Thermoelectric Device Fabrication Using Thermal Spray and Laser Micromachining

Thermoelectric generators (TEGs) are solid-state devices that convert heat directly into electricity. They are used in many engineering applications such as vehicle and industrial waste-heat recovery systems to provide electrical power, improve operating efficiency and reduce costs. State-of-art TEG manufacturing is based on prefabricated materials and a labor-intensive process involving soldering, epoxy bonding, and mechanical clamping for assembly. This reduces their durability and raises costs. Additive manufacturing technologies, such as thermal spray, present opportunities to overcome these challenges. In this work, TEGs have been fabricated for the first time using thermal spray technology and laser micromachining. The TEGs are fabricated directly onto engineering component surfaces. First, current fabrication techniques of TEGs are presented. Next, the steps required to fabricate a thermal spray-based TEG module, including the formation of the metallic interconnect layers and the thermoelectric legs are presented. A technique for bridging the air gap between two adjacent thermoelectric elements for the top layer using a sacrificial filler material is also demonstrated. A flat 50.8 mm × 50.8 mm TEG module is fabricated using this method and its performance is experimentally characterized and found to be in agreement with expected values of open-circuit voltage based on the materials used.     

“Smart” coatings: A technical note

The integration of sensors into thermally sprayed coatings can provide feedback about the functional status and operating history of the coatings as well as of the coated structures and surrounding environments. Sensors can be spray- formed directly on coatings using masking techniques. Production of coatings that contain embedded sensors opens up a new dimension for thermal spray technology: “smart” coatings. This paper describes the results of initial experiments to spray- form thermocouples, humidity sensors, strain gages, and sensor arrays.     

Fabrication of Thermoelectric Devices Using Thermal Spray: Application to Vehicle Exhaust Systems

Thermoelectric devices produce electricity directly from heat; they are small, have no moving parts, and are quiet. Commercially available thermoelectric devices, however, are expensive and labor intensive to produce, and come in very limited form factors. This article presents initial results for the use of thermal spray to directly fabricate thermoelectric devices. The target application is automotive exhaust systems and other high-volume heat sources. In this work, FeSi₂ and Mg₂Si metal silicides were sprayed. Characterization of the Mg₂Si deposits indicates that both the thermal conductivity and the Seebeck coefficient are roughly one half the values of bulk Mg₂Si. The electrical conductivity, however, is several orders of magnitude lower than bulk measurements in the literature, with likely reasons including impurities in the starting powder, oxidation during spraying, and using an undoped material. Fe _x Co_4?x Sb₁₂ skutterudite material has also been sprayed; however, not enough powder was available to fabricate samples large enough for characterization. The steps required to fabricate a thermoelectric device are presented, including the formation of the bottom and top metallic layers and the thermoelectric legs using thermal spray and laser micromachining. A technique for bridging the air gap between adjacent thermoelectric elements for the top layer based on a sacrificial filler material has also been demonstrated.     

Laser Processing of Multilayered Thermal Spray Coatings: Optimal Processing Parameters

Laser processing offers an innovative approach for the fabrication and transformation of a wide range of materials. As a rapid, non-contact, and precision material removal technology, lasers are natural tools to process thermal spray coatings. Recently, a thermoelectric generator (TEG) was fabricated using thermal spray and laser processing. The TEG device represents a multilayer, multimaterial functional thermal spray structure, with laser processing serving an essential role in its fabrication. Several unique challenges are presented when processing such multilayer coatings, and the focus of this work is on the selection of laser processing parameters for optimal feature quality and device performance. A parametric study is carried out using three short-pulse lasers, where laser power, repetition rate and processing speed are varied to determine the laser parameters that result in high-quality features. The resulting laser patterns are characterized using optical and scanning electron microscopy, energy-dispersive x-ray spectroscopy, and electrical isolation tests between patterned regions. The underlying laser interaction and material removal mechanisms that affect the feature quality are discussed. Feature quality was found to improve both by using a multiscanning approach and an optional assist gas of air or nitrogen. Electrically isolated regions were also patterned in a cylindrical test specimen.     

