VTechWorks Repository :: Browsing by Author "Burton, Larry C."

Browsing by Author "Burton, Larry C."

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Boundary-layer analysis and measurement of Newtonian and non-Newtonian fluids
Kim, Byung Kyu (1984)
The velocity fields around a circular cylinder in a crossflow of drag-reducing polymeric solutions and water were experimentally investigated using a laser-Doppler velocimeter. Measured boundary-layer velocity profiles indicated that the flow parameter controlling the drag on a bluff body in drag-reducing flows is the turbulence intensity rather than the Reynolds number. For turbulence intensity less than 0.7% polymer addition induced delayed separation. For turbulence intensity over 1% the opposite effect was true. Time-averaged velocity profiles of water did not show any significant difference between self-induced and forced oscillatory flows. Heat, mass and momentum transfer of Newtonian and power-law non-Newtonian fluids were theoretically investigated using an implicit finite-difference scheme. The results clearly· indicated that shear-dependent non-Newtonian viscosity controls the entire transport processes of the power-law fluids. For the major portion of the boundary layer, it was found that the more shear thinning the material exhibits, the lower the skin friction and the higher the heat transfer result. Accounting for the motion of the stagnation point provided an improved prediction of heat transfer for Newtonian fluid.
A capability for continuous topology transient analysis in SCR switching-mode power supplies
Avant, Roger Lonzo (Virginia Polytechnic Institute and State University, 1983)
A general purpose computer model for the SCR is developed. The model, consisting of both a circuit analog and parameter estimation procedure, is uniformly applicable to popular computer aided design and analysis programs such as SPICE2 and SCEPTRE. The circuit analog is based on the intrinsic three PN junction structure of the SCR and is similar to Nienhus' model. The parameter estimation procedure requires only manufacturer's specification sheet quantities as a database. It employs some of the concepts developed by Hu for a SPICE2 SCR model. This uniform model, denoted the J³ SCR model, is shown to be a useful design aid through computer simulation of fault transients which may occur in a"Schwarz" converter. The transients simulated would not be observable without use of a highly accurate continuous topology non-linear SCR model such as is developed here.
Composition Measurements related to the CU2S-ZNXCD1-XS Heterojunction
Burton, Larry C. (AIP Publishing, 1979)
Composition of zinc and cadmium have been measured for three key regions related to the formation of Cu2S on Zn x Cd1_x X by aqueous ion exchange. These are the ion_exchange solution, the resulting Cu2S, and the Zn x Cd1_x S near the Cu2S_Zn x Cd1_x interface. It is found that the participation of zinc in the ion_exchange reaction is incomplete. Zinc is thus retained in the p_n junction region, and is found mainly in the Zn x Cd1_x S just under the interface. This phenomenon helps to account for certain differences between electro_optical properties of Cu2S/CdS and Cu2S/Zn x Cd1_x S solar cells.
Compositional depth profiles of chemiplated Cu2S/(Zn,Cd)S heterojunction solar cells
Uppal, P. N.; Burton, Larry C.; Rivaud, Lydia; Greene, J. E. (American Institute of Physics, 1983)
Atomic absorption spectroscopy combined with controlled chemical etching and Auger electron spectroscopy profiling with ion beametching have been used to obtain composition versus depth analyses of Cu2S/(Zn,Cd)S heterojunctionsolar cells formed by an aqueous cation exchange, or chemiplating, process. The Cu2S films, ranging from 0.5 to 6 _m in thickness, were polycrystalline and exhibited a (001) preferred orientation on either textured polycrystalline or cleaved single crystal (Zn,Cd)S substrates. The profiling results showed that the interfacial regions were compositionally graded over very large distances ranging from tens to hundreds of nonometers depending on the Cu2S film thickness. This is much wider than observed for comparable Cu2S/CdS cells. Moreover, excess Zn in the form of both elemental Zn and ZnS was always found in the interfacial region and may be responsible for the short circuit current being lower than expected for these cells.
