Microsyst Technol (2014) 20:585–592 DOI 10.1007/s00542-013-1993-7
TECHNICAL PAPER
Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples Silvia Pelegrini · Andrea Adami · Cristian Collini · Paolo Conci · Clodoaldo I. L. de Araújo · Vittorio Guarnieri · Saulo Güths · André A. Pasa · Leandro Lorenzelli
Received: 31 July 2013 / Accepted: 21 November 2013 / Published online: 30 November 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract This work reports the development and the characterization of a microthermoelectric generator (µTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO2/Si3N4/SiO2 thermally insulating membrane to create a thermal gradient. The µTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N2) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm2 and of 1.1 µW/cm2, respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N2 flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10−3 µW/cm2 K2. 1 Introduction Nowadays rechargeables and non-rechargeables batteries with lifetime limitation power most of the electronic devices. However, different alternatives to conventional S. Pelegrini · C. I. L. de Araújo · S. Güths · A. A. Pasa (*) Universidade Federal de Santa Catarina, Trindade, Florianópolis, SC 88040-970, Brazil e-mail:
[email protected] S. Pelegrini · A. Adami · C. Collini · P. Conci · V. Guarnieri · L. Lorenzelli Fondazione Bruno Kessler, via Sommarive 18, 38123 Trento, Italy
batteries have been considered, since many applications need high reliability and long life power. In addition, we have to consider the environmental impact of producing and disposing of chemical batteries. One approach is to convert heat from the environment into electricity using microthermoelectric generators. Others possibilities are scavenging energy from the environment with photovoltaic cells and vibrational generators. By comparing these approaches, it is clear that photovoltaics have the greatest energy density, but direct access to sun irradiation is not always available in innumerous applications. On the other hand, vibrational MEMS suffer from limitations that are regulated by the mass of the resonator. Thermoelectric generators (TEGs), usually composed by an array of thermocouples exploiting the Seebeck voltage generated by temperature gradients between hot and cold areas of the device, are a good alternative to produce energy for applications where photovoltaic cells are not of practical use. TEGs started to be developed in 1960 to deliver power to space vehicles (Riffat and Ma 2003; Rowe 2006). Nowadays, there are many reports on devices powered by ambient energy such as battery-free wireless sensors (Salerno 2010), and biomedical and other devices powered by human body energy (Boniche et al. 2009; Leonov and Vullers 2009; Sue and Tsai 2012). Integrated and flexible µTEGs using low-cost plating process with different thermoelectric pillars, in one case Ni and Cu interconnected by Cu and in the other case p and n-type Bi2Te3 interconnected by Au, were already fabricated (Glatz et al. 2009; Schowyter et al. 2008). In our case, the thermoelements are constantan and copper connected in series in a planar architecture. Constantan is a copper– nickel alloy, which can be plated directly on the surface of the silicon wafer (Delatorre et al. 2003). The Seebeck coefficient of constantan is −37.3 µV/K at temperature of
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273 K, a factor 2 higher than nickel of −19.5 µV/K (Genix et al. 2009). The output voltage (Uo) of thermoelectric generators based on serial arrangement of thermocouples is dependent on the voltage generated by each thermocouple; in other words, it is proportional to the total number of thermocouples (Kasap 2001). According to the Seebeck effect, the open-circuit output voltage (Uo) is given by,
Uo = nα∆TG ,
(1)
where n is the number of thermocouples, α is the relative Seebeck coefficient, i.e. the difference between the absolute Seebeck coefficients of Cu (+1.7 µV/K) and constant in our case, and ∆TG is the temperature difference between the hot and cold thermoelectric junctions. The output power (Po) is given by,
Po =
Uo2 , 4R
(2)
where the load R is equal to the internal resistance of the generator for maximum power transfer. The performance of the devices are usually compared by the power efficiency factor (ϕ) (Xie et al. 2010), defined as,
ϕ=
Po , AG ∆TG2
(3)
where AG is the surface area of the generator. In this work, we present a microthermoelectric generator (µTEG) based on planar technology using constantan (CuNi) and copper (Cu) thermocouples deposited electrochemically (ECD) on a silicon-based substrate. Thin film implementation (~1 µm) was selected because it can be manufactured into large area and also on flexible substrates, with low cost production. The device can be used to exploit waste heat from equipments, body heat or hot surfaces in general, using the environment temperature and heat exchange by convection at cold side of the device. In the current implementation, a silicon structure has been designed and optimized with finite element (FE) simulation in order to exploit the different thermal conductivity of silicon and air gaps to produce the maximum temperature difference on a planar surface.
