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Ferromagnetism and diamagnetism behaviors of MgO synthesized via thermal decomposition method
Accepted Manuscript Ferromagnetism and diamagnetism behaviors of MgO synthesized via thermal decomposition method Atchara Khamkongkaeo, Napat Mothaneeyachart, Panus Sriwattana, Thanachai Boonchuduang, Thanawat Phetrattanarangsi, Chonnakan Thongchai, Burimpak Sakkomolsri, Adulphan Pimsawat, Sujittra Daengsakul, Santi Phumying, Narong Chanlek, Pinit Kidkhunthod, Boonrat Lohwongwatana PII:
S0925-8388(17)30607-2
DOI:
10.1016/j.jallcom.2017.02.170
Reference:
JALCOM 40898
To appear in:
Journal of Alloys and Compounds
Received Date: 2 June 2016 Revised Date:
12 February 2017
Accepted Date: 16 February 2017
Please cite this article as: A. Khamkongkaeo, N. Mothaneeyachart, P. Sriwattana, T. Boonchuduang, T. Phetrattanarangsi, C. Thongchai, B. Sakkomolsri, A. Pimsawat, S. Daengsakul, S. Phumying, N. Chanlek, P. Kidkhunthod, B. Lohwongwatana, Ferromagnetism and diamagnetism behaviors of MgO synthesized via thermal decomposition method, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.02.170. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Ferromagnetism and Diamagnetism Behaviors of MgO Synthesized Via Thermal Decomposition Method Atchara Khamkongkaeo1, Napat Mothaneeyachart2, Panus Sriwattana2, Thanachai Boonchuduang1, Thanawat Phetrattanarangsi1, Chonnakan Thongchai1, Burimpak
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Sakkomolsri3, Adulphan Pimsawat3,4,5, Sujittra Daengsakul3,4,5, Santi Phumying6, Narong Chanlek7, Pinit Kidkhunthod7 and Boonrat Lohwongwatana1∗ 1
Innovative Metals Research Unit, Department of metallurgical, Faculty of Engineering, Chulalongkorn
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University, Phayathai Road, Wangmai, Patumwan, Bangkok 10330, Thailand. 2
Department of Nano-Engineering, Faculty of Engineering, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand. 4
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002, Thailand
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Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen, 40002, Thailand
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The Integrated Nanotechnology Research Center, Khon Kaen University, Khon Kaen 40002, Thailand
School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
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Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District,
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Nakhon Ratchasima 30000, Thailand.
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Corresponding author: Asst. Prof. Dr. Boonrat Lohwongwatana Tel.: +66-2-218-6939 (office); +66-1-343-8886 (mobile) e-mail: [email protected]
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ACCEPTED MANUSCRIPT ABSTRACT MgO powders were synthesized by a thermal decomposition method which was determined by differential scanning calorimetry coupled with thermogravimetric analysis (TG/DSC). The starting material, Mg(OH)2, was calcined at 400°C, 450°C and 500°C for 1
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hour each to obtain the MgO powders. Phase composition, morphology and magnetic properties at room temperature of calcined powders were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron
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spectroscopy (XPS), fourier transform infrared Spectroscopy (FTIR) and vibrating sample magnetometer (VSM), respectively. Both Mg(OH)2 and MgO phases were found in all
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calcined powders. The combination of ferromagnetism and diamagnetism at room temperature of all powders were observed. It is believed to be attributed to Mg vacancies and defect in the MgO and Mg(OH)2 structures.
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Keywords: MgO, Mg(OH)2, Magnetic properties, Mg vacancies, XPS, Rietveld analysis
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1. INTRODUCTION Magnesium oxide (MgO) is one of many advanced engineering materials that has been investigated intensively in recent years because of their potentials for many applications in bulk, powder, and thin film forms. For instance, MgO can be used as an additive in refractory
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for investment casting process in the jewelry industry [1-5]. Moreover, MgO is also used in medical and pharmaceutical products [6-8] and toxic waste remediation [9, 10]. Since MgO is a good insulator due to its wide band gap of 7.8 eV [4, 11, 12], combined with diamagnetic
structure which Mg is surrounded by six O atoms
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properties [13-17], it is also commonly used in an electronic industry. MgO takes a rock salt [12, 18, 19]. The structure and
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completeness of the structure determine the diamagnetic property of this material. However, ferromagnetism in MgO is reported in both theoretical and experimental [15, 20-26] but not with a clear explanation on the cause. Recently, room temperature ferromagnetism combined with diamagnetism can be found in MgO powder [18, 19, 26-28]. In these publications,
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several authors suggested that this behavior was attributed to the induction of magnetic moment from the 2p orbitals of the nearest O atoms surrounding the Mg vacancies but aside from the calculation simulation, no further evidence can support this claim. Our paper aims to
ferromagnetism.
