全文获取类型
收费全文 | 197篇 |
免费 | 3篇 |
国内免费 | 1篇 |
专业分类
化学 | 124篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 41篇 |
物理学 | 25篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 8篇 |
2013年 | 12篇 |
2012年 | 10篇 |
2011年 | 14篇 |
2010年 | 8篇 |
2009年 | 8篇 |
2008年 | 5篇 |
2007年 | 11篇 |
2006年 | 5篇 |
2005年 | 11篇 |
2004年 | 6篇 |
2003年 | 4篇 |
2002年 | 9篇 |
2001年 | 2篇 |
2000年 | 7篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 7篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有201条查询结果,搜索用时 46 毫秒
1.
p palan BS 《Physical review letters》1987,58(16):1597-1599
2.
Maria do Carmo B. Pimentel Eduardo Henrique M. Melo José Luiz Lima Filho William M. Ledingham Nelson Durán 《Applied biochemistry and biotechnology》1997,66(2):185-195
This work is a study of lipase production by a Brazilian strain ofPenicillium citrinum using an inexpensive and simple medium without organic nitrogen sources and of some important industrial properties, including
thermostability in relation to ionic strength. The maximal lipase activity (1585 U/L) was obtained whenPenicillium citrinum was cultured on 0.75% ammonium sulfate complemented with minerals salts instead of yeast extract. Although this activity
was about 55% lower than that produced in medium with yeast extract (2850 U/L), the specific activity (7.8 U/mg proteins)
was higher than that obtained with the yeast extract (4.9 U/mg proteins). The morphology of fungus changed totally, with yeast
extract there are smooth, solid, and spherical pellets whereas on ammonium sulfate there are small “hairy” pellets uniformly
suspended in the medium. The effect of ferrous (Fe++) ions was carried out using medium MA with and without Fe++ ions. Lipase production byPenicillium citrinum in medium MA requires Fe++ ions, the absence of which caused a decreased of about 50% in the specific activity (3.5 U/mg proteins). The utilization
of commercial, locally available oils as carbon sources, such as soybean oil (236 U/L) and corn oil (74 U/L) resulted in lower
activity compared to olive oil, showing that lipase production byPenicillium citrinum is specifically induced by olive oil. Potassium concentration in the medium can effects the production of lipase (1 mM (1585
U/L), 10 mM (1290 U/L), and 30 mM (1238 U/L), 50 mM (195 U/L), and 100 mM (2 U/L). The crude culture filtered was susceptable
to thermal deactivation. It was stable at pH 6.0, but was not stable at the optimum pH (8.0-8.5) at 50 mM. At the low ionic
concentration (1-25 mM) this lipase was stable at low pH (3.5-4.0). The activation energy was 22.4 ±2.2 Kcal. mol 1. 相似文献
3.
Maria do Carmo Rangel Varela Milton Francisco de Jesus Filho Fernando Galembeck 《Hyperfine Interactions》1994,83(1):159-167
Mössbauer spectroscopy was used to follow the aging of ferrihydrite with different amounts of acetate, to achieve more crystalline products. Mössbauer spectra of fresh samples did not show any magnetic components. After two years aging, the solids presented crystalline fractions. Hematite formation was inhibited as acetate content increased in solids. For an acetate/iron molar ratio equal to 0.63 goethite was formed instead of hematite. X-ray diffraction confirmed these results. This work shows that synthetic samples can be used for elucidating the anti-hematitic effect of organic matter already detected in soils. 相似文献
4.
R. F. B. Serpa E. F. O. De Jesus M. J. Anjos R. T. Lopes M. G. T. do Carmo S. Moreira M. S. Rocha A. M. B. Martinez 《Journal of Radioanalytical and Nuclear Chemistry》2006,269(3):647-652
Summary The study of trace element levels is of great importance due to their relevance in agingand several neurodegenerative diseases.
