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Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study
Accepted Manuscript Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study Sanghamitra Dash, Rakesh K. Sahoo, Arya Das, Shubhra Bajpai, Debidutta Debasish, Saroj K. Singh PII:
S0925-8388(17)32813-X
DOI:
10.1016/j.jallcom.2017.08.085
Reference:
JALCOM 42842
To appear in:
Journal of Alloys and Compounds
Received Date: 31 March 2017 Revised Date:
17 July 2017
Accepted Date: 10 August 2017
Please cite this article as: S. Dash, R.K. Sahoo, A. Das, S. Bajpai, D. Debasish, S.K. Singh, Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.085. 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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GRAPHICAL ABSTRACT
Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study
Singh
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Sanghamitra Dash, Rakesh K. Sahoo∗, Shubhra Bajpai, Arya Das, Debidutta Debasish and Saroj K.
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Technology, Bhubaneswar -751013, India
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Advanced Materials Technology Department, CSIR - Institute of Minerals and Materials
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Figure (a) Peak shift of the (311) peak of XRD, (b) normalized FTIR spectra in the range of 450 to 900 cm-1 and (c) Load-depth profile of samples
∗
Corresponding author. Tel.:+ 916742379456; E-mail: [email protected] (Rakesh K. Sahoo).
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Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study
Singh
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Sanghamitra Dash, Rakesh K. Sahoo∗, Arya Das, Shubhra Bajpai, Debidutta Debasish and Saroj K.
Advanced Materials Technology Department, CSIR - Institute of Minerals and Materials
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Technology, Bhubaneswar -751013, India
Abstract: Bulk quantity of the MgAl2O4 spinels with specific composition has been synthesized by thermal plasma method. The process time and compositions of the feed are optimized for a desired stoichiometry. A structural co-relation has been derived from several set of experiments by major variation in the α-Al2O3 content in the feed material. Formation of desired phase with
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particular feed stoichiometry has been studied by X-ray diffraction pattern (XRD) analysis which is corroborated by Raman scattering analysis. A good structural co-relation between the XRD, Raman and Fourier transform infrared spectroscopy (FTIR) analysis was derived. Additionally,
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the electron probe microanalysis (EPMA) of the specific sample has been carried out to study the
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exact compositional distribution and the presence of additional impurity phases in the assynthesized spinels. Finally, the micro mechanical behaviour of the as-synthesized spinels has been studied and well co-related with the structural data. Keywords: MgAl2O4;thermal plasma processing, structure; mechanical behaviour
∗
Corresponding author. Tel.:+ 916742379451; E-mail: [email protected] (Rakesh K. Sahoo). 1
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1. Introduction Magnesium aluminium oxide spinel (MgAl2O4) is a ternary oxide having chemical formula
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AB2O4, where A represents a divalent metal cation and B represents a trivalent metal cation. A occupies tetrahedral and B octahedral position in cubic packed AB2O4 crystal structure [1]. In recent years, researchers have focused on the synthesis of MgAl2O4 spinel due to its good
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mechanical properties, high chemical stability, low density and low electrical conductivity, Also, it has showed transparency in visible and infrared wavelength region [2-3].This material finds
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broad applications in refractory materials, humidity sensors, catalyst support, electroceramic materials, etc. [4-5].
Several methods have been adopted for synthesis of MgAl2O4 spinel such as flash pyrolysis method [6], sol-gel auto combustion method [7], microwave-assisted combustion synthesis
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(MWCS) [8], simple sol-gel method [3], self heat sustained (SHS) technique [9], organic precursor's method [10-11], molten salt technique [11], spark plasma sintering of powder to form bulk [12] and mechanochemical processing [5]. Abdi et al. [13] used this mechnochemical
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method for the synthesis of nano MgAl2O4 spinel. Jorgen et al. prepared MgAl2O4 by Pechini method to study the mechanical behaviour of agglomerates [14]. Also, the synthesis of MgAl2O4
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ceramic particles by gel-combustion [15]. and effect of glycine–starch mixing ratio on the structural characteristics of MgAl2O4 nano-particles synthesized by sol–gel combustion has been studied[16]. Likewise, modified sol-gel[3], gel casting[17], slip casting [18] methods have been used for the synthesis of MgAl2O4 spinel. Mean while, hollow sphere and 3D hierarchical structures different morphologies of MgAl2O4 spinel has been synthesized by Bin et al. using simple hydrothermal method[19].
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Recently, low-temperature synthesis of mesoporous nanocrystalline [20], Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions techniques was reported [21]. Additionally, the structural beauty of this material draws attention of researchers to synthesize
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the material from industrial wastes arising from aluminium and magnesium scraps [22]. Keeping an eye to the interesting physical and chemical properties of this materials, recently several groups have synthesized such spinels for various applications like: luminescent material by
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doping with Eu2+, Tb3+[23], as a purifier of Selenium from ground water[24] and as a microwave
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dielectric media [25].
In the present study, we synthesize MgAl2O4 powder by a simple plasma processing route (thermal plasma reactor). Since MgAl2O4 spinel phase formation require heat treatment above 900 °C in open atmosphere, which tempted the utilization of thermal plasma reactor to attain a
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very high temperature that can be achieved in fraction of minutes, for the synthesis of such spinels with accelerated reaction rates of the chemically reactive species and easy liquidation of solid feed materials. Additionally, the processing time is very low (10 minutes) compared to
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other reported synthesis methods in a bigger scale of production. In the plasma feed the weight percentage of the alumina content has been varied and their structural variations have been
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analyzed. The change in structural behaviour of the synthesized samples have been analyzed using X-ray diffraction spectroscopy (XRD), Raman Spectroscopy and Fourier transform infrared spectroscopy (FTIR) was used. In addition, the change in surface composition and morphologies are analyzed using Electron probe microanalysis (EPMA) and Field emission scanning electron microscope (FESEM) in coordination with FTIR. Finally, the nano-mechanical behaviour of the materials have been analyzed and compared.
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2. Experimental Procedure 2.1. Synthesis of MgAl2O4 spinel The nano-sized MgAl2O4 spinel was prepared via plasma processing route by using
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commercially available magnesite (MgCO3, HIMEDIA, minimum assay 40.0%), aluminium oxide (Al2O3, HIMEDIA, minimum assay 98.0%) and dextrin white binder. Stoichiometrically 1:1 molar ratio of magnesite and aluminium oxide.(represented as S1). With three other
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compositions were taken by varying the MgCO3 composition. To maintain the required magnesite and alumina stoichiometry weight percentage was varied and mixed with the dextrin
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binder. The samples were named based on their change in weight percentage mentioned in Table-1. Samples of total weight 200 g each were thoroughly mixed with 25 g of dextrin and little amount of water in agate mortar for about 30 minutes. Then, these powders are granulized to small spherical form and dried. These granules were used as the plasma feed. For plasma
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processing the plasma current and voltage between two electrode was maintained at 300A and 50 V, respectively. Argon was used as the plasma gas and flowed through the axial hole of the reactor for arcing and the reaction was carried out for 10 min. After this the sample was collected
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from the crucible of plasma reactor. Then these fused samples were taken for characterization.
28].
