全文获取类型
收费全文 | 832篇 |
免费 | 41篇 |
国内免费 | 1篇 |
专业分类
化学 | 736篇 |
晶体学 | 5篇 |
力学 | 2篇 |
数学 | 48篇 |
物理学 | 83篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 34篇 |
2021年 | 41篇 |
2020年 | 27篇 |
2019年 | 34篇 |
2018年 | 16篇 |
2017年 | 26篇 |
2016年 | 37篇 |
2015年 | 31篇 |
2014年 | 36篇 |
2013年 | 44篇 |
2012年 | 74篇 |
2011年 | 81篇 |
2010年 | 37篇 |
2009年 | 33篇 |
2008年 | 47篇 |
2007年 | 63篇 |
2006年 | 51篇 |
2005年 | 43篇 |
2004年 | 23篇 |
2003年 | 23篇 |
2002年 | 27篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 7篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1990年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1943年 | 1篇 |
排序方式: 共有874条查询结果,搜索用时 15 毫秒
1.
2.
Carolina Pereira De Souza Margareth Spangler Andrade Bernardo Ruegger Almeida Neves 《Microscopy and microanalysis》2002,8(6):509-517
In this work, effective, yet simple, recycling mechanisms for used scanning probe microscopy (SPM) tips were implemented. Comprising a tip profile characterization methodology and specific cleaning procedures, which decontaminate SPM tips whether the contamination nature is known or not, such routes were optimized during numerous tests with brand new, previously used, and already discarded categories of SPM tips. The results show that if the used tip suffered contamination only, but no physical damages, during its scanning lifetime, it becomes readily available for reutilization after the cleansing process, characterizing a recycling route. On the other hand, if the tip went through wear and breakages during its utilization, it still can be decontaminated, but may not be directly reutilized due to its inadequate physical profile. Nevertheless, the methodology developed in this work may yet be applied as part of a more complex recycling route. 相似文献
3.
Fernández-Rivas C Méndez M Nieto-Oberhuber C Echavarren AM 《The Journal of organic chemistry》2002,67(15):5197-5201
The intramolecular reaction of allylsilanes and allylstannanes with alkynes proceeds catalytically in the presence of Pt(II), Pd(II), Ru(II), and Au(III) chlorides. Although more limited, AgOTf also catalyzes the cyclization. Usually, PtCl2 as the catalyst in methanol or acetone gives the best results. The reaction proceeds by exo attack of the allyl nucleophile on the alkyne to form five- or six-membered ring carbocycles. The reaction generally proceeds with anti stereoselectivity. However, a terminally substituted trimethylsilyl derivative reacts by a syn-type addition. The intermediate alkenylpalladium complex has been trapped with allyl chloride to form an allylated derivative with an additional carbon-carbon bond. 相似文献
4.
Cabrerizo FM Petroselli G Lorente C Capparelli AL Thomas AH Braun AM Oliveros E 《Photochemistry and photobiology》2005,81(5):1234-1240
Pterins are heterocyclic compounds with important biological functions, and most of them may exist in two acid-base forms in the pH range between 3 and 13 in aqueous solution. In this work, the photophysical properties of acid and basic forms of six compounds of the pterin family (6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT], N-methylfolic acid [MFA], and pteroic acid [PA]) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety and the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single-photon-counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) and short hydrocarbon chain (4 carbon atoms) (RPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (Phi(F)) of the first group of compounds are relatively high (>/=0.4), whereas MFA and PA exhibit very small Phi(F) values (=0.01). 相似文献
5.
The phosphoramidate-like reaction of 1,3-dithiole derived N-(diethoxyphosphinyl)hydrazones with formyl derivatives of 1,3-dithiole affords the longest aza-analogues of extended tetrathiafulvalenes with a polyenic spacer reported to date. Their structural and electrochemical properties are discussed. 相似文献
6.
Carolina A. Pinto P. M. Büchler J. Dweck 《Journal of Thermal Analysis and Calorimetry》2007,87(3):715-720
The catalyst
used in fluidized catalytic cracking (FCC) units of refineries after several
recovery cycles in regeneration units, reduces its activity and it is partially
substituted by new catalyst in the process. As it has a high silicon and aluminum
oxides content, the pozzolanic properties of a Brazilian FCC spent residual
catalyst, used in different substitution degrees to cement, were evaluated
by three thermal analysis techniques during the early stages of hydration
of a type II Portland cement. NCDTA curves show in real time that the residual
catalyst, accelerates the stages of cement hydration. TG and DSC curves of
respective pastes after 24 h of hydration evidence the pozzolanic activity
of the waste, respectively, by the lower water mass loss during the dehydroxylation
of the residual calcium hydroxide and by the lower dehydroxylation endothermal
effect. Within the analyzed period, the higher is the cement substitution
degree, the higher is the pozzolanic activity of the residual catalyst. 相似文献
7.
