Expensive and weighty metals play crucial roles in heat transfer applications because of their high thermal conductivity. For example, cases for cellular phones and some other electronics are made of metal rather than inexpensive materials with lower thermal conductivity because the metal is necessary to dissipate induced internal heat. Similarly, seawater desalination treatments use expensive titanium heat exchangers instead of exchangers made of more economical metals because titanium withstands corrosive characteristics of saltwater.
Until recently, polymers, which are cost-effective, lightweight and corrosion-resistant materials, were not alternatives to metals in heat transfer applications because their thermal conductivity was too low. A few years ago, a group of MIT researchers developed nanofibers of polyethylene with thermal conductivity of 100 watts per meter kelvin. This value of thermal conductivity is higher than the values for half of the metals. However, sheets of the polyethylene were needed to implement the material for practical applications. An engineering professor at the University of Houston and his collaborators at MIT have developed a method for large-scale production of the high thermal conductivity polyethylene.
Hadi Ghasemi, assistant professor of mechanical engineering at the UH Cullen College of Engineering, published a paper in the journal, Technology, about a process for continuous fabrication of highly aligned polyethylene sheets with high thermal conductivity.
“Replacing titanium heat exchangers with polyethylene heat exchangers will have a significant impact in cost reduction of desalination,” Ghasemi said. “Also, this new material will lead to lighter and more cost-effective electronics.”
Polymers contain the same types of molecular chains, or carbon-carbon bonds, as graphene and graphite, which are materials with high thermal conductivity. However, misalignment of the bonds in bulk polymers scatters the heat carriers resulting in reduced thermal conductivity. The team of researchers discovered that realignment of the bonds in the direction of heat flux produced high values of thermal conductivity.
The newly developed process includes three-steps: sol-gel preparation, extrusion and drawing. The developed platform accomplishes the steps automatically and fabricates polyethylene sheets with thermal conductivity of more than 30 W/(mK).
Polyethylene, which accounts for approximately 50 percent of the polymer industry, is already used widely in applications ranging from chemical to biotechnological. The new platform opens a window for use of polyethylene in heat transfer applications with advantages including lower cost, lighter weight and higher energy efficiency during manufacturing than metals, Ghasemi said.