全文获取类型
收费全文 | 3063篇 |
免费 | 79篇 |
国内免费 | 9篇 |
专业分类
化学 | 1921篇 |
晶体学 | 21篇 |
力学 | 58篇 |
数学 | 459篇 |
物理学 | 692篇 |
出版年
2023年 | 19篇 |
2022年 | 41篇 |
2021年 | 72篇 |
2020年 | 66篇 |
2019年 | 77篇 |
2018年 | 74篇 |
2017年 | 43篇 |
2016年 | 99篇 |
2015年 | 68篇 |
2014年 | 97篇 |
2013年 | 141篇 |
2012年 | 169篇 |
2011年 | 177篇 |
2010年 | 104篇 |
2009年 | 115篇 |
2008年 | 126篇 |
2007年 | 117篇 |
2006年 | 109篇 |
2005年 | 107篇 |
2004年 | 79篇 |
2003年 | 67篇 |
2002年 | 74篇 |
2001年 | 31篇 |
2000年 | 32篇 |
1999年 | 43篇 |
1998年 | 21篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 31篇 |
1994年 | 20篇 |
1993年 | 32篇 |
1992年 | 30篇 |
1991年 | 23篇 |
1990年 | 16篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1986年 | 17篇 |
1984年 | 28篇 |
1982年 | 17篇 |
1981年 | 21篇 |
1979年 | 30篇 |
1978年 | 24篇 |
1977年 | 22篇 |
1976年 | 31篇 |
1975年 | 30篇 |
1974年 | 15篇 |
1956年 | 16篇 |
1933年 | 15篇 |
1932年 | 13篇 |
1927年 | 14篇 |
排序方式: 共有3151条查询结果,搜索用时 0 毫秒
991.
A detailed study of the elution behaviour, apparent molecular characteristics and hydrodynamic properties of amylopectin-type fraction (isolated from normal corn starch) in aqueous media employing asymmetrical flow field-flow fractionation (AF4) was undertaken by systematically varying the channel flow (Fch), cross flow (Fcr) and Fcr/Fch ratios. Distributions of apparent molar masses and radii of gyration, mass recoveries and hydrodynamic radii decreased as a function of increasing Fcr at a fixed Fch, due to the increase in the retention of amylopectin-type fraction in the AF4 channel. Increased retention of the amylopectin-type fraction in the AF4 channel was also observed at low Fch and high Fcr/Fch ratios. Large amylopectin-type molecules/particles (possibly aggregates) eluted at high Fch, low Fcr and low Fcr/Fch ratios. 相似文献
992.
Sanhao Ji Bernd Bruchmann Frederik Wurm Harm‐Anton Klok 《Journal of polymer science. Part A, Polymer chemistry》2012,50(1):25-34
Hyperbranched polyesters are among the most common hyperbranched polymers. One of the interesting features of hyperbranched polyesters is that they contain unreacted hydroxyl and carboxylic acid groups at the linear and terminal structural units, which can be postmodified to adjust thermal, solubility, or mechanical properties, or to prepare core–shell type architectures. This article reports on the synthesis of a novel class of hyperbranched polyesters via an A2 + B3 type Baylis–Hillman polymerization of 2,6‐pyridinedicarboxaldehyde and trimethylolpropane triacrylate. Baylis–Hillman polymerization generates highly functional polyesters that contain not only unreacted aldehyde and/or acrylate groups at the linear and terminal structural units but also chemically orthogonal vinyl and hydroxyl groups along the polymer backbone. Using 3‐hydroxyquinuclidine as the catalyst, hyperbranched polymers with number‐average molecular weights up to 7500 g/mol and degrees of branching up to 0.81 were obtained. To demonstrate the versatility of these hyperbranched polyesters to act as platforms for further derivatization, the orthogonal postpolymerization modification of the hydroxyl, vinyl, and pyridine functional moieties with phenyl isocyanate, methyl‐3‐mercaptopropionate, and methyl iodide is presented. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012. 相似文献
993.
Mamdooh Alqathami Anton Blencowe Greg Qiao Duncan Butler Moshi Geso 《Radiation Physics and Chemistry》2012,81(7):867-873
The aim of this study is to investigate the effect of trihalomethane radical initiators on the radiological properties, radiation dose sensitivity and post response photo-stability of the PRESAGE dosimeter. Different PRESAGE dosimeters containing 50 and 100 mM of iodoform (CHI3), bromoform (CHBr3) or chloroform (CHCl3) radical initiators where fabricated and irradiated with 6 MV photons for a range of radiation doses from 0 to 30 Gy. A comparison between sensitivity and radiological properties of the PRESAGE dosimeters with the different radical initiators was carried out. Optical density changes of the dosimeters before and after irradiation were measured using a spectrophotometer. The incorporation of different radical initiators in the composition of the PRESAGE dosimeter resulted in variation of the radiation dose sensitivity and radiological properties of the dosimeters depending on the type and concentration of the radical initiator used, with iodoform showing the highest dose-response slope followed by bromoform and chloroform. However, at 100 mM iodoform, the effective atomic number was significantly higher than water (Zeff=16). This enhancement in dose-response was found to be directly related to the carbon–halogen bond dissociation energy and to the radiological properties of each individual radical initiator used in this study. Furthermore, the post-response stability of the PRESAGE dosimeters over two weeks remained stable regardless of the trihalomethane radical initiator employed, with negligible change in the post-response stability and linearity of the PRESAGE dosimeters. 相似文献
994.
