Chapter 2. High performance thermoelectric painting
2.2 Characterize of thermoelectric ink
2.2.2 Thermoelectric properties of n-and p-type sintered sample
Dense and well-sintered grains in painted TE samples resulted in excellent TE properties in both n- and p-type samples. TE properties of painted n- and p-type samples annealed at 450 oC were characterized at measuring temperatures ranging from 25 oC to 125 oC.
The temperature dependence of electrical conductivities of n-and p-type samples in Figure 2.18. The electrical conductivity of the n-and p-type samples gradually decreased as the ambient temperature increased to 125 oC, which indicates a metallic conducting behavior. The maximum electrical conductivity of the n-and p-type samples 650–750 S cm-1 at room temperature which were similar with the properties of hot-pressed or spark plasma sintered Bi2Te3 based TE materials with 700~1100 S cm-
1.These high electrical conductivities arise from the appropriately high carrier mobility of 149 cm2 V-1 s-1 for the n-type and 141 cm2 V-1 s-1 for the p-type materials.
Fig. 2.18. Temperature dependence of electrical conductivity for TE n- and p-type samples solder w/o solder solder w/o solder
0 100 200 300 400 500 600 700
800 N-type
P-type
Electrical conductivity (S cm-1 )
25 50 75 100 125
200 300 400 500 600 700 800
Electrical conductivity (S cm-1 )
Temperature (oC)
N-type P-type
３１
The temperature dependence of electrical conductivities of n-and p-type samples in Figure 2.19. The absolute seebeck coefficient value of TE n- and p-type samples were increased as the temperature increased at 100 oC and then began to decrease. This behavior is characteristic feature of a highly degenerate semiconductor due to the reduction of carrier concentration result from the Fermi level is moving up to valence band maximum with temperature increased. The Seebeck coefficient of n- and p- type samples were 115 μV/K and 170μV/K at room temperature and the peak value reached 135 μV/K and 190 μV/K at 375 K, respectively. The high carrier concentrations of 3.0 x 1019 cm-3 for n-type samples and 2.9 x 1019 cm-3 for p-type samples14,15 result in those relative low Seebeck coefficients. As well, It was founded that the annealing temperature affected the electrical properties of n-and p-type painted samples. The figure 2.20 shows SEM images of n- and p-type painted samples with increasing annealing temperatures that could be attributed to the increased grain sizes and densities arising from Sb2Te3-ChaM effect to interconnect grains with the annealing temperatures.
Fig. 2.19. Temperature dependence of absolute Seebeck coefficient for TE n- and p-type samples
25 50 75 100 125
40 80 120 160 200 240
Temperature (
oC)
Absolute seebeck coefficient (V K-1 )
N-type P-type
３２
Fig. 2.20. SEM images of n-type (a) and p-type (p) TE paints sintered at various temperatures.
３３
The most significant effect of molecular ChaM-assisted sintering is seen in the great reduction in the thermal conductivities of the n- and p-type samples (Figure 2.21), i.e., 0.5–0.6 W m-1 K-1 in comparison with the 1.5–2.5 W m-1 K-1 of bulk Bi2Te3-based materials16.
Fig. 2.21 Temperature dependence of thermal conductivity for TE n- and p-type samples
The thermal conductivity is the sum of electronic and phonon contribution, which is called the lattice thermal conductivity (κl). The lattice thermal conductivity can be calculated by subtracting the electronic contribution to the thermal conductivity (κe) from total thermal conductivity (κ), which was estimated by using the Wiedemann-Franz Law (κe= LTσ, where T is the absolute temperature, L is the Lorenz number, σ is the electrical conductivity). The Lorenz number of 2.0×10-8 V2 K-2 is the typically used value for a degenerate semiconductor. However, in the recently published papers, the more reliable calculated value of ~1.6×10-8 V2 K-2 was widely used in Bi2Te3 related materials. Based on this value, the minimum calculated κl was 0.19 W m-1 K-1 for n-type and 0.20 W m-1 K-1 for p-type painted materials.
