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Highly enhanced thermoelectric energy harvesting from a high-temperature heat source by boosting thermal interface conduction
Energy Conversion and Management 183 (2019) 360–368
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Highly enhanced thermoelectric energy harvesting from a high-temperature heat source by boosting thermal interface conduction ⁎
Duckjong Kima, , Chihyun Kima,b, Jinsung Parkb, Tae Young Kimc,
T
⁎
a
Department of Applied Nano Mechanics, Korea Institute of Machinery & Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea Department of Control and Instrumentation Engineering, Korea University, 2511 Sejong-ro, Sejong 30019, Republic of Korea c Division of Mechanical System Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea b
A B S T R A C T
Thermoelectricity is regarded as one of the promising waste heat recovery candidates. A fundamental but effective method for the best use of thermoelectric generators (TEGs) is to maximize the heat flow crossing thermoelectric materials. The main focus of the present study was to develop a thermal interface material (TIM) with high thermal conductivity and temperature resistance as the key to minimizing the overall thermal resistance of the heat flow path of a TEG module operating under high-temperature conditions. In combination with a polyimide matrix and a multi-dimensional filler compound, a new TIM having stable heat conduction behavior at high temperatures was produced. The developed TIM was stable without losing mass up to ∼500 °C, and its thermal conductivity reached 81.4 W/m·K. It was applied to the interface between a TEG and a heat source whose temperature ranged from 100 to 300 °C and, the effect of the thermal conductivity and interface thermal resistance of the TIM on thermoelectric power generation performance during onsite curing of the TIM was investigated. Reduction of the interface thermal resistance by the new TIM improved the power generation by, at most, 132.3 and 38.6% compared to cases without a TIM and with a conventional graphite foil TIM, respectively. In terms of energy conversion efficiency, the TEG with the new TIM showed maximum improvements of 73.2 and 20.9% over the cases without a TIM and with a graphite TIM for the same temperature difference across the TEG, respectively. In addition, thermal cyclic testing confirmed the longlasting heat-conducting feature of the developed TIM. The present work clearly shows the potentially significant influence that TIMs have on the waste heat recovery performance of TEGs.
1. Introduction As one of the promising thermal energy harvesting technologies, thermoelectricity has garnered considerable attention. It has been shown that a huge amount of waste heat from inefficient parts of transportation vehicles and industrial plants operating in high-temperature ranges can potentially be recovered using thermoelectric power generation [1]. The increasing tendency of the waste heat recovery performance of thermoelectric generators (TEGs) according to the increase in the temperature difference across TEGs has boosted advances in high-temperature thermoelectric materials and systems [2]. Most of research works have pursued a phonon-glass electroncrystal material by means of phonon scattering structures [3], highly mismatched isoelectronic doping [4], etc. to maximize ZT, the thermoelectric figure of merit. Cheikh et al. [5] shed new light on praseodymium telluride by modifying its density of states for an improved Seebeck coefficient and lower thermal conductivity, resulting a ZT value of 1.7 even at 1200 K. Zhang et al. [6] reported an enhancement in the thermal stability of a TEG consisting of n- and p-type SiGe elements that peaked their figures of merit at ∼1073 K by means of TaAlN thin films resistant to high-temperature oxidization. Even though most
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Corresponding authors. E-mail addresses: [email protected] (D. Kim), [email protected] (T.Y. Kim).
https://doi.org/10.1016/j.enconman.2018.12.108 Received 10 September 2018; Accepted 24 December 2018 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
of research works on thermoelectric materials have pursued a high ZT value, there is another way to increase the energy harvesting performance. A TEG is typically located between a heat source and a heat sink. Reduction in the overall thermal resistance of the path from the heat source to the heat sink can allow larger heat transfer rate through the TEG, leading to enhanced TEG power. Thermal interface material (TIM) is applied to the interface between the TEG and the heat source/ sink to reduce the thermal resistance. In comparison with the bulky and robust configuration of heat sources and TEGs, a thin layer of TIM is mostly developed for electronics cooling and tends to be vulnerable to high-temperature conditions [7]. This challenge raises the need for temperature resistant TIMs. The higher the interface thermal resistance, the larger the loss of the temperature difference across the thermoelectric materials, and this has led to an unexpectedly lower power output [8,9]. As such, keeping the tight coupling between a TEG and adjacent components (heat source and heat sink) is another concern for non-adhesive TIMs. For this reason, several types of TIM including metal alloys, vertically aligned carbon nanotubes (VACNTs), 3D-boron nitride/graphene, and polymer-based materials have been proposed for high-temperature applications. Metal alloys containing In, Bi, Sn, and Au are the materials most commonly
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Nomenclature A c CNT Cp DSC h I IA k max MWCNT NP P PI Q q
R s sc sh t TEG TGA TIM T ΔT VACNT ZT
area (m2) cooling block carbon nanotube specific heat (J/kg·K) differential scanning calorimetry heating block interface between TEG and heating block interface between TEG and cooling block thermal conductivity (W/m·K) maximum value multiwalled carbon nanotube nanoparticle power output (W) polyimide heat transfer rate (W) heat flux (W/m2)
thermal resistance (K/W) surface; spacing (m) surface of cooling block in contact with TEG surface of heating block in contact with TEG thickness (m) thermoelectric generator thermogravimetric analysis thermal interface material temperature (K) temperature difference (K) vertically aligned carbon nanotube figure of merit
Greek α ρ
thermal diffusivity (m2/s) density (kg/m3)
used. Even though their thermal conductivities are usually high (∼50 W/m·K), the use of these TIMs is limited because of their limited integration ability with other nonmetal materials due to poor wetting properties [10]. As mentioned, VACNTs have also been considered as candidate TIMs because of their extremely high thermal conductivity (thousands of W/m·K). However, the synthesis of CNTs requires a hightemperature environment (∼800 °C) and when exposed to the air, they can be oxidized at around 400–500 °C [11]. Recently, 3D foam-like graphene and hexagonal boron nitride have been proposed as TIMs. Even though the 3D foam-like structures have high cross-plane thermal conductivity (62–86 W/m·K), the highly complicated, time-consuming, and expensive fabrication processes for these materials comprise a serious bottleneck [10]. Polymer-based composite materials have been widely used as TIMs because of their excellent wetting properties. For high-temperature applications, the thermally stable polymer polyimide (PI) has been considered as a promising matrix material, although the industrial use of the PI-based TIMs is seriously limited due to their low thermal conductivity (∼1 W/m·K) [12]. In addition, it is hard to find a report on TIM development that considers practical application conditions (mating surfaces, onsite curing, etc.) for high-temperature TEGs, and the required interface thermal resistance and thermal properties of TIMs for sufficient thermoelectric power generation have not yet been well-investigated. This letter presents a new TIM paste by introducing a multi-dimensional filler design composed of 3D microscale Ag flakes, 1D multiwalled carbon nanotubes (MWCNTs), and 0D Ag nanoparticles (Ag NPs) proposed in [13] that significantly enhances the thermal conductivity into a thermally stable PI matrix (hereafter, referred to as MFPI TIM). The excellence of the new TIM is confirmed in terms of printability, thermal conductivity, and thermal stability. The MFPI TIM paste was applied to the interface between a TEG and a heat source and the effect of the interface thermal resistance on thermoelectric power generation performance was investigated during onsite curing of the TIM using a customized experimental setup. From the experimental work, appropriate thermal conductivity of the TIM for sufficient thermoelectric power generation is discussed. In addition, the interface thermal resistance of the MFPI TIM and corresponding thermoelectric power generation characteristics are compared with those for cases without a TIM and with a commercial graphite foil TIM. The reliability of the MFPI TIM is also validated from consistent levels of the power output of the TEG throughout the thermal cyclic test performed by varying the heat source temperature between 100 and 300 °C. Fig. 1. MFPI TIM preparation procedure.
