Marit E. Meyer

Glenn Research Center, Cleveland, Ohio

Particle Morphology and Elemental Composition 

of Smoke Generated by Overheating Common 

Spacecraft Materials

NASA/TM—2015-218891

December 2015
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Marit E. Meyer

Glenn Research Center, Cleveland, Ohio

Particle Morphology and Elemental Composition 

of Smoke Generated by Overheating Common 

Spacecraft Materials

NASA/TM—2015-218891

December 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The author would like to gratefully acknowledge the microscopy performed by Victoria Bryg, SEM, High Resolution TEM and 

EDS (SEM) and Kristin Bunker and Traci Lersch of RJ Lee Group: High Resolution SEM/STEM Microscopy and EDS. 

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NASA/TM—2015-218891 1

Particle Morphology and Elemental Composition of Smoke 

Generated by Overheating Common Spacecraft Materials

Marit E. Meyer

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Fire safety in the indoor spacecraft environment is concerned with a unique set of fuels which are 

designed to not combust. Unlike terrestrial flaming fires, which often can consume an abundance of 

wood, paper and cloth, spacecraft fires are expected to be generated from overheating electronics 

consisting of flame resistant materials. Therefore, NASA prioritizes fire characterization research for 

these fuels undergoing oxidative pyrolysis in order to improve spacecraft fire detector design. A thermal 

precipitator designed and built for spacecraft fire safety test campaigns at the NASA White Sands Test 

Facility (WSTF) successfully collected an abundance of smoke particles from oxidative pyrolysis. A 

thorough microscopic characterization has been performed for ten types of smoke from common 

spacecraft materials or mixed materials heated at multiple temperatures using the following techniques: 

SEM, TEM, high resolution TEM, high resolution STEM and EDS. Resulting smoke particle 

morphologies and elemental compositions have been observed which are consistent with known thermal 

decomposition mechanisms in the literature and chemical make-up of the spacecraft fuels. Some 

conclusions about particle formation mechanisms are explored based on images of the microstructure of 

Teflon smoke particles and tar ball-like particles from Nomex fabric smoke.

1.0 Introduction

Smoke particles were collected via a thermal precipitator (TP) during test campaigns at the NASA 

White Sands Test Facility (WSTF) (Meyer et al. 2013). A description of the device is given in 

NASA/TM—2015-218746 (Meyer 2015). Particles collected were subsequently examined with a Hitachi 

S-5500 scanning electron microscope (SEM), Hitachi HD-2300 scanning transmission electron 

microscope (STEM), and a Philips Model CM20 transmission electron microscope (TEM). The 

microscopes are equipped with energy dispersive X-ray spectroscopy (EDS) detectors that allow for 

elemental information of individual particles to be collected. Four potential substrates were incorporated 

into the collection surface for multiple microscopy options. A strip of conductive carbon tape was placed 

onto the surface of the SEM stub to allow for viewing of the particles on both the aluminum stub substrate 

as well as a neutral carbon background. TEM grids with a) continuous carbon film as well as a b) holey 

carbon film were placed at the edge of the carbon tape to allow sample collection for higher magnification 

examination. Smoke particle morphology gives insight into the thermal decomposition mechanisms of the 

spacecraft materials as well as the particle formation mechanisms, both important elements of smoke

characterization. Elemental information on smoke particle composition gained from EDS provides 

additional information on individual particle characteristics. The polymeric fuel materials in this study 

had accompanying material data sheets, however, the specific formulations are proprietary and thus some

additional constituents may be present such as plasticizers or colorants, which have the potential to affect 

the smoke production. Thus a number of elements are often observed in the EDS spectra which do not 

appear in the fuel chemical formula. In addition, there are other potential sources of elements, particularly 

Cu, Al and Si. The presence of Cu can be attributed to the TEM grid itself and a low Al peak may be 
NASA/TM—2015-218891 2

present owing to the sample holder of the microscope (therefore known as a system peak). The WSTF 

smoke generation process relies on a mica liner to hold the fuel during heating, which is another potential 

source of Si and Al in the smoke particles.

Thermal decomposition of the common spacecraft materials shown in this work mostly follow a 

common pattern. Under oxidative pyrolysis, a solid matrix of polymers will degrade and give off gaseous 

and liquid fuels which can be combined in high temperature reactions. These pyrolysis products are 

molecular fragments ranging in size from polymer chains down to the size of the monomer subunit of the 

polymer (Mulholland et al. 2015). Lower molecular weight materials can produce volatile species by 

evaporation or sublimation, whereas high molecular weight species which are cross-linked must undergo 

thermal decomposition at higher temperatures to produce gases. Polymers release low molecular weight 

gases upon initial heating followed by higher molecular weight pyrolysis products (Durlak et al. 1998).

