Sintering behavior and thermoelectric properties of LaCoO₃ ceramics with Bi₂O₃–B₂O₃–SiO₂ as a sintering aid

Abstract

Recently, LaCoO₃ has received increasing attention as a promising thermoelectric oxide. However, its sintering temperature is too high from the practical point of view. This study investigated the effect of Bi₂O₃–B₂O₃–SiO₂ (BBS) as a sintering aid on the low-temperature sintering behaviors, microstructures and thermoelectric properties of LaCoO₃ ceramics. The XRD results revealed that the LaCoO₃ ceramics with 1–3 wt% BBS could form a single crystalline phase without visible secondary phases. The sintering temperature of LaCoO₃ ceramics was reduced remarkably from 1200 °C to 950 °C by adding 3 wt% BBS, with a relative density higher than 95%. The process of densification and the appearance of rod-like grains in LaCoO₃ with BBS aids were observed by SEM. Meanwhile, the thermoelectric properties of LaCoO₃ ceramics were strongly dependent on the densification and microstructures. For the samples sintered at 1050 °C, the electrical conductivity increased significantly with the increase of BBS content (x, wt%) up to x = 2.0, and the Seebeck coefficients were almost the same when the test temperatures were higher than 200 °C. Moreover, the sample with x = 2.0 had the optimum thermoelectric properties with ZT = 0.05 at 400 °C, which was about 66.7% higher than that of pure LaCoO₃ ceramics sintered at 1200 °C, indicating that the addition of BBS could effectively promote the densification and further improve thermoelectric properties of LaCoO₃ ceramics without using complicated processes.

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Introduction

Thermoelectric (TE) materials have attracted much attention in recent years because of their great application potential to transfer energy between heat and electricity without mechanical devices. The efficiency of a TE material is usually described by the dimensionless figure of merit ZT, defined as S²σT/k, where S, T, σ, and k are the Seebeck coefficient, absolute temperature, electrical conductivity, and thermal conductivity, respectively. Therefore, a good TE material should have a high electrical conductivity σ, a high Seebeck coefficient S, and a low thermal conductivity k.

Most thermoelectric oxides have been investigated so far, including CaMnO₃, SrTiO₃ and LaCoO₃. LaCoO₃ system with a perovskite structure has large Seebeck coefficient (S) of 600 μV K⁻¹ in the vicinity of room temperature,¹ however, its TE properties are still very low for practical application.² Recently, many strategies to enhance the thermoelectric performance have been developed in the LaCoO₃ system, such as La-site or Co-site substitutions,^3–7 and microstructure control.^8,9 For example, Vulchev et al.⁶ evaluated the effect of double substitution with Ni and Fe on the thermoelectric efficiency of LaCoO₃. They reported that the composition with LaCo_0.8Ni_0.1Fe_0.1O₃ exhibited the best thermoelectric activity with ZT = 0.16 at room temperature mainly due to the decrease in the thermal conductivity (k) induced by the synergic effect of the double substitution. Li et al.⁸ designed a core–shell structure of LaCoO₃ ceramics to enhance its electrical conductivity while keeping low thermal conductivity, in which the Ni-doped LaCoO₃ with high Seebeck coefficient was selected as the interior, and the Ni and Sr co-doped LaCoO₃ with high electrical conductivity was used as the surface coating layer. They found that electrical and thermal transport properties could be separately controlled according to the requirement for enhancing thermoelectric properties by introducing the composite microstructure. Wang et al.¹⁰ reported that the creation of vacancies in La/O sites could effectively reduce the thermal conductivity by strong point-defect scattering, thus leading to an increase in the dimensionless figure of merit ZT of La_1−xCoO_3−y, which provided an effective pathway for the design of better thermoelectric oxides. In addition, our previous work improved the thermoelectric properties of LaCoO₃ ceramics by the combination of the ultrafine microstructure controlled by the low-temperature liquid-phase sintering and Cu²⁺ substitution provided by the B₂O₃–CuO addition.⁹ The results showed that the sintering temperature of LaCoO₃ ceramics could be reduced significantly from 1200 °C to 1000 °C by using B₂O₃–CuO additions and the ZT_max = 0.073 was obtained at 100 °C, which was about 1.5 time higher than that of pure LaCoO₃ sintered at 1200 °C. However, the optimization of the thermoelectric performance and the reduction of the sintering temperature are still challenges for the LaCoO₃ system. Thus, more sintering aids are needed to investigate in order to control the complicated microstructure with grain boundary phases and the reduced grain size, which could lower the lattice thermal conductivity by the enhanced boundaries phonon scattering, while the thermoelectric power factor (P = S²σ) is not affected.

