Electrochromic heat modulator successfully tested in space

Common electrochromic devices (ECDs) work in the visible and the near-IR region (0.4–2.5μm) and modulate transmitted or reflected light intensities upon application of low voltages. Depending on their configuration—i.e., whether transparent, reflective, or both—these devices can be used to control light to advantage. Examples include dimmable rearview mirrors in cars, and smart energy-saving windows, sunglasses, and sport goggles. The technology can be used wherever an optical shutter or modulator is required. When applied in the long-wave IR region or in the reflective mode, electrochromic technology can also provide a variable emissivity (VE) surface.

Figure 1. Schematic cross-section of an EclipseVED.

A fundamental challenge is how to cope with extreme temperature changes of hot and cold for both internal and external heat loads on fixed surfaces. For example, an ideal means for thermal control of satellites is to be able to change the absorbance or emittance properties of the radiator depending on changing thermal conditions. The hot (around 7μm), in-sun condition requires low solar absorptivity but high emissivity surfaces. During cold (around 12μm), eclipse conditions, the satellite surface must have low emissivity. The goal is to keep the satellite at room temperature. Because of their light weight (5g/m²), low power requirement (1mW), and wide range of reflectance modulation (0.8 change), ECDs are an excellent thermal management technology for satellites and spacecraft.

Figure 2. Reflectance spectra for two typical EclipseVEDs designed for maximum responses at 8 and 10μm.

Figure 3. Spectral emittance plot of an EclipseVED. Red curve: Colored state (high-e). Blue curve: Bleached state (low-e).

The optically and electrochemically active layers of ECDs become optically absorbing or nonabsorbing by injection or extraction of electron-ion pairs. Their location determines which mode the device is in. When the pairs are located in the ion-storage (IS) layer, the ECD is in the nonabsorbing and high-reflective mode. When they are in the electrochromic (EC) layer, the ECD is in absorptive and less-reflective mode. An electrolyte that acts both as an ion conductor and as an electronic insulator film separates the two layers and keeps them distinct from one another. The ECD has two outside electrodes each of which has contact with a surface of the EC and IS layers. These conductors switch the system from colored to bleached or vice versa by applying a low charge-driving voltage to them.

Figure 4. Optical (left) and thermal camera (right) pictures of the EclipseVED corresponding to the sample emittance spectra given in Figure 3.

Figure 5. Results of the MidSTAR-1 experiment to measure heat dissipation rates of colored and bleached EclipseVED devices. Thin solid lines represent calculated data. Reference gold plate and black surface represent low-e (emissivity=0) and high−e (emissivity=1) surfaces, respectively.

The IR reflective properties of all known conductive materials, including transparent oxide conductors, make it impossible for ordinary ECDs to tune light intensity beyond the near−IR region (2.5μm). Much work has been done on IR ECDs to modulate the long−wave IR (LWIR). But success is elusive owing to a lack of LWIR transparent conductive windows for the devices.

We have developed a coating called EclipseTECTM that enables creation of electrodes with transparency ranging from 0.4 to 27 μm.¹ We applied this technology to our EclipseVEDTM to obtain variable emissivity thermal control devices with unique EC and IS layers that exhibit optical activity in the mid- and LWIR regions. Figure 1 shows a cross-section of an EclipseVED that acts as a switchable or dimmable mirror.

The optical response of an ECD can be evaluated by FTIR (Fourier transfer IR) reflectance spectra. Figure 2 shows bleached (nonabsorbing states of the EC and IS layers) and colored states (absorbing modes) of the spectral reflectance of each EclipseVED. The coloring and bleaching voltage of the device is around ±1V.

Figure 3 shows spectra of a typical EclipseVED. Emittance is calculated by 1-reflectance data. The high reflective state of the device corresponds to low emittance (low-e), and highly absorbing low reflectance corresponds to high emittance (high-e). Figure 4 shows optical and thermal camera photographs of four EclipseVEDs deposited simultaneously. The samples had been placed on a hot plate heated to 50° C. Colored (bottom two samples) and uncolored samples were the same for each respective type of sample. The right side of Figure 4 corresponds to the thermal camera images of identical samples.

These results show that low-e states of the EclipseVED are dark (top two samples), and high-e states (lower two samples) are bright and about the same color as the heated hot plate devices. The similarity between the hot plate samples and the high-e EclipseVEDs, and the contrast with the low-e samples, clearly indicates that the EclipseVED is an excellent emissivity or heat modulator. The colored mode of our device constitutes its cooling state, and the bleached state represents heating.

In summary, we have developed an electrochromic system that works in the 7–12μm far-IR region. Eclipse Energy Systems Inc. intends to use the EclipseVED as a thermal control coating for the exterior surface of spacecraft and satellites. The first space-tested samples were sent aboard the MidSTAR-1 satellite in March 2007.² Figure 5 compares the cooling rates of the colored and bleached states of the EclipseVED placed aboard the MidSTAR-1 in space with reference black (high-e) and gold (low-e) samples. The remaining challenge is to test the system in an actual space environment and protect the sample surface accordingly without compromising optical performance.