Effect of Impurity Gas Additions on the Superconducting Critical Current of Vapor-Deposited Nb[sub 3]Sn

Abstract

The addition of various gaseous impurities into the process gases used in the chemical vapor deposition of superconducting Nb3normalSn has been found to alter the grain size of Nb3normalSn and thereby the critical current (Ic ). Increasing concentrations of CO2 , CO , and N2 cause a progressive decrease in the particle size and a concomitant increase of Ic up to three times the value found for undoped specimens. In contrast, O2 and CH4 increase the particle size and reduce Ic . Ic is linearly dependent on the reciprocal of the grain size, 1/D, to values of D as small as 400Aå. The present work suggests that impurities serve to control the grain size, presumably by the formation of small second‐phase precipitates which interfere with the growth of Nb3normalSn grains.

Effect of carbon on the structure and superconducting properties of Nb3Sn coatings produced by electrocodeposition

“…It is of interest to compare our microstructural data with earlier reports on the microstructure of Nb 3 Sn coatings produced by CVD via hydrogen reduction of niobium and tin chlorides [8][9][10][11]. To control the microstructure of niobium stannide deposits, carbon-containing gases were introduced into the hydrogen or chlorine flow at the inlet of the CVD reactor.…”

“…To control the microstructure of niobium stannide deposits, carbon-containing gases were introduced into the hydrogen or chlorine flow at the inlet of the CVD reactor. C 2 H 6 , C 3 H 8 , CO 2 , CO, and CCl 4 additions were found to reduce the average grain size of the superconducting layer by almost a factor of 3 and, accordingly, to ensure a threefold increase in critical current [8,10]. The Nb 3 Sn surface became smoother and changed from dull to glossy.…”

“…Under appropriate conditions, carbon atoms may dissolve in the A-15 phase, forming a solid solution [8]. Moreover, at high concentrations of carbon, it may be present as an additional phase, primarily in the form of submicron-sized carbide inclusions, which are capable of inhibiting Nb 3 Sn grain growth during CVD [10].…”

“…Moreover, partial tantalum substitution for niobium in the A-15 structure markedly increases the upper critical field, without reducing T c [6]. In producing niobium stannide by chemical vapor deposition (CVD), it was found that carbon impurity may also notably enhance the critical current of this material [8][9][10][11].…”