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Reduction of microcracking in YMnO3 ceramics by Ti substitution
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Scripta Materialia 67 (2012) 427–430 www.elsevier.com/locate/scriptamat
Reduction of microcracking in YMnO3 ceramics by Ti substitution M. Tomczyk,a A.M.O.R. Senos,a I.M. Reaneyb and P.M. Vilarinhoa,⇑ a
Department of Materials and Ceramic Engineering, Centre for Research in Ceramics and Composite Materials, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK Received 26 February 2012; accepted 25 April 2012 Available online 18 May 2012
Microcracking in YMnO3 ceramics originates from a combination of 3.7% volume change and strong anisotropy in the thermal expansion coefficient of the hexagonal structure. In this paper, the effect of Ti doping on the microstructure, structure and dielectric response of YMnO3 ceramics is studied. Lattice anisotropy of the hexagonal cell markedly decreased with increasing Ti, reducing the residual stresses created at the high-temperature phase transition and commensurately decreasing the crack density (3.2 vol.% YMnO3 to 0.9 vol.% YMn0.9Ti0.1O3). These results represent the first systematic work to reduce microcracking in YMnO3-based ceramics. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: YMnO3; Ti doping; Microstructure; Microcracking
Materials possessing two or more of the ferroic properties of ferroelasticity, ferroelectricity and (anti)ferromagnetism are known as multiferroics [1]. Because of their potential technological applications in modern electronic devices such as memory elements, sensors and spintronics, they have become the subject of intense research [2]. Among the known multiferroic materials, extensive studies have been performed on hexagonal manganites, especially yttrium manganese oxide, which exhibits high-temperature ferroelectric transition (TC 900 K) along with a low-temperature antiferromagnetic transition (TN 70 K) [3]. YMnO3 exhibits a noncentrosymmetric space group P63cm at room temperature, which transforms to the centrosymmetric equivalent, P63/mmc, at 1270 K [4]. The Mn3+ ions form MnO5 bipyramids with three planar oxygen (O3 and O4) and two apical oxygen (O1 and O2) atoms. The MnO5 bipyramids are corner linked to form a triangular lattice layer in the ab plane. Along the c direction, the crystal structure is composed of alternate Y3+ ions and corner-shared MnO5 layers [5]. Single crystal data have shown that the permittivity, er, is 20 at room temperature and increases slowly with temperature. Clear ferroelectric hysteresis is observed along the c-axis with spontaneous polarization PS 5
⇑ Corresponding
author. Tel.: +351 234 370354; fax: +351 234 425300; e-mail: [email protected] (P.M. Vilarinho).
lC cm2 [6,7]. However, these properties have not yet been realized in bulk polycrystalline samples. Microcracking and porosity observed in YMnO3 ceramics [8] are considered to strongly affect the ferroelectric response. Recently, it was shown that microcracking originates from a combination of a 3.7% volume change and strong anisotropy in the thermal expansion coefficient (a) of the hexagonal structure (|aa ac| 4.57 105 °C1) between 600 and 1000 °C [9]. It is therefore a matter of practical interest to prevent or reduce anisotropy/microcracking in YMnO3 ceramics. Commonly, the fabrication of dense and crack free ceramics is achieved by reducing the stresses that are generated during processing using doping and/or decreasing the grain size below a critical value [10]. For instance, the fabrication of crack-free PbTiO3 ceramics is achieved by modifications with CaO or Sm2O3, which inhibit microcracking by reducing lattice anisotropy [11]. Aikawa et al. [12] studied the effect of the Ti substitution on the electric and magnetic properties in YMnO3 and observed that magnetocapacitance is enhanced around x = 0.175. Moreover, for the Mn site substitution, electron diffraction and dark-field imaging revealed that doping of Ti4+ in YMnO3 resulted in the formation of a coherent rhombohedral paraelectric phase within the P63cm matrix at around x = 0.20, with the rhombohedral phase dominant above x = 0.30. As the volume fraction of the rhombohedral phase increased to x > 0.20, the ferroelectric and magnetocapacitive response was suppressed
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.04.042
