Elizabeth N. Hoffman

Double-Layer Capacitance of Carbide Derived Carbons in Sulfuric Acid

Nanoporous carbons obtained by selective leaching of Ti and Al from Ti2AlC, as well as B from B4C, were investigated as electrode materials in electric double-layer capacitors. Cyclic voltammetry tests were conducted in 1 M H2SO4 from 0-250 mV on carbons synthesized at 600, 800, 1000, and 1200°C. Results show that the structure and pore sizes can be tailored and that the optimal synthesis temperature is 1000°C. Specific capacitances for Ti2AlC CDCs and B4C CDCs were 175 and 147 F/g, respectively, compared to multiwall carbon nanotubes and two types of activated carbon, measured herein to be 15, 52, and 125 F/g, respectively.

On the spontaneous growth of soft metallic whiskers

The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free solder technology. Recently it was shown that the driving force is most probably a reaction between oxygen and the sprouting metal (Barsoum, 2004). The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains observations on In and Sn whiskers as well as many past observations. Herein further evidence is presented for, and discussion of, the proposed model. Stresses, calculated using finite element modeling, are reasonable and in line with measured values. Based on this work, a potential solution to the whisker problem is in principle simple: either slow or prevent the diffusion of oxygen into the soft metal or, more practically and effectively, work with larger grained solder, which should reduce the magnitude of the compressive stresses.

Diffusion, Thermal Properties and Chemical Compatibilities of Select MAX Phases with Materials For Advanced Nuclear Systems

The demands of Gen IV nuclear power plants for long service life under neutron irradiation at high temperature are severe. Advanced materials that would withstand high temperatures (up to 1000+ oC) to high doses in a neutron field would be ideal for reactor internal structures and would add to the long service life and reliability of the reactors. The objective of this work is to investigate the chemical compatibility of select MAX with potential materials that are important for nuclear energy, as well as to measure the thermal transport properties as a function of neutron irradiation. The chemical counterparts chosen for this work are: pyrolytic carbon, SiC, U, Pd, FLiBe, Pb-Bi and Na, the latter 3 in the molten state. The thermal conductivities and heat capacities of non-irradiated MAX phases will be measured.

Neutron Damage and MAX Phase Ternary Compounds

The Demands of Gen IV nuclear power plants for long service life under neutron radiation at high temperature are severe. Advanced materials that would withstand high temperatures (up to 1000+ C) to high doses in a neutron field would be ideal for reactor internal structures and would add to the long service life and reliability of the reactors. The objective of this work is to investigate the response of a new class of machinable, conductive, layered, ternary transition metal carbides and nitrides - the so-called MAX phases - to low and moderate neutron dose levels.