Mechanodynamic diffusion of helium atoms into porous copper

Mechanodynamic penetration of helium atoms into porous copper compressively strained at 4.2 K is studied. Porous copper is obtained by vaporizing zinc out of brass in vacuum at a temperature of 800°C for 8 h. The number of helium atoms which penetrated into the sample increased monotonically with strain to reach 2.9 × 10¹⁶ atoms/cm² at ? = 42%. This amount of helium is two and even more orders of magnitude larger than that obtained from the data available thus far on mechanodynamic penetration of atoms of an external medium into crystalline and amorphous materials under strain. The relations obtained suggest that specific types of helium traps determine the kinetics of mechanodynamic diffusion of helium into solids.     

Specific features of helium penetration into single-crystal and nanocrystalline copper under deformation in liquid helium

This paper reports on experimental data on the penetration of helium atoms into single-crystal and nanocrystalline copper samples subjected to tensile and compressive strains at T=4.2 K, respectively. The dependences of the helium concentration N in the samples on the strain ? and the curves of helium extraction in the temperature range 300–1000 K at different strains ? are determined. It is found that the dependences N(?) and σ(?) correlate qualitatively with each other for single-crystal copper and do not correlate for nanocrystalline copper. This is associated with the different mechanisms of deformation in these samples. The deformation proceeds through the dislocation mechanism in single-crystal copper and through the jumpwise (twinning, rotational) mechanisms in nanocrystalline copper during local heating in regions of plastic shears. These factors are also responsible for the considerable difference between the curves of helium extraction from samples of both types. The curves of helium extraction exhibit two maxima for single-crystal copper and five maxima for nanocrystalline copper samples. The results obtained are discussed in terms of both the dynamic dislocation pipe diffusion and grain-boundary mechanisms of particle penetration from the surrounding medium into copper through different-type moving defects under applied stresses and due to the gradient of the chemical potential at the metal-surrounding medium interface.     

The thermoluminescence (TL), phosphorescence and cryoluminescence of n-type hexagonal (6H) SiC crystals

The low temperature (down to liquid helium temperature) TL, phosphorescence and cryoluminescence of n-type 6H SiC crystals is described. The crystals contained nitrogen as the major impurity at concentrations of about 10¹⁶ cm^-3. The glow curves exhibited peaks at about 25, 45, 70 and 90°K (in addition to a peak at 250°K). Thermal activation energies for the above peaks ranged between 0.02 and 0.14 eV (0.30 eV for the 250°K peak). These are much lower than energies reported earlier for nitrogen donor levels in 6H SiC. The values obtained for the 70–90°K peaks (0.08–0.14 eV) fit quite well those obtained by electrical transport measurements and Raman scattering.The crystals exhibited strong phosphorescence even at liquid helium temperature. This was shown to be only partly due to thermal release from traps, the other components being due to pair-recombination and optical release from the shallow traps by the black body radiation (BBR) from the walls of the cryostat. This BBR was found to be responsible also for the observed cryoluminescence.     

Dynamic diffusion of helium into various types of solids during their deformation and dispersion

Quantitative relations governing the penetration of helium atoms into various types of solids in the course of their plastic deformation in liquid ³He (T = 0.6–1.8 K) and ⁴He (T = 4.2 K) and dispersion in gaseous helium at 300 K were obtained and analyzed. Experiments were carried out on metals with different lattice types, ionic single crystals, amorphous alloys, and barite and titanium dioxide powders dispersed in helium. Curves illustrating helium extraction from deformed specimens under dynamic annealing were obtained. The temperature range of helium extraction was found to correlate with the melting temperature and the initial and deformed structures of a material, which determine the number and character of helium traps present in the material. The dependence of helium penetration intensity on the type of defects forming under plastic deformation for various materials, as well as the formation of chemical bonds of helium atoms to the defected structure of these materials, is discussed.     

