TiO2−X based thermoelectric generators enabled by additive and layered manufacturing

Applied Energy 192 (2017) 24–32

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TiO2
X based thermoelectric generators enabled by additive and layered manufacturing Hwasoo Lee, Ramachandran Chidambaram Seshadri, Su Jung Han, Sanjay Sampath ⇑ Center for Thermal Spray Research, Stony Brook University, Stony Brook, NY, USA

h i g h l i g h t s  Design and fabrication of TEGs for waste heat application.  Scalable and additive manufacturing demonstrated using thermal spray.  Multi-layer deposits composed of distinct functionality.  TEG produces 2.43 mW electrical power with a max efficiency of 0.85% (1.52 V).

a r t i c l e

i n f o

Article history: Received 20 November 2016 Received in revised form 14 January 2017 Accepted 1 February 2017

Keywords: Thermoelectric power generation Waste-heat energy harvesting Sub-stoichiometric TiO2 Atmospheric plasma spray

a b s t r a c t Traditional thermoelectric modules are acquired as separate components and then integrated by mechanical attachment into the engineering systems. There is, however, an interest and opportunity to manufacture thermoelectric device and basic electronics directly onto engineering structures. Recent studies have shown that plasma spray synthesized sub-stoichiometric titanium oxide (TiO2
x) deposits show reasonable thermoelectric figure-of-merit, are capable of operating at relatively high temperatures (500 °C), and can be easily and cost effectively deposited onto both planar and cylindrical substrates over large areas with the capability to produce patterned and multilayer assemblies to optimize the power harvesting. This study demonstrates the fabrication and performance of such thermoelectric generators based on n-type TiO2
x and Ni as the surrogate p-type and interconnect structures embedded within ceramic deposits. Up to 72 thermocouple modules were prepared incorporating both series and parallel connections to augment the performance resulting in a max efficiency of 0.85% for the couple and electric power of 2.43 mW at temperature of 723 K. Preliminary experiments were conducted with Li:Co3O4 as the p-type material with significant performance improvement. The methodologies described in this paper represents a potential pathway for large scale synthesis and fabrication of thermoelectric system directly in waste heat systems over large areas. Ó 2017 Published by Elsevier Ltd.

1. Introduction Energy from heat generated through turbocharger, exhausts, power plants and steam pipes can be a significant source of energy for waste heat conversion [1]. Solid state waste-heat to power conversion based on thermoelectric devices are attractive solution for such a strategy due to their passive operation, and simplicity of having no moving parts. A thermoelectric generator relies on the Seebeck effect that generates voltage in materials in the presence of a temperature gradient. Although many materials demonstrate Seebeck effect, the efficiency of power generation relies on a com⇑ Corresponding author at: Center for Thermal Spray Research, Heavy Engineering Bldg., Rm 130, Stony Brook University, Stony Brook, NY 11794-2275, USA. E-mail address: [email protected] (S. Sampath). http://dx.doi.org/10.1016/j.apenergy.2017.02.001 0306-2619/Ó 2017 Published by Elsevier Ltd.

plex interplay among absolute temperature, temperature gradient, intrinsic Seebeck coefficient, electrical and thermal conductivities [2]. Often these properties do not scale in the same direction leading to challenges in efficient thermoelectric power generation. Altenkirch derived the thermoelectric efficiency, ZT, mostly known as the thermoelectric figure of merit [3], to describe the thermoelectric energy conversion capability of a material. This value is given as

ZT ¼

a2 rT j

ð1Þ

where a is the Seebeck coefficient, r is the electrical conductivity, T is the absolute temperature, and j is the thermal conductivity. For a power generation device with both n-type and p-type legs, the maximum ZT can be shown as

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25

Nomenclature Symbols D I T TC TE TEG TS V Z ZT

j m q r

change in property output current (A) absolute temperature (K) commercially available K-type thermocouple thermoelectric thermoelectric generator thermal sprayed output voltage (V) figure of merit (K
1) dimensionless figure of merit

thermal conductivity (W m
1 K
1) velocity (m s
1) electrical resistivity (X m) electrical conductivity (S m
1)

Subscripts avg average c cold side h hot side max maximum n n-type material p p-type material

Greek symbols a Seebeck coefficient (lV K
1) g conversion efficiency

ðap
an Þ ZT max ¼ pffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffi 2 ð jp qp þ jn qn Þ

ð2Þ

where p is p-type semiconductor and n is n-type semiconductor, and q is electrical resistivity, respectively. The max efficiency of power conversion is

gmax

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ZT av g
1 DT ¼  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T c Th 1 þ ZT av g þ T h

