全文获取类型
收费全文 | 23656篇 |
免费 | 4630篇 |
国内免费 | 6903篇 |
专业分类
化学 | 17692篇 |
晶体学 | 820篇 |
力学 | 1596篇 |
综合类 | 536篇 |
数学 | 3146篇 |
物理学 | 11399篇 |
出版年
2024年 | 60篇 |
2023年 | 335篇 |
2022年 | 788篇 |
2021年 | 847篇 |
2020年 | 954篇 |
2019年 | 952篇 |
2018年 | 841篇 |
2017年 | 1026篇 |
2016年 | 1019篇 |
2015年 | 1193篇 |
2014年 | 1471篇 |
2013年 | 1965篇 |
2012年 | 2161篇 |
2011年 | 2278篇 |
2010年 | 2012篇 |
2009年 | 2049篇 |
2008年 | 2240篇 |
2007年 | 1989篇 |
2006年 | 1836篇 |
2005年 | 1500篇 |
2004年 | 1197篇 |
2003年 | 879篇 |
2002年 | 875篇 |
2001年 | 817篇 |
2000年 | 886篇 |
1999年 | 537篇 |
1998年 | 327篇 |
1997年 | 258篇 |
1996年 | 263篇 |
1995年 | 193篇 |
1994年 | 217篇 |
1993年 | 190篇 |
1992年 | 168篇 |
1991年 | 120篇 |
1990年 | 138篇 |
1989年 | 117篇 |
1988年 | 94篇 |
1987年 | 72篇 |
1986年 | 59篇 |
1985年 | 47篇 |
1984年 | 42篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 27篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1974年 | 6篇 |
1965年 | 11篇 |
1959年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
991.
<正>The C-Br bond of 2-bromothiophene was successfully cleavaged by tetrakis(trimethylphosphine)cobalt(0). Penta-coordinate cobalt(II) bromide complex 1 (α-C_4H_3S)CoBr(PMe_3)_3 as C-Br bond activation product was obtained. The molecular and crystal structures of complex 1 were determined by single-crystal X-ray diffraction. Complex 1 belongs to the orthorhombic crystal system, space group Pnma with a = 20.428(4) , b =11.036(2) , c = 9.201(2), V = 2074.3(7)~3, Z = 4, D_c = 1.442 g/cm~3, μ = 0.401 mm~(-1), F(000) = 924 and T = 273 K. Complex 1 was also characterized by elemental analysis, IR, UV-Vis, photoluminescence (PL) spectroscopy and TGA. 相似文献
992.
Xiang Ying Shi Jue Lin Ziyang Li Tao Jin-Wang Huang Liang-Nian Ji 《Liquid crystals》2013,40(2):211-214
We observed that the planar aligned nematic liquid crystal (5CB) doped with a volume fraction of 1% of FeTPPCl [5,10,15,20-tetraphenylporphyriniron(III)chloride] or MnTPPCl [5,10,15,20-tetraphenylporphyrinmanganese(III)chloride] dramatically decreased the critical magnetic field for the magnetic field induced Freedericksz transition, while 5CB doped with ZnTPP [5,10,15,20-tetraphenylporphyrinzinc(II)] revealed no such effect, when compared with pure 5CB. In the guest-host (5CB) system, FeTPPCl and MnTPPCl as guests are both strong paramagnetic materials with an interaction through coordination of the -CN group in 5CB onto the metal ion of the porphyrin. As a result, the 5CB molecules are dragged to reorientate under a static magnetic field, while ZnTPP is a diamagnetic material without such a property. This phenomenon concerning magneto-optical components could be useful in liquid crystal displays. 相似文献
993.
Cong-Shan Zhou Ping Xu Ke-Wen Tang Xin-Yu Jiang Tao Yang Pan-Liang Zhang 《Chemical Papers》2013,67(2):155-163
Enantioselective extraction of hydrophilic 2-chloromandelic acid (CMA) enantiomers from organic to aqueous phase with hydroxypropyl-??-cyclodextrin (HP-??-CD) as the selector was investigated. Equilibrium of the extraction system was modeled using a reactive extraction model with a homogeneous aqueous phase reaction. The influence of important process variables on the extraction efficiency, such as the type of the organic solvent and ??-cyclodextrin derivatives (??-CDs), concentration of the selector, pH and temperature, was investigated by experiment and modeling. Important parameters of this model were determined experimentally. Results showed that the experimental data agree with the model prediction perfectly and the model was further applied to accurately predict the extraction efficiency influenced simultaneously by pH and the concentration of HP-??-CD. Combining the experiment and the model data, the best extraction conditions were: pH of 2.5, HP-??-CD concentration of 0.05 mol L?1, and temperature of 5°C, providing the enantioselectivity of 1.285 and the performance factor (pf) of 0.011. 相似文献
994.
995.
Yuhui Kou Derong Cao Hongqi Tao Lingyun Wang Jianquan Liang Zhizhao Chen Herbert Meier 《Journal of inclusion phenomena and macrocyclic chemistry》2013,77(1-4):279-289
A detailed study of the reaction conditions revealed that a quantitative cyclocondensation of 1,4-dialkoxy-2,5-bis(alkoxymethyl)-benzenes to pillar[n]arenes can be achieved by catalysis of p-toluenesulfonic acid in CH2Cl2. Major product of this new reaction is in each case a cyclopentamer (n = 5), but small amounts of the pillar[n]arenes with n = 6, 7 and 10 can be obtained as well. Different alkoxy groups in 1- and 4-position lead to regioisomers. All cyclooligomers exist in pillar structures as pair of enantiomers, which show a racemisation at room temperature, which is fast in terms of the NMR time scale. The racemisation process occurs by rotation of the 1,4-phenylene segments in the macrocyclic rings. Pillar[n]arenes exhibit novel host–guest behavior. 相似文献
996.
