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Ultrathin AlN barrier coatings for enhancing surface chemical stability and suppressing electrochemical migration in immersion silver–finished printed circuit boards
(2026-08-15)
Kaewbuadee, Woraprach
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Theekhasuk, Nattharika
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Khumtong, Thanakorn
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Rudradawong, Chalermpol
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Sakdanuphab, Rachsak
Immersion silver-finished printed circuit boards (PCB-ImAg) provide low contact resistance and excellent solderability, but their chemical instability in humid and sulfur-containing environments can compromise long-term reliability. In this study, ultrathin aluminum nitride (AlN) films (20–60 nm) were deposited on PCB-ImAg substrates by reactive DC magnetron sputtering as inorganic barrier layers. Their protective performance was evaluated by accelerated H<inf>2</inf>S exposure, long-term ambient air exposure, tape testing, electrical resistance measurements, electrochemical migration (ECM) testing under a 3 V bias at 30 °C and 80% RH, and surface characterization. Uncoated PCB-ImAg samples showed severe tarnishing, Ag<inf>2</inf>S formation, dendritic corrosion, and a marked increase in electrical resistance after both H<inf>2</inf>S and prolonged air exposure. In contrast, AlN-coated samples retained a cleaner surface, remained adherent after the tape test, and showed much smaller resistance changes. X-ray photoelectron spectroscopy detected sulfur-related chemical states only on the uncoated surfaces, indicating suppression of sulfide formation by the AlN layer. A 20 nm AlN coating was sufficient for anti-tarnish and ambient air stability, whereas coatings of 40 nm or greater were required for robust ECM suppression. These results demonstrate that ultrathin AlN films effectively improve the corrosion resistance and ECM reliability of PCB-ImAg surfaces.
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Enhancement of thermoelectric performance and mechanical reliability in electrodeposited chitosan nanofiber-bismuth telluride nanocomposite
(2026-07-01)
Tian, Jianghan
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Gobpant, Jakrit
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Van Toan, Nguyen
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Theekhasuk, Nattharika
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Pham, Cong Kha
Thermoelectric generators (TEGs) offer a promising route for converting waste heat into electrical energy; however, the practical implementation of high-performance micro-TEGs (μTEGs) is limited by the material performance and mechanical fragility of thick bismuth telluride (Bi<inf>2</inf>Te<inf>3</inf>) films. Although thick thermoelectric layers are required to sustain sufficient temperature gradients, conventional fabrication often induces residual stress, leading to cracking and structural failure. In this work, we introduce a sustainable synthesis strategy by incorporating bio-derived chitosan nanofibers (ChNFs) into the electrodeposition process. The amino and hydroxyl functional groups of ChNFs promote interfacial bonding and nucleation, enabling the rapid growth of dense, crack-free composite films with thicknesses up to 1000 μm. The introduction of ChNFs also creates abundant phonon-scattering interfaces, significantly reducing lattice thermal conductivity from 1.48 to 0.29 W m−1 K−1 and resulting in a 303% increase in the room-temperature figure of merit (ZT), from 0.12 to 0.50. At an optimal loading of 0.0123 wt%, the composite films exhibit a 15% increase in hardness while maintaining structural integrity. These results demonstrate a multifunctional materials design strategy that simultaneously enhances thermoelectric performance, mechanical robustness, and fabrication scalability. The ChNF–Bi<inf>2</inf>Te<inf>3</inf> nanocomposites provide a viable green pathway for developing reliable thick films for next-generation self-powered electronics and compact waste-heat harvesting systems.
