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Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties O. Nirmala a,⇑, P. Sreedhara Reddy b, V. Diwakar Reddy c a
Dept. of Mechanical Engineering, SVU College of Engineering, Tirupati 517502, India Department of Physics, SV University, Tirupati 517502, India c Department of Mechanical Engineering, SVU College of Engineering, Tirupati 517502, India b
a r t i c l e
i n f o
Article history: Received 23 March 2019 Accepted 10 May 2019 Available online xxxx Keywords: Nanocrystalline Hexagonal YMnO3 Sol-gel citrate route X-ray diffraction Dielectric properties
a b s t r a c t Nanocrystalline perovskite hexagonal yttrium manganese oxide (YMnO3) sample have been synthesized through a sol-gel citrate route and then calcination has been done at 700 °C for one hour. The prepared sample was subjected to structural and morphological studies through X-ray diffraction (XRD) technique and scanning electron microscope (SEM). The XRD pattern of the prepared sample confirmed the formation of a pure hexagonal phase of YMnO3. The energy-dispersive X-ray analysis (EDX) results proved the elemental composition of pure yttrium, manganese, and oxygen. The dielectric properties of the YMnO3 nanocrystals were studied using a precision LCR meter at different temperature range 28 °C–500 °C. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Recent Advances in Materials, Manufacturing & Energy Systems.
1. Introduction The rare-earth manganite, yttrium manganese oxide (YMnO3) is a multiferroic material possessing ferroelectricity and magnetism properties simultaneously, which attracted much research interest from the past two decades. YMnO3 exists in three phases; such as amorphous, hexagonal phase and an orthorhombic phase whose chemical compositions are identical [1]. Hexagonal YMnO3 is one of the most intensively studied materials, with space group P63cm, having a high ferroelectric transition temperature (TC 900 K) and a low anti-ferromagnetic transition temperature (TN 70 K) [2–5]. Orthorhombic YMnO3, with space group Pbnm/ Pnma, exhibits ferroelectric transition temperature (TC = 30 K) and a low anti-ferromagnetic transition temperature (TN = 42 K) [6,7]. The transition of YMnO3 from hexagonal phase to orthorhombic phase occurs by changing the ionic radius of Y ion in perovskite structure and by high-pressure synthesis [1,6]. YMnO3 is a pyroelectric material which gives electric polarization response with temperature [8–10]. YMnO3 does not have any volatile elements such as Bi and Pb and has a single spontaneous polarization axis [11]. Due to these absolute properties of hexagonal YMnO3, it became a promising candidate in many applications such as information storage process, multi-state memories [2], ⇑ Corresponding author. E-mail address: [email protected] (O. Nirmala).
non-volatile ferroelectric random access memories, ferroelectricgate field-effect transistors [11–13], magneto electric sensors [2,12,13] and as H2S gas sensor [14]. From the past two decades, several articles were reported on the synthesis of hexagonal YMnO3 ceramics and their characterizations. In those studies, solid-state synthesis [5,15,16] and various chemical methods like sol-gel synthesis route[2,4,14,17], polyacrylamide gel route [11,12] were used to synthesize hexagonal YMnO3 ceramics. And several studies concentrated on magnetic properties [2,6,13,17,18], optical properties [11,12] and dielectric properties [3,4,16,17]. However, the study of dielectric properties was still inadequate. In the present work, a sol-gel citrate route, followed by grinding and calcination was used to prepare multiferroic hexagonal YMnO3 nanocrystals and their structural, morphological characteristics were studied. In addition to that dielectric properties also studied in wide temperature range 28 °C–500 °C and frequency range 50 Hz–5 MHz.
2. Experimental work The perovskite hexagonal YMnO3 sample was prepared by sol–gel citrate process. The raw materials are yttrium oxide Y2O3 (99.9%) (0.02 mol), manganese acetate tetra hydrate Mn(CH3COO)2 (0.02 mol), citric acid monohydrate C6H8O7H2O (0.04 mol) and
https://doi.org/10.1016/j.matpr.2019.05.391 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Recent Advances in Materials, Manufacturing & Energy Systems.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
2
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
nitric acid HNO3 (0.02 mol). Stoichiometric amounts of above mentioned raw materials were calculated by using the formula in Eq. (1).
