Modeling of ultrasonic processing

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.Includes bibliographical references (leaves 53-55).This paper presents a finite element analysis (FEA) of ultrasonic processing of an aerospace-grade carbon-epoxy composite laminate. An ultrasonic (approximately 30 kHz) loading horn is applied to a small region at the laminate surface, which produces a spatially nonuniform strain energy field within the material. A fraction of this strain energy is dissipated during each ultrasonic loading cycle depending on the temperature- dependent viscoelastic response of the material. This dissipation produces a rapid heating, yielding temperature increases over 100⁰C in approximately Is and permitting the laminate to be consolidated prior to full curing in an autoclave or other equipment. The spatially nonuniform, nonlinear, and coupled nature of this process, along with the large number of experimental parameters, makes trial-and-error analysis of the process intractable, and the FEA approach is valuable in process development and optimization.by Wenguang Zhao.S.M

Signatures of Emerging Subsurface Structures in Acoustic Power Maps

We show that under certain conditions, subsurface structures in the solar interior can alter the average acoustic power observed at the photosphere above them. By using numerical simulations of wave propagation, we show that this effect is large enough for it to be potentially used for detecting emerging active regions before they appear on the surface. In our simulations, simplified subsurface structures are modeled as regions with enhanced or reduced acoustic wave speed. We investigate the dependence of the acoustic power above a subsurface region on the sign, depth, and strength of the wave speed perturbation. Observations from the Solar and Heliospheric Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the emergence of NOAA active region 10488 are used to test the use of acoustic power as a potential precursor of magnetic flux emergence.Comment: 7 pages, 5 figures, accepted for publication in Solar Physics on 21 March 201

Photographic style transfer

© 2018, The Author(s). Image style transfer has attracted much attention in recent years. However, results produced by existing works still have lots of distortions. This paper investigates the CNN-based artistic style transfer work specifically and finds out the key reasons for distortion coming from twofold: the loss of spatial structures of content image during content-preserving process and unexpected geometric matching introduced by style transformation process. To tackle this problem, this paper proposes a novel approach consisting of a dual-stream deep convolution network as the loss network and edge-preserving filters as the style fusion model. Our key contribution is the introduction of an additional similarity loss function that constrains both the detail reconstruction and style transfer procedures. The qualitative evaluation shows that our approach successfully suppresses the distortions as well as obtains faithful stylized results compared to state-of-the-art methods

Large oxygen-isotope effect in Sr_{0.4}K_{0.6}BiO_{3}: Evidence for phonon-mediated superconductivity

Oxygen-isotope effect has been investigated in a recently discovered superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with ^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K, corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c dependencies of alpha_{O} observed in bismuthates and cuprates suggest that bismuthates should belong to conventional phonon-mediated superconductors while cuprates might be unconventional supercondutors.Comment: 9 pages, 5 figure

Approximate Solution of the effective mass Klein-Gordon Equation for the Hulthen Potential with any Angular Momentum

The radial part of the effective mass Klein-Gordon equation for the Hulthen potential is solved by making an approximation to the centrifugal potential. The Nikiforov-Uvarov method is used in the calculations. Energy spectra and the corresponding eigenfunctions are computed. Results are also given for the case of constant mass.Comment: 12 page

Comparison of 35 and 50 {\mu}m thin HPK UFSD after neutron irradiation up to 6*10^15 neq/cm^2

We report results from the testing of 35 {\mu}m thick Ultra-Fast Silicon Detectors (UFSD produced by Hamamatsu Photonics (HPK), Japan and the comparison of these new results to data reported before on 50 {\mu}m thick UFSD produced by HPK. The 35 {\mu}m thick sensors were irradiated with neutrons to fluences of 0, 1*10^14, 1*10^15, 3*10^15, 6*10^15 neq/cm^2. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particles (MIPs) from a 90Sr \b{eta}-source. The leakage current, capacitance, internal gain and the timing resolution were measured as a function of bias voltage at -20C and -27C. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both. Within the fluence range measured, the advantage of the 35 {\mu}m thick UFSD in timing accuracy, bias voltage and power can be established.Comment: 9 pages, 9 figures, HSTD11 Okinawa. arXiv admin note: text overlap with arXiv:1707.0496

Time--Distance Helioseismology Data Analysis Pipeline for Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results

