全文获取类型
收费全文 | 207559篇 |
免费 | 2375篇 |
国内免费 | 647篇 |
专业分类
电工技术 | 4195篇 |
综合类 | 131篇 |
化学工业 | 28931篇 |
金属工艺 | 7720篇 |
机械仪表 | 5634篇 |
建筑科学 | 4468篇 |
矿业工程 | 800篇 |
能源动力 | 4954篇 |
轻工业 | 15027篇 |
水利工程 | 1799篇 |
石油天然气 | 2942篇 |
武器工业 | 7篇 |
无线电 | 27898篇 |
一般工业技术 | 39041篇 |
冶金工业 | 47120篇 |
原子能技术 | 3655篇 |
自动化技术 | 16259篇 |
出版年
2021年 | 1389篇 |
2020年 | 1042篇 |
2019年 | 1388篇 |
2018年 | 2279篇 |
2017年 | 2255篇 |
2016年 | 2361篇 |
2015年 | 1655篇 |
2014年 | 2857篇 |
2013年 | 8636篇 |
2012年 | 4820篇 |
2011年 | 6570篇 |
2010年 | 5279篇 |
2009年 | 6162篇 |
2008年 | 6572篇 |
2007年 | 6573篇 |
2006年 | 6003篇 |
2005年 | 5615篇 |
2004年 | 5534篇 |
2003年 | 5337篇 |
2002年 | 5140篇 |
2001年 | 5472篇 |
2000年 | 5075篇 |
1999年 | 5593篇 |
1998年 | 15834篇 |
1997年 | 10492篇 |
1996年 | 8090篇 |
1995年 | 5917篇 |
1994年 | 5154篇 |
1993年 | 5156篇 |
1992年 | 3508篇 |
1991年 | 3362篇 |
1990年 | 3322篇 |
1989年 | 3179篇 |
1988年 | 2878篇 |
1987年 | 2350篇 |
1986年 | 2413篇 |
1985年 | 2735篇 |
1984年 | 2461篇 |
1983年 | 2171篇 |
1982年 | 2007篇 |
1981年 | 2132篇 |
1980年 | 1874篇 |
1979年 | 1754篇 |
1978年 | 1736篇 |
1977年 | 2118篇 |
1976年 | 2790篇 |
1975年 | 1503篇 |
1974年 | 1451篇 |
1973年 | 1388篇 |
1972年 | 1185篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Recent advances in control systems analysis and design have implied new uses for the Lyapunov equation of the form AX +XA T+Q =0. Implementation requirements for the incorporation of the use of Lyapunov equations in practical design, however, point out the need for a set of specialized numerical procedures. This special set of numerical procedures must efficiently solve large, sparse Lyapunov equations, solve sets of Lyapunov equations that differ only in the coefficient matrix Q , and provide good low rank estimates of the Lyapunov equation solution X in the case where low rank approximations are applicable. Discussions of the motivations for the solution of these problems and of candidate solution approaches are provided 相似文献
962.
963.
The specific heat of 4He condensed on an evaporated gold surface has been measured for coverages between 0.007 and 0.096 Å?2 and temperatures between 0.4 and 3.0 K. There are at least two types of physisorption site on the gold surface with binding energies ?83 ± 7 and ?94 ± 7 K. For coverages below 0.06 Å?2 (~0.5 monolayers), the 4He is a two-dimensional classical gas at sufficiently high temperatures. At lower temperatures and higher coverages, the 4He forms condensed phases, including liquids, solids commensurate with the gold lattice, and incommensurate solids. The phase diagram of 4He/evaporated gold is similar to that for 4He/graphite, but the phase boundaries are shifted and not well defined, nor is the diagram as rich in structure. 相似文献
964.
Taking into consideration the knock-on cascade mechanism and the free volume theory, the cooling condition dependentR
s (ion etching rate) is investigated for a liquid-quenched Fe-10at%Si-15at%B alloy glass. A highR
s is found in a slow cooled glass. 相似文献
965.
