Effects of interstitial additions on the structure of Ti 5 Si 3
- PDF / 222,346 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 81 Downloads / 230 Views
S.K. Malik Tata Institute of Fundamental Research, Bombay 400005, India, and University of Missouri Research Reactor, Columbia, Missouri 65211 (Received 3 August 1999; accepted 24 May 2000)
Changes in the structure of Ti5Si3 were measured by x-ray and neutron diffraction as carbon, nitrogen, or oxygen atoms were systematically incorporated into the lattice. Additionally, the lattice parameters and variable atomic positions of pure Ti5Si3 were determined to be a ⳱ 7.460 Å, c ⳱ 5.152 Å, xTi ⳱ 0.2509, and xSi ⳱ 0.6072. The measured trends in lattice parameters as carbon, nitrogen, or oxygen atoms were added to Ti5Si3 showed that most of the previous studies on supposedly pure Ti5Si3 were actually contaminated by these pervasive light elements. Also, oxygen and carbon additions were shown to strongly draw in the surrounding titanium atoms—evidence for bonding between these atoms. The bonding changes that occurred on addition of carbon, nitrogen, or oxygen acted to decrease the measured anisotropic properties of Ti5Si3, such as thermal expansion.
I. INTRODUCTION
Published values of the lattice, thermodynamic quantities, and thermal properties of Ti5Si3 from 1985 to present show considerable scatter. For example, published lattice parameters vary by 0.5%,1–6 thermal expansion coefficients by 90%,4–8 and enthalpy of formation by 15%,2,9,10 all of which are significant variations. Without question, a primary reason for this scatter in properties is the presence of interstitial impurities. Two recent studies by Radhakrishnan et al.11 and Thom and Akinc12 highlight the improbability of synthesizing and consolidating Ti5Si3 without interstitial contamination of carbon, nitrogen, and oxygen. Particularly, studies that use metal powder as a starting material are highly likely to result in oxygen impurity of at least one to two weight percent. Not only titanium powder, but yttrium, zirconium, and other early transition metal powders that have a high affinity for carbon, nitrogen, and oxygen are expected to yield silicides with a significant impurity content after synthesis and processing. Furthermore, studies on Ti5Si3 with carbon, nitrogen, or oxygen intentionally added show a very dramatic effect on crystal and thermal properties, an effect that does account for some of the scatter in the literature data. For example, Thom et al.5 have shown a reduction in thermal expansion anisotropy by 20% when carbon is intentionally added to Ti5Si3. In conjunction with contaminated starting materials, an additional reason for the difficulty in synthesizing Ti5Si3 without impurities is the presence of large, unocJ. Mater. Res., Vol. 15, No. 8, Aug 2000
http://journals.cambridge.org
Downloaded: 12 Mar 2015
cupied interstices in the lattice. Figure 1(a) shows a (001) orthographic projection of the hexagonal Ti5Si3 lattice, which has Mn5Si3 as its prototype structure (space group ⳱ P63/mcm). The occupied atomic sites for pure Ti5Si3 are Ti at 4d sites at (1⁄3, 2⁄3, 0), Ti at 6g sites at (xTi, 0, 1⁄4), where xTi ≈ 0.25, and Si at 6g sites
Data Loading...
