Towards a Fundamental Understanding of the Improved Electrochemical Performance of Silicon–Carbon Composites
Juliette A. SaintMathieu MorcretteDominique LarcherLydia LaffontS. D. BeattieJean Paul PeresDavid TalagaM. CouziJ. M. Tarascon
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Abstract Silicon–carbon composites consisting of Si particles embedded in a dense and non‐porous carbon matrix are prepared by the pyrolysis of intimate mixtures of poly(vinyl chloride) (PVC) and Si powder at 900 °C under a flow of N 2 . In contrast to bare micrometer‐sized (1–10 μm) and nanometer‐sized (10–100 nm) Si powders, which show poor cycling behavior with almost no capacity remaining after 15 cycles, the texture of the composite is seen to greatly enhance the reversibility of the alloying reaction of Si with Li. For instance, a capacity of ca. 1000 mA h g –1 is achieved for 20 cycles (0–2.0 V vs. Li + /Li) for a silicon–carbon composite containing nanometer‐sized Si particles. We also demonstrate that a mild manual grinding treatment degrades the cycling performance of the composites to levels as low as the parent Si, even though free Si is not released. The electrochemical measurements in conjunction with Raman spectroscopy data indicate that a huge stress is exerted on the Si domains by the in situ formed carbon. This carbon‐induced stress is found to disappear during the milling of the composites, indicating that the carbon‐induced pressure, along with the accompanying improvement in electrical connectivity, are the key parameters for the improved cycling behavior of Si versus Li.Keywords:
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By comparing the electroless composite coating of (Ni P) (TiO 2 nanometer particles) with the electroless composite coating of (Ni P) (SiC micrometer particles) and electroless plating Ni P alloy coating, the tribological characteristic of the electroless composite coating of (Ni P) (TiO 2 nanometer particles) has been discussed in this paper. The result shows that because of the good structure and mechanic properties, the electroless composite coating of Ni P (TiO 2 nanometer particles) has good wear resistance and low friction coefficient in the sliding friction, especially in high load condition.
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The experiments of Ni-P electroless coating and Ni-P-SiO_2 (micrometer), Ni-P-SiO_2 (nanometer) electroless composite coating were done to study how SiO_2particles affect the coating rate and properties of the film. The results show that coating rate increased, hardness and wear of the film comparing to the film of Ni-P and micrometer particles composite coating are promoted notably when the nanometer particles SiO_2 were put into the solution.
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In Raman detection, the most popular solution for the samples is tri-distilled water. But the effect of aqueous solution is barely studied in Raman spectroscopy. In fact Raman spectroscopy of solid-state and liquid-state are obvious different. In addition, FWHM of Raman spectral peaks also change evidently. In this paper, several samples were selected for the experiment; including sodium nitrate, sodium nitrite, glucose and caffeine. By comparing the Raman spectroscopy of samples at different concentrations, it is found that the concentration of the sample can affect the strength of Raman spectroscopy, but it can hardly impact FWHM of Raman spectral peaks. By comparing the Raman spectroscopy of liquid-state with the Raman spectroscopy of solid-state, it is observed that the FWHM of some Raman spectral peaks varied obviously; that may be because when the sample was dissolved into the water, the crystal lattice structure was broken, and for some samples atom form became ion form in aqueous solution. Those structural variations caused the variation of the FWHM. The Raman spectroscopy of caffeine aqueous solution at very low concentration was also detected and analyzed. Compared with the Raman spectra of solid-state samples, it is found that some Raman spectral peaks disappeared when the sample was dissolved in water. It is possible that the low concentration of the sample result in the weakening of Raman signals and the disappearing of some weak Raman spectral peaks. Then Ag nanoparticles were added into the caffeine aqueous solution, the results suggest that surface enhanced Raman spectroscopy (SERS) not only can enhance the Raman spectral signal, but also can reduce the effect of aqueous solution. It is concluded that the concentration of sample only affects the strength of Raman spectroscopy; the aqueous solution can affect the FWHM of Raman spectral peaks; and SERS can reduce the effect of aqueous solution.
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The Raman spectra of nanometer-sized Cr2O3 particles (annealed in air and in vacuum at different temperatures) at room temperature are reported. The results show that the correlation between the change of the Raman spectrum of nanometer Cr2O3 and its crystal size is different from that of other nanometer-sized materials. It was shown that the shifting and broadening of the Raman spectra were related to the defect structures produced by an oxygen deficiency in the material and was not due to a grain size effect.
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In this paper,thermogravimetry/differential scanning calorimetry(TG/DSC) and XPS were used to characterize the thermo-reaction properties of nanometer aluminum powders and micro-aluminum under air,Ar and O2.The different oxidation conditions of these two kinds of powders were studied.The experimental results confirm the difference of thermo-reaction properties between nanometer aluminum powders and micrometer aluminum.Nanometer aluminum powders would not be oxidized under 550℃,and micrometer aluminum would not be oxidized markedly till 1000℃ at atmosphere.
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The properties of nanometer TiO2 were briefly introduced.The modification of nanometer TiO2 and its application in Leather Industry were also reviewed significantly.The development direction of the modification of Nanometer TiO2 was indicated,according to the comparisons between different modifyingmethods.In the end,the application prospect ofthe nanometer TiO2was predicted.
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The DSC/TG、XRD and SEM were used to characterize the thermo-reaction properties of nanometer aluminum powders and micro-aluminum under air.The different oxidation condition of these two kinds of powders was studied.The experimental results confirm the different thermo-reaction properties between nanometer aluminum powders and micrometer aluminum.Nanometer aluminum powders would not be oxidized under 550℃,and micrometer aluminum would not be oxidized markedly till 950℃.Nanometer aluminum powders could be obviously oxidized after sintered at 1050℃ for 30min in air and α-Al2O3 particles could be formed at the particle surface.The obtained α-Al2O3 powders has no obvious agglomeration,spherical shape and a mean particle of 100 nm.
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Techniques and reaction mechanism for preparation of silicon carbide nanometer whiskers are presented and reviewed. It is indicated that the silicon carbide nanometer whiskers are synthesized by VS and VLS reaction mechanism,and the reactant, catalyst, nucleant which have the nanometer dimension is the key for synthesizing silicon carbide nanometer whiskers.
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