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High thermoelectric figure of merit ZT > 1 in SnS polycrystals
Journal Pre-proof High thermoelectric figure of merit ZT > 1 in SnS polycrystals Asfandiyar, Bowen Cai, Li-Dong Zhao, Jing-Feng Li PII:
S2352-8478(19)30262-X
DOI:
https://doi.org/10.1016/j.jmat.2019.12.003
Reference:
JMAT 251
To appear in:
Journal of Materiomics
Received Date: 7 December 2019 Revised Date:
12 December 2019
Accepted Date: 15 December 2019
Please cite this article as: Asfandiyar , Cai B, Zhao L-D, Li J-F, High thermoelectric figure of merit ZT > 1 in SnS polycrystals, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Graphical abstract
High thermoelectric figure of merit ZT > 1 in SnS polycrystals Asfandiyar a , Bowen Cai a , Li-Dong Zhao b , Jing-Feng Li a, * a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua
University, Beijing, 100084, China b
School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
Abstract Eco-friendly tin sulfide (SnS) has attracted increasing attention in the thermoelectric community because of its elemental abundance and analogous crystal structure to SnSe as a new thermoelectric material. However, so far no high dimensionless thermoelectric figure of merit ZT > 1 was reported in SnS polycrystals. This work found an effective strategy for enhancing the thermoelectric performance of p-type polycrystalline SnS by Ag doping and vacancy engineering, leading to three orders of magnitude increase in carrier concentration and optimized effective mass and carrier mobility. As a result of the enhanced electrical conductivity, three times higher power factor ~ 3.85 µW/cm K2 at 877 K is realized in Sn0.995Ag0.005S sample. Interestingly, nanostructuring with Ag nano-precipitates were formed in the Ag-doped SnS sample. Moreover, with introducing Sn vacancies in the crystal structure of Sn0.995-vacAg0.005S, the power factor further enhanced to ~ 4.25 µW/cm K2. In addition to the low-frequency phonons scattering by Ag nano-precipitates, dislocations strengthens the scattering of mid-frequency phonon, leading to an ultralow lattice thermal conductivity < 0.5 W/m K above 800 K and a record high ZT up to 1.1 at 877 K in Sn0.99Ag0.005S polycrystals.
Keywords Thermoelectric tin sulfide; spark plasma sintering; nanoprecipitation; dislocation *
Corresponding author. E-mail address: [email protected] (J.-F Li).
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1. Introduction Thermoelectricity is one of environmental-friendly physical phenomenon, which enables invertible conversion between thermal and electrical energy directly and provides a green route for power generation through harvesting waste heat [1,2]. The conversion efficiency of thermoelectric technology is determined by the temperature dependent dimensionless figure of merit for a given thermoelectric material, ZT = S2σT/κT = S2σT/(κE + κL), where S, σ, S2σ, T, κT, κE and κL are the Seebeck coefficient, the electrical conductivity, the power factor, the absolute temperature in Kelvin, the total thermal conductivity, the charge carrier and phonon contributions to the total thermal conductivity, respectively [3]. The thermoelectric parameters, particularly S, σ and κE are intertwined with each other, therefore, manipulating the complex interrelationship is challenging to improve the ultimate ZT value. For example, S inversely, and σ and κE are directly proportional to the carrier concentration. Several outstanding strategies have been proposed to decouple these parameters and to promote the thermoelectric performance. Such as, optimizing power factors through improving carrier concentration [4-6], band flattening [7,8], band convergence [9,10] and reducing thermal conductivity through all-scale hierarchical architecture [11]. Besides, a unique major strategy for achieving high ZTs is to reduce the independent material property, lattice thermal conductivity (κL) [12], which can be accomplished through introducing phonons scattering by a few approaches that include nanostructuring [13-15], mesoscale grains and boundaries [16], intentional-induced dislocations [17] and atomic-scale point defects [18]. Tin sulfide (SnS), with an analogous crystal structure to SnSe has suggested that potentially it should also has ultralow lattice thermal conductivity, high Seebeck coefficient and high carrier mobility in its crystalline form [19,20]. Conversely, the recent report using crystalline undoped and Na-doped SnS [21] samples was rather disappointing in its outcome as it resulted in a relatively low ZT of ~ 0.38 and ~ 1.1 at 870 K, respectively. The inferior performance of SnS crystals compared to its counterpart (SnSe) [22,23], suffered mainly from the low power factor and high thermal conductivity. However, SnS and SnSe crystals are not well suited for thermoelectric devices due to the high cost for production, weak cleaving properties and special demands of crystal-growth technique [24,25]. Therefore, polycrystalline SnS [20] and SnSe [26,27] thermoelectric materials have become a promising alternative candidate for practical applications due to machinability and scalability. However, the highest ZTs of SnS and SnSe in its polycrystalline forms are much lower because polycrystalline samples possess low carrier mobility, which stem from the effects of grain boundaries [28-30]. Polycrystalline SnS [31] perform much poorly as a result of its low power factor and high thermal conductivity than that of the SnSe [32,33]. Therefore, it attracted less attention of the thermoelectric researchers and in the past very few strategies including Ag [20] and Na [34-36] doping at Sn sites have been used to promote its electrical conductivity and to reduce the thermal conductivity. As a result of the above strategies, the power factor has been optimized to some extent, but it is still challenging to reduce the high thermal conductivity. A high ZT of ~ 1.0 at 875 K was achieved in Na-doped PbS thermoelectric material [37]. However, the environmental hazards of Pb in the atmosphere limit its thermoelectric applications and samples doped with Na are not stable upon repeated heating and cooling [38]. Therefore, the ecofriendly Sn, low-cost and more earthabundant S elements than Se, and the prospect of achieving much higher thermal properties and the
2
lack of information regarding their transport properties have motivated us to perform a comprehensive study of the thermoelectric performance of polycrystalline form of SnS. In this work, we produced bulk SnS polycrystals using mechanical alloying and spark plasma sintering (MA+SPS) technique. We target to upsurge the electrical transport properties through optimizing hole concentration by silver (Ag) doping on the sites of tin (Sn). Of note, the doping fraction of 0.5% was fixed such as to investigate the Sn vacancies effects on the thermoelectric properties [20]. As a result, the power factor at 877 K was improved from ~ 1.4 for undoped SnS to 3.85 µW/cm K2, for Sn0.995Ag0.005S sample, which was further enhanced to ~ 4.25 µW/cm K2 for doped-vacancy-containing Sn0.99Ag0.005S sample. The structural and morphological characterizations including scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (STEM-EDS) and atomic resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) indicate that Ag doping can result in single crystalline coherent nanoprecipitates at the grain boundaries. In addition to Ag nano-precipitates, dislocations induced by Sn vacancies exhibits an important effect on the lattice thermal conductivity and further leads to an extra low lattice thermal conductivity and in turn a high peak ZT ~ 1.1 in Sn0.99Ag0.005S at 877 K, which is a record value for the polycrystalline SnS based thermoelectric materials so far. More importantly, polycrystalline SnS could compete the thermoelectric performance of its single crystalline counterpart.
