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排序方式: 共有193条查询结果,搜索用时 15 毫秒
81.
A novel approach to fabricate highly graphitic carbon nanostructures such as carbon nanotubes (CNTs), metal/graphitic-shell nanocrystals and hollow carbon nanospheres (HCNSs) in a very short time is demonstrated. 相似文献
82.
A series of well-aligned arrays of copper chalcogenide nanostructures, including Cu(7)S(4) and Cu(2-x)Se nanotubes with double walls have been successfully prepared by using Cu(OH)(2) nanorods as sacrificial templates. This new method is based on layer-by-layer chemical conversion and inward etching of the sacrificial templates, which is essentially a kind of lithography inside the Cu(OH)(2) nanorods. The key step of the process involves repeated formation of the copper chalcogenide wall and the dissolution of the Cu(OH)(2) core for two consecutive cycles. A large difference of the solubility product (Ksp) between the copper chalcogenide wall and the Cu(OH)(2) core materials is crucial for the direct replacement of one type of anions by the other. Our work provides a novel and general approach to the controllable synthesis of the arrays of copper chalcogenide nanotubes with double walls and complex hierarchies. 相似文献
83.
Peng Wu Hongyuan Tan Jianfeng Zhan Weixin Wang Ting Hu Shiming Li 《Molecules (Basel, Switzerland)》2020,25(24)
Poria cocos mushroom is widely used as a food and an herb in East Asian and other countries due to its high nutritional value. Research has demonstrated that Poria cocos polysaccharides (PCP) are the major bioactives and possess antioxidation, anti-inflammation, immunoregulation, and other health promoting properties. However, the efficient preparation of PCP has been a challenge, particularly in large scale for industry. Herein, we investigated the biotransformation of PCP from Poria cocos, catalyzed by β-glucanase from Aspergillus niger and focused on optimizing the most four influencing parameters: Temperature, time, pH, and enzyme dosage in this study. After numerous optimizations with the assistance of response surface optimization methodology, we have established that the optimal conditions for the biotransformation PCP preparation were as following: Enzymolysis temperature 60 °C, time 120 min, pH 5.0 and enzyme dose 20 mL. Under these conditions, the extraction yield of PCP reached as high as 12.8%. In addition, the antioxidant activities of PCP were evaluated by reducing power assay and 1,1-diphenyl-2-picryl-hydrazyl, superoxide anion, and hydroxyl radicals scavenging assays. Resulting data showed that PCP presented outstanding antioxidant capacity. Thus, these findings indicate that PCP could be produced as a natural antioxidant for further development. 相似文献
84.
Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) were synthesized by thermal decomposition of pyridine and iron phthalocyanine over an iron catalyst in an atmosphere of ammonia. The N-MWCNTs thus obtained were analyzed by X-ray photoelectron spectroscopy. They were found to contain three types of nitrogen (N) atoms, namely pyridine-like, graphite-like, and molecular N. The effect of the pyridine-like N and the graphite-like N was investigated. The pyridine-like N is absorbing nitric oxide (NO) more easily than the graphite-like N. The N-MWCNTs with higher N content (especially the pyridine-like N) have higher catalytic activity (in terms of electrooxidation of NO) than those containing less N. The N-MWCNTs with high levels of pyridine N were incorporated into an electrode which suitable for sensing NO and for removal of NO due to its excellent electrocatalytic activity. 相似文献
85.
86.
Amareshwar Kumar Rai Weixin Fei Zhiwen Lu Zijing Lin 《Theoretical chemistry accounts》2009,124(1-2):37-47
Microsolvation and combined microsolvation-continuum approaches are employed to investigate the structures and energies of canonical and zwitterionic histidine conformers. The effect of hydration on the relative conformational stability is examined. The strategy of exploring singly and doubly hydrated structures and the possible microsolvation patterns are described. We find that bonding water molecule may significantly change the relative conformational stabilities. In gas phase, the singly and doubly hydrated canonical forms are more stable than their zwitterionic counterparts. In solution, the continuum solvent model shows that bare zwitterionic form is more stable than bare canonical form by 1.1 kcal/mol. This energy separation is increased to 2.2 and 3.4 kcal/mol with inclusion of one and two explicit water molecules, respectively. We have also observed that the doubly hydrated structures obtained by combining two water molecules simultaneously to the solute molecule are preferred over the stepwise hydration. Hydrogen bond energies for the most stable hydrated histidine tautomers are determined by the atoms in molecules theory. The infrared (IR) spectra for the most stable singly and doubly hydrated structures of both histidine tautomers in gas phase are characterized. The stretching frequencies for NH of imidazole ring and OH of COOH are red shifted due to the hydrations. The IR spectra for the most stable zwitterionic tautomers in solution are also presented and discussed in connection with the comparison to the experiments. The pKa values obtained for the ring protonated zwitterions with two explicit water molecules appear to be in good agreement with the experiments. 相似文献
87.
