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Molten salt synthesis and formation mechanism of magnesium aluminate spinel using different shape Al2O3 as the templets
Ceramics International 45 (2019) 14397–14403
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Molten salt synthesis and formation mechanism of magnesium aluminate spinel using different shape Al2O3 as the templets
T
Xiaolong Houa, Xibao Lia,∗∗, Mingqiang Liua, Zhijun Fenga, Zhihui Hua, Meng Zhanga, Zhi Chena, Juntong Huanga,b,∗ a b
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, Jiangxi Province, 330063, PR China The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium aluminate spinel Different shape Al2O3 Molten salt synthesis Template formation mechanism Gradient growth
In order to verify the “Al2O3-template-formation” mechanism of magnesium aluminate (MA) spinel proposed previously using Al2O3 and MgO micro-powders as raw materials, in this work, MA spinel was synthesized by nanograined, plate-like, and fibrous Al2O3 in LiCl molten salt at 1150 °C for 3 h, respectively. The products were characterized by XRD, SEM, TEM, and EDS techniques, and the grain size of the products and raw materials were analyzed. The results showed clearly that the MA spinel was initially nucleated and subsequently developed to octahedral crystal. When the reaction further took place, when using nanograined Al2O3, the newly formed MA spinel seeds initially moved and attached to the surface of the large octahedral MA spinel crystal, and they were subsequently engulfed by the large MA spinel crystal, which further grew via layer-by-layer to become microsized crystal. Using plate-like or fibrous Al2O3 raw material, MgO diffused continuously into the interior of Al2O3 to form MA spinel with a gradient growth from surface to depth. These revealed that whatever shape of Al2O3 was used, the synthesis of MA spinel was governed by “Al2O3-template formation mechanism”, i.e., by the reaction of MgO diffusion into the Al2O3 templet in the molten salt.
1. Introduction Ceramics and refractory materials have been widely used in many fields, and with the in-depth study for them over several decades, highquality raw materials have become the guarantee for the preparation of excellent performance ceramics and refractory materials [1,2]. Because of its excellent properties including good mechanical properties, high chemical stability, high thermal shock resistance, wide energy band, low electrical conductivity and low thermal conductivity [3–6], magnesium aluminate (MA) spinel has important applications in the field of refractory [7,8], optical materials [9,10], transparent ceramics and armor [11,12], and electronic ceramic materials, etc [13]. Synthesizing high quality MA spinel powder is a key step for its application. The conventional method to prepare MA spinel is through solid-state reaction of Al2O3 and MgO at around 1500 °C. This method is easy to operate and can be used in large-scale production [14], but the reaction temperature required is very high and the final product does not reach the expected purity and particle size. The vapor deposition method is complex and the reaction conditions are harsh. Sol-gel method is an effective and widely used method for synthesizing MA ∗
spinel [15–19], the required synthesis temperature is low and the purity is high. Unfortunately, the equipment and raw materials are expensive and the preparation cost is very high [20,21]. Therefore, during past years, MA spinel powders has been commercially synthesized using several novel processing routes, mainly including flash pyrolysis method [22], self heat sustained technique [23], microwave-assisted combustion synthesis [24,25], combustion method [26], hydrothermal method [27], co-precipitation method [28], mechanochemical processing [29–31], etc. Among these methods, whatever method is used to prepare MA spinel, the purpose is to enhance the reactive activity, reduce the synthesis temperature and increase surface area [32–35]. In recent years, molten salt synthesis (MSS) technology has aroused considerable attention [36–38]. During in the MSS liquid/solid system, the raw materials could be mixed more homogeneously and diffused more rapidly than in the conventional mixed oxide (CMOS) solid–solid system, resulting in a significant reduction in the synthesis temperature and reaction time [39–41]. Zhang et al. synthesized MA spinel by the MSS technology with Al2O3 and MgO micro-powders as raw materials in alkali chloride salt (LiCl, NaCl or KCl) at 1150 °C for 3 h, indicating that the morphology and size of particle was similar to Al2O3, and
Corresponding author. School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, Jiangxi Province, 330063, PR China. Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (J. Huang).
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https://doi.org/10.1016/j.ceramint.2019.04.157 Received 1 April 2019; Received in revised form 15 April 2019; Accepted 17 April 2019 Available online 21 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of products synthesized from MgO powders and different shape Al2O3 (a) nanograins, (b) platelets, (c) fibers in molten salt LiCl at 1150 °C for 3 h.
suggested that the formation of MA spinel was through the "Al2O3template formation mechanism" in the MSS process [40]. In order to verify the rationality and accuracy of this mechanism, in this work, a similar MSS method was used to synthesize MA spinel by using nanograined, plate-like, fibrous Al2O3 with MgO powder as raw materials, respectively. Their morphologies and microstructures were observed and analyzed in-depth by FESEM, TEM, HRTEM, along with EDS, especially for the polished surface of products. Finally, their corresponding formation mechanisms were proposed.
2. Experimental Al2O3 nanograins (A800207, Shanghai Macklin Biochemical Co., Ltd. China), plate-like Al2O3 powders (AR, Xilong Chemical Co., Ltd. China) or Al2O3 fibers (AR, Xilong Chemical Co., Ltd. China) as well as MgO powders (AR, Xilong Chemical Co., Ltd. China) were used as the raw materials. LiCl (AR, Shanghai Macklin Biochemical Co., Ltd. China) was chosen as salt.
