全文获取类型
收费全文 | 584383篇 |
免费 | 14266篇 |
国内免费 | 4801篇 |
专业分类
电工技术 | 13638篇 |
技术理论 | 2篇 |
综合类 | 5014篇 |
化学工业 | 88013篇 |
金属工艺 | 23616篇 |
机械仪表 | 19441篇 |
建筑科学 | 17751篇 |
矿业工程 | 4150篇 |
能源动力 | 16136篇 |
轻工业 | 51259篇 |
水利工程 | 6605篇 |
石油天然气 | 12438篇 |
武器工业 | 656篇 |
无线电 | 67363篇 |
一般工业技术 | 108289篇 |
冶金工业 | 106084篇 |
原子能技术 | 11268篇 |
自动化技术 | 51727篇 |
出版年
2022年 | 5356篇 |
2021年 | 7800篇 |
2020年 | 5698篇 |
2019年 | 6284篇 |
2018年 | 9615篇 |
2017年 | 9794篇 |
2016年 | 9860篇 |
2015年 | 8398篇 |
2014年 | 12704篇 |
2013年 | 28162篇 |
2012年 | 18998篇 |
2011年 | 25001篇 |
2010年 | 20052篇 |
2009年 | 21813篇 |
2008年 | 22129篇 |
2007年 | 21334篇 |
2006年 | 18776篇 |
2005年 | 16843篇 |
2004年 | 15280篇 |
2003年 | 14729篇 |
2002年 | 13887篇 |
2001年 | 13953篇 |
2000年 | 12820篇 |
1999年 | 13542篇 |
1998年 | 33562篇 |
1997年 | 23528篇 |
1996年 | 18039篇 |
1995年 | 13582篇 |
1994年 | 11711篇 |
1993年 | 11317篇 |
1992年 | 8149篇 |
1991年 | 7628篇 |
1990年 | 7400篇 |
1989年 | 7123篇 |
1988年 | 6559篇 |
1987年 | 5822篇 |
1986年 | 5706篇 |
1985年 | 6298篇 |
1984年 | 5922篇 |
1983年 | 5193篇 |
1982年 | 4920篇 |
1981年 | 4982篇 |
1980年 | 4725篇 |
1979年 | 4621篇 |
1978年 | 4410篇 |
1977年 | 5337篇 |
1976年 | 6975篇 |
1975年 | 3798篇 |
1974年 | 3552篇 |
1973年 | 3666篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Mobility management for VoIP service: Mobile IP vs. SIP 总被引:4,自引:0,他引:4
Wireless Internet access has gained significant attention as wireless/mobile communications and networking become widespread. The voice over IP service is likely to play a key role in the convergence of IP-based Internet and mobile cellular networks. We explore different mobility management schemes from the perspective of VoIP services, with a focus on Mobile IP and session initiation protocol. After illustrating the signaling message flows in these two protocols for diverse cases of mobility management, we propose a shadow registration concept to reduce the interdomain handoff (macro-mobility) delay in the VoIP service in mobile environments. We also analytically compute and compare the delay and disruption time for exchanging signaling messages associated with the Mobile IP and SIP-based solutions. 相似文献
992.
The ray-tracing (RT) algorithm has been used for accurately predicting the site-specific radio propagation characteristics, in spite of its computational intensity. Statistical models, on the other hand, offers computational simplicity but low accuracy. In this paper, a new model is proposed for predicting the indoor radio propagation to achieve computational simplicity over the RT method and better accuracy than the statistical models. The new model is based on the statistical derivation of the ray-tracing operation, whose results are a number of paths between the transmitter and receiver, each path comprises a number of rays. The pattern and length of the rays in these paths are related to statistical parameters of the site-specific features of indoor environment, such as the floor plan geometry. A key equation is derived to relate the average path power to the site-specific parameters, which are: 1) mean free distance; 2) transmission coefficient; and 3) reflection coefficient. The equation of the average path power is then used to predict the received power in a typical indoor environment. To evaluate the accuracy of the new model in predicting the received power in a typical indoor environment, a comparison with RT results and with measurement data shows an error bound of less than 5 dB 相似文献
993.
E.A. Soliman A.K. Abdelmageed M.A. El-Gamal 《AEUE-International Journal of Electronics and Communications》2002,56(3)
In this paper a new artificial neural network (ANN) based model for the calculation of the method of moments (MoM) matrix elements is presented. Training sets that characterize the matrix elements are first constructed. These sets are then utilized to effectively train two radial basis function (RBF) neural networks to accurately estimate all the elements of the MoM matrix for any mesh used. The potential of the proposed approach is demonstrated in the case of a narrow microstrip line. The current distribution on the microstrip line produced by the trained RBF networks agrees very well with the exact distribution. In addition, the proposed ANN model is much faster than the conventional MoM procedure. 相似文献
994.
Nicholas Biery Marc De Graef Jack Beuth Rafel Raban Andrew Elliott Tresa M. Pollock Curt Austin 《Metallurgical and Materials Transactions A》2002,33(10):3127-3136
Tensile experiments have been performed on specimens of four different investment-cast TiAl-based alloys with variations in
casting conditions. The average ductilities obtained in these experiments vary between approximately 0.8 to 2.0 pct plastic
strain to failure in tension. By using the three-parameter form of the Weibull relation, with the 0.2 pct offset yield strength
as the minimum failure strength, the resulting variability in the data can be quantified and is found to be similar for those
alloys with similar microstructural scale (grain size). Large variations in lamellar volume fraction, segregation, and phase
distribution have a minor influence on property variability, compared to changes in the scale of the grain structure caused
by either variations in cooling rate during casting or the addition of grain refiners. 相似文献
995.
