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Technology Roadmap Sections and Deliverables

4VEM - Variable Emissivity Materials for Spacecraft

Thermochromic variable emissivity materials (VEMs) can be used for a wide range of applications, from spacecraft radiators to windows used on Earth. For this technology roadmap, the use of VEMs for spacecraft radiators will be the focus.

Roadmap Overview

The only way that orbiting spacecraft can reject heat is through radiation. Because of this, the thermo-optical properties of spacecraft radiators are important. The thermo-optical properties of radiators, such as emissivity, determine how much heat is radiated away.

There is a way to calculate how much heat is radiated from a surface, and it is shown in the equation below. Q is heat being radiated away from a surface, A is the area of the radiating surface, σ is the Stefan-Boltzmann constant, ε is the emissivity of the surface material, and T is the temperature of the surface.

Q=A\sigma\varepsilon T^4

The emissivity of a material is directly proportional to the heat radiated away. So, when a material has high emissivity, more heat is radiated away. Most materials have a constant emissivity, but there are some materials whose emissivity can change based on environmental conditions or whether they are powered. These materials are called variable emissivity materials (VEMs).

There are active (electrochromic) and passive (thermochromic) VEMs. Electrochromic VEMs require power input to change emissivity, unlike thermochromic VEMs which change their emissivity based on their temperature. The technology of thermochromic VEMs whose emissivity is lower at low temperatures and higher at high temperatures is expected to be widely used in radiators for spacecraft, because compared to constant-emissivity radiators, it reduces spacecraft heater power requirements and temperature swings, all without power or human input. Figure 1 shows how a thermochromic VEM can help spacecraft more efficiently manage their temperature.

Figure 1: Diagram of how VEMs affect spacecraft, from [1]

This roadmap will explore how thermochromic VEMs have evolved, their Figures of Merit, and what is expected in the future.

Design Structure Matrix (DSM) Allocation

Figure 2 below shows the interactions between various technologies and the 4VEM variable emissivity technology. The x's in the matrix signify interaction.

Figure 2: DSM Matrix

VEMs are part of a larger technology tree, with the 1st level being the technology of a general spacecraft:

1SPC Spacecraft

The 1SPC Spacecraft technology has a variety of subsystem technologies; all of the subsystems of a spacecraft interact with each other and make the spacecraft work, and they make up the second level:

2STR Structures Subsystem
2POW Power Subsystem
2CDH Command and Data Handling Subsystem
2ADC Attitude Determination and Control Subsystem
2PAY Payload Subsystem
2PRO Propulsion Subsystem
2COM Communication Subsystem
2THE Thermal Subsystem.

VEMs are part of the 2THE thermal subsystem of a spacecraft. There are many technologies that are used for spacecraft thermal subsystems, and they are in the 3rd level:

3HEP Heat Pipes
3LOU Louvers
3HEA Heaters
3CRY Cryocoolers
3PCM Phase-change materials
3TIM Thermal interface materials
3RAD Radiators

VEMs are used in the 3RAD Radiators technology. The 3RAD technology can be split into two groups as well, which comprise the 4th level:

4CEM Constant emissivity materials used for radiators
4VEM Variable emissivity materials used for radiators.

So, VEMs are a small part of the whole spacecraft technology. Figure 3 below shows where the 4VEM technology is inside a tree.

Figure 3: 4VEM Technology Tree Placement

Roadmap Model using OPM

The Object-Process Diagram (OPD) and Object-Process Language (OPL) represented in Figure 4 and Figure 5, respectively, were generated by OPCloud. This model describes the processes and Figures of Merit (FOM) involved with passive spacecraft VEMs, as well as their object interactions. It highlights the thermochromic VEMs discussed in this roadmap, which require no electrical energy to operate.

Figure 4: VEM OPD

Figure 5: VEM OPL

Figures of Merit (FOM)

There are several Figures of Merit (FOMs) that describe VEMs. In order to describe the FOMs, an emissivity vs. temperature graph of several VEMs made from vanadium dioxide doped with tungsten will be shown in Figure 6, which is from [1].

Figure 6: Emissivity versus Temperature for Some VEMs, from [1]

There are four types of VEMs shown in Figure 6; each VEM is made from vanadium dioxide doped with different percentages of tungsten. The first VEM is doped with 3.5% tungsten (the MPB W(3.5%) VEM), the second VEM is doped with 2.5% tungsten, the third VEM is doped with 1.5% tungsten, and the last VEM is doped with 0% tungsten.

For each type of VEM, there is an up curve and a down curve. The up curve corresponds to the emissivity profile when the material is being heated up, and the down curve corresponds to the emissivity profile when the material is being cooled down. The logistic fit curve fits the data to a logistic equation, and the midpoint line is in between the up and down curves.

Because there is a difference in the emissivity vs. temperature curves for when the material is being heated up and when the material is being cooled down, there is hysteresis. One FOM of VEMs is seeing how much hysteresis there is in the material. The less hysteresis, the better because it is easier and quicker to model VEMs with no hysteresis.

