全文获取类型
收费全文 | 1754篇 |
免费 | 59篇 |
国内免费 | 208篇 |
专业分类
化学 | 1707篇 |
晶体学 | 37篇 |
力学 | 5篇 |
综合类 | 9篇 |
数学 | 1篇 |
物理学 | 262篇 |
出版年
2024年 | 2篇 |
2023年 | 18篇 |
2022年 | 14篇 |
2021年 | 28篇 |
2020年 | 23篇 |
2019年 | 45篇 |
2018年 | 33篇 |
2017年 | 45篇 |
2016年 | 39篇 |
2015年 | 42篇 |
2014年 | 70篇 |
2013年 | 151篇 |
2012年 | 103篇 |
2011年 | 106篇 |
2010年 | 91篇 |
2009年 | 81篇 |
2008年 | 82篇 |
2007年 | 121篇 |
2006年 | 106篇 |
2005年 | 109篇 |
2004年 | 106篇 |
2003年 | 91篇 |
2002年 | 70篇 |
2001年 | 43篇 |
2000年 | 36篇 |
1999年 | 42篇 |
1998年 | 37篇 |
1997年 | 33篇 |
1996年 | 39篇 |
1995年 | 38篇 |
1994年 | 19篇 |
1993年 | 33篇 |
1992年 | 28篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有2021条查询结果,搜索用时 31 毫秒
51.
碘化钾-乙基紫-水体系固相浮选分离镉的研究 总被引:4,自引:0,他引:4
研究了碘化钾 乙基紫 水体系固相浮选分离镉的行为及与常见金属离子分离的最佳条件。试验表明,将碘化钾溶液、乙基紫溶液和盐酸溶液混合稀释,生成稳定的CdI2-4·(EV+)2三元缔合物可很快浮于水相,使Cd(Ⅱ)被定量浮选。控制适当条件,可实现Cd(Ⅱ)与常见金属离子Mn(Ⅱ)、Al(Ⅲ)、Ni(Ⅱ)、Zn(Ⅱ)之间的分离。对合成水样进行了浮选分离测定,结果满意。 相似文献
52.
The charge transfer (CT) band maximum of N-alkyl pyridinium iodide (NAPI) has been studied as a function of the composition of binary mixed dipolar aprotic solvents. The deviation from linearity of the energy maximum (E12) and the mole fraction (of a component solvent) plot is explained as due to a preferential solvation by the more polar cosolvent in the binary mixture. The extent of preferential solvation has been observed to vary with the composition, the maximum being towards the less-polar end. The role of hydrogen bond donating ability of a solvent in preferential solvation is discussed. 相似文献
53.
Mohammad Ali Zolfigol Khodabakhsh Niknam Mojtaba Bagherzadeh Arash Ghorbani‐Choghamarani Nadiya Koukabi Maryam Hajjami Eskandar Kolvari 《中国化学会会志》2007,54(5):1115-1118
Tribromoisocyanuric acid (TBCA) and Oxone®‐MX systems were used as effective oxidizing agents for the oxidation of thiols to their corresponding disulfides under mild conditions at room temperature with good to excellent yields. 相似文献
54.
Carla M. Bossu Edilene C. Ferreira Fernanda S. Chaves Eveline A. Menezes Ana Rita A. Nogueira 《Microchemical Journal》2006,84(1-2):88-92
A flow injection system was proposed to evaluate the transient product of a colorimetric reaction between hydrolysable tannin and potassium iodate (KIO3) solution. The system optimization was accomplished by using statistical methods based on experimental design. Flow rate of KIO3 solution, sample volume, carrier flow rate, and reaction coil were the selected factors for evaluation. On screening step, complete factorial 24 was used and two levels for each selected factor were studied. For the optimization phase, a centered face composite design 22 + star was employed to evaluate sample volume and flow rate of KIO3 solution, which were the factors identified in the screening phase as having more influence on the absorbance signal. After optimization, the proposed system was compared with batch determination. Some characteristics, such as analytical frequency, reagent consumption and chemical residues generation presented better results by the use of the proposed system if compared with batch method. The system presented good repeatability with standard deviation lower than 3%, for n = 10, linearity (R2 = 0.9974) for tannic acid standard, analytical frequency of 15 injections h− 1 and limit of quantification of 24 mg L− 1 of tannic acid. Good results were obtained when the proposed system was applied to hydrolysable tannin determination in Stryphnodendron barbatimão, Eucalyptus citriodora and Phyllanthus niruri, samples of plants commonly used in popular medicine. 相似文献
55.
