Microstructure and thermoelectric properties of p and n type doped β-FeSi2 fabricated by mechanical alloying and pulse plasma sintering

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ScienceDirect Materials Today: Proceedings 8 (2019) 531–539

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Microstructure and thermoelectric properties of p and n type doped β-FeSi2 fabricated by mechanical alloying and pulse plasma sintering Franciszek Dąbrowski*, Łukasz Ciupiński, Joanna Zdunek, Jakub Kruszewski, Rafał Zybała, Andrzej Michalski, Krzysztof Jan Kurzydłowski Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska St. 141, 02-507 Warsaw, Poland

Abstract N and p type doped, β-FeSi2 was obtained by pulse plasma sintering (PPS) of mechanically alloyed Fe, Si with Mn, Co, Al, P as dopants. The consolidated samples were subsequently annealed at 1123 K for 36 ks. SEM observations proved that the samples consolidated by PPS preserve fine grain size of the mechanically alloyed βFeSi2 and the low porosity. The results of XRD measurements confirmed for all samples a nearly complete transformation from α- and ε- into β-FeSi2 phase after annealing. Their thermal conductivity decreases significantly with the increase of the test temperature in the entire rage of the temperatures of practical meaning. With the exception of the Mn-doped, all samples exhibited a high Seebeck coefficient, with its highest value for FeSi1.95P0.05 exceeding -400 μV/K up to 550 K. The Mn and Co – Fe site dopants revealed a stronger effect on the thermoelectric properties with 0.15 ZT parameter at 773 K for Fe0.97Co0.03Si2 alloy. The thermoelectric properties of PPS sintered samples were compared to the previously reported consolidated by hot pressing and spark plasma sintering. It has been concluded that the pulse plasma sintering offers an alternative to the already explored methods of production of thermoelectric materials. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 15th European Conference on Thermoelectrics. Keywords: iron disilicide; doping; thermoelectricity; Seebeck effect; pulse plasma sintering;

1. Introduction Thermoelectric generators (TEGs) made of materials exhibiting a pronounced Seebeck effect allow to directly convert heat into electricity [1-4]. With their efficiency increasing, nowadays TEGs find more and more applications in recovering energy from waste heat. This in turn spurred further research efforts to enhance TEGs performance, among other by nanostructuring and implementing novel fabrication methods. Iron disilicide (β-FeSi2), which has been attracting attention as a potential thermoelectric material since 1960s [5], is a semiconductor with a band gap of 0.85 eV at room temperature. This disilicide is characterized by a * Corresponding author. Tel.: +48 22 234 8156; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 15th European Conference on Thermoelectrics.

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high Seebeck coefficient, oxidation resistance and high working temperature range (up to 1200 K)[6-8]. As a result of non-toxicity and availability of the substrates used to fabricate β-FeSi2, this compound draws much attention in the context of TEGs applications [9,10]. However, the thermoelectric properties of β-FeSi2 are quite low, because of the high thermal conductivity. Also, economic methods for a production of β-FeSi2 with the sufficiently high thermoelectric properties have not yet been established, despite the enormous research efforts, in which among others, the effects of nitrogenizing [11], fine particle dispersion [12], and grain refinement [13], were explored. In this work, a novel fabrication route of β-FeSi2 has been investigated, which combines the mechanical alloying (MA) of the substrates and their pulse plasma sintering [14] - a variant of the spark plasm sintering (SPS) used in the powder metallurgy. This choice of fabrication route was based on the earlier results indicating that PPS preserves the nano-structure of consolidated materials, which is of particular importance in the case of the materials for thermoelectric applications. PPS and SPS methods use electric current as source of the heat, which is generated by the Joule heating by a high-density current passing through graphite punches, die and the sintered powder. The PPS as well as SPS methods feature fast heating rates and short times of the consolidation due to the diffusion being enhanced by the electric field. The major characteristics of the both techniques are given in Table 1. Table 1. The characteristics of the PPS and SPS consolidation methods Parameter

