全文获取类型
收费全文 | 428篇 |
免费 | 4篇 |
国内免费 | 5篇 |
专业分类
化学 | 251篇 |
晶体学 | 1篇 |
力学 | 14篇 |
数学 | 94篇 |
物理学 | 77篇 |
出版年
2022年 | 8篇 |
2021年 | 12篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 32篇 |
2012年 | 20篇 |
2011年 | 27篇 |
2010年 | 11篇 |
2009年 | 16篇 |
2008年 | 17篇 |
2007年 | 29篇 |
2006年 | 18篇 |
2005年 | 17篇 |
2004年 | 20篇 |
2003年 | 15篇 |
2002年 | 17篇 |
2001年 | 4篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 8篇 |
1994年 | 4篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1970年 | 2篇 |
1967年 | 2篇 |
1966年 | 2篇 |
1960年 | 2篇 |
1942年 | 1篇 |
排序方式: 共有437条查询结果,搜索用时 46 毫秒
51.
A classical system for the preparation of carbonyl compounds from alkenes relies upon ozonolysis of the double bond, followed by reductive cleavage of the ozonide so formed. Among the reagents of choice for such a reduction, triphenylphosphine certainly has enjoyed a widespread use.1 However, in spite of the simplicity of the method, often one can face problems in the purification of the carbonyl compound from unreacted triphenylphosphine, especially if the polarities of both products are very similar. We have encountered such a problem during the preparation of the (20 S)-6β -ethoxy-3 α, 5-cyclo-5α -pregnane-20-carbaldehyde 4 from the suitably protected stigmasterol32. In this case, when triphenylphosphine was used for reduction of ozonide we never could isolate the aldehyde 2 uncontaninated by tripherylphosphine. 相似文献
52.
Alessandra Bianco Prevot Debora Fabbri Edmondo Pramauro Claudio Baiocchi Claudio Medana Enzo Montoneri Vittorio Boffa 《Journal of photochemistry and photobiology. A, Chemistry》2010,209(2-3):224-231
To promote bio-based products for the industry, six bio-organic substances (cHALi) isolated from yard trimmings (green) and food (humid) urban residues aged under aerobic digestion for 0–60 days were investigated for their potential to perform as sensitizers for azo-dyes photodegradation. Ethylorange (EO) was used as probe molecule at 5 mg L?1 starting concentration and irradiated in a closed Pyrex® cell with a Xenon (1500 W) lamp and a cut-off filter for wavelengths below 340 nm or in a cylindrical photochemical reactor equipped with a 125 W medium pressure Hg lamp. The cHALi/EO ratio in the starting EO solution varied in the 0–200 (w/w) range. The % dye abatement was found a function of the irradiation time, of the type of cHALi substance and of the cHALi/EO ratio. The best results were achieved with cHALi isolated after 7 days biomass aerobic digestion. Total dye abatement was achieved within relatively short few hours. A progressive dye mineralization was observed under the same experimental conditions. On the contrary no significant degradation was evident for the cHALi substances within the same irradiation time. Several hydroxylated azo compounds were identified as likely responsible of residual color after total EO abatement. The results, coupled to the previously reported good performance of the same cHALi substances as auxiliaries for textile dyeing, augur well for the development of both efficient and environmentally friendly textile dyeing processes. 相似文献
53.
We consider in this paper random flights in ℝ
d
performed by a particle changing direction of motion at Poisson times. Directions are uniformly distributed on hyperspheres
S
1
d
. We obtain the conditional characteristic function of the position of the particle after n changes of direction. From this characteristic function we extract the conditional distributions in terms of (n+1)−fold integrals of products of Bessel functions. These integrals can be worked out in simple terms for spaces of dimension
d=2 and d=4. In these two cases also the unconditional distribution is determined in explicit form. Some distributions connected with
random flights in ℝ3 are discussed and in some special cases are analyzed in full detail. We point out that a strict connection between these
types of motions with infinite directions and the equation of damped waves holds only for d=2.
Related motions with random velocity in spaces of lower dimension are analyzed and their distributions derived. 相似文献
54.
Sergio Mantegani Tiziano Bandiera Enzo Brambilla Gabriella Traquandi 《Journal of heterocyclic chemistry》1992,29(2):455-459
The synthesis of a series of compounds indolo[4,3-fg]quinazoline related to ergoline (indolo[4,3-fg]quinoline) is reported. The key step of the synthetic sequence is the ring cleavage of the ketone IV. The stereochemistry of the chiral centers is established by the chirality of dihydrolysergic acid I. 相似文献
55.
Element specificity is one of the key factors underlying the widespread use and acceptance of X-ray absorption spectroscopy (XAS) as a research tool in the environmental and geo-sciences. Independent of physical state (solid, liquid, gas), XAS analyses of metal(loid)s in complex environmental matrices over the past two decades have provided important information about speciation at environmentally relevant interfaces (e.g. solid–liquid) as well as in different media: plant tissues, rhizosphere, soils, sediments, ores, mineral process tailings, etc. Limited sample preparation requirements, the concomitant ability to preserve original physical and chemical states, and independence from crystallinity add to the advantages of using XAS in environmental investigations. Interpretations of XAS data are founded on sound physical and statistical models that can be applied to spectra of reference materials and mixed phases, respectively. For spectra collected directly from environmental matrices, abstract factor analysis and linear combination fitting provide the means to ascertain chemical, bonding, and crystalline states, and to extract quantitative information about their distribution within the data set. Through advances in optics, detectors, and data processing, X-ray fluorescence microprobes capable of focusing X-rays to micro- and nano-meter size have become competitive research venues for resolving the complexity of environmental samples at their inherent scale. The application of μ-XANES imaging, a new combinatorial approach of X-ray fluorescence spectrometry and XANES spectroscopy at the micron scale, is one of the latest technological advances allowing for lateral resolution of chemical states over wide areas due to vastly improved data processing and detector technology. 相似文献
56.
57.
Salice P Maity P Rossi E Carofiglio T Menna E Maggini M 《Chemical communications (Cambridge, England)》2011,47(32):9092-9094
This communication demonstrates a straightforward continuous-flow method for efficient exohedral functionalisation of carbon nanotubes which affords soluble samples in a much shorter time over conventional batch processing. 相似文献
58.
Yeast cell wall particles: a promising class of nature-inspired microcarriers for multimodal imaging
Figueiredo S Moreira JN Geraldes CF Rizzitelli S Aime S Terreno E 《Chemical communications (Cambridge, England)》2011,47(38):10635-10637
This communication demonstrates that yeast cell wall particles (YCWPs) are a promising class of nature-inspired biocompatible microcarriers for the delivery of amphipathic/lipophilic imaging reporters. When a paramagnetic MRI agent is loaded, the longitudinal relaxivity per particle at 0.5 T is the highest ever reported for Gd-based systems. 相似文献
59.
60.