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M. Tomczyk et al. / Scripta Materialia 67 (2012) 427–430
[12]. This paper aims to illustrate that Ti substitution also reduces anisotropy and microcracking in YMnO3 ceramics, thereby facilitating the establishment of accurate bulk microstructure/properties relations. YMn1xTixO3 (0 6 x 6 0.25) powders were prepared by solid-state reaction at ambient pressure [9]. Pure oxides Y2O3 (ABCR, 99.99%), Mn2O3 (ABCR 99%) and TiO2 (MERCK, 99%) were weighted according to the stoichiometry of YMn1xTixO3. The starting materials were milled for 5 h in a planetary mill using Teflon pots with zirconia balls and ethanol. The milled powders were calcined at 1100 °C for 10 h and then milled again for 5 h. The calcined and milled powders were uniaxially pressed into 10 mm diameter discs at 80 MPa and isostatically pressed at 200 MPa. Sintering was conducted at 1400 °C for 5 and 10 h, depending on the composition. The structure, microstructure and phase assemblage of the sintered ceramics were analyzed by X-ray diffraction (XRD; Rigaku D/Max-B, Cu Ka), scanning electron microscopy (SEM; Hitachi S-4100) and transmission electron microscopy (TEM; Hitachi H9000). A CELREF program (Laugier & Bochu) was used to refine the lattice parameters. Samples for TEM were prepared by grinding and polishing to 50 lm, followed by ion milling, using a Gatan PIPS 691 ion beam miller operating at 5 kV and a combined gun current, at an incidence angle 10°. The thermal expansion behavior of the sintered bodies was analyzed by dilatometric measurements at a constant heating rate of 5 °C min1 up to 1400 °C and then cooling down to room temperature, using a computer-assisted dilatometer (Linseis, model 4L702000). For the dielectric measurements, sintered samples were polished to a thickness of 1 mm and silver electrodes were painted on both sides of the ceramic disks. The complex impedance was measured at different temperatures from 25 to 700 °C with a heating rate of 4 °C min1 in the frequency range from 100 Hz to 1 MHz, using a Precision LCR Meter HP 4284A.
Figure 1 depicts the SEM images of YMn1xTixO3 ceramics as a function of Ti concentration. The microstructure of undoped YMnO3 ceramics is dominated by a large volume fraction of inter- and intragranular pores and cracks [8,9]. The latter is promoted by the large volume change (3.7%) at the high-temperature TC, which gives rise to 200 MPa residual stress, and also by the highly anisotropic thermal expansion of the hexagonal unit cell parameters [9]. However, as the Ti concentration increases, the volume fraction of pores and microcracks decreases markedly. To clarify the role of Ti doping on structural evolution, XRD patterns were obtained from YMn1xTixO3 ceramics for 0 6 x 6 0.25 (Fig. 2a). For x < 0.175, the structure remained single phase hexagonal (P63cm). However, for x > 0.175, peaks not associated with the hexagonal phase could be detected at 2h of approx. 28, 24, 47 and 64° that matched a rhombohedral (R 3c) structure [12]. Although the peaks may be indexed according to a rhombohedral structure, their intensities are weak and indicate the coexistence of two distinct phases (hexagonal and rhombohedral) over the range of x investigated, as described in Ref. [12]. The rhombohedral phase is reported to arise due to the formation of a superlattice with a 3 d001 periodicity along the c-axis [13]. The dependence of the cell parameters as a function of x for YMn1xTixO3 (0 6 x 6 0.25) is plotted in Figure 2b. For compositions with x < 0.175 and x P 0.175, cell parameters were refined using P63cm symmetry and R3c symmetry, respectively. To directly compare the changes in cell parameters in the two phases, c/3 of the R3c cell was plotted along with the c-axis of the P63cm phase. As the Ti concentration increased, the a-axis increased and the c-axis decreased. No anomaly was observed at the P63cm/R3c boundary, consistent with a gradual increase in volume fraction of a coherent rhombohedral phase in a hexagonal matrix. The structural changes as a function of x were further studied by TEM. Figure 3a displays the [1 1 0]h zone axis electron diffraction patterns of YMn1xTixO3 at room
Figure 1. SEM micrographs of YMn1xTixO3, where (a) x = 0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.15, (e) x = 0.175, (f) x = 0.2 and (g) x = 0.25. As the Ti concentration increases, the concentration of microcracks and intragranular porosity decrease.