2. Experimental High-purity elements Sn (≥ 99.99%), S (≥ 99.99%) and Ag (99.5%) in powder form were weighed in Argon (Ar) glovebox. The weighted elements in the form of compositions SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S were subjected to the sealed stainless steel jars containing stainless steel balls as shown in Fig. S1. The weight ratio of ball to raw powder was ~ 20:1. All the compositions were ball-milled continuously at 450 revolutions/minutes for 900 minutes. Then, all the sealed jars, and graphite molds of outer and inner diameter ~ 60 mm and 13.5 mm were transferred to the glovebox, respectively. Opening of the jars and filling of molds with obtained ball-milled compositions were done in Argon atmosphere such as to avoid samples from oxidations. Finally, all the molds one after another transferred to spark plasma sintering system and were sintered at 850 K for 7 minutes under an axial pressure of 50 MPa in vacuum. The resulted SPS-ed specimens were cylindrical shape with dimensions of Φ ~ 13 mm × 12 mm as shown in Fig. 1c. Phase structure was investigated by XRD (D8 Advance, Bruker, Germany) with Cu–Kα radiation. The morphology of all the bulk samples was observed through a field emission scanning electron microscope (FE/SEM, JSMe7001 JEOL, Japan) and the microstructure was confirmed by transmission electron microscopy (TEM, JEOL, JEM 2010F, Japan). Two bulk specimens were achieved via cutting and grinding; one is rectangular-shaped about ~ 12 mm × 10.2 mm × 1 mm and/or 0.9 mm for thermal conductivity measurements and the second is strip-like about ~ 2.2 mm × 2.1 mm × 10 mm for Seebeck coefficient/electrical resistivity measurement. The electrical and thermal transport properties were calculated along the sample section perpendicular to the SPS pressurizing direction (in-plane). The electrical resistivity and Seebeck coefficient were measured in the temperature range 300–877 K in the helium atmosphere using an 3
electric resistance/Seebeck coefficient measuring system (ZEM-3, Ulvac-Riko, Japan). The total thermal conductivity κT was calculated by the relationship of κT = D×Cp×d where D is the diffusivity, Cp is the specific heat capacity and d is the density of the bulk specimens. D was measured with a laser flash method (NETZSCH Laser Flash Apparatus LFA 457, Germany) as shown in Fig. S2. The specific heat Cp was taken from our previously reported data for doped sample [20]. d for all the samples were measured by the Archimedes method as shown in Table S1. Using the Weidman-Franz law (κE= LσT) the electronic thermal conductivity (κE) was calculated, where L is the Lorentz, σ is the electrical conductivity and T is the temperature. The lattice thermal conductivities (κL) were calculated according to the relation, κL = κT – κE = κT – LσT. Hall coefficient (RH) was measured under a reversible magnetic field of 0.52 T by the Van der Pauw method using a Hall measurement system (ResiTest 8340 DC, Tokyo, Japan) nearly at room-temperature (~ 300 K). The charge Carrier concentration was calculated by using an equation, nH= 1/(eRH) and carrier mobility (µH) was obtained by µH = σRH.
3. Results and discussion Fig. 1 illustrates the two temperature-dependent crystal structures of SnS. Fig. 1a depicts the low
temperature orthorhombic structure with Pnma space group ( = 4.33 ± 0.02Å, = 11.99 ± 0.02Å and = 3.98 ± 0.02Å), which has a layered crystal structure with two Sn–S bilayers [39]. Similar to SnSe, the crystal structure of SnS features a distorted Sn polyhedral coordination with three shorts and four long Sn–S bonds [20]. Covalent bonds exist in its crystal structure and joined the atoms in each layer with the three neighbor atoms, forming accordion shaped corrugated slabs. The three covalent bonds among the corresponding atoms are stronger, while the other four covalent bonds (two in the same bilayer and two in the next bilayer) are weaker bonds. The Pnma orthorhombic structure of SnS transforms to nearly high symmetric orthorhombic structure (Cmcm) at elevated temperature as shown in Fig. 1b [40]. Indeed, the Pnma crystal structure at low temperature is a subgroup of the Cmcm crystal structure at elevated temperature [41]. All the thermoelectric properties were measured in the temperature range ~ 300-877K along the direction perpendicular to SPS pressurizing direction (in-plane) as shown in Fig. 1c. Here the in-plane direction is chosen because the two neighbored bilayers associated with weaker Sn-S bonding along the a-direction results in easy cleavage in the b-c plane. Fig. 1d displays the room temperature (~ 300K) carrier concentration ( ) and mobility ( ) of the undoped SnS, 0.5% Ag-doped and doped-vacancycontaining Sn0.995-vacAg0.005S (vac= 0, 0.0025, 0.005, 0.0075) samples, where “vac” stands for the amounts of vacancy generation. At ~ 300 K, of the undoped SnS and Sn0.995Ag0.005S samples are 17 −3 18 −3 ~ 0.0162 × 10 cm and ~ 3.73 × 10 cm , respectively. The significant enhancement in confirms the effective substitution of Ag in the Sn atomic sites of SnS acting as acceptor dopants, as follows: ′
+ ℎ° 1
The Ag doping in the SnS crystal structure was confirmed by high-resolution X–ray photoelectron spectroscopic spectra, in which Ag 3d5/2 and Ag 3d3/2 can be observed at ~ 367 and ~ 373 eV, respectively as shown in Fig. 1e. Therefore, we attribute the significant enhancement in is further to hole (h°) produced by the above reaction. Compared to the 0.5% Ag-doped SnS, 4
slightly enhanced by the intentional introduction of Sn vacancies, which act as acceptors. Besides the significant enhancement in hole concentration, the Ag-doped sample exhibits impressively high (slightly lower than that of the undoped SnS) at ~ 300 K. further decreases by incorporating Sn vacancies into the lattice structure. greatly deteriorates in Sn0.9875Ag0.005S sample compared with in Sn0.995Ag0.005S sample and then it is further other doped samples. We have discussed the high decreasing trend in vacancy-containing samples in Fig. S3. In summary, the improved thermoelectric transport properties lead to a record significant enhancement of ~ 204% in power factor (PF) and ~ 424% in thermoelectric figure of merit (ZT) as shown in Fig. 1f. Fig. 2a depicts the electrical conductivities (σ) as a function of temperature for the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S bulk samples, respectively. As shown, σ of the undoped SnS increases sharply after doping with Ag. The room-temperature (~ 300 K) σ for Sn0.995Ag0.005S sample is ~ 7.33 S/cm, which is much higher than that of the undoped SnS. Here the significant enhancement in σ can be attributed to the optimized and high . σ of the Sn0.995Ag0.005S sample further increases by introducing Sn vacancy. In particular, the roomtemperature σ values are ~ 8.7, 11.2 and 10.12 S/cm for doped-vacancy-containing Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples, respectively. As the Sn0.9875Ag0.005S (vac ≥ 0.0075) sample exhibit very low , so the electrical conductivity further decreased. The electrical conductivities (except for undoped SnS) slightly increase from 300 to ~ 450 K and then decrease with rising temperature (before ~ 648 K). Previously, this type of variation in the σ value was also observed in Na/K doped SnSe polycrystals [33]. The conductivities above ~ 648 K increase abruptly with rising temperature due to a rapid increase in thermally activated carrier concentration. The Seebeck coefficient (S) of the undoped SnS, 0.5% Ag-doped and doped-vacancy-containing samples as a function of temperature is shown in Fig. 2b. For the Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples, the room-temperature S values are ~ 350.2, 344.3, 339.4 and 334.3 µV/K, respectively. Consistent with the increase in , S values at ~ 300 K for the 0.5% Ag-doped and doped-vacancy-containing samples are lower than that of the undoped SnS (~ 525.3 µV/K), but still maintains impressively higher levels. S for the doped samples increase with rising temperature and are consistent with heavily doped semiconductor behavior [33]. After intrinsic excitation begins, S decreases with further increasing temperature due to a rapid increase in caused by the intrinsic excitation. The temperature corresponding to the maximum S of doped samples (~ 648 K) is higher than that of the undoped one (~ 598 K) because of the high . To witness the excellent electrical transport properties in doped samples, we calculated the effective mass ( ∗ ) using the following equations [42,43]: = where *3 (/) is Fermi integral
!" (& + 5/2)*+,-/. (/) $ − /2 2 # (& + 3/2)*+,0/. (/)
*3 (/) = 4 The effective mass (
∗
9
:
53 ;5 3 1 + exp (5 − /)
) of hole is calculated using Hall carrier concentration (
5
)
(2 ∗ !" <)-/. *0/. (/) = 4 2= . ħ& ∗
where & is Hall factor
1 2= . ħ- & = ? !" < *0/. (/)
& =
./-
@
5
3 *0/. (/)*A0/. (/) 6 B*: (/)C. 4
Among them, r is the scattering rate, kB is the Boltzmann's constant, e is the electron charge, / is the reduced Fermi energy level, ħ is the reduced Planck's constant and T is the temperature. Table 1 shows the variations of effective mass ( ∗ ) to the electron mass ( E ) in Sn0.995∗ sharply increases in the 0.5% Ag-doped vacAg0.005S (vac = 0, 0.0025, 0.005 and 0.0075) samples. sample and then varies negligibly in the doped-vacancy-containing samples. The Seebeck coefficient (S) depends on the carrier concentration (n) and effective mass ( ∗ ) as follows: =
8= . !" 3#ℎ.