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The details of the plasma reactor and plasma processing are described in our previous works [26-
2.2 Characterization Technique The X-ray diffraction patterns of the samples was characterised by using PAN analytical X-pert pro diffractometer over the range of 10°< 2θ <80° with scan rate 2°/min and the CuKα source (λ = 1.54 Å). The Raman spectra of all the samples were recorded by using Reneshaw inVia
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instrument and Ar-Kr ion used as a laser with the wavelength of 514 nm. For microstructural analysis Field Emission Scanning Electron Microscopy (FESEM) was carried out using ZEISS Supra 55A. The electron probe micro analyser (EPMA) was carried out using JEOL, JXA
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8200.The nanomechanical properties were characterized with a nanoindentation system (UMIS, Fischer- Cripps, Australia) consisting Berkovich diamond indentor (face angle 65.3°) having a tip radius of 150 nm. The indentation tests were carried out at an applied load of 30 mN with 12
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tests on each sample in order to understand the average response of the material. Indents on the same sample were separated apart by 15 µm. Young’s modulus and hardness were calculated
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from the load–displacement curve using Oliver–Pharr analysis. Detailed methodology can be found elsewhere [29].
3.Results and Discussions
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3.1. XRD Analysis
The XRD patterns of as-synthesized MgAl2O4 samples are shown in Fig.1a. All the diffraction patterns corresponds to cubic crystal system of MgAl2O4 with space group Fd-3m and well
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matched the JCPDS File 073-1959 [30](for S1 and S2), 075-1796 [31]and 075-1798 (for S3 and S4) samples. The X-ray diffraction patterns of all the peaks at 2θ value - 19.1◦, 31.4◦, 37.03◦, 45◦,
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55.9◦, 59.5◦, 65.4◦, 68.9◦, 74.5◦ and 77.6◦ were indexed to the corresponding (hkl): (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (5 3 1), (6 2 0) and (5 3 3) planes. In all four samples, a pure crystalline phase of MgAl2O4 spinel and no other phase of magnesite and aluminium oxide were observed. The most intense diffraction peak was observed at 37.3° for (3 1 1) plane. The magnified short scan image of this most intense peak, in the 2θ range of 36 to 38° is shown in Fig.1b. In this diffraction pattern (Fig. 1b), all the peaks monotonically shifts towards lower
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angel side. As the alumina content varied the 2θ value shifted by0.09, 0.08 and 0.34 ° for sample S2, S3 and S4 with respect to S1, respectively. Additionally, one interesting feature was observed in the peak shape and peak intensity. Changing the alumina content from 45 to 55 wt
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%, the FWHM of the peaks almost remain equal with Gaussian shapes. However, further adding the alumina change the peak intensity two fold, reduces the FWHM to half and peak shape changes to non-symmetrical type. Thus it can be summarized that addition of alumina above 55
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wt%, induces crystallinity in the MgAl2O4 spinel matrix. Our observation is an good agreement
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with Jing et. al. report [32].
To further confirm above observations the average crystallite size of MgAl2O4 samples was calculated by using Debye-Scherrer equation (1) [33] which is given by D = Kλ /β1/2 Cos θ (1)
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Where K is constant, λ represents the wavelength of the XRD, β1/2 is the full width half maximum of diffraction peak and θ is the Bragg's angle. The change in 2θ, FWHM value, lattice parameter and average crystallite size are shown in supplementary data Table-1A. This results
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confirmed and well matched with our previous observations.
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3.2. Raman Analysis
The Raman spectroscopy of as-synthesized MgAl2O4 samples are shown in Fig. 2. The MgAl2O4 spinel is a closed packed FCC crystal structure with space group Fd-3m. To know the symmetry of individual atoms the irreducible representation of normal modes vibration Brillouin zone centre is as follows: Γ = A1g ( R ) + E g ( R ) + 3T2 g ( R ) + 4T1u ( IR ) + T1g + 2 A2u + 2 Eu + 2T2u
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Where (R) represents Raman and (IR) represents Infrared active modes and the other modes are optically silent [34]. The normal modes of vibration of each individual atom Mg, Al and O at the centre of the Brillouin zone are noted as D3d, Td and C3v [35]. Among these 16 optical phonons,
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five modes are Raman active (1A1g, 1Eg and 3T2g) whereas four (T1u ) modes are IR active [34]. According to the above selection rule MgAl2O4 spinel has 5 Raman active modes but in our Raman data of MgAl2O4, we observe total four Raman active modes. Similar, type of result was
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also observed by other groups [36-38]. The Raman bands observed at 311, 407, 667 and 767 cmcorresponds to T2g(1), Eg, T2g(2) and A1g modes, respectively [34]. The band observed at 311
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cm-1 corresponds to the T2g(1) mode and appeared due to transition of Mg atoms in tetrahedral sites [37]. The Eg mode can be credited to bending mode of Al ions in tetrahedral sites[26]. The other two bands observed at 667 and 767 cm-1 are assigned to T2g(2) and A1g modes. These bands can be attributed to bending motion in octahedral sites of Mg atoms [39]. The A1g mode is the
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symmetric stretching of the AlO4 in tetrahedral sites [40]. Further, to identify and IR active modes present in the samples and to derive the structural correlations with Raman modes, FTIR
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analysis was carried out.
3.3. FTIR Analysis
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The FTIR spectra of synthesized MgAl2O4 spinels were captured in the range of 400-4000 cm-1 and illustrated in Fig. 3. As expected the vibration bands due to water absorbed on the surface and the hydroxide species are observed at 1633 cm-1 and 3350 cm-1 for all samples [41]. Additionally, several common bands are observed for all the samples at 1098, 1500 and 2926 cm-1. The narrow band observed at 1098 cm
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and 1500 cm-1 are stretching modes of carbonate
group [42] and the band at 2926 cm-1 is stretching mode of C-H group[43]. The broad band in
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the range of 400-800 cm-1 of the FTIR spectra (shown in Fig. 3b) reveals peaks typically due to metal–oxygen bonds and gives information about the MgAl2O4 phase. The spectra are normalized and shown in Fig 3b. In this broad band, several absorptions peaks centered at 463,
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588, 679 and 803.cm-1 are observed. In this broad band, the absorption peaks centered at 803 cm−1 and 679 cm−1 are attributed to Al–O stretching frequencies of AlO6 octahedral and AlO4 tetrahedral, respectively [44]. As the alumina loading in the feed increases the intensity of the
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band centered at 803 cm−1 remains almost equal, whereas the peak centered at 679 cm−1 decreases. This indicates that, as the alumina content in the feed increases, there are fewer Al2+
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ions in tetrahedral sites and more in octahedral site. In a normal spinel such as MgAl2O4, the divalent cation resides in tetrahedral sites and the trivalent cation is in octahedral sites. Additionally, The bands observed at 588 and 679 cm-1 are considered as the signature band of Al-O-Mg [3] and the peak observed at 463 cm-1 corresponds to the lattice vibrations of Al-O
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bending mode [45]. Intensity of these three bands (463, 588, 679 cm-1)decrease as the alumina content in the feed increases up to 55 wt % (Fig. 3b). However, further increase in alumina content the intensity of these three peaks increases. The FTIR data suggest that, as the alumina
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loading was increased, the Al3+ cations occupation of crystallographic sites resembles more
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closely that of bulk MgAl2O4 spinel which is consistent with the XRD data.