Cubic Aluminum Nitride Coating Through Atmospheric Reactive Plasma Nitriding

Aluminum nitride is a promising material for structural and functional applications. Cubic AlN (c-AlN) is expected to have higher thermal conductivity due to their high symmetry; however, its fabrication is difficult. In this study, c-AlN was synthesized by atmospheric plasma spray process through the reaction between Al feedstock powder and nitrogen plasma. Al powders were supplied to the plasma stream by Ar carrier gas and reacted with surrounding N₂ plasma, then deposit onto substrate. The obtained coatings were c-AlN/Al mixture at 150 mm of spray distance, and the nitride content was improved by increasing the spray distance. The coatings almost consist of c-AlN at 300 mm of spray distance. The coatings thickness decreased from 100 to 10 μm with increasing spray distance from 150 to 300 mm. Using carrier gas, N₂ enable to fabricate thick c-AlN coating with hardness 1020 Hv.     

Thermal Spray Applications in Electronics and Sensors: Past,Present, and Future

Thermal spray has enjoyed unprecedented growth and has emerged as an innovative and multifaceted deposition technology. Thermal spray coatings are crucial to the enhanced utilization of various engineering systems. Industries, in recognition of thermal spray’s versatility and economics, have introduced it into manufacturing environments. The majority of modern thermal spray applications are “passive” protective coatings, and they rarely perform an electronic function. The ability to consolidate dissimilar material multilayers without substrate thermal loading has long been considered a virtue for thick-film electronics. However, the complexity of understanding/controlling materials functions especially those resulting from rapid solidification and layered assemblage has stymied expansion into electronics. That situation is changing: enhancements in process/material science are allowing reconsideration for novel electronic/sensor devices. This review critically examines past efforts in terms of materials functionality from a device perspective, along with ongoing/future concepts addressing the aforementioned deficiencies. The analysis points to intriguing future possibilities for thermal spray technology in the world of thick-film sensors.     

Characterization of a hybrid laser-assisted mechanical micromachining (LAMM) process for a difficult-to-machine material

Mechanical micro-cutting is emerging as a viable alternative to lithography based micromachining techniques for applications in optics, semiconductors and micro-mold/dies. However, certain factors limit the types of workpiece materials that can be processed using mechanical micromachining methods. For difficult-to-machine materials such as mold and die steels or ceramics, limited cutting tool/machine stiffness and strength are major impediments to the efficient use of mechanical micromachining methods. In addition, at micron length scales of cutting, the effect of tool/machine deflection on the dimensional accuracy of the machined feature can be significant. This paper presents experimental characterization of a novel hybrid laser assisted mechanical micromachining (LAMM) process designed for 3D micro-grooving that involves highly localized thermal softening of the hard material by focusing a solid-state continuous wave laser beam in front of a miniature cutting tool. Micro-scale grooving experiments are conducted on H-13 mold steel (42 HRc) in order to understand the influence of laser variables and cutting parameters on the cutting forces, groove depth and surface finish. The results show that the laser variables significantly influence the process response. Specifically, the mean thrust force is found to decrease by 17% and the 3D average surface roughness increases by 36% when the laser power is increased from 0 to 10 W. The groove depths are found to be influenced by the machine (stage) deflection and tool thermal expansion, which affect the actual depth of cut, in the presence of laser heating. In particular, it is found that the accuracy of groove depth improves with laser heating. Explanations for the observed trends are given.     

A Study on Microstructure and Dielectric Performances of Alumina/Copper Composites by Plasma Spray Coating

In this study, surfaces of copper plates were coated with a thick alumina layer by the plasma spray coating to fabricate a composite with a dielectric performance that made them suitable as substrates in electronic devices with high thermal dissipation. The performance of alumina dielectric layer fabricated by the plasma spray coating and traditional screen-printing process was compared, respectively. Effects of the spraying parameters and size of alumina particles on the microstructure, thickness, and the surface roughness of the coated layer were explored. In addition, the thermal resistance perpendicular to the interface of copper and alumina and the breakdown voltage across the alumina layer of the composite were also investigated. Experimental results indicated that alumina particles with 5-22 μm in diameter tended to form a thicker layer with a poorer surface roughness than that of the particles with 22-45 μm in diameter. The thermal resistance increased with the surface roughness of the alumina layer, and the breakdown voltage was affected by the ambient moisture, the microstructure and the thickness of the layer. The optimal parameters for plasma spray coating were an alumina powder of particles size between 22 and 45 μm, a plasma power of 40 kW, a spraying velocity of 750 m/s, an argon flow rate of 45 L/min, a spraying distance of 140 mm, and a spraying angle of 90°. It can be concluded that an alumina layer thickness of 20 μm provided a low surface roughness, low thermal resistance, and highly reliable breakdown voltage (38 V/μm).     

