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In thermoelectric modules, multiple n-type and p-type thermoelectric elements are electrically connected in series on a Cu electrode that is bonded to a ceramic substrate. Defects in the bond between the thermoelectric elements and the Cu electrode could impact the performance of the entire thermoelectric module. This study investigated the effect of plating layers on the bonding strength of p-type Bi–Te thermoelectric elements. Ni and Pd electroplating was applied to Bi–Te thermoelectric elements; further, electroless Ni–P immersion gold (ENIG) plating was applied to Cu electrodes bonded to ceramic substrates. Forming a Pd/Ni electroplating layer on the surface of thermoelectric elements and an ENIG plating layer on the surface of the Cu electrode improved the bonding strength by approximately 3.5 times. When the Pd/Ni and ENIG plating layers were formed on Bi–Te elements and Cu substrates, respectively, the solderability greatly increased; as the solderability increased, the thickness of the diffusion layer formed with the solder layer increased. The improved bonding strength of the Pd/Ni plated thermoelectric element bonded on the ENIG plated substrate is attributed to the enhanced solderability due to the rapid inter-diffusion of Pd and Au into the solder layer and the formation of a stable and non-defected solder reaction interface layer.

An effective diffusion barrier layer was coated onto the surface of BiTe-based materials to avoid the formation of brittle intermetallic compounds (IMCs) by the diffusion of the constituents of Sn-based solder alloys into the BiTe-based alloys. In this study, the electrochemical deposition of multi-layers, i.e., electroless nickel/electroless palladium/immersion gold (ENEPIG) was explored to enhance the bonding strength of BiTe materials with Cu electrodes. The thermoelectric modules with the ENEPIG plating layer exhibited high bonding strengths of 8.96 MPa and 7.28 MPa for the n- and p-type, respectively that increased slightly to 9.26 MPa and 7.76 MPa, respectively after the thermoelectric modules were heated at 200 °C for 200 h. These bonding strengths were significantly higher than that of the thermoelectric modules without a plating layer.

Zn–Ni alloy plating is widely applied in manufacturing of automobile and construction material components because it provides better corrosion resistance and wear resistance than Zn plating. Furthermore, chromate coating treatment is gaining attention with respect to improving the corrosion resistance of Zn–Ni alloys. In this study, we investigated the effects of NaF additive concentration and treatment time on trivalent chromate coating, which has been developed as an alternative to hexavalent chromate coating. The chromate post-treatment solution used in this study comprises Cr(NO3)3·9H2O (360 g/L), CoSO4·7H2O (40 g/L), and HNO3 (35 mL/L), to which NaF is added in the concentration range of 0–30 g/L. The as-formed coating films at 1.6 pH and 60 °C treatment temperature for deposition times ranging from 30 to 120 s demonstrated a decreasing corrosion rate as the NaF concentration increased. The electrochemical and morphological analyses inferred that NaF acted as a catalyst, enhancing the rate of film formation. Furthermore, the film thickness increased with the treatment time, and the film color changed in the order of yellow, purple, and green.

We investigated the effects and interactions of the organic additives Polyquaternium-2 (PUB) and 1-benzyl pyridinium-3-carboxylate (BPC) in alkaline non-cyanide zinc electroplating. As PUB and BPC were added, the cathode potential of the polarization curve shifted in the negative direction at the same current density that occurred in the electrochemical experiment, and as confirmed by a scanning electron microscopy, the particles on the plating surface in the zinc deposits became finer, and the grain size decreased. Moreover, strong (101) and (002) peaks appeared in the X-ray diffraction pattern when no additive was added. However, as PUB and BPC were added, the intensity of the two peaks decreased, and an increase in the intensity of the (100) peak changed to a crystallographic orientation. With the addition of PUB and BPC, the gloss and whiteness gradually increased, and the surface roughness decreased. Finally, the throwing power tended to increase as PUB and BPC were added.

