Physical properties of superhard and ultrahard fullerites created from solid C60 by high-pressure-high-temperature treatment

60 in the pressure range and the temperature range were studied. These properties are the specific gravity, specific heat in the range , sound velocities, elastic moduli, electrical properties, resistance to uniaxial stress and stability against oxidation. These properties are different from those of diamond and other carbon forms. The hardness of ultrahard fullerites exceed the hardness of diamond. The structures of the samples are described on the basis of X-ray data. Received: 27 September 1996/Accepted: 19 December 1996     

On the thermodynamic properties of higher and smaller fullerites

The temperature dependences of the saturation vapor pressure of C₉₆ and C₃₆ fullerites and their properties along the sublimation curves are calculated using a correlation method of unsymmetrized self-consistent field that allows for strong anharmonicity of the lattice vibrations. The calculation is performed in terms of the Girifalco intermolecular potential with parameters recently determined for these fullerenes. Since experimental data on C₉₆ and C₃₆ fullerites are unavailable, the results of our calculations are compared with our results obtained earlier for the most commonly encountered fullerite C₆₀. The specific features in the dependences of the properties of C₉₆ and C₃₆ fullerites on the number of atoms per molecule are revealed.     

Converters for ultrahard x-rays

A linear theory is constructed to explain the operation of single- and multipass converters for transforming the energy of beam electrons into bremmstrahlung within the spectral interval 10–100 keV (ultrahard x-rays, or UHX-radiation) in a cold-target approximation. Expressions are obtained that make it possible to calculate the spectral and energy characteristics of such radiation. The completed calculations make it possible to give recommendations on choosing target material and thickness and deceleration regimes from the viewpoint of maximizing the output of UHX-radiation energy. Institute of High-Current Electronics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 73–78, December, 1995.     

Femtosecond ablation of ultrahard materials

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm² fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 20 March 2002     

Elastic properties of CsCN

The temperature dependence of the elastic constants of CsCN exhibiting the CsCl structure was measured with ultrasonic and neutron techniques. The room temperature values were found to bec ₁₁=18.8,c ₁₂=10.7 andc ₄₄=2.95 10¹⁰ dyn/cm². The sound waves inT _2g andE _g symmetries exhibit anomalous temperature effects which are similar but definitively weaker than those in the NaCl type cyanides. In addition we measured the acoustic phonon dispersion along [100] and [110]. These results are compared with recent molecular dynamics calculations.     

Elastic properties of FeSi

Measurements of the sound velocities in a single crystal of FeSi were performed in the temperature range 4–300 K. Elastic constants C ₁₁ and C ₄₄ deviate from a quasiharmonic behavior at high temperature; on the other hand, elastic constants C ₁₂ increases anomalously in the entire temperature range, indicating a change in the electron structure of this material.     

Channeling of fast particles in fullerites

A theory of channeling of relativistic electrons and positrons as well as positively and negatively charged ions in molecular crystals of fullerenes (fullerites) is developed. The crystal potentials are calculated, and the spatial and angular distributions of beams of particles propagating along principal crystallographic directions are determined. A method is developed for taking into account the effect of incoherent scattering on the channeling process.     

Polyamorphic transition in amorphous fullerites

Samples of amorphous fullerites have been prepared by mechanoactivation (grinding in a ball mill), and their structure has been studied using neutron diffraction. It has been found that the amorphous fullerites subjected to high-temperature (600–1700°C) annealing undergo a polyamorphic transition from the molecular glass to the atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles.     

Elastic properties of magnetic materials

This review discusses the theoretical aspects of magnetoelastic coupling with emphasis on the magnetic perturbation of elastic properties. The basic theory of magnetostriction is set out with application to ferromagnets, ferrimagnets and antiferromagnets, and is followed by a discussion of the physical origin of the magnetoelastic coupling coefficients in both localized and itinerant magnetic systems. Magnetic contributions to elastic compliance are then discussed and sound velocity anomalies near magnetic phase transitions investigated, including the cooperative Jahn-Teller limit for which the acoustic mode itself drives a structural transition even when magnetic ordering does not occur. The review concludes with discussion of magnetoelastic (or mixed magnon-phonon) waves in low temperature magnetically ordered phases and with a study of local striction phenomena in magnetically dilute materials. The latter leads to a recognition of internal rearrangement modes which may also be present in concentrated magnetic systems, and which may or may not couple significantly to bulk homogenous strain.     

Elastic properties of aluminum nitride

The elastic moduli and their temperature and pressure derivatives for sintered, isotropic, polycrystalline aluminum nitride ceramic have been determined. The temperature derivatives of the elastic moduli are somewhat smaller than those of other wurtzite structure semiconductors, and the pressure derivative of the shear modulus is very small. The mode Grüneisen gamma of the shear mode is essentially zero, while the elastic Grüneisen constant agrees well with the thermodynamic Grüneisen constant. Both the bulk and shear modulus conform to the Keyes relationship.     

Elastic properties of covalent glasses

The dependence of bulk moduli on compositions has been studied for chalcogenide glasses and a-Si:H films. The modulus is nearly constant when the average coordination number (Z) is smaller than 2.7, and increases drastically when Z ⪆ 2.7. This dependence differs from the theoretical result, which predicts the elastic phase transition at Z = 2.4. Origins of this disagreement are discussed.     

Elastic properties of lanthanum manganites

We give a brief overview of the data on elastic properties of the CMR manganites published during last decade. The main emphasis is put on the results obtained for single crystals.     

Elastic properties of layered crystals

The characteristic features of the elastic properties of layered crystals and their dependence on temperature and pressure are analyzed. The relations between the elastic constants of hexagonal layered crystals are given. It is shown that the anomalous behavior of the elastic constants in the temperature region of a phase transition affects both the magnitude and sign of the thermal expansion coefficients of layered crystals. From analyzing the pressure and temperature dependences of the elastic constants, it is found that the anharmonicity of the bonding forces between the layers is much greater than the anharmonicity of the intralayer forces. The contribution from thermal expansion to the variations of the elastic constants with temperature is estimated.     

Elastic properties of HTSC ceramics

Elastic properties of ceramics of various high-temperature superconductor (HTSC) types are considered based on experimental data. The effect of porosity of HTSC ceramics is analyzed.     

单壁纳米管的弹性性质

基于第一性原理计算了一系列单壁碳纳米管(椅型、锯齿型)和氮化硼锯齿型纳米管的杨氏模量.用实际的数值结果显示了杨氏模量与纳米管管径之间的依赖关系.还讨论了各类纳米管卷曲形变能以及平衡基矢长度a随管型的变化. 关键词： 单壁纳米管 平衡基矢长度 杨氏模量 形变能     

Elastic properties of liquid crystals

The structural and elastic properties of 4-n′-pentyl-4′-cyanobiphenyl (5CB) in the nematic liquid-crystal phase are investigated in the framework of the statistical-mechanical theory and the molecular dynamics method.