2 Experimental 2.1 Design Figure 1 shows an illustration of the µTEG developed in this work with a silicon substrate covered with an insulating multilayer, and a planar array of thermocouples connected in series and integrated on top of the structure. The thermoelectric pair is Cu/Constantan interconnected by Al, which
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Fig. 1 Illustration of the micro-thermoelectric generator based on planar technology with thermocouples in series
was also used for the pads. The light area in the center of the device corresponds to the region of the SiO2/Si3N4/SiO2 multilayer that is suspended forming a membrane. The generator was thought to be placed on a hot surface to generate the thermal gradient and the appearance of the output voltage at the terminals (pads). On each wafer 14 generators were fabricated, 6 with 180 and 8 with 16 thermoelements, with areas of 1.3 × 3.5 and 1.0 × 1.0 cm2, respectively. 2.2 Electroplating of copper and constant alloy There are many reports in the literature on electrochemical deposition of Cu100–x Nix alloy, where x is the Nicontent (%) (Green et al. 1998; Iraj et al. 2008; Sartorelli et al. 2001). The proportion of Cu to Ni varies according to deposition parameters (composition of electrolyte, temperature, and applied potential). In this work, the aqueous electrolyte used for the deposition of constantan alloy was 0.171 mol/l NiSO4, 0.019 mol/l CuSO4, 0.190 mol/l C6H5Na3O7, at room temperature and prepared with deionized water with a resistivity of 18 M cm. The thermocouples were plated from this single electrolyte by applying −0.60 V for copper and −1.05 V for constantan (Cu40Ni60) (Delatorre et al. 2003). The counter-electrode used was a Pt grid and the reference electrode was Ag/AgCl with 1 M of KCl. The working electrode was Cr/Au (3 nm/20 nm) seed layer evaporated on the surface of the SiO2/Si3N4/SiO2 multilayer illustrated in Fig. 1. The Cr film was applied to improve the adherence of the Au on the oxide. In the voltammogram showed in Fig. 2a are indicated the potentials that resulted in Cu and constantan deposits. Figure 2b shows a scanning electron microscopy (SEM) image of the compact constantan layer 1 µm thick used in
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Fig. 2 a Voltammogram on Au/Cr/SiO2 surface at a scanning rate of 20 mV/s with electrolyte containing 0.019 M CuSO4, 0.171 M NiSO4 and 0.19 M C6H5Na3O7. The regions of the voltammogram corresponding to reduction of Cu and constantan (Cu and Ni simultaneously) are indicated. b SEM image of constantan alloy with 60 at.% of Ni and thickness of 1 µm (color figure online)
Table 1 Quantities used in Eq. 4 (Genix et al. 2009) Materials
t (nm)
σ (Ω m)−1
α (µV/K)
Au Cr CuNi
20 3 1,000
Cu
1,000
4 × 107 7.8 × 106 2.0 × 106
+1.8 +18.5 −37.3
2.5 × 106
+1.7
the thermoelements with a composition of 60 at.% of Ni, as determined by energy dispersive X-ray spectroscopy (EDX). The contribution of Cr/Au layers to the output voltage can be determined by calculating the Seebeck coefficient (S//) for planar structures in parallel (Herin and Théry 1992), described by the equation below,
S// =
tCr σCr αCr + tAu σAu αAu + tCuNi σCuNi αCuNi , tCr σCr + tAu σAu + tCuNi σCuNi
(4)
where t, σ and α are the thickness, electrical conductivity and Seebeck coefficient for each materials, shown in Table 1. The calculated S//is equal to −25.0 µV giving an equivalent Seebeck coefficient of −26.7 µV/K, which is about 70 % of the Seebeck coefficient presented above for Cu/constantan without the Au/Cr layer. 2.3 Microfabrication The starting material was a p-type Si wafer (100), Fig. 3a), where a layered structure of Si2O/Si3N4/SiO2 was grown in both sides of the sample for the future fabrication of the membrane. The first 500 nm thick oxide layer was grown by thermal wet oxidation, which was followed by 150 nm silicon nitride (Si3N4) and 500 nm oxide, both deposited by low pressure chemical vapor deposition (LPCVD), as displayed in Fig. 3b. The seed layer of Cr/Au was sequentially deposited on the polished side of the wafer by e-beam evaporation on top of the trilayer SiO2/Si3N4/SiO2, as seen in Fig. 3c. The photoresist AZ4562 (6,000 nm thick) was
Fig. 3 Cross section illustration of the principal steps of the microfabrication of the thermoelectric generator. The total number of process steps was 54
spin coated on the surface of the seed layer and exposed to light after the alignment of the mask. Figure 3d presents the process step after the developing of the photoresist for opening the copper regions with a length of 2 mm and width of 40 µm. Cu was electrochemically deposited and AZ4562 layer was removed by solvent and O2 plasma, Fig. 3e, f. The same steps were followed for constantan
