全文获取类型
收费全文 | 23955篇 |
免费 | 3833篇 |
国内免费 | 2633篇 |
专业分类
化学 | 16791篇 |
晶体学 | 306篇 |
力学 | 1427篇 |
综合类 | 247篇 |
数学 | 2692篇 |
物理学 | 8958篇 |
出版年
2024年 | 41篇 |
2023年 | 496篇 |
2022年 | 684篇 |
2021年 | 770篇 |
2020年 | 981篇 |
2019年 | 891篇 |
2018年 | 742篇 |
2017年 | 714篇 |
2016年 | 1083篇 |
2015年 | 994篇 |
2014年 | 1242篇 |
2013年 | 1625篇 |
2012年 | 2068篇 |
2011年 | 2090篇 |
2010年 | 1432篇 |
2009年 | 1374篇 |
2008年 | 1541篇 |
2007年 | 1414篇 |
2006年 | 1356篇 |
2005年 | 1092篇 |
2004年 | 880篇 |
2003年 | 737篇 |
2002年 | 638篇 |
2001年 | 524篇 |
2000年 | 523篇 |
1999年 | 609篇 |
1998年 | 503篇 |
1997年 | 436篇 |
1996年 | 442篇 |
1995年 | 388篇 |
1994年 | 363篇 |
1993年 | 327篇 |
1992年 | 272篇 |
1991年 | 232篇 |
1990年 | 239篇 |
1989年 | 160篇 |
1988年 | 120篇 |
1987年 | 84篇 |
1986年 | 97篇 |
1985年 | 65篇 |
1984年 | 37篇 |
1983年 | 40篇 |
1982年 | 37篇 |
1981年 | 21篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 885 毫秒
81.
A numerical study of confined jets in a cylindrical duct is carried out to examine the performance of two recently proposed turbulence models: an RNG-based K-? model and a realizable Reynolds stress algebraic equation model. The former is of the same form as the standard K-? model but has different model coefficients. The latter uses an explicit quadratic stress-strain relationship to model the turbulent stresses and is capable of ensuring the positivity of each turbulent normal stress. The flow considered involves recirculation with unfixed separation and reatachment points and severe adverse pressure gradients, thereby providing a valuable test of the predictive capability of the models for complex flows. Calculations are performed with a finite volume procedure. Numerical credibility of the solutions is ensured by using second-order-accurate differencing schemes and sufficiently fine grids. Calculations with the standard K-? model are also made for comparison. Detailed comparisons with experiments show that the realizable Reynolds stress algebraic equation model consistently works better than does the standard K-? model in capturing the essential flow features, while the RNG-based K-? model does not seem to give improvements over the standard K-? model under the flow conditions considered. 相似文献
82.
83.
84.
Zhu Da xin 《数学年刊B辑(英文版)》1994,15(3):293-298
ACOMPLETEMETRICOFPOSITIVECURVATUREONR~nANDEXISTENCEOFCLOSEDGEODESICS¥ZHUDAXIN(DepartmentofMathematics,TianjinUniversitylTianj?.. 相似文献
85.
The impact of fabrication errors on a planar waveguide demultiplexer is analyzed based on an analytical method. The explicit
expression of the transfer function taking into account phase and amplitude errors is presented in order to analyze the loss
and crosstalk of the demultiplexer caused by fabrication errors. A basic requirement for the demultiplexer with a certain
crosstalk criterion can be easily obtained. Using an etched diffraction grating demultiplexer as an example, it is shown that
the analytical results have a good agreement with results from a numerical method. 相似文献
86.
87.
The new phenylpropanoid diglycoside ligusinenoside A ( 1 ), and the two new 8,4′‐oxyneolignan(‘8‐O‐4′‐neolignan’) diglycosides ligusinenosides B ( 2 ) and C ( 3 ), together with nine known compounds, were isolated from the rhizomes of Ligusticum sinensis Oliv. The structures of 1 – 3 were elucidated on the basis of spectroscopic analyses. 相似文献
88.
全固态多波长飞秒脉冲激光系统 总被引:1,自引:1,他引:0
利用棱镜对引进频谱空间啁啾来补偿飞秒脉冲激光二次谐波产生中的相位失配,提高了倍频效率建立了一套全固态、多波长(1065nm, 532nm,823.1nm, 402nm)飞秒脉冲激光系统自制的Nd:YVO4激光器输出532nm绿光激光,最高平均功率可达5.6W当用2.5W绿光激光泵浦时,从自制的钛宝石激光器及经BBO倍频可分别输出中心波长为823.1nm和402nm,平均功率300mW和73mW,谱宽32.3nm和5.1nm,脉宽22fs和33.3fs、重复率108MHz的近红外和蓝光激光整个系统具有结构紧凑、倍频效率高、运行稳定的特点. 相似文献
89.
Raman scattering studies were performed on hot-wall chemical vapor deposited (heteroepitaxial) silicon carbide (SiC) films grown on Si substrates with orientations of (1 0 0), (1 1 1), (1 1 0) and (2 1 1), respectively. Raman spectra suggested that good quality cubic SiC single crystals could be obtained on the Si substrate, independent of its crystallographic orientation. Average residual stresses in the epitaxially grown 3C-SiC films were measured with the laser waist focused on the epilayer surface. Tensile and compressive residual stresses were found to be stored within the SiC film and in the Si substrate, respectively. The residual stress exhibited a marked dependence on the orientation of the substrate. The measured stresses were comparable to the thermal stress deduced from elastic deformation theory, which demonstrates that the large lattice mismatch between cubic SiC and Si is effectively relieved by initial carbonization. The confocal configuration of the optical probe enabled a stress evaluation along the cross-section of the sample, which showed maximum tensile stress magnitude at the SiC/Si interface from the SiC side, decreasing away from the interface in varied rate for different crystallographic orientations. Defocusing experiments were used to precisely characterize the geometry of the laser probe in 3C-SiC single crystal. Based on this knowledge, a theoretical convolution of the in-depth stress distribution could be obtained, which showed a satisfactory agreement with stress values obtained by experiments performed on the 3C-SiC surface. 相似文献
90.