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41.
Online Search with Time-Varying Price Bounds 总被引:1,自引:0,他引:1
Online search is a basic online problem. The fact that its optimal deterministic/randomized solutions are given by simple formulas (however with difficult analysis) makes the problem attractive as a target to which other practical online problems can be transformed to find optimal solutions. However, since the upper/lower bounds of prices in available models are constant, natural online problems in which these bounds vary with time do not fit in the available models.We present two new models where the bounds of prices are not constant but vary with time in certain ways. The first model, where the upper and lower bounds of (logarithmic) prices have decay speed, arises from a problem in concurrent data structures, namely to maximize the (appropriately defined) freshness of data in concurrent objects. For this model we present an optimal deterministic algorithm with competitive ratio \(\sqrt{D}\), where D is the known duration of the game, and a nearly-optimal randomized algorithm with competitive ratio \(\frac{\ln D}{1+\ln2-\frac{2}{D}}\). We also prove that the lower bound of competitive ratios of randomized algorithms is asymptotically \(\frac{\ln D}{4}\).The second model is inspired by the fact that some applications do not utilize the decay speed of the lower bound of prices in the first model. In the second model, only the upper bound decreases arbitrarily with time and the lower bound is constant. Clearly, the lower bound of competitive ratios proved for the first model holds also against the stronger adversary in the second model. For the second model, we present an optimal randomized algorithm. Our numerical experiments on the freshness problem show that this new algorithm achieves much better/smaller competitive ratios than previous algorithms do, for instance 2.25 versus 3.77 for D=128. 相似文献
42.
The increased popularity of tablets in general has led to uptake in education. We critically review the literature reporting use of tablets by primary and secondary school children across the curriculum, with a particular emphasis on learning outcomes. The systematic review methodology was used, and our literature search resulted in 33 relevant studies meeting the inclusion criteria. A total of 23 met the minimum quality criteria and were examined in detail (16 reporting positive learning outcomes, 5 no difference and 2 negative learning outcomes). Explanations underlying these observations were analysed, and factors contributing to successful uses of tablets are discussed. While we hypothesize how tablets can viably support children in completing a variety of learning tasks (across a range of contexts and academic subjects), the fragmented nature of the current knowledge base, and the scarcity of rigorous studies, makes it difficult to draw firm conclusions. The generalizability of evidence is limited, and detailed explanations as to how, or why, using tablets within certain activities can improve learning remain elusive. We recommend that future research moves beyond exploration towards systematic and in‐depth investigations building on the existing findings documented here. 相似文献
43.
Reliability-based design optimization (RBDO) incorporates probabilistic analysis into optimization process so that an optimum
design has a great chance of staying in the feasible design space when the inevitable variability in design variables/parameters
is considered. One of the biggest drawbacks of applying RBDO to practical problem is its high computational cost that is often
impractical to industries. In search of the most suitable RBDO method for industrial applications, we first evaluated several
existing RBDO approaches in details such as the double-loop RBDO, the sequential optimization and reliability assessment,
and the response surface method. Then, based on industry needs, a platform incorporating/integrating the existing algorithm
of optimization and reliability analysis is built for a practical RBDO problem. Effectiveness of the proposed RBDO approach
is demonstrated using a simple cantilever beam problem and a more complicated industry problem. 相似文献
44.
Sang Hyun Lee Sung Ho Ha Chun-Yeol You Yoon-Mo Koo 《Korean Journal of Chemical Engineering》2007,24(3):436-437
Recovery of 1-butyl-3-methylimidazolium tetrachloroferrate ([bmim]FeCl4) from its mixture with water was investigated. The [bmim]FeCl4 rich phase in the mixture forming two-phase was successfully separated, while homogeneous mixtures could not be separated.
However, the concentration of the homogeneous mixture varied as a function of the magnetic field strength. Therefore, a combination
of magnetic field and conventional methods to recover magnetic ILs from reaction mixtures will be very useful and have great
potential. 相似文献
45.
46.
47.
