|Acquisition Program: ||PEO Ground Combat Systems|| Objective: ||Develop and demonstrate a process for the growth of large aluminum oxynitride (AlON) transparent single crystals in the cubic crystal class, with high purity and perfection such as those suitable for optical or microelectronic applications. A process for the manufacture of large ALON single crystals will also play a vital role in enabling the validation of ab initio and molecular dynamics calculations of AlON anisotropic elastic constants.
|| Description: ||This effort would develop and demonstrate a process for the formation of large single crystals (50 mm diameter x 25 mm thick) of aluminum oxynitride, AlON, material which is a dense transparent armor ceramic being considered for windows in armored vehicles and semiconductor applications [1,2]. Current AlON material is formed from pressed, cast or molded powder which is then densified by heating in an oven. In this polycrystalline form, individual crystals are on the order of 200 microns in size, and are of cubic spinel structure. Polycrystalline aluminum oxynitride is commercially known under the name ALON(trademark) (see e.g. www.surmet.com). Large single crystals of gamma-AlON are needed that appear on the pseudo-binary AlN-Al203 (aluminum nitride-aluminum oxide) phase equilibrium diagram as described in .
|| ||PHASE I: The phase I objective will develop and demonstrate a process for growing large single crystals of AlON with stoichiometry (about 30-35% mole percent AlN) ideally centered at Al8(CN IV) Al15(CN VI) AluminumVacancy(AlVI)O27 N5 as described in [3,4,5]. Crystals grown in Phase I should be sufficiently large (> 2 mm3), optically transparent and isotropic, and nearly free from defects so that resonant ultrasound spectroscopic (RUS) methods  can be used to independently determine the ambient cubic elastic constants, (C11, C12, C44), for the material. Deliverables for Phase I will include:
1) Develop and demonstrate a process for growing single crystals of AlON (> 2 cubic millimeters)
2) Crystals should have real in-line visible light transmission of 80% and the degree of optical transparency using a UV-VIS-IR transmission curve with > 70 percent transmittance.
3) Demonstrate that the single crystals are optically isotropic.
4) Measure cubic elastic constants (C11, C12, C44) for the single crystal material using RUS or other method.
5) Write a final report describing the Phase I crystal growth process specifications, which incorporates all information in 1) – 4) above.
6) Deliver five (5) single crystals (> 2 cubic millimeters) to ARL.
|| ||PHASE II: Phase II will demonstrate that the process can be scaled up to grow larger single crystals of AlON (50 millimeter diameter x 25 millimeter thick).
Deliverables for Phase II will include:
1) Develop a process capable of scaling up the Phase I effort to grow AlON single crystals (50 mm diameter x 25 mm thick).
2) Crystals should have real in-line visible light transmission of 85% and the degree of optical transparency using a UV-VIS-IR transmission curve with > 70 percent transmittance.
3) Demonstrate that the Phase II single crystals are optically isotropic.
4) Measure and compare the cubic elastic properties of crystals grown using Phase I (>2 cubic millimeter) using RUS techniques with Phase II, (50 mm diameter x 25 mm thick) using pulse echo or other techniques.
5) Measure and compare the dielectric properties of crystals grown using Phase I (>2 mm3) with those in Phase II, (50 mm diameter x 25 mm thick).
6) Write a final report describing the Phase II crystal growth process specifications, which incorporates all information in 1) – 5) above.
7) Develop a viable technology transition plan.
|| ||PHASE III: The technology developed under this program if successful can be used on a number of Army and commercial applications that require large optical windows, domes, and plates, substrates for microelectronic applications, and the solution will be applicable/usable by any and all aircraft in the Department of Defense inventory.
|| References: ||
1. E.K. Graham, W.C. Munly, J.W. McCauley, and N.D. Corbin, “Elastic properties of polycrystalline aluminum oxynitride spinel and their dependence on pressure, temperature, and composition,” J. American Ceramic Soc., V. 71, No. 10, 897- 812, 1998.
2. N.D. Corbin, “Aluminum oxynitride spinel: A review,” J. Eur. Ceram. Soc., V. 5, 143-154, 1989.
3. J.W. McCauley, “Structure and properties of AlN and AlON ceramics,” Elsevier’s Encyclopedia of Materials: Science and Technology, Elsevier Science, Ltd. 127-132, 2001.
4. C.M. Fang, R. Metselaar, H. T. Hintzen, and G. de With, “Structure models for gamma-aluminum oxynitride from ab initio calculations,” J. Am. Ceram. Soc., V. 84, No. 11, 2633–2637, 2001.
5. I.G. Batyrev, B.M. Rice, and J.W. McCauley, “First principles calculations of nitrogen atomic position effects on elastic properties of aluminum oxynitride (AlON) Spinel, MRS Fall Meeting, Boston, MA, 30 Nov – 3 Dec, 2009.
|Keywords: ||Cubic crystal class, aluminum oxynitride, resonant ultrasound spectroscopy, AlON, transparent armor ceramic, large single crystals.|