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Raphael Fortulan, Suwei Li, Michael John Reece, [Illia Serhiienko](https://orcid.org/0000-0002-3072-9412), [Takao Mori](https://orcid.org/0000-0003-2682-1846), Sima Aminorroaya Yamini

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 125, 203903 (2024) and may be found at https://doi.org/10.1063/5.0235499.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Enhanced thermoelectric performance of <i>p</i>-type BiSbTe through incorporation of magnetic CrSb](https://mdr.nims.go.jp/datasets/c83ae6ca-dc72-407c-94a6-5e096312b1d3)

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Microsoft Word - MDI-Manuscriptrev-Raphael-TM Enhanced Thermoelectric Performance of p-type BiSbTe Through Incorporation of Magnetic CrSb  Raphael Fortulan a,b,∗, Suwei Lic, Michael John Reecec, Illia Serhiienkod,e, Takao Morid,e, Sima Aminorroya Yaminib,f,∗ aMaterials and Engineering Research Institute, Sheffield Hallam University, Sheffield, UK bPresent address: Unconventional Computing Laboratory, University of the West of England, Bristol, UK cSchool of Engineering and Material Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK dInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, Tsukuba, Japan eGraduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Japan fSchool of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, 2006, Australia   Abstract  This study investigates the thermoelectric properties of p-type Bi0.5Sb1.5Te3 with added 10% Te composites containing a varying ratio of the ferromagnetic semiconductor of CrSb (0, 0.125, 0.5, and 1 wt.%) as a secondary phase. Samples were synthesized by a combination of ball milling and spark plasma sintering techniques. The incorporation of CrSb particles resulted in an increase in thermopower due to an increase in the effective mass of the charge carriers, indicating that there is a drag effect originating from the magnetic particles. However, this was at the expense of reduced electrical conductivity due to  ∗Corresponding authors Email addresses: raphael.vicentefortulan@uwe.ac.uk (Raphael Fortulan ), s.aminorroaya@uq.edu.au (Sima Aminorroya Yamini)    Preprint submitted to Elsevier May 6, 2024 2    reduced carrier mobility. While overall only marginal improvements in power factors were observed, the multiphase samples exhibited significantly lower thermal conductivity compared to the single-phase material. As a result, a peak zT value of ∼1.4 was achieved at 325 K for the sample with 0.125 wt.% CrSb. The synergistic effects of magnetically induced thermopower enhance- ment and thermal conductivity reduction resulted in showing the potential of incorporating magnetic secondary phases to optimize the thermoelectric performance in this system.  1. Introduction  Thermoelectric materials have been extensively studied as a green energy source due to their ability to directly convert thermal energy into electrical energy. The dimensionless figure of merit (zT ) is defined as zT = α2σT/κ, where α, σ, T , and κ are the thermopower, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively, is a metric that can be used to evaluate the performance of thermoelectric materials [1, 2]. To create a high-performance thermoelectric material, it is necessary to simultaneously achieve a high power factor (PF = α2σ) and a low total thermal conductivity (κ). However, α, σ, and κ exhibit strong correlations [3– 5], making the task of optimizing one parameter independently, while keeping the others constant, extremely challenging. Bismuth telluride alloys are among the most efficient thermoelectric mate- rials for near-room-temperature applications. Alloying Bi2Te3 with Sb2Te3 optimizes parameters such as carrier concentration, lattice thermal conduc- 3    tivity, and electronic band structure to improve the figure of merit zT [6, 7]. However, the thermoelectric performance of these alloys is limited by issues such as their relatively low thermopower. Utilization of magnetism has emerged as a promising strategy to in- crease thermopower through mechanisms such as paramagnon drag and spin-dependent effects [8–11]. The concept that magnetism can possibly enhance thermoelectric properties dates back decades, with