3.4 TG- DSC Analysis
Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis were used to identify the decomposition temperature of precursor and surface absorbed carbon to avoid the residual carbon in the product and to study the thermal effects in a diffusion bonded compound formation after plasma processing. First, feed powder containing 50 wt% of MgCO3 and 50 wt% alumina mixture was analyzed thermogravimetrically (TG) in a nitrogen atmosphere from room 8
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temperature to 900 °C(shown in Fig 4a). The TG spectrum indicate the weight loss in four different zones (indicated by arrow marks in Fig. 4a). Further, to study the phase formation and thermodynamics of the phase formation, DSC curves of TG analyzed sample (feed powder50
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wt% of MgCO3 and 50 wt% alumina mixture) and samples after plasma processing (S2,S3 and S4) was compared and presented in Fig. 4b.
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To explain the transition region in the DSC curve in more simplified form we categorize and
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name the regions as region-i, ii, iii and iv (marked in Fig. 4b).
Region -i: The early weight loss of about 10% can be ascribed to the removal of physically adsorbed water molecules and decomposition and evaporation of hydroxyl species (Fig. 4a). For sample before plasma processing (Fig.4b-i), the endothermic peak was observed at 239 °C,
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assigned to the dehydration reaction of Al(OH)3 and vaporization of physically bound absorbed water which is close to the transitional temperature of Al(OH)3 to Al2O3 i.e. 230 °C [5]. However, in case of samples after plasma processing (Fig. 4b-ii, iii and iv) no such transitional
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behaviour in DSC curve is observed.
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Region-ii: The second step of weight loss of about 8.5% at around 355 °C may be due to the transformation of Mg(OH)2 to MgO phase after heat treatment (Fig. 4a). In the DSC curve (Fig.4b-i), the second endothermic peak observed at about 355°C is attributed to the dehydration of Mg(OH)2 and double layer hydrates [5]. 2 Mg (OH ) 2 → MgO + H 2 ↑ ....................................................................................(2)
N atmospher /
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In this region also no transitional behaviour of the DSC curves was observed for the samples after plasma processing (Fig. 4b-ii, iii and iv).
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Region-iii: The third step of weight loss of about 16.8 % was observed in the temperature range of 468-800 °C which may be due to the decomposition of MgCO3 to MgO phase after heat treatment (Fig. 4a). For sample before plasma processing (Fig.4b-i), DSC peak observed at 468 °
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C may be due to the decomposition of carbonate, nitrates or un-reacted metal nitrates [3, 8].
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This region is considered as the magnesia formation region which follows the following reaction 2 MgCO3 + Al2 O3 → MgO + Al2 O3 + CO2 ↑
N Atmospere
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As expected the weight loss in his region is very low for plasma processed samples indicating the pre presence of this phase. Meanwhile, this transitional region gives the indication of the
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precursor decomposition temperature and phase formation.
Region- iv: The fourth step of weight loss of about 7.9 % for the sample after heat treatment in N2 atmosphere in the temperature range of 800- 900 °C (Fig. 4a). There observed a exothermic
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peak in DSC spectrum at 835 °C as shown in Fig. 4b-i. Wajler et al.[46] carefully analyzed this
there
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peak and in-correlation with XRD data confirmed that in the temperature range of 500-800 °C, occurred
the
formation
of
non-stoichimetric
magnesium
aluminates
(Mg0.64Al0.36)(Al0.82Mg0.18)2O4. Above 900 °C, the dominating phase is MgAl2O4 with additional phases of MgO and Al2O3. As reported by Amini et al. this exothermic peak assigned to the oxidation of the residue carbon [47]. Interestingly, this peak was observed for all samples; precursor mixture sample (Fig. 4b-i) and samples after plasma processing (Fig. 4b-ii, iii and iv). Thus, it is anticipated that the alloy phase formation is more acceptable for the precursor mixture
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sample. On the other hand, in case of plasma processed samples, this peak is the joint contribution from alloy phase formation and oxidation of residual carbon present. This carbon comes from the graphite crucible used in plasma processing and the residue MgO and Al2O3
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remained after plasma reaction participated in alloy formation. Additionally, in this region weight gain of the plasma processed samples are very low in comparison to the precursor mixture sample because the ions density participated in the alloy phase formation is very low
carbon free of the MgAl2O4 spinel remains above 900 °C.
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3.4. Scanning electron microscopic analysis
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[46,48] (residual only). Thus, the exact temperature for phase formation and contaminated
The surface morphology of the magnesium aluminate flakes after plasma processing are analyzed under electron microscope. The surface morphology of the sample named S1 shows pores on the surface. Theses pores are of millimetre order diameter shown in Fig. 5a and 5b. As
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the alumina content in the feed increases, (Sample-S2), the pores on the surface reduces, the grains looks as if melted and sintered with each other in a typical melted form with size in the range of 20-30 µm was observed (Fig 5c). Subsequently, changing the alumina content to 50 wt
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%. (Sample-S3) resulted increase in the grain size of the sample. Figure 5d show the SEM images of the sample with 55 wt % of alumina with bigger grains in sintered form and the grains
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appear in cubic shape with average size in the range 40-50 µm. Further, increase in alumina content to 60 wt % (Sample-S4), grain surface morphology changes to triangular shape (Fig. 5e) and the grain size increases to average size 50-70 µm. (shown in the magnified image Fig. 5f). Figure 5f shows the magnified image acquired from the same imaging area marked square shape in red label (Fig. 5g) indicated the finer particles in an assembled form. This results well matches
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with the XRD result indicating the increase in crystallite size with increase in the alumina content.
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Additionally, the EDS elemental mapping of all the samples were obtained and presented (shown in supplementary data Fig.1A,1B,1C and 1D). Although the elemental map in EDS is not considered as the standard but it gives an abstract idea of the elements present inside the surface.
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In all the images the major elemental content of Al, Mg and O are confirmed. To observe the
3.5. Electron probe micro analysis
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exact elemental distribution, sample S4 was taken for electron probe microanalysis.
The microstructure and corresponding elemental maps of synthesized MgAl2O4 (S4) are obtained by electron probe micro analysis (EPMA) (shown in Fig. 6A),). Elemental maps of aluminium,
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magnesium, oxygen and silicon are shown in Fig 6A with their corresponding colour scale bars indicating the relative concentration of each element. The aluminum, and magnesium were all distributed homogeneously on the imaging area. The elemental concentration of aluminum on
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the surface was the highest, while that of magnesium. Also, the WDS spectrum from the same imaging area was obtained (Fig. 6B). This spectra indicates the presence of Al, Mg, O and Si.
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The qualitative spectra confirms the presence of Al, Mg and O. The other elements (C and Au) detected correspond to carbon tape attached for mounting the sample and the conducting gold layer coated on the sample surface [49].
