全文获取类型
收费全文 | 1962篇 |
免费 | 118篇 |
国内免费 | 53篇 |
专业分类
化学 | 1298篇 |
晶体学 | 12篇 |
力学 | 78篇 |
综合类 | 2篇 |
数学 | 176篇 |
物理学 | 273篇 |
无线电 | 294篇 |
出版年
2024年 | 3篇 |
2023年 | 12篇 |
2022年 | 38篇 |
2021年 | 57篇 |
2020年 | 73篇 |
2019年 | 106篇 |
2018年 | 95篇 |
2017年 | 81篇 |
2016年 | 147篇 |
2015年 | 92篇 |
2014年 | 136篇 |
2013年 | 228篇 |
2012年 | 157篇 |
2011年 | 145篇 |
2010年 | 122篇 |
2009年 | 100篇 |
2008年 | 130篇 |
2007年 | 68篇 |
2006年 | 72篇 |
2005年 | 33篇 |
2004年 | 50篇 |
2003年 | 24篇 |
2002年 | 21篇 |
2001年 | 11篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 3篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1970年 | 9篇 |
1969年 | 4篇 |
1937年 | 2篇 |
1927年 | 3篇 |
排序方式: 共有2133条查询结果,搜索用时 78 毫秒
991.
Mehdi Adib Ehsan Sheikhi Morteza Karimzadeh Hamid Reza Bijanzadeh Massoud Amanlou 《Helvetica chimica acta》2012,95(5):788-794
A simple synthesis of N2‐alkyl‐N3‐[2‐(1,3,4‐oxadiazol‐2‐yl)aryl]benzofuran‐2,3‐diamines 5 via a one‐pot four‐component reaction is described (Scheme 1). A mixture of N‐(isocyanoimino)triphenylphosphorane ( 1 ), a 2‐aminobenzoic acid 2 , a 2‐hydroxybenzaldehyde 3 , and an isocyanide 4 in absolute EtOH at room temperature undergoes a smooth reaction to afford 5 in excellent yields (Table). 相似文献
992.
Bahram Ghalami‐Choobar Hamid Dezhampanah Paria Nikparsa Ali Ghiami‐Shomami 《International journal of quantum chemistry》2012,112(10):2275-2280
In this work, calculations of pKa values have been performed on benzoic acid and its para‐substituted derivatives and some drugs by using Gaussian 98 software package. Gas‐phase energies were calculated with HF/6‐31 G** and B3LYP/6‐31 G** levels of theory. Free energies of solvation have been computed using the polarizable continuum model (PCM), conductor‐like PCM (CPCM), and the integral equation formalism‐PCM at the same levels which have been used for geometry determination in the gas‐phase. The results that show the calculated pKa values using the B3LYP are better than those using the corresponding HF. In comparison to the other models, the results obtained indicate that the PCM model is a suitable solvation model for calculating pKa values. For the investigated compounds, a good agreement between the experimental and the calculated pKa values was also observed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
993.
Pradeep D. Lokhande Kamal Hasanzadeh Hamid Khaledi Hapipah Mohd Ali 《Journal of heterocyclic chemistry》2012,49(6):1398-1406
The treatment of 3,3a,4,5‐tetrahydro‐3‐aryl‐2‐phenyl‐2H‐benzo[g]indazoles 4 with I2/DMSO led to the oxidation of the five‐member rings ( 5 ) as well as the iodination of N‐phenyl moieties along with oxidation of the five‐member rings ( 6 ). However, the reactions of 4 with CuCl2/DMSO gave only compounds 5 . The reaction of N‐bromosuccinimide (NBS) with compounds 4 resulted in fully aromatization along with bromination at C‐5 of the indazole rings ( 7 ). The indazole six‐member rings in compounds 5 and 6 also underwent aromatization along with bromination by using NBS ( 7 and 8 ). 相似文献
994.
An efficient synthesis of new barbiturate salts consist of 2-aminopyridinium moiety by a?n uncatalyzed three-component condensation reaction of C–H acids, pyrimidine-tetraones, and 2-aminopyridines? in refluxing chloroform is reported. The reaction of pyrimidine-tetraones and 2-aminopyridines with ninhydrin in the formation of new barbiturate salts consist of 2-aminopyridinium moiety is also investigated. 相似文献
995.
Midori Motegi Alfarius Eko Nugroho Yusuke Hirasawa Takashi Arai A.Hamid A. Hadi Hiroshi Morita 《Tetrahedron letters》2012,53(10):1227-1230
Two novel indole alkaloids and new quinazoline alkaloids, namely leucomidines A–C (1–3), were isolated from the barks of Leuconotis griffithii. The structures including the absolute configuration were elucidated on the basis of spectroscopic data, chemical means, and CD calculations. The quinazoline alkaloid, 3, is proposed to be derived from the same biogenetic precursor as the indole alkaloids 1 and 2. 相似文献
996.
