全文获取类型
收费全文 | 2405篇 |
免费 | 100篇 |
国内免费 | 22篇 |
专业分类
化学 | 1743篇 |
晶体学 | 26篇 |
力学 | 112篇 |
综合类 | 3篇 |
数学 | 273篇 |
物理学 | 370篇 |
出版年
2023年 | 15篇 |
2022年 | 105篇 |
2021年 | 111篇 |
2020年 | 64篇 |
2019年 | 73篇 |
2018年 | 66篇 |
2017年 | 55篇 |
2016年 | 96篇 |
2015年 | 83篇 |
2014年 | 92篇 |
2013年 | 218篇 |
2012年 | 161篇 |
2011年 | 167篇 |
2010年 | 96篇 |
2009年 | 92篇 |
2008年 | 92篇 |
2007年 | 93篇 |
2006年 | 82篇 |
2005年 | 65篇 |
2004年 | 60篇 |
2003年 | 58篇 |
2002年 | 64篇 |
2001年 | 42篇 |
2000年 | 48篇 |
1999年 | 20篇 |
1998年 | 23篇 |
1997年 | 17篇 |
1996年 | 26篇 |
1995年 | 21篇 |
1994年 | 23篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 17篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 19篇 |
1982年 | 17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2527条查询结果,搜索用时 343 毫秒
991.
Three supramolecular coordination polymers (SCPs) [(AgSCN)2L] {L = 4,4′-bipyridine (bpy) (1), trans-1,2-bis(4-pyridyl)ethylene (tbpe) (2) and phenazine (phenz) (3)} have been synthesized and structurally characterized by single-crystal X-ray diffraction. Synthesis was affected in H2O/acetonitrile/NH3 media at room temperature. The bpy, tbpe and phenz bipodal ligands adopt different conformations which would affect the skeleton of the (AgSCN)n building blocks that allow the interconnection of the (AgSCN)n fragments and propagation of the network structure in three dimensions. Supramolecular interactions such as hydrogen-bonding, argentophilic interaction and π–π stacking play an important role in the assembly of these coordination polymers. 相似文献
992.
A tetrad field that is homogeneous and anisotropic which contains two unknown functions A(t) and B(t) of cosmic time is applied to the field equations of f(T), where T is the torsion scalar, T = T~μ_(νρ)S_μ~(νρ). We calculate the equation of continuity and rewrite it as a product of two brackets, the first is a function of f(T) and the second is a function of the two unknowns A(t) and B(t). We use two different relations between the two unknown functions A(t) and B(t) in the second bracket to solve it. Both of these relations give constant scalar torsion and solutions coincide with the de Sitter one. So,another assumption related to the contents of the matter fields is postulated. This assumption enables us to drive a solution with a non-constant value of the scalar torsion and a form of f(T) which represents ΛCDM. 相似文献
993.
Ibrahim A. Abbas 《基于设计的结构力学与机械力学》2015,43(3):265-276
For a free vibration problem of a thermoelastic hollow sphere into the context of the generalized thermoelasticity theory with one relaxation time, exact analytic solutions are obtained with the use of eigenvalue approach. Both the inner and outer curved surfaces of the sphere are considered stress-free and isothermal surfaces. The dispersion relations for the existence of various types of possible modes of vibrations in the considered hollow sphere are derived. The numerical results have been presented graphically in respect of natural frequencies, thermoelastic damping, and frequency shift. 相似文献
994.
S. K. Sadasivuni W. Malalasekera S. S. Ibrahim 《Russian Journal of Physical Chemistry B, Focus on Physics》2010,4(3):465-474
The unsteady flamelet/progress variable approach has been developed for the prediction of a lifted flame to capture the extinction
and re-ignition physics. In this work inclusion of the time variant behavior in the flamelet generation embedded in the large
eddy simulation technique, allows better understanding of partially premixed flame dynamics. In the process sufficient simulations
to generate unsteady laminar flamelets are performed, which are a function of time. These flamelets are used for the generation
of the look-up table and the flamelet library is produced. This library is used for the calculation of temperature and other
species in the computational domain as the solution progresses. The library constitutes filtered quantities of all the scalars
as a function of mean mixture fraction, mixture fraction variance and mean progress variable. Mixture fraction and progress
variable distributions are assumed to be β-PDF and δ-PDF respectively. The technique used here is known as the unsteady flamelet
progress variable (UFPV) approach. One of the well known lifted flames is considered for the present modeling which shows
flame lift-off. The results are compared with the experimental data for the mixture fraction and temperature. Lift off height
is predicted from the numerical calculations and compared with the experimentally given value. Comparisons show a reasonably
good agreement and the UFPV combustion model appears to be a promising technique for the prediction of lifted and partially
premixed flames. 相似文献
995.
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
996.
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
997.
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
998.
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
999.
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
1000.
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