Hair color changes and protein damage caused by ultraviolet radiation 总被引:12,自引:0,他引:12
Santos Nogueira AC Joekes I 《Journal of photochemistry and photobiology. B, Biology》2004,74(2-3):109-117
Ultraviolet and visible radiations are known to damage hair. However, quantitative data relating damage to hair type, proteins and color to the radiation wavelength are missing. We studied the effect of UV plus visible, UVA plus visible, visible mercury-vapor lamp radiation and sunlight on (blended) virgin dark-brown, blond and red hair and (one head) virgin black and curly dark-brown hair. All hair types showed a substantial increase in protein loss in water after lamp and sun irradiation. The damaging effect of UVB was about 2-5 times higher than that of UVA plus visible radiation, depending on the hair type. Significant color changes were also observed in every hair type, after lamp and sun irradiation, being more pronounced for the light colored hairs. The luminosity difference parameter was the major contributor to the hair color changes, but significant changes in the red-green and yellow-blue parameters of every hair were observed. In this case, the damaging effect is ascribable mainly to UVA radiation. No significant changes in the mechanical properties or topography were observed in any case. We discuss these results in terms of hair type and composition and melanin types. 相似文献
8.
Carolina M Escobar-Llanos David Díaz Guillermo Mendoza-Díaz 《Journal of inclusion phenomena and macrocyclic chemistry》1998,31(4):333-349
The formation of inclusion complexes of -, -, hydroxypropyl-- (HP--) and -cyclodextrins with phenylbutazone and oxyphenbutazone has been studied in aqueous buffer solution (pH 7.5 and 0.1 mol dm-3 NaCl), dimethylsulfoxide, and 25, 50 and 75% dimethylsulfoxide/water mixtures. These complexation reactions have been followed by UV electronic absorption spectroscopy. In addition, 1D and 2D 1H NMR spectra were recorded to obtain structural information about the inclusion complexes formed in solution; 136 binding constant values were determined at five different temperatures (288, 293, 298, 303 and 310 K) from the electronic absorption data and, from these Hbinding and Sbinding values were obtained. At the studied cyclodextrin and guest concentration ranges, 1 : 1 inclusion complexes were detected. Only in three cases were 1 : 2 complexes detected, those of phenylbutazone and oxyphenbutazone with -cyclodextrin in aqueous, and oxyphenbutazone with hydroxypropyl--cyclodextrin in 75% dimethylsulfoxide/water solutions. 相似文献
9.
Carolina Menna Natalia Calonghi Lanfranco Masotti Paolo Neyroz 《Journal of fluorescence》1993,3(1):33-43
This report describes a method to conjugate lucifer yellow to the external surface of liposomes. The heterobifunctional cross-linking reagentN-succinimidyl 3-(2-pyridyldithio)propionate has been used to activate DMPE molecules. The DMPE-dithiopyridine product has been mixed with DMPC to prepare liposome vesicles. These have been reduced by DTT and finally reacted with lucifer yellow-iodoacetamide to produce the fluorescence-labeled vesicles. The quenching of their fluorescence intensity by Kl is consistent with fully exposed fluorophores. The decay of the fluorescence intensity of the lipid-bound lucifer yellow is biexponential (1=7.9 ns; 2=1.1 ns), with a relative yield of 0.16. When the fluorescent liposomes are mixed with cells, the lucifer yellow-DMPE derivative is transferred. Boar spermatozoa and peripheral human blood lymphocytes have been used as cellular models. The extent of incorporation is dependent on the incubation time and temperature. At 36°C, lucifer yellow fluorescence appears in the spermatozoa cells after 10 min of incubation and reaches its maximum at about 60 min. The fluorescent phospholipid derivative seems to incorporate specifically into membrane structures. The highest labeling ratio is observed with integer, scarcely motile, spermatozoa. A poorer labeling yield (15%) is found with lymphocytes. Interestingly, photobleaching due to epiillumination of the labeled cells is apparently negligible and cells are clearly visible after irradiation times ranging from several minutes to few hours.A preliminary account of this work was presented at the Quarto Simposio su Biotecnologie Biochimiche, Capri, 28–30 June 1992. 相似文献
10.
Rigol C Olea-Azar C Mendizábal F Otero L Gambino D González M Cerecetto H 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):2933-2938
Cyclic voltammetry and electron spin resonance (ESR) techniques were used in the investigation of several potential antiprotozoal thiosemicarbazones nitrofurane derivatives. A self-protonation process involving the protonation of the nitro group due to the presence of an acidic proton in the thiosemicarbazone moiety was observed in the first step of a CEE(rev) reduction mechanism of these derivatives. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. AM1 methodology was used to obtain the optimized geometries and UB3LYP calculations were performed to obtain the theoretical hyperfine coupling constants. The theoretical study exhibited an unusual assignment of the spin densities showing a free radical centered in the thiosemicarbazone moiety rather than the nitro which are in agreement with the experimental hyperfine pattern. 相似文献