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Manipulating the combustion wave during self-propagating synthesis for high thermoelectric performance of layered oxychalcogenide Bi1-xPbxCuSeO Dongwang Yang, Xianli Su, Yonggao Yan, Tiezheng Hu, Hongyao Xie, Jian He, Ctirad Uher, Mercouri G. Kanatzidis, and Xinfeng Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01291 • Publication Date (Web): 11 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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Chemistry of Materials

Manipulating the combustion wave during self-propagating synthesis for high thermoelectric performance of layered oxychalcogenide Bi1-xPbxCuSeO Dongwang Yang†, Xianli Su†*, Yonggao Yan†, Tiezheng Hu†, Hongyao Xie†, Jian He‡, Ctirad Uherξ, Mercouri G. Kanatzidis# and Xinfeng Tang†* †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. ‡

Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA.

ξ

Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA.

#

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA

Supporting Information ABSTRACT: Novel time- and energy-efficient synthesis methods, especially those adaptable to large-scale industrial processing, are of vital importance for broader applications of thermoelectrics. We herein reported a case study of layerstructured oxychalcogenides Bi1-xPbxCuSeO (x = 0 to 10%) with emphases on the reaction mechanism of self-propagating high-temperature synthesis (SHS) and the impact of SHS conditions on the thermoelectric properties. The combined results of X-ray powder diffraction, differential scanning calorimetry, and quenching experiments corroborated that the SHS process of BiCuSeO consisted two fast binary SHS reactions (2Bi+3Se→Bi2Se3 and 2Cu+Se→Cu2Se) intimately coupled with two relatively slow solid-state diffusion reactions (2Bi2Se3+B2O3→3Bi2SeO2 and then Bi2SeO2+Cu2Se→2BiCuSeO). The formation rate of the reaction intermediate Bi2SeO2 was the bottleneck in the SHS process of BiCuSeO. Importantly, we found that adding PbO in the starting materials has (i) facilitated the formation of Bi2SeO2 and thus significantly reduced the SHS reaction time; (ii) improved the phase purity and sample homogeneity; (iii) increased the power factor via increasing both carrier concentration and effective mass; and (iv) reduced the lattice thermal conductivity via more point defect phonon scattering. As a result, a state-of-the-art ZT value ~ 1.2 has been attained at 923 K for Bi0.94Pb0.06CuSeO. These results not only open a new avenue for mass production of single phased multinary thermoelectric materials but also inspire more investigation into the SHS mechanisms of multinary materials in diverse fields of material science and engineering.

1. INTRODUCTION Thermoelectric (TE) materials convert waste heat into electricity via the Seebeck effect and they function as solidstate Peltier coolers, thereby ensuring an important role in our global solution package of renewable energy1-3. The efficiency of a TE material is gauged by its dimensionless figure of merit ZT, defined as ZT=α2σT/κ, where α, σ, κ and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity and the absolute temperature, respectively. Significant advances in the efficiency of thermoelectric materials have raised prospects for commercialization4-8. Toward

broader commercial applications of thermoelectrics, there have been two long standing grand challenges: first, developing novel higher performance TE materials based on naturally abundant and environmentally friendly elements9-15; second, developing novel time- and cost-efficient material synthesis procedures adaptable to large-scale industrial production 16-24. Recently, lead and tellurium free compounds have drawn much attention due to their excellent thermoelectric properties6-14, 25, 26. In the same vein, BiCuSeO-based compounds are interesting. BiCuSeO adopts a layer structured “hybrid” crystal structure with conductive layers of (Cu2Se2)2-

