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91.
Wang Y Metzger K Stephens DN Williams G Brownlie S O'Donnell M 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2003,50(7):805-823
In this paper, a scheme called coded excitation with spectrum inversion (CEXSI) is presented. An established optimal binary code whose spectrum has no s and possesses the least variation is encoded as a burst for transmission. Using this optimal code, the decoding filter can be derived directly from its inverse spectrum. Various transmission techniques can be used to improve energy coupling within the system pass-band. We demonstrate its potential to achieve excellent decoding with very low (<80 dB) side-lobes. For a 2.6 /spl mu/s code, an array element with a center frequency of 10 MHz and fractional bandwidth of 38%, range side-lobes of about 40 dB have been achieved experimentally with little compromise in range resolution. The signal-to-noise ratio (SNR) improvement also has been characterized at about 14 dB. Along with simulations and experimental data, we present a formulation of the scheme, according to which CEXSI can be extended to improve SNR in sparse array imaging in general. 相似文献
92.
Sepiolite of Turkish origin was used as Si precursor in the syntheses of silicon nitride (Si3N4) powders by carbothermal reduction-nitridation (CRN) by mixing with several reducing agents i.e. charcoal, carbon black and petroleum coke as discrete particles and acrylonitrile as an intercalation medium. Purified sepiolite samples with a pre-determined C/SiO2 ratio of 4 yielded Si3N4 powders after firing at temperatures 1300–1475°C under continuous nitrogen flow. The various sepiolite-reducing agent combinations were evaluated. The / ratio and secondary phase content of the powders after CRN were found to depend on temperature, time, heating rate and on the physicochemical properties of the precursor used such as, surface area and mixing of the reactants. 相似文献
93.
Sepiolite-PAN intercalation used as Si3N4 forming precursor 总被引:2,自引:0,他引:2
Silicon nitride (Si3N4) formation was investigated using sepiolite and polyacrylonitrile as silicon and carbon source, respectively. It was found that purified sepiolite could readily adsorb a sufficient amount of acrylonitrile (AN) reagent without pre-treatment. Polymerisation of sepiolite-AN mixtures and subsequent cyclization of the polymerised complex yielded a precursor, which was found ideal as a starting material in carbothermal reduction-nitridation (CRN) for the formation of silicon nitride powders. The quantity of silicon nitride, grain size and morphology were found to be dependent on the reaction conditions and starting reagent. Fine grain size, high surface area (up to approx. 12.4 m2/g) powders of mainly -Si3N4 were obtained via pyrolysis of sepiolite-PAN complex after 4 h heating at 1400°C in 1000 ml/min nitrogen flow with a heating rate of 300°C/h. Mg retained in the molecular structure of the mineral must have promoted the formation of -grains in CRN process. 相似文献
94.
Cory J. Rupp Anton Evgrafov Kurt Maute Martin L. Dunn 《Structural and Multidisciplinary Optimization》2007,34(2):111-121
We develop a topology optimization approach to design two- and three-dimensional phononic (elastic) materials, focusing primarily
on surface wave filters and waveguides. These utilize propagation modes that transmit elastic waves where the energy is contained
near a free surface of a material. The design of surface wave devices is particularly attractive given recent advances in
nano- and micromanufacturing processes, such as thin-film deposition, etching, and lithography, which make it possible to
precisely place thin film materials on a substrate with submicron feature resolution. We apply our topology optimization approach
to a series of three problems where the layout of two materials (silicon and aluminum) is sought to achieve a prescribed objective:
(1) a grating to filter bulk waves of a prescribed frequency in two and three dimensions, (2) a surface wave device that uses
a patterned thin film to filter waves of a single or range of frequencies, and (3) a fully three-dimensional structure to
guide a wave generated by a harmonic input on a free surface to a specified output port on the surface. From the first to
the third example, the resulting topologies increase in sophistication. The results demonstrate the power and promise of our
computational framework to design sophisticated surface wave devices. 相似文献
95.
