Ultrasonic effect on multicomponent nanoheterostructures

The ultrasonic effect on the characteristics of GaP and AlGaInN multicomponent nanohetero-structures has been studied. It is found that the ultrasonic irradiation at frequencies of ∼10⁵ Hz for several hours leads to a significant degradation of the characteristics of multicomponent nanoheterostructures and shifts the luminescence spectral peak of LEDs based on these structures. The results obtained are qualitatively explained.     

Anomalous paramagnetism in pressure quenched CdS

Very large “paramagnetic” on positive magnetization has been observed in “pressure quenched” samples of CdS. Pressure quenching is a formative process involving pressure release rates ≈10⁶ bar s⁻¹.

The pressure quenched samples were prepared by pressure quenching at room temperature from above 30 kbar, i.e. from above the insulation-to-metal like transition. magnetization as a function of magnetic field was measured at 293 and 77 K using a vibrating specimen magnetometer. A linear M versus H behavior is observed in fields above a few hundred gauss, with values of χ = (M/H) ≈ 10^-4 cgs units. In some specimens saturation occurs, while in others the magnetization passes through a maximum. The maximum value of the magnetic moment M observed is of the order of tens of gauss.     

Amorphization from the quenched high-pressure phase of silicon and germanium

The phase transitions from the quenched high-pressure phase, including amorphous state, have been investigated for crystalline silicon and germanium at various pressures. X-ray diffraction patterns have been measured at pressures up to 15 GPa and temperatures down to 90 K by an energy-dispersive method using synchrotron radiation and a diamond anvil cell. The quenched β-Sn phase undergoes an amorphous phase transition when heated at 1.5 GPa for c-Si and 2.0 GPa for c-Ge. On the other hand, the quenched β-Sn phase transforms into a metastable crystalline phase when heated at higher pressures. The phase behavior is discussed in relation to the pressure dependence of the height of potential barrier between the β-Sb and amorphous phases and that between the β-Sn and metastable phases. The coordination number for the pressure-induced amorphous germanium, obtained through amorphization from the quenched high-pressure phase, is estimated to be about 4.     

The effect of electron irradiation on the luminescence of cadmium and lead tungstates

It is shown that the intensity of luminescence of cadmium tungstate crystals changes nonmonotonically under electron irradiation. At low irradiation doses, an increase in the luminescence intensity can be observed. However, with an increase in the irradiation dose, the luminescence intensity decreases. The effect observed can be explained by the competition of two processes: increase in the number of luminescence centers and nonradiative recombination under the action of irradiation.     

Glass-to-crystalline transformation in rapidly quenched Fe78B9Si13 ferromagnetic alloy

Calorimetric studies of the ferromagnetic Curie temperature T_c, and the crystallization kinetics of the metallic glass Fe₇₈B₉Si₁₃ have been performed in an attempt to elucidate the possibility of reversible relaxation processes near T_c and the crystallization mechanisms taking place. From the change of T_c with heating rate and on annealing it appears that ageing irreversibly increases the Curie temperature. Crystallization is thermally activated following an Arrhenius behaviour and proceeds in two stages, the best fit to the experimental data for each stage of crystallization has been obtained by use of a Johnson-Mehl-Avrami-Erofe'ev equation. The effective activation energy and the kinetic exponent are respectively E = (4.7 ± 0.1) eV, n = 2.0 ± 0.2 for the first and E = (4.5 ± 0.1) eV, n = 4.0 ± 0.2 for the second stage of crystallization. From these results it appears that the mechanisms of crystallization are quite different in both stages.     

Ultrasonic relaxation studies in the liquid crystal 80CB

The variation of the ultrasonic velocity with temperature in the liquid crystal 4′-n-octyloxy-4-cyanobiphenyl (80CB) at four different fixed ultrasonic frequencies is studied. From these studies various relaxation parameters like the adiabatic relaxation time, velocity at zero frequency, in the vicinity of isotropic-nematic phase transition and velocity at infinite frequency are estimated. From these parameters the relaxation mechanism in the vicinity of isotropic-nematic phase transition is studied and the results are discussed.     

