全文获取类型
收费全文 | 2180篇 |
免费 | 59篇 |
国内免费 | 2篇 |
专业分类
化学 | 1444篇 |
晶体学 | 21篇 |
力学 | 59篇 |
数学 | 301篇 |
物理学 | 416篇 |
出版年
2023年 | 20篇 |
2022年 | 78篇 |
2021年 | 77篇 |
2020年 | 38篇 |
2019年 | 48篇 |
2018年 | 44篇 |
2017年 | 43篇 |
2016年 | 79篇 |
2015年 | 64篇 |
2014年 | 86篇 |
2013年 | 129篇 |
2012年 | 141篇 |
2011年 | 170篇 |
2010年 | 105篇 |
2009年 | 134篇 |
2008年 | 163篇 |
2007年 | 125篇 |
2006年 | 107篇 |
2005年 | 76篇 |
2004年 | 72篇 |
2003年 | 57篇 |
2002年 | 50篇 |
2001年 | 41篇 |
2000年 | 30篇 |
1999年 | 21篇 |
1998年 | 31篇 |
1997年 | 16篇 |
1996年 | 22篇 |
1995年 | 23篇 |
1994年 | 14篇 |
1993年 | 13篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 12篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1976年 | 8篇 |
1974年 | 3篇 |
1902年 | 2篇 |
1897年 | 2篇 |
1894年 | 2篇 |
排序方式: 共有2241条查询结果,搜索用时 15 毫秒
981.
Ferreira A Ananias D Carlos LD Morais CM Rocha J 《Journal of the American Chemical Society》2003,125(47):14573-14579
The synthesis and structural characterization of microporous lanthanide silicates (Na(1.08)K(0.5)Ln(1.14)Si(3)O(8.5).1.78H(2)O, Ln = Eu, Tb, Sm, Ce) are reported. The structure of these solids is closely related with the structure of hydrated calcium silicate minerals known as tobermorites and was solved by powder X-ray diffraction ab initio (direct) methods and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, (23)Na and (29)Si MAS NMR and luminescence spectroscopy. These materials combine microporosity with interesting photoluminescence properties, and their structural flexibility allows fine-tuning of luminescence properties, by introducing a second type of lanthanide ion in the framework. Thus, they may find applications in new types of sensors. 相似文献
982.
Oligomeric proanthocyanidins: naturally occurring O-heterocycles 总被引:1,自引:0,他引:1
This review covers the flavan-3-ols (catechins), flavan-4-ols/flavan-3,4-diols (leucoanthocyanidins), A-type proanthocyanidins, B-type proanthocyanidins including the procyanidins, prodelphinidins, propelargonidins, proteracacinidins, promelacacinidins, procassinidins, probutinidins, and non-proanthocyanidins with flavan-3-ol constituent units. Newly isolated proanthocyanidins, structure elucidation, syntheses, HPLC/MS analysis, NMR/ conformational analysis, and the effects of proanthocyanidins on human nutrition and health are reported. The literature from January 1999 to December 2001 is reviewed, and 130 references are cited. 相似文献
983.
M. G. Ferreira da Silva J. C. Waerenborgh 《Journal of Radioanalytical and Nuclear Chemistry》2007,272(1):177-185
Samples of the composition of 10Fe2O3·10CaO·80SiO2 were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe3+ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation. 相似文献
984.
