全文获取类型
收费全文 | 2269篇 |
免费 | 88篇 |
国内免费 | 32篇 |
专业分类
化学 | 1385篇 |
晶体学 | 12篇 |
力学 | 63篇 |
数学 | 550篇 |
物理学 | 379篇 |
出版年
2023年 | 27篇 |
2022年 | 58篇 |
2021年 | 62篇 |
2020年 | 54篇 |
2019年 | 43篇 |
2018年 | 51篇 |
2017年 | 38篇 |
2016年 | 81篇 |
2015年 | 94篇 |
2014年 | 96篇 |
2013年 | 146篇 |
2012年 | 153篇 |
2011年 | 149篇 |
2010年 | 109篇 |
2009年 | 97篇 |
2008年 | 139篇 |
2007年 | 125篇 |
2006年 | 123篇 |
2005年 | 109篇 |
2004年 | 88篇 |
2003年 | 50篇 |
2002年 | 71篇 |
2001年 | 18篇 |
2000年 | 23篇 |
1999年 | 16篇 |
1998年 | 22篇 |
1997年 | 16篇 |
1996年 | 16篇 |
1995年 | 8篇 |
1994年 | 11篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 18篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1985年 | 9篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 22篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1956年 | 6篇 |
排序方式: 共有2389条查询结果,搜索用时 15 毫秒
31.
32.
Homogeneous and unimolecular gas‐phase thermal decomposition kinetics of methyl benzoylformate: experimental and theoretical study 下载免费PDF全文
Yeljair Monascal Alexis Maldonado José R. Mora Tania Córdova Gabriel Chuchani 《Journal of Physical Organic Chemistry》2015,28(1):40-46
The kinetics of the gas‐phase thermal decomposition of the α‐ketoester methyl benzoylformate was carried out in a static system with reaction vessel deactivated with allyl bromide, and in the presence of the free radical inhibitor propene. The rate coefficients were determined over the temperature range of 440–481 °C and pressures from 32 to 80 Torr. The reaction was found to be homogenous, unimolecular and obey a first‐order rate law. The products are methyl benzoate and CO. The temperature dependence of the rate coefficient gives the following Arrhenius parameters: log10 k (s?1) = 13.56 ± 0.31 and Ea (kJ mol?1) = 232.6 ± 4.4. Theoretical calculations of the kinetic and thermodynamic parameters are in good agreement with the experimental values using PBE1PBE/6‐311++g(d,p). A theoretical Arrhenius plot was constructed at this level of theory, and the good agreement with the experimental Arrhenius plot suggests that this model of transition state may describe reasonably the elimination process. These results suggest a concerted non‐synchronous semi‐polar three‐membered cyclic transition state type of mechanism. The most advanced coordinate is the bond breaking Cδ+‐‐‐δ‐OCH3 with an evolution of 66.7%, implying this as the limiting factor of the elimination process. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
33.
Gas‐phase elimination kinetics of selected aliphatic α,β‐unsaturated aldehydes catalyzed by hydrogen chloride 下载免费PDF全文
Libia L. Julio José R. Mora Alexis Maldonado Gabriel Chuchani 《Journal of Physical Organic Chemistry》2015,28(4):261-265
The gas‐phase elimination of 2‐methyl‐2‐propenal catalyzed by HCl yields propene and CO gas, while E‐2‐pentenal with the same catalyst gives butene and CO gas. The kinetics determinations were carried out in a static system with the reaction vessels deactivated with allyl bromide and the presence of the free radical inhibitor toluene. Temperature and pressure ranges were 350.0–410.0 °C and 34–76 Torr. The elimination reactions are homogeneous and unimolecular, and follow a first‐order rate law. The rate coefficients for the reactions are expressible by the following Arrhenius equations: Data from the kinetic and thermodynamic parameters of these catalyzed elimination reactions implies a mechanism of a concerted five‐membered cyclic transition state structure for the formation of the corresponding olefin and carbon monoxide. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
34.
