Willem Perold

Willem Perold
Stellenbosch University | SUN · Department of Electrical & Electronic Engineering

PhD

About

94
Publications
23,174
Reads
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412
Citations
Citations since 2016
22 Research Items
209 Citations
20162017201820192020202120220102030405060
20162017201820192020202120220102030405060
20162017201820192020202120220102030405060
20162017201820192020202120220102030405060
Introduction
Skills and Expertise

Publications

Publications (94)
Article
In this study, we investigated the usage of Fourier-transform near-infrared (FT-NIR) spectroscopy as a fast, non-destructive method. FT-NIR spectroscopy was used over a spectral range of 800-2500 nm to develop multivariate prediction models for physical, chemical, and phytochemical parameters of dried pomegranate arils ('Wonderful'). Results from t...
Article
Soaring demand for high-quality pomegranate seed oil because of the health benefits associated with its consumption has gained global attention. The assessment of quality characteristics associated with pomegranate oil is known to be expensive, labor intensive and impractical on a commercial scale. Alternative ways of assessing quality attributes o...
Article
Full-text available
The pomegranate kernel oil has gained global awareness due to the health benefits associated with its consumption; these benefits have been attributed to its unique fatty acid composition. For quality control of edible fats and oils, various analytical and calorimetric methods are often used, however, these methods are expensive, labor-intensive, a...
Article
The large-scale surface modification of polyethylene terephthalate track-etched membranes (TM) using industrial roll-to-roll planar magnetron sputtering of Ti and TiO2 for use in advanced technologies such as separation, biotechnology, and microelectronics was investigated. The Ti and TiO2-Ti coated TM surface characterisation was done using a comb...
Article
Full-text available
This review covers recent developments in the field of non-invasive techniques for the quality assessment of processed horticultural products over the past decade. The concept of quality and various quality characteristics related to evaluating processed horticultural products are detailed. A brief overview of non-invasive methods, including spectr...
Article
In this work, thermally stimulated spectroscopy methods are applied to PET films to analyse the electret state. The thermally stimulated discharge currents, or thermally stimulated depolarization of track-etched like membrane samples obtained at different technological stages of their manufacture, have been investigated. The analysis of thermally s...
Article
Full-text available
Bruise damage is a very commonly occurring defect in apple fruit which facilitates disease occurrence and spread, leads to fruit deterioration and can greatly contribute to postharvest loss. The detection of bruises at their earliest stage of development can be advantageous for screening purposes. An experiment to induce soft bruises in Golden Deli...
Article
This study investigates the surface modification of polyethylene terephthalate track-etched membranes by planar magnetron sputtering of titanium and titanium dioxide thin films to improve its hydrophilicity and photocatalytic activity. After cold plasma treatment to enhanced adhesion, Ti thin films were deposited by planar magnetron sputtering and...
Article
Highlights Measurements of relaxation times in intact banana at micro-Tesla field was achieved. Bulk spin-spin relaxation time highly correlated with best descriptors of banana ripening. A basis for quasi-continuous distribution of spin-spin relaxation in banana was given. Abstract . Achieving fast, low-cost, and non-destructive internal quality t...
Article
Highlights In the Emission Head (EH) configuration differences in apple bruise severity were well captured. A good representation of new samples variability, in calibration, ensured robust quantitative PLS-DA models. EH mode with PLS-DA is an attractive spectroscopic option for inline apple sorting based on bruise damage. Abstract. Bruise damage...
Article
Resistive sensing techniques have recently emerged as a powerful tool to sensitively detect proteins of interest. However, detecting intracellular proteins present in cell-derived lysates remains largely unexplored. Here, a resistive sensing technique was utilized to develop a biosensor for the detection of the autophagy-related LC3 protein using t...
Article
Spectrometric methods based on near infrared radiation (NIR) are commonly used effectively in the agricultural and food industry. However, these methods still face limitations whereby meeting requirements for application such as nondestructive quality testing of large fruits and automated sorting and grading is still a challenge. A Fourier transfor...
Article
Chemical and microbial contamination of natural water supplies is one of the biggest challenges facing developing nations, especially communities in rural areas. An estimated 1.6 million children die each year due to water related deaths. Low-cost, portable and point-of-use biosensors are required to monitor water quality and improve living standar...