∗
= ./G 7 3
Therefore, the lower Seebeck coefficients at ~ 300 K of the 0.5% Ag-doped and doped-vacancycontaining samples with respect to undoped SnS sample are due to the significantly increased in . S of the doped-vacancy-containing samples are slightly lower than that of the Sn0.995Ag0.005S sample due to the slight increase in as shown in enlarged Fig. 2c. Due to the highest value of , Sn0.9875Ag0.005S sample show the lowest S value at ~ 300 K. has significant effects on the S, i.e., when the is not high enough it always results in lower S value [44]. However, here the in doped samples is three order higher than that of the undoped SnS, but the S values still maintain higher levels due to the enhanced ∗ , as S directly proportional to ∗ (Eq. 7). Moreover, for a given thermoelectric material a dimensionless quality factor (B) is characterized by the following relation [45,46]:
where
∗
I ∝ (
∗ )-/.
8
is the effective mass and µ is the carrier mobility.
It is challenging to optimize B, because ∗ and are contrariwise with each other [47,48]. However, the Sn0.995Ag0.005S sample exhibits improved ∗ and also impressively high (negligible ∗ deteriorations in the as compared to their sharp increase in ). For example, the large surfaces in 0.5% Ag-doped sample provide uninterrupted way for carrier transport, which compensate for the improved ∗ as shown in Fig. S3. Fig. 2d shows the power factor (PF = S2σ) values for undoped SnS and Sn0.995-vacAg0.005S samples versus 0.5% Ag doping and vacancy (vac=0, 0.0025, 0.005 and 0.0075) in the temperature range of ~ 300–877 K, respectively. As a result of the improved σ in doped samples, S2σ of the undoped SnS has been significantly enhanced. We attribute the high , and optimized m* and . The high electrical performance in Sn0.995Ag0.005S to the increase in 6
temperature S2σ for Sn0.99Ag0.005S (~ 4.25 µW/cm K2 at 877 K) is about three times higher than that for undoped SnS (~ 1.4 µW/cm K2). Here, the significant enhanced S2σ values for Sn0.995-vacAg0.005S samples are higher and/or comparable to other typically reported SnS polycrystals in the temperature range of ~ 300–877 K [20,34,35]: Fig. 3 shows the variation of total thermal conductivity (κT) for the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples as a function of temperature, respectively. κT of all the samples decreases with increasing temperature from ~ 300 to 877 K. For the undoped SnS, κT of ~ 0.58 W/m K at 877 K is obtained. κT decrease upon 0.5% Ag doping and the minimum thermal conductivity ~ 0.46 W/m K at 877 K was realized in Sn0.995Ag0.005S sample. Then, in the whole range of temperature, κT further decreases with increase in vacancy (vac) and the lowest value of ~ 0.34 W/m K is achieved for the Sn0.99Ag0.005S sample. So far, κT for the optimized doped-vacancy-containing sample is significantly lower than those of the previously reported data as shown in the right top inset of Fig. 3a [20,34,35,49]. The temperature dependent lattice thermal conductivities (κL) of all the samples were calculated according to κL = κT – κE = κT – LσT, where L is the Lorentz number, σ is the electrical conductivity and T is the absolute temperature. As shown in Fig. 3b, the lattice part (κL) of thermal conductivity is comparable to that of total thermal conductivity. Therefore, the electronic part (κE) is negligible and the dominant lattice part accounts for total thermal conductivity. In the whole temperature range, κL of the 0.5% Ag-doped and dopedvacancy-containing samples is much lower than that of the undoped SnS. Of note, κL for the Sn0.995Ag0.005S and Sn0.99Ag0.005S samples are 28% and 74% lower than that of the undoped SnS sample at 877 K. Due to the abundant deficiencies such as dislocations, Ag nano-precipitates, vacancies and grain boundaries, which contribute to strong scattering of phonons, we use the Debye-Callaway model to calculate the lattice thermal conductivity [50]: KLMN
TU
!" !" < - V RS# 3 = . F G 4 PNQN (R) 3 ; 9 2= O ħ (# − 1). 3 : R=
ℏX 10 !" <
where kB is the Boltzmann’s constant, O is the average sound velocity, ħ is reduced Planck’s constant, YZ is the Debye temperature, and X is the frequency of phonons. PNQN is the total relaxation time and calculated according to the Matthiessen’s rule [51,52]: A0 A0 A0 PNQN = P[A0 + P\A0 + P]" + P^A0 + P^Z + PZA0 11
which relates the scattering from Umklapp process, Normal process, grain boundaries, precipitates, point defects and dislocations. Umklapp process and Normal process relaxation time are acquired by fitting the experimental data of undoped SnS. The density and size of nano-precipitates, the dislocation density are acquired from the TEM observations. The relaxation time of point defects is given by: XS _: A0 (` + ` ) 12 P^Z = 4=O - a 7
V0 is the average volume per atom. `a and ` are the disorder scattering parameters of mass and strain field fluctuations, respectively. For Sn0.99Ag0.005S sample, the disorder is caused by Ag doping and Sn vacancies, so the both contribution must be considered. Assume that the kth atom of the ith sublattice has mass bcd , radius &cd , and fractional ecd . The fc ) and radius (&̅c ) on the ith sublattice is b fc = ∑d ecd bcd and &̅c = ∑d ecd &cd . The average mass (b f = ∑ci0 c b fc ⁄∑ci0 c . ci are the degeneracy (here average atomic mass of the compound is b c1=c2=1). Then the `a and ` are as follow [53,54]: lll b ∑ci0 c ( fk ). `ac b `a = 13 ∑ci0 c
`ac = m ecd (1 − d
bcd . ) 14 fc b
lll b ∑ci0 c ( fk ). ` c b ` = 15 ∑ci0 c
` c = m ecd 5(1 − d
&cd . ) 16 &̅c
ε is a phenomenological adjustable parameter. The other specific expressions of the individual relaxation time are included in Table S2, which could be found in the reports [55-57]. All the parameters for calculation are also listed in Table S2. The calculated result for Sn0.99Ag0.005S sample is plotted in Fig. 3b (red hollow stars). It can be seen that a good consistency with the experimental value (red filled stars), which reinforces the validity of our analysis. We also apply the Cahill model to estimate the minimum lattice thermal conductivity of Sn0.99Ag0.005S sample as follows [58]: Knc
= 0 = ( )- !" 6
Tp . < . V R-# 3 - m oc ( ) 4 ;R 17 Yc (# 3 − 1). : c
where the Debye temperature is θi = vi(ћ/kB)(6π2n)1/3, n is the number density of atoms and vi is the sound velocity for each polarization mode. Here we use 3368 m/s for longitudinal and 1537, 2368 m/s for transverse branches, as indicated in literature [36]. The calculated results are also included in Fig. 3b. The calculated values are more closely with the experimental data and the results derived from the Debye-Callaway model at 900 K, which further proves the validity of our argument. To study microstructures and compositional analyses of the Sn0.995-vacAg0.005S, we first performed scanning transmission electron microscopy (STEM) and scanning transmission electron microscopyenergy dispersive X–ray spectroscopy (STEM-EDS) characterization. Then, to determine the detailed crystalline information, selected area electron diffraction (SAED) analyses and atomic resolution high angle annular dark field-scanning transmission electron microscopy (HAADFSTEM) were used. Fig. 4a is the STEM image of the stoichiometric Sn0.995Ag0.005S sample. Beside the effective substitution in the Sn atomic sites of SnS, Ag elements also accumulate at the grain boundaries and exhibit brighter contrast than the matrix as shown in Fig. 4a. Fig. 4b shows STEM8
EDS elemental mapping taken on the framed area (solid red) in Fig. 4a, revealing that elements tin (Sn), Silver (Ag) and sulfur (S) distribute homogeneously in the matrix. The matrix SAED pattern can be indexed to SnS taken along [100] zone axis as shown in Fig. S4. A typical HAADF-STEM image of the framed area (solid orange) in Fig. 4a for the two small-sized nano-precipitates at grain boundaries is shown in Fig. 4c. The observed nano-precipitates of bright-contrast are in the sizes of ~ 7–12 nm. These nano-precipitates have a coherent feature i.e., without any structure defects and exhibit single crystalline nature. Right top inset of Fig. 4c is the STEM-EDS elemental mapping taken on the framed area (solid black), confirming that the nano-precipitates are Ag elements since they show brighter contrast than the matrix. Ag single crystalline coherent nano-precipitates scatter the low-frequency phonons in Sn0.995Ag0.005S sample [32]. The lattice thermal conductivity for Sn0.995Ag0.005S sample is 28% lower than that of the undoped SnS sample at 877 K, which confirms that nanostructuring plays an important role in the reduction of κL. Fig. 4d is the HAADF-STEM image of the Sn0.99Ag0.005S sample, which exhibit dislocations inside the grains. Dislocation is a strategy to suppress the intermediate frequency phonon propagations [59]. Fig. 4e is the zoom-in image of the framed area (solid yellow), which shows dislocations in the crystal structure. Fig. 4f is another HAADF-STEM image for the corresponding sample, in which plenty amount of sinusoidal atomic arrays were observed. Dislocations and sinusoidal atomic structure strengthened phonons scattering in Sn0.99Ag0.005S sample. Conversely, dislocations distress the hole transport in dopedvacancy-containing samples and as a result mobility decreased. For example the sample Sn0.9875Ag0.005S exhibits the lowest µ value. However, dislocations exhibit an imperative effect on κL compared to the effect on its electrical transport properties. Therefore, the very low κL in dopedvacancy-containing samples suggests that mid-frequency phonons scattering have been remarkably enhanced. Hence, a remarkable 74% lattice thermal conductivity has been suppressed in Sn0.99Ag0.005S sample so far. A marked enhancement in the thermoelectric figure of merit (ZT) was obtained for SnS polycrystals via enhancing power factor enabled by Ag doping and the simultaneously reduced thermal conductivity by introducing dislocations (induced by Sn vacancies) in addition to Ag nanoprecipitates. The ZT of undoped SnS is ~ 0.21 at 877 K as shown in Fig. 5a. More importantly, a significant increase by ~ 424% the maximum ZT of 1.1 at 877 K was achieved for the Sn0.99Ag0.005S bulk sample, which is the highest ZT value for polycrystalline SnS based materials reported at this temperature so far. More exciting, the maximum ZT of Na-doped SnS [21] single crystal was reproduced in its polycrystalline form, and is much higher than polycrystalline Na-doped SnSe [60] and Ag-doped SnS [20] prepared by the same synthesis method and/or different method [34,35].
