Effect of TiB2 nano-inclusions on the thermoelectric properties of boron rich boron carbide

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ScienceDirect Materials Today: Proceedings 5 (2018) 10306–10315

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Effect of TiB2 nano-inclusions on the thermoelectric properties of boron rich boron carbide Prasanna Ponnusamya,b *, Bing Fenga, Hans-Peter Martina, Pim Groenb a

Fraunhofer Institute for Cermamic Technologies, Winterbergstrasse 28, 01277 Dresden, Germany bDelft University of Technology, Kluyverweg 1, 2629 HS Delft, Netherlands * Corresponding author E-mail address: [email protected]

Abstract Boron carbide has exceptional thermoelectric properties due to its unique crystal structure. The effects of nano-TiB2 inclusions on the thermoelectric properties of boron rich boron carbide is presented here. Boron rich boron carbide-TiB2 composites were prepared in-situ by sintering of wet ball milled B, B4C and nano-TiO2 powders through Spark Plasma Sintering (SPS) at 1900 °C under 50 MPa pressure. Five different compositions of the composite were prepared- with 1, 2.5, 5, 7.5 and 10 wt.% TiO2. The dependence of densification and thermoelectric properties on TiO2 content were studied. The microstructure was observed through Field Emission Scanning Electron Microscope (FESEM) and phases were analyzed using X-ray diffraction (XRD). Relative densities of 94.8 % to 99. 3% were obtained and an almost homogeneous distribution of TiB2 was observed. The electrical property was found to be high for sample with 1 wt% TiO2 and reduced subsequently with higher wt% of TiO2. The overall dimensionless figure of merit was found to be higher than B4C-TiB2 composites, due to the formation of boron rich phase but the effect of TiO2 was not significant. The increase in lattice parameter of boron carbide phase for different TiO2 amounts added showed that the final composition of the composite depends on the amount of TiO2 added. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS. Keywords: Boron-rich boron carbide;BxC- TiB2 composite; thermoelectric properties;densification.

2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.

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1. Introduction Thermoelectric materials available today suffer from poor efficiency, high cost and rarity of constituent materials (eg. Tellurides) [1]. High temperature thermoelectric conversion is very promising, since the thermoelectric efficiency is given by [2],

=

where Tm is the mean temperature given by

−

−1 + 1 +

Eq. 1

+ 1+

and

is the Carnot efficiency and

is the dimensionless

figure of merit of the material ( =∝ / ). Boron carbide is one of the most widely researched materials for high ) owing to its excellent temperature thermoelectric applications, ( = 200 − 300 , = 5 − 30 / thermoelectric properties arising from its unique crystal structure, and stability at higher temperatures [3]. Boron rich compositions have much higher figure of merit due to defects arising from boron substitution. A of about 0.85x10-3/K was reported by Bouchacourt and Thevenot at 1250 K for 13.3% of carbon content[3], while Wood reported a value of about 0.5x10-3/K at 1300 K for B9C composition [4]. Even though, these results have not been reproducible yet, indeed, the study on the properties of boron rich compositions show promise in improving the [5, 6]. Roszeitis et al. reported a power factor of 1.8x10-4 W/mK-2 for 13.3% C prepared from sintering of elemental boron and carbon powders [7]. One of the main drawbacks with synthesis of boron rich boron carbides is the requirement of high purity of starting powders to synthesize it. Feng et al.[8] obtained a 100% increase in over plain B4C with B4C1-x-TiB2 composites prepared in-situ from reactive sintering of B4C and nano-TiO2 powders resulting in the formation of non-stoichiometric carbon deficient (hence, boron rich) boron carbide. TiB2 addition not only improves the densification in boron carbide but also improves its thermoelectric properties by increasing the electrical conductivity due to its metallic nature [9] and reducing thermal conductivity by inhibiting grain growth during sintering [10]. Hence, it would be interesting to see the combined effect of boron-rich boron carbide and TiB2. In the present experiment, an effort to synthesize the B7C-nano TiB2 composite, in-situ, from B, B4C and nano-TiO2 as starting powders has been made. The thermoelectric and densification behaviour of the composite have been studied and the results are discussed in the following sections.

