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ScienceDirectMaterials Today: Proceedings 00 (2018) 0000–0000 www.materialstoday.com/proceedings
AFM2 2017
Effect of synthesis parameters on the morphology of nanocrystal thermoelectric M(= Mg, Mn, Cu) doped Ca3Co4O9 fibers by
electrospinning
Yun Oua,b,c,*, Daifeng Zoud, Fang Wanga, Juanjuan Chenga
aHunan Provincial Key Laboratory of Health Maintenance for Mechanical Equipment, Hunan University of Science and Technology, Xiangtan City, Hunan Province, China 411201b Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, Guangdong, China cEngineering Research Center of Nano-Geo Materials of Ministry of Education,China University of Geosciences, Wuhan 430074,China dSchool of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan, China 411201
Abstract
The thermoelectric M (=Mg, Mn, Cu) doped Ca3Co4O9 (CCO) fibers were synthesized by electrospinning based on sol-gel method. The effect of various substrates, electrospinning time, and different doping on the morphology of M doped CCO fibers were studied systematically. The results showed that the morphology of calcined fibers with substrate was kept better and the grain size was smaller than that of without substrate, simultaneously. It is observed that the shorter electrospinning time, the better preserved nanofiber configuration, and hollow structure was appeared in the manganese doped CCO fibers.
© 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.
Keywords: sol-gel, electrospinning, thermoelectric, layered oxide;
** Corresponding author. Tel.: 0731-58290265.E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.
2 Author name / Materials Today: Proceedings 00 (2018) 0000–0000
Introduction
Thermoelectric (TE) material is a kind of functional material which can convert exhaust heat energy into
electrical energy directly,[1-4] and the figure of merit can evaluate the properties of the TE materials. A
promising TE material requires a large S and σ , and a low κ simultaneously, which includes the contribution
from phonon thermal conductivity and electronic thermal conductivity. It has been shown that the phonon thermal
conductivity can be reduced significantly in nanocomposites due to the increased phonon interface and boundary
scattering.[5] It was reported that the ZT values reach 2.4 in layered nanoscale structures at 300 K, [6] and 3.2 for a
bulk semiconductors with nanoscale inclusions at about 600 K.[7] Since a large Seebeck coefficient was found in
layered oxide NaCo2O4 exhibiting a promising TE performance, p-type-layered cobalt oxides have attracted more
and more attention owing to having similar structure with NaCo2O4, high thermal stability, low cost and low
toxicity. CCO is a distorted CaO-CoO-CaO rock-salt-type layer and a CdI2-type CoO2 layer stacked along the c-axis
direction.[8-10] However, at present the ZT value of CCO is too low to be used practically. In order to improve the
thermoelectric performance of CCO, nanocrystal CCO ceramics were manufactured by different techniques.[11-14]
In recent years, one-dimensional nanostructured materials have attracted much attention owing to their
significant potential in nanodevices.[9,10] Among the techniques fabrication one-dimensional nanofiber,
electrospinning is one effective approach with high efficiency and convenient operation. Encouraged by these
developments and our recent works on spark plasma sintering nanocrystalline nanofibers, [11,15] The effect of various
substrates, electrospinning time, different doping on the morphology of nanocrystalline thermoelectric oxide M
doped CCO nanofibers were investigated.
Experimental
The M doped CCO sol-gel precursor was prepared by dissolving C4H6O4Ca·H2O, Co(CH3COO)2·4H2O,
Cu(NO3)2·3H2O, MgCl2·6H2O, and C4H6MnO4·4H2O with stoichiometric proportions into a hybrid solution of
methanol (CH3OH) and propionic acid (CH3CH2COOH) with a volume ratio of 3/7. Poly (vinyl pyrrolidone) (PVP,
Mw= 1300000 g·mol-1) was then added to the sol-gel precursor with 0.03 g·mL-1 concentration, and stirred
continuously to form a 0.2 mol·L-1 homogeneous M doped CCO electrospinning precursor. The solution was loaded
into a plastic syringe equipped with a stainless steel needle connected to a high-voltage supply (Spellman
SL40P300), and then electrospun with the feed rate of the solution controlled at 0.015 mL·min-1 and the electric
field set around 1.4 kV·cm-1. The as-spun nanofibers were collected by a glass flake with aluminum foil or
Pt/Ti/SiO2/Si (PT) substrate, dried at 120 °C for 4 h, and then calcinated at 750 °C for 2 h. For comparison, CCO
ceramic powders were also synthesized using the conventional sol-gel process, by drying the CCO precursor
solution without PVP in an oven at 120 °C for 2 days to obtain xerogel, and then sintered the xerogel at 800 °C for 2
h. M doped CCO powders with different doping have been synthesized by sol-gel method.
