全文获取类型
收费全文 | 4217篇 |
免费 | 201篇 |
国内免费 | 1037篇 |
专业分类
化学 | 4392篇 |
晶体学 | 29篇 |
力学 | 11篇 |
综合类 | 30篇 |
数学 | 16篇 |
物理学 | 182篇 |
综合类 | 795篇 |
出版年
2024年 | 8篇 |
2023年 | 17篇 |
2022年 | 53篇 |
2021年 | 64篇 |
2020年 | 56篇 |
2019年 | 96篇 |
2018年 | 58篇 |
2017年 | 78篇 |
2016年 | 108篇 |
2015年 | 103篇 |
2014年 | 134篇 |
2013年 | 310篇 |
2012年 | 188篇 |
2011年 | 223篇 |
2010年 | 183篇 |
2009年 | 220篇 |
2008年 | 261篇 |
2007年 | 244篇 |
2006年 | 266篇 |
2005年 | 286篇 |
2004年 | 272篇 |
2003年 | 222篇 |
2002年 | 239篇 |
2001年 | 191篇 |
2000年 | 139篇 |
1999年 | 152篇 |
1998年 | 172篇 |
1997年 | 152篇 |
1996年 | 140篇 |
1995年 | 159篇 |
1994年 | 135篇 |
1993年 | 112篇 |
1992年 | 87篇 |
1991年 | 90篇 |
1990年 | 58篇 |
1989年 | 61篇 |
1988年 | 33篇 |
1987年 | 27篇 |
1986年 | 19篇 |
1985年 | 10篇 |
1984年 | 15篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
排序方式: 共有5455条查询结果,搜索用时 15 毫秒
931.
Jordi Casanovas Dr. Julien Preat Dr. David Zanuy Dr. Carlos Alemán Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(18):4676-4684
Tuning sensing abilities! The affinity of three different [15]crown‐5 ether functionalized polythiophenes for alkali ions has been explored (see figure). Ab initio and DFT quantum mechanical calculations show that the binding energy between neutral conducting polymers and metallic ions, which interact attractively, decreases as the size of the ion increases. Oxidation of these polythiophene derivatives significantly reduces their affinity towards alkali ions, becoming low or even nonexistent.
932.
通过在磺化聚醚醚酮(SPEEK,DS=61.68%)中分别混入酚酞型聚醚砜(PES-C)、磺化酚酞型聚醚砜(SPES-C,DS=53.7%)制备出SPEEK/PES-C、SPEEK/SPES-C共混质子交换膜.结果表明,共混的两种聚合物之间均具有较好的相容性.PES-C、SPES-C的混入能有效降低膜的溶胀及甲醇透过,且随着共混量的增加,这种作用越趋明显.纯SPEEK膜在75℃左右溶解,而SPEEK/PES-C(30wt%)、SPEEK/SPES-C(30wt%)共混膜在80℃时溶胀度仅为22.5%、26.32%.在室温至80℃范围内,纯SPEEK及共混膜的甲醇透过系数都在10-7cm2.s-1数量级上,远小于Nafion115膜.在饱和湿度下,温度大于90℃时,SPEEK/PES-C(20wt%)共混膜电导率超过Nafion115膜;温度大于110℃时,SPEEK/SPES-C(30wt%)共混膜电导率与Nafion115膜相当,达到0.11S.cm-1.高电导率,低透醇系数以及明显提高了的可使用温度表明该类共混膜有望在DMFC中使用. 相似文献
933.
13-Crown-4,16-crown-5,dibenzo-12-crown-4 and dibenzo-14-crown-4 were synthesized by a one-pot microwave-assisted procedure in good yields.Irradiation of diols and dichlorides in the presence of sodium hydroxide in DMSO gave title crown ethers presumably within a template effect. 相似文献
934.
Dimethyl ether (DME) was synthesized from methane through a two-step process, in which CH3Br was prepared from the oxidative bromination reaction of methane in the presence of HBr and oxygen over a Rh-SiO2 catalyst and then, in the second step, CH3Br was hydrolyzed to DME over a silica supported metal chloride catalyst. 12 mol%ZnCl2/SiO2 catalyst was found to be the most active, but it deactivated because of Cl− losing. 相似文献
935.
Environmental analysis of higher brominated diphenyl ethers and decabromodiphenyl ethane 总被引:1,自引:0,他引:1
Methods for environmental analysis of higher brominated diphenyl ethers (PBDEs), in particular decabromodiphenyl ether (BDE209), and the recently discovered environmental contaminant decabromodiphenyl ethane (deBDethane) are reviewed. The extensive literature on analysis of BDE209 has identified several critical issues, including contamination of the sample, degradation of the analyte during sample preparation and GC analysis, and the selection of appropriate detection methods and surrogate standards. The limited experience with the analysis of deBDethane suggests that there are many commonalities with BDE209. The experience garnered from the analysis of BDE209 over the last 15 years will greatly facilitate progress in the analysis of deBDethane. 相似文献
936.
