全文获取类型
收费全文 | 4725篇 |
免费 | 981篇 |
国内免费 | 685篇 |
专业分类
化学 | 1938篇 |
晶体学 | 16篇 |
力学 | 413篇 |
综合类 | 72篇 |
数学 | 1664篇 |
物理学 | 1185篇 |
无线电 | 1103篇 |
出版年
2024年 | 28篇 |
2023年 | 197篇 |
2022年 | 231篇 |
2021年 | 368篇 |
2020年 | 412篇 |
2019年 | 292篇 |
2018年 | 258篇 |
2017年 | 238篇 |
2016年 | 281篇 |
2015年 | 240篇 |
2014年 | 235篇 |
2013年 | 370篇 |
2012年 | 210篇 |
2011年 | 254篇 |
2010年 | 214篇 |
2009年 | 276篇 |
2008年 | 253篇 |
2007年 | 256篇 |
2006年 | 247篇 |
2005年 | 175篇 |
2004年 | 202篇 |
2003年 | 154篇 |
2002年 | 163篇 |
2001年 | 129篇 |
2000年 | 114篇 |
1999年 | 109篇 |
1998年 | 85篇 |
1997年 | 74篇 |
1996年 | 46篇 |
1995年 | 52篇 |
1994年 | 32篇 |
1993年 | 22篇 |
1992年 | 21篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有6391条查询结果,搜索用时 897 毫秒
71.
The threat and global concern of energy crises have significantly increased over the last two decades. Because solar light and water are abundant on earth, photocatalytic hydrogen evolution through water splitting has been considered as a promising route to produce green energy. Therefore, semiconductor photocatalysts play a key role in transforming sunlight and water to hydrogen energy. To date, various photocatalysts have been studied. Among them, TiO2 has been extensively investigated because of its non-toxicity, high chemical stability, controllable morphology, and high photocatalytic activity. In particular, 1D TiO2 nanofibers (NFs) have attracted increasing attention as effective photocatalysts because of their unique 1D electron transfer pathway, high adsorption capacity, and high photoinduced electron–hole pair transfer capability. However, TiO2 NFs are considered as an inefficient photocatalyst for the hydrogen evolution reaction (HER) because of their disadvantages such as a large band gap (~3.2 eV) and fast recombination of photoinduced electron–hole pairs. Therefore, the development of a high-performance TiO2 NF photocatalyst is required for efficient solar light conversion. In recent years, several strategies have been explored to improve the photocatalytic activity of TiO2 NFs, including coupling with narrow-bandgap semiconductors (such as ZnIn2S4). Recently, microwave (MW)-assisted synthesis has been considered as an important strategy for the preparation of photocatalyst semiconductors because of its low cost, environment-friendliness, simplicity, and high reaction rate. Herein, to overcome the above-mentioned limiting properties of TiO2 NFs, we report a 2D/1D ZnIn2S4/TiO2 S-scheme heterojunction synthesized through a microwave (MW)-assisted process. Herein, the 2D/1D ZnIn2S4/TiO2 S-scheme heterojunction was constructed rapidly by using in situ 2D ZnIn2S4nanosheets decorated on 1D TiO2 NFs. The loading of ZnIn2S4 nanoplates on the TiO2 NFs could be easily controlled by adjusting the molar ratios of ZnIn2S4 precursors to TiO2 NFs. The photocatalytic activity of the as-prepared samples for water splitting under simulated solar light irradiation was assessed. The experimental results showed that the photocatalytic performance of the ZnIn2S4/TiO2 composites was significantly improved, and the obtained ZnIn2S4/TiO2 composites showed increased optical absorption. Under optimal conditions, the highest HER rate of the ZT-0.5 (molar ratio of ZnIn2S4/TiO2= 0.5) sample was 8774 μmol·g-1·h-1, which is considerably higher than those of pure TiO2 NFs (3312 μmol·g-1·h-1) and ZnIn2S4nanoplates (3114 μmol·g-1·h-1) by factors of 2.7 and 2.8, respectively. Based on the experimental data and Mott-Schottky analysis, a possible mechanism for the formation of the S-scheme heterojunction between ZnIn2S4 and TiO2 was proposed to interpret the enhanced HER activity of the ZnIn2S4/TiO2heterojunctionphotocatalysts.
相似文献
72.
73.
In the light of the plate tectonics the outline of paleogeography, sedimentary properties and facies patterns in the depositional processes of the Wutong Formation at the Lower Yangtze area are discussed. The argument that the Wutong Formation contains not only terrestrial sediments but also marine-terrestrial transitional or marine sediments is expounded to cover the development, evolution and paleoclimate of the Wutong depositional regions. 相似文献
74.
