Reaction products of Sm2Zr2O7 with calcium–magnesium– aluminum–silicate (CMAS) and their evolution

During flight, many silicates (sand, dust, debris, fly ash, etc.) are ingested by an engine. They melt at high operating temperatures on the surface of thermal ...     

Phase relations in ZrO2–La2 Zr2O7–Y2 Zr2O7system

Abstract

The phase relations in the system ZrO₂–La₂Zr₂O₇– Y₂Zr₂O₇ have been investigated using X-ray diffraction. Mixed oxide phase assemblages were prepared by hydrolysing zirconium butoxide with solutions of Y and La nitrates, followed by drying, calcining, and sintering. The cubic zirconia phase can accept into solid solution the larger, non-cubic stabilising lanthanum ion in the presence of a suitable proportion of the cubic stabilising oxide of yttrium. As the amount of the larger rare earth element ion is increased formation of pyrochlore and tetragonal type compounds is favoured.     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

(MgO·Y2O3)稳定ZrO2的研究

本研究在MgO-PSZ基础上引入Y2O3共同稳定ZrO2,采用溶胶-凝胶法制得MgO-Y2O3-ZrO2超微细粉,经成型、烧结制得(Mg、Y)-PSZ陶瓷材料.经性能及微观结构测试,结果表明用此法制得的材料,其烧结温度从1600℃降至1450℃,晶粒尺寸减小至1～2μm,抗弯强度及断裂韧性明显提高.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Mechanical properties of highly toughened ZrO2Y2O3

Y-PSZ (partially stabilized zirconia) and Y-TZP (tetragonal zirconia polycrystals) containing from 1·5 to 5·0 mol% Y₂O₃ were prepared by hot isostatic pressing. The K_IC value nonlinearly increases with the decrease from 2·5 to 2·0 mol % Y₂O₃, and has a maximum K_IC value of 20 MPa √m at 2·0 mol% Y₂O₃. In order to clarify the toughness creating mechanism of the above materials, the experimental results of K_IC, σ_f and transformation zone size were examined, with emphasis on Y₂O₃ composition, Al₂O₃ additives and grain size.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

ZrO2(3Y)包裹Al2O3纳米复合粉体的制备

以pH值缓冲溶液为沉淀剂,用非均匀成核和液相共沉淀相结合的方法合成出了ZrO₂（3Y）包裹Al₂O₃纳米复合粉体.用XRD、TEM对所制备粉体进行了表征.研究表明：加入分散剂使Al₂O₃悬浮液的等电点向酸性区移动,Zeta电位绝对值增大,其中pH值为9.56对应的Zeta电位绝对值最大,悬浮液最稳定.Al₂O₃悬浮液pH值为9.5,ZrOCl₂和YCl₃混合溶液浓度为0.2 mol•L^-1时前驱体的收得率最高.600℃煅烧后得到的粉体中只有α-Al₂O₃、t-ZrO₂两种物相,在Al₂O₃颗粒表面附着10 nm左右的ZrO₂（3Y）颗粒.     

Structural parameters and oxygen ion conductivity of Y2O3–ZrO2 and MgO–ZrO2 at high temperature

The oxygen ion conductivity of zirconia-based solid electrolytes doped with 8 mol% Y₂O₃–ZrO₂ (YSZ) and 9 mol% MgO–ZrO₂ (Mg-PSZ) at high temperature was investigated in terms of their thermal behavior and structural changes. At room temperature, YSZ showed a single phase with a fluorite cubic structure, whereas Mg-PSZ had a mixture of cubic, tetragonal and some monoclinic phases. YSZ exhibited higher ionic conductivity than Mg-PSZ at temperatures from 600 °C to 1250 °C because of the existence of the single cubic structure and low activation energy. A considerable increase in the conductivity with increasing temperature was observed in Mg-PSZ, which showed higher ionic conductivity than YSZ within the higher temperature range of 1300–1500 °C. A monoclinic-to-tetragonal phase transformation was found in Mg-PSZ and the lattice parameter of the cubic phase increased at 1200 °C. The phase transformation and the large lattice free volume contributed to the significant enhancement of the ionic conductivity of Mg-PSZ at high temperatures.     

Phase formation in the ZrO2 — HfO2 — Gd2O3 and ZrO2 — HfO2 — Yb2O3 systems

The formation of phases and properties of solid solutions and Gd₂Zr₂O₇ and Yb₄Zr₃O₁₂ chemical compounds in the ZrO₂ — Gd₂O₃ and ZrO₂ — Yb₂O₃ systems are studied. The compounds undergo a phase transformation of the type“order — disorder” and melt incongruently. The data on the parameters of the elementary cells, the melting temperatures, and the TCLE of phases in the ZrO₂ — Gd₂O₃ and ZrO₂ — Yb₂O₃ systems are presented. These data are compared with similar data for phases based on HfO₂. The considered solid solutions are shown to be promising for the production of high-temperature, heat-insulating ceramics for growing single crystals.     

Synthesis and thermal stability of Re2Zr2O7, (Re = La,Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La₂Zr₂O₇, Gd₂Zr₂O₇, La₂Ce₂O_7?γ and La₂(Zr_0.7Ce_0.3)O_7?γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400 °C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar_(g)/3%H_2(g) atmosphere.     

Obtaining ZrO2–3 mol % Y2O3 Ceramics with Various Degrees of Tetragonality and Studying Low Temperature Degradation

Glass Physics and Chemistry - Nanocrystalline ceramic samples of the ZrO2–3 mol % Y2O3 composition with varying degrees of tetragonality c/a (1.4337 and 1.0425) are obtained. The process of...     