Optical and FTIR studies of CuO-doped lead borate glasses and effect of gamma irradiation

CuO doped lead borate glasses of the composition PbO 70%–B₂O₃ 30% with varying CuO contents were prepared. UV–visible and infrared spectroscopic studies were measured before and after successive gamma irradiation with two different doses namely 2 and 8 Mrad. The experimental results indicate that the undoped sample reveals strong UV–near visible absorption while copper doped samples show additional broad visible band due to (Cu^2 +) ions.FT infrared absorption spectrum reveals vibrational bands due to triangular and tetrahedral borate groups together with the sharing of Pb–O vibrations.CuO-doped glasses have been found to show a shielding behavior towards the effects of progressive gamma irradiation causing the maintenance of the spectral curves. The changes in the UV–visible and infrared spectral data are discussed in relation to the states of copper ions and structural evolution caused by the change in glass composition including the CuO.     

Synthesis of magnesium oxysulfate whiskers in the presence of sodium dodecyl benzene sulfonate

Uniform magnesium oxysulfate (5Mg(OH)₂·MgSO₄·3H₂O) whiskers with a length of 10‐15 µm and a diameter of 0.4‐1.0 µm were synthesized in the presence of sodium dodecyl benzene sulfonate (Na‐SO₃‐C₆H₄‐C₁₂H₂₅) at 200°C for 1 h, using MgSO₄·7H₂O and NaOH as the reactants. Mg(OH)₂ precursor with poor crystallization and small crystal size was formed owing to the adsorption of sodium dodecyl benzene sulfonate on the Mg(OH)₂ surface. The quick dissolution of Mg(OH)₂ precursor in the subsequent hydrothermal reaction inhibited the occurrence of the sector‐like byproduct and promoted the formation of magnesium oxysulfate whiskers with uniform morphology. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structures of the benzene and ethanol solvates of neotame

The benzene and ethanol solvates of neotame crystallized from solutions of neotame anhydrate in benzene and ethanol, respectively. The crystal structures of the two solvates were determined by single-crystal X-ray diffraction using synchrotron radiation. The benzene solvate crystallizes in the monoclinic space group, P2₁, Z = 2, with one neotame molecule and one benzene molecule per asymmetric unit. The cell constants are a = 13.060 (6) Å, b = 5.582 (2) Å, c = 17.954 (9) Å, and = 102.079 (15)°. The ethanol solvate crystallizes in the orthorhombic space group, P2₁2₁2₁ with Z = 8 (Z = 2). The cell constants are a = 10.047 (4) Å, b = 17.001 (4) Å, and c = 28.948 (7) Å. Intermolecular hydrogen bonding among neotame molecules is evident in the two crystals. The benzene solvate has a nonpolar region containing the benzene molecules, with the benzene rings and alkyl chains of the neotame molecules.     

Ultrasonic relaxation studies in potassium-borate melts

The potassium-borate system, as other alkali-borates, shows a peculiar viscosity behavior. As potassium oxide is added to boron trioxide, the isothermal viscosity first decreases to a minimum then increases to a maximum. Ultrasonic relaxation studies show that the minimum and maximum are due to the behavior of the most probable relaxation time and the width of the distribution of relaxation times. These results support the model proposed by Beekenkamp when the effect of his alkali-borate structures on the relaxation times is interpreted by an environmental relaxation model developed by two of these authors. The relaxation measurements appear to be insensitive to the alleged phase transition in this system.     

Application of the model of locally nonequilibrium solidification to the process of structure formation in alloys rapidly quenched by spinning

Solidification of a binary alloy rapidly quenched by spinning [1] has been studied by mathematical simulation.     

Ultrasonic Investigations in the Liquid Crystal p-Cyanophenyl-trans-4-Propylocyclohexane Carboxylate

The variation of ultrasonic velocity with temperature in the liquid crystal p-cyanophenyl-trans-4-propylcyclohexane carboxylate at four different fixed frequencies is studied. From the observed temperature variation of the ultrasonic velocities, relaxation parameters like adiabatic relaxation time, velocity at infinite frequency in the isotropic as well as in the nematic liquid crystalline phases are determined. The results are discussed.     

