In order to detect habitable planets beyond our solar system, scientists measure the intensity and spectra of light reflected from a planet’s surface to discern the types of gases contained in the exoplanet’s atmosphere. The challenge lies in the extreme contrast ratio required to do this successfully. NASA’s planned Habitable Worlds Observatory is targeting a contrast ratio of 1:1 billion. This means that the necessary telescope has to be 1000× more stable than state-of-the-art observatories like the James Webb Space Telescope and the forthcoming Nancy Grace Roman Space Telescope. To achieve this, a team from the company ALLVAR has been working in collaboration with NASA’s Marshall Space Flight Center and the Jet Propulsion Laboratory to show how a material with negative thermal expansion characteristics can create ultra-stable telescope structures. Although current materials used in telescope mirrors and struts have drastically improved the dimensional stability of modern observatories, they still fall short of the 10 pm level stability over several hours that will be required for the Habitable Worlds Observatory. NASA’s Nancy Grace Roman Space Telescope sits atop the support structure and instrument payloads. The long black struts holding the telescope’s secondary mirror will contribute roughly 30% of the wave front error while the larger support structure underneath the primary mirror will contribute another 30%. Courtesy of NASA/Chris Gunn. ALLVAR received a Small Business Innovative Research (SBIR) grant from NASA to scale and integrate a new alloy material into telescope structure demonstrations for potential use on future NASA missions like the Habitable Worlds Observatory. The alloy, called ALLVAR Alloy 30, shrinks when heater and expands when cooled — a property known as negative thermal expansion (NTE). It exhibits a -30 ppm/ºC coefficient of thermal expansion (CTE) at room temperature. This means that a 1-m long piece of this alloy will shrink 0.03 mm for every 1 ºC increase in temperature. For comparison, aluminum expands at +23 ppm/°C. Because it shrinks when other materials expand, ALLVAR Alloy 30 can be used to strategically compensate for the expansion and contraction of other materials. The alloy’s unique NTE property and lack of moisture expansion could enable optic designers to address the stability needs of future telescope structures. Calculations have indicated that integrating ALLVAR Alloy 30 into certain telescope designs could improve thermal stability up to 200× compared to only using traditional materials like aluminum, titanium, carbon fiber reinforced polymers, and the nickel–iron alloy, Invar. To demonstrate that negative thermal expansion alloys can enable ultra-stable structures, the ALLVAR team developed a hexapod structure to separate two mirrors made of a commercially available glass ceramic material with ultra-low thermal expansion properties. Invar was bonded to the mirrors and flexures made of Ti-6Al-4V — a titanium alloy commonly used in aerospace applications — were attached to the Invar. To compensate for the positive CTEs of the Invar and Ti-6Al-4V components, an NTE ALLVAR Alloy 30 tube was used between the Ti-6Al-4V flexures to create the struts separating the two mirrors. The natural positive thermal expansion of the Invar and Ti-6Al-4V components is offset by the negative thermal expansion of the NTE alloy struts, resulting in a structure with an effective zero thermal expansion. The stability of the structure was evaluated at the University of Florida Institute for High Energy Physics and Astrophysics. The hexapod structure exhibited stability well below the 100 pm/√Hz target and achieved 11 pm/√Hz. This first iteration is close to the 10 pm stability required for the future Habitable Worlds Observatory. The ALLVAR enabled ultra-stable hexapod assembly undergoing interferometric testing between 293K and 265K (right). On the left, the root mean square changes in the mirror’s surface shape are visually represented. The three roughly circular red areas are caused by the thermal expansion mismatch of the invar bonding pads with the ZERODUR mirror, while the blue and green sections show little to no changes caused by thermal expansion. The surface diagram shows a less than 5 nm root mean square change in mirror figure. Courtesy of NASA’s X-Ray and Cryogenic Facility. Furthermore, a series of tests run by NASA Marshall showed that the ultra-stable struts were able to achieve a near-zero thermal expansion that matched the mirrors in the above analysis. This result translates into less than a 5-nm root mean square change in the mirror’s shape across a 28K temperature change. Beyond ultra-stable structures, the NTE alloy technology has enabled enhanced passive thermal switch performance and has been used to remove the detrimental effects of temperature changes on bolted joints and infrared optics. These applications could influence technologies used in other NASA missions. For example, these new alloys have been integrated into the cryogenic sub-assembly of Roman’s coronagraph technology demonstration. The addition of NTE washers enabled the use of pyrolytic graphite thermal straps for more efficient heat transfer. ALLVAR Alloy 30 is also being used in a high-performance passive thermal switch incorporated into the University of California, Berkeley Space Science Laboratory’s Lunar Surface Electromagnetics Experiment-Night (LuSEE Night) project aboard Firefly Aerospace’s Blue Ghost Mission 2, which will be delivered to the Moon through NASA’s Commercial Lunar Payload Services initiative. The NTE alloys enabled smaller thermal switch size and greater on-off heat conduction ratios for LuSEE Night. Through another recent NASA SBIR effort, the ALLVAR team worked with NASA’s Jet Propulsion Laboratory to develop detailed datasets of ALLVAR Alloy 30 material properties. These large datasets include statistically significant material properties such as strength, elastic modulus, fatigue, and thermal conductivity. The team also collected information about less common properties like micro-creep and micro-yield. With these properties characterized, ALLVAR Alloy 30 has cleared a major hurdle towards space-material qualification. As a spinoff of this NASA-funded work, the team is developing a new alloy with tunable thermal expansion properties that can match other materials or even achieve zero CTE. Thermal expansion mismatch causes dimensional stability and force-load issues that can impact fields such as nuclear engineering, quantum computing, aerospace and defense, optics, fundamental physics, and medical imaging. The potential uses for this new material will likely extend far beyond astronomy. For example, ALLVAR-developed washers and spacers are now commercially available to maintain consistent preloads across extreme temperature ranges in both space and terrestrial environments. These washers and spacers excel at counteracting the thermal expansion and contraction of other materials, ensuring stability for demanding applications.