The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) provides continuous full-disk observations of solar oscillations. We develop a data-analysis pipeline based on the time-distance helioseismology method to measure acoustic travel times using HMI Doppler-shift observations, and infer solar interior properties by inverting these measurements. The pipeline is used for routine production of near-real-time full-disk maps of subsurface wave-speed perturbations and horizontal flow velocities for depths ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic maps for the subsurface properties are made from these full-disk maps. The pipeline can also be used for selected target areas and time periods. We explain details of the pipeline organization and procedures, including processing of the HMI Doppler observations, measurements of the travel times, inversions, and constructions of the full-disk and synoptic maps. Some initial results from the pipeline, including full-disk flow maps, sunspot subsurface flow fields, and the interior rotation and meridional flow speeds, are presented.Comment: Accepted by Solar Physics topical issue 'Solar Dynamics Observatory

Modeling phonon-polariton generation and control in ferroelectric crystals

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 101-109).In this thesis, we present simulations, using Finite Element Method (FEM), of phonon-polariton generation and coherent control in ferroelectric crystals LiNbO₃ and LiTaO₃ through nonlinear electro-optic interactions with ultrashort laser pulses. This direct space-time monitoring platform is used to investigate the nature of the excitation mechanism, the science of propagation in patterned structure, and the waveform control via multi-dimensional pulse shaping. Compared with previous simulation methods, this platform demonstrates considerable improvement in complex domain by achieving varied accuracy over space based on the level of interest of the region, which may facilitate scientific exploration in high power terahertz generation and polaritonic signal processing.by Zhao Chen.S.M

Electric field engineering in GaN high electron mobility transistors

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2008.Includes bibliographical references (leaves 66-70).In the last few years, AlGaN/GaN high electron mobility transistors (HEMTs) have become the top choice for power amplification at frequencies up to 20 GHz. Great interest currently exists in industry and academia to increase the frequency to mm-wave frequencies. The goal of this thesis has been to identify new solutions to some of the main challenges to increase this frequency performance even further. Electron velocity is a critical parameter affecting the transistor performance. In standard GaN transistors, the extremely high electric fields present in the channel of the device reduce the average electron velocity well below the peak electron velocity, resulting in low cutoff frequencies. In this thesis, we introduced a partial recess in the drain access region of the transistor to engineer the electric field along the channel of the device without introducing parasitic capacitances. By reducing the peak electric field, the average electron velocity is increased by 50%. This new technology has the potential to improve not only the cutoff frequencies, but also the breakdown voltage of GaN transistors. To successfully engineer the electric field in GaN devices, an accurate, reliable and low damage etching technology is needed. However none of the traditional GaN dry etching technologies meets these requirements. This lack of suitable technology has motivated us to develop a new atomic layer etching technique of AlGaN/GaN structures. This technology has been shown to be a self limited process with very high reliability and low damage, which will be very useful both in electric field engineering and gate recess. Finally, another factor hindering GaN HEMTs from competing with InGaAs devices at high frequencies are their high parasitic capacitances and resistances. In this thesis, ohmic drain contacts are replaced with Schottky drain contacts to reduce the drain access resistance.(cont) ADS simulations predict a very significant increase in the cutoff frequencies by virtue of the lowered parasitic resistances. In conclusion, the theoretical and experimental work developed during this project has demonstrated the great potential of three new technologies to overcome the main challenges of mm-wave GaN HEMTs. The application of these technologies to actual devices is under way and it will represent an important element of the ultra-high GaN transistors of the future.by Xu Zhao.S.M

Formation of delta ferrite in 9 wt.% Cr steel investigated by in-situ X-ray diffraction using synchrotron radiation

In-situ X-ray diffraction (XRD) measurements using high energy synchrotron radiation were performed to monitor in real time the formation of delta ferrite in a martensitic 9 wt pct chromium steel under simulated weld thermal cycles. Volume fractions of martensite, austenite, and delta ferrite were measured as a function of temperature at a 10 K/s heating rate to 1573 K (1300 °C) and subsequent cooling. At the peak temperature, the delta ferrite concentration rose to 19 pct, of which 17 pct transformed back to austenite on subsequent cooling.Max Kade Foundation, Inc.Austrian Academy of SciencesUnited States. Dept. of Energy (Division of Materials Sciences and Engineering, Office of Science, and Office of Basic Energy Sciences