全文获取类型
收费全文 | 66574篇 |
免费 | 10799篇 |
国内免费 | 2611篇 |
专业分类
化学 | 61735篇 |
晶体学 | 673篇 |
力学 | 1698篇 |
综合类 | 92篇 |
数学 | 5203篇 |
物理学 | 10583篇 |
出版年
2023年 | 247篇 |
2022年 | 593篇 |
2021年 | 848篇 |
2020年 | 1853篇 |
2019年 | 3099篇 |
2018年 | 1515篇 |
2017年 | 1178篇 |
2016年 | 4185篇 |
2015年 | 4236篇 |
2014年 | 4379篇 |
2013年 | 5798篇 |
2012年 | 5280篇 |
2011年 | 4765篇 |
2010年 | 4413篇 |
2009年 | 4172篇 |
2008年 | 4466篇 |
2007年 | 3677篇 |
2006年 | 3265篇 |
2005年 | 3309篇 |
2004年 | 2915篇 |
2003年 | 2487篇 |
2002年 | 3109篇 |
2001年 | 2169篇 |
2000年 | 1930篇 |
1999年 | 841篇 |
1998年 | 393篇 |
1997年 | 403篇 |
1996年 | 405篇 |
1995年 | 319篇 |
1994年 | 322篇 |
1993年 | 327篇 |
1992年 | 314篇 |
1991年 | 252篇 |
1990年 | 193篇 |
1989年 | 167篇 |
1988年 | 174篇 |
1987年 | 151篇 |
1986年 | 118篇 |
1985年 | 195篇 |
1984年 | 132篇 |
1983年 | 109篇 |
1982年 | 139篇 |
1981年 | 100篇 |
1980年 | 95篇 |
1978年 | 91篇 |
1977年 | 92篇 |
1976年 | 102篇 |
1975年 | 113篇 |
1974年 | 84篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Zi‐Niu Wu 《国际流体数值方法杂志》2005,48(5):541-564
The widely used locally adaptive Cartesian grid methods involve a series of abruptly refined interfaces. In this paper we consider the influence of the refined interfaces on the steady state errors for second‐order three‐point difference approximations of flow equations. Since the various characteristic components of the Euler equations should behave similarly on such grids with regard to refinement‐induced errors, it is sufficient enough to conduct the analysis on a scalar model problem. The error we consider is a global error, different to local truncation error, and reflects the interaction between multiple interfaces. The steady state error will be compared to the errors on smooth refinement grids and on uniform grids. The conclusion seems to support the numerical findings of Yamaleev and Carpenter (J. Comput. Phys. 2002; 181: 280–316) that refinement does not necessarily reduce the numerical error. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
73.
Mao‐Chuan Yuan Ping‐I Shih Chen‐Han Chien Ching‐Fong Shu 《Journal of polymer science. Part A, Polymer chemistry》2007,45(14):2925-2937
We have synthesized a blue‐light‐emitting polyfluorene (PF) derivative ( PF‐CBZ‐OXD ) that presents bulky hole‐transporting carbazole and electron‐transporting oxadiazole pendent groups functionalized at the C‐9 positions of alternating fluorene units. The results from photoluminescence and electrochemical measurements indicate that both the side chains and the PF main chain retain their own electronic characteristics in the copolymer. An electroluminescent device incorporating this polymer as the emitting layer was turned on at 4.5 V; it exhibited a stable blue emission with a maximum external quantum efficiency of 1.1%. Moreover, we doped PF‐CBZ‐OXD and its analogue PF‐TPA‐OXD with a red‐light‐emitting iridium phosphor for use as components of phosphorescent red‐light emitters to investigate the effect of the host's HOMO energy level on the degree of charge trapping and on the electrophosphorescent efficiency. We found that spectral overlap and individual energy level matching between the host and guest were both crucial features affecting the performance of the electroluminescence devices. Atomic force microscopy measurements indicated that the dipolar nature of PF‐CBZ‐OXD , in contrast to the general nonpolarity of polydialkylfluorenes, provided a stabilizing environment that allowed homogeneous dispersion of the polar iridium triplet dopant. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2925–2937, 2007 相似文献
74.
