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Title: Theoretical and experimental investigation of carbide-bonded graphene-assisted hot embossing of plano-convex glass microlens arrays
Authors: Yang, Gao
Degree: Ph.D.
Issue Date: 2020
Abstract: Glass plano-convex microlens arrays (MLAs) show great potential in serving imaging, sensing, illuminating, photovoltaic, digital optical processing, and optical communicating systems. Hot embossing is a promising technology for the production of glass MLAs, because of its high manufacturing efficiency, high replication fidelity and simplicity in tool development. The conventional approach for realizing high temperatures in hot embossing is by using electrical cartridge tubes, which requires the mold and workpiece to be heated simultaneously. As a result, the throughput of replication is limited by the high energy consumption and the long thermal cycle time. This study attempts to address these drawbacks by the replacement of the traditional bulk heating method with a novel carbide-bonded graphene (CBG) based Joule heating technique. The highlight of the proposed glass hot embossing configuration is that the CBG thin film on the silicon mold insert can serve as a film heater as well as a protective mold coating. The CBG-based Joule heating system mainly consists of a CBG thin film and an intrinsic silicon wafer. The former is a new material, while the latter is a semiconductor. Therefore, the heating characteristics of the CBG-based Joule heating system are expected to be complicated and deserve investigation. First, the influence of gaseous environment on the heating performance of a CBG-coated silicon wafer was quantitatively evaluated by controlled trials. The electrical and thermal responses of CBG-coated intrinsic silicon was analyzed. Furthermore, the effects of input voltage and current-limiting threshold on its thermal profiles were evaluated. Moreover, it was noted that the resistance of CBG-coated intrinsic silicon was strongly temperature-dependent, and a general pattern of the temperature dependence was discussed. After that, the relation between the temperature change of the sample and the input energy of power supply was analyzed. The heating characteristics of CBG film on fused silica and doped silicon were evaluated as well. The findings in this study can facilitate the precise temperature control of the CBG-based Joule heating system in glass hot embossing. In the second part, a tailor-made micro hot embossing tool equipped with a modified CBG-based Joule heating system was designed and developed for transferring MLAs from the CBG-coated silicon mold insert into the optical glass (P-SK57). Initially, the surface topographies of a typical 3Ă—3 MLA from a bare silicon mold to an embossed glass replica were compared for the purpose of evaluating form errors which arose during different preparation stages. The feasibility of the CBG-based Joule heating technique for the non-equilibrium thermal forming of optical glass was evaluated by the surface integrity and replication fidelity of the embossed MLA features. The thermally induced residual stress and imaging performance of the embossed glass lens was assessed as well. The experimental results indicate that the presented CBG-assisted embossing process has the capability of manufacturing high-quality microstructures on optical glass, and it allows notably fine replication of surface shapes at the microscale, as well as roughness information at the nanoscale. Consequently, the imaging performance of the embossed glass MLAs can be ensured.
Warpage is inevitably generated on the global surface of the glass replica during the hot embossing process, which may deteriorate its optical performance. The third part of this study attempted to numerically evaluate the sensitivity of warpage shape to different process parameters, which is expected to provide guidelines for minimizing or even eliminating the warpage during the CBG-assisted hot embossing process. First, the material properties of glass and molds were studied by either experiment or referring to archive literature. Material properties, together with interaction effects and boundary conditions, served as the input information of the finite element (FE) simulation model. In the establishment of the FE simulation models, the generalized Maxwell, William-Landel-Ferry (WLF), and Tool­Narayanaswamy-Moynihan (TNM) models are used to characterize the viscoelastic, thermo-rheological simple, and structural relaxation behaviors of glass, respectively. With the predefined process parameters, a simulation of the hot embossing process was undertaken by employing commercially available FE software named ABAQUS. The convergence of the FE simulation model was validated by using different mesh methods. The results under different simulation conditions were then analyzed to quantify the influences of different process parameters (e.g., embossing temperature, soaking time, pressing rate, pressing time, annealing rate, and fast cooling point) on the warpage shape. Apart from the provision of the sensitivity results, the simulation models also provided a better understanding of the shape evolution of warpage on the replica surfaces in the course of glass hot embossing. The present study investigated the heating characteristics of CBG-coated intrinsic silicon and successfully verified the utility of the CBG-based Joule heating technique in the hot embossing of glass micro optics for the first time. Furthermore, simulating the warpage formation on the global surface of the embossed glass replica was conducted for optimizing the glass hot embossing process. Therefore, the outcome of this study significantly contributes to the state-of-the-art level of advanced optical fabrication technology.
Subjects: Glass embossing
Hong Kong Polytechnic University -- Dissertations
Pages: xxiv, 291 pages : color illustrations
Appears in Collections:Thesis

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