全文获取类型
收费全文 | 3319篇 |
免费 | 357篇 |
国内免费 | 280篇 |
专业分类
化学 | 2527篇 |
晶体学 | 55篇 |
力学 | 148篇 |
综合类 | 15篇 |
数学 | 317篇 |
物理学 | 894篇 |
出版年
2023年 | 34篇 |
2022年 | 49篇 |
2021年 | 99篇 |
2020年 | 97篇 |
2019年 | 118篇 |
2018年 | 95篇 |
2017年 | 76篇 |
2016年 | 140篇 |
2015年 | 165篇 |
2014年 | 177篇 |
2013年 | 244篇 |
2012年 | 296篇 |
2011年 | 273篇 |
2010年 | 190篇 |
2009年 | 195篇 |
2008年 | 231篇 |
2007年 | 188篇 |
2006年 | 166篇 |
2005年 | 161篇 |
2004年 | 112篇 |
2003年 | 106篇 |
2002年 | 107篇 |
2001年 | 65篇 |
2000年 | 45篇 |
1999年 | 47篇 |
1998年 | 38篇 |
1997年 | 26篇 |
1996年 | 26篇 |
1995年 | 28篇 |
1994年 | 29篇 |
1993年 | 15篇 |
1992年 | 19篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1980年 | 25篇 |
1978年 | 23篇 |
1977年 | 48篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1970年 | 5篇 |
1968年 | 5篇 |
1943年 | 10篇 |
排序方式: 共有3956条查询结果,搜索用时 15 毫秒
981.
Hsin‐Tsung Chen Jee‐Gong Chang Shin‐Pon Ju Hui‐Lung Chen 《Journal of computational chemistry》2010,31(2):258-265
We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first‐principle density‐functional theory (DFT) calculations. Calculations on selected model show that the low‐coordinated Au atoms of the Au29 nanoparticle carry slightly negative charges, which enhance the O2 binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH). The overall LH reaction O2(ads) + CO(gas) → O2(ads) + CO(ads) → OOCO(ads) → O(ads) + CO2(gas) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states ( TSLH1 and TSLH2 ) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au29 + O2(gas) + CO(gas) → Au29–O2(ads) + CO(gas) → Au29–CO3(ads) → Au29–O(ads) + CO2(gas) requires an overall activation barrier of 0.19 eV, and the formation of Au29–CO3(ads) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au29 nanoparticle is likely to occur at or even below room temperature. To gain insights into high‐catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
982.
新型含氟姜黄素类似物的合成、晶体结构及抗肿瘤活性 总被引:7,自引:0,他引:7
采用活性基团拼接法将含氟苯甲醛与酮反应, 合成了5个含氟单羰基姜黄素类似物, 结构经IR, 1H NMR及ESI-MS确认. 对所合成的化合物进行了抗肿瘤活性测试(MTT法), 结果表明, 部分化合物对肿瘤细胞有较强的选择性抑制活性, 其中2,6-二(4-氟亚苯基)环己酮(3a)和2,6-二(2-氟亚苯基)环己酮(3b)的抗肿瘤谱广, 活性较好, 就此对化合物3b作了单晶培养, 并就晶体结构进行了X衍射分析. 结构表明化合物3b属三斜晶系, 空间群 P-1. 晶胞参数 a=0.9222(2) nm, b=0.9732(2) nm, c=1.0127(2) nm, α=88.920(4)°, β=75.672(3)°, γ=62.404(3)°, V=0.7755(3) nm3, Z=2, Dc=1.329 Mg•m-3, μ=0.097 mm-1, F(000)=324, 最终偏离因子R=0.0590, wR=0.1841. 相似文献
983.