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alternating with insulating (Bi2O2)2+ layers (Fig.1)27, allowing for optimizing the electrical and thermal properties somewhat individually. BiCuSeO-based compounds possess very low lattice thermal conductivity28-33, favorable for thermoelectrics, we shall discuss the underlying mechanisms in section 3.6. On the other hand, due to the strong ionic bonds between the conducting and insulating layers, the electronic properties of undoped (pristine) BiCuSeO are rather poor30. Cu vacancies34,35, dual vacancies36, alkali metal dopants37,38, alkaline-earth dopants39-42,and heavy element dopants such as Pb on the Bi site29,43-45, Te doping on the Se-site46,47 have proved effective in improving the electrical properties of BiCuSeO-based compounds. As a result, the thermoelectric figure of merit ZT above unity has been achieved in the intermediate temperature range. Despite the promising TE performance and being made of naturally abundant elements, the traditional synthesis methods of BiCuSeO-based compounds are involved with multi-step high-temperature solid-state reactions and/or high-energy ball milling processes27-30, 34, 36-44, 46-48. These traditional procedures have a propensity for the formation of impurity phases and also quite time- and energy-consuming by the standard of industrial mass production. In this work, we employed the self-propagating high-temperature synthesis (SHS) technique, known for its time- and energyefficiency, to synthesize BiCuSeO-based compounds. Since 1960s, the SHS technique has been utilized to fabricate refractory inorganic compounds49,50. Once ignited at one end of the pellet made of starting materials, selfsustained combustion wave propagates through the pellet driven by exothermic reactions, neither external energy input nor sample container is needed. The SHS process only takes a few seconds to finish, the requirement for environmental control is minimal. The SHS equipment is thus simple and easy to scale up49. Finally, the propagation of combustion wave front tends to self-purify the as formed compound in a way similar to the zone refining process49. The adiabatic combustion temperature (Tad), the experimentally measured combustion temperature (Tc), the speed of combustion wave (υ), and the reaction activation energy (E) are the most important SHS parameters51. While a Tad value in excess of 1800 K usually guarantees a SHS process, many SHS processes can be realized at lower temperaures52. We have recently demonstrated the viability of low temperature SHS of binary TE materials such as Bi2Te3, Bi2Se3, Cu2Se, PbS, PbSe, SnTe, Mg2Si, Mg2Sn and a few ternary TE materials such as Cu2SnSe3 and ZrNiSn17-19. Notwithstanding the success in binary and ternary TE materials, the SHS of single phased higher multinary TE materials is scarce45; more importantly, the underlying reaction mechanism is inadequately understood. We in this work extended the SHS technique from binary and ternary compounds to quaternary and quinary Bi1-xPbxCuSeO compounds. As a caveat, many experimental methods proved effective in the study of SHS of binary and ternary compounds turned out to be inadequate for Bi1-xPbxCuSeO. The high level of complexity warranted a concerted effort of X-ray diffraction, differential scanning calorimetry, and quenching experiments. Our main objective is to clarify the reaction mechanism underlying the SHS synthesis of single phased quinary Bi1-xPbxCuSeO (c.f. section 3.1

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to 3.4). The results will improve our understanding of SHS of higher multinary compounds.

2. EXPERIMENTAL SECTION Synthesis. Bi2O3 (5N, 200 mesh), PbO (5N, 200 mesh), Bi (5N, 200 mesh), Cu (4N, 200 mesh), and Se (5N, 100 mesh) powders were weighed according to Eq.1 (x=0, 2%, 4%, 6%, 8%, 10%), (1-x)/3Bi2O3+xPbO+(1-x)/3Bi+Cu+ Se = Bi1-xPbxCuSeO

(1)

The thoroughly mixed and hand ground powders were cold pressed into pellets at a pressure of 5 MPa, 10 MPa, 15 MPa, and 20 MPa for 10 min to achieve different packing densities (porosities), respectively. The packing density helped control the kinetics of SHS process and extract the reaction activation energy E. The pellet was ignited by Joule heating a carbon foil in vacuum in a homemade SHS apparatus (Fig. 2a). ADC power supply (HAP 30-30, Hua Tai®, China) provided a current of 80 A and a voltage of 15 V to the carbon foil. The two most important SHS parameters Tc and υ were measured by a thermocouple (K-type) centered in the pellet (Fig. 2a) and a high-speed camera (i-SPEED-3, OLYMPUS®, Japan) at a sampling frequency of 400 fps, respectively. Figure 2(b) shows different stages of a typical SHS process recorded by high-speed camera, namely (i) local ignition; (ii) combustion wave propagation; (iii) product cooling. The as reacted product was hand ground and consolidated by spark plasma sintering (SPS) (SPS1050, Sumimoto®, Japan) under a pressure of 40 MPa in vacuum at 973 K for 7 min. The resulting cylindrical pellets with a diameter of 15 mm and a height of 12 mm were cut into different shapes for the TE property measurements. Electrical Transport Properties. The electrical conductivity and Seebeck coefficient were measured on barshaped samples cutting along the directions both perpendicular to and parallel with the SPS pressing direction40. Since the TE properties measured in the perpendicular direction are superior to those in the parallel direction, the TE properties discussed here after are referred to the former, if not otherwise noted. The measurements were conducted on an ULVAC-RIKO ZEM-3 (Ulvac®, Japan) instrument under a helium atmosphere from 300 to 923 K. The Hall coefficient (RH) at 300 K, was measured using a 5-wire configuration on a Physical Properties Measurement System (PPMS-9, Quantum Design®, USA) with the magnetic field strength sweeping between +/-1.0 T. The effective carrier concentration (pH) was calculated by the formulas: pH=1/eRH, with e being the electron charge. Thermal Conductivity. The thermal conductivity was calculated using the relation κ =D·Cp·ρ, where the thermal diffusivity D was measured on a LFA457 (Netzsch®, Germany) laser flash apparatus. Thermal diffusivity (D) measurements were performed on square-shaped samples cutting along the direction parallel with the SPS pressing direction to ensure that all TE properties are measured in the same direction. High temperature specific heat capacity Cp (300-923 K) was determined by a Differential Scanning Calorimetry (DSC) apparatus (Q20, Thermal Analysis®, USA), while low temperature Cp(2-300 K) was measured on the


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PPMS-9 system. The packing density ρ of the sample was measured by the Archimedes method. X-ray Diffraction and Microscopy. Phase purity of all samples was inspected by X-ray powder diffraction (Empyrean, Cu Kα line, PANalytical®, Holland). Images of freshly fractured surfaces were taken by field emission scanning electron microscopy (FESEM) (SU8000, Hitachi®, Japan) with energydispersive X-ray spectroscopy (EDS) (XFlash6160, BRUKER®, Germany). Secondary electron images and back scattering images were taken by EPMA (JXA-8100, JEOL®, Japan).