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study the Mg vacancies in MgO and link their effects to the transition from diamagnetism to
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Several methods have been reported on the synthesis of MgO powder such as sol–gel [29-31], hydrothermal [5, 10, 32], wet chemical [3, 4], polymer pyrolysis [27] and coprecipitation [12]. However, most of these methods have complicated synthetic steps with high reaction temperatures and long reaction times. Starting with Mg(OH)2, the focus of our work was to produce nanocrystalline MgO powder by thermal decomposition method under different calcination temperatures. Such method allowed for repeatability and controllability of the nanocrystalline MgO powder production. Characterization techniques including TG/DSC, XRD and TEM were applied to characterize the obtained powders. Moreover,
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ACCEPTED MANUSCRIPT chemical species, bonding and existing compounds of MgO powders were analyzed by XPS to investigate the Mg-O bond on samples surface. In addition, the functional groups present in the powders were studied using the FTIR. Magnetic properties of the final products at room temperature were studied to understand the transition from diamagnetism to
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ferromagnetism. The results were discussed and compared with other reported experiment.
2. EXPERIMENTAL DETAILS
MgO powders were prepared by a simple thermal decomposition method using Mg(OH)2
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(Magnesium hydroxide, extra pure, >95.0%) commercial powder (HiMedia Laboratories Pvt. Ltd, India) as a starting material. The Mg(OH)2 powder was characterized by Differential
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scanning calorimetry (DSC) coupled with Thermogravimetry analyzer (TGA) (Netzsch STA 449 F3 Jupiter thermal analyzer, Germany). The temperature scan was from 30°C to 800°C using the heating rate of 5°C/min to monitor the thermal decomposition in air. Then the starting material was calcined at 400°C, 450°C and 500°C for 1 hour in air to obtain the MgO
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powder of different crystallinity. The calcined powders and the as-received Mg(OH)2 powder were characterized by X-ray diffractometer (PANalytical, EMPYREAN, The Netherlands). The percentage of phase content, lattice parameters and site occupancy factor were observed
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by Rietveld refinement. The morphology and selected area electron diffraction of the calcined powders were investigated by transmission electron microscopy (TEM, FEI Tecnai G2 20).
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XPS spectra were measured by using ULVAC-PHI, Japan with a standard Al Kα excitation source (1486.6 eV) with the dimension of a spot size on the sample around 500 µm x 500 µm and 10 nm deep. The C 1s peak at 284.8 eV was used as the calibration reference for all XPS spectra. The functional groups present in the samples were studied using the Fourier transform infrared Spectroscopy (FTIR, Nicolet 6700, Waltham, MA). The samples were incorporated in KBr pellets. The FTIR spectra were obtained in the 4000–400 cm−1. Magnetic properties of the final products were studied at room temperature with a vibrating sample
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3. RESULTS AND DISCUSSIONS The TG/DSC curves for Mg(OH)2 powder are shown in Fig. 1. The endothermic peak
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and the small weight loss corresponded to the removal of adsorbed moisture approaching 100°C. At 382°C the second endothermic peak and the weight loss of 28.5 % were the result of the decomposition of Mg(OH)2. Almost no weight loss could be observed at above 400°C
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suggesting that the formation of crystalline MgO was completed.
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Fig. 2 shows XRD patterns of the Mg(OH)2 commercial powder, and of the calcined powders with the following conditions: 400°C/1h, 450°C/1h and 500°C/1h. The diffraction peaks of the starting material (Fig. 2(a)) was indexed as only hexagonal structure of Mg(OH)2 (Fig. 2(a)), compatible with the ICDD No. 01-084-2163. The phase analysis of all powders
compositions.
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display both Mg(OH)2 (ICDD No. 01-078-3952) and MgO (ICDD No. 01-076-6597)
The phase formation of MgO that was calcined with 400°C/1h, 450°C/1h and 500°C/1h
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conditions was intended not to be completed. Several articles reported the formation of MgO after heat treatments. Verma et al. [4] have observed MgO powder after the Mg(OH)2
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precursor which was calcined at 500°C for 2 hours. The transformation from Mg(OH)2 to MgO completed at 1100°C for 10 hours have been reported by Jain et al. [3]. Moreover, Park et al. [33] and Sabet et al. [32] have found MgO powder after the calcinations of Mg(OH)2 at 500°C for 5 hours and 800°C for 8 hours, respectively. Shown in Table 1., the percentage of phase contents, lattice parameters and site occupancy factor were analyzed using Rietveld refinement. The MgO content of 400°C/1h, 450°C/1h and 500°C/1h were found to be at 79.3wt%, 80.1wt% and 96.9wt%, respectively. Lattice parameters of MgO content were observed to be increasing with calcination temperature. The higher calcination
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ACCEPTED MANUSCRIPT temperature resulted in higher degree of crystallinity of MgO. The result is in good agreement with other studies reported in the literatures [3-5, 32, 33]. From the Rietveld analysis, the site occupancy factor (SOF) is the value that indicates the fraction of an atomic coordinate (x,y,z) occupied by a specific atom. The analyses could lead
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to the detection of vacancies such as cation vacancies [34] and location of vacancies [35]. In this work the SOF of Mg specie in MgO were between 0.93 and 0.94. The SOF of Mg specie in Mg(OH)2 were found to be different among all the samples. The SOF of Mg in Mg(OH)2
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structure drops drastically when the calcination temperature reached 450°C. This may be attributed to the purity of starting material and the degree of crystallinity of the calcined
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powders.
