To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure firstname.lastname@example.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
An apparatus design is described. It is for detecting VHE gamma ray point sources by means of the atmospheric Cerenkov technique. Obviously, the improvement of flux sensitivity and discrimination between gamma ray and isotropic proton showers is still a key problem. Of course, it is necessary to set up more observatories and to track an object continuously with several facilities. With this in mind, we decided to develop an experiment for observing VHE gamma ray sources in China. As a first step, we will set up an apparatus which consists of three 1.5 m diameter searchlight morrors at Xinglong station of Beijing Observatory, Xinglong county, Hebei province (40°. 4N, 117°.5E, altitude 940 m). The observation will start in 1988. Then, the second apparatus will be set up at Delingha station of Purple Mountain Observatory, Delingha county, Qinghai porvince (37°.22N, altitude 3204 m). Both the sites are far from air and light pollution, and have suitable meteological condition for Cerenkov light detection as well as quite convenient facilities for transportation. Some probable technical improvements are also discussed in this paper.
In this study, we investigated thermotolerance, several physiological responses and damage to reproductive cells in chlorpyrifos-resistant (Rc) and -susceptible (Sm) strains of the diamondback moth, Plutella xylostella subjected to heat stress. The chlorpyrifos resistance of these strains was mediated by a modified acetylcholinesterase encoded by an allele, ace1R, of the ace1 gene. Adults of the Rc strain were less heat resistant than those of the Sm strain; they also had lower levels of enzymatic activity against oxidative damage, higher reactive oxygen species contents, weaker upregulation of two heat shock protein (hsp) genes (hsp69s and hsp20), and stronger upregulation of two apoptotic genes (caspase-7 and -9). The damage to sperm and ovary cells was greater in Rc adults than in Sm adults and was temperature sensitive. The lower fitness of the resistant strain, compared with the susceptible strain, is probably due to higher levels of oxidative stress and apoptosis, which also have deleterious effects on several life history traits. The greater injury observed in conditions of heat stress may be due to both the stronger upregulation of caspase genes and weaker upregulation of hsp genes in resistant than in susceptible individuals.
This is the first report of microsatellite markers (simple sequence repeats, SSR) for fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), an important quarantine pest in some European and Asian countries. Here, we developed 48 microsatellite markers for H. cunea from SSR enrichment libraries. Sequences isolated from libraries were sorted into four categories and analyzed. Our results suggest that sequences classified as Grouped should not be used for microsatellite primer design. The genetic diversity of microsatellite loci was assessed in 72 individuals from three populations. The number of alleles per locus ranged from 2 to 5 with an average of 3. The observed and expected heterozygosities of loci ranged from 0 to 0.958 and 0 to 0.773, respectively. A total of 18 out of 153 locus/population combinations deviated significantly from Hardy–Weinberg equilibrium. Moreover, significant linkage disequilibrium was detected in one pair of loci (1275 pairs in total). In the neutral test, two loci were grouped into the candidate category for positive selection and the remainder into the neutral category. In addition, a complex mutation pattern was observed for these loci, and FST performed better than did RST for the estimation of population differentiation in different mutation patterns. The results of the present study can be used for population genetic studies of H. cunea.
This study investigates characteristics of p-n junction diodes fabricated by silicidation through a silicon buffer layer and dopant drive-out process. The purpose of using the buffer silicon layer is to reduce silicon consumption from the Si substrate during silicidation, and thus reduce the effective junction depth. The resulting structure is suitable for elevated CoSi2 source/drain contact in a metal-oxide-semiconductor field effect transistor or a silicided polysilicon emitter in a bipolar junction transistor. It was found that boron diffusion is enhanced by these buffer layers comparing to silicided diodes without silicon buffer layers. The sheet resistance of the CoSi2/polysilicon/Si structure does not degrade as seriously as CoSi2/polysilicon/oxide structure. The diode leakage current density is higher compared to diodes without buffer layers, especially when thinner buffer layers and high temperature 1000°C anneal are used.
Phase formations in Co thin films (200Å in thickness) reacting with atomically clean Si(100), Ge(100), and Si0.80Ge0.20 epitaxial layer (800Å in thickness on Si(100) substrates) in UHV have been investigated. For the Co/Si system, it is found that CoSi (FeSi structure) is formed at 375°C through a very disordered CoSi phase, and the final CoSi2 phase is formed at 575°C. On the other hand, the Co5Ge7 phase was identified for the Co/Ge samples annealed at 300°C and 450°C and the final CoGe2 phase is formed at 600°C. For the Co/Si0.8 Ge0.20 samples annealed from 400°C to 600°C, Co(Si1−yGey) phases with y∼0.10 were detected, and for annealing at 700°C, only the CoSi2 phase was formed. These results indicate a preferential Co- Si reaction when annealing the Co/SiGe structure. It was also found that the sheet resistance of the reacted thin films depend strongly on annealing temperature.
