全文获取类型
收费全文 | 2725篇 |
免费 | 98篇 |
国内免费 | 1篇 |
专业分类
化学 | 1779篇 |
晶体学 | 20篇 |
力学 | 94篇 |
数学 | 380篇 |
物理学 | 551篇 |
出版年
2023年 | 25篇 |
2022年 | 29篇 |
2021年 | 54篇 |
2020年 | 58篇 |
2019年 | 52篇 |
2018年 | 80篇 |
2017年 | 74篇 |
2016年 | 156篇 |
2015年 | 112篇 |
2014年 | 125篇 |
2013年 | 210篇 |
2012年 | 200篇 |
2011年 | 233篇 |
2010年 | 139篇 |
2009年 | 92篇 |
2008年 | 195篇 |
2007年 | 177篇 |
2006年 | 134篇 |
2005年 | 98篇 |
2004年 | 78篇 |
2003年 | 81篇 |
2002年 | 55篇 |
2001年 | 35篇 |
2000年 | 32篇 |
1999年 | 26篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 22篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有2824条查询结果,搜索用时 15 毫秒
981.
982.
Sousa C Lopes G Pereira DM Taveira M Valentão P Seabra RM Pereira JA Baptista P Ferreres F Andrade PB 《Combinatorial chemistry & high throughput screening》2007,10(5):377-386
The changes in antioxidant compounds of Brassica oleracea L. var. costata DC seeds were monitored during the first twelve days of seedling development. Sprouts were screened at time intervals of two days for phenolic compounds and organic acids. The identified phenolic compounds included esters of sinapic acid with glucose, gentiobiose and kaempferol, as well as sinapoylcholine. The organic acids were oxalic, aconitic, citric, pyruvic, malic, shikimic, and fumaric acids. During germination, a depletion of phenolic compounds was observed, although no qualitative changes were seen. Among individual compounds, kaempferol, choline and glucose esters of sinapic acid showed a marked decrease between days two and six, whereas the changes in gentiobiose esters of sinapic acid were smaller. The total organic acids content increased rapidly during the first four days, with less significant variations thereafter. Malic acid, the major organic acid found in sprouts, greatly contributed to this result though oxalic, pyruvic, and fumaric acids also increased in the same manner. In contrast, aconitic, citric and shikimic acids showed decreases between days two and twelve of germination. 相似文献
983.
A. Suwattanamala A. L. Magalhães J. A. N. F. Gomes 《Theoretical chemistry accounts》2007,117(3):431-440
A comparative DFT study of distally dibromo-dipropoxythiacalix[4]arene and dipropoxythiacalix[4]arene was performed. The hydrogen
bonds OH O at the lower rim play an important role in stabilizing all the conformers, and cone is predicted to be the most stable. The structure and energetic ordering of the conformers are not affected by the bromination
at upper rim. The inclusion of the solvent effect keeps the energetic orderings unchanged, but leads to a reduction in the
energy gaps between conformers. Concerning the 1:1 neutral Zn2+ complexes, only one binding mode was found which presented the metal ion coordinated to four oxygen atoms at lower rim. This
is in good agreement with our previous study on the Zn2+ complex with thiacalix[4]arene carrying opposite phenolate groups. The results thus support the idea that the position of
the phenolate groups at lower rim is crucial to define the structural binding mode pattern of metal complexes. 相似文献
984.
Contieri Abad F Winck PR Benvenutti EV do Carmo Ruaro Peralba M Bastos Caramão E Alcaraz Zini C 《Journal of separation science》2007,30(13):2109-2116
A new material for matrix solid phase dispersion (MSPD) was synthesized -- p-nitro-N-propylaniline/silica (pNNPASi) by grafting reactions, characterized by elemental analysis and N(2)-adsorption-desorption isotherms, and tested for multiclass multiresidue analysis of pesticides in wet and freeze-dried carrots. Results obtained applying this new solid phase sorbent to MSPD extraction of ten pesticides (trichlorphon, trifluralin, dicloran, chlorothalonil, prometryn, linuron, captan, procymidone, prochloraz, and deltametrin) in wet carrots showed better results than the ones obtained for freeze-dried samples. Recoveries were in the range of 48-106% and precisions varied from 6 to 20% when wet samples were employed. Comparison between pNNPASi sorbent and C(18) showed better performance of pNNPASi for eight out of ten pesticides tested. The LOQs show that the developed method can be used to detect the pesticides investigated in carrots at concentrations below the maximum residue levels (MRL) established by EU, USEPA, and National Sanitary Surveillance Agency (ANVISA). Linuron, captan, prochloraz, and deltamethrin were found in at least one of the two commercial samples studied in concentrations above the LOQ of this method. Concentrations of the last three pesticides were above the European MRL in one of the commercial samples. 相似文献
985.
