Low-temperature specific heat of compacted vitreous silica

We present low-temperature specific-heat measurements of two types of irreversibly compacted vitreous silica (Suprasil W and Suprasil I, differing in their OH⁻ content). In the whole temperature range investigated (0.1 K<T<35 K), the specific heatC is reduced by up to 50% compared to that of uncompacted samples but exhibits a similar temperature dependence, with the characteristic maximum inC/T ³ shifted to higher temperatures. This coherent change ofC, which is roughly the same for both types of vitreous silica, gives strong evidence that the low-energy excitations characteristic of amorphous materials have a common structural origin. Possible relations to the microscopic changes upon compaction are discussed, giving support to the coupled-rotation model of SiO₄ tetrahedra. Phenomenologically, the change ofC upon compaction can be understood by the soft-potential model. The relationship between height and position of theC/T ³ maximum observed in normal and compacted states for both types of vitreous silica and, surprisingly, also for α-quartz hints at a correlation between the mechanisms leading to theC/T ³ maximum for amorphous and crystalline materials with similar short-range order.     

Thermo-sensitive N-n-propylacrylamide gels

Thermo-sensitive gels were prepared by irradiating aqueous solutions of poly(N-n-propylacrylamide) (NNPA) and its copolymers with acrylic acid (AA) with γ-rays from a ⁶⁰Co source. The equilibrium swelling volume of the gel in water was determined as a function of temperature. NNPA gel showed a discontinuous and reversible volume phase transition. The transition temperature and the volume change at the transition decreased with irradiation time. The transition temperature was approximately 12°C lower than that of poly(N-isopropylacrylamide) gel. A discontinuous volume transition was also observed in the copolymer gels of NNPA and AA. The dependence of the transition temperature on the concentration of carboxyl groups revealed a marked difference depending on whether they were protonated or dissociated. For gels having side groups of COONa, the transition temperature rose and the volume change at the transition was elevated as the COONa concentration increased. In contrast, an increase in the COOH concentration resulted in a decrease in the transition temperature. These results are discussed in terms of an equation of state constructed based on scaling theory.     

Magnetic studies with μSR at high pressures

A newly developed high pressure-low temperature μSR spectrometer was employed for two types of magnetic studies. Firstly we measured the pressure dependence of the local magnetic field B_μ in Fe, Co and Ni at 77 K up to 7 kbar. From the pressure derivative dlnB_μ/dP the volume derivative dlnB _Hf /dlnV was deduced. In connection with previous room temperature data we calculated the temperature dependence of B_hf at constant volume. It deviates markedly from the temperature dependence of the host magnetization. Secondly, we looked at the pressure dependence of the muonic Knight shift in Sb at 30 K for polycrystalline and single crystal samples. A strong pressure dependence was observed which depends on the orientation of the magnetic field relative to the c-axis. The pressure coefficients of the isotropic and the axial term of the Knight shift were deduced.     

Ultrasonic study of molecular interaction in binary liquid mixtures at 30°C

Densities ρ and ultrasonic speeds u of the binary mixtures of tetrahydrofuran (THF) with 1-butanol and tert-butanol, at 30°C, over the entire composition range were measured. From these data isentropic compressibility, K _s, intermolecular free length L _f, relative association R _A, acoustic impedance Z, molar sound speed R _m, deviations in isentropic compressibility ΔK _s, and excess volume V ^E were calculated. The variation of these parameters with composition of the mixture helps us in understanding the nature and extent of interaction between unlike molecules in the mixtures. Further, theoretical values of ultrasonic speed were evaluated using theories and empirical relations. The relative merits of these theories and relations were discussed.     