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2. Materials and methods
(Branson Ultrasonic Corp., Branson 5510). The mixture solution of the MWCNTs and Ag NPs was vacuum-filtered to collect the MWCNTs functionalized with the Ag NPs and dried for 12 h in a vacuum chamber (JeioTech, VDR-25G) at room temperature. Finally, the Ag flakes (Metalor, SA-31812, 7.5–55.4 vol%) and MWCNTs coated with Ag NPs (3.5–4.2 vol%) were mixed in 33 wt% polyimide (Huntsman Chemical Company, Matrimid® 5218) dissolved in 1-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, 328634, 99.5%) by stirring for 5 min to obtain the TIM paste. To prepare the coin specimens for the laser flash analysis (LFA), the TIM paste was poured into a Teflon mold and cured for 4 h at 180 °C and then for 1 h at 260 °C, unless otherwise stated. Before pouring the paste into the mold, a releasing agent (NABAKEM, FLEX-A) was applied to the surface of the mold to detach the coin specimens from the mold
The MFPI TIM was prepared using a protocol similar to that previously published [13] as summarized in Fig. 1. The TIM was composed of polyimide (matrix material), Ag flakes (primary filler), and MWCNTs coated with Ag NPs (secondary filler). First, 400 mL of AgNO3 solution in ethanol (Kojundo Chemical, 0.02 mol/L) and 3.2 mL of benzyl mercaptan solution in ethanol (Sigma-Aldrich, B25401, 99%, 0.1 mol/L) were stirred for 48 h to synthesize the Ag NPs functionalized with a phenyl group (diameter: 3–5 nm). In the next step, 50 mg of MWCNTs (Nano solution, TMC-05010) were dispersed in 100 mL of ethanol by tip sonication (Sonics & Materials Inc, Vibra-Cell VCX 750) and then mixed with the prepared Ag NP solution through 3 h of bath sonication
Fig. 2. Schematic diagrams of (a) the generator-level waste heat recovery performance tester and (b) the entire experimental setup. 362
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where kAl is the thermal conductivity of the aluminum heating block. The area-averaged heat flux is calculated in a similar fashion to the area-averaged surface temperature. The rate of heat transferred to the hot surface of the TEG is the product of the area-averaged heat flux and the horizontal cross-sectional area of the heating block. In an analogous manner, the averaged surface temperature and heat flux on the surface of the cooling block in contact with the TEG were obtained. By performing an error analysis, the uncertainties with a confidence of 95% for experimental results were estimated by combining the bias and precision errors [14]. The bias errors were calculated using accuracies of data acquisition systems and sensitivity coefficients of experimental variables. The precision errors were determined by using standard deviation of three repeats for each physical quantity (thermal conductivity, interface thermal resistance, TEG power, conversion efficiency and TEG internal resistance) and t-distribution (for more detailed information, see Supplementary Table A1). Steady-state conditions were assumed to have been reached when the variation in timeaveraged surface temperatures of TEG measured every 1 s was smaller than 0.1 K during consecutive durations of 120 s. Once steady-state conditions had been reached, the resistance of electric load connected to the two end terminals of the TEG varied from 0.18 to 60.00 Ω, which obtained corresponding voltage output and current output values used to analyze the waste heat recovery performance of the TEG. The effect of the TIMs on the energy harvesting performance of TEG was examined by forming three different interface thermal conduction conditions at the interface between the surface of the test TEG and the heating block: a 0.3-mm-thick MFPI TIM (proposed in this study), a 0.04-mm-thick commercial graphite foil TIM (Hi-lim Electronics Ltd., HGS40), and no TIM (where the ceramic cover plate of the TEG was in direct contact with the bare surface of the heating block). The variations in interface thermal resistances were tracked as a function of the curing time and the type of TIM. The detailed procedure for calculating the thermal resistance is given in Supplementary Fig. A1.
easily. The cross-plane thermal diffusivity (α) of the coin specimen made of the MFPI TIM was measured using LFA (Netzsch, LFA-447), and the specific heat (Cp) was measured at 10 °C min−1 using differential scanning calorimetry (DSC, Mettlertoledo, DSC1 STARe System) under a nitrogen atmosphere in the range of 25–300 °C. The density (ρ) of the coin specimen was measured with a gas pycnometer (InstruQuestInc, Humipyc model 2) under a helium atmosphere. The cross-plane thermal conductivity (k) was calculated using k = ρ ⋅ Cp ⋅ α. The phase transformation of the Ag NPs on the surface of the MWCNTs was characterized using DSC under a nitrogen atmosphere with a preset temperature profile consisting of three stages: heating (10 °C/min) from 50 to 350 °C, holding at 350 °C for 1 min, and cooling (−10 °C/min) from 350 to 50 °C. The thermal stability was investigated via a thermogravimetric analysis (TGA, Scinco, TGA N-1500) in the range of 25–800 °C at 20 °C/min under a nitrogen atmosphere. The viscosity was measured at a shear rate of 2–100 s−1 using a viscometer (Thermo scientific, HAAKE MARS III). The thermal resistance of the MFPI TIM and its effect on the generator-level thermoelectric energy conversion performance was examined by means of a custom-made test setup consisting of a set of aluminum heating and cooling blocks, as shown in Fig. 2. A 40 (W) × 40 (L) × 4 (H) mm3 test TEG (Custom Thermoelectric, 1991G7L31-12CQ) was sandwiched between the heating and cooling blocks and compressed by tightening via a clamp with a torque of 5 kg·f·cm (during the curing of the MFPI TIM) or a weight of 30 kg·f (during the energy harvesting experiment). Each of the aluminum heating and cooling blocks had 6 holes tapped into their sides for the thermocouple temperature measurements. The heating block raised the surface temperature of the test TEG using eight 60-mm-long cartridge heaters inserted in the holes made in its sides. The cartridge heaters were controlled by a power supply equipped with a proportional-integralderivative (PID) temperature controller. An additional hole formed on the side of the heating block, indicated as 'feedback' in the inset of Fig. 2(a), was used to mount a thermocouple that provided the power supply with temperature feedback for PID control. Using the feedback control, the temperature of the heating block is set to a target value and is maintained for steady-state experiments. The bottom and side surfaces of the heating block, except the surface where thermocouples and heaters are mounted, are shrouded in polyether ether ketone housing (k = 0.28 W/m·K) to reduce heat loss to the ambient. To maintain a specified temperature difference across the TEG, water at ∼303 K was supplied to the cooling block via a water pump (Wilo, RS 20/6). Passing through a serpentine-shaped passage inside the cooling block, water took the heat from the TEG; it was then cooled by an air-cooled heat exchanger (Lytron, 4220G10) and a chiller (Lab Companion, RW3025G/HTBC-2330AT), which kept the water temperature at the inlet of the cooling block almost constant. All of the temperature measurements were logged in a data acquisition system (Graphtec Corp., GL820) and processed to calculate the rate of heat transferred to and the time-averaged surface temperature of the TEG. The local temperature of the heating block surface in contact with the TEG, Tshj, was calculated based on the continuity of the heat flux as follows:
Tshj = Th1j −
sh1 (Th2j − Th1j ), sh2
for j = 1, 2, and 3
3. Results and discussion The rheological and thermal properties of the MFPI TIM were checked to confirm its usability. To investigate the printability of the TIM, the thixotropic index of the material was obtained. Thixotropy refers to the rheological behavior that a material exhibits when flowing under stress [15], and the thixotropic index is a time-dependent rheological property that describes the extent of the thixotropic behavior. To obtain the thixotropic index, viscosity of the TIM according to the shear rate was measured, as shown in Fig. 3a. The viscosity is approximately independent of the shear rate when the shear rate is low, showing the Newtonian behavior. On the other hand, the viscosity apparently decreases with the shear rate at high shear rate. It could be ascribed to alignment of suspended fillers in the direction of flow [16]. The rate of viscosity decrease according to the increase in shear rate apparently increased when the shear rate was larger than 30 s−1. Since the thixotropic index is the ratio of viscosity values at two shear rates that are different by a factor of 10 [17], the thixotropic index was larger than 5 when the minimum shear rate was larger than 10 s−1, which was higher than those of commercial TIMs [18,19]. The effect of the volume fraction of the primary filler (Ag flake) on the thermal conductivity of the TIM was also investigated, as shown in Fig. 3b (for more detailed information, see Supplementary Table A2). The thermal conductivity abruptly increases when the volume fraction of the Ag flake increases from 15.5 to 23.6%, indicating that the percolation threshold of the primary and secondary fillers must lie in this range. When the Ag flake volume fraction was larger than 23.6%, the thermal conductivity monotonically increased with the volume fraction and reached 81.4 W/ m·K when the volume fraction was 55.4% which was the volume fraction of the Ag flake in the TIM paste used in the TEG experiment. To check the thermal stability of the TIM, a TGA was conducted, the results of which in Fig. 3c show that the 1 wt% loss temperature of the TIM was
(1)
where Th1j and Th2j are temperatures measured with thermocouples mounted into the heating block, and sh1 and sh2 are spaces between the thermocouples, respectively, as illustrated in Fig. 2(a). The area-averaged surface temperature is an arithmetic mean of the three local surface temperatures. The local heat fluxes on the hot surface of the TEG, qhj, are calculated as
qhj = −kAl ⎛ ⎝
⎜
Tshj − Th1j ⎞ , sh1 ⎠ ⎟
for j = 1, 2, and 3
(2) 363
