全文获取类型
收费全文 | 78762篇 |
免费 | 8479篇 |
国内免费 | 5207篇 |
专业分类
电工技术 | 5954篇 |
技术理论 | 3篇 |
综合类 | 6737篇 |
化学工业 | 11077篇 |
金属工艺 | 4863篇 |
机械仪表 | 5371篇 |
建筑科学 | 5725篇 |
矿业工程 | 2808篇 |
能源动力 | 2259篇 |
轻工业 | 7397篇 |
水利工程 | 2111篇 |
石油天然气 | 3957篇 |
武器工业 | 1133篇 |
无线电 | 8872篇 |
一般工业技术 | 7965篇 |
冶金工业 | 3279篇 |
原子能技术 | 1224篇 |
自动化技术 | 11713篇 |
出版年
2024年 | 520篇 |
2023年 | 1557篇 |
2022年 | 3115篇 |
2021年 | 4008篇 |
2020年 | 2971篇 |
2019年 | 2156篇 |
2018年 | 2407篇 |
2017年 | 2600篇 |
2016年 | 2259篇 |
2015年 | 3463篇 |
2014年 | 4381篇 |
2013年 | 5012篇 |
2012年 | 6049篇 |
2011年 | 6278篇 |
2010年 | 5786篇 |
2009年 | 5196篇 |
2008年 | 5417篇 |
2007年 | 5247篇 |
2006年 | 4783篇 |
2005年 | 3839篇 |
2004年 | 2693篇 |
2003年 | 1966篇 |
2002年 | 1864篇 |
2001年 | 1720篇 |
2000年 | 1473篇 |
1999年 | 1226篇 |
1998年 | 820篇 |
1997年 | 626篇 |
1996年 | 592篇 |
1995年 | 496篇 |
1994年 | 408篇 |
1993年 | 281篇 |
1992年 | 265篇 |
1991年 | 190篇 |
1990年 | 139篇 |
1989年 | 119篇 |
1988年 | 95篇 |
1987年 | 59篇 |
1986年 | 58篇 |
1985年 | 43篇 |
1984年 | 32篇 |
1983年 | 31篇 |
1982年 | 29篇 |
1981年 | 27篇 |
1980年 | 38篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1973年 | 13篇 |
1951年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
23.
Ring‐opening polymerization of D,L ‐lactide (LA) has been successfully carried out by using rare earth 2,6‐dimethylaryloxide (Ln(ODMP)3) as single component catalyst or initiator for the first time. The effects of different rare earth elements, solvents, monomers and catalyst concentration as well as polymerization temperature and time on the polymerization were investigated. The results show that La(ODMP)3 exhibits higher activity to prepare poly(D,L ‐lactide) (PLA) with a viscosity molecular weight of 4.5 × 104 g mol?1 and the conversion of 97 % at 100 °C in 45 min. The catalytic activity of Ln(ODMP)3 has following sequence: La > Nd > Sm > Gd > Er > Y. A kinetic study has indicated that the polymerization is first order with respect to both monomer and catalyst concentration. The apparent activation energy of the polymerization of LA with La(ODMP)3 is 69.6 kJ mol?1. The analyses of polymer ends indicate that the LA polymerization proceeds according to ‘coordination–insertion’ mechanism with selective cleavage of the acyl–oxygen bond of the monomer. Copyright © 2004 Society of Chemical Industry 相似文献
24.
均匀设计法分析制备过程对钴钼硫化物催化剂机械强度的影响 总被引:1,自引:1,他引:0
建立了CoMoP/Al2O3加氢精制催化剂机械强度及其可靠性在制备过程中的数学模型,采用均匀设计考察了浸渍、干燥、煅烧、硫化4个制备过程中,浸渍时间、干燥温度、干燥时间、煅烧温度、煅烧时间、硫化温度和硫化升温速率7个实验因子对催化剂强度均值、Weibull模量的影响,同时考察了这些因素对颗粒密度的影响。方差分析表明所有模型都是充足的。实验结果表明,硫化过程是影响催化剂强度均值的主要过程。在实验范围内提高硫化温度、降低硫化时的升温速率有利于提高催化剂的强度。影响催化剂Weibull模量的因素主要为各个制备过程的交互效应。要提高催化剂强度的可靠性就必须全面考虑催化剂制备的各个过程,特别是各个制备过程的交互效应。 相似文献
25.
