High Temperature Electric Conductivity of Pure Silicon

Abstract

The electric conductivity of high purity silicon has been measured at elevated temperatures. Thin silicon rods were heated in a horizontal electric resistance furnace up to temperatures of 1120 °C. By passing a current through the rods and by measuring the resulting voltage drop, the conductivity was determined in the temperature range from 550 to 1120 °C. The data compare very well with reported literature values.     

THE NITRIDING OF SILICON POWDER COMPACTS

Abstract

The reaction-bonding process to prepare silicon nitride by nitriding silicon compacts was studied, and an examination made of the influence of raw material and process variables on the properties of the resulting silicon nitride. The silicon powder grain size and the impurities content were considered as powder variables, and the green density and thermal cycles as process parameters. The examination of green-density effects indicates that, under the experimental conditions, the gas permeation of nitrogen through the silicon compacts was the rate-determining step of the reaction-bonding process. Regarding the effect of nitriding temperature, the final conversion, Si to Si₃N₄, is an increasing function of the temperature in the range 1300–1400°C. As to the composition of silicon nitride obtained, α-phase formation is favoured when oxygen is present as an impurity in silicon powder. Finally, physical, chemical, and thermomechanical tests showed that reaction-bonded silicon nitride has good bending strength (21 kgf/mm²) and can be used in very severe conditions up to 1200°C.     

Structural properties of vapor deposited silicon nitride

Some of the chemical and physical properties of silicon nitride films have been studied to determine the effects of deposition process variables and wafer preparation prior to deposition. The boat temperature and the reaction gas mixture were changed to optimize the quality of the silicon nitride films. This resulted in amorphous films free of pinholes, cracks, and impurities, together with good electrical properties such as an effective barrier against sodium ions and a low fast and fixed surface state density. Most of the work has been done on silicon nitride deposited over silicon dioxide films or silicon dioxide steps on a siliconsubstrate. The silane-ammonia and the silicon tetrachloride-ammonia reactions resulted in silicon nitride of comparable physical and chemical properties. Cleaning procedures are most effective if they include an etching step to take off 50 to 100Å of the oxide prior to nitride deposition. The influence of subsequent heat treatments (up to 1200°C) on cracking and etch rates of the silicon nitride films has been studied. Films thicker than 2000Å deposited over oxide steps were found to crack after heat treatments above 1000°C. Films in the lower thickness range of 500 to 1000Å are most suitable because they have good resistance against cracking after heat treatments up to 1200°C, do not need excessively long etch times, and are still an excellent barrier against a heavy sodium contamination applied at 535°C.     

Effect of mechanical activation on the formation of Si3N4-SiC composite powders

The paper examines how the mechanical activation of an initial silicon-carbon powder mixture and the temperature of its subsequent treatment influence the specific surface and phase composition of the end products. It is established that the preliminary mechanical activation of Si-C mixtures significantly increases the nitration rate of silicon and decreases the temperatures of formation of silicon carbide and silicon nitride (1200–1300 °C). The products resulting from the nitration of mechanically activated mixtures consist of α-Si₃N₄ whiskers with an aspect ratio of 10–100, silicon, carbide, and silicon nitride particles to 100 nm in size, and their agglomerates.     

MECHANICAL STRENGTH AND THERMAL-FATIGUE CHARACTERISTICS OF SILICON NITRIDE

Abstract

The mechanical properties and the thermal-shock and thermal-fatigue characteristics of three reaction-sintered (density 1·9-2·6 g/ml) and one hot-pressed (density 3·00-3·18 g/ml) samples of silicon nitride showed significant differences according to composition and method of manufacture. The resistance to creep (at 1000-1200°C) and to thermal fatigue (at 1000°C), particularly of the dense grades, can be considerably superior to that of creep-resistant alloys. However, the appreciable variation in performance of all grades indicates that further development of the manufacturing technique is necessary to ensure consistent behaviour.     

Chemical stability of silicon nitride powders in biochemical media

We have used chemical and x-ray analysis to study the stability and phase composition of silicon nitride powders with different panicle size ranges and morphology in biochemical media (physiological solution, gastric juice, blood serum) at 37°C. We have established the high solubility of Si₃N₄ in physiological solution and blood serum. The solubility increases with the dispersity of the powder in all the media. We explain this behavior of silicon nitride in biochemical media using modern theories of bioinorganic catalysis.     