Cross diffusion of Cd and Zn in Cu2S formed on Zn x Cd1_x S thin films
Burton, Larry C.; Uppal, P. N.; Dwight, D. W. (American Institute of Physics, 1982)
Cadmium and zinc compositions in Cu 2S formed on Zn x Cd1_x S films (0
Cu₂S/ZnCdS thin film heterojunction solar cell studies
Chang, Shang-wen (Virginia Polytechnic Institute and State University, 1985)
Cu₂S/CdS solar cells have been studied extensively for the past two decades due to their potentially high efficiencies per unit cost. The operation and characteristics of Cu₂S/CdS solar cells are fairly well understood. However, the properties of the newer Cu₂S/ZnCdS cell type are not well understood. The main goals of this thesis were to compare Cu₂S/CdS and Cu₂S/ZnCdS cells using Cu₂S/CdS cells as a reference, and to understand the operation and properties of Cu₂S/ZnCdS cells in order to improve cell performance. Four different measurements were used in this research to achieve these goals. They were; electrical, spectral, capacitance and deep trap measurements. I-V measurements give important electrical parameters of the cells; cell efficiency, fill factor, short circuit current, open circuit voltage, shunt resistance and series resistance are reported. From a In(I_SC) versus V_OC measurement, the diode factor, A, was found to be about 1 for Cu₂S/CdS, Cu₂S/Zn_0.11Cd_0.89S, and about 1.2 for Cu₂S/Zn_0.25Cd_0.75S cells. The relation between In(J_oo) (current density) and ϕ (potential barrier height) is linear for both types of cells. The slope of this linear relationship increases as the content of Zn increases in Zn_xCd_1-xS. Under air mass 1 (100 mW/cm²) illumination, it was found that V_OC decays and capacitance increases for Cu₂S/ZnCdS cells. This is attributed to electron relaxation from deep traps near the junction. Spectral response with and without bias light were measured for both Cu₂S/CdS and Cu₂S/ZnCdS cells. White and blue bias light enhance the spectral response, while red bias light quenches the response. This is attributed to ionization and filling of deep traps near the junction. Capacitance measurements on both cell types show that 1/C² versus voltage is quite flat, which indicates the existence of an i-layer (insulation layer) in the CdS or ZnCdS near the junction. Three methods–photocapacitance, space-charge-limited current, and thermally stimulated. current techniques–were used for deep trap measurements. Photocapacitance measurements indicate one deep donor energy and two deep acceptor energy levels. These trap energies become larger as the content of Zn in ZnCdS increases. Space-charge-limited current measurements give a trap density of the order of 10¹⁶ cm³ for both cell types. The shallow energy trap is found to be 0.26 eV below the conduction band edge of CdS. The occurrence of a current-saturated region for Cu₂S/ZnCdS is attributed to the filling of the interface traps near the junction. Thermally stimulated current measurements give two energy levels below the conduction band of CdS; 0.05 eV and 0.26 eV. From the above results, several differences between the Cu₂S/CdS and the Cu₂S/ZnCdS cells can be seen. The Cu₂S/ZnCdS cells show stronger red quenching, smaller electron lifetime at the interface near the junction, and deeper traps than the Cu₂S/CdS cells. These differences can account for the decline of I_SC and the V_OC decay. The smaller I_SC for the Cu₂S/ZnCdS cells can also possibly result from smaller electron lifetime at the interface, larger interface recombination velocity, different deep trap levels, and enhanced Zn concentration near the junction. The V_OC decay for the Cu₂S/ZnCdS cells is mostly due to long decay of charge. Longer decay could be attributed to deeper traps.
