Single-Phase Polycrystalline Ceramics

In the case of single-phase polycrystalline ceramics, in addition to porosity, it is necessary to determine the amount, size, shape, and distribution of the other constituents to characterize the microstructure completely. The microstructure of polycrystalline ceramics develops as grains that meet at faces whose intersections form angles of 120°. In some materials. 

Permeability coefficient for viscous gas flow in beryllia ware of differing porosities. 

Crystal-liquid structure of a forsterite composition (150x). 

These have definite structural characteristics such as specific orientation, concentration, and impurity segregations. In addition, the properties 

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Some examples of the grain-size effect in ceramics are illustrated below Ti3SiC2 was chosen as one exemplar, since this ternary compound exhibits a unique combination of properties. It is a layered material that is as machinable as graphite. At the same time, CG (100-300 pm) samples of Ti3SiC2 have been observed to be damage-tolerant, not susceptible to thermal shock and oxidation resistant. The specimens are fully dense, bulk, single-phase polycrystalline samples of Ti3SiC2. This material exhibits brittle failure characteristics at RT, but is plastic at 1,300 °C with yield points of 300 and 100 MPa under compression and flexure, respectively. 

Houle, M.C., Coble, R.L., 1962. Ceramographic techniques, I. Single phase polycrystalline hard materials. Bull. Am. Ceram. Soc. 41, 378. 

The relation between conductivity, chemical and phase compositions, and the structure of separate phases of solid electrolytes is very important In many systems, the maximum in conductivity closely coincides with the existing F-phase (fluorite-phase) limit TEM image of Figure 13.3 shows the formation of a dense single-phase polycrystalline microstructure for the sintered ceramics [1]. X-ray diffraction and analysis of electron diffractograms of this sintered ceramic confirmed the fluorite structure. 

The quality of a ceramic sample is a function of the degree to which it consists of the desired product. An essential tool for the characterization of a polycrystalline sample is powder X-ray diffraction. The powder pattern is a fingerprint of the sample. For a sample to be declared single phase, all low angle peaks (below 60 20 for CuKq radiation) which are above the noise must be accounted for. Powder X-ray diffraction is often unable to see impurity phases present below the 5% level. Visual inspection (using a microscope)... 

In the past, most solids were prepared on a large scale by standard ceramic techniques, in which accurate control of the composition, as well as uniform homogeneity of the product, were not readily achieved. Unfortunately, this has sometimes led to uncertainty in the interpretation of the physical measurements. In recent years more novel methods have been developed to facilitate the reaction between solids. This is particularly true for the preparation of polycrystalline samples, on which the most measurements have been made. It is of utmost importance to prepare pure single-phase compounds, and this may be very difficult to attain. Even for a well-established reaction, careful control of the exact conditions is essential to ensure reproducible results. For any particular experiment, it is essential to devise a set of analytical criteria to which each specimen must be subjected. It will be seen from the solid-state syntheses included in this volume that one or more of the following common tests of purity are used to characterize a product. 

Silicon nitride-based polycrystalline ceramic materials have the potential for structural applications at both room temperature and elevated temperatures, despite containing silicate/silicate-rich oxynitride grain boundary phases. These are generally much weaker compared to the silicon nitride single crystal grains of which the... 

In addition to the high k of many relaxor compositions they also have a broad peak in the permittivity versus temperature range, even in the absence of additives and even in the form of single crystals. This behavior is attributed to nanoscale (-lOnm)-ordered regions, which are too small to yield the sharp phase transition of normal ferroelectrics. As a result, spontaneous polarization and associated ferroelectric properties are retained over a very broad temperature range. Another attractive feature of relaxors is that dense polycrystalline ceramics are achievable at relatively low sintering temperatures (<900°C), which allows a significant reduction in the amount of Pd used in Ag-Pd metallizations for electrodes in multilayer capacitors (see Section 31.7). 

When there is no distortion in the structure or no change in orientation throughout a crystal, we refer to the structure as a single crystal. A solid consisting of many single crystals with different orientations is called a polycrystalline aggregate. There are also polycrystalline aggregates of multiple phases, as in metals, rocks, and ceramics. 

Many ceramics are used in a random polycrystalline form and thus, it is useful to be able to predict the elastic constants from those of the single crystals. The approaches outlined in the last two sections are used for this procedure by considering the random polycrystal as an infinite number of phases with all possible orientations. For example, Voigt and Reuss used a technique based on averaging the stiffness or compliance constants and obtained upper and lower bounds. The Voigt upper bounds for the bulk (B) and shear (/i) moduli of the composite can be written as... 

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