films was similar to the values of the sintered body. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of a small amount of Al2O3 addition on the transformation of Y2O3-partially stabilized ZrO2 during annealing

Tetragonal-to-monoclinic phase transformations in 2 and 3 mol% Y2O3–ZrO2 ceramics and their composites with 5 vol% Al2O3 during annealing in water and in vacuum at 353–623 K were investigated to explore the effect of a small quantity of Al2O3 addition on the transformation. The dispersion of Al2O3 particles into the ZrO2(Y2O3) matrix was found to be effective to suppress the transformation directly induced by the attack of H2O during annealing in water, even though the amount was as small as 5 vol%. However, the transformation predominantly caused by thermal activation during annealing in vacuum was not affected by the limited amount of Al2O3 addition. The effect of suppression of Al2O3 on the water-induced phase transformation was considered to be realized through the hydroxydation of Al2O3 particles, by which the sample surface was effectively "protected" from further attack of H2O, which accelerated the low-temperature degradation transformation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electrical conductivity of Cr2O3-doped Y2O3-stabilized ZrO2

The electrical conductivity of Cr₂O₃-doped Y₂O₃-stabilized ZrO₂ (YSZ) has been studied as functions of composition, temperature and oxygen pressure. The specimens have been prepared by hot preoning of co precipitated oxides to yield >99.7% density. The Cr₂O₃ added above the solubility limit ( 0.7 mol %) precipitated as a secondary phase at the grain boundaries. The conductivity of Cr₂O₃-doped YSZ was almost independent of the oxygen pressure in the range 10¹⁸ to 10⁵ Pa, indicating a dominant ionic condition. The electronic conductivity of dopant CR₂O₃ would be hindered by the higher ionic conductivity in thep _O2 ranges studied. The conductivity and the activation energy for conduction decreased slightly with the addition of Cr₂O₃. These phenomena seemed to be caused by vacancy trapping or polarization at the grain boundaries with the Cr₂O₃ precipitates. The samples with 1 mol % Cr₂O₃ addred to zirconia containing various Y₂O₃ contents showed similar conduction behaviour to those without Cr₂O₃ addition; that is, the conductivity maxima are observed at around 8 mol % Y₂O₃ addition to zirconia, and the activation energies increased with tha Y₂O₃ addition.     

Oxygen ionic conduction in Y2O3-stabilized Bi2O3 and ZrO2 composites

Oxygen-ion conductivity was measured in two-phase mixed composites in the system f c c Bi₂O₃ (BY)-fcc ZrO₂ (ZY) stabilized with Y₂O₃ (Y = 20 at%). The composites appeared in the range 0.02 < × < 0.90 for BY_xZY_1-x (mole fraction). Conductivity decreased with increasing ZY concentration. The composition dependence of conductivity could be explained by using an effective-medium approximation, although the conductivity was slightly higher than the calculated value in specimens containing ZY above 50 vol %.     

Preparation, Phase Composition, and Optical Properties of Thin ZrO2–Y2O3 Films

ZrO₂–Y₂O₃ films were prepared over the entire composition range. Their phase composition was determined, and the conditions for the formation of cubic ZrO₂-based solid solutions were established. The optical properties of the films were found to depend on their thickness and phase composition and the nature of the substrate. The composition dependence of the refractive index was shown to correlate with the phase composition of the films.     

Synthesis and characterization of rod-like Y2O3 and Y2O3:Eu3+

Y₂O₃ rods 100 to 200 nm in diameter and 10 to 20 m in length are accessible via polyol-mediated synthesis of a precursor material with similar shape. By heating of Y(CH₃COO)₃ · xH₂O and a defined amount of water at 190°C in diethylene glycol, the rod-like precursor material is formed. Infrared spectroscopy (IR), differential thermal analysis (DTA) and thermal gravimetry (TG) evidence that this precursor material still contains acetate. However, the precursor material can be transformed to Y₂O₃ by sintering at 600°C without destruction of the rod-like shape. According to X-ray powder diffraction analysis, the rods are well crystallized. They can be assumed to be with [100] orientation. By doping with Eu³⁺ (5 mol%), red emitting phosphor rods can be realized. With optical spectroscopy the typical line emission of Eu³⁺ is observed. Diffuse reflectance of Y₂O₃:Eu³⁺ rods is determined to be higher than 95% in the visible. While exciting at 254 nm (Hg-discharge), a quantum efficiency of 38.5% is proven for the prepared Y₂O₃:Eu³⁺ rods.     

Preparation of tetragonal ZrO2-Gd2O3 powders

ZrO₂-Gd₂O₃ alloys containing 2,3,5 and 8 mol.% Gd₂O₃ have been prepared by mixed oxide (MO), hybrid sol-gel (SG), and co-precipitation (CP) routes. No tetragonal (t) phase is retained in the MO method, while 100% t phase is obtained in the calcined CP samples; the SG method leads to only partial stabilization of the t phase. Washing of the CP powders with propan-2-ol leads to unagglomerated powders with increased specific surface area (145 versus 89 m²g^–1) and sintered density (98% versus 79%). Cubic and t phase also appear on sintering the samples with >2 mol.% Gd₂O₃.     

橡胶等静压成型纳米ZrO2(3Y)粉素坯

对橡胶等静压成型(Rubber isostatic pressing,RIP)制备纳米Y-TZP陶瓷作了初步研究.研究结果表明,通过RIP成型,可以获得相对密度较高、体积较大的ZrO₂(3Y)素坯,并在较低温度下无压烧结得到纳米Y-TZP陶瓷.在1100℃下烧结2h所得的Y-TZP陶瓷的相对密度可达97％,晶粒仅为70nm左右.相对密度较高、平均孔径小是RIP成型素坯烧结温度低的主要原因.