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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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Optimal AIN film deposition conditions for enhancing surface corrosion resistance and silver appearance
(2026-05-01)
Sanguanmak, Pattarapol
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Sakdanuphab, Rachsak
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Sakulkalavek, Aparporn
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Rudradawong, Chalermpol
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Somjaijaroen, Natthawirot
This study investigates the development of AlN thin-film protective coatings for enhancing the corrosion resistance, surface hardness, and color stability of Ag 925 substrates used in decorative and wearable applications. AlN coatings were deposited by reactive DC magnetron sputtering, and the influences of working pressure and film thickness on bonding chemistry, oxygen incorporation, optical appearance, and artificial-sweat corrosion resistance were systematically evaluated. Increasing the working pressure promoted Al–O bond formation and noticeable surface discoloration, consistent with enhanced oxygen uptake that disrupted the near-surface Al–N bonding network. In contrast, films deposited within a moderate pressure window (2.5–7.5 × 10<sup>−1</sup> Pa) exhibited minimal color change (ΔE < 3), maintained stable Al–N bonding characteristics, and achieved a hardness of approximately 5.2 GPa. Film thickness further governed the protective performance: ultrathin AlN layers provided only limited resistance, whereas coatings thicker than ~ 300 nm significantly improved surface resistance, as supported by the reduced chloride-induced discoloration (ΔE decreased from ~ 33.6 to ~ 11) and the effective suppression of Ag and Cu oxidation, as confirmed by FE-SEM/EDS analysis. These results demonstrate that both an optimal working pressure regime and a sufficiently thick coating (> 300 nm) are essential for achieving long-term color stability and corrosion resistance in wearable silver applications.
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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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Sputter-deposited AlN coatings for enhanced tarnish resistance and mechanical durability of silver jewelry
(2026-04-01)
Sudsawad, Kanyarat
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Somjaijaroen, Natthawirot
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Somdock, Nuttakrit
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Sakdanuphab, Rachsak
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Sakulkalavek, Aparporn
Silver jewelry is prone to surface tarnishing caused by sulfur-containing species in ambient environments. In this study, a transparent aluminum nitride–based thin film was deposited by magnetron sputtering and evaluated as a protective barrier against silver tarnishing. Sheets of 99.9% pure silver were coated under various nitrogen flow conditions to optimize film composition and performance. An appropriate nitrogen flow rate of 25 standard cubic centimeters per minute (sccm), corresponding to an N₂/Ar gas ratio of approximately 1:1, was identified for forming AlN-rich films, while an AlN-based film thickness in the range of 80–110 nm was found to be suitable for jewelry protection. X-ray photoelectron spectroscopy analysis showed that insufficient nitrogen availability suppresses complete nitridation, resulting in residual metallic aluminum, which readily reacts with residual oxygen and moisture, increasing the oxygen content in the films. At an N₂ flow rate of 25 sccm, metallic aluminum was suppressed and the films were dominated by Al–N bonding with minor oxygen incorporation, accompanied by the development of a polycrystalline structure. Nanoindentation measurements performed on the 110 nm-thick film yielded a peak hardness of approximately 3.6 GPa, indicating enhanced mechanical durability compared with uncoated silver. Comparative evaluation of color difference, mechanical hardness, tarnish resistance, and environmental durability demonstrated improved performance of the AlN-coated silver. Finally, the practical applicability of the coating was demonstrated by depositing AlN-based films onto large and intricately designed silver jewelry items, indicating compatibility with industrial-scale processing.
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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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Advancing enterprise credit rating through multi-head self-attention and transfer learning
(2026-01-01)
Zhang, Rui
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Li, Qing
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Chen, Jiangchuan
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Li, Jie
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Sakdanuphab, Rachsak
This study introduces the multi-head self-attention rotary-positional encoding Informer (MHSA-RPEI), a novel enterprise credit rating model that leverages self-correlation weights and positional information of financial indicators to enhance rating accuracy. Corporate credit rating presents challenges such as temporal dynamics, complex non-linear relationships, and cross-regional variations, which traditional and deep learning models struggle to address. The MHSA-RPEI model overcomes these limitations through four innovations: (1) objective MHSA-RPEI model, which is specifically designed for credit rating, achieving high precision using only financial data and serving as an objective and validated tool; (2) revolutionary rotary positional encoding (RoPE), which enhances temporal modeling, improving dynamic risk assessment; (3) strategic transfer learning (TL), which facilitates cross-regional adaptations, mitigating data scarcity and domain variations; and (4) optimized informer architecture, which improves long-term risk forecasting based on extensive financial time series. Experiments on the U.S., Chinese, and Japanese financial data showed that MHSA-RPEI achieved 92.16% accuracy (U.S.), 90.32% (China), and 92.50% (Japan), with area under the receiver operating characteristic curve scores ranging from 88.47% to 91.95%, surpassing traditional and state-of-the-art models. Furthermore, TL experiments highlighted the exceptional adaptability of the MHSA-RPEI for cross-regional applications, yielding performance gains of 5–6%. Ablation studies confirmed that the RoPE and self-correlation weight modules were crucial to the effectiveness of the model.
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Microwave-assisted biosynthesis of silver nanoparticles using two marine microalgal extracts and their antimycobacteriosis activity against bacteria isolated from Betta splendens
(2025-12-01)
Manklinniam, Piyapan
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Phunpruch, Saranya
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Sakulkalavek, Aparporn
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Sakdanuphab, Rachsak
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Worananthakij, Worakrit
This study explores the synthesis and antibacterial properties of silver nanoparticles (AgNPs) as a safer, eco-friendly alternative to traditional chemical treatments for bacterial infections. AgNPs were synthesized using aqueous extracts of marine microalgae, Isochrysis galbana and Chaetoceros calcitrans, via conventional and microwave-assisted methods, with the latter accelerating nanoparticle production. Extracts in ethanol, hexane, and acetone were tested, with the ethanolic extract of I. galbana showing the strongest antibacterial effects. The AgNPs exhibited broad-spectrum antibacterial activity against pathogens such as Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, and fish pathogens like Aeromonas veronii. Microwave-assisted synthesis with ethanolic extracts resulted in the highest inhibition, particularly against fish and tuberculosis-related pathogens, including Mycobacterium marinum. Nanoparticle formation was confirmed using various characterization methods, including ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), which revealed crystalline structures. Transmission electron microscopy (TEM) analysis revealed that AgNPs varied in size, with an average diameter of less than 50 nm and all particles being smaller than 100 nm. This research demonstrates the potential of AgNPs as an effective alternative to antibiotics, offering targeted bacterial inhibition while reducing the risk of antibiotic resistance. This makes it a promising approach for treating bacterial infections in ornamental fish.
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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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