全文获取类型
收费全文 | 3363篇 |
免费 | 133篇 |
国内免费 | 33篇 |
专业分类
化学 | 2460篇 |
晶体学 | 16篇 |
力学 | 143篇 |
数学 | 300篇 |
物理学 | 610篇 |
出版年
2023年 | 31篇 |
2022年 | 149篇 |
2021年 | 123篇 |
2020年 | 120篇 |
2019年 | 114篇 |
2018年 | 123篇 |
2017年 | 94篇 |
2016年 | 152篇 |
2015年 | 115篇 |
2014年 | 155篇 |
2013年 | 342篇 |
2012年 | 219篇 |
2011年 | 231篇 |
2010年 | 181篇 |
2009年 | 143篇 |
2008年 | 168篇 |
2007年 | 127篇 |
2006年 | 120篇 |
2005年 | 68篇 |
2004年 | 87篇 |
2003年 | 81篇 |
2002年 | 57篇 |
2001年 | 30篇 |
2000年 | 35篇 |
1999年 | 29篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 20篇 |
1992年 | 24篇 |
1991年 | 23篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有3529条查询结果,搜索用时 15 毫秒
101.
A high-order leap-flog based non-dissipative discontinuous Galerkin timedomain method for solving Maxwell's equations is introduced and analyzed. The proposed method combines a centered approximation for the evaluation of fluxes at the interface between neighboring elements, with a Nth-order leap-frog time scheme. Moreover, the interpolation degree is defined at the element level and the mesh is refined locally in a non-conforming way resulting in arbitrary level hanging nodes. The method is proved to be stable under some CFL-like condition on the time step. The convergence of the semi-discrete approximation to Maxwell's equations is established rigorously and bounds on the global divergence error are provided. Numerical experiments with highorder elements show the potential of the method. 相似文献
102.
Abdelmalek Zahra Khan Sami Ullah Waqas Hassan Riaz Arshad Khan Israr Ali Tlili Iskander 《Journal of Thermal Analysis and Calorimetry》2021,144(1):205-217
Journal of Thermal Analysis and Calorimetry - The significant bioconvection phenomenon with the utilization of nanoparticles encountered fundamental industrial and technological applications in... 相似文献
103.
Fernando Dures Patrícia M. A. Silva Pedro Novais Isabel Amorim Luís Gales Ctia I. C. Esteves Samuel Guieu Hassan Bousbaa Madalena Pinto Emília Sousa 《Molecules (Basel, Switzerland)》2021,26(11)
Thioxanthones are bioisosteres of the naturally occurring xanthones. They have been described for multiple activities, including antitumor. As such, the synthesis of a library of thioxanthones was pursued, but unexpectedly, four tetracyclic thioxanthenes with a quinazoline–chromene scaffold were obtained. These compounds were studied for their human tumor cell growth inhibition activity, in the cell lines A375-C5, MCF-7 and NCI-H460. Photophysical studies were also performed. Two of the compounds displayed GI50 values below 10 µM for the three tested cell lines, and structure–activity relationship studies were established. Three compounds presented similar wavelengths of absorption and emission, characteristic of dyes with a push-pull character. The structures of two compounds were elucidated by X-ray crystallography. Two tetracyclic thioxanthenes emerged as hit compounds. One of the two compounds accumulated intracellularly as a bright fluorescent dye in the green channel, as analyzed by both fluorescence microscopy and flow cytometry, making it a promising theranostic cancer drug candidate. 相似文献
104.
Cellulose - The electrical conductivity of cotton fabrics was enhanced using an in situ synthesis of graphene or polymerization of conductive polymer. The impact of conductive materials... 相似文献
105.
Afaq Shagufta Khan Sadia Kazmi Mehdi Hassan Fatima Itrat Malik Abdul Inamullah Farah Farhad Zahid Abbas Tanveer Tareen Rasool Bakhsh 《Chemistry of Natural Compounds》2021,57(2):269-273
Chemistry of Natural Compounds - Armatans A (1) and B (2), new isoflavans, have been isolated from the EtOAc-soluble fraction of the MeOH extract of Colutea armata Hemsl. & Lace, along with... 相似文献
106.
Santhosh Kumar Podiyanachari Maciej Barłóg Marc Comí Salahuddin Attar Saeed Al-Meer Mohammed Al-Hashimi Hassan S. Bazzi 《Journal of polymer science. Part A, Polymer chemistry》2021,59(24):3150-3160
Ring-opening metathesis polymerization (ROMP)-derived poly(oxanorbornene imide)s bearing bay-linked mono - alkoxy -M1 and 1,7-di-alkoxy M2 functionalized perylene diimides (PDIs) were synthesized using Grubb's third ( G3 ) and Hoveyda-Grubbs second generation ( HG2 ) ruthenium-alkylidene metathesis initiators. The mono-alkoxy-derived PDI-based non-ladderphane polymer poly M1 displayed 67% to 77% of the trans olefin content in the polymer chain depending on the initiator used for the polymerization. When using the symmetrical 1,7-di-alkoxy-derived PDI-based polymer poly M2 having the ladderphane type-structure, this displayed a significant amount of cis and trans olefin contents in the polymer chains, irrespective of the type of initiators used for the polymerization. ROMP of both monomers M1 and M2 proceeded in a well-controlled manner with a linear dependence of molecular weight on the monomer/initiator ratio using G3 as initiator. Optical properties of the ladderphane-based poly M2 and non-ladderphane-based poly M1 were characterized in both solution and the film state. X-ray diffraction (XRD) analysis for all the polymers showed significant π-stacking in the thin film state with ordered molecular packing and closer values of d-spacing for both poly M1 and poly M2 . Film morphology examined by AFM elucidated homogenous smooth polymer surface for both polymers in general, but with some irregularities observed for poly M1 . In addition, CV analysis revealed both polymers could be good candidates as electron-accepting materials, with excellent film-forming ability, and thermal stability. 相似文献
107.
108.
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
![点击此处可从《应用有机金属化学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
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
109.
Hassan Amirhashchi Anirudh Pradhan Rekha Jaiswal 《International Journal of Theoretical Physics》2013,52(8):2735-2752
Some new exact solutions of Einstein’s field equations have come forth within the scope of a spatially homogeneous and anisotropic Bianchi type-III space-time filled with barotropic fluid and dark energy by considering a variable deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect fluid as well as direct interaction with it. Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution of the universe, the equation of state (EoS) for dark energy ω (de), in both cases, tends to ?1 (cosmological constant, ω (de)=?1), by displaying various patterns as time increases, which is consistent with recent observations. The cosmic jerk parameter in our derived models are in good agreement with the recent data of astrophysical observations under appropriate condition. It is observed that the universe starts from an asymptotic Einstein static era and reaches to the ΛCDM model. So from recently developed Statefinder parameters, the behaviour of different stages of the universe has been studied. The physical and geometric properties of cosmological models are also discussed. 相似文献
110.