Hydrate formation rate plays an important role in making hydrates for the storage and transport of natural gas. Micellar surfactant solutions were found to increase gas hydrate formation rate and storage capacity. With the presence of surfactant, hydrate could form quickly in a quiescent system and the energy costs of hydrate formation reduced. Surfactants (an anionic surfactant, a non‐ionic surfactant and their mixtures) and liquid hydrocarbons (cyclopentane and methylcyclohexane) were used to improve hydrate formation. The experiments of hydrate formation were carried out in the pressure range 3.69–6.82 MPa and the temperature range 274.05–277.55 K. The experimental pressures were kept constant during hydrate formation in each experimental run. The effect of anionic surfactant (sodium dodecyl sulphate (SDS)) on natural gas storage in hydrates is more pronounced compared to a non‐ionic surfactant (dodecyl polysaccharide glycoside (DPG)). The induction time of hydrate formation was reduced with the presence of cyclopentane (CP). Cyclopentane and methylcyclohexane (MCH) could increase hydrate formation rate, but reduced hydrate storage capacity The higher methylcyclohexane concentration, the lower the hydrate storage capacity. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
26.
van Driel W.D. van Gils M.A.J. Xuejun Fan Zhang G.Q. Ernst L.J. 《Components and Packaging Technologies, IEEE Transactions on》2008,31(2):260-268
Exposed pad packages were introduced in the late 1980s and early 1990s because of their excellent thermal and electrical performance. Despite these advantages, the exposed pad packages experience a lot of thermo-hygro-mechanical related reliability problems during qualification and testing. Examples are die lift, which occurs predominantly after moisture sensitivity level conditions, and die-attach to leadframe delamination leading to downbond stitch breaks during temperature cycling. In this chapter, nonlinear finite element (FE) models using fracture mechanics based J-integral calculations are used to assess the reliability problems of the exposed pad package family. Using the parametric FE models any geometrical and material effects can be explored to their impact on the occurrence diepad delamination, and dielift. For instance the impact of diepad size is found to be of much less importance as the impact of die thickness is. Using the fracture mechanics approach, the starting location for the delamination from thermo-hygro-mechanical point of view is deducted. The results indicate that when diepad delamination is present, cracks are likely to grow beneath the die and dielift will occur. The interaction between dielift and other failure modes, such as lifted ball bonds, are not found to be very significant. The FE models are combined with simulation-based optimization methods to deduct design guidelines for optimal reliability of the exposed pad family. 相似文献
27.
Fan Zhang To-yat Cheung 《IEEE transactions on pattern analysis and machine intelligence》2003,29(1):1-14
The fault-state detection approach for blackbox testing consists of two phases. The first is to bring the system under test (SUT) from its initial state to a targeted state t and the second is to check various specified properties of the SUT at t. This paper investigates the first phase for testing systems specified as observable nondeterministic finite-state machines with probabilistic and weighted transitions. This phase involves two steps. The first step transfers the SUT to some state t' and the second step identifies whether t' is indeed the targeted state t or not. State transfer is achieved by moving the SUT along one of the paths of a transfer tree (TT) and state identification is realized by using diagnosis trees (DT). A theoretical foundation for the existence and characterization of TT and DT with minimum weighted height or minimum average weight is presented. Algorithms for their computation are proposed. 相似文献
28.
FAN Xi-zhi 《半导体光子学与技术》2004,10(3)
Based on Chun-Ching Shih's idea, the basic transform was substituted and the quasi-ChunChing Shih's fractional Fourier transform with periodicity of 2, 3 and M was deduced. The two former transforms and the Chun-Ching Shih's fractional Fourier transform were only the particular cases of quasiChun-Ching Shih's fractional Fourier transform with periodicity of M. 相似文献
29.
The simulation and analysis of S-shaped waveguide bend are presented.Bend radius larger than 30 mm assures less than 0.5 dB radiation loss for a 4-μm-wide silicon-on-insulator waveguide bend with 2-μm etch depth.Intersection angle greater than 20° provides negligible crosstalk (<-30 dB) and very low insertion loss.Any reduction in bend radius and intersection angle is at the cost of the degradation of characteristics of bent waveguide and intersecting waveguide, respectively. 相似文献
30.