The morphology and crystal structure of all powders were further studied by TEM as shown in Fig 3. From the TEM images, agglomerated cluster with a diameter less than 50 nm were observed in all experimental powders. All SAED patterns in Figs. 3(a)-3(d) indicated
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the polycrystalline nature in all powder samples. Fig 3(a) the yellow indexed planes correspond to Mg(OH)2. Both ring patterns of Mg(OH)2 (yellow) and MgO (white) were shown in Fig. 3(b) for the 400°C/1h powder. Fig. 3(c) and 3(d) showed the diffraction rings
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of MgO (white indexes) calcined at the 450°C/1h and 500 °C/1h conditions respectively. The
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results were in good agreement with XRD results. XPS was used to characterize the chemical composition of the MgO powders. Fig. 4(a)4(c) revealed the survey scans of 400°C/1h, 450°C/1h and 500°C /1h powders. The peaks corresponding to Mg 1s, Mg 2s, Mg 2p, O 1s and C 1s were clearly observed. There are no magnetic impurities in all powders. Fig. 5(a)-5(c) showed Mg 2p peaks in the XPS spectra of 400°C/1h, 450°C/1h and 500°C/1h powders, respectively. The Mg 2p peak could be deconvoluted into 2 peaks. The peak at the binding energy of 51.06-51.46 eV was attributed to MgO [36-38], while the peak at the binding energy of 49.04-49.22 eV was attributed to
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ACCEPTED MANUSCRIPT Mg(OH)2 [36, 37, 39]. For 400°C/1h, 450°C/1h and 500°C/1h powders, both of MgO and Mg(OH)2 phases were found at different ratios.This result indicates that all calcination temperatures in this work are incomplete to achieving 100% MgO conversion. The relative atomic concentration of Mg–O bond in MgO structure for 400°C/1h, 450°C/1h and 500°C/1h
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powders were 77.7%, 89.5% and 94.1%, respectively. It can also be found that 400°C/1h, 450°C/1h and 500°C/1h powders exhibit Mg-deficiency. Although the amounts of Mg–O bond were measured, the information obtained was from the surface of all samples due to the
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limitation on XPS technique, the information corresponded and complimented the XRD result and Rietveld analysis.
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FTIR spectra of the Mg(OH)2, 400°C/1h, 450°C/1h and 500°C /1h powders in the wavenumber range from 4000 cm−1 to 400 cm−1 are shown in Fig. 6. The strong adsorption bands for 400°C/1h, 450°C/1h and 500°C /1h powders around 3696 cm−1 and 3438 cm−1 are corresponding to the stretching mode of –OH vibration [10, 33, 40-43]. The presence of
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absorption peak around 1646 cm−1 confirms the bending surface modes of physisorbed water [40, 41]. These observations may be involved in (i) the reaction between all calcined powders and humidity at ambient environment during samples preparation for FTIR measurement or
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(ii) the incompleteness of powders after calcined process. However, the present of –OH vibration is in good agreement with the XRD, TEM and XPS results as described above. In
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addition, the replacement of H− impurities on oxygen vacancies of 400°C/1h, 450°C/1h and 500°C /1h powders were found at 1080 cm−1 which is corresponding to several reports [40, 41, 43]. Moreover, the bands in between 900 – 400 cm−1 are due to characteristic stretching of the Mg–O bond in MgO structure were observed [40, 43]. Fig. 7 represented the magnetic properties of all powders at room temperature as a function of applied field. M-H curves of the raw material and calcined powders with different conditions exhibited diamagnetic with small ferromagnetic behavior. The inset showed the
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ACCEPTED MANUSCRIPT ferromagnetic behavior of all powders after subtraction of diamagnetic component. The magnetization (M) at ± 2 kOe after subtraction of diamagnetic component as summarized in Table 1. The 450°C/1h sample produced the largest amplitude of M-H curve with magnetization value around 1.89 m·emu/g while the magnetization value of 400°C/1h,
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500°C/1h and Mg(OH)2 commercial powders were 1.14, 0.62 and 0.52 m·emu/g, respectively. Several reports have tried to explain the origin of ferromagnetism in MgO. It is believed that the ferromagnetism behavior is related to Mg vacancies and/or oxygen
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vacancies, crystal direction and lattice distortion in the MgO structure [14, 15, 20-26, 43-49]. Recently, Chandran et al. [50] have studied the nature and concentration of defects using
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photoluminescence and positron annihilation spectroscopy in ferromagnetic MgO. They found that the positron annihilation spectroscopy presence of only Mg vacancies that responsible for ferromagnetism in nanoparticles of MgO. Moreover, the ab initio electronic structure suggested that the origin of ferromagnetism in MgO is due to Mg vacancies, while
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all of O vacancies cannot was observed by Kuang et al. in 2014 [13]. Although some articles are reported that the H− impurities on oxygen vacancies that confirmed by FTIR spectra can be promoted ferromagnetism in MgO, Choudhury et al. [40] have found that Mg vacancy is
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initiating ferromagnetism in MgO.
