全文获取类型
收费全文 | 27011篇 |
免费 | 3933篇 |
国内免费 | 2881篇 |
专业分类
化学 | 15374篇 |
晶体学 | 280篇 |
力学 | 1145篇 |
综合类 | 101篇 |
数学 | 2456篇 |
物理学 | 6809篇 |
无线电 | 7660篇 |
出版年
2024年 | 135篇 |
2023年 | 730篇 |
2022年 | 890篇 |
2021年 | 1180篇 |
2020年 | 1219篇 |
2019年 | 1089篇 |
2018年 | 933篇 |
2017年 | 846篇 |
2016年 | 1288篇 |
2015年 | 1387篇 |
2014年 | 1471篇 |
2013年 | 2005篇 |
2012年 | 2212篇 |
2011年 | 2186篇 |
2010年 | 1545篇 |
2009年 | 1571篇 |
2008年 | 1713篇 |
2007年 | 1471篇 |
2006年 | 1462篇 |
2005年 | 1272篇 |
2004年 | 961篇 |
2003年 | 785篇 |
2002年 | 815篇 |
2001年 | 636篇 |
2000年 | 524篇 |
1999年 | 529篇 |
1998年 | 370篇 |
1997年 | 304篇 |
1996年 | 319篇 |
1995年 | 271篇 |
1994年 | 246篇 |
1993年 | 187篇 |
1992年 | 171篇 |
1991年 | 162篇 |
1990年 | 154篇 |
1989年 | 113篇 |
1988年 | 109篇 |
1987年 | 69篇 |
1986年 | 64篇 |
1985年 | 73篇 |
1984年 | 55篇 |
1983年 | 38篇 |
1982年 | 27篇 |
1981年 | 27篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1977年 | 13篇 |
1976年 | 19篇 |
1975年 | 23篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
71.
72.
73.
74.
Scaffold based tissue engineering strategies use cells, biomolecules and a scaffold to promote the repair and regeneration of tissues. Although scaffold-based tissue engineering approaches are being actively developed, most are still experimental, and it is not yet clear what defines an ideal scaffold/cell construct. Solid free form fabrication (SFF) techniques can precisely control matrix architecture (size, shape, interconnectivity, branching, geometry and orientation). The SFF methods enable the fabrication of scaffolds with various designs and material compositions, thus providing a control of mechanical properties, biological effects and degradation kinetics. This paper reviews the application of micro-robotics and MEMS-based fabrication techniques for scaffold design and fabrication. It also presents a novel robotic technique to fabricate scaffold/cell constructs for tissue engineering by the assembly of microscopic building blocks. 相似文献
75.
Dong Han Kim Arthur A. Santilli Richard A. Fieber 《Journal of heterocyclic chemistry》1972,9(6):1347-1354
Syntheses of 11-acety1-2-phenylpyrimido[5,4-c][1,5]benzoxazepin-5(11H)one ( 16a ) and analogs ( 16b,c, 22 ) were described. The reaction of 4-chloro-2-phenyl-5-pyrimidinecarboxylic acid ethyl ester ( 7 ) with 2-aminophenol afforded 4-(2-hydroxyanilino)-2-phenyl-5-pyrimidine-carboxylic acid ethyl ester ( 8a ). The latter was also prepared by catalytic reduction of 4-(2-nitrophenoxy)-2-phenyl-5-pyrimidinecarboxylic acid ethyl ester ( 9 ), which was obtained from 7 and 2-nitrophenol. Involvement of 4-(2-aminophenoxy)-2-phenyl-5-pyrimidinecarboxylic acid ethyl ester ( 12a ) in this reduction as an intermediate was demonstrated by an independent synthesis of 12a and its subsequent rearrangement to 8a. Hydrolysis of 8a or 12a gave 4-(2-hydroxyanilino)-2-phenyl-5-pyrimidinecarboxylic acid ( 15a ). Reaction of 15a with acetic anhydride afforded 16a , the first member of a novel ring system, the pyrimido[5,4- c ][1,5]-benzoxazepin. Additional examples ( 16b,c ) were prepared similarly. The corresponding 11-ethyl derivative ( 22 ) was prepared in similar fashion, starting with 7 and 2-ethylaminophenol. A possible reaction mechanism for the formation of 16a-c from 15a-c and acetic anhydride was discussed. 相似文献
76.
Xu QM Wang D Han MJ Wan LJ Bai CL 《Langmuir : the ACS journal of surfaces and colloids》2004,20(8):3006-3010
Scanning tunneling microscopy (STM) combined with cyclic voltammetry has been employed to investigate the adsorption of cinchonine on Cu(111). Similar to cinchonidine, cinchonine forms a long-range ordered adlayer with (4 x 4) symmetry on the substrate. The structural details on molecular adsorption were obtained by high-resolution STM images. On the basis of the previous results and obtained STM images, the quinoline ring is proposed to lie parallel to Cu(111) and serve as an anchoring group. The chiral quinuclidine moiety extends out of the surface to facilitate the interaction with the prochiral reactants. 相似文献
77.
Bong Ban S. Baker Curtis L. Long Dale D. 《IEEE transactions on bio-medical engineering》1976,(1):75-77
A stimulator circuit is presented which is capable of generating pulse train waveforms suitable for neurophysiological experiments are available, as well as special test conditions such as dishabituation. Digital and linear integrated circuits are used to provide precise control over the stimulus parameters. 相似文献
78.
A new high performance series-resonance energy-recovery circuit (ERC) for a plasma display panel (PDP) is proposed. Two different ERCs are used for both sides of the PDP, and the slow falling and fast rising times are employed. Therefore, it features the zero voltage switching, low electromagnetic interference, low current stress, no severe voltage notch, and high energy-recovery capability. 相似文献
79.
HACCP体系的原理 总被引:8,自引:0,他引:8
论述了美国从1997年12月18日零时起执行的并被国际认同的HACCP体系的原理,对我国水产品加工企业出口产品与编制产品的HACCP计划有重要指导作用。 相似文献
80.
雾霾天气下拍摄的图像,由于大气中混浊悬浮物对光的吸收和散射的影响,导致"透光"强度减弱,图像能见度严重降低,许多特征被覆盖或模糊,限制和影响了可见光视觉系统工作效用的发挥.研究针对低能见度天气下,可见光视觉系统工作受限的现实,从雾天图像退化模型出发,通过发掘颜色分布与透射率两者之间的约束关系构建透射率求解方程.之后,通... 相似文献