Weight ¼ No:of moles molecular weight
V 1000
characteristics were evaluated for the prepared hexagonal YMnO3 pellet with Precision LCR meter (Wayne-Kerr electronics-6400B, high precision impedance analyzer) in the wide temperature range 28 °C500 °C and frequency range 50 Hz–5 MHz.
ð1Þ
where V is the volume of de-ionized water taken to prepare the solution. The schematic diagram of the synthesis process was shown in Fig. 1. Calculated amounts of raw materials were carefully weighed and mixed in a ‘V’ amount of de-ionized water. This total mixture was stirred for two hours at room temperature to get the clear solution. Then ammonium hydroxide (about 25%) was added to this mixture for adjusting its pH value up to 7, which was required to accomplish complex reaction between metal ions and citric acid. After reaching required pH value, 0.04 mol of ethylene glycol (CH2OH)2 (98%), which acts as a polymerization agent and a catalytic amount of C-TAB, which acts as structure directing agent were added to the mixture. After that, the final mixture was again stirred at room temperature for half an hour. Next, the mixture was placed on a hot plate, heated along with stirring until the mixture was boiled, evaporated and converted into a black powder (precursor). The obtained powder was cooled to room temperature and then grounded for ½ hour in a mortar. Next, the grounded powder was subjected to calcination at 700 °C for 2 h in a muffle furnace to get required hexagonal YMnO3 nanocrystals. The structural characteristics of as-synthesized precursor powder and the calcined YMnO3 nanocrystals were studied by an X-ray powder Diffraction (Rigaku, Miniflex 300/600, Cu Ka radiation). SEM with EDX has been used to observe the morphological characteristics and elemental presence in the prepared YMnO3 nanocrystals sample using SEM-Carl Zeiss (EVOMA15 model) with EDSOxford Instruments (Inca PENTAFET X3). The YMnO3 nanocrystals were uni-axially pressed to make a pellet (13 mm diameter) at pressure 75 Kg/cm2 in a hydraulic press and then the pallet was heated at 200 °C for one hour. The dielectric and dissipation
3. Results and discussion Fig. 2 shows the X-ray diffraction results of as-synthesized precursor powder and the powder calcined at 700 °C using an X-ray
Fig. 2. XRD patterns of as-synthesized precursor powder and the powder calcined at 700 °C.
Fig. 1. Synthesis process to get hexagonal YMnO3 nanocrystals.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
diffractometer with scan range 10°–80° (2h). The strong intensity XRD peaks reveal that the as-synthesized precursor particles have better crystallinity even before calcination. The most of the XRD peaks of YMnO3 nanocrystals (after calcination) are indexed with hexagonal YMnO3 with space group: P63cm, JCPDS 25-1079, which proves the formation of pure phase hexagonal YMnO3 particles. The disappearance of small peaks (impurity peaks) in the XRD pattern of particles calcined at 700 °C at 21.32 and 23.98 ‘2h’ angles, indicates the crystallinity reaction on calcination. The grain size was calculated by using Scherrer’s formula in Eq. (2) [2,12].
D¼
Kk b cos h
ð2Þ
where, D is the mean size of the crystal, in nm, K is the crystallite shape factor which is constant and equal to 0.89, k is the X-rays wavelength, 0.15406 nm from JCPDS 25-1079, b is the full width half maximum (FWHM) at predominant peak and h is the Bragg’s angle at which predominant peak occurs. The average grain size of YMnO3 nanocrystals was calculated as 40 nm. From XRD data results, the lattice parameters of YMnO3 nanocrystals are a = b = 6.120 °A and c = 11.331 °A. The standard lattice parameter values of YMnO3 particles are a = b = 6.136 °A and c = 11.4 °A (from JCPDS 25-1079) which are closer to the obtained results. Through SEM, the surface morphology of the synthesized YMnO3 nanocrystals was studied and shown in Fig. 3. The SEM observation unveils that YMnO3 nanocrystals are varying in size from small to medium; some crystals are joined with other crystals as clusters and leaving some space with other clusters of crystals. It
Fig. 3. SEM image of as-synthesized powder calcined at 700 °C.
