全文获取类型
收费全文 | 40105篇 |
免费 | 1511篇 |
国内免费 | 66篇 |
专业分类
电工技术 | 412篇 |
综合类 | 31篇 |
化学工业 | 7783篇 |
金属工艺 | 801篇 |
机械仪表 | 842篇 |
建筑科学 | 2099篇 |
矿业工程 | 139篇 |
能源动力 | 1124篇 |
轻工业 | 3269篇 |
水利工程 | 471篇 |
石油天然气 | 123篇 |
武器工业 | 5篇 |
无线电 | 2807篇 |
一般工业技术 | 6610篇 |
冶金工业 | 7843篇 |
原子能技术 | 317篇 |
自动化技术 | 7006篇 |
出版年
2023年 | 212篇 |
2022年 | 359篇 |
2021年 | 711篇 |
2020年 | 482篇 |
2019年 | 644篇 |
2018年 | 818篇 |
2017年 | 731篇 |
2016年 | 868篇 |
2015年 | 784篇 |
2014年 | 1078篇 |
2013年 | 2568篇 |
2012年 | 1783篇 |
2011年 | 2214篇 |
2010年 | 1777篇 |
2009年 | 1648篇 |
2008年 | 1953篇 |
2007年 | 1917篇 |
2006年 | 1714篇 |
2005年 | 1575篇 |
2004年 | 1288篇 |
2003年 | 1243篇 |
2002年 | 1149篇 |
2001年 | 775篇 |
2000年 | 630篇 |
1999年 | 701篇 |
1998年 | 850篇 |
1997年 | 726篇 |
1996年 | 686篇 |
1995年 | 676篇 |
1994年 | 607篇 |
1993年 | 592篇 |
1992年 | 556篇 |
1991年 | 333篇 |
1990年 | 471篇 |
1989年 | 444篇 |
1988年 | 382篇 |
1987年 | 415篇 |
1986年 | 356篇 |
1985年 | 468篇 |
1984年 | 465篇 |
1983年 | 371篇 |
1982年 | 353篇 |
1981年 | 325篇 |
1980年 | 298篇 |
1979年 | 309篇 |
1978年 | 292篇 |
1977年 | 272篇 |
1976年 | 280篇 |
1975年 | 234篇 |
1973年 | 214篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
41.
Dongsu Kim Yoonsu Choi Ahn M. Allen M.G. Stevenson Kenney J. Marry P. 《Microwave and Wireless Components Letters, IEEE》2003,13(10):434-436
Continuously variable ferroelectric (BST on sapphire) phase shifters based on all-pass networks are presented. An all-pass network phase shifter consists of only lumped LC elements, and thus the total size of the phase shifter is kept to less than 2.2 mm /spl times/ 2.6 mm at 2.4 GHz. The tunability (C/sub max//C/sub min/) of a BST interdigital capacitor is over 2.9 with a bias voltage of 140 V. The phase shifter provides more than 121/spl deg/ phase shift with the maximum insertion loss of 1.8 dB and the worst case return loss of 12.5 dB from 2.4 GHz to 2.5 GHz. By cascading two identical phase shifters, more than 255/spl deg/ phase shift is obtained with the maximum insertion loss of 3.75 dB. The loss figure-of-merit of both the single- and double-section phase shifters is over 65/spl deg//dB from 2.4 GHz to 2.5 GHz. 相似文献
42.
This article introduces a special section on the use of taxometrics to examine the categorical versus the dimensional structure of various forms of psychopathology. Paving the way into the special section, this introduction briefly describes 3 taxometric methods--mean above minus below a sliding cut (MAMBAC), maximum covariation (MAXCOV), and maximum eigenvalue (MAXEIG)--and discusses possible threats to statistical conclusion validity that often emerge when such techniques are applied in psychopathology research. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
43.
