Kinetics of the solid-state reaction for the formation of amorphous ZrCo studied by electrical conductance measurements
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I. INTRODUCTION During the last years it was shown that many combinations of early and late transition metals form an amorphous alloy, when a multilayer configuration is annealed at moderate temperatures.' A characteristic of all these combinations is a large negative heat of mixing, which acts as a driving force for the reaction, and an asymmetry of the mobility of the reactants. The amorphous phase forms as a planar interlayer between the elemental layers2 showing a diffusion controlled growth for longer reaction times. 34 For short reaction times, deviations from a x// law were already noticed.s'6 It is the aim of this work to study the early stages of the solidstate reaction in a more systematic way. This will be done by the measurement of the electrical conductance during the solid-state reaction. This proves to be a sensitive indicator for the kinetics of the reaction.
II. EXPERIMENTAL DETAILS Bilayers of Zr and Co were condensed onto glass substrates by electron-beam evaporation in an UHV system (base pressure 10 7 Pa). The use of two mechanical shutters excluded any possibility of a simultaneous deposition of both metals. Two quartz balances were used to determine the thicknesses of the single layers (standard thickness: Zr 33.5 nra, Co 17.7 nm). The substrate was fixed on a heated copper block. The temperature was measured close to the substrate on the copper block by means of a thermocouple. The temperature was regulated at the reaction temperature of TR = 473, 496, and 523 K, respectively, with an accuracy of + 1 K. A standard four-probe technique was used to measure the electrical conductance of the layers during the deposition and the subsequent annealing. It is advantageous to deposit the layers on the substrate regulated at the reaction temperature because no changes of the conductance due to a temperature change have to be considJ. Mater. Res. 3 (3), May/Jun 1988
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ered and no time lack due to the heating of the substrate up to the reaction temperature occurs. To make certain that an amorphous phase forms with time when the layers are condensed on a substrate at the reaction temperature, 12 alternating layers of Co and Zr were deposited onto a silicon wafer held at 523 K. This sample was analyzed by cross-sectional transmission electron microscopy (CS-TEM) using a Philips 420 ST electron microscope. The preparation of the TEM specimen has already been described6 and is similar to the method of Garulli et al?
III. RESULTS Figure 1 shows a CS-TEM picture of alternating layers of Zr and Co that were condensed onto a substrate at 523 K and then annealed for 24 h. The layers of Co and Zr show a contrast that is typically for polycrystalline layers emerging from the various Bragg conditions of the single grains. At every Zr/Co interface an amorphous phase has formed. This is in agreement with layers that were condensed at 77 K and then annealed at 523 K.6 Independent of the specimen orientation in the TEM, the amorphous phase appears structureless, showing a broad ring in t
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