The direct current-direct current (DC-DC) converter is designed for 1 T static random access memory (SRAM) used in display
driver integrated circuits (ICs), which consists of positive word-line voltage (V
PWL), negative word-line voltage (V
NWL) and half-V
DD voltage (V
HDD) generator. To generate a process voltage temperature (PVT)-insensitive V
PWL and V
NWL, a set of circuits were proposed to generate reference voltages using bandgap reference current generators for respective
voltage level detectors. Also, a V
PWL regulator and a V
NWL charge pump were proposed for a small-area and low-power design. The proposed V
PWL regulator can provide a large driving current with a small area since it regulates an input voltage (VCI) from 2.5 to 3.3
V. The V
NWL charge pump can be implemented as a high-efficiency circuit with a small area and low power since it can transfer pumped
charges to V
NWL node entirely. The DC-DC converter for 1 T SRAM were designed with 0.11 μm mixed signal process and operated well with satisfactory
measurement results. 相似文献
48.
文章介绍了国内小电流接地系统故障选线方法的的发展简况,在分析小电流接地故障暂态特征的基础上对近几年提出的几种较新的基于暂态特征信息的选线方法进行了简单的评述,同时筒略说明了不足之处.文章最后提出了一种基于暂态能量函数的小电流接地选线方法,并有待于进一步研究. 相似文献
49.
K. Rózga-Wijas U. Mizerska W. Fortuniak J. Chojnowski R. Hałasa W. Werel 《Journal of Inorganic and Organometallic Polymers and Materials》2007,17(4):605-613
Three types of silica particles modified with vinyl groups were obtained: (i) xerogel formed by hydrolytic polycondensation
of the mixture of tetramethoxysilane (TMOS) and 1,1,1,7−tetramethoxy-3,5,7-trimethyl-3,5,7-trivinyltetrasiloxane, (ii) mesoporous
silica obtained from the same precursors in the presence of the cetyltrimethylammonium bromide (CTAB), and (iii) commercial
Fluka silica gel 60A with a vinyltriethoxysilane-treated surface. Vinyl groups on these silica materials were transformed
into silyl chloride by hydrosilylation with HMe2SiCl. These groups were used to graft living polysiloxane that was synthesized by anionic ring-opening polymerization of 2,4,6-tri(3-chloropropyl)-2,4,6-trimethylcyclotrisiloxane
and initiated by BuLi. Chloropropyl groups on the grafted polymer were used to quaternize N,N-dimethyl-n-octylamine. Silica particles with grafted polysiloxane having quaternary ammonium salt (QAS) groups pendant to polymer chains
were obtained. Silica material with QAS groups directly attached to the surface were generated by the action of N,N-dimethyl-n-octylamine on particles obtained by the sol–gel process involving tetraethoxysilane (TEOS) with 3-chloropropyltriethoxysilane.
The bacteriocidal properties of all these materials were tested in water suspension against five representative strains for
Gram-positive and Gram-negative bacteria. Some of the silica–polysiloxane hybrid materials have good antibacterial properties
against Gram-positive strains, but not as good as the non-tethered QAS-substituted polysiloxane in water solution. The QAS
groups that are directly bonded to the silica material surface are inactive. 相似文献
50.
Harukazu Yoshino Zeynel Bayindir Joydeep Roy Ben Shaw Heon-ick Ha Andrei Lebed Michael J. Naughton 《Journal of Low Temperature Physics》2006,142(3-4):323-326
A new TTP donor, Me-DH-TTP (2-methyl-5-(1,3-dithiolan-2-yliden)-1,3,4,6-tetrathiapentalene), was designed to realize a system
with large on-site Coulomb repulsion as compared with the previously known bis-fused type TTP donors. Probably as a consequence,
(Me-DH-TTP)2AsF6 exhibits semiconducting behavior from room temperature to liquid helium temperature. By increasing pressure, metallic behavior
appears below 300 K, and with distinct metal-insulator (M-I) transition up to 2.2 GPa. This M-I transition suddenly disappears
beyond 2.5 GPa, and metallic state is stabilized down to 2.6 K. We discuss the possibility of quantum critical point around
2.4 GPa. 相似文献