ALBERT

Notes: Ash fusion, sintering, and deposition may impose serious operational difficulties in conventional and advanced coal-combustion systems. Conventional ash fusion techniques (e.g., ASTM methods) determine the qualitative behavior of ash samples at atmospheric pressure. Presently, there is no known available technique that can measure the behavior of coal ash at elevated temperatures and pressures. In the literature, methods based on electrical resistance or shrinkage of coal ash have been reported at atmospheric pressure (elevated temperatures) conditions. A high-pressure microdilatometer (HPMD) has been developed to investigate ash fusion and sintering behavior at elevated pressures and temperatures by the simultaneous measurement of the temperature of initial contraction and electrical resistivity of samples. This novel technique facilitates the measurement of ash properties over a wide range of temperature, pressure, and gas atmosphere (oxidizing, reducing, or inert). The operating principle of the HPMD includes measuring the temperature at which there is a significant "shift'' in the electrical resistivity (and/or sample volume) that represents ash sintering and fusion. Sintering occurs through the formation of solid-state, particle-to-particle "necks'' or the appearance of a molten phase, which allows a path for electrical conductance. The ability to perform both resistivity and shrinkage measurements simultaneously or independently at elevated pressures makes the HPMD truly unique. The HPMD can also be used to investigate the swelling and softening behavior of pyrolyzing coal at elevated pressures and relatively rapid heating rates. The HPMD can provide insights into the sintering/fusion of coal ash or coal swelling at a range of conditions: (a) the influences of various gas atmospheres can be investigated, (b) the effects of pressure can be studied, (c) different temperature/heating rate schemes can be used (constant rates, isothermal holds below or above the sintering temperature, etc.), and (d) studies can be performed to investigate the influence of increased heating rate at elevated pressures (which were not performed previously) on coal swelling and plasticity.It can also be applied to other fossil fuels such as oil shale or carbonaceous materials (e.g., graphite) to measure their electrical conductivity or expansion and contraction behavior at ambient or elevated pressures and temperatures. The HPMD allows researchers to gather novel data and thereby facilitate a fundamental understanding of the behavior of coal or coal ash at actual processing conditions.

Notes: Transmission line techniques are used to calculate the traversal time of electrons in resonant tunneling structures. It is suggested that the real part of the quantum-mechanical wave impedance Re[(2(h-dash-bar)/m*)ψ'/ψ], at resonance, can be used to calculate the electron traversal time. Furthermore, it is shown that for symmetric structures the lifetime and evolution time of electrons are the same. The sum of the lifetime and the evolution time equals the electron traversal time. Traversal time variations for the asymmetric well are discussed.

Notes: Optical measurements are performed near the fundamental absorption edge for single-crystal AlxGa1−x N epitaxial layers in the composition range of 0≤x≤0.4. The dependence of the energy band gap on composition is found to deviate downwards from linearity, the bowing parameter being b=1.0±0.3 eV. The origin of the large bowing is discussed in terms of the pseudopotential of Al and Ga based on the pseudopotential of the Heine–Abarenkov type. With increasing x the absorption edges broaden, which is attributed to the increase of the compositional nonuniformity.

Notes: This paper presents a simple yet accurate method for solving the Schrödinger equation to calculate the quantum mechanical transmission probability across arbitrary potential barriers. It is based on the concept of wave impedance analogous to transmission line theory. The quantum mechanical transmission probability in a resonant tunneling device can be easily calculated using this method.