全文获取类型
收费全文 | 4155篇 |
免费 | 215篇 |
国内免费 | 88篇 |
专业分类
化学 | 2983篇 |
晶体学 | 17篇 |
力学 | 196篇 |
数学 | 472篇 |
物理学 | 790篇 |
出版年
2023年 | 23篇 |
2022年 | 61篇 |
2021年 | 99篇 |
2020年 | 126篇 |
2019年 | 162篇 |
2018年 | 223篇 |
2017年 | 171篇 |
2016年 | 308篇 |
2015年 | 199篇 |
2014年 | 280篇 |
2013年 | 502篇 |
2012年 | 352篇 |
2011年 | 322篇 |
2010年 | 223篇 |
2009年 | 200篇 |
2008年 | 247篇 |
2007年 | 210篇 |
2006年 | 116篇 |
2005年 | 115篇 |
2004年 | 120篇 |
2003年 | 89篇 |
2002年 | 60篇 |
2001年 | 27篇 |
2000年 | 25篇 |
1999年 | 23篇 |
1998年 | 11篇 |
1997年 | 12篇 |
1996年 | 11篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 12篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1975年 | 6篇 |
1938年 | 1篇 |
1931年 | 1篇 |
排序方式: 共有4458条查询结果,搜索用时 468 毫秒
71.
In this paper we, present some MATLAB and GAP programs and use them to find the automorphism group of the Euclidean graph
of the C80 fullerence with connectivity and geometry of I
h
symmetry point group. It is proved that this group has order 120 and is isomorphic to I
h
≊Z2×A5, where Z2 is, a cyclic group of order 2 and A5 is the alternating group on five symbols. 相似文献
72.
Reaction of Telluriumhexafluoride and Trimethylamine, Structures of the TeF5? and SeF5? Anions The reaction of TeF6 and (CH3)3N is of the redox kind, resulting in reduction of tellurium: X-ray single crystal analysis reveals the compounds (CH3)2N? CH2? N(CH3)2+TeF5? and [(CH3)3NH+]5(TeF5?)3(HF2?)2. By comparison with published data it can be shown that this mixture is identical to previously published [(CH3)3N]2TeF6. The latter was supposed to be one of the few examples of tellurium in a coordination state of eight. (CH3)4N+TeF5? and (CH3)4N4SeF5? are obtained and their structure is investigated by single crystal x-ray methods also. The anions SF5?, SeF5? and TeF5? are discussed in terms of weak interactions. 相似文献
73.
Azizian J Karimi AR Kazemizadeh Z Mohammadi AA Mohammadizadeh MR 《The Journal of organic chemistry》2005,70(4):1471-1473
[reaction: see text] 11-(1H-Pyrrol-1-yl)-11H-indeno[1,2-b]quinoxaline and 3-(1H-pyrrol-1-yl)indolin-2-one derivatives have been synthesized in good yields in a novel, one-pot, and efficient process by condensation of 11H-indeno[1,2-b]quinoxalin-11-one or isatin derivatives with 4-hydroxyproline on solid-support montmorillonite K10 under microwave irradiation. 相似文献
74.
A pseudo-first-order catalytic mechanism in which both reactant and product of a redox reaction are strongly immobilized
on an electrode surface is theoretically analysed under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV).
A mathematical procedure is developed under diffusionless conditions. The relationships between the properties of the voltammetric
response and both the kinetic parameters of the redox reaction and the parameters of the excitation signal are studied. The
phenomenon of the quasi-reversible maximum is discussed. A comparative study between SWV and SCV is presented and the limitations
and advantages of both techniques, from analytical and kinetic points of view, are discussed. The theoretical predictions
are experimentally confirmed by the redox reaction of azobenzene in the presence of hydrogen peroxide as an oxidizing agent.
Electronic Publication 相似文献
75.
An aniline-based polymer was electrochemically prepared and applied as a new fiber coating for solid phase microextraction (SPME) of some priority phenols from water samples. The polyaniline (PANI) film was directly electrodeposited on the platinum wire surface in sulfuric acid solution using cyclic voltammetry (CV) technique. The efficiency of new coating was investigated using a laboratory-made SPME device and gas chromatography with flame ionization detection for the extraction of some phenols from the headspace of aqueous samples. The scanning electron microscopy (SEM) images showed the homogeneity and the porous surface structure of the film. The results obtained proved the ability of this polymer as a suitable SPME fiber coating for trapping the selected phenols. Influential parameters affecting the extraction process were optimized and an extraction time of 50 min at 50 °C gave maximum efficiency, when the aqueous sample was saturated with NaCl and adjusted at pH 2. This new coating can be prepared easily in a reproducible manner and it is rather inexpensive and stable against most of organic solvents. The PANI thickness can be precisely controlled by the number of CV cycles. At the optimum conditions, the R.S.D. for a double distilled water spiked with phenol and chlorophenols at ppb level were 4.8-17% (n = 3) and detection limits for the studied compounds were between 0.69 and 3.7 ng ml−1, except for phenol and 4-chlorophenol. The optimized method was successfully applied to some real-life water samples. 相似文献
76.
