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.     

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.     

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.     

Limits on Superconductivity in Nanoparticles and Nanowires

Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter 5–10 nm, length 150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e² 6.5 k.     

TEM observation of iron oxide pillars in montmorillonite

The microstructures of iron oxide-pillared montmorillonite have been investigated by highresolution transmission electron microscopy. The iron oxide is incorporated in the interlayer space of montmorillonite as Fe₂O₃ crystallites. The crystallites are present in two different morphologies: isolated particles 10 nm long and 4 nm thick, and bands (possibly plates) with a thickness 1 nm running parallel to the basal plane.     

Sol-gel prepared Ni-alumina composite materials

The sol-gel method has been utilized for the preparation of dense, homogeneous ceramic-metal composites with up to 50% Ni in Al₂O₂. Examination by SEM and TEM shows that the materials consist of micrometre-size Al₂O₃ with metallic Ni in isolated regions from 50 m down to nanometre size. The density ranges from 97% (10% Ni) to 74% (50% Ni) of the theoretical number. The hardness decreases from 18 GPa for pure alumina to 10 GPa for alumina containing 50% Ni. The fracture toughness increases significantly from K _1c=3–4 MPa m^1/2 to K _1c=8.5 MPa m^1/2. The elastic and shear moduli decrease from E=400 GPa and G=160 GPa for pure alumina to E=320 GPa and G=135 GPa when containing 50% Ni. The electrical resistivity is 10⁶m with 10 to 40% Ni but decreases drastically at 50% Ni content.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

The structure of sea urchin spines, large biogenic single crystals of calcite

Sea urchin spines are porous, single crystal Mg-rich calcite [(Mg, Ca)(CO₃)] with a three dimensional meshwork architecture. The crystallographic orientation of large (10 cm in length) spines from Heterocentrotus trigonarius was determined using the X-ray back-reflection Laue technique; the long axis of all spines is parallel to the 00·1 direction of calcite. The internal structure was studied using SEM and TEM. The macropore structure dominated the SEM images, whereas many small (80 nm) protein occlusions can be observed within the single crystal spines using TEM. The spines appeared crystallographically perfect in the TEM. The bend strengths of a group of spines were also determined and varied between 13 and 41 MPa.     

Widening the Scope of Working of Solid‐State Nd‐Glass Lasers

With the example of the serial model of a pulsed Ndglass laser, it is shown that the use of an unstable resonator with a semireflecting homogeneous exit mirror makes it possible to significantly decrease the laserbeam divergence and increase the uniformity of the radiationintensity distribution in the near zone. We were the first to obtain a laserbeam quality of (40–50) mm·mrad for technological glass lasers and to attain a depth of fusing of of 6.3 mm for steel (aspect ratio 10) for an energy of 23 J. The obtained uniformity of the radiationintensity distribution was estimated at the level of ±10%. The beamquality level attained for the Ndglass laser beam allows us to recommend it for both realization of deepfusion regimes and laser hardening without the use of external integrating optical elements.     

New high permittivity and low loss ceramics in the BaO–TiO2–Nb2O5 composition

New dielectric ceramics with formula BaTi3Nb4O17 and Ba6Ti14Nb2O39 have been prepared and characterized. BaTi3Nb4O17 was densified to 92% of TD after firing at 1310 °C for 4 h. However, Ba6Ti14Nb2O39 fired under optimized conditions (1260 °C for 4 h) showed only 85% TD together with secondary phase. The crystal system of both of the compositions is orthorhombic. The BaTi3Nb4O17 has r56, Qu2100 (at 4.402 GHz), f+86 p.p.m. K-1 and Ba6Ti14Nb2O39 as r50, Qu650 (4.359 GHz) and f+165 p.p.m. K-1. © 1998 Kluwer Academic Publishers     

Measurement of the thermomechanical coefficient in confined normal liquid3He

The thermomechanical coefficient P/T has been measured for liquid³He from 2–20 mK. The temperature difference was established across³He in a porous plug with 90 nm pore diameter. The coefficient was 5 kPa K^–1 and only weakly temperature dependent; in terms of the heat capacity per unit volume, the coefficient varies from 4C at 2 mK to C at 20 mK.     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