4. Conclusion In summary, the thermoelectric performance of polycrystalline SnS was optimized. The intentional-induced dislocations are effective in decreasing the thermal conductivity. As a result, very low thermal conductivity ~ 0.34 W/m K at 877 K for Sn0.99Ag0.005S sample was obtained. Ag doping increases the carrier concentration of SnS, and optimizes the effective mass and carrier mobility, which results in ultimately high power factor. And a record ZT of 1.1 at 877 K is achieved for Sn0.99Ag0.005S sample. Here, the high thermoelectric performance advocates that SnS based polycrystals have plentiful room to advance and ultimately reach the performance of the SnSe 9
polycrystals. Moreover, the high performance in doped SnS polycrystals also encourages SnS single crystals to improve and reach the performance of its denser analogous SnSe single crystals.
Authors Contributions Jing-Feng Li and Asfandiyar designed the project; Asfandiyar performed the experiments and drafted the manuscript. Jing-Feng Li, Bowen Cai and Li-Dong Zhao revised the manuscript.
Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work is supported by the Basic Science Center Project of Natural Science Foundation of China (51788104) and the National Key R&D Program of China (2018YFB0703603).
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Table and Figures
Table 1 Represent the room-temperature (~ 300 K) Hall carrier concentration, the ratio of effective mass ( ∗) to the electron mass ( E ) and the carrier mobility for the Sn0.995-vacAg0.005S bulk samples. All samples are measured along the sample section perpendicular to the SPS pressurizing direction.
Samples
nH (1018 cm−3)
Sn0.995Ag0.005S Sn0.9925Ag0.005S Sn0.99Ag0.005S Sn0.9875Ag0.005S
3.73 4.12 4.34 4.42
∗
/
1.097 1.119 1.114 1.092
E
µH (cm 2/Vs) 5.28 4.56 3.78 2.16
Fig. 1. (a) The room-temperature and (b) High-temperature crystal structure of SnS, gray atom represents Sn and yellow one is S, (c) Schematic illustration of the measurement direction of thermoelectric transport properties, (d) Room-temperature carrier concentration and carrier mobility for the pure SnS, doped Sn0.995Ag0.005S and doped-vacancy-containing Sn0.995-vacAg0.005S samples, (e) X–ray photoelectron spectrum of the 3d orbitals for the Ag element of 0.5% Ag-doped SnS sample and (f) Power factor (S2σ) and thermoelectric figure of merit (ZT) for the corresponding samples at 877 K.
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Fig. 2. (a) Temperature dependent electrical conductivities, (b) Seebeck coefficients, (c) Enlarged Figure for doped samples (temperature range ~ 498 to 800 K) and (d) Power factors of the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S bulk samples in the temperature range ~ 300–877 K along the direction perpendicular to SPS pressurizing direction, respectively. For comparisons the optimized data of the previous reports are also plotted [20,34,35].
Fig. 3. Temperature dependent (a) total thermal conductivity of the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples (right top inset show comparisons of the lowest thermal conductivity achieved in this work with other previously reported work) and (b) Experimental lattice thermal conductivity of the corresponding samples and the calculated κL for the Sn0.99Ag0.005S sample.
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Fig. 4. (a) STEM annular bright field (ABF) image showing nano-precipitates at the grain boundary with bright contrast in Sn0.995Ag0.005S sample, (b) The EDS elemental map images of the framed area (solid red) in (a), (c) HAADF-STEM image of the framed area (solid orange) in (a) containing two nanoscale precipitates, reveals continuous lattice between grain boundaries and nano-precipitates (right top inset is the STEM-EDS elemental mapping taken on the framed area (solid black), confirms that the nano-precipitates are Ag elements), (d) HAADF-STEM image of the Sn0.99Ag0.005S sample, confirm nanoscale dislocations, (e) Zoomin image of the framed area (solid yellow), witness a dislocation and (f) HAADF-STEM image for the corresponding sample, which exhibit sinusoidal atomic arrays (right top inset is the zoom-in image).
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Fig. 5. (a) Temperature dependent ZTs for all the samples in this work and (b) Comparisons of the ZT in this work with other reported optimized data [20,34,35,49].
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Highlights • • • •
Stoichiometric and Ag-doped Sn-deficient SnS samples were fabricated by ball milling combined with spark plasma sintering. A high ZT up to 1.1 at 877 K is achieved for Sn0.99Ag0.005S sample. Ag doping and Sn vacancies were revealed to be effective for increasing power factors in SnS. The mechanism of ultralow thermal conductivity was discussed in association with nanostructure and dislocation.
Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Asfandiyar is a Ph.D. candidate in the School of Materials Science and Engineering, Tsinghua University, China. He obtained his MSc. in Physics from Islamia College University of Peshawar (Pakistan) in 2010 and BSc. in Physics from Peshawar University (Pakistan) in 2008. His current research interests include SnS-based thermoelectric materials.
Bowen Cai is now a Postdoctoral fellow working at the School of Materials Science and Engineering in Tsinghua University. His current research interests include thermoelectric materials and high pressure technology. He holds a B.E. from Xi’an University of Technology, a Ph.D from Yanshan University in Qinhuangdao, China.
Dr. Li-Dong Zhao is a full professor of the School of Materials Science and Engineering at Beihang University, China. He received and his Ph.D. degree from the University of Science and Technology Beijing, China in 2009. He was a postdoctoral research associate at the Université Paris-Sud and Northwestern University from 2009 to 2014. His research interests include electrical and thermal transport behaviors in the compounds with layered structures. Group website: http://shi.buaa.edu.cn/zhaolidong/zh_CN/index.htm
Jing-Feng Li is a professor in Tsinghua University, China. He graduated from Huazhong University of Science and Technology (China) in 1984, and obtained his doctor degree from Tohoku University (Japan) in 1991. After working in Tohoku University as assistant professor and associate professor from1992 to 2002, he joined Tsinghua University as a full professor in 2002. His research interests include thermoelectric materials and devices, piezoelectric ceramics, composites and films for MEMS applications.