Article
Near infrared spectroscopy (NIRS) has become a versatile tool for probing the properties of biomaterials such as fruit. Employing chemometric methods has allowed the effective use of NIRS to study complex materials including foodstuff and horticultural products. However, the complexity of the NIR spectral information may require that selective meth...
Article
Full-text available
Polyclonal antibodies against Escherichia coli and fluorescent, secondary, antibodies were immobilized on borosilicate glass fibers pre-treated with 3-glycidyloxypropyl trimethoxysilane (GPS). Light with an average wavelength of 627 nm, emitted by a diode placed at one end of the glass fiber, was detected by an ultrasensitive photodiode with peak s...
Article
Full-text available
The supply of safe potable water, free from pathogens and chemicals, requires routine analyses and the application of several diagnostic techniques. Apart from being expensive, many of the detection methods require trained personnel and are often time-consuming. With drastic climate changes, severe droughts, increases in population and pollution of...
Patent
A system and method for the detection and quantification of biomolecules by measuring a piezoelectric signal is described. The system comprises a plurality of elongate zinc oxide nanowires mounted generally parallel to each another on a semi conductive silicon substrate. The free ends of the nanowires are provided with biomolecules that are capable...
Article
In microfiltration, caking is a major problem. Organic molecules get absorbed on the track-etched membrane TM surface during water purification. This leads to a loss of efficiency and changes in TM selectivity. A solution devised to solve this problem is the creation of self-cleaning, low-absorptive TM coatings. The TM surface was modified by depos...
Article
The ensemble Monte Carlo particle simulation technique is used to determine the upper operational frequency limit of the transferred electron mechanism in bulk GaAs and GaN empirically. This mechanism manifests as a decrease in the average velocity of the electrons in the bulk material with an increase in the electric field bias, which yields the c...
Article
An analytical model is proposed for the calculation of the polarization fields of point-to-plane electrospraying geometries. The electrospraying process is limited by corona discharges, due to these highly inhomogeneous electric fields generated by point-to-plane geometries. The results of this work agree with an earlier obtained model, which was d...
Article
Zinc Oxide (ZnO) nanowires were produced by using the seed layer deposition, RF cylindrical magnetron sputtering and sol-gel spin coating techniques. The ZnO seed layer films were characterized by atomic force microscopy (AFM) and ellipsometry. The ZnO nanowires were characterized by scanning electron microscopy (SEM), energy dispersive spectroscop...
Article
Lysozyme was covalently immobilized onto self-assembled monolayer (SAM) 3-mercaptopropanoic acid, linked to hydrothermally synthesized ZnO nanowires. Immobilization was confirmed by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and fluorescence microscopy. Adherence of antibodies to immobilized lysozyme ZnO nanowire...
Conference Paper
Full-text available
Superconducting Quantum Interference Devices (SQUIDs) have made the detection of low-field (LF) and ultra-low field nuclear magnetic resonance (ULF-NMR) a reality. The latter has been proven to be a potential tool for non-destructive quality testing of horticultural products, amongst many other applications. High-Temperature Superconductor (HTS) dc...
Article
The manufacturing process of a zinc oxide piezoelectric nanogenerator is discussed in detail. The manufacturing of the nanogenerator on solid and flexible substrate is discussed. It is shown that the choice of substrate greatly affects the output voltage of the nanogenerator. Two main reasons for the difference in output voltage of the different na...
Article
Zinc oxide (ZnO) nanowires were synthesised through two different methods; the vapour liquid solid (VLS) method and an aqueous solution method. Each method had its own set of parameters and changes in those parameters influenced the output voltage of a nanogenerator. Changes in output voltage occurred as a result of variations in the resistance and...
Article
Full-text available
Different techniques to increase the output voltage of a zinc oxide (ZnO) nanogenerator are investigated. As a first optimization step, the ZnO nanowires were annealed in different gases after growth, and this led to a threefold increase in the output voltage. The effect of the different gases on the ZnO nanowires, and hence on the output voltage,...
Article
Full-text available
Three different output circuits for a zinc oxide (ZnO) nanogenerator are investigated. A full-bridge rectifier, synchronized switch harvesting on inductor (SSHI) and the LTC3588 IC are investigated through simulations and practical experimentation. A nanogenerator supplying a maximum of 5 V at 1.25 μA is used for practical tests. The full-bridge re...