3
also shows that crystal size was compatible with that size calculated from XRD analysis. The EDX result of synthesized YMnO3 nanocrystals was shown in Fig. 4. Each element weight % and atomic weight % were also shown in the Fig. 4 itself. No other elements were detected, other than yttrium, manganese, and oxygen. The dielectric properties of synthesized YMnO3 nanocrystals were measured using LCR bridge by preparing the pellet from synthesized YMnO3 nanocrystals. The precision LCR meter gives capacitance (C), and dissipation factor (tan d) values. The Dielectric constant (e) of a dielectric material can be defined as the ratio of the capacitance using the material as the dielectric in a capacitor to the capacitance of vacuum. The Dielectric constant (e) was determined by using the following Eqs. (3) and (4) [19].
e¼
C C0
ð3Þ
and
C0 ¼
eo A t
ð4Þ
where ‘C’ is the capacitance using the prepared pallet (YMnO3 nanocrystals) as dielectric, ‘C0’ is capacitance using vacuum as dielectric, ‘e0’ is the permittivity of free space (8.85 1012 F/m), ‘A’ is the area of the pellet and ‘t’ is the thickness of the pellet. The dissipation factor or dielectric loss is the loss of electrical energy in the form of heat, in the presence applied ac field in the polarization process. The temperature dependent dielectric constant (e) and dissipation factor (tan d) of YMnO3 nanocrystals at different frequencies were shown in Fig. 5(a), and (b). In Fig. 5(a) at 50 Hz frequency (normal condition) the dielectric anomaly was observed at 350 °C which is described as rising and fall of dielectric constant and it was accompanied by the anomaly in the dielectric loss at the same temperature at 50 Hz frequency shown in Fig. 5 (b). After this anomaly, the dielectric constant (e) was increased continuously and reached a maximum value and the corresponding rise was observed in dielectric loss also. An apparent dielectric constant relaxation was noticed in Fig. 5(a) and a corresponding relaxation in the dielectric loss in Fig. 5(b) in the whole temperature range at all frequencies other than 50 Hz, 500 Hz, and 5 kHz. At the temperature above 450 °C the dielectric constant was increased quickly and correlate with the rapid increase in dissipation factor at the higher temperature at frequencies of 50 Hz, 500 Hz, and 5 kHz only. Fig. 6(a) and (b) shows the logarithmic frequency dependent dielectric constant (e) of YMnO3 nanocrystals at temperature range 28 °C–500 °C. In Fig. 5(a) at 500 Hz frequency dielectric constant value increases up to 200 °C temperature and then goes to negative value from 250 °C to 450 °C temperature and corresponding
Fig. 4. EDS results of as-synthesized powder calcined at 700 °C.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
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O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 5. The temperature dependent dielectric constant ‘e’ (a), and dissipation factor (tan d) (b) of YMnO3 nanocrystals at different frequencies.
Fig. 6. The frequency dependent dielectric constant ‘e’ from temperature 28 °C to 250 °C (a), and from temperature 300 °C to 500 °C (b) of YMnO3 nanocrystals.
negative value of dielectric constant was clearly visible in Fig. 6(a) and (b) from temperature range 250 °C–450 °C. These negative dielectric constant values are unexpected, not observed in previous studies and a novel negative dielectric constant material is an essential key for preparing metamaterials, or artificial negative index materials (NIMS) [20]. The logarithmic frequency dependent dissipation factor (tan d) of YMnO3 nanocrystals at temperature range 28 °C–500 °C were shown in Fig. 7(a) and (b). At lower temperature from 28 °C to 150 °C no change in dielectric constant was perceived in the entire frequency range in Fig. 6(a), but a corresponding rise and gradual decrease of dissipation factor were observed in Fig. 7(a). At 200 °C a relaxation is noticed as a gradual decrease in dielectric constant in Fig. 6(a) and correlates with a peak and then relaxation in dissipation factor in Fig. 7(a). At 500 °C temperature, the dielectric constant reached a maximum value at the lower frequency and fall down to a lower value at the higher frequency which was shown in Fig. 6(b). In Fig. 7(a) and (b) from temperature 250 °C–500 °C, the dissipation factor
values gradually increased and also fluctuating up and down from lower to medium frequency range and then fall down at the higher frequency. From the above dielectric characteristics of YMnO3 nanocrystals, the dielectric constant decreases to the least value with the increase in frequency, which is the characteristic of the dielectric material. The permanent dipoles align themselves along the direction of the field at lower frequency region and cause the total polarization of the dielectric material. And at the higher frequency the dipoles can’t follow the field due to the too rapid variation of the field, so dipoles do not contribute total polarization and hence it becomes negligible.