Nomenclature

Cp

Seebeck Coefficient (µV/K) Electrical conductivity (S/cm) Thermal conductivity (W/mK) Dimensionless figure of merit Specific heat at constant pressure

2. Experiment

The weight percentages of the individual powders to prepare B7C were calculated according to the stoichiometric + → and 1 wt.%, 2.5 wt.%, 5 wt.%, 7.5 wt.% and 10 wt.% TiO2 were added to it. The reaction required amount of B4C, B and TiO2 powders were weighed and milled using SiC balls (∅1.25-2 mm) in ethanol using planetary ball mill (PM 400, Retsch, Germany) for one hour at 200 rpm, for homogeneous mixing of the

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powder. These are commercially available powders- B4C grade HD20 ( = 0.3 − 0.6 , H.C. Starck, Germany), boron grade I (average particle size 1 , H.C.Starck Germany) and TiO2 (5-10 nm). 1 wt.% KM 1500 (dispersant) was added for better dispersion. The wear of the balls constitute about 0.1-0.3 % to the impurity in the samples. After milling, ethanol was evaporated from the suspension using Nutzung rotary evaporator. The powder was then dried in the oven overnight at 80 °C and then sieved in a 315μm sieve. The dried powders were sintered using Spark Plasma Sintering technique (SPS) (model HP d25/1; FCT Systeme GmbH, Germany). Graphite foil coated with boron nitride was used to prevent the reaction between the graphite die and the powders. A heating rate of 100 K/min and an initial pressure of 10 MPa was used until the temperature was raised to 1400 °C and maintained at that temperature for half an hour to evaporate any volatiles and oxides formed on the surface and also for the in-situ reactions to take place. The temperature was then raised to 1900 °C at the same heating rate (100 K/min) and within one minute after reaching this temperature the pressure was increased to 50 MPa. These conditions were maintained for 10 minutes to form tablets of 30 mm diameter and approximately 12g in weight. The entire process was carried out under vacuum atmosphere. In order to find the reaction taking place in the sample during sintering, thermogravimetric analysis (TG, DTG) and Differential Thermal Analysis (DTA) were performed in Argon atmosphere with powder containing 5 wt% TiO2 (nomenclature: TEKS 3.4) using a NETZSCH STA 449 F1 thermal analyzer. As there are three starting components (B, B4C and TiO2), the reaction study is more complicated. Hence, to understand the reactions better, the powders of B and B4C in stoichiometric proportion to form B7C was thermally treated at 1400 °C for 2 hours and 5 wt % of TiO2 was added to it. This powder was named as TEKS 3.7 and the thermal analysis on this powder was performed. A heating rate of 20 K/min was used to raise the temperature until 1800 °C. Further studies on the powder were made using x-ray diffraction (XRD) (Bruker D8) with Cu- radiation with a measurement rate of 3 s for every 0.03° until 80°. The humidity of all the samples were measured and the density was determined by Archimedes principle. The phases in the sintered samples were analyzed using x-ray diffraction (XRD) and the lattice parameters were determined using Rietveld analysis. The microstructure was studied using Field Emission Scanning Electron Microscope (FESEM) (ULTRA 55; CARL ZEISS, Germany) with in-built energy-dispersive x-ray (EDX) detector. Chemical polishing gives a clear view of the grain structure but removes the TiB2 from the samples. Hence, the samples were observed again by polishing them with ion-beam to see the homogeneity of TiB2 distribution in the samples. Samples of size 20 mm x 5 mm x 0.5 mm, 20 mm x 5 mm x 5 mm and 10 mm x 10 mm x 1.5 mm were used for Seebeck coefficient, electrical conductivity and thermal diffusivity measurements respectively. The Seebeck coefficient was measured using inhouse measurement facility available at Fraunhofer IKTS (PhysTech GmbH, Germany) which has a capability to measure Seebeck coefficient until 600 °C with an error percentage of 10. Gold wires were used for measurement. Four point method was used to measure the electrical conductivity from room temperature until 800 °C using Keithley 2750, also available in-house. Thermal diffusivity was measured by laser flash method (LFA427; NETZSCH) and Cp of B7C was used for the thermal conductivity calculation.

