全文获取类型
收费全文 | 448篇 |
免费 | 27篇 |
国内免费 | 3篇 |
专业分类
化学 | 357篇 |
力学 | 11篇 |
数学 | 23篇 |
物理学 | 53篇 |
无线电 | 34篇 |
出版年
2023年 | 7篇 |
2022年 | 36篇 |
2021年 | 28篇 |
2020年 | 22篇 |
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 20篇 |
2014年 | 21篇 |
2013年 | 43篇 |
2012年 | 29篇 |
2011年 | 22篇 |
2010年 | 21篇 |
2009年 | 11篇 |
2008年 | 21篇 |
2007年 | 25篇 |
2006年 | 14篇 |
2005年 | 14篇 |
2004年 | 18篇 |
2003年 | 16篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 10篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有478条查询结果,搜索用时 15 毫秒
131.
This work provides an insight into the effect of incorporating of magnetite nanoparticles on the rheology of fluids. In this respect, polymer-stabilized magnetite nanoparticles were obtained using sodium salt of poly (2-acrylamido-2-methylpropanesulfonate (PAMPS-Na). Monodisperse polymer coated magnetite nanoparticles Fe3O4/poly(styrene-AMPS) copolymer nanoparticles with diameters of 50–300 nm were prepared by radical polymerization in the presence of a ferrofluid coated with PAMPS-Na. The magnetic nanoparticles were easily separated in a magnetic field. The structure of the obtained magnetic nanoparticles was characterized by Fourier transform infrared spectroscopy (FTIR). The morphology and size of the magnetic nanoparticles were determined by transmission electron microscopy (TEM). FTIR and TEM revealed that the Fe3O4 nanoparticles were incorporated into the shells of poly(styrene-AMPS). Aqueous dispersed solutions of a charged hydrophobically modified Fe3O4/poly(styrene-AMPS) copolymer nanoparticles exhibit high viscosities even at low polymer concentrations (0.1 wt %), which is an interesting feature in connection with enhanced oil recovery. Effects of temperature and addition of sodium chloride on the viscosity properties of a semidilute dispersed solution of Fe3O4/poly(styrene-AMPS) copolymer nanoparticles are examined. The results indicated that Fe3O4/poly(styrene-AMPS) copolymer nanoparticles disclose strong interactions between magnetite and coated polymers of both PAMPS-Na and styrene-AMPS copolymers. 相似文献
132.
Firas F. Awwadi Deeb Taher Alaa Maabreh Fadel Z. Alwedian Hamdan Al-Ebaisat Tobias Rüffer Heinrich Lang 《Structural chemistry》2013,24(2):401-408
The analogy of chloride–chloride contacts in compounds containing Fe–Cl1···Cl2–Fe synthons with well-studied organic C–Cl1···Cl2–C interactions has been investigated. The crystal structures of the two tetrahaloferrate(III) salts, [(2-iodopyridinium)2FeX4]X (X = Cl, Br) have been determined. Analysis of these two isomorphous structures and related published structures shows that the arrangement of Fe–Cl1···Cl2–Fe synthons is similar to that of C–Cl1···Cl2–C with the Fe–Cl1···Cl2 and Cl1···Cl2–Fe angles being ~150°. While inter-chlorine distances are less than the sum of van der Waals radii in C–Cl1···Cl2–C units, they are equal to, or longer, than the sum of van der Waals radii in the corresponding Fe–Cl1···Cl2–Fe contacts. This might indicate that the arrangement of Fe–Cl1···Cl2–Fe synthons occurs predominately to reduce repulsive forces rather than as a result of attractive forces. However, it is observed that the halide–halide distance in [(2-iodopyridinium)2FeBr4]Br is shorter than in the isostructural chloride species, which can be explained by the fact that bromine is softer than chlorine. Several intermolecular forces unite the cations and anions within the crystalline lattice of [(2-iodopyridinium)2FeX4]X including N–H···X?, C–I···X–Fe, N(π)···X–Fe, N(π)···I–C, and Fe–X1···X2–Fe contacts. The calculated electron density and electrostatic potential of the [FeX4]? anions and the organic iodopyridinium cations was used to describe the arrangement of these synthons and the hierarchy of the strengths of the respective contacts. 相似文献
133.
A. S. Aly Kh. M. Abu-Zied Alaa El-Din M. Gaafar 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):3063-3078
In one-pot synthesis 2-arylidene-5,6,7,8-tetrahydrothiazolo[3,2-a] cyclopenteno-thieno[2,3-d] pyrimidine-3,5-diones (3) were prepared via the reaction of a ternary mixture of 2-thioxo-1,3,4,5,6,7-hexahydr cyclopentinothieno [2,3-d]-4-one (2), chloroacetic acid and a proper aldehyde. Compound 2 reacted with 3-chloropent-2, 4-dione in ethanolic potassium hydroxide yielding the S-acetyl acetone derivative 5f . The latter compound reacted with hydrazine hydrate and phenyl hydrazine yielded the 2-pyrazolthio derivative 10a, b. Compound 5f also underwent cyclization on heating with acetic acid—pyridine solution to give 11. The 2-methylthio derivative 5a, when treated with hydrogen peroxide gave the corresponding oxidized product 9. 相似文献
134.