Fig. 3. SEM images of products synthesized by nano Al2O3 with MgO powders in molten salt.
Al2O3 and MgO were dry-mixed at a molar ratio of 1:1, respectively. The salt was added into the each mixed powder in a mass ratio of 5:1, and each powder mixture was placed in an alumina crucible with an alumina lid. Then all the sets were placed in alumina tube-furnace and heated from room temperature to 600 °C at a heating rate of 10 °C·min−1, then to 1150 °C at a heating rate of 5 °C·min−1 for 3 h. After furnace-cooling to room temperature, the solidified sample was rinsed with hot distilled water to remove the LiCl salt. This process was repeated several times until Cl− could not be detected by dripping AgNO3 solution into the filtrate, i.e., no white AgCl precipitation was observed. Finally, the obtained products were oven-dried at 100 °C for 6 h before further characterization. The crystal phase of samples was identified by X-ray diffraction (XRD, X-ray diffractometer, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5418 Å). The morphology and microstructure were observed and analyzed by a field emission scanning electron microscope (FESEM, NovaNanoSEM450, FEI), a high-resolution transmission electron microscopy (HRTEM, Talos F200X, FEI, USA). The energy
Fig. 2. SEM image of (a, b) nano Al2O3, (c, d) plate-like Al2O3, (e, f) Al2O3 fiber and (g, h) MgO powder. 14398
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dispersive X-ray spectroscopies (EDS) linked with the FESEM and the HRTEM, along with selected area electron diffraction (SAED), were employed to assist the phase identification. The size distributions of the products were measured by laser particle size analyzer (LA-950, Horiba company, Japan).
3. Results and discussion
Fig. 4. SEM images of products synthesized by plate-like Al2O3 with MgO powders in molten salt.
Fig. 5. SEM images of products synthesized by fibrous Al2O3 with MgO powders in molten salt.
Fig. 1 shows the XRD patterns of the samples obtained in the molten salt of LiCl at 1150 °C for 3 h. The main crystal phase of the three samples was MA spinel (JCPDS Card No. 21-1152), indexed to a pure cubic phase (Space Group: Fd-3m) with the lattice parameter a = 8.083 Å. In addition to MA spinel, a small amount of MgO (JCPDS Card No. 45-0946) was also detected in all three samples. There was the trace of residual Al2O3 (JCPDS Card No. 46-1212) in the samples using nanograined and plate-like Al2O3 while the residual Al2O3 was the other main phase in the sample using fibrous Al2O3. These results revealed that the reaction was incomplete, but does not affect to investigate the formation mechanism of MA spinel. In order to illustrate the change of morphologies before and after the reaction, we began with the observation for morphologies of the raw materials by SEM, shown in Fig. 2. The nanograined Al2O3 formed many aggregates with the size of 3–6 μm (Fig. 2a), and their nanograins had flat surface and irregular hemispherical shape with a uniform size of ∼120 nm (Fig. 2b). The plate-Like Al2O3 was also easy to agglomerate into spherical particles (Fig. 2c). Their platelets had a size in the range of 8–25 μm, and each of them had a smooth surface (Fig. 2d). Al2O3 fibers also had a smooth surface and uniform diameter of about 5–8 μm, and they were up to several hundred micrometer long (Fig. 2e& f). MgO powders possessed various particle sizes, rough surfaces and agglomerates (Fig. 2g&h). Fig. 3 indicates the SEM images of products synthesized by nanograined Al2O3 with MgO powders in molten salt LiCl at 1150 °C for 3 h. New octahedral MA spinel crystals were produced by the reaction of irregular hemispherical nano Al2O3 and MgO powders (Fig. 3a&b). As shown in Fig. 3b–d, many small particles of ∼120 nm were attached to the surface of large octahedral crystals with growth step, having the size of 2–3 μm, (Fig. 3c&d). The small MA spinel particles (EDS not shown here) on the surface of octahedral crystals had a similar size and morphology to the nano Al2O3 raw materials (Figs. 2b and 3d). This suggested that the small MA spinel crystals may be governed by the well documented "template formation mechanism", which declared that the MA, to a large extent, retained the size and morphology of the Al2O3. But this formation mechanism should not be applicable for the overall growth process of well-developed octahedral MA spinel crystals. We suggested that the subsequent evolution process to form octahedral MA spinel crystals was through step-bunched or layer-by-layer growth. Fig. 4 is the SEM images of products synthesized by plate-like Al2O3 with MgO powders. The product basically retained the spherical appearance of Al2O3 large particles (Fig. 4a&b), which were made up by
Fig. 6. Particle size distributions of (a) nanograined Al2O3, (b) plate-like Al2O3, (c) fibrous Al2O3 as well as MgO, and MA synthesized at 1150 °C for 3 h in LiCl. 14399
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Table 1 Comparison of grain sizes of original Al2O3 raw materials and the resultant MA spinel. Raw material
Agglomerates
Single grain
Product
Agglomerates
Single grain
Nano Al2O3 Plate-like Al2O3 Fibrous Al2O3
3–10 μm 10–150 μm –
0.1–0.2 μm 1–20 μm 6–8 μm (diameter)
Nano MA Plate-Like MA Fibrous MA
– 3–25 μm 10–20 μm (diameter)
0.1–2 μm 0.2–0.6 μm 1–3 μm
Fig. 7. TEM microstructure characterization of product synthesized using nanograined Al2O3, (a) Magnified TEM image, (b) HRTEM image, (c) SAED pattern, (d) SEM image in BSE mode, (e–h) EDS full spectrum of elements and their individual mapping images.
