全文获取类型
收费全文 | 15525篇 |
免费 | 1248篇 |
国内免费 | 1877篇 |
专业分类
化学 | 7069篇 |
晶体学 | 76篇 |
力学 | 935篇 |
综合类 | 234篇 |
数学 | 6214篇 |
物理学 | 4122篇 |
出版年
2024年 | 41篇 |
2023年 | 277篇 |
2022年 | 334篇 |
2021年 | 407篇 |
2020年 | 500篇 |
2019年 | 450篇 |
2018年 | 401篇 |
2017年 | 503篇 |
2016年 | 635篇 |
2015年 | 461篇 |
2014年 | 738篇 |
2013年 | 1300篇 |
2012年 | 766篇 |
2011年 | 1043篇 |
2010年 | 888篇 |
2009年 | 1065篇 |
2008年 | 1022篇 |
2007年 | 1132篇 |
2006年 | 898篇 |
2005年 | 803篇 |
2004年 | 688篇 |
2003年 | 655篇 |
2002年 | 513篇 |
2001年 | 440篇 |
2000年 | 391篇 |
1999年 | 323篇 |
1998年 | 340篇 |
1997年 | 286篇 |
1996年 | 230篇 |
1995年 | 188篇 |
1994年 | 141篇 |
1993年 | 124篇 |
1992年 | 117篇 |
1991年 | 98篇 |
1990年 | 55篇 |
1989年 | 63篇 |
1988年 | 46篇 |
1987年 | 28篇 |
1986年 | 41篇 |
1985年 | 40篇 |
1984年 | 26篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Marc Baboulin Jack Dongarra Serge Gratton Julien Langou 《Numerical Linear Algebra with Applications》2009,16(7):517-533
In this paper, we address the accuracy of the results for the overdetermined full rank linear least‐squares problem. We recall theoretical results obtained in (SIAM J. Matrix Anal. Appl. 2007; 29 (2):413–433) on conditioning of the least‐squares solution and the components of the solution when the matrix perturbations are measured in Frobenius or spectral norms. Then we define computable estimates for these condition numbers and we interpret them in terms of statistical quantities when the regression matrix and the right‐hand side are perturbed. In particular, we show that in the classical linear statistical model, the ratio of the variance of one component of the solution by the variance of the right‐hand side is exactly the condition number of this solution component when only perturbations on the right‐hand side are considered. We explain how to compute the variance–covariance matrix and the least‐squares conditioning using the libraries LAPACK (LAPACK Users' Guide (3rd edn). SIAM: Philadelphia, 1999) and ScaLAPACK (ScaLAPACK Users' Guide. SIAM: Philadelphia, 1997) and we give the corresponding computational cost. Finally we present a small historical numerical example that was used by Laplace (Théorie Analytique des Probabilités. Mme Ve Courcier, 1820; 497–530) for computing the mass of Jupiter and a physical application if the area of space geodesy. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
972.
Andriy Romanyuk Viktor Melnik Yaroslav Olikh Johannes Biskupek Ute Kaiser Martin Feneberg Klaus Thonke Peter Oelhafen 《Journal of luminescence》2010,130(1):87-91
Silicon clusters embedded in a silicon dioxide matrix were prepared by ultrasound-assisted implantation resulting in a modified concentration of suboxide states as revealed by high-resolution photoelectron spectroscopy. It is suggested that ultrasound treatment results in formation of different interface structure between silicon cluster and silicon dioxide matrix which is characterized by a distinctly reduced concentration of the suboxide states. It is observed that photoluminescence properties are strongly correlated with the concentration of the suboxide states thereby providing an evidence that besides a quantum confinement effect a closer look at the chemical composition of the nc-Si/SiO2 system is important. 相似文献
973.
974.
Ana Erica Paramo Susana Palmero Aranzazu Heras Alvaro Colina David Ibañez 《Electroanalysis》2016,28(4):890-897
The purpose of this work is to develop simple and reproducible ways to fabricate carbon nanofibers (CNFs) electrodes. Disposable CNFs electrodes were fabricated after filtering CNFs through polytetrafluoroethylene filters (PTFE), using directly as electrodes these filters with the CNFs film. The electrochemical behaviour of CNFs/PTFE electrodes were tested with ferrocenemethanol and tris(2,2′‐bypyridyl)dichlororuthenium(II) (Ru(bipy)32+). As a proof of concept of the analytical usefulness of the CNFs/PFTE electrodes, the study of vitamin B12 was carried out. Multivariate calibrate has been successfully used to determine vitamin B12 in a complex medium with vitamins B1 and B6 as interferents. 相似文献
975.
976.
We combined scanning tunneling microscopy and density functional theory to establish the structure-functionality relationship for nanometer-sized defects on TiO2(1 1 0). Three-angstrom high topographically distinct dots are ascribed to stoichiometric TiO2 nanoclusters with low coordination numbers. The under-coordinated O atoms of the nanocluster, with surface O atoms, provide exceptionally strong binding sites for Au nanoparticles. Our atomistic model elucidates a number of characteristics salient to low temperature CO oxidation by Au nanoparticles. 相似文献
977.
Cornelis van der Mee Pietro Contu Paolo Pintus 《Journal of Quantitative Spectroscopy & Radiative Transfer》2010,111(1):214-225
In this article we present a method to determine the band spectrum, band gaps, and discrete energy levels, of a one-dimensional photonic crystal with localized impurities. For one-dimensional crystals with piecewise constant refractive indices we develop an algorithm to recover the refractive index distribution from the period map. Finally, we derive the relationship between the period map and the scattering matrix containing the information on the localized modes. 相似文献
978.
关于反中心对称矩阵的某些性质探讨 总被引:2,自引:0,他引:2
利用反中心对称矩阵的定义以及翻转矩阵等技巧,给出了反中心对称矩阵的伴随矩阵、特征值及特征向量的一些新结论. 相似文献
979.
《Angewandte Chemie (International ed. in English)》2017,56(40):12340-12343
Detailed knowledge of the electronic structure of vanadium oxide clusters provides the basis for understanding and tuning their significant catalytic properties. However, already for the simple four‐atom V2O2 molecule, there are contradictory reports in the literature regarding the electronic ground state and a possible vanadium–vanadium bond. We herein show through a combination of experimental (matrix isolation) studies and theoretical results that there is a multiple vanadium–vanadium bond in this benchmark vanadium oxide molecule. 相似文献
980.
Ganesh P. Sanganwar Ram B. Gupta Alexandre Ermoline James V. Scicolone Rajesh N. Dave 《Journal of nanoparticle research》2009,11(2):405-419
Due to the increased use of nanocomposites, mixing at nanoscale has become important. Current mixing techniques can be classified
into: (a) dry mixing (mechanical mixing), (b) wet mixing, and (c) simultaneous production of mixed nanoparticles (when possible).
Dry mixing is in general not effective in achieving desired mixing at nanoscale, whereas wet mixing suffers from different
disadvantages like nanomaterial of interest should be insoluble, has to wet the liquid, and involves additional steps of filtration
and drying. This paper examines the use of pressurized carbon dioxide having high density and low viscosity to replace the
liquids (e.g., n-hexane, toluene). Ultrasound is applied to the suspension of nanopowders in gaseous and supercritical carbon dioxide where
high impact collisions during sonication help mixing and the final mixture is obtained by simple depressurization. The method
is tested for binary mixture of alumina/silica, silica/titania, MWNT (multiwalled carbon nanotubes)/silica, and MWNT/titania.
The effects of sonication intensity and pressure on the degree of mixing are studied. Comparative study is also done with
liquid n-hexane as a mixing media. Quantitative characterization (e.g., mean composition standard deviation, intensity of segregation)
of mixing of alumina/silica and silica/titania is done with energy-dispersive X-ray spectroscopy, and that of MWNT/silica
and MWNT/titania is done using field-emission scanning electron microscopy and day-light illumination spectrophotometry. Results
show that mixing in carbon dioxide at higher ultrasound amplitudes is as good as in liquid n-hexane, and the final mixed product does not contain any residual media as in the case of liquid n-hexane. 相似文献