全文获取类型
收费全文 | 2367篇 |
免费 | 104篇 |
国内免费 | 186篇 |
专业分类
化学 | 1302篇 |
晶体学 | 5篇 |
力学 | 882篇 |
综合类 | 65篇 |
数学 | 131篇 |
物理学 | 272篇 |
出版年
2024年 | 5篇 |
2023年 | 16篇 |
2022年 | 36篇 |
2021年 | 37篇 |
2020年 | 53篇 |
2019年 | 52篇 |
2018年 | 40篇 |
2017年 | 75篇 |
2016年 | 81篇 |
2015年 | 68篇 |
2014年 | 80篇 |
2013年 | 111篇 |
2012年 | 76篇 |
2011年 | 126篇 |
2010年 | 97篇 |
2009年 | 117篇 |
2008年 | 88篇 |
2007年 | 105篇 |
2006年 | 126篇 |
2005年 | 93篇 |
2004年 | 86篇 |
2003年 | 78篇 |
2002年 | 54篇 |
2001年 | 51篇 |
2000年 | 55篇 |
1999年 | 42篇 |
1998年 | 47篇 |
1997年 | 38篇 |
1996年 | 34篇 |
1995年 | 44篇 |
1994年 | 27篇 |
1993年 | 27篇 |
1992年 | 27篇 |
1991年 | 21篇 |
1990年 | 34篇 |
1989年 | 26篇 |
1988年 | 42篇 |
1987年 | 54篇 |
1986年 | 61篇 |
1985年 | 103篇 |
1984年 | 70篇 |
1983年 | 28篇 |
1982年 | 46篇 |
1981年 | 28篇 |
1980年 | 29篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1971年 | 1篇 |
排序方式: 共有2657条查询结果,搜索用时 31 毫秒
171.
Ferritic/martensitic (F/M) steels are good candidate structural materials for the future fusion reactors and spallation sources. However, irradiation of steels is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature (DBTT) and reduction of fracture toughness and creep resistance starting at low doses. Helium (He), produced by transmutation during the irradiation, also impacts mechanical properties. Numerous experimental and theoretical studies on the evolution of the microstructure of steels under irradiation have been conducted until now. We review the effect of irradiation-induced point defects and in particular of He on the mechanical properties of F/M steels. To cite this article: R. Schäublin et al., C. R. Physique 9 (2008). 相似文献
172.
Ibrahim I. A. ElSersy H. H. Abadir M. F. 《Journal of Thermal Analysis and Calorimetry》2004,76(3):713-718
Thermal analysis was first used to investigate the pattern of dissociation of hydrated ordinary Portland cement. Portlandite
(Ca(OH)2) decomposes at about 500°C. This was confirmed by kinetic calculations. Thermal analysis was then performed to establish
the effect of varying the cement content on the percent mass loss associated with the decomposition of Ca(OH)2 in cement mortar cured for 28 days. An increasing relation was obtained. Standard concrete cubes were then prepared with
cement contents ranging from 200 to 450 kg m-3. The loss in mass on heating, up to 750°C, of concrete samples cured for 28 days was then related to the cement content in
concrete. The relation obtained was tested for concrete cubes of known cement content and found to be in better agreement
than the results obtained by conventional chemical analysis. This method can be used for an approximate determination of the
cement content in concrete.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
173.
174.
175.
176.
177.
178.
Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials. 相似文献
179.
Ollerenshaw J McClung RE 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,144(2):255-265
A magnetic resonance imaging method is presented for imaging of heterogeneous broad linewidth materials. This method allows for distortionless relaxation weighted imaging by obtaining multiple phase encoded k-space data points with each RF excitation pulse train. The use of this method, turbo spin echo single-point imaging-(turboSPI), leads to decreased imaging times compared to traditional constant-time imaging techniques, as well as the ability to introduce spin-spin relaxation contrast through the use of longer effective echo times. Imaging times in turboSPI are further decreased through the use of low flip angle steady-state excitation. Two-dimensional images of paramagnetic doped agarose phantoms were obtained, demonstrating the contrast and resolution characteristics of the sequence, and a method for both amplitude and phase deconvolution was demonstrated for use in high-resolution turboSPI imaging. Three-dimensional images of a partially water-saturated porous volcanic aggregate (T(2L) approximately 200 ms, Deltanu(1/2) approximately 2500 Hz) contained in a hardened white Portland cement matrix (T(2L) approximately 0.5 ms, Deltanu(1/2) approximately 2500 Hz) and a water-saturated quartz sand (T(2) approximately 300 ms, T(2)(*) approximately 800 microseconds) are shown. 相似文献
180.
I. I. Bashter 《辐射效应与固体损伤》2013,168(3-4):351-364
Abstract The fast neutron and gamma ray spectra measured behind different thickness of steel scrap concrete with density of 4 g/cm3 have been studied. The mix proportions by weight of this type of concrete were 1 cement: 6.89 steel scrap: 2.9 sand and 0.5 Water. Comparison with a standard ordinary concrete of density 2.3 g/cm3 have been carried out. The measurements were made using a collimated beam of both gamma rays and neutrons emitted from one of the horizontal channel of the Egyptian Research Reactor-1. A fast neutron and gamma ray spectrometer with a stilbene crystal was used to measure the spectra of fast neutrons and gamma rays. Pulse shape discrimination using the zero cross over technique was used to separate the photon pulses from the electron pulses. The equation due to Schmidt has been modified and applied for determining the neutron effective removal cross sections (~R) for steel scrap, ordinary, hematite–serpentine, ilmenite–limonite and ilmenite concretes. This equation gives results which are in good agreement with the measured values. The derived empirical equation in a previous work to calculate the neutron integral flux behind different thicknesses of different types of concretes, gives good results for steel scrap concrete under investigation comparing with the corresponding experimental data. Total neutron macroscopic cross sections, linear attenuation coefficients for gamma rays and the half-value layers for both radiations at different energies have been obtained for steel scrap concrete and comparing with the corresponding values of ordinary concrete. The results show that steel scrap concrete is better than ordinary, hematite–serpentine, ilmenite–limonite and ilmenite concretes from the radiation shielding point of view. 相似文献