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Optimization of ZnO sheets dimension in terms of ductility, micro-indentation, mechanical resistance, Amlouk-Boubaker optothermal expansivity and crystallites size
Materials Science and Engineering A 528 (2011) 1455–1457
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Optimization of ZnO sheets dimension in terms of ductility, micro-indentation, mechanical resistance, Amlouk-Boubaker optothermal expansivity and crystallites size K. Boubaker Unité de physique des dispositifs à semi-conducteurs, Faculté des sciences, de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia
a r t i c l e
i n f o
Article history: Received 4 October 2010 Received in revised form 14 October 2010 Accepted 15 October 2010
a b s t r a c t ZnO sheets with different thicknesses have been prepared using a simple spray pyrolysis technique. Parallel to classical characterization techniques like common XRD and AFM, a critical thickness has been proposed on the bases of micro-indentation related hardness, mechanical ductility, optothermal expansivity and crystallite sizes measurements. © 2010 Elsevier B.V. All rights reserved.
PACS: 62.20.Qp 64.70.ph 68.35.Ct 81.40.Ef Keywords: Semiconductors Spray technique Amlouk-Boubaker optothermal expansivity AB
Ductility Optimization Micro-indentation
1. Introduction Zinc oxide, as a direct wide band-gap semi-conductor, is getting increasing attention due to its various applications in several fields as optoelectronics, thermoelectric devices, transparent electrodes, selective surfaces, piezo-electric devices and gas sensor sensors. Zinc oxide main advantages are undoubtedly its high energy gap (Eg ≈ 3.437 eV at low temperatures) and large exciton binding energy (60 meV) [1]. This compound has been prepared by the mean of different methods [2–14] including chemical vapour deposition (CVD), magnetron sputtering, pulsed laser deposition (PLD) and sol–gel technique. Among these techniques, spray pyrolysis has proved to be a low cost and easy-to-implement method, particularly useful for industrial large scale applications. In this study work, ZnO thin sheets have been prepared by spray pyrolysis technique using zinc acetate. The thickness-dependent
E-mail address: [email protected] 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.049
performance of the different as-grown layers has been investigated in terms of morphological, opto-thermal and micro-hardness behaviours. Thickness effects analysis protocol was inspired by similar works carried out by Asserin-Lebert et al. [15] and applied in the same context, but to another material (6056-aluminum alloy). Additional investigations in terms of ductility and crystallites size allowed conjecturing the existence of an optimal thickness.
2. Samples preparation Zinc oxide sheets have been prepared by spray pyrolysis technique, which is detailed in recent publications [16–19]. ZnO sheets were fabricated at different thicknesses: d = 0.16, 0.4, 0.58, 0.64, 0.84 and 1.03 m on glass substrates by spraying aqueous solutions (mixture of water and propanol-2) containing zinc acetate (Zn(CH3 CO2 )2 ) in acid medium. Substrate temperature was fixed at the optimal value (Ts = 460 ◦ C). This optimality has been conjectured and recently proved by Amlouk et al. [20–22].
1456
K. Boubaker / Materials Science and Engineering A 528 (2011) 1455–1457
Fig. 2. Micro-hardness and mechanical ductility versus thickness d. Fig. 1. XRD patterns of the deposited ZnO layers.
3.3. Nano-scale crystallite size and optothermal expansivity analysis 3. Results and analysis 3.1. Precedent analysis Precedent studies presented XRD, mechanical and opto-thermal characterization of the as-grown sheets [21,22]. Obtained spectra have been analysed using Spectrum software in order to obtain the -dependent absorbance profiles as well as the band gap energies Eg . XRD patterns of the deposited ZnO layers are shown in Fig. 1. Diagram analysis (Fig. 1) shows that the thinner sheets (d ∈ {0.16, 0.4, 0.58, 0.64}) develop an preferred orientation of the crystallites with respect to the (0 0 2) reflection. Differently, the thicker ones (d ∈ {0.84, 1.03}), present, in addition to the (0 0 2) peak, two extra XRD peaks: (1 0 1) and (1 0 0). This structure results in low transparency along with an increasing level of surface roughness, which is a characteristic of thick sheets.
3.2. Microindentation resistance and ductility investigations
Parallel to the XRD analyses, the FWHM data provided by XRD apparatus were used to calculate and estimate the crystallite size D using Debye–Scherrer [25,26] formula: D=
K ˇ cos
(2)
where = 1.5418 A˚ for Cu radiation, K = 0.9, is the diffraction angle, ˇe2 − ˇ02 , where ˇe is measured from and ˇ is the FWHM with ˇ = the film and ˇ0 is a reference value [26]. The opto-thermal expansivity AB , which is a thermo-physical parameter defined in precedent studies [27,28], is assimilated to a 3D expansion velocity of the transmitted heat inside material. It is expressed in m3 s−1 , and calculated by: AB
=
D ˛ ˆ
(3)
where D is the thermal diffusivity and ˛ ˆ is the effective absorptivity [27,28].
The microindentation tests of the as-grown samples have been carried out by means of hardness quasi-static testing setup using a pyramidal indenter, under loads varying in the range of [200, 400] mN. Indentation related microhardness of an elaborated sheet is defined as its resistance to plastic deformation. It is quantified as maximum indentation load-to-actual projected contact area ratio:
Hv = k
P d2
where P is the maximum load, d is mean imprint projected dimension and k is indenter shape constant (k = 1.8544). An average value of eight indentations for each sample was taken. Ductility, which traduces material ability to support plastic deformation just before failure [23,24], has been measured by the elongation to fracture ratio (A %). Microhardness and ductility results are gathered in Fig. 2.
Fig. 3. Crystallite size D and optothermal expansivity
AB
versus thickness d.
K. Boubaker / Materials Science and Engineering A 528 (2011) 1455–1457
The values of the calculated opto-thermal expansivity AB , along with crystallite size D, for the different sheets, are gathered in Fig. 3. The recently conjectured [29] critical thickness (dcr. ≈ 0.656 m) ensured optimality in terms of optothermal-hardness conjoint performance. It was interesting to observe here that this thickness lies inside the critical zone for both mechanical and optothermal properties (Figs. 2 and 3). 4. Conclusion We have discussed heterogeneous behaviours of ZnO sheets deposited at different thicknesses. The already emitted conjecture of the critical thickness has been discussed in terms of mechanical resistance and photo thermal performance along with crystallite size. Measurements and analysis were qualitatively and quantitatively favourable to the conjectured criticality hypothesis. Establishment of the physical validity of this conjecture needs thorough crystallographic, atomic-scale and nano-mechanical investigations [15,30–32]. References [1] D.Q. Yu, L.Z. Hu, J. Li, H. Hu, H.Q. Zhang, Z.W. Zhao, Mater. Lett. 62 (2008) 4063. [2] H. Guosheng, M. Yalin, W. Biaobing, Mater. Sci. Eng. A 504 (2009) 8. [3] J. Shi, Y. Wang, L. Liu, H. Bai, J. Wu, C. Jiang, Z. Zhou, Mater. Sci. Eng. A 512 (2009) 109. [4] K.K. Phani, D. Sanyal, Mater. Sci. Eng. A 490 (2008) 305. [5] F. Karimzadeh, M.H. Enayati, M. Tavoosi, Mater. Sci. Eng. A 486 (2008) 45. [6] Y. Wang, J. Shi, L. Han, F. Xiang, Mater. Sci. Eng. A 501 (2008) 220.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
1457
H.J. Liu, Y.C. Zhu, Mater. Lett. 62 (2008) 255. Y.Y. Xi, T. Cheung, H.L.Ng. Dickon, Mater. Lett. 62 (2008) 1282. Wang, R. Liu, A. Pan, S. Xie, B. Zou, Mater. Lett. 61 (2007) 4459–4462. J. Montero, J. Herrero, C. Guillén, Sol. Energy Mater. Sol. Cells 94 (2010) 612–616. A. Alkaya, R. Kaplan, H. Canbolat, S.S. Hegedus, Renew. Energy 34 (2009) 1595–1599. Y. Khoptiar, I. Gotman, Mater. Lett. 57 (2002) 72. G.X. Liu, F.K. Shan, W.J. Lee, B.C. Shin, H.S. Kim, J.H. Kim, Ceram. Int. 34 (2008) 1011–1015. J. Bandara, C.M. Divarathne, S.D. Nanayakkara, Sol. Energy Mater. Sol. Cells 81 (2004) 429–437. A. Asserin-Lebert, J. Besson, A.F. Gourguesn, Mater. Sci. Eng. A 395 (2005) 186. A. Amlouk, K. Boubaker, M. Amlouk, J. Alloys Compd. 