全文获取类型
收费全文 | 84031篇 |
免费 | 6804篇 |
国内免费 | 9599篇 |
专业分类
化学 | 36719篇 |
晶体学 | 1930篇 |
力学 | 2667篇 |
综合类 | 544篇 |
数学 | 4286篇 |
物理学 | 13354篇 |
综合类 | 40934篇 |
出版年
2024年 | 269篇 |
2023年 | 833篇 |
2022年 | 1652篇 |
2021年 | 1836篇 |
2020年 | 1958篇 |
2019年 | 1733篇 |
2018年 | 1507篇 |
2017年 | 1983篇 |
2016年 | 2446篇 |
2015年 | 2675篇 |
2014年 | 3797篇 |
2013年 | 4701篇 |
2012年 | 5768篇 |
2011年 | 4927篇 |
2010年 | 3965篇 |
2009年 | 4733篇 |
2008年 | 5127篇 |
2007年 | 6153篇 |
2006年 | 5632篇 |
2005年 | 5142篇 |
2004年 | 4705篇 |
2003年 | 4103篇 |
2002年 | 3960篇 |
2001年 | 2909篇 |
2000年 | 2742篇 |
1999年 | 2220篇 |
1998年 | 1925篇 |
1997年 | 1662篇 |
1996年 | 1501篇 |
1995年 | 1262篇 |
1994年 | 1183篇 |
1993年 | 977篇 |
1992年 | 906篇 |
1991年 | 627篇 |
1990年 | 555篇 |
1989年 | 455篇 |
1988年 | 413篇 |
1987年 | 279篇 |
1986年 | 193篇 |
1985年 | 158篇 |
1984年 | 124篇 |
1983年 | 86篇 |
1982年 | 86篇 |
1981年 | 81篇 |
1980年 | 72篇 |
1979年 | 76篇 |
1978年 | 54篇 |
1977年 | 61篇 |
1974年 | 43篇 |
1973年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
81.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献
82.
H.S RagabA Shehap M.S Abo EllilW.H Osman F.H Abd El-Kader 《Journal of Physics and Chemistry of Solids》2002,63(10):1839-1847
Ternary-phase ceramic system of Li2O Al2O3 4SiO2 doped with CuO, FeO and TiO2 has been prepared and subjected to dc electrical conductivity and thermally stimulated depolarization current (TSDC) measurements as a function of temperature (30-250 °C) and field strength. The electrical conductivity results are explained by assuming both ionic and electronic conduction mechanisms coexist with different contributions over the whole temperature range of experiments. TSDC spectra have been found to be characterized by a broad intense relaxation peak, which can be attributed to an ionic charge polarization. The broad relaxation transitions are apparently a result of the nonuniform nature of this process. Activation energies are calculated for both dc electrical conductivity and TSDC according to Arrhenius equation and initial rise method, respectively. 相似文献
83.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
84.
网络环境下的物理实验教学模式 总被引:4,自引:2,他引:2
目前网络环境下的物理实验教学新模式主要有物理实验专题网站、实验教学网络管理、实验类网络课程及远程控制物理实验.本文简要介绍了各个模式的特点及实施方法. 相似文献
85.
运用层析法、结晶法从狭苞橐吾全草中分离得到艾里莫芬烷型倍半萜8β-hydroxyeremophil-7(11)-ene-12,8α(4β,6α)-diolide化合物,并通过质谱、核磁共振氢谱和碳谱、氢核-氢核相关谱、异核多量子相干谱和异核多键相干谱进行结构鉴定.其单晶经X射线衍射测试表明,其晶体属正交晶系,空间群为P212121,晶胞参数为: a=6.8519(5),b=10.7191(8),c=18.5942(14)A,V=1365.67(18) A3,Z=4,Dc=1.354 mg/m3.F(000)=592,μ=0.101 mm-1 相似文献
86.
C. Maurel 《Surface science》2006,600(2):442-447
Light emitted in the tunneling junction of a scanning tunneling microscope has been used to establish the electrical characteristics of nanojunctions made of Au islands deposited on flat MoS2 surfaces. It is shown that these characteristics are those of rectifying contacts when the gold islands are isolated and that they evolve toward those of ohmic contacts when the island density increases. It is observed that the rectifying behavior also evolves over time as on infinite metal/semiconductor contacts. Using the STM tip, single gold islands can be manipulated on the MoS2 surface so that their electrical behavior can be changed depending on their position with regard to the other islands. 相似文献
87.
Th. Seyller K.V. Emtsev F. Speck A. Tadich J.D. Riley O. Rader A.M. Shikin 《Surface science》2006,600(18):3906-3911
Photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy were used to investigate the electronic and structural properties of graphite layers grown by solid state graphitization of SiC(0 0 0 1) surfaces. The process leads to well-ordered graphite layers which are rotated against the substrate lattice by 30°. On on-axis 6H-SiC(0 0 0 1) substrates we observe graphitic layers with up to several 100 nm wide terraces. ARUPS spectra of the graphite layers grown on on-axis 6H-SiC(0 0 0 1) surfaces are indicative of a well-developed band structure. For the graphite/n-type 6H-SiC(0 0 0 1) layer system we observe a Schottky barrier height of ?B,n = 0.3 ± 0.1 eV. ARUPS spectra of graphite layers grown on 8° off-axis oriented 4H-SiC(0 0 0 1) show unique replicas which are explained by a carpet-like growth mode combined with a step bunching of the substrate. 相似文献
88.
本文针对炉体温度对象建模较难,常规算法难以发挥良好的作用,阐述了将模糊控制与变结构控制结合控制的原理及方法,并成功的将该设计方法应用于成都砂轮厂树脂硬化炉的温度控制中,取得良好效果。 相似文献
89.
90.