The use of nitride intermediates in the preparation of metals. A study of the reduction of Nb2O5 with NH3

A kinetics study of the reduction of Nb₂O₅ with NH₃ was conducted at 600° to 1300°C, using vertical fixed-bed, flow-through reactors, with the goal of using the nitride as an interme-diate in the preparation of niobium (columbium) metal via a thermal decomposition step. The effects of reactor materials (stainless steel, nickel, molybdenum, graphite, alumina, and Vycor) upon ammonia reactivity toward Nb₂O₅ were investigated. At low temperatures, the metal reactor systems were more catalytically reactive, yielding faster rates of reac-tion and a greater degree of nitride conversion, whereas at high temperatures, the non-metal reactor systems performed better. In general, the initial reaction rate-temperature data exhibited a maximum, associated with oxynitride formation, near 700°C for the metal reactor systems and 800° to 900°C for the nonmetal reactor systems, followed by a mini-mum, associated with NbO₂ formation, at 800° to 850°C for the metal reactor systems and 950° to 1000°C for the nonmetal reactor systems where NbN formation commences. A sec-ond maximum, associated with the hexagonal NbN phase, occurred at 1200°C. The ranges of activation energies for these regions were from 15 to 30 kcal/mole for region I, 8 to 22 kcal/mole for region II, and 10 to 22 kcal/mole for region III.     

Transformation of Ba-Al-Si precursors to celsian by high-temperature oxidation and annealing

Celsian (monoclinic BaO · A1₂O₃ · 2SiO₂) is being considered as a matrix material for ceramic composites used in high-temperature structural applications. The present article describes the synthesis of celsian by the oxidation and annealing of solid, malleable, metallic Ba-Al-Si precursors. The phase and microstructural evolution after various stages of oxidation at 300 °C to 1260 °C in pure oxygen at 1 atm pressure have been examined by X-ray diffraction (XRD) and electron microprobe analyses (EPMA). Barium peroxide, BaO₂, formed rapidly during oxidation at 300 °C, with aluminum and silicon remaining largely as unoxidized particles in a BaO₂ matrix. Between 300 °C and 500 °C, barium orthosilicate, Ba₂Si0₄, formed by a solid-state reaction between barium peroxide and unoxidized silicon. Further exposure to temperatures between 500 °C and 1200 °C resulted in the oxidation of aluminum and of residual silicon. The oxidized silicon reacted with the barium orthosilicate matrix to yield higher silica-containing barium silicates that, in turn, reacted with alumina or mullite to form metastable hexacelsian (hexagonal BaO-A1₂O₃ · 2SiO₂). Celsian was then obtained by further exposure to peak temperatures ≤1260°C.     

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.     

Powder reaction mechanism in fabrication of high silicon iron alloy

Abstract

In the present paper, the reaction mechanism of silicon and iron powders under different sintering conditions during the fabrication of high silicon iron sheet (～6·5 mass-%Si) is clarified. It is indicated that the phases, Fe₃Si (Si) and FeSi, play an important role in the reaction between iron and silicon powders. Two temperature regions of the powder reaction are very important for producing commercial high silicon iron sheets: the temperature region of ～1000°C in which the ductile composite structure can be produced, and the temperature region of ～1200°C in which the density and homogeneity can be improved.     

Phase equilibria in the metal-rich side of the Ta-N system

The region of the Ta?N system with nitrogen content less than 20 at. pct and temperature between 350° and 1200°C was investigated using electron microscopy and diffraction and X-ray diffraction. For temperatures above 790°C, the hexagonal nitride Ta₂N was observed to form. Below this temperature two interstitially ordered cubic nitrides were observed. The higher temperature cubic nitride was found to be an equilibrium phase of approximate composition Ta₉N₂; a crystal structure, for this phase consistent with the data is presented. The terminal solubility was measured, and a partial phase diagram is given. In addition, the precipitation of these nitrides from supersaturated solution is described.     

Metallurgical reactions in two industrially strip-cast aluminum-manganese alloys

Precipitation, phase transformation, subgrain growth, and recrystallization that occur during heat treatment of two strip-cast, cold-rolled, high manganese aluminum alloys have been studied mainly by transmission electron microscopy (TEM). The alloys differ in silicon content. The isothermal heat treatments have been performed in a salt bath at temperatures between 330 °C and 530 °C for times up to 1000 hours. Size distributions for each type of secondary particle have been determined. After short annealing times, small quasicrystals precipitated and subsequently transformed to α phase. The densities of these precipitates controlled dislocation movement and regulated subgrain sizes. Prolonged heating resulted in peritectoid reactions to Al₆Mn or Al₁₂Mn. Recrystallization, which is associated with the formation of Al₁₂Mn, is advanced by increasing the silicon content; the nucleation and growth of Al₁₂Mn occurs only at the expense of other phases that stabilize the subgrain network. Formerly with SINTEF, Oslo, Norway     

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT

The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30?min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.     

