Coefficient-of-Thermal-Expansion Tester: Measurement Principle and Applications

A coefficient-of-thermal-expansion tester measures specimen length change during heating and calculates average and instantaneous linear coefficients of thermal expansion. Thermal expansion mismatch can generate stress between joined materials and cause cracking, solder-joint failure, or seal failure.

The instrument uses push-rod or optical measurement. In a push-rod system, the specimen is held in a quartz or corundum tube; expansion moves a rod, and a linear differential transformer or grating sensor converts displacement into a signal. A nearby thermocouple records temperature, and software synchronizes length and temperature data. The system includes a furnace, displacement and temperature controls, specimen holder, and acquisition software. The tube can use inert gas or vacuum.

Fused quartz, high-purity alumina, and zirconia are used for holders and push rods. Quartz has low thermal expansion but softens above its service range; alumina supports higher temperatures but requires correction for its own expansion.

Prepare cylindrical or prismatic specimens with parallel, smooth ends. Mount the specimen, place the push rod within its measurement range, set the heating rate, maximum temperature, and hold time, and record the expansion curve. The average coefficient is obtained from expansion over a temperature interval; the instantaneous coefficient is the curve slope at a selected temperature. Glass-transition temperature, softening point, and expansion anomalies can be identified from curve inflections.

Applications include thermal-expansion measurements for metals, ceramics, glass, composites, electronic materials, and coatings. Thermal history and internal stress affect results. Excessive heating rate can create a temperature gradient and apparent expansion lag; excessive rod force can indent a soft specimen. Use vacuum or inert gas for materials that release gas or react during heating.