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Thermoelectric Ceramics

Liang, L., Romo-De-La-Cruz, C.-O., Carvilo, P., Jackson, B., Gemmen, E., Paredes-Navia, S. A., . . . Song, X. (2019). Difference between transition metal cation substitution and Nonstoichiometric addition on nanostructure and thermoelectric performance of complex oxide ceramics. Journal of Solid State Chemistry, 277, 427-433.

$Right figure: Seebeck coefficient S as a function of temperature K. Left figure: High resolution images and corresponding fourier transformation from [110]b2 axis of the Ca3Co4CuyO9 samples showing the grain interior is free of secondary phase and the presence of extra diffraction spots in the sample Ca3Co4Cu0.05O9. (a) y ¼ 0.01, and (b) y ¼ 0.05.$

Unit cell structures of crystals with different Cu doping. (a) pure Ca3Co4O9; (b) crystal with stoichiometric substitution Ca3Co4-xCuxO9, (x ¼ 0.5 and 0.1) having the Cu substation in the CoO2 layer; (c) Crystal with nonstoichiometric addition Ca3Co4CuyO9 (y ¼ 0.5 and 0.1) having Cu substitution in the Ca2O1O3 layer. Meanwhile, there is existence of additional (Co1-yCux)O2 polytype that is presumably with larger b2 lattice parameter and alternating with the primary pure CoO2 subsystems along the c-axis of the unit cell

This work presents the impact of different methodologies in introducing the transition metal dopants into the thermoelectric $C a_{3} C o_{4} O_{9}$ ceramics and their resultant nanostructure and electrical transport properties. The polycrystalline ceramics samples are with designed nominal composition of cation substitution of $C a_{3} C o_{4 - x} C u_{x} O_{9}$ (x = 0, 0.01, 0.05, 0.1) and cation non-stoichiometric addition of $C a_{3} C o_{4} C u_{y} O_{9}$ (y = 0, 0.01, 0.05, 0.1), respectively. At low-temperature regime, the electrical resistivity decreased to the lowest values for each set of samples, upon the minute Cu doping with x = 0.01, and the electrical resistivity increase with Cu doping level. The Seebeck coefficient presents little change among $C a_{3} C o_{4 - x} C u_{x} O_{9}$ samples. By contrast, the Seebeck coefficient was significantly increased especially in the low-temperature regime in samples of $C a_{3} C u_{y} C o_{4} O_{9}$ , when the Cu addition level is over y = 0.05. While the grain size and crystal texture status are almost unchanged among $C a_{3} C o_{4 - x} C u_{x} O_{9}$ samples, non-stoichiometric Cu addition substantially triggered the grain growth, and essentially change the misfit relationship between the $C a_{2} C o O_{3}$ and $C o O_{2}$ layers within the unit cell. The present study demonstrates a novel approach for hierarchically modifying the structure of complex oxide ceramics to tune their thermoelectric properties through cation non-stoichiometric addition.

Song, X., Paredes Navia, S. A., Liang, L., Boyle, C., Romo-De-La-Cruz, C.-O., Jackson, B., . . . Chen, Y. (2018). Grain Boundary Phase Segregation for Dramatic Improvement of the Thermoelectric Performance of Oxide Ceramics. Acs Applied Materials & Interfaces, 10 (45), 39018-39024. doi:10.1021/acsami.8b12710

Evolution of the grain morphology and formation of the grain boundary network upon Cu addition in Ca1–xBixMnCuyO3−δ samples. (a) CaMnO3−δ; (b) Bi-doped CaMnO3−δ with Cu addition

Temperature dependence of ZT of Ca1–xBixMnCuyO3−δ samples

HRTEM imaging taken from the Bi-doped CaMnO3−δ samples and Fourier transformation from the corresponding nanodomains with different orientations

HRTEM imaging taken from the Cu-added Bi-doped CaMnO3−δ samples and Fourier transformation from the corresponding nanodomains with different orientations

