Open Conductor Research Articles

Ab initiomodeling of quantum transport properties of molecular electronic devices

We report on a self-consistent ab initio technique for modeling quantum transport properties of atomic and molecular scale nanoelectronic devices under external bias potentials. The technique is based on density functional theory using norm conserving nonlocal pseudopotentials to define the atomic core and nonequilibrium Green's functions (NEGF's) to calculate the charge distribution. The modeling of an open device system is reduced to a calculation defined on a finite region of space using a screening approximation. The interaction between the device scattering region and the electrodes is accounted for by self-energies within the NEGF formalism. Our technique overcomes several difficulties of doing first principles modeling of open molecular quantum coherent conductors. We apply this technique to investigate single wall carbon nanotubes in contact with an Al metallic electrode. We have studied the current-voltage characteristics of the nanotube-metal interface from first principles. Our results suggest that there are two transmission eigenvectors contributing to the ballistic conductance of the interface, with a total conductance $G\ensuremath{\approx}{G}_{0}$ where ${G}_{0}{=2e}^{2}/h$ is the conductance quanta. This is about half of the expected value for infinite perfect metallic nanotubes.

TIVA and SEI developments for enhanced front and backside interconnection failure analysis

Thermally-Induced Voltage Alteration (TIVA) and Seebeck Effect Imaging (SEI) are newly developed techniques for localizing shorted and open conductors from the front and backside of an IC. Recent improvements have greatly increased the sensitivity of the TIVA/SEI system, reduced the acquisition times by more than 20X, and localized previously unobserved defects. The system improvements, non-linear response of IC defects to heating, modeling of laser heating and examples using the improved system are presented.

Thick-Film Processing for Tl-Oxide Wire and Tape

Thick-film processing techniques such as electrodeposition, electrophoresis, spray pyrolysis, and ink spraying offer promising and economic (<$10/kAm) approaches for fabricating high-temperature superconducting (HTS) wire or tape for the Tl-oxide superconductors. The performance of the Tl system exceeds that for the Bi-oxides and offers easier fabrication than YBCO. Scale up, however, may be difficult because of the need for open tape conductors with aligned microstructures. The Tl-oxides, moreover, offer the potential for operation at 77K in practical magnetic fields of 3-5 T, which is supported by measurements on the irreversibility behavior of the Tl-1223 single layer compound with Pb, Bi, and Sr substitution. Recent advances in the development of biaxial textured electrodeposited films on Ag coated single crystal substrates are discussed. This paper will review the merits for a near-term Tl-oxide based conductor for use in HTS electric power applications, currently under development, such as the transformer and transmission cable. Recent results for the buffer layer development that would provide an oriented ceramic layer or template on a flexible textured nickel substrate utilizing the “RABiTS” substrates developed by ORNL will then be discussed with a projection for the cost of a “long length” thick-film process using electrodeposition.

New Rapid Failure Analysis Techniques for Multilevel Metallization

Open interconnections on integrated circuits (ICs) are a serious and ubiquitous problem throughout the micro-electronics industry. The efforts to understand the mechanisms responsible for producing open interconnections and to develop analytical methods to localize them demonstrate the concern manufacturers have for this problem. Multiple layers of metallization not only increase the probability that an open conductor or via will occur because of the increased number of interconnections and vias but also increase the difficulty in localizing the site of the failure because upper layers may mask the failure site.Rapid failure analysis of open-conductor defects is critical in new product development and reliability assessment of ICs where manufacturing and product development delays can cost millions of dollars a day. In this article, we briefly review some standard failure analysis approaches and then concentrate on new techniques to rapidly locate open-conductor defects that would have been difficult or impossible to identify using earlier methods. Each method is described in terms of the physics of signal generation, application, and advantages and disadvantages when compared to existing methods.