Saturated vapors emitted from the fuel either homogeneously nucleate into small molecular clusters as the 

gas cools, becoming internally homogeneous particles, or gases can condense onto existing particles 

which then appear as internally heterogeneous particles with inclusions or coatings. These formation 

mechanisms are distinctly different from soot formation in flaming combustion, which can be generalized 

into a three step formation mechanism consisting of 1) precursor species formation (when the complex 

fuel molecules are broken down into low molecular weight radicals which participate in many reactions), 

2) particle inception (in which solid particle nuclei form and undergo surface growth by absorbing 

available gas phase molecules and becoming 20 to 24 nm spherules which agglomerate under Brownian 

motion in the flame), and finally 3) further agglomeration and surface growth, until the soot particle exits 

the flame when it cools and can potentially adsorb organic surface coatings (Moosmüller et al. 2009).

2.0 Overview of Heated Materials and Microscopy Results

A summary of the SEM analyses performed using a field emission SEM (Hitachi S-5500) is given in 

Table 1 which describes the fuel, heating temperature, particle coverage obtained by the TP sampling, and 

qualitative descriptions of the particle sizes and morphologies observed. Two different thumbnail images 

are also provided in the table to show representative smoke particles from each sample at 5000x and 

10,000x magnification. Images of smoke particles collected on the carbon tape have a somewhat bubbled 

texture in the background, most likely from the non-uniform adhesive under the carbon membrane.

Images of smoke particles collected on the aluminum SEM stub surfaces often have a background pattern 

of faint parallel lines which are striations in the aluminum surface, as the stubs were not polished before 

use. Additional images and analyses from SEM, TEM, high resolution TEM and high resolution STEM 

are shown below for smoke particles from the following fuels: printed circuit board, wire insulation, 

Teflon, Kapton and Nomex. 

Run numbers refer to the WSTF testing and can be cross-referenced to Meyer et al. 2013, where 

material descriptions and fuel preparation methods are outlined in detail. Wire insulation tests outlined 

here used PFPI insulation (partially fluorinated polyimide manufactured by TRW).

3.0 Smoke Particles From Printed Circuit Board Populated With 

Components

A spacecraft fire could originate by overheating electronics, so circuit board materials are an 

important candidate for smoke characterization. As circuit boards are heated to above 250 °C, scission of 

high energy bonds takes place and small fragments are liberated along with monomers of the constituent 

materials. Above 500 °C, aromatic condensed ring systems (native to the original polymer or formed by 

reactions) will thermally decompose (Lambert, 1993).
TABLE 1.—SUMMARY OF SEM ANALYSES FOR WSTF FUELS

NASA/TM—2015-218891 3
TABLE 1.—CONTINUED.

NASA/TM—2015-218891 4
TABLE 1.—CONTINUED.

NASA/TM—2015-218891 5
TABLE 1.—CONTINUED.

NASA/TM—2015-218891 6
TABLE 1.—CONTINUED.

NASA/TM—2015-218891 7
TABLE 1.—CONTINUED.

NASA/TM—2015-218891 8
NASA/TM—2015-218891 9

The construction of the FR-4 begins with a woven glass fabric (E-glass, an alumino-borosilicate glass 

(Baker et al. 2004)) which is impregnated with epoxy resin. Circuit boards can be plated with tin, gold, 

nickel or copper, and the circuit board fuel in these tests was granulated and included components 

(integrated circuits [IC] microcircuits attached to the board with tin-lead solder) which can contain many 

other materials: silicone encapsulent; aluminum, gold or copper wires; copper or nickel alloy leadframes, 

among others. Printed circuit boards and components thermally decompose at different rates, with IC 

components having higher thermal stability owing to the prominence of Si which enhances char formation 

(Duan and Li, 2010). The printed circuit board fuel used in the smoke tests is the composite material FR4, which has been developed to be flame retardant (hence the abbreviation ‘FR’). Halogens are added to 

the glass-reinforced epoxy laminate to make the boards flame resistant, or self-extinguishing, and studies 

on thermal recycling of circuit board waste show that bromine is present as high as 8.5 weight percent 

(Chien et al. 2000).

In several tests, collection of printed circuit board smoke particles on an aluminum SEM stub in the 

thermal precipitator often resulted in a film layer with distinct embedded particles and clusters of particles 

as small as 150 nm to 200 nm, as seen in Figure 1.

Most particles observed on the SEM stub were somewhat spherical or compact agglomerates, 

however, the TEM grid analyses revealed other distinct types and features, including encapsulated and 

coated particles, char particles as well as crustal particles. Sheridan et al. (1994) defined the 

compositional category of crustal particles for atmospheric aerosols, to include O and at least two 

elements common in the Earth’s crust: Si, Al, Ca, and Fe. It has since become a common particle 

characterization category, with some authors including additional crustal elements (Ramirez-Leal et al. 

