Nb/Al and Nb/Al(Cu) multilayer thin films: the enthalpy of formation of NbAl3

Using differential scanning calorimetry (DSC), the heat of formation of NbA l3 was measured from the reaction of sputter-deposited Nb/Al and Nb/Al(Cu) multilayer thin-films with different bilayer thickness. The measured heat o f reaction of the Nb/Al samples was 40.0+/-41 kJ/g-atom. The Nb/Al(Cu) film s, with 0.5 and 1.0 wt.% Cu, yielded heats of formation of 40.8+/-0.5 and 4 0.5+/-0.6 kJ/g-atom, respectively. The measured heats of reaction in all th ree sets of samples were consistent with earlier investigations and reveale d that, within experimental errors, the addition of the small amounts of Cu did not affect the formation enthalpy of NbAl3. Thus, combining the data f rom all three types of samples gives a value of 40.6+/-2.9 kJ/g atom for th is enthalpy. Enthalpy measurements as a function of multilayer periodicity further indicated that no significant interdiffusion or intermixing took pl ace prior to NbAl3 formation. This lack of intermixing was also confirmed b y fabricating a multilayer sample with a 10 nm thick codeposited layer, hav ing a nominal composition of Nb42Al58. interleaved between the Nb and Al la yers. The presence of this intentionally intermixed layer caused a reductio n in reaction enthalpy to 33.1+/-0.5 kJ/g-atom. (C) 2000 Elsevier Science B .V. All rights reserved.