全文获取类型
收费全文 | 3350篇 |
免费 | 572篇 |
国内免费 | 354篇 |
专业分类
化学 | 2493篇 |
晶体学 | 57篇 |
力学 | 139篇 |
综合类 | 4篇 |
数学 | 357篇 |
物理学 | 1226篇 |
出版年
2024年 | 2篇 |
2023年 | 66篇 |
2022年 | 63篇 |
2021年 | 124篇 |
2020年 | 149篇 |
2019年 | 130篇 |
2018年 | 115篇 |
2017年 | 116篇 |
2016年 | 160篇 |
2015年 | 171篇 |
2014年 | 202篇 |
2013年 | 282篇 |
2012年 | 358篇 |
2011年 | 337篇 |
2010年 | 199篇 |
2009年 | 241篇 |
2008年 | 197篇 |
2007年 | 213篇 |
2006年 | 192篇 |
2005年 | 132篇 |
2004年 | 126篇 |
2003年 | 85篇 |
2002年 | 75篇 |
2001年 | 45篇 |
2000年 | 55篇 |
1999年 | 65篇 |
1998年 | 42篇 |
1997年 | 35篇 |
1996年 | 51篇 |
1995年 | 24篇 |
1994年 | 26篇 |
1993年 | 29篇 |
1992年 | 25篇 |
1991年 | 18篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 13篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有4276条查询结果,搜索用时 15 毫秒
41.
Interfacial bonding mechanism and bonding strength of AlTiCrN coating by cathodic arc ion plating 下载免费PDF全文
AlTiCrN coating was prepared on the surface of YT14 tungsten carbide cutting tools by cathodic arc ion plating with Ti, Al and Cr as targets. The surface morphologies, interface energy spectrum, phase and elements' binding energy of the coatings were observed with SEM, EDS, XRD and XPS, respectively, and bonding strength of the coating interface was measured with scratch tester. The results show that the phases of AlTiCrN coating are mainly composed of AlN, CrN and TiN, the crystal plane of (111) has a strong preferred orientation. The concentrations of Al, Ti, Cr, N in the coating are higher than those in the substrate, showing the gradient diffusion distribution at the bonding interface, while C atoms of the substrate have diffused into the lattices of TiN, AlN and CrN to form an obvious interdiffusion layer, and the average bonding strength of coating interface is 57.65 N. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
42.
通过酰氯制备异硫氰酸酯,与5-(4-氨基苄基)-2,4-咪唑啉二酮反应合成了含2,4-咪唑啉二酮的N-苯甲酰基-N'-苯基硫脲,然后在三乙胺存在下再与溴丙酮发生碱催化缩合反应以中等以上收率合成了新型含2,4-咪唑啉二酮的4-甲基-3-苯基-2-酰亚胺噻唑啉,它们的化学结构经1H NMR,IR,HR-ESI-MS和化合物3g和4g的X-ray单晶衍射表征,硫脲与溴丙酮的反应机理通过化合物3g和4g的晶体结构得到进一步确证.初步生物活性测定结果表明:部分目标化合物对供试昆虫及菌种显现出良好的抑制活性,如在200 mg/L浓度下化合物3i和4q对小菜蛾的死亡率分别为86%和100%,在50 mg/L浓度下化合物4n对油菜菌核菌的抑制率为82.6%,而它们对油菜和稗草显示出微弱但并不特征的除草活性. 相似文献
43.
Solventless Formation of G‐Quartet Complexes Based on Alkali and Alkaline Earth Salts on Au(111) 下载免费PDF全文
Chi Zhang Likun Wang Lei Xie Huihui Kong Prof. Dr. Qinggang Tan Liangliang Cai Qiang Sun Prof. Dr. Wei Xu 《Chemphyschem》2015,16(10):2099-2105
Template cations have been extensively employed in the formation, stabilization and regulation of structural polymorphism of G‐quadruplex structures in vitro. However, the direct addition of salts onto solid surfaces, especially under ultra‐high‐vacuum (UHV) conditions, to explore the feasibility and universality of the formation of G‐quartet complexes in a solventless environment has not been reported. By combining UHV‐STM imaging and DFT calculations, we have shown that three different G‐quartet‐M (M: Na/K/Ca) complexes can be obtained on Au(111) using alkali and alkaline earth salts as reactants. We have also identified the driving forces (intra‐quartet hydrogen bonding and electrostatic ionic bonding) for the formation of these complexes and quantified the interactions involved. Our results demonstrate a novel route to fabricate G‐quartet‐related complexes on solid surfaces, providing an alternative feasible way to bring metal elements to surfaces for constructing metal–organic systems. 相似文献
44.
