Homologous temperature expresses the

thermodynamic temperature Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expres ...

of a material as a fraction of the thermodynamic temperature of its

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

(i.e. using the

Kelvin scale The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

T_H = \frac

For example, the homologous temperature of lead at room temperature (25 °C) is approximately 0.50 (T_H = T/T_mp = 298 K/601 K = 0.50).

Significance of the homologous temperature

The homologous temperature of a substance is useful for determining the rate of steady state creep (diffusion-dependent deformation). A higher homologous temperature results in an exponentially higher rate of diffusion dependent deformation. Additionally, for a given fixed homologous temperature, two materials with different melting points would have similar diffusion-dependent deformation behaviour. For example, solder (T_mp = 456 K) at 115 °C would have comparable mechanical properties to copper (T_mp = 1358 K) at 881 °C, because they would both be at 0.85T_mp despite being at different absolute temperatures. In electronics applications, where circuits typically operate over a −55 °C to +125 °C range, eutectic tin-lead (Sn63) solder is working at 0.48T_mp to 0.87T_mp. The upper temperature is high relative to the melting point; from this we can deduce that solder will have limited

mechanical strength Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...

(as a bulk material) and significant creep under stress. This is borne out by its comparatively low values for tensile strength, shear strength and modulus of elasticity. Copper, on the other hand, has a much higher melting point, so foils are working at only 0.16T_mp to 0.29T_mp and their properties are little affected by temperature.

References

Scales of temperature {{Measurement-stub