Linfa Peng

Micro hot embossing of thermoplastic polymers: a review

Micro hot embossing of thermoplastic polymers is a promising process to fabricate high precision and high quality features in micro/nano scale. This technology has experienced more than 40 years development and has been partially applied in industrial production. Three modes of micro hot embossing including plate-to-plate, roll-to-plate and roll-to-roll have been successively developed to meet the increasing demand for large-area patterned polymeric films. This review surveys recent progress of micro hot embossing in terms of polymeric material behavior, embossing process and corresponding apparatus. Besides, challenges and innovations in mold fabrication techniques are comprehensively summarized and industrial applications are systematically cataloged as well. Finally, technical challenges and future trends are presented for micro hot embossing of thermoplastic polymers.

Fabrication of Metallic Bipolar Plates for Proton Exchange Membrane Fuel Cell by Flexible Forming Process-Numerical Simulations and Experiments

PEM fuel cell nowadays is expensive for widespread commercialization, though it has obvious advantages, such as high efficiency, high power density, fast startup and high system robustness. As one of the most important and costliest components in the PEM fuel cell stack, bipolar plates (BPPs) account for more than 80% of the weight and 30% of the cost of the whole stack. By replacing the conventional graphitic or machined thick metal plates with the lightweight and low-cost thin metallic sheet BPP with sustainable coating, PEM fuel cell will become an attractive choice for manufacturers. In this study, the fabrication of micro-channel features by flexible forming process (FFP) are studied first, which demonstrates the feasibility of using FFP to manufacture thin metallic BPPs. Then, the obtained knowledge is applied onto the fabrication of real thin metallic BPPs with some process amendment. The first investigation of this study focuses on the forming of micro-channel features with 100 μm thickness stainless steel sheet. A finite element analysis (FEA) model is built and key process parameters (hardness of soft tools used in FFP, friction coefficients between contact surfaces) associated with the formability of BPPs are studied. The FEA is partly validated by the experiments. In the second investigation, finite element analysis method is adopted in the design of the BPP forming process. Based on the numerical simulation results, the die setup is prepared and some process amendments are made to improve the formability of BPPs. As a result, high quality metallic BPPs are obtained in the latter experiments, which demonstrates the feasibility to manufacture the metallic bipolar plate by FFP.

A Numerical Model for Predicting Gas Diffusion Layer Failure in Proton Exchange Membrane Fuel Cells

Gas diffusion layer (GDL) is one of the critical components in proton exchange membrane fuel cells (PEMFCs) and plays several important roles, such as structural support, reactants permeation, water removal, electrons, and heat conduction. The assembly pressure on bipolar plate is an important factor that affects the performance of PEMFC stack. Not enough assembly pressure leads to leakage of fuels and high contact resistance. Too much pressure, on the other hand, results in damage to the GDL, which increases the GDL Ohmic resistance and interfacial contact resistance, and in turn influences the reactant transport and water removal. The objective of the present study is to develop a numerical model to predict the onset of GDL failure and obtain the maximum assembly pressure on bipolar plate. Composite micromechanical model is applied to calculate the effective elastic properties of GDL; strength failure criterion is established to judge GDL damage with the stress distribution; finite element method model is developed to show the failure zone and the failure propagation in GDL combining the estimated elastic properties and strength failure criterion. Toray TGP-H-060 carbon paper is introduced as a numerical example and the numerical results show good agreements with experimental results. This numerical prediction model is beneficial to understand the basic mechanism of GDL failure and helpful to guide the assembling of PEMFC stack.

Design, Optimization, and Fabrication of Slotted-Interdigitated Thin Metallic Bipolar Plates for PEM Fuel Cells

Thin metallic sheet bipolar plates (BPPs) with sustainable coating are promising candidates to replace conventional graphitic or machined thick metal plates due to their lightweight and low cost. Interdigitated flow field design is easier for two stamped thin metallic sheets joined together to compose reactant flow fields in both sides and serpentine coolant flow field in the middle. Unfortunately, this kind of BPP inevitable brings two main defects: rupture of material during forming process and uneven flow distribution in practical operation. First, we propose a slotted-interdigitated configuration of the flow field for proton exchange membrane fuel cell with consideration of the characteristics of the metallic sheet forming process. In order to relieve the uneven flow distribution, an analytic model is introduced to analyze the reactant gas flow based on the similarity between the gas flow and the electrical current. Furthermore, an optimization model is proposed. The depth of the slot on the channel rib is optimized to eliminate the uneven flow distribution to obtain high reaction performance. Second, we studied the BPPs from the manufacturability perspective because it is also another important factor that should be considered in the design stage. The key geometric dimensions of flow field section, where the rupture occurs, are extracted and parametrized. Finite element analysis model is established to analyze the formability of BPP by flexible forming process (FFP). In addition, different dies with various flow channel sections are prepared and experiments are performed. Some design principles about material selection and key geometric dimension definition are proposed to improve the formability of BPPs. In the end, based on the design principles and experimental results, the dies are carefully design and fabricated experimental setup for FFP is prepared and practical experiments are performed. High quality metallic BPPs are achieved eventually by FFP. DOI: 10.1115/1.4002229

Multilayered TiAlN films on Ti6Al4V alloy for biomedical applications by closed field unbalanced magnetron sputter ion plating process

Ti6Al4V alloy has been widely used as a suitable material for surgical implants such as artificial hip joints. In this study, a series of multilayered gradient TiAlN coatings were deposited on Ti6Al4V substrate using closed field unbalanced magnetron sputter ion plating (CFUBMSIP) process. Taguchi design of experiment approach was used to reveal the influence of depositing parameters to the film composition and performance of TiAlN coatings. The phase structure and chemical composition of the TiAlN films were characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Mechanical properties, including hardness, Youngs modulus, friction coefficient, wear rate and adhesion strength were systematically evaluated. Potentiodynamic tests were conducted to evaluate the corrosion resistance of the coated samples in Ringers solution at 37°C to simulate human body environment. Comprehensive performance of TiAlN films was evaluated by assigning different weight according to the application environment. S8, deposited by Ti target current of 8A, Al target current of 6A, bias voltage of -60V and nitrogen content with OEM (optical emission monitor) value of 45%, was found to achieve best performance in orthogonal experiments. Depositing parameters of S8 might be practically applied for commercialization of surgical implants.

Experimental investigation on the large-area fabrication of micro-pyramid arrays by roll-to-roll hot embossing on PVC film

Large-area polymeric components with micro-pyramids have been widely applied in the fields of optics, optoelectronics, biology and chemistry, etc. Roll-to-roll (R2R) hot embossing is regarded as a promising approach to fulfil high throughput fabrication of patterned polymeric films. In this study, an R2R hot embossing system has been developed in-house and effective and continuous production of the polymeric component with micro-pyramids is demonstrated by R2R hot embossing. The influence of processing parameters has been firstly investigated by using the one-variable-at-a-time method. Afterwards, a series of experiments based on the central composite design approach have been conducted for the analysis of variance and the establishment of empirical models of the R2R hot embossing process. As a result, a 90 mm × 90 mm PVC sample with a feature height of 65 µm was successfully fabricated and the height consistency reached 94.5%. Additionally, a process window with a mold temperature of 150–160 °C, an applied force of 18–25 kgf and a feeding speed of 0.3–0.5 m min−1, was established to achieve 100% passable micro-pyramid arrays. The processing rules and the concrete ranges of parameter values can guide the process production of large-area micro-pyramids.

Flow behavior of polymers during the roll-to-roll hot embossing process

The roll-to-roll (R2R) hot embossing process is a recent advancement in the micro hot embossing process and is capable of continuously fabricating micro/nano-structures on polymers, with a high efficiency and a high throughput. However, the fast forming of the R2R hot embossing process limits the time for material flow and results in complicated flow behavior in the polymers. This study presents a fundamental investigation into the flow behavior of polymers and aims towards the comprehensive understanding of the R2R hot embossing process. A three-dimensional (3D) finite element (FE) model based on the viscoelastic model of polymers is established and validated for the fabrication of micro-pyramids using the R2R hot embossing process. The deformation and recovery of micro-pyramids on poly(vinyl chloride) (PVC) film are analyzed in the filling stage and the demolding stage, respectively. Firstly, in the analysis of the filling stage, the temperature distribution on the PVC film is discussed. A large temperature gradient is observed along the thickness direction of the PVC film and the temperature of the top surface is found to be higher than that of the bottom surface, due to the poor thermal conductivity of PVC. In addition, creep strains are demonstrated to depend highly on the temperature and are also observed to concentrate on the top layer of the PVC film because of high local temperature. In the demolding stage, the recovery of the embossed micro-pyramids is obvious. The cooling process is shown to be efficient for the reduction of recovery, especially when the mold temperature is high. In conclusion, this research advances the understanding of the flow behavior of polymers in the R2R hot embossing process and might help in the development of the highly accurate and highly efficient fabrication of microstructures on polymers.

Continuous fabrication of nanostructure arrays for flexible surface enhanced Raman scattering substrate

Surface-enhanced Raman spectroscopy (SERS) has been a powerful tool for applications including single molecule detection, analytical chemistry, electrochemistry, medical diagnostics and bio-sensing. Especially, flexible SERS substrates are highly desirable for daily-life applications, such as real-time and in situ Raman detection of chemical and biological targets, which can be used onto irregular surfaces. However, it is still a major challenge to fabricate the flexible SERS substrate on large-area substrates using a facile and cost-effective technique. The roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the continuous fabrication of flexible SERS substrate due to its high-speed, large-area, high-resolution and high-throughput. In this paper, we presented a facile and cost-effective method to fabricate flexible SERS substrate including the fabrication of polymer nanostructure arrays and the metallization of the polymer nanostructure arrays. The polymer nanostructure arrays were obtained by using R2R UV-NIL technique and anodic aluminum oxide (AAO) mold. The functional SERS substrates were then obtained with Au sputtering on the surface of the polymer nanostructure arrays. The obtained SERS substrates exhibit excellent SERS and flexibility performance. This research can provide a beneficial direction for the continuous production of the flexible SERS substrates.

Mechanism of forming defects in roll-to-roll hot embossing of micro-pyramid arrays I: experiments

Roll-to-roll (R2R) hot embossing is a promising process for the continuous fabrication of micro-structures on polymers with high accuracy and high throughput. This paper presents an experimental investigation into forming defects for micro-pyramid arrays during the R2R hot embossing process, aiming at a comprehensive understanding of the mechanism of forming defects as well as providing effective guidance for large-area fabrication of micro-structures on polymers. Four defect modes, the platform-like defect, the collapse-like defect, the high-low defect, and the bubble-like defect, have been discovered so far and the corresponding assessment methods have also been preliminarily established using Polypropylene (PP) as an example. The influence of two key process parameters including mold temperature and feeding speed on forming defects have been systematically investigated as well. It is demonstrated that the forming defects change from a platform-like defect to a collapse-like defect, then to a high-low defect, and finally disappear as the mold temperature increases. In the meantime, a bubble-like defect may occur if the feeding speed exceeds 1.2 m min−1. Besides, the forming defects are also observed in the R2R hot embossing of other polymer materials, such as poly(vinyl chloride), polymethyl-methacrylate and polycarbonate. Analysis of the mechanism of forming defects is beneficial to the understanding of the flow behavior in the R2R hot embossing process and to the realization of process control for high-accuracy replication of large-area polymer films with micro-structures.

Continuous Fabrication of Multiscale Compound Eyes Arrays With Antireflection and Hydrophobic Properties

The multiscale hierarchical structures inspired by moths compound eyes offer multifunctional properties in optoelectronic devices. However, it is still a major challenge to fabricate these hierarchical structures on large-area substrates using a simple and cost-effective technique. The roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) technique provides a solution for the continuous fabrication of multiscale compound eyes arrays due to its high-speed, large-area, high-resolution, and high-throughput. In this paper, the R2R UV-NIL technique was used to fabricate the multiscale compound eyes arrays on the PET substrate. The mold used in the R2R UV-NIL process was acquired by anodic aluminum oxide process and then the multiscale compound eyes arrays were directly obtained via one-step R2R imprinting. The obtained multiscale compound eyes arrays exhibit excellent antireflective performance within the wavelength 400-800 nm. Besides, the compound eyes arrays also equip the surface of the microlens with excellent hydrophobic characteristics. These multifunctional properties enable the multiscale compound eyes arrays to retain their superior optical properties in real-time environmental conditions. This report can provide a beneficial direction for the continuous production and widespread applications of the multiscale compound eyes arrays.