全文获取类型
收费全文 | 244034篇 |
免费 | 3257篇 |
国内免费 | 1241篇 |
专业分类
电工技术 | 4892篇 |
综合类 | 269篇 |
化学工业 | 35573篇 |
金属工艺 | 10891篇 |
机械仪表 | 7085篇 |
建筑科学 | 6260篇 |
矿业工程 | 481篇 |
能源动力 | 6304篇 |
轻工业 | 24192篇 |
水利工程 | 1842篇 |
石油天然气 | 1335篇 |
武器工业 | 3篇 |
无线电 | 31137篇 |
一般工业技术 | 44998篇 |
冶金工业 | 50292篇 |
原子能技术 | 3880篇 |
自动化技术 | 19098篇 |
出版年
2021年 | 1575篇 |
2019年 | 1436篇 |
2018年 | 2330篇 |
2017年 | 2231篇 |
2016年 | 2458篇 |
2015年 | 1921篇 |
2014年 | 3169篇 |
2013年 | 10826篇 |
2012年 | 5471篇 |
2011年 | 7830篇 |
2010年 | 6301篇 |
2009年 | 7004篇 |
2008年 | 7590篇 |
2007年 | 7681篇 |
2006年 | 7053篇 |
2005年 | 6550篇 |
2004年 | 6268篇 |
2003年 | 6287篇 |
2002年 | 6252篇 |
2001年 | 6394篇 |
2000年 | 5824篇 |
1999年 | 6434篇 |
1998年 | 16618篇 |
1997年 | 11651篇 |
1996年 | 8980篇 |
1995年 | 6712篇 |
1994年 | 5924篇 |
1993年 | 5744篇 |
1992年 | 4063篇 |
1991年 | 4009篇 |
1990年 | 3585篇 |
1989年 | 3512篇 |
1988年 | 3449篇 |
1987年 | 2915篇 |
1986年 | 2848篇 |
1985年 | 3419篇 |
1984年 | 3060篇 |
1983年 | 2750篇 |
1982年 | 2554篇 |
1981年 | 2576篇 |
1980年 | 2439篇 |
1979年 | 2276篇 |
1978年 | 2242篇 |
1977年 | 2817篇 |
1976年 | 4187篇 |
1975年 | 1903篇 |
1974年 | 1796篇 |
1973年 | 1787篇 |
1972年 | 1470篇 |
1971年 | 1319篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
H. Tsukamoto T.D. Boone J. Han J.M. Woodall 《Photonics Technology Letters, IEEE》2005,17(7):1411-1413
We present a novel optical switching technique utilizing emission packet positioning of semiconductor heterostructure. A modulation-doped p-AlGaAs-GaAs heterostructure is employed to control spontaneous emission packet positioning with electric fields. Emission packets generated by optical input signals are brought over 150 /spl mu/m with electric fields, so the output fibers can detect the emission intensity as signals. The first-order analysis indicates that the drift velocity of minority electrons in GaAs limits the detectable maximum data rate and nanoseconds timescale signal routing operation at 20 Gb/s is possible at an electron drift velocity of 2/spl times/10/sup 7/ cm/s. 相似文献
992.
We study deterministic finite automata (DFA) with recursive calls, that is, finite sequences of component DFAs that can call each other recursively. DFAs with recursive calls are akin to recursive state machines and unrestricted hierarchic state machines. We show that they are language equivalent to deterministic pushdown automata (DPDA). 相似文献
993.
Experiments in micro-flow devices almost always show deviations compared to the corresponding situations in macro-systems. Often special “micro-effects” are proposed to explain these unexpected results. However, based on a nondimensional form of the problem formulation these “micro-effects” can be identified as scaling effects referred to a standard analysis in macro-dimensions. Thus many “unexpected results” can be explained. This is demonstrated for a specific example recently published in this journal. 相似文献
994.
An automotive engine oil viscosity sensor 总被引:1,自引:0,他引:1
For the evaluation of the condition of automotive engine oil, the oil's viscosity is one of the most important parameters. Using microacoustic viscosity sensors, an oil-viscosity measurement can be performed on-board. In this contribution, we discuss the behavior of the viscosity of engine oil, its temperature dependence, and the resulting representation in terms of output signals of microacoustic viscosity sensors. These considerations are illustrated by means of measurement results obtained for used oil samples, which have been obtained from test cars and fresh oil samples out of different viscosity classes. Finally, the detection of the viscosity increase due to soot contamination is demonstrated. 相似文献
995.
