Chapter 3. Direct ink writing of three-dimensional thermoelectric microarchitectures
3.4 Experimental section
69
demonstrate the practicability of 3D direct writing for manufacturing high-performance μ-TEGs that can be integrated into electronic systems.
70
240 ml of a zirconia milling jar. The particles with the Dmedian of 8.759 μm were synthesised by mechanical alloying using SPEX Mill (8000M Mixer-Mill, SPEX) with stainless steel balls including two balls (φ = 12.7 mm) and four balls (φ = 6.35 mm) for 5 h. In all samples, agglomerated particles were removed by sieving the particles to < 45 μm.
3.4.2 Synthesis of all-inorganic (Bi,Sb)2(Te,Se)3-based ink
The Sb2Te4 ChaM was synthesised by dissolving 0.32 g of Sb powder and 0.64 g of Te powder in a co-solvent of 2 ml of ethanethiol and 8 ml of ethylenediamine at room temperature in a N2–filled glove box. The Sb2Te4 solution was stirred for over 24 h until the particles fully dissolved to produce a dark purple colour. Anti-solvent of acetonitrile was added to the solution with the volume ratio of 8:1, which is followed by centrifugation at 6615 g for 10 min to precipitate out the precursor. The
precipitate was dried under vacuum for 30 minutes to produce Sb2Te4 ChaM powder. A mixture of 4 g of the TE powder and desired amount of the ChaM were dispersed in 4 g of glycerol, and the solution was mixed with a planetary centrifugal mixer (ARM-100, Thinky) for 2 h to fully homogenise the ink,
3.4.3 Rheological properties of the ink
The rheological properties of TE inks were measured using a rotational rheometer (Haake MARS III, Thermo Scientific) equipped with a coaxial cylinder geometry at 25 °C. The stress sweep tests were conducted over the range of 0.005 – 300 Pa at a frequency of 1 rad s−1 and the frequency sweep tests were carried out before and after stress sweep test at a constant stress of 1 Pa to assess the phase stability of ink. The three-interval thixotropy tests (3ITTs) at various stresses also conducted as reported elsewhere40. The 1st interval was conducted under constant shear stress of 0.1 Pa (linear viscoelastic range) for 120 s, the 2nd interval was done at various stresses (1, 5, 10, 50, and 200 Pa) for 120 s. The 3rd interval was attempted at a shear stress of 0.1 Pa for 300 s in order to evaluate the structural recovery of ink after deposition during 3D printing.
3.4.4 Direct 3D writing process
Direct writing was executed using an in-house pneumatic extrusion-based 3D printer with a nozzle connected to an ink reservoir, a pressure controller, a compressor unit and a three-axis stage with a stepper motor. Ink was loaded in a 5 ml syringe barrel (Saejong) with a metal needle of inner diameters ranging from 80 to 310 μm and deposited in a z-axis at 0.25 mm s-1. The pressure control
71 unit was adjusted to extrude a high aspect ratio filament.
3.4.5 Materials characterization
XRD patterns were collected by using X’pert Pro (PANalytical) with a Cu K𝛼 X-ray source (wavelength of 1.5418 nm), operating at 40 kV and 30 mA equipped with an X’Celerator detector.
The microstructure was characterised by using a field-effect SEM (Nova-NanoSEM230, FEI and S- 4800, Hitachi High-Technologies) operated at 10 kV. The optical microscopic images were obtained using an Olympus BX51M. The CCD images were obtained by a CCD camera (MicroPublisher 5.0 RTV, QImaging). Particle size distributions of the TE powders were determined with Laser particle size analyser (LS13 320, Beckman Coulter). ζ-potential of the particles was determined by a Zetasizer (Nano ZS, Malvern). The surface functional groups of the particles were obtained using a FT-IR spectrometer (Varian 670/620) in attenuated total reflectance (ATR) mode over the scanning range of 650-4000 cm−1. XPS measurements were performed by an X-ray photoelectron spectrometer (K-alpha, Thermo Scientific).
