Ceramic temperature dependence

Sihca and aluminosihcate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobaUte (1). Cristobahte is a form of crystalline siUca that can cause siUcosis, a form of pneumoconiosis. lARC has deterrnined that cristobaUte should be classified as 2A, a probable carcinogen. The amount of cristobahte formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobahte. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Fig. 107. Temperature dependence of the dielectric permittivity r determined at various frequencies for a ceramic sample ofRbsNb3OF,H.

Recently, interesting composite materials incorporating polymeric materials into the sol-gel glasses have been reported by Wilkes and his co-workers [9]. These materials are named ceramers . The properties of ceramers strongly depend on the reaction conditions, i.e., acidity, water content, reaction temperature, the amount of organic polymer, the molecular weight of polymer, solvent, and so on. 

Morscher, G, Pirouz, P. and Hener, A.H. (1990). Temperature dependence of interfacial shear strength in SiC-fiher-reinforced RBSN. J. Am. Ceram. Soc. 73, 713-720. 

Figure 4.47 Schematic representation of temperature dependence of (a) oxygen vacancy and (b) oxygen diffusivity in oxygen-deficient oxide ceramics. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Primenko, V. L, Theoretical method of determining the temperature dependence of the thermal conductivity of glasses. Glass Ceram., 37, 240-242 (1980). 

Figure 5.116 Temperature dependence of bending strength for SiC-reinforced lithium aluminosilicate (LAS) CMC. Reprinted, by permission, from R. W. Davidge and J. J. R. Davies, in Mechanical Testing of Engineering Ceramics at High Temperatures, B. F. Dyson, R. D. Lohr, and R. Morrell, eds., p. 264. Copyright 1989 by Elsevier Science Publishers, Ltd.

Attenuation in solids due to viscosity may be treated by a similar analysis. There may well be other damping mechanisms, such as heat conduction (i.e. imperfectly adiabatic conditions) which also gives an f2 law, and other phenomena associated with solid state defects that may have more complicated frequency and temperature dependence. In polycrystalline solids, especially metals and alloys and also ceramics, elastic grain scattering may cause much greater attenuation than any inelastic damping (Papadakis 1968 Stanke and Kino 1984). 

Lankford J., 1983, Comparative study of the temperature dependence of hardness and compressive strength in ceramics, J. Mater. Sci., 18, 1666. 

The oxidation behaviour of Si3N4 ceramics strongly depends on impurities in the gas atmosphere. Impurities like alkaline or alkaline earth metals, S02, and vanadium drastically decrease oxidation [431, 433, 434]. The main influence of the different impurities is caused by a change of the viscosity or the destruction of the oxide scale, accelerating the diffusion of oxygen or water vapour into the ceramic and increasing the corrosion. Of coarse, the effect strongly depends on temperature and gas composition. 

As the temperature dependence of the CTL spectrum has information about the type of vapor, the present authors and coworkers [17] reported a method to recognize organic vapors by means of spectrum-temperature imaging. For this purpose, a system to simultaneously measure the CTL spectra at various temperatures was developed (Fig. 27). The sintered layer of the CTL catalyst is laid on a ceramic heater substrate of 5 x 60 mm2, which has a temperature distribution ranging from 440 to 530 °C along the stream of a sample gas in a quartz tube. A mask with an optical slit of 0.3 mm width is placed on a quartz tube. The CTL emission passing through the slit is focused on... 

Figure 1.15 BaTiOj Temperature dependence of the permittivity for (a) bulk ceramics with different grain sizes and (b) thin films with different grain sizes and (c) microstructure of thin films.

Fortunately, they are several species of low-loss dielectric ceramics with tailored temperature coefficient of dielectric constant, which can be made lower than 1 ppm/K for a certain temperature window around room temperature. Physically, this can be accomplished either by intrinsic compensation of the temperature dependence of thermal volume expansion V(T) and lattice polarizability a(T) via the Clausius-Mossotti relation ... 

The present work summarizes opportunities of using high-resolution synchrotron and standard xrd techniques for structural characterization as well as for investigations of structure-property-relationships. xrd will be used to determine quantitatively the phase content of morphotropic pzt. Temperature dependent measurements provide information about the phase transformation of morphotropic donor doped pzt ceramics and high-resolution synchrotron X-ray diffraction gives information about the extrinsic and intrinsic contributions to the electric field induced strain, xrd results are finally compared with electrical measurements to analyze the interactions among microstructure, phase content and properties. 

Figure 7.7 Temperature dependence of the relative permittivity for PZT ceramics near the mpb. pzt (54/46) undergoes a partial Fr Ft phase transition between 40 and 80°C.

The perovskite structure is, of course, of special significance in the electroceramics context since the ferroelectric perovskites are dominant in the ceramic capacitor, PTC thermistor and electromechanical transducer industries. The structure favours the existence of soft modes (low frequency phonons) as evidenced by its tendency to instability, for example the ferroelectric-paraelectric transition. Instability is evident in the case of the T23 compound which exhibits a tetragonal-orthorhombic transition in the region of 700 °C (the exact temperature depends on the oxygen content). Extensive twinning, very reminiscent of ferroelectric domain structures, is observed. 

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