Low temperature electronic transport properties of boron carbide

Seebeck-coefficient, dc conductivity, ac conductivity from 10kHz to 1 GHz and dielectric constant measurements have been made on hot-pressed samples of boron carbide B/sub 1-x/C/sub x/ in the compositional range 0.13< x <0.2, between room temperature and 4.2 K. The dielectric constant measurements, performed over five orders of magnitude of the frequency, permit to separate bulk effects from grain or twin boundary effects. The positive Seebeck coefficient was always found to decrease with decreasing temperature down to values of 10 to 30 μV/K around 5 to 20 K depending on the sample. The dc conductivity was found in most of the cases to be of the order of 10-9 to 10-11 (Ωxcm)-1 at the same temperatures. The small thermopowers and small hopping conductivities are both consistent with the motion of a large density of holes hopping close to the Fermi level. This conclusion is in agreement with the frequency dependence of the conductivity measured at low temperatures (4.2 K): the conductivity follows, over 5 orders of magnitude of frequency, the usual sigma = sigma0 ω/sup s/ law with sapprox. =0.9, independent of the composition. The prefactor sigma0 is only 6 times lower at the composition x = 0.13 than at the composition x = 0.2. On the contrary, the room temperature conductivity is 23 times higher at the composition 0.13 (B13C2) than at the composition 0.2 (B3$C). Presumably inhomogeneities (grain and twin boundaries) play a non negligeable role in the transport properties of hot pressed samples of boron carbide