全文获取类型
收费全文 | 2071篇 |
免费 | 70篇 |
国内免费 | 12篇 |
专业分类
化学 | 1470篇 |
晶体学 | 42篇 |
力学 | 28篇 |
数学 | 129篇 |
物理学 | 484篇 |
出版年
2023年 | 10篇 |
2022年 | 11篇 |
2021年 | 14篇 |
2020年 | 34篇 |
2019年 | 25篇 |
2018年 | 25篇 |
2017年 | 18篇 |
2016年 | 40篇 |
2015年 | 37篇 |
2014年 | 57篇 |
2013年 | 75篇 |
2012年 | 110篇 |
2011年 | 127篇 |
2010年 | 77篇 |
2009年 | 64篇 |
2008年 | 128篇 |
2007年 | 107篇 |
2006年 | 126篇 |
2005年 | 90篇 |
2004年 | 73篇 |
2003年 | 91篇 |
2002年 | 71篇 |
2001年 | 58篇 |
2000年 | 41篇 |
1999年 | 32篇 |
1998年 | 22篇 |
1997年 | 21篇 |
1996年 | 21篇 |
1995年 | 26篇 |
1994年 | 43篇 |
1993年 | 29篇 |
1992年 | 30篇 |
1991年 | 30篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1986年 | 25篇 |
1985年 | 39篇 |
1984年 | 28篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 14篇 |
1977年 | 20篇 |
1976年 | 27篇 |
1975年 | 17篇 |
1973年 | 11篇 |
排序方式: 共有2153条查询结果,搜索用时 15 毫秒
61.
Murahashi S Imada Y Kawakami T Harada K Yonemushi Y Tomita N 《Journal of the American Chemical Society》2002,124(12):2888-2889
A chiral titanium complex, Ti(O-i-Pr)(4)/BINOL/tert-butylcatechol, catalyzes enantioselective addition reaction of ketene silyl acetals to nitrones to give optically active beta-amino acid derivatives which are biologically active compounds and useful synthetic intermediates of natural products and pharmaceuticals such as beta-lactam antibiotics. The combined process of catalytic oxidation of secondary amines and enantioselective carbon-carbon bond formation of nitrones thus obtained with ketene silyl acetals provides a useful two-step method for the synthesis of optically active beta-amino acid derivatives and related nitrogen compounds. 相似文献
62.
Saito K Masuyama T Oyaizu K Nishide H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4240-4246
Depolymerization of an engineering plastic, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M(n)=1.0x10(4), M(w)/M(n)=1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M(n)=4.9x10(2) (M(w)/M(n)=1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth. 相似文献
63.
Polypeptides with two histidines and an iron porphyrin (1H40-7H46) were synthesized with a variety of positions of a histidine. In 4H43, histidine (H43) was in the hydrophobic region of an α-helix. The other polypeptides were of slightly or substantially distorted conformation. In the pH 7.2 buffer solution, two histidines of the polypeptide coordinated the iron porphyrin regardless of their positions. Some polypeptides (1H40, 3H42, and 5H44) showed an enhanced catalytic activity in the peroxidase reaction using cumene hydroperoxide compared to that of 4H43, whereas some polypeptides (2H41 and 6H45) were ineffective catalysts. The distortion of the peptide conformation by the addition of MeOH was also effective for the peroxidase reaction. 相似文献
64.
Naozumi Teramoto Toyoki MotoyamaRyutoku Yosomiya Mitsuhiro Shibata 《European Polymer Journal》2002,38(7):1365-1369
Amylose was etherified with 1-bromopropane in DMSO. The degree of substitution (DS) was varied by altering the feed ratio of 1-bromopropane. The structures of the products were characterized by IR and 1H-NMR spectroscopy. When the molar feed ratio of 1-bromopropane to hydroxyl groups of amylose was beyond 7.5, the hydroxyl groups were completely substituted with propyl ether groups. The etherified amylose with DS 1.9 showed a glass transition temperature (Tg), and that with DS 2.3 or 3.0 showed both Tg and melting temperature (Tm) (DS 3.0 means complete substitution). The etherification imparted melt processability and solubility in nonpolar organic solvent to amylose. 相似文献
65.
