全文获取类型
收费全文 | 4890篇 |
免费 | 559篇 |
国内免费 | 404篇 |
专业分类
化学 | 2852篇 |
晶体学 | 57篇 |
力学 | 199篇 |
综合类 | 23篇 |
数学 | 461篇 |
物理学 | 994篇 |
无线电 | 1267篇 |
出版年
2024年 | 9篇 |
2023年 | 74篇 |
2022年 | 80篇 |
2021年 | 161篇 |
2020年 | 151篇 |
2019年 | 164篇 |
2018年 | 132篇 |
2017年 | 121篇 |
2016年 | 205篇 |
2015年 | 245篇 |
2014年 | 288篇 |
2013年 | 373篇 |
2012年 | 405篇 |
2011年 | 395篇 |
2010年 | 283篇 |
2009年 | 299篇 |
2008年 | 334篇 |
2007年 | 273篇 |
2006年 | 237篇 |
2005年 | 236篇 |
2004年 | 162篇 |
2003年 | 168篇 |
2002年 | 161篇 |
2001年 | 101篇 |
2000年 | 75篇 |
1999年 | 89篇 |
1998年 | 72篇 |
1997年 | 55篇 |
1996年 | 42篇 |
1995年 | 48篇 |
1994年 | 37篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 25篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1980年 | 25篇 |
1978年 | 25篇 |
1977年 | 48篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1943年 | 10篇 |
排序方式: 共有5853条查询结果,搜索用时 906 毫秒
991.
Ju Gu Qiang Xiong Demin Jia Yuanfang Luo Rongshi Cheng 《Frontiers of Chemistry in China》2009,4(1):75-82
A modified nano-calcium carbonate (R-CCR) was prepared by coating a layer of unsaturated hydroxylfatty acid on the surface
of CCR powders using a solid state method; the latter were commercial nano-CaCO3 modified with stearic acid. FTIR studies indicate that the modifier is combined on the surface of CaCO3. PP/EPDM/nano-CaCO3 ternary composites were prepared by a melt-mixing method. SEM and TEM were utilized to examine the morphology of the composites.
The tensile fractured surface of PP/EPDM/R-CCR showed a fibroid morphology and large-scale yield deformation. The impact fractured
surface showed that the amount of cavities in the PP/EPDM/R-CCR system was increased, however their size diminished obviously.
R-CCR particles were dispersed uniformly in the PP matrix, and their compatibility was distinctly improved as compared to
CCR when the amount of R-CCR was 15 h−1. The tensile strength remained nearly constant (reduced from 27.6 MPa to 27.5 MPa), while the impact strength increased from
9.6 kJ/m2 to 15.4 kJ/m2 as CCR was replaced by R-CCR. Meanwhile, the bending strength and bending modulus also increased correspondingly. Furthermore,
the impact strength of PP/EPDM/R-CCR was maintained at a high level (15.4 kJ/m2), which was more than the sum of that of PP/EPDM and PP/R-CCR (6.6 kJ/m2 and 6.1 kJ/m2 respectively). This indicates that the R-CCR and EPDM have a significant synergistic toughening effect on PP while maintaining
the strength and modulus of virgin PP. Both the storage modulus G′ and loss modulus G″ of PP/EPDM and PP/EPDM/R-CCR composites increase with increasing frequency, but the values of G′ and G″ of the tertiary composite are relatively higher than those of the binary system. The loss factor and viscosity decrease with
increasing frequency, but there is little difference between tertiary and binary composites. The apparent viscosity η of the tertiary system containing R-CCR is lower than that of the tertiary system containing CCR and virgin PP. The viscosity
of the composites sig-nificantly decreases with increasing shear rate. The mea-sured mechanical properties of the composites
indicate that replacing CCR with R-CCR for binary composites could simultaneously enhance the toughness and strength of PP.
__________
Translated from Acta Polymerica Sinica, 2008, 4 (in Chinese) 相似文献
992.
Hyun‐Ju Lee U‐Hwang Lee Dr. Jin‐Young Park Sang‐Hoon Yoo Sungho Park Dr. Young‐Uk Kwon Dr. 《化学:亚洲杂志》2009,4(8):1284-1288
The synthesis of Pt thin films with a controlled nanoscopic architecture that can support surface enhanced Raman scattering (SERS) is reported. The syntheses are achieved by replicating the pores of a type of mesoporous silica thin film whose pore structure could be described as a regular array of vertical channels of ~9 nm in diameter and their interconnections, forming a 3‐dimensional pore network. Electrochemical deposition into the pores followed by the removal of the templates produced Pt films composed of arrays of vertically standing Pt nanorods with narrow gaps between them. The 3‐dimensional nanostructure increases the surface area and enables the Pt film to absorb visible light. SERS studies of rhodamine 6G and benzenethiol on such Pt films as substrates reveals that the control of the nanostructure is critical for the SERS effect. 相似文献
993.