3. RESULTS AND DISCUSSION 3.1 Single phased BiCuSeO by SHS technique Figure 2(c) displays the time dependent temperature profile of the SHS process of BiCuSeO recorded by the embedded thermocouple. The recorded temperature quickly increased and reached the maximum temperature at 723 K within 10 seconds before decreasing monotonically. The speed of combustion wave υ was determined to be 0.94 mm/s for BiCuSeO, using the built-in video analysis software. This υ value falls within the range of literature data49. Figure 2(d) presents the XRD pattern of the final product. Indeed, single phased BiCuSeO was formed.

3.2 Multi-staged and coupled combustion reactions in the SHS of BiCuSeO In previous study of binary and ternary combustion reactions, DSC has been extensively used to “simulate” the SHS process53,54. For BiCuSeO, we adopted the same methodology. A cold pressed disc of stoichiometric mixture of Bi2O3, Bi, Cu and Se was cycled in a DSC, a fastest possible heating rate of 85 K/min and a fastest possible cooling rate of 50 K/min at an Ar flow of 50 mL/min between Tmin = 273 K and Tmax = 753 K. It is imperative to note such simulation is inherently limited by the heating rate of DSC that is orders of magnitude lower than that of SHS, thus the reaction mechanism deduced from DSC simulation may not always be the case in an actual SHS process. Extra caution should be taken in dealing with the SHS of higher multinary compounds. As shown in Figure 3(a), there are three endothermic peaks, Q1 (494 K), Q3 (545 K), and Q4 (665 K), and one exothermic peak Q2 (504 K). To verify the nature of each peak, we varied Tmax = 443 K, 490 K, 540 K and 643 K that are respectively lower than Q1, Q2, Q3, and Q4, and conducted the same heating-cooling DSC cycle. Figures 3(b)-(c) show the XRD pattern of the final product after cycling to different Tmax. At about 494 K, Se melts as featured by an endothermic peak (Q1), which activates the system18. Liquid Se rapidly reacts with Bi and Cu and form Bi2Se3 and Cu2Se, which is featured by a high exothermic peak (Q2) at 504 K. At 545 K, Bi melts, featured by an endothermic peak (Q3). As the temperature rises further, Bi2O3, Bi2Se3 and Cu2Se react to form the target product, the title compound BiCuSeO (Q4).The following reaction scheme is suggested: +Q1:Se(s)=Se(l) -Q2:2Bi(s) +3Se(l)=Bi2Se3(s) 2Cu(s)+Se(l)=Cu2Se(s)

(2) (3) (4)

+Q3:Bi(s)=Bi(l) +Q4:Bi2Se3(s)+2Bi2O3(s)+3Cu2Se(s)=6BiCuSeO(s)

(5) (6)

The “s” and “l” in the parenthesis stand for “solid” and “liquid”, the “+” and “-”in front of Q refer to endothermal and exothermal, respectively. To verify the reaction scheme (2)-(6), especially the validity of reaction (6), we have crosschecked the results of combustion reactions between the starting materials Bi2O3, Bi, Cu and Se at the same molar ratio as given in Eq. (1). Basically we broke the quaternary reaction (1) down into a number of binary and ternary combustion reactions to clarify the interplay between the reactants. We first look at the results of binary combustion reactions. As shown in Figs. 4(a) and 4(b), both reaction (3) and reaction (4) are self-sustained with a combustion wave speed of 6.0 mm/s and 5.6 mm/s18, respectively. In contrast, Bi2O3 doesn’t react with Bi, Cu or Se, and Bi doesn’t react with Cu in a self-sustained manner (c.f. Figs. S2(a-d) in the Supporting Information). Next, we inspect the results of ternary combustion reactions. The reaction Bi2O3+Cu+Se yields only Cu2Se, with Bi2O3 staying unreacted (c.f. Fig. S2(e) in the Supporting Information). Figure 4(c) suggests that Bi2O3+Bi+Se react and form a reaction intermediate Bi2SeO2 along with some unreacted Bi2O3, the combustion wave speed is 1.0 mm/s. We note that Bi2O3 doesn’t react with Bi2Se3 unless they are heated at 773 K or above for 6 hours or longer (Fig. 4(d)). All these results point toward the following reaction, 2Bi2O3 +2Bi +3Se ⇌3Bi2SeO2

(7)

The presence of the reaction intermediate Bi2SeO2 questions the validity of the reaction (6). Actually we have tried a combustion reaction of (6) but to no avail. The reaction (7) poses an immediate question as to how the title compound BiCuSeO is derived from the reaction intermediate Bi2SeO2. To answer, we first experimentally verified that Bi2SeO2 doesn’t react with either Cu or Se, but it can react with a mixture of Cu and Se and a combustion wave speed of 1.5 mm/s, yielding single phased BiCuSeO (c.f. Figs. S2(f) and 4(e)). For completeness, we have also verified that the formation of BiCuSeO requires heating Cu2Se and Bi2SeO2 at 1123 K or above for at least 5 hours (Fig. 4(f)). Thus it is plausible to argue that final stage of the SHS process is: 2Cu+Se +Bi2SeO2 =2BiCuSeO

