Solicitud de datos de medidas

The TEM grids were examined with a Philips™ CM20 Transmission Electron Microscope at 200KV and images were collected using an Olympus™ Veleta camera. The digital camera was calibrated using a MAG*I*CAL® calibration reference which is a NIST traceable standard.

Figure 2.8 TEM images of Kapton particle morphology from ISS high temperature testing, unaged on the left, aged on the right. Sample heating temperature was 574 °C, reference length scale = 5 μm.

Kapton is a low outgassing polyimide film that survives a wide temperature range and is used

in electrical wire insulation and other spacecraft applications. Kapton smoke particles are the

smallest of the five materials tested and are rarely agglomerated. The spherical shape and

uniform density indicate growth by condensation in the saturated vapor of the pyrolysis products.

Figure 2.8 shows the effect of aging, with the unaged (left) having a higher population of very

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Figure 2.9. Kapton ISS samples after heating to 420 oC.

The sample carousel shown in Figure 2.2 holds six samples within wire heating coils. After

the experiments were performed on the ISS, these were returned to Earth for examination. It can

be observed from the optical microscope images of Kapton heated samples in Figure 2.9 that the

material discolors in the center of the heating coil and the film layers become brittle after

heating. This is evident in the cross-section (lower right image) in which the fracture exhibits

brittle failure of the layers where the sample experienced the most heat. The amber colored

lighter ends of the layers are notably separate i.e., not fused. Higher temperature test specimens

examined showed more discoloration and warping of the Kapton film.

Lamp wick smoke aerosols (Figure 2.10, left image) are known to be primarily spherical

droplet-type particles that grow by condensation of pyrolysis gases (Mulholland et al, 1995).

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observed: uniformly dense or lighter in the center which suggests that they arrive at the carbon

film of the TEM grid as a liquid. Some TEM images display additional faint particles that

covered only one or two pixels.

Figure 2.10. TEM images of unaged smoke from ISS tests: lamp wick, 265 °C (left) and Pyrell, 242 °C (center) and Teflon, 514 °C (right), reference length scale = 5 μm.

Pyrell® _{is used for stowage foam to cushion instruments and other payloads during launch into}

space. Its widespread use made it a strong candidate in the survey of potential sources of smoke

in spacecraft fires. Pyrell smoke particles consist of agglomerates made up of primary particles

ranging from 30 nm to 100 nm (Figure 2.10, center image). Teflon® is present on the

International Space Station in many forms, but predominantly in wire insulation. Teflon primary

particles are much smaller than Pyrell and are fractal agglomerates (Figure 2.10, right image).

The darker agglomerates are more electron-dense and indicate that the fainter particles may have

partially evaporated in the electron beam. In addition, some particles were not completely

adhered to the TEM grid and movement could be observed as the force of the electron beam

33 Figure 2.11. Lamp wick ISS sample after heating.

Photographs of a lamp wick sample after heating are shown in Figure 2.11. The central part of

the lamp wick within the heating coil undergoes charring as a result of oxidation, while the ends

experienced less heat and are less discolored. The mass loss after heating this sample was

approximately 3 mg.

Figure 2.12. Pyrell foam ISS sample after heating to 245 oC.

Pyrell samples did not appear significantly different after heating, however, the sample shown

in Figure 2.12 had 0.5 mg mass loss after heating to 245 oC. The Teflon sample in Figure 2.13 shows evidence of the polymer expanding during heating and discoloration of the heating coil.

This particular sample had 1.3 mg mass loss after heating, while others at higher temperatures

34 Figure 2.13. Teflon ISS sample after heating to 515 o_C.

Silicone particles were not wholly preserved on the TEM grids owing to the volatile nature of

the pyrolysis products. Only very small and faint particles remained after the return flight to

earth, as seen in Figure 2.14. Note that the magnification in this figure is nearly double that of

the other particle images shown. Figure 2.15 shows two different silicone rubber samples after

heating. Note that the heating coil is not discolored but the silicone tends to swell and become

brittle. The sample on the left had 1.5 mg mass loss after heating to 349 oC and the right sample had 3.5 mg mass loss after heating to 380 o_C.

Figure 2.14. TEM images of residual silicone smoke particles from SAME pyrolysis at approximately twice the magnification. Sample heating temperature was 380 °C, reference length scale = 2 μm.

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Figure 2.15. Silicon rubber ISS samples after heating to 349 oC (left) and 380 oC (right). Morphology results show that only Kapton and lamp wick are spherical aerosols so they are

better suited to calculating the particle diameters from TEM images. Although the TEM images

of silicone do not reflect spherical morphology, it is considered a spherical smoke aerosol as it

consists of liquid droplets (Mulholland, 1995). Meaningful values of dav can be calculated

regardless of shape, and the material-specific calibration of the DustTrak with fundamental

aerosol mass measurements provides moment method values of dm which are valid for the

nonspherical materials Pyrell and Teflon (by equations (3) and (4)). No significant discernable

difference was noted between the morphology of the pyrolysis particles sampled in low gravity

vs. normal gravity for typical SAME flow conditions. A specific set of test points were run in

low gravity with no flow through the SAME smoke generation duct, which resulted in

significantly larger spherical particles. Details of these tests are outlined in Mulholland et al.

(2015).