Search

A series of Zn 1- x Cu x ZrNb 2 O 8 (x = 0.000, 0.003, 0.010, and 0.050) ceramics with low dielectric loss and intermediate dielectric constants were synthesized by a conventional solid-phase reaction method. The terahertz dielectric performances of Zn 1- x Cu x ZrNb 2 O 8 (x = 0.000, 0.003, 0.010, and 0.050) ceramics have been investigated by using the Rietveld refinement method, scanning electron microscopy, Raman spectroscopy, and terahertz time-domain spectroscopy. The microscopic morphology analysis shows that the appropriate amount of Cu2+ content can make the specimen have the most uniform and medium-sized grains, which can minimize the dielectric loss in the terahertz range. Raman mapping reveals that the stress distribution is not uniform in different grains or in different regions of the same grain. The internal stresses affect the interatomic distances and influence the electronic properties, thus increasing the lattice anharmonicity and dielectric losses. In addition, the dielectric constant of the specimen in the terahertz range is mainly determined by the ionic polarization. For Zn 0.997 Cu 0.003 ZrNb 2 O 8 sintered at 1240 °C, low dielectric losses (< 4 × 10−3 @ 0.40 THz) and intermediate dielectric constant (∼22.95 @ 0.40 THz) were obtained, which make it promising for applications in terahertz dielectric filters. [ABSTRACT FROM AUTHOR]

ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics were synthesized using a conventional solid phase reaction technique. The terahertz dielectric performances of ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics have been investigated using Rietveld refinement, scanning electron microscopy, first principles, and terahertz time-domain spectroscopy. XRD and mapping data indicate that the replacement of Nb5+ ions with V5+ ions induces variations in inter-ion interactions. Combined analysis with scanning electron microscopy indicated that the authigenic low melting point phase (ZnV 2 O 6) had a positive effect on the densification of the specimen (ZnNb 1.8 V 0.1 O 5.75) at low temperatures. However, more low melting point phases (ZnV 2 O 6 and Zn 2 V 2 O 7) adversely affect the densification and terahertz dielectric performances of the specimens as the V5+ content increases. In addition, the dielectric constant of the specimen is mainly affected by the ionicity of the Nb–O bond, while the dielectric loss is jointly affected by the covalency of the Nb–O bond and the grain size. The developed materials are expected to be used in LTCC terahertz application due to the low sintering temperature (950 °C) and excellent dielectric performance (dielectric constant of ∼20 and dielectric loss of 0.02–0.04). [ABSTRACT FROM AUTHOR]

Microwave dielectric ceramics play a vital role in wireless communication systems. However, it is a great challenge to obtain ideal performances of single-phase ceramics. In this work, (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 3 (MoO 4) 9 (abbreviated as L5ZMO) high-entropy ceramics were successfully prepared by the solid-state reaction method, the pure trigonal L5ZMO structure can be synthesized in the mixture calcined at 700 °C. Rietveld refinement based on XRD data reveals that lattice parameters of L5ZMO ceramics are affected by the radius of the five rare earth elements, which indicates that the rare earth elements have been solubilized and randomly distributed in the Ln-site. The relative density illustrates a strong dependence on sintering temperature, which gradually increases with increasing sintering temperature. Based on the analysis of bond characteristics, the Mo–O bond exhibits a vital effect on the dielectric properties of L5ZMO ceramics. Besides, the infrared reflection spectrum and terahertz time-domain spectroscopy analysis show that ionic polarization is the dominant polarization in the microwave/terahertz bands. Meanwhile, the tan δ can be further reduced by optimizing process parameters. Notably, the L5ZMO high-entropy ceramics sintered at 750 °C exhibit outstanding dielectric properties of ε r = 10.46, Q · f = 59,713 GHz, and τ f = −21.94 ppm/°C, indicating that the high-entropy design optimizes τ f (−38.8 ppm/°C) of the simple La 2 Zr 3 (MoO 4) 9 ceramics. The attempt provides theoretical guidance for the application of high-entropy design concepts in microwave/terahertz bands. [ABSTRACT FROM AUTHOR]

In this work, spinel-structured MgAl 2- x (Zn 0.5 Mn 0.5) x O 4 (0 ≤ x ≤ 0.08) single-phase ceramics were prepared through a solid-state reaction route. The substitution of (Zn 0.5 Mn 0.5)3+ for Al3+ at the octahedral site affected the degree of inversion of A/B lattice sites, bond length/strength/valence, and covalency of metal-oxygen bond in the tetrahedron and hence microwave dielectric properties of MgAl 2 O 4. The variation in ε r and tanδ of ceramics is investigated in the millimeter wave-terahertz frequency band by combining infrared reflection spectrum and terahertz time-domain spectroscopy. A high Q × f value of 111,010 GHz @ 12.01 GHz, low ε r = 8.3, and slightly lower τ f = −60 ppm/°C is obtained for MgAl 1.98 Zn 0.01 Mn 0.01 O 4 ceramics, which is tuned by adding a small amount of SrTiO 3. The composite ceramics exhibited a near-zero τ f (2.8 ppm/°C), high Q × f (55,400 GHz @ 11.15 GHz), and low ε r (= 8.5), showing a great potential application prospect for 5G/6G wireless communication. [ABSTRACT FROM AUTHOR]

Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 (x = 0.1–0.5; R = Mg, Ca) ceramics were prepared via solid-state reaction method. The effects of Mg2+ (∼0.89 Å) and Ca2+ (∼1.12 Å) substitution at the dodecahedral site for Y3+ (∼1.019 Å) on the microstructure, crystal structure and microwave dielectric properties of Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 garnet ceramics were investigated and discussed using the P–V-L theory, Raman spectrum, bond valence theory, Terahertz time-domain spectroscopy and infrared reflectance spectrum. Different from normal garnet structure of ceramics, the dielectric constant is abnormally increased in Y 3- x Mg x Al (Oct)2 Al (Tet)3- x Si x O 12 (x = 0.1–0.5) ceramics due to the increase of ion polarizability caused by the rattling effect. There is a dependence of bond energy, bond valence and dodecahedral distortion on τ f value, which provides new possibility for tuning the temperature coefficient of garnet ceramics. There are strong effects of grain size distribution, average lattice energy, packing fraction, FWHM of Raman peak on their Q×f values of Y 3- x R x Al (Oct)2 Al (Tet)3- x Si x O 12 isostructural ceramics caused by the difference of Mg2+ and Ca2+ ionic radius. Above work provides deeper insights into the understanding of the microstructural characteristics and performance relationship of garnet-type microwave ceramics, and lays a foundation for future applications in the 5G/6G frequency bands. [ABSTRACT FROM AUTHOR]

Bidirectional mode-locked fiber lasers are able to deliver two outputs of pulses in the clockwise (CW) and counterclockwise (CCW) directions, having important applications in dual-comb spectroscopy, asynchronous optical sampling and terahertz time-domain spectroscopy. In this paper, we have experimentally demonstrated an all-fiber Er/Yb-doped bidirectional passively mode-locked Mamyshev oscillator (MO) without an external seed. Two stable mode-locked states with remarkable different characteristics are observed by adjusting the pump power. The transition process between the two mode-locked states shows chaotic behaviors, which are further investigated with numerical simulations based on the Ginzburg-Landau equation. The bidirectional MO proposed in this work is not only a good laser source, but also a good platform to study soliton dynamics, system bifurcation, and chaos in ultrafast fiber laser systems. [ABSTRACT FROM AUTHOR]