To produce BiTe-based thermoelectric modules, BiTe-based species with an electroless NiP (EN) coating layer acting as a diffusion barrier were connected to Cu electrodes by a soldering process. In order to investigate the thermal stability of the EN diffusion barrier on BiTe-based thermoelectric materials, Ni-coated BiTe-based modules were annealed at 200 °C for 200, 500, and 1000 h. The bonding strength of the BiTe species was >8 MPa, and it remained stable after 200, 500, and 1000 heating hours. The interfacial structure indicated the formation of a Sn–Cu intermetallic compound (IMC) below the EN-plated region. The morphology of Sn–Cu IMC before and after heating was observed. As the current flow was applied through the BiTe-based thermoelectric module, the temperatures on the hot and cold side of the modules were recorded. Owing to the thermal stability of the EN layer, the 200 h heated modules presented heating and cooling efficiencies similar to those of non-heated modules.

Aluminium (Al)-based alloys have attracted as promising materials for structural applications using metal 3D printing due to their excellent specific mechanical strength, formability, and durabilit...

This study investigates a brazing method for manufacturing PbTe thermoelectric modules using a Ag-based filler metal with a melting point of about 650 °C. To improve the bonding strength between the Ag-based brazing layer and the PbTe thermoelectric module, an electroless Ni-P plating layer is formed on the surface of the thermoelectric module as a diffusion barrier layer. The bonding strength of the PbTe thermoelectric module manufactured by the electroless Ni-P plating and Ag-based brazing has a high value of approximately 8.3 MPa. No defects such as pores or cracks were observed at the bonding interface between the thermoelectric element and the brazing layer. Furthermore, because of the high bonding strength of the manufactured thermoelectric module, fractures occur inside the thermoelectric element rather than at the bonding interface. Accordingly, the electroless Ni-P plating and Ag-based brazing method proposed in this study is found to be effective in manufacturing PbTe-based thermoelectric modules with high bonding strength.

In the present study, an acidic palladium-nickel alloy plating solution was prepared with ethylenediamine as the complexing agent, and the physical properties of the alloy plating layer were investigated. The palladium-nickel alloy plating layer could be deposited over a broad range of current densities as the deposition potential of palladium in the solution containing ethylenediamine significantly shifted toward the base direction and the value was similar to that of nickel. X-ray diffraction patterns confirmed that the palladium-nickel alloy existed as a single-phase solid solution in the prepared alloy. The hardness of the palladium-nickel alloy plating layer increased with the increase in nickel content. The surface and cross-sectional images of the palladium-nickel alloy plating layer indicated a dense and uniform morphology without defects such as cracks and pores.

In this study, the effects of etching time and temperature on the adhesion of plated layers of acrylonitrile butadiene styrene (ABS) were investigated. The ABS surface micropores, which act as anchors to improve the adhesion of the plated layer, increased in numbers as the etching temperature increased. Adhesion was maximum at the etching time of 9 min at etching temperatures of 60 and 70 °C. For the etching times of 12 and 15 min, micropores on the ABS surface were incompletely filled during electroplating because the pores were both deep and narrow. The adhesion strength was decreased by these unfilled micropores, which further reduced the anchor effect. The ratio of the increased surface area ratio of the ABS to that of the plated surface was maximized at 9 min etching time, at which point the plating adhesion was also maximized.

A method for directly bonding thermoelectric elements onto copper electrodes without applying a solder paste was developed in this study. A tin coating of thickness approximately 50 μm was deposited via electroplating onto the surface of a Bi₂Te₃-based thermoelectric element, which had a nickel diffusion barrier layer. The resulting structure was subsequently subjected to direct thermocompression bonding at 250 °C on a hotplate for 3 min at a pressure of 1.1 kPa. Scanning electron microscopy imaging confirmed that a strong and uniform bond was formed at the copper electrode-thermoelectric element interface, and the melted or solidified tin layer remained defect-free. The thermoelectric module fabricated using tin plating had an average bonding strength similar to that fabricated using soldering.

In this study, a thermoelectric module substrate was fabricated by subjecting an aluminum plate to a surface treatment process. To achieve this, the aluminum-based substrate was carried out to electrolytic etching, anodization, and Ni plating. The anodization of aluminum created an oxide film, which served as an insulation layer, while the Ni plating formed a conductive circuit layer. The substrate fabricated in this study exhibited excellent insulation performance, demonstrating its potential for future use in thermoelectric module substrates. Its adhesion properties were verified using a cross-cut adhesion test; microstructures of the surface and cross-section revealed the successful formation of the oxide film and Ni circuit layers on the aluminum base. From the results of these, it is clearly confirmed that the anodized aluminum substrate developed in this study provides suitable insulating performances and bonding nature with Ni electrode.

