Serrated plastic deformation in LiF single crystals at high temperatures

The serrated plastic flow in LiF single crystals has been studied in the mode of active deformation at high temperatures (T = 573–1093 K). The parameters of the jumps in the deforming stresses (normalized amplitude and relaxation time of stress oscillations) were determined at the stage of strain softening under conditions of uniaxial compression and tension. It was shown that the jump parameters are essentially dependent on the type of the stressed state and the deformation temperature. The activation energy of serrated deformation in shear bands was established to be close to the migration energy for cation vacancies.     

Dislocation behaviour in annealed and aged LiF crystals

Parallel investigation of the magnitude of the critical strear stress τ₀ and dynamic characteristics of individual screw dislocations at temperatures 4.2, 77, and 300 K has been carried out in LiF crystals after annealing and after long aging at room temperature. It has been established that in impure LiF crystals, even at such low temperatures as room temperature (0.25T_melt), diffusion processes take place which lead to quantitative and qualitative changes of the structure of obstacles determining macroscopic and microscopic mechanical properties.     

Dielectric relaxation in vitreous borates at low temperatures

Dielectric relaxation measurements have been made in vitreous alkali borates from 4.2 to 300 K, and from 10² to 10⁵ s^?1. Two types of relaxation are present: the first, at about 25 K (10³ s^?1), is intimately related to the presence of tetrahedral BO₄ units in distorted positions, and the second, at about 160 K (10³ s^?1), is correlated with water that is present in small quantities in glass.     

Specific heat of ethanol at low temperatures

We present here new specific heat measurements at low temperatures (2–20 K) of the different phases of ethanol, characterized by the same calorimetric set-up at higher temperatures. We have extended and improved earlier measurements by implementing higher-accuracy calorimetric methods at low temperatures (using two complementary versions of the thermal relaxation method), as well as at higher temperatures (using a quasi-adiabatic, continuous method). The quantitatively very similar low temperature properties and glass-transition features of both structural glass and orientationally-disordered crystal of ethanol provide clear evidence that the lack of long-range crystalline order typical of amorphous solids is an unimportant factor regarding the universal properties of glasses. We have also employed these new measuring methods to study the possible effect of water impurities on the specific heat of the different solid phases of ethanol, and to study possible variations in the specific heat between different found phases of the monoclinic crystal of ethanol.     

Some features of plastic deformation of LiF and NaCl single crystals at high temperatures

The deformation of the (001) plane of LiF and NaCl by indentation and scratching at high temperatures was performed. The dislocation patterns arising from deformation were sharply different from those observed at room temperature. It was proposed the explanation of these results. The model of plastic flow in (001) indentation and scratching were considered. Anisotropy of hardness by the scratch method was revealed clearly at high temperatures. It was proposed the explanation of the anisotropy of hardness. The correlation was established between the dislocation structures observed near the indentations and scratches.     

Thermal relaxation calorimetry on ethanol at low temperatures

Ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal which by quenching becomes an orientationally-disordered crystal with glassy properties, and the ordinary amorphous glass. Therefore, it appeared as a good model system to investigate low-temperature properties of glasses, including the role played by orientational vs translational disorder. In the present work, we extend and improve previous measurements by implementing higher-accuracy calorimetric methods at low temperatures (especially, two complementary versions of the thermal relaxation method). We have employed these new measuring methods to study the possible effect of water impurities on the calorimetric and thermodynamic behavior of the different solid phases of ethanol.     

Thermal properties of sodium silicate glasses at low temperatures

The heat capacity and the thermal expansion coefficient have been measured at low temperatures for vitreous silica containing 0, 10, 20, 30 and 40% soda. The heat capacity per gram is not much affected at 4 K, but is reduced by up to 30% at 10 K by addition of soda. The effect on the expansion coefficient is much greater; the negative values shown by pure silica below 150 K largely disappear with the addition of {10% soda. Below 10 K the coefficient remains negative until 25% soda is added. The Grüneisen function γ(T) is calculated for the various compositions.     

DC electrical conductivity of carbon black polymer composites at low temperatures

The understanding of the charge transport mechanism of the conducting polymers and their composites is important for the successful fabrication of semiconductor devices.In this work, the DC electrical conductivity of carbon black particles in a polystyrene matrix, in the temperature range of 80 to 250 K is examined, for concentrations below the critical percolation concentration. We show that three-dimensional variable range hopping is observed, that is, the temperature behavior of the DC conductivity can be expressed by . From the hopping parameter we obtained the mean hopping distance, the density of active centers and the energy associated with it.     

