Jerzy F. Janik

Spin fluctuation scattering of neutrons in magnetite

Abstract Observations on the peak widths and intensities of inelastically scattered neutrons from Fe3O4 are found to agree with spin-wave theory over a wide range of temperatures. The quadratic dispersion relation for ferrimagnetic spin waves is verified, and the value of JAB − 2 × 10−3 eV is in agreement with an earlier result by Brockhouse (Phys. Rev. 106, 859 (1957)). The statistical behaviour of spin waves (magnons) is found to be well described by Bose-Einstein statistics. The maximum temperature for the validity of the spin-wave approximation is seen to be a function of the magnon energy. At higher temperatures fluctuations are also found to exist in the z-component of the spins.

SiOC glasses produced from silsesquioxanes by the aerosol-assisted vapor synthesis method

In this paper, we describe a new method based on aerosol-assisted vapor synthesis for making glass materials by pyrolysis of readily available silsesquioxanes CH3Si(OCH3)3 and CH3Si(OC2H5)3. Combined powder X-ray diffraction (XRD) and spectroscopic studies in the far infrared region (FIR) showed that under applied conditions the method yielded amorphous materials. Subsequent structural studies with the application of the (29)Si and (13)C MAS NMR, Raman, and middle infrared (MIR) techniques led to the conclusion that the pyrolysis of the silsesquioxane precursors resulted in glass materials with a structure of amorphous silica v-SiO2. In the case of certain glasses prepared from CH3Si(OCH3)3, they were also shown to possess in the structure some Si-C bonds (black glasses), whereas those originated from CH3Si(OC2H5)3 were composites that in addition to the silica glass phase contained domains of free/unbound carbon.

Preparation of Phase Pure Cubic Gallium Nitride, c-GaN, by Ammonothermal Conversion of Gallium Imide, {Ga(NH)3/2}n

Gallium imide, {Ga(NH)3/2}n, was sealed in quartz tubes with anhydrous NH3 and NH4X (X=Cl, Br, I) and heated vertically in an autoclave with the hot-zone (bottom) temperature ranging from 300–530°C. With NH4I mineralizer, the imide was converted to primarily the zinc-blende phase of GaN (c-GaN), usually in the form of micron sized or smaller tetrahedra. With NH4Cl mineralizer, hexagonal GaN nanoparticles formed instead, and NH4Br facilitated the conversion of imide to irregular h-GaN nanoparticles at lower temperatures, and larger particles above 500°C. At the higher temperatures, chemical transport took place and GaN crystals grew on the walls at the middle to the top of the tube. Most of the deposited GaN was cubic, usually in a heavily etched triangular prismatic morphology. Several experiments were performed in a Pt lined pressure vessel in order to demonstrate that the synthesis method can be scaled up.

SOME RECENT DEVELOPMENTS IN PRECURSOR ROUTES TO CERAMIC NANOCOMPOSITES

Reactive inorganic polymers and copolymers have recently been used to prepare homogeneously mixed crystalline ceramic solid solutions and composites. The polymers have considerable processing advantages over classical powders, and they may be used to obtain composite fibres, coatings, and dense or porous near net shape forms.

XRD and 29Si MAS NMR spectroscopic studies of carbon materials obtained from pyrolyses of a coal tar pitch modified with various silicon-bearing additives

A typical coal tar pitch used in manufacturing of graphite electrodes was individually modified with various silicon-bearing additives yielding mixtures that were pyrolyzed (carbonized) stepwise at increased temperatures up to 1650 °C. Three commercially available additives such as the elemental Si, silica SiO2, and poly(carbosilane) {–CH2–SiH(CH3)–}n as well as reference silicon carbide SiC were used. The resulting intermediate and final materials were studied with the XRD and 29Si MAS NMR methods. For two additives, i.e., elemental Si and poly(carbosilane), the beginning of their conversion to SiC was detected at 1300 °C while for SiO2 higher temperatures were required under applied conditions.

Silicon Carbide Modified Carbon Materials. Formation of Nanocrystalline SiC from Thermochemical Processes in the System Coal Tar Pitch/Poly(carbosilane)

Poly(carbosilane) or PCS, {–CH2–SiH(CH3)–}n, is used as a Si-bearing precursor in combination with a coal tar pitch to study thermally induced transformations toward SiC-modified carbon composites. Following mixing of the components in the molten pitch at 160°C, the mixture is heated under argon atmosphere at 500°C yielding a solid carbonizate that is further subjected to separate pyrolysis experiments at 1300°C or 1650°C. At temperatures up to 500°C, the PCS reacts with suitable pitch components as well as undergoing decomposition reactions. At higher temperatures, clusters of prevailingly nanocrystalline β-SiC are confirmed after the 1650°C pyrolysis step with indications that the formation of the compound starts at 1300°C. 29Si MAS NMR, XRD, FT-IR, XPS, and elemental analysis are used to characterize each pyrolysis step, especially, from the viewpoint of transformation of silicon species to silicon carbide in the carbon matrix evolved from the pitch.