M. G. Ferreira da silva J. M. Fernández Navarro 《Journal of Sol-Gel Science and Technology》1996,6(2):169-177
The present investigation was carried out in order to study the incorporation of iron oxide in several samples with a general composition xFeO-(100-x)SiO2 (x between 0.1 and 9.5) and 5RnO-xFeO-(95-x)SiO2 (R=Li, Na or Ca) prepared by the sol-gel method. Samples were gelated and dried at 60°C and thermally treated between 200 and 1000°C in oxidizing and reducing atmospheres. The color change and structural evolution were studied by VIS and NIR spectrophotometry, TEM and X-ray diffraction.When the temperature increases, the color changes from yellow to brown-reddishness, going through a bright ruby-red, at the same time the absorption edge shifts to higher wavelengths and at elevated temperatures to smaller wavelengths newly. A part of iron oxide is incorporated into the glass network and the remaining as colloidal particles, producing a double mechanism of coloration by optical absorption and dispersion.In the samples heat-treated in oxidizing conditions all iron is present in the ferric state. When the heat-treatment is carried out in reducing conditions the presence of iron (II) was detected at temperatures of nearly 1000°C. 相似文献
985.
In the present paper, a simultaneous pre-concentration procedure for the sequential determination of cadmium and lead in table salt samples using flame atomic absorption spectrometry is proposed. This method is based on the liquid-liquid extraction of cadmium(II) and lead(II) ions as dithizone complexes and direct aspiration of the organic phase for the spectrometer. The sequential determination of cadmium and lead is possible using a computer program. The optimization step was performed by a two-level fractional factorial design involving the variables: pH, dithizone mass, shaking time after addition of dithizone and shaking time after addition of solvent. In the studied levels these variables are not significant. The experimental conditions established propose a sample volume of 250 mL and the extraction process using 4.0 mL of methyl isobutyl ketone. This way, the procedure allows determination of cadmium and lead in table salt samples with a pre-concentration factor higher than 80, and detection limits of 0.3 ng g−1 for cadmium and 4.2 ng g−1 for lead. The precision expressed as relative standard deviation (n = 10) were 5.6 and 2.6% for cadmium concentration of 2 and 20 ng g−1, respectively, and of 3.2 and 1.1% for lead concentration of 20 and 200 ng g−1, respectively. Recoveries of cadmium and lead in several samples, measured by standard addition technique, proved also that this procedure is not affected by the matrix and can be applied satisfactorily for the determination of cadmium and lead in saline samples. The method was applied for the evaluation of the concentration of cadmium and lead in table salt samples consumed in Salvador City, Bahia, Brazil. 相似文献
986.
Silva WC Lima JB Moreira IS Neto AM Gandra FC Ferreira AG McGarvey BR Franco DW 《Inorganic chemistry》2003,42(21):6898-6906
The compounds [Ru(NH(3))(5)(dtdp)](TFMS)(3), [Os(NH(3))(5)(dtdp)](TFMS)(3), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](TFMS)(6), [(NH(3))(5)Os(dtdp)Ru(NH(3))(5)](TFMS)(3)(PF(6))(2), and [(NH(3))(5)Os(dtdp)Fe(CN)(5)] (dtdp = 4,4'-dithiodipyridine, TFMS = trifluoromethanesulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and (1)H NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os(NH(3))(5)(dtdp)](3+) as a function of the solution pH enable us to calculate the pK(a) for the [Os(NH(3))(5)(dtdpH)](4+) and [Os(NH(3))(5)(dtdpH)](3+) acids as 3.5 and 5.5, respectively. The comparison of the above pK(a) data with that for the free ligand (pK(1) = 4.8) provides evidence for the -S-S- bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](6+), is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the -S-S- bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin-spin interaction parameter of 150-200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the pi orbitals of the dttp ligand relative to that found in [Os(NH(3))(5)(dtdp)](3+). The comproportionation constant, K(c) = 2.0 x 10(5), for the equilibrium equation [Os(II)Os(II)] + [Os(III)Os(III)] right harpoon over left harpoon 2[Os(II)Os(III)] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](5+) (lambda(MMCT) = 1665 nm, epsilon = 3.5 x 10(3) M(-)(1) cm(-)(1), deltanu(1/2) = 3.7 x 10(3) cm(-)(1), alpha = 0.13, and H(AB) = 7.8 x 10(2) cm(-)(1)), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH(3))(5)Os(III)(dtdp)Ru(II)(NH(3))(5)](5+) (lambda(MMCT) = 965 nm, epsilon = 2.2 x 10(2) M(-)(1) cm(-)(1), deltanu(1/2) = 3.0 x 10(3) cm(-)(1), and H(AB) = 2.2 x 10(2) cm(-)(1)) and [(NH(3))(5)Os(III)(dtdp)Fe(II)(CN)(5)] (lambda(MMCT) = 790 nm, epsilon = 7.5 x 10 M(-)(1) cm(-)(1), deltanu(1/2) = 5.4 x 10(3) cm(-)(1), and H(AB) = 2.0 x 10(2) cm(-)(1)) also suggest a considerable electron delocalization through the S-S bridge. As indicated by a comparison of K(c) and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order: Fe < Ru < Os. 相似文献
987.
