Heat treatment of 6H-SiC under different gaseous environments

Silicon carbide is a useful material for the reactors in chemical processes. In recent years, microreactors have gained significant attentions due to the high demand for process miniaturization. As heat and mass-transfer are highly improved inside the gas flow channels in microreactors, any change on the surface of inner channels under heating becomes critical to the performance of microreactors. To investigate the surface changes of silicon carbide during the heat treatment, 6H-SiC coupons were processed in five different gases—Ar, N₂, air, 0.9% O₂ in Ar and 50% H₂O in air—that are commonly encountered in high temperature chemical processes. While the formation of oxide film was found to be dependent on the partial pressure of oxidizing gas, surface decomposition was found from the treatment in nitrogen environment. Characterization of the SiC surface by Raman spectroscopy and SEM–EDX revealed that a graphitic layer has formed at the oxide film/SiC interface. Crystallinity of graphitic layer at the interface seemed to be dependent on the partial pressure of oxidizing gas, which was revealed by the relationship between G peak position and R(I_D/I_G). The intensity ratio of FTO(0)/FTO(2/6) bands showed that stacking faults on the surface of SiC coupons were reduced after heat treatment.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Effect of fluorine chemistry in the remote plasma enhanced chemical vapor deposition of silicon films from Si2H6-SiF4-H2

SiF₄ was added into Si₂H₆-H₂ to deposit polycrystalline silicon films at low temperatures around 400°C in a remote plasma enhanced chemical vapor deposition reactor. It was found out that the fluorine chemistry obtained from SiF₄ addition had an influence on the chemical composition, crystallinity, and silicon dangling bond density of the film. The fluorine chemistry reduced the amount of hydrogen and oxygen incorporated into the film and also suppressed the formation of powders in the gas phase, which helped the crystallization at low temperatures. Effect of SiF₄ concentration as well as the deposition temperature was also significant.     

Electrochemical characteristics of silicon coated graphite prepared by gas suspension spray method for anode material of lithium secondary batteries

Silicon-coated graphite particles were tested as anodes for lithium ion rechargeable batteries. The synthetic graphite particles were first coated with silicon precursor containing solution by gas suspension spray method and then calcined at heat treatment temperature at 500 °C under hydrogen atmosphere. The silicon-coated graphite showed high specific capacity and good cycle performance due to the formation of amorphous silicon-carbon black composite layer on the surface of the graphite particles. It has stable structure under repeated volume expansion and contraction. The silicon-coated graphite still has high irreversible capacity due to the solid electrolyte interface (SEI) formation during the 1st cycle. However, the capacity loss could be lessened to a certain level by controlling the composition of the solvent mixture in the electrolyte.     

An Investigation of Porous Silicon by Means of Positron Annihilation

Positron lifetime spectroscopy has been used to investigate a porous silicon film subjected to heat treatments up to 1170°C. Annealings between 300 and 500°C resulted in a 17% mass increase of the film due to oxygen uptake following the effusion of hydrogen. The positron data also indicate that vacancy clusters are formed in the silicon oxide layer or the silicon oxide—silicon interface surrounding the nanocrystallites as oxygen replaces the effusing hydrogen. The vacancy cluster concentration, which may have a bearing on the photoluminescent properties, increased by a factor of three with heating to 500°C and then decreased to one-third the original value at higher temperatures. Above 900°C vacancy migration and clustering occurred, accompanied by visible deterioration of the film.     

The Role of Infiltration Temperature in the Reaction Bonding of Boron Carbide by Silicon Infiltration

The present paper is concerned on the effect of infiltration temperature on the components, microstructure, and mechanical properties of reaction‐bonded boron carbide (RBBC) ceramics. RBBC ceramics were fabricated by reactive infiltration of molten silicon (Si) into porous preforms containing boron carbide (B₄C) and free carbon. It has been found that infiltration temperatures have significant influence on the infiltration reactions involved and therefore the evolution of different phases formed in the RBBC ceramics. An increase in grain size of boron carbide particles through the coalescence of neighboring grains was observed at certain infiltration temperatures. The morphology of silicon carbide (SiC) phases developed from discontinuous and cloud‐like SiC to continuous and integrated SiC zones with the increase of infiltration temperatures. With increasing temperatures up to 1600°C, the hardness, flexural strength, and fracture toughness all increased. When the temperatures exceeded 1600°C, while the hardness and flexural strength decreased, the fracture toughness continued to increase.     

