Ceramics measurement

Grain boundaries have a significant effect upon the electrical properties of a polycrystalline solid, used to good effect in a number of devices, described below. In insulating materials, grain boundaries act so as to change the capacitance of the ceramic. This effect is often sensitive to water vapor or other gaseous components in the air because they can alter the capacitance when they are absorbed onto the ceramic. Measurement of the capacitance allows such materials to be used as a humidity or gas sensor. 

Typically, the sample or object to be activated is placed in a container for irradiation. This may be aluminium foil (as aluminium cannot easily be measured by this technique) or silica glass tubes. Several samples and standards are placed in the same container for irradiation, to ensure the same conditions for samples and standards. For example, in the case of ceramics measured at the British Museum laboratories (Hughes et al. 1991), powdered... 

Nickel KG (ed) (1994) Corrosion of Advanced Ceramics - Measurement and Modelling. Kluwer Academic Publishers, Dordrecht... 

Figure 2.3 The magnitude of da as a function of temperature in a PZT-5H ceramic measured using a large amplitude electric field, after [3].

Figure 13.1 Examples of the field dependence of piezoelectric coefficients (a) direct effect in ferroelectric ceramics, measured with a dynamic press (b) converse effect in rhombohedral 60/40 pzt thin films with different orientations, measured with an optical interferometer [1], correspond to pseudocubic axes.

E. Thellier and O. Thellier (1959), who measured the magnetic field in ancient ceramics and then reheated them beyond the Curie point and allowed them to cool in the same fashion as originally. They then remeasured the magnetic field. These early ceramic measurements have been supplemented by reconstructions based on igneous rocks and deep-sea cores, which extend back many radiocarbon half-lives (see Bard (1998) for an excellent summary). 

For most polycrystalline ceramics, measured strengths are in the range of /100 to /1000 or even less. Why is there such a large discrepancy between theoretical and measured strengths The reason is the presence of preexisting cracks on the surface or inside the ceramic and sharp corners that may be introduced during processing. The presence of cracks does not mean that samples will fracture spontaneously our teeth are full of cracks. 

Luthra, K.L. 1994. Theoretical aspects of the oxidation of silica-forming ceramics. Pp. 23-34 in Corrosion of Advanced Ceramics Measurement and Modeling Proceedings of the NATO Advanced Research Workshop, Tubingen, Germany August 30-September 3, 1993. Klaus G. Nickel (ed.). Norwell, Mass. Kluwer Academic Publications. 

Another micro-hardness test is the Knoop Hardness Test (henceforth KHT, discussed below). It is worth mentioning that superficial Rockwell tests are also used for ceramics measurements. Hardness tests have been upgraded by the application of instrumented Knoop and Vickers hardness measurements. 

Shetty D.K., Rosenfield A.R. Duckworth W.H. 1985. Fracture toughness of ceramics measured by a chevron-notched diametral compression specimen. Journal of the American Ceramic Society 68 325-327. 

Figure 1. Typical result of instrumented indentation on alumina ceramics, measured with Vickers indenter.

Sangiorgi, R. (1994), in Corrosion of Advanced Ceramics Measurements and Modelling, Nickel, K. G. (Ed.). Dordrecht, The Netherlands Kluwer Academic Publishers, pp. 261-284. 

Figure 7. Relative tensile modulus of porous ceramics (measured values, predictions and master fit) HS upper bound (thin solid curve), predictions for spherical pores (thin dotted special case of the Spriggs relation Eq. (110), thin dashed Coble-Kingeiy Eq. (117), thick solid modified exponential relation Eq. (114)), experimentally measured values (squares alumina, diamonds ZTA, triangles ATZ, circles zirconia, empty potato starch as a pore-forming agent, full com starch as a pore-forming agent) and master curve (thick dotted curve obtained by fitting with the Pabst-Gregorova relation, Eq. (121), critical porosity 0.729).

PLASTIC DEFORMATION AND CRACKING RESISTANCE OF SIC CERAMICS MEASURED BY INDENTATION... 

Plastic Deformation and Cracking Resistance of SiC Ceramics Measured by Indentation... 

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