THE PREPARATION AND PROPERTIES OF SELF-BONDED SILICON CARBIDE

Abstract

Dense self-bonded silicon carbide, prepared by impregnating with silicon pressed mixtures of silicon carbide and graphite, has been improved by attention to grading and composition. Treatment at 2000°C for ½ h has given a much greater degree of self-bonding. Porous self-bonded silicon carbide with a dense surface has also been prepared.     

Liquid phase sintering,heat treatment and properties of ultrahigh carbon steels

Abstract

Thermo-Calc modelling was employed to predict liquid phase amounts for Fe–0·85Mo–(0·4–0·6)Si–(1·2–1·4)C in the temperature range of 1285–1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4–0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc^?1 were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.     

Iron‐Rich Iron‐Titanium‐Silicon Alloys with Strengthening Intermetallic Laves Phase Precipitates

Two‐phase ternary Fe‐Ti‐Si alloys with Si contents from 2 to 16 at.% and Ti contents from 2 to 28 at.% were studied with respect to room temperature hardness, fracture strain and yield stress at room and higher temperatures up to 1150 °C. In addition oxidation was checked at temperatures between 400 and 1150 °C. The alloys are strengthened by precipitation of the stable Laves phase (Fe,Si)₂Ti which is a hard and brittle intermetallic phase. The yield stress as well as the brittle‐to‐ductile transition temperature (BDTT) increase with increasing Ti content. Yield stresses up to about 1400 MPa and BDTT between 100 °C and 600 °C with fracture strains of the order of 1 % below BDTT were achieved. The observed short‐term oxidation performance at temperatures up to 1150 °C compares favourably with that of Fe‐AI alloys with high Al contents.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

High-temperature tensile deformation and thermal cracking of ferritic spheroidal graphite cast iron

Ferritic spheroidal graphite (SG) cast irons of different silicon contents were used to study the tensile behavior in the temperature range of 500 °C to near Ac ₁. The thermal-cracking behavior under cyclic heating to various temperatures from 650 °C to 850 °C was also explored. According to the tensile data, the temperature dependence of the flow stress is concave upward, and that of the elongation is concave downward with drastic descent after reaching the maximum. The temperature range of ascending stress and descending elongation is above Ac ₁, in which the eutectic cell-wall region transforms to austenite. Intergranular fracture with serious ductility loss can take place at 500 °C, if the silicon content is too high (3.9 wt pct in this test). This brittle phenomenon can be eliminated through microstructure refining. As to the thermal-cycling test, it indicates that the thermal cracking occurs through intergranular fracture. Whereas the susceptibility to thermal cracking increases with increasing silicon content, it can be reduced by refining the microstructure. Unlike the cast irons heated above Ac ₁ with phase transformation, the heating temperature of about 750 °C leads to the most severe thermal cracking. In addition, the specimens heated in air have lower thermal-cracking resistance than those heated in a neutral salt bath.     

Effects of second-phase particles on coarsening of austenite in 0.15 Pct carbon steels

The effects of second-phase particles formed by the addition of vanadium, nitrogen, and aluminum on the austenite grain coarsening behavior of 0.15 pct carbon steels were studied. The oxidation and etching technique has been adopted to reveal the prior austenite grain boundaries. The specimens were austenitized at intervals of 50°C within the range of 900°C to 1150°C under high vacuum (<10⁻⁴ torr) for half an hour, toward the end of which they were oxidized for about one minute by introducing oxygen at about 250 mm Hg to reveal the grain boundaries, and then quenched into iced water. The variation of prior austenite grain size with temperature in these steels indicates that vanadium carbonitride, V(C, N), is much more effective in austenite grain refinement than vanadium carbide, VC, at all temperatures. The effect of vanadium carbonitride in austenite grain refinement is more or less the same as that of aluminum nitride. AlN, at temepratures below 1000°C, but this effect of vanadium carbonitride in austenite grain refinement decreases with increasing temperature. Above 1000°C, aluminum nitride is a much better grain refiner than vanadium carbonitride. The presence of the V (C, N) and AlN particles in the same steel causes moderate grain growth of austenite. MD. Mohar Ali Bepari, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Associate Professor of Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.