[Display omitted] The process analytical technology (PAT) framework is well established and integral to facilitate process understanding, enable a transition from batch to continuous manufacturing, and improve product quality. Near-infrared (NIR) spectroscopy has been established as a standard PAT tool for many process analytical challenges, including monitoring powder blend homogeneity. However, alternative technologies for monitoring powder blending are of interest due to the importance of the blending step in manufacturing solid oral dosage forms. Terahertz time-domain spectroscopy (THz-TDS) is therefore explored in this study as an alternative tool for monitoring blend homogeneity with the potential for endpoint control in a batch blending process. Powder blends of microcrystalline cellulose (MCC) and dibasic calcium phosphate dihydrate and blends of MCC and granulated α-lactose monohydrate were investigated non-invasively at various compositions using THz-TDS in transmission mode for acquiring spectra from samples enclosed in the blending container. It was found that attenuation- and phase-related parameters acquired with THz-TDS could reliably resolve physical changes related to the homogeneity of the blend. Further evaluations revealed that changes in the bulk density of the blend, in addition to the intrinsic optical properties of the materials, played a critical role in the observed trends for both systems. In contrast, the scattering contribution of the powder was mainly crucial for the attenuation-related parameter in blends with materials of high refractive indices. Finally, THz-TDS measurements were acquired throughout a blending process mimicking a continuous acquisition. The method could follow blending dynamics and resulted in reasonable predictive errors of the content of 0.5 – 2.5 %. Relative standard deviations for high content blends (20 %) were acceptable (3 – 7 %) whereas at low contents (5 %) significantly higher values (9 – 35 %) were found. Based on these findings, THz-TDS is a feasible PAT tool for monitoring blend homogeneity and controlling high content blend processes, although precision and accuracy is considered to improve with a more suitable interface. [ABSTRACT FROM AUTHOR]

Quartz fiber-reinforced polymer (QFRP) is a vital non-polar material used in aviation wave-transparent structural components. Automatic characterization of delamination defects in QFRP is critical to aviation structural component safety. Terahertz time-domain spectroscopy (THz-TDS) is one of the new non-destructive testing (NDT) methods with highly accurate characterization of internal defects in non-polar material. Hence, attempts to extract features of THz time-domain signals for automatic characterization have been made by using deep learning algorithms. In this work, a Transformer-based neural network to classify the THz time-domain signals collected from a QFRP curved structure for automatic characterization of pre-embedded delamination defects has been reported. A THz-TDS system combined with a collaborative robot for collecting the THz signals from QFRP curved structure has been built. An automatic characterization method framework is developed. Results show that the precision rates of Transformer-based neural network for 1 s t delamination to 5 t h delamination are 1.0, 1.0, 1.0, 0.985, 1.0, and F 1 score of it is 0.982. During the process of testing, delamination defects inside the QFRP curved structure were visualized using pixels with different colors. Results indicate that the Transformer-based neural network can characterize all pre-embedded delamination defects while minimizing false identification of non-defective areas, performing outstanding generalization. [ABSTRACT FROM AUTHOR]

Cables are one of the most critical load-bearing components in cable-supported bridges. The condition of the cables is essential to the scientific management and safe operation of bridges. In contrast to the surface defect, the internal defect of cables is hidden, and the damage caused by it is sudden and difficult to predict, which remains a critical and challenging aspect of bridge maintenance. The commonly used internal defect detection methods, such as the magnetic leakage method and the magnetostrictive guided wave method, have difficulties in accurately locating defects. It is desirable to develop a practical nondestructive testing method for bridge cables. Therefore, this study proposes a method based on terahertz time-domain spectroscopy (THz-TDS) technology and deep learning (DL) algorithms. Firstly, the propagation theory of terahertz waves in the cable is introduced. Then, the methodology of cable defect detection based on THz-TDS and DL is proposed, including the measurement of optical properties using the transmission mode and the detection of sheath thickness and internal corrosion using the reflection mode. Finally, a series of experiments are conducted to verify the proposed method. The results indicate that the proposed method can effectively identify and locate internal corrosion in the cable. In addition, the proposed method can measure the thickness of the sheath at any position of the cable with high accuracy, showing the potential to rapidly evaluate the uniformity of the sheath thickness. • A non-destructive method to measure cable sheath thickness is proposed. • THz-TDS technology is introduced for detecting internal cable corrosion for the first time. • Corrosion defects are identified by classifying terahertz signals. [ABSTRACT FROM AUTHOR]

• Plastic particle testing has been successfully conducted using a rapid and non-destructive approach. • By integrating LDA with THz spectral detection, there is an enhancement in the precision of substance identification. • We also conducted a comparative analysis between LDA and PCA in relation to the identification of data in higher dimensions. For fast and accurate non-destructive differentiation of plastic pellets with similar appearance during import and export inspections, as well as for quality control purposes. Terahertz technology offers a non-destructive analysis approach for samples from diverse fields. By integrating deep learning algorithms with terahertz time-domain spectroscopy (THz-TDS) and linear discriminant analysis (LDA), it is possible to establish a highly accurate terahertz spectroscopy qualitative identification model. Additionally, the incorporation of principal component analysis (PCA) enables the application of this model to higher dimensional data. Notable differences in absorption coefficient, phase difference, and refractive index are observed among different plastic particles. The implementation of this rapid, accurate, and non-destructive detection method can greatly facilitate the testing and identification of plastic particles in customs and quality inspection departments. [ABSTRACT FROM AUTHOR]

• Three distinct non-invasive inspection methods are used, including infrared thermography, terahertz time-domain spectroscopy, and air-coupled ultrasound. • YOLOv7, as an automatic defect detection deep learning network, is applied. • To obtain a sufficient dataset to train the network, the finite element analysis and finite-difference time-domain method are introduced. Due to fungal growth and mishandling in the book, there are various types of defects as they age such as foxing, tears, and creases. It is important to develop novel non-invasive inspection techniques and defect recognition algorithms. In this work, three non-invasive inspection techniques, including infrared thermography (IRT), terahertz time-domain spectroscopy (THz-TDS), and air-coupled ultrasound (ACU), were employed for the detection of defects in an ancient book cover. To improve the image quality and defect contrast, principal component analysis, fast Fourier transform, and partial least squares regression algorithms are used as the post-processing methods. Furthermore, the YOLOv7 network is deployed for defect automatic detection. Finite element analysis and finite-difference time-domain methods were employed for generating training dataset of YOLOv7 network. Experimental results demonstrate that IRT and THz-TDS has excellent detection capability for surface and subsurface defects, respectively. By employing YOLOv7 network with simulation datasets, defects can be effectively identified. [ABSTRACT FROM AUTHOR]

The effects of Sn4+ substitution on the structure–property relationship of Co 0.5 (Ti 1− x Sn x) 0.5 NbO 4 ceramics were studied systemically using the dielectric theory of complex crystals, and the intrinsic dielectric properties were examined. A rutile Co 0.5 (Ti 1− x Sn x) 0.5 NbO 4 solid solution was formed, and the properties were examined via X-ray diffraction and transmission electron microscopy. According to Rietveld refinement analysis, Sn4+ increased the axis length and unit-cell volume. The dielectric theory of complex crystals showed that the introduction of Sn4+ leads to a consistent variation between the average bond covalency value and dielectric constant, indicating a decline in dielectric polarizability. However, an evaluation of the lattice energy of chemical bonds revealed a decrease in the structural stability, with a concomitant increase in the dielectric loss. Moreover, far-infrared reflectivity analysis, complex dielectric function analysis, and Terahertz time-domain spectroscopy of the intrinsic dielectric properties of Co 0.5 (Ti 1− x Sn x) 0.5 NbO 4 (x = 0.2) ceramic revealed similar results between the theoretical fitting and experimental test. [ABSTRACT FROM AUTHOR]

