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Journal of Alloys and Compounds 754 (2018) 131e138

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Effect of synthesis processes on the thermoelectric properties ofBiCuSeO oxyselenides

Bo Feng a, b, Guangqiang Li a, b, Zhao Pan a, b, Hu Xiaoming a, b, Liu Peihai a, b, He Zhu a, b,Li Yawei a, b, Xi'an Fan a, b, *

a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, Chinab National-Provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology,Wuhan 430081, China

a r t i c l e i n f o

Article history:Received 13 March 2018Received in revised form26 April 2018Accepted 27 April 2018Available online 30 April 2018

Keywords:Thermoelectric materialsBiCuSeOBall millingSinteringThermoelectric properties

* Corresponding author. The State Key Laboratory oWuhan University of Science and Technology, Wuhan

E-mail address: 15671676079@163.com (X. Fan).

https://doi.org/10.1016/j.jallcom.2018.04.3070925-8388/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

The BiCuSeO based ceramics have been prepared by mechanical alloying (MA) and resistance pressingsintering (RPS) process. The effects of the parameters of annealing, ball milling and sintering on themicrostructure, thermoelectric properties, and mechanical properties are investigated systematically.The results indicates that the samples prepared by MA exhibit higher carrier concentration due to higherCu vacancies concentration, smaller grain size resulting in stronger phonon scattering, and corre-spondingly higher thermoelectric properties than those prepared by annealing. By prolonging millingtime and increasing ball milling strength, it can further refine grains, increase the carrier concentration,enhance phonon scattering and further improve thermoelectric properties. Coupled with Ca/Pb doping,the electrical conductivity at room temperature increased to 445 Scm�1. The total thermal conductivity isbelow 0.90 Wm�1K�1 in the whole measured temperature range, which is attributable to the increaseddefects and gain boundaries. The maximum power factor of 0.75 mWm�1K�2 and ZT value of 1.15 wereobtained for the Bi0.84Ca0.08Pb0.08CuSeO with BM time (t)¼ 16 h at 873 K.

© 2018 Elsevier B.V. All rights reserved.

1. Introduction

Thermoelectric materials, which are capable of convertingwaste heat into electrical power, are currently receiving significantscientific attention. However, the widespread use of thermoelectrictechnology is constrained by the relatively low conversion effi-ciency of present available thermoelectric materials. Thermoelec-tric oxides are vigorous candidates in terms of large-scaleapplication because of their low cost, less toxicity and good stabilityin relatively high temperature range. Thermoelectric oxides includeseveral types of oxide ceramics such as Ca3Co4O9 [1], NaCo2O4 [2],and SrTiO3 [3], which have been studied extensively over the pastdecades. However, the ZT values still remain low for the low elec-trical conductivity and high thermal conductivity because of theionic nature of oxides, which limits the application.

Recently, the layered oxychalcogenides BiCuSeOwith a ZrSiCuAsstructure (space group P4/nmm) was reported to be a promising

f Refractories and Metallurgy,430081, China.

thermoelectric materials due to its extremely low thermal con-ductivity and large Seebeck coefficient. Especially, the highest ZTvalue of substituted BiCuSeO can be as high as 1.5, which is superiorto all the other thermoelectric oxides [4]. However, the thermo-electric properties of pristine BiCuSeO are not desirable due to theintrinsic low carrier concentration (~1.0� 1018 cm�3) and carriermobility (~20 cm2V�1s�1) [5]. In the past few years, tremendousefforts have been made to modify the electrical/thermal transportproperties by monovalent elements (K [6], Na [7] and Ag [8]),divalent elements (Mg [9], Ca [10], Sr [11], Ba [12], Pb [13], Cd [14]and Zn [15]), trivalent elements (Sb [16] and La [17]) and tetravalentelement (Sn [18]) doping at Bi site, monovalent element (Ag [19])doping at Cu site, dual doping with Ca and Pb at Bi site [4], dualdoping with Pb at Bi site and Te at Se site [20], Cu-deficient [21] andtexturation [22], 3D Modulation Doping [23] and compositestructure [24]. Up to now, the highest ZT value of 1.5 was obtainedfor the Ca/Pb-dual-doped BiCuSeO oxychalcogenides coupled withall-scale structural optimization [4].

