全文获取类型
收费全文 | 1634篇 |
免费 | 105篇 |
国内免费 | 50篇 |
专业分类
化学 | 1265篇 |
晶体学 | 12篇 |
力学 | 77篇 |
综合类 | 2篇 |
数学 | 167篇 |
物理学 | 266篇 |
出版年
2024年 | 3篇 |
2023年 | 9篇 |
2022年 | 28篇 |
2021年 | 48篇 |
2020年 | 61篇 |
2019年 | 88篇 |
2018年 | 84篇 |
2017年 | 69篇 |
2016年 | 125篇 |
2015年 | 82篇 |
2014年 | 114篇 |
2013年 | 202篇 |
2012年 | 138篇 |
2011年 | 134篇 |
2010年 | 108篇 |
2009年 | 87篇 |
2008年 | 119篇 |
2007年 | 56篇 |
2006年 | 56篇 |
2005年 | 26篇 |
2004年 | 46篇 |
2003年 | 16篇 |
2002年 | 18篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1996年 | 4篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1970年 | 1篇 |
1964年 | 1篇 |
1941年 | 1篇 |
1937年 | 2篇 |
1927年 | 3篇 |
排序方式: 共有1789条查询结果,搜索用时 46 毫秒
101.
In the present work,an easy solid phase extraction method using alumina modified with polyethylenimine as a new adsorbent was applied to the simultaneous extraction of copper,silver,and palladium ions prior to their determination with electrothermal atomic absorption spectrometry.The analytical procedure involved the complex formation of these cations with polyethylenimine as a chelating agent in buffer media of pH 7.0.Under the optimum conditions,a preconcentration factor of200,150,and 200,precision of ±5.4%,±4.7%,and ±5.2%and linear calibration ranges of 15.0-140,4.0-93,and 7.5-125 ng/L(in original solution) for Cu,Ag,and Pd were obtained,respectively.Also detection limits of3.9,1.1,and 2.0 ng/L were obtained for Cu,Ag,and Pd,respectively.The proposed method was applied to the determination of copper,silver,and palladium in some real samples with satisfactory results. 相似文献
102.
A convenient and efficient solvent-free procedure is described for preparation of 6-amino-4-aryl-3-methyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles by four-component reaction of hydrazine hydrate, ethyl acetoacetate, aryl aldehyde, and malononitrile in the presence of a catalytic amount of titanium dioxide nano-sized particles. Short reaction times, high yields under ambient conditions, simple reaction, clean work-up, and reusability of the nano heterogeneous catalyst are the advantages of this method. 相似文献
103.
Uncatalyzed one-pot pseudo-four-component reaction of ethyl pyruvate, anilines, and aldehydes in n-hexane as solvent, under reflux, affords a variety of 3-amino-1,5-dihydro-2H-pyrrol-2-ones in high yield. n-Hexane is an excellent driving force in preparation of the desired products. These compounds have biological and pharmacological properties and are also used in medicinal chemistry. Use of a non-toxic and inexpensive solvent, simple and efficient synthesis, clean work-up, and high yields of the products are the advantages of this method. We report the first catalyst-free method for synthesis this class of compounds. 相似文献
104.
Hafiz Hamid Raza G. Murtaza Umm-e-Hani Nawaz Muhammad Shahid M. Ramay 《International journal of quantum chemistry》2020,120(24):e26419
Hydrogen can be utilized as an energy source; therefore, hydrogen storage has received the most appealing examination interest in recent years. The investigations of hydrogen storage applications center fundamentally around the examination of hydrogen capacity abilities of recently presented compounds. XSrH3 (X = K and Rb) compounds have been examined by density functional theory (DFT) calculations to uncover their different characteristics, as well as hydrogen capacity properties, for the first time. Studied compounds are optimized in the cubic phase, and optimized lattice constants are obtained as 4.77 and 4.99 Å for KSrH3 and RbSrH3, respectively. These hydrides have shown negative values of formation enthalpies as they are stable thermodynamically. XSrH3 might be used in hydrogen storage applications because of high gravimetric hydrogen storage densities, which are 2.33 and 1.71 wt% for KSrH3 and RbSrH3, respectively. Moreover, electronic properties confirm the semiconductor nature of these compounds having indirect band gaps of values 1.41 and 1.23 eV for KSrH3 and RbSrH3, respectively. In addition, mechanical properties from elastic constants such as Young modulus and Pugh's ratio, also have been investigated, and these compounds were found to satisfy born stability conditions. Furthermore, Pugh's ratio and Cauchy pressure show that these hydrides have a brittle nature. Furthermore, thermodynamic properties such as entropy and Debye temperature have been examined using the quasiharmonic Debye model for different temperatures and pressures. 相似文献
105.
Hamid Arvinnezhad Fatemeh Ghorbani Hormoz Khosravi Khosrow Jadidi Behrouz Notash Soheila Naderi 《Journal of heterocyclic chemistry》2020,57(8):3222-3229
Using both classical reflux and microwave-mediated conditions, a series of new spiroindoloindolizidines was synthesized by multicomponent 1,3-dipolar cycloaddition of azomethine ylides in unprecedented exo/endo stereocontrolled. Both conditions easily afforded two identical and separable exo/endo diastereomeric ratios of cycloadducts. However, the ratio of two diastereomeric products obtained from conventional conditions was reversed in all examined cases when the reactions were explored under microwave-mediated conditions. As expected, utilizing the microwave-assisted conditions produced higher yields and reaction rates compared to classical conditions. The structure and exact stereochemistry of synthesized cycloadducts were determined by applying various 2D-NMR spectroscopic techniques and single-crystal X-ray diffraction. Finally, the mechanism of the reaction has been briefly investigated by using density functional theory (DFT) calculations. 相似文献
106.
Hosseini Fahimeh Mohammadi-Khanaposhtani Maryam Azizian Homa Ramazani Ali Tehrani Maliheh Barazandeh Nadri Hamid Larijani Bagher Biglar Mahmoud Adibi Hossein Mahdavi Mohammad 《Structural chemistry》2020,31(3):999-1012
Structural Chemistry - A new series of 4-oxobenzo[d]1,2,3-triazin-pyridinium-phenylacetamide hybrids 8a–p was designed, synthesized, and screened as the potential cholinesterase inhibitors... 相似文献
107.
This article concerns the analysis of an unsteady stagnation point flow of Eyring-Powell nanofluid over a stretching sheet. The influence of thermophoresis and Brownian motion is also considered in transport equations. The nonlinear ODE set is obtained from the governing nonlinear equations via suitable transformations. The numerical experiments are performed using the Galerkin scheme. A tabular form comparison analysis of outcomes attained via the Galerkin approach and numerical scheme (RK-4) is available to show the credibility of the Galerkin method. The numerical exploration is carried out for various governing parameters, namely, Brownian motion, steadiness, thermophoresis, stretching ratio, velocity slip, concentration slip, thermal slip, and fluid parameters, and Hartmann, Prandtl and Schmidt numbers. The velocity of fluid enhances with an increase in fluid and magnetic parameters for the case of opposing, but the behavior is reversed for assisting cases. The Brownian motion and thermophoresis parameters cause an increase in temperature for both cases (assisting and opposing). The Brownian motion parameter provides a drop-in concentration while an increase is noticed for the thermophoresis parameter. All the outcomes and the behavior of emerging parameters are illustrated graphically. The comparison analysis and graphical plots endorse the appropriateness of the Galerkin method. It is concluded that said method could be extended to other problems of a complex nature. 相似文献
108.
This paper describes a new method for prediction of the Chapman-Jouguet detonation pressures of CHNOFCl explosives using the heat of detonation, Qdet, the number of moles of gaseous products of detonation per gram of explosive, α, and the average molecular weight of gaseous products, M. The equation has the form: PCJ=15.88α(MQdet)1/2ρ02−11.17, where PCJ is the Chapman-Jouguet detonation pressure and ρ0 the loading density. Calculated PCJ by this procedure show good result with respect to measured detonation pressure for any pure or mixture of ideal and some of less ideal CHNOFCl explosives at ρ0>0.8 g/cm3. 相似文献
109.
A rapid and simple procedure was developed for selective and sensitive determination of ultra‐trace silver in biological and environmental samples using the electrodeposition on a graphite probe modified with palladium followed by electrothermal atomic absorption spectrometry. Several experimental parameters for the electrodeposition, such as deposition potential, electrolyte concentration, pH of solution and deposition time were optimized. The calibration graph after preconcentration was linear in the range of 10‐250 ngL?1 with correlation coefficient of 0.9989 under the optimum conditions for procedure. The limits of detection (LOD) and quantification (LOQ) base on (3σ) and (10σ) were 2.8 ngL?1 and 9.4 ngL?1 respectively. Related standard deviation (RSD) for eight replicate measurements of 100 ngL?1 silver was 4.3%. Samples were digested completely in a closed microwave digestion system using only perchloric acid, and interference owing to various cations was also investigated. The proposed procedure was successfully applied to determine silver in blood, urine and some environmental samples with satisfactory analytical results. 相似文献
110.
Hexamethylene bis(N-methylimidazolium) bis(dichloroiodate) (HMBMIBDCI), an ionic liquid iodinating reagent, have been prepared and characterized. Its ability to perform iodination reactions with a variety of substrates has been explored. In general, iodination reactions of aromatic and heteroaromatic amines proceed with good yields in the absence of solvent. Reactions of terminal alkynes in the presence of 1,8-diazabicyclo [5.4.O] undec-7-ene and tetrahydrofuran have been investigated as well. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource: Full experimental and spectral details.] 相似文献