全文获取类型
收费全文 | 5445篇 |
免费 | 338篇 |
国内免费 | 36篇 |
专业分类
化学 | 3715篇 |
晶体学 | 65篇 |
力学 | 206篇 |
数学 | 886篇 |
物理学 | 947篇 |
出版年
2023年 | 36篇 |
2022年 | 133篇 |
2021年 | 167篇 |
2020年 | 116篇 |
2019年 | 200篇 |
2018年 | 170篇 |
2017年 | 117篇 |
2016年 | 276篇 |
2015年 | 170篇 |
2014年 | 283篇 |
2013年 | 573篇 |
2012年 | 326篇 |
2011年 | 338篇 |
2010年 | 242篇 |
2009年 | 232篇 |
2008年 | 246篇 |
2007年 | 237篇 |
2006年 | 226篇 |
2005年 | 182篇 |
2004年 | 170篇 |
2003年 | 146篇 |
2002年 | 194篇 |
2001年 | 77篇 |
2000年 | 117篇 |
1999年 | 66篇 |
1998年 | 48篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 44篇 |
1994年 | 38篇 |
1993年 | 38篇 |
1992年 | 33篇 |
1991年 | 28篇 |
1990年 | 16篇 |
1989年 | 34篇 |
1988年 | 37篇 |
1987年 | 22篇 |
1986年 | 20篇 |
1985年 | 31篇 |
1984年 | 28篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 27篇 |
1980年 | 17篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 14篇 |
1975年 | 17篇 |
1974年 | 9篇 |
排序方式: 共有5819条查询结果,搜索用时 62 毫秒
981.
We have studied the effect of subsequent gamma (γ) irradiation on the absorption spectra and the optical energy gap of ZnO thin films doped with Li (ZnO:Li). The optical transmission (T) and optical reflection (R) in the wavelength range 190∼800 nm of films deposited at 300 °C on sapphire, MgO or quartz substrates were measured. The dependence of the absorption coefficient α on photon energy hν was determined as a function of γ‐doses. The films show direct allowed interband transition that influenced by the gamma doses. Both the optical energy gap Eoptg and the absorption coefficient (α) were found to be γ‐dose dependent. The results can be discussed on the basis of γ‐irradiation‐induced defects in the film and on the film structure. The absorption coefficient exhibits exponential dependence on photon energy obeying Urbach's rule in the absorption edge. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
982.
Free Sulfate is a major parameter affecting gypsum crystallization during phosphoric acid production. Gypsum crystal size, shape and filtration rate are significantly affected by the concentration of free sulfate. It is, therefore, important to evaluate the effectiveness of different sulfate levels from 1.5% to 3.5% on induction time and gypsum morphology. The crystallization of gypsum was carried out under simulated conditions of phosphoric acid production by the dihydrate process. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80 °C, and the turbidity of the reaction mixture was measured at different time periods to calculate the induction time of gypsum crystals formation. With increasing free sulfate concentration, the induction time was significantly decreased. Chemical processing of Central and South Florida phosphate concentrates under different concentrations of free sulfate from 1.5% to 5.5% was carried out. The change on crystal size distribution and filtration rate were traced with free sulfate concentrations. The results show that, filtration rate of phosphogypsum was correlated to the mean diameter of crystals. In addition, induction time and co‐crystallized (lattice) P2O5 % in gypsum are decreased with increasing free sulfate content from 1.5% to 3.5%. Morphology of formed gypsum crystals at different sulfate contents and different supersaturation ratios are investigated. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
983.
Youssef Boudabbous 《Mathematical Logic Quarterly》1999,45(3):421-431
Let T and T1 be tournaments with n elements, E a basis for T, E′ a basis for T′, and k ≥ 3 an integer. The dual of T is the tournament T” of basis E defined by T(x, y) = T(y, x) for all x, y ε E. A hemimorphism from T onto T′ is an isomorphism from T onto T” or onto T. A k-hemimorphism from T onto T′ is a bijection f from E to E′ such that for any subset X of E of order k the restrictions T/X and T1/f(X) are hemimorphic. The set of hemimorphisms of T onto itself has group structure, this group is called the group of hemimorphisms of T. In this work, we study the restrictions to n – 2 elements of a tournament with n elements. In particular, we prove: Let k ≥ 3 be an integer, T a tournament with n elements, where n ≥ k + 5. Then the following statements are equivalent: (i) All restrictions of T to subsets with n – 2 elements are k-hemimorphic. (ii) All restrictions of T to subsets with n – 2 elements are 3-hemimorphic. (iii) All restrictions of T to subsets with n – 2 elements are hemimorphic. (iv) All restrictions of T to subsets with n – 2 elements are isomorphic, (v) Either T is a strict total order, or the group of hemimorphisms of T is 2-homogeneous. 相似文献
984.
El Watik L. Claire Y. Zineddine H. Rossi C. Kaloustian J. Sergent M. Périchaud A. 《Journal of Thermal Analysis and Calorimetry》1999,58(1):19-28
The study of the thermal degradation of the fireproof polystyrene—butadiene copolymer by an intumescent system: ammonium polyphosphate (APP)—pentaerythritol (PER)—talc (TAL), was done through an experimental process. Thermogravimetry under air sweeping was used. We observed for the degradation rate of the mixture with fireproofing agents, in the field of 20 to 50% mass loss, a better linear increasing when this mixture contains more TAL and the ratio APP/PER is smaller.
