Subsequent High-temperature Treatment Research Articles

Ion-beam synthesis of InAs nanocrystals in crystalline silicon

The formation of nanodimensional InAs crystallites on Si wafers was studied by the method of high-fluence implantation of As and In ions with subsequent high-temperature treatment. It was found that the size and depth distributions of the crystallites depend on both the implantation temperature and the annealing conditions. A broad band in an energy range of 0.75–1.1 eV was recorded in the photoluminescence spectra of the samples.

Microstructure and Mechanical Properties Determined in Compressive Tests of Quasi-Rapidly Solidified NiAl-Cr(Mo)-Hf Eutectic Alloy After Hot Isostatic Pressure and High Temperature Treatments

The effect of high cooling rate of approximately 102 K/s and subsequent hot isostatic pressure (HIP) and high temperature (HT) treatment on the microstructure and mechanical properties of NiAl-based intermetallic alloys was investigated. The results reveal that rapid solidification refines the microstructure of the NiAl-Cr(Mo)-0.5Hf eutectic alloy and transforms the Ni2AlHf Heusler phase, which is present in the equilibrium state to a metastable Hf(Ni, Al, Cr) solid solution phase. Simultaneously, the shape and distribution of the Hf(Ni, Al, Cr) solid solution phase were considerably improved. After the HIP treatment, the Hf(Ni, Al, Cr) solid solution phase has changed from continuous distribution along eutectic cell boundaries into semicontinuous distribution, and the primary NiAl(Cr, Mo) phase has coarsened. The HT treatment reduces the volume fraction of the primary NiAl(Cr, Mo) phase and optimizes the distribution of the Hf(Ni, Al, Cr) solid solution phase. Rapid solidification and the resulting fine-grained microstructure will significantly improve the mechanical properties of the alloy in compression tests. However, additional HIP and HT treatments enhance the high-temperature strength properties obviously.

Simulation of nanochannel membrane formation

Monte Carlo simulations of atomic processes on the surface of silicon nanochannel membranes during molecular-beam epitaxy and subsequent thermal oxidation are performed. It is demonstrated that silicon deposition on Si(001) wafers with 1–100 nm cylindrical pores results in constriction of channel inlets. The rates of reduction of the nanochannel diameter are estimated as functions of the wafer temperature, silicon deposition rate, and initial nanochannel diameter. Optimal conditions of silicon deposition on nanochannel membranes are determined: the wafer temperature of 250–450°C and silicon flux intensity of 10−2 to 10 monolayers (ML) per second. Under these conditions, the rate of reduction of the nanochannel inlet diameter is 0.13–0.15 nm/ML, which allows membrane channel modifications over a wide range down to several nanometers. Simulations of nanochannel membrane oxidation in an oxygen flux shows that precise reduction of nanochannel inlet diameters down to complete sealing of the channel due to oxide growth is only possible for small diameters of the initial pores. For channels with large lateral sizes, the effect of reduction of the channel inlet diameter due to oxidation is insignificant. Oxidation of pores enhances their stability to subsequent high-temperature treatment.

Investigation of the feasibility of pyrolytic obtaining of porous biomorphic SiC ceramics

Today's nature increasingly becomes a model for innovation in structural design. For this reason, biomorphic ceramics prepared by the biotemplating technique, where natural grown structures are used as bulk templates for fast high-temperature conversion into ceramics, has attracted a lot of attention. For example, the introduction of silicon in wood opens up the possibility to obtain porous silicon carbide ceramics during high-temperature treatment in an inert atmosphere. Pine ( Pinus silvestris) wood with shaped sample dimensions of 20 mm × 20 mm × 5 mm (axial) was selected as the raw material. SiO 2 sol was prepared, and samples were impregnated under different vacuum/pressure conditions. Impregnated samples were pyrolysed at 500 °C under oxygen-free atmosphere with the subsequent high-temperature treatment at 1600 °C in an Ar atmosphere—the obtained ceramic materials retained a tubular pore structure of pinewood. Biomorphic SiC ceramics and its precursors were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). An experimental result shows that the optimised vacuum/pressure impregnation technique is highly effective for introduction of SiO 2 in the wood. Staged pyrolysis treatment decreases the amount of vapours in a high-temperature oven, although the first pyrolysis step cannot exceed 600 °C, while the resulting template cooling gives some quantities of hard melting SiO 2 crystallic modification.

