全文获取类型
收费全文 | 310284篇 |
免费 | 3913篇 |
国内免费 | 714篇 |
专业分类
电工技术 | 6197篇 |
综合类 | 560篇 |
化学工业 | 46857篇 |
金属工艺 | 10336篇 |
机械仪表 | 8825篇 |
建筑科学 | 7945篇 |
矿业工程 | 865篇 |
能源动力 | 8582篇 |
轻工业 | 31142篇 |
水利工程 | 2535篇 |
石油天然气 | 3669篇 |
武器工业 | 8篇 |
无线电 | 39781篇 |
一般工业技术 | 58551篇 |
冶金工业 | 56832篇 |
原子能技术 | 5470篇 |
自动化技术 | 26756篇 |
出版年
2021年 | 2286篇 |
2019年 | 2150篇 |
2018年 | 3613篇 |
2017年 | 3456篇 |
2016年 | 3547篇 |
2015年 | 2564篇 |
2014年 | 4464篇 |
2013年 | 14296篇 |
2012年 | 7389篇 |
2011年 | 10138篇 |
2010年 | 7926篇 |
2009年 | 9150篇 |
2008年 | 9633篇 |
2007年 | 9605篇 |
2006年 | 8525篇 |
2005年 | 7903篇 |
2004年 | 7768篇 |
2003年 | 7512篇 |
2002年 | 7348篇 |
2001年 | 7552篇 |
2000年 | 7214篇 |
1999年 | 7561篇 |
1998年 | 17720篇 |
1997年 | 12866篇 |
1996年 | 10128篇 |
1995年 | 7886篇 |
1994年 | 7220篇 |
1993年 | 6920篇 |
1992年 | 5292篇 |
1991年 | 5079篇 |
1990年 | 4939篇 |
1989年 | 4791篇 |
1988年 | 4681篇 |
1987年 | 3892篇 |
1986年 | 4015篇 |
1985年 | 4721篇 |
1984年 | 4279篇 |
1983年 | 4028篇 |
1982年 | 3606篇 |
1981年 | 3763篇 |
1980年 | 3464篇 |
1979年 | 3420篇 |
1978年 | 3232篇 |
1977年 | 3817篇 |
1976年 | 4858篇 |
1975年 | 2783篇 |
1974年 | 2664篇 |
1973年 | 2677篇 |
1972年 | 2219篇 |
1971年 | 1964篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
971.
D.R Hayhurst D.A Lavender N.G Worley A Salim 《International Journal of Pressure Vessels and Piping》1985,20(4):289-317
Constant stress creep data for a wrought 1%Cr, , steel obtained from six tests carried out at 565°C have been fitted and extrapolated using the θ-projection method. Creep curves for high stresses were fitted well, but curves extrapolated to stress levels below those used in the θ-projection did not represent well the available data for primary-secondary creep. To enable a satisfactory extrapolation of results, it was found necessary to redefine the failure condition to be less sensitive to the shape of the latter sections of the tertiary creep curve. This was achieved by defining lifetime as the point on the curve at which the strain rate reached a fixed multiple of the creep rate at half the rupture time. The extrapolated data compare well with the results of tests carried out independently on a cast 1%Cr, , steel. The extrapolation procedure not only predicts the ‘knee’ in the log stress-log lifetime curve but allows extrapolation of lifetimes by over a factor of thirty. In addition, acceptable predictions have been made of constant load experimental data. 相似文献
972.
Sidhartha R. Das Basheer M. Khumawala 《International Journal of Flexible Manufacturing Systems》1991,3(2):121-147
Flexible manufacturing systems (FMSs) are a class of automated systems that can be used to improve productivity in batch manufacturing. Four stages of decision making have been defined for an FMS—the design, planning, scheduling, and control stages. This research focuses on the planning stage, and specifically in the area of scheduling batches of parts through the system.The literature to date on the FMS planning stage has mostly focused on the machine grouping, tool loading, and parttype selection problems. Our research carries the literature a step further by addressing the problem of scheduling batches of parts. Due to the use of serial-access material-handling systems in many FMSs, the batch-scheduling problem is modeled for a flexible flow system (FFS). This model explicitly accounts for setup times between batches that are dependent on their processing sequence.A heuristic procedure is developed for this batch-scheduling problem—the Maximum Savings (MS) heuristic. The MS heuristic is based upon the savings in time associated with a particular sequence and selecting the one with the maximum savings. It uses a two-phase method, with the savings being calculated in phase I, while a branch-and-bound procedure is employed to seek the best heuristic solution in phase II. Extensive computational results are provided for a wide variety of problems. The results show that the MS heuristic provides good-quality solutions. 相似文献
973.
On the basis of a geological - volcanological - geophysical survey, integrated with petrological and geochronological data, a structural model, including a shallow magmatic body, was defined for the Ribeira Grande area, Sao Miguel island (Azores). Two series of production tests were carried out in the Pico Vermelho 1 and Ribeira Grande 1 wells, which led to a preliminary definition of the deep fluid circulation. The data obtained from these studies were used to reconstruct a preliminary model of the Ribeira Grande geothermal field. 相似文献
974.
