Hungyen Lin

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.

3D objects with integrated electronics are produced using an additive manufacturing approach relying on multiphoton fabrication (direct laser writing, (DLW)). Conducting polymer‐based structures (with micrometer‐millimeter scale features) are printed within exemplar matrices, including an elastomer (polydimethylsiloxane, (PDMS)) have been widely investigated for biomedical applications. The fidelity of the printing process in PDMS is assessed by optical coherence tomography, and the conducting polymer structures are demonstrated to be capable of stimulating mouse brain tissue in vitro. Furthermore, the applicability of the approach to printing structures in vivo is demonstrated in live nematodes (Caenorhabditis elegans). These results highlight the potential for such additive manufacturing approaches to produce next‐generation advanced material technologies, notably integrated electronics for technical and …

Terahertz time-domain spectroscopy has experienced significant progress in imaging, spectroscopy, and quality inspection, e.g., for semiconductor packaging or the automotive industry. Additive manufacturing alloys (also known as alloys for use in 3D printing) have risen in popularity in aerospace and biomedical industries due to the ability to fabricate intricate designs and shapes with high precision using materials with customized mechanical properties. However, these 3D-printed elements need to be polished thereafter, where the surface roughness is inspected using techniques such as the laser scanning microscope. In this study, we demonstrate the use of terahertz time-domain spectroscopy to assess the average roughness profile and height leveling of stainless steel for comparisons against the same parameters acquired using laser scanning microscopy. Our results highlight the potential of the proposed …

Giron Rodriguez et al. [ACS Sustainable Chem. Eng., 2023, 11, 1508] previously showed that radiation-grafted anion-exchange membranes containing N-benzyl-N-methylpiperidinium headgroups (MPIP-RG-AEM) are promising for use in CO2 electrolysis (cf. commercial and other RG-AEM types). For a more sustainable synthesis, MPIP-RG-AEMs have now been fabricated using a reduced 1.1 times excess of amine reagent (historically made using >5 times excess). A resulting RG-AEM promisingly had a bulk amination level that was comparable to those made with the traditional large excess. Unexpectedly, however, it had a significantly reduced water content, with two further batches showing that this observation was repeatable (and reproducible via measurements collected on a single batch using different techniques in different labs). The ionic conductivities of the RG-AEM made with a controlled 1.1 excess of …

Terahertz time-domain spectroscopy has been used to extract the water states of proton exchange membranes prepared at different relative humidities using saturated salt solutions and the results obtained show good agreement with literature.

Polyethylene glycol (PEG) hydrogel are commonly used as both scaffolds and drug delivery systems to aid in cellular differentiation, thus developing a good understanding of their water properties is key to PEG optimisation. Terahertz (THz) imaging has previously been used to analyse the water content inside polymer films in ambient environment. Here, we apply this approach to extract the desorption profiles of PEG hydrogels of four concentrations producing results that indicate confinement effects due to porosity on the diffusional properties of water. These preliminary data therefore highlight the broad applicability of THz imaging for the analysis of water properties of PEG hydrogels.

Fourier domain optical coherence tomography (FD-OCT) is a well-established imaging technique that provides high-resolution internal structure images of an object at a fast speed. Modern FD-OCT systems typically operate at speeds of 40,000–100,000 A-scans/s, but are priced at least tens of thousands of pounds. In this study, we demonstrate a line-field FD-OCT (LF-FD-OCT) system that achieves an OCT imaging speed of 100,000 A-scan/s at a hardware cost of thousands of pounds. We demonstrate the potential of LF-FD-OCT for biomedical and industrial imaging applications such as corneas, 3D printed electronics, and printed circuit boards.

Terahertz (THz) spectrum and technology are envisioned to be a highly promising solution for enabling 6G and beyond wireless networks. This demand coincides with an urgent need to develop efficient electromagnetic interference shields to alleviate electromagnetic pollution in the THz range. Fully bioderived THz shields could be a sustainable solution to transcend towards a biocarbon economy. Herein, we report an environmentally benign and facile approach to fabricating 3D porous ultra-light aerogels and flexible nanopapers from cellulose nanofibers and highly conductive biocarbon fully derived from biomass. These functional constructs showcased excellent broadband THz shielding performance in both sub THz (W band) and THz region (0.4–2.0 THz). The THz shielding effectiveness of a 600 µm thick nanopaper and 3.00 mm aerogel was found to be 46 dB and 70 dB, with superior THz absorption …

Perfluorinated sulfonic-acid ionomers are the most common proton exchange membrane material whose structure underpins their unique water and chemical/mechanical stability properties. Understanding this performance-stability trade-offs is therefore vital for realising optimal membranes. Terahertz time-domain spectroscopy has been demonstrated to resolve molecular water states and retention properties inside thick Nafion membranes. By developing a parametric-based algorithm for data analysis, we demonstrate the broad applicability of this technique to industrially relevant thin ionomers (13–70 μm) prepared under various processing conditions where results are supported by conventional gravimetric analysis and prior demonstrations. Using this technique therefore opens up opportunities for rapid future parameter space investigation for membrane optimisation.

