全文获取类型
收费全文 | 593篇 |
免费 | 11篇 |
国内免费 | 4篇 |
专业分类
化学 | 331篇 |
晶体学 | 26篇 |
力学 | 12篇 |
数学 | 84篇 |
物理学 | 155篇 |
出版年
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 5篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 4篇 |
2016年 | 7篇 |
2015年 | 9篇 |
2014年 | 16篇 |
2013年 | 23篇 |
2012年 | 32篇 |
2011年 | 38篇 |
2010年 | 11篇 |
2009年 | 21篇 |
2008年 | 23篇 |
2007年 | 35篇 |
2006年 | 49篇 |
2005年 | 28篇 |
2004年 | 11篇 |
2003年 | 18篇 |
2002年 | 22篇 |
2001年 | 12篇 |
2000年 | 18篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1992年 | 6篇 |
1989年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1974年 | 3篇 |
1971年 | 3篇 |
1960年 | 3篇 |
1947年 | 6篇 |
1946年 | 5篇 |
1944年 | 4篇 |
1943年 | 7篇 |
排序方式: 共有608条查询结果,搜索用时 0 毫秒
81.
In this paper, we first prove that for any connected graph G with at least two vertices, there is an integer m for which the strong product X⌅Gm has pancyclic ordering from each vertex. After characterizing the graphs G for which GX⌅K2 is Hamiltonian, we determine a criterion for extendability of cycles. We also prove that if G is a connected, K1.3-free graph with δ ≥ 2, then GX⌅XK2 is fully cycle extendable as well as 1-edge Hamiltonian. © 1996 John Wiley & Sons, Inc. 相似文献
82.
Panicker Devyani Ramachandran Mahesh Doddadasarahalli Muniyappa Sreelekha Kanapadinchareveetil Suresh Narayanan Nair Karapparambu Gopalan Ajithkumar Sujith Samraj Anoopraj Rajappan Anju Varghese Deepa Chundayil Kalarickal Reghu Ravindran Srikanta Ghosh Sanis Juliet 《Molecules (Basel, Switzerland)》2021,26(16)
Prostaglandins are a group of important cell-signaling molecules involved in the regulation of ovarian maturation, oocyte development, egg laying and associated behaviors in invertebrates. However, the presence of prostaglandin E2 (PGE2), the key enzymes for PGE2 biosynthesis and its interference by drugs were not investigated previously in the ovary of ticks. The present study was undertaken to assess the modulation of the PGE2-mediated pathway in the eclosion blocking effect of flumethrin and terpenoid subfraction isolated from Artemisia nilagirica in Rhipicephalus annulatus ticks. The acaricidal activities and chemical profiling of the terpenoid subfraction were performed. The localization of the cyclooxygenase1 (COX1) and prostaglandin E synthase (PGES) enzymes and the quantification of PGE2 in the ovaries of the ticks treated with methanol (control), flumethrin and terpenoid subfraction were also undertaken. In addition, the vitellogenin concentration in hemolymph was also assayed. Both flumethrin and the terpenoid subfraction of A. nilagirica elicited a concentration-dependent inhibition of fecundity and blocking of hatching of the eggs. The COX1 could not be detected in the ovaries of treated and control ticks, while there was no significant difference observed in the concentration of vitellogenin (Vg) in them. The presence of PGES in the oocytes of control ticks was confirmed while the immunoreactivities against PGES were absent in the vitellogenic oocytes of ticks treated with flumethrin and terpenoid subfraction. The levels of PGE2 were below the detection limit in the ovaries of the flumethrin-treated ticks, while it was significantly lower in the ovaries of the terpenoid subfraction-treated ticks. Hence, the prostaglandin E synthase and PGE2 were identified as very important mediators for the signaling pathway for ovarian maturation and oviposition in ticks. In addition, the key enzyme for prostaglandin biosynthesis, PGES and the receptors for PGE2 can be exploited as potential drug targets for tick control. The detection of PGES by immunohistochemistry and quantification of PGE2 by LC-MSMS can be employed as valuable tools for screening newer compounds for their eclosion blocking acaricidal effects. 相似文献
83.
The boron-atom insertion reaction of nido-9,11-I(2)-7,8-C(2)B(9)H(9)(2-), with the HBCl(2):SMe(2) complex yields closo-4,7-I(2)-1,2-C(2)B(10)H(10), 1, in excellent yield. Although the two boron atoms (B3 and B6) nearest to the carbon atoms in 1 are equally available for attack by nucleophiles, the boron-degradation reaction of 1 with alkoxide ion occurs only at the B6 vertex, yielding regioselectively [(CH(3))(3)NH][nido-2,4-I(2)-7,8-C(2)B(9)H(10)], 2. The molecular structures of 1 and 2 have been determined by X-ray diffraction studies. Crystallographic data are as follows. For 1, monoclinic, space group P2(1)/n, a = 6.9199(19) Angstroms, b = 23.9560(7) Angstroms, c = 7.2870(2) Angstroms, beta = 94.081(4) degrees, V = 1204.9(6) Angstroms(3), Z = 4, rho(calcd) = 2.18 g cm(-3), R = 0.020, R(w) = 0.0610; for 2, orthorhombic, space group Pca2(1), a = 14.1141(7) Angstroms, b = 7.0276(4) Angstroms, c = 16.4602(9) Angstroms, V = 1632.7(15) Angstroms(3), Z = 4, rho(calcd) = 1.81 gcm(-3), R = 0.022, R(w) = 0.0623. 相似文献
84.
