全文获取类型
收费全文 | 9998篇 |
免费 | 1543篇 |
国内免费 | 1390篇 |
专业分类
化学 | 7787篇 |
晶体学 | 85篇 |
力学 | 456篇 |
综合类 | 101篇 |
数学 | 1255篇 |
物理学 | 3247篇 |
出版年
2024年 | 23篇 |
2023年 | 150篇 |
2022年 | 168篇 |
2021年 | 235篇 |
2020年 | 353篇 |
2019年 | 367篇 |
2018年 | 324篇 |
2017年 | 263篇 |
2016年 | 408篇 |
2015年 | 451篇 |
2014年 | 508篇 |
2013年 | 706篇 |
2012年 | 839篇 |
2011年 | 789篇 |
2010年 | 635篇 |
2009年 | 603篇 |
2008年 | 687篇 |
2007年 | 606篇 |
2006年 | 569篇 |
2005年 | 496篇 |
2004年 | 445篇 |
2003年 | 410篇 |
2002年 | 453篇 |
2001年 | 315篇 |
2000年 | 293篇 |
1999年 | 262篇 |
1998年 | 185篇 |
1997年 | 157篇 |
1996年 | 146篇 |
1995年 | 113篇 |
1994年 | 119篇 |
1993年 | 85篇 |
1992年 | 91篇 |
1991年 | 79篇 |
1990年 | 58篇 |
1989年 | 62篇 |
1988年 | 53篇 |
1987年 | 41篇 |
1986年 | 58篇 |
1985年 | 50篇 |
1984年 | 43篇 |
1983年 | 34篇 |
1982年 | 27篇 |
1981年 | 18篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1977年 | 17篇 |
1976年 | 17篇 |
1975年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
942.
943.
Ümit Akbey Robert Graf Yuan G. Peng Peter P. Chu Hans W. Spiess 《Journal of Polymer Science.Polymer Physics》2009,47(2):138-155
NMR studies of the structure and dynamics of a system composed of the acidic polymer poly(acrylic acid) (PAA) and the basic polymer poly(4‐vinyl pyridine) (P4VP) are presented. This system aims at the application of anhydrous proton‐conducting membranes that can be used at elevated temperatures at which the proton conduction of hydrated membranes breaks down. The 1H NMR measurements have been preformed under fast magic angle spinning (MAS) conditions to achieve sufficient resolution and the applied 1H NMR methods vary from simple 1H MAS to double‐quantum filtered methods and two‐dimensional 1H double‐quantum spectroscopy. The dynamic behavior of the systems has been investigated via variable temperature 1H MAS NMR. 13C cross‐polarization MAS NMR provides additional aspects of dynamic and structural features to complete the picture. Different types of acidic protons have been identified in the studied PAA‐P4VP systems that are nonhydrogen‐bonded free acidic protons, hydrogen‐bonded dicarboxylic dimers, and protons forming hydrogen bonds between carboxylic protons and ring nitrogens. The conversion of dimer structures in dried PAA to free carboxylic acid groups is accomplished at temperatures above 380 K. However, the stability of hydrogen‐bonding strongly depends on the hydration level of the polymer systems. The effect of hydration becomes less apparent in the complexes. An inverse proportionality between hydrogen‐bonding strength and proton conduction in the PAA‐P4VP acid–base polymer blend systems was established. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 138–155, 2009 相似文献
944.
This paper considers the problem of coordinating a single-vendor multi-buyer inventory system when there are privacy restrictions in the information required to solve the problem. The objective function and cost parameters of each facility are regarded as private information that no other facilities in the system have access to. Moreover, each facility is responsible to specify its own replenishment policy. The objective is to minimize the total average setup/ordering and inventory-related cost. Solution methodologies under private and global information are developed to find two types of nested power-of-two stationary policies. The first policy assumes all the buyers must replenish simultaneously. The second policy is a more general case where the common replenishment assumption is relaxed. A simple form of information exchange is uncovered that allows the solution methodologies for private and global information yield the same results. The experimental results suggest that the performance of the proposed heuristics is comparable or better than an existing method. 相似文献
945.
