全文获取类型
收费全文 | 1244篇 |
免费 | 31篇 |
国内免费 | 4篇 |
专业分类
化学 | 921篇 |
晶体学 | 5篇 |
力学 | 22篇 |
数学 | 75篇 |
物理学 | 256篇 |
出版年
2022年 | 6篇 |
2021年 | 11篇 |
2020年 | 11篇 |
2019年 | 16篇 |
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 21篇 |
2015年 | 20篇 |
2014年 | 27篇 |
2013年 | 69篇 |
2012年 | 58篇 |
2011年 | 59篇 |
2010年 | 34篇 |
2009年 | 31篇 |
2008年 | 65篇 |
2007年 | 57篇 |
2006年 | 66篇 |
2005年 | 64篇 |
2004年 | 54篇 |
2003年 | 56篇 |
2002年 | 45篇 |
2001年 | 40篇 |
2000年 | 26篇 |
1999年 | 18篇 |
1997年 | 14篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 17篇 |
1993年 | 7篇 |
1992年 | 25篇 |
1991年 | 17篇 |
1990年 | 26篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1986年 | 23篇 |
1985年 | 31篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1968年 | 5篇 |
排序方式: 共有1279条查询结果,搜索用时 309 毫秒
41.
M. Karppinen Y. Morita I. Grigoraviciute R.S. Liu 《Journal of solid state chemistry》2005,178(11):3464-3470
Superconductivity characteristics have been systematically evaluated for a two-CuO2-plane copper oxide system, (Cu,Mo)-12s2, upon increasing the number of fluorite-structured layers, s, between the two CuO2 planes. Essentially single-phase samples of (Cu0.75Mo0.25)Sr2YCu2O7+δ (s=1), (Cu0.75Mo0.25)Sr2(Ce0.45Y0.55)2Cu2O9+δ (s=2) and (Cu0.75Mo0.25)Sr2(Ce0.67Y0.33)3Cu2O11+δ (s=3) were synthesized through a conventional solid-state route in air. To make the samples superconductive an additional high-pressure oxygenation (HPO) treatment was required. Such treatment (carried out at 5 GPa and 500 °C in the presence of 75 mol% Ag2O2 as an oxygen source to maximize the Tc) compressed the crystal lattice for the three members of the (Cu0.75Mo0.25)-12s2 series equally, i.e., by 0.01 Å for the a parameter and by 0.07 Å for the c parameter per formula unit. From both Cu L-edge and O K-edge XANES spectra the s=1 sample was found to possess the highest overall hole-doping level among the HPO samples. Accordingly it exhibited the best superconductivity characteristics. With increasing s, both the Tc (s=1: 88 K, s=2: 61 K, s=3: 53 K) and Hirr values got depressed, being well explained by the trend of decreasing CuO2-plane hole concentration with increasing s as revealed from O K-edge XANES spectra for the same samples. Hence, the present results do not suggest any significant (negative) impact on the superconductivity characteristics from the gradually thickened fluorite-structured block itself. 相似文献
42.
Octadecapeptides carrying a ferrocene moiety at the molecular terminal were self-assembled on gold, and long-range electron transfer from the ferrocene moiety to gold was investigated by electrochemical methods. Effects on electron transfer of dipole moment of helical peptides, linkers connecting the peptide to gold, and chromophores introduced into the side chains were discussed. Cyclic voltammetry of the monolayers in an aqueous solution revealed that long-range electron transfer over 40 A occurred along the peptide molecule. Chronoamperometry showed that the long-range electron transfer should be ascribed to a hopping mechanism with use of amide groups as hopping sites. Electron transfer through the long peptide was not significantly accelerated by the dipole moment. However, the linker remarkably affected electron transfer depending on whether it was a methylene chain or a phenylene group, suggesting that local electron transfer between gold and the peptides should be the slowest step to determine the overall rate. Pyrenyl groups introduced into the side chains in the middle of the peptide molecule did not noticeably change electron transfer, probably because pyrenyl groups were too distant to allow direct electron transfer between them. Electrostatic potential profiles across the peptide monolayers were also calculated to explain reasonably the several interesting features in the present peptide systems. 相似文献
43.
