ZrO2 Fibers Research Articles

Oxidation Behavior of SiCN–ZrO2 Fiber Prepared from Alkoxide‐Modified Silazane

The oxidation behavior of SiCN–ZrO2 fibers and SiCN at 1350°C are compared. The as‐measured parabolic rate constants for the two materials are nearly the same (15–20 × 10−18 m2/s). However, after implementing a correction for the difference in the compositions, the rate constant is 13.2 × 10−18 m2/s for the fiber, and 29.4 × 10−18 m2/s for SiCN. The lower oxidation rate of the fiber is ascribed to the lower carbon content in the fiber material.

Sintering behaviour of composite materials borosilicate glass-ZrO2 fibre composite materials

The characteristics and properties of mixtures of a borosilicate glass and ZrO2 fibres obtained from compacts isothermally sintered are presented. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and the glass matrix viscosity on sintering. The fundamental developed aspects are the sintering kinetics, the dense material characterisation and the study of the sintering mechanisms. The composite materials are suitable for sealing molten carbonate fuel cells (MCFC). # 2002 Elsevier Science Ltd. All rights reserved.

Structure and properties of ceramic composites based on ZrO2

Monodisperse fine powders of high purity obtained by sol-gel method are used for production of high technical data ceramics. The fiber reinforcement is used for hardening of composite materials. It was of interest to study production possibility of reinforced composite material based on ZrO2 obtained by sol-gel method with filler from fibers of partially stabilized zirconia. ZrO2 powders were obtained by precipitation of its hydrated gel from aqueous zirconium oxychloride solution by ammonium hydroxide followed by thermal treatment. For composite reinforcement ceramic partially stabilized (8 mole Y2O3) ZrO2 fibers 0.16–0.67 mm in length and 5–7 μm in diameter were used. Content of the fibers in composite was 20 wt.%. From powders and their mixtures with fibers, the samples were pressed as disks, beams and cylinders, and anneal in air at 1100–1600°C temperature range. The investigation has shown that the fibers of partially stabilized zirconia change the composite structure, increase the content of tetragonal modification that promotes its hardening. Treatment temperature of precursor determines physical chemical properties of compositions with fibers. Their high specific surface and reaction ability provides a workability of forming and sintering processes into strong composite material. The ceramics was increased by 2.5–3 times as strength after fibrous filler introduction into ZrO2 hydrogel matrix.

Mechanical and Electromechanical Properties of Monoclinic ZrO<sub>2</sub> Fiber/PZT Composites

Mechanical and electromechanical properties of monoclinic ZrO2 fiber/PZT composites fabricated by slip casting and ordinary sintering were examined. Bending strength of the composites decreased by increasing the fiber content from 0 to 20vol%. However, the composite containing 5vol% ZrO2 fibers yielded a fracture toughness KIC of 1.4MPa⋅m1/2, 75% higher than that of PZT matrix. The measurement of thermal expansion and observation of microstructure suggest that microcracking, crack bowing and crack deflection contribute to the increase in KIC. Addition of ZrO2 fibers reduced both permittivity and coupling factor K31 of the composites. At 5vol% ZrO2, the K31 value of the composite was 25% lower than that of PZT matrix.

Slip Casting of Monoclinic ZrO<sub>2</sub> Fiber/PZT Composites

Monoclinic ZrO2 fiber/PZT composites were fabricated by slip casting and pressureless sintering. The isoelectric points of ZrO2 and PZT were 4.8 and 7.6, respectively. Homogeneous composites were obtained by adjusting pH of slurries at 10. Although densification was inhibited by addition of fibers, a composite with 20vol% ZrO2 fibers attained a relatively high bulk density of 93%. The grain size of PZT matrix was almost invariant with addition of fibers. EDS (Energy Dispersion X-Ray Spectroscopy) and XRD (X-Ray Diffractometer) analyses verified that no macroscopic reaction between ZrO2 fibers and PZT occurred.

Influence of Sintering Behavior on Mechanical Properties and Thermal Shock Resistance at Temperature Difference of 600K of Fiber-Dispersed ZrO<sub>2</sub> Ceramics

Thermal shock resistant zirconia ceramics reinforced with zirconia fibers have been developed. Zirconia ceramics with 12wt% fiber having an average length of 2.5 or 0.2mm and without fibers were studied. Sintered bodies with different bulk densities were obtained by firing in air at temperatures between 1250 and 1650°C. Dispersed ZrO2 fibers remarkably improved the thermal shock resistance of the ZrO2 ceramics. The bending strength and thermal shock resistance of the samples in which ZrO2 fibers were spheroidized by diffusion during sintering decreased, even though they had high densities. The bending strength did not change after water quenching from 625 to 25°C for the zirconia ceramics reinforced with long fibers.

Processing of Al2O3-ZrO2Composites, ZrO2Fibres and Films

Some of the work carried out on ZrO2 ceramics in the author's laboratory is described. Al2O3—ZrO2, composites containing CeO2 stabilised tetragonal zirconia were processed by a hybrid sol-gel route starting with Al2O3 powder and a Zr-alkoxide sol. A uniform dispersion of tetragonal zirconia particles was obtained and K10 values exceeding 10 MPa.m were achieved. A colloidal approach to the processing of these composites is also outlined in which the heteroflocculation of Al2O3 and ZrO2 particles was encouraged by coating the ZrO2 particles with a thin layer of SiO,. In addition, the preparation of ZrO2 fibres by unidirectional freezing of a hydrogel and high temperature stability of ZrO2, film, prepared by sol-gel technique, on sapphire are also described. The length of the zirconia fibres was found to depend on the period of ageing of the hydrogel; due to low contact angle between ZrO2 and Al2O3, thin films of ZrO2 on sapphire developed porosity on heating to high temperatures.

