全文获取类型
收费全文 | 2862篇 |
免费 | 229篇 |
国内免费 | 678篇 |
专业分类
化学 | 2515篇 |
晶体学 | 86篇 |
力学 | 100篇 |
综合类 | 32篇 |
数学 | 274篇 |
物理学 | 762篇 |
出版年
2024年 | 7篇 |
2023年 | 32篇 |
2022年 | 71篇 |
2021年 | 100篇 |
2020年 | 88篇 |
2019年 | 65篇 |
2018年 | 68篇 |
2017年 | 80篇 |
2016年 | 106篇 |
2015年 | 106篇 |
2014年 | 131篇 |
2013年 | 254篇 |
2012年 | 163篇 |
2011年 | 155篇 |
2010年 | 136篇 |
2009年 | 176篇 |
2008年 | 168篇 |
2007年 | 200篇 |
2006年 | 225篇 |
2005年 | 199篇 |
2004年 | 176篇 |
2003年 | 148篇 |
2002年 | 108篇 |
2001年 | 97篇 |
2000年 | 131篇 |
1999年 | 92篇 |
1998年 | 70篇 |
1997年 | 73篇 |
1996年 | 48篇 |
1995年 | 56篇 |
1994年 | 63篇 |
1993年 | 42篇 |
1992年 | 41篇 |
1991年 | 15篇 |
1990年 | 21篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有3769条查询结果,搜索用时 15 毫秒
121.
Purification and partial characterization of a lipase from Bacillus coagulans ZJU318 总被引:1,自引:0,他引:1
An extracellular lipase was purified from the fermentation broth of Bacillus coagulans ZJU318 by CM-Sepharose chromatography, followed by Sephacryl S-200 chromatography. The lipase was purified 14.7-fold with
18% recovery and a specific activity of 141.1 U/mg. The molecular weight of the homogeneous enzyme was (32 kDa), determined
by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. The enzyme activity was maximum at pH 9.0 and was stable
over a pH range of 7.0–10.0, and the optimum temperature for the enzyme reaction was 45°C. Little activity loss (6.2%) was
observed after 1 h of incubation at 40°C. However, the stability of the lipase decreased sharply at 50 and 60°C. The enzyme
activity was strongly inhibited by Ag+ and Cu2+, whereas EDTA caused no inhibition. SDS, Brij 30, and Tween-80 inhibited lipase, whereas Triton X-100 did not significantly
inhibit lipase activity. 相似文献
122.
A new infrared-spectroscopic method to characterize acid sites of zeolites using small and weakly basic molecules such as diatomic and monoatomic molecules is reviewed. It has been revealed that N2 is an effective probe molecule to characterize both Brønsted acidity and Lewis acidity of H-form zeolites. The characteristics of the N 2 probe are discussed in detail in comparison with the CO probe. O2 and rare gases have also been applied to monitor the strong acid sites in the H-form zeolites. Further, the studies of the adsorption of water on H-form zeolites are shortly reviewed: a recent IR study of the H2
18O adsorption on H-ZSM-5 has given direct experimental evidence that the main feature of the observed IR bands is due to the hydrogen-bonded adsorption of water on the Brønsted acid sites. 相似文献
123.
3-苯氧甲基-6-(2,4-二氟苯基)-7H-1,2,4-三唑并[3,4-b][1,3,4]噻二嗪的合成与晶体结构研究 总被引:2,自引:0,他引:2
由苯氧乙酸出发,经多步反应,制得3-苯氧甲基-4-氨基-5-巯基-1,2, 4-三唑,它与2-氯-2',4'-二氟苯乙酮进行环化反应,生成3-苯氧甲基-6-( 2,4-二氟苯基)-7H-1,2,4-三唑并[3,4-b][1,3,4]噻二嗪,通过元素分析、红外 光谱、核磁共振氢谱与碳谱、质谱进行表征,并利用单晶X射线衍射法测定其结构 。晶体属单斜晶系、P2_(1/c)空间群,a = 1.339(3) nm, b = 1.4683(4) nm, c = 0.8606(2) nm, β = 108.49(2)°, Z = 4, F(000) = 736, R_1 = 0.0509。还 对均三唑并噻二唑两类稠杂环的晶体结构作了比较。 相似文献
124.
125.