Solid phase amorphization has been found to occur in all refractory metal and a number of rare—earth and platinum group metal thin film on silicon systems. For Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si and Ta/Si systems, the growth of amorphous interlayer (a—interlayer) was found to follow a linear law in the initial stage. Si atom was found to be the dominant diffusing species in the solid phase amorphization in Ti/Si, Zr/Si and Hf/Si systems. For the Y/Si system, the stability of amorphous interlayer depends critically on the composition of the amorphous films.
Auto–correlation function analysis was utilized to determine the structure of the amorphous interlayers. HRTEM in conjunction with the fast Fourier transform were applied to determine the first nucleated crystalline phase. Simultaneous presence of multiphases was observed to occur in a number of refractory metal/Si systems. Interesting electrical properties of amorphous interlayers were found for Ti/Si, Zr/Si and Hf/Si systems.
The high temperature solid phase reaction of Ti with SixGe1−x produces a low resistivity titanium germanosilicide which is isomorphic with the C54 phase of TiSi2 and TiGe2. The composition of the final C54 Ti(SiyGe1−y)2 film is dependent on the composition of the initial Si-Ge alloy and on the annealing conditions. The intermediate phases of the Ti-Si and Ti-Ge reactions are C49 TiSi2 and Ti6Ge5 respectively. The reaction path of Ti - SixGe1−x shifts from that of Ti-Si to that of Ti-Ge as the SixGe1−x alloy composition changes (×=1→0). Phase separations were observed at low temperatures for Ti reactions with Si-Ge alloys and the C54 formation temperature was observed to decrease as the Si-Ge alloy composition approached Si.5Ge.5. Surface and interface morphologies were examined using SEM and TEM. The formation of smooth, large grain, low resistivity films has been observed for the reaction of Ti with low Ge content alloys (x≥0.7). As germanium content is increased the formation of faceted islands is observed. Reactions with high Ge content alloys (x≤0.3) produce films with morphologies similar to those of the Ti-Ge reaction.
Light-emitting diodes (LEDs) based on p-GaN/ZnO heterojunction were fabricated. GaN was deposited on sapphire using metal-organic chemical vapor deposition (MOCVD), and two kinds of ZnO i.e. ZnO thin film deposited by sputtering and ZnO nanorods (NRs) grown by hydrothermal method were used as n-type layer respectively. MgO film with the thickness around 10 nm was deposited by electron-beam deposition to act as an interlayer between GaN and ZnO. Photoluminescence, electroluminescence and I-V curves were measured to compare the properties of GaN based heterojunction LEDs with different architectures. The existence of MgO interlayer as well as the morphology of ZnO obviously influenced the electrical and optical properties of GaN based LEDs. The effect of MgO interlayer on ZnO growth, properties and I-V curves and emission spectra of LEDs is discussed in detail.
The thermodynamic La–Sr–Mn–Cr–O oxide database is obtained as an extension of thermodynamic descriptions of oxide subsystems using the calculation of phase diagrams approach. Concepts of the thermodynamic modeling of solid oxide phases are discussed. Gibbs energy functions of SrCrO4, Sr2.67Cr2O8, Sr2CrO4, and SrCr2O4 are presented, and thermodynamic model parameters of La–Sr–Mn–Chromite perovskite are given. Experimental solid solubilities and nonstoichiometries in La1−xSrxCrO3−δ and LaMn1−xCrxO3−δ are reproduced by the model. The presented oxide database can be used for applied computational thermodynamics of traditional lanthanum manganite cathode with Cr-impurities. It represents the fundament for extensions to higher orders, aiming on thermodynamic calculations in noble symmetric solid oxide fuel cells.
The high Al content AlGaN epilayers have been obtained by metalorganic chemical vapor deposition (MOCVD), and the optical property has been investigated by photoluminescence (PL) spectroscopy. Longitudinal-optic (LO) phonon mode has been studied by Raman scattering. Further analysis shows that the edge dislocation is an important factor influencing optical quality of AlGaN epilayers, and it also shows that the correlation between the A1 (LO) polar modes and the edge dislocation is intensive, which may be expected to become a characterization method of the related crystal defects.
The influence of pressure on the MOCVD grown InAlN/AlN/GaN heterostructure has been investigated by high-resolution X-ray diffraction, Hall measurement and atomic force microscopy. High pressure is beneficial to increase indium incorporation efficiency. The electrical properties of InAlN/AlN/GaN heterostructure become better with the pressure decreasing from 100 Torr to 50 Torr. Indium droplets tend to form on the InAlN surface at high pressure. The edge of the indium droplet is the Al-rich region while the interior is the In-rich region, demonstrated by the phase-contrast mode. Phase contrast across the V-defect is strong on the surface of InAlN grown at low pressure (50 Torr) whereas it is not evident at high pressure (100 Torr), indicating that large stress in the InAlN film will enhance the compositional variation.
