全文获取类型
收费全文 | 3547篇 |
免费 | 233篇 |
国内免费 | 179篇 |
专业分类
化学 | 2549篇 |
晶体学 | 48篇 |
力学 | 146篇 |
综合类 | 27篇 |
数学 | 363篇 |
物理学 | 826篇 |
出版年
2023年 | 42篇 |
2022年 | 77篇 |
2021年 | 117篇 |
2020年 | 117篇 |
2019年 | 120篇 |
2018年 | 102篇 |
2017年 | 62篇 |
2016年 | 141篇 |
2015年 | 145篇 |
2014年 | 144篇 |
2013年 | 211篇 |
2012年 | 312篇 |
2011年 | 327篇 |
2010年 | 186篇 |
2009年 | 202篇 |
2008年 | 234篇 |
2007年 | 189篇 |
2006年 | 173篇 |
2005年 | 155篇 |
2004年 | 98篇 |
2003年 | 97篇 |
2002年 | 103篇 |
2001年 | 41篇 |
2000年 | 56篇 |
1999年 | 44篇 |
1998年 | 37篇 |
1997年 | 28篇 |
1996年 | 40篇 |
1995年 | 27篇 |
1994年 | 17篇 |
1993年 | 27篇 |
1992年 | 28篇 |
1991年 | 17篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1988年 | 11篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1972年 | 7篇 |
排序方式: 共有3959条查询结果,搜索用时 15 毫秒
1.
Nonlinear Dynamics - The nonlinear modes of a non-conservative nonlinear system are sometimes referred to as damped nonlinear normal modes (dNNMs). Because of the non-conservative... 相似文献
2.
3.
近年来,机器学习等人工智能技术被应用于蛋白质工程,其在蛋白质结构、功能预测、催化活性等研究中具有独特优势。在未知蛋白质结构的情况下,将蛋白质序列和功能特性与机器学习相结合,基于序列-活性关系(innovative sequence-activity relationship,ISAR)算法,将蛋白质氨基酸序列数字化,用快速傅里叶变换(fast four transform,FFT)进行预处理,再进行偏最小二乘回归建模,可在数据集较少情况下拟合得到最佳模型。通过机器学习对紫色球杆菌视紫红质(gloeobacter violaceus rhodopsin,GR)的突变体蛋白质氨基酸序列与光谱最大吸收波长进行建模,获得了最佳模型。用最佳索引LEVM760106建模得到的确定系数R2 为0.944,均方误差E为11.64。用小波变换进行的预处理,其R2 虽也约为0.944,但E大于11.64,不及FFT进行的预处理。方法较好地解决了蛋白质序列与功能特性之间的数学建模问题,在蛋白质工程中可为预测更优的突变体提供支持。 相似文献
4.
This study examined how selected U.S. and Asian mathematics curricula are designed to facilitate students' understanding of the arithmetic average. There is a consistency regarding the learning goals among these curriculum series, but the focuses are different between the Asian series and the U.S. reform series. The Asian series and the U.S. commercial series focus the arithmetic average more on conceptual and procedural understanding of the concept as a computational algorithm than on understanding the concept as a representative of a data set; however, the two U.S. reform series focus the concept more on the latter. Because of the different focuses, the Asian and the U.S. curriculum series treat the concept differently. In the Asian series, the concept is first introduced in the context of “equal‐sharing” or “per‐unit‐quantity,” and the averaging formula is formally introduced at a very early stage. In the U.S. reform series, the concept is discussed as a measure of central tendency, and after students have some intuitive ideas of the statistical aspect of the concept, the averaging algorithm is briefly introduced. 相似文献
5.
A new high-birefringence (Hi-Bi) fiber torsion sensor demodulated by a Hi-Bi fiber Bragg grating (FBG) is proposed in this study. The twisted Hi-Bi fiber sensor characteristics are analyzed using the Jones matrix. The intensity ratio from two reflected Bragg wavelengths is associated with the twist angle of the measured Hi-Bi fiber. It is found that the twist angle resolution is estimated at around 0.3° under ±0.1 dB readout from an optical spectrum analyzer if the polarization state of the light source is stable. The advantages of this new torsion sensor are: (1) insensitivity to intensity variations from the light source, (2) insensitivity to the torsion gauge length, and (3) absolute measurement in the twist angle. However, the polarization state of light in the proposed method needs to be controlled, and any birefringence change in the twisted Hi-Bi fiber needs to be prevented. 相似文献
6.
R. Pizzoferrato L. Lagonigro T. Ziller A. Di Carlo R. Paolesse F. Mandoj A. Ricci C. Lo Sterzo 《Chemical physics》2004,300(1-3):217-225
We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φIR=4×10−4 is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er3+. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R0, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials. 相似文献
7.
8.
Journal of the Operational Research Society - Serious environmental problems have accompanied China's remarkable economic growth for decades, which also have direct and indirect impacts on its... 相似文献
9.
Jianhu Yang Shixun Dai Nengli Dai Lei Wen Lili Hu Zhonghong Jiang 《Journal of luminescence》2004,106(1):9-14
Nonradiative decay of 4I13/2→4I15/2 transition of Er3+ has been investigated in a series of oxide glasses. For Er3+-doped glass samples, the Judd-Ofelt analysis on absorption spectra was performed and the fluorescence lifetime was determined by extrapolating to zero Er3+ concentration limit. Infrared spectra were measured in order to investigate the influence of OH− groups in different glasses. The effects of glass matrix on the decay rate were discussed from the viewpoint of phonon energy, variations of effective fields, and OH− groups. Compared to other glasses, phosphate glass presents low quantum efficiency and large nonradiative decay rate due to its high phonon energy and hygroscopic behavior. 相似文献
10.
Tm3+/Yb3+-codoped germanate-niobic (GN) and germanium-bismuth (GB) glasses have been synthesized by conventional melting and quenching method. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions 1G4→3H6 and 1G4→3H4, respectively, were observed at room temperature. The possible up-conversion mechanisms are discussed and estimated. GN glass showed a weaker up-conversion emission than GB glass, which is inconsistent with the prediction from the difference of maximum phonon energy between GN and GB glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Compared with phonon side-band spectroscopy, Raman spectroscopy extracts more information including both phonon energy and phonon density. For the first time, our results reveal that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency. 相似文献