an early work proposing that magnon scattering could be the origin of an increase in the thermopower of some magnetic elements at low temperatures [Sorry, I think I missed writing this reference in the previous many corrections here that I made, it is: M. Bailyn: Phys. Rev., 126, 2040 (1962). ]. The recent advancements include (1) using intrinsically magnetic semiconductors, such as MnTe [12], CuFeS2 [13, 14], and Cr2Ge2Te6 [15]; (2) doping non-magnetic thermoelectric materials with magnetic elements resulting in increased thermopower due to interactions between charge carriers and local magnetic moments [8, 16– 19]; and (3) introducing magnetic secondary phases, such as nanoparticles or inclusions, into non-magnetic thermoelectric matrices [20–23]. Studies on Ba0.3In0.3Co4Sb12 [20, 24] and Ti0.25Zr0.25Hf0.5NiSnSb [21] materials have shown that the inclusion of coherent magnetic particles can simultaneously enhance thermopower and carrier mobility. Magnetic secondary phases may al- low the tuning of properties through the composition, size, and microstructure of the materials [25–29]. In this study, ball-milled stoichiometric Bi0.5Sb1.5Te3 10 at.% Te-rich and CrSb (0, 0.125, 0.5, and 1 wt.%) were fabricated by spark plasma sintering (SPS). Excess Te was added to the system as Te-rich bismuth telluride and bismuth antimony telluride alloys showed high thermoelectric performance 4  with a significant reduction in lattice thermal conductivity and suppression of 5    defects in the system [30–33]. The inclusion of excess Te suppresses the defects caused by its easy volatilization during the SPS process [34] and this strategy has been shown to result in more efficient thermoelectric materials [33]. CrSb is a ferromagnetic semiconductor with a Curie temperature of approximately 700 K [35], which makes it a promising magnetic secondary phase candidate for improving the thermoelectric performance of bismuth antimony telluride over a wide temperature range. By incorporating varying concentrations of magnetic CrSb particles, we were able to increase the thermopower through indicated drag effect, while maintaining a relatively high electrical conductivity. The addition of a secondary phase introduces additional phonon scattering mechanisms that reduce thermal conductivity. This, combined with the high thermopower, enabled a high peak of zT ≈ 1.4 at 325 K.  2. Experimental details  2.1. Synthesis A set of multiphase polycrystalline samples of Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) were synthesized by ball milling cast ingots of Bi0.5Sb1.5Te3+0.3 and CrSb. First, Bi0.5Sb1.5Te3+0.3 samples were synthesized by direct reaction of stoichiometric amounts of high purity elements of Bi shots (99.999%, Alfa Aesar), Te pieces (99.999%, Alfa Aesar), and Sb shots (99.999%, Alfa Aesar) in vacuum-sealed quartz ampules loaded in an inert atmosphere glove box. The ampule was heated to 850 ◦C for 12 h, homogenized at 1000 ◦C, quenched in cold water, and then annealed at 400 ◦C for 72 h. Second, a pristine CrSb sample was also synthesized by loading stoichiometric amounts of Cr 60 mesh powder (99.95%, Alfa Aesar) and Sb shots (99.999%, 6    Alfa Aesar) into a vacuum-sealed quartz ampule. The ampule was heated to 850 ◦C for 24 h, mixed every 4 h, homogenized at 1160 ◦C for 1 h, and allowed to cool naturally in a furnace. The resulting ingot was hand-ground using an agate mortar and pestle housed inside a glove box, loaded into a vacuum-sealed quartz tube, and annealed at 900 ◦C for 24 hours to eliminate the presence of a CrSb2 impurity phase. The annealed powder was then loaded into a graphite die and sintered under vacuum to produce 11-mm-diameter rods by SPS (KCE FCT-H HP D-25 SD, FCT Systeme GmbH, Rauenstein, Germany) at a pressure of 50 MPa for 20 min at 1173 K. The density of the sample was 99.9% of its nominal value. In sequence, the cast ingots were manually pre-milled using an agate mortar and pestle housed inside a glovebox. The powders were weighed and placed in a 250 mL agate jar along with 20 mm agate balls and absolute ethanol (99.97%, VWR). The ball-to-powder ratio was 15:1, and the solvent-to-powder ratio used was 100 ml to 10 g [36]. The milling process was conducted using a Retsch Planetary Ball Mill PM 100 at 300 rpm for 4 h at 15 min intervals with a 5 min break and a change in direction halfway through the milling process. The jar was then placed in a desiccator for at least 15 h. The dried powders were then placed into a graphite die and sintered under vacuum to fabricate 11 mm diameter rods using SPS. The samples were sintered at 50 MPa for 5 min at 400 ◦C. The densities of all the samples were measured based on the rod dimensions and weight. The average density of the samples was approximately 95% of their nominal density. 