Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies
Introduction
Silicon carbide (SiC) fibre-reinforced SiC matrix composite materials (SiC/SiC) are considered to be the attractive candidates as materials for advanced energy systems, such as high performance combustion systems, fuel-flexible gasification systems, fuel cell/turbine hybrid systems, nuclear fusion reactors and high temperature gas-cooled fission reactors [1]. Recently, a review of state-of-the-art achievements in production and application of SiC/SiC composites was published [2].
It has long been known that SiC is a polytypic substance. But the formation of a phase diagram is very difficult, for annealing is slow; different forms may grow under almost identical conditions, and even small quantities of impurities may have significant effects. From previous studies, it was found that especially the cubic form grows under conditions where one of the hexagonal polytypes is more stable. First explanations of this fact were given in terms of a stacking reversal at a surface and bulk polytype energies [3]. A later extension of this idea was based on a distinction between the two different (0 0 0 1) surfaces and application of bulk-derived parameters at a surface [4].
The positron affinity is a fundamental bulk quantity of a solid, which does not depend on the surface orientation of a crystalline sample, and it has already been calculated for 3C–SiC and 6H–SiC polytypes [5]. At the same time, an experimental estimation of the electron work function of 6H–SiC, combined with independent positron work-function measurements on the same specimen, allowed the evaluation of the positron affinity and its comparison with the theoretical value. This comparison has prompted suggestions for improvements in the theoretical calculations to be confirmed by future work.
The observation of copious positron re-emission from crystalline 6H–SiC, due to a negative positron work function and with no pre-treatment and without the need for ultra-high vacuum conditions, suggests this material may form the basis of an important new moderator for the production of monoenergetic positron beams [6].
Furthermore, SiC in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel applications in electronics [7]. An essential step in most of the state-of-the-art technologies is ion implantation, which is used to confine the lateral dimensions of an area of a crystal wafer, or film on a substrate, to be modified. Therefore, the detection and characterization of lattice defects is an essential need and challenge for materials science.
Positron annihilation spectroscopy (PAS) is generally suited to detect, distinguish, and eventually identify open volume defects in solids, including semiconductors [8]. Slow positron implantation spectroscopy (SPIS), based on the generation, implantation and subsequent annihilation of monoenergetic positrons in a sample, is well suited to study depth dependent vacancy-type damage in silicon carbide [9]. In addition, atomic force microscopy (AFM) [10], [11] is a suitable method to investigate the surface morphology of a sample.
Recently, systematic SPIS and AFM studies of various 6H–SiC samples, differing in their conductivity type, crystal quality, ion implantation conditions and annealing, were conducted in order to see if and how these parameters may influence the formation of continuous long furrows (undulations) running in one direction across the wafer surface [12]. It was found that the observed changes in surface morphology are primarily the result of thermal activation during annealing and thus occur independent of conductivity type, crystal quality and ion implantation. Moreover, it was observed that the changes in surface morphology have no influence on the defect depth profiling by SPIS.
Based on the experience in studying basic properties and near surface defects in single-crystalline 6H–SiC [5], [6], [9], [12], it is challenging to investigate a SiC/SiC composite made from nano-crystalline 3C–SiC. Following the specification of the preparation conditions of such a composite, results of various experimental investigations, namely X-ray diffraction (XRD), AFM, SPIS and the re-emission of positrons, will be presented and discussed. In addition, some experimental and theoretical results for graphite are presented to complement these discussions. Conclusions are drawn at the end of the paper.
Section snippets
Preparation of a SiC/SiC composite
The preparation of a sample having the dimensions 10 mm × 10 mm × 1 mm was performed by the nano-infiltration transient eutectic phase sintering (NITE) process [13] in four steps as follows: (1) selection of a 3C–SiC nano-powder (∼30 nm diameter; Marketech International Inc., Port Townsend/WA, USA, as determined by XRD and transmission electron microscopy (TEM)) and sintering additives (Al2O3 + Y2O3 = 12 wt.% (Al2O3:Y2O3 = 60:40) and SiO2 = 3 wt.%); (2) preparation of a matrix slurry (3C–SiC nano-powder and
X-ray diffraction
A standard phase analysis was performed by XRD in Bragg-Brentano geometry using a D8-Advance instrument (Bruker AXS). Fig. 1 shows the diffraction pattern measured with Cu Kα radiation (λ = 0.154 nm). The positions of the diffraction lines according to the powder diffraction database (PDF) with (h k l) = (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) are indicated for 3C–SiC (PDF 29-1129) and graphite with (h k l) = (0 0 2) (PDF 41-1487). Non-indicated lines are formed by the sintering additive YAlO3 (PDF 38-0222).
Conclusions
It has been experimentally demonstrated by XRD that a macroscopic SiC/SiC composite sintered from nano-crystalline 3C–SiC consists exclusively of 3C–SiC containing still some graphite inclusions.
AFM measurements on different areas of the composite sample reveal that micrometer sized 2D plies of SiC fibres are embedded in a matrix of 3D crystallites with diameters in the range between 30 and 90 nm.
SPIS investigations underline the perfectness of the composite at an atomic size level due to the
Acknowledgement
The authors express their gratitude to Dr. V. Heera (FZ Rossendorf) for valuable discussions of various aspects of this work.
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