(8)

in place of Eq. (6).Though Eq. (3), (4), (7) and (8) represent a self consistent reaction scheme of the SHS process of BiCuSeO, the presence of Bi2SeO2 was deduced from indirect evidences. Direct XRD determination of Bi2SeO2 was to no avail, suggesting the transient nature of Bi2SeO2 or the limitation of DSC simulation. We are conducting further investigations to pinpoint the reaction scheme.

3.3 Quenching to gain new insight into the SHS process of BiCuSeO SHS process is a spatiotemporal process in the pellet, quenching the pellet55,56 in a stainless steel die (Fig. 5(a)) allows us to take a snap shot. The results of quenching experiments provide more insights into the kinetics of SHS process. Upon quenching, the combustion wave is extinguished



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as the heat is effectively shunted into the die. Figs. 5(b) displays the typical morphology of a quenched pellet, the effectiveness of quenching is reflected by the sharp boundary between the reacted and unreacted portion. Fig. 5(c) shows the cross-sectional area of a quenched pellet cut along the direction in which the combustion wave propagates. For the sake of discussion, we divided the cross sectional area into a few regions individually numbered. Each of these regions was examined using FESEM and XRD techniques. The XRD patterns are shown in Figs. 5(d), while the results of regional composition analysis are presented in Table S1 of the Supporting Information. The combined results of FESEM and XRD measurements show that (i) region 1 is the mixture zone with unreacted raw materials; (ii) regions 2-5 are the preheated zone, the average stoichiometry is close to Bi:Se = 2:3 despite some spot-to-spot inhomogeneity; (iii) regions 6-21 are the reaction zone behind the combustion wave front, where single phased BiCuSeO of various amount is formed. The region between the preheating zone and the reaction zone is designated as the combustion wave front. Region 22 is the product zone, formed purely by single phased BiCuSeO. The regional compositional analysis (Table S1) shows that BiCuSeO is formed immediately after the combustion wave front without detectable Cu2Se or Bi2SeO2. This observation suggests that as soon as Bi2SeO2 forms, it reacts with the Cu and Se and transforms into BiCuSeO right away. On the other hand, Bi2O3 is detected over a large portion of the reaction zone, suggesting the reactions involving Bi2O3 are slow. In light of Eq. (7), we conclude that the formation rate of reaction intermediate Bi2SeO2 is the bottleneck in the SHS process of BiCuSeO. As shown above, the SHS process of BiCuSeO is complex, it is thus instructive to inspect the as formed microstructures in the quenched sample. Figures 5(e) through (h) show the microstructure and EDS results obtained by FESEM, spanning from (e) the mixture zone 1, (f) the preheating zone 3, (g) the reaction zone 11, (h) the product zone 22. In the mixture zone 1, all raw materials maintain their morphology, and are mixed thoroughly (Fig. 5(e)). When the mixture of reactants is preheated in the proximity of the combustion wave front, ribbon-like structures are formed (Fig. 5(f)). EDS results show that Bi-Se compounds, form by Bi dissolving in molten Se, with the average stoichiometry Bi:Se = 2:3. After the combustion wave front passes, the ribbon-like Bi2Se3 in the reaction zone 11 rapidly reacts with surrounding Bi2O3, Se and Cu and form BiCuSeO in the form of sub-μm thin platelets (Fig. 5(g)). As the temperature is sufficiently high and for sufficiently long time after the passage of combustion wave front, grain coarsening of BiSeCuO occurs in the production zone 22 (Fig. 5(h)).

3.4 Lowering the reaction activation energy by Pb doping We have determined that the overall speed of the reaction (1) is only 0.94 mm/s, much slower compared to the counterpart of 6 mm/s and 5.6 mm/s for the reaction (3) and (4), respectively18. The bottleneck is thus the low speed of the reaction (7), which is about 1 mm/s. In other words, the for-

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mation rate of the reaction intermediate Bi2SeO2 restricts the overall SHS reaction rate of BiCuSeO. It is highly desired to facilitate the formation of Bi2SeO2. Our preliminary study of adding PbO in the starting materials showed promising results: upon the addition of PbO, the combustion temperature Tc has risen dramatically from 748 K for the Pb-free pellet to 920 K for the pellet (60% relatively dense) containing 10% of Pb. Meanwhile, the average speed of the combustion wave υ substantially increased from 0.94 mm/s to 1.28 mm/s (c.f. Fig. S3 and Table S2 in the Supporting Information). These observations corroborate that the addition of PbO lowers the reaction activation energy E. The reminder of this section will be devoted to experimentally determine the E value in the SHS process of BiCuSeObased materials. A SHS process generally consists a self-sustained combustion wave coupled with mass transport, which is hard to be fully described theoretically51. Nonetheless, assuming the heat release rate being proportional to the diffusion rate of reactants through the product layer, one can drive a reaction kinetics equation in which the speed of combustion wave is given by51:

