Crystal 6H Research Articles

Fabrication, thermal treatment and microstructure development in SiC-AIN-Al2OC ceramics

Samples containing equimolar amounts of SiC, AIN and Al2OC were fabricated by hot pressing a mixture of SiC, AIN, Al2O3 and Al4C3. The predominant constituent in the hot pressed material was of 2H crystal type. The samples were subsequently annealed at a temperature up to 2050‡ C for up to 153 h in an atmosphere of nitrogen. Samples made with IbigawaΒ-SiC exhibited formation of needle-type precipitates. Carbon deficient specimens contained substantial amounts of Al3O3N. Principal characterization techniques employed consisted of optical microscopy, electron microscopy, X-ray diffraction and chemical analysis.

The effects of N 2 + and B+ ion implantation on the hardness behaviour and near-surface structure of SiC

The effects of ion implantation on the near-surface deformation behaviour and structure of both single crystal (6H) silicon carbide and a reaction-bonded SiC composite (REFEL) have been studied using 80 keV N 2 + and 40 keV B+ ions in the dose range 1 to 1 × 1017 ions cm−2. The ion species were chosen to have different effects on the polytype stability of SiC but produce similar distributions of damage. Subsequent room temperature, load-variant, indentation hardness tests showed that N 2 + implantations produced a marked near-surface softening above a critical dose of ∼ 4 × 1017 ions cm−2 . The effect, which appeared specific to N 2 + implanation is possibly attributable to the rapid thickening of the surface amorphized damage layer by nitrogen ges bubble formation. Implantation with both species suppressed the breakout of the lateral cracks produced by indentation fracture and this is presumed due to the production of a compressive surface stress. Scratch tests established that aven the lowest doses of both species suppressed chipping to the extent that scratch tracks appeared plastically deformed only Subsequent TEM examination showed that the microcrystalline or amorphous material produced by implantation with either species had transformed during deformation to the low-temperature-stable cubic (3C) polytype despite the differing expected influences of boron and nitrogen on polytype stability.

Superconducting energy gap of NbS2

The superconducting energy gaps of small particles (1–10 μm) of 2H–NbS2 and 2H–NbSe2 have been measured by far-infrared (FIR) transmission spectroscopy. Comparison of these results with previous FIR measurements made on millimetre-size single crystals of 2H–NbSe2 validates the use of the novel method of sample preparation used here. Comparison of the gap frequencies (νg) with the superconducting transition temperature (Tc) determined by magnetic susceptibility measurements indicates that 2H–NbS2, unlike 2H–NbSe2, exhibits non-Bardeen–Cooper–Schrieffer (BCS)-like behaviour, with hνg/kTc = 2.3.

Stacking faults and structural transformations in Zn xMn 1−xS single crystals grown from vapour

This paper reports an X-ray diffraction study of the stacking faults and solid state transformations observed in Zn x Mn 1− x S (0.9 < x < 1) single crystals grown from the vapour phase at 1100°C in the presence of H 2S. The crystals were found to contain 2H, 3C, 2H + 3C and polytypes as well as structures with considerable disorder due to random stacking faults. Needle shaped 2H crystals were annealed in vacuum in the temperature range from 300 to 1100°C to induce structural transformations. Nearly 75% of the crystals transformed to a disordered twinned 3C structure on annealing around 600°C. The rest of the crystals transformed first to a disordered 6H structure and then, on further annealing at higher temperatures, to the disordered twinned 3C structure. Most of the transformed 3C structures showed an intensity enhancement near the positions of the 6H reflections on X-ray diffraction photographs. All the 3C crystals transform back to a disordered 2H structure on annealing above 1050°C. To determine the nature of stacking faults involved in the 2H–6H transformation we have investigated the broadening of X-ray diffraction maxima along reciprocal lattice rows with H - K ≠ 3 n produced by crystals undergoing the transformation. The point intensity distribution along the 10. L reciprocal lattice row of several partially transformed crystals were recorded on a four circle single crystal diffractometer in steps of Δ L = 0.01. A study of the experimental profiles obtained by plotting the diffractometer record of intensity versus L in reciprocal space shows that the transformation occurs by the non-random insertion of deformation faults in the 2H structure. It is shown that the one-parameter model proposed by Pandey, Lele and Krishna [Proc. Roy. Soc. (London) A369 (1980) 451 is not applicable to 2H–6H solid state transformation in Zn x Mn 1− x S crystals. A two-parameter model involving different fault probabilities for the nucleation (α) and the growth (β) of the 6H phase in the 2H structure is suggested.