(d)
(e) (f)
(c)
(b)
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X-ray diffraction (XRD) (Rigaku D/max-rA) with CuKa radiation was used to identify their crystalline
structures, and scanning electron microscopy (SEM) (FEI Sirion) was used to examine the morphology of M doped
CCO nanofibers and powders with different substrates and collecting time.
Results and discussion
The morphology of CCO fibers retained on the two different substrates and lifted off the substrate is shown in
Fig. 1. It is observed in Fig. 1(a) that the diameter of the green fibers on the PT substrate is in the range of 400–2000
nm with straight and smooth surface, each nanofiber is rather uniform across its length, and almost fibers are column
shape. The morphology of green fibers on aluminum foil substrate is shown in Fig. 1(b), while some are belts and
majority are nanofibers, the width of the belts is between 2 and 5 μm with nanosize thickness, the diameter of
nanofibers is clearly less than that of nanobelts and their size distribution is more inhomogenous than that of PT
substrate due to the inhomogeneous electric field distribution of aluminum foil. However, when the fibers were
lifted off the substrate and then dried at 120 °C for 4 h, the continuous fibers change to short stick, and their length
is in the range of 1-20 μm with nanometer diameter as seen in Fig. 1(c), which indicates that substrate can provide
binding force to keep fiber with stable morphology, and stress is large in the evaporation of the solvent, the huge
tensile stress shuts off the long nanofibers.
Fig. 1. SEM images of CCO green fibers collected on different substrate and ceramic fibers with different collecting time(a) PT (b) Al foil (c) lift off PT (d) 1 min (e) 30 min. (f) 2 h.
The SEM images of calcined CCO ceramic fiber collected on PT substrate with different collecting time are
shown in Figs. 1(d)-(f). It is shown that after being calcined the diameter of nanofibers on PT substrate is reduced to around 100–300 nm, there are partial continuous fibers when collecting only 1
(g) (f)
(c)(a) (b)
(e)
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min as shown in Fig. 1(d), fractional nanofibers are broken and the grain size is typically nanocrystal. When
collecting time extend to 30 min, the CCO ceramic preserved fiber structure, but the diameter of fibers increased to
about 200–400 nm as some fibers are bonding together, which enhanced the average grain size of ceramic as shown in Fig. 1 (e). However, in Fig. 1 (f) few fibers structure was kept in 2 h collecting time
sample, because the mutual binding of nanofibers increased with increasing collecting time, which results the grown
of grain size and the disappear of fiber morphology.
Fig. 2 compared the crystal size of the fibers on or without the PT substrate with sol-gel powders, and the
particle size distribution was analyzed by image J. It is confirmed that the grain size derived by fibers is much
smaller than that of sol-gel, and the crystal size of fibers on PT is the smallest among the three samples. The average
particle size of the CCO powders by sol-gel is the largest, which is in the range of 200-1100 nm with wider
distribution. The average particle size of the powders grinded by calcined fibers without PT substrate is around 80
nm with normal distribution. The average size of the particle with PT substrate is reduced to approximately 60 nm
with centralized distribution. Fig.2 also reveals the lamellar morphology of CCO powders expected from the layered
lattice structure.
Fig. 2. SEM images of M doped CCO sol-gel powders and fibers(a) sol-gel (b) fibers without PT (c) fibers on PT
(e) (f) (g) corresponding crystal size distribution of (a) (b) (c).
Fig. 3 gives the SEM images of the fibers doped with different elements and corresponding XRD results. It is
obvious that the morphology of fibers various with different doping. The morphology of Ca2.8Mg0.2Co4O9 fiber (Fig.
