[Image above] A rendering of NASA’s planned DAVINCI mission to Venus in the 2030s. Missions to Venus will require the development of sensors and other instruments capable of operating in extremely high temperatures. Credit: NASA Goddard, YouTube
While the moon and Mars remain top priorities for space agencies during the 2020s, planetary scientists who study Venus are gearing up for the 2030s, when three new missions to Earth’s long-neglected neighboring planet are scheduled to take place.
Two of the missions, announced in June 2021, will be headed by NASA in the United States. DAVINCI will be the first mission to study Venus using both spacecraft flybys and a descent probe, while VERITAS will create the first global, high-resolution topographic and radar images of Venus.
To prepare for these missions, scientists will need to develop sensors and other instruments capable of operating in extremely high temperatures. That is because Venus, despite being the second planet from the sun, is the hottest planet in the solar system. The thick atmosphere that surrounds Venus traps sunlight and heats up the planet’s surface to 465°C (870°F).
Electronic circuits based on silicon carbide (SiC) are expected to play a role in these upcoming missions. SiC is a semiconductor that offers higher thermal conductivity, higher electron mobility, and lower power losses than silicon, the main semiconductor used in modern electronic devices. These properties allow SiC-based electronics to operate efficiently at higher temperatures than silicon-based electronics.
SiC-based electronics are already entering the market as components for hybrid and electric vehicles. However, that application only requires SiC to withstand temperatures of on average 95°C (203°F)—a far cry from the temperatures expected on Venus.
In 2015 and 2017, researchers led by NASA Glenn Research Center demonstrated that SiC-based piezoresistive pressure sensors and integrated circuits could operate efficiently up to 800°C (1,472°F), well above the expected temperatures on Venus.
Above this temperature, however, the researchers observed increasing signal instabilities. They hypothesized that this instability was partially due to leakage of the current through aluminum nitride, the material used to package the SiC-based electronics.
Ensuring signal stability up to and greater than 800°C would safeguard SiC-based electronics against higher-than-expected temperatures during a mission. It would also allow SiC-based electronics to be designed specifically for use in hotter environments, such as spacecraft operating near the sun.
In a new paper, NASA Glenn Research Center research electronics engineer Robert S. Okojie and intern Thomas M. Deucher investigated the electrical properties of several ceramic and glass packaging materials to determine which may perform better than aluminum nitride at temperatures of more than 800°C.
The four ceramics tested in this study were aluminum nitride, silicon nitride, beryllium oxide, and yttrium oxide. The one glass was silicon dioxide.
Okojie and Deucher begin by acknowledging the extensive literature on the electrical bulk resistivity of these materials in response to elevated temperature.
“However, substantial data on surface resistivity that is crucial to understanding package leakage current-induced instabilities in devices are lacking,” they write.
Due to this gap in the literature, they provided a detailed description of their experimental approach for testing bulk and surface resistivities at temperatures up to 1,200°C (2,192°F), including sample preparation and testing parameters. In brief, samples were thermally cycled twice in nitrogen while the resistivities were measured in situ.
Based on these experiments, Okojie and Deucher determined that beryllium oxide maintained the highest bulk and surface resistivity values at 1,200°C, with 28 and 34 kΩ, respectively. In contrast, aluminum nitride had the lowest resistivity values, with 6.8 kΩ cm for bulk and 0.64 kΩ cm for surface.
However, silicon nitride was identified as the most promising candidate for packaging material, despite its lower thermal conductivity relative to aluminum nitride. It received this designation because of its low mismatch in coefficient of thermal expansion with SiC and relative resistivity stability compared to aluminum nitride.
While this study focused specifically on packaging for SiC-based pressure sensors, “the characterization of these materials at elevated temperature may prove useful in [other SiC-based] applications,” the researchers conclude.
The paper, published in Journal of the American Ceramic Society, is “Temperature effects on electrical resistivity of selected ceramics for high-temperature packaging applications” (DOI: 10.1111/jace.19548).