全文获取类型
收费全文 | 3732篇 |
免费 | 250篇 |
国内免费 | 646篇 |
专业分类
化学 | 2483篇 |
晶体学 | 51篇 |
力学 | 74篇 |
综合类 | 40篇 |
数学 | 14篇 |
物理学 | 184篇 |
综合类 | 1782篇 |
出版年
2024年 | 16篇 |
2023年 | 50篇 |
2022年 | 98篇 |
2021年 | 124篇 |
2020年 | 103篇 |
2019年 | 109篇 |
2018年 | 87篇 |
2017年 | 127篇 |
2016年 | 151篇 |
2015年 | 136篇 |
2014年 | 175篇 |
2013年 | 285篇 |
2012年 | 180篇 |
2011年 | 190篇 |
2010年 | 183篇 |
2009年 | 198篇 |
2008年 | 179篇 |
2007年 | 220篇 |
2006年 | 207篇 |
2005年 | 198篇 |
2004年 | 177篇 |
2003年 | 175篇 |
2002年 | 152篇 |
2001年 | 137篇 |
2000年 | 121篇 |
1999年 | 112篇 |
1998年 | 101篇 |
1997年 | 71篇 |
1996年 | 91篇 |
1995年 | 67篇 |
1994年 | 74篇 |
1993年 | 64篇 |
1992年 | 66篇 |
1991年 | 42篇 |
1990年 | 34篇 |
1989年 | 44篇 |
1988年 | 28篇 |
1987年 | 17篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1959年 | 2篇 |
排序方式: 共有4628条查询结果,搜索用时 15 毫秒
991.
We study an homogeneous aquifer where fresh and salt water are separated by an interface T. The flow is governed by a nonlinear Darcy' law. Using suitable approximated problems, we prove existence of a solution and establish some properties. 相似文献
992.
以4-甲基-5-羟乙基噻唑和对氰基氯化苄为原料微波合成一种新型噻唑盐,3-(对氰基苄基)-4-甲基-5-(2-羟乙基)噻唑氯化物,再用此噻唑盐替代VB1作为安息香缩合反应的催化剂,检验其催化性能,发现催化苯甲醛得苯偶姻收率为74.5%、催化糠醛得糠偶姻收率为76.1%,此噻唑盐不仅具有天然VB1的无毒性,且可回收利用;考察了物质的量投料比、反应时间、反应温度及微波反应器功率对此噻唑盐收率的影响;并用HPLC测定产物的纯度,通过熔点、元素分析、IR及LC-MS对噻唑盐性质和及苯偶姻结构进行了表征. 相似文献
993.
《应用有机金属化学》2017,31(11)
Efficient and recyclable novel nano tetra‐2,3‐pyridiniumporphyrazinato‐oxo‐vanadium tricyanomethanide, {[VO(TPPA)][C(CN)3]4}, as a vanadium surface‐free phthalocyanine‐based molten salt catalyst was successfully designed, produced and used for the Strecker synthesis of α‐aminonitrile derivatives through a one‐pot three‐component reaction between aromatic aldehydes, trimethylsilyl cyanide and aniline derivatives under neat conditions at 50 °C. This catalyst was well characterized using Fourier transform infrared, UV–visible, X‐ray photoelectron and energy‐dispersive X‐ray spectroscopies, X‐ray diffraction, scanning and high‐resolution transmission electron microscopies, inductively coupled plasma mass spectrometry and thermogravimetric analysis. The catalyst can be simply recovered and reused several times without significant loss of catalytic activity. 相似文献
994.
Liuchen Zhao Hui Zhang Wanxu Wang 《Journal of Dispersion Science and Technology》2017,38(10):1465-1470
Surface tension, steady-shear viscosity, and antibacterial activity were investigated when salicylate 2-hydroxybenzoate (NaSal) was added to the micellar solution of N-hexadecane-N-methylpyrrolidinium bromide (C16MPB). It was found that for the mixed system, two flat portions appeared on the surface tension curve with the existence of NaSal in the low-concentration range. The equilibrium contact angle of droplets was decreased with the increase of NaSal concentration. In addition, the steady-shear viscosity abruptly increased with the increase of the concentration of mixture, even if the concentration of NaSal is low. However, this high-viscosity solution exhibited temperature-sensitive property, and white precipitate formation below 18°C. Moreover, the bactericidal capability of C16MPB was affected severely by the mixture of high concentration. All of these features could be attributed to the special interactions between C16MPB and NaSal molecules, among which intramolecular hydrogen bonding of NaSal plays a key role, which have been primely demonstrated by the package of Gaussian09 in this work. 相似文献
995.
