CVD of LTS Materials

The NbiSn phase was first determined to be superconducting at 18.0 K in 1954 by Matthias et al. [39]. NbiSn adopts the cubic [Cr.iSij structure (Fig. 2-5) with an -axis spacing of about 5.29 A. NbjSn is a nonductile material with a CTE of 9.8 ppm/K as determined using high temperature X-ray diffraction from 25 to 700 °C (Table 2-4). The CVD synthesis of Nb Sn was first realized in 1964 by G. W. Cullen et al. [40]. This was a landmark contribution as it was the first LTS superconducting material to be synthesized by CVD. Two reviews by researchers at RCA provide additional details and specialized information on Nb Sn CVD [40, 41]. 

Crystal structure [CrjSi] or A 1.5 [CriSi] or A15 [NaCl] orBl [NaCI] orBl 

Typical precursors NbCl, -f SnCh + NbCl, -f GeCl, -I- NbCh/Hz -b CH4 NbCF -f H2 -f N2 

The common reaction pathway for the formation of Nb Sn films is hydrogen reduction of niobium chloride and tin chloride precursors (Eq. 2.1). There are several schemes 

For CVD film growth, materials such as Mo. Ta, W, Pt, Ni, Hastelloy (Ni-alloy) and ceramics such as magnesium silicates have been employed as substrates for Nb. Sn 

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This subsection summarizes the CVD of LTS materials where only brief details of film growth and characterization have been reported. These reports include the A15 materials Nb Si, V Si, ViGe, Nb Ga, as well asTiC yNy, Wi vGey, andTa materials. 

Table 2-4. Physical and optimum electrical properties of CVD-derived LTS materials.

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