Influence of pore volume on laser performance of Nd : YAG ceramics

For present study, 1.1 at% Nd-doped YAG ceramics with a controlled pore volume (150–930 vol ppm) were fabricated by a solid-state reaction method using high-purity powders. The scattering coefficients of Nd:YAG ceramics, obtained from Fresnel' equation, increased simply with increases in the pore volume. The cw laser output power of Nd:YAG ceramics was clearly related to the scattering coefficients of the specimens examined in the present works, which in turn were affected on the pore volume. The effective scattering coefficients of Nd:YAG ceramics with a pore volume of 150 vol ppm were nearly equivalent to those of a 0.9 at%Nd:YAG single crystal by Czochralski method. As the exciting power was increased under excitation by an 808-nm diode laser, however, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal because of the fairly large amount of Nd additives. The lasing performance of the Nd:YAG ceramics changed drastically with change in pore volume. On the other hand, lasing performance was not affected by the existence of grain boundaries in the polycrystalline Nd:YAG ceramics.     

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.     

X-ray-Induced Paramagnetic Centers in SiO2Xerogels

SiO₂xerogels prepared by a sol–gel process followed by sintering at 350–1100°C were irradiated with 30-keV x-rays at a dose rate of 80 R/s. The room-temperature electron spin resonance (ESR) spectra of the xerogels showed signals arising from radiation-induced intrinsic defects (E" centers and terminal oxygens) and organic radicals. By recording ESR spectra in a vacuum of 10^–2Pa, surface E" centers differing in relaxation time from the volume centers were identified for the first time. The ESR signal from the E" centers was measured as a function of x-ray dose. The possible mechanisms for the formation of E" centers under x-ray irradiation are discussed.     

Synthesis, crystal structure and X-ray powder diffraction data of the phosphor matrix 4SrO·7Al2O3

The white phosphor matrix 4SrO·7Al₂O₃ has been synthesized by firing the appropriate mixture of SrCO₃, Al(OH)₃ and H₃BO₃ in the molar ratios 1:3.5:0.135 at 1300°C for 4–7 h. The crystal structure of 4SrO·7Al₂O₃ has been determined as a orthorhombic Pmma space group with a=24.7451(2)Å, b=8.4735(6)Å, c=4.8808(1)Å, V=1023.41(3)ų, Z=2, and D=3.66 g cm^–3 by the Rietveld analysis. The refinement figures of merit are R_p=8.26, R_wp=11.60, R_bragg=4.44 and s=2.61 for 844 reflections with 2<119.94°. And the corresponding X-ray powder diffraction data are presented for search/match analysis.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

Spray atomization of two Al–Fe binary alloys: solidification and microstructure characterization

The effect of solidification history on the resultant microstructures in atomized Al–3Fe and Al–7Fe powders is studied, with particular emphasis on the relationships between droplet size, undercooling and phase stability. The atomized Al–Fe powders exhibit four microstructural features, i.e. Al3Fe phase, Al + Al6Fe eutectic, -Al dendrite and a predendritic structure. The presence of these is noted to depend on a kinetic phase competitive growth mechanism, which was determined by the initial undercooling experienced by the powders. The results of scanning electron microscope analysis demonstrate that the content of Fe in the -Al phase increases with decreasing powder particle size, i.e. for Al–3 wt% Fe powders, the content of Fe in the -Al phase is 2.21 and 2.56 wt% corresponding to powder particle sizes of 90 and 33 m, respectively; for Al–7 wt% Fe powders, the content of Fe in the -Al phase is 5.51 and 5.98 wt% corresponding to powder sizes of 90 and 33 m, respectively. In the present study, homogeneous nucleation undercooling, corresponding to the -Al phases, is also estimated using an existing correlation.     

A study of electron paramagnetic resonance and optical absorption in calcium manganese phosphate glasses containing praseodymium

This paper reports on electron paramagnetic resonance (EPR) and optical absorption studies for Mn ions in praseodymium calcium manganese phosphate glasses. The EPR spectra exhibit three resonance signals at g2.0, g3.3 and g4.8. A six line hyperfine structure spectrum centered at g2.0 has been observed in the EPR spectra of all the glasses at lower concentration of Mn ions. The effects of the concentration of Mn ions and praseodymium oxide on resonance signals have been studied. The temperature dependence of EPR signals were also studied. The intensity of the resonance signals decreases with increase in temperature whereas the linewidths are found to be independent of temperature. As temperature is increased small fluctuations in hyperfine splitting are observed. From the optical absorption spectrum, the crystal field parameters and optical band gap energy were evaluated. The optical band gap energy is found to depend quite sensitively on added manganese content. The theoretical values of optical basicity were also evaluated.     