Optimized vacuum/pressure sol impregnation processing of wood for the synthesis of porous, biomorphic SiC ceramics

Pine ( Pinus silvestris) wood with shaped sample dimensions of 20 mm × 20 mm × 5 mm (axial) was selected as the raw material. Samples were dried and, for a half of the samples, resin extraction from the sample was applied. SiO 2 sol was prepared, and samples were impregnated under different vacuum/pressure conditions. Relative impregnation efficiency was calculated for impregnated samples and varied from 95 up to 105% of the theoretical value for different samples and impregnation conditions. Impregnation and drying procedures were repeated up to three times to increase the SiO 2 amount introduced in the sample. Impregnated samples were pyrolyzed at 500 °C under oxygen free atmosphere with the subsequent high temperature treatment at 1600 °C in an Ar atmosphere. Biomorphic SiC ceramics and its precursors were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). An experimental result shows that the optimized vacuum/pressure impregnation technique is highly effective for the introduction of SiO 2 in the wood.

One-dimensional α-MnO 2: Trapping chemistry of tunnel structures, structural stability, and magnetic transitions

Highly crystalline one-dimensional (1D) α-MnO 2 nanostructures were synthesized by a hydrothermal method. All samples were characterized by X-ray diffraction, transmission electron microscope, thermogravimetric and differential scanning calorimeter, and infrared spectroscopy. During the formation reactions, the tunnel structure of 1D α-MnO 2 was simultaneously modified by NH 4 + species and water molecules. The amount of NH 4 + species that were trapped in the tunnels is almost independent on the reaction temperature, while the total water content increased with the reaction temperature. The average diameter of α-MnO 2 nanorods increased from 9.2 to 16.5 nm when the reaction temperature increased from 140 to 220 °C. 1D α-MnO 2 was destabilized by a subsequent high-temperature treatment in air, which is accompanied by a structural transformation to 1D Mn 2O 3 of a cubic structure. At low temperatures, all 1D α-MnO 2 nanorods showed two magnetic transitions that were characterized by a decreased Néel temperature with rod diameter reduction. According to the effective magnetic moments experimentally measured, Mn ions presented in the nanorods were determined to be in a mixed valency of high spin state Mn 4+/Mn 3+.

Thermal Treatment and Light Illumination Effects in EPR Spectra of KTaO3:Cu

In order to understand the manner of incorporation of oxygen vacancies into perovskite structure oxides, KTaO3 crystals containing Cu2+ impurity ions were investigated following thermal treatments in atmosphere of H2, air, and vacuum. The results of thermal treatments were controlled by EPR. The influence of light illumination on copper centres in these crystals was studied also. The thermal treatment and light illumination effects in EPR spectra of Cu impurity centres are studied for the incipient perovskite-like ferroelectric KTaO3 single crystals. It was found that thermal treatment at ∼950°C in hydrogen atmosphere and in vacuum strongly depresses EPR signal of both types of Cu2+(1) and Cu2+(2) axial centres substituting Ta5+ in the B-sublattice positions. Subsequent high-temperature treatment in air restores EPR signals of Cu2+ centres, but changes the ratio between Cu2+(1) and Cu2+(2) centres concentration. These effects, together with UV light illumination effects observed for T < 80 K, are connected with the possibility of the Cu2+ ←→ Cu+ thermal and photo-induced transformations of the Cu impurity centres.

Improved Firmness in Calcified Diced Tomatoes by Temperature Activation of Pectin Methylesterase

ABSTRACT: The effects of temperature and calcium on pectin methylesterase (PME) activity and texture in tomato pericarp material were examined. Heating thin slices of pericarp to temperatures between 50°C and 75°C led to the rapid evolution of methanol from the material, indicating an activation of PME. This activity was further stimulated when CaCl2 (up to 2.0% w/v) was added. When applied to half‐inch diced tomato pericarp, the same conditions that led to the activation of PME also improved firmness. Diced tomatoes treated for 5 min with 0.5% CaCl2 at 70°C were 2.5 times firmer than diced tomatoes treated with CaCl2 at room temperature. This improvement in texture by treating with CaCl2 at elevated temperatures was only apparent when the tomatoes received a subsequent 100°C treatment. Heating tomatoes to 70°C either before or after the CaCl2 treatment also improved firmness through a subsequent high‐temperature treatment, but to a lesser extent than heating during the CaCl2 treatment. These results are consistent with the model that heating to 70°C greatly increases PME activity, leading to extensive pectin de‐esterification and increased calcium cross‐linking of the pectins in the middle lamella. Production of thermally processed diced tomatoes with improved firmness should be possible by increasing the temperature during and after calcium treatment.