We study topological boundary conditions in abelian Chern–Simons theory and line operators confined to such boundaries. From the mathematical point of view, their relationships are described by a certain 2-category associated to an even integer-valued symmetric bilinear form (the matrix of Chern–Simons couplings). We argue that boundary conditions correspond to Lagrangian subgroups in the finite abelian group classifying bulk line operators (the discriminant group). We describe properties of boundary line operators; in particular we compute the boundary associator. We also study codimension one defects (surface operators) in abelian Chern–Simons theories. As an application, we obtain a classification of such theories up to isomorphism, in general agreement with the work of Belov and Moore. 相似文献
995.
996.
Damage accumulation and creep-fatigue interaction are investigated and depicted for a continuum damage model proposed for the lifetime prediction of bonded steel joints with the structural adhesive BETAMATE 1496V™. The nonlinearity of damage accumulation is caused by the nonlinearity of damage interaction as a result of the identified parameters. In consequence of the nonlinear accumulation, the model allows for the loading sequence effect, which is the influence of the chronological order of the load values on the lifetime. Although the nonlinearity of the damage accumulation is not very pronounced, the model prediction is in good agreement with lifetimes from tests with service loading of the adhesive at hand. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
997.
The noise generation of turbulent flames is governed by temporal changes of the total flame volume due to local heat release
fluctuations. On the basis of the wave equation an expression for the relationship between the acoustic power and the heat
release fluctuations is derived and a correlation function is obtained which reveals that the sound pressure level of flames
is governed by the spatial coherence. Noise models rely on precise coherence information in terms of characteristic length
scales, which are the measure of the acoustic efficiency of the flame. Since the published length scale information is scarce
and inconsistent, length scales were measured for a number of laboratory flames using two measurement techniques developed
for this purpose: A planar LIF-system with a repetition rate of 1 kHz acquires the instantaneous flame front position and
heat release, whereas two chemiluminescence probes with an optics confining the measurement volume to a line of sight provide
further spatial correlation data. For all flames investigated the length scales are smaller than the height of the burner
exit annulus and they are of the order of the local flame brush thickness. Using the measured length scales, the coherent
volume and the efficiency of the noise generation are calculated, which are three orders of magnitude higher than measured.
However, the proper order of magnitude is obtained, if only the measured fluctuating part of the thermal power is used in
the model and if the periodic formation of local zones with heat release overshoot and deficit are properly incorporated. 相似文献
998.
A digital image measurement (DIM) system is used to study the plastic damage accumulation around a notch under conditions
of low-cycle fatigue. This system incorporates a contrast correlation method to evaluation the level of plastic damage at
each point of the studied area from two images acquired before and after the introduction of fatigue deformation. A compact
tension specimen of 304 stainless steel with a notch radius of 1 mm is analyzed during the stages of fatigue crack initiation
and growth. The results obtained using this measurement system are compared with those attained by means of a recrytallization
technique. 相似文献
999.
Anton Freund 《PAMM》2016,16(1):903-904
A fundamental question in mathematical logic asks: What are the minimal assumptions and deduction principles required to prove a particular theorem? Now consider the special case of a theorem that can be established by checking a finite number of decidable cases — think of a single instance of the finite Ramsey theorem. In this particular situation the answer to our question is trivial: The theorem can be demonstrated by an explicit verification, thus without the use of any “strong” proof principles. This answer, however, is not very satisfying: An explicit verification may be unfeasible if there is an enormous number of cases to check. At the same time there might be a short and meaningful proof using stronger proof methods. Such a situation suggests a modified question: What are the minimal assumptions and deduction principles required for a reasonably short proof of the given theorem? Our contribution explores this question for instances of the Paris-Harrington principle. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1000.
Nicusor Fifere Anton Airinei Constantin Gaina Viorica Gaina Mircea Grigoras 《Journal of fluorescence》2014,24(2):345-353
A fluorophore-photochrome system incorporating an aryleneimine type fluorophore and an azobismaleimide photochrome was developed and the photochemical properties of this system were investigated. The photoisomerization of trans-azoaromatic chromophore leads to the increase of the fluorescence intensity of fluorophore. The cis azobismaleimide isomers revert photochemically to the trans form and the emission intensity decreases. The fluorescence intensity of the imine fluorophore can be modulated under irradiation with UV and visible (436 nm) light due to reversible trans-cis-trans photoisomerization of azobismaleimide partner. The photoisomerization kinetics was obeyed a first-order relationship with a rate constant of 1.95?×?10?2 s?1 for azobismaleimide/imine fluorophore system and for polyazothioetherimide/imine derivative the kinetics was described by a biexponential equation. 相似文献