Carlos G. Lopez Sarah E. Rogers Ralph H. Colby Peter Graham João T. Cabral 《Journal of Polymer Science.Polymer Physics》2015,53(7):492-501
We report a small angle neutron scattering (SANS) and rheology study of cellulose derivative polyelectrolyte sodium carboxymethyl cellulose with a degree of substitution of 1.2. Using SANS, we establish that this polymer is molecularly dissolved in water with a locally stiff conformation with a stretching parameter . We determine the cross sectional radius of the chain ( 3.4 Å) and the scaling of the correlation length with concentration (ξ = 296 c?1∕2Å for c in g/L) is found to remain unchanged from the semidilute to concentrated crossover as identified by rheology. Viscosity measurements are found to be in qualitative agreement with scaling theory predictions for flexible polyelectrolytes exhibiting semidilute unentangled and entangled regimes, followed by what appears to be a crossover to neutral polymer concentration dependence of viscosity at high concentrations. Yet those higher concentrations, in the concentrated regime defined by rheology, still exhibit a peak in the scattering function that indicates a correlation length that continues to scale as . © 2014 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 492–501 相似文献
986.
An N‐Acetyl Cysteine Ruthenium Tricarbonyl Conjugate Enables Simultaneous Release of CO and Ablation of Reactive Oxygen Species 下载免费PDF全文
Dr. João D. Seixas Dr. Miguel Chaves‐Ferreira Diana Montes‐Grajales Ana M. Gonçalves Dr. Ana R. Marques Dr. Lígia M. Saraiva Prof. Jesus Olivero‐Verbel Prof. Carlos C. Romão Dr. Gonçalo J. L. Bernardes 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(42):14708-14712
We have designed and synthesised a [Ru(CO)3Cl2(NAC)] pro‐drug that features an N‐acetyl cysteine (NAC) ligand. This NAC carbon monoxide releasing molecule (CORM) conjugate is able to simultaneously release biologically active CO and to ablate the concurrent formation of reactive oxygen species (ROS). Complexes of the general formulae [Ru(CO)3(L)3]2+, including [Ru(CO)3Cl(glycinate)] (CORM‐3), have been shown to produce ROS through a water–gas shift reaction, which contributes significantly, for example, to their antibacterial activity. In contrast, NAC‐CORM conjugates do not produce ROS or possess antibacterial activity. In addition, we demonstrate the synergistic effect of CO and NAC both for the inhibition of nitric oxide (formation) and in the expression of tumour‐necrosis factor (TNF)‐α. This work highlights the advantages of combining a CO‐releasing scaffold with the anti‐oxidant and anti‐inflammatory drug NAC in a unique pro‐drug. 相似文献
987.
Phototherapy on the Treatment of Burning Mouth Syndrome: A Prospective Analysis of 20 Cases 下载免费PDF全文
Lúcia de Fátima C. dos Santos Samantha C. de Andrade Gessé E. C. Nogueira Jair C. Leão Patrícia M. de Freitas 《Photochemistry and photobiology》2015,91(5):1231-1236
The aim of this study was to report the effect of laser phototherapy (LPT) on the treatment of burning mouth syndrome (BMS). This prospective clinical study reports on preliminary outcomes of twenty volunteers diagnosed with BMS who have undergone the conventional treatment prior to laser phototherapy. LPT consisted of weekly sessions of LPT (660 nm), for a period of 10 weeks. The laser protocol consisted of the following parameters: 40 mW, 10 J cm2 and 0.4 J per point, irradiation time of 10 s. In all sessions, the burning intensity was evaluated with a 10 cm Visual Analogue Scale (VAS). The burning intensity evaluation by VAS was performed immediately before and after each LPT session. Nonparametric test of Wilcoxon was used for statistical analysis, considering a significance level of 5%. All volunteers reported reduced burning intensity in all sessions when compared to the previous one and reduction in VAS scores by up to 49% in the last clinical session when compared to the first session. When only the VAS baseline of the first session was compared with the consecutive sessions, there was a statistically significant reduction in VAS scores in almost all sessions. The LPT may be an alternative treatment for the relief of oral burning symptoms in patients with BMS. 相似文献
988.
