全文获取类型
收费全文 | 33221篇 |
免费 | 3184篇 |
国内免费 | 3919篇 |
专业分类
化学 | 26270篇 |
晶体学 | 476篇 |
力学 | 1622篇 |
综合类 | 213篇 |
数学 | 4522篇 |
物理学 | 7221篇 |
出版年
2024年 | 42篇 |
2023年 | 337篇 |
2022年 | 578篇 |
2021年 | 897篇 |
2020年 | 1099篇 |
2019年 | 1070篇 |
2018年 | 957篇 |
2017年 | 1158篇 |
2016年 | 1267篇 |
2015年 | 1162篇 |
2014年 | 1438篇 |
2013年 | 2307篇 |
2012年 | 1788篇 |
2011年 | 2065篇 |
2010年 | 1748篇 |
2009年 | 2230篇 |
2008年 | 2047篇 |
2007年 | 2028篇 |
2006年 | 1857篇 |
2005年 | 1634篇 |
2004年 | 1679篇 |
2003年 | 1278篇 |
2002年 | 1613篇 |
2001年 | 1004篇 |
2000年 | 826篇 |
1999年 | 711篇 |
1998年 | 608篇 |
1997年 | 538篇 |
1996年 | 562篇 |
1995年 | 522篇 |
1994年 | 499篇 |
1993年 | 344篇 |
1992年 | 379篇 |
1991年 | 224篇 |
1990年 | 172篇 |
1989年 | 173篇 |
1988年 | 176篇 |
1987年 | 137篇 |
1986年 | 106篇 |
1985年 | 96篇 |
1984年 | 96篇 |
1983年 | 39篇 |
1982年 | 90篇 |
1981年 | 113篇 |
1980年 | 122篇 |
1979年 | 134篇 |
1978年 | 113篇 |
1977年 | 81篇 |
1976年 | 71篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
981.
Shandong Yuan Sharifah Bee Derouane-Abd Hamid Yongxue Li Pinliang Ying Qin Xin Eric G Derouane Can Li 《Journal of molecular catalysis. A, Chemical》2002,180(1-2):245-258
The conversion of n-C4H10 was undertaken on MoO3/HZSM-5 catalyst at 773–973 K and the phases of molybdenum species were detected by XRD. The XRD results show that bulk MoO3 on HZSM-5 can be readily reduced by n-C4H10 to MoO2 at 773 K and MoO2 can be gradually carburized to molybdenum carbide above 813 K. The molybdenum carbide formed from the carburization of MoO2 with n-C4H10 below 893 K is -MoC1−x with fcc-structure, while hcp-molybdenum carbide formed above 933 K. During the evolution of MoO3 to MoO2 (>773 K) or the carburization of MoO2 to molybdenum carbide (>813 K), deep oxidation, cracking and coke deposition are serious, in particular at higher reaction temperatures, these lead to the poor selectivity to aromatics. Aromatization of n-C4H10 can proceed catalytically on both Mo2C/HZSM-5 and MoO2/HZSM-5, the distribution of the products for the two catalysts is similar below 813 K, but the activity for Mo2C/HZSM-5 is much higher than that for MoO2/HZSM-5. 相似文献
982.
Anna B. DenisovaVyacheslav Ya. Sosnovskikh Wim Dehaen Suzanne ToppetLuc Van Meervelt Vasiliy A. Bakulev 《Journal of fluorine chemistry》2002,115(2):183-192
Reaction of 2-hydrazinothiazoles 1 with 1-thienyl- and 1-furyl-1,3-butanediones 2a,b in methanol in the presence of hydrochloric acid mainly leads to a mixture of pyrazoles 3 and pyrazolines 4 or pyrazoles 3 and 5 in strong acidic conditions. Isomeric hydrazones 6 and pyrazolines 4 were formed and isolated in these reactions in the absence of hydrochloric acid. It has been shown that the regioselectivity in the reaction of diketones 2 with hydrazine 1 is governed by both the concentration of acid and the nature of substituents in the 1,3-diketones 2. Cyclization of hydrazones 6 is shown to occur under milder conditions than dehydration for pyrazolines 4. The new heterocyclic compounds were prepared and fully characterized by NMR spectra and by X-ray analysis for 3c. 相似文献
983.
E. I. Basaldella J. C. Tara G. Aguilar Armenta M. E. Patiño-Iglesias E. Rodríguez Castellón 《Journal of Sol-Gel Science and Technology》2006,37(2):141-146
Samples of mesoporous silica, SBA-15, were prepared under hydrothermal conditions and Cu cations were incorporated into the
framework by two different impregnation techniques. The corresponding adsorption/desorption isotherms of propylene, propane,
and N2 were measured to evaluate the material's effectiveness in the separation of propane/propylene mixtures.
Adsorption uptake of propylene increased and that of propane decreased in Cu containing samples as compared to the uptakes
observed in undoped SBA-15 samples. It was demonstrated that the presence of Cu atoms in the adsorbent lattice led to a greater
selectivity towards propylene. Furthermore, the highest level of Cu(I) were obtained in samples prepared by equilibrium impregnation,
which in turn improved the olefin/paraffin uptake ratio. Under some working conditions, the amount of propylene adsorbed in
selected samples is totally reversible while propane uptake was negligible. 相似文献
984.
985.
