Significant Enhancement in Thermoelectric Power Factor of Bulk

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Significant enhancement in thermoelectric power factor of bulk nanostructured calcium cobalt oxide ceramics Nidhi Puri, Ram Pal Tandon, and Ajit Kumar Mahapatro ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01205 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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ACS Applied Energy Materials

Significant Enhancement in Thermoelectric Power Factor of Bulk Nanostructured Calcium Cobalt Oxide Ceramics

Nidhi Puri, Ram P. Tandon, and Ajit K. Mahapatro* Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India

*

Corresponding author contact details:

Email: [email protected]

Table of Content Figure:

1 ACS Paragon Plus Environment

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Abstract

An elementary and economical approach is demonstrated for achieving bulk nanostructured calcium cobalt oxide (CCO) ceramics with enhanced thermoelectric power factor (PF) of 64.2 10-3 Wm-1K-2 at 770 K. Nanostructures of CCO are realized by high-energy ball milling of the as-synthesized CCO, and are consolidated using hot-press (HP) technique to form fully dense pellets. The morphological, crystallographic, and spectroscopic properties suggest the formation of highly pure nanostructures and ceramics of CCO. The significant improvement of ~ 11 times in PF for bulk nanostructured CCO ceramics compared to the pellets of as-synthesized CCO microstructures is attributed to the enhancement of Seebeck coefficient (S) triggered due to structural deformations originated from the re-arrangement of atomic contents, and development of oxygen vacancies during HP process of ceramic formation with application of high pressure at elevated temperatures and introduction of enlarged interface area at grain boundaries in bulk nanostructuring. This concept of structural modulation triggered enhancement in S could be adopted for understanding the thermoelectric properties in other nanostructured materials and the currently achieved bulk nanostructured CCO ceramics could be routinely utilized in developing efficient thermoelectric power generation devices for large scale applications of converting the waste heat into usable form of electrical energy.

Keywords: Bulk nanostructuring, thermoelectrics, calcium cobalt oxide, hot-press, structural modulation


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1. Introduction

Energy harvesting technologies focus on developing resources with usable form of energies generated from the waste energies by adopting sustainable and ecofriendly techniques. Globally, the major portion of total waste energy produced from various natural (solar heat and geothermal heat) and artificial (automobiles and industry) sources are in the form of heat. The possible conversion of heat into electrical energy using the concept of thermoelectric (TE) power generation based on the principle of Seebeck effect, has been considered as one of the most promising and emerging green energy approach to harvest electricity from waste heat sources [1,2].

Considerable attention has been devoted in engineering TE module devices with the advantages of blocking direct emission of greenhouse gases for real world applications. The widespread adaptation of thermoelectricity is currently limited by the low heat to electricity conversion efficiency, gauged by the dimensionless quantity called figure of merit (ZT), which at a given temperature T is related to Seebeck coefficient ( ), electrical resistivity (

, and thermal conductivity ( ) of TE material by the relation,

. The efficiency of converting the temperature difference (∆T) to electric potential difference (V) is governed by

⁄

. The total thermal conductivity κ = κl + κe includes the contributions from lattice

thermal conductivity (κl) and electronic thermal conductivity (κe). In the process of engineering materials for improving ZT values, few reported TE materials with values of ZT ~ 1 [1] deal with the fundamental issues of inherent toxicity and chemical instabilities due to degradation or evaporation of materials content at elevated temperatures, and restrict their practical usage. The development of efficient TE devices requires synthesis of thermally and chemically stable materials with larger power factor

) and minimal κ values that

could lead to higher ZT values [2].

In bulk materials, the electrical conductivity is represented as, charge carriers flowing through the materials, mobility of the charge carriers, leading to inherent trade-off TE behavior of higher

is the electronic charge, and [ (

]⁄ )

leads to reduced

efforts have been contributed to find materials with enhanced

(decrease in

, where,

is the number of

is the velocity with ( ⁄ )

as the

. Here, considering the

or increase in ) and vice-versa,

value compared to the reducing factor in

Both in combination provide improved PF value but have to compromise for

, since reduction in

.

leads to

increase in , resulting reduction in ZT. These unavoidable selection rules in engineering materials with optimal parameters of

or , S, and κ, raise difficulty and hinder achievement of TE materials with higher PF and ZT



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values. This major practical issue for producing suitable TE materials with simultaneous requirement of high S, high  (or low increased

), and minimal κ, could be resolved by considering low dimensional materials, where, the

with consideration of semi-classical formulation of freezing

and concept of limited localised

charges with confinement effect leads to possible achievement of higher PF. In addition, the increased phonon scatterings at the boundaries of the low-dimensional structures [3] forming the pellet, provides much reduced , and the bulk nanostructuring could provide favorable parameters for achieving enhanced TE properties.