2014, Li et al. 2011, Furutani et al. 2011, Mogo et al. 2005, Chen et al. 1998).

Figure 2 shows a limited number of particles in the left image (highlighted with arrows) with 

diameters on the order of 200 to 400 nm, and a detail of the particle (red box) in the right image 

(approximately 350 nm in diameter) with its corresponding EDS spectrum below. Note that in the EDS 

spectra shown, some Cu can be attributed to the copper TEM grid which supports the lacey carbon, 

although this peak is relatively large compared to other circuit board particles and it logically is expected 

to appear in the circuit board smoke particles from copper layers in the board or the clad laminate.

Figure 2 has a prominent bromine peak, as well as Pb, which is also common in many circuit board 

smoke particles sampled with the thermal precipitator.

Figure 1.—SEM images of circuit board smoke particles. (a) At 640 °C (Run 33). (b) At 340 °C (Run 27).
NASA/TM—2015-218891 10

Figure 2.—Lacey carbon TEM grid with circuit board smoke particles at 540 °C (Run 31). (a) Typical rounded multicomponent particles on the order of 200 to 400 nm diam. (b) Detail of particle in red box, approximately 350 nm 

diam. (c) Corresponding EDS spectrum showing Br, Cu, and Pb, which are typical constituents of circuit board and 

component materials. Cu can also be attributed to the TEM grid.

Figure 3 is an example of an irregular particle with inclusions (the particle is distinctly internally 

heterogeneous). The inclusions range from 40 to 60 nm, and some may be agglomerated to the surface, 

but most appear to be bound within the coating of the particle. Elements in the EDS spectrum include Cu,

Br, Pb and Si. Cu can be attributed to the copper TEM grid which supports the lacey carbon grid and the 

Si signal may be attributed to the mica sheet in the smoke generator.

Figure 4 shows High resolution STEM (Hitachi HD-2300) images of inclusions in circuit board 

smoke particles, which demonstrate a Pb/Br-rich particle with a Br-rich coating, with Si often present in 

the coating as well. Note that the inclusions in the right image appear to be crystalline, with recognizable 

angular crystal faces.

Figure 5 is an image of a crustal particle along with its EDS spectrum showing a significant Si peak 

and a minor Al peak (both can attributed to the E-glass in the circuit board, although the Al may be a 

system peak). Elements detected within circuit board smoke particles (including other EDS spectra not 

shown) are C, O, F, Fe, Cu, Al, and Si. Gases evolved from overheating circuit board include CO, HF, 

HCl and off-line ion chromatography analyses also show HBr from these tests (Meyer et al. 2013).
NASA/TM—2015-218891 11

Figure 3.—EDS spectrum of a circuit board smoke particle at 540 °C with TEM grid image (inset) (Run 31).

Figure 4.—High resolution STEM image details on inclusions within circuit board smoke particles at 540 °C (Run 31).

Figure 5.—EDS spectrum of circuit board smoke particle (inset) at 540 °C (Run 31) and TEM image. The jagged 

crustal particle is much larger than the typical rounded particles and EDS shows a significant Si peak as well as Cu 

and Al, all typical constituents of electronics (circuit board and components).
NASA/TM—2015-218891 12

4.0 Smoke Particles From Wire Insulation 

Along with circuit boards, thermal degradation of wire insulation has high potential as a source of 

smoke in spacecraft. Samples of partially fluorinated polyimide (PFPI) wire insulation manufactured by 

TRW were overheated and smoke particles collected with the thermal precipitator. PFPI polymers were 

invented under NASA Lewis Research Center sponsorship in the late 1970s and were targeted as superior 

candidates for high temperature wire insulation products (Jones, 1994). This PFPI wire insulation was 

shown to generate smoke in previous testing within 35 sec at current levels from 10 to 15 A, and smoke 

reportedly vented from bubbles and imperfections in the insulation (Hammoud et al. 1995). Figure 6

shows two distinct PFPI smoke particle morphologies from thermal precipitator sampling, spherical and 

crystalline.

The majority of the wire insulation smoke particles captured are spherical, some appearing as 

doublets, as in the left image of Figure 6. Larger square and angular crystalline particles are less common, 

as shown in the center and right images of Figure 6.

Some notable yet infrequent large spherical particles were observed ranging in diameter from 4.0 to

4.6 μm, as shown in Figure 7. These larger particles appear to have somewhat irregular surfaces which are 

most likely smaller particles agglomerating on the surface, as evident in the right image of Figure 7. The 

advantage of collecting SEM images is the ability to observe particle surface features and morphology 

with distinct details, including the 300 nm particle and other particles adhered to the sphere in the right 

image. Striations in the aluminum surface finish SEM stub are evident in the right image.

