Systematic Changes of the Nb-Sn Reaction With Time, Temperature, and Alloying in Restacked-Rod-Process (RRP) ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strands

Abstract

Reaction heat treatments spanning 605 to 750degC and 24 to ~400 hours were applied to several sets of RRP strands. Magnetization and transport measurements were used to track the changes of superconducting properties and the amounts of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn formed. The experiments showed that temperature increases of 15degC produced equivalent properties in half the time within the temperature range 620-680degC. This result was the same whether Ta or Ti was used to alloy the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn. The bulk pinning force <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fp</i> for Ta-alloyed wires displayed a significant drop for temperatures outside the range above, due to tin gradients at low temperatures and grain growth at high temperatures. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fp</i> drop at high reaction temperatures prevents wire technology from taking advantage of the significantly higher Kramer-plot intercept <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</sub> for high-temperature reactions. On the other hand, Ti alloying provides a quick and potent means to increase <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</sub> at reaction temperatures for which <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Fp</i> remains high.