Thermoelectric Power, Electrical Resistance, and Crystalline Structure of Carbons

Abstract

The thermoelectric power $\Theta$ and the electric resistance $R$ of soft and hard carbons were investigated as functions of heat-treatment temperature $T_{\mathrm{ht}}$ from 1000°C to 3100°C at three ambient temperatures $T$ equal to 90°K, 305°K, and 573°K. The crystalline structure was studied by means of x-ray powder diffraction technique. For a given $T$, with increase of $T_{\mathrm{ht}}$ the thermoelectric power of soft carbons goes through a flat minumum at $T_{\mathrm{ht}}\sim 1400°$C, increases quite fast to a maximum at about 2100°C, and subsequently drops down again and continues to do so up to the highest $T_{\mathrm{ht}}$. The electric resistance is only very slowly decreasing (plateau) up to $T_{\mathrm{ht}}\sim 2000°$C, with a subsequent rapid decrease up to the highest $T_{\mathrm{ht}}$ investigated. The positions of the maximum in $\Theta$ and of the corresponding knee in resistance (end of plateau) shifts to higher heat treatments with decrease of ambient temperature $T$. The thermoelectric power is proportional to temperature $T$ in the range of heat treatments below the $T_{\mathrm{ht}}$ corresponding to the maximum in $\Theta$. The observed dependence of $\Theta$ and of $R$ on $T_{\mathrm{ht}}$ and $T$ are in good agreement with Mrozowski's energy band scheme for carbons and graphites. A hard carbon prepared from phenol-benzaldehyde resin shows a two-stage graphitization with $\frac{1}{8}$ of the crystalline mass beginning to graphitize at $T_{\mathrm{ht}}\sim 2200°$C and the remainder only above $T_{\mathrm{ht}}=2800\mathrm{}°$C. Correspondingly the resistance curves show two plateaus ending one at $T_{\mathrm{ht}}\sim 2200°$C and the other at $T_{\mathrm{ht}}\sim 2800°$C. The dependence of $\Theta$ on $T_{\mathrm{ht}}$ is somewhat similar to that found in soft carbons.