Monte Carlo study of multibarrier heterostructure switch

A multibarrier heterostructure GaAs/AlAs current switching diode has been investigated by Monte Carlo method. The switching phenomenon is based on electron tunnelling and thermoemission from the GaAs wells to the AlAs barriers, electron drift across the thin AlAs barriers followed by a subsequent impact ionisation in the undoped GaAs layers. The calculated switching voltage is close to 100 V for the diode involving four AlAs barriers at 300 K lattice temperature. The estimated switching time is in the order of 10 ps

Shot Noise in Mesoscopic Conductors

Theoretical and experimental work concerned with dynamic fluctuations has developed into a very active and fascinating subfield of mesoscopic physics. We present a review of this development focusing on shot noise in small electric conductors. Shot noise is a consequence of the quantization of charge. It can be used to obtain information on a system which is not available through conductance measurements. In particular, shot noise experiments can determine the charge and statistics of the quasiparticles relevant for transport, and reveal information on the potential profile and internal energy scales of mesoscopic systems. Shot noise is generally more sensitive to the effects of electron-electron interactions than the average conductance. We present a discussion based on the conceptually transparent scattering approach and on the classical Langevin and Boltzmann-Langevin methods; in addition a discussion of results which cannot be obtained by these methods is provided. We conclude the review by pointing out a number of unsolved problems and an outlook on the likely future development of the field.Comment: 99 two-column pages; 38 .eps figures included. Submitted to Physics Reports. Many minor improvements; typos corrected; references added and update

Optical pump rectification emission: route to terahertz free-standing surface potential diagnostics

We introduce a method for diagnosing the electric surface potential of a semiconductor based on THz surface generation. In our scheme, that we name Optical Pump Rectification Emission, a THz field is generated directly on the surface via surface optical rectification of an ultrashort pulse after which the DC surface potential is screened with a second optical pump pulse. As the THz generation directly relates to the surface potential arising from the surface states, we can then observe the temporal dynamics of the static surface field induced by the screening effect of the photo-carriers. Such an approach is potentially insensitive to bulk carrier dynamics and does not require special illumination geometries

Disclosing water-energy-economics nexus in shale gas development

Shale gas has gained importance in the energy landscape in recent decades. However, its development has raised environmental concerns, especially, those associated with water management. Thus, the assessment of water management aspects, which inevitably impact the economic aspects, is crucial in evaluating the merits of any project exploiting this energy source. This paper provides a review of the economic and environmental implications of shale gas development around the world. Furthermore, to demonstrate the interplay between the various technical, environmental and economic factors in concrete terms, we report on a specific set of case studies conducted using an integrated decision-support tool that has been implemented to model and optimize shale gas development projects. The case study results confirm that the gas breakeven price decreases with expansion in scale of the shale gas development, i.e. increasing the number of well-pads in the system. However, scale also increases the options for water re-use and recycle in drilling and fracturing operations, which can result in lower freshwater withdrawal intensity. Moreover, under water scarcity scenarios, the choice of well-pad designs that are inherently less water intensive was found to be more cost-effective than water re-use or/and recycle strategies at reducing net freshwater demand. Similar trends were observed when the impact of wastewater quality, i.e. total dissolved solids concentration, on the optimal development strategy of shale gas plays was investigated. The results of these case studies reveal that greater efforts are needed at characterizing freshwater availability and wastewater quality for the evaluation of both the economic and environmental aspects of shale gas development

Capsule-Based Dropwise Additive Manufacturing with Pharmaceutical Suspensions

Current manufacturing of pharmaceutical products focuses on creating a standard dosage of the active pharmaceutical ingredient (API); however, dosages often need to be altered or customized to account for a patient’s age, weight, comorbidity, and other genetic factors. A potential method for dispensing precise dosages of API suspensions through dropwise addition is detailed in the following paper. By using a drop-on-demand printing rig, a series of suspensions comprised of varying volume fractions of a micron-scale API in a carrier fluid were printed, and individual drop volumes were analyzed using high-resolution imaging. From this, capsules with 1 mg dosages and 100 mg dosages were manufactured. Completed trials yielded respective means of 1.043 mg and 99.946 mg of API being deposited across varying suspension compositions. The relative standard deviations of the 1 mg capsules averaged to be 1.51% and 0.30% for the 100 mg capsules. Further combinations of APIs and carrier fluids are continuing to be tested. The relative standard deviations of both dosage sizes are well under the 6% maximum variability imposed by the US Food and Drug Administration to regulate dosages of API, which provides evidence for the feasibility of printing pharmaceutical suspensions to create customized dosages for patient consumption

Resilience and risk analysis of fault-tolerant control design in continuous pharmaceutical manufacturing

PresentationThe effects of the paradigm shift from batch to continuous manufacturing on pharmaceutical industry, in terms of process safety and product quality, e.g., danger of dust explosions and risk of off-spec products, are of major concerns in the recent research progress in control system design. Specifically, a fault-tolerant control of critical process parameters (CPPs) and critical quality attributes (CQAs) is of paramount importance for the continuous operation with built-in safety and quality. In this study, a systematic framework for fault-tolerant control design, analysis, and evaluation for continuous pharmaceutical solid-dosage manufacturing is proposed, consisting of system identification, control design and analysis (controllability, stability, resilience, etc.), hierarchical three-layer control structures (model predictive control, state estimation, data reconciliation, etc.), risk mapping, assessment and planning (Risk MAP) strategies, and control performance evaluation. The key idea of the proposed framework is to identify the potential risks in the control design, material variance, and process uncertainties, under which the control strategies are evaluated. The framework is applied to a continuous direct compaction process, specifically the feeding-blending system wherein the major source of variance in the process operation and product quality arises. It can be demonstrated that the process operation failures and product quality variances in the feeding-blending system can be mitigated and managed through the proposed systematic fault-tolerant control system design and risk analysis framework