Design and automation of MEDUSA (Materials and Electronic Device Universal System Analyzer)
Johnson, Phillip (Virginia Tech, 1990-02-05)
MEDUSA (Materials and Electronic Devices Universal System Analyzer) is a computer controlled automated workstation capable of conducting eight different experiments, under different independent parameters, and plotting twenty-eight different graphs representing basic semiconductor diode and transport characteristics. This thesis discusses the methodology of computer automation, and the development of the MEDUSA experimental test station. MEDUSA is divided into four different sections: a controlling batch file, a parameter selection routine (PARAMETER), an experimental running routine (RUNIT), and a data manipulation/plotting routine (GRAPHICS). MEDUSA conducts these eight experiments (capacitance and conductance versus time, voltage, current versus voltage, van der Pauw, and four-point resistivity) over a temperature range of 10-600K, with minimal operator interaction. The graphics routines, using elemental semiconductor equations, process the data, and plots high quality graphs suitable for publication. Device and material results are shown to substantiate the validity of this automated system.
Development of a high-density, off-line, quasi-resonant converter using hybrid techniques
Hopkins, Douglas Charles (Virginia Polytechnic Institute and State University, 1989)
The advancement of Very Large Scale Integration (VLSI) technology has reduced the size and increased the speed of information processing circuits. Consequently, power supplies for such circuits have had to meet increasing demands for power, yet simultaneously decrease in size. This need for higher power density in the supplies can be met with higher circuit operating frequencies and by using high-density circuit fabrication techniques. Generally, when the conversion frequency of conventional Pulse-Width-Modulated (PWM) supplies approaches 1 MHz, the switching loss becomes very large. This sharply reduces the efficiency of the supply. A quasi-resonant topology reduces much of this loss. For a Zero-Current-Switched (ZCS) Quasi-Resonant Converter (QRC) the turn-off loss is nearly eliminated. It was the objective of the research reported here to combine the quasi-resonant technology with thick-film hybrid microelectronics technology to produce a high density dc-dc converter. For this research endeavor an off-line, half-bridge ZCS-QRC was used. The circuit processed 300V and up to 20A with switching frequencies in the 1MHz to 2MHz range. The voltage and current levels exemplify the high electric field and current densities that must be considered in the design of most QRC circuits that process power up to 100W. Only available materials for thick-film hybrid processing were used although some characteristics were modified. No special magnetic or capacitive components, or semiconductors were developed. To combine technologies the following were performed: 1. identification of critical power electronic circuit and hybrid component parameters such as maximum voltages and currents, thermal and electrical component impedances; 2. assessment of thick-film hybrid microelectronic materials and their compatibility in circuits having high voltage and current levels; 3. development of a complete thick-film power hybrid process; and 4. design, fabrication and evaluation of a power hybrid QRC that has high power-processing density.
Dynamic electromechanical measurements of carbon black loaded SBR
Hwang, Yawlin (Virginia Tech, 1988-07-14)
The major objectives of this study were to examine electrical and electromechanical properties of SBR filled with carbon black in the 0-70 phr range. The experiments were divided into four parts: dielectric measurement, loss modulus and phase angle measurements, temperature rise measurement during stress cycling, and dynamic conductivity measurement. It is established that there are three distinct conduction regimes existing at carbon black loadings below, at, and above the percolation threshold. Characteristics of dielectric dispersion depend strongly on carbon black loading and frequency. Dielectric and AC conductivity measurements are shown to provide a nondestructive method to explore the carbon black network inside the rubber. Both loss modulus and phase angle are related to hysteresis properties, and to temperature rise due to compressive cycling. Measurements of these parameters will be discussed in detail, as functions of carbon black loading, stress and strain amplitudes, and oscillation frequency. These and other results can be understood in terms of the mechanics of the carbon black network. The variation of conductivity with strain amplitude is related directly to the interplay between the "persistent" and "transient" fractions of carbon black network. It is shown that, owing to its experimental accuracy and great sensitivity to carbon black network changes, the dynamic conductivity measurement is preferable to traditional modulus measurements for determining certain dynamic properties of carbon black filled rubbers.
