全文获取类型
收费全文 | 162105篇 |
免费 | 12591篇 |
国内免费 | 16652篇 |
专业分类
化学 | 110420篇 |
晶体学 | 1893篇 |
力学 | 5175篇 |
综合类 | 1741篇 |
数学 | 25493篇 |
物理学 | 46626篇 |
出版年
2024年 | 178篇 |
2023年 | 1183篇 |
2022年 | 3559篇 |
2021年 | 4146篇 |
2020年 | 4100篇 |
2019年 | 4064篇 |
2018年 | 3501篇 |
2017年 | 4582篇 |
2016年 | 5497篇 |
2015年 | 4839篇 |
2014年 | 6444篇 |
2013年 | 12076篇 |
2012年 | 9656篇 |
2011年 | 9121篇 |
2010年 | 7859篇 |
2009年 | 10192篇 |
2008年 | 10348篇 |
2007年 | 10966篇 |
2006年 | 9496篇 |
2005年 | 7719篇 |
2004年 | 7150篇 |
2003年 | 6157篇 |
2002年 | 8176篇 |
2001年 | 4809篇 |
2000年 | 4289篇 |
1999年 | 3872篇 |
1998年 | 3436篇 |
1997年 | 2706篇 |
1996年 | 2441篇 |
1995年 | 2222篇 |
1994年 | 1979篇 |
1993年 | 1782篇 |
1992年 | 1706篇 |
1991年 | 1184篇 |
1990年 | 996篇 |
1989年 | 884篇 |
1988年 | 841篇 |
1987年 | 607篇 |
1986年 | 554篇 |
1985年 | 614篇 |
1984年 | 526篇 |
1983年 | 321篇 |
1982年 | 547篇 |
1981年 | 690篇 |
1980年 | 598篇 |
1979年 | 646篇 |
1978年 | 536篇 |
1977年 | 418篇 |
1976年 | 365篇 |
1973年 | 255篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
801.
测量了Cr^4+,YAG、Cr^4+,Mg2SiO4晶体在室温和液氮温度下的荧光光谱,吸收光谱和激发态寿命,讨论了温度变化时,两种晶体中Cr^4+近红外辐射积分强度变化与激光发态寿命变化的关系,得出结论:在77K ̄300K范围内,Cr^4+的^3T2能级荧光辐射截面本身受温度影响不大,Cr^4+辐射荧光的变化,主要是由无辐射弛豫速率随温度变化而引起的。 相似文献
802.
The electron-stimulated desorption (ESD) of D− and H− ions from condensed D2O and H2O films is investigated. Three low-energy peaks are observed in the ESD anion yield, which are identified as arising from excitation of 2B1, 2A1 and 2B2 dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a1 orbital. The ion yield resulting from excitation of the 2B1 resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption. 相似文献
803.
Ken Ono 《Mathematische Annalen》1998,312(2):251-260
804.
侯旻 《新疆大学学报(理工版)》1991,(1)
本文比较了循环图类{c_p(n_1,…,n_p)}和{c-p(n_1…,n_p,p/α)}的直径下界。对于p和α满足一定条件的循环图类{c_p(n_1,n_2,p/α)},本文给出了达到或几乎达到此图类直径下界的一类几乎最优循环图{c_p(m,m+1,p/α)}。 相似文献
805.
本文研究了包括二阶离化过程的强激光场下的双光子自电离。导出了光电子谱的解析表达式,并对一些特殊情况下的光电子谱进行了分析。 相似文献
806.
Philip T. Keenan 《Numerical Algorithms》1994,7(2):269-293
We describe a new family of discrete spaces suitable for use with mixed methods on certain quadrilateral and hexahedral meshes. The new spaces are natural in the sense of differential geometry, so all the usual mixed method theory, including the hybrid formulation, carries over to these new elements with proofs unchanged. Because transforming general quadrilaterals into squares introduces nonlinearity and because mixed methods involve the divergence operator, the new spaces are more complicated than either the corresponding Raviart-Thomas spaces for rectangles or corresponding finite element spaces for quadrilaterals. The new spaces are also limited to meshes obtained from a rectangular mesh through the application of a single global bilinear transformation. Despite this limitation, the new elements may be useful in certain topologically regular problems, where initially rectangular grids are deformed to match features of the physical region. They also illustrate the difficulties introduced into the theory of mixed methods by nonlinear transformations. 相似文献
807.
Poly (n-butylisocyanate)-benzene solutions prepared by solubilization at 45°C, followed by aging at room temperature were found to be metastable for months, although, eventually, they separated into a birefringent polymer-rich phase and an isotropic solution. These metastable solutions, as well as isothermally phase-separated biphasic samples, flowed and exhibited dynamic moduli indicative of low polymer connectivity. By contrast samples prepared by a freeze-thaw cycle were uniformly and highly birefringent and showed network (gel) behavior at room temperature. The mechanism of gel formation is most likely the exclusion of the polymer from the benzene crystal during crystallization, forcing the polymer to align and exist at grain boundaries. Films formed from solutions have different moduli than those formed from gels, and are consistent with the proposed mechanism. 相似文献
808.
Laser gas-assisted material processing finds wide application in industry. The modelling of heating, elastic response of the substrate material, and the wave analysis gives insight into the laser workpiece interaction. In the present study, laser gas-assisted heating of steel is considered. The normal component of the thermal stress is taken as the source of load for the flexural wave generation in the material. The flexural wave generated is simulated and the wave characteristics are analyzed at four locations at the workpiece surface. The numerical scheme employing a control volume approach is introduced when solving the governing equations of flow and heat transfer while finite element and spectran element methods are used when solving the stress and wave equations. It is found that the normal component of the stress is tensile. The dispersion effect of the workpiece material, interference of the reflected beam, and partial overlapping of second mode of the travelling wave enable to identify a unique pattern in the travelling wave in the substrate. 相似文献
809.
810.
Olaf Steinbach 《Numerische Mathematik》2002,90(4):775-786
Summary. In this paper we investigate a stability estimate needed in hybrid finite and boundary element methods, especially in hybrid
coupled domain decomposition methods including mortar finite elements. This stability estimate is equivalent to the stability
of a generalized projection in certain Sobolev spaces. Using piecewise linear trial spaces and appropriate piecewise constant test spaces,
the stability of the generalized projection is proved assuming some mesh conditions locally.
Received April 11, 2000 / Revised version received February 15, 2001 / Published online July 25, 2001 相似文献