Recently, the combination of nickel (Ni) and cobalt (Co) binary hydroxides have been regarded as a promising class of materials due to their excellent redox reversibility and relatively high capacity. In this work, we demonstrate Ni and Co layered double hydroxides (NiCo–LDHs) on Ni–coated textile for wearable supercapacitor (SC) applications. Ni–coated textile was used as flexible current collector and NiCo–LDH nanosheets were deposited on textile as a pseudocapacitive materials. The electrochemical performances of NiCo–LDHs have been evaluated and optimized by varying the mole ratio of Ni and Co via hydrothermal process. The NiCo–LDH electrodes with Ni:Co ratio of 3:7 has the maximum specific capacitance, which is attributed to the CoOOH by improving the conductivity and enlarged interlayer spacing with intercalation of NO3−. Moreover, solid–state NiCo–LDH SC exhibits a maximum volumetric energy density of 1.25 mWh cm−3 at a power density of 47.4 mW cm−3.

Aluminum powders are known to provide outstanding volumetric exothermic enthalpy energy during thermal oxidation. However, the amount of energy released tends to be limited by the dense surface oxide (Al2O3) layer of the powder. Hence, a prerequisite for improving the reactivity of passivated Al particles is to remove the Al2O3 film from the surface. Considering that the self-propagating high-temperature synthesis (SHS) reaction of Ni and Al can generate additional exothermic heat in Al powder, Ni can be considered as a promising alternative to the surface oxide layer. Here, we report oxide-layer-free fine Al particles with a characteristic Ni/Al interface, where a Ni layer replaces the Al2O3 film. The microstructure of the synthesized powder consists of a 200-nm-thick Ni layer homogeneously coated on the Al surface, which has nanosized craters caused by the geometrical removal of Al2O3. Thermal analysis and in-situ heating transmission electron microscopy (TEM) results clearly show that active interdiffusion of atoms through the Ni/Al interface results in the formation of intermetallic compounds to provide additional exothermic energy, compared to the result for simply mixing Ni and Al powders. Hence, these findings provide new routes for the design and application of reactive metallic particles using the SHS reaction.

Fifty-pair Bi2Te3-based thermoelectric (TE) modules were produced by spreading Sn alloy solder on Cu electrodes before heating at 270℃ for 10 min to connect Bi2Te3-based legs with Cu electrodes. To suppress unexpected reactions between solder and TE materials, a conventional electroless nickel (EN) layer and a novel coating encompassing EN, electroless palladium, and immersion gold (ENEPIG) layers were adopted for Bi2Te3 materials as diffusion barriers. Influence of variant diffusion barriers, namely, EN and ENEPIG, on TE performance of module was ascertained. Interfacial structure at Bi2Te3/solder/Cu electrode interface revealed that intermetallic compounds were generated owing to interdiffusion among Cu, Sn, and coating elements, and they mostly concentrated at Bi2Te3/solder and solder/Cu electrode interfaces. Additionally, ENEPIG coating exhibited much better wettability with solder than the EN coating, resulting in fewer defect sites at Bi2Te3/solder interface. When current flowed through TE modules, the ENEPIG-coated module yielded lower temperatures on both hot and cold sides than the EN-coated module. While the modules had temperature differences between two substrates, output power generated by the ENEPIG-coated module was higher than that of the EN-coated module. In addition, the ENEPIG-coated module showed a smaller internal resistance than the EN-coated module. After heating at 200℃ for 200 h, both modules presented markedly reduced output power, whereas internal resistance was escalated.

