Sa-21. F. Schmid, C. P. Khattak, H. H. Rogers, D. M. Felt, J.
Askinazi and R.V. Wientzen, "Current status of very large
sapphire crystal growth for optical applications," SPIE's
13th Annual Int'l Symposium on Aerospace/Defense Sensing, Simulation,
and Controls, Orlando, FL, April 1-5, 1999.
Abstract
Sapphire is an ideal visible-MWIR window material due to its excellent optical and mechanical properties and its availability in large sizes up to 340-mm diameter boules. Anticipated applications for new, high performance optical systems call for even larger, 450 - 750-mm diameter, windows. The present effort has focused on producing 500-mm diameter sapphire boules using the Heat Exchanger Method (HEM). Three experimental growth runs demonstrated the feasibility of producing 500-mm diameter sapphire boules. Completely crack-free boules have not been grown, but large size sapphire pieces up to 400 mm x 280 mm have yielded from these experimental runs.
Sa-20. K. Schmid, F. Schmid and C. P. Khattak, and D. C. Harris, "High Temperature Compression and Ring-on-Ring Testing of Sapphire," SPIE's 13th Annual Int'l Symposium on Aerospace/Defense Sensing, Simulation, and Controls, Orlando, FL, April 1-5, 1999.
Abstract
Sapphire's strength at elevated temperature is highly dependent on the test conditions. Tests that involve compressive forces at localized contact points can cause failure at low strengths due to rhombohedral twinning. High contact stress can result at the load points due to roughness of surfaces. A thin sheet of Grafoil serves as a compliant layer between the load surface and the specimen and reduces the contact stress, and this increased the compression strength by a factor of 4 and the biaxial flexure of c oriented specimens by 2X at 600C. The strength reported by different testing facilities was comparable when Grafoil was used. The use of Grafoil has made it possible to evaluate the effect of process parameters on the compressive and biaxial flexure strength at 600C.
Sa-19. F. Schmid, C. P. Khattak, and K. Schmid, and D. C. Harris, "Increasing the Strength of Sapphire by Heat Treatments," SPIE's 13th Annual Int'l Symposium on Aerospace/Defense Sensing, Simulation, and Controls, Orlando, FL, April 5-7, 1999.
Abstract
Heat treatments of finished sapphire compression and biaxial flexure specimens increased sapphire's high temperature strength. Heat treatments of sapphire specimens at 1450C for 48 hours in an air atmosphere enriched with oxygen increased the compression strength by 60% and biaxial flexure strength at 600C by 45% over untreated samples.
Sa-18. M. Smith, K. Schmid, F. Schmid and C. P. Khattak, and J. Lambropoulos, "Correlation of Crystallographic Orientation with Processing of Sapphire Optics," SPIE's 13th Annual Int'l Symposium on Aerospace/Defense Sensing, Simulation, and Controls, Orlando, FL, April 5-7 1999.
Abstract
Sapphire is an ideal optical material and is being used for window and dome applications. The anisotropic properties of sapphire affect the production of high-quality components. Out of the three major orientations, c-axis or [0001], a-axis or , m-axis or , the c-[0001] axis is preferred for optical applications as it is the zero birefringence orientation. This orientation is difficult to grow with high quality. Therefore, components are fabricated by sectioning from the sides of a- or m- boules. The anisotropic properties also present problems in grinding and polishing windows for precision optical applications. The degree of difficulty varies with the orientation selected. For hemispherical domes involving polishing of several orientations, it is difficult to achieve a good figure. The choice for larger diameter windows is limited to a- or m- orientation; the m- orientation may be preferable due to the geometry of fabrication-induced stress.
Sa-17. F. Schmid, H. H. Rogers, C. P. Khattak and D. M. Felt, "Growth of Very Large Sapphire Crystals for Optical Applications," SPIE Proc. #3424 05, San Diego, CA, July 1998.
Abstract
Recent interest in monitoring systems requires very large optical windows that are transmitting over a wide spectral range. Some of the other requirements involve durability, high strength and robustness to withstand severe environments. Therefore, sapphire has been required for these applications. The Heat Exchanger Method (HEM)? has been used to produce very large sapphire crystals primarily for optical applications. Crystals of 20 cm and 25 cm diameter have been produced in production for over 20 years. Presently, 34 cm diameter boules have been adopted in production, and 50 cm diameter sapphire growth is currently in development. Results of progress and characterization data of the boules will be presented.
Sa-16. M.B. Smith, K. Schmid, F. Schmid, C.P. Khattak and J. C. Lambropoulos, "Controlling stress in sapphire optics," Proc. Optical Manufacturing and Testing II, (H. P. Stahl, Chair/Editor) SPIE Vol. 3134 pp. 284-292 (1997).
Abstract
Precision optical fabrication is often influenced by surface stress introduced during processing. Various steps, such as lapping, grinding, polishing and coating, can influence optical figure and transmitted wave front in sapphire optics. The Twyman effect was used as a tool to measure the variation in stress from different processes and to investigate annealing treatments. Compressive stresses were generated by all fabrication techniques; however, the magnitude of stress varied considerably. The highest stress was generated during the transition from the brittle to ductile mode of removal; the lowest stress was observed during polishing with colloidal silica. Heat treatments were successful in removing machining stress from the parts. After heat treatment at 1450°C, the remaining grinding-induced stress levels were too small to measure accurately.
Sa-15. F. Schmid, C. P. Khattak, M. B. Smith and D. M. Felt, "Current Status of Sapphire for Optics," Proc. #CR67-09, SPIE, (in press) (1997).
