全文获取类型
收费全文 | 49460篇 |
免费 | 2932篇 |
国内免费 | 49篇 |
专业分类
电工技术 | 324篇 |
综合类 | 41篇 |
化学工业 | 10148篇 |
金属工艺 | 859篇 |
机械仪表 | 914篇 |
建筑科学 | 1593篇 |
矿业工程 | 108篇 |
能源动力 | 1169篇 |
轻工业 | 7522篇 |
水利工程 | 450篇 |
石油天然气 | 236篇 |
武器工业 | 4篇 |
无线电 | 2743篇 |
一般工业技术 | 8235篇 |
冶金工业 | 11141篇 |
原子能技术 | 234篇 |
自动化技术 | 6720篇 |
出版年
2023年 | 448篇 |
2022年 | 784篇 |
2021年 | 1352篇 |
2020年 | 996篇 |
2019年 | 990篇 |
2018年 | 1781篇 |
2017年 | 1754篇 |
2016年 | 1860篇 |
2015年 | 1337篇 |
2014年 | 1804篇 |
2013年 | 3404篇 |
2012年 | 2909篇 |
2011年 | 2855篇 |
2010年 | 2246篇 |
2009年 | 2101篇 |
2008年 | 2284篇 |
2007年 | 2005篇 |
2006年 | 1508篇 |
2005年 | 1260篇 |
2004年 | 1149篇 |
2003年 | 1036篇 |
2002年 | 897篇 |
2001年 | 584篇 |
2000年 | 582篇 |
1999年 | 741篇 |
1998年 | 3575篇 |
1997年 | 2165篇 |
1996年 | 1427篇 |
1995年 | 845篇 |
1994年 | 719篇 |
1993年 | 694篇 |
1992年 | 251篇 |
1991年 | 243篇 |
1990年 | 235篇 |
1989年 | 244篇 |
1988年 | 216篇 |
1987年 | 202篇 |
1986年 | 202篇 |
1985年 | 234篇 |
1984年 | 200篇 |
1983年 | 143篇 |
1982年 | 169篇 |
1981年 | 187篇 |
1980年 | 164篇 |
1979年 | 134篇 |
1978年 | 124篇 |
1977年 | 206篇 |
1976年 | 430篇 |
1975年 | 92篇 |
1973年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Xueming Huang Gordon I. McCalla Jim E. Greer Eric Neufeld 《User Modeling and User-Adapted Interaction》1991,1(1):87-115
A user/student model must be revised when new information about the user/student is obtained. But a sophisticated user/student model is a complex structure that contains different types of knowledge. Different techniques may be needed for revising different types of knowledge. This paper presents a student model maintenance system (SMMS) which deals with revision of two important types of knowledge in student models: deductive knowledge and stereotypical knowledge. In the SMMS, deductive knowledge is represented by justified beliefs. Its revision is accomplished by a combination of techniques involving reason maintenance and formal diagnosis. Stereotypical knowledge is represented in the Default Package Network (DPN). The DPN is a knowledge partitioning hierarchy in which each node contains concepts in a sub-domain. Revision of stereotypical knowledge is realized by propagating new information through the DPN to change default packages (stereotypes) of the nodes in the DPN. A revision of deductive knowledge may trigger a revision of stereotypical knowledge, which results in a desirable student model in which the two types of knowledge exist harmoniously. 相似文献
112.
Gérard Battail 《电信纪事》1987,42(9-10):499-509
We propose to redefine weighted decoding of a redundant code as consisting of reestimating a given prior probability distribution in order to take into account the code constraints. The optimum solution of this problem results from using the « minimum cross-entropy » principle and is shown to coincide with the computation of the probabilities to be compared in order to perform maximum likelihood decoding when the prior distribution is exactly known. The principle however also holds when the prior distribution is approximate or biased, which explains the robustness of decoding with respect to weighting information. Such a redefinition also enables using the intermediate steps of decoding, in the case of codes associated by concatenation or product, as a source of weighting information for the subsequent steps. One is led for simplicity’s sake to an estimate of the posterior probability in separable form with respect to the information symbols ; its definition is given and certain of its applications are discussed. 相似文献
113.
114.
During large scale wildfires, suppression activities are carried out under the direction of an Incident Management Team (IMT). The aim of the research was to increase understanding of decision processes potentially related to IMT effectiveness. An IMT comprises four major functions: Command, Operations, Planning, and Logistics. Four methodologies were used to study IMT processes: computer simulation experiments; analyses of wildfire reports; interviews with IMT members; and cognitive ethnographic studies of IMTs. Three processes were important determinants of IMT effectiveness: information management and cognitive overload; matching component function goals to overall goals; and team metacognition to detect and counter task‐disruptive developments. These processes appear to be complex multi‐person analogues of individual Incident Command processes identified previously. The findings have implications for issues such as: creating IMTs; training IMTs; managing IMTs; and providing decision support to IMTs. 相似文献
115.
