Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics

Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics

Optical Materials xxx (2016) 1e4 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Effec...

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Optical Materials xxx (2016) 1e4

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics Dan Han a, b, Jian Zhang a, *, Peng Liu c, Shiwei Wang a, ** a The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, PR China b Graduate University of the Chinese Academy of Sciences, Beijing, 100049, PR China c School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou, 221116, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 March 2016 Received in revised form 10 May 2016 Accepted 12 June 2016 Available online xxx

Transparent MgAl2O4 ceramics were fabricated by reactive sintering in air followed by hot isostatic press treatment using commercial Al2O3 powder (g-Al2O3 or a-Al2O3) and MgO powder as raw materials. The densification rate, microstructure and optical properties of the ceramics were investigated. Densification temperature of the sample from g-Al2O3/MgO was lower than that from a-Al2O3/MgO. However, in-line transmission (2 mm thick) of the sample from a-Al2O3/MgO at the wavelength of 600 nm and 1100 nm were respectively 77.7% and 84.3%, higher than those (66.7%, 81.4%) of the sample from g-Al2O3/MgO. SEM observation revealed that the sample from a-Al2O3/MgO exhibited a homogeneous and pore-free microstructure, while, the sample from g-Al2O3/MgO showed an apparent bimodal microstructure containing pores. © 2016 Elsevier B.V. All rights reserved.

Keywords: Spinel Reactive sintering Microstructure Optical property Transparent ceramic

1. Introduction Since the first appearance in the late 1960s [1], transparent magnesium aluminate spinel (MgAl2O4) ceramics have received a great deal of attention due to the excellent mechanical properties and high optical transmission (0.2e6 mm) [2,3]. Over the last ten years, with the improvement of fabrication technology and raw powder quality, MgAl2O4 ceramics with high optical quality and large size have been produced, which renders them suitable for transparent armor, domes and windows for ultraviolet (UV), visible (VIS), and infrared (IR) application [4e6]. Most of the transparent MgAl2O4 ceramics were produced by hot pressing (HP) [7,8], HP/HIP (Hot Isostatic Press) [9,10] or spark plasma sintering (SPS) [11e13] using MgAl2O4 powders doped with LiF or other sintering aids [1,7,14]. The powder characteristics including purity, particle size, size distribution, morphology obviously affect sintering process and properties of the ceramics. The limited availability and fairly high cost of high quality raw powder

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (S. Wang).

have restricted the widely application of transparent MgAl2O4 ceramics. In fact, reactive sintering using widely produced Al2O3 and MgO powders is a feasible method to prepare transparent MgAl2O4 ceramics. A few articles [15e17] have reported the production of transparent MgAl2O4 ceramics by solid-state reactive sintering and almost all of them choose a-Al2O3 as raw materials. It is known that g-Al2O3 owns high activity which can promote the reaction and densification process, but it is barely reported to be used in producing transparent MgAl2O4 ceramics. In the present work, transparent MgAl2O4 ceramics were fabricated by reactive sintering in air and further by HIPing, starting from commercial Al2O3 powder (g-Al2O3 or a-Al2O3) and MgO powder. The effects of polymorphism of Al2O3 on the densification rate, microstructure and optical properties were investigated.

2. Experimental procedures The raw powders were high-purity g-Al2O3 (purity, 99.99%; particle size, 100 nm), a-Al2O3 (purity, 99.99%; particle size, 150 nm) and MgO (purity, 99.99%; particle size, 150 nm). The combinations of starting powders were g-Al2O3 with MgO (gAl2O3/MgO) and a-Al2O3 with MgO (a-Al2O3/MgO). As the powders were easy bibulous, they were dried at 120  C for 12 h before

http://dx.doi.org/10.1016/j.optmat.2016.06.016 0925-3467/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: D. Han, et al., Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.016

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weighed. The powders were weighted according to the stoichiometry of MgAl2O4, and then mixed by ball milling for 12 h using ethanol as medium. The mixtures were dried at 60  C for 12 h and then sieved through 80-mesh screen. After calcined at 800  C for 6 h to remove the organic component, the powders were dry pressed at about 20 MPa, and then further cold isostatically pressed at 200 MPa. Relative density of the pellet from g-Al2O3/MgO is about 35%, much lower than that from a-Al2O3/MgO (~50%). All pellets were pre-sintered in air up to a relative density of 95e98% followed by HIPing to obtain transparent samples. Finally, the resultant ceramics were double-sides polished for further tests. The phase compositions of samