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11.
Örn Helgason Haraldur P. Gunnlaugsson Siguröur Steinthorsson Steen Mørup 《Hyperfine Interactions》1992,70(1-4):981-984
Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (JS-T) curves with Curie temperatures of about 580°C, has revealed considerable amounts of maghemite (γ-Fe2O3) in many samples. In view of the expected instability, of maghemite at temperatures above 350°C, this reversibility is rather surprising. Here we report Mössbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600°C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mössbauer spectroscopy at room temperature. The Mössbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes. 相似文献
12.
We consider a multiperiod mean-variance model where the model parameters change according to a stochastic market. The mean
vector and covariance matrix of the random returns of risky assets all depend on the state of the market during any period
where the market process is assumed to follow a Markov chain. Dynamic programming is used to solve an auxiliary problem which,
in turn, gives the efficient frontier of the mean-variance formulation. An explicit expression is obtained for the efficient
frontier and an illustrative example is given to demonstrate the application of the procedure. 相似文献
13.
Let G be a locally compact Abelian group. Following Ruy Exel, we view Fell bundles over the Pontrjagin dual group of G as continuous spectral decompositions of G-actions on C*-algebras. We classify such spectral decompositions using certain dense subspaces related to Marc Rieffel's theory of square-integrability. There is a unique continuous spectral decomposition if the group acts properly on the primitive ideal space of the C*-algebra. But there are also examples of group actions without or with several inequivalent spectral decompositions. 相似文献
14.
In this paper, it is shown that some of the results of torus actions on Poincaré duality spaces, Borel’s dimension formula
and topological splitting principle to local weights, hold if ‘torus’ is replaced by ‘pro-torus’. 相似文献
15.
Simple and convenient methods for determining surface chemical composition of lignocellulosic materials are described. The
methods are based on vapor phase fluorine surface derivatization with either trifluoro acetic anhydride (TFAA), tri-fluoro
ethanol (TFE) or pentafluorophenyl hydrazine (PFPH) and subsequent Electron Spectroscopy for Chemical Analysis (ESCA). Model
cellulosic surfaces with well defined functionalities were used to optimize the derivatization reaction conditions. Detection
and accessibility of surface hydroxyl functional groups were investigated in cotton and regenerated cellulose as models. Carboxymethyl
cellulose (CMC) was used as a model surface for detection and quantification of carboxylic acid groups. Theoretical conversion
curves for derivatization reactions were calculated and used to evaluate the extent of the reactions on the model surfaces.
It was found that the conversion was higher for the regenerated cellulose and CMC than for cotton. The protocols developed
using the model surfaces were applied to a case study on wood fibers with different degrees of complexity, namely dissolving
and chemithermomechanical (CTMP) pulp. Untreated and oxygen-plasma modified pulps were compared with respect to the surface
composition of functional groups. According to the derivatization reactions, functionalities containing oxygen were significantly
increased on the plasma-treated samples. The effect of the treatment was found to be dependent on the type of pulp. Fluorine
derivatization is shown to be an unambiguous method for clear assessment of the chemical functionalities of cellulosic surfaces. 相似文献
16.
Ralf Quadt 《Foundations of Physics》1989,19(8):1027-1035
It is shown in this paper that a classical way of speaking about the past can be rejected when quantum systems without superselection rules are considered. To show this, use is made of a formal quantum language. The noncontextuality of quantum measurements is a presupposition of the quantum language. In addition, it is shown that introspective measurements, in contrast to the claims of Albert et al., do not violate the noncontextuality, and hence the result of rejecting the classical way of speaking remains still applicable to introspective measurements. 相似文献
17.
İ. B. Özdemir 《Experiments in fluids》1997,22(4):271-280
This paper is concerned with an experimental investigation of the mixing inside the vortex ring formed by the gravity slumping
motion of a dense cloud in a less dense atmosphere. The dynamics of the spreading and instantaneous structures of the turbulent
flow were examined by visualization, single and multi-point measurements of velocity and concentration for two heavy gases,
carbondioxide (CO2) and dichlorodifluoromethane (CCl2F2), in a configuration in that heavy gas, initially trapped in a reservoir, was released with the rise of a shutter into calm
air of a sector-shaped dispersion channel.
Visualization of the cloud as a whole showed a spreading motion in which an advancing frontal structure was followed by a
stratified flow with a layer of dense fluid of higher velocities near the wall and, on top of it, a layer of dilute fluid
whose concentration is controlled by the mixing mechanisms within the head. During the course of spreading, there was always
a phase in which the head attained to a constant speed of advance, which occurred as 0.13 m/s for CO2 and 0.48 m/s for CCl2F2. It was interesting to observe for CO2 that the phase of constant speed took place in between two acceleration phases; the former was due to the initial slumping
of the cloud at the exit of the reservoir, and the latter was attributed to the collapse of the head on the transition to
the passive dispersion phase.
Instantaneous two-dimensional velocity field, measured with particle image velocimeter (PIV), showed that the cloud overran
the ambient air which caused the approaching dense fluid deflected away from the wall with significant vertical velocities
and downstream-moving separation, and the air trapped under the head resulted in the density inversion which introduced further
intricacy to the turbulent structure of the head. Instabilities at the upper free shear layer due to density and velocity
discontinuity rolled into periodic array of vortices which engulfed a considerable amount of air as they were convected backwards
over the head, but the incorporation of heavy and light fluids was completed with the appearance of microscales after the
collapse on the stratified layer. Analyses of the cloud head at different downstream locations also revealed that its size
remained unchanged when the speed of advance was constant, allowing the rate of change of the cloud volume being modeled with
the rate of spreading.
Contours of concentration obtained from digitized PIV pictures confirmed the kinematic features of the mixing revealed by
the velocity field and that the concentration values within the large structures were higher than those at the upper part
of the stratified layer. Motivated by the experimental observations, a semi-empirical analysis was presented to describe the
results and based on local values of the Richardson and Reynolds numbers.
Received: 4 October 1995 / Accepted: 4 July 1996 相似文献
18.
19.
Solubility of carbon dioxide in aqueous solutions of sodium chloride: Experimental results and correlation 总被引:2,自引:0,他引:2
The solubility of carbon dioxide in aqueous solutions of sodium chloride was measured in the temperature range from 40 to 160°C, up to 6 mol-kg salt solutions and total pressures up to 10 MPa. Pitzer's(1) equations as well as the Chen and Evans(2) model were used to correlate the new data. Results are reported and compared to literature data and correlations. 相似文献
20.
Steven Schmidt Gü nter Lattermann Ralf Kleppinger Joachim H. Wendorff 《Liquid crystals》1994,16(4):693-702
Liquid crystalline complexes with chromium, molybdenum and with tungsten as metallic centres are reported. 1,4,7-Trisubstituted 1,4,7-triazacyclononane and three carbonyl groups are coordinated in an octahedral geometry. The observed mesophases are characterized as disordered rectangular columnar of a pyramidic type. 相似文献