Electroless deposition and silicidation of Ni contacts into p-type Porous Silicon

Porous Silicon (PS) has attracted much attention since the discovery of its photo luminescent behavior. It has also been used for various other applications such as electroluminescent light emitting-diodes (LEDs), photodetectors and solar cells. For such devices, it is important to make good metallic Ohmic contacts to the PS in order to maximize the efficiency. In order to produce buried contacts, barrier layers, Schottky devices, etc. in PS, it is advantageous to deposit metal that covers not only the surface of the porous layer, but also the inside walls and the bottom of the pores. In this work experiments were performed to examine the morphology and properties of electroless deposition of Nickel into p-type PS and subsequent formation of Nickel silicide after heat treatment. Circular PS samples of 6 mm diameter were produced by anodizing p-type Silicon wafers for 15 min and were subsequently plated with Ni using three different plating baths. The pores are on average 20 μm deep and 4 μm wide. Two samples of each type were heat treated in an nitrogen atmosphere for one hour at 400 and 600°C respectively to produce Nickel silicide. Reference samples were made by means of electron beam evaporation of Ni. SEM micrographs show that the best pore coverage was achieved using the Ni plating bath containing hypophosphite. I–V characterization shows that different rectifying and Ohmic contacts can be formed between electroless deposited Ni and PS depending on the conditions of the heat treatment. XRD and EDX characterizations show that both the NiSi and Ni₂Si phases exist in the sample at the same time.      

SUPPRESSING HYDROGEN EVOLUTION BY AQUEOUS SILICON POWDER DISPERSIONS THROUGH THE INTRODUCTION OF AN ADDITIONAL CATHODIC REACTION

Silicon powder reacts with water, liberating hydrogen gas, which poses an explosion risk. Adding metal ions with a high reduction potential suppresses hydrogen generation. Copper (II) ions are particularly effective in this regard. In their presence the reaction features three distinct stages. In the initial phase copper is deposited on the silicon surface concomitant with a rapid drop in the solution pH. Most of the hydrogen evolves during a second active stage with the pH showing a slight upward drift. Finally, in the third stage, the silicon surface passivates and hydrogen evolution comes to a halt. A comparison of this method and two other methods previously reported, i.e., controlled air oxidation of the silicon powder before slurrying and adding organic corrosion inhibitors, shows that silane surface modification of silicon is the most effective method in terms of decreasing the greatest amount of hydrogen released and increasing silicon reactivity in a typical pyrotechnic composition.     

Silicon surface wet cleaning and chemical oxide growth by a novel treatment in aqueous chlorine solutions

A few new reactants for the wet cleaning of silicon surfaces, for example, ozone dissolved in ultrapure water (UPW), have been proposed to replace the original RCA process using H₂O₂ solutions. In the present work we describe, for the first time, the mechanism of silicon surface oxidation by dilute solutions of elemental chlorine. Upon reaction with this highly oxidizing agent, the open circuit potential (OCP) shifted immediately to positive values, the effect being identical for both n- and p-type Si substrates. The surface transformation was firstly investigated by electrochemical impedance spectroscopy which showed successive semicircles representing RC equivalent circuits, showing a gradual growth of an insulating layer. X-ray photoelectron spectroscopy (XPS) recordings demonstrated unequivocally the formation of a pure and uniform chemical oxide layer, the possible contamination by Cl element being negligible.Moreover, the strong oxidizing and complexing properties of chlorinated aqueous solutions afford this reactant a high efficiency in surface cleaning from metal impurities. The effectiveness of this treatment was proven by AFM observation of the surface which was beforehand intentionally contaminated by Cu nuclei. The original flat surface was easily restored after a short cleaning using this new chemical reactant.This novel technique of surface treatment is promising with respect to the economy and environmental requirements, and also for the possible subsequent growth of multi-layer high-k dielectric structures.     

Joining of sintered SiC ceramics at a lower temperature using borosilicate glass with laser cladding Si modification layer

The silicon carbide whose surface had been modified by laser cladding silicon layer was soldered by borosilicate glass. And the borosilicate glass solder had a thermal expansion coefficient similar to that of the silicon carbide substrate. The laser cladding silicon layer significantly improved the wettability between molten glass solder and silicon carbide and could reduce the soldering temperature. A sandwich-like joining structure (SiC/Si/solder/Si/SiC) was made after the borosilicate glass slurry put on the laser cladding silicon layers. The microstructure, compositions, and interfacial properties were studied. Results showed that good adhesion between silicon carbide and the glass soldering layer was achieved. The flexural strength of the connection structure prepared at 900 °C in the air reached 110 MPa. This research provides an effective technical solution for realizing local heat treatment soldering of large silicon carbide components.     

High‐Temperature Na2SO4 Deposit‐Assisted Corrosion of Silicon Carbide – I: Temperature and Time Dependence

Time and temperature dependence of Na₂SO₄‐induced hot corrosion were studied for sintered‐α (Hexoloy) as well as CVD‐SiC at temperatures between 900°C and 1100°C and at times from 0.75 to 96 h. The extent of corrosion was quantified using mass change measurements, removal of corrosion products using sequential water, HCl, and HF dissolution steps followed by ICP‐OES analysis, and by optical profilometry of corroded materials to characterize pitting on the sample surface. In addition, SEM, EDS, and XRD were used to better understand the morphology, distribution, and phase composition of corrosion products. It was found that hot corrosion of Hexoloy was more severe than that of high‐purity CVD‐SiC. Hot corrosion is initially rapid until a continuous silica layer is formed underneath the mixed silicate layer. Once a continuous silica layer was formed the temperature dependence of the corrosion rate was consistent with diffusion of oxygen through the silica layer.     

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO₂ scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO₂ surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO₂ scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO₂ layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO₂ layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition k_l governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.     

