全文获取类型
收费全文 | 135905篇 |
免费 | 20587篇 |
国内免费 | 13718篇 |
专业分类
化学 | 71264篇 |
晶体学 | 1218篇 |
力学 | 6920篇 |
综合类 | 533篇 |
数学 | 11909篇 |
物理学 | 41736篇 |
无线电 | 36630篇 |
出版年
2024年 | 549篇 |
2023年 | 3277篇 |
2022年 | 4047篇 |
2021年 | 5312篇 |
2020年 | 5209篇 |
2019年 | 4885篇 |
2018年 | 4128篇 |
2017年 | 4071篇 |
2016年 | 5983篇 |
2015年 | 6139篇 |
2014年 | 7494篇 |
2013年 | 9605篇 |
2012年 | 11416篇 |
2011年 | 11644篇 |
2010年 | 8163篇 |
2009年 | 8106篇 |
2008年 | 8564篇 |
2007年 | 7759篇 |
2006年 | 7267篇 |
2005年 | 6323篇 |
2004年 | 4375篇 |
2003年 | 3670篇 |
2002年 | 3290篇 |
2001年 | 2743篇 |
2000年 | 2593篇 |
1999年 | 3017篇 |
1998年 | 2658篇 |
1997年 | 2317篇 |
1996年 | 2565篇 |
1995年 | 2032篇 |
1994年 | 1930篇 |
1993年 | 1551篇 |
1992年 | 1448篇 |
1991年 | 1276篇 |
1990年 | 988篇 |
1989年 | 692篇 |
1988年 | 559篇 |
1987年 | 442篇 |
1986年 | 428篇 |
1985年 | 371篇 |
1984年 | 276篇 |
1983年 | 197篇 |
1982年 | 165篇 |
1981年 | 114篇 |
1980年 | 88篇 |
1979年 | 53篇 |
1978年 | 40篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
用自旋-晶格弛豫时间(T1)研究了溶胀的交联聚丙烯酰胺-丙烯酸网络和线型聚苯乙烯溶液中质子的弛豫行为。交联网络中,随着交联度增大,T1CH/T1CH2的值由1.17逐渐趋近于1;而线型聚苯乙烯溶液中,T1CH/T1CH2的值由最稀浓度下的1.7过渡到1。说明在交联网络中,交联度很低时,链段的运动已经相当受约束;但交联度很大时,充分溶胀的交联网络中链段运动仍有一定自由度。而在线型高分子浓溶液中,链段的运动严重受阻,导致自旋扩散效应非常完全,彻底平均掉了各质子间T1时间的差异。 相似文献
972.
Phanstiel O IV Price HL Wang L Juusola J Kline M Shah SM 《The Journal of organic chemistry》2000,65(18):5590-5599
An efficient five-step synthetic method was developed to access a homologous series of spermidine-acridine and spermidine-anthracene conjugates. The derivatives were comprised of a spermidine fragment covalently tethered at its N4 position to either an acridine or anthracene nucleus via an aliphatic chain (e.g., spermidine-[aliphatic tether]-acridine). The distance separating the spermidine and aromatic nucleus was altered by using different tethers comprised of four or five methylene units, respectively. These ligands (2-5) were shown to inhibit human DNA topoisomerase-II (TOPO-II) activity at 10 microM. Enzymatic activity was assessed as the ability to unknot (decatenate) and cleave kinetoplast DNA (kDNA). Polyamine conjugation did not disrupt the ability of the acridine-spermidine conjugates 2 and 3 to inhibit TOPO-II activity as compared with the 9-aminoacridine and 9-(N-butyl)aminoacridine controls (at 10 microM). In general, the acridine derivatives (2 and 3) showed higher TOPO-II inhibitory activity than their anthracene counterparts (4 and 5). However, this trend was reversed in a whole cell assay with L1210 (murine leukemia) cells, wherein the anthracene analogues were more potent than their acridine counterparts. In this regard the qualitative enzyme-based assay did not predict the trends in the corresponding IC(50) values. Within either series insertion of an additional methylene unit did not significantly alter activity. While the appended spermidine unit did not disrupt TOPO II inhibition by the tethered DNA intercalator, it did provide an alternative mode of entry into the cell as demonstrated by spermidine protection assays. These results were compared with a spermine-intercalator analogue. Of all the conjugates tested the N(4)-(4-(9-aminoacridinyl)butyl)spermine hexahydrochloride (conjugate 16)resulted in the highest degree of L1210 cell rescue upon cotreatment of the cells with exogenous spermidine. It was concluded that the monoalkylated spermine motif present in 16 holds promise as a better vector than its N4 monoalkylated spermidine counterpart. 相似文献
973.
There have been remarkable progresses in manipulating heterogeneous catalysts' nanostructures in the past decade. The concept of single atom alloy (SAA) was firstly proposed in 2012 when researchers successfully stabilized single Pd atoms on the Cu(111) surface. However, earlier work in 2009, which focused on replacing one Au atom with a Pd atom in thiolate protected Au25 nanoclusters, could also be considered as the pioneer work of single atom alloy. Both kinds of single atom alloys exhibited the potential of maximum utilization of scarce elements and attractive catalytic performances. The well‐defined structures of SAA catalysts make accurate modeling possible, which further realizes the rational design of single atom alloy catalysts. In this review, we summarize the research trajectory of single atom alloys as well as recent achievements in this field. We also introduce several commonly adopted characterization methods for SAA catalysts such as scanning tunneling microscopy (STM), temperature programmed reaction (TPR), extended X‐ray absorption fine structure (EXAFS) spectra, matrix assisted laser desorption/ionization mass spectrum (MALDI‐MS) and differential pulse voltammetry (DPV). Through discussing recent progresses in SAA catalysts, we propose that future researches in this filed should be focused on exploring new kinds of metal nanocrystals and controlling the nanostructure of SAA even more precisely. 相似文献
974.
975.
合成了六种轻希土硫氰酸盐与标题不饱和冠醚的新固体配合物,进行了元素分析,摩尔电导率、DTA、UV、IR和XPS等表征.对Ln(NCS)3·L和Ln(NCS)3的XPS研究中,得到它们组成原子芯能级电子(Ln3d5/2,Ols,Nls,S2p)的结合能信息;观察到除Y3d5/2外Ln3d5/2(Ln=La、Ce、Pr、Nd、和Sm)的伴峰结构,Ln3d5/2结合能与原子序数呈线性关系,并推断类似化合物Pm3d5/2结合能在1035eV附近. 相似文献
976.
977.
978.
979.
μ-酰基与μ-烯基铁硫原子簇配合物的合成及结构 总被引:3,自引:0,他引:3
由芳香族酰氯或α,β-不饱和酰氯与十二羰基三铁-硫醇-三乙胺体系反应,制得了通式为[(μ-RS)(μ-R′CO)Fe,(CO)_6](A)的六个铁硫原子簇配合物,以及通式为((μ-RS)(μR″CH=CR′)Fe_2(CO)_6](B)的六个铁硫配合物。对两种类型的配合物生成的过程及转化进行了初步的讨论。 相似文献
980.