PPS

SPS 4

Pulse voltage

Up to 1×10 V

Usually up to 12 V

Pulse current

Up to 100 kA

Usually up to 10 kA

Pulse frequency

Up to 4 Hz

From Hz to kHz

Pulse duration

150 – 250 ms

From a few to tens of ms

The PPS has already been successfully used to fabricate metal [15,16] and ceramic [17] matrix composites. It has been also used for welding [18,19] and, most importantly in the context of the present paper, preserving the nanocrystalline structure of the sintered substrates [20-24]. This is because the PPS offers a unique combination of rapid heating/cooling and short processing time, the features essential for preserving nanometer size of the consolidated powders. The rapid heating and short time of sintering prevent the grain growth and the nanorefinement of the sintered substrates. The research objective of the current study was to investigate to what degree it opens new perspectives for processing β-FeSi2 powders with enhanced thermoelectric properties. In order to assess the efficiency of the proposed technological path, based on MA and PPS,a series of specimens have been prepared of mechanically alloyed and pulse plasma sintered n (Co, P) and p (Mn, Al) type doped β-FeSi2. The obtained samples were characterized in terms of their structure and properties, including the thermoelectric power. The results are compared with the ones reported for β-FeSi2 fabricated with the previously used technologies.

1. Experimental procedure Elemental powders of Fe (purity 99.99%), Si (purity 99.999%), Co (purity 99.5%), Mn (purity 99.95%), Al (purity 99.95%) and Fe2P (purity 99.5%) were used to prepare n- and p- type FeSi2 powders using MA method. The mechanical alloying was conducted in Retsch planetary ball mill PM 100 in argon atmosphere. The mixtures of substrates were sealed in 500 ml steel containers with 50 balls, inside glove box in argon atmosphere. The mixtureto-ball weight ratio was 1:10 for all samples. Parameters of milling are given in Table 2. Preliminary milling was employed to disperse the elemental powders before the final milling. Selected compounds were manufactured: FeSi2, Fe0.92Mn0.08Si2, Fe0.97Co0.03Si2, FeAl0.07Si1.93, FeP0.05Si1.95.

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Table 2. Milling conditions and parameters Type

Mode

Effective milling period

Break

Interval

Rotation speed

Preliminary

Alternating

2 min

0

1 min

100 rpm

Main

Alternating

40h

15 min

15 min

300 rpm

It has been found experimentally that under the conditions of the main milling, 40h were needed to assure a full transformation of the substrates into α and ε phases, which form β phase after the annealing. The mixtures of the mechanically alloyed powders were PPS sintered in graphite dies with the diameter of 10 mm. The equipment used for the pulse plasma sintering has been described in details in [14]. The post-sintering annealing has been carried out in vacuum, using sealed quartz tubes, at 1123K for 10h. The density, ρ, of the samples subjected to post sintering annealing was measured using the Archimedes method. Phase analyses were performed by X-ray diffraction (XRD, Bruker, D8 Advance, Cu Kα). The XRD spectra were obtained in the 2Θ range of 15-60° where most characteristic peaks of Fe-Si system are located. Microstructure of powders and sintered samples were evaluated using scanning electron microscopy (SEM, Hitachi, SU-70) and the chemical composition of bulk samples with energy-dispersive spectroscopy (EDS). The Seebeck coefficient, S, as well as the electrical conductivity, σ, were measured using the standard four-probe method, in vacuum. The thermal diffusivity, D, was measured by the laser flash method (LFA, Netzsch, 457 MicroFlash) using samples with diameter of 10 mm and height of ~1 mm. The thermal conductivity, κ, was calculated according to the formula κ = D Cp ρ, where Cp – specific heat and ρ – density [25]. All measurements were performed within the temperature range from 323 to 773 K. 2. Results and discussion The morphology of the mechanically alloyed powders is shown in Figure 1. SEM observations revealed that the agglomerates size ranges from less than 1 to 10 μm. On the other hand, the agglomerates consist of much smaller crystal grains, with of sub-micron size. The size of the agglomerates was found acceptable for an efficient sintering with the pulse plasma device used. Thus, no efforts have been undertaken to de-agglomerate the mixtures. The relative densities of the sintered samples after the post-sintering annealing have been calculated under the assumption of the theoretical density of 4.93 g/cm3 for un-doped iron silicide, taking into account true compositions of the doped samples and atomic mass differences between Fe, Si and dopants. The obtained values are listed in Table 3. Table 3. The results of relative density measurements Sample

Relative density [%]

FeSi2

94.93

Fe0.92Mn0.08Si2

96.21

Fe0.97Co0.03Si2

95.87

FeAl0.07Si1.93

92.16

FeP0.05Si1.95

95.21

With the exception of Al doped samples, the densities varied in the range of ~95-96%. Such values agree with the results reported for SPS in [13,26]. Also, a conclusion can be drawn that Al retards consolidation of FeSi2. This effect, however, is placed outside the main thrust of the research efforts reported here. The results of XRD measurements of the samples before and after the post-sintering annealing are shown in Figure 2.