Romanato Filippo Berti Marina Mazzer Massimo Drigo Antonio Vittorio Lazzarini Laura Franzosi Paolo Salviati Giancarlo Bertone Daniele 《Mikrochimica acta》1994,114(1):431-440
A combined method for structural characterization of strained epitaxial heterostructures involving different techniques such as Rutherford backscattering spectrometry (RBS), multiple crystal X-ray diffractometry (MCD) and transmission electron microscopy (TEM) is presented. In order to obtain a complete characterization of the analysed structure, three different quantities are measured independently: the epilayer thickness, the density of misfit dislocations which may appear at the interface, and the significant components of the strain tensor, mainly the tetragonal distortion, affecting the epilayer lattice. In this way the thermodynamic state and the mechanisms of plastic deformation of the structures can be fully investigated. In this contribution we present and discuss the experimental results concerning a set of InP/GaAs samples having different layer thicknesses ranging from 5 to 500 nm. The thickness of the samples has been determined by RBS. Measurements of in-plane strain and tetragonal distortion have been performed by MCD and RBS-channelling respectively, finally TEM has been used for determining the defects densities and distribution. 相似文献
126.
合成了新的O-(硫杂蒽酮-[2]-基)-氧乙酸及其稀土配合物. 通过元素分析, IR, 1H NMR, UV, DTA-TG和13C NMR谱对其结构进行了表征. 研究表明: 配体羧羰基脱质子后与金属离子配位, 2位氧原子也与金属离子配位, 配合物中含有一定量的配位水, 配合物为非电解质类型. 同时, 研究了O-(硫杂蒽酮-[2]-基)-氧乙酸稀土配合物对质粒DNA的切割作用. 结果表明: 铕的配合物对DNA的切割较明显, 且当配合物浓度增加时, 质粒DNA的超螺旋构型逐渐减少, 而缺刻、开环型构型逐渐增多. 在相同条件下, Eu(III)离子对质粒pBR322DNA几乎没有切割作用; 配体O-(硫杂蒽酮-[2]-基)-氧乙酸对质粒pBR322DNA也有切割作用, 但配合物EuL3对质粒pBR322DNA的切割作用明显强于配体, 表明稀土离子Eu(III)与配体生成配合物后有较好的协同切割作用. 相似文献
127.
Pradip K. Bhowmik Robert W. Lenz 《Journal of polymer science. Part A, Polymer chemistry》1994,32(4):651-659
A series of fully aromatic, thermotropic polyesters based on 1,1′-binaphthyl-4,4′-diol, BND, was prepared by the melt polycondensation method and characterized for their thermotropic behavior by a variety of experimental techniques. The homopolymer of BND with terephthalic acid formed a nematic melt at 353°C. In contrast, the polyester from BND and 2,6-naphthalenedicarboxylic acid had a melting transition, Tm, above 400°C, so it was not possible with the equipment available to determine whether it formed a nematic melt. All of the copolymers of BND formed nematic melts at much lower Tm values than those of its respective homopolymers, as expected, because of the copolymerization effect of the added monomer. Moreover, all of the copolymers had higher glass transition temperatures, Tg, than those of other liquid crystalline polyesters and higher thermal stabilities. © 1994 John Wiley & Sons, Inc. 相似文献
128.
Investigation of pyrolysis of chinese coals using thermal analysis/mass spectrometry 总被引:2,自引:0,他引:2
Li X. Matuschek G. Herrera M. Wang H. Kettrup A. 《Journal of Thermal Analysis and Calorimetry》2003,71(2):601-612
TA/MS (thermal analysis coupled with mass spectrometry) was applied to the pyrolysis of Chinese coals with different ranks.
A total of 13 Chinese coals were investigated. The samples were deliberately chosen to represent the 13 types of Chinese coals
according to the Chinese coal classification system. The experiments were carried out in an argon atmosphere with a flow rate
of 150 ml min-1. The samples were heated from 40 up to 1200°C with a constant heating rate of 10 k min-1. The main evolved pyrolysis products were identified through the on-line recorded mass spectra. The thermal and evolution
behavior was compared between the coals. The results showed a strong thermal and evaluation behavior dependence on the coal
rank. Different aliphatic fragments and also some aromatic substances, which are of environmental concern (BTX, PAHs), were
found to be released depending on the different types of coal.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
129.
130.
水热一步法合成Ti-SBA-15分子筛及其催化性能研究 总被引:14,自引:1,他引:14
采用钛酸丁酯和乙酰丙酮作用后的产物作为钛的前驱体,水热法一步合成出了 Ti-SBA-15分子筛。通过X射线衍射、N2气吸附脱附、紫外—可见漫反射等表征手段 ,证明在Si/Ti=50,25,20时,钛原子成功地取代了SBA-15中的硅原子而没有改 变SBA-15高度有序的介孔二维六角结构,其中钛全部以四配位状态存在,而且高度 分散。当Si/Ti=12.5时,钛的分散度降低,部分钛聚集生成二氧化钛。与用两步 法合成的Ti-SBA-15比较,一步法合成的Ti-SBA-15中钛分散度好,添加量高,对催 化氧化环己烯有较高的催化活性。对一步法和两步法合成Ti-SBA-15的机理以及相 应产物在结构以及催化活性上的差异进行了探讨。 相似文献