ð3Þ

where DT is the temperature difference, Th is the hot side temperature, Z is the figure of merit, Tavg is the average of hot and cold side temperature, and Tc is the cold side temperature, respectively. Materials that offer intrinsically high ZT value are rare and generally comprised of rare-earth based compounds notably selenides and tellurides based alloys. SiGe [4], skutterudites [5], clathrates [6] and half-heusler [7] alloys are other promising systems. Among them, skutterudites e.g. p-Zn4Sb3, p-CeFe3.5Co0.5Sb12 and n-CoSb3, are actively pursued by many groups for potential power generation as offer high figure-of-merit in the intermediate temperature range from 500 K to 973 K with conversion efficiency of 9.5% [5,8]. One approach to improve ZT is by nano-structuring by introducing superlattices [9] or nanostructures [10,11] as it was recognized to have reduction in phonon thermal conductivity. Venkatasubramanian et al. [9] reported that p-type Bi2Te3/Sb2Te3 superlattices exhibit an exceptionally high ZT of 2.4 at room temperature with a remarkably low thermal conductivity of 0.22 W m
1 K
1. There are other reports on superlattices of Si/Ge [12], GaAs/AlAs [13], and PbTe/PbSe [14] that have lower lattice thermal conductivity than their alloy counterparts. However, many of these materials have poor thermal and chemical stability, difficult to produce into useful forms and are expensive, low availability and in some cases, high toxicity. There is considerable research around the world to not only seek to find potential thermoelectric alloys and oxides, but also to engineer multifunctional properties through microstructural engineering. Expanding the applicability of thermoelectric will require development of low cost, abundant materials as well as viability or applicability of scalable manufacturing technologies. One potential source of scalable thermoelectric materials is those based on transition metal oxides which offer reasonable electrical conductivity and Seebeck coefficient, while being cost-effective, and environmentally friendly. Oxides based on Ti, Mg, W, Zn, Cu, V, Co, Rh, and Mo oxides are a vast, but conventionally less widely investigated in TE materials [15]. Metal oxides can show a wide range of electronic properties ranging from insulating to semiconducting

and conducting. Their electronic properties can be engineered by changing their morphology [16], doping and stoichiometry [17]. Many transition metal oxide materials offer high Seebeck coefficients, some with reasonable thermal and electrical conductivities, at targeted temperatures that can be potentially engineered into applications. In particular, sub-stoichiometric TiO2
x has been explored as a potential thermoelectric material [18]. Tsuyomoto et al. have reported that TiO1.94 with an orthorhombic crystal structure prepared by reducing the anatase in H2, exhibits a peak a of -518 mV K
1 and r of 1.9  103 S m
1 at 343 K [19]. Evidently, the introduction of oxygen vacancies in the TiO2 crystal lead to increase in carrier concentration resulting enhancement of r [20]. However, an excessive oxidation of TiO2 at elevated temperature lowered r and a, resulting in low PF and ZT [21]. Thus, obtaining a fabrication process for TiO2
x that allows maintaining the stoichiometry is key to their utilization in thermoelectric systems and should be chosen to maximize the high-power factor while maintaining the low thermal conductivity. Recently, we reported promising thermoelectric properties in plasma spray synthesized TiO2
x with ZT values of up to 0.13 at an operating temperature of 750 K [22]. The key element of this finding is the ability to achieve the requisite combination of phase, microstructure and stoichiometry to yield Seebeck values of approximately 230 mV K
1 along with electrical conductivities of 5.5  103 S m
1 and low thermal conductivities of 1.65 W m
1 K
1. The high electrical conductivity is related to controlled deoxidation and quenching which leads to oxygen vacancies while the low thermal conductivities are also resultant from the layered assemblage of the thermal sprayed deposit and associated increase in interfaces. A key benefit of the plasma spray approach is the ability to directly apply oxide deposits onto thermo-structural components in either blanket or patterned form (either through masking or via direct writing) which reduces the heating-up of component due to the high thermal exposure of the rapidly quenched oxide. As such it is feasible to produce functional thermoelectric elements [23,24] and systems through mesoscale (20 mm  1 mm) layered engineering of active films, dielectrics and interconnects. Of further importance is the capability to apply these layers on non-flat (conformal) geometries eliminating the need for major design modifications to harness waste heat energy. Ueno et al. [23] and Sodeoka et al. [24] reported fabrication of the thermoelectric device using thermal spray process. The TEG fabricated by the aforementioned researchers were able to produce voltages ranging between 8  45 mV. In this paper, we present such a concept involving a thermal spray based layered engineering strategy to

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produce thermoelectric devices with reasonable efficiencies. Thermal spray is a well-accepted industrial manufacturing process for application of deposits in harsh environments and it is known for its cost effectiveness, flexibility and scalability. Our prior work through additive layered fabrication and additive-subtractive fabrication of thermopiles enables novel integration of functional materials within thermo-structural deposits [25,26]. In this work, we demonstrate several engineering approaches to harness the capabilities of the manufacturing process along with unique process induced material functionalities. It is envisioned that such a manufacturing enabled material/device design will provide a framework for development of applied energy systems from waste heat resources. 2. Thermoelectric device design for thermal spray engineering Since thermoelectric devices are made from individual thermoelectric modules, a key element of this work focuses on multilayer device design that works with the capabilities of the process to produce multitude of p-n junctions to maximize the efficiency and power output. A thermoelectric module is an array of thermocouples connected electrically in series, but thermally in parallel configurations [27,28]. The module consists of different components such as thermoelectric elements, interconnectors, and packaging enclosures. Thermoelectric elements are n- and p-type semiconductor legs, which are connected electrically in series because the voltage drop across on couple is only on the order of millivolts. Interconnectors between n- and p-type legs are usually made with metals for high electrical and thermal conductivity. Packaging enclosures are selected to reduce thermal leakage and provide protection from harsh environments. However, these conventional thermoelectric leg configurations have limited implementation and performance capabilities due to their restricted structures. Due to the above-mentioned reasons, there have been attempts to modify the architecture from other groups to improve on performance as well as making it practical [29–34]. Thus, the aim is to fabricate the entire TE device using thermal spray technology to take advantage of the ability to apply the active devices onto existing engineering structures as deposits. Figs. 1 and 2 shows the illustration of thermal sprayed thermocouple in 2D and 3D perspective. Figs. 1(a) and 2(a) shows the deposited pattern of the single legs of TE materials on the

