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排序方式: 共有315条查询结果,搜索用时 484 毫秒
71.
Sea-Fue Wang Yuh-Ruey Wang Thomas C. K. Yang Che-Fu Chen Chun-An Lu Chi-Yuen Huang 《Journal of magnetism and magnetic materials》2000,220(2-3)
The mixing of (Ni0.38Cu0.12Zn0.50)Fe2O4 powders with Bi2O3 was performed using the solid-state mixing as well as wet chemical coating processes such as ammonia precipitation coating, urea precipitation coating, and solution coating. Ferrites prepared from the wet chemical coating processes could be densified at a lower sintering temperature without significant impact on the microstructural evolution compared with that prepared by solid-state mixing. In addition, samples prepared from the wet chemical coating process have a higher Br and Bs and a lower Hc compared with that from solid-state mixing. Considering both the effects of sintering temperature and sintered density (>95% T.D.), ferrites with 1.5 wt% Bi2O3 addition by ammonia precipitation coating sintered at 900°C can provide the best permeability and quality factor (191 and 68.2, respectively) among all the cases studied. 相似文献
72.
Czechoslovak Mathematical Journal - Let $$T(q) = \sum\limits_{k = 1}^\infty {d(k){q^k},\,\,\,\,\left| q \right| < 1,} $$ where d(k) denotes the number of positive divisors of the natural... 相似文献
73.
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75.
Lu G Kwong FY Ruan JW Li YM Chan AS 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(15):4115-4120
The direct addition of in situ prepared arylzinc to aldehydes with chiral binaphthyl-derived amino alcohols as catalysts can afford optically active diarylmethanols in high yields and with excellent enantioselectivities (up to 99 % ee, ee = enantiomeric excess). By using a single catalyst, both enantiomers of many pharmaceutically interesting diarylmethanols can be obtained by the proper combination of various arylzinc reagents with different aldehydes; this catalytic system also works well for the phenylation of aliphatic aldehydes to give up to 96 % ee. 相似文献
76.
Qiu L Kwong FY Wu J Lam WH Chan S Yu WY Li YM Guo R Zhou Z Chan AS 《Journal of the American Chemical Society》2006,128(17):5955-5965
A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed. 相似文献
77.
Lam WW Yiu SM Lee JM Yau SK Kwong HK Lau TC Liu D Lin Z 《Journal of the American Chemical Society》2006,128(9):2851-2858
The oxidation of alkanes and arylalkanes by KMnO(4) in CH(3)CN is greatly accelerated by the presence of just a few equivalents of BF(3), the reaction occurring readily at room temperature. Carbonyl compounds are the predominant products in the oxidation of secondary C-H bonds. Spectrophotometric and kinetics studies show that BF(3) forms an adduct with KMnO(4) in CH(3)CN, [BF(3).MnO(4)](-), which is the active species responsible for the oxidation of C-H bonds. The rate constant for the oxidation of toluene by [BF(3).MnO(4)](-) is over 7 orders of magnitude faster than by MnO(4)(-) alone. The kinetic isotope effects for the oxidation of cyclohexane, toluene, and ethylbenzene at 25.0 degrees C are as follows: k(C6H12)/k(C6D12) = 5.3 +/- 0.6, k(C7H8)/k(C7D8) = 6.8 +/- 0.5, k(C8H10)/k(C8D10) = 7.1 +/- 0.5. The rate-limiting step for all of these reactions is most likely hydrogen-atom transfer from the substrate to an oxo group of the adduct. A good linear correlation between log(rate constant) and C-H bond energies of the hydrocarbons is found. The accelerating effect of BF(3) on the oxidation of methane by MnO(4)(-) has been studied computationally by the Density Functional Theory (DFT) method. A significant decrease in the reaction barrier results from BF(3) coordination to MnO(4)(-). The BF(3) coordination increases the ability of the Mn metal center to achieve a d(1) Mn(VI) electron configuration in the transition state. Calculations also indicate that the species [2BF(3).MnO(4)](-) is more reactive than [BF(3).MnO(4)](-). 相似文献
78.
The full LaCo1−xRhxO3 solid solution was investigated utilizing structural, electrical transport, magnetic, and thermal conductivity characterization. Strong evidence for at least some conversion of Rh3+/Co3+ to Rh4+/Co2+ is found in both structural and electrical transport data. The crystal structure is that of a rhombohedrally distorted perovskite over the range 0.0≤x≤0.1. The common orthorhombic distortion of the perovskite structure is found over the range 0.2≤x≤1.0. A crossover of all three orthorhombic cell edges occurs at x=0.5 giving the appearance of a cubic structure, which actually remains orthorhombic. The octahedra in the orthorhombic structure must be distorted for x values less than 0.5, and the observed distortion suggests orbital ordering for Co2+. Electrical resistivity measurements as a function of temperature show semiconducting-like regions for all compositions. There is a steady increase in electrical resistivity as the Rh content increases. Large positive thermopower values are generally obtained above 475 K. With increasing Rh substitution there is a decrease in thermal conductivity, which slowly rises with increasing temperature due to increased electrical conductivity. The electronic part of the thermal conductivity is suppressed significantly upon Rh substitution. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo0.5Rh0.5O3 at 775 K, and is expected to reach 0.15 at 1000 K. 相似文献
79.
Chi Kwong Chang 《中国化学快报》2010,21(3):373-375
<正>DNA photocleavage properties of halogenated mono-hydroxyl corrole 1-5 were investigated.It was found that these corroles were able to photocleavage supercoiled pBR 322 DNA(SC) into nicked-circular DNA(NC).The activity of these corroles follows an order of 432≈15.The photosensitized singlet oxygen(Φ_△) quantum yield by these corroles also follows that same order,showing the photocleavage activity is related to the heavy atom effect of halogen atoms on corroles. 相似文献
80.
A facile one-pot procedure for copper-catalyzed PhI(OAc)2-mediated asymmetric alkene aziridination had been developed. Commercially available PhI(OAc)2 and sulfonamides were used to generate the nitrene precursors (PhINR) in situ for olefin aziridination. This one-pot procedure had been optimized using 4-nitrobenzenesulfonamide as the nitrene source. With 5 mol % of the chiral copper catalyst, these conditions afforded 94% yield of the isolated product with 75% ee. We had also developed a simple and rapid method to monitor the rate of this one-pot aziridination. 相似文献