Influence of the defect and structural state of FCC and BCC metals on the intensity of mechanodynamic penetration of helium atoms

The specific features of the mechanodynamic penetration of helium under plastic deformation into fcc (Cu) and bcc (Fe, Nb) metals with different initial defect structures (single-crystal, nanocrystalline, and porous samples) are investigated. The intensity of mechanodynamic penetration into these metals is shown to depend on the type of bonding (metallic or covalent), which determines the degree of localization of the plastic flow of these metals, as well as on the type of defect structure and on the character of plastic flow (dislocation deformation, twinning, grain-boundary sliding). Curves of helium extraction from samples at different strains are obtained. It is found that the helium release exhibits a wide variety of peaks depending on the degree and character of plastic deformation of the metals under investigation. This suggests that the metals contain different types of helium traps, which determine the content of helium and the specific features of its release in the temperature range studied.     

Mechanodynamic penetration of helium atoms into aluminum and its alloys under strain in liquid helium

The specific features of helium penetration into aluminum and its alloys, V95 and D16T, at a temperature of 4.2 K under uniaxial tension, compression, and reversal of the sign of the load are investigated. The role played by serrated strain in the intensity of the effect under consideration and the influence of impurities on the number of helium atoms penetrating into strained samples are elucidated. It is shown that the condition of additivity of the effect observed under successive reversal of the sign of the load depends on the specific features of the tensile and compressive strains.     

Use of the helium medium for preparation of nano-sized powder materials by the example of industrial cement

Based on the phenomenon of the mechanodynamic diffusion of particles of the external medium in solids, a new in principle method has been proposed for the first time for producing nano-sized powder materials using industrial cement milled in a helium medium as an example. The temperature dependences of the extraction rate and the amount of helium in powders upon their heating in a temperature range of 20–1200°C have been obtained using mass spectrometry. It has been shown that milling of the cement powder of the M-400 brand using an MK-1 laboratory mill in helium leads to a considerable shift of its extraction curve towards lower temperatures compared with the air medium. The particle sizes of the powder milled in helium lie in a range of 5–10 nm, which is smaller than the powder size (～500 nm) after milling in the air medium by a factor of 100. The compression strength of cement samples obtained from the powders milled in helium increased by a factor of 2 compared with the strength of the samples from the initial material. The activation energies of helium extraction from the cement powders milled in helium and in air have been analyzed. The obtained results indicate a high efficiency of the method for producing nano-sized powder materials in the helium medium. The method can be used in the industrial scale based on the existing mill equipment with its minimal modernization.     

Plastic properties and defect structure of LiF-LiF:Mg layered single crystals at T = 4.2 K

The compressive stress-strain diagrams are obtained for layered single crystals of the LiF-LiF:Mg type with different orientations of the reinforcing layers at T = 4.2 and 300 K. The strength characteristics and specific features of the defect and dislocation structures of the crystals strained in liquid helium are studied and compared with those for the crystals strained at 300 K. It is established that the layered single crystals remain plastic at T = 4.2 K. Under a strain ε > 1%, there arise microcracks and macrocracks responsible for complete fracture of the crystals. It is revealed that long-term (for ten years) storage of the layered single crystals at 300 K substantially affects the magnesium impurity structure in the reinforcing layers and leads to a considerable enhancement of their strength characteristics. This effect is taken into account when analyzing the strength properties of the layered single crystals in the temperature range T = 300–4.2 K. The inference is made that the results obtained in studying the defect microstructure of the model layered single crystals at the initial stage of plastic deformation can be used to predict the strength properties of the composites at T = 4.2 K.     

Mechanochemical penetration of helium atoms into Pd84.5-Si15.5 and Ni78-Si8-B14 eutectic amorphous films extended in liquid 3He and 4He

The penetration of helium atoms into amorphous films extended to fracture in liquid helium has been investigated. It is found that helium atoms penetrate into the eutectic alloy films Pd_84.5-Si_15.5 in ³He (T=0.5 K) and Ni₇₈-Si₈-B₁₄ in ⁴He (T=4.2 K). The spectra of helium liberation from these materials after deformation are obtained upon dynamic (4–5 K/min) annealing at T=293–1323 K. The maximum amount of helium is observed in the regions of local plastic microshears running across the whole width of films and also in the sample regions containing fracture macrocracks and isolated groups of slip bands. The spectra of helium liberation from different regions of destroyed samples show several peaks that correlate with the temperatures of crystallization and melting of the studied films. The data obtained are interpreted within the model of mechanochemical penetration of helium atoms through the dynamically excited dislocation-like defects, which are typical of the amorphous films under consideration.     

