Laser thermogravimetric analysis (laser TGA) is a technique that yields time-resolved data on the thermal decomposition of a specimen of a solid material exposed to a heat flux comparable to the heat flux in a typical rocket engine. Like the technique described in the preceding article, laser TGA involves heating the specimen with a continuous-wave laser beam to obtain the required high heat flux. The utility of laser TGA is not restricted to rocket-engine materials; laser TGA could be used to study high-heating-rate thermal decomposition of almost any high-temperature insulating material.

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CO2-Laser Beams in Tubes enter the test chamber through salt windows on opposite sides. A third salt window can be used for observation by an infrared camera, or optical pyrometer.

The specimen is fabricated as a circular wafer. The thickness of the wafer is chosen according to the thermodynamic properties of the specimen material. The specimen should be made as thin as practicable. The exposure of both sides of the very thin specimen to equal irradiance provides essentially uniform heating with no appreciable thermal gradients across the thickness.

During an experiment, the temperature and the weight-loss data are recorded and plotted against each other to obtain an industry-standard density-vs.-temperature plot. The experiment can be conducted at various heating rates by changing the power output of the laser.

This work was done by Tim Johnson of Thiokol Corp. for Marshall Space Flight Center.

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