3 Ionization of Gases

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3 Ionization of Gases STUDY OF GAS IONIZATION IN A GLOW DISCHARGE AND DEVELOPMENT OF A MICRO GAS IONIZER FOR GAS DETECTION AND ANALYSIS THÈSE NO 2919 (2004) PRÉSENTÉE À LA FACULTÉ SCIENCES ET TECHNIQUES DE L'INGÉNIEUR Institut de microélectronique et microsystèmes SECTION DE MICROTECHNIQUE ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES TECHNIQUES PAR Ralf G. LONGWITZ Diplom-Ingenieur, Technische Universität Carolo-Wilhelmina, Braunschweig, Allemagne et de nationalité allemande acceptée sur proposition du jury: Prof. Ph. Renaud, directeur de thèse Dr Ch. Hollenstein, rapporteur Prof. J.-F. Loiseau, rapporteur Dr G. Stehle, rapporteur H. van Lintel, rapporteur Lausanne, EPFL 2004 Abstract In the pursuit of a portable gas detector/analyser we studied the components of an ion mobil- ity spectrometer (IMS), which is a device that lends itself well to miniaturisation. The com- ponent we focused on was the ionizer. We fabricated a series of micro ionizers with micro electromechanical systems (MEMS) technology, which had a gap spacing between 1 and 50 µm and a thickness from 0.3 to 50 µm. They were used to examine micro discharges as such and as a means of ionization. In our measurements of electrical breakdown in small gaps we confirmed the deviation from Paschen's law for breakdown voltages in gaps below 5 µm. One important result is the identification of conditions for stable DC glow discharge in micro gaps. With planar electrodes we observed stable glow for factors of pressure times gap dis- tance pd up to 0.2 Pa×m in N2, and up to 0.14 Pa×m in Ar. With thick electrodes the glow range was extended: up to 4 Pa×m in Ar, and 10 Pa×m in laboratory air at atmospheric pres- sure. The advantage of using discharges in micro gaps as the ionization principle is the low voltage and power that is necessary to drive a discharge. A prerequisite for using an ionizer in an ion mobility spectrometer is the possibility to operate such an ionizer at high, up to atmospheric, pressure. Our final micro discharge devices were operated in laboratory air for several hours without significant deterioration. A miniature ion mobility spectrometer was set up, in which miniature and micro discharge ionizers were applied as the ion sources. We extracted ions from micro ionizers and meas- ured mobility spectra of gases and mixtures of gases (air, N2, Ar). The measured peaks in the mobility spectra varied depending on the gas. In the conclusions we suggest improvements that should increase the resolution and stability of our ion mobility spectrometer, so it may become useful for gas detection. The most important improvements will be a better control of the measurement conditions and of the initial extension of the ion pulse. In addition to our experimental results we present in this work an overview of the research that has already been done in our area of interest. As the basis of our research, the involved physical theory has been worked out. Consequently, the first chapters contain a compilation of relevant subjects, from the basics of electrostatics to the theory of DC glow discharge as far as we believe it can serve the reader in understanding our results. Zusammenfassung Im Bestreben nach einem tragbaren Gas Detektor/Analysator, wurden die Komponenten eines Ionen-Mobilitäts-Spektrometers (IMS) untersucht. Derartige Spektrometer lassen sich vorteilhaft miniaturisieren. Die Komponente, auf die wir uns konzentrierten war der Ioni- sator. Eine Reihe von Mikroionisatoren wurden mit Methoden der Mikrosystemtechnik her- gestellt. Die Elektroden dieser Ionisatoren hatten einen Abstand zwischen 1 und 50 µm und eine Dicke von 0.3 bis 50 µm. Sie wurden zur Untersuchung von Mikroentladungen als sol- chen und ihrer Verwendung zur Ionisation von Gasen im Besonderen verwendet. Unsere Messungen elektrischer Durchschläge in kleinen Spalten bestätigten die Abweichung von Paschens Gesetz für Durchschlagspannungen in Spalten kleiner 5 µm. Ein wichtiges Ergebnis sind unsere Erkenntnisse über die Bedingungen unter denen eine stabile Glimm- entladung in Mikrospalten erreicht werden kann. Mit ebenen Elektroden beobachteten wir stabile Glimmentladungen für Faktoren von Druck mal Spaltabstand pd von bis zu 0.2 Pa×m in N2, und bis zu 0.14 Pa×m in Ar. Mit dicken Elektroden erweiterte sich der Glimmbereich: Bis zu 4 Pa×m in Ar, und 10 Pa×m in Laborluft bei Atmosphärendruck. Der Vorteil elektrischer Entladungen als Ionisationsprinzip in Mikrospalten liegt in der vergleichsweise geringen Spannung und Leistung, die zu deren Betrieb notwendig ist. Eine Vorraussetzung für die Anwendung eines Ionisators in einem Ionen-Mobilitäts-Spektrometer ist die Möglichkeit, einen solchen Ionisator bei hohem, bis zu atmosphärischem Druck ver- wenden zu können. Die