M. Tomczyk et al. / Scripta Materialia 67 (2012) 427–430
Figure 2. X-ray powder diffraction patterns of YMn1xTixO3 ceramics at room temperature. d indicates the new peaks associated with the rhombohedral phase (a) and the dependence of a and c lattice parameters on Ti concentration for YMn1xTixO3 (b).
Figure 3. [1 1 0] zone axis electron diffraction patterns of YMn1xTixO3 for x = 0, 0.175 and 0.25. For x > 0.175 a new coherent rhombohedral phase coexists within the hexagonal P63cm matrix (a) and a two-beam dark-field superlattice image was obtained using unique rhombohedral reflections along the [1 1 0] direction (b).
429
Figure 4. Thermal expansion hysteresis of YMn1xTixO3 ceramics.
temperature for x = 0.0, 0.175 and 0.25. For x = 0 and 0.175, the reflections are uniquely associated with the hexagonal YMnO3 phase. However, when the Ti concentration increases to x > 0.175, superstructure reflections appear at the ±1/3{hkl} positions, thereby confirming the coexistence of the hexagonal P63cm and rhombohedral R3c phases [12,13]. The associated two-beam dark-field superlattice image shown in Figure 3b reveals the domain size and distribution of the rhombohedral phase in the hexagonal matrix. It is proposed that the decrease in the lattice anisotropy decreases the residual stresses at the phase transition and inhibits microcracking in Ti-doped YMnO3. Accordingly, the thermal expansion/contraction curves obtained during heating and cooling of sintered YMn1xTixO3 (x = 0, 0.1 and 0.2) ceramics are depicted in Figure 4. Upon thermal cycling, microcracked materials show large hysteresis in the thermal expansion curve [14]. Undoped YMnO3 shows low thermal expansion on heating. This phenomenon can be explained in terms of cracks healing. On cooling, the sample first shrinks in a manner characteristic of a bulk ceramic but then expands at 620 °C when the cracks reopen. For Ti-doped YMnO3, the thermal hysteresis behavior is significantly modified during the heating and cooling cycles. YMn0.9Ti0.1O3 shows less hysteresis and a higher thermal expansion than YMnO3, while YMn0.8Ti0.2O3 contracts normally when cooling to room temperature, with no perturbation in the contraction curve. However, there is an anomaly in the expansion curve for YMn0.9Ti0.1O3 at 450 °C, the origin of which is unknown. From the curves of Figure 4, an apparent thermal expansion coefficient can be determined during heating, whereas the thermal contraction of the body, without any microcracking, can be estimated from the thermal contraction curve until the microcracking temperature,
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Table 1. Crack density, microcracking temperature and thermal expansion coefficients for YMn1xTixO3 ceramics.
a
Composition
Crack density (vol.%)
Microcraking temperature, Tm (°C)
Apparent thermal expansion coefficient, aA [RT–1100 °C] (105 °C1)
Crack free thermal expansion coefficient, aF [1100–Tm °C] (105 °C1)
YMnO3 YMn0.9Ti0.1O3 YMn0.8Ti0.2O3
3.2 0.9 –
620 540 –
0.34 0.95 1.31
1.16 1.18 1.21a
Calculated between 1100 °C and room temperature (RT).