small platelets (Fig. 4b&c). Each small platelet was comprised of large number of crystals with different sizes. By further enlarging the image, the small crystals were octahedral MA spinel crystals (EDS not shown here, Fig. 4d). It is notable that the surface roughness between the raw materials and products was different, i.e., each of the Al2O3 platelets was smooth and dense (Fig. 2d), but the surface of the product was rough (Fig. 4c&d). Regarding it as a whole, the shape and size of the original plate-like Al2O3 were largely preserved, indicating the growth of MA spinel with the "template formation mechanism". Fig. 5 is the SEM images of products synthesized by fibrous Al2O3 with MgO powders. Some fibrous rods were covered with small octahedral structural MA spinel particles, but still remained some residual Al2O3 fibrous core (EDS not shown here, Fig. 5a&b). Some octahedral
MA spinel particle clusters were agglomerated and kept a contour of fibers (Fig. 5c&d). Interestingly, the MA contour fiber rods of products had a rough surface with an increased diameter of 10–20 μm and had been cracked into a hollow state (Fig. 5a–c), whereas Al2O3 original fibers had smooth surface and uniform diameter of about 5–8 μm (Fig. 2c&d). This should be due to the decrease in density of MA spinel phase formed from Al2O3 and MgO (Al2O3: 3.99 g·cm−3; MgO: 3.58 g·cm−3; MA: 3.58 g·cm−3), associated with the volume expansion of ∼8% and the formation of pores [42]. As the reaction diffused into the interior of the fiber rod, the center of the fiber rod became hollow and the diameter was increased. Consequently, the agglomerated MA spinel crystal-clusters just kept a contour of fibers. Fig. 6 shows the particle size distributions of the synthesized MA
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Fig. 8. SEM elemental line scanning microstructure characterization of product synthesized using plate-like Al2O3 after being embedded and polished in phenolic resin (PR), (a) high magnification SEM image, (b–d) Line scanning results of Mg, Al and O elements, (e) EDS results from different location points in SEM image of Fig. 8a.
Fig. 9. SEM elemental mapping microstructures characterization of products synthesized using fibrous Al2O3 after being embedded and polished in phenolic resin (PR), (a) partly reacted fibrous MA with their individual mapping images (b) full-reacted fibrous MA with their individual mapping images.
Fig. 10. Schematic of proposed mechanisms for the MA spinel formation using different shape Al2O3 as the templets (I) nanograins, (II) platelets, (III) fibers with MgO powder in molten salt LiCl at 1150 °C for 3 h. 14401
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spinel as well as the original Al2O3 and MgO raw materials. However, the detected size of particle was not as big as that of crystal, seen from the SEM images in Figs. 2–5. Therefore, the crystal sizes of original Al2O3 raw material and the resultant MA spinel were also evaluated from the SEM images and listed in Table 1. The original Al2O3 nanograins with the crystal size of 0.1–0.2 μm were highly agglomerate to form particles with the sizes of 3–10 μm, whereas the resultant MA spinel was in the form of single crystals with the sizes of 0.1–2 μm (Figs. 3 and 6a and Table 1), so that the tested average particle size of the nano Al2O3 raw materials was larger than that of the produced MA powders by Fig. 6a. As for using the plate-like Al2O3, the produced MA spinel with the single octahedral crystal size of 0.2–0.6 μm kept the plate-like shape with the sizes of 3–25 μm (Fig. 2c&d, Fig. 4 and Table 1). Since the synthetic MA spinel agglomerates became small and loose, and it was easily dispersed after ultrasonication, so that the size of the MA spinel was slightly smaller than that of the Al2O3 raw material, and the particle size distribution of the resultant plate-like MA was different from that of the MgO, but similar to that of the plate-like Al2O3 (Fig. 6b). As seen from Fig. 6c, there were only the particle size distribution of MgO raw material and the synthesized MA, because the fibrous Al2O3 with the diameter of ∼8 μm was too long to test its particle size distribution, just marked by dotted line. The particle size distribution of the synthesized MA seem to be similar to that of the agglomerated MgO, however, seen from the SEM images (Fig. 5), some of the generated MA with crystal size of 1–3 μm existed in a short rodlike form, to an extent, keeping with the morphology of original fibrous Al2O3. In order to illustrate the formation mechanism of MA spinel, the detailed structures of different MA products were further characterized. Firstly, the microstructures of product synthesized by nanograined Al2O3 were characterized by TEM, HRTEM, SAED