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wnloaded from https://www.cambridge.org/core. Squire Law Library, on 18 Dec 2019 at 04:43:35, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-230-61

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Table I. Composition, periodicity and layer thicknesses of Nb/Al multilayer films Composition at.% Al 33 25

Periodicity

dAl

dNb

dNb + dA,

nm

nm

143 500 70 143 330 500

45 158 16 34 77 117

98 342 54 109 253 383

layer thickness. The overall compositions of the films were controlled by varying the individual layer thicknesses and were selected at the stoichiometric composition of the two intermetallic phases Al5 (25 at.% Al) and sigma (33 at.% Al) [3]. The A15 and sigma phases have a range of compositions and become stoichiometric only at very high temperatures. In particular the composition 25 at.% Al is in the two phase region of A15 and sigma for the temperatures of this study (< 1100 0C). The multilayer compositions and periodicities are presented in Table I. Calorimetry results of Coffey et al. [2] were used to determine the best annealing conditions for mapping the phase formation sequence. The samples were annealed either isothermally or at a constant heating rate in the calorimeter furnace. Additional long isothermal anneals were carried out in a tube furnace. The details of cross-sectional transmission electron microscopy (XTEM) sample preparation are given elsewhere [4]. The superconducting transition temperature T, was measured by a four point resistive technique. RESULTS AND DISCUSSION As Stage 2 progresses, the Nb and NbAI3 are consumed and the Al5 and sigma grow. The presence of all four phases can clearly be seen in Fig. 1. The Nb grains have grown to give one row of grains per remaining layer of Nb. The A15 phase has coalesced into a continuous layer approximately 100 nm inthickness with approximately 100 nm lateral grain size. The sigma phase is a highly twinned phase with grain size 1030 nm and has grown by penetrating the grain boundaries of the NbAI3 phase. Thus the effective area of contact between the sigma and NbAI3 depends on the grain boundary area of the NbAI3. We believe that this effective grain boundary area will be proportional to the thickness of the NbAI3. The same four phases are observed for a 25 at.% Al film annealed isothermally for 4 hours at 750 0C. We found the same sequence of phases for both 25 and 33 at.% Al films, whether annealing was carried out isothermally or at a constant heating rate. Thus Stage 2, with all four phase present, is a universal behavior for the system. Although the phase formation sequence for films of 25 and 33 at. % Al is the same through Stage 2, the composition of the A15 shows a notable difference. Figure 2 shows the superconducting transition temperature T, versus the maximum-anneal-temperature Tm for films of 143 nm periodicity annealed into Stage 2. Since Tc is sensitively dependent on the composition of the superconducting Al 5 phase [5], Fig. 2 qualitatively represents the composition of the Al5 phase for the two