The effects of urea content on the structural, thermal and morphological properties of MgO nanopowders

Author’s Accepted Manuscript The effects of urea content on the structural, thermal and morphological properties of MgO nanopowders Ismail Ercan, Omer Kaygili, Tankut Ates, Bayram Gunduz, Niyazi Bulut, Suleyman Koytepe, Imren Ozcan www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)31194-5 https://doi.org/10.1016/j.ceramint.2018.05.068 CERI18243

To appear in: Ceramics International Received date: 17 April 2018 Revised date: 8 May 2018 Accepted date: 9 May 2018 Cite this article as: Ismail Ercan, Omer Kaygili, Tankut Ates, Bayram Gunduz, Niyazi Bulut, Suleyman Koytepe and Imren Ozcan, The effects of urea content on the structural, thermal and morphological properties of MgO nanopowders, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.05.068 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 galley proof before it is published in its final citable 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.

The effects of urea content on the structural, thermal and morphological properties of MgO nanopowders Ismail Ercan1, Omer Kaygili2, Tankut Ates2, Bayram Gunduz3, Niyazi Bulut2,*, Suleyman Koytepe4, Imren Ozcan4 1

Department of Biophysics, Institute for Research and Medical Consultations, Imam Abdulrahman Bin Faisal University, 34441, Dammam, Saudi Arabia 2 Department of Physics, Faculty of Science, Firat University, 23119 Elazig, Turkey 3 Department of Science Education, Faculty of Education, Mus Alparslan University, 49250, Mus, Turkey 4 Department of Chemistry, Faculty of Science & Arts, Inonu University, 44280 Malatya, Turkey

*

Corresponding author: Tel: +90 424 2370000 / 3695; fax: +90 424 2330062,

[email protected]

Abstract In the present study, we have looked into the effects of the amount of the combustion fuel of urea on the morphological, thermal and morphological properties of MgO structure. To this goal, three Magnesium Oxide (MgO) samples were synthesized by the combustion method and the as-mentioned properties were characterized by the experimental analysis techniques of

X-Ray Diffraction (XRD),

Differential

Thermal

Analysis

(DTA),

Thermogravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM). The functional groups in the samples were also determined using Fourier Transform Infrared (FTIR) spectrometry. The crystallinity percent values for each sample were found to be about in the range of 94-96%. The crystallite sizes of the samples were computed to be in the ranges of 26-37 nm and 23-32 nm for Scherrer and Williamson-Hall methods, respectively. The increasing quantity of urea caused a gradual growth in the lattice parameter of Magnesium Oxide structure. The thermal properties and morphology were affected by the urea content. Keywords: MgO; X-Ray Diffraction (XRD); Differential Thermal Analysis (DTA); Thermogravimetric Analysis (TGA); Combustion method

1

1. Introduction Due to its ionic property, simple stoichiometry and crystal structure, Magnesium Oxide (MgO) is a distinctive inorganic material. This material consists of Mg2+ and O2− ions. d-orbitals in this material are empty owing to the electronic configurations of these ions. Nonexistence of free electrons makes this material an excellent insulator with a band gap energy of 7.8 eV [1-3]. On account of this property, Magnesium Oxide is used in heatresistant, high temperature insulating materials and in fuel oil additives [4, 5]. In addition, Magnesium Oxide is a well-known metal oxide having numerous implementations in many fields, including, microelectronics [6], heterogeneous catalysis [7, 8], display panels [9], photonic devices [10], biotechnology [11], electrochemical cells [12], drug delivery systems [13] luminescence [14], high temperatures conductor composites [15] and fibers [16]. Generally, Magnesium Oxide is obtained by thermal decomposition of various magnesium salts [17-19]. The disadvantage of this method is that the large crystallite size with low surface area–to-volume ratio limits its applications for nanotechnology. Therefore, the formation of Magnesium Oxide nanostructures with a small crystalline size of less than 100 nm, has importance and opened its applications in many fields due to their physiochemical properties. The morphology and properties of MgO depend on the synthesis route and working terms [20]. However, the properties of MgO are enhanced when used as nano-size powder. There are a lot of methods to synthesize MgO nanostructure such as precipitation [21], solvothermal [22], chemical vapor deposition [23], electrochemical [24], sonochemical [25], microwave [26], electron spinning [27], combustion [28], template [29], and carbothermic reduction [30]. Each method itself has advantages and disadvantages. The morphology of nanoparticles should be controlled in the synthesis and preparation of nanostructured MgO.

