Thermoelectric properties of metashale geopolymer mortar doped with graphite powder

Building materials with favorable thermoelectric properties can become a supplementary source of clean energy due to their ability to convert waste heat into electric energy. Depending on the thermoelectric conversion effectivity defined by the Seebeck coefficient, constructions made of these materials can serve as civil engineering energy harvesters. Since the conversion effectivity of common calcium(alumino)silicates (cementitious materials, geopolymers) is low, doping with electrically conductive admixtures is a crucial step to handle the issue. The paper is focused on the design of metashale mortar doped with graphite powder (3 wt.%), determination of its common material properties, as well as experimental determination of thermoelectric properties. The maximum thermoelectric voltage (161.65 mV, ΔT = 130 °C), Seebeck coefficient (538 μV K-1), and figure of merit (∼ 10-9) revealed significantly better thermoelectric performance than cement pastes or alkali-activated slags doped with multi-walled carbon nanotubes and if of promising thermoelectric conversion potential.


Introduction
Considering favorable material properties, environmental impact, and arising sustainability, geopolymers are highly competitive materials to dominantly used cementitious counterparts.Possessing favorable material properties, enhanced chemical and high-temperature resistance, and the fact that they can be designed using locally available wastes or industrial byproducts, they are ideal candidates for civil engineering applications [1,2].Optimization of electrical, piezoelectric, or thermoelectric properties can further increase their application potential due to new material functions and capabilities dependent on the abovementioned properties.Geopolymer composites with thermoelectric property can be utilized in the process of waste heat conversion to electrical energy which can partially cover increasing energy demands and decrease the share of the use of unclean energy sources [3].
Research on thermoelectric properties of building materials is being studied for approximately 20 years since Sun et al. [4] proved the thermoelectric effect in cementitious materials.Thermoelectric function can be enhanced by doping the material matrix with electrically conductive admixtures, such as graphite powder (GP), carbon fibers (CF), carbon powder (CP), carbon black (CB), carbon nanotubes (CNT), etc. [5][6][7].The potential and efficiency for thermoelectric function and power generation are expressed by the power factor PF where is the thermal conductivity and T [K] is the absolute temperature.
The energy conversion (thermal to electrical) is given by movement of electrically charged particles and relation between the temperature difference and corresponding thermoelectric voltage is defined by If the particles are negatively charged (electrons), the Seebeck coefficient is negative, and the material is referred to as n-type.Otherwise, positively charged carriers (holes) identify the positive Seebeck coefficient, and the material is identified as p-type (metashale).
This paper deals with the design of a geopolymer mortar doped with GP and verification of its thermoelectric properties.

Additional water 86
Samples were prepared followingly: First, 30 % GP-water suspension was prepared with the presence of Triton X-100 surfactant and nonionic siloxane-based air-detraining Lukosan S agent.The suspension was homogenized for 15 min by ULTRA-TURRAX homogenizer and further 5 min using ultrasonic treatment.The dry blend of metashale and aggregate was then mixed with the prepared graphite suspension, potassium activators, and additional water to ensure good workability (w/b = 0.47).The whole content was thoroughly mixed by a standard stirrer and placed into the moulds.Solidified samples were demoulded after 24 h and left in laboratory conditions (22 °C, 50 % RH) for further aging.
The bulk density ρv [kg•m -3 ] of the designed geopolymer was determined on (40 × 40 × 160) mm 3 prisms by the gravimetric method and the matrix density ρmat [kg•m -3 ] was evaluated on small material pieces by the helium pycnometry (Pycnomatic ATC EVO).Based on the bulk and matrix density data, the total open porosity ψ [%] was calculated.
The bending strength test carried out on (40 × 40 × 160) mm 3 samples was followed by the compressive strength testing on halves of the prisms originating from the bending experiment.Both experiments were carried out according to ČSN EN 196-1 standard [14] at 7 and 28 days using FP100 and ED60 mechanical presses.
Thermoelectric measurements were performed on (50 × 50 × 50) mm 3 sample that was pre-treated to ensure a proper determination of temperature difference and corresponding thermoelectric voltage.Therefore, two opposite sides (bottom and top) of the cube were painted with a carbon paste in a thin layer to smoothen the surface and pasted over by copper tape to provide good electrical contact of measuring voltmeter with the material.Thermocouples used for temperature measurements were placed on electrodes and insulated by a silicon pad and thermally conductive tape.The sample was placed into a thermally insulative calcium silicate board to avoid heat losses on lateral sides.The voltage and temperature measurements were performed using a Fluke 8846A device and Comet MS6D datalogger, respectively.HG PZ 28-2 hotplate device was used for heating the bottom side of the sample ensuring the dynamic change in the thermal difference between the top and bottom side of the sample (figure 1).The hotplate heated up the bottom side of the sample to 160 °C in approximately 10 min (heating part of the experiment).Being then turned off, the cooling of the sample lasted approximately 5h (cooling part of the experiment).During the whole experiment, the thermoelectric voltage and differences in temperature were recorded.The whole experiment was carried out under laboratory conditions at a temperature of 22 °C and based on the measured data further calculations were performed according to equation ( 1) and (2).

