A Thermal Switch

June 1, 2015

As knowledgeable homeowners realize when paying their winter heating bills, thermal conductivity is an important material property. Home insulation is identified by a quality measure called the R-value. Since people like to relate better things with higher numbers, the R-value is used instead of its reciprocal, called the thermal transmittance (U-value). The U-value is the rate of heat transfer in a square meter area divided by the temperature difference across the insulator, so it has units of W/m²-K.

The R-value is the reciprocal of this, so it has units m²-K/W. Since the US still clings to older measurement units, R-values are expressed here as °F-ft²/(Btu/hr). I was part of my former employer's metrication team in the early 1980s, so there's been a lot of talk, but no progress, in US metrication in three decades. There are times, however, when customary units are better; e.g., weather reports in Fahrenheit express more accuracy than those in Celsius.

According to folklore, the customary length unit, the yard was defined as the distance from nose to fingertips of the outstretched arm of King Henry I of England (1068-1135). (Modified Wikimedia Commons image.)

Typical fiberglass batting for home insulation is sold as R-30. Thirty is what you get when you multiply the R-value of fiberglass batts (about 4) by the thickness in inches. Highly energy-efficient homes will even have exterior walls of greater thickness than the traditional 3-1/2 inch width of a wooden stud in order to accommodate more insulation. Of course, much of the heat from buildings is lost through windows, so multiple glass panes are important.

There's a story (Wikipedia "citation needed" folklore, but probably true) that the Tasmanian house of radio astronomy pioneer, Grote Reber (1911-2002), was so well insulated that he wasn't able to use his cooking oven, since it would overheat the house. Here's a table of R-values for common building materials.

Material	R-value (°F-ft2/(Btu/hr)
Poured concrete	0.08
Glass	0.14
Hardwood (typical)	0.71
Snow	1
Brick	1.5
Vermiculite (loose-fill)	2.25
Plywood	2.55
Cellulose (loose-fill)	3.5
Cotton batts	3.7
Home foam	3.9
Fiberglass batts	4
Vacuum panel	40

I've written many articles about thermal conductivity on this blog, one of which, Thermal Diodes, February 19, 2014, was about devices designed to conduct heat in just one direction. One thermal diode has been proven by experiment,[1-2] while another has been shown to be feasible in theory.[3-4] The trick to making such a device is manipulation of phonons, the quantized acoustic vibrations in solids principally responsible for heat conduction in solids.

A thermal switch is just as valuable as a thermal diode. An automatic mechanical thermal switch was patented by Carl E. Weller in 1960 as a mean of maintaining the temperature of a soldering iron.[5] Some applications would benefit from an electrically-actuated, solid state thermal switch with no moving parts.

Scientists at Sandia National Laboratories (Albuquerque, New Mexico), the University of Virginia (Charlottesville, Virginia), and Pennsylvania State University (University Park, Pennsylvania) have recently reported a thermal switch based on the common material, lead zirconate titanate (Pb(Zr_0.3Ti_0.7)O₃.[6-7]

Carl Weller patented a mechanical thermal switch for keeping a soldering iron tip at the proper temperature. The design has a spring-loaded magnet (shown here attracting a nickel-coated wire). The magnetism vanishes at the Curie temperature, throwing the thermal switch.[5] (Photo by the author)

A solid-state thermal switch would enable such exotic technologies as phononic logic, as well as offering improvements in thermal management and energy harvesting.[6] PZT is a common material that's used in ceramic and thin film form for actuators, speakers, and spark generators for lighting barbecue grills.[7]

The thermal switch operates by controlling the nanoscale ferroelectric domain walls in the material, and this leads to control of the heat transmitting phonons. The demonstration device has a subsecond response time, and the room temperature thermal conductivity could be changed by 11%.[6-7]

It takes just the voltage from a nine-volt battery to effect the 11% thermal conductivity change at room temperature.[7] Says Jon Ihlefeld of Sandia National Laboratories,

"We can alter PZT's thermal conductivity over a broad temperature range, rather than only at the cryogenic temperatures achieved by other research groups... and we can do it reversibly: When we release our voltage, the thermal conductivity returns to its original value... We feel this groundbreaking work will advance the field of phononics."[7]

The research team investigated the mechanism for the thermal switching action through use of an atomic force microscope and a scanning electron microscope. A change in the shape and length of the the ferroelectric domain walls is what alters the phonon transport in the PZT material, and this affects the thermal conductivity.[7]

Thermal experiments require care. Note the insulated cabinet for this Sandia atomic-force microscope. (Sandia photo by Randy Montoya.[7])

This research was supported by Sandia's Laboratory Directed Research and Development office, the Air Force Office of Scientific Research, and the National Science Foundation.[7]

References:

1. W. Kobayashi, Y. Teraoka and I. Terasaki, "An oxide thermal rectifier," Applied Physics Letters, vol. 95, no. 17 (October, 2009), Document No. 171905 (3 pages); available, also, on arXiv.
2. Heat Diode Paves the Way For Thermal Computing, Technology Review, October 9, 2009.
3. Yan Wang, Ajit Vallabhaneni, Jiuning Hu, Bo Qiu, Yong P. Chen and Xiulin Ruan, "Phonon Lateral Confinement Enables Thermal Rectification in Asymmetric Single-Material Nanostructures," Nano Letters, Article ASAP (January 6, 2014), DOI: 10.1021/nl403773f.
4. Emil Venere, "Research could bring new devices that control heat flow," Purdue University Press Release, January 27, 2014.
5. Carl E. Weller, "Electric Soldering Iron," US Patent No. 2,951,927, September 6, 1960.
6. Jon F. Ihlefeld, Brian M. Foley, David A. Scrymgeour, Joseph R. Michael, Bonnie B. McKenzie, Douglas L. Medlin, Margeaux Wallace, Susan Trolier-McKinstry, and Patrick E. Hopkins, "Room-Temperature Voltage Tunable Phonon Thermal Conductivity via Reconfigurable Interfaces in Ferroelectric Thin Films," Nano Lett., vol. 15, no. 3 (March 11, 2015), pp. 1791-1795, DOI: 10.1021/nl504505t.
7. Jon Ihlefeld, "Phonons, arise! - Small electric voltage alters conductivity in key materials," Sandia Labs Press Release, April 22, 2015.