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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Multi-Wavelength Dielectrometer (MWD) Sensor For Planetary Subsurface Exploration

NASA's mission to Moon, Mars, and Beyond envisions landing of a light weight measurement platform on the planetary surface. The Multi-Wavelength Dielectrometer (MWD) on-board consists of essential electronics and metallic plates acting as electrodes attached to the body of such platform. An electric signal applied to one of the electrodes acting as a cathode sets up electric field pattern (in the soil medium) between the cathode and other electrodes acting as anodes. The electrodes are swept through multiple wavelengths (1Hz-1MHz) and the electric current drawn by the electrodes is measured at each frequency. The measured current whose amplitude and phase depend upon electrode spacing, dielectric constant of the subsurface soil, and the frequency is then used to estimate electrical properties of the soil. In this paper the MWD sensor that will measure the dielectric properties of Moon/Mars s soil is presented. A procedure to process the MWD measured data for extracting the soil properties is also described. Assuming the subsurface soil structure as multilayer strata having varying electric properties, an electric equivalent circuit of the multiple electrodes configuration placed on a multi-layer soil sample is obtained. The current drawn by the equivalent circuit from the low frequency signal generator is then calculated. By minimizing the difference between the model s estimated current and measured MWD data the electric properties of soil samples are extracted. Experimental and simulated results will be presented to validate the proposed procedure for extracting soil properties.

Deshpande, Manohar↗
NASA NTRS1993

Analysis of microstrip dipoles and slots transversely coupled to a microstrip line using the FDTD method

Printed dipole elements and their complement, linear slots, are elementary radiators that have found use in low-profile antenna arrays. Low-profile antenna arrays, in addition to their small size and low weight characteristics, offer the potential advantage of low-cost, high-volume production with easy integration with active integrated circuit components. The design of such arrays requires that the radiation and impedance characteristics of the radiating elements be known. The FDTD (Finite-Difference Time-Domain) method is a general, straight-forward implementation of Maxwell's equations and offers a relatively simple way of analyzing both printed dipole and slot elements. Investigated in this work is the application of the FDTD method to the analysis of printed dipole and slot elements transversely coupled to an infinite transmission line in a multilayered configuration. Such dipole and slot elements may be used in dipole and slot series-fed-type linear arrays, where element offsets and interelement line lengths are used to obtain the desired amplitude distribution and beam direction, respectively. The design of such arrays is achieved using transmission line theory with equivalent circuit models for the radiating elements. In an equivalent circuit model, the dipole represents a shunt impedance to the transmission line, where the impedance is a function of dipole offset, length, and width. Similarly, the slot represents a series impedance to the transmission line. The FDTD method is applied to single dipole and slot elements transversely coupled to an infinite microstrip line using a fixed rectangular grid with Mur's second order absorbing boundary conditions. Frequency-dependent circuit and scattering parameters are obtained by saving desired time-domain quantities and using the Fourier transform. A Gaussian pulse excitation is applied to the microstrip transmission line, where the resulting reflected signal due to the presence of the radiating element is used to determine the equivalent element impedance.

Tulintseff, A. N.↗
NASA NTRS1995

On the modeling and optimization of Schottky varactor frequency multipliers at submillimeter wavelengths

Schottky varactor frequency multipliers are used to generate local oscillator power at millimeter and submillimeter wavelengths. The equivalent circuit of the Schottky varactor contains a junction capacitance, a junction conductance, a series resistance and a model for electron velocity saturation. A millimeter wavelengths the equivalent circuits is affected by the edge effects, which are due to the small-area circular anode. The correction factors due to the edge effect for the junction capacitance and for the series resistance are available in the literature. In this work the electron velocity saturation is modeled by limiting the velocity of the transition front between the depleted and undepleted layer. By using this model the maximum current of the diode is given by the actual area of the transition front between depleted and undepleted layers, and is therefore related to the capacitance correction factor. The new model has been tested by analyzing a two diode balanced doubler for 160 GHz presented earlier in the literature. The agreement between the theoretical results and the measurements is excellent. The new diode model is useful in optimization of varactors for high millimeter and submillimeter wave frequencies.

TRT-THEORETICAL↗
NASA NTRS2009

Resonance-induced sensitivity enhancement method for conductivity sensors

Methods and systems for improving the sensitivity of a variety of conductivity sensing devices, in particular capacitively-coupled contactless conductivity detectors. A parallel inductor is added to the conductivity sensor. The sensor with the parallel inductor is operated at a resonant frequency of the equivalent circuit model. At the resonant frequency, parasitic capacitances that are either in series or in parallel with the conductance (and possibly a series resistance) is substantially removed from the equivalent circuit, leaving a purely resistive impedance. An appreciably higher sensor sensitivity results. Experimental verification shows that sensitivity improvements of the order of 10,000-fold are possible. Examples of detecting particulates with high precision by application of the apparatus and methods of operation are described.

Tai, Yu-Chong↗
DOE OSTI2024

Impacts of Non-Ideal Back Contact on Capacitance Measurements in CdTe Solar Cells

CdTe solar cells suffer from a non-ideal back contact which can strongly affect the equivalent circuit model and complicate capacitance measurements. Here, five different back contact materials are deposited on identical CdTe absorbers and their influence on capacitance measurements is characterized. A five-element equivalent circuit model is shown to represent a CdTe solar cell with a non-ideal back contact, and capacitance-frequency (C-f) measurements on each sample clearly show the influence of this contact. Capacitance-voltage (C-V) measurements on each sample show the influence of frequency on measured capacitance. In the AlGaOx/Au sample, temperature dependent C-fs show the back barrier height to be 240 meV, and ongoing measurements will give a comparison of back barrier heights for all samples. This study provides insight into the efficacy of various back contacts and highlights potential errors in the interpretation of capacitance measurements due to the presence of the back contact.

14 SOLAR ENERGY↗
DOE OSTI2024

Revealing the role of redox reaction selectivity and mass transfer in current–voltage predictions for ensembles of photocatalysts

Photocatalysts are conceptually simple reaction units where nanoscale semiconductors integrated with catalysts drive a pair of redox reactions on illumination. However, the proximity of reaction sites performing cathodic and anodic reactions poses dire challenges to realize large light-to-fuel conversion efficiencies. In this study, a powerful, yet straightforward, equivalent-circuit detail-balance modeling framework is developed and applied to evaluate the performance of photocatalytic systems featuring multiple light absorbers. Specifically, low bandgap iridium-doped strontium titanate is modeled as a Z-scheme photocatalyst to achieve desirable hydrogen evolution and iron-based redox shuttle oxidation reactions. Our model has unique capabilities to simulate competing redox reactions and address mass-transfer limitations. In a significant departure from state-of-the-art circuit models, our study develops tools to perform load-line analyses by incorporating a net electrochemical load curve that includes both desired and competing redox reactions. Consequently, reaction selectivity is predicted from equivalent circuit models for photocatalytic and photoelectrochemical systems. Our investigation into ensembles comprised of multiple, semi-transparent light absorbers reveals their potential to outperform a single, optically thick light absorber, particularly when operated under mass-transfer-limited conditions. However, this outcome hinges on minimizing mass-transfer rates of select redox species to prevent undesired reactions of hydrogen oxidation and/or redox shuttle reduction. Our findings demonstrate that reaction selectivity can be achieved by tuning asymmetry in redox species mass-transfer even with perfectly symmetric electrocatalytic charge-transfer coefficients. The influences of various kinetic, mass-transfer, and thermodynamic parameters are explored to offer crucial insights for synthesis of the next-generation of photocatalysts and selective coatings, and reactor designs.

25 ENERGY STORAGE↗
NASA NTRS1984

Barrier height determination on Schottky contacts formed at the back contact-semiconductor interface of degraded solar cells

A method is described of determining an equivalent circuit for solar cells which have degraded as a result of the formation of a rectifying Schottky barrier at the back contact. An excellent fit of experimental data has been achieved using SCEPTRE with an equivalent circuit derived from the shape of the measured current voltage characteristics. One key parameter of the Schottky barrier diode, the reverse saturation current, can be used to determine the barrier potential. The barrier potential increases as the cell is stressed with 0.5 volts being a typical experimentally determined value for a degraded cell.

Misiakos, K.↗
NASA NTRS1992

Periodic structures with time-varying loads

A new method is described for generating and predicting the power scattered at multiple frequencies from a periodic array excited by a monochromatic incident plane wave. The scattered power is predicted by developing an equivalent circuit representation from the exact method of moment solution of the unloaded periodic problem and then employing a modified method of moments approach to predict the scattered fields due to time-varying loads. An advantage of this method is that the generality of the equivalent circuit can be exploited to study the effects of different loads on the power scattered in the harmonic frequencies with a minimal amount of effort. The method can also be applied to multiple incident frequencies.

Epp, Larry W.↗
NASA NTRS1992

Theoretical and experimental investigation of a rectenna element for microwave power transmission

A microstrip measurement system has been designed to analyze packaged GaAs Schottky barrier diodes under small and large signal conditions. The nonlinear equivalent circuit parameters of the diode are determined using a small signal test method that analyzes the diode's scattering parameters at various bias levels. The experimental results of a 2.45 GHz diode are verified using a nonlinear circuit simulation program based on a multireflection algorithm. A 35 GHz rectenna has been built using a microstrip patch antenna and Ka-band mixer diode. The measured efficiency was 29 percent at 120 mW input power. A frequency selective surface is designed using an equivalent circuit model to reduce the second harmonic radiations for a 2.45 GHz rectenna. Theoretical results are found to be in fairly good agreement with experiments.

Mcspadden, James O.↗
NASA NTRS2021

Mathematical Characterization of Battery Models

The purpose of this document is to demonstrate the use of the Extended Kalman Filter as a tool for battery state estimation and the estimation of battery state of charge. The mathematical details based on the equivalent circuit model are presented followed by an electrochemical engineering model. A simplified first-order model is used to demonstrate the procedure followed by second and third-order models. Next a simplified electrochemistry model is presented along with observer development. State observability is calculated for the simpler equivalent circuit models and the simplified electrochemistry model. An outline of the battery model parameter identification method is presented, and model performance based on experimental and flight data is demonstrated.

