全文获取类型
收费全文 | 16654篇 |
免费 | 1810篇 |
国内免费 | 1548篇 |
专业分类
化学 | 12532篇 |
晶体学 | 491篇 |
力学 | 1501篇 |
综合类 | 47篇 |
数学 | 384篇 |
物理学 | 5057篇 |
出版年
2024年 | 55篇 |
2023年 | 348篇 |
2022年 | 539篇 |
2021年 | 567篇 |
2020年 | 870篇 |
2019年 | 592篇 |
2018年 | 517篇 |
2017年 | 558篇 |
2016年 | 818篇 |
2015年 | 843篇 |
2014年 | 925篇 |
2013年 | 1206篇 |
2012年 | 860篇 |
2011年 | 1096篇 |
2010年 | 961篇 |
2009年 | 1043篇 |
2008年 | 1053篇 |
2007年 | 1130篇 |
2006年 | 973篇 |
2005年 | 810篇 |
2004年 | 765篇 |
2003年 | 679篇 |
2002年 | 463篇 |
2001年 | 385篇 |
2000年 | 323篇 |
1999年 | 235篇 |
1998年 | 243篇 |
1997年 | 200篇 |
1996年 | 179篇 |
1995年 | 104篇 |
1994年 | 102篇 |
1993年 | 79篇 |
1992年 | 74篇 |
1991年 | 60篇 |
1990年 | 57篇 |
1989年 | 50篇 |
1988年 | 28篇 |
1987年 | 33篇 |
1986年 | 31篇 |
1985年 | 27篇 |
1984年 | 25篇 |
1983年 | 22篇 |
1982年 | 27篇 |
1981年 | 12篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1973年 | 4篇 |
1971年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
Composite materials were processed by casting a mixture of aqueous suspensions of latex and microfibrils. These microfibrils, or whiskers, are extracted from a sea animal and are monocrystals of cellulose, with an aspect ratio around 100 and an average diameter of 20 nm. It has been found that the mechanical properties (shear modulus) are increased by more than two orders of magnitude in the rubbery state of the polymeric matrix, when the whisker content was 6% (w/w). This very large effect is discussed on the basis of different types of mechanical models and it is concluded that these whiskers form a rigid network, probably linked by hydrogen bonds. The formation of this network is assumed to be governed by a percolation mechanism. 相似文献
104.
Several derivatization procedures with o-phthaldialdehyde-N-acetylcysteine (OPA-NAC) were compared for a rapid analysis of primary aliphatic short-chain monoamines in water samples by HPLC using a LiChorospher analytical separation column (100RP18 mm i.d., 5 μm). Both the solution and the solid-support assisted off-line derivatization on C18 SPE cartridges were inadequate options because of beginning degradation processes of the instable isoindol derivatives during their transfer to the analytical column. This problem was precluded with the on-column or solid-support assisted on-line derivatization. In the last mentioned procedure, the derivatization took place in a Hypersil C18 precolumn ( mm i.d., 30 μm) connected with an additional preconcentration step resulting in better detection limits (0.002-0.040 μg ml−1 requiring only 150 μl of water sample) than in the on-column procedure (0.08-0.16 μg ml−1). The improved sample handling, the better control of parameters affecting reaction rates, the fully automation of this method with only 10 min analysis time for each sample are further advantageous. The potential of the solid-support assisted on-line derivatization was outlined and applied to water samples from several sources. Recovery values near 100% were obtained. 相似文献
105.
J.A. ColladoM.A.G. Aranda A. CabezaP. Olivera-Pastor S. Bruque 《Journal of solid state chemistry》2002,167(1):80-85
The La2W2−xMoxO9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La2W2O9 has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La2W1.7Mo0.3O9 has been analyzed by the Rietveld method in P213 space group. The final R-factors were RWP=9.0% and RF=5.6% with a structure similar to that of β-La2Mo2O9. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10−6 and 9.7×10−6°C−1 for α-La2W2O9 and β-La2W1.2Mo0.8O9, respectively. 相似文献
106.
Pure copolymer nanoparticles from 8-aminoquinoline (AQ) and 2-ethylaniline (EA) were easily synthesized by a chemically oxidative polymerization in three different aqueous media. The potential and temperature of polymerization solution were used to successfully follow the polymerization progress. The molecular and morphological structures of the resulting AQ/EA copolymer particles were systematically characterized by IR, UV/Vis, NMR, gel permeation chromatography, laser particle-size analysis, atomic force and transmission electron microscopy. The oxidation potential of the monomers as well as the polymerization yield, structure, and properties of the particles were found to significantly depend on AQ/EA ratio, polymerization temperature and medium. It is surprisingly found that AQ homopolymerization and AQ/EA (50:50) copolymerization at 5 degrees C in HCl simply afford nano-ellipsoids with the major/minor axis diameters of 24/14 nm and 80/67 nm, respectively. A simple method of synthesizing semiconducting pure nanoparticles by introducing the AQ units with positively charged quaternary ammonium groups but in the absence of adscititious stabilizer or sulfonic substituent on the monomers is established first. Both the molecular weight and bulk electroconductivity of the copolymers exhibit a maximum at AQ content of 10 mol %. The solubility and film formability of the copolymers are good in highly polar solvents and reach the optimal at the AQ content of 20 and 10 mol %, respectively. 相似文献
107.
108.
Godehard Sutmann 《Journal of Electroanalytical Chemistry》1998,450(2):1946
We have performed molecular dynamics computer simulations of water in homogeneous external electric fields which were varied in a wide range of field strengths. The dielectric response is found to be linear up to fields E0≈0.01 V/Å from where dielectric saturation effects become important. At fields of E0≈3 V/Å a phase transition into an ordered, ice-like structure is observed, which is stabilized through hydrogen-bonds. With an increasing external electric field, the frequency spectrum of the water dynamics shows a remarkable red shift of the intramolecular modes and a blue shift of the librational motions, where the frequency varies quadratically with the field strength. A simple analytical model is discussed which reproduces the observed behavior. 相似文献
109.
A reversed-phase ion-pair HPLC with indirect photometric detection for the simultaneous determination of sulfite, sulfate, hydroxymethanesulfonate (HMS), and other inorganic anions in atmospheric water has been developed. Separations were accomplished in less than 10 min in a cetylpyridinium-coated C18 column with 0.5 mM potassium hydrogen phthalate-0.015% triethanolamine-3% methanol at pH 7.9 as mobile phase. Quantitation was carried out by the peak area method, with detection limits in the pmol range. UV light absorption responses were linear over a wide concentration range from several hundred μmoles to the detection limits of each anion. The application of the method provides a rapid and efficient technique for the quantitative determination of sulfur and other inorganic species in atmospheric liquids. 相似文献
110.
K. Surendra 《Tetrahedron letters》2006,47(13):2133-2136
A new and convenient procedure for the synthesis of homoallylic alcohols directly from aromatic acetals and dioxolanes has been developed with very good yields under biomimetic conditions using a Zn-mediated Barbier-type allylation in the presence of β-cyclodextrin in water. 相似文献