224 research outputs found
In situ power-loss estimation of IGBT modules
No full textA fault detection and prediction method for insulated-gate bipolar transistors (IGBTs) has been improved over the past decades to reduce system downtime. In situ lifetime estimation of IGBT modules has been challenging due to a number of requirements: the necessity to operate at high voltage in the switching environment and the measurement precision of the gate-threshold voltage or collector-to-emitter voltage. This thesis presents a wear-fatigue estimation framework that consists of collector-to-emitter measurement, power loss calculation, and thermal lifetime prediction model. The measurement circuit enables the estimation of power loss across a variety of IGBT modules with minimum impact on system reliability.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2023-05-01The student, Qichen Jin, accepted the attached license on 2021-04-29 at 10:53.The student, Qichen Jin, submitted this Thesis for approval on 2021-04-29 at 10:58.This Thesis was approved for publication on 2021-04-29 at 15:19.DSpace SAF Submission Ingestion Package generated from Vireo submission #16596 on 2021-09-16 at 20:14:39Made available in DSpace on 2021-09-17T04:06:56Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-04-29Embargo set by: Seth Robbins for item 118716
Lift date: 2023-09-17T04:07:01Z
Reason: Author requested closed access (OA after 2yrs) in Vireo ETD systemAuthor requested closed access (OA after 2yrs) in Vireo ETD systemLimite
Ab initio study of electron mean free paths and thermoelectric properties of lead telluride
No full textLast few years have witnessed significant enhancement of thermoelectric figure of merit of lead telluride (PbTe) via nanostructuring. Despite the experimental progress, current understanding of the electron transport in PbTe is based on either band structure calculation using first principles with constant relaxation time approximation or empirical models, both relying on adjustable parameters obtained by fitting experimental data. Here, we report parameter-free first-principles calculation of electron and phonon transport properties of PbTe, including mode-by-mode electron-phonon scattering analysis, leading to detailed information on electron mean free paths and the contributions of electrons and phonons with different mean free paths to thermoelectric transport properties in PbTe. Such information will help to rationalize the use and optimization of nanostructures to achieve high thermoelectric figure of merit
Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers
No full textModern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material-the combination of electrical conductivity and Seebeck coefficient-measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron-acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.United States. Department of Energy. Office of Science. Basic Energy Sciences (Award # SC0001299/DE-FG02-09ER46577 (for fundamental research on electron–phonon interaction in thermoelectric materials))United States. Defense Advanced Research Projects Agency. Materials for Transduction program (Grant HR0011-16-2-0041 (for code development to support practical thermoelectric devices)
Electron mean-free-path filtering in Dirac material for improved thermoelectric performance
No full textRecent advancements in thermoelectric materials have largely benefited from various approaches, including band engineering and defect optimization, among which the nanostructuring technique presents a promising way to improve the thermoelectric figure of merit (zT) by means of reducing the characteristic length of the nanostructure, which relies on the belief that phonons’ mean free paths (MFPs) are typically much longer than electrons’. Pushing the nanostructure sizes down to the length scale dictated by electron MFPs, however, has hitherto been overlooked as it inevitably sacrifices electrical conduction. Here we report through ab initio simulations that Dirac material can overcome this limitation. The monotonically decreasing trend of the electron MFP allows filtering of long-MFP electrons that are detrimental to the Seebeck coefficient, leading to a dramatically enhanced power factor. Using SnTe as a material platform, we uncover this MFP filtering effect as arising from its unique nonparabolic Dirac band dispersion. Room-temperature zT can be enhanced by nearly a factor of 3 if one designs nanostructures with grain sizes of ∼10 nm. Our work broadens the scope of the nanostructuring approach for improving the thermoelectric performance, especially for materials with topologically nontrivial electronic dynamics. Keywords: Dirac material; electrom mean-free-path filtering; thermoelectrics; nanostructuring approach; electron-phonon interaction
Tailoring Superconductivity with Quantum Dislocations
No full textDespite the established knowledge that crystal dislocations can affect a material’s superconducting properties, the exact mechanism of the electron-dislocation interaction in a dislocated superconductor has long been missing. Being a type of defect, dislocations are expected to decrease a material’s superconducting transition temperature (T[subscript c]) by breaking the coherence. Yet experimentally, even in isotropic type I superconductors, dislocations can either decrease, increase, or have little influence on T[subscript c]. These experimental findings have yet to be understood. Although the anisotropic pairing in dirty superconductors has explained impurity-induced T[subscript c] reduction, no quantitative agreement has been reached in the case a dislocation given its complexity. In this study, by generalizing the one-dimensional quantized dislocation field to three dimensions, we reveal that there are indeed two distinct types of electron-dislocation interactions. Besides the usual electron-dislocation potential scattering, there is another interaction driving an effective attraction between electrons that is caused by dislons, which are quantized modes of a dislocation. The role of dislocations to superconductivity is thus clarified as the competition between the classical and quantum effects, showing excellent agreement with existing experimental data. In particular, the existence of both classical and quantum effects provides a plausible explanation for the illusive origin of dislocation-induced superconductivity in semiconducting PbS/PbTe superlattice nanostructures. A quantitative criterion has been derived, in which a dislocated superconductor with low elastic moduli and small electron effective mass and in a confined environment is inclined to enhance T[subscript c]. This provides a new pathway for engineering a material’s superconducting properties by using dislocations as an additional degree of freedom.