Conference Paper
A nanogenerator is developed using zinc oxide (ZnO) nanowires. Design of experiments is used to optimise the growth parameters of the ZnO nanowires, with an increase in output voltage set as the optimising parameter. After the optimisation of the nanowire growth, focus is shifted to the optimisation of the manufacturing of the nanogenerator. The na...
Conference Paper
Full-text available
Background: External and internal quality is an important aspect in the fruit industry. In recent years, consumers have become increasingly aware of the importance of quality. As a result of socioeconomic changes, the concept of quality is continuously evolving. Therefore, there is an increasing need in the main fruit producing countries to rapidly...
Article
Full-text available
A novel method to measure the output voltage of a zinc oxide nanowire nanogenerator is proposed. Various tests are performed to verify that the output voltage does indeed originate from the nanogenerator and not from environmental noise. Although noise does influence the output voltage measurements, the output voltage is easily distinguishable from...
Article
Full-text available
The output voltage of a zinc oxide (ZnO) nanogenerator is investigated. Different growth methods are used to synthesis the ZnO nanowires that are used to manufacture the nanogenerator. The influence of various growth parameters on the output voltage of the nanogenerator is also investigated. It was found that the aqueous solution method yields a sl...
Article
The critical current (Ic) of YBa2Cu3O7-x (YBCO) AFM plough micro-constrictions is measured as a function of temperature, width and the magnetic flux density (B), which was applied perpendicular to the YBCO ab-plane and surface of the bridges. C-axis oriented thin films of YBa2Cu3O7-x were deposited on MgO substrates using an inverted cylindrical ma...
Article
Using atomic force microscope (AFM) as a lithography tool, we ploughed (a) constant thickness nanoconstriction ( ∼ 492 nm) and (b) variable thickness bridges ( ∼ 25 to 50 nm) on YBa2Cu3O7−x thin films. We measured I-V characteristics and observed Shapiro steps, giving positive evidence of Josephson effect in these nanobridges. Further the linear te...
Article
Full-text available
The RF impedance of nanocrystalline Ni (nc-Ni), and oxygen stabilized Ni (OS-Ni), nano crystalline powders are measured at 1 MHz as a function of temperature. Nanocrystalline Ni particles are obtained by ball milling technique while OS-Ni particles are synthesized via a chemical route and followed by annealing at 400 °C for 1 hour. The impedance of...
Article
Measuring pressure with zinc oxide (ZnO) nanowires was investigated. ZnO exhibits the piezoelectric effect, generating a voltage when pressure is applied to the material. This relationship was used to make a pressure sensor. Simulations with specialized software were conducted to get a better understanding of how the ZnO nanowires would work. A Vap...
Article
Full-text available
The effect of applying a high electric field in-situ during the deposition and growth of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> (YBCO) thin films is investigated. The YBCO films are deposited from a stoichiometric YBCO target by pulsed laser deposition (PLD) on (100) MgO single crystal substrates at 726degC, in an oxygen atmosphere. The elect...
Article
Full-text available
Using an atomic force microscope (AFM), we successfully ploughed micron size planar constriction type junctions on YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> thin films. The 100 nanometer (nm) thin films are deposited on MgO substrates by an inverted cylindrical magnetron (ICM) sputtering technique. The films are then patterned into 8-10 micron s...
Article
The design and simulation of a complementary output switching logic (COSL) compatible, voltage state, switching logic comparator (SLC), and a flash analog-to-digital converter (ADC) for high-speed applications, with multigigahertz input bandwidth, is presented. A detailed design for the SLC is provided and verified with simulations. The comparator...
Article
Full-text available
A novel high-Tc superconducting (HTS) buffered step-edge Josephson junction is fabricated. A 250 nm PrBa2Cu3O7 (PBCO) layer was epitaxially grown on a (001) MgO substrate by PLD, which acts as a buffered template for a step-edge. Argon-ion milling was used to obtain a step-edge with a step angle of 25°. The step-edge is analysed in terms of an exte...
Article
Full-text available
Taking advantage of a New Wave 213 nm laser system, equipped with high-precision submicron resolution sample mosaic navigation, we are able to laser-etch and fabricate planar submicron-size Josephson junctions on YBa2Cu3O7−x thin films successfully. The entire process of laser etching and sample navigation is software controlled. The widths were me...