Mikroionisatoren, zu denen wir schliesslich gelangt sind, wurden in Laborluft während mehrerer Stunden betrieben, ohne einen wesentlichen Schaden davon- zutragen. Ein Miniatur-Ionen-Mobilitäts-Spektrometer wurde aufgebaut, in welchem wir Miniatur- und Mikroionisatoren als Ionenquellen eingesetzt haben. Wir extrahierten Ionen von Mikro- ionisatoren und maßen Mobilitätsspektren von Gasen und Gasmischungen (Luft, N2, Ar). Eine Gasabhängigkeit der gemessenen Peaks in den Spektren wurde gezeigt. In unseren Schlussfolgerungen schlagen wir Verbesserungen vor, welche die Auflösung und Stabilität unseres Miniatur-Ionen-Mobilitäts-Spektrometers so weit erhöhen sollten, daß es zur Gas- detektion nützlich wird. Wesentliche Verbesserungen werden von einer besseren Kontrolle der Messbedingungen und der Anfangsausdehnung des Ionenpulses erwartet. Zusätzlich zu unseren experimentellen Ergebnissen präsentieren wir in dieser Arbeit einen Überblick über Forschung die auf diesem Gebiet bereits durchgeführt wurde. Als Grundlage unserer Untersuchungen erarbeiteten wir uns die notwendige physikalische Theorie. Die ersten Kapitel enthalten entsprechend eine Zusammenstellung relevanter Themen, von den Grundlagen der Elektrostatik bis zur Theorie von Gleichspannungsglimmentladungen, soweit sie dem Leser zum Verständnis unserer Ergebnisse dienen können. Contents Abstract ...................................................................................................................................... i Zusammenfassung ....................................................................................................................ii Contents...................................................................................................................................... i 1 Introduction....................................................................................................................... 1 1.1 The scope of this thesis ............................................................................................... 1 1.2 Spectrometric gas analysis .......................................................................................... 2 1.3 State of the art ............................................................................................................. 3 1.3.1 Micro discharges .......................................................................................... 3 1.3.2 Miniature ion mobility spectrometers .......................................................... 4 1.3.3 Micro mass spectrometers............................................................................ 5 2 Basics of electrostatics, electrodynamics, and gas dynamics ........................................ 7 2.1 Electrostatics ............................................................................................................... 7 2.1.1 Basic expressions of electrostatics............................................................... 7 2.1.2 Calculation of fields ..................................................................................... 8 2.1.3 Charges and ions ........................................................................................ 10 2.2 Gas dynamics and charged particle kinetics.............................................................. 10 2.2.1 Movements of electrons and ions in electric fields.................................... 10 2.2.2 The mean free path..................................................................................... 12 2.2.3 Mobility and ion filtering by mobility ....................................................... 13 3 Ionization of gases........................................................................................................... 19 3.1 Principles of gas ionization ....................................................................................... 19 3.2 Electrical discharges in gases and in vacuum ........................................................... 20 3.2.1 Electric breakdown..................................................................................... 20 3.2.2 Types of DC discharges ............................................................................. 33 3.3 Choice of an ionization method ................................................................................ 39 3.3.1 A comparison of ionization methods ......................................................... 39 3.3.2 Industrial ion sources ................................................................................. 41 3.3.3 Choice of a method .................................................................................... 42 4 The DC glow discharge................................................................................................... 45 4.1 Characteristics
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