Figure 6.—SEM images of wire insulation smoke particles at 640 °C (Run 6). (a) Smaller particle populations are 

mostly spherical. (b) and (c) Particles with square crystalline morphology.

Figure 7.—SEM images of wire insulation smoke particles at 640 °C (Run 6). These large spheres range from 4.0 to 

4.6 μm diameter. (a) Three spherical particles in proximity. (b) Note the smaller particles adhered to the large 

particle.
NASA/TM—2015-218891 13

5.0 Smoke Particles From Teflon

Teflon is the DuPont Company trade name of polytetrafluoroethylene (PTFE, (C2F4)n), a crystalline 

fluoropolymer commonly used in spacecraft for wire insulation, water storage bladders, sampling bags, 

suits, and cargo liners. The thermal decomposition of Teflon differs from the other polymers analyzed in 

spacecraft fire characterization testing. Teflon polymer fragments are released during the pyrolysis of 

Teflon, as opposed to polymers which release low and high molecular weight gases. These fragments, 

ejected under the same temperatures as the heating range of these tests, grow through nucleation, 

condensation and coagulation (Mulholland et al. 2015).

Teflon smoke particles collected with the thermal precipitator had nearly spherical morphology of 

varying sizes, as seen in Table 1. Figure 8 and Figure 9 show high resolution SEM images (Hitachi S5500) of typical Teflon smoke particles which are nearly spherical. Although these particles are fairly 

large (typically greater than 500 nm), it is evident that many smaller particles have agglomerated to the 

surfaces, some as small as 50 nm in diameter.

Smoke particles in Figure 8 and Figure 9 were collected on two different types of TEM grids that 

were attached to carbon tape on the SEM stubs, one with continuous carbon film between the copper grid 

matrix, and the other with lacey carbon between the copper grid openings.

Figure 8.—High resolution SEM images of Teflon smoke particles at 540 °C collected onto a carbon film TEM grid. (a) 

250 nm to 1.5 μm diam. range. (b) 500 nm and 700 nm diam. (c) 700 nm diam.

Figure 9.—High resolution SEM images of Teflon smoke particles at 540 °C collected onto a lacey carbon TEM grid. 

(a) Representative particles. (b) 700 nm and approximately 400 nm diam.
NASA/TM—2015-218891 14

An EDS spectrum and image of a Teflon smoke particle are shown in Figure 10. The prominent 

fluorine peak is expected, and indicates that the material did not completely thermally decompose during 

heating, substantiating that intact fragments of polymers are liberated in the smoke. 

However, the elements Al and Si are not expected to appear in Teflon smoke particles. The Al is most 

likely a system peak, Cu can be attributed to the copper TEM grid which supports the lacey carbon, and 

Si is potentially from the mica liner in the WSTF smoke generator.

Several typical Teflon smoke particles were examined at higher magnifications in a TEM (Philips 

Model CM20) in order to reveal some chemical information about the structure and order of the particle 

material. Figure 11 shows high resolution TEM images of a Teflon smoke particle (640 °C) 

approximately 400 nm in diameter adhered to the lacey carbon. The irregular rounded particle on the left 

appears to be composed of layers and/or agglomerated smaller particles, as seen by the varying density 

(darkness) and has a tail which is magnified in the center and right images. The particle tail is thinner than 

the particle itself and it is hanging over the edge of the lacey carbon support structure, which allows 

imaging of the microstructure. The center image shows smaller particles in the tail which have either 

agglomerated or become encapsulated. The far right image shows the amorphous interior structure of the 

tail.

Figure 10.—High resolution SEM image of Teflon smoke particle (inset) at 540 °C collected on a lacey carbon TEM 

grid and corresponding EDS spectrum with a prominent F peak.

Figure 11.—High resolution TEM images of a Teflon smoke particle at 640 °C (test 15) adhered to a lacey carbon 

TEM grid. (a) The irregular rounded particle with an extended tail is approximately 400 nm in diameter. (b) Particle 

tail detail which contains smaller particles. (c) Amorphous interior structure of the tail.
NASA/TM—2015-218891 15

Figure 12 shows a 250 nm spherical Teflon smoke particle, which appears to consist of many smaller 

particles bound together by amorphous material as seen in the right image of Figure 12, which is at higher 

magnification and interior particles are on the order of 20 to 30 nm. This primary particle size has been

confirmed by initial particle size measurements of fresh smoke in the test chamber, however, within 

seconds, the extremely small particles rapidly combine into larger agglomerates in the high concentrations 

typically seen in the smoke chamber. Teflon particles are known to escape detection by photoelectric 

smoke detectors owing to their small size. 