Inverted EPR signal from nitrogen defects in a synthetic diamond single crystal at room temperature

Electron paramagnetic resonance (EPR) in diamond single crystals was studied. The crystals were grown using apparatuses of the “split-sphere” type in a Ni-Fe-C system using the temperature gradient method with a subsequent high-temperature high-pressure treatment. It was found that, after the high-temperature high-pressure treatment of a diamond sample, the EPR signal from the lattice defects containing nitrogen atoms became inverted with the growth of the microwave power in an H102 resonator. In a constant polarizing magnetic field, when the microwave power applied to the diamond was low, a resonance absorption by the nitrogen defects took place, whereas, when the microwave power was high, an emission was observed. The inversion of the EPR lines of a single nitrogen atom substituting for a carbon atom at a diamond lattice site could be caused by the presence of a nickel atom with an uncompensated magnetic moment at the adjacent tetrahedral interstitial site. In synthetic diamond crystals that were not subjected to high-temperature high-pressure treatment, the inversion of the EPR signal from nitrogen atoms (P1 centers, nitrogen in the C form) was absent.

A model for the formation of lattice defects at silicon oxide precipitates in silicon

The critical size of silicon oxide precipitates and the formation of lattice defects by the precipitates are discussed. An expression is derived allowing estimation of self-interstitial emission by spherical precipitates as well as strain build-up during precipitate growth. The predictions are compared with published experimental data. A model for stacking fault nucleation at oxide precipitates is developed based on strain and self-interstitial accumulation during the thermal history of the wafer. During a low-temperature treatment high levels of strain develop. During subsequent high-temperature treatment, excess strain energy in the precipitate is released by self-interstitial emission leading to favourable conditions for stacking fault nucleation.

Crystallization of mechanically alloyed amorphous W–Mg alloy under high pressure

Pure metal powder mixtures of W and Mg at the desired composition were milled in conventional high-energy ball mill, and amorphous alloy W 50Mg 50 was obtained after milling for 20 h. The structure evolution of elemental powder mixtures was studied following milling and subsequent high pressure and high temperature treatment. The amorphous alloy transform into a nanocrystalline material below 1050 °C at 4.0 GPa. On increasing the temperature, it transforms into a mixture of several new crystal phases under high-pressure condition. It also found that both mechanical alloying and high pressure treatment are the two necessary processes to form the nanocrystalline and the new phases.

Synthesis, structure and reactive mechanism of intermetallic W 3Mg

Assisted by mechanical alloying and high-pressure technique, a new W 3Mg intermetallic was formed. W 3Mg amorphous mixture was obtained by mechanically alloying the pure metal powder mixtures at designated composition for 20 h. A new compound was found after the subsequent high pressure and high temperature treatment. W 3Mg intermetallic was identified as a tetragonal structure and the lattice parameter was a=0.7880 nm, c=0.7070 nm. The synthesis mechanism is also discussed in this paper.

Synthesis, Structure and Reactive Mechanism of Intermetallic W4Mg.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

A promising reusable sorbent for desulfurizing hot coal gas is being developed in the form of core-in-shell pellets which consist of a highly reactive CaO core encased in a porous protective shell made largely of an inert material. The spherical pellets are made by pelletizing plaster of Paris and then applying a coating of alumina (80%) and limestone (20%) particles. Subsequent high temperature treatment converts the cores to CaO and sinters the coating material to form the porous shells. The pellets absorb and react with H2S at high temperatures (e.g., 800−900 °C) to form CaS and are regenerated by applying a cyclic oxidation/reduction method. The rate of sulfidation does not appear to be controlled by chemical reaction but, instead, seems to be controlled by one or more of the following diffusion resistances: external gas film, porous shell, micropores between CaO grains in the core, and product layer surrounding CaO within each grain. After considering various models, a brief review of which is inclu...

