全文获取类型
收费全文 | 1876篇 |
免费 | 490篇 |
国内免费 | 201篇 |
专业分类
化学 | 586篇 |
晶体学 | 84篇 |
力学 | 694篇 |
综合类 | 31篇 |
数学 | 83篇 |
物理学 | 1089篇 |
出版年
2024年 | 8篇 |
2023年 | 42篇 |
2022年 | 64篇 |
2021年 | 58篇 |
2020年 | 50篇 |
2019年 | 49篇 |
2018年 | 54篇 |
2017年 | 86篇 |
2016年 | 93篇 |
2015年 | 60篇 |
2014年 | 116篇 |
2013年 | 136篇 |
2012年 | 144篇 |
2011年 | 141篇 |
2010年 | 90篇 |
2009年 | 102篇 |
2008年 | 91篇 |
2007年 | 135篇 |
2006年 | 120篇 |
2005年 | 123篇 |
2004年 | 108篇 |
2003年 | 98篇 |
2002年 | 63篇 |
2001年 | 74篇 |
2000年 | 61篇 |
1999年 | 63篇 |
1998年 | 57篇 |
1997年 | 49篇 |
1996年 | 28篇 |
1995年 | 27篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 6篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 14篇 |
1986年 | 2篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 19篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1969年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有2567条查询结果,搜索用时 93 毫秒
71.
Ahmad Shaabani Heshmatollah Sepahvand Seyyed Emad Hooshmand Mahmoud Borjian Boroujeni 《应用有机金属化学》2016,30(6):414-421
The synthesis and characterization of an efficient and reusable nanocatalyst, Cu/GA/Fe3O4@SiO2, obtained by ultrasonic‐assisted grafting of guanidineacetic acid on modified Fe3O4@SiO2 core–shell nanocomposite spheres and subsequent immobilization of Cu(II), are described. The catalyst was characterized by means of X‐ray diffraction, scanning and transmission electron microscopies, energy‐dispersive X‐ray spectroscopy, elemental analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, vibrating sample magnetometry and inductively coupled plasma optical emission spectrometry. The prepared nanocatalyst facilitated an efficient and straightforward friendly procedure for the synthesis of benzodiazepines and imidazoles in ethanol and under solvent‐free conditions, respectively. The nanocatalyst can be easily recovered using a magnet and reused several times without any significant loss of activity. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
72.
High diffusion barrier and piezoelectric nanocomposites based on polyvinylidene fluoride‐trifluoroethylene copolymer and hydrophobized clay 下载免费PDF全文
Sara Dalle Vacche Fabiane Oliveira Olha Sereda Antonia Neels Alex Dommann Dragan Damjanovic Yves Leterrier 《Journal of Polymer Science.Polymer Physics》2017,55(24):1828-1836
Nanocomposites based on polyvinylidene fluoride–trifluoroethylene copolymer and up to 4 vol % of hydrophobized clay nanoparticles are investigated. The structure, piezoelectric properties, and oxygen permeability of solvent cast films are analyzed before and after annealing above the Curie temperature of the polymer. Exfoliation of the clay takes place at concentrations up to 1 vol %, beyond which it rapidly drops and is absent at a concentration of 4 vol %. The presence of clay does not change the crystallinity of the polymer, but leads to a threefold decrease of the oxygen permeability at a concentration of 0.5 vol %. Annealing at 130 °C increases the crystallinity, the proportion of β phase up to 94%, and the piezoelectric coefficient by 20–40% at clay fractions below 1 vol %. Annealing also leads to a remarkable 3‐ to 10‐fold decrease of O2 permeability and to intriguing changes of the activation energy for O2 transport, which decreases from 56 kJ/mol for the as‐cast polymer to below 10 kJ/mol for the polymer and exfoliated composite. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1828–1836 相似文献
73.
Nitrogen doped nano porous graphene as a sorbent for separation and preconcentration trace amounts of Pb,Cd and Cr by Ultrasonic assisted in‐syringe dispersive micro solid phase extraction 下载免费PDF全文
Nitrogen doped nano porous graphene was used as an efficient sorbent in solid‐phase extraction process for simultaneous separation and pre‐concentration of metal ions lead (II), cadmium(II), and chromium(III)) in biological samples. Ultrasonic assisted in‐syringe dispersive micro solid phase extraction coupled with micro sampling atomic absorption spectrometry was utilized for the determination of metal ions. Nitrogen doped nano porous graphene was synthesized as a nano sorbent by chemical vapour deposition method. Methane and aniline were used as carbon and nitrogen sources. The characterization of sorbent was performed by field emission scanning electron microscope, transmission electron microscopy, atomic force microscope, fourier transform infrared, chemical element analysis and raman analysis. Effective parameters on the extraction efficiency such as pH, sorbent dosage, eluent volume and eluent concentration were optimized by central composite design and desirability function. Experimental results indicate that the optimal conditions for this extraction were pH = 6.4, 1.42 mg of sorbent, 100 μL of eluent, and 0.84 mol L‐1 of eluent concentration. The detection limits are as low as 1.5, 0.3 and 0.9 μg L‐1 for lead, cadmium, and chromium, respectively. The intra‐day precisions were 3.6, 4.38 and 2.94 and Inter‐day precision were 4.83, 5.26 and 4.52 for lead, cadmium, and chromium, respectively. Method performance was investigated by determination of mentioned heavy metals in complicated biological matrixes such as human plasma, urine and saliva samples with good recoveries. 相似文献
74.