2

In this study, Magnesium Oxide nanostructures have been successfully synthesized by conventional combustion method using urea as the fuel because it was reported as the optimal fuel for metal nitrates in solution combustion synthesis [31]. The effects of the amount of urea used in the synthesis on their structural, thermal and morphological properties have been investigated.

2. Materials and method 2.1. Synthesis The reaction used for synthesizing of magnesium oxide nanopowders can be written as 6Mg(NO3)2•6H2O(aq)+10CO(NH2)2(aq)→6MgO(s)+10CO2(g)+38H2O(g)+13N2(g). In this study the distilled water was used as the solvent. Three 50 ml of 0.4 M magnesium nitrate hexahydrate (Mg(NO3)2•6H2O, Merck) solutions were prepared in different beakers, and then 5, 10 and 15 ml of 50 ml of 0.6 M urea solution were added to these beakers, respectively. Hereafter, according to the urea content, the as-synthesized samples were referred to as MgOU5, MgO-U10 and MgO-U15. These solutions were stirred in a magnetic stirrer for 90 minutes without heating. After that, each solution was put onto a heat plate at 500 °C. The solutions were boiled and dried at this temperature for 20 minutes, and during this time the ignition and intense smoke outflow were observed. Finally, the as-dried samples were heated in an electric furnace at 1150 °C for 2 hours, and white Magnesium Oxide powders were obtained.

2.2. Characterization of the as-synthesized Magnesium Oxide powders The as-prepared Magnesium Oxide powders were characterized experimentally using the following experimental analysis methods. To make a detailed crystal structure analysis, Xray diffraction (XRD) measurements were used. The diffractometer is a Rigaku RadB-DMAX

3

II model with a step size of 0.02°. Furthermore, a Cu Kα radiation having the wavelength of 0.154056 nm were used. In order to determine the functional groups in the as-synthesized samples, Fourier transform infrared (FTIR) data in the range of 400-4000 cm-1 were recorded by a PerkinElmer Spectrum One spectrophotometer using the KBr pellet method. Differential thermal analysis (DTA, Shimadzu DTA 50) and thermogravimetric analysis (TGA, Shimadzu TGA 50) were used to investigate the thermal properties in the temperature range from room temperature to 1000 °C at a heating rate of 10 °C min−1 for ~1.5 h. A LEO EVO 40xVP model scanning electron microscope operated at an acceleration voltage of 20 kV, equipped with a Röntech xflash energy dispersive X–ray (EDX) detector was used for investigating of the morphology and elemental composition of the as-synthesized powders.

3. Results and discussion 3.1. X-ray diffraction analysis By analyzing each X-ray diffraction pattern, belonging to the as-prepared Magnesium Oxide samples, illustrated in Fig. 1, following results have been achieved. All the patterns are in a very good harmony with the standard data for Magnesium Oxide (JCPDS pdf no: 870652), which is known as the periclase phase and has the cubic crystal structure. For each sample, Magnesium Oxide is the single phase, and its purity is 100%. The observed and calculated values belonging to the interplanar distance (d) between two adjacent parallel planes of the lattice and 2θ angles for all the as-observed Miller indices (e.g., (111), (200), (220), (311) and (222)) are tabulated in Table 1 for making a comparison between both values. The lattice parameter (a) and unit cell volume (V) were estimated using the following relations, respectively [32]:

a  d h2  k 2  l 2

(1)

4

V  a3

(2)

The calculation of the crystallinity percent (XC%) can be found elsewhere [33]. As can be seen from the results given in Table 2, experimentally, the value of the lattice parameter of MgO increases by stages with increasing amount of urea. Depending on the increase in the lattice parameter, the unit cell volume increases as a matter of course. The crystallinity percent values of all the samples are higher than that of 94%, that is, each sample has a very high crystallinity. The Scherrer and Williamson-Hall equations were used to estimate the crystallite size values (DS and DWH) for each sample, respectively [32, 34]

DS 

0.9  cos 

 cos  

(3)

0.9  4 sin  DWH

(4)

where β is the full width at half maximum (FWHM), λ is the X-ray wavelength and ε is the lattice strain. The crystallite size values calculated from Scherrer and Williamson-Hall methods are quite close to each other for all the samples. According to Eq. (4), the slope of the  cos  vs. 4 sin  graph shown in Fig. 2 gives the ε value. The lattice strain value is found positive for the MgO-U5 sample, which has the lower urea content, whereas for the-rest two, this value is negative. Positive and negative values mean that tensile and compressive forces influence on the crystal structure, respectively [35, 36].

