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排序方式: 共有59条查询结果,搜索用时 15 毫秒
1.
Zolper J.C. Shenvin M.E. Baca A.G. Shul R.J. Klem J.F. Hietala V.M. 《Electron Device Letters, IEEE》1994,15(12):493-495
C ion implantation has been employed, for the first time, to form the buried p-layer in GaAs, self-aligned, ion implanted JFETs. Comparable DC performance was seen for JFETs with C or Mg implants; however, C-backside JFETs showed superior high-frequency performance. High dose C-backside devices had a ft of 28.3 GHz and a fmax of 43.2 GHz for a 0.5 μm gate length that were 28% and 46% higher, respectively, than comparable Mg-implanted JFETs. This enhancement is a result of the lower Cgs in the C-backside device resulting from he inherently low activation of the implanted C below the channel while the C still effectively compensated the tail of the Si-channel implant. This approach relaxes the trade-off between optimizing the DC and the AC performance for the buried p-implant in GaAs JFETs and MESFET's 相似文献
2.
Martens J.S. Hietala V.M. Zipperian T.E. Kurtz S.R. Ginley D.S. Tigges C.P. Philips J.M. Newman N. 《Applied Superconductivity, IEEE Transactions on》1992,2(2):111-113
Far-infrared (IR) focal plane arrays are becoming increasingly important for terrestrial and space-based imaging applications. For noise reasons, it is desirable to have transresistance preamplifiers at the focal plane, and, therefore, they need to operate over wide temperature ranges. High-temperature superconducting (HTS) amplifiers that have a high gain, relatively low noise, and response times of <200 ps over at least a 10-80-K temperature range have been developed 相似文献
3.
Susan L. Hietala Douglas M. Smith C. Jeffrey Brinker Alan J. Hurd Altaf H. Carim Neal Dando 《Journal of the American Ceramic Society》1990,73(10):2815-2821
This paper compares the structure ofAl2 O3 -SiO2 xerogels prepared with Al2 O3 contents ranging from 21 to 75 wt% (13 to 64 mol%). The 47 wt% Al2 O3 xerogel (Al/Si ≅ 1) exhibits anomalously low surface area (≅1 m2 /g) and skeletal density compared with other Al2 O3 -Si2 compositions. Based on the results of nitrogen adsorption/condensation, helium displacement, high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS), we attribute the low surface area and skeletal density (density of the solid phase which is inaccessable to helium) to the formation of closed micropores, whereas the higher surface area materials exhibit a slightly coarser texture comprising open pores with radii of ≅1 nm. X-ray diffraction (XRD) and 29 SI and 27 Al magic-angle spinning nuclear magnetic resonance (MASNMR) indicate no anomalous behavior in the 47% sample on molecular length scales. HRTEM indicates the presence of a small fraction of crystallites which is supported by the SAXS results, but it is unknown if this crystallinity is related to low surface area. Low-field 1 H NMR spin-lattice relaxation measurements show that the physical structure of all of the 'wet' gels is similar implying that pore closure occurs during drying. Consistent with this idea, gel surface area and density increased significantly when the pore fluid (water) was replaced with a lower surface tension fluid (formamide, dioxane, ethanol. water/surfactant). 相似文献
4.
Kari mnkknen Tuula T. Pakkanen Joni Hietala Esko J. Pkknen Pertti Pkknen Timo Jskelinen Terho Kaikuranta 《Polymer Engineering and Science》2002,42(7):1600-1608
A comprehensive experimental study was carried out to replicate sub‐micron features using the injection molding technique. For the experiments, five different plastic materials were selected according to their flow properties. The materials were polycarbonate (PC), styrene‐butadiene block copolymer (SBS), impact modified poly(methyl methacrylate), methyl methacrylate‐acrylonitrile‐butadiene‐styrene polymer (MABS), and cyclic olefin copolymer (COC). Nanofeatures down to 200‐nm line width and with aspect ratios (aspect ratio = depth/width) of 1:1 could be replicated. In all selected materials, the greatest differences between the materials emerged when the aspect ratio increased to 2:1. The most favorable results were obtained with the use of high flow polycarbonate as the molding material. The best replication results were achieved when melt and mold temperatures were higher than normal values. 相似文献
5.
Martens J.S. Hietala V.M. Zipperian T.E. Vawter G.A. Ginley D.S. Tigges C.P. 《Applied Superconductivity, IEEE Transactions on》1992,2(2):74-78
The authors report the fabrication of long, step-edge junctions using TlCaBaCuO with associated control lines that demonstrate strong modulation of critical current, usable current gains, large power gains, and fairly large bandwidths. The devices are operational at up to 95 K and RF measurements have been made at 77 K at frequencies up to a probe-limited 10 GHz. These four terminal devices have output impedances of 2-20 Ω, small signal current gains greater than 2.5, available power gains of over 10 dB from 0.5 to 10 GHz, and minimum noise figures of less than 1 dB 相似文献
6.