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property at the interface between the heat source and the TEG apparently increased the TEG power under the same conditions. To measure the thermal conductivity of the TIM during the course of the curing, the paste in the coin specimen mold was dried at 80 °C and cured in the TEG test setup. Fig. 4b clearly shows that the TEG power ratio significantly depended on the thermal resistance of the interface between the heating block and the power generator. As the interface thermal resistance decreased, the TEG power ratio increased. The interface thermal resistance (RI) is the sum of the thermal resistance for conduction through the TIM (RTIM = tTIM/kTIM/ATIM where tTIM, kTIM, and ATIM represent the thickness, thermal conductivity, and area of the TIM, respectively) and the thermal contact resistance of the interfaces between the TIM and the thermoelectric device (RC1) or the heating block (RC2), as shown in inset of Fig. 4b. Since the heat source temperature and the heat sink temperature were kept almost identical during the experiment, more heat flow could cross the thermoelectric device as the interface resistance decreased. The increased heat flow increased the temperature difference across the thermoelectric materials and led to an increase in the TEG power. Fig. 4b shows that both resistances (RTIM, RC1 + RC2) comprising the interface resistance decreased during the curing process and that, initially, RTIM was comparable to RC1 + RC2. For a curing time of less than 80 s, the thermal resistance reduction mainly came from the thermal conductivity increase of the TIM. For the curing time between 80 s and 200 s, the reduction in RC1 + RC2 mainly contributed to the decrease of RI, and for the curing time larger than 200 s, only RTIM decreased due to the increase in the thermal conductivity of the TIM during the curing for a while. Although RC1 + RC2 also depended on the thermal conductivity of the TIM, the dependence weakened as the thermal conductivity of the TIM increased [13,21] and in this study, almost disappeared when the thermal conductivity of the TIM reached around 20 W/m·K. For the curing time of 200 s and over, since RC1 + RC2 was dominant in RI, the increase in the thermal conductivity of the TIM did not increase the TEG power ratio significantly, as shown in Fig. 4b. The TEG power ratio reached 90% when the TIM thermal conductivity is around 20 W/m·K and further increase in the thermal conductivity did not significantly enhance the TEG power. A TIM with thermal conductivity reaching about 80 W/m·K was used for the TEG test, which means that the filler was excessively used in the tested TIM. Fig. 3b shows that 23.6 vol% primary filler, for which the thermal conductivity of the TIM is larger than 20 W/m·K, was enough to maximize the TEG power. The DSC analysis shows that the thermal conductivity increase of the MFPI TIM came from the coalescence between the Ag NPs functionalized on the surface of MWCNTs and the Ag flakes [21], facilitating inelastic energy exchanges across the solid/solid interfaces [22]. Fig. 4c shows that an endothermic valley and an exothermic peak can be observed at around 175 °C during the heating and cooling of the uncured filler, respectively (for all heating and cooling curves, see Supplementary Fig. A2), the former of which is attributable to the melting of the Ag NPs on the MWCNTs [23]. In addition, there is an exothermic peak at around 175 °C in the heating curve for the uncured filler and that cured for just 3.5 min, indicating that coarsening of the Ag NPs occurred [24]. The overlapping exothermic peak disappeared for the samples cured over 2 h. The exothermic peak in the temperature range 200–275 °C, which could be related to a reduction in the crystallographic defects of the Ag NPs, disappeared when the filler was cured for over 4 h [24,25]. Hence, it could be inferred that the coalescence between the Ag NPs and the Ag flakes, which was completed after a short time of curing, mainly contributed to the reduction in RTIM, and that the recrystallization of the Ag NPs, which lasted for longer time, apparently decreased RC1 + RC2. We also compared RI for the various interface materials, as shown in Fig. 4d. The experiments on the waste heat recovery performance of the TEG were carried out under fixed heat source temperature conditions of 100–300 °C at intervals of 50 °C. As the temperature of the surface of the cooling block facing the test TEG was not precisely controlled
Fig. 3. (a) The viscosity of the MFPI TIM paste, and (b) the effect of the Ag flake volume fraction on thermal conductivity and (c) the TGA results of the cured TIM.
576.7 °C. This indicates that the TIM could be used up to around 500 °C without losing mass. Even if mass of fillers is not considered, the 1 wt% loss temperature is 508.6 °C which is much higher than that of the PI without fillers (434.0 °C). The improved thermal stability can be ascribed to the tortuous path formed by the fillers that hinders the release of the volatiles generated by thermal decomposition of the matrix material, delaying the degradation [20]. The MFPI TIM was applied to the interface between the heating block and the thermoelectric device, and the TEG power and the interface thermal resistance were monitored. As shown in Fig. 4a, the TEG power increased with the TIM curing. Comparing the TEG power during the curing process with that at the same temperature after curing the TIM, the power ratio between them increased with the curing time and reached 90% with the curing time of 350 s. The power ratio mostly increased as the TIM temperature increased to a curing temperature of 180 °C, which was determined based on the melting temperature of the Ag NPs in the multi-dimensional filler [13]. Fig. 4a also shows that the thermal conductivity of the TIM increased during the curing process, which indicates that the enhancement in the thermal conduction 364
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Fig. 4. (a) The TEG power and thermal conductivity of the TIM, and (b) the thermal resistance during the curing process. (c) The DSC results of the MFPI TIM according to curing time. (d) The interface thermal resistance (RI) of the various interface materials.
graphite TIM, respectively, under the same temperature difference conditions. For thermoelectric energy conversion efficiency, the TEG with the graphite TIM, which is widely known for its high in-plane thermal conductivity, was superior to the one without a TIM. The TEG with the MFPI TIM showed the best thermoelectric energy conversion efficiency because the isotropic high thermal conductivity of the TIM formed the most uniform temperature distribution on the surface of TEG, making full use of all thermoelectric junctions in the TEG. To investigate the effect of the TIM on the energy harvesting efficiency of the waste heat recovery system in the field, the total waste heat rate from the heat source could be used instead of the heat transfer rate through the TEG in the calculation of the conversion efficiency. The modified efficiency would be more improved by the TIM because, in addition to the effect of the uniform temperature profile on the surface of the TEG, more heat energy would be used for the thermoelectric energy conversion due to the enhanced thermal conduction to the TEG. Fig. 5b shows the internal resistance of the tested TEG, which can be estimated from the resistance of the load connected to both of its end terminals when its output power is maximized for each heat source temperature. It can be seen that at the same heat source temperature, similar electrical resistances were found regardless of the type of TIM. This implies that the thermoelectrical properties of the test TEG rarely changed during the experiments and even during the curing process of the MFPI TIM, indicating that the change in the TEG power ratio observed during the curing process (Fig. 4a) was primarily due to the variation in the interface thermal resistance RI. A thermal cyclic test using the same device was also carried out. Each cycle consisted of 15 min heating from 100 to 300 °C followed by 12 min cooling from 300 to 100 °C. Fig. 5c shows that the TEG power was well maintained over 30 cycles (continuous operation for ∼14 h). The uniformity in the TEG power output in the thermal cyclic test can
because of the limited cooling capacity of the chiller used in this study, it is not reasonable to compare the experimental results obtained with the three different cases of TIM use under the same heat source temperature conditions. For this reason, the heat transfer rate to the TEG and power output from the TEG for the same difference between Tsh and Tsc were obtained by using polynomial curve fits for the heat transfer rate and the power output as a function of the temperature difference. The polynomial curve fits are in good agreement with the experimental data (see Supplementary Figs. A3 and A4). By using the curve fits, RI was calculated for the same temperature difference between the heat source and the heat sink. The two numbers shown above each set of bar graphs indicate the percentage improvement by the MFPI TIM compared to the case without any TIM and the case with graphite TIM, respectively. The interface thermal resistance was reduced by the MFPI TIM by 80.8–90.8% and 67.2–80.9% compared to the cases without TIM and with the commercial graphite TIM, respectively. The effect of the TIMs on the power output of the test TEG was also investigated, as presented in Fig. 5a. Clearly, the lower the RI (which increased the heat flow rate crossing the TEG), the higher the power output because of the increased heat transfer rate from heat source to the TEG for thermoelectric energy conversion. The MFPI TIM allows the TEG to generate the highest power output than its two counterparts over the entire tested temperature range by reducing the interface thermal resistance, showing 101.5–132.3% and 29.7–38.6% improvements over the cases without TIM and with the graphite TIM, respectively. Similar to heat engines, the TEG converts heat energy input into useful electrical energy. The energy conversion efficiency can be defined as the ratio of the TEG power to the rate of heat transferred to the TEG [26]. As shown on the right side of Fig. 5(a), the use of the MFPI TIM resulted in 40.3–73.2% and 7.0–20.9% enhancements in conversion efficiency compared to the cases without TIM and with the 365
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Fig. 5. (a) The TEG power output and conversion efficiency according to the type of TIM at a fixed temperature difference across the test TEG. (b) The electrical resistance of the test TEG during the experiments with the three different cases of TIM usage at fixed heat source temperature conditions. (c) The TEG power measured during the thermal cyclic testing. (d) The temperature dependence of the TIM thermal conductivity.
