全文获取类型
收费全文 | 10435篇 |
免费 | 429篇 |
国内免费 | 87篇 |
专业分类
化学 | 7181篇 |
晶体学 | 94篇 |
力学 | 270篇 |
综合类 | 1篇 |
数学 | 1746篇 |
物理学 | 1659篇 |
出版年
2023年 | 77篇 |
2022年 | 101篇 |
2021年 | 194篇 |
2020年 | 209篇 |
2019年 | 250篇 |
2018年 | 159篇 |
2017年 | 161篇 |
2016年 | 386篇 |
2015年 | 290篇 |
2014年 | 322篇 |
2013年 | 609篇 |
2012年 | 616篇 |
2011年 | 654篇 |
2010年 | 456篇 |
2009年 | 407篇 |
2008年 | 589篇 |
2007年 | 630篇 |
2006年 | 499篇 |
2005年 | 547篇 |
2004年 | 443篇 |
2003年 | 352篇 |
2002年 | 337篇 |
2001年 | 163篇 |
2000年 | 145篇 |
1999年 | 101篇 |
1998年 | 103篇 |
1997年 | 112篇 |
1996年 | 114篇 |
1995年 | 97篇 |
1994年 | 86篇 |
1993年 | 101篇 |
1992年 | 82篇 |
1991年 | 80篇 |
1990年 | 77篇 |
1989年 | 67篇 |
1988年 | 54篇 |
1987年 | 59篇 |
1986年 | 73篇 |
1985年 | 107篇 |
1984年 | 105篇 |
1983年 | 83篇 |
1982年 | 69篇 |
1981年 | 72篇 |
1980年 | 68篇 |
1979年 | 64篇 |
1978年 | 68篇 |
1977年 | 42篇 |
1976年 | 40篇 |
1975年 | 51篇 |
1974年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Inhomogeneous Markov chains are a relevant framework for analysing event histories. The fundamental estimator in the presence
of incompletely observed data is the Nelson-Aalen estimator of the multivariate cumulative hazards. It may be summarised in
terms of probability estimates via the empirical transition matrix. The empirical transition matrix has only slowly entered
applications, one reason being previous lack of software. In a number of applications, further summary measures are desired.
We illustrate how they may be computed from the empirical transition matrix and why bootstrapping their variance works. In
contrast, computing such summaries outside the present framework has typically led to biased results. As an example, we consider
in more detail hospital stay following infectious complication. This summary quantity is often considered by clinical decision
makers, but reliable estimates require modelling the timing of infection as in the present set-up. In this context, we also
derive new summary measures that further distinguish between patients discharged and patients deceased. 相似文献
993.
994.
Excimer laser‐induced photolysis of gaseous diethyl selenium and diethyl tellurium (C2H5)2M (M = Se, Te) is controlled by cleavage of both M? C bonds, it yields C1–C4 hydrocarbons (ethene as major product) and results in chemical vapour deposition of selenium films and nanosized tellurium powder. The selenium and tellurium properties were characterized by X‐ray photoelectron spectroscopy and Scanning electron Microscopy techniques. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
995.
Frank Lenzen Florian Becker Jan Lellmann Stefania Petra Christoph Schnörr 《Computational Optimization and Applications》2013,54(2):371-398
We introduce a class of adaptive non-smooth convex variational problems for image denoising in terms of a common data fitting term and a support functional as regularizer. Adaptivity is modeled by a set-valued mapping with closed, compact and convex values, that defines and steers the regularizer depending on the variational solution. This extension gives rise to a class of quasi-variational inequalities. We provide sufficient conditions for the existence of fixed points as solutions, and an algorithm based on solving a sequence of variational problems. Denoising experiments with spatial and spatio-temporal image data and an adaptive total variation regularizer illustrate our approach. 相似文献
996.
Murari L. Gupta Stefanie A. Sydlik Jan M. Schnorr Dong Jin Woo Sebastian Osswald Timothy M. Swager Dharmaraj Raghavan 《Journal of Polymer Science.Polymer Physics》2013,51(6):410-420
Several solvent-free processing methods to disperse multiwalled carbon nanotubes (MWCNTs) in bisphenol F-based epoxy resin were investigated, including the use of a microfluidizer (MF), planetary shear mixer (PSM), ultrasonication (US) and combinations. The processed mixture was cured with diethyl toluene diamine. Three complimentary techniques were used to characterize the dispersion of the MWCNTs in cured composite samples: optical microscopy, micro Raman spectroscopy, and scanning electron microscopy (SEM). For sample MF + PSM, optical micrographs and Raman images showed reduced agglomeration and a homogeneous distribution of MWCNTs in the epoxy matrix. SEM analysis of fractured specimen after tensile testing revealed breakage of nanotubes along the fracture surface of the composite. A comparison of the MWCNT dispersion in the epoxy samples processed using different methods showed that a combination of MF and PSM processing yields a more homogeneous sample than the PSM or US + PSM processed samples. Mechanical testing of the composites showed about 15% improvement in the tensile strength of samples processed by the MF + PSM method over other methods. Thermogravimetric analysis (TGA) results showed a small decrease in the onset degradation temperature for poorly dispersed samples produced by PSM compared with the well-mixed samples (MF + PSM). These results strongly suggest that the MF + PSM processing method yield better-dispersed and stronger MWCNT/epoxy composites. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 相似文献
997.
998.
Steady-state nonlinear differential equations govering the stem curve of a wind-loaded pine are derived and solved numerically. Comparison is made between the results computed and the data from photographs of a pine stem during strong wind. The pine breaking is solved at the end. 相似文献
999.
Marcin Płosiński Tomasz Laskowski Paweł Sowiński Jan Pawlak 《Magnetic resonance in chemistry : MRC》2012,50(12):818-822
The absolute configurations of all the stereogenic centers of the antibiotic mycoheptin A2 were established upon previously elaborated general procedure, consisting of DQF‐COSY, NOESY, ROESY, HSQC and HMBC experiments as major tools. The structure of mycoheptin A2 without stereochemistry of its aglycone has been reported before. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
1000.
Transmission of light through a left-handed slab is studied theoretically. The slab consists of randomly distributed electric and magnetic scatterers. In a practical realization these could be wire and split ring resonators forming a disordered metamaterial. Enhanced transmission close to the middle of the slab is demonstrated. It is explained canalization of source image through self-collimated channels. Existence of self-collimated channels localized on the surface of the slab is demonstrated and a possibility of sub-wavelength imaging by such a system is discussed. 相似文献