Dynamic Elemental Analysis during Thermal Decomposition using Radiometric Techniques

An investigation to find an instrumental technique for measuring the instantaneous distribution of mass fractions of the elements within materials composed only of hydrogen, carbon and oxygen is described. The investigation was prompted by the need to plot changes in elemental distribution in isothermal layers of solids during their thermal de composition as a function of time, in order to develop an understanding of the processes involved during combustion of natural and synthetic polymers. A computer-controlled apparatus was designed and built which allowed measurement of the attenuation of the intensity of a narrow, collimated beam of y-rays by solids of section up to 50 mm?. The y-rays came from one of four selectable radio-active sources. Radiant heat, orthogonal to the y-ray beam, was applied to one face of the sample and thermocouples buried in the sample were used to follow the changing temperature profile. The solids were supported on a load cell so that overall weight loss could also be followed. The attenuation could be automatically measured at different distances from the heated face. It was expected that comparison of the relative attenuations at different energies would provide sufficient information to calculate the mass fractions in thin layers of the sample. It was found that the constancy of the energy of the radiation from the sources imposed intolerable restrictions on the nature and size of sample that could be analysed. Selecting only one of the sources, }*’Cs , the apparatus was used to measure density changes during thermal decomposition of flame-retardant treated and un-treated Douglas Fir. First-order kinetics were assumed to apply and pre-exponential factor, A and activation energy, E were both measured at different intensities of radiant heat, Ip, to assess the efficacy of the flame-retardant used to treat the wood. Average values were found to be for virgin Douglas Fir A = 6.2 10*8 sec-!, FE = 113 kJ (mole)-! for flame-retarded Douglas Fir A = 2.1 10'° sec-1, E = 83 kJ (mole)! Mass fractions, Fx, can be calculated from a knowledge of mass attenuation coefficients for each element, X, as a function of energy, (4(£)),, and solution of simultaneous equations developed from (FE) sample = Fa E(E)) 4 + Fo(T(E))¢ + Fo(7(E)) when (4(£)) is measured at three different energies. sample In an attempt to provide three selectable energies of electromagnetic radiation for the elemental analysis, three orders of Bragg diffraction of the Bremsstrahlung from an Xray tube were sought from a crystal of rock salt. The required lines were found to be obtainable only at 5.81 keV, 11.62 keV and 17.44 keV using a tube fitted with a molybdenum target. Finally, a technique using a broad band spectrum of X-radiation was developed which allowed smoothing of statistical variations of measured values to produce consistent results in measuring the mass fraction of carbon in polymethyl methacrylate. Measurement time was sufficiently short to suggest that a successful instrumental technique will be developed using this approach