Characteristics of Alumina Diffusion Barrier Films on Hastelloy
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The diffusion behavior of elements constituting Hastelloy C-276 (C, Si, Mn, Co, W, Fe, Cr, Mo, and Ni) in alumina films was investigated using secondary ion mass spectroscopy. The films were deposited by ion-beam-assisted deposition and annealed in vacuum over a temperature range of 500–1000 °C. Characterization of film microstructure was performed using transmission electron microscopy and selected area diffraction analyses. The films were predominantly amorphous with alumina nanocrystallites nonuniformly dispersed throughout the volume both before and after annealing. A relatively wide interface region between the Hastelloy substrate and alumina film was formed in the as-deposited sample due to ion beam mixing. No diffusion of any of the substrate elements was observed after annealing, except for Mn, Cr, and Ni. The impurity depth distributions consisted of two components, which differed by several orders of magnitude with respect to diffusion coefficient and solubility. Activation energies and temperature dependencies of the diffusion coefficients were determined, and a diffusion mechanism was discussed.
I. INTRODUCTION
Development of biaxially textured magnesia (bt-MgO) template layers suitable for subsequent deposition of high-temperature superconducting (HTS) Y1Ba2Cu3O7−␦ (YBCO) films offers a viable production route for high critical current, long-length, coated conductors.1,2 The advantage of bt-MgO over biaxially textured yttriastabilized zirconia (bt-YSZ) is that only approximately 10 nm of the former is required compared to 500–700 nm for the latter material. A large reduction in template film thickness is important to HTS coated conductor production because it can significantly reduce the processing time and related costs for producing this layer. However, a YBCO film epitaxially deposited on a multilayer film stack of strontium ruthenate/magnesia/yttria (SRO/btMgO/Y2O3) deposited on a Hastelloy C-276 substrate was not superconducting at a temperature of 75 K.3 (The Y2O3 layer served as a nucleation layer for bt-MgO and the SRO layer served as a buffer layer for a better lattice match to YBCO.) Secondary ion mass spectroscopy (SIMS) studies revealed that strong intermixing between the YBCO film and Hastelloy substrate took place. It is well known that deviations in stoichiometry of YBCO4 and/or the presence of even small quantities of impurities5 may have deleterious effects on its superconducting properties. During our standard YBCO pulsed-laser deposition (PLD) conditions, the substrate is held at a DOI: 10.1557/JMR.2004.0152 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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temperature of 750–800 °C. The thick bt-YSZ is thought to serve as a barrier against diffusion of substrate elements into YBCO films and allows fabricating films with excellent superconducting properties.6 However, the much thinner bt-MgO layers do not have such diffusion barrier capability. Thus, an additional layer of material that serves as a diffusion barrier is necessary. It w
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