One way to calculate the amount of hysteresis is to determine at what temperature the material is when emissivity is at its midpoint and compare that temperature between the heating curve and the cooling curve. So, for the MPB W(0%) down curve, the midpoint temperature T_{down}^\ast is around 55 degrees Celsius; for the MPB W(0%) up curve, the midpoint temperature T_{up}^\ast is around 80 degrees Celsius. So, the difference between T_{up}^\ast and T_{down}^\ast is a FOM, which will be called \Delta T_{hys} .

∆T_{hys}=T_{up}^*-T_{down}^*

For the MPB W(0%) curve,

∆Thys=80-55 degrees C=25 degrees C

The lower ∆Thys is, the less hysteresis. An increase in tungsten doping decreases hysteresis and ∆Thys as shown in Figure 6. The nominal value of ∆Thys is 0 degrees Celsius.

There is also the minimum emissivity \varepsilon_L and maximum emissivity \varepsilon_H of a VEM. The minimum emissivity corresponds to when the material is relatively cold, and the maximum emissivity corresponds to when the material is relatively hot. The theoretical limit for minimum emissivity is an emissivity of 0, and the theoretical limit for maximum emissivity is an emissivity of 1. Engineers and scientists have generally been trying to make the minimum emissivity as close to 0 as possible and the maximum emissivity as close to 1 as possible.

So, two FOMs are the minimum emissivity \varepsilon_L and maximum emissivity \varepsilon_H of a VEM. For the MPB W(0%) midpoint curve, \varepsilon_L is around 0.32, and \varepsilon_H is around 0.8.

Another FOM engineers and scientists have been using in analyzing VEM technology is the difference between the minimum emissivity \varepsilon_L and the maximum emissivity \varepsilon_H of a VEM; this FOM will be called ∆ε. Engineers and scientists have been trying to maximize ∆ε and trying to get ∆ε as close to 1 as possible; the nominal value for ∆ε is 1.

∆ε=εH- εL

So, for the MPB W(0%) midpoint curve, with an \varepsilon_L of around 0.32 and an \varepsilon_H of around 0.8,

∆ε=0.8- 0.32=0.48

The closer \varepsilon_L, \varepsilon_H, and ∆ε are to their nominal values, the easier it should be for spacecraft to passively regulate their temperature.

The below table presents the FOM that can be used to evaluate various VEMs.

Figure of Merit	Units	Description
Temperature Hysteresis (\Delta T_{hys} )	[K]	This describes how large hysteresis is for the VEM, and it is a difference of temperatures. The smaller the quantity, the better. The nominal value is 0 Kelvin (or 0 degrees Celsius).
Minimum Emissivity (\epsilon_L )	[-]	This is the minimum emissivity of a VEM. The nominal value is 0.
Maximum Emissivity (\epsilon_H )	[-]	This is the maximum emissivity of a VEM. The nominal value is 1.
Emissivity Range (\Delta\epsilon_H )	[-]	This is the difference between the maximum and minimum emissivities of VEMs. The nominal value is 1.
Lifetime (n )	[Cycles]	This is the number of emissivity state switches the material can undergo before failure. The higher this value, the better the VEM.
Response Time (t )	[s]	This is the amount of time it takes for the device to switch between different emissivity states. The lower the response time, the better the thermal control.

Figure 7 displays a diverse spread of variability in emissivity ranges for active and passive types of modern VEMs over time using data from [2] and [3], which also include the specific VEM technologies plotted. The distribution of the emissivity ranges is a reminder that emissivity range may not necessarily be the most important figure of merit for a specific VEM application, hence why there is not a clear upward trend in the data over time. Although the data is very disperse, linear regression models were applied, and they show an improvement for electrochromic and thermochromic VEMs of 0.02/year and 0.03/year, respectively. Due to physics principles, the theoretical emissivity range for a given material is 1, with the maximum emissivity being 1 and the minimum being 0. This cannot exceed 1 due to the law of conservation of energy. That it, an object cannot emit more than 100% of its energy. In terms of application to passive spacecraft thermal control, thermochromic VEMs are at the incubation stage of the technology lifecycle due to only one VEM technology, Thermal Control Tiles designed by MPB Communications, being sent to space [4][5]. However, there are initiatives for other thermochromic VEMs to be sent to space in the near future by the Air Force Research Laboratory [6].

Figure 7: Modern VEM Emissivity Ranges with Data Adapted from [2] and [3]

Sources

[1] I. Foster, "Variable Emissivity Materials for Thermal Radiators: Introduction to Characterizing Thermochromic Infrared Surfaces in Space," in AIAA SciTech 2024 Forum, 2024. https://arc-aiaa-org.libproxy.mit.edu/doi/abs/10.2514/6.2024-1295

[2] https://arc.aiaa.org/doi/10.2514/1.T6555

[3] https://doi.org/10.1016/j.xcrp.2022.101198

[4] https://umstarlab.ca/index.php/past-projects/

[5] https://ttu-ir.tdl.org/items/4b815052-4a4a-4031-9624-48605cae9c89

[6] https://afresearchlab.com/technology/space-power-beaming/