There are some foods that contain mutagenic or carcinogenic agents, some of which occur naturally and others that may be formed during preparation or cooking. Several foods such as legumes, also contain natural antimutagens and/or anticarcinogens. Lupine is one such legume that contains high amounts of protein (40%) and oils (14%). About 90 species of lupine have been reported throughout Mexico. However, the use of this crop as a source of food has been limited by the presence of antinutritional agents such as phenolic compounds (PC), carbohydrates (CH) and quinolizidinic alkaloids (Qas). It has also been suggested that consuming these compounds can affect human health and may even reduce the risk of disease. The objective of this work was to determine the effect of PC, CH and Qas, isolated and quantified from Lupinus campestris on the mutagenicity of 1-nitropyrene (1-NP) as a model mutagen and we used the Salmonella typhimurium tester strain YG1024 by the Kado microsuspension method. The results indicate that L. campestris seeds have 11 mg (+)catechin equivalent g(-1) seed coat; 120.3 mg g(-1) seeds and 2.13 mg g(-1) seeds of PC, CH and Qas, respectively. 1-NP mutagenicity was inhibited by 86% for PC, 76% for CH and 75% for Qas at concentrations of 200, 512 and 13.6 microg/tube, respectively. 相似文献
116.
Daniel X. Gouveia Odair P. Ferreira Antonio G. Souza Filho M. G. da Silva J. A. C. de Paiva Oswaldo L. Alves Josué Mendes Filho 《Journal of Materials Science》2007,42(2):534-538
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution. 相似文献
117.
Mamiko Hori Cécile Pagnoux Jean-François Baumard Masayuki Nogami 《Journal of Materials Science》2007,42(1):80-86
A new method is proposed to produce gold nanoparticles (GNP) by in situ reduction of a gold salt dissolved in water. The reducing
agent used is Tiron instead of the citrate anion most often mentioned in literature. The influence of various parameters has
been investigated, such as the content of Tiron with respect to that of the precursor of gold HAuCl4, or the initial pH of the solution after mixing of reactants. It is shown that Tiron also exerts a positive influence as
a dispersant, which impedes agglomeration of gold nanoparticles. The typical average size of GNP synthesized in the present
work is close to 7 nm. 相似文献
118.
A. Hernández O. Arés C. Hart D. Domínguez H. Pastoriza A. Butera 《Journal of Low Temperature Physics》2004,135(1-2):119-122
The response of mesoscopic superconductors to an ac magnetic field is investigated both experimentally and with numerical simulations. We study small square samples with dimensions of the order of the penetration depth. We obtain the ac susceptibitity χ=χ′+iχ″ at microwave frequencies as a function of the dc magnetic field H dc. We find that the dissipation, given by x″, has a non monotonous behavior in mesoscopic samples. In the numerical simulations we obtain that the dissipation increases before the penetration of vortices and then it decreases abruptly after vortices have entered, the sample. This is verified experimentally, where we find that χ″ has strong oscillations as a function of H dc in small squares of Pb. 相似文献
119.
这个最新设计实例介绍了一种以少量无源器件来设计简单的高频LC振荡器的方法。但为获得最佳结果,稳定振荡器的实际硬件设计需要更多的器件且更为复杂。图1显示一种具有自动电平输出幅度控制以及能提供具有较低谐波含量正弦波输出缓冲的18MHz稳定振荡器(参考文献2)。此外,本设计实例还用英飞凌科技公司(Infineon Technologies)的廉价BF998型双栅极MOSFET替换了原来的JFET振荡器,该双栅极MOSFET可从DigiKey及其它公司购买。 相似文献
120.
David Alleysson Sabine Süsstrunk Jeanny Hérault 《IEEE transactions on image processing》2005,14(4):439-449
There is an analogy between single-chip color cameras and the human visual system in that these two systems acquire only one limited wavelength sensitivity band per spatial location. We have exploited this analogy, defining a model that characterizes a one-color per spatial position image as a coding into luminance and chrominance of the corresponding three colors per spatial position image. Luminance is defined with full spatial resolution while chrominance contains subsampled opponent colors. Moreover, luminance and chrominance follow a particular arrangement in the Fourier domain, allowing for demosaicing by spatial frequency filtering. This model shows that visual artifacts after demosaicing are due to aliasing between luminance and chrominance and could be solved using a preprocessing filter. This approach also gives new insights for the representation of single-color per spatial location images and enables formal and controllable procedures to design demosaicing algorithms that perform well compared to concurrent approaches, as demonstrated by experiments. 相似文献