全文获取类型
收费全文 | 4628篇 |
免费 | 582篇 |
国内免费 | 443篇 |
专业分类
化学 | 3403篇 |
晶体学 | 50篇 |
力学 | 257篇 |
综合类 | 36篇 |
数学 | 492篇 |
物理学 | 1415篇 |
出版年
2024年 | 18篇 |
2023年 | 73篇 |
2022年 | 125篇 |
2021年 | 167篇 |
2020年 | 152篇 |
2019年 | 169篇 |
2018年 | 152篇 |
2017年 | 128篇 |
2016年 | 216篇 |
2015年 | 186篇 |
2014年 | 230篇 |
2013年 | 326篇 |
2012年 | 323篇 |
2011年 | 365篇 |
2010年 | 252篇 |
2009年 | 245篇 |
2008年 | 275篇 |
2007年 | 248篇 |
2006年 | 250篇 |
2005年 | 178篇 |
2004年 | 169篇 |
2003年 | 146篇 |
2002年 | 174篇 |
2001年 | 131篇 |
2000年 | 103篇 |
1999年 | 110篇 |
1998年 | 95篇 |
1997年 | 84篇 |
1996年 | 52篇 |
1995年 | 71篇 |
1994年 | 60篇 |
1993年 | 50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5653条查询结果,搜索用时 15 毫秒
991.
Moliner M Willhammar T Wan W González J Rey F Jorda JL Zou X Corma A 《Journal of the American Chemical Society》2012,134(14):6473-6478
A new molecular sieve, ITQ-38, containing interconnected large and medium pores in its structure has been synthesized. The rational combination of dicationic piperidine-derivative molecules as organic structure directing agents (OSDAs) with germanium and boron atoms in alkaline media has allowed the synthesis of ITQ-38 zeolite. High-resolution transmission electron microscopy (HRTEM) has been used to elucidate the framework topology of ITQ-38, revealing the presence of domains of perfect ITQ-38 crystals as well as very small areas containing nanosized ITQ-38/ITQ-22 intergrowths. The structure of ITQ-38 is highly related to ITQ-22 and the recently described polymorph C of ITQ-39 zeolite. It shares a common building layer with ITQ-22 and contains the same building unit as the polymorph C of ITQ-39. All three structures present similar framework density, 16.1 T atoms/1000 ?(3). 相似文献
992.
Wan Qiang Cao 《Physica A》2012,391(4):1105-1110
The shoving model and the Vogel-Fulcher relation are employed to derive correlation of the fragility with the mechanical moduli for glass-forming simple liquids. The result shows that a liquid with smaller fragility will have larger ratio of K∞/G∞ in dilute liquid system. Based on radial distribution function with the Lennard-Jones potential modified by the Gaussian potential with a second minimum, fragility of the supercooled simple liquid is derived from the correlation between viscosity and shear modulus via configurational entropy. The results demonstrate that the fragility is determined by two parts: thermodynamic components and mechanical moduli. For a weak Gaussian potential liquid, the fragility is proportional to the Tg, while for a strong one, the fragility is inversely proportional to the Tg, and the Gaussian potential will increase fragility. 相似文献
993.
Qing-Bo Wang Guang Zheng Qi-Li Chen Miao Wan Xi-Cheng Wang 《Physica B: Condensed Matter》2012,407(4):719-723
We calculated the formation energy of single vacancy in V-doped ZnO in different conditions (oxygen or zinc rich) by first principles. Effect of an intrinsic vacancy on the electronic density of states and magnetic moment of V-doped ZnO (Zn15VO16) with and without single vacancy was also calculated. Our calculation was performed by the CASTEP program within spin-polarized GGA approximation implemented in materials studio software. The formation energy showed that oxygen vacancy inclined to stay far from vanadium (V) and zinc vacancy preferred to stay at a position near V. The calculated formation energy also showed that a zinc vacancy may automatically occur but an oxygen vacancy may not appear automatically. Vanadium doping introduced spin-polarization around Fermi level. For an energy favorable vacancy, an oxygen vacancy had little effect on the electronic density of states. A zinc vacancy made the spin-polarization peaks around Fermi level broaden and decreased their magnitude. For the magnetic moment in energy favorable configurations, an oxygen vacancy had little effect on the magnetic moment; a zinc vacancy significantly decreased the magnetic moment (as high as 63.7%). Changes in magnetic moments were consistent with electronic density of states. Our calculation may interpret various experimental magnetic moment values. Our work also provided a reference for preparing V-doped ZnO-based dilute magnetic semiconductors. 相似文献
994.
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
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.
研究设计了一个有效的可扩展的二维刻槽离子芯片。为了减少激光在离子芯片表面的散射,使被囚禁离子更加稳定,并使激光容易控制和探测成行的被囚禁离子,在每两个平行的射频电极中间刻槽使冷却光和探测光路径可穿过芯片。把控制离子运动的直流电极跟射频电极分开,减轻了不同电压对被囚禁离子的干扰,改进了对离子的控制。用有限元分析的方法对芯片表面上方的电势分布做了计算模拟。模拟结果表明,在这种新型的刻槽可扩展芯片上可以生成一个可扩展的离子阱阵列。这种结构提供了一个新颖的刻槽二维平面离子芯片,被囚禁其上的线形离子阵列可用来进行大型的量子信息处理。 相似文献