全文获取类型
收费全文 | 1559篇 |
免费 | 46篇 |
国内免费 | 2篇 |
专业分类
化学 | 1151篇 |
晶体学 | 19篇 |
力学 | 31篇 |
数学 | 149篇 |
物理学 | 257篇 |
出版年
2023年 | 8篇 |
2022年 | 43篇 |
2021年 | 50篇 |
2020年 | 42篇 |
2019年 | 37篇 |
2018年 | 21篇 |
2017年 | 18篇 |
2016年 | 37篇 |
2015年 | 56篇 |
2014年 | 46篇 |
2013年 | 102篇 |
2012年 | 113篇 |
2011年 | 108篇 |
2010年 | 58篇 |
2009年 | 76篇 |
2008年 | 111篇 |
2007年 | 85篇 |
2006年 | 70篇 |
2005年 | 50篇 |
2004年 | 68篇 |
2003年 | 53篇 |
2002年 | 43篇 |
2001年 | 30篇 |
2000年 | 31篇 |
1999年 | 23篇 |
1998年 | 17篇 |
1997年 | 28篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1991年 | 5篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1985年 | 8篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1975年 | 6篇 |
1974年 | 3篇 |
1973年 | 6篇 |
1924年 | 2篇 |
1918年 | 2篇 |
排序方式: 共有1607条查询结果,搜索用时 15 毫秒
111.
Ohne Zusammenfassung 相似文献
112.
The flow activation free energy ( ΔG *) is the minimum energy necessary to induce a flow in a system that is at rest. It can be calculated from the system viscosity using the Andrade--Eyring theory. In the present work the flow activation free energy of cholesteric lyotropic liquid crystals was studied as a function of D-(+)-mannose concentration added to a nematic mesophase. The results obtained showed that all the systems can be characterized as Newtonian, but a smooth time-dependent effect can be observed mainly in systems with a lower chiral inductor concentration. It has also been observed that an increase of cholestericity leads to a decrease of ΔH * values until a limit is reached at 1 mol % of inductor. From a positive ΔS * variation it was possible to verify the existence of a locally less organized transient state during the process of micellar diffusion. When the cholesterization process was considered, the increase of the inductor concentration leads to a decrease of ΔS * and consequently to a system, as a whole, more orderly, possibly as a result of the restriction of movement caused by the presence of chiral interactions. 相似文献
113.
Paula Amaral Luís M. Fernandes Joaquim Júdice Hanif D. Sherali 《Journal of Global Optimization》2009,45(4):645-666
This paper addresses the problem of finding an optimal correction of an inconsistent linear system, where only the nonzero
coefficients of the constraint matrix are allowed to be perturbed for reconstructing a consistent system. Using the Frobenius
norm as a measure of the distance to feasibility, a nonconvex minimization problem is formulated, whose objective function
is a sum of fractional functions. A branch-and-bound algorithm for solving this nonconvex program is proposed, based on suitably
overestimating the denominator function for computing lower bounds. Computational experience is presented to demonstrate the
efficacy of this approach. 相似文献
114.
Christian J.L. Hermes Robson O. Piucco Jader R. Barbosa Cláudio Melo 《Experimental Thermal and Fluid Science》2009,33(2):371-379
The present study advances a theoretical and experimental investigation of the frost growth and densification on flat surfaces. This study focuses on the most important factors affecting the frost formation process, i.e. the surrounding air temperature, humidity and velocity, and the surface temperature. The processes of frost growth and densification were investigated experimentally in order to provide a physical basis for the development of a theoretical model to predict the variation of the frost layer thickness and mass with time. The mathematical model was based on mass and energy balances within the frost layer, assuming the frost as a porous medium and accounting for the supersaturation of the moist air on the frost surface. The governing equations for mass and heat diffusion were integrated analytically, giving rise to a semi-algebraic formulation which requires numerical integration of only one time dependent ordinary differential equation. When compared with experimental data, the model predictions of the frost thickness as a function of time agreed to within ±10% error bands. The experimentally-validated model was then used to predict the frost layer growth and densification with respect to the operation conditions such as plate surface temperature, air stream temperature, humidity and velocity. 相似文献
115.