The ultra-rough surface of the 3D-printed metal produced by laser powder bed fusion technique is one of the major challenges faced in commercializing 3D-printed metallic components in a wider context. The electrochemical polishing method is currently considered a promising method to overcome this obstacle. However, evaluating the average roughness of a 3D printed surface before and after polishing with the current state-of-the-art laser scanning microscopy suffers incomplete datasets due to highly scattered signals. Here we successfully evaluate the rms roughness of the metallic surfaces with different roughness levels using terahertz time-domain spectroscopy (THz-TDS) for comparison against roughness values acquired using a commercial laser scanning microscope.

In-line terahertz pulsed imaging has been utilized to measure the film coating thickness of individual tablets during the coating process in a production-scale pan coater. A criteria-based waveform selection algorithm (WSA) was developed to select terahertz signals reflected from the surface of coating tablets and determine the coating thickness. Since the WSA uses many criteria thresholds to select terahertz waveforms of sufficiently high quality, it could reject some potential candidate tablet waveforms that are close but do not reach the threshold boundary. On the premise of the availability of large datasets, we aim to improve the efficiency of WSA with machine learning. This article presents a recurrent neural network approach to optimize waveform selection. In comparison with the conventional method of WSA, our approach allows more than double the number of waveforms to be selected while maintaining …

Terahertz time-domain spectroscopy (THz-TDS) has been recently demonstrated to resolve water states inside industrially thin fuel cell membranes. In this work, we apply THz-TDS to anion exchange membranes (AEMs) where detailed water information is crucial to their stability and operational performance. By performing independent ionic conductivity measurement on said membranes, we show a good correlation against the extracted conductivities and resolved bulk water of three AEMs produced with different amination time.

Water plays an important role in the degradation of polymers. Diffusion across bulk polymers is commonly investigated by gravimetric and capacitive techniques. Here, the water diffusion in polycarbonate (PC) and poly(methyl methacrylate) (PMMA) is studied by terahertz time-domain spectroscopy (THz-TDS) and gravimetric measurements. Time-resolved measurements of the THz refractive index and absorption coefficients show a good agreement to the gravimetrics and allow the contact-free extraction of the diffusion coefficients.

Successful water management is critical to achieving high performance in proton exchange membrane fuel cells (PEMFCs). The relatively high sensitivity of terahertz radiation to liquid water is promising as a complementary alternative for flooding detection in PEMFCs. Using a novel, commercially available terahertz source and camera, this paper investigates the feasibility of a compact and low‐cost terahertz imaging system for visualizing water build‐up inside an operating PEMFC, supported by simultaneous high‐resolution optical imaging. Several phenomena of water accumulation and transport, such as membrane hydration, main droplet appearance, water pool formation, growth, and eventual flush out by gases are imaged by terahertz imaging. These results agree with optical imaging and cell voltage readings. The proposed setup is also able to distinguish the effect of different air flow rates.

Seed germination and uniform plant stand in the field are the most critical crop growth stages determining the final yield. Pea (Pisum sativum L.) seeds production is often hampered due to the seed dormancy caused by the hard seed coat. Such effect is mainly attributed to poor or uneven germination and unsynchronised seedling emergence. Understanding the time course of water intake and several critical germination indicators can reveal many features of seed germination such as rate and uniformity. This paper used optical coherence tomography (OCT), a noninvasive and cross‐sectional imaging technique, to monitor the inner structural changes throughout the germination process. A sequence of cross‐sectional OCT images of pea (P. sativum L.) seeds, together with additional microscopic optical images, was recorded continuously and in situ for over 40 h. OCT and microscopic images revealed the …

The dynamic behavior of polycarbonate and poly(methyl methacrylate) upon water immersion at temperatures of 40 °C and 70 °C was studied by gravimetric measurements and by terahertz time-domain spectroscopy. Time-resolved measurements of the terahertz refractive index and absorption coefficient show a good agreement to the gravimetric measurements. Assuming a Fickian diffusion mechanic the diffusion coefficients were extracted from both methods. It was found that the terahertz measurements provide a reliable non-contact way to study the water diffusion in polymers. Especially, the THz absorption coefficient seems to be a robust method to determine the diffusion coefficients.

The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz …

Water states in proton exchange membranes have previously been extracted using terahertz time-domain spectroscopy (THz-TDS) for thick Nafion membranes. In this work, we have developed a parametric based algorithm for data analysis and applied it to thin industrially relevant membranes (13-70μm), processed under different conditions, producing results consistent with conventional gravimetric analysis and prior demonstrations. This therefore opens up opportunities for rapid membrane investigations and understanding.

Waveform selection plays an important role in the processing of in-line terahertz measurements of pharmaceutical tablet coating processes. This paper presents an approach to optimise waveform selection by utilising an artificial recurrent neural network and transfer learning. The results show that the averaged coating thickness gradually increases throughout the coating process. In comparison with the conventional method, our approach allows more than double the number of waveforms to be selected without compromising on the accuracy when compared against off-line measurements. Moreover, the processing time of waveform selection decreases so that it can be applied for real-time coating monitor in the pharmaceutical industry.