Olaf D. Kinzel Edith Monteagudo Ester Muraglia Federica Orvieto Giovanna Pescatore Maria del Rosario Rico Ferreira Michael Rowley Vincenzo Summa 《Tetrahedron letters》2007,48(37):6552-6555
An efficient synthesis of methyl 9-amino-3-hydroxy-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-2-carboxylate as a late stage intermediate for a new class of HIV-1 integrase inhibitors is described. After construction of the bicyclic core scaffold, a stereoselective introduction of the chiral amino group in the 9-position is achieved. 相似文献
988.
Marcos de A. Bezerra André L. B. Conceição Sérgio L. C. Ferreira 《Mikrochimica acta》2006,154(1-2):149-152
This paper describes the development of a cloud point extraction procedure for the determination of manganese in saline effluents
by flame atomic absorption spectrometry (FAAS). The optimization step was performed using the Doehlert matrix involving the
following variables: buffer concentration, pH and centrifugation time. The validation process was assessed as: parameters
of the analytical curve, precision, effect of other ions in the proposed procedure, robustness test and accuracy. The proposed
procedure allows the determination of manganese with a detection limit (3δ/S) of 0.60 μg L−1, and a precision expressed as relative standard deviation (RSD) of 2.2 (n = 8) and 1.5% (n = 8) for a manganese concentration
of 1 and 5 μg L−1, respectively. The pre-concentration factor obtained was 84. The recovery achieved for the determination of manganese in
the presence of several other metal ions demonstrated that this procedure could be satisfactorily applied to the analysis
of environmental samples. The accuracy was confirmed by the analysis of CRM trace elements in water (NIST 1643d). This procedure
was applied to the determination of manganese in saline effluents of a petroleum refinery. For three analyzed samples the
manganese content varied between 44.9 and 67.9 μg L−1. 相似文献
989.
Clostridium botulinum organisms generally produce 1 of 4 neurotoxin types (A, B, E, and F) associated with human illness. Neurotoxin type determination is important in identification of the bacterium. A polymerase chain reaction (PCR) method was developed to identify 24 h botulinal cultures as potential types A, B, E, and F neurotoxin producers as well as other clostridial species which also produce neurotoxins. Components of the PCR and amplification conditions were adjusted for optimal amplification of toxin gene target regions to enable simultaneous testing for types A, B, E, and F in separate tubes using a single thermal cycler. Each primer set was specific for its corresponding toxin type. A DNA extraction procedure was also included to remove inhibitory substances that may affect amplification. This procedure is rapid, sensitive, and specific for identification of toxigenic C. botulinum. 相似文献
990.
A flow-injection method has been developed for the sequential spectrophotometric determination of iron and titanium using 3,4 dihydroxybenzoic acid as chromogenic reagent. The system involves the sequential measurement of the absorbances of the complexes at 380 and 570 nm. The system is designed using a simultaneous injection of sample and reagent into separate carrier streams. The proposed method is characterized by a precision of about 2%, a sampling rate of about 50 samples per hour, and a reagent consumption of 200 mul (0.50% solution) per sample. It is relatively free of interferences and was used for the sequential determination of titanium and iron in rocks. 相似文献