4. Conclusion Hexagonal YMnO3 nanocrystals were synthesized by citrate solgel route and the structural morphological characterization proved
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
5
Fig. 7. The frequency dependent dissipation factor (tan d) from temperature 28 °C to 250 °C (a), and from temperature 300 °C to 500 °C (b) of YMnO3 nanocrystals.
the formation of hexagonal phase YMnO3 nanocrystals with grain size 40 nm. This synthesis process was able to produce h-YMnO3 nanocrystals at lower calcination temperature 700 °C. Dielectric anomalies are observed at higher temperatures and at 50 Hz, 500 Hz, and 5 kHz only. A dielectric relaxation, as no change in its value, was identified throughout the temperature range at all other frequencies. And an unexpected negative dielectric constant was identified at 500 Hz frequency between temperatures 250 °C– 450 °C which can be used for developing artificial negative index materials (NIMS). Acknowledgments The present work was financially supported by the Technical Education Quality Improvement Program (TEQIP)-1.2. The authors are thankful to Dr.Y. VenkataSubbaiah for his help in conducting XRD analysis. The authors are also thankful to Osmania University for extending facility of LCR meter for dielectric study. References [1] I. Iliescu, M. Boudard, L. Rapenne, O. Chaix-Pluchery, H. Roussel, MOCVD selective growth of orthorhombic or hexagonal YMnO3 phase on Si (1 0 0) substrate, Appl. Surf. Sci. 306 (2014) 27–32. [2] Nagesh Kumar, Anurag Gaur, G.D. Varma, Enhanced magnetization and magnetoelectric coupling in hydrogen treated hexagonal YMnO3, J. Alloys Compounds 509 (2011) 1060–1064. [3] Yan Ma, Wu Yong Jun, Xiang Ming Chen, Ji Peng Cheng, Yi Qi Lin, In situ synthesis of multiferroic YMnO3 ceramics by SPS and their characterization, Ceram. Int. 35 (2009) 3051–3055. [4] Chao Zhang, Su. Jie, Xiaofei Wang, Fengzhen Huang, Junting Zhang, Yaoyang Liu, Liang Zhang, Kangli Min, Zhijun Wang, Lu. Xiaomei, Feng Yan, Jinsong Zhu, Study on magnetic and dielectric properties of YMnO3 ceramics, J. Alloy. Compd. 509 (2011) 7738–7741. [5] M. Tomczyk, A.M.O.R. Senos, I.M. Reaney, P.M. Vilarinho, Reduction of microcracking in YMnO3 ceramics by Ti substitution, Scr. Mater. 67 (2012) 427–430.
[6] Z. Brankovic, G. Brankovic, M. Pocuca-Nesic, Z. Marinkovic Stanojevic, M. Zunic, D. Lukovic Golic, R. Tararam, M. Cilense, M.A. Zaghete, Z. Jaglicic, M. Jagodic, J. A. Varela, Hydrothermally assisted synthesis of YMnO3, Ceram. Int. 41 (2015) 14293–14298. [7] S.A. Nikolaev, V.G. Mazurenko, A.N. Rudenko, Influence of magnetic order on phonon spectra of multiferroic orthorhombic YMnO3, Solid State Commun. 164 (2013) 16–21. [8] Richard Laneand Benjamin Craig, ‘‘Material E A S E”, The AMPTIAC Quarterly, Volume 7, Number 2. [9] Yan R. Kucherov, Thermodyne, Inc., ‘‘Piezo-Pyroelectric energy converter and method”, United States Patent, Patent Number: 5,644,184, [45] Date of Patent: Jul. 1, 1997. [10] A.J.C. Buurma, G.R. Blake, T.T.M. Palstra, U. Adem, Multiferroic materials: physics and properties, Ref. Module Mater. Sci. Mater. Eng. (2016), https://doi. org/10.1016/B978-0-12-803581-8.09245-6. [11] S.F. Wang, H. Yang, T. Xian, X.Q. Liu, Size-controlled synthesis and photocatalytic properties of YMnO3 nanoparticles, Catal. Commun. 12 (2011) 625–628. [12] Aijun Han, Minchun Zhao, Mingquan Ye, Juanjuan Liao, Zhimin Zhang, Nan Li, Crystal structure and optical properties of YMnO3 compound with high nearinfrared reflectance, Sol. Energy 91 (2013) 32–36. [13] R. Dhinesh Kumar, R. Jayavel, Low-temperature hydrothermal synthesis and magnetic studies of YMnO3 nanorods, Mater. Lett. 113 (2013) 210–213. [14] C. Balamurugan, D.W. Lee, Perovskite hexagonal YMnO3 nanopowder as p-type semiconductor gas sensor for H2S detection, Sens. Actuators, B 221 (2015) 857–866. [15] Monika Tomczyk, Ana Maria Senos, Paula Maria Vilarinhoand Ian Michael Reaney, Origin of microcracking in YMnO3 ceramics, Scr. Mater. 