Silicon carbide grain sizes

An optical microscopy examination, represented in Figure 2, revealed that the material is made from coarse silicon carbide grains of some millimetres size, bonded by an intergranular phase that also contains smaller SiC grains. Some porosity can be observed in the intergranular phase, as well as within the coarse SiC grains. 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

In the United States, a number of physical tests are performed on silicon carbide using standard AGA-approved methods, including particle size (sieve) analysis, bulk density, capillarity (wettability), friability, and sedimentation. Specifications for particle size depend on the use for example, coated abrasive requirements (134) are different from the requirements for general industrial abrasives. In Europe and Japan, requirements are again set by ISO and JSA, respectively. Standards for industrial grain are approximately the same as in the United States, but sizing standards are different for both coated... 

Abrasive material green silicon carbide SiC, grain size 30(630-500fim) V Abrasive material noble alundum EA, grain size 30(630-500(im)... 

Hoppe P., Strebel R., Eberhardt P., Amari S., and Lewis R. S. (2000) Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5—1.5 p,m. Meteorit. Planet. Sci. 35, 1157-1176. 

Silicon carbide. Analyses of grain size separates of SiC have shown that the sensitive Kr isotopic ratios vary as a function of size of the SiC grains (Lewis et al. 

Figure 9. Correlation between Ne/ °Xe and Kr/ Kr in the G component carried by silicon carbide. Both ratios increase with average grain size of the SiC samples (Lewis et al. 1994).

Initial explanations of the extraordinary high-temperature stability of precursor-derived Si-B-C-N ceramics were presented previously by Jalowiecki et al. The authors investigated the microstmcture of boron-doped silicon carbonitride composites by HR-TEM and fotmd turbostratic BN(C) segregation occuring along grain boundaries of nano-sized silicon carbide and silicon nitride crystals. We suppose... 

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