全文获取类型
收费全文 | 12936篇 |
免费 | 2295篇 |
国内免费 | 3048篇 |
专业分类
化学 | 10060篇 |
晶体学 | 270篇 |
力学 | 695篇 |
综合类 | 356篇 |
数学 | 1721篇 |
物理学 | 5177篇 |
出版年
2024年 | 17篇 |
2023年 | 125篇 |
2022年 | 298篇 |
2021年 | 319篇 |
2020年 | 372篇 |
2019年 | 454篇 |
2018年 | 357篇 |
2017年 | 417篇 |
2016年 | 520篇 |
2015年 | 606篇 |
2014年 | 735篇 |
2013年 | 971篇 |
2012年 | 1084篇 |
2011年 | 1157篇 |
2010年 | 1028篇 |
2009年 | 1034篇 |
2008年 | 1167篇 |
2007年 | 1112篇 |
2006年 | 1080篇 |
2005年 | 898篇 |
2004年 | 690篇 |
2003年 | 633篇 |
2002年 | 599篇 |
2001年 | 508篇 |
2000年 | 419篇 |
1999年 | 296篇 |
1998年 | 201篇 |
1997年 | 148篇 |
1996年 | 140篇 |
1995年 | 116篇 |
1994年 | 106篇 |
1993年 | 110篇 |
1992年 | 99篇 |
1991年 | 79篇 |
1990年 | 81篇 |
1989年 | 59篇 |
1988年 | 54篇 |
1987年 | 42篇 |
1986年 | 36篇 |
1985年 | 26篇 |
1984年 | 17篇 |
1983年 | 21篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1973年 | 3篇 |
1970年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Catalytic direct dehydrogenation of methanol to formaldehyde was carried out over Ag‐SiO2‐MgO‐Al2O3 catalysts prepared by sol‐gel method. The optimal preparation mass fractions were determined as 8.3% MgO, 16.5% Al2O3 and 20% silver loading. Using this optimum catalyst, excellent activity and selectivity were obtained. The conversion of methanol and the selectivity to formaldehyde both reached 100%, which were much higher than other previously reported silver supported catalysts. Based on combined characterizations, such as X‐ray diffraction (XRD), scanning electronic microscopy (SEM), diffuse reflectance ultraviolet‐visible spectroscopy (UV‐Vis, DRS), nitrogen adsorption at low temperature, temperature programmed desorption of ammonia (NH3‐TPD), desorption of CO2 (CO2‐TPD), etc., the correlation of the catalytic performance to the structural properties of the Ag‐SiO2‐ MgO‐Al2O3 catalyst was discussed in detail. This perfect catalytic performance in the direct dehydrogenation of methanol to formaldehyde without any side‐products is attributed to its unique flower‐like structure with a surface area less than 1 m2/g, and the strong interactions between neutralized support and the nano‐sized Ag particles as active centers. 相似文献
892.
A novel array silver nanoparticles and Rutin complex film modified paraffin‐impregnated graphite electrode was proposed in this work (denoted as Ag/Rutin/WGE). The characteristics were investigated by the field emission scanning electron microscopy (FE‐SEM), infrared spectra (IR), UV‐visible (UV), X‐ray photoelectron spectroscopy (XPS) and electrochemical techniques. Silver ions were gradually chelated by polyrutin film at 4′‐oxo‐5′‐OH and 5‐OH‐4‐oxo sites accompanying adsorption, then. Silver nanoparticles were highly‐dispersed electrodeposited on polyrutin film. The electrochemical behaviors of tyrosine (Tyr) and tryptophan (Trp) were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The Ag/Rutin/WGE electrode shows overlapping catalysis for the oxidation of Tyr and Trp. The linear response of Tyr and Trp were 0.3–10.0 and 0.7–70.0 μM with detection limit of 0.07 and 0.1 μM in a signal‐to‐noise ratio of 3. 相似文献
893.
