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排序方式: 共有6469条查询结果,搜索用时 15 毫秒
51.
52.
At one time, supercomputing, a term that was synonymous with the Cray-1, was considered to be the best deal in town. However, by the 1990s, manufacturers began to question the need to continue the development of high-speed computers, turning their attention instead to the potential for developing a "high-capacity and high-speed national research and education computer network." 相似文献
53.
On the cramer-rao bound for carrier frequency estimation in the presence of phase noise 总被引:1,自引:0,他引:1
We consider the carrier frequency offset estimation in a digital burst-mode satellite transmission affected by phase noise. The corresponding Cramer-Rao lower bound is analyzed for linear modulations under a Wiener phase noise model and in the hypothesis of knowledge of the transmitted data. Even if we resort to a Monte Carlo average, from a computational point of view the evaluation of the Cramer-Rao bound is very hard. We introduce a simple but very accurate approximation that allows to carry out this task in a very easy way. As it will be shown, the presence of the phase noise produces a remarkable performance degradation of. the frequency estimation accuracy. In addition, we provide asymptotic expressions of the Cramer-Rao bound, from which the effect of the phase noise and the dependence on the system parameters of the frequency offset estimation accuracy clearly result. Finally, as a by-product of our derivations and approximations, we derive a couple of estimators specifically tailored for the phase noise channel that will be compared with the classical Rife and Boorstyn algorithm, gaining in this way some important hints on the estimators to be used in this scenario 相似文献
54.
55.
The computation of covariance and correlation matrices are critical to many data mining applications and processes. Unfortunately
the classical covariance and correlation matrices are very sensitive to outliers. Robust methods, such as Quadrant Correlation
(QC) and the Maronna method, have been proposed. However, existing algorithms for QC only give acceptable performance when
the dimensionality of the matrix is in the hundreds; and the Maronna method is rarely used in practice because of its high
computational cost. In this paper we develop parallel algorithms for both QC and the Maronna method. We evaluate these parallel
algorithms using a real data set of the gene expression of over 6000 genes, giving rise to a matrix of over 18 million entries.
In our experimental evaluation, we explore scalability in dimensionality and in the number of processors, and the trade-offs
between accuracy and computational efficiency. We also compare the parallel behaviours of the two methods. From a statistical
standpoint, the Maronna method is more robust than QC. From a computational standpoint, while QC requires less computation,
interestingly the Maronna method is much more parallelizable than QC. After a thorough experimentation, we conclude that for
many data mining applications, both QC and Maronna are viable options. Less robust, but faster, QC is the recommended choice
for small parallel platforms. On the other hand, the Maronna method is the recommended choice when a high degree of robustness
is required, or when the parallel platform features a large number of processors (e.g., 32). 相似文献
56.
Langley Cynthia M.; Riley Donald A.; Bond Alan B.; Goel Namni 《Canadian Metallurgical Quarterly》1996,22(2):139
The experiments reported here were designed to test the suggestion of many researchers that selective attention to visual features of a prey can account for search-image effects. In 3 experiments pigeons ate wheat and vetch grains presented on multicolored and gray gravel trays. In Experiment 1 search-image effects were evident when grains were cryptic but not when they were conspicuous. Experiment 2 demonstrated that search images can be activated when the grains encountered are either cryptic or conspicuous but that search images affect search performance only when the grains are cryptic. Experiment 3 demonstrated that search images are short-term in nature: A 3-min delay between successive encounters with a type of grain disrupted an activated search image. The discussion addresses how these results further develop a model in which search images are viewed as selective attention to visual features of a prey. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
57.
W. Jill Harrison K. R. Pearson Alan A. Powell E. John Small 《Computational Economics》1994,7(3):203-223
General equilibrium moels are usually represented as a system of levels equations (e.g., in North America) or a system of linearized equations (e.g., in Australia). Either representation can be used to obtain accurate solutions. General-purpose software is available in both cases-GAMS or MPS/GE is typically used by levels modellers and GEMPACK by linearizers. Some equations (notably accounting identities) are naturally expressed in the levels while others (especially behavioural equations) are naturally expressed in a linearized form. This paper describes the new GEMPACK facility for solving models represented as a mixture of levels and linearized equations and discusses the advantages to modellers of using such a representation. 相似文献
58.
This paper surveys recent research in deliberative real-time artificial intelligence (AI). Major areas of study have beenanytime algorithms, approximate processing, and large system architectures. We describe several systems in each of these areas, focusing both on progress within the field, and the costs, benefits and interactions among different problem and algorithm complexity limitations used in the surveyed work. 相似文献
59.
Mechanical damping and dynamic modulus measurements in alumina and tungsten fibre-reinforced aluminium composites 总被引:2,自引:0,他引:2
Simultaneous measurements of mechanical damping, or internal friction (Q
–1 ), and dynamic Young's modulus (E) were made near 80 kHz and at strain amplitudes () in the range 10–8 to 10–4 on small specimens of continuous or chopped fibre-reinforced metal matrix composites (MMCs): 6061 aluminium reinforced with alumina (Al/Al2O3) and 6061 aluminium reinforced with tungsten (Al/W). Baseline experiments were also done on 99.999% aluminium (pure Al). The strain amplitude dependence of damping and the temperature dependence of dynamic modulus were of particular interest in this study. The temperature (T) dependence of the modulus from room temperature up to 475° C was determined for the Al/Al2O3 and pure Al specimens and a highly linear decrease in modulus with increasing temperature was observed. The rate of modulus loss (dE/dT –80 M Pa° C–1 ) was the same for both materials and the reduction in modulus of the Al/Al2O3 was attributed to the reduction in modulus of the alu minium matrix, not the alumina fibres. The size, type, and amount of fibre reinforcement were found to have a significant effect on the strain amplitude dependence of the damping in both MMCs. Unreinforced aluminium exhibited classical dislocation damping trends with a region of strain amplitude independent damping at low strains (less than 10–5) followed by a non linear, strain amplitude dependent region at higher strains. The addition of alumina fibres (chopped or continuous), while increasing stiffness, resulted in a significant reduction in damping capacity for the MMC relative to that for aluminium and near complete suppression of the amplitude dependent response. The damping levels increased as the volume fraction of fibre, and therefore, the amount of fibre/matrix (FM) interface decreased, indicating that the matrix, not factors such as increased dislocation densities at the FM interface, was the dominant influence on the damping. Analysis of the Al/Al2O3 results by Granato-Lücke (GL) theory indicated that dislocation densities were increased relative to those in aluminium, but the dis locations were well pinned and unable to increase damping levels effectively. Analysis of the Al/W results by GL theory also revealed high dislocation densities, but, unlike the Al/Al2O3 specimens, the Al/W specimens (continuous fibres) exhibited strong amplitude dependent damping (starting near strain levels of 2 × 10–6) with damping levels approximately twice those of pure aluminium. Trends showed increased damping with increased fibre diameter, not with increased FM interface area. There was some evidence that it was the tungsten fibre itself that dominated the damping behaviour in Al/W composites, not the aluminium matrix or the FM interface. 相似文献