Monday, July 6, 2015
SpectroscopyNOW- Last Month's Most Accessed Feature: The heat is on: Thermal behaviour of MOFs
The first measurements of the thermoelectric behaviour of a nanoporous metal-organic framework (MOF), including infrared temperature measurements, have been carried out and could point the way to an entirely new class of materials for a wide range applications from heatsinks for computer chips and cameras to energy harvesting.
Thermoelectric devices are able to convert heat to electricity without resorting to moving parts and so could be used for either cooling or energy-harvesting applications. If thermoelectric MOFs were to exist, these materials could bring the design flexibility and improved performance capacity to this area. MOFs have a crystalline structure consisting of rigid organic molecules linked together through metal ions, which gives rise to many hybrid properties including nanoporosity, enormous surface area to volume ratio and high thermal stability. Those pores can, of course, be filled with other molecules to make "Guest@MOF" compounds, according to researchers at Sandia National Laboratories, USA. These guests can be used to impart entirely new properties to the MOF or fine tune existing ones for a given application simply by changing the guest.
Previously, the team had demonstrated electrical conductivity in MOFs by feeding tetracyanoquinodimethane, TCNQ, into the pores, which made the researchers hopeful of observing thermoelectricity, although team member Mark Allendorf says this was by no means a given. “These results introduce MOFs as a new class of thermoelectric materials that can be tailored and optimized,” explains physicist François Léonard.“This discovery brings us a step closer to realizing the potential of MOFs in practical applications.” Allendorf adds that, “We found that not only is the material thermoelectric but also the Seeback coefficient exceeds that of the best thermoelectric materials, such as bismuth telluride."
Once they had filled the pores of their MOF with TCNQ, to make TCNQ@MOF, the researchers had to find a way to measure the anticipated thermoelectric properties. As such, Léonard, Alec Talin and Kristopher Erickson, created a thermoelectric device by connecting Peltier heaters and coolers to each end of a thin film of TCNQ@MOF to generate a tiny temperature gradient. They accurately measured the temperature gradient with an infrared camera whilst at the same time recording the voltage output. From the data they could obtain the Seebeck coefficient, the voltage per unit of temperature change. The thermal conductivity, which is needed to estimate the efficiency of thermoelectric energy conversion, was measured by Patrick Hopkins and Brian Foley from the University of Virginia using a laser technique. Time Dependent Thermal Reflectance or TDTR, in which a laser pulse is used to heat a metal film deposited on top of the material in question and to measure the reflectance. A second laser pulse delays by a short delta-t is then used to measure the change in reflectance. The metal reflectance is strongly dependent on the temperature, so by measuring delta-R you can measure delta-T, from which you can calculate the thermal conductivity using a model.
TCNQ@MOF has a high Seebeck coefficient and low thermal conductivity, which are two important prerequisites for efficient thermoelectricity. The Seebeck coefficient is on a par with that measured for bismuth telluride, a leading solid state thermoelectric material.
The measurements have also given the researchers a clearer understanding of the electronic structure of TCNQ@MOF. Team members Catalin Spataru and Mike Foster conducted detailed electronic structure calculations and Reese Jones performed thermal conductivity simulations. "We were trying to understand the role of the guest molecule, TCNQ in this case, when it infiltrates the pore of a MOF," says Spataru. "Finding a representative configuration for the combined TCNQ@MOF system via computer simulations was particularly challenging, as we don't expect guest molecules to form an ordered structure." Nevertheless, the simulations revealed how charge transport occurs in the material and shows that the TCNQ@MOF is a p-type material.
The obvious next step, having revealed that latter fact, is to find a small molecule to combine with a MOF to create an n-type semiconductor but with similar properties to TCNQ@MOF. "Once we find that, we’ll be at the early stage of creating a full thermoelectric device," says said Léonard.
Efficient Guest@MOF materials could replace existing cooling technology in devices where size and weight matter, such as in the cameras mounted on satellites or in a more mundane situation to replace the noisy fans needed for cooling by high-power computer chips. Conversely, energy-harvesting thermoelectric devices would capture waste heat for countless sources to generate electricity. A thermoelectric device fitted to a car engine or exhaust system could grab the waste heat and use it to power the car's electronics. "Another potential application is using temperature gradients in the ground to power sensors in remote areas," adds Léonard. "Thermoelectrics could be quite ideal for this application, as you could set up a device and leave it to run for long periods of time."
"The next step is how do we make [them] better?" asks Allendorf. "The energy conversion is not competitive yet with solid state materials, but we think we can improve that with better electrical conductivity."