66 (2012) 288–291. [16] Monika Tomczyk, Paula Maria Vilarinho, Agostinho Moreira, Abílio Almeida, High-temperature dielectric properties of YMnO3 ceramics, J. Appl. Phys. 110 (2011) 064116. [17] Tai-Chun Han, Wei-Lun Hsu, Wei-Da Lee, Grain size-dependent magnetic and electric properties in nanosized YMnO3 multiferroic ceramics, Nanoscale Res. Lett. 6 (2011) 201. [18] Aboalqasim Alqat, Zohra Gebrel, Vladan Kusigerski, Vojislav Spasojevic, Marian Mihalik, Matus Mihalik, Jovan Blanusa, Synthesis of hexagonal YMnO3 from precursor obtained by the glycine-nitrate process, Ceram. Int. 39 (2013) 3183–3188. [19] C. Kittel, ‘‘Introduction to solid state physics”, Wiley Eastern Limited (5th Edition). [20] NASA Technology Transfer Program, ‘‘Materials and Coatings Negative Dielectric Constant Material”, NASA patent, PUBLICATIONS Patent No: 8,696,940; 9,734,932.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties O. Nirmala a,⇑, P. Sreedhara Reddy b, V. Diwakar Reddy c a
Dept. of Mechanical Engineering, SVU College of Engineering, Tirupati 517502, India Department of Physics, SV University, Tirupati 517502, India c Department of Mechanical Engineering, SVU College of Engineering, Tirupati 517502, India b
a r t i c l e
i n f o
Article history: Received 23 March 2019 Accepted 10 May 2019 Available online xxxx Keywords: Nanocrystalline Hexagonal YMnO3 Sol-gel citrate route X-ray diffraction Dielectric properties
a b s t r a c t Nanocrystalline perovskite hexagonal yttrium manganese oxide (YMnO3) sample have been synthesized through a sol-gel citrate route and then calcination has been done at 700 °C for one hour. The prepared sample was subjected to structural and morphological studies through X-ray diffraction (XRD) technique and scanning electron microscope (SEM). The XRD pattern of the prepared sample confirmed the formation of a pure hexagonal phase of YMnO3. The energy-dispersive X-ray analysis (EDX) results proved the elemental composition of pure yttrium, manganese, and oxygen. The dielectric properties of the YMnO3 nanocrystals were studied using a precision LCR meter at different temperature range 28 °C–500 °C. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Recent Advances in Materials, Manufacturing & Energy Systems.
1. Introduction The rare-earth manganite, yttrium manganese oxide (YMnO3) is a multiferroic material possessing ferroelectricity and magnetism properties simultaneously, which attracted much research interest from the past two decades. YMnO3 exists in three phases; such as amorphous, hexagonal phase and an orthorhombic phase whose chemical compositions are identical [1]. Hexagonal YMnO3 is one of the most intensively studied materials, with space group P63cm, having a high ferroelectric transition temperature (TC 900 K) and a low anti-ferromagnetic transition temperature (TN 70 K) [2–5]. Orthorhombic YMnO3, with space group Pbnm/ Pnma, exhibits ferroelectric transition temperature (TC = 30 K) and a low anti-ferromagnetic transition temperature (TN = 42 K) [6,7]. The transition of YMnO3 from hexagonal phase to orthorhombic phase occurs by changing the ionic radius of Y ion in perovskite structure and by high-pressure synthesis [1,6]. YMnO3 is a pyroelectric material which gives electric polarization response with temperature [8–10]. YMnO3 does not have any volatile elements such as Bi and Pb and has a single spontaneous polarization axis [11]. Due to these absolute properties of hexagonal YMnO3, it became a promising candidate in many applications such as information storage process, multi-state memories [2], ⇑ Corresponding author. E-mail address: [email protected] (O. Nirmala).