S2352-8478(19)30262-X
DOI:
https://doi.org/10.1016/j.jmat.2019.12.003
Reference:
JMAT 251
To appear in:
Journal of Materiomics
Received Date: 7 December 2019 Revised Date:
12 December 2019
Accepted Date: 15 December 2019
Please cite this article as: Asfandiyar , Cai B, Zhao L-D, Li J-F, High thermoelectric figure of merit ZT > 1 in SnS polycrystals, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Graphical abstract
High thermoelectric figure of merit ZT > 1 in SnS polycrystals Asfandiyar a , Bowen Cai a , Li-Dong Zhao b , Jing-Feng Li a, * a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua
University, Beijing, 100084, China b
School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
Abstract Eco-friendly tin sulfide (SnS) has attracted increasing attention in the thermoelectric community because of its elemental abundance and analogous crystal structure to SnSe as a new thermoelectric material. However, so far no high dimensionless thermoelectric figure of merit ZT > 1 was reported in SnS polycrystals. This work found an effective strategy for enhancing the thermoelectric performance of p-type polycrystalline SnS by Ag doping and vacancy engineering, leading to three orders of magnitude increase in carrier concentration and optimized effective mass and carrier mobility. As a result of the enhanced electrical conductivity, three times higher power factor ~ 3.85 µW/cm K2 at 877 K is realized in Sn0.995Ag0.005S sample. Interestingly, nanostructuring with Ag nano-precipitates were formed in the Ag-doped SnS sample. Moreover, with introducing Sn vacancies in the crystal structure of Sn0.995-vacAg0.005S, the power factor further enhanced to ~ 4.25 µW/cm K2. In addition to the low-frequency phonons scattering by Ag nano-precipitates, dislocations strengthens the scattering of mid-frequency phonon, leading to an ultralow lattice thermal conductivity < 0.5 W/m K above 800 K and a record high ZT up to 1.1 at 877 K in Sn0.99Ag0.005S polycrystals.
Keywords Thermoelectric tin sulfide; spark plasma sintering; nanoprecipitation; dislocation *
Corresponding author. E-mail address: [email protected] (J.-F Li).
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1. Introduction Thermoelectricity is one of environmental-friendly physical phenomenon, which enables invertible conversion between thermal and electrical energy directly and provides a green route for power generation through harvesting waste heat [1,2]. The conversion efficiency of thermoelectric technology is determined by the temperature dependent dimensionless figure of merit for a given thermoelectric material, ZT = S2σT/κT = S2σT/(κE + κL), where S, σ, S2σ, T, κT, κE and κL are the Seebeck coefficient, the electrical conductivity, the power factor, the absolute temperature in Kelvin, the total thermal conductivity, the charge carrier and phonon contributions to the total thermal conductivity, respectively [3]. The thermoelectric parameters, particularly S, σ and κE are intertwined with each other, therefore, manipulating the complex interrelationship is challenging to improve the ultimate ZT value. For example, S inversely, and σ and κE are directly proportional to the carrier concentration. Several outstanding strategies have been proposed to decouple these parameters and to promote the thermoelectric performance. Such as, optimizing power factors through improving carrier concentration [4-6], band flattening [7,8], band convergence [9,10] and reducing thermal conductivity through all-scale hierarchical architecture [11]. Besides, a unique major strategy for achieving high ZTs is to reduce the independent material property, lattice thermal conductivity (κL) [12], which can be accomplished through introducing phonons scattering by a few approaches that include nanostructuring [13-15], mesoscale grains and boundaries [16], intentional-induced dislocations [17] and atomic-scale point defects [18]. Tin sulfide (SnS), with an analogous crystal structure to SnSe has suggested that potentially it should also has ultralow lattice thermal conductivity, high Seebeck coefficient and high carrier mobility in its crystalline form [19,20]. Conversely, the recent report using crystalline undoped and Na-doped SnS [21] samples was rather disappointing in its outcome as it resulted in a relatively low ZT of ~ 0.38 and ~ 1.1 at 870 K, respectively. The inferior performance of SnS crystals compared to its counterpart (SnSe) [22,23], suffered mainly from the low power factor and high thermal conductivity. However, SnS and SnSe crystals are not well suited for thermoelectric devices due to the high cost for production, weak cleaving properties and special demands of crystal-growth technique [24,25]. Therefore, polycrystalline SnS [20] and SnSe [26,27] thermoelectric materials have become a promising alternative candidate for practical applications due to machinability and scalability. However, the highest ZTs of SnS and SnSe in its polycrystalline forms are much lower because polycrystalline samples possess low carrier mobility, which stem from the effects of grain boundaries [28-30]. Polycrystalline SnS [31] perform much poorly as a result of its low power factor and high thermal conductivity than that of the SnSe [32,33]. Therefore, it attracted less attention of the thermoelectric researchers and in the past very few strategies including Ag [20] and Na [34-36] doping at Sn sites have been used to promote its electrical conductivity and to reduce the thermal conductivity. As a result of the above strategies, the power factor has been optimized to some extent, but it is still challenging to reduce the high thermal conductivity. A high ZT of ~ 1.0 at 875 K was achieved in Na-doped PbS thermoelectric material [37]. However, the environmental hazards of Pb in the atmosphere limit its thermoelectric applications and samples doped with Na are not stable upon repeated heating and cooling [38]. Therefore, the ecofriendly Sn, low-cost and more earthabundant S elements than Se, and the prospect of achieving much higher thermal properties and the
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lack of information regarding their transport properties have motivated us to perform a comprehensive study of the thermoelectric performance of polycrystalline form of SnS. In this work, we produced bulk SnS polycrystals using mechanical alloying and spark plasma sintering (MA+SPS) technique. We target to upsurge the electrical transport properties through optimizing hole concentration by silver (Ag) doping on the sites of tin (Sn). Of note, the doping fraction of 0.5% was fixed such as to investigate the Sn vacancies effects on the thermoelectric properties [20]. As a result, the power factor at 877 K was improved from ~ 1.4 for undoped SnS to 3.85 µW/cm K2, for Sn0.995Ag0.005S sample, which was further enhanced to ~ 4.25 µW/cm K2 for doped-vacancy-containing Sn0.99Ag0.005S sample. The structural and morphological characterizations including scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (STEM-EDS) and atomic resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) indicate that Ag doping can result in single crystalline coherent nanoprecipitates at the grain boundaries. In addition to Ag nano-precipitates, dislocations induced by Sn vacancies exhibits an important effect on the lattice thermal conductivity and further leads to an extra low lattice thermal conductivity and in turn a high peak ZT ~ 1.1 in Sn0.99Ag0.005S at 877 K, which is a record value for the polycrystalline SnS based thermoelectric materials so far. More importantly, polycrystalline SnS could compete the thermoelectric performance of its single crystalline counterpart.
