全文获取类型
收费全文 | 8681篇 |
免费 | 456篇 |
国内免费 | 140篇 |
专业分类
化学 | 6599篇 |
晶体学 | 44篇 |
力学 | 457篇 |
数学 | 904篇 |
物理学 | 1273篇 |
出版年
2024年 | 11篇 |
2023年 | 58篇 |
2022年 | 240篇 |
2021年 | 400篇 |
2020年 | 438篇 |
2019年 | 432篇 |
2018年 | 461篇 |
2017年 | 423篇 |
2016年 | 586篇 |
2015年 | 416篇 |
2014年 | 552篇 |
2013年 | 1010篇 |
2012年 | 748篇 |
2011年 | 657篇 |
2010年 | 449篇 |
2009年 | 426篇 |
2008年 | 428篇 |
2007年 | 344篇 |
2006年 | 235篇 |
2005年 | 197篇 |
2004年 | 139篇 |
2003年 | 134篇 |
2002年 | 112篇 |
2001年 | 39篇 |
2000年 | 22篇 |
1999年 | 29篇 |
1998年 | 19篇 |
1997年 | 25篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 15篇 |
1986年 | 5篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 18篇 |
1981年 | 8篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1971年 | 3篇 |
排序方式: 共有9277条查询结果,搜索用时 390 毫秒
981.
Mohammad Pervez Mughal 《Heat and Mass Transfer》2006,42(7):577-586
Many of the ceramics used in the manufacture of the metal-ceramic composites are available as powders, which has encouraged
investigators to develop tool less manufacturing techniques. Innovative processes like combustion synthesis and laser sintering
processes offer great potential for specific material combinations. These processes involve heat and mass transfer in porous
matrix with melting, solidification and may be chemical reaction. Since the resulting transport is relatively complex, there
are few mathematical models available. In the present study, melting and transport of metal is demonstrated for a one-dimensional
bed with uniform heat generation and convectively cooled boundaries. The effects of different Bond, Biot and Prandtl numbers
are discussed using constant and temperature varying material properties. At low bond number the transport is controlled by
capillary forces and temperature and saturation distribution is symmetric about the center of the bed. For Biot number larger
than ten the process is internally controlled. Effect of Prandtl number is opposite of that of the Bond number. Use of constant
properties overpredicts the process rate by 13%. 相似文献
982.
983.
Efficient Solvent‐Free Synthesis of Benzothiazine‐Fused Pyrrolo[3,4‐c]coumarins: Cycloaddition Reactions between Coumarin‐Based Dihydrobenzothiazoles and Isocyanides
下载免费PDF全文
![点击此处可从《Helvetica chimica acta》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Mehdi Khoobi Ali Ramazani Mohammad Mahdavi Alireza Foroumadi Saeed Emami Sang Woo Joo Katarzyna Ślepokura Tadeusz Lis Abbas Shafiee 《Helvetica chimica acta》2014,97(6):847-853
A new and unusual synthesis of benzothiazine‐fused pyrrolo[3,4‐c]coumarins, involving the ring‐opening of coumarin‐based dihydrobenzothiazoles and subsequent [4+1] cycloaddition reaction with isocyanides, was described. Thus, simple heating of various 3‐(2,3‐dihydro‐2‐methylbenzo[d]thiazol‐2‐yl)coumarins with isocyanides produced the title compounds in good yields under solvent‐free conditions. 相似文献
984.
Reinvestigation of the Synthesis of Ketanserin (5) and its Hydrochloride Salt (5.HCl) via 3‐(2‐Chloroethyl)‐2,4‐(1H,3H)‐quinazolinedione (2) or Dihydro‐5H‐oxazole(2,3‐b)quinazolin‐5‐one (1)
下载免费PDF全文
![点击此处可从《Journal of heterocyclic chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The synthesis of ketanserin ( 5 ) and its hydrochloride salt ( 5.HCl ) using respectively equimolar amounts of 3‐(2‐chloroethyl)‐2,4‐(1H,3H)‐quinazolinedione ( 2 ) with 4‐(parafluorobenzoyl)piperidine ( 3 ) and dihydro‐5H‐oxazole(2,3‐b)quinazolin‐5‐one ( 1 ) with hydrochloride salt of 4‐(parafluorobenzoyl)piperidine ( 3.HCl ) is reinvestigated. The one‐pot reaction of ethyl‐2‐aminobenzoate with ethyl chloroformate and ethanol amine has afforded 3‐(2‐chloroethyl)‐2,4‐(1H,3H)‐quinazolinedione ( 2 ) (86%) that was then refluxed with 4‐(parafluorobenzoyl)piperidine ( 3 ) in ethyl methyl ketone in the presence of sodium carbonate to obtain free base of ketanserin (87%). In another attempt, a very pure hydrochloride salt of ketanserin ( 5.HCl ) was synthesized using equimolar amounts of dihydro‐5H‐oxazole(2,3‐b)quinazolin‐5‐one ( 1 ) and hydrochloride salt of 4‐(parafluorobenzoyl)piperidine ( 3.HCl ) by a solvent‐less fusion method. Thus, under optimized conditions, 180°C and a reaction time of 30 min, the powder mixture was transformed into glassy crystals that were initially readily soluble in chloroform but were transformed afterwards over time (2 h) to white precipitates ( 5.HCl ) suspended in chloroform with a yield of 72%. 相似文献
985.