Table 1 Summary of results. Properties of MFPI TIM Thixotropic Index greater than 5
1 wt% loss temperature 576.7 °C
Thermal Conductivity (W/m·K) 81.4
Power output (W)
Conversion efficiency (%)
TEG test results Temperature difference (°C)
150 200 250
Interface thermal resistance (K/W) No TIM
Graphite
MFPI
No TIM
graphite
MFPI
No TIM
graphite
MFPI
0.144 0.173 0.202
0.0691 0.0983 0.1180
0.0132 0.0322 0.0388
1.16 2.09 3.29
1.94 3.36 5.11
2.69 4.54 6.63
0.397 0.536 0.710
0.568 0.734 0.931
0.687 0.843 0.996
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Acknowledgements
be ascribed to the high-temperature stability of the device and the TIM (Fig. 3c). Fig. 5d shows that the thermal conductivity of the TIM hardly changed throughout the three thermal cycles ranging from room temperature to 300 °C, implying that the newly developed TIM has robust thermal conduction capability. Fig. 5d also shows that the TIM’s thermal conductivity slightly decreased with temperature rise. The TIM’s thermal conductivity is the sum of the lattice thermal conductivity and electron thermal conductivity, and it could be expected that the electron thermal conductivity would decrease with increasing temperature due to phonon-electron scattering (∼T−1.2) in the tested temperature range, while there would be negligible temperature dependency of the lattice thermal conductivity [27]. The experimental results were well fitted by an equation comprising the sum of a constant term on the lattice thermal conductivity and a power function term on the electron thermal conductivity (represented by the red broken curve in Fig. 5d). This showed that the TIM thermal conductivity reduction at the elevated temperature could be attributed to the electron scattering by the thermal vibrations of the lattice. In addition, the fitting equation indicates that the lattice thermal conductivity (estimated as 59.2 W/ m·K) is a major part of the TIM thermal conductivity, which confirms the finding in our previous work [13]. The large lattice thermal conductivity would guarantee high thermal-conduction performance even at temperatures above 300 °C.
This work was jointly supported by the Center for Advanced MetaMaterials (CAMM) funded by the Ministry of Science and ICT as a Global Frontier Project (CAMM-N0. 2014063701, 2014063700), the “Exergy and pumping loss analyses based system design matching method for net power gain maximization of energy harvesting system” Project under the auspices of National Research Foundation (NRF, 2018085824), and World Premium Materials (WPM) program by Ministry of Trade, Industry & Energy, KOREA (10037890). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.enconman.2018.12.108. References [1] Ovik R, Long BD, Barma MC, Riaz M, Sabri MFM, Said SM, et al. A review on nanostructures of high-temperature thermoelectric materials for waste heat recovery. Renew Sustain Energy Rev 2016;64:635–59. [2] Kim TY, Negash A, Cho G. Direct contact thermoelectric generator (DCTEG): a concept for removing the contact resistance between thermoelectric modules and heat source. Energy Convers Manag 2017;142:20–7. [3] Zhu T, Swaminathan-Gopalan K, Stephani K, Ertekin E. Thermoelectric phononglass electron-crystal via ion beam patterning of silicon. Phys Rev B 2018;97:174201. [4] Lee J-H, Wu J, Grossman JC. Enhancing the thermoelectric power factor with highly mismatched isoelectronic doping. Phys Rev Lett 2010;104:16602. [5] Cheikh D, Hogan BE, Vo T, Von Allmen P, Lee K, Smiadak DM, et al. Praseodymium telluride: a high-temperature High-ZT thermoelectric material. Joule 2018;2:698–709. [6] Zhang B, Zheng T, Wang Q, Guo Z, Kim MJ, Alshareef HN, et al. Stable and low contact resistance electrical contacts for high temperature SiGe thermoelectric generators. Scr Mater 2018;152:36–9. [7] Karthick K, Joy GC, Suresh S, Dhanuskodi R. Impact of thermal interface materials for thermoelectric generator systems. J Electron Mater 2018;47:5763–72. https:// doi.org/10.1007/s11664-018-6496-y. [8] Díaz-Chao P, Muñiz-Piniella A, Selezneva E, Cuenat A. Precise measurement of the performance of thermoelectric modules. Meas Sci Technol 2016;27. https://doi. org/10.1088/0957-0233/27/8/085002. [9] Wang S, Xie T, Xie H. Experimental study of the effects of the thermal contact resistance on the performance of thermoelectric generator. Appl Therm Eng 2018;130:847–53. https://doi.org/10.1016/j.applthermaleng.2017.11.036. [10] Loeblein M, Tsang SH, Pawlik M, Phua EJR, Yong H, Zhang XW, et al. High-density 3D-boron nitride and 3D-graphene for high-performance nano–thermal interface material. ACS Nano 2017;11:2033–44. [11] Hao M, Huang Z, Saviers KR, Xiong G, Hodson SL, Fisher TS. Characterization of vertically oriented carbon nanotube arrays as high-temperature thermal interface materials. Int J Heat Mass Transf 2017;106:1287–93. [12] Yorifuji D, Ando S. Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure. J Mater Chem 2011;21:4402–7. [13] Suh D, Moon CM, Kim D, Baik S. Ultrahigh thermal conductivity of interface materials by silver-functionalized carbon nanotube phonon conduits. Adv Mater 2016;28:7220–7. [14] Beckwith TG, Marangoni RD. JH Lienhard V, Mechanical Measurements, 6th 2007. [15] Lin C, Chung DDL. Nanoclay paste as a thermal interface material for smooth surfaces. J Electron Mater 2008;37:1698–709. [16] Genovese DB. Shear rheology of hard-sphere, dispersed, and aggregated suspensions, and fi ller-matrix composites. Adv Colloid Interface Sci 2012;171–172:1–16. https://doi.org/10.1016/j.cis.2011.12.005. [17] D2196-10 A. Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield type) Viscometer; 2010. [18] I. AI Technology. Technical data sheet for ME 8512, ME 8638-UT, and ME 7519-LB from AI Technology 2012; 2012. [19] I. AI Technology. Technical data sheet for TC-2022 from Dow Corning; 2016. [20] Ren H, Zhou Y, He M, Xu R, Ding B, Zhong X. Enhanced mechanical properties of silica nanoparticle-covered cross-linking graphene oxide filled thermoplastic polyurethane composite. New J Chem 2018:3069–77. https://doi.org/10.1039/ c7nj03503a. [21] Prasher RS. Surface chemistry and characteristics based model for the thermal contact resistance of fluidic interstitial thermal interface materials. J Heat Transfer 2001;123:969–75. [22] Giri A, Braun L, Hopkins PE. Implications of interfacial bond strength on the spectral contributions to thermal boundary conductance across solid, liquid, and gas. Interfaces: Mol Dyn Study 2016. https://doi.org/10.1021/acs.jpcc.6b08124. [23] Liu M, Wang RY. Phase change nanocomposites with tunable melting temperature and thermal energy storage density. Nanoscale 2013;5:7234–7. [24] Li M, Xiao Y, Zhang Z, Yu J. Bimodal sintered silver nanoparticle paste with
4. Conclusions In this study, the effect of TIMs on the interface thermal resistance and overall waste heat recovery performance of a TEG operating under high-temperature conditions was investigated. Table 1 summarizes the key findings of this study. A new TIM paste referred to as MFPI TIM was developed from highly thermal-conducting hybrid fillers composed of Ag flakes and MWCNTs coated with Ag NPs and a temperature-resistant PI matrix with good printability. The excellence of the MFPI TIM was confirmed in terms of printability, thermal conductivity, and thermal stability. The thixotropic index of the TIM was larger than 5 when the minimum shear rate was larger than 10 s−1. The TIM was stable without losing mass up to around 500 °C and its thermal conductivity reached 81.4 W/ m·K. Using a custom-built experimental setup, the enhancement of the waste heat recovery performance of the TEG using the MFPI TIM was explored comparatively with cases without a TIM and with a conventional graphite foil TIM. The TEG power significantly increased as the thermal resistance of the interface between the heating block and the power generator was reduced by the MFPI TIM. From measurements during the onsite curing of the TIM, the required thermal conductivity of the TIM for sufficient thermoelectric power generation was found to be 20 W/m·K. The experimental results show that under the same temperature difference conditions, the MFPI TIM outperformed its two counterparts in thermal conduction at the interface, providing at most 132.3 and 38.6% improvements in power output compared to the cases without a TIM and with the graphite TIM, respectively. The use of the MFPI TIM was also beneficial in energy conversion, resulting in at most 73.2 and 20.9% rises in conversion efficiency compared to without a TIM and with the graphite TIM, respectively. The power output of the test TEG kept increasing with the rising temperature of a heating block even to 300 °C, which is close to the allowable maximum temperature (320 °C) of the test TEG. In addition, a thermal cyclic test consisting of a series of heating and cooling procedures between 100 and 300 °C demonstrated the long-lasting heat conduction performance of the developed TIM in a TEG module experiencing large-amplitude temperature variations. This pioneering study has verified the importance of the role that the TIM plays in reducing the interface thermal resistance by building the most favorable heat flow paths inside TEG modules for optimal waste heat recovery from them.
