• Terahertz Characterisation of Lead-Free Dielectrics for Different Applications

      Zhang, Man; Zhang, Hangfeng; Jiang, Qinghui; Gao, Feng; Chen, Riqing; Zhang, Dou; Reece, Michael J.; Yang, Bin; Viola, Giuseppe; Yan, Haixue; et al. (American Chemical Society, 2021-11-02)
      In this Spotlight on Applications, we describe our recent progress on the terahertz (THz) characterization of linear and non-linear dielectrics for broadening their applications in different electrical devices. We begin with a discussion on the behavior of dielectrics over a broadband of frequencies and describe the main characteristics of ferroelectrics, as an important category of non-linear dielectrics. We then move on to look at the influence of point defects, porosity and interfaces, including grain boundaries and domain walls, on the dielectric properties at THz frequencies. Based on our studies on linear dielectrics, we show that THz characterization is able to probe the effect of porosities, point defects, shear planes and grain boundaries to improve dielectric properties for telecommunication applications. Further, we demonstrate that THz measurements on relaxor ferroelectrics can be successfully used to study the reversibility of the electric field-induced phase transitions, providing guidance for improving their energy storage efficiency in capacitors. Finally, we show that THz characterization can be used to characterize the effect of domain walls in ferroelectrics. In particular, our studies indicate that the dipoles located within domain walls provide a lower contribution to the permittivity at THz frequencies than the dipoles present in domains. The new findings could help develop a new memory device based on non-destructive reading operations using a THz beam.
    • Terahertz Characterisation of UV Offset Lithographically Printed Electronic-Ink

      Zeng, Yang; Edwards, Marc R.; Stevens, Robert; Bowen, John; Donnan, Robert S.; Yang, Bin; University of London; National University of Defense Technology; University of Chester; Nottingham Trent University; University of Reading (Elsevier, 2017-06-10)
      Inkjet-printed electronics are showing promising potential in practical applications, but methods for real-time, non-contact monitoring of printing quality are lacking. This work explores Terahertz (THz) sensing as an approach for such monitoring. It is demonstrated that alterations in the localised dielectric characteristics of inkjet-printed electronics can be qualitatively distinguished using quasi-optically-based, sub-THz reflection spectroscopy. Decreased reflection coefficients caused by the sintering process are observed and quantified. Using THz near-field scanning imaging, it is shown that sintering produces a more uniform spatial distribution of permittivity in the printed carbon patterns. Images generated using THz-TDS based imaging are presented, demonstrating the combination of high resolution imaging with quantification of complex permittivities. This work, for the first time, demonstrates the feasibility of quality control in printed electronic-ink with THz sensing, and is of practical significance to the development of in-situ and non-contact commercial-quality characterisation methods for inkjet-printed electronics.
    • Terahertz Probing Irreversible Phase Transitions Related to Polar Clusters in Bi0.5Na0.5TiO3-based Ferroelectric

      Yang, Bin; University of Chester (Wiley-VCH, 2020-02-16)
      Electric-field-induced phase transitions in Bi0.5Na0.5TiO3 (BNT)-based relaxor ferroelectrics are essential to the controlling of their electrical properties and consequently in revolutionizing their dielectric and piezoelectric applications. However, the fundamental understanding of these transitions is a long-standing challenge due to their complex crystal structures. Given the structural inhomogeneity at the nanoscale or sub-nanoscale in these materials, dielectric response characterization based on terahertz (THz) electromagnetic-probe beam-fields, is intrinsically coordinated to lattice dynamics during DC-biased poling cycles. The complex permittivity reveals the field-induced phase transitions to be irreversible. This profoundly counters the claim of reversibility, the conventional support for which, is based upon the peak that is manifest in each of four quadrants of the current-field curves. The mechanism of this irreversibility is solely attributed to polar clusters in the transformed lattices. These represent an extrinsic factor which is quiescent in the THz spectral domain.
    • Terahertz probing of low temperature degradation in zirconia bioceramics

      Ahmed, Shafique; Zhang, Man; Koval, Vladimir; Zou, Lifong; Shen, Zhijian; Chen, Riqing; Yang, Bin; Yan, Haixue; Queen Mary University of London; Mehran University of Engineering and Technology; Institute of Materials Research; Stockholm University; Fujian Agriculture and Forestry University; University of Chester (Wiley, 2021-09-19)
      ZrO2 based ceramics are widely used in biomedical applications due to its colour, biocompatibility, and excellent mechanical properties. However, low temperature degradation (LTD) introduces a potential risk for long-term reliability of these materials. The development of innovative non-destructive techniques, which can explore LTD in zirconia-derived compounds, is strongly required. Yttria stabilised zirconia, 3Y-TZP, is one of the well-developed ZrO2 based ceramics with the improved resistance to LTD for dental crown and implant applications. Here, 3Y-TZP ceramic powders were pressed and sintered to study the LTD phenomenon by phase transition behaviour. The LTD driven tetragonal-to-monoclinic phase transition was confirmed by XRD. XPS analysis demonstrated the LTD induced the reduction of oxygen vacancies to support these findings. It is proved that after the degradation the 3Y-TZP ceramics show the decreased dielectric permittivity at terahertz frequencies due to the crystallographic phase transformation. Terahertz non-destructive probe is a promising method to investigate LTD in zirconia ceramics.
    • Terahertz reading of ferroelectric domain wall dielectric switching