In order to elucidate the mechanisms of blue light damage on ocular tissues, the transepithelial transport, electrical characteristics and ultrastructural properties of irradiated isolated bovine retinal pigment epithelium (RPE) were investigated. Blue light (430 nm) irradiation at 20 mW/cm2 significantly reduced the transepithelial potential and short circuit current of RPE. During blue light exposure, a decrease in chloride transport was observed, and this decrease appeared to be closely coupled to changes in the electrical properties of the pigment epithelium. A decrease in leucine transport was also noted, but the effect required 10-30 min of exposure to be manifested on some occasions. Utilizing the observed depolarizing effect of blue light, an action spectrum was determined which encompasses the absorption spectrum of the oxidized and reduced forms of cytochrome c oxidase. O2 uptake studies on isolated pigment epithelial cells verified the reduction of respiration by exposure to blue light, which is observed in other cells. Ultrastructural studies revealed that the major cytopathology observed up to 60 min after blue light exposure was a blistering of the mitochondria which progressed to a swollen, disrupted state within the post irradiation period of 1 h. Comparison of these results with those of other studies suggests that the mechanism of UV-A damage differs substantially from that of blue light. 相似文献
44.
N Otsuka M Fukunaga K Morita S Ono K Nagai T Furukawa S Yanagimoto T Tomomitsu H Kawahara H Tanaka 《Radioisotopes》1988,37(6):330-335
In order to detect the varicocele, scrotal scintigraphies by using 99mTc-HSA-D were performed in 14 patients with male infertile or palpable mass in left scrotum on physical examinations. Abnormal pooling of 99mTc-HSA-D, indicative of varicocele lesion, could be found in left scrotum in 9 cases, confirmed surgically or clinically. Compared with 99mTc-HSA, 99mTc-HSA-D was superior in high uptake ratio of varicocele to soft tissue and in nonvisualization of bladder. Thus, 99mTc-HSA-D scrotal scintigraphy seemed to be of a great use to detect the varicocele. 相似文献
45.
Kaname Takagi Tomoji Aotsuka And Hikari Morita Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1986,23(5):1443-1449
Reactions of 4-ethoxycarbonylamino-1H-1,5-benzodiazepine-3-carbonitrile (2) with aliphatic primary amines gave 1-substituted 4-(2-aminoanilino)pyrimidin-2(1H)-one-5-carbonitriles 3. Analogous reactions of 2 with aromatic primary amines afforded 2-(2′-anilino-1′-cyanovinyl)benzimidazoles 5 and 6. Upon treatment with triethylamine, 3 underwent intramolecular cyclization to give 3-substituted 5-aminopyrimidino[4,5-b]-[1,5]benzodiazepin-2(3H,11H)-ones 8 . Heating of 3 with p-toluenesulfonic acid in ethanol gave 2-substituted pyrimidino[1,6-a]benzimidazol-1(2H)one-4-carbonitriles 9 . Reactions of 2 with hydrazines were also described. Mechanistic pathways are proposed to account for the products. 相似文献
46.
[structure: see text] Two cytotoxic quaternary Daphniphyllum alkaloids with an unprecedented fused-polycyclic skeleton containing a 1-azoniatetracyclo[5.2.2.0.(1,6)0.(4,9)]undecane ring system, daphniglaucins A (1) and B (2), have been isolated from the leaves of Daphniphyllum glaucescens. Their structures and relative stereochemistry were elucidated on the basis of spectroscopic data. 相似文献
47.
The paper presents a procedure for the multi-element inorganic speciation of As(III, V), Se(IV, VI) and Sb(III, V) in natural water with GF-AAS using solid phase extraction technology. Total As(III, V), Se(IV, VI) and Sb(III, V) were determined according to the following procedure: titanium dioxide (TiO2) was used to adsorb inorganic species of As, Se and Sb in sample solution; after filtration, the solid phase was prepared to be slurry for determination. For As(III), Se(IV) and Sb(III), their inorganic species were coprecipitated with Pb-PDC, dissolved in dilute nitric acid, and then determined. The concentrations of As(V), Se(VI) and Sb(V) can be calculated by the difference of the concentrations obtained by the above determinations. For the determination of As(III), Se(IV) and Sb(III), palladium was chosen as a modifier and pyrolysis temperature was 800 °C. Optimum conditions for the coprecipitation were listed for 100 ml of sample solution: pH 3.0, 15 min of stirring time, 40.0 μg l−1 Pb(NO3)2 and 150.0 μg l−1 APDC. The proposed method was applied to the determination of trace amounts of As(III, V), Se(IV, VI) and Sb(III, V) in river water and seawater. 相似文献
48.