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Fig. 4 Optical micrographs of a device with 16 thermocouples, a before and b after the fabrication of the membrane. The region delimited by the blue square in a is magnified to show in detail the thermoelectric stripes and the Al interconnections (color figure online)
plating (sequence not shown in the figure). The seed layer was removed, Fig. 3g, from the regions where no Cu and constantan were deposited by using chemical etchants. The evaporation of Al on the patterned surface for the pads and interconnections and lift-off removal of Al from areas not pre-defined are illustrated in Figs. 3h, i respectively. Figure 3i, j show the opening of hard mask (SiO2/Si3N4/SiO2) on the backside of the wafer for the release of the insulating membrane by wet etching. Figure 3j also displays the application of SU-8, a thick photoresist (6,000 nm), to protect the surface against the anisotropic etching of the wafer giving additional mechanical resistance to the membrane. Finally, Fig. 3l shows the TMAH (Tetra methyl ammonium hydroxide) bulk micromachining of the substrate, leaving the suspended insulating membrane. Figure 4 shows optical micrographs of a device with 16 thermocouples, Fig. 4a before and Fig. 4b after the fabrication of the membrane. In Fig. 4b the area of the membrane is clearly seen. To optimize the output of the microgenerator two parallel lines of thermocouples were fabricated, in this case with 8 thermoelements each side. The marked square region in Fig. 4a is amplified to presents in detail the electric connection with Al (light brown) between both sides and between the Cu (orange) and constantan (black) stripes. 2.4 Experimental setup Figure 5 shows the testing setup with the microthermoelectric generator on top of a hot plate with all the electric connections for measuring resistance, voltage and temperature. The inset is an illustration of the device with an acrylic cover leaving a cavity of 5 mm × 2 mm of cross section for the force nitrogen flow. The open circuit potential was measured at different flow velocities for increasing temperature difference (∆T ) between the hot plate and room temperature (~300 K).
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2.5 Thermal modelling and simulation The thermal model of the device can be implemented by considering the heat transfer equations for conduction and convection conditions and the Seebeck effect. Thermal conduction can be calculated from Fourier’s law (Reynolds 1972),
q˙ = −kA
∆T12 l
(5)
where q˙ is the rate of heat transfer in units of (W), k is the thermal conductivity (W/mK), A is the heat transfer area (m2), ∆T12 = T2 − T1 is the temperature (K) difference between a region 2 (hot) and a region 1 (cold), and l is the length (m) between these 2 regions. Typical thermal conductivities for materials in the device are reported in Table 2, showing the very low thermal conductivity of air with respect to the other materials, which are used to create the temperature gradient across thermocouples. Thermal convection conditions on the surface of the device can be calculated using Newton’s law (Reynolds 1972),
˙ = hA∆T12 Q
(6)
˙ is the heat transfer rate (J/s), and h is the convecwhere Q tive heat transfer coefficient in units of (W/m2 K). The convective heat transfer at the bottom part of the device is neglected, since the size of the cavity is too small (see Fig. 3l). The heat transfer coefficient (h) depends upon physical properties of the fluid and the geometry of the device; i. e., if convection is on a planar surface or in a tube, for example, and can be calculated from the equations of applied thermodynamics (Holman 1992), Nu =
hL (Nusselt number), k
(7)
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Fig. 5 Testing setup and inset showing the cross section of the acrylic cap with cavity of 5 mm × 2 mm for the force N2 flow
Table 2 Thermal conductivity of the materials used in the device (Xie et al. 2010)
Table 3 Physical properties of the fluid (N2) used in thermal finiteelement simulation (FE) (Lemmon and Jacobsen 2004)
Materials
Si
SiO2
CuNi
Air
Physical properties of the fluid
Values
Thermal conductivity (W/mK)
149
1.25
19.5
0.026
Density Dynamic viscosity Specific heat Thermal conductivity
1.22 kg/m3 17.8 × 10−6 Pa.s 1.005 kJ/KgK 0.026 W/mK
Temperature
300 K
Re =
ωxρ (Reynolds number), µ
(8)
Pr =
cρ µ (Prandtl number), k
(9)
