全文获取类型
收费全文 | 295479篇 |
免费 | 7101篇 |
国内免费 | 3531篇 |
专业分类
电工技术 | 7597篇 |
技术理论 | 5篇 |
综合类 | 3305篇 |
化学工业 | 42433篇 |
金属工艺 | 13305篇 |
机械仪表 | 10806篇 |
建筑科学 | 9902篇 |
矿业工程 | 2282篇 |
能源动力 | 8154篇 |
轻工业 | 26212篇 |
水利工程 | 2597篇 |
石油天然气 | 4943篇 |
武器工业 | 416篇 |
无线电 | 40017篇 |
一般工业技术 | 53131篇 |
冶金工业 | 50826篇 |
原子能技术 | 4180篇 |
自动化技术 | 26000篇 |
出版年
2022年 | 2420篇 |
2021年 | 3665篇 |
2020年 | 2640篇 |
2019年 | 2849篇 |
2018年 | 3647篇 |
2017年 | 3828篇 |
2016年 | 3824篇 |
2015年 | 3903篇 |
2014年 | 5689篇 |
2013年 | 13660篇 |
2012年 | 8526篇 |
2011年 | 11303篇 |
2010年 | 9308篇 |
2009年 | 10066篇 |
2008年 | 10642篇 |
2007年 | 10877篇 |
2006年 | 10294篇 |
2005年 | 9360篇 |
2004年 | 8220篇 |
2003年 | 7781篇 |
2002年 | 7407篇 |
2001年 | 7510篇 |
2000年 | 7277篇 |
1999年 | 7880篇 |
1998年 | 16726篇 |
1997年 | 12016篇 |
1996年 | 9888篇 |
1995年 | 7495篇 |
1994年 | 6614篇 |
1993年 | 6151篇 |
1992年 | 4580篇 |
1991年 | 4398篇 |
1990年 | 4230篇 |
1989年 | 3988篇 |
1988年 | 3712篇 |
1987年 | 3066篇 |
1986年 | 3034篇 |
1985年 | 3451篇 |
1984年 | 3111篇 |
1983年 | 2879篇 |
1982年 | 2741篇 |
1981年 | 2652篇 |
1980年 | 2393篇 |
1979年 | 2320篇 |
1978年 | 2143篇 |
1977年 | 2596篇 |
1976年 | 3490篇 |
1975年 | 1857篇 |
1974年 | 1681篇 |
1973年 | 1696篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
A novel class of narrow-band tunable wavelength filters is proposed and evaluated. Wavelength selectivity of the proposed filters Is derived from the finite time response of an optical nonlinearity. The nonlinearity is gain saturation in semiconductor optical amplifier structures. The filters are shown to have very narrow passbands tunable over the entire semiconductor gain bandwidth. The key to filter implementation is a device configuration in which the wave-mixing products can be isolated from the amplified inputs. Three integrated optics compatible configurations are considered and shown to have high filter throughputs 34 to 180% and subangstrom bandwidths 相似文献
42.
It is not known whether impaired hematopoiesis noted during human immunodeficiency virus (HIV) infection results from infection of stem/progenitor cells or of cells of the bone marrow microenvironment. Normal adherent primary stromal layers were exposed to HIV to determine which of this mixture of endothelial cells, fibroblasts, and macrophages are susceptible to the virus. Viral p24 in supernatants was noted with monocytotropic HIV-1Ada, HIV-1Ba-L, and HIV-1JR-FL but not with lymphotropic HIV-1LAI nor HIV-1MN strain, and only stromal macrophages expressed the viral antigens. Coculture of the layers with PHA-activated normal lymphocytes failed to rescue lymphotropic virus. No p24 was produced when macrophage-depleted stromal cells were exposed to either HIV-1Ba-L or HIV-1LAI; proviral DNA was then amplified by PCR in cells exposed to either virus, though coculture with lymphocytes rescued only HIV-1Ba-L. Altogether, these data indicate that macrophages are the major targets of HIV in cultured stromal layers. As virus replication in macrophages did not affect the profile of major cytokines involved in regulating hematopoiesis, HIV infection could alter hematopoiesis by other as yet unspecified mechanisms. 相似文献
43.
J. Bartonicek W. Zaiss W. Hienstorfer H. Kocklemann F. Schckle 《Nuclear Engineering and Design》1995,153(2-3)
At GKN, fatigue monitoring of important components has been conducted since 1979. The monitoring methods depend on the mechanisms of damage; quasi-static loads are regarded as well as dynamic loads. The components were selected for monitoring on the basis of a system analysis. The data resulting from monitoring are used to optimise operation mode steadily. Experience shows that the use of monitoring data as input for fatigue assessment is the most realistic and cost-effective way. This fatigue assessment uses global and local sensitivity studies to evaluate the load-stress relation for each component. These relations can be programmed to produce stress vs. time curves. These are processed according to ASME rules to give a realistic fatigue usage. 相似文献
44.
