Applied Physics at the APS March Meeting
By Michael Lucibella
APS March Meeting, Denver — This year’s March Meeting included physics with big commercial potential. Here are just a few highlights of the research presented with serious industrial applications.
Thermoelectric generators can convert waste heat into electricity and are poised to make a big splash in the green-energy industry. Gregory Meisner of General Motors Global R & D in Warren, Michigan is developing a device to harvest waste heat from auto exhaust systems. Meisner and his team received a four-year Department of Energy (DOE) grant in 2012 to develop a workable prototype that can increase the fuel economy of a vehicle by 5%.
“If we can convert heat to electricity, we can improve vehicle economy,” Meisner said. “This will be the first application of high-temperature thermoelectric materials for high-volume use.” Thermoelectric generators use materials that convert temperature differences into an electric potential.
Meissner said that earlier devices simply bolted onto the exhaust pipe, away from the heat of the catalytic converter. His first prototype, still a bolt-on device, was able to convert about 2.5% of the heat it absorbed into electricity. He is planning future prototypes that will be “more integrated into the system.”
Thermoelectrics could have a big impact on the auto industry. As much as 75% of the energy generated by a car’s internal combustion engine ends up lost as waste heat.
“Thermoelectrics have been around for more than 100 years,” Meisner said, adding that NASA has been using the technology for decades to power its space probes [see page 2]. “It’s only been relatively recently that the performance of thermoelectric materials entered the range of significant power generation.”
Though Meisner hopes to have a working prototype by 2016, it would likely still take several years of commercialization before cars start rolling off the lot with these generators.
Faster 3D Printing
The 3D printing revolution is just beginning and physicists are finding ways to speed up the process.
Most 3D printers squirt a thin layer of resin onto a surface, then add more layers on top of it until the object is printed. Printers creating metal objects work essentially the same way, but produce a jet of fine metallic grains and fuse them together with a powerful laser.
These printers can take many hours to fabricate an object. The time could be shortened by increasing the flow rate with smaller grains, but small grains also have a propensity to clog at the printer’s nozzle, known as a hopper.
Guo-Jie Gao of Osaka University said, “We can reduce the jamming probability and increase the flow rate, if we put an occlusion in the hopper.”
It sounds counterintuitive, but Gao found that he could reduce jams by controlling the flow with a strategically placed obstacle right in front of the nozzle. The blockage redirects the granular flow around it so particles don’t clog at the narrow bottleneck.
Gao simulated the dynamics of a hypothetical material, and is planning next on investigating the properties of actual grains used in 3D printing.
“We want to study the effect of the friction constants of different materials,” Gao said.
De-Icing Wind Turbines
Snow and ice hamper a turbine’s ability to harvest wind energy. The trick to efficiently clearing ice off a wind turbine might be to keep its surfaces just a little bit wet. Researchers have developed a coating for metal that traps nano-sized air and water bubbles so that ice slides off with almost no effort.
“Our strategy is inspired by ice skating,” said Jianjun Wang from the Chinese Academy of Sciences. He added that ice skaters slide along a thin layer of liquid water underneath their blades. “Could we introduce such a water layer under the ice?”
Wang and his team created a coating from hygroscopic polymers, materials that absorb water. The trapped water droplets stay liquid, creating a thin wet layer that easily sloughs off ice.
“Hygroscopic polymers will absorb and hold water, thus an aqueous lubricating layer forms,” Wang said.
Wind turbine companies have already started inquiring about applying the team’s coating even though the researchers have only carried out preliminary proof-of- concept tests.
Better Flexible Solar Cells
Researchers found that a dose of graphene improves the efficiency of flexible solar cells. They were able to almost triple the energy conversion efficiency of a solar cell made of polymers by mixing the right concentration of nano-sized graphene flakes into the solar cell.
“Graphene is a promising additive to a polymer solar cell,” said Yan Jin of the University of Cincinnati.
Polymer solar cells have huge commercial potential because of their flexibility and durability compared to silicon cells, but have been held back because of their low energy-conversion efficiency.
Jin and her team extracted the graphene flakes from graphite and then mixed them in with the polymer that makes up the cell’s active layer, which converts light into electricity. She found that the optimal mix of graphene is about 0.1 milligrams per milliliter of the solar cell polymer.
“We expect that it can also be used for other systems,” Jin said.
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