Structure, nonuniformity, and certain properties of an Al2O3-ZrO2(Y2O3) ceramic produced in processes of oriented crystallization

During solidification of Al2O3-ZrO2(Y2O3) molten materials, redistribution of the basic components of the material occurs, which leads to the occurrence of nonuniformities. The character of entry of yttrium oxide into it has the strongest influence on the structure and properties of the ceramic. A low Y2O3 content in the charge leads to incomplete stabilization of certain ZrO2 grains and to cracks in the material. With a Y2O3 content of more than 8% new phases — 2Y2O3·Al2O3, 3Y2O3·5Al2O3, and others — almost always occur and with an yttrium oxide content of more than 10–12% the ZrO2(Y2O3) grains match poorly with the sapphire matrix. Work with a material of eutectic composition does not solve the problems of uniformity of the ceramic since during the melting-solidification process redistribution of the components occurs in the liquid phase as a result of gravitation, the difference in vapor pressure, and the influence on the composition of the synthesized material of the crystallographic orientation of the matrix. In formation on the front of crystallization of flat facets a ceramic with well ordered distribution of the ZrO2(Y2O3) fibers in the matrix is formed in a quite broad range of % compositions of ZrO2(Y2O3):Al2O3≈0.5–1.

Sol-gel-derived CaO- and CeO2-stabilized ZrO2 fibers—Conversion process of gel to oxide and tensile strength

Abstract The 11CaO.89ZrO2 (in mol%) fibers were fabricated by the sol-gel method using Zr(O-n-C3H7)4 and Ca(CH3COO)2.H2O or Ca(NO3)2.4H2O. The maximum tensile strength of 1000°C-treated fibers was 0.8–1.0 GPa at diameters of 10–15 μm. However, the strength of the alkoxide/acetate-derived fibers decreased with increasing diameter at a greater rate than that of the alkoxide/nitrate-derived fibers. This was associated with the sudden removal of acetate groups at a unique temperature, and was explained by the increased difficulty in the removal of acetate groups through the oxide matrix without forming microcracks in the fibres having increased diameters. For the CeO2-ZrO2 fibers made from Zr(O-n-C3H7)4 and Ce(NO3)3.6H2O, a maximum tensile strength of 1.0–1.2 GPa was attained at diameters of 10–15 μm. The addition of a small amount of CuO as a sintering aid much improved the mechanical properties of the fibers, resulting in fibers with tensile strength as high as 1.8–2.0 GPa.

Superplastic Flow in Ceramic Microfiber Specimens

ABSTRACTTensile superplastic deformation was studied using ceramic specimens that had a cross section of approximately 1 μm × 10–50 μm, and a length of 4 mm. These fibers were prepared by physical vapor deposition of oxides on a predefined pattern made by lithography.Experiments with Y2O3 stabilized ZrO2 fibers and spinel fibers showed two characteristic features of flow behavior: serrations and a threshold stress. The serrations were explained by a flow model based on discrete sliding events distributed in time and space throughout the polycrystal. These fluctuations in flow stress were detected in our microfiber specimens due to relatively few grains in the gage section. Experimental results of the effect of sample size, grain size and strain rate on the serration behavior are discussed in the frame of the new model.

Preparation of Fibrous ZrO<sub>2</sub> and CaO-ZrO<sub>2</sub> from Zirconium Alkoxide by Sol-Gel Method

The Zr(O-nC3H7)4 and Ca(CH3COO)2⋅H2OH2O-C2H5OH-HCl solutions were prepared and heated at 80°C for hydrolysis-condensation reaction of the alkoxide. The spinnability of the solutions was examined in the course of reaction. Following results have been obtained.(1) The Zr(O-nC3H7)4 solutions with the H2O/alkoxide molar ratio ranging from 0.7 to 1.5 and C2H5OH/alkoxide molar ratio larger than 4 exhibited spinnability within 2 to 9 hours. The ZrO2 gel fibers were successfully drawn.The CaO-ZrO2 gel fibers could also be drawn from the corresponding alkoxide-acetate solution with H2O/alkoxide molar ratio of about 1 and C2H5OH/alkoxide ratio of 27.(2) The solution composition region for spinnability is considerably limited, compared to the cases of making SiO2 and TiO2 gel fibers from Si(OC2H5)4 and Ti(OC3H7)4 solutions.(3) Transformation of the tetragonal ZrO2 to monoclinic ZrO2 occurring in ZrO2 fibers above 600°C caused the fracture of the fibers on heating. The addition of CaO to fibrous ZrO2 increased the stability of precipitating tetragonal ZrO2 and resulted in the avoidance of fracture of fibers by heating.(4) It is considered that the linear type zirconoxane polymers consisting of ZrO6 octahedra connected with each other by sharing their edges are formed in the spinnable Zr(O-nC3H7)4 solutions.