The effects of ion bombardment on the chemical reactivity of GaAs(100)
Epp, June Miriam (Virginia Polytechnic Institute and State University, 1989)
The effects of ion bombardment on the chemical reactivity of GaAs(100) were investigated by X-ray photoelectron spectroscopy. The enhancement in reactivity was shown to be related to the energy and mass of the bombarding ion. The oxidation results were compared to chemically cleaned (1:1 HCI(conc)/H₂O) and IHT (simultaneous ion/heat treatment) prepared GaAs(100). Before ion bombardment, GaAs(100) was chemically cleaned with 1:1 HCI(conc)/H₂O to remove surface oxides. Chemically cleaned GaAs was bombarded with 0.5-3 KeV Ar⁺ ions (fluences = 10¹⁶-10¹⁷ ions/cm²) and with 3 KeV Xe⁺, Ar⁺, ²⁰Ne⁺, and ³He⁺ ions (fluence =10¹⁷ ions/cm²) to investigate the effect of ion bombardment energy and mass on chemical reactivity. Ion bombardment results in the preferential sputtering of As and the amount of As depletion is dependent upon ion bombardment energy and mass. Following chemical cleaning and ion bombardment, GaAs was exposed to 10⁷-10¹³ L O2, 10⁹-10¹³ L H₂O,10⁶-10⁸ L NO, and 10⁷-10¹¹ L N₂O (1 Langmuir (L) = 1.3x10⁻⁴ Pa•sec). Chemically cleaned GaAs produced equivalent amounts of Ga₂O₃ and As₂O₃ upon O₂ exposure. Oxygen exposure of ion bombarded GaAs resulted in the formation of Ga₂O₃, As₂O₃, and As₂O₅. Nitric oxide exposure produced Ga₂O₃ and As₂O₃, and N₂O exposure produced only Ga₂O₃. Gallium oxide was preferentially formed for ion bombarded material and the relative amount of Ga₂O₃ increased with increasing ion energy. 3 KeV Xe⁺ ion-bombarded GaAs exhibited the greatest reactivity to O₂ and NO. Exposure of ion bombarded GaAs to NO produced the greatest amounts of Ga₂O₃. Ion bombarded GaAs was the least reactive to N₂O. Exposure of ion bombarded GaAs to H₂O resulted in the formation of GaOOH and Ga(OH)₃, with Ga(OH)₃ formation occurring only on 2 KeV Ar⁺ and 3 KeV Ar⁺ and Xe⁺ ion-bombarded material at exposures above 10¹⁰ L. It was shown that defects were responsible for the increased reactivity and that preferential formation of Ga₂O₃ on ion bombarded material was not determined by the Ga/As surface ratio. Exposing IHT prepared GaAs to O₂ produced equivalent amounts of Ga₂O₃ and As₂O₃ when the Ga/As ratio was 1.23±0.07. The damage caused by ion bombardment was investigated by optical reflectivity in the visible and near-ultraviolet region (1.6-5.6 eV), Raman spectroscopy, and capacitance-voltage measurements. Ion bombardment forms a damaged layer that is amorphous. The depth of damage is proportional to the energy of the bombarding ion and inversely proportional to the mass of the bombarding ion. The shallow damage depth for 3 KeV Xe⁺ ion-bombarded GaAs offers some explanation for increased chemical reactivity. The increased reactivity of ion bombarded GaAs with O₂ and NO is attributed to surface defects (broken surface bonds). It is suggested that these broken bonds are in the form of singly occupied dangling bonds. A model for the surface and possible reaction pathways for O₂ and NO reactions are discussed.