Taking Ni(2‐mpac)2(H2O)2 as molecular building block, two novel coordination polymers with the formula of [Ni(2‐mpac)2(4,4′‐bpy)·(4,4′‐bpy)]n ( 1 ) and [Ni(2‐mpac)2(py)·4H2O]n ( 2 ) (2‐mpac = 5‐methyl‐2‐pyrazinecarboxylic acid; 4,4′‐bpy = 4,4′‐bipyridine; py = pyrazine) have been synthesized and structurally characterized by X‐ray single crystal diffraction method. In addition, elemental analysis, IR and magnetism characterization are presented. 相似文献
894.
Weihua Wang Nana Wang Ping Li Yuxiang Bu Xiaoyan Xie Rui Song 《Theoretical chemistry accounts》2008,121(1-2):21-31
The influences of thioketo substitution on the properties of uracil monomer and dimer and their interactions with Zn2+ have been systematically investigated at the B3LYP/6-311+G*level of theory. Those properties include the structural characteristics, acidities, ionization potentials, and singlet–triplet energy gaps of SU monomers and their dimers, where SU=2-thiouracil, 4-thiouracil, and 2,4-dithiouracil, respectively. Computational results suggest that thioketo substitution leads to an increase in the acidities of the N-H groups for both uracil and its dimer, where the N1–H group is still the most acidic site relative to that of N3–H group. However, the opposite behaviors are true for the ionization potentials and the singlet–triplet energy gaps of uracil monomer and its dimer, suggesting that thiouracils are more susceptible to radiation damage relative to the unsubstituted uracil. For uracil and 2-thiouracil, the corresponding triplet excited-state geometries are predicted to be highly nonplanar compared with the planar geometries of the ground state as well as 4-thiouracil and 2,4-dithiouracil upon triplet excitation. As a rule, the intermolecular H-bonds involving the sulfur atom directly have been influenced more significant than those the oxygen atom directly involved for U::U and SU::SU base pairs upon ionization and excitation. Additionally, Zn2+ binding is expected to lead to an increase in the stability of U::U and SU::SU base pairs. 相似文献
895.
Ping Yang Yonghong Chen Qiyong Zhu Fengwu Wang Lun Wang Yongxin Li 《Mikrochimica acta》2008,163(3-4):263-269
Glutathione (GSH), l-cysteine (l-Cys) and 6-mercaptopurine (6-MP) inhibit the CL reaction of luminol–H2O2 catalyzed by gold colloids. In order to explore this, GSH, l-Cys and 6-MP were injected into the chemiluminescence system of luminol and H2O2 catalyzed by gold colloids. The results showed that gold colloids interact with GSH, l-Cys and 6-MP and decrease the CL emission. Based on this phenomenon, a simple, sensitive and convenient flow injection CL method was developed for the determination of GSH, l-Cys and 6-MP. This method provides a novel, and effective CL assay for GSH, l-Cys and 6-MP that has been applied to the determination of GSH in human serum. 相似文献
896.
胺是有机分子中最常见且重要的结构之一,广泛存在于天然产物分子、药物、染料、塑料等的分子结构中,开发合成胺的有效方法在化学工业中具有很高的需求但仍充满挑战。在诸多已知有机胺的合成方法中,以氰基为前体还原衍生化制备胺被认为是一条原子经济且绿色可行的方案。然而,氰基中存在稳定的碳氮三重键,还原过程涉及较多中间体,而部分中间体活性比氰基更高,因此反应受还原剂、催化剂、反应条件的影响很大,由氰基选择性制备目标胺结构仍充满挑战。本文从氰基的基本电子结构与反应活性出发,综述了近年来以氰基为原料的官能团胺基化反应,对部分重要反应的催化剂和反应机理做了简要讨论,为设计氰基制胺催化剂和应用工艺提供了新的视角。 相似文献
897.
