The ability of Raman spectroscopy to nondestructively evaluate thermal degradation in graphite reinforced epoxy composites was examined. A series of composite samples, exposed to temperatures ranging from 150 to 400 °C for periods of 2 to 20 minutes, were analyzed by Fourier transform Raman and reflectance infrared spectroscopies. The intensity of the Raman and infrared polymeric backbone vibration at 1 600 cm' diminishes with increasing thermal exposures and can be correlated to failure strain and flexural strength measured by four point bending tests, as well as acoustic emission events. These data, along with infrared transmission spectra of species evolved from composite pyrolysis, suggest that thermal degradation occurs in three stages: 1) polymeric fragmentation (possibly microcracking), 2) advanced polymer degradation observed as delamination between the four ply layers, and 3) final composite failure with fiber fracture.
This research has performed preliminary in situ Fourier transform infrared (FTIR) measurements during the plasma deposition of amorphous silicon (a-Si:H). Experiments demonstrate both gas phase and film measurements within a simple SiH4 plasma reactor using a specially modified FTIR spectrometer. Films are deposited on substrates of either gold (mirror finish) or stainless steel (matte finish). In particular, in situ emission/reflection FTIR of the film yields information about surface temperature, film thickness, and film composition. We have measured surface temperature to ±5 K and detected the onset of poor film growth at a thickness of 500–1000 Å using the 2080 cm−1 absorption feature. A simple model for the reflectance of a film on a metal is employed to determine the thickness of the films. In situ emission/transmission FTIR of the plasma determines the gas composition and average gas temperature. Measurements show that the silane conversion is ∼11% within the plasma region for a typical deposition at 250 °C and roughly doubles for a deposition at room temperature. The FTIR spectra show that most of this converted silane reappears as disilane (Si2H6). Before starting the plasma, the silane gas is ∼30 K cooler than the nominal substrate temperature of 250 °C; starting the plasma raises the average temperature another 20 °C.
A bench-scale instrument has been constructed which provides a fire hazard and combustion product profile of polymers and composites used in advanced construction. Novel to the instrument is that a standardized test of material flammability (oxygen index) can be measured as a function of sample exposure temperature up to 800°C. Sample mass loss is measured concurrently ; and Fourier transform infrared (FT-IR) spectroscopy is used for on-line quantification of evolved gas species during heating, burning, and smoldering of the sample. The instrument has applications for industrial quality control of the fire characteristics of processed materials, and research and development of new building materials. This article describes the instrument and presents measurement results for several materials.
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