全文获取类型
收费全文 | 5443篇 |
免费 | 437篇 |
国内免费 | 1792篇 |
专业分类
化学 | 6782篇 |
晶体学 | 50篇 |
力学 | 108篇 |
综合类 | 83篇 |
数学 | 54篇 |
物理学 | 595篇 |
出版年
2024年 | 16篇 |
2023年 | 90篇 |
2022年 | 199篇 |
2021年 | 236篇 |
2020年 | 274篇 |
2019年 | 226篇 |
2018年 | 178篇 |
2017年 | 247篇 |
2016年 | 273篇 |
2015年 | 249篇 |
2014年 | 280篇 |
2013年 | 497篇 |
2012年 | 350篇 |
2011年 | 313篇 |
2010年 | 303篇 |
2009年 | 313篇 |
2008年 | 357篇 |
2007年 | 379篇 |
2006年 | 376篇 |
2005年 | 335篇 |
2004年 | 338篇 |
2003年 | 283篇 |
2002年 | 231篇 |
2001年 | 199篇 |
2000年 | 193篇 |
1999年 | 156篇 |
1998年 | 137篇 |
1997年 | 116篇 |
1996年 | 96篇 |
1995年 | 90篇 |
1994年 | 88篇 |
1993年 | 70篇 |
1992年 | 43篇 |
1991年 | 25篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 18篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
排序方式: 共有7672条查询结果,搜索用时 15 毫秒
991.
催化动力学测定食品中痕量甲醛 总被引:15,自引:0,他引:15
在H2SO4介质中,甲醛可灵敏催化溴酸钾氧化中性红的褪色反应,研究了该指标反应的动力学参数,建立了动力学光度法测定痕量甲醛的新方法。该方法的检出限为4.08×10-9g/mL,线性范围在0-1.0μg/25mL。用水蒸汽蒸馏法蒸馏甲醛用于食品中甲醛的测定,结果满意。 相似文献
992.
Thallada Bhaskar Kazuya Murai Toshiki Matsui Mihai Adrian Brebu Md. Azhar Uddin Akinori Muto Yusaku Sakata Katsuhide Murata 《Journal of Analytical and Applied Pyrolysis》2003,70(2):369-381
The thermal degradation of acrylonitrile-butadiene-styrene copolymer (ABS-Br; 10 g) containing brominated flame retardant (Br: 9.59 wt.%) was carried out at 450 °C using a semi batch operation using two different temperature programs. The heating rate was found to affect the quality of the degradation oil and yield of products (liquid, gas and residue). Data on the effect of the temperature program on the accumulation of liquid products was presented. It was found that the majority of the bromine was concentrated in the carbon residue and while majority of the nitrogen accumulates in the liquid products irrespective of degradation mode. The use of a one step constant heating rate process (I) produced a higher liquid yield (39%), than a two step process (29%). Differences were also noted in the Br and N contained in the liquids produced by the two processes. 相似文献
993.
V. N. Kuznetsov A. A. Lisachenko I. V. Ivanova V. A. Konnov 《Journal of Applied Spectroscopy》1997,64(5):678-682
Using the spectroscopic method of diffuse reflection in the region between 0.5 and 6 eV, we investigate the degradation of
reflectivity in thermal vacuum treatment of white anodic-oxide coatings on an Al alloy (Al–AOC) that are used as thermoregulating
coatings of space vehicles. It is established that at T≥350 K absorption bands at 4.05 and ∼4.7 eV resembling those induced
by UV irradiation in vacuum appear in the diffuse reflection spectra of Al–AOC. The kinetics of the increase of absorption
in heating Al–AOC with a constant rate correlates with the rate of gas liberation from specimens. We assume that a thermally
activated reaction with the formation of color centers identical to those produced by the photoeffect occur in Al–AOC in vacuum
at T≥350 K. This process is accompanied by desorption of molecular products.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 664–667, September–October, 1997. 相似文献
994.
G. Evan Roberts Johan P. A. Heuts Thomas P. Davis 《Journal of polymer science. Part A, Polymer chemistry》2003,41(6):752-765
The cobaloxime‐mediated catalytic‐chain‐transfer polymerization of styrene at 60 °C was studied with an emphasis on the effects of monomer purification and polymerization conditions. Commonly used purification methods, such as column chromatography and simple vacuum distillation, were not adequate for obtaining kinetic data to be used in mechanistic modeling. A purification regime involving inhibitor removal with basic alumina, followed by polymerization of the styrene in the presence of the cobaloxime and subsequent vacuum distillation, was found to be essential to this end. It was then possible to quantitatively investigate effects such as the initiator concentration and conversion dependencies of the apparent chain‐transfer constant that resulted from the occurrence of cobalt–carbon bond formation. A value of about 9 × 103 was found for the true chain‐transfer constant to cobaloxime boron fluoride, that is, its value in the absence of cobalt–carbon bond formation. Furthermore, previous predictions were confirmed: the measured chain‐transfer constant decreased with increasing initiator concentration and conversion. Finally, it was confirmed that the presence of light increased the amount of free Co(II) catalyst in agreement with other studies. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 752–765, 2003 相似文献
995.