substrate. One layer of p-type material is deposited using a prepatterned mask, followed by n-type layer using a second mask at an angle while making a junction at one end. More legs can be deposited simultaneously, and by utilizing the mask, materials can be connected electrically in series. Figs. 1(b) and 2(b) shows how multiple TE legs are connected in series. In this process, an insulating layer (Al2O3) is deposited in between p- and n-type TE materials to enlarge the thermal gradient by reducing the overlapping contact areas. Furthermore, multilayer concept is introduced to enhance the electrical performance output by increasing the number of legs in connection as shown in Fig. 1(c) and 2(c). These multiple TE legs can be stacked, connected in series or in parallel by using an appropriately designed mask. In this experiment, layers were connected in parallel circuit to reduce the overall resistance to increase the current output. The architecture fabricated in this work maintains an in-plane heat flux across the p-type and n-type legs. This includes the bottom and top metallic conducting layers as well as both p- and n-type thermoelectric materials. In the configurations described in Fig. 2(a, b, c), the heating and cooling were linearly separated. An additional circular device configuration was also contemplated and designed with ‘‘star” type thermopile radiating from a central heat source, the schematic of such a design is shown in Fig. 1(d). Similar to the previous design, p- and n-type TE materials are connected in series, and each series circuits are connected in parallel by stacking multi-layers. The realization of the devices via thermal spraying is showing in the form of prototype TEG modules as shown in Figs. 3 and 4. Fig. 3 (a) shows the Ni layer deposited in a pattern and Fig. 3(b) TiO2
x layer deposited in a pattern on Al2O3 and Ni layer, making junctions with Ni at the tips, while isolating the remaining regions with an insulating oxide (Al2O3). This process is repeated for stacking up the multilayers. The TiO2
x/Ni multilayer consists of TiO2
x legs with 60  2.5  0.2 (Length  Width  Height mm3) and Ni of 65  1.8  0.05 (Length  Width  Height mm3) dimensions respectively. The tilt angle of each TiO2
x/Ni legs was 6° for the linear design. Fig. 4(a) shows the picture of the circular TEG, made of 72 couples (24 couples in each stack) of TiO2
x and Ni legs. Similar dimension was used as the linear design though the tilt angle was modified to 3°. Each stack is composed of 24 couples in series, and these stacks were tripled, and then connected in parallel. The cross-section of the TEG module is shown in Fig. 4(b).

Fig. 1. Schematic design of the thermoelectric legs in (a) couple, (b) series (linear), (c) series and parallel (linear), and (d) series and parallel (circular).

H. Lee et al. / Applied Energy 192 (2017) 24–32

27

ensure the reliability on insulators, conductors and the active TE layer which are well established methods developed at Center for Thermal Spray Research [35,36]. All of the said devices were rigorously tested multiple times.

3. Experimental 3.1. Fabrication of the thermoelectric deposits

Fig. 2. Schematic design of the thermoelectric legs in (a) couple, (b) series (linear), and (c) series and parallel (linear).

Repeatability tests were conducted by fabricating multiple tiles of both single layer and multilayer thermopiles for both the linear and star configurations. The deposition process was optimized to

Table 1 shows the feedstock materials used to be fabricate insulators, active elements and interconnects, which were yttria stabilized zirconia (#204, Saint-Gobain Coating Solutions Thermal Spray Powders), TiO1.9 (Metco 102, Oerlikon Metco), Ni (Metco 56C-NS, Oerlikon Metco), Al2O3 (WCA-30, Microgrit), and Li doped Co3O4 (Laboratory made, Alfa Aesar). Deposits were fabricated using two plasma torch variants (F4MB-XL and SinplexProTM. both from Oerlikon Metco, Westbury, NY) with the 8 and 9 mm nozzle diameter respectively. Both plasma gas composition (H2 content) and process conditions were varied to identify the most appropriate conditions to achieve the best thermoelectric properties. Details of this effort and results are reported in a recent paper on the process-structure-thermoelectric properties of the TiO2
x [22]. The selected conditions repeatedly produced a n-type TiO2
x deposit that had a Seebeck Coefficient of
230 uV K
1, electrical conductivity of 5500 S m
1 and thermal conductivity of 1.65 W m
1K
1 together providing a thermoelectric figure of merit of 0.132 and power factor of 2.9 mW cm
1 K
2. Since Li doped Co3O4 was not commercially available in sprayable form, only a small quantity of the material was prepared within the laboratory for feasibility assessment. The Li:Co3O4 feedstock was made by blending LiCoO2 (Alfa Aesar) and Co3O4 (Alfa Aesar) powders in a rotary blending machine. The mixture is then place in a recrystallized alumina crucible and sintered in box furnace at 1000 °C. The commercial compound powders were mixed in a ratio to yield a solid solution with 5% of the Co atoms replaced by Li. The sintered mixture is then crushed and sieved to achieve a sprayable powder (average particle size of the powder was 10 mm). The same parameter that was used for TiO2
x was applied here with reasonable initial results. Yttria stabilized zirconia (YSZ) and alumina (Al2O3) were used as insulator materials between the active layers while Ni was used as an interconnect and as a surrogate for the 2nd leg of the thermoelectric device (in absence of sufficient quantities of sprayable p-type alternative) as a way to demonstrate the device capability and performance assessment. Thermoelectric devices were fabricated to meet the multilayer design requirements that was proposed in Figs. 1 and 2. Atmospheric plasma spray process was used for all of the layers. A stainless-steel substrate served as the structural material which was insulated initially using YSZ. A Ni-5%Al bond coat layer was used which is standard practice in thermal spray technologies to

Fig. 3. Picture of the plasma sprayed (a) Ni and (b) TiO2
x legs from the top view.

