Figure 1 illustrates the XRD patterns obtained in Ca3Co4O9 + x wt.% B4C powdered samples. As it can be easily observed in the figure, all samples present the same patterns, independently of the B4C addition. Moreover, all peaks can be associated to the diffraction planes (indicated in the graph) of Ca3Co4O9 phase with monoclinic symmetry [18, 19]. Furthermore, the most intense diffractions correspond to its ab-planes, which could be associated to preferential grain orientation. However, it should be highlighted that XRD patterns were obtained on powdered samples and, consequently, this grain orientation is produced by the samples preparation, as observed in previous works . On the other hand, no B4C has been identified with this technique, probably due to its small amount, and the fact that its most intense peak appears at 39.4 degrees , together with a possible partial decomposition during sintering, and it can be masked by the (020) diffraction of Ca3Co4O9 phase.
Figure 2 presents several representative SEM micrographs performed on the samples surfaces. These images show that all samples are composed of flake-like particles with no preferential orientation, which is the characteristic situation in this system, as reported in previous works [22, 23]. Moreover, all grains composition has been determined, through EDS, as Ca3Co4O9 phase, confirming the XRD results previously presented. On the other hand, the Ca3Co4O9 grain sizes are decreased, when the amount of B4C in the samples is increased, while porosity seems to be increased. In order to confirm the raise in porosity when the B4C content is larger, the samples density was evaluated and the mean results, together with their standard error, are displayed in Table 1. As it is clear from these data, samples density is slightly decreased when the amount of B4C is increased, corroborating the SEM observations. In any case, the relative density values are in the order of the typically reported in materials prepared through the classical solid-state method [23–25]. This low densification can be explained when considering the phase equilibria diagram , which shows that Ca3Co4O9 phase is stable up to 926 ºC. Consequently, sintering procedure has to be performed below this temperature, which is, in turn, much lower than the eutectic temperature (1350 ºC), drastically limiting the samples densification.
Figure 3 presents the three point bending tests results for the Ca3Co4O9 and their standard errors, as a function of B4C content. In spite of the slight decrease of density induced by B4C addition, mechanical properties are drastically increased, when compared to the pure samples. It is increased in around 50% for 0.1% B4C addition, and more than 100% for higher contents. These results seem to be contradictory and, consequently, the fractured sections of samples were microstructurally studied. Figure 4 shows a representative micrograph at high magnification of 0.75 B4C sample. As it can be observed in the picture, besides the grey contrast (Ca3Co4O9 phase), a new contrast (dark grey) is appearing. EDS analysis has shown that this contrast contains B, C, O, Ca, and Co, and it is connecting thermoelectric grains. This behaviour can be explained when considering that B4C starts to be oxidized under air at around 450 ºC, producing CO2 and B2O3 . This effect could lead to the total B4C oxidation, however, the formed B2O3 has melting point of 450 ºC  and, between 600 and 1000 ºC, forms a protective layer on the particles surface avoiding further oxidation . On the other hand, the presence of this liquid phase, making close contact with the thermoelectric grains, lead to Ca-B-O , and Co-B-O  compounds formation on the B4C particles surface. Consequently, these compounds form bridges between the thermoelectric grains, enhancing the samples mechanical properties.
Electrical resistivity variation with temperature, as a function of B4C content is presented in Fig. 5. As shown in the plot, all samples possess very similar behaviour in the whole measured temperature range. The samples display a minimum at around 450 ºC, which corresponds to the change between semiconducting (dρ/dT < 0) and metallic (dρ/dT > 0) behaviour. This temperature indicates the change from a hole hopping from Co4+ to Co3+  to a charge carriers transport in the valence or conduction band . This is a very common behaviour observed in Ca3Co4O9 sintered materials [23, 34]. On the other hand, B4C addition up to 0.25 wt.% decreases electrical resistivity, while further addition drastically increases it, when compared to samples without additions. This behaviour with B4C is similar to the one observed in thin films, where a slightly lower Co content, with respect to the stoichiometric one, leads to lower electrical resistivity . Moreover, this effect also induces a lower grain size, which agrees with the previous observations. On the other hand, the decrease of electrical resistivity of B4C containing samples, when compared to the pure ones, is lower than the observed in , which can be associated to a lower reduction of Co content. Furthermore, the formation of B-containing phases in these samples can also affect the electrical transport properties. Consequently, the lowest electrical resistivity values have been obtained in samples with 0.25 wt.% B4C. The minimum resistivity values measured in these samples at 800 ºC (14 mΩ cm) are slightly lower than the best values reported for Ca3Co4O9 samples sintered or textured through spark plasma sintering (15–18 mΩ cm)  and around 20% lower than in pure samples. However, they are higher than the measured in highly dense materials prepared using alternative methods (10 mΩ cm) .