Interfacial segregation of Ti in the brazing of diamond grits onto a steel substrate using a Cu-Sn-Ti brazing alloy 总被引:1,自引:0,他引:1
Wen-Chung Li Shun-Tian Lin Cheng Liang 《Metallurgical and Materials Transactions A》2002,33(7):2163-2172
Diamond grits were brazed onto a steel substrate using a prealloyed Cu-10Sn-15Ti (wt pct) brazing alloy at 925 °C and 1050
°C. Due to the relatively high concentration of Ti in the brazing alloy, the braze matrix exhibited a composite structure,
composed of β-(Cu,Sn), a Cu-based solid solution, and various intermetallic compounds with different morphologies. The reaction of Ti with
diamond yielded a continuous TiC layer on the surfaces of the diamond grits. On top of the TiC growth front, an intermetallic
compound, composed of Sn and Ti, nucleated and grew into a randomly interwoven fine lacey structure. An interfacial structure
developed as the interwoven fine lacey phase was semicoherently bonded to the TiC layer, with the Cu-based braze matrix filling
its interstices. The thickness of such a composite layer was increased linearly with the square root of isothermal holding
time at 925 °C, complying with the law of a diffusion-controlled process. However, at 1050 °C, the segregation behavior of
Ti and Sn to the interfaces between the TiC layer and the braze matrix diminished, due to the increased solubility of Ti in
the Cu-based liquid phase. The enhanced dissolution of Ti in the Cu-based liquid phase at 1050 °C also caused the precipitation
of rod-like CuTi with an average diameter of about 0.2 μm during cooling. SnTi3 was the predominant intermetallic compound and existed in three different forms in the braze matrix. It existed as interconnected
grains of large size which either floated to the surface of the braze matrix or grew into faceted grains. It also exhibited
a nail-like structure with a mean diameter of about 1 μm for the rod section and a lamellar structure arising from a eutectic reaction during cooling. 相似文献
996.
A theory related to the statistics of the structure and properties of inhomogeneous materials has been proposed, dealing with the distribution of properties from sampling of inhomogeneous structures by line transects. It is demonstrated that the empirical cumulative distribution functions of the intercepts from the microstructural constituents and their variances are important fingerprints of the inhomogeneous structures.Simulated sampling of the images of the microstructure by line transects has been proposed as a powerful method for characterization the distribution of properties of inhomogeneous structures. One of the advantages of the method based on the expected values of the order statistics of the intercepts is that the probability bounds of the property are determined without prior knowledge of the functional relationship between the property and the intercepts from the structural constituents.The concept intercept variance has an important application in determining the minimum transect length that stabilizes the variation of the intercept at a low value. Additionally, the concept ‘intercept variance’ can be used for topological optimization of the microstructure regarding the risk of intercepting a large amount of the weaker constituent. Equations regarding the variance of the intercepted fraction characterizing transect lengths with a specified distribution have also been derived. 相似文献
997.
G. M. Kulikov S. V. Plotnikova 《International journal for numerical methods in engineering》2002,55(10):1167-1183
The precise representation of rigid body motions in the displacement patterns of curved Timoshenko–Mindlin (TM) shell elements is considered. This consideration requires the development of the strain–displacement relationships of the TM shell theory with regard to their consistency with the rigid body motions. For this purpose a refined TM theory of multilayered anisotropic shells is elaborated. The effects of transverse shear deformation and bending‐extension coupling are included. The fundamental unknowns consist of five displacements and eight strains of the face surfaces of the shell, and eight stress resultants. On the basis of this theory the simple and efficient mixed models are developed. The elemental arrays are derived using the Hu–Washizu mixed variational principle. Numerical results are presented to demonstrate the high accuracy and effectiveness of the developed 4‐node shell elements and to compare their performance with other finite elements reported in the literature. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
998.
999.
Fathy A.E. Pendrick V.A. Geller B.D. Perlow S.M. Tormey E.S. Prabhu A. Tani S. 《Microwave Theory and Techniques》2002,50(10):2247-2252
We have developed an innovative semianalytical technique for various substrate material characterization. The developed technique is a measurement procedure and data-reduction formulation that takes into consideration the radiation loss in a resonant structure, allowing for a more effective means of dielectric- and conductor-loss determination for a microstrip ring resonator and its substrate material. We separate dielectric and conductor loss precisely, evaluate the contribution of each term in the overall loss performance, and analytically predict the error in their respective predicted value. 相似文献
1000.
Tsu-Hua Ai Jiann-Fuh Chen Tsorng-Juu Liang 《Industrial Electronics, IEEE Transactions on》2002,49(3):595-597
To overcome the problem of unequal switching loss in power switches, in conventional hybrid pulse width modulation (HPWM) full-bridge inverters, a random switching method for HPWM full-bridge inverters is proposed. The proposed method equalizes switching losses of the four switches, while also providing good output performance 相似文献