The efficiency of acousto-optic interaction in single-mode strip silica waveguide is analyzed theoretically by determining the overlap integral between the optical and acoustic field distributions. The results show that there is a good overlap of the optical and SAW fields in the low SAW frequency range. At high acoustic frequencies, the overlap integral decreases with increasing acoustic frequency. At 216 MHz, the maximum of 0.8544 for the overlap integral is obtained, provided the H/Λ equals 0.02. At last, the diffraction efficiencies for acoustic frequency of 216 MHz are calculated as a function of the square root of acoustic power for different acoustic apertures.
The I-V characteristics of AlGaN/GaN high electron mobility transistors in the temperature range between 100 K and 300 K are studied. It is found that both the maximum drain-source current and transconductance decrease with the increase of temperature. Decrease of the electron mobility with increasing temperature is considered to be the main cause for that condition. The threshold voltage shows a forward shift, which can be explained by the increase of Schottky barrier with increasing temperature. It is found that at VGS = 0 V the drain-source current reduces with the ascending temperature, which should be due to the variation of the electron mobility with the temperature. While at VGS = −5 V the drain-source current is found to increase with the ascending temperature, it is suggested to be caused by the positive temperature coefficient of the electron transport in the depleted region.
The temperature dependence of the I-V characteristics on Au/Ni-HEMT Schottky contacts was measured and analyzed. Large deviations from the thermionic emission and thermionic-field emission model were observed in the I-V-T characteristics. The thin surface barrier model only fits the measured curves in the high bias region, but deviates drastically in the low bias region. Using a revised thin surface barrier model, the calculated curves match well with the measured curves. It is also found that tunneling emission model is the dominant current transport mechanism at low temperature, yet thermionic-field emission model is the dominant current transport mechanism at high temperature.
Controlling the characteristics of nanowires in order to later construct nanoarchitecture and nanocomponents for nanodevice and nanosensor applications is essential. Metal oxide nanowires are aligned using the Langmuir-Blodgett (LB) technique to uniaxially compress the nanowires. A surfactant monolayer of metal oxide nanowires is fabricated, and then compressed on an aqueous subphase. The compression yields an array of aligned nanowires, which is transferred to a planar substrate. Cutting areas of the array are defined by electron beam nanolithography. After an etching process, it is shown that the metal oxide nanowires have been successfully cut. With further refinement of this technique, the nanowires can be used to construct basic building blocks of nanodevices and nanosensors.
We are of the opinion that students of an introductory materials science and engineering course should gain a thorough understanding of crystallographic core concepts by applying them quasi-experimentally in computer simulation sessions that run parallel to the lectures. Software simulations of goniometry of direct lattice vectors in a transmission electron microscope (TEM) will serve two purposes at once: to introduce students to practical aspects of electron microscopy and support their comprehension of crystallographic core concepts. We use the programming software Matlab and Java (Jmol applets) on a PC platform for the creation of software simulations that demonstrate this methodology and complement already existing software simulations. The newly created software is used in classroom demonstrations of an introductory materials science and engineering course at Portland State University and will become freely accessible over the internet. This software will also support and promote image-based nanocrystallography in TEM.
The National Science Foundation's National Science Digital Library (NSDL) Program is a premier collective portal of authoritative scientific resources supporting education and research. With funding from NSF, the Materials Digital Library (MatDL) is a collaborative project being developed by the National Institute of Standards and Technology's Materials Science and Engineering Laboratory (NIST/MSEL), the Department of Materials Science and Engineering at the Massachusetts Institute of Technology (MIT), the Department of Chemical Engineering and the Department of Materials Science and Engineering at the University of Michigan (U-M), with Kent State University and University of Colorado at Boulder providing the materials science informatics and workflow technology backbone. As part of the NSDL program, MatDL aims to supports the interface of materials science information and its cognate disciplines, with an emphasis on soft matter. Initial content of MatDL begins with resources selected from NIST/MSEL. Students and faculty in three types of materials science and engineering (MSE) courses at MIT and U-M are taking part in a pilot to use and contribute to MatDL utilizing domain-specific authoring tools. Given the central and interdisciplinary role of materials science in science and engineering, two goals of MatDL are to: 1.) expand its founding partnership with additional participants from the MSE community; and 2.) facilitate the flow of digital materials related knowledge from laboratories where the most recent research discoveries are taking place to the classrooms where new scientists are being trained.
We describe the development of a graduate level course designed to teach computational materials science and its application to nanoscale science and engineering. We discuss the use of MatDL, a web-based digital library and materials science resource, as a collaborative learning tool within the context of the course.
Shape memory alloys (SMAs) are unique materials that effectively capture the attention of students due to their dramatic phase transformations that result from temperature or stress. The fascinating properties and intriguing applications of SMAs can entice students to learn about the materials field, as well as relatively complicated materials concepts. SMAs have been incorporated into a range of courses under a variety of topics, such as crystal structures, phase transformations, kinetics, constitutive relations, and smart materials. The concepts can be presented at different levels of knowledge, appropriate to the learning objectives for the particular audience. Several educational activities using NiTi shape memory alloys have been developed, such as web-based videos, shape setting of new designs, classroom demonstration of actuation, visualization of stress-induced transformations, and heat treatments to change transformation temperatures.