(530b) A Microplasma Glow Discharge For Fire Detection

Abstract

In this study the main objective was to develop and demonstrate a glow discharge microplasma coupled to a miniature spectrometer for detection of fire signatures from pyrolyzing and burning spacecraft materials. Our experimental results demonstrate that combustion-produced carbonaceous aerosols can serve to identify the burning materials. Demonstrating versatility for chemistry analysis, the plasma detector could differentiate carbonaceous aerosols with different C/H ratios and distinguish inorganic samples such as salts and metal oxides from carbonaceous aerosols. In addition, in-situ analysis of aerosol samples validated the microplasma’s analytical utility by linearity of its optical emission intensity with aerosol elemental composition.

Introduction

Plasmas are used as direct analytical tools for optical emission spectroscopy.  ICP (inductively coupled plasma) [1] and DCP (direct-current plasma) [2] instruments create high temperature plasmas that perform the dual analytical function of atomizing and electronically exciting the constituent elements whose spectra uniquely identify the material composition. Liquids are readily analyzed and even solids, if sample digestion is first performed using strong acids. Glow discharge plasmas are also used in commercial laboratory instruments for atomization of solid samples. Typically a kW power supply powers the glow discharge under a low-pressure inert gas to atomize near surface layers of a material for subsequent mass spectrometry analysis [3]. Nevertheless, mass spectrometry is difficult to calibrate, particularly over a wide range of atomic masses. 

In recent years the hollow cathode glow discharge has been miniaturized [4] thereby testing physical scaling relationships.  Specifically these relate the operating pressure to the relevant (hollow cathode) physical dimension, i.e. the “hollow” diameter [5].  By following White’s relation [6], atmospheric pressure operation could be achieved with hollow cathode spatial dimensions on the order of 100 microns diameter.  Significantly the need for low-pressure operation and vacuum hardware [7] is negated.  A second advantage is the drastic reduction of operating power. Both factors greatly simplify the supporting hardware.  Single and arrayed elements have been successfully demonstrated [8].  To-date, research of such microdischarges has focused upon their applications to photonics and their use as VUV light sources [9]. 

Uniquely, in this work we synergize several of these concepts towards a micro-plasma fire detector. Though other microplasmas have been developed and demonstrated for detecting gases [10-12], their geometry is not well suited for analysis of solids or particulate.  Relative to flowing configurations, the micro hollow glow discharge (MHGD) offers unique capability for analysis of combustion-produced particulate.  Despite recent reviews, microplasma application to detection and analysis of aerosols, suspended or deposited, was not identified, though detailed studies of traditional glow discharges applied to their analysis as deposit, and for that matter, more generally to powder and solids.

Using recent advancements in micro-hollow cathode technology for operation at atmospheric pressure, we utilize the traditional sputtering capability of the glow discharge to both atomize and electronically excite samples for stand-alone analytical analysis, akin to dedicated laboratory scale instruments but now on a microscale with Watt-level power usage.  Moreover we analyze carbonaceous materials, notorious for their traditional analytical challenges not least of which is sample atomization/digestion and light element detection, in particular hydrogen content, quantitatively.  Combustion produced soot can vary widely in composition, in particular C/H ratio, reflecting both its molecular origins and temperature of evolution [13,14]. Such samples are of interest for a variety of reasons ranging from atmospheric aerosols and their impact upon global warming [15], health impacts associated with particulate emissions from combustion sources [16] and finally as potential fires signatures [17].

One of the primary goals of the fire detection element of NASA’s Fire Safety Program is to provide recommendations for the development of more effective and efficient fire sensors to be installed in space vehicles and future habitats. Detectors that can detect minute amounts of soot and additionally analyze its composition could not only provide early warning but also act as a “smart detector” by identifying the material producing the soot, whether in smoldering or flaming combustion.  Clearly low detection limits are desirable. Moreover, false-positives, “detection” of inorganic mineral matter leading to a fire alarm must be minimized given their disruptive nature.  To satisfy these dual constraints a multifunctional glow discharge analyzer was developed to not only detect and differentiate soot particles generated from the combustion of spacecraft materials but also to distinguish them from inorganic samples such as salts and metal oxides.

Material and Methods

Materials approved for use aboard spacecraft were subjected to pyrolysis and combustion in a combustion system described elsewhere[18]. Concisely, the combustion system consists of a tubular electric furnace mounted vertically and fitted with a quartz tube. The furnace is equipped with a temperature controller and the quartz tube is connected to different gas lines. The sample mass varied, depending on the particulate yield from the material. The combustion tests for all samples were performed at 1000 ^oC with an air flow of 150 sccm, upwards. Soot and condensables were collected on a copper disc of 0.127 mm thickness and then analyzed in the micro-hollow glow discharge. Materials evaluated included polypropylene pellets, polyurethane foam, silicone rubber, Kapton sheet, Teflon and Halar wire insulations that conform to current NASA flammability standards as outlined in by the 6001D Materials Outgasing and Flammability test [19].

Conclusion

Based on our studies we have demonstrated that the glow-discharge, plasma-based detection system is capable of detecting and identifying soots produced from combustion of six materials approved for use in space vehicles and habitats. Based upon the distinctiveness of the spectra, the (material) source of the fire event can be identified by analyzing the MHGD plasma emission spectrum of the corresponding soot. In addition, the micro-plasma analysis method is able to determine the C/H elemental ratio of carbonaceous aerosols and distinguish these combustion-produced emissions from ambient inorganic (nuisance) aerosols. Our detector is highly specific to smoldering or combustion events (by selective detection of emission products) and can reduce if not eliminate false alarms due to ambient dust and detritus. Finally, we have successfully demonstrated a linear correlation between the spectral intensity versus sample mass, further extending the analytical capabilities of the MHGD plasma instrument as a “smart” fire sensor.

References

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