Flexible ternary carbon black/Bi2Te3 based alloy/polylactic acid thermoelectric composites fabricated by additive manufacturing

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Flexible ternary carbon black/Bi

₂

Te

₃

based alloy/polylactic acid

thermoelectric composites fabricated by additive manufacturing

Yong Du

a,b,*

, Jiageng Chen

a

, Qiufeng Meng

a

, Jiayue Xu

a

, Biplab Paul

b

, Per Eklund

b a_{School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China}

b_{Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Link€oping University, SE-58183, Link€oping, Sweden}

a r t i c l e i n f o

Article history:

Received 30 September 2019 Received in revised form 8 February 2020 Accepted 16 February 2020 Available online 17 February 2020 Keywords: Flexible Bi2Te3based alloy Additive manufacturing Thermoelectric

a b s t r a c t

Flexible ternary carbon black/Bi2Te3based alloy/polylactic acid (CB/BTBA/PLA) composites were

fabri-cated by additive manufacturing and their thermoelectric properties were investigated from 300 K to 360 K. At 300 K, as the mass ratios of BTBAs in the composites increased from 38.5% to 71.4%, both the electrical conductivity and Seebeck coefficient of the composites increased from 5.8 S/cm to 13.3 S/cm, and from 60.2mV/K to 119.9mV/K, respectively, and the thermal conductivity slightly increased from 0.15 W m1K1to 0.25 W m1K1, as a result, the ZT value of the composites increased from 0.004 to 0.023. As the temperature increased from 300 K to 360 K, the electrical conductivity of all the composites slightly decreased, while the thermal conductivity slowly increased, and a highest ZT value of 0.024 was achieved for the composites with 71.4% BTBAs at 320 K. Unlike traditional sterolithography, fused deposition modeling, selective laser melting, etc., this additive manufacturing process can directly print the solutions which contain inorganicfillers and polymer matrixes into almost any designed intricate geometries of thermoelectric composites, therefore this process has great potential to be used for fabrication offlexible polymer based thermoelectric composites and devices.

1. Introduction

Flexible thermoelectric (TE) generators, which have the ability to convert waste heat to electrical power, have been widely studied

for the development of wearable electronic devices [1]. The ef

ﬁ-ciency ofﬂexible TE generators depends on the TE properties of

materials. The main methods for preparation ofﬂexible TE

mate-rials and TE generators are screen printing [1], vacuumﬁltration [2], and spray printing [3]. For example, Kim et al. [1] fabricated a ﬂexible glass fabric-based TE generator using a screen printing method, and an output power per unit mass of 28 mW/g at a temperature difference of 50 K was obtained. Du et al. [2] fabricated

a TE generator using 5 strips of BieTe based alloy nanosheet/

poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)

(PEDOT:PSS) TE nanocomposite _{ﬁlms prepared by a vacuum}

ﬁltration method, and an output power of 16.9 nW at a temperature difference of 47.2 K was achieved. Bae et al. [3] prepared aﬂexible

TE generator by spray printing 6 strips of TeeBi2Te3/PEDOT:PSS

composites on a polyimide substrate, and an open circuit voltage of 1.54 mV at a temperature difference of 10 K was obtained.

Additive manufacturing (3D printing), has been widely used in aerospace, building, education, and transportation, etc. areas, mainly due to its advantages of saving raw materials and time [4,5].

However, studies on TE materials prepared via additive

manufacturing are very limited [6e11]. While ﬂexible

polymer-based TE composites in general are widely studied [12e20],

in-vestigations on additive manufacturing of such composites are more limited [7,9,11]. Recently, we [11] reported aﬂexible n-type tungsten carbide (WC)/polylactic acid (PLA) TE composites pre-pared by additive manufacturing, and a ZT value (_{¼ S}2

_s

_T/

_k

_{, where S}

is the Seebeck coefﬁcient,

s

is the electrical conductivity, T is the

absolute temperature, and

k

is the thermal conductivity) of

~6.7 104at 300 K for the composites with ~60 vol % WC was

achieved. The relatively low ZT is mainly due to the low Seebeck coefﬁcient (e 11.4

m

V/K -e 12.3

m

V/K at 300 K) and low electrical conductivity (10.6 S/cm - 42.2 S/cm at 300 K), although the thermal

conductivities of the composites are very low (0.20 W m1K1

-* Corresponding author. School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China.

