A series of Bi

Republic of China

2 Department of Physical Science and Engineering, Yulin Normal University, Yulin 541004, People’s Republic of China

3 Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

4 Author to whom any correspondence should be addressed E-mail:gracylady@163.com

Keywords:Bi_1−xSr_xMnO₃, solid-state reaction, hot press, thermoelectric properties

Abstract

A series of Bi

_1−x

Sr

x

MnO

3(x=0.40, 0.45, 0.50, 0.55)

samples labeled as BSMO040, BSMO045, BSMO050, and BSMO055, respectively, have been fabricated by the modified solid-state reaction method. The crystal structural, microstructures, and chemical states of the elements and the thermoelectric properties were investigated with respect to the partial substitution of Sr

²⁺

for Bi

³⁺

. The samples were characterized by x-ray diffraction

(XRD)

at 723 K, scanning electron microscopy

(SEM), and x-ray photoelectron spectroscopy(XPS). Moreover, their electrical conductivities(σ),

Seebeck coefficients

(S), and thermal conductivities(κ)

were determined. All the samples exhibited orthorhombic structure. The partial substitution of Sr

²⁺

for Bi

³⁺

caused valence shift of some Mn ions from

+3 to+4 to maintain electric charge balance. The change in electric charge led to an increase in

electron concentration, and thus, the electrical conductivity as well as the absolute value of Seebeck coefficient increased. Consequently, the power factor also increased. The highest power factor

(0.3×10⁻⁴

Wm

⁻¹

K

⁻¹)

was obtained for BSMO055 at 1023 K. Moreover, the highest dimensionless

figure-of-merit(ZT)

obtained in this study was 0.015 for BSMO055 at 1073 K. It can be concluded that the partial substitution of Sr

²⁺

for Bi

³⁺

in the Bi

_1−x

Sr

x

MnO

3

samples

(x=0.40, 0.45, 0.50, and 0.55)

improved the thermoelectric properties effectively.

1. Introduction

In recent years, thermoelectric materials, which can directly and reversibly convert heat energy into electricity, have become a hot topic[1–5]. The performance of thermoelectric materials is characterized by the

dimensionlessfigure-of-merit, defined asZT=S Ts² k,whereσ,S,κ, andTare the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively. In order to achieve high energy conversion efficiency of thermoelectric materials, a high value ofZTis required. Thus, the thermoelectric materials should have highσandS, and a lowκ. However, the three parameters are interrelated. Besides, a good thermoelectric material for commercial application should have highZT, good stability, low cost, and safety.

In the past several decades, researchers have developed thermoelectric materials with relatively highZT, such as those based on Pb[6,7], Cu2Se[8–12], SnSe[13,14], NaxCoO2[15–17], and Ca3Co4O9[18,19]. In addition, new thermoelectric materials with highZTcomposed of naturally abundant nontoxic elements have been explored, especially the oxide thermoelectric materials. The oxide thermoelectric materials having high durability at high temperatures in air, non-toxicity, low cost, and minimal environmental impact have attracted the researchers’attention[20].

Many oxides have been studied such as NaxCoO2[15–17], Bi2Sr2Co2Ox[21], SrTiO3-based oxides[22–24], CaMnO3-based oxides[25,26], ZnO-based oxides[27,28], and In2O3-based oxides[29,30]. The currently

18 April 2018

© 2018 IOP Publishing Ltd

researched oxides have a general shortcoming of low electrical conductivity. Therefore, most researches focused on improving the electrical conductivity of oxide thermoelectric materials.

In n-type materials, the dominant charge carriers are electrons. Therefore, increasing the electron concentration, either by doping or by other strategies, is the main way to increase the electrical conductivity of oxides[31,32]. Among the oxides, ABO3compounds(A: alkaline earth metal, B: transition metal)with a perovskite structure(figure1)have received much attention for their potentially useful electrical properties.

BiMnO_3,an n-type semiconductor at high temperatures, has a perovskite structure and a narrow band gap (Eg=1.23 eV)[33], indicating good electrical conductivity. It has been extensively studied in the ferroelectricity field[34–38]. However, there are few researches on the thermoelectric properties of BiMnO3-based materials. In the present study, we replace the Bi³⁺ions in BiMnO3by Sr²⁺ions to induce more n-type carriers to achieve higher electrical conductivity and Seebeck coefficient.