A. Zabaleta 《European Polymer Journal》2009,45(2):466-473
Rubber toughening of poly(ether imide) (PEI) has been elusive up to now due to the high processing temperature of PEI, which leads to degradation of the rubber. In this study, by profiting from the miscibility between PEI and poly(butylene terephthalate) (PBT), and the low Tg of PBT, we prepared a blend by melt extrusion with 20 wt% PBT in an attempt to render it toughenable by decreasing its Tg and processing temperature. The PEI-rich blend was subsequently mixed with maleic anhydride (0.9 wt%) grafted poly(ethylene-octene) copolymer (mPEO) up to 30 wt%. The decrease in Tg and processing temperature resulted in no observable degradation of the mPEO, and to the formation of a homogeneous morphology of rubber particles with a fine particle size, indicating that compatibilization was achieved. Upon rubber addition, stiffness decreased, while a very large toughness increase occurred with only 15% mPEO (impact strength more than 10-fold that of the PEI-PBT matrix). Upon observation of the fracture surface, the increase in impact strength was attributed partially to the cavitation and debonding of the rubber particles, and mostly to the deformation and yielding of the PEI-PBT matrix. 相似文献
937.
Chiral recognition of phenylalanine (Phe) was achieved in the gas phase by electrospray ionization Q-TOF tandem mass spectrometry.
In this method, two central ions, i.e. proton and divalent copper, were used and chiral crown ether, (+)-2,3,11,12-tetracarboxylic
acid-18-crown-6 (18-C-6-TCA), was used as a chiral host. Dimeric complexes were readily formed by electrospray ionization
of a methanol/water (50/50, V/V) solution containing central ions, Phe and 18-C-6-TCA. The dimeric complex included proton-bound (18-C-6-TCA)(Phe)H+ and
copper-bound deprotonated [Cu2+(18-C-6-TCA)(Phe)-H]+ ions were mass selected and then collided with Ar in the CID experiments. The chiral recognition capability of these complexes
was evaluated using the relative abundance of daughter ion to parent ion. A higher degree of chiral recognition ability was
observed with Cu2+ compared to that of H+. Different central ions exhibited distinctive dissociation pathways and unique chiral recognition characteristics. The chiral
recognition mechanism was also discussed in detail with the help of the structure of copper-bound complex predicted by theoretical
calculation. 相似文献
938.
设计了含改性松香无苯环的环氧固化体系和无松香的对照体系,分别是无松香体系Ⅰ:丁二醇二缩水甘油醚(BDGE)和甲基六氢苯酐(MeHHPA);无规体系Ⅱ:丙烯酸松香(AR)、BDGE和MeHHPA;有规体系Ⅲ:丙烯酸松香基环氧树脂(ARE,由AR和BDGE预聚而来)和MeHHPA。对三体系的固化反应应用动态示差扫描量热仪(DSC)进行了研究,利用KAS法求得不同转化率下的表观活化能,通过整个反应过程反应活化能的变化,得到了三体系反应的内在机理,结果发现松香基的引入及引入顺序都对固化反应产生影响。 相似文献
939.
Serena Staccioli Maria Altamura Gianluca Giorgi Antonio Triolo 《Journal of mass spectrometry : JMS》2009,44(7):1081-1086
Di(hydroxybutyl) ether (DHBE), a liver protecting drug, is composed of a mixture of three regioisomers: 4‐(3‐hydroxybutoxy)‐2‐butanol (1), 3‐(4‐hydroxy‐2‐butoxy)‐1‐butanol (2), and 3‐(3‐hydroxybutoxy)‐1‐butanol (3). Unequivocal differentiation of each regioisomer of DHBE was rapidly obtained without isolation of the single components, using GC‐MS with electron ionization (EI). The mass spectrum of 1 showed a rearrangement ion at m/z 118, characteristic of the 3‐hydroxybutyl chain, deriving from loss of acetaldehyde from the molecular ion, whereas 2 and 3 were characterized by the ion at m/z 117, expected from α‐cleavage of the 4‐hydroxy‐2‐butyl chain. The species at m/z 118, in turn, loses a water molecule via a mechanism involving both alcohol hydrogens, as shown by deuterium exchange experiments. Both this finding and theoretical calculations support a mechanism in which the loss of acetaldehyde in 1 occurs via a cyclic intermediate, stabilized by a strong hydrogen bond between the alcohol oxygen bearing the charge and the other alcohol oxygen, and involves initial hydrogen transfer from the former to the latter. The EI spectrum of 2, having two 4‐hydroxy‐2‐butyl chains, showed the fragmentations expected from classical fragmentation rules of aliphatic ethers and alcohols, whereas the EI spectrum of 3, bearing one 4‐hydroxy‐2‐butyl and one 3‐hydroxybutyl chain, showed essentially the characteristic fragments of both chains. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
940.
Simon Reitz Menekse Cebi Philipp Reiß Gregor Studnik Uwe Linne Ulrich Koert Prof. Dr. Lars‐Oliver Essen Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(26):4853-4857
The attachment of modulators to a trimeric porin ion channel was investigated (see picture of the trimer with a crown ether modulator (orange)). The interplay of modulator and protein is essential for the conformational heterogeneity of the hybrid channel. Single‐site attachment in large pores is not sufficient to change the electrophysiological characteristics of the pores—such change requires additional noncovalent interactions or second‐site attachments.