Fabrication of Al2O3–ZrO2 ceramics with high porosity and strength

This study describes the synthesis of ceramics, in which a micrometre-sized Al₂O₃–ZrO₂ nanopowder was used as an oxide base for the hardening of the materials. To a suspension of this mixed metal oxide, the pore-forming crystallisation additives camphor and carbamide were added to produce ceramics with thin permeable pores. Camphor crystallised in the oxide suspension in the form of single pentagonal stars and сarbamide crystallised in the form of thin elongated needles. The use of the different crystallisation additives allowed the formation of ceramics after sintering that have both permeable and complex pore morphologies, where anisotropic properties were observed using carbamide as an additive but not when camphor was used. The total porosity of the resulting ceramics was 51.3%, with a compressive strength in the range of 17.3–92.3 MPa.     

Fabrication of novel ZrO2(Y2O3)–Al2O3 ceramics having high strength and toughness utilising pulsed electric current pressure sintering (PECPS)

Abstract

Pulsed electric current pressure sintering (PECPS) makes it possible to produce novel monolithic and composite materials with well controlled microstructure or nanostructure, which has not been realised before. ZrO₂ based ceramics containing 25 mol.-%Al₂O₃ and 0·90–1·125 mol.-%Y₂O₃, i.e. ZrO₂(1·2–1·5 mol.-%Y₂O₃)–25 mol.-%Al₂O₃, revealing high bending strength σ_b≧1 GPa and high fracture toughness K_IC ≧15·5 MPa m^1/2 simultaneously, have been fabricated by PECPS of cubic ZrO₂ solid solution (ss) powders containing both Al₂O₃ and Y₂O₃ prepared via a sol–gel process. Tetragonal ZrO₂ (t-ZrO₂) phase composite ceramics sintered at 1573 to 1623 K (1300–1350°C) for 10 min under 60 MPa in Ar were composed of ～φ200 nm grains with high relative density ≧97·5%. Transmission electron microscopy observation cleared that these mechanical properties might originate from homogeneous distribution of α-Al₂O₃ particles in the dense t-ZrO₂ matrix; the precipitation of α-Al₂O₃ could be achieved by adopting the ss powders and PECPS.     

CaO-MgO-Al2O3-SiO2 corrosion behavior of air-plasma-sprayed (LaxYb1−x)2Zr2O7

The CaO-MgO-Al₂O₃-SiO₂ (CMAS) corrosion of thermal barrier coatings (TBCs) is a crucial problem for the lifetime of blades and vanes of jet engine and gas turbine at high operating temperature. Although many new alternative materials for TBCs have been developed in recent years, their application is limited by the CMAS corrosion. On the other hand, the composition difference of CMAS between regions makes solving this problem very difficult. Therefore, in this study, the yearly composition changes of sand-dust around Beijing area were investigated. The high-temperature corrosion behavior of air-plasma-sprayed 8YSZ and newly developed (La_xYb_1−x)₂Zr₂O₇ TBCs by the representative sand-dust of Beijing was investigated. In comparison, a universally used CaO-riched composition of simulated silicate deposit was also adopted for the TBCs corrosion test. It is found that the (La_xYb_1−x)₂Zr₂O₇ TBCs performs much better anti-corrosion behavior than that of 8YSZ, both by Beijing sand-dust and simulated one. Particularly, Yb₂Zr₂O₇ TBCs appear to be the optimum material against silicate deposits attack. The mechanism of silicate deposits corrosion has also been discussed.     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

粘结剂对喷雾造粒ZrO2(Y2O3)粉末特性的影响

采用喷雾造粒工艺对及ZrO2(Y2O3)粉体进行造粒，研究喷雾造粒过程中粘结剂含量对粉料流动性、填充性、造粒团聚体强度的影响，并对坯体成形过程中团聚体的破碎行为进行了分析。结果表明：喷雾造粒过程中粘结剂含量较高时，造粒团聚体粒子具有良好的填充性，但粒子强度高，在成形过程中不易破碎，将阻碍致密化烧结。     

Ionic conductivity of CaO–Y2O3–ZrO2 materials with constant oxygen vacancy concentration

Structural modification of the fully stabilised zirconia is a possible way to improve its electrical properties. Electrical properties, especially ionic conductivity, of cubic zirconia solid solutions are strictly related to the ionic radius and valency of cations incorporated into the zirconia structure. Nanopowders with a constant oxygen vacancy concentration of 8 and 10 mol% were prepared by a hydrothermal treatment of co-precipitated zirconia hydrogels in a NaOH environment. The desired oxygen vacancy concentrations were obtained by introducing calcia and yttria, at different ratios, to the zirconia solid solutions. Phase compositions and lattice parameters of the respective phases were determined using X-ray diffraction analysis. Electrical properties of the samples were described on the basis of complex impedance spectroscopy analysis. It has been stated that substitution of calcia for yttria or yttria for calcia in zirconia solid solutions leads to ionic conductivity enhancement. Samples with a cubic structure, close to the stabilisation threshold, had the highest conductivity.     

Production of Chemically Pure Zirconia-Based Nanoceramics in the ZrO2(Y2O3)–Al2O3 System for Restorative Dentistry

Theoretical Foundations of Chemical Engineering - The synthesis technology of a chemically pure nanodisperse precursor powder (10–12 nm) based on a tetragonal solid zirconia (t-ZrO2) solution...