3. Results and Discussion 3.1 Thermal analysis The results of the thermal analysis of powders containing 5 wt.% TiO2 are shown in Fig. 1. As it can be seen from the figure, the reaction mechanisms happening in B4C+B+5wt.% TiO2 (TEKS 3.4) and B7C+5 wt.%TiO2 (TEKS 3.7) are similar. There is no significant calorific change in the samples as seen from DTA graph.

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TG of B7C+5 wt.% TiO2

Fig. 1: Comparison of TG-DTG results of B7C+ 5 wt% TiO2 (blue) and B4C+B+5wt.% TiO2 (red) is shown on the left and the DTA results of both the powders is shown on the right side.

The initial drop in DTG curve at 200 °C corresponds to the evaporation of volatiles. The exothermic drop from 1400°C until 1800 °C, indicates the occurrence of a reaction. This could correspond to the conversion of TiO2 to TiB2 based on the available literature [8, 11, 12]. In order to confirm this, the B4C+B+5 wt.% TiO2 powder was heat treated at three different temperatures, 1400 °C, 1600 °C and 1800 °C for half an hour each and the phases formed at these temperatures were analyzed using XRD. TiB2 and BxC phases were identified at all the three temperatures as can be seen from Fig. 2. The lattice constants ‘a’ and ‘c’ of the BxC phase formed at 1400 °C, 1600 °C and 1800°C were calculated using Rietveld analysis and are presented in Table 1. From the previous studies on the lattice constants of boron carbide [13, 14], it can be concluded that these lattice constants correspond to different carbon content (or different boron content) indicating the conversion of TiO2 to TiB2 at different temperatures. This is again evident from the increasing peak intensities of TiB2 phase corresponding to increasing amounts of TiB2 formed with increasing temperatures. TiO2 could be converted to TiB2 through two means- reaction of B4C and TiO2 as studied by Levin et al.[11], and the reaction of free boron and TiO2 [12]. When the second reaction happens, the expected stoichiometric boron rich boron carbide cannot be formed. Another reaction which could happen simultaneously is the reaction of free boron with TiO2 resulting in Ti2O3 and TiBO3 which happens at temperatures less than 780 °C. These compounds later react with boron to form B2O3 and TiB2[12]. The weak exothermic peak near 600 °C in B4C+B+5wt% TiO2 (TEKS 3.4) in TG-DTG could correspond to the formation of these intermediate products Ti2O3 and TiBO3. Moreover, the free boron forms boron oxides which evaporates at higher temperatures leading to loss of boron content [15]. This again affects the expected stoichiometry of boron carbide in the final product. The mass loss is more in B7C+5 wt.% TiO2 sample compared to B4C+B+5 wt.% TiO2. This could correspond to the CO/CO2 loss formed as a by-product of reaction, indicating that the reaction in B7C+ TiO2 sample occurs as suggested by Levin et al. [11]. Table 1: Lattice parameters of powders annealed at different temperatures (1400 °C, 1600 °C and 1800 °C) Temperature (°C)

Lattice constant ‘a’ ( )

Lattice constant ‘c’ ( )

1400

5.6141

12.172

1600

5.6233

12.184

1800

5.6318

12.204

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Fig. 2: XRD analysis of B4C+3B+5 wt.% TiO2 powders annealed at three different temperatures

Fig. 3: XRD analysis of sintered samples with different amounts of TiO2

3.2 XRD analysis and lattice parameter determination The XRD results of the sintered samples showed the presence of two phases- BxC and TiB2 (Fig. 3). The EDX analysis confirmed the presence of the same. The lattice parameters determined using Rietveld analysis showed a decrease in lattice constant ‘a’ until 5 wt% and continued to remain the same for 7.5 wt% and 10 wt% of TiO2. Table 2 shows the weight percentages of the starting powders, the lattice parameters of the corresponding boron carbide phase formed and the estimated content of carbon based on the lattice parameters. As it can be seen, the boron content decreases as TiO2 content increases. On account of all these reactions happening simultaneously during sintering as explained in section 3.1, the overall boron content is reduced than intended and hence the expected B7C composition is not obtained. Table 2: The weight percentages of starting powders along with the lattice parameters of the boron carbide phase formed and the amount of TiB2 formed for each composition. TiO2 (wt.%)

