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161.
MuShanxiang JiXiaoli LiMing LiXingguo 《电子科学学刊(英文版)》2003,20(6):407-413
Linear Modulated Frequency (LMF) Costas Stepped Frequency Radar (SFR) signal is introduced. Its ambiguity function is derived and analyzed in detail and its feasibility is validated in theory. The scheme of the proposed signal processing is also presented. The results of theoretic analysis and simulation show that, by using the proposed signal and increasing the bandwidth of the total stepped frequency, the ambiguity sidelobe is well suppressed and the range-velocity coupling in the stepped frequency radar is also greatly weakened. 相似文献
162.
ZhaoHongzhong ZhouJianxiong LiWeimin FuQiang 《电子科学学刊(英文版)》2003,20(6):401-406
High acceleration of radar targets is analyzed using Acceleration Ambiguity Function(AAF). The acceleration resolution based on AAF is defined. The AAF and acceleration resolution of rectangle pulse signal are derivated and the conclusion that its acceleration resolution is in inverse proportion with the square of its duration is drawn. In the end, these conclusions are applied to the parameter designing and performance evaluation for a certain type of pulse Doppler radar. 相似文献
163.
Sungho Jin 《Journal of Electronic Materials》2003,32(12):1366-1370
In packaging of microelectromechanical systems (MEMS), optical, and electronic devices, there is a need to directly bond a
wide variety of inorganic materials, such as oxides, nitrides, and semiconductors. Such applications involve hermetic-sealing
components, three-dimensional MEMS assembly components as well as active semiconductor or optical components, dielectric layers,
diffusion barriers, waveguides, and heat sinks. These materials are known to be very difficult to wet and bond with low melting-point
solders. New Sn-Ag- or Au-Sn-based universal solders doped with a small amount of rare-earth (RE) elements have been developed,
which now allow direct and powerful bonding onto the surfaces of various MEMS, optical, or electronic device materials. The
microstructure, interface properties, and mechanical behavior of the bonds as well as the potential packaging applications
of these new solder materials for MEMS and optical fiber devices are described. Various packaging-related structural, thermal,
or electrical issues in MEMS are also discussed. 相似文献
164.
Fretting fatigue is one of the most important phenomena for inducing a significant reduction of fatigue strength and consequently,
leading to unexpected failure accidents of the engineering structures even at very low stresses. In the present study, both
plain and fretting fatigue tests with zero mean stress were carried out on two different types of steel, low-carbon steel
and martensitic stainless steel, by means of a reversed bending fatigue testing machine. The drop in the fatigue strengths
through fretting at vise clamp-specimen interface were significant for both tested steels. The fretting processes produced
a reduction in fatigue strength of about 27% for low-carbon steel and 16% for martensitic stainless steel. 相似文献
165.
166.
Co-evolution as a computational and cognitive model of design 总被引:6,自引:0,他引:6
Co-evolutionary design has been developed as a computational model that assumes two parallel search spaces: the problem space
and the solution space. The design process iteratively searches each space using the other space as the basis for a fitness
function when evaluating the alternatives. Co-evolutionary design can also be developed as a cognitive model of design by
characterizing the way in which designers iteratively search for a design solution, making revisions to the problem specification.
This paper presents the computational model of co-evolutionary design and then describes a protocol study of human designers
looking for evidence of co-evolution of problem specifications and design solutions. The study shows that co-evolutionary
design is a good cognitive model of design and highlights the similarities and differences between the computational model
and the cognitive model. The results show that the two kinds of co-evolutionary design complement each other, having strengths
in different aspects of the design process.
Electronic Publication 相似文献
167.
The microstructure and thermal behavior of the Sn-Zn-Ag solder were investigated for 8.73–9% Zn and 0–3.0% Ag. The scanning
electron microscopy (SEM) analysis shows the Ag-Zn compound when the solder contains 0.1% Ag. X-ray diffraction (XRD) analysis
results indicate that Ag5Zn8 and AgZn3 become prominent when the Ag content is 0.3% and above. Meanwhile, the Zn-rich phase is refined, and the Zn orientations
gradually diminish upon increase in Ag content. The morphology of the Ag-Zn compound varies from nodular to dendrite structure
when the Ag content increases. The growth of the Ag-Zn compounds is accompanied by the diminishing of the eutectic structure
of the Sn-9Zn solder. Differential scanning calorimetry (DSC) investigation reveals that the solidus temperature of these
solders exists at around 198°C. A single, sharp exothermic peak was found for the solders with Ag content less than 0.5%.