The magnetic behavior of all samples in this work exhibited a combination of
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ferromagnetism and diamagnetism at room temperature. Normally, Mg(OH)2 exhibit diamagnetic property and no article reports the ferromagnetism in this material. However, the Mg(OH)2 commercial powder in this experiment has >95% purity with SOF of Mg at 0.95. This suggested that there were some Mg vacancies in Mg(OH)2 structure. Mg(OH)2 has hexagonal structure, of which Mg is surrounded by six O atoms like Mg-O in MgO structure. We believed that the ferromagnetism of Mg(OH)2 was due to the induction of magnetic moment from the 2p orbitals of the nearest O atoms surrounding the Mg vacancies. This
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ACCEPTED MANUSCRIPT mechanism is similar to the origin of ferromagnetism in MgO. For the 400°C/1h, 450°C/1h and 500°C /1h powders, Mg vacancies in Mg(OH)2 and MgO structures are the main reason that induce the ferromagnetism. Although the SOF at Mg in MgO of 400°C/1h and 500°C /1h samples were the same value, the magnetization at ± 2 kOe of these samples were quite
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different. It was due to the low content of Mg(OH)2 and the different SOF at Mg in Mg(OH)2 structure. For the 450°C/1h sample, the completion of Mg(OH)2 structure was very low at 0.1 SOF with 19.9% of phase composition which led to the highest degree of ferromagnetism. In
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conclusion, the Mg vacancies and the percentages of MgO and Mg(OH)2 played an important role for magnetic properties of all calcined powders in this work.
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4. CONCLUSION
MgO nanopowders were prepared by a thermal decomposition method using Mg(OH)2 commercial powder as a starting material. The raw material was then calcined at 400 °C, 450 °C and 500 °C for 1 hour in air. Phase composition showed both of MgO and Mg(OH)2 in all
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calcined samples. TEM images showed the agglomeration of nanoparticles with size of less than 50 nm. Magnetic properties of Mg(OH)2 and calcined powders exhibited diamagnetic with small ferromagnetic behavior. The Mg vacancies, and the ratios of MgO and Mg(OH)2
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compositions and degree of crystallinity caused a combination of ferromagnetism and
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diamagnetism at room temperature.
ACKNOWLEDGMENTS The authors would like to thank The Center for Scientific and Technological Equipment, Suranaree University of Technology, Synchrotron Light Research Institute (Public Organization, SUT-NANOTEC-SLRI XPS, Nakhon Ratchasima, Thailand, National Electronics and Computer Technology Center (NECTEC) Thailand for providing for providing TEM facility, XPS facility, photoluminescence measurement facility. We thank the Department of Physics, Khon Kaen University for providing the VSM and XRD facilities.
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ACCEPTED MANUSCRIPT Adulphan Pimsawat and Sujittra Daengsakul would like to thank Nanotec-KKU Excellence Center on Advanced Nanomaterials for Energy Production and Storage and the Integrated Nanotechnology Research Center, Khon Kaen University, Thailand. This research is supported by Rachadapisek Sompote Fund for Postdoctoral Fellowship, Chulalongkorn
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University.
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List of Figure Captions Fig. 1 TG/DSC measurement of Mg(OH)2. The red curve corresponds to the weight change (TG) and the blue curve corresponds to the enthalpic change (DSC – exothermic down).
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Fig. 2 X-ray diffraction patterns of (a) Mg(OH)2 commercial (b) 400°C/1h, (c) 450°C/1h and (d) 500°C/1h powders.
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Fig. 3 TEM images with corresponding SAED patterns of the (a) Mg(OH)2 commercial (b) 400°C/1h, (c) 450°C/1h and (d) 500 °C/1h powders. The yellow indexed planes are from Mg(OH)2 while the white indexed planes are from MgO.
Fig. 4 XPS survey scan of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders.
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Fig. 5 Mg 2p XPS data of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders. Fig. 6 FTIR spectra of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders. Fig. 7 M-H curves at room temperature for (a) Mg(OH)2 commercial (b) 400°C/1h, (c)
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450°C/1h and (d) 500°C/1h powders. Inset shows the M-H curves after subtraction of
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diamagnetic component.
List of Table
Table 1 Summary of percentage of phase composition, lattice parameters, site occupancy factor and magnetization (M) at room temperature of all samples.
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 7
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Rp (%)
Rwp (%)
Rexp (%)
Goodness of fit ( 2)
Atom
Site
Site occupancy factor (S.O.F)
M at ± 2 kOe (m·emu/g)
Mg(OH)2 Commercial 100%
a = 3.1474 c = 4.7762
5.71
8.80
4.51
1.95
Mg1 O1 H1
1a 2d 2d
0.95 1.00 0.70
0.52
1.05
Mg O Mg O H
4a 4b 1a 2d 6i
0.93 1.00 1.00 0.98 0.53
1.14
1.05
Mg O Mg O H
4a 4b 1a 2d 6i
0.94 1.00 0.10 0.18 1.00
1.89
4a 4b 1a 2d 6i
0.93 0.99 0.32 0.79 1.00
0.62
500°C /1h MgO 96.9% Mg(OH)2 3.1%
4.69
a = 4.2295 a = 3.1480 c = 4.7790
3.81
5.00
a = 4.2307 a = 3.1480 c = 4.7790
3.74
4.94
4.77
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4.93
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Mg(OH)2 19.9%
3.74
4.72
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450°C /1h MgO 80.1%
a = 3.1465 c = 4.7724
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Mg(OH)2 20.7%
a = 4.2286
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400°C /1h MgO 79.3%
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Sample
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Mg O Mg O H
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Highlights - Series of thermal decomposition of Mg(OH)2 experiments were carried out. - Different levels of crystallinity/vacancy of MgO were systematically achieved.
