β-SiC Crystals Research Articles

The Effect of Oxygen Content on Pyrolysis-Suppression of Si-Ti-C-O Fibers Coated with Surface Oxide Layers

The pyrolysis of two types of amorphous Si-Ti-C-O fibers (Tyranno T-1 (18%O) and T-2 (13%O) coated with oxide layers of various thicknesses was investigated in an argon stream at 1673K. The oxide layers thicknesses of 0.12, 0.21, 0.42 and 0.65μm were formed on the surface of the fibers by the heat treatment in an oxygen stream at 1673K. The fibers with oxide layers of various thicknesses were heated rapidly at 1673K and kept at this temperature in an argon stream continuously monitoring the weight change with a thermoba-lance. After cooling the crystal structure and morphology of the fibers were examined with X-ray diffraction and SEM observation, respectively. The pyrolysis of all T-2 fibers with oxide layers of the above thicknesses was suppressed significantly. On the other hand, the T-1 fiber with an oxide layer 0.12μm thick pyrolyzed completely, and growth and coalescence of β-SiC crystal were observed. The pyrolysis of the T-1 fiber with oxide layers of 0.21, 0.42 and 0.65μm thick was suppressed significantly, and this pyrolytic behavior was similar to that of the T-2 fiber. The oxide layer of fibers was removed by chemical etching treatment, then the microstructure of the etched fibers was observed with TEM and their tensile strength was measured at room temperature. As the thickness of oxide layer decreased, crystallization of β-SiC in the fiber was inhibited and the tensile strength of the fiber increased. At the same thickness of oxide layer, the tensile strength of the T-2 fiber was larger than that of the T-1 fiber.

Understanding Nicalon® fibre

Abstract From structural and nanotextural studies by transmission electron microscopy, Nicalon fibre was found to be a microcomposite. Its composition by weight is 55% β-SiC crystals (1·6 nm average diameter), 40% SiO1·15C0·85 intergranular phase and 5% free carbon. The free carbon is composed of isolated aromatic entities less than 1 nm in diameter, with 2–3 aromatic layers stacked in turbostratic order. Taking into account the peculiar texture and applying the rule of mixtures, the density and porosity of the as-received fibre have been calculated, and found to be in agreement with measured values. From the nanotextural and chemical events occurring during a series of increasing heat-treatments (argon flow), a chemical balance accounting for the gaseous species released and fibre weight loss is proposed. Qualitative and quantitative changes in elastic modulus and electrical resistivity have been modelled.

Hydrothermal Corrosion of Si-Ti-C-O System (Tyrano) Fibers.

The wet corrosion by high temperature-high pressure water has been studied on the three types of Si-Ti-C-O system (Tyrano) fibers at 300∼800°C under 100 MPa for 2h. The X-ray diffraction analysis and SEM observation of the scale layers and the measurement of the mass change of reaction products revealed that the pyrolysis together with forming β-SiC crystal took place in lower temperatures, while the hydro-thermal oxidation with following the formation of amorphous silica films propagated in higher temperatures. The oxidation reaction was sharply reduced as the amor-phous silica crystallizes to form cristobalite, whence the porous oxide layer is densified. The densification hindered the gaseous diffusion through the oxide film. The fiber containing 18% O (rich type), where atomic arrangement was disordered, showed a comparatively rapid pyrolytic rate. On the contrary, the fiber containing 12% O (poor type) showed a rapid oxidation rate.

Raman Scattering Characterization of Ultrathin Films of β-SiC

AbstractRaman scattering has been used to characterize ultrathin films of β-SiC, ranging in thickness from 38 nm to 240 nm. These films were prepared on the surface of a <100> Si substrate by a carbonization process at a temperature of 1300°C. In each case, the LO phonon near 970 cm−1 and the TO phonon near 795 cm−1 are observed, indicating the formation of β-SiC crystal. The Raman linewidths and peak positions indicate evidence of nonuniform stress and disorder. The Raman intensity of the TO phonon is nearly twice the intensity of the LO phonon measured both with and without the Si substrate, which indicates that the crystal growth was not entirely confined in the <100> direction.