3.6 Nanoindentation analysis
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From nanoindentation testing, using Oliver - Pharr method, measured hardness (H) and elastic modulus (E) of the samples were in the range of 3.3-8.2GPa and 88-218GPa respectively. Khasanov et al showed that the SPS processed transparent MgAl2O4 consisted microhardness of
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18.52 GPa, Young's modulus of 212 GPa [50]. These values were achieved by optimizing the sintering duration and pressing force. Ganesh et al. compared the various processing methods such as slip casting (SC), gel-casting (GC), hydrolysis assisted solidification (HAS) and
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hydrolysis induced aqueous gel casting (GCHAS) route and found that hardness was ranging between 6.43- 12.5 GPa [51]. HIP processed MgAl2O4 showed the hardness ranging between
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2.89 and 7.79 GPa [52].
Representative P- h curve obtained in this study are given in Fig. 7. With increased alumina content nanoindentation results showed that initially hardness and elastic modulus of the
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corresponding samples increases and attain a maximum but then subsequently reduces. Sample 3 showed the highest hardness and elastic modulus and therefore can be assume to be the optimum atomic ratio content of the precursor materials for the given processing conditions. The ratio
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H3/E2, as concluded in the Table-2, represents the proportionality of material’s resistance to plastic deformation [29]. The ratio (H3/E2) illustrated the increased resistance of the synthesized
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MgAl2O4 with increased alumina content. It is crucial to report that with slightly increased alumina ratio, in case of S2 and S3, material’s ability to resist the plastic deformation has increased (~2 times). Such an effect may be attributed to the better fracture toughening offered by un reacted alumina retained in case of S4. To understand the elastic –plastic behaviour of the synthesized materials in all cases, plastic deformation energies have been estimated. Elastic recovery (hmax- hf) was also determined. Trend in the plastic deformation energy illustrates that
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samples produced with the necessary ratios of precursor powders, showed moderate values of it. This is observed in S2 and S3 samples which also showed higher elastic modulus. Furthermore, it is evident that the elastic recovery of both the samples is very similar. As a consequence,
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measured elastic modulii of these samples are close to each other. Furthermore, elasticity ratio (ER), which is graphically the ratio of recovered area to total area in a P-h curve, was estimated. In terms of energy, ER is the ratio of the consumed energy for elastic deformation to the total
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consumed deformation energy, Ut. Here, Ut is the summation of elastic deformation energy (Ue) and plastic deformation energy (Up). Results summarized in Table - 2 indicated that the sample
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S3, which showed the superior mechanical properties, also showed the largest elasticity ratio (48.43%). 4. Conclusions:
1. Magnesium aluminate powders were synthesized via thermal plasma using MgCO3 and
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Al2O3 as starting materials. The effects of changing the alumina content in the plasma feed on formation MgAl2O4 spinel were investigated. 2. Optimum processing time for samples was 10 min that was significantly lower than that
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required by the solid state method. Less processing time was a very effective factor for cot effective large scale production .
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3. XRD and Raman data confirms the formation of single phase of MgAl2O4 without any other impurity phases in the samples.
4. Increasing the Al2O3 : MgCO3 stichiometric ratio there occurred grain growth phenomenon thus causing increase in the particle size. Increase in grain size with increase in alumina content in the feed was confirmed form the XRD and SEM data.
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5. EPMA maps reflecting uniform elemental distribution, in particular, Al and Mg in the compound lump in a particular imaging area without any elemental zoning. 6. The nano-indention measurements revealed that an increase in the alumina concentration
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in the plasma feed from 45 wt% to 55 wt %, enhances the mechanical properties (hardness, resistance to plastic deformation and Elastic recovery) of the samples significantly by order of 3. However, further increasing the alumina content to 60 wt%
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the above values decrease.
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[22] E.M.M. Ewais, A.A.M. El-Amir, D.H.A. Besisa, M. Esmat, B.E.H. El-Anadouli, Synthesis of nanocrystalline MgO/MgAl2O4 spinel powders from industrial wastes, J. Alloys Compd. , 691
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[29] S. Bajpai, A. Gupta, S.K. Pradhan, T. Mandal, K. Balani, Crack Propagation Resistance of α-Al2O3 Reinforced Pulsed Laser-Deposited Hydroxyapatite Coating on 316 Stainless Steel,
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order-disorder behavior in the MgAl2O4 spinel, Phy. Rev. B, 45 (1992) 500-502. [37] A. Chopelas, A.M. Hofmeister, Vibrational spectroscopy of aluminate spinels at 1 atm and of MgAl2O4 to over 200-kbar, Phys. Chem. Minerals, 18 (1991) 279-293.
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[46] A. Wajler, H. Tomaszewski, E. Drozdz-Ciesla, H. Weglarz, Z. Kaszkur, Study of magnesium aluminate spinel formation from carbonate precursors, J. Eur. Ceram. Soc., 28 (2008) 2495-2500.
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(MgAl2O4) nanopowder with high surface area with a novel and simple sol-gel method, J. Porous Mater., 22 (2015) 481-485.
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[48] Ji-Guang Li, T. Ikegami, Jong-Heun Lee, T. Mori, Y. Yajima, Synthesis of Mg±Al spinel powder via precipitation using ammonium bicarbonate as the precipitant, J. Eur. Ceram. Soc., 21 (2001) 139-148.
[49] R. K.Sahoo, S.K. Singh, B.K. Mishra, Surface and bulk 3D analysis of natural and processed ruby using electron probe micro analyzer and X-ray micro CT scan, J. Electron. Spectrosc. Relat. Phenom., 211 (2016) 55-63.
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[50] O. Khasanov, E. Dvilis, A. Khasanov, E. Polisadova, A. Kachaev, Optical and mechanical properties of transparent polycrystalline MgAl2O4 spinel depending on SPS conditions, physica status solidi (c), 10 (2013) 918-920.
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[51] I. Ganesh, G. Jaganatha Reddy, G. Sundararajan, S.M. Olhero, P.M.C. Torres, J.M.F. Ferreira, Influence of processing route on microstructure and mechanical properties of MgAl2O4 spinel, Ceramics International, 36 (2010) 473-482.
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spinel and related composites, Ceramics International, 28 (2002) 153-158.
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Figure captions Fig. 1. (a) XRD patterns of powder MgAl2O4 samples in the range of 20–70° (b) (b) magnified
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view of the (3 1 1) peak of the MgAl2O4 samples where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4
Fig. 2. Comparative Raman spectra of MgAl2O4 samples spectra in the range of 200–900 cm−1
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where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4
Fig. 3. FTIR spectra of MgAl2O4 powder samples (a) in the range of 450–4000 cm−1(b) in the
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range of 450–900 cm−1 where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4 Fig. 4. (a) TG curves for raw feed powder 50 wt% of MgCO3 and 50 wt% alumina mixture, obtained on heating at 5 K/min in nitrogen atmosphere. (b) DSC spectra of (i) TG sample,
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(ii)Sample S2, (iii) Sample S3 and (iv) Sample S4
Fig.5. SEM images of the MgAl2O4 samples synthesized in different conditions, (a) low magnification image of sample S1, (b) higher magnification image of the sample S1 from same
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imaging area of the same sample, (c) Sample S2, (d) Sample S3, (e) low magnification image of sample S4, (f) high magnification of single grain and (g) higher magnification image from the
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same imaging area marked by square in red label in Fig. 5(e). Fig. 6A. WDS elemental map of the S4 sample Fig. 6B. Qualitative WDS spectra obtained from S4 sample as shown in Fig.6A and the element detected by crystal TAP, LIF, LDE2 and PETH. Fig. 7. Nanoindentation load-depth profile of MgAl2O4 samples where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4 23
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Sample
Al2O3 (wt %)
MgCO3( wt %)
45.00
55.00
S2
50.00
50.00
S3
55.00
45.00
S4
60.00
40.00
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ER (hmax-hf) (µm)
%Elasticity Ratio
3.9571
26.74
base
0.20391
45.63
181.99±40.28
2.7768
32.15
1.182379
0.14098
35.59
8.23±2.77
218.55±58.67
2.4597
26.56
2.52058
0.1459
48.43
6.67±2.16
149.84±54.98
3.2327
22.47
2.854464
0.18074
41.30
Elastics modulus (GPa)
U(P) (nJ)
S1
3.31±0.49
88.50±17.51
S2
5.66±2.12
S3 S4
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Hardness (GPa)
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E/H
Sample
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Research Highlights A minimal time processing rout for MgAl2O4 spinel powder in large scale.