Li‐Ming Tang Ji‐Qian Wu Yi‐Qun Duan Li Pan Yan‐Guo Li Yue‐Sheng Li 《Journal of polymer science. Part A, Polymer chemistry》2008,46(6):2038-2048
Five novel vanadium(III) complexes [PhN = C(R2)CHC(R1)O]VCl2(THF)2 ( 4a : R1 = Ph, R2 = CF3; 4b : R1 = t‐Bu, R2 = CF3; 4c : R1 = CF3, R2 = CH3; 4d : R1 = Ph, R2 = CH3; 4e : R1 = Ph, R2 = H) have been synthesized and characterized. On activation with Et2AlCl, all the complexes, in the presence of ethyl trichloroacetate (ETA) as a promoter, are highly active precatalysts for ethylene polymerization, and produce high molecular weight and linear polymers. Catalyst activities more than 16.8 kg PE/mmolV h bar and weight‐average molecular weights higher than 173 kg/mol were observed under mild conditions. The copolymerizations of ethylene and norbornene or 1‐hexene with the precatalysts were also explored, which leads to high molecular weight copolymers with high comonomer incorporation. Catalyst activity, comonomer incorporation, and polymer molecular weight as well as polydispersity index can be controlled over a wide range by the variation of precatalyst structure and the reaction parameters such as Al/V molar ratio, comonomer feed concentration, and polymerization temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2038–2048, 2008 相似文献
946.
对a∈(0,1/2)和r∈(0,1),Ramanujan广义模方程中的广义Grǒtzsch环函数μa(r)定义如下:μa(r)=[πκa^1(r)]/[2sin(πa)κa(r)].该文通过研究μs(r)和μ(r)的关系,以及μa(r)和一些初等函数的组合的单调性和凹凸性,获得了μa(r)的几个精确不等式,从而把μ(r)的一些熟知的性质推广到μa(r)上. 相似文献
947.
948.
S为1/2体系粉末或冷冻玻璃态电子自旋共振谱的模拟 总被引:1,自引:1,他引:0
本文较详细地介绍了模拟s为1/2体系粉末或冷冻玻璃态电子自旋共振谱的理论基础。给出了具体计算公式,利用这些公式完全可以在微机上进行计算机模拟电子自旋共振谱。 相似文献
949.
Copolymers that separately contained thiophosphate–polysiloxane, thiophosphate–amino, and thiophosphate–polysiloxane–amino groups were synthesized and characterized. The performances of these synthesized copolymers on metal surfaces under a high load and rotary velocity were examined by the measurement of the oil temperature, frictional coefficient, and electrical contact resistance between two metal surfaces. The configuration of the adsorption layer was studied with energy‐dispersive spectrometry (EDS) and scanning electron microscopy (SEM). Copolymers that contained amino–thiophosphate groups reduced the frictional coefficient between two metal surfaces and markedly limit the increase in the oil temperature. Notably, a higher content of polysiloxane groups in the copolymer corresponded to an increased operating time to establish the adsorption layer on the metal surface. However, a higher content of amino groups reduced the time required for the layer to form. The adsorption layers that formed on the metal surfaces were investigated by EDS as phosphides produced by a chemical reaction of the thiophosphate‐containing copolymer with the metal surface. These adsorption layers on the metal surface were directly observed with SEM. The layer that formed on the metal surface of copolymers containing less polysiloxane and more amino groups was the thickest layer for all the synthesized copolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1795–1803, 2002 相似文献
950.
本文制备了三种在1,4-bis[2-(4-pyridyl)ethenyl]-benzene(bp-eb)上接枝不同烷基链长度的热致变色材料DC8、DC12、DC16. 在365 nm激发光下,随着温度升高,它们呈现出荧光颜色的改变,这种改变来自于晶体态与无定形态之间的转变. 此外,DC16也呈现出光致变色的性质. 通过差示扫描量热法测试得到的相转变温度高于实验过程中荧光颜色改变时的温度. 因此,这种变色行为来自于光与热共同作用的结果. 乙醇可以使粉末变回起始的晶体状态,从而使荧光颜色恢复,实现热致变色行为的可逆. 本研究对理解热致变色分子的结构-性质关系,指导热致变色分子设计具有重要意义. 相似文献