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Figure 1. SEM images of the mechanically alloyed substrate powders: a) Fe0.92Mn0.08Si2; b) Fe0.97Co0.03Si2; c) FeAl0.07Si1.93; d) FeP0.05Si1.95

Figure 2. The XRD patterns for the samples: before (a) and after (b) annealing at 1123K for 36 ks

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As the XRD patterns provide information on phase content, it can be concluded that with the exception of Fe0.92Mn0.08Si2, these patterns reveal significant presence neither of the residual α-Fe2Si5 nor ε-FeSi phases. This implies that the post sintering annealing at 1123 K for 36 ks transforms the sintered powders into β-FeSi2. Since the residual ε phase negatively impacts the thermoelectric performance of β-FeSi2, this finding gives additional rational to the new technological route investigated here. It should be noted also that the post sintering annealing at 1123 K for 36 ks transforms α- and ε- into β-FeSi2 in all samples despite the differences in the doping type and amount. This is an additional advantage of the technology proposed here as it has been reported in the past that the post processing annealing conditions need to be adjusted to the dopant type and amount [12,13,26,27]. Kinetics of the α + ε →β conversion has been known to be sluggish [9]. This peritectoid reaction is controlled by the diffusion, indicating that the transformation rate from β to α is much lower than α to β [28]. Therefore, a long-time annealing after the sintering is usually required to obtain dominant β phase in a Fe – 2Si compound. The XRD patterns have also been used to estimate the size of crystalline domains from the measurements of the peak widening (Sherrer-Debye method). The obtained estimates, given in Table 4, show that the annealing used in the present study preserves sub-micron size of the crystallites. This is because, this time is relatively short, in comparison to the one reported in [9,11,12,29]. Table 4. Crystallite size calculated using Scherrer equation Compound

Crystallite size [nm]

FeSi2

55.6

Fe0.92Mn0.08Si2

52.6

Fe0.97Co0.03Si2

53.5

FeAl0.07Si1.93

51.8

FeP0.05Si1.95

44.0

The SEM images representative of the PPS consolidated and post-sintering annealed samples are shown in Figure 3. The SEM images show that before annealing samples contain fine size grains of ε phase (nano to few micrometers in diameter). These grains are imaged as bright, while grains of α phase are imaged as dark areas. After the annealing ε phase is found only in Fe0.92Mn0.08Si2 and Fe0.97Co0.03Si2 samples. In the case of Mn doped sample post-annealing image well agrees with XRD data. In the case of Co doped sample some residual ε phase was found. In general, the SEM images confirm, nearly complete transformation of the compacts into β-FeSi2. The SEM observations agree with the results reported for the materials consolidated by hot pressing [25,30] and SPS [26]. The microstructures obtained after PPS consolidation reveal fine, evenly distributed grains of ε phase similar to that reported by Nogi et al, who defined the optimum sintering and annealing conditions for β-FeSi2 formed by slip casting [31]. One should also note that the current technological route allows to obtain much lower contents of the residual silicon and ε phase. Thermoelectric properties of the investigated samples after the post sintering annealing are shown in Figure 4. Below 800K, the thermal conductivity (Figure 4a.) of all samples decreases with the temperature, in agreement with the enhanced lattice vibrations. The n-type dopants strongly decreased thermal conductivity especially at low temperatures. The samples Fe0.97Co0.03Si2 and FeP0.05Si1.95, containing residual ε phase, reveal positive impact of this phase on conductance, because of its metallic character. It can be also concluded that Al doping impacts thermal conductivity to lesser degree than the Mn doping. As expected, each dopant used here decreased thermal conductivity via introducing defects scattering phonons [1].

