全文获取类型
收费全文 | 101374篇 |
免费 | 4319篇 |
国内免费 | 2748篇 |
专业分类
化学 | 40884篇 |
晶体学 | 1073篇 |
力学 | 8109篇 |
综合类 | 109篇 |
数学 | 34556篇 |
物理学 | 23710篇 |
出版年
2023年 | 494篇 |
2022年 | 695篇 |
2021年 | 814篇 |
2020年 | 999篇 |
2019年 | 976篇 |
2018年 | 10999篇 |
2017年 | 10785篇 |
2016年 | 7098篇 |
2015年 | 1924篇 |
2014年 | 1577篇 |
2013年 | 2042篇 |
2012年 | 6036篇 |
2011年 | 12703篇 |
2010年 | 7107篇 |
2009年 | 7414篇 |
2008年 | 8119篇 |
2007年 | 10129篇 |
2006年 | 1554篇 |
2005年 | 2415篇 |
2004年 | 2333篇 |
2003年 | 2582篇 |
2002年 | 1600篇 |
2001年 | 681篇 |
2000年 | 719篇 |
1999年 | 662篇 |
1998年 | 613篇 |
1997年 | 578篇 |
1996年 | 645篇 |
1995年 | 472篇 |
1994年 | 380篇 |
1993年 | 326篇 |
1992年 | 286篇 |
1991年 | 259篇 |
1990年 | 219篇 |
1989年 | 201篇 |
1988年 | 155篇 |
1987年 | 166篇 |
1986年 | 144篇 |
1985年 | 128篇 |
1984年 | 89篇 |
1983年 | 79篇 |
1982年 | 76篇 |
1981年 | 56篇 |
1980年 | 57篇 |
1979年 | 53篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
G. Z. Kaziev A. A. Dutov S. I. Quinones A. de Ita S. N. Sychkin 《Russian Journal of General Chemistry》2005,75(1):18-20
Indium dodecatungstosilicate of the composition [In(OH)⋅5H2O]2 [SiW12O40]⋅H2O is synthesized and studied by means of IR spectroscopy, thermogravimetry, and X-ray phase analysis. The crystals of this compound are triclinic, space group P1, a 13.079(3), b 13.795(3), c 13.967(3) Å, α 90.08(3)°, β 103.76(3)°, ψ107.76(3)°, Z 2, and πcalc 4.900 g cm−3.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 1, 2005, pp. 21–23.Original Russian Text Copyright © 2005 by Kaziev, Dutov, Quinones, Ita, Sychkin. 相似文献
973.
Protein tyrosine phosphatase 1B (PTP1B) functions by removing the phosphoryl group from tyrosinephosphorylated proteins in
insulin signaling and metabolism. The regeneration of the active site involves a sulphenylamide intermediate derived from
the intrastrand cross-linking between the catalytic serine and the neighboring backbone nitrogen. Two mechanisms have been
proposed for the formation of the sulphenylamide intermediate and the subsequent reactivation of the catalytic site. In the
current work, the proposed mechanisms have been investigated by the use of density functional theory calculations. Our results
suggest that these two mechanisms have similar overall energy barriers and that the preferred route will be determined by
the availability of hydrogen peroxide or other oxidizing reagents. 相似文献
974.
Summary. A highly efficient, selective, fast, and cheap protocol is developed for oxidation of aromatic amines and alcohols utilizing
34% hydrogen peroxide in water catalyzed by some W- and Mo-based heteropolyoxometalates. Findings showed that dodecatungstophosphoric
acid, H3PW12O40, was the most efficient catalyst in the examined oxidation reactions. This methodology may prove to be a valuable alternative
for eco-friendly green oxidation. Inherent simplicity, easy work up, and using regenerable catalysts were other key aspects
of this oxidation protocol. 相似文献
975.
A chiral liquid chromatographic method for determination of the enantiomeric purity of both l-carnitine and acetyl-l-carnitine is described. Separation of the enantiomers of dl-carnitine and acetyl-dl-carnitine was achieved on a commercial chiral column (Chiralcel OD-R) after derivatization with (alpha-bromo)methyl phenyl ketone. Introduction of this lipophilic UV chromophoric group to the carnitine and acetylcarnitine molecules improved their retention, resolution, and UV detection. The mobile phase was 74:26 (v/v) 0.5 mol L-1 sodium perchlorate–acetonitrile, pH 3.8, and the flow rate was 0.4 mL min-1. Detection was performed at 235 nm. The method is selective and reliable for determination of the enantiomeric purity of bulk drug substances l-carnitine and acetyl-l-carnitine. 相似文献
976.
A simple and sensitive fluorimetric method for the determination of ascorbic acid (AA) is described. The method is based on the condensation reaction between AA and o-phenylenediamine (OPDA) in the absence of the oxidant. The fluorescence intensity is measured at excitation and emission wavelengths of 360 and 430 nm, respectively. Under optimum condition, a linear relationship is obtained between the fluorescence intensity and the concentration of AA in the range of 0.05-40 μg ml−1. The detection limit is 0.006 μg ml−1, which is obviously lower than that of other fluorimetric methods reported. 相似文献
977.