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H. Lee et al. / Applied Energy 192 (2017) 24–32

Fig. 4. (a) Picture of the star configuration TEG from the top view and (b) cross-sectional microstructure of the thermoelectric module.

Table 1 Plasma sprayed parameters for fabrication of the deposits.

a

Materials-torch Hardware

Feed rate (g min
1)

Ar flow (L min
1)

H2 flow (L min
1)

Current (A)

Power (kW)

Mean particle velocity (m s
1)

Mean particle temperature (K)

YSZ-F4 8 mm nozzle Ni-F4 8 mm nozzle TiO1.9-Sinplex 9 mm nozzle Al2O3-F4 8 mm nozzle LixCo3
xO4-Sinplex 9 mm nozzlea

30 15 30 30 –

45 45 45 45 45

6 6 0 6 0

550 550 380 550 380

36.7 37.4 29.3 36.9 29.3

139 ± 3 83 ± 1 230 ± 2 268 ± 4 –

2829 ± 26 2808 ± 44 2813 ± 24 3009 ± 19 –

Laboratory made powder.

achieve adhesion of a ceramic. The choice of zirconia was due to its higher expansion coefficient for a ceramic which more closely matched to stainless steel than alumina. Substrates were sand blasted (80 psi with 200 mm standoff) to get rough surface and carefully cleaned with alcohol to remove the contaminants. A 150 mm Ni-5%Al bond coat was deposited on the grit blasted substrates and then a 250 mm of YSZ was sprayed over it. TiO2
x was deposited on the YSZ deposit with thickness of 200 mm. The substrate dimensions were 228  25.4  1.6 mm3. Prior to deposition of the active thermoelectric layers, an embedded thermocouple was fabricated within the YSZ deposits using methodologies developed in our group and reported in literature [26,37–40]. Essentially, NiCr and NiAl materials which simulate a K-type thermocouple were sprayed through a mask to form junctinon and then overlayed with the YSZ deposit to isolate the thermocouple. Details about the concept and experimental process of the embedded thermocouple can be found in Longtin et al. [38,40]. Subsequently, active elements of Ni and TiO2
X were deposited using line masks to produce linear patterns and junctions in ways similar to the embedded thermocouple. Between the spraying of Ni and TiO2
x, the process was interrupted with introduction of an alumina insulator layer and additional masking was used to separate most of the active layers except for the junction regions. The resultant multi-functional layers of Al2O3/Ni/Al2O3/TiO2
x deposits with total thickness of 350 lm produced the TE legs in series. This process was repeated to fabricate additional device layers through thickness allowing parallel connection between the TE elements. The production of three such multifunctional layers resulted in a total thickness of approximately 1.2 mm. Al2O3

deposit was sprayed on top of the TEG to prevent the oxidation of the metal and to retain the sub-stoichiometric nature of TiO2
x. Two variants of aforementioned concepts were manufactured. A linear TE system comprised of four TiO2
x and three Ni lines were produced in a strip configuration with multilayers in each. Many such samples were produced that allowed externally connecting them for increasing power output. Here the goal was to heat one side of the strip and cool the other side. The star configuration allowed 24 junctions within each multifunctional layer with a total of 72 junctions. Here the heat source was placed in the center of the star while the cooler side was on the periphery. The star approach was contemplated for use in exhaust heat pipes of automobiles. The strip TE devices were experimentally produced 12 times successfully demonstrating repeatability in manufacturing and performance in each case, the measured voltage was ranging from 0.191 to 0.217 at 750 K. The star device was also produced 3 times with good repeatability of voltage performance ranging from 0.149 to 0.154 at 723 K. 3.2. Experimental setup to measure the thermoelectric properties, temperature and electrical performance of the TEG Thermogravimetric analysis was performed to calculate the stoichiometry of TE material using a thermal balance (TG/DSC, STA 449C, Netzsch). The temperature (Tc and Th) and electrical (voltage, current and resistivity) measurements were performed using digital multimeter (Kethley 2700) from room temperature to 773 K to calculate Seebeck coefficient, electrical conductivity, and electrical output. Seebeck coefficient was calculated using the measured

