全文获取类型
收费全文 | 2805篇 |
免费 | 126篇 |
国内免费 | 377篇 |
专业分类
化学 | 2917篇 |
晶体学 | 18篇 |
力学 | 8篇 |
数学 | 2篇 |
物理学 | 363篇 |
出版年
2024年 | 3篇 |
2023年 | 63篇 |
2022年 | 57篇 |
2021年 | 54篇 |
2020年 | 53篇 |
2019年 | 56篇 |
2018年 | 66篇 |
2017年 | 87篇 |
2016年 | 132篇 |
2015年 | 138篇 |
2014年 | 151篇 |
2013年 | 210篇 |
2012年 | 168篇 |
2011年 | 276篇 |
2010年 | 217篇 |
2009年 | 290篇 |
2008年 | 252篇 |
2007年 | 216篇 |
2006年 | 164篇 |
2005年 | 161篇 |
2004年 | 134篇 |
2003年 | 51篇 |
2002年 | 41篇 |
2001年 | 34篇 |
2000年 | 22篇 |
1999年 | 34篇 |
1998年 | 24篇 |
1997年 | 13篇 |
1996年 | 24篇 |
1995年 | 29篇 |
1994年 | 11篇 |
1993年 | 6篇 |
1992年 | 18篇 |
1991年 | 12篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有3308条查询结果,搜索用时 46 毫秒
991.
992.
Xiaogang Xiong Jingyi Wang Hongbing Jia Eryuan Fang Lifeng Ding 《Polymer Degradation and Stability》2013
In this report, we demonstrate that both the thermal stability and the thermal conductivity of bromobutyl rubber (BIIR) nanocomposites could be improved by incorporating the ionic liquids (ILs) modified graphene oxide (GO-ILs) using a solution compounding method. The structure, thermal stability and thermal conductivity of this newly modified BIIR nanocomposites were systematically analyzed and studied. The X-ray diffraction (XRD) analysis of GO-ILs showed that ILs had been effectively intercalated into the interlayer of GO, which was found to be able to raise the exfoliation degree of GO. The increased exfoliation degree facilitated a good dispersion of GO-ILs in the BIIR matrix, as revealed by the scanning electron microscope (SEM) images. The glass transition temperatures (Tg) of the GO-ILs/BIIR nanocomposites were also raised by the addition of GO-ILs, which indicates the strong interfacial adhesion between GO-ILs and the rubber. Most importantly, the incorporation of GO-ILs in the BIIR matrix could effectively improve the thermal stability of the rubber nanocomposites according to our thermogravimetric analysis (TGA). The activation energy (Ea) of thermal decomposition of GO-ILs/BIIR nanocomposites increases with the addition of GO-ILs. Besides, the thermal conductivity of GO-ILs/BIIR nanocomposite with 4 wt% of GO-ILs had 1.3-fold improvement compared to that of unfilled BIIR. 相似文献
993.
Xin Zhao Yuxia Gao Chenhui Zhang Yanqiu Zhu Jinmei Lei Yue Ma 《Journal of Dispersion Science and Technology》2013,34(12):1820-1828
AbstractDue to the important use of pesticide formulation, it is necessary to make it clear how ionic surfactant effect the wettability at leaf surface. In this work, we used the sessile drop method to study the wettability of SDS and DTAB on wheat leaf surfaces at different leaf stages, and reveal the relationship between surfactants structures and leaf stages of wheat leaf surfaces on wettability behavior. Results showed that few surfactant molecules adsorbed at the interface at low concentrations. With the concentration increased, the surfactant replaced the air layer partially within the nano/micro structure of leaf surfaces. When the concentration exceeded to CMC, the adsorption of surfactant molecules was saturated at both air-liquid interface and solid-liquid interface, the wetting state was still the transitional state between Cassie-Baxter’s and Wenzel’s state. In all concentrations, the adhesional tension and surface tension showed the linear relationship and the slope values were all below ?1, suggesting there were more surfactant molecules adsorbed at the solid-liquid interface than the liquid-air interface. As SDS is a common wetting agent and DTAB is a common fungicide in agrochemical, this study will provide potential guidance in practical application of pesticide solutions in leaf surface wetting. 相似文献
994.
