Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 92-94). Also available via the Internet.

dissertatic,11 submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering, I.: Johannesburg, 1998 / Corona stabilisation is not a well understood phenomenon, consequently a rigorous theoretical description of how the merhanism operates does not exist. Furthermore, postulates on the prcJ'.tse physical processes that occur during corona stabilised breakdown are varied and conflicting. 1t was thus essential to develop a qualitative theory of how the mechanism operates around which a quantitative model could be constructed. To this aim, a detailed analysis of available modelling methods and empirical data from published works was carried out. This analysis together with additional finite elements modelling done as part of this work, leads to a proposed qualitative model which serves as a basis for a quantitative model developed to simulate corona stabilized breakdown from a fixed panicle defect. There is a favorable comparison between the predictions of the model and published measurements, thus imbuing optimism in the proposed method. The problem of measuring t: .;true charge deployed in a discharge is discussed and an experiment is designed to circumvent this problem.

No description available.

<p>As electronic device dimensions decrease to micro and nanoscale, Paschen’s law (PL)—the standard theory used to predict breakdown voltage (<i>V_b</i>) governed by Townsend avalanche (TA)—fails due to ion-enhanced field emission (FE). Analytic models to predict <i>V_b</i>at these scales are necessary to elucidate the underlying physics driving breakdown and electron emission in these regimes. Starting from a previously-derived breakdown criterion coupling TA and FE, this dissertation derives a universal (true for any gas) breakdown equation. Further simplifying this equation using a matched asymptotic analysis, dependent on the product of the ionization coefficient and the gap distance, yields an analytic theory for dimensionless <i>V_b</i>. This analytic model unifies the coupled FE/TA regime to a universal PL derived by applying scaling parameters to the standard PL. This model enables parametric analyses to assess the effects of different parameters (such as pressure, gap distance, and field enhancement factor) on breakdown and quantify the relative contribution of FE and TA to identify the transition to the universal PL. This dissertation applies this general theory to experimental cases of different gap width, gap pressure and electrode surface roughness before exploring unification across electron emission regimes, validation with molecular dynamics simulations, and extensions to alternating current (AC).</p> <p> </p> <p>One application of this theory to experimental data used data from a collaborator at Xi’an Jiaotong University, who used an electrical-optical measurement system to measure the breakdown voltage and determine breakdown morphology as a function of gap width. An empirical fit showed that the breakdown voltage varied linearly with gap distance at smaller gaps as in vacuum breakdown. This dissertation demonstrates that applying the matched asymptotic theory in the appropriate limits recovers this scaling with the slope as a function of field emission properties. </p> <p> </p> <p>Pressure also plays a critical role in gas breakdown behavior. This dissertation derives a new analytic equation that predicts breakdown voltage <i>V_b</i> within 4% of the exact numerical results of the exact theory and new experimental results at subatmospheric pressure for gap distances from 1-25 . At atmospheric pressure, <i>V_b</i> transitions to PL near the product of pressure and gap distance, <i>pd</i>, corresponding to the Paschen minimum; at lower pressures, the transition to PL occurs to the left of the minimum. We further show that the work function plays a major role in determining whether <i>V_b</i> transitions from the coupled FE/TA equation back to the traditional PL to the right or the left of the Paschen minimum as pressure increases, while field enhancement and the secondary emission coefficient play smaller roles. These results indicate that appropriate combinations of these parameters cause <i>V_b</i> to transition to PL to the left of the Paschen minimum, which would yield an extended plateau similar to some microscale gas breakdown experimental observations. </p> <p> </p> <p>Finally, the importance of electrode surface structure on microscale gas breakdown remains poorly understand. This dissertation provides the next step at assessing this by applying the asymptotic theory to microscale gas breakdown measurements for a pin-to-plate electrode setup in air at atmospheric pressure with different cathode surface roughness. Multiple discharges created circular craters on the flat cathode up to 40 μm deep with more pronounced craters created at smaller gap sizes and greater cathode surface roughness. The theory showed that breakdown voltage and ionization coefficient for subsequent breakdown events followed our earlier breakdown theory when we replaced the gap distance <i>d</i> with an effective gap distance <i>d_eff</i> defined as the sum of cathode placement distance and crater depth. Moreover, the theory indicated that <i>d_eff</i> could become sufficient large to exceed the Meek criterion for streamer formation, motivating future studies to assess whether the cathode damage could drive changes in the breakdown mechanism could for a single electrode separation distance or the Meek criterion requires modification at microscale. </p> <p> </p> <p>We next unified field emission with other electron emission mechanisms, including Mott-Gurney (MG), Child-Langmuir (CL), and quantum space-charge-limited current (QSCL) to develop a common framework for characterizing electron emission from nanoscale to the classical PL. This approach reproduced the conditions for transitions across multiple mechanisms, such as QSCL to CL, CL to FE, CL to MG to FE, and microscale gas breakdown to PL using a common nondimensional framework. Furthermore, we demonstrated the conditions for more complicated nexuses where multiple asymptotic solutions matched, such as matching QSCL, CSCL, MG, and FE to gas breakdown. A unified model for radiofrequency and microwave gas breakdown will be compared to experimental results from Purdue University to elucidate breakdown mechanism. </p> <p>The results from this dissertation will have applications in microscale gas breakdown for applications including microelectromechanical system design, combustion, environmental mitigation, carbon nanotube emission for directed energy systems, and characterizing breakdown in accelerators and fusion devices.</p>