Effects of ion processing and substrate variables on electrical characteristics of GaAs
Sen, Sidhartha (Virginia Tech, 1991)
The main objective of this study was to determine fundamental information related to ionbeam-induced damage to gallium arsenide (GaAs). The study covers experimental results concerning defect creation in GaAs versus parameters such as implantation energy, nature of GaAs substrate, crystalline orientation, and annealing. Transport and deep level transient spectroscopy (DLTS) results are presented for 50 keV Si-implanted and RTA (rapid thermal annealing) GaAs with (100) and (211) substrate orientations. Several electron traps are identified and their possible origins discussed. It is observed that (211) GaAs, after Si-implantation and RTA, has higher residual damage than (100) oriented GaAs. The electrical properties of active GaAs on Cr-doped and undoped GaAs substrates are compared. The DLTS response of active layers on Cr-doped GaAs is significantly different from those on undoped GaAs. A viable explanation that accounts for this difference is presented. The effects of furnace annealing on electrical properties of 50 keV, 4 x 10¹³ cm⁻² Si-implanted GaAs are addressed. A correlation between the structural recovery and electrical activation is established. The effects of 2 and 6 MeV Si implantation followed by RTA on the electrical characteristics of GaAs are investigated in detail. MeV Si-implantation and RTA generates active buried layers in GaAs. The buried layer quality is found to be at least comparable to a similarly processed keV Si-implanted active GaAs layer. The deep traps in MeV-implanted GaAs are identified and explained in terms of their probable origins. The deep level behavior of MeV Si-implanted and RTA GaAs is distinctly different from keV Si-implanted and RTA GaAs. This difference is largely due to the dynamic annealing occurring during MeV implantation. MESFETs formed on MBE-grown Al_.35Ga_.65As and low temperature MBE-grown GaAs buffer layers have shown peculiar characteristics (improved transconductance, sharper carrier profile, variability in threshold voltage, significant backgating, etc.). The effects of Al_.35Ga_.65As buffers and low temperature GaAs buffers on the electrical properties of the overlying active GaAs are investigated. Transport, DLTS, and SIMS (Secondary Ion Mass Spectroscopy) measurements are employed to explain the abnormalities in buffered MESFETs. Deep states and impurities are identified in buffers; they appear to migrate toward the channel-buffer interface during processing. The defects originating from the buffer are correlated to the performance of MESFETs formed on them. The effects of ion processing parameters, substrate chemistry, buffer layers, and annealing on the electrical characteristics of active GaAs layers are identified. An understanding of these effects is extremely critical to obtain reproducible devices with desirable characteristics.
Electrical analysis of low energy argon ion bombarded GaAs
Cole, Eric D. (Virginia Polytechnic Institute and State University, 1988)
An electrical analysis was done on A1 and Au Schottky diodes fabricated on n-type (100) GaAs which had been bombarded with low energy Ar ions. The purpose of this study was to quantify electrically damage caused by the Ion Beam Etching (IBE) as functions of energy and fluence. Electrical studies included Deep Level Transient Spectroscopy (DLTS), Current-Voltage (I-V), Capacitance-Voltage (C-V), ConductanceVoltage (G-V), Capacitance-Temperature (C-T), and Activation Energy Analysis. These electrical measurements were carried out on GaAs which had been exposed to a variety of treatments after IBE (such as chemical etch removal) to determine damage depth. At the lowest energy studied, 0.5keV, Schottky reverse saturation currents (I_sat) increased by over 4 orders of magnitude from the virgin case. The ideality factor, n, increased slightly while the breakdown voltage decreased. The most prominent changes occurred in the DLTS spectrum where it was observed that the native arsenic defect EL2 peak disappeared completely after ion etching. Concurrently a sharp increase in the diode conductivity with temperature was seen. It was found that chemical removal of 100Å of GaAs by chemical means could restore most of the diode parameters and the EL2 peak. It is proposed that the loss of EL2 is not related to a true physical reduction (i.e. an arsenic depletion) since calculations showed that the As loss would have extended beyond 3000Å for detectable DLTS changes. Also, the EL2 peak could be made to artificially disappear on a virgin sample with an external diode shunting resistor. The loss of the EL2 peak is, rather, attributed to a thin low resistivity surface layer having a partly amorphous nonstoichiometric crystal structure which can desensitize or mask the DLTS measurement. Surface chemical etch studies over the top of the Schottky diodes recovered 25% of the EL2 peak supporting this conclusion. Lower fluences had no effect at 0.5keV. Increasing ion bombardment energy showed a steady degradation in diode ideality factors. The reverse breakdown voltage increased past the unetched value and the DLTS spectrum began to show a very slight return of EL2. At 3keV the ideality factor was large, indicating the presence of a somewhat thicker high resistance layer. In fact recovery of diode parameters and EL2 did not occur until after 100Å removal. This was much deeper than expected at this energy, according to theory. Physical and lumped R-C electrical models are reported with an accompanying computer simulation of experimental DLTS results. The simulation used both thin low resistance and thick high resistance top layers to show that EL2 could be removed artificially. The models were also somewhat successful in explaining previously reported capacitance dispersion found in IBE GaAs.
