全文获取类型
收费全文 | 428622篇 |
免费 | 6206篇 |
国内免费 | 2247篇 |
专业分类
电工技术 | 8411篇 |
技术理论 | 5篇 |
综合类 | 1318篇 |
化学工业 | 66281篇 |
金属工艺 | 17610篇 |
机械仪表 | 12851篇 |
建筑科学 | 10788篇 |
矿业工程 | 2834篇 |
能源动力 | 10056篇 |
轻工业 | 39361篇 |
水利工程 | 4812篇 |
石油天然气 | 9116篇 |
武器工业 | 252篇 |
无线电 | 46833篇 |
一般工业技术 | 82483篇 |
冶金工业 | 79431篇 |
原子能技术 | 9646篇 |
自动化技术 | 34987篇 |
出版年
2021年 | 3809篇 |
2019年 | 3415篇 |
2018年 | 5507篇 |
2017年 | 5555篇 |
2016年 | 5999篇 |
2015年 | 4351篇 |
2014年 | 7096篇 |
2013年 | 19080篇 |
2012年 | 11799篇 |
2011年 | 15956篇 |
2010年 | 12800篇 |
2009年 | 13985篇 |
2008年 | 14811篇 |
2007年 | 15009篇 |
2006年 | 13069篇 |
2005年 | 11755篇 |
2004年 | 10824篇 |
2003年 | 10321篇 |
2002年 | 10015篇 |
2001年 | 10118篇 |
2000年 | 9706篇 |
1999年 | 9835篇 |
1998年 | 22617篇 |
1997年 | 16456篇 |
1996年 | 12766篇 |
1995年 | 9479篇 |
1994年 | 8675篇 |
1993年 | 8447篇 |
1992年 | 6627篇 |
1991年 | 6290篇 |
1990年 | 6190篇 |
1989年 | 6228篇 |
1988年 | 5871篇 |
1987年 | 5050篇 |
1986年 | 4991篇 |
1985年 | 5703篇 |
1984年 | 5452篇 |
1983年 | 5050篇 |
1982年 | 4470篇 |
1981年 | 4747篇 |
1980年 | 4340篇 |
1979年 | 4628篇 |
1978年 | 4520篇 |
1977年 | 4865篇 |
1976年 | 6455篇 |
1975年 | 3987篇 |
1974年 | 3754篇 |
1973年 | 3797篇 |
1972年 | 3182篇 |
1971年 | 2893篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
We studied experimentally the influence of the degree of water emulsification ofM-40 mazut heated to the water boiling point on the self-ignition time with its pulsed high-pressure injection into heated air. It is shown that water addition does not change the chemistry of the reaction and heating the water-fuel emulsion leads to the disappearance of differences in the ignition character of the emulsion and the moisture-free fuel. It is confirmed that the features of selfignition are determined mostly by conditions of mixture formation. Indirect evidence is obtained that the dynamic tensile strength of the fluid strongly influences the primary dispersion of the jet during its outflow from the sprayer nozzle.Institute of Theoretical and Applied Mechanics, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 4, pp. 20–25, July–August, 1995. 相似文献
992.
We consider the problem of allocating bandwidth fairly to each node in a shared, unidirectional bus network. We focus on the pi persistent protocol, since these are open loop policies designed to operate well in high speed networks, which have a very large bandwidth-delay product and feedback in the upstream direction is not available in a timely manner. First, we introduce an improvement to the basic pi persistent protocol, in which we replace random coin tosses with a deterministic counting algorithm, and thereby reduce the delays for all nodes for any given choice of {pi}. We then describe an exact method for calculating average packet delays and queue lengths in both the pi persistent and our new deterministic n out of m protocols, based on the regenerative approach of Georgiades et al. (1987). These delay results, together with simulation measurements, show that both of these protocols still waste some bandwidth. After presenting a lower bounding argument to show that some wasted bandwidth is inevitable in all such distributed access control schemes, assuming a passive bus without feedback in the upstream direction, we show that changing the bus to unidirectional point-to-point links between (very simple) active interfaces at each node allows us to construct distributed access schemes that require no upstream feedback and are both work conserving and fair. To illustrate how this can be done, we introduce the pi preemptive protocol, in which each node randomly inserts its own packets into the traffic arriving from upstream. We derive a simple and effective heuristic for calculating the preemption probability for each node, and use simulation to show how well it equalizes the delays at each node 相似文献
993.