2. Experimental High-purity elements Sn (≥ 99.99%), S (≥ 99.99%) and Ag (99.5%) in powder form were weighed in Argon (Ar) glovebox. The weighted elements in the form of compositions SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S were subjected to the sealed stainless steel jars containing stainless steel balls as shown in Fig. S1. The weight ratio of ball to raw powder was ~ 20:1. All the compositions were ball-milled continuously at 450 revolutions/minutes for 900 minutes. Then, all the sealed jars, and graphite molds of outer and inner diameter ~ 60 mm and 13.5 mm were transferred to the glovebox, respectively. Opening of the jars and filling of molds with obtained ball-milled compositions were done in Argon atmosphere such as to avoid samples from oxidations. Finally, all the molds one after another transferred to spark plasma sintering system and were sintered at 850 K for 7 minutes under an axial pressure of 50 MPa in vacuum. The resulted SPS-ed specimens were cylindrical shape with dimensions of Φ ~ 13 mm × 12 mm as shown in Fig. 1c. Phase structure was investigated by XRD (D8 Advance, Bruker, Germany) with Cu–Kα radiation. The morphology of all the bulk samples was observed through a field emission scanning electron microscope (FE/SEM, JSMe7001 JEOL, Japan) and the microstructure was confirmed by transmission electron microscopy (TEM, JEOL, JEM 2010F, Japan). Two bulk specimens were achieved via cutting and grinding; one is rectangular-shaped about ~ 12 mm × 10.2 mm × 1 mm and/or 0.9 mm for thermal conductivity measurements and the second is strip-like about ~ 2.2 mm × 2.1 mm × 10 mm for Seebeck coefficient/electrical resistivity measurement. The electrical and thermal transport properties were calculated along the sample section perpendicular to the SPS pressurizing direction (in-plane). The electrical resistivity and Seebeck coefficient were measured in the temperature range 300–877 K in the helium atmosphere using an 3
electric resistance/Seebeck coefficient measuring system (ZEM-3, Ulvac-Riko, Japan). The total thermal conductivity κT was calculated by the relationship of κT = D×Cp×d where D is the diffusivity, Cp is the specific heat capacity and d is the density of the bulk specimens. D was measured with a laser flash method (NETZSCH Laser Flash Apparatus LFA 457, Germany) as shown in Fig. S2. The specific heat Cp was taken from our previously reported data for doped sample [20]. d for all the samples were measured by the Archimedes method as shown in Table S1. Using the Weidman-Franz law (κE= LσT) the electronic thermal conductivity (κE) was calculated, where L is the Lorentz, σ is the electrical conductivity and T is the temperature. The lattice thermal conductivities (κL) were calculated according to the relation, κL = κT – κE = κT – LσT. Hall coefficient (RH) was measured under a reversible magnetic field of 0.52 T by the Van der Pauw method using a Hall measurement system (ResiTest 8340 DC, Tokyo, Japan) nearly at room-temperature (~ 300 K). The charge Carrier concentration was calculated by using an equation, nH= 1/(eRH) and carrier mobility (µH) was obtained by µH = σRH.
3. Results and discussion Fig. 1 illustrates the two temperature-dependent crystal structures of SnS. Fig. 1a depicts the low
temperature orthorhombic structure with Pnma space group ( = 4.33 ± 0.02Å, = 11.99 ± 0.02Å and = 3.98 ± 0.02Å), which has a layered crystal structure with two Sn–S bilayers [39]. Similar to SnSe, the crystal structure of SnS features a distorted Sn polyhedral coordination with three shorts and four long Sn–S bonds [20]. Covalent bonds exist in its crystal structure and joined the atoms in each layer with the three neighbor atoms, forming accordion shaped corrugated slabs. The three covalent bonds among the corresponding atoms are stronger, while the other four covalent bonds (two in the same bilayer and two in the next bilayer) are weaker bonds. The Pnma orthorhombic structure of SnS transforms to nearly high symmetric orthorhombic structure (Cmcm) at elevated temperature as shown in Fig. 1b [40]. Indeed, the Pnma crystal structure at low temperature is a subgroup of the Cmcm crystal structure at elevated temperature [41]. All the thermoelectric properties were measured in the temperature range ~ 300-877K along the direction perpendicular to SPS pressurizing direction (in-plane) as shown in Fig. 1c. Here the in-plane direction is chosen because the two neighbored bilayers associated with weaker Sn-S bonding along the a-direction results in easy cleavage in the b-c plane. Fig. 1d displays the room temperature (~ 300K) carrier concentration ( ) and mobility ( ) of the undoped SnS, 0.5% Ag-doped and doped-vacancycontaining Sn0.995-vacAg0.005S (vac= 0, 0.0025, 0.005, 0.0075) samples, where “vac” stands for the amounts of vacancy generation. At ~ 300 K, of the undoped SnS and Sn0.995Ag0.005S samples are 17 −3 18 −3 ~ 0.0162 × 10 cm and ~ 3.73 × 10 cm , respectively. The significant enhancement in confirms the effective substitution of Ag in the Sn atomic sites of SnS acting as acceptor dopants, as follows: ′
+ ℎ° 1
The Ag doping in the SnS crystal structure was confirmed by high-resolution X–ray photoelectron spectroscopic spectra, in which Ag 3d5/2 and Ag 3d3/2 can be observed at ~ 367 and ~ 373 eV, respectively as shown in Fig. 1e. Therefore, we attribute the significant enhancement in is further to hole (h°) produced by the above reaction. Compared to the 0.5% Ag-doped SnS, 4
slightly enhanced by the intentional introduction of Sn vacancies, which act as acceptors. Besides the significant enhancement in hole concentration, the Ag-doped sample exhibits impressively high (slightly lower than that of the undoped SnS) at ~ 300 K. further decreases by incorporating Sn vacancies into the lattice structure. greatly deteriorates in Sn0.9875Ag0.005S sample compared with in Sn0.995Ag0.005S sample and then it is further other doped samples. We have discussed the high decreasing trend in vacancy-containing samples in Fig. S3. In summary, the improved thermoelectric transport properties lead to a record significant enhancement of ~ 204% in power factor (PF) and ~ 424% in thermoelectric figure of merit (ZT) as shown in Fig. 1f. Fig. 2a depicts the electrical conductivities (σ) as a function of temperature for the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S bulk samples, respectively. As shown, σ of the undoped SnS increases sharply after doping with Ag. The room-temperature (~ 300 K) σ for Sn0.995Ag0.005S sample is ~ 7.33 S/cm, which is much higher than that of the undoped SnS. Here the significant enhancement in σ can be attributed to the optimized and high . σ of the Sn0.995Ag0.005S sample further increases by introducing Sn vacancy. In particular, the roomtemperature σ values are ~ 8.7, 11.2 and 10.12 S/cm for doped-vacancy-containing Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples, respectively. As the Sn0.9875Ag0.005S (vac ≥ 0.0075) sample exhibit very low , so the electrical conductivity further decreased. The electrical conductivities (except for undoped SnS) slightly increase from 300 to ~ 450 K and then decrease with rising temperature (before ~ 648 K). Previously, this type of variation in the σ value was also observed in Na/K doped SnSe polycrystals [33]. The conductivities above ~ 648 K increase abruptly with rising temperature due to a rapid increase in thermally activated carrier concentration. The Seebeck coefficient (S) of the undoped SnS, 0.5% Ag-doped and doped-vacancy-containing samples as a function of temperature is shown in Fig. 2b. For the Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples, the room-temperature S values are ~ 350.2, 344.3, 339.4 and 334.3 µV/K, respectively. Consistent with the increase in , S values at ~ 300 K for the 0.5% Ag-doped and doped-vacancy-containing samples are lower than that of the undoped SnS (~ 525.3 µV/K), but still maintains impressively higher levels. S for the doped samples increase with rising temperature and are consistent with heavily doped semiconductor behavior [33]. After intrinsic excitation begins, S decreases with further increasing temperature due to a rapid increase in caused by the intrinsic excitation. The temperature corresponding to the maximum S of doped samples (~ 648 K) is higher than that of the undoped one (~ 598 K) because of the high . To witness the excellent electrical transport properties in doped samples, we calculated the effective mass ( ∗ ) using the following equations [42,43]: = where *3 (/) is Fermi integral
!" (& + 5/2)*+,-/. (/) $ − /2 2 # (& + 3/2)*+,0/. (/)
*3 (/) = 4 The effective mass (
∗
9
:
53 ;5 3 1 + exp (5 − /)
) of hole is calculated using Hall carrier concentration (
5
)
(2 ∗ !" <)-/. *0/. (/) = 4 2= . ħ& ∗
where & is Hall factor
1 2= . ħ- & = ? !" < *0/. (/)
& =
./-
@
5
3 *0/. (/)*A0/. (/) 6 B*: (/)C. 4
Among them, r is the scattering rate, kB is the Boltzmann's constant, e is the electron charge, / is the reduced Fermi energy level, ħ is the reduced Planck's constant and T is the temperature. Table 1 shows the variations of effective mass ( ∗ ) to the electron mass ( E ) in Sn0.995∗ sharply increases in the 0.5% Ag-doped vacAg0.005S (vac = 0, 0.0025, 0.005 and 0.0075) samples. sample and then varies negligibly in the doped-vacancy-containing samples. The Seebeck coefficient (S) depends on the carrier concentration (n) and effective mass ( ∗ ) as follows: =
8= . !" 3#ℎ.