Battery↗
NASA NTRS2020

Composite Piezoelectric Resonator 1D Modeling with Loss

Abstract – Maximizing the benefit of applying piezoelectric materials in composite structures, which are a mix of piezoelectric and elastic elements, requires analytical modeling. In this paper we describe two main classes of resonators and how to model using network equivalent including material losses. These include extensional devices where the layer areas are parallel to the direction of wave travel as found in a variety of thickness or length mode transducers (BC. Stress T1 = T2 at boundary) and transverse mode resonators (BC. Strain S1 = S2 at boundary) where the major layer area is perpendicular to the wave direction as is found in radial mode or length thickness mode composite resonators. In order to illustrate these models we will use Mason’s equivalent circuits with elastic, dielectric and piezoelectric loss. It is noted that in order to maintain consistency with the linear equations of piezoelectricity and the wave equation care is required when applying complex loss coefficients to the models. Although these techniques are applicable to other models we use Mason’s network equivalent circuit because it is intuitive for composite modeling due to the frequency independent turns ratio and the independence of the equations describing the elastic layers from the adjoining layer properties which aids in understanding the boundary conditions to impose on the layer boundaries. We will present network models for a variety of applications with loss and show how to calculate their impedance spectra, interface stress, strain velocity and displacement spectra.

Sherrit, Stewart↗
NASA NTRS1975

Factors in the design of spacecraft utilizing multiple electric thrusters

An analysis has been performed which describes the electrical interactions between simultaneously operating ion thrusters and between the ion thrusters and a spacecraft bus. The problem is reduced to a relatively simple equivalent circuit including all leakage paths. The influence of the various circuits elements on a spacecraft potential is studied. A thruster grounding scheme is introduced which is designed to minimize the electrical interactions and includes protection for both thrusters and spacecraft from possible damage due to neutralizer failure. The paper includes the results of various experiments which have been conducted to quantitatively define the elements of the equivalent circuit.

Fitzgerald, D. J.↗
NASA NTRS1987

Fast simulation techniques for switching converters

Techniques for simulating a switching converter are examined. The state equations for the equivalent circuits, which represent the switching converter, are presented and explained. The uses of the Newton-Raphson iteration, low ripple approximation, half-cycle symmetry, and discrete time equations to compute the interval durations are described. An example is presented in which these methods are illustrated by applying them to a parallel-loaded resonant inverter with three equivalent circuits for its continuous mode of operation.

King, Roger J.↗
DOE OSTI2021

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic–inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI3)0.85(MAPbBr3)0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

36 MATERIALS SCIENCE↗
NASA NTRS1989

Computer-aided simulation study of photomultiplier tubes

A computer model that simulates the response of photomultiplier tubes (PMTs) and the associated voltage divider circuit is developed. An equivalent circuit that approximates the operation of the device is derived and then used to develop a computer simulation of the PMT. Simulation results are presented and discussed.

Zaghloul, Mona E.↗
NASA NTRS1996

Design of a Mode Conversion Ultrasonic Motor for Position Control

The many useful characteristics of ultrasonic motors, such as high holding torques, and high torque at low speeds, have made them the subject of increasing interest. In addition, several of their characteristics make them attractive for aerospace applications: they have a torque to weight ratio, and they require neither gearing mechanisms nor lubrication. Moreover, they create negligible magnetic fields, and conversely, they are not affected by external magnetic fields. Ultrasonic motors based on bolt-tightened structures offer simplicity and high stress capability. They use the inverse piezoelectric effect in the stator to produce vibrational energy, which is transferred to the rotor by friction. We designed a bolt-tightened ultrasonic motor using numerical modelling tools (finite element and electromechanical circuit analyses), creating an equivalent circuit model that takes into account the electromechanical energy conversion in the stator and the contact between the stator and the rotor. Analysis of the circuit gives insight into the behavior of the motor and allows its performance to be calculated. Two prototypes of the motor were built; their transient responses and other quantities, such as starting torque, were measured. In this paper, we discuss the numerical and the experimental results, and demonstrate the usefulness of numerical analysis in designing ultrasonic motors and estimating their performance.

LeLetty, Ronan↗
DOE OSTI2021

Modeling and Simulation of Power-Electronic Inverters in Analog Electronic Circuit Simulators

This paper demonstrates how equivalent-circuit representations of grid-following power-electronic inverters can be realized within a SPICE-based development environment using common circuit components and VerilogA code. This facilitates computationally lean simulations of inverter networks leveraging the strengths of SPICE in large-scale simulations. We validate the approach with time-domain simulations for a modified version of the IEEE standard 118-bus system modeled in Virtuoso (a SPICE-based solver). Simulation results are compared-focusing on accuracy and computation speed-with results from a commonly used power-electronics simulation package. We note a significant decrease in simulation time with comparable signal resolution when simulating the network in SPICE.

Billmeyer, Ryan↗