Electron microscopic study on new phases of BaMnO3ȡx formed by electron irradiation

New phases in the BaMnO3ȡx system were investigated by high resolution electron microscopy. A hexagonal cell with two atomic layers (2H, a = 0.57 nm, c = 0.48 nm) was found to have an oxygen-deficiency to some degree, though its X-ray powder pattern gave a single phase of a 2H crystal. Observed structure images showed that a 2H phase was transformed into a 10H (6L + 4L) phase under a low beam intensity and into a 6H (4L + 2L) phase under a very high beam intensity. A mechanism of phase transformation is proposed that oxygen-deficiency in face-sharing octahedra induces the displacement of the Mn ions which reduce the coordination number and the disruption of face-sharing octahedra.

Dielectric relaxation in cyano complexes of the K 4Me 11(CN) 6·3 H 2O type. I. Ferroelectric behaviour

Several K 4Me 11 (CN) 6·3 H 2O type cyano complexes belonging to the C 2h crystal class were studied to examine their ferroelectricity and the dielectric-relaxation process. Isomorphous substances obtained through replacing the Fe 2− ion by Ru 2+, Os 2+ and Mn 2+ ions show ferroelectric behaviour and dielectric relaxation, in both the ferro- and para-electric phases. It was found, through a careful analysis of the shape of the dielectric-absorption band, that a polydispersive nature appears in the whole temperature range explored and becomes very pronounced in the immediate vicinity of the transition temperature. The distribution of the relaxation times has been described by means of the Williams-Watts decay function.

Electrointercalation of silver into 2H–NbSe2

The electrointercalation of Ag into single crystals of 2H–NbSe2 was studied. The distribution of Ag in the samples was measured with an electron beam probe. Intercalation generally proceeded in two stages, with the Van der Waals gaps, near the sample surface, appearing to intercalate first, while a critical concentration, [Formula: see text] of Ag, was required before intercalation in the bulk took place. The Ag concentration profiles were generally characterized by sharp fronts which remained stationary for long periods of time after intercalation was discontinued. Radioactive tracer measurements gave Ag self-diffusion coefficient of [Formula: see text] for [Formula: see text].

X-ray diffraction from one-dimensionally disordered 2H crystals undergoing solid state transformation to the 6H structure. I. The layer displacement mechanism

The 2H or AB. . . close-packed structure may be transformed to 6H or ABCACB. . . structure if layer displacement faults occur preferentially on every third close-packed layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H → 6H structural transformation by such a layer displacement mechanism is developed. It is shown that it is necessary to consider that the faults do not occur entirely at random but prefer to occur at three-layer separations from each other in order to statistically create a 6H structure. The diffraction theory, as developed by earlier workers for 2H crystals containing a completely random distribution of stacking faults, cannot therefore be applied to the present case. An exact expression for the diffracted intensity from crystals undergoing the 2H → 6H transformation has been obtained and the different observable diffraction effects (like change in integrated intensity, peak shift, integral breadth, peak asymmetry) have been predicted.

X-ray diffraction from one-dimensionally disordered 2H crystals undergoing solid state transformation to the 6H structure II. The deformation mechanism

The 2H or AB Structure may be transformed to 6H or ABCACB structure if deformation faults occur preferentially after every third close-packed layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H → 6H structural transformation by such a deformation mechanism is developed. For this, it is necessary to consider that the faults are not distributed entirely at random but tend to occur in such a manner as to statistically create a 6H structure. An exact expression for the diffracted intensity for crystals undergoing the 2H → 6H transformation by the deformation mechanism has been obtained and the different observable diffraction effects have been predicted. These results are very different from those obtained of 2H crystals containing an entirely random distribution of deformation faults, especially for large fault probabilities.

X-ray diffraction from one-dimensionally disordered 2H crystals undergoing solid state transformation to the 6H structure. III. Comparison with experimental observations on SiC

The diffraction effects predicted theoretically in the preceding two papers for 2H crystals undergoing solid state transformation to the 6H structure by the layer displacement mechanism and the deformation mechanism are compared with those experimentally observed on SiC. It is shown that the observed diffraction characteristics can be explained in terms of the layer displacement mechanism and not the deformation mechanism. A simple estimate of the layer displacement fault probability in two transformed 6H SiC crystals has been made by analysing the halfwidth of the experimentally obtained intensity profiles of the 10. L reflexions. It is also shown that the presence of a small concentration of growth faults in the as-grown 2H SiC crystal does not alter the basic diffraction characteristics predicted in part I of this series of papers.

