Home Search Collections Journals About Contact us My IOPscience Thermoelectric properties of p-type Bi2Sr2Co2O9 glass-ceramics This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Semicond. Sci. Technol. 29 124011 (http://iopscience.iop.org/0268-1242/29/12/124011) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 176.9.124.142 This content was downloaded on 20/11/2014 at 09:03 Please note that terms and conditions apply. Semiconductor Science and Technology Semicond. Sci. Technol. 29 (2014) 124011 (6pp) doi:10.1088/0268-1242/29/12/124011 Invited Article Thermoelectric properties of p-type Bi2Sr2Co2O9 glass-ceramics Matthias Jost1,2, Julian Lingner1,2, Martin Letz2 and Gerhard Jakob1 1 Department of Physics, Johannes Gutenberg-University Mainz, Staudinger Weg 7, D-55128, Mainz, Germany 2 SCHOTT AG, Material Development, Hattenbergstraße 10, D-55122, Mainz, Germany E-mail: jost@imp.tu-darmstadt.de Received 30 September 2014 Accepted for publication 15 October 2014 Published 14 November 2014 Abstract In the oxide system of Bi–Sr–Co glass melts have been prepared by adding a small amount of glass formers. A crystallization leads to crystalline phases of Bi8Sr8Co4O25, BiSrCo2Ox and Bi2Sr2Co2O9 (BC-222) densely embedded into a residual glass phase. This work shows the possibility of obtaining microstructured bulk material with low thermal conductivity and relatively high electrical conductivity via such a glass ceramic approach. Furthermore the stability of these materials under thermal cycling for temperatures up to 700 °C is shown. A characterization of the thermoelectric properties leads to a ﬁgure of merit (ZT) between 0.008 and 0.018. Keywords: Bi2Sr2Co2O9, oxide thermoelectrics, thermal cycling, glass-ceramic (Some ﬁgures may appear in colour only in the online journal) Introduction Although having high ZT values up to unity, well established thermoelectric materials suffer from a limited stability at elevated temperatures (e.g. Bi2Te3) [2], consist of rare and expensive raw materials (e.g. SiGe) or environmentally harmful materials like lead (e.g. PbTe), for which they will be forbidden in the EU for applications in the automotive industry [3, 4]. In contrast, oxide thermoelectric materials with ZT close to unity, which were ﬁrst investigated by Ichiro Terasaki in 1997 [5], are neither toxic nor harmful to the environment, the raw materials are cost-efﬁcient as well as available in large quantities and they have a high chemical and thermal stability. This makes them competitive due to economic reasons even despite the lower performance, reaching values of about ZT = 0.65 for polycrystalline ZnO [6] or ZT = 0.87 for Ca3Co4O9 single crystals [7]. Recent development on oxide thermoelectric materials can be found in [8] and [9]. Most of today's bulk oxide thermoelectric materials are produced via a ceramic route. The advantage of such ceramics in comparison to glass-ceramics is the high electrical conductivity, resulting from the purity of the thermoelectric In recent decades the demand for renewable energy has grown steadily since more than 60% of the energy from fossil fuels is dissipated as waste heat during the combustion process. Based on the Seebeck effect [1] thermoelectric devices can convert this waste heat into electrical energy. By this, the consumption of fossil fuels as well as the emission of CO2 can be reduced. A thermoelectric module is composed of two different materials, for example, a p-type semiconductor and an n-type semiconductor, which are connected electrically in series and thermally in parallel. The quality of a thermoelectric material is determined by the dimensionless ﬁgure of merit ZT = S 2σT , κ (1) where S is the Seebeck coefﬁcient (a material dependent constant), σ the electrical conductivity, κ the thermal conductivity and T the absolute temperature. 0268-1242/14/124011+06$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK M Jost et al Semicond. Sci. Technol. 