The effect of the surface boundary between free space and a conducting medium on the excitation properties of neurons by magnetic fields are analyzed. The electric field and the spatial derivative of the induced field generated by a coil mounted both parallel and perpendicular to the surface of a semi-infinite conducting medium were calculated using the method of images. An imaginary axon is located in the same relative position from the coil in both configurations and the excitation properties are compared. The calculations are expressed in terms of the activating function for the electrical stimulation of axons. The calculations indicate that the activating function for magnetic stimulation is biphasic as opposed to triphasic for electrical stimulation. The large spatial extent of the magnetically induced electric field compared to the electric field generated by point source electrode suggests a different mode of excitation for neuronal structures in the CNS. The field distribution have been verified experimentally and are important for the understanding of the mechanisms of magnetic stimulation of neural tissue. 相似文献
966.
We have examined the applicability of this simple and effective test to polymer systems widely used in the medical industry. First, it was found that not every polymer examined exhibited a distinct onset of autocatalytic oxidation detectable by DSC. However, for cases where distinct onsets are found, powerful insights can be obtained. These included activation energy of the degradation, thermal oxidative shelf life prediction, and correlation, with product performances. Actual examples are presented to illustrate the utility of the test. 相似文献
967.
Kim S. Jeong S. Lee Y.T. Kim D.-H. Lim J.-S. Seo K.-S. Nam S. 《Electronics letters》2002,38(13):622-623
A new ultra-wideband, low-loss and small-size coplanar waveguide (CPW) to coplanar strip (CPS) transition which can be used from DC to 110 GHz is presented. The proposed transition connects CPW with CPS by the reformed air-bridge. Two ground planes of CPW are tied at their ends by a line and the centre of the line is connected to the ground strip of CPS by another line. Owing to the symmetry of the proposed structure, the currents of two ground planes of CPW are combined with the same phase and transferred to the ground strip of CPS. With height of 3 μm, the signal line of CPW passes over two connecting lines and is connected to the signal strip of CPS. For the back-to-back transition structure, insertion loss <1 dB and return loss >15 dB are obtained from 0.5 to 110 GHz 相似文献
968.
T. M. Holden J. W. L. Pang R. A. Holt 《Metallurgical and Materials Transactions A》2002,33(3):749-755
The development of the understanding of the intergranular stresses in ZIRCALOY-2 is reviewed. Neutron diffraction measurements
of the intergranular strains were made on rod-textured material and highly textured plate. The elastoplastic self-consistent
(EPSC) model provides a sound theoretical foundation for our understanding of the behavior. The strain response of ZIRCALOY-2
to applied tensile stress is well described for two very different textures with the same slip and hardening parameters. It
is almost certain that tensile twinning is the explanation for the response to compressive stress and rolling that is, as
yet, incompletely understood.
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
969.
Courteville A. Gharbi T. Cornu J.Y. 《IEEE transactions on bio-medical engineering》1998,45(2):145-150
A high-sensitivity human muscle-vibration measurement (MMG) sensor adapted to clinical use is presented. The muscle vibration phenomenon is modeled and investigated to optimize the measurement technique. The sensor uses an acoustic impedance adaptation technique to convert the skin surface vibration in terms of acoustic pressure, which is sensed by a microphone. The device is calibrated and gives the real amplitude of the vibration. It is also well fitted to measure other physiological vibrations in the 2 Hz-1 kHz range 相似文献
970.
This paper describes a new write/erase method for flash memory to improve the read disturb characteristics by means of drastically reducing the stress leakage current in the tunnel oxide. This new write/erase operation method is based on the newly discovered three decay characteristics of the stress leakage current. The features of the proposed write/erase method are as follows: 1) the polarity of the additional pulse after applying write/erase pulse is the same as that of the control gate voltage in the read operation; 2) the voltage of the additional pulse is higher than that of a control gate in a read operation, and lower than that of a control gate in a write operation; and 3) an additional pulse is applied to the control gate just after a completion of the write/erase operation. With the proposed write/erase method, the degradation of the read disturb life time after 106 write/erase cycles can be drastically reduced by 50% in comparison with the conventional bipolarity write/erase method used for NAND type flash memory. Furthermore, the degradation can he drastically reduced by 90% in comparison with the conventional unipolarity write/erase method fur NOR-, AND-, and DINOR-type flash memory. This proposed write/erase operation method has superior potential for applications to 256 Mb flash memories and beyond 相似文献