H. Lee et al. / Applied Energy 192 (2017) 24–32

voltage generated from the temperature gradient. The temperature difference between the hot and cold side of the sample was generated by heating and cooling each end of the substrate, using 600 W IR-heater or commercial combustion-torch as a heater and cold air gun (Vortex) as a chiller. The surface temperature of the deposit was measured using K-type thermocouple attached to the apparatus. Electrical resistivity was measured by four-point probe technique from room temperature to 750 K. The open circuit was made to measure voltage and current as a function of temperature to calculate the power output. Thermal sprayed embedded thermocouple was utilized to measure the inner temperature of the TEG. Through-thickness thermal conductivity of the TE material was measured using laser flashing method (FlashlineTM System X-PlatformTM, Anter Corporation) from room temperature to 750 K and Archimedes method was used to measure the density. The measurements were made in the perpendicular direction as that’s the only straightforward direction of measurements with the flat sample instrument. More details about the thermoelectric property measurements can be found in Lee et al. [22]. Cross-section microstructure of as-sprayed deposits were investigated by scanning electron microscopy (SEM, TM3000, Hitachi). Commercialized Peltier module (CP20351) from Cui IncÒ was used for the comparison of the lab-made TEG. The commercialized Peltier module had dimension of 30  30  5.1 mm, which consists of an array of Bismuth Telluride semiconductor pellets that have been doped either positive or negative. The pairs of p/n pellets are configured so that they are connected electrically in series, but thermally in parallel. The connection between thermoelectric pairs are done with solder that can last max. up to 138 °C. 4. Results and discussion In this section, we assess the performance of both individual and integrated TE elements produced by the plasma spray process and assessed for device performance for design strategies outlined in Section 2. This provided a way to assess not only the design aspects but manufacturing repeatability and scalability of the process to produce these multilayers. Although plasma sprayed deposits are produced in layers, there is significant ‘‘chaos” in the assembly and eliminating some of the anisotropy. This is particularly true for this situation where the selected process condition leads to some fraction of unmelted particles which not only show isotropic response within themselves, but also extend tortuosity in the deposits. Majority of the reported data is for the TiO1.93/Ni thermoelectric couple as these two materials were available in sufficient quantities in sprayable form to demonstrate the device engineering strategy, assess performance and bench mark with anticipated results. This allowed realization of the designmanufacturing interplays. However, Ni is not an ideal surrogate for the p-type leg of the thermocouple and as such represented parasitic losses. Limited work was conducted on a more suitable p-type material Li:Co3O4 to demonstrate that the concepts can be applied with more appropriate systems. This material was synthesized in small quantities in plasma spray sizes (average 10 mm) and characterized within the laboratory as no commercial source is available. This test validated the device strategy and performance and pointed to future pathways to build multilayer devices. 4.1. Single layer, single thermocouple The calculated TE quantities of TiO2
x/Ni together with measured TE quantities (Seebeck coefficient a, electrical conductivity r, thermal conductivity j, figure of merit ZT, and max efficiency g) of TiO2
x, and Ni at 750 K are summarized in Table 2. Note that the values of TiO2
x, and Ni are those of individually deposited

29

samples fabricated under the same condition as that for fabricating TiO2
x/Ni couples. Seebeck coefficient of single and couple of TiO2
x and Ni as a function of Th is shown in Fig. 5(a). The negative values of the Seebeck coefficients reveal TiO2
x, and Ni are n-type and Seebeck coefficient of plasma sprayed TiO2
x operated in the range of about
224 to
243 mV K
1 whereas properties of Ni was found to be in the range of about
27 to
30 mV K
1. The Seebeck coefficient of TiO2
x/Ni was calculated as 202 mV K
1 (table 2). As a n-type material, ZT for TiO2
x was measured to be 0.132 at 750 K with max g of 1.89% while Ni (n-type) was measured with ZT of 0.02 and max g of 0.31% at 750 K, the coupling of these two n-type material’s calculated ZT and max g of TiO2
x/Ni couple was 0.057, and 0.85% at 750 K, respectively. However, actual measurement for the Seebeck differed from the calculation as shown in Fig. 5(a). The Seebeck coefficient for the couple was measured to be 164 mV K
1 at 750 K, for which the ZT was calculated to be 0.038 and max g was found to be 0.57%. As noted earlier, much of this paper relies on using Ni as the coupling material with the active thermoelectric element being TiO2
x. Although there was not a significant opportunity to optimize the deposit, initial results with the laboratory synthesized powder was encouraging. The calculated TE quantities of TiO2
x/LixCo3
xO4 together with measured TE quantities of TiO2
x, and LixCo3
xO4 are summarized in Table 3. As-sprayed LixCo3
xO4 doesn’t show p-type behavior due to the quenched disordered structure produced during rapid solidification, and therefore the deposit needed to be annealed at 973 K for an hour for crystallization to enable it to transform into spinel-like structure. Following separate annealing of the LixCo3
xO4 prior to connecting with TiO2
x allowed assessment of the junction. Seebeck coefficient of thus produced single and couple of TiO2
x and LixCo3
xO4 as a function of Th is shown in Fig. 5(b). The single leg of LixCo3
xO4 had ZT of 0.163 and max g of 2.29% at 750 K. The couple TiO2
x/LixCo3
xO4 was processed by spraying TiO2
x after the annealing of the as-sprayed LixCo3
xO4. Seebeck coefficient of TiO2
x/LixCo3
xO4 was calculated to be 816 mV K
1, but the measured value was 859 mV K
1. Using the calculated values, the ZT of TiO2
x/LixCo3
xO4 at 750 K can be estimated to be 0.153 and max g to be 2.17%. When using the Seebeck measured from the couple shown in Fig. 5(b), the ZT was achieved to be 0.169 and max g of 2.39% for TiO2
x/LixCo3
xO4 couple. This encouraging result indicated two important results: that it would be feasible to produce a p-n junction with high Seebeck coefficient and reasonable thermoelectric performance along with the multilayer, multi-material device concepts. The present TiO2
x-Ni system can be enhanced with availability of better thermoelectric materials suitable for plasma spray. One of the challenges with the LixCo3
xO4 is for the need for post-deposition crystallization annealing which can limit some of the abilities in multilayer device fabrication. Additional experimental researches are in process to find a substitute for p-type material or optimize process conditions to deposit LixCo3
xO4. 4.2. Series and parallel connections Following the successful demonstration of individual and couple device performance, efforts were focused to increase the power output of the system. Through addition of junctions in series, efforts were sought to increase the Seebeck voltage, while connections in parallel of multiple elements enabled improvements in current. Lastly combining both series and parallel connections should improve both attributes with enhanced device performance. The voltage and current output is plotted as a function of number of TE legs at temperature gradient of 461 K shown in Fig. 6(a).