BaXO4 (X = Mo, W) nanobelts and a variety of hierarchical superstructures assembled from the nanobelts have been synthesized in a catanionic reverse‐micelle system. The effects of various factors, such as the mixing ratio (r) between the anionic and cationic surfactants, the temperature, and the presence of polymeric additives, on the formation of the nanobelts and their hierarchical assembly have been examined in detail. In particular, r has been shown to be powerful in modulating the formation and assembly of the BaMoO4 and BaWO4 nanobelts. Architectural control of the penniform nanobelt superstructures has been readily achieved by changing the experimental parameters. A plausible two‐stage growth mechanism has been proposed for the formation of penniform BaXO4 nanobelt superstructures in catanionic reverse micelles. 相似文献
996.
997.
998.
Sanghyun Ju Jianye Li Pimparkar N. Alam M.A. Chang R.P.H. Janes D.B. 《Nanotechnology, IEEE Transactions on》2007,6(3):390-395
Nanorod field-effect transistors (FETs) that use multiple Mg-doped ZnO nanorods and a SiO2 gate insulator were fabricated and characterized. The use of multiple nanorods provides higher on-currents without significant degradation in threshold voltage shift and subthreshold slopes. It has been observed that the on-currents of the multiple ZnO nanorod FETs increase approximately linearly with the number of nanorods, with on-currents of ~1 muA per nanorod and little change in off-current (~4times10-12). The subthreshold slopes and on-off ratios typically improve as the number of nanorods within the device channel is increased, reflecting good uniformity of properties from nanorod to nanorod. It is expected that Mg dopants contribute to high n-type semiconductor characteristics during ZnO nanorod growth. For comparison, nonintentionally doped ZnO nanorod FETs are fabricated, and show low conductivity to compare with Mg-doped ZnO nanorods. In addition, temperature-dependent current-voltage characteristics of single ZnO nanorod FETs indicate that the activation energy of the drain current is very low (0.05-0.16 eV) at gate voltages both above and below threshold 相似文献
999.
Masturzo S.A. Yarrison-Rice J.M. Jackson H.E. Boyd J.T. 《Nanotechnology, IEEE Transactions on》2007,6(6):622-626
Grating couplers with nanoscale periodicity have been fabricated on silicon-on-insulator (SOI) substrates by electron beam lithography and reactive ion etching. A versatile experimental apparatus has been implemented to measure the efficiency of these gratings in coupling free-space radiation into planar waveguides. This coupling efficiency has been measured as a function of grating depth and the angle and wavelength of incident radiation. Coupling efficiencies of at least 5% and as high as 20% are demonstrated for wavelengths in the vicinity of 1550 nm and incident angles around 45deg. 相似文献
1000.
H. Oesterreicher 《Journal of Superconductivity and Novel Magnetism》2007,20(3):201-213
The degree of the isolation of the CuO2 planes (e.g. distance or bond valence to the apical coordination) has been shown by quantitative algorithms to be the major
factor in determining aspects of the doping curves. They include the magnitude of the optimal number of doped holes (hop) and the corresponding T
cop. It is shown that the roots of these phenomenological laws lie in a related structural dependence of super-exchange. The
latter is expressed in the pseudo-gap or Neel temperature of the undoped parent compound. A fruitful language can be developed
which deals with a buildup of complex quantum chemical features by bringing two holes into vicinity of a super-exchange O,
forming a “local” Cu2O7 pair. Structural considerations also dictate that stress is relieved by alternate orthogonal pair orientation. This leads
to plaid patterns with primary and secondary channels of charge. The presence of these two types of charge channels is involved
in the mechanism of superconducting charge transport. Similar structuring of doped charge into plaid patterns of “local” pairs
has been proposed for “all” high T
c superconductivity. STM now gives pictorial representation of the remnants of such an electronic crystal structure. The response
of these bond-ordering motifs to structural details is further discussed. These ideas supply organization to the manifold
experimental situation and provide opportunities for a unifying theory for high T
c superconductivity in terms of real space structuring of “local” pairs, largely on crystal-chemical principles. 相似文献