[Display omitted] • EMI shielding efficiencies of ultrathin films made of low-density B, C, and N atoms have been measured in the THz range. • Maximum shielding efficiency of 30dB and 55dB observed at 1.72 THz for films with 4.5 nm and 6.5 nm thickness. • Ultrathin films of carbon-doped hexagonal BN on Cu substrates using Atmospheric Pressure Chemical Vapor deposition. • Carbon doping of BN films was achieved using intrinsic carbon impurities present in the Cu substrate. We have investigated the electromagnetic interference (EMI) shielding efficiency of carbon-doped hexagonal boron nitride (hBN) films in 0.3–1.8 THz regions using transmission-based terahertz time-domain spectroscopy (THz TDS). The hBN films were synthesized on Copper foils by atmospheric pressure chemical vapor deposition (APCVD) utilizing ammonia borane (AB) powder as a precursor. Carbon doping of BN films was achieved by using the intrinsic carbon dissolved in the Copper substrate. The EMI shielding efficiencies and the optical, dielectric, and electrical properties of the films were determined using THz TDS. The Absorbance is much greater than the Reflectance indicating that absorption is the dominant mechanism of shielding in these films. These carbon-doped hBN films show a high EMI shielding effectiveness, with 99.99 % EM radiation blocked. A maximum value of 30 dB and 55 dB at 1.72 THz were observed for films with 4.5 nm and 6.5 nm thicknesses. This is equivalent to shielding efficiency per unit thickness of 6667 dB/µm and 8460 dB/µm, which is among the highest reported for 2D materials. [ABSTRACT FROM AUTHOR]

Aminobutyric acid, a non-proteinogenic amino acid, is present in the human body and influences physiological functions, exhibiting promising pharmaceutical prospects. γ -Aminobutyric Acid (GABA) and its isomer, L-2-Aminobutyric Acid (L-AABA), exhibit unique characteristic absorption peaks in the 0.6 to 2.5 THz range. To elucidate the mechanisms behind these absorption peaks, this study employed Density Functional Theory (DFT) for theoretical simulations of the crystal cell models of L-AABA and GABA, achieving terahertz spectra consistent with experimental results. Additionally, the study utilized Vibrational Mode Automatic Relevance Determination (VMARD) and Interaction Region Indicator (IRI) methodologies to interpret the vibrational modes corresponding to different absorption peaks and to detail the locations and types of intermolecular interactions within the crystals. Research indicates that the combination of terahertz time-domain spectroscopy (THz-TDS) and theoretical calculations can serve as an effective method for identifying isomers of Aminobutyric Acid and detecting intermolecular weak interactions such as hydrogen bonds within these crystals. • Measurement of THz spectra and calculation of absorption coefficients. • Classification and description of intramolecular vibrational modes ssing the VMARD method. • Visualization of weak interactions within crystals based on periodic boundary conditions. [ABSTRACT FROM AUTHOR]

Fluidized bed drying (FBD), as a novel dehydration method, has been widely applied in drying various heat-sensitive high-value food products, in which the pulsed fluidized bed drying (PFBD) technique addresses the ineffective fluidized limitation of the traditional FBD when dealing with cohesive products. However, achieving in situ real-time monitoring of dynamic product moisture content (MC) change during both FBD and PFBD without sampling is still challenging. This paper presents an innovative integration of a whisk-broom terahertz time-domain spectroscopy (THz-TDS) imaging system with a fluidized bed dryer, facilitating in situ , real-time monitoring of MC reduction of products without process pausing for sampling. The reliability, and robustness of the established system and prediction models were tested by drying peppercorn and sweetcorn under various drying settings, including different temperatures, flow patterns, and durations. Implementing the convolutional neural networks (CNN) and long short-term memory (LSTM) for correlating terahertz time-domain image data with reference datasets in this research provided reliable MC reduction prediction results for the mixture dataset from peppercorn and sweetcorn drying. The successful application of this combined sensing and analytical method presents a promising approach for enhancing industrial processing control and expands the utility of THz-TDS in the agriculture and food industrial fields. • THz-TDS imaging system was integrated with FBD system for product MC loss prediction. • Time-series spectra were acquired by applying a whisk broom THz imaging scanner. • The system performance was validated by diverse drying settings and samples. • Proposed deep learning model is well suitable for the developed THz-FBD system. • Developed sensing method shows potential application for other bioprocesses control. [ABSTRACT FROM AUTHOR]

[Display omitted] • The analytical method for calculating the THz parameters of thin films is inadequate. • A numerical method to extract the THz optical parameters is elaborated. • Applications to water and hydrogel thin films in a "cuvette" set-up is presented. • THz-TDS gives insights into the hydration state of agarose hydrogels. • Codes used in this work are made freely available to promote the use of THz-TDS. Terahertz time-domain spectroscopy (THz-TDS) is an emerging optical technique that has potential applications in the characterization of (bio)materials. However, the complicated extraction of optical parameters from multi-layered and optically thin samples is a barrier towards its acceptance by applied scientists. Therefore, the aim of this work is to provide a straightforward approach for the extraction of the THz absorption coefficient and index of refraction profiles of aqueous thin films in a window-sample-window configuration, which is ubiquitous in many laboratories (i.e., sample in a cuvette). A numerical approach-based methodology that accounts for multiple layers, Fabry–Pérot effect, and sample thickness is elaborated which involves an optical interference model based on a tri-layer structure and a simple thickness estimation technique. This method was validated on water samples where a good agreement was found with the THz optical parameters of water reported in the literature, while the use of a commercial software resulted in erroneous optical parameters estimates when used without due regard to its limitations. A case study was then performed to demonstrate the ability of the proposed method to characterize agarose hydrogels with varying degree of sulfation. It was demonstrated that THz-TDS can provide insight into the hydration state of the agarose hydrogels, including the relative number of the hydrogen bonds between the hydroxyl moieties of water and the polysaccharide network which is perturbed by the presence of sulfate. The trend in the index of refraction profiles suggested microstructural differences between the agarose hydrogels, which were confirmed by visualizing the agarose network morphology using cryo-SEM imaging. [ABSTRACT FROM AUTHOR]

The identification of different rubber materials is crucial to ensuring the quality of rubber products. In order to quickly and effectively identify the types of rubber, reduce the impact of counterfeit rubber on the market. This study proposes a rubber identification method based on terahertz time-domain spectroscopy (THz-TDS), Chemometry, and Improved Honey Badger Algorithm (IHBA). Initially, the absorption spectra of eight types of rubber within the 0.2–1.6 THz range are obtained and calculated using THz-TDS. This is followed by data preprocessing using Savitzky-Golay and Principal component analysis(PCA). The optimization effects of genetic algorithm (GA), grid optimization algorithm (GRID), particle swarm optimization algorithm (PSO) and honey badger algorithm (HBA) on support vector machine (SVM) model parameters were compared respectively. The HBA-SVM model achieves 96.88 % recognition accuracy on the prediction set, which is higher than other models and shows excellent parameter optimization ability.To further improve accuracy, Bernoulli chaotic mapping, cosine density factor, and Cauchy mutation are introduced for improvement. Compared with the original model, the IHBA-SVM model improves the accuracy of rubber recognition from 96.88 % to 98.96 %. Furthermore, compared with other models, the IHBA-SVM model achieved the highest classification accuracy. In summary, this study provides technical support and reference for the rapid identification of rubber, which is of great significance for ensuring the quality of rubber products. • A nondestructive and fast rubber classification method. • Classification of rubber using support vector machine combined with terahertz spectroscopy. • An improved honey badger algorithm is proposed in this paper for optimal support vector machine parameter selection. [ABSTRACT FROM AUTHOR]