This paper analyzes probability of bit-error (Pe) performance of asynchronous bandlimited direct-sequence code-division multiple-access systems with binary phase-shift keying spreading. The two present methods of Pe analysis under bandwidth-efficient pulse shaping: the often-cited standard Gaussian approximation and the characteristic function (CF) method suffer from either a low accuracy in regions of low Pe (< 10-3) or a prohibitively large computational complexity. The paper presents an alternate method of Pe analysis with moderate computational complexity and high accuracy based on a key observation. A sequence of chip decision statistics (whose sum yields a bit statistic) forms a stationary, m-dependent sequence when conditioned on the chip delay and phase offset of each interfering signal. This observation permits the generalization of the improved Gaussian approximation previously derived for the rectangular pulse and the derivation of a numerically efficient approximation based on the CF method. Numerical examples of systems using the square-root raised-cosine and IS-95 pulses illustrate THE P e performance, user capacity and the accuracy of the proposed method 相似文献
45.
46.
Field surveys of a series of conjugated diunsaturated straight-chain primary alcohols, acetates, and aldehydes (dienes), including the 3,5-dode-cadienyl, 8,10-dodecadienyl, 3,5-tetradecadienyl, 8,10-tetradecadienyl, 9,11-tetradecadienyl, and 10,12-tetradecadienyl primary alcohols, acetates, and most aldehydes, and the related monounsaturated straight-chain primary alcohols, acetates, and aldehydes (monoenes), including the 3-dodecadienyl alcohol and acetates, attracted the following species from the title families. Diene attractants have been demonstrated for at least onePhyllonorycter sp.,Chionodes lugubrella (Fabr.),Leptostales ferruminaria (Zell.),Drepana bilineata (Pack.),Pyrausta fodinalis (Led.),Notocelia purpurissatana Heinr.,Phaneta alterana Heinr. Monoene attractants were demonstrated forHedya separatana (Kft.),Cydia fletcherana (Kft.),Barbara mappana Free.,Ancylis burgessiana (Zell.),Ancylis nubeculana Clem.,Evippe prunifoliella Cham.,Phlyctaenia coronata tertialis (Gn.), aChionodes sp., a Gelechiidae sp., and an unidentifiedPhyllonorycter sp. Replicated field experiments showed attraction-inhibition relationships for most species. Electroantennagram data are presented forLeptostales ferruminaria andDrepana bilineata. 相似文献
47.
Summary Starting with Z-2, 3-diamino-2-butenedinitrile as diamine component the synthesis of the low molecular N,N,O,O-chelate 3.Co and its ligand 3 is optimized. An easy synthesis of the chelate 8.Co covalently bound on macroreticular resins is presented. The chelates 8.Co are the most active heterogenous catalysts for the valence isomerization of quadricyclane to norbornadiene today. 相似文献
48.
Liang Y.C. Wenjiang Zeng Pick Hong Ong Zhaoxia Gao Jun Cai Balasubramanian N. 《Electron Device Letters, IEEE》2002,23(12):700-703
In this letter, a concise process technology is proposed for the first time to enable the fabrication of good quality three-dimensional (3-D) suspended radio frequency (RF) micro-inductors on bulk silicon, without utilizing the lithography process on sidewall and trench-bottom patterning. Samples were fabricated to demonstrate the applicability of the proposed process technology. 相似文献
49.
Ahn Jaeshin Stromsmoe Keith A. Lawson Ronald P. W. 《Industrial Electronics, IEEE Transactions on》1985,(4):405-409
A microprocessor-based system with 32 A/D, 24 D/A, and 16 ac load controllers, has been designed and built to monitor and control an ion beam thin-film deposition system. The A/D and D/A channels have electrical isolation of 7.5 kV between channels and between input and output. The microprocessor system keeps the ion beam deposition parameters stable for extended periods of operation and it is proposed as a means to greatly simplify switching from one deposition species to another to grow thin multilayer or alloy films. 相似文献
50.
Poly(vinyl alcohol) is crosslinked in dilute solution (c=0.1 wt%) with glutaraldehyde. The reaction product is characterized by viscometry and gel permeation chromatography (g.p.c.). The intrinsic viscosity decreases with increasing degree of crosslinking and does not depend on temperature. G.p.c. reveals that the reaction product is not homogeneous, but consists of a mixture of particles with different sizes, possibly both intra- and intermolecularly crosslinked molecules. The intramolecularly crosslinked molecules are smaller in size than the initial polymer molecules and their size depends on the degree of crosslinking. They possess a narrow particle size distribution even if the initial polymer sample had a broad molecular weight distribution. 相似文献