Electrical conduction transport mechanisms of barium titanate- based multilayer ceramic capacitors
Zhang, Tong (Virginia Tech, 1988-08-15)
The major objectives of this study were to examine electrical conduction properties of BaTiO3-based multilayer ceramic ( MLC ) capacitors in order to gain a better understanding of the conduction transport mechanisms inside the devices. The experiments involved mainly leakage current versus time measurements under both low temperature-low voltage stress and high temperature-high voltage stress. It was established that leakage current conduction in a MLC capacitor under temperature-voltage stress can be divided into three different conduction regions due to different mechanisms. Those regions are polarization current, DC conduction current and degradation current. The polarization current decreases with time as a power law relation, i.e. Ic(t) t-m where the exponent value m is strongly dependent on the type of capacitor and temperature, but is only weakly dependent on the applied voltage. It has been proposed that two degradation models ( a charge carrier concentration model and a reduction of grain boundary barrier height model ) can explain the degradation behavior for the Z5U devices tested. Degradation measurements indicate that the lifetime for Z5U capacitors can be described by Minford's expression. However, these models account only partly for X7R degradation. X7R behavior, is characterized by an early power law time dependence, followed by exponential voltage dependence. The most probable conduction transport mechanism in X7R capacitors is small polaron hopping, while grain boundary transmission may be the predominant conduction transport mechanism in Z5U capacitors.
Electrical crosstalk in multilayer ceramic substrates
Yoo, In Kyeong (Virginia Tech, 1990)
Electrical crosstalk is becoming increasingly important as size shrinks and component densities increase in hybrid circuits. This occurs especially in multilayer thick film circuits. Crosstalk studies at high frequencies apply transverse electromagnetic (TEM) or quasi-TEM mode. However, the TEM mode theory is not applicable at low frequencies and low frequency-crosstalk is still a concern for circuit designers. In this research, crosstalk equations were derived in a simpler way, which can be used for low frequency applications such as automotive electronics. Test patterns were designed in both single and multilayer substrates in order to study crosstalk parameters such as line separation, line width, ground distance, ground type, and multilayer thickness. The mutual inductance and the mutual capacitance were calculated in order to use them in predicting crosstalk for particular transmission line geometrical structures. A conventional technique was used for the mutual inductance, and a new conformal mapping technique was developed for the mutual capacitance. Frequency dependence of crosstalk was confirmed by the equations developed. There is good agreement between the experimental mutual inductance and mutual capacitance and the calculated values. Finally, crosstalk prediction ( simulated by combining crosstalk equations with calculated mutual parameters ) fits well with the measured values.