3.2. Fourier Transform Infrared Spectroscopy analysis The Fourier Transform Infrared Spectroscopy spectra of Magnesium Oxide samples synthesized via the combustion method using urea as the fuel at various amounts are shown in Fig. 3. The weak band at 866 cm–1 is associated to the vibrational mode of metal-oxygen bond of MgO [37, 38]. The band at 1058 cm–1 is related to the stretching mode of C-O bond [39].

5

The carbonate group related band is observed at 1409 cm–1, and as known the chemisorption of the atmospheric carbon dioxide causes the formation of this group [40, 41] The bands detected at 2902 and 2977 cm–1 are due to C-H stretching modes [42]. The band observed at 3676 cm–1 is due to the hydroxyl groups at the low-coordination sites or defects [43]. The observation of the bands related to H2O and CO2 groups in the Fourier Transform Infrared Spectrometry can be explained as follows: Magnesium Oxide chemisorbs these molecules once it is exposed to the atmosphere, as known [44, 45].

3.3. Thermal analysis results The Differential Thermal Analysis and Thermogravimetric Analysis curves, shown in Fig. 4 and Fig. 5 respectively, help for investigating of the thermal behaviors of the asprepared Magnesium Oxide powders. An endothermic peak, which is associated with the decomposition of the carbonate caused the release of CO2, is detected at 771, 789 and 809 °C for MgO-U5, MgO-U10 and MgO-U15, respectively [46, 47]. This peak temperature increases with increasing amount of the urea. Additionally, a second endothermic peak is also detected only for MgO-U5. When analyzing the TGA curves illustrated in Fig. 5, it is seen that a mass gain for each sample observed and these detected increases are reached at the maximum values of 2.25% at 836 °C, 2.04% at 870 °C and 1.97% at 994 °C for MgO-U5, MgO-U10 and MgO-U15, respectively. The location of this peak shifts to the right and the mass gain decreases with increasing amount of urea. These mass gains can be associated with the oxidation processes [48]. It is seen that the amount of the fuel (or urea) significantly affects the thermal behavior of Magnesium Oxide.

6

3.4. Morphological investigations Scanning electron microscope images and EDX spectra for all the samples are illustrated in Fig. 6. As can be seen from this figure, the morphologies of MgO-U10 and MgO-U15 samples are almost similar to each other, whereas the morphology of MgO-U5 differs from both samples. For all the samples, the stoichiometric ratio of Mg to O has almost been retained for each region where EDX analysis performed. Addition to, no impurity has been detected in all the as-prepared samples.

4. Conclusions Three Magnesium Oxide samples, with high crystallinity and without any impurity, were produced via the combustion method. The crystallite sizes of the samples were calculated to be in the range of 26-37 nm for Scherrer method and in the interval of 23-32 nm for Williamson-Hall method. The lattice parameter dramatically increased with increasing urea content. While the lattice strain was found to be a positive value for MgO-U5 sample having the lower urea content, this was computed to be a negative value for the samples with the higher urea content. Positive and negative values imply that tensile and compressive forces influence on the crystal structure of Magnesium Oxide. The effects of the amount of urea on the thermal properties of Magnesium Oxide were observed. The morphology was affected by the amount of urea. The stoichiometry of Mg: O ratio was estimated to be 1 for each sample. To summarize, the amount of the combustion fuel has caused some notable changes in the structural, thermal and morphological properties of Magnesium Oxide structure and can be used for controlling of these properties.

7

Acknowledgments This work was supported by Management Unit of Scientific Research projects of Firat University (FÜBAP) (Project Number: FF.15.03).

References [1] S.H. Tamboli, R. Patil, S. Kamat, V. Puri, R. Puri, Modification of optical properties of MgO thin films by vapour chopping, J. Alloys Compd. 477 (2009) 855–859. [2] A. Lempicki, The electrical conductivity of MgO single crystals at high temperatures, Proc. Phys. Soc. B 66 (1953) 281–283. [3] I.O. Wilson, Magnesium oxide as a high-temperature insulant, IEE Proc. 128 (1981) 159164. [4] W. Zhang, H.L. Tay, S.S. Lim, Y. Wang, Z. Zhong, R. Xu, Supported cobalt oxide on MgO: Highly efficient catalysts for degradation of organic dyes in dilute solutions, Appl. Cat. B 95 (2010) 93–99. [5] A. Poullikkas, Cost-benefit analysis for the use of additives in heavy fuel oil fired boilers, Energy Convers. Manage. 45 (2004) 1725-1734. [6] J. Zhang, L. Zhang, Intensive green light emission from MgO nanobelts, Chem. Phys. Lett. 363 (2002) 293-297. [7] T. Selvamani, A. Sinhamahapatra, D. Bhattacharjya, I. Mukhopadhyay, Rectangular MgO microsheets with strong catalytic activity, Mater. Chem. Phys. 129 (2011) 853-861. [8] L. Shang, B.J. Li, W.J. Dong, B.Y. Chen, C.R. Li, W.H. Tang, Hetero nanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, J. Hazard Mater. 178 (2010) 1109-1114.