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Process optimization for a particular stiochiometry.
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Structural and morphological correlation with mechanical properties.
S0925-8388(17)32813-X
DOI:
10.1016/j.jallcom.2017.08.085
Reference:
JALCOM 42842
To appear in:
Journal of Alloys and Compounds
Received Date: 31 March 2017 Revised Date:
17 July 2017
Accepted Date: 10 August 2017
Please cite this article as: S. Dash, R.K. Sahoo, A. Das, S. Bajpai, D. Debasish, S.K. Singh, Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.085. 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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GRAPHICAL ABSTRACT
Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study
Singh
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Sanghamitra Dash, Rakesh K. Sahoo∗, Shubhra Bajpai, Arya Das, Debidutta Debasish and Saroj K.
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Technology, Bhubaneswar -751013, India
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Advanced Materials Technology Department, CSIR - Institute of Minerals and Materials
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Figure (a) Peak shift of the (311) peak of XRD, (b) normalized FTIR spectra in the range of 450 to 900 cm-1 and (c) Load-depth profile of samples
∗
Corresponding author. Tel.:+ 916742379456; E-mail: [email protected] (Rakesh K. Sahoo).
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Synthesis of MgAl2O4 spinel by thermal plasma and its synergetic structural study
Singh
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Sanghamitra Dash, Rakesh K. Sahoo∗, Arya Das, Shubhra Bajpai, Debidutta Debasish and Saroj K.
Advanced Materials Technology Department, CSIR - Institute of Minerals and Materials
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Technology, Bhubaneswar -751013, India
Abstract: Bulk quantity of the MgAl2O4 spinels with specific composition has been synthesized by thermal plasma method. The process time and compositions of the feed are optimized for a desired stoichiometry. A structural co-relation has been derived from several set of experiments by major variation in the α-Al2O3 content in the feed material. Formation of desired phase with
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particular feed stoichiometry has been studied by X-ray diffraction pattern (XRD) analysis which is corroborated by Raman scattering analysis. A good structural co-relation between the XRD, Raman and Fourier transform infrared spectroscopy (FTIR) analysis was derived. Additionally,
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the electron probe microanalysis (EPMA) of the specific sample has been carried out to study the
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exact compositional distribution and the presence of additional impurity phases in the assynthesized spinels. Finally, the micro mechanical behaviour of the as-synthesized spinels has been studied and well co-related with the structural data. Keywords: MgAl2O4;thermal plasma processing, structure; mechanical behaviour
∗
Corresponding author. Tel.:+ 916742379451; E-mail: [email protected] (Rakesh K. Sahoo). 1
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1. Introduction Magnesium aluminium oxide spinel (MgAl2O4) is a ternary oxide having chemical formula
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AB2O4, where A represents a divalent metal cation and B represents a trivalent metal cation. A occupies tetrahedral and B octahedral position in cubic packed AB2O4 crystal structure [1]. In recent years, researchers have focused on the synthesis of MgAl2O4 spinel due to its good
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mechanical properties, high chemical stability, low density and low electrical conductivity, Also, it has showed transparency in visible and infrared wavelength region [2-3].This material finds
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broad applications in refractory materials, humidity sensors, catalyst support, electroceramic materials, etc. [4-5].
Several methods have been adopted for synthesis of MgAl2O4 spinel such as flash pyrolysis method [6], sol-gel auto combustion method [7], microwave-assisted combustion synthesis
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(MWCS) [8], simple sol-gel method [3], self heat sustained (SHS) technique [9], organic precursor's method [10-11], molten salt technique [11], spark plasma sintering of powder to form bulk [12] and mechanochemical processing [5]. Abdi et al. [13] used this mechnochemical
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method for the synthesis of nano MgAl2O4 spinel. Jorgen et al. prepared MgAl2O4 by Pechini method to study the mechanical behaviour of agglomerates [14]. Also, the synthesis of MgAl2O4
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ceramic particles by gel-combustion [15]. and effect of glycine–starch mixing ratio on the structural characteristics of MgAl2O4 nano-particles synthesized by sol–gel combustion has been studied[16]. Likewise, modified sol-gel[3], gel casting[17], slip casting [18] methods have been used for the synthesis of MgAl2O4 spinel. Mean while, hollow sphere and 3D hierarchical structures different morphologies of MgAl2O4 spinel has been synthesized by Bin et al. using simple hydrothermal method[19].
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Recently, low-temperature synthesis of mesoporous nanocrystalline [20], Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions techniques was reported [21]. Additionally, the structural beauty of this material draws attention of researchers to synthesize
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the material from industrial wastes arising from aluminium and magnesium scraps [22]. Keeping an eye to the interesting physical and chemical properties of this materials, recently several groups have synthesized such spinels for various applications like: luminescent material by
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doping with Eu2+, Tb3+[23], as a purifier of Selenium from ground water[24] and as a microwave
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dielectric media [25].
In the present study, we synthesize MgAl2O4 powder by a simple plasma processing route (thermal plasma reactor). Since MgAl2O4 spinel phase formation require heat treatment above 900 °C in open atmosphere, which tempted the utilization of thermal plasma reactor to attain a
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very high temperature that can be achieved in fraction of minutes, for the synthesis of such spinels with accelerated reaction rates of the chemically reactive species and easy liquidation of solid feed materials. Additionally, the processing time is very low (10 minutes) compared to
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other reported synthesis methods in a bigger scale of production. In the plasma feed the weight percentage of the alumina content has been varied and their structural variations have been
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analyzed. The change in structural behaviour of the synthesized samples have been analyzed using X-ray diffraction spectroscopy (XRD), Raman Spectroscopy and Fourier transform infrared spectroscopy (FTIR) was used. In addition, the change in surface composition and morphologies are analyzed using Electron probe microanalysis (EPMA) and Field emission scanning electron microscope (FESEM) in coordination with FTIR. Finally, the nano-mechanical behaviour of the materials have been analyzed and compared.