E-mail addresses:ydu@sit.edu.cn(Y. Du),176081125@mail.sit.edu.cn(J. Chen),

mengqiufenhpu@163.com(Q. Meng),xujiayue@sit.edu.cn(J. Xu),biplab.paul@liu. se(B. Paul),per.eklund@liu.se(P. Eklund).

Peer review under responsibility of The Chinese Ceramic Society.

Contents lists available atScienceDirect

Journal of Materiomics

j o u r n a l h o m e p a g e : w w w . j o u rn a l s . e l s e v i e r . c o m / jou rn al-of - ma teriomics/

https://doi.org/10.1016/j.jmat.2020.02.010

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0.28 W m1K1 at 300 K). Therefore, the primary issue for improving the ZT value of the polymer-based TE composites fabricated via additive manufacturing is to enhance its electrical

conductivity and Seebeck coefﬁcient, while retaining low thermal

conductivity.

BieTe based alloy (BTBA) bulk materials and ﬁlms are

commonly studied, due to exhibited high ZT value at room tem-perature. For instance, a BieTe based alloy (BTBA) bulk material was prepared by a spark plasma sintering process [21]; a Bi2Te2.7Se0.3

ﬁlm was prepared by a 3D conformal printing and photonic sin-tering method [22]. BTBA can also be used asﬁllers for preparation of polymer-based TE composites, e.g. a BTBA/polyaniline

compos-ites was prepared by a mechanical blending method [23], and a

BTBA/PEDOT:PSS composites was prepared by a drop casting

technique [24]. Carbon black (CB) is much cheaper when compared

to graphene and carbon nanotubes, which is typically used as the

conductingﬁllers for the polymer matrixes [25]. Thus, a composite

consisting of BTBA with high Seebeck coefﬁcient and electrical

conductivity, CB with high electrical conductivity asﬁllers, and PLA with low thermal conductivity as polymer matrix, a high ZT value may be achieved by combining the advantages of the BTBA, CB, and PLA. In particular, combining this with additive manufacturing

would allow easy fabrication of such composites. Here, ﬂexible

ternary CB/BTBA/PLA composites were fabricated by additive

manufacturing, and the inﬂuence of BTBA content on the

morphology and TE properties of CB/BTBA/PLA composites are investigated.

2. Experimental details 2.1. Materials

BTBA (Bi0.4Sb1.6Te3, 200 mesh) was obtained from Shanghai

Liuzu New Material Science & Technology, Inc. CB (particle size:

30e45 nm) was purchased from Nanjing XFNANO Materials Tech

Co., Ltd. Chloroform (CHCl3) was purchased from Sinopharm

Chemical Reagent Co., Ltd. PLA (1.75 mm 3Dﬁlament) was obtained

from Shenyang Gein Technology Co., Ltd. All the materials have been used in their as-received state without further treatment or puriﬁcation.

2.2. Preparation of CB/BTBA/PLA composites

The preparation of CB/BTBA/PLA composites is as follows: 0.5 g of PLA was dissolved in 5 mL of CHCl3, and then 0.3 g CB was added

to form Solution I. A determined amount of BTBAs (to obtain the mass ratios of PLA:CB:BTBA of 0.5:0.3:0.5, 0.5:0.3:1, 0.5:0.3:1.5, and 0.5:0.3:2, corresponding to nominal mass ratios of BTBAs ~ 38.5%, 55.6%, 65.2%, and 71.4%, respectively) was added to Solution I and stirred for 2 h to produce Solution II. Solution II was inhaled in the 10 mL syringes and then printed using a 3D solution printer

(Shenzhen Polymer Science& Technology LTD. Model: PLM-I) with

nozzle diameter of 0.5 mm and print speed of 300 mm/min. After

drying at room temperature for 12 h, the CHCl3was evaporated

from the mixed solution, to form the CB/BTBA/PLA composites. 2.3. Characterization

The compositions and morphologies of the samples were characterized by X-ray diffraction (XRD) (Bruker D8 Advance, Germany) and scanning electron microscopy (SEM; FEI Quanta200 FEG, Holand) equipped with energy dispersive X-ray spectrometry (EDS). In-plane Seebeck coefﬁcients and electrical conductivities of the CB/BTBA/PLA composites were measured simultaneously in an

MRS-3L thin-ﬁlm thermoelectric test system in a low-vacuum

at-mosphere (40 Pa) from 300 K to 360 K with instrument test errors

of 6% and 5% for Seebeck coefﬁcient and electrical conductivity,

respectively (Wuhan Giant Instrument Technology Co., Ltd, China). Out-of-plane thermal conductivities of the samples were measured by a transient hot-wire method from 300 K to 360 K (TC3000E thermal conductivity meter, Xiatech Electronics Co., Ltd., China).

