Mariusz Drygaś

Magnetic and optical properties of (GaMn)N nanocrystalline powders prepared by the aerosol-assisted vapour phase synthesis and anaerobic imide route methods

Herein, we report a study on magnetic and optical properties of hexagonal (GaMn)N nanocrystalline powders obtained by two methods: aerosol-assisted vapour phase synthesis and anaerobic imide route. Measurements of the magnetization as a function of temperature and magnetic field for the powders show a typical paramagnetic behaviour. In addition, antiferromagnetic contribution originating from a residual MnO by-product and a small ferromagnetic contribution are observed. Magnetization measured as the function of magnetic field shows a smaller saturation effect than expected for non-interacting Mn-ions. Electron paramagnetic resonance (EPR) measurements reveal a single, structureless resonance line with a g-factor equal to 2.008±0.003 indicating a presence of Mn 2+ -centres in the samples. Both magnetic and EPR measurements suggest weak AF interactions between Mn-ions incorporated in (GaMn)N.

Morphology and surface properties of carbonizates (C/SiC nanocomposites) obtained via pyrolysis of a coal tar pitch modified with selected silicon-bearing precursors

Morphology of carbon-based composite materials obtained from pyrolysis of a coal tar pitch admixed with selected silicon-bearing additives is discussed based on SEM/EDX observations, mercury porosimetry, BET surface area determinations, and helium density data. The silicon precursors used in the study included elemental silicon Si, silica SiO2, poly(carbomethylsilane) {–CH2–SiH(CH3)–} n , and commercial SiC. Each individual binary mixture, i.e. pitch/silicon additive, was first repeatedly homogenized at 160°C in the liquid medium of the molten pitch followed by carbonization at 500°C. In all cases, one part of the initial 500°C solid carbonizate was further pyrolyzed at 1300°C and another part at 1650°C under an argon flow resulting in nanocomposite products C/SiC. Differences in properties and morphology of the products were linked to specific chemical changes taking place in the reaction systems.

Ammonolytical conversion of microcrystalline gallium antimonide GaSb to nanocrystalline gallium nitride GaN: thermodynamics vs. topochemistry

Reaction of microcrystalline powders of readily available gallium antimonide GaSb with ammonia gas at elevated temperatures afforded in one step high yields of nanocrystalline powders of the semiconductor, gallium nitride GaN. In particular, temperatures of 900–1000 °C and suitable reaction times of 36–170 hours resulted in complete nitridation in the system. GaN was prepared as a mixture of the major stable hexagonal and the minor metastable cubic polytypes. Formation of the cubic GaN was consistent with topochemistry playing a meaningful role in the ammonolysis of the cubic GaSb substrate. Specific experimental conditions, including variations in the reaction temperature/time and manual grinding or high energy ball milling of the substrate, had a significant impact on the final GaN polytype make-up and average crystallite size, the latter ranging from a few to a few tens of nanometers. Under the applied conditions, all by-products were conveniently removed from the reaction mixture as volatile species, thus affording chemically pure GaN nanopowders of very good quality.

Ammonolysis of polycrystalline and amorphized gallium arsenide GaAs to polytype-specific nanopowders of gallium nitride GaN

Convenient single-step N-for-As metathesis reactions of gallium arsenide GaAs with ammonia NH3 at temperatures in the range 650–950 °C for 6–90 hours afforded in this oxygen-free system high yields of pure nanocrystalline powders of the wide bandgap semiconductor gallium nitride GaN. High energy ball milling via noticeable amorphization of the monocrystalline cubic GaAs substrate enabled complete ammonolysis and nitride preparation at lower temperatures and shorter times relative to manual grinding. Under the applied conditions, all by-products were removed as volatiles affording pure GaN nanopowders. Reaction-controlled average crystallite sizes ranged from a few to a few tens of nanometers. When compared to the related ammonolysis reactions of cubic GaP and cubic GaSb which yielded, respectively, either the hexagonal polytype only or mixtures of mostly hexagonal with some cubic GaN polytypes, here, the nitride could be made both as solely hexagonal and as a mixture of two polytypes in a wide composition range. All this supports diverse reaction pathways which were found to be closely correlated with substrate grain size characteristics. The ball milled fine GaAs particles afforded only hexagonal or hexagonal GaN-enriched mixtures pointing to predominantly thermodynamic reaction control. Under similar conditions, the manually ground coarser GaAs particles yielded cubic GaN-enriched mixtures, instead, consistent with prevailing topochemical control.