This work presents a novel approach of dramatically increasing the energy conversion efficiency of thermoelectric $C a M n O_{3 - δ}$ ceramics through the combination of lattice dopants substitution and secondary phase segregation at the grain boundaries. The oxide ceramic samples are with the nominal composition of $C a_{1 - x} B i_{x} M n C u_{y} O_{3 - δ}$ ( x = 0, 0.02, 0.03; y = 0.02, 0.04). When Cu is introduced into the $C a_{1 - x} B i_{x} M n C u_{y} O_{3 - δ}$ samples, the grain growth from Bi-doped $C a M n O_{3 - δ}$ grains is accompanied by the limited solubility of Cu ions in the grain interior, whereas Cu mainly formed a CuO secondary phase at the grain boundaries. Cu nonstoichiometry addition subsequently resulted in the increase of the Seebeck coefficient and decrease of electrical resistivity simultaneously. The sample with designed chemistry of $C a_{0.97} B i_{0.03} M n C u_{0.04} O_{3 - δ}$ exhibits the power factor of 2.4 $m W m^{- 1} K^{- 2}$ at 337 K and figure of merit ZT of 0.67 at 773 K. This ZT of 0.67 is by far the highest ZT reported for various perovskites oxide ceramics. Such enhancements in electrical power factor and the overall ZT are attributed to the synergistic effect of decreasing the carrier concentration to increase the Seebeck coefficient and simultaneously increasing the carrier mobility through the existence of CuO phase at the grain boundaries.

Romo-De-La-Cruz, C., Liang, L., Navia, S. A. P., Chen, Y., Prucz, J., & Song, X. (2018). Role of oversized dopant potassium on the nanostructure and thermoelectric performance of calcium cobaltite ceramics. Sustainable Energy & Fuels, 2 , 876-881. doi:10.1039/C7SE00612H

Temperature dependence of electrical properties for Ca3Co4O9Kx (x = 0, 0.05, 0.1, 0.15, and 0.2): (a) ρ - T, (b) S - T, and (c) S 2 /ρ - T.

Schematic showing the crystal texture development and dopants K segregation at the grain boundaries of sample Ca3Co4O9K0.1.

The impact of the non-stoichiometric addition of potassium (K) on the nanostructure and thermoelectric performance of misfit layered calcium cobaltite $(C a_{3} C o_{4} O_{9})$ ceramics is reported. The samples were prepared with the designed nominal composition of $C a_{3} C o_{4} O_{9} K_{x}$ ( x = 0, 0.05, 0.1, 0.15, and 0.2). The K addition promoted the crystal growth and improved the crystal texture. The nanostructure and chemical analysis revealed the segregation of K at the $C a_{3} C o_{4} O_{9}$ grain boundaries, while the $C a_{3} C o_{4} O_{9}$ grain interior was free of K. At the optimal doping level, the dopant K grain boundary segregation reduced the electrical resistivity and simultaneously increased the Seebeck coefficient, resulting in a large increase in the power factor. At 320 K, the sample $C a_{3} C o_{4} O_{9} K_{0.1}$ achieved the power factor of 930 $μ W K^{- 2} m^{- 1}$ , which is 2.25 times higher than 412 $μ W K^{- 2} m^{- 1}$ from pristine $C a_{3} C o_{4} O_{9}$ and by far, the highest power factor at room temperature regime for the $C a_{3} C o_{4} O_{9}$ ceramics. The impact of the dopant segregation on the ionic diffusion along the grain boundaries and its resultant thermoelectric performance enhancement of $C a_{3} C o_{4} O_{9}$ ceramics are discussed.

Boyle, C., Liang, L., Romo-De-La-Cruz, C.-O., Johnson, R., Chen, Y., Prucz, J., . . . Song, X. (2018). Improving the thermoelectric performance and thermal stability of $C a_{3} C o_{4} O_{9 + δ}$ ceramics by sintering in oxygen atmosphere. Journal of Sol-Gel Science and Technology, 85 (3), 712-722.