Three measurements of the Seebeck coefﬁcients, electrical

con-ductivities, and thermal conductivities were performed for each sample, and the average values are reported.

3. Results and discussion

Fig. 1is a schematic of the additive manufacturing process of the CB/BTBA/PLA composites. The CB/BTBA/PLA composites with mass ratios of PLA:CB:BTBA of 0.5:0.3:0.5, 0.5:0.3:1, 0.5:0.3:1.5, and 0.5:0.3:2 (corresponding to nominal mass ratios of BTBAs ~ 38.5%,

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55.6%, 65.2%, and 71.4%, respectively) were denoted as S1, S2, S3,

and S4, respectively.Fig. 2 shows the XRD patterns of the pure

BTBAs and as-prepared composites. All the peak positions in the S1, S2, S3, and S4 agree with the pure BTBAs (Bi0.4Sb1.6Te3,

PDF#72-1836). As the content of BTBAs increased, no new peaks were detected.

Fig. 3a - d show the SEM surface images of the composites. As the content of BTBAs increased up to 71.4%, no obvious difference in the morphologies was observed. The BTBAs can be clearly seen in the fracture surface images of the composites (the pink arrows in Fig. 3e - h), due to BTBAs show a layered structure, which is the typical morphology of BTBAs [24].Fig. 3i shows the SEM image of a

fracture surface of the S3 composite, andFig. 3j - l show SEM-EDS

mappings of the Bi, Sb, and Te elements for the corresponding area inFig. 3i, respectively. It can be seen that Bi, Sb, and Te are homo-geneously distributed in the EDS mappings, demonstrating uniform distribution of BTBAs particles in the composites. There are some voids in the composites, which might be caused by the

volatiliza-tion of solvent soluvolatiliza-tion (CHCl3) and shrinkage of PLA component

after the drying process. The sample thickness is ~185

m

m (the inset inFig. 3g).

Fig. 4(a) - (e) show the temperature dependence of electrical conductivity, Seebeck coefﬁcient, power factor (S2

_s

_{), thermal}

con-ductivity, ZT of the composites with different BTBA content, and Fig. 4(f) - (i) show the mechanicalﬂexibility of the S3. At 300 K, as the mass ratios of BTBAs increased from 38.5% to 71.4%, the elec-trical conductivity of the composites slightly increased from 5.8 S/

cm to 13.3 S/cm, and the Seebeck coefﬁcient signiﬁcantly rose from

60.2

m

V/K to 119.9

m

V/K. The reasons for this phenomenon are as

below: (1) BTBA has a superior TE property (high electrical con-ductivity and Seebeck coefﬁcient [26]); (2) The Seebeck coefﬁcient for nondegenerate semiconductors is directly proportional to

scattering factors of carriers based on the following equation (1)

[27]: S¼kB e " ð2:5 þ rÞ þ ln2ð2

p

m*kBTÞ1:5 h3_n # (1)

where kB, h, r, and m* are the Boltzmann constant, Planck constant,

scattering parameter, and effective mass of the carrier, respectively.

It can be seen that Seebeck coefﬁcient is proportional to the

scat-tering parameter. As the mass ratios of BTBAs in the composites increase, more interfaces between BTBA, CB, and PLA (BTBA-BTBA, BTBA-PLA, CB-CB, and BTBA-CB interfaces) were formed, which led to more scattering of carriers (holes) in the composites. Further-more, since more interfaces were formed in the composites, and more nanometer-sized barriers were existed on the interfaces, the Fig. 2. XRD patterns of CB/BTBA/PLA composites with different BTBA loadings.