Liquidus temperatures were identified with the DSC analysis to vary from 206°C to 215°C when the Ag content ranges from 1.0%
to 3.0% 相似文献
168.
It is generally appreciated that the mechanical behavior of granular media depends fundamentally on the interaction of the
constituent particles, and that the validity of numerical models of granular media would be greatly improved with knowledge
of the grain-scale mechanics. However, most supporting experimental work has been conducted on highly idealized materials,
and a limited amount of information exists on grain-scale force–displacement relationships for naturally occurring materials.
To address this shortcoming, we are conducting a program that integrates laboratory experiments on grains of naturally occurring
aggregate with the discrete element modeling method, with the goal of relating the grain-scale physical and mechanical properties
of granular media to bulk behavior. The paper describes the equipment and methods that have been developed to conduct close-loop
controlled, grain-scale experiments under monotonic and cyclic loading conditions, and presents results from an initial set
of experiments on unbonded grains. The implications of the grain-scale results to the discrete element model are discussed.
Discussions center on the applicability of a physically based approach to the mechanics of granular media in general. In light
of future exploration missions and the resulting need to predict the mechanical properties of lunar and planetary regoliths,
the paper examines the potential usefulness of our physically based approach to the problem of predicting the behavior of
the types of materials found in those environments. 相似文献
169.
Barbara K. Reck Robert B. Gordon 《JOM Journal of the Minerals, Metals and Materials Society》2008,60(7):55-59
Nickel and chromium are essential ingredients in alloys increasingly important for energy-efficient, environmentally friendly
modern technology. Quantitative assessment of the flows of these metals through the world economy from resource extraction
to final disposal informs resource policy, energy planning, environmental science, and waste management. This article summarizes
the worldwide technological cycles of nickel and chromium in 2000. Stainless steel is the major use of these metals, but they
serve numerous other special needs, as in superalloys for high-temperature service, as plating materials, and in coinage.
Because they are used primarily in alloys, novel recycling issues arise as their use becomes more widespread.
“... the great New York and St. Louis double track, nickel plated railroad...”
— Norwalk, Ohio, Chronicle 10 March 1881 announces arrival of surveyors for the future Nickel Plate Railway
“Later [1911] I formed an alloy of Iron and Chromium, which showed remarkable resistance to rust and tarnish ... [It was]
rediscovered by an Englishman named Brearley, in 1914.”
—Elwood Haynes to Stephen F. Roberts, 17 January 1925 相似文献
170.
Shafeeque G. Ansari Mushtaq Ahmad Dar Young-Soon Kim Hyung-Kee Seo Gil-Sung Kim Rizwan Wahab Zubaida A. Ansari Jae-Myung Seo Hyung-Shik Shin 《Korean Journal of Chemical Engineering》2008,25(3):593-598
A comparative study for the nucleation of diamond was carried out using surface treatment like (i) surface scratching with
1 μm diamond paste and (ii) surface etching using chlorine plasma at different RF powers (50, 100 and 150 W). Atomic force
microscopic study shows variation in roughness from 31 nm to 110 nm. Scratching results in random scratches, whereas plasma
etches a surface uniformly. Scanning electron microscopic observations show well faceted crystallites with a predominance
of angular shaped grains corresponding to 〈100〉 and 〈110〉 crystallite surfaces for the scratched as well as plasma etched
substrate. Surface etching at 150 W plasma power results in a better growth in comparison with 50 and 100 W plasma powers.
Chlorine-radical is found responsible for the changes in the growth morphology. Raman spectroscopy shows a sharp peak at 1,332
cm−1 and a peak at ∼1,580 cm−1 for both samples. 相似文献