ZT of samples sintered a in air and b in oxygen, respectively

TEM images showing the morphology of Ca3Co4O9+δ nano-lamella from the pellets sintered in oxygen at different
temperatures. a 1193 K, b 1233 K, c 1253 K and d 1273 K

We report on the influence of sintering gas atmosphere on the thermoelectric (TE) performance of $C a_{3} C o_{4} O_{9 + δ}$ ceramics made from precursor powders synthesized using chemical sol–gel route. Two sets of polycrystalline pellets were sintered in air and oxygen atmosphere, respectively. Sintering gas atmosphere and sintering temperature were found to have great influence on the $C a_{3} C o_{4} O_{9 + δ}$ crystal orientation and formation of the $C o_{3} O_{4}$ secondary phase. All samples, regardless of sintering atmosphere, had similar Seebeck coefficients S that increased with measurement temperature. By contrast, the electrical resistivity ρ changed dramatically as the sintering temperature and atmosphere changed. For samples sintered in air, when the sintering temperature is above 1193 K, ρ increased as the sintering temperature increased. For samples sintering in oxygen, ρ decreased monotonically as the sintering temperature increased up to 1253 K. Benefited from the reduced ρ, the electrical power factor of the sample sintered in the oxygen at 1253 K is 0.539 $m W m^{- 1} K^{- 2}$ , and 40% greater than that from the sample sintered at 1193 K in air. Dimensionless figure-of-merit ZT of the sample sintered at 1253 K in oxygen is 0.28 at 1073 K, which is 20% greater than that of samples sintered in air at 1193 K. Overall, the highest TE performance was obtained for $C a_{3} C o_{4} O_{9 + δ}$ ceramics sintered in oxygen at 1253 K, which is significantly higher than the $C a_{3} C o_{4} O_{9 + δ}$ decomposition temperature of 1199 K in air.

Talley, K., Barron, S., Nguyen, N., Wong-Ng, W., Martin, J., Zhang, Y., Song, X. (2017). Thermoelectric properties of the $L a C o O_{3} - L a C r O_{3}$ system using a high-throughput combinatorial approach. Solid State Sciences, 64 , 7-12.

Schematic and image for PLD processing of a LaCo1-xCrxO3 combinatorial library

A combinatorial film of the $L a C o_{1 - x} C r_{x} O_{3}$ system was fabricated using the $L a C o O_{3}$ and $L a C r O_{3}$ targets at the NIST Pulsed Laser Deposition (PLD) facility. As the ionic size of $C r^{3 +}$ is greater than that of $C o^{3 +}$ , the unit cell volume of the series increases with increasing x. Using a custom screening tool, the Seebeck coefficient of $L a C o_{1 - x} C r_{x} O_{3}$ approaches a measured maximum of 286 μV/K, near to the cobalt-rich end of the film library (with x ≈ 0.49). The resistivity value increases continuously with increasing x. The measured power factor, PF, of this series, which is related to the efficiency of energy conversion, also exhibits a maximum at the composition of x ≈ 0.49, which corresponds to the maximum value of the Seebeck coefficient. Our results illustrate the efficiency of applying the high-throughput combinatorial technique to study thermoelectric materials

Boyle, C., Liang, L., Chen, Y., Prucz, J., Cakmak, E., Watkins, T. R., . . . Song, X. (2017). Competing dopants grain boundary segregation and resultant seebeck coefficient and power factor enhancement of thermoelectric calcium cobaltite ceramics. Ceramics International, 43 (14), 11523-11528.