Pyrolysis of Low Oxygen SiC Fiber Prepared by Electron-Irradiation Curing Method

The pyrolysis of the low oxygen (1.7% O) SiC fiber prepared by the electron-irradiation curing method has been investigated. The pyrolytic rate was measured with a thermobalance in Ar at temperatures from 1673 to 1973K. The pyrolyzed fiber was examined by X-ray diffraction, SEM observation, specific resistance measurement and tensile test. Lowering of oxygen content in the fiber was found to be effective for the suppression of pyrolysis. The pyrolyzed fiber was composed of β-SiC and amorphous carbon. The growth and coarsening of SiC crystals were observed on the skin of the fiber, while the core was smooth and remained in the microcrystalline state. The tensile strength of the fiber, which was heated at 1973K for 3.6ks, retained 34% of that of the original fiber. The strength was correlated closely with the size of the β-SiC crystal in the fiber.

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

Si-Ti-C-O系繊維の高温熱分解に及ぼす表面酸化被覆層の効果

The pyrolysis of Si-Ti-C-O amorphous fibers (Tyranno fiber) coated with oxide layers of various thicknesses was investigated in an argon stream at 1673 K. Oxide layers were formed on the surface of the fiber in an oxygen stream at 1673 K. The thicknesses of the oxide layers were 0.12, 0.21, 0.42 and 0.65 μm. The fibers with the oxide layers of various thicknesses were heated rapidly and kept in an argon stream at 1673 K. During this heat treatment, the mass change of the coated fibers was measured continuously with a thermobalance. The heat-treated fibers were examined with X-ray diffraction, AES, SEM and TEM observation.The fiber with oxide layer of 0.12 μm thickness pyrolyzed completely, and the growth and coarsening of the β-SiC crystal were observed. The pyrolysis of the fibers with oxide layers 0.21, 0.42 and 0.56 μm in thickness was inhibited significantly. The degree of crystallization to β-SiC in the inner area of the fiber increased as the thickness of the oxide layer increased. The oxide layer of the fiber was removed by chemical etching treatment, and it was prepared in order to measure its tensile strengh. The tensile strength of the fiber was inversely proportional to the size of β-SiC crystallite. The tensile strength of the fiber with the oxide layer of 0.21 μm thickness was 58% as large as that of the original fiber.If the Tyranno fiber is coated with an oxide layer of more than 0.21 μm thickness, the high-temperature strength of the fiber can be improved fairly.

Phase equilibria in the Si-C-N-O system and the kinetic analysis of silicon carbide whisker growth

The significant binary, ternary and quaternary phase equilibria in the Si-C-N-O system are reviewed particularly with regard to the relative stabilities of α and β forms of silicon carbide and nitride. The standard Gibbs free energies of formation of the relevant high-temperature phases are compared, and for the α-form of SiC and Si3N4 the values are derived from the empirical results. From these free energy values, the phase boundaries in the Si-C-N-O system have been calculated and are plotted as typical RTInPO2 versus 1/T plots. The significance of various phase fields in relation to the processing and fabrication of carbide, nitride and oxynitride ceramics is discussed. The rate of chemical reaction has been analysed and the mechanism of reduction of silica to SiC whisker has been proposed. The energetics of whisker growth is also discussed and the derived value of activation energy is compared with the surface self-diffusivity of carbon in pure β-SiC crystals.

Mechanical properties and microstructure of ?-SiAION-?-SiC composites by pressureless sintering

β-SiAION-β-SiC composites containing up to 12 wt% β-SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (KIC) for composites was higher than that for β-SiAION ceramics. The maximum value was 5.4 MPa m1/2 for 6 wt% β-SiC, and this was an improvement of 15% in comparison with β-SiAION ceramics. From SEM observations, an improvement inKIC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in β-SiAION. From TEM observations, β-SiAION crystals were nanocomposites, within which existed the fine crystals in β-SiAION crystal. For composite, β-SiAION and β-SiC crystals were directly in contact. The mismatching zone was observed in β-SiC.