Structure and Shape Memory Properties of Polyurethane Copolymers Having Urethane Chains as Soft Segments

Shape memory polyurethanes are usually fabricated with low-molecular weight polyols through a two-step copolymerization, which often results in difficulty attaining both desired shape memory switch temperature and optimal thermomechanical properties. Here we present a series of shape memory polyurethane copolymers having urethane chains as soft segments. The structure and shape memory properties of copolymers were investigated with differential scanning calorimetry, dynamic mechanical analysis, small angle x-ray scattering, and thermomechanical tests. Increasing the length of the urethane soft segments enhanced phase separation, while it brought little change to the glass transition temperature (T _g). Based on the urethane soft segments, some rigid chain extenders could be readily introduced into the backbone of copolymers, resulting in better phase separation. All polyurethane copolymers exhibited more than 90% of shape recovery. The shape recovery of the materials was proved to be inversely proportional to the fraction of hard phase and directly proportional to the stability of hard domains. The copolymers containing longer soft and hard segments and rigid chain extenders exhibited higher deformation stress and thus larger recovery stress. The copolymerization employing urethane chains as soft segments can greatly expand flexibility for molecular design and favor the optimization of shape memory properties.     

Magnetostriction studies of magnetic phase transitions in the copper metaborate CuB2O4

The field dependences of the longitudinal and transversal magnetostriction of the copper metaborate CuB₂O₄ were measured at various temperatures below the Néel point in magnetic fields directed along the tetragonal axis or in the basal plane. Magnetostriction was found to exhibit jumps at magnetic-field-induced phase transitions to a commensurate weak ferromagnetic state, as well as to grow smoothly in fields above and below the critical level. The magnetostriction observed in a magnetic field directed along the tetragonal axis is shown to be primarily caused by volume dilatation of the crystal. The experimental data obtained were used to construct the magnetic phase diagram of copper metaborate magnetized along the tetragonal axis.     

Positron lifetime measurement in NaNO2

Positron lifetimes in NaNO₂ were measured as a function of temperature. The lifetime spectra consisted of one component, and could not be resolved into two component. The positron trapping model is not applicable in this case. Observed changes of lifetimes around the transition temperature could not be understood only by the volume dilatation, but they must be affected by the ferro-para electric phase transition     

A new method for studying the transport of radon and thoron in various building materials using CR-39 and LR-115 solid state nuclear track detectors

Radon (²²²Rn) and thoron (²²⁰Rn) α-activities per unit volume were measured inside and outside different building materials by using two types of solid state nuclear track detectors (SSNTD) (CR-39 and LR-115 type II). In addition, the radon and thoron emanation coefficients of the studied materials were evaluated. Based on these data, the transport of radon and thoron across parallelepipedic blocks of the building materials could be investigated and radon and thoron global α-activities per unit volume outside different building material blocks were determined. Moreover, the diffusion length and the effective diffusion coefficient of radon in the building materials were evaluated and the total alpha activity due to radon in the atmospheres of different rooms consisting of different building materials was studied.     

Applications of photoconductivity measurements in n-InSb under crossed fields

The photocurrent in n-InSb at 85K was measured as function of the applied longitudinal voltage and an additionally applied transverse magnetic field. In the driftconfiguration the photoionized electron-hole plasma was driven into the sample volume, and negative photocurrents resulted from the negative feedback of this motion. With the magnetic field being reversed, in the Suhl configuration, only positive photocurrents were measured. All results are in agreement with the theory in [1]. They were used to determine recombination coefficients in n-InSb, and, principally, the transverse diffusion coefficientD _⊥. As first observed in [13],D _⊥ proved to be anomalously enhanced above classical values if plasma instabilities were generated in the samples.     

Energy dispersive analysis of X-ray radiation from the inhomogeneous surface layer

The intensities of theK -line of phosphorus as a function of the energy of primary electron beam and of the change in take-off angle were measured on the iron sample with implanted phosphorus by means of the EDS analysis in the scanning electron microscope. The observed dependences are confronted with model conception of the concentration distribution of the implanted element in the iron. The possibility of analysing the element composition in the region smaller than the excited volume is shown.Dedicated to Academician Vladimír Hajko on the occasion of his 65th birthday.     

