全文获取类型
收费全文 | 885篇 |
免费 | 29篇 |
国内免费 | 2篇 |
专业分类
化学 | 512篇 |
晶体学 | 16篇 |
力学 | 38篇 |
数学 | 70篇 |
物理学 | 280篇 |
出版年
2023年 | 11篇 |
2022年 | 31篇 |
2021年 | 24篇 |
2020年 | 37篇 |
2019年 | 27篇 |
2018年 | 22篇 |
2017年 | 31篇 |
2016年 | 33篇 |
2015年 | 15篇 |
2014年 | 41篇 |
2013年 | 93篇 |
2012年 | 70篇 |
2011年 | 55篇 |
2010年 | 55篇 |
2009年 | 41篇 |
2008年 | 38篇 |
2007年 | 38篇 |
2006年 | 30篇 |
2005年 | 21篇 |
2004年 | 16篇 |
2003年 | 7篇 |
2002年 | 13篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 18篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1967年 | 3篇 |
1962年 | 2篇 |
排序方式: 共有916条查询结果,搜索用时 15 毫秒
51.
Vinod K. Tiwari Archana Singh Hakkim A. Hussain Bhuwan B. Mishra Vyasji Tripathi 《Monatshefte für Chemie / Chemical Monthly》2007,138(7):653-658
Summary. A convenient and high yielding method for the synthesis of diverse dithiocarbamates having various substituents including
alkyl, aryl, heteroaryl, and alkylaryl at the thiol chain or at the amine chain or at both thiol and amine chains were developed
by the one-pot reaction of mercaptans, amines, and bis(benzotriazolyl)methanethione in presence of amidine base under mild
reaction conditions. 相似文献
52.
Kumar A Lal Tamta M Negi N Chandrasekhar K Singh Negi D 《Natural product research》2011,25(18):1680-1686
The essential oil of Premna latifolia Roxb. was obtained by hydrodistillation of fresh leaves of the plant having an oil yield of 0.05%, both non-polar and essential oil were analysed by GC and GC-MS. Hexane fraction of the leaves of P. latifolia was transesterified and analysed by GC and GC-MS, 40 non-polar components were identified comprising 89.3%. The most abundant fatty acid constituents were hexadecanoic acid (25.04%), 8,11,14-docosatrienoic acid (13.62%), octadecanoic acid (6.82%), 9,12-octadecadienoic acid (4.19%) and 29 components were investigated in the essential oil which comprises 78.1%. The most abundant oil constituents were 1-octen-3-ol (35.69%), terpendiol II (7.19%), δ-guaiene (7.49%) 2-undecanone (4.80%) and α-pinene (3.27%). Different extracts were also tested against polyphagous crop pest Spodoptera litura for antifeedant activity. Essential oil showed maximum growth reduction of 56.83% followed by chloroform extract of 43.93%. 相似文献
53.
Richa Upadhyay Jitendra Kumar Chaurasia Kavindra Nath Tiwari Karuna Singh 《Applied biochemistry and biotechnology》2013,171(8):2153-2164
Phyllanthus fraternus is widely used in the cure of various liver diseases and possess antiviral properties especially against hepatitis virus. In the present study, evaluation of the antioxidant activity of stem and calli induced from stem has been done by different assays. Extraction was done by standard method in water and ethanol. Total antioxidant capacity was measured by 1, 1-diphenyl-2-picrylhydrazyl free radical scavenging method. Lipid peroxidation was measured in terms of thiobarbituric acid-reactive substances (TBARS) by using egg yolk homogenates as lipid-rich media, and superoxide radical scavenging activity was measured using riboflavin–light–nitro blue tetrazolium assay. Reducing power was determined on the basis of Fe3+–Fe2+ transformation in the presence of the extract. In addition to the antioxidant activity, polyphenolic compounds like total phenolics and flavonoids were also measured by spectroscopic method. Results showed that the ethanolic extract of stem is more potent in antioxidant activity than its aqueous extract and ethanolic extract of calli. A significant correlation between antioxidant capacity and polyphenolic content and reducing potential was observed, indicating that phenolic compounds and reducers present in extract are major contributors to the antioxidant potential. Thus, this plant extract could be used as a potent natural antioxidant. 相似文献
54.
Tiwari AD Mishra AK Mishra SB Mamba BB Maji B Bhattacharya S 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,79(5):1050-1056
The thiocarbohydrazone Schiff-base ligand with a nitrogen and sulphur donor was synthesized through condensation of pyridine-2-carbaldehyde and thiocarbohydrazide. Schiff-base ligands have the ability to conjugate with metal salts. A series of metal complexes with a general formula [MCl2(H2L)]·nH2O (MNi, Co, Cu and Zn) were synthesized by forming complexes of the N1,N5-bis[pyridine-2-methylene]-thiocarbohydrazone (H2L) Schiff-base ligand. These metal complexes and ligand were characterized by using ultraviolet-visible (UV-Vis), Fourier Transform Infrared (FT-IR), 1H and 13C NMR spectroscopy and mass spectroscopy, physicochemical characterization, CHNS and conductivity. The biological activity of the synthesized ligand was investigated by using Escherichia coli DNA as target. The DNA interaction of the synthesized ligand and complexes on E. coli plasmid DNA was investigated in the aqueous medium by UV-Vis spectroscopy and the binding constant (Kb) was calculated. The DNA binding studies showed that the metal complexes had an improved interaction due to trans-geometrical isomers of the complexes than ligand isomers in cis-positions. 相似文献
55.
