全文获取类型
收费全文 | 3282篇 |
免费 | 132篇 |
国内免费 | 3篇 |
专业分类
化学 | 1721篇 |
晶体学 | 62篇 |
力学 | 149篇 |
数学 | 832篇 |
物理学 | 653篇 |
出版年
2024年 | 6篇 |
2023年 | 28篇 |
2022年 | 41篇 |
2021年 | 56篇 |
2020年 | 105篇 |
2019年 | 103篇 |
2018年 | 167篇 |
2017年 | 146篇 |
2016年 | 205篇 |
2015年 | 133篇 |
2014年 | 182篇 |
2013年 | 519篇 |
2012年 | 177篇 |
2011年 | 198篇 |
2010年 | 182篇 |
2009年 | 147篇 |
2008年 | 156篇 |
2007年 | 144篇 |
2006年 | 95篇 |
2005年 | 40篇 |
2004年 | 48篇 |
2003年 | 41篇 |
2002年 | 47篇 |
2001年 | 46篇 |
2000年 | 23篇 |
1999年 | 31篇 |
1998年 | 31篇 |
1997年 | 30篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 19篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1975年 | 10篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1927年 | 5篇 |
排序方式: 共有3417条查询结果,搜索用时 62 毫秒
71.
Hasan Kırmızıbekmez Yiğit İnan Rengin Reis Hande Sipahi Ahmet C. Gören Erdem Yeşilada 《Natural product research》2019,33(17):2541-2544
Phytochemical investigations on the EtOH extract of Clematis viticella led to the isolation of six flavonoid glycosides, isoorientin (1), isoorientin 3′-O-methyl ether (2), quercetin 7-O-α-L-rhamnopyranoside (3), quercetin 3,7-di-O-α-L-rhamnopyranoside (4), manghaslin (5) and chrysoeriol 7-O-β-D-glucopyranoside (6), one phenylethanol derivative, hydroxytyrosol (7), along with three phenolic acids, caffeic acid (8), (E)-p-coumaric acid (9) and p-hydroxybenzoic acid (10). The structures of the isolates were elucidated on the basis of NMR and HR-MS data. All compounds were isolated from C. viticella for the first time. Compounds 7 and 8 showed significant anti-inflammatory activity at 100 μM by reducing the release of NO in LPS-stimulated macrophages comparable to positive control indomethacin. Compounds 3 and 7 exhibited anti-inflammatory activity through lowering the levels of TNF-α while 1, 3 and 5 decreased the levels of neopterin better than the positive controls. 相似文献
72.
A microorganism Agrobacterium tumefacients as an immobilized cell on a solid support was presented as a new biosorbent for the enrichment of Fe(III), Co(II), Mn(II) and Cr(III) prior to flame atomic absorption spectrometric analysis. Amberlite XAD-4 was used as a support material for column preconcentration. Various parameters such as pH, amount of adsorbent, eluent type and volume, flow rate of sample solution, volume of sample solution and matrix interference effect on the retention of the metal ions have been studied. The optimum pH for the sorption of above mentioned metal ions were about 6, 8, 8 and 6, respectively. The loading capacity of adsorbent for Co(II) and Mn(II) were found to be 29 and 22 μmol g−1, respectively. The recoveries of Fe(III), Co(II), Mn(II) and Cr(III), under the optimum conditions were found to be 99 ± 3, 99 ± 2, 98 ± 3 and 98 ± 3%, respectively, at the 95% confidence level. The limit of detection was 3.6, 3.0, 2.8 and 3.6 ng ml−1 for Fe(III), Co(II), Mn(II) and Cr (III), respectively, by applying a preconcentration factor of 25. The proposed enrichment method was applied for metal ion determination from water samples, alloy samples, infant foods and certified samples such as whey powder (IAEA-155) and aluminum alloy (NBS SRM 85b). The analytes were determined with a relative error lower than 10% in all samples. 相似文献
73.
Nuray Ulusoy Muammer Kiraz Ömer Küçükbasmacı 《Monatshefte für Chemie / Chemical Monthly》2002,133(10):1305-1315
Summary. New 4-alkyl/aryl-1-((6-(4-bromophenyl)-imidazo[2,1-b]thiazol-3-yl)-acetyl)-3-thiosemicarbazides and 3-alkyl/aryl-2-(((6-(4-bromophenyl)-imidazo[2,1-b]thiazol-3-yl)-acetyl)-hydrazono)-4-thiazolidinones were synthesized from 6-(4-bromophenyl)-imidazo[2,1-b]thiazole-3-acetic acid hydrazide. Their structures were elucidated by elemental analyses and spectroscopic data. All compounds
were tested for antibacterial and antifungal activities. The antimicrobial activities of the compounds were assessed by the
microbroth dilution technique. The compounds were also evaluated for antituberculosis activity against Mycobacterium tuberculosis H37Rv (ATCC 27294); they exhibited varying degrees of inhibition in the in vitro primary screening at 6.25 μg · cm−3. The most active compound was 3-(4-nitrophenyl)-2-(((6-(4-bromophenyl)-imidazo[2,1-b]thiazol-3-yl)-acetyl)-hydrazono)-4-thiazolidinone.