Prof. Dr. Santanu Bhattacharya Dr. Suman K. Samanta 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(52):16632-16641
The synthesis, hydrogelation, and aggregation‐induced emission switching of the phenylenedivinylene bis‐N‐octyl pyridinium salt is described. Hydrogelation occurs as a consequence of π‐stacking, van der Waals, and electrostatic interactions that lead to a high gel melting temperature and significant mechanical properties at a very low weight percentage of the gelator. A morphology transition from fiber‐to‐coil‐to‐tube was observed depending on the concentration of the gelator. Variation in the added salt type, salt concentrations, or temperature profoundly influenced the order of aggregation of the gelator molecules in aqueous solution. Formation of a novel chromophore assembly in this way leads to an aggregation‐induced switch of the emission colors. The emission color switches from sky blue to white to orange depending upon the extent of aggregation through mere addition of external inorganic salts. Remarkably, the salt effect on the assembly of such cationic phenylenedivinylenes in water follow the behavior predicted from the well‐known Hofmeister effects. Mechanistic insights for these aggregation processes were obtained through the counterion exchange studies. The aggregation‐induced emission switching that leads to a room‐temperature white‐light emission from a single chromophore in a single solvent (water) is highly promising for optoelectronic applications. 相似文献
996.
《Tetrahedron letters》2019,60(21):1434-1436
A novel approach to convert aldehydes into nitriles using NH3·H2O/FeCl2/NaI/Na2S2O8 has been developed. Both alkyl and aryl nitriles were obtained in good to excellent yields. Electron-withdrawing and electron-donating groups, such as fluoro, chloro, bromo, nitro, ester, cyano, trifluoromethyl and alkoxy were tolerated. Notably, febuxostat and its intermediate, ethyl 2-[3-cyano-4-(2-methylpropoxy)phenyl]-4-methyl-5-thiazolecarboxylate, were obtained in excellent yields. 相似文献
997.
Hui Qin Yuanyuan Miao Ke Zhang Jian Xu Haopeng Sun Wenyuan Liu Feng Feng Wei Qu 《Tetrahedron》2018,74(47):6809-6814
A practical convenient conversion of oxindoles into the corresponding spirocyclopropyl oxindoles is achieved efficiently using bromoethylsulfonium salt, which is easily prepared on a large scale and is stable crystalline. This reaction of bromoethylsulfonium salt with different substituted unprotected oxindoles proceeded under mild condition and provided moderate yields. 相似文献
998.
999.
Zidan Zhang Dachey Lin Venkat Ganesan 《Journal of polymer science. Part A, Polymer chemistry》2022,60(2):199-213
We used atomistic simulations to study the mechanisms of ion transport in salt-doped polymeric ionic liquid systems at higher salt concentrations. Consistent with the experimental observations, our simulations indicate that at higher salt concentrations, the anion mobilities become lower than that of the lithium cations. Further, the anion mobilities become relatively insensitive to the salt concentration, while the mobilities of lithium increase with increasing salt concentration. We rationalize the results for the anion mobilities by considering the fractions of anions which are exclusively coordinated with the polycations (Type1); co-coordinated with cations and lithium (Type2); and those exclusively coordinated with lithium (Type3). By considering the coordination characteristics of the different types of anions and their hopping motions, we demonstrate that the net anion mobilities results from a compensation effect of the salt concentration dependence of the mobilities of the different anions. With respect to the mobilities of the lithium ions, we demonstrate that the latter moves primarily by a structural diffusion mechanism involving refreshing of the solvation shell during hopping. Further, for the majority of the lithium ions, the solvation shell is shown to be comprised of co-coordinated Type2 anions, and that the number of polycations and the unique polymer chains involved in such coordination decreases with increasing salt concentration. Such changes are shown to weaken the solvation shell around the lithium, thereby facilitating faster ion motion. Together, our results suggest that systems in which the anion which exhibits a stronger coordination to the polycation in comparison to that of the lithium can facilitate higher transference numbers without a concomitant reduction in the mechanical strength. 相似文献
1000.
Synthesis and Characterization of a Helicene‐Based Imidazolium Salt and Its Application in Organic Molecular Electronics
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Jan Storch Dr. Jaroslav Zadny Dr. Tomas Strasak Prof. Dr. Martin Kubala Dr. Jan Sykora Dr. Michal Dusek Dr. Vladimir Cirkva Prof. Dr. Pavel Matejka Dr. Milos Krbal Prof. Dr. Jan Vacek 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2343-2347
Herein we demonstrate the synthesis of a helicene‐based imidazolium salt. The salt was prepared by starting from racemic 2‐methyl[6]helicene, which undergoes radical bromination to yield 2‐(bromomethyl)[6]helicene. Subsequent treatment with 1‐butylimidazole leads to the corresponding salt 1‐butyl‐3‐(2‐methyl[6]helicenyl)‐imidazolium bromide. The prepared salt was subsequently characterized by using NMR spectroscopy and X‐ray analysis, various optical spectrometric techniques, and computational chemistry tools. Finally, the imidazolium salt was immobilized onto a SiO2 substrate as a crystalline or amorphous deposit. The deposited layers were used for the development of organic molecular semiconductor devices and the construction of a fully reversible humidity sensor. 相似文献