This work compares the elemental concentrations in different postnatal ages and between the temporal cortex, entorhinal cortex
and hippocampus from Wistar rats, using X-ray total reflection fluorescence with synchrotron radiation. Ten elements were
determined in brain samples: Ti, Cr, Mn, Fe, Cu, Zn, (at trace level) and P, S, Cl and K (at major levels). The elements that
increased with aging in cortical areas were: S, K, Fe, Cu and Zn. Ca and Zn levels decreased with advancing age in the hippocampus.
In addition to this, Ti, Mn and Fe levels were more conspicuous in the entorhinal cortex. 相似文献
5.
de Magalhães Padilha P Rocha JC Moreira JC de Sousa Campos JT do Carmo Federici C 《Talanta》1997,45(2):317-323
Cellulose phosphate (CELLPHOS) was studied as a collector for analytical preconcentration of traces of Cd(II), Cr(III), Cu(II) and Ni(II) from aqueous sample solution. It has been proved that using chromatographic columns packed with CELLPHOS for preconcentration and 1.0 mol 1(-1) HCl for elution the adsorbed analytes are quantitatively enriched. An enrichment factor of 20 (100 ml sample, 5 ml concentrate) was achieved by this separation procedure, which was applied to a series of water analyses (river, sea, bog water). 相似文献
6.
M. Carmo Freitas 《Analytical and bioanalytical chemistry》1995,352(1-2):58-60
The IAEA-331 spinach material NIST SRM 1570a Spinach Leaves, submitted to an intercomparison run by the IAEA, has been analysed by k0-based INAA with counting using both low and high energy photon detectors. The results have been compared with (i) the certified values of the NIST SRM 1570a; the agreement is good, taking into account the uncertainties; and (ii) the certified and “consensus” values of the former NIST SRM 1570 Spinach Leaves, their composition in minor and trace-elements are quite similar; however a lower content has been observed for Co, Fe and Sc in the IAEA-331 and for Zn in the SRM 1570. For quality control, the NIST 1573 Tomato Leaves and the NIST 1575 Pine Needles have been analysed using the same conditions as for the IAEA-331. The results agree quite well with the certified and “consensus” values given in the literature. 相似文献
7.
Abstract Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F2 and F3 + aggregate color centres produced by electron irradiation were performed. 相似文献
8.
As part of an ongoing program directed toward the preparation of antraciclinones1 and aryl substituted lapachones2, which are important anticancer and antibiotic3 agents, we became interested in a efficient synthesis of substituted 1,4-naphthoquinones, such as juglone (3). This compound has received some attention due its activity as an allelophatic compound4. An extensive analysis of the literature procedures indicated that several methods are available starting from 1,5-dihydroxynaphthalene (1). All the procedures explored the oxidation of carbon four of the naphthyl ring by several known reagents, such as, chromium trioxide-piridine5, thallium trinitrate5, chromic acid6, peracetic acid7, dichromate8, iron (III) chloride9 and mercuric oxide10. None of the several known methods for the preparation of (3) proceeded in satisfactory yield (except for the thallium trinitrate method which gave 642 yield) and most of them gave a mixture of 1,2 and 1,4-naphthoquinone. 相似文献
9.
Viviana Heguaburu Hugo do Carmo Florencia Parpal María Eugenia Amorós Andrés González 《Tetrahedron letters》2017,58(18):1738-1741
The synthesis of 3-hydroxy-2-hexanone and 2,3-hexanediol, two components of the aggregation pheromone of several cerambycid species, is disclosed in here. Starting from 2-hexanone, through an α-hydroxylation using (diacetoxyiodo)benzene, 3-hydroxy-2-hexanone is obtained in good yield. Further reduction of this compound, gives 2,3-hexanediol in excellent yield. A study of the α-hydroxylation reaction of several alkylketones using an hypervalent iodine reagent is also disclosed in here. The synthesis of optically active compounds (R)- and (S)-3-hydroxy-2-hexanone was achieved starting from 2-hexanone with nitrosobenzene and l- and d-proline respectively, in several reaction media. 相似文献
10.