These values are lower or comparable than the predicted minimum κl of 0.31 W m-1 K-1 in n-type Bi2Te3
and 0.20 W m-1 K-1 and p-type (Bi,Sb)2Te3. It using the Debye-Callaway model17, whereas the densities of painted materials are at most 50~55% of full densities. (Figure 2.22)
25 50 75 100 125
0.2 0.3 0.4 0.5 0.6 0.7
0.8 N-type
P-type
Temperature (
oC)
Thermal conductivity (W m-1 K-1 )
３４
Fig. 2.22. Temperature dependence of calculated lattice thermal conductivities of n- and p-type painted samples using the modified formulation of the effective medium theory
The improvement of ZT values in nanostructured TE materials originates in the falloff of thermal conductivity by phonon-grain boundary scattering arising from the increased interface density. The grain sizes of the painted samples range from several hundred nanometres to several micrometres so that their ultralow lattice thermal conductivities are difficult to be explained solely by grain boundary scattering.
One possible explanation for the ultralow lattice thermal conductivity is the porosity of materials because the porosity in the painted samples reach 45~50%, which suggest that the phonon scattering at multiple pore sites can be a crucial factor to the reduced thermal conductivity.
The Brunauer Emmett and Teller (BET) measurement was conducted to analyse the porosity(Figure 2.23). Both n-and p-type samples have pores with the size less than 50 nm, which should be located at the interfaces formed from Sb2Te4 sintering aids rather than grains insides. These small pores can act as scattering sites for phonons with short wavelengths. However, the volumes of these small pores are responsible for only 2~3% porosity in the painted samples, calculated based on the measured pore volumes, which suggests the existence of micro-scale pores.
３５
Fig. 2.23. Nitrogen adsorption-desorption isotherms of the painted (a) n-and (b) p-type samples.
The inset shows the pore size distributions.
Figure 2.24. Low-magnification SEM images of fractured structure (a) and surface of painted sample.
(b) n-type, (c) p-type. The red circles show the micro-scale pores in the samples.
To confirm the micro-scale pores, the microstructure of the painted samples was analysed by the SEM.
As shown in Figure 2.24, the multiple pores with the size ranging from several tens of nanometres to several microns were clearly observed in the SEM images. Given that the presence of multi-scale pores can reduce the thermal conductivity by phonon scattering with a broad range of wavelength at pore sites,
３６
the ultralow thermal conductivities of painted TE materials can be explained by the porosity.
To quantitatively estimate the porosity effect on the thermal transport, The thermal conductivity was compute the using the effective medium theory proposed by Lee et al.18 κl = κh((2-2Φ))/((2+Φ)), where κh and Φ is the lattice thermal conductivity of host materials and the porosity respectively. The calculated minimum κl of the n-and p-type painted samples are 0.44 W m-1 K-1 and 0.47 W m-1 K-1 (Figure 2.22) respectively,which are comparable to those of typical nanostructured bulk materials prepared from ball-milled Bi2Te3-based TE materials. Consequently, these results suggest that the porosity of painted samples can be a crucial factor to the reduced latticed thermal conductivity for boundary scattering of phonons at pore sites, rather than grain boundaries.
Generally, the porosity of solid materials strongly affects the charge carrier transport due to scattering of carriers at the pore sites.19 A charge carrier passing near a pore is scattered due to the potential perturbation18, degrading the carrier mobility and eventually the electrical conductivity. The carrier scattering effect on mobility can be qualitatively described by the Matthiessen’s rule18
1 𝜇tot=𝜇1
bulk+𝜇 1
impurity+𝜇 1
boundary+𝜇1
pore (1) Accordingly, the total scattering is the sum of the contribution of different carrier scattering mechanism. For example, μbulk is the mobility induced solely by the carrier scattering with acoustic phonons. In the painted materials, considering no additional impurity element except Bi, Sb and Te, μboundary and μpore should be the critical factors to determine the overall mobility. Lee et al. suggested that the porosity effect on electrical properties become weaker for larger grains18. Since the material with larger grains necessarily has larger pores with the lower number density under the same porosity, the scattering rate is reduced and mobility is enhanced for larger grain sizes. The fact that the grain size is in the range of several micrometres and the pores are mainly macro-scale in the painted materials (less than 3% of micro-pores in volume) suggests that the moderately high mobility is attributed to the lower number density of the pore.
