全文获取类型
收费全文 | 26027篇 |
免费 | 3981篇 |
国内免费 | 3896篇 |
专业分类
化学 | 15448篇 |
晶体学 | 377篇 |
力学 | 1570篇 |
综合类 | 189篇 |
数学 | 1553篇 |
物理学 | 14767篇 |
出版年
2024年 | 77篇 |
2023年 | 262篇 |
2022年 | 637篇 |
2021年 | 675篇 |
2020年 | 843篇 |
2019年 | 705篇 |
2018年 | 721篇 |
2017年 | 785篇 |
2016年 | 961篇 |
2015年 | 891篇 |
2014年 | 1209篇 |
2013年 | 2242篇 |
2012年 | 1428篇 |
2011年 | 1567篇 |
2010年 | 1339篇 |
2009年 | 1724篇 |
2008年 | 1686篇 |
2007年 | 1858篇 |
2006年 | 1813篇 |
2005年 | 1384篇 |
2004年 | 1282篇 |
2003年 | 1228篇 |
2002年 | 1000篇 |
2001年 | 905篇 |
2000年 | 864篇 |
1999年 | 798篇 |
1998年 | 692篇 |
1997年 | 584篇 |
1996年 | 526篇 |
1995年 | 492篇 |
1994年 | 423篇 |
1993年 | 336篇 |
1992年 | 368篇 |
1991年 | 265篇 |
1990年 | 179篇 |
1989年 | 164篇 |
1988年 | 159篇 |
1987年 | 123篇 |
1986年 | 102篇 |
1985年 | 95篇 |
1984年 | 75篇 |
1983年 | 50篇 |
1982年 | 65篇 |
1981年 | 52篇 |
1980年 | 50篇 |
1979年 | 53篇 |
1978年 | 24篇 |
1977年 | 32篇 |
1976年 | 30篇 |
1974年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
251.
Abstract The emission spectra for LaOBr : Eu were measured at pressures up to 13 GPa and room temperature. The pressure dependences of levels of 7F0,1,2,3,4 and 5D0,1,2 are given. The crystal field parameters Bk q were computed by fitting the experimental levels. The strength of crystal field decreases with increasing pressure. A brief discussion on the observed phenomena is presented. 相似文献
252.
Die Gammaspektren von Radionuklidgemischen, die als Folge der Folge der momentanen Spaltung von235 U mit thermischen Neutronen, 235U und 239 Pu mit Spaltneutronen und 238 U mit 14-MeV -Neutronen entstehen, wurden berechnet. Die Angaben über Zerfallsreihen, Spaltausbeuten, Halbwertszeiten und Nuklidgammaspektren für etwa 200 Radionuklide basieren vol allem auf einer Zusammenstellung von Davies. Es sind die Gammaspektren, die Gammaemissionsraten, die Expositionsdosisleistungen und die mittleren Energien für ein Alter der Spaltproduktgemische von einer Minute bis zu 100 Jahren angegeben. 相似文献
253.
The application of polarized X-rays in energy dispersive X-ray fluorescence analysis (EDXRF) is prooved to be one method for improvement the sensitivity of trace analysis at rock- and soil-samples. The use of a high power X-ray tube in connection with an amorphous Barkla-scattering target for polarization makes possible the increase of peak-to-background ratios in a wide energy range (5 keV < E < 40 keV) in comparison to direct excitation. The influence of the polarization on the peak-to-background ratios at trace analysis at geological samples will be shown. 相似文献
254.
《Composite Interfaces》2013,20(5-6):349-361
Properties of the fiber/matrix interface in SiO2/epoxy and SiC/epoxy composite are investigated using the slice compression test (SCT) for the single fiber, where the specimen is loaded and unloaded between a plate which has different mechanical properties. It is found that the interfacial debonding proceeds from the polished surface at a soft plate side and that the fiber protrusion occurs after unloading. The fiber-protrusion length is directly measured at each applied stress level using a scanning electron microscope. Interfacial shear-sliding stress is obtained based on the constant shear-sliding stress analysis employing the obtained protrusion length. It is demonstrated that the value of interfacial shear-sliding stress shows good agreement with that obtained from another technique, the push-out test, on the same system. The relation between the fiber-protrusion length and applied stress is proportional to a certain extent. From this result, it is analytically pointed out that the applied stress has a limiting value in this SCT because of Poisson's effect. Also, two interfacial debonding criteria, which are determined analytically for the PMC, are discussed. 相似文献
255.