Enantiomerically pure (+)-(2S,6R)-2-propyl-6-methyl-piperidine 6 has been obtained in 4 steps, 66% overall yield and without separation of diastereomers from the chiral 2-cyano-6-oxazolopiperidine synthon 1c derived from (+)-1,2-diphenyl-2-aminoethanol 相似文献
995.
The anilines could be easily and selectively N-alkylated with alcohols in the presence of a small amount of Raney nickel and with a greatly shortened period under microwave irradiation. A purely non-thermal effect of microwave was observed in the reaction. 相似文献
996.
A new, environmentally benign two-step synthesis of 5H[1,3]-thiazolo[2,3-b]quinazoline-3,5-(2H)-dione and 5H-thiazolo[2,3-b]quinazolin-5-one derivatives has been accomplished stepwise. The substituted 2-aminobenzoic acid upon condensation with thiourea in 1-butyl-3-methylimidazolium bromide at moderate temperature under nitrogen atmosphere yielded 2-thioxo-1H-4-quinazolinones. The resulting intermediate 2-thioxo-1H-4-quinazolinones, when reacted with 2-chloroethanoic acid/2-chloropropanal underwent cyclization to yield the desired product in excellent yields. 相似文献
997.
A new approach to the synthesis and 1,3-dipolar cycloadditions of nitrones has been described from 2,3-dihydro-4H-pyran and various hydroxylamines, with electron-deficient alkynes for the synthesis of isoxazoline derivatives. Significant rate acceleration and improved yields of exclusively exo isoxazolines in 1-butyl-3-methylimidazolium based ionic liquids have been observed. Novel isoxazolines may be used as a precursor for the synthesis of variety of peptides. 相似文献
998.
An unprecedented multi-component reaction for the synthesis of thiosugar-annulated 1,3-thiazines is reported. The envisaged synthetic strategy involves the reaction of d-glucose/d-xylose and 2-methyl-2-phenyl-1,3-oxathiolan-5-one with AcONH4/RNH2 in task specific ionic liquid (TSIL), [bmim]SCN which afforded thiosugar annulated 1,3-thiazines in excellent yields (83–93%). The reaction is effected via ionic liquid promoted Michael addition followed by mercaptoacetylative ring transformation in a one-pot procedure and the ionic liquid, [bmim]OH could be easily recycled for further use without any loss of efficiency and be used for the synthesis of [bmim]SCN, thus allowing recycling of the TSIL for further use. 相似文献
999.
A series of 3-(3-hydroxy-6-methyl-4-oxo-4H-pyran-2-yl)-3-phenylpropanamide derivatives were synthesized efficiently by one-pot condensation of aromatic aldehydes, meldrum’s acid, allomaltol, and ammonium acetate in ionic liquid [bmim]BF4. The novel synthesis method offers the advantage of operational simplicity, mild reaction conditions, short reaction time, higher yields, and environmental friendliness. 相似文献
1000.
In this study, the applicability of a chiral ionic liquid (CIL) as the sole chiral selector in CE was investigated for the first time. In particular, five amino acid ester‐based CILs were synthesized and used as additives in the BGE in order to evaluate their chiral recognition ability. The performance of these CILs as the sole chiral selectors was evaluated by using 1,1′‐binaphthyl‐2,2‐diylhydrogenphosphate (BNP) as the analyte and by comparing the resolution values. Different parameters were examined, such as the alkyl group bulkiness and the configuration of the cation, the anion type of the CIL and its concentration, and the pH of the BGE, in order to optimize the separation of the enantiomers and to demonstrate the effect that each parameter has on the chiral‐recognition ability of the CIL. Baseline separation of BNP within 13 min was achieved by using a BGE of 100 mM Tris/10 mM sodium tetraboratedecahydrate (pH 8) and a chiral selector of 60 mM l ‐alanine tert butyl ester lactate. The run‐to‐run and batch‐to‐batch reproducibilities were also evaluated by computing the %RSD values of the EOF and the two enantiomer peaks. In both cases, very good reproducibilities were observed, since all %RSD values were below 1%. 相似文献