Low‐k dielectrics have been developed as an interlayer insulating material in the large scale integrated circuitry devices by the copolymerization of various weight percentages (10, 20, 30, and 40 wt%) of cyanate ester tethered POSS (POSS‐OCN) and bisphenol‐A cyanate ester (BACY) to obtain BACY/POSS‐OCN nanocomposites. The reinforcement of POSS‐OCN significantly contributes to the reduction in the value of dielectric constant and dielectric loss as well, which might be due to the presence of porous structured POSS‐OCN. The 30 wt% POSS‐OCN/BACY nanocomposite possesses the lowest value of dielectric constant of 1.81 at 1 MHz. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
85.
This paper experimentally investigates the noise from a large wind turbine (GE 1.5 MW) with a compact microphone array (OptiNav 24) using advanced deconvolution based beamforming methods, such as DAMAS and CLEAN-SC beamforming algorithms, for data reduction. Our study focuses on the ability of a compact microphone array to successfully locate both mechanical and aerodynamic noise sources on the wind turbine. Several interesting results have emerged from this study: (i) A compact microphone array is sufficient to perform a detailed study on wind turbine noise if advanced deconvolution methods are applied. (ii) Noise sources on the blade and on the nacelle can clearly be separated. (iii) Noise of the blades is dominated by trailing edge noise which is frequency dependent and is distributed along the length of the blade with the dominant noise source closer to the tip of the blade. (iv) The LP and DAMAS algorithms represent the distributed trailing edge noise source better than CLEAN-SC and classical beamforming. (v) Additional tonal noise produced during yawing operation is believed to be radiating from the tower of the wind turbine that acts like a resonator. (vi) Ground reflection is not believed to have a significant effect on noise source location estimates in this study. 相似文献
86.
Azhakar R Ghadwal RS Roesky HW Hey J Stalke D 《Dalton transactions (Cambridge, England : 2003)》2012,41(5):1529-1533
The reaction of N-heterocyclic silylene (NHSi) L [L = CH{(C[double bond, length as m-dash]CH(2))(CMe)(2,6-iPr(2)C(6)H(3)N)(2)}Si] with benzoylhydrazine, 1,2-dicarbethoxyhydrazine, 1,2-diacetylhydrazine and 1,2-bis(tert-butoxycarbonyl)hydrazine in 1 : 1 molar ratio resulted in compounds 1-4 with an almost quantitative yield and five coordinate silicon atoms. Compounds 1-4 were formed by double N-H bond activation by deliberate selection of N,N'-bis-substituted hydrazine compounds bearing the -C(O)NHNH- unit. Compounds 1-4 were characterized by NMR spectroscopy, EI-MS and elemental analysis. The molecular structures of compounds 1-3 were unambiguously established by single crystal X-ray structural analysis. 相似文献
87.
Mn inclusion in the oxide based diluted magnetic semiconductor Zn1−x
Mn
x
O (x = 0.04, 0.06, 0.08, and 0.10) grown by standard high temperature solid state reaction technique has been studied. The local
and average structure of Zn1−x
Mn
x
O was characterized by the super resolution technique maximum entropy method and pair distribution function analysis using
the X-ray powder data. Magnetic studies on this material using a Vibrating Sample Magnetometer were also carried out to ascertain
the doping level in Zn1−x
Mn
x
O. 相似文献
88.
We demonstrate polarization-sensitive measurement of the modal content of waveguides by generalizing the classic rotating wave-plate-based polarimeter to wide-field optical low-coherence interferometry. The spatial phases of the modes are retrieved with principal component analysis. By applying this polarization-sensitive cross-correlation (C2) imaging technique to the characterization of a few-mode fiber, we reveal that different modes experience distinct bend-induced birefringence in optical fibers. This polarization-resolved C2 imaging is well suited for analyzing the impact of polarization on wave propagation in high-power fiber lasers as well as in mode-division-multiplexed communications systems. 相似文献
89.
The ethyl acetate extracts of the bark and leaves of Ficus coronata were separated by column chromatography and the resulting fractions tested for their bioactivity toward methicillin-resistant-Staphylococcus aureus (MRSA) and M. luteus. The bioactive column chromatography fractions were further separated by preparative thin layer chromatography (TLC) and the resulting bands investigated by high-performance liquid chromatography-electrospray ionization-ion trap mass spectrometry (HPLC-ESI-MS(n) ) and ESI-MS(n) . The resulting retention times, molecular masses, their fragmentation patterns, and the chemnet database (www.chemnetbase.com) were then used in the dereplication process by structural elucidation of some of the compounds when compared with known structures of natural origin. Some molecular masses and the corresponding fragmentations were found that did not correlate with any known compounds thus revealing potentially novel natural products that could be investigated on a larger scale and could ultimately find application as new drugs against MRSA and other multidrug-resistant microorganisms. Structures are also proposed for known compounds that have not been previously reported for F. coronata. 相似文献
90.
Parthasarathy Ramachandran 《理论物理通讯》2009,52(4):646-652
In this article, we study traffic flow in the presence ofspeed breaking structures. The speed breakers are typically used toreduce the local speed of vehicles near certain institutions such asschools and hospitals. Through a cellular automata model we study the impact of such structures on global traffic characteristics. The simulation results indicate that the presence of speed breakers could reduce the global flow under moderate global densities. However, under low and high global density traffic regime the presence of speed breakers does not have an impact on the global flow. Further the speed limit enforced by the speed breaker createsa phase distinction. For a given global density and slowdown probability, as the speed limit enforced by the speed breaker increases, the traffic moves from the reduced flow phase to maximum flow phase. This underlines the importance of proper design of these structures to avoid undesired flow restrictions. 相似文献