by James A. Monroe, Ph.D., ALLVAR
Almost all materials expand when heated and contract when cooled, known as positive thermal expansion. Some materials expand and contract a lot while others expand and contract a little. This thermal expansion difference between materials can cause problems in assemblies when cooling to cryogenic temperatures. For example, aluminum fixtures are often fastened with stainless steel bolts. The steel bolt contracts less than the aluminum when cooled. The resulting thermal expansion mismatch can cause the bolt to loosen by more than 60 percent when cooling to liquid nitrogen temperatures. If this causes a failure in cryogenic instrumentation that takes days to cool, it can break testing schedules. If this occurs in on-orbit payloads or space environments, it can be catastrophic.
ALLVAR alloys are the only metals that shrink when heated and expand when cooled. This opens the door for engineers to overcome the cryogenic thermal expansion mismatch problem like never before. Over the last decade, ALLVAR alloys have been inserted into optics, aerospace systems, scientific equipment and specialty applications where compensating for thermal expansion mismatch is critical. For example, ALLVAR Alloy 30 washers can compensate for the mismatch between steel bolts and aluminum, as described above. Beyond bolts, ALLVAR alloys enable engineers to break free from traditional metals’ thermal expansion properties and create components with thermal expansion properties to meet a specific application’s needs. This new tool opens the current trade space for engineers designing and building cryogenic systems.
The negative coefficient of thermal expansion (CTE) of ALLVAR Alloy 30 is -30 ppm/K at room temperature. In comparison, regular titanium, steel and aluminum have positive CTEs of 9 ppm/K, 16 ppm/K and 23 ppm/K at room temperature respectively. ALLVAR Alloy 30’s large CTE magnitude allows a small amount to easily compensate for CTE mismatch between traditional materials.
For room temperature applications, the linear CTE assumption is fine, but the colder you go, the more non-linear the thermal response. The CTE of normal metals like aluminum, steel and titanium is non-linear and approaches zero CTE when approaching 0 K. ALLVAR Alloy 30’s CTE is also non-linear when going to cryogenic temperatures but remains negative as it approaches zero CTE around 0 K. ALLVAR has recently collected CTE data down to 20 K so engineers can integrate Alloy 30 into their systems with confidence.
ALLVAR Alloy 30 offers three new engineering tools to design cryogenic systems: 1) control over displacements such as optics where lenses and mirrors need to maintain specific distances, 2) control over forces such as the bolts and mechanical assemblies described above and 3) control over passive actuation such as thermal switches.
ALLVAR Alloy washers are already helping cryogenic thermal straps maintain thermal conductance. For example, ALLVAR Alloy 30 washers have been integrated into the cryogenic sub-assembly in the Nancy Grace Roman Space Telescope’s coronagraph payload. Alloy 30 washers enabled the use of very delicate thermal straps made from pyrolytic graphite. Pyrolytic graphite is a perfect thermal strap material for space applications because it has high thermal conductivity and low density but it is very delicate. ALLVAR Alloy 30 washers gently but firmly hold the pyrolytic graphite so thermal conduction can be maintained while cooling the coronagraph’s detectors.
Elastomers and polymers lose their compliant elastic properties when cooled, presenting an issue with cryogenic seals. Alternative metal-to-metal seals require very tight tolerances where components are lapped or ground and gaps can form if a bolted seal loses its preload. Allvar Alloy washers can maintain bolt preload or even tighten a bolt to a desired clamping force when cryogenically cooled. The result is a tighter seal.
Passive thermal switches use thermal expansion mismatch between components to connect and disconnect a thermally conductive path. The large negative CTE magnitude and low thermal conductivity of Allvar Alloy 30 enable large reductions in switch size while maintaining large turndown ratios that approach 1200:1 in studies. This far exceeds currently available thermal switches with turndown ratios around 70:1.
Allvar Alloy 30 and its tunable variants provide a simple, reliable CTE-mismatch solution that enables cryogenic systems built at room temperature to perform predictably in extreme cold.
From seals and scientific instruments to space-based systems, the negative thermal expansion properties of ALLVAR Alloy 30 turn the design paradigm on its head. For the first time, engineers have a simple solution to the CTE mismatch problem. Engineers can assemble a system at room temperature and be confident it will work at cryogenic temperatures. The next generation of ALLVAR Alloys can even be tuned to match a desired CTE. These unique capabilities, unlock new thermal management architectures and materials to enable future cryogenic applications. http://www.allvaralloys.com
