Enhanced Measurement of Paper Basis Weight Using Phase Shift in Terahertz Time-Domain Spectroscopy

THz time-domain spectroscopy has evolved as a noncontact, safe, and efficient technique for paper characterization. Our previous work adopted peak amplitude and delay time as features to determine paper basis weight using terahertz time-domain spectroscopy. However, peak amplitude and delay time tend to suffer from noises, resulting in degradation of accuracy and robustness. This paper proposes a noise-robust phase-shift based method to enhance measurements of paper basis weight. Based on Fresnel Formulae, the physical relationship between phase shift and paper basis weight is formulated theoretically neglecting multiple reflections in the case of normal incidence. The established formulation indicates that phase shift correlates linearly with paper basis weight intrinsically. Subsequently, paper sheets were stacked to fabricate the samples with different basis weights, and experimental results verified the developed mathematical formulation. Moreover, a comparison was made between phase shift, peak amplitude, and delay time with respect to linearity, accuracy, and noise robustness. The results show that phase shift is superior to the others

Defect feature extraction of marine protective coatings by terahertz pulsed imaging

Feature extraction of marine protective coatings using Hilbert transform (HT) and wavelet transform modulus maximum (WTMM) on terahertz pulsed imaging (TPI) of the coatings was proposed. For TPI-based marine protective coating detection, it is difficult to locate exactly the reflected echoes form internal structure of coating system by the faint feature in the time domain due to the interference of background noise. However, those faint features were usually caused by the interface between two different medium layers whose refractive indices are very similar or caused by micro defects beneath the coating. The proposed algorithm was validated by simulated and experimental TPI waveform obtained from marine paint samples with or without defects. To extract the structure feature more clearly and intuitively, Hilbert transform procedure was carried out on detected terahertz signal to get Hilbert envelope for further processing. Subsequently, the modulus maximum of the stationary wavelet transform approximation coefficients were employed as the criteria for feature extraction of internal interfaces and defect features, according to the relationship between WTMM and signal singularity. The results demonstrated that the combination of HT and WTMM algorithms could be used to exactly extract the structure feature and to evaluate the position of defects beneath coatings

Sensing water accumulation and transport in proton exchange membrane fuel cells with terahertz radiation

Fuel cells are like batteries in the sense that they are electrochemical cells whose main components are two electrodes (anode and cathode) and an electrolyte material. They differ from most batteries as they require a continuous stream of fuel and oxidant, generating electricity and heat for as long as these are supplied. Perfluorinated sulfonic-acid ionomers such as Nafion are the most common proton exchange membrane material (solid electrolyte) whose structure underpins its unique water and chemical/mechanical stability properties. Pure hydrogen and air are typically used as the fuel and oxidant, respectively, and by-products are water and waste heat. Due to their high efficiency, low temperature operation and capacity to quickly vary their output to meet shifting demands, these fuel cells are attractive to the automobile industry, although they can also be used for stationary power production. Water management is a prominent issue in proton exchange membrane fuel cell technology. Strategies in this topic must maintain a delicate balance between adequate hydration levels in the Nafion proton electrolyte membrane to maximise proton conductivity, and minimal flooding, which hinders mass transport to active sites. The complex nature of water transport in these fuel cells can be investigated via in situ or ex situ diagnostics with visualisation techniques such as neutron imaging or optical diagnostics. Despite the wealth of information provided by these techniques, they suffer from issues such as limited availability, excessive cost, limited sensitivity, and penetration depth. Terahertz radiation has been growing in popularity for contactless and non-destructive testing across various industrial sectors, including pharmaceutical coating analysis, defect identification, and gas pipeline monitoring. The ability of terahertz waves to penetrate through dielectric materials such as plastics or ceramics combined with strong attenuation by liquid water provides the necessary contrast to image water presence in proton exchange membrane fuel cells and their components. Motivated by the recent commercial availability of a compact terahertz source and video-rate terahertz camera, a simple terahertz imaging system in transmission geometry was realised. First, as a first step towards flooding inspection in an operating fuel cell, the feasibility of the imaging system for visualising and quantifying liquid water during an ambient air desorption process for Nafion membranes of a wide range of thicknesses – NRE-212 (50 µm), N-115 (127µm), N-117 (180 µm) and N-1110 (254 µm) was investigated. It was demonstrated that the imaging system was able to quantify liquid water in the 25-500 µm thickness range, estimate membrane weight change related to liquid water desorption, which correlated well against simultaneous gravimetric analysis and visualise the room temperature liquid water desorption process of a partially hydrated Nafion N-117 membrane. Further work consisted in imaging water build-up inside an operating proton exchange membrane fuel cell using the terahertz imaging system, combined with high-resolution optical imaging. Using a custom-built, laboratory-scale, terahertz, and optically transparent fuel cell, two-phase flow phenomena of water accumulation and transport, such as membrane hydration, main droplet occurrence, water pool formation, growth, and eventual flush out by gases were imaged. Results of the terahertz agree with simultaneous optical imaging and electrochemical readings. To demonstrate the potential used of the proposed imaging modality, the effect of air gas flow rates on flooding was demonstrated