A. V. Dolbin

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.

The specific heat and the radial thermal expansion of bundles of single-walled carbon nanotubes

The specific heat at constant pressure C(T) of bundles of single-walled carbon nanotubes (SWNTs) closed at their ends has been investigated in the temperature interval of 2–120 K. It is found that the curve C(T) has features near 5, 36, 80, and 100 K. The experimental results on the C(T) and the radial thermal expansion coefficient αR(T) of bundles of SWNTs oriented perpendicular to the sample axis have been compared. It is found that the curves C(T) and αR(T) exhibit a similar temperature behavior at T > 10 K. The temperature dependence of the Gruneisen coefficient γ(T) has been calculated. The curve γ(T) also has a feature near 36 K. Above 36 K the Gruneisen coefficient is practically independent of temperature (γ ≈ 4). Below 36 K, γ(T) decreases monotonically with lowering temperature and becomes negative at T < 6 K.

Tunneling effects in the kinetics of helium and hydrogen isotopes desorption from single-walled carbon nanotube bundles

The kinetics of desorption both helium isotopes and molecules of hydrogen and deuterium from open-ended or γ-irradiated single-walled carbon nanotube bundles was investigated in temperature range of 10–300 K. The gases desorption rates obey the Arrhenius law at high temperatures, deviate from it with temperature reduction and become constant at low temperatures. These results indicate the quantum nature of gas outflow from carbon nanotube bundles. We had deduced the crossover temperature below which the quantum corrections to the effective activation energy of desorption become significant. This temperature follows linear dependence against the inverse mass of gas molecule and is consistent with theoretical prediction.

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 500 °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 16 K 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.

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 (1.2 MeV) of 60Co ((1.5–1.7) × 107 rad) radiation at room temperature in deuterium, nitrogen, and oxygen atmosphere (P = 1 atm), 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 12–1270 K. 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.

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 290 K. It is found that a thermally activated mechanism with an estimated activation energy Ea ≈ 466 K predominates in the H2 sorption kinetics of an MCM-41 sample for temperatures of 60–290 K. For temperatures of 17–60 K 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 8–17 K 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 Em ≈ 21.2 K. At temperatures below 8 K the diffusion coefficients of H2 depend weakly on temperature. This presumably corresponds to a cha...

Novel carbon materials: New tunneling systems (Review Article)

This review covers recent achievements in the studies of quantum properties of the novel carbon materials (fullerite C60 and bundles of single-walled nanotubes (SWNT)) saturated with such light-mass species as helium isotopes, the homonuclear molecular hydrogens, and neon. It is shown that even some heavy dopants demonstrate kinetic phenomena, in which coherent effects play an essential role. Two theoretical concepts are surveyed which have been suggested for the explanation of the anomalous phenomena in saturation kinetics and linear thermal expansion of doped C60. Most unusual effects have been also observed in the low-temperature radial expansion of bundles of single-walled carbon nanotubes saturated with the helium isotopes. First, it was shown that low-temperature radial expansion of pure SWNT is negative, i.e., a nanotube shrinks with warming. Second, saturation of SWNT bundles with the helium isotopes entails a huge increase of the negative expansion effect, when the dopant is He. So far, no detail...

The effect of the thermal reduction on the kinetics of low-temperature 4He sorption and the structural characteristics of graphene oxide

The kinetics of the sorption and the subsequent desorption of 4He by the starting graphite oxide (GtO) and the thermally reduced graphene oxide samples (TRGO, Treduction = 200, 300, 500, 700 and 900 °C) have been investigated in the temperature interval 1.5–20 K. The effect of the annealing temperature on the structural characteristics of the samples was examined by the x-ray diffraction (XRD) technique. On lowering the temperature from 20 to 11–12 K, the time of 4He sorption increased for all the samples, which is typically observed under the condition of thermally activated diffusion. Below 5 K the characteristic times of 4He sorption by the GtO and TRGO-200 samples were only weakly dependent on temperature, suggesting the dominance of the tunnel mechanism. In the same region (T < 5 K) the characteristic times of the TRGOs reduced at higher temperatures (300, 500, 700 and 900 °C) were growing with lowering temperature, presumably due to the defects generated in the carbon planes on removing the oxygen f...

Sorption of hydrogen by silica aerogel at low-temperatures

The programmed thermal desorption method is used at temperatures of 7–95 K to study the sorption and subsequent desorption of hydrogen by a sample of silica aerogel. Physical sorption of hydrogen owing to the weak van-der-Waals interaction of hydrogen molecules with the silicon dioxide walls of the pores of the sample was observed over the entire temperature range. The total capacity of the aerogel sample for hydrogen was ∼1.5 mass %. It was found that when the sample temperature was lowered from 95 to 60 K, the characteristic sorption times for hydrogen by the silica aerogel increase; this is typical of thermally activated diffusion (Ea ≈ 408 K). For temperatures of 15–45 K the characteristic H2 sorption times depended weakly on temperature, presumably because of the predominance of a tunnel mechanism for diffusion over thermally activated diffusion. Below 15 K the characteristic sorption times increase somewhat as the temperature is lowered; this may be explained by the formation of a monolayer of H2 mo...