Investigation of structural transformations in pyridine nitrate at low temperatures and high pressures

The structural transformations in pyridine nitrate PyHNO₃ (C₅D₅NHNO₃) are investigated by neutron diffraction in the temperature range 16–300 K at normal pressure and in the high-pressure range 0–3.5 GPa at room temperature. A new high-pressure phase with a monoclinic structure (space group P2₁/c) is revealed in the PyHNO₃ compound at pressures P > P_tr ～ 1 GPa. The geometry of hydrogen bonds and the coordination of the PyH⁺ and NO ₃ ^? ions in the structure of the PyHNO₃ compound are studied as a function of the temperature and pressure.     

Mössbauer spectroscopic study of potassium borate glasses at low temperatures

Mössbauer studies of potassium borate glasses containing a small amount of iron were performed at dry ice and liquid nitrogen temperatures in order to investigate the physical properties of non-bridging oxygens in the glasses. The Mössbauer spectra at the temperature of dry ice were almost the same as those at room temperature. On the other hand, the spectra at the temperature of liquid nitrogen consisted of quadrupole doublet and hyperfine structure due to Fe³⁺ ions with tetrahedral symmetry. Magnetic suceptibility measurements revealed that the hyperfine structure was observed because of a paramagnetic relaxation effect. Isomer shift values for both the quadrupole doublet and the hyperfine structure were constant in the alkali region below 20 mol.%, and continuously decreased when the alkali content of the glasses was in the region ? 20 mol.%. The absorption areas for the hyperfine structure were also constant in the alkali region below 20 mol.%, and linearly decreased with alkali content in the region ? 20 mol.%. The internal magnetic field for the hyperfine structure also showed the same tendency as the absorption area. These results were attributed to the formation of non-bridging oxygen at the site adjacent to the iron, and the decrease of the absorption area for the hyperfine structure seemed to be directly related to the fraction of non-bridging oxygen in the oxygens constituting FeO₄ tetrahedra.     

Deformation mechanism and dislocation structure of high-purity molybdenum single crystals at low temperatures

High purity molybdenum single crystals were deformed in tension and compression along the symmetric double slip orientation [110] in the temperature range from 300 K down to 0.5 K with strain rates between 10⁻³ and 10⁻⁵ s⁻¹. The activation volume was measured by stress relaxation tests. The dislocation structure of the deformed crystals (T → 1.85 K) was examined by high voltage electron microscopy. It was established that the low temperature increase of the critical shear stress exhibits three distinct temperature regimes with different temperature dependences. These non-uniformities are discussed in terms of recently developed theories of kink-pair formation and kink-kink interactions on screw dislocations in bcc metals. The HVEM observations suggest that the mobility of screw dislocations at very low temperatures should be determined by the combined effects of the PEIERLS barriers and the jog dragging.     

Ultrasonic studies of the amorphous binary alloy Pd0.82Si0.18 at low temperatures

Ultrasonic attenuation and velocity changes are measured in an amorphous Pd_0.82Si_0.18 binarys alloy at frequencies from 10 MHz to 200 MHz (transverse) and temperatures down to 0.27 K. The attenuation is found to contain an amplitude dependent (saturable) component. These observations, as well as the temperature dependence of the velocity changes, are consistent with the predictions of the two level tunnelling systems (TLS) theory. Contrary to the predictions of this theory, however, linear frequency dependence and essentially no temperature dependence of saturable attenuation changes were observed at very low temperatures.     

Thermal conductivity of Tb2(MoO4)3 crystals at low temperatures

Thermal conductivity of the improper ferroic Tb₂(MoO₄)₃ was measured in the range 0.3 K–100 K by the method of the stationary heat flow. An additional maximum in thermal conductivity observed at 0.45 K is due to the anomaly of specific heat corresponding to the antiferromagnetic phase transition. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the instability boundaries of plastic flow in Cu + 14 at.% Al single crystals at low temperatures

The authors experimentally revealed the existence of the temperature-strain rate region of jumplike deformation in Cu + 14 at.% Al single crystals between 4.2 and 25 K. For each particular temperature in this temperature range, there are a lower and an upper critical strain rates, between which deformation is jumplike. The character of transients occurring during strain rate variation was investigated by increasing the strain rate to the lower critical value. The experimental data are explained in terms of the thermomechanical instability concept and inertial effects of the sample-machine system.