3.4.6 TE properties of 3D-printed samples
The room-temperature electrical conductivity of the filaments was measured using the sheet resistance of the samples by a four-point method (Keithley 2400 multimeter controlled Lab trace 2.0 software, Keithley Instrument, Inc.) The room-temperature Seebeck coefficienct was obtained by using home-built set-up where the temperature gradient and the open circuit voltage were measured using a Keithley 2400 source-meter. Seebeck coefficient was calculated by the slope of the measured voltages versus the applied temperature differences ranging from 1K to 15K, which was measured by two T-type thermocouples.
The temperature-dependent electrical conductivity and the Seebeck coefficients of the bulk
materials were measured simultaneously under Ar atmosphere in the temperature range from 300 K to 525 K using thermal analyzer (SBA458 Nemesis, Netzsch, Germany). A typical sample for
measurement had cuboid shape with the dimension of 10×10×2 mm3. The thermal conductivity (κ) was calculated from the relationship κ = ρCpD, where ρ is the density, Cp is the specific heat capacity and D is the thermal diffusivity. The thermal diffusivity was measured in the temperature range 300 K to 525 K by using laser flash analyzer (LFA-457, Netzsch, Germany). The specific heat capacity was determined using the rule of mixtures with the previously reported specific heat capacity values of
72
Bi2Te3 and Sb2Te352,53. The density was estimated by measuring the volume and weight of the cuboid shape. The carrier concentrations and mobilities were measured at room temperature by using a Hall measurement system (HMS-8400, Lake Shore) with the magnetic field of ±10T.
3.4.7 Fabrication and output power characterization of the μ-TEG
Figure 3.13. Schematic illustration of the screen-printing of patterned Ag electrodes.
Ag electrodes were screen-printed on a SiO2 wafer using Ag-containing adhesive (Pyro-Duct 597- A, Aremco, USA) as illustrated in Figure 3.13. On top of the Ag electrodes, 3 pairs of n-type and p- type TE filaments were extruded out of a metal needle with the inner diameter of 210 μm. The printed filaments were initially dried at 110 °C on a hot plate for 5 minutes, and annealed at 450 °C for 30 minutes in a tube furnace flowing a gas mixture of 15% H2 and 85% Ar. Each pair of TE filaments was electrically connected in series and thermally in parallel by bridging Ag-containing adhesive on top of the annealed filaments. To measure the temperature difference across each pair of TE filaments, two T-type thermocouples were attached to the surface of the SiO2 wafer (hot side) and on top of the Ag electrodes (cold side). The ceramic heater (10 mm×10 mm) was used as a heat source and the cold side temperature was maintained below 48oC by attaching polyacrylamide (PAAm) hydrogel on top of the μ-TEG. The PAAm hydrogel was separately fabricated by radical polymerisation of 10 % (w/v) acrylamide and 0.5 % (w/v) N,N’-methylenebisacrylamide, with 0.2 % (w/v) ammonium persulfate and 0.2 % (w/v) N,N,N′,N′-tetramethylethylenediamine as co-initiators54. For the measurement of power generation, the TEG was connected to a Keithley 2400 at each temperature differences and the maximum power output (P) was calculated using P=V2/4R. Thermal silicone grease (YYTG-201, Youngyiel) was applied between the heater, and μ-TEG to minimise thermal contact resistance.
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78 Acknowledgement
2016년 처음으로 UNIST 에 오던 날이 아직도 생생합니다. 한국 생활에 적응할 틈도 없이 바쁘
게 지내보니 6년이라는 세월이 훌쩍 지난 것 같습니다. 이 짧고도 긴 6년이란 세월 안에 수많은 감정들이 녹아 들어있는 것 같습니다. 평생 간직할 추억을 만들기도 하고 잊지 못할 경험도 해본 것 같습니다. 또 한 저의 부족함으로 인하여 많이 아파도 봤고 많은 이들에게 실망을 안겨주기도 하였습니다. 하지만 이 런 고난이 있을 때 여러분들이 함께 해주였기에 잘 이겨낼 수 있었고 제가 성장을 하는데 큰 발판이 된 것 같습니다. 제 마음을 완벽하게 전달할 수 없겠지만, 이 글로 모든 분들에게 감사의 말씀을 전하고 싶 습니다.