Selected layered clay minerals of smectite group with different chemical composition (e.g. synthetic hectorite, synthetic smectite and natural smectite) were modified by rare earth cations such as Er3+ and a stochiometric mixture of Er3+/Yb3+. Raw and modified smectites were systematically heated up to temperatures ensuring their dehydration (400 °C), dehydroxylation (800 °C) and formation of new high temperature phases (1200 °C). The products of heating were investigated by terahertz-time domain spectroscopy to follow changes of their dielectric properties in the far-infrared region. The frequency-dependent refractive index of raw and modified clays was determined in the range of 0.2–3.8 THz (6.6–126.5 cm−1). The heating at 800 °C and above resulted in increase frequency dependent index of refraction and decrease absorption coefficient in comparison to the raw clays. The ion-exchanged smectites by rare earth cations further decreased the absorption coefficient of the samples heated at 1200 °C. Raw natural smectite sample revealed a low frequency band at 0.86 THz, already reported in the literature, which shifted to 0.67 THz after sample modification by rare earth cations. Heating of modified JP to 1200 °C resolved a new absorption band at ∼3.40 THz in the ceramic phase produced from raw natural smectite. • Cation exchange reaction was used for modification of clays by selected rare earth elements. • Effect of sample heating up to 400, 800 and 1200 °C was studied. • Terahertz time domain spectroscopy was used to study the refractive index dispersion. • The frequency dependence of absorption in THz region resolved new typical absorption bands. • The high temperature treatment can be used for synthesis of new ceramic phosphor materials. [ABSTRACT FROM AUTHOR]

Liquid moulded polymer matrix composites (LM-PMCs) are increasingly used in aerospace, automotive and other industrial applications. Liquid moulding processes featuring dry fibre preforms provide flexibility and enable cost reductions for secondary load-bearing structures. However, preform variability and manufacturing reproducibility remain major obstacles to wider use in primary structures. The open literature records only marginal use of non-destructive inspection (NDI) methods for dry multilayer preforms due to technological limitations and cost of NDI methods. In this work, terahertz time-domain spectroscopy (THz-TDS) is used for inspecting three dry multilayer glass fibre preforms featuring different defects, for the first time. A novel time-domain enhancement method is compared with classical image processing methods, aiming at improving image contrast and detecting potential defects. Furthermore, THz B-Scan is used for verifying the accuracy of interply defect detection. Finite difference time domain is simulated for analyzing THz magnitude variation in time-domain. Finally, quantitative evaluation is applied to further illustrate the significant potential of THz-TDS for the inspection of dry fibre preforms. The results show that the errors on defects lengths, widths, angles, and diameters for THz-TDS are in the ranges 8.2 %–34.3 %, 18.67 %–75 %, 0.29 %–6.67 %, and 1.33 %–10 % respectively. [Display omitted] • Terahertz technology is used for multilayer dry glass fibre preforms inspection for the first time. • A novel time-domain image enhancement method is proposed. • A new method based on disturbances is used to verify the interply detection accuracy. • Quantitative evaluation is performed and the potential of industrial use is discussed. [ABSTRACT FROM AUTHOR]

As a promising approach, terahertz time-domain spectroscopy (THz-TDs) provides the powerful assistance for quantitatively characterizing the solid-liquid synergistic lubrication. The hydration ratio in nanoadditive solutions was calculated by terahertz dielectric spectrum, and the exact threshold for superlubricity was found as 40%. The high hydration ratio also promoted the formation of high-quality tribofilm. Besides, not all hydrogen bonds, but the solid-liquid hydrogen bond was the true answer to the nanoadditive superlubricity. Finally, a synergistic lubrication model was proposed and it gave a detailed description of the molecular states with different lubrication. This work exerts the sensitivity of THz-TDs on hydration and liquid lubrication, providing avenues for the quick assessment of superlubricity and optimal design of nanoadditive superlubricity in the future. [Display omitted] • The threshold of hydration ratio was proposed as 40% for the nanoadditive superlubricity. • Solid-liquid hydrogen bond, instead of liquid-liquid hydrogen bond, contributes to the superlubricity and stability. • Poor hydration and weak hydrogen bond would lead to water evaporation and even dry friction. [ABSTRACT FROM AUTHOR]

In this paper, the terahertz time-domain spectroscopy (THz-TDS) is proposed to investigate the behavior underlying kerosene-water two-phase flow in Y-shaped rectangular microchannels. The microchannel is initially saturated with water and water forms an external continuous phase at low kerosene flow velocities (v k), while a dispersed phase is formed at high v k. The flow process of kerosene-water two-phase flow in microchannel was in situ monitored using a THz-TDS peak intensity, and four flow patterns such as droplet flow, plug flow, slug flow and deformed interface flow are identified over a wide range of v k from 2.08 to 43.13 cm/s at selected water flow velocity (v w) of 13.96 cm/s and 27.71 cm/s. Due to the strong dependence of flow patterns on the v w , only droplet, plug and slug flows are observed at high v w. In addition, with the v w increasing from13.96 cm/s to 27.71 cm/s the ranges of v k for droplet flow and plug flow patterns are expended by ∼100%. A parameter β = k / v w , where k is the slop of the peak intensity of THz-TDS against v k , was proposed to characterize the kerosene-water two-phase flow patterns, and each β only corresponded to one phase for various v k at different v w. These results render THz-TDS a feasible technology for investigating the behavior of oil-water two-phase flow inside microfluidic devices and lay the foundation for experimental and numerical investigations on multiphase flow in microchannels for further study. [Display omitted] • Terahertz spectroscopy was applied to characterize the flow patterns of kerosene-water two-phase flow in microchannels. • Four flow patterns were identified over a wide range of kerosene flow velocities. • Only droplet, plug and slug flows are observed at a high water flow velocity. [ABSTRACT FROM AUTHOR]

[Display omitted] • A rapid and non-destructive approach for discrimination the hair and nails of people from different regions in forensic anthropology. • Terahertz time-domain spectroscopy, when coupled with advanced chemometric techniques, can effectively establish robust classification models. • Fisher discriminant analysis algorithm, exhibited the best recognition ability, achieving a 100 % accuracy rate and F-value. Hair and fingernails, emerging as crucial biological physical evidence, play a prominent role in geographical traceability studies within the forensic anthropology. This study explored the potential of terahertz spectroscopy and chemometrics techniques in identifying hair and fingernail samples from various geographical regions. A total of 196 hair and fingernail samples underwent scanning with terahertz time-domain spectroscopy. Different chemometric tools, including multilayer perceptron, support vector machine, and Fisher discriminant analysis, were employed for model building on fingernail dataset, hair dataset, and fusion data, respectively. The performance of models constructed based on different datasets was compared and evaluated using metrics such as accuracy, precision, recall, and F-score. Results revealed that the fusion data, employing the Fisher discriminant analysis (FDA) algorithm, exhibited the best recognition ability, achieving a 100 % accuracy rate and F-value. Terahertz spectroscopy combined with FDA algorithm emerged as an effective and convenient tool for differentiating hair and fingernail samples from diverse geographical regions. This innovative technique boasts environmental friendliness, rapid analyses, and minimal sample preparation requirements compared to traditional methods. [ABSTRACT FROM AUTHOR]