This paper proposes a system architecture, related design approaches for autonomous mobile systems and guidelines for self-sufficient autonomy. Development of a tiered layout for a hybrid-state control in a series of stages as well as the integration of such a controller in the overall autonomy structure are proposed and demonstrated as part of multiple examples, including The Ohio State University participation in DARPA Urban Challenge 2007. The hierarchical layout and the iterative design methodology enable a certain level of design flexibility for the overall system and preparation for various contingencies, as illustrated on specific development cycles. 相似文献
96.
The preparation of graft copolymers of poly(methyl methacrylate) with some alkyl methacrylates were carried out via atom transfer radical polymerization method catalyzed by CuCl/2,2′-bipyridine and using a macroinitiator, poly[(methyl methacrylate)-co-(3,5-bis(chloroacetoxy)phenyl methacrylate)], including an amount of 1 mol % having α-halogeno carbonyl group in the side groups. Although the number-average molecular weights of a graft copolymer series of n-butyl methacrylate (n-ButMA) ended at different times increased from 55,700 to 99,500, the polydispersities decreased from 1.85 to 1.39 with time. The thermal degradation kinetics of macroinitiator and a two-armed graft copolymer of n-ButMA with this macroinitiator, PMMA-g-PnButMA: 4% (by mol), were carried out at different heating rates by thermogravimetric analysis and the results were compared. Using both the Flynn–Wall–Ozawa and Kissinger methods, the decomposition activation energies for macroinitiator were determined as 168 and 162 kJ/mol, respectively; they were also calculated as 233 and 239 kJ/mol for PMMA-g-PnButMA: 4%. The solid state thermodegradation mechanisms of both macroinitiator and PMMA-g-PnButMA: 4% are R1-type mechanism, a phase boundary-controlled reaction, and F1-type mechanism, a random nucleation with one nucleus on the individual particle, respectively. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
97.
98.
Lead film electrodes (PbFEs) deposited in situ on glassy carbon or carbon paste supports have recently found application in adsorptive stripping voltammetric determination of inorganic ions and organic substances. In this work, the PbFE, prepared in ammonia buffer solutions, was investigated using scanning electron microscopy, atomic force microscopy and various voltammetric techniques. The microscopic images of the lead films deposited on the glassy carbon substrate showed a considerable variability in microstructure and compactness of the deposited layer depending on the selected experimental conditions, such as the concentration of Pb(II) species, the nucleation and deposition potential, and the time applied. The catalytic adsorptive systems of cobalt and nickel in a solution containing 0.1 ammonia buffer, 2.5 × 10−5 M nioxime and 0.25 M NaNO2 were employed to investigate the electrochemical characteristics and utility of the in situ prepared lead films.The optimal parameters, i.e. the lead concentration in the solution, the procedure of film removal, and the time and potential of lead nucleation and film deposition for the adsorptive determination of metal traces, were selected, resulting in the very good reproducibility (RSD = 4.2% for 35 scans) of recorded signals. The voltammetric utility of the lead film electrode was compared to that of glassy carbon, mercury film and bismuth film electrodes, and was subsequently evaluated as superior. 相似文献
99.
The mechanism of corrosion of 99.99% purity aluminum in alkaline solutions was investigated, through detailed examination of open-circuit potential transients. These transients displayed a characteristic time dependence, in which the potential first decreased over a few seconds to a minimum of −1.7 to −1.9 V vs. Ag/AgCl, and then slowly increased over a period of hours. The value of the minimum potential of electropolished foils, along with its dependence on pH and aluminate ion concentration, indicated that it was determined by the Nernst potential for the oxidation of surface aluminum hydride (AlH3). This finding supports the direct role of hydride in the dissolution process. The increase of anodic polarization after the minimum potential occurred in two stages, the first correlated with the buildup of surface hydride, and the second with surface enrichment of Cu and Fe impurities. 相似文献
100.