7    3. Materials characterization  The phase purity and crystal structure of the sintered samples were characterized by Powder X-ray Diffraction (PXRD) using a PANalytical X’Pert Pro diffractometer with CuKα1 radiation (λ = 0.154 06 nm, 40 kV, 40 mA). Electrical conductivity (σ) and thermopower (α) were measured perpendicular to the sintering direction of the samples by cutting ≈2 × 2 × 8 mm3 bar specimens from the rods. Measurements were conducted from room temperature to 523 K under helium atmosphere using a Linseis LSR-3 apparatus. The thermal diffusivity (D) of all the samples was measured by the LFA method using a NETZSCH LFA 467 HyperFlash® instrument. Slab-shaped samples were also cut to measure the room-temperature Hall coefficient (RH) under a ±0.55 T magnetic field using an ECOPIA 3000 Hall Effect Measurement System. The Hall carrier concentration (nH) was calculated as nH = 1/(e · RH). Heat capacity measurements were conducted from room temperature to 473 K using a PerkinElmer DSC 8000 according to the ASTM sapphire standard method E1269 and from 473 K to 773 K using a TA Instruments SDT 650 simultaneous thermal analyzer according to the ASTM modulated DSC method E2716. The measured heat capacity values are shown in the Supporting Information.  4. Results and discussion  4.1. Structural and phase analysis The phase purity and crystal structure of all samples were determined by indexing their PXRD patterns, as shown in figure 1. All samples matched 8    a single-phase rhombohedral Bi0.5Sb1.5Te3 phase, with no impurity phases detected. It should be noted that the typical detection range for XRD phase analysis is 5 ∼ 10% [37]. Therefore, given the low concentrations of this set of samples, it was expected that no CrSb peaks would be detected. The phase  10 20 30 40 50 60 70 80 90 100 110 2θ∘  Figure 1: Powder X-ray diffraction patterns of Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples.  purity and crystal structure of the CrSb sample were determined by indexing its PXRD pattern (see fig. S2 in the Supporting Information). 4.2. Transport properties Figure 2 shows the thermopower, electrical conductivity, and power factor of Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1), measured perpendicular to the direction of sintering. All samples showed a positive Intensity (a.u.)      1 wt.% 0.5 wt.% 0.125 wt.% 0 wt.% 9  α (µ V.K )   thermopower (fig. 2(a)), indicating a p-type semiconductor behavior. The electrical conductivity exhibited metallic behavior with decreasing values, as shown in figure 2(b).  (a)   300   250   200   150   100 (b)  4  3.5  3  2.5  2  1.5  1 300 350 400 450 500 550 Temperature (K) 300 350 400 450 500 550 Temperature (K)  (c) 16  14  12  10  8  6  4 300 350 Te 400 450 500 550 mperature (K)  Figure 2: Temperature dependence of the (a) thermopower, (b) electrical conductivity, and (c) power factor of ball milled Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples.             PF (mW.m-1.K-2σ (× 104 S.m-110    The room-temperature values of the thermopower increase with the ad- dition of the second phase, rising from ∼238 µV.K−1 for the single-phase sample and from ∼215 µV.K−1 to ∼270 µV.K−1 for the 1 wt.% CrSb. With the opposite effect on electrical conductivity, the electrical conductivity of the single-phase sample decreased from ∼2.7 × 104 S.m−1 to ∼1.8 × 104 S.m−1. Table 1 lists the Hall carrier concentrations and mobilities of the samples. The carrier concentrations are lower than those observed in the literature (e.g. ∼1 × 1019 cm−3 [33]) which the overall lower electrical conductivity of the samples. The mobility values of 232 cm2.V−1.s−1 of the single-phase samples are similar to those reported in the literature (e.g., 248 cm2.V−1.s−1 [30]). The values of nH ranged from 7.7 ∼ 7.1 1018 cm−3, indicating that the addition of Te to the system stabilized the carrier concentration of the samples. However, the mobility decreases with the inclusion of the secondary phase, which is consistent with the behavior of multiphase materials [38, 39].   