The efficiency of the TE material is represented by the dimensionless figure of merit(ZT) and ZT=1 correspond to about 10% of total energy conversion efficiency. To reach the high value of ZT, It needs to have high Seebeck coefficient, high electrical conductivity and low thermal conductivity. The ZT value was calculated using the thermal conductivity, electrical conductivity and thermal conductivity.
Figure 2.25 shows the temperature dependence of ZT for the n- and p-type sample. The highest values of ZT = 0.68 for n-type at 375 K and 1.15 for p-type at 350 K were achieved. It is noteworthy that those values were much higher than those of Bi2Te3 based bulk ingots with single crystal or Polycrystalline of ZT ≈ 0.8–1.016 and comparable to those of recent nanostructured TE materials with ZT ≈ 1.1–1.720-
23. Especially, as far as we know, these exceedingly high ZT values in current painted materials are 3–4
３７
times higher than the best reported values of screen-printed TE pastes24. The high electrical conductivities and ultra-low thermal conductivities led to remarkably high TE ZT values of n-and p- type painted samples.
Fig. 2.25. Temperature dependence of ZT for TE n- and p-type samples
2.3 Fabrication of in-plane type thermoelectric devices via painting process.
2.3.1 The process of thermoelectric painting
The synthesized n-and p-type TE paints were painted on glass, aluminum, polyimide, and alumina substrates with a flat painting brush. Painted layers on a substrate were sequentially dried on a hot- plates at 90 oC for 30 min ~ 60 min, 120 oC for 30 min ~ 60 min, and 150 oC for 30 min ~ 60 min and then they were annealed at desired temperatures with 350~450 oC for 10~30 min. To obtain thick painted layers with several hundreds of micrometers in thickness, painting and drying processes were repeated by several times, followed by annealing. In-plane type TE devices were fabricated by painting five couples of Sb2Te3-ChaM assisted n-and p-type TE paints with the size of 5 mm x 10 mm on various substrates such as flat glass and polyimide, and curved hemi-cylindrical convex and concave glasses, and alumina hemi-sphere. Glass and alumina substrates were hydrophilized by a UV plasma treatment for 1 h before painting. Silver paste was painted electrically in series and thermally in parallel in order to interconnect n-and p-type TE legs. Nitrogen-filled glovebox is used for all experiment.
2.3.2 Output characteristics of in-plane thermoelectric devices on flat substrates
In the first case of all-painted TE devices, n-and p-type TE paints were painted on a flexible polyimide substrate with a brush and painted layers were dried. Surprisingly, the painted layers formed continuously uniform films with the thickness of ~50μm during annealing. It seemed to smooth out the uneven surfaces composed of TE particles via filling up void spaces with molecular Sb2Te3-ChaM.
25 50 75 100 125
0.0 0.3 0.6 0.9 1.2
1.5 N-type
P-type
ZT
Temperature (oC)
３８
Metal electrodes were also prepared by painting of Ag paste on pre-painted and annealed TE layers.
This all-painted TE device was comprised of 5 couples of n-and p-type legs with the lateral dimension of 5 mm × 10 mm and the average thickness of ~50μm. (Figure 2.26)
Fig. 2.26. (a) Scheme and photographs of an in-plane type TE devices composed of painted legs with silver electrodes on a (b) polyimide substrate and (c) glass substrate. (d) Cross-sectional SEM image
of painted TE device
In manufacturing the TE device, TE materials well attached to the electrode is highly important factor for determine the power of device. The internal resistance of this device was 25.8 Ω, higher than the expected resistance in relation with the electrical properties. It might be due to the high contact resistance between the Ag electrode and the TE leg. In order to investigate the cause for high resistance of the TE device, The contact resistance between the Ag electrode and the painted TE leg was measured by the transmission line method. The measured contact resistance is quite high at 4.8 × 10-2 Ω cm2.(Figure 2.27) That value is three or four orders of magnitude higher than the contact resistance observed in conventional module composed of Bi2Te3-based TE legs25.
