As a functional material used for the conversion between thermal and electric energy, thermoelectric materials are investigated widely.^[1–4] The electrical transport is usually evaluated by the power factor α²/ρ, where α and ρ are the Seebeck coefficient and the electrical resistivity, respectively.^[5]

Ca₃Co₄O_9+δ has attracted continuous attention because of both the coexisting high Seebeck coefficient and thermal stability.^[6] The CCO is composed of two misfit-layered subsystems of CoO₂ layer and Ca₂CoO₃ layer along the c axis.^[7] Co³⁺ and Co⁴⁺ have three kinds of spin states, respectively, including low spin state (LS), intermediate-spin state (IS) and high spin state (HS).^[8] The special structure as described above makes it possible to optimize electrical conductivity and Seebeck coefficient simultaneously.^[9]

The rare earth substitutions at Ca-sites are effective in improving the Seebeck coefficient α. For instance, Nong et al. reported that the α values of CCO can be enhanced significantly by rare earth substitution at Ca-site, such as Lu, Dy and Ho.^[10]

Wang et al. reported that Sm doping can effectively improve thermoelectric properties of CCO materials.^[11] The substitutions of the high valence ions for calcium sites may improve α from the reduced number p of holes in p-type semiconductor. To consider the similar ionic radius and average higher valence between Sm ions and Ca ions, we choose to utilize Sm elements to substitute Ca sites.

The misfit layered structure of CCO shows highly anisotropic properties, which leads to high resistivity for randomly oriented poly-crystalline materials. To decrease resistivity, many efforts have been taken to enhancement of the textured degree. Most of them improve the textured extent of ceramic materials by spark plasma sintering or hot pressing.^[12,13] Additionally, Torres and Chen et al. have gained textured CCO materials by reducing the sizes of precursor powders.^[14,15] Recently, more and more attentions focus on tuning physical properties of materials by inducing magnetic field in prepared process. Due to the CCO’s magnetic anisotropy and the coexistence of different spin states, applying uniform magnetic field is more helpful to form textures. Huang et al. reported that the textured degree of CCO can be obviously improved by applying high magnetic field (4 T and 8 T) in the sample sintering process.^[9] However, in such a strong magnetic field environment it is difficult to obtain CCO generally and the cost is expensive. Several reports have described that the textures may be produced and the spin can reorient by inducing weaker magnetic field during the process of chemical reaction.^[16,17] For example, Du et al.^[18] synthesized Fe₃O₄ nanoparticles with orderly arrangement under 0.4 T magnetic field. In addition, the defect splitting and electron pumping towards higher energy level can happen as magnetic field reach 0.1 T in CCO materials.^[19]

Therefore, we prepared the precursor powders of Ca_3–xSm_xCo₄O_9+δ firstly by sol-gel method under 0.5 T uniform magnetic field. Secondly, the CCO powders were cold pressed to pellets in 0.3 T uniform magnetic field. Finally, the above pellets were further hot-pressed and sintered. The effects of Sm doping and magnetic field utilizing on the structure and electric properties of CCO ceramics are studied in detail.

To synthesize Ca_3–xSm_xCo₄O_9+δ powders, we used distilled water to dissolve nitrates of Ca, Sm and Co. Drop 1 mol/l citric acid into the above nitrate solution slowly until the pH value equals 2. In 0.5 T magnetic field, the above mixed liquid was stirred at 353 K until forming colloid, and then the colloid was dried. To decompose the organic molecules and nitrates, the dry gel was burned by self-propagating method. The precursor was annealed for 6 h at 1073 K to form CCO powders. The grinding CCO powders were cold pressed firstly in magnetic field (0.3 T). The dense CCO bulks were prepared by further hot-pressing at 493 K for 1.5 h (40 MPa), and then sintering at 1123 K for 12 h. For convenience, the x = 0–0.25 Sm-doped samples are named as CCO, CSCO5, CSCO10, CSCO15 and CSCO25, and the corresponding samples prepared under magnetic field are named as CCOM, CSCO5M, CSCO10M, CSCO15M and CSCO25M, respectively.