In this paper we consider the numerical approximation of steady and unsteady generalized Newtonian fluid flows using divergence free finite elements generated by the Powell–Sabin–Heindl elements. We derive a priori and a posteriori finite element error estimates and prove convergence of the method of successive approximations for the steady flow case. A priori error estimates of unsteady flows are also considered. These results provide a theoretical foundation and supporting numerical studies are to be provided in Part II. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
75.
Sungyong Kim Lingxin Chen Sangyeop Lee Gi Hun Seong Jaebum Choo Eun Kyu Lee Chil-Hwan Oh Sanghoon Lee 《Analytical sciences》2007,23(4):401-405
A rapid DNA analysis has been developed based on a fluorescence intensity change of a molecular beacon in a PDMS microfluidic channel. Recently, we reported a new analytical method of DNA hybridization involving a PDMS microfluidic sensor using fluorescence energy transfer (FRET). However, there are some limitations in its application to real DNA samples because the target DNA must be labelled with a suitable fluorescent dye. To resolve this problem, we have developed a new DNA microfluidic sensor using a molecular beacon. By monitoring the change in the restored fluorescence intensity along the channel length, it is possible to rapidly detect any hybridization of the molecular beacon to the target DNA. In this case, the target DNA does not need to be labelled. Our experimental results demonstrate that this microfluidic sensor using a molecular beacon is a promising diagnostic tool for rapid DNA hybridization analysis. 相似文献
76.
Ana Luísa Daniel‐da‐Silva João Carlos Moura Bordado José Miguel Martín‐Martínez 《Journal of Polymer Science.Polymer Physics》2007,45(22):3034-3045
The degree of phase separation in several moisture‐cured poly(urethane urea)s (PUUs) was studied by FTIR spectroscopy, wide angle X‐ray diffraction (WAXD), and temperature‐modulated differential scanning calorimetry (TMDSC). This latter technique was shown to be particularly useful in analysing the degree of phase separation in PUU polymers. Both phase mixing and phase segregation coexisted in the PUUs and the degree of phase separation increased as the urea hard segment (HS) content in the PUU increased. The maximum solubility of urea HSs into the polyol soft segments (SSs) was achieved for 50 wt % urea HS content in diol‐based PUUs, whereas for triol‐based PUUs the highest solubility between HS and SS was reached for lower urea HS amount. Finally, the higher the urea HS content the higher the extent of phase separation in the PUU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3034–3045, 2007 相似文献
77.
J. M. Martínez‐Burgos R. Benavente E. Prez M. L. Cerrada 《Journal of Polymer Science.Polymer Physics》2003,41(11):1244-1255
Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003 相似文献
78.
79.
C. Foces‐Foces M. Lpez‐Rodríguez C. Prez 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(2):o95-o97
The title compound, C12H21NO3, crystallizes in two polymorphic forms, viz. the tetragonal form described here and the monoclinic form described previously [Foces‐Foces, López‐Rodríguez, Pérez, Martín & Pérez‐Hernández (2007). Cryst. Growth Des. 7 , 905–911]. The differences in the conformations of the hydroxymethyl and methylaminocarbonyl substituents have important consequences in the hydrogen‐bond interaction motifs and, therefore, in the packing arrangements. These forms are concomitant polymorphs with melting points differing by 3 K. 相似文献
80.
Anti-BZ-Structure in Effect Algebras 总被引:1,自引:0,他引:1
The definitions of sharply approximating effect algebras, anti-BZ-effect algebras, central approximating effect algebras, and S-anti-BZ-effect algebras are given, the relationships between sharply approximating effect algebras and anti-BZ-effect algebras, between central approximating effect algebras and anti-BZ-effect algebras are established, and the set of anti-BZ-sharp elements in S-anti-BZ-effect algebras is proved to be an orthomodular lattice. 相似文献