HeeDong Lee

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.

Processing and transparency of polycrystalline yttrium aluminum garnet (YAG) fibers for optical applications

Polycrystalline YAG fibers are of interest for both optical and structural applications. Various processing routes of YAG fibers for structural applications have been explored; however, processing routes for optical quality polycrystalline YAG fiber have not been investigated intensively despite the potential of the material to enable high power lasers. Recent results in the processing of YAG fiber for laser applications are presented and detailed relationship between processes, microstructures, and optical properties of YAG fibers are discussed. Specifically, details of the processes for green fiber preparation, sintering methods, and transparencies depending on the process variables are shown. Our recent advancement in fiber processing prior to sintering has improved the transparency of YAG fiber significantly. Vacuum or air sintering followed by Hot Isostatic Press (HIP) produced fibers with transparency comparable to that of single crystal YAG fiber.

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.

Densification and Microhardness of Spark Plasma Sintered ZrB2+SiC Nano-Composites

Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC nanofiller are useful as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Here, we study the densification, grain growth, and microhardness of ZrB2 nanocomposites with 15% and 20% SiC consolidated using SPS.

Optical characterizations on surface-polished polycrystalline YAG fibers

The superior thermal and optical properties of transparent polycrystalline ceramics make them attractive alternatives to glass-based materials for laser gain media. Fibers have other advantages of compactness, vibration-resistance, and reduced cooling requirements. Recently it was found that surface roughness caused by grain boundary grooving dominated optical scattering even though there were other scattering sources in the fiber. Therefore, a lot of effort went to fabrication of fibers with smooth surfaces. A mechanical polishing method for polycrystalline YAG fibers was developed. The fiber surface roughness was reduced, while maintaining a circular cross-section. Surface-polished 1.5% Ho-doped polycrystalline YAG fiber, 62 mm long with 31 μm diameter, was fabricated, and lasing was demonstrated from this fiber. Effects of surface-polishing on the surface roughness and scattering coefficient are presented, and lasing characteristics are discussed.

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.

Effect of Argon Gas Purging of Spark Plasma Sintered ZrB2+SiC Nano-Powder Composites

Spark Plasma Sintering (SPS) consolidated ZrB2+SiC composites using nano-powders (around 40 nm) showed smaller grains compared to those using micron size powders and segregation of SiC into islands is minimal but with higher oxidation of ZrB2 to form ZrO2 in nano-composites. Argon-gas purging prior to SPS consolidation at around 2000 °C and 40 MPa of ZrB2+20vol.%SiC nano-powders was used to minimize the oxidation and obtain fine granules with high densification. The densification of the Argon-gas purged nano-composites is higher compared to those consolidated without Argon gas purging. The EDX analysis showed a strong reduction in the oxygen peak for the Argon gas purged composites. The XRD spectra also support this observation with less ZrO2 phase composition in Argon gas purged composites. The Vickers micro-hardness showed slightly lower values for Argon gas purged composites though they have higher densification.

Comparative Study between Vickers and Knoop Micro-hardness of Ultra High Temperature Ceramics

Ultra-high temperature ceramics such as ZrB2 and HfB2 with small percentage of SiC are useful as structural materials for applications in leading edge of hypersonic vehicles [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Fine grain, low porosity, and high densification yield higher micro-hardness ceramics those can be used for high temperature oxidation resistant materials. Here we made a comparative study between Vickers and Knoop micro-hardness of SPS consolidated UHTC composites starting with micronand nano-powders.

High-energy Ball-milling of ZrB 2 and HfB 2 Powders: Effect on Particle Size and Crystalline Grain Distribution

Introduction: Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC are being considered as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. We used high energy ball milling of the precursor powders to increase lattice distortion enhanced inter-diffusion, uniform distribution of SiC, and reduce grain growth during Spark plasma sintering (SPS). Here, we study the effect of high energy ball milling of ZrB2 or HfB2 with 20 vol% SiC on the particle size and crystalline-grain distribution.

Recent progress in ceramic YAG cladding technology for fiber laser applications

The advent of high energy, high power fiber lasers hinges on the availability of new fibers and suitable cladding materials. New fiber materials with the needed thermal and optical properties are urgently needed for high powered fiber lasers; a viable cladding process using suitable cladding materials is equally critical. These two fundamental technologies are being developed. Most recently, the technical feasibility of applying optically transparent undoped YAG as the fiber cladding was explored via a single source electron beam deposition process. The technical feasibility of depositing such fiber claddings was successfully demonstrated on various model fibers. Amorphous YxAl1-xO3 coatings with various chemistries and high optical transmittances were deposited and shown to be stable even after high temperature annealing, while retaining the optical quality.