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Passive Thermal Management

November 21, 2022

Before thermostats became just another of the many electronic gadgets used in our homes, their operating principle was mechanical. Electrical contacts were opened and closed by action of liquid mercury, a conductive metal, inside a sealed glass bulb. The switch state was controlled by temperature responsive motion of a bimetallic metal strip. The differential thermal expansion of two metal strips bonded together, typically by welding, causes the bimetallic strip to bend (see figure). Temperature control was improved in thermally sluggish home heating systems by the addition of a small in-circuit adjustable resistor inside the thermostat. This so-called anticipation heater applied a small amount of heat to the thermostat so it would turn off somewhat earlier.

Portion of fig. 1 of US Patent No. 3,284,002, 'Control Apparatus,' by Walter E. Edelman and David J. Sutton, October 26, 1964.

Portion of fig. 1 of United States Patent No. 3,284,002, "Control Apparatus," by Walter E. Edelman and David J. Sutton, October 26, 1964.

This is the iconic Honeywell T87 Round Thermostat containing a bimetallic element (11) that moves a mercury switch (13).

(Via Google Patents.)

Fortunately for the thermostat application, the thermal expansion coefficient of metals varies over a wide range (see table), as does that of metal alloys. A common bimetallic couple is steel (12 x 10-6/K) paired with brass (18.7 x 10-6/K) or copper (16.5 x 10-6/K). In most other applications, thermal expansion is a problem that causes dimensional change of mechanical components and delamination of coatings.

Thermal expansion coefficients near room temperature for many elemental metals in units of 10-6 per kelvin.

(Data from Wikipedia data page.)

lithium46   |  beryllium11.3
magnesium24.8   |  aluminum23.1
silicon2.6   |  titanium8.6
vanadium8.4   |  chromium4.9
manganese21.7   |  iron11.8
cobalt13   |  nickel13.4
copper16.5   |  zinc30.2
germanium6   |  yttrium10.6
zirconium5.7   |  niobium7.3
molybdenum4.8   |  ruthenium6.4
rhodium8.2   |  palladium11.8
silver18.9   |  cadmium30.8
tin22   |  antimony11
hafnium5.9   |  tantalum6.3
tungsten4.5   |  rhenium6.2
osmium5.1   |  iridium6.4
platinum8.8   |  gold14.2
lead28.9   |  bismuth13.4
thorium11   |  uranium13.9

As a consequence of their different atomic bonding, polymers have thermal expansion coefficients that are many time larger than those of metals (see table). A bipolymeric strip made from bonding two dissimilar polymers will produce a much larger mechanical change with temperature than a bimetallic strip.

Average thermal expansion coefficients near room temperature for some common polymer materials in units of 10-6 per kelvin.

(Data from Ref. 1.[1])

EVA - Ethylene Vinyl Acetate180   |  PI - Polyimide55
PS - Polystyrene (Crystal)65   |  CPVC - Chlorinated Polyvinyl Chloride70
PET - Polyethylene Terephthalate70   |  PMMA - Polymethylmethacrylate Acrylic70
SAN - Styrene Acrylonitrile70   |  PEEK - Polyetheretherketone77.5
PBT - Polybutylene Terephthalate80   |  PC - Polycarbonate80
ASA - Acrylonitrile Styrene Acrylate85   |  HDPE - High Density Polyethylene85
PA 6 - Polyamide 6 (nylon 6)85   |  XLPE - Crosslinked Polyethylene100
XABS - Acrylonitrile Butadiene Styrene110   |  PVDF - Polyvinylidene Fluoride115
HIPS - High Impact Polystyrene125   |  POM - Polyoxymethylene (Acetal)125
PVC - Polyvinyl Chloride125   |  CA - Cellulose Acetate130
CAB - Cellulose Acetate Butyrate135   |  PTFE - Polytetrafluoroethylene (Teflon)135
LDPE - Low Density Polyethylene150   |  PVDC - Polyvinylidene Chloride150
PCL - Polycaprolactone165   |  

As researchers from Nankai University (Tianjin, China) have shown, bipolymeric strips can be used in a device for passive thermal management.[2-4] The strips uncoil as temperature is increased to emit infrared radiation and thereby cool a lower thermochromic material.[2,4] This thermochromic material will change color when lower temperature causes the strips to coil and allow passage of sunlight.[2,4] The color change allows more sunlight to be absorbed to heat the thermochromic material.[2,4] As the authors write, "The combination of visible and infrared “thermochromism” enables this device to freely switch between solar heating and radiative cooling modes by spontaneously perceiving the temperature without any external energy consumption."[2] In one mode, the device captures heat from the sun; and, in the other mode, it radiates heat into the cold of outer space.

Uncoiling of bipolymeric strip during heating

Uncoiling of bipolymeric strip during heating.

As the research team writes in its paper, the threshold temperature for switching between the coiled and uncoiled states can be adjusted by careful choice of the bipolymeric materials and the thermochromic material coating the substrate.[2]

I haven't read the full research paper, since it's paywalled, but the bipolymeric layers are evidently bonded together on a cylindrical mandrel to achieve their initial coiled state.

(Still images from supplementary video for Ref. 2.[3])

The paper authors state that they were inspired by biological stress responses in nature, such as the opening-folding movement of the leaves of the Mimosa pudica, which modulates their infrared emissivity.[2] The leaves of this plant fold inward and droop when touched to defend them from harm, and they return to their original state a few minutes later. Its foliage also closes in darkness and it reopens in light, giving a thermal response like that of the Chinese device.

Mimosa plant and mimosa cocktail

Mimosa plant and mimosa cocktail. My wife had a potted mimosa pudica in our apartment when we were first married. The mimosa cocktail is named after a different plant of the same color, the yellow-flowered Acacia dealbata. A mimosa cocktail is usually mixed from champagne and chilled orange juice, and it's a common party drink that's not too potent in usual volumes. (Left, portion Wikimedia Commons image by Vengolis. Right, portion of a Wikimedia Commons image by Sarah Stierch.)

Nearly half of global energy consumption comes from heating and cooling.[4] Older office buildings were built with huge glass windows, and this was the case for a corporate campus at which I worked in the 1980s. At one point, it was decided to apply a thermochromic film to the windows of several buildings as an energy conservation measure, possibly motivated by some tax incentives. Color-changing windows such as these can do either heating or cooling, but not both as in the Chinese device.[4] Laboratory tests of a four centimeter by four centimeter device demonstrated survival for 500 coiling cycles.[4] Other measurements revealed that the device has a solar heating power of about 250 W/m2 and a cooling power of about 60 W/m2.[4]


  1. Table of polymer thermal expansion coefficients at omnexus.specialchem.com.
  2. Quan Zhang, Yufeng Wang, Yiwen Lv, Shixiong Yu, and Rujun Ma, "Bioinspired zero-energy thermal-management device based on visible and infrared thermochromism for all-season energy saving," Proc. Natl. Acad. Sci., vol. 119, no. 38 (September 12, 2022), Article no. e2207353119, https://doi.org/10.1073/pnas.2207353119.
  3. Supplementary Information for Ref. 2 (PDF file). Two videos of the device action can be found here and here.
  4. Prachi Patel, "Zero-Energy Tech Heats When Cold and Cools When Hot," IEEE Spectrum, September 22, 2022.

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