2. Experimental

2.1. Sample preparation

Bi_1−xSrxMnO3(x=0.40, 0.45, 0.50, and 0.55)samples designated as BSMO040, BSMO045, BSMO050, and BSMO055, respectively, were prepared by the modified solid-state reaction technique using Bi2O3(Kermel,

>99.9%), SrO(Alfa Aesar,>97%), and MnO(Alfa Aesar,>99%)as the starting powders. The powders were weighed in stoichiometric ratios, and the mixture of Bi2O3, SrO, and MnO powders with ethyl alcohol was milled for 10 h in a planetary mill(Nanjing University)using ZrO2balls as the grinding media. The weight ratio of ZrO2balls and the powders was 3:1. The obtained slurries were dried at 323 K for 12 h. The dried powders were calcined for 2 h in a ceramic crucible at different temperatures: 1073 K for BSMO030 and BSMO035, 1133 K for BSMO040, and 1173 K for BSMO045, BSMO050, and BSMO055. The calcination temperature increased with increasing Sr²⁺content in the Bi1−xSrxMnO3samples. Thereafter, the calcined powders were ground in a high-energy ball-milling machine(SPEX 8000M Mixer/Mill, Metuchen, NJ, USA)for 10 h. The as- prepared powders were placed into a columnar chamber die in air and sintered using a hot press(DCHP-2000A- 03, HLSTAR, USA)under a pressure of 80 MPa. The sintering temperature was 1073 K for all the samples. the holding time was 3 min. Finally, ingots with 12.7 cm diameter were obtained.

2.2. Characterization

The crystal structures of samples were analyzed using an x-ray diffractometer(XRD, Bruker, D8 Advance, Germany)at 723 K with Cu K_αradiation at 40 kV and 40 mA. The microstructures of the as-sintered samples were observed with afield emission scanning electron microscope(FE-SEM, Zeiss Ultra 55, Germany), and the elemental chemical states were investigated by x-ray photoelectron spectroscopy(XPS, Thermofisher, K-Alpha, America). Bars(2 mm×2 mm×12 mm)and disks(12.5 mm in diameter and 2 mm in thickness)were cut and polished forσ,S(ZEM-3, Ulvac-Riko, Japan), andκ(LFA457, Netzsch, Germany)measurements in the temperature range from 723 K to 1073 K.ZTvalues were then calculated using the measured values of the aforementioned properties.

Figure 1.Perovskite structure of ABO₃compounds. The small B cation(red sphere)is at the center of an octahedron of oxygen anions (green spheres), and the large A cations(blue spheres)occupy the unit-cell corners.

3. Results and discussions

3.1. Phase analysis

Figure2shows the XRD patterns of BSMO040, BSMO045, BSMO050, and BSMO055 at 723 K. All of the samples crystallize into an orthorhombic structure. The impurity in BSMO040 is only Mn3O4. BSMO045, BSMO050, and BSMO055 have pure phases.

3.2. SEM measurement

The SEM images of the sintered samples are shown infigure3. It is obvious that the grain size decrease with increasing doped Sr²⁺content. The average grain sizes of BSMO040, BSMO045, BSMO050, and BSMO055 are around 300, 250, 200, and 100 nm, respectively. The ionic radius of Sr²⁺(0.112 nm)is larger than that of Mn³⁺ (0.0645 nm)and Mn⁴⁺(0.053 nm); thus, the grain sizes should increase. However, surprisingly, it shows an opposite trend. It is possible that the doped Sr inhibits the growth of particles of the BSMO system[39]. And in [39], it also points out that the addition of alkaline earth metals suppresses the necking of grains due to reduced grain boundary mobility and decreases the grain size. Besides, the grain sizes gradually become more uniform, which maybe because the doping of Sr.

Figure 2.XRD patterns of Bi_1−xSr_xMnO₃samples at 723 K.

Figure 3.SEM images of Bi_1−xSr_xMnO₃samples:(a)BSMO040,(b)BSMO045,(c)BSMO050, and(d)BSMO055.

