Investigation of Thermoelectric Performance and Power Generation

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Investigation of Thermoelectric Performance and Power Generation Characteristics of Dual-Doped Ca1xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 # x # 0.1) Rapaka S C Bose, and Abanti Nag ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00368 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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ACS Applied Energy Materials

Investigation of Thermoelectric Performance and Power Generation Characteristics of Dual-Doped Ca1-xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 ≤ x ≤ 0.1)

Rapaka S C Bose1 and Abanti Nag1

1

Materials Science Division, CSIR-National Aerospace Laboratories, Bangalore 560017, India

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

The dual-doped Ca1-xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 ≤ x ≤ 0.1) based n-type thermoelectric oxides were synthesized by the sol-gel methodology. The sol-gel method of synthesis lowered the sintering temperature; this enhanced the density of the system close to the theoretical value. Rare-earth doping at the Ca-site drastically reduced the electrical resistivity by two orders of magnitude, compared to that of undoped CaMnO3. This was due to the formation of Mn3+ (t2g3eg1) ions with eg1 electrons in the Mn4+ (t2g3eg0) matrix of Ca1-xRE'x/2RE"x/2MnO3. The temperature dependence of electrical resistivity revealed a change from semiconductor to metallic at lower

doping

levels

(Ca0.95RE'0.025RE"0.025MnO3)

while

at

higher

doping

levels

(Ca0.9RE'0.05RE"0.05MnO3) the ρ(T) curve was semiconducting nature in the entire temperature range. In contrast, the Seebeck coefficient showed linear temperature dependence for all the compositions. The power factor (PF) of Ca1-xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 ≤ x ≤ 0.1) was much higher than the PF of undoped CaMnO3 and the highest PF obtained were 530 µW m-1 K-2 for Ca0.95Dy0.025Yb0.025MnO3 and 580 µW m-1 K-2 for Ca0.9Lu0.05Yb0.05MnO3 at 950 K. The proof-of-concept experiment of power generation with Ca3Co4O9 as p-type element resulted in a power output of 160 µW at 500 ºC for uni-couple module.

Keywords: thermoelectric; transition metal oxides; perovskites; oxychalcogenides; layered cobalt oxides


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INTRODUCTION

Thermoelectric materials and devices are areas of great current research interest. This is a consequence of the proposed future scarcity of fossil fuels on the one hand, which puts pressure on the search for alternate fuels, and global warming arising from green house gas emissions on the other hand, which leads to an increased demand for cleaner forms of energy. Thermoelectric generators constructed from n- and p-type thermoelectric materials directly convert industrial and automobile waste heat into electricity, without involving any moving parts, with simple configuration and maintenance-free operation for thousands of hours.1-4 Studies indicate that more than 60 % of energy is lost worldwide as waste heat that can be converted to electrical energy through

environmentally

friendly

thermoelectric

energy

conversion

technologies.5

The

thermoelectric performance is defined by the dimensionless figure of merit, ZT = S2σTκ-1, where Z and T are the figure of merit and absolute temperature, respectively. High ZT values are realized for thermoelectric materials with low thermal conductivity (κ), high electrical conductivity (σ), and large Seebeck coefficient (S). According to Ioffe principles of efficient thermoelectric energy conversion, intermetallic alloys (Bi-Te or Pb-Te based compositions) with carrier concentrations (n) of the order of 1018–1020 cm-3 and bandgaps of 10kBT are the best materials.6 However, these alloys have limited practical application at high temperatures as their constituents can decompose, vaporize, melt, or get oxidized in air. In this context, oxide thermoelectric materials are non-toxic, earth-abundant, chemically and thermally stable, and do not contain hazardous Pb and expensive Te. However, low electrical conductivity and porous structures are the bottleneck for oxide-based thermoelectric materials to realize practical application.7-14

The discovery of NaxCoO2 crystals with large positive Seebeck coefficient (S ~ 100 µV K-1), high electrical conductivity (σ ~ 500 S m-1), and low thermal conductivity (κ ~ 4–5 W m-1 K-1) resulting 3 ACS Paragon Plus Environment