Persistent efforts have been devoted to improve ZT valuesthrough boosting power factor while most of them neglectinglowering thermal conductivity. In particular, nanostructuring has

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138132

been proven to be an effective approach to reduce the thermalconductivity by embedding nanoscale precipitates in the matrix[25]. Besides, it is well known that the preparation process has agreat influence on the properties of materials, especially for thethermoelectric materials which are extremely sensitive to thecomposition and structure. However, few studies have paid atten-tion to the preparation process (ball milling [26,27], ultra-fast self-propagating high-temperature synthesis [28,29], and flux synthesis[30]) of BiCuSeO. And no reports have ever studied the mechanicalproperties of BiCuSeO before. However, mechanical properties havea decisive influence on the machinability during the use of ther-moelectric materials. Herein, we synthesize BiCuSeO by ball millingand annealing, respectively, followed by sintering, and report theeffect of the parameters of annealing, ball milling and sintering onthe thermoelectric andmechanical properties of BiCuSeO ceramics.

2. Experimental sections

2.1. The synthesis of powder

A series of samples with the chemical composition of BiCuSeO(called as BCSO below) and Bi1-2xCaxPbxCuSeO (x¼ 0, 0.01, 0.02,0.04, 0.06, 0.08) (called as BCPCSO below) were prepared fromcommercial powders (Bi2O3 (99.99%), Bi (99.999%), Cu (99.999%), Se

Fig. 1. XRD patterns from (a) annealing at 350 �C, (b) annealing at 800

(99.99%), CaO (99.99%), Pb (99.99%)). We synthesize the powder byball milling and annealing, respectively.

1) The stoichiometric mixture of powders were ground: high-energy ball milling was performed at 300 or 400 rpm in argonatmosphere for 2 h/4 h/6 h/8 h/12 h and 4 h/8 h/12 h/16 h/20 h,respectively, followed by MA with analytical reagent (AR)ethanol absolute (�99.7%) for 1 h.

2) The stoichiometric mixture of powders were placed in a quartzglass tube, vacuum sealed, then annealed at 300 or 800 �C for3 h/8 h in a muffle furnace. Take out the powder, and grind withagate mortar.

2.2. The sintering for bulk

Then, the powder with high phase purity were sintered byresistance pressing sintering (RPS) at 823/873/923 K for 10minunder the axial compressive pressure of 40/50/60MPa under argonatmosphere to form disk-shaped sample of F20� 13mm.

2.3. Characterization

The phase identification were characterized by X-ray diffraction

�C, (c) balling milling at 300 rpm, (d) balling milling at 400 rpm.

Fig. 2. SEM of the fractured surfaces for BCSO bulks ((a) stands for 800 �C annealing; (b)e(d) stand for ball milling time t¼ 8 h, 12 h, 16 h, respectively).

Table 1The relative density (r), carrier concentration (n), mobility (m) and m* for the BCSOsamples at room temperature.

Sample r (%) S (mVK�1) n (1019cm�3) m (cm�2V�1s�1) m*/mo

800 �C 97.6 400 0.84 13.96 0.81400 rpm, t¼ 8 h 98.4 371 1.5 8.85 0.97400 rpm, t¼ 12 h 97.9 355 1.9 7.73 1.24400 rpm, t¼ 16 h 97.6 334 2.1 5.15 1.36

Fig. 3. The average grain size and proportion of grain (<500 nm) counted by NanoMeasurer from SEM of the fractured surfaces for BCSO samples (samples 0 stands for800 �C annealing; 1e3 stand for ball milling time t¼ 8 h, 12 h, 16 h, respectively).

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138 133

with an X'Per Pro diffractometer using Cu Ka radiation(l¼ 1.5418Å). The microstructures were studied by field emissionscanning electron microscopy (FESEM) (FEI, Nova 400 Nano SEM)and the elemental compositions were determined using energy-dispersive X-ray spectroscopy (EDS). The sintered bulks were cutinto bar-shaped specimens of 3mm� 3mm� 15mm along theradical direction to test the electrical transport properties. Theelectrical resistivity (r) and Seebeck coefficient (S) were measuredby commercial equipment (ZEM-2, Ulvac-Riko, Japan). The bulksamples were cut into small wafer with an approximate dimensionof f12.7mm� 2mm for measuring the thermal transport proper-ties. The thermal diffusivity (D) and specific heat (Cp) weremeasured by a Laser Flash apparatus (LFA457, Netzsch). The volumedensity (d) was measured by the Archimedes method. The thermalconductivity (k) was calculated from the relationship k¼DCpd. Bothof the electrical transport properties and the thermal diffusivitieswere measured along the direction perpendicular to the RPS pressdirection. The carrier concentration and mobility at room temper-ature were measured by the van der Pauw method using the Hall-effect measurement system (HMS-5500, Ekopia). For the mea-surement of the band gap between the valence band and conduc-tion band, the infrared spectroscopy was obtained by Fouriertransform infrared/Raman spectrometer (VERTEX 70, Bruker, Ger-many). All the specimens were polished with two parallel planesand the plane close to the central part was utilized for Vickershardness (HV) measurement, which were conducted on HV-10008with a load of 25 g and a loading time of 15 s. Every sample wastested with 5 data points, and the average HV was used for char-acterizing mechanical properties.