Mohammad Behbahani Hamid Sadeghi Abandansari Mani Salarian Meysam Babapour Akbar Bagheri Mohammad Reza Nabid 《Mikrochimica acta》2014,181(1-2):129-137
We have designed and synthesized a thermosensitive tri-block copolymer for selective trace extraction of Pb(II) ions from biological and food samples. The polymer was characterized by Fourier transform IR and NMR spectroscopy, and by gel permeation chromatography. The critical aggregation concentration and lower critical solution temperature were determined via fluorescence and UV spectrophotometry, respectively. The effects of solution pH value, amount of copolymer, of the temperature on extraction and on phase separation, and of the matrix on the extraction of Pb(II) were optimized. Pb(II) ions were then quantified by FAAS. The use of this copolymer resulted in excellent figures of merit including a calibration plot extending from 0.5 to 160 μg L?1 (with an R2 of >0.99), a limit of detection (LOD) as low as 90 pg L?1, an extraction efficiency of >98 %, and relative standard deviations of <4 % for eight separate extraction experiments. Figure
In this paper, for the first time an intelligent system using a thermosensitive tri-block copolymer for selective trace removal of Pb(II) in biological and food samples was designed and its determination was carried out by flame atomic absorption spectrometry. 相似文献
986.
Screen-printing is one of the most promising approaches towards simple, rapid and inexpensive production of biosensors. Disposable biosensors based on screen printed electrodes (SPEs) including microelectrodes and modified electrodes have led to new possibilities in the detection and quantitation of biomolecules, pesticides, antigens, DNA, microorganisms and enzymes. SPE-based sensors are in tune with the growing need for performing rapid and accurate in-situ analyses and for the development of portable devices. This review (with 226 refs.) first gives an introduction into the topic and then is subdivided into sections (a) on DNA sensors (including methods for the detection of hybridization and damage), (b) on aptasensors (for thrombin, OTA, immunoglobulins and cancer biomarkers), (c) on immunosensors (for microorganisms, immunoglobulins, toxins, hormones, lactoferrin and biomarkers), (d) on enzymatic biosensors (for glucose, hydrogen peroxide, various pharmaceuticals, neurotransmitters, amino acids, NADH, enzyme based sensors). Figure
This review first gives an introduction into the topic of screen-printed electrodes for biosensing and is subdivided into sections (a) on DNA sensors, (b) on aptasensors, (c) on immunosensors, (d) on enzymatic biosensors. It contains 226 references 相似文献
987.
ZnO–SnO2 nanoparticles were prepared by coprecipitation method; then Mg, with different molar ratios and calcination temperatures, was loaded on the coupled nanoparticles by impregnation method. The synthesized nanoparticles were characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) techniques. Based on XRD results, the ZnO–SnO2 and Mg/ZnO–SnO2 nanoparticles were made of ZnO and SnO2 nanocrystallites. According to DRS spectra, the band gap energy value of 3.13 and 3.18 eV were obtained for ZnO–SnO2 and Mg/ZnO–SnO2 nanoparticles, respectively. BET analysis revealed a Type III isotherm with a microporous structure and surface area of 32.051 and 49.065 m2 g?1 for ZnO–SnO2 and Mg/ZnO–SnO2, respectively. Also, the spherical shape of nanocrystallites was deduced from TEM and FESEM images. The photocatalytic performance of pure ZnO–SnO2 and Mg/ZnO–SnO2 was analyzed in the photocatalytic removal of methyl orange (MO). The results indicated that Mg/ZnO–SnO2 exhibited superior photocatalytic activity to bare ZnO–SnO2 photocatalyst due to high surface area, increased MO adsorption and larger band gap energy. Maximum photocatalytic activity of Mg/ZnO–SnO2 nanoparticles was obtained with 0.8 mol% Mg and calcination temperature of 350°C. 相似文献
988.
Reza Ghiasi Taherh Jahan Ara Amir Hossein Hakimyoun 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(4):616-624
The electronic structure and properties of the platinanaphthalenes, and ring-fused B-N platinanaphthalenes isomers have been explored using the hybrid density functional mpw1pw91 theory. The energetic aspect shows that stability of Ptb and Pta isomers are isoenergetic in platinanaphthalenes. On the other hand, BNa1 isomer is the most stable isomer of ring-fused B-N platinanaphthalenes. This is compatible with principles of minimum energy and minimum polarizability in ring-fused B-N platinanaphthalenes. Molecular orbital analysis shows increasing of hardness in ring-fused B-N platinanaphthalenes isomers. Also, electronic spectra analysis indicates that, in all the molecules HOMO-1 → LUMO transition makes the major contribution in most intense electronic transition. 相似文献
989.
A novel synthesis of thioxo-quinazolino[3,4-a]quinazolinone framework was developed through a four-step reaction starting from isatoic anhydride. The resulting 2-aminobenzamides from the reaction of isatoic anhydride and different amines underwent coupling–cyclization reaction with 2-nitrobenzaldehydes, reduction of nitro group, and then cyclization reaction with carbon disulfide (CS2). All steps were carried out under easy and user-friendly conditions in a short time without using expensive catalysts or reagents.
[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.] 相似文献
990.
The three-component reactions of phenanthridine, activated acetylenes, and ethyl bromopyruvate through both of simultaneous and stepwise process are surveyed. The reactions afforded the corresponding pyrrolo[1,2-f]phenanthridine derivatives in good yields without using any catalyst and activation.
[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.] 相似文献