Beryllium Materials for National Ignition Facility Targets LDRD Final Report

McEachern, R L

Lawrence Livermore National Laboratory

关键词: Microstructure;    Alloys;    Angular Distribution;    Hydrogen Isotopes;    70 Plasma Physics And Fusion Technology;   

DOI  :  10.2172/15013519
RP-ID  :  UCRL-ID-142818
RP-ID  :  W-7405-ENG-48
RP-ID  :  15013519

美国|英语

来源: UNT Digital Library

【 摘 要 】

The National Ignition Facility (NIF) will require spherical ignition capsules approximately 2 mm in diameter with a 120- to 150-pm-thick ablator. Beryllium-based alloys are promising candidates for an ablator material due to their combination of low opacity and relatively high density (compared to polymer coatings). For optimum performance, the Be-coated capsules require a smooth surface finish, uniform thickness, microscopic homogeneity, and preferably high strength. The coatings must contain on the order of 1 at.% of a high-Z dopant (such as Cu) and permit the capsule to be filled with fuel, which will be a mixture of hydrogen isotopes. These demanding requirements can be met through a synthesis method with a focus on the control of microstructure. In our experiments, the sputter deposition process has been manipulated so as to decrease the grain size, thereby reducing roughness and improving homogeneity. The material properties of sputter-deposited coatings are sensitive to their microstructure and growth morphology. To meet the requirements for Be coated capsules, the goal of this project was to optimize the microstructure and growth morphology through the control of deposition process parameters. Prior experimental studies of evaporation and sputter deposition revealed that the grain size of 99.8 at.% pure Be can be reduced by adding insoluble metal impurities such as Fe or Ti. These higher atomic weight elements can replace the requirement of adding 1 at.% Cu to the Be. Grain size can also be reduced by using additives that are metallic-glass formers, such as boron. Finally, the microstructure can be modified by changing the energy or angular distribution of the depositing flux. Our initial experiments focused on the development of Be-B-X alloys, where X is Fe and/or Cu. This work was successful in reducing the grain size of coatings deposited on planar substrates from microns to at least nanometers. TEM cross sections of these films showed no discernible grain structure, so for the purposes of this project the material was a glass. Deposition of a few pm of this alloy onto stationary capsules produced extremely smooth films: rms roughnesses of about 1 nm were observed using atomic force microscopy. As the coating thickness was increased, however, intrinsic stress in the film became a serious problem. Buckling and delamination were the typical symptoms of this phenomenon. Although there are techniques for mitigating stress build-up in deposited films, we concluded it would pose a formidable problem for the very thick coatings required for NIF capsules. For this reason, we shifted our efforts away from glassy alloys and concentrated on modifying the energy and angular distribution of the depositing Cu-doped beryllium flux.