Boron carbide (wt.%)

Boron (wt.%)

Lattice constant ‘a’ ( )

Lattice constant ‘c’ ( )

Estimated carbon content based on literature[13],[14] (wt.%)

TiB2 (wt.%)

0

62.99

36.973

5.6239

12.170

13.5

0

1

62.36

36.6

5.6196

12.150

14.75

2.145

2.5

61.41

36.04

5.6122

12.136

16.25

1.97

5

59.84

35.12

5.6082

12.128

17.37

1.94

7.5

58.26

34.2

5.6093

12.148

17.37

1.86

10

56.69

33.2

5.6081

12.141

17.37

1.86

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3.3 Density The density was measured according to DIN-EN 623-2 standards used for ceramic materials. It was observed that the porosity increased with TiO2 content from 2.5 wt.% of TiO2 (Fig 4 b). This can be attributed to a number of reasons such as variations occurring during milling and sintering, the trapping of gas molecules formed as a byproduct of the reaction during sintering.

Fig. 4: a) The variation of density with TiO2 content, b) The relative density and porosity dependence on TiO2 content.

For 1wt.% TiO2 sample 99.3% of theoretical density was obtained. This increase in density in 1 wt.% TiO2 sample from sample with no TiB2 content can be attributed to the effect of TiB2 on increasing the densification of boron carbide ceramic [16]. But the trend is not maintained further as the TiO2 content is increased. This could be due to the porosity increase due to the gaseous by-products of the reactions happening during sintering, or due to sintering conditions. Moreover, the B2O3 layer formed with higher TiO2 content could serve as a barrier to densification [17]. Nevertheless, the densities of samples were high compared to density of plain boron carbide and the values of relative density varied from 94% until 99.3% as shown in Fig 4.These results are consistent with the previous efforts to form B4C-TiB2 composites [9, 11, 18], showing the positive effect of TiB2 on densification of boron carbide. Densification is also enhanced by the disorder in boron rich boron carbide [16]. 3.4 Microstructure The microstructure of the samples are shown in Fig. 5. The Fig. 5a is the SEM image of chemically polished sample with no TiO2 addition. The grain boundaries are clearly visible and the pores can be seen. The Fig. 5b and 5c shows the sample with 10 wt.% and 5 wt.% TiO2 addition respectively, polished with ion beam and it can been seen that TiB2 (white) is homogeneously distributed in BxC matrix (grey). Twin boundaries which are typical of BxC formation were observed at few places. The grain size of the samples were calculated using linear analysis and the for sample with no TiB2 and 0.58 for sample with 10 wt.% average grain size of BxC was found to be 0.45 TiO2. The TiB2 particles were almost homogeneously distributed as shown in Fig. 5d (5 wt.% TiO2 sample at a lower magnification), with few agglomerates at some places as can be seen from the micrographs. Grains as minimum as 0.04 were also observed.

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Fig. 5: SEM micrographs of BxC samples, a) without TiO2 addition, the chemically polished sample showing grain boundaries and pores, b) with 10 wt.% TiO2 , c) with 5wt. % TiO2 d) 5wt.% TiO2 sample at a lower magnification showing homogeneous distribution of TiB2