and EDS, shown in Fig. 7. Several mono-grains were agglomerated in clusters (Fig. 7a). The product was well crystalline with a measured lattice fringe spacing of 2.9 Å, matching well with the (220) plane of MA spinel (Fig. 7b). The SAED pattern indicated the (220), (2¯02¯ ) and (022¯ ) lattice planes, revealing a cubic structure with Fd3¯m symmetry (Fig. 7c). The EDS spectrum indicated the existence of Al, Mg, and O elements (Fig. 7d&e), and the EDS mapping of Mg, Al and O elements with the uniform distributions as well as corresponding SEM (BSE mode) image in Fig. 7d,fh revealed that the closely contacted grain clusters were the formed MgAl2O4 grains. However, some areas with Mg and O elements but without Al element pointed by arrows showed the residue of some MgO, corresponding to the XRD results (Fig. 1c). Second, in order to discriminate the formation of MA spinel based on MgO or Al2O3 shape, the products synthesized by plate-like and fibrous Al2O3 were embedded and solidified in thermosetting phenolic resin (PR). After being polished, they were investigated by SEM-EDS analysis (Fig. 8 and Fig. 9). Fig. 8a is the high magnification SEM image of polished platelike particle with a 2 μm thickness rectangular section, which was consistent with the above SEM result in Fig. 6c. The linear composition profiles revealed that the distribution of Mg, Al and O elements on the micro-structural scale was very inhomogeneous within the particle (Fig. 8a–d). Mg was enriched in the boundary layer of the particle and gradually reduced into the central part (Fig. 8b), which was also supported by the EDS data (Fig. 8e). The high magnification SEM images of polished fibrous product sections with Mg, Al and O elemental maps in the delineated rectangle area are shown in Fig. 9. The elemental maps of Fig. 9a indicated that Al and O elements were well dispersed in full fibrous oblique section, while Mg was just rich in outer layer but not clearly visible in central part. However, the elemental maps of Fig. 9b showed that Mg, Al and O elements were very homogeneous in fibrous oblique section. This should be due to partial reaction (Fig. 9a) and full reaction (Fig. 9b) between Al2O3 and MgO. The above results were strong evidence revealing that the formation of MA was through the reaction of MgO diffusion into Al2O3 from outside to inside. The above results suggested that the ‘‘template formation mechanism’’ played a
key role in the MSS process. This mechanism documented that the more soluble reactant dissolved in the salt, diffused onto the surface of the insoluble or less soluble reactant, and then reacted to form the product which kept the morphology and size of the insoluble or less soluble reactant [40]. Based on the results described and discussed above, the formation mechanism of the MA spinel using different shape Al2O3 with MgO as raw materials in LiCl molten salt can be schematically illustrated in Fig. 10 and discussed in the follows. Whatever shape of Al2O3 was used, the formation of MA spinel should be through the reaction of MgO diffusion into the Al2O3 templet in the molten salt (Fig. 10a&b). First, MgO should be dissolved as Mg2+ and O2− in LiCl melt at high temperature. In liquid LiCl, MgO migrated to the surface of undissolved Al2O3 in the form of Mg2+ and O2− and reacted with Al2O3, and the MA spinel was nucleated by the reaction (1), and afterwards was developed to octahedral crystal (Fig. 10b). When the reaction further took place, as for the sample using nanograined Al2O3 raw material, the newly formed MA spinel seeds initially moved and attached to the surface of the large octahedral MA spinel crystal, and they were subsequently engulfed by the large MA spinel crystal, which further grew in a layered manner to become micro-sized crystal (Figs. 3d and 10Ic), namely this was governed by “epitaxy formation mechanism”. As for the sample using plate-like or fibrous Al2O3 raw material, MgO diffused continuously into the interior of Al2O3 to form MA spinel with a gradient decrease x from the surface to the depth (Figs. 8–9). Due to the increased migration distance and the resistance from the MA spinel layer, the reaction rate would be slowed down, resulting in some fibers with MA shell and unreacted Al2O3 core (Figs. 8a, 9a and 10IIIc) When the reaction proceeded, the internal Al2O3 reacted to form MA spinel totally (Fig. 9b). Finally, the formed MA spinel with a rough surface basically retained the plate-like or short-fibrous appearance of original Al2O3 large particles (Fig. 10IIc&IIIc). Nevertheless, due to the difference in crystal structure and volume expansion between the MA spinel and the original Al2O3 fiber, some of resultant MA spinel became porous and their diameters were increased (Figs. 5c and 10IIIc).
