An overview of a research program to screen material candidates for more durable windows that operate over a broad temperature and wavelength range is presented. The hardness, high melting points, and oxidation resistance of cubic oxides make them logical material choices to screen. Empirical and density functional theory modeling along with extensive datamining of literature and on-line databases are used to screen materials for window relevant properties. Promising materials are processed and characterized for optical transparency at room and high temperature by high-throughput methods to validate predicted properties. Potential window materials identified by these methods are presented and discussed.
Fluorite structure oxides include cubic-stabilized ZrO2, HfO2, ThO2, UO2, and some rare-earth compositions. Many rare-earth oxides also form cubic bixbyite (Ln2O3) structures. These materials are some of the most refractory oxides known. The optical and thermomechanical properties of fluorite structured oxides and rare-earth bixbyites are reviewed. Existing data on transmittance in the visible and infrared is summarized and compared with theoretical predictions from density functional theory and other physics-based models. Properties such as thermal conductivity, thermal expansion, melting point, modulus, hardness, refractive index, dielectric constant, thermochemical stability, and the trade-offs between these properties and optical properties are also discussed. New results for optical and thermomechanical properties for selected bixbyite and fluorite compositions will be presented, compared with existing data, and with model predictions.
Yttrium Aluminum Garnet (YAG) is a promising material for solid state fiber lasers. The cladding surrounding the YAG core must have a tightly controlled refractive index, high thermal conductivity, and be fully dense. Ca3Ga2Ge3O12 (CGG) is a potential cladding material that meets these criteria, and has a low melting-temperature, allowing for unique processing methods. This presentation will discuss application of a dense CGG cladding onto a YAG fiber core using a Laser Heated Pedestal Growth (LHPG) apparatus. In-situ melt/solidification of the CGG in the LHPG will be shown, as well as characterization of cladded fibers via SEM, EDS, and TEM.
Doped single-crystal YAG fibers used as single-mode lasers require claddings with precise refractive index and high thermal conductivity. Three cladding materials that use coextrusion of green cladding on fiber cores as an initial processing step are described: 1. Undoped YAG cladding, followed by sintering or hot isostatic pressing. 2. Ca3Ga2Ge3O12 garnet cladding that melts beneath 1400°C. 3. LiCa2Mg2As3xV3-3xO12 garnet cladding that melts beneath 1100°C. Microstructures are characterized by TEM. Equipment and procedures are described. Garnet refractive index models are developed and validated to predict cladding refractive index. Advantages and disadvantages of the different claddings are compared.
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