An electroless nickel (EN) layer and EN, along with electroless palladium (EP), and immersion gold (IG) (ENEPIG) multilayers were two variant diffusion barriers employed for Bi2Te3-based thermoelectric materials. After the electroless plating process, coated Bi2Te3-based species were bonded with Cu electrodes using a soldering method with Sn-based alloy as a solder. Heat treatment was conducted at 200 °C for 200, 500, and 1000 h to investigate the thermomechanical stability of the Bi2Te3/solder interface adopting different coatings. First, the Bi2Te3/solder interface characteristics were analyzed by compositional identification and morphology observation. Intermetallic compounds (IMCs) were generated below the plating area in both EN and ENEPIG. The phase composition of these IMCs was identified by qualitative analysis. Additionally, the thickness of IMCs with ENEPIG was larger than those with EN regardless of annealing duration. The IMC morphology without and with 200 h of heating showed columnar grains, while those with heating over 500 h exhibited equiaxed grains. The ENEPIG coating generally yielded a higher bonding strength for the Bi2Te3/solder interface than the EN layer. Meanwhile, the bonding strength for EN and ENEPIG was constant after 200 h of heating, followed by a significant drop at 500 h, and it finally leveled off after heating for 1000 h. Furthermore, the fracture caused by the shear test was characterized by the brittle fracture mode. Ultimately, the IMC morphology was found to influence the bonding strength along with the increase in annealing duration rather than IMC thickness and composition.

In this study, we devised a method to bond thermoelectric elements directly to copper electrodes by plating indium with a relatively low melting point. A coating of indium, ~30 μm in thickness, was fabricated by electroplating the surface of a Bi2Te3-based thermoelectric element with a nickel diffusion barrier layer. They were then subjected to direct thermocompression bonding at 453 K on a hotplate for 10 min at a pressure of 1.1 kPa. Scanning electron microscopy images confirmed that a uniform bond was formed at the copper electrode/thermoelectric element interface, and the melted/solidified indium layer was defect free. Thus, the proposed novel method of fabricating a thermoelectric module by electroplating indium on the surface of the thermoelectric element and directly bonding with the copper electrode can be used to obtain a uniformly bonded interface even at a relatively low temperature without the use of solder pastes.

The diffusion barrier layers in thermoelectric generators (TEGs) are widely used to improve their mechanical/thermal stabilities and energy generation performance; however, thickness dependence of diffusion barrier was rarely reported. In this study, we prepared Bi2Te3-based thermoelectric (TE) modules and Ni diffusion barrier with a thickness of 0–30 µm to investigate the correlation between the thickness of Ni layers and TE output performance. A TE module with 1 μm-thick Ni layer harvests a maximum output power of 2.79 mW under a temperature difference (ΔT) of 200 K, which is a higher value than that of a module without a diffusion barrier. Regarding the dependence of TE output performance on the diffusion barrier’s thickness, TEGs based on the thicker Ni layers showed lower output performance for all ΔT ranges compared with a 1 µm-Ni layer TEG. The degradation of TE conversion efficiency was related to the increment of contact resistance (Rc) with the thickness of the diffusion barrier. This Ni thickness-dependent Rc was explored using the transmission line method. The results from this study prove that the thickness of optimized diffusion barriers would maximize the output performance of TEGs, which could be used as a guide for improving the performance of TEGs.

We investigated the effect of thermal aging on the contact resistance and solderability of Au-Sn and Au-Co alloy films. The Au-Sn alloy exhibited lower contact resistance and better solderability compared to the conventional Au-Co alloy. In the Au-Sn alloy, an Au-Sn–Ni intermetallic compound layer was formed at the interface between the electroplated Ni and Au-Sn layers during the thermal aging process. The resulting Au-Sn–Ni layer effectively blocked the diffusion of underlying nickel to the surface, leading to improved solderability and enhanced thermal stability of the contact resistance. Meanwhile, the results revealed that a significant amount of nickel diffused to the surface of the Au-Co alloy during the thermal treatment, and the nickel atoms on the surface were oxidized to NiO or Ni2O3, which increased the contact resistance and deteriorated solderability by acting as a barrier layer.

In this study, an electroless nickel (EN), electroless palladium (EP), and immersion gold (IG) (ENEPIG) multilayer coat was developed for application on BiTe-based thermoelectric materials to increase the bonding strength between BiTe-based materials and Cu electrodes in thermoelectric modules. The ENEPIG-plated thermoelectric module exhibited a bonding strength of over 11 MPa, which was approximately 40% higher than those of conventional EN-plated modules. According to the interfacial analysis of the bonding areas via a field emission electron probe micro analyzer (FE-EPMA), the Ni layers in ENEPIG and EN acted as diffusion barriers to inhibit the formation of brittle Sn-Te intermetallic compounds (IMCs). Sn-Cu-Pd and Sn-Cu IMCs were formed below the Ni-plated area at the bonding interface of the ENEPIG and EN-plated modules, respectively. However, Sn-Cu-Pd IMCs in the ENEPIG interface exhibited thicker and more homogeneous IMC layers after heat treatment compared to the Sn-Cu IMCs in the conventional EN interface. The wettability of the ENEPIG-plated BiTe surface was significantly greater than that of the EN-plated surface. This directly induced fewer and smaller pores on the bonding interface of the BiTe-based materials with ENEPIG plating compared to those with EN plating, thereby resulting in the higher bonding strength of ENEPIG-plated modules.