Figure 13 shows a portion of a much larger agglomerate with distinctly angular shaped protrusions.

The center and right images are magnified portions of the angular agglomerate and the morphology shows 

evidence of crystallinity. This is not unexpected for Teflon, which can be up to 90 percent crystalline.

Images in Figure 14 are the highest magnification images which illustrate the individual carbon fringe 

structures or nanostructures of the agglomerate particle of Figure 13. The linear pattern in the interior of 

the particle shows the alignment of crystal fringes within an amorphous carbon coating. This substantiates 

that the original crystalline material fragments have persisted through the heating cycle and were 

subsequently coated by amorphous products condensing on the surface. 

Figure 12.—High resolution TEM images of a Teflon smoke particle at 640 °C (test 15) adhered to a lacey carbon 

TEM grid. (a) Compact agglomerate, approximately 250 nm diam. contains smaller particles bound together by 

amorphous material. (b) Particle detail.

Figure 13.—High resolution TEM images of a Teflon smoke particle at 640 °C (test 15).
NASA/TM—2015-218891 16

Figure 14.—High resolution TEM images of a Teflon smoke particle at 640 °C (test 15) showing a crystalline interior 

with an amorphous coating.

Figure 15.—Teflon smoke particles collected with the 

SAME thermal precipitator on the ISS, showing vastly 

different morphology compared to WSTF samples 

(TEM, Philips Model CM20).

Teflon smoke particles from the WSTF thermal precipitator are very different from the morphology 

observed in the NASA Smoke Aerosol Measurement Experiment (SAME), performed on the 

International Space Station (ISS), and pictured in Figure 15, most likely because of different heating 

methods and fuel preparation (Meyer et al. 2015).
NASA/TM—2015-218891 17

6.0 Smoke Particles From Kapton

Kapton, a polyimide with chemical formula (C22H10N2O5)n performs well at both high and low 

temperatures and has excellent insulating properties and fire resistance. Spacecraft applications of Kapton 

include thin-film heaters, wire insulation, space suits, tape and multi-layer insulation (MLI). Thermal 

decomposition of Kapton results in liquid aromatic products, and the general spherical shape of the 

particles observed with the TEM is consistent with the particles starting as a liquid solution with many 

components (Mulholland et al. 2015). Kapton smoke particles under heating conditions of these tests can 

form chain-like agglomerate structures.

Figure 16 shows SEM images of Kapton smoke particles generated at 540 °C (left image) which are

either spherical or angular, ranging from 100 to 500 nm in diameter. The right image of Figure 16 shows 

particles generated at 640 °C in the same size range, which exhibit chain-like agglomerate structures and 

are coated by condensed pyrolysis products. These particles are on the aluminum SEM stub, and the 

striations in the unpolished surface are evident in the images.

Figure 17 shows high resolution SEM images of Kapton smoke particles on the lacey carbon TEM 

grid. These particles are in the same size ranges as the SEM images in Figure 16 although at higher 

magnification. Some particles appear to have angular edges and uneven surfaces. In the lower left image 

of Figure 17, the string of primary spherical particles ranging from 100 to 350 nm seem to be held 

together by liquid bridges that solidified.

Figure 18 shows an angular Kapton smoke particle 640 °C with its EDS spectrum with peaks for O, 

Si and Al. In all the spectra shown, Cu is attributed to the copper TEM grid which supports the lacey 

carbon, Al may be a system peak and the presence of Al and Si may be attributed to the mica liner in the 

WSTF smoke generator.

Figure 19(a) shows the coverage achieved with the thermal precipitator during a 640 °C test. The Cu

grid around the carbon film appears on either side of the left image. The middle image shows a 450 nm 

spherical particle with a surface similar to previous particles, but with multiple smaller particles adhered 

to it. The right image of Figure 19 shows a chain of three particles all close to 350 nm in diameter, one 

irregularly shaped which is attached to two more spherical particles with rough surfaces, most likely from 

smaller agglomerated particles. Figure 20 shows a 400 nm Kapton smoke particle with a similar rough 

surface texture.

Figure 16.—SEM images of Kapton smoke particles on the Al SEM stub. (a) At 540 °C are compact and either 

spherical or angular. (b) At 640 °C exhibit chain-like agglomerate structures, some doublets and longer chains are 

coated.
NASA/TM—2015-218891 18

Figure 17.—High resolution SEM analysis of Kapton smoke particles at 640 °C on a lacey carbon TEM grid showing 

irregular surfaces and angular edges. (a) Typical coverage. (b) Approximately 700 nm long. (c) The coating on 

spherical particle chain appears to have primaries connected by liquid bridges that solidified and connected primary 

particles ranging from 100 to 350 nm. (d) 500 nm particle.

