The incomplete depletion approximation in semiconductor heterojunctions

Under equilibrium the region near the heterojunction interface is not completely depleted of carriers. As the forward bias is increased, carriers are transported out of the region and the charge density is increased. The well-known depletion approximation is a special case of the more general incomplete depletion approximation.     

Decentralized Swarm Coordination: A Combined Coverage/Connectivity Approach

Decentralized motion coordination for coverage optimization purposes in mobile sensor networks is the scope of this paper. Coordination is performed based on spatial Voronoi tessellation, while taking into consideration the limited sensing capabilities of the agents. Each node performs an independent optimization in order to increase network??s area coverage via its motion, while it attains information from its current and future Delaunay neighbors. A decentralized algorithm is proposed in order to achieve optimal network??s coverage, based on local information. Connectivity issues are analyzed in detail, while a lower bound on the communication radius of the nodes is derived, in order to attain sufficient information for performing the corresponding optimization. An agent moves inside its region of responsibility in a way that the total area surveyed by the network is a monotonically increasing function of time. The online control action makes the network adaptive to possible changes in the environment.     

Revisiting active perception

Despite the recent successes in robotics, artificial intelligence and computer vision, a complete artificial agent necessarily must include active perception. A multitude of ideas and methods for how to accomplish this have already appeared in the past, their broader utility perhaps impeded by insufficient computational power or costly hardware. The history of these ideas, perhaps selective due to our perspectives, is presented with the goal of organizing the past literature and highlighting the seminal contributions. We argue that those contributions are as relevant today as they were decades ago and, with the state of modern computational tools, are poised to find new life in the robotic perception systems of the next decade.     

Transformation of stainless steel slag toward a reactive cementitious binder

Argon oxygen decarburization (AOD) slag represents more than 50 wt% of the slag from stainless steel production. Although some applications are available, e.g., as aggregates for road constructions or fertilizers, they are characterized by low economic value and limited applicability. In order to increase the economic value of AOD slag, alternative applications have been proposed, e.g., as partial or full replacement for Ordinary Portland Cement (OPC). The work presented here investigates whether the adaptation of the AOD slag chemistry within a high temperature process leads to an improvement of its hydraulic properties and thereby can demonstrate its potential to be converted into a hydraulic binder suitable for OPC replacement. For this purpose, three synthetic AOD slags with basicities (CaO/SiO₂) of 2.0, 2.2, and 2.4 were synthesized, and the effect of the CaO/SiO₂ ratio on the material stability, the amount of tricalcium silicate formed, and their hydraulic properties investigated. X‐ray diffraction, scanning electron microscope (SEM), and isothermal calorimetry analysis were used to characterize the microstructure and the hydraulic activity. The results show that the proposed method is indeed a promising way to stabilize a stainless steel AOD slag and convert it into a hydraulic binder.     

Influence of the fuel structure on the flame synthesis of carbon nanomaterials

Carbon nanomaterials (CNMs) were grown on nickel-coated stainless-steel mesh grids using a combustion synthesis method. Three fuel feedstocks of dissimilar chemical structures, ethylene, ethyl benzene and ethyl alcohol, were burned at fuel-rich equivalence ratios to generate disparate combustion products. The chemical compositions of these products were assessed to identify and contrast the species that contribute to the growth of CNMs. The generated CNMs were characterized using SEM and TEM. Each set of conditions produced a distinct distribution (bi-modal or mono-modal) of CNMs, including carbon nanotubes and carbon nanofibers, both of varying diameters and lengths. The effects of combustion-generated light aliphatic hydrocarbons and light aromatic hydrocarbons on CNM growth and structure were ascertained. It was found that ethylene and ethyl alcohol generated tubular CNMs under all tested conditions, whereas ethyl benzene generated only small amounts of CNMs and under much more limited conditions. It was also determined that concentrations of aliphatic species, notably methane and acetylene, at mole fractions as low as 500 ppm have a thickening effect on the resulting CNMs. Linear correlations were found between the concentrations of these species and the mean diameter of the CNMs.     

Soot surface area evolution during air oxidation as evaluated by small angle X-ray scattering and CO2 adsorption

The total surface area of two diesel engine produced soots, a high volatile content NIST standard (termed NIST) and a low volatile content soot (termed NEU), were determined with CO₂ adsorption and small angle X-ray scattering (SAXS), as a function of the extent of oxidation. During initial volatilization of condensables of the NIST and NEU soots in a thermogravimetric analyzer, in helium at 1073 K, their CO₂ surface areas increased sharply from 49 m²/g to 273 m²/g and from 96 m²/g to 367 m²/g, respectively. During oxidation, the CO₂ surface area increased by an additional 100-150 m²/g, until 50% conversion was reached. Thereafter, the CO₂ surface area was relatively constant with conversion for the NIST soot, but decreased to 150 m²/g for the NEU soot. Three porosity regimes were assumed for the calculation of SAXS areas; they were based on (a) constant density (shrinking core), (b) constant diameter, and (c) an observed (with a TEM) diameter variation. The best agreement between the CO₂ and SAXS surfaces area occurred for the constant density assumption, in contrast to the actual measured diameter variation. By applying fractal surface analysis to the SAXS data, this discrepancy is ascribed to the opening up of internal volume to reaction volatilization of condensables and oxidation.     

Single particle ignition and combustion of anthracite,semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O₂/79%N₂) and four simulated dry oxy-fuel conditions: 21%O₂/79%CO₂, 30%O₂/70%CO₂, 35%O₂/65%CO₂ and 50%O₂/50%CO₂. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N₂ gas of air with CO₂ (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO₂ to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO₂. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.