먼저 저에게 배움과 성장의 기회를 주신 지도 교수님이신 손재성 교수님께 진심으로 깊은 감사 의 말씀을 전하고 싶습니다. 부족함이 많은 저를 잘 이끌어 주시고 항상 진심을 다해 연구에 대한 지혜뿐 만 아니라 인생을 살아가는데 있어서의 태도와 자세를 알려주셔서 그 누구보다도 저에게 큰 귀감이 되 었습니다. 좋은 연구를 할 수 있는 기회를 주시고 저를 항상 믿어주셔서 정말 감사합니다. 그동안 해주신 진심 어린 조언과 격려의 말씀은 평생 잊지 않고 마음속에 새기면서 교수님을 본받아 정말 멋진 연구자 그리고 어른이 되고 싶습니다.
바쁘신 와중에도 박사 학위 논문 심사를 맡아주신 차채녕 교수님, 채한기 교수님, 이지은 교수 님, 그리고 김경태 박사님께 다시 한번 감사의 인사 올립니다. 교수님들 그리고 박사님과 함께 공동 연구 를 진행할 수 있어서 큰 영광이었고 그 과정에서 많은 것을 배울 수 있던 것 같습니다. 그리고 학위 심사 때 주신 귀중한 조언들은 잊지않고 항상 고민하면서 살겠습니다. 제 첫 논문과 두번째 논문에 큰 도움을 주신 UNIST 채한기 교수님, 그리고 부경대학교 엄영호 교수님. 두 분의 열정 덕분에 제가 상상도 하지 못하였던 좋은 논문을 쓸 수 있었다고 생각합니다. 대학원 초창기에 한국전기연구원에 방문할 때마다 항상 반갑게 맞아주시고 친절하게 대해주신 전남대학교 이지은 교수님, 교수님 덕분에 제가 부담감 없 이 실험 할 수 있었고 멋진 논문을 쓸 수 있었습니다. 한국재료연구원에 김경태 박사님, 좋은 소재를 제 공해 주셔서 제가 좋은 논문을 쓰는데 큰 도움이 되었습니다. 그리고 항상 저에게 웃음을 주신 UNIST 차 채녕 교수님, 학위 논문 심사를 맡아 주셔서 감사하고 만나면 항상 유쾌하게 해주셔서 감사합니다. 멋진 논문을 쓰는데 큰 도움을 주신 주혜진 연구원님, 김선태 연구원님께도 진심으로 감사의 말씀 드립니다.
그동안 같은 공간에서 함께 동고동락한 NSE 실험실 구성원들에게도 고맙다는 말을 전합니다.
한국에 와서 우왕좌왕 하고 있을 때 버팀목이 되어준 우리 1기 멤버 그리고 뿔뿔이 흩어진 우리 동기들 과 함께했던 시간들이 기억에 많이 남습니다. 저의 사수인 성훈이형! 형 덕분에 제가 많은 것을 배울 수 있었고 제가 부사수로서 부족한 점을 많이 보여줬는데도 절 잘 가르쳐주고 이끌어주셔서 정말 감사합니 다. 그리고 학위과정에서 정신적으로 힘이 되어준 조승기 박사님! 형 덕분에 잊지 못할 추억들도 많이 만 들고 제가 힘이 들 때 의지 할 수 있어서 형한테 너무 감사합니다. 한국재료연구원에 취업하신 것 너무 축하드리고 저도 형을 본받아서 멋진 연구원으로 자리잡고 싶습니다. 그리고 꾸준하고 침착하게 연구를