29 (2014) 124011 phase. The disadvantages are the porosity and the associated instability due to exposing a larger surface to chemical inﬂuences or the high thermal conductivity. Nanostructuring, which offers a further possibility to enhance the thermoelectric properties [10], is also very challenging, due to grain growth during the sintering process. These problems can be overcome by using a glass-ceramic route for the fabrication process. The glass-ceramics represent a promising new class of materials for thermoelectrics and are also recently considered as novel optical materials [11–14]. They are essentially pore free, have a low thermal conductivity and both a higher chemical and mechanical stability, due to their additional glassy phase. A high amount of the thermoelectric phase is still important to ensure the interconnection of these crystals, which is mandatory for providing a sufﬁcient electrical conduction. Nanostructuring can be expected by applying a special heat treatment since the thermoelectric crystals are grown inside the glass matrix. By varying the parameters of this heat treatment the phase or crystal size can be inﬂuenced. One of the ﬁrst themoelectric glass-ceramic systems was investigated by [15]. The n-type SrTiO3 was fabricated using a glass-ceramic route. Although the material had a very high absolute Seebeck-coefﬁcient of 480 μV K−1, a thermal conductivity lower than 1.6 W m−1 K−1 and showed thermal cycling up to 650 °C, the electrical conductivity was very low, because the percolation threshold was not reached. This work represents the ﬁrst attempt of synthesizing a ptype thermoelectric glass-ceramic. Based on [16], a glass melt was prepared with the aim of obtaining a Ca3Co4O9 glassceramic. Due to the impurity of the material, the second melt was varied in an attempt to reduce the amount of secondary phases and to get a Bi2Sr2Co2O9 glass-ceramic, which is also known as a good misﬁt-structured thermoelectric material [17]. Due to the complete crystallization during the melting processes, the growth of speciﬁc phases could not be affected. Nevertheless, thermoelectric characterizations were carried out. forming range of the composition. The mixture was melted in a platinum–iridium crucible at 1500 °C for 30 min. After that it was casted on a copper plate, quenched with a second copper plate, and cooled down very slowly again. Scanning electron microscopy (SEM) images were taken to identify the structure of the material. For that a Zeiss LEO 1530 Gemini was used. The x-ray diffraction (XRD) patterns were measured with a PANalytical X’Pert Pro MPD x-ray Diffractometer. The speciﬁc heat cp(T) of the materials was determined by Difference-Scan Calorimetry (DSC) with a NETZSCH DSC 404 Pegasus. For the measurements of the temperature dependent electrical conductivities and Seebeck coefﬁcients rods with a geometry of 10 mm × 2 mm × 2 mm were prepared. For the measurements of the thermal conductivities disks with a diameter of 10 mm and a thickness of 1 mm were prepared. The measurements for the BSCO-1 were done at the EMPA in Switzerland. For the Seebeck coefﬁcient and the electrical conductivity an Ozawa Science RZ2001i unit was used. The measurements were carried out in an O2 atmosphere in a temperature range between 50 °C and 700 °C. According to [18], the measurement error originates from the inaccuracy of the thermocouples, the geometry of the samples and the averaging of ﬁve data points, which were received at each measurement. The relative error of the Seebeck coefﬁcient and electrical conductivity can therefore be assumed to be 5%. The measurements of the thermal diffusivity α were carried out using a NETSCH LFA 457 MicroFlash®. Together with the density of the material and its speciﬁc heat cp, the thermal conductivity κ can be calculated via κ = α ⋅ c p ⋅ ρ. (2) Due to the fact that each of these measurements is errorprone, the relative error of the thermal conductivity amounts to about 7.5%. The measurements for the BSCO-2 sample were carried out at the Johannes Gutenberg-University in Mainz. The Seebeck coefﬁcient and the electrical conductivity were measured using a Linseis LSR-3. In contrast to the measurements of the BSCO-1 sample, these measurements were carried out in air. The error is assumed to be 5–10%. Again, the thermal diffusivity measurements were done using a NETSCH LFA 457 MicroFlash®. Experimental procedures The samples were prepared by a conventional melting process. For the ﬁrst melt (BSCO-1), powders of CoO, CaCO3, Bi2O3 and SrCO3 were mixed in a molar ratio of 2:1:1:1, as in [16]. Unlike in [16], a platinum–iridium crucible instead of an alumina crucible was used and a well deﬁned amount of Al2O3 was added to the composition, to simulate the decomposing of the alumina crucible. The mixture was melted at 1500 °C for 30 min, before it was casted on a copper plate and quenched with another copper plate. To reduce the amount of stress inside the material, it was cooled down very slowly after the quenching. For the second melt (BSCO-2), powders of CoO, Bi2O3 and SrCO3 were mixed in a molar ratio of 2:1:2 and some additional B2O3 was added to the mixture. These changes were done due to the phases grown in BSCO-1. Since B2O3 is known to be a good glass former, it should increase the glass Results and discussion The ﬁrst melt (BSCO-1) showed a very low viscosity and the surface appeared to be homogeneous and clear of crystals. Therefore we assume that the components melted without any residuals. Nevertheless, it completely crystallized upon casting. Because of this, the crystallization was only barely controlled, resulting in a number of secondary crystalline phases in the material. The XRD pattern of the BSCO-1 (ﬁgure 1) shows that the main phase is Bi8Sr8Co4O25 (JCPDS 85-1501), which is a 2 M Jost et al Semicond. Sci. Technol. 29 (2014) 124011 Figure 1. XRD pattern of BSCO-1, measured at 25 °C. Figure 3. SEM image of BSCO-1 containing (1, 3) CaSrBi2O5, CoO and Ca3Al2O6, (2, 6) Ca3Al2O6, CaSrBi2O5 and CoO, and (4, 5) Bi8Sr8Co4O25, Bi2Sr2Co4O5+δ and a small amount of the Ca3Al2O6. Figure 2. XRD pattern of BSCO-2, measured at 25 °C. phase with good thermoelectric properties. In addition to this phase three other phases crystallized, including CoO (JCPDS 48-1719), which is known to be a insulator with a band gap of about 2.5 ± 0.3 eV [19]. The presence of CoO will thus decrease the thermoelectric properties of the material. The remaining phases are Bi2Sr2Co4O5+δ (JCPDS 49-0691) and CaSrBi2O5 (JCPDS 49-1539). The ﬁrst result shows that in this experiment it was not possible to receive a clear glass by melting down the batch, based on ‘Example 1’ in the patent of R Funahashi. The idea behind the second melt (BSCO-2) was to stabilize it by adding a network forming material (B2O3) to the batch, with the intention to obtain a glass with more resilience against devitriﬁcation. It was further necessary to suppress the secondary phases by excluding the components of Al2O3 and CaCO3. This time the targeted crystal phase was supposed to be Bi2Sr2Co2O9, which is known to be a good thermoelectric material [17]. To fulﬁll these efforts CaCO3 was replaced by SrCO3 in the batch. The growth of the BC-222 phase should therefore be supported. This melt had a very low viscosity as well and also crystallized completely upon casting, which made a controlled crystallization in a second step impossible. The XRD pattern in ﬁgure 2 shows the existence of at least four different crystalline phases in the material. The main phase is Bi2Sr2Co4O5+δ (JCPDS 49-0691). The secondary phases are SrBi4O7 (JCPDS 46-0752), CoO (JCPDS 48-1719) and Sr3B2O6 (JCPDS 31-1343). The SEM images of the two samples are given in ﬁgures 3 and 4. These images show again the existence of different crystalline phases inside the materials. The different phases are represented in a variety of gray shades in the SEM images. Both materials do not show any pores. Additionally emission spectra were measured via Figure 4 SEM image of BSCO-2 containing (1) and (2) CoO, (3) and (4) Bi2Sr2Co2O9 and (5) and (6) SrBi4O7 and CoO. Figure 5. EDX measurement of BSCO-2(4). The carbon detected in the EDX spectrum results from the coating of the sample. energy-dispersive x-ray spectroscopy (EDX) at six different points of each sample. In the BSCO-1, the region around 4 and 5 contains Bi8Sr8Co4O25, Bi2Sr2Co4O5+δ and a small amount of Ca3Al2O6, which was not detected by the XRD. Regions 1 and 3 contain CaSrBi2O5, CoO and Ca3Al2O6 as well. Area 2 and 6 contain Ca3Al2O6, CaSrBi2O5 and CoO. Hence the bright regions in ﬁgure 3 contain most of the desired thermoelectric phase, whereas big parts of the material consist of secondary phases, suppressing the good thermoelectric properties of the main phase. All of the examined points in the BSCO-1 sample contain a certain amount of alumina, which is a hint that it is present as an amorphous phase. The BSCO-2 contains secondary phases as well. However, the different areas of the crystals are much larger compared to BSCO-1. Regions 1 and 2 consist of pure CoO, the areas of 3 and 4 of pure Bi2Sr2Co2O9 (BC-222). The 3 M Jost et al Semicond. Sci. Technol. 