Laser patterning of thin film sensors on 3-D surfaces

Thin film strain sensors applied directly on machine components provide high reliability. However, sensors patterned by standard photolithographic processes are limited to planar surfaces. To overcome these limitations we developed a 3-D capable direct patterning process for NiCr thin film sensors based on ultrafast lasers and galvanometer scanners. Our investigations showed that strain sensors with spatial resolution of 30 μm can be patterned on 3-D shaped machine tool components even at extremely tilted surfaces (up to 70°). First machining results and sensor tests indicate that laser thin film patterning enables efficient and automatable production of novel sensor concepts, e.g. for automotive, medical and machine tool applications.     

Modified Steels for Cold-Forming U-Bolts Used In Leaf Springs Systems

In this work, a low alloy steel and a fabrication process were developed to produce U-Bolts for commercial vehicles. Thus, initially five types of no-heat treated steel were developed with different additions of chrome, nickel, and silicon to produce strain hardening effect during cold-forming processing of the U-Bolts, assuring the required mechanical properties. The new materials exhibited a fine perlite and ferrite microstructure due to aluminum and vanadium additions, well known as grain size refiners. The mechanical properties were evaluated in a servo-hydraulic test machine system—MTS 810 according to ASTM A370-03; E739 and E08m-00 standards. The microstructure and fractography analyses of the cold-formed steels were performed by using optical and scanning electronic microscope techniques. To evaluate the performance of the steels and the production process, fatigue tests were carried out under load control (tensile-tensile), R = 0.1 and f = 30 Hz. The Weibull statistic methodology was used for the analysis of the fatigue results. At the end of this work the 0.21% chrome content steel, Alloy 2, presented the best fatigue performance.     

Influence of vibration on micro-tool fabrication by electrochemical machining

Recent trend in societies is to have micro products in limited space. Efficient micromachining technologies are essential to fabricate micro products which in turn will be helpful in saving material, energy and enhancing functionality. For micromachining, micro tool is very much essential. This paper is aimed at finding the most suitable and quickest method of micro tool fabrication by electrochemical machining. Tungsten micro tools were fabricated at different machining conditions to know the influences of voltage, frequency of tool vibration, amplitude of vibration of tungsten tool, concentrations of electrolyte and dipping length of tool inside the electrolyte. Fabrication of uniform diameter of micro tool is possible at each applied voltage starting at 2 V to higher volt utilizing vibration with appropriate amplitude. Good quality micro tools with different shapes can be fabricated by controlling a proper diffusion layer thickness within a very short time introducing the vibrations of micro tool. Finally, the fabricated micro tools were applied for machining precise micro holes and micro channel using electrochemical micromachining (EMM).     

Sensors in Spray Processes

This paper presents what is our actual knowledge about sensors, used in the harsh environment of spray booths, to improve the reproducibility and reliability of coatings sprayed with hot or cold gases. First are described, with their limitations and precisions, the different sensors following the in-flight hot particle parameters (trajectories, temperatures, velocities, sizes, and shapes). A few comments are also made about techniques, still under developments in laboratories, to improve our understanding of coating formation such as plasma jet temperature measurements in non-symmetrical conditions, hot gases heat flux, particles flattening and splats formation, particles evaporation. Then are described the illumination techniques by laser flash of either cold particles (those injected in hot gases, or in cold spray gun) or liquid injected into hot gases (suspensions or solutions). The possibilities they open to determine the flux and velocities of cold particles or visualize liquid penetration in the core of hot gases are discussed. Afterwards are presented sensors to follow, when spraying hot particles, substrate and coating temperature evolution, and the stress development within coatings during the spray process as well as the coating thickness. The different uses of these sensors are then described with successively: (i) Measurements limited to particle trajectories, velocities, temperatures, and sizes in different spray conditions: plasma (including transient conditions due to arc root fluctuations in d.c. plasma jets), HVOF, wire arc, cold spray. Afterwards are discussed how such sensor data can be used to achieve a better understanding of the different spray processes, compare experiments to calculations and improve the reproducibility and reliability of the spray conditions. (ii) Coatings monitoring through in-flight measurements coupled with those devoted to coatings formation. This is achieved by either maintaining at their set point both in-flight and certain spray parameters (spray pattern, coating temperature…), or defining a good working area through factorial design, or using artificial intelligence based on artificial neural network (ANN) to predict particle in-flight characteristics and coating structural attributes from the knowledge of processing parameters.     

