全文获取类型
收费全文 | 158篇 |
免费 | 5篇 |
专业分类
化学 | 143篇 |
力学 | 1篇 |
数学 | 8篇 |
物理学 | 11篇 |
出版年
2023年 | 1篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 5篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 4篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 6篇 |
2013年 | 16篇 |
2012年 | 20篇 |
2011年 | 20篇 |
2010年 | 4篇 |
2009年 | 14篇 |
2008年 | 11篇 |
2007年 | 11篇 |
2006年 | 7篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 4篇 |
2002年 | 4篇 |
2001年 | 1篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1995年 | 1篇 |
1991年 | 1篇 |
排序方式: 共有163条查询结果,搜索用时 15 毫秒
31.
Arroyo M Miguel D Villafañe F Alegria EC Pombeiro AJ 《Dalton transactions (Cambridge, England : 2003)》2012,41(23):7017-7025
Molybdenum and tungsten complexes containing the pypzH (3-(2-pyridyl)pyrazole) ligand as a chelating bidentate are prepared: [Mo(CO)(4)(pypzH)], cis-[MoBr(η(3)-allyl)(CO)(2)(pypzH)], cis-[MoCl(η(3)-methallyl)(CO)(2)(pypzH)], [MI(2)(CO)(3)(pypzH)] (M = Mo, W) from [Mo(CO)(4)(NBD)] or the adequate bis(acetonitrile) complexes. The deprotonation of the molybdenum allyl or methallyl complexes affords the bimetallic complexes [cis-{Mo(η(3)-allyl)(CO)(2)(μ(2)-pypz)}](2) or [cis-{Mo(η(3)-methallyl)(CO)(2)(μ(2)-pypz)}](2) (μ(2)-pypz = μ(2)-3-(2-pyridyl-κ(1)N)pyrazolate-2κ(1)N). The allyl complex was subjected to an electrochemical study, which shows a marked connection between both metallic centres through the bridging pyridylpyrazolates. 相似文献
32.
Olga Ershova Elisabete V. da Costa António J. S. Fernandes M. Rosário Domingues Dmitry V. Evtuguin Herbert Sixta 《Cellulose (London, England)》2012,19(6):2195-2204
The effect of urea on the cellulose degradation under conditions of alkaline pulping has been studied using purified cellulose powder. The increased cellulose yield in the presence of urea was assigned essentially to the carbamation reactions which were confirmed by elemental analysis, UV-resonance Raman and solid-state 13C nuclear magnetic resonance spectroscopy. The stabilizing effect of urea on the cellulose peeling reactions during heating up period of pulping process was suggested based on kinetic studies and additionally confirmed in model reactions using cellobiose. The reaction products formed in alkaline urea solutions were analysed by tandem electrospray ionization mass spectrometry and the occurrence of Maillard type reactions between reducing end groups of cellobiose and urea were evidenced. Both Maillard type reactions and carbamation of reducing end groups were proposed to be a part of cellulose protection mechanism against peeling under the conditions of alkaline pulping. 相似文献
33.
Cláudia Freitas Elisabete Neves Alberto Reis Paula C. Passarinho Teresa Lopes da Silva 《Applied biochemistry and biotechnology》2012,168(6):1501-1515
Bioethanol produced from lignocellulosic materials has been considered a sustainable alternative fuel. Such type of raw materials have a huge potential, but their hydrolysis into mono-sugars releases toxic compounds such as weak acids, which affect the microorganisms' physiology, inhibiting the growth and ethanol production. Acetic acid (HAc) is the most abundant weak acid in the lignocellulosic materials hydrolysates. In order to understand the physiological changes of Saccharomyces carlsbergensis when fermenting in the presence of different acetic acid (HAc) concentrations, the yeast growth was monitored by multi-parameter flow cytometry at same time that the ethanol production was assessed. The membrane potential stain DiOC6(3) fluorescence intensity decreased as the HAc concentration increased, which was attributed to the plasmic membrane potential reduction as a result of the toxic effect of the HAc undissociated form. Nevertheless, the proportion of cells with permeabilized membrane did not increase with the HAc concentration increase. Fermentations ending at lower external pH and higher ethanol concentrations depicted the highest proportions of permeabilized cells and cells with increased reactive oxygen species levels. Flow cytometry allowed monitoring, near real time (at-line), the physiological states of the yeast during the fermentations. The information obtained can be used to optimize culture conditions to improve bioethanol production. 相似文献
34.
Camila Elias Elisabete A. De Nadai Fernandes Márcio Arruda Bacchi 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):245-250
Brazil holds the second largest population of domestic dogs in the world, with 33 million dogs, only behind the United States.
The annual consumption of dog food in the country is 1.75 million tons, corresponding to the World’s sixth in trade turnover.
