全文获取类型
收费全文 | 8193篇 |
免费 | 1237篇 |
国内免费 | 747篇 |
专业分类
化学 | 5680篇 |
晶体学 | 132篇 |
力学 | 484篇 |
综合类 | 72篇 |
数学 | 785篇 |
物理学 | 3024篇 |
出版年
2024年 | 33篇 |
2023年 | 179篇 |
2022年 | 309篇 |
2021年 | 312篇 |
2020年 | 373篇 |
2019年 | 350篇 |
2018年 | 300篇 |
2017年 | 253篇 |
2016年 | 411篇 |
2015年 | 401篇 |
2014年 | 528篇 |
2013年 | 605篇 |
2012年 | 672篇 |
2011年 | 693篇 |
2010年 | 457篇 |
2009年 | 374篇 |
2008年 | 498篇 |
2007年 | 425篇 |
2006年 | 379篇 |
2005年 | 321篇 |
2004年 | 214篇 |
2003年 | 218篇 |
2002年 | 166篇 |
2001年 | 124篇 |
2000年 | 154篇 |
1999年 | 151篇 |
1998年 | 129篇 |
1997年 | 131篇 |
1996年 | 152篇 |
1995年 | 110篇 |
1994年 | 108篇 |
1993年 | 89篇 |
1992年 | 78篇 |
1991年 | 69篇 |
1990年 | 72篇 |
1989年 | 47篇 |
1988年 | 49篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1970年 | 12篇 |
1937年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
151.
A detailed study of the various cosmological aspects in massive gravity theory has been presented in the present work. For the homogeneous and isotropic FLRW model, the deceleration parameter has been evaluated, and, it has been examined whether there is any transition from deceleration to acceleration in recent past, or not. With the proper choice of the free parameters, it has been shown that the massive gravity theory is equivalent to Einstein gravity with a modified Newtonian gravitational constant together with a negative cosmological constant. Also, in this context, it has been examined whether the emergent scenario is possible, or not, in massive gravity theory. Finally, we have done a cosmographic analysis in massive gravity theory. 相似文献
152.
在高压、高应变率加载条件下,孪晶变形对材料的塑性变形具有重要的贡献,而目前孪晶对金属材料的动态屈服强度、冲击响应等的影响还没有被充分揭示.为此,本文考虑孪晶变形和晶粒碎化,针对铍(Be)材料在高应变率加载下的动态力学响应发展了含孪晶的热弹-黏塑性晶体塑性模型.经过和实验结果的对比,发现该模型可以更准确地预测Be材料在动态加载下,尤其是高压动态加载下的屈服强度.进一步,基于该塑性模型研究了Be材料在冲击加载下的准弹性卸载行为,结果表明剪切波速随着压力和剪应变的变化而发生变化是材料产生准弹性卸载现象的主要原因.此外,研究了冲击波卸载过程中Be材料孪晶的演化过程,发现Be材料卸载过程中也伴随着孪晶的产生. 相似文献
153.
Qianqian Sun Yuwei Long Hongxi Li Shuang Pan Jidong Yang Shaopu Liu Xiaoli Hu 《Journal of fluorescence》2018,28(2):523-531
Caffeic acid (CA), a familiar color stabilizing reagent, has aroused general concern due to its uncontrolled addition, and thus the detection of CA is increasingly important. In our report, the bright carbon dots (CDs) were prepared via hydrothermal treatment with urea and citric acid act as raw material and their characteristics were discussed through X-ray diffraction (XRD), transmission electron microscopy (TEM) and so on. Impressively, the strong emission of the as-prepared CDs (Quantum Yield: 24.3%) decreased sharply upon a full reaction with the added CA. Hence, we first present an improved strategy for determining CA based upon the quenching of the strong emission of CDs. In this strategy, 0.79–100.0 µmol L??1 caffeic acid could be simply detected, and a detection limit of 0.24 µmol L??1 was allowed. Additionally, CA in red wine samples can be successfully detected by this method and the exploration of the quenching mechanism of the CA-CDs system was done. 相似文献
154.
Phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls 下载免费PDF全文
The phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls are investigated by using a cell dynamics simulation of the time-dependent Ginzburg-Landau theory.The morphological dependence of the wall-block interaction and the distance between walls(confinement degree) has been systematically studied,and the effect of repulsive interactions between different monomers is also discussed.It is interesting that multiple novel morphological transitions are observed by changing these factors,and various multilayered sandwich structures are formed in the mixture.Furthermore,the parametric dependence and physical reasons for the microdomain growth and orientational order transitions are discussed.From the simulation,we find that much richer morphologies can form in a binary mixture of diblock copolymers than those in a pure diblock copolymer.Our results provide an insight into the phase behaviors under parallel wall confinement and may provide guidance for experimentalists.This model system can also give a simple way to realize orientational order transition in soft materials through confinement. 相似文献
155.
