Defects boron carbides

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

The mechanical strength of sintered boron carbide is 15-20 % lower than that of hot-pressed samples this may be owing to intercrystalline defects in the former case, and transcrystalline ones in the latter case. 

Because boron carbide can be used as the control rod material in a nuclear reactor, in order to interpret its performance it is necessary to establish nature of grown-in and neutron-radiation-induced lattice defects in boron carbide. It was found that the dose received by the irradiated specimen corresponds to transmutation of about eight B nuclei per unit cell in equal number of both Li and " He nuclei (Ashbee 1971). It is believed that the formation of the partial dislocation loops resulting from the agglomeration of point defects are introduced during neutron irradiation. 

Ashbee, K. H. G. 1971. Defects in boron carbide before and after neutron irradiation. Acta Metall. 19 ... 

Storm, R. S. 1980. One-step curing method for manufacture of neutron absorbing plates. U.S. Patent 4198322. Stoto, T., L. Zuppiroli, and J. Pelissier. 1985. Absence of defect clusters in electron irradiated boron carbide. Radiat. Effects 90 161-170. 

Table 4.7 Boron carbide Comparison between theoretical electronic properties, experimental characterization and intrinsic point defects determined experimentally. 

Wang,). and Marshall, D.S. (2001) Defect formation in boron carbide-an ab-initio electronic structure study. Proceedings, Fall Meeting MRS, Boston, 26-30 November, Symposium G, paper G8.6. Materials Research Society. Available at http // www.mrs.org/s mrs/... 

On a 300mm etch tool, boron carbide coated chamber was also used in a 1,000 wafer marathon. The boron carbide coated chamber meets all the requirements including aluminum etch rate and etch rate non-uniformity, etch profiles, defects and particles, metal contamination [49]. The particle performance at 0.12 pm or larger is the critical requirement. It is obvious that the boron carbide coated chamber can meet the requirement. The up limit of particle allowance at 0.12 pm or larger is defined as 50 adders/ per wafer. 

After the detail study through a thorough process qualification, the new boron carbide coated chamber wall is used to replace the previously anodized aluminum surface. The new ceramic material such as YAG or Y2O3 is used to replace original high purity alumina. This configuration was introduced to semiconductor wafer fabrication for evaluation. Excellent etch performance, enhanced defect and particle reduction, and 50 to 100 times chamber lifetime improvement are reported. The production yield of the wafer fabrication also improved about 7% in production at the customer site (see Fig.l9) [41]. The following data provide some of the information. The sequence of the data collection is as follows ... 

Table 8 shows that, for a-rhombohedral boron, p-rhombohedral boron and boron carbide, the electron deficiencies determined by electronic band structure calculations are correlated with the densities of defects in the structures. Since calculations of the electronic properties of defects in icosahedral boron-rich solids are missing, the following assumptions (202) were made in accordance with general results obtained on defects in semiconductors. 

The idealized, most energetically favorable structure of boron carbide B13C2 (structure formula Bi2(CBC)) (206,207) was taken for reference to determine the composition dependent electron deficiency and the concentration of structural defects (202). 

Unfortunately, there are no calculations of the electronic band structure available. However, the high defect concentration of 17 at.% is at least qualitatively correlated with the very strong tail of the absorption edge (Fig. 48) exceeding that of boron carbide by far. 

WC is the technically most important transition metal carbide. It adopts a very simple structure (Fig. 11, space group P6m2) [44,45], which may be described as a defect-AlB2 structure in which every other boron position is unoccupied. This results in a symmetry reduction. In space group P6m2 (a translationengleiche subgroup of index 2 (t2) of P6/mmm) the two-fold 2d position of P6/mmm is reduced to two one-fold positions. This allows the introduction of ordered defects. The... 

Chapter 6 addresses the stracture of non-polymer fibers. A wide range of non-polymer fibers, such as carbon, glass, silicon carbide, boron, asbestos, and metal fibers, now is available commercially. Compared with polymer fibers, nonpolymer fibers often are stronger, stiffer, more heat resistant, and nonflammable. However, except for metal fibers, non-polymer fibers also are characterized by their brittleness. These property characteristics are directly related to the atomic arrangement and the defect stracture of non-polymer fibers. Chapter 6 discusses the stracture of two most used non-polymer fibers caibon and glass fibers. 

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