Steam-exploded corn stover biomass was used as the substrate for fed-batch separate enzymatic hydrolysis and fermentation
(SHF) to investigate the solid concentration ranging from 10% to 30% (w/w) on the lignocellulose enzymatic hydrolysis and fermentation. The treatment of washing the steam-exploded material was also
evaluated by experiments. The results showed that cellulose conversion changed little with increasing solid concentration,
and fermentation by Saccharomyces cerevisiae revealed a nearly same ethanol yield with the water-washed steam-exploded corn stover. For the washed material at 30% substrate
concentration, i.e., 30% water insoluble solids (WIS), enzymatic hydrolysis yielded 103.3 g/l glucose solution and a cellulose
conversion of 72.5%, thus a high ethanol level up to 49.5 g/l. With the unwashed steam-exploded corn stover, though a cellulose
conversion of 70.9% was obtained in hydrolysis at 30% solid concentration (27.9% WIS), its hydrolysate did not ferment at
all, and the hydrolysate of 20% solid loading containing 3.3 g/l acetic acid and 145 mg/l furfural already exerted a strong
inhibition on the fermentation and ethanol production. 相似文献
984.
Fluorescent hyperbranched polymers (FHBPs), which combine the versatile fluorescent property with unique characteristics of hyperbranched architecture, are desirable candidates for stimulus responsive materials. This review demonstrates the structure and environment-dependent emission behaviors of a series of FHBPs. AEE active FHBPs showing opposite performance to ACQ effect are used to sensitively detect explosives and a superamplification effect is found. Specially designed FHBPs can complex with metal ions, leading to fluorescence turn-off due to complex quenching effect. The protonation of amino-containing FHBPs exhibits pH-dependent fluorescence responses to solution acidity. Some FHBPs containing responsive moieties are photo- and thermo-sensitive, and show potential applications as smart materials. 相似文献
985.
Hyo Sung Kim Ju Chan Yang Yong Sung Koo Hyoun Woo Kim 《Journal of solid state chemistry》2010,183(10):2445-2450
Nanowires consisting of GaN/Mn3O4 were prepared using a two-step approach that involved dipping the as-synthesized GaN nanowires into an aqueous manganese acetate solution. To examine the effects of annealing, GaN/Mn3O4 core-shell nanowires were heated thermally to 700 °C in N2 ambient. Transmission electron microscopy showed that the continuous Mn3O4 shell layer had agglomerated to expose a bare GaN core surface after thermal annealing. The magnetic measurements showed that the ferromagnetic behavior of the GaN nanowires had been suppressed by coating with the Mn3O4 shell, without significant change by the subsequent thermal annealing. The GaN/Mn3O4 core-shell nanowires exhibited blue, green, and red photoluminescence (PL) emission. The red emission was enhanced by thermal annealing. This paper discusses the associated mechanism for the variations in PL and magnetic properties of GaN/Mn3O4 core-shell nanowires. 相似文献
986.
Pseudo-SMB, often called “J-O process”, is a modified SMB process to completely separate a ternary mixture with two discrete steps per one cycle. For improved separation, two new design parameters, the position of step 1 (χS1) and the number of port switches during step 2 (nSMB), were introduced. A multi-objective optimization method was used to optimize the operating conditions of the pseudo-SMB process with four average zone flow-rate ratios for one cycle. Nadolol isomers were selected for the model solutes and the global objective for the design of the pseudo-SMB was to collect 99% of the intermediate retained solute. The separation was optimized for 8-column pseudo-SMB system with three column lengths (2.5, 5.0, and 10 cm) and three feed composition ratios (1/1/1, 1/2/1, and 2/1/2). The simulation results showed that productivity was increased 4.3 times (nSMB = 20, χS1 = 0.5, 1/1/1) and desorbent to feed ratio D/F was decreased 45% (nSMB = 16, χS1 = 0.5, 1/1/1) compared to normal operation (nSMB = 8, χS1 = 0.5, 1/1/1). Productivity and D/F were significantly improved when short columns were used in the pseudo-SMB process. The pseudo-SMB was compared with recycle chromatography and SMB cascades for the same total amount of adsorbent. Recycle chromatography and 8-column SMB cascades using 20 cm and 40 cm of total column lengths were not able to separate the intermediate component with the target purity and the same feed rate of the pseudo-SMB process. 相似文献
987.