[Display omitted] • To improve the accuracy of the model and reduce the computational effort, three algorithms including UVE, CARS and PCA are used to extract the spectral features from terahertz absorption spectra and refractive index spectra. • Because of the large data volume of terahertz time-domain spectra, temporal features, including maximum value, minimum values, variance, are calculated to subsequent data processing. • To further enhance classification accuracy of classifiers, the data fusion method of feature-level fusion is employed. • The results show that the performance of data fusion model is best than that of the model using single-type spectra. The model with the highest prediction accuracy is the KNN model, with an accuracy of 97.37%, when the refractive index spectra and time-domain spectra are fused as inputs. Rubber, an essential industrial product, plays a crucial role in industry and human life. However, some manufacturers of rubber use inferior, recycled or low-cost rubber materials to replace high-quality rubber products. Therefore, in order to ensure industrial production, protect natural environment and ensure personal safety, it is of great economic and social significance to enable rapid and non-destructive qualitative identification of rubber products. In this paper, eight different types of black rubber, including Nitrile Butadiene Rubber (NBR), Choloroprene Rubber (CR) and Ethylen Propylen Diene Monomer (EPDM), were selected as research objects. Terahertz Time-Domain Spectroscopy (THz-TDS) technology was utilized to get the absorption spectra, refractive index spectra and time-domain spectra of these samples for exploring the differences between different types of rubber in terahertz spectra. To reduce the data volume, three band selection algorithms were employed to extract features from the absorption spectra and refractive index spectra, and temporal features of time-domain spectra were extracted for subsequent data processing. In the selection of classification model algorithms, K-Nearest Neighbor (KNN), Random Forest (RF) and Supervised Kohonen Neural Networks (S-Kohonen) were employed to recognize these spectral data. To fully exploit the latent information within these data and considering the possibility of the loss of partial information in single-type signal, a feature-level data fusion method was applied to different optical parameter signals to obtain more information about terahertz spectra during the process of data modeling. The results of the research indicated that the model with the highest prediction accuracy was the KNN model, with an accuracy of 97.37%, when refractive index spectra and time-domain spectra were fused as inputs, the F1-score reached 99.64. The performance of data fusion was noticeably better than that of a single-type signal. This study validates the feasibility of using terahertz time-domain spectroscopy for qualitative identification of different types of rubber. [ABSTRACT FROM AUTHOR]

Water-sensitive nature of terahertz radiation has been exploited here to discriminate the hydration states of different hydrated salts, such as Cobalt Chloride Hexahydrate (CoCl 2.6H 2 O), Copper Sulfate Pentahydrate (CuSO 4.5H 2 O), Magnesium Chloride Hexahydrate (MgCl 2.6H 2 O), and Calcium Chloride Dihydrate (CaCl 2.2H 2 O). Hydration states of these salts have been prepared by heating them in an oven at different temperature for specific time on the basis of thermo-gravimetric analysis. The prepared samples were analyzed using terahertz time-domain spectroscopy (THz-TDS). The variation in absorption coefficient and refractive indices of these hydrated salts in terahertz spectral regime (0.2–1.6 THz) are linked with the change of hydration states, which has been used for their discrimination. The results have been validated with X-ray diffraction analysis. The present work indicates that THz-TDs can be considered a promising technique to fulfill the ever increasing demand of the analysis of these hydrated salts. [Display omitted] • THz spectroscopy, an efficient technique to identify the hydration states of salts. • Different hydration states of salts were obtained by heating them. • The verification of hydration states has been done by XRD analysis. • Refractive index and absorption coefficient were measured in THz spectral regime. • Hydration states were discriminated on the base of these optical parameters. [ABSTRACT FROM AUTHOR]

Bone is among the main by-products of the meat and dairy livestock. Currently, in many countries, it is valorised in pet food, disposed through incineration or exported. However, thanks to its biological and mechanical properties, it could be used in several bio-based products such as coatings, packaging, phosphorous, calcium and magnesium supply, flame retardants, etc. Successful valorisations routes of discarded bones need a comprehensive characterization of the bone tissue variability with major biological factors such as age of the animal and the anatomy of the bone itself. In this study, this variability was assessed through Terahertz time-domain spectroscopy and energy dispersive X-ray microanalysis. Bones optical properties (refractive index and absorption coefficient) and main minerals (calcium and phosphorous), were determined for adult cows' femurs and tibias, from the same breed and breeding system and in the age range 93–120 months. Clustering analysis was then carried out, by unsupervised statistical tools (Principal Component Analysis and Agglomerative Hierarchical Clustering). Results showed that the bones considered can be grouped in two classes, represented by each anatomy; besides, the anatomy was most significant than age. The refractive index increased with age and was higher for femur; the cluster of tibias was more homogeneous than femurs. The THz radiation showed to be a useful and non-destructive tool to assess bovine bones variability and also their optical properties. This radiation could be applied to the predict animal bone components (collagen and minerals) extractability so as to valorise each cluster according to its potential. [ABSTRACT FROM AUTHOR]

[Display omitted] • We have utilized THz-TDS and DFT studies to explore low-frequency vibrational properties of ADNP (4-Amino 3,5 Dinitro Pyrazole) within the 0.1 to 3.0 THz range. • We employed a Toptica-Teraflash handheld terahertz system in reflection mode. The refractive index and absorption coefficients are found to be 1.8 and 10–180 cm−1, respectively. • The optical and dielectric properties were experimentally measured and compared with DFT predictions, demonstrating good agreement. • The observed vibrational absorption bands in our study can serve as signature spectra for detecting ADNP in bulk and at trace levels. Pyrazoles have recently received significant attention due to their unique and potential applications in the medical field, agriculture and are also known to be highly stable explosives. The present work describes the terahertz time-domain spectroscopy (THz-TDS) based study of 4-Amino 3,5 Dinitro Pyrazole(ADNP) in between the 0.1 and 3.0 THz ranges. A Toptica-Teraflash fibre-coupled handheld terahertz system has been employed in reflection mode configuration. We ascertained complex refractive index, absorption coefficients, and complex dielectric constants from 0.1 THz to 3.0 THz. The value of the refractive index and absorption coefficients are found to be 1.8 and 10–––180 cm −1, respectively. Also, we have analyzed the structural, vibrational, and optical properties of ADNP using the plane-wave pseudopotential method based on Density Functional Theory (DFT) calculations. We have observed six low-frequency optical phonon modes, located at 0.36, 1.20, 1.52, 1.77, 2.40, and 2.75 THz, respectively, exhibiting a redshift compared to the values predicted by the DFT calculations. The possible reasoning for the above might be due to the anharmonicity that is not considered in the DFT calculations. The theoretical calculations align with the experimental results and deliver direction for further investigations and the futuristic application of ADNP. [ABSTRACT FROM AUTHOR]

Polyvinyl chloride (PVC) composites have been studied using terahertz time-domain spectroscopy (THz-TDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to highlight the prospects for their applications. The terahertz dielectric properties and conductivity of PVC composites in the frequency window of 0.2–1.6 THz have been investigated. The universal dielectric response model has been applied to fit the real part of THz conductivity. It shows that a super-linear behavior is presented in the conductivity, and the conductivity can be tuned by varying the content of PVC and the type of composite. In addition, relationships are established among the types and concentrations as well as the dielectric properties and conductivity. These results are expected to be applied to the preparation of terahertz devices and to the monitoring of materials in industrial production. [Display omitted] • The dielectric properties and conductivity of PVC in THz regime have been tuned by blending plastics and fillers. • These properties of PVC/plastic composites vary with the concentration and type of plastic. • The dielectric loss and real conductivity of PVC/filler composites increase with increasing filler content. • THz-conductivity has been applied to fit the UDR model to investigate the conductivity mechanism. [ABSTRACT FROM AUTHOR]

• Terahertz (THz) magneto-optical (MO) properties of N-doped diamond (N-D) is studied by using THz time-domain spectroscopy in the presence of magnetic field in Faraday geometry. • The right- and left-handed circular transmission coefficients and MO conductivities are obtained. By fitting the experimental results with theoretical formulas, we determine magneto-optically the key electronic parameters of N-D. • The dependence of these parameters in N-D upon magnetic field from 0 to 8 T at a temperature of 80 K is examined and analyzed. Nitrogen-doped diamond (N-D) is one of the most important carbon-based electronic and optical materials. Here we study the terahertz (THz) magneto-optical (MO) properties of N-D grown by microwave plasma-enhanced chemical vapor deposition. The optical microscope, SEM, XRD, Raman spectrum, FTIR spectroscopy and XPS are used for the characterization of N-D samples. Applying THz time-domain spectroscopy (TDS), in combination with the polarization test and the presence of magnetic field in Faraday geometry, THz MO transmissions through N-D are measured from 0 to 8 T at 80 K. The complex right- and left-handed circular transmission coefficients and MO conductivities for N-D are obtained accordingly. Through fitting the experimental results with theoretical formulas of the dielectric constant and MO conductivities for an electron gas, we are able to determine magneto-optically the key electronic parameters of N-D, such as the static dielectric constant ε b , the electron density n e , the electronic relaxation time τ , the electronic localization factor α and, particularly, the effective electron mass m* obtained under non-resonant condition. The dependence of these parameters upon magnetic field is examined and analyzed. We find that the MO conductivities of N-D can be described rightly by the MO Drude-Smith formulas developed by us previously. It is shown that N-doping and the presence of the magnetic field can lead towards the larger ε b and heavier m* in diamond, while n e / τ / α in N-D decreases/increases/decreases with increasing magnetic field. The results obtained from this work are benefit to us in gaining an in-depth understanding of the electronic and optoelectronic properties of N-D. [ABSTRACT FROM AUTHOR]