non-volatile ferroelectric random access memories, ferroelectricgate field-effect transistors [11–13], magneto electric sensors [2,12,13] and as H2S gas sensor [14]. From the past two decades, several articles were reported on the synthesis of hexagonal YMnO3 ceramics and their characterizations. In those studies, solid-state synthesis [5,15,16] and various chemical methods like sol-gel synthesis route[2,4,14,17], polyacrylamide gel route [11,12] were used to synthesize hexagonal YMnO3 ceramics. And several studies concentrated on magnetic properties [2,6,13,17,18], optical properties [11,12] and dielectric properties [3,4,16,17]. However, the study of dielectric properties was still inadequate. In the present work, a sol-gel citrate route, followed by grinding and calcination was used to prepare multiferroic hexagonal YMnO3 nanocrystals and their structural, morphological characteristics were studied. In addition to that dielectric properties also studied in wide temperature range 28 °C–500 °C and frequency range 50 Hz–5 MHz.
2. Experimental work The perovskite hexagonal YMnO3 sample was prepared by sol–gel citrate process. The raw materials are yttrium oxide Y2O3 (99.9%) (0.02 mol), manganese acetate tetra hydrate Mn(CH3COO)2 (0.02 mol), citric acid monohydrate C6H8O7H2O (0.04 mol) and
https://doi.org/10.1016/j.matpr.2019.05.391 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Recent Advances in Materials, Manufacturing & Energy Systems.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
2
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
nitric acid HNO3 (0.02 mol). Stoichiometric amounts of above mentioned raw materials were calculated by using the formula in Eq. (1).
Weight ¼ No:of moles molecular weight
V 1000
characteristics were evaluated for the prepared hexagonal YMnO3 pellet with Precision LCR meter (Wayne-Kerr electronics-6400B, high precision impedance analyzer) in the wide temperature range 28 °C500 °C and frequency range 50 Hz–5 MHz.
ð1Þ
where V is the volume of de-ionized water taken to prepare the solution. The schematic diagram of the synthesis process was shown in Fig. 1. Calculated amounts of raw materials were carefully weighed and mixed in a ‘V’ amount of de-ionized water. This total mixture was stirred for two hours at room temperature to get the clear solution. Then ammonium hydroxide (about 25%) was added to this mixture for adjusting its pH value up to 7, which was required to accomplish complex reaction between metal ions and citric acid. After reaching required pH value, 0.04 mol of ethylene glycol (CH2OH)2 (98%), which acts as a polymerization agent and a catalytic amount of C-TAB, which acts as structure directing agent were added to the mixture. After that, the final mixture was again stirred at room temperature for half an hour. Next, the mixture was placed on a hot plate, heated along with stirring until the mixture was boiled, evaporated and converted into a black powder (precursor). The obtained powder was cooled to room temperature and then grounded for ½ hour in a mortar. Next, the grounded powder was subjected to calcination at 700 °C for 2 h in a muffle furnace to get required hexagonal YMnO3 nanocrystals. The structural characteristics of as-synthesized precursor powder and the calcined YMnO3 nanocrystals were studied by an X-ray powder Diffraction (Rigaku, Miniflex 300/600, Cu Ka radiation). SEM with EDX has been used to observe the morphological characteristics and elemental presence in the prepared YMnO3 nanocrystals sample using SEM-Carl Zeiss (EVOMA15 model) with EDSOxford Instruments (Inca PENTAFET X3). The YMnO3 nanocrystals were uni-axially pressed to make a pellet (13 mm diameter) at pressure 75 Kg/cm2 in a hydraulic press and then the pallet was heated at 200 °C for one hour. The dielectric and dissipation
3. Results and discussion Fig. 2 shows the X-ray diffraction results of as-synthesized precursor powder and the powder calcined at 700 °C using an X-ray
Fig. 2. XRD patterns of as-synthesized precursor powder and the powder calcined at 700 °C.
Fig. 1. Synthesis process to get hexagonal YMnO3 nanocrystals.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
diffractometer with scan range 10°–80° (2h). The strong intensity XRD peaks reveal that the as-synthesized precursor particles have better crystallinity even before calcination. The most of the XRD peaks of YMnO3 nanocrystals (after calcination) are indexed with hexagonal YMnO3 with space group: P63cm, JCPDS 25-1079, which proves the formation of pure phase hexagonal YMnO3 particles. The disappearance of small peaks (impurity peaks) in the XRD pattern of particles calcined at 700 °C at 21.32 and 23.98 ‘2h’ angles, indicates the crystallinity reaction on calcination. The grain size was calculated by using Scherrer’s formula in Eq. (2) [2,12].
