Low temperature microhardness of Xe-intercalated fullerite C₆₀

The Vickers microhardness of Xe-intercalated polycrystalline fullerite C₆₀ (XexC₆₀, x ≃ 0.35) is measured in a moderately low temperature range of 77 to 300 K. A high increase in the microhardness of the material (by a factor of 2 to 3) as compared to that of pure C₆₀ single crystals is observed. It is shown that the step-like anomaly in the temperature dependences of the microhardness of pure C₆₀ single crystals recorded under the orientational fcc-sc phase transition (Tc ≃ 260 K) is also qualitatively retained for XexC₆₀, but its onset is shifted by 40 K towards lower temperatures and the step becomes less distinct and more smeared. This behavior of ̅NV(T) correlates with x-ray diffraction data, the analysis of which revealed a considerable influence of xenon interstitial atoms on the peculiar features of fullerite thermal expansion due to orientational phase transitions (see the paper by A.I. Prokhvatilov et al. in this issue)

Intercalation of fullerite C₆₀ with N₂ molecules. An investigation by x-ray powder diffraction

The lattice parameter a of fullerite C₆₀ intercalated with N₂ molecules is investigated in the temperature interval 6–295 K by x-ray diffraction. It is found that the interstitial molecular N₂ has a considerable effect on both the temperatures, Tc of the orientational phase transition and Tg of the orientational glass formation. Hysteresis of a(T) has been detected in the Tc and Tg regions, besides, the abrupt change in the volume over the region defining Tc. Complete intercalation of C₆₀ with N₂ molecules results in a 0.2% increase in the lattice parameter, which persists over the whole temperature range. Evidence is also obtained that the interstitial guest molecular N₂ induces a slight deformation of the cubic symmetry of the host C₆₀ lattice

Low-temperature heat capacity of fullerite C₆₀ doped with nitrogen

The heat capacity Cm of polycrystalline fullerite C₆₀ doped with nitrogen has been measured in the temperature interval 2–13 K. The contributions to the heat capacity from translational lattice vibrations (Debye contribution), orientational vibrations of the C₆₀ molecules (Einstein contribution), and from the motion of the N₂ molecules in the octahedral cavities of the C₆₀ lattice have been estimated. However, we could not find (beyond the experimental error limits) any indications of the first-order phase transformation that had been detected earlier in the dilatometric investigation of the orientational N₂–C₆₀ glass. A possible explanation of this fact is proposed

The effect of the noncentral impurity-matrix interaction upon the thermal expansion and polyamorphism of solid CO–C₆₀ solutions at low temperatures

Orientational glasses with CO molecules occupying 26 and 90% of the octahedral interstitial sites in the C₆₀ lattice have been investigated by the dilatometric method in a temperature interval of 2.5–22 K. At temperatures 4–6 K the glasses undergo a first-order phase transition which is evident from the hysteresis of the thermal expansion and the maxima in the temperature dependences of the linear thermal expansion coefficients α (T), and the thermalization times τ₁(T) of the samples. The effect of the noncentral CO–C₆₀ interaction upon the thermal expansion and the phase transition in these glasses was clarified by comparing the behavior of the properties of the CO–C₆₀ and N₂–C₆₀ solutions

Hysteretic phenomena in Xe-doped C₆₀ from x-ray diffraction

Polycrystalline fullerite С₆₀ intercalated with Xe atoms at 575 K and a pressure of 200 MPa was studied by powder x-ray diffraction. The integrated intensities of a few brighter reflections have been utilized to evaluate the occupancy of the octahedral interstitial sites in С₆₀ crystals, which turned out to be (34±4) %, and in good agreement with another independent estimate. It is found that reflections of the (h00) type become observable in Xe-doped С₆₀. The presence of xenon in the octahedral sites affects both the orientational phase transition as well as the glassification process, decreasing both characteristic temperatures as well as smearing the phase transition over a greater temperature range. Considerable hysteretic phenomena have been observed close to the phase transition and the glassification temperature. The signs of the two hysteresis loops are opposite. There is reliable evidence that at lowest temperatures studied the thermal expansion of the doped crystal is negative under cool-down

On the polyamorphism of fullerite-based orientational glasses

The dilatometric investigation in the temperature range of 2–28 K shows that a first-order polyamorphous transition occurs in the orientational glasses based on C₆₀ doped with H₂, D₂ and Xe. A polyamorphous transition was also detected in C₆₀ doped with Kr and He. It is observed that the hysteresis of thermal expansion caused by the polyamorphous transition (and, hence, the transition temperature) is essentially dependent on the type of doping gas. Both positive and negative contributions to the thermal expansion were observed in the low-temperature phase of the glasses. The relaxation time of the negative contribution occurs to be much longer than that of the positive contribution. The positive contribution is found to be due to phonon and libron modes, whilst the negative contribution is attributed to tunneling states of the C₆₀ molecules. The characteristic time of the phase transformation from the low-T phase to the high-T phase has been found for the C₆₀–H₂ system at 12 K. A theoretical model is proposed to interpret these observed phenomena. The theoretical model proposed, includes a consideration of the nature of polyamorphism in glasses, as well as the thermodynamics and kinetics of the transition. A model of noninteracting tunneling states is used to explain the negative contribution to the thermal expansion. The experimental data obtained is considered within the framework of the theoretical model. From the theoretical model the order of magnitude of the polyamorphous transition temperature has been estimated. It is found that the late stage of the polyamorphous transformation is described well by the Kolmogorov law with an exponent of n = 1. At this stage of the transformation, the two-dimensional phase boundary moves along the normal, and the nucleation is not important

Low-temperature thermal expansion of pure and inert gas-doped fullerite C₆₀

The low temperature (2-24 K) thermal expansion of pure (single-crystal and polycrystalline) C₆₀ and polycrystalline C₆₀ intercalated with He, Ne, Ar, and Kr has been investigated using the high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations ΔL(t) on heating by ΔT shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C₆₀ molecules. It is assumed that the reorientation is of a quantum character. The inert gas impurities affect the reorientation of the C₆₀ molecules very strongly, especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr- and He-C₆₀ solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems

The optimization of hull forms using viscous and wave resistance theory

SIGLELD:6026.982(NMI-R--151) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The use of resistance component measurements as diagnostic tools in assessing the resistance qualities of ship models

SIGLELD:6026.982(NMI-R--153) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A convenient method for estimating wave resistance, and its variation with small changes of hull shape, for a wide range of ship types

SIGLELD:6026.982(NMI-R--129). / BLDSC - British Library Document Supply CentreGBUnited Kingdo