全文获取类型
收费全文 | 285051篇 |
免费 | 4039篇 |
国内免费 | 1313篇 |
专业分类
电工技术 | 4894篇 |
综合类 | 525篇 |
化学工业 | 42208篇 |
金属工艺 | 11203篇 |
机械仪表 | 8619篇 |
建筑科学 | 6671篇 |
矿业工程 | 1585篇 |
能源动力 | 7285篇 |
轻工业 | 23645篇 |
水利工程 | 3108篇 |
石油天然气 | 5821篇 |
武器工业 | 43篇 |
无线电 | 34025篇 |
一般工业技术 | 57634篇 |
冶金工业 | 51177篇 |
原子能技术 | 7535篇 |
自动化技术 | 24425篇 |
出版年
2021年 | 2496篇 |
2020年 | 1841篇 |
2019年 | 2325篇 |
2018年 | 3875篇 |
2017年 | 3932篇 |
2016年 | 4247篇 |
2015年 | 2759篇 |
2014年 | 4755篇 |
2013年 | 12643篇 |
2012年 | 7608篇 |
2011年 | 10342篇 |
2010年 | 7940篇 |
2009年 | 9198篇 |
2008年 | 9478篇 |
2007年 | 9487篇 |
2006年 | 8090篇 |
2005年 | 7479篇 |
2004年 | 7403篇 |
2003年 | 7124篇 |
2002年 | 6874篇 |
2001年 | 6924篇 |
2000年 | 6548篇 |
1999年 | 6830篇 |
1998年 | 16383篇 |
1997年 | 11635篇 |
1996年 | 8973篇 |
1995年 | 6892篇 |
1994年 | 6059篇 |
1993年 | 6097篇 |
1992年 | 4427篇 |
1991年 | 4215篇 |
1990年 | 4105篇 |
1989年 | 4093篇 |
1988年 | 3772篇 |
1987年 | 3417篇 |
1986年 | 3414篇 |
1985年 | 3761篇 |
1984年 | 3492篇 |
1983年 | 3228篇 |
1982年 | 3041篇 |
1981年 | 3045篇 |
1980年 | 3045篇 |
1979年 | 2833篇 |
1978年 | 2871篇 |
1977年 | 3147篇 |
1976年 | 4104篇 |
1975年 | 2422篇 |
1974年 | 2384篇 |
1973年 | 2398篇 |
1972年 | 2018篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
951.
Wood A.C.G. O'Neill A.G. Phillips P.J. Biswas R.G. Whall T.E. Parker E.H.C. 《Electron Devices, IEEE Transactions on》1993,40(1):157-162
Delta-doped MOSFETs have been fabricated in MBE-grown silicon using for the first time boron as the dopant within the delta layer. Current-voltage characteristics have been measured, and secondary ion mass spectrometry (SIMS) is used to confirm the location of the delta layer and the extent of layer broadening by diffusion during processing. Precise threshold voltages of the devices are difficult to determine since the devices (which all operate in depletion mode) take several volts to switch off. Transconductances of the devices have been measured, and it is shown how analysis of these results can yield estimates of the carrier mobility for transport along the delta layers despite the uncertainty in the threshold voltage. A clear transition is observed in the results which is attributed to the formation of a parasitic surface-channel field-effect transistor, providing conclusive evidence that the devices are conducting along a delta channel for part of the measured range of applied gate biases 相似文献
952.
Internationalization of technology remains a subject of considerable interest to analysts and policy makers in the 1990s. This paper is a modest attempt at providing some new empirical observations for debate and discussion on one dimension of the subject, namely, the nature and extent of the production of technology undertaken by large firms outside their home base. The evidence, based on the US patenting activities of the world's largest 539 firms (based in 13 countries and covering 16 product groups), shows that for an overwhelming majority of them technology production remains close to the home base. It also shows that firms devoting a large proportion of their resources to technology are amongst the least internationalized. When these firms do go abroad, there is no systematic relationship between their relative presence in a technical field and the relative strength of the host country. The analysis points to the dangers of generalizing on the basis of anecdotal evidence from a small sample of firms from a particular country or sector 相似文献
953.
954.