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- Root causes of ferromagnetism and diamagnetism were investigated.
- Rietveld analysis and site occupancy factor were used to determine the vacancies.
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- Additional FTIR and PL experiments were performed.
S0925-8388(17)30607-2
DOI:
10.1016/j.jallcom.2017.02.170
Reference:
JALCOM 40898
To appear in:
Journal of Alloys and Compounds
Received Date: 2 June 2016 Revised Date:
12 February 2017
Accepted Date: 16 February 2017
Please cite this article as: A. Khamkongkaeo, N. Mothaneeyachart, P. Sriwattana, T. Boonchuduang, T. Phetrattanarangsi, C. Thongchai, B. Sakkomolsri, A. Pimsawat, S. Daengsakul, S. Phumying, N. Chanlek, P. Kidkhunthod, B. Lohwongwatana, Ferromagnetism and diamagnetism behaviors of MgO synthesized via thermal decomposition method, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.02.170. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Ferromagnetism and Diamagnetism Behaviors of MgO Synthesized Via Thermal Decomposition Method Atchara Khamkongkaeo1, Napat Mothaneeyachart2, Panus Sriwattana2, Thanachai Boonchuduang1, Thanawat Phetrattanarangsi1, Chonnakan Thongchai1, Burimpak
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Sakkomolsri3, Adulphan Pimsawat3,4,5, Sujittra Daengsakul3,4,5, Santi Phumying6, Narong Chanlek7, Pinit Kidkhunthod7 and Boonrat Lohwongwatana1∗ 1
Innovative Metals Research Unit, Department of metallurgical, Faculty of Engineering, Chulalongkorn
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University, Phayathai Road, Wangmai, Patumwan, Bangkok 10330, Thailand. 2
Department of Nano-Engineering, Faculty of Engineering, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand. 4
Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen, 40002, Thailand
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Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen, 40002, Thailand
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The Integrated Nanotechnology Research Center, Khon Kaen University, Khon Kaen 40002, Thailand
School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
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Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District,
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Nakhon Ratchasima 30000, Thailand.
∗
Corresponding author: Asst. Prof. Dr. Boonrat Lohwongwatana Tel.: +66-2-218-6939 (office); +66-1-343-8886 (mobile) e-mail: [email protected]
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ACCEPTED MANUSCRIPT ABSTRACT MgO powders were synthesized by a thermal decomposition method which was determined by differential scanning calorimetry coupled with thermogravimetric analysis (TG/DSC). The starting material, Mg(OH)2, was calcined at 400°C, 450°C and 500°C for 1
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hour each to obtain the MgO powders. Phase composition, morphology and magnetic properties at room temperature of calcined powders were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron
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spectroscopy (XPS), fourier transform infrared Spectroscopy (FTIR) and vibrating sample magnetometer (VSM), respectively. Both Mg(OH)2 and MgO phases were found in all
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calcined powders. The combination of ferromagnetism and diamagnetism at room temperature of all powders were observed. It is believed to be attributed to Mg vacancies and defect in the MgO and Mg(OH)2 structures.
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Keywords: MgO, Mg(OH)2, Magnetic properties, Mg vacancies, XPS, Rietveld analysis
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1. INTRODUCTION Magnesium oxide (MgO) is one of many advanced engineering materials that has been investigated intensively in recent years because of their potentials for many applications in bulk, powder, and thin film forms. For instance, MgO can be used as an additive in refractory
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for investment casting process in the jewelry industry [1-5]. Moreover, MgO is also used in medical and pharmaceutical products [6-8] and toxic waste remediation [9, 10]. Since MgO is a good insulator due to its wide band gap of 7.8 eV [4, 11, 12], combined with diamagnetic
structure which Mg is surrounded by six O atoms
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properties [13-17], it is also commonly used in an electronic industry. MgO takes a rock salt [12, 18, 19]. The structure and
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completeness of the structure determine the diamagnetic property of this material. However, ferromagnetism in MgO is reported in both theoretical and experimental [15, 20-26] but not with a clear explanation on the cause. Recently, room temperature ferromagnetism combined with diamagnetism can be found in MgO powder [18, 19, 26-28]. In these publications,
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several authors suggested that this behavior was attributed to the induction of magnetic moment from the 2p orbitals of the nearest O atoms surrounding the Mg vacancies but aside from the calculation simulation, no further evidence can support this claim. Our paper aims to
ferromagnetism.
