全文获取类型
收费全文 | 4117篇 |
免费 | 981篇 |
国内免费 | 595篇 |
专业分类
化学 | 2930篇 |
晶体学 | 107篇 |
力学 | 181篇 |
综合类 | 33篇 |
数学 | 58篇 |
物理学 | 2384篇 |
出版年
2024年 | 10篇 |
2023年 | 31篇 |
2022年 | 105篇 |
2021年 | 126篇 |
2020年 | 123篇 |
2019年 | 105篇 |
2018年 | 113篇 |
2017年 | 154篇 |
2016年 | 208篇 |
2015年 | 164篇 |
2014年 | 263篇 |
2013年 | 620篇 |
2012年 | 312篇 |
2011年 | 331篇 |
2010年 | 273篇 |
2009年 | 246篇 |
2008年 | 257篇 |
2007年 | 319篇 |
2006年 | 291篇 |
2005年 | 285篇 |
2004年 | 253篇 |
2003年 | 203篇 |
2002年 | 217篇 |
2001年 | 106篇 |
2000年 | 103篇 |
1999年 | 84篇 |
1998年 | 63篇 |
1997年 | 47篇 |
1996年 | 45篇 |
1995年 | 41篇 |
1994年 | 26篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 3篇 |
排序方式: 共有5693条查询结果,搜索用时 218 毫秒
121.
122.
123.
掺铁纳米TiO_2的制备及其光催化性能 总被引:2,自引:0,他引:2
以TiC l4为原料,采用微波加热与常规加热2步合成法制备掺铁纳米TiO2。用X射线粉末衍射(XRD)、透射电镜(TEM)、紫外可见吸收光谱(UV-V is)、光电子能谱(XPS)等测试技术对其进行了表征。结果表明,所制得的掺铁纳米TiO2是以锐钛矿为主相的混晶,平均粒径约为10 nm,适量Fe3+的掺杂能促进金红石相变,抑制锐钛矿晶粒的生长,使Ti2p电子结合能升高0.3 eV,并使TiO2吸光能力增强,带边吸收向可见光区移动。在UVA段(320~400 nm)光照下,通过对维生素VB12的光催化降解,发现掺入摩尔分数为0.5%的Fe3+的纳米TiO2能明显提高其光催化活性,使维生素B12的降解速率提高2.3倍。 相似文献
124.
ZuPeiLIANG YaQingFENG ShuXianMENG ZhiYanLIANG 《中国化学快报》2005,16(1):135-138
Urease was immobilized onto the glutaraldehyde cross-linked chitosan beads that were prepared under microwave irradiation. The activity and the yield of activity of immobilized urease was 10.83 U/g B and 47.7%, respectively. The conditions of urease immobilization were optimized. The properties of the immobilized urease were investigated and compared with that of the free enzyme. 相似文献
125.
3-(3-Aryl-4-formyl-1-pyrazolyl)propionic acids are converted by Knoevenagel condensation under conditions of microwave activation
into 3-[3-aryl-1-(2-ethoxycarbonyl)-4-pyrazolyl]acrylic acids. Reduction of the latter with hydrazine hydrate in the presence
of Raney nickel gives 3-[3-aryl-1-(2-ethoxycarbonyl)-4-pyrazolyl]propionic acids.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 686–690, May, 2006. 相似文献
126.
The title compound (C25H24N4O3) has been prepared by the cyclocondensation of 5-amino-3-methyl-l-phenypyrazol, m-nitrobenaldehyde and dimedone in glycol under microwave irradiation without catalyst. The crystal structure was determined by single-crystal X-ray diffraction to be orthorhombic, space group Pbca with a=16.3331(10), b=13.8329(9), c=19.4163(12) A, V= 4386.8(5)A3, Z=8, Dc=1.298g/cm^3,μ=0.087 mm^-1, F(000)=1808, Mr=428.48, the final R=0.0519 and wR=0.1019. X-ray analysis revealed that the pyridine ring is of boat conformation and the six-membered ring fused with it adopts twist boat conformation. 相似文献
127.