Phase transformation and electrical behaviour of lead iodide polytypes

The electrical resistivity behaviour of lead iodide polytypic crystals has been investigated. The polytypes were obtained by inducing phase transformation through vacuum annealing (10-4 Torr, temperature 200-290 degrees C) in the as-grown crystals corresponding to 2H structures. The resistivities of the lead iodide polytypes exhibited two types of variations. The resistivities of the 4H crystals arising from the interpolytypic transformation of the 2H to 4H type were found to be higher than the initial 2H structures, a representative resistivity variation of this type being a change from 3.29*1011 Omega cm(2H) to 2.56*1012 Omega cm (4H). On the other hand, the resistivities of higher polytypes originating from the occurrence of the 2H to nH (n>4) transformation were always lower than the initial 2H structure. As for example, the resistivity of a 12H polytype was found to be 4.10*108 Omega cm whereas the initial 2H crystal had a resistivity of 2.83*1012 Omega cm. Arguments and evidence have been given to show that the foregoing resistivity variations become intelligible in terms of the changes in the interactions between layers and also the changes in the band structures and Brillouin zones, which result due to the formation of the polytypes consequent to interpolytypic conversion.

Recrystallization of SiC thin films

Thin films of SIC have been prepared by electron-beam evaporation. By being pulse-annealed in the electron microscope, as-deposited noncrystalline films recrystallize and grow, first in polycrystalline 2H and 3C phases and subsequently into syntactic 2H and 3C single crystals.

Investigation of exciton complexes in 6H-SiC by isotope substitution

Unintentionally doped single crystals of 6H|SiC generally exhibit at low temperature a luminescence spectrum consisting of three zero phonon lines and a series of strong phonon replicas at lower energies including lines due to a localized vibrational mode. The fact that this localized mode shows no change for crystals doped with 15N, 10B or 18O is regarded as strong evidence that this spectrum is not due to the recombination of excitons at ionized nitrogen donors as has been reported by other authors, nor to recombination at centres involving the aforementioned impurities.

Low-Temperature Solid-State Phase Transformations in 2H Silicon Carbide

Single crystals of 2H SiC were observed to undergo phase transformations at temperatures as low as 400°C. Some 2H crystals transformed to a structure with one-dimensional disorder along the crystal c axis. Others transformed to a faulted cubic/6H structure. The transformation is time and temperature dependent, and is greatly enhanced by dislocations. Our observations indicate that the transformation takes place by means of a slip process perpendicular to the c axis. Cubic SiC crystals were observed to undergo a solid-state transformation above 1400°C.

The growth of silicon carbide needles by the vapor-liquid-solid method

Single crystal needles of alpha (2H-wurtzite) silicon carbide have been grown under controlled conditions by the hydrogen reduction of purified methyltrichlorosilane onto silicon carbide coated graphite susceptors. Previous work of Ryan et al. has been extended and more precise specifications of growth conditions of the 2H modifications have been determined. The 2H crystals were grown several millimeters in length by one millimeter diameter. They were optically clear and had sharply defined face. Their growth morphology was compatible with the VLS mechanism. The crystals were characterized by optical microscopy, electron microscopy and electron microprobe techniques. A detailed X-ray investigation of the anomalous behavior of these 2H crystals under thermal annealing is presented in a companion paper (Krishna and Marshall, this meeting).

Phase transformations, habit changes and crystal growth in SiC

The phase transitions and habit changes of SiC crystals of the polytypes 2H, 3C, 4H and 15R have been studied in the conditions of a semiclosed SiC/C system, by annealing in argon at 1 atm pressure. In the temperature range of investigation (1500–2830 °C) all types transform to tabular crystals of the 6H structure. The transition of 2H whiskers into 6H crystals takes place via the metastable 3C modification. The experimental data are discussed in terms of models assuming either in situ volume transitions or surface mechanisms. The observed transition rates of normal crystals of 3C, 4H and 15R to 6H are considerably lower than the rate of the 2H→3C transformation of whiskers. From Arrhenius plots of the transition rates activation energies were derived which, within the limits of experimental accuracy, are equal (≈ 140 kcal/mole), and identicall with the activation energies of evaporation and of crystal growth by recrystallization of 6H. It is concluded that both for the transformations, where near-surface transport takes place, and for evaporation one and the same molecular step is rate controlling.

Absorption line spectrum in SiC 6H nitrogen doped crystals

Two sharp doublets have been observed in absorption on SiC 6H crystals heavily doped with nitrogen (5×10 18/cm 3). One of these doublets corresponds to previously observed emission peaks. The observation of absorption lines in SiC is of some importance as this compound may be formed in interstellar space and could be responsible for the absorption lines observed in interstellar clouds. The above observed lines do, however, not correspond to known lines observed in these clouds. SiC forms about 40 polytypes and only one has been studied in this paper.