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and heat recovery using a heat pipe assisted thermoelectric generator system. Energy Convers Manag 2015;91:110–9. https://doi.org/10.1016/j.enconman.2014. 12.001. [27] Okhotin AS, Zhmakin LI, Ivanyuk AP. The temperature dependence of thermal conductivity of some chemical elements. Exp Therm Fluid Sci 1991;4:289–300.
ultrahigh thermal conductivity and shear strength for high temperature thermal interface material applications. ACS Appl Mater Interfaces 2015;7:9157–68. [25] Moon K-S, Dong H, Maric R, Pothukuchi S, Hunt A, Li Y, et al. Thermal behavior of silver nanoparticles for low-temperature interconnect applications. J Electron Mater 2005;34:168–75. [26] Fairuz M, Tan L, Date A, Singh B, Akbarzadeh A. Simultaneous power generation
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Highly enhanced thermoelectric energy harvesting from a high-temperature heat source by boosting thermal interface conduction ⁎
Duckjong Kima, , Chihyun Kima,b, Jinsung Parkb, Tae Young Kimc,
T
⁎
a
Department of Applied Nano Mechanics, Korea Institute of Machinery & Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea Department of Control and Instrumentation Engineering, Korea University, 2511 Sejong-ro, Sejong 30019, Republic of Korea c Division of Mechanical System Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea b
A B S T R A C T
Thermoelectricity is regarded as one of the promising waste heat recovery candidates. A fundamental but effective method for the best use of thermoelectric generators (TEGs) is to maximize the heat flow crossing thermoelectric materials. The main focus of the present study was to develop a thermal interface material (TIM) with high thermal conductivity and temperature resistance as the key to minimizing the overall thermal resistance of the heat flow path of a TEG module operating under high-temperature conditions. In combination with a polyimide matrix and a multi-dimensional filler compound, a new TIM having stable heat conduction behavior at high temperatures was produced. The developed TIM was stable without losing mass up to ∼500 °C, and its thermal conductivity reached 81.4 W/m·K. It was applied to the interface between a TEG and a heat source whose temperature ranged from 100 to 300 °C and, the effect of the thermal conductivity and interface thermal resistance of the TIM on thermoelectric power generation performance during onsite curing of the TIM was investigated. Reduction of the interface thermal resistance by the new TIM improved the power generation by, at most, 132.3 and 38.6% compared to cases without a TIM and with a conventional graphite foil TIM, respectively. In terms of energy conversion efficiency, the TEG with the new TIM showed maximum improvements of 73.2 and 20.9% over the cases without a TIM and with a graphite TIM for the same temperature difference across the TEG, respectively. In addition, thermal cyclic testing confirmed the longlasting heat-conducting feature of the developed TIM. The present work clearly shows the potentially significant influence that TIMs have on the waste heat recovery performance of TEGs.
1. Introduction As one of the promising thermal energy harvesting technologies, thermoelectricity has garnered considerable attention. It has been shown that a huge amount of waste heat from inefficient parts of transportation vehicles and industrial plants operating in high-temperature ranges can potentially be recovered using thermoelectric power generation [1]. The increasing tendency of the waste heat recovery performance of thermoelectric generators (TEGs) according to the increase in the temperature difference across TEGs has boosted advances in high-temperature thermoelectric materials and systems [2]. Most of research works have pursued a phonon-glass electroncrystal material by means of phonon scattering structures [3], highly mismatched isoelectronic doping [4], etc. to maximize ZT, the thermoelectric figure of merit. Cheikh et al. [5] shed new light on praseodymium telluride by modifying its density of states for an improved Seebeck coefficient and lower thermal conductivity, resulting a ZT value of 1.7 even at 1200 K. Zhang et al. [6] reported an enhancement in the thermal stability of a TEG consisting of n- and p-type SiGe elements that peaked their figures of merit at ∼1073 K by means of TaAlN thin films resistant to high-temperature oxidization. Even though most
⁎
Corresponding authors. E-mail addresses: [email protected] (D. Kim), [email protected] (T.Y. Kim).
https://doi.org/10.1016/j.enconman.2018.12.108 Received 10 September 2018; Accepted 24 December 2018 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
of research works on thermoelectric materials have pursued a high ZT value, there is another way to increase the energy harvesting performance. A TEG is typically located between a heat source and a heat sink. Reduction in the overall thermal resistance of the path from the heat source to the heat sink can allow larger heat transfer rate through the TEG, leading to enhanced TEG power. Thermal interface material (TIM) is applied to the interface between the TEG and the heat source/ sink to reduce the thermal resistance. In comparison with the bulky and robust configuration of heat sources and TEGs, a thin layer of TIM is mostly developed for electronics cooling and tends to be vulnerable to high-temperature conditions [7]. This challenge raises the need for temperature resistant TIMs. The higher the interface thermal resistance, the larger the loss of the temperature difference across the thermoelectric materials, and this has led to an unexpectedly lower power output [8,9]. As such, keeping the tight coupling between a TEG and adjacent components (heat source and heat sink) is another concern for non-adhesive TIMs. For this reason, several types of TIM including metal alloys, vertically aligned carbon nanotubes (VACNTs), 3D-boron nitride/graphene, and polymer-based materials have been proposed for high-temperature applications. Metal alloys containing In, Bi, Sn, and Au are the materials most commonly
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Nomenclature A c CNT Cp DSC h I IA k max MWCNT NP P PI Q q
R s sc sh t TEG TGA TIM T ΔT VACNT ZT
area (m2) cooling block carbon nanotube specific heat (J/kg·K) differential scanning calorimetry heating block interface between TEG and heating block interface between TEG and cooling block thermal conductivity (W/m·K) maximum value multiwalled carbon nanotube nanoparticle power output (W) polyimide heat transfer rate (W) heat flux (W/m2)
thermal resistance (K/W) surface; spacing (m) surface of cooling block in contact with TEG surface of heating block in contact with TEG thickness (m) thermoelectric generator thermogravimetric analysis thermal interface material temperature (K) temperature difference (K) vertically aligned carbon nanotube figure of merit
Greek α ρ
thermal diffusivity (m2/s) density (kg/m3)
used. Even though their thermal conductivities are usually high (∼50 W/m·K), the use of these TIMs is limited because of their limited integration ability with other nonmetal materials due to poor wetting properties [10]. As mentioned, VACNTs have also been considered as candidate TIMs because of their extremely high thermal conductivity (thousands of W/m·K). However, the synthesis of CNTs requires a hightemperature environment (∼800 °C) and when exposed to the air, they can be oxidized at around 400–500 °C [11]. Recently, 3D foam-like graphene and hexagonal boron nitride have been proposed as TIMs. Even though the 3D foam-like structures have high cross-plane thermal conductivity (62–86 W/m·K), the highly complicated, time-consuming, and expensive fabrication processes for these materials comprise a serious bottleneck [10]. Polymer-based composite materials have been widely used as TIMs because of their excellent wetting properties. For high-temperature applications, the thermally stable polymer polyimide (PI) has been considered as a promising matrix material, although the industrial use of the PI-based TIMs is seriously limited due to their low thermal conductivity (∼1 W/m·K) [12]. In addition, it is hard to find a report on TIM development that considers practical application conditions (mating surfaces, onsite curing, etc.) for high-temperature TEGs, and the required interface thermal resistance and thermal properties of TIMs for sufficient thermoelectric power generation have not yet been well-investigated. This letter presents a new TIM paste by introducing a multi-dimensional filler design composed of 3D microscale Ag flakes, 1D multiwalled carbon nanotubes (MWCNTs), and 0D Ag nanoparticles (Ag NPs) proposed in [13] that significantly enhances the thermal conductivity into a thermally stable PI matrix (hereafter, referred to as MFPI TIM). The excellence of the new TIM is confirmed in terms of printability, thermal conductivity, and thermal stability. The MFPI TIM paste was applied to the interface between a TEG and a heat source and the effect of the interface thermal resistance on thermoelectric power generation performance was investigated during onsite curing of the TIM using a customized experimental setup. From the experimental work, appropriate thermal conductivity of the TIM for sufficient thermoelectric power generation is discussed. In addition, the interface thermal resistance of the MFPI TIM and corresponding thermoelectric power generation characteristics are compared with those for cases without a TIM and with a commercial graphite foil TIM. The reliability of the MFPI TIM is also validated from consistent levels of the power output of the TEG throughout the thermal cyclic test performed by varying the heat source temperature between 100 and 300 °C. Fig. 1. MFPI TIM preparation procedure.