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138134

3. Results and discussion

X-ray diffraction patterns at room-temperature for all the BCSOsamples are shown in Fig. 1. It can be seen from Fig. 1(a) that theformation of BiCuSeO phase happened at 350 �C annealing, but thegeneration efficiency is very low. After 8 h, there are still a large

Fig. 4. Temperature dependence of (a) Electrical conductivity, (b) Seebeck coefficient, (c) PowBCSO.

number of strong impurity peaks as shown in Fig. 1(b), indicatingthat the kinetic condition of the reaction is too poor. When theannealing temperature is up to 800 �C, the generation efficiency ofBiCuSeO is greatly improved. After 3 h, the intensity of the impuritypeak is very weak. It's contributed to the improvement of the ki-netic conditions with increased temperature based on the

er factor (d) Thermal conductivity, (e) Lattice thermal conductivity and (f) ZT values of

Fig. 5. Vickers hardness of the BCSO samples (samples 0 stand for the annealedsample; sample 1e3 stand for the ball milled samples with t¼ 8 h, 12 h, 16 h,respectively).

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138 135

unreacted core kinetics model [31]. After 8 h, there is no detectableimpurity peak. In the process of ball milling, as shown in Fig. 1(c)and (d), the reaction efficiency of BiCuSeO is lower than that of theannealing method. When the ball milling speed is 300 rpm, there isno apparent heterozygosity after 12 h. When the milling speedincreased to 400 rpm, the reaction efficiency of BiCuSeO isincreased, no apparent heterozygosity after 8 h. This is contributingto the increased milling intensity with faster ball milling speed formore accelerated cold welding, welding process [32]. Moreover,with the further prolongation of t, the XRD peaks become wider,indicating that the grains get smaller, and a small amount of het-erophase (CuSe) is found.

The relative density of the samples sintering at 823 K, 873 K, and923 K is 96%, 98.2%, 93%, respectively. The sample sintering at 823 Khas micro cracks and is easily broken in cutting. During cracking,bridges formed by matrix grains remain intact behind the crack tip.Such bridges are formed by crack bridging of microcrack [33].When the sintering temperature is too low, the bonding betweengrains is not intact enough. The bridge is weakened greatly,resulting in easy cracking. Melting out was found on themoldwhenthe sample sintering at 923 K was finished, indicating that thesintering temperature was too high. And the relative density forboth of them is lower than that of 873 K, so the most suitable sin-tering temperature is 873 K.When the sintering pressure is 40MPa,the mechanical property of the sample is very poor, and it can bepinched by hand. When the sintering pressure is 60MPa, thesample is cracked when removed from the mold. When the pres-sure is low, bridges of microcrack are not easily formed, thesubcritical crack growth (SCG) or creep damage accumulation arenot being effectively curbed [34]. When the pressure is too high, theSCG or creep damage accumulation increases instead. As a result,the most appropriate sintering pressure is 50MPa.

Considering the high production efficiency of 800 �C annealingand ball milling with 400 rpm, the corresponding powder is sin-tered (the sintering temperature is 873 K and the sintering pressureis 50MPa) into bulk. Fig. 2 shows the SEM images of the fracturedsurfaces of BCSO bulks. The samples consist of lath-like micro-grains which is stacked densely and there are few pores after theprocess of densification by RPS. The relatively densities of all thesamples are above 97% obtained by the Archimedes principle asshown in Table 1. The grain size of the annealed sample is obviouslylarger than that of the BM sample. The average grain size andproportion of grain (<500 nm) counted by Nano Measurer fromSEM of the fractured surfaces for BCSO bulks are shown in Fig. 3.The average grain size of the annealed sample is 1481 nm, muchlarger than 952 nm for the BM sample with t¼ 8 h. With the pro-longation of ball milling time, the average size of grain significantlydecreases, from 952 nm for t¼ 8 h to 598 nm for t¼ 12 h, andfurther down to 400 nm for the 16 h. And the proportion of finegrains (<500 nm) increases markedly, from 24% for t¼ 8 h to 44%for t¼ 12 h, and further up to 62% for the 16 h. The grain size tendsto decrease monotonically withmilling time towards a steady-stategrain size before the saturation in stored enthalpy, which is in linewith the previous literature on the ball mill [35].