L De Petrocellis P Orlando M Gavagnin M Ventriglia G Cimino V Di Marzo 《Canadian Metallurgical Quarterly》1996,52(9):874-877
Five novel 1,2-sn-diacylglycerols with diterpenoid acyl moieties in the sn-1 position were isolated and characterized, together with the corresponding 1,3-sn-diacylglycerols, from three species of dorid nudibranchs molluscs. Their potent activity as morphogens in vivo in the Hydra tentacle regeneration assay and their parallel activity as activators of rat brain protein kinase C (PKC) in vitro are reported here. Our findings promote the use of these compounds as useful molecular probes for both in vivo and in vitro studies on the participation of PKC in cell development. 相似文献
955.
The three-dimensional solution structure of des-[Phe(B25)] human insulin has been determined by nuclear magnetic resonance spectroscopy and restrained molecular dynamics calculations. Thirty-five structures were calculated by distance geometry from 581 nuclear Overhauser enhancement-derived distance constraints, ten phi torsional angle restraints, the restraints from 16 helical hydrogen bonds, and three disulfide bridges. The distance geometry structures were optimized using simulated annealing and restrained energy minimization. The average root-mean-square (r.m.s.) deviation for the best 20 refined structures is 1.07 angstroms for the backbone and 1.92 angstroms for all atoms if the less well-defined N and C-terminal residues are excluded. The helical regions are more well defined, with r.m.s. deviations of 0.64 angstroms for the backbone and 1.51 angstroms for all atoms. It is found that the des-[Phe(B25)] insulin is a monomer under the applied conditions (4.6 to 4.7 mM, pH 3.0, 310 K), that the overall secondary and tertiary structures of the monomers in the 2Zn crystal hexamer of native insulin are preserved, and that the conformation-averaged NMR solution structure is close to the structure of molecule 1 in the hexamer. The structure reveals that the lost ability of des-[Phe(B25)] insulin to self-associate is caused by a conformational change of the C-terminal region of the B-chain, which results in an intra-molecular hydrophobic interaction between Pro(B28) and the hydrophobic region Leu(B11)-Leu(B15) of the B-chain alpha-helix. This interaction interferes with the inter-molecular hydrophobic interactions responsible for the dimerization of native insulin, depriving the mutant of the ability to dimerize. Further, the structure displays a series of features that may explain the high potency of the mutant on the basis of the current model for the insulin-receptor interaction. These features are: a change in conformation of the C-terminal region of the B-chain, the absence of strong hydrogen bonds between this region and the rest of the molecule, and a relatively easy accessibility to the Val(A3) residue. 相似文献
956.
957.
S. Bismo P. Duverneuil L. Pibouleau S. Domenech J. -P. Couderc 《Materials and Manufacturing Processes》1995,10(2):241-266
This paper presents the results of our work on a new type of CVD reactor, annular reactor. This equipment is able to deposit pure silicon and also in-situ phosphorus doped silicon on a large number of substrates 相似文献
958.
M. Bethmont Y. Meyzaud P. Soulat 《International Journal of Pressure Vessels and Piping》1996,65(3):221-229
Primary piping elbows of ligh water reactors are made of cast austeno-ferritic materials (grade CF8 or CF8M) which are susceptible to embrittlement by thermal ageing in service. Ageing kinetics depends on chromium content and on the temperature. The effect of ageing on mechanical properties is determined by the decreasing of Charpy-U energy. It is shown that the rupture of such aged material comes from a combination of cleavage of the ferrite and shearing or ductile tearing of austenite ligaments. The fracture toughness is determined by J-R curves, which exhibit a large scatter related to the solidification macrostructure of the steel. It is shown that J-R curves are independent of the geometry of the specimens. 相似文献
959.
Piquemal F.P.M. Trapon G.R. Geneves G.P.J. 《IEEE transactions on instrumentation and measurement》1996,45(6):918-922
AC measurements of the longitudinal resistance, Rxx, of a quantum Hall effect (QHE) sample have been made in a frequency range from 10 Hz to 10 kHz. The results show no frequency effect on the minimum value of Rxx corresponding to the quantum numbers i=2 and i=4, within the measurement resolution of 0.5 mΩ. Therefore, the influence of frequency on the value of the quantized Hall resistance, RH, should not exceed a few parts in 109 . Some unwanted effects detected during the development of the resistance bridge have been pointed out 相似文献
960.
Summary Small oscillations of a rotating elasticum with a mass at the free end are investigated with Poincaré-Lindstedt series. It is shown that the mass moves on a figure-eight shaped curve in a direction determined by the sign of the angular velocity and hence that the Coriolis force influences the motion. 相似文献