Jing Cao Tong Mou Bingbao Mei Pengfei Yao Ce Han Xue Gong Ping Song Prof. Zheng Jiang Prof. Thomas Frauenheim Prof. Jianping Xiao Prof. Weilin Xu 《Angewandte Chemie (International ed. in English)》2023,62(43):e202310973
Full understanding to the origin of the catalytic performance of a supported nanocatalyst from the points of view of both the active component and support is significant for the achievement of high performance. Herein, based on a model electrocatalyst of single-iridium-atom-doped iron (Fe)-based layered double hydroxides (LDH) for oxygen evolution reaction (OER), we reveal the first completed origin of the catalytic performance of such supported nanocatalysts. Specially, besides the activity enhancement of Ir sites by LDH support, the stability of surface Fe sites is enhanced by doped Ir sites: DFT calculation shows that the Ir sites can reduce the activity and enhance the stability of the nearby Fe sites; while further finite element simulations indicate, the stability enhancement of distant Fe sites could be attributed to the much low concentration of OER reactant (hydroxyl ions, OH−) around them induced by the much fast consumption of OH− on highly active Ir sites. These new findings about the interaction between the main active components and supports are applicable in principle to other heterogeneous nanocatalysts and provide a completed understanding to the catalytic performance of heterogeneous nanocatalysts. 相似文献
898.
Luyi Chen Ping Xiao Bin Hu Yiwen Wang Jianmao Yang Jianyun Liu 《Electroanalysis》2023,35(5):e202200521
3-dimensional (3D) Fe−Co−LDH/MXene composite was synthesized by in-situ synthesis and assembly of Fe−Co−LDH rod around MXene under hydrothermal condition. Due to the unique 3D configuration and good conductivity, the obtained Fe−Co−LDH/MXene modified glassy carbon electrode (Fe−Co−LDH/MXene/GCE) showed excellent electrochemical activity for As(III) detection. Via square-wave anodic stripping voltammetry, the response current on Fe−Co−LDH/MXene/GCE had good linear relationship with As(III) concentrations (1∼1000 ppt) with superior sensitivity (0.22 μA ppt−1 cm−2) and low detection limit (0.9 ppt). The mechanism of As(III) adsorption was demonstrated. The electrode showed excellent anti-interference ability. Real water sample analysis demonstrated the Fe−Co−LDH/MXene/GCE was deployable in aqua-system. 相似文献
899.
900.
Zhuxing Luo Jinhong Yang Jie Zhang Gang Meng Qingjun Lu Xi Yang Ping Zhao Youshan Li 《Molecules (Basel, Switzerland)》2022,27(6)
Mulberry leaf is an excellent protein resource that can be used as feed additive for livestock and poultry. Nevertheless, the use of mulberry leaves in animal diets is limited by its protease inhibitors, tannic acid and other anti-nutritional factors. This study systematically analyzed the type and activity of serine protease inhibitors (SPIs) from the leaves of 34 mulberry varieties, aiming to reveal the physicochemical properties and inactivation mechanism of SPIs. The types and activities of trypsin inhibitors (TIs) and chymotrypsin inhibitors (CIs) exhibited polymorphisms among different mulberry varieties. The highest number of types of inhibitors was detected in Jinshi, with six TIs (TI-1~TI-6) and six CIs (CI-1~CI-6). TIs and CIs exhibited strong thermal and acid–base stability. High-temperature and high-pressure treatment could reduce the activities of TIs and CIs to a certain extent. β-mercaptoethanol treatment could completely abolish TIs and CIs, suggesting that the disulfide bridges were critical for their inhibitory activities. The Maillard reaction could effectively eliminate the inhibitory activities of TI-1~TI-4 and CI-1~CI-4. This study reveals the physicochemical properties and inactivation mechanisms of the anti-nutritional SPIs from mulberry leaves, which is helpful to exploit mulberry-leaf food with low-activity SPIs, promote the development and utilization of mulberry-leaf resources in animal feed and provide reference for mulberry breeding with different functions. 相似文献