A re-examination of the Kraft and Flinn diffraction data for Cr2B, (Cr,Fe)2B, and the boride phase in Fe + 18.5 wt % Ni + 20 wt % Cr + B alloys

Kraft and Flinn's [2] powder X-ray diffraction data obtained from excess-phase particles in Fe + 18.5 wt% Ni + 20 wt% Cr + B alloys have been analyzed. The major boride phase was determined to be (Cr,Fe)2B (orthorhombic, Fddd, isomorphous with Cr2B). Its lattice parameters, a = 1.4585 ± 0.0010 nm, b = 0.7331 ± 0.0005 nm, c = 0.4223 ± 0.0003 nm, are indicative of the composition (Cr1.04±0.08Fe0.96 ± 0.08)B. It was not possible to identify the phase(s) giving rise to a much weaker set of diffraction lines in the patterns. Of the two alloys used by Kraft and Flinn as standards, the one intended to be Cr2B was found to be a mixture of Cr2B and Cr5B3. The second, intended to be Cr1.0Fe1.0B was determined to be Cr0.67± 0.07Fe1.33± 0.07B.     

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from to -SiC reveals a dependency on the initial -content and the sintering atmosphere. The transformation rate decreases with an increasing -content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure -SiC starting powders when the additive system consists of 10.34 wt% Y2O3 and 2.95 wt% AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial /-ratio (>1:9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.     

Effect of hydrogen and temperature on the resistivity of an aluminum-2 wt % copper thin film

Thin metallic films are required to provide interconnection between contacts on devices and between devices. As device dimensions decrease, the low electrical resistivity becomes the most important issue in microelectronics industry. Recently in thin metallic films, hydrogen has been shown to lower the electrical resistivity of thin metallic films, reduce the rate of electromigration in interconnect lines, and lower the stress generated in thin metallic films during annealing. These effects are referred to as hydrogen effects. Low temperature resistivity measurements were performed to determine the effect of hydrogen and temperature on the resistivity of an aluminum-2 wt% copper film. Low temperature control experiments were performed in helium since it has a similar thermal conductivity as hydrogen but is chemically inert. The electrical resistivity of an aluminum-2 wt% copper alloy is lowered by hydrogen even at low temperature. The temperature coefficient of resistivity (TCR) values are identical within the experimental error (±0.005 cm/K). This means that phonon scattering is identical. So the decrease of the electrical resistivity is due to the decrease in the residual resistivity. Hydrogen effects are reversible for an aluminum-2 wt% copper alloy at room temperature because the adsorption and desorption of hydrogen occur. Hydrogen effects are slowly reversible for an aluminum-2 wt% copper alloy at low temperature because the adsorption of hydrogen occurs, but the desorption of hydrogen does not occur readily.     

Characterization of strength and microstructure in deformation processed Al-Mg composites

The microstructures, mechanical properties and electrical resistivity have been evaluated for deformation processed Al-20 vol%Mg and Al-13 vol%Mg composites. The Mg second phase adopts a convoluted, ribbon shape filamentary morphology after deformation. Both the size and spacing of these filaments decreases with deformation. The strength of these composites increases exponentially with reduced spacing of Mg filaments. The electrical resistivity of these Al-Mg composites is slightly higher than that of pure Al.     