３９
Fig. 2.27. Contact resistance measurement by the transmission line method
The output voltage of all-painted TE device on a polyimide substrate exhibited voltage of 79.4 mV and output power of 60.8 μW at temperature difference of 50 oC. (Figure 2.28a) The output power density reached 2.43 mW/cm2(Figure 2.28b), which considerably exceeded the best reported values of in-plane typed TE devices by ~2 times. Also, 60 μW-level output power offer the potential for wearable TE devices for harvesting energy from human body as an auxiliary power supply. All-painted TE devices on a hydrophilized glass substrate were prepared under same preparation conditions. The internal resistance, output voltage, out power and density (Figure 2.28c and d) were ~20 Ω, 79 mV, 59 μW, and 2.4 mW/cm2 respectively, which were almost identical to those of all-painted TE devices on a polyimide substrate, suggesting the consistency of TE painting process on various substrates.
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Fig. 2.28. Output characteristics of in-plane TE devices painted on flat substrates.
Output characteristics of in-plane TE devices painted on plate substrates. (a,b) Output power and power density of a polyimide substrate TE device (c,d) Output power and power density of a glass substrate TE
2.3.3 Output characteristics of in-plane thermoelectric devices on curved substrates
Such versatility of TE paints allowed us to apply TE layers on curved surfaces to directly prepare TE devices on curved heat sources. As shown in Figure 2.29, TE paints were painted on concave and convex surfaces of hydrophilized glass hemi-cylinder and were annealed at 450 oC, followed by painting of Ag pastes to interconnect n- and p-type layers. The resulting all-painted TE devices on curved surfaces were composed of 5 couples of n- and p-type layers with the lateral dimension of 5 mm × 10 mm and the expected thickness of ~50 μm (Figure 2.29a and 2.29b). Internal resistances of TE devices on convex and concave surfaces were identical values of ~20 Ω, which was consistent with those of the painted TE devices on flat substrates. Under temperature difference of 30 oC, these devices produced the output voltage of 31~35 mV and output power of 17~18 μW, respectively, leading to comparable values of 0.7４１
mW/cm2 to those of the painted TE devices on flat glass substrates under same temperature difference (Figure 2.29c and 2.29d for a concave device, and 2.29e and 2.29f for a convex device). Especially, output power densites of all TE devices painted on flat and curved substrates with same dimensions of TE layers reached into the same line, which demonstrated the applicability of TE paints on any-shaped surfaces.
４２
Fig. 2.29. Output characteristics of in-plane TE devices painted on curved substrates. (a) Photo of a concave device, (b) Photo of a convex device (c,d) Output power and power density of a concave
device (e,f) Output power and power density of a convex device
2.4 Fabrication of hemispherical thermoelectric device via Painting process
New-typed all-painted TE devices were prepared comprising 5.5 couples of triangle-shaped TE layers with ~20 mm in base and 25 mm in height on large-sized ceramic hemi-sphere (diameter~50 mm) which collected heat energy from a bottom side. Internal resistance of this TE device was ~32 Ω, higher than those of other devices due to large-sized TE layers with 25% higher aspect ratio.
