Experimental determination of vapor species from laser-ablated carbon phenolic composites

The thermal loadings approximating those to be encountered by heat-shield materials at peak heating during vehicle entry into the atmospheres of the outer planets have been simulated with laser irradiation. The relative concentrations of the resulting vapors as they emanate from the ablating material have been measured, and the technique for vaporizing the surface of bulk samples by laser irradiation while measuring in situ the vapor species by mass spectrometry is described. Results from carbon phenolic show that vapor composition varies with irradiance level and with depth of heating (or extent of pyrolysis). Attempts are made to compare these experimental results with the theoretical predictions from computer codes. N view of the paucity of measurement data identifying the chemical species generated by vaporization of heat-shield materials, the objective in this work has been to determine experimentally the composition of blow-off gasses produced by carbon phenolic composites when subjected to the level of incident thermal energy encountered during entry into planetary atmospheres. This information is important as a basis for predicting the blockage factors for incident thermal radiation, and as a means of verifying the computer codes that predict chemical mole fractions on the basis of ther- modynamic and chemical equilibrium. The approach in this work has been to measure the vapors in situ as they emerge from the surface of the heated material prior to striking container walls or condensing on any surfaces, thus making possible a more complete determination of the chemical composition of the gaseous components including the free radicals and condensables as well as the more stable molecules. Although carbon/phenolic heat shield materials are not new, heretofore the amount of experimental work that has been done to define the vaporization products of the com- posite material has been limited both in extent and in in- strumentation employed. In most cases, the heating has been accomplished by placing the sample on a resistively heated surface or by putting it in a furnace, and the gas analysis carried out subsequently by gas chromatography or mass spectrometry. This inherently limits analysis to the stable vapor species. The major contribution of the work reported here lies both in the results and in the technique used to heat ablation materials by incident thermal radiation in an attempt to simulate the thermal levels encountered under vehicle flight conditions, while simultaneously sampling the gaseous products by instrumentati on that can best determine the initial pyrolysis species, including the transient intermediates. Experimental As in earlier work,1 the technique used for studying the materials of interest has been to vaporize an area of the sample surface by laser irradiation while measuring the vapor products using a mass spectrometer as the in situ diagnostic tool; the sample and spectrometer are within the same high- vacuum chamber and are positioned so that the ion source of the mass spectrometer is in a line-of-sight, collision-free path from the heated surface. The instrumentation used in this work is shown in Fig. 1; it has been described previously along with the associated data acquisition system.2 A time-of-flight (TOP) mass spec- trometer was selected because of its unique ability to present up to 50 complete mass spectra per millisecond plus its capability of measuring quantitatively several vapor species simultaneously, thereby permitting an in situ analysis during each laser-induced vaporization process. When the pulsed laser was employed, mass spectra were recorded only as the initial vapor cloud passed through the ion source; this insured equal measurement of the condensables and short-lived species along with the fixed gases. Although absolute calibration of the instrument has not been achieved at this time, sensitivity factors for the various gases were derived from the literature and from experimentation so that relative concentrations could be determined. Two types of laser heating have been employed: 1) a pulsed Ndrglass laser in the normal (or burst) mode, and 2) a con- tinuous wave (cw) carbon dioxide laser operating at a wavelength of 10.6 pm. Although pulsed lasers operating in the burst mode characteristically produce a highly irregular time-intensity distribution of output energy, their lower power densities and longer radiating periods better simulate the heating encountered by planetary probes than would those operating in the Q-switched mode. Furthermore, it is necessary to minimize any laser beam-vapor interactions that produce plasma, etc.—typically a result of the very high power intensities associated with Q-switched lasers. The pulsed laser heating conditions used here are shown in Table 1. Earlier work3 has shown that under the given conditions a laser beam having a uniform cross section and focused onto a planar surface—thereby vaporizing material in a circular spot within a reduced ambient pressure (or vacuum)—produces a gas-dynamic phenomenon which is a quasisteady, supersonic, free-jet expansion of the vapor products. Thus, the spot acts as a sonic orifice for the vapors and produces isentropic flow with streamlines appearing to radiate from a point source.