Thermal processing of spray-pyrolyzed and electrodeposited Tl superconductor films and optimization of the transport critical current density

Employing a flow-through "two-zone" thermal processing system to control thallous oxide vapor pressure, we have investigated two different types of precursors and explored their potential for producing open tape conductors with high critical current densities. The two methods examined were: (1) precursor Pb-Sr-Ba-Ca-Cu-O(Ag) sprayed as a colloidal ink onto a heated substrate; (2) precursor material deposited by electrodeposition from an electrolytic solution containing the metal nitrates. Although both of these methods produce precursors with the proper stoichiometric ratios, vast differences in other properties such as reactivity, density, thickness, and morphology exist. The effects of these precursor properties upon subsequent superconducting characteristics such as phase development, final morphology, and electrical transport are characterized by XRD, SEM/optical microscopies, R vs. T and J/sub c/ measurements, respectively. Both types of precursors have produced superconducting films with high J/sub c/'s. Phase-pure textured films have been obtained on a variety of substrates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Negative-sequence overcurrent element application and coordination in distribution protection

Negative-sequence overcurrent elements do not respond to balanced load and can thus be set to operate faster and more sensitively than phase overcurrent elements for phase-to-phase faults. It is demonstrated that on a radial distribution system negative-sequence overcurrent elements need only be coordinated with downstream phase overcurrent devices for phase-to-phase faults. Coordination for other fault types is then achieved with no further analysis required. The effects of negative-sequence load current and open phase conductors on negative-sequence overcurrent elements are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Three dimensional finite element analysis of high power microwave devices

A three-dimensional nodal-based finite element formulation for electric or magnetic fields is presented. The formulation includes open boundary problems and conductor interfaces. The open boundary is modeled using the Engquist-Majda absorbing boundary condition. Current sheet excitation is used to excite modes in the waveguide. A validation using an infinitesimal current dipole is shown, and an example of modeling a circular waveguide illustrates the use of the method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Open conductor detector system

An open phase conductor detector system consisting of one or more transmitters and a receiver which communicate through the distribution line neutral conductor is described. The design holds a promise to provide reliable protection for most down and open 5 to 35 kV distribution phase conductors in contact with high impedance surfaces. The transmitter(s) detects the open phase conductor by monitoring the phase conductor voltage using redundant inputs. When an open phase conductor is detected, the transmitter injects a signal to the distribution line neutral conductor by means of a coupling current transformer. The signal is frequency and duration coded. The receiver decodes the signal and generates a trip signal which may activate a breaker, a ground switch, or a recloser, causing isolation of the distribution line with the open (downed) phase conductor. The system is presently implemented for single distribution lines. A prototype open conductor detector system has been constructed and field tested on a Georgia Power Company distribution system. >

Scattering theory of thermal and excess noise in open conductors.

Thermal fluctuations at equilibrium and excess fluctuations in the presence of transport in open multiprobe conductors are related to the scattering matrix of the conductor. The fluctuation-dissipation theorem for multiprobe conductors is discussed. A general expression for the excess noise in the presence of transport is derived. These results are applied to conductors which exhibit the quantized Hall effect. If backscattering is suppressed, excess noise is also suppressed.

Transmission probabilities and the quantum Hall effect

A scattering approach to magneto-transport permits the discussion of open conductors with current and voltage terminals. This approach leads to generalized longitudinal and generalized Hall resistances which are determined by the current carrying states at the Fermi energy. In samples small compared to the distance over which current carrying states equilibrate, the contact properties are crucial in determining whether the Hall voltage is quantized or not. Simple situations in which both the longitudinal and the Hall resistance are simultaneously quantized at values unrelated to the bulk properties of the sample are emphasized. The departure from the quantum Hall plateaus in narrow conductors is discussed in terms of intrinsic Breit-Wigner resonances.

