全文获取类型
收费全文 | 4103篇 |
免费 | 738篇 |
国内免费 | 235篇 |
专业分类
化学 | 1179篇 |
晶体学 | 27篇 |
力学 | 264篇 |
综合类 | 28篇 |
数学 | 298篇 |
物理学 | 3280篇 |
出版年
2024年 | 5篇 |
2023年 | 54篇 |
2022年 | 83篇 |
2021年 | 148篇 |
2020年 | 149篇 |
2019年 | 102篇 |
2018年 | 107篇 |
2017年 | 149篇 |
2016年 | 167篇 |
2015年 | 139篇 |
2014年 | 258篇 |
2013年 | 292篇 |
2012年 | 264篇 |
2011年 | 304篇 |
2010年 | 243篇 |
2009年 | 243篇 |
2008年 | 281篇 |
2007年 | 255篇 |
2006年 | 253篇 |
2005年 | 202篇 |
2004年 | 189篇 |
2003年 | 155篇 |
2002年 | 137篇 |
2001年 | 109篇 |
2000年 | 121篇 |
1999年 | 98篇 |
1998年 | 92篇 |
1997年 | 80篇 |
1996年 | 60篇 |
1995年 | 47篇 |
1994年 | 41篇 |
1993年 | 36篇 |
1992年 | 32篇 |
1991年 | 26篇 |
1990年 | 26篇 |
1989年 | 15篇 |
1988年 | 22篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有5076条查询结果,搜索用时 31 毫秒
91.
《Surface and interface analysis : SIA》2005,37(1):42-52
Polyatomic primary ions have been applied recently to the depth profiling of organic materials by secondary ion mass spectrometry (SIMS). Polyatomic primary ions offer low penetration depth and high damage removal rates in some polymers, but the relationship between polymer chemistry and degradation under polyatomic primary ion bombardment has not been studied systematically. In this study, positive and negative ion time‐of‐flight SIMS (ToF‐SIMS) was used to measure the damage of ~100 nm thick spin‐cast poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA) and poly(methacrylic acid) (PMAA), films under extended (~2 × 1014 ions cm?2) 5 keV SF5+ bombardment. These polymers were compared to determine the effect of the main chain and pendant methyl groups on their degradation under SF5+ bombardment. The sputter rate of PMMA was approximately twice that of PMA or PMAA and the rate of damage accumulation was higher for PMA and PMAA than PMMA, suggesting that the main chain and pendant methyl groups played an important role in the degradation of these polymers under SF5+ bombardment. These results are consistent with the literature on the thermal and radiation‐induced degradation of these polymers, which show that removal of the main chain or pendant methyl groups reduces the rate of depolymerization and increases the rate of intra‐ or intermolecular cross‐linking. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
92.
V.L. Furer A.E. Vandyukov J.P. Majoral A.M. Caminade V.I. Kovalenko 《Vibrational Spectroscopy》2006,40(2):155-160
The Raman (10–3500 cm−1) and infrared (150–3500 cm−1) spectra have been recorded for tris(4-oxibenzaldehyde)thiophosphate. This compound includes structural parts of elementoorganic dendrimers: a core and terminal aldehyde groups. The structural optimization and normal mode analysis are performed for elementoorganic dendrimer on the basis of the ab initio density functional theory. It is found that the dendrimer exist in a single stable conformation with planar C6H4CHO fragments. Our calculations show that conformer with one trans and two gauche 4-oxibenzaldehyde groups is realized. All these observations suggest that steric congestion does not disturb the construction of dendrimers even for the highest generations, and that terminal groups are readily available for further reactions. Relying on DFT calculations a complete vibrational assignment is proposed for different parts of the studied dendrimers. 相似文献
93.
Fu Qiang Huang 《Journal of solid state chemistry》2003,174(2):334-341
Three new compounds, Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5, have been synthesized from the respective elements and a reactive flux Cs2S3 at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å3 for Cs2Bi2ZnS5; a=15.817(2), b=4.1782(6), c=18.473(3) Å, V=1220.8(3) Å3 for Cs2Bi2CdS5; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å3 for Cs2Bi2MnS5. The structure is composed of two-dimensional ∞2[Bi2MS52−] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs+ cations. The layers consist of edge-sharing ∞1[Bi2S66−] and ∞1[MS34−] chains built from BiS6 octahedral and MS4 tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs2Bi2MS5, is related to that of Na2ZrCu2S4 and those of the AMM′Q3 materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs2Bi2ZnS5 and Cs2Bi2CdS5 are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs2Bi2CdS5 is ≈1.7 eV, as derived from its optical absorption spectrum. 相似文献
94.
95.