∗
= ./G 7 3
Therefore, the lower Seebeck coefficients at ~ 300 K of the 0.5% Ag-doped and doped-vacancycontaining samples with respect to undoped SnS sample are due to the significantly increased in . S of the doped-vacancy-containing samples are slightly lower than that of the Sn0.995Ag0.005S sample due to the slight increase in as shown in enlarged Fig. 2c. Due to the highest value of , Sn0.9875Ag0.005S sample show the lowest S value at ~ 300 K. has significant effects on the S, i.e., when the is not high enough it always results in lower S value [44]. However, here the in doped samples is three order higher than that of the undoped SnS, but the S values still maintain higher levels due to the enhanced ∗ , as S directly proportional to ∗ (Eq. 7). Moreover, for a given thermoelectric material a dimensionless quality factor (B) is characterized by the following relation [45,46]:
where
∗
I ∝ (
∗ )-/.
8
is the effective mass and µ is the carrier mobility.
It is challenging to optimize B, because ∗ and are contrariwise with each other [47,48]. However, the Sn0.995Ag0.005S sample exhibits improved ∗ and also impressively high (negligible ∗ deteriorations in the as compared to their sharp increase in ). For example, the large surfaces in 0.5% Ag-doped sample provide uninterrupted way for carrier transport, which compensate for the improved ∗ as shown in Fig. S3. Fig. 2d shows the power factor (PF = S2σ) values for undoped SnS and Sn0.995-vacAg0.005S samples versus 0.5% Ag doping and vacancy (vac=0, 0.0025, 0.005 and 0.0075) in the temperature range of ~ 300–877 K, respectively. As a result of the improved σ in doped samples, S2σ of the undoped SnS has been significantly enhanced. We attribute the high , and optimized m* and . The high electrical performance in Sn0.995Ag0.005S to the increase in 6
temperature S2σ for Sn0.99Ag0.005S (~ 4.25 µW/cm K2 at 877 K) is about three times higher than that for undoped SnS (~ 1.4 µW/cm K2). Here, the significant enhanced S2σ values for Sn0.995-vacAg0.005S samples are higher and/or comparable to other typically reported SnS polycrystals in the temperature range of ~ 300–877 K [20,34,35]: Fig. 3 shows the variation of total thermal conductivity (κT) for the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples as a function of temperature, respectively. κT of all the samples decreases with increasing temperature from ~ 300 to 877 K. For the undoped SnS, κT of ~ 0.58 W/m K at 877 K is obtained. κT decrease upon 0.5% Ag doping and the minimum thermal conductivity ~ 0.46 W/m K at 877 K was realized in Sn0.995Ag0.005S sample. Then, in the whole range of temperature, κT further decreases with increase in vacancy (vac) and the lowest value of ~ 0.34 W/m K is achieved for the Sn0.99Ag0.005S sample. So far, κT for the optimized doped-vacancy-containing sample is significantly lower than those of the previously reported data as shown in the right top inset of Fig. 3a [20,34,35,49]. The temperature dependent lattice thermal conductivities (κL) of all the samples were calculated according to κL = κT – κE = κT – LσT, where L is the Lorentz number, σ is the electrical conductivity and T is the absolute temperature. As shown in Fig. 3b, the lattice part (κL) of thermal conductivity is comparable to that of total thermal conductivity. Therefore, the electronic part (κE) is negligible and the dominant lattice part accounts for total thermal conductivity. In the whole temperature range, κL of the 0.5% Ag-doped and dopedvacancy-containing samples is much lower than that of the undoped SnS. Of note, κL for the Sn0.995Ag0.005S and Sn0.99Ag0.005S samples are 28% and 74% lower than that of the undoped SnS sample at 877 K. Due to the abundant deficiencies such as dislocations, Ag nano-precipitates, vacancies and grain boundaries, which contribute to strong scattering of phonons, we use the Debye-Callaway model to calculate the lattice thermal conductivity [50]: KLMN
TU
!" !" < - V RS# 3 = . F G 4 PNQN (R) 3 ; 9 2= O ħ (# − 1). 3 : R=
ℏX 10 !" <
where kB is the Boltzmann’s constant, O is the average sound velocity, ħ is reduced Planck’s constant, YZ is the Debye temperature, and X is the frequency of phonons. PNQN is the total relaxation time and calculated according to the Matthiessen’s rule [51,52]: A0 A0 A0 PNQN = P[A0 + P\A0 + P]" + P^A0 + P^Z + PZA0 11
which relates the scattering from Umklapp process, Normal process, grain boundaries, precipitates, point defects and dislocations. Umklapp process and Normal process relaxation time are acquired by fitting the experimental data of undoped SnS. The density and size of nano-precipitates, the dislocation density are acquired from the TEM observations. The relaxation time of point defects is given by: XS _: A0 (` + ` ) 12 P^Z = 4=O - a 7
V0 is the average volume per atom. `a and ` are the disorder scattering parameters of mass and strain field fluctuations, respectively. For Sn0.99Ag0.005S sample, the disorder is caused by Ag doping and Sn vacancies, so the both contribution must be considered. Assume that the kth atom of the ith sublattice has mass bcd , radius &cd , and fractional ecd . The fc ) and radius (&̅c ) on the ith sublattice is b fc = ∑d ecd bcd and &̅c = ∑d ecd &cd . The average mass (b f = ∑ci0 c b fc ⁄∑ci0 c . ci are the degeneracy (here average atomic mass of the compound is b c1=c2=1). Then the `a and ` are as follow [53,54]: lll b ∑ci0 c ( fk ). `ac b `a = 13 ∑ci0 c
`ac = m ecd (1 − d
bcd . ) 14 fc b
lll b ∑ci0 c ( fk ). ` c b ` = 15 ∑ci0 c
` c = m ecd 5(1 − d
&cd . ) 16 &̅c
ε is a phenomenological adjustable parameter. The other specific expressions of the individual relaxation time are included in Table S2, which could be found in the reports [55-57]. All the parameters for calculation are also listed in Table S2. The calculated result for Sn0.99Ag0.005S sample is plotted in Fig. 3b (red hollow stars). It can be seen that a good consistency with the experimental value (red filled stars), which reinforces the validity of our analysis. We also apply the Cahill model to estimate the minimum lattice thermal conductivity of Sn0.99Ag0.005S sample as follows [58]: Knc
= 0 = ( )- !" 6
Tp . < . V R-# 3 - m oc ( ) 4 ;R 17 Yc (# 3 − 1). : c
where the Debye temperature is θi = vi(ћ/kB)(6π2n)1/3, n is the number density of atoms and vi is the sound velocity for each polarization mode. Here we use 3368 m/s for longitudinal and 1537, 2368 m/s for transverse branches, as indicated in literature [36]. The calculated results are also included in Fig. 3b. The calculated values are more closely with the experimental data and the results derived from the Debye-Callaway model at 900 K, which further proves the validity of our argument. To study microstructures and compositional analyses of the Sn0.995-vacAg0.005S, we first performed scanning transmission electron microscopy (STEM) and scanning transmission electron microscopyenergy dispersive X–ray spectroscopy (STEM-EDS) characterization. Then, to determine the detailed crystalline information, selected area electron diffraction (SAED) analyses and atomic resolution high angle annular dark field-scanning transmission electron microscopy (HAADFSTEM) were used. Fig. 4a is the STEM image of the stoichiometric Sn0.995Ag0.005S sample. Beside the effective substitution in the Sn atomic sites of SnS, Ag elements also accumulate at the grain boundaries and exhibit brighter contrast than the matrix as shown in Fig. 4a. Fig. 4b shows STEM8
EDS elemental mapping taken on the framed area (solid red) in Fig. 4a, revealing that elements tin (Sn), Silver (Ag) and sulfur (S) distribute homogeneously in the matrix. The matrix SAED pattern can be indexed to SnS taken along [100] zone axis as shown in Fig. S4. A typical HAADF-STEM image of the framed area (solid orange) in Fig. 4a for the two small-sized nano-precipitates at grain boundaries is shown in Fig. 4c. The observed nano-precipitates of bright-contrast are in the sizes of ~ 7–12 nm. These nano-precipitates have a coherent feature i.e., without any structure defects and exhibit single crystalline nature. Right top inset of Fig. 4c is the STEM-EDS elemental mapping taken on the framed area (solid black), confirming that the nano-precipitates are Ag elements since they show brighter contrast than the matrix. Ag single crystalline coherent nano-precipitates scatter the low-frequency phonons in Sn0.995Ag0.005S sample [32]. The lattice thermal conductivity for Sn0.995Ag0.005S sample is 28% lower than that of the undoped SnS sample at 877 K, which confirms that nanostructuring plays an important role in the reduction of κL. Fig. 4d is the HAADF-STEM image of the Sn0.99Ag0.005S sample, which exhibit dislocations inside the grains. Dislocation is a strategy to suppress the intermediate frequency phonon propagations [59]. Fig. 4e is the zoom-in image of the framed area (solid yellow), which shows dislocations in the crystal structure. Fig. 4f is another HAADF-STEM image for the corresponding sample, in which plenty amount of sinusoidal atomic arrays were observed. Dislocations and sinusoidal atomic structure strengthened phonons scattering in Sn0.99Ag0.005S sample. Conversely, dislocations distress the hole transport in dopedvacancy-containing samples and as a result mobility decreased. For example the sample Sn0.9875Ag0.005S exhibits the lowest µ value. However, dislocations exhibit an imperative effect on κL compared to the effect on its electrical transport properties. Therefore, the very low κL in dopedvacancy-containing samples suggests that mid-frequency phonons scattering have been remarkably enhanced. Hence, a remarkable 74% lattice thermal conductivity has been suppressed in Sn0.99Ag0.005S sample so far. A marked enhancement in the thermoelectric figure of merit (ZT) was obtained for SnS polycrystals via enhancing power factor enabled by Ag doping and the simultaneously reduced thermal conductivity by introducing dislocations (induced by Sn vacancies) in addition to Ag nanoprecipitates. The ZT of undoped SnS is ~ 0.21 at 877 K as shown in Fig. 5a. More importantly, a significant increase by ~ 424% the maximum ZT of 1.1 at 877 K was achieved for the Sn0.99Ag0.005S bulk sample, which is the highest ZT value for polycrystalline SnS based materials reported at this temperature so far. More exciting, the maximum ZT of Na-doped SnS [21] single crystal was reproduced in its polycrystalline form, and is much higher than polycrystalline Na-doped SnSe [60] and Ag-doped SnS [20] prepared by the same synthesis method and/or different method [34,35].