酸化皮膜で被覆されたSi-Ti-C-O系非晶質繊維の熱分解挙動

The pyrolysis of Si-Ti-C-O amorphous fiber (Tyranno fiber) coated with oxide film was investigated at temperatures between 1573 and 1773 K. After the oxide film was formed around the fiber in an O2 gas stream, the atmosphere gas was changed from O2 to Ar. During this heat treatment, the mass change of the fiber was measured by means of a thermobalance. The heat-treated fiber was examined with X-ray diffraction, SEM and TEM observation.At 1773 K, the fiber coated with the oxide film pyrolyzed completely, and the growth and coarsening of β-SiC crystal was observed. At the temperatures lower than 1723 K, the size of β-SiC crystallite, to which the amorphous fiber was crystallized before it was coated with the oxide film in the O2 gas stream, remained unchanged in the Ar gas stream. The tensile strength of this fiber was from 1/5 to 1/4 as large as that of the original fiber.If adequate control is provided in the formation of the oxide film, the high-temperature strength of Tyranno fiber can be improved.

SiC Crystals Derived from β‐SiC Powder Using a Conically Converging Shock‐Wave Technique

α‐SiC and β‐SiC crystals are prepared from vapor by sublimation of β‐SiC powder subjected to a conically converging shock‐wave generated by detonating an explosive charge. α‐SiC crystals are mainly 6H modifications and their features are plates, pyramids, needles, and filaments grown in the [0001] direction; 4H‐SiC filaments are also obtained. α‐SiC rods having a triangular cross section develop parallel to the [111] direction.

Stardust in the TEM

We report here high resolution TEM and diffraction studies of two phases, diamond and silicon carbide, recently discovered in dissolution residues from two carbonaceous meteorites (Allende and Murray). Isotopic anomalies of Xe, Ne, C, N, and Si in the residues indicate a presolar origin for these phases in carbon-rich circumstellar environments. Lattice column images and azimuthally-averaged diffraction ring profiles confirm previous reports of the fcc diamond structure, but ring broadening indicates that the number-average crystallite size is ≈ 1 nm. The diamond crystals were therefore likely quenched from the solid phase, or nucleated from the vapor phase under high supersaturation conditions, and given little chance to grow thereafter. The silicon carbide crystals, ranging in size typically from tens of nm to several μm, are fcc β-SiC crystals. High resolution images show frequent 111; twins in the form of 2 nm laths on the twin plane, and occasional “bulk” twins with boundaries near perpendicular to the plane of twinning. Neither dislocations or facets associated with growth, nor structural damage associated with exposure to radiation, are evident from examinations do far. Size distributions for SiC and diamond show little overlap, and thereby cluster only at opposite ends of the 1 μm to 5 nm range observationally associated with dust between stars.

Characterization of the free-carrier concentrations in doped β-SiC crystals by Raman scattering

LO phonon-overdamped plasmon coupled modes in n-type epitaxial films of β-SiC have been measured in the carrier concentration range from 6.9×1016 to 2×1018 cm−3. The carrier concentrations and damping constants are determined by line-shape fitting of the coupled modes and compared with the values derived from Hall measurements. The concentrations obtained from the two methods agree fairly well. The Faust–Henry coefficient determined from the fitting is 0.35. The line-shape analysis of the coupled mode has shown that the dominant scattering mechanisms in β-SiC are deformation-potential and electro-optic mechanisms.

Reaction mechanism in carbon-liquid silicon systems at elevated temperatures

The mechanism of reactions which occur in the carbon-liquid silicon system at elevated temperatures is far from being understood although the reactions are utilised for producing manu interesting SiC materials. In the present paper the formation of fibrous β-SiC in this system, which occurs with the SILCOMP SiC/Si material, has been investigated by observing the evolution of the microstructure at successive stages of the reaction and by measuring the thermal effects accompanying the reaction. A model has been developed which permits a rational explanation of the main features of the reaction rate and of the microstructure of the products formed at the first and intermediate stages of the reaction; the microstructure is of a decisive importance for the formation of fibrous β-SiC in the final product. An explanation is given for: (1) the preferential formation of small β-SiC crystals disseminated at random in the Si matrix in localised zones which were previously occupied by the carbon fibres; (2) the high rate of reaction in spite of the low solubility of carbon in silicon at the temperatures, attained by external heating, which are usually used to produce SILCOMP (1680–1880 K). Because analogous reactions occur in the production of SILCOMP, REFEL SiC and other similar materials, the model developed may have a wider range of applications.