Keywords: Dislocations; disordered superconductor; effective field theory; electron-dislocation interactionUnited States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0001299)United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-FG02-09ER46577)United States. Defense Advanced Research Projects Agency (Award HR0011-16-2-0041
Characterisation of prognostic and cardiovascular markers in coronavirus disease 19
No full textCoronavirus disease 19 (COVID-19) is responsible for one of the worst pandemics of our time. Clinical risk stratification plays a pivotal role in guiding patient care decisions, such as admission vs. discharge from the hospital and the allocation of therapeutic resources. The development of novel biomarkers for assessing the prognostic impact of COVID-19 on patients is a clinical priority. This retrospective observational project identified cardiac, inflammatory, and risk-score-based biomarkers, and tested their prognostic value in a UK population of COVID-19 patients encountered during the first wave of the pandemic. The biomarkers included high-sensitivity cardiac troponin T (hs-cTnT), lymphocyte-CRP ratio (LCR), ferritin-lymphocyte ratio (FLR), and the non-invasive pneumonia severity score CRB-65. The results showed that hs-cTnT achieved a high negative predictive value for ruling out inpatient mortality. LCR and FLR were not superior to CRP for predicting adverse outcomes in COVID-19. Low CRB-65 scores showed high negative predictive values for ruling out both fatal and non-fatal outcomes, independent of chest X-ray findings. Five markers were shown to be independent predictors of inpatient mortality (hs-cTnT, oxygen requirement, CRB-65, FLR, and history of ischaemic heart disease). These markers were combined into a new risk score which performed well for predicting mortality in COVID-19 patients. Oxygen requirement was the only independent predictor of escalation to non-invasive ventilation, intubation/ventilation and intensive care unit admissions. Cardiac troponins, CRB-65 and the combined risk score with oxygen requirement deserve further validation for translating into clinically viable risk stratification tools in COVID-19
Ab initio study of electron transport in lead telluride
No full textThesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 91-97).Last few years have witnessed significant enhancement of thermoelectric figure of merit of lead telluride (PbTe) via nanostructures. Despite the experimental progress, current understanding of the electron transport in PbTe is based on either band structure simulated using first-principles in combination with constant relaxation time approximation or empirical models, both requiring adjustable parameters obtained by fitting experimental data. This thesis aims to compute thermoelectric properties of PbTe all from first-principles. We start by discussing the formalism based on Boltzmann transport equation to calculate the electron transport properties in PbTe using first principles and identify the importance to calculate electron-phonon interaction accurately. We then discuss the challenges in studying electron-phonon interaction in semiconductors using first-principles and introduce electron-phonon Wannier interpolation which allows us to calculate the strength of electron-phonon coupling on a very fine mesh. In polar materials like PbTe, the Fröhlich interaction due to long-range dipole field of longitudinal optical phonons contributes to the electron-phonon coupling as well. As the long-range nature of the dipole field makes the standard Wannier interpolation fail, we have discussed the detailed procedures for correction. Next, we study the screening effect of free carriers on electron transport by modulating the polar scattering. These considerations enabled us to report parameter-free first-principles calculation of electron and phonon transport in PbTe, including mode-by-mode electron-phonon scattering, leading to detailed information on electron mean free paths and the cumulative contributions by electrons and phonons with different mean free paths to thermoelectric transport properties in PbTe. Such information will help to rationalize the use and optimization of nanostructures to achieve high thermoelectric figure of merit.by Qichen Song.S.M
Simultaneously high electron and hole mobilities in cubic boron-V compounds: BP, BAs, and BSb
No full textThrough first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs, and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression of polar scattering and the reduction of intervalley transition mediated by large-wave-vector optical phonons, both of which significantly facilitate charge transport. We also discover that BAs simultaneously has a high hole mobility (2110cm[superscript 2]/Vs) and electron mobility (1400cm[superscript 2]/Vs) at room temperature, which is rare in semiconductors. Our findings present insight into searching high mobility polar semiconductors, and point to BAs as a promising material for electronic and photovoltaic devices in addition to its predicted high thermal conductivity.United States. Office of Naval Research. Multidisciplinary University Research Initiative (Grant N00014-16-1-2436
Design and analysis on trusted network equipment access authentication protocol
No full textabstract: Cloud security is a system engineering problem. A common approach to address the problem is to adapt existing Trusted Network Connection (TNC) framework in the cloud environment, which can be used to assess and verify end clients’ system state. However, TNC cannot be applied to network equipment attached to the cloud computing environment directly. To allow the network devices to access the trusted network devices safely and reliably, we first developed a Trusted Network Equipment Access Authentication Protocol (TNEAAP). We use the BAN logic system to prove that TNEAAP is secure and credible. We then configure the protocol in an attack detection mode to experimentally show that the protocol can withstand attacks in the real network. Experiment results show that all the nine goals that decide the protocol’s security have been achieved.NOTICE: this is the author's version of a work that was accepted for publication in SIMULATION MODELLING PRACTICE AND THEORY. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in SIMULATION MODELLING PRACTICE AND THEORY, 51, 157-169. DOI: 10.1016/j.simpat.2014.10.01
Enhanced photocatalytic activity of Fe-doped TiO2 coated on N-doped activated carbon composites for photocatalytic degradation of dyeing wastewater
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