Stimulation von ZnS-Phosphoren mit langwelligem Infrarot

Stimulation experiments with ZnS phosphors, using IR wavelengths from 2 to 15μ, were performed at the temperature of liquid helium. For this purpose a cryostat was constructed which allowed to keep the samples and the screening device at the temperature of liquid helium. Moreover glow curves after different decay times at 6 °K were taken. After excitation of ZnS phosphors a strong release of carriers from relatively deep traps is taking place although the phosphor is kept at the temperature of liquid helium. This emptying of traps is accompanied by a luminous recombination of the released electrons with the activator levels producing an intense afterglow which can be observed over a long time. This phenomenon cannot be explained by thermal release of trapped electrons into the conduction band, followed by recombination with the activator levels because of the depth of the emptied traps. The rate of thermal carrier release was calculated to be about 10^?29 sec^?1 for 0,05 eV deep traps, but the observed rate was of the order of 10^?4 sec^?1. An emptying of traps by IR-stimulation can be excluded because the phosphor was surrounded by a concentric screening device kept at 4.2 °K. The effect can be explained by luminous tunneling of trapped electrons to the activator levels. An estimate of the tunneling rate gives a value agreeing with the experimental results. If such a “tunnel afterglow” does exist a spectral shift to longer wavelengths compared to the usual fluorescence and phosphorescence is to be expected. This could be actually observed. Additional experiments included IR-stimulated emission after various decay times, glow curves taken after such stimulation, and studies of the influence of temperature. The results rule out the possibility that the carriers were released from the traps by IR light. Apparently, IR radiation affects the potential barrier between the traps and the activators so that the rate of recombination by tunneling increases.     

Dielectric relaxations in undoped,Ce-doped and Ce,Zr-codoped Lu3Al5O12 single crystals

Dielectric spectroscopy was performed on single crystals of pure, Ce-doped or Ce,Zr-codoped Lu₃Al₅O₁₂, before and after UV- or X-irradaiation, at various frequencies within the range 100 Hz–1 MHz as the temperature was scanned from 110 to 353 K. All samples previously subjected to ionising radiations gave spectra showing loss peaks with Arrhenius characteristics of permanent dipoles relaxation. We attribute the dipoles to defect-stabilised pairs of anion–anion vacancies (oxygen ions and oxygen vacancies) that have captured holes and photo-electrons separately, thus forming O^?- and F⁺-like centers. The dielectric relaxation peaks disappeared in undoped or doped samples annealed at 573 K, suggesting that charge carrier traps are relatively deep. UV–visible absorption spectra have also been measured, which tend to support our proposed interpretation. Further evidence for deep traps has come from thermally stimulated luminescence experiments.     

Thermally stimulated luminescence and infrared studies of CaS : Bi,Tm phosphors

Calcium sulphide phosphors doped with bismuth and thulium are prepared from Indian minerals. The glow curves are recorded in the temperature range of 96–320 °K. The activation energies are determined by analyzing the glow peaks after thermal cleaning, using different methods. The results show that, in these phosphors, the electron traps responsible for thermoluminescence are present prior to irradiation. The infrared absorption spectra are recorded in the range of 4000-250 cm^-1. It is concluded that the traps are due to host lattice defects which may arise from S^-2 ion vacancies, created during phosphor preparation.     

Microhardness of armco iron samples with different initial structures after rolling in a helium or nitrogen medium

We have analyzed the dependences of microhardness H on load P for the surface layers of Armco iron samples with different initial structures, i.e., annealed and after equal-channel angular pressing (ECAP). The microhardness was measured in air after rolling of the sample in helium or nitrogen medium. It has been found that these mediums produce different effects on the H(P) dependences for preliminarily annealed samples compared to those subjected to ECAP. This is due to the differences in their initial defects structures and in the types of binding forces of helium atoms and nitrogen molecules with dislocations, which determine the intensity of their penetration into the surface layer of the samples under investigation. We have obtained curves that describe the release of helium from samples rolled in different mediums and have measured the amount of helium.