8

[9] C.H. Jung, J.Y. Lee, D.H. Yoon, Characterization of the MgO nano powder synthesized by using a liquid phase precursor method for plasma display panels protecting layer, J. Nanosci. Nanotechnol. 12 (2012), 1700-1704. [10] X.L. Xu, S.A. Asher, Synthesis and utilization of monodisperse hollow polymeric particles in photonic crystals, J. Am. Chem. Soc. 126 (2004) 7940–7945. [11] H.X. Zhao, W.J. Dong, Y.Y. Zheng, A.P. Liu, J.M. Yao, C.R. Li, The structural and biological properties of hydroxyl apatite modified titanate nanowire scaffolds, Biomaterials 32 (2011) 5837–5846. [12] W.J. Dong, B.J. Li, Y. Li, X.B. Wang, L.N. An, C.R. Li, General approach to well defined perovskite MTiO3 (M ¼ Ba, Sr, Ca, and Mg) nanostructures, J. Phys. Chem. C 115 (2011) 3918–3925. [13] W.R. Zhao, H.R. Chen, Y.S. Li, L. Li, M.D. Lang, J.L. Shi, Uniform rattle type hollow magnetic mesoporous spheres as drug delivery carriers and their sustained-release property, Adv. Funct. Mater. 18 (2008) 2780–2788. [14] P.B. Devaraja, D.N. Avadhani, S.C. Prashantha, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, H.P. Nagaswarupa, H.B. Premkumar, MgO: Eu3+‫ ‏‬red nanophosphor: low temperature synthesis and photoluminescence properties, Spectrochim. Acta A 121 (2014) 46–52. [15] P. Yang, C.M. Lieber, Nanorod-superconductor composites: A pathway to high critical current density materials, Science 273 (1996) 1836-1840. [16] X. Jin, K. Yuan, X. Lin, X. Wang, G. Zhang, L. Zhu, N. Que, D. Xu, Effects of water vapor on the crystallization and microstructure manipulation of MgO ceramic fibers, Ceram. Inter. 44 (2018) 5257-5265. [17] P.J. Anderson, R.F. Horlock, Thermal decomposition of magnesium hydroxide, Trans. Faraday Soc. 58 (1962) 1993-2004.

9

[18] J. Green, Calcination of precipitated Mg(OH)2 to active MgO in the production of refractory and chemical grade MgO, J. Mater. Sci. 18 (1983) 637-651. [19] M.G. Kim, U. Dahmen, A.W. Searcy, Structural transformations in the decomposition of Mg(OH)2 and MgCO3, J. Am. Ceram. Soc. 70 (1987) 146-154. [20] A. Kumar, J. Kumar, On the synthesis and optical absorption studies of nano-size magnesium oxide powder, J. Phys. Chem. Solids 69 (2008) 2764-2772. [21] S. Veldurthi, C.H. Shin, O.S. Joo, K.D. Jung, Synthesis of mesoporous MgO single crystals without templates, Microporous Mesoporous Mater. 152 (2012) 31-36. [22] Z. Zhao, H. Dai, Y. Du, J. Deng, L. Zhang, F. Shi, Solvo – or hydrothermal fabrication and excellent carbon dioxide adsorption behaviors of magnesium oxide with multiple morphologies and porous structures, Mater. Chem. Phys. 128 (2011) 348-356. [23] H. Li, M. Li, X. Wang, X. Wu, F. Liu, B. Yang, Synthesis and optical properties of single –crystal MgO nanobelts, Mater. Lett. 102-103 (2013) 80-82. [24] R. Hahn, J.G. Brunner, J. Kunze, P. Schmuki, S. Virtanen, A novel approach for the formation of Mg(OH)2/MgO nanowhiskers on magnesium: rapid anodization in chloride containing solutions, Electrochem. Commun. 10 (2008) 288-292. [25] M.A. Alavi, A. Morsali, Synthesis and characterization of Mg(OH)2 and MgO nanostructures by ultrasonic method, Ultrason. Sonochem. 17 (2010) 441-446. [26] R. Al-Gaashani, S. Radiman, Y. Al-Douri, N. Tabet, A.R. Daud AR, Investigation of the optical properties of Mg(OH)2 and MgO nanostructures obtained by microwave-assisted methods, J. Alloys Compd. 521 (2012) 71-76. [27] R. Murugan, K. Ramamoorthy, S. Sundarrajan, S. Ramakrishna, Magnesium oxide nanotubes: synthesis, characterization and application as efficient recyclable catalyst for pyrazolyl 1,4-dihydropyridine derivatives, Tetrahedron 68 (2012) 7196-7201.