Effect of small amount of alumina doping on superplastic behavior of tetragonal zirconia

The effect of Al₂O₃ doping of around 0.1 wt% on superplastic behavior was studied in 3 mol% yttria-stabilized tetragonal zirconia polycrystal (TZP) which was free from SiO₂ contamination and had a grain size of 0.4 m. Compression creep tests revealed that high-purity TZP with less than 0.07 wt% Al₂O₃ had two deformation regions: the low stress region had a stress exponent of three and an apparent activation energy of 640 kJ/mol, and the high stress region had two and 460 kJ/mol. On the other hand, TZP containing more than 0.12 wt% Al₂O₃ had only one region which had a stress exponent of two and an activation energy of 480 kJ/mol. The region of diffusion control with a stress exponent of one was not observed in any samples. High resolution transmission electron microscopy revealed that no amorphous grain boundary phase was produced even with 0.18 wt% Al₂O₃ doping. Energy dispersive X-ray spectroscopy near grain boundaries revealed that yttrium was segregating at the grain boundaries with denuded zones of 30 nm width, which were created during slow cooling from the sintering temperature.     

Carrier Mobility in GaSb–V2Ga5and GaSb–GaV3Sb5 Eutectic Alloys

Experimental evidence is presented that semiconductor–metal eutectics with a low content of the metallic phase (4 vol %) are similar in electronic structure to inhomogeneous semiconductors. The microstructure of undoped and Te-doped GaSb–V₂Ga₅ and GaSb–GaV₃Sb₅ eutectic alloys is examined, and the Hall mobility of carriers in these alloys is determined. The anomalous temperature variation of Hall mobility in GaSb–V₂Ga₅ ( T ²) and GaSb–GaV₃Sb₅ ( T ⁵) is interpreted in terms of infinite clusters ofn-type metallic inclusions embedded in a p-type semiconductor matrix and interconnected through overlapping inhomogeneous interfaces. It seems likely that the difference in conductivity type between the semiconductor matrix and the infinite clusters gives rise to a random large-scale potential relief. Te compensation of GaSb in the eutectic alloys causes the Hall mobility to vary more rapidly with temperature, T ³ to T ¹⁰, which is interpreted as due to an increase in the amplitude of the random large-scale potential relief, the formation of infinite clusters, and partial compensation of unintentional acceptor doping in the semiconductor matrix.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

Viscosity of the Grain-Boundary Phase and Sintering of TiB2-Modified Alumina

A model is proposed for the sintering of powders via the sliding of particles facilitated by a grain-boundary phase. Data on the initial-stage sintering kinetics of TiB₂-modified alumina are used to assess the temperature-dependent viscosity of the grain-boundary phase, formed by Al₂O₃, oxidation products of TiB₂, and impurities present on the surface of alumina particles. In the range 1570–1620 K, the temperature variation of viscosity is fitted well by the expression 25.9exp(143308/RT).     

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     

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.     

Mechanical and Corrosion Characteristics of Zr + 2.5% Nb Zirconium Reactor Alloy

The results of structural and phase hardening of pipes made of Zr + 2.5% Nb alloy show that ultrahigh-frequency thermal treatment of pipes (fast heating to the temperature of existence of the -phase followed by sharp cooling and annealing in the high-temperature range of the -phase) destroys the texture and forms a fine-grained structure (the grain diameter is about 1 m) with numerous transitional twins and a high density of precipitations of the secondary -niobium phase ( 10¹⁶cm^–3). In this state, the alloy is rather strong and plastic (at room temperature, _u 650 MPa, _0.2 550 MPa, and 20% both in the longitudinal and transverse directions). The efficiency of hardening by ultrahigh-frequency thermal treatment is not reduced with increase in the temperature of testing up to 500°C. Corrosion tests of channel pipes made of Zr + 2.5% Nb alloy subjected to ultrahigh-frequency thermal treatment in water containing various amounts of oxygen (from 0.1–0.3 to 600 mg/kg) at temperatures of 285–350°C for 700–6600 h under static conditions and in reactor water of the Ignalina Nuclear Power Plant for 5000 h under dynamic conditions showed that the corrosion resistance of this alloy is on a par with the corrosion resistance of the material of assembly channels of high-power channel reactors subjected to a standard treatment.     

Investigation into the growth and electromechanical properties of Pb(Mg1/3Nb2/3)O3 thin films

Pulsed laser deposition (PLD) has been used to fabricate relaxor thin films and thin film capacitors based on the Pb(Mg_1/3Nb_2/3)O₃ system. Best capacitor structures show dielectric constants (_r) of 1000 and losses (tan ) 0.02 at 1 kHz at 300 K. Electromechanical investigations show that tensile longitudinal strains of up to 0.2% can be achieved in these films.     

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.