Table 1: Hall carrier concentration and mobility of ball milled Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples  x nH(× 1018 cm−3) µH (cm2.V−1.s−1) 0 7.7 232.0 0.125 7.7 203.9 0.5 7.3 185.5 1 7.1 161.6  The thermopower of the CrSb-added samples increased overall with the inclusion of the secondary phase. Since the carrier concentration of all samples 11    has similar values, these changes cannot simply be attributed to changes in the values of nH. The behavior of the band structure with CrSb inclusion was analyzed by modeling the thermopower and Hall carrier concentration using the single parabolic band (SPB) model [40] (see Supporting Information for more details). The SPB model used here may not fully and accurately determine the behavior of multiphase samples due to the presence of bipolar conduction, complex scattering processes, and the non-parabolic nature of the valence band. However, this approach can indicate a trend in the calculated effective mass m∗ of samples [41]. The calculated effective masses for all the samples are listed in table 2. The effective mass of the samples increased  Table 2: Calculated effective mass of ball milled Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples using the single parabolic band model  x m∗/m0 0 0.90 0.125 1.16 0.5 1.06 1 1.21  in the presence of the secondary phase, in alignment with the trend of the thermopower. This trend is visible in both table 2 and in the dependence between the thermopower α, carrier concentration n, and m∗ shown in the Pisarenko plot in figure 3. The thermopower for a degenerate semiconductor, with a parabolic band and energy-independent scattering approximation, can 12    be written as [40]  8π2kB  ∗   π  2/3 α =  3qh2 m T 3n , (1) where m∗ is the effective mass of the DOS.  300  250  200  150  100  50 1019 1020 Carrie r concentration, n (cm-3)  Figure 3: The Pisarenko plots (thermopower versus Hall carrier concentration) at room temperature for ball milled Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1). The dashed lines represent the calculated values from the SPB model using the effective masses as shown in the inset.  Equation 1 shows a direct proportionality between the thermopower and effective mass. This suggests that the presence of a magnetic phase in the material can lead to an increase in the effective mass of the sample and consequently an increase in the thermopower, similar to observations of studies with magnetic dopants [16, 42]. Interestingly, the results show that despite the presence of a magnetic secondary phase with poor thermoelectric performance and low thermoelectric power of approximately 12.5 µV.K−1 at 0 wt.% 0.125 wt.% 0.5 wt.% 1 wt.%     The rmopowe r, α (µV.K-1 ) 13    room temperature and 4.38 µV.K−1 at 545 K, the electrical performance of the samples can be improved rather than degraded (see Supporting Information for CrSb data). However, the combined decrease in carrier mobility due to the presence of an additional phase and the possible dragging effect caused by the magnetic phase severely degrades the electrical conductivity of multiphase samples [16]. Consequently, there was only a marginal increase in the power factor PF (as shown in fig. 2(c)). The thermal conductivity (κ) of the samples is shown in figure 4(a). As the temperature increased, the thermal conductivity (κ) of all samples started to increase. The electronic contribution to the thermal conductivity (κe) is shown in figure 4(b) and it was estimated using the Wiedemann-Franz law [43] (κe = LσT ) where L is the Lorenz number, and it was calculated using the SPB model (see Supporting Information for more details). Since multiphase samples have lower electrical conductivity, κe is reduced accordingly. The lattice (κl) contributions to thermal conductivity were calculated as κl = κ − κe, as shown in figure 4(c). All samples exhibited optimized behavior when compared to the single-phase sample. 