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Single Crystal Self-Assembly


OBJECTIVE: Produce and characterize uniform barium titanate seed crystals suitable for templated grain growth. Develop and demonstrate a fabrication method to array the seeds in a matrix with orientational control of two crystal axes. Demonstrate the consolidation of the seeded matrix into a single crystal in the solid state. DESCRIPTION: Single crystals are conventionally produced by directional solidification from the melt. They are difficult to scale up and usually require extensive machining to form near net shaped components. Significant cost reductions could be achieved by combining powder metallurgy with single crystal growth in the solid state. Advances in templated grain growth suggest the possibility for production of true single crystals in the solid state. A key need to make this a reality is the production of uniform, faceted seed crystals that can be oriented (with 2 crystal axes) in a powder matrix and consolidated via sintering followed by secondary grain growth to form single crystals. Barium titanate (BaTiO3) is proposed as a model material because of its desirable ferroelectric and photorefractive properties. Processes with potential for chemical synthesis of seed crystals include hydrothermal, sol-gel, or other methods that allow control of the particle surface. Success in this endeavor may be transitioned to other single crystal materials with additional functional (i.e., electronic, magnetic, acoustic or optical) properties. The BaTiO3 seed crystals should be uniform in size and shape, with a nominal diameter of 50 microns and a narrow size distribution. The growth of uniform size and shaped seed crystals will be facilitated by development of synthesis methods with fine control of nucleation and growth processes. Precipitating crystals in polymer gels for example, has been used by Henish to suppress nucleation rates (7). Reactor design may also be important in achieving the topic goals. Plug flow reactors for example provide a uniform time temperature profile for the product. Fluidic Self Assembly technology (8) of chips provides an example of how seed crystals might be placed in ordered arrays. Processes that use surface tension to orient the crystal may also be feasible. Pick and place methods would be challenging at small size scales. Successful development of processes to grow single crystals via self-assembly would enable growth of shaped crystals not easily made by conventional melt processes. PHASE I: Develop a reliable and reproducible process to produce uniform sized faceted barium titanate single crystals, with a narrow size distribution centered around 50 micron diameter. Characterize the crystal facet and polar orientation of the seed crystals. Design a method to capture and array the seed crystals for characterization and for oriented placement in further fabrication steps. PHASE II: Scale-up the process for growing seed crystals and demonstrate the reproducibility of the process by producing 3 identical batches of seed crystals. Determine a suitable process to place the seed crystals in ordered and crystalographically oriented arrays within powder preforms. They may be surrounded by structured or unstructured BaTiO3 material. Required Phase II deliverables will include: (1) adequate seed crystals for fabrication of a large (millimeter-scale) single crystal; (2) a reproducible method to place and orient seed crystals, and (3) a model for consolidation of the array into a dense solid. PHASE III: Commercial and military/DoD applications include lead free sonar transducers and electro-optical modulators. Barium titanate single crystals have applications as lead free ultrasonic actuators useful in medical imaging. The manufacturing technology can be extended to single crystal turbine blades, crystal textured magnets and semiconductor suitable for gamma ray scintillators. REFERENCES: 1) PK Gallagher. Chemical Synthesis, in Engineered Materials Handbook Volume 4: Ceramics and Glasses, ASM International, 1991, pp. 52-64. 2) A Jana, S. Ram, and TK Kundu. BaTiO3 nanoparticles of orthorhombic structure following a polymer precursor. Phil. Mag., 2007, 87 (35), pp 5485-5504. 3) T Sato and T Kimura. Preparation of 111-textured BaTiO3 ceramics by templated grain growth method using novel template particles. Ceramics International, 2008, 34(4) pp. 757-760. 4) Y Chen, B Yu, J Wang, R Cochran and J Shyue. Template-based fabrication of SrTiO3 and BaTiO3 nanotubes. Inorg. Chem., 2009, 48 (2), pp 681686. 5) DB Hovis and KT Faber. Textured microstructures in barium hexaferrite by magnetic field assisted gelcasting and templated grain growth. Scripta Met., 2000, 44, pp. 2525-2529. 6) I Soten, H Miguez, SM Yang, S Petrov, N Coombs, N Tetreault, N Matsuura, HE Ruda, and GA Ozin. Barium titanate inverted opals3/4Synthesis, characterization, and optical properties. Advanced Functional Materials, 2002, 12 (1), pp. 71-77. 7) H. K. Henish, Crystal Growth in Gels, Pennsylvania University Press, Pennsylvania (1970). 8) Microfluidic self-assembly; see US Patents 5824186 and 6281038. 9) GM Whitesides and B Grzybowski. Self-assembly at all scales. Science, 2002, 295, pp. 2418-2421. 10) M Bunzendahl, P Lee-Van Schaick, JFT Conroy, CE Daith and PM Norris, Convective self-assembly of Stoeber sphere arrays. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001, 182, pp 275-283.
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