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2. Materials and methods
(Branson Ultrasonic Corp., Branson 5510). The mixture solution of the MWCNTs and Ag NPs was vacuum-filtered to collect the MWCNTs functionalized with the Ag NPs and dried for 12 h in a vacuum chamber (JeioTech, VDR-25G) at room temperature. Finally, the Ag flakes (Metalor, SA-31812, 7.5–55.4 vol%) and MWCNTs coated with Ag NPs (3.5–4.2 vol%) were mixed in 33 wt% polyimide (Huntsman Chemical Company, Matrimid® 5218) dissolved in 1-methyl-2-pyrrolidone (NMP, Sigma-Aldrich, 328634, 99.5%) by stirring for 5 min to obtain the TIM paste. To prepare the coin specimens for the laser flash analysis (LFA), the TIM paste was poured into a Teflon mold and cured for 4 h at 180 °C and then for 1 h at 260 °C, unless otherwise stated. Before pouring the paste into the mold, a releasing agent (NABAKEM, FLEX-A) was applied to the surface of the mold to detach the coin specimens from the mold
The MFPI TIM was prepared using a protocol similar to that previously published [13] as summarized in Fig. 1. The TIM was composed of polyimide (matrix material), Ag flakes (primary filler), and MWCNTs coated with Ag NPs (secondary filler). First, 400 mL of AgNO3 solution in ethanol (Kojundo Chemical, 0.02 mol/L) and 3.2 mL of benzyl mercaptan solution in ethanol (Sigma-Aldrich, B25401, 99%, 0.1 mol/L) were stirred for 48 h to synthesize the Ag NPs functionalized with a phenyl group (diameter: 3–5 nm). In the next step, 50 mg of MWCNTs (Nano solution, TMC-05010) were dispersed in 100 mL of ethanol by tip sonication (Sonics & Materials Inc, Vibra-Cell VCX 750) and then mixed with the prepared Ag NP solution through 3 h of bath sonication
Fig. 2. Schematic diagrams of (a) the generator-level waste heat recovery performance tester and (b) the entire experimental setup. 362
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where kAl is the thermal conductivity of the aluminum heating block. The area-averaged heat flux is calculated in a similar fashion to the area-averaged surface temperature. The rate of heat transferred to the hot surface of the TEG is the product of the area-averaged heat flux and the horizontal cross-sectional area of the heating block. In an analogous manner, the averaged surface temperature and heat flux on the surface of the cooling block in contact with the TEG were obtained. By performing an error analysis, the uncertainties with a confidence of 95% for experimental results were estimated by combining the bias and precision errors [14]. The bias errors were calculated using accuracies of data acquisition systems and sensitivity coefficients of experimental variables. The precision errors were determined by using standard deviation of three repeats for each physical quantity (thermal conductivity, interface thermal resistance, TEG power, conversion efficiency and TEG internal resistance) and t-distribution (for more detailed information, see Supplementary Table A1). Steady-state conditions were assumed to have been reached when the variation in timeaveraged surface temperatures of TEG measured every 1 s was smaller than 0.1 K during consecutive durations of 120 s. Once steady-state conditions had been reached, the resistance of electric load connected to the two end terminals of the TEG varied from 0.18 to 60.00 Ω, which obtained corresponding voltage output and current output values used to analyze the waste heat recovery performance of the TEG. The effect of the TIMs on the energy harvesting performance of TEG was examined by forming three different interface thermal conduction conditions at the interface between the surface of the test TEG and the heating block: a 0.3-mm-thick MFPI TIM (proposed in this study), a 0.04-mm-thick commercial graphite foil TIM (Hi-lim Electronics Ltd., HGS40), and no TIM (where the ceramic cover plate of the TEG was in direct contact with the bare surface of the heating block). The variations in interface thermal resistances were tracked as a function of the curing time and the type of TIM. The detailed procedure for calculating the thermal resistance is given in Supplementary Fig. A1.
easily. The cross-plane thermal diffusivity (α) of the coin specimen made of the MFPI TIM was measured using LFA (Netzsch, LFA-447), and the specific heat (Cp) was measured at 10 °C min−1 using differential scanning calorimetry (DSC, Mettlertoledo, DSC1 STARe System) under a nitrogen atmosphere in the range of 25–300 °C. The density (ρ) of the coin specimen was measured with a gas pycnometer (InstruQuestInc, Humipyc model 2) under a helium atmosphere. The cross-plane thermal conductivity (k) was calculated using k = ρ ⋅ Cp ⋅ α. The phase transformation of the Ag NPs on the surface of the MWCNTs was characterized using DSC under a nitrogen atmosphere with a preset temperature profile consisting of three stages: heating (10 °C/min) from 50 to 350 °C, holding at 350 °C for 1 min, and cooling (−10 °C/min) from 350 to 50 °C. The thermal stability was investigated via a thermogravimetric analysis (TGA, Scinco, TGA N-1500) in the range of 25–800 °C at 20 °C/min under a nitrogen atmosphere. The viscosity was measured at a shear rate of 2–100 s−1 using a viscometer (Thermo scientific, HAAKE MARS III). The thermal resistance of the MFPI TIM and its effect on the generator-level thermoelectric energy conversion performance was examined by means of a custom-made test setup consisting of a set of aluminum heating and cooling blocks, as shown in Fig. 2. A 40 (W) × 40 (L) × 4 (H) mm3 test TEG (Custom Thermoelectric, 1991G7L31-12CQ) was sandwiched between the heating and cooling blocks and compressed by tightening via a clamp with a torque of 5 kg·f·cm (during the curing of the MFPI TIM) or a weight of 30 kg·f (during the energy harvesting experiment). Each of the aluminum heating and cooling blocks had 6 holes tapped into their sides for the thermocouple temperature measurements. The heating block raised the surface temperature of the test TEG using eight 60-mm-long cartridge heaters inserted in the holes made in its sides. The cartridge heaters were controlled by a power supply equipped with a proportional-integralderivative (PID) temperature controller. An additional hole formed on the side of the heating block, indicated as 'feedback' in the inset of Fig. 2(a), was used to mount a thermocouple that provided the power supply with temperature feedback for PID control. Using the feedback control, the temperature of the heating block is set to a target value and is maintained for steady-state experiments. The bottom and side surfaces of the heating block, except the surface where thermocouples and heaters are mounted, are shrouded in polyether ether ketone housing (k = 0.28 W/m·K) to reduce heat loss to the ambient. To maintain a specified temperature difference across the TEG, water at ∼303 K was supplied to the cooling block via a water pump (Wilo, RS 20/6). Passing through a serpentine-shaped passage inside the cooling block, water took the heat from the TEG; it was then cooled by an air-cooled heat exchanger (Lytron, 4220G10) and a chiller (Lab Companion, RW3025G/HTBC-2330AT), which kept the water temperature at the inlet of the cooling block almost constant. All of the temperature measurements were logged in a data acquisition system (Graphtec Corp., GL820) and processed to calculate the rate of heat transferred to and the time-averaged surface temperature of the TEG. The local temperature of the heating block surface in contact with the TEG, Tshj, was calculated based on the continuity of the heat flux as follows:
Tshj = Th1j −
sh1 (Th2j − Th1j ), sh2
for j = 1, 2, and 3
3. Results and discussion The rheological and thermal properties of the MFPI TIM were checked to confirm its usability. To investigate the printability of the TIM, the thixotropic index of the material was obtained. Thixotropy refers to the rheological behavior that a material exhibits when flowing under stress [15], and the thixotropic index is a time-dependent rheological property that describes the extent of the thixotropic behavior. To obtain the thixotropic index, viscosity of the TIM according to the shear rate was measured, as shown in Fig. 3a. The viscosity is approximately independent of the shear rate when the shear rate is low, showing the Newtonian behavior. On the other hand, the viscosity apparently decreases with the shear rate at high shear rate. It could be ascribed to alignment of suspended fillers in the direction of flow [16]. The rate of viscosity decrease according to the increase in shear rate apparently increased when the shear rate was larger than 30 s−1. Since the thixotropic index is the ratio of viscosity values at two shear rates that are different by a factor of 10 [17], the thixotropic index was larger than 5 when the minimum shear rate was larger than 10 s−1, which was higher than those of commercial TIMs [18,19]. The effect of the volume fraction of the primary filler (Ag flake) on the thermal conductivity of the TIM was also investigated, as shown in Fig. 3b (for more detailed information, see Supplementary Table A2). The thermal conductivity abruptly increases when the volume fraction of the Ag flake increases from 15.5 to 23.6%, indicating that the percolation threshold of the primary and secondary fillers must lie in this range. When the Ag flake volume fraction was larger than 23.6%, the thermal conductivity monotonically increased with the volume fraction and reached 81.4 W/ m·K when the volume fraction was 55.4% which was the volume fraction of the Ag flake in the TIM paste used in the TEG experiment. To check the thermal stability of the TIM, a TGA was conducted, the results of which in Fig. 3c show that the 1 wt% loss temperature of the TIM was
(1)
where Th1j and Th2j are temperatures measured with thermocouples mounted into the heating block, and sh1 and sh2 are spaces between the thermocouples, respectively, as illustrated in Fig. 2(a). The area-averaged surface temperature is an arithmetic mean of the three local surface temperatures. The local heat fluxes on the hot surface of the TEG, qhj, are calculated as
qhj = −kAl ⎛ ⎝
⎜
Tshj − Th1j ⎞ , sh1 ⎠ ⎟
for j = 1, 2, and 3
(2) 363