Tm. The values of Tm, the apparent thermal expansion coefficient (aA) and crack-free thermal expansion coefficient (aF) are presented in Table 1. YMnO3 has an apparent low thermal expansion, with aA = 0.34 105 °C1 on heating in the temperature range 25–1100 °C, while YMn0.9Ti0.1O3 and YMn0.8Ti0.2O3 show greater thermal expansion, at aA = 0.95 105 °C1 and aA = 1.31 105 °C1, respectively. In YMnO3, the solid volume of the specimen expands into the microcracks on heating and consequently the macroscopic dimensions remain almost unchanged. As a result, the material expands very little. However, the crack-free thermal expansion coefficient, aF (Table 1), is equivalent for all compositions (aF = 1.16 105 °C1 for x = 0, aF = 1.18 105 °C1 for x = 0.1 and aF = 1.21 105 °C1 for x = 0.2), meaning that Ti doping does not have a significant effect on it but, instead, prevents the microcracking phenomenon. Microcracking elimination must reduce the difference between aF and aA, as was observed with increasing x (Table 1); for x = 0.2, close values of expansion coefficients are obtained from the heating and cooling curves, which confirms that no appreciable microcracking exists. The crack density is calculated by the extrapolation to room temperature of the intrinsic thermal contraction before microcracking. The difference between the extrapolated line and the actual expansion of the specimen at room temperature represents the expansion caused by grain boundary microcracking. Multiplying this linear expansion by three gives the grain boundary crack density at room temperature. The crack densities calculated in this study for YMnO3 and for YMn0.9Ti0.1O3 are 3.2 and 0.9 vol.%, respectively (Table 1). The decrease in crack density should manifest itself not only in the mechanical integrity of the bulk ceramic bodies but also in their electrical properties. Two peaks in the permittivity were observed at T1 = 340 °C and T2 = 550 °C, respectively, for YMnO3, accompanied by a single peak in the dielectric losses at 390 °C [15]. The low-temperature dielectric anomaly at T1 is associated with the relaxation of double-ionized oxygen vacancies (Vo¨) [15]. The origin of the second anomaly at T2 is suggested to be associated with free carriers [15]. However, for Ti-doped YMnO3 ceramics only one peak can be observed at T = 450 °C in the dielectric constant and one peak in dielectric losses at the same temperature. This peak correlates with the anomaly observed in the thermal expansion curve of YMn0.9Ti0.1O3 (Fig. 4), and thus most likely arises from a structural phase transition. In addition to reducing microcracking, it is anticipated that Ti will act as donor dopant within
the YMnO3 hexagonal lattice. Typically, Ti4+ substituted for Mn3+ would suppress the formation of oxygen vacancies, according to the defect equation: 2TiO2 þ V €o ! 2TiMn þ 4Ox0 Hence, the anomalies associated with Vo¨ in YMnO3 sintered in air become progressively weaker as the Ti concentration increases. Ti doping has been shown to be an effective way to reduce the microcracking in YMnO3-based ceramics. Ti-doped YMnO3 ceramics presented decreased crack density and intragranular porosity compared with undoped YMnO3. The improvement in crack density is attributed to a decrease in the c/a ratio of the hexagonal cell, which reduces the anisotropic expansion at the high-temperature phase transition, considered responsible for microcracking in YMnO3. However, for x > 0.175, there is a transformation from a ferroelectric hexagonal to a paraelectric rhombohedral phase. The authors acknowledge the financial support from FEDER, QREN, COMPETE, and FCT within the project PTDC/CTM/67575/2006. [1] D. Khomskii, Physics 2 (2009) 20. [2] K.F. Wang, J.M. Liu, Z.F. Ren, Adv. Phys. 58 (2009) 321. [3] B.B. van Aken, T.T.M. Palstra, A. Filippetti, N.A. Spaldin, Nat. Mater. 3 (2004) 164. [4] K. Lukaszewicz, J. Karut-Kalicinska, Ferroelectrics 7 (1974) 81. [5] A.S. Gibbs, K.S. Knight, P. Lightfoot, Phys. Rev. B 83 (2011) 094111. [6] G.A. Smolenskii, V.A. Bokov, J. Appl. Phys. 35 (1964) 915. [7] T. Choi, Y. Horibe, H.T. Yi, Y.J. Choi, W. Wu, S.-W. Cheong, Nat. Mater. 9 (2010) 253. [8] B. Fu, W. Huebner, M.F. Trubelja, V.S. Stubican, J. Mater. Res. 9 (1994) 2645. [9] M. Tomczyk, A.M. Senos, P.M. Vilarinho, I.M. Reaney, Scripta Mater. 66 (2012) 288. [10] S.Y. Chu, T.Y. Chen, Sensor. Actuat. A-Phys. 107 (2003) 75. [11] J.A. Kuszyk, R.C. Bradt, J. Amer. Cer. Soc. 56 (1973) 420. [12] Y. Aikawa, T. Katsufuji, T. Arima, K. Kato, Phys. Rev. B 71 (2005) 184418. [13] T. Asaka, K. Nemoto, K. Kimoto, T. Arima, Y. Matsui, Phys. Rev. B 71 (2005) 014114. [14] Y. Ohya, Z. Nakagawa, J. Mater. Sci. 31 (1996) 1555. [15] M. Tomczyk, P.M. Vilarinho, J.A. Moreira, A. Almeida, J. Appl. Phys. 110 (2011) 064116.