Oxide based thermoelectrics have the advantages of materials with high thermal and chemical stabilities in ambient conditions at elevated temperatures, environmetal friendly, oxidation resistant, safe to use with absence of toxic consitituents, long durability, ease in synthesis using simple and cost-effective processing recipes, and remarkable TE performance with relatively high ZT values, suggest its practical usage and possible commercialization with development of TE modules. With the discovery of single-crystal sodium cobaltite NaCo2O4 [4], the devoted efforts lead to recent progress in various oxide alloy materials including layered cobalt-based oxides (CCO) [5], perovskite-type oxide materials (calcium manganite, CaMnO3, strontium titanate, SrTiO3) [6,7], transparent conductive oxides (zinc oxide, ZnO) [8], and others (bismuth copper selenium oxide, BiCuSeO, copper aluminate, CuAlO2, dibismuth palladate, Bi2PdO4),, have demonstrated their potential as oxide materials for utilization in thermoelectric applications [9-11].

The primarily interest focuses on synthesizing perfectly composed and well-engineered nanostructures [1214] with tuned materials properties favorable for exhibiting enhanced TE behavior by following low cost processing techniques with mass-scale production capability. In this context, CCO with molecular formula of Ca3Co4O9 or (Ca2CoO3)q(CoO2), where q=b1/b2 is the misfit ratio between two sublattices of Ca2CoO3 and CoO2, has been demonstrated its competency as a suitable TE material at high temperature for utilization in TE power generation applications. This work presents the preparation of CCO nanostructures at sub-10 nm dimensions through high-energy ball milling (HEBM) processing of as-synthesized micro-plates and consolidation of resulting nanostructures for achieving fully dense pellets using hot-press (HP) technique. Enhanced S with PF of 64.2 10-3 Wm-1K-2 is demonstrated for the first time in the resulting bulk nanostructured CCO ceramics.


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2. Materials and experimental methods

2.1. Preparation of CCO nanostructures

Calcium carbonate (CaCO 3) and cobalt oxide (Co3O4) were procured from Sigma Aldrich & Co., USA. These precursors in a stoichiometric ratio of 3:4:9 for Ca, Co, and O were processed with solid state HEBM technique for 48 h, followed by drying on hot plate and calcined at 1123 K for 6 h to produce misfit-layered CCO powder (CCO-BP). The detailed optimized recipe with various ball milling (BM) and calcination time process to synthesize highly pure CCO-BP powder has been discussed in our earlier work [15]. The resulting as-synthesized CCO-BP powder was further ball-milled for 2, 6, 12, and 24 h and nomenclatured as CCO-NP2, CCO-NP6, CCO-NP12, and CCO-NP24, respectively.

2.2. Powder processing of CCO

As-synthesized CCO-BP and its ball-milled counterpart CCO-NP24 powders were loaded separately in the graphite die with an inner diameter of 20 mm, and hot-pressed into bulk disk-shaped pellets using direct current induced HP technique in a vacuum chamber, by raising the temperature to 1173 K under a hydraulic pressure of 200 MPa for 25 min, and subsequent cooling passively down to room temperature. Bar shaped samples of 12

6

2 mm3 were cut and polished from the hot-pressed disk-shaped pellets for pursuing

measurements to study the materials and TE properties.

