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991.
992.
The thermal motion of a long-chain molecule dispersed in a solvent is examined in terms of the velocity autocorrelation, in a reference frame attached to a subunit of the chain. 相似文献
993.
994.
Israel Goldberg Lon-Tang W. Lin Harold Hart 《Journal of inclusion phenomena and macrocyclic chemistry》1984,2(1-2):377-389
This paper reports crystalline complexes of the new hosts N,N'-ditritylurea (DTU) and N-tritylurea (NTU) with various uncharged molecular guests. The crystal structures of the following complexes were elucidated by single crystal X-ray diffraction analysis at 115oK: (I) 1:1 DTU-propanamide — space group C2/c, a=15.839Å, b=9.088Å, c=24.584Å, =111.05o, Z=4; (II) 1:1 DTU-ethyl N-acetylglycinate — space group P1, a=9.010Å, b=10.800Å, c=19.810 Å, =105.29o =94.33o, =93.03o, Z=2; (III) 2:1 NTU-N, N-dimethylformamide — space group Cc, a=29.614Å, b=8.906Å, c=16.127Å, =121.04o, Z=4. The three crystal structures are stabilized mainly by a cooperative effect of hydrogen bonding between amide fragments displaced along the shortest axis of each crystal. This interaction occurs between host and guest in complexes I and II, and between host and host in complex III. The latter also represents a cage-type clathrate in which the guest molecules are accommodated in voids between the hydrophobic fragments of four neighboring NTU hosts. On the other hand, complexes of DTU are characterized by a more specific interaction between the two components, each guest molecule being inserted between two adjacent hosts (related by translation) and strongly bound to them via hydrogen bridges. These results illustrate a useful concept in the design of molecular species which can be potential hosts upon crystallization with neutral molecular guests.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82022 (7 pages). 相似文献
995.
A formulation of the kinetic theory of dilute, classical polyatomic gases is given which parallels the Waldmann development for structureless molecules. In the first section the Boltzmann equation is written in terms of the specific rates of inelastic collision processes and then the properties of these rates and those of the corresponding collision cross sections are examined. The dependence of the distribution function on the dynamical variables is discussed and the equations of change for the gas are derived. Finally, a study is made of the properties of the linearized Boltzmann collision operation. In the second section the Boltzmann equation is deduced from a rigorous statistical-mechanical point of view and discussed in terms of the basic ideas of Bogoliubov. The computationally important special case of impulsive interactions is then considered.This research was supported in part by a grant from the National Science Foundation and in part by the Ames Laboratory of the U. S. Atomic Energy Commission. Contribution No. 2554. 相似文献
996.
A new paramagnetic center, AfsAg?Si, in silver-diffused α-quartz is described and its spin-Hamiltonian parameters are presented. The center is formulated as a silver atom situated in a c-axis channel, bonded weakly to a normal silicon cation, with four oxygen anions as neighbors. AAg?Si is created by X-irradiation and is unstable at room temperature. 相似文献
997.
Design of a large,rigid, modular optical bed for versatile optical and spectroscopic experimentation
David M. Coleman John P. Walters 《Spectrochimica Acta Part B: Atomic Spectroscopy》1978,33(5):127-142
Described is a mechanically-rigid and referenceable, vibrationally-isolated, modular optical support bed which has demonstrated significant utility in recent research on the physical—chemical nature of the atmospheric pressure spark discharge. By interconnecting an assembly of pre-aligned, truss-like sub-structures, a complete instrumental complex has been fabricated which exhibits high mechanical stability. This stability is sufficient to allow alignment of, for example, a 5 m focal length, high-dispersion optical spectrometer, to be maintained for months without detectable degradation. The approach is versatile and can be easily extended to include a wide variety of optical and other research applications. Supporting mechanical, vibration, and optical data are included as well as documentation of fabrication details. 相似文献
998.
Bruce J. West Ary L. Goldberger Galina Rovner Valmik Bhargava 《Physica D: Nonlinear Phenomena》1985,17(2):198-206
Under physiologic conditions, the AV junction is traditionally regarded as a passive conduit for the conduction of impulses from the atria to the ventricles. An alternative view, namely that subsidiary pacemakers play an active role in normal electrophysiologic dynamics during sinus rhythm, has been suggested based on nonlinear models of cardiac oscillators. A central problem has been the development of a simple but explicit mathematical model for coupled nonlinear oscillators relevant both to stable and perturbed cardiac dynamics. We use equations describing an analog electrical circuit with an external d.c. voltage source (V0) and two nonlinear oscillators with intrinsic frequencies in the ratio of 3:2, comparable to the SA node and AV junction rates. The oscillators are coupled by means of a resistor. 1:1 (SA:AV) phase-locking of the oscillators occurs over a critical range of V0. Externally driving the SA oscillator at increasing rates results in 3:2 AV Wenckebach periodicity and a 2:1 AV block. These findings appear with no assumptions about conduction time or refractoriness. This dynamical model is consistent with the new interpretation that normal sinus rhythm may represent 1:1 coupling of two or more active nonlinear oscillators and also accounts for the appearance of an AV block with critical changes in a single parameter such as the pacing rate. 相似文献
999.
Bruce M. Hasch Melchior A. Meilchen Sang-Ho Lee Mark A. Mchugh 《Journal of Polymer Science.Polymer Physics》1993,31(4):429-439
Cloud-point data to 180°C and 2800 bar are presented for polyethylene, poly(methyl acrylate), and two poly(ethylene-co-methyl acrylate) copolymers (10 and 31 mol % methyl acrylate) in propane and chlorodifluoromethane with two cosolvents, acetone and ethanol. The addition of small amounts of either cosolvent to the copolymer–solvent mixtures shifts the cloud-point curve to lower pressures and temperatures, as both cosolvents provide favorable polar interactions with the acrylate group in the backbone of the copolymer. Ethanol has a larger effect than acetone since ethanol hydrogen bonds to the acrylate group. However, if the concentration of ethanol is increased above ca. 10 wt %, it self-associates and reverts to antisolvent behavior, forcing the copolymer out of solution. For nonpolar polyethylene–propane mixtures, the polar cosolvents behave as traditional an-tisolvents. In poly(methyl acrylate)–chlorodifluoromethane mixtures, both polar cosolvents enlarge the single-phase region. The cloud-point curves for the (co)polymer–propane–acetone mixtures are modeled reasonably well using the Sanchez–Lacombe equation of state with two adjustable mixture parameters. No attempt is made to model the mixtures that exhibit hydrogen bonding. © 1993 John Wiley & Sons, Inc. 相似文献
1000.
在循环流化床锅炉炉膛内,分为湍流床和快床2个区域。论述了由鼓泡流化床与从气力输送状态向循环流化床转化过程中的炉内流体动力学现象。研究了湍流床开始出现到完全转化为湍流床及快床时,炉内气体速度变化的规律和相应的计算公式。对转化过程中的主要影响因素,如床的温度及固体颗粒供给速度等进行了深入试验研究,对循环流化床锅炉的设计和运行有一定的指导意义。 相似文献