Priority compositions of boron carbide crystals obtained by self-propagating high-temperature synthesis
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Priority Compositions of Boron Carbide Crystals Obtained by Self-Propagating High-Temperature Synthesis V. I. Ponomarev, S. V. Konovalikhin, I. D. Kovalev, and V. I. Vershinnikov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, 142432 Russia e-mail: [email protected] Received December 16, 2014
Abstract—Splitting of reflections from boron carbide has been found for the first time by an X-ray diffraction study of polycrystalline mixture of boron carbide В15 – хСх, (1.5 ≤ x ≤ 3) and its magnesium derivative C4B25Mg1.42. An analysis of reflection profiles shows that this splitting is due to the presence of boron carbide phases of different compositions in the sample, which are formed during crystal growth. The composition changes from В12.9С2.1 to В12.4С2.6. DOI: 10.1134/S1063774515050089
INTRODUCTION It was shown in [1, 2] that carbon atoms in boron carbide crystals В15 – хСх, (1.5 ≤ x ≤ 3) obtained by magnesium thermal self-propagating high-temperature synthesis (SHS) can be incorporated into five out of fifteen structural sites. Carbon incorporation into any of five sites does not lead to a change in the structural motif [3–9]. According to the X-ray diffraction (XRD) data, the structural units of boron carbide crystals are В12 – z icosahedra (0 ≤ z ≤ 1) and С3 – yBy groups (1 ≤ y ≤ 2). Carbon atoms can be incorporated into any of six crystallographically equivalent sites of icosahedron. The values of diffraction reflection half-widths were considered evidence for the diversity in carbon incorporation into boron carbide. According to the XRD data on boron carbide polycrystals with 2 ≤ x ≤ 3, the reflection half-width changes from 0.11° to 0.4° [2]. This change is due to the simultaneous presence of crystals of different compositions and different versions of carbon incorporation into the structure in the reaction products. On the other hand, it is known that other factors (for example, crystal mosaicity and defects) can also affect the reflection half-width. In this study we obtained for the first time experimental data confirming the hypothesis, according to which the increase in the half-width of in boron carbide reflections is related to the presence of crystals of different compositions in the synthesis products. Crystals of the C4B25Mg1.42 composition were obtained in [10, 11] by SHS. XRD analysis of the reaction products revealed that these crystals are a mixture of C4B25Mg1.42 and boron carbide crystals (figure 1). According to the unit-cell parameters, the composi-
tion of boron carbide is close to В12.5С2.5. Some specific features were revealed in X-ray diffraction patterns. In particular, the intensity of reflection 101 in some experiments was higher than that of reflections 003 and 102 (figure 1). This situation was never observed previously in boron carbide X-ray diffraction patterns [2, 5, 7, 12]. The reflection half-width is also unusual. For boron carbide crystals of the В12.5С2.5 composition, it is in the range of 0.
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