Lee HJ Svahn E Swanson JM Lepp H Voth GA Brzezinski P Gennis RB 《Journal of the American Chemical Society》2010,132(45):16225-16239
Cytochrome c oxidase (CytcO), the final electron acceptor in the respiratory chain, catalyzes the reduction of O(2) to H(2)O while simultaneously pumping protons across the inner mitochondrial or bacterial membrane to maintain a transmembrane electrochemical gradient that drives, for example, ATP synthesis. In this work mutations that were predicted to alter proton translocation and enzyme activity in preliminary computational studies are characterized with extensive experimental and computational analysis. The mutations were introduced in the D pathway, one of two proton-uptake pathways, in CytcO from Rhodobacter sphaeroides . Serine residues 200 and 201, which are hydrogen-bonded to crystallographically resolved water molecules halfway up the D pathway, were replaced by more bulky hydrophobic residues (Ser200Ile, Ser200Val/Ser201Val, and Ser200Val/Ser201Tyr) to query the effects of changing the local structure on enzyme activity as well as proton uptake, release, and intermediate transitions. In addition, the effects of these mutations on internal proton transfer were investigated by blocking proton uptake at the pathway entrance (Asp132Asn replacement in addition to the above-mentioned mutations). Even though the overall activities of all mutant CytcO's were lowered, both the Ser200Ile and Ser200Val/Ser201Val variants maintained the ability to pump protons. The lowered activities were shown to be due to slowed oxidation kinetics during the P(R) → F and F → O transitions (P(R) is the "peroxy" intermediate formed at the catalytic site upon reaction of the four-electron-reduced CytcO with O(2), F is the oxoferryl intermediate, and O is the fully oxidized CytcO). Furthermore, the P(R) → F transition is shown to be essentially pH independent up to pH 12 (i.e., the apparent pK(a) of Glu286 is increased from 9.4 by at least 3 pK(a) units) in the Ser200Val/Ser201Val mutant. Explicit simulations of proton transport in the mutated enzymes revealed that the solvation dynamics can cause intriguing energetic consequences and hence provide mechanistic insights that would never be detected in static structures or simulations of the system with fixed protonation states (i.e., lacking explicit proton transport). The results are discussed in terms of the proton-pumping mechanism of CytcO. 相似文献
988.
硅烯为卡宾的硅类似物,在有机光和热化学中作为一类重要的反应活性中间体引起了化学界的广泛兴趣.杂环型硅烯是实验合成的稳定硅烯,因其在有机硅化学中的重要作用,十几年来在实验和理论上均有研究报道.本文基于单重态杂环型硅烯的特殊电子性质和丰富的化学反应性,系统探讨了几种杂环型硅烯的插入反应.使用密度泛函理论(DFT),在B3LYP/6-311++G(d,p)水平上研究了三种氮杂环型不饱和硅烯(1,3,4)和两种氮杂环型饱和硅烯(2,5)分别与H—X(X=F,OH,NH2)键的插入反应,阐明了插入反应的机理并对各反应的结果进行了比较.杂环型硅烯与H—X的插入反应机理类似于简单硅烯,体现了硅烯的亲电亲核的双重反应性.由反应势垒和反应热看,五种硅烯与H—X键的插入反应均为HF最容易,H2O次之,NH3最难.饱和硅烯的反应性比不饱和硅烯的反应活性高,这也间接验证了杂环型饱和硅烯在实验中较难合成. 相似文献
989.
Xuan Yun Wang Su Qin Liu Ke Long Huang Qiu Ju Feng De Lai Ye Bing Liu Jin Long Liu Guan Hua Jin 《中国化学快报》2010,21(8):987-980
<正>With high surface area,open porosity and high efficiency,a catalyst was prepared and firstly employed in electrocatalytic reduction of CO_2 and electrosynthesis of dimethyl carbonate(DMC).The electrochemical property for electrocatalytic reduction of CO_2 in ionic liquid was studied by cyclic voltammogram(CV).The effects of various reaction variables like temperature,working potential and cathode materials on the electrocatalytic performance were also investigated.80%yield of DMC was obtained under the optimal reaction conditions. 相似文献
990.