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Post-annealing effects on (00l) texture, Cl/Se ratio, and electrical and glass-like thermal transport in Bi₄O₄SeCl₂
(2026-04-25)
Somdock, Nuttakrit
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Theekhasuk, Nattharika
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Voraud, Athorn
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Limsuwan, Pichet
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Naemchanthara, Kittisakchai
Bi₄O₄SeCl₂ is a heteroanionic layered material with intrinsically low lattice thermal conductivity and anisotropic charge transport. In this work, the effects of post-annealing temperature on the crystallographic texture, anion chemistry, defect evolution, and transport properties of Bi₄O₄SeCl₂ were systematically investigated. Polycrystalline Bi₄O₄SeCl₂, synthesized by solid-state reaction combined with high-energy ball milling, was post-annealed at 400–700 °C. X-ray diffraction and electron microscopy revealed that post-annealing eliminated the residual BiOCl precursor phase, enhanced the (00 l) preferred orientation, and promoted grain growth up to 600 °C, followed by partial texture degradation at 700 °C due to recrystallization. Energy-dispersive spectroscopy showed progressive Se and Cl volatilization during annealing, leading to an increased Cl/Se ratio. The carrier mobility and electrical conductivity reached maximum values at 600 °C, consistent with improved texture and layered-domain connectivity. Thermal transport remained lattice-dominated and only weakly temperature-dependent. The phonon mean free path, estimated using kinetic theory, was in the sub-nanometer range (∼0.25–0.57 nm), comparable to the interatomic spacing, indicating glass-like phonon transport. Representative HRTEM observations also revealed dislocation-related lattice defects and locally distorted regions, suggesting that vacancy disorder and local strain fields may provide additional phonon scattering. These results demonstrate that post-annealing optimizes electrical transport through phase purification, texture development, and defect-mediated carrier regulation, while the lattice thermal conductivity remains fundamentally limited by intrinsic glass-like phonon transport in Bi₄O₄SeCl₂.
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Advances in flexible telluride thin-film thermoelectric generators: Deposition, performance, and module fabrication
(2026-04-01)
Theekhasuk, Nattharika
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Sakdanuphab, Rachsak
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Kianwimol, Supasak
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Khumtong, Thanakorn
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Toan, Nguyen Van
Flexible thermoelectric generators (TEGs) are gaining increasing attention for wearable and skin-attachable electronics due to their ability to harvest low-grade heat from the human body. In this work, p-type Sb<inf>2</inf>Te<inf>3</inf> and n-type Bi<inf>2</inf>Te<inf>3</inf> thin films were deposited by direct current (DC) magnetron sputtering, and the effects of post-deposition annealing on their structural, electrical, and thermal transport properties were systematically evaluated. X-ray diffraction revealed that n-Bi<inf>2</inf>Te<inf>3</inf> exhibited higher crystallinity than p-Sb<inf>2</inf>Te<inf>3</inf>, attributed to its lower formation energy. Annealing at 250 °C markedly enhanced the electrical conductivity of both films, with p-Sb<inf>2</inf>Te<inf>3</inf> showing a larger improvement due to concurrent increases in carrier concentration and mobility driven by Te volatilization and the formation of acceptor-type vacancies and antisite defects. In contrast, n-Bi<inf>2</inf>Te<inf>3</inf> exhibited a slight reduction in carrier concentration as a result of compensating donor defects. Consequently, maximum ZT values of approximately 0.5 for p-Sb<inf>2</inf>Te<inf>3</inf> and 0.25 for n-Bi<inf>2</inf>Te<inf>3</inf> were achieved at 513 K. Notably, the compatibility factor (S) values of both films differed by less than a factor of two, indicating favorable thermodynamic pairing for p–n module assembly. A flexible TEG module with 12 thermocouples was subsequently fabricated and delivered an output power of 0.65 μW at ΔT < 25 K, demonstrating promising potential for scalable self-powered wearable electronics.
