Phase Growth Mechanism Research Articles

Crystallization Behavior of High Purity Amorphous Silicon Nitride Fiber

The crystallization of high purity amorphous polymer-derived Si3N4 fiber, which was heat-treated with and without BN powders, was investigated. In the case without BN powder, the crystallization proceeds by vapor phase growth mechanism through SiO gas. The activation energy of 602kJ/mol was calculated. In the case with BN powder, the crystalline growth and crystallization through vapor phase was fairly suppressed. The crystallization proceeds by solid phase growth mechanism. The activation energy of 1273kJ/mol was calculated.Durability of Si3N4 fiber in a ceramic matrix composite was estimated. Si3N4 fiber may remain amorphous at 1300°C for 5×104h and at 1400°C for 6×10h, respectively.

Effects of varying initial Bi stoichiometry on phase formation of partial melt-processed Bi 2Sr 2CaCu 2O x

Phase formation and growth mechanisms of polycrystalline Bi 2Sr 2CaCu 2O x have been investigated over a broad range of sintering times at the partial melting temperature by scanning electron microscopy (SEM) with quantitative energy dispersive X-ray spectroscopy (EDX), optical microscopy, and magnetization hysteresis measurements. It is observed that the initial powder experienced varying amounts of Bi loss during sintering, and microstructural studies showed the formation of non-superconducting phases (Sr 1− y Ca y ) n CuO x ( n=1, 2) in addition to the superconducting 2212 phase in all samples. The effects of varying initial Bi stoichiometry on the reduction of secondary phases are investigated. By increasing the Bi content in the starting powder, the number and size of the (Sr 1− y Ca y ) n CuO x grains was reduced and the volume fraction of 2212 phase was increased.

Phase formation and growth mechanisms in Bi 2Sr 2CaCu 2O 8 glass ceramics

Devitrification of Bi 2Sr 2CaCu 2O 8 (Bi-2212) glasses, including phase formation sequences and mechanisms, has been investigated using both thermal analysis and high-temperature X-ray diffraction (HTXRD). Differential scanning calorimetry (DSC) was used to determine the activation energy for crystallization to be 395 kJ/mol, and in addition showed that the crystallization of these glasses occurs via a bulk mechanism. Differential thermal analysis (DTA) and simultaneous differential thermal and thermogravimetric analysis (STA), in conjunction with HTXRD, were used to establish the phase evolution of both bulk and powdered glass samples heated under O 2 and N 2 atmospheres. The results clearly indicate that Bi 2Sr 2Cu 1O 6 (Bi-2201) crystallizes directly from the glass first, and that Bi-2212 forms at temperatures above ~800°C by the diffusion of Ca, Cu and O into the Bi-2201 lattices.

Interfacial reactions between rhodium and GaAs in bulk and thin film form

Interfacial reactions between rhodium and GaAs in both thin film and bulk forms have been investigated in the temperature range 200–1000 °C using transmission electron microscopy, energy-dispersive X-ray analysis, scanning electron microscopy and electron probe microanalysis. Thin film reaction behavior was consistent with the bulk diffusion couples which were annealed at higher temperatures and for longer times. In the thin film case, where the rhodium supply is limited, the final configuration is RhGa/RhAs 2/GaAs. The diffusion path for bulk Rh/GaAs was determined to be Rh/ RhGa/Rh 2As/Rh 12As 7/RhAs/RhAs 2/GaAs; however, before this final configuration was reached, a series of transient paths were observed which included supersaturated phases and two-phase structures. The diffusion path and reaction morphology have been rationalized using the phase diagram, kinetic observations and growth mechanisms observed during the reactions.

Interfacial reactions in the Ir/GaAs system

Interfacial reactions between iridium and gallium arsenide in both the thin-film and bulk forms have been investigated in the temperature range 400 to 1000 °C using transmission electron microscopy, energy dispersive x-ray analysis, and electron probe microanalysis. The diffusion path for Ir/GaAs has been determined to be Ir/IrGa/IrAs2 /GaAs and is consistent with the phase diagram between the initial stages of reaction (thin-film) and long-term annealing (bulk). In the thin film case where the Ir supply is limited, the final configuration is Ir3 Ga5/ IrAs2 /GaAs. The diffusion path and reaction morphology has been rationalized using the phase diagram, kinetic observations, and growth mechanisms observed during the reactions.