D¼
Kk b cos h
ð2Þ
where, D is the mean size of the crystal, in nm, K is the crystallite shape factor which is constant and equal to 0.89, k is the X-rays wavelength, 0.15406 nm from JCPDS 25-1079, b is the full width half maximum (FWHM) at predominant peak and h is the Bragg’s angle at which predominant peak occurs. The average grain size of YMnO3 nanocrystals was calculated as 40 nm. From XRD data results, the lattice parameters of YMnO3 nanocrystals are a = b = 6.120 °A and c = 11.331 °A. The standard lattice parameter values of YMnO3 particles are a = b = 6.136 °A and c = 11.4 °A (from JCPDS 25-1079) which are closer to the obtained results. Through SEM, the surface morphology of the synthesized YMnO3 nanocrystals was studied and shown in Fig. 3. The SEM observation unveils that YMnO3 nanocrystals are varying in size from small to medium; some crystals are joined with other crystals as clusters and leaving some space with other clusters of crystals. It
Fig. 3. SEM image of as-synthesized powder calcined at 700 °C.
3
also shows that crystal size was compatible with that size calculated from XRD analysis. The EDX result of synthesized YMnO3 nanocrystals was shown in Fig. 4. Each element weight % and atomic weight % were also shown in the Fig. 4 itself. No other elements were detected, other than yttrium, manganese, and oxygen. The dielectric properties of synthesized YMnO3 nanocrystals were measured using LCR bridge by preparing the pellet from synthesized YMnO3 nanocrystals. The precision LCR meter gives capacitance (C), and dissipation factor (tan d) values. The Dielectric constant (e) of a dielectric material can be defined as the ratio of the capacitance using the material as the dielectric in a capacitor to the capacitance of vacuum. The Dielectric constant (e) was determined by using the following Eqs. (3) and (4) [19].
e¼
C C0
ð3Þ
and
C0 ¼
eo A t
ð4Þ
where ‘C’ is the capacitance using the prepared pallet (YMnO3 nanocrystals) as dielectric, ‘C0’ is capacitance using vacuum as dielectric, ‘e0’ is the permittivity of free space (8.85 1012 F/m), ‘A’ is the area of the pellet and ‘t’ is the thickness of the pellet. The dissipation factor or dielectric loss is the loss of electrical energy in the form of heat, in the presence applied ac field in the polarization process. The temperature dependent dielectric constant (e) and dissipation factor (tan d) of YMnO3 nanocrystals at different frequencies were shown in Fig. 5(a), and (b). In Fig. 5(a) at 50 Hz frequency (normal condition) the dielectric anomaly was observed at 350 °C which is described as rising and fall of dielectric constant and it was accompanied by the anomaly in the dielectric loss at the same temperature at 50 Hz frequency shown in Fig. 5 (b). After this anomaly, the dielectric constant (e) was increased continuously and reached a maximum value and the corresponding rise was observed in dielectric loss also. An apparent dielectric constant relaxation was noticed in Fig. 5(a) and a corresponding relaxation in the dielectric loss in Fig. 5(b) in the whole temperature range at all frequencies other than 50 Hz, 500 Hz, and 5 kHz. At the temperature above 450 °C the dielectric constant was increased quickly and correlate with the rapid increase in dissipation factor at the higher temperature at frequencies of 50 Hz, 500 Hz, and 5 kHz only. Fig. 6(a) and (b) shows the logarithmic frequency dependent dielectric constant (e) of YMnO3 nanocrystals at temperature range 28 °C–500 °C. In Fig. 5(a) at 500 Hz frequency dielectric constant value increases up to 200 °C temperature and then goes to negative value from 250 °C to 450 °C temperature and corresponding
Fig. 4. EDS results of as-synthesized powder calcined at 700 °C.
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
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Fig. 5. The temperature dependent dielectric constant ‘e’ (a), and dissipation factor (tan d) (b) of YMnO3 nanocrystals at different frequencies.
