### 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(Zr0.3Ti0.7)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:

Linked Keywords: Owner-occupier; homeowner; winter; HVAC; heating; invoice; bill; thermal conductivity; material property; building insulation; home insulation; quality measure; R-value; multiplicative inverse; reciprocal; thermal transmittance; heat transfer; square meter; area; temperature; thermal insulation; insulator; watt; kelvin; United States customary units; Fahrenheit; square foot; British thermal unit; Btu; metrication; 1980s; decade; weather forecasting; weather report; Fahrenheit; accuracy; Celsius; folk etymology; folklore; yard; nose; fingertip; King Henry I of England (1068-1135); Wikimedia Commons; building insulation material; fiberglass batting; multiplication; multiply; inch; energy conversion efficiency; energy-efficient; home; wall; wall stud; wooden stud; window; multiple glass panes; citation needed; Tasmania; Tasmanian; radio astronomy; innovator; pioneer; Grote Reber (1911-2002); cooking oven; building materials; thermal diode; experiment; theory; phonon; quantization; quantized; acoustics; acoustic vibration; solid; thermal conduction; heat conduction; temperature; thermal; switch; automatic; mechanical; Carl E. Weller; soldering iron; electricity; electrical; actuator; actuate; scientist; Sandia National Laboratories (Albuquerque, New Mexico); University of Virginia (Charlottesville, Virginia); Pennsylvania State University (University Park, Pennsylvania); lead zirconate titanate; soldering iron; spring; magnet; nickel; wire; magnetism; Curie temperature; technology; phonon; phononic; logic gate; logic; thermal management; energy harvesting; ceramic; thin film; actuator; loudspeaker; speaker; electric spark; barbecue grill; nanoscopic scale; nanoscale; ferroelectricity; ferroelectric; domain wall; second; response time; room temperature; voltage; nine-volt battery; cryogenic; research; reversible process; phononics; atomic force microscope; scanning electron microscope; Randy Montoya; Laboratory Directed Research and Development office; Air Force Office of Scientific Research; National Science Foundation; Carl E. Weller, "Electric Soldering Iron," US Patent No. 2,951,927, September 6, 1960.

Latest Books by Dev Gualtieri
Previews Available
at Tikalon Press

STEM-themed novel for middle-school students

Mathematics-themed novel for middle-school students

Complete texts of LGM, Mother Wode, and The Alchemists of Mars

Other Books