Pingsheng He

Cure kinetics of epoxy-based nanocomposites analyzed by Avrami theory of phase change

Abstract The effect of montmorillonite on the cure kinetics and the structure of epoxy resin/organo-montmorillonite (Org-MMT)/methyltetrahydrophthalic anhydride (MeTHPA)/2-ethyl-4-methyl-imidazole (2,4-EMI) nanocomposites are investigated by X-ray diffraction (XRD) and the dynamic torsional vibration method (DTVM). The XRD result indicates that an exfoliated nanocomposite was obtained. The experimental data obtained from DTVM are explained by the Avrami theory of phase change, and show that the theoretical prediction is in good agreement with the experimental results. No special formation mechanism for epoxy resin/Org-MMT/MeTHPA/2,4-EMI nanocomposite is observed. The addition of Org-MMT reduces the gelation time tg, and increases the rate of the curing reaction and the value of the kinetic constant k. The half-time t1/2 of cure after the gel point decreases with increasing cure temperature, while the values of kinetic constant k rise as the cure temperature increases. The nanocomposite has a larger value of k and smaller value of n comparing with the epoxy resin/MeTHPA/2,4-EMI system without Org-MMT.

Thin SiC films prepared by pyrolysis of polyimide Langmuir–Blodgett films on silicon

The quasi‐single‐crystal SiC thin films were grown on silicon (111) by pyrolysis of polyimide LB films at 1000 °C in vacuum. The chemical and structural characterizations were studied by x‐ray diffraction, infrared absorption, x‐ray photoelectron spectroscopy, and Laue photography.

PbS/epoxy resin nanocomposite prepared by a novel method

PbS/epoxy resin nanocomposite is prepared by a novel method. The epoxy resin microemulsion is taken as a microreactor for the formation of PbS nanocrystals. After the reaction, the collected epoxy proved to be a composite with nano-PbS embedded in. The morphological observation of cured PbS/epoxy resin nanocomposite by tunnel electronic microscopy (TEM) indicates that the PbS nanocrystals are dispersed in cured epoxy resin matrix homogenously. X-ray diffraction (XRD) and TEM were used to characterize the PbS nanocrystals.

Growth of β‐SiC film by pyrolysis of polyimide Langmuir–Blodgett films on silicon

The heteroepitaxial growth of β‐SiC has been studied by pyrolysis of polyimide Langmuir–Blodgett films on silicon substrate in the temperature range 500–1000 °C in vacuum. Thin SiC films have been grown at temperatures above 700 °C. Both the onset temperature of the SiC formation and the growth rate of the SiC films were affected by the orientation of the silicon substrates. The growth rate of the SiC films increased with temperature and was controlled by the reaction of Si with C‐containing reactants. By comparison with dip‐coating polyimide films, it was found that the crystallinity of the SiC layers depended on the degree of order of the molecular arrangement in the polyimide films. Highly ordered superlattice structures of polyimide LB films favored the growth of single crystal SiC films.

Stable blue-green and ul traviolet photoluminescence from silicon carbide on porous silicon

β-Silicon carbide layers have been prepared by high temperature pyrolysis of polyimide Langmuir-Blodgett films on porous silicon substrate in vacuum. The formation of silicon carbide is confirmed by the IR and XRD spectra. It is found that photoluminescence still exists and appears in the blue-green and ultraviolet regions after thermal treatment at 900°C. These results indicate that the silicon carbide layers, which are formed, are responsible for the blue-green luminescence.

SiC formation at the interface of polyimide Langmuir–Blodgett film and silicon

X‐ray photoelectron spectroscopy has been used to explore the process of the interaction between the polyimide film made by the Langmuir–Blodgett method and the substrate Si(111). It is evident that the process includes three stages: The polymer degrades below temperature of about 500 °C resulting in some hydrocarbon species on the surface; at higher temperatures the residual hydrocarbons convert to some state of elemental carbon and then diffuse into the substrate to form ‘‘C–Si alloy’’ which is regarded as a precursor of SiC formation; SiC starts to form at about 700 °C and grows at higher temperatures.

SiC films grown on silicon by pyrolysis of polyimide Langmuir–Blodgett films containing dispersed silicon nanoparticles

Abstract By combining nano-particles and the Langmuir–Blodgett method of depositing films, SiC thin films were successfully grown on Si substrates, the growth temperature for the synthesis of SiC/Si composites is lowered to 700°C. The growth process based on the fourier transform-infrared (FTIR) spectra is discussed. The films on Si(111) substrates exhibit preferred orientation and weak photo-luminescence is detected from samples grown on p-type Si substrates.

The growth mechanism of SiC film from polyimide LB film

The growth mechanism of SiC film produced by pyrolysis polyimide LB film was discussed. AES result showed that the atoms of C and Si form a gradient distribution in the pyrolyzed SiC films. Parameters including the diffusion co-efficiency of carbon atoms, growth rate of SiC film, and the activation energy were estimated. We suggest that the SiC formation process is controlled by the diffusion of silicon atoms, and the whole growth rate depends on the reaction of Si and C.