Temperature linear expansion

Figure 12 Dep>endences of lattice parameter at 300 K (a), temperature linear expansion coefficient at 573-1073 K (b), and thermoconductivity coefficient T = 300 K) (c) on (SiC) ,(AIN)i-, solid solution composition.

Thermal Stresses. When the wak of a cylindrical pressure vessel is subjected to a temperature gradient, every part expands in accordance with the thermal coefficient of linear expansion of the steel. Those parts of the cylinder at a lower temperature resist the expansion of those parts at a higher temperature, so setting up thermal stresses. To estimate the transient thermal stresses which arise during start-up or shutdown of continuous processes or as a result of process intermptions, it is necessary to know the temperature across the wak thickness as a function of radius and time. Techniques for evaluating transient thermal stresses are available (59) but here only steady-state thermal stresses are considered. The steady-state thermal stresses in the radial, tangential, and axial directions at a point sufficiently far away from the ends of the cylinder for there to be no end effects are as fokows ... 

Thermal Expansion. Coefficients of linear thermal expansion and linear expansion during transformation are listed in Table 7. The expansion coefficient of a-plutonium is exceptionally high for a metal, whereas those of 5- and 5 -plutonium are negative. The net linear increase in heating a polycrystalline rod of plutonium from room temperature to just below the melting point is 5.5%. 

Phase Temperature range, °C Linear expansion coefficient X 10 /K Transformation expansion, 100 AL/L... 

The linear expansion of aluminum over several temperature ranges can be calculated from equations 1—3... 

The coefficient of linear expansion of these alloys in the temperature range of 21 to 100°C (70 to 212°F) is 12.2 X lO C (6.8 X 10"V°F), which is slightly above that of cast iron (National Bureau of Standards). Since these loys have practically no elasticity, it is necessary to use expansion joints in relatively short pipe hnes. Connections for flanged pipe, fittings, valves, and pumps are made to 125-lb American Standarci drilling. 

Bimetallie elements are widely used in instruments sueh as thermostats to sense or eon-trol temperatures. There are several bimetallie element types available, sueh as straight strips, eoils and dises, but all rely on the same working prineiple. In its most basie form, the bimetallie strip eomprises of two dissimilar metal strips bonded together, usually of the same surfaee area, but not neeessarily of the same thiekness thermostat. The eom-posite metal strip is elamped at one end to aet as a eantilever beam, and is horizontal at a partieular temperature. When the temperature is inereased, the strip defleets in the direetion of the metal with the least eoeffieient of linear expansion. Its working prineiple relies on the faet that the metals will expand at different rates as the strip is heated. The purpose of this defleetion is to typieally eause the strip to make eontaet with a switeh or eomplete an eleetrie eireuit at a partieular setpoint temperature above the ambient. 

Deflection temperature under load (1.8 MPa) Coefficient of linear expansion (-30°C to +30°C Specific heat 20-300°C >300°C... 

The thermal expansion and contraction of solids can also have safety implications. For a given material the amount of its linear expansion, or contraction, in one direction is directly related to temperature and its original size (i.e. length, diameter, circumference). Thus ... 

Metal A lomic number Atomic weight Lattice structure Density at 20°C (g/em ) Melting point (°C) Thermal conductivity at 0-l00°C (W/m°C) Specific heat at 0°C (J/kg C) Coefficient of linear expansion at 20-iOO°C X 70 Thermal neutron cross-section (barns) (10-- m ) Resistivity at 0°C (fiil em) Temperature coefficient of resistance o-ioo°c X 10 ... 

Lemington H.26X., made by the General Electric Co., is a very hard borosilicate glass of high softening temperature. Tte Mg point is 780°C. It is used in high pressure mercury vapour lamps. The linear expansion coefficient is 4-6 x 10 from 20 to 580°C. Sodium and potassiiun are absent and alumina is present in quantity in this glass. 

Substrate material Price per unit area (arb. units) Dielectric permittivity of insulator Maximum working temperature (K) Resistivity of dielectric layer (fl-cm) Density (g/cm3) Linear expansion coefficient x i[Pg.490]

Several methods of measurement of the thermal expansion have been developed as a function of the material, dimension and shape of the sample, temperature range and requested accuracy. The measurement of the linear expansion coefficient a = 1/L (AL/A7) of a sample is done by recording the length change AL (in a definite direction) due to a temperature variation AT. 

One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

---