3(a)) is similar with that of CCO fiber as the Mg ions substituting the Ca site not Co site.[16] The Ca3Co3.8Mn0.2O fiber
(Fig. 3(b)) presents a obvious hollow structure, that is because the Mn substitutes for Co in CoO2 layers and the
melting point of MnO2 is much lower than that of CoO2, phase separation promotes the formation of hollow
Author name / Materials Today: Proceedings 00 (2018) 0000–0000 5
structure. The different morphology between Fig. 3(b) and Fig. 3(c) is being Mn ions mainly occupy Co sites in
CoO2 layers whereas Cu ions substitutes for Co sites in Ca2CoO3 layers.[17] It is indicated that different doping
elements results various morphology owing to different doping sites, which results different structure.
The crystalline structure of M doped CCO ceramic fibers with different doping were examined by XRD, as
shown in Fig. 3(d). All the diffraction peaks of different samples are identical to the standard JCPDS card 21-0139
of CCO. Excellent crystallinity was observed in the four kinds nanofibers calcinated at 750 °C, and no visible
impurity phase was detected because of the low doping ratio.
Fig. 3. SEM images and XRD of the fibers with different doping (a) Ca2.8Mg0.2Co4O9
(b) Ca3Co3.8Mn0.2O9 (c) Ca3Co3.8Cu0.2O9 (d) XRD of the M doped CCO
Conclusions
In conclusion, nanocrystalline M doped CCO fibers have been successfully fabricated by electrospinning, and
the effect of major process parameters on the morphology of the fibers were investigated systematically. It is found
that the particle sizes with limited collecting time and PT substrate are much smaller than that of without substrate
and longer collecting time, and the particle size of fibers derived powders is also much smaller than the powders
prepared by sol-gel method. The average particle size of the fibers derived powders with the substrate is about 80
nm. Our experiments further demonstrate that the particle size synthesized by electrospinning is much smaller than
the particles by sol-gel method.
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Acknowledgments
We acknowledge support from Natural Science Foundation of China (Approval No. 11702092 and 11762016)
and Opening Project of Engineering Research Center of Nano-Geo Materials of Ministry of Education of China
University of Geosciences (NGM2018KF005).
References
[1] Arunachalam VS, Fleischer EL. MRS Bulletin 2008; 33: 264-288.
[2] Yang JH, Caillat T. MRS Bulletin 2006; 31: 224-229.
[3] DiSalvo FJ. Science 1999; 285: 703-706.
[4] Tritt TM, Subramanian MA. MRS Bulletin 2006; 31: 188-198.
[5] Chen G, Shakouri A. Journal of heat transfer 2002; 124: 242-252.
[6] Venkatasubramanian R, Siivola E, Colpitts T, O'Quinn B. Nature 2001; 413: 597-602.
[7] Harman TC, Taylor PJ, Walsh MP, LaForge BE. Science 2002; 297: 2229-2232.
[8] Masset AC, Michel C, Maignan A, Hervieu M, Toulemonde O, Studer F, et al. Physical Review B 2000; 62: 166-175.
[9] Hu JT, Ouyang M, Yang PD, Lieber CM. Nature 1999; 399: 48-51.
[10] Heath JR, Kuekes PJ, Snider GS, Williams RS. Science 1998; 280: 1716-1721.
[11] Ma FY, Ou Y, Yang Y, Liu YM, Xie SH, Li JF, et al. The Journal of Physical Chemistry C 2010; 114: 22038-22043.
[12] Qi XL, Fan YY, Zhu DS, Zeng LK. Rare metals 2011; 30: 111-115.
[13] Thomas EL, Song XY, Yan YG, Martin J, Wong-Ng W, Ratcliff M, et al. MRS proceedings 2010; 1267.
[14] Kang MG, Cho KH, Oh SM, Kim JS, Kang CY, Nahm S, et al. Applied Physics Letters 2011; 98: 142102.
[15] Yin TF, Liu DW, Ou Y, Ma FY, Xie SH, Li JF, et al. The Journal of Physical Chemistry C 2010; 114: 10061-10065.
[16] Sun DY, Sung WP, Chen R. Applied Mechanics and Materials 2011; 71-78: 959-962.
[17] Wang Y, Sui Y, Ren P, Wang L, Wang XJ, Su WH, et al. Chemistry of Materials 2010; 22: 1155-1163.
Highlights
1. The M doped thermoelectric Ca3Co4O9 (CCO) fibers with nanocrystal size were synthesized by
electrospinning, hollow structure were formed in Mn doped fiber.
2. The substrate can provide binding force to keep the small crystal in electrospinning, results the formation of
nanocrystal fibers.