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study the Mg vacancies in MgO and link their effects to the transition from diamagnetism to
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Several methods have been reported on the synthesis of MgO powder such as sol–gel [29-31], hydrothermal [5, 10, 32], wet chemical [3, 4], polymer pyrolysis [27] and coprecipitation [12]. However, most of these methods have complicated synthetic steps with high reaction temperatures and long reaction times. Starting with Mg(OH)2, the focus of our work was to produce nanocrystalline MgO powder by thermal decomposition method under different calcination temperatures. Such method allowed for repeatability and controllability of the nanocrystalline MgO powder production. Characterization techniques including TG/DSC, XRD and TEM were applied to characterize the obtained powders. Moreover,
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ACCEPTED MANUSCRIPT chemical species, bonding and existing compounds of MgO powders were analyzed by XPS to investigate the Mg-O bond on samples surface. In addition, the functional groups present in the powders were studied using the FTIR. Magnetic properties of the final products at room temperature were studied to understand the transition from diamagnetism to
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ferromagnetism. The results were discussed and compared with other reported experiment.
2. EXPERIMENTAL DETAILS
MgO powders were prepared by a simple thermal decomposition method using Mg(OH)2
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(Magnesium hydroxide, extra pure, >95.0%) commercial powder (HiMedia Laboratories Pvt. Ltd, India) as a starting material. The Mg(OH)2 powder was characterized by Differential
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scanning calorimetry (DSC) coupled with Thermogravimetry analyzer (TGA) (Netzsch STA 449 F3 Jupiter thermal analyzer, Germany). The temperature scan was from 30°C to 800°C using the heating rate of 5°C/min to monitor the thermal decomposition in air. Then the starting material was calcined at 400°C, 450°C and 500°C for 1 hour in air to obtain the MgO
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powder of different crystallinity. The calcined powders and the as-received Mg(OH)2 powder were characterized by X-ray diffractometer (PANalytical, EMPYREAN, The Netherlands). The percentage of phase content, lattice parameters and site occupancy factor were observed
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by Rietveld refinement. The morphology and selected area electron diffraction of the calcined powders were investigated by transmission electron microscopy (TEM, FEI Tecnai G2 20).
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XPS spectra were measured by using ULVAC-PHI, Japan with a standard Al Kα excitation source (1486.6 eV) with the dimension of a spot size on the sample around 500 µm x 500 µm and 10 nm deep. The C 1s peak at 284.8 eV was used as the calibration reference for all XPS spectra. The functional groups present in the samples were studied using the Fourier transform infrared Spectroscopy (FTIR, Nicolet 6700, Waltham, MA). The samples were incorporated in KBr pellets. The FTIR spectra were obtained in the 4000–400 cm−1. Magnetic properties of the final products were studied at room temperature with a vibrating sample
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3. RESULTS AND DISCUSSIONS The TG/DSC curves for Mg(OH)2 powder are shown in Fig. 1. The endothermic peak
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and the small weight loss corresponded to the removal of adsorbed moisture approaching 100°C. At 382°C the second endothermic peak and the weight loss of 28.5 % were the result of the decomposition of Mg(OH)2. Almost no weight loss could be observed at above 400°C
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suggesting that the formation of crystalline MgO was completed.
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Fig. 2 shows XRD patterns of the Mg(OH)2 commercial powder, and of the calcined powders with the following conditions: 400°C/1h, 450°C/1h and 500°C/1h. The diffraction peaks of the starting material (Fig. 2(a)) was indexed as only hexagonal structure of Mg(OH)2 (Fig. 2(a)), compatible with the ICDD No. 01-084-2163. The phase analysis of all powders
compositions.
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display both Mg(OH)2 (ICDD No. 01-078-3952) and MgO (ICDD No. 01-076-6597)
The phase formation of MgO that was calcined with 400°C/1h, 450°C/1h and 500°C/1h
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conditions was intended not to be completed. Several articles reported the formation of MgO after heat treatments. Verma et al. [4] have observed MgO powder after the Mg(OH)2
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precursor which was calcined at 500°C for 2 hours. The transformation from Mg(OH)2 to MgO completed at 1100°C for 10 hours have been reported by Jain et al. [3]. Moreover, Park et al. [33] and Sabet et al. [32] have found MgO powder after the calcinations of Mg(OH)2 at 500°C for 5 hours and 800°C for 8 hours, respectively. Shown in Table 1., the percentage of phase contents, lattice parameters and site occupancy factor were analyzed using Rietveld refinement. The MgO content of 400°C/1h, 450°C/1h and 500°C/1h were found to be at 79.3wt%, 80.1wt% and 96.9wt%, respectively. Lattice parameters of MgO content were observed to be increasing with calcination temperature. The higher calcination
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to the detection of vacancies such as cation vacancies [34] and location of vacancies [35]. In this work the SOF of Mg specie in MgO were between 0.93 and 0.94. The SOF of Mg specie in Mg(OH)2 were found to be different among all the samples. The SOF of Mg in Mg(OH)2
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structure drops drastically when the calcination temperature reached 450°C. This may be attributed to the purity of starting material and the degree of crystallinity of the calcined
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powders.