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Dual optimization of ZT and output power in bulk Bi2Te3 through metal-assisted chemical etching
(2026-03-01)
Theekhasuk, Nattharika
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Sakulkalavek, Aparporn
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Ono, Takahito
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Sakdanuphab, Rachsak
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Nguyen, Duc Nam
Thermoelectric materials offer a promising route for sustainable energy harvesting by directly converting waste heat into electricity, enabling compact, solid-state, and environmentally friendly energy solutions. Among them, bismuth telluride (Bi₂Te₃) stands out as the benchmark material for near-room-temperature applications due to its excellent electronic transport properties and commercial maturity. However, achieving high-performance in bulk or thick-film Bi₂Te₃ remains a formidable challenge. Conventional strategies such as doping, alloying, and nanoinclusion, while successful in thin films, often fail to translate effectively to bulk systems due to issues like pore collapse, poor uniformity, and degraded electrical connectivity. These limitations hinder the formation of efficient phonon-scattering architectures without compromising charge transport, resulting in limited improvement in the thermoelectric figure of merit (ZT). In this study, we present a novel and scalable nanoengineering strategy that applies metal-assisted chemical etching (MACE) to fabricate nanoporous surface layers on bulk Bi₂Te₃ for the first time. Unlike conventional nanostructuring techniques, MACE enables the formation of oriented nanostructures via a simple wet-chemical process, offering high tunability, low cost, and compatibility with large-area substrates. To reduce interfacial resistance, nickel was subsequently electrodeposited onto the nanostructured surface, forming a conformal contact layer that improves charge extraction and output performance. By systematically tuning the MACE duration, the optimized nanostructured Bi₂Te₃ sample exhibited a 2.3-fold improvement compared to the pristine bulk sample. Furthermore, due to the increased surface area from the nanoporous architecture, the internal resistance and output power of the nanostructured Bi₂Te₃ devices demonstrated 25-fold and 5.8-fold improvments, respectively, relative to the untreated sample. These remarkable improvements are attributed to the synergistic effect of enhanced phonon scattering within the nanoporous layer and improved charge transport enabled by the conformal nickel coating. This work not only introduces a powerful nanostructuring route for Bi₂Te₃ but also establishes a practical platform for high-performance, thick-film thermoelectric devices. The findings offer deep insight into the structure, property, and performance relationships governing thermoelectric efficiency and pave the way toward the scalable fabrication of next-generation thermoelectric modules for real-world applications such as industrial waste heat recovery and self-powered electronics.
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Enhanced antimony telluride thermoelectric generators: From material synthesis to device applications
(2025-12-01)
Theekhasuk, Nattharika
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Sakdanuphab, Rachsak
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Voraud, Athorn
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Limsuwan, Pichet
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Sakulkalavek, Aparporn
This study investigates the effect of Bi₄O₄SeCl₂ (BOSC) addition (0–4 wt%) on the thermoelectric performance of p-type Bi₀.₅Sb₁.₅Te₃ synthesized via high-energy ball milling. XRD analysis revealed lattice incorporation at 1 wt% BOSC, while higher concentrations led to phase separation. The 1 wt% BOSC sample exhibited a significantly reduced total thermal conductivity of 0.28 W/m·K, compared to 0.46 W/m·K in the undoped sample, attributed to enhanced phonon scattering. Despite moderate decreases in electrical conductivity and Seebeck coefficient, a peak ZT of 1.02 at 50 °C was achieved—representing a ∼54 % improvement over the undoped material. Furthermore, a prototype thermoelectric module fabricated with BOSC-doped legs produced a power density of 17.6 mW/cm² under a 150 °C temperature gradient. These results demonstrate that BOSC is an effective additive for reducing thermal conductivity and enhancing overall thermoelectric performance, offering potential for energy harvesting applications at moderate temperatures.
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Advanced AlN/SiO2/AlN multilayer coatings for protecting gold-like decorative surfaces: Improved hardness and color stability
(2025-12-01)
Raengroeng, Sitanan
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Theekhasuk, Nattharika
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Sakulkalavek, Aparporn
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Sakdanuphab, Rachsak
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Somdock, Nuttakrit
This study explores the development of multilayer AlN/SiO<inf>2</inf>/AlN thin film coatings to enhance tarnish resistance, surface hardness, and color stability of gold-coated silver substrates for decorative use. Gold films were deposited via electroplating, followed by multilayer coatings using reactive magnetron sputtering. The SiO<inf>2</inf> thickness was systematically varied while maintaining fixed AlN layers. Optical evaluations using CIE Lab parameters confirmed that the specimen with a 110-min SiO<inf>2</inf> layer exhibited acceptable color difference (ΔE < 5). X-ray photoelectron spectroscopy (XPS) revealed stable Al–N and Si–O bonds with minimal oxidation. Nanoindentation tests showed a significant hardness increase, reaching 7.02 ± 0.62 GPa. After 240 days of ambient exposure, multilayer-coated samples showed no visible discoloration or sulfur-induced degradation, unlike uncoated and electrochemically coated samples. These results confirm that AlN/SiO<inf>2</inf>/AlN multilayers effectively improve the durability and aesthetic stability of gold-like surfaces, offering a promising solution for long-term decorative applications.
