全文获取类型
收费全文 | 729篇 |
免费 | 25篇 |
国内免费 | 11篇 |
专业分类
化学 | 495篇 |
力学 | 15篇 |
数学 | 112篇 |
物理学 | 143篇 |
出版年
2023年 | 3篇 |
2022年 | 2篇 |
2021年 | 16篇 |
2020年 | 18篇 |
2019年 | 11篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 27篇 |
2013年 | 48篇 |
2012年 | 47篇 |
2011年 | 45篇 |
2010年 | 22篇 |
2009年 | 30篇 |
2008年 | 28篇 |
2007年 | 49篇 |
2006年 | 32篇 |
2005年 | 42篇 |
2004年 | 44篇 |
2003年 | 35篇 |
2002年 | 29篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 15篇 |
1989年 | 3篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1908年 | 1篇 |
排序方式: 共有765条查询结果,搜索用时 31 毫秒
61.
62.
A. Lopez-Molinero A. Castro J. Pino J. Perez-Arantegui J. R. Castillo 《Analytical and bioanalytical chemistry》2000,367(6):586-589
Artificial neural networks with unsupervised learning strategy known as Self-Organizing Maps were applied to classify ancient Roman glazed ceramics. Their clay ceramic bodies were analyzed by Inductively Coupled Plasma-Atomic Emission Spectroscopy and the chemical composition obtained was processed by this neural algorithm. The results obtained provide two types of information: firstly, classification of ceramic samples with identification of several groups and secondly, differentiation between the elemental chemical information. It was found that there are certain chemical elements which can be considered as principal and which can serve to differentiate between ceramics, whereas other elements give redundant information and do not contribute to sample differentiation. Seven chemical elements were considered principal and provide the necessary information. Two types of element were identified: 1 – a group formed by common elements, such as: Ca, Fe, Mg, Mn and 2 – another formed by optional elements: K or Na and Ba or Sr and Al or Ti. 相似文献
63.
Barry C. Arnold Enrique Castillo Jos María Sarabia 《Journal of multivariate analysis》2008,99(7):1383-1392
It is well known that full knowledge of all conditional distributions will typically serve to completely characterize a bivariate distribution. Partial knowledge will often suffice. For example, knowledge of the conditional distribution of X given Y and the conditional mean of Y given X is often adequate to determine the joint distribution of X and Y. In this paper, we investigate the extent to which a conditional percentile function or a conditional mode function (of Y given X), together with knowledge of the conditional distribution of X given Y will determine the joint distribution. Finally, using this methodology a new characterization of the classical bivariate normal distribution is given. 相似文献
64.
A.A. Lemus-Santana L.F. del Castillo M. Basterrechea 《Journal of solid state chemistry》2009,182(4):757-766
The materials under study, T[Ni(CN)4]·2pyz with T=Mn, Zn, Cd, were prepared by separation of T[Ni(CN)4] layers in citrate aqueous solution to allow the intercalation of the pyrazine molecules. The obtained solids were characterized from chemical analyses, X-ray diffraction, infrared, Raman, thermogravimetry, UV-Vis, magnetic and adsorption data. Their crystal structure was solved from ab initio using direct methods and then refined by the Rietveld method. A unique coordination for pyrazine to metal centers at neighboring layers was observed. The pyrazine molecule is found forming a bridge between Ni and T atoms, quite different from the proposed structures for T=Fe, Ni where it remains coordinated to two T atoms to form a vertical pillar between neighboring layers. The coordination of pyrazine to both Ni and T atoms minimizes the material free volume and leads to form a hydrophobic framework. On heating the solids remain stable up to 140 °C. No CO2 and H2 adsorption was observed in the small free spaces of their frameworks. 相似文献
65.
G. F. Torres Del Castillo 《Letters in Mathematical Physics》1986,12(2):149-155
By using the expressions for the solutions of the Einstein-Maxwell equations in terms of potentials, valid in the case where the spacetime admits a shear-free geodesic null congruence and the electromagnetic field is aligned to it, we show that a pair of complex potentials generates simultaneous perturbations of the gravitational and the electromagnetic fields. We also show that if the background electromagnetic field is null, then the pair of complex potentials is determined by a pair of coupled, linear, second-order differential equations. 相似文献
66.
Juan C. Castillo Rodrigo Abonía Michael B. Hursthouse Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(10):o495-o497
In the title compound, C23H22N4O, there is evidence for some bond fixation in the aryl component of the quinolinone unit. Pairs of molecules related by inversion are linked into R22(8) dimers by almost linear N—H...O hydrogen bonds, and dimers related by inversion are linked into chains by a single aromatic π–π stacking interaction. 相似文献
67.
