全文获取类型
收费全文 | 7487篇 |
免费 | 1308篇 |
国内免费 | 1134篇 |
专业分类
化学 | 6143篇 |
晶体学 | 105篇 |
力学 | 367篇 |
综合类 | 117篇 |
数学 | 866篇 |
物理学 | 2331篇 |
出版年
2024年 | 25篇 |
2023年 | 146篇 |
2022年 | 269篇 |
2021年 | 290篇 |
2020年 | 353篇 |
2019年 | 349篇 |
2018年 | 323篇 |
2017年 | 283篇 |
2016年 | 371篇 |
2015年 | 414篇 |
2014年 | 466篇 |
2013年 | 579篇 |
2012年 | 654篇 |
2011年 | 689篇 |
2010年 | 531篇 |
2009年 | 483篇 |
2008年 | 531篇 |
2007年 | 415篇 |
2006年 | 367篇 |
2005年 | 320篇 |
2004年 | 250篇 |
2003年 | 238篇 |
2002年 | 273篇 |
2001年 | 221篇 |
2000年 | 159篇 |
1999年 | 132篇 |
1998年 | 95篇 |
1997年 | 62篇 |
1996年 | 102篇 |
1995年 | 87篇 |
1994年 | 60篇 |
1993年 | 49篇 |
1992年 | 45篇 |
1991年 | 34篇 |
1990年 | 43篇 |
1989年 | 25篇 |
1988年 | 23篇 |
1987年 | 24篇 |
1986年 | 12篇 |
1985年 | 23篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1977年 | 6篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1972年 | 11篇 |
1971年 | 6篇 |
排序方式: 共有9929条查询结果,搜索用时 46 毫秒
991.
采用常见元器件等效实现一个广义忆阻器, 进而制作出一个电路特性可靠的非线性电路, 有助于忆阻混沌电路的非线性现象的实验展示及其所产生的混沌信号的实际工程应用. 基于忆阻二极管桥电路, 构建了一种无接地限制的、易物理实现的一阶有源广义忆阻模拟器; 由该模拟器并联电容后与RC桥式振荡器线性耦合, 实现了一种无电感元件的忆阻混沌电路; 建立了无感忆阻混沌电路的动力学模型, 开展了相应的耗散性、平衡点、稳定性和动力学行为等分析. 结果表明, 无感忆阻混沌电路在相空间中存在分布2个不稳定非零鞍焦的耗散区和包含1个不稳定原点鞍点的非耗散区; 当元件参数改变时, 无感忆阻混沌电路有着共存分岔模式和共存吸引子等非线性行为. 研制了实验电路, 该电路结构简单、易实际制作, 实验测量和数值仿真两者结果一致, 验证了理论分析的有效性. 相似文献
992.
折射是影响辐射传输的重要因素. 为分析大气折射对辐射传输的影响, 基于Monte Carlo方法, 给出了考虑大气折射的矢量辐射传输模型, 实现了均匀气层和耦合面处光子随机运动过程的模拟, 实现了直射光及漫射光Stokes矢量、偏振度和辐射通量等参数的计算. 在考虑和不考虑大气折射两种条件下, 验证了模型的准确性; 在纯瑞利散射条件下, 讨论了大气折射对不同方向漫射光Stokes矢量的影响; 在不同太阳天顶角、大气廓线、气溶胶及含云大气条件下, 分析了大气折射对辐射传输过程的影响. 结果表明: 大气折射对漫射光Stokes矢量的影响主要体现在天顶角70°–110°区间, 且随着太阳入射角增大, 其影响更为显著; 不同大气廓线情形下, 大气折射对Stokes矢量的影响不一致, 其原因是不同大气廓线对应的折射率廓线存在差异. 含云及含气溶胶大气条件下, 大气折射对辐射传输的影响变弱, 沙尘型及海盐型气溶胶条件下, 折射对辐射传输的影响强于可溶型气溶胶情形; 不同形状气溶胶条件下, 大气折射对辐射传输的影响也存在显著差异; 不同云高条件下, 大气折射对漫射光Stokes矢量的影响无显著差异, 但随着云光学厚度增大, 大气折射的影响减弱. 相似文献
993.
994.
