Surface modification of inorganic materials with graphene oxide : From solution to high-temperature synthesis

Sammanfattning: The use of graphene as an additive in materials is often challenging due to the agglomeration of the two-dimensional graphene sheets. An alternative additive is graphene oxide (GO), an oxidized form of graphene, which is easily dispersed in water. In this thesis, a method based on electrostatic interactions has been developed to coat powder particles with GO. The GO surface has a negative charge at most pH values, while most inorganic materials form a thin oxide surface layer with a pH-dependent surface charge. Below a certain pH, a powder surface can have a positively charged surface oxide, which can interact with the negatively charged GO sheets and form a coated particle. The pH range for a successful coating process can be predicted based on the ionic potential (z/r) of the surface oxides and the oxide stability regions seen in Pourbaix diagrams. In the thesis, GO-coatings were obtained on powders of Cu, Fe, 316L stainless steel, MnAl(C) and AlSi7Mg. The coated powders showed reduced reflectance, long-term oxidation stability and in most cases improved flowability.The effect of the GO-coating on the processability of Cu, MnAl(C), 316L stainless steel and AlSi7Mg in laser powder bed fusion (L-PBF) was also investigated. For Cu, the reduced reflectance at the wavelength of the laser led to printing of fully dense parts compared to a 10 % porosity of the printed uncoated Cu powder using the same printing parameters. L-PBF printing of MnAl(C) has a problem with cracking but printing with the GO-coated powder resulted in a 35 % crack reduction. Fully dense parts could also be printed with GO-coated AlSi7Mg and 316L stainless steel powders. Significant changes in texture compared to the uncoated reference as well as moderately improved mechanical properties were observed.TiB2-SiC composites can be formed by reactive hot pressing of a TiC-Si-B4C powder mixture. In agreement with predictions, it was very difficult to coat powders of Si and B4C due to their acidic surface oxides. In the thesis, the influence of reduced GO (rGO) and carbon black (CB) on the microstructure and properties of the TiB2-SiC composite was investigated using ball milled mixtures of the powders. After reactive hot pressing at 1850 °C, sheet-like structures could be observed at fractured surfaces suggesting that graphene-like sheets survive the high temperatures. The rGO containing samples showed increased thermal conductivity, hardness, and fracture toughness by 50 %, 16 % and 20 %, respectively. The improved properties were attributed to a good dispersion of the rGO additive, a change in the microstructure and a decreased oxygen content of the composite.

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