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2. Experimental Procedure 2.1. Synthesis of MgAl2O4 spinel The nano-sized MgAl2O4 spinel was prepared via plasma processing route by using
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commercially available magnesite (MgCO3, HIMEDIA, minimum assay 40.0%), aluminium oxide (Al2O3, HIMEDIA, minimum assay 98.0%) and dextrin white binder. Stoichiometrically 1:1 molar ratio of magnesite and aluminium oxide.(represented as S1). With three other
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compositions were taken by varying the MgCO3 composition. To maintain the required magnesite and alumina stoichiometry weight percentage was varied and mixed with the dextrin
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binder. The samples were named based on their change in weight percentage mentioned in Table-1. Samples of total weight 200 g each were thoroughly mixed with 25 g of dextrin and little amount of water in agate mortar for about 30 minutes. Then, these powders are granulized to small spherical form and dried. These granules were used as the plasma feed. For plasma
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processing the plasma current and voltage between two electrode was maintained at 300A and 50 V, respectively. Argon was used as the plasma gas and flowed through the axial hole of the reactor for arcing and the reaction was carried out for 10 min. After this the sample was collected
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from the crucible of plasma reactor. Then these fused samples were taken for characterization.
28].
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The details of the plasma reactor and plasma processing are described in our previous works [26-
2.2 Characterization Technique The X-ray diffraction patterns of the samples was characterised by using PAN analytical X-pert pro diffractometer over the range of 10°< 2θ <80° with scan rate 2°/min and the CuKα source (λ = 1.54 Å). The Raman spectra of all the samples were recorded by using Reneshaw inVia
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instrument and Ar-Kr ion used as a laser with the wavelength of 514 nm. For microstructural analysis Field Emission Scanning Electron Microscopy (FESEM) was carried out using ZEISS Supra 55A. The electron probe micro analyser (EPMA) was carried out using JEOL, JXA
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8200.The nanomechanical properties were characterized with a nanoindentation system (UMIS, Fischer- Cripps, Australia) consisting Berkovich diamond indentor (face angle 65.3°) having a tip radius of 150 nm. The indentation tests were carried out at an applied load of 30 mN with 12
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tests on each sample in order to understand the average response of the material. Indents on the same sample were separated apart by 15 µm. Young’s modulus and hardness were calculated
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from the load–displacement curve using Oliver–Pharr analysis. Detailed methodology can be found elsewhere [29].
3.Results and Discussions
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3.1. XRD Analysis
The XRD patterns of as-synthesized MgAl2O4 samples are shown in Fig.1a. All the diffraction patterns corresponds to cubic crystal system of MgAl2O4 with space group Fd-3m and well
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matched the JCPDS File 073-1959 [30](for S1 and S2), 075-1796 [31]and 075-1798 (for S3 and S4) samples. The X-ray diffraction patterns of all the peaks at 2θ value - 19.1◦, 31.4◦, 37.03◦, 45◦,
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55.9◦, 59.5◦, 65.4◦, 68.9◦, 74.5◦ and 77.6◦ were indexed to the corresponding (hkl): (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (5 3 1), (6 2 0) and (5 3 3) planes. In all four samples, a pure crystalline phase of MgAl2O4 spinel and no other phase of magnesite and aluminium oxide were observed. The most intense diffraction peak was observed at 37.3° for (3 1 1) plane. The magnified short scan image of this most intense peak, in the 2θ range of 36 to 38° is shown in Fig.1b. In this diffraction pattern (Fig. 1b), all the peaks monotonically shifts towards lower
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angel side. As the alumina content varied the 2θ value shifted by0.09, 0.08 and 0.34 ° for sample S2, S3 and S4 with respect to S1, respectively. Additionally, one interesting feature was observed in the peak shape and peak intensity. Changing the alumina content from 45 to 55 wt
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%, the FWHM of the peaks almost remain equal with Gaussian shapes. However, further adding the alumina change the peak intensity two fold, reduces the FWHM to half and peak shape changes to non-symmetrical type. Thus it can be summarized that addition of alumina above 55
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wt%, induces crystallinity in the MgAl2O4 spinel matrix. Our observation is an good agreement
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with Jing et. al. report [32].
To further confirm above observations the average crystallite size of MgAl2O4 samples was calculated by using Debye-Scherrer equation (1) [33] which is given by D = Kλ /β1/2 Cos θ (1)
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Where K is constant, λ represents the wavelength of the XRD, β1/2 is the full width half maximum of diffraction peak and θ is the Bragg's angle. The change in 2θ, FWHM value, lattice parameter and average crystallite size are shown in supplementary data Table-1A. This results
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confirmed and well matched with our previous observations.
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3.2. Raman Analysis
The Raman spectroscopy of as-synthesized MgAl2O4 samples are shown in Fig. 2. The MgAl2O4 spinel is a closed packed FCC crystal structure with space group Fd-3m. To know the symmetry of individual atoms the irreducible representation of normal modes vibration Brillouin zone centre is as follows: Γ = A1g ( R ) + E g ( R ) + 3T2 g ( R ) + 4T1u ( IR ) + T1g + 2 A2u + 2 Eu + 2T2u
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Where (R) represents Raman and (IR) represents Infrared active modes and the other modes are optically silent [34]. The normal modes of vibration of each individual atom Mg, Al and O at the centre of the Brillouin zone are noted as D3d, Td and C3v [35]. Among these 16 optical phonons,
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five modes are Raman active (1A1g, 1Eg and 3T2g) whereas four (T1u ) modes are IR active [34]. According to the above selection rule MgAl2O4 spinel has 5 Raman active modes but in our Raman data of MgAl2O4, we observe total four Raman active modes. Similar, type of result was
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also observed by other groups [36-38]. The Raman bands observed at 311, 407, 667 and 767 cmcorresponds to T2g(1), Eg, T2g(2) and A1g modes, respectively [34]. The band observed at 311
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cm-1 corresponds to the T2g(1) mode and appeared due to transition of Mg atoms in tetrahedral sites [37]. The Eg mode can be credited to bending mode of Al ions in tetrahedral sites[26]. The other two bands observed at 667 and 767 cm-1 are assigned to T2g(2) and A1g modes. These bands can be attributed to bending motion in octahedral sites of Mg atoms [39]. The A1g mode is the
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symmetric stretching of the AlO4 in tetrahedral sites [40]. Further, to identify and IR active modes present in the samples and to derive the structural correlations with Raman modes, FTIR
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3.3. FTIR Analysis
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The FTIR spectra of synthesized MgAl2O4 spinels were captured in the range of 400-4000 cm-1 and illustrated in Fig. 3. As expected the vibration bands due to water absorbed on the surface and the hydroxide species are observed at 1633 cm-1 and 3350 cm-1 for all samples [41]. Additionally, several common bands are observed for all the samples at 1098, 1500 and 2926 cm-1. The narrow band observed at 1098 cm
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and 1500 cm-1 are stretching modes of carbonate
group [42] and the band at 2926 cm-1 is stretching mode of C-H group[43]. The broad band in
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the range of 400-800 cm-1 of the FTIR spectra (shown in Fig. 3b) reveals peaks typically due to metal–oxygen bonds and gives information about the MgAl2O4 phase. The spectra are normalized and shown in Fig 3b. In this broad band, several absorptions peaks centered at 463,
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588, 679 and 803.cm-1 are observed. In this broad band, the absorption peaks centered at 803 cm−1 and 679 cm−1 are attributed to Al–O stretching frequencies of AlO6 octahedral and AlO4 tetrahedral, respectively [44]. As the alumina loading in the feed increases the intensity of the
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band centered at 803 cm−1 remains almost equal, whereas the peak centered at 679 cm−1 decreases. This indicates that, as the alumina content in the feed increases, there are fewer Al2+
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ions in tetrahedral sites and more in octahedral site. In a normal spinel such as MgAl2O4, the divalent cation resides in tetrahedral sites and the trivalent cation is in octahedral sites. Additionally, The bands observed at 588 and 679 cm-1 are considered as the signature band of Al-O-Mg [3] and the peak observed at 463 cm-1 corresponds to the lattice vibrations of Al-O
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bending mode [45]. Intensity of these three bands (463, 588, 679 cm-1)decrease as the alumina content in the feed increases up to 55 wt % (Fig. 3b). However, further increase in alumina content the intensity of these three peaks increases. The FTIR data suggest that, as the alumina
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loading was increased, the Al3+ cations occupation of crystallographic sites resembles more
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closely that of bulk MgAl2O4 spinel which is consistent with the XRD data.