where x is a characteristic dimension (m), L in this case is the membrane length (m), ω the fluid velocity (m/s), ρ is the density (Kg/m3), µ the dynamic viscosity (Pa.s), and cp the specific heat (kJ/kgK) at constant pressure. In real operation conditions, the surface of the membrane was exposed to N2 forced flow using a closed pipe with characteristic length (x) equal to the hydraulic diameter (d). For the experimental setup used in this work (see Sect. 2.4), a duct with a rectangular cross section of area (Ac) equal to 10 mm2 (5 mm × 2 mm) was used, and a hydraulic diameter of ~2.9 mm, calculated from equation,
d=4
Ac , P
(10)
where P is the perimeter of the rectangular duct (Holman 1992). Table 3 shows the physical properties used for the calculation of the heat coefficients considering a fluid (N2) under
Table 4 Calculated heat transfer coefficients (h) and simulated ∆TG values velocities in the range of 1–6 m/s ∆T = 10 K between the bottom (310 K) and top (300 K) Velocity (m/s)
h (W/m2 K)
∆TG (K)
6 5 4 3 2
20.89 19.07 17.06 14.77 12.06
2.84 2.65 2.44 2.18 1.85
1
8.53
1.39
normal temperature and pressure (STP), in accordance with experimental conditions. The Reynolds numbers obtained from Eq. 8 considering velocities in the range of 1 a 6 m/s are smaller than 2,300, which corresponds to the laminar regime, and Nu can be calculated from equation (Holman 1992),
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Fig. 6 a Temperature profile as simulated by ANSYS for a half cross-section of the generator with lateral size of 3,000 µm and a height of 502 µm. The insulating membrane and thermocouple stripes were 1 µm thick each. A forced N2 convection of 6 m/s was considered in the simulation. b Amplification of the rectangular region showed in a to illustrate in detail the simulated structure
Fig. 7 a Open-circuit voltage (mV/cm2) and b output power (µW/cm2), both versus temperature difference (∆T) between the hot plate and room temperature for N2 velocities from 1 up to 6 m/s and 180 thermocouples
Fig. 8 a, b The results of Fig. 7 plotted against the simulated temperature difference between hot and cold junctions of the thermocouples (∆TG)
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Microsyst Technol (2014) 20:585–592 1
1
Nu = 0.664Pr 3 Re 2 .
591
(11)
By introducing the results of Eq. 11 for the different velocities and using Eq. 7, it is possible to obtain the coefficient (h), as seen in Table 4. The thermal simulation was performed using the software ANSYS with thermal finite-element analysis (FEA) (Huesgen et al. 2008). To simplify the analysis, a two-dimensional (2-D) model was considered. Figure 6a shows a half crosssection of the generator with a lateral size of 3,000 µm and a height of 502 µm (thickness of the insulating layer and thermocouple stripes, 1 µm each, and of the silicon substrate of 500 µm). The distribution of temperatures shown in this figure was obtained solving the conduction equations with the following boundary condition: fixed temperature at bottom side (310 K), room temperature of 300 K, and heat transfer coefficient from Table 4. The simulation in Fig. 6 was obtained for an h = 20.8 W/m2 K and forced 6 m/s nitrogen flow velocity. A temperature difference ∆TG of 2.8 K between hot (Thot) and cold (Tcold) junctions is obtained. The ∆TG values for the 6 different velocities are presented in Table 4.
3 Results and discussion The generated voltage and power as a function of temperature difference between hot plate and room temperature ∆T, gas velocities in the range of 0–6 m/s for devices with internal resistances of ~3 KΩ and 180 thermocouples are shown in Fig. 7. The temperature of the hot plate was varied in order to obtain temperature differences as high as 75 K for constant room temperature of 300 K. The power was calculated using Eq. 2. For the highest ∆T of 75 K and maximum velocity of 6 m/s an open voltage of 118 mV/ cm2 and power of 1.1 µW/K2cm2 were generated. Figure 8 shows the open-circuit voltage and the power as a function of the simulated ∆TG (this quantity was not measured experimentally) using the conditions of Fig. 7, that is, ∆T varying from 15 to 75 K. We see that all the curves from Fig. 7 collapse into one curve for voltage and also for power, as expected, since the Seebeck coefficient does not dependent on the gas velocity. However, the higher values of voltage and power were only attained by increasing gas velocity. Equation 1 allow us now to calculate the effective Seebeck coefficient of the device, which for a voltage of 118 mV/cm2 corresponding to a simulated ∆TG of 22 K gives raise to a value of −30.0 µV/K, which is consistent with the equivalent Seebeck coefficient calculated in Sect. 2.2 of −26.7 µV/K for Cu/constantan thermocouples with a Cr/Au seed layer. The maximum efficiency (performance) was 2 × 10−3 µW/cm2 K2, as calculated from Eq. 4. This results is comparable to the one found in the literature for Cu/Ni TEGs produced by e-beam evaporation (Glatz et al. 2009).