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H. Lee et al. / Applied Energy 192 (2017) 24–32

Table 2 Summary of the TE parameters (a, j, r, ZT, and g at 750 K) of TiO2
x and Ni single legs and TiO2
x/Ni couple. Properties at 750 K (n-type) TiO2
x (n-type) Ni TiO2
x/Nia a

a (lV K
1)
230
30 200

r (S m
1) 3

5.49  10 4.17  105 –

j (W (mK)
1)

ZT

g (%)

1.65 13.71 –

0.132 0.020 0.057

1.89 0.31 0.85

Values calculated using Eqs. (2) and (3).

Fig. 5. The Seebeck coefficient of the single legs and the couple of (a) TiO2
x and Ni (b) TiO2
x and LixCo3
xO4.

Table 3 Summary of the TE parameters (a, j, r, ZT, and g at 750 K) of TiO2
x and LixCo3
xO4a single legs and TiO2
x/LixCo3
xO4a couples. Properties at 750 K (n-type) TiO2
x (p-type) LixCo3
xO4a TiO2
x/LixCo3
xO4a,b a b

a (lV K
1)
230 586 816

r (S m
1) 3

5.4910 7.89102 –

j (W (mK)
1)

ZT

g (%)

1.65 1.25 –

0.132 0.163 0.153

1.89 2.29 2.17

LixCo3
xO4 as-deposit was annealed at 973 K for 2 h and tested. Values calculated using Eqs. (2) and (3).

For the test, 6 modules were fabricated, each consisting of 4 legs of TiO2
x and 3 legs of Ni as illustrated in Fig. 2(b). All the samples were measured concurrently, including modules connected in series. When modules were connected in series, the increase in voltage output was measured from 0.195 V (7 TE legs in series) to 0.899 V (28 TE legs in series). Simultaneous decrease in the current was measured from 0.25 mA (7 TE legs) to 0.06 mA (28 TE legs) where series connection of the TE legs have raised the internal resistance of the TEG which have reduced the current output. Further enhancement on the output performance of the TEG was conducted by introducing the multi-layer concept to reduce the internal resistance by fabricating multiple stacks connected in parallel circuit. Deposited layers were stacked above original layers of Al2O3/Ni/Al2O3/TiO2
x and were connected in parallel as illustrated in Fig. 2(c) and the output performance of the module is shown in Fig. 6(b). Voltage output values from stack 1, 2 and 3 were measured to be 0.196 V, 0.196 V and 0.197 V, respectively. Increase in the current output was observed from 0.25 mA to 0.75 mA, indicating that each stack is connected in parallel. Adding TE legs in series provided expected voltage increase and current decrease while adding TE legs in parallel increased in current with voltage maintained at a constant value. Theoretical voltage output of a module in Fig. 6(a) was calculated from Table 2. Four legs of TiO2
x and three legs of Ni had theoretical Seebeck coefficient of 101 mV K
1. At DT of 461 K, the expected voltage output is calculated to be 0.327 V, which reaches about 60% of

theoretical voltage measured in the module. This is due to the electrical contact made to the n-n type legs of TiO2
x/Ni, where 18% decrease in the Seebeck coefficient was observed. In the case of the module, since more electrical contact were made of the n-n type, this led to additional decrease in the Seebeck coefficient. For a given thermal input power, the temperature difference and the output performance are influenced by the length, the cross-sectional area (width and height), and the number of TE couples. Difference in the length of TE legs lead to different temperature gradient, which results in variances in the output performance of the TEG. The voltage output was measured to be higher as length of the TE leg increased, which was due to higher thermal gradient when equivalent heat was provided along the sample. The Seebeck coefficient dependent on the length from 10 mm to 130 mm, were measured to be the equivalent at same temperature gradient. In the same fashion the width of the legs varied from 500 mm to 4 mm and the height was varied between 33 mm to 200 mm. The resultant measured voltage values were same for all the variables while increase in currents were observed as cross-sectional area was increased. Therefore, at a given thermal input power, optimizing the length of the leg to control the thermal gradient for higher voltage output and cross-sectional area of the leg for higher current output can enhance the overall power output. In this study, we have only been able to report the electrical performance based on the temperature as it is difficult to reliably measure the thermal losses or conduction through the substrate.

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Fig. 6. The output voltage and current of TEG with different amount of (a) TE legs connected in series and (b) TE stacks connected in parallel.

Following the successful demonstration of the multilayer thermoelectric-thermopile concept, the tests were repeated on the alternate star configuration. The advantage here is many more junctions were possible in one unit in situations where localized heat source is available. The output power of the TEG module from Fig. 4(a) and its maximum efficiency (calculated using Eq. (2)) of TiO2
x/Ni thermoelectric couple is shown as a function of temperature in Fig. 7. In this configuration, the added embodiment is that the temperatures were measured from the surface and from the inside of the sample using K-type and the embedded thermocouple, respectively. This embedded sensor is also built by thermal spray incorporating classic K-type materials NiCr-NiAl although their exact compositions do not match that of commercial thermocouples. However, past work [39] has shown that through a-priori calibration of the sprayed thermocouple it is possible to measure the temperature changes within the embedded deposits. Medium temperature from the hot side was calculated using the average from embedded and surface temperatures and was used as the temperature for hot side. The maximum voltage was measured to be 1.52 V from 24 couples of TiO2
x and Ni at 723 K, producing average of 0.063 V per couple. The current in the first stack of TEG was measured to be 0.66 mA at 723 K, giving average of 27.5 mA per couple. Same configuration of TE layers were deposited over stack 1, connected in parallel to enhance the current output. The electrical performances (voltage and current) were