Dog food is supposed to be a complete and balanced diet, formulated with high quality ingredients. All nutrients and minerals
required for an adequate nutrition of dogs are added to the formulation to ensure longevity and welfare. In this context,
the present study aimed at assessing the chemical composition of dry dog foods commercialized in Brazil. Thirty-four samples
were acquired in the local market of Piracicaba and analyzed by instrumental neutron activation analysis (INAA) to determine
the elements As, Br, Ca, Co, Cr, Cs, Fe, K, La, Na, Rb, Sb, Sc, Se, U, and Zn. In general, the concentrations of Ca, Fe, K,
Na, and Zn complied with the values required by the Association of American Feed Control Officials (AAFCO). To evaluate the
safety of dog food commercialized in Brazil, further investigation is necessary to better understand the presence of toxic
elements found in this study, i.e. Sb and U. INAA was useful for the screening analysis of different types and brands of dry
dog foods for the determination of both essential and toxic elements. 相似文献
35.
Luís Gustavo Cofani dos Santos Márcio Arruda Bacchi Elisabete A. De Nadai Fernandes Simone Silveira Nery da Silva Cofani dos Santos 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):179-185
As a non-destructive and multi-element technique, with high-level metrological properties, instrumental neutron activation analysis (INAA) has an important role to determine chemical elements in food. However, its use may be limited when looking for mass fractions near the detection limits. The Compton scattering of higher energy gamma-rays raised the spectrum baseline thus impairing the determination of several elements. Therefore, the gamma-ray spectrometry with Compton suppression becomes an alternative for improving the performance of INAA, since it can reduce the uncertainty of measurements and the detection limits by increasing the proportion between photopeak area and baseline. Here the performance of a Compton suppression system set by Ortec, with 50% relative efficiency and 2.04?keV resolution (FWHM) for the 1,332?keV photopeak, was evaluated for food analysis. Samples of beans, chickpeas, lentils, peas, and rice were irradiated with neutrons and measured in the suppression system. Detection limits calculated from suppressed and unsuppressed spectra were compared. The suppression factor achieved by the system for 137Cs was 5.88?±?0.11 (n?=?20) in the plateau region (358 to 382?keV), which was stable along a 20?week period and similar to the data provided in literature for other systems. Amongst fifteen elements determined, the detection limits for Br, Co, La, Na, Sc, and Se were not improved by the use of Compton suppression. On the other hand, the variable improvement obtained for As, Ca, Cd, Cr, Fe, Hg, K, Rb, and Zn corroborated the idea that the performance of the Compton suppressor must be individually assessed for each type of sample. 相似文献
36.
Tassiane C. M. Matsubara Márcio A. Bacchi Elisabete A. De Nadai Fernandes 《Journal of Radioanalytical and Nuclear Chemistry》2012,291(1):201-205
Phosphorus is an essential element for plants and animals, playing a fundamental role in the production of biochemical energy.
Despite its relevance, phosphorus is not commonly determined by instrumental neutron activation analysis (INAA), because 32P does not emit gamma-rays in its decay. There are alternative methods for the determination of phosphorus by INAA, such as
the use of beta counting or the measurement of bremsstrahlung originated from the high energy beta particle from 32P. Here the determination of phosphorus in plant materials by measuring the bremsstrahlung production was further investigated, to optimize an analytical protocol for minimizing interferences and overcoming the poor
specificity. Eight certified reference materials of plant matrices with phosphorus ranging between 171 and 5,180 mg kg−1 were irradiated at a thermal neutron flux of 9.5 × 1012 cm−2 s−1 and measured with a HPGe detector at decay times varying from 7 to 60 days. Phosphorus solutions added to a certified reference
material at three levels were used for calibration. Counts accumulated in the baseline at four different regions of the gamma-ray
spectra were tested for the determination of phosphorus, with better results for the 100 keV region. The Compton scattering
contribution in the selected range was discounted using an experimental peak-to-Compton factor and the net areas of all peaks
in the spectra with energies higher than 218 keV, i.e. Compton edge above 100 keV. Amongst the interferences investigated,
the production of 32P from sulfur, and the contribution of Compton scattering should be considered for producing good results. 相似文献
37.