Figure 18.—Angular Kapton smoke particle (inset), approximately 270 nm at 640 °C with its EDS spectrum showing 

O, Si, and Al. The Al is most likely a system peak, Cu can be attributed to the copper TEM grid, and Si is potentially 

from the mica liner in the WSTF smoke generator.
NASA/TM—2015-218891 19

Figure 19.—High resolution SEM images of Kapton smoke particles at 640 °C. (a) Typical coverage. (b) 450 nm 

spherical particle with multiple smaller particles adhered to it. (c) Chain of three particles all close to 350 nm diam.

Figure 20.—EDS spectrum of a 400 nm Kapton smoke particle (inset) at 640 °C on a carbon film TEM grid showing 

Cu, Si, and Al. The Al is most likely a system peak, Cu can be attributed to the copper TEM grid and Si is 

potentially from the mica liner in the WSTF smoke generator.

Both the compact irregular Kapton smoke particle in Figure 18 and the more spherical particle with a 

textured surface in Figure 20 contain the same elements, although the former has a higher C peak. The Al 

is most likely a system peak, Cu can be attributed to the copper TEM grid and Si is potentially from the 

mica liner in the WSTF smoke generator. Kapton smoke particles shown here compare well to the 

morphology observed in the NASA Smoke Aerosol Measurement Experiment (SAME), in spite of very 

different heating methods and fuel preparation (Meyer et al. 2015). Figure 21 shows aged Kapton 

particles collected with the SAME thermal precipitator on the ISS, which have similar spherical 

morphology and connected as doublets, triplets and longer chains and some appearing to have a 

somewhat rough surface. Furthermore, all the diameters of single particles shown in the WSTF

micrographs are within the ranges of the smoke particle size distributions measured with the Scanning 

Mobility Particle Sizer Spectrometer (SMPS) in the ground testing of SAME flight hardware heating 

Kapton and the size distribution obtained from image analysis (projected area equivalent diameter) of 

pyrolysis smoke particles collected on a TEM grid on board the ISS.
NASA/TM—2015-218891 20

Figure 21.—Aged Kapton smoke particles collected with 

the SAME thermal precipitator on the ISS showing 

similar morphology to WSTF samples (TEM, Philips 

Model CM20).

7.0 Smoke Particles From Nomex

Nomex cloth is a heat and flame-resistant textile woven from continuous meta-aramid polymer fibers 

(DuPont chemical company). It is an aromatic polyamide produced by condensation reaction from two 

monomers, m-phenylenediamine and isophthaloyl chloride (Chanda and Roy, 2008). In spacecraft 

applications, Nomex cloth is used for acoustic insulation, cargo bags, thermal blankets and pressure suits.

Nomex affords a longer heat-resistance owing to carbonization during thermal decomposition (VilarRodil et al. 2001). Zhang (2010) performed analysis of the thermal degradation of Nomex fibers using 

TGA-DTA/FT-IR (Thermogravimetric analysis—differential thermal analysis/Fourier Transform—

Infrared) and reported that above 500 °C, complex depolymerization, random rupture, restructuring and 

other chemical reactions take place. As the Nomex is heated without combustion (oxidative pyrolysis), 

many organic species are liberated leaving a carbon-rich ash or char.

Nomex smoke particles were observed with four types of morphology: irregular char-like, irregular 

with inorganics, rounded irregular and spherical. Figure 22 shows the Nomex smoke particles on the 

aluminum SEM stub, with many spherical particles, some irregular rounded particles, and one large 

crustal particle in addition to several other smaller irregular crustal particles.

Figure 23 shows what appear to be a char particles as the accompanying EDS spectra show very large 

carbon peaks. In all the spectra shown, Cu is attributed to the copper TEM grid which supports the lacey 

carbon. The upper spectrum shows the presence of Al which is a system peak and Si is potentially from 

the mica liner in the WSTF smoke generator.

Figure 24 shows an irregular Nomex smoke particle with the EDS spectrum showing a large oxygen 

peak along with the inorganic elements Mg, Ca and Si, which appears larger than other system peaks.
NASA/TM—2015-218891 21

Figure 22.—SEM image showing spherical Nomex smoke 

particles in the 300 to 600 nm range and a large irregular 

particle (on the right), possibly char. Light areas indicate 

charging by the electron beam, however, the spherical 

particles are stable and did not volatilize under the beam.

