Triplicane A. Parthasarathy

Development of ceramic fibers for high-energy laser applications

Polycrystalline ceramics offer a number of advantages relative to single crystal materials such as lower processing temperatures, improved mechanical properties, and higher doping levels with more uniform distribution of dopants for improved laser performance. Ceramic YAG (Y3Al5O12) and rare earth sesquioxide (RE2O3) fibers promise to enable a number of high power laser devices via high thermal conductivity and higher allowable dopant concentration; however, these materials are not currently available as fine diameter optical-quality fibers. Powder processing approaches for laser quality polycrystalline ceramic fibers are in development at AFRL. Current processing techniques will be reviewed. The effects of a number of processing variables on the resulting fibers as well as preliminary optical characterization will also be presented.

Determination of Fiber/Matrix Interface Mechanical Properties in Brittle-Matrix Composites

It has been evident for some time that the mechanical properties of the fiber/matrix interface play an important role in determining the mechanical behavior of ceramic composites (for reviews, see [1], [2], and [3[). Recently there has been a growing interest in the role of the fiber/matrix interface in intermetallic matrix composites. While ceramic and intermetallic composites are certainly very different materials, understanding the behavior of one will provide insight into the other. Furthermore, the basic issues regarding the determination of interface properties are the same. The accuracy of micromechanics models of any composite system is dependent upon the accuracy of all the constituent and interface properties. It is far preferable to measure actual materials constants rather than test-specific quantities. The tests described here are intended to measure the interfacial shear strength (or mode II toughness) and the interfacial tensile strength. The objective of this work is to briefly outline a few of the approaches which are being evaluated for and applied to ceramic composites, and which may be of interest to investigators working in intermetallic composites.

Towards optical quality yttrium aluminum garnet (YAG) fibers: recent efforts at AFRL/RX

Traditional silica fibers currently are unlikely to be able to sustain the powers needed for future Air Force applications. The low thermal conductivity of silica makes it difficult to control thermal gradients within the fibers resulting in failure or degradation in beam quality. While some of these problems can be ameliorated by using longer fibers, this results in problems with nonlinear effects such as stimulated Raman and Brillouin scattering (SRS and SBS). Yttrium aluminum garnet (Y3Al5O12, YAG) has the potential for overcoming these problems due to 1) higher thermal conductivity, 2) reduced thermal lensing, and 3) higher SBS threshold. Polycrystalline YAG has been demonstrated to be a highly efficient and economical laser host material in slab form. Polycrystalline YAG can be doped more uniformly and at higher levels than single-crystals with no dopant loss by zone refinement, has higher fracture toughness than single-crystals, and supports higher power densities. Despite the anticipated advantages, polycrystalline YAG has never been demonstrated in high-power fiber lasers. The development and characterization of YAG fibers for high energy laser applications is the primary goal of our research. Recent results in the processing of optical quality polycrystalline YAG fibers will be presented and discussed.

Fiber Coating Design Parameters for Ceramic Composites as Implied by Considerations of Debond Crack Roughness

The key technological challenge to the use of ceramic composites in high temperature structural applications is oxidation resistant control of fiber/matrix interface properties. A number of approaches based on oxide coatings are being investigated (for a brief review and current references, see Refs. 1 and 2, respectively) and, while the principle role of these coatings is to promote debonding, the actual material and geometric requirements they must satisfy have been poorly understood. As the level of understanding of composite behavior has been increasing, so has appreciation for the potential complexities of fiber coating design. This has led to recognition of several potential design parameters; characteristics that must be controlled in the design of the coating or accommodated in the design of the overall composite. A good deal of our awareness of these coating design parameters stems from the study of the role and importance of surface topography.

Lasing of surface-polished polycrystalline Ho: YAG (yttrium aluminum garnet) fiber

A polycrystalline 1.5% Ho: YAG fiber with a diameter of 31 µm was prepared. Surface roughness from grain boundary grooving was reduced by polishing, which decreased the fiber scattering coefficient from 76 m-1 to 35 m-1. Lasing tests were done on this fiber with a SF57 Schott glass cladding. Lasing was confirmed by spectrum narrowing with threshold pump power lower than 500 mW and a slope efficiency of 7%. To our knowledge, this is the first lasing demonstration from a small diameter polycrystalline ceramic fiber.

5.6 Mechanical Behavior of Non-Oxide Fiber-Reinforced CMCs at Elevated Temperature: Environmental Effects

Non-oxide ceramic composites will be subject to elevated temperatures in oxidizing conditions while at appreciable stresses in future engine applications. It is imperative to understand the nature of stressed-oxidative interactions for these types of composites in order to design composite constituent content and architecture as well as to attempt to model life-properties. Various stressed-oxidation mechanisms are summarized followed by models relating degradation of composites subject to different stress/temperature/environmental conditions for several different composite types. Finally, some approaches to improve stressed-oxidative life are discussed.

Recent developments in polycrystalline oxide fiber laser materials: production of Yb-doped polycrystalline YAG fiber

Laser quality, polycrystalline oxide fibers offer significant advantages over state-of-the-art silica fiber for high energy lasers. Advanced ceramic processing technology, along with a novel powder production process, has potential to produce oxide fibers with an outstanding optical quality for use in the fiber laser applications. The production of contaminant-free green fibers with a high packing density, as well as uniform packing distribution, is a key factor in obtaining laserquality fibers. High quality green fibers are dependent on the powder quality combined with the appropriate slurry formulation. These two fundamental technologies were successfully developed at UES, and used to produce Yb-doped yttrium aluminum garnet (YAG) fibers with high optical quality, high chemical purity, and suitable core diameters down to 20-30 microns.