This work presents a variational formulation of the material failure process, idealized as strain or displacement discontinuities, by weak, strong, or discrete embedded discontinuities into a continuum. It is shown that the solution of the proposed variational formulation may be approximated by different types of finite elements with embedded discontinuities. The developed displacement approximation of a finite element split by the discontinuity leads to a symmetric stiffness matrix, which considers not only the continuity of tractions but also the rigid body relative motions of the portions in which the element is split. The variational formulation of a continuum with more than one discontinuity in its interior is developed. It is shown that this formulation may lead to finite elements with embedded discontinuities that can be classified as displacement, force, mixed, and hybrid models. To show the effectiveness of the proposed formulation, the classical example of a bar under tension is solved using one and 2D finite element approximations. © 2008 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2009 相似文献
994.
Using the variational method, it is shown that the set of all strong peak functions in a closed algebra A of Cb(K) is dense if and only if the set of all strong peak points is a norming subset of A. As a corollary we can induce the denseness of strong peak functions on other certain spaces. In case that a set of uniformly strongly exposed points of a Banach space X is a norming subset of , then the set of all strongly norm attaining elements in is dense. In particular, the set of all points at which the norm of is Fréchet differentiable is a dense Gδ subset. In the last part, using Reisner's graph-theoretic approach, we construct some strongly norm attaining polynomials on a CL-space with an absolute norm. Then we show that for a finite dimensional complex Banach space X with an absolute norm, its polynomial numerical indices are one if and only if X is isometric to . Moreover, we give a characterization of the set of all complex extreme points of the unit ball of a CL-space with an absolute norm. 相似文献
995.
基于谱微分矩阵方法,给出MKdV方程的多辛Fourier拟谱格式及其相应多辛离散守恒律,证明了它等价于通常的Fourier拟谱格式.数值结果表明,格式对于长时间计算具有稳定性与高精度. 相似文献
996.
997.
998.
The effects of overlapping levels and concentration ratios of overlapping components, and of scan rates of the mass spectrometer, on the capability of the automated mass spectral deconvolution and identification system (AMDIS) in pesticide residue analysis were studied. To investigate the capability of AMDIS in removing interferences from the overlapping peaks, this system was applied to data files obtained from the gas chromatography/mass spectrometry (GC/MS) analysis of two overlapping (co-eluting) pesticides (beta-HCH and PCNB) in full scan mode. Differences in overlap levels, the concentration ratios of the two overlapping components and the scan rates of the instrument were studied. When the difference in scan number of overlapping compounds was equal to 1 scan, AMDIS incompletely extracted 'purified' mass spectra but as the difference increased to 3 or more scans, complete correct spectra could be extracted. The results also show that when the scan rate was in the range of 0.4-0.90 s/scan and the concentration ratios of the target compound/interference were above 1/5, there were ideal deconvolution results for this approach. To further study the application of AMDIS to pesticide residue analysis, AMDIS was applied to the identification of pesticides spiked in real samples (cabbage and rice). Typical pesticides being evaluated were identified using AMDIS at concentrations >50 ng/g in the extracts. 相似文献
999.
Hydrogen oxidation on Pt (111) surface is modeled by density functional theory (DFT). Previous DFT calculations showed too large O2 dissociation barriers, but we find them highly coverage dependent: when the coverage is low, dissociation barriers close to experimental values (approximately 0.3 eV) are obtained. For the whole reaction, a new pathway involving hydroperoxyl (OOH) intermediate is found, with the highest reaction barrier of only approximately 0.4 eV. This may explain the experimental observation of catalytic water formation on Pt (111) surface above the H2O desorption temperature of 170 K, despite that the direct reaction between chemisorbed O and H atoms is a highly activated process with barrier approximately 1 eV as previous calculations showed. 相似文献
1000.
Solid phase microextraction and capillary gas chromatography-mass spectrometry were used for the determination of seven terpenes in tequila. The method was selected based on the following parameters: coating selection (PA, PDMS, CW/DVB, and PDMS/DVB), extraction temperature, addition of salt, and extraction time profile. The extraction conditions were: PDMS/DVB fiber, Headspace, 100% NaCl, 25 degrees C extraction temperature, 30 min extraction time and stirring at 1200 rpm. The calibration curves (50-1000 ng/ml) for the terpenes followed linear relationships with correlation coefficients (r) greater than 0.99, except for trans,trans-farnesol (r = 0.98). RSD values were smaller than 10% confirmed that the technique was precise. Samples from 18 different trade brands of "Aged" tequila analyzed with the developed method showed the same terpenes in different concentrations. The analytical procedure used is selective, robust (more than 100 analyses with the same fiber), fast and of low-cost. 相似文献