Zaragoza-Contreras EA Hernández-Escobar CA Navarrete-Fontes A Flores-Gallardo SG 《Micron (Oxford, England : 1993)》2011,42(3):263-270
In this study, carbon black/polystyrene electrically conductive composites were obtained by suspension polymerization technique. The composite was characterized using transmission electron microscopy, which indicated two outstanding features concerning to the carbon black; first, that the carbon particles were adsorbed onto the surface of the polystyrene particles, similarly as in the Pickering emulsion phenomenon and second, that the primary aggregate structure of the carbon black was significantly affected by the dispersion process. On the other hand, the composite resistivity was in the order of 200 Ωcm, which was attributed to the direct contact of primary carbon black particles (percolation) and not to the tunneling effect. The obtained composite was evaluated as the electrically conductive element in SBR matrix. 相似文献
35.
36.
The standard Vicsek model (SVM) is a minimal non-equilibrium model of self-propelled particles that appears to capture the essential ingredients of critical flocking phenomena. In the SVM, particles tend to align with each other and form ordered flocks of collective motion; however, perturbations controlled by a noise term lead to a noise-driven continuous order–disorder phase transition. In this work, we extend the SVM by introducing a parameter α that allows particles to be individualistic instead of gregarious, i.e. to choose a direction of motion independently of their neighbors. By focusing on the small-noise regime, we show that a relatively small probability of individualistic motion (around 10%) is sufficient to drive the system from a Vicsek-like ordered phase to a disordered phase. Despite the fact that the α-extended model preserves the O(n) symmetry and the interaction range, as well as the dimensionality of the underlying SVM, this novel phase transition is found to be discontinuous (first order), an intriguing manifestation of the richness of the non-equilibrium flocking/swarming phenomenon. 相似文献
37.
We report the use of nanotube field-effect transistor devices for chemical sensing in a conducting liquid environment. Detection of ammonia occurs through the shift of the gate voltage dependence of the source-drain current. We attribute this shift to charge transfer from adsorbed ammonia molecules, with the amount of charge estimated to be as small as 40 electrons for the smallest shift detected. Using the concentration dependence of the response as an adsorption isotherm, we are able to measure the amount of charge transfer to be 0.04 electron per ammonia molecule. 相似文献
38.
Hunter M Backman V Popescu G Kalashnikov M Boone CW Wax A Gopal V Badizadegan K Stoner GD Feld MS 《Physical review letters》2006,97(13):138102
Light scattered from biological tissues can exhibit an inverse power law spectral component. We develop a model based on the Born approximation and von Karman (self-affine) spatial correlation of submicron tissue refractive index to account for this. The model is applied to light scattering spectra obtained from excised esophagi of normal and carcinogen-treated rats. Power law exponents used to fit dysplastic tissue site spectra are significantly smaller than those from normal sites, indicating that changes in tissue self-affinity can serve as a potential biomarker for precancer. 相似文献
39.
We propose a numerical method to accurately discriminate the influence of the different intrachannel nonlinear effects occurring in 40 Gbit/s optical transmissions, following an analogy with methods used to discriminate WDM interchannel effects. In contrast to other studies showing the predominance of intrachannel cross-phase modulation when low-dispersion fibers are used, in our study intrachannel four-wave mixing is the most penalizing effect in all investigated cases. 相似文献
40.
We derive the coupling of a localized hexadecapolar mode to conduction electrons in tetragonal symmetry. The derivation can be easily adapted to arbitrary multipoles in an arbitrary environment. We relate our model to the two-channel Kondo (2CK) model and show that for an f(2) configuration a relevant crystal field splitting in addition to the 2CK interaction is intrinsic to tetragonal symmetry. We discuss possible realizations of a hexadecapolar Kondo effect in URu(2)Si(2). Solving our model we find good agreement with susceptibility and specific heat measurements in Th(1-x)U(x)Ru(2)Si(2) (x?1). 相似文献