Robert E. Buxbaum

Hydrogen transport through non-porous membranes of palladium-coated niobium, tantalum and vanadium

Experiments are reported for hydrogen transport in palladium-coated membranes of niobium, tantalum, and vanadium. Two geometries are studied, coated discs 2 mm thick, and coated tubes of 0.25 mm wall thickness. The hydrogen is extracted at 100% purity. Fluxes are much higher than with current palladium alloy and polymeric membranes at these pressures, and are somewhat higher than with palladium-coated porous ceramics. Palladium-coated niobium discs show effective permeabilities about 0.32 μmol/m-sec-Pa12 at 425°C and 2 atm pressure (including gas-phase resistance). The coated tubes have about 12 this permeability, and thus mass transport resistances about 1450 m2-sec-Pa12/mol. Coated tantalum discs have effective permeabilities about 0.1 μmol/m-sec-Pa12, but are less susceptible to hydrogen embrittlement. Since these coated-metals are stronger and more durable than palladium, ceramics of polymers, considerable design flexibility is possible. Based on these measurements, the flux for advanced membranes would be 2000 scf/hr-m2 for a driving force of 3 atm, suggesting attractive economics for petrochemical plant use.

Mechanical tension can specify axonal fate in hippocampal neurons

Here we asked whether applied mechanical tension would stimulate undifferentiated minor processes of cultured hippocampal neurons to become axons and whether tension could induce a second axon in an already polarized neuron. Experimental tension applied to minor processes produced extensions that demonstrated axonal character, regardless of the presence of an existing axon. Towed neurites showed a high rate of spontaneous growth cone advance and could continue to grow out for 1–3 d after towing. The developmental course of experimental neurites was found to be similar to that of unmanipulated spontaneous axons. Furthermore, the experimentally elongated neurites showed compartmentation of the axonal markers dephospho-tau and L-1 in towed outgrowth after 24 h. Extension of a second axon from an already polarized neuron does not lead to the loss of the spontaneous axon either immediately or after longer term growth. In addition, we were able to initiate neurites de novo that subsequently acquired axonal character even though spontaneous growth cone advance began while the towed neurite was still no longer than its sibling processes. This suggests that tension rather than the achievement of a critical neurite length determined axonal specification.

Cytomechanics of axonal development

AbstractMechanical tension is a robust regulator of axonal development of cultured neurons. We review work from our laboratory, using calibrated glass needles to measure or apply tension to chick sensory neurons, chick forebrain neurons, and rat PC12 cells. We survey direct evidence for two different regimes of tension effects on neurons, a fluid-like growth regime, and a nongrowth, elastic regime. Above a minimum tension threshold, we observe growth effects of tension regulating four phases of axonal development:1.Initiation of process outgrowth from the cell body;2.Growth cone-mediated elongation of the axon;3.Elongation of the axon after synaptogenesis, which normally accommodates the skeletal growth of vertebrates; and4.Axonal elimination by retraction. Significantly, the quantitative relationship between the force and the growth response is suprisingly similar to the simple relationship characteristic of Newtonian fluid mechanical elements: elongation rate is directly proportional to tension (above the threshold), and this robust linear relationship extends from physiological growth rates to far-above-physiological rates. Thus, tension apparently integrates the complex biochemistry of axonal elongation, including cytoskeletal and membrane dynamics, to produce a simple “force input/growth output” relationship. In addition to this fluid-like growth response, peripheral neurons show elastic behaviors at low tensions (below the threshold tension for growth), as do most cell types. Thus, neurites could exert small static forces without diminution for long periods. In addition, axons of peripheral neurons can actively generate modest tensions, presumably similar to muscle contraction, at tensions near zero. The elastic and force-generating capability of neural axons has recently been proposed to play a major role in the morphogenesis of the brain.

Development of membranes for hydrogen separation: Pd coated V–10Pd

Abstract Numerous group IVB and VB alloys were prepared and tested as potential membrane materials, but most of these materials were brittle or exhibited cracking during hydrogen exposure. One of the more ductile alloys, V–10Pd (at.-%), was fabricated into a thin foil (107 μm thick) composite membrane coated with 100 nm of Pd on each side. The material was tested for hydrogen permeability, resistance to hydrogen embrittlement, and long term hydrogen flux stability. The hydrogen permeability ϕ of the V–10Pd membrane was 3·86×10–8 mol m–1 s–1 Pa–0·5 (average of three different samples) at 400°C, which is slightly higher than the permeability of Pd–23Ag at that temperature. A 1400 h hydrogen flux test at 400°C demonstrated that the rate of metallic interdiffusion was slow between the V–10Pd foil and the 100 nm thick Pd coating on the surface. However, at the end of testing, the membrane cracked at 118°C because of hydrogen embrittlement.

