Development of a 1 L/h Scale Liquid Hydrogen System

A small-scale hydrogen liquefaction plant consists of cryocooler, cryostat, liquid nitrogen (LN2) precooling bath, two ortho–para (o–p) hydrogen converters, a heat pipe, multilayer insulations (MLI), sensors and auxiliary components. The liquefaction plant was designed as both a cryocooler-cooled liquefier and a long-term zero boil-off liquid storage reservoir. A Gifford–McMahon (GM) cryocooler was selected as a cold sink and purchased from Cryomech Inc. A vacuum-jacketed 200 L capacity cryostat was designed to provide highly efficient thermal protection to LH2 storage space. A heat pipe was uniquely designed and installed in the cryostat to enhance heat transfer between the cryocooler and hydrogen gas and liquid. A LN2-cooled precooling bath was designed to act as a precooler and an o–p hydrogen converter. MLT blankets with hydrogen getter under high vacuum in the cryostat minimize heat loss into LH2 storage. All the components were carefully assembled in dust, oil and solvent-free environment with extra caution of vacuum tightness followed by dry nitrogen and helium purge. After initial cool-down of the cryocooler under vacuum, 99.999% of 300 K hydrogen gas was introduced to the liquefaction plant. From a series of liquefaction tests with or without LN2 precooling, the hydrogen liquefaction plant successfully demonstrated 1.4 L/h of hydrogen liquefaction with LN2 precooling and two o–p hydrogen converters, and 0.7 L/h without LN2 precooling.

Stress Induced Void Nucleation in Narrow Aluminum Alloy Lines

The stress induced failure of passivated aluminum based metallizations is the combined result of the stress induced void nucleation and growth of voids. The present work investigates the dependence of the void nucleation on aging temperature and cooling rate. Narrow Al, Al(0.2%Cu) and Al(2%Si) lines of different widths were fabricated by lift-off and passivated with various thicknesses of Si3N4. After a 400°C anneal, the specimens were cooled directly to the selected aging temperature and aged for up to 1100 hours. Void nucleation was found to occur predominantly during the initial cool-down. The rate of nucleation varied in quite different ways with the combination of aging temperature and cooling rate for the different alloys, which is consistent with a grain boundary sliding controlled nucleation mechanism.

Stress Induced Void Nucleation in Narrow Aluminum Alloy Lines

ABSTRACTThe stress induced failure of passivated aluminum based metallizations is the combined result of the stress induced void nucleation and growth of voids. The present work investigates the dependence of the void nucleation on aging temperature and cooling rate. Narrow Al, Al(0.2%Cu) and Al(2%Si) lines of different widths were fabricated by lift-off and passivated with various thicknesses of Si3N4. After a 400°C anneal, the specimens were cooled directly to the selected aging temperature and aged for up to 1100 hours. Void nucleation was found to occur predominantly during the initial cool-down. The rate of nucleation varied in quite different ways with the combination of aging temperature and cooling rate for the different alloys, which is consistent with a grain boundary sliding controlled nucleation mechanism.