Elevated concentrations of mercury in soils are quite hazardous to flora and fauna and water bodies near these soils. This makes continuous monitoring of mercury very essential. This work compares two potential spectroscopic methods LIBS and SIBS at their optimum experimental conditions for mercury monitoring. The experimental conditions for Hg measurements with LIBS and SIBS were determined and calibration was developed. The limits of detection (LODs) of Hg in soil were calculated from the Hg calibration curves. The LOD for mercury (Hg) in soil calculated using LIBS and SIBS are 483 parts-per-million and 20 parts-per-million, respectively. The present study indicates that SIBS is more efficient with powder samples in a low concentration region for quantification of mercury in soils while LIBS is efficient in the region of higher concentrations using pellet samples. Both these techniques can be further investigated and improved for in-situ analysis of soils.

<p>Dielectric breakdown in gases is an important criterion for device reliability when designing various electronic devices such as sensors, medical plasma jets and fusion applications. As devices become smaller and more compact, microscale gap distances need to be considered and Paschen’s law, which dictates typical breakdown behavior when electron avalanche dominates, fails. The stronger electric fields for microscale gaps induce field emission, which generates additional electrons that further enhance the electric field at the cathode and the resulting secondary emission to reduce breakdown voltages below those predicted by Paschen’s law. Field emission is governed by the Fowler-Nordheim equation, which mathematically describes the quantum tunneling that occurs. Many studies have examined breakdown voltage in the Paschen’s and field emission regimes but recent theories have unified the two regimes for DC gas breakdown at microscale gaps [A. L. Garner, A. M. Loveless, J. N. Dahal, and A. Venkattraman (2020)]. However, although microwave and RF fields are used in many microelectronics systems and microplasmas, they have been less studied. This thesis derives a breakdown condition that unifies avalanche and field emission for RF fields. The derivation includes analysis of potential secondary emission representations for AC fields. The breakdown condition is then benchmarked to simulations that accounts for both avalanche and field emission for user-defined AC fields.</p> <p>We use a modified version of XPDP1, which is a one-dimensional in space and three-dimensional in velocity (1D/3v) particle-in-cell (PIC) code that incorporates field emission. The resulting breakdown depends on several parameters, most notably gap size d, pressure p and frequency f. We determine breakdown voltages in terms of d and pd scalings for 1-10 𝜇m gap distances, 10-1000 GHz frequencies, and 180-760 Torr pressures. Additional scalings that were studied include the work function, field enhancement factor, secondary emission, and ionization coefficients. PIC demonstrated that the breakdown voltage varied linearly with gap distance up to ~4 𝜇m from DC to 10 GHz for a secondary electron emission coefficient 𝛾𝑆𝐸=0.05. For DC fields, the breakdown voltage decreases with increasing gap distance; the breakdown voltage increases with increasing frequency, approaching linearly with increasing gap distance.</p>

No / The effects of the three principal possible exact breakdowns which may occur using BiCGStab are discussed. BiCGStab is used to solve large sparse linear systems of equations, such as arise from the discretisation of PDEs. These PDEs often involve a parameter, say . We investigate here how the numerical error grows as breakdown is approached by letting tend to a critical value, say c, at which the breakdown is numerically exact. We found empirically in our examples that loss of numerical accuracy due stabilisation breakdown and Lanczos breakdown was discontinuous with respect to variation of around c. By contrast, the loss of numerical accuracy near a critical value c for pivot breakdown is roughly proportional to |¿c|¿1.

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In this work, we have investigated numerical techniques to determine the breakdown behavior of complex semiconductor devices using two-dimensional simulation. In particular, we have augmented the device simulator SEPSIP with a capability for handling single and multiple floating field rings, and for handling devices with slanted edges. We have furthermore improved the grid width selection algorithm in SEPSIP. A capability for plotting equi-field contours was added to the code. Finally, all system dependencies were removed from the SEPSIP code, and a new version of SEPSIP (Version 2.0) was generated which can be executed on any PC/XT, PC/AT, or PC/386 compatible computer. This eliminates the need for transfering files back and forth between the PC, which had formerly been used as an I/O processor, and the VAX, which was used for numerically intensive computations. It also makes the code more accessible to scientists and engineers who are working in this important research area.

No description available.