Electrical properties of polyimides modified with metal salts
Rancourt, James David (Virginia Tech, 1987)
Polyimides, due to their high thermal stability, excellent chemical stability, useful mechanical properties, and extremely high electrical resistivity, are utilized in aerospace, electronic, and specialty consumer markets. However, in some applications, lower electrical resistivity is preferable. Toward this goal, polyimide films have been modified with metal salts and metal complexes. Depending upon processing conditions, the films contain ionic species uniformly distributed throughout their bulk, or are highly anisotropic structures containing near-surface metal or metal oxide. Evaluation of solvent cast films by a variety of analytical techniques has been used to develop structure-property-process correlations in cobalt chloride modified polyimides. To date, no interaction between the additive and the matrix has been indicated by ultraviolet, visible, or infrared spectroscopy, though by differential scanning calorimetry and a specialized thermogravimetry technique, some interaction is implied. Elemental analysis has verified that polyimides having a metal oxide surface also have residual bulk metal ion content. A major controller of the bulk resistivity of metal ion modified polyimide films was found to be the polymer glass transition temperature; a lower glass transition temperature resulted in lower electrical resistivity at a particular temperature. Central to this research work was the design and construction of a sensitive and reliable electrical resistivity measurement system. The system was also found useful for probing polymer contamination and molecular motion. Further, the electrical measurements indicated that uniformity between samples was poor. Modification of an inert gas oven, allowing processing in controlled atmosphere, proved that film surface conductivity is critically influenced by humidity. A model has been proposed that supports both the d.c. electrical properties and surface spectroscopic data. The model and details in the ceramic literature were the primary factors in pursuing a specific codoped polyimide system. With the codoped system, the electrical resistivity and activation energy for conduction, compared with either of the singly doped polyimide films, were predictably and favorably reduced.
Electrical studies on ion-etched n-GaAs(100) surfaces
Sen, Sidhartha (Virginia Tech, 1987-12-15)
The major objective of this thesis was to evaluate electrically the damage caused by a low energy (< 4keV) Ar⁺ bombardment on n-GaAs(100) surfaces. Electrical measurements were performed on Schottlky diodes formed on the virgin and the ion-etched surfaces. The l-V measurements show deterioration of diode parameters by ion etching. The ion etched diodes have a strong component of surface leakage current. The high frequency capacitance of ion-etched diodes is less than that of the virgin diodes. The low frequency capacitance of ion-etched diodes was found to be frequency dispersive. The extent of frequency dispersion diminishes at low temperatures and at low reverse biases. Virgin diode capacitance, on the other hand, was found to be independent of frequency. The electrical characteristics of ion-etched diodes are explained by means of an amorphous layer and a donor-like damaged layer formed as a result of ion etching. The depth of the top amorphous layer increases with etch energy. The damaged layer containing the ion induced traps superimposes over the amorphous layer and extends deep into the bulk semi-conductor. The density of such traps is very bias sensitive and also temperature dependent. A possible equivalent circuit model for the ion-etched material is proposed. Low temperature isochronal annealing (< 450°C, 10mins.) was not found effective in causing complete recovery of the ion-damaged surface.
Electrical transport properties of barium titanate-based capacitor ceramics
Lee, Hee Young (Virginia Polytechnic Institute and State University, 1987)
Electrical conduction mechanisms in BaTiO₃-based ferroelectric capacitor ceramics with an emphasis on the X7R type were studied. Dominant charge carriers in this material were identified as conduction band electrons below a temperature of 850°C. This was substantiated by the following results: negative Seebeck coefficients, zero galvanic cell voltage, and evidence of space charge-limited currents in MLC capacitors and related ceramic. Effects of chip thickness on the electrical parameters, as well as the I-V characteristics, were studied. Chip electrical parameters such as resistivity, dielectric constant, and activation energy were found to be independent of chip thickness. Effects of ambient were also studied and differences in current-voltage behavior were attributed to surface effects. Complex impedance spectroscopy proved to be a useful technique in separating grain, grain boundary, and contact contributions to the total impedance. Impedance plots for X7R ceramic revealed negligible contact impedance. The most probable electrical transport mechanism in X7R ceramic is small polaron hopping, although the possibility of combining small polaron hopping and grain boundary transmission cannot be excluded.