Corresponding author. E-mail: nurayulusoy@yahoo.com
Received December 10, 2001. Accepted (revised) March 1, 2002 相似文献
74.
A. Ugur M. M. Saç G. Yener Ü. Altinbaş Y. Kurucu M. Bolca B. Özden 《Journal of Radioanalytical and Nuclear Chemistry》2004,259(2):265-270
Vertical distributions of 137Cs and 210Pb in soil profiles were examined to study their availability in soil erosion at Gökova region where there exists intensive agricultural activities on sloppy fields. Since the mobility of these radionuclides depend on soil characteristics, soil samples were analyzed also for their physical and chemical properties. From 137Cs inventories measured, erosion rates for cultivated and disturbed (no cultivation) soils were calculated to range from 79.1 to 6.5 t.ha-1.y-1 and from 79.9 to 3.5 t.ha-1.y-1, respectively. The 210Pb technique is found to be not suitable for erosion determination for this area, presumably due to the coal-fired power plants operating in the region. 相似文献
75.
Naki Çolak Yılmaz Yıldırır Mustafa Kavutcu Nilhan Nurlu 《Monatshefte für Chemie / Chemical Monthly》2007,138(12):1283-1287
Summary. Methotrexate (MTX) is a folate antagonist used in treatment of several chronic inflammatory and neoplastic conditions. In this study, new MTX-like compounds that may-be potential anticancer agents were synthesized and their structures were determined by IR, UV, GC-MS,
1H NMR, and 13C NMR spectra. Also, in this study, a series structurally related to MTX or folate analogous compounds were evaluated whether they have inhibitory properties on the dihydrofolate reductase activity
(DHFR). 相似文献
76.
The ligand has been
prepared from ω-chloroisonitrosoacetophenone and 1,4-phenylenediamine
in the presence of NaHCO3. The ligand have a C=O group
and an –NH–R group adjacent to the oxime group. The Ni(II), Cu(II)
and Co(II) complexes of the synthesized ligand were prepared and their structures
were identified using FTIR, UV-Vis spectral data, elemental analysis, thermal
analysis and magnetic moment measurements. The metal-ligand ratios were found
to be 3:2. The ligands were found to coordinate to the metal ions via the
oxime nitrogen, oxime oxygen, one of the carbonyl group and amide nitrogen
atoms. Thermal analyses data reveals that the water in the complexes was found
to be non-coordinated to the metal ions. In the trinuclear structures, the
metals have the tetrahedral environments. 相似文献
77.
This work reports on the synthesis of new unsymmetrically substituted phthalocyanines (M = Zn, Cu, Co, Ni) bearing three benzo-15-crown-5 units through oxy bridges and a nitro group. Phthalocyanines were prepared by a statistical condensation of 4-nitro phthalonitrile and 1-{[(benzo-15-crown-5)-4′-yl]oxy}phthalonitrile in the presence of anhydrous metal salts. All the target unsymmetrical phthalocyanines were separated by column chromatography and characterized elemental analyses, 1H NMR, IR, mass and UV–Vis spectral data. Electrochemical behaviors of Cu (II) phthalocyanine by cyclic voltammetry and differential pulse voltammetry techniques gave two common phthalocyanine ring reductions and one ring oxidation processes. Peak-to-peak separation of the processes II and III (388 mV) and the measure of gap (1.672 V) between the HOMO and LUMO for the complex, fits a phthalocyanine with electrochemically inactive metal center. 相似文献
78.
Rotation matrices were expressed in terms of Gaunt coefficients and complex spherical harmonics. The rotation matrices were calculated using two different ways. In the first, Gaunt coefficients and normalized complex spherical harmonics were directly calculated using binomial coefficients; in the second, Gaunt coefficients and complex spherical harmonics were recursively calculated. The methods were compared with respect to accuracy and computation time (CPU) for low and very high quantum numbers. 相似文献
79.
80.
The thermal behaviours of the Ti(II), Mn(II), Fe(II), Ni(II), Cu(II) and Zn(II) complexes of triethanolamine were studied by means of thermogravimetry, differential thermogravimetry, differential thermal analysis infrared spectrophotometry and elemental analysis. The sequence of thermal stability of the metal complexes, determined by using the initial decomposition temperature, was found to be Ti(II)?Mn(II)>Fe(II)>Ni(II)>Zn(II)>Cu(II). Some of the kinetic parameters, such as the activation energy and order of reaction for the initial decomposition reaction, were calculated and the relationship between the thermal stability and the chemical structure of the complexes is discussed. 相似文献