υ 2 = f (n)(c pκ / Q)(RTc2 / E )K 0 exp(− E / RTc ) (9) Here, υ is the speed of combustion wave, f(n) a function

of the reaction order n, c p the average mass capacity, κ the thermal conductivity, Q the heat of reaction per unit mass, R the gas content, Tc the combustion temperature (in practice we use T* , the separatrix temperature, to approximate Tc, c.f. Fig. S1 and the discussion in the Supporting Information), E the (apparent) reaction activation energy, and K0 a constant. The value of E of the SHS reaction can be derived from the slope of the Arrhenius plot (-In(υ/Tc) vs. (1/Tc)). In this work, we used the packing density ρ to systematically vary the Tc and υ value (c.f. Fig. S3 and Table S2 of the Supporting Information). We found that the value of E is reduced from 79 kJ/mol for pure BiCuSeO to 39 kJ/mol for Bi0.9Pb1.0CuSeO (Fig. 6(a)).

3.5 Phase compositions and bulk microstructure Pb doped BiCuSeO compounds synthesized by the SHS technique remained phase pure. As shown in Fig. 6(b), the final products of Bi1-xPbxCuSeO are well indexed into ICSD#98-015-9474. The lattice constants a and c consistently increase with increasing Pb content (Fig. 6(c)), consistent with Vegard’s law and as expected from an ionic radius of Pb2+ (119pm) larger than Bi3+ (103pm). This is a piece of evidence that Pb enters the lattice of BiCuSeO. Pb doping modifies the electronic charge of each layer to become [(Bi1(2-2x)+ and [Cu2Se2]2-, using a simple electron count. xPbx)2O2] Therefore, the doping led to a decrease of Coulomb attraction between the layers28,30，and a larger increase of the caxis lattice constant than the a-axis lattice constant. Figures 7(a) and 7(b) show the fracture surface morphology of BiCuSeO and Bi0.94Pb0.06CuSeO after SHS+SPS. The samples are well compacted with a relative density higher than 99%. The grain size has increased to 2-10 µm, and the


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grain surface and boundary are rather clean. Figures 7(c) and 7(d) depict the secondary electron images and back scattering images of a polished surface of Bi0.94Pb0.06CuSeO obtained by EPMA. The consistent composition contrast image documents the high phase purity of the final product. The inset in Fig. 7(d) shows the DSC heat flow curve lacking endothermic and exothermic peaks in the range between room temperature and 823 K, and that is a further evidence of fully completed phase transitions during the SHS process.

3.6 Thermoelectric properties of Bi1-xPbxCuSeO Figure 8 presents the electrical transport properties as a function of temperature for SHS+SPS Bi1-xPbxCuSeO (x=0, 2%, 4%, 6%, 8%, 10%) samples. Pb doping facilitates a rapid crossover from an intrinsic semiconducting behavior to a degenerate semiconductor behavior in both electrical conductivity (Fig. 8(a)) and Seebeck coefficient (Fig. 8(b)) in terms of the temperature dependence and magnitude. Pristine BiCuSeO attains the maximum Seebeck coefficient of about 450 µV/K between 400-500 K. The effective carrier concentration, assuming a single carrier system, is obtained from the relation pH=1/RHe. At 300K, the hole concentration drastically increases from 2.03×1017 cm-3 for the x = 0 sample to 1.51×1021 cm-3 for the x=10% sample, showcasing the efficacy of hole doping. The experimentally measured carrier concentration follows with the theoretical expectation (i.e., one substitutional Pb donates one hole), as shown in Fig. 8(c). We note that pH in the pristine sample synthesized by a two-step solid state reaction28 is on the order of 1018 cm-3, an order of magnitude higher than our pristine SHS sample. In light of the fact that the SHS+SPS samples tend to have high crystallinity (sharp XRD peaks in Fig. 6(b) and tens of µm grain size with clean grain boundaries in Fig. 7), this observation suggests the pristine SHS sample has less defects, charged or neutral. Figure 8(d) presents the dependence of the Seebeck coefficient on the carrier concentration, which is a graphical representation of the Pisarenko relation given in Eq. 10 that assumes a single parabolic band dominated by acoustic phonon scattering3,

8π 2κ B2 T *  π m  α= 3eh2  3 pH

  

2 3

(10)