The structural, elastic, and electronic properties of NiAl alloyed with rare earth elements Pr, Pm, Sm, and Eu are investigated by using density functional theory (DFT). The study suggests that Pr, Pm, Sm, and Eu are all tend to be substituted for Al site. Ni8Al7Pm possesses the largest ductility. Only the hardness and ductility of Ni8Al7Eu are enhanced simultaneously. The covalency strength of Ni-Al bond in Ni8Al7Pm is higher than that in Ni8Al7Eu. The covalency strength of Al-Al bond and that of Ni-Ni bond in Ni8Al7Eu are higher than that in Ni8Al7Pm. Ni-Pm bond and Ni-Eu bond are covalent, and the covalency strength of Ni-Pm bond is greater. Al-Pm bond and Al-Eu bond show great covalency strength and ionicity, respectively. 相似文献
156.
Yan Xia Zhinong Gao Xueming Liao Wanying Wei Chenchen Pan 《Journal of nanoparticle research》2017,19(8):290
Acanthosphere-like gold microstructures (AGMs) were synthesized using a facile, two-step, seed-mediated method and butanediyl-1,4-bis(dimethylhexadecylammonium bromide) (16-4-16) as a structure-directing agent. The morphologies and sizes of the products were controlled during the synthesis process by adjusting the concentrations of 16-4-16, the AgNO3 feed, HAuCl4, ascorbic acid, the amount of Ag seeds and the types of gemini surfactants used through systematic inquiry; particle sizes ranging from 130 to 800 nm were well prepared. Correspondingly, the morphology of the products changed between regular and irregular AGMs, and the products presented a number of new morphologies, such as open-mouthed submicrostructures and ribbon nanowires. In particular, with the increase in the 16-4-16 concentration, the structural morphology of the thorns clearly changed from a tip to a lamellar structure. A UV-vis spectroscopic analysis indicated that the localized surface plasmon resonance (LSPR) peak of the AGMs could be adjusted by changing the above factors, which extended from 500 to 1350 nm in the near-infrared (NIR) region, enabling a tremendous potential for using the AGMs as platforms for various biomedical applications. Based on the intermediate products, we propose a two-stage growth mechanism for the AGMs in which their solid cores and tips are generated successively. Surface-enhanced Raman scattering (SERS) measurements indicate that the AGMs can serve as sensitive SERS substrates; a SERS detection limit of 5 × 10?7 M is presented for rhodamine B molecules. 相似文献
157.
158.
Coplanar Phase Shifters Based on Ferroelectric Thin Films 总被引:2,自引:0,他引:2
Chunlin Fu Fusheng Pan Hongwei Chen Shucheng Feng Wei Cai Chuanren Yang 《International Journal of Infrared and Millimeter Waves》2007,28(3):229-235
Barium strontium titanate (Ba0.6Sr0.4TiO3, short for BST) thin films were prepared by RF-magnetron sputtering. The dielectric tunability, loss tangent, remanent polarization
(2Pr) and coercive electric field (Ec) of the BST films are respectively 29.5%, 0.013, 2.29 μC/cm2 and 22.27 kV/cm at 1 kHz and 20 V. The designed coplanar waveguide (CPW) phase shifter consists of 56 same sections. It is
shown that the fabricated lines of electrodes are smooth and the widths at the tip of bottom electrodes are about 5 μm. At
the central frequency of 28 GHz and a bias voltage of 20 V, the figure of merit is approximately 13°/dB. 相似文献
159.
160.
采用低压金属有机化合物气相沉积法(LP-MOCVD)生长并制作了1.6—1.7μm大应变InGaAs/InGaAsP分布反馈激光器.采用应变缓冲层技术,得到质量良好的大应变InGaAs/InP体材料.器件采用了4个大应变的量子阱,加入了载流子阻挡层改善器件的温度特性.1.66μm和1.74μm未镀膜的3μm脊型波导器件阈值电流低(小于15mA),输出功率高(100mA时大于14mW).从10—40℃,1.74μm激光器的特征温度T0=57K,和1.55μm InGaAsP分布反馈激光器的特征温度相当.
关键词:
MOCVD
InGaAs/InGaAsP
应变量子阱
分布反馈激光器 相似文献