xMgO + Al2 O3 → Mgx Al2 O3 + x
(1)
4. Conclusions MA spinel was synthesized from nanograined, plate-like, and fibrous Al2O3 with MgO powders in LiCl molten salt at 1150 °C for 3 h, respectively. The nanograined Al2O3-resultant MA spinel was octahedral single grain with the sizes of 0.1–2 μm, and it kept the plate-like and short rod-like morphology when using plate-like and fibrous Al2O3. These results and analysis determined that “Al2O3-template formation mechanism” played a pivotal role in MSS process of MA spinel, i.e., the reaction of MgO diffusion into the Al2O3 templet in the molten salt. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772140 and 51862024), Natural Science Foundation of Jiangxi Province (20171ACB21033), Aeronautical Science Foundation of China (2016ZF56019), Science and Technology Innovation Fund of Shanghai Aerospace (SAST2017116), Science and Technology Project of the Education Department of Jiangxi Province (GJJ170573), and Graduate Innovation Special Fund of Nanchang Hangkong University (YC2018003). References
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Molten salt synthesis and formation mechanism of magnesium aluminate spinel using different shape Al2O3 as the templets
T
Xiaolong Houa, Xibao Lia,∗∗, Mingqiang Liua, Zhijun Fenga, Zhihui Hua, Meng Zhanga, Zhi Chena, Juntong Huanga,b,∗ a b
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, Jiangxi Province, 330063, PR China The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium aluminate spinel Different shape Al2O3 Molten salt synthesis Template formation mechanism Gradient growth
In order to verify the “Al2O3-template-formation” mechanism of magnesium aluminate (MA) spinel proposed previously using Al2O3 and MgO micro-powders as raw materials, in this work, MA spinel was synthesized by nanograined, plate-like, and fibrous Al2O3 in LiCl molten salt at 1150 °C for 3 h, respectively. The products were characterized by XRD, SEM, TEM, and EDS techniques, and the grain size of the products and raw materials were analyzed. The results showed clearly that the MA spinel was initially nucleated and subsequently developed to octahedral crystal. When the reaction further took place, when using nanograined Al2O3, the newly formed MA spinel seeds initially moved and attached to the surface of the large octahedral MA spinel crystal, and they were subsequently engulfed by the large MA spinel crystal, which further grew via layer-by-layer to become microsized crystal. Using plate-like or fibrous Al2O3 raw material, MgO diffused continuously into the interior of Al2O3 to form MA spinel with a gradient growth from surface to depth. These revealed that whatever shape of Al2O3 was used, the synthesis of MA spinel was governed by “Al2O3-template formation mechanism”, i.e., by the reaction of MgO diffusion into the Al2O3 templet in the molten salt.
1. Introduction Ceramics and refractory materials have been widely used in many fields, and with the in-depth study for them over several decades, highquality raw materials have become the guarantee for the preparation of excellent performance ceramics and refractory materials [1,2]. Because of its excellent properties including good mechanical properties, high chemical stability, high thermal shock resistance, wide energy band, low electrical conductivity and low thermal conductivity [3–6], magnesium aluminate (MA) spinel has important applications in the field of refractory [7,8], optical materials [9,10], transparent ceramics and armor [11,12], and electronic ceramic materials, etc [13]. Synthesizing high quality MA spinel powder is a key step for its application. The conventional method to prepare MA spinel is through solid-state reaction of Al2O3 and MgO at around 1500 °C. This method is easy to operate and can be used in large-scale production [14], but the reaction temperature required is very high and the final product does not reach the expected purity and particle size. The vapor deposition method is complex and the reaction conditions are harsh. Sol-gel method is an effective and widely used method for synthesizing MA ∗
spinel [15–19], the required synthesis temperature is low and the purity is high. Unfortunately, the equipment and raw materials are expensive and the preparation cost is very high [20,21]. Therefore, during past years, MA spinel powders has been commercially synthesized using several novel processing routes, mainly including flash pyrolysis method [22], self heat sustained technique [23], microwave-assisted combustion synthesis [24,25], combustion method [26], hydrothermal method [27], co-precipitation method [28], mechanochemical processing [29–31], etc. Among these methods, whatever method is used to prepare MA spinel, the purpose is to enhance the reactive activity, reduce the synthesis temperature and increase surface area [32–35]. In recent years, molten salt synthesis (MSS) technology has aroused considerable attention [36–38]. During in the MSS liquid/solid system, the raw materials could be mixed more homogeneously and diffused more rapidly than in the conventional mixed oxide (CMOS) solid–solid system, resulting in a significant reduction in the synthesis temperature and reaction time [39–41]. Zhang et al. synthesized MA spinel by the MSS technology with Al2O3 and MgO micro-powders as raw materials in alkali chloride salt (LiCl, NaCl or KCl) at 1150 °C for 3 h, indicating that the morphology and size of particle was similar to Al2O3, and
Corresponding author. School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang, Jiangxi Province, 330063, PR China. Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (J. Huang).
∗∗
https://doi.org/10.1016/j.ceramint.2019.04.157 Received 1 April 2019; Received in revised form 15 April 2019; Accepted 17 April 2019 Available online 21 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of products synthesized from MgO powders and different shape Al2O3 (a) nanograins, (b) platelets, (c) fibers in molten salt LiCl at 1150 °C for 3 h.
suggested that the formation of MA spinel was through the "Al2O3template formation mechanism" in the MSS process [40]. In order to verify the rationality and accuracy of this mechanism, in this work, a similar MSS method was used to synthesize MA spinel by using nanograined, plate-like, fibrous Al2O3 with MgO powder as raw materials, respectively. Their morphologies and microstructures were observed and analyzed in-depth by FESEM, TEM, HRTEM, along with EDS, especially for the polished surface of products. Finally, their corresponding formation mechanisms were proposed.
2. Experimental Al2O3 nanograins (A800207, Shanghai Macklin Biochemical Co., Ltd. China), plate-like Al2O3 powders (AR, Xilong Chemical Co., Ltd. China) or Al2O3 fibers (AR, Xilong Chemical Co., Ltd. China) as well as MgO powders (AR, Xilong Chemical Co., Ltd. China) were used as the raw materials. LiCl (AR, Shanghai Macklin Biochemical Co., Ltd. China) was chosen as salt.