Fig. 6. The frequency dependent dielectric constant ‘e’ from temperature 28 °C to 250 °C (a), and from temperature 300 °C to 500 °C (b) of YMnO3 nanocrystals.
negative value of dielectric constant was clearly visible in Fig. 6(a) and (b) from temperature range 250 °C–450 °C. These negative dielectric constant values are unexpected, not observed in previous studies and a novel negative dielectric constant material is an essential key for preparing metamaterials, or artificial negative index materials (NIMS) [20]. The logarithmic frequency dependent dissipation factor (tan d) of YMnO3 nanocrystals at temperature range 28 °C–500 °C were shown in Fig. 7(a) and (b). At lower temperature from 28 °C to 150 °C no change in dielectric constant was perceived in the entire frequency range in Fig. 6(a), but a corresponding rise and gradual decrease of dissipation factor were observed in Fig. 7(a). At 200 °C a relaxation is noticed as a gradual decrease in dielectric constant in Fig. 6(a) and correlates with a peak and then relaxation in dissipation factor in Fig. 7(a). At 500 °C temperature, the dielectric constant reached a maximum value at the lower frequency and fall down to a lower value at the higher frequency which was shown in Fig. 6(b). In Fig. 7(a) and (b) from temperature 250 °C–500 °C, the dissipation factor
values gradually increased and also fluctuating up and down from lower to medium frequency range and then fall down at the higher frequency. From the above dielectric characteristics of YMnO3 nanocrystals, the dielectric constant decreases to the least value with the increase in frequency, which is the characteristic of the dielectric material. The permanent dipoles align themselves along the direction of the field at lower frequency region and cause the total polarization of the dielectric material. And at the higher frequency the dipoles can’t follow the field due to the too rapid variation of the field, so dipoles do not contribute total polarization and hence it becomes negligible.
4. Conclusion Hexagonal YMnO3 nanocrystals were synthesized by citrate solgel route and the structural morphological characterization proved
Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391
O. Nirmala et al. / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 7. The frequency dependent dissipation factor (tan d) from temperature 28 °C to 250 °C (a), and from temperature 300 °C to 500 °C (b) of YMnO3 nanocrystals.
the formation of hexagonal phase YMnO3 nanocrystals with grain size 40 nm. This synthesis process was able to produce h-YMnO3 nanocrystals at lower calcination temperature 700 °C. Dielectric anomalies are observed at higher temperatures and at 50 Hz, 500 Hz, and 5 kHz only. A dielectric relaxation, as no change in its value, was identified throughout the temperature range at all other frequencies. And an unexpected negative dielectric constant was identified at 500 Hz frequency between temperatures 250 °C– 450 °C which can be used for developing artificial negative index materials (NIMS). Acknowledgments The present work was financially supported by the Technical Education Quality Improvement Program (TEQIP)-1.2. The authors are thankful to Dr.Y. VenkataSubbaiah for his help in conducting XRD analysis. The authors are also thankful to Osmania University for extending facility of LCR meter for dielectric study. References [1] I. Iliescu, M. Boudard, L. Rapenne, O. Chaix-Pluchery, H. Roussel, MOCVD selective growth of orthorhombic or hexagonal YMnO3 phase on Si (1 0 0) substrate, Appl. Surf. Sci. 306 (2014) 27–32. [2] Nagesh Kumar, Anurag Gaur, G.D. Varma, Enhanced magnetization and magnetoelectric coupling in hydrogen treated hexagonal YMnO3, J. Alloys Compounds 509 (2011) 1060–1064. [3] Yan Ma, Wu Yong Jun, Xiang Ming Chen, Ji Peng Cheng, Yi Qi Lin, In situ synthesis of multiferroic YMnO3 ceramics by SPS and their characterization, Ceram. Int. 35 (2009) 3051–3055. [4] Chao Zhang, Su. Jie, Xiaofei Wang, Fengzhen Huang, Junting Zhang, Yaoyang Liu, Liang Zhang, Kangli Min, Zhijun Wang, Lu. Xiaomei, Feng Yan, Jinsong Zhu, Study on magnetic and dielectric properties of YMnO3 ceramics, J. Alloy. Compd. 509 (2011) 7738–7741. [5] M. Tomczyk, A.M.O.R. Senos, I.M. Reaney, P.M. Vilarinho, Reduction of microcracking in YMnO3 ceramics by Ti substitution, Scr. Mater. 67 (2012) 427–430.
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Please cite this article as: O. Nirmala, P. Sreedhara Reddy and V. Diwakar Reddy, Synthesis of hexagonal YMnO3 nanocrystals, characterization and study of their dielectric properties, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.391