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property at the interface between the heat source and the TEG apparently increased the TEG power under the same conditions. To measure the thermal conductivity of the TIM during the course of the curing, the paste in the coin specimen mold was dried at 80 °C and cured in the TEG test setup. Fig. 4b clearly shows that the TEG power ratio significantly depended on the thermal resistance of the interface between the heating block and the power generator. As the interface thermal resistance decreased, the TEG power ratio increased. The interface thermal resistance (RI) is the sum of the thermal resistance for conduction through the TIM (RTIM = tTIM/kTIM/ATIM where tTIM, kTIM, and ATIM represent the thickness, thermal conductivity, and area of the TIM, respectively) and the thermal contact resistance of the interfaces between the TIM and the thermoelectric device (RC1) or the heating block (RC2), as shown in inset of Fig. 4b. Since the heat source temperature and the heat sink temperature were kept almost identical during the experiment, more heat flow could cross the thermoelectric device as the interface resistance decreased. The increased heat flow increased the temperature difference across the thermoelectric materials and led to an increase in the TEG power. Fig. 4b shows that both resistances (RTIM, RC1 + RC2) comprising the interface resistance decreased during the curing process and that, initially, RTIM was comparable to RC1 + RC2. For a curing time of less than 80 s, the thermal resistance reduction mainly came from the thermal conductivity increase of the TIM. For the curing time between 80 s and 200 s, the reduction in RC1 + RC2 mainly contributed to the decrease of RI, and for the curing time larger than 200 s, only RTIM decreased due to the increase in the thermal conductivity of the TIM during the curing for a while. Although RC1 + RC2 also depended on the thermal conductivity of the TIM, the dependence weakened as the thermal conductivity of the TIM increased [13,21] and in this study, almost disappeared when the thermal conductivity of the TIM reached around 20 W/m·K. For the curing time of 200 s and over, since RC1 + RC2 was dominant in RI, the increase in the thermal conductivity of the TIM did not increase the TEG power ratio significantly, as shown in Fig. 4b. The TEG power ratio reached 90% when the TIM thermal conductivity is around 20 W/m·K and further increase in the thermal conductivity did not significantly enhance the TEG power. A TIM with thermal conductivity reaching about 80 W/m·K was used for the TEG test, which means that the filler was excessively used in the tested TIM. Fig. 3b shows that 23.6 vol% primary filler, for which the thermal conductivity of the TIM is larger than 20 W/m·K, was enough to maximize the TEG power. The DSC analysis shows that the thermal conductivity increase of the MFPI TIM came from the coalescence between the Ag NPs functionalized on the surface of MWCNTs and the Ag flakes [21], facilitating inelastic energy exchanges across the solid/solid interfaces [22]. Fig. 4c shows that an endothermic valley and an exothermic peak can be observed at around 175 °C during the heating and cooling of the uncured filler, respectively (for all heating and cooling curves, see Supplementary Fig. A2), the former of which is attributable to the melting of the Ag NPs on the MWCNTs [23]. In addition, there is an exothermic peak at around 175 °C in the heating curve for the uncured filler and that cured for just 3.5 min, indicating that coarsening of the Ag NPs occurred [24]. The overlapping exothermic peak disappeared for the samples cured over 2 h. The exothermic peak in the temperature range 200–275 °C, which could be related to a reduction in the crystallographic defects of the Ag NPs, disappeared when the filler was cured for over 4 h [24,25]. Hence, it could be inferred that the coalescence between the Ag NPs and the Ag flakes, which was completed after a short time of curing, mainly contributed to the reduction in RTIM, and that the recrystallization of the Ag NPs, which lasted for longer time, apparently decreased RC1 + RC2. We also compared RI for the various interface materials, as shown in Fig. 4d. The experiments on the waste heat recovery performance of the TEG were carried out under fixed heat source temperature conditions of 100–300 °C at intervals of 50 °C. As the temperature of the surface of the cooling block facing the test TEG was not precisely controlled
Fig. 3. (a) The viscosity of the MFPI TIM paste, and (b) the effect of the Ag flake volume fraction on thermal conductivity and (c) the TGA results of the cured TIM.
576.7 °C. This indicates that the TIM could be used up to around 500 °C without losing mass. Even if mass of fillers is not considered, the 1 wt% loss temperature is 508.6 °C which is much higher than that of the PI without fillers (434.0 °C). The improved thermal stability can be ascribed to the tortuous path formed by the fillers that hinders the release of the volatiles generated by thermal decomposition of the matrix material, delaying the degradation [20]. The MFPI TIM was applied to the interface between the heating block and the thermoelectric device, and the TEG power and the interface thermal resistance were monitored. As shown in Fig. 4a, the TEG power increased with the TIM curing. Comparing the TEG power during the curing process with that at the same temperature after curing the TIM, the power ratio between them increased with the curing time and reached 90% with the curing time of 350 s. The power ratio mostly increased as the TIM temperature increased to a curing temperature of 180 °C, which was determined based on the melting temperature of the Ag NPs in the multi-dimensional filler [13]. Fig. 4a also shows that the thermal conductivity of the TIM increased during the curing process, which indicates that the enhancement in the thermal conduction 364
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Fig. 4. (a) The TEG power and thermal conductivity of the TIM, and (b) the thermal resistance during the curing process. (c) The DSC results of the MFPI TIM according to curing time. (d) The interface thermal resistance (RI) of the various interface materials.
graphite TIM, respectively, under the same temperature difference conditions. For thermoelectric energy conversion efficiency, the TEG with the graphite TIM, which is widely known for its high in-plane thermal conductivity, was superior to the one without a TIM. The TEG with the MFPI TIM showed the best thermoelectric energy conversion efficiency because the isotropic high thermal conductivity of the TIM formed the most uniform temperature distribution on the surface of TEG, making full use of all thermoelectric junctions in the TEG. To investigate the effect of the TIM on the energy harvesting efficiency of the waste heat recovery system in the field, the total waste heat rate from the heat source could be used instead of the heat transfer rate through the TEG in the calculation of the conversion efficiency. The modified efficiency would be more improved by the TIM because, in addition to the effect of the uniform temperature profile on the surface of the TEG, more heat energy would be used for the thermoelectric energy conversion due to the enhanced thermal conduction to the TEG. Fig. 5b shows the internal resistance of the tested TEG, which can be estimated from the resistance of the load connected to both of its end terminals when its output power is maximized for each heat source temperature. It can be seen that at the same heat source temperature, similar electrical resistances were found regardless of the type of TIM. This implies that the thermoelectrical properties of the test TEG rarely changed during the experiments and even during the curing process of the MFPI TIM, indicating that the change in the TEG power ratio observed during the curing process (Fig. 4a) was primarily due to the variation in the interface thermal resistance RI. A thermal cyclic test using the same device was also carried out. Each cycle consisted of 15 min heating from 100 to 300 °C followed by 12 min cooling from 300 to 100 °C. Fig. 5c shows that the TEG power was well maintained over 30 cycles (continuous operation for ∼14 h). The uniformity in the TEG power output in the thermal cyclic test can
because of the limited cooling capacity of the chiller used in this study, it is not reasonable to compare the experimental results obtained with the three different cases of TIM use under the same heat source temperature conditions. For this reason, the heat transfer rate to the TEG and power output from the TEG for the same difference between Tsh and Tsc were obtained by using polynomial curve fits for the heat transfer rate and the power output as a function of the temperature difference. The polynomial curve fits are in good agreement with the experimental data (see Supplementary Figs. A3 and A4). By using the curve fits, RI was calculated for the same temperature difference between the heat source and the heat sink. The two numbers shown above each set of bar graphs indicate the percentage improvement by the MFPI TIM compared to the case without any TIM and the case with graphite TIM, respectively. The interface thermal resistance was reduced by the MFPI TIM by 80.8–90.8% and 67.2–80.9% compared to the cases without TIM and with the commercial graphite TIM, respectively. The effect of the TIMs on the power output of the test TEG was also investigated, as presented in Fig. 5a. Clearly, the lower the RI (which increased the heat flow rate crossing the TEG), the higher the power output because of the increased heat transfer rate from heat source to the TEG for thermoelectric energy conversion. The MFPI TIM allows the TEG to generate the highest power output than its two counterparts over the entire tested temperature range by reducing the interface thermal resistance, showing 101.5–132.3% and 29.7–38.6% improvements over the cases without TIM and with the graphite TIM, respectively. Similar to heat engines, the TEG converts heat energy input into useful electrical energy. The energy conversion efficiency can be defined as the ratio of the TEG power to the rate of heat transferred to the TEG [26]. As shown on the right side of Fig. 5(a), the use of the MFPI TIM resulted in 40.3–73.2% and 7.0–20.9% enhancements in conversion efficiency compared to the cases without TIM and with the 365
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Fig. 5. (a) The TEG power output and conversion efficiency according to the type of TIM at a fixed temperature difference across the test TEG. (b) The electrical resistance of the test TEG during the experiments with the three different cases of TIM usage at fixed heat source temperature conditions. (c) The TEG power measured during the thermal cyclic testing. (d) The temperature dependence of the TIM thermal conductivity.
Table 1 Summary of results. Properties of MFPI TIM Thixotropic Index greater than 5
1 wt% loss temperature 576.7 °C
Thermal Conductivity (W/m·K) 81.4
Power output (W)
Conversion efficiency (%)
TEG test results Temperature difference (°C)
150 200 250
Interface thermal resistance (K/W) No TIM
Graphite
MFPI
No TIM
graphite
MFPI
No TIM
graphite
MFPI
0.144 0.173 0.202
0.0691 0.0983 0.1180
0.0132 0.0322 0.0388
1.16 2.09 3.29
1.94 3.36 5.11
2.69 4.54 6.63
0.397 0.536 0.710
0.568 0.734 0.931
0.687 0.843 0.996
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Acknowledgements
be ascribed to the high-temperature stability of the device and the TIM (Fig. 3c). Fig. 5d shows that the thermal conductivity of the TIM hardly changed throughout the three thermal cycles ranging from room temperature to 300 °C, implying that the newly developed TIM has robust thermal conduction capability. Fig. 5d also shows that the TIM’s thermal conductivity slightly decreased with temperature rise. The TIM’s thermal conductivity is the sum of the lattice thermal conductivity and electron thermal conductivity, and it could be expected that the electron thermal conductivity would decrease with increasing temperature due to phonon-electron scattering (∼T−1.2) in the tested temperature range, while there would be negligible temperature dependency of the lattice thermal conductivity [27]. The experimental results were well fitted by an equation comprising the sum of a constant term on the lattice thermal conductivity and a power function term on the electron thermal conductivity (represented by the red broken curve in Fig. 5d). This showed that the TIM thermal conductivity reduction at the elevated temperature could be attributed to the electron scattering by the thermal vibrations of the lattice. In addition, the fitting equation indicates that the lattice thermal conductivity (estimated as 59.2 W/ m·K) is a major part of the TIM thermal conductivity, which confirms the finding in our previous work [13]. The large lattice thermal conductivity would guarantee high thermal-conduction performance even at temperatures above 300 °C.