Structural and magnetic properties of GaN/Mn nanopowders prepared by an anaerobic synthesis route†

A new oxygen-free molecular precursor system based on (i) ammonolysis in refluxing/liquid NH3 of selected mixtures of gallium tris(dimethyl)amide Ga(NMe2)3 and manganese bis(trimethylsilyl)amide Mn[N(SiMe3)2]2 (Me = CH3, initial Mn-contents = 0.1, 5, 20, 50 at.%) followed by (ii) pyrolysis under flowing ammonia gas at 500, 700, and 900 °C afforded a range of nanocrystalline powders in the GaN/Mn system. The nanopowders were characterized mainly by powder XRD diffraction, FT-IR spectroscopy, Raman spectroscopy, SEM/EDX morphology examination, and XRF elemental analysis. Magnetization measurements as a function of magnetic field and temperature were carried out with a SQUID magnetometer. Structurally, the materials were shown to be single-phases based on the gallium nitride lattice. The presence of small quantities of residual amorphous Mn/N/Si/C species due to an incomplete transamination/removal of the trimethylsilylamide groups during ammonolysis was deduced from the XRF, FT-IR, Raman, and magnetization data. Magnetic properties for all nanopowders consistently pointed to a paramagnetic GaMnN phase with antiferromagnetic interactions among Mn-centers that under favorable circumstances reached the level of 3.8 at.% Mn. The paramagnetic phase was accompanied by a residual antiferromagnetic phase due to a facile oxidation in air of excessive Mn-containing by-products.

N-bridged and H-bridged aminoalanes: Single-crystal X-ray structure determinations for the planar dimer {[(Me3Si)2N]2Al(μ-H)}2 and the puckered four-membered ring compound (Me3Si)2N(Me2N)Al(μ-NMe2)2Al(H)N(SiMe3)2

Abstract The known bis(bis(trimethylsilyl)amino)alane {[(Me 3 Si) 2 N] 2 Al( μ -H)} 2 (1) is obtained from disproportionation of the base-stabilized monomeric bis(trimethylsilyl)aminoalane [(Me 3 Si) 2 NAlH 2 ] · NMe 3 in refluxing toluene. The single-crystal X-ray structure determination for 1 provides a H-bridged planar dimer with exo-cyclic N(SiMe 3 ) 2 groups. The combination of 1 with HNMe 2 in a 1:1 ratio results in competing aminolysis at the Al–H site and transamination at the Al–N(SiMe 3 ) 2 site of 1, as illustrated by the molecular structure of (Me 3 Si) 2 N(Me 2 N)Al( μ -NMe 2 ) 2 Al(H)N(SiMe 3 ) 2 (2) which is isolated from this system. The characteristic structural feature of 2 is a puckered {Al–N–Al–N} ring with different ligands on two aluminum atoms.

Preparation and characterization of bis-(trimethylsilyl) aminoalanes [H2AlN(SiMe3)2] · NMe3 and {HAl[N(SiMe3) 2]2}n crystal structure determination for tris-(trimethylsilyl) alane trimethylamine adduct (Me3Si) 3Al · NMe3

Abstract The adduct [H 2 AlN(SiMe 3 ) 2 ] · NMe 3 ( 1 ) is obtained in high yield from the reaction between H 3 Al · NMe 3 and HN(SiMe 3 ) 2 . Thermal decomposition of 1 results in disproportionation with formation of hydrogen, free NMe 3 , elemental aluminum, and {HAl[N(SiMe 3 ) 2 ] 2 } n ( 2 ). Attempted synthesis of a copolymer from the reaction of [H 2 AlN(SiMe 3 ) 2 ] · NMe 3 ( 1 ) with [(Me 3 Si) 2 AlNH 2 ] 2 results in the unexpected formation of tris-(trimethylsilyl)alane trimethylamine adduct, (Me 3 Si) 3 Al · NMe 3 ( 3 ), for which the crystal structure was determined: monoclinic, space group P 2 1 / n , a = 8.9770(10)A, b = 15.930(3)A, c = 15.980(3)A, V = 2285.1(10)A 3 , Z = 8, D = 0.888 gcm −3 , R = 7.07%.

The systems LiAlH4/NH4X and N2H5Cl as precursor sources for AlN

The reactions of LiAlH,, with NH,X (X = Cl, Br), N,H, - HCl, and N,H, .2HC1 in ethereal solvents have been investigated as potentially simple routes to a processible preceramic polymer. The polymer obtained from NH4Br is easily converted by pyrolysis at 950°C to crystalline AlN in high ceramic yield. The polymer obtained from N,H,Cl provides an ether-soluble fraction that can be used to form AlN coatings.