4 Conclusion A microthermoelectric generator was successfully fabricated on a micromachined Si substrate using electrodeposited Cu/constantan thermocouples to generate the voltage. The maximum output voltage and power of 118 V/cm2 and of 1.1 µW/cm2, respectively, were obtained for a device with 180 thermocouples, 3 KΩ of internal resistance, and under a force flow of N2 of 6 m/s. References Boniche I, Masilamani S, Durscher RJ, Morgan BC, Arnold DP (2009) Design of a miniaturized thermoelectric generator using micromachined silicon substrates. J Electr Mater 38(7):1293– 1302. doi:10.1007/s11664-009-0764-9 Delatorre RG, Sartorelli ML, Schervenski AQ, Güths S, Pasa AA (2003) Thermoelectric properties of electrodeposited CuNi alloys on Si. J Appl Phys 93(10):6154–6158. doi:10.1063/1.1569432 Genix M, Vairac P, Cretin B (2009) Local temperature surface measurement with intrinsic thermocouple. Int J Therm Sci 48:1679– 1682. doi:10.1016/j.ijthermalsci.2009.01.020 Glatz W, Schowyter E, Durrer L, Hierold C (2009) Bi2Te3: based flexible micro thermoelectric generator with optimized design. J Microelectromech Syst 18(3):763–772. doi:10.1109/JM EMS.2009.2021104 Green TA, Russell AE, Roy S (1998) The development of a stable citrate electrolyte for the electrodeposition of copper–nickel alloys. J Electrochem Soc 145(3):875–881. doi:10.1149/1.1838360 Herin Ph, Théry P (1992) Measurements on the thermoelectric properties of thin layers of two metals in electrical contact. Application for designing new heat-flow sensors. Meas Sci Technol 3:495. doi:10.1088/0957-0233/3/5/009 Holman JP (1992) Heat transfer, 7th edn. Metric Editions, London, p 713 Huesgen T, Woias P, Kockmann N (2008) Design and fabrication of MEMS thermoelectric generators with high temperature efficiency. Sens Actuators A 145–146:423–429. doi:10.1016/j.sna.2007.11.032 Iraj K, Elham K, Mansoor F (2008) Fabrication and nanostructure study of ultra thin electroplating constantan film on GaAs as a thermopower sensor. J Phys Conf Ser 100:052025. doi:10.1088/1742-6596/100/5/052025 Kasap SO (2001) Thermoelectric effects in metals: thermocouples, principles of electronic materials and devices, 2nd edn. e-Booklet Lemmon EW, Jacobsen RT (2004) Viscosity and thermal conductivity equations for nitrogen, oxygen, argon, and air. Int J Thermophys 25(1):21–69. doi:210195-928x/04/0100-0021/02004 Leonov V, Vullers RJM (2009) Wearable electronics self-powered by using human body heat: the state of the art and the perspective. J Renew Sustain Energy 1:062701. doi:10.1063/1.3255465 Reynolds AJ (1972) Thermofluid dynamics. Wiley, London, p 680 Riffat SB, Ma X (2003) Thermoelectrics: a review of present and potential applications. Appl Therm Eng 23:913–935. doi:10.1016/ S1359-4311(03)00012-7 Rowe DM (2006) Review thermoelectric waste heat recovery as a renewable energy source. Int J Innov Energy Syst Power 1(1):13–23 Salerno D (2010) Ultralow voltage energy harvester uses thermoelectric generator for battery-free wireless sensors. J Analog Innov 20(3):1–11 Sartorelli ML, Schervenski AQ, Delatorre RG, Klauss P, Maliska AM, Pasa AA (2001) Cu–Ni thin films electrodeposited on Si:
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Microsyst Technol (2014) 20:585–592 Xie J, Lee C, Feng H (2010) Desing, fabrication, and characterization of CMOS MENS-based thermoelectric power generators. J Microelectromech Syst 19(2):317–324. doi:10.1109/JM EMS.2010.2041035