     

Molecular Beam Epitaxy of Si–Ge–Sn Heterostructures for Monolithically Integrated Optoelectronic Devices Based on Silicon

Elastically strained metastable Ge_1–xSn _x layers with molar fractions of tin of up to 0.15 are grown on Si (001) substrates. To analyze the optical properties of the samples, photoluminescence (PL) spectra are measured at room temperature and IR transmission spectra are measured at the liquid helium temperature. The room temperature direct intrinsic absorption edge at 0.71–0.72 eV is visible in the spectra of the studied structures with tin contents of 12–13%.     

Inelastic deformation of C60 single crystals in the temperature range 80–300 K

The rate spectrum of small inelastic strains of the C₆₀ single crystal in the temperature range 80–300 K has been obtained with a precision laser interferometer. It is revealed that the spectrum exhibits two large peaks in the glass formation (90–100 K) and phase transition (250–260 K) ranges. A small strain acceleration is also observed at ～220 and 240 K. The first two maxima are attributed to the changes in strain resistance upon transitions, and the strain acceleration at 220 and 240 K is associated with the annealing of the defects formed upon rapid cooling of the crystal. It is demonstrated that the peak at 250–260 K broadens with an increase in the stress. The spectrum of strain rates is compared with the calorimetric curve for the same single crystal.     

Numerical study on optical and electrical properties of strained GaInSb/GaInAlSb mid-infrared quantum well laser

The optical and electrical properties of compressively strained GaInSb/GaInAlSb mid-infrared quantum well lasers are numerically studied solving one-dimensional Schrödinger equation using finite difference method. The simulation results demonstrate that band-mixing effects and effective mass of hole are reduced when the well is compressively strained. The strain-dependent optical and differential gains are evaluated for 0.6, 0.9, 1.21, and 1.52% compressively strained quantum well and found maximum when well is strained by 1.52%. The emission wavelength for the proposed laser can be tuned from 2.40 to 2.26 μm due to change in compressive strain from 0.60 to 1.52% at temperature 300 K. For the range of strain, the shift in wavelength is found from 2.38 to 2.24 μm at temperature 275 K. The results obtained from PSPICE simulation indicate that, the optical output power and threshold current are strongly depend on the number of wells and found to be almost constant for the number of wells three and above.     

The influence of strain rate on the visibility of dislocations in transmission electron microscopy images of deformed Ti–6 wt% Al–4 wt% V and in Timet 550

Samples of Ti–6?wt%?Al–4?wt%?V and Timet 550 (Ti–4?wt%?Al–4?wt%?Mo–2?wt%?Sn–0.5?wt%?Si) have been subjected to strain rates between 10^?1 and 10³?s^?1and detailed examination of the dislocation structure in the α grains has been carried out using transmission electron microscopy (TEM). For samples deformed to a strain of 0.1 at 10^?1?s^?1, detailed analysis of the defects can be carried out using all diffracting vectors and the presence of (c +?a) dislocations and a dislocations thus confirmed. In contrast, for samples strained to the same strain of 0.1 but at 5?s^?1, it is not possible to obtain images of dislocations when using any diffracting vectors other than 0002. Thus the presence of dislocations which have a Burgers vector containing a c component can be confirmed in the samples strained at 5?s^?1 but the presence of a-component dislocations can only be inferred from TEM of these samples because of the difficulty of obtaining images with diffracting vectors other than 0002. Limited observations on samples strained at 10³?s^?1 show that similar difficulties are found in imaging dislocations as are found in samples deformed at 5?s^?1 but at this strain rate, the highest used, the difficulties are reduced since images can be obtained in some grains using diffracting vectors other than 0002. These results are discussed in terms of the nature of damage as a function of strain rate and the factors that influence contrast from dislocations in crystals.     

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b ³, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.     

Trapping centers and their distribution in Tl2In2Se3S layered single crystals

Thermally stimulated current (TSC) measurements have been carried out on Tl₂In₂Se₃S layered single crystals in the temperature range of 10–175 K. The TSC spectra reveal the presence of two peaks (A and B). The electronic traps’ distributions have been analyzed by different light illumination temperature techniques. It was revealed that the obtained traps’ distribution can be described as an exponential one. The variations of one order of magnitude in the traps’ density for every 30 meV (A peak) and 59 meV (B peak) were estimated. Moreover, the mean activation energy, attempt-to-escape frequency, capture cross section and concentration of the traps were determined.     

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.