14  0 wt.% 0.125 wt.% 0.5 wt.% 1 wt.% 0 wt.% 0.125 wt.% 0.5 wt.% 1 wt.% 0 wt.% 0.125 wt.% 0.5 wt.% 1 wt.% κ l (W.m-1.K-1κ e (W.m-1.K-1     (a)   2.5  2  1.5  1  0.5 (b)    0.3  0.25  0.2  0.15  0.1  0 300 350 400 450 500 550 Temperature (K) 0.05 300 350 400 450 500 550 Temperature (K)  (c)   2   1.5   1   0.5  0 300 350 400 450 500 550 Temperature (K)  Figure 4: Temperature dependence of the (a) thermal conductivity, (b) electronic thermal conductivity, and (c) lattice thermal conductivity of Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples. κ (W.m-1.K-115    The figure of merit (zT ) of the samples is shown in Figure 5. The combination of significantly reduced thermal conductivity, aligned with an increase in thermal power due to the incorporation of the magnetic secondary phase, contributed to the high zT values for the multiphase samples compared to the single-phase Bi0.5Sb1.5Te3+0.3 material. The 0.125 wt.% CrSb sample exhibited the highest zT, reaching a peak value of ∼1.4 at 325 K. This represents a remarkable ∼400% improvement over the pristine sample, which only achieved zT ≈ 0.35 at the same temperature. The other multiphase compositions also showed promising zT improvements, albeit to a lesser extent. The 0.5 wt.% CrSb had the next highest zT of ∼0.92 at 525 K, followed by the 1 wt.% at ∼0.68. This trend suggests that similar to doping, there is likely to be an optimum doping level beyond which the benefits diminish due to the excessive degradation of the electrical properties. The high zT values result from the synergistic effects of magnetically induced thermopower enhancement and thermal conductivity reduction due to interfacial scattering in the multiphase samples. These results suggest that the incorporation of magnetic phases may be an effective strategy to optimize the thermoelectric performance of bulk materials such as Bi0.5Sb1.5Te3+0.3. 16      1.4 1.2 1 0.8 0.6 0.4 0.2 0  300 350 400 450 500 550 Temperature (K)  Figure 5: Temperature dependence of zT of Bi0.5Sb1.5Te3+0.3 with x wt.% CrSb (x = 0, 0.125, 0.5, and 1) samples.  5. Conclusions  In this study, a series of multiphase Bi0.5Sb1.5Te3+0.3 samples with varying concentrations of CrSb magnetic secondary phase (0, 0.125, 0.5, and 1 wt. %) were successfully synthesized by a combination of ball milling and spark plasma sintering techniques.  The results showed that the incorporation of small amounts of the CrSb magnetic phase significantly enhanced the thermopower of the samples by increasing the carriers’ effective mass, which is consistent with previous findings for magnetic dopants. However, the electrical 0 wt.% 0.125 wt.% 0.5 wt.% 1 wt.% zT 17    conductivity is adversely affected by the reduced carrier mobility caused by the presence of the secondary phase. As a result, only marginal improvements in power factor were observed. Despite the limited power factor improvement, the multiphase samples exhibited significantly lower thermal conductivity than the single-phase material. This effect, combined with the increased power factors, resulted in a relatively high figure of merit (zT ) values for multiphase compounds, particularly at around room temperature. These results confirm the potential benefits of incorporating magnetic secondary phases into thermoelectric materials to modulate their electronic and thermal transport properties favorably.  Acknowledgements  This study was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 801604. This work also received support from the Henry Royce Institute for Advanced Materials, funded through EPSRC grants EP/R00661X/1, EP/S019367/1, EP/P025021/1, and EP/P025498/1. TM would like to thank JST Mirai Program Grant Number JPMJMI19A1. IS was also supported by JST SPRING, Grant Number JPMJSP2124. 18    References  1.  Tang, Y. et al. Convergence of Multi-Valley Bands as the Electronic Origin of High Thermoelectric Performance in CoSb3 Skutterudites. Nature Materials 14, 1223–1228. issn: 1476-4660. (2024) (Dec. 2015). 2.  Wu, Y. et al. Lattice Strain Advances Thermoelectrics. Joule 3, 1276– 1288 (2019). 3. Zhang, M. et al. 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