The morphology and crystal structure of all powders were further studied by TEM as shown in Fig 3. From the TEM images, agglomerated cluster with a diameter less than 50 nm were observed in all experimental powders. All SAED patterns in Figs. 3(a)-3(d) indicated
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the polycrystalline nature in all powder samples. Fig 3(a) the yellow indexed planes correspond to Mg(OH)2. Both ring patterns of Mg(OH)2 (yellow) and MgO (white) were shown in Fig. 3(b) for the 400°C/1h powder. Fig. 3(c) and 3(d) showed the diffraction rings
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of MgO (white indexes) calcined at the 450°C/1h and 500 °C/1h conditions respectively. The
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results were in good agreement with XRD results. XPS was used to characterize the chemical composition of the MgO powders. Fig. 4(a)4(c) revealed the survey scans of 400°C/1h, 450°C/1h and 500°C /1h powders. The peaks corresponding to Mg 1s, Mg 2s, Mg 2p, O 1s and C 1s were clearly observed. There are no magnetic impurities in all powders. Fig. 5(a)-5(c) showed Mg 2p peaks in the XPS spectra of 400°C/1h, 450°C/1h and 500°C/1h powders, respectively. The Mg 2p peak could be deconvoluted into 2 peaks. The peak at the binding energy of 51.06-51.46 eV was attributed to MgO [36-38], while the peak at the binding energy of 49.04-49.22 eV was attributed to
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powders were 77.7%, 89.5% and 94.1%, respectively. It can also be found that 400°C/1h, 450°C/1h and 500°C/1h powders exhibit Mg-deficiency. Although the amounts of Mg–O bond were measured, the information obtained was from the surface of all samples due to the
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limitation on XPS technique, the information corresponded and complimented the XRD result and Rietveld analysis.
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FTIR spectra of the Mg(OH)2, 400°C/1h, 450°C/1h and 500°C /1h powders in the wavenumber range from 4000 cm−1 to 400 cm−1 are shown in Fig. 6. The strong adsorption bands for 400°C/1h, 450°C/1h and 500°C /1h powders around 3696 cm−1 and 3438 cm−1 are corresponding to the stretching mode of –OH vibration [10, 33, 40-43]. The presence of
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absorption peak around 1646 cm−1 confirms the bending surface modes of physisorbed water [40, 41]. These observations may be involved in (i) the reaction between all calcined powders and humidity at ambient environment during samples preparation for FTIR measurement or
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(ii) the incompleteness of powders after calcined process. However, the present of –OH vibration is in good agreement with the XRD, TEM and XPS results as described above. In
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addition, the replacement of H− impurities on oxygen vacancies of 400°C/1h, 450°C/1h and 500°C /1h powders were found at 1080 cm−1 which is corresponding to several reports [40, 41, 43]. Moreover, the bands in between 900 – 400 cm−1 are due to characteristic stretching of the Mg–O bond in MgO structure were observed [40, 43]. Fig. 7 represented the magnetic properties of all powders at room temperature as a function of applied field. M-H curves of the raw material and calcined powders with different conditions exhibited diamagnetic with small ferromagnetic behavior. The inset showed the
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ACCEPTED MANUSCRIPT ferromagnetic behavior of all powders after subtraction of diamagnetic component. The magnetization (M) at ± 2 kOe after subtraction of diamagnetic component as summarized in Table 1. The 450°C/1h sample produced the largest amplitude of M-H curve with magnetization value around 1.89 m·emu/g while the magnetization value of 400°C/1h,
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500°C/1h and Mg(OH)2 commercial powders were 1.14, 0.62 and 0.52 m·emu/g, respectively. Several reports have tried to explain the origin of ferromagnetism in MgO. It is believed that the ferromagnetism behavior is related to Mg vacancies and/or oxygen
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vacancies, crystal direction and lattice distortion in the MgO structure [14, 15, 20-26, 43-49]. Recently, Chandran et al. [50] have studied the nature and concentration of defects using
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photoluminescence and positron annihilation spectroscopy in ferromagnetic MgO. They found that the positron annihilation spectroscopy presence of only Mg vacancies that responsible for ferromagnetism in nanoparticles of MgO. Moreover, the ab initio electronic structure suggested that the origin of ferromagnetism in MgO is due to Mg vacancies, while
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all of O vacancies cannot was observed by Kuang et al. in 2014 [13]. Although some articles are reported that the H− impurities on oxygen vacancies that confirmed by FTIR spectra can be promoted ferromagnetism in MgO, Choudhury et al. [40] have found that Mg vacancy is
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initiating ferromagnetism in MgO.
The magnetic behavior of all samples in this work exhibited a combination of
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ferromagnetism and diamagnetism at room temperature. Normally, Mg(OH)2 exhibit diamagnetic property and no article reports the ferromagnetism in this material. However, the Mg(OH)2 commercial powder in this experiment has >95% purity with SOF of Mg at 0.95. This suggested that there were some Mg vacancies in Mg(OH)2 structure. Mg(OH)2 has hexagonal structure, of which Mg is surrounded by six O atoms like Mg-O in MgO structure. We believed that the ferromagnetism of Mg(OH)2 was due to the induction of magnetic moment from the 2p orbitals of the nearest O atoms surrounding the Mg vacancies. This
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different. It was due to the low content of Mg(OH)2 and the different SOF at Mg in Mg(OH)2 structure. For the 450°C/1h sample, the completion of Mg(OH)2 structure was very low at 0.1 SOF with 19.9% of phase composition which led to the highest degree of ferromagnetism. In
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conclusion, the Mg vacancies and the percentages of MgO and Mg(OH)2 played an important role for magnetic properties of all calcined powders in this work.