Dendrimer‐encapsulated Cu(Π) nanoparticles immobilized on superparamagnetic Fe3O4@SiO2 nanoparticles as a novel recyclable catalyst for N‐arylation of nitrogen heterocycles and green synthesis of 5‐substituted 1H‐tetrazoles 下载免费PDF全文
In this study, dendrimer‐encapsulated Cu(Π) nanoparticles immobilized on superparamagnetic Fe3O4@SiO2 nanoparticles were prepared via a multistep‐synthesis. Then, the synthesized composite was fully characterized by various techniques such as fourier transform infrared (FT‐IR) spectroscopy, X‐ray diffraction (XRD), dynamic light scattering (DLS), UV‐vis spectroscopy, energy dispersive X‐ray analysis (EDX), thermogravimetric analysis (TGA) and vibration sample magnetometer (VSM). From the information gained by FE‐SEM and TEM studies it can be inferred that the particles are mostly spherical in shape and have an average size of 50 nm. Also, the amount of Cu is determined to be 0.51 mmol/g in the catalyst by inductively coupled plasma (ICP) analyzer. This magnetic nano‐compound has been successfully applied as a highly efficient, magnetically recoverable and stable catalyst for N‐arylation of nitrogen heterocycles with aryl halides (I, Br) and arylboronic acids without using external ligands or additives. The catalyst was also employed in a one‐pot, three‐component reaction for the efficient and green synthesis of 5‐substituted 1H‐tetrazoles using various aldehydes, hydroxylamine hydrochloride and sodium azide in water. The magnetic catalyst can be easily separated by an external magnet bar and is recycled seven times without significant loss of its activity. 相似文献
77.
Preparation and characterization of Cu (II) supported on poly(8‐hydroxyquinoline‐p‐styrene sulphonate) and its application as catalyst for the synthesis of hexahydroquinolines 下载免费PDF全文
Ardeshir Khazaei Mahsa Tavasoli Vahid Jamshidi Fatemeh Gohari Ghalil Ahmad Reza Moosavi‐Zare 《应用有机金属化学》2018,32(7)
Cu ( II ) supported on poly(8‐hydroxyquinoline‐p‐styrenesulfonate) (Cu ( II )@PHQSS) was prepared and fully characterized by the different techniques including fourier transform infrared spectroscopy (FT‐IR), 1H NMR, 13C NMR, thermal gravimetric analysis (TGA), differential thermal gravimetric (DTG), differential thermal analysis (DTA), scanning electron microscope (SEM) and energy dispersive X‐ray analysis (EDS). Afterward, the Cu ( II )@PHQSS as nanostructured catalyst was used as catalyst for the synthesis of hexahydroquinolines. 相似文献
78.
Seyedeh Hoda Jazayeri Amir Amiri-Yekta Salahadin Bahrami Hamid Gourabi Mohammad Hossein Sanati Mohammad Reza Khorramizadeh 《Applied biochemistry and biotechnology》2018,185(4):986-1003
The rapid growth of global biopharmaceutical market in the recent years has been a good indication of its significance in biotechnology industry. During a long period of time in recombinant protein production from 1980s, optimizations in both upstream and downstream processes were launched. In this regard, one of the most promising strategies is expression vector engineering technology based on incorporation of DNA opening elements found in the chromatin border regions of vectors as well as targeting gene integration. Along with these approaches, cell line engineering has revealed convenient outcomes in isolating high-producing clones. According to the fact that more than 50% of the approved therapeutic proteins is being manufactured in mammalian cell lines, in this review, we focus on several approaches and developments in vector and cell line engineering technologies in mammalian cell culture. 相似文献
79.
Reza Mahjoub Fredrik Odegaard Gregory S Zaric 《The Journal of the Operational Research Society》2014,65(4):588-604
Many new drugs, such as biologics and cancer drugs, are very costly. However, their effectiveness outside of clinical trial settings is often uncertain at the time they gain market approval. This uncertainty may reflect a lack of real-world outcomes data, as opposed to clinical trials data, for a typical patient population. A risk-sharing agreement is a contract between a drug manufacturer and a healthcare payer to help manage uncertainties regarding the cost and effectiveness of those drugs. In this paper, we model a risk-sharing agreement in which a proportion of total sales is rebated. We model disease progression using a continuous time Markov chain with uncertain transition rates. We examine the performance of this risk-sharing agreement from the manufacturer’s perspective and investigate the conditions under which the manufacturer will make a profit. We illustrate with a numerical model parameterized using data from a Phase 2 clinical trial of an oncology drug that was subjected to a risk-sharing agreement in the UK. 相似文献
80.