Chapter 2. 3D printing of shape-conformable thermoelectric materials using all-inorganic
2.4 Experimental section
2.4.1 Materials
Ethanethiol (>97%), ethylenediamine (>99.5%), acetonitrile (>99.8%), and glycerol (>99.5%), were purchased from Aldrich Chemical Co. Elemental granules of Bi, Sb, Te and Se (99.999%) were purchased from 5N Plus. All elements and chemicals were used without further purification.
2.4.2 Synthesis of all-inorganic Bi2Te3-based ink
The p-type and n-type TE powders were prepared with the stoichiometric composition of
Bi0.4Sb1.6Te3 and Bi2Sb2.7Se0.3, respectively, by high-energy ball milling of Bi, Sb, Te, and Se for 5 h to produce grain sizes smaller than 45 μm. Agglomerated particles were removed by sieving the particles to < 45 μm. The soluble ionic Sb2Te4 ChaM binder was synthesised by dissolving 0.32 g of Sb powder and 0.68 g of Te powder in a co-solvent of 2 mL of ethanethiol and 8 mL of ethylenediamine at room temperature. The solution was then stirred for over 6 h to produce a dark purple solution. The ChaM binder was precipitated by adding 40 mL of acetronitrile into the solution, followed by a
centrifugation at 7500 rpm for 10 min. The precipitated ChaM binder was dried for 30 min under vacuum. 2 g of the TE powder and the desirable amount of ChaM were dispersed in 2 g of glycerol, and the solution was mixed with a planetary centrifugal mixer (ARM-100, Thinky) for 2 h to fully homogenise the ink. 5 zirconium oxide grinding balls of 5 mm in diameter were added to expedite the homogenising process. The whole synthesis process was carried out under an N2 atmosphere
2.4.3 3D printing process
3D printing was performed by a home-built air-powered extrusion-based 3D printer with programmable control of temperature and pressure (Figure 2.8). The TE ink was placed in a 5 mL syringe barrel (Saejong). The syringe had a metal nozzle of 260 μm inner diameter. Using a design software, the 3D TE material was fabricated by printing a grid structure consisting of alternating layers of parallel printing lines, the lines in each layer being perpendicular to those of the previous layer. The printing process was carried out at 105 oC, and the layers were printed at intervals of 2 min.
The printed material was dried at 105 oC for 12 h and then annealed at 450 oC for 1 h under a N2
46 atmosphere.
2.4.4 Rheological properties of the ink
The dynamic rheological properties of the TE inks were measured with a rheometer (Haake MARS
Ⅲ, Thermo Scientific) at 25 oC in a parallel-plate geometry with a diameter of 35 mm. The plate gap and strain level were 1 mm and 1%, respectively. The rheological responses of BST100 and
BST100/12.5, BST100/25 and BST100/37.5 were assessed to verify the effect of the ChaM on the viscoelastic properties of the TE inks.
2.4.5 Fabrication and output power measurement of the cylindrical TEG
Figure 2.17 Temperatures on the hot and cold sides of the cylindrical TEG under a flow of hot water.
Three pairs of Cu electrodes were attached on top of the alumina pipe of 5 mm inner diameter and 8 mm outer diameter. Then three pairs of 3D printed half–ring-shaped TE materials of 8 mm inner diameter, 15 mm outer diameter, and ~1.53 mm average thickness were attached onto the Cu
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electrodes by using Ag containing epoxy adhesives (Elcoat A-200, CANS, Inc.). Each pair of half–
ring-shaped TE material was electrically connected in series and thermally in parallel by electrodes on the top and bottom sides of the half rings. Two T-type thermocouples were attached at the surface of the alumina pipe (hot side) and on the Cu electrode (cold side) to measure their temperature difference.
For measuring the TEG output characteristics, I assembled a measurement set-up where hot water constantly circulated inside the alumina pipe thanks to an electric water pump, increasing the hot-side temperature from 40 to 70 oC, while maintaining the cold side at a constant temperature of 30 ± 4 oC (Figure 2.17). The hot water flow rate was set at 6.9 × 10-6 m3·s-1 and the temperature difference between the hot water and the module itself was 2.3 oC. For a reliable measurement of power generation, the TEG was connected to a Keithley 2400 sourcemeter to measure an I-V curve and the output power density at each temperature difference. For the measurement of the internal resistance (R), the device was connected to an ammeter in series and to a voltmeter in parallel. The maximum power output (P) was calculated by the equation: P = V2/4R.