Four new phenolic dimers and trimers that contain ferulic acid moieties were isolated from the alkaline hydrolyzate of insoluble maize bran fiber and their structures were established by 1D/2D NMR and mass spectrometry. The biological role of one dimer remains unclear whereas the dimeric vanillin-ferulic acid-cross-product probably represents an oxidative degradation product from the corresponding diferulate. Both ferulic acid dehydrotrimers are able to cross-link polysaccharide chains. However, the 5-5/8-O-4(H2O)-triferulic acid may be a cross-link in its identified structure whereas we assume that the identified 8-O-4/8-5(non-cyclic)-triferulic acid arose from a natural 8-O-4/8-5(cyclic)-triferulate analog during the saponification process. 相似文献
996.
ROMPgel-supported tris(triphenylphosphine)rhodium(I) chloride has been prepared and the immobilised catalyst has been effectively employed in selective hydrogenations of a variety of alkenes and terminal alkynes. 相似文献
997.
含羧酸共生稀土光敏剂的低密度聚乙烯膜紫外光氧化降解 总被引:3,自引:0,他引:3
含羧酸共生稀土光敏剂的低密度聚乙烯膜紫外光氧化降解林宜超(福建省测试技术研究所福州350003)近年来,许多作者先后研究并开发出含硬脂酸铈(CeSt3)光敏剂的可控光降解低密度聚乙烯(LDPE)[1,2]、高密度聚乙烯[3]、聚丙烯[4]、聚苯乙烯... 相似文献
998.
Jun‐Feng Wu Dong‐Mei Li Li‐Feng Cui Chang‐Fu Zhuang Sai‐Nan Song Tie‐Gang Wang Ji‐Qing Xu Heng‐Qing Jia Ning‐Hai Hu 《应用有机金属化学》2007,21(12):1033-1040
Mo2O2S2(HGly)(Gly)2 1 and K6[Mo2O2S2(nta)2][Mo2O2S2(ntaH)2]·4H2O 2 were synthesized by the reactions of (NH4)2MoS4 and amino acids L (L = glycine, nitrilotriacetic acid) in ethanol–water medium at ambient temperature. The two complexes were characterized by elemental analysis, infrared spectra, UV–visible spectra, TG–DTA and XPS. X‐ray crystallographic structural analyses revealed that compound 1 is a binuclear Mo? S? glycinate complex, a glycinate ligand is coordinated to each molybdenum atom through its amine nitrogen and carboxylato oxygen, respectively, and the third glycinate acts as a bridge through its two carboxylato oxygens linking the two molybdenum atoms. Compound 2 is also a binuclear Mo? S complex with two nitrilotriacetate ligands, each of which is coordinated to a molybdenum atom via its two β‐carboxylato oxygens and a nitrogen atom. Simultaneously, each molybdenum atom in 1 and 2 is chelated to a terminal oxygen and two bridging sulfurs to complete the octahedral configuration. Their catalytic activities in the reduction from C2H2 to C2H4 as well as other binuclear Mo? S? polycarboxylate complexes, a [Fe4S4] single cubane and a chainlike Mo? Fe? S compound were investigated and it was found that 1 exhibited relatively good catalytic activity. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
999.
L. El Watik P. Antonetti Y. Claire H. Zineddine J. Kaloustian C. Rossi A. Perichaud 《Journal of Thermal Analysis and Calorimetry》1998,53(1):297-308
The authors studied the ignifugation and the kinetic of thermal degradation of the Styrene-Butadiene copolymer with an intumescent
system Ammonium polyphosphate-Pentaerythrinol-Melamine. For that, they used the thermogravimetric and oxygen index techniques.
The best formulation of intumescent system which can give the optimum results is proposed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
1000.
The physicochemical and catalytic properties of cobalt- and silicon-substituted AlPO-5 molecular sieves were studied by means of temperature-programmed reduction, temperature-programmed desorption of ammonia and the reaction of toluene disproportionation. 相似文献