Alam MS Kim IJ Nemati B O'Neill JJ Romero V Severini H Sun CR Wang P Zoeller MM Crawford G Fulton R Gan KK Kagan H Kass R Lee J Malchow R Morrow F Sung M White C Whitmore J Wilson P Butler F Fu X Kalbfleisch G Lambrecht M Skubic P Snow J Bortoletto D Brown DN Dominick J McIlwain RL Miao T Miller DH Modesitt M Schaffner SF Shibata EI Shipsey IP Battle M Ernst J Kroha H Roberts S Sparks K Thorndike EH Wang C Artuso M Goldberg M Haupt T Horwitz N Kennett R Moneti GC Playfer S Rozen Y Rubin P 《Physical review D: Particles and fields》1992,46(11):4822-4827
66.
Kenzo Miyazaki Toru Fukatsu Ichiro Yamashita Toshifumi Hasama Kawakatsu Yamada Takuzo Sato 《Applied physics. B, Lasers and optics》1991,52(1):1-7
Improvements in output pulse energy and efficiency of a conventional capacitor-transfer-type discharge excimer laser with automatic preionization have been achieved by extending the discharge volume and resulting moderate pumping of the active medium. The discharge laser produces a pulse energy of more than 1 J for XeCl, KrF, and ArF lasers in square beams of about 2×2 cm2, and the maximum overall efficiency observed is 2.9% for XeCl, 3.2% for KrF and 1.8% for ArF. The laser device has been involved in a picosecond ( 32 ps) XeCl laser amplification system, and was operated as an amplifier at a repetitive frequency of 10 Hz. Saturation fluence for XeCl laser was measured to be 1.4 mJ/cm2, and the picosecond pulse energy of 40 mJ was extracted from the amplifier.On leave from Ebara Corp., 6-6-7, Ginza, Chuo-ku, Tokyo 104, JapanOn leave from Mitsubishi Heavy Industries, LTD., 4-6-22, Kan-on shinmachi, Nishi-ku, Hiroshima 733, Japan 相似文献
67.
68.
Fulton R Jensen T Johnson DR Kagan H Kass R Morrow F Whitmore J Wilson P Bortoletto D Chen W Dominick J McIlwain RL Miller DH Ng CR Schaffner SF Shibata EI Shipsey IP Yao W Battle M Sparks K Thorndike EH Wang C Alam MS Kim IJ Li WC Romero V Sun CR Wang P Zoeller MM Goldberg M Haupt T Horwitz N Jain V Mestayer MD Moneti GC Rozen Y Rubin P Sharma V Skwarnicki T Thulasidas M Zhu G Csorna SE Letson T Alexander J Artuso M Bebek C Berkelman K Browder T Cassel DG Cheu E Coffman DM Crawford G DeWire JW 《Physical review D: Particles and fields》1991,43(3):651-663
69.
Kubota Y Nelson JK Perticone D Poling R Schrenk S Crawford G Fulton R Jensen T Johnson DR Kagan H Kass R Malchow R Morrow F Whitmore J Wilson P Bortoletto D Brown D Dominick J McIlwain RL Miller DH Modesitt M Ng CR Schaffner SF Shibata EI Shipsey IP Battle M Kroha H Sparks K Thorndike EH Wang C Alam MS Kim IJ Li WC Nemati B Romero V Sun CR Wang P Zoeller MM Goldberg M Haupt T Horwitz N Jain V Mestayer MD Moneti GC Rozen Y Rubin P Skwarnicki T Stone S Thusalidas M Yao W Zhu G Barnes AV Bartelt J 《Physical review D: Particles and fields》1991,44(3):593-600
70.
High concentrations of arsenic were observed in the blubber of ringed seals (Pusa hispida) in our previous study. To better understand the arsenic accumulation in blubber of marine mammals, arsenicals in the blubber of ringed seal were characterized using high performance liquid chromatography–inductively coupled plasma mass spectrometry (HPLC–ICPMS). The most predominant water-soluble arsenical in the blubber was dimethylarsinic acid (DMA), in spite of the predominance of arsenobetaine in other tissues. Lipid-soluble fraction was hydrolyzed under mild (tetraethylammonium hydroxide (TEAH) hydrolysis) and strong (NaOH hydrolysis) conditions, and then an aliquot of hydrolysate was injected onto HPLC–ICPMS. Both TEAH-labile and TEAH-stable/NaOH-labile lipid-soluble fractions contained precursors of DMA. These results suggest that the blubber might be the pool of DMA and DMA-containing precursors in ringed seals. 相似文献