全文获取类型
收费全文 | 4308篇 |
免费 | 335篇 |
国内免费 | 423篇 |
专业分类
化学 | 3390篇 |
晶体学 | 60篇 |
力学 | 218篇 |
综合类 | 28篇 |
数学 | 336篇 |
物理学 | 1034篇 |
出版年
2024年 | 8篇 |
2023年 | 44篇 |
2022年 | 97篇 |
2021年 | 89篇 |
2020年 | 102篇 |
2019年 | 97篇 |
2018年 | 82篇 |
2017年 | 85篇 |
2016年 | 106篇 |
2015年 | 119篇 |
2014年 | 152篇 |
2013年 | 325篇 |
2012年 | 194篇 |
2011年 | 183篇 |
2010年 | 173篇 |
2009年 | 251篇 |
2008年 | 247篇 |
2007年 | 266篇 |
2006年 | 237篇 |
2005年 | 215篇 |
2004年 | 204篇 |
2003年 | 162篇 |
2002年 | 469篇 |
2001年 | 154篇 |
2000年 | 139篇 |
1999年 | 100篇 |
1998年 | 111篇 |
1997年 | 88篇 |
1996年 | 77篇 |
1995年 | 76篇 |
1994年 | 61篇 |
1993年 | 63篇 |
1992年 | 49篇 |
1991年 | 42篇 |
1990年 | 28篇 |
1989年 | 27篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 22篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有5066条查询结果,搜索用时 15 毫秒
91.
Yuki Yamamoto Jun‐ya Hasegawa Yoshikatsu Ito 《Journal of Physical Organic Chemistry》2012,25(3):239-247
The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D2O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO2 source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation kapp, and the rate constants for the formation k1 and the breakdown k?1 of the carbamate were estimated. Plots of log k1 or log k?1 versus pKa of amino acids indicated that the formation rate k1 increased with the basicity (pKa) of amino acid, while the decomposition rate k?1 decreased. A Br?nsted β value of 0.39 was obtained from the former plot, being in good agreement with the previously reported ones (0.26–0.43). The observed negative pKa dependence of log k?1 (Br?nsted α = 0.34) is reasonable, because the carbamate decomposition is acid‐catalyzed and the steady‐state concentration of H+ should be higher for weaker basic amines. The charge (σ) and the lone‐pair energy (EN) at the nitrogen atom of the amino group were calculated. Although log k1 correlated with σ and EN, log k?1 was unrelated with both of these parameters. Considering that the carbamate formation (k1) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and EN in addition to pKa is rational. The irrelevance of log k?1 to σ and EN is not surprising, because σ and EN are not a direct measure of [H+] of the solution. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
92.
The first-order thermodynamic perturbation theory of Wertheim (TPT1) is extended to treat ring aggregates, formed by inter- and intramolecular association. The expression for the residual association contribution to the Helmholtz free energy for ring aggregates, incorporating the appropriate terms in Wertheim's fundamental graph sum of the TPT1 density expansion, is derived to calculate the distribution of the molecular bonding states. This requires the introduction of two new parameters to characterise each possible ring type: the ring size τ, which is equal to one in the case of intramolecular association, and a parameter W that captures the likelihood of two ring-forming sites bonding. The resulting framework can be incorporated in equations of state that account for the residual association contribution to the free energy, such as the statistical associating fluid theory (SAFT) family, or the cubic plus association (CPA) equation of state. This extends the applicability of these equations of state to mixtures with an arbitrary number of association sites capable of hydrogen bonding to form intramolecular and intermolecular rings. The formalism is implemented within SAFT-VR Mie to calculate the fluid-phase equilibria of model chain-like molecules containing two associating sites A and B, allowing for the formation of open-chain aggregates and intramolecular bonds. The effect of adding a second component that competes for the association sites that mediate intramolecular association in the chain is also examined. Accounting for intramolecular bonding is shown to have a significant impact on the phase equilibria of such systems. 相似文献
93.
94.
Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed. 相似文献
95.
Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed. 相似文献
96.