29 (2014) 124011 Figure 6. XRD pattern of BSCO-2 with the identiﬁcation of the Bi2Sr2Co2Oy phase of [21]. Figure 7. Temperature dependent electrical conductivities of BSCO1 and BSCO-2. regions in 5 and 6 consist of SrBi4O7 and CoO, while Sr3B2O6 as determined by the XRD, was not found at these points. The EDX spectrum of point number 4 of the BSCO-2 is shown in ﬁgure 5. It matches the one presented by Wang et al [20]. The small additional peak of carbon on ﬁgure 5 results from the coating of the SEM sample. These measurements show the presence of the BC-222 phase inside the sample, although it was not possible to identify it by XRD evaluations. This can be explained by the fact, that there is no JCPDS ﬁle existing for the BC-222. Therefore we assume the peaks of the Bi2Sr2Co4O5+δ phase in the XRD to be related to the BC-222 phase. Comparing the measured XRD pattern in ﬁgure 2 with the one in [21], it is possible to match the BSC-222 hkl reﬂexes, shown in ﬁgure 6. The line shows the XRD measurement results, while the numbers represent the identiﬁcation of the Bi2Sr2Co2Oy phase according to [21]. In ﬁgure 7, the temperature dependent electrical conductivities of the samples are presented. Both conductivities increase with temperature, which is a typical behavior for semiconductors, whereas the slope of the BSCO-2 is less steep than the one of BSCO-1. The BSCO-1 reaches a value up to 16 S cm−1 at 700 °C, the BSCO-2 only reaches about 5 S cm−1. The different slopes might have their origin in the different doping levels of the Co defect band. The electrical conductivities of these samples are about one (BSCO-1)/two (BSCO-2) orders of magnitude smaller than the electrical conductivities known from common oxidic thermoelectric ceramics (e.g. Ca3Co4O9 ceramics [22]). In ﬁgure 8 the temperature dependent Seebeck coefﬁcient is shown. For the BSCO-1 it is nearly constant over the temperature range and reaches a value of about 86 μV K−1 at 700 °C. However the Seebeck coefﬁcient of the BSCO-2 is increasing from about 100 μV K−1 at 100 °C up to 233 μV K−1 at 700 °C. Hence it was possible to increase the Seebeck coefﬁcient signiﬁcantly from one melt to the other. The resulting power factors of the two samples are shown in ﬁgure 9. The power factor of BSCO-2 has been improved compared to BSCO-1, despite its lower electrical conductivity. The thermal conductivities (ﬁgures 10 and 11) of the two samples do not differ very much. Only in the temperature Figure 8. Temperature dependent Seebeck coefﬁcients of BSCO-1 and BSCO-2. Figure 9. Temperature dependent power factors of BSCO-1 and BSCO-2. range between 100 °C–250 °C and at 700 °C, signiﬁcant differences can be identiﬁed. The BSCO-1 shows an almost constant value for the thermal conductivity over the temperature range up to 650 °C. The value at 700 °C, however, has a much lower value than it would be expected from the general trend. This arises from the cp measurement, because the speciﬁc heat shows a dip around 700 °C. The thermal diffusivity decreases continuously with increasing temperature. From this, we obtain a 4 M Jost et al Semicond. Sci. Technol. 