Ceramic resistance, determination

M. G. Jenkins, Ceramic Crack Growth Resistance Determination Utilizing Laser Interferometry, PhD Dissertation, University of Washington, Seattle, WA, 1987. 

Of the experimental methods used to determine Q, an often-used method is the resistivity technique resistivity is measured, since electrical conductivity in most ceramics is determined by the number and type of defects present. This technique involves quenching a specimen and then measuring and evaluating its resistivity, as given by ... 

Because ceramics are brittle, they are susceptible to catastrophic failure under mechanical load. The useful strength of a ceramic is determined by the flaw population stresses are concentrated at flaws, which cause cracks to propagate to failure. The critical property for ceramics in load-bearing uses is not the strength, but the fracture toughness—the resistance of the ceramic to crack propagation. The fracture surface of a ceramic bears the evidence of its failure. One must read the features in a fracture surface to understand the origin and path of the fracture. The case study by E. K. Beauchamp shows how much practical information can be obtained from ceramic fracture analysis. 

Tetrahedron. See silicate structures. Textile Ceramics. The achievement of artistic effects with combinations of textiles and ceramics, in particular by using viscose-silica fibre, a cellulose fibre containing up to 33% silica, used for fire-resistant clothing and upholstery. Bowls, wall hangings and other ornaments of delicate texture can be produced. Texture. (1) The physical property of a ceramic product determined by the shapes and sizes of the pores and the grading of the solid constituents. The texture can to some extent be evaluated in terms of porosity and permeability additional information is provided by pore size measurement and the total... 

DIN 51068-1-1976. Testing of ceramic materials determination of resistance to thermal shock water quenching method for refractory bricks. 

Impact resistance is determined usiag flyer plate impact tests, long rod impact tests, Hopkinson bar tests (50), and the Hquid jet technique (51). Impact damage resistance is often quantified by measuring the postimpact strength of the ceramic. 

Generally the harder the ceramic, the better its wear resistance however, other properties such as fracture toughness may play the dominant role. If a ceramic is mated with a metal hardness is the determining factor, but when a ceramic is mated with another ceramic fracture toughness appears to determine the wear rate (54). 

Hardness is determined by hardness tests which involve the measurement of a material s resistance to surface penetration by an indentor with a force applied to it The indentation process occurs by plastic deformation of metals and alloys. Hardness is therefore inherently related to plastic flow resistance of these materials. Brittle materials, such as glass and ceramics at room temperature, can also be subjected to hardness testing by indentation. This implies that these materials are capable of plastic flow, at least at the microscopic level. However, hardness testing of brittle materials is frequently accompanied by unicrack formation, and this fact makes the relationship between hardness and flow strength less direct than it is for metals. 

The dissipation factor of capacitors at high frequencies is determined by the series resistance. For low frequencies there may be losses caused by leakage currents as well as by slow components in the polarizability, especially of high e ceramics and polymer dielectrics. The dissipation factor of the SIKO at room temperature is below 10-4. At 200 °C it is still very low (2X10-4). 

One final note is appropriate for this section. Dne to the fact that many oxide ceramics are used as insulating materials, the term thermal resistivity is often used instead of thermal conductivity. As will be the case with electrical properties in Chapter 6, resistivity and conductivity are merely inverses of one another, and the appropriateness of one or the other is determined by the context in which it is used. Similarly, thermal conductance is often used to describe the thermal conductivity of materials with standard thicknesses (e.g., building materials). Thermal condnctance is the thermal conductivity divided by the thickness (C = k/L), and thermal resistance is the inverse of the prodnct of thermal conductance and area R = 1/C A). 

Blondel R., 1942, Methode simple pour la determination de la resistance a l abrasion a froid des produits refractaires, L lnd. Ceram., 205-210. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

The ceramic is required in the form of dense, polycrystalline layers, as thin as practicable to minimise cell resistance, consistent with the need to ensure mechanical integrity. Adopted layer thickness is, of course, also determined by the operating temperature of the cell. Self-supporting plates 150 pm thick can be used at 900-1000 °C and supported thick films (down to 15/rm) to as low as 700°C. 

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