Abstract
Sapphire has been used for many optical applications. However, smaller sizes have been used, even though the Heat Exchanger Method (HEM) has produced 20 cm diameter crystals. New generation systems require outstanding optical properties, high strength, and abrasion and thermal shock resistance. Therefore, the choice is limited to sapphire. Crystals up to 34 cm diameter, 65 kg have been grown by HEM, and it is planned to scale up the size to 50 cm diameter. In addition to larger size, the optical quality has been improved to cover the vacuum ultraviolet (VUV) and the near infrared wavelengths. Fabrication technology was advanced to fabricate larger size, higher precision optics cost effectively. Improved transmitted wavefronts and higher quality surfaces have been produced to address current applications.
Sa-14. C.P. Khattak, F. Schmid, M.B. Smith, "Correlation of Sapphire Quality with Uniformity and Optical Properties," Proc. Window and Dome Technologies and Materials V, (R.Tustison, Chair/Editor), SPIE Vol. 3060 pp. 250-257 (1997).
Abstract
Large sapphire boules, up to 34cm diameter, 65kg, are being grown by the Heat Exchanger Method (HEM) and even larger sizes are sought to meet future requirements of advanced optical systems. These boules, especially in large sizes, exhibit lattice distortion and light scatter in a very narrow range. A qualitative grading system has been developed to characterize sapphire. Windows of five grades and different orientations were prepared and measured for refractive index homogeneity to evaluate transmitted wavefront distortion. The data showed that the refractive index homogeneity for all samples was in the 10-7 (0.1 ppm) range. The fact that lattice distortion does not affect the transmitted wavefront allows fabrication of large sapphire windows in production mode at low cost.
Sa-13. F. Schmid, C.P. Khattak and D. M. Felt, "Sapphire Sparkles in Many Optical Elements," Laser Focus World, 32 (6) pp. 167-174 (1996).
Abstract
Over the last 25 years, advances in laser, optical, and electro-optical systems have placed stringent demands on optical materials. Progress in detectors and signal processing have made midwave infrared (IR) systems attractive for scientific, industrial, and military applications resulting in a need for advanced optical materials. When stronger, tougher, harder, low-scatter material with a multispectral range, high thermal conductivity, or higher temperature capability is required for advanced applications, sapphire is becoming the preferred choice.
Sa-12. M. Smith, F. Schmid, C.P. Khattak and Keil Schmid, "Sapphire Fabrication for Precision Optics," Center for Optics Manufacturing University of Rochester (1996).
Abstract
Large sapphire components are required to meet the challenging needs of commercial and military optical applications. However, the desirable properties of sapphire also make it difficult to grind and polish. Fabrication costs can represent 50% of the price of large sapphire components. Cutting and grinding studies on sapphire were carried out with four types of diamond tools to correlate tool characteristics and process parameters with the grinding mechanism. The Twyman effect was also investigated to relate to crystallographic properties of sapphire with fabrication concerns. If grinding and microgrinding techniques can be optimized, costs associated with fabrication of sapphire and other optical materials will ultimately be reduced.
Sa-11. F. Schmid, M.B. Smith, C.P. Khattak, "Current Status of Sapphire Dome Production," Proc. Window and Dome Technologies and Materials IV, SPIE 2286, (14) (1994).
Abstract
Sapphire is an ideal choice for the transparent dome element in higher speed missile systems because of its high transmission, high strength and thermal shock resistance. Sapphire domes for various missile systems are being produced, but fabrication technology development is required for cost reduction and improved performance. Major costs of dome production are in fabrication. It has been shown that grinding to near-finished size is important to reduce fabrication costs and improve quality of finished domes. With this approach, optical distortion problems related to the anisotropic properties and subsurface damage can be minimized. Subsurface damage has been shown to reduce sapphire's strength. The depth of subsurface damage has been quantified, and it has been shown that room temperature strength can be increased with a post-polish heat treatment.
Sa-10. F. Schmid, C. P. Khattak, D. Mark Felt, "Producing Large Sapphire for Optical Applications," American Ceramic Society Bulletin, 73 (2) (1994).
Abstract
Sapphire has a unique combination of desirable mechanical and optical properties. The Heat Exchanger Method has been developed to produce large-sized, high-quality sapphire for industrial applications. Fabrication technology has been developed to produce large optical components with super flat and smooth surfaces for IR and VUV optics.
Sa-9. D.C. Harris, F. Schmid, J.J. Mecholsky, Jr. and Y.L. Tsai, "Mechanism of Mechanical Failure of Sapphire at High Temperature," Proc. Window and Dome Technologies and Materials IV SPIE 2286 (1994).
Abstract
The strength of sapphire decreases more rapidly with increasing temperature than does the strength of polycrystalline alumina and many other ceramics. Twinning on the rhombohedral plane (1102) at elevated temperature induced by compression along the crystallographic c-axis [0001] appears to initiate failure and accounts for the decreased strength. The tensile strength of sapphire along the a- [1120] or c-axes is constant to within approximately 30% between 20° and 800°C. Compressive strength along the a-axis is also constant to within approximately 20%. However, compressive strength along the c-axis falls by >95% (from 2000 MPa to less than 100 MPa) between 20° and 800°C.
Sa-8. C.P. Khattak and F. Schmid, "Production of Near-Net-Shaped Sapphire Domes Using the Heat Exchanger Method (HEM™)," Proc. Window and Dome Technologies and Materials III, SPIE 160 pp. 41-47 (1992)
Abstract
Transparent dome elements for future higher speed missile systems place stringent requirements on the mechanical properties of the dome material. Currently, no material meets all the requirements. Among the five candidate oxide materials, sapphire appears to be the best choice based upon material properties, thermal shock resistance and status of material production. A disadvantage of sapphire is that it must be produced in single crystal form because of its anisotropic structure. A new approach to produce near-net-shaped sapphire domes from the melt is discussed. Multiple near-net-shaped sapphire domes of various sizes and curvature have been produced.