This work was jointly supported by the Center for Advanced MetaMaterials (CAMM) funded by the Ministry of Science and ICT as a Global Frontier Project (CAMM-N0. 2014063701, 2014063700), the “Exergy and pumping loss analyses based system design matching method for net power gain maximization of energy harvesting system” Project under the auspices of National Research Foundation (NRF, 2018085824), and World Premium Materials (WPM) program by Ministry of Trade, Industry & Energy, KOREA (10037890). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.enconman.2018.12.108. References [1] Ovik R, Long BD, Barma MC, Riaz M, Sabri MFM, Said SM, et al. A review on nanostructures of high-temperature thermoelectric materials for waste heat recovery. Renew Sustain Energy Rev 2016;64:635–59. [2] Kim TY, Negash A, Cho G. Direct contact thermoelectric generator (DCTEG): a concept for removing the contact resistance between thermoelectric modules and heat source. Energy Convers Manag 2017;142:20–7. [3] Zhu T, Swaminathan-Gopalan K, Stephani K, Ertekin E. Thermoelectric phononglass electron-crystal via ion beam patterning of silicon. Phys Rev B 2018;97:174201. [4] Lee J-H, Wu J, Grossman JC. Enhancing the thermoelectric power factor with highly mismatched isoelectronic doping. Phys Rev Lett 2010;104:16602. [5] Cheikh D, Hogan BE, Vo T, Von Allmen P, Lee K, Smiadak DM, et al. Praseodymium telluride: a high-temperature High-ZT thermoelectric material. Joule 2018;2:698–709. [6] Zhang B, Zheng T, Wang Q, Guo Z, Kim MJ, Alshareef HN, et al. Stable and low contact resistance electrical contacts for high temperature SiGe thermoelectric generators. Scr Mater 2018;152:36–9. [7] Karthick K, Joy GC, Suresh S, Dhanuskodi R. Impact of thermal interface materials for thermoelectric generator systems. J Electron Mater 2018;47:5763–72. https:// doi.org/10.1007/s11664-018-6496-y. [8] Díaz-Chao P, Muñiz-Piniella A, Selezneva E, Cuenat A. Precise measurement of the performance of thermoelectric modules. Meas Sci Technol 2016;27. https://doi. org/10.1088/0957-0233/27/8/085002. [9] Wang S, Xie T, Xie H. Experimental study of the effects of the thermal contact resistance on the performance of thermoelectric generator. Appl Therm Eng 2018;130:847–53. https://doi.org/10.1016/j.applthermaleng.2017.11.036. [10] Loeblein M, Tsang SH, Pawlik M, Phua EJR, Yong H, Zhang XW, et al. High-density 3D-boron nitride and 3D-graphene for high-performance nano–thermal interface material. ACS Nano 2017;11:2033–44. [11] Hao M, Huang Z, Saviers KR, Xiong G, Hodson SL, Fisher TS. Characterization of vertically oriented carbon nanotube arrays as high-temperature thermal interface materials. Int J Heat Mass Transf 2017;106:1287–93. [12] Yorifuji D, Ando S. Enhanced thermal conductivity over percolation threshold in polyimide blend films containing ZnO nano-pyramidal particles: advantage of vertical double percolation structure. J Mater Chem 2011;21:4402–7. [13] Suh D, Moon CM, Kim D, Baik S. Ultrahigh thermal conductivity of interface materials by silver-functionalized carbon nanotube phonon conduits. Adv Mater 2016;28:7220–7. [14] Beckwith TG, Marangoni RD. JH Lienhard V, Mechanical Measurements, 6th 2007. [15] Lin C, Chung DDL. Nanoclay paste as a thermal interface material for smooth surfaces. J Electron Mater 2008;37:1698–709. [16] Genovese DB. Shear rheology of hard-sphere, dispersed, and aggregated suspensions, and fi ller-matrix composites. Adv Colloid Interface Sci 2012;171–172:1–16. https://doi.org/10.1016/j.cis.2011.12.005. [17] D2196-10 A. Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield type) Viscometer; 2010. [18] I. AI Technology. Technical data sheet for ME 8512, ME 8638-UT, and ME 7519-LB from AI Technology 2012; 2012. [19] I. AI Technology. Technical data sheet for TC-2022 from Dow Corning; 2016. [20] Ren H, Zhou Y, He M, Xu R, Ding B, Zhong X. Enhanced mechanical properties of silica nanoparticle-covered cross-linking graphene oxide filled thermoplastic polyurethane composite. New J Chem 2018:3069–77. https://doi.org/10.1039/ c7nj03503a. [21] Prasher RS. Surface chemistry and characteristics based model for the thermal contact resistance of fluidic interstitial thermal interface materials. J Heat Transfer 2001;123:969–75. [22] Giri A, Braun L, Hopkins PE. Implications of interfacial bond strength on the spectral contributions to thermal boundary conductance across solid, liquid, and gas. Interfaces: Mol Dyn Study 2016. https://doi.org/10.1021/acs.jpcc.6b08124. [23] Liu M, Wang RY. Phase change nanocomposites with tunable melting temperature and thermal energy storage density. Nanoscale 2013;5:7234–7. [24] Li M, Xiao Y, Zhang Z, Yu J. Bimodal sintered silver nanoparticle paste with
4. Conclusions In this study, the effect of TIMs on the interface thermal resistance and overall waste heat recovery performance of a TEG operating under high-temperature conditions was investigated. Table 1 summarizes the key findings of this study. A new TIM paste referred to as MFPI TIM was developed from highly thermal-conducting hybrid fillers composed of Ag flakes and MWCNTs coated with Ag NPs and a temperature-resistant PI matrix with good printability. The excellence of the MFPI TIM was confirmed in terms of printability, thermal conductivity, and thermal stability. The thixotropic index of the TIM was larger than 5 when the minimum shear rate was larger than 10 s−1. The TIM was stable without losing mass up to around 500 °C and its thermal conductivity reached 81.4 W/ m·K. Using a custom-built experimental setup, the enhancement of the waste heat recovery performance of the TEG using the MFPI TIM was explored comparatively with cases without a TIM and with a conventional graphite foil TIM. The TEG power significantly increased as the thermal resistance of the interface between the heating block and the power generator was reduced by the MFPI TIM. From measurements during the onsite curing of the TIM, the required thermal conductivity of the TIM for sufficient thermoelectric power generation was found to be 20 W/m·K. The experimental results show that under the same temperature difference conditions, the MFPI TIM outperformed its two counterparts in thermal conduction at the interface, providing at most 132.3 and 38.6% improvements in power output compared to the cases without a TIM and with the graphite TIM, respectively. The use of the MFPI TIM was also beneficial in energy conversion, resulting in at most 73.2 and 20.9% rises in conversion efficiency compared to without a TIM and with the graphite TIM, respectively. The power output of the test TEG kept increasing with the rising temperature of a heating block even to 300 °C, which is close to the allowable maximum temperature (320 °C) of the test TEG. In addition, a thermal cyclic test consisting of a series of heating and cooling procedures between 100 and 300 °C demonstrated the long-lasting heat conduction performance of the developed TIM in a TEG module experiencing large-amplitude temperature variations. This pioneering study has verified the importance of the role that the TIM plays in reducing the interface thermal resistance by building the most favorable heat flow paths inside TEG modules for optimal waste heat recovery from them.
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and heat recovery using a heat pipe assisted thermoelectric generator system. Energy Convers Manag 2015;91:110–9. https://doi.org/10.1016/j.enconman.2014. 12.001. [27] Okhotin AS, Zhmakin LI, Ivanyuk AP. The temperature dependence of thermal conductivity of some chemical elements. Exp Therm Fluid Sci 1991;4:289–300.
ultrahigh thermal conductivity and shear strength for high temperature thermal interface material applications. ACS Appl Mater Interfaces 2015;7:9157–68. [25] Moon K-S, Dong H, Maric R, Pothukuchi S, Hunt A, Li Y, et al. Thermal behavior of silver nanoparticles for low-temperature interconnect applications. J Electron Mater 2005;34:168–75. [26] Fairuz M, Tan L, Date A, Singh B, Akbarzadeh A. Simultaneous power generation
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