Metastable phase formation in rapidly quenched Zr80Pt20 alloys: Linkage to oxygen

Melt spun Zr₈₀Pt₂₀ ribbons having oxygen contents ranging from <200 to ≈5000 mass ppm are shown to form various metastable phases during quenching. In the presence of sufficient oxygen, a big cube Zr₆Pt₃O phase forms, but this structure is de-stabilized towards an icosahedral quasicrystalline structure at lower oxygen levels. Further reduction of oxygen promotes increasingly stable ZrPt bonds, and a metastable β-Zr(Pt) structure is formed.     

The effect of electron irradiation on electrophysical properties of CdS,CdSe and ZnTe thin films

The effect of electron irradiation on electrophysical properties of CdS, CdSe and ZnTe thin films and CdS-PbTe and CdS-Te heterojunctions were studied. It was found that the effect of irradiation on thin films and devices strongly depend on the electron-irradiation doses and the preparation conditions.     

A positron annihilation study of vacancies and their clusters in diluted aluminium alloys quenched or neutron-irradiated

Pure Al and Al alloyed with Mg, Zn, Ge, and Si was quenched from 600°C in water at room temperature or irradiated with fast neutrons. With the help of positron lifetime and angular correlation peak height measurements vacancies, their clustering and their annealing behaviour were studied.     

Supramolecular benzene structure and its changes under the action of dissolved fullerenes

A model of the supramolecular benzene structure and its changes induced by the introduction of fullerenes into benzene is proposed based on the complex consideration of previously calculated and experimental data.     

Short-range and medium-range order in rapidly quenched Al50Mg50 alloy

The rapid quenching processes of a larger-scale Al₅₀Mg₅₀ alloy system consisting of 100,000 atoms have been simulated by using molecular dynamics method. The formation and structure of short-range order (SRO) and medium-range order (MRO) in the rapidly quenched Al₅₀Mg₅₀ alloy are investigated by means of several structural analysis methods. It is found that the massive icosahedra in the Al₅₀Mg₅₀ supercooled liquid prevent it from crystallizing and play a critical role in the formation of metallic glass. The SRO in Al₅₀Mg₅₀ metallic glass cannot be modeled by a uniquely prescribed stereo-chemical structure or five Bernal polyhedra, but various types of basic clusters in which the icosahedron is dominant. The MRO is characterized by some certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or rings, which is different from the FCC and icosahedral building schemes for the MROs in metallic glasses with significant chemical SRO. The size distributions of these MRO clusters exhibit a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41….     

Behaviour of Ultrasonic Velocity in Cholesteric Liquid Crystals

Ultrasonic velocities (V) have been determined, employing a fixed path double crystal interferometer, in three cholesteric liquid crystals, namely cholesteryl propionate, cholesteryl laurate and cholesteryl myristate in their isotropic and anisotropic phases including the region of the phase transition. The variation of specific volume (v) is also studied in the same temperature range by a special dilatometer constructed for this purpose. Anomalous behaviour of ultrasonic velocity is observed near the isotropic-cholesteric phase transition in all three compounds. In the two polymesomorphic liquid crystals, namely cholesteryl laurate and cholesteryl myristate, contrary to the ultrasonic behaviour of cholesteryl stearate, prominent velocity dips are observed at cholesteric-smectic transition temperatures. The parameters adiabatic compressibility (β_ad) and molar sound velocity (R) are estimated and they are found to exhibit sudden jumps at cholesteric-smectic and isotropic-cholesteric transitions. The thermal expansion and temperature co-efficient of compressibility are found to show abnormal increase near the phase transition indicating the existence of large-magnitude pre-transitional effects near the phase transition. A comparative study of the ultrasonic behaviour of six aliphatic esters of cholesterol has shown that the magnitude of the velocity dip observed at the isotropic-cholesteric transition increased with increase of molecular weight and only cholesteryl acetate shows deviation.