Influence of dopants, particularly carbon, onβ-rhombohedral boron

Abstract

Due to the high affinity of carbon to boron, the preparation of carbon-free boron is problematic. Even high-purity (6 N) β-rhombohedral boron contains 30-60 ppm of C. Hence, carbon affects the boron physical properties published so far more or less significantly. We studied well-defined carbon-doped boron samples based on pure starting material carefully annealed with up to about 1% C, thus assuring homogeneity. We present and discuss their electrical conductivity, optical absorption, luminescence and phonon spectra. Earlier attempts of other authors to determine the conductivity of C-doped boron are revised. Our results allow estimating the effects of oxygen and iron doping on the electrical conductivity using results taken from literature. Discontinuities at low T impair the electronic properties.

Thermoelectric Borides: Review and Future Perspectives

“…Boron has two allotropes referred to as 𝛼 and 𝛽 in which the 𝛼allotrope is unstable and dissociates into 𝛽-boron at high temperatures. The peak zT of 𝛽-boron is obtained at high temperatures, so that it may be used at temperatures above 1700 K. [46,48] The rest of this review deals with 𝛽-boron and its possible thermoelectric applications.…”

“…Moreover, atoms tend to fill some specific vacancies depending on the size of the vacancy and the charge of the dopant. [46,177] For example, occupation of A 1 sites is directly (Cu,Co,V,Mg,Li,Mn/Sc,Cr; [ 181] Al, Si; [46] Fe; [ 46,179] Zr [45] ) correlated with 𝜎, while S is affected by the atoms in the D site. On the other hand, by comparing the variations in the electrical and thermal transport properties, the location of each atom can be determined.…”

“…[47,177] Seebeck coefficient can be either changed from negative to positive, or its absolute value can be increased by doping. [46] Since Cr and Fe occupy A 1 and D sites, they can be both doped to form Cr 0.8 Fe 0.2 B 105 with enhanced Seebeck coefficient. Moreover, CrB 105 is n-type, which is promising for thermoelectric applications.…”

“…[45] The most striking discovery in this family is the switching from p-to n-type conduction upon doping with V, Zr, or Cr. [46][47][48] In addition, elemental boron is a promising dopant for commercially available thermoelectric materials. [49][50][51][52][53][54] This review provides a detailed analysis of synthetic routes, thermoelectric properties of different families of borides as well as studies on state-of-the-art TE materials that have used elemental boron and boron compounds as dopants.…”

Phase Transitions in Boron Carbide

“…A few samples, whose C content was clearly below B 11 C, were unambiguously characterized as boron carbide, albeit with a large error margin of lattice parameters (see [13] and references cited therein). Otherwise, β-rhombohedral boron with clearly higher C content has been obtained (see [14] and references cited therein). Therefore, the coexistence of both structures in a certain range cannot be excluded.…”