M Kuoppam?ki EP P?lvim?ki E Syv?lahti J Hietala 《Canadian Metallurgical Quarterly》1994,255(1-3):91-97
Chronic treatment with clozapine (14 days; 10 and 25 mg/kg/day) decreases 5-HT1C receptor density but not affinity in rat choroid plexus measured with [3H]mesulergine. We now report the effects of the same clozapine treatment regimens on the function of 5-HT1C receptors (measured by maximal stimulation of 5-HT1C receptor-mediated phosphoinositide hydrolysis) in relation to receptor changes in rat choroid plexus. Quantitative 5-HT1C receptor autoradiography indicated that chronic clozapine treatment decreased, in a dose-related manner, 5-HT1C receptor binding sites labeled by antagonist ([3H]mesulergine) and agonist ([125I](+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, [125I]DOI) radioligands. However, only the higher dose of clozapine decreased statistically significantly the maximal 5-HT1C receptor-mediated phosphoinositide hydrolysis response. Chronic administration of haloperidol (0.5 mg/kg/day) did not change any of the 5-HT1C receptor parameters. In conclusion, chronic clozapine treatment is able to modulate the function of 5-HT1C receptors. This further strengthens the possibility that 5-HT1C receptors may contribute to some of the atypical effects of clozapine. 相似文献
7.
Sherwin M.E. Zolper J.C. Baca A.G. Shul R.J. Howard A.J. Rieger D.J. Klem J.F. Hietala V.M. 《Electron Device Letters, IEEE》1994,15(7):242-244
An all implanted self-aligned n-channel JFET fabrication process is described where Zn implantation is used to form the p+ gate region. A refractory metal (W) gate contact is used to allow subsequent high temperature activation of the self-aligned Si source and drain implant. 0.7 μm JFET's have a maximum transconductance of 170 mS/mm with a saturation current of 100 mA/mm at a gate bias of 0.9 V. The p+/n homojunction gate has a turn on voltage of 0.95 V at a current of 1 mA/mm. The drain-source breakdown voltage is 6.5 V. Microwave measurements made at a gate bias of 1 V show an ft of 19 GHz with an fmax of 36 GHz. These devices show promise for incorporation in both DCFL and complementary logic circuits 相似文献
8.
Martens J.S. Hietala V.M. Zipperian T.E. Ginley D.S. Tigges C.P. Phillips J.M. 《Microwave Theory and Techniques》1991,39(12):2018-2025
Microwave two-port S-parameter measurements and modeling of superconducting flux flow transistors are presented. The transistors, based on the magnetic control of flux flow in any array of high temperature superconducting weak links, exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive while the output impedance is both resistive and inductive. It is shown that the characteristics of these devices are useful in numerous applications including matched amplifiers, phase shifters and active impedance convertors 相似文献
9.
Rheological properties of aqueous solutions and hydrogels formed by an amphiphilic star block copolymer, poly(acrylic acid)-block-polystyrene (PAA54-b-PS6)4, were investigated as a function of the polymer concentration (Cp), temperature, and added salt concentration. The water-soluble polymer synthesised by atom transfer radical polymerization (ATRP) was found to form hydrogels at room temperature at polymer concentrations, Cp, over 22 g/L due to the interpolymer hydrophobic association of the PS blocks. Increasing Cp leads to stronger elastic networks at room temperature that show a gel-to-solution transition with increasing temperature. Increase of ionic strength decreases the moduli compared with the pure hydrogel but did not affect the gel-sol transition temperature significantly. Small-angle X-ray experiments showed two distinct scattering correlation peaks for samples above the gelling Cp, which indicates the aggregates formed due to hydrophobic association. Upon heating the intensity of the scattering correlation peaks was found to decrease indicating the loss of the network structure due to thermal motion. 相似文献
10.
Tommy Munk Stefania Baldursdottir Sami Hietala Thomas Rades Markus Nuopponen Katriina Kalliomäki Heikki Tenhu Jukka Rantanen Clare J. Strachan 《Polymer》2013
The use of vibrational spectroscopy to investigate complex structural changes in polymers yields chemically rich data, but interpretation can be challenging and subtle but meaningful spectral changes may be missed through visual inspection alone. Multivariate analysis is an efficient approach to gain an oversight of small but systematic spectral differences anywhere within the spectra, providing further insight into structural changes and associated transformation mechanisms. In this study, the novel analytical approach of infrared spectroscopy combined with principal component analysis and Gaussian peak fitting was used to investigate the structural changes in aqueous solutions of a polymer, using poly(N-isopropyl acrylamide) (PNIPAM) in the atactic form and with controlled tacticity as a model system. Subtle spectral changes associated with the dehydration and phase separation upon heating included peak shifts, an area ratio change of the amide I band to the amide II band and formation of a new peak in the amide I band were efficiently detected. Dehydration and phase separation of PNIPAM occurred in two temperature ranges, one for the atactic and one for isotactic rich part, both involving a complex re-organization of the hydrogen bonds and change of the hydration layer. The changes agreed with existing results from other techniques, and new insights were gained into the effect of controlled tacticity on phase transformation behaviour. The study demonstrates that infrared spectroscopy combined with the multivariate analytical method principal component analysis and Gaussian peak fitting is an efficient approach to probing structural change in polymers during heating. The simplicity of the presented approach could find excellent use in analysing and understanding the molecular environment of a range of stimuli-responsive polymers, for instance block or grafted types of polymers, as well as those with controlled tacticity. 相似文献