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排序方式: 共有101条查询结果,搜索用时 15 毫秒
41.
42.
J.W. Cockburn M.S. Skolnick J.P.R. David R. Grey G. Hill M.A. Pate 《Superlattices and Microstructures》1992,12(4)
At high electric fields, hot electrons injected into the undoped regions of n-i-n structures can give rise to impact ionization of the host lattice and electron-hole pair production. The holes created by impact ionization recombine with majority carrier electrons, leading to electroluminescence (EL) from the device at fields in excess of 105V/cm. In the present work, we show that the study of such EL in GaAs/AlGaAs/GaAs single barrier tunnelling structures provides a simple and effective quantitative method to determine impact ionization coefficients in GaAs. Structures with undoped regions of length 100, 150 and 200nm are shown to give very similar results for the impact ionization coefficient. The values obtained for the impact ionization coefficient are shown to be consistent with those obtained by carrier multiplication methods. 相似文献
43.
The tracklengths in mica have been measured for iodine and gold ions in the energy range 0 to 1 MeV/amu. These data and those from previous studies have been compared with the Lindhard-Scharff-Schiøtt theory for the stopping of heavy ions in matter. Discrepancies from the theory are discussed. 相似文献
44.
Thomas H. Pate 《Linear algebra and its applications》1976,14(3):285-292
Suppose each of m, n, and k is a positive integer, k ? n, A is a (real-valued) symmetric n-linear function on Em, and B is a k-linear symmetric function on Em. The tensor and symmetric products of A and B are denoted, respectively, by A ?B and A?B. The identity is proven by Neuberger in [1]. An immediate consequence of this identity is the inequality In this paper a necessary and sufficient condition for is given. It is also shown that under certain conditions the inequality can be considerably improved. This improvement results from an analysis of the terms 6A?qB6, 1?q?n, appearing in the identity. 相似文献
45.
Nikolaos Politakos Eleftherios Ntoukas Apostolos Avgeropoulos Vahik Krikorian Brian D. Pate Edwin L. Thomas Randal M. Hill 《Journal of Polymer Science.Polymer Physics》2009,47(23):2419-2427
We report the observation of a cubic phase consistent with the double gyroid structure in strongly segregated diblock copolymers of PS‐b‐PDMS over a volume fraction (φPDMS) range of ~0.39 to 0.45. The samples have respective molecular weights of 127 kg/mol and 73 kg/mol and degree of segregation Nχ equal to 187 and 106, respectively, at annealing temperature of 130 °C. It is important to highlight that two out of the total four samples investigated, exhibited hexagonally close packed cylindrical domains of PDMS and alternating lamellae at φPDMS = 0.39 and 0.45, respectively, indicating the possible narrow range of the DG morphology for the specific diblock copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2419–2427, 2009 相似文献
46.
Effect of Film Thickness on the Columnar Packing Structures of Discotic Supramolecules in Thin Films
Hyo‐Sik Kim Sung‐Min Choi Prof. Brian D. Pate Dr. Po Gyu Park Dr. 《Chemphyschem》2009,10(15):2642-2646
The effects of film thickness on the columnar packing structure of discotic supramolecules in a thin supported film have been investigated by grazing‐incidence small‐angle X‐ray scattering technique using magnetically aligned cobalt octa(n‐decylthio)porphyrazine (CoS10) films on octadecyltrichlorosilane (OTS)‐functionalized substrates as model systems. Magnetically aligned CoS10 films with a range of film thicknesses (49–845 nm) form uniaxially oriented ‘edge‐on’ columnar superstructures with their columnar directors perpendicular to the applied magnetic field. However, the orientational ordering of the columnar packing in the plane perpendicular to the applied magnetic field is strongly dependent on the film thickness. While being damped by the elasticity of the side chains of CoS10, the strong interfacial interaction at the film‐substrate interface propagates up to 50–100 nm from the substrate, maintaining the orientation of columnar packing in the plane perpendicular to the applied magnetic field. When the distance from the film‐substrate interface becomes larger than about 100 nm, symmetric tilting of columnar layer orientation, which saturates at 11.5°, occurs due to longitudinal edge dislocations induced by accumulated elastic deformation. 相似文献
47.
Nathan A. Seifert Daniel P. Zaleski Cristbal Prez Justin L. Neill Brooks H. Pate Montserrat Vallejo‐Lpez Alberto Lesarri Emilio J. Cocinero Fernando Castao Isabelle Kleiner 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(12):3274-3277
Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi‐isotopic broadband rotational spectra. This heterodimer is held together by a primary C H⋅⋅⋅π hydrogen bond, assisted by multiple weak C H⋅⋅⋅F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum. 相似文献
48.
A 2 MHz resolution electric-resonance optothermal spectrometer and a microwave-sideband CO2 laser have been used with microwave-infrared double resonance to investigate high-order torsional couplings in the 10 μm infrared spectrum of 3,3,3-trifluoropropene. Three normal mode vibrations are studied with band origins at 963.4, 980.2 and 1025.2 cm−1. The 963.4 cm−1 band is well characterized by an asymmetric-top Hamiltonian, except for the presence of a weak perturbation for J′ = 7, Ka′ = 2 affecting only the A-symmetry internal-rotor state. Microwave-infrared double resonance is used to study the microwave spectrum of the perturbing or ‘dark’ state. The observed dark-state K-doublet asymmetry splittings and rotational-state selection rules indicate that the perturbing state has five quanta of excitation in the torsional mode (ν21) built upon the A″ ν19 fundamental. The precise frequency determined for 5 β21 of 421(2) cm−1 leads to the first accurate determination of the barrier to CF3 internal rotation as 641(5) cm−1. In contrast to the 963.4 cm−1 vibration, the 980.2 and 1025.2 cm−1 modes show a large number of J′ and Ka′ perturbations which differentially affect the A and E symmetry internal-rotor states. The magnitude of the perturbation-induced A/E splittings indicate that the perturbing states must have at least four quanta of torsional excitation. The present results suggest that high-order vibrational interactions are important in the vibrational dynamics of molecules at low levels of overall vibrational excitation. 相似文献
49.
Neill JL Steber AL Muckle MT Zaleski DP Lattanzi V Spezzano S McCarthy MC Remijan AJ Friedel DN Widicus Weaver SL Pate BH 《The journal of physical chemistry. A》2011,115(24):6472-6480
Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical-radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope. 相似文献
50.
Kisiel Z Lesarri A Neill JL Muckle MT Pate BH 《Physical chemistry chemical physics : PCCP》2011,13(31):13912-13919
The rotational spectrum of the cyclic (HCl)(2)H(2)O cluster has been identified for the first time in the chirped pulse, Fourier transform microwave spectrum of a supersonically expanded HCl/H(2)O/Ar mixture. The spectrum was measured at frequencies 6-18.5 GHz, and transitions in two inversion-tunneling states, at close to 1?:?3 relative intensity, have been assigned for the parent species. The two single (37)Cl isotopic species, and the double (37)Cl species have been assigned in the natural abundance sample, and the (18)O and HDO species of the cluster were identified in isotopically enriched samples. The rich nuclear quadrupole hyperfine structure due to the presence of two chlorine nuclei has been satisfactorily fitted and provided useful information on the nonlinearity of intermolecular bonds in the cluster. The r(s) heavy atom geometry of the cluster was determined and the strongest bond in the intermolecular cycle r(O···HCl) = 3.126(3) ?, is found to be intermediate in length between the values in H(2)O···HCl and (H(2)O)(2)HCl. The fitted spectroscopic constants and derived molecular properties are compared with ab initio predictions, and a discussion of complexation effects in these three clusters is made. 相似文献