Solar-array panels will experience very low temperatures of 50 K for the proposed Europa Clipper Mission (ECM). Solar-array panels will also undergo thermal cycling from 50 K to 133 K during the mission due to Jovian eclipsing. Unfortunately, there was no knowledge of the physical properties of the materials planned to be used down to ≈50 K, making it difficult to assess their reliability under such extreme cryogenic temperature conditions.

The thermomechanical analyzer unit on the left consists of the TMA measuring head, furnace lift control, cryostat, and computer-controlled power supply. The dilatometer and its associated equipment are shown on the right.
A commercial materials characterization system that works down to 123 K was modified to perform characterizations down to ≈20 K. Two separate custom designed and manufactured equipment systems were procured: a thermomechanical analyzer (TMA) with a three-point bend probe (TPBP) module and a dilatometer. The TMA is used to measure Young’s modulus (or e-modulus). The dilatometer is used to measure coefficient of thermal expansion (CTE). The test configuration was optimized to measure the physical properties of radiated and nonradiated material samples for CTE and Young’s modulus of solar array materials.

The TMA was used to measure the Young’s modulus of a given material. The deflection of the test sample under load in the TPBP was measured by means of a special sample holder set-up and a knife-edge piston. The effective sample load is the sum of the static sample load and momentary value of the dynamic sample load. The change in deflection is recorded as a function of time and temperature. One can evaluate using the data of Delta deflection, temperature, dimensions of the materials, etc., to determine Young’s modulus for a given material and temperature.

The dilatometer basic unit consists of a measuring head, furnace lift control, and the computer-controlled power supply for the furnace. Dilatometers measure either change in length, width, or thickness of a test coupon as a function of temperature. This change in length (DL) can be a reversible change, or a sum of reversible and irreversible changes in length in the cases where there are phase transformations. Principally, samples lying in a sample holder are linearly heated while the case may be cooled. The change in length is transmitted by means of a push rod from the furnace on a linear variable differential transducer (LVDT). The sample temperature is recorded by a diode temperature sensor. Measurements were carried out under a partial vacuum of helium. CTE and Young’s modulus were studied to less than 20 K for solar array materials. At the time of this reporting, no such data exists in the published literature for such extreme cryogenic temperature conditions. The existing methods are only capable of measuring CTE and Young’s modulus to 123 K.

This work was done by Rajeshuni Ramesham, Stephen F. Dawson, and Antonio Ulloa-Severino of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49666