• The temperature dependence of absorption coefficient and phase matching angle of Potassium titanyl arsenate (KTiOAsO 4) with temperatures from 5.8 to 300 K was determined by terahertz time-domain spectroscopy. • It is demonstrated that the absorption coefficient of KTA crystal exhibits a decrease with decreasing temperature, particularly the absorption coefficient along the Z axis changes more pronounced. • It is calculated the relationship between the phase matching angle and the output wavelength with temperatures of 300 K and 5.8 K, and the phase matching angle changes by less than 1° at the same output wave. • The results demonstrate that KTiOAsO 4 has low absorption coefficient and can achieve phase matching with output wavelengths in the terahertz range at low temperatures. The temperature dependence of absorption coefficient and phase matching (PM) angle of Potassium titanyl arsenate (KTiOAsO 4 , KTA) with temperatures from 5.8 to 300 K was determined by terahertz time-domain spectroscopy. It is demonstrated that the absorption coefficient of KTA crystal exhibits a decrease with decreasing temperature, particularly the absorption coefficient along the Z axis changes more pronounced, which can be attributed to the reduction in the mobility of K+ at low temperatures. The Sellmeier equation was employed to fit the refractive indices along the X , Y , and Z axes with temperatures from 300 to 5.8 K to obtain the refractive index dispersion equation. The refractive index dispersion equation was substituted into the PM equation to calculate the relationship between the PM angle and the output wavelength with temperatures of 300 K and 5.8 K, and the PM angle changes by less than 1° with temperature at the same output wavelength. The results demonstrate that KTA can achieve PM at low temperatures with output wavelengths in the THz range. Moreover, the THz absorption coefficient decreases and the PM angle remains relatively stable at low temperatures provide a foundation for efficient THz wave generation using KTA under low temperature conditions. [ABSTRACT FROM AUTHOR]

• Terahertz time-domain spectroscopy contains noises and uncertainties. • The noises and uncertainties add difficulty to spectral signal analysis. • A recurrent quantum neural network was proposed as an adaptive filter. • The proposed network can eliminate the noise and maintain spectrum characteristics. System interferences such as noise and background information in terahertz time-domain spectroscopy (THz-TDS) add difficulty to spectral signal analysis and affect the accuracy of component identification. In this paper, we proposed an adaptive filter based on a recurrent quantum neural network (RQNN) to process the THz-TDS signal. RQNN is a network architecture guided by quantum mechanics, for adaptive filtering of THz-TDS signal. Six pure substances, ferric sulfate, ferrous sulfate, Prussian blue, agarose, polylysine and sodium of polyaspartic acid were used as test samples. The TDS signals of the six samples were filtered with RQNN and five competing filters. Optical constants including refractive index and absorption coefficient were calculated from raw THz signal and filtered THz signal. Peak signal-to-noise ratio (PSNR) and structural similarity (SSIM) were calculated between the optical constants calculated before and after filtering as evaluation indicators. Results revealed that RQNN could better eliminate the noise in THz-TDS signals, and retain the characteristic peaks in the calculated optical constants simultaneously. In terms of PSNR and SSIM, RQNN-filtered THz-TDS signals demonstrated higher values than those of the competing filters. The RQNN proposed in this paper can be used as an adaptive filter for noise filtering and signal extraction of THz-TDS signal, with advantages above traditional filtering algorithms. [ABSTRACT FROM AUTHOR]

Dielectric constants of Al 2 O 3 (alumina) ceramic samples have been measured with non-destructive terahertz time-domain spectroscopy. Using an effective medium approximation, we demonstrated that the relative permittivity can be used to evaluate the porous alumina samples, with porosity fractions ranging from 0% to 20% per volume. This result makes it possible to control sample porosity, a key material parameter related to mechanical strength under compression load, hardness and wear resistance, or thermal stability. We also employed terahertz time-domain spectroscopy to differentiate alumina samples fabricated with different grain sizes, or reveal a small fraction of impurities, such as 5% vol of ZrO 2 , embedded in non-porous alumina. These results demonstrated the potential of terahertz radiation for non-destructive characterization of low loss high dielectric constant ceramics. [ABSTRACT FROM AUTHOR]

[Display omitted] • THz-TDS technology was used to identify rice grains infested with Sitophilus oryzae at different growth stages. • CDAE was developed to reconstruct preprocessed THz spectra for eliminating noise. • RFC model was constructed to analysis original and reconstructed THz spectra. Rice grains are often infected by Sitophilus oryzae due to improper storage, resulting in quality and quantity losses. The efficacy of terahertz time-domain spectroscopy (THz-TDS) technology in detecting Sitophilus oryzae at different stages of infestation in stored rice was employed in the current research. Terahertz (THz) spectra for rice grains infested by Sitophilus oryzae at different growth stages were acquired. Then, the convolutional denoising autoencoder (CDAE) was used to reconstruct THz spectra to reduce the noise-to-signal ratio. Finally, a random forest classification (RFC) model was developed to identify the infestation levels. Results showed that the RFC model based on the reconstructed second-order derivative spectrum with an accuracy of 84.78%, a specificity of 86.75%, a sensitivity of 86.36% and an F1-score of 85.87% performed better than the original first-order derivative THz spectrum with an accuracy of 89.13%, a specificity of 91.38%, a sensitivity of 88.18% and an F1-score of 89.16%. In addition, the convolutional layers inside the CDAE were visualized using feature maps to explain the improvement in results, illustrating that the CDAE can eliminate noise in the spectral data. Overall, THz spectra reconstructed with the CDAE provided a novel method for effective THz detection of infected grains. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detection by terahertz time-domain spectroscopy combined with chemometrics. • Whale Optimization Algorithm was used to improve RF and ELM model. • Different concentrations of PDE5 Inhibitor can be detected. Combating the illicit use of PDE5 inhibitor drugs is a focal point in forensic science research. In order to achieve rapid identification of such drugs, this study applies terahertz time-domain spectroscopy combined with chemometrics to establish a fast and accurate detection method for PDE5 inhibitors. The optimal detection method is determined by comparing the spectral performance of three optical parameters, namely absorption coefficient, refractive index, and dielectric constant. Linear discriminant models based on different spectral parameters, whale optimization algorithm optimized extreme learning machine models, and whale optimization algorithm optimized random forest models are established. The effectiveness and performance of principal component analysis and competitive adaptive reweighted sampling algorithm for spectral feature data selection are also investigated. The PDE5 inhibitor identification model based on the competitive adaptive reweighted sampling - whale optimization algorithm - random forest (CARS-WOA-RF) model achieves an accuracy of 98.61%, and the identification model for two concentrations of Sildenafil achieves 100% accuracy. The results demonstrate that terahertz time-domain spectroscopy combined with chemometrics can effectively detect various common types of PDE5 inhibitor drugs and different concentrations. [ABSTRACT FROM AUTHOR]