490 (2010) 602. A. Amlouk, K. Boubaker, M. Amlouk, M. Bouhafs, J. Alloys Compd. 485 (2009) 887. S. Dabbous, T. Ben Nasrallah, J. Ouerfelli, K. Boubaker, M. Amlouk, S. Belgacem, J. Alloys Compd. 487 (2009) 286–292. B. Ouni, J. Ouerfelli, A. Amlouk, K. Boubaker, M. Amlouk, J. Non-Cryst. Solids 356 (2010) 1294. M. Amlouk, S. Belgacem, N. Kamoun, H.EL. Houichet, R. Bennaceur, Ann. Chim. Fr. 19 (1994) 469. M. Amlouk, F. Touhari, S. Belgacem, N. Kamoun, D. Barjon, R. Bennaceur, Phys. Status Solidi (a) 163 (1999) 73. S. Belgacem, R. Bennaceur, Rev. Phys. Appl. 25 (1990) 1245. H.Y. Yue, W.D. Fei, L.D. Wang, Mater. Sci. Eng. A 472 (2008) 231. M. Joshy Jose, Abdul Khadar, Mater. Sci. Eng. A 304 (2008) 810. B.D. Cullity, Elements of X-ray Diffraction, A.W. Pub. Comp. Inc., 1978. E.F. Kaeble, Handbook of X-rays, McGraw-Hill, New York, 1967. S. Lazzez, K.B. Ben Mahmoud, S. Abroug, F. Saadallah, M. Amlouk, Curr. Appl. Phys. 9 (2009) 1129. K.B. Ben Mahmoud, M. Amlouk, Mater. Lett. 63 (2009) 991. A. Amlouk, K. Boubaker, M. Bouhafs, M. Amlouk, J. Alloys Compd. (2010), submitted under reference:JALCOM-S-10-07401. G. Hu, Y. Ma, B. Wang, Mater. Sci. Eng. A 504 (2009) 8. X. Hou, F. Zhou, B. Yu, W. Liu, Mater. Sci. Eng. A 452 (2008) 732. S. Illy-Cherrey, O. Tillement, J.M. Dubois, F. Massicot, Y. Fort, J. Ghanbaja, S. Bégin-Colin, Mater. Sci. Eng. A 338 (2008) 70.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Optimization of ZnO sheets dimension in terms of ductility, micro-indentation, mechanical resistance, Amlouk-Boubaker optothermal expansivity and crystallites size K. Boubaker Unité de physique des dispositifs à semi-conducteurs, Faculté des sciences, de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia
a r t i c l e
i n f o
Article history: Received 4 October 2010 Received in revised form 14 October 2010 Accepted 15 October 2010
a b s t r a c t ZnO sheets with different thicknesses have been prepared using a simple spray pyrolysis technique. Parallel to classical characterization techniques like common XRD and AFM, a critical thickness has been proposed on the bases of micro-indentation related hardness, mechanical ductility, optothermal expansivity and crystallite sizes measurements. © 2010 Elsevier B.V. All rights reserved.
PACS: 62.20.Qp 64.70.ph 68.35.Ct 81.40.Ef Keywords: Semiconductors Spray technique Amlouk-Boubaker optothermal expansivity AB
Ductility Optimization Micro-indentation
1. Introduction Zinc oxide, as a direct wide band-gap semi-conductor, is getting increasing attention due to its various applications in several fields as optoelectronics, thermoelectric devices, transparent electrodes, selective surfaces, piezo-electric devices and gas sensor sensors. Zinc oxide main advantages are undoubtedly its high energy gap (Eg ≈ 3.437 eV at low temperatures) and large exciton binding energy (60 meV) [1]. This compound has been prepared by the mean of different methods [2–14] including chemical vapour deposition (CVD), magnetron sputtering, pulsed laser deposition (PLD) and sol–gel technique. Among these techniques, spray pyrolysis has proved to be a low cost and easy-to-implement method, particularly useful for industrial large scale applications. In this study work, ZnO thin sheets have been prepared by spray pyrolysis technique using zinc acetate. The thickness-dependent
E-mail address: [email protected] 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.049
performance of the different as-grown layers has been investigated in terms of morphological, opto-thermal and micro-hardness behaviours. Thickness effects analysis protocol was inspired by similar works carried out by Asserin-Lebert et al. [15] and applied in the same context, but to another material (6056-aluminum alloy). Additional investigations in terms of ductility and crystallites size allowed conjecturing the existence of an optimal thickness.
