全文获取类型
收费全文 | 4549篇 |
免费 | 693篇 |
国内免费 | 514篇 |
专业分类
化学 | 1268篇 |
晶体学 | 13篇 |
力学 | 73篇 |
综合类 | 33篇 |
数学 | 377篇 |
物理学 | 2190篇 |
无线电 | 1802篇 |
出版年
2024年 | 7篇 |
2023年 | 42篇 |
2022年 | 67篇 |
2021年 | 91篇 |
2020年 | 91篇 |
2019年 | 125篇 |
2018年 | 102篇 |
2017年 | 122篇 |
2016年 | 137篇 |
2015年 | 160篇 |
2014年 | 302篇 |
2013年 | 326篇 |
2012年 | 291篇 |
2011年 | 304篇 |
2010年 | 291篇 |
2009年 | 293篇 |
2008年 | 325篇 |
2007年 | 326篇 |
2006年 | 340篇 |
2005年 | 250篇 |
2004年 | 240篇 |
2003年 | 239篇 |
2002年 | 174篇 |
2001年 | 145篇 |
2000年 | 180篇 |
1999年 | 120篇 |
1998年 | 115篇 |
1997年 | 81篇 |
1996年 | 83篇 |
1995年 | 68篇 |
1994年 | 43篇 |
1993年 | 44篇 |
1992年 | 47篇 |
1991年 | 41篇 |
1990年 | 23篇 |
1989年 | 28篇 |
1988年 | 18篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有5756条查询结果,搜索用时 15 毫秒
101.
《Physics and Chemistry of Liquids》2012,50(3):149-166
Abstract The structure factor for liquid lead at the temperatures 613 K, 643 K, 863 K and 1163 K are measured by neutron diffraction. Extensive comparisons to earlier measurements are made as well as comparisons to different hard-core models. 相似文献
102.
A. Pélisson‐Schecker H. J. Hug J. Patscheider 《Surface and interface analysis : SIA》2012,44(1):29-36
Accurate charge referencing in XPS of insulating specimens is a delicate issue. This difficulty is illustrated in the case of Al‐Si‐N composite thin films deposited by reactive magnetron sputtering with variable composition from pure aluminum nitride to pure silicon nitride. The samples were mounted with Au‐coated metallic clamps. Argon sputter cleaning was required to remove a surface native oxide before analysis. For charge referencing implanted argon atoms from the sputter gas and a small amount of gold re‐deposited from the metallic clamps onto the specimen surface during sputter cleaning were evaluated. For the argon atoms, a surprisingly large chemical shift (~1 eV) and a significant peak broadening (0.6 eV) of the Ar 2p3/2 photoelectron line were found with varying the Si content of the films. This could be related to chemical and structural changes of the Al‐Si‐N films. Hence implanted argon could not be used for charge referencing of Al‐Si‐N samples. In contrast to the implanted argon, the Au 4f7/2 line width of the gold re‐deposited onto the sample surface did not depend on the Si content of Al‐Si‐N films. A constant energy shift (~1.2 eV) of the Au 4f7/2 line as compared with bulk gold was, however, found, which was related to the size of gold particles formed on the insulating films. Therefore gold could be reliably used to study chemical shifts of sample‐relevant species in Al‐Si‐N films, but the absolute binding energies of Al 2p, Si 2p and N 1s photoelectrons could not be determined. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
103.
To evaluate the reactivity of new difluroroaromatic compounds in nucleophilic substitution, the positive charges on carbon
atoms of C−F bonds were calculated using the quantum-chemical semiempirical PM3 method. A correlation between the charges
calculated and the chemical shifts in the19F NMR spectra was established.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 623–625, April, 1998. 相似文献
104.
Dobson G Murrell J Despeyroux D Wind F Tabet JC 《Journal of mass spectrometry : JMS》2004,39(11):1295-1304
Electrospray ionization mass spectra obtained from different scan directions are observed to be dependent on the axial modulation potential amplitudes used for resonant ejection and on the positive deviation caused by higher even-multipole fields present in most commercial ion traps. The axial modulation voltage influences the dissociation of ions during resonant ejection and the observed mass shifts. The higher even-multipole fields in commercial ion traps are known to influence resonant ejection from the ion trap and can cause a loss in mass resolution for peaks in reverse scan mass spectra compared with that obtained by the forward scan. However, along with the dissociation of ions during resonant ejection causing a loss in resolution, the possibility of resolving an isotopic distribution is also shown to be influenced by the mass shifts caused by the space charge. These mass shifts differ depending on the scan direction employed. A significant loss in resolution can also result from resonant ejection using non-optimal axial modulation voltages. We also present results showing the ejection of ions at betaz = 1/2 using the reverse scan mode without the axial modulation voltage. Ion ejection at betaz = 1/2 is uncommon in commercial (stretched ion traps) with the conventional analytical scan without the use of a frequency of the axial modulation voltage corresponding to this non-linear resonance. 相似文献
105.
