Thermoelectric Power, Electrical Resistance, and Crystalline Structure of Carbons

Abstract

The thermoelectric power $\ensuremath{\Theta}$ and the electric resistance $R$ of soft and hard carbons were investigated as functions of heat-treatment temperature ${T}_{\mathrm{ht}}$ from 1000\ifmmode^\circ\else\textdegree\fi{}C to 3100\ifmmode^\circ\else\textdegree\fi{}C at three ambient temperatures $T$ equal to 90\ifmmode^\circ\else\textdegree\fi{}K, 305\ifmmode^\circ\else\textdegree\fi{}K, and 573\ifmmode^\circ\else\textdegree\fi{}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 ${T}_{\mathrm{ht}}\ensuremath{\sim}1400\ifmmode^\circ\else\textdegree\fi{}$C, increases quite fast to a maximum at about 2100\ifmmode^\circ\else\textdegree\fi{}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 ${T}_{\mathrm{ht}}\ensuremath{\sim}2000\ifmmode^\circ\else\textdegree\fi{}$C, with a subsequent rapid decrease up to the highest ${T}_{\mathrm{ht}}$ investigated. The positions of the maximum in $\ensuremath{\Theta}$ 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 $\ensuremath{\Theta}$. The observed dependence of $\ensuremath{\Theta}$ 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 ${T}_{\mathrm{ht}}\ensuremath{\sim}2200\ifmmode^\circ\else\textdegree\fi{}$C and the remainder only above ${T}_{\mathrm{ht}}=2800\ifmmode^\circ\else\textdegree\fi{}$C. Correspondingly the resistance curves show two plateaus ending one at ${T}_{\mathrm{ht}}\ensuremath{\sim}2200\ifmmode^\circ\else\textdegree\fi{}$C and the other at ${T}_{\mathrm{ht}}\ensuremath{\sim}2800\ifmmode^\circ\else\textdegree\fi{}$C. The dependence of $\ensuremath{\Theta}$ on ${T}_{\mathrm{ht}}$ is somewhat similar to that found in soft carbons.