全文获取类型
收费全文 | 3620篇 |
免费 | 82篇 |
国内免费 | 769篇 |
专业分类
化学 | 3507篇 |
晶体学 | 9篇 |
力学 | 33篇 |
数学 | 10篇 |
物理学 | 912篇 |
出版年
2024年 | 15篇 |
2023年 | 94篇 |
2022年 | 144篇 |
2021年 | 76篇 |
2020年 | 120篇 |
2019年 | 91篇 |
2018年 | 83篇 |
2017年 | 111篇 |
2016年 | 126篇 |
2015年 | 95篇 |
2014年 | 139篇 |
2013年 | 190篇 |
2012年 | 174篇 |
2011年 | 249篇 |
2010年 | 208篇 |
2009年 | 285篇 |
2008年 | 251篇 |
2007年 | 316篇 |
2006年 | 262篇 |
2005年 | 177篇 |
2004年 | 173篇 |
2003年 | 144篇 |
2002年 | 115篇 |
2001年 | 108篇 |
2000年 | 81篇 |
1999年 | 78篇 |
1998年 | 73篇 |
1997年 | 77篇 |
1996年 | 61篇 |
1995年 | 47篇 |
1994年 | 44篇 |
1993年 | 37篇 |
1992年 | 28篇 |
1991年 | 29篇 |
1990年 | 26篇 |
1989年 | 31篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有4471条查询结果,搜索用时 31 毫秒
101.
We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation. 相似文献
102.
The dynamics of dissociative hydrogen adsorption on clean polycrystalline uranium has been studied using temperature programmed desorption and supersonic molecular beams. The initial sticking probability was measured as a function of incident kinetic energy between 3 and 255 meV. Two adsorption channels were observed; a non-activated direct channel was shown to be active over the entire energy range studied and a low energy indirect channel that was characterised by a decrease in sticking probability with increasing beam energy, and an insensitivity to both surface temperature and a range of hydrogen coverages. Together these results suggest the existence of an unaccommodated molecular precursor that has sufficient lifetime and mobility to locate favourable sites and dissociatively adsorb. 相似文献
103.
Miros?aw Kwiatkowski 《Applied Surface Science》2011,257(21):8912-8922
The aim of this work has been to analyze the problems related to the identification of microporous structure parameters of carbonaceous materials. The new methods for microporous structure parameters identification have been explored with special focus on the influence of the analyzed relative pressure range on the reliability of parameters identification. For that purpose, the adsorption isotherm of nitrogen on active carbon for different ranges of relative pressures p/p0 was analyzed. The conducted research was to provide for an answer to the question of whether the range of the analyzed relative pressures has any effect on the quality of adsorption system parameters identification, as well as what range of the relative pressure permits execution of the reliable identification of microporous structure parameters. 相似文献
104.
Farouq Ahmed Ryo NagumoRyuji Miura Suzuki AiHideyuki Tsuboi Nozomu HatakeyamaAkira Endou Hiromitsu TakabaMomoji Kubo Akira Miyamoto 《Applied Surface Science》2011,257(24):10503-10513
The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface. 相似文献
105.
106.
In this work, we performed density functional calculations to examine the molecular adsorption states of thiophene on β-SiC(0 0 1)-2×1 surface. A number of possible adsorption geometries are considered into two groups as the polymeric thiophene chain and the individual molecules covalently bonded onto the surface. The results show that the polymeric chain on the surface is the less stable adsorption case and individual arch like adsorption case structure is more stable than others. In all adsorption cases, the adsorbed SiC surfaces are characterized as different semiconductors. 相似文献
107.
Lihui SunJifan Hu Feng GaoYongjia Zhang Hongwei Qin 《Physica B: Condensed Matter》2011,406(21):4105-4108
The adsorption of NO molecule on the LaFeO3 (0 1 0) surface was studied using first-principle calculations based on density functional theory. The calculated results indicate that the Fe-top site is the most favorable for NO adsorption. The N-O bond length, Mulliken charge, and the N-O vibration frequency of the NO molecule are discussed after adsorption. The analysis results of the density of the states show that when NO is adsorbed with the Fe-NO configuration, the bonding mechanism is mainly from the interaction between the NO and the Fe d orbit. 相似文献
108.
109.
Copper is one of the most toxic heavy metals having significant effects on the living organisms and hence effective removal of copper from waste water is crucial. The current work investigates the application of activated watermelon shell based biosorbent for the removal of copper from aqueous solution. The effect of activation using calcium hydroxide and citric acid as well as the effect of operating parameters like contact time, adsorbent dosage, temperature, pH, initial concentration and ultrasonic power on the extent of removal has been investigated. Experiments performed in the presence of ultrasound to investigate the degree of intensification as compared to the conventional agitation based treatment revealed that the adsorption rate significantly increases in the presence of ultrasound and also the time required for reaching the equilibrium reduces from 60 min in conventional approach to only 20 min in the presence of ultrasound. The extent of adsorption of Cu(II) on adsorbents was found to increase with an increase in the operating pH till an optimum value of 5. The extent of adsorption also increased with a decrease in the initial concentration and particle size as well as with an increase in ultrasonic power till an optimum. Kinetics and isotherm study revealed that all the experimental data was found to best fit the pseudo second order kinetics and Langmuir adsorption isotherm model respectively. Maximum adsorption capacity was found to be 31.25 mg/g for watermelon treated with calcium hydroxide and 27.027 mg/g for watermelon treated with citric acid. Overall present study established that activated watermelon is an environmentally friendly, low cost and highly efficient biosorbent that can be successfully applied for the removal of copper from aqueous solution with intensification benefits based on the ultrasound assisted approach. 相似文献
110.
Wenhua Chen Alan L. Stottlemyer Payam Kaghazchi Theodore E. Madey Robert A. Bartynski 《Surface science》2009,603(20):3136-16391
We report on the adsorption and decomposition of NO on O-covered planar Ir(2 1 0) and nanofaceted Ir(2 1 0) with variable facet sizes investigated using temperature programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT). When pre-covered with up to 0.5 ML O, both planar and faceted Ir(2 1 0) exhibit unexpectedly high reactivity for NO decomposition. Upon increasing the oxygen coverage to 0.7 ML O, planar Ir(2 1 0) has little activity while faceted Ir(2 1 0) still remains active toward NO decomposition, although NO decomposition is completely inhibited when both surfaces are pre-covered by 1 ML O. NO molecularly adsorbs on O-covered Ir at 300 K. At low NO and oxygen coverage, NO adsorbs on the atop sites of planar Ir(2 1 0) while on the bridge and atop sites of faceted Ir(2 1 0) composed of (1 1 0) and {3 1 1} faces. No evidence for size effects in the decomposition of NO on O-covered faceted Ir(2 1 0) is observed for average facet size in the range 5-14 nm. Our findings should be of importance for development of Ir-based catalysts for NO decomposition under oxygen-rich conditions. 相似文献