Fig. 3. SEM images of S1 (a), S2 (b), S3 (c), and S4 (d), fracture surface of S1 (e), S2 (f), S3 (g)& (i), and S4 (h), respectively. (j), (k) & (l) is the corresponding SEM-EDS mapping (Bi, Sb, and Te element) image of the panel (i), respectively.

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holes with low energy cannot pass through the interfaces, while the

holes with high-energy can pass (namely, the energyﬁltering

ef-fects in the composites were enhanced) (seeFig. 5) [24,28], leading

to the improvement of Seebeck coefﬁcient. Note that there are

some voids in the composite ﬁlms, which may be beneﬁcial to

enhance the scattering of charge carriers and thus Seebeck coef

ﬁ-cient, while they are adverse to enhance the composites’ electrical

conductivity. In order to decrease the voids in the composites, so as to enhance the electrical conductivities, varieties of methods such as cold pressing [29], hot pressing [30], post-sintering [31] may be

used to treat the as-prepared compositeﬁlms. As the temperature

increased from 300 K to 360 K, the electrical conductivity of all the

composites slightly decreased, while the Seebeck coefﬁcient slowly

increased, for example, the electrical conductivity decreased from

13.3 S/cm to 11.4 S/cm, while the Seebeck coefﬁcient slowly

increased from 119.9

m

V/K to 128.6

m

V/K for the S4.

As the mass ratios of BTBAs increased from 38.5% to 71.4%, the

power factor of the composites increased from 2.1

m

W m1K2to

19.2

m

W m1K2at 300 K, mainly due to the same trends of

elec-trical conductivity and Seebeck coefﬁcient. This value

(19.2

m

W m1K2) is much higher than that of a WC/PLA

(0.64

m

W m1K2at 300 K) with 60 vol% WC [11], a Bi0.5Sb1.5Te3/

PLA composite (6.8

m

W m1K2at room temperature) with 87.5 wt

% Bi0.5Sb1.5Te3 [32]; however this value is lower than that of a

Cu1.75Te/polyvinylidene ﬂuoride (PVDF) composite

(23

m

W m1K2at room temperature) with 66.7 wt% Cu1.75Te [33],

a poly(3,4-ethylenedioxythiophene)/single walled carbon

nano-tube (PEDOT/SWCNT) compositeﬁlm (44.1

m

W m1K2at 294 K)

with 35 wt% SWCNT [34], a Bi0.5Sb1.5Te3nanosheet

(NS)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (BST NS/

PEDOT:PSS) compositeﬁlm (32.26

m

W m1K2at room

tempera-ture) with 4.10 wt % BST NSs [24].

The thermal conductivity of the composites increased as mass ratios of BTBAs increased from 38.5% to 71.4%, or as the temperature

increased from 300 K to 360 K, e.g. from 0.15 W m1K1for S1 to

0.25 W m1K1for S4 at 300 K, and from 0.25 W m1K1at 300 K to

0.34 W m1K1at 360 K for the S4. The value (0.15 W m1K1

-0.25 W m1K1at 300 K) is lower than that of a WC/PLA

(0.28 W m1K1at 300 K) with 60 vol% WC [11], a Bi0.5Sb1.5Te3/PLA

composite (0.34 W m1K1 at room temperature) with 87.5 wt%

Bi0.5Sb1.5Te3[32], a Cu1.75Te/PVDF composite (~0.85 W m1K1at

room temperature) with 66.7 wt% Cu1.75Te [33], and almost the

same value when compared to a BST NS/PEDOT:PSS compositeﬁlm

(~0.2 W m1K1at room temperature) with 4.10 wt% BST NSs [24]. The reasons why the thermal conductivity of the composites is much lower than that of BTBAs ( 1.5 W m1K1) [35] are as below: (1) the PLA has a low thermal conductivity ( 0.13 W m1K1[36]); (2) the voids caused in the composites by the volatilization of

sol-vent solution (CHCl3) and shrinkage of PLA component after the

drying process, and the interfaces formed by BTBA, CB, and PLA scattered the phonons and charge carriers, lead to a decrease of the

electronic thermal conductivity (

k

e) and lattice thermal

Fig. 4. Electrical conductivity (a), Seebeck coefﬁcient (b), power factor (c), thermal conductivity (d), ZT (e) of the CB/BTBA/PLA thermoelectric composites. Flexible display digital photos of the S3 (f) - (i).

