全文获取类型
收费全文 | 17990篇 |
免费 | 3281篇 |
国内免费 | 2991篇 |
专业分类
化学 | 7954篇 |
晶体学 | 409篇 |
力学 | 715篇 |
综合类 | 267篇 |
数学 | 1443篇 |
物理学 | 5336篇 |
无线电 | 8138篇 |
出版年
2024年 | 78篇 |
2023年 | 345篇 |
2022年 | 667篇 |
2021年 | 668篇 |
2020年 | 673篇 |
2019年 | 667篇 |
2018年 | 645篇 |
2017年 | 768篇 |
2016年 | 759篇 |
2015年 | 995篇 |
2014年 | 1198篇 |
2013年 | 1500篇 |
2012年 | 1638篇 |
2011年 | 1573篇 |
2010年 | 1471篇 |
2009年 | 1416篇 |
2008年 | 1456篇 |
2007年 | 1347篇 |
2006年 | 1253篇 |
2005年 | 1099篇 |
2004年 | 808篇 |
2003年 | 555篇 |
2002年 | 522篇 |
2001年 | 461篇 |
2000年 | 371篇 |
1999年 | 227篇 |
1998年 | 112篇 |
1997年 | 116篇 |
1996年 | 101篇 |
1995年 | 97篇 |
1994年 | 76篇 |
1993年 | 81篇 |
1992年 | 90篇 |
1991年 | 72篇 |
1990年 | 55篇 |
1989年 | 61篇 |
1988年 | 36篇 |
1987年 | 34篇 |
1986年 | 25篇 |
1985年 | 19篇 |
1984年 | 17篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 19篇 |
1980年 | 19篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1965年 | 6篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Investigation of remelting and cladding processing with laminar plasma jets on several metals has been conducted looking for possible development of a new surface modification technique. The remelting tests illustrated that the new method could evidently improve the material microstructure and properties of cast iron. The cladding was done with Al2O3 ceramic powder on stainless steel. The energy dispersive spectra (EDS) analysis was used to determine the distribution of the major cladding element in the plasma-processed layers, for which the microstructure observations and hardness measurements were also performed. 相似文献
52.
The solid-liquid interface motion of NaBi(WO4)2 (NBWO) melt crystal growth is observed in an in situ system, in which the whole processes of interface transition from fiat interface and cellular to dendrite are visualized. The spacing of the dendrite under smaller temperature gradient turns out to be larger than that under larger temperature gradient, which is found to be sensitive to the temperature distribution. The mechanism of dendrite growth of NBWO is studied based on the model of the growth units of anion coordination polyhedra. The { 001} face has two apex links, so it shows higher stability and has high growth rate and forms the arm of dendrite, whereas the {010} face has only one apex link, and thus shows relative slower growth rate and firstly forms the branches. 相似文献
53.
Jin Hu Gang Liu Jixiu Zhang Lingchao Kong Weidong Fei 《Applied Surface Science》2006,253(5):2792-2795
The microstructure of a laser treated Al18B4O33w/2024Al composite has been investigated using transmission electron microscope (TEM), low-angle (glancing angle) X-ray diffraction (XRD) techniques. Various surface microstructures were observed in the laser treated composite. The Al18B4O33 whisker on the surface of the composite was decomposed during laser surface melting, various decomposition products were studied in the laser treated composite. Eutectic phases and the precipitation in the matrix of the composite with laser-treated were observed. The main phases detected in the molten zone were aluminum and decomposition products Al2O3. The effect of laser treatment on the hardness of the composite was also examined. A surface hardness of 400 Hv was noted. 相似文献
54.
本文对几种模糊传递关系进行比较 ,从一个侧面说明了用于构造偏好关系的模糊关系应是互逆关系 .文中证明了对任意的 n阶互逆传递模糊关系 R,必有 Rn=Rn+1 . 相似文献
55.
56.
57.
Qingfa Wang Li Wang Yaquan Wang Fei He Zhenhua Li Zhentao Mi 《Reaction Kinetics and Catalysis Letters》2004,81(2):297-304
Deactivated palladium catalysts in the hydrogenation of anthraquinione were regenerated with ethanol, nitric acid, hydrogen
peroxide, boiling water and steam, respectively. The deactivated and regenerated catalysts were characterized by XPS, ICP,
TG, FTIR, TPD, XRD, etc., and studied in the hydrogenation of anthraquinone. The results showed that the main cause of catalyst deactivation is the
coverage of the active component by deposits. The treatments by hydrogen peroxide and boiling water can effectively regenerate
the deactivated catalysts.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
58.
通过对非线性规划问题的K-T点的光滑路径跟踪方法研究,得出一种针对于既有不等式约束条件又有等式约束条件的优化问题光滑路径跟踪方法,证明了光滑跟踪路径的存在性。 相似文献
59.
60.
Flowers G.T. Fei Xie Bozack M.J. Malucci R.D. 《Components and Packaging Technologies, IEEE Transactions on》2004,27(1):65-71
Single frequency vibration tests were used to induce fretting corrosion in tin alloy plated contacts. The samples used in this study were connectors consisting of 25 pairs of mated pin and socket contacts. Experimental results for a variety of vibration levels, frequencies, and wiring tie-off lengths are presented. The experiments consisted of running a series of vibration tests at each frequency where the excitation level was stepped through a range of g-levels. During each test run contact resistance was monitored as a performance characteristic. The results exhibit threshold behavior at each frequency for the onset of fretting degradation. Typically a plateau region was observed where similar g-levels produced similar fretting rates. It was also found that outside the plateau region the g-levels varied according to the dynamic behavior of the mechanical system. In addition, a transfer matrix model was used to analyze these results. An empirical fit of the data correlated well with the model when damping was used. This analysis revealed the importance of the bending moment induced at the contact interface as a result of excitation levels and tie-off configurations. Consequently, it is concluded that dynamic response of the mechanical system under various g-levels and tie off configurations can greatly impact the performance of a connector system subjected to vibration stresses. 相似文献