Synthesis, structure and reactive mechanism of intermetallic W 4Mg

Assisted by a mechanical alloying and high-pressure technique, a new W 4Mg intermetallic was formed. W 4Mg amorphous mixture was obtained by mechanically alloying the pure metal powder mixtures at designated composition for 20 h. A new compound was found after the subsequent high-pressure and high-temperature treatment. W 4Mg intermetallic was identified as a cubic structure and the lattice parameter was a=0.4150 nm. The synthesis mechanism is also discussed in this paper.

Interfacial Gas Desorption and Diffusion during the Low Thermal Stress Fusion of III/V Materials to Si

AbstractA Low Thermal Stress (LTS) process, involving the removal of most of the InGaAs(P) material, was used to allow for wafer-fusion of InGaAs(P) samples to Si. The formation and behavior of bubbles of trapped gas between the thinned layer of InGaAs(P) material and the thick Si substrate were studied due to the yield problems such bubbles represent. Observations revealed a low temperature regime &lt;300°C, during which significant gas generation occurred but only weak, reversible bonds were formed. The higher temperature regime, 300°-650°C, was characterized by no significant increase in bubble density, but resulted in a reduction or complete blockage of gas escape during storage. Further investigation of the higher temperature regime indicated continued production of gas, but with a tendency of the gases produced above 300°C to escape from the interface without the formation of bubbles under normal conditions. This information led to the development of a scalable LTS process requiring no modification of the wafers prior to bonding and utilizing sample pre-heating to 300°C before InGaAs(P) removal and subsequent high temperature treatment to 650°C for permanent bond formation.

Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

A DRIFT study of ZnHZSM-5 zeolites with Si/Al ratios of 15 or 41 and a Zn loading of 0.8 wt% revealed a high thermal stability of bridging OH groups that was practically the same as in the pure hydrogen forms. It was concluded that the incipient wetness impregnation of NH4ZSM-5 zeolite with zinc nitrate and the subsequent high-temperature treatment results only in a minor amount of ion exchange. A considerable part of the modifying zinc forms nanometric ZnO clusters inside the channels of the zeolite. The use of the low-temperature adsorption of dihydrogen as a probe indicated the appearance, after high-temperature vacuum pretreatment, of three different Lewis acid sites connected with coordinatively-unsaturated Zn2+ ions. The strongest Lewis sites, with an H–H stretching frequency of adsorbed molecular hydrogen of 3940 cm−1, dissociatively adsorbed hydrogen, methane and propane at both room and elevated temperatures. These sites are represented either by Zn2+ ions on the walls of the main channels of the zeolite (α sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxide.

Intensification of thermooxidation of polyacrylonitrile fibres

It was found that in thermooxidation of PAN fibres in air by the conductive (contact) method, the density of 1.38–1.40 g/cm3, required for subsequent high-temperature treatment, is attained in 30–60 min. The chemical composition and equilibrium sorption of the PAN fibre oxidized by convective tempering do not differ significantly from the fibre fabricated in industrial conditions in convective tempering.

Effects of thermochemical treatment on diamond polycrystal sintering

A study has been made on the effects of thermochemical treatment applied to a diamond micropowder and designed to reduce the number of sharp edges at the surfaces of the diamond particles, which influences the high-pressure sintering of polycrystalline diamond. That treatment increases the poured density of the powder and increases the shrinkage, while reducing the number of particles that break under the high pressure at room temperature, and also accelerating the compaction on subsequent high-temperature treatment.

Control of the degree of long-range compositional order in ceramics of the compound perovskites Pb(B′1/2B″1/2)O3 using recrystallization

It is established that an ordered state forms in Pb(Yb1/2Nb1/2)O3 at fairly low temperatures (650–800°C). Subsequent high-temperature heat treatment of the ceramic without additives (sintering or additional annealing) does not produce any significant change in the degree of long-range compositional order s because of the low diffusion rate of the Yb and Nb ions. The addition of Li2CO3, which forms a liquid phase, creates conditions for the dissolution of grains with a high value of s, the nucleation of new crystallization centers, and the growth of grains with a new equilibrium value of s at the sintering (annealing) temperature.