Publications

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Herein we present details of the design, simulation, and performance of a 100-GW linear transformer driver (LTD) cavity at Sandia National Laboratories. The cavity consists of 20 "bricks." Each brick is comprised of two 80 nF, 100 kV capacitors connected electrically in series with a custom, 200 kV, three-electrode, field-distortion gas switch. The brick capacitors are bipolar charged to ±100 kV for a total switch voltage of 200 kV. Typical brick circuit parameters are 40 nF capacitance (two 80 nF capacitors in series) and 160 nH inductance. The switch electrodes are fabricated from a WCu alloy and are operated with breathable air. Over the course of 6,556 shots the cavity generated a peak electrical current and power of 1.03 MA (±1.8%) and 106 GW (±3.1%). Experimental results are consistent (to within uncertainties) with circuit simulations for normal operation, and expected failure modes including prefire and late-fire events. New features of this development that are reported here in detail include: (1) 100 ns, 1 MA, 100-GW output from a 2.2 m diameter LTD into a 0.1 Ω load, (2) high-impedance solid charging resistors that are optimized for this application, and (3) evaluation of maintenance-free trigger circuits using capacitive coupling and inductive isolation.

This Part 2 study examined the microstructural characteristics of braze joints made between two KOVar^TM base materials using the filler metals, Ag-xAl, having x = 0, 2, 5, and 10 wt.% Al additions. Brazing processes had temperatures of 965°C (1769°F) and 995°C and brazing times of 5 and 20 min. All brazing was performed under high vacuum.

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The run-out phenomenon was observed in Ag-Cu-Zr active braze joints made between the alumina ceramic and Kovar™ base material. Run-out introduces a significant yield loss by generating functional and/or cosmetic defects in brazements. A prior study identified a correlation between run-out and the aluminum (Al) released by the reduction/oxidation reaction with alumina and aluminum's reaction with the Kovar™ base material. A study was undertaken to understand the fundamental principles of run-out by examining the interface reaction between Ag-xAl filler metals (x = 2,5, and 10 wt-%) and Kovar™ base material. Sessile drop samples were fabricated using brazing temperatures of 965° (T769°F) or 995°C 0823°F) and times of 5 or 20 min. The correlation was made between the degree of wetting and spreading by the sessile drops and the run-out phenomenon. Wetting and spreading increased with Al content (x) of the. Ag-xAl filler metal, but was largely insensitive to the brazing process parameters. The increased Al concentration resulted in higher Al contents of the (Fe, Ni, Co)xAly reaction layer. Run-out was predicted when the filler metal has a locally elevated Al content exceeding 2-5 wt-%. Several mitigation strategies were proposed, based upon these findings.

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Similar to other refractory metals, commercially pure niobium is difficult attach using soldering processes without first plating with nickel-gold, nickel-tin or similar materials that are directly solderable. Currently used procedures require the aforementioned plating process or a step-brazing process in which copper substrates are brazed at a lower temperature onto the niobium surfaces eliminating the plating requirements. A solder-dipping process is then used to pre-tin the exposed copper surfaces, preparing them for next-assembly soldering steps. As part of a product development effort to reduce or eliminate entire processes or processing steps, a project was initiated to replace commercially pure niobium sheet material with explosively bonded niobium-copper sheet. The exposed copper surfaces could then be subsequently coated using a solder dipping procedure. To simulate the component brazement geometry, explosively bonded niobium and copper metal sheets were actively brazed to 94% alumina ceramic test specimens. The thickness of the explosively bonded substrates was 0.5 mm and the thickness of the niobium metal approximately twice that of the copper. ASTM F19 tensile buttons were fabricated using the explosively bonded niobium-copper material as the interlayers. The test samples were active brazed using a commercially available gold-based active brazing filler metal of the composition 35Au-62Cu-2Ti-1Ni (wt %). Brazing peak temperatures and soak times at peak temperatures were varied to assess the process robustness. Finite element analysis (FEA) simulations were performed to determine the theoretical residual stresses in the braze samples. Helium mass spectrometer leak detection data, brazed sample tensile strengths and scanning electron microscope image analysis of the niobium-copper, niobium-alumina and copper-alumina interfaces will be presented. Copyright 2012 ASM International® All rights reserved.

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Low Temperature Cofired Ceramic (LTCC) has proven to be an enabling medium for microsystem technologies, because of its desirable electrical, physical, and chemical properties coupled with its capability for rapid prototyping and scalable manufacturing of components. LTCC is viewed as an extension of hybrid microcircuits, and in that function it enables development, testing, and deployment of silicon microsystems. However, its versatility has allowed it to succeed as a microsystem medium in its own right, with applications in non-microelectronic meso-scale devices and in a range of sensor devices. Applications include silicon microfluidic ''chip-and-wire'' systems and fluid grid array (FGA)/microfluidic multichip modules using embedded channels in LTCC, and cofired electro-mechanical systems with moving parts. Both the microfluidic and mechanical system applications are enabled by sacrificial volume materials (SVM), which serve to create and maintain cavities and separation gaps during the lamination and cofiring process. SVMs consisting of thermally fugitive or partially inert materials are easily incorporated. Recognizing the premium on devices that are cofired rather than assembled, we report on functional-as-released and functional-as-fired moving parts. Additional applications for cofired transparent windows, some as small as an optical fiber, are also described. The applications described help pave the way for widespread application of LTCC to biomedical, control, analysis, characterization, and radio frequency (RF) functions for macro-meso-microsystems.