engleski

Thermal stability of amorphous Al-W alloys

Binary Al-W amorphous alloys seem to be a promising materials according to the specific properties of respective constituents(1,2). However, their thermal stability is essential for any application at the sustained elevated temperatures. In this work the crystallization temperatures of amorphous Al-W alloys are reported. The Al-W thin films were prepared by simultaneous d.c sputtering of both pure Al and pure W in a multiple source sputtering apparatus(3,4). Tha working gas was argon at 0.7 Pa in a continuous flow mode. The area-averaged cathode current densities were 3-7 mA/cmý for tungsten and 7-13 mA/cmý for aluminum, allowing for a wide variation of film chemical composition. The film deposition rate was 0.1-0.3 nm/s, while the final film thickness was about few ćm. In order to assure a lateral compositional homogeneity of the deposited films, the substrate holder rotated during the deposition. Several sets of samples were deposited onto circular (1 cm dia) glass, fused silica, monocrystalline silicone and alumina ceramic substrates. The prepared films with composition in the range from Al80W20 to Al62W38 were completely amorphous(5). Their electric resistivity at the room temperature was in the 150-200 ćęcm range, while the corresponding temperature coefficient of resistance (TCR) negative values increased with the aluminum content up to -5.5ú10-4 K-1. Thermal stability of completely amorphous films was investigated by the continuous in situ electric resistance measurements in a vacuum better than 10-? mbar. A standard a.c. technique has been employed. The samples were radiatively heated up to 750řC at a ramp rate of 1, 2.2 and 4 K/min, respectively. The initial and final structure of the film submitted to the up/down temperature cycle was determined by the XRD method/ analysis. An example of the film electrical resistance variation with the temperature in a continuous up/down cycle is given in Fig.1. As seen, below about 150řC no noticeable change with respect to the sub-zero temperature behaviour occurred. The increase in the electrical resistance between 150řC and 540řC is presumably due to some still unresolved structural relaxation/ change in the film. The TCR value in that temperature range retains its negative value upon cooling. Tha phase transformation started at 540řC and resulted in a crystalline phase with a positive TCR. In Fig.2 the XRD patterns of the as-deposited and crystallized film are shown. Three intermetallic compounds (Al12W, Al5W, Al4W) and pure alloy constituents (Al, ŕ-W, á-W) are considered as possible end-products of the phase transformation. As seen, a dominant component after crystallization is the Al4W intermetallic compound. The crystallization temperature only slightly varies with the composition in the Al-W amorphicity range examined here, generally increasing with the increase of the tungsten content. A dominant crystallization product is always Al4W. However, on substrates containing silicone (mono-Si, fused silica) the reaction between Si and W from the film occured at temperatures slightly lower than crystallization temperature, thus yielding WSi2 and pure Al as the end-products. The observed crystallization temperatures of the Al-W amorphous films indicate that the transformation process is probably diffusion controlled/ limited(6). Therefore, to establish application suitability of the examined amorphous alloys up to here determined crystallization temperatures, the isothermal annealing studies are further required.

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