Hot-press sintering and the properties of lanthanum-rich calcium lanthanum sulfide ceramic
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The lanthanum-rich calcium lanthanum sulfide (CLS) was formed by a carbonate coprecipitating method followed by sulfurizing in a CS 2 atmosphere at 950 °C for 3 h sulfurization. This powder then was densified by hot-press sintering at the temperature range from 1050 to 1150 °C for 0.5-2 h. The sintered pellets were resulfurized in a CS 2 atmosphere at 950 °C for 1 h to restore the sulfur loss. The pellet sintered at 1050 °C for 0.5 h with 1.3 mm thickness had about 46% transmission at 13
I. INTRODUCTION Calcium lanthanum sulfide (CLS) is a potential material for an 8-14 yttm spectral optical window. Saunders et al.x indicated that the general formula of
CLS
is
Cai-ILa(lll)2+(6/9)x-(8/%La(ll)(8/9)3,S4-(4/9)>,
[ 1(4/9))-. where [ ] represents a sulfur vacancy and 0 =s x,y =£ 1. In the early studies,2'3 the formation of pure CLS is very difficult since the second phase was present. The CaS is the general one with a structure quite different from that of CLS. The existence of CaS would hinder the ir transmission of CLS. From 1986,4"6 lanthanum-rich CLS was prepared to avoid the formation of CaS; however, /3-La 2 S 37 - 8 appeared in such a powder. Essentially /3-La 2 S 3 9 is an oxysulfide with a tetragonal structure with a general formula Lai 0 Si 4 O x Si- x (0 =£ x =£ 1). It appears to be a viable material to investigate for its potential as an ir transmitting material. The range of transmittance for /3-La 2 S 3 is from 0.5 to 10 /Am.10 Kumta et al.,l0~u who studied the formation of lanthanum sulfide powder with an alkoxide route method, pointed out that /3-La 2 S 3 is the intermediate phase of cubic-La 2 S 3 . La 2 S 3 belonging to the rare earth sulfide has three important polymorphs; i.e., a - L a 2 S 3 is a low-temperature form (450-600 °C), and the intermediate phase, tetragonal-La2S3 (/3-La 2 S 3 ), which is stable at 900 °C, would phase transform to cubic-La2S3 (y-La 2 S 3 ) at above 1300 °C. Cubic-La 2 S 3 exhibits a very good transmission in infrared spectra and is an important optical material. In the studies of Kumta et al., La-isopropanol alkoxide was used to form the lanthanum sulfide precursor and y - L a 2 S 3 could be formed by sulfurizing in an H 2 S atmosphere at 1000 °C for 8 h. The alkoxide method can help to reduce the formation temperature of y - L a 2 S 3 . However, lanthanum alkoxide is expensive and that process is more complicated than that of coprecipitating method. In our previous study,15"17 the lanthanum-rich CLS powder has J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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been synthesized successfully by CS 2 as a sulfurizing agent and treating at 950 °C for 3 h. The property of lanthanum-rich CLS is similar to y-La 2 S 3 , 1 5 showing a better thermal stability17 than that of CaLa 2 S 4 powder. In this present study, a lanthanum-rich CLS precursor (La/Ca = 15) was formed by the coprecipitation method, followed by sulfurizing in a CS 2 atmosphere at 950 °C for 3 h. The sulfided powder then was densified by hot-press sintering. The inf
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