Scripta Materialia 67 (2012) 427–430 www.elsevier.com/locate/scriptamat
Reduction of microcracking in YMnO3 ceramics by Ti substitution M. Tomczyk,a A.M.O.R. Senos,a I.M. Reaneyb and P.M. Vilarinhoa,⇑ a
Department of Materials and Ceramic Engineering, Centre for Research in Ceramics and Composite Materials, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK Received 26 February 2012; accepted 25 April 2012 Available online 18 May 2012
Microcracking in YMnO3 ceramics originates from a combination of 3.7% volume change and strong anisotropy in the thermal expansion coefficient of the hexagonal structure. In this paper, the effect of Ti doping on the microstructure, structure and dielectric response of YMnO3 ceramics is studied. Lattice anisotropy of the hexagonal cell markedly decreased with increasing Ti, reducing the residual stresses created at the high-temperature phase transition and commensurately decreasing the crack density (3.2 vol.% YMnO3 to 0.9 vol.% YMn0.9Ti0.1O3). These results represent the first systematic work to reduce microcracking in YMnO3-based ceramics. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: YMnO3; Ti doping; Microstructure; Microcracking
Materials possessing two or more of the ferroic properties of ferroelasticity, ferroelectricity and (anti)ferromagnetism are known as multiferroics [1]. Because of their potential technological applications in modern electronic devices such as memory elements, sensors and spintronics, they have become the subject of intense research [2]. Among the known multiferroic materials, extensive studies have been performed on hexagonal manganites, especially yttrium manganese oxide, which exhibits high-temperature ferroelectric transition (TC 900 K) along with a low-temperature antiferromagnetic transition (TN 70 K) [3]. YMnO3 exhibits a noncentrosymmetric space group P63cm at room temperature, which transforms to the centrosymmetric equivalent, P63/mmc, at 1270 K [4]. The Mn3+ ions form MnO5 bipyramids with three planar oxygen (O3 and O4) and two apical oxygen (O1 and O2) atoms. The MnO5 bipyramids are corner linked to form a triangular lattice layer in the ab plane. Along the c direction, the crystal structure is composed of alternate Y3+ ions and corner-shared MnO5 layers [5]. Single crystal data have shown that the permittivity, er, is 20 at room temperature and increases slowly with temperature. Clear ferroelectric hysteresis is observed along the c-axis with spontaneous polarization PS 5
⇑ Corresponding
author. Tel.: +351 234 370354; fax: +351 234 425300; e-mail: [email protected] (P.M. Vilarinho).
lC cm2 [6,7]. However, these properties have not yet been realized in bulk polycrystalline samples. Microcracking and porosity observed in YMnO3 ceramics [8] are considered to strongly affect the ferroelectric response. Recently, it was shown that microcracking originates from a combination of a 3.7% volume change and strong anisotropy in the thermal expansion coefficient (a) of the hexagonal structure (|aa ac| 4.57 105 °C1) between 600 and 1000 °C [9]. It is therefore a matter of practical interest to prevent or reduce anisotropy/microcracking in YMnO3 ceramics. Commonly, the fabrication of dense and crack free ceramics is achieved by reducing the stresses that are generated during processing using doping and/or decreasing the grain size below a critical value [10]. For instance, the fabrication of crack-free PbTiO3 ceramics is achieved by modifications with CaO or Sm2O3, which inhibit microcracking by reducing lattice anisotropy [11]. Aikawa et al. [12] studied the effect of the Ti substitution on the electric and magnetic properties in YMnO3 and observed that magnetocapacitance is enhanced around x = 0.175. Moreover, for the Mn site substitution, electron diffraction and dark-field imaging revealed that doping of Ti4+ in YMnO3 resulted in the formation of a coherent rhombohedral paraelectric phase within the P63cm matrix at around x = 0.20, with the rhombohedral phase dominant above x = 0.30. As the volume fraction of the rhombohedral phase increased to x > 0.20, the ferroelectric and magnetocapacitive response was suppressed
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.04.042
428
M. Tomczyk et al. / Scripta Materialia 67 (2012) 427–430