The burning and extinction characteristics of isolated small nonane droplets are examined in a buoyant convective environment and in an environment with no external axial convection (as created by doing experiments at low gravity) to promote spherical droplet flames. The ambience is air and a mixture of 30%O2/70%He to assess the influence of soot formation. The initial droplet diameter (Do) ranges from 0.4 to 0.95 mm. Measurements are reported of the extinction diameter and time to extinction, and of the evolution of droplet diameter, flame diameter, soot shell diameter, burning rate, and broadband radiative emissions.In a buoyancy-free environment for air larger droplets burn slower than smaller droplets for the range of Do examined, which is attributed to the influence of soot. In the presence of a buoyant flow in air, no influence of Do is observed on the burning rate while the buoyant flames are still heavily sooting. The effect of Do is believed to be due to a combination of dominance of the nonluminous, nonsooting, portion of the buoyant flame around the forward half of the droplet on heat transport and the secondary role of the luminous wake portion of the flame. In a non-sooting helium inert at low gravity, no effect of Do is found on the evolution of droplet diameter.Flame extinction is observed only in the 30%O2/70%He ambience. For all of the observations, extinction appears to occur before the disappearance of the droplet which is then followed by a period of evaporation. The extinction diameter and time to extinction increases with Do and an empirical correlation is presented for these two variables. 相似文献
97.
** Email: decentsp{at}for.mat.bham.ac.uk The coalescence of two viscous liquid drops in an inviscid gasor in a vacuum is studied using the interface formation model.In the very early stages of coalescence during the formationof the liquid-bridge connecting the two drops,this model predicts a moving contact line and a dynamic contactangle. This paper examines the dynamic evolution of this contactangle, and for small Reynolds number and small Capillary number,relevant particularly in micro-fluidics, a non-linear differentialequation is derived for the contact angle and solved computationally.It is found that the contact angle evolution can only be evaluatedby determining information about the flow away from the contactline. This is a manifestation of so-called hydrodynamic assist,studied experimentally in the context of curtain coating byBlake et al. (1999 Experimental evidence of non-local hydrodynamicinfluence on the dynamic contact angle. Phys. Fluids, 11, 19952007).For small Capillary number and small Reynolds number, the free-surfaceevolution is determined for the coalescence of two cylindersof equal radius. Finally, some comments are made on experimentsin coalescence, as well as on issues arising in a computationalsolution of the full model described here. 相似文献
98.
Skin formation in drying a film of soft matter solutions:Application of solute based Lagrangian scheme 下载免费PDF全文
When a film of soft matter solutions is being dried, a skin layer often forms at its surface, which is a gel-like elastic phase made of concentrated soft matter solutions. We study the dynamics of this process by using the solute based Lagrangian scheme which was proposed by us recently. In this scheme, the process of the gelation(i.e., the change from sol to gel) can be naturally incorporated in the diffusion equation. Effects of the elasticity of the skin phase, the evaporation rate of the solvents, and the initial concentration of the solutions are discussed. Moreover, the condition for the skin formation is provided. 相似文献
99.
Antonius Indarto Anna Giordana Giovanni Ghigo Glauco Tonachini 《Journal of Physical Organic Chemistry》2010,23(5):400-410
Polyynes of general formula H? (C?C? )nH are known to play a significant role in combustion and pyrolysis, possibly being intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. They have also been detected in astrophysical investigations. The key step in the polyyne‐based radical breeding mechanism for PAH growth is a cyclization, put forward by Krestinin, which implies disruption of electron couples, plausibly expected to be energy demanding. We explore the electronic features and energy requirements of such a process by quantum mechanical multiconfiguration methods (CASSCF and CASPT2). The features of the wavefunction are analyzed, and the free energy barriers are estimated over a wide range of temperatures, for three molecular models. The initial radical adduct A, generated by H . , HC?C . (ethynyl), or HC?C? C . H2 ( propargyl ) addition to butadiyne (BD, HC?C? C?CH), undergoes a cyclization with the generation of two new radical centers. However, in most of the cases, one of these new singly occupied sp2 orbitals has some overlap with the unpaired electron lobe already existent in A: some sort of bonding builds up and consequently the triradical character cannot be large. Only one model suggests a possible role of the radical breeding mechanism during combustion. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
100.
Using the first-principles methods,we study the formation energetics properties of intrinsic defects,and the charge doping properties of extrinsic defects in transparent conducting oxides CuCrO2.Intrinsic defects,some typical acceptortype,and donor-type extrinsic defects in their relevant charge state are considered.By systematically calculating the formation energies and transition energy,the results of calculation show that,V Cu,O i,and O Cu are the relevant intrinsic defects in CuCrO2 ;among these intrinsic defects,V Cu is the most efficient acceptor in CuCrO2.It is found that all the donor-type extrinsic defects have difficulty in inducing n-conductivity in CuCrO2 because of their deep transition energy level.For all the acceptor-type extrinsic defects,substituting Mg for Cr is the most prominent doping acceptor with relative shallow transition energy levels in CuCrO2.Our calculation results are expected to be a guide for preparing promising n-type and p-type materials in CuCrO2. 相似文献