全文获取类型
收费全文 | 1038篇 |
免费 | 22篇 |
国内免费 | 1篇 |
专业分类
化学 | 568篇 |
晶体学 | 19篇 |
力学 | 35篇 |
数学 | 52篇 |
物理学 | 299篇 |
无线电 | 88篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 15篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2018年 | 23篇 |
2017年 | 15篇 |
2016年 | 32篇 |
2015年 | 16篇 |
2014年 | 34篇 |
2013年 | 54篇 |
2012年 | 64篇 |
2011年 | 77篇 |
2010年 | 54篇 |
2009年 | 55篇 |
2008年 | 71篇 |
2007年 | 77篇 |
2006年 | 57篇 |
2005年 | 47篇 |
2004年 | 33篇 |
2003年 | 36篇 |
2002年 | 27篇 |
2001年 | 16篇 |
2000年 | 19篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1973年 | 3篇 |
1918年 | 2篇 |
排序方式: 共有1061条查询结果,搜索用时 15 毫秒
921.
Computational investigation on the low-lying photo-excited states of N-tert-butyl-α-phenylnitrone (PBN), a well-known spin-trap agent, has revealed its photo-product (oxaziridine) formation channel. The S0-S2 vertical excitation in PBN is subsequently followed by a non-radiative decay pathway through S2/S1 and S0/S1 conical intersections (CIs) with CNO-kinked structures, situated around 23 kcal/mol and 45 kcal/mol below the vertically excited S2 state, respectively. The reverse photo-process of PBN formation involves photo-excitation of oxaziridine to its S2 and S3 photo-excited states. The forward photo-isomerization leads to the trans-oxaziridine with a backside CNO kink (trans-OXB) while the reverse path studied by us, connects its front-side CNO-kinked analogue (trans-OXF) with the PBN. Our search for the reverse thermal reaction paths from these two oxazirdines has led to their corresponding transition states, one at 35 kcal/mol and the other at 27 kcal/mol above trans-OXF and trans-OXB geometries, respectively. They lead to two different isomers (E and Z) of PBN which supports the reported nature of products from the trans-oxaziridine in this thermal reaction. The inversion path of the chiral nitrogen atom of this N-tert-butyl-oxaziridine (barrier 21 kcal/mol) has also been tracked. This reaction path has been compared with that of the N-methyl (barrier 30 kcal/mol) and N-acyl (barrier 10.5 kcal/mol) oxaziridine analogues. 相似文献
922.
Mainul Haque Joydev Chattopadhyay 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(2):163-178
The role of disease in ecological systems is a very important issue from both mathematical and ecological points of view. This paper deals with the qualitative analysis of a prey-dependent predator – prey system in which a disease is spreading among the prey species only. We have analysed the behaviour of the system around each equilibrium and obtained conditions for global stability of the system around an equilibrium by using suitable Lypunov functions. We have also worked out the region of parametric space under which the system enters a Hopf bifurcation and a transcritical bifurcation but does not experience either saddle-node bifurcations or pitchfork bifurcations around the disease-free equilibrium E 2. Finally, we have given an example of a real ecological situation with experimental data simulations. 相似文献
923.
Ghosh K Chattopadhyay S Banerjee M Bhattacharya S 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,65(3-4):659-666
The absorption spectra of the electron donor-acceptor complexes of [60]fullerene with five different aromatic hydrocarbon (AH) molecules containing flexible phenyl substituents have been investigated in toluene medium. An absorption band due to charge transfer (CT) transition is observed in each case in the visible region. The experimental CT transition energies are well correlated with the vertical ionization potentials of the AHs studied (through Mulliken's equation) from which we extract degrees of charge transfer, oscillator and transition dipole strengths of the CT complexes. The degrees of CT in the ground state of the complexes have been found to be very low (0.49-0.55%). The formation constants (K) for the complexes of [60]fullerene with the aromatic hydrocarbons have been determined by UV-vis spectroscopy. Both K values and PM3 calculations on [60]fullerene/AH complexes reveal that nature of substitution in the donor moiety as well as steric compatibility with the acceptor molecule govern the process of EDA complex formation. 相似文献
924.
Sen S Talukder P Dey SK Mitra S Rosair G Hughes DL Yap GP Pilet G Gramlich V Matsushita T 《Dalton transactions (Cambridge, England : 2003)》2006,(14):1758-1767
A series of Zn(II), Cd(II), Co(II), Co(III) and Mn(III) complexes with the Schiff base [(CH3)2NCH2CH2N=CHC6H3(OH)(OMe)], LH, derived from 2-dimethylaminoethylamine and o-vanillin, has been synthesised and structures of all the products have been established by X-ray crystallography. In the cases of zinc and cadmium, dimeric complexes [Zn(LH)2(NCS)] [Zn2(L)(mu(1,1)-CH3COO)(NCS)3] (1), [Cd2(L)2(Cl)2] (2) and [Cd2(L)2(NCS)2] (3), and for cobalt and manganese, monomeric complexes [Co(LH)2(NCS)]2 [Co(NCS)4] (4), [Co(LH)2(NCS)]ClO4 (5), [Co(L)(N3)(o-vanillinate)] x 0.5 MeOH (6) and [Mn(LH)2(MeOH)2](ClO4)3 (7), are formed with various terminal ligands. All the complexes have been characterised by elemental analysis and IR spectra. UV-Vis and NMR spectroscopy, magnetic, and electrochemical studies, were also carried out where feasible. The Schiff base functions as a bi-, tri- or tetra-dentate chelating agent and coordinates via the protonated or deprotonated phenolic oxygen, amine and imine nitrogens, and only in case of 1 with the methoxy oxygen atoms, to the metal ion leading to the formation of mono- or bi-metallic complexes. 相似文献
925.