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Tuning the thermoelectric performance of flexible copper selenide thin films through sputtering pressure and hybrid microwave annealing
(2025-10-10)
Khuncharoen, Wasan
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Theekhasuk, Nattharika
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Rudradawong, Chalermpol
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Voraud, Athorn
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Sakdanuphab, Rachsak
Flexible copper selenide (Cu₂₋ₓSe) thin films were deposited on polyimide substrates by direct current magnetron sputtering under varying pressures (0.8–4.0 × 10⁻² mbar) and subsequently annealed using hybrid microwave irradiation at 250 °C for 10–30 min. Increasing sputtering pressure raised the copper content (62.2–63.8 at%) and suppressed the formation of Cu₃Se₂ impurities. Hybrid microwave annealing promoted the transformation to stoichiometric β-Cu₂Se, removed oxide phases such as selenium dioxide and copper oxide, and improved crystallinity, as confirmed by x-ray diffraction and x-ray photoelectron spectroscopy. Field-emission scanning electron microscopy revealed microstructural densification at 10–20 min, whereas 30 min induced cracks and porosity that degraded transport properties. The optimized 20-minute annealed film achieved a peak power factor of 81.5 × 10⁻⁵ W/m·K² at 300 °C—over 130 times higher than that of the as-deposited film and comparable to other flexible Cu₂Se systems. Stability tests confirmed excellent retention after three months of ambient storage. These results establish sputtering pressure control and hybrid microwave annealing as scalable strategies for high-performance, stable Cu₂Se thermoelectric films.
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Post-ball-milling-assisted solid-state synthesis of Bi4O4SeCl2: A low thermal conductivity material
(2025-02-01)
Theekhasuk, Nattharika
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Somdock, Nuttakrit
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Voraud, Athorn
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Ounrit, Iyarat
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Limsuwan, Pichet
This study investigates the synthesis of Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> through a cost-effective ball-milling-assisted solid-state reaction method. The as-grown samples predominantly consisted of the Bi<inf>12</inf>O<inf>15</inf>Cl<inf>6</inf> phase, with minor contributions from BiOCl and Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf>. A systematic post-ball-milling process was applied to enhance the formation of the Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> phase. Prolonged milling time led to the progressive dominance of the Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> phase, resulting in significant improvements in electrical conductivity and reductions in thermal conductivity. After 30 min of milling, the carrier concentration increased notably from −2.23 × 1016 cm−3 (as-grown) to −1.01 × 1018 cm−3, while electrical conductivity rose from 0.14 S/cm (as-grown) to 2.26 S/cm. Simultaneously, thermal conductivity decreased from 0.65 W m−1 K−1 (as-grown) to 0.35 W m−1 K−1. These findings demonstrate that post-ball-milling is a scalable and economical method for synthesizing Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> with low thermal conductivity, highlighting its potential as a promising material for thermal barrier coatings and thermoelectric applications.
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Enhanced thermoelectric properties of bismuth telluride via Ultra-Low thermal conductivity BOSC compound addition
(2024-12-01)
Theekhasuk, Nattharika
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Somdock, Nuttakrit
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Voraud, Athorn
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Limsuwan, Pichet
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Sakdanuphab, Rachsak
This study aimed to enhance the thermoelectric properties of bismuth telluride by adding Bi<inf>4</inf>O<inf>4</inf>SeCl<inf>2</inf> (BOSC). Commercial N-type bismuth telluride was mixed with BOSC powder in varying concentrations. As the BOSC content increased, the carrier concentration also rose due to chlorine atoms acting as donor impurities. Despite this increase, the power factor values of the samples with BOSC additives did not significantly differ from those of the bare bismuth telluride sample. However, the total thermal conductivity decreased significantly with the addition of BOSC, reaching a minimum value of 0.54 W·m–1·K–1 at 150 °C for the sample with 1 wt% BOSC. Notably, the ZT value for the sample with 1 wt% BOSC was about 0.86, which is four times higher than that of the bare bismuth telluride sample. Our findings demonstrate superior thermoelectric performance, indicating a more efficient modification of thermoelectric properties through the addition of BOSC to the bismuth telluride matrix.

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