A fault-induced transient analysis of unbalanced distribution systems with harmonic distortion

Abstract This paper presents a new method to calculate a fault-induced transient in an unbalanced three-phase distribution system, subjected to a shunt fault and/or a series fault, using the three-phase bus impendance matrix. The method is also applicable to multiple faults which involve a shunt fault and an open conductor. The imbalance caused by large single-phase loads, untransposed feeders, conductor bundles, etc., is reflected in the polyphase bus impedance matrix. Consequently, line removals, additions, impedance changes, conductor and phase openings can be simulated by modifying the base-case impedance matrix in the complex frequency domain. Thus, the proposed method makes it possible to analyze any type of single or multiple fault-induced transient occurring anywhere in the system.

Transmission probabilities and the quantum Hall effect.

Abstract A scattering approach to magneto-transport permits the discussion of open conductors with current and voltage terminals. This approach leads to generalized longitudinal and generalized Hall resistances which are determined by the current carrying states at the Fermi energy. In samples small compared to the distance over which current carrying states equilibrate, the contact properties are crucial in determining whether the Hall voltage is quantized or not. Simple situations in which both the longitudinal and the Hall resistance are simultaneously quantized at values unrelated to the bulk properties of the sample are emphasized. The departure from the quantum Hall plateaus in narrow conductors is discussed in terms of intrinsic Breit-Wigner resonances.

Absence of backscattering in the quantum Hall effect in multiprobe conductors.

Under certain conditions, high magnetic fields in a two-dimensional conductor lead to a suppression of both elastic and inelastic backscattering. This, together with the formation of edge states, is used to develop a picture of the integer quantum Hall effect in open multiprobe conductors. We consider both ideal contacts without elastic scattering and also disordered contacts. Ideal contacts populate edge states equally whereas disordered contacts lead to an initial nonequilibrium population of the edge states. In Hall samples much larger than an inelastic length, and in the presence of disordered contacts, the sample edges become equipotential lines only an inelastic scattering length away from the current source and current drain contacts. Samples so small that the carriers can travel from one contact to the other without inelastic relaxation do not exhibit exact quantization if the contacts are disordered. In all cases we find that the quantum Hall effect occurs only if the sample exhibits at least two sets of equilibrated edge states which do not interact via elastic or inelastic scattering. The onset of interaction between the two sets of edge states leads to deviations from exact quantization and eventually to a breakdown of the quantum Hall effect.

Synchronous generator stability due to multiple faults on unbalanced power systems

Abstract This paper investigates the dynamic behavior of a synchronous generator connected to an unbalanced three-phase power system which is subjected to a shunt fault at one location and blown fuses or open conductors at another location. The method involves phase frame representation of the line. The imbalance created by large single-phase loads, untransposed lines, conductor bundles, etc., is reflected in polyphase impedance matrices. Consequently, line removals, additions, impedance changes, conductor and faulted phase openings can be simulated based on the boundary conditions. Thus, the proposed method makes it possible to study the synchronous generator stability due to any combination of single or multiple asymmetrical series and shunt faults occurring anywhere in the system and the results of several case studies are presented. A significant finding is that certain faults cause the generator to lose synchronism only after several cycles of the rotor angle swing curve. In such cases it is therefore no longer correct to judge the system's stability only on the ‘first swing cycle’ of the rotor angle.

Induction motor transient response to multiple faults in unbalanced power systems

Although the effects of an unbalanced supply on overheating induction motors have been known for some time, little or no attention has been paid to the dynamic response of such a system. A recent investigation though has evaluated the effects of an unbalanced distribution system, unbalanced single-phase loads, unbalanced shunt faults, and series faults (open conductor) on the transient behavior of a three-phase induction motor. This paper extends that work to the case of motor transient response due to multiple asymmetrical faults in an unbalanced power system. The paper investigates the dynamic behavior of an induction motor connected to an unbalanced three-phase power system subjected to a shunt fault at one location and blown fuses or open conductor at another location. The method involves phase frame representation of the line. A significant finding is that certain multiple faults cause the induction motor to lose stability.