Fig. 3. SEM images of products synthesized by nano Al2O3 with MgO powders in molten salt.
Al2O3 and MgO were dry-mixed at a molar ratio of 1:1, respectively. The salt was added into the each mixed powder in a mass ratio of 5:1, and each powder mixture was placed in an alumina crucible with an alumina lid. Then all the sets were placed in alumina tube-furnace and heated from room temperature to 600 °C at a heating rate of 10 °C·min−1, then to 1150 °C at a heating rate of 5 °C·min−1 for 3 h. After furnace-cooling to room temperature, the solidified sample was rinsed with hot distilled water to remove the LiCl salt. This process was repeated several times until Cl− could not be detected by dripping AgNO3 solution into the filtrate, i.e., no white AgCl precipitation was observed. Finally, the obtained products were oven-dried at 100 °C for 6 h before further characterization. The crystal phase of samples was identified by X-ray diffraction (XRD, X-ray diffractometer, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5418 Å). The morphology and microstructure were observed and analyzed by a field emission scanning electron microscope (FESEM, NovaNanoSEM450, FEI), a high-resolution transmission electron microscopy (HRTEM, Talos F200X, FEI, USA). The energy
Fig. 2. SEM image of (a, b) nano Al2O3, (c, d) plate-like Al2O3, (e, f) Al2O3 fiber and (g, h) MgO powder. 14398
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dispersive X-ray spectroscopies (EDS) linked with the FESEM and the HRTEM, along with selected area electron diffraction (SAED), were employed to assist the phase identification. The size distributions of the products were measured by laser particle size analyzer (LA-950, Horiba company, Japan).
3. Results and discussion
Fig. 4. SEM images of products synthesized by plate-like Al2O3 with MgO powders in molten salt.
Fig. 5. SEM images of products synthesized by fibrous Al2O3 with MgO powders in molten salt.
Fig. 1 shows the XRD patterns of the samples obtained in the molten salt of LiCl at 1150 °C for 3 h. The main crystal phase of the three samples was MA spinel (JCPDS Card No. 21-1152), indexed to a pure cubic phase (Space Group: Fd-3m) with the lattice parameter a = 8.083 Å. In addition to MA spinel, a small amount of MgO (JCPDS Card No. 45-0946) was also detected in all three samples. There was the trace of residual Al2O3 (JCPDS Card No. 46-1212) in the samples using nanograined and plate-like Al2O3 while the residual Al2O3 was the other main phase in the sample using fibrous Al2O3. These results revealed that the reaction was incomplete, but does not affect to investigate the formation mechanism of MA spinel. In order to illustrate the change of morphologies before and after the reaction, we began with the observation for morphologies of the raw materials by SEM, shown in Fig. 2. The nanograined Al2O3 formed many aggregates with the size of 3–6 μm (Fig. 2a), and their nanograins had flat surface and irregular hemispherical shape with a uniform size of ∼120 nm (Fig. 2b). The plate-Like Al2O3 was also easy to agglomerate into spherical particles (Fig. 2c). Their platelets had a size in the range of 8–25 μm, and each of them had a smooth surface (Fig. 2d). Al2O3 fibers also had a smooth surface and uniform diameter of about 5–8 μm, and they were up to several hundred micrometer long (Fig. 2e& f). MgO powders possessed various particle sizes, rough surfaces and agglomerates (Fig. 2g&h). Fig. 3 indicates the SEM images of products synthesized by nanograined Al2O3 with MgO powders in molten salt LiCl at 1150 °C for 3 h. New octahedral MA spinel crystals were produced by the reaction of irregular hemispherical nano Al2O3 and MgO powders (Fig. 3a&b). As shown in Fig. 3b–d, many small particles of ∼120 nm were attached to the surface of large octahedral crystals with growth step, having the size of 2–3 μm, (Fig. 3c&d). The small MA spinel particles (EDS not shown here) on the surface of octahedral crystals had a similar size and morphology to the nano Al2O3 raw materials (Figs. 2b and 3d). This suggested that the small MA spinel crystals may be governed by the well documented "template formation mechanism", which declared that the MA, to a large extent, retained the size and morphology of the Al2O3. But this formation mechanism should not be applicable for the overall growth process of well-developed octahedral MA spinel crystals. We suggested that the subsequent evolution process to form octahedral MA spinel crystals was through step-bunched or layer-by-layer growth. Fig. 4 is the SEM images of products synthesized by plate-like Al2O3 with MgO powders. The product basically retained the spherical appearance of Al2O3 large particles (Fig. 4a&b), which were made up by
Fig. 6. Particle size distributions of (a) nanograined Al2O3, (b) plate-like Al2O3, (c) fibrous Al2O3 as well as MgO, and MA synthesized at 1150 °C for 3 h in LiCl. 14399
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Table 1 Comparison of grain sizes of original Al2O3 raw materials and the resultant MA spinel. Raw material
Agglomerates
Single grain
Product
Agglomerates
Single grain
Nano Al2O3 Plate-like Al2O3 Fibrous Al2O3
3–10 μm 10–150 μm –
0.1–0.2 μm 1–20 μm 6–8 μm (diameter)
Nano MA Plate-Like MA Fibrous MA
– 3–25 μm 10–20 μm (diameter)
0.1–2 μm 0.2–0.6 μm 1–3 μm
Fig. 7. TEM microstructure characterization of product synthesized using nanograined Al2O3, (a) Magnified TEM image, (b) HRTEM image, (c) SAED pattern, (d) SEM image in BSE mode, (e–h) EDS full spectrum of elements and their individual mapping images.
