Porosity polycrystalline ceramic

If the second phase is porosity, as is often the case in polycrystalline ceramics, then intuitively we realize that there will be a decrease in the elastic modulus. A pore has zero stiffness. Several relationships have been developed... 

The measured properties of polycrystalline ceramics are greatly influenced by the microstructure of the material. Of particular importance are the grain size of the crystallites, porosity, voids or gas bubbles within the ceramic and any impurity phases present. In addition, chemical defects such as point defects and mobile charge carriers within grains and the physics and chemistry of the grain botmdaries will all have an influence on the measured properties of the solid. Because of this, ceramics are subjected to carefully controlled fabrication routes and the sintering temperature and time have a considerable effect upon the measured properties (Figure 6.3c). 

This equation is valid for polycrystalline ceramics with equilibrium shapes of the isolated pores to be nearly spherical, i.e., dihedral angles are larger than 150°. Once the pores become nonspherical, the expression of (j) could have very complicated form. If the shapes of the pores are dramatically changed, as shown in Fig. 5.25b [1], although the volume is the same, the value of (j) could be changed, because it is now dependent on not only the porosity but also the shape of the pores. When the pore shape is not spherical, the dihedral angles are reduced, so that the actual area of the grain boundary is decreased, therefore, (f> will be decreased. 

Deformation at elevated temperatures is the commonly observed case in ceramics. Once again using the example of polycrystalline ceramic MgO, the following stress-strain curves are illustrated (Fig. 4.8). One of the possible differences in these stress-strain curves reflects the difference in grain-size the different grain size before deformation is shown in Fig. 4.9. The composition and porosity were also different in the otherwise nominally pure and dense specimens, as seen in Table 4.1. 

When steps are taken during fabrication to substantially reduce the amount of residual porosity, the ceramic undergoes a transformation from opaque to translucent or even to nearly transparent. Such a transition can be observed in special polycrystalline alumina ceramics. Normally, sintered alumina bodies are observed to be opaque to somewhat translucent. However, an alumina material is produced under the trade name of Lucalox (General Electric Co.) in which there is low residual porosity. The ceramic is almost transparent to light and is used in the production of lamp envelopes for exterior lighting. 

Too great a load chosen for deep penetration to overcome or minimize the sample surface factors can produce erratic hardness results as brittle ceramics crack locally around the indent and energy is expended on crack propagation. Deliberate overloading has now been shown to be a nondestructive microscopic way of determining important properties of ceramic systems, and as such occupies the whole of Chapter 5. However, when a polycrystalline ceramic is not obviously cracked and the indented area extends over many grains, then other microstructural features become dominant. For example, equation (1.2) is found to relate hardness to porosity in sintered materials. ... 

In order to compare these theoretically calculated values with experimentally measured or extrapolated values for the respective polycrystalline ceramics one has to take account of the fact that real polycrystalline materials always contain a certain amount of porosity. 

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. 

The foregoing treatment assumes that at least one of the reactants is a single crystal and the reactant/product geometry is well-defined. Many technologically important ceramic reactions, on the other hand, are usually carried out between polycrystalline powders. The reaction kinetics in these cases depend on several physical factors such as particle size, packing density, porosity, and so on. Jander (1927) and Carter (1961) have proposed models for powder reactions making several simplifying assumptions. 

Glass-ceramics are an important class of materials that have been commercially quite successful. They are polycrystalline materials produced by the controlled crystallization of glass and are composed of randomly oriented crystals with some residual glass, typically between 2 and 5 percent, with no voids or porosity. 

PZT and PLZT. The development of polycrystalline lead lanthanum zir-conate-titanate (PLZT) electronic ceramic monoliths, which fully transmit incident light, requires methods for controlling stoichiometry, impurity content, porosity, grain size, and so on. Alkoxy-derived PLZT powders were prepared by hydrolytic decomposition of mixed-metal alkoxides [40]. The Zr and Ti alkoxides were synthesized by the ammonia method, and the lanthanum tris-isopropoxide was synthesized by the metal/alcohol reaction method, which were described in the previous section. The Pb alkoxide was prepared by the reaction of anhydrous lead acetate, Pb(C2H302)2, with sodium isoamyloxide, NaOCsHn [40] ... 

As a result of carefully controlled thermal treatment, the initial glass is converted into a polycrystalline material in which the final properties depend on the nature of the precipitated phases, the final degree of crystallinity, the size of the crystallites, etc. The material is generally opaque, although translucent and even transparent glass-ceramics have been produced. The small size of the grains and the absence of porosity are characteristics of glass-ceramics. These result in excellent mechanical properties. This is explained in part by the action of the microcrystal-... 

Sintering General process by which powders react and density to form polycrystalline compacts or ceramics. Such compacts density by grain growth and porosity reduction. Sintering can be reactive, i.e., involve the reaction of two or more solid components to form a product or products. 

Dunn, M L. 1995. Effects of grain shape anisotropy, porosity, and microcracks on the elastic and dielectric constants of polycrystalline piezoelectric ceramics. Journal of Applied Physics 78 [3] 1533-1541. 

E P. Knudsen, Dependence of mechcuiical strength of brittle polycrystalline specimens on porosity cuid grcun size. /. Amer. Ceramic Soc. 42, 376-387 (1959). 

F.P. Knud sen, Dependence of Mechanical Strength of Brittle Polycrystalline Specimens on Porosity and Grain Size, Am. Ceram. Soc., 42 [8] 376-87 (1959). 

Hasselman DPH. Relation between the effects of porosity on strength and Young s modulus of elasticity of polycrystalline materials. J Am Ceram Soc. 1963 46 564-5. 

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