Palladium-catalyzed oxidative diffusion for tritium extraction from breeder-blanket fluids at low concentrations

Oxidative diffusion can extract hydrogen from metal solutions at extremely low partial pressures. The hydrogen diffuses through a metal membrane and is oxidized to water. The oxidation reaction produces the very low downstream pressures that drive the flux. This method is attractive because the flux can be proportional to the square-root of upstream pressure. For fusion reactors with liquid lithium or lithium-lead alloy breeder blankets, permeation windows provide a simple, cheap tritium extraction method. Interdiffusion rates, separation flux, window size, helium contents, tritium holdup costs, and overall costs are calculated for membranes of palladium-coated zirconium, niobium, vanadium, nickel and stainless-steel. For extracting tritium from liquid lithium using the cheapest windows, Zr-Pd, the material and labor cost is 8.0 M

Hydrogen transport and embrittlement for palladium coated vanadium—chromium—titanium alloys

at 1 wppm, and is inversely proportional to tritium concentration in the lithium. The tritium holdup cost for the windows is 4.8 M

The Use of Zirconium-Palladium Windows for the Separation of Tritium from the Liquid Metal Breeder-Blanket of a Fusion Reactor

, and for the blanket it is proportional to the blanket volume and concentration. An overall economic optimization suggests that 1 to 1.5 wppm in lithium is optimal. For extracting tritium from 17Li83Pb at 0.26 wppb, the cheapest window is V-Pd; the cost is 2.6 M

MEMBRANE REACTOR ADVANTAGES FOR METHANOL REFORMING AND SIMILAR REACTIONS

, and the tritium holdup is negligible.

A chemical theory analysis of the solution thermodynamics of oxygen, nitrogen and hydrogen in lead-rich LiPb mixtures

Vanadium based alloys have been identified as a leading candidate material for fusion first-wall blanket structure application because they exhibit favorable safety and environmental characteristics, good fabricability, potential for high performance and long-time operation lifetime in a fusion environment. As part of a study of the thermodynamics, kinetics and embrittlement properties of hydrogen in vanadium based alloys, experiments were conducted to determine the rate of hydrogen transport through the vanadium reference alloys, V-7.5Cr-15Ti and V-4Cr-4Ti, and to determine these alloys` hydrogen embrittlement, they were exposed to hydrogen pressures of 3 and 300 kPa (0.03--3 atm) at temperatures between 380 and 475 C. To facilitate hydrogen entry and egress, tubes of these alloys were coated with palladium on the inside and outside faces. Observed permeabilities were 0.015 to 0.065 {micro}moles/(m{sup 2}sPa{sup 0.5}) for the V-7.5Cr-15Ti alloy and 0.02 to 0.05 {micro}moles/m{sup 2}sPa{sup 0.5} for the V-4Cr-4Ti alloy depending on the quality of the coat and the operating temperature. At 1.7 atm hydrogen, V-7.5Cr-15Ti embrittled at temperatures below 380 C while V-4Cr-4Ti embrittled around 330 C.

Measurement of diffusive and surface transport resistances for deuterium in palladium-coated zirconium membranes

Abstract Any first generation fusion power reactor must be surrounded by a heat tranfer medium and a tritium breeder-blanket containing lithium in some form. Liquid lithium and liquid lithium alloys are considered to be particularly attractive choices for both purposes; the main disadvantage of these materials is the difficulty of tritium separation. The use of zirconium-palladium windows for tritium separation is shown to have many advantages over the best alternative devices. This is based on the materials properties and costs as they stand in 1983. The design of a typical permeation window resembles that of a shell and tube heat exchanger; liquid lithium or lithium alloy loaded with tritium from the reactor, flows through on the zirconium side of the window, while a purge stream of argon and oxygen flows through on the side with the palladium coating. Oxygen is present so that sufficiently low tritium pressures may be maintained on the downstream side of the window. Tritium is recovered as T2O. Data ar...