Jocelyn Jalbert Roland Gilbert Pierre Tétreault Brigitte Morin Denise Lessard-Déziel 《Cellulose (London, England)》2007,14(4):295-309
In this study, headspace gas chromatography/mass spectrometry has been used to assess the volatile by-products generated by
the ageing of oil-impregnated paper insulation of power transformers. Sealed-glass ampoules were used to age under oxidative
conditions 0.5-g specimens of insulating paper in 9 mL of inhibited mineral oil in a temperature range of 60–120 °C and moisture
of 0.5, 1 and 2% (w/w). A linear relationship between one of the oil-soluble degradation by-products, i.e. methanol, and the
number of ruptured 1,4-β-glycosidic bonds of cellulose, regardless of the type of paper (ordinary Kraft or thermally-upgraded
(TU) Kraft paper), was established for the first time in this field. Ageing at 130 °C of model compounds of the Kraft paper
constituents (α-cellulose, hemicellulose and lignin) and two cellulosic breakdown by-products (D-(+)-glucose and 1,6-anhydro-β-d-glucopyranose) confirmed that the α-cellulose degradation was mostly responsible for the presence of this molecule in the
system. Furthermore, additional 130 °C-tests with six different papers and pressboard samples under a tight control of initial
moisture indicated that at least one molecule of methanol is formed for each rupture of 1,4-β-glucosidic bond of the molecular
chains. Stability tests showed that the ageing indicator is stable under the oxygen and temperature conditions of open-breathing
transformers. The presence of methanol was detected in 94% of oil samples collected from over than 900 in-service pieces of
equipment, confirming the potential for this application. Lastly, the tests have shown that oil-oxidation by-products and
TU-nitrogenous agents modify the methanol partitioning coefficients in the paper/oil/air system, which makes their study essential
over a range of field conditions encountered by power transformers. Results are presented and discussed in comparison with
2-furfuraldehyde, which is the current reference in the domain. 相似文献
978.
Muhammad Ilyas Sarwar Sonia Zulfiqar Zahoor Ahmad 《Colloid and polymer science》2007,285(15):1733-1739
Fluoro-aramid-based sol/gel-derived nanocomposites were synthesized by condensing a mixture of 4,4′-(hexafluoro-isopropylidene)dianiline
and 1,3-phenylenediamine with terephthaloylchloride (TPC) in dimethylacetamide. TPC was added in slight excess to produce
amide chains with carbonyl chloride end groups and then replaced with alkoxy groups using aminophenyltrimethoxysilane to develop
bonding with the silica network. Mechanical, dynamic mechanical thermal, water absorption and morphological measurements were
carried out on the thin hybrid films. Increase in the tensile strength and modulus was observed as compared to pristine polyamide.
The thermal decomposition temperature was found in the range of 400–500 °C. The water absorption was found to be reduced with
higher silica content. The glass transition temperature and the storage moduli increased with increasing silica concentration.
The maximum increase in the T
g value (345 °C) was observed with 20 wt% silica. Scanning electron microscopy revealed the uniform distribution of silica
in the matrix with an average particle size ranging from 8 to 50 nm. 相似文献
979.
Anna Wang Wei Zhang Jin Sun Jianfang Liu Yanshuang Sang Shuo Gao Zhonggui He 《Chromatographia》2007,66(9-10):741-745
A simple, rapid, and sensitive liquid chromatography–mass spectrometric (LC–MS) method was developed and validated for the determination of isoniazid in dog plasma. Plasma samples were deproteined with methanol and separated on a C18 column interfaced with a single quadrupole mass spectrometer, using 0.1% formic acid–acetonitrile (91:9 v/v) as mobile phase. Detection was performed by positive electrospray ionization with selected ion monitoring at m/z 138 for isoniazid and 152 for entecavir maleate internal standard. Linearity was obtained over the range of 25–5,000 ng mL?1, with a lower limit of quantification of 25 ng mL?1. The intra- and inter-day precision was less than 2.7% in terms of relative standard deviation. Accuracy, expressed as relative error, ranged from ?2.0 to 8.0%. Plasma samples were analysed within 5 min. The method was successfully applied to the evaluation of the pharmacokinetics of isoniazid in dog plasma. 相似文献
980.
吲哚乙酸与磷钼黄反应及其极谱分析的研究 总被引:1,自引:0,他引:1
在NH3-NH4Cl缓冲溶液中(pH=9.5),吲哚乙酸与磷钼黄的还原产物在滴汞电极上于-1.10V(vs.SCE)处产生一灵敏的极谱波,该波波高与吲哚乙酸的浓度在1.0×10-7-5.0×10-6mol/L范围内成线性关系,检测下限为5.0×10-8mol/L。如在上述还原产物中加入偶氮胭脂红B、孔雀绿、甲基紫等染料及聚乙烯醇,可使极谱波的灵敏度提高,使吲哚乙酸的浓度在5.0×10-8-1.0×10-6mol/L范围内与波高成线性关系,检测下限降低至1.0×10-8mol/L。 相似文献