Joice Jaqueline Kaschuk Talita Martins Lacerda Véronique Coma Elisabete Frollini 《Cellulose (London, England)》2017,24(6):2437-2453
Enzymatic saccharification of sisal cellulosic pulp has been investigated. Brazil leads global production of lignocellulosic sisal fiber, which has high cellulose content, an important property for producing glucose via saccharification. Hence, sisal pulp can be a good alternative for use in biorefineries. Prior to enzymatic hydrolysis, the starting pulp [85 ± 2% α-cellulose, 15 ± 2% hemicelluloses, 1.2 ± 2% insoluble lignin, viscometric average molar mass (MMvis) 19,357 ± 590 g mol?1, crystallinity index (CI) 74%] was pretreated with alkaline aqueous solution (mercerization, 20 g of pulp L?1, 20% NaOH, 50 °C). The changes in the properties of the cellulosic pulp during this pretreatment were analyzed [α-cellulose content, MMvis, CI, pulp fiber dimensions, and scanning electron microscopy (SEM)]. The unmercerized and mercerized (97.4 ± 2% α-cellulose, 2.6 ± 2% hemicelluloses, 0.3 ± 0.1% insoluble lignin, MMvis 94,618 ± 300 g mol?1, CI 68%) pulps were subjected to enzymatic hydrolysis (48 h, commercial cellulase enzymes, 0.5 mL g?1 pulp); during the reactions, aliquots consisting of unreacted pulp and liquor were withdrawn from the medium at certain times and characterized (unreacted pulp: MMvis, CI, fiber dimensions, SEM; liquor: high-performance liquid chromatography). The changes in pulp properties observed during mercerization facilitated access of enzymes to cellulose chains, and the yield of the hydrolysis reaction increased from 50.2 (unmercerized pulp) to 89.0% (mercerized pulp). These initial results for enzymatic hydrolysis of sisal pulp indicate that it represents a good alternative biomass for bioethanol production. 相似文献
38.
Gabriela de Matuoka e Chiocchetti Elisabete A. De Nadai Fernandes Márcio Arruda Bacchi Rogério Augusto Pazim Silvana Regina Vicino Sarriés Thaís Melega Tomé 《Journal of Radioanalytical and Nuclear Chemistry》2013,297(3):399-404
Brazil is one of the largest producers of fruits cropping 40 million tons per year. In agro-food processing, approximately 50 % of raw material is discarded generating large amounts of by-products. The lack of information on the nutritional quality of agroindustrial by-products precludes their potential use in the manufacture of food products accessible to all. In this context, the objective of this work was to investigate the nutritional quality of by-products of the industrial processing of fruits. Samples of bagasse, peel and seeds of several fruits (banana, camu camu, coconut, cupuaçu, guava, jackfruit, mango, orange, papaya, pineapple, and soursop) were analysed by neutron activation analysis for the determination of Br, Ca, Co, Cr, Cs, Fe, K, La, Na, Rb, Sc and Zn. In general, higher levels of minerals were found in the by-products rather than in the pulps of fruits. This indicates that the use of the by-products should be encouraged, thereby reducing the economic and environmental impact of waste generated by agroindustrial processing. 相似文献
39.
Bárbara S.S. Lorca Evelin S. Bessa Marcio Nele Elisabete P. Santos José Carlos Pinto 《Macromolecular Symposia》2012,319(1):246-250
Summary: The present paper analyzes the production of poly(methyl methacrylate) – PMMA – nanoparticles loaded with benzophenone-3 through miniemulsion polymerization. The obtained product is homogeneous and stable, allowing for preparation of photo-protective formulations. It is observed in particular that bezophenone-3 interacts with the reacting system, promoting the growth of the PMMA chains produced in miniemulsion. 相似文献
40.
Luís Rodrigues Elisabete Coelho Renata Madeira Pedro Teixeira Isabel Henriques Manuel A. Coimbra 《Molecules (Basel, Switzerland)》2022,27(8)
Essential oil (EO), hydrolate, and nondistilled aqueous phase (decoction) obtained from the hydrodistillation of lemongrass byproducts were studied in terms of their potential as food ingredients under a circular economy. The EO (0.21%, dry weight basis) was composed mainly of monoterpenoids (61%), the majority being citral (1.09 g/kg). The minimal inhibitory concentrations (MIC) of lemongrass EO against Escherichia coli, Salmonella enterica, and Staphylococcus aureus, were 617, 1550, and 250 μg/mL, respectively. This effect was dependent on the citral content. Particularly for Gram-negative bacteria, a synergism between citral and the remaining EO compounds enhanced the antimicrobial activity. The polymeric material obtained from the nondistilled aqueous phase was composed of phenolic compounds (25% gallic acid equivalents) and carbohydrates (22%), mainly glucose (66 mol%). This polymeric material showed high antioxidant activity due to bound phenolic compounds, allowing its application as a functional dietary fiber ingredient. Matcha green tea formulations were successfully mixed with lemongrass hydrolate containing 0.21% EO (dry weight basis) with 58% of monoterpenoids, being citral at 0.73 g/kg, minimizing matcha astringency with a citrus flavor and extending the product shelf life. This holistic approach to essential oils’ hydrodistillation of Cymbopogon citratus byproducts allows for valorizing of the essential oil, hydrolate, and decoction for use as food ingredients. 相似文献