In the 21st century, hydrogen energy is a novel energy source. Its use is expected to mitigate the problems of environmental pollution and global warming caused by the excessive use of conventional fossil fuels. The hydrogen evolution reaction (HER) for water splitting has attracted considerable attention because of its environmental friendliness. To improve electrocatalyst performance and reduce operation cost, carbon-based metal hybrid materials exhibiting high efficiency and catalytic activity have been developed. Among them, carbon dots (CDs) have garnered significant research attention and have been widely applied in biosensing, bioimaging, and energy conversion/storage because of their facile synthesis, biocompatibility, tunable photoluminescence, excellent stability, and good electronic properties. CDs are widely used as carriers in the construction of electrocatalysts prepared from carbon-based metal hybrid materials. At present, it is believed that CDs exhibit excellent confinement effects, which can effectively inhibit the growth and agglomeration of metal nanoparticles, thereby preparing well-distributed carbon-based metal hybrid materials with a uniform and controllable size. However, the formation process of the small-molecule raw materials of CDs has not been elucidated. In this study, CDs and small-molecule raw materials from synthetic CDs were used as precursors to prepare nitrogen-doped CD-supported ruthenium nanoparticle (Ru@CDs) and small-molecule-supported ruthenium nanoparticle (Ru@Molecule) hybrid materials, respectively. The interaction between the small molecules and Ru in the process of CD formation and the effect on HER performance were explored. Moreover, we prepared different carriers such as metal organic frameworks(MOF), carbon nanotubes (CNTs), and graphene (GO)-supported ruthenium nanoparticle hybrid materials. Among them, Ru@CDs exhibited controllable size and excellent dispersibility and exhibited outstanding HER activity and good stability. Ru@CDs were found to require a low overpotential of 22 mV to reach a current density of 10 mA·cm−2. Moreover, we observed the presence of an intermediate state between the molecules and CDs and demonstrated that the intermediate state exhibits no confinement effect. Furthermore, we found that with increasing calcination temperature, the intermediate state gradually changes to CDs. The unique spatial confinement between CDs and metal ions is key to the formation of monodisperse Ru nanoparticles. Our results confirmed that Ru@CDs serve as excellent HER catalyst supports. This work not only reveals the effect of the unique spatial confinement of CDs on the supported metals and their promoting effect on electrocatalytic activity but also provides guides the future development of CD-based metal hybrid electrocatalysts. 相似文献
75.
During electrolysis of molten KF-2HF, strongly adherent fluorine bubbles are generated at the surface of carbon anode. The current was passing even if the horizontal anode was fully covered with gas film. The formation and growth of gas bubbles were studied by transient electrochemical techniques. It was observed that the fluorine bubbles do not have the spherical cap shape predicted by the classical theory. The curvature radius of the interface profile is not constant, the edge of the bubble being flat with a contact angle close to zero. The results are interpreted in the frame of a model which takes into account the predominant role of the interfacial properties. 相似文献
76.
Detection of single molecules, particles, and rapid redox events is a challenge of electrochemical investigations and requires either an amplification strategy or significant averaging for the electrochemical current to exceed the noise level. We consider the minimum number of electrons required to reach the limit of quantification in these electrochemical measurements. A survey of the literature indicates that the state-of-the-art limit in current detection for different types of measurements (e.g. voltammetry, single-molecule redox cycling, ion channel recordings of single molecules, metal nanoparticle collision, and phase nucleation) is independent of the nature of the measurement and increases linearly with reciprocal response time, Δt?1, over ~5 orders of magnitude (from ~10 to ~106 s?1). We demonstrate that the practical limit of quantification requires cumulative measurement of ~2100 electrons during Δt and is determined by statistics of counting electrons, that is, the shot noise in the current. 相似文献
77.
Chen Luo Chenggang Zhou Jinping Wu T. J. Dhilip Kumar Naduvalath Balakrishnan Robert C. Forrey Hansong Cheng 《International journal of quantum chemistry》2007,107(7):1632-1641
Structures and physical properties of small palladium clusters Pdn up to n = 15 and several selected larger clusters were studied using density functional theory under the generalized gradient approximation. It was found that small Pdn clusters begin to grow 3‐dimensionally at n = 4 and evolve into symmetric geometric configurations, such as icosahedral and fcc‐like, near n = 15. Several isomers with nearly degenerate average binding energies were found to coexist and the physical properties of these clusters were calculated. For several selected isomers, relatively moderate energy barriers for structural interchange for a given cluster size were found, implying that isomerization could readily occur under ambient conditions. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
78.
《中国化学快报》2020,31(4):988-991
Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction(HER) is urgently required for renewable and sustainable energy conversion.Here,we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti_3C_2T_x MXene and [PtCl_4]~2 in H_2 PtCl_6 aqueous solution.By taking advantage of homogeneously distributed Pt nanoparticulates on highly electrically conductive porous Ti_3C_2T_x/Ti_3AlC_2 monolith,the as-prepared electrocatalysts show high catalytic performance for hydrogen evolution.Specifically,the binder-free electrocatalysts have Pt loadings as low as 8.9 μg/cm~2,with low overpotential of 43 mV at a curre nt density of 10 mA/cm~2 and low Tafel slope that three times lower than porous Ti_3C_2T_x/Ti_3AlC_2 without Pt loading.This strategy offers a new approach to constructing ultra-low Pt-loading HER catalysts on the basis of in situ redox reaction between noble metal ions and MXenes. 相似文献
79.
硫酸介质中Ti/SnO2/PbO2析氧阳极的研究 总被引:13,自引:0,他引:13
由于硫酸溶液具有高的电导、一般条件下性质稳定、价格相对低廉等优点,所以电化学合成大多在硫酸溶液中进行.阳极析氧是电化学合成不可避免的阳极过程.但因硫酸的强腐蚀性,以及从阳极析出的氧的强氧化性,使得能满足工业生产的阳极材料很缺乏.自1950年荷兰Hen... 相似文献
80.
基于构建的数字经济发展水平评价指标体系与2010~2018年全国30个省市(除西藏、港澳台)的面板数据,对中国数字经济发展水平的时空特征、动态演化与区域差异原因进行探究。研究发现:全国及三大区域的数字经济发展水平均呈现稳步上升态势,并伴有明显的空间相关性、空间集聚性和区域差异性,且东部相较于中西部区域内差异更大。数字经济发展速度动态演化显示,整体上中国各省市数字化发展水平变化速度状态表现为上升趋势,相较于中西部,东部拥有更大的速度变化状态。地区前期数字化基础、经济发展的一阶滞后、政府科技投入均会促进区域数字经济的发展,对外开放水平会抑制数字经济发展,地区规模对数字经济的影响表现为非线性。此外,不同线性影响因素在不同区域内对数字经济发展水平具有不同的影响力。 相似文献