García-Couceiro U Castillo O Cepeda J Lanchas M Luque A Pérez-Yáñez S Román P Vallejo-Sánchez D 《Inorganic chemistry》2010,49(24):11346-11361
We report herein the synthesis and physicochemical characterization of eight new manganese-oxalato compounds with 1,2-bis(4-pyridyl)ethylene (bpe): {(Hbpe)(2)[Mn(2)(μ-ox)(3)]·~0.8(C(2)H(5)OH)·~0.4(H(2)O)}(n) (1), {[Mn(μ-ox)(μ-bpe)]·xH(2)O}(n) (2), [Mn(2)(μ-ox)(2)(μ-bpe)(bpe)(2)](n) (3), [Mn(μ-ox)(μ-bpe)](n) (4a and 4b), and {[Mn(4)(μ-ox)(3)(μ-bpe)(4)(H(2)O)(4)]·(X)(2)·mY}(n) with X = NO(3)(-) (5a), Br(-) (5b), and ClO(4)(-) (5c) and Y = solvation molecules. The appropriate selection of the synthetic conditions allowed us to control the crystal structure and to design extended 2D and 3D frameworks. Compound 1 is obtained at acid pH values and its crystal structure consists of stacked [Mn(2)(μ-ox)(3)](2-) layers with cationic Hbpe(+) molecules intercalated among them. Compound 2 was obtained at basic pH values with a manganese/bpe ratio of 1:1, and the resulting 3D structure consists of an interpenetrating framework in which metal-oxalato chains are bridged by bpe ligands, leading to a microporous network that hosts a variable number of water molecules (between 0 and 1) depending on the synthetic conditions. Compound 3, synthesized with a manganese/bpe ratio of 1:3, shows a 2D framework in which linear metal-oxalato chains are joined by bis-monodentate 1,2-bis(4-pyridyl)ethylene ligands. The thermal treatment of compound 3 permits the release of one of the bpe molecules, giving rise to two new 2D crystalline phases of formula [Mn(μ-ox)(μ-bpe)](n) (4a and 4b) depending on the heating rate. The open structures of 5a-5c were synthesized in a medium with a high concentration of nitrate, perchlorate, or bromide salts (potassium or sodium as cations). These anions behave as templating agents directing the crystal growing toward a cationic porous network, in which the anions placed in the voids and channels of the structure present high mobility, as inferred from the ionic exchange experiments. Variable-temperature magnetic susceptibility measurements show an overall antiferromagnetic behavior for all compounds, which are discussed in detail. 相似文献
68.
Dmitrii A. Guschin John Castillo Nina Dimcheva Wolfgang Schuhmann 《Analytical and bioanalytical chemistry》2010,398(4):1661-1673
The design of polymers carrying suitable ligands for coordinating Os complexes in ligand exchange reactions against labile
chloro ligands is a strategy for the synthesis of redox polymers with bound Os centers which exhibit a wide variation in their
redox potential. This strategy is applied to polymers with an additional variation of the properties of the polymer backbone
with respect to pH-dependent solubility, monomer composition, hydrophilicity etc. A library of Os-complex-modified electrodeposition
polymers was synthesized and initially tested with respect to their electron-transfer ability in combination with enzymes
such as glucose oxidase, cellobiose dehydrogenase, and PQQ-dependent glucose dehydrogenase entrapped during the pH-induced
deposition process. The different polymer-bound Os complexes in a library containing 50 different redox polymers allowed the
statistical evaluation of the impact of an individual ligand to the overall redox potential of an Os complex. Using a simple
linear regression algorithm prediction of the redox potential of Os complexes becomes feasible. Thus, a redox polymer can
now be designed to optimally interact in electron-transfer reactions with a selected enzyme. 相似文献
69.
Fernando de la Peña Moreno Gracia Patricia BlanchGema Flores Maria Luisa Ruiz del Castillo 《Journal of chromatography. A》2010,1217(7):1083-1088
A method based on the use of the through oven transfer adsorption–desorption (TOTAD) interface in on-line coupling between reversed phase liquid chromatography and gas chromatography (RPLC–GC) for the determination of chiral volatile compounds was developed. In particular, the method was applied to the study of the influence of methyl jasmonate (MJ) treatment on the production and enantiomeric composition of selected aroma compounds in strawberry. The compounds studied were ethyl 2-methylbutanoate, linalool and 4-hydroxy-2,5-dimethyl-3(2H)-furanone (i.e. furaneol), which were examined on days 3, 6 and 9 after treatment. The method developed resulted in relative standard deviations (RSDs) of 21.6%, 8.1% and 9.8% and limits of detection (LD) of 0.04, 0.07 and 0.02 mg/l for ethyl 2-methylbutanoate, linalool and furaneol, respectively. The application of the RPLC–TOTAD–GC method allowed higher levels of ethyl 2-methylbutanoate, linalool and furaneol to be detected, particularly after 9 days of treatment. Besides, MJ demonstrated to affect the enantiomeric distribution of ethyl 2-methylbutanoate. On the contrary, the enantiomeric composition of linalool and furaneol kept constant in both control and MJ-treated strawberries throughout the study. These results are discussed. 相似文献
70.
Castillo AM Patiny L Wist J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,209(2):123-130
The computational cost for the simulation of NMR spectra grows exponentially with the number of nuclei. Today, the memory available to store the Hamiltonian limits the size of the system that can be studied. Modern computers enable to tackle systems containing up to 13 spins [1], which obviously does not allow to study most molecules of interest in research. This issue can be addressed by identifying groups of spins or fragments that are not or only weakly interacting together, i.e., that only share weakly coupled spin pairs. Such a fragmentation is only permitted in the weak coupling regime, i.e., when the coupling interaction is weak compared to the difference in chemical shift of the coupled spins. Here, we propose a procedure that removes weak coupling interactions in order to split the spin system efficiently and to correct a posteriori for the effect of the neglected couplings. This approach yields accurate spectra when the adequate interactions are removed, i.e., between spins only involved in weak coupling interactions, but fails otherwise. As a result, the computational time for the simulation of 1D spectra grows linearly with the size of the spin system. 相似文献