3.4 TG- DSC Analysis
Thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis were used to identify the decomposition temperature of precursor and surface absorbed carbon to avoid the residual carbon in the product and to study the thermal effects in a diffusion bonded compound formation after plasma processing. First, feed powder containing 50 wt% of MgCO3 and 50 wt% alumina mixture was analyzed thermogravimetrically (TG) in a nitrogen atmosphere from room 8
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temperature to 900 °C(shown in Fig 4a). The TG spectrum indicate the weight loss in four different zones (indicated by arrow marks in Fig. 4a). Further, to study the phase formation and thermodynamics of the phase formation, DSC curves of TG analyzed sample (feed powder50
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wt% of MgCO3 and 50 wt% alumina mixture) and samples after plasma processing (S2,S3 and S4) was compared and presented in Fig. 4b.
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To explain the transition region in the DSC curve in more simplified form we categorize and
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name the regions as region-i, ii, iii and iv (marked in Fig. 4b).
Region -i: The early weight loss of about 10% can be ascribed to the removal of physically adsorbed water molecules and decomposition and evaporation of hydroxyl species (Fig. 4a). For sample before plasma processing (Fig.4b-i), the endothermic peak was observed at 239 °C,
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assigned to the dehydration reaction of Al(OH)3 and vaporization of physically bound absorbed water which is close to the transitional temperature of Al(OH)3 to Al2O3 i.e. 230 °C [5]. However, in case of samples after plasma processing (Fig. 4b-ii, iii and iv) no such transitional
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behaviour in DSC curve is observed.
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Region-ii: The second step of weight loss of about 8.5% at around 355 °C may be due to the transformation of Mg(OH)2 to MgO phase after heat treatment (Fig. 4a). In the DSC curve (Fig.4b-i), the second endothermic peak observed at about 355°C is attributed to the dehydration of Mg(OH)2 and double layer hydrates [5]. 2 Mg (OH ) 2 → MgO + H 2 ↑ ....................................................................................(2)
N atmospher /
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In this region also no transitional behaviour of the DSC curves was observed for the samples after plasma processing (Fig. 4b-ii, iii and iv).
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Region-iii: The third step of weight loss of about 16.8 % was observed in the temperature range of 468-800 °C which may be due to the decomposition of MgCO3 to MgO phase after heat treatment (Fig. 4a). For sample before plasma processing (Fig.4b-i), DSC peak observed at 468 °
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C may be due to the decomposition of carbonate, nitrates or un-reacted metal nitrates [3, 8].
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This region is considered as the magnesia formation region which follows the following reaction 2 MgCO3 + Al2 O3 → MgO + Al2 O3 + CO2 ↑
N Atmospere
..............................................................(3)
As expected the weight loss in his region is very low for plasma processed samples indicating the pre presence of this phase. Meanwhile, this transitional region gives the indication of the
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precursor decomposition temperature and phase formation.
Region- iv: The fourth step of weight loss of about 7.9 % for the sample after heat treatment in N2 atmosphere in the temperature range of 800- 900 °C (Fig. 4a). There observed a exothermic
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peak in DSC spectrum at 835 °C as shown in Fig. 4b-i. Wajler et al.[46] carefully analyzed this
there
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peak and in-correlation with XRD data confirmed that in the temperature range of 500-800 °C, occurred
the
formation
of
non-stoichimetric
magnesium
aluminates
(Mg0.64Al0.36)(Al0.82Mg0.18)2O4. Above 900 °C, the dominating phase is MgAl2O4 with additional phases of MgO and Al2O3. As reported by Amini et al. this exothermic peak assigned to the oxidation of the residue carbon [47]. Interestingly, this peak was observed for all samples; precursor mixture sample (Fig. 4b-i) and samples after plasma processing (Fig. 4b-ii, iii and iv). Thus, it is anticipated that the alloy phase formation is more acceptable for the precursor mixture
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sample. On the other hand, in case of plasma processed samples, this peak is the joint contribution from alloy phase formation and oxidation of residual carbon present. This carbon comes from the graphite crucible used in plasma processing and the residue MgO and Al2O3
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remained after plasma reaction participated in alloy formation. Additionally, in this region weight gain of the plasma processed samples are very low in comparison to the precursor mixture sample because the ions density participated in the alloy phase formation is very low
carbon free of the MgAl2O4 spinel remains above 900 °C.
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3.4. Scanning electron microscopic analysis
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[46,48] (residual only). Thus, the exact temperature for phase formation and contaminated
The surface morphology of the magnesium aluminate flakes after plasma processing are analyzed under electron microscope. The surface morphology of the sample named S1 shows pores on the surface. Theses pores are of millimetre order diameter shown in Fig. 5a and 5b. As
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the alumina content in the feed increases, (Sample-S2), the pores on the surface reduces, the grains looks as if melted and sintered with each other in a typical melted form with size in the range of 20-30 µm was observed (Fig 5c). Subsequently, changing the alumina content to 50 wt
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%. (Sample-S3) resulted increase in the grain size of the sample. Figure 5d show the SEM images of the sample with 55 wt % of alumina with bigger grains in sintered form and the grains
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appear in cubic shape with average size in the range 40-50 µm. Further, increase in alumina content to 60 wt % (Sample-S4), grain surface morphology changes to triangular shape (Fig. 5e) and the grain size increases to average size 50-70 µm. (shown in the magnified image Fig. 5f). Figure 5f shows the magnified image acquired from the same imaging area marked square shape in red label (Fig. 5g) indicated the finer particles in an assembled form. This results well matches
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with the XRD result indicating the increase in crystallite size with increase in the alumina content.
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Additionally, the EDS elemental mapping of all the samples were obtained and presented (shown in supplementary data Fig.1A,1B,1C and 1D). Although the elemental map in EDS is not considered as the standard but it gives an abstract idea of the elements present inside the surface.
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In all the images the major elemental content of Al, Mg and O are confirmed. To observe the
3.5. Electron probe micro analysis
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exact elemental distribution, sample S4 was taken for electron probe microanalysis.