Article
Full-text available
Utilizing atomic force microscope (AFM) with a diamond tip, we were able to successfully plough nano-constrictions on epitaxially grown YBa2Cu3O(7-x) thin films deposited on MgO substrates. The thickness, width, and length of the obtained constrictions were in the range of a few 100 nm. Furthermore, we managed to produce a new S-type constriction,...
Article
Ultra-high speed microprocessors, clocked at hundreds of Gigahertz, remain the dreams of processor designers and consumers alike. However, such processors would require designs and clocking schemes that differ radically from that of current processors. This paper discusses superconductive asynchronous processors as one alternative to mainstream pro...
Article
The elevated operating temperatures of high-Tc superconductor (HTS) rapid single flux quantum (RSFQ) logic cells lead to higher thermal noise and this can be a major limiting factor in the development of working circuits of this kind. Determining the thermally induced bit error probability (BEP) of a circuit is therefore extremely important. If the...
Article
Rapid Single Flux Quantum (RSFQ) logic is a digital circuit technology that in recent years has presented itself as an alternative to semiconductors in the application of ultra high speed, very low power applications. The optimal timing of digital circuits operating at hundreds of Gigahertz is still a complex problem for both RSFQ and semiconductor...
Article
A new RSFQ latch, the DC-Resettable Latch (DCRL), is introduced. The DCRL functions like a standard nondestructive readout register (NDRO), except that it requires a dc current or current pulse to reset. This allows the DCRL to be used as a memory element in large memory blocks, where the entire block can be erased with a current pulse from a singl...
Article
Full-text available
Manufacturing tolerances influence circuit parameters, and inductance is no exception. A computer application was developed to fully automate inductance calculation as part of a layout extraction suite. InductEx takes a GDSII layout file as input, finds the inductance ports, extracts structures, applies mask-to-wafer offsets and random process tole...
Article
Asynchronous timing is very important in the design of large-scale ultra-high speed superconducting digital electronic (SDE) circuits. This paper presents an optimized asynchronous self-timing scheme that will simplify and speed up the design of large digital circuits. Rapid Single Flux Quantum Asynchronous Timing (RSFQ-AT) is compared to other tim...
Article
Although published research in COSL has stagnated, it remains a useful logic family for interfacing the much faster RSFQ family to hot-logic circuits. COSL has always been plagued by the absence of latches, and previous attempts at implementing such latches were clumsy. A new COSL Set-Reset flip-flop is discussed here, and results shown. This async...
Article
Currently, when a new fabrication process becomes available, all logic cells have to be re-designed, re-optimized and laid out from scratch. This is a manual and time intensive process, which hampers the migration of RSFQ circuits to better fabrication processes. The paper investigates the parameters that are required to specify logic gates that ar...
Article
Over the last decade, Monte Carlo simulations have emerged as the most useful way of predicting the yield of RSFQ circuits, as they consider all manufacturing tolerance effects on a circuit, and are not restricted to bias current variations. Here we finally present a comprehensive definition of layout-extracted Monte Carlo model creation for lumped...
Conference Paper
Rapid single flux quantum (RSFQ) logic is a digital circuit technology that in recent years has presented itself as an alternative to semiconductors in the application of ultra high speed, very low power applications. The optimal timing of digital circuits operating at hundreds of GHz is still a complex problem for both RSFQ and semiconductor techn...
Conference Paper
Full-text available
As the clock speeds of experimental digital electronic circuits increase beyond the measurement ranges of commercial oscilloscopes, the testing and verification thereof becomes a significant problem. Superconducting circuits add further complications, such as 4 Kelvin operating temperatures, output signal levels of around 300 muV, and the need for...
Article
Even simple superconducting logic gates can contain tens of inductors, resistors and Josephson junctions. In order to increase the yield and reliability of such suboptimal circuits to acceptable levels, it is often needed to adjust all the element values. The search space is therefore very large, and genetic algorithms have been used with remarkabl...
Article
Several programs exist that calculate inductance in line structures. Some can even handle corners and holes in the ground plane. However, real structures in superconducting electronics are often more complex, and more elaborate techniques are needed to provide a reliable estimate of inductance, especially over very short intra-gate distances. A tec...
Article
Novel logic devices in the RSFQ and COSL superconducting logic families are most often sub-optimal. Before such devices can be incorporated into physical designs, they have to be optimized for high theoretical yield, and preferably for highest possible yield. Even simple logic gates can contain numerous inductors, resistors and Josephson junctions....