Bi₂O₃–B₂O₃–SiO₂ (BBS) eutectic with glass transition temperature around 300 °C (ref. 11) is a kind of novel sintering aid used in the field of low-temperature co-fired ceramics.¹² In this study, the sintering behavior and the thermoelectric properties of LaCoO₃ ceramic were investigated with an emphasis on the influence of BBS in relation to the relative density, microstructure and thermoelectric properties of the sintered ceramics.

Experimental procedures

Materials synthesis

LaCoO₃ powders were prepared starting from La(NO₃)₃·6H₂O and Co(CH₃COO)₂·4H₂O through the polyacrylamide gel method as reported in our previous work.⁹ BBS additive was prepared by milling a mix of Bi₂O₃, B₂O₃ and SiO₂ with a molar ratio of 6 : 3 : 1 in a planetary agitator, and then added to the 600 °C pre-calcinated LaCoO₃ powder with different contents x (wt%, x = 0, 1.5, 2.0, 3.0). The mixed powders were milled with absolute ethyl alcohol for 24 h in a planetary agitator. After being dried and grinded, the powders were granulated with a 4 wt% polyvinyl alcohol (PVA) solution and then sieved through a 60-mesh. Finally, the mixed powers were compacted into disk-shaped specimens of 20 mm in diameter and 3–4 mm in thickness under the pressure of about 100 MPa. Sintering of the pellets was carried out in air at temperatures ranging from 950 °C to 1150 °C for 6 h with a heating rate of 5 °C min⁻¹ and being cooled inside the furnace to room temperature.

Characterization

The X-ray diffraction XRD (Rigaku D/MAX-2550V) of the sample was measured using a standard diffract meter with Cu Kα radiation as an X-ray source. Densities of the samples were measured by the Archimedes method. The morphology was studied by observing the fractured cross section of the samples using a field emission scanning electron microscope (Camscan, MX2600FE).

Thermoelectric properties were measured along a specimen surface perpendicular to the pressing direction. The electrical conductivity and Seebeck coefficient were recorded simultaneously as a function of temperature from 50 °C to 500 °C in He atmosphere by using a standard four-probe method on ULVAC ZEM-3 system. Thermal conductivity was calculated from bulk density, specific heat, and thermal diffusivity measured by a laser flash method (Netzsch, LFA457, Germany).

Results and discussion

Fig. 1 shows the relative densities of the LaCoO₃ ceramics sintered at different temperatures as function of the Bi₂O₃–B₂O₃–SiO₂ (BBS) contents x. The relative densities of all samples increase with the increasing sintering temperatures and BBS contents. It is noted that the relative density of the sample with x = 0 is still below 85% even the sintering temperature is up to 1150 °C. However, the relative density of the samples sintered at 950 °C increases from 76.3% to 95.8% as x rises from 0 to 3.0, and the samples sintered at 1000–1150 °C show the similar trend. It could also be seen that the relative densities of the samples with x ≥ 2.0 sintered above 1000 °C are around 96–98%, approaching that of pure LaCoO₃ (∼97%) sintered at 1200 °C in our previous study.⁹ This suggests an unusually fast densification process due to the formation of some BBS liquid phases in the LaCoO₃ matrix,¹³ which could enhance mass transfer and speed up the grain growth. Thus, the densification temperature of LaCoO₃ ceramics with x = 3.0 can be reduced from 1200 °C to 950 °C by the liquid phase sintering.

Fig. 1 Relative densities of LaCoO₃ ceramics sintered at different temperatures as function of the content of BBS additives.

The composition and crystallinity of the fabricated materials were checked with XRD technology. As shown in Fig. 2 for the samples sintered at 1050 °C, there are three major diffraction peaks at 32.88°, 33.30° and 47.50° observed for the LaCoO₃ crystals, which can be assigned to (110), (104) and (024) lattice planes of LaCoO₃ (JCPDS card no. 48-0123), respectively. It is determined that there is a single crystalline phase of LaCoO₃ presenting in all the specimens, without any clear evidence of secondary phases. This may be attributed to the low content of BBS in LaCoO₃ ceramics, which is difficult to be detected by XRD. Moreover, the XRD results of the other samples sintered at 950 °C or 1150 °C (not shown here) are consistent with what mentioned above.

