Improved Thermal Switch Electrically Controls Heat Flow

What you’ll learn:

Why is this type of thermal switch needed?
How the ceramic material and structure was developed for superior performance.
The results achieved with this material and arrangement.

Many engineers are naturally focused on getting heat away from their components, circuit boards, and systems. However, situations arise where it’s desirable and even necessary to dynamically manage the heat-flow path, change it, channel it, and maintain a desired setpoint temperature.

That’s where a thermal switch is needed. Applications for this function include thermal shutters, advanced displays, regulation of infrared heat transfer, and enhanced waste-heat recovery.

[Note that this thermal switch isn’t a thermally controlled electrical switch that flips its current-flow on-off state when a temperature threshold or trip point is reached (such as shown in “Graphene-Infused Foam Provides Dynamically Tunable Path for Thermal Management” and “Managing Distributed Power For Safety and Redundancy.”) Instead, it’s an electrically controlled switch that impedes or allows for the flow of thermal energy. In short: these thermal switches turn heat flow on and off. As often happens in technology, we have the same name for different or complementary functions.]

“Switching” to CeO₂ Thin Films

A research team at the Research Institute for Electronic Science, Hokkaido University (Japan), devised a novel approach to implementing a thermal switch by using cerium oxide (CeO₂) thin films as the active material. It provides a highly efficient and sustainable alternative to existing materials with performance that exceeds prior benchmarks.

For a variety of materials-science reasons, they focused on the electrochemical reduction/oxidation of CeO₂ as an active material for thermal switches. For instance, CeO₂ is an abundant material that’s widely used in practical applications such as polishing powders, catalysts, and sunscreens.

The structure of their solid-state electrochemical thermal switch has a 103-nm-thick CeO₂ epitaxial film on a 0.5-mm-thick yttrium-stabilized zirconia (YSZ) substrate sandwiched between platinum (Pt) films. (If you’re not familiar with YSZ, it’s an inexpensive and widely used oxide-crystal substrate for the epitaxial growth of thin films.) The electrochemical reduction/oxidation treatments were performed at 280°C in the air.

When a negative/positive voltage is applied to the top Pt electrode, electrochemical reduction/oxidation of the CeO₂ layer occurs. The top Pt electrode is also used as a transducer for thermal conductivity measurements (Fig. 1).

Schematic device structure of a CeO2-based solid-state electrochemical thermal switch — 1. The schematic device structure of a CeO₂-based solid-state electrochemical thermal switch shows that when a negative/positive voltage is applied to the top Pt electrode, it causes electrochemical reduction/oxidation of the CeO₂ layer. The top Pt electrode is also used as a transducer for thermal-conductivity measurements.

Their 5- × 5-mm device was placed on a heater at 280°C, and a negative/positive voltage driving a constant current of ±10 µA was injected at the top Pt electrode (Fig. 2). These treatments were performed by applying an electron density (Q) of 1 × 10²¹/cm³.

CeO2-based thermal switch operating at 280°C in air. — 2. This photograph shows a CeO₂-based thermal switch operating at 280°C in air. The surface area of the device is ~25 mm², and a constant current of ±10 μA was applied for oxidation (reduction).

Switch Measurements and Results

Measurement of thermal and related behaviors of these ceramic devices requires a range of advanced instrumentation, especially as they wanted to assess both macroscopic thermal performance as well as microscopic crystalline behavior. Their tests involved investigation of in-plane and out-of-plane x-ray diffraction (XRD) pattern of the device using high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) observations, as well as thermal flow using time-domain thermoreflectance (TDTR).

Their device achieved an on/off thermal conductivity ratio of 5.8 and a thermal conductivity (κ)-switching width of 10.3 W per meter-Kelvin (W/m·K). The thermal conductivity in its minimal state (off-state) is 2.2 W/m·K, but in the oxidized state (on-state), it significantly rises to 12.5 W/m·K (Fig. 3).

Thermal stability of the reduced CeO2-based thermal switch — 3. The thermal stability of the reduced CeO₂-based thermal switch is an important factor. **(A)** Change in the out-of-plane XRD patterns of the reduced CeO₂-based thermal switch while kept at 150°C in the air. After the reduction (Q = −1.5 × 10²² cm⁻³, 0 hours), the diffraction peak of 002 CeO_2−δ (δ ~ 0.24, b) is seen together with 002 CeO₂ (a). The peak position of b shifts to the higher *q_z*/2π side, and the peak b becomes weaker with time. On the other hand, peak a becomes stronger with time. After 190 hours, the peak b disappears. (c) Change of lattice parameter, c, of the CeO₂ (a) and CeO_2−δ (b) with time. Parameter c of the b phase gradually decreases with time while that of the a phase is almost constant. **(B)** XRD peak intensity ratio [I_b/(I_a + I_b)], which reflects the volume fraction of the b phase. The results at 100°C in air are also shown. At 150°C, the intensity ratio is ~0.8 at the beginning and decreases almost linearly with time, reaching zero after 190 hours. In contrast, the intensity ratio is almost constant at 100°C.

Other tests investigated the switching speeds and relaxation times, as well as performance over many cycles. The researchers were especially proud of the cyclic results, as the performance metrics remained consistent after 100 cycles of reduction and oxidation, which they say demonstrated remarkable durability and reliability for extended usage in practical applications.

The work is detailed in their readable eight-page paper “High-performance solid-state electrochemical thermal switches with earth-abundant cerium oxide” published in Science Advances.