全文获取类型
收费全文 | 399394篇 |
免费 | 3965篇 |
国内免费 | 1399篇 |
专业分类
电工技术 | 7154篇 |
综合类 | 367篇 |
化学工业 | 60343篇 |
金属工艺 | 19854篇 |
机械仪表 | 12424篇 |
建筑科学 | 9019篇 |
矿业工程 | 2349篇 |
能源动力 | 8666篇 |
轻工业 | 31780篇 |
水利工程 | 4418篇 |
石油天然气 | 9934篇 |
武器工业 | 15篇 |
无线电 | 43876篇 |
一般工业技术 | 80511篇 |
冶金工业 | 75096篇 |
原子能技术 | 10554篇 |
自动化技术 | 28398篇 |
出版年
2021年 | 3014篇 |
2019年 | 2906篇 |
2018年 | 5147篇 |
2017年 | 5184篇 |
2016年 | 5654篇 |
2015年 | 3486篇 |
2014年 | 5954篇 |
2013年 | 16637篇 |
2012年 | 9400篇 |
2011年 | 12762篇 |
2010年 | 10375篇 |
2009年 | 11472篇 |
2008年 | 12033篇 |
2007年 | 12049篇 |
2006年 | 10756篇 |
2005年 | 9907篇 |
2004年 | 9520篇 |
2003年 | 9454篇 |
2002年 | 9349篇 |
2001年 | 9494篇 |
2000年 | 8853篇 |
1999年 | 9274篇 |
1998年 | 22460篇 |
1997年 | 15987篇 |
1996年 | 12251篇 |
1995年 | 9220篇 |
1994年 | 8160篇 |
1993年 | 8165篇 |
1992年 | 6148篇 |
1991年 | 6143篇 |
1990年 | 5843篇 |
1989年 | 5685篇 |
1988年 | 5595篇 |
1987年 | 4980篇 |
1986年 | 4962篇 |
1985年 | 5575篇 |
1984年 | 5153篇 |
1983年 | 4771篇 |
1982年 | 4421篇 |
1981年 | 4677篇 |
1980年 | 4402篇 |
1979年 | 4458篇 |
1978年 | 4574篇 |
1977年 | 5155篇 |
1976年 | 7097篇 |
1975年 | 4076篇 |
1974年 | 4040篇 |
1973年 | 4067篇 |
1972年 | 3562篇 |
1971年 | 3189篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Sheldon H. Jacobson Julie L. Virta Jon M. Bowman John E. Kobza John J. Nestor 《IIE Transactions》2003,35(3):259-269
Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide. 相似文献
52.
The main objective of this present study was to evaluate, for a standard mortar, the drying effect on its mechanical behaviour. Numerous uniaxial compression tests were thus performed with loading-unloading cycles. They were carried out on different samples previously preserved under various conditions of conservation: preserved from desiccation, air drying and rapid drying at 60°C. The obtained results showed significant influences of these conditions on the material behaviour (increase in strength, decrease in Young's modulus and Poisson's ratio) and the necessity of taking into account the coupling effects between mechanical—poromechanical behaviours and drying. 相似文献
53.
A.L. Pan H.G. ZhengZ.P. Yang F.X. Liu Z.J. Ding Y.T. Qian 《Materials Research Bulletin》2003,38(5):789-796
Small Ag particles or clusters dispersed mesoporous SiO2 composite films were prepared by a new method: First the matrix SiO2 films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO3 solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO2 films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements. 相似文献
54.
A new topology for a photonic signal processor, which overcomes the basic recursive frequency response problem that limits the passband range, is presented. The structure is based on a new multiple-wavelength offset-cavity structure that is cascaded with a series of unbalanced delay line structures. This not only can synthesize a very narrow notch response with good shape factor but also permits a multifold extension of the free spectral range (FSR) and passband width. Results on the interference mitigation filter demonstrate a stopband of 1% of center frequency and a fourfold increase in the FSR and passband width, while also having a very small shape factor, in excellent agreement with predictions. 相似文献
55.
D. Vignolles D. Smirnov G. Rikken B. Raquet H. Rakoto C. Proust M. Nardone J. Léotin F. Lecouturier M. Goiran O. Drachenko J. M. Broto L. Brossard A. Audouard 《Journal of Low Temperature Physics》2003,133(1-2):97-120
An overview over past and present activities and future developments at the Toulouse pulsed magnetic field facility is given, both as far as technical developments of the infrastructure, as well as low temperature physics performed at the LNCMP are concerned. 相似文献
56.
H. Yamamoto N. Heyamoto T. Matsui N. Murayama J. Shibata 《International Journal of Thermophysics》2003,24(5):1385-1394
Polyvinylalcohol (PVA) polymer gel is a temperature sensitive polymeric gel, with a critical transition temperature (with H2O) of 310 K. At higher than 310 K, this temperature sensitive polymer gel shrinks because of discharging water, whereas at lower temperatures, the gel swelled because of absorbing water. The reversibility of the gel's volume change was confirmed by temperature swing. The adsorption behavior of an organic compound onto the PVA polymer gel in water was tested at various temperatures. The amount of adsorbed organic compound increased remarkably at temperatures higher than about 310 K. Then, it was confirmed that the organic compound in PVA polymer gel could be reversibly adsorbed and desorbed by a temperature change between 293 and 323 K. The mechanism of adsorption of the organic compound onto the PVA polymer gel could be explained by hydration and dehydration of polymer gel. 相似文献
57.
58.
McConaghy T. Leung H. Bosse E. Varadan V. 《IEEE transactions on instrumentation and measurement》2003,52(6):1771-1779
Radial basis function (RBF) neural networks are used to classify real-life audio radar signals that are collected by a ground surveillance radar mounted on a tank. Currently, a human operator is required to operate the radar system to discern among signals bouncing off tanks, vehicles, planes, and so on. The objective of this project is to investigate the possibility of using a neural network to perform this target recognition task, with the aim of reducing the number of personnel required in a tank. Different signal classification methods in the neural net literature are considered. The first method employs a linear autoregressive (AR) model to extract linear features of the audio data, and then perform classification on these features, i.e, the AR coefficients. AR coefficient estimations based on least squares and higher order statistics are considered in this study. The second approach uses nonlinear predictors to model the audio data and then classifies the signals according to the prediction errors. The real-life audio radar data set used here was collected by an AN/PPS-15 ground surveillance radar and consists of 13 different target classes, which include men marching, a man walking, airplanes, a man crawling, and boats, etc. It is found that each classification method has some classes which are difficult to classify. Overall, the AR feature extraction approach is most effective and has a correct classification rate of 88% for the training data and 67% for data not used for training. 相似文献
59.
O. A. Lambri J. I. Prez-Landazbal J. A. Cano V. Recarte 《Materials Science and Engineering: A》2004,370(1-2):459-463
Mechanical spectroscopy, neutron diffraction and differential scanning calorimetry (DSC) were performed on commercial Fe–6 wt.% Si alloy after quenching from high temperature. The damping spectrum shows a peak at around 800 K and an associated modulus defect. The modulus shows an increase during the second and subsequent heating runs. In addition, an anomaly in the modulus behavior has been found at around 400 K. Different thermal treatments allows to obtain two different recovery degrees of the quenched-in defects. The influence of the recovery degree on the 800 K internal friction peak and on the anelastic modulus has been evaluated and confirm the validity of the grain boundary mechanism associated to this peak. Experimental results are discussed on the basis of recovery and ordering processes. 相似文献
60.