A technology focus area of the Cryogenics Test Laboratory is thermal insulation systems. The development of cost-effective, robust cryogenic insulation systems that operate at soft-vacuum level is a primary target from the energy and economics point of view. This applied research and development work includes the test, evaluation, characterization, and application of silica aerogel beads produced by Cabot Corporation. The aerogel bead material has many potential applications for cryogenic and other higher-performance insulation needs in industry. Evaluation activities include novel composite constructions and larger-scale applications such as cold boxes. The material has been proposed for insulating cryogenic umbilical connections for new commercial launch platforms, retrofitting perlite-insulated storage dewars, and insulating a miles-long cryogen transfer line. Over 100 liquid nitrogen boiloff tests of the aerogel products using research cryostats have been performed. Characterization information, such as evacuation, outgassing, and ease of use, is also being obtained. The thermal performance data are being used in the preliminary development of future space launch and exploration applications.

The aerogel beads have a bulk density of about 80 kilograms per cubic meter (kg/m³) and a mean particle diameter of 1 millimeter (mm). The typical pore diameter of the particles is about 120 angstroms. Production of the aerogel beads employs a continuous spray process of manufacturing. The ambient drying step replaces the costly supercritical drying step characteristic of most aerogels produced by solution-and-gelation (sol-gel) methods. The beads are treated to remain hydrophobic, but a hydrophilic (untreated) product is also available for oxygen service. The properties of aerogel beads are given as follows:

Properties of Aerogel Beads

Steady-state liquid nitrogen boiloff methods were used to characterize the thermal performance of aerogel beads in comparison with conventional insulation products such as perlite powder and multilayer insulation (MLI). Test articles are heated and evacuated to below 10^-5 torr to begin a test series. The cold vacuum pressure (CVP) is adjusted for the desired vacuum level using nitrogen as the residual gas. A thermal shroud maintains the insulation outer surface (warm boundary temperature [WBT]) at approximately 293 K. (See figure 1.) The cold mass (cold boundary temperature [CBT]) is kept at approximately 80 K. After coincident stability of the vacuum level, all layer temperatures, and the evaporation (boiloff) rate are achieved, the apparent thermal conductivity (k-value) is determined from Fourier’s law of heat conduction for a cylindrical wall.

The materials tested were aerogel beads (81 kg/m3), opacified aerogel beads (94 kg/m³, carbon black R300), perlite powder (115 kg/m3, 50 x 50 mesh), and MLI (92 kg/m3, 60 layers aluminum foil and fiberglass paper). All test specimens were made in a cylindrical configuration at a typical thickness of 25 mm. A summary graph of the k-value as a function of CVP is given in figure 2. The experimental curves for perlite and MLI compare well with similar thermal performance data from the literature. The aerogel beads gave superior performance for all CVP’s above 0.1 torr. The carbon black opacifier improved the performance of the aerogel beads for CVP’s below 10 torr. The high-vacuum (1 x 10^-4 torr) performance of the plain white aerogel beads is approximately 1.1 milliwatts per meter-kelvin (mW/m-K), while that of the opacified specimen is approximately 0.6 mW/m-K. The added radiation shielding effect of the opacifier yielded almost 50-percent improvement. The k-values for the aerogel beads were found to be comparable to the performance of other aerogel-based products under similar cryogenic vacuum conditions. The results showed the performance of the aerogel beads was significantly better than the conventional materials in the soft-vacuum to no-vacuum range. Opacified aerogel beads performed better than perlite powder under high-vacuum conditions.