[Display omitted] • Form I and II of vanillin are distinguished using terahertz time-domain spectroscopy. • THz spectra of both forms are simulated using harmonic and anharmonic methods. • Couplings between O H bond stretch and hydrogen bond motions are uncovered. • Electronic energy mainly determines the stability order of two forms. Polymorphism commonly exists in organic molecular crystals. The fingerprint features in low-frequency vibrational range are important information reflecting different intermolecular interactions of polymorphs. Interpreting these features is very helpful to understand vibrational property of polymorphs and reveal the thermodynamic stability. In this work, the low-frequency vibrations of form I and II of vanillin are investigated using terahertz time-domain spectroscopy. Static DFT calculation and ab initio molecular dynamics (AIMD) are employed to interpret their low-frequency vibrations of both forms in harmonic and anharmonic ways, respectively. Their low-frequency vibration characteristics in harmonic calculations are discussed, and anharmonic mode couplings between O H bond stretch and the stretching and bending motion of hydrogen bonds are uncovered. Moreover, the thermodynamic energies including electronic potential energy and vibrational/kinetic energy arising from nuclear motions are calculated. The result reveals that the stability order of the two forms is mainly dependent on their electric potential energy difference. [ABSTRACT FROM AUTHOR]

• Automatic defect detection in glass composites using terahertz radiation. • Time-domain spectroscopy reflective scanner with an electronically controlled optical sampling scheme. • Pre-selection of C-scans with defect-related features. • Thresholding method based on Gaussian histogram fitting. • Size and depth of inclusions and delaminations determined with high accuracy. We developed a method for automatic detection of defects in glass fibre reinforced polymer (GFRP) based on a terahertz time-domain spectroscopy (TDS) system. Two GFRP specimens with round defects of different sizes (inclusions and delaminations) were measured by means of a high-speed TDS reflective scanner with an electronically controlled optical sampling scheme. The algorithm provided a series of C-scans, which were then pre-selected to identify only those with features potentially associated with defects. A thresholding method based on Gaussian fitting of the histogram was developed. The results showed that 87 % of defects were detected for the specimen with inclusions and 100 % for the specimen with delaminations. The algorithm also provided information about the depth of defects, as each C-scan was assigned to a time slice. The results demonstrate that the TDS-based system, along with the proposed histogram-based method, can automatically detect defects in GFRP. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

Optical pump-terahertz probe spectroscopy (OPTP) was employed to investigate the photocarrier dynamics and the optical characteristics of the semiconductor Si. A bulk Si with a relatively high resistivity of ∼1 kΩcm was polished to a thickness of about 10 ± 3 μm. This thickness is approximately equivalent to the penetration depth of the pump beam in OPTP. We investigated the photocarrier dynamics with the condition that the influence of the inhomogeneous density distribution of the photocarrier is suppressed by using Si with thickness approximately equals to the penetration depth of the pump beam in OPTP. Our results showed a difference in the transmitted terahertz (THz) field amplitude as well as in the peak delay time of the bulk Si and the polished Si which indicates a difference in the photocarrier dynamics, under various applied fluences, between the two types of Si. The observed propagation characteristics of the THz pulse and the obtained optical constants from Fourier analysis illustrate that the thick Si film behaves as a homogeneous material with carrier density generated by photoexcitation. • Si with thickness approximately equal to penetration depth of pump beam (Thick Si film). • Photocarrier dynamics in thick Si film investigated by optical pump-terahertz probe spectroscopy. • Suppression of inhomogeneous density distribution of photocarriers. [ABSTRACT FROM AUTHOR]

With a growing interest on inorganic ceramics based solid-state electrolytes for all-solid-state batteries, there is a need to maximise their density to optimise electrochemical performance and fuel impermeability. In this paper, we demonstrate the sensitivity of terahertz time-domain spectroscopy (THz-TDS) combined with effective medium theory to quantify the porosity or density of sodium superionic conductor (NaSICON)-based solid-state electrolyte (SSEs) pellets prepared at densities in the range of 2.2–2.9 g cm−3, corresponding to 50–90 % relative densities sintered at 900–1150 °C. The results of which, have been validated against complementary Archimedes analysis and mercury porosimetry highlighting the potential of THz-TDS for rapid, contactless, non-destructive electrolyte characterisation. • Demonstrated sensitivity of terahertz radiation to NaSICON-based electrolyte. • NaSICON densities quantified using effective medium theory. • Estimated densities correlates to Archimedes analysis. • Estimated densities agrees with to mercury porosimetry data. [ABSTRACT FROM AUTHOR]

• Terahertz imaging was suitable for ceramic ballistic protection armor. • Terahertz XY raster scanner with terahertz time-domain spectroscopy was used. • The transfer matrix method simulated the terahertz signals. • Interfaces and regular pattern of hexagonal alumina tiles were clearly visible. • The position and shape of puncture defects matched the reference X-ray images. We prepared an alumina-based ceramic ballistic protection armor sample consisting of hexagonal alumina tiles arranged in a honeycomb structure, glued with Kevlar®, and covered with a polyurethane-based resin. The sample was shot with four projectiles, strongly hit from the back, and locally covered with water. The terahertz complex refractive index of the materials constituting the sample was measured. A terahertz time-domain spectroscopy (THz-TDS) setup in a reflection configuration was used to raster scan the XY sample. The results in the form of time signals, top views, and cross-sections enabled an understanding the internal structure of the sample. THz-TDS was proved to be suitable for non-destructive testing of the alumina-based armors with depth-discriminant 3D imaging. Transfer matrix method (TMM) correctly simulated terahertz signals reflected from the sample, calculated the thickness of the sample layers, and help determine the type of hidden defects. [ABSTRACT FROM AUTHOR]

[Display omitted] Salicylic acid is a raw material for preparing aspirin and holds an important position in medical history. Studying the crystallization of these two drugs is of great significance in improving their dissolution rate, bioavailability, and physical stability. Although various techniques have been used for structural characterization, there is still a lack of information on the collective vibrational behavior of aspirin and salicylic acid eutectic compounds. Firstly, two starting materials (salicylic acid and aspirin) were ground in a 1:1 M ratio to prepare eutectic compounds. The eutectic composition was studied using vibrational spectroscopy techniques, such as X-ray powder diffusion (XRPD), terahertz time-domain spectroscopy (THz-TDS), and Raman spectroscopy. Additionally, the structure of the aspirin and salicylic acid eutectic was simulated and optimized using density functional theory. It was found that the eutectic type II was the most consistent with the experiment, and the corresponding vibration modes of each peak were provided. These results offer a unique method for characterizing the structural composition of eutectic crystals, which can be utilized to enhance the physical and chemical properties, as well as the pharmacological activity, of specific drugs at the molecular level. [ABSTRACT FROM AUTHOR]

With an increasing demand for THz electronics applications, it is essential to obtain the electrical properties of semiconductor materials used in their fabrication. This study presents an approach to estimate the electrical properties of semi-insulating compound semiconductors using terahertz time-domain spectroscopy (THz-TDS). By employing THz-TDS in transmission mode, the resistivity and carrier concentration of semi-insulating Silicon Carbide (SiC) and Indium Phosphide (InP) wafers are non-destructively measured. The study utilizes the simplified Drude model and the Nelder-Mead algorithm to estimate the electrical properties from the THz-TDS transmission (S21) measurement data. The resistivity values of the SiC and InP samples, (1.42 ± 0.15) × 105 Ω cm and (3.0 ± 0.1) × 107 Ω cm, respectively, are in accordance with the manufacturer specifications. Moreover, the feasibility of non-destructive mapping of the electrical properties of the wafers is demonstrated, offering a novel tomographic inspection approach for online monitoring. This technique holds significant promise for enhancing production yields in the semiconductor industry. [ABSTRACT FROM AUTHOR]

