Polycrystalline ceramics

The most commercially important application that takes advantage of the pyroelectric effect ia polycrystalline ceramics is iafrared detection, especially for wavelengths ia excess of 2.5 p.m. AppHcations range from radiometry and surveillance to thermal imaging, and pyroelectric materials work under ambient conditions, unlike photon detectors, which require cooling. 

DiCarlo, J. A., Creep Stress Relaxation Modeling of Polycrystalline Ceramic Fibers, NASA, 1994. 

Figure 6.7 Complex impedance of a polycrystalline ceramic sample (a) representation of the equivalent circuit of a component (b) the impedance spectrum of the equivalent circuit in (a) (c) the impedance spectrum of a typical ceramic sample. Each semicircular arc represents one component with an equivalent circuit as in (a) that at the highest frequency corresponds to the repose of the bulk, that at middle frequencies to the grain boundary response, and that at lowest frequencies to the electrodes.

Figure 6.13 Surface of thermally etched La0.5Sr0.5Fe0.5Co0.5O3> a polycrystalline ceramic material.

In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature shaped by casting, film formation, or fiber drawing and then consolidated to furnish dense glasses or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon, boron, titanium, and aluminum. In alcohol water solution, the alkoxide groups are removed stepwise by hydrolysis under acidic or basic catalysis and... 

Related to the attack of polycrystalline ceramic materials by aqueous media is the hydrolysis of silicate glasses. The following relationship has been developed to describe the effect of time and temperature on the acid corrosion (10% HCl) of silicate... 

Figure 13.11—Scanning electron microscopy (SEM) accompanied by X-ray fluorescence analysis. Secondary electron image of a cross-section of a supraconducting polycrystalline ceramic with oriented grains of oxide BiPbiSriCaiCurO, (Philips instrument, model XL30FEG). Energy emission spectra corresponding to the matrix and to a 5 pm-long inclusion (bottom). It should be noted that it is possible with this technique to obtain the composition at a precise point on the sample (Link-Oxford analyser) (study by V. Rouessac, reproduced by permission of CRISMAT. University of Caen).

Finally we can conclude that the strength of polycrystalline ceramics depends on many factors, mainly on their chemical composition (a measure for the strength of the bonds between the atoms), their microstructure and their surface condition. 

Clarke and co-workers developed a model to calculate the thickness of the amorphous film observed in polycrystalline ceramics.37,38 The model is based on a force balance between an attractive van der Waals dispersion force that acts across the grain boundaries, any capillary forces present, and repulsive disjoining forces (such as steric forces and electrical double-layer forces) in the amorphous film.37,38 The repulsive steric force is based on the... 

Clarke, D.R., Shaw, T.M., Philipse, A.P. and Horn, R.G., Possible electrical doublelayer contribution to the equilibrium thickness of intergranular glass films in polycrystalline ceramics ,/. Am. Ceram. Soc., 1993 76(5) 1201-1204. 

Gorges et al. [155] introduced anodic spark deposition (ASD), a modification of anodic oxidation, for the formation of polycrystalline ceramic oxide layers on passivating metals (Figure 2.99). The method is therefore limited to Ti, Al and Zr... 

The piezoelectric coefficients are third rank tensors, hence the piezoelectric response is anisotropic. A two subscript matrix notation is also widely used. The number of non-zero coefficients is governed by crystal symmetry, as described by Nye [2], In most single crystals, the piezoelectric coefficients are defined in terms of the crystallographic axes in polycrystalline ceramics, by convention the poling axis is referred to as the 3 axis. 

The frequency response of a real polycrystalline ceramic carrying metallic electrodes may yield three well defined semicircles representing respectively polarization processes associated with the interior of the grains, with the grain boundary regions and with the electrode-ceramic interfacial region. Such a clear-cut situation is illustrated in Fig. 2.56. 

Careful attention has to be given to the purity of the precursors to avoid detrimental effects on conductivity. In a polycrystalline ceramic the conductivities of grain boundaries and bulk contribute to overall conductivity. In the case of polycrystalline YSZ, because of its unusually high intrinsic (bulk) conductivity the grain boundaries are far less conductive than the crystal, typically by a factor of 100. The effect the grain boundaries have on overall conductivity will depend on grain size and, of course, on impurity content (e.g. silica), since impurities tend to concentrate there. It is the effort to understand more of the various contributors to overall conductivity which has led to the application of impedance spectroscopy (see Section 2.7.5). 

Rutile is anisotropic, with the values of er at room temperature being approximately 170 and 90 in the c and a directions respectively. In the polycrystalline ceramic form er averages to intermediate values with a... 

Polycrystalline materials in which the crystal axes of the grains are randomly oriented all behave electrostrictively whatever the structural class of the crystallites comprising them. If the crystals belong to a piezoelectric class and their crystal axes can be suitably aligned, then a piezoelectric polycrystalline ceramic becomes possible. 

Not all the tensor components are independent. Between Eqs (6.29a) and (6.29b) there are 45 independent tensor components, 21 for the elastic compliance sE, six for the permittivity sx and 18 for the piezoelectric coefficient d. Fortunately crystal symmetry and the choice of reference axes reduces the number even further. Here the discussion is restricted to poled polycrystalline ceramics, which have oo-fold symmetry in a plane normal to the poling direction. The symmetry of a poled ceramic is therefore described as oomm, which is equivalent to 6mm in the hexagonal symmetry system. 

A complete description of the electro-optic effect for single crystals necessitates full account being taken of the tensorial character of the electro-optic coefficients. The complexity is reduced with increasing symmetry of the crystal structure when an increasing number of tensor components are zero and others are simply interrelated. The main interest here is confined to polycrystalline ceramics with a bias field applied, when the symmetry is high and equivalent to oomm (6 mm) and so the number of tensor components is a minimum. However, the approach to the description of their electro-optic properties is formally identical with that for the more complex lower-symmetry crystals where up to a maximum of 36 independent tensor components may be required to describe their electro-optic properties fully. The methods are illustrated below with reference to single-crystal BaTi03 and a polycrystalline electro-optic ceramic. 

In the case of polycrystalline ceramic (6mm) the form of the electro-optic tensor is the same as that for m3m symmetry except that R66 = (Rn - RX2). Therefore, when a field is applied along the x3 axis, the induced birefringence is again... 

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