Shuang-Shuang Kong Wei-Kai Liu Xiao-Xia Yu Ya-Lin Li Liu-Zhu Yang Yun Ma Xiao-Yong Fang 《Frontiers of Physics》2023,18(4):43302
Silicon carbide nanosheets (SiCNSs) have a very broad application prospect in the field of new two-dimensional (2D) materials. In this paper, the interlayer interaction mechanism of bilayer SiCNSs (BL-SiCNSs) and its effect on optical properties are studied by first principles. Taking the charge and dipole moment of the layers as parameters, an interlayer coupling model is constructed which is more convenient to control the photoelectric properties. The results show that the stronger the interlayer coupling, the smaller the band gap of BL-SiCNSs. The interlayer coupling also changes the number of absorption peaks and causes the red or blue shift of absorption peaks. The strong interlayer coupling can produce obvious dispersion and regulate the optical transmission properties. The larger the interlayer distance, the smaller the permittivity in the vertical direction. However, the permittivity of the parallel direction is negative in the range of 150-300 nm, showing obvious metallicity. It is expected that the results will provide a meaningful theoretical basis for further study of SiCNSs optoelectronic devices. 相似文献
45.
46.
Yongyue Wang Zhirong Cui Jun Chi Pengfei Tang Meihui Zhang Jixin Li Yongyi Li Hao Zhang Jun Luo Lingyi Kong 《中国化学》2021,39(1):129-136
Sarcaglarols A—D ( 1 — 4 ), two pairs of lindenane?monoterpene heterodimers fused by a 1,2‐dioxane moiety, were discovered and isolated from the leaves of Sarcandra glabra guided by MS/MS molecular networking‐based strategy. Their planar structures, absolute configurations of basic skeleton and flexible polyhydric side chain were established by analysis of HRESIMS, NMR spectroscopic data, ECD spectrum, and the X‐ray diffraction study of isopropylidene derivatives. An intermolecular [2+2+2] cycloaddition may play a key role in the biosynthesis pathway of the 1,2‐dioxane moiety fused lindenane?monoterpene heterodimer skeleton, which can be recognized as the biogenetic precursors of our previous reported lindenane?normonoterpene conjugates. In addition, compounds 1 , 3 and 4 exhibited moderate inhibitory effects of lipid accumulation in free fatty acid‐exposed L02 cells. 相似文献
47.
Aiqin Xiang Shuai Xie Fei Pan Hongchang Jin Yiheng Zhai Yanwu Zhu Xianghua Kong Hengxing Ji 《中国化学快报》2021,32(2):830-833
Electrical double-laye r capacitors are widely concerned fo r their high power density,long cycling life and high cycling efficiency.However,their wide application is limited by their low energy density.In this study,we propose a simple yet environmental friendly method to synthesize cobalt and nitrogen atoms co-doped porous carbon(CoAT-NC) material.Cobalt atoms connected with primarily pyridinic nitrogen atoms can be uniformly dispersed in the amorphous carbon matrix,which is benefit for improving electrical conductivity and density of states of the carbon material.Therefore,an enhanced perfo rmance is expected when CoAT-NC is served as electrode in a supercapacitor device.CoAT-NC displays a good gravimetric capacitance of 160 F/g at 0.5 A/g combing with outstanding capacitance retention of 90% at an extremely high current density of 100 A/g in acid electrolyte.Furthermore,a good energy density of30 Wh/kg can be obtained in the organic electrolyte. 相似文献
48.
Recent advances in the applications of transition metal chalcogenides/graphene (TMC/graphene) nanocomposites in future energy storage and conversion are reviewed. The synthesis processes and structures of TMC/graphene, workingpriciple of evergy energy device, and the electrochemical performances are summarized. 相似文献
49.
本文建立了一种同时检测DNA样品中Ethylidene-dG和Propano-dG含量的液相色谱-串联质谱(LC-MS/MS)方法,方法的精密度较好(RSD6%),检出限分别为0.010ng/mL和0.005ng/mL,回收率在97.2%~101.6%之间。同时,以体外小牛胸腺DNA为模型,选取不同乙醛暴露剂量(0、0.001、0.01、0.05、0.1、0.5和1.0mmol/L)和不同的暴露时间(0、2、4、10、12、20和24h),结果显示在体外Propano-dG加合物的生成需有氨基酸作为催化剂,且小牛胸腺DNA中乙醛-DNA加合物的含量随着染毒剂量和染毒时间的增加而升高,存在剂量-效应和时间-效应关系。 相似文献
50.
Stimuli-responsive polymers have undoubtedly been of great interest in the past decades due to a variety of their potential applications in biomedical territory. However, their non-degradability limits their in vivo applications. Herein, we report a novel pH and temperature dual-stimuli responsive-poly(β-amino ester). The pH/temperature sensitivities are interrelated and can be easily tuned by changing PEG-diacrylate chain length and the percentage of biamines in the feed ratio. These dual-responsive polymers are very useful in drug delivery. Reaction of methyl ether poly(ethylene glycol)(MPEG) and poly(β-amino ester) resulted in an amphiphilic MPEG-PBAE block copolymer which could form nanoparticles by selfassembly. A hydrophobic drug(DOX) was loaded in the self-assembled nanoparticles at low temperature without using organic solvents. The loaded drug was released very slowly and steadily at 37 ℃ under physiological conditions(p H 7.4), but rapidly released from the micelles in weakly acidic environments(pH 6.4 and 5.0) for intracellular drug release. Thus, these poly(β-amino ester) polymers constitute ideal drug carriers since their thermal sensitivity allows the drug loadings without using organic solvent and their pH sensitivity permits fast intracellular drug release. 相似文献