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

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

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

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

Perspectives on Nanomaterials and Nanotechnology for Sustainable Bioenergy Generation

The issue of global warming calls for a greener energy production approach. To this end, bioenergy has significant greenhouse gas mitigation potential, since it makes use of biological products/wastes and can efficiently counter carbon dioxide emission. However, technologies for biomass processing remain limited due to the structure of biomass and difficulties such as high processing cost, development of harmful inhibitors and detoxification of produced inhibitors that hinder widespread usage. Additionally, cellulose pre-treatment is often required to be amenable for an enzymatic hydrolysis process. Nanotechnology (usage of nanomaterials, in this case) has been employed in recent years to improve bioenergy generation, especially in terms of catalyst and feedstock modification. This review starts with introducing the potential nanomaterials in bioenergy generation such as carbon nanotubes, metal oxides, silica and other novel materials. The role of nanotechnology to assist in bioenergy generation is discussed, particularly from the aspects of enzyme immobilization, biogas production and biohydrogen production. Future applications using nanotechnology to assist in bioenergy generation are also prospected

Enhanced mechanical properties of 3D printed graphene-polymer composite lattices at very low graphene concentrations

The advent of 3D printing has enabled the rapid prototyping of complex structures with relatively shorter production times and lower material wastage. Despite these advantages, it is still a challenge to fabricate nanofiller-reinforced lattices using 3D printing. Here, we report for the first time, the successful 3D printing of graphene-polymer octet-truss lattices using the stereolithography (SLA) technique. The factors influencing the mechanical properties of the printed graphene-polymer composite, such as filler concentration, solvent addition and post-fabrication baking temperature and duration were investigated in detail. Our results showed that stereolithographic 3D printing can confer the same improvement in material modulus with ~ 10 times less graphene concentration compared to other processing techniques reported in literature. Our calculations suggest that this was due to a unique characteristic of stereolithography, which enabled the selection and incorporation of aligned graphene platelets into the polymer matrix during the 3D printing process. These exceptional mechanical properties of SLA fabricated polymer-graphene composites are indicative of their potential for use in various applications such as aerospace, automotive and sports equipment.Accepted versionThe authors would like to acknowledge funding for this project by the Temasek Research Fellowship

Anomalous elastic response of a 3D anti-tetrachiral metamaterial

The elastic modulus and Poisson's ratio of a 3D anti-tetrachiral (3ATC) metamaterial design was investigated using an exact analytical model, finite element simulations and experiments on additively manufactured Ti6Al4V lattices. The 3ATC structure was found to undergo a unique symmetric-to-asymmetric transition as the number of unit cells in the lattice decreases, an observation that has not been reported to date. A reduced lattice size also increases the influence of shear forces introduced by the fixed boundary conditions, which can lead to a higher elastic modulus in certain orientations and reduce it in others. These shear forces also drive the joints in small lattices into an out-of-plane rotation that causes the Poisson's ratio of such structures to range from -1.2 to 1 for different relative densities, in contrast to a constant value of -0.5 for bulk 3ATC lattices that do not undergo this joint twisting. Our results strongly indicate that the 3ATC structure belongs to a new ‘rotation-dominated’ geometric class in the Ashby framework for cellular materials, in addition to the well-established bending- and stretch- dominated topologies. The main contributor of strain for this class of materials is rigid joint rotation, with novel, distinctive traits such as a nonlinear elastic stress-strain response and multiple relative modulus vs. relative density relationships. For the 3ATC structure, one of these relations is linear, similar to stretch-dominated structures, while the other is disjointed and does not follow the power law, which is atypical of a cellular material.Accepted versionTemasek Research FellowshipFunding for this project was partially provided by C.Q.L.’s Temasek Research Fellowship, for which he gives thanks. Samples for experimental testing were provided by SLM Solutions Singapore Pte Ltd

Effect of reinforcement bending on the elastic properties of interpenetrating phase composites