      Zhang, Man; Chen, Zhe; Yue, Yajun; Chen, Tao; Yan, Zhongna; Jiang, Qinghui; Yang, Bin; Eriksson, Mirva; Tang, Jianhua; Zhang, Dou; et al. (American Chemical Society, 2021-03-08)
      Ferroelectric domain walls (DWs) are important nano scale interfaces between two domains. It is widely accepted that ferroelectric domain walls work idly at terahertz (THz) frequencies, consequently discouraging efforts to engineer the domain walls to create new applications that utilise THz radiation. However, the present work clearly demonstrates the activity of domain walls at THz frequencies in a lead free Aurivillius phase ferroelectric ceramic, Ca0.99Rb0.005Ce0.005Bi2Nb2O9, examined using THz time domain spectroscopy (THz-TDS). The dynamics of domain walls are different at kHz and THz frequencies. At low frequencies, domain walls work as a group to increase dielectric permittivity. At THz frequencies, the defective nature of domain walls serves to lower the overall dielectric permittivity. This is evidenced by higher dielectric permittivity in the THz band after poling, reflecting decreased domain wall density. An elastic vibrational model has also been used to verify that a single frustrated dipole in a domain wall represents a weaker contribution to the permittivity than its counterpart within a domain. The work represents a fundamental breakthrough in understanding dielectric contributions of domain walls at THz frequencies. It also demonstrates that THz probing can be used to read domain wall dielectric switching.
    • Terahertz Signatures of Hydrate Formation in Alkali Halide Solutions

      Ligang, Chen; Ren, Guanhua; Liu, Liyuan; Guo, Pan; Wan, Endong; Zhou, Lu; Zhu, Zhonglie; Zhang, Jianbing; Yang, Bin; Zhang, Wentao; et al.
      We systematically studied the ability of 20 alkali halides to form solid hydrates in the frozen state from their aqueous solutions by terahertz time-domain spectroscopy combined with density functional theory (DFT) calculations. We experimentally observed the rise of new terahertz absorption peaks in the spectral range of 0.3-3.5 THz in frozen alkali halide solutions. The DFT calculations prove that the rise of observed new peaks in solutions containing Li+, Na+ or F‾ ions indicates the formation of salt hydrates, while that in other alkali halide solutions is caused by the splitting phonon modes of the imperfectly crystallized salts in ice. As a simple empirical rule, the correlation between the terahertz signatures and the ability of 20 alkali halides to form a hydrate has been established.
    • Titanium Dioxide Engineered for Near-dispersionless High Terahertz Permittivity and Ultra-low-loss

      Chuying, Yu; Zeng, Yang; Yang, Bin; Donnan, Robert S.; Huang, Jinbao; Xiong, Zhaoxian; Mahajan, Amit; Shi, Baogui; Ye, Haitao; Binions, Russell; et al. (Nature Publishing Group, 2017-07-26)
      Realising engineering ceramics to serve as substrate materials in high-performance terahertz(THz) that are low-cost, have low dielectric loss and near-dispersionless broadband, high permittivity, is exceedingly demanding. Such substrates are deployed in, for example, integrated circuits for synthesizing and converting nonplanar and 3D structures into planar forms. The Rutile form of titanium dioxide (TiO2) has been widely accepted as commercially economical candidate substrate that meets demands for both low-loss and high permittivities at sub-THz bands. However, the relationship between its mechanisms of dielectric response to the microstructure have never been systematically investigated in order to engineer ultra-low dielectric-loss and high value, dispersionless permittivities. Here we show TiO2 THz dielectrics with high permittivity (ca. 102.30) and ultra-low loss (ca. 0.0042). These were prepared by insight gleaned from a broad use of materials characterisation methods to successfully engineer porosities, second phase, crystallography shear-planes and oxygen vacancies during sintering. The dielectric loss achieved here is not only with negligible dispersion over 0.2 - 0.8 THz, but also has the lowest value measured for known high-permittivity dielectrics. We expect the insight afforded by this study will underpin the development of subwavelength-scale, planar integrated circuits, compact high Q-resonators and broadband, slow-light devices in the THz band.
    • Ultrafast Electric Field-induced Phase Transition in Bulk Bi0.5Na0.5TiO3 under High Intensity Terahertz Irradiation

