全文获取类型
收费全文 | 91334篇 |
免费 | 6917篇 |
国内免费 | 13212篇 |
专业分类
化学 | 56789篇 |
晶体学 | 1919篇 |
力学 | 869篇 |
综合类 | 605篇 |
数学 | 3762篇 |
物理学 | 15796篇 |
综合类 | 31723篇 |
出版年
2024年 | 281篇 |
2023年 | 1337篇 |
2022年 | 2727篇 |
2021年 | 2652篇 |
2020年 | 2634篇 |
2019年 | 2432篇 |
2018年 | 2122篇 |
2017年 | 2782篇 |
2016年 | 3026篇 |
2015年 | 2914篇 |
2014年 | 4091篇 |
2013年 | 6458篇 |
2012年 | 5767篇 |
2011年 | 6169篇 |
2010年 | 5184篇 |
2009年 | 6029篇 |
2008年 | 5845篇 |
2007年 | 6548篇 |
2006年 | 5850篇 |
2005年 | 5144篇 |
2004年 | 4810篇 |
2003年 | 4138篇 |
2002年 | 3377篇 |
2001年 | 2789篇 |
2000年 | 2483篇 |
1999年 | 2135篇 |
1998年 | 1677篇 |
1997年 | 1478篇 |
1996年 | 1288篇 |
1995年 | 1240篇 |
1994年 | 1094篇 |
1993年 | 906篇 |
1992年 | 845篇 |
1991年 | 688篇 |
1990年 | 528篇 |
1989年 | 427篇 |
1988年 | 348篇 |
1987年 | 250篇 |
1986年 | 162篇 |
1985年 | 129篇 |
1984年 | 111篇 |
1983年 | 45篇 |
1982年 | 80篇 |
1981年 | 94篇 |
1980年 | 68篇 |
1979年 | 72篇 |
1978年 | 45篇 |
1977年 | 50篇 |
1976年 | 31篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
31.
以乙酸四氢呋喃甲酯为原料,通过自由基氯化反应,合成双脱氧核苷类抗艾滋病药物的重要中间体化合物5—0—乙酰基—2.3—双脱氧—D.L—吹喃戊糖基氯.对于此种不稳定的中间体化合物,用~1HNMR跟踪监测反应物的方法,研究了改变反应时间、反应温度和反应物摩尔比对反应产物的影响,获得了最佳的反应条件,使其收率从10%提高到50%左右. 相似文献
32.
33.
用程序降温法生长了SrFCl:Eu~(2+)晶体,并测量了它的电子自旋共振谱。应用最小二乘法拟合技术对g因子及自旋哈密顿参数进行了精确拟合。报道了各向异性的g因子及有关自旋哈密顿参量。发现并归属了Eu~(2+)的高阶禁阻跃迁。 相似文献
34.
35.
Semi-empirical molecular orbital calculations were carried out for the compounds (C2H5)3As, (C2H5)3Ga and RAsH2 (R = C2H5, i-C3H7, i-C4H9, and t-C4H9) by using the CNDO/2-U program, and their capability of β-elimination reaction is compared on the basis of the torsion energy to the transition state, electrostatic interactions and orbital overlapping between the central atom and the β-hydrogen, and bond order of the metal-carbon, and carbon-hydrogen bond. In the comparison of (C2H5)3As with (C2H5)3Ga, we found that the β-elimination of (C2H5)3As could hardly be expected to take place in the thermal decomposition. The capability of β-elimination would be smaller in C2H5AsH2 than that in (C2H5)3As. Moreover when the ethyl group is replaced by a t-butyl group in RAsH2, the β-elimination reaction appears to become more difficult and a large possibility for a radical process is suggested. 相似文献
36.
Q. Gong R. N tzel P.J. van Veldhoven T.J. Eijkemans J.H. Wolter 《Journal of Crystal Growth》2005,280(3-4):413-418
We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux. 相似文献
37.
Nanocomposites (NC) were formed using cationic poly(L ‐lysine) (PLL), a semicrystalline polypeptide, that was reinforced by sodium montmorillonite (MMT) clay via solution intercalation technique. By varying solution conditions such as pH, temperature, and polypeptide concentration in the presence of clay platelets, the secondary structure of PLL was controllably altered into α‐helical, β‐sheet, and random coil. The high molecular weight polypeptide shows a strong propensity to fold into the β‐sheet structure when cast as films, irrespective of the initial secondary structure in solution. Nanocomposite local morphology confirms intercalated MMT platelets with PLL over a wide range of compositions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 239–252, 2007. 相似文献
38.
YU Jianqiao LIAO Jianwei 《武汉大学学报:自然科学英文版》2007,12(5):887-892
We propose a multiple-tree overlay structure for resource discovery in unstructured P2P systems. Peers that have similar interests or hold similar type of resources will be grouped into a tree-like cluster. We exploit the heterogeneity of peers in each cluster by connecting peers with more capacities closer to the root of the tree. The capacity of a peer can be defined in different ways (e.g. higher network bandwidth, larger disk space, more data items of a certain type etc.) according to different needs of users or applications. 相似文献
39.
Information extraction techniques on the Web are the current research hotspot. Now many information extraction techniques based on different principles have appeared and have different capabilities. We classify the existing information extraction techniques by the principle of information extraction and analyze the methods and principles of semantic information adding, schema defining,rule expression, semantic items locating and object locating in the approaches. Based on the above survey and analysis,several open problems are discussed. 相似文献
40.
Leong‐Huat Gan Palaniswamy Ravi Bao Wei Mao Kam‐Chiu Tam 《Journal of polymer science. Part A, Polymer chemistry》2003,41(17):2688-2695
Polymerization of 2‐(diethylamino)ethyl methacrylate (DEAEMA) via homogeneous atom transfer radical polymerization under various reaction conditions is described. The effects of the initiators and solvents were examined. With 1,1,4,7,10,10‐hexamethyl triethylenetetramine/copper(I) chloride/p‐toluenesulfonyl chloride as the ligand/catalyst/initiator system in methanol, poly(DEAEMA) with a polydispersity index as low as 1.07 was synthesized. Kinetic studies demonstrated the polymerization was very well controlled and exhibited the living characteristic of the process. Well‐defined block copolymers of DEAEMA and tert‐butyl methacrylate (tBMA) were successfully synthesized. The copolymers could be synthesized with equally good results by starting with either p(DEAEMA) or p(tBMA) as the macroinitiators. However, only the macroinitiators terminated with chlorine should be used. The corresponding macroinitiators with bromine as a transferable group did not yield well‐defined copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2688–2695, 2003 相似文献