全文获取类型
收费全文 | 4677篇 |
免费 | 622篇 |
国内免费 | 459篇 |
专业分类
化学 | 3483篇 |
晶体学 | 50篇 |
力学 | 259篇 |
综合类 | 37篇 |
数学 | 492篇 |
物理学 | 1437篇 |
出版年
2024年 | 18篇 |
2023年 | 76篇 |
2022年 | 131篇 |
2021年 | 166篇 |
2020年 | 158篇 |
2019年 | 172篇 |
2018年 | 153篇 |
2017年 | 131篇 |
2016年 | 222篇 |
2015年 | 190篇 |
2014年 | 231篇 |
2013年 | 332篇 |
2012年 | 326篇 |
2011年 | 370篇 |
2010年 | 252篇 |
2009年 | 250篇 |
2008年 | 283篇 |
2007年 | 254篇 |
2006年 | 260篇 |
2005年 | 186篇 |
2004年 | 172篇 |
2003年 | 151篇 |
2002年 | 176篇 |
2001年 | 133篇 |
2000年 | 104篇 |
1999年 | 113篇 |
1998年 | 95篇 |
1997年 | 86篇 |
1996年 | 52篇 |
1995年 | 71篇 |
1994年 | 63篇 |
1993年 | 51篇 |
1992年 | 51篇 |
1991年 | 31篇 |
1990年 | 36篇 |
1989年 | 15篇 |
1988年 | 18篇 |
1987年 | 12篇 |
1986年 | 26篇 |
1985年 | 24篇 |
1984年 | 17篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1977年 | 4篇 |
1974年 | 7篇 |
1973年 | 7篇 |
1969年 | 4篇 |
排序方式: 共有5758条查询结果,搜索用时 15 毫秒
991.
Z.-C. Fu J. Dai T. Li H.-Y. Liu Q.-F. Dai L.-J. Wu S. Lan S.-L. Tie X. Wan A. V. Gopal V. A. Trofimov T. M. Lysak 《Applied physics. B, Lasers and optics》2012,108(1):61-66
Femtosecond laser ablation based on two-photon absorption was employed to cut ZnO nanorods into uniform ZnO nanoparticles of deep subwavelength size. The fabricated ZnO nanoparticles possess a shorter mean transport length for photons at the emission wavelength and a much smaller scattering cross section at the pump wavelength, leading to highly efficient two-photon-pumped random lasing with a low threshold of ??8?mJ/cm2. It was demonstrated that the significant enhancement in two-photon luminescence after the irradiation of femtosecond laser pulses could also be utilized for realizing optical data storage. 相似文献
992.
In this paper, we consider there exist two types of fundamental quasihole excitation in the fractional quantum spin Hall state and investigate their topological properties by both Chern-Simons field theory and the Berry phase technique. By the two different techniques, we obtain the identical charge and statistical angle for each type of quasihole, as well as the identical mutual statistics between two different types of quasihole excitation. 相似文献
993.
The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire. 相似文献
994.
We report an experimental/theoretical study of single-crystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50?mK; (2) strongly field-dependent coefficients of the low-temperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W)?≈?53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80?K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7). 相似文献
995.
Ji ZF You JL Simon P Wang YY Hou M Wang LH Zhang GC Wan SM Fu PZ Wu YC Yin ST 《光谱学与光谱分析》2012,32(1):127-131
测量了BiB3O6晶体从常温到熔融态的拉曼光谱,考察了其温致结构变化和预测了熔体的结构基元。同时,采用量子化学从头计算和密度泛函理论对拉曼光谱进行了计算和结构分析。结果表明,随着温度的升高,对包含阳离子Bi的外部晶格和[BO3]3-硼氧三角形基团结构的温致结构影响有限,却有效地削弱了[BO4]5-硼氧四面体基团结构,其松弛度增加,稳定性变差,并在熔体结构中彻底消失。研究表明BiB3O6熔体主要以硼氧六元环[B6O12]6-的形式存在,阳离子Bi起电荷平衡作用,其氧配位数为3,有别于晶体中的6。 相似文献
996.