[Display omitted] • The vibrational spectra of gallic acid and gallic acid monohydrate at 0.2–3.0 THz were analyzed. • The origin of each absorption peak was analyzed by the method of VMADR. • Hirshfeld surface and AIM topological analyses were used to study the weak intermolecular interactions between gallic acid and gallic acid monohydrate. The terahertz (THz) spectroscopy of gallic acid (GA) and gallic acid monohydrate (GAMD) in the frequency range of 0.2–3.0 THz were investigated using terahertz time-domain spectroscopy (THz-TDS) system. The THz absorption spectrum of GAMD exhibited a red-shift compared to that of GA. To resolve the absorption spectra of the two substances, density functional theory (DFT) was employed to calculate and simulate the theoretical spectra of the two substances in this frequency range, while the vibration mode automatic relevance determination (VMARD) method combined with GaussView and VMD software was utilized to determine the origin of each absorption peak. Additionally, Hirshfeld surface analysis and the Atoms-in-Molecules (AIM) topological analysis were conducted to explore the intermolecular interactions between GA and GAMD. The results revealed that the characteristic absorption peaks of both GA and GAMD are attributed to overall atomic and molecular motions, whereas those of GAMD are accompanied by water molecule translation. Furthermore, H...O/O...H hydrogen bonding was identified as the main contributor to the interaction between GA and GAMD. The addition of water molecules led to differentiation in the position and strength of intermolecular interactions; specifically, hydrogen bonding in GA primarily involved the carboxyl group with the hydroxyl group, while hydrogen bonding in GAMD occurred between the water molecule and the hydroxyl group of the central molecule. [ABSTRACT FROM AUTHOR]

The fluorescent carbon quantum dots (FCQDs) are promising for wide applications especially in the field of optoelectronic materials and devices. For these applications, it is essential to examine the optoelectronic properties of FCQDs not only in a solution but also in the form of dry particles. Here we study the optoelectronic properties of dry FCQDs by using terahertz (THz) time-domain spectroscopy (TDS). The THz response of dry FCQDs, which can emit blue (B-CQDs) and red (R-CQDs) lights in solutions under optical excitation, has been measured in temperature range from 80 to 280 K. We find that R-CQDs behaves like an optical insulator in 0.2–1.2 THz range, while B-CQDs experiences insulator-to-semiconductor transition with increasing THz frequency and temperature. The optical conductance and the key sample parameters of B-CQDs are determined. This is the first research work in applying the THz TDS to study the dry CQDs. The findings from this study can be helpful in gaining an in-depth understanding of the physical properties of dry FCQDs. Image 1 • Carbon quantum dots (CQDs): proposed as materials for the next generation of LED and display. • Optoelectronic properties: dry CQDs innovatively examined by terahertz (THz) time-domain spectroscopy in 0.2 – 1.2 THz, 80 – 280 K regime. • Dry CQDs with red fluorescence in solution (R-CQDs): THz transmittance ∼1, Fig. (b) , are in an insulator phase optically. • Dry CQDs with blue fluorescence in solution (B-CQDs) experience optically an insulator-to-semiconductor transition. • Optical conductance and key sample parameters are determined optically; Temperature dependence is examined. [ABSTRACT FROM AUTHOR]

The optical constants (complex dielectric function) of CuO and ZnO particles in polyethylene pellets are measured by terahertz time-domain spectroscopy. The determination of the dielectric properties of these materials is of interest for energy-storage applications, for instance. Maxwell-Garnett theory is used to extract the contribution to the frequency-dependent optical constants of the oxide powders. The validity of the assumptions of Maxwell-Garnett theory are experimentally verified and self-consistency of the results of the model confirmed. On this basis, experimental complex permittivity values for isotropic CuO and ZnO oxide powders are reported in the 100 GHz-3 THz range. [ABSTRACT FROM AUTHOR]

The phonon dispersion spectrum, eigenvector and lattice vibration frequency of zincblende cadmium telluride (CdTe) are studied by density functional theory (DFT). The vertical or parallel relationship between the phonon propagating wave vector atomic vibration direction is analyzed, and the frequency of the CdTe transverse (TO)/longitudinal (LO) optical phonon is obtained. The Boltzmann transport equation clearly expresses the phonon scattering process, which is combined with the Huang Kun equation to obtain the three-phonon collision frequency (γ). The terahertz time-domain spectroscopy system is used to measure the reflectivity and dielectric constant of CdTe single crystal. The experimental results verify that the theoretical calculation based on DFT can give the correct terahertz band dielectric constant with frequency dependence characteristics completely independent of the experiment. [ABSTRACT FROM AUTHOR]

We demonstrate a method for examining the effects of thermal aging of cellulose pressboard using terahertz time-domain spectroscopy. The change of refractive indexes at the frequencies of pulsed terahertz waves while applying heat reveals that the degree of thermal aging and the condition of thermal breakdown can be clearly analyzed by performing terahertz measurements. The visual examination method only allows evaluation of the degree of blackness on the external surfaces of cellulose pressboard. However, using terahertz techniques for non-destructive inspection enables determination of the dielectric integrity to check the strength of the electrical insulation inside the cellulose pressboard. The results indicate that the terahertz techniques could be very useful, especially in the electric power industry, which requires a variety of methods for internal non-destructive testing to ensure the safety and reliability of insulation materials in electric power facilities and components. • THz spectroscopy as a new non-destructive testing tool. • Thermal aging of cellulose pressboard measured by THz wave. • THz technique for non-destructive internal inspection in electric power industry. [ABSTRACT FROM AUTHOR]

Dual-frequency liquid crystals (DFLCs), the mixture of positive and negative liquid crystal (LC) molecules, exhibit unique alternating (AC) frequency dependent anisotropic properties. Carbon nanotube (CNT), as a novel nanomaterial with strong anisotropy, has attracted much attention in recent years. Herein, we investigate the tunable terahertz birefringence and phase shift characteristics of the DFLC doped with CNT (CNT-LC) by using the terahertz time-domain polarization spectroscopy. The results show that the CNT-LC (1.5 wt%) can reach 0.5π at 0.793 THz, which can be used as a tunable THz phase shifter that is 0.12π higher than the pure DFLCs. Furthermore, through measuring the output polarization state, it is confirmed that the dielectric anisotropy enhancement mechanism of CNT-LC originates from the surface interaction between CNTs and LC molecules. Therefore, a tunable quarter-wave plate with the active polarization conversion from linearly polarized (LP) to LP or LP to circularly polarized (CP) can be realized at 0.925 THz in the CNT-LC, while the pure DFLC cannot be achieved at the same frequency. The dielectric anisotropy enhancement of CNT-LC shows its utility in the improvement of various tunable terahertz LC phase shifter and wave plate. Image 1 [ABSTRACT FROM AUTHOR]

• THz spectroscopy was studied to predict wheat gluten contents for the 1st time. • GPR and SVM methods for THz spectra to detect gluten contents were proposed. • GPR model performed better than SVM model in predicting gluten contents. • The study provides a novel technique to ensure gluten-free food samples. The purpose of this study was for the first time to explore the feasibility of terahertz (THz) spectral imaging for the detection of gluten contents in food samples. Based on the obtained 80 THz spectrum data, Gaussian process regression (GPR) and support vector machine (SVM) models were established to predict wheat gluten concentrations in 40 potato starch mixture samples. The prediction performances of GPR and SVM obtained were R 2 = 0.859 and RMSE = 0.070, and R 2 = 0.715 and RMSE = 0.101 in the gluten concentration range of 1.3%-100%, respectively, showing that the linear SVM algorithm had better prediction performance. The results indicated that THz spectral imaging combined with GPR could be used to predict the gluten content in food samples. It is thus hoped that this research should provide a novel technique for gluten content detection to ensure gluten-free food samples. [ABSTRACT FROM AUTHOR]