全文获取类型
收费全文 | 734篇 |
免费 | 27篇 |
国内免费 | 4篇 |
专业分类
化学 | 572篇 |
晶体学 | 22篇 |
力学 | 22篇 |
数学 | 44篇 |
物理学 | 105篇 |
出版年
2024年 | 3篇 |
2023年 | 3篇 |
2022年 | 32篇 |
2021年 | 40篇 |
2020年 | 25篇 |
2019年 | 28篇 |
2018年 | 18篇 |
2017年 | 10篇 |
2016年 | 27篇 |
2015年 | 16篇 |
2014年 | 34篇 |
2013年 | 62篇 |
2012年 | 62篇 |
2011年 | 66篇 |
2010年 | 41篇 |
2009年 | 31篇 |
2008年 | 40篇 |
2007年 | 27篇 |
2006年 | 27篇 |
2005年 | 23篇 |
2004年 | 25篇 |
2003年 | 15篇 |
2002年 | 15篇 |
2001年 | 3篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 1篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有765条查询结果,搜索用时 15 毫秒
81.
The sonolysis of an aqueous solution of Fe(CO)(5) in the presence of sodium dodecyl sulfate leads to the formation of a stable hydrosol of amorphous Fe(3)O(4) nanoparticles. The amorphicity of iron oxide nanoparticles was determined by X-ray diffraction and differential scanning calorimetry. The nanoparticles were characterized by elemental analysis, EDX, transmission electron microscopy, dynamic light scattering, Raman spectroscopy, XPS, and spot test. 相似文献
82.
R. M. Shubietah A. Z. Abu Zuhri A. G. Fogg 《Fresenius' Journal of Analytical Chemistry》1994,348(11):754-757
Guanine is determined at the 5.0×10–10 –2.0×10–7 mol/l level by differential-pulse adsorptive stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper (II) complex at –0.21 V vs. Ag-AgCl electrode. The optimum analytical conditions were found to be Britton-Robinson buffer solution (pH 4.8), an accumulation potential of 0.0 V and an accumulation time of 3 min. Under these conditions, the detection limit is 5.0×10–10 mol/l and the relative standard deviation 2.6% for 1.0×10–7 mol/l guanine. The method is compared with the previous voltammetric methods. The presence of some purine derivatives does not interfere. 相似文献
83.
Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr2+ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr2+ content on optical parameters of (NH4)2ZnCl4: x Sr2+ single crystals (x?=?0.00, 0.020, 0.039, 0.087 or 0.144?wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH4)2ZnCl4: x Sr2+ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (E g ) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x?=?0.087 or 0.144?wt.% Sr2+. The results can be discussed on the basis of γ-irradiation-induced defects and Sr2+ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed. 相似文献
84.
This study evaluates the effects of gamma radiation on structure, electrochemical corrosion behavior and Vickers hardness of commercial dental Nikkeli–Kromi–Polttosekoitus [Ni65.2Cr22.5Mo9.5X2.8 (X=Nb, Si, Fe and Mn)] alloy. The corrosion rate of Ni65.2Cr22.5Mo9.5X2.8 (X=Nb, Si, Fe and Mn) alloy with 0.5 M HCl is increased with increasing the exposure rate of gamma radiation. The corrosion resistance of Ni65.2Cr22.5Mo9.5X2.8 (X=Nb, Si, Fe and Mn) is varied and reaches a minimum value at 30 KGy. The corrosion potential value also is varied and reaches its highest value at 30 KGy. The Vickers hardness value of Ni65.2Cr22.5Mo9.5X2.8 (X=Nb, Si, Fe and Mn) alloy is decreased by increasing the gamma radiation dose. Also it is obvious from our results that the effects of gamma radiation at the surface are much higher as compared with deeper parts and the structure of the alloy is changed due to its exposure to gamma radiation. 相似文献
85.
A.Y. Bani Hashim N.A. Abu Osman W.A.B. Wan Abas L. Abdul Latif 《Applied Mathematical Modelling》2013,37(1-2):258-264
This study examined the foot biomechanics that are utilized when foot is in contact with the ground during a stance phase. The purpose of the study was to investigate the normalized ground reaction forces that impacted certain sections and points on the footprint, and to identify patterns in the degrees to which these forces occurred. Foot was modeled in such a manner that a vertex represented a bone and an edge represented a joint, and a graph that depicted the foot was created. Twelve nodes were marked on the footprint and these were linked together to create a gait path. By fusing the graph and the gait path and by manipulating the mathematical models, a profile for an ideal bipedal walking locomotion was developed. A male subject performed bipedal walking through a force plate system in order to obtain the profile that reflected actual bipedal walking. The actual and the proposed profiles were compared and there were significant similarities between the two profiles, with both exhibiting an double-bump pattern. It is therefore viable that the approximation techniques proposed in this work may provide an alternative means over the application of a force plate system to generate a profile for bipedal walking. However, the accuracy and reliability of the results yielded from this technique need further investigation. 相似文献
86.