Zainul Akmar Zakaria Wan Azlina Ahmad Zainoha Zakaria Firdausi Razali Norsuhada Abdul Karim Mohamad Md Sum Mohd. Saufi Mohd. Sidek 《Applied biochemistry and biotechnology》2012,167(6):1641-1652
The bacterial reduction of Cr(VI) from industrial wastewater was evaluated using a 2.0-m3 bioreactor. Liquid pineapple waste was used as a nutrient for the biofilm community formed inside the bioreactor. The use of rubber wood sawdust as packing material was able to immobilize more than 106?CFU?mL?1 of Acinetobacter haemolyticus cells after 3?days of contact time. Complete reduction of 15?C240?mg?L?1 of Cr(VI) was achieved even after 3?months of bioreactor operation. Cr(VI) was not detected in the final effluent fraction indicating complete removal of Cr from solution from the flocculation/coagulation step and the unlikely re-oxidation of Cr(III) into Cr(VI). Impatiens balsamina L. and Gomphrena globosa L. showed better growth in the presence of soil?Csludge mixture compared to Coleus scutellarioides (L.) Benth. Significant amounts of Cr accumulated at different sections of the plants indicate its potential application in Cr phytoremediation effort. The bacterial-based system was also determined not to be detrimental to human health based on the low levels of Cr detected in the hair and nail samples of the plant operators. Thus, it can be said that bacterial-based Cr(VI) treatment system is a feasible alternative to the conventional system especially for lower Cr(VI) concentrations, where sludge generated can be used as growth supplement for ornamental plant as well as not detrimental to the health of the workers. 相似文献
997.
Wan Kamarul Zaman Wan Safwani Suzana Makpol Somasundaram Sathapan Kien Hui Chua 《Applied biochemistry and biotechnology》2012,166(8):2101-2113
Human adipose-derived stem cells (ASCs) have generated a great deal of excitement in regenerative medicine. However, their safety and efficacy issue remain a major concern especially after long-term in vitro expansion. The aim of this study was to investigate the fundamental changes of ASCs in long-term culture by studying the morphological feature, growth kinetic, surface marker expressions, expression level of the senescence-associated genes, cell cycle distribution and ß-galactosidase activity. Human ASCs were harvested from lipoaspirate obtained from 6 patients. All the parameters mentioned above were measured at P5, P10, P15 and P20. Data were subjected to one-way analysis of variance with a Tukey post hoc test to determine significance difference (P?0.05). The data showed that growth of ASCs reduced in long-term culture and the ß-galactosidase activity was significantly increased at later passage (P20). The morphology of ASCs in long-term culture showed the manifestation of senescent feature at P15 and P20. Significant alteration in the senescence-associated genes expression levels was observed in MMP1, p21, Rb and Cyclin D1 at P15 and P20. Significant increase in CD45 and HLA DR DQ DP surface marker was observed at P20. While cell cycle analysis showed significant decrease in percentage of ASCs at S and G2/M phase at later passage (P15). Our data showed ASCs cultured beyond P10 favours the senescence pathway and its clinical usage in cell-based therapy may be limited. 相似文献
998.
The effect of ultrahigh molecular weight polyacrylonitrile (UHMWPAN) on the shear and extensional rheological behavior of
PAN solutions were studied. The PAN solutions were prepared by dissolving medium molecular weight polyacrylonitrile in dilute
UHMWPAN/dimethyl sulfoxide solutions. The results of shear rheological measurements indicated that the existence of UHMWPAN
reduced the shear-thinning but increased the characteristic relaxation time and the elasticity of PAN solutions. Moreover,
the PAN solutions containing UHMWPAN exhibited much more evident strain-hardening behavior than the solution without UHMWPAN.
It was found from the results of extensional rheological measurements that the strain hardening and elasticity of PAN solutions
increased greatly with the increase of molecular weight or content of UHMWPAN in the solutions. PAN solutions containing a
small amount of UHMWPAN have better drawability and favor the increase of jet stretch ratio in dry-jet wet spinning of PAN
precursor fibers. 相似文献
999.
1000.
随着能源危机的加剧,太阳能电池作为开发和利用太阳能的一种普遍形式, 日益受到世界各国的重视.随着太阳能电池向着高效率、薄膜化、无毒性和原材料丰富的方向发展, 单纯的硅系太阳能电池已经无法达到这样的要求,因此新的材料和工艺的开发利用迫在眉睫. 本文研究了碳材料在硅异质节上实现光伏效应的改善及其可能在太阳能电池上的应用. 采用脉冲激光沉积方法制备的Co2-C98/Al2O3/Si异质结构在标准日光照射 (AM1.5, 100 mW/cm2)条件下,可获得0.447 V的开路电压和18.75 mA/cm2的电流密度, 转换效率可达3.27%.通过电容电压特性和暗条件下的电输运性能测量, 证明了氧化铝层的引入不但对单晶硅的表面起到了物理钝化作用,减小了反向漏电流, 使异质结界面缺陷、界面能级和复合中心减少,还起到了场效应钝化作用, 增加了异质结界面的势垒高度,增加了开路电压,使异质结的光伏效应显著增强. 相似文献