The elastic modulus of interpenetrating phase composites (IPCs) was analyzed through a theoretical model that accounted for bending deformation of the reinforcement phase. The model was validated against literature data, as well as simulation and experimental results of IPCs that were constructed from 3D-printed polymeric reinforcements embedded in a polydimethylsiloxane (PDMS) matrix. The reinforcements were in the form of Octet Truss and Kelvin Cell lattices, which are known to exhibit very different degrees of bending during elastic deformation. When the matrix modulus was relatively low, the model was able to explain how the bending of reinforcement struts caused the overall IPC modulus to be much lower than those predicted by other theoretical models. As the matrix modulus increased to beyond 20% that of the reinforcement material, however, the different lattice designs were found to have no significant influence on the IPC modulus. Further increase in matrix modulus pushed the elastic response of IPCs towards the isostrain limit, as the matrix helped to distribute the load more evenly and suppress the bending of struts, especially for lower density lattices. The model was able to account for a wide range of different constituent moduli and was also applicable to IPCs which utilized stochastic foams for reinforcement. The insights derived in this study is expected to be particularly useful for designing polymer-based IPCs where the elastic moduli of the reinforcement and matrix can differ over several orders of magnitude.Accepted versionThe authors would like to acknowledge funding for this project by the Temasek Research Fellowship (grant no. M4061969.680)

The Effect of Different Sintering Strategies on Properties of YSZ Reinforced Graphene Composites

Partial, single and double step sintering has been carried out to densify YSZ reinforced 1 wt. % graphene composites at temperature as low as 40% of the homologous temperature. The influence of sintering conditions on resulting density, microstructure, porosity and contact angles were determined. The microstructure of the composites showed that amount of micro pores was reduced upon double step sintering and infiltrating with PDC resin. This study has improved upon existing sintering methods resulting in the use of low temperature for sintering ceramics yet achieving relative density >95% with porosity as low as 0.15 using double step sintering

Mechanical anisotropy of graphene nanocomposites induced by graphene alignment during stereolithography 3D printing

Stereolithographically 3D-printed graphene-PMMA nanocomposites were previously found to be mechanically stiffer and stronger in the print axis, suggesting that the graphene filler was selectively oriented. Here, using polarized light microscopy, we confirm experimentally for the first time the presence of aligned graphene platelets in these nanocomposites. The alignment appears to be weak, however, as anisotropy of the storage modulus and quasistatic failure strength was only ~ 10% – 30%, about 100 × lower than the maximum anisotropy possible and 10 × smaller than that of other 3D-printed anisotropic composites. The optimal graphene concentration for maximum anisotropy was 0.02wt%– 0.05wt%, as graphene agglomeration at higher concentrations reduced anisotropy and beyond 0.2wt% it prevented 3D printing altogether. Using finite element simulations, which were experimentally verified, it was also shown that the anisotropy of the bulk nanocomposites could be fully imparted to more complex 3D-printed parts such as Octet Truss structures. Graphic abstract: [Figure not available: see fulltext.].Nanyang Technological UniversitySubmitted/Accepted versionThis work was partially funded by C.Q.L’s startup grant (#020868-00001)

Enhanced Tribological Behaviour of Hybrid MoS2@Ti3C2 MXene as an Effective Anti-Friction Additive in Gasoline Engine Oil

Hybrid molybdenum disulfide (MoS2)-MXene (Ti3C2) was added as an additive in SAE 5W-40-based engine oil in an attempt to reduce interfacial friction between contact surfaces. It was found that the coefficient of friction (COF) and wear scar diameter (WSD) were reduced by 13.9% and 23.8%, respectively, with the addition of 0.05 wt.% MoS2-Ti3C2 compared to base engine oil due to the interlaminar shear susceptibility of MXene. However, we postulate that the high surface energy and presence of -OH, -O and -F functional groups on the surfaces limited the dispersibility and stability of MXene in base oil, while high activity of MoS2 nanoparticles due to large surface area and vigorous Brownian motion prompted fast settling of nanoparticles due to gravitational force. As such, in the present study, hybrid MoS2-Ti3C2 were amine-functionalized to attain stability in SAE 5W-40-based engine oil. Experimental findings indicate that amine-functionalized 0.05 wt.% MoS2-Ti3C2 exhibited higher COF and WSD, i.e., 12.8% and 12.3%, respectively, compared to base oil added with 0.05 wt.% unfunctionalized MoS2-Ti3C2. Similarly, Noack oil volatility was reduced by 24.6% compared to base oil, indicating reduced oil consumption rate, maximal fuel efficiency and enhanced engine performance for a longer duration