3.3. Elemental chemical states of as-sintered samples

XPS analysis reveals the chemical states of Mn and Bi ions in the representative samples. The Mn 2p3/2and Bi 4f_7/2core level spectra of the different specimens are shown infigures4(a)and(b), respectively. Infigure4(a), the peaks around 640.7–640.9 eV are assigned to Mn³⁺[40], while those around 642.2 eV and 643.2 eV are attributed to Mn⁴⁺; these are in good agreement with the results reported in[37]. Further, the peak at 644.1 eV is attributed to Mn3O4[41]. These are consistent with the existing of Mn3O4in BSMO040 in XRD test. The peak

Figure 4.Mn2p_3/2(a), Bi4f_7/2(b)core level spectra, and survey spectra of Mn2p(c)of the as-sintered samples.

areas of Mn⁴⁺for different specimens increase with increasing doped Sr²⁺content, indicating that more Mn³⁺ cations convert into Mn⁴⁺cations with increase doped Sr²⁺content. Infigure4(b), the peaks at

158.2∼158.4 eV correspond to Bi³⁺. These values are comparable to those in[42]. The small differences between the peak values arise from structural differences. The peaks confirm that Bi is present only in+3 state, and not in+5 state. The charge imbalance is compensated by Mn³⁺, Mn⁴⁺. Moreover, infigure4(c), a satellite peak around 647.33 eV confirms the presence of mixed state of Mn³⁺and Mn⁴⁺[37].

3.4. Thermoelectric properties 3.4.1. Electrical conductivity

The temperature dependences of the electrical conductivity of BSMO040, BSMO045, BSMO050, and BSMO055 are shown infigure5. The electrical conductivities of the samples increased with increasing temperature

indicating a typical semiconducting behavior. It got a maxim value around 1.0×10⁴S/m and 1.2×10⁴S/m at 923 K for BSMO040 and BSMO045, 1.6×10⁴S/m and 1.5×10⁴S/m at 973 K for BSMO050 and BSMO055, respectively. These maxim values are higher than those CaMnO3-based oxides[25,26]. And then their electrical conductivities decline due to the scattering between electrons and the scattering by phonons. Furthermore, the electrical conductivities of different samples increased with increasing doped Sr²⁺content except for BSMO050.

This is mainly because of the partial substitution of Sr²⁺for Bi³⁺, which causes a valence shift of some Mn ions from+3 to+4 state to maintain the electric charge balance thereby increasing the carrier concentration and promoting electron transport between Mn³⁺and Mn⁴⁺[43]. This significantly increases the electrical

conductivity. Infigure4(a), the Mn2p3/2spectra show the presence of Mn³⁺and Mn⁴⁺. With increasing doped Sr²⁺content, the more Mn³⁺convert into Mn⁴⁺, the charge carrier concentration become higher. Therefore, the electrical conductivities increase with increasing doped Sr²⁺content. We can see that the electrical conductivity of BSMO055 is lower than that of BSMO050. As we known, the electrical conductivity(σ)can be expressed by the following equation:σ=neμ, wherenis the carrier concentration,eis the electron charge, and μis the carrier mobility. Thus it needs high carrier concentration and mobility to obtain high electrical

conductivity.μis defined as:m= ^t_*,

m

e whereτis the time between scattering events of electron carriers,m^*is the charge carrier effective mass. On the one hand, doping of Sr²⁺leads to a decrease in density and grain size(see SEM images infigure3); soτdecreases; On the other hand, with increasing doped Sr²⁺content, the more Mn³⁺ convert into Mn⁴⁺,m^*become heavier, as reported in[44]. The two factors lead the electron mobility to decrease. As discussed above, BSMO055 has a smallest grain size and heaviest carrier. This is the reason why the electrical conductivity of BSMO055 is lower than that of BSMO050.

Besides, the magnitude of increase in electrical conductivity of BSMO040 is much smaller than the

magnitude of increase in electrical conductivity of BSMO045. This is related to the low charge concentration and impurity which scatter the charge carriers in BSMO040.

3.4.2. Seebeck coefficient

Figure6shows the temperature dependence of Seebeck coefficients in the range of 723–1073 K. The sign of Seebeck coefficient for BSMO040, BSMO045, BSMO050, and BSMO055 is negative over the entire temperature range, indicating n-type conduction. For each sample, the absolute value of S increased as temperature rises.

Based on the Mott equation[45,46]:







 p

m

= + m

=

( ) ( )

S k T

e n dn E

dE

d E dE 3

1 1

B ,

E E 2 2

F

wherekBis the Boltzmann constant,Eis the carrier energy,EFis the Fermi energy, ande,n, andμhave been defined previously. From thefirst term in the Mott equation, it is believed that|S|for a semiconductor should

decease as the carrier concentration increase because of thermal excitation. Moreover,|S|also depends on the second term which is related to the mobility. So, the absolute value ofSincreases as temperature rises in Bi1−xSrxMnO3, which is not due to the carrier concentration, but due to the mobility which needs to be further studied. And the absolute value ofSincreased with increasing doped Sr²⁺content. For a heavy doped

semiconductor[47]:











p * p

S= m k T

eh n

8

3 ^B 3 ,

2 2

2 3

wherehis Planck constant,kB, ande,n,m^*,Thave been defined previously. That isS∝m^*. As discussed previously, with increasing doped Sr²⁺content, the more Mn³⁺convert into Mn⁴⁺,m^*become heavier. So|S| increases with increasing doped Sr²⁺content, as reported in[44].

