Growth and luminescence characteristics of indium nitride for optoelectronics applications in the 1.55-micron region

Ph.DDOCTOR OF PHILOSOPH

Commercialization of group III nitrides-on-silicon technologies

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2010Cataloged from PDF version of thesis.Includes bibliographical references (pages 35-39).While group Ill nitride materials have been commercialized for many years, there is recent interest in growing these materials on silicon substrates as a cost effective alternative to more expensive sapphire and silicon carbide technologies. Therefore, it is necessary to determine how group Ill nitride-on-silicon technologies can be positioned in way for them to be effective in their respective applications, thereby enabling their commercialization. This thesis is a systematic evaluation of the epitaxial growth on silicon carbide, sapphire and silicon substrates, focusing on their lattice-mismatches, thermal expansion mismatches, and thermal conductivity. The subsequent analysis of important commercial applications determined that GaN-on-Si technology is ready for commercialization in the near future. These applications include the InGaN/GaN white light emitting diode and the blue laser diode, as well as the AIGaN/GaN high electron mobility transistor, each with its own unique requirements for the technology and the implementation. It was recommended that start-up firms interested in commercializing GaN-on- Si technology focus on the growth of GaN on silicon substrates and engage device manufacturers proactively. InN and In-rich nitrides can complement maturing GaN and Ga-rich nitrides technologies, resulting in new applications and products in future. While the growth of InN films is currently very challenging, it is believed that the experience and revenue obtained from the commercialization of GaN-on-Si technology can benefit InN-on-Si technology, speeding up the latter's commercialization. A brief business strategy aimed at translating the findings into a feasible approach for commercialization is also provided.by Ian Peiyuan Seetoh.M. Eng.M.Eng. Massachusetts Institute of Technology, Department of Materials Science and Engineerin

Influence of microstructure topology on the mechanical properties of powder compacted materials

Powder compaction is an important technique for fabricating engineering materials as it offers good resolution and is compatible with complex stoichiometry and geometries. It forms the basis of important manufacturing processes such as powder bed 3D printing, powder metallurgy and metal injection moulding. However, a major disadvantage is that the presence of porosity in the resultant material can lead to a drastic deterioration of its mechanical properties. To improve the stiffness and strength of these powder compacts, it is imperative to pinpoint the main cause of these weakening effects. Here, we attempt to do so by examining the mechanics of different topologies that the microstructures of powder compacted materials can adopt. General structure – property relationships were first derived for (i) compression/ stretch – dominated (CD) (ii) compression, shear and bending (CSB) and (iii) compression, shear and joint rotation (CSR) topologies, for the range of relative densities between 0 and ~ 0.9. Using the Face-Centered Cubic (FCC), Body-Centered Cubic (BCC) and 3D Anti-Tetrachiral (3ATC) geometries to represent the CD, CSB and CSR topologies respectively, the analytical and simulated relative stiffness vs. relative density and relative strength vs. relative density trends were compared against experimental data in the literature. It was found that the mechanical properties of powdered materials typically fall within an exclusive range of values exhibited by the 3ATC lattice, which is much lower than that expected of FCC and BCC lattices. A closer examination of the analytical equations indicated that the low modulus of 3ATC lattices and powder compacted materials is caused by joint (i.e. particulate) rotation, while their weak strength is the result of thin beams, which manifest as narrow neck-like interparticle connections in powder compacted materials. These results are supported by previous studies, which showed that powder compacted materials have eccentric microstructures similar to 3ATC unit cells and the compression of granular material usually results in extensive particulate rotations. Higher coordination number of the particles is expected to reduce these rotations, thus illuminating the strategy for improving the modulus of powder compacted materials. The material strength, on the other hand, has already been shown to improve with a thickening of the neck regions, which can be achieved through higher sintering temperature, compressive pressure and/ or longer compaction time.Ministry of Defence (MINDEF)Accepted versionFunding for this project was provided by C.Q.L.’s Temasek Research Fellowship, for which he gives thanks

Machine learning assisted investigation of defect influence on the mechanical properties of additively manufactured architected materials

Additive manufacturing techniques can introduce defects that worsen the mechanical properties of 3D printed parts. Current techniques for quantifying the detrimental effects of these defects can only provide detailed analysis for a small number of geometries. Here, we investigate the effect of each defect feature (surface roughness and void position, number density and size) on the mechanical properties of a large number of truss lattices belonging to the stretch-dominated and bending-dominated topology. This is done by reducing each truss lattice into a single-beam sub-unit cell and conducting finite element simulations on it. The generated data is subjected to machine learning algorithms to identify the most important defect and design features that determine the mechanical properties of the overall structure. Our results indicate that surface roughness, Rmax (i.e. peak-to-trough height), exceeding 10% of the beam diameter strongly reduces the specific modulus and strength of lattice structures, especially for bending-dominated geometries. Interior voids, on the other hand, adversely affect stretch-dominated geometries but improve the specific properties of bending-dominated structures by removing under-stressed material in the core of the beams and causing them to become more “tube-like”. These insights are supported by first-principles analytical modeling and experimental data of additively manufactured metal lattices in the literature.Nanyang Technological UniversitySubmitted/Accepted versionThis work was partially funded by the Temasek Labs Innovation Grant (TLIG21-02) for which the authors are grateful for

Effect of geometrical design on the latent heat cooling properties of a lightweight two-phase composite