Mikhail I. Kodess Marina A. Ezhikova Galina L. Levit Victor P. Krasnov Valery N. Charushin 《Journal of organometallic chemistry》2005,690(11):2766-2768
Experimental conditions for determination of enantiomeric composition of 1-substituted 3-aminocarboranes by 1H and 13C NMR spectroscopy using chiral shift reagent Eu(hfc)3 have been found. 相似文献
106.
107.
利用 1H NMR方法, 研究了高浓度的P123(PEO20PPO70PEO20)在重水溶液中的溶胶-凝胶转变过程. 升高温度使得体系发生溶胶-凝胶转变, 进一步升高温度, 体系发生凝胶-溶胶转变. 通过对不同质子基团的谱线宽度和化学位移偏移的分析, 同时结合流变学频率扫描和同步辐射(SR)研究, 发现质量分数为30%的P123的重水溶液在凝胶化过程中, 结构经历了由立方相(cubic)-六角柱状相(hcp)-层状相(lamellar)的转变过程, 其中立方相为面心立方(fcc)和六角密堆积球状相(hcps)的混合相. 高温时从凝胶到溶胶的转变主要体现为P123形成富集区与水发生相分离的过程. 相似文献
108.
Mika Tiainen Hannu Maaheimo Pasi Soininen Reino Laatikainen 《Magnetic resonance in chemistry : MRC》2010,48(2):117-122
The one‐ and two‐bond 13C isotope shifts, typically ?1.5 to ?2.5 ppb and ?0.7 ppb respectively, in non‐cyclic aliphatic systems and up to ?4.4 ppb and ?1.0 ppb in glucose cause effects that need to be taken into account in the adaptive NMR spectral library‐based quantification of the isotopomer mixtures. In this work, NMR spectral analyses of some 13C‐labelled amino acids, D ‐glucose and other small compounds were performed in order to obtain rules for prediction of the 13C isotope effects on 1H chemical shifts. It is proposed that using the additivity rules, the isotope effects can be predicted with a sufficient accuracy for amino acid isotopomer applications. For glucose the effects were found strongly non‐additive. The complete spectral analysis of fully 13C‐labelled D ‐glucose made it also possible to assign the exocyclic proton signals of the glucose. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
109.
Mikhail Elyashberg Kirill Blinov Yegor Smurnyy Tatiana Churanova Antony Williams 《Magnetic resonance in chemistry : MRC》2010,48(3):219-229
The accuracy of 13C chemical shift prediction by both DFT GIAO quantum‐mechanical (QM) and empirical methods was compared using 205 structures for which experimental and QM‐calculated chemical shifts were published in the literature. For these structures, 13C chemical shifts were calculated using HOSE code and neural network (NN) algorithms developed within our laboratory. In total, 2531 chemical shifts were analyzed and statistically processed. It has been shown that, in general, QM methods are capable of providing similar but inferior accuracy to the empirical approaches, but quite frequently they give larger mean average error values. For the structural set examined in this work, the following mean absolute errors (MAEs) were found: MAE(HOSE) = 1.58 ppm, MAE(NN) = 1.91 ppm and MAE(QM) = 3.29 ppm. A strategy of combined application of both the empirical and DFT GIAO approaches is suggested. The strategy could provide a synergistic effect if the advantages intrinsic to each method are exploited. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
110.
Hsin‐Chung Cheng Chau‐Hsiang Wang Chiung‐Fang Huang Yung‐Kang Shen Yi Lin Dong‐Yea Sheu Yi‐Han Hu 《先进技术聚合物》2010,21(9):632-639
In this study, master of the microlens arrays is fabricated using micro dispensing technology, and then electroforming technology is employed to replicate the Ni mold insert of the microlens arrays. Finally, micro hot embossing is performed to replicate the molded microlens arrays from the Ni mold insert. The resin material is used as the dispensing material, which is dropped on a glass substrate. The resin is exposed to a 380 W halogen light. It becomes convex under surface tension on the glass substrate. A master for the microlens arrays is then obtained. A 150‐nm‐thick copper layer is sputtered on the master as an electrically conducting layer. The electroforming method replicates the Ni mold insert from the master of the microlens arrays. Finally, micro hot embossing is adopted to replicate the molded microlens arrays. The micro hot embossing experiment employs optical films of polymethylmethacrylate (PMMA) and polycarbonate (PC). The processing parameters of micro hot embossing are processing temperature, embossing pressure, embossing time, and de‐molding temperature. Taguchi's method is applied to optimize the processing parameters of micro hot embossing for molded microlens arrays. An optical microscope and a surface profiler are utilized to measure the surface profile of the master, the Ni mold insert and the molded microlens arrays. AFM is employed to measure the surface roughness of the master, the Ni mold insert and the molded microlens arrays. The sag height and focal length are determined to elucidate the optical characteristics of the molded microlens arrays. Copyright © 2009 John & Sons, Ltd. 相似文献