Fracture Behavior of Woven Silicon Carbide Fibers Exposed to High‐Temperature Nitrogen and Oxygen Plasmas

High‐temperature aero‐thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two‐dimensional woven SiC fibers. Hi‐Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high‐enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas‐surface interactions and high‐temperature oxidizing processes involving the formation of SiO₂ bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.     

Purification of silicon powder by the formation of thin porous layer followed by photo-thermal annealing

ABSTRACT: Porous silicon has been prepared using a vapor-etching based technique on a commercial silicon powder. Strong visible emission was observed in all samples. Obtained silicon powder with a thin porous layer at the surface was subjected to a photo-thermal annealing at different temperatures under oxygen atmosphere followed by a chemical treatment. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) results indicate that silicon purity is im-proved from 99.1 to 99.994 % after annealing at 900 degreesC.     

Sol-Gel Alumina Coating for Improved Cyclic Oxidation Resistance of Silicon Nitride

A 25 nm thick α-alumina layer was deposited on a turbine-grade silicon nitride by sol-gel dip coating and subsequent heat treatment in air at 1200°C. This layer had a nanometer grain structure. Silicon nitride protected by this thin layer showed a significant improvement in oxidation resistance over its uncoated counterpart after 200 cyclic exposures in air at 1250°C. The oxide layer grown on the coated silicon nitride also exhibited superior surface morphology, compared with the uncoated silicon nitride.     

Grinding performance improvement of silicon nitride ceramics by utilizing nanofluids

Silicon nitride ceramics are widely used as advanced structural components because of their excellent thermal and mechanical properties at ambient and elevated temperatures. In manufacturing industries, grinding is an efficient and productive technique for finishing ceramic workpieces. However, high wheel-workpiece friction and the extreme hardness associated with silicon nitride cause large heat generation during grinding. The heat produced during grinding impairs the workpiece quality by inducing surface and sub-surface damages, tensile residual stresses etc. The damages can critically limit the applications of ground ceramic components. Extensive experimental studies have been carried out to find the effect of dry and nano MQL (Graphite, WS₂ and MoS₂) grinding conditions on silicon nitride using resin bonded diamond wheel at different parametric (wheel speed, depth of cut and table speed) combinations. Results indicate that the use of nanofluids considerably improve the process performance in terms of grinding forces, surface finish and sub-surface damage. The ground surface is characterized by optical microscopy, SEM/EDX and XRD.     

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 10¹⁹ cm^-3 to 3.7 × 10¹⁷ cm^-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.     

Novel synthesis and characterization of silicon carbide nanowires on graphite flakes

Silicon carbide nanowires were synthesized on the surface of graphite by partially reacting with silicon powders in NaF–NaCl based salt at 1150–1400 °C in argon. The effects of temperature and time of heat treatment as well as Si/graphite ratio on synthesis of SiC nanowires were studied. The results showed that the formation of SiC nanowires started at about 1200 °C, and the amounts of SiC nanowires increased in the resultant powders with increasing temperature. Their morphologies were characterized by scanning electron microscopy and high-resolution transmission electron microscopy. It was found that β-SiC nanowires with diameter of 10–50 nm and various lengths grew along their preferred direction perpendicular to (111). The zeta potential of graphite was also increased after coating with silicon carbide nanowires. SiC nanowires that formed on the graphite surface acted as an anti-oxidant to a certain extent, and they protected the inner graphite from oxidation.     

Chemical Etching of Silicon (100) by Hydrogen

The reactive etching of silicon (100) by hydrogen was studied in the temperature range 1325–1600 K. Rectangular etch pits formed from inverted pyramids were predominant, although more complex shapes revealing (111) planes were noted. The observed etch rates were found to lie near the thermodynamic limit, of one accepts the most positive reported enthalpy of formation of silylene. An activation energy of 314 ± 42 kj/mol for the etching reaction was determined from the rates at 1325–1550 K. At temperatures >1550 K, the etch rated decreased with increasing temperature, indicating a possible transfer of material from the silicon carbide-coated reaction chamber.     

Nanostructural Characterization of Silicon Oxycarbide Glasses and Glass-Ceramics

Silicon oxycarbide glasses were synthesized by the sol-gel process using precursors such as methyl-, propyl-, and phenyltrimethoxysilanes. The final products contained 14–38 wt% carbon. A TEM study on the nanometer scale revealed that all of the materials were amorphous and monophasic, and that it was not possible to detect any crystalline or otherwise distinct carbon phases. Hot-pressing the glasses led to the crystallization of graphite and silicon carbide within the amorphous matrix. X-ray and electron diffraction showed increasing crystallinity at the higher hot-pressing temperatures. Hot pressing at 1400°C resulted in the appearance of fine-grained silicon carbide, whereas at the highest temperature (1650°C), graphite and both hexagonal and cubic silicon carbide were produced. Subsequent heat treatment of the hot-pressed glasses under an argon atmosphere at 1400°C resulted in the formation of cristobalite. The glass-ceramics produced at the highest hot-pressing temperatures were more resistant to the crystallization of cristobalite during subsequent heat treatments.