2.3. Characterization techniques

The morphology was imaged through field emission scanning electron microscopy (FESEM, Zeiss, GeminiSEM 500, Germany) and transmission electron microscopy (TEM, FEI, Tecnai G2 T-30 U-TWIN, USA). The crystallinity for all the CCO samples were characterized by studying the crystallographic structure through Xray diffraction (XRD, Rigaku, Ultima IV X-ray diffractometer, Japan) patterns utilizing Cu-Kα excitation wavelength of 1.541 Å recorded in the angular range of 2θ = 15˚ to 70˚ at a scanning rate of 2˚/min, and selected area electron diffraction (SAED) patterns captured through TEM. The compositional details were studied using energy-dispersive X-ray (EDX) spectroscopy attachment to the FESEM. Thermal stability was tested with thermogravimetric and differential thermal analysis (TG-DTA, TA Instruments, SDT Q600, USA), scanned from room temperature to 1273 K at a heating rate of 5˚/min and performed in nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR, Perkin Elmer, USA) was recorded in spectral range from 400 to 4000



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cm−1. Raman spectra was recorded using Renishaw InVia Raman microscope, UK, equipped with an Ar laser excitation wavelength of 514.5 nm and grating with 2400 lines/mm.

The temperature dependent S and

were measured using the ―Seebeck Coefficient and Electrical

Resistivity‖ setup (Marine India, India) over the range of 300 K to 770 K. The instrument estimates the value of S by measuring the resultant voltage drop (V) produced due to difference in temperatures ( between the two ends of the pellets using the relation,

⁄

. The

)

measurements were performed

by passing constant current (I) from a current source through the pellets of cross sectional area

and

length ( ), and the resulting voltage drop was measured using a micro-voltmeter.

3. Results and discussion

FESEM image of the as-synthesized CCO-BP powder (Figure 1a) reveal plate-like microstructures of dimensions 400 – 800 nm placed randomly without agglomeration. The nanostructures of CCO are prepared by further ball-milling the as-synthesized CCO-BP for (a) 2, (b) 6, (c) 12, and (d) 24 h and the captured FESEM images are presented in Figure S1 of the Supporting Information. The onset of nanostructure formation with reduced size of featured particles is observed after grinding the CCO-BP microstructures mechanically for atleast 12 h and uniformly distributed nano-features are observed after 24 h (in CCO-NP24, Figure 1b). The TEM imaging reveals micro-sized features for CCO-BP (Figure 1c) and nanostructures in sub-10 nm dimensions for CCO-NP24 (Figure 1d). EDX patterns for both the CCO-BP micro-plates and CCO-NP24 nanostructures (Figures 1e and 1f, respectively), reveal compositions of Ca, Co, and O in a stoichiometric ratio close to 3:4:9, suggesting no chemical degradation during the mechanical grinding process. In addition, the crystallographic properties with presence of (h,k,l,m) planes observed through XRD (Figure S2 in Supporting Information) and appearance of molecular bonds in FTIR (Figure S3 in Supporting Information), agree with the inherent CCO structure [15], indicating the materials properties remain intact with milling the as-synthesized CCO-BP into nanosized CCO-NP24 powders. The Tauc plot for the UV-Vis spectra (Figure S4 and Table S1 in Supporting Information) estimates increase in the energy gap with increasing BM time from 2 to 24 h and reflects the dominance of confinement effect due to reduction in feature size of CCO structures. The above observations suggest the formation of pure and stable nanostructures with inherent CCO


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crystallographic and materials properties for 24 h ball milling of as-synthesized powders (CCO-NP24) that has been utilized for further testing of the TE behaviors.

Figure 1. (a,b) FESEM and (c,d) TEM images, and (e,f) EDX patterns for CCO-BP (a,c,e) and CCO-NP24 (b,d,f) powders of CCO. Insets are the zoomed images of respective FESEM images captured at higher magnifications. Inset tables in (e) and (f) are the compositional details of CCO estimated from analysis of the respective EDX patterns.

FESEM images of CCO-BP-HPP (Figures 2a and 2b) and CCO-NP24-HPP (Figures 2c and 2d) formed by consolidating the CCO-BP and CCO-NP24 powders, captured at low (Figures 2a and 2c) and high (Figures 2b and 2d) magnifications, respectively, show textures of partially aligned lamella-like platelet structures. During HP process, the simultaneous application of high pressure (200 MPa) and temperature (1173 K) assist the consolidation of micro/nano-structures of CCO-BP/CCO-NP24, to form highly dense pellets. The volumetric mass density (D) estimated from the Archimedes principle by calculating the ratio of weight of the pellet (W) to it‘s apparent weight in water (W w) compared to air



(Wa), and 3 mm thickness with

. The currently developed disc shaped pellets of dimensions 20 mm diameter = 4.22 g,

= 40.48 g, and

= 39.57 g, estimate the value of D = 4.65 ±

0.05 gcm−3 for both the CCO-BP-HPP and CCO-NP24-HPP, and closely agree with the theoretical density of 4.68 gcm−3 assigned for CCO [16]. This indicates a relative density of 99.2 ± 0.5 % for the HPP samples, and suggests effective fusion and rearrangement of micro/nanostructures during the hotpressing process to develop fully dense CCO ceramics. EDX patterns for both the CCO-BP-HPP and CCO-NP24-HPP shown in Figures 2e and 2f, respectively, estimate the contents of Ca, Co, and O with atomic percentage (at. %) closely agreeing to a stoichiometric ratio of 3:4:9. The FTIR spectra (Figure S5 in the Supporting Information) provides invariant bonding for Co-O and Ca-O during nanostructuring and hot-pressing, and eliminates the possibility of any residual contents postpelletization, and supports the EDX mapping as processed using the FESEM image for CCO-NP24HPP (Figure S6 in Supporting Information).

Figure 2. FESEM images for (a,b) CCO-BP-HPP and (c,d) CCO-NP24-HPP captured at low (a,c) and high (b,d) magnifications. EDX patterns for (e) CCO-BP-HPP and (f) CCO-NP24-HPP. Inset tables in (e) and (f) are the tabulated values for elemental contents of Ca, Co, and O evaluated through EDX.


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Figure 3 represents the high resolution TEM (HRTEM) images (top row) and the SAED patterns (bottom row) for CCO-BP, CCO-NP24, CCO-BP-HPP, and CCO-NP24-HPP. The observed fringes with aligned patches in larger dimensions of 60 – 100 nm for CCO-BP (Figure 3a) and CCO-BP-HPP (Figure 3c), compared to the regions of ~ 5-30 nm for CCO-NP24 (Figure 3b) and CCO-NP24-HPP (Figure 3d), are due to reduction of the crystallite sizes during HEBM. Both the HPP samples (CCO-BP-HPP and CCO-NP24-HPP) with closely packed domains support the formation of fully dense pellets with multiple regions of prominent crystallinity and grain boundaries (as marked in Figure 3c-d). The presence of closely packed amorphous regions and crystalline domains in sub-30 nm dimensions are the salient features of the newly developed CCO-NP24-HPP. The SAED patterns of CCO-BP (Figure 3e) and CCO-NP24 (Figure 3f) show diffraction spots corresponding to (hklm) planes, and the patterns of CCO-BP-HPP (Figure 3g) and CCO-NP24-HPP (Figure 3h) show (00l0) planes of the CCO crystal structure. In addition, the appearance of more diffraction spots forming diffused rings in the SAED patterns of CCO-NP24 indicates presence of substantial polycrystals with HEBM processing. The limited circularly arranged bright spots in CCO-BP-HPP compared to large numbers of bright spots in SAED pattern of CCO-NP24-HPP reveals the existence of sizable number of crystalline domains within the same volume after HEBM processing. The d-spacings in different fringe patterns in the HRTEM are identical with the distances between the diametrically opposite bright spots of identical intensities placed equidistant from centre of the SAED patterns of CCO (as represented in Table S2 of the Supporting Information).

Figure 3. (a,b,c,d) HRTEM images and (e,f,g,h) SAED patterns for (a,e) CCO-BP, (b,f) CCO-NP24, (c,g) CCO-BP-HPP, and (d,h) CCO-NP24-HPP. In (d) the yellow and red lines represent crystalline and amorphous regions, respectively.



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XRD patterns for the as-synthesized CCO-BP and nanopowders of CCO-NP24 represent (hklm) planes (Figure 4a) and agree well with previous reports for single phase CCO [17] . The appearance of only (00l0) planes (l = 2, 3, 4, and 5) in both the HPP processed CCO-BP-HPP and CCO-NP24-HPP samples, support the texturing features observed in the corresponding FESEM images (Figures 2a-d) of CCO ceramics due to reorientation of the crystallographic planes [18]. The degree of texturing could be ⁄

evaluated from the Lotgering factor [19],

, where,

of XRD peak intensities along (00l0) and (hklm) planes, i.e., ∑

and

⁄∑

are the ratio of sum , observed in the

perfectly oriented HPP samples (CCO-BP-HPP and CCO-NP24-HPP) and their respective randomly oriented CCO powders (CCO-BP and CCO-NP24), respectively. The values of CCO-BP-HPP, and

= 1 and

= 1 and

= 0.6 for

= 0.54 for CCO-NP24-HPP, are estimated from the peak intensities

represented in Figure 4a. This estimates

and indicates the achievement of highly oriented

crystallographic planes in [00l0] directions for the pellets of CCO, and favours the texturing structures in FESEM images of both the kinds of hot-pressed pellets.

The signature peak at 2θ = 16.5 for (0020) plane of CCO is observed in XRD patterns of both the powder and pellet forms and plotted in Figure 4b. Here, the broadening of diffraction peak in CCO-NP24 compared to CCO-BP validates the reduction in the size of CCO features with formation of polycrystalline structures after mechanical milling for 24 h, and complements the SAED patterns (in Figures 3e-h). Shifting of the highly prominent (0020) peak towards higher 2θ values after nanostructuring is attributed to the lattice strain originated from displacement of the in-plane surface atoms. The additional shifting of peaks to higher 2θ values in HPP samples compared to their powder forms is due to the extra strain produced during interaction of surface atoms with their adjacent micro/nano-structures during consolidation process to form fully dense pellets. This explains the observed increase in full width half maximum (FWHM) of the X-ray diffraction peak ( ) and decrease in average crystallite size (

) for CCO-NP24 and CCO-NP24-HPP (Figure 4c) compared to their micro-plate

counterparts, estimated using the Debye Scherrer‘s formula [20], wavelength of X-ray excitation, K = 0.9 is a constant, and

⁄

is the Bragg‘s angle.


, where, λ = 1.54 Å is the

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Figure 4. (a) XRD patterns of CCO-BP, CCO-NP24, CCO-BP-HPP, and CCO-NP24-HPP with the indexed (hklm) planes assigned for CCO, (b) Zoomed portion of the diffraction peak at 2θ = 16.5˚ representing the (0020) plane (c) The average crystallite size (

) and FWHM ( ) of the (0020) peak

appeared at 16.5˚ in the XRD patterns for various CCO structures.

TG-DTA measurements in temperature range from 300 to 1273 K are performed in nitrogen environment and represented in Figure 5. The TGA provides total weight losses upto 8 % for CCO-BP and 19 % for CCO-NP24 (Figure 5a). The DTA with prominent endothermic peaks (Figure 5b) only in the powder forms of CCO (at 1130 K for CCO-BP and at 917 K for CCO-NP24), are explained as the conversion of the decomposed traces of unreacted Co 3 O4 into CoO [15] and subsequent formation of CCO. The observation of the endothermic peak at lower temperature in CCO-NP24 compared to the CCO-BP coincides with the concept of lowering the decomposition temperature due to reduction in feature size of the structures and indicates formation of nanostructures in HEBM processed CCO. The rapid weight loss in intermediate temperatures (800-1140 K) followed by very small change in weight



loss at elevated temperatures (> 1140 K) in TGA and appearance of endothermic peak in DTA in both the powder forms agrees with the inherent thermal stabilities of polycrystalline CCO [15,21,22]. In contrast, the negligible weight losses of 1.1 % for CCO-BP-HPP and 3 % for CCO-NP24-HPP in TGA with diminished endothermic peak in DTA observed for the HPP samples, imply the realization of higher thermal stability after hot-pressing the CCO powders at optimized pressure and temperature (Figure S7 in Supporting Information).

Figure 5. (a) TGA and (b) DTA plots for CCO-BP (blue), CCO-NP24 (green), CCO-BP-HPP (red), and CCO-NP24-HPP (pink).

The above extensive experimental results for materials characterization provide evidences for production of polycrystalline nanostructures and retention of high purity without compromising the inherent crystal properties of CCO, after grinding the as-synthesized micro-sized CCO structures for 24 h using HEBM, and further consolidation of the resulting nanostructures produce textured and thermally stable bulk nanostructured CCO ceramics. TE properties of the newly developed CCO-NP24-HPP ceramics are tested in temperature range of 300-770 K and compared with CCO-BP-HPP (Figure 6). The decrease in ‗ ‘ with increasing temperature in the range of 300-770 K (Figure 6a) for both the CCO-BP-HPP and CCO-NP24-HPP, indicates semiconducting behaviour, and agrees with the inherent electronic properties of CCO pellets formed using various techniques [23-25]. Here, the negligible (≈ 0.08 times) increase in ‗ ‘ in the bulk nanostructured pellet could be due to increased electron scattering reinforced at the grain boundaries, and supports the appearance of more grains in the morphology of CCO-NP24-HPP captured through FESEM and HRTEM (Figures 2 and 3, respectively). The CCO-BPHPP and CCO-NP24-HPP show lower ‗ ‘ values at high temperatures, as compared to the reported


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values of ‗ ‘ for CCO pellets prepared using various processing techniques (Table S3, Supporting Information).

The observed positive value for S (Figure 6b) throughout the temperature range indicates holes as the dominant charge carriers, exhibiting p-type characteristics for both the CCO ceramics. Further, the value of S for CCO-NP24-HPP is higher than CCO-BP-HPP in the entire temperature range 300-770 K. The observed highest value of S = 804 μV/K for CCO-NP24-HPP is nearly 4 times higher compared to S = 223 μV/K for CCO-BP-HPP, and is the highest value reported in CCO based materials and alloys [23-32] at 770 K. Generally, for metals and highly degenerate semiconductors, the thermopower could be mathematically expressed by Mott–Jones relation [33], as

(

(

(

(

where,

is the electronic charge of the carrier,

and

)

)

)

(

) )

is the Boltzmann‘s constant,

is the energy dependent electrical conductivity with

is the Fermi level, is the mobility, and

is the density of the charge carriers associated with the Fermi distribution function and density of states

. Considering these mathematical formulations, the observed

enhancement in S is explained with the energy dependence in

or

. This concept describes the

performance of intrinsic bulk TE materials and composites with external doping of atomic contents used for tailoring the crystal arrangements and band structure tuning that subsequently control the phonon and electron transports, respectively, and influences the Seebeck coefficient. The HEBM processed nanostructures contain reduce sized domains (~ 5 nm) and lead to quantum confinement effect with discretized energy states. On consolidation (CCO-NP24-HPP), the existence of domains with dimensions in sub-30 nm (as imaged through FESEM and TEM) and crystallographic study suggests rearrangement of crystallites influence the density of energy states near (

(

)

)

, increase the slope factor

in the expression for S, and assist enhancement in thermopower [3, 34–36].



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Figure 6. Thermoelectric parameters of (a) electrical resistivity (ρ), (b) Seebeck coefficient (S), plotted for CCO-BP-HPP (blue open squares) and CCO-NP24-HPP (red solid spheres) in the temperature range 300-770 K. Insets in (a) and (b) represents the schematics of the respective experimental measurement setup connecting the pellets of dimension

= thickness and

= width. M is the metal

electrode, TC is the temperature controller, ΔT is the temperature difference, H and C are the hot and cold ends respectively, I is the current source, and V is the voltmeter.

Since the current study utilizes nanostructured CCO, the observed enhancement in thermopower would include the existing concepts of S dependence in crystallography and energy spectrum of the materials [37], and possible effect from the less discussed structural modulation due to defects triggered by spin variation, vacancy mediated charge transfer, and surface deformation. All these factors lead to produce configurational entropy (ζC) in addition to the thermal entropy (ζT) and provides a total entropy of ζ = ζC+ ζT for the system, which proportionately effect the value of S, defined as the entropy density associated with unit charge carrier in motion.

The intrinsic nature of the CCO in CCO-NP24-HPP restrains the concept of increase in S due to alteration in the compositional entropy with changing concentrations (x) in

of the doped CCO

systems [25,38,39]. Further, the absence of additional peaks in DSC measurement (Figure S8 in


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Supporting Information) eliminate the role of spin state (low, intermediate, and high) transition in Co ions with unpaired 3d electrons (Co4+, Co3+, and Co2+) and expects the existence oxygen vacancy [40] created during HP processing. Also, the concept of electronic charge transitions due to spin, the temperature dependent resistivity should have shown sudden decrease in resistivity with insulator to metal phase transition [37,41], which is absent in CCO-NP24-HPP ceramics (Figure 6a). The above analysis excludes the contribution of spin in describing the enhancement of S with temperature in the nanostructure-based CCO-NP24-HPP ceramics.

Further, the additional factors contributing to thermoelectric behaviours are understood by measuring the materials contents and their inherent structural arrangements using extensive crystallographic and spectroscopic studies. Generally, the intrinsic misfit layer structure of CCO comprises assembly of two monoclinic sub-lattices containing the rock-salt (RS) type Ca2CoO3 layer (SL1) separated by alternately stacked CdI 2 type CoO2 layers (SL2) along the c-direction, with lattice constants a1=a2=a, c1=c2=c, angle between the ‗a‘ and ‗c‘ axis as β L1= βL2=βL, and b1≠b2 [33]. Rietveld refinement on the experimentally obtained XRD data of CCO-BP, CCO-NP24, CCO-BP-HPP, and CCO-NP24-HPP were processed using Topaz program by considering the general consensus of monoclinic symmetry in the superspace group Cm (0 1-p 0) for CCO structure [17]. The resulting lattice parameters (‗a‘ and ‗c‘) and the misfit ratio (b 1/b2) of the two subsystems of CCO unit cell are tabulated in Table S4 of the Supporting Information. The observed lattice parameters (a, b1/b2, and c) for all the samples agrees well with the reported values of a = 4.8339 Å, c = 10.8436 Å, and b1/b2 = 1.6142, for CCO [17]. The lattice parameters ‗a‘ and ‗c‘ show larger values for nanostructured CCO-NP24 and CCO-NP24-HPP compared to their respective micro-plate counterparts (CCO-BP and CCO-BP-HPP, respectively), and tabulated in Table S4. Here, the expansion of lattice parameters in CCO nanostructured powders and subsequent pellets is attributed to the surface restructuring with enlargement of surface area due to reduction in particle sizes to sub-30 nanometer dimensions [42], and accord with the shifting of Bragg‘s diffraction angle to higher values and increasing FWHM (Figure 3c) of the dominated (0020) plane in the XRD patterns. Interestingly, the observed substantial decrease in βL for CCO unit cell in nanostructures and subsequent pellets is due to expansion of lattice parameters ‗a‘ and ‗c‘, and lattice rearrangement triggered during application of external pressure and temperature on the nanoscale features, respectively. The HRTEM images (Figures 7a and 7b) with line separations of 1.2 and 1.3 nm correspond to the c-axis periodicity in CCO-BP-HPP and CCO-NP24-HPP,



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respectively. The above study indicates release in strain during nanoparticle formation produces pronounced distortion in atomic arrangements, which intensifies with evolution of structural rearrangement during HP process.

Figure 7c represents Raman spectra of CCO powder (CCO-BP and CCO-NP24) and ceramic (CCO-BPHPP and CCO-NP24-HPP) structures (Table S5 in Supporting Information). The peak at 194 cm-1 is attributed to vibrational modes from the heavier Co atom [43,44]. The prominent Raman bands at 270, 516, 615, and 685 cm-1 correspond to vibrational modes of oxygen atoms. The hot-pressed pellets show noticeable changes in characteristic Raman bands compared to the spectra of powder forms, including (i) band splitting at 270 cm-1 with rise in intensity, (ii) band broadening at 516 cm-1 assigned to the in-plane E1g vibrations of oxygen atoms, (iii) increase in band intensity and width at 615 cm-1 corresponding to the out-of-plane A1g vibration modes originating from oxygen atoms, and (iv) diminishing of band at 685 cm-1 indicates increase in number of oxygen vacancies [45] and supports the significant reduction in oxygen content in CCO-NP24-HPP observed in EDX. These modifications of increased intensities, band splitting and shifting, and band width variations reflecting in the Raman modes of hot-pressed samples suggest the introduction of local strain fluctuations, distortions, and oxygen vacancies by hot-pressing the as-prepared micro and nano-sized CCO-NP24-HPP powders without significantly transforming the inherent crystal structure of CCO. The oxygen vacancies influence internal stress between the two subsystems and support the observed structural rearrangements in XRD, which causes enhancement in S [46]. The associated phonon wavelengths ( Raman bands centred at, the source and

