S.G. Buga

Structures and physical properties of superhard and ultrahard 3D polymerized fullerites created from solid C60 by high pressure high temperature treatment

Abstract Superhard and ultrahard phases of C60 were synthesized by quenching at high pressures up to 13 GPa and high temperatures in the 300–2100 K range. The structures of the samples are discussed on the basis of X-ray and Raman spectra and electron microscopy data. The following physical properties of hard samples were studied: specific gravity; specific heat in the range 400–600 K; sound velocities; elastic moduli; electrical properties; resistance to uniaxial stress; stability against oxidation. These properties are different from those of diamond and other carbon forms. The hardness of ultrahard fullerites exceeds the hardness of diamond.

Electronic properties of single-crystal diamonds heavily doped with boron

Single-crystal diamonds with characteristic sizes of 2–7 mm doped with boron in the concentration range 1019–1020 cm−3 have been grown by the temperature gradient method at high static pressures. The temperature dependence of the resistance R of the synthesized single crystals has been measured in the range 0.5 K < T < 297 K. An activated dependence R(T) with an activation energy of about 50 meV is observed in the range from room temperature to T ≈ 200 K. At temperatures below approximately 50 K, the temperature dependence of the conductivity for heavily doped crystals is proportional to T1/2, which is characteristic of degenerate semiconductors with a high number of defects.

Thermoelectric properties of bismuth telluride nanocomposites with fullerene

A nanocomposite material consisting of Bi2Te3 nanocrystals 30 nm in size coated with a C60 molecule layer is obtained. The Hall effect and thermoelectric properties of the composite are studied. The acceptor effect of fullerene in both p- and n-type Bi2Te3 is detected. Nanocomposite material properties are modified due to the formation of layers of fullerene charged molecules: the nanocomposite thermopower increases, while the thermal conductivity decreases.

Fast-response thermistors made of synthetic single-crystal diamonds

Miniature thermistors are produced from boron-doped synthetic single-crystal diamonds grown under pressure using the thermal gradient method. It is shown that heavily doped diamonds with a boron concentration of 1019 cm−3 or higher are most suitable for this purpose. In the temperature range of 300–700 K, coefficient β = ln(R1/R2)/(1/T1 − 1/T2) is 2500 K. The characteristic response time of temperature-sensitive elements based on crystals with dimensions of 1 × 1 × 0.3 mm is ∼100 µs; i.e., they can be used in monitoring systems with a response speed of up to 10 kHz.

Heat capacity of bulk boron-doped single-crystal HPHT diamonds in the temperature range from 2 to 400 K

The heat capacity, Cp, of boron-doped single-crystal diamonds grown by the temperature gradient method was studied. The boron contents were < 1016, ~ 1018, and ~ 1020 cm–3. The heat capacity data for all tested crystals match well (within the measurement accuracy 1%) in the temperature range of 150–400 K and obey the Debye law. At low temperatures the heat capacity follows linear law possibly due to metallic inclusions in diamond bulk. Using this data the amount of metal can be calculated for each sample.

Thermoelectric properties of nanostructured Bi-Sb-Te doped with C60

New nanocomposite thermoelectric material composed from nanocrystallites of Bi-Sb-Te alloys covered by C60 molecules has been synthesized and studied. The fullerene molecules provide additional thermal phonons blocking. The observed effects give opportunities for optimizations of transport properties of thermoelectric materials. The figure of merit ZT has increased in relation to starting material by 30% and equals to 1.16 for the optimized thermoelectric nanocomposite of p-type.