For the first time, a diffusive gradients in thin films (DGT) device using molecularly imprinted polymer (MIP) as the binding agent and nylon membrane (NM) as the diffusive layer (NM-MIP-DGT) has been developed for sampling 4-chlorophenol (4-CP) in water. The MIP was prepared by precipitation polymerization with methacrylic acid as monomer and ethyleneglycoldimethacrylate as cross-linker. The diffusion coefficient of 4-CP through NM was obtained to be 0.788 ± 0.040 μ cm2 s−1 by diffusion cell method. The ratio was 1.01 ± 0.05 (mean ± standard deviation) for the concentration of 4-CP sampled by NM-MIP-DGT and analyzed by HPLC method to the total concentration of 4-CP in the synthetic solution where free 4-CP species dominated. The results showed that NM-MIP-DGT could sample 4-CP in synthetic solution accurately. The performance of NM-MIP-DGT for sampling 4-CP was independent of pH in the range of 3–7 and ionic strength in the range of 0.0001–0.1 mol L−1 NaCl solution. The concentration of free form of 4-CP sampled by NM-MIP-DGT decreased with the increasing concentration of dissolved organic carbon in different water samples due to the electrostatic interaction of natural organic compounds with 4-CP. 1.8 mg L−1 of the free form of 4-CP was determined by HPLC which was sampled by NM-MIP-DGT in an intermediate untreated industrial effluent. The NM-MIP-DGT can be a potential passive tool for sampling the free form of 4-CP in water. 相似文献
995.
A new eudesmanolide, 1‐oxo‐11αH‐eudesma‐2,4(14)‐dien‐12,8β‐olide ( 1 ), and four new guaianolides, 9β,10β‐epoxy‐4α‐hydroxy‐1βH,11αH‐guaian‐12,8α‐olide ( 2 ), 9β,10β‐epoxy‐4α‐hydroxy‐1βH,11βH‐guaian‐12,8α‐olide ( 3 ), 4α,9α‐dihydroxy‐1βH,11αH‐guai‐10(14)‐en‐12,8α‐olide ( 4 ), and 4α,9α‐dihydroxy‐1βH,11βH‐guai‐10(14)‐en‐12,8α‐olide ( 5 ), together with one known eudesmanolide and two known germacranolides, were isolated from the whole plants of Carpesium triste. Their structures and relative configurations were elucidated on the basis of spectroscopic methods, including 2D‐NMR techniques. 相似文献
996.
The Crystal Structure and Synthesis Mechanism of 3,6‐Bis(3,5‐dimethylpyrazol‐1‐yl)‐1,4‐dihydro‐1,2,4,5‐tetrazine (BDT): A Key Precursor of S‐tetrazine
下载免费PDF全文
![点击此处可从《Journal of heterocyclic chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jian‐Guo Zhang Jin‐Ting Wu Mou Sun Jin‐Ling Feng Tong‐Lai Zhang Zun‐Ning Zhou 《Journal of heterocyclic chemistry》2014,51(Z1):E234-E240
The single crystal structure of 1,1′‐bis (3,5‐dimethyl‐pyrazole) methenehydrazine (BDM) was determined by X‐ray single crystal diffraction for the first time. The obtained experimental results indicated that BDM was the intermediate of 3,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,4‐dihydro‐1,2,4,5‐tetrazine (BDT), which was the key precursor of s‐tetrazine. By this evidence, the preparation mechanism of BDT was proved: At 318.15 K, triaminoguanidine and pentanedione reacted to achieve the intermediate (BDM) by molecular nucleophilic addition and intramolecular nucleophilic substitution; when heated to 363.15 K, BDT was then generated by two molecules of BDM with nucleophilic substitution reaction. Furthermore, the thermal decomposition properties and also the non‐isothermal kinetic parameters have been investigated in the present work. 相似文献
997.