in a ZT value of 1, shifted the focus onto oxides as potential thermoelectric materials.8 Although efficient p-type materials based on oxychalcogenides with a figure-of-merit up to 1.0 have been reported,15 the performance of n-type thermoelectric materials are still very poor with the highest reported figure-of-merit of 0.3 for CaMnO3 based oxides.16 The main reasons for the low performance of CaMnO3 based oxides are high electrical resistance leading to a low power factor and the low density of the system which has its origin in the conventional solid-state synthesis route resulting in fragile nature thermoelectric module. Therefore, in this paper, we attempted to synthesize n-type dual-lanthanide-doped CaMnO3 based oxides through soft-chemical sol-gel methodology. Several reports are available in the literature where extensive investigations have been carried out on electron-doped CaMnO3.15-29 Few reports also show that dual-doping at both Ca and Mn-sites can enhance the thermoelectric performance.27-34 Herein, we also carried out investigations to enhance the performance of the CaMnO3 based oxides via dual-doping by rareearth ions at the A-site that can help to enhance the electron density of the system and reduce the electrical resistance of CaMnO3. Further, the dual-doped composition is synthesized using a softchemical method that results in relatively higher solubility of the rare-earth ions, reduce the particle size, and enhance the density of the system. Thus, we report an efficient soft-chemical route to prepare high density n-type dual-doped Ca1-xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 ≤ x ≤ 0.1) thermoelectric oxides with densities close to the theoretical values of the crystallographic density.

EXPERIMENTAL

The dual-doped Ca1-xRE'x/2RE"x/2MnO3 (RE'/RE" = Dy, Gd, Yb, Lu; 0.05 ≤ x ≤ 0.1) oxides were prepared by the soft-chemical sol-gel method. For this purpose, stoichiometric ratios of Ca(NO3)2⋅4H2O (Alfa Aesar), Mn(NO3)2⋅4H2O (Alfa Aesar), rare-earth nitrates, and anhydrous 4 ACS Paragon Plus Environment

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citric acid (C6H8O7, Merck) were used as the starting materials. The metal nitrates and citric acid (chelating agent) were dissolved in distilled water with a molar ratio of 1:2, to obtain transparent sol without any precipitation. The precursor solution was obtained by homogenization and subsequent polymerization at 80 °C, under constant stirring. This resulted in the formation of polymeric complex called xerogel. The obtained xerogel was heated at 300 °C in air for a few hours, ground to powder, and calcined in air at 800 °C for 5 h. The resultant powders were pressed in rectangular as well as circular pellets and sintered at 1200 °C for 5 h. The soft-chemical route involved sol formation by hydrolysis polymerization of the metal citrate followed by gelation into a cross-linked polymeric chain. Upon undergoing evaporation, calcination, and sintering this polymeric chain formed the dense thermoelectric elements of Ca1-xRE'x/2RE"x/2MnO3.

Powder X-ray diffraction (XRD, M/s. Bruker D8 Advance X-ray diffractometer, Ni-filtered Cu Kα radiation) was used for the determination of the phase composition and crystal structure of Ca1xRE'x/2RE"x/2MnO3

(RE'/RE" = Dy, Gd, Yb, Lu). Rietveld refinement of the powder XRD data was

carried out by the least-squares refinement method (GSAS). The lattice parameters, atomic coordinates, isothermal temperature factors (Uiso), scale factor, background (shifted Chebyshev background function), and pseudo-Voigt profile function (U, V, W, and X) were refined. The morphology of the sintered pellets was investigated by field emission scanning electron microscopy (FESEM) using a FEI Quanta 200 F SEM. The cationic composition of randomly selected particles was analyzed by energy dispersive X-ray spectroscopy (EDX) coupled to SEM. The oxygen content of the compositions was determined by the Murray titration.[35] The relative bulk density was measured by the Archimedes method. Electrical resistivity (ρ) and Seebeck coefficient (S) measurements were carried out between 300 and 950 K using the standard 4-probe and steady-state techniques, respectively.



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RESULTS AND DISCUSSIONS

The soft-chemical sol-gel methodology yielded pure Ca1-xRE'x/2RE"x/2MnO3 at a lower sintering temperature in comparison with the conventional solid-state route. Further, the sol-gel route of synthesis yielded materials with densities close to the theoretical value. The orthorhombic Pnma phase had formed at 800ºC for sol-gel synthesized Ca1-xRE'x/2RE"x/2MnO3 (Figure S1). The 800ºC sintered sample did not give higher density; therefore the sintering temperature raised to 1200ºC to obtain density close to theoretical one. The density of the sintered thermoelectric blocks was one of the crucial parameters from the application point of view, considering the durability of the thermoelectric

module.

The

microstructural

features

of

the

sintered

pellets

of

Ca0.9RE'0.05RE"0.05MnO3 (RE'/RE" = Dy, Gd, Yb, Lu) were analyzed using a scanning electron microscope as shown in Figure 1.

