全文获取类型
收费全文 | 13872篇 |
免费 | 1582篇 |
国内免费 | 1374篇 |
专业分类
化学 | 7161篇 |
晶体学 | 109篇 |
力学 | 504篇 |
综合类 | 113篇 |
数学 | 1209篇 |
物理学 | 3088篇 |
综合类 | 4644篇 |
出版年
2024年 | 61篇 |
2023年 | 241篇 |
2022年 | 414篇 |
2021年 | 423篇 |
2020年 | 426篇 |
2019年 | 396篇 |
2018年 | 326篇 |
2017年 | 340篇 |
2016年 | 541篇 |
2015年 | 560篇 |
2014年 | 719篇 |
2013年 | 859篇 |
2012年 | 1016篇 |
2011年 | 1063篇 |
2010年 | 751篇 |
2009年 | 712篇 |
2008年 | 844篇 |
2007年 | 729篇 |
2006年 | 607篇 |
2005年 | 550篇 |
2004年 | 489篇 |
2003年 | 497篇 |
2002年 | 666篇 |
2001年 | 573篇 |
2000年 | 385篇 |
1999年 | 441篇 |
1998年 | 287篇 |
1997年 | 248篇 |
1996年 | 243篇 |
1995年 | 200篇 |
1994年 | 195篇 |
1993年 | 183篇 |
1992年 | 147篇 |
1991年 | 131篇 |
1990年 | 107篇 |
1989年 | 99篇 |
1988年 | 76篇 |
1987年 | 53篇 |
1986年 | 45篇 |
1985年 | 35篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1963年 | 7篇 |
1959年 | 12篇 |
1958年 | 11篇 |
1957年 | 15篇 |
1956年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
最优化法在计算平台破损后浮态及初稳性中的应用 总被引:2,自引:0,他引:2
季春群 《上海交通大学学报》1991,(4)
提出了一种用优化方法求解平台破损进水后浮态和初稳性的方法,并用所编制的计算机程序对平台破损后的浮态和初稳性进行了计算。文中给出了某一平台在下沉就位过程中的计算实例,计算结果表明:利用优化方法来求解平台破损进水后的浮态和初稳性是切实可行的. 相似文献
72.
通过对气吹式精密播种机田间试验资料的相关分析,发现合格指数A与|D-M|(重播指数与漏播指数之差的绝对值)之间里极显著负相关(r=-0.9312)。在对数据作进一步回归分析的基础上,本文建立了一套“由排种粒数预测排种质量”的方法。 相似文献
73.
The laser cladding of Fe-based alloys on a medium carbon steel substrate was performed using a CO2 laser and Ar shielding gas that was blown into a molten pool. The microstructure and cracking susceptibility of the laser-clad layers were studied in terms of carbon additions. Results show that the small change of the carbon content in the alloy powders can obviously change the microstructure and properties of the layers. When the carbon content is in the range of 0.3–0.4 wt.%, the decrease of the carbon content in alloy powders will increase the hardness and toughness of the layers simultaneously under the same process parameters. As a result, crack-free coatings with high hardness can be obtained. As the carbon content increases from 0.2 wt.% to 0.4 wt.%, the segregation ratio of chromium increases, while the segregation ratios of nickel, manganese, and silicon first decrease and then increase. At the same time, a new designing principle concerning the composition and microstructure has been put forward, and the principal mechanisms of strengthening and toughening of the layers are fine-grain strengthening and low carbon martensitic phase transformation strengthening. 相似文献
74.
75.
Do Young Kim Ji Sim Jung Young Rae Jang Kun Ho Yoo Jin Jang 《Physica E: Low-dimensional Systems and Nanostructures》2003,16(3-4):400
We studied the growth of nanocrystalline silicon (nc-Si) thin film exhibiting a strong room temperature photoluminescence (PL) at 1.81–2.003 eV. The amorphous silicon was crystallized by Ni silicide mediated crystallization (Ni SMC) and then Secco-etched to exhibit the PL. The PL peak energy and intensity increase with increasing the metal density on the a-Si because of the reduction in the grain size down to 2 nm. The photoluminescence energy and peak intensity depend strongly on the Secco etch time because the grain size is reduced by etching the grain boundaries. 相似文献
76.
1-Propyl-3-methylimidazolium chloride ([C3MIm]Cl) ionic liquid and sulfamic acid (NH2SO3H), as a synergetic catalytic medium, were used for the transesterification of acetoacetate with alcohols of different structures. It shows the good ability for the chemoselective transesterificatin of β-ketoesters and maintains its catalytic activity in the reuse. 相似文献
77.
78.
丝瓜种子萌芽及子叶生长过程中谷氨酰胺合成酶同工酶的变化 总被引:2,自引:1,他引:1
测定了丝瓜种子萌芽及子叶不同生长阶段的谷氨酰胺合成酶(GS)的活性及其同工酶的变化.除了发育上的变化外,光和外源氮对GS活性及其同工酶影响也是显著的.无论外源氮(N)是否存在,GS活性在种子萌芽和子叶生长初期是逐渐升高的,于第6天达到最大,其后活性逐步降低.在光照下,外源氮对GS1诱导作用大于GS2;在无氮下,光对GS2的刺激大于GS1;在暗处,外源氮对GS1和GS2两者都有诱导作用. 相似文献
79.
Ru Ju Wang Feng Ying LiTing Zhang Deng Ji Fang WangHong Chen Chang Qing Jin 《Solid State Communications》2003,127(1):47-50
Using pulse echo overlap measurement, the elastic behavior of amorphous carbon has been studied at ambient and low temperatures. The smaller ratio B/G of the bulk modulus to shear modulus and smaller Poisson's ratio σ at room temperature indicate that there is an intrinsic stiffening of transverse acoustic phonons in the amorphous carbon. The acoustic velocity and attenuation for longitudinal modes have been measured between 2.1 and 300 K at three frequencies of 7, 21 and 35 MHz, respectively. Their frequency and temperature dependence are observed. The elastic constant C11 increases with decreasing temperature and show enhanced stiffening at low temperatures. In the 130-220 K region, the abnormal change and effect of longitudinal velocity and attenuation with temperature and frequency, and a phase transition associated with structure relaxations are discussed. 相似文献
80.
The paper presents results of perturbative calculations for nucleon Compton scattering. The results are given for the real incoming photon and for the virtual incoming photon. 相似文献