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Title:Development of a ferroelectric plasma source through material degradation studies and characterization of volume and surface discharges
Author(s):Masters, Benjamin Charles
Director of Research:Eden, James G.
Doctoral Committee Chair(s):Eden, James G.
Doctoral Committee Member(s):Stubbins, James F.; Uddin, Rizwan; Allain, Jean Paul
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
ferroelectric plasma source
ferroelectric electron emission
electron emission
ion emission
lead zirconate titanate
cold cathode
materials degradation
Abstract:The efficiency and lifetime of a plasma source or charged particle emitter is partially limited by the materials used. A particular case of this is a ferroelectric plasma source (FPS) or ferroelectric electron emitter (FEE), in which a strongly polarizable material, capable of generating large surface currents (10^0 – 10^5 A/cm2), is pulsed to emit electrons and ions. The ferroelectric material uniquely affects emission, which in turn uniquely affects the ferroelectric surface. Lifetimes are typically limited to 10^4 to 10^5 emissions due to poorly understood degradation mechanisms. Understanding the causes of ferroelectric degradation can help minimize these effects and extend its service lifetime. It is hypothesized that degradation is caused by (1) thermal degradation from dielectric and hysteresis heating, (2) removal of oxygen, and (3) local deviation from stoichiometric ratios. Navy Type II lead zirconate titanate (PZT) was used as the emitter material. Microcavities for emission sites were made using a micromilling tool, in contrast with traditional photolithography methods described in the literature. Both the bulk ferroelectric properties and surface structure of PZT were characterized. Materials characterization takes place through the use of X-ray Photoelectron Spectroscopy (XPS), scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX); and X-ray Diffraction (XRD) is used for the crystallographic properties of the ferroelectric material. The purpose of these measurements is to try to quantify the degradation of the emitter surface. In addition, information about oxygen concentration, and the relative ratios of lead, zirconium and titanium will show if the material is still PZT, or simply a mixture of oxides and alloys. The emitter is made by micromachining small cavities into the PZT surface. A series of 1 ms duration, 2.5 kVpp pulses were applied every 1.1 seconds for at least 100000 pulses. Emission is characterized with a Faraday cup, a fast photodiode, an optical emission spectrometer (OES), a residual gas analyzer (RGA), and a quartz crystal microbalance (QCM) that all monitor output during the emission process. The Faraday cup serves as an excellent detector of the amount of charged species being produced, and the timing of emission with respect to the applied pulse. Spectroscopy is used to indentify materials ejected from the cathode in real time. The RGA independently confirms oxygen loss even when there is no optical emission. The cathode material in this work was found to be able to produce emission current densities on the order of 1 – 10 A/cm^2. Experiments explored the effect of different amounts of pulses and the strength of the extraction grid voltage. Results indicate that most useful emission occurs within the first 25000 pulses, and that optical emission persists even after electron emission becomes unreliable and weak. Several species were identified via spectroscopy; the most pervasive were neutral (406.21 nm) and singly ionized (560.88 nm) lead, and the electrode material, silver (520.90 nm). The RGA detected large quantities of atomic oxygen at irregular intervals, pointing to explosive emission as a possible cause. XPS and EDX respectively revealed the composition of witness plate debris and of the microcavities after emission. XPS indicated that there were disproportionately higher atomic percentages of lead than the undamaged sample, and EDX confirmed that the damaged samples had much lower atomic percentages of lead. Finite difference modeling revealed that the bulk temperature of the cathode rises to approximately 130 °C and may cause partial depolarization. The surface temperature is expected to be higher still due to the large current densities.
Issue Date:2018-01-10
Rights Information:© 2018 Benjamin Charles Masters. All Rights Reserved
Date Available in IDEALS:2018-09-04
Date Deposited:2018-05

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