全文获取类型
收费全文 | 2192篇 |
免费 | 75篇 |
国内免费 | 15篇 |
专业分类
化学 | 1423篇 |
晶体学 | 6篇 |
力学 | 75篇 |
数学 | 231篇 |
物理学 | 547篇 |
出版年
2023年 | 8篇 |
2022年 | 30篇 |
2021年 | 29篇 |
2020年 | 31篇 |
2019年 | 39篇 |
2018年 | 24篇 |
2017年 | 34篇 |
2016年 | 86篇 |
2015年 | 41篇 |
2014年 | 70篇 |
2013年 | 130篇 |
2012年 | 119篇 |
2011年 | 113篇 |
2010年 | 74篇 |
2009年 | 68篇 |
2008年 | 128篇 |
2007年 | 103篇 |
2006年 | 86篇 |
2005年 | 84篇 |
2004年 | 92篇 |
2003年 | 87篇 |
2002年 | 77篇 |
2001年 | 54篇 |
2000年 | 59篇 |
1999年 | 46篇 |
1998年 | 16篇 |
1997年 | 23篇 |
1996年 | 40篇 |
1995年 | 57篇 |
1994年 | 46篇 |
1993年 | 48篇 |
1992年 | 51篇 |
1991年 | 24篇 |
1990年 | 31篇 |
1989年 | 19篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 10篇 |
1982年 | 19篇 |
1981年 | 16篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有2282条查询结果,搜索用时 62 毫秒
101.
102.
S. R. de Miguel J. C. Heinen A. A. Castro O. A. Scelza 《Reaction Kinetics and Catalysis Letters》1989,40(2):331-335
The enhancement of the activity of a carbon treated with HF is due to the drastic diminution of its inorganic impurities, which produces an increase of the surface area of the carbon, a stronger Pt-C interaction and a higher Pt dispersity.
, HF, , , Pt-C Pt.相似文献
103.
The use of photochemical reactions in flow injection (FI) is reported. The irradiation of an FI reactor with a suitable source facilitates the development of the iron(III)-oxalate reaction, allowing the amperometric determination of the anion in the range 1.0-13.0 micrograms ml-1, with a relative standard deviation of 1.1% and a sampling frequency of 40 h-1. The proposed method was applied successfully to the determination of oxalate in urine samples. 相似文献
104.
The joint action of trisdimethylaminophosphine (TDAP) and carbon tetrachloride on some vicinal diols
The joint action of trisdimethylaminophosphine and carbon tetrachloride on vicinal diols affords either trans epoxides or spirophosphoranes. The mutarotation of these spirophosphoranes is described by 1H NMR and 31P NMR spectra. The mechanism of reaction providing either epoxides or spirophosphoranes is discussed. 相似文献
105.
Liliana Castro Patricio Reyes Consuelo Montes de Correa 《Journal of Sol-Gel Science and Technology》2002,25(2):159-168
Fe-ZrO2 and Cu-ZrO2 xerogels were prepared by a sol-gel method. The effect of the hydrolysis catalyst during the gelation step, namely H2SO4 or NH4OH, on the properties of the resulting materials was investigated by XRD, BET, TGA/DTA, TPD of ammonia, FTIR, and TPR. Fe-ZrO2 and Cu-ZrO2 xerogels, with sulfuric acid introduced as the hydrolysis catalyst, mainly crystallyzed in the tetragonal phase and exhibited larger surface area and acid amount than those obtained with NH4OH. Ammonia TPD shows that copper promoted sulfated zirconia is the most acidic material. TGA and FTIR reveal that under oxidizing conditions sulfated zirconia promoted with iron and copper retains more sulfate species than unpromoted sulfated zirconia. Regardless of the hydrolysis catalyst employed, copper promoted catalysts calcined at 600°C, contain a large fraction of copper oxide specieseasily reduced at low temperatures. These copper oxide species are believed to have different environment and interactions with the surface oxygen vacancies of the zirconia support. A FeO-like phase appears to be the most probable one after reduction of Fe-ZrO2 catalysts prepared with NH4OH as the hydrolysis catalyst. The formation of Fe° species may be hindered by the high dispersion and interaction of Fe2+ ions with the zirconia support. On the other hand, the reduction peaks of iron oxide and sulfate species exhibit a considerable overlap in the TPR profiles of sulfated Fe-ZrO2 samples. Hence, the nature of the supported phase in the latter samples is rather uncertain. 相似文献
106.
Eigenstates of a particle in a localized and unconfined harmonic potential well are investigated. Effects due to the variation of the potential parameters as well as certain results from asymptotic expansions are discussed. 相似文献
107.
Raul Aguayo Felipe Arias Alvaro Cañete Carolina Zuñiga Enrique A. Castro Paulina Pavez José G. Santos 《国际化学动力学杂志》2013,45(3):202-211
The reactions of the title compounds with phenoxides, secondary alicyclic (SA) amines, and pyridines, in 44 wt% ethanol–water, at 25°C and an ionic strength of 0.2 M, were subjected to kinetic and product studies. From analytical techniques (HPLC and NMR), two pathways were detected (nucleophilic attack at the phosphoryl center and at the C‐1 aromatic carbon) for the reactions of all the nucleophiles with the phosphate ( 2 ) and for the pyridinolysis of the thionophosphate ( 1 ). Only aromatic nucleophilic substitution was found for the reactions of 1 with phenoxides and SA amines. For the dual reactions, the nucleophilic rate constants (kN) were separated in two terms: $k_{\rm N}^{\rm P}$ and $k_{\rm N}^{{\rm Ar}}$, which are the rate constants for the corresponding electrophilic centers. The absence of a break in the Brønsted‐type plots for the attack at P is consistent with concerted mechanisms. The Brønsted slopes, βAr 0.32–0.71, for the attack at the aromatic C‐1, are in agreement with stepwise mechanisms where formation of a Meisenheimer complex is the rate‐determining step. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 202–211, 2013 相似文献
108.
109.
A simple, quick and novel method for the determination of diffusion properties through polymer films, based on Quantum Resistive Sensors made of Conductive Polymer nanoComposites is presented. The integral time lag method is employed for the calculation of diffusion coefficient, and the results are compared simultaneously with that of Fourier transform infrared spectroscopy and sorption method. Two model polymers, a semi‐crystalline poly(lactic acid) and an amorphous poly(isobutylene‐co‐isoprene), are used to validate the study. A good correlation is established between the diffusion coefficient values derived from all techniques demonstrating the interest of such reliable, simple and cheap nanosensors for the quick determination (several minutes) of diffusion properties in polymer films. Our first results suggest that this technique is meaningful for the determination of barrier properties in nanocomposite membranes filled with platelets of graphene or clay. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
110.
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