Figure 1. SEM micrographs showing sintered pellets of (a) Ca0.95Dy0.025Yb0.025MnO3 , (b) Ca0.95Gd0.025Lu0.025MnO3,

(c)

Ca0.9Dy0.05Gd0.05MnO3, 6

(d)

Ca0.9Gd0.05Yb0.05MnO3 and (f) Ca0.9Yb0.05Lu0.05MnO3 ACS Paragon Plus Environment

Ca0.9Dy0.05Lu0.05MnO3,

(e)

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The grains are polygonal in nature without noticeable pores. The average grain size varies from 1.5 µm for Ca0.9Dy0.05Gd0.05MnO3 to 4.5 µm for Ca0.9Yb0.05Gd0.05MnO3. The average grain sizes also increases slightly upon increasing the dual-doping concentration, such as 2.0 µm for Ca0.95Dy0.025Yb0.025MnO3 to 2.75 µm for Ca0.95Gd0.025Lu0.025MnO3. All the compositions of Ca1xRE'x/2RE"x/2MnO3

were prepared by the sol-gel route followed by sintering under the same

conditions, indicating that both the dual-doping and the dopant concentration may have little effect on the grain size. Further, the relative densities of the dual-doped compositions are between 92 % and 96 % of the theoretical density indicating good compactness (Table 1). The density increases upon increasing the dopant concentration from 92 % (x = 0.025) to 96 % (x = 0.05) for Ca0.9RE'0.05RE"0.05MnO3. However, the densities are irrespective of the nature of the rare-earth ions present in the system. Therefore, the densities of the sintered Ca0.9RE'0.05RE"0.05MnO3 and the grain distribution are process dependent. Both these parameters are highly modified through sol-gel methodology, when compared to the conventional solid-state route of synthesis.

Table 1. Refined lattice parameters, R-factors, bond distances, bond angles, calculated tolerance factors (t), average ionic radii of rare-earths and density for Ca0.9RE'0.05RE"0.05MnO3 (RE'/RE" = Dy, Gd, Yb, Lu) Ca0.9RE'0.05RE"0.05MnIV0.86MnIII0.14O3

a (Å) b (Å) c (Å) b/√2 V (Å3) Rwp Rp RF

χ2

˂Mn–O˃ Mn–O– Mn (°) t Average ionic radii (Å) Density g/cc (%)

Yb/Lu 5.2923(3) 7.4511(5) 5.2665(4) 5.269 207.676(1) 5.84 3.16 17.48 23.66

Dy/Lu 5.3049(2) 7.4608(3) 5.2699(2) 5.276 208.576(2) 3.89 2.45 6.45 9.51

Dy/Yb 5.3059(2) 7.4634(3) 5.2718(2) 5.278 208.763(2) 3.57 2.13 4.62 8.53

Gd/Lu 5.3063(3) 7.4643(5) 5.2735(3) 5.279 208.871(3) 4.79 3.00 16.69 15.77

Gd/Yb 5.3065(4) 7.4682(8) 5.2817(4) 5.281 209.313(2) 4.50 2.66 14.62 13.28

Dy/Gd 5.3069(1) 7.4743(3) 5.2822(2) 5.285 209.520(1) 3.85 2.48 14.56 10.82

1.9059(6)

1.9068(3)

1.9079(2)

1.9087(3)

1.9098(7)

1.9109(6)

151.645(2) 158.858(8) 0.9127 0.981

153.648(5) 158.530(5) 0.9134 1.002

152.118(1) 158.572(4) 0.9135 1.006

154.084(2) 158.846(7) 0.9140 1.015

152.903(3) 158.844(1) 0.9141 1.019

152.670(4) 158.840(5) 0.9149 1.04

4.6569 94.37

4.6760 95.18

7 4.7178 94.31

4.6795 4.6852 4.7890 94.28 94.53 96.78 ACS Paragon Plus Environment


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Figure 2 shows the Rietveld refinement plots of the XRD data for Ca0.9RE'0.05RE"0.05MnO3 (RE'/RE" = Dy, Gd, Yb, Lu). Rietveld refinement was carried out using a GdFeO3-type orthorhombic perovskite structure model with Pnma space group (ao ≈ co ≈ ac√2 and bo ≈ 2ac, where o and c denote orthorhombic and cubic, respectively).36 The refinement confirms single-phase formation for Ca1-xRE'x/2RE"x/2MnO3 in the composition range 0.05 ≤ x ≤ 0.1. The low intensity (

Investigation of Thermoelectric Performance and Power Generation

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