The crystal structure of the synthesized Ca_3–xSm_xCo₄O_9+δ samples was measured (Fig. 1(a)) by an x-ray diffractometer (Rigaku, D/MAX 2200). Cu Kα radiation is used at 40 kV and 100 mA. X-ray photoelectron spectroscopy (XPS) measurements were performed on a KRATOS (Axis Ultra^DLD) electron spectrometer at a base pressure of 9.8 × 10⁻¹⁰ Torr using Al Kα radiation (hν = 1486.6 eV).

Fig. 1.

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	Fig. 1. Schematic diagrams of the samples for the measurements of (a) x-ray diffraction, (b) electric resistivity, (c) Seebeck coefficient, and (d) Hall coefficient.

The morphological characteristics were tested by a scanning electron microscope (SU-70), and the energy dispersed spectrum (EDS) was measured. We measured the density using the Archimedes principle. The samples with the size of 13 mm × 2 mm × 2 mm were used for testing resistivity, which were carried using the conventional DC four-probe method (Fig. 1(b)). According to the relation α = ΔE/ΔT, α values were obtained by testing the thermal voltage ΔE and the temperature difference ΔT through the bar bulk as shown in Fig. 1(c). We simultaneously measured α and ρ in quartz protection tube. At room temperature, the amount of hole p and mobility μ were determined by Hall instrument (HL5500PC). The used four-probe method is shown in Fig. 1(d).

The x-ray diffraction patterns of the Sm-doped samples are presented in Fig. 2(a). As x ≤ 0.15, only CaCo₄O_9+δ (PDF#21-0139) phase was detected, which is in agreement with Ref. [20]. With further increase of x to 0.25, weak Sm₂O₃ peaks (PDF#43-1029) can be observed (Fig. 2(b)). As displayed in Fig. 2(c), a small shift towards lower 2θ angle appears with the increasing Sm concentration from 0 to 0.15, which suggests that Sm ions with average larger radii substitute Ca sites (reference Bragg equation 2d sinθ = nλ). As well known, Sm²⁺ (1.22 Å) is larger and Sm³⁺ (0.96 Å) is smaller compared with Ca²⁺ (0.99 Å). Thus, as x ≤ 0.15, more Sm²⁺ with larger radius occupied Ca-sites. Increasing x from 0.15 to 0.25, the peaks shift towards higher angles, which is attributed to the fact that more and more Sm³⁺ ions substitute Ca sites. This will be further discussed in XPS part (Fig. 3). The peaks shift towards lower angles from the samples obtained under external magnetic field, which mostly results from the different radii of cobalt ions as analyzed in Ref. [21].

Fig. 2.

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	Fig. 2. (a) XRD patterns of Sm-doped bulk materials, (b) the enlarged partial diffraction peaks of the CSCO25 sample, (c) the enlarged (004) diffraction peaks of the samples studied.

Fig. 3.

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	Fig. 3. XPS spectra of Sm 3d of CSCO10, CSCO15 and CSCO25.

Table 1 shows the lattice parameters of Sm-doped samples obtained by the least-square method.^[22] With the increasing Sm concentration, the lattice parameters increase slightly due to larger ionic radii of Sm²⁺ than that of Ca²⁺. Compared with the CSCO15 (M) samples, with further increase of x to 0.25, the lattice parameters decrease conversely. In addition, the lattice parameters also increase with using external magnetic field.

Table 1.

Table 1.

Table 1. Summaries of lattice parameters, Lotgering factor F and relative density (RD) of the Sm-doped samples. . Samples Composition Abbreviation a/Å b/Å c/Å β/(°) F RD/% Without magnetic field Ca3Co4O9+δ CCO 4.8232 4.5406 10.7739 98.0025 0.496 87.65 Ca2.95Sm0.05Co4O9+δ CSCO5 4.8293 4.5569 10.7823 98.1932 0.462 81.38 Ca2.90Sm0.10Co4O9+δ CSCO10 4.8429 4.5702 10.8871 98.2004 0.424 79.96 Ca2.85Sm0.15Co4O9+δ CSCO15 4.8436 4.5769 10.9304 98.3475 0.386 77.53 Ca2.75Sm0.25Co4O9+δ CSCO25 4.8234 4.5635 10.7783 98.0468 0.307 75.51 With magnetic field Ca3Co4O9+δ CCOM 4.8236 4.5411 10.7816 98.0085 0.513 92.71 Ca2.95Sm0.05Co4O9+δ CSCO5M 4.8338 4.5600 10.9039 98.3474 0.497 86.44 Ca2.90Sm0.10Co4O9+δ CSCO10M 4.8816 4.5746 10.9905 99.0365 0.487 81.58 Ca2.85Sm0.15Co4O9+δ CSCO15M 4.8890 4.5882 11.0766 99.2068 0.474 79.35 Ca2.75Sm0.25Co4O9+δ CSCO25M 4.8298 4.5680 10.7919 98.0701 0.461 78.14

