Low-temperature chemical routes to formation and IR properties of lanthanum sesquisulfide (La 2 S 3 ) ceramics
- PDF / 2,656,303 Bytes
- 17 Pages / 576 x 792 pts Page_size
- 38 Downloads / 183 Views
Subhash H. Risbud Division of Materials Science and Engineering, The University of California, Davis, California 95616 (Received 1 July 1992; accepted 26 January 1993)
Lanthanum sulfide (La2S3) exists in three different allotropic forms in the crystalline state. The low-temperature orthorhombic phase ( a ) transforms to the intermediate tetragonal phase (/3) which further undergoes transformation to the high-temperature cubic phase (y). The intermediate tetragonal phase (/3) is an oxysulfide with possible use as a phosphor, and the cubic phase (y) of La 2 S 3 shows potential for application as a far IR (8-14 /tm) window material. We report metal-organic synthesis routes for the preparation of the /3- and y-phases of La 2 S 3 by processing modifications and choice of precursor chemistry. The alkoxide-derived precursors and the transformed sulfide powders were characterized for morphology, microstructure, and thermal stability. Phase evolution studies were also conducted, and the results are discussed in the context of formation of the /3- and y-phases. Results of densification, microstructure, and infrared transmission characteristics are also presented.
i. INTRODUCTION Materials that are transparent in the 8-14 /xm spectral window play an important role in many modern optical and opto-electronic applications.1"3 Requirements of high melting point, high hardness, low thermal expansion, and optical transmission restrict the choice of available materials for infrared window applications. Chalcogenide materials exhibit good infrared transmission characteristics, but are soft with low melting points and high thermal expansion coefficients. Semiconductors (Si and Ge) exhibit good transmission and fulfill the strength criterion, but at temperatures above 100 °C, free electron absorption renders these materials opaque and, hence, limits their use. Recent research has shown that one of the materials that satisfies many requirements for IR windows is ZnS, particularly in the 8—14 fim region. Zinc sulfide (ZnS) does suffer from low hardness and poor resistance to severe rain erosion conditions as do other developing optical materials. Table I lists some popular optical materials currently being considered for windows and domes along with the means of preparation, transmission range, and level of usefulness.4 Many of these materials are currently at an advanced stage of development, while some are still being pursued as candidate materials. Diamond, however, has been identified as the best candidate for the far infrared, while SiC is still being researched as a possible mid-IR optical material. There is more interest in silicon carbide as a 1394 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 6, Jun 1993 Downloaded: 05 Jul 2014
wide band gap semiconductor for electronic devices, and, hence, it has received much less attention as an optical material. In this paper we report novel synthesis routes using metal-organics for the synthesis of the cubic and tetragonal phases of lanthanum sulfide. Judicious use of the me
Data Loading...
