Temperature dependence of the coefficient of thermal expansion of different artificial graphites and the dimensional change during heat treatment of carbonized specimens

The linear thermal expansion of two artificial graphite products, an isotropic graphite and an extruded nipple electrode, was measured during heating up to 2400 °C by a newly developed device with a non-contact laser micro-gage. The temperature dependence of the coefficient of thermal expansion (CTE) of two graphites was investigated. The test piece was 100 mm long a round rod of 20 mm diameter, which is large enough to reflect the different textures of these products. In the nipple electrode, two specimens taken from parallel or perpendicular to the extrusion direction were used. For the isotropic graphite and the parallel specimen of the nipple electrode, temperature dependences of the CTE below 500 °C were different. In the case of the parallel nipple specimen, a negative value of CTE was found in the temperature region from 150 to 400 °C, probably reflecting the negative CTE along the basal planes of a graphite crystal. In situ measurement of the dimensional changes of the specimens before graphitization process of these products during heat treatment up to 2400 °C was also attempted. For the isotropic product, two-step of thermal shrinkage was observed, but for the nipple electrode, growth above 1400 °C was clearly observed, resulting from the “puffing” of calcined needle cokes. For the puffing, it was found that there was clear anisotropy in the extruded direction of the nipple electrode.