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conductivity (

k

p), as a result, the total thermal conductivity

(

k

¼

k

eþ

k

p) decreased.

The ZT values of the composites were estimated based on the

measured in-plane Seebeck coefﬁcients and electrical

conductiv-ities, and the out-of-plane thermal conductivities. The ZT values of the composites also increased from 0.004 to 0.023 at 300 K as the mass ratios of BTBA increased from 38.5% to 71.4%, and the ZT value of the composites were slightly increased and then decreased as the temperature increased from 300 K to 360 K. A highest ZT value of 0.024 was achieved for the S4 at 320 K, which is much higher than

that of a WC/PLA composite (~6.7 104at 300 K with 60 vol % WC)

prepared by additive manufacturing [11], indicating using BTBA

with high Seebeck coefﬁcient and electrical conductivity, CB with

high electrical conductivity asﬁllers, and PLA with low thermal

conductivity as matrixes, can signiﬁcantly enhance the ZT value of

the composites fabricated by additive manufacturing. This value is higher than that of a Bi0.5Sb1.5Te3/PLA composite (ZT¼ 0.006 at

room temperature) with 87.5 wt% Bi0.5Sb1.5Te3[32], a Cu1.75Te/PVDF

composite (ZT¼ ~0.01 at room temperature) with 66.7 wt% Cu1.75Te

[33], and a BST NS/PEDOT:PSS compositeﬁlm (ZT ¼ ~0.01 at room

temperature) with 4.10 wt% BST NSs [24]; however this value is

lower than that of a PEDOT/SWCNT compositeﬁlm (ZT ¼ 0.028 at

294 K) with 35 wt% SWCNT [34], mainly due to the PLA matrix used

in our samples is an insulator. The advantages for using PLA as matrix are as followings: low-price, good processing ability, and suitable for industrial production. Furthermore, most of the con-ducting polymers are normally insoluble and infusible, which are not suitable for fabrication of conducting polymer based TE

com-posites via additive manufacturing [11].Table 1summarizes room

temperature TE properties of the CB/BTBA/PLA composites and those of composite materials previously reported.

Fig. 4(f) - (i) show the mechanicalflexibility of the S3. After being bent to the bending radius of 13 mm, 11 mm, and 8.1 mm for 100 times, respectively, the resistance for all the samples were increased. However, the variation in the resistance values were lower than 4%; After being bent for 300 times at bending radius of 13 mm, the variations of electrical conductivity and Seebeck coef-ficient for all the composite films were less than 4%. Note that, as

the content of BTBAs increased, theﬂexibility of the composites

decreased, and S4 was broken after bent at the bending radius of Fig. 5. Schematic illustration of CB/BTBA/PLA thermoelectric composites with low (a) and high (b) content of BTBAs, and phonons and carriers transport across the interfaces (c).

Table 1

TE properties of CB/BTBA/PLA composites and those of composite materials previously reported.

Samplea _s_(S/cm) _{S (}_m_V/K) _{PF (}_m_Wm1_K2₎ _k_(Wm1_K1₎ _ZT _Ref. WC/PLA 42.2 12.3 0.64 0.28 ~6.7 104 _[₁₁_] BST NS/PEDOT:PSS 1295.21 ~16 32.26 [24] Bi0.5Sb1.5Te3/PLA 1.73 199 6.8 0.34 0.006 [32] Bi0.5Sb1.5Te3/MWCNTs/PLA 3.54 178.7 11.3 0.31 0.011 [32] Cu1.75Te/PVDF 2490 9.6 23 ~0.85 0.01 [33] PEDOT/SWCNT 318 37.2 44.1 0.475 0.028 [34]