Figure 23.—SEM images and corresponding spectra from Nomex smoke of what appear to be char particles owing to 

the high C peaks. Cu in both spectra are attributed to the copper TEM grid in the upper spectrum.
NASA/TM—2015-218891 22

Figure 24.—Spectrum of an irregular Nomex smoke particle with distinct angular inclusions (inset), containing O as 

well as the elements, Si, Ca, Al, and Mg. The Al may be a system peak and Cu can be attributed to the copper 

TEM grid.

Figure 25 is an irregular rounded Nomex smoke particle that is amorphous, and appears to have 

arrived at the TEM grid in a partially-solidified viscous liquid state. The EDS spectrum shows the particle 

contains C, O, N, Al, Si and Cl. The presence of Cl in the Nomex smoke particles is explained by the 

polymer formation process which uses a condensation reaction with isophthaloyl chloride, and 

furthermore, the reaction is catalyzed with LiCl, and the solution used in spinning Nomex fibers typically 

contains more than 3 percent calcium chloride (CaCl2) by weight (Fink, 2008, Gabara et al. 2006). The 

Nomex used for fuel in these tests emits HCl gas in WSTF and GASP (Gases & Aerosols from 

Smoldering Polymers) laboratory testing at levels ranging from 20 to 115 ppm, depending on the fuel 

mass, while HCN and HF are also emitted in lesser concentrations (Briggs et al. 2015, Meyer et al. 2013). 

The process of carbonization during Nomex pyrolysis can be seen as a parallel to the carbonization 

taking place in making charcoal and to biomass burning in general, as well as to coal pyrolysis. Tar has 

been generally defined in the literature as any condensable product emitted from the heating of solid fuel 

(Bond and Bergstrom 2006). More specifically, in coal combustion research, it is referred to as aromatic 

cluster fragments with lower molecular weights (Wang et al. 2013, Wang et al. 2015). In the biomass 

combustion/pyrolysis literature, tar is referred to as high-molecular weight water-insoluble organic 

polymer species, and a liquid from biomass distillation products (Hand et al. 2005, Tóth et al. 2014, Fang 

et al. 2014). The definition of tar with regards to thermal decomposition of organic materials (coal, 

biomass and polymers) is of importance in the classification of particles as ‘tar balls’. Tar balls have been 

observed in wildfire emissions as well as laboratory generated smoldering and combustion, in both fresh 

and aged smoke (Pósfai et al. 2003, Li et al. 2003, Pósfai et al. 2004, Tivanski et al. 2007, Chakrabarty et 

al. 2010, Turmolva et al. 2010, Adachi and Buseck, 2011, China et al. 2013, Tóth et al. 2014). 

Characteristic descriptions of tar balls in the above-mentioned literature vary somewhat, depending on the 

smoke source sampled and analyzed, but most include the following traits: carbonaceous, perfectly 

spherical to near-spherical, amorphous and with homogeneous composition (without inorganic inclusions, 

crystalline structure, graphene microstructures or internal cores). Tar balls are stable/resistant to electron 

beam damage under high magnification and are classified optically into two types: dark and bright, based 

on degree of oxidation and coating. Their absorption Ångström coefficients indicate they can be 

categorized as brown carbon, which is a type of organic carbon that is brownish or yellowish in 

appearance and has an imaginary refractive index that varies with wavelength (Moosmüller et al. 2009). 

Tar ball EDS spectra reportedly consist of C, N, O, (some sources include inorganic species S, Cl, K and 
NASA/TM—2015-218891 23

Si as well) and have a high C/O atomic ratio. Tar balls appear externally mixed, that is, they do not 

agglomerate with other particle types. They are reportedly produced from smoldering combustion, 

nucleated from the gas phase within smoke plumes (gas to particle formation mechanism), and vary based 

on environmental conditions: fuel type, formation temperature and transport/aging in the atmosphere 

(Pósfai et al. 2003, Li et al. 2003, Pósfai et al. 2004, Tivanski et al. 2007, Chakrabarty et al. 2010, 

Turmolva et al. 2010, Adachi and Buseck, 2011, China et al. 2013, Tóth et al. 2014).

Tóth et al. (2014) created the first laboratory-generated tar ball particles by droplet emissions of 

liquid tar obtained by dry distillation of wood. After aerosolization of the liquid, the tar droplets had a 

residence time of about 0.3 sec in a heated zone between 560 and 630 °C which solidified the droplets 

into particles with all the characteristics of tar balls. They hypothesize that tar balls may be formed by 

expulsion of liquid tar from biomass pores upon pyrolysis which are solidified by a short residence time 

in the flame zone of a fire.

The thermal decomposition of Nomex creates liquid smoke particles, so surface tension dictates the 

final shape. Some particles arrive at the TEM grid mostly solidified, as spheres, while some adhere to the 

lacey carbon grid before solidifying, as seen in Figure 26.