Plots of electrical properties for the Ca3−xBixBayCo4O9 (x = 0 and 0.1, and y = 0, 0.02, 0.05, 0.07, and 0.1). (a) ρ as a function of temperature T; (b) S as a function of temperature T; and (c) power factor S2/ρ as a function of temperature T

$TEM diffraction contrast images (a and b) and high resolution TEM image (c) from the sample of Ca2.9Bi0.1Ba0.07Co4O9. In Fig. a, individual circled numbers depict the spots where the EDS spectra and data (in Table 4) were taken from$
Table 4. Chemical compositions of two neighboring grains and in between GB area as shown in Fig. 4a. Total six set of EDS data were collected, and each data set includes three data points across the GB area. All the data indicate that GB area is Ba enriched but Bi depleted

Table 4. Chemical compositions of two neighboring grains and in between GB area as shown in Fig. 4a. Total six set of EDS data were collected, and each data set includes three data points across the GB area. All the data indicate that GB area is Ba enriched but Bi depleted

The present work demonstrates the feasibility of increasing the values of Seebeck coefficient S and power factor of calcium cobaltite $C a_{3} C o_{4} O_{9}$ ceramics through competing dopant grain boundary segregation. The nominal chemistry of the polycrystalline material system investigated is $C a_{3 - x} B i_{x} B a_{y} C o_{4} O_{9}$ with simultaneous stoichiometric substitution of Bi for Ca and non-stoichiometric addition of minute amounts of Ba. There is continuous increase of S due to Bi substitution and Ba addition. The electrical resistivity also changes upon doping. Overall, the power factor of best performing Bi and Ba co-doped sample is about 0.93 $m W m^{- 1} K^{- 2}$ , which is one of the highest power factor values ever reported for $C a_{3} C o_{4} O_{9}$ , and corresponds to a factor of 3 increase compared to that of the baseline composition $C a_{3} C o_{4} O_{9}$ . Systematic nanostructure and chemistry characterization was performed on the samples with different nominal compositions. When Bi is the only dopant in $C a_{3} C o_{4} O_{9}$ , it can be found at both the grain interior and the grain boundaries GBs as a result of segregation. When Bi and Ba are added simultaneously as dopants, competing processes lead to the segregation of Ba and depletion of Bi at the GBs, with Bi present only in the grain interior. Bi substitution in the lattice increases the S at both the low and high temperature regimes, while the segregation of Ba at the GBs dramatically increase the S at low temperature regime.

Boyle, C., Carvillo, P., Chen, Y., Barbero, E. J., McIntyre, D., & Song, X. Y. (2016). Grain boundary segregation and thermoelectric performance enhancement of bismuth doped calcium cobaltite. Journal of the European Ceramic Society, 36 (3), 601-607. doi:10.1016/j.jeurceramsoc.2015.10.042

Temperature dependence of figure of merit ZT of Ca3−xBixCo4O9 with x = 0, 0.1, 0.2, 0.3 and 0.4. The maximum relative error for the repeated measurements was evaluated as (ZT) = ± 10%

$Nanostructure examination of Ca2.7Bi0.3Co4O9. (a), low magnification TEM diffraction contrastimage shows typical grain boundary and the adjacenttwo grains. The pink arrow C1 and the green arrow C2 indicate the different c-axis orientations of such two neighboring grains and the misorientation angle is about 6◦. (b), Low magnification STEM Z-contrast image from the same grain boundary as indicated in (a). And (c), High magnification STEM Z-contrast image of the same grain boundary indicated in b, showing two segregation sites$

The effect of Bi doping on the nanostructure and thermoelectric performance of the polycrystalline calcium cobaltite $C a_{3 - x} B i_{x} C o_{4} O_{9}$ ( x = 0, 0.1, 0.2, 0.3 and 0.4) is reported. The samples were prepared using a chemical sol-gel route. Increasing Bi concentrations up to x = 0.3 enhance the grain growth and improve the crystal texture. Through nanostructural and chemical analyses, significant Bi segregation at grain boundary was observed for the first time. From 318 K up to 1073 K, the Seebeck coefficient increases and the electrical resistivity decreases as Bi increases to 0.3, resulting in high power factor of 0.95 $m W m^{- 1} K^{- 2}$ at 318 K, which is so far the highest power factor for Calcium Cobaltite ceramics. Combined with low thermal conductivity of 1.9 $W m^{- 1} K^{- 1}$ , $C a_{2.8} B i_{0.2} C o_{4} O_{9}$ shows the peak ZT value of 0.43 at 1073 K. The Bi grain boundary segregation improves the texture development and acts as carrier filter in increasing the Seebeck coefficient.

