Local structure and defects in ultrahigh-temperature materials of borosilicon carbonitride
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Yoshio Hasegawa Art Kagaku Co., Ltd., Tohkai-Mura, Naka-Gun, Ibaraki 319-1112, Japan (Received 10 October 2007; accepted 19 February 2008)
We have investigated structural changes of amorphous borosilicon carbonitride materials with atomic ratios of B/Si/C of 2/3/6 and 4/3/6 calcined at several temperatures. The boron K-edge x-ray absorption spectra showed that the structures of both hexagonal boron nitride ([BN3] unit) with nitrogen-void defects ([BN2] and [BN1] units) and boron oxide existed in the samples, and the relative peak intensity due to the [BN3] unit became stronger by increasing the calcined temperature. It is thought that the well-developed B–N chain and the borosilicate glass coating lead to the high resistance to oxidation at high temperature. X-ray diffraction and infrared measurements followed the x-ray absorption near-edge spectroscopy findings.
I. INTRODUCTION
Borosilicon carbonitride (BSiCN) materials composed of four light elements of boron, silicon, carbon, and nitrogen have been reported to be quite stable, amorphous materials at high temperatures.1–9 The local structure and the coordination state around these elements have been studied by several methods, including x-ray diffraction (XRD) and neutron diffraction,3 x-ray absorption spectroscopy,4 scanning electron microscopy, and transmission electron microscopy (TEM),5,6 and magnetic resonance.7–9 Recently, Hasegawa and colleagues10,11 prepared BSiCN materials with a distinctive method using the precursor, and investigated the excellent resistance of the materials against oxidation at high temperatures with a view toward their application in space materials such as heat insulators and rocket engines. These materials lose small amounts of mass up to 1700 °C under ambient pressure and have outstanding resistance to oxidation under a partial pressure of oxygen ranging from 0.4 to 2 × 105 Pa. Hasegawa and colleagues10,11 have investigated their structure by XRD, TEM, electron energy loss spectroscopy (EELS), electron probe microanalysis (EPMA), and infrared (IR) spectroscopy. However, there have been few reports on the local structure around the boron atom using x-ray absorption near-edge structure
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0224 1642
J. Mater. Res., Vol. 23, No. 6, Jun 2008
(XANES) measurements.4 In general, XANES is sensitive to the local structural changes of an x-ray-absorbing atom because XANES spectra reflect the electronic states of the x-ray-absorbing atoms and the surrounding atoms, and the three-dimensional configuration just around the x-ray-absorbing atom. In the present study, we have clarified mainly the boron-related local structure and the defects in BSiCN materials as functions of calcination and composition, from the results of B K-edge XANES and electron spin resonance (ESR) spectra as well as XRD and IR absorption spectra. II. EXPERIMENTAL A. Synthesis of BSiCN materials
Dimethyldichlorosilane (Si[CH 3 ] 2 Cl 2 ; Shin-Etsu Chemical, Tokyo, Japa
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