Processing and characterization of atz and ysz-graphene composites for fabrication of mems scale microthruster

Markandan, Kalaimani (2017) Processing and characterization of atz and ysz-graphene composites for fabrication of mems scale microthruster. PhD thesis, University of Nottingham.

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Structural ceramics such as zirconia and alumina are widely used in the materials industry owing to their high hardness, chemical inertness and electrical insulation properties. However, they bear the disadvantage of low fracture toughness that has limited their further applications. As such, ceramics with improved fracture toughness are desired in many engineering fields.

As for silicones, they are extensively used in current micro-electromechanical system (MEMS) components such as fuel cells and combustion engines. However, in hot and aggressive environments, silicon reduces functionality and efficiency of the components. Besides, low hardness and toughness of silicone material is undesirable in MEMS components. The ideal MEMS components require that the material has excellent structural strength at high temperature, exceptional thermal shock resistance and resistant to chemical corrosion. In view of these limitations, ceramics are currently being used to fabricate MEMS components.

This PhD project sets out to tackle the disadvantages of ceramics such as brittleness and low electrical conductivity by developing ceramic nanocomposites using nanostructured fillers. Graphene nanoplatelets (GNPs) - a newly emerging carbon nanomaterial and alumina were chosen as the reinforcing fillers. Two types of composites namely alumina toughened zirconia (ATZ) and yttria stabilized zirconia- graphene (YSZ-Gr) composites were fabricated and their properties were investigated. These composites were produced by gel-casting route on PDMS soft molds. There were two main problems in fabrication of YSZ-Gr composites: (i) achieving homogenous dispersion of graphene in ceramic matrix and (ii) production of good quality graphene. The first problem was solved by dispersing commercially available graphene platelets in a surfactant;cetyltrimethylammonium bromide (CTAB). Scanning electron microscopy (SEM) ascertains homogenously dispersion of graphene in the ceramic matrix. In order to solve the second problem, processing parameters such as stirring speed and duration were optimised before fabricating the composites.

Nearly dense ATZ and YSZ-Gr ceramic composites were obtained after sintering and infiltration with PDC resin (RD-212a). The prepared ceramic composites were characterized and their properties were studied. Analysis shows that hardness and fracture toughness of composites increased with the addition of fillers. For ATZ composites, there was an improvement of ≈25.26 % in fracture toughness and ≈16.29 % in hardness with 20 wt. % alumina. For YSZ-Gr composites, there was an improvement of ≈20.31 % in fracture toughness and ≈25.78 % in hardness with 1 wt. % GNP. Electrical conductivity of YSZ-Gr composites was increased by ≈9 orders of magnitude with 2 wt. % GNP compared to pure YSZ.

The idea of this research is to use the two materials (ATZ and YSZ-Gr) to fabricate a MEMS scale chemical microthruster for space applications. Graphene provides conductive paths to decompose propellant (hydroxylammonium nitrate, HAN) and hence produces thrust. The characterization and testing of microthruster revealed that the proposed design is a success where; optimum thrust of 180.5 mN was achieved at a propellant flow rate of 60 µl/min. Future work should focus on optimization of chamber geometry, nozzle geometry and fuel choices to enable thruster to produce larger forces while decreasing the power consumption. Modelling and simulation of MEMS microthrusters can be used to verify the experimental results obtained.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Chin, Jit Kai
Tan, Michelle
Keywords: graphene composites, micro-electromechanical system (MEMS), alumina toughened zirconia (ATZ), yttria stabilized zirconia- graphene (YSZ-Gr)
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UNMC Malaysia Campus > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 43403
Date Deposited: 10 Oct 2017 08:49
Last Modified: 13 Oct 2017 05:04

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