Thomas Höche Wolfgang NeumannSaeid Esmaeilzadeh Reinhard UeckerMarkus Lentzen Christian Rüssel 《Journal of solid state chemistry》2002,166(1):15-23
Sr2TiSi2O8 single crystals were grown by Czochralski pulling and from a high-temperature solution. X-ray diffractometry revealed the modulated crystal structure of Sr2TiSi2O8 to belong to the 5D superspace group P4bm (−α, α, 1/2; α, α, 1/2) with α=0.3. Atomic positions, anisotropic displacement factors and positional modulation parameters for Sr2TiSi2O8 are determined and discussed. The positional modulation is further investigated by electron diffraction and high-resolution transmission electron microscopy. In the latter experiments, the 2D modulation appears to be superimposed by some 1D modulation waves. This effect is discussed in terms of growth conditions. 相似文献
96.
Alan C. Hopkinson Min H. Lien Keith Yates Imre G. Csizmadia 《Theoretical chemistry accounts》1977,44(4):385-398
The geometries of the 2-chloroethyl and ethylenechloronium cations, two possible intermediates in the electrophilic addition of chlorine to ethylene, have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set.These geometries were then used to compute more accurate wave functions using Dunning's double-zeta basis set. The bridged chloronium ion was found to be more stable by 9.35 kcal/mole, the opposite order of stability from the C2H4F+ ions. Interconversion of the two C2H4Cl+ cations was computed to have a barrier of 6.25 kcal/mole.The activation energy for this chlorination reaction, using the ethylenechloronium cation and a chlorine anion at infinite separation as the model for the activated complex, was computed to be 128.7 kcal/mole, showing that this is not a feasible gas phase reaction. 相似文献
97.
The influence of Stokes shift in optosensing was discussed. Then, the current status of large Stokes shift-based optosensing was reviewed here. 相似文献
98.
In on-column or in splitless injection with recondensation of the solvent, the length of the flooded zone in the column inlet depends primarily on the wettability of the internal wall of the inlet. For columns with a coated inlet this explains why peak distortion due to band broadening in space by a certain sample volume is pronounced in one case and hardly observable in another. Glass or fused silica capillaries silylated with diphenyltetramethyldisilazane were found to give optimal retention gaps. They combine thorough and thermostable deactivation with good wettability and low retention power. On the other hand it is very easy to deactivate fused silica capillaries with Carbowax. The resulting retention gaps are suitable for a wide range of applications and are particularly attractive for the analysis of dirty samples which require frequent replacement of the inlet. 相似文献
99.
Di Li Naoki Ohashi Shunichi Hishita Taras Kolodiazhnyi Hajime Haneda 《Journal of solid state chemistry》2005,178(11):3293-3302
An overall comparative study was carried out on N-doped, F-doped, and N-F-codoped TiO2 powders (NTO, FTO, NFTO) synthesized by spray pyrolysis in order to elucidate the origin of their visible-light-driven photocatalysis. The comparisons in their experimentally obtained characteristics were based on the analysis of XPS, UV-Vis, PL, NH3-TPD and ESR spectra. The comparisons in their theoretically predicted properties were based on the analysis of the calculated electronic structures. As the results, N-doping into TiO2 resulted in not only the improvement in visible-light absorption but also the creation of surface oxygen vacancies. F-doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity and the increase of Ti3+ ions. Doped N atoms formed a localized energy state above the valence band of TiO2, whereas doped F atoms themselves had no influence on the band structure. The photocatalytic tests indicated that the NFTO demonstrated the highest visible-light activity for decompositions of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of several beneficial effects induced by the N-F-codoping. 相似文献
100.
P. G. Royall D. Q. M. Craig M. Reading T. J. Lever 《Journal of Thermal Analysis and Calorimetry》2000,60(3):795-805
The purpose of this study was to investigate the feasibility of modulating the temperature programme of a conventional DSC
by use of an alternating gas-flow system. Modulated temperature differential scanning calorimetry (MTDSC) is an important
thermal analysis technique but suffers from a limited applicable frequency range due to the mass of the sample and DSC cell
leading to the impingement of thermal conductivity effects. We suggest that the frequency limit can be increased by replacing
the cell as the source of temperature modulation with an external gaseous source, directed towards the sample and reference
pans. In this evaluation, an alternating gas-flow was passed through a line to a forced gas-flow accessory (FGFA). The FGFA
consisted of two matched cylinders containing chambers that allowed pre-temperature-equilibration of the stream of gas before
it was passed over the sample and reference pans. The development of this device revealed the essential practical requirements
of gas-flow modulation for high-frequency temperature modulation. These include the following: an appropriately sealed tunable
gas supply to both sample and reference pans, an effective method for high-frequency cycling of the gas-flow rate, a small
aperture to deliver the flowing gas directly over the pan and a temperature equilibration chamber. The results from samples
of quenched PET and amorphous Saquinavir indicate that gas-flow modulation is indeed feasible, with the FGFA able to raise
the attainable temperature modulation frequency by an order of magnitude compared to conventional MTDSC.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献