全文获取类型
收费全文 | 2364篇 |
免费 | 98篇 |
国内免费 | 21篇 |
专业分类
化学 | 1761篇 |
晶体学 | 25篇 |
力学 | 110篇 |
综合类 | 1篇 |
数学 | 262篇 |
物理学 | 324篇 |
出版年
2023年 | 16篇 |
2022年 | 108篇 |
2021年 | 108篇 |
2020年 | 64篇 |
2019年 | 74篇 |
2018年 | 72篇 |
2017年 | 60篇 |
2016年 | 98篇 |
2015年 | 83篇 |
2014年 | 90篇 |
2013年 | 211篇 |
2012年 | 166篇 |
2011年 | 165篇 |
2010年 | 91篇 |
2009年 | 81篇 |
2008年 | 87篇 |
2007年 | 92篇 |
2006年 | 76篇 |
2005年 | 59篇 |
2004年 | 56篇 |
2003年 | 57篇 |
2002年 | 63篇 |
2001年 | 41篇 |
2000年 | 48篇 |
1999年 | 17篇 |
1998年 | 22篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 15篇 |
1986年 | 10篇 |
1985年 | 22篇 |
1984年 | 18篇 |
1983年 | 19篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1973年 | 5篇 |
排序方式: 共有2483条查询结果,搜索用时 62 毫秒
951.
Raafat M. Shaker Hisham A. Abd El-Naby Essam Kh. Ahmed Mahmoud A. A. Ibrahim Sageda A. Gedamy 《Phosphorus, sulfur, and silicon and the related elements》2019,194(1-2):147-155
A simple, facile, and convenient practical method for the one-pot synthesis of pharmaceutically interesting 5,5`-(1,4-phenylenebis-(methanylylidene))bis-thiohydantoins via a three-component condensation reaction of terephthalaldehyde, α-amino acids and isothiocyanates had been developed. The S-alkylated derivatives 6a and 6b were obtained by the alkylation of the bis-thiohydantoins 4a with methyl iodide and/or benzyl chloride in a basic media. The molecular structures of the synthesized compounds were confirmed by their elemental analyses and spectral data (IR, 1H, 13C NMR and MS). The assignment of more stable Z- or E-isomers as the major form of 4a, 6a, and 8a was investigated by DFT calculations at B3LYP/6-31+G* level. Some of the prepared compounds were screened for their in-vitro antimicrobial activity. Compounds 4a, 6 b, 8 b and 8c exhibited low antibacterial activity against gram-positive bacteria Bacillus cereus. (5Z,5′Z)-5,5′-(1,4-Phenylenebis(methanylylidene))bis(3-benzyl-2-thioxoimidazolidin-4-one) (4b) exhibited good fungicidal activity against Fusarium oxysporum. 相似文献
952.
Swabna Vivekanandam Selvakumar Muniraj Lydia Rhyman Ibrahim A. Alswaidan Ponnadurai Ramasami 《高分子科学杂志,A辑:纯化学与应用化学》2019,56(1):52-62
Melanoidins are composite biopolymers which consist of amino carbonyl compounds which are the major coloring and polluting elements of distillery effluent. In this study, a synthetic melanoidin was used as a model for natural melanoidins, the chief colorant of the effluent. Deportations of melanoidins were investigated using both natural adsorbents and constructed microbial consortia based adsorbents and denoting it using the adsorption isotherms and SEM analysis. A review enabled us to assess about the valuation of the prepared low cost adsorbent and microbial coated adsorbents and for the treatment of the synthetic melanoidin solution. Based on the performances, both Microbial Coated Commercially Activated Carbon (MCCAC) and Commercially Activated Carbon (CAC) were better when compared with the other adsorbents at lower melanoidin concentrations; however, at higher melanoidin concentration, MCCAC performed better. The utilization of CAC for the removal of melanoidins may be cost competitive; hence Microbial Coated Procured Prosopsis Activated Carbon (MCPPAC) can be used as a substitute for the removal systems. Therefore, the microbial coated adsorbents can be a remedy for the removal of color through expatriation of melanoidin from the distillery effluents. 相似文献
953.
Shimeles Addisu Kitte Tadesse Haile Fereja Mohamed Ibrahim Halawa Baohua Lou Haijuan Li Guobao Xu 《Electrophoresis》2019,40(16-17):2050-2057
This review gives a summary of applications of different nanomateials, such as gold nanoparticles (AuNPs), carbon‐based nanoparticles, magnetic nanoparticles (MNPs), and nano‐sized metal organic frameworks (MOFs), in electrophoretic separations. This review also emphasizes the recent works in which nanoparticles (NPs) are used as pseudostationary phase (PSP) or immobilized on the capillary surface for enhancement of separation in CE, CEC, and microchips electrophoresis. 相似文献
954.
The novel 6‐ethyl‐4‐hydroxy‐2,5‐dioxo‐5,6‐dihydro‐2H‐pyrano[3,2‐c]quinoline‐3‐carboxaldehyde ( 2 ) was efficiently synthesized from Vilsmeier–Haack formylation of 3‐(1‐ethy1‐4‐hydroxy‐2‐oxo‐(1H)‐quinolin‐3‐yl)‐3‐oxopropanoic acid ( 1 ). The aldehyde 2 was allowed to react with some nitrogen nucleophiles producing a variety of hydrazones 3 – 7 . Reaction of aldehyde 2 with hydrazine hydrate and hydroxylamine hydrochloride afforded pyrazole and isoxazole annulated pyrano[3,2‐c]quinoline‐2,5(6H)‐dione, respectively. The reactivity of aldehyde 2 was examined toward some active methylene nitrile, namely, malononitrile, ethyl cyanoacetate, and cyanoacetamide leading to 2‐iminopyrano[2′,3′:4,5]pyrano[3,2‐c]quinolines 10 – 12 , respectively. Also, some novel pyrazolo[4″,3″:5′,6′]pyrano[2′,3′:4,5]pyrano[3,2‐c]quinolines ( 13 , 14 ) and thiazolo[5″,4″:5′,6′]pyrano[2′,3′:4,5]pyrano[3,2‐c]quinolines ( 15 , 16 ) were synthesized. Structures of the new synthesized products were deduced on the basis of their analytical and spectral data. 相似文献
955.
