Variation of low temperature internal friction at microplastic deformation of high purity molybdenum single crystals

Amplitude and temperature spectra of ultrasound absorption in weakly deformed high purity molybdenum single crystals of different orientations were measured. The results were discussed in terms of parameter changes related to quasiparticle- or dislocation oscillations, respectively, dislocation-point defect interactions as well as defect generation at microplastic deformation.     

Unstable internal friction in Bismuth and Antimony single crystals

In the temperature range of 6 to 300 K an effect of the high amplitude ultrasound waves on amplitudedependences of decrement and modulus defects in the antimony and bismuth single crystals of different orientation is studied. The character of changes of the decrement and modulus defect amplitude dependences under the influence of the high amplitude ultrasound indicates that dislocation multiplication, obstacle break-through by dislocation pile-ups or deformation twinning may be observed in crystals depending on the ultrasound propagation direction and the temperature interval investigated.     

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.     

Dislocation formation and motion in germanium single crystals

The dislocation structure of germanium single crystals used in IR optics is investigated. The effect of high-temperature annealing on the dislocation distribution in Ge single crystals is shown.     

Dislocation activity in single crystals of stannic iodide

Single crystals of stannic iodide (SnI₄) have been grown employing the controlled-reaction between SnCl₂ and KI solutions by diffusion process in silica gels. As-grown (111) surfaces of the crystals have been optically studied. Characteristic growth spirals and hopper growth patterns have been observed on them. By successively etching (111) surfaces in a 0.2 N HCl solution, it is established that the pits indicate the sites of dislocations in the crystals. This is further confirmed by comparing the etch patterns before and after chemically polishing (111) surfaces. The average dislocation in the SnI₄ crystals have been determined and found to be 7.3 × 10² cm⁻². Observations of polygonisations are described and the implications are discussed.     

Dislocation etching of tin iodide single crystals

Single crystals of tin-iodide (SnI₂) have been grown using the controlled reaction between SnCl₂ and KI by diffusion process in gel medium. As grown (010) surfaces of the crystals have been optically studied. Characteristic etch pits have been observed on them. This suggests that SnI₂ crystals might go into dissolution in the acid-set gel. By successively etching (010) surfaces in a mixture of ammonia, acetic acid, and CdCl₂ solution, it is established that the pits indicate the site of dislocations in the crystals. This is further confirmed by comparing the etch patterns before and after chemically polishing (010) surfaces. The average dislocation density in the crystals have been evaluated and found to be 3.2 × 10³ cm⁻² and the implications are discussed.     

Dislocation densities in copper single crystals after thermal treatment

Copper crystals of high purity and doped with nickel were subjected to a long time thermal treatment at 1050°C, in order to reduce the as-grown dislocation density of about 10⁷ cm⁻². A dissolution of the dislocations during their motion to the crystal surface was observed. Regions of different dislocation densities arose. Finally the crystals exhibited constant dislocation densities of 10² cm⁻². Such a perfection or a better one of the crystals is reached only by a long time treatment, since the dislocation velocity without external stresses cannot exceed 1 · 10⁻⁷ cm · s⁻¹ in copper single crystals of high purity.     

Concentration dependence of the critical resolved shear stress of Cd-Zn single crystals in the temperature interval 1.5 K—50 K

Cd—Zn single crystals were deformed in creep at temperatures between 1.5 K and 50 K. The from the creep experiments established critical resolved shear stress (CRSS) increases with cⁿ where c is the atomic concentration of zinc as solute. The exponent n was found to be close to 2/3, i.e. the concentration dependence of the CRSS can be explained according to the theory of LABUSCH .     

Formation conditions and temperature dependence of forbidden reflections during resonant X-ray diffraction in zinc oxide single crystals

Different mechanisms of formation of purely resonant reflections hhl (h = 2n + 1) during resonant diffraction of X-ray synchrotron radiation in zinc oxide, whose intensity very strongly changes with an increase in temperature, are considered. Such reflections, forbidden in the dipole-dipole scattering, can be excited owing to the dipole-quadrupole scattering and the anisotropy of X-ray susceptibility induced by thermal motion of atoms and point defects. Different contributions are theoretically considered and it is shown that they can be separated owing to their different temperature dependences.     

Growth of high-purity ZnTe single crystals by the sublimation travelling heater method

A travelling heater method (THM) was developed to grow high-purity ZnTe from the vapor phase. This crystal growth method is called the “sublimation THM”. The temperature of the sublimation interface was set at 815°C and the temperature of the growth interface was varied from 785 to 800°C. The growth rate was 3 mm/day. Under these conditions, it was found that the growth process was mainly due to surface nucleation. Characteristics of the crystals were compared with those of solution-THM and vapor phase epitaxially grown crystals. The free-exciton line at 2.381 eV strongly appears and a doublet structure in neutral- acceptor bound exciton at 2.375 eV is clearly resolved with splitting energy of about 0.7 meV. We thus conclude that the THM from the vapor phase is suitable for preparing ZnTe single crystals which have excellent luminescent properties.     