88.
A one‐pot synthesis of alkynyl sulfide from terminal alkyne has been reported via lithiation of the alkyne, oxidative addition of sulfur, consecutively followed by the nucleophilic substitution of lithium alkynyl thiolate to various halides. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 23:105–110, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.20745 相似文献
89.
Weixin Zheng Yangfeng Wu Fenfen Zheng Linfeng Hu Ya Hong 《Tetrahedron letters》2010,51(36):4702-11640
α-Methylenyl zirconacyclopentenes are synthesized regio- and stereoselectively via reductive intermolecular cross-coupling of alkynes and allenes promoted by zirconocene species ‘Cp2Zr’. An interesting reductive intramolecular coupling of the α-methylenyl zirconacyclopentene has been observed in the presence of DMAD/CuCl, resulting in the generation of cyclobutene with an exocyclic double bond. Polysubstituted 1,4-dienes can be given with high selectivity and good yields. 相似文献
90.
CuOx/CeO2催化剂在CO氧化反应中表现出高催化活性和显著结构敏感性.文献报道中CuOx/CeO2催化剂体系的合成条件差异较大,从而导致观察到的CuOx-CeO2相互作用存在较大争议.因此,系统研究并阐明CuOx/CeO2催化剂中CuOx-CeO2相互作用对于理解复杂的CuOx-CeO2界面催化作用具有重要的研究意义.近期发现,氧化物纳米晶的形貌可作为一种新的结构参数,在不改变氧化物催化剂组成的条件下实现其结构和性能的调控.本文以不同形貌CeO2纳米晶为载体,包括优先暴露{110}+{100}晶面的CeO2纳米棒、优先暴露{100}晶面的CeO2纳米立方体和优先暴露{111}晶面的CeO2纳米多面体,采用等体积浸渍方法合成了Cu担载量为0.025%~5%的CuOx/CeO2纳米晶催化剂,结合谱学和电镜表征方法,以及CO吸附原位红外光谱,系统研究了CuOx物种在不同形貌CeO2纳米晶上的结构演化及其催化CO氧化的构-效关系.结构表征结果表明, CuOx物种结构不仅依赖于Cu的担载量,也依赖于载体CeO2的形貌.随着Cu担载量的增加, CuOx物种优先沉积在CeO2的表面缺陷位,然后聚集和长大;同时伴随着CuOx物种从孤立Cu离子到与载体强/弱相互作用的CuOx团簇,高分散Cu O颗粒和大尺寸Cu O颗粒.孤立Cu^+离子和与载体弱相互作用CuOx团簇主要形成于CeO2纳米立方体的表面,这可能与CeO2纳米立方体暴露的氧终止CeO2{100}晶面相关.CO吸附原位红外结果表明, CuOx团簇与不同CeO2表面相互作用的强度顺序为:CeO2纳米棒暴露的{110}面>CeO2纳米多面体暴露的{111}面>CeO2纳米立方体暴露的{100}面.CeO2纳米立方体与Cu2+离子间相互作用弱于与Cu^+之间的,因此CeO2纳米立方体负载的CuOx物种在CO还原过程中容易停留在稳定的Cu^+中间物种;而CeO2纳米棒与Cu2+离子之间的相互作用强于与Cu^+之间的相互作用,因此CeO2纳米棒负载的CuOx物种在CO还原过程中容易形成金属铜.因此CO吸附原位红外光谱观察到CeO2纳米立方体负载CuOx催化剂中吸附在Cu^+的CO物种远远多于CeO2纳米棒负载CuOx催化剂.CO氧化反应结果表明, CuOx/CeO2催化剂表现出同时依赖于CuOx物种结构和CeO2形貌的结构敏感性.CuOx/CeO2催化剂活性表现出与CuOx/CeO2催化剂的CO还原性能的正相关性,说明中CuOx/CeO2催化CO氧化反应遵循Mv K反应机理.这些结果系统地关联了CeO2形貌, CuOx-CeO2相互作用, CuOx物种结构和CeO2还原性能, CuOx/CeO2催化CO氧化反应活性. 相似文献