Crystal growth of oxide superconductors

The general problems of crystal growth of the cuprate superconductors yttrium barium cuprate (YBCO), bismuth calcium strontium cuprates, and thallium calcium barium cuprates are thermal instability, chemical instability, and structural instability. Crucible corrosion and resulting contamination are other obstacles to obtaining large crystals with high transition temperatures. Recent studies of phase diagrams, growth mechanisms, and effects of impurities and crucibles on growth of YBCO will be discussed. Also, the reproducible growth of large free crystals of YBCO will be revealed.

Growth mechanism and magnetic properties of AlMn metastable phase formed by ion beam mixing

The kinetics of ion beam mixing and the growth mechanism of metastable phases have been investigated for the Al-Mn system. It is revealed that the residual diffusive motion induced by a collision cascade is the decisive factor for metastable phase formation. The residual diffusivity depends strongly on the substrate temperature during ion irradiation. Its relaxation time at temperatures of 350–550 K is estimated to be of the order of 10 −7−10 −6 s. More rapid quenching produces an amorphous phase, whereas slower cooling leads to thermally stable phases. The metastable τ phase with ferromagnetism is formed directly by ion beam mixing in the above temperature range. The dependences of mixing temperature and ion dose on the magnetic properties are accounted for by the kinetics of ion beam mixing and the induced internal stress. Finally, a model describing the ion beam mixing effect is proposed.

Crystallization Behavior of Amorphous Silicon Nitride Powder

Amorphous silicon nitride powder (particle size: 0.04μm) obtained by the vapor phase reaction of Si(CH3)4-NH3 system was compacted into pellet and heat-treated in N2 to investigate the crystallization behavior. The amorphous powder crystallized to 100% α-Si3N4 by the heat treatment for 1h at 1550°C. The equiaxed crystalline particles having the size of 1-2μm were obtained when powder was compacted, whereas acicular particles were produced when powder was not compacted. When Si3N4 powder was used as powder bed which covered pellet, the crystalline particles grew extensively and the crystallinity increased with a decrease in the packing fraction of particles in pellet. The heat-treated powder with a low crystallinity consisted of fine amorphous particles and coarse crystalline particles. The crystallization seemed to proceed by a vapor phase growth mechanism, in which SiO vapor takes part as silicon carrier between amorphous particles and crystalline ones. When BN powder was used as powder bed, the crystallization and grain growth were retarded, and the crystal phase contained a small amount of β-Si3N4. Crystallization inhibitor, e. g. boron oxide, may have penetrated into pellet from BN powder.

A study of the conditions for the electrodeposition of polythiophen in acetonitrile

The oxidation of thiophen and bithiophen in CH 3CN+Bu 4NBF 4 has been studied at polished Pt disc electrodes. It is shown that, as in the case of polypyrrole, the deposition of polythiophen occurs via a nucleation and phase growth mechanism akin to the deposition of a metal onto a foreign substrate. It is also demostrated that H 2O in the medium can have a strongly adverse effect on the electrodeposition process and this may explain some of the conflicting reports in the literature concerning the influence of the electrolyte and its concertration.

CoSi 2 growth: Kinetics, phase sequence and mechanism

The growth kinetics of CoSi 2 formation has been studied in the presence and absence of Au. Very small amounts of Au (as little as 14 Å at the Si Co interface) were found to increase the growth rate of CoSi 2 dramatically and also caused a decrease in the activation energy from 2.3 to 1.2 eV. A change in phase sequence and growth mechanism was also observed.

P-T-X phase diagram of the Pb-Se system and vapor phase growth mechanism of lead selenide single crystals

The mechanism and conditions for growth of PbSe crystals from the vapor phase have been determined and related to the P-T-X phase diagram. Observations of the growth process, morphological features of the growth surfaces, and the composition of as-grown crystals indicate that the investigated conditions growth seems to proceed via the vapor-liquid-solid mechanism. The real temperature difference between the charge and growth zone is 15–20°.