Conference Paper
Even simple superconducting logic gates can contain tens of inductors, resistors and Josephson junctions. In order to increase the yield and reliability of such circuits to acceptable levels, it is often needed to adjust all the element values. The search space is therefore very large, and genetic algorithms have been used with remarkable success t...
Conference Paper
Determining the inductance of thin film superconducting structures accurately is extremely important in sub-Terahertz superconducting logic circuits. For numeric analysis, both the structure and ground plane need to be sufficiently segmented. The method of images significantly reduces the number of segments, but the selection of the position of the...
Article
A procedure is described where the mask dimensions of superconducting components are used in SPICE simulations to predict the performance of a device. Thickness tolerances of the fabrication process, as well as mask bias offsets and mask tolerances are included in the component models. This makes it possible to predict circuit yield by Monte Carlo...
Article
MC-PVM, an efficient parallel implementation of the Monte Carlo device simulation on a network of personal computers, is presented. A more efficient leap-frog derivative of the MC-PVM algorithm is also discussed. The network of personal computers has been employed into a parallel virtual machine (PVM). To demonstrate the viability of employing a ne...
Article
This paper describes the design, analysis and test results for a 4-bit pseudo-random bit-sequence generator (PRBSG) implemented with complementary output switching logic (COSL) gates. The PRBSG was optimized using a Monte Carlo simulation method for 10-GHz operation. The circuit has been fabricated using niobium technology with critical current den...
Conference Paper
A critical control system evaluation is presented of basic flux-locked loop systems. The development of a new superconducting quantum interference device (SQUID) linearization method is then described, where no magnetic flux feedback is necessary to cancel the applied flux. It is shown that a dual SQUID configuration will be able to produce a true...
Conference Paper
The simulation of existing and newly developed RSFQ components is discussed, and the results interpreted. These components represent data with picosecond voltage pulses. Amongst superconducting logic families, RSFQ promises to be the fastest. Theoretical operational limits are predicted as 300 GHz, 100 times faster than conventional semiconducting...
Conference Paper
We have designed and simulated a comparator, based on the quasi-one-junction superconducting quantum interference device (SQUID) and a non-symmetrical two-junction SQUID read-out system. This comparator can be utilized to construct an n-bit analog-to-digital convertor with only n comparators. The theoretical input bandwidth is in the GHz range
Conference Paper
The optimization of millimeter wave Gunn oscillators comprises two avenues, namely the use of heterojunctions for hot electron injection and multidomain operation Gunn diodes. A combination of these two techniques is proposed resulting in multiple hot electron launchers spaced throughout the diode. The results presented here suggest that this confi...
Conference Paper
Complementary output switching logic (COSL) is used to define building blocks for ultra-high speed (~20 GHz) logic circuits. The family consists of the standard logic functions OR, NOR, AND, NAND and XOR. The building blocks include a new single gate inverter and various latch functions. Changes in the layout of the COSL gates are proposed to impro...
Article
Full-text available
A computer model is used to simulate the DC performance of the Gray transistor. A procedure for the approximation of the recombination lifetime, the phonon pair-breaking time constant and the phonon escape time is described. The accuracy of the model is verified by comparison of the simulation results with measured values obtained by Gray (see Appl...
Article
A computer model is described which simulates the injection and detection of quasiparticles in superconducting tunnel barriers. The model makes provision for non-equilibrium phenomena such as superconductor band-gap suppression and the transport of excess quasiparticles in a superconductor. The model is a powerful tool in the study of superconducto...
Conference Paper
Full-text available
A short tutorial on the Gunn diode is presented. The principles underlying Gunn oscillations are discussed briefly and illustrated by relevant simulations. The simulation of a typical Gunn diode in a cavity is also presented. In conclusion, the fabrication process of low-power Gunn diodes is discussed
Conference Paper
The development of a new superconducting logic family, complementary output switching logic (COSL), is described. It is described how the two main problems inhibiting superconducting operation at microwave frequencies, namely magnetic flux trapping and large variations of the nominal component values due to the fabrication process, are solved. A br...
Article
Full-text available
The authors have previously proposed a new superconducting voltage-state logic family called complementary output switching logic (COSL). This logic family has been designed using a Monte Carlo optimization process such that circuits have a high theoretical yield at 5-10 Gb/s clock speeds in spite of existing Josephson process variations. In the pr...