Decomposition Of Al2O3 Research Articles

Formation of Different Si3N4 Nanostructures by Salt-Assisted Nitridation.

Silicon nitride (Si3N4) products with different nanostructure morphologies and different phases for Si3N4 ceramic with high thermal conductivity were synthesized by a direct nitriding method. NaCl and NH4Cl were added to raw Si powders, and the reaction was carried out under a nitrogen gas flow of 100 mL/min. The phase composition and morphologies of the products were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. At 1450 °C, the NaCl content was 30 wt %, the NH4Cl content was 3 wt %, and the maximum α-Si3N4 content was 96 wt %. The process of Si nitridation can be divided into three stages by analyzing the reaction schemes: in the first stage (25-900 °C), NH4Cl decomposition and the generation of stacked amorphous Si3N4 occurs; in the second stage (900-1450 °C), NaCl melts and Si3N4 generates; and in the third stage (>1450 °C), α-Si3N4 → β-Si3N4 phase change and the evaporation of NaCl occurs. The products are made of two layers: a thin upper layer of nanowires containing different nanostructures and a lower layer mainly comprising fluffy, blocky, and short needlelike products. The introduction of NaCl and NH4Cl facilitated the evaporation of Si powders and the decomposition of Al2O3 from porcelain boat and furnace tube, which resulted in the mixing of N2, O2, Al2O, and Si vapors and generated Al xSi yO z nanowires with rough surfaces and lead to thin Si3N4 nanowires, nanobranches by the vapor-solid (VS), vapor-liquid-solid (VLS), and the double-stage VLS base and VS tip growth mechanisms.

Colloidal processing of Al2O3 and BST materials

The paper presents the analysis of thermal stability and decomposition of Al2O3 and (Ba,Sr)TiO3 green bodies as well as commercially available organic additives which are used in shaping of ceramics by colloidal processing. The first analyzed ceramic material was Al2O3 green body obtained by gelcasting. The second examined material was elastic ceramic–polymer composite based on ferroelectric powder Ba0.8Sr0.2TiO3 with NiO obtained by tape casting method. This flexible and durable material is characterized by high value of dielectric constant and finds the application in devices operating at very high frequencies up to sub-THz. Most organic additives are indispensable just in forming step but must be then eliminated in order to obtain pure ceramic phase, additionally some of them have to be stable in a wide temperature range. The analyzed organic substances were as follows: 2-carboxyethyl acrylate, SYNTRAN 8250, DURAMAX D-3005, DURAMAX B-1000. The DTA/TG measurements coupled with mass spectrometry allowed to gain new knowledge concerning degradation of selected organic additives, thermal stability of ceramic–polymer composite and type of gases released from samples during thermal treatment.

Numerical Simulation of Detonation in Fine Aluminum-Oxygen Mixtures with Hybrid Combustion Model

The detonation of fine aluminum-oxygen mixtures was numerical simulated using a hybrid combustion model. The emphasis was laid on the influence of the decomposition of Al2O3 on the detonation structure. Results showed that including the decomposition of Al2O3 will limit the gas temperature behind the detonation wave below a certain value. On the other hand, a double-wave structure will appear in the pressure profiles in cases with rich-dust condition if the decomposition of Al2O3 is not included.

Investigation on the Decomposition of Al2O3 Using an Electrochemical Method

Investigation on the Decomposition of Al2O3 Using an Electrochemical Method

Interfacial reactions and oxidation behavior of Al2O3 and Al2O3/Al coatings on an orthorhombic Ti2AlNb alloy

The uniform and dense Al2O3 and Al2O3/Al coatings were deposited on an orthorhombic Ti2AlNb alloy by filtered arc ion plating. The interfacial reactions of the Al2O3/Ti2AlNb and Al2O3/Al/Ti2AlNb specimens after vacuum annealing at 750°C were studied. In the Al2O3/Ti2AlNb specimens, the Al2O3 coating decomposed significantly due to reaction between the Al2O3 coating and the O-Ti2AlNb substrate. In the Al2O3/Al/Ti2AlNb specimens, a γ-TiAl layer and an Nb-rich zone came into being by interdiffusion between the Al layer and the O-Ti2AlNb substrate. The γ-TiAl layer is chemically compatible with Al2O3, with no decomposition of Al2O3 being detected. No internal oxidation or oxygen and nitrogen dissolution zone was observed in the O-Ti2AlNb alloy. The Al2O3/Al/Ti2AlNb specimens exhibited excellent oxidation resistance at 750°C.

Mass-Transfer Mechanism of Alumina Ceramics under Oxygen Potential Gradients at High Temperatures

The oxygen permeability of an undoped polycrystalline α-Al2O3 wafer that was exposed to oxygen potential gradients was evaluated at temperatures up to 1973 K. Oxygen preferentially permeated through the grain boundaries of α-Al2O3. The main diffusion species, which were attributed to oxygen permeation, depended on oxygen partial pressures (PO2), forming oxygen potential gradients. Under oxygen potential gradients generated by PO2 below about 1 Pa, oxygen permeation occurred by oxygen diffusing from regions of higher PO2 to regions of lower PO2. By contrast, under oxygen potential gradients generated by PO2 above about 1 Pa, oxygen permeation proceeded by aluminum diffusing from regions of lower PO2 to regions of higher PO2. In other words, O2 molecules were adsorbed onto a surface at higher PO2 and subsequently dissociated into oxygen ions (forming Al2O3), while oxygen ions on the opposite surface at lower PO2 were desorbed by association into O2 molecules (decomposition of Al2O3). The grain-boundary diffusion coefficients of oxygen and aluminum as a function of PO2 were determined from the oxygen permeation constants.

Fusion and friction stir welding of aluminum-metal-matrix composites

Microstructure evolutions and degradations of aluminum-metal-matrix composites during fusion welding were studied and compared with thermodynamic calculations. In fusion welds of Al2O3-reinforced composites, the decomposition of Al2O3 was observed. In fusion welds of SiC whisker-reinforced composites, the decomposition of SiC to Al4C3+Si by reaction with molten aluminum occurred. These phenomena led to unacceptable fusion welds in aluminum metal-matrix composites. Successful welds were produced in the same composites by friction stir welding (FSW). Significant reorientation of SiC whiskers close to the boundary of the dynamically recrystallized and thermomechanically affected zone (TMAZ) was observed. The small hardening in the dynamically recrystallized region was attributed to the presence of dislocation tangles in between SiC whiskers.

Laser-induced surface decomposition of Al2O3 excited in the V-center absorption band

The primary products of desorption from Al2O3 surface excited by laser pulses (pulse duration τ∼15 ns; wavelength λ=354 nm, radiant power density P/S<108 W/cm2) in the V-center absorption band were studied by the time-of-flight (TOF) spectroscopy. The TOF spectra show evidence of the desorption of one “ cold” (T1=300 K) and two “hot” (T2=1000 K, T3=4300 K) groups of oxygen molecules with the Maxwell velocity distributions, as well as of the hot Al and O atoms with nonequilibrium energy distributions (E1=0.37 eV, E2=0.38 eV). A model describing the oxygen desorption as initiated by the electron transitions is suggested, in which escape of the cold O2 molecules from the surface is related to discharge of the O 2 − anions adsorbed on the V-centers, desorption of the hot atoms is attributed to discharge of the surface O− anions, and the appearance of the hot O2 molecules is related to the associative desorption of two O− anions localized at the same V-center discharged by a pair of excitons.