Cold Spraying of TiO<Subscript>2</Subscript> Photocatalyst Coating With Nitrogen Process Gas

Titanium dioxide (TiO₂) is a promising material for photocatalyst coatings. However, it is difficult to fabricate a TiO₂ coating with anatase phase by conventional thermal spray processes due to a thermal transformation to rutile phase. In this paper, anatase TiO₂ coatings were fabricated by the cold spray process. To understand the influence of process gas conditions on the fabrication of the coatings, the gas nature (helium or nitrogen) and the gas temperature are investigated. It was possible to fabricate TiO₂ coatings with an anatase phase in all spraying conditions. The process gas used is not an important factor to fabricate TiO₂ coatings. The thickness of the coatings increased with the process gas temperature increasing. It indicates that the deposition efficiency of the sprayed particles can be enhanced by controlling the spray conditions. The photocatalytic activity of the coatings is similar or better than the feedstock powder due to the formation of a large reaction area. Concludingly, cold spraying is an ideal process for the fabrication of a TiO₂ photocatalyst coating.     

CO2 laser micromachined crackless through holes of Pyrex 7740 glass

The drilling of glass through holes with a high aspect ratio is crucial for microsystems application, especially in the inlet/outlet connection of microfluidic devices for biological analysis or for the anodic bonded silicon-glass ones. Traditional glass drilling using mechanical processing and laser processing in air would produce many kinds of defects such as bulges, debris, cracks and scorch. In this paper, we have applied the method of liquid-assisted laser processing (LALP) to reduce the temperature gradient, bulges and heat affected zone (HAZ) region for achieving crack-free glass machined holes. The nominal diameters of circles from 100 to 200 μm were drawn for through glass machining test. Through-hole glass etching can be obtained by LALP for 10 passes of circular scanning in several seconds on conditions of a 6 W laser power, 76 μm spot size and 11.4 mm/s scanning speed. The ANSYS software was also used to analyze the temperature distribution and thermal stress field in air and water ambient during glass hole machining. The higher temperature gradient in air induced higher stress for crack formation while the smaller temperature gradient in water had less HAZ and eliminated the crack during processing. CO₂ laser micromachining under water has merits of high etching rate, easy fabrication and low cost together with much improved surface quality compared to that in air.     

Coating Bores of Light Metal Engine Blocks with a Nanocomposite Material using the Plasma Transferred Wire Arc Thermal Spray Process

Engine blocks of modern passenger car engines are generally made of light metal alloys, mostly hypoeutectic AlSi-alloys. Due to their low hardness, these alloys do not meet the tribological requirements of the system cylinder running surface—piston rings—lubricating oil. In order to provide a suitable cylinder running surface, nowadays cylinder liners made of gray cast iron are pressed in or cast into the engine block. A newer approach is to apply thermal spray coatings onto the cylinder bore walls. Due to the geometric conditions, the coatings are applied with specifically designed internal diameter thermal spray systems. With these processes a broad variety of feedstock can be applied, whereas mostly low-alloyed carbon steel feedstock is being used for this application. In the context of this work, an iron-based wire feedstock has been developed, which leads to a nanocrystalline coating. The application of this material was carried out with the Plasma Transferred Wire Arc system. AlMgSi0.5 liners were used as substrates. The coating microstructure and the properties of the coatings were analyzed.     