Ion bombardment induced phase transformations and inert gas mobility in the semiconductors Ge,Si, and GaAs

Abstract

The present study contributes some new aspects to the general understanding of the ion implantation behaviour of 3 common semiconductor materials, and of diffusion processes in these materials. Single crystals of Si, Ge, and GaAs were bombarded with Kr- or Xe-ions at energies of 40 or 500 keV and doses between 10¹¹ and 2 × 10¹⁶ ions/cm². Gas release measurements and Rutherford scattering of 1 MeV He⁺-ions combined with channeling were used to study bombardment damage (amorphization) and inert gas diffusion. At low bombardment doses (10¹¹ ions/cm²) and energy (40 keV), no damage was observed and the gas release was compatible with volume diffusion resembling Group I and VIII behaviour. Hence, the pre-exponential terms, D ₀, were low (range 10-^5±1 cm² sec^?1) and the activation enthalpies, Δ H, were much lower than those of self-diffusion or of diffusion of Group III and V elements. The Δ H's for gas diffusion followed the relation Δ H = (1.05±0.1) × 10^?3 T_m eV with the melting point, T_m , in °K. The mechanism of gas mobility might be the Turnbull dissociative mechanism. Rutherford scattering and channeling data indicated that part of the gas occupied lattice sites.

At higher doses, the bombarded layers turned amorphous. Channeling experiments showed a coincidence in temperatures for a gas release process different from the above one of volume diffusion, and recrystallization of the disordered layer to the single crystalline state. Both processes occurred in the temperature range 0.60 to 0.65 T_m . The gas release indicated a (partial) single jump character with implied Δ H's following the relation Δ H = (2.1±0.1) × 10^?3 T_m eV. Contrary to previous results on oxides, this new gas release occurred at temperatures near to those or even above those of volume diffusion of the gas.

Due to the easy formation of an amorphous layer it was difficult to observe the retarded release (trapping of gas) that has been found in many materials at high gas and damage concentrations. However, in a separate series of experiments with 500 keV Kr-ions, a release retarded with respect to volume diffusion of the gas was observed in Si and Ge.     

Examination of optical properties and estimation of mechanical energies of irradiated polypropylene and teflon

The optical absorption and mechanical yielding energy has been studied under exposure of γ-radiation. Two crystalline polymers, polypropylene (PP) and Teflon, were irradiated with a ⁶⁰Co source, with doses ranging up to 6 kGy. The observed optical energy gap (E _opt) and energy gap tail (Δ E) for irradiated thin sheets of PP were determined from the measured absorption spectra. The average values of (E _opt) and (Δ E) were 5.85 and 0.5 eV, respectively. There is no detectable change in the optical energy gap under the applied γ-ray doses. On the other hand, the effect of γ-radiation on mechanical properties of irradiated Teflon was much pronounced. It was found that Young’s modulus and yield stress increase with radiation dose, whereas the yield strain decreases. The calculated yield energy increases with radiation dose from 0.27 to 0.35 Mpa per unit volume. The enhancement in the mechanical properties of irradiated Teflon was attributed mainly to crosslinking process and other structural changes occuring during irradiation with γ-rays.     

Melt crystalfization and crystal morphology of two low molecular weight linear polyethylene fractions

Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G _b) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17°C (at 117°C and 120°C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122°C and 124°C), at which the cusps of the G _b data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments, it is found that within a 2°C temperature region, the G _b change is accompanied by a sharp long period increase and a drastic change in single-crystal morphology from a truncated lozenge with curved (200) and (110) planes to a lenticular-shape crystal. The morphological change may result from a sudden increase in the G _b coupled with a smaller change in the growth rate along the a axis with undercooling. This implies that, within this temperature region (2°C), the crystals may undergo substantial changes in the geometry of the (110) and (200) crystal growth fronts and chain folding behavior.     

Far-infrared donor spectroscopy of MBE-GaAs

Magneto-optical spectra of shallow donors in MBE-GaAs were measured in the far-infrared using Fourier transform spectroscopy. In the purest sample (N _I=9×10¹⁴cm^–3) silicon was observed to be the dominating donor impurity. In a less pure sample sulfur was observed to dominate. The increase of the purity and the elimination of the sulfur contamination was correlated with the change from water cooling to liquid nitrogen cooling of the cooling shrouds around the effusion cells. An increase of the growth temperature was probably also one reason for the increase of the purity.     