10

[28] N.C.S. Selvam, R.T. Kumar, L.J. Kennedy, J.J. Vijaya, Comparative study of microwave and conventional methods for the preparation and optical properties of novel MgO-micro and nano-structures, J. Alloys Compd. 509 (2011) 9809-9815. [29] R.Q. Sun, L.B. Sun, Y. Chun, Q.H. Xu, H. Wu, Synthesizing nanocrystal-assembled mesoporous magnesium oxide using cotton fibres as exotemplate, Microporous Mesoporous Mater. 111 (2008) 314-322. [30] M. Nusheh, H. Yoozbashizadeh, M. Askari, H. Kobatake, H. Fukuyama, Mechancially activated synthesis of single crystalline MgO nanostructures, J. Alloys Compd. 506 (2010) 715-720. [31] Y. Gao, F. Meng, K. Ji, Y. Song, Z. Li, Slurry phase methanation of carbon monoxide over nanosized Ni–Al2O3 catalysts prepared by microwave-assisted solution combustion, Appl. Catal. A: Gen. 510 (2016) 74–83. [32] B.D. Cullity, Elements of X-ray Diffraction, 2nd edition, Addison–Wesley Publishing Company, Massachusetts, 1978. [33] O. Kaygili, Synthesis and characterization of Na2O–CaO–SiO2 glass–ceramic, J. Therm. Anal. Calorim. 117 (2014) 223-227. [34] O. Kaygili, S. Keser, Sol–gel synthesis and characterization of Sr/Mg, Mg/Zn and Sr/Zn co-doped hydroxyapatites, Mater. Lett. 141 (2015) 161–164. [35] H. M. Moghaddam, S. Nasirian, Dependence of activation energy and lattice strain on TiO2 nanoparticles?, Nanoscience Methods, 1 (2012) 201-212. [36] P. Ghosh, J. Oliva, E. De la Rosa, K.K. Haldar, D. Solis, A. Patra, Enhancement of upconversion emission of LaPO4:Er@Yb core−shell nanoparticles/nanorods, J. Phys. Chem. C 112 (2008) 9650–9658.

11

[37] J.P. Singh, M.J. Ji, M. Kumar, I.J. Lee, K.H. Chae, Unveiling the nature of adsorbed species onto the surface of MgO thin films during prolonged annealing, J. Alloys Compds. 748 (2018) 355-362. [38] V. Rajendran, B. Deepa, R. Mekala, Studies on structural, morphological, optical and antibacterial activity of Pure and Cu-doped MgO nanoparticles synthesized by coprecipitation method, Mater. Today-Proc. 5 (2018) 8796–8803. [39] J. Sharma, M. Sharma, S. Basu, Synthesis of mesoporous MgO nanostructures using mixed surfactants template for enhanced adsorption and antimicrobial activity, J. Environ. Chem.l Eng. 5 (2017) 3429-3438. [40] H. Niu, Q. Yang, K. Tang, Y. Xie, Large-scale synthesis of single-crystalline MgO with bone-like nanostructures, J. Nanoparticle Res. 8 (2006) 881–888. [41] W.N.R.W. Isahak, Z.A.C. Ramli, M.W.M. Hisham, M.A. Yarmo, The formation of a series of carbonates from carbon dioxide: Capturing and utilisation, Renew. Sust. Energ. Rev. 47 (2015) 93-106. [42] M. Li, N. Zhou, X. Luo, G. Zhang, Z. Xie, L. Xu, P. Liu, Macroporous MgO monoliths prepared by sol–gel process with phase separation, Ceram. Int. 42 (2016) 16368-16373. [43] A. Kumar, J. Kumar, On the synthesis and optical absorption studies of nano-size magnesium oxide powder, J. Phys. Chem. Solids 69 (2008) 2764–2772. [44] N.C.S. Selvam, R.T. Kumar, L.J. Kennedy, J.J. Vijaya, Comparative study of microwave and conventional methods for the preparation and optical properties of novel MgO-micro and nano-structures, J. Alloys Compd. 509 (2011) 9809-9815. [45] K. Mageshwari, S.S. Mali, R. Sathyamoorthy, P.S. Patil, Template-free synthesis of MgO nanoparticles for effective photocatalytic applications, Powder Technol. 249 (2013) 456-462.

