Intrinsic and mechanically modified thermal stabilities of α-, β- and γ-aluminum trihydrides AlH3

The intrinsic and mechanically modified thermal stabilities of the α-, β-, and γ-phases of AlH₃ have been experimentally determined. The thermogravimetric profiles of the α- and γ-phases exhibit dehydriding reactions in the temperature range of 370–450 K. The amounts of hydrogen released are nearly 9 mass %. The profile of the β-phase shows the continuous dehydriding reactions, which differs from the other two phases. The values of the enthalpy of dehydriding reactions ΔH_dehyd. are determined to be 6.0±1.5, -3∼-5 and 1.0±0.5 kJ/mol H₂ for the α-, β- and γ-phases, respectively. The milling-time dependences of the powder X-ray diffraction measurement and thermal analyses indicate the occurrence of the dehydriding reactions both in the α- and γ-phases during milling, but there is no drastic change in the β-phase. PACS 84.60.Ve; 51.30.+i; 81.70.Pg     

Excited state absorption and stimulated emission of Nd3+ in crystals. Part 2: YVO4, GdVO4, and Sr5(PO4)3F

3+ ion in the crystals YVO₄, GdVO₄, and Sr₅(PO₄)₃F. The measurements were performed in the spectral region of the main laser transitions ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2by a continuous wave pump and probe technique. The calibrated gain and ESA spectra are presented and possible implications of ESA on the laser performance are estimated. It is shown that ESA can be a small loss factor to the laser emission near 1060 nm but does considerably diminish the effective emission cross sections near 1340 nm especially in Nd:YVO₄ and Nd:GdVO₄. Received: 29 January 1998/Revised version: 8 May 1998     

Network-theoretical model of the gyrotron oscillator part I: Empty cavity oscillation modes

A network-theoretical model of the gyrotron has been elaborated which is both conspicious and rigorous. The problem of self-consistently representing the field in the presence of the electron beam is attacked by expansion into the empty structure oscillation modes. In this first part, a method is presented to determine the empty gyrotron cavity oscillation modes, i.e. theQ factors, resonance frequencies, and field distributions in axisymmetric, but otherwise quite general cavities. It is based on the field representation in terms of local normal waveguide modes. Matrix equations in these modes for different type of cavity sections (building blocks) are formulated. Along with the open resonator boundary conditions, these equations form a nonstandard eigenvalue problem; the eigenvalues are the complex eigenfrequencies, the eigenvectors correspond to the field distributions. By way of example, the method is applied to a simple and a complex cavity.     

GC/MS-FID analysis of BSTFA derivatized polar components of diesel particulate matter (NBS SRM-1650) extract

Summary Hydroxy and acid compounds, present in the polar HPLC fraction number 8 from the NBS-SRM 1650 (diesel particulate extract) were derivatized with N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) to form the trimethylsilane (TMS) derivatives. This improved the gas-chromatographic separation to give an improved characterisation using GC-MS. TMS derivatives were only observed in the acid extract of an aliquot of HPLC fraction number 8 which had undergone an acid-base fractionation. A number of previously unidentified compounds in fraction 8 could be characterised using mass spectroscopy. Ion mass chromatograms, recorded at m/z 73, 89 and 147 were correlated with the total ion current (TIC) to identify derivatized compounds. Only single, two and three ring derivatized aromatics were observed. The compounds identified in this polar fraction of the extract included p-nitrophenol, benzoic acid, hydroxy-benzoic acids, hydroxy-benzaldehyde, hydroxy-phthalimide, benzene dicarboxylic acids, naphthalene dicarboxylic acids, anthracene/phenanthrene dicarboxylic acids and, for some of these, the C1 to C3 alkyl homologues. Several methyl esters were also observed including biphenyl dimethyl ester and benzene trimethyl ester.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Majorants and Minorants for Elliptic Measures on

Let μ(· ; Σ, G₁) and μ(· ; Ω, G₂) be elliptically contoured measures on ^k centered at 0, having scale parameters (Σ, Ω) and radial cdf′s (G₁, G₂). Elliptical measures v_m(·) and v_M(·), depending on (Σ, Ω, G₁, G₂), are constructed such that V_m(C) ≤ {μ(C; Σ, G₁), μ(C; Ω, G₂)} for every symmetric convex set C ^k with equality for certain sets. These in turn rely on the construction of spectral lower and upper matrix bounds for (Σ, Ω). Extensions include bounds for certain ensembles and mixtures, including versions having star-shaped contours. The lindings specialize to give envelopes for some nonstandard distributions of quadratic forms, with applications to stochastic characteristics of ballistic systems.     

The Migration of High Angle Grain Boundaries during Recrystallization

When plastically deformed metallic materials are annealed, new strain free grains emerge from the microstructure and grow by means of grain boundary migration until the deformation microstructure is eliminated. This process is called recrystallization. In this paper the various methods by which grain boundary migration rates are measured stereologically in order to characterize the growth process are described and compared using illustrations from recrystallization experiments on commercial AA1050 aluminum. It seems abundantly clear that during recrystallization of cold-deformed materials, isothermal grain boundary migration rates decrease with time and reasons for such a decrease are discussed. A new methodology whereby migration rates of the individual recrystallization texture components may be quantified by combining stereology and orientation imaging by the electron back scattered pattern analysis is outlined. By illustration, recent experiments on aluminum and copper are summarized documenting the slight growth rate advantage the cube texture component (001)[100] possesses during recrystallization of cold rolled material. The role of orientation pinning effects on grain boundary migration is described briefly. It appears that such pinning effects allow recrystallized grains emerging from the weaker deformation texture components to enjoy an average growth rate advantage over those emerging from the stronger deformation texture components.