Photoluminescence and Raman scattering in spatially inhomogeneous heteroepitaxial InGaN layers

Spatial inhomogeneities of the indium distribution in In _x Ga_1–x N epitaxial layers grown on sapphire substrate with a GaN buffer layer were investigated using photoluminescence (PL) in addition to confocal scanning Raman spectroscopy (RS) and PL. Broad emission bands from In-enriched InGaN nanoclusters (700–900 nm) and from the volume outside the clusters (about 460 nm) were observed in PL spectra of an epitaxial InGaN layer with an average In content of 25.7%. It was established that larger micro-PL intensities corresponded to energetically shallower clusters. The observed broadly asymmetric A₁(LO) RS band of InGaN confirmed that the In concentration in the layer was highly variable. Modeling the LO phonon band by two Lorentzian curves gave an average In concentration of 21% in the volume outside the clusters and 37% in the nanoclusters, which was considerably higher than the average concentration in the layer and agreed well with their PL band positions.     

Parametric interaction of magnetostatic waves with a nonstationary local pump

A solution is obtained for the general problem of the nonstationary interaction of backward volume magnetostatic waves in films of yttrium-iron garnet with local parametric pumping. In the case of a large pump region, l≫λ, where λ is the wavelength of the backward volume magnetostatic waves, the problem reduces to a system of truncated equations for two packets of counter propagating waves. In the opposite case, l<λ, the exact problem of parametric interactions of the eigenmodes of a ferrite film (both counterpropagating and in the same direction) is solved numerically. Both cases are studied experimentally and good qualitative and quantitative agreement is obtained with the theory. For the first time, the reversal of a wave front and the time reversal of the shape of backward volume magnetostatic wave pulses are observed and a change in the propagation time for the peak of the signal pulse and a reduction in its width owing to pumping are recorded. Two operating regimes are identified for a nonstationary parametric backward volume magnetostatic wave amplifier with local pumping, which differ in the ratio of the duration of the pump pulse to the transit time for the wave through the local pump region, and the effect of the parametric excitation of two-dimensional spin waves on the interaction of backward volume magnetostatic waves with a local nonstationary parametric pump is determined. Zh. éksp. Teor. Fiz. 116, 2192–2211 (December 1999)     

Compressibility determination by electrical resistance measurement: a universal method for both crystalline and amorphous solids

A new method is introduced for investigating the compressibility of solids under high pressure by in situ electrical resistance measurement of a manganin wire, which is wrapped around the sample. This method does not rely on the lattice parameters measurement, and the continuous volume change of the sample versus pressure can be obtained. Therefore, it is convenient to look at the compressibility of solids, especially for the X-ray diffraction amorphous materials. The I–II and II–III phase transition of Bi accompanying with volume change of 4.5% and 3.5% has been detected using the method, respectively, while the volume change for the phase transition of Tl occurring at 3.67 GPa is determined as 0.5%. The fit of the third-order Birch–Murnaghan equation of state to our data yields a zero-pressure bulk modulus K ₀=28.98±0.03 GPa for NaCl and 6.97±0.02 GPa for amorphous red phosphorus.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

Probing oxygen in high Tc superconductor LaSrCuO by negative muons

A novel microscopic method of muon spin rotation for negative muon (μ⁻) attached to oxygen is applied on highT _c materials LaSrCuO. The paramagnetic shift and relaxation rate are measured as a function of Sr concentration and temperature. Analysis shows the large difference in shift and relaxation between two different sites. A large anisotropy was also observed by preliminary measurements along different crystalline axes.     

Depletion effect on partial organization of atactic polymer chain segments in microcells

Glass transition for atactic poly(methyl methacrylate) (a-PMMA) prepared in nano-cells by microemulsion polymerization was measured at a faster heating rate after slow cooling of the sample from a temperature above T_g. An additional enthalpy relaxation and glass transition were observed at higher temperatures for the a-PMMA sample due to the partial organization of the chain segments which occurred during microemulsion polymerization. The re-precipitated a-PMMA did not show any self-organization under the same thermal conditions, although there are no changes in molecular weight or tacticity of the polymer chains. A depletion-interaction phenomenon was understood to provide entropic force for the self-organization of polymer chains inside the walls of the microemulsion cells.