Electromagnetic radiation calorimetry of thermoplastics, elastomers and composites systems
Chen, Ming (Virginia Polytechnic Institute and State University, 1989)
The application of microwave radiation for processing of glassy and semicrystalline thermoplastics, elastomeric polymers and composites was investigated. The goal of this research was to reveal the relationship between polymer structure and microwave absorptivity, and hence processability. The specimens were subjected to an electric field at 2.45 GHz either inside a rectangular waveguide or in a cylindrical resonant cavity applicator with less than 100 watts applied power. Both travelling wave modes and standing wave modes were examined. Temperatures, powers and times were recorded, leading to the concept of "microwave calorimetry." Low frequency dynamic mechanical and dielectric frequency-temperature spectra were obtained on the materials and combined to conveniently extrapolate structure-property relationships into the GHz region. A correlation was found between the dielectric properties of various polymers and the dipole moments of small molecule analogues. Evaluating heatability was most accurately found to be determined by the magnitude of (ε_S - ε_∞), the oscillator strength. The value of (ε_S - ε_∞) should be used together with the distribution of relaxation times and the activation energies of dipolar dispersion to predict heatability for microwave processing. The critical temperatures, T_C, of dielectric loss were obtained from the intercepts of positive slope tangents of heating rate versus temperature plots at 2.45 GHz for polymers. Microwave processing was rapid above the critical temperature where the maximum dielectric loss fell in the 2.45 GHz frequency domain for efficient coupling of energy to the polymers. Shifting the dielectric relaxation spectrum into the microwave region by directly or indirectly increasing the temperature of each sample was unique and of key importance to processability. A schematic model was proposed to explain the behavior of two-phase materials subjected to microwave heating. Combining the heatability, (ε_S - ε_∞), and the dielectric relaxation spectral response was found to be helpful in evaluating formulations of two-phase materials for electromagnetic radiation processing at high frequencies.
Employment of metal-modified polyimide to achieve optimum conductance at an aluminum joint
Madigan, Elizabeth A. (Virginia Tech, 1984-03-03)
Earlier research relating to the use of polyimides modified with metal-ion complexes and metal particles indicate that enhanced conductivity and adhesive strength can be achieved. This research evaluated the employment of metal-modified polyimides to achieve optimum conductance at an aluminum joint. Condensation and addition polyimides were employed. The modification of the polyimides occurred in two ways. The first method involved homogeneous doping of the condensation polyimides with metal-ion complexes. The second modification method involved heterogeneous doping of condensation or addition polyimides with particles of a nickel-aluminum alloy.
High-frequency off-line power conversion using quasi-resonant and multi-resonant techniques
Jovanović, Milan Miodrag (Virginia Polytechnic Institute and State University, 1988)
Three recently-proposed power conversion technologies, the zero-current-switching (ZCS) and zero-voltage-switching (ZVS) quasi-resonant techniques, and the zero-voltage-switching multi-resonant technique, are evaluated for high-frequency, off-line applications. The study is performed with emphasis on the conversion-frequency range, efficiency, load range, input-voltage range, output power, dynamic response, and power density. A comprehensive dc analysis of the half-wave and full-wave, half-bridge zero-current-switched quasi-resonant converters (QRCs) is presented. Design procedures for closed-loop design of the converters are also derived. The procedures are used to design and fabricate half-wave and full-wave converters operating in the low-megahertz range and experimentally assess their suitability for high-frequency, off-line power conversion. The zero-voltage-switching technique is employed to further increase the conversion frequency. First, the half-bridge zero-voltage-switched quasi-resonant converter is analyzed and the trade-offs between its frequency range, load range, and efficiency are examined in detail. The multi-resonant-switch concept is applied to this converter to improve its characteristics, primarily its load range. A complete dc analysis of the zero-voltage-switched multi-resonant converter (MRC) is given and the dc voltage-conversion-ratio characteristics are derived. A graphic design procedure for the converter is established and is used to build an experimental 100 W, off-line converter operating in the frequency range of 2 MHz to 8 MHz. Finally, a comprehensive comparison of the QRCs and MRCs is performed and conditions where their applications appears most desirable are defined.

Browsing by Author "Burton, Larry C."

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