Here, α is the Seebeck coefficient, kB the Boltzmann constant, T the absolute temperature, e the elementary charge, h the Planck constant, m* the density of states effective mass, and pH the carrier concentration. It should be noted that single parabolic band model is a semi-quantitative approximation to the actual multiple band conduction in Pbdoped BiCuSeO (the inset of Fig. 8(d)). Hence we restrict our discussion to the trend of variation of m* with the Pb content, rather than the precise value of m*. The Pisarenko plot thus semi-quantitatively describes the impact of Pb doping on the m*. Figure 8(d) shows the Pisarenko plot at 300 K. The m* systematically increases from 0.5 me to 6.5 me with increasing pH. These observations are consistent with the electron band structure of BiCuSeO. As shown in the inset of Fig. 8(d), the valence band structure30,37-39,57 consists of a light band lying along the Γ-M line, and two heavier bands

lying about 0.15 eV below this band at point Z and along the Γ-X line30,39. The electrical transport in lightly doped BiCuSeO is thus dominated by the light hole band. As the doping ratio increases, the Fermi level moves deeper into the valence band, the two closely lying heavier bands start to participate in the electrical transport. Figure 9(a) displays the temperature dependent total thermal conductivity κtot of SHS+SPS Bi1-xPbxCuSeO (x=0, 2%, 4%, 6%, 8%, 10%) samples. The κtot of the x=0 sample decreases with the increasing temperature from 0.96 Wm-1K-1 at 300 K to 0.46 Wm-1K-1 at 923 K, in good agreement with literature data28.The κtot increases with the increasing level of Pb doping, but remains low in the entire temperature range (< 1.2 Wm-1K-1). According to the Wiedemann-Franz relation, κel=LσT, where the Lorenz constant L can be obtained by combining the Seebeck coefficient and Hall coefficients and assuming a single band of carriers dominated by acoustic phonon scattering. Temperature dependence of as obtained Lorenz number is given in Fig. S6. The temperature dependent lattice thermal conductivity κL calculated by directly subtracting the electronic thermal conductivity κel from the total thermal conductivity κtot is displayed in the inset of Fig. 9(a). Again, a single band parabolic model is over-simplified, the derived L value is thus a rough estimation. Nonetheless, the results can qualitatively reflect the impact of Pb-doping on the κL. Upon Pb doping, the κL decreases due to a strong phonon scattering by mass fluctuations and strain field fluctuations. All Bi1-xPbxCuSeO (x=0, 2%, 4%, 6%, 8%, 10%) samples have a very low lattice thermal conductivity over the entire temperature range, reaching values as low as 0.37~0.41 Wm-1K-1 at 923 K. The intrinsically low thermal conductivity is the most important attribute of Bi1-xPbxCuSeO compounds, largely responsible for their excellent TE properties30-33. To elucidate the origin of κL, we measured the isobaric specific heat Cp of Bi1-xPbxCuSeO(x=0, 2%, 4%, 6%, 8%, 10%) samples between 2K and 300K. The Cp data are shown in Fig. S7. In the context of the Debye model, we fit the Cp data between 2 K and 5 K into the formula Cp=γT+βT3, Here, β and γ are coefficients of the lattice and electronic specific heat, respectively. The Debye temperature ΘD can be calculated from the formula 4 3 β =36π R/5ΘD . Notably the fitting to the Cp of pristine sample consistently leads to negative γ values, c.f. the inset of Fig. 9(b), which suggest contributions other than those in the standard Debye model. Often, low-frequency optical modes are the culprit. Hence we incorporate an Einstein term into the Cp formula58:

C pE = Z

(Θ E T )2 e Θ

(e

ΘE T

E

)

−1

2

T

(11)

where Z is the vibration strength and ΘE the Einstein temperature. While the Bi3+ ion with its 6s2 valence configuration contributes little to the valence band edge of BiCuSeO59, its large valence shell leads to a large Grüneisen parameter30. At the same time, Bi is the heaviest element in the compound, so it is expected to vibrate with a lower frequency. The anharmonicity and low vibration frequencies suggest strong optical-acoustic phonon scattering, which can explain the intrinsically low κL32. Including Eq. (11) in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Debye model, C p

= γT + β T 3 + C pE , and assuming all Bi

atoms act as Einstein oscillators (the remainder acts a Debye lattice) led to a ΘE ~ 72 K. Notably, we can verify the ΘE value derived from Cp fitting by the atomic displacement parameter (ADP) of the Bi atom Uiso derived from temperaturedependent XRD60:

U iso =

h 2 coth (Θ E 2T ) 2m κ B Θ E

(12)