2.4.6 TE properties measurement
The room-temperature electrical conductivity of the 3D printed samples was measured by a Van der Pauw 4-terminal method (Keithley 2,400 source-meter controlled Lab trace 2.0 software, Keithley Instrument, Inc.). The Seebeck coefficients of the 3D printed samples at room temperature were determined by measuring the open-circuit voltage at the applied temperature gradient by a commercial Peltier cooler in contact with the samples. Both the voltage and temperature difference across the samples were measured by T-type thermocouples (a Keithley 2,400 source-meter and a Keithley 2,000 multimeter). Typically, six data points were obtained with temperature differences ranging from ±1 oC to ±5 oC, and the Seebeck coefficient was estimated by the slope of the voltage-temperature difference curve. The accuracy of this experimental set-up was confirmed by measuring the Seebeck coefficient of n-type Bi2Te3 and p-type BiSbTe ingot samples, and the accuracy was found to be within ±3%. The measurements of the temperature-dependent electrical conductivity and Seebeck coefficient were performed using a commercial equipment (ZEM-3, Ulvac-Riko) in the temperature range from 25 °C to 225 °C under a low-pressure helium atmosphere. The thermal conductivity (κ) was calculated by using κ = ρCpD, where ρ is the density, Cp the heat capacity and D the thermal diffusivity. The thermal diffusivity was measured in the temperature range from 38 °C to 218 °C using laser flash analysis (LFA 457, Netzsch). The density of the 3D printed samples was estimated by measuring their volume and weight. Assuming the law of mixtures, the specific heat capacity of the samples was determined by using the specific heat capacity values of Bi0.4Sb1.6Te4, Sb2Te3, Bi2Te3 and Bi2Se345-47. The carrier
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concentration and mobility were measured at room temperature by a Hall measurement system.
2.4.7 Simulation study
I developed a three-dimensional FEM to calculate the temperature and electrical potential distributions in the TEGs. As in the experiments, the TEGs in the FEM were attached on top of an alumina circular tube in which hot water flowed at a temperature Tw. I assumed that the water had a uniform temperature along the tube and its flow was considered as fully developed laminar flow in terms of heat dissipation (Nusselt number, Nu, of 3.66, or equivalent, convection coefficient, ht, of 448 W·m-2·K-1). The convective heat transfers at the outer surface of the entire system due to the convection of air at the temperature Ta = 27 oC and with the convection coefficient ha was also considered. The coefficient ha used in the calculation was varied to produce the same cold-side temperature of the half-ring-based TEG as in the experiment (~ 30 oC). When Tw changed from 40 oC to 70 oC, ha increased from 20 W·m-2·K-1 to 95 W·m-2·K-1, which is equivalent to increasing the air flow rate around the TEG during the experiment.
For comparative simulation, a planar TEG with a substrate area of 144 mm2 was used since the half-ring-based TEG was in contact with the alumina tube over an area of ~115 mm2. This regular TEG consisted of 1 mm thick alumina substrates, 100 m thick copper electrodes, and 17 pairs of rectangular TE elements of 1 mm2 cross-sectional area and 2 mm length. Based on the properties of the commercial TE materials, I assumed that the TE element had a Seebeck coefficient of 197 V·K-
1, electrical resistivity of 0.96 mΩ·cm, and thermal conductivity of 1.7 W·m-1·K-1. As the contact area between the regular TEG and the heated tube may also be important for the TEG performance, the planar TEG was attached to an alumina pipe in my model by an epoxy with a thermal conductivity of
~0.43 W·m-1·K-1. Two different contact areas were considered by setting the epoxy width (wEpoxy) as 1 mm and 4 mm. For the determination of ΔTTE, three thermal resistances (Rths) were taken into account:
Rth between water and the bottom of the TE elements (Rth,hot), Rth across the TE elements (Rth,TE), and Rth between the top of the TE elements and air (Rth,cold). ΔTTE was approximated by ΔTTE ~ (Tw-Ta) Rth,TE /( Rth,hot + Rth,TE + Rth,cold). The conformal TEG achieved a larger ΔTTE than the planar TEGs due to a smaller Rth,hot, as the conformal TEG had not only a larger contact area but also no insulating material such as the epoxy with the heated tube. To obtain a larger ΔTTE in the conventional planar TEG, Rth,hot should be reduced by employing a highly conductive adhesive and maximising wEpoxy.
49 2.4.7 Material characterization
XRD patterns were obtained by using X’pert Pro, PANalytical with a Cu Ka C-ray source, wavelength of 1.5418 nm, operating at 40KV and 30mA equipped with an X’Celerator detector. The SEM images were obtained by using a field effect SEM (Nova-NanoSEM230, FEI and S-4800 Hitachi High-Technologies) operated at 10 KV. The optical microscopy image was obtained by using Olympus BX51M.
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