Structural characterization of a coarse-grained transparent silicon carbide powder by a combination of powder diffraction techniques

Diffraction of hard synchrotron radiation as well as constant-wavelength and time-of-flight neutron diffraction were used for the structural characterization of a silicon carbide powder having extremely low levels of chemical impurities, high perfection of the crystalline lattice and a grain size of up to 150 μm. The presence of three polytypes was ascertained and the ratios of their mass fractions were determined to be w15R : w6H = 0.002,3(8) and w4H : w6H = 0.000,6(2). Introduction Silicon carbide powders having chemical impurity levels as low as a few mg per kg or even less, high perfection of the crystalline lattice and a crystallite size exceeding several tens or even hundred μm show not only high hardness but also exceptional chemical resistance (insolubility in acids), as well as unique electronic and optical properties. One of the main challenges to a reliable structural characterization of such a material is its rather large crystallite size considerably exceeding the value acceptable for reliable X-ray powder diffraction measurements. Any grinding bears a high risk of changing/destroying the original real structure characteristics of the sample, e.g. the degree of stacking disorder and their polytype composition. Thus, grinding can significantly distort the outcome of the structural investigation and, 62 European Powder Diffraction Conference, EPDIC 11 therefore, should be avoided. Consequently, highly penetrating radiation has to be used for any quantitative diffraction analysis and measures have to be taken to reduce the unfavourable influence of insufficient crystal orientation statistics as much as possible. Sample The element composition of the sample was extensively examined mainly by atomic absorption spectrometry (AAS) and by inductively coupled plasma optical emission spectrometry with electrothermal evaporation of the sample (ETV-ICP OES). The content of the following trace elements were determined: Al, B, Ca, Cr, Cu, Fe, Mg, Na, Ni, Ti, V, Zr. The mass fraction of the Al component incorporated into the bulk of the material is about 44 mg/kg, and that of B about 4 mg/kg. The other trace metals are mainly adhered to the surface of the particles. For that part of their mass fractions that is dissolved in the silicon carbide crystals values below 1 mg/kg were determined. According to SEM the most frequently occurring particle size is about 20 μm. The smallest particles are ca. 8 μm while a considerable number of particles have dimensions between 100 and 180 μm (see figure 1). TEM in combination with the FIB preparation technique showed that the powder particles are quite perfect single crystals. Dislocations were found only in a surface layer less than 1 μm thick but not in the interior of crystals. Figure 1. SEM pictures of individual crystallites of the silicon carbide powder sample. The largest dimensions in the 2D-projection of the displayed crystallites are ca. 140 μm (left) and 110 μm (right). Data collection and data analysis Instrumentation and conditions of data collection used in the present diffraction investigation as well as computer programs applied for data evaluation are summarized in table 1. Results and discussion All three diffraction patterns have FWHM values very close to the instrument contributions to the line profiles, see figure 2. The three patterns were evaluated by the Rietveld method [1-2] describing the sample as 6H-SiC, either completely pure or with traces of other SiC Z. Kristallogr. Suppl. 30 (2009) 63 Table 1. Characteristics of the three diffraction patterns of the silicon carbide material. pattern #1 pattern #2 pattern #3 diffracted radiation monochromatic neutrons λ ≈ 1.7967 A pulsed neutrons from spallation source monochromatized synchrotron radiation λ ≈ 0.8003 A instrument; facility E9; HMI, BENSC HRPD ; ISIS. RAL ID31; ESRF type of pattern from a single specimen from a single specimen synthesized from the patterns of 10 specimen type of specimen capillary, O 16 mm, vanadium capillary, O 8 mm, vanadium capillary, O 1 mm, borosilicate specimen rotation none none 3000 rpm mass of specimen 12 g 4 g 80 mg (= 10 • 8 mg) total measuring time 17 h 5 h 10 h = (10 • 1h) range of d-values 34.0 A– 0.916 A 2.4 A 0.674 A 22.9 A 0.825 A FWHM 2 0.016 A 0.002 A 0.001 A signal-to-Bkg 2 33 350 650 noise-to-Bkg 3 ± 0.125 ± 1 ± 0.05 Rietveld programs used FullProf.2k V.4.30 (TOPAS V.2.1) TOPAS V.4.1 TOPAS V.2.1 TOPAS V.4.1 1 pattern #2 was collected before the major upgrade of the neutron guide in 2007 2, 3 FWHM and signal-to-background ratio for the 103 reflection of 6H-SiC at ~ 2.36 A; noise-to-background ratio near that line.