Pingting He Jie Tao Xianli Huang Jianjun Xue 《Journal of Sol-Gel Science and Technology》2013,68(2):213-218
Nitrogen doped TiO2 (N-TiO2) nanoparticles with about 30 nm in size were produced by a sol–gel method and characterized respectively by UV–vis, X-ray diffraction (XRD), Transmission electron microscopy, X-ray photoelectron spectroscopy (XPS). Their photocatalytic antibacterial properties were evaluated by the antibacterial ratio against Escherichia coli in dark and under simulated sunlight respectively. The XRD pattern showed that the doped nano-TiO2 was mainly composed of anatase phase. The XPS spectra of the N-TiO2 sample indicated that TiO2 was doped by nitrogen atom. The nitrogen doping created a new N 2p state slightly above the valence band top consists of O 2p state, and this pushes up the valence band top and decreased the band gap. Which leaded to the absorption edge was red-shifted to the visible light region of UV–vis spectra of nitrogen doped nano-TiO2 comparing with pure nano-TiO2. The antibacterial percentage of N-TiO2 against E. coli reached to 90 % under simulated sunlight for 2 h, which was much better than that in dark, also than that of pure nano-TiO2. The photo-catalytic antibacterial activity was activated under visible light. The structure and integrity of cell wall and cell membrane were destructed, and even caused the bacteria death. 相似文献
997.
Preparation and characterization of ZrCO/C composite aerogels 总被引:1,自引:0,他引:1
Li Ye Wenfeng Qiu Hao Li Aijun Zhao Tao Cai Tong Zhao 《Journal of Sol-Gel Science and Technology》2013,65(2):150-159
Zr-containing organic aerogels were synthesized by ligand substitution reaction of polyzirconoxone and 2, 4-dihydroxybenzoic acid, followed by polymerization with formaldehyde, and then supercritical drying using CO2. After carbonization and carbothermal reduction under an argon atmosphere, ZrCO/C composite aerogels with controllable zirconium content (47.8–78.6 wt%) were obtained. The carbothermal reduction was substantially completed at 1,500 °C, and the obtained ZrCO/C composite aerogels exhibit low oxygen contents (9.4–6.7 wt%) and high surface areas (589–147 m2/g). Pore morphologies of the ZrCO/C composite aerogels were investigated in detail by nitrogen sorption measurements, scanning electron microscopy and its associated energy-dispersive X-ray microanalysis measurements. The results show that the aerogels are composed of carbon framework and Zr-conglomerations, and the surface area of aerogel is severely affected by its zirconium content. The presence of reductive ZrC crystals can greatly enhance the oxidation resistance ability of amorphous carbon framework and prevent collapse. 相似文献
998.
Tao Wang Shuai Shi Dr. Mie Højer Vilhelmsen Tuo Zhang Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(37):12512-12516
New and chemoselective gold(I)‐catalyzed transformations of 1‐(arylethynyl)‐7‐oxabicyclo[4.1.0]‐ heptan‐2‐ones were developed. Two completely different products—6,7‐dihydrobenzofuran‐4(5H)‐ones and benzofurans—could be obtained from the same starting material. The selectivity is determined by the ligand of the gold catalyst: triphenylphosphine delivers 6,7‐dihydrobenzofuran‐4(5H)‐ones, and 1,3‐bis(diisopropylphenyl)imidazol‐2‐ylidene leads to benzofurans. Eleven examples of each case are provided. The mechanistic suggestions for the pathways to both product types are supported by isotope labeling experiments. 相似文献
999.
1000.
Tao Yang Prof. Dr. Xiang Zhao Prof. Dr. Shigeru Nagase 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2649-2654
Like C60, C70 is one of the most representative fullerenes in fullerene science. Even though there are 8149 C70 isomers, only two of them have been found before: the conventional D5h and an isolated pentagon rule (IPR)‐violating C2v(7854). Through the use of quantum chemical methods, we report a new unconventional C70 isomer, C2(7892), which survives in the form of dimetallic sulfide endohedral fullerene Sc2S@C70. Compared with the IPR‐obeying C70 and the C2v(7854) fullerene with three pairs of pentagon adjacencies, the C2(7892) cage violates the isolated pentagon rule and has two pairs of pentagon adjacencies. In Sc2S@C2(7892)‐C70, two scandium atoms coordinate with two pentalene motifs, respectively, presenting two equivalent Sc? S bonds. The strong coordination interaction, along with the electron transfer from the Sc2S cluster to the fullerene cage, results in the stabilization of the non‐IPR endohedral fullerene. The electronic structure of Sc2S@C70 can be formally described as [Sc2S]4+@[C70]4?; however, a substantial overlap between the metallic orbitals and cage orbitals has also been found. Electrochemical properties and electronic absorption, infrared, and 13C NMR spectra of Sc2S@C70 have been calculated theoretically. 相似文献