2. Samples preparation Zinc oxide sheets have been prepared by spray pyrolysis technique, which is detailed in recent publications [16–19]. ZnO sheets were fabricated at different thicknesses: d = 0.16, 0.4, 0.58, 0.64, 0.84 and 1.03 m on glass substrates by spraying aqueous solutions (mixture of water and propanol-2) containing zinc acetate (Zn(CH3 CO2 )2 ) in acid medium. Substrate temperature was fixed at the optimal value (Ts = 460 ◦ C). This optimality has been conjectured and recently proved by Amlouk et al. [20–22].
1456
K. Boubaker / Materials Science and Engineering A 528 (2011) 1455–1457
Fig. 2. Micro-hardness and mechanical ductility versus thickness d. Fig. 1. XRD patterns of the deposited ZnO layers.
3.3. Nano-scale crystallite size and optothermal expansivity analysis 3. Results and analysis 3.1. Precedent analysis Precedent studies presented XRD, mechanical and opto-thermal characterization of the as-grown sheets [21,22]. Obtained spectra have been analysed using Spectrum software in order to obtain the -dependent absorbance profiles as well as the band gap energies Eg . XRD patterns of the deposited ZnO layers are shown in Fig. 1. Diagram analysis (Fig. 1) shows that the thinner sheets (d ∈ {0.16, 0.4, 0.58, 0.64}) develop an preferred orientation of the crystallites with respect to the (0 0 2) reflection. Differently, the thicker ones (d ∈ {0.84, 1.03}), present, in addition to the (0 0 2) peak, two extra XRD peaks: (1 0 1) and (1 0 0). This structure results in low transparency along with an increasing level of surface roughness, which is a characteristic of thick sheets.
3.2. Microindentation resistance and ductility investigations
Parallel to the XRD analyses, the FWHM data provided by XRD apparatus were used to calculate and estimate the crystallite size D using Debye–Scherrer [25,26] formula: D=
K ˇ cos
(2)
where = 1.5418 A˚ for Cu radiation, K = 0.9, is the diffraction angle, ˇe2 − ˇ02 , where ˇe is measured from and ˇ is the FWHM with ˇ = the film and ˇ0 is a reference value [26]. The opto-thermal expansivity AB , which is a thermo-physical parameter defined in precedent studies [27,28], is assimilated to a 3D expansion velocity of the transmitted heat inside material. It is expressed in m3 s−1 , and calculated by: AB
=
D ˛ ˆ
(3)
where D is the thermal diffusivity and ˛ ˆ is the effective absorptivity [27,28].
The microindentation tests of the as-grown samples have been carried out by means of hardness quasi-static testing setup using a pyramidal indenter, under loads varying in the range of [200, 400] mN. Indentation related microhardness of an elaborated sheet is defined as its resistance to plastic deformation. It is quantified as maximum indentation load-to-actual projected contact area ratio:
Hv = k
P d2
where P is the maximum load, d is mean imprint projected dimension and k is indenter shape constant (k = 1.8544). An average value of eight indentations for each sample was taken. Ductility, which traduces material ability to support plastic deformation just before failure [23,24], has been measured by the elongation to fracture ratio (A %). Microhardness and ductility results are gathered in Fig. 2.
Fig. 3. Crystallite size D and optothermal expansivity
AB
versus thickness d.