Recent Heterocyclic Compounds from (1,3‐Dioxo‐2,3‐dihydro‐1H‐inden‐2‐ylidene)propanedinitrile 下载免费PDF全文
This review summarizes the literatures dealing with the synthesis of some important nitrogen or nitrogen/sulfur heterocyclic, spiroheterocyclic, and fused heterocyclic compounds. 相似文献
135.
136.
Rasha M. Al‐Makhlasawy Alaa A. Hefnawy Mustafa M. Abd Elnaby Fathi E. Abd El‐Samie 《International Journal of Communication Systems》2020,33(13)
This paper investigates a vital issue in wireless communication systems, which is the modulation classification. A proposed framework for modulation classification based on deep learning (DL) is presented in the presence of adjacent channel interference (ACI). This framework begins with the generation of constellation diagrams from the received data. These constellation diagrams are fed to convolutional neural networks (CNNs) for modulation classification. The objective of this process is to eliminate the manual feature extraction from the received data and make feature extraction process as a built‐in step with CNNs. Three types of CNNs are considered in this paper and compared for this objective. These types are AlexNet, VGG‐16, and VGG‐19. The proposed classifier is applied on Rayliegh and Rician fading channels. 相似文献
137.
Shakeel Ahmad Khandy Ishtihadah Islam Kulwinder Kaur Atif Mossad Ali Alaa F. Abd El-Rehim 《Molecules (Basel, Switzerland)》2022,27(12)
This paper presents the strain effects on the structural, electronic and phonon properties of a newly proposed SrBaSn half Heusler compound. Since it is stable considering chemical thermodynamics, we tested its strength against uniform strain w.r.t phonon spectrum and it produces a direct bandgap of 0.7 eV. The direct bandgap reduces to 0.19 eV at −12% strain beyond which the structure is unstable. However, an indirect gap of 0.63 eV to 0.39 eV is observed in the range of +5% to +8% strain and afterwards the strain application destabilizes the structure. From elastic parameters, the ductile nature of this material is observed. 相似文献
138.
Alaa Z. Omar Najla A. Alshaye Tawfik M. Mosa Samir K. El-Sadany Ezzat A. Hamed Mohamed A. El-Atawy 《Molecules (Basel, Switzerland)》2022,27(12)
A new N,N′-disubstituted piperazine conjugated with 1,3,4-thiadiazole and 1,2,4-triazole was prepared and the chemical structures were identified by IR, NMR and elemental analysis. All the prepared compounds were tested for their antimicrobial activity. The antimicrobial results indicated that the tested compounds showed significant antibacterial activity against gram-negative strains, especially E. coli, relative to gram-positive bacteria. Docking analysis was performed to support the biological results; binding modes with the active site of enoyl reductase amino acids from E. coli showed very good scores, ranging from −6.1090 to −9.6184 kcal/mol. Correlation analysis was performed for the inhibition zone (nm) and the docking score. 相似文献
139.
Hamad M. Alkahtani Amer Alhaj Zen Ahmad J. Obaidullah Mohammed M. Alanazi Abdulrahman A. Almehizia Siddique Akber Ansari Fadilah Sfouq Aleanizy Fulwah Yahya Alqahtani Rana M. Aldossari Raghad Abdullah Algamdi Lamees S. Al-Rasheed Sami G. Abdel-Hamided Alaa A.-M. Abdel-Aziz Adel S. El-Azab 《Molecules (Basel, Switzerland)》2023,28(1)
140.
Jiqian Liu Lijuan Han Wenzheng Han Linsheng Gui Zhenzhen Yuan Shengzhen Hou Zhiyou Wang Baochun Yang Sayed Haidar Abbas Raza Abdulaziz Faisal Saleh Alowais Alaa Ahmed Alraddadi Anwar Mohammed Alanazi 《Molecules (Basel, Switzerland)》2023,28(1)
There are limited reports about the effect of different heat treatments on the quality and flavor of Black Tibetan sheep meat. The current study examined the effect of pan-frying, deep-frying, baking, and boiling treatment on the quality of Black Tibetan sheep meat; the amino acid, fatty acid, and volatile flavor compounds (VFCs) were investigated by a texture analyzer, ultra-high-performance liquid chromatography (UHPLC), gas chromatography (GC), and headspace-gas chromatography-ion mobility (HS-GC-IMS). The key VFCs were identified through orthogonal partial least squares discrimination analysis (OPLS-DA), and variable importance projection (VIP) values. In addition, Pearson’s correlations between meat quality parameters and key VFCs were examined. The sensory scores, including texture, color, and appearance, of baked and pan-fried meat were higher than those of deep-fried and boiled meat. The protein (40.47%) and amino acid (62.93 µmol/g) contents were the highest in pan-fried meat (p < 0.05). Additionally, it contained the highest amounts of monounsaturated and polyunsaturated fatty acids, such as oleic, linoleic, and α-linolenic acids (p < 0.05). Meanwhile, pan-fried and deep-fried meat had higher amounts of VFCs than baked meat. The OPLS-DA similarity and fingerprinting analyses revealed significant differences between the three heat treatment methods. Aldehydes were the key aroma compounds in pan-fried meat. Importantly, 3-methylbutyraldehyde and 2-heptanone contents were positively correlated with eicosenoic, oleic, isooleic, linoleic, α-Linolenic, and eicosadiene acids (p < 0.05). To sum up, pan-fried Black Tibetan sheep meat had the best edible, nutritional, and flavor quality. 相似文献