29 (2014) 124011 2 decreased compared to BSCO-1, while the Seebeck coefﬁcient increased, as well as the power factor of BSCO-2. The thermal conductivity increased only slightly, resulting in an overall higher ZT value. The corresponding error bars are shown in each ﬁgure. They were calculated with the help of the error assumptions, shown in the 'Experimental' part of this work. This results in relative errors for ZT of about 16%. The ZT values of these glass-ceramics are 1–2 orders of magnitude lower than the values of common bulk oxidic ceramics. To improve these further, the melt has to be stabilized in a way that a clear glass can be obtained after casting. Only with a true glass a controlled crystallization can be realized via a special heat treatment. Figure 10. Temperature dependent thermal conductivity of BSCO-1. Conclusion Two glass melts have been produced, one which was aimed at obtaining a Ca3Co4O9 single-crystal like it was claimed in [16], the other one with the goal to obtain a Bi2Sr2Co2O9 glass-ceramic. Both of the melts completely crystallized upon casting therefore showing a variety of crystalline phases, due to the uncontrolled crystallization. The main phases of the samples were Bi8Sr8Co4O25/Bi2Sr2Co4O5+δ for the BSCO-1 and Bi2Sr2Co2O9 for the BSCO-2. The second material could be signiﬁcantly improved compared to the ﬁrst material, resulting in a ZT of 0.018 at 700 °C. To increase this value even further, the electrical conductivity and the Seebeck coefﬁcient have to be improved by increasing the amount of crystalline phases with good thermoelectric properties. A single-phase glass-ceramic is a goal for the further development. In addition it should be mentioned, that the microstructured material BSCO-2 offers high-temperature stability and that at least one thermal cycle is possible. The present work represents a good starting point for further glass-ceramic development of oxide thermoelectric materials. Figure 11. Temperature dependent thermal conductivity of BSCO-2. Acknowledgments Figure 12. ZT of BSCO-1 and BSCO-2. The authors would like to thank Dr Sascha Populoh and Professor Dr Anke Weidenkaff from the EMPA in Dübendorf as well as Dr Benjamin Balke and Dr Michael Schwall from the Johannes Gutenberg-University of Mainz for the help and support during the thermoelectric measurements. value of 1.28 W m−1 K−1 for the thermal conductivity at 700 °C. The values for the BSCO-2 sample on the other hand, show a decreasing slope with increasing temperature, for which a value of about 1.44 W m−1 K−1 at 700 °C is reached. The corresponding ZT values are shown in ﬁgure 12. For the BSCO-1 a value of 0.008 is reached at 700 °C. At the same temperature BSCO-2 reaches a value of 0.018, which is more than twice as high as the value of BSCO-1, although it was only measured in air instead of an O2 atmosphere. Thus it was possible to improve the material by varying the composition of the melt. The results at 700 °C are the most interesting results, as oxidic thermoelectric materials work best at high temperatures. Here, the electrical conductivity of BSCO- References [1] Seebeck T J 1822 Abh. K. Akad. Wiss. 1820–21 289–373 [2] Sommerlatte J, Nielsch K and Boettner H 2007 Phys.-J. 6 35–41 [3] 2003 Ofﬁcial J. Eur. Union L37 19–23 [4] 2011 Ofﬁcial J. Eur. Union L85 3–7 [5] Terasaki I, Sasago Y and Uchinokura K 1997 Phys. Rev. B 56 R12685–7 5 M Jost et al Semicond. Sci. Technol. 29 (2014) 124011 [15] Lingner J, Letz M and Jakob G 2013 J. Mater. Sci. 48 2812–16 [16] Funahashi R et al 2003 US Patent US6,544,444B2 [17] Funahashi R, Matsubara I and Sodeoka S 2000 Appl. Phys. Lett. 76 2385–7 [18] Populoh S, Aguirre M H, Brunko O C, Galazka K, Lu Y and Weidenkaff A 2012 Scr. Mater. 66 1073–6 [19] van Elp J, Wieland J L, Eskes H, Kuiper P, Sawatzky G A, de Groot F M F and Turner T S 1991 Phys. Rev. B 44 6090–103 [20] Wang S, He L, Elhadj D, Chen J, Wang J, Chen M, Yu W and Fu G 2010 Thin Solid Films 518 6829–32 [21] Wang S, Zhang Z, He L, Chen M, Yu W and Fu G 2009 Appl. Phys. Lett. 94 162108 [22] Mikami M, Guilmeau E and Funahashi R 2005 J. Mater. Res. 20 2491–7 [6] Ohtaki M, Araki K and Kiyoshi Y 2009 J. Electron. Mater. 38 1234–8 [7] Shikano M and Funahashi R 2003 Appl. Phys. Lett. 2 1851 [8] Snyder G J and Toberer E S 2008 Nat. Mater. 7 105–14 [9] Koumoto K, Wang Y, Zhang R, Kosuga A and Funahashi R 2010 Annu. Rev. Mater. Res. 40 363–94 [10] Dresselhaus M and Hicks L D 1993 Phys. Rev. B 47 12727–31 [11] Tanabe S, Fujita S, Yoshihara S, Sakamoto A and Yamamoto S 2005 Proc. SPIE 5941, 5th Int. Conf. on Solid State Lighting 5941 594112 [12] Engel A, Letz M, Zachau T, Pawlowski E, Seneschal-Merz K, Korb T, Enseling D, Hoppe B, Peuchert U and Hayden J S 2007 Proc. SPIE 6486 64860Y [13] Chen D 2014 J. Eur. Ceram. Soc. 34 4069–75 [14] Chen D and Chen Y 2014 Ceram. Int. B 40 15325–9 6