Azza M. El-Kazak Nasser M. El-Gohary Al-Shimaa Badran Magdy A. Ibrahim 《Tetrahedron》2019,75(29):3923-3932
Vilsmeier-Haack formylation of 3-acetyl-4-hydroxycoumarin (1) afforded the unexpected 3-chloro-3-(4-chlorocoumarin-3-yl)prop-2-enal (3). Compound 3 reacted with p-toluidine and benzylamine producing Schiff base 4 and enamine 6, respectively. Treatment of compound 3 with hydrazine hydrate produced bis-coumarin 15 which upon treating with hydrazine hydrate afforded bis chromeno[4,3-c]pyrazole derivative 16. Compound 3 reacted with cyanoacetohydrazide under different conditions. Condensation of compound 3 with some heterocyclic amines and 1,2-N,N-binucleophiles was studied. Structures of the new synthesized products were deduced on the basis of their analytical and spectral data. 相似文献
956.
Mohamed A. El‐Borai Hala F. Rizk Seham A. Ibrahim Amira K. Fares 《Journal of heterocyclic chemistry》2019,56(10):2787-2795
5‐Amino‐3‐phenyl‐1‐(2,4,6‐trichlorophenyl)‐1H‐thieno[3,2‐c]pyrazole‐6‐carbonitrile ( 2 ) was designed and synthesized by one‐pot multicomponent reaction. Compound 2 was reacted with different reagents to obtain new condensed moieties with our thienopyrazole skeleton. The compounds were prepared by using environmentally benign techniques as microwave irradiation, ultrasonic irradiation, and ball‐milling. The structure of the prepared compounds was elucidated through spectroscopic methods. The new compounds were evaluated for their in vitro antibacterial and antifungal potentialities. 相似文献
957.
Maha S. M. Shehata Hadi Rezazadeh Emad H. M. Zahran Eric Tala-Tebue Ahmet Bekir 《理论物理通讯》2019,71(11):1275-1280
In this article, we employ the perturbed Fokas-Lenells equation(FLE), which represents recent electronic communications. The Riccati-Bernoulli Sub-ODE method which does not depend on the balance rule is used for thefirst time to obtain the new exact and solitary wave solutions of this equation. This technique is direct, effective and reduces the large volume of calculations. 相似文献
958.
In the present paper, the two-dimensional quantum Zakharov-Kuznetsov (QZK) equation, three-dimensional quantum Zakharov-Kuznetsov equation and the three-dimensional modified quantum Zakharov-Kuznetsov equation are analytically investigated for exact solutions using the modified extended tanh-expansion based method. A variety of new and important soliton solutions are obtained including the dark soliton solution, singular soliton solution, combined dark-singular soliton solution and many other trigonometric function solutions. The used method is implemented on the Mathematica software for the computations as well as the graphical illustrations. 相似文献
959.
Omar B. Ibrahim El-Sayed A. Manaaa M.M. AL-Majthoub Ahmed M. Fallatah Abdel Majid A. Adam Mha M. Alatibi Jehan Y. Al-Humaidi Moamen S. Refat 《光谱学与光谱分析》2018,38(11):3622-3630
Metformin (Met) is a drug developed for the treatment of patients with type Ⅱ diabetes. Recently, Met estimation in pharmaceutical formulations and human fluids has gained a growing interest. To extend requisite data that can be used to assessment of Met quantitatively based on charge-transfer (CT) complexation, the present study describes the synthesis and characterization of CT complexes that formed between drug Met and the organic π-acceptors picric acid (PA), chloranilic acid (CLA), chloranil (CHL), 7,7’,8,8’-tetracyanoquinodimethane (TCNQ), and dichlorodicyanobenzoquinone (DDQ). The properties of the formed CT complexes were investigated by elemental, spectral (UV-visible, IR, and Raman spectroscopies), thermal (TG) and morphological (SEM) studies. IR results indicated that the complexation of Met with either PA or CLA acceptors occurs through proton transfer interaction, whereas its complexation with CHL, TCNQ, or DDQ acceptors occurs through n→π* interaction. 相似文献
960.
The transitional separated–reattached flow on a flat plate with a blunt leading edge under 2% free-stream turbulence (FST) is numerically simulated using the Large-eddy simulation (LES) approach. The Reynolds number based on the free-stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed and the LES results compare well with the available experimental data.It is well known that FST enhances shear-layer entrainment rates, reduces the mean reattachment distance, and causes early transition to turbulence leading to an early breakdown of the separated boundary layer. Many experimental studies have shown that different vortex shedding frequencies exist, specially the so called low-frequency flapping when there is a separation bubble but its mechanism is still not completely understood. The previous study by us without free-stream turbulence (NFST) did not show the existence of such a low-frequency flapping of the shear layer and it is not clear what the effects of FST will have on these shedding modes. Detailed analysis of the LES data has been presented in the present paper and the low-frequency flapping has not been detected in the current study. 相似文献