Devendra Kumar Uday Razdan Alka D. Gupta 《Journal of polymer science. Part A, Polymer chemistry》1992,30(7):1477-1487
Various melt-polymerizable bisimido-bisphthalonitrile polymer precursors were synthesized by the reaction of 4-aminophthalonitrile (4-APN) with bis(3,4-dicarboxyphenyl) dimethylsilane dianhydride (SIDA), 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), bis(3,4-dicarboxyphenyl)ether dianhydride (ODPA), and 3,3′, 4,4′-tetracarboxylichenzophen+ne dianhydride (BTDA) in an aprotic solvent. The synthesized monomers showed crystalline melting at 269 and 271°C. Elemental analysis, differential thermal analysis (DTA), infrared (IR), nuclear magnetic resonance (1H-NMR) and mass spectral studies were carried out to characterize the synthesized monomers. Thermogravimetric analysis (TGA) of the synthesized monomers showed their thermal stability at 410–400°C. A detailed study and NMR investigation of the first step of condensation reaction was carried out and indicated the formation of a transient charge transfer complex. Thermal cyclization of the formed intermediate, however, gave the required monomers. A preliminary study demonstrated that melt-polymerization of the synthesized monomers gave thermallystable, tough polymers. 相似文献
56.
Summary The halogen oxidation and nitrosylation of cis-[(SB)M-(CO)4] [M = Cr or Mo, SB = N,N-ethylenebis(p-tolualideneimine), N,N-ethylenebis(p-N,N-dimethylaminobenzalideneimine) or N,N-ethylenebis(methyl-p-methoxyphenylketimine); M = Mo, SB = N,N-ethylenebis-(cinnamylideneimine) or N,N-ethylenebis(methyl-p-methylphenylketimine)] have been studied. Halogenation of [(SB)Cr(CO)4] yielded [(SB)CrX2] (X = Cl, Br or I) where-as [(SB)Mo(CO)4] gave [(SB)Mo(CO)3X2] (X = Br or I) and [(SB)MoX
x
] (X = I, n = 2; X = Cl or Br, n = 4). NOCl produced [(SB)Cr(NO)2Cl2] and [(SB)Mo(CO)2(NO)Cl] when reacted with the corresponding [(SB)M(CO)4]. The complexes were characterized by physico-chemical and spectroscopic measurements. 相似文献
57.
Aqueous polymerization of methyl methacrylate (MMA), initiated by the potassium bromate-thioglycollic acid (TGA) redox system, has been studied at 30 ± 0.2° C under positive pressure of nitrogen. The rate is given by K[MMA] [TGA] 0[KBrO3]x where × = 1 for lower KBrO3 concentrations and 0.5 for higher KBrO3 concentrations. The reaction has been studied over the 20–45°C range. The activation energy was found to be 65.72 kJ/mol (15.71 kcal/mol) in the investigated range of temperature. Inorganic electrolytes except MnSO4·4H2O and Na2C2O4 depress both the rate of polymerization and the maximum conversion. All the alcohols (viz., MeOH, EtOH, iso-PrOH, tert-BuOH) and acetone depress the rate of polymerization as well as the maximum conversion. 相似文献
58.
Devendra Kumar 《Journal of polymer science. Part A, Polymer chemistry》1985,23(6):1661-1670
A heat-resistant polybismaleimide was obtained by the thermal polymerization of bis(maleimidophenoxy)- tetrakis(phenoxy)cyclotriphosphazene. The thermal stabilities of the polybismaleimide were evaluated in nitrogen and in air by thermogravimetric analysis. The polybismaleimide was stable to 340°C and has char yield of 70% at 800°C in nitrogen and of 60% at 700°C in air. The monomer bismaleimide was obtained by the reaction of bis(4-aminophenoxy)tetrakis(phenoxy)cyclotriphosphazene with maleic anhydride. The diamine was synthesized by a stepwise reaction of hexachlorocyclotriphosphazene with phenol and 4-nitrophenol to give bis(4-nitrophenoxy)tetrakis(phenoxy)cyclotriphosphazene and reducing the nitro groups. The structure of the cyclotriphosphazenes were characterized using Fourier transform infrared (IR), proton nuclear magnetic resonance (1H-NMR) spectroscopy, and elemental analysis. 相似文献
59.
We have obtained relations between certain components of the metric and the electromagnetic potentials for source-free Brans-Dicke-Maxwell fields described by a nonstatic cylindrically symmetric Einstein-Rosen metric. These are important, in the sense that they generate a class of solutions that in a way can be said to belong to the class generated by similar relations obtained by Majumdar [1] and Papapetrou [2] for generalized static Einstein-Maxwell fields. The relations have further been used to reduce the B-D Maxwell equations to B-D vacuum equations and vice versa. 相似文献
60.