Table 1. Summaries of lattice parameters, Lotgering factor F and relative density (RD) of the Sm-doped samples. .

Lotgering factor F was explored on that basis of x-ray diffraction data, which was summarized in Table 1. Generally, F was expressed as F = (P – P₀)/(1 – P₀), P = ΣI(00l)/ΣI(hkl). Here P is counted from the prepared samples and P₀ is obtained from the standard card.^[23] Obviously, the F values for the bulks prepared in magnetic field are larger. With the increasing Sm doping concentration, the degree of textures decreases.

The interest to the samarium compounds gives rise due to the existence of mixed valence state of samarium ions. According to Brunckova et al.,^[24] the XPS spectrum of Sm 3d consists two components related to Sm³⁺ (4f⁵) and Sm²⁺(4f⁶). To identify the valence state of samarium ions, the XPS investigation is carried out. For the x = 0.05 sample, the Sm concentration is too low to identify the diffraction peaks of Sm ions. Thus, the XPS spectra of the Ca_3–xSm_xCo₄O_9+δ (x = 0.1–0.25) samples are given in Fig. 3. For the x = 0.1 sample (CSCO10), only the strongest peak of Sm²⁺ (3d_5/2) located at about 1071.0 eV is observed.^[24,25] As x = 0.15, the peak of Sm²⁺ (3d_5/2) has dropped and the strongest peak of the Sm³⁺ component located at 1083.0 eV (3d_5/2) is identified clearly. With further increase of x to 0.25, the peak of Sm²⁺ (3d_5/2) nearly disappears and the doublet of the Sm³⁺ component located at 1083.0 eV (3d_5/2) and 1109.4 eV (3d_3/2) is detected obviously.^[24–26] The Sm 3d states are split into two lines Sm 3d_5/2-Sm 3d_3/2 with energy difference 26.4 eV, which is due to the spin orbit interaction. Thus, as x = 0.1, Sm²⁺ is the dominant substituted component at Ca²⁺ site. With the increasing Sm concentration x from 0.15 to 0.25, more and more Sm³⁺ ions substitute for Ca²⁺ ions. The results are well in accord with the analysis of previous XRD and lateral holes concentration data.

Figure 4 gives the fractured scanning pictures of the Ca_3–xSm_xCo₄O_9+δ samples. Due to the CCO’s layer-structural feature, laminated microstructure is clearly seen as shown in Figs. 4(a)–4(g). With the increasing x, less laminated structures are observed, and the average crystal size decreases (CCO: 1–2 μm; CSCO5: ∼1 μm; CSCO15: ∼0.8 μm). In the samples prepared under magnetic field, the degree of orientation and grain sizes are increased obviously. Table 1 also gives the relative densities. When taking theoretical density as 4.94 g/cm³, the relative densities of the samples rang from 87.65% to 75.51% with the increasing Sm concentrations x from 0 to 0.25. However, relative densities of the bulk ceramics prepared in magnetic field are larger. The EDS (Fig. 4(h)) displays that the Sm-doped sample has all the expected elements. The quantitative analysis of the spectrum reveals that the atomic ratio of the bulk is very close to the (Ca + Sm):Co:O = 9 : 12 : 28 (the inset in Fig. 4(h)).

Fig. 4.

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	Fig. 4. Fractured cross-sectional SEM images of bulk materials: (a) CCO, (b) CCOM, (c) CSCO5, (d) CSCO5M, (e) CSCO15, (f) CSCO15M, (g) CSCO15, and (h) EDS of CSCO15.