CB/BTBA/PLA 13.1 125.5 20.7 0.28 0.024 This work

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11 mm, although it can be bent at the bending radius of 13 mm. This indicates that there is a limit to the BTBA content that can be

incorporated in the composites before the mechanicalﬂexibility is

reduced. This solution additive manufacturing can directly print the

solutions which contain inorganicﬁllers and polymer matrixes into

almost any designed intricate geometries of thermoelectric com-posites, and therefore this process is different to the common used sterolithography apparatus, fused deposition modeling, selective laser melting, etc., indicating that this process has a great potential

to be used for fabrication ofﬂexible polymer based thermoelectric

composites and devices. 4. Conclusions

Flexible ternary carbon black/Bi2Te3based alloy/polylactic acid

(CB/BTBA/PLA) composites were fabricated by additive

manufacturing. As the mass ratios of BTBAs increased from 38.5% to

71.4%, the power factor of the composites signiﬁcantly increased

from 2.1

m

W m1K2 to 19.2

m

W m1K2, and the thermal

con-ductivity increased from 0.15 W m1K1to 0.25 W m1K1, as a result, the ZT value of the composites increased from 0.004 to 0.023 at 300 K. The ZT value of the composites were slightly increased and then decreased as the temperature increased from 300 K to 360 K. A highest ZT value of 0.024 was achieved for the composite with 71.4% of BTBAs at 320 K. The additive manufacturing process has great potential to be used for fabrication

of_{ﬂexible polymer based thermoelectric composites and devices.}

Declaration of competing 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 has been supported by the Shanghai Innovation Ac-tion Plan Project (17090503600), the NaAc-tional Natural Science Foundation of China (11811530636, 61504081, 61611530550), and the Program for Professor of Special Appointment (Young Eastern Scholar Program) at Shanghai Institutions of Higher Learning (QD2015039). We also acknowledge support from the Swedish Research Council under project no. 2016-3365, the Swedish Energy Agency under project 46519-1, the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program, and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Link€oping University (Faculty Grant SFO-Mat-LiU No. 2009 00971).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2020.02.010.

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Yong Du currently holds position as professor in the School of Materials Science and Engineering at Shanghai Institute of Technology, China. Previously, he was an Alfred Deakin Postdoctoral Research Fellow at Institute for Frontier Ma-terials, Deakin University, Australia. He obtained his Ph.D. at Tongji University, China in 2013. He is interested in ﬂexible thermoelectric nanocomposites and devices. He has published more than 50 journal papers, two reviews and applied 10 patents.

Jiageng Chen received his B.S. degree in Materials Science and Engineering from Shanghai Institute of Technology, China in 2017. He is currently pursuing his Master’s degree in Materials Science and Engineering at Shanghai Institute of Technology, under the supervision of Prof. Yong Du. He is interested in polymer/inorganic thermoelectric materials.

Qiufeng Meng currently works in the School of Materials Science and Engineering at Shanghai Institute of Tech-nology, China. She obtained her Ph.D. at Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2019. She is interested in ﬂexible organic/inorganic nanocomposites

and their applications in energy generating and harvesting devices.

Jiayue Xu received his BS degree in Semiconductor Mate-rials from Jilin University in 1988, and then received his Master and PhD degree in Materials Science from Shanghai Institute of Ceramics, Chinese Academy of Sciences (SIC-CAS) in 1991 and 1999. He has worked in the Crystal Center of SICCAS for 20 years. Currently, Dr. Xu is a pro-fessor and Dean of the School of Materials Science and Engineering, Shanghai Institute of Technology. His research interests include semiconductor, scintillation and piezoelectric crystals and their applications. He has auth-ored and co-authauth-ored above 240 peer-reviewed journal ar-ticles, 5 books and held more than 50 patents.

Biplab Paul currently holds a position as assistant profes-sor in Thin Film Physics Division at Link€oping University, Sweden. Previously, he was Postdoctoral Researcher in Universitat Autonoma de Barcelona (UAB), Spain. He ob-tained his Ph.D. at Indian Institute of Technology Khar-agpur, India in 2011. He is an expert in thermoelectric materials and device design and fabrication. He has developed and implemented novel designs for advanced characterization of thermoelectric materials and published his research in more than 30 journal papers.

Per Eklund is associate professor and head of the Energy Materials Unit of the Thin Film Physics Division at €oping University, established his research group in Link-€oping in 2009, recruited from a postdoc at Århus, Denmark. His research interests include thin-ﬁlm synthe-sis and in the general area of thin-ﬁlm ceramics for energy applications within thermoelectrics, MAX phases, and fuel cells. He has received numerous prestigious grants and awards, including the ERC Starting Grant, Future Research Leaders from the Swedish Foundation for Strategic Research, and Wallenberg Academy Fellow. He has pub-lished ~160 papers including 4 reviews. He is also Editor of Vacuum (IF¼ 2.5).