Fig. 4 (a) shows the temperature dependence of the electricalconductivity (s) for BCSO samples. No obvious bipolar conductionis observed for all the BCSO samples. And the s increases withincreasing testing temperature, typical of a semiconductingbehavior. The s of the sample prepared by annealing is much lowerthan that of ball milling in the whole temperature range, which iscontributing to the larger grain size (as shown in Figs. 2 and 3), andthe corresponding lower concentration of Cu vacancies. The s isknown to depend on the carrier concentration and mobility asshown in the relationship s¼ nme (e is carrier charge, n carrierconcentration and mmobility) [4]. The Hall carrier concentration (n)

and mobility (m) of BCSO bulks at room temperature are shown inTable 1. The n of the sample prepared by annealing is far lower thanthat of the BM sample. With the prolongation of t, the s shows anupward trend and increases with increasing testing temperature.Specifically, the room temperature s significantly increases from~11 Scm�1 for t¼ 8 h to ~16 Scm�1 for t¼ 12 h, and further up to~20 Scm�1 for t¼ 16 h. This is attributed to the increase of n asshown in Table 1. With the prolongation of t, and the severe plasticdeformation during ball-milling process could promote the for-mation of Cu vacancies, which act as electron acceptor for BiCuSeO,and correspondingly the increase of n [36]. The hole mobility de-creases with the prolongation of t, from 8.85 cm�2V�1s�1for t¼ 8 hto 5.15 cm�2V�1s�1for t¼ 16 h. Generally, the inverse of the effec-tive hole mobility of the whole material can be expressed byRef. [37]:

1m¼ 1

m0þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8kBTpm*

pe

exp�fb

kBT

�1l

(1)

where m' is the charge carrier mobility within the individual grains,m* is the effective mass at Fermi level, 4b is the potential barrierheight at the grain boundaries and l is the average size of the grains.l decreases with the prolongation of t as shown in Figs. 2e3, and m'decreases for that the n increases, which would both touch off thedecrease of m.

The Seebeck coefficient (S) of BCSO samples are shown in Fig. 4(b). It can be found that the annealed BCSO exhibits large S from400 mV K�1 near room temperature to 322 mVK-1 at 873 K,contributing to the unique superlattice structure and the corre-sponding 2D carrier confinement effect of BiCuSeO [38]. The See-beck coefficients of all the BM samples are lower than that of theannealed samples in the whole temperature range. With the pro-longation of t, the S exhibits a decreasing trend. Specifically, theroom temperature S significantly decreases from ~ þ 371 mVK�1 fort ¼ 8 h to ~ þ 355 mVK�1 for t ¼ 12 h, and further down to ~ þ334 mVK�1 for t¼ 16 h. For degenerate semiconductors, the Seebeckcoefficient can be expressed roughly by the Pisarenko relation,assuming parabolic band structure and an acoustic phonon scat-tering approximation [2]:

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138136

S ¼ 8p2k2BT

3eh2 m*� p

3n

�2=3(2)

where S is Seebeck coefficient, kB is Boltzman constant, T is theabsolute temperature, e is carrier charge, h is Plank constant, m* isthe effective mass at Fermi level, and n is the carrier concentration.

Fig. 6. Temperature dependence of (a) Electrical conductivity, (b) Seebeck coefficient, (c) PowBCPCSO.

That is to say, the Seebeck coefficient is inversely proportional tothe carrier concentration. As mentioned above, the carrier con-centration increased significantly as the prolongation of t, whichhas touched off the decrease of the Seebeck coefficient.

The power factor (PF) of BCSO samples are shown in Fig. 4 (c). Ascan be seen, the PF of the annealed BCSO increases as the tem-perature rises, and the maximum value is 0.26 mWm�1K�2. The PF

er factor (d) Thermal conductivity, (e) Lattice thermal conductivity and (f) ZT values of

B. Feng et al. / Journal of Alloys and Compounds 754 (2018) 131e138 137

of the ball milled samples are higher than the annealed sample overthe entire temperature range, which is aroused by the significantlyincreased electrical conductivity. The highest power factor of 0.31mWm�1K�2 is obtained at 873 K for the BCSO with t¼ 16 h.

