Radioisotope Thermoradiative Cell Power Generator
A deep-space power source that generates electricity by radiating heat to the cold of space, the thermodynamic inverse of a solar cell. My primary contribution is the design, construction, and commissioning of the cryogenic measurement system used to characterize prototype thermoradiative cells as well as helping with testing and characterizing device contacts.
This work is funded by NASA's Innovative Advanced Concepts (NIAC) program under a Phase II award, in collaboration with the NASA John Glenn Research Center COMPASS team, which is studying mission applications including a CubeSat in Uranus orbit.
Uranus Cubesat: TITUS
(Technology Innovation Thermoradiative Uranus Smallsat)
A conventional solar cell generates electricity by absorbing high-energy photons from the hot sun (~5800 K) and re-emitting lower-energy photons to the environment. A thermoradiative cell (TRC) reverses this principle, the device sits at an elevated temperature and emits infrared photons toward the cold environment of deep space (~3 K), extracting electrical work from that temperature differential.
Paired with a radioisotope heat source, a General Purpose Heat Source (GPHS) module containing a single 238Pu pellet producing 62.5 W of thermal power, a TRC can generate electricity with no moving parts, no solar panels, and no sunlight. This makes it ideal for missions to the outer solar system, permanently shadowed polar lunar craters, or any environment where solar power is impractical.
The target material, InAs0.91Sb0.09 (bandgap 0.28 eV), can theoretically yield 8 W of electrical power from a single GPHS pellet with a mass-specific power of 12.7 W/kg, over a 4.5× improvement as well as providing 10× mass-specific power and 1000× volume reduction over the heritage Multi-Mission RTG.
Characterizing a thermoradiative cell requires replicating the device's actual operating environment, a hot sample (~575 K) emitting infrared radiation toward an extremely cold "sky." The measurement system must simultaneously hold the device at high temperature while maintaining a cold detector stage at cryogenic temperatures, all under high vacuum. I led the design, construction, thermal analysis, and commissioning of this system.
The measurement enclosure and sample stage were designed in SolidWorks to mount inside a Cryosystems, inc number 22.
The original design was to use a Janis Research ST-500-1 cryostat but unfortunately previous work on the project resulted in severe damage to the system.
SolidWorks CAD of measurement enclosure
A key design constraint is maximizing the radiative flux from the hot TRC device onto the cold detector while minimizing background radiation from the warm chamber walls. I modeled these competing heat fluxes as a function of cold plate diameter to inform the enclosure geometry.
At a 1 mm device-to-detector gap, the heat load from the 575 K source remains roughly constant at ~0.6 W regardless of plate size, while parasitic heat from the 300 K walls grows steeply with plate area. The crossover point at 1.71" diameter defines the maximum cold plate size beyond which wall background dominates the signal.
This analysis directly determined the cold plate dimensions used in the SolidWorks design, ensuring the detector is sized to maximize device signal without being swamped by ambient background.
A VIGO Systems PVI-5-1 HgCdTe photovoltaic detector (5.5 μm cutoff, hyperhemispherical GaAs lens, TO-39 package with no window) is used to measure infrared emission from InAsSb test devices. System temperature is monitored at multiple points using a LakeShore 331 temperature controller with silicon diode sensors.
During commissioning, the cold stage reached 10.684 K.
For a thermoradiative cell to operate at practical power output, the device must run at temperatures up to 650 K. Standard ohmic contact metals for III-V semiconductors degrade rapidly above ~350°C due to interdiffusion, which causes the contact resistance to rise dramatically and ultimately destroys the device.
I helped to fabricated ring Transmission Line Model (TLM) test structures on heavily-doped n-type GaSb using photolithography and e-beam metal evaporation, then measured the specific contact resistivity ρc after stepwise annealing at temperatures up to 400°C. Three contact metallization stacks were evaluated: Ti/Pt/Ag, Pd/Ge/Pd, and Ag, more planned. Contact resistance was extracted by fitting total resistance as a function of ring geometry using a custom Python analysis script.
The ring-TLM geometry was selected to eliminate edge current crowding effects that would introduce systematic errors in the resistivity extraction, particularly important given the small feature sizes patterned on GaSb.
TLM testing
Graphs made by Brian O'Neil graduate researcher
Ti/Pt/Ag maintains a specific contact resistivity of ~1×10−6 Ω·cm² and remains stable throughout extended annealing at 400°C, making it the strongest candidate for a high-temperature TRC ohmic contact on n-GaSb tested so far. Pd/Ge/Pd shows higher and more variable resistivity, while Ag contacts degrade significantly at elevated temperatures.
Creating the TLM test structures used for contact characterization requires patterning metal features at the micrometer scale using photolithography. We developed and optimized the photoresist process for GaSb substrates, including systematic characterization of development time in CD-26 developer to achieve clean, residue-free pattern transfer.
A key challenge was the sensitivity of the development process to ambient humidity. Exposure and development results varied significantly with lab conditions, requiring careful control and systematic testing across a range of development times to identify robust process windows before committing to metallization.
Metal stacks (Ti/Pt/Ag, Pd/Ge/Pd) were deposited by e-beam evaporation in the CHA Industries e-beam system, followed by liftoff in solvent to produce the ring-TLM test structures. The development issues led to further problems down the line with lift off where some of the rings would get stuck and require sonication to remove them which risked damaging the rest of the device and contact.
Development and lift off
Finished contacts
Stuck rings
NPRL research group