Figure 6 shows the S variation with temperature and B4C content. In all cases S shows positive values, indicating that conduction is mainly produced by holes. Moreover, the behaviour is the same for all samples, the values are increased when the temperature rises, and these values are the same for all samples within the measurement errors. These very close S values, independently of B4C content can be explained by the very small compositional modification produced in the samples discussed previously. Furthermore, the cations drainage induced by the liquid B2O3 is probably limited to the zones close to the grain boundaries, maintaining the core of the grains unchanged. Consequently, no significant modifications in S values should be expected. The highest values at 800 ºC (195 µV/K) are higher than the reported in pure Ca3Co4O9 sintered or textured through spark plasma sintering (170–175 µV/K) , but lower than the reported in very dense materials obtained by alternative methods (205 µV/K) .
Total κ consists in the addition of two components, κ = κl + κe, where κl is the lattice thermal conductivity contribution and κe is the electronic counterpart. κe can be estimated from the Wiedemann-Franz’s law , which is expressed as κe = L σ T, where L, and σ are Lorenz number (2.45 × 10− 8 V2/K2), and electrical conductivity, respectively. The variation with temperature and B4C content of the electronic and total thermal conductivity are presented in Fig. 7. When comparing both graphs, it is clear that electronic thermal conductivity is increased when the temperature rises. In contrast, the total thermal conductivity decreases from room temperature to around 600–650 ºC induced by the increase of lattice vibrations which enhance phonon scattering. At higher temperatures, the total thermal conductivity increases due to the larger influence of electronic thermal conductivity. Moreover, B4C addition leads to a clear decrease of total thermal conductivity, when compared to the pure samples, especially at high temperatures. These results are in agreement with previous works in Ca3 − xBxCo4O9 which showed that thermal conductivity is decreased with B-content up to 0.5 . At 800 ºC, the minimum total thermal conductivity value determined in 0.25 wt.% B4C samples (1.2 W/K m) is about 20% higher than the lowest measured in these samples (1.0 W/K m at 650 ºC). Moreover, these values are comparable to the reported in classically sintered materials (0.9 W/K m with ∼ 60% theoretical density) which show higher amount of porosity  than the samples in this work. On the other hand, it is lower than the best values obtained by spark plasma sintering (2.1 W/K m) , and much lower than the measured in textured materials by hot-uniaxial pressing (2.2, 3, and 4.7 W/K m) [13, 40, 42].
With all the previously discussed data, ZT has been calculated and plotted in Fig. 8. As displayed in the graph, ZT values are increased with temperature in the whole measured temperature range, and also with B4C content up to 0.25 wt.%, decreasing for higher additions. The highest value has been measured at 800 ºC in 0.25 wt.% B4C samples (0.24), which is more than two times higher than the calculated for pure ones. Moreover, it is higher than the measured in classically (0.08 ), or spark plasma sintered samples (0.15 ), and even than the determined in textured samples (0.18, and 0.16 [13, 40]). However, this value is lower than the reported one for materials prepared through non-conventional methods (0.35 ) or in Ca3 − xSrxCo4O9 textured materials (0.29 ).
Thermal expansion coefficient has been determined in all samples and presented in Table 2. As it can be observed in this table, thermal expansion of pure samples (10.32 ppm/K) is in the range of the reported value for Ca3Co4O9 materials (10.6 ppm/K ) and lower than the measured in Bi-substituted materials (12.8 ppm/K ). On the other hand, B4C addition, up to 0.25 wt.%, decreases thermal expansion coefficient, increasing with further additions. The minimum value (9.3 ppm/K) measured in this work corresponds to a decrease of only around 10% but closer to the one reported for Al2O3 (7.5 ppm/K ). This reduction, even if it does not seem to be significant, may reduce the differential thermal expansion between two of the main components of thermoelectric modules, decreasing the internal stresses at working temperatures, and increasing the life span of these devices.