４３
Fig. 2.30. Photo of hemispherical TE device via Painting process
2.4.1 Simulation study on the power output.
The comparative simulation study was investigated on the power output of the painted TE generator and the conventional module on a curved heat source using COMSOL. The model calculates the temperature distribution and generated power of the TE generators integrated with a heated hemispherical alumina substrate.The TE generator composed of one pair of n-and p-types of TE layers and conductive paste layers where the thickness was assumed as 50 m. Each triangular TE layer has a width of 20 mm and a height of 60 mm such that the substrate has sufficient area for 5.5 couples of TE layers. The conductive paste was assumed to have the thermal conductivity of 9 W m-1·K-1 and the electrical conductivity of 103 S cm-1. Based on the geometry and the material properties, the electrical resistance was estimated as 3.5 per a pair of the TE layers. Based on the survey of the material properties and the geometry for commercial TE modules, It was defined that a standard TE module has a substrate area of 40 mm 40 mm and ~100 pairs of Bi-Te materials. The considered module has the thermal conductance of 0.65 W K-1, the electrical resistance of 2.3 , and the Seebeck coefficient of 52.8 mV/K. For the simple modeling, the FEM for the conventional module includes only one leg of TE material. The heat loss in the FEM was considered by including the convective heat transfer. To simulate the natural convection over all the surfaces that are exposed to air, the convection heat transfer coefficient was 10 W m-2 K-1 with an ambient temperature of 25 oC. [ref. 26]
The temperature distribution of the conventional module is greatly non-uniform (Figure 2.31) since the contact area (d) with a hemisphere is narrow which results in a significantly low output voltage of 13.3 mV for d = 1 mm and 4.5 mV for d = 0.1 mm. Thus, conventional module generates the output power of 76.9 W and the output power density of 15 W cm-2 when d = 1mm. when d = 0.1mm, the output power of 8.6 W and the output power density of 1.7 W cm-2 which are considerably reduced values compared with the reported values of 4-10 mW cm-2 obtained on a flat heat source27.
４４
Fig. 2.31. Calculated (a,c) temperature and (b,d) electrical potential distribution of a conventional TE module that is contact on a heated hemispherical substrate.
On the other hand, the temperatures distribution of a hemispherical type module was kept at 45 oC and 25 oC (Figure 2.32) the uniform temperature distribution and electrical potential field on the painted generator result in an order of magnitude higher power density of 205 W cm-2. (Figure 2.33)
Fig. 2.32. A finite element model for a hemispherical heated substrate.
(a) A meshed substrate that is subject to a uniform heat flux of 1.5 kW/m2, and natural convection (h = 10 W m-2 K-1). The bottom surface is set at 25 oC. (b, c) Temperature distribution along an arc A-B. A and B indicates the apex and the bottom of an alumina hemisphere, respectively. Red arrows indicate the heat flux and blue arrows present the temperature on the bottom of hemisphere.
Temperature (
oC)
Temperature (
oC)
< 38 41 45
< 30 38 45
Electrical Potential (mV) 0 104 207.9
Electrical Potential (mV) 0 37.2 74.5
a b
c d
10 mm
４５
Fig. 2.33. Calculated (a) temperature and (b) electrical potential distribution of a pair of p- and n- types of painted TE generator.
2.4.2 Output characteristics of hemispherical thermoelectric devices
Figure 2.34 show output characteristics of painted hemispherical TE devices. Under temperature difference of 19.7oC, the output voltage was 18 mV and output power was 2.6 μW. A little lower output voltage than those of other devices could come from the deviation between actual temperatures applied to TE legs and temperatures at the hot and cold sides due to heat loss arising from small contact area of TE devices to the hot side. Lower output power was attributed to higher resistance and lower output voltage. Although this hemi-spherical TE device exhibited lower output power density due to large- sized TE layers, these results validate that TE painting process facilitated shape engineering of TE materials on any-shaped surfaces while maintaining their high TE performance, which should be the most effective heat energy collection from any heat sources.
a
b
45 30 25 Temperature (oC)
2.7 1.5 0
Electrical Potential (mV)
Conductive paste TE paint
10 mm p-type n-type
４６
Fig. 2.34. Output characteristics of hemispherical TE devices
The output power density of the hemispherical generator was calculated with the assumption of same dimensions of TE legs with others. The fact that the output power density of the TE devices painted on flat and curved substrates with the same dimension of TE legs merges into the same line validates the applicability of the TE paints to any-shaped surfaces. (Figure 2.35)
Figure. 2.35. Comparison of output power densities of painted TE devices