This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
985.
Magdy A. M. Ibrahim Hamdy H. Hassan Sayed S. Abd El Rehim Mohamed A. Amin 《Journal of Solid State Electrochemistry》1999,3(7-8):380-386
The electrochemical behaviour of polycrystalline silver electrodes in Na2CO3 solutions was studied under potentiodynamic and potentiostatic conditions and complemented with X-ray diffraction analysis.
Potentiodynamic E/i anodic curves exhibit active passive transition prior to an oxygen evolution reaction. The active region involves a small
peak AI followed by a major peak AII before the passive region. Peak AI is assigned to the formation of an Ag2O layer while peak AII is due to the formation of an Ag2CO3 layer. The height of the anodic peaks increases with increasing Na2CO3 concentration, scan rate and temperature. The effect of increasing additions of NaClO4 on the electrochemical behaviour of Ag in Na2CO3 solutions was investigated. The perchlorate ions stimulate the active dissolution of Ag, presumably as a result of the formation
of soluble AgClO4 salt. In the passive region, ClO−
4 ions tend to break down the dual passive film, leading to pitting corrosion at a certain critical pitting potential. The
pitting potential decreases with ClO−
4 concentration. Potentiostatic current/time transients showed that the formation of Ag2O and Ag2CO3 layers involves a nucleation and growth mechanism under diffusion control. However, in the presence of ClO−
4 ions, the incubation time for pit initiation decreases on increasing the anodic potential step.
Received: 3 July 1998 / Accepted: 10 March 1999 相似文献
986.
Novel polysubstituted 1,5‐benzodiazepine 5 , 2,2‐bis(methylthio)benzoxazoles 8a–d , 2,2‐bis‐ (acetyl)benzoxazole 8e , 2‐(3‐methyl‐1‐phenylpyrazolo‐ 4‐yl)benzoazole derivatives 16a–c , as well as the previously reported 2‐di[cyano(acetyl)‐methylene]benzothiazoles 7a,b have been obtained via a new utility of ketene dithioacetals 1a,b and 12 with aniline derivatives 2 . Rationales for the reactions pathways are presented. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:407–412, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20031 相似文献
987.
The aim of this paper is to get a good approximation of damping properties of viscoelastic damped sandwich structures. Firstly we will compare the classical models based on the kinematics of Kirchhoff-Love, Mindlin, Reddy or Touratier. In the second stage these models are used to establish the zig-zag models which describe a piecewise continuous displacement field. These models are compared in the static and the dynamic fields. In all cases the Finite-Element-based solution is considered as reference. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
988.
Mohamed I. Hegab Nasser A. Hassan Emad M. El‐Telbani Ibrahim S. Ahmed Farag Farouk M. E. Abdel‐Megeid 《Heteroatom Chemistry》2003,14(3):223-228
Hydrazones 12a–c and ketazines 13a–c were prepared by the reaction of ketones 11a–c with hydrazine hydrate depending on the temperature and the reaction time. Some ketone (aryl)hydrazone derivatives 14a,c,e reacted with thionyl chloride to afford the chlorothiadiazoline derivatives 15a–c . Surprisingly, the chlorine atom in the latter compounds was found to undergo smooth nucleophilic substitution, and by boiling these compounds in absolute ethanol gave the corresponding ethoxythiadiazoline derivatives 16a–c . The structure of the ethoxythiadiazoline 16b was confirmed by single crystal X‐ray determination. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:223–228, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10125 相似文献
989.
Mahmoud G. Metwally Abdelaziz H. Benhawy Reda M. Khalifa Rasha M. El Nashar Marek Trojanowicz 《Molecules (Basel, Switzerland)》2021,26(21)
The increase of the global population and shortage of renewable water resources urges the development of possible remedies to improve the quality and reusability of waste and contaminated water supplies. Different water pollutants, such as heavy metals, dyes, pesticides, endocrine disrupting compounds (EDCs), and pharmaceuticals, are produced through continuous technical and industrial developments that are emerging with the increasing population. Molecularly imprinted polymers (MIPs) represent a class of synthetic receptors that can be produced from different types of polymerization reactions between a target template and functional monomer(s), having functional groups specifically interacting with the template; such interactions can be tailored according to the purpose of designing the polymer and based on the nature of the target compounds. The removal of the template using suitable knocking out agents renders a recognition cavity that can specifically rebind to the target template which is the main mechanism of the applicability of MIPs in electrochemical sensors and as solid phase extraction sorbents. MIPs have unique properties in terms of stability, selectivity, and resistance to acids and bases besides being of low cost and simple to prepare; thus, they are excellent materials to be used for water analysis. The current review represents the different applications of MIPs in the past five years for the detection of different classes of water and wastewater contaminants and possible approaches for future applications. 相似文献
990.
A k‐star is the graph K1,k. We prove a general theorem about k‐star factorizations of Cayley graphs. This is used to give necessary and sufficient conditions for the existence of k‐star factorizations of any power (Kq)s of a complete graph with prime power order q, products C × C ×··· × C of k cycles of arbitrary lengths, and any power (Cr)s of a cycle of arbitrary length. © 2001 John Wiley & Sons, Inc. J Graph Theory 36: 59–66, 2001 相似文献