On the temperature dependence of the London penetration depth in a superconductor

In the framework of the electron-phonon theory of superconductivity [1], it is shown that the magnetic field penetration depth _L increases like a certain power of temperature atTT _c due to the low-lying excitations in the phonon spectrum. For the acoustic phonons with the density of states ² the penetration depth increases T ⁵. The origin of such a high power ofT is the same as that in the case of resistivity of the normal metal: the phonon corrections to the electromagnetic vertex should be taken into account, and major terms (T ³) cancel, the surviving ones having a higher power ofT. The possibility of linear and quadratic terms in _L(T) is discussed in a model of electrons interacting with two-level centers [2].     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Structure of alkali dimers at the surface of liquid helium

Calculations are presented of the equilibrium configuration (dimple) of a Na₂ or Li₂ molecule absorbed on the surface of liquid³H e or liquid⁴He. The computed aimer binding energies are somewhat greater than those of the monomers. The lowest energy occurs when the molecule lies flat, but the energy in the erect orientation is only 1K higher (implying relatively free rotation). The center of mass lies 4Åabove the liquid surface and the dimple has a depth 3Å. An exceptional case is Li₂ on liquid³H e, for which the surface state is unstable relative to solvation in the bulk.     

Coherent Brillouin Spectroscopy in Solid Parahydrogen

We applied coherent Brillouin spectroscopy to solid parahydrogen, and measured the Brillouin spectra of longitudinal acoustic modes at 5.6K. It was found that the linewidth of these spectra is 1.5MHz. From the observed Brillouin shift and the crystal orientation, the elastic stiffness was determined as C ₁₁=0.355±0.016GPa and C ₃₃=0.432±0.022GPa.     

The frictional behaviour of LiF single crystals

A study has been made of the influence of initial surface roughness, renewable and non-renewable surface contaminants, and irradiation hardening on the coefficient of friction for one LiF single crystal (A) sliding on another (B) in {100}_A<010>_A{100}_B 010_B orientation at 295 K. The normal load was 1 N, the nominal contact pressure 0.1 MPa, the sliding velocity 0.2 to 0.6 mm sec^–1, and the amplitude of the (reciprocate) motion a few millimetres. Any influence of non-renewable contaminants persisted only for cumulative relative displacements 0.1 m, and that of micrometre-scale initial surface roughness only for a few metres. At steady state in the presence of renewable contaminants the coefficient of friction varied only from a high of 0.45 in ultra-high vacuum ( 7.5 × 10^–8 Pa) and dry nitrogen-rich air ( 10⁵ Pa, relative humidity 15%) to a low of 0.38 in moist nitrogen-rich air ( 10⁵ Pa, relative humidity 50%). Irradiation hardening had no effect on the coefficient of friction at steady state. The worn surfaces created by steady-state sliding always exhibited a grooved topography partly obscured by more-or-less adherent layers of variously consolidated equiaxed debris particles 100 nm in size. Owing to the action of image forces, these particles contained no dislocations. It is suggested that the coefficient of friction was determined at steady state by the stress needed to shear these tiny particles past one another as near-rigid bodies.     

Low-temperature synthesis,pyrolysis and crystallization of tantalum oxide gels

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅C₂H₅OHH₂OHCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide water HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase -Ta₂O₅, which transformed into the high-temperature tetragonal phase -Ta₂O₅ when further heated to 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The -Ta₂O₅ converted back into the low-temperature phase, -Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.     

Low temperature internal friction on γ-irradiated polyvinylidene fluoride (PVDF)

A least-squares fitting of the below room temperature part of the internal friction spectra, obtained by the torsion pendulum technique on as-received and-irradiated (up to 1 Grad) strips and fibres of polyvinylidene fluoride [-(CH₂-CF₂-) _n -.; PVDF] by a superposition of single Debye functions, reveals that the spectral component features are determined not only by purely amorphous chain characteristics but also by the dosedependence of crystallinity. A careful analysis of the relaxation spectra confirms that at least one relaxation effect (236 K) is created upon irradiation. The analysis of the dose dependence of the characteristics of the (glass transition; 220 K) and _u (apparent upper glass transition; 270 K) relaxations, suggests the probable influence of crystallinity on the molecular motion in the amorphous phase. The increase of the intensity of the relaxation (190 K) is related to the irradiation-induced crystallite degradation.Fellow of the Interuniversitair Instituut voor Kernwetenschappen     

Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

The deformation behaviour of bulk polycrystalline zirconium hydrides in the composition range ZrH_1.27 to ZrH_1.66 has been investigated by compressive loading at temperatures between room temperature and 500° C. Single-phase -zirconium hydride is brittle below 100° C. Analyses of slip traces on specimens deformed at temperatures between 100 and 250° C have shown that the glide planes are {111} types. The deformation characteristics of and ( + ) alloys at temperatures between 100 and 500° C are consistent with the hydrogen vacancies in the -phase providing significant lattice friction to the movement of dislocations in the zirconium lattice of the hydride structure. The room temperature fracture stress of ( + ) alloys increases with the volume fraction of the -phase and this can be related to the resistance offered by platelets to the propagation of cleavage cracks in the matrix. In a ( + + ) alloy the resistance to crack propagation at room temperature is further increased by the soft -zirconium phase.     

Preparation of YBa2Cu3O7 ? δ Thin Films by the Self-Template Sputtering Method

Using the self-template technique, c-axis-oriented epitaxial YBa₂Cu₃O_{7 –} thin films have been prepared in situ on LaAlO₃ substrate by the d.c. magnetron sputtering method. The properties of thin film dependence on the deposition conditions of the two-step self-template method have been systematically investigated. By optimizing the parameters, high-quality YBCO thin films with T _c0 90 K, T _c 1 K, R _s (77 K, 10 GHz)500 were reproducibly obtained. The best sample grown under optimal conditions gave a low R _s of 330 at 77 K, 10 GHz, which can be used in a microwave field.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

Tilted and Crossing Vortex Chains in Layered Superconductors

No Heading In presence of the Josephson vortex lattice in layered superconductors, small c-axis magnetic field penetrates in the form of vortex chains. In general, structure of a single chain is determined by the ratio of the London [] and Josephson [_J] lengths, = /_J. The chain is composed of tilted vortices at large s (tilted chain) and at small s it consists of crossing array of Josephson vortices and pancake-vortex stacks (crossing chain). We study chain structures at the intermediate s and found two types of phase transitions. For 0.6 the ground state is given by the crossing chain in a wide range of pancake separations a [2–3]_J. However, due to attractive coupling between deformed pancake stacks, the equilibrium separation can not exceed some maximum value depending on the in-plane field and . The first phase transition takes place with decreasing pancake-stack separation a at a = [1 – 2]_J, and rather wide range of the ratio , 0.4 0.65. With decreasing a, the crossing chain goes through intermediate strongly-deformed configurations and smoothly transforms into the tilted chain via the second-order phase transition. Another phase transition occurs at very small densities of pancake vortices, a [20 – 30]_J, and only when exceeds a certain critical value 0.5. In this case small c-axis field penetrates in the form of kinks. However, at very small concentration of kinks, the kinked chains are replaced with strongly deformed crossing chains via the first-order phase transition. This transition is accompanied by a very large jump in the pancake density.PACS numbers: 74.25.Qt, 74.25.Op, 74.20.De     

High-Rate IR Laser Evaporation of Silica Glass

IR (10.6 m) laser irradiation of silica glass preforms ensures rapid evaporation of the surface layer. The resulting deposit consists of amorphous SiO₂ with a particle size of 70 nm.     

High temperature deformation behaviour of MoSi2 and WSi2 single crystals

High temperature deformation behaviour of MoSi₂ and WSi₂ single crystals, which both oriented near 001 and near 100, have been studied by compression tests over the temperature range of 1100 to 1500° C in a high vacuum of less than 6×10^–4 Pa. At elevated temperatures, several per cent compression deformation is possible in both MoSi₂ and WSi₂. Slips on {110 and {013 planes, the dislocation with the direction of Burgers vector 331 and the stacking fault on {110 plane are observed in both deformed MoSi₂ and WSi₂. In MoSi₂, the 0.2% offset stress of the sample oriented <001> is higher than that of the sample oriented <100>. The higher strength of the sample oriented <001> is related to the higher CRSS for the main slip plane of it. The reverse orientation dependence of the strength in WSi₂ is also correlated with the difference in CRSS on {110 and {013 planes, which shows the opposite result to MoSi₂. The higher CRSS on {110 plane in WSi₂ compared to that on {013 may be caused by the formation of a large number of stacking faults on {110 plane.     

