Supersonic Cluster Beam Synthesis of Nanophase Materials
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ABSTRACT We present the characterization of supersonic cluster beam deposition as a viable technique for the synthesis of nanostructured materials. Stable and intense cluster beams can be obtained with a pulsed microplasma cluster source. This technique has been applied to produce TiNi nanostructured thin films on various substrates at room temperature. The morphology and the structure of the film are strongly influenced by the precursor clusters. Films characterized by crystallite sizes of a few tens of nanometers can be grown without recrystallization by thermal annealing. The stoichiometry of the original TiNi alloy is maintained. INTRODUCTION Recently low energy supersonic cluster beam deposition (LESCBD) has gained a considerable attention as a viable technique for the synthesis of nanostructured thin films [1]. The properties of clusters as building blocks can be adjusted by changing their size prior to deposition; if the clusters retain their individuality once deposited, it is possible to synthesize a material with tunable physico-chemical properties. An essential requisite for the use of LESCBD, in alternative to more traditional synthesis routes, is the capability of producing beams with high intensity, stability and tunability in terms of cluster mass distribution. Here we present a source of pulsed supersonic cluster beams able to provide long-time stability, high deposition rates and control on cluster mass distribution suitable for an efficient production of refractory material nanostructured films. We have used LESCBD to deposit TiNi nanostructured thin films. The synthesis of thin films of shape memory alloys, with well-controlled and reproducible properties, is a key factor for the development and fabrication of micromechanical devices [2-4]. TiNi alloys are known to exhibit shape memory properties and are considered promising candidates for a variety of applications such as microactuators and micromachines [5-7]. The understanding and control of martensitic transformations in thin films is more difficult than in bulk samples mainly because of the role played by the film-substrate interaction. Together with film-interface effects other parameters as degree of crystallinity, the stoichiometry and the presence of contaminants affect the martensitic transformation [2, 7-9]. The most widely adopted technique for the deposition of TiNi thin films is sputtering under controlled atmosphere [2-4, 10]. This method allows a good control of stoichiometry but produces amorphous films which must be annealed (temperature range 700-900 K) during or after deposition to induce the crystallization necessary to obtain the shape memory effect [2, 3]. Compared to sputtering, LESCBD allows the deposition on films characterized by a nanocrystalline structure with a substrate at room temperature. Cluster mass distribution and kinetic energies influence nanocrystalline film growth. The refinement of grain size can strongly modify the structural and thermodynamic properties of shape memory alloys [11].
283 Mat. Res. Soc. S
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