On the change of dislocation structure during the postdeformation of cold worked molybdenum single crystals at elevated temperature

Molybdenum single crystals predeformed at low temperature (T₁ ≦ 293 K) exhibit during tensile reloading at T₂ = 493 K pronounced work softening accompanied by stress serrations and localized slip. It is shown that the initial state for the development of coarse slip bands is the formation of narrow obstacle-free slip channels. The change from the homogeneous dislocation structure to the cell structure proceeds in a heterogeneous and discontinuous manner.     

Temperature dependence of lattice parameters of langasite single crystals

To determine the coefficient of thermal expansion of trigonal langasite (La₃Ga₅SiO₁₄) the two independent lattice parameters a and c are measured over a temperature range of 800 °C using X‐ray diffraction on single crystal samples. From the given nonlinear temperature dependence the linear and quadratic thermal coefficients of expansion α₁₁, β₁₁ and α₃₃, β₃₃ for the two lattice parameters a and c could be deduced. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature,frequency, and amplitude dependence of internal friction of metallic glass

The internal friction Q^?1 and Young’s modulus E of Zr–Ti–Cu–Ni–Be bulk metallic glass specimens were measured using a dynamical mechanical spectrometer. Temperature dependence measurements were carried out for differently heat-treated specimens from room temperature up to a temperature above the crystallization temperature using several vibration frequencies (0.2–25 Hz). Two types of Q^?1 peaks appeared: a high-temperature peak in the supercooled region, and a medium-temperature peak in the glassy state region. Both of them were shown to be Arrhenius-type anelastic peaks. From the obtained results, the kinetics of movable atoms in the material were discussed. The amplitude dependence of the internal friction was measured at a given frequency at various temperatures. Fluctuations and rapid changes were observed in Q^?1 when the amplitude was gradually increased. The fluctuations were especially large near the glass transition temperature. The results were considered on the basis of the dynamics of the atoms in glass-forming materials near the glass transition.     

Radial and axial impurity distribution in high-purity germanium crystals

To grow high purity germanium (HPGe) crystals in an underground environment for ultra-low background experiments is being studied. In the present work, HPGe crystals along 〈100〉 direction have been grown by the Czochralski method. In order to investigate the distribution of the impurities as a function of length for a grown crystal, i.e. the axial direction, we fabricated a system to measure the resistivity along the axial direction at both room temperature and liquid nitrogen temperature. The distribution of the impurities along the radial direction was measured with a Hall Effect System. The results show that the carrier concentration in some crystals grown in a hydrogen atmosphere has an impurity level of about 10¹⁰/cm³, which meets the requirements of detector-grade crystals.     

Investigation of carrier scattering mechanisms in molybdenum diselenide single crystals by hall effect measurements

Molybdenum diselenide (MoSe₂) belong to the large family of layered transition metal dichalcogenides. It consists of weakly coupled sandwiched layers i.e. Se – Mo – Se in which a Mo atom layer is enclosed within two Se layers. This structure makes MoSe₂ extremely anisotropic in character and leads to unusual structural properties. In addition, MoSe₂ possess flexible nature along with good carrier mobility to make them potential candidate for fabricating flexible high mobility electronic devices such as Schottky barrier devices, FETs, solar cell etc. In context of this authors made an effort to study the low temperature (12 < T < 300 K) electronic transport properties of Molybdenum diselenide (MoSe₂). Through the investigation the temperature dependent Hall mobility study revealed that the grown crystals of MoSe₂possess a mixed scattering mechanism. It has been found that observed temperature dependant mobility has at least two transitions from lattice to impurity scatterings showing an imprint of multicarrier nature of this semiconductor originating from its complex band structure. It has been observed that the studied crystals have at least two group of carriers of differing origins in which transition between dominant scattering mechanisms occur at different temperatures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Orientation dependence of the NMR signal of deformed NaCl single crystals

The ²³Na signal of different deformed NaCl crystals was measured as function of the angle of rotation of the crystal with respect to the static magnetic field. The second moment of the NMR line is changed periodically determined by the slip mechanism. It is shown that the measured second moment can be obtained by a superposition of the parts originating from two-fold and four fold slip, respectively. The portion of the two slip mechanisms can be calculated quantitatively by fitting the experimental points.     

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,piezoelectric, elastic properties and internal friction of TGSe crystals with large unipolarity

In this work we have studied influence of temperature and external electric field on dielectric, elastic, piezoelectric properties and internal friction of D, L-α-alanine doped triglycine selenate crystals. Experimental result analysis allows to conclude that ferroelectric materials with stable physical properties may be created on doping crystals foundation.