全文获取类型
收费全文 | 517篇 |
免费 | 8篇 |
专业分类
化学 | 351篇 |
晶体学 | 11篇 |
力学 | 9篇 |
数学 | 54篇 |
物理学 | 100篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 10篇 |
2020年 | 6篇 |
2019年 | 11篇 |
2018年 | 5篇 |
2017年 | 3篇 |
2016年 | 18篇 |
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 29篇 |
2012年 | 16篇 |
2011年 | 30篇 |
2010年 | 24篇 |
2009年 | 39篇 |
2008年 | 40篇 |
2007年 | 23篇 |
2006年 | 19篇 |
2005年 | 29篇 |
2004年 | 27篇 |
2003年 | 12篇 |
2002年 | 19篇 |
2001年 | 26篇 |
2000年 | 16篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 10篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 1篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1917年 | 2篇 |
排序方式: 共有525条查询结果,搜索用时 15 毫秒
521.
Peroxidase immobilized on Amberlite IRA-743 resin for on-line spectrophotometric detection of hydrogen peroxide in rainwater 总被引:1,自引:0,他引:1
The importance of atmospheric hydrogen peroxide (H2O2) in the oxidation of SO2 and other compounds has been well established. A spectrophotometric method for the determination of hydrogen peroxide in rainwater is proposed. This method is based on selective oxidation of hydrogen peroxide using an on-line tubular reactor containing peroxidase immobilized on Amberlite IRA-743 resin. The hydrogen peroxide in the presence of phenol, 4-aminoantipyrine and peroxidase, produces a red compound (λ = 505 nm). Beer's law is obeyed in a concentration range of 1–100 μmol l−1 hydrogen peroxide with an excellent correlation coefficient (r = 0.9991), at pH 7.0, with a relative standard deviation (R.S.D.) <2%. The detection limit of the method is 0.7 μmol l−1 (4.8 ng of H2O2 in a 200 μl sample). Measurements of hydrogen peroxide in rain samples were carried out over the period from November 2003 to January 2005, in the central area of the Juiz de Fora city, Brazil. The concentration of H2O2 varied from values lower than the detection limit to 92.5 μmol l−1. The effects of the presence of nonseasalt (NSS) SO42−, NO3− and H+ in the concentration of hydrogen peroxide in the rainwater had been evaluated. The average concentrations of H2O2, NO3−, NSS SO42− and SO42− are 23.4, 18.9, 7.9 and 10.3 μmol l−1, respectively. The pH values for 82% of the collected samples are greater than 5.0. The spectrophotometeric method developed in this work that uses enzyme immobilized on the resin ion-exchange compared with the amperometric method did not present any significant difference in the results. 相似文献
522.
Fabiana S. Felix L. C. Cides da Silva L. Angnes J. R. Matos 《Journal of Thermal Analysis and Calorimetry》2009,95(3):877-880
The thermal decomposition of salbutamol (β2 — selective adrenoreceptor) was studied using differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry
(TG/DTG). It was observed that the commercial sample showed a different thermal profile than the standard sample caused by
the presence of excipients. These compounds increase the thermal stability of the drug. Moreover, higher activation energy
was calculated for the pharmaceutical sample, which was estimated by isothermal and non-isothermal methods for the first stage
of the thermal decomposition process. For isothermal experiments the average values were E
act=130 kJ mol−1 (for standard sample) and E
act=252 kJ mol−1 (for pharmaceutical sample) in a dynamic nitrogen atmosphere (50 mL min−1). For non-isothermal method, activation energy was obtained from the plot of log heating rates vs. 1/T in dynamic air atmosphere (50 mL min−1). The calculated values were E
act=134 kJ mol−1 (for standard sample) and E
act=139 kJ mol−1 (for pharmaceutical sample). 相似文献
523.
W. K. Yoshito J. R. Matos V. Ussui D. R. R. Lazar J. O. A. Paschoal 《Journal of Thermal Analysis and Calorimetry》2009,97(1):303-308
A porous nickel–8 mol% yttria stabilized zirconia (Ni–8YSZ) composite, used as anode for solid oxide fuel cell, was obtained
by reduction of NiO–8YSZ cermet. The first goal was the evaluation of the temperature effect of powder processing by thermogravimetry.
In addition, the influence of porosity in the reduction kinetic of the sample sintered at 1450 °C was evaluated. The final
porosity produced in NiO–8YSZ composite by pore former was 30.4 and 37.9 vol.%, respectively, for 10 and 15 mass% of corn
starch. The sample with 15 mass% of corn starch promotes a reduction rate almost twice higher than sample with 10 mass% of
corn starch. The porosity introduced by the reduction of NiO was 23 vol.%. 相似文献
524.
R. M. A. Pinto M. Ramos Silva A. Matos Beja J. A. R. Salvador J. A. Paixo 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(5):o214-o216
In the title compound, C23H34O4, which is an intermediate in the synthesis of pregnane derivatives with a modified skeleton that show potent abortion‐inducing activity, the conformation of ring B is close to half‐chair due to the presence of both the C=C double bond and the axial 5β‐methyl group. Rings A and C have conformations close to chair, while ring D has a twisted conformation around the bridgehead C—C bond. Molecules are hydrogen bonded via the hydroxyl and acetoxy groups into infinite chains. Quantum‐mechanical ab initio Roothan Hartree–Fock calculations show that crystal packing might be responsible for the low values of the angles between rings A and B, and between ring A and rings C and D, as well as for a different steric position of the methyl ketone side chain compared to the geometry of the free molecule. 相似文献
525.
Fbio Rodrigues de Oliveira Ngila Monteiro da Silva Moiss Hamoy Maria Elena Crespo-Lpez Irlon Maciel Ferreira Edilene Oliveira da Silva Barbarella de Matos Macchi Jos Luiz Martins do Nascimento 《Molecules (Basel, Switzerland)》2022,27(11)
Seizures and epilepsy are some of the most common serious neurological disorders, with approximately 80% of patients living in developing/underdeveloped countries. However, about one in three patients do not respond to currently available pharmacological treatments, indicating the need for research into new anticonvulsant drugs (ACDs). The GABAergic system is the main inhibitory system of the brain and has a central role in seizures and the screening of new ACD candidates. It has been demonstrated that the action of agents on endocannabinoid receptors modulates the balance between excitatory and inhibitory neurotransmitters; however, studies on the anticonvulsant properties of endocannabinoids from plant oils are relatively scarce. The Amazon region is an important source of plant oils that can be used for the synthesis of new fatty acid amides, which are compounds analogous to endocannabinoids. The synthesis of such compounds represents an important approach for the development of new anticonvulsant therapies. 相似文献