Correlation between magnetic hysteresis and magnetic relaxation in YBaCuO single crystals

The dependence of the magnetic momentm obtained from the hysteresis loops on the speed of the magnetic field sweep =dH _ext/dt is explained on the basis of Anderson's interpretation of the magnetic flux creep. In addition, a phenomenological model is suggested which predicts a linear dependence ofm on ln with the slope m/ ln , numerically equal to the relaxation rate m/ ln(t) from the usual magnetic relaxation. Such linear relations betweenm and ln were observed experimentally in single crystals of YBaCuO. Preliminary experiments on the complementary time dependent relaxation ofm after a simulated step change ofH _ext gave mostly relaxation rates close to the predicted values. The model here presented also enables one to compare the critical state in the superconductor at a field sweep rate with the critical state at some timet _eff after a step change ofH _ext. The values of analyzed in our experiments actually correspond to the critical state at timest _eff between0.04 and4 sec after an imaginary large step change ofH _ext.     

Hydrothermal Synthesis and Spectroscopic Properties of Crystalline ZnO Powders and Films

Polycrystalline zinc oxide is synthesized under mild hydrothermal conditions (150–250°C). The phase composition, morphology, and pulsed cathodoluminescence (PCL) spectra of ZnO powders, polycrystalline films, and single crystals are studied as a function of synthesis conditions (the nature of the precursor, the type and concentration of the solvent, solution temperature, and the redox potential of the system). The crystallite size in the resultant materials varies from 100 nm to 10 m, depending on the synthesis conditions. The PCL spectra measured under pulsed electron-beam ( 2 ns) or laser (Nd:YAG, fourth harmonic, = 266 nm, 10 ns) excitation show two bands in the UV and visible spectral regions. The intensity ratio between these bands depends on the ZnO preparation conditions. The influence of the precursor and solvent on the PCL spectrum of ZnO is studied. The redox potential of the system is shown to have a strong effect on the luminescent properties of the resultant material.     

Kinetics of Solid State Reactions With Fractal Reagent

In the present research we theoretically studied the kinetics of nucleation-limited solid state reactions as influenced by the fractal properties of solid reagent. We consider the model of equal-sized primary particles assembled in fractal cluster. The geometry of such an object is assumed to be described solely by its fractal dimension D and by upper (R _max) and lower (R _min) cutoffs of fractality further identified with the overall size of the object and the size of the primary particle correspondingly. Depending on the ratio between R _max, R _min and the radius of the critical nucleus R _nucl the following cases are considered: (1) R _max R _nucl. In this case the reaction kinetics is described by the equation: = 1 – B{ln(k + 1)}^D/(D–3), where B, k are constants. Numerical solution of this equation gives rise to n-order reaction kinetics with n > 1. (2) R _min R _nucl R _max. In this case under certain conditions there can exist non-trivial critical density _crit 0, 1 that favors the formation of the critical nuclei of the new phase. The asymptotic kinetic equation for large times corresponds to n-order reaction with n = (D + 3)/(D + 1). (3) R _min R _nucl R _max. In this case the reaction follows the first-order kinetics with D-dependent rate constant.     

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.     

Effect of Air Oxidation on the Thermophysical Properties of a Zirconium Alloy

The results of an experimental study of the effect of air oxidation on the temperature dependence of the heat capacity and normal spectral emissivity (for the wavelength of 0.65 m) of the zirconium alloy E635 (1.3 mass% Sn, 0.3 mass% Fe, 1 mass% Nb) are presented. The subsecond resistive pulse heating technique has been used. The samples were heated in ambient air by single and multiple cyclic pulses.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Microhardness response of nitrogen-implanted and X-irradiated lithium fluoride

The microhardness behaviour of {001} faces of LiF has been investigated before and after both nitrogen-ion implantation and implantation followed by X-irradiation. Implantation alone produced no changes in either hardness or dislocation etch-pit rosette sizes on the surface, though radial indentation fracture was promoted. Implantation followed by X-irradiation produced high densities of colour centres deep into the crystal, much deeper than the implantation damage depth (0.2m). Hardness and rosette sizes were affected to depths of 150m, which was less than the colouration depth. Thus the colour centres producing the colour are probably different from those impeding dislocation motion.