A novel coordination polymer [(C10H16N)2(Pb2I6)·2DMF·H2O]n (C10H16N=N-butyl-2-Methy-Pyridinium) was synthesized by the reaction of Pb(NO3)2 with C6H10NI at room temperature in DMF solvent and structurally characterized by means of X-ray single diffraction. The title compound crystallizes in triclinic system, space group P1 with a=1.1237(2) nm, b=1.25330(16) nm, c=0.808 00(12) nm, α=102.523(4)°, β=92.475(5)°, γ=95.712(10)°, V=1.102 9(3) nm3, Z=1, Dc=2.470 Mg·m-3, F(000)=738, chemical formula C26H48N4O3Pb2I6 and Mr=1 640.46, μ(MoKα)=11.849 mm-1, the final R=0.057 8 and wR=0.166 5 for 3716 observed reflections with I > 2σ(I). The title compound consists of cations ([(C10H16N)+] and anion chain(PbI3-),they are combined by static attracting forces in the crystal. DMF and H2O locate between the organic and inorganic moiety. CCDC: 210812. 相似文献
56.
Single crystals of potassium iron hydrogen phosphate, KFe3(HPO4)2(H2PO4)6 · 4 H2O, were prepared hydrothermally by heating a mixture of Fe2O3, H3PO4 and K2CO3 with a small amount of water. It crystallizes monoclinic, space group C2/c (N° 15 Int. Tab.) with Z = 4 and a = 1701(2), b = 960.4(5), c = 1750(1) pm, β = 90.88(7)°. The crystal structure was solved by using 1716 unique reflections F0 > 4σ(F0) with a final wR2 value of 0.126 (SHELXL-93). The main feature of the crystal structure are layers formed by PO4-tetrahedra around the FeO6-octahedra parallel to (001). K+ and H2O molecules connect these layers. Effective Coordination Numbers (ECoN), Mean Fictive Ionic Radii (MEFIR), Charge Distribution (CHARDI) and the Madelung Part of Lattice Energy (MAPLE) are calculated for the title compound. The existence of hydrogen bonds is confirmed by these calculations. 相似文献
57.
Nitration of aromatic halides proceeded smoothly in the presence of catalytic amounts of Cu bronze and N,N′-dimethylethylenediamine. Sodium nitrite-18-crown-6, or tetra-n-butylammonium nitrite (n-Bu4NNO2) turned out to be efficient nitrating agents. The aromatic nitro compounds were synthesized under essentially neutral conditions. 相似文献
58.
Regioselective oxidative bromination of electron-rich aromatic rings has been studied using potassium bromide as a bromine source in the presence of benzyltriphenylphosphonium peroxymonosulfate as oxidant under nearly neutral reaction conditions. In most cases we obtained monobrominated derivatives regioselectively and in good to high yields without the aid of strong acids. 相似文献
59.
D. V. Aleksanyan V. A. Kozlov I. L. Odinets P. V. Petrovskii T. A. Mastryukova 《Russian Chemical Bulletin》2007,56(5):1049-1052
Diethoxyphosphorylthioacetamide in the reaction with 2,3-dichloroquinoxaline acted as a thionating reagent, giving diethoxyphosphorylacetonitrile
and 2-chloro-3-[(3-chloro-2-quinoxalinyl)thio]quinoxaline. Reactions of phosphorylthioacetamides with N-methylquinoxalinium iodide in alcohol media in the presence of a base proceeded stereoselectively to afford cis-3-phosphoryl-1,3,3a,4,9,9a-hexahydro-2H-pyrrolo[2,3-b]quinoxaline-2-thiones.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1010–1013, May, 2007. 相似文献
60.
The C-C coupling reaction between ethyl acetoacetate and aryl halides in the presence of CuI is described. The effects of solvent, ligands such as vicinal diamines and amino acids, base and temperature are reported. The arylated acetoacetate ester is deacylated under the reaction conditions resulting in the generation of 2-arylacetic acid esters, constituting a mild alternative to direct arylation of carboxylate esters. 相似文献