Corrosion resistance and microstructure of stainless steel modified by pulsed high energy density plasma

Pulsed high energy density plasma has been used to deposit aluminum nitride films on the surface of the 1Cr18Ni9Ti stainless steel (0.1% C, 18% Cr, 9% Ni, 0.8% Ti). The formed films are composed of nanocrystalline-structured aluminum nitride phase, whose crystal sizes are about 10 nm. Transition areas are formed across the film to the substrate and strengthen the adhesion between them. The nanocrystalline-structured aluminum nitride films contribute to the improvement of the corrosion resistance of the modified stainless steel, whose corrosion rate has been reduced by about ten times compared with that of the unmodified stainless steel.     

Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

High resolution TEM study of ceramic BaO·Pr2O3·4TiO2

Ceramic specimens of BaO·Pr₂O₃·4TiO₂ (Ba_4.5Pr₉Ti₁₈O₅₄) were prepared by the mixed oxide-route. The single phase products were examined by High Resolution Transmission Electron Microscopy (HRTEM). Orthorhombic symmetry, with cell parameters a22.2 Å, b12.2 Å and c7.6 Å, was confirmed. On the basis of space group pnam (No. 62) and the crystal structural data of Rawn et al., good agreement was obtained between the experimental HRTEM images and images simulated by the multislice method.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Phase formation kinetics of nanoparticle-seeded strontium bismuth tantalate powder

Strontium bismuth tantalate (SBT) having composition of Sr_0.7Bi_2.3Ta₂O₉ has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt% of nanometer-sized SBT particles to the sol followed by pyrolysis. Differential thermal analyses (DTA) were performed on the unseeded and seeded powder and Aurivillius phase formation temperatures were found to be reduced by 60°C in the seeded ones. Non-isothermal kinetic analyses were applied to the DTA results to obtain activation energy and Avrami exponent values for the Aurivillius phase formation in unseeded and seeded samples. The activation energy for the Aurivillius phase formation was found to be 268 kJ/mol for the seeded ones, while 375 kJ/mol for the unseeded ones, which plays a major role for the enhanced kinetics in the seeded ones. The Avrami exponent values for the Aurivillius phase formation in unseeded and seeded ones were determined as 2.80 and 1.15, respectively, revealing different nucleation and crystal growth mechanisms.     

Effect of pH on 980 °C spinel phase-mullite formation of Al2O3-SiO2 gels

Different reaction paths of mullite formation via sol-gel processing techniques are reviewed. These variations are due to differences in hydrolysis/gelation behaviours of the silica and alumina components used. Variations of pH during processing without altering other variables follow three different routes of mullite formation. In the highly acidic region(pH 1), the gel does not exhibit a 980 °C exotherm but forms -Al₂O₃. Mullite forms at high temperature by diminution of -Al₂O₃ and -cristobalite, respectively. In the pH range of 3–4.5, gels exhibit a 980 °C exotherm and develop only mullite. In the highly alkaline region (pH 14), the gel produces a Si-Al spinel phase at the 980 °C exotherm and mullite formation at the 1330 °C exotherm takes place from the intermediate Si-Al spinel phase.     

Formation of Friction Layers and Wear Resistance of Steels under Conditions of Boundary Friction

We studied the influence of the lubricant compositions Grafitol with 10% graphite (1), Aerol containing 17% MoS₂ (2), Limol containing 60% MoS₂ (3), Limol + 10% chlorine-paraffin (4), and Limol + 10% copper powder (5) on the wear rate and formation of the fine structure of surface friction layers of structural steels. We established a correlation between the tribological characteristics of steels and lubricants. The abrasive wear of 40KhFA steel was minimum if it was lubricated with Limol + 10% copper powder. In this case, its wear was smaller by a factor of 10, 2, 1.25, and 7.25 as compared with lubricant compositions 1–4, respectively. In the course of minimum wear of 40KhFA steel, in the surface friction layers, we observed the minimum values of second-kind distortions (a/a) and of the true size of domains of X-ray coherent scattering (D) as well as the minimum difference between the crystal lattice constants (a) of steel before and after friction.