Latent heat storage materials undergo phase changes to maintain a constant temperature environment and are fast emerging as a passive “green” technology for thermal management. Phase-change materials (PCMs) typically have poor thermal conductivities; however, their response to rapid fluctuations in temperature can be sluggish. Here, we explore the feasibility of adding various aluminum alloy (AlSi10Mg) structures to speed up the thermal response. The cooling performance of various geometries with the same mass density was first investigated, and the best performing geometries were then further optimized to investigate the possible weight savings. Our results indicate that, for unidirectional heat flux, designs with 3D periodicity, such as triply periodic minimal surfaces, do not perform as well as those with 1D (parallel plates) and 2D (honeycombs) periodicity. Furthermore, a strong correlation was found between the cooling performance and the interfacial area density. An expanding melt front, which leads to an increase in the interfacial area for heat transfer over time, and even heat distribution were also observed to be advantageous. After optimization, the honeycomb design with tapered triangular rods surrounded by the PCM matrix was able to achieve greatest weight savings for a given performance requirement. Compared to a thermal management panel consisting solely of the PCM, it was able to keep a heated surface cooler by 90% and also outperformed a pure Al panel despite being more than 40% lighter.Published versio

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)

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)

Strength and energy absorption characteristics of Ti6Al4V auxetic 3D anti-tetrachiral metamaterials

The strength and energy absorption properties of the auxetic 3D anti-tetrachiral (3ATC) lattice, a member of the newly formalized joint-rotation-dominated cellular topology, were studied for the first time, using analytical modelling, simulations and experiments employing additively manufactured titanium alloy (Ti6Al4V) lattices. A limited ductility failure model was employed in the simulations to accurately account for the enhanced brittleness of 3D printed Ti6Al4V. The 3ATC lattices exhibited 2 distinct relative strength vs. relative density relationships. If the relative density was varied through changes in strut length, the relationship was linear, a result that has, until now, been a distinguishing trait of rigid stretch-dominated lattices, rather than auxetic lattices, which normally experience strut bending. On the other hand, if the relative density was varied through changes in strut width, the relationship was highly nonlinear and does not follow the power law trend that is typical of many cellular solids. The strength of 3ATC lattices can be higher than that of stochastic foams, but were lower than those of stretch-dominated designs. In addition, 3ATC bulk lattices exhibited highly uniform properties across the three different orthogonal loading directions, despite individual unit cells being highly orthotropic. The failure of 3ATC lattices was characterized by a series of bumps in the plateau regime, which corresponded to the sequential failure and compaction of individual unit cell layers in the lattice. The specific energy absorbed during the failure process was found to be linearly related to the failure strength. The mechanical characteristics of the 3ATC lattices are dependent on 3 normalized design parameters: i) strut length, ii) strut width, and iii) eccentricity, offering enhanced flexibility for design optimization over conventional lattice designs. When manufactured with a material of high specific strength like Ti6Al4V, it was shown that 3ATC lattices can be useful as lightweight and high strength auxetic architected metamaterials that provide good energy absorption capabilities.Submitted/Accepted versio

Extremely stiff and lightweight auxetic metamaterial designs enabled by asymmetric strut cross-sections

Negative Poisson's ratio has been shown to enhance packing efficiency, impact absorption, indentation resistance, fracture resistance and acoustics attenuation in auxetic materials, making them useful for applications such as protective cases and stents. However, these benefits are usually realized at the expense of a low specific modulus and poor loading efficiencies, as auxetic structures rely on ‘soft’ strut bending and/ or joint rotation deformation modes to reach perceptible levels of negative Poisson's ratios. Here, a general 3D Anti-Tetrachiral (3ATC) structure was analyzed and shown that, in the limit of low relative density (i.e lightweight structure), the trade-off between specific relative stiffness and auxeticity is linear and the maximum attainable specific relative stiffness is 1/3. If the strut cross-section is symmetric, the maximum attainable Poisson's ratio is -0.5, while that for an asymmetric cross-section depends on the specific geometry of the cross-section. Importantly, our analysis shows that the non-zero product of inertia for asymmetric cross-sections can lead to an additional twist of the joint, thereby increasing the auxeticity of the 3ATC structure. This allows the 3ATC lattice to exhibit a large specific relative stiffness for a given Poisson's ratio, which can be more than an order of magnitude higher when compared to other auxetic designs in the literature. Our analysis was validated by finite element simulations and experiments on 3ATC lattices with strut cross-sections that were square (symmetric) and L-shaped (asymmetric).Nanyang Technological UniversitySubmitted/Accepted versionFunding for this project was partially provided by C.Q.L’s startup, Singapore grant (award no.: 020868–00001

Mechanical properties and failure behaviour of architected alumina microlattices fabricated by stereolithography 3D printing

Alumina microlattices with solid struts and different topologies were fabricated by the stereolithography 3D printing method. Mechanical analysis shows that specific stiffness and strength were highest for Simple Cubic lattices, followed by Octet Truss, then Kelvin Cell lattices. The mechanical properties followed Ashby’s power law well at small relative densities ( ≤ 0.3), but deviated from it at higher relative densities due to the increased importance of joint deformation. Failure in the Simple Cubic lattices proceeded in a column-by-column manner from the boundaries inwards to the centre, while fracture in Octet Truss and Kelvin Cell lattices took place predominantly along the diagonal (111) and (110) planes respectively. The underlying mechanism controlling these mechanical responses has been thoroughly discussed using finite element simulation analysis. Because lattice strength was limited by the tensile strength of alumina, which was an order of magnitude lower than its compressive strength, the microlattices were weaker than Ashby’s predictions. Nevertheless, they were still able to exhibit better specific modulus and strength than many current engineering materials, as well as some degree of ductility in the form of pseudoplastic strains (0.1 % - 0.5 %).Agency for Science, Technology and Research (A*STAR)Accepted versionThe authors would like to acknowledge with thanks the financial support of the work by A*STAR AME IRG grant with project number of A1883c0009 and the project with PA number of POD0713727