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4. CONCLUSION
MgO nanopowders were prepared by a thermal decomposition method using Mg(OH)2 commercial powder as a starting material. The raw material was then calcined at 400 °C, 450 °C and 500 °C for 1 hour in air. Phase composition showed both of MgO and Mg(OH)2 in all
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calcined samples. TEM images showed the agglomeration of nanoparticles with size of less than 50 nm. Magnetic properties of Mg(OH)2 and calcined powders exhibited diamagnetic with small ferromagnetic behavior. The Mg vacancies, and the ratios of MgO and Mg(OH)2
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compositions and degree of crystallinity caused a combination of ferromagnetism and
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diamagnetism at room temperature.
ACKNOWLEDGMENTS The authors would like to thank The Center for Scientific and Technological Equipment, Suranaree University of Technology, Synchrotron Light Research Institute (Public Organization, SUT-NANOTEC-SLRI XPS, Nakhon Ratchasima, Thailand, National Electronics and Computer Technology Center (NECTEC) Thailand for providing for providing TEM facility, XPS facility, photoluminescence measurement facility. We thank the Department of Physics, Khon Kaen University for providing the VSM and XRD facilities.
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University.
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[48] K. Tamanoi, M. Sato, M. Oogane, Y. Ando, T. Tanaka, Y. Uehara, T. Uzumaki, J. Magn Magn Mater. 320 (2008) 2959-2962. [49] J. Hu, Z. Zhang, M. Zhao, H. Qin, M. Jiang, Appl. Phys. Lett. 93 (2008) 192503. [50] A. Chandran, J. Prakash, K.K. Naik, A.K. Srivastava, R. Dąbrowski, M. Czerwiński,
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List of Figure Captions Fig. 1 TG/DSC measurement of Mg(OH)2. The red curve corresponds to the weight change (TG) and the blue curve corresponds to the enthalpic change (DSC – exothermic down).
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Fig. 2 X-ray diffraction patterns of (a) Mg(OH)2 commercial (b) 400°C/1h, (c) 450°C/1h and (d) 500°C/1h powders.
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Fig. 3 TEM images with corresponding SAED patterns of the (a) Mg(OH)2 commercial (b) 400°C/1h, (c) 450°C/1h and (d) 500 °C/1h powders. The yellow indexed planes are from Mg(OH)2 while the white indexed planes are from MgO.
Fig. 4 XPS survey scan of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders.
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Fig. 5 Mg 2p XPS data of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders. Fig. 6 FTIR spectra of (a) 400°C/1h, (b) 450°C/1h and (c) 500°C/1h powders. Fig. 7 M-H curves at room temperature for (a) Mg(OH)2 commercial (b) 400°C/1h, (c)
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450°C/1h and (d) 500°C/1h powders. Inset shows the M-H curves after subtraction of
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List of Table
Table 1 Summary of percentage of phase composition, lattice parameters, site occupancy factor and magnetization (M) at room temperature of all samples.
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Fig. 3
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Fig. 4
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Fig. 5
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Rp (%)
Rwp (%)
Rexp (%)
Goodness of fit ( 2)
Atom
Site
Site occupancy factor (S.O.F)
M at ± 2 kOe (m·emu/g)
Mg(OH)2 Commercial 100%
a = 3.1474 c = 4.7762
5.71
8.80
4.51
1.95
Mg1 O1 H1
1a 2d 2d
0.95 1.00 0.70
0.52
1.05
Mg O Mg O H
4a 4b 1a 2d 6i
0.93 1.00 1.00 0.98 0.53
1.14
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4a 4b 1a 2d 6i
0.94 1.00 0.10 0.18 1.00
1.89
4a 4b 1a 2d 6i
0.93 0.99 0.32 0.79 1.00
0.62
500°C /1h MgO 96.9% Mg(OH)2 3.1%
4.69
a = 4.2295 a = 3.1480 c = 4.7790
3.81
5.00
a = 4.2307 a = 3.1480 c = 4.7790
3.74
4.94
4.77
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Mg(OH)2 19.9%
3.74
4.72
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450°C /1h MgO 80.1%
a = 3.1465 c = 4.7724
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Mg(OH)2 20.7%
a = 4.2286
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400°C /1h MgO 79.3%
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Highlights - Series of thermal decomposition of Mg(OH)2 experiments were carried out. - Different levels of crystallinity/vacancy of MgO were systematically achieved.
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- Root causes of ferromagnetism and diamagnetism were investigated.
- Rietveld analysis and site occupancy factor were used to determine the vacancies.
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- Additional FTIR and PL experiments were performed.