The microstructure and corresponding elemental maps of synthesized MgAl2O4 (S4) are obtained by electron probe micro analysis (EPMA) (shown in Fig. 6A),). Elemental maps of aluminium,
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magnesium, oxygen and silicon are shown in Fig 6A with their corresponding colour scale bars indicating the relative concentration of each element. The aluminum, and magnesium were all distributed homogeneously on the imaging area. The elemental concentration of aluminum on
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the surface was the highest, while that of magnesium. Also, the WDS spectrum from the same imaging area was obtained (Fig. 6B). This spectra indicates the presence of Al, Mg, O and Si.
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The qualitative spectra confirms the presence of Al, Mg and O. The other elements (C and Au) detected correspond to carbon tape attached for mounting the sample and the conducting gold layer coated on the sample surface [49].
3.6 Nanoindentation analysis
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From nanoindentation testing, using Oliver - Pharr method, measured hardness (H) and elastic modulus (E) of the samples were in the range of 3.3-8.2GPa and 88-218GPa respectively. Khasanov et al showed that the SPS processed transparent MgAl2O4 consisted microhardness of
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18.52 GPa, Young's modulus of 212 GPa [50]. These values were achieved by optimizing the sintering duration and pressing force. Ganesh et al. compared the various processing methods such as slip casting (SC), gel-casting (GC), hydrolysis assisted solidification (HAS) and
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hydrolysis induced aqueous gel casting (GCHAS) route and found that hardness was ranging between 6.43- 12.5 GPa [51]. HIP processed MgAl2O4 showed the hardness ranging between
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2.89 and 7.79 GPa [52].
Representative P- h curve obtained in this study are given in Fig. 7. With increased alumina content nanoindentation results showed that initially hardness and elastic modulus of the
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corresponding samples increases and attain a maximum but then subsequently reduces. Sample 3 showed the highest hardness and elastic modulus and therefore can be assume to be the optimum atomic ratio content of the precursor materials for the given processing conditions. The ratio
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H3/E2, as concluded in the Table-2, represents the proportionality of material’s resistance to plastic deformation [29]. The ratio (H3/E2) illustrated the increased resistance of the synthesized
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MgAl2O4 with increased alumina content. It is crucial to report that with slightly increased alumina ratio, in case of S2 and S3, material’s ability to resist the plastic deformation has increased (~2 times). Such an effect may be attributed to the better fracture toughening offered by un reacted alumina retained in case of S4. To understand the elastic –plastic behaviour of the synthesized materials in all cases, plastic deformation energies have been estimated. Elastic recovery (hmax- hf) was also determined. Trend in the plastic deformation energy illustrates that
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samples produced with the necessary ratios of precursor powders, showed moderate values of it. This is observed in S2 and S3 samples which also showed higher elastic modulus. Furthermore, it is evident that the elastic recovery of both the samples is very similar. As a consequence,
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measured elastic modulii of these samples are close to each other. Furthermore, elasticity ratio (ER), which is graphically the ratio of recovered area to total area in a P-h curve, was estimated. In terms of energy, ER is the ratio of the consumed energy for elastic deformation to the total
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consumed deformation energy, Ut. Here, Ut is the summation of elastic deformation energy (Ue) and plastic deformation energy (Up). Results summarized in Table - 2 indicated that the sample
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S3, which showed the superior mechanical properties, also showed the largest elasticity ratio (48.43%). 4. Conclusions:
1. Magnesium aluminate powders were synthesized via thermal plasma using MgCO3 and
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Al2O3 as starting materials. The effects of changing the alumina content in the plasma feed on formation MgAl2O4 spinel were investigated. 2. Optimum processing time for samples was 10 min that was significantly lower than that
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required by the solid state method. Less processing time was a very effective factor for cot effective large scale production .
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3. XRD and Raman data confirms the formation of single phase of MgAl2O4 without any other impurity phases in the samples.
4. Increasing the Al2O3 : MgCO3 stichiometric ratio there occurred grain growth phenomenon thus causing increase in the particle size. Increase in grain size with increase in alumina content in the feed was confirmed form the XRD and SEM data.
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5. EPMA maps reflecting uniform elemental distribution, in particular, Al and Mg in the compound lump in a particular imaging area without any elemental zoning. 6. The nano-indention measurements revealed that an increase in the alumina concentration
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in the plasma feed from 45 wt% to 55 wt %, enhances the mechanical properties (hardness, resistance to plastic deformation and Elastic recovery) of the samples significantly by order of 3. However, further increasing the alumina content to 60 wt%
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the above values decrease.
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spinel and related composites, Ceramics International, 28 (2002) 153-158.
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Figure captions Fig. 1. (a) XRD patterns of powder MgAl2O4 samples in the range of 20–70° (b) (b) magnified
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view of the (3 1 1) peak of the MgAl2O4 samples where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4
Fig. 2. Comparative Raman spectra of MgAl2O4 samples spectra in the range of 200–900 cm−1
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where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4
Fig. 3. FTIR spectra of MgAl2O4 powder samples (a) in the range of 450–4000 cm−1(b) in the
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range of 450–900 cm−1 where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4 Fig. 4. (a) TG curves for raw feed powder 50 wt% of MgCO3 and 50 wt% alumina mixture, obtained on heating at 5 K/min in nitrogen atmosphere. (b) DSC spectra of (i) TG sample,
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(ii)Sample S2, (iii) Sample S3 and (iv) Sample S4
Fig.5. SEM images of the MgAl2O4 samples synthesized in different conditions, (a) low magnification image of sample S1, (b) higher magnification image of the sample S1 from same
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imaging area of the same sample, (c) Sample S2, (d) Sample S3, (e) low magnification image of sample S4, (f) high magnification of single grain and (g) higher magnification image from the
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same imaging area marked by square in red label in Fig. 5(e). Fig. 6A. WDS elemental map of the S4 sample Fig. 6B. Qualitative WDS spectra obtained from S4 sample as shown in Fig.6A and the element detected by crystal TAP, LIF, LDE2 and PETH. Fig. 7. Nanoindentation load-depth profile of MgAl2O4 samples where (i) Sample-S1, (ii) Sample-S2, (iii) Sample-S3 and (iv) Sample-S4 23
ACCEPTED MANUSCRIPT Table-1:- Experimental Specification of Samples
Sample
Al2O3 (wt %)
MgCO3( wt %)
45.00
55.00
S2
50.00
50.00
S3
55.00
45.00
S4
60.00
40.00
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ACCEPTED MANUSCRIPT Table-2:- Comparison of the mechanical properties of all the Samples H3/E2
ER (hmax-hf) (µm)
%Elasticity Ratio
3.9571
26.74
base
0.20391
45.63
181.99±40.28
2.7768
32.15
1.182379
0.14098
35.59
8.23±2.77
218.55±58.67
2.4597
26.56
2.52058
0.1459
48.43
6.67±2.16
149.84±54.98
3.2327
22.47
2.854464
0.18074
41.30
Elastics modulus (GPa)
U(P) (nJ)
S1
3.31±0.49
88.50±17.51
S2
5.66±2.12
S3 S4
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Hardness (GPa)
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E/H
Sample
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Research Highlights A minimal time processing rout for MgAl2O4 spinel powder in large scale.
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Process optimization for a particular stiochiometry.
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Structural and morphological correlation with mechanical properties.