Two- and Three-dimensional Arrays of Magnetic Microspheres
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Two- and three-dimensional arrays of magnetic microspheres Weijia Wen Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095-1595
Ning Wang Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong
D. W. Zheng, C. Chen, and K. N. Tu Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095-1595 (Received 14 September 1998; accepted 22 December 1998)
A novel fabrication approach for two- and three-dimensional arrays of magnetic microspheres is presented in this paper. The magnetic microsphere is made from 47 mm size Al2 O3 spheres onto which a 2–3 mm thick nickel layer is coated through electroless plating. After proper anneal, the outer nickel layer is converted to exhibit a crystalline structure. As an example for utilizing such magnetic microspheres, a two-dimensional, anisotropically conductive matrix is made by transferring the magnetic microsphere array from a template to a transparent adhesive tape using a magnetic attractive force. In addition, a three-dimensional array has also successfully been constructed on a metal plate. The two-dimensional conductor array may be useful for high-density circuit packaging applications in the semiconductor industry, and the three-dimensional array may open up a possibility for constructing three-dimensional photonic crystals. Although few papers report to date about major industrial applications of magnetic microspheres, its application is well considered in various fields such as visual communication device,1 ferrofluid,2 enhanced solder alloy,3 nonlinear phenomena,4 low temperature physics,5 and macroscopic simulation of magnetic materials and theoretical modeling of colloid.6 The magnetic materials used in previous experiments are often of irregular shape, even though the theoretical modeling is based on spherical particles. The reason for this is due to the difficulty in fabrication of magnetic microsphere in such a small size. In this paper, a simple method for fabricating magnetic microspheres in the size range of 5 to 100 mm in diameter is introduced. In addition, the magnetization of the sphere can be easily controlled by adjusting the nickel thickness of the coating in the composite sphere. As an example, we have fabricated magnetic microspheres utilizing Al2 O3 spheres on which a 2–3 mm thick nickel layer is coated by electroless plating. Under proper annealing conditions, the coated nickel layer is made to have a crystalline structure. For potential applications of such magnetic microspheres, two- and three-dimensional microsphere arrays are built up on an adhesive tape and metal plate using a magnetic attractive force. This novel method, for example, the two-dimensional array structure, can be applied to fabricate an anisotropically conductive adhesive tape that may be useful for high-density electronic packaging devices. The three-dimensional matrix presented here provides a route for constructing photonic crystals with un
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