Microstructure analysis of Ni-SiC mixed layers

The techniques of high-resolution and microanalytical electron microscopy and RBS have been applied to the study of the composition and structural variation of mixed regions obtained by pulsed-laser or ion-beam irradiation of Ni on single crystals of β-SiC. The results indicate that mixing takes place with a distribution depending on the ion dose or pulsed-laser energy used.

OBSERVATION OF DEFECTS IN β-SILICON CARBIDE EPITAXIAL FILM

The defects in β-SiC epitaxial film are studied by means of etching method. The etchant used is the fused alkali hydroxide. The triangular etch pits with sharp bottom correspond to the dislocations. The perfect dislocations in β-SiC crystal are 73° dislocations and 60° dislocations. In β-SiC crystal there exists {111} boundary stacking fault formed by the contact of two β-SiC growing layer having different stacking sequences. The etching figure of the stacking fault is straight line running parallel to the 〈110〉 direction. The 60° dislocations can dissociate into two 1/6〈112〉 Schockley dislocat'ions and these two partial dislocations with a {111} stacking fault form an extensive dislocation. Three 1/6(110} stair-rod dislocations and three {111} stacking faults construct a stacking fault pyramid. Two parts of β-SiC film having positive and negative stacking sequence can produce a twin with the twin plane (111). The results of etching method and X-ray Laue method confirme the existence of the twin of this type.

Structure and annealing properties of silicon carbide thin layers formed by implantation of carbon ions in silicon

Abstract An investigation was made of the formation of crystalline SiC thin layers by implantation of carbon ions in silicon and their development. Electron microscopy and diffraction studies showed that α-SiC was formed in some regions of asimplanted layers. IR absorption analysis showed that the formation and the development of β-SiC crystals by annealing began at lower temperatures for layers implanted with lower energy carbon ions. Complete conversion to β-SiC was observed on annealing at 900 °C for 30 min irrespective of the implanted ion energy. Well-defined rings and/or well-ordered spots due to β-SiC crystals were observed in the electron diffraction patterns of these annealed layers. The distribution of the implanted carbon ions was investigated using Auger electron spectroscopy combined with ion etching as well as the spherical drilling technique.

Heteroepitaxial growth of β-SiC on silicon substrate using SiCl 4-C 3H 8-H 2 system

Single crystals of β-SiC were prepared on Si substrates at a temperature around 1390°C with the standard conditions: H 2 ≈ 1 1/min, SiCl 4≈3 ml/min, C 3H 8≈1 ml/min, deposition period≈10 min. The dependences of the growth rate and the crystallinity on the substrate temperature were studied. By detailed reflection electron diffraction analyses, the crystallinity of β-SiC with 1 μm thickness was found to be better for the layer on the (100) and (110)Si substrates than for that on the (111)Si substrate. An activation energy of 25kcal/mole was obtained for the formation of β-SiC. Optimum conditions to obtain thicker β-SiC films are discussed.

Optical Properties of β-SiC Crystals Prepared by Chemical Vapor Deposition

Optical Properties of β-SiC Crystals Prepared by Chemical Vapor Deposition

Surface polarities of β-Sic crystals with hexagonal pillar shape

Cubic SiC crystals (β-modification) were grown from Si solution in a graphite crucible at about 1550 °C. Several crystals with hexagonal pillar shape were found in these crystals, and are believed to have grown under quasi-equilibrium conditions. The side surfaces of the hexagonal pillar consisted of four {111} and two {100} planes, and the long axis was in the 〈110〉 direction. The crystallographic polarities of the four {111} planes were determined using the anomalous X-ray dispersion effect and the chemical etching technique. It was confirmed that the neighboring two{111} planes are the complementary planes of opposite polarities.