Song, X., McIntyre, D., Chen, X., Barbero, E. J., & Chen, Y. (2015). Phase evolution and thermoelectric performance of calcium cobaltite upon high temperature aging. Ceramics International, 41 (9), 11069-11074. doi:10.1016/j.ceramint.2015.05.052

Morphologies of the pressed surface taken from (a) the baseline sample and (b) the
aged sample.

$TEM images of the aged sample. (a) TEM diffraction contrast image showing co-containing CaO in between the Ca3Co4O9 nano-lamellas in the aged sample; (b) the electron pattern collected from the blue-dotted circle area indicated in Fig. 4 (a), and (c) the electron pattern can be indexed as [010] Ca3Co4O9 main phase and [110] CaO secondary phase (labeled using red circles)$
TEM images of the aged sample showing Ca2Co2O5 and nano-lamella Ca3Co4O9.

TEM images of the aged sample showing Ca2Co2O5 and nano-lamella Ca3Co4O9.

Crystal phase transformation and their nanostructure evolutions were studied for baseline thermoelectric ceramic $C a_{3} C o_{4} O_{9}$ , and for a sample aged at 1283 K, which is significantly higher than the $C a_{3} C o_{4} O_{9}$ decomposition temperature of 1199 K. X-ray diffraction from both samples only reveals peaks from $C a_{3} C o_{4} O_{9}$ phase. TEM examination reveals that in the aged sample, CaO nano-phase exists between neighboring $C a_{3} C o_{4} O_{9}$ nano-lamella grains. Moreover, a small amount of $C o_{3} O_{4}$ and $C a_{2} C o_{2} O_{5}$ phase were also observed in the aged sample. The $C a_{2} C o_{2} O_{5}$ phase does not exhibit the same lamella nanostructure as $C a_{3} C o_{4} O_{9}$ and displayed few crystal defects. Except for $C a_{3} C o_{4} O_{9}$ , $C a_{2} C o_{2} O_{5}$ , and $C o_{3} O_{4}$ , no other Ca–Co–O cobaltite phase, such as $C a_{3} C o_{2} O_{6}$ , was found in the aged sample. Electrical and thermal properties of both baseline and aged sample were measured over the temperature range of 320–1033 K.

Carvillo, P., Chen, Y., Boyle, C., Barnes, P. N., & Song, X. (2015). Thermoelectric Performance Enhancement of Calcium Cobaltite through Barium Grain Boundary Segregation. Inorganic Chemistry, 54 (18), 9027-9032. doi:10.1021/acs.inorgchem.5b01296

Left image: Plots of electrical properties for the Ca3BaxCo4O9+δ (x = 0, 0.01, 0.05, and 0.1) samples. (a) Electrical resistivity ρ as a function of temperature T; (b) Seebeck coefficient S as a function of temperature T. Right image: High resolution TEM image showing the Ca3Co4O9+δ grain boundary is free of secondary phase

$Cross-sectional SEM images from the fractured surface of the pellets showing crystal texture of the samples with different Ba concentrations. (a) Ca3Co4O9+δ. (b) Ca3Ba0.01Co4O9+δ. (c) Ca3Ba0.05Co4O9+δ. (d) Ca3Ba0.1Co4O9+δ.$

