全文获取类型
收费全文 | 141544篇 |
免费 | 17927篇 |
国内免费 | 14685篇 |
专业分类
化学 | 122461篇 |
晶体学 | 3019篇 |
力学 | 3997篇 |
综合类 | 639篇 |
数学 | 17494篇 |
物理学 | 26546篇 |
出版年
2024年 | 226篇 |
2023年 | 1319篇 |
2022年 | 2793篇 |
2021年 | 2858篇 |
2020年 | 3475篇 |
2019年 | 4867篇 |
2018年 | 4307篇 |
2017年 | 5287篇 |
2016年 | 5696篇 |
2015年 | 7493篇 |
2014年 | 8276篇 |
2013年 | 12887篇 |
2012年 | 9780篇 |
2011年 | 10804篇 |
2010年 | 8727篇 |
2009年 | 9557篇 |
2008年 | 9670篇 |
2007年 | 9038篇 |
2006年 | 8243篇 |
2005年 | 7610篇 |
2004年 | 6955篇 |
2003年 | 5733篇 |
2002年 | 5843篇 |
2001年 | 3779篇 |
2000年 | 3455篇 |
1999年 | 2354篇 |
1998年 | 1617篇 |
1997年 | 1407篇 |
1996年 | 1356篇 |
1995年 | 1297篇 |
1994年 | 1200篇 |
1993年 | 932篇 |
1992年 | 894篇 |
1991年 | 629篇 |
1990年 | 465篇 |
1989年 | 428篇 |
1988年 | 388篇 |
1987年 | 299篇 |
1986年 | 242篇 |
1985年 | 250篇 |
1984年 | 221篇 |
1983年 | 104篇 |
1982年 | 201篇 |
1981年 | 221篇 |
1980年 | 207篇 |
1979年 | 192篇 |
1978年 | 156篇 |
1977年 | 128篇 |
1976年 | 103篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
Nadya Dencheva Teresa G. Nunes M. Jovita Oliveira Zlatan Denchev 《Journal of Polymer Science.Polymer Physics》2005,43(24):3720-3733
The crystalline structure of polyamide‐12 (PA12) was studied by solid‐state 13C nuclear magnetic resonance (NMR) as well as by synchrotron wide‐ and small‐angle X‐ray scattering (WAXS and SAXS). Isotropic and oriented PA12 showed different NMR spectra ascribed to γ‐ and γ′‐crystalline modifications, respectively. On the basis of the position of the first diffraction peak, the isotropic γ‐form and the oriented γ′‐form were shown to be with hexagonal crystalline lattice at room temperature. When heated, the two PA12 polymorphs demonstrated different behaviors. Above 140 °C, the isotropic γ‐PA12 partially transformed into α‐modification. No such transition was observed with the oriented γ′‐PA12 phase even after annealing at temperatures close to melting. A γ′–γ transition was observed here only after isotropization by melting point. Various structural parameters were extracted from the WAXS and SAXS patterns and analyzed as a function of temperature and orientation: the degree of crystallinity, the d‐spacings, the Bragg's long spacings, the average thicknesses of the crystalline (lc) and amorphous (la) phases, and the linear crystallinity xcl within the lamellar stacks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3720–3733, 2005 相似文献
992.
993.
使用分子图形软件设计出多种CnP-4(n =1~ 7)的结构模型 ,并进行B3LYP密度泛函几何构型优化和振动频率计算 .最稳定的CP-4和C2 P-3 都是平面环状结构 .最稳定的CnP-4(n =3 ,5 ,7)结构在直碳链的一端连接 1个磷原子且另一端是P3 C的四元环的平面结构 .最稳定的CnP-4(n =4,6)结构在直碳链的一端连接 1个磷原子且另一端是P3 的三元环的锄状结构 .直碳链可与平面环的磷原子生成大π键 .大多数构型是由C2 ,C3 ,C4子结构以环状或链状方式组成的 .碳原子与磷原子以交替方式排列的结构数量少、能量高 . 相似文献
994.