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) can be used to quantify water absorption in hygrothermally aged simple and commercial epoxy systems supported by conventional gravimetric analysis.

In the paper, synthesis of chitosan-based hydrogels modified with Aloe vera juice is presented. The novelty of the research was a combination of hydrogel materials with properties beneficial in viewpoint of their use as modern wound dressings and Aloe vera juice supporting the wound healing process. Hydrogels have been obtained via UV radiation. The impact of the amount of the crosslinking agent as well as the introduction of the Aloe vera juice into the hydrogel matrix has been determined. Performed measurements involved analysis of the swelling ability, characteristics of the surface roughness, determining the release profile of Aloe vera and the contact angles of hydrogels. Furthermore, the analysis of the dehydration process of the polymer membrane, investigations on the cytotoxicity of hydrogels via MTT reduction assay and the neutral red uptake assay as well as the studies on the pro-inflammatory …

The eggshell membrane (ESM) is an abundant resource with innate complex structure and composition provided by nature. With at least 60 million tonnes of hen eggs produced globally per annum, utilisation of this waste resource is highly attractive in positively impacting sustainability worldwide. Given the morphology and mechanical properties of this membrane, it has great potential as a biomaterials for wound dressing. However, to date, no studies have demonstrated nor reported this application. As such, the objective of this investigation was to identify and optimise a reproducible extraction protocol of the ESM and to assess the physical, chemical, mechanical and biological properties of the substrate with a view to use as a wound dressing. ESM samples were isolated by either manual peeling (ESM-strip) or via extraction using acetic acid [ESM-A0.5] or ethylenediaminetetraacetic acid, EDTA [ESM-E0.9 …

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

the realization of terahertz external amplitude modulators with a carrier frequency of 0.8 THz is presented for application in the next generation near-field wireless communications.

In the field of non-destructive testing, terahertz sensing has been used to analyze a wide range of materials where the most successful applications have involved materials that are semi-transparent to terahertz radiation. In this work, we demonstrate the sensitivity of terahertz time-domain spectroscopy to quantify water absorption in hygrothermally aged simple and commercial epoxy systems supported by conventional gravimetric analysis.

Terahertz time domain spectroscopy (THz-TDS) has been demonstrated to be capable of quantifying water uptake and retention properties of Nafion proton exchange membranes (PEMs). With a growing interest in thinner membranes, we analysis thin membranes to reveal its water uptake and retention properties.

We demonstrate how reflection terahertz time-domain spectroscopy (THz-TDS) can be used to measure conductivity of large area graphene films through silicon substrate. This through-substrate approach in turn allows an unhindered access to the graphene top surface and thus, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

We demonstrate how terahertz time-domain spectroscopy (THz-TDS) operating in reflection geometry can be used for quantitative conductivity mapping of large area chemical vapor deposited graphene films through silicon support. We validate the technique against measurements performed using the established transmission based THz-TDS. Our through-substrate approach allows unhindered access to the graphene top surface and thus, as we discuss, opens up pathways to perform in situ and in-operando THz-TDS using environmental cells.

The performance and durability of proton exchange membrane fuel cells are closely linked to effective water management strategies. Inadequate water balance results in polymer electrolyte membrane (PEM) dehydration or water flooding, both leading to performance loss and possible cell failure. Exploiting the contrast from liquid water absorption and transparency of dielectric materials, such as polymers, at terahertz (THz) frequencies, we apply a simple, compact THz imaging system to image water build-up in the flow channels of a THz-transparent fuel cell supported by high resolution optical gauging.

In-line terahertz sensing has been demonstrated to measure the coating thickness of individual pharmaceutical tablets during coating operation. As coating uniformity is highly dependent on tablet mixing, this study presents our research progress to date on using in-line terahertz sensing to investigate the effects of baffle design, drum rotational speed and batch size, on tablet mixing inside a laboratory-sized perforated pan coater.

Terahertz pulsed imaging (TPI) was introduced approximately fifteen years ago and has attracted a lot of interest in the pharmaceutical industry as a fast, non-destructive modality for quantifying film coatings on pharmaceutical dosage forms. In this topical review, we look back at the use of TPI for analysing pharmaceutical film coatings, highlighting the main contributions made and outlining the key challenges ahead.

Nafion membranes are considered as the industry standard electrolyte material for proton exchange membrane fuel cells. These membranes require adequate hydration in order to reach a high proton conductivity. The relatively high sensitivity of terahertz radiation to liquid water enables contrast to be observed for inspecting water presence in Nafion electrolyte membranes. Utilising a commercially available terahertz source and camera, this paper investigates the feasibility of a compact terahertz 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 (127 μm), N-117 (180 μm) and N-1110 (254 μm). We demonstrate that the terahertz imaging system is able to quantify liquid water in the 25–500 μm thickness range, estimate membrane weight change related to liquid water desorption …