Piezoelectric cat-eye modulating retro-reflector (MRR) is the core component of an acousto-optic retro-identification system. The influence of the optical parameters (link distance l, lens focal length f and the detector diameter r) to the cat-eye effect was analyzed theoretically and experimentally. A piezoelectric cat-eye retro-identification (PCERI) system was built, and an information transmission ratio of 2 kHz was achieved which can satisfy an access control system. 相似文献
75.
76.
Santhosh Govindaraju Sumaiya Tabassum 《Phosphorus, sulfur, and silicon and the related elements》2017,192(3):292-299
Silica iodide catalyzed, competent synthesis of 15 novel substituted-1,4,5,6-tetrahydropyridine-3-carboxylate derivatives was achieved by a one-pot four-component cyclocondensation of Meldrum's acid, aryl aldehydes, aromatic amines and ethyl acetoacetate under ultrasonic irradiation. In comparison to the conventional methods, our protocol is convenient, eco-friendly and offers several benefits such as: mild conditions, high yields, shorter reaction time and ease in product isolation. 相似文献
77.
《Journal of separation science》2017,40(3):697-701
An effective method for the simultaneous determination of docosahexaenoic acid and eicosapentaenoic acid in common seafood by gas chromatography was developed and validated. Total docosahexaenoic acid and eicosapentaenoic acid were extracted from seafood by ultrasonic cell crusher assisted extraction and methyl esterified for gas chromatography analysis in the presence of the internal standard. The linearity was good (r > 0.999) in 9.59 ∼ 479.5 μg/mL for docosahexaenoic acid and 9.56 ∼ 477.8 μg/mL for eicosapentaenoic acid. The intrarun and interrun precisions were both within 4.8 and 6.1% for the two analytes, while the accuracy was less than 5.8%. The developed method was applied for determination of docosahexaenoic acid and eicosapentaenoic acid in six kinds of seafood. The result showed the content of docosahexaenoic acid and eicosapentaenoic acid was all higher than 1 mg/g in yellow croaker, hairtail, venerupis philippinarum, mussel, and oyster. Our work may be helpful for dietary optimization and production of docosahexaenoic acid and eicosapentaenoic acid. 相似文献
78.
Abstract The pulse-echo-overlap method was used to measure the longitudinal and transverse wave velocities in single-crystal (100) and (111) samples of InSb up to 3 GPa at room temperature. The peculiar variations of the elastic constants were observed near the covalent-metallic transition. 相似文献
79.
Weimin Xia Zhuo Xu Qiuping Zhang Zhicheng Zhang Yuanqing Chen 《Journal of Polymer Science.Polymer Physics》2012,50(18):1271-1276
Thermal processing at various temperatures has been used to fabricate poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐co‐TrFE)] films with varied crystalline properties in an attempt to improve their piezoelectric properties. Although the dielectric constant of the films annealed at higher temperature is smaller than that of cooled and quenched ones, it has been shown that the annealed films possess larger crystallinity and stacked lamellar crystal grain size. The ferroelectric domains deriving from crystal region in all the samples are effectively improved by hot polarization. As a result, the remnant polarizations (Pr) and coercive electric field (Ec) of the corresponding films are improved at a low frequency due to the response of dipoles in crystal phase, and the largest piezoelectric constant in the longitudinal thickness mode (d33=?25 pC/N) is obtained in an annealed copolymer film. The results illustrate improving the crystal structure of P(VDF‐co‐TrFE) is an effective way to realize high electromechanical properties, which provides broadly applied scenery for this kind of copolymer in piezoelectric components. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012 相似文献
80.
Zinc sulfide (ZnS) thin films have been deposited on Si (1 0 0) substrate using ultrasonic spray pyrolysis. X-ray diffraction (XRD) analysis revealed that the films are (0 0 2) preferentially oriented with c-axis-oriented wurtzite structure. The crystallinity has been found to improve with film thickness in the 180–6000 nm range. Film structure has been analyzed by XRD, scanning electron microscope, FTIR, and Raman spectroscopies, while the stoichiometry has been verified by energy-dispersive spectroscopy and particle-induced X-ray emission techniques. Electrical properties of the grown films were characterized by current–voltage and capacitance–voltage measurements where, the films show better conducting behavior at higher thickness. 相似文献