This paper describes the synthesis of four tricyclic heterocycles, furo[2,3–6:4,5-c']- ( 5a ), furo[3,2-b:4,5-c']- ( 5b ), furo[2,3-c:4,5-c']- ( 5c ) and furo[3,2-c:4,5-c']dipyridine ( 5d ). Starting with 2-formylfuropyridines ( 1a-d ), β-(2-furopyridyl)acrylic acids 2a-d were obtained by condensing with malonic acid. The acrylic acids were converted to the acid azides by reaction with ethyl chloroformate and the subsequent reaction with sodium azide. Heating of the acid azides at 230–240° with diphenylmethane and tributylamine gave tricyclic pyridinones 3a-d , which were converted to the respective chloro derivatives 4a-d by reaction with phosphorus oxychloride. Reduction of the chloro compounds over palladium-charcoal yielded compounds 5a-d respectively. All the compounds 2 to 5 were characterized by elemental analysis and spectral data. The H and 13C nmr and electronic spectral features of the furodipyridines were discussed comparing with those of the parent furopyridines. 相似文献
49.
A simple synthesis of furo[2,3-c]pyridine and its 2- and 3-methyl derivatives from ethyl 3-hydroxyisonicotinate ( 2 ) is described. The hydroxy ester 2 was O-alkylated with ethyl bromoacetate or ethyl 2-bromopropionate to give the diester 3a or 3b . Cyclization of compound 3a afforded ethyl 3-hydroxyfuro [2,3-c]pyridine-2-carboxylate ( 4 ) which was hydrolyzed and decarboxylated to give furo[2,3-c]pyridin-3(2H)-one ( 5a ). Cyclization of 3b gave the 2-methyl derivative 5b . Reduction of 5a and 5b with sodium borohydride yielded the corresponding hydroxy derivative 6a and 6b , respectively, which were dehydrated with phosphoric acid to give furo[2,3-c]pyridine ( 7a ) and its 2-methyl derivative 7b . 4-Acetylpyridin-3-ol ( 8 ) was O-alkylated with ethyl bromoacetate to give ethyl 2-(4-acetyl-3-pyridyloxy) acetate ( 9 ). Saponification of compound 9 , and the subsequent intramolecular Perkin reaction gave 3-methylfuro[2,3-c]pyridine ( 10 ). Cyclization of 9 with sodium ethoxide gave 3-methylfuro[2,3-c]pyridine-2-carboxylic acid, which in turn was decarboxylated to give compound 10 . 相似文献
50.
Tsuneyuki Sato Naoki Morita Hitoshi Tanaka Tadatoshi Ota 《Journal of polymer science. Part A, Polymer chemistry》1989,27(8):2497-2508
Solvent effect on the polymerization of di-n-butyl itaconate (DBI) with dimethyl azobisisobutyrate (MAIB) was investigated at 50 and 61°C. The solvents used were found to affect significantly the polymerization. The polymerization rate (Rp) and the molecular weight of the resulting polymer are lower in more polar solvents. The initiation rate (Ri) by MAIB, however, shows a trend of being rather higher in polar solvents. The stationary state concentration of propagating poly(DBI) radical was determined by ESR in seven solvents. The rate constants of propagation (kp) and termination (kt) were evaluated by using Rp, Ri, and the polymer radical concentration observed. The kp value decreases fairly with increasing polarity of the solvent used, whereas kt is not so influenced by the solvents. The solvent effect on kp is explained in terms of a difference in the environment around the terminal radical center of the growing chain. Copolymerization of DBI with styrene (St) was also examined in three solvents with different physical properties. The poly(DBI) radical shows a lower reactivity toward St in a more polar solvent. 相似文献