[12]. This paper aims to illustrate that Ti substitution also reduces anisotropy and microcracking in YMnO3 ceramics, thereby facilitating the establishment of accurate bulk microstructure/properties relations. YMn1xTixO3 (0 6 x 6 0.25) powders were prepared by solid-state reaction at ambient pressure [9]. Pure oxides Y2O3 (ABCR, 99.99%), Mn2O3 (ABCR 99%) and TiO2 (MERCK, 99%) were weighted according to the stoichiometry of YMn1xTixO3. The starting materials were milled for 5 h in a planetary mill using Teflon pots with zirconia balls and ethanol. The milled powders were calcined at 1100 °C for 10 h and then milled again for 5 h. The calcined and milled powders were uniaxially pressed into 10 mm diameter discs at 80 MPa and isostatically pressed at 200 MPa. Sintering was conducted at 1400 °C for 5 and 10 h, depending on the composition. The structure, microstructure and phase assemblage of the sintered ceramics were analyzed by X-ray diffraction (XRD; Rigaku D/Max-B, Cu Ka), scanning electron microscopy (SEM; Hitachi S-4100) and transmission electron microscopy (TEM; Hitachi H9000). A CELREF program (Laugier & Bochu) was used to refine the lattice parameters. Samples for TEM were prepared by grinding and polishing to 50 lm, followed by ion milling, using a Gatan PIPS 691 ion beam miller operating at 5 kV and a combined gun current, at an incidence angle 10°. The thermal expansion behavior of the sintered bodies was analyzed by dilatometric measurements at a constant heating rate of 5 °C min1 up to 1400 °C and then cooling down to room temperature, using a computer-assisted dilatometer (Linseis, model 4L702000). For the dielectric measurements, sintered samples were polished to a thickness of 1 mm and silver electrodes were painted on both sides of the ceramic disks. The complex impedance was measured at different temperatures from 25 to 700 °C with a heating rate of 4 °C min1 in the frequency range from 100 Hz to 1 MHz, using a Precision LCR Meter HP 4284A.
Figure 1 depicts the SEM images of YMn1xTixO3 ceramics as a function of Ti concentration. The microstructure of undoped YMnO3 ceramics is dominated by a large volume fraction of inter- and intragranular pores and cracks [8,9]. The latter is promoted by the large volume change (3.7%) at the high-temperature TC, which gives rise to 200 MPa residual stress, and also by the highly anisotropic thermal expansion of the hexagonal unit cell parameters [9]. However, as the Ti concentration increases, the volume fraction of pores and microcracks decreases markedly. To clarify the role of Ti doping on structural evolution, XRD patterns were obtained from YMn1xTixO3 ceramics for 0 6 x 6 0.25 (Fig. 2a). For x < 0.175, the structure remained single phase hexagonal (P63cm). However, for x > 0.175, peaks not associated with the hexagonal phase could be detected at 2h of approx. 28, 24, 47 and 64° that matched a rhombohedral (R 3c) structure [12]. Although the peaks may be indexed according to a rhombohedral structure, their intensities are weak and indicate the coexistence of two distinct phases (hexagonal and rhombohedral) over the range of x investigated, as described in Ref. [12]. The rhombohedral phase is reported to arise due to the formation of a superlattice with a 3 d001 periodicity along the c-axis [13]. The dependence of the cell parameters as a function of x for YMn1xTixO3 (0 6 x 6 0.25) is plotted in Figure 2b. For compositions with x < 0.175 and x P 0.175, cell parameters were refined using P63cm symmetry and R3c symmetry, respectively. To directly compare the changes in cell parameters in the two phases, c/3 of the R3c cell was plotted along with the c-axis of the P63cm phase. As the Ti concentration increased, the a-axis increased and the c-axis decreased. No anomaly was observed at the P63cm/R3c boundary, consistent with a gradual increase in volume fraction of a coherent rhombohedral phase in a hexagonal matrix. The structural changes as a function of x were further studied by TEM. Figure 3a displays the [1 1 0]h zone axis electron diffraction patterns of YMn1xTixO3 at room
Figure 1. SEM micrographs of YMn1xTixO3, where (a) x = 0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.15, (e) x = 0.175, (f) x = 0.2 and (g) x = 0.25. As the Ti concentration increases, the concentration of microcracks and intragranular porosity decrease.
M. Tomczyk et al. / Scripta Materialia 67 (2012) 427–430
Figure 2. X-ray powder diffraction patterns of YMn1xTixO3 ceramics at room temperature. d indicates the new peaks associated with the rhombohedral phase (a) and the dependence of a and c lattice parameters on Ti concentration for YMn1xTixO3 (b).
Figure 3. [1 1 0] zone axis electron diffraction patterns of YMn1xTixO3 for x = 0, 0.175 and 0.25. For x > 0.175 a new coherent rhombohedral phase coexists within the hexagonal P63cm matrix (a) and a two-beam dark-field superlattice image was obtained using unique rhombohedral reflections along the [1 1 0] direction (b).
429
Figure 4. Thermal expansion hysteresis of YMn1xTixO3 ceramics.
temperature for x = 0.0, 0.175 and 0.25. For x = 0 and 0.175, the reflections are uniquely associated with the hexagonal YMnO3 phase. However, when the Ti concentration increases to x > 0.175, superstructure reflections appear at the ±1/3{hkl} positions, thereby confirming the coexistence of the hexagonal P63cm and rhombohedral R3c phases [12,13]. The associated two-beam dark-field superlattice image shown in Figure 3b reveals the domain size and distribution of the rhombohedral phase in the hexagonal matrix. It is proposed that the decrease in the lattice anisotropy decreases the residual stresses at the phase transition and inhibits microcracking in Ti-doped YMnO3. Accordingly, the thermal expansion/contraction curves obtained during heating and cooling of sintered YMn1xTixO3 (x = 0, 0.1 and 0.2) ceramics are depicted in Figure 4. Upon thermal cycling, microcracked materials show large hysteresis in the thermal expansion curve [14]. Undoped YMnO3 shows low thermal expansion on heating. This phenomenon can be explained in terms of cracks healing. On cooling, the sample first shrinks in a manner characteristic of a bulk ceramic but then expands at 620 °C when the cracks reopen. For Ti-doped YMnO3, the thermal hysteresis behavior is significantly modified during the heating and cooling cycles. YMn0.9Ti0.1O3 shows less hysteresis and a higher thermal expansion than YMnO3, while YMn0.8Ti0.2O3 contracts normally when cooling to room temperature, with no perturbation in the contraction curve. However, there is an anomaly in the expansion curve for YMn0.9Ti0.1O3 at 450 °C, the origin of which is unknown. From the curves of Figure 4, an apparent thermal expansion coefficient can be determined during heating, whereas the thermal contraction of the body, without any microcracking, can be estimated from the thermal contraction curve until the microcracking temperature,
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M. Tomczyk et al. / Scripta Materialia 67 (2012) 427–430
Table 1. Crack density, microcracking temperature and thermal expansion coefficients for YMn1xTixO3 ceramics.
a
Composition
Crack density (vol.%)
Microcraking temperature, Tm (°C)
Apparent thermal expansion coefficient, aA [RT–1100 °C] (105 °C1)
Crack free thermal expansion coefficient, aF [1100–Tm °C] (105 °C1)
YMnO3 YMn0.9Ti0.1O3 YMn0.8Ti0.2O3
3.2 0.9 –
620 540 –
0.34 0.95 1.31
1.16 1.18 1.21a
Calculated between 1100 °C and room temperature (RT).
Tm. The values of Tm, the apparent thermal expansion coefficient (aA) and crack-free thermal expansion coefficient (aF) are presented in Table 1. YMnO3 has an apparent low thermal expansion, with aA = 0.34 105 °C1 on heating in the temperature range 25–1100 °C, while YMn0.9Ti0.1O3 and YMn0.8Ti0.2O3 show greater thermal expansion, at aA = 0.95 105 °C1 and aA = 1.31 105 °C1, respectively. In YMnO3, the solid volume of the specimen expands into the microcracks on heating and consequently the macroscopic dimensions remain almost unchanged. As a result, the material expands very little. However, the crack-free thermal expansion coefficient, aF (Table 1), is equivalent for all compositions (aF = 1.16 105 °C1 for x = 0, aF = 1.18 105 °C1 for x = 0.1 and aF = 1.21 105 °C1 for x = 0.2), meaning that Ti doping does not have a significant effect on it but, instead, prevents the microcracking phenomenon. Microcracking elimination must reduce the difference between aF and aA, as was observed with increasing x (Table 1); for x = 0.2, close values of expansion coefficients are obtained from the heating and cooling curves, which confirms that no appreciable microcracking exists. The crack density is calculated by the extrapolation to room temperature of the intrinsic thermal contraction before microcracking. The difference between the extrapolated line and the actual expansion of the specimen at room temperature represents the expansion caused by grain boundary microcracking. Multiplying this linear expansion by three gives the grain boundary crack density at room temperature. The crack densities calculated in this study for YMnO3 and for YMn0.9Ti0.1O3 are 3.2 and 0.9 vol.%, respectively (Table 1). The decrease in crack density should manifest itself not only in the mechanical integrity of the bulk ceramic bodies but also in their electrical properties. Two peaks in the permittivity were observed at T1 = 340 °C and T2 = 550 °C, respectively, for YMnO3, accompanied by a single peak in the dielectric losses at 390 °C [15]. The low-temperature dielectric anomaly at T1 is associated with the relaxation of double-ionized oxygen vacancies (Vo¨) [15]. The origin of the second anomaly at T2 is suggested to be associated with free carriers [15]. However, for Ti-doped YMnO3 ceramics only one peak can be observed at T = 450 °C in the dielectric constant and one peak in dielectric losses at the same temperature. This peak correlates with the anomaly observed in the thermal expansion curve of YMn0.9Ti0.1O3 (Fig. 4), and thus most likely arises from a structural phase transition. In addition to reducing microcracking, it is anticipated that Ti will act as donor dopant within
the YMnO3 hexagonal lattice. Typically, Ti4+ substituted for Mn3+ would suppress the formation of oxygen vacancies, according to the defect equation: 2TiO2 þ V €o ! 2TiMn þ 4Ox0 Hence, the anomalies associated with Vo¨ in YMnO3 sintered in air become progressively weaker as the Ti concentration increases. Ti doping has been shown to be an effective way to reduce the microcracking in YMnO3-based ceramics. Ti-doped YMnO3 ceramics presented decreased crack density and intragranular porosity compared with undoped YMnO3. The improvement in crack density is attributed to a decrease in the c/a ratio of the hexagonal cell, which reduces the anisotropic expansion at the high-temperature phase transition, considered responsible for microcracking in YMnO3. However, for x > 0.175, there is a transformation from a ferroelectric hexagonal to a paraelectric rhombohedral phase. The authors acknowledge the financial support from FEDER, QREN, COMPETE, and FCT within the project PTDC/CTM/67575/2006. [1] D. Khomskii, Physics 2 (2009) 20. [2] K.F. Wang, J.M. Liu, Z.F. Ren, Adv. Phys. 58 (2009) 321. [3] B.B. van Aken, T.T.M. Palstra, A. Filippetti, N.A. Spaldin, Nat. Mater. 3 (2004) 164. [4] K. Lukaszewicz, J. Karut-Kalicinska, Ferroelectrics 7 (1974) 81. [5] A.S. Gibbs, K.S. Knight, P. Lightfoot, Phys. Rev. B 83 (2011) 094111. [6] G.A. Smolenskii, V.A. Bokov, J. Appl. Phys. 35 (1964) 915. [7] T. Choi, Y. Horibe, H.T. Yi, Y.J. Choi, W. Wu, S.-W. Cheong, Nat. Mater. 9 (2010) 253. [8] B. Fu, W. Huebner, M.F. Trubelja, V.S. Stubican, J. Mater. Res. 9 (1994) 2645. [9] M. Tomczyk, A.M. Senos, P.M. Vilarinho, I.M. Reaney, Scripta Mater. 66 (2012) 288. [10] S.Y. Chu, T.Y. Chen, Sensor. Actuat. A-Phys. 107 (2003) 75. [11] J.A. Kuszyk, R.C. Bradt, J. Amer. Cer. Soc. 56 (1973) 420. [12] Y. Aikawa, T. Katsufuji, T. Arima, K. Kato, Phys. Rev. B 71 (2005) 184418. [13] T. Asaka, K. Nemoto, K. Kimoto, T. Arima, Y. Matsui, Phys. Rev. B 71 (2005) 014114. [14] Y. Ohya, Z. Nakagawa, J. Mater. Sci. 31 (1996) 1555. [15] M. Tomczyk, P.M. Vilarinho, J.A. Moreira, A. Almeida, J. Appl. Phys. 110 (2011) 064116.