Chakraborty S Chattopadhyay J Peng H Chen Z Mukherjee A Arvidson RS Hauge RH Billups WE 《The journal of physical chemistry. B》2006,110(49):24812-24815
Single-walled carbon nanotubes have been functionalized and the specific surface areas of the functionalized nanotubes measured. Contrary to expectations, functionalization leads to a decrease in specific surface area compared to that of the unfunctionalized nanotubes. Treatment with a concentrated 1:1 nitric/sulfuric acid mixture followed by high-temperature baking at 1000 degrees C was found to increase the specific surface area of the nanotubes. For the unfunctionalized SWNTs, this treatment increases the specific surface area (SSA) by 20%. In the case of SWNTs functionalized by n-butyl groups the increase in the SSA was nearly 2-fold with the value increasing from 410 (drying at 110 degrees C) to 770 m2/gm (acid and bake treatment followed by drying at 110 degrees C). For the ozonized SWNTs, the SSA increases more than 3-fold from 381 (drying at 110 degrees C) to 1068 m2/gm (acid and bake treatment followed by drying at 110 degrees C). SEM images indicate that the nanotubes rebundle in the solid state with an average bundle size of 10-30 nm. AFM studies show that the ozonized tubes have been cut to short bundles after ozonolysis. Hydrogen uptake studies carried out on the baked ozonized tubes led to a 3 wt % hydrogen uptake at 77 K and 30 bar. 相似文献
926.
Fluorescence recovery after photobleaching (FRAP) is one of the most widely used approaches to quantitatively estimate diffusion
characteristics of molecules in solution and cellular systems. In general, comparison of the diffusion times (t
1/2) from a FRAP experiment provides qualitative estimates of diffusion rates. However, obtaining consistent and reliable quantitative
estimates of mobility of molecules using FRAP is hindered by the lack of appropriate standards for calibrating the FRAP set-up
(microscope configuration and data fitting algorithms) used in a given experiment. In comparison with other fluorescent markers,
the green fluorescent proteins (GFP) possess characteristics that are ideal for use in such experiments. We have monitored
the mobility of pure enhanced green fluorescent protein (EGFP) in a viscous solution by confocal FRAP experiments. Our experimentally
determined diffusion coefficient of EGFP in a glycerol–water mixture is in excellent agreement with the value predicted for
GFP in a solution of comparable viscosity, calculated using the Stokes–Einstein equation. The agreement in the experimentally
determined diffusion coefficient and that predicted from theoretical framework improves significantly when one takes into
account the effective size of the bleached spot in such experiments. Our results therefore validate the use of GFP as a convenient
standard for FRAP experiments. Importantly, we present a simple method to correct for artifacts in the accurate determination
of diffusion coefficient of molecules measured using FRAP arising due to the underestimation in the effective size of the
bleached spot. 相似文献
927.
Abelev BI Aggarwal MM Ahammed Z Amonett J Anderson BD Anderson M Arkhipkin D Averichev GS Bai Y Balewski J Barannikova O Barnby LS Baudot J Bekele S Belaga VV Bellingeri-Laurikainen A Bellwied R Benedosso F Bhardwaj S Bhasin A Bhati AK Bichsel H Bielcik J Bielcikova J Bland LC Blyth SL Bonner BE Botje M Bouchet J Brandin AV Bravar A Burton TP Bystersky M Cadman RV Cai XZ Caines H Calderón de la Barca Sánchez M Castillo J Catu O Cebra D Chajecki Z Chaloupka P Chattopadhyay S Chen HF Chen JH 《Physical review letters》2006,97(13):132301
We report the measurements of Sigma(1385) and Lambda(1520) production in p+p and Au+Au collisions at sqrt[s{NN}]=200 GeV from the STAR Collaboration. The yields and the p(T) spectra are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central Au+Au collisions. Our results indicate that there may be a time span between chemical and thermal freeze-out during which elastic hadronic interactions occur. 相似文献
928.
Tanay Chattopadhyay 《Optics & Laser Technology》2010,42(6):1014-1021
929.
Tanay Chattopadhyay 《Optik》2010,121(7):617-622
Multi-valued logic can be viewed as an alternative approach to solving many problems in transmission, storage and processing of large amount of information in digital signal processing. For the first time to our knowledge, the principal of possibilities of design of all-optical quaternary multi-valued literals circuit (truncated sum, truncated difference and down literals) are proposed and described. Here the different quaternary logical states are represented by different polarized state of light. Terahertz optical asymmetric demultiplexer (TOAD) based interferometric switch can take an important role here. Computer simulation result (by Mathcad-7.0) confirming described methods and conclusion are given in this paper. 相似文献
930.
Based on "amide-triazole bioequivalence" principle, 1,2,3-triazole-fused chiral medium ring benzo-heterocycles capable of mimicking benzolactams were designed. Their syntheses were accomplished by cycloaddition of different sugar-derived azidoalkynes. While triazole-fused eight-membered benzo-heterocycles were formed by exclusive intramolecuclar [3 + 2] cycloaddition, attempted preparation of seven-membered analogues led to some intermolecular cycloaddition resulting in a dimeric macrocyclic product, in addition to intramolecular cycloaddition furnishing the expected heterocycle. 相似文献