Multilayer coatings have been produced using unbalanced magnetron physical vapour deposition in which discrete layers of TiN and ZrN were co-deposited. It has been shown that the multilayers exhibited two-fold periodicity having values of 13 and 2.8 nm. This periodicity, measured both by cross-sectional transmission electron microscopy and low-angle X-ray diffraction, has been precisely related to the process parameters used. Analysis has shown that a third, previously unreported, phase existed within the coating having a lattice parameter comparable to that of TiN and a well-defined orientation relationship with the primary phases. The existence of this phase has been explained in terms of variations in the stoichometry of the coating due to non-uniform metal to nitrogen deposition rates within the chamber. Textural studies revealed that preferred orientation existed in the coating; the degree of which was greater in the ZrN layers than the TiN layers. 相似文献
994.
Force modulation microscopy (FMM) is used to characterize the external surface and internal fracture surface morphologies of three different block copolymer samples. A roll-cast poly(styrene-butadiene-styrene) triblock copolymer film, spin-coated poly(styrene-b-methyl methacrylate) thin films, and an ultrathin poly(styrene-b-hexyl isocyanate) rod-coil block copolymer film were investigated. For each sample, height and elasticity images were obtained for the same areas allowing direct comparison. The elasticity images obtained using force modulation microscopy were independent of surface roughness and found to exhibit better contrast and spatial resolution of the respective block copolymer domains than the height images. The lateral resolution of the elasticity images was sufficient to show microphase separated domains having length scales as small as about 10 nm. The poly(styrene-b-methyl methacrylate) samples demonstrate that FMM can even be successfully used to study block copolymers in which both blocks are glassy under the conditions of measurement. 相似文献
995.
Oommen B.J. Zgierski J.R. 《IEEE transactions on pattern analysis and machine intelligence》1993,15(2):185-192
Let Λ be a finite plaintext alphabet and V be a cypher alphabet with the same cardinality as Λ. In all one-to-one substitution cyphers, there exists the property that each element in V maps onto exactly one element in Λ and vice versa. This mapping of V onto Λ is represented by a function T *, which maps any v ∈V onto some λ∈Λ (i.e., T *(v )=λ). The problem of learning the mapping of T * (or its inverse (T *)-1) by processing a sequence of cypher text is discussed. The fastest reported method to achieve this is a relaxation scheme that utilizes the statistical information contained in the unigrams and trigrams of the plaintext language. A new learning automaton solution to the problem called the cypher learning automaton (CLA) is given. The proposed scheme is fast, and the advantages of the scheme in terms of time and space requirements over the relaxation method have been listed. Simulation results comparing both cypher-breaking techniques are presented 相似文献
996.
The analysis and scanning characteristics of an infinite array of rectangular microstrip patches each loaded with a varactor diode is presented. The analysis is based on full-wave moment method theory and uses attachment modes to accurately model the current through the feed and the diode. The effect of the biased varactor diode on the scan performance is presented, and it is shown that the impedance mismatch caused by a scan blindness can be eliminated. Other characteristics are also given such as: the active element gain, the efficiency of each loaded microstrip element, and the level of cross polarization generated by the loading of the patch. The effect of the diode biasing level as well as the position of the diode on each radiating element on these scan characteristics is also considered 相似文献
997.
L. K. Seah
J. Rhodes
B. S. Lim
《Thin》1993,16(1-4):145-177Turning the free edge of an unstiffened flange inwards or outwards to form a ‘lip’, can substantially improve the local buckling resistance of a member. The lip is the most common type of edge stiffener used in cold-rolled, thin-walled sections. In this paper the behaviour of plate elements of thin-walled sections stiffened by compound lips (i.e. lips which are folded twice to form ‘lips on the lips’) is examined both theoretically and experimentally. An outline of a series of tests on compound edge-stiffened thin-walled sections of various geometries is given and some load-end compression displacement paths are compared with the theoretical predictions. Reasonably good agreement is obtained between the experimental and theoretical results. Comparisons of the theoretical predictions with experimental results of other researchers are also presented in this paper. 相似文献
998.
C Serveau L Juliano P Bernard T Moreau R Mayer F Gauthier 《Canadian Metallurgical Quarterly》1994,76(2):153-158
A discontinuous borate/formate buffer system is presented for horizontal polyacrylamide gel electrophoresis of DNA fragments. The resolution potential of the system could be altered by changing the total monomer concentration (5-9%T), the concentration of the crosslinker piperazine diacrylamide (2-5%CPDA), as well as the concentration of formate in the gel (40-120 mM), the leading ion of the buffer system. The separation of DNA fragments would be improved by increasing the migration distance from 22 to 28 cm. This discontinuous polyacrylamide gel electrophoresis system proved highly reproducible. 相似文献
999.
1000.
The data recovered from compact discs are contaminated with errors that exhibit a bursty behavior. A channel model is developed to characterize the error bursts and good-data gaps that occur in actual data recovered from compact discs. The probability of having m erroneous symbols in a data block of length n symbols is calculated using the channel model. This probability can be used to evaluate the performance of the error-correction code (ECC) used in the compact disc system 相似文献