K. Boubaker / Materials Science and Engineering A 528 (2011) 1455–1457
The values of the calculated opto-thermal expansivity AB , along with crystallite size D, for the different sheets, are gathered in Fig. 3. The recently conjectured [29] critical thickness (dcr. ≈ 0.656 m) ensured optimality in terms of optothermal-hardness conjoint performance. It was interesting to observe here that this thickness lies inside the critical zone for both mechanical and optothermal properties (Figs. 2 and 3). 4. Conclusion We have discussed heterogeneous behaviours of ZnO sheets deposited at different thicknesses. The already emitted conjecture of the critical thickness has been discussed in terms of mechanical resistance and photo thermal performance along with crystallite size. Measurements and analysis were qualitatively and quantitatively favourable to the conjectured criticality hypothesis. Establishment of the physical validity of this conjecture needs thorough crystallographic, atomic-scale and nano-mechanical investigations [15,30–32]. References [1] D.Q. Yu, L.Z. Hu, J. Li, H. Hu, H.Q. Zhang, Z.W. Zhao, Mater. Lett. 62 (2008) 4063. [2] H. Guosheng, M. Yalin, W. Biaobing, Mater. Sci. Eng. A 504 (2009) 8. [3] J. Shi, Y. Wang, L. Liu, H. Bai, J. Wu, C. Jiang, Z. Zhou, Mater. Sci. Eng. A 512 (2009) 109. [4] K.K. Phani, D. Sanyal, Mater. Sci. Eng. A 490 (2008) 305. [5] F. Karimzadeh, M.H. Enayati, M. Tavoosi, Mater. Sci. Eng. A 486 (2008) 45. [6] Y. Wang, J. Shi, L. Han, F. Xiang, Mater. Sci. Eng. A 501 (2008) 220.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
1457
H.J. Liu, Y.C. Zhu, Mater. Lett. 62 (2008) 255. Y.Y. Xi, T. Cheung, H.L.Ng. Dickon, Mater. Lett. 62 (2008) 1282. Wang, R. Liu, A. Pan, S. Xie, B. Zou, Mater. Lett. 61 (2007) 4459–4462. J. Montero, J. Herrero, C. Guillén, Sol. Energy Mater. Sol. Cells 94 (2010) 612–616. A. Alkaya, R. Kaplan, H. Canbolat, S.S. Hegedus, Renew. Energy 34 (2009) 1595–1599. Y. Khoptiar, I. Gotman, Mater. Lett. 57 (2002) 72. G.X. Liu, F.K. Shan, W.J. Lee, B.C. Shin, H.S. Kim, J.H. Kim, Ceram. Int. 34 (2008) 1011–1015. J. Bandara, C.M. Divarathne, S.D. Nanayakkara, Sol. Energy Mater. Sol. Cells 81 (2004) 429–437. A. Asserin-Lebert, J. Besson, A.F. Gourguesn, Mater. Sci. Eng. A 395 (2005) 186. A. Amlouk, K. Boubaker, M. Amlouk, J. Alloys Compd. 490 (2010) 602. A. Amlouk, K. Boubaker, M. Amlouk, M. Bouhafs, J. Alloys Compd. 485 (2009) 887. S. Dabbous, T. Ben Nasrallah, J. Ouerfelli, K. Boubaker, M. Amlouk, S. Belgacem, J. Alloys Compd. 487 (2009) 286–292. B. Ouni, J. Ouerfelli, A. Amlouk, K. Boubaker, M. Amlouk, J. Non-Cryst. Solids 356 (2010) 1294. M. Amlouk, S. Belgacem, N. Kamoun, H.EL. Houichet, R. Bennaceur, Ann. Chim. Fr. 19 (1994) 469. M. Amlouk, F. Touhari, S. Belgacem, N. Kamoun, D. Barjon, R. Bennaceur, Phys. Status Solidi (a) 163 (1999) 73. S. Belgacem, R. Bennaceur, Rev. Phys. Appl. 25 (1990) 1245. H.Y. Yue, W.D. Fei, L.D. Wang, Mater. Sci. Eng. A 472 (2008) 231. M. Joshy Jose, Abdul Khadar, Mater. Sci. Eng. A 304 (2008) 810. B.D. Cullity, Elements of X-ray Diffraction, A.W. Pub. Comp. Inc., 1978. E.F. Kaeble, Handbook of X-rays, McGraw-Hill, New York, 1967. S. Lazzez, K.B. Ben Mahmoud, S. Abroug, F. Saadallah, M. Amlouk, Curr. Appl. Phys. 9 (2009) 1129. K.B. Ben Mahmoud, M. Amlouk, Mater. Lett. 63 (2009) 991. A. Amlouk, K. Boubaker, M. Bouhafs, M. Amlouk, J. Alloys Compd. (2010), submitted under reference:JALCOM-S-10-07401. G. Hu, Y. Ma, B. Wang, Mater. Sci. Eng. A 504 (2009) 8. X. Hou, F. Zhou, B. Yu, W. Liu, Mater. Sci. Eng. A 452 (2008) 732. S. Illy-Cherrey, O. Tillement, J.M. Dubois, F. Massicot, Y. Fort, J. Ghanbaja, S. Bégin-Colin, Mater. Sci. Eng. A 338 (2008) 70.