where the isotropic displacement parameters Biso follows from Biso=8π2/Uiso. Substituting Biso=1.08 Å2 into the above formula48 yielded a well consistent ΘE=72 K. As shown in Figure 9(b), the fitting to the low temperature Cp of Bi1xPbxCuSeO is very good. All fitting parameters are listed in Table S3 in the Supporting Information. As shown in Figure S8,the parameter γ increases nearly linearly with the carrier concentration, indicating that Pb doping contributes significantly to the density of states of holes near the Fermi level, consistent with our Pisarenko analysis above. In contrast, ΘD decreases gradually with increasing Pb content, indicating that Pb doping reduces the lattice rigidity and thus lowers the velocity of sound. Hence the synergy of anharmonicity, low frequency vibrations, doping induced lattice softening, and phonon scattering by point defects account for the observed low κL. Figure 9(c) plots the temperature dependence of the ZT. The ZT value initially increases with the increasing Pb content and then decreases. The maximum value of ZT=1.2 is achieved at 923 K for the x= 6% sample. This corresponds to the optimal carrier concentration of about 7×1020 cm-3. Notably, the pristine BiCuSeO compound exhibits a reasonably high TE performance with a ZTmax=0.66 at 873 K. It is instructive to compare these results with the previous reported Bi1-xPbxCuSeO29,43-45. Apparently, the thermoelectric properties of Pb-doped BiCuSeO samples are sensitive to the microstructure and purity of as-formed phase, which are in turn hinged upon the SHS conditions. The SHS procedure herein reported is to date the best in terms of the ZT values and the facileness. Finally, to check on the reproducibility and thermal stability of the samples, the powder of the optimized Bi0.94Pb0.06CuSeO compound after the SHS reaction was cold pressed into pellets and annealed in vacuum at a temperature 50 K higher than the maximum measurement temperature, i.e., 973 K， for 3 and 7 days， respectively. After annealing, the pellets were hand ground and densified using the same SPS sintering conditions and cut into the standard shape for TE measurements. As shown in Fig. S9, the electrical conductivity, the Seebeck coefficient, the thermal conductivity and the ZT are well repeated within the error bar range. The maximum ZT of 1.2 at 923 K was reproduced for all three samples annealed for 0, 3, and 7 days, as shown in Fig. 9(d). These results corroborate that the SHS+SPS procedure not only yields high TE performance materials in a time- and energy-efficient way, and the materials as prepared are of good reproducibility and good thermal stability.

4. SUMMARY

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In this work, we focused on the reaction mechanism of the SHS process of quinary Bi1-xPbxCuSeO compounds. We found that the formation of single phased Bi1-xPbxCuSeO compounds is the result of two fast binary SHS reactions coupled with two relatively slower solid-state reactions. Importantly, a small amount of PbO addition greatly reduced the reaction activation energy and promoted the speed of combustion process, in addition, Pb doping synergistically increased the carrier concentration and the density of states effective mass, and reduced the lattice thermal conductivity, thereby enhancing the ZT. The pristine BiCuSeO compound with high phase purity attains a rather high thermoelectric performance with the ZTmax=0.66 at 873 K. The optimal carrier concentration at 7×1020 cm-3 with 6mol% of Pb doping yielded a maximum ZT of 1.2 at 923 K. Our work opens a new avenue for fast, low cost, mass production fabrication of Bi1xPbxCuSeO compounds, and the technical insights on the SHS process should be applicable to the synthesis of other functional materials.

ASSOCIATED CONTENT Supporting Information. Thermodynamic and Kinetic calculations of SHS process, reaction mechanism of PbO, Lorentz number calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] * Email: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge support from the National Basic Research Program of China (973 program) under project 2013CB632502, the fundamental research funds for central campus (2015-Ⅲ-061), the Natural Science Foundation of China (Grant No. 51402222, 51172174, 51521001) and the 111 project of China (Grant No.B07040). C.U. acknowledges support from the CERC-CVC joint U.S.-China Program supported by the U.S. Department of Energy under the Award Number DE-PI0000012 in verification of high temperature transport property measurements. J. H. acknowledges support from NSF DMR 1307740.

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A Table of Contents graphic Figure 1. Crystal structure of BiCuSeO. Figure 2. (a) schematic diagram of a customer designed SHS system; (b) different stages of the reaction process; (c) temperature profile of the SHS reaction; (d) XRD pattern of the SHS product. Figure 3. (a)heat flow at a heating rate of 85K/min and a cooling rate of 50K/min; (b)(c) XRD patterns of the products with temperature cutting off at 443K, 490K, 540K, 573K, 643K, 753K respectively. Figure 4. XRD patterns of (a) Bi+Se(SHS); (b) Cu+Se(SHS); (c) Bi2O3+Bi+Se(SHS); (d)Bi2Se3+Bi2O3(solid state reaction at 773K/6h); (e) Bi2SeO2+Cu+Se(SHS); (f) Bi2SeO2+Cu2Se(solid state reaction at 1123K/5h). Figure 5. (a)Diagram of the combustion wave quenching; (b)the surface morphology of the combustion wave quenching sample;(c)small portions diagram of the half bold after combustion wave quenching; (d)XRD patterns for typical regions; The microstructure and composition analysis by FESEM from reactants to products along the axis of the quenching sample. (e) mixture zone 1, (f) preheating zone 3, (g) reaction zone 11, (h) product area 22. Figure 6. (a) Arrhenius curves(–In(υ/Tc) vs 1/Tc) of Bi1-xPbxCuSeO(x=0,10%) in SHS process; (b) XRD patterns of the SHS+SPS products of Bi1-xPbxCuSeO(x=0, 2%,4%, 6%, 8%, 10%); (c) the lattice parameters of Pb-doping samples. Figure 7. The fracture surface morphology of SHS+SPS (a)BiCuSeO and(b)Bi0.94Pb0.06CuSeO. The samples are densely compacted with a relative density higher than 99%. The grain size has increased to 2-10µm after SPS, and the grain surfaces and boundaris are fairly clean;(c) and (d) depict the secondary electron images and back scattering images ofapolished surface of Bi0.94Pb0.06CuSeO.The inset in (d) shows the DSC heat flow curve with neither endothermic nor exothermic peaks between the room temperature and 823 K, a solid evidence of the completeness of the SHS process and the phase stability through the SPS process. Figure 8. Electrical properties of Bi1-xPbxCuSeO(x=0, 2%,4%, 6%, 8%, 10%) samples. (a) electrical conductivity, (b) Seebeck coefficient, (c) the nominal and experimental carrier concentration(at 300K) as a function of Pb content, (d) Pisarenko plot at 300K, literature data are listed for comparison, the inset shows the diagram of hole pockets in the valence band Figure 9. Thermoelectric performance of Bi1-xPbxCuSeO(x=0, 0.02,0.04,0.06,0.08,0.1) (a) the total thermal conductivity, the inset is the lattice thermal conductivity; (b) low temperature Cp, fitting with a combined Debye and Einstein model, the inset is the Cp of undoped sample fitting with Debye model, the fitting consistently leads to negative γ values, even though we resynthesized the high purity BiCuSeO compound again; (c) ZT of Bi1-xPbxCuSeO samples compared with the best results in literatures43-45; (d) ZT curves of Bi0.94Pb0.06CuSeO samples annealed at 973K for 0d/3d/7d.