Farshad Darvishi Jacqueline Destain Iraj Nahvi Philippe Thonart Hamid Zarkesh-Esfahani 《Applied biochemistry and biotechnology》2012,168(5):1101-1107
The extracellular lipase of Yarrowia lipolytica presents numerous potentialities for biotechnological applications. This work describes the development and storage of powders obtained from supernatants containing Y. lipolytica lipase by freeze-drying as downstream process that is important in obtaining a stable lipase powder with high enzymatic activity. Lipase was produced by Y. lipolytica U6 mutant strain in 20-L bioreactor. Non-concentrated cell-free culture supernatant samples were supplemented with different concentrations (0.5?C1?%) of maltodextrin and glycerol as additives to freeze-drying. Effects of additives, temperature, pH, and storage time on lipase powders were determined. After addition of additives, freeze-drying yield increased 3.5-fold compared to supernatant without additive. Maltodextrin with 0.5?% concentration gave the best protection of lipase during dehydration treatment and its freeze-drying yield (77?%) is better than other formulations. Lipase powders were stored at 4 and 25?°C for 46?weeks without loss of lipase activity. A common impediment to the production of commercial enzyme is their low-stability aqueous solutions. The present study shows that freeze-dried lipase powders of Y. lipolytica have good stability for storage and various applications. 相似文献
997.
Co3S4 nanocrystals were synthesized via a hydrothermal technique by adding thioglycolic acid and Co(CH3COO)2·4H2O as the precursors. The effect of some parameters, such as the reaction time and temperature and concentration of reactants, on the growth and morphology of the nano-structures has been studied. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and Fourier transform infrared (FT-IR) spectra. 相似文献
998.
Iman?Jabbari Zahir Abadi Omid?Sadeghi Hamid?Reza?Lotfizadeh Zhad Najmeh?Tavassoli Vahid?Amani Mostafa?M.?AminiEmail author 《Journal of Sol-Gel Science and Technology》2012,61(1):90-95
Nanoporous silica SBA-15 was prepared to evaluate its application as an oral drug delivery system. A series of surface-functionalized
nanopore materials as efficient clarithromycin delivery carriers was investigated. An efficient pH-responsive carrier system
was constructed by hydrogen bond interaction between carboxyl and hydroxyl groups in the clarithromycin and the amine group
in modified SBA-15. HPLC analyses of clarithromycin were run on a C18 column using a mobile phase comprised of potassium dihydrogen phosphate, acetonitrile and methanol (30:40:30, v/v/v). Active
molecules such as clarithromycin could be stored and released from the pore voids of SBA-15 by changing the pH. The amount
of clarithromycin stored in the pores of nanoporous silica based on TREN [tris(2-aminoethyl) amine]-modified SBA-15 rods was
up to 46 ± 4.8 wt% at pH 8. In addition, when the pH was below 4, clarithromycin was steadily released from the pores of SBA-15
(up to 97 wt% in simulated gastric medium). 相似文献
999.
Five organotin(IV) complexes,were obtained by reaction of SnR2Cl2(R = Ph,Me,Bu) with ONO donor Schiff bases.The synthesized complexes have been investigated by elemental analysis and IR,1H NMR,and 119Sn NMR spectroscopy.These data show that the Schiff base acts as a tridentate dianionic ligand and coordinates via the imine nitrogen and two oxygen atoms.The Xray crystallography of complex 4 shows a dimeric structure for this molecule.The in vitro antibacterial activities of the Schiff bases and their complexes have been evaluated against Gram-positive(Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria and compared with the standard antibacterial drugs. 相似文献
1000.
Amin Aziznia Hamid Reza Bozorgzadeh Naser Seyed-Matin Morteza Baghalha Ali Mohamadalizadeh 《天然气化学杂志》2012,(4):466-475
In the current study,the hybrid effect of a corona discharge and γ-alumina supported Ni catalysts in CO2 reforming of methane is investigated.The study includes both purely catalytic operation in the temperature range of 923-1023K,and hybrid catalytic-plasma operation of DC corona discharge reactor at room temperature and ambient pressure.The effect of feed flow rate,discharge power and Ni/γ-Al2O3 catalysts are studied.When CH4/CO2 ratio in the feed is 1/2,the syngas of low H2/CO ratio at about 0.56 is obtained,which is a potential feedstock for synthesis of liquid hydrocarbons.Although Ni catalyst is only active above 573K,presence of Ni catalysts in the cold corona plasma reactor(T≤523K) shows promising increase in the conversions of methane and carbon dioxide.When Ni catalysts are used in the plasma reaction,H2/CO ratios in the products are slightly modified,selectivity to CO increases whereas fewer by-products such as hydrocarbons and oxygenates are formed. 相似文献