Fig. 2 XRD patterns of LaCoO₃ ceramics with different amount of BBS additives sintered at 1050 °C.

Fig. 3 displays the SEM micrographs of the fresh fractures for the samples with x = 2.0 and x = 3.0 sintered at temperatures from 950–1150 °C. As shown in Fig. 3(a) and (b), the microstructure of the samples sintered at 950 °C indicates that the grains size of the sample with x = 2.0 are around 1.0 μm, and there are many pores existing due to an insufficient densification. With the increasing of BBS up to x = 3.0, it can be seen that the microstructure is more dense and the grain size becomes larger as well from ∼1.0 μm for x = 2.0 to ∼2 μm for x = 3.0. This grain growth and morphology change may originate from the presence of the liquid phase formed by BBS, which speeds up the particle rearrangement behavior at the early period of sintering and the mass transportation at the middle period during the sintering process.¹⁴ However, the samples sintered at 1050 °C have the different trend as showed in Fig. 3(c) and (d). The microstructure became denser while the grains size was not larger with the increase of x from x = 2.0 to x = 3.0. This can be explained by a simple power law¹⁵ as follows:

Gⁿ − G₀ⁿ = Kt (1)

where G₀ is the initial grain size after the liquid formation transients, G is the grain size during liquid phase sintering, and t is the sintering time, n is near to 3, K is the grain growth rate constant. K is normalized to the Ostwald ripening model where the grain growth rate constant is given as¹⁶where D_S is the solid diffusivity in the liquid, C is the solubility of the solid in the liquid, Ω is the solid molar volume, γ_SL is the solid–liquid interface energy, R is the gas constant, and T is the absolute temperature. The thermal stability of LaCoO₃ ceramics is excellent because LaCoO₃ is a high melting point compound. The changes of D_S, C, Ω, γ_SL are subtle with small temperature difference and a little amount of liquid phase so that the grain growth rate constant K is mainly depended on temperature. For the samples sintered at 950 °C and 1050 °C, the sample sintered at 950 °C has larger K compared to that sintered at 1050 °C according to eqn (2). Simultaneously, the sintering time t is consistent. Thus, the increase of grain size is more obvious for the samples sintered at 950 °C according to eqn (1).

Fig. 3 SEM photographs of the fractured surface of the LaCoO₃ ceramics with different BBS contents and sintering temperature. (a) x = 2.0, 950 °C (b) x = 3.0, 950 °C (c) x = 2.0, 1050 °C (d) x = 3.0, 1050 °C (e) x = 2.0, 1100 °C (f) x = 2.0, 1150 °C. The inset of Fig. 3(d) shows amplified rod-like grain structure of the circled area.

Moreover, some rod-like grains could be observed from the circled area amplified in the inset of Fig. 3(d). The reasons may be as follows: the BBS phase is around the grain boundary and the small grains of LaCoO₃ are dissolved away from contact points between adjacent solid particles separated by BBS, leading to the rod-like grain growth. Nevertheless, this is an open issue yet and needs further clarification. In addition, all LaCoO₃ samples with BBS addition show inter-granular fracture morphology, indicating that the grain boundaries are mechanically weaker than the grains. This is quite different with that of LaCoO₃ samples with B₂O₃–CuO addition in our previous work.⁹

Fig. 3(a), (c), (e) and (f) indicate the grain morphology of specimens with x = 2.0 sintered at 950 °C, 1050 °C, 1100 °C and 1150 °C, respectively. Obvious grain growth and reduction in porosity are observed for the samples with the increasing sintering temperatures. However, some pores could be found on the surface of grains, especially for the samples sintered at 1100 °C and 1150 °C, originating from the volatility of BBS at higher temperatures.