A Catalyst‐Free Aqueous Synthesis of 2‐Amino‐7,9‐dihydrothieno[3′,2′:5,6]pyrido[2,3‐d]pyrimidine‐4,6(3H,5H)‐dione Derivatives
下载免费PDF全文
![点击此处可从《Journal of heterocyclic chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Changsheng Yao Ting Lu Chenxia Yu Xiangshan Wang Tuanjie Li 《Journal of heterocyclic chemistry》2014,51(Z1):E163-E166
A series of novel 7,9‐dihydrothieno[3′,2′:5,6]pyrido[2,3‐d]pyrimidine‐4,6(3H,5H)‐dione derivatives were synthesized efficiently via the catalyst‐free reaction of aldehyde, ethyl 2,4‐dioxotetrahydrothiophene‐3‐carboxylate, and 2,6‐diaminopyrimidine‐4(3H)‐one through the sequence of deethoxycarbonylation and three‐component condensation in aqueous media. This protocol featured mild reaction conditions, high yields, easy work‐up, and environmentally friendly procedure. 相似文献
998.
We have studied the trans-membrane electron transfer in human red blood cells (RBCs) immobilized in a chitosan film on a glassy carbon electrode (GCE). Electron transfer results from the presence of hemoglobin (Hb) in the RBCs. The electron transfer rate (k s) of Hb in RBCs is 0.42 s?1, and <1.13 s?1 for Hb directly immobilized in the chitosan film. Only Hb molecules in RBCs that are closest to the plasma membrane and the surface of the electrode can undergo electron transfer to the electrode. The immobilized RBCs displayed sensitive electrocatalytic response to oxygen and hydrogen peroxide. It is believed that this cellular biosensor is of potential significance in studies on the physiological status of RBCs based on observing their electron transfer on the modified electrode.
The transmembrane electron transfer rate of Hb in RBCs is slower than hemoglobin molecules directly immobilized on the chitosan film. Only those hemoglobin in RBCs closest to the plasma membrane and electrode could exchange electrons with the electrode. The immobilized RBCs showed sensitive electrocatalytic response to O2 and H2O2. 相似文献
999.
Ting Liu Xian Chen Cheng-Yi Hong Xiao-Ping Xu Huang-Hao Yang 《Mikrochimica acta》2014,181(7-8):731-736
Electrochemiluminescence (ECL) integrates the advantages of electrochemical detection and chemiluminescent techniques. The method has received particular attention because it is highly sensitive and selective, has a wide linear range but low reagent costs. The use of nanomaterials with their unique physical and chemical properties has led to new kinds of biosensors that exhibit high sensitivity and stability. Compared to other nanomaterials, DNA nanostructures are more biocompatible, more hydrophilic, and thus less prone to nonspecific adsorption onto the electrode surface. We describe here a label-free and ultrasensitive ECL biosensor for detecting a cancer-associated microRNA at a femtomolar level. We have designed two auxiliary probes that cause the formation of a long-range self-assembly in the form of a μm-long 1-dimensional DNA concatamer. These can be used as carriers for signal amplification. The intercalation of the ECL probe Ru(phen)3 2+ into the grooves of the concatamers leads to a substantial increase in ECL intensity. This amplified sensor shows high selectivity for discriminating complementary target and other mismatched RNAs. The biosensor enables the quantification of the expression of microRNA-21 in MCF-7 cells. It also displays very low limits of detection and provides an alternative approach for the detection of RNA or DNA detection in diagnostics and gene analysis. Figure
The long-range self-assembly DNA concatamers were used as carriers for signal amplification by the intercalation of numerous ECL probe (Ru(phen)3 2+) into the grooves of the DNA concatamers. Such signal amplification strategy lead to a substantial increase in ECL intensity and sensitivity. 相似文献
1000.
Bin Dong Di Qin Hong L. Shi Yui R. Fang Wen Z. Wang Yang Y. He Bao S. Cao Yan X. Liu Yong Ding 《Mikrochimica acta》2014,181(7-8):791-795
We report on the synthesis and optical spectra of silver nanorice particles. Two strong absorption bands are resolved in the near UV and near-IR region, and the dark field scattering spectra are consistent with the absorption spectra. Finite-difference time-domain simulations reveal that the peak in the IR region can be attributed to the E field that is parallel to the long axis, while the peak in the UV can be attributed to the E field perpendicular to the short axis of the silver nanorice particles. Figure
This paper reports on the synthesis of Ag nanorices, and their characterization by optical absorption, TEM, dark field scattering microscopy of single Ag nanorice and theoretical simulations using time domain finite-difference time-domain method. Electric field distribution of Ag nanorice dimer excited by polarization along long axis. 相似文献