The structural and optical properties of thin films of polyimide composites with nanosilica particle content of 15?wt%, prepared via sol–gel process, were studied as a function of the gamma dose. The resultant effect of gamma irradiation on the properties of polyimide/silica nanocomposite has been investigated using X-ray diffraction and UV spectroscopy. Absorption and reflectance spectra were collected by a spectrophotometer giving UV-radiation of wavelength range 200–800?nm. The optical data obtained were analyzed and the calculated values of the optical energy gap exhibited gamma dose dependence. The direct optical energy gap for the nonirradiated polyimide/silica nanocomposite is about 2.41?eV, and increases to a value of 2.65?eV when irradiated with gamma doses up to 300?kGy. It was found that the calculated refractive index of the polyamide/silica increases with the gamma dose in the range 50–300?kGy. 相似文献
256.
The optical absorption spectra (d-d transition bands) and covalent effect of Ni2+ ions in octahedral sites of Ca3Sc2Ge3O12 crystal have been investigated by the full energy matrix based on the two spin–orbit coupling parameters model. The bond length of octahedral site is Ri?=?2.19 Å, which can be determined by the cubic crystal-field parameter and optical spectral data. The lattice distortion of the Ni2+ center in Ca3Sc2Ge3O12 crystal is also obtained from the calculations. In addition, the result has shown that the covalent effect of Ni2+ ion in the octahedral site of Ga3Sc2Ge3O12 is obvious and cannot be ignored. The calculated d-d transition bands agree well with that of the experimental findings, suggesting that the present methods can explain reasonably the optical spectral data and covalent effect of 3d8 ions in octahedral lattices. 相似文献
257.
Anisotropic energy distribution of sputtered atoms induced by low energy heavy ion bombardment 总被引:1,自引:0,他引:1
Lai Zhang 《辐射效应与固体损伤》2013,168(8):337-347
The theory of anisotropic sputtering published in Phys. Rev. B 71(2), 026101 (2005) and Radiat. Effects Defects Solids 159(5), 301 (2004) has been modified and used to calculate the sputtering yield energy distributions for copper, tungsten, and aluminum targets bombarded by low-energy argon ion. As usual, the electronic stopping is ignored in the analysis. The present theory (modified Sigmund’s theory) has been shown to fit the corresponding experimental results of sputtering yield energy distributions well, except for the cases where the larger ion incident angle and larger sputtering emission angles were considered. The larger discrepancy between the present theory and the experimental result in the latter cases is probably due to the influence of direct recoil atoms on the energy spectrum. Compared with Falcone’s analytical theory, the present theory can reproduce much better experimental results of sputtering phenomena. The fact clearly demonstrates the intrinsic relation between the ion–energy dependence of the total sputtering yield and the sputtering yield energy distribution and suggests the great importance of momentum deposited on the target surface in the physical sputtering 相似文献
258.
Effects of the high pressure on the emission spectra of Eu3+-activated L-EuBO3 were considered at room temperature up to 100 kbar. The position of five 0–2 lines in the 5D0→7F2 transition region was determined. The pressure does not have the same effect on all these lines. In four of them, high pressure induced a red shift with different shift rates:+0.0022,+0.0035,+0.0034 and+0.0027 nm kbar?1, respectively, whereas in the last one, high pressure induced a blue shift with shift rate?0.0034 nm kbar?1. Possible reasons for the mentioned pressure effects on the line positions were considered. 相似文献
259.
The performance and detailed near-wake characteristics of a vertical axis, cross-flow turbine (CFT) of aspect ratio 1 were measured in a large cross-section towing tank. The near-wake at one turbine diameter downstream was examined using acoustic Doppler velocimetry, where essential features regarding momentum, energy, and vorticity are highlighted. Dominant scales and their relative importance were investigated and compared at various locations in the measurement plane. Estimates for the terms in the mean streamwise momentum and mean kinetic energy equation were computed, showing that the unique mean vertical velocity field of this wake, characterised by counter-rotating swirling motion, contributes significantly more to recovery than the turbulent transport. This result sheds light on previous CFT studies showing relatively fast downstream wake recovery compared to axial-flow turbines. Finally, predictions from a Reynolds-averaged Navier–Stokes simulation with the commonly used actuator disk model were compared with the experimental results, evaluating its use as an engineering tool for studying flow in CFT arrays. Unsurprisingly, the model was not able to predict the near-wake structure accurately. This comparison highlights the need for improved parameterised engineering models to accurately predict the near-wake physics of CFTs. 相似文献
260.
We report observations of the Meyer–Neldel rule for the non-isothermal crystallization of glassy Se85? x Te15Sb x (x =?0, 2, 4, 6, 8, 10) alloys. We found a strong co-relation between the pre-exponential factor K 0 of the rate constant K(T) for crystallization and the activation energy of crystallization E c. This indicates the presence of a compensation effect for the non-isothermal crystallization process in this glassy system. The composition dependence of the crystallization temperature T c and the activation energy for crystallization E c is discussed. 相似文献