Warm Spraying: An improved spray process to deposit novel coatings

Warm Spray is an atmospheric coating process through continuous impact and deposition of solid particles heated and accelerated by a supersonic jet controlled between 800~1900 K and 900~1600 m s^− 1. This paper introduces successful fabrication of dense and less-oxidized metallic titanium (Ti) coatings by Warm Spray and clarification of phenomena occurring upon the spray process. Temperature and velocity of an in-flight Ti particle were compared between measurement by the diagnostic instrument and calculation based on the fluid dynamics simulation. Deformation behaviour of particle from impact to deposition was analyzed through the finite element method (FEM). Densification of stacking particles was attained by applying bi-modal size distribution to the feedstock Ti powder. Qualitative restriction of changes in chemical composition of Ti coating obtained was demonstrated by elemental analysis and by calculation based on the oxidation model. Warm Spray enables various materials to fabricate coatings without thermal deterioration of the original characteristics such as purity and crystallographic phase.     

A flame-sprayed resistance strain gage for high-temperature applications

The thermal spray technique is often employed for sensor attachments, especially for high-temperature applications, because flame spray techniques usually produce a denser film than ceramic cements. This article introduces a newly developed electrical resistance strain sensor that is installed on the test article by means of a flame spray technique. The gage is made of a specially developed alloy, palladium/13 wt% chromium (Pd13Cr), and is temperature-compensated with a platinum element. A flame-sprayed Pd13Cr-based gage is demonstrated to be a viable sensor candidate for static strain measurement in the temperature range from room temperature to 800 °C (1470 °F). The flame spray technique used for in-stallation of this strain gage is described, and the characteristics of the gage are presented.     

Role of mechanical strain on thermal conductivity of nanoscale aluminum films

Thin film components of conventional and flexible solid-state devices experience mechanical strain during fabrication and operation. At the bulk scale, small values of strain do not affect thermal conductivity, but this may not true for grain sizes comparable with the electron and phonon mean free paths and for higher volume fraction of grain boundaries. To investigate this hypothesis, thermal and electrical conductivity of nominally 125-nm-thick aluminum films (average grain size 50 nm) were measured as functions of tensile thermo-mechanical strain, using a modified version of the 3-ω technique. Experimental results show pronounced strain–thermal conductivity coupling, with ～50% reduction in thermal conductivity at ～0.25% strain. The analysis shows that mechanical strain decreases the mean free path of the thermal conduction electrons, primarily through enhanced scattering at the moving grain boundaries. This conclusion is supported by similar effects of mechanical loading observed on the electrical conduction in the nanoscale aluminum specimens.     

Laser micromachining for biomedical applications

Laser micromachining is becoming a common method for fabrication of microstructured medical devices. Developments in pulsed laser technology have made it possible to achieve precision machining of sub-micrometer features with minimal damage to the surrounding material. Several aspects of laser micromachining, including machining methods, types of lasers used in micromachining, and laser-material interaction, are discussed in this article. Biomedical applications of laser micromachining are also reviewed. The ablation behavior of silicon was examined as a function of laser energy, aperture, and repetition rate. In vitro studies showed that microscale grooves on silicon substrates may be used to orient human aortic vascular smooth muscle cells. We anticipate that the use of laser micromachining for modifying medical and dental devices will become more significant over the coming years.     

Characterization of the drilling alumina ceramic using Nd:YAG pulsed laser

Laser micromachining can replace mechanical removal methods in many industrial applications, particularly in the processing of difficult-to-machine materials such as hardened metals, ceramics, and composites. It is being applied across many industries like semiconductor, electronics, medical, automotive, aerospace, instrumentation and communications. Laser machining is a thermal process. The effectiveness of this process depends on thermal and optical properties of the material. Therefore, laser machining is suitable for materials that exhibit a high degree of brittleness, or hardness, and have favourable thermal properties, such as low thermal diffusivity and conductivity. Ceramics which have the mentioned properties are used extensively in the microelectronics industry for scribing and hole drilling.Rapid improvement of laser technology in recent years gave us facility to control laser parameters such as wavelength, pulse duration, energy and frequency of laser. In this study, Nd:YAG pulsed laser (with minimum pulse duration of 0.5 ms) is used in order to determine the effects of the peak power and the pulse duration on the holes of the alumina ceramic plates. The thicknesses of the alumina ceramic plates drilled by laser are 10 mm. Average hole diameters are measured between 500 μm and 1000 μm at different drilling parameters. The morphologies of the drilled materials are analyzed using optical microscope. Effects of the laser pulse duration and the peak power on the average taper angles of the holes are investigated.