1 February 2002
New panels block high heat
By Jim Strothman
Thermal-protection panels made by impregnating fibrous ceramic substrates with organic polymers can temporarily protect against temperatures up to 3,500°C or heat fluxes up to 16 megawatt/meter2 (MW/m2), researchers discovered at NASA's Ames Research Center.
Like ablative thermal-protection panels of older types made of other combinations of materials, the panels absorb and dissipate incident heat through depolymerization and charring resins, transpiration (blowing gaseous pyrolysis products from boundary layers), and radiation from charred surface layers.
Its developers said the new panel type, compared with older panel types, is more porous, and gaseous pyrolysis products can percolate more freely. The new panels also dissipate heat through additional mechanisms that include vaporizing ceramic fibers and radiation from the ceramic substrates' heated surfaces.
Asked about real-life application potential, Huy Tran, one Ames Research Center researcher working on the project, said the panels might protect workers in steel mills or other high-heat plants. He also said the materials could see use in oil fields.
At a heat flux in the approximate range of 1–2.5 MW/m2, the surface layer that forms on the Ames panel includes not only a char formed by decomposition of the organic material but also the coalesced ends of ceramic fibers. The resulting char layer that builds up on the surface is stronger than the char layers that build up on ablative thermal-protection panels of older types. The surface recession rate for the panel is therefore lower, the researchers said.
Another contribution to thermal protection arises from a complex of additional mechanisms that also occur at a heat flux in the approximate range of 1–2.5 MW/m2. If, for example, the ceramic fibers are made of silica, then during pyrolysis, the organic material can react with the fibers to form a layer of silicon carbide protected by a thin, glassy layer of silicon dioxide. The glassy layer makes the panel less catalytic, reducing the rate of convective heating. The surface is also highly emissive, so most of the energy absorbed at the surface reradiates. The combination of reduced convective heating and reradiation helps suppress loss of mass and recession of the surface.
At a heat flux in the approximate range of 4.3–16 MW/m2, the panel dissipates incident heat almost entirely by reradiation and microspallation, or the ceramic substrate's evaporation.
For further information, access the Technical Support Package free online at www.nasatech.com under the Materials category. IT
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