small platelets (Fig. 4b&c). Each small platelet was comprised of large number of crystals with different sizes. By further enlarging the image, the small crystals were octahedral MA spinel crystals (EDS not shown here, Fig. 4d). It is notable that the surface roughness between the raw materials and products was different, i.e., each of the Al2O3 platelets was smooth and dense (Fig. 2d), but the surface of the product was rough (Fig. 4c&d). Regarding it as a whole, the shape and size of the original plate-like Al2O3 were largely preserved, indicating the growth of MA spinel with the "template formation mechanism". Fig. 5 is the SEM images of products synthesized by fibrous Al2O3 with MgO powders. Some fibrous rods were covered with small octahedral structural MA spinel particles, but still remained some residual Al2O3 fibrous core (EDS not shown here, Fig. 5a&b). Some octahedral
MA spinel particle clusters were agglomerated and kept a contour of fibers (Fig. 5c&d). Interestingly, the MA contour fiber rods of products had a rough surface with an increased diameter of 10–20 μm and had been cracked into a hollow state (Fig. 5a–c), whereas Al2O3 original fibers had smooth surface and uniform diameter of about 5–8 μm (Fig. 2c&d). This should be due to the decrease in density of MA spinel phase formed from Al2O3 and MgO (Al2O3: 3.99 g·cm−3; MgO: 3.58 g·cm−3; MA: 3.58 g·cm−3), associated with the volume expansion of ∼8% and the formation of pores [42]. As the reaction diffused into the interior of the fiber rod, the center of the fiber rod became hollow and the diameter was increased. Consequently, the agglomerated MA spinel crystal-clusters just kept a contour of fibers. Fig. 6 shows the particle size distributions of the synthesized MA
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Fig. 8. SEM elemental line scanning microstructure characterization of product synthesized using plate-like Al2O3 after being embedded and polished in phenolic resin (PR), (a) high magnification SEM image, (b–d) Line scanning results of Mg, Al and O elements, (e) EDS results from different location points in SEM image of Fig. 8a.
Fig. 9. SEM elemental mapping microstructures characterization of products synthesized using fibrous Al2O3 after being embedded and polished in phenolic resin (PR), (a) partly reacted fibrous MA with their individual mapping images (b) full-reacted fibrous MA with their individual mapping images.
Fig. 10. Schematic of proposed mechanisms for the MA spinel formation using different shape Al2O3 as the templets (I) nanograins, (II) platelets, (III) fibers with MgO powder in molten salt LiCl at 1150 °C for 3 h. 14401
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spinel as well as the original Al2O3 and MgO raw materials. However, the detected size of particle was not as big as that of crystal, seen from the SEM images in Figs. 2–5. Therefore, the crystal sizes of original Al2O3 raw material and the resultant MA spinel were also evaluated from the SEM images and listed in Table 1. The original Al2O3 nanograins with the crystal size of 0.1–0.2 μm were highly agglomerate to form particles with the sizes of 3–10 μm, whereas the resultant MA spinel was in the form of single crystals with the sizes of 0.1–2 μm (Figs. 3 and 6a and Table 1), so that the tested average particle size of the nano Al2O3 raw materials was larger than that of the produced MA powders by Fig. 6a. As for using the plate-like Al2O3, the produced MA spinel with the single octahedral crystal size of 0.2–0.6 μm kept the plate-like shape with the sizes of 3–25 μm (Fig. 2c&d, Fig. 4 and Table 1). Since the synthetic MA spinel agglomerates became small and loose, and it was easily dispersed after ultrasonication, so that the size of the MA spinel was slightly smaller than that of the Al2O3 raw material, and the particle size distribution of the resultant plate-like MA was different from that of the MgO, but similar to that of the plate-like Al2O3 (Fig. 6b). As seen from Fig. 6c, there were only the particle size distribution of MgO raw material and the synthesized MA, because the fibrous Al2O3 with the diameter of ∼8 μm was too long to test its particle size distribution, just marked by dotted line. The particle size distribution of the synthesized MA seem to be similar to that of the agglomerated MgO, however, seen from the SEM images (Fig. 5), some of the generated MA with crystal size of 1–3 μm existed in a short rodlike form, to an extent, keeping with the morphology of original fibrous Al2O3. In order to illustrate the formation mechanism of MA spinel, the detailed structures of different MA products were further characterized. Firstly, the microstructures of product synthesized by nanograined Al2O3 were characterized by TEM, HRTEM, SAED and EDS, shown in Fig. 7. Several mono-grains were agglomerated in clusters (Fig. 7a). The product was well crystalline with a measured lattice fringe spacing of 2.9 Å, matching well with the (220) plane of MA spinel (Fig. 7b). The SAED pattern indicated the (220), (2¯02¯ ) and (022¯ ) lattice planes, revealing a cubic structure with Fd3¯m symmetry (Fig. 7c). The EDS spectrum indicated the existence of Al, Mg, and O elements (Fig. 7d&e), and the EDS mapping of Mg, Al and O elements with the uniform distributions as well as corresponding SEM (BSE