V. Kavan Kumar R. Mahendiran P. Subramanian S. Karthikeyan A. Surendrakumar 《印度化学会志》2022,99(3):100384
One of the most significant issues of the last few decades has been tracing for renewable energy sources. Animal fleshing (ANFL) is the most common proteinaceous solid waste accured during the production of leather and it must be disposed of in an environmentally responsible manner. This paper is attempts to assess the biogas production from solid waste originating from the chrome based tannery. Anaerobic digestion of these wastes will be a viable option for waste stabilization and energy production in the form of biogas to be utilized in the industry. The bio-methane potential of the wastes were examined by mixing these wastes with various sources of inoculum and different inoculum to substrate (I/S) ratio considered. The batch experiments were carried out in 2.5 l glass reactors with a various source of inoculumviz., Cow Dung (CD), Elephant Dung (ED) and Bio-Digested Slurry (BDS) with varied inoculum to substrate (LFs) ratios for a retention time of 50 days with replications. The results obtained from the experiments showed that BDS:LF (25:75) had the highest gas production of 14505 ml (651.85 ml CH4g?1 VS) followed by CD:LF (50:50) produced 12072.5 ml (789.36 ml CH4g?1 VS) and ED:LF (75:25) produced 11252.5 ml (1492.08 ml CH4g?1 VS)with a methane content of 63.77, 61.92 and 62.72%, respectively. 相似文献
986.
André Deratani Thierry Maraldo Estelle Renard 《Journal of inclusion phenomena and macrocyclic chemistry》1995,23(2):137-146
Reduction of-cyclodextrin (-CD) aromatic ketone (acetophenone and acetonaphthones) inclusion compounds were carried out in the presence of a large number of chemically inert species as potential co-guests. In several cases, it was observed that stoichiometric molar ratios of these compounds to ketone significantly modify the chiral induction yielding the inverted alcohol enantiomer and increasing the face selectivity. The results were found to depend strongly on the respective structure and shape of both the ketone and the additive, and on the molar ratio of-CD:ketone:third compound. These observations suggest the formation of a three-component inclusion complex in which the geometry of binding of the substrate and its mobility are changed with respect to the binary system. 相似文献
987.
R. Bruce King 《Theoretical chemistry accounts》1980,56(4):269-296
The following procedure is described for investigating the qualitative dynamics of simple chemical systems: 1) A so-called influence diagram is generated representing the relationships between the reference reactants (phase-determining intermediates); 2) This influence diagram is used to generate a truth table indicating possible transitions between state vectors representing the signs of the time derivatives of of the reference reactant concentrations; 3) The truth table is used to determine a state transition diagram representing the flow topology around unstable equilibrium points; 4) The characteristic equation of the adjacency matrix of the influence diagram is solved in order to determine the presence of such unstable equilibrium points. The two types of qualitative dynamics possible for chemical systems containing two reference reactants and one feedback circuit are bifurcation between two attracting regions (bistability) and limit cycle oscillation. However, in two reference reactant systems oscillation requires an additional self-activating loop to generate the unstable equilibrium point required for its realization. Bistability and limit cycle oscillation are also two of the possible types of qualitative dynamics for chemical systems containing three reference reactants. However, chemical systems with three reference reactants and two or more feedback circuits can also contain interlocking limit cycles, which can lead to toroidal oscillations or chaos. The influence diagrams are given for the systems exhibiting these various types of dynamic behavior along with a summary of the important properties of all 729 possible influences for simple chemical systems containing three reference reactants. 相似文献
988.
热处理对Co(Ni)-Mo/Al2O3催化剂加氢脱硫活性的影响 总被引:9,自引:0,他引:9
介绍了硫化Co-Mo/Al2O3催化剂的热处理方法及其结果。实验表明,硫化催化剂经热处理后,加氢脱硫活性明显增加,以塞吩转化率表示的催化活性随处理温度升高而变化,在450℃时观察到催化活性的极大值。 相似文献
989.
严继民 《中国科学B辑(英文版)》1990,(7)
An ab initio calculation of the hydrogen bond complex (HF)_2 is given with the 6-311 G~(**) basis set, according to which the potential surface around the balance point of the distancès and the orientations between two HF molecules is obtained. The atomic charges in the system are calculated with the PD/LSF method (potential-derived/least-square-fitting method) and then an analysis of the hydrogen bond interaction between two HF molecules is given with the (exp-6-1) potential function, by means of which it is shown that the main interaction between them is not an electro-static but a charge transfer one. The potential curve between two HF molecules is like a Morse function. 相似文献
990.
CuO/TiO2的制备及对NO+CO反应性能的研究 总被引:2,自引:1,他引:2
以三种不同的TiO2为载体,负载6%CuO,在色谱-微反装置上考察了三种Cu6-Ti催化剂对NO+CO反应的活性。结果表明,由于制备方法不同,活性差别很大,三者活性顺序依次为:Cu6-Ti(T)> Cu6-Ti(S)> Cu6-Ti(nm),TiO2的比表面积及晶型对活性的影响不明显。H2-TPR结果表明,Cu6-Ti在整个TPR过程中出现了3或4个还原峰,表明有多种CuO物种存在。不同预处理Cu6-Ti的TPR显示,在反应中Cu物种可能以多种价态同时存在。NO-TPD结果亦证明,Cu6-Ti对NO+CO的反应活性与NO在催化剂表面的解离难易密切相关,而与NO的吸附量关系不大。 相似文献