We present a low cost paper‐based electronic tongue capable of discriminating forged water samples. System comprises of 4 paper‐based potentiometric sensors (sensitive to Cl?, Na+/K+, Ca2+/Mg2+, ) and a traditional Ag/AgCl reference electrode. Different electrode materials and methods of insulation were tested with best results obtained for pencil graphite and lamination. The presented electronic tongue was able to distinguish tap and lake water from mineral water samples (PCA – Principal Component Analysis and KNN ? K‐nearest neighbour). In total 14 different water samples were used in this study. Sensors presented good signal repeatability, selectivity and reasonable sensitivity. 相似文献
989.
Controlling the Formation of Ionic‐Liquid‐based Aqueous Biphasic Systems by Changing the Hydrogen‐Bonding Ability of Polyethylene Glycol End Groups 下载免费PDF全文
Prof. Dr. Jorge F. B. Pereira Dr. Kiki A. Kurnia Dr. Mara G. Freire Prof. Dr. João A. P. Coutinho Prof. Dr. Robin D. Rogers 《Chemphyschem》2015,16(10):2219-2225
The formation of aqueous biphasic systems (ABS) when mixing aqueous solutions of polyethylene glycol (PEG) and an ionic liquid (IL) can be controlled by modifying the hydrogen‐bond‐donating/‐accepting ability of the polymer end groups. It is shown that the miscibility/immiscibility in these systems stems from both the solvation of the ether groups in the oxygen chain and the ability of the PEG terminal groups to preferably hydrogen bond with water or the anion of the salt. The removal of even one hydrogen bond in PEG can noticeably affect the phase behavior, especially in the region of the phase diagram in which all the ethylene oxide (EO) units of the polymeric chain are completely solvated. In this region, removing or weakening the hydrogen‐bond‐donating ability of PEG results in greater immiscibility, and thus, in a higher ability to form ABS, as a result of the much weaker interactions between the IL anion and the PEG end groups. 相似文献
990.
Insights on beer volatile profile: Optimization of solid‐phase microextraction procedure taking advantage of the comprehensive two‐dimensional gas chromatography structured separation 下载免费PDF全文
Cátia Martins Tiago Brandão Adelaide Almeida Sílvia M. Rocha 《Journal of separation science》2015,38(12):2140-2148
The aroma profile of beer is crucial for its quality and consumer acceptance, which is modu‐lated by a network of variables. The main goal of this study was to optimize solid‐phase microextraction experimental parameters (fiber coating, extraction temperature, and time), taking advantage of the comprehensive two‐dimensional gas chromatography structured separation. As far as we know, it is the first time that this approach was used to the untargeted and comprehensive study of the beer volatile profile. Decarbonation is a critical sample preparation step, and two conditions were tested: static and under ultrasonic treatment, and the static condition was selected. Considering the conditions that promoted the highest extraction efficiency, the following parameters were selected: poly(dimethylsiloxane)/divinylbenzene fiber coating, at 40ºC, using 10 min of pre‐equilibrium followed by 30 min of extraction. Around 700–800 compounds per sample were detected, corresponding to the beer volatile profile. An exploratory application was performed with commercial beers, using a set of 32 compounds with reported impact on beer aroma, in which different patterns can be observed through the structured chromatogram. In summary, the obtained results emphasize the potential of this methodology to allow an in‐depth study of volatile molecular composition of beer. 相似文献