The temperatures used in the generation of tar balls from liquid tar by Tóth et al. (2014) are similar to 

the conditions of the Nomex heating test (640 °C). Nomex smoke samples collected with the thermal 

precipitator had some perfectly spherical particles which have a striking resemblance to tar balls, and it 

can be assumed that the tar droplets had long enough residence time in the heated zone of the smoke 

generating tube furnace to solidify, however, there are a number of particles that were clearly liquid when 

they were captured on the TEM grid.

Figure 27 shows a tar ball-like Nomex smoke particle which is attached to the lacey carbon by a small 

liquid bridge. The EDX spectrum shows the elements C, N, O, as reported in tar ball descriptions in the 

literature, but also the presence of Al, which is a system peak from the specimen holder. All the 

magnified smoke particles in Figure 26 have the same elements in their EDX spectra as the one shown in 

Figure 27 (C, N, O and Al), and the upper right image has Cl in its spectrum as well.

Figure 25.—EDS spectrum of a non-spherical 700 nm Nomex smoke particle with a rounded irregular shape (inset)

containing C, O, N, Al, Si, and Cl. The Al may be a system peak, Cu can be attributed to the copper TEM grid and 

Si is potentially from the mica liner in the WSTF smoke generator.
NASA/TM—2015-218891 24

Figure 26.—High resolution SEM images of Nomex smoke particles that appear on the TEM grid as liquid droplets of 

varying solidity, some with perfectly spherical tar ball morphology. Diameters of single particles range from 300 to 

800 nm. (a) Large scale image showing coverage (b) 800 nm. (c) 300 nm. (d) 450 nm.

Figure 27.—EDS spectrum of a tar ball-like 600 nm Nomex smoke particle (inset) with peaks of C, O, N, Al, Si, and Cl.
NASA/TM—2015-218891 25

Figure 28.—Nomex fuel tests. (a) Nomex snips before heating (0.5 g fuel sample). (b) After heating, the Nomex snips 

became one or two compact pieces of char/ash, as shown in the remains of five different tests. (c) TEOM filters 

after mass concentration measurements of Nomex smoke particles show the characteristic yellowish color of brown 

carbon.

Figure 28 shows the Nomex fuel before and after heating. The left image shows 0.5 g of the fluffy 

unburned fabric, randomly snipped into pieces 1 cm or less per side. The fuel undergoes carbonization in 

the furnace, reducing the small pieces of fibrous Nomex to individual clumps of char as the fibers thicken 

and combine when subjected to intense heat (Nomex Technical Guide, 2001), as seen in the center image 

of Figure 28. A Tapered Element Oscillating Microbalance (TEOM) was used as a reference instrument 

for mass concentration in these experiments. This instrument uses consumable filters which must be 

replaced when the instrument pressure drop becomes too high. Upon removal, most TEOM filters from 

other polymer fuels were barely discolored (mostly white), with the exception of PVC smoke, which was 

very dark brown, nearly black. Another notable exception was Nomex smoke which made the TEOM 

filters yellowish brown, the color typically observed for brown carbon. The used TEOM filters from the 

WSTF testing in which these particles were collected are shown in Figure 28(c). This is additional 

evidence to support the hypothesis that Nomex smoke from oxidative pyrolysis includes tar balls.

8.0 Conclusions

The thermal precipitator successfully collected an abundance of smoke particles for microscopic 

analysis. A comprehensive characterization has been performed for each material, although there are 

many additional sampled particles which have not been examined. From the analyses to date, the 

following general conclusions are made:

• Ten types of common spacecraft materials or mixed materials underwent oxidative pyrolysis at 

different temperatures and the resulting smoke particles were characterized according to 

morphology and elemental composition.

• Results are consistent with known thermal decomposition mechanisms in the literature and 

chemical make-up of spacecraft fuels.

• Teflon particles show evidence of native polymer shards liberated from the bulk material based 

on elemental mapping and high resolution TEM micrographs showing a crystalline interior with 

an amorphous coating.

• Under the heating conditions of these tests, Teflon particles coagulate into compact spherical 

agglomerates, unlike SAME Teflon smoke particles which were fractal agglomerates in both 

ground testing and ISS tests.
NASA/TM—2015-218891 26

• Kapton smoke particles are consistent in size and morphology with SAME smoke from ISS 

experiments and ground testing.

• Crustal particles, rich in inorganic species, were observed in circuit card smoke.

• Tar ball morphology was observed for Nomex smoke particles.

Ongoing spacecraft smoke characterization research takes place in GASP (Gases & Aerosols from 

Smoldering Polymers) laboratory at the NASA Glenn Research Center. Current experiments focus on 

quantifying smoke aerosol and acid gas concentrations generated with these and other spacecraft fuels.

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