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Figure 3. Representative SEM images of the samples before and after post sintering annealing: a) FeSi2; b) Fe0.92Mn0.08Si2; c) Fe0.97Co0.03Si2; d) FeAl0.07Si1.93; e) FeP0.05Si1.95

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Figure 4. Thermoelectric properties of the investigated samples after the post-sintering annealing in the function of temperatur: a) thermal conductivity; b) electrical conductivity; c) Seebeck coefficient; d) ZT;

The results of the measurements of electrical conductivity are plotted in Figure 4b. The Fe site dopants: Mn and Co, despite the fact of their opposed conduction type, both strongly enhanced σ to the highest value of 158 S/cm at 773 K for Fe0.92Mn0.08Si2. The impact on electrical conductivity is nearly linear in the whole investigated temperature range with the directional factor 0.22 and 0.32 for Co and Mn respectively. Smallest impact on electrical conductance revealed n-type P. This was most probably caused by lower concentration of charge carriers. The FeAl0.07Si1.93 sample revealed a higher σ than non-doped sample, reaching 53 S/cm in 773 K. From the Seebeck (S) coefficient graph (Figure 4c.) it can be concluded, that pure FeSi2 obtained, using the technological route proposed here, exhibits p-type conductance with the highest Seebeck coefficient reaching 392 µV/K at 448 K. The highest S value, of all samples, exhibits FeSi1.95P0.05 at low and mid temperature range up to 700 K.

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The results of calculations of figure of merit are shown in Figure 4d. The highest ZT exhibited sample Fe0.97Co0.03Si2 in entire investigated temperature range, while the smallest un-doped FeSi2 followed by FeP0.05Si1.95. Those results directly ensue from previously presented and discussed graphs. The highest observed ZT value was achieved at 773 K reaching 0.15 for Co doped sample. For the non-doped sample ZT reaches 0.01 at 673 K, which is 15 times smaller than the value for Co doped one. The Al doped samples exhibit nearly linear growth of ZT values reaching 0.06 at 773 K, at temperature which other p-type doped sample - Fe0.92Mn0.08Si2 overtakes, with ZT higher by 0.001. The results show that P doping does not have a significant influence on the figure of merit. The results obtained in the present study agree in general with those previously reported for samples synthesized with alternative techniques, as shown in Table 5. Table 5. A comparison of the S values of the specimens obtained, with the earlier results obtained for the doped FeSi2 Reference

Compound

Consolidation method

Highest |S| [μV/K]

Current study

FeSi2 Fe0.92Mn0.08Si2 Fe0.97Co0.03Si2 FeSi1.93Al0.07 FeSi1.95P0.05

Pulse Plasma Sintering

392 in 448 K 222 in 445 K 298 in 523 K 354 in 448 K 440 in 447 K

[11]

FeAl0.1Si

Hot Pressing

302 in 700 K

[12]

(Fe0.95Co0.05Si2)0.75 - (Si0.8Ge0.2)0.25

Hot Pressing

270 in 898 K

[13]

Fe0.98Co0.02Si2

Spark Plasma Sintering

242 in 673 K

[25]

FeSi1.98P0.02

Hot Pressing

280 in 450 K

[26]

Fe0.91Mn0.09Si2

Spark Plasma Sintering

207 in 673 K

[30]

Fe0.92Mn0.08Si2

Hot Pressing

279 in 442 K

Most of the samples fabricated in the present study, exhibit higher values of the Seebeck coefficient. Thus, we suggest that there is rational in further development of plasma assisted sintering combined with MA. 4. Conclusions The microstructures and thermoelectric properties of p and n-type doped β-FeSi2 obtained by mechanical alloying and pulse plasma sintering prove that this fabrication route offers an alternative to the earlier explored fabrication methods. The technological path proposed here allowed to obtain samples with relative density in general above ~95 %, (with only FeAl0.07Si1.93 exhibiting 92 %) and with the microstructure nearly fully transformed into β-FeSi2 (with the exception of Fe0.92Mn0.08Si2 sample which contained unreacted Si). The best combination of thermoelectric properties has been measured for Fe0.97Co0.03Si2 with a high Seebeck coefficient resulting in ZT reaching 0.15 at 773 K. The results show that Fe site dopants exert a stronger influence on the electrical conductivity resulting in the best thermoelectric performance at 773 K. The Si site dopants showed only one of the three σ / S / κ thermoelectric parameters high, so in their case further research is needed to optimize other two parameters. Finally, it can be concluded that the manufacturing method proposed here, combining the mechanical alloying with pulse plasma sintering, shows an advantage in comparison to the previously developed technologies in terms of shortening the post-consolidation annealing time and preventing an excessive grain growth. 3. Acknowledgements This work was conducted at Faculty of Materials and Engineering Warsaw University of Technology within the National Science Centre, Poland PRELUDIUM 7 grant entitled „The influence of dopants, nanoparticles, texture and manufacturing methods on thermoelectric properties of carbon disilicide” (2014/13/N/ST8/00619).

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