Effect of the Electroless Nickel, Electroless Palladium, and Immersion Gold Multilayer as a Diffusion Barrier on the Bonding Strength of BiTe-Based Thermoelectric Modules.

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.

Effect of Plating Layers on the Bonding Strength of p-Type Bi-Te Thermoelectric Elements.

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.

Self-Propagating Heat Synthetic Reactivity of Fine Aluminum Particles via Spontaneously Coated Nickel Layer.

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.

Effects of the Process Conditions on the Color of Dye-Treated Anodized AA5052.

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.

Acid Palladium-Nickel Electroplating Using Ethylenediamine as Complexing Agent.

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.

Effect of Etching Time and Temperature on Plating Adhesion of Acrylonitrile Butadiene Styrene (ABS)-Based Material.

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.

Fabrication of an Aluminum-Based Thermoelectric Module Substrate via Anodization and Nickel Plating.

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.

Fabrication of a Bi2Te3-Based Thermoelectric Module Using Tin Electroplating and Thermocompression Bonding.

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.

Wet Etching Method for Electroless Ni-P Plating of Bi-Te Thermoelectric Element.

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.

Fabrication of Aluminum-Based Thermal Radiation Plate for Thermoelectric Module Using Aluminum Anodic Oxidization and Copper Electroplating.

In this study, electrolytic etching, anodic oxidation, and copper electroplating were applied to aluminum to produce a plate on which a copper circuit for a thermoelectric module was formed. An oxide film insulating layer was formed on the aluminum through anodic oxidation, and platinum was coated by sputtering to produce conductivity. Finally, copper electroplating was performed directly on the substrate. In this structure, the copper plating layer on the insulating layer served as a conductive layer in the circuit. The adhesion of the copper plating layer was improved by electrolytic etching. As a result, the thermoelectric module fabricated in this study showed excellent adhesion and good insulation characteristics. It is expected that our findings can contribute to the manufacture of plates applicable to thermoelectric modules with high dissipation performance.

A Novel Fabrication Method of Bi2Te3-Based Thermoelectric Modules by Indium Electroplating and Thermocompression Bonding.

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.