3.5 Thermoelectric properties Due to the variation of a number of parameters because of the three starting powders, there is not a clear trend in thermoelectric properties with respect to TiO2 content. The electrical conductivity was found to be higher for 1 wt.% TiO2 sample compared to the rest of the samples. This increase could be the result of high density, less porosity and the metallic nature of TiB2. The decline in electrical conductivity of samples with higher percentages of TiO2 can be explained based on the porosity increase with higher TiO2 addition. Moreover, polaron hopping being the primary conduction mechanism in boron carbide, it can be conceived that at lower boron concentrations, the electrical conductivity is reduced due to less polarons, resulting from less disorder in the crystal structure at lower boron concentrations [5]. Nevertheless, the observed electrical conductivity for all the samples is higher compared to the boron rich boron carbide prepared through SPS [7] and also higher than the B4C-TiB2 composites reported previously [8, 19] due to the combined effect of TiB2 and boron rich composition. The thermal conductivity almost remains constant with temperature as the major conduction mechanism is through the covalent bonds and hence less dependent on temperature [4]. The variation of thermal conductivity did not show a clear trend with TiO2 content. This can be attributed to a number of parameters- porosity, grain size and TiB2 distribution and TiB2 particle size, all of these parameters differing for different samples due to the complexity in reactions during sintering. TiB2 increases the thermal conductivity due to its metallic nature as is evident from Fig 6b. The highest thermal conductivity of 1 wt.% TiO2 sample is consistent with the highest electrical conductivity of the same. The lowest conductivity of 7.5 wt.% TiO2 sample among all other samples could be ascribed to overcoming effect of the smaller grain size of the sample and the increase in porosity for this composition. The observed values of thermal conductivity of all the samples is comparable to the thermal conductivity values reported for plain boron carbide compositions [3, 4], indicating that the effect of metallic TiB2 is reduced by the phonon scattering effect of smaller grains and nano-particles of TiB2.

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The Seebeck coefficient can be seen to have a mixed-up trend as well as often observed. A Seebeck coefficient, as high as 275 was observed at 473 K. The Seebeck coefficient reduces until about 400 °C for all the samples as can be seen from Fig 6c. This is in agreement with the increase in electrical conductivity due to TiB2 addition. Moreover, the introduction of TiB2 introduces negative carriers resulting in reduction of the positive Seebeck coefficient of boron carbide [20]. The temperature average values computed from these measurements showed that the Seebeck coefficient decreased until 2.5 wt.% TiO2 and increased from then on until 10 wt.% TiO2. At higher temperatures, the Seebeck coefficient of samples with higher TiB2 content shows an increasing trend. The thermally assisted hopping at higher temperatures could be one of the reasons for this increasing trend, surpassing the effect due to TiB2 addition [4]. The overall dimensionless figure of merit in samples with TiB2 addition is not high compared to the one without it, as shown in Fig 6d. But indeed, the overall zT of all the samples is high compared to the reported zT values of plain B4C, boron rich boron carbide and B4C-TiB2 composites reported previously [7, 8, 19], due to the combined positive impact of boron rich composition and that of TiB2. Also, the zT shows an increasing trend with temperature for 5 wt.%, 7.5 wt.% and 10 wt. % TiO2 addition, showing a promise for higher zT values at higher temperatures.

Fig. 6: The variation of thermoelectric properties with temperature for different amounts of TiO2 content a) Electrical conductivity vs temperature b) Thermal conductivity vs temperature c) Seebeck coefficient vs temperature d) zT vs temperature.

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4. Conclusion In-situ synthesis of boron rich-TiB2 composites from boron, boron carbide and nano-TiO2 as starting powders resulted in samples with high density. The thermal analysis predicted that the in-situ reactions start around 1400 °C and the predictions were confirmed with XRD analysis. Since the synthesis involved three starting powders, there were simultaneous reactions which reduced the boron content in the anticipated boron rich composition. The two main reasons for not obtaining the desired B7C composition being, the formation of intermediate oxides with boron which evaporates at higher temperature, and the reaction of boron and TiO2 to form TiB2. The lattice parameter determination using XRD analysis of the sintered samples confirmed the same. The microstructure observed through FESEM showed homogeneous distribution of TiB2 with agglomerates of TiB2 only at few places. Even though, the dimensionless figure of merit did not improve as much as expected due to the above mentioned reasons, the research indeed shows a promise in improving the figure of merit in boron rich boron carbide, especially at higher temperatures. The measured thermoelectric properties were higher than that of the B4C- TiB2 composites. Further work involving the formation of boron rich boron carbide by annealing the powders of boron and boron carbide and then adding TiB2 could yield interesting results. Acknowledgements I would like to thank and acknowledge the funding for this project from the Free state of Saxony (Grant No. SAB 100234923) References: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

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