Yongwu Zhao

Mechanical and thermal properties of graphene oxide/ultrahigh molecular weight polyethylene nanocomposites

Graphene oxide (GO) was prepared according to a modified Hummers method, and a range of GO/ultrahigh molecular weight polyethylene (UHMWPE) composites were fabricated using liquid-phase ultrasonication mixing followed by hot-pressing. The thermal performances of the GO/UHMWPE composites were characterized by TGA and DSC. The dispersion of GO in GO/UHMWPE composites was investigated by FTIR and XRD. Moreover, the mechanical properties, including micro-hardness, tensile properties, and impact strength of GO/UHMWPE composites were also studied and the fractured surfaces were observed under SEM. The results show that the melting temperature of these composites was about 135 °C and the crystallinity was improved with the addition of GO. Moreover, the initial decomposition temperature was about 472 °C and the addition of GO improved the thermal performance of GO/UHMWPE. Furthermore, not only was the impact strength increased substantially with the addition of GO, but the micro-hardness was also improved gradually and the tensile properties were improved with the addition of GO. The thermal and mechanical performances of the GO/UHMWPE composites are influenced by the free-space effect and interaction-force effect.

Chemical Mechanical Planarization from Macro-Scale to Molecular-Scale

Chemical mechanical planarization (CMP) has been an essential method to fabricate wafer surfaces in IC industry. This article provides a relatively comprehensive review on the state-of-the-art and recent progress in the modeling of CMP and material removal mechanism, and addresses the limitations and further research directions of the modeling work and material removal mechanism.

Effects of gamma irradiation and accelerated aging on GO/UHMWPE nanocomposites

ABSTRACT This article investigates irradiated and accelerated aged graphene oxide (GO)/ultrahigh molecular weight polyethylene (UHMWPE) nanocomposites. The prepared GO/UHMWPE nanocomposites are gamma-irradiated at a high irradiation dose in a vacuum and then accelerated aging procedure is performed at 80°C in an air oven for 21 days. Irradiated and aged samples are characterized by Raman spectrum, Fourier transform infrared (FT-IR) spectrum, differential scanning calorimetry, contact angle, and gel content. Filling GO reduces the intensity of Raman spectrum of UHMWPE and irradiation or aging cannot affect vibrational modes of UHMWPE and GO/UHMWPE. The result of the FT-IR spectrum shows that UHMWPE and GO/UHMWPE basically have the same oxidation index values, whether with irradiation or accelerated aging. Irradiation or aging can slightly increase the melting temperature. GO, irradiation, or aging can significantly increase the crystallinity and improve wetting properties. In irradiated GO/UHMWPE, GO is able to maintain the efficiency of the cross-linking. However, after aging, the cross-linking density of GO/UHMWPE is reduced significantly. According to the above results, it is proposed that GO shows a very weak scavenging free radicals capacity in GO/UHMWPE composites and cannot display antioxidant capacity.

The role of interactions between abrasive particles and the substrate surface in chemical-mechanical planarization of Si-face 6H-SiC

The interactions between abrasive particles and the wafer surface play a significant role in the chemical-mechanical planarization (CMP) process. The influence of interactions between silica or ceria nanoparticles and the substrate surface on the CMP of Si-face (0001) 6H-SiC in different slurries with varied pH values was investigated using zeta potential measurements, SEM observations, friction tests, polishing experiments and XPS analysis. Meanwhile, the interaction forces between the substrate surfaces and the abrasive nanoparticles were also estimated using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Silica particles are prone to adhere to the Si-face 6H-SiC surface below pH 5 and are repelled above pH 5, while ceria particles tend to adhere to the similarly charged and oppositely charged 6H-SiC surfaces. This can be attributed to the fact that the ceria particle possesses a chemical tooth and Si–O–Ce bonds are formed between ceria particles and the 6H-SiC surface. The friction coefficient and material removal rate during the CMP of 6H-SiC could be reduced significantly by the adhesion of silica particles on the 6H-SiC surface resulting from the electrostatic interaction at pH 2 and 4, while this phenomenon was not observed when ceria particles were adsorbed. The XPS analysis indicated that more oxidized species (e.g. Si–C–O, Si–Ox–Cy, Si–O2, Si4–C4−x–O2, Si4–C4–O4 and C–O) were formed during immersion in aq. KMnO4 solution. Finally, an ideal electrostatic interaction between the abrasive particles and the 6H-SiC substrate surface during the CMP process was proposed.

