全文获取类型
收费全文 | 34311篇 |
免费 | 9567篇 |
国内免费 | 1388篇 |
专业分类
化学 | 40232篇 |
晶体学 | 310篇 |
力学 | 678篇 |
综合类 | 1篇 |
数学 | 1917篇 |
物理学 | 2128篇 |
出版年
2022年 | 15篇 |
2021年 | 147篇 |
2020年 | 1221篇 |
2019年 | 2577篇 |
2018年 | 1012篇 |
2017年 | 649篇 |
2016年 | 3355篇 |
2015年 | 3508篇 |
2014年 | 3409篇 |
2013年 | 4007篇 |
2012年 | 2846篇 |
2011年 | 2037篇 |
2010年 | 2776篇 |
2009年 | 2750篇 |
2008年 | 2242篇 |
2007年 | 1647篇 |
2006年 | 1335篇 |
2005年 | 1539篇 |
2004年 | 1343篇 |
2003年 | 1247篇 |
2002年 | 1940篇 |
2001年 | 1330篇 |
2000年 | 1248篇 |
1999年 | 340篇 |
1998年 | 34篇 |
1997年 | 21篇 |
1993年 | 67篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1982年 | 15篇 |
1981年 | 21篇 |
1980年 | 21篇 |
1979年 | 20篇 |
1977年 | 18篇 |
1976年 | 25篇 |
1975年 | 21篇 |
1974年 | 29篇 |
1973年 | 18篇 |
1972年 | 20篇 |
1971年 | 12篇 |
1970年 | 57篇 |
1969年 | 35篇 |
1968年 | 18篇 |
1967年 | 13篇 |
1963年 | 18篇 |
1958年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
981.
Sonia Dopico‐García Ana Ares‐Pernas Jorge Otero‐Canabal Mar Castro‐López José M. López‐Vilariño Victoria González‐Rodríguez María J. Abad‐López 《先进技术聚合物》2013,24(8):723-731
Nowadays, there is a growing availability of biodegradable industrial materials intended to food contact applications whose service life behavior needs to be further investigated. This article is focused on the degradation of two materials based on polylactic acid. The correlation between the rate of degradation and the amount of trapped degradation products was investigated applying three characterization techniques in parallel, namely rheology, high‐performance liquid chromatography (HPLC), and matrix‐assisted laser desorption/ionization (MALDI). The rate of degradation was studied through the evaluation of their rheological properties and calculation of the number of average molecular weights, and weight‐average molecular weights. Water‐soluble oligomers and lactic acid were quantified by HPLC‐ultraviolet. Changes in cyclic and linear oligomers were monitored by MALDI‐time‐of‐flight mass spectrometry. Specimens of 4‐mm thickness of each biopolymer were subjected to hydrolysis in deionized water up to 6 months at two temperatures, simulating service conditions of food packaging. The diminution in viscosity and consequently in molecular weight distribution (20–60%) showed the degradation of the molecular structure of both polylactic acids. The chain scission was followed through the increasing values of lactic acid and hydrolyzed oligomers (twofold to eightfold), and the predominant signal of the linear oligomers over the cyclic ones with aging. Rheology, HPLC, and MALDI showed to be complementary tools to better understand the changes in the molecular structure. The obtained results showed the necessity of adding suitable stabilizers for each particular food packaging application. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
982.
Adrian M. Tomuta Xavier Fernández‐Francos Francesc Ferrando Xavier Ramis Àngels Serra 《先进技术聚合物》2013,24(11):962-970
A hyperbranched aromatic polyester (HBPOH) has been synthesized, and poly(ε‐caprolactone) arms have been grown on some of its end hydroxyl groups (HBPCL). These modifiers have been used in cationic diglycidyl ether of bisphenol A formulations cured with ytterbium triflate as cationic initiator. The effect of HBPOH and HBPCL on the curing kinetics has been studied using differential scanning calorimetry (DSC). The obtained materials have been characterized by dynamomechanical analysis, DSC, thermogravimetric analysis and mechanical tests. The modifiers are incorporated into the thermosetting network because of the participation of the end hydroxyl groups in the cationic curing of epoxides by the activated monomer mechanism. Homogeneous thermosets have been obtained with a remarkable increase in impact strength without sacrificing elastic modulus or hardness. A compromise between the rigid structure of the aromatic hyperbranched core and the flexibilizing effect of the poly(ε‐caprolactone) arms is believed to be responsible for the overall thermal and mechanical properties of the materials. The use of these polymeric modifiers increases the thermal stability of the resulting materials because of the low degradability of the aromatic ester groups in the hyperbranched core and the incorporation of the modifier into the network structure. However, the presence of such ester groups makes them reworkable by hydrolysis or alcoholysis in an alkaline medium, thus opening a way for recovery of valuable substrates. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
983.
Edith Chow Burkhard Raguse Karl‐H. Müller Lech Wieczorek Avi Bendavid James S. Cooper Lee J. Hubble Melissa S. Webster 《Electroanalysis》2013,25(10):2313-2320
The porosity of 1‐hexanethiol‐functionalised gold nanoparticle films was assessed and utilised as chemiresistor sensors. Electrochemical capacitance measurements showed that the accessibility of electrolytes of different ionic strengths into the pores depended on the thickness of the electric double layer formed. A large variation in capacitance was measured in 0.01–1000 mM NaClO4, implying a wide pore size distribution. The change in morphology of the nanoparticle films upon storage in air, water and ethanol for two weeks was investigated. There was a significant decrease in the electrochemical capacitance at high electrolyte concentrations for the ethanol‐stored films compared to the freshly‐prepared films suggesting a decrease in the number of small pores of radii in the range of 0.3–3 nm. This was further supported by optical topographical measurements where a decrease in the thickness of ethanol‐stored films was observed relative to the freshly‐prepared films. The porous nature of the nanoparticle films was found to have an effect on the chemical sensing behaviour. When used as chemiresistor sensors, for the detection of heptane in water, the ethanol‐stored films provided larger resistance changes and longer response times. This suggests that the more densely packed ethanol‐stored films provided more sites that enabled film swelling, and that diffusion of the analyte occurred through the narrower water‐filled pores. This demonstrates the effect of different storage conditions on film morphology and subsequently sensor response. 相似文献
984.
985.
986.
987.
988.
989.