全文获取类型
收费全文 | 6207篇 |
免费 | 1119篇 |
国内免费 | 1092篇 |
专业分类
化学 | 5326篇 |
晶体学 | 60篇 |
力学 | 267篇 |
综合类 | 81篇 |
数学 | 772篇 |
物理学 | 1912篇 |
出版年
2024年 | 19篇 |
2023年 | 102篇 |
2022年 | 110篇 |
2021年 | 147篇 |
2020年 | 232篇 |
2019年 | 271篇 |
2018年 | 228篇 |
2017年 | 172篇 |
2016年 | 287篇 |
2015年 | 320篇 |
2014年 | 351篇 |
2013年 | 477篇 |
2012年 | 530篇 |
2011年 | 507篇 |
2010年 | 420篇 |
2009年 | 415篇 |
2008年 | 435篇 |
2007年 | 399篇 |
2006年 | 376篇 |
2005年 | 336篇 |
2004年 | 320篇 |
2003年 | 301篇 |
2002年 | 355篇 |
2001年 | 228篇 |
2000年 | 191篇 |
1999年 | 192篇 |
1998年 | 124篇 |
1997年 | 103篇 |
1996年 | 72篇 |
1995年 | 59篇 |
1994年 | 59篇 |
1993年 | 33篇 |
1992年 | 37篇 |
1991年 | 43篇 |
1990年 | 30篇 |
1989年 | 22篇 |
1988年 | 17篇 |
1987年 | 11篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1968年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有8418条查询结果,搜索用时 15 毫秒
971.
本文研究奇性两点狄利克雷问题,在f(t,x,y)≤α(t)g(x)+β(t)h(x)|y|ε,0<ε≤2情形下,得到了解的存在性定理,推广了Tineo(1992)的结果. 相似文献
972.
A new type of macrocyclic polyethers has been synthesized. It consists of an azacrown ether as mother ring, e.g. 1,7-dioxa-4,10-diaza-cyclododecane (1a) or 1,7,10,16-tetraoxa-4,13-diazacycloocatadecane (1b), and two side chains attached on the two nitrogen atoms of 1a or 1b. A number of these new crown ethers are obtained by alkylation of the two secondary amino groups of 1a or 1b with corresponding halides, BrCH2(CH2OCH2)nCH2OR, in the presence of potassium carbonate. The crown-alkali metal complex thus obtained is hydrolyzed by acid. In order to obtain pure crown ether the reaction mixture is treated with tetramethylammonium hydroxide and followed by solvent extraction. The ability of complexing alkali cations of macrocyclic polyethers in terms of the equilibrium constant have been studied by the method of solubilities of salts in chloroform. It is shown that the size of the mother ring, the number of oxygen atoms either in the ring or in the side-chains, and the ionic radius of the alkali metal are the factors governing the stability of the metal complexes. Most of these new crown ethers possess high ability for alkali metal complexation some of them, such as N,N′- di-β-methoxyethyl-1,7-dioxa-4,10-diaza-cyclododecanc (13a), possesses higher selectivity for Na+ and K+ ions than 18-crown-6- and 4,4′(5′)-dimethylbenz-30-crown-10. 相似文献
973.
Recently,thestudyofrelationshipsbetweenstructureandpropertyforseriesofcompoundhavingenhancedopticalnonlinearityhasincreaseourunderstandingofwhatmolecularleveIdesigncriteriaareimPortantformaximizingthesecondandthethird-ordernonlinearity'.lngeneral,thesestudieshavefocusedonmanipulationoftheeffectiveconjugationlengthforavarietyofn-electronbackbones,mostlyrelatedto.-PvariousorganiccomPoundscontainingdonorandacceptorgroups'.InthispaPer,wewillstudytheinteractionamongdonor-metal-donorinx-conjugation… 相似文献
974.
We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg1?x V x )Sr2(Y1?y Ca y )Cu2O z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed. 相似文献
975.
976.
Tian Sheng MEI Li Zeng PENG Tao ZHANG Yu Lin LINational Laboratory of Applied Organic Chemistry Institute of Organic Chemistry Lanzhou University Lanzhou 《中国化学快报》2004,15(7)
A concise and stereoselective synthesis of cathasterone's side chain using methyl isopropyl ketone 3 as the fragment is described. 相似文献
977.
本文用样条函数对GM(1,1)模型的残差序列进行插值拟合,然后作用于二阶线性微分方程,并以此修正原模型,得到一种新的预测模型的数值解. 相似文献
978.
借助于Contingent切锥和集值映射的上图而引入的有关集值映射的Contingent切导数,对约束集值优化问题的超有效解建立了最优性Kuhn Tucker必要及充分性条件,借此建立了向量集值优化超有效解的Wolfe型和Mond Weir型对偶定理. 相似文献
979.
The application of digital phase shift holographic interferometry to flow-visualization and flow temperature measurement of a 2D thermal flow field was experimentally investigated. An improved recording system for two reference beam phase shift holographic interferometry is set up for this experiment, and details of this experimental technique are described. The phase distribution obtained by this technique presents an excellent picture of flow visualization. From this phase image, temperature distribution of the 2D thermal flow field is also calculated and compared with some measured values obtained using thermocouple probes. 相似文献
980.
Ming Jin Ran Lu Qing Xin Yang Chun Yan Bao Ren Sheng Ting Hua Xu YingYing Zhao 《Journal of polymer science. Part A, Polymer chemistry》2007,45(15):3460-3472
The preparation of a series of novel homopolymers and copolymers containing bisazobenzene chromophores with side‐on structure in the side chains via atom transfer radical polymerization (ATRP) were presented. UV–vis spectra of the thin films of these polymers under irradiation of 488 nm Ar+ laser suggested that the photoisomerization of the bisazobenzene chromophores happened mainly on one of the two azo groups in the bisazobenzene chromophores with similar probability due to their side‐on structure. Good photoalignment behaviors of these polymers were proved by photoinduced birefringence measurements because side‐on structure permitted the two azo groups in the bisazobenzene chromophores both participated in the trans–cis–trans photoisomerization cycles equally to induce the whole chromophore reorientation. Furthermore, the reorientation axis located at the middle of chromophore decreased the sweep volume during photoalignment. The impetus for this study was to evaluate the photoisomerization and photoalignment process of side‐on bisazobenzene‐containing polymers and to find possible applications in the photosensitive devices. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3460–3472, 2007 相似文献