Apparatus for testing gas-surface reactions for epicatalysis.

Recently, a new mode of gas-surface heterogeneous catalysis (epicatalysis) has been identified, having potential applications ranging from industrial and green chemistry to novel forms of power generation. This article describes an inexpensive, easily constructed, vacuum-compatible apparatus by which multiple candidate gas-surface combinations can be rapidly screened for epicatalytic activity. In exploratory experiments, candidate surfaces (teflon, kapton, glass, and gold) and gases (helium, argon, cyclohexane, water, methanol, formic acid, and acetic acid) were tested for epicatalytic activity. Kapton and teflon displayed small but reproducible differences in formic acid and methanol dimer desorption, thereby demonstrating the first examples of room-temperature epicatalysis. Other gas-surface combinations showed smaller or inconclusive evidence for epicatalysis.

Nonequilibrium heterogeneous catalysis in the long mean-free-path regime.

It is shown that a standard principle of traditional catalysis-that a catalyst does not alter the final thermodynamic equilibrium of a reaction-can fail in low-pressure, heterogeneous gas-surface reactions. Kinetic theory for this epicatalysis is presented, and two well-documented experimental examples are detailed: surface ionized plasmas and hydrogen dissociation on refractory metals. This phenomenon should be observable over a wide range of temperatures and pressures, and for a broad spectrum of heterogeneous reactions. By transcending some constraints of equilibrium thermodynamics, epicatalysis might provide additional control parameters and synthetic routes for reactions, and enable product streams boosted in thermochemical energy or desirable species.

Orthogonally-oriented nanotube arrays: experiment I.

Recently a new type of self-assembling surface has been proposed that, in theory, possesses a number of desirable tribological, electrical, and thermal characterstics. The surface consists of arrays of carbon nanotubes partially embedded lengthwise in a substrate such that when two arrayed surfaces are brought together orthogonally, the areal contact between them is small, limited to a lattice of nearly point-like contacts. These orthogonally-oriented nanotube arrays (ONAs) are predicted to exhibit: (i) surface adhesion (stiction) 10-100 times less than for Teflon or other advanced perfluorocarbons; (ii) frictional coefficients up to 1000 times less than for conventional solids; (iii) ultra-low wear; and (iv) superior thermal and electrical conductivity. In this paper, laboratory methods are described for embedding nanotubes in trenched substrates. Using microscopically trenched substrates and a custom ultrasonic atomization source, experiments show that individual nanotubes can spontaneously and controllably entrench themselves via interfacial forces (capillary and surface tension). Results indicate ONAs might be relatively simply and inexpensively fabricated. More decisive experiments are proposed.

Orthogonally-oriented nanotube arrays: theory.

A novel surface involving ordered arrays of partially-embedded carbon nanotubes is developed theoretically. Analysis indicates it should exhibit ultra-low values for friction, adhesion and wear, and also possess superior thermal and electrical properties. The surface consists of orthogonally-oriented, self-assembling arrays of carbon nanotubes, partially embedded lengthwise in a solid substrate. Calculations indicate that stiction forces due to van der Waals interactions can be made small, perhaps more than an order of magnitude less than for Teflon and other advanced perfluorocarbons. Static and kinetic frictional forces could be three orders of magnitude less than for conventional solids.

Intrinsically biased electrocapacitive catalysis.

We propose the application of the contact potential from metal-metal junctions or the built-in potential of semiconductor p-n junctions to induce or catalyze chemical reactions. Free of external sources, this intrinsic potential across microscale and nanoscale vacuum gaps establishes electric fields in excess of 10(7) Vm. The electrostatic potential energy of these fields can be converted into useful chemical energy. As an example, we focus on the production of superthermal gas ions to drive reactions. Analysis indicates that this intrinsically biased electrocapacitive catalysis can achieve locally directed ion energies up to a few electron volts and local gas temperature boosts in excess of 10(4) K. Practical considerations for implementation and experimental tests are considered.

Modeling a submicrometer electrostatic motor.

Numerical models are developed for a recently proposed submicrometer device that uses the electric field energy of a biased parallel-plate semiconducting capacitor to propel a piston through the open capacitor gap. Through variation of design parameters or applied external bias, actuator forces on the order of hundreds of piconewtons are developed for device size scales ranging from 10(-7) m to 10(-4) m per side. A rotary configuration of the device is also presented.