Pak Yan (Daisy) Yuen
Research Interests
Devices are composed of materials that need to operate under various conditions, such as increased temperature, humidity and mechanical stresses. These conditions may change the mechanical and fracture properties of the materials, and lead to a compromise in the safety and performance of the device.
I study the synergistic effects of these “stressing” parameters on the mechanical and fracture properties. I relate these properties to the underlying material chemical composition and microstructure, and provide insight into material property, device design and fabrication processes improvements.
Mechanical Characterization of Novel Lithium Metal Battery Materials
Lithium metal batteries have been considered the “holy grail” in battery research because of their high capacity. One of the biggest challenges to overcome is the suppression of lithium dendrites. During battery cycling, lithium dendrites may grow and puncture the electrolyte/separator, causing internal short circuit and detrimental safety problems.
It has been shown that a high modulus material can mechanically suppress lithium dendrites growth. Mechanical characterization thus becomes a critical part of material discovery for lithium metal batteries.
Fig. 1: A schematic showing the suppression of lithium dendrites by a solid electrolyte in a lithium metal battery.
I examine the mechanical properties, including elastic modulus and hardness, of novel solid electrolytes materials. I apply mechanics models to explain the role of microstructure and composition on the overall mechanical behavior, shedding insights into strategies to further improve on the mechanical properties.
Effect of Damp Heat on Backsheet Structures in Photovoltaic Modules
Photovoltaic (PV) modules are composed of several key components including glass, solar cell, encapsulant and backsheet structure. Even though PV modules are designed to operate for at least 20 years, tearing of the backsheet is observed after only a few years of use.
A backsheet structure, or “backsheet” for short, is a composite film laminated on the back of a PV module. Backsheets are composed of several layers of polymeric film each serving a specific barrier function such as electrical insulation and moisture barrier. The integrity of the backsheets is critical to the performance and safety of a PV module.
Fig. 2: Cross-section of a PV module in which tearing of the backsheet is illustrated.
In my study, I use a wide selection of backsheets with various types of fluoropolymer layer and adhesion layer. I determine the role of damp heat (85°C/85% relative humidity) on the tearing properties and correlate the change in mechanical properties to the change in chemical composition and microstructure.
I also analyze the complex failure modes in the backsheet during tearing. I find that due to the interfacial interaction between the constituent layers, the tearing resistance of a backsheet is more than simply that of the “sum” of its constituent layers.
Effect of Mechanical Constraint on Tearing of Proton Exchange Membranes in Fuel Cells
The thermomechanical reliability of low temperature polymer proton exchange membranes (PEMs) in hydrogen fuel cells is one of the biggest challenges in improving fuel cell reliability. The reliability of the PEMs is controlled by several factors, including the molecular structure, the operation conditions (e.g. relative humidity, temperature), and possible contamination of the membrane. In particular, the role of mechanical constraint on the tearing resistance is especially relevant because membranes are always constrained by the fuel cell hardware such as the bipolar plates.
Fig. 3: Cross-section of a proton exchange membrane fuel cell in which the PEM is compressed between the bipolar plates.
I studied salient mechanical properties, including stiffness, strength, tearing resistance and stress, of several types of perfluorosulfonic acid polymer (PFSA) PEMs. I used an unconstrained tearing test to examine the effect of temperature and water content on local plasticity during tearing.
In addition, I developed a constrained tearing test to investigate the effect of mechanical constraint on the tearing resistance. I demonstrated that mechanical constraints can significantly decrease the tearing resistance because the constraints limit the formation of local plasticity during tearing. I also showed that the rate dependency in tearing is suppressed when under mechanical constraints.
Fig. 4: Schematic and cross-polarized image of a) an unconstrained tearing specimen and b) a constrained tearing specimen, and c) a plot showing the tearing force as a function of total tear length of an unconstrained and constrained tearing specimen. The tearing force of the constrained specimen is significantly lower.
Publications
- P.Y. Yuen, R.H. Dauskardt, “Effect of Mechanical Constraint on Tearing Energy of Polymer Membranes”, Macromol. Mater. Eng., 2016.
- Y.Y. Liu, D.C. Lin, P.Y. Yuen, K.Liu, J. Xie, R.H. Dauskardt, Y. Cui, “An Artificial Solid Electrolyte Interphase with High Li-Ion Conductivity, Mechanical Strength, and Flexibility for Stable Lithium Metal Anodes”, Advanced Materials, 2016.
Conference Proceedings and Presentations
- P.Y. Yuen and R. H. Dauskardt, “Reliability of Polymer Backsheet Structures in Photovoltaic Modules”, ACS 253rd National Meeting, San Francisco, CA, April 5, 2017.
- P.Y. Yuen and R. H. Dauskardt, “Mechanics of Proton Exchange Membrane in Fuel Cells,“ MRS Spring Meeting, San Francisco, CA, April 9, 2015.
Awards
- Stanford Graduate Fellowship – Charles H. Kruger Fellow (2013-2016)
Activities
- Mentorship Committee, Association for Women in Science – Palo Alto (2017)
- Media Chair, Stanford Mechanical Engineering Women’s Group (2014-2016)
Professional Portfolio