Anti-corrosion performance of chemically bonded phosphate ceramic coatings reinforced by nano-TiO2

To promote anti-corrosion property of chemically bonded phosphate ceramic coatings (CBPCs), the nano-TiO2 is selected as the reinforcement. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy Dispersive Spectrometer (EDS) and the electrochemical analysis are carried out to clarify the role of nano-TiO2 on the improvement of anti-corrosion performance. The experiments show that with the addition of nano-TiO2, the curing temperature and the activation energy of the curing process increase, which allows longest reaction and positively drives curing reactions at elevated conversions. The enhancement of anti-corrosion performance of CBPCs reinforced by nano-TiO2 particles is based on three main mechanisms. Firstly, more bonded phase (AlPO4) can be formed with the addition of nano-TiO2, which can help CBPCs to get more compact microstructure. Additionally, AlPO4 particles possess the low density and good corrosion resistance, which leads to the increase in the corrosion resistance of CBPCs. Secondly, increasing content of nano-TiO2 can also strengthen the compactness of CBPCs to protect the substrates from the penetration of aggressive electrolyte and prolong electrolyte diffusion path. Thirdly, through the analysis of microstructure of CBPCs, it is found that most of the hydrophobic nano-TiO2 particles homogeneously distribute on the surface of CBPCs. Therefore, CBPCs show well hydrophobic performance, which can further improve the anti-corrosion property of themselves.

The Influence of Irradiation and Accelerated Aging on the Mechanical and Tribological Properties of the Graphene Oxide/Ultra-High-Molecular-Weight Polyethylene Nanocomposites

Graphene oxide/ultra-high-molecular-weight polyethylene (GO/UHMWPE) nanocomposite is a potential and promising candidate for artificial joint applications. However, after irradiation and accelerated aging, the mechanical and tribological behaviors of the nanocomposites are still unclear and require further investigation. GO/UHMWPE nanocomposites were successfully fabricated using ultrasonication dispersion, ball-milling, and hot-pressing process. Then, the nanocomposites were irradiated by gamma ray at doses of 100 kGy. Finally, GO/UHMWPE nanocomposites underwent accelerated aging at 80°C for 21 days in air. The mechanical and tribological properties of GO/UHMWPE nanocomposites have been evaluated after irradiation and accelerated aging. The results indicated that the incorporation of GO could enhance the mechanical, wear, and antiscratch properties of UHMWPE. After irradiation, these properties could be further enhanced, compared to unirradiated ones. After accelerated aging, however, these properties have been significantly reduced when compared to unirradiated ones. Moreover, GO and irradiation can synergistically enhance these properties.

Investigation of irradiated graphene oxide/ultra-high-molecular-weight polyethylene nanocomposites by ESR and FTIR spectroscopy

ABSTRACT Graphene oxide/ultra-high-molecular-weight polyethylene (GO/UHMWPE) nanocomposite has a potential application for artificial joints. However, free radicals and antioxidative properties of irradiated GO/UHMWPE were not clearly clarified. In this paper, GO/UHMWPE nanocomposites were prepared and irradiated by gamma-irradiation with a dose of 100 kGy. Afterward, test samples were aged in air. Free radicals and molecular structures of test samples were investigated by electron spin resonance (ESR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy, respectively. These studies indicated that irradiation enhanced GO radical concentrations. And GO radicals were stable in air and even could not be quenched after accelerated aging. GO radicals were superimposed on free radicals of irradiated UHMWPE. Although GO showed free radical-scavenging capacity, the influence of GO on free radicals of irradiated UHMWPE was too weak to be observed in ESR spectroscopy. Free radicals concentrations of irradiated GO/UHMWPE nanocomposites were gradually decayed with aging time evolving in air. Observing the oxidation index values of test samples, it was proposed that irradiated GO/UHMWPE might show very weak antioxidative properties.