      Yang, Bin; Zhang, Man; McKinnon, Ruth A.; Viola, Giuseppe; Zhang, Dou; Reece, Michael J.; Abrahams, Isaac; Yan, Haixue; University of Chester; Queen Mary University of London; Central South University (American Chemical Society, 2020-12-22)
      Ultrafast polarization switching is being considered for the next generation of ferroelectric based devices. Recently, the dynamics of the field-induced transitions associated with this switching have been difficult to explore, due to technological limitations. The advent of terahertz (THz) technology has now allowed for the study of these dynamic processes on the picosecond (ps) scale. In this paper, intense terahertz (THz) pulses were used as a high-frequency electric field to investigate ultrafast switching in the relaxor ferroelectric, Bi0.5Na0.5TiO3. Transient atomic-scale responses, which were evident as changes in reflectivity, were captured by THz probing. The high energy THz pulses induce an increase in reflectivity, associated with an ultrafast field-induced phase transition from a weakly polar phase (Cc) to a strongly polar phase (R3c) within 20 ps at 200 K. This phase transition was confirmed using X-ray powder diffraction and by electrical measurements which showed a decrease in the frequency dispersion of relative permittivity at low frequencies.
    • Verification of calculation code THERM in accordance with BS EN ISO 10077-2

      Nammi, Sathish K.; Shirvani, Hassan; Shirvani, Ayoub; Edwards, Gerard; Whitty, Justin P. M.; Anglia Ruskin University, Anglia Ruskin University, Anglia Ruskin University, University of Chester, University of Central Lancashire (Anglia Ruskin Research Online, 2014)
      Calculation codes are useful in predicting the heat transfer features in the fenestration industry. THERM is a finite element analysis based code, which can be used to compute thermal transmittance of windows, doors and shutters. It is important to verify results of THERM as per BS EN ISO 10077-2 to meet the compliance requirements. In this report, two-dimensional thermal conductance parameters were computed. Three versions of THERM, 5.2, 6.3 and 7.1, were used at two successive finite element mesh densities to assess their comparability. The results were all compliant with the aforementioned British Standard.
    • Visual-Inertial 2D Feature Tracking based on an Affine Photometric Model

      Aufderheide, Dominik; Edwards, Gerard; Krybus, Werner; South Westphalia University of Applied Sciences, University of Chester, South Westphalia University of Applied Sciences (Springer, 2015-04-08)
      The robust tracking of point features throughout an image sequence is one fundamental stage in many different computer vision algorithms (e.g. visual modelling, object tracking, etc.). In most cases, this tracking is realised by means of a feature detection step and then a subsequent re-identification of the same feature point, based on some variant of a template matching algorithm. Without any auxiliary knowledge about the movement of the camera, actual tracking techniques are only robust for relatively moderate frame-to-frame feature displacements. This paper presents a framework for a visual-inertial feature tracking scheme, where images and measurements of an inertial measurement unit (IMU) are fused in order to allow a wider range of camera movements. The inertial measurements are used to estimate the visual appearance of a feature’s local neighbourhood based on a affine photometric warping model.
    • Volatile Liquid Detection by Terahertz Technologies

      Baxter, Harry W.; Worrall, Adam A.; Pang, Jie; Chen, Riqing; Yang, Bin; University of Chester; Fujian Agriculture and Forestry University (Frontiers Media, 2021-04-08)
      The prospect of being able to move through security without the inconvenience of separating liquids from bags is an exciting one for passengers, and there are important operational benefits for airports as well. Here, two terahertz (THz) systems, 100 GHz sub-THz line scanner and attenuation total reflection-based THz time domain spectroscopy (TDS), have been used to demonstrate the capability of identifying different liquid samples. Liquid samples’ THz complex permittivities are measured and their differences have contributed to the variation of 100 GHz 2D images of volatile liquids with different volumes inside of cannister bottles. The acquired attenuation images at 100 GHz can easily be used to distinguish highly absorbed liquids (Water, Ethanol, Fuel Treatment Chemicals) and low loss liquids (Petrol, Diesel, Kerosene and Universal Bottle Cleaner). The results give a promising feasibility for mm-wave imager and THz spectroscopy to efficiently identify different volatile liquids.
    • Wasserstein GAN based Chest X-Ray Dataset Augmentation for Deep Learning Models: COVID-19 Detection Use-Case

      Hussain, B. Zahid; Andleeb, Ifrah; Ansari, Mohammad Samar; Joshi, Amit Mahesh; Kanwal, Nadia; Aligarh Muslim University; University of Chester; Malaviya National Institute of Technology Jaipur; Keele University (IEEE, 2022-09-08)
      The novel coronavirus infection (COVID-19) is still continuing to be a concern for the entire globe. Since early detection of COVID-19 is of particular importance, there have been multiple research efforts to supplement the current standard RT-PCR tests. Several deep learning models, with varying effectiveness, using Chest X-Ray images for such diagnosis have also been proposed. While some of the models are quite promising, there still remains a dearth of training data for such deep learning models. The present paper attempts to provide a viable solution to the problem of data deficiency in COVID-19 CXR images. We show that the use of a Wasserstein Generative Adversarial Network (WGAN) could lead to an effective and lightweight solution. It is demonstrated that the WGAN generated images are at par with the original images using inference tests on an already proposed COVID-19 detection model.