Fig. 4 shows the temperature dependence of the electrical conductivity (a), Seebeck coefficient (b), and power factor (c) for all samples sintered at 1050 °C with different BBS contents. Data for pure LaCoO₃ sample sintered at 1200 °C in our previous study,⁹ which is abbreviated as LaCoO₃(C), are given by dashed lines for comparison. As shown in Fig. 4(a), σ for all the samples increases with the measuring temperature, being indicative of semiconducting behavior. There is an obvious σ transition at about 200 °C for all samples, i.e. σ increases slightly with temperatures from 50 °C to 200 °C, however, σ increases remarkably until reaching maximum when temperature is 500 °C. This mutation of different temperature section suggests a different conductive mechanism, which is related to the temperature induced spin-state transition of Co³⁺.^17,18 In addition, the σ increases obviously with increasing x up to x = 2.0, and then decreases after a maximum value. For example, σ at 500 °C increases remarkably from about 531.21 S cm⁻¹ for x = 0 to ∼1046.5 S cm⁻¹ for x = 2.0, which is about 11% higher than that of LaCoO₃(C), and then to ∼874.5 S cm⁻¹ for x = 3.0. For the samples sintered at 1050 °C with x = 0–2.0, the porosity of the samples decreases gradually due to the formation of liquid phase and the increases of grain sizes with increasing x, resulting in the enhancement of σ. For the sample with x = 3.0, whose relative density is around 98%, the decrease of σ could be related to the segregation of thick BBS phase with low electrical conductivity at grain boundaries and appearance of rod-like grains around boundaries.

Fig. 4 Temperature dependences of electrical conductivity σ (a), Seebeck coefficient S (b) and power factor P (c) of the LaCoO₃ ceramics sintered at 1050 °C in the temperature range from 50 to 500 °C. The inset shows the plots of σ/S/P versus BBS contents x for the samples sintered at different temperatures.

Furthermore, the plots of σ tested at 400 °C versus BBS contents x for the samples sintered at different temperatures are presented in the inset of Fig. 4(a). For the samples with x = 0, the σ increases obviously from 117.2 S cm⁻¹ to 501.8 S cm⁻¹ with the increasing of sintering temperature from 950 °C to 1100 °C, which is mainly attributed to the enhancement of relative density. For the samples with x = 1.5 and x = 2.0, the σ increases with the increasing of sintering temperature up to 1050 °C and then decreases slightly. This may be attributed to the carrier scattering caused by the pores observed on the surface of grain sintered at 1100 °C and 1150 °C as shown in Fig. 3(e) and (f). For the samples with x = 3.0, the σ decreases with increasing sintering temperatures due to the volatilization of BBS and appearance of rod-like grain as shown in Fig. 3(d), which may inhibit carrier transmission or change transmission path, leading to a reduction of the electrical conductivity.

All the samples display a positive Seebeck coefficient at the measured temperature ranging from 50 °C to 500 °C as shown in Fig. 4(b), being indicative of p-type conduction with holes as the major carriers. As temperature increases from 50 °C to 200 °C, the Seebeck coefficient of samples decreases significantly, then it decreases slowly to ∼30 μV K⁻¹ as temperature increases further to 500 °C. This temperature behavior of S can be explained by the modified Heikes' equation as discussed in previous studies,^19–21 which indicates that the Seebeck coefficient depends on the degeneration of electronic states of Co³⁺ and Co⁴⁺ ions as well as the ratio between them. In addition, as temperature increases from 50 °C to 200 °C, S of pure LaCoO₃ (LaCoO₃(C) or x = 0) is higher than that of samples with BBS due to the potential barrier to filter low energy carriers,²² while they are close at temperature ranging from 200 °C to 500 °C because the holes produced by the thermally activated low-spin Co⁴⁺ from low-spin Co³⁺ are saturated.²³ Moreover, the bar chart of S at 400 °C versus BBS content x for the samples sintered at different temperatures are presented in the inset of Fig. 4(b). It could be seen that the influence of sintering temperature and BBS content for S tested at 400 °C is unobvious.

Combining the electrical conductivity and Seebeck coefficient, temperature dependence of the power factor (P = S²σ) for the samples sintered at 1050 °C are calculated and shown in Fig. 4(c). It can be seen that the P of all the samples increases with elevated temperature from 50 °C to 400 °C, and then decreases gradually. For the samples with BBS addition, P increases with x up to x = 2.0 and then decreases as x = 3.0 due to the change of σ. The highest P is 1.15 × 10⁻⁴ W m⁻¹ K⁻² measured at 400 °C for the sample with x = 2.0, which is about 94% higher than that of sample with x = 0 sintered at 1050 °C and 21% higher than that of LaCoO₃(C). Furthermore, the change trend of P at 400 °C versus BBS content x for the samples sintered at different temperatures is in accord with that of σ, as shown in the inset of Fig. 4(c).