Results and discussion
The bulk and matrix density and the total porosity of the material were 2016 kg•m -3 , 2418 kg•m -3 , and 16.6 % which is in accordance with the data of similar geopolymer composites with GP admixture (3.33 % GP, bulk density approx.2000 kg•m -3 , matrix density approx.2500 kg•m -3 and the porosity approx.20 % [15]).Mechanical properties were determined at 7d and 28d.It was observed approximately 1.5 increase between the 7-day (22.1 MPa) and 28-day (34.24MPa) compressive strength.The 7-day and 28-day flexural strengths were of 2.99 MPa and 3.07 MPa, respectively.
The thermal conductivity λ [W•m -1 ·K -1 ] and specific heat capacity c [J·kg -1 ·K -1 ] of the sample in wet state (undried sample cured in laboratory conditions) were of 1.1 W•m -1 ·K -1 and 891 J·kg -1 ·K -1 , respectively.Thermoelectric properties of the geopolymer composite were determined in its dry state.The thermal and electrical conductivity of the dried material were λd = 0.86 W•m -1 ·K -1 and σd = 2.23•10 -6 S•m -1 , respectively.The thermoelectric measurement results (thermoelectric voltage and Seebeck coefficient) are presented in figure 2. The thermoelectric data logged during the experiment are represented by solid lines (red: heating, blue: cooling).The Seebeck coefficients (dashed lines) were calculated according to equation (3) using adjacent control red/blue points (10 °C temperature difference, corresponding difference in thermoelectric voltage).The maximum thermoelectric voltage of -161.65 mV was observed for the temperature difference of 130 °C during the cooling phase.During the heating phase, the Seebeck coefficient ranged between 111 -920 μV•K -1 with an average value of 538 μV•K -1 .The average ZT was of 3.01×10 -9 .During the cooling phase, the Seebeck coefficient ranged between 122 -1200 μV•K -1 and ZT fluctuated around the value of 1.28×10 -9 .
Based on observations, the designed geopolymer composite exhibited significantly higher thermoelectric (energy harvesting) potential than cement pastes, Wen S and Chung D D L [16] reported their maximal Seebeck coefficient of 2.69 μV•K -1 and Wei J. et al. [17] stated even more negligible values of around 10 -4 μV•K -1 [17].The designed geopolymer is also of higher thermoelectric performance compared to alkali-activated slag doped with electrically conductive admixtures, such as multi-walled carbon nano tubes (MWCNTs) studied by Park H. et al. [18].Different amounts (1-3% of MWCNTs) led to maximum Seebeck coefficient values of 22.5 μV•K -1 at 50 °C temperature difference.It should be noted that thermoelectric potential is greatly influenced by moisture content [19].Therefore, the thermoelectric potential of the designed geopolymer can be further increased in different wet states.

Conclusions
In this work, the material properties (basic physical, mechanical, thermal, electrical) and thermoelectric behavior of the designed metashale geopolymer composite with GP were investigated.The main following conclusions can be drawn: -Mechanical properties of the material are sufficient for various civil engineering applications in the field of smart structures.-The maximum thermoelectric voltage of 161.65 mV was observed for the temperature difference of 130 °C during the cooling phase.-The Seebeck coefficient of the designed geopolymer (538 μV•K -1 ) is significantly higher than that of cement pastes (2.69 μV•K -1 ) or alkali-activated slag doped with MWCNTs (22.5 μV•K -1 at 50 °C).

Figure 2 .
Figure 2. Thermoelectric voltage and Seebeck coefficient dependence on temperature difference