The electrical resistivity ρ of Ca_3–xSm_xCo₄O_9+δ bulk materials is plotted in Fig. 5(a). With the increasing Sm concentration, the ρ increases monotonously. The ρ can be described by 1/ρ = peμ,^[27] where p is the amount of holes and μ is mobility. Thus, the increase of ρ with the increasing x is mainly from the decrease of both the p and μ (Figs. 5(b) and 5(c)). With Sm increasing, the decrease of p means that Sm³⁺ has partly substituted Ca ions. The reduction of μ is owed to the declines in texture, relative density and grain size. Thus, the increase of ρ comes from the combined effects among the holes concentration, grain size, density and extent of texture. Compared with other lanthanide components, Sm doping has almost the same influence on the ρ of CCO ceramic materials.^[10] However, the ρ values for the bulk materials fabricated in magnetic field are smaller. For example, at 323 K, the ρ = 15.56 mΩ⋅cm (CSCO15M) is much lower than that of CSCO15 (ρ = 19.42 mΩ⋅cm).

Fig. 5.

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	Fig. 5. Electrical properties of Sm-doped CCO: (a) electrical resistivity ρ versus temperature T, (b) and (c) relationship of hole concentration p and mobility μ with doping amount x.

Figure 6 gives the changes of ln(T/ρ) with 1000/T of Ca_3–xSm_xCo₄O_9+δ. As T > 673 K, the CCO system complies with the small polaron hopping conduction (SPHC) model, which is in agreement with Ref. [28]. According to the SPHC model, the resistivity ρ is depressed as functions of hole concentration p, activation energy E_a and temperature T:^[27]

Fig. 6.

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	Fig. 6. Changes of ln(T/ρ) with 1000/T for Sm-doped bulk materials.

Figure 7 shows the change of activation energy versus the Sm concentration, and the temperature range is 673–1073 K. The activation energy increases from 70.87 meV to 94.57 meV with increasing x from 0 to 0.15, which is due to the fact that Sm impurities in the Ca₂CoO₃ layer would disturb the conduction route. On the other hand, the activation energy reduces slightly with using external magnetic field, which is mainly from the energy that has been stored in the samples during the preparing process.

Fig. 7.

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	Fig. 7. Variations of activation energy with the Sm-doping content.

Figure 8 gives the Seebeck coefficient α of the Sm-doped samples. The specimens show positive α values, confirming the p-type semiconductor behavior. The α increases from 109.9 μV/K (CCO) to 136.3 μV/K (CSCO15) at 423 K. According to the Mott formula,^[29] α of the CCO system is described by the amount of hole p,

Fig. 8.

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	Fig. 8. Changes of Seebeck coefficient α with temperature.

In the Ca₃Co₄O_9+δ system, α may be expressed by the modified Heikes equation:^[31]

The power factor PF for all the specimens increases with temperature because of the enhancement in both 1/ρ and α (Fig. 9). It shows that Sm-doping can evidently enhance PF, especially at higher temperatures. For example, at 1073 K, the PF of CSCO15 could reach up to 0.16 mW⋅m⁻¹⋅K⁻², which exceeds 23.86% compared with CCO. The result is well accord with the report by Wang et al.^[11] Applying external magnetic field can cause significant increases in the both 1/ρ and α, therefore the PF is improved remarkably. Among all the Sm-doped series, CSCO15M has the largest PF value (0.20 mW⋅m⁻¹⋅K⁻² at 1073 K). The conclusion agrees with the report by Huang et al.^[9] However, Huang et al. mainly studied the effects of magnetic field in the sample sintering process on the thermoelectric properties of CCO in the temperature range of 2–350 K.

Fig. 9.

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	Fig. 9. Changes of power factor with temperature.

Ca_3–xSm_xCo₄O_9+δ bulk materials with high orientation have been obtained by the inducement of magnetic field, and the structure and electrical performance are studied in detail. The results show that samarium doping can cut down the texture and the number of the holes. Thus, the 1/ρ decreases and α increases, which results in high PF. Utilizing magnetic field to synthesize Ca_3–xSm_xCo₄O_9+δ bulk materials can improve the PF effectively. The present study describes that the electrical properties of Ca₃Co₄O_9+δ can be enhanced by applying magnetic field and doping samarium.