Fig. 5 presents the temperature dependence of thermal conductivity (a) and ZT (b) for LaCoO₃ with x = 2.0 and 3.0 sintered at 950 °C, 1050 °C and 1150 °C, compared with LaCoO₃(C). All the samples shown in Fig. 5 have high relative densities around 95–97% so that the comparisons of k and ZT are reliable and intrinsically meaning. The data of the LaCoO₃(C) are also shown by a dashed line for comparison. As shown in Fig. 5(a), the thermal conductivity k of the samples shows an increasing trend as temperature increases, which contributes to the increase in heat transport by the conductive carrier at higher temperatures. In addition, over the measured temperature range, k decreases with the decrease of the sintering temperatures and k for samples with BBS sintered in the range from 950 °C to 1150 °C is much lower than that of LaCoO₃(C), which is attributed to the enhanced phonon scattering by the complicated microstructure with BBS grain boundary phases and the reduced grain size caused by the low temperature sintering.

Fig. 5 Temperature dependences of k (a) and ZT (b) for LaCoO₃ sintered at different temperatures.

In Fig. 5(b), it could be seen that the ZT of all the samples increases with elevated temperature up to 400 °C, and then decreases gradually. The ZT of the samples with BBS is much higher than that of LaCoO₃(C), which originates from the higher k of LaCoO₃(C). The maximum ZT value reaches 0.05 at 400 °C for the sample with x = 2.0 sintered at 1050 °C, which is about 24% higher than that of sample with x = 3.0 sintered at 950 °C and 67% higher than that of LaCoO₃(C) sintered at 1200 °C. Note that the LaCoO₃ ceramics with BBS sintering aid, like pure LaCoO₃ ceramics, have the maximum ZT value at 400 °C. However, most of the La-site or Co-site substituted LaCoO₃ ceramics and the LaCoO₃ ceramics with B₂O₃–CuO addition⁹ have the maximum ZT value around 100 °C. The different temperature dependence of ZT is useful for the design of functional graded thermoelectric materials. Though the maximum ZT of the samples with BBS addition is a little lower than the ZT value at 400 °C of the LaCoO₃ ceramics with B₂O₃–CuO addition,⁹ it is much higher than most of the reported ZT value at 400 °C, such as those reported by Li et al.²⁴ for LaCo_0.85Cu_0.15O_3−δ (ZT = 0.022 at 400 °C) and reported by Kun et al.²⁵ for La_0.99Ba_0.03CoO₃ (ZT = 0.025 at 400 °C). Therefore, the low temperature sintering strategy opens a new possibility with low cost for improving the thermoelectric performance of oxides. Some optimizations of composition and the added amount for sintering aids are needed to obtain dense ultra-fine or nano-structure thermoelectric oxide ceramics at lower sintering temperatures, which may have the enhanced power factor and the reduction of the thermal conductivity due to the enhancement of scattering effects originating from the higher grain boundary density.

Conclusions

The low-temperature sintering behaviors, microstructures and thermoelectric properties of the LaCoO₃ ceramics with Bi₂O₃–B₂O₃–SiO₂ addition were investigated in the temperature range from 50 °C to 500 °C in this study. The sintering temperature of LaCoO₃ ceramics was reduced remarkably from 1200 °C to 950 °C by adding BBS aids (x = 3.0), with a relative density higher than 95%, originating from the formation of a low-melting-point liquid phase of BBS. The addition of BBS in LaCoO₃ ceramics improved the electrical conductivity due to the increase of densification while decreased the thermal conductivity because of the enhanced phonon scattering by the complicated microstructure with BBS grain boundary phases and the reduced grain size caused by the low temperature sintering. For the samples sintered at 1050 °C, the electrical conductivity increased significantly with the increase of BBS content up to x = 2.0, and the Seebeck coefficient were almost the same when the test temperatures were higher than 200 °C. The sample with x = 2.0 had the maximum ZT = 0.05 at 400 °C, which was about 66.7% higher than that of LaCoO₃(C) ceramics sintered at 1200 °C, indicating that the low temperature sintering strategy provides a low cost and facile approach to improve the sinterability and thermoelectric performance of oxides.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (grant no. 50772026).

Notes and references

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