全文获取类型
收费全文 | 49929篇 |
免费 | 5554篇 |
国内免费 | 3074篇 |
专业分类
电工技术 | 3876篇 |
技术理论 | 2篇 |
综合类 | 3888篇 |
化学工业 | 7402篇 |
金属工艺 | 2722篇 |
机械仪表 | 3265篇 |
建筑科学 | 4180篇 |
矿业工程 | 1480篇 |
能源动力 | 1482篇 |
轻工业 | 3424篇 |
水利工程 | 1272篇 |
石油天然气 | 2312篇 |
武器工业 | 497篇 |
无线电 | 6364篇 |
一般工业技术 | 5690篇 |
冶金工业 | 2066篇 |
原子能技术 | 814篇 |
自动化技术 | 7821篇 |
出版年
2024年 | 286篇 |
2023年 | 878篇 |
2022年 | 1671篇 |
2021年 | 2437篇 |
2020年 | 1776篇 |
2019年 | 1542篇 |
2018年 | 1605篇 |
2017年 | 1710篇 |
2016年 | 1541篇 |
2015年 | 2109篇 |
2014年 | 2739篇 |
2013年 | 3026篇 |
2012年 | 3431篇 |
2011年 | 3763篇 |
2010年 | 3525篇 |
2009年 | 3223篇 |
2008年 | 3057篇 |
2007年 | 2919篇 |
2006年 | 2774篇 |
2005年 | 2203篇 |
2004年 | 1728篇 |
2003年 | 1776篇 |
2002年 | 2025篇 |
2001年 | 1699篇 |
2000年 | 1165篇 |
1999年 | 963篇 |
1998年 | 621篇 |
1997年 | 479篇 |
1996年 | 389篇 |
1995年 | 312篇 |
1994年 | 250篇 |
1993年 | 217篇 |
1992年 | 164篇 |
1991年 | 106篇 |
1990年 | 103篇 |
1989年 | 84篇 |
1988年 | 78篇 |
1987年 | 32篇 |
1986年 | 27篇 |
1985年 | 16篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1959年 | 10篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
The vapour phase compositions of a series of pack powder mixtures containing elemental Al and Hf or W powders as depositing sources and CrCl3·6H2O or AlF3or CrF3as activators were analysed in an attempt to further develop the pack cementation process to codeposit Al and Hf or W to form diffusion coatings on nickel base superalloys. The results suggested that Al could be codeposited with Hf, but not with W, from the vapour phase. Compared with both AlF3and CrF3, CrCl3·6H2O has been shown to be a more suitable activator for codepositing Al with Hf. The optimum coating temperature was identified to be in the range of 1050°C to 1150°C. Based on the thermochemical analysis, a series of coating deposition studies were undertaken, which confirmed that codeposition of Al and Hf could be achieved at a deposition temperature of 1100°C in the CrCl3·6H2O activated packs containing elemental Al and Hf powders. The coating obtained had a multilayer structure consisting of a Ni7Hf6Al16top layer and a NiAl layer underneath, followed by a diffusion zone, which revealed that the coating was formed by the outward Ni diffusion. It is suggested that the compositions suitable for codeposition of Al and Hf could be effectively identified by comparing the vapour pressures of HfCl4and HfCl3with that of AlCl in the packs activated by chloride salts. It has also been experimentally demonstrated that, although W could not be deposited from the vapour phase, a high volume of fine W particles can be entrapped into the outer NiAl coating layer formed by the outward Ni diffusion using a modified pack configuration. This leads to the formation of a composite coating layer with W particles evenly distributed in a matrix of NiAl. It is suggested that this modified pack process could be similarly applied to develop nickel aluminide coatings containing other refractory metals that may not be codeposited with Al from the vapour phase. 相似文献
33.
Xiang Cao Sheng Zhong 《Communications Letters, IEEE》2006,10(8):580-581
Lin et al., (2003) proposed a remote user authentication scheme for multi-server architecture. In this paper, we breaks this scheme by giving an attack. Our attack allows an adversary to impersonate any user in the system, as long as a single authentication message of that user is observed. 相似文献
34.
H. L. Du S. R. Rose Z. D. Xiang P. K. Datta X. Y. Li 《Materialwissenschaft und Werkstofftechnik》2003,34(4):421-426
The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl3 diffusion coating was studied in an environment of H2/H2S/H2O at 850oC. The kinetic results demonstrate that the TiAl3 coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al2O3 scale on the surface of the TiAl3‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate. 相似文献
35.
Performance limitation due to statistical Raman crosstalk in a WDM system with multiple-wavelength bidirectionally pumped Raman amplification 总被引:1,自引:0,他引:1
A general theoretical model of statistical Raman crosstalk and its impact on system performance in a multiwavelength bidirectionally pumped Raman fiber amplifier (RFA) is developed for the first time, where we have taken modulation statistics, dispersion-induced pulse walk-off and signal-induced pump depletion into account. Two kinds of statistical Raman crosstalk, from signal-induced forward-pump depletion and from signal-signal Raman interaction, are included in one model. Formulas for normalized Raman crosstalk, Raman crosstalk-induced relative intensity noise spectral density, and its variance and system performance impact in terms of Q penalty are presented for both a single-span system and a dispersion-compensated multispan wavelength-division-multiplexed (WDM) link. Based on these formulas, we numerically investigate the impact of Raman crosstalk on system performance in a three-wavelengths bidirectionally pumped 40 /spl times/ 40-Gb/s WDM system for various fiber types. In addition, Raman crosstalk in a four-wavelength bidirectionally pumped RFA was experimentally measured. The results agree well with our theory. 相似文献
36.
求解二维结构-声耦合问题的一种直接方法 总被引:1,自引:1,他引:0
本文基于传递矩阵法(TMM)和虚拟边界元法(VBEM),提出了一种求解在谐激励作用下二维结构-声耦合问题的直接法。文中对任意形状的二维弹性环建立了一阶非齐次运动微分方程组,便于用齐次扩容精细积分法求解,对于含有任意形状孔穴的无穷域流体介质的Helmholtz外问题,采用复数形式的Burton-Miller型组合层势法建立了虚拟边界元方程,保证了声压在全波数域内存在唯一解。根据叠加原理并结合最小二乘法,提出了一种耦合方程的直接解法,由于该方法不存在迭代过程,因而具有较高的计算精度和效率。文中给出了二个典型弹性环在集中谐激励力作用下声辐射算例,计算结果表明本文方法较通常采用的混合FE/BE法更为有效。 相似文献
37.
38.
39.
一种基于三次样条函数求离子浓度的自动算法 总被引:1,自引:0,他引:1
本文提出了一种用三次样条函数模拟双次标准加入法测量方程,直接求解离子浓度的自动算法。比较了三咱不同边界条件下用要池数计算离子浓度的结果。造出节点区间两极端点的二阶导数为零时的三次样条函数为最佳模拟函数。并讨论了该方法在实际分析中误差的来源及消除办法。经对一系列文献数据的验算对比,表明本法完全可代替传统的迭代法和查图法,且能方便地设置在智能化的电位分析系统中。 相似文献
40.