is the wavelength of

and

could be estimated from location of the

, where

is the excitation wavelength of

. The prominent multiple Raman bands provide phonon wavelengths in

the range 5-18 nm (Table S6, Supporting Information). This includes a wide range of short to long wavelength phonons contributing to the thermal conductivity, indicating effective scattering for achieving low thermal conductivity [3]. The above analysis attributes the increase of S in nanostructured CCO ceramics to the development of structural distortion during nanoparticle formation and rearrangement of atomic contents due to application of high pressure at elevated temperatures during HP powder processing, leading to introduction of significant contribution from the configurational entropy in addition to the quantization effect with nanostructuring.


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Figure 7. The line separation corresponding to c-axis with a periodicity of 1.2 and 1.3 nm in (a) CCOBP-HPP and (b) CCO-NP24-HPP, respectively, in HRTEM image, and (c) Raman spectra recorded for CCO-BP (blue), CCO-NP24 (green), CCO-BP-HPP (pink), and CCO-NP24-HPP (red).

Figure 8 shows the variation of

with temperature, recorded for CCO-NP24-HPP, and

the observed value of 64.2 10-3 Wm-1K-2 at 770 K is almost 11 times higher than CCO-BP-HPP (5.8 10-3 Wm-1K-2). This enhanced value of PF is validated with giant 4 times (400 %) increase in S (Figure 6b) and negligible 0.08 times (8 %) increase in resistivity or decrease in conductivity (Figure 6a). Figure 9 shows the comparative bar diagram of S and PF values at 300, 550, and 770 K [22-31], reported for CCO as TE material. The values of S and PF in various bulk nanostructured TE materials at 300 K and 770 K are summarized and compared with observed values (Table S7, Supporting Information). Since, the PF describes the ZT value at unit thermal conductivity, the enhancement in PF noticed for CCO-NP24-HPP suggests potentially increase in the heat to electricity conversion efficiency. The currently observed improvement in thermoelectric power generation is exhibited for the first time in nanostructured CCO-NP24-HPP. This result in combination with the general consensus of observing reduced thermal conductivity in bulk nanostructured pellets due to increase in phonon scattering at nanograin interfaces, suggest the possible achievement of relatively high ZT factors.



Figure 8. Thermoelectric power factor (PF) plotted for CCO-BP-HPP (blue open squares) and CCO-NP24HPP (red solid spheres) in the temperature range 300-770 K. Inset shows the magnified PF in the temperature range of 300-450 K.

Figure 9. Bar diagrams for the (a) Seebeck coefficient (S) and (b) power factor (PF) at 300, 550, and 770 K, mentioning enhancement in the S and PF in the current study compared to the previous reports.


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4. Conclusion Enhanced thermoelectric response with power factor of 64.2×10-3 Wm-1K-2 is demonstrated for the first time in hot-pressed pellets of nanostructures formed by simple high-energy ball milling of the as-synthesized CCO powders. The enhancement of over one order in power factor achieved in pellets of HEBM processed CCO nanostructures compared to the pellets of as-synthesized micro-plate counterparts, is attributed to the structural modulation and development of oxygen vacancies during hot pressing process in addition to the contribution from the quantized energy states due to inherent confinement effect. The currently reported bulk nanostructured CCO ceramics prepared using HP technique could be utilized commercially for mass scale production at a lower production cost in relatively short processing time, and could be routinely used as a simple and potential approach for electrical power generation from the waste heat for achieving sustainable energy.

Supporting Information

Additional experimental details, extensive material characterization for optimization of bulk nanostructured CCO, and evidences for structural modulation in CCO ceramics are included in the Supporting Information.

Acknowledgements

The authors would like to thank the Defence Research and Development Organization (DRDO), Delhi, India, for providing financial support through the extramural research funding (ERIP/ER/1103992/M/01/1515, 2013) and under the Contract for Acquisition of Research Services (CARS) scheme (1115/CARS-53/TS/SPL/16, 2017) of Solid State Physical Laboratory, DRDO, Delhi, India.



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Significant Enhancement in Thermoelectric Power Factor of Bulk

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