Nanodiamond-dispersed Bi2Te2.7Se0.3 (ND/BTSe) matrix composites were fabricated by a high energy ball milling followed by spark plasma sintering process. The fabricated ND/BTSe composites show that ND powders with 5 nm in size are dispersed in the BTSe matrix grain rather than agglomerated at the grain boundary. It was found that atomically disordered-lattice structure present nearby the newly formed ND/BTSe interfaces. This interfacial region artificially formed by addition of ND powders plays a role as atomic-scaled defects increasing electron concentration. Electric conductivities of all the ND/BTSe composites show significantly increased values compared to that of pure BTSe in the temperature range from 298 K to 473 K. However, total thermal conductivity of the composites exhibit higher values than the BTSe due to superior electric conductivity even though active lattice phonon scattering at the interfaces affect lowering thermal conductivity. The maximum ZT, 0.97 was obtained from 0.5vol%ND/BTSe composite at 473 K and enhancement in ZT values was clearly revealed above 348 K. Therefore, these results elucidate that addition of ND powders into n-type BTSe matrix is promising method to improve thermoelectric performances.

In the present study, electroless Ni–P plating was applied to Bi–Te-based thermoelectric materials as a barrier layer and the effect of the Ni–P plating on the bonding strength of the thermoelectric module was investigated. The bonding strength of the n- and p-type modules increased after being subjected to the electroless Ni–P plating treatment. In the case of the thermoelectric module that was not subjected to electroless Ni–P plating, Sn and Te were interdiffused and formed a brittle Sn–Te-based metallic compound. The shearing mostly occurred on the bonding interface where such an intermetallic compound was formed. On the other hands, it was found from the FE-EPMA analysis of the bonding interface of thermoelectric module subjected to electroless Ni-P plating that the electroless Ni-P plating acted as an anti-diffusion layer, preventing the interdiffusion of Sn and Te. Therefore, by forming such an anti-diffusion layer on the surface of the Bi–Te based thermoelectric element, the bonding strength of the thermoelectric module could be increased.

This study investigated the effects of process conditions (anodization time, coloring treatment time, dye concentration) on the color of dye-treated anodized aluminum alloy 5052 (AA5052). The color change of the anodic layers and amount of dye adsorbed were quantitatively analyzed using a UV-vis spectrophotometer. The color of the anodic layer turned darker as the anodizing time, coloring treatment time, and dye concentration increased due to the amount of dye adsorbed by the layers. As the anodizing time increased, the thickness of the anodic layers also increased, and a growth in the number of nanopore sites resulted in a greater amount of dye being adsorbed, causing the darkening of the color. Increase in the coloring treatment time and dye concentration led to a greater amount of dye being adsorbed in the anodic layers, thereby contributing to the darkening of the color. Quantitative analysis in the depth direction of the anodic layer using glow discharge optical emission spectroscopy revealed that the dye was mostly adsorbed on the outer region of the anodic layer.

In this study, a method for electroless Ni-P plating with excellent adhesion via chemical wet etching to fabricate Bi-Te thermoelectric modules is proposed. The electroless Ni-P plating formed through the proposed method showed excellent adherence without peeling, even under heat treatment of 200 °C for 24 h. Wet etching and electroless Ni-P plating was performed on a Bi-Te thermoelectric module, which showed the excellent bond strength of approximately 10 MPa. The surface roughness of the Bi-Te thermoelectric element was increased significantly by the wet etching process, which secured the adherence of the Ni-P plating by anchoring to this induced surface roughness.

A method for directly bonding thermoelectric elements onto copper electrodes without applying a solder paste was developed in this study. A tin coating of thickness approximately 50