I. V. Gervas’eva D. P. Rodionov Yu. V. Khlebnikova N. F. Vil’danova A. S. Shtyrlov G. A. Dosovitskii A. R. Kaul’ 《The Physics of Metals and Metallography》2007,103(6):600-608
The texture, structure, and magnetic and mechanical properties of thin ribbons of nickel-palladium alloys have been studied. It has been demonstrated that in Ni94Pd6 and Ni92.5Pd5W2.5 alloys, a sharp cube texture can be formed that is stable up to rather high annealing temperatures. Segregation of palladium atoms to surface layers of ribbons has been found, which gives additional advantages for using these ribbons as substrates for epitaxial deposition of high-temperature superconductors. 相似文献
975.
On the basis of Odum's ecological economic measure of emergy as embodied solar energy, a system account of biomass resource exploitation by agriculture in China 2004 is developed in this paper, which supplements a former study on corresponding long-term historical trends during 1978 to 2000 (Chen et al., 2006. Emergy-based analysis of the Chinese agriculture. Agriculture, Ecosystems and Environment 115, 161–173). The aggregate fluxes and indicators for biomass resource exploitation in China 2004 are calculated and illustrated when compared with those for 2000 to elucidate the latest status of the Chinese agriculture as the exploitation sector for biomass resource. Data sources and algorithm are presented in detail as basic references for related analysis involving the ecological economy of biomass exploitation in agriculture. 相似文献
976.
Abstract: This article presents a review of dynamic fracture studies on functionally graded materials. A brief literature review on the fracture mechanics of graded materials is presented first. This is followed by a discussion on the higher-order asymptotic analysis of the transient elastic field surrounding the tip of a dynamically growing crack in a functionally graded material. A comprehensive experimental study of dynamic crack growth in model functionally graded material using the optical method of reflection photoelasticity and high-speed photography is then presented. The results are analysed to establish a generalised relationship between the crack velocity and the dynamic mode-I stress intensity factor (SIF). This relationship is found to be unique and is distinctly different from that previously established for the matrix material (polyester). Finally, an innovative experimental procedure is used to demonstrate the necessity of employing a fully transient stress-field representation in the analysis of optical data for an accurate prediction of the dynamic SIF history. 相似文献
977.
R. Dinesh T. Fujiwara T. Watanabe K. Byrappa M. Yoshimura 《Journal of Materials Science》2006,41(5):1541-1546
A well-crystallized AMO4 (A=Ba, Ca, Sr; M=W, Mo) films have been prepared at room temperature through a simple solution reaction in respective alkaline
solution at higher pH ranging from 12–14. Adopting the corrosion principle for oxidation of metal substrate, these double
oxide films were carried out in presence of chemical driving force without any special apparatus or devices. Hydrogen peroxide
was used to enhance the dissolution rate of metal substrates. The driving force for the film formation and growth were high
concentration of A2+, MO42− ions with high pH conditions. Average grain sizes of 8–10 μm with bipyramidal shaped particle were grown to the thickness
of about 10–14 μm after 3–6 hours treatment. The crystallization of AMO4 was characterized by three-dimensional nucleation. This work demonstrates the possibility of fabrication of functional ceramic
films directly from the aqueous solution in a single step by solution reactions. 相似文献
978.
R. B. Vasil’ev S. G. Dorofeev M. N. Rumyantseva L. I. Ryabova A. M. Gas’kov 《Semiconductors》2006,40(1):104-107
The impedance spectra of nanocrystalline SnO2 ceramics with an average grain size d ranging from 3 to 43 nm were investigated in the frequency range 1–106 Hz at temperatures from 25 to 300°C in a dry oxygen atmosphere. Analysis of the experimental data by the graphoanalytical method made it possible to separate the contributions of grain bulk and grain boundaries to the conductivity. It is shown that the samples investigated can be arbitrarily divided into two groups with respect to the character of their conductivity. For the samples with an average grain size d < 25 nm, the charge transfer processes are almost completely determined by the grain boundaries. In samples with a larger grain size, the contribution of grain bulk to the conductivity is comparable with that of grain boundaries. 相似文献
979.
The tree representation of evolutionary relationship oversimplifies the view of the process of evolution as it cannot take into account the events such as horizontal gene transfer, hybridization, homoplasy and genetic recombination. Several algorithms exist for constructing phylogenetic networks which result from events such as horizontal gene transfer, hybridization and homoplasy. Very little work has been published on the algorithmic detail of phylogenetic networks with constrained recombination. The problem of minimizing the number of recombinations in a phylogenetic network, constructed using binary DNA sequences, is NP-hard. In this paper, we propose a pattern recognition-based O(n2) time approach for constructing the phylogenetic network, where n is the number of nodes or sequences in the input data. The network is constructed with the restriction that no two cycles in the network share a common node. 相似文献
980.
Moeyersoon B. Morthier G. Bockstaele R. Baets R. 《Photonics Technology Letters, IEEE》2005,17(10):2032-2034
We show that the wavelength switching behavior of semiconductor tunable lasers can be improved through optical feedback from a stable reference filter. The filter is based on a novel prism-based implementation of a Fox-Smith resonator and has a response consisting of periodically spaced peaks, both in reflection and transmission. The improvement of the wavelength switching behavior stems from the suppression of the thermally induced wavelength drift associated with the switching of the tuning currents. 相似文献