Three-phase modeling of unbalanced distribution systems during open conductors and/or shunt fault conditions using the bus impedance matrix

Abstract This paper presents a digital simulation technique for unbalanced three-phase distribution systems subjected to a shunt or a series fault. The method is also applicable to multiple faults which involve a shunt fault at one location and blown fuses or open conductors at another. The method utilizes the phase frame representation of network elements. The imbalance conditioned by large single-phase loads, untransposed feeders, conductor bundles, etc., is reflected in the polyphase impedance matrix. Consequently, line removals, additions, impedance changes, conductor and phase openings can be simulated by modifying the base-case impedance matrix. Thus, the proposed method makes it possible to analyze any type of single or multiple fault occurring anywhere in the system.

Predetermination of lateral length to prevent overvoltage problems due to open conductors in three-phase systems

The opening of one or two conductors in three-phase circuits supplying an ungrounded transformer produces a steady-state overvoltage due to ferroresonance between the lateral capacitance and the transformer nonlinear inductance. Available methods of determining the conditions under which such an overvoltage may occur are mostly based on experimental studies, and hence are limited in application to systems whose design parameters are similar to those used in the tests. A simple and elegant approach, presented here, reveals that an incorrect choice of overvoltage limit leads to the selection of an unsafe lateral length, resulting in a potentially dangerous high voltage across the connected apparatus and protective equipment. Generalised mathematical equations are developed to determine the safe lateral length as a function of transformer size for the prevention of the overvoltage problem due to open conductors in three-phase systems. The computed results are in good agreement with experimental observations.

Lightning and Power Surge Damage to Appliances

Lightning rarely damages appliances by direct stroke to the appliance. Damage is usually caused by direct strokes to the power lines or by induced surges. Induced surges are caused by the release of bound charges on the line when overhead clouds lose charge because of a strike to objects other than the line. The direct stroke produces a large voltage, while induced surges usually have a lower magnitude voltage. Lightning usually damages appliance electrical insulation which leads to a continuation of the damage by the 60-Hz power that has the appearance of a non- lightning-related failure. Lightning or power surge damage can probably be ruled out if the equipment has failed because of certain problems such as an open conductor or a bad bearing. Electric lamps can provide a clue as to whether a high-voltage surge occurred in the electrical system because the filaments often open when subjected to such surges. Equipment can be protected by a design to withstand the highest normal voltage spikes expected. Proper grounding and bonding also is important. For critical components or equipment, surge arresters may be required.

Exact Calculation of Simultaneous Faults Involving Open Conductors and Line-to-Ground Short Circuits on Inherently Unbalanced Power Systems

The analysis of simultaneous faults, espec ially involving mutually coupled lines on inherently unbalanced power systems using the symmetrical components technique, may not be practically feasible because of the difficulties encountered in the assembly f sequence networks and their solution. To al-le- these problems, this paper presents an alternative method for digitally simulating a basically ubnlanced three-phase power system which is subjected to a shunt fault at one location and blown fuses or open conductors at another location. The method involves phase frame representation of network elements and the use of the distributed source solution technique in conjunction with the principle of superposition. e phase coordinate model of power system ele- is modified/enlarged to simulate inherent system imbalances as well as the unbalancing contributed by the change in network configuration. The unbalance conditioned by large single-phase loads, untransposed transmission lines, conductor bundles, etc. is re- flected in the polyphase admittance matrix. Consequently, line removals, additions, impedance changes, conductor and phase openings can be simulated quite easily by simply modifying the base case admittance matrix. Even the treatment of mutually coupled lines presents no computational problems, unlike the symmetrical component approach. Thus, the proposed method makes it possible to analyze simultaneous faults on basically unbalanced power systems with the same ease and simplicity as unsymmetrical faults on a balanced system with the aid of symmetrical components. The method is applicable to any combination of series and shunt faults occurring anywhere in the system.

Exact Calculation of Simultaneous Faults Involving Open Conductors and Line-to-Ground Short Circuits on Inherently Unbalanced Power Systems

Exact Calculation of Simultaneous Faults Involving Open Conductors and Line-to-Ground Short Circuits on Inherently Unbalanced Power Systems