Three-dimensional models, or pharmacophores, describing Euclidean constraints on the location on small molecules of functional
groups (like hydrophobic groups, hydrogen acceptors and donors, etc.), are often used in drug design to describe the medicinal
activity of potential drugs (or ‘ligands’). This medicinal activity is produced by interaction of the functional groups on
the ligand with a binding site on a target protein. In identifying structure-activity relations of this kind there are three
principal issues: (1) It is often difficult to “align” the ligands in order to identify common structural properties that
may be responsible for activity; (2) Ligands in solution can adopt different shapes (or `conformations’) arising from torsional
rotations about bonds. The 3-D molecular substructure is typically sought on one or more low-energy conformers; and (3) Pharmacophore
models must, ideally, predict medicinal activity on some quantitative scale. It has been shown that the logical representation
adopted by Inductive Logic Programming (ILP) naturally resolves many of the difficulties associated with the alignment and
multi-conformation issues. However, the predictions of models constructed by ILP have hitherto only been nominal, predicting
medicinal activity to be present or absent. In this paper, we investigate the construction of two kinds of quantitative pharmacophoric
models with ILP: (a) Models that predict the probability that a ligand is “active”; and (b) Models that predict the actual
medicinal activity of a ligand. Quantitative predictions are obtained by the utilising the following statistical procedures
as background knowledge: logistic regression and naive Bayes, for probability prediction; linear and kernel regression, for
activity prediction. The multi-conformation issue and, more generally, the relational representation used by ILP results in
some special difficulties in the use of any statistical procedure. We present the principal issues and some solutions. Specifically,
using data on the inhibition of the protease Thermolysin, we demonstrate that it is possible for an ILP program to construct
good quantitative structure-activity models. We also comment on the relationship of this work to other recent developments
in statistical relational learning.
Editors: Tamás Horváth and Akihiro Yamamoto 相似文献
44.
The effects of parameter uncertainty on optimal policy have been a matter of interest for academics, and even for some policymakers,
for a long time. Two lines of literature have developed analytical results on this matter. The first line uses static models
and the second dynamic models. In this dynamic line most of the results are confined to models with a single state and a single
control variable.
In this paper we want to encourage the analysis of more general dynamic cases. To do so, the results in the dynamic line are
extended from one-state and one-control finite horizon models to models with a pair of control variables. We then discuss
some of the hurdles which must be surmounted for the results to be made more general and suggests some lines for further research.
JEL classification: C61; E61 相似文献
45.
Hambleton P.J. Ng B.K. Plimmer S.A. David J.P.R. Rees G.J. 《Electron Devices, IEEE Transactions on》2003,50(2):347-351
The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions. 相似文献
46.
S. David Graber 《Canadian Metallurgical Quarterly》2004,130(1):67-80
Numerous types of pipes and channels with spatially increasing flows in environmental engineering applications are identified by type and function and referred to as collection conduits. An overview of methods for designing and analyzing collection conduits is provided. Full conduits with nonuniform and uniform inflow are first considered. Dimensional analysis is then employed to demonstrate the relationship between variables for open channels; that leads to the identification of possibilities for generalized numerical solutions. Prior collection conduit applications are discussed within the framework of the dimensional analysis (which also pertains to some constant-flow applications). A previously unpublished generalized numerical solution for rectangular collection conduits is presented. Subsurface drains are addressed with particular emphasis, including the use of numerical methods to develop a new generalized chart and relation to other design methods. Among the important conclusions for subsurface drains is that the somewhat common practice of using Manning’s equation alone for such problems is not generally adequate. Examples and practical design suggestions are included, and the use of computer-based numerical methods is discussed more generally. 相似文献
47.
Mark Parsons Tony Allen 《今日电子》2006,(4):44-45
对于绝大多数微控制器时钟电路而言,硅振荡器是一种简单且有效的解决方案。与晶体和陶瓷谐振器不同,基于硅材料的定时器具有抗振动、抗撞击和抗电磁干扰的优点。同时,硅振荡器不需要严格匹配的定时元件和线路板走线。 相似文献
48.
49.
A.S. Fomichev I. David S.M. Lukyanov Yu.E. Penionzhkevich N.K. Skobelev O.B. Tarasov 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》1994,350(3):605-607
The mass and charge identification of secondary particles with Z < 4 by a large CsI(T1) scintillation detector is performed using pulse shape analysis and time-of-flight methods. The dependence of the light output on E, A and Z is studied in the energy range of 1–20 MeV/A and special attention is paid to the integration time of the photomultiplier anode signal. It is found that the behaviour of the calibration curves strongly depends on the choice of the integration time interval. 相似文献
50.
A Trattner Z Reizis M David A Ingber J Hagler M Sandbank 《Canadian Metallurgical Quarterly》1993,129(5):590-593
Interferon alpha-2a, which has antineoplastic, antiviral, immunomodulatory, and antiangiogenic effects, was evaluated in a prospective study, treating 10 lesions of patients with classical Kaposi's sarcoma. Ten patients received injections of interferon alpha-2a intralesionally in a dose of 3 million units three times weekly for 4 weeks, and in a variable dose for 4 more weeks. Two of the patients had a complete response, and in one of these, distant lesions also responded. Seven had a partial response, and one did not respond. The treatment was generally well tolerated. The results of our study support the use of interferon in the therapy of classical Kaposi's sarcoma, although it would appear that to achieve maximum efficacy, a longer period of treatment is needed. 相似文献