Elvis J. De França Elisabete A. De Nadai Fernandes Márcio A. Bacchi Peter Bode Rob T. M. van Soldt 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(1):125-132
For environmental quality assessment, INAA has been applied for determining chemical elements in small (200 mg) and large
(200 g) samples of leaves from 200 trees. By applying the Ingamells’ constant, the expected percent standard deviation was
estimated in 0.9–2.2% for 200 mg samples. Otherwise, for composite samples (200 g), expected standard deviation varied from
0.5 to 10% in spite of analytical uncertainties ranging from 2 to 30%. Results thereby suggested the expression of the degree
of representativeness as a source of uncertainty, contributing for increasing of the reliability of environmental studies
mainly in the case of composite samples. 相似文献
116.
Alam Gustavo Trov Silene Alessandra Santos Melo Raquel Fernandes Pupo Nogueira 《Journal of photochemistry and photobiology. A, Chemistry》2008,198(2-3):215-220
Photodegradation of the pharmaceuticals amoxicillin (AMX), bezafibrate (BZF) and paracetamol (PCT) in aqueous solutions via the photo-Fenton process was investigated under black-light and solar irradiation. The influences of iron source, initial H2O2 concentration and matrix (distilled water and sewage treatment plant effluent) on degradation efficiency were discussed in detail. The results showed that (i) the degradation of the drugs was favored in the presence of potassium ferrioxalate (FeOx) in comparison to Fe(NO3)3; (ii) the increase of the H2O2 concentration improved the efficiency of AMX and BZF oxidation; however, the same was not observed for PCT; (iii) the influence of the matrix was observed for the degradation of BZF and PCT; (iv) under solar irradiation, the oxidation of the BZF and PCT is faster than under black-light irradiation. All these pharmaceuticals can be efficiently degraded employing the process evaluated. 相似文献
117.
Caitlin F. Zipp Manuel A. Fernandes Joseph P. Michael Susan Winks 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(11):o553-o554
The title compound, C7H13NO, forms R22(8) N—H...O hydrogen‐bonded dimers and C4 N—H...O‐linked chains, which are further stabilized by a C—H...O interaction. The combination of these interactions results in a hydrogen‐bonded network parallel to (100), with a motif that can be described by the secondary graph set R46(16). The existence of the same hydrogen‐bonding motif in 1‐phenylcyclopentanecarboxamide and 1‐(2‐bromophenyl)cyclohexanecarboxamide [Lemmerer & Michael (2008). CrystEngComm, 10 , 95–102 indicates that replacing the H atom on position 1 with a more bulky group does not necessarily disrupt the observed hydrogen‐bonding pattern. The presence of a C—H...O interaction to stabilize the R46(16) network does, however, seem to be required. In addition, the title compound is isomorphous with a previously published structure of cyclopentanecarboxamide [Winter et al. (1981). Acta Cryst. B 37 , 2183–2185]. 相似文献
118.
Ester Buchaca Siud Pui Man Majid Motevalli Lesley Robson Alice Sullivan John Wilson 《Polyhedron》2009,28(18):4115-4121
The compounds diethyl-N-carbazolylmethylphosphonate 1 and diethyl-2-oxymethylphosphonate carbazole 2 have been synthesised and characterized by a range of techniques including NMR, absorption and emission spectroscopy, infrared, and mass spectroscopy. Compound 1 forms a 2:1 complex 3 with calcium nitrate and the single crystal X-ray diffraction structure of 3 is described. Titrations of 1 with Zn2+ and Ca2+ in ethanol reveal that the intrinsic fluorescence is only slightly perturbed in the presence of these metal ions in micromolar ethanol solutions. Compounds 1 and 2 are readily taken up and visualized in L929 fibrosarcoma and DRG (Dorsal root ganglia) cell lines. 相似文献
119.
We discuss the bi-Lipschitz geometry of an isolated singular point of a complex surface with particular emphasis on when it
is metrically conical.
相似文献
120.
We compute Hölder Complexes, i.e. the complete bi-Lipschitz invariants, for germs of real weighed homogeneous algebraic or semialgebraic surfaces. 相似文献