Fig. 4 (d) shows the total thermal conductivity (k) as a functionof testing temperature for the BCSO bulks. The k of all the samplesdecreases with increasing testing temperature over the entiretemperature range, which is aroused by the enhancing phononscattering. The k of the annealed BCSO decreases from 0.92Wm�1K�1 at 300 K to 0.61 Wm�1K�1 at 873 K. The intrinsically lowthermal conductivity is related to the weak chemical bonds,phonon confinement due to the layered structure, and the presenceof heavy element. The k of all the ball milled samples is lower thanthe annealed one for finer grains and stronger interface scattering.With the prolongation of t, the k shows an downward trend, mainlyattributed to the decrease of lattice thermal conductivity (kL) asshown in Fig. 4 (e). The calculated kL follows the relationship askLfT�1, which suggests that the phonon-phonon Umklapp scat-tering is predominant in phonon transport. In general, kL can bequalitatively expressed as:

kL ¼13CnР ¼ 1

3Cn2tðР ¼ ntÞ (3)

or

kL ¼13

ZCðuÞn2ðuÞtðuÞdu (4)

where C, n, P and t are specific heat, speed of sound, phonon meanfree path, and phonon relaxation time, respectively. As t is pro-longed, the grain size becomes smaller as shown in Figs. 2 and 3,the phonon scattering increases, the average free path of thephonon and the phonon relaxation time decreases, leading to thedecrease of the kL.

The dimensionless thermoelectric figure of merits (ZT) all theBCSO bulks are plotted in Fig. 4 (f). It can be seen that the ZT value ofthe annealed BCSO is notably low, whose peak is at 873 K with ZT of0.43. The ZT values of all the ball milled samples are much higherthan that of the annealed one, and dramatically rise up with t,ranging from 0.51, 0.55 to 0.62 of peak value at 873 K with t¼ 8 h,12 h, 16 h, respectively.

Besides the thermoelectric properties, mechanically robust isalso very important for applications in TEGs. Up to now, few studies[39] have been reported to investigate the mechanical properties ofBiCuSeO oxyselenides. In this work, the HV of the BCSO sampleswere measured as shown in Fig. 5. It can be seen that the HV of theannealed BCSO is ~171 HV. In addition, the hardness of the ballmilled bulks is larger than the annealed one and show an increasingtrend with the increase of t, which might be caused by the fine-grained strengthening [40]. Besides, as we can see from Fig. 2, ast is prolonged, the grain size becomes smaller, the size of theporosity is smaller, the number of porosity decreases, which in-dicates that the microstructure is more compact. This may also beone of the reasons for the higher hardness. The highest hardness of270 HV is obtained for the ball milles sample with t¼ 16 h.

In fact, a series of BCPCSO samples were also obtained via thebest optimal synthetic parameters (BM: 400 rpm, 16 h; RPS: 873 K,50MPa). Fig. 6 shows the temperature dependence of the ther-moelectric properties for BCPCSO samples. It can be seen that thethermoelectric properties have been greatly improved with theincrease of the amount of Ca/Pb doping. Ca2þ/Pb2þ doping at Bi3þ

site introduces holes, and correspondingly increase n and s. The Sdecreased to a certain extent upon Ca/Pb doping, which is attrib-uted to the increase n according to (5). The k increases obviously

mainly driven by the increase of the electrical thermal conductivity.Finally, the maximum power factor reached 0.75 mWm�1K�2 andthe highest ZT value reached 1.15.

4. Conclusions

In this work, we have proposed an effective and feasible methodto improve the thermoelectric/mechanical performance of BiCuSeObased thermoelectric materials through preparation process opti-mization and Ca/Pb co-doping. A series of p-type polycrystallineBiCuSeO samples with different preparation process parametersand different contents of Ca/Pb (x¼ 0, 0.01, 0.02, 0.04, 0.06, 0.08)were prepared by a combined method of MA and RPS. The resultsshow that the sample prepared by MA has higher carrier concen-tration due to higher Cu vacancies concentration, smaller grain sizeresulting in stronger scattering for phonons, and thus higherthermoelectric properties than those prepared by annealing. Byprolonging milling time and increasing ball milling strength, it canfurther refine grains, increase carrier concentration, enhancephonon scattering and further improve thermoelectric properties.The optimum process parameters are as follows: 400 rpm for ballmilling speed, 16 h for ball milling time, 873 K for sintering tem-perature, 50MPa for sintering pressure. The correspondingmaximum power factor reached 0.31 mWm�1K�2 and the highestZT value reached 0.62 for the pristine BiCuSeO. Through Ca/Pbdoping, the maximum power factor increased to 0.75 mWm�1K�2

and the highest ZT value increased to 1.15. This significantimprovement in thermoelectric properties coupled with goodmechanically robust underlines that fine-grained polycrystallineBiCuSeO obtained by MA is very promising for thermoelectricapplications.

Acknowledgement

This work was supported by the National Natural ScienceFoundation of China (51674181, 11074195) and Key Project ofDepartment of Education of Hubei Provincial (D20151103).

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