Akira Sakakura 《Tetrahedron》2004,60(33):7067-7075
To obtain information on the chemical modification of biomolecules with jolkinolide D, a bioactive diterpenoid of plant origin, jolkinolide D pharmacophore was synthesized, and its reactivity toward amino acids, nucleosides, and DNA was investigated. 相似文献
995.
2-Cyanoethyl anilines were controllably cleaved by 1 equiv of potassium t-butoxide in dry THF with a reaction that can be used in situ with the Wittig reaction to synthesize conjugated primary and secondary anilines. 相似文献
996.
In this paper, a projection method is presented for solving the flow problems in domains with moving boundaries. In order to track the movement of the domain boundaries, arbitrary‐Lagrangian–Eulerian (ALE) co‐ordinates are used. The unsteady incompressible Navier–Stokes equations on the ALE co‐ordinates are solved by using a projection method developed in this paper. This projection method is based on the Bell's Godunov‐projection method. However, substantial changes are made so that this algorithm is capable of solving the ALE form of incompressible Navier–Stokes equations. Multi‐block structured grids are used to discretize the flow domains. The grid velocity is not explicitly computed; instead the volume change is used to account for the effect of grid movement. A new method is also proposed to compute the freestream capturing metrics so that the geometric conservation law (GCL) can be satisfied exactly in this algorithm. This projection method is also parallelized so that the state of the art high performance computers can be used to match the computation cost associated with the moving grid calculations. Several test cases are solved to verify the performance of this moving‐grid projection method. Copyright © 2004 John Wiley Sons, Ltd. 相似文献
997.
This paper presents an operator‐splitting method (OSM) for the solution of the universal Reynolds equation. Jakobsson–Floberg–Olsson (JFO) pressure conditions are used to study cavitation in liquid‐lubricated journal bearings. The shear flow component of the oil film is first solved by a modified upwind finite difference method. The solution of the pressure gradient flow component is computed by the Galerkin finite element method. Present OSM solutions for slider bearings are in good agreement with available analytical and experimental results. OSM is then applied to herringbone grooved journal bearings. The film pressure, cavitation areas, load capacity and attitude angle are obtained with JFO pressure conditions. The calculated load capacities are in agreement with available experimental data. However, a detailed comparison of the present results with those predicted using Reynolds pressure conditions shows some differences. The numerical results showed that the load capacity and the critical mass of the journal (linear stability indicator) are higher and the attitude angle is lower than those predicted by Reynolds pressure conditions for cases of high eccentricities. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
998.
The interfacial dynamics‐based cavitation model, developed in Part‐1, is further employed for unsteady flow computations. The pressure‐based operator‐splitting algorithm (PISO) is extended to handle the time‐dependent cavitating flows with particular focus on the coupling of the cavitation and turbulence models, and the large density ratio associated with cavitation. Furthermore, the compressibility effect is important for unsteady cavitating flows because in a water–vapour mixture, depending on the composition, the speed of sound inside the cavity can vary by an order of magnitude. The implications of the issue of the speed of the sound are assessed with alternative modelling approaches. Depending on the geometric confinement of the nozzle, compressibility model and cavitation numbers, either auto‐oscillation or quasi‐steady behaviour is observed. The adverse pressure gradient in the closure region is stronger at the maximum cavity size. One can also observe that the mass transfer process contributes to the cavitation dynamics. Compared to the steady flow computations, the velocity and vapour volume fraction distributions within the cavity are noticeably improved with time‐dependent computations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
999.
1000.
A new numerical method called linearized and rational approximation method is presented to solve non‐linear evolution equations. The utility of the method is demonstrated for the case of differentiation of functions involving steep gradients. The solution of Burgers' equation is presented to illustrate the effectiveness of the technique for the solution of non‐linear evolution equations exhibiting nearly discontinuous solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献