12

[46] H. Dong, C. Unluer, E.H. Yang, A. Al-Tabbaa, Recovery of reactive MgO from reject brine via the addition of NaOH, Desalination 429 (2018) 88-95. [47] C. Unluer, A. Al-Tabbaa, Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements, Cem. Concr. Res. 54 (2013) 87-97. [48] M. Seifert, M.L. Leite, G. Motz, Formation of Mg-silicates by reaction of perhydropolysilazane with MgO during pyrolysis in air, J. Ceram. Soc. Jpn 124 (2016) 10031005.

13

Fig. 1. XRD patterns of the as-synthesized MgO samples having the different urea contents of 5, 10 and 15 ml Fig. 2. The  cos  vs. 4 sin  plot of the as-produced MgO samples Fig. 3. FTIR spectra of MgO samples Fig. 4. DTA thermograms for each MgO sample Fig. 5. TGA curves for each MgO powder Fig. 6. Scanning electron microscope images of the as-synthesized MgO samples taken at magnification of ×10,000

14

Intensity (a.u.)

MgO-U15

(222)

MgO-U5

(311)

(111)

(220)

(200)

MgO-U10

JCPDS (87-0652)

10

20

30

40

50

2 () Fig. 1

15

60

70

80

0.007

MgO-U15

0.006 0.005 0.004 0.003 0.007

MgO-U10

 cos

0.006 0.005 0.004 0.003 0.007

MgO-U5

0.006 0.005 0.004 0.003 1.0

1.2

1.4

1.6

1.8

4sin Fig. 2

16

2.0

2.2

2.4

2.6

2.8

100 80

MgO-U15

60

Transmittance (%)

40 20 100 80

MgO-U10

60 40 20 100

MgO-U5

80 60 40

20 4000 3600 3200 2800 2400 2000 1600 1200 -1

Wavenumber (cm ) Fig. 3

17

800

400

15

MgO-U5 MgO-U10 MgO-U15

10

DTA (V/mg)

5 0 -5 -10 -15 -20 -25 -30 0

100

200

300

400

500 

T ( C) Fig. 4

18

600

700

800

900 1000

103

MgO-U5 MgO-U10 MgO-U15

Mass (%)

102

101

100

99 0

100

200

300

400

500 

T ( C) Fig. 5

19

600

700

800

900 1000

Fig. 6

20

Table 1. The comparison of the observed and calculated values of both d and 2θ for each of the as-observed Miller indices Sample

MgO-U5

MgO-U10

MgO-U15

h

k

l

dobs (nm)

dcal (nm)

2θobs (°)

2θcal (°)

1

1

1

0.243143

0.243124

36.94

36.94

2

0

0

0.210549

0.210551

42.92

42.92

2

2

0

0.148870

0.148882

62.32

62.31

3

1

1

0.126969

0.126967

74.70

74.70

2

2

2

0.121565

0.121562

78.64

78.64

1

1

1

0.244420

0.243535

36.74

36.88

2

0

0

0.211205

0.210908

42.78

42.84

2

2

0

0.149171

0.149134

62.18

62.20

3

1

1

0.127114

0.127182

74.60

74.55

2

2

2

0.121694

0.121768

78.54

78.48

1

1

1

0.244164

0.243633

36.78

36.86

2

0

0

0.211300

0.210992

42.76

42.82

2

2

0

0.149214

0.149194

62.16

62.17

3

1

1

0.127172

0.127233

74.56

74.52

2

2

2

0.121772

0.121816

78.48

78.45

21

Table 2. The as-calculated crystal structure related parameters for each sample Sample

a (nm)

V (nm3)

DS (nm) DWH (nm)

ε

XC% 95.3

MgO-U5

0.421102 0.074673

31.17

31.51

0.27×10–4

MgO-U10

0.421815 0.075053

25.81

22.62

–3.71×10–4 94.4

MgO-U15

0.421984 0.075143

33.71

30.74

–1.85×10–4 96.3

22