measured after each stacking, and as a result, 48 and 72 couples produced 1.05 and 1.60 mA at 723 K, averaging 21.9 and 22.2 mA per couple, respectively. Power output and max efficiency were 2.43 mW and 0.85% with total of 72 couples. Fig. 8 shows the voltage and current output of the starconfiguration TEG and commercialized TEG as a function of temperature. Both TEG used 72 legs of TE. The plasma-sprayed TEG was able to produce equal amount of voltage, but much lower current at different operation temperature. The low power output of the present TiO2
x/Ni TEG is a step from which to improve considering two reasons: first Ni will be substituted to p-type material for beneficial increase in ZT and second the amount of heat created in many industrial facilities can be voluminous and significant, that cost-effective materials may present some advantage for such applications. Moreover, there are certain design advantages of the star configuration TEG beyond the energy conversion performance. These advantages include the compatibility with low-cost processing, additive manufacturing which allows to directly apply thermoelectric elements onto thermostructural components, and ability to accomplish this at relatively low processing temperatures which allows for the multilayer fabrication to enable direct assembly of thermoelectric modules on to the waste heat systems. The above results have shown that plasma spray is a flexible, scalable and potentially cost effective way to synthesize thermoelectric devices. Innovative attributes of this work lie in the integration of design, materials and processes, where device design itself was enabled by novel manufacturing attributes. First, the

Fig. 7. The output power and maximum efficiency of star configuration TEG as a function of hot temperature.

Fig. 8. The voltage (solid) and current (open) output of the star shape configuration TEG and commercially available TEG.

4.3. Star configuration

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plasma spray process itself was used to engender appropriate phase, stoichiometry and microstructure in TiO2
x to enable reasonable thermoelectric performance. Second, mesoscale, multilayer device engineering was made possible through judicious combination of patterning (using masking), movable registry of the mask to produce multiple active junctions and sequential/ layered manufacturing to optimize series and parallel connections to enhance both thermoelectric voltage and current. Future efforts will be aimed at increasing junction density (example through use of laser micromachined isolations rather than masking [26]), better materials for both p- and n-type systems, reducing system losses through use of thermal barrier and thermal conductive layers at appropriate locations and packaging for harsh environment performance. 5. Conclusions In the present work, thermoelectric power harvesting p-n junction devices were created with a TiO2
x/Ni thermocouple. Ni was used as a surrogate p-type material although it’s not the ideal sample but allowed proving the capability of the concept to produce devices. The goal was to showcase a widely used scalable, manufacturing technology: plasma spray, coupled with novel control of material exposure in the processing environment to tune the thermoelectric properties of this unique system. Conclusions obtained from the results are: (1) Star configuration TEG presented the same voltage output at a higher operation temperature comparing to the commercialized TEG. (2) With the TEG applied with p-type (substitution of Ni) such as LixCo3
xO4; power output and conversion efficiencies can be improved on star configuration (3) In TiO2
x/Ni couples; the actual conversion efficiencies were slightly lower. For instance, calculated conversion efficiencies were 0.85% and 0.57% for the calculation (individual) and measurement (couple) respectively. (4) In order to reduce internal resistance of the device, stacking in parallel circuit or increasing the volume of the material can be done. The results point to a potential pathway for a large-scale fabrication of lowcost oxide based thermoelectric with potential applicability using additive manufacturing of plasma spray. Acknowledgements This work was supported by the National Science Foundation Partnership for Innovation program under award IIP-114205. Support through the Stony Brook Industrial Consortium for Thermal Spray Technology is also acknowledged. References [1] Scherrer H, Scherrer S, Rowe D. Thermoelectric handbook-macro to nano. In: Rowe DM, editor. Taylor & Francis; 2006. [2] Snyder GJ, Toberer ES. Complex thermoelectric materials. Nat Mater 2008;7:105–14. [3] Telkes M. The efficiency of thermoelectric generators. I. J Appl Phys 1947;18:1116–27. [4] Wang XW, Lee H, Lan YC, Zhu GH, Joshi G, Wang DZ, et al. Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy. Appl Phys Lett 2008;93:193121. [5] El-Genk MS, Saber HH, Caillat T. Performance tests of skutterudites and segmented thermoelectric converters. AIP Conf Proc 2004;699:541–52. [6] Shi X, Yang J, Bai S, Yang J, Wang H, Chi M, et al. On the design of highefficiency thermoelectric clathrates through a systematic cross-substitution of framework elements. Adv Funct Mater 2010;20:755–63. [7] Yang J, Li H, Wu T, Zhang W, Chen L, Yang J. Evaluation of half-Heusler compounds as thermoelectric materials based on the calculated electrical transport properties. Adv Funct Mater 2008;18:2880–8. [8] Sales BC, Mandrus D, Williams RK. Filled skutterudite antimonides: a new class of thermoelectric materials. Science 1996;272:1325–8. [9] Venkatasubramanian R, Siivola E, Colpitts T, O’quinn B. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 2001;413:597–602.

[10] Hicks L, Dresselhaus M. Thermoelectric figure of merit of a one-dimensional conductor. Phys Rev B 1993;47:16631. [11] Hicks L, Dresselhaus M. Effect of quantum-well structures on the thermoelectric figure of merit. Phys Rev B 1993;47:12727. [12] Liu W, Borca-Tasciuc T, Chen G, Liu J, Wang K. Anisotropic thermal conductivity of Ge quantum-dot and symmetrically strained Si/Ge superlattices. J Nanosci Nanotechnol 2001;1:39–42. [13] Capinski WS, Maris HJ. Thermal conductivity of GaAs/AlAs superlattices. Physica B 1996;219:699–701. [14] Caylor J, Coonley K, Stuart J, Colpitts T, Venkatasubramanian R. Enhanced thermoelectric performance in PbTe-based superlattice structures from reduction of lattice thermal conductivity. Appl Phys Lett 2005;87:023105. [15] Walia S, Balendhran S, Nili H, Zhuiykov S, Rosengarten G, Wang QH, et al. Transition metal oxides – thermoelectric properties. Prog Mater Sci 2013;58:1443–89. [16] Koumoto K, Wang Y, Zhang R, Kosuga A, Funahashi R. Oxide thermoelectric materials: a nanostructuring approach. Annu Rev Mater Res 2010;40:363–94. [17] Sootsman JR, Chung DY, Kanatzidis MG. New and old concepts in thermoelectric materials. Angew Chem Int Ed 2009;48:8616–39. [18] Masayuki I, Nobukazu N, Hiroo T, Hiroshi I. Hall effect and thermoelectric power in semiconductive TiO2. Jpn J Appl Phys 1967;6:311. [19] Tsuyumoto I, Hosono T, Murata M. Thermoelectric power in nonstoichiometric orthorhombic titanium oxides. J Am Ceram Soc 2006 [0:060427083300013???]. [20] Sharma A, Gouldstone A, Sampath S, Gambino RJ. Anisotropic electrical conduction from heterogeneous oxidation states in plasma sprayed TiO[sub 2] coatings. J Appl Phys 2006;100:114906. [21] Harada S, Tanaka K, Inui H. Thermoelectric properties and crystallographic shear structures in titanium oxides of the Magnèli phases. J Appl Phys 2010;108:083703. [22] Lee H, Han SJ, Chidambaram Seshadri R, Sampath S. Thermoelectric properties of in-situ plasma spray synthesized sub-stoichiometry TiO2
x. Sci Rep 2016;6:36581. [23] Ueno K, Sodeoka S, Suzuki M, Tsutsumi A, Kuramoto K, Sawazaki J, et al. Fabrication of large size FeSi2 thermoelectric device by thermal spraying process. In: Proceedings ICT 98 XVII International Conference on Thermoelectrics, IEEE; 1998. p. 418–21. [24] Sodeoka S, Inoue T, Ruckriem J. Thermoelectric device fabricated by plasma spray. Thermal spray 2007: global coating solutions, proceedings from the international thermal spray conference. Beijing, China; 2007. p. 313–8. [25] Chen Q, Longtin J, Tankiewicz S, Sampath S, Gambino R. Ultrafast laser micromachining and patterning of thermal spray multilayers for thermopile fabrication. J Micromech Microeng 2004;14:506. [26] Tong T, Li J, Chen Q, Longtin JP, Tankiewicz S, Sampath S. Ultrafast laser micromachining of thermal sprayed coatings for microheaters: design, fabrication and characterization. Sens Actuators, A 2004;114:102–11. [27] Min G, Rowe D. Optimisation of thermoelectric module geometry for ‘waste heat’electric power generation. J Power Sources 1992;38:253–9. [28] Sahin AZ, Yilbas BS. The thermoelement as thermoelectric power generator: effect of leg geometry on the efficiency and power generation. Energy Convers Manage 2013;65:26–32. [29] Bejan A, Lorente S, Kang D-H. Constructal design of thermoelectric power packages. Int J Heat Mass Transf 2014;79:291–9. [30] Kim H, Park S-G, Jung B, Hwang J, Kim W. New device architecture of a thermoelectric energy conversion for recovering low-quality heat. Appl Phys A 2014;114:1201–8. [31] Owoyele O, Ferguson S, O’Connor BT. Performance analysis of a thermoelectric cooler with a corrugated architecture. Appl Energy 2015;147:184–91. [32] Bell LE. Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 2008;321:1457–61. [33] Antonik M, O’Connor BT, Ferguson S. Performance and design comparison of a bulk thermoelectric cooler with a hybrid architecture. J Therm Sci Eng Applicat 2016;8:021022-. [34] Takahashi K, Kanno T, Sakai A, Tamaki H, Kusada H, Yamada Y. Bifunctional thermoelectric tube made of tilted multilayer material as an alternative to standard heat exchangers. Sci Rep 2013;3. [35] Dwivedi G, Wentz T, Sampath S, Nakamura T. Assessing process and coating reliability through monitoring of process and design relevant coating properties. J Therm Spray Technol 2010;19:695–712. [36] Sampath S, Srinivasan V, Valarezo A, Vaidya A, Streibl T. Sensing, control, and in situ measurement of coating properties: an integrated approach toward establishing process-property correlations. J Therm Spray Technol 2009;18:243–55. [37] Chen Q, Longtin J, Sampath S, Gambino R. Ultrafast laser micromachining and patterning of thermal spray multilayers for novel sensor fabrication. In: ASME 2003 Heat Transfer Summer Conference: American Society of Mechanical Engineers; 2003. p. 63–70. [38] Longtin J, Sampath S, Tankiewicz S, Gambino RJ, Greenlaw RJ. Sensors for harsh environments by direct-write thermal spray. IEEE Sens J 2004;4:118–21. [39] Gutleber J, Brogan J, Gambino RJ, Gouldstone C, Greenlaw R, Sampath S, et al. Embedded temperature and heat flux sensors for advanced health monitoring of turbine engine components. 2006 IEEE Aerospace Conference: IEEE; 2006. p. 9 pp. [40] Longtin J, Sampath S, Gambino R, Greenlaw R, Brogan J. Direct-write thermal spray for sensors and electronics: an overview. NIP & Digital fabrication conference: society for imaging science and technology; 2006. p. 116–9.