Fluid Mechanics of Micro Cold Spray Direct Write Process
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Cold spray, also known as the gas dynamic spray process, was first discovered in the 1980s while doing high speed two phase wind tunnel experiments. The principle underlying this process is that if a metal particle is accelerated to a velocity above a certain critical velocity, upon impact on a substrate the particle and substrate will undergo rapid plastic deformation and form a “splat”. This process is currently being used for coatings applications. In this process, metal particles of diameter 5 μm to 50 μm are accelerated to a very high velocity (>500 m/s) and are deposited on substrates. Based on principles similar to cold spray process, we have developed a novel direct write process known as the Micro Cold Spray Direct Write (MCS-DW) process. Initial results from our experimental study have shown that conductive patterns of copper, tin and aluminum can be printed on flexible and rigid substrates using this process. The smallest feature size that can be printed using this process is 50 μm. In order to improve the deposition efficiency of the MCS-DW process, numerical studies were carried out to simulate the flow of aerosol particles through different nozzle geometries. It was found that a convergent capillary nozzle with a linear converging section of length 19 mm and a straight capillary of length 14 mm can be used to accelerate and focus silver particles of diameter 2 μm. Copper particles of diameter 3 μm can accelerate to their critical velocity by using a longer straight section of length 30 mm.