V. G. Gavrilko

On the polyamorphism of fullerite-based orientational glasses.

The dilatometric investigation in the temperature range of 2–28 K shows that a first-orderpolyamorphous transition occurs in the orientational glasses based on C60 doped with H2, D2 andXe. A polyam ...

Radial thermal expansion of single-walled carbon nanotube bundles at low temperatures

For the first time, the linear coefficient of radial thermal expansion is measured on a system of carbon single-walled nanotube (SWNT) bundles at low temperatures (2.2–120K). The measurements are performed using a dilatometer with a sensitivity of 2×10−9cm. A cylindrical sample 7mm high and 10mm in diameter was obtained by compressing powder. The resulting bundles of nanotubes are oriented perpendicular to the sample axis. The starting powder consisted of over 90% SWNTs with outer diameter 1.1nm, the length varying in the range 5–30μm. A change of sign of the radial thermal expansion coefficient at 5.5K is observed.

Thermal expansion and polyamorphism of N2–C60 solutions

The linear coefficients alpha(T) of N-2-C-60 solutions with 9.9% and 100% of the C-60 lattice thermal expansion interstitials filled with N-2 are investigated in the interval 2.2-24 K. The dependen ...

Kinetics of 3He, 4He, H2, D2, Ne, and N2 sorption by bundles of single-walled carbon nanotubes. Quantum effects

This is a study of the low temperature kinetics of the sorption and subsequent desorption of 3He, 4He, H2, D2, Ne, and N2 by bundles of single-wall carbon nanotubes with closed ends (c-SWNT). The characteristic times of the sorption and desorption processes are the same to within the experimental error. Annealing a sample of bundles at 500u2009°C greatly reduces the characteristic times and changes their temperature dependences. The effect of annealing decreased with increasing molecular weight of the dissolved gas. At temperatures below 16u2009K the sorption times for 3He, 4He, H2, and D2 depends weakly on temperature, which suggests a tunnel character of the sorption for these impurities by nanotube bundles. The effect of γ-irradiation of nanotube bundles on the sorption and desorption of hydrogen is qualitatively similar to the effect of annealing.

Diffusion of H2 and Ne impurities in fullerite C60. Quantum effects

The kinetics of sorption and subsequent desorption of normal H2 and Ne gases in C60 powder has been investigated in the temperature range 12–292u2009K using the method of direct pressure measurement. The measured characteristic times for filling the octahedral interstitial sites of fullerite with gas molecules were used to obtain the temperature dependences of the diffusion coefficients in fullerite for H2 and Ne. Upon cooling down below room temperature the diffusion coefficients of the impurities were found to initially decrease in both solid state solutions and then rise steeply below 90u2009K (H2) or 100u2009K (Ne). At the lowest temperatures reached in the experiments, the temperature dependence of the diffusion leveled. The features revealed are attributed to the competition between the thermally activated diffusion, which is dominant at comparatively high temperatures, and the quantum diffusion, which prevails at low temperatures.

Hydrogen sorption by the bundles of single-wall carbon nanotubes, irradiated in various gas media

The effect of radioactive irradiation on H2 sorption by bundles of single-wall carbon nanotubes (SWNTs) has been investigated in various gas media. The samples were irradiated with γ-quanta (1.2u2009MeV) of 60Co ((1.5–1.7)u2009×u2009107u2009rad) radiation at room temperature in deuterium, nitrogen, and oxygen atmosphere (Pu2009=u20091u2009atm), and in a vacuum. The processes of H2 sorption and desorption in the SWNT bundles were investigated before and after irradiation in the temperature interval 12–1270u2009K. It is found that irradiation in a gas environment has a significant effect both on the low-temperature H2 sorption induced by the weak physical interaction, and the chemical H2 sorption by the SWNT bundles. A phenomenological model has been proposed to explain the defect generation in carbon nanotubes irradiated in gas media.

The effect of glass transition in fullerite C60 on Ar impurity diffusion

The kinetics of sorption and subsequent desorption of argon gas by powdered fullerite C60 has been investigated in the temperature interval 58–290u2009K. The temperature dependence of the Ar diffusion coefficients in fullerite has been obtained using measured characteristic times of sorption. The diffusion coefficients for Ar decrease monotonically with decreasing temperature in the entire temperature range, which corresponds to the thermally activated diffusion of Ar atoms in fullerite. The glass transition in fullerite induces an order-of magnitude decrease in the activation energy of Ar diffusion in fullerite. This appears to be due to new paths that appeared as a result of the glass transition, in which the barriers separating the interstitial voids in the C60 lattice are significantly lower.

Quantum effects in the sorption of hydrogen by mesoporous materials

The sorption and desorption of hydrogen by mesoporous MCM-41 silicate material is studied at temperatures ranging from 6.8 to 290u2009K. It is found that a thermally activated mechanism with an estimated activation energy Eau2009≈u2009466u2009K predominates in the H2 sorption kinetics of an MCM-41 sample for temperatures of 60–290u2009K. For temperatures of 17–60u2009K the diffusion coefficient of H2 molecules in MCM-41 is almost entirely temperature independent, which is typical when a tunneling diffusion mechanism predominates over the thermally activated mechanism. Within the 8–17u2009K range, a change in the mobility of H2 molecules in the channels of MCM-41 is observed that appears to correspond to the formation of a monolayer (or its destruction during heating) and subsequent layers of hydrogen which have condensed on the inner surfaces of the channels. This process has an activation energy Emu2009≈u200921.2u2009K. At temperatures below 8u2009K the diffusion coefficients of H2 depend weakly on temperature. This presumably corresponds to a cha...

The effect of the temperature of graphene oxide reduction on low-temperature sorption of 4He

The sorption of 4He by graphene oxide powders thermally reduced at T = 200, 300, 500, 700, 900u2009°C has been investigated in the interval 1.5–290u2009K. The measured dependence of the quantity of sorbed helium upon the reduction temperature shows up as a nonmonotonic curve. The highest quantities of helium were sorbed by the samples reduced at T = 300 and 900u2009°C. It is assumed that the thermal reduction of graphite oxide by heating it to 300u2009°C causes evaporation of the water intercalated in the spacings of the carbon layers, this results in exfoliation of the graphene planes, which enhances the sorptive capacity. Heating the samples to 900u2009°C generates numerous defects in the carbon planes, as a result, the interlayer spacings become accessible for sorption, which enhances the sorptive capacity.

Quantum effects in the sorption kinetics of 4He by mesoporous materials

Sorption and desorption of 4He by a mesoporous silicate material MCM-41 was studied in the temperature range of 1.5–290u2009K. It was shown that for T = 25–290u2009K the thermal activation mechanism is dominant in the sorption kinetics of 4He atoms by an MCM-41 sample. Its activation energy was estimated as Ea ≈ 164.8u2009K. For T = 12–23u2009K, the diffusion of 4He atoms in the MCM-41 was practically independent of temperature, which typically occurs when the tunnelling mechanism of diffusion dominates over the thermally activated one. A change in the mobility of 4He atoms in MCM-41 channels was observed at T = 6–12u2009K, which may be indicative of the formation upon cooling (or decay upon heating) of a 4He monolayer and subsequent multilayers on the inner surfaces of the channels. Below 6u2009K, the diffusion coefficients of 4He are only weakly temperature dependent, which may be attributed to the behavior of quantum 4He liquid in the MCM-41 channels covered with several layers of 4He atoms.