9


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Table of Contents Graphic 80x21mm (300 x 300 DPI)


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Figure 1. Crystal structure of BiCuSeO. 80x80mm (300 x 300 DPI)



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Figure 2. (a) schematic diagram of a customer designed SHS system; (b) different stages of the reaction process; (c) temperature profile of the SHS reaction; (d) XRD pattern of the SHS product. 161x138mm (300 x 300 DPI)


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Figure 3. (a)heat flow at a heating rate of 85K/min and a cooling rate of 50K/min; (b)(c) XRD patterns of the products with temperature cutting off at 443K, 490K, 540K, 573K, 643K, 753K respectively. 80x240mm (300 x 300 DPI)



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Figure 4. XRD patterns of (a) Bi+Se(SHS); (b) Cu+Se(SHS); (c) Bi2O3+Bi+Se(SHS); (d)Bi2Se3+Bi2O3(solid state reaction at 773K/6h); (e) Bi2SeO2+Cu+Se(SHS); (f) Bi2SeO2+Cu2Se(solid state reaction at 1123K/5h). 160x241mm (300 x 300 DPI)


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Figure 5. (a)Diagram of the combustion wave quenching; (b)the surface morphology of the combustion wave quenching sample;(c)small portions diagram of the half bold after combustion wave quenching; (d)XRD patterns for typical regions; The microstructure and composition analysis by FESEM from reactants to products along the axis of the quenching sample. (e) mixture zone 1, (f) preheating zone 3, (g) reaction zone 11, (h) product area 22. 131x149mm (300 x 300 DPI)



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Figure 6. (a) Arrhenius curves(–In(υ/Tc) vs 1/Tc) of Bi1-xPbxCuSeO(x=0,10%) in SHS process; (b) XRD patterns of the SHS+SPS products of Bi1-xPbxCuSeO(x=0, 2%,4%, 6%, 8%, 10%); (c) the lattice parameters of Pb-doping samples. 80x233mm (300 x 300 DPI)


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Figure 7. The fracture surface morphology of SHS+SPS (a)BiCuSeO and(b)Bi0.94Pb0.06CuSeO. The samples are densely compacted with a relative density higher than 99%. The grain size has increased to 2-10µm after SPS, and the grain surfaces and boundaris are fairly clean;(c) and (d) depict the secondary electron images and back scattering images ofa polished surface of Bi0.94Pb0.06CuSeO.The inset in (d) shows the DSC heat flow curve with neither endothermic nor exothermic peaks between the room temperature and 823 K, a solid evidence of the completeness of the SHS process and the phase stability through the SPS process. 299x230mm (300 x 300 DPI)



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Figure 8. Electrical properties of Bi1-xPbxCuSeO(x=0, 2%,4%, 6%, 8%, 10%) samples. (a) electrical conductivity, (b) Seebeck coefficient, (c) the nominal and experimental carrier concentration(at 300K) as a function of Pb content, (d) Pisarenko plot at 300K, literature data are listed for comparison, the inset shows the diagram of hole pockets in the valence band. 161x161mm (300 x 300 DPI)


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Figure 9. Thermoelectric performance of Bi1-xPbxCuSeO(x=0, 0.02,0.04,0.06,0.08,0.1) (a) the total thermal conductivity, the inset is the lattice thermal conductivity; (b) low temperature Cp, fitting with a combined Debye and Einstein model, the inset is the Cp of undoped sample fitting with Debye model, the fitting consistently leads to negative r values, even though we re-synthesized the high purity BiCuSeO compound again; (c) ZT of Bi1-xPbxCuSeO samples compared with the best results in literatures43-45; (d) ZT curves of Bi0.94Pb0.06CuSeO samples annealed at 973K for 0d/3d/7d. 160x160mm (300 x 300 DPI)


Manipulating the Combustion Wave during Self ... - ACS Publications

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