3.4.3. Power factor

The power factors are shown infigure7. The power factor of each sample increased with increase in temperature as well as with increasing Sr²⁺content, which is related to the increase in electrical conductivity and Seebeck coefficient. Although the electrical conductivity of BSMO050 is higher than that of BSMO055, but the Seebeck coefficient of BSMO050 is lower than that of BSMO055. So the highest value of power factor

(0.3×10⁻⁴Wm⁻¹K⁻¹)is obtained in BSMO055 at 1023 K, which is comparable to that in[48]. It is nearly three times larger than that of BSMO040 at 1023 K. This implies that the partial substitution of Sr²⁺for Bi³⁺is effective for improving the power factor of BiMnO3.

3.4.4. Thermal conductivity

Figure8shows the thermal conductivity as function of temperature. With increasing temperature, the thermal conductivity of each sample increased gradually. In addition, the thermal conductivities of the samples also increased with increasing doped Sr²⁺content except for BSMO050. The thermal conductivity of BSMO055 is lower than that of BSMO045 and BSMO050 owing to the decrease in density and grain size with increasing doping content. The grain size of BSMO055 is the smallest; thus, it has the most grain boundaries and the lowest density compared to other samples. The grain boundaries serve as carriers and phonons scattering centers, thus decrease the thermal conductivity. Generally, the thermal conductivity can be expressed asκ=κph+κel, whereκphis the lattice thermal conductivity andκelis the electronic thermal conductivity, which can be estimated from the Wiedemann-Franz law as follows:κel=L0σT, whereL0is the Lorenz number

Figure 6.Seebeck coefficients of Bi_1−xSrxMnO3samples as function of temperature.

Figure 7.Power factors of Bi_1−xSr_xMnO₃samples as a function of temperature.

(L0=2.44×10⁻⁸V²/K²). The value ofκelis less than one-tenth of the totalκ. So the thermal conductivity is dominated by thermally activated phonons.

3.4.5. ZT

The temperature dependence ofZTis shown infigure9. TheZTof all the samples increased monotonically with increasing temperature, as well as with increasing Sr²⁺doping content. This enhancement results from the increase power factor with increasing Sr²⁺doping content. Therefore, the highestZTobtained in this study was 0.015 at 1073 K for BSMO055, which is higher than 0.003 of Sr(Mn1−xMox)O3(0.01„x„0.25)[49]. And it is also comparable to the values obtained in[43]. The highestZTof BSMO055 also attributes to its relatively low thermal conductivity. These results confirm that the partial substitution of Sr²⁺for Bi³⁺is highly effective for improving the thermoelectric performance of BiMnO3.

4. Conclusions

Hot-press sintered samples of Bi1−xSrxMnO3(x=0.40, 0.45, 0.50, and 0.55)were fabricated by the modified solid-state reaction method. BSMO040, BSMO045, BSMO050, and BSMO055 all crystallized into an orthorhombic structure at 723 K. The partial substitution of Sr²⁺for Bi³⁺in BiMnO3decreased the grain size and density and increased the effective mass of electrons. So the carrier mobility of electrons decreased. The electrical conductivities of samples increased with increasing doped Sr²⁺content except for BSMO055; this is mainly because of the valence shifts of some Mn ions from+3 to+4 state, which increased the electron concentration. The electrical conductivity of BSMO055 was lower than that of BSMO050 was due to its low mobility and heavy effective mass of electrons. Moreover, the substitute Sr²⁺contributed to increasing the absolute value of Seebeck coefficient, leading to a remarkable increase in power factor. The maximum value of power factor(0.3×10⁻⁴Wm⁻¹K⁻¹)was obtained in BSMO055 at 1023 K. And the highest value ofZT (ZT=0.015)was got in BSMO055 at 1073 K. The partial substitution of Sr²⁺for Bi³⁺in BiMnO3is effective for enhancing its thermoelectric performance. And this study supplies a candidate oxide material which has the potential for thermoelectric application in high temperature.

Figure 9.ZTsof Bi_1−xSr_xMnO₃samples as function of temperature.

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