Microscale sample deposition onto hydrophobic target plates for trace level detection of neuropeptides in brain tissue by MALDI-MS 总被引:1,自引:0,他引:1
Wei H Dean SL Parkin MC Nolkrantz K O'Callaghan JP Kennedy RT 《Journal of mass spectrometry : JMS》2005,40(10):1338-1346
A sample preparation method that combines a modified target plate with a nanoscale reversed-phase column (nanocolumn) was developed for detection of neuropeptides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A gold-coated MALDI plate was modified with an octadecanethiol (ODT) self-assembled monolayer to create a hydrophobic surface that could concentrate peptide samples into a approximately 200-500-microm diameter spot. The spot sizes generated were comparable to those obtained for a substrate patterned with 200-microm hydrophilic spots on a hydrophobic substrate. The sample spots on the ODT-coated plate were 100-fold smaller than those formed on an unmodified gold plate with a 1-microl sample and generated 10 to 50 times higher mass sensitivity for peptide standards by MALDI-TOF MS. When the sample was deposited on an ODT-modified plate from a nanocolumn, the detection limit for peptides was as low as 20 pM for 5-microl samples corresponding to 80 amol deposited. This technique was used to analyze extracts of microwave-fixed tissue from rat brain striatum. Ninety-eight putative peptides were detected including several that had masses matching neuropeptides expected in this brain region such as substance P, rimorphin, and neurotensin. Twenty-three peptides had masses that matched peaks detected by capillary liquid chromatography with electrospray ionization MS. 相似文献
128.
129.
Microwave-assisted acyl chloride-isonitrile condensation and CaCO3-mediated hydrolysis constitute a one-pot, 2-minute process to prepare α-ketoamides. 相似文献
130.
Peter Gajdoš Soňa Pavlíková Filip Bureš Alžbeta Krutošíková 《Central European Journal of Chemistry》2005,3(2):311-325
The synthesis and reactions of methyl 2-[3-(trifluoromethyl)phenyl]-4H-furo[3,2-b]pyrrole-5-carboxylate (1a) are described. Upon reaction with methyl iodide, benzyl chloride, or acetic anhydride, this compound gave N-substituted products 1b-d. By hydrolysis of compounds 1a-c, the corresponding acids 2a-c were formed, or by reaction with hydrazine-hydrate, the corresponding carbohydrazides 3a-c were formed. By heating 2-[3-(trifluoromethyl)phenly]-4H-furo[3,2-b]pyrrole-5-carboxylic acid (2a) in acetic anhydride, 4-acetyl-2-[3-(trifluoromethyl)phenyl]furo[3,2-b]pyrrole (4) was formed. By hydrolysis of 4, 2-[3-(trifluoromethyl)phenyl]-4H-furo[3,2-b]pyrrole (5a) was formed, and reactions with methyl iodide or benzyl chloride gave N-substituted products 5b-c. The reaction of 4 with dimethyl butynedioate gave substituted benzo[b]furan 6. Compound 3a reacted with triethyl orthoesters giving 7a-c, which afforded with phosphorus (V) sulphide the corresponding thiones 8a-c. The thiones 8a-c reacted with hydrazine hydrate to form hydrazine derivatives 9a-c. The reaction of triethyl orthoformiate with compounds 9a-c led to furo[2′,3′: 4,5]pyrrolo[1,2-d][1,2,4]triazolo[3,4-f][1,2,4]triazines 10a-c. Hydrazones 11a-c were formed from 3a-c and 5-[3-(trifluoromethyl)phenyl]furan-2-carboxaldehyde. The effect of microwave irradiation on some condensation reactions
was compared with “classical” conditions. The results showed that microwave irradiation shortens the reaction time while affording
comparable yields. 相似文献