全文获取类型
收费全文 | 342704篇 |
免费 | 5185篇 |
国内免费 | 1099篇 |
专业分类
化学 | 187331篇 |
晶体学 | 4591篇 |
力学 | 13633篇 |
综合类 | 7篇 |
数学 | 41492篇 |
物理学 | 101934篇 |
出版年
2020年 | 2359篇 |
2019年 | 2245篇 |
2018年 | 2310篇 |
2017年 | 2208篇 |
2016年 | 4603篇 |
2015年 | 3842篇 |
2014年 | 5109篇 |
2013年 | 15549篇 |
2012年 | 11836篇 |
2011年 | 14737篇 |
2010年 | 9041篇 |
2009年 | 8873篇 |
2008年 | 13672篇 |
2007年 | 13850篇 |
2006年 | 13360篇 |
2005年 | 12188篇 |
2004年 | 10867篇 |
2003年 | 9563篇 |
2002年 | 9382篇 |
2001年 | 10651篇 |
2000年 | 8210篇 |
1999年 | 6500篇 |
1998年 | 5234篇 |
1997年 | 5083篇 |
1996年 | 5219篇 |
1995年 | 4752篇 |
1994年 | 4544篇 |
1993年 | 4423篇 |
1992年 | 4847篇 |
1991年 | 4585篇 |
1990年 | 4318篇 |
1989年 | 4093篇 |
1988年 | 4372篇 |
1987年 | 4036篇 |
1986年 | 3899篇 |
1985年 | 5680篇 |
1984年 | 5753篇 |
1983年 | 4701篇 |
1982年 | 5080篇 |
1981年 | 5129篇 |
1980年 | 4864篇 |
1979年 | 4953篇 |
1978年 | 4983篇 |
1977年 | 4935篇 |
1976年 | 4879篇 |
1975年 | 4751篇 |
1974年 | 4595篇 |
1973年 | 4777篇 |
1972年 | 2678篇 |
1971年 | 2002篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
991.
We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sn, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. 相似文献
992.
993.
Karla J. Moreno Miroslaw Maczka Ulises Amador 《Journal of solid state chemistry》2006,179(12):3805-3813
The structural and microstructural characteristics of metastable Gd2(Ti1−yZry)2O7 powders prepared by mechanical milling have been studied by a combination of XRD and Raman spectroscopy. Irrespective of their Zr content, as-prepared powder phases present an anion-deficient fluorite-type of structure as opposed to the pyrochlore equilibrium configuration obtained for the same solid solution by other synthetic routes. These fluorites are stable versus thermal activation, at least up to temperatures of 800 °C. For the Ti-rich compositions, thermal treatments at higher temperatures facilitate the rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects whereas for compositions with high Zr content, the fluorite crystal structure is retained even at temperatures as high as 1200 °C. Interestingly enough, transient pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process. 相似文献
994.
Summary Vezetéknév 相似文献
995.
The system design must be started from the concept with low cost and high performance. In this point, the topological shape of the structure is very important in the view of the structural rigidities and light-weight design.In this paper, the optimization methodology is presented in the design stage of the large optical mirror. We obtain the optimal layout through the topology optimization and then design the details through the size or shape optimization for structural rigidity. 相似文献
996.
Statistical Analysis of the Effect of Mineral Admixtures on the Strength of the Interfacial Transition Zone 总被引:1,自引:0,他引:1
Microscopic evidence indicates that the thickness of the interfacial transition zone (ITZ) between the aggregate and the cement paste is modified when mineral admixtures partially replace portland cement. Unfortunately, there is limited information on the significance of these microscopic modifications to the mechanical properties of the ITZ. This research reports experimental results on the shear and tensile strength of the ITZ as affected by the addition of the following mineral admixtures: silica fume, fly ash, and natural pozzolan. Statistical analysis was used to identify the significant parameters affecting the tensile and shear strength of the ITZ. Experimental results indicate that not only does the incorporation of silica fume and fly ash increase the strength of the ITZ, these mineral admixtures have a greater influence on the strength increase of the ITZ than in the strength increase of the cement paste. 相似文献
997.
The state of the art iterative method for solving large linear systems is the conjugate gradient (CG) algorithm. Theoretical convergence analysis suggests that CG converges more rapidly than steepest descent. This paper argues that steepest descent may be an attractive alternative to CG when solving linear systems arising from the discretization of ill-posed problems. Specifically, it is shown that, for ill-posed problems, steepest descent has a more stable convergence behavior than CG, which may be explained by the fact that the filter factors for steepest descent behave much less erratically than those for CG. Moreover, it is shown that, with proper preconditioning, the convergence rate of steepest descent is competitive with that of CG.This revised version was published online in October 2005 with corrections to the Cover Date. 相似文献
998.
S Watanabe R R Anderson S Brorson G Dalickas J G Fujimoto T J Flotte 《Photochemistry and photobiology》1991,53(6):757-762
Threshold radiant exposures for grossly apparent immediate whitening and ultrastructural alterations of melanosomes in black guinea pig skin were determined for a series of red visible laser pulses ranging from 4 x 10(-4) to 6.5 x 10(-14) s. Threshold exposures for melanosomal injury were found to be independent of pulsewidth when the pulsewidths were below the estimated thermal relaxation time of melanosomes. Threshold radiant exposures for melanosomal injury were found to increase when the pulsewidths were approximately equal to or above the thermal relaxation time of melanosomes. At longer pulse durations, fracturing of melanosomes was not observed despite the longer exposures necessary for injury. Instead, perimelanosomal vacuoles were noted. These findings are consistent with the theory of selective photothermolysis and provide evidence for the thermal initiation of melanosomal disruption. 相似文献
999.
1000.
Standard ODE methods such as linear multistep methods encounter difficulties when applied to differential-algebraic equations (DAEs) of index greater than 1. In particular, previous results for index 2 DAEs have practically ruled out the use of all explicit methods and of implicit multistep methods other than backward difference formulas (BDFs) because of stability considerations. In this paper we embed known results for semi-explicit index 1 and 2 DAEs in a more comprehensive theory based on compound multistep and one-leg discretizations. This explains and characterizes the necessary requirements that a method must fulfill in order to be applicable to semi-explicit DAEs. Thus we conclude that the most useful discretizations are those that avoid discretization of the constraint. A freer use of e.g. explicit methods for the non-stiff differential part of the DAE is then possible.Dedicated to Germund Dahlquist on the occasion of his 70th birthdayThis author thanks the Centro de Estadística y Software Matemático de la Universidad Simón Bolivar (CESMa) for permitting her free use of its research facilities.Partial support by the Swedish Research Council for Engineering Sciences TFR under contract no. 222/91-405. 相似文献