Isothiochromene[3,4‐d] pyrimidine derivatives 2 , 3 , and 4a , b were synthesized from the reaction of 3‐amino‐1‐(pyridin‐4‐yl)‐5‐(pyridin‐4‐ylmethylene)‐5,6,7,8‐tetrahydro‐1H‐isothiochromene‐4‐carbonitrile 1 with acetic anhydride, formamide, urea, or thiourea in appropriate experimental conditions. Combination of 1 with carbon acid derivatives afforded isothiochromene [3,4‐b]pyridine 6 – 8 in good yield. A simple approach for N‐substituted fused isothiochromene derivatives has been explored. A POCl3‐mediated direct amination of isothiochromene amide 2 with NH2‐heterocycles, secondary amines, and carbohydrazides is described and compared with classical method, yielding 10 – 14 . The structures of the newly synthesized compounds were elucidated on the basis of elemental analysis, and spectral data. 相似文献
87.
S. Hassan F. Adam M.R. Abu Bakar S.K. Abdul Mudalip 《Journal of Saudi Chemical Society》2019,23(2):239-248
Solubility of active pharmaceutical ingredient (API) in solvents is very important for drug development and manufacturing. Solubility data may provide further information such as thermochemical properties and intermolecular interactions that may lead to a better understanding of the formation of API crystals. In this study, solubility of ascorbic acid was determined by gravimetric method in four different commonly used polar protic solvents: water, methanol, ethanol and 2-propanol. The solubility of ascorbic acid crystal was also predicted using Conductor-like Screening Model – Realistic Solvent (COSMO-RS) approach. In this computational analysis, the generated ΔG values are based on the solubilities that were experimentally obtained to simulate the intermolecular forces. The intermolecular interaction data from COSMO-RS provide an insight into the relationship between the intermolecular interactions and its crystal habit across four different polar protic solvents. The habit of the crystals was then determined using light microscopy and scanning electron microscopy techniques, while the polymorphic form of the crystals was identified by X-ray powder diffraction and single X-ray diffraction techniques. The solubility and characterization data showed that the solvents with high polarity increased the solubility of ascorbic acid. The data also showed that different solvent polarity influenced the crystal habit, but did not change the crystal structure to form a new polymorph. 相似文献
88.
Enzymatic inhibitory activity of Ficus deltoidea leaf extract on matrix metalloproteinase-2, 8 and 9
Amirul Ridzuan Abu Bakar Adiratna Mat Ripen Amir Feisal Merican 《Natural product research》2019,33(12):1765-1768
Dysregulation of matrix metalloproteinases (MMPs) activity is known in many pathological conditions with which most of the conditions are related to elevate MMPs activities. Ficus deltoidea (FD) is a plant known for its therapeutic properties. In order to evaluate the therapeutic potential of FD leaf extract, we study the enzymatic inhibition properties of FD leaf extract and its major bioactive compounds (vitexin and isovitexin) on a panel of MMPs (MMP-2, MMP-8 and MMP-9) using experimental and computational approaches. FD leaf extract and its major bioactive compounds showed pronounced inhibition activity towards the MMPs tested. Computational docking analysis revealed that vitexin and isovitexin bind to the active site of the three tested MMPs. We also evaluated the cytotoxicity and cell migration inhibition activity of FD leaf extract in the endothelial EA.hy 926 cell line. Conclusively, this study provided additional information on the potential of FD leaf extract for therapeutical application. 相似文献
89.
90.
Nor Aiyyuhal Jemali Hasan Abu Kassim V. Radhika Devi Keshav N. Shrivastava 《Journal of Non》2008,354(15-16):1744-1750
We have used the density functional theory to make the models of GexSe1?x glass for which the energy is a minimum. The clusters, Ge2Se2, Ge2Se3, Ge3Se, Ge3Se2, Ge4Se, GeSe3, GeSe4, chain mode zig-zag Ge4Se3, corner sharing GeSe4, and edge sharing Ge2Se6, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, GexSe1?x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, ν < 100 cm?1, are found. The corner sharing GeSe4 has low frequency modes at 54 cm?1 and 93 cm?1 whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C4 symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge3, Ge4Se3, Ge2Se3, GeSe3 and Ge3Se2 are actually present in the Raman spectra of the glass, GexSe1?x(0 < x < 0.3). 相似文献