We report the dramatic increase of the Seebeck coefficient S and thermoelectric performance of calcium cobaltite $C a_{3} C o_{4} O_{9 + δ}$ ceramics through non-stoichiometric addition of minute amount of Ba. The nominal chemistry of polycrystal pellets are $C a_{3} B a_{x} C o_{4} O_{9 + δ}$ ( x = 0, 0.01, 0.05, and 0.1). At 323 K, S of $C a_{3} C o_{4} O_{9 + δ}$ is 135 $μ V K^{- 1}$ , whereas S of Ba incorporated $C a_{3} B a_{0.05} C o_{4} O_{9 + δ}$ is 162.5 $μ V K^{- 1}$ , which is the highest S value near room temperature regime reported for calcium cobaltite. The increase of S for $C a_{3} B a_{0.05} C o_{4} O_{9 + δ}$ sample is accompanied by the decrease of the electrical resistivity ρ, resulting in high power factor $S^{2} / ρ$ of 843 $μ W m^{- 1} K^{- 2}$ at 1007 K. Moreover, the thermal conductivities κ of $C a_{3} B a_{x} C o_{4} O_{9 + δ}$ decrease with the increase of the Ba addition. The figure-of-merit ZT for $C a_{3} B a_{0.05} C o_{4} O_{9 + δ}$ reaches 0.52 at 1073 K and a factor of 2.5 increment in comparison with undoped $C a_{3} C o_{4} O_{9 + δ}$ . Nanostructure examinations show that the added Ba segregated at the $C a_{3} C o_{4} O_{9 + δ}$ grain boundaries, while the $C a_{3} C o_{4} O_{9 + δ}$ grain interior is free of Ba. Performance enhancement is attributed to the carrier filtering effect caused by the Ba segregation. In addition, Ba segregation promotes the better crystal alignment and the development of crystal texture.

Song, X., Chen, Y., Chen, S., Barbero, E., Thomas, E. L., & Barnes, P. (2012). Significant enhancement of electrical transport properties of thermoelectric $C a_{3} C o_{4} O_{9 + δ}$ through Yb doping. Solid State Communications, 152 (16), 1509-1512. doi:http://dx.doi.org/10.1016/j.ssc.2012.06.014

$SEM images showing the fractured cross sections of Ca3xYbxCo4O9þd, (a) x=0.1 and pressed at 0.5 GPa, and (b) x=0.3 and pressed at 2 GPa. Inserted images show morphologies of the corresponding pressed planes.$

Electrical transport properties of Ca3xYbxCo4O9þd, x=0, 0.1, 0.3 and 0.5. Columns a and b indicate the two sets of samples pressed at (a) 0.5 GPa and (b) 2 GPa, respectively

TEM image for the sample doped with 0.3 Yb and pressed at 2 GPa. Insert: HRTEM image shows the microstructure near GB region

We report the significant enhancement of the power factor of $C a_{3} C o_{4} O_{9 + δ}$ through Yb doping. The pellets were prepared by pressing under 0.5 GPa and 2 GPa. The highest power factor of 553 $μ W m^{- 1} K^{- 2}$ due to the significant increase of electrical conductivity was obtained for $C a_{2.9} Y b_{0.1} C o_{4} O_{9 + δ}$ pressed at 0.5 GPa. This is 2.3 times higher than that of $C a_{3} C o_{4} O_{9 + δ}$ (246 $μ W m^{- 1} K^{- 2}$ ). Nanostructure examinations show that the pellets pressed at 0.5 and 2 GPa have different nano-lamella structures. This work suggests that Yb is an effective doping element for enhancing the electrical transport properties of $C a_{3} C o_{4} O_{9 + δ}$ , and the optimum doping level is related to the nanostructure of the bulk pellets.

Chen, S., Song, X., Chen, X., Chen, Y., Barbero, E. J., Thomas, E. L., & Barnes, P. N. (2012). Effect of precursor calcination temperature on the microstructure and thermoelectric properties of $C a_{3} C o_{4} O_{9}$ ceramics . Journal of Sol-Gel Science and Technology, 64 (3), 627-636. doi:10.1007/s10971-012-2894-4

Temperature dependence of figure of merit ZT for the samples calcined at 923 and 1,073 K

High resolution TEM image taken from pellet made of CCO923 K powder

$C a_{3} C o_{4} O_{9}$ (CCO) powder precursors were prepared by the chemical sol–gel route and calcined at various temperatures between 923 K (CCO-923 K) and 1,073 K (CCO-1,073 K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100 nm in the powders calcined at 923 K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923 K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923 K is weakly dependent on operating temperature. Compared to the CCO-1,073 K sample, the CCO-923 K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity.