Yuh-Jin Chao

Degradation Characteristics of Elastomeric Gasket Materials in a Simulated PEM Fuel Cell Environment

Polymer electrolyte membrane (PEM) fuel cell stack requires gaskets and seals in each cell to keep the reactant gases (hydrogen and oxygen) within their respective regions. The stability of the gaskets/seals is critical to the operating life as well as the electrochemical performance of the fuel cell. The time-dependant chemical and mechanical degradation of two commercially available silicones-based elastomeric gasket materials in a simulated fuel cell environment was investigated in this work. Two temperatures based on actual fuel cell operation were selected and used in this study. Using optical microscopy, the topographical damage on the sample surface due to the acidic environment was revealed. Atomic adsorption spectrometer analysis shows that silicon, calcium, and magnesium were leached from the materials into the soaking solution. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to study the surface chemistry of the elastomeric gasket materials before and after exposure to the simulated fuel cell environment over time. The ATR-FTIR and XPS test results indicate that the surface chemistry changed significantly and the chemical degradation mechanism is de-crosslinking and chain scission in the backbone. The microindentation test results show that the mechanical properties of the silicone materials changed significantly after exposure to the simulated PEM fuel cell environment over time.

In situ Nanomechanical Characterization of Single-Crystalline Boron Nanowires by Buckling†

One-dimensional (1D) nanostructures have attracted tremendous attention because of their various intriguing properties and potential applications in mesoscopic research as well as in the development of nanodevices. Boron is the third lightest element in the solid form with a high melting temperature ( 2300 8C) and hardness close to that of diamond. The mechanical strength, chemical and thermal stabilities, and electrical conductivity of 1D boron nanostructures are predicted to be comparable with or even better than those of carbon nanotubes. Previous studies have shown that crystalline boron nanowires are semiconducting and exhibit electrical properties consistent with those of elemental boron. Amorphous boron nanowires have been used as templates to synthesize crystalline MgB2 nanowires. [11] The superconductivity of those MgB2 nanowires was confirmed. Moreover, the electronic properties and diameters of boron and boride nanowires are amenable to synthetic control. Hence, both conducting and semiconducting components necessary to construct functional nanodevices can be developed from boron and boride nanowires. Due to these unique features, boron and boride nanowires are emerging as the most promising nanoscale building blocks. Many efforts have been devoted to the synthesis of boron nanowires. To date, boron nanowires have been synthesized by chemical vapor deposition (CVD), thermal vapor transport processes, laser ablation,

Numerical Analysis of Joint Temperature Evolution During Friction Stir Welding Based on Sticking Contact

A three-dimensional numerical model for friction stir welding was developed by using the ABAQUS software based on a fully sticking friction. The temperature measurement was performed to validate the reliability of the model. The simulated thermal histories are in good agreement with the experiments. Simulated results show that the rotation speed has no influence on the time to reach the peak temperature in the workpiece, while the welding speed has significant effect on the time to reach the peak temperature at points away from the plunging center. The value of this peak temperature also changes somewhat. Moreover, the peak temperature in the workpiece tends to reach a quasi-steady state at the beginning of the moving stage; but the temperature at some distance away from the weld does not reach the quasi-steady state during the welding.

Modelling of entire friction stir welding process by explicit finite element method

Abstract A coupled thermomechanical three-dimensional finite element model was developed for friction stir welding in the ABAQUS environment using Johnson–Cook material law and Johnson–Cook failure model. The temperature evolution during the plunge, dwell and moving stages of a friction stir welded 7050 aluminium alloy and the effect of heat conduction by the back plate were investigated. Results show that the temperature almost symmetrically distributes across the plate cross-section, and the temperature contour in the weld nugget zone presents a V type shape after the plunge stage. In the dwell stage, the frictional heat conducts around to preheat the plate. While in the moving stage, the heat gradually accumulates until a quasi-stable temperature field is formed. Moreover, it is shown that the heat conduction through the back plate has a significant effect on the temperature field. With the increasing heat convective coefficient of the back plate, the temperature field remarkably shrinks.

Microindentation Test for Assessing the Mechanical Properties of Silicone Rubber Exposed to a Simulated Polymer Electrolyte Membrane Fuel Cell Environment

The elastomeric materials used as seals and gaskets in polymer electrolyte membrane (PEM) fuel cells are exposed to acidic environment, humid air, and hydrogen, and subjected to mechanical compressive load. The long-term mechanical and chemical stability of these materials is critical to both sealing and the electrochemical performance of the fuel cell. In this paper, mechanical degradation of two elastomeric materials, Silicone S and Silicone G, which are potential gasket materials for PEM fuel cells, was investigated. Test samples were subjected to various compressive loads to simulate the actual loading in addition to soaking in a simulated PEM fuel cell environment. Two temperatures, 80°C and 60°C, were selected and used in this study. Mechanical properties of the samples before and after exposure to the environment were studied by microindentation. Indentation load, elastic modulus, and hardness were obtained from the loading and unloading curves. Indentation deformation was studied using Hertz contact model. Dynamic mechanical analysis was conducted to verify the elastic modulus obtained by Hertz contact model. It was found that the mechanical properties of the samples changed considerably after exposure to the simulated environment over time. The temperature and the applied compressive load play a significant role in the mechanical degradation. The microindentation method is proved to provide a simple and efficient way to evaluate the mechanical properties of gasket materials.

Effect of tensile offset angles on micro/nanoscale tensile testing

For one-dimensional (1D) structures such as tubes, wires, and beams, tensile testing is a simple and reliable methodology for measuring their mechanical properties. The tensile offset angle effect on mechanical property measurement has long been ignored. In this study, theoretical and finite-element analysis (FEA) models for analyzing the tensile offset angle effect have been established. It is found that longitudinal stress decreases with increasing offset angles. The theoretically calculated elastic modulus relative errors reach 4.45% at the offset angle of 10°, whereas the experimentally measured elastic modulus relative errors are 45.4% at the offset angle of 15°. The difference in elastic modulus relative errors between the theoretical analysis and the experimental results is discussed with reference to the sensing system in the experimental instrumentation. To accurately measure the mechanical properties using the tensile testing technique, perfect alignment with a zero or small offset angle less th...

Degradation of Gasket Materials in a Simulated Fuel Cell Environment

A Polymer Electrolyte Membrane (PEM) fuel cell stack requires elastomeric gaskets in each cell to keep the reactant gases within their respective regions. If any gasket degrades or fails, the reactant gases (O2 and H2 ) can leak overboard or mix with each other directly during operation or during standby, and affect the overall operation and performance of the fuel cell. The degradation of four commercial gasket materials was investigated in a simulated fuel cell environment in this study. In an effort towards predicting lifetime of fuel cells, two solutions and two temperatures were used in the short-term, accelerated aging tests. Bend-strip environment crack resistance tests were performed on samples with various bend angles. Weight loss was monitored and surface structure changes were examined using optical microscopy on the samples exposed to the simulated fuel cell environment for selected periods of time. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy was employed to study surface chemistry of the gasket materials before and after exposure to the simulated fuel cell environment over time. Stress and strain analysis was conducted using finite element method (FEM) to quantify the stress/state in test samples. The test results reveal that two silicone materials were degraded significantly while the other two did not show much degradation up to 42 weeks exposure to the simulated fuel cell environment. Optical microscopy and ATR-FTIR spectroscopy analysis indicate that the surface chemistry altered gradually via mechanisms involving de-cross linking and chain scission in the backbone. From experimental and numerical results, it is concluded that there is an interaction between chemistry and stress that appears to accelerate the degradation of the gasket materials in fuel cell environment.Copyright

Variation of Compression of Seals in PEM Fuel Cells

Seals or gaskets under compressive stress are used in PEM fuel cells (PEMFC) or stacks to prevent leaking of the liquid and gas inside the cell. The fuel cells are normally assembled with bolts or a combination of bolts and springs. As the seal is typically made of polymers, the level of the compressive stress on the seal during long term operation of the fuel cell depends on the stress relaxation property and any potential chemical degradation of the seal materials. In addition, the amount of compression applied to the seal may vary due to temperature changes during the fuel cell operation which causes thermal expansion and contraction of all components in the cell. To understand the sealing force existed in a fuel cell during operation, all these factors must be fully understood. In this study, the compression of the seal in a PEMFC was investigated experimentally. Specifically the compressive amount was measured in-situ, i.e. immediately after the assembly and during the normal operation of the PEMFC. The objective of this study is to gain an understanding of the variation of compressive strain applied to the seal as the temperature of the PEMFC changes and cycles. This information can then be used to estimate the sealing force in the cell and consequently the life prediction of the seal. Both the temperature and pressure are monitored during the tests. An interesting observation is that both the gap spacing and the outside dimensions jumped initially and did not follow the cell temperature’s later rise to a maximum of 80oC. It’s effect by the first gas inlet pressure.

Compression of Seals in PEM Fuel Cells

Seals or gaskets are used in PEM fuel cells (PEMFC) or stacks to prevent leaking of the liquid and gas inside the cell. The fuel cells or the stacks are normally assembled with nuts and bolts or a combination of nuts, bolts, and springs. As the seal is typically made of polymers, the level of the compressive stress applied to the seal during long term operation of the fuel cell relaxes. In addition, the amount of compression applied to the seal may vary due to temperature changes during the fuel cell operation which arises from thermal expansion and contraction of all components in the cell. To understand the sealing force existed in a fuel cell during operation, all these factors must be fully understood. In this study, the compression of the seal in a PEMFC was investigated experimentally. Specifically the amount of compression was measured in-situ, i.e. immediately after the assembly and during the normal operation of the PEMFC. The objective of this study is to gain an understanding of the variation of compressive strain applied to the seal as the temperature of the PEMFC changes and cycles. This information is useful in estimating the sealing force in the cell and consequently the life of the seal.

Service Life Prediction of Seal in PEM Fuel Cells

Proton exchange membrane fuel cell (PEMFC) is a promising power source for automobiles in the near future. During operation, there are gases and liquids inside the fuel cell. Sealing around the peripherals of the cell is therefore required to prevent the gases/liquids inside the cell from leaking. Polymers are usually used as the sealing or gasket materials. They in general possess the property of viscoelasticity. The stress relaxation behavior of liquid silicon rubber, a type of polymer, is studied in this article. Applying the time-temperature superposition, master curve is generated for prediction of service life of this material used as seals in PEMFC.