全文获取类型
收费全文 | 15377篇 |
免费 | 1846篇 |
国内免费 | 1689篇 |
专业分类
化学 | 7970篇 |
晶体学 | 152篇 |
力学 | 2334篇 |
综合类 | 97篇 |
数学 | 5351篇 |
物理学 | 3008篇 |
出版年
2023年 | 210篇 |
2022年 | 202篇 |
2021年 | 345篇 |
2020年 | 607篇 |
2019年 | 462篇 |
2018年 | 426篇 |
2017年 | 478篇 |
2016年 | 696篇 |
2015年 | 629篇 |
2014年 | 824篇 |
2013年 | 1377篇 |
2012年 | 889篇 |
2011年 | 980篇 |
2010年 | 672篇 |
2009年 | 960篇 |
2008年 | 967篇 |
2007年 | 1009篇 |
2006年 | 907篇 |
2005年 | 773篇 |
2004年 | 692篇 |
2003年 | 649篇 |
2002年 | 517篇 |
2001年 | 499篇 |
2000年 | 417篇 |
1999年 | 325篇 |
1998年 | 323篇 |
1997年 | 281篇 |
1996年 | 267篇 |
1995年 | 227篇 |
1994年 | 180篇 |
1993年 | 166篇 |
1992年 | 109篇 |
1991年 | 103篇 |
1990年 | 81篇 |
1989年 | 65篇 |
1988年 | 62篇 |
1987年 | 53篇 |
1986年 | 64篇 |
1985年 | 39篇 |
1984年 | 55篇 |
1983年 | 16篇 |
1982年 | 29篇 |
1981年 | 30篇 |
1980年 | 20篇 |
1979年 | 29篇 |
1978年 | 42篇 |
1977年 | 43篇 |
1976年 | 40篇 |
1975年 | 15篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
21.
假设纤维方向角沿层合板的长度方向线性变化,研究了变角度纤维复合材料层合斜板的颤振.通过坐标变换将斜板变换为正方形板,采用层合板表面连续变化的速度环量来模拟空气对其的作用,速度环量分布利用Cauchy积分公式计算.建立了系统的Lagrange方程并采用Ritz法得到了层合板的自振频率和颤振/不稳定性分离临界速度.通过数值算例验证了本文模型和方法的正确性和收敛性,分析了各个铺层内纤维方向角的变化对自振频率和颤振/不稳定性分离临界速度的影响.研究结果表明,通过纤维的变角度铺设,可有效地提高层合板的基频和颤振/不稳定性分离临界速度.经合理设计的变角度复合材料层合板具有抑制颤振的作用. 相似文献
22.
23.
Motor Imagery Electroencephalography (MI-EEG) has shown good prospects in neurorehabilitation, and the entropy-based nonlinear dynamic methods have been successfully applied to feature extraction of MI-EEG. Especially based on Multiscale Fuzzy Entropy (MFE), the fuzzy entropies of the τ coarse-grained sequences in τ scale are calculated and averaged to develop the Composite MFE (CMFE) with more feature information. However, the coarse-grained process fails to match the nonstationary characteristic of MI-EEG by a mean filtering algorithm. In this paper, CMFE is improved by assigning the different weight factors to the different sample points in the coarse-grained process, i.e., using the weighted mean filters instead of the original mean filters, which is conductive to signal filtering and feature extraction, and the resulting personalized Weighted CMFE (WCMFE) is more suitable to represent the nonstationary MI-EEG for different subjects. All the WCMFEs of multi-channel MI-EEG are fused in serial to construct the feature vector, which is evaluated by a back-propagation neural network. Based on a public dataset, extensive experiments are conducted, yielding a relatively higher classification accuracy by WCMFE, and the statistical significance is examined by two-sample t-test. The results suggest that WCMFE is superior to the other entropy-based and traditional feature extraction methods. 相似文献
24.
25.
26.
Reza Abbasi Jafar Mostafavi Amjad Hamed Nosrati Ramin Mohammadkhani Hosein Danafar 《应用有机金属化学》2020,34(10):e5825
Nowadays, nanostructures have been given significant attention in medical and biological fields. Among these nanostructures, graphene oxide (GO) has been widely used in drug delivery systems, because of its unique properties, and the ability to connect to other nanostructures such as magnetic nanoparticles (NPs) as well as polymers by its functional groups. In this research, first, GO was prepared by exfoliating graphite according to the modified Hummer’s method, and then the Fe3O4 NPs were synthesized by a simple co-precipitation method on GO nanosheets. In the next step, with the help of the ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide coupling reagents, the polyethylene glycol (PEG) polymer was bonded to the GO-Fe3O4 nanocomposite. Finally, anti-cancer drug, curcumin (Cur) was loaded onto the nanocomposite and the Cur loading ratio was measured at about 8%. The samples were evaluated using Fourier transform-infrared, differential scanning calorimtery, vibrating-sample magnetometry, atomic force microscopy and dynamic light scattering techniques. The results show that the prepared nanocomposite is an appropriate candidate for biomedical applications. 相似文献
27.
Olga Bakhanovich Dr. Petr Beier 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(4):773-782
Organic α-fluorinated azidoalkanes appeared in the literature for the first time half a century ago. However, for a long time they remained undeveloped and were regarded as chemical curiosities. Recent advances in the preparation of α-fluorinated azidoalkanes as well as studies on their stability and reactivity opened up their broader synthetic potential for the preparation of valuable fluorinated and non-fluorinated compounds. 相似文献
28.
Fang Xu Longjian Xie Akira Yoneda Nicolas Guignot Andrew King Guillaume Morard 《高压研究》2020,40(2):257-266
ABSTRACTTiC-MgO composite was developed as a heating element for X-ray study in the multi-anvil high pressure apparatus. We synthesized TiC-MgO blocks (50–70 wt.% of TiC) by compression in a cold isostatic press followed by baking in a gas flow furnace. Heaters of tubular shape were manufactured from the synthesized blocks either by lathe or numerically controlled milling machine. The so-produced heating elements have been proved to generate temperatures up to 2250?K at 10?GPa, condition where classical graphite heaters are not suitable anymore due to graphite-diamond transition. These new heaters have been successfully used for in situ X-ray radiography and diffraction measurements on liquid Fe alloys, exploiting excellent X-ray transparency. 相似文献
29.
We calculate the energy levels corresponding to the Roper resonance based on a two-flavor chiral effective Lagrangian for pions, nucleons, deltas, and the Roper resonance at the leading one-loop order. We show that the Roper mass can be extracted from these levels for lattice volumes of moderate size. 相似文献
30.