4. Conclusion In summary, the thermoelectric performance of polycrystalline SnS was optimized. The intentional-induced dislocations are effective in decreasing the thermal conductivity. As a result, very low thermal conductivity ~ 0.34 W/m K at 877 K for Sn0.99Ag0.005S sample was obtained. Ag doping increases the carrier concentration of SnS, and optimizes the effective mass and carrier mobility, which results in ultimately high power factor. And a record ZT of 1.1 at 877 K is achieved for Sn0.99Ag0.005S sample. Here, the high thermoelectric performance advocates that SnS based polycrystals have plentiful room to advance and ultimately reach the performance of the SnSe 9
polycrystals. Moreover, the high performance in doped SnS polycrystals also encourages SnS single crystals to improve and reach the performance of its denser analogous SnSe single crystals.
Authors Contributions Jing-Feng Li and Asfandiyar designed the project; Asfandiyar performed the experiments and drafted the manuscript. Jing-Feng Li, Bowen Cai and Li-Dong Zhao revised the manuscript.
Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work is supported by the Basic Science Center Project of Natural Science Foundation of China (51788104) and the National Key R&D Program of China (2018YFB0703603).
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Table and Figures
Table 1 Represent the room-temperature (~ 300 K) Hall carrier concentration, the ratio of effective mass ( ∗) to the electron mass ( E ) and the carrier mobility for the Sn0.995-vacAg0.005S bulk samples. All samples are measured along the sample section perpendicular to the SPS pressurizing direction.
Samples
nH (1018 cm−3)
Sn0.995Ag0.005S Sn0.9925Ag0.005S Sn0.99Ag0.005S Sn0.9875Ag0.005S
3.73 4.12 4.34 4.42
∗
/
1.097 1.119 1.114 1.092
E
µH (cm 2/Vs) 5.28 4.56 3.78 2.16
Fig. 1. (a) The room-temperature and (b) High-temperature crystal structure of SnS, gray atom represents Sn and yellow one is S, (c) Schematic illustration of the measurement direction of thermoelectric transport properties, (d) Room-temperature carrier concentration and carrier mobility for the pure SnS, doped Sn0.995Ag0.005S and doped-vacancy-containing Sn0.995-vacAg0.005S samples, (e) X–ray photoelectron spectrum of the 3d orbitals for the Ag element of 0.5% Ag-doped SnS sample and (f) Power factor (S2σ) and thermoelectric figure of merit (ZT) for the corresponding samples at 877 K.
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Fig. 2. (a) Temperature dependent electrical conductivities, (b) Seebeck coefficients, (c) Enlarged Figure for doped samples (temperature range ~ 498 to 800 K) and (d) Power factors of the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S bulk samples in the temperature range ~ 300–877 K along the direction perpendicular to SPS pressurizing direction, respectively. For comparisons the optimized data of the previous reports are also plotted [20,34,35].
Fig. 3. Temperature dependent (a) total thermal conductivity of the SnS, Sn0.995Ag0.005S, Sn0.9925Ag0.005S, Sn0.99Ag0.005S and Sn0.9875Ag0.005S samples (right top inset show comparisons of the lowest thermal conductivity achieved in this work with other previously reported work) and (b) Experimental lattice thermal conductivity of the corresponding samples and the calculated κL for the Sn0.99Ag0.005S sample.
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Fig. 4. (a) STEM annular bright field (ABF) image showing nano-precipitates at the grain boundary with bright contrast in Sn0.995Ag0.005S sample, (b) The EDS elemental map images of the framed area (solid red) in (a), (c) HAADF-STEM image of the framed area (solid orange) in (a) containing two nanoscale precipitates, reveals continuous lattice between grain boundaries and nano-precipitates (right top inset is the STEM-EDS elemental mapping taken on the framed area (solid black), confirms that the nano-precipitates are Ag elements), (d) HAADF-STEM image of the Sn0.99Ag0.005S sample, confirm nanoscale dislocations, (e) Zoomin image of the framed area (solid yellow), witness a dislocation and (f) HAADF-STEM image for the corresponding sample, which exhibit sinusoidal atomic arrays (right top inset is the zoom-in image).
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Fig. 5. (a) Temperature dependent ZTs for all the samples in this work and (b) Comparisons of the ZT in this work with other reported optimized data [20,34,35,49].
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Highlights • • • •
Stoichiometric and Ag-doped Sn-deficient SnS samples were fabricated by ball milling combined with spark plasma sintering. A high ZT up to 1.1 at 877 K is achieved for Sn0.99Ag0.005S sample. Ag doping and Sn vacancies were revealed to be effective for increasing power factors in SnS. The mechanism of ultralow thermal conductivity was discussed in association with nanostructure and dislocation.
Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Asfandiyar is a Ph.D. candidate in the School of Materials Science and Engineering, Tsinghua University, China. He obtained his MSc. in Physics from Islamia College University of Peshawar (Pakistan) in 2010 and BSc. in Physics from Peshawar University (Pakistan) in 2008. His current research interests include SnS-based thermoelectric materials.
Bowen Cai is now a Postdoctoral fellow working at the School of Materials Science and Engineering in Tsinghua University. His current research interests include thermoelectric materials and high pressure technology. He holds a B.E. from Xi’an University of Technology, a Ph.D from Yanshan University in Qinhuangdao, China.
Dr. Li-Dong Zhao is a full professor of the School of Materials Science and Engineering at Beihang University, China. He received and his Ph.D. degree from the University of Science and Technology Beijing, China in 2009. He was a postdoctoral research associate at the Université Paris-Sud and Northwestern University from 2009 to 2014. His research interests include electrical and thermal transport behaviors in the compounds with layered structures. Group website: http://shi.buaa.edu.cn/zhaolidong/zh_CN/index.htm
Jing-Feng Li is a professor in Tsinghua University, China. He graduated from Huazhong University of Science and Technology (China) in 1984, and obtained his doctor degree from Tohoku University (Japan) in 1991. After working in Tohoku University as assistant professor and associate professor from1992 to 2002, he joined Tsinghua University as a full professor in 2002. His research interests include thermoelectric materials and devices, piezoelectric ceramics, composites and films for MEMS applications.