mode) image in Fig. 7d,fh revealed that the closely contacted grain clusters were the formed MgAl2O4 grains. However, some areas with Mg and O elements but without Al element pointed by arrows showed the residue of some MgO, corresponding to the XRD results (Fig. 1c). Second, in order to discriminate the formation of MA spinel based on MgO or Al2O3 shape, the products synthesized by plate-like and fibrous Al2O3 were embedded and solidified in thermosetting phenolic resin (PR). After being polished, they were investigated by SEM-EDS analysis (Fig. 8 and Fig. 9). Fig. 8a is the high magnification SEM image of polished platelike particle with a 2 μm thickness rectangular section, which was consistent with the above SEM result in Fig. 6c. The linear composition profiles revealed that the distribution of Mg, Al and O elements on the micro-structural scale was very inhomogeneous within the particle (Fig. 8a–d). Mg was enriched in the boundary layer of the particle and gradually reduced into the central part (Fig. 8b), which was also supported by the EDS data (Fig. 8e). The high magnification SEM images of polished fibrous product sections with Mg, Al and O elemental maps in the delineated rectangle area are shown in Fig. 9. The elemental maps of Fig. 9a indicated that Al and O elements were well dispersed in full fibrous oblique section, while Mg was just rich in outer layer but not clearly visible in central part. However, the elemental maps of Fig. 9b showed that Mg, Al and O elements were very homogeneous in fibrous oblique section. This should be due to partial reaction (Fig. 9a) and full reaction (Fig. 9b) between Al2O3 and MgO. The above results were strong evidence revealing that the formation of MA was through the reaction of MgO diffusion into Al2O3 from outside to inside. The above results suggested that the ‘‘template formation mechanism’’ played a
key role in the MSS process. This mechanism documented that the more soluble reactant dissolved in the salt, diffused onto the surface of the insoluble or less soluble reactant, and then reacted to form the product which kept the morphology and size of the insoluble or less soluble reactant [40]. Based on the results described and discussed above, the formation mechanism of the MA spinel using different shape Al2O3 with MgO as raw materials in LiCl molten salt can be schematically illustrated in Fig. 10 and discussed in the follows. Whatever shape of Al2O3 was used, the formation of MA spinel should be through the reaction of MgO diffusion into the Al2O3 templet in the molten salt (Fig. 10a&b). First, MgO should be dissolved as Mg2+ and O2− in LiCl melt at high temperature. In liquid LiCl, MgO migrated to the surface of undissolved Al2O3 in the form of Mg2+ and O2− and reacted with Al2O3, and the MA spinel was nucleated by the reaction (1), and afterwards was developed to octahedral crystal (Fig. 10b). When the reaction further took place, as for the sample using nanograined Al2O3 raw material, the newly formed MA spinel seeds initially moved and attached to the surface of the large octahedral MA spinel crystal, and they were subsequently engulfed by the large MA spinel crystal, which further grew in a layered manner to become micro-sized crystal (Figs. 3d and 10Ic), namely this was governed by “epitaxy formation mechanism”. As for the sample using plate-like or fibrous Al2O3 raw material, MgO diffused continuously into the interior of Al2O3 to form MA spinel with a gradient decrease x from the surface to the depth (Figs. 8–9). Due to the increased migration distance and the resistance from the MA spinel layer, the reaction rate would be slowed down, resulting in some fibers with MA shell and unreacted Al2O3 core (Figs. 8a, 9a and 10IIIc) When the reaction proceeded, the internal Al2O3 reacted to form MA spinel totally (Fig. 9b). Finally, the formed MA spinel with a rough surface basically retained the plate-like or short-fibrous appearance of original Al2O3 large particles (Fig. 10IIc&IIIc). Nevertheless, due to the difference in crystal structure and volume expansion between the MA spinel and the original Al2O3 fiber, some of resultant MA spinel became porous and their diameters were increased (Figs. 5c and 10IIIc).
xMgO + Al2 O3 → Mgx Al2 O3 + x
(1)
4. Conclusions MA spinel was synthesized from nanograined, plate-like, and fibrous Al2O3 with MgO powders in LiCl molten salt at 1150 °C for 3 h, respectively. The nanograined Al2O3-resultant MA spinel was octahedral single grain with the sizes of 0.1–2 μm, and it kept the plate-like and short rod-like morphology when using plate-like and fibrous Al2O3. These results and analysis determined that “Al2O3-template formation mechanism” played a pivotal role in MSS process of MA spinel, i.e., the reaction of MgO diffusion into the Al2O3 templet in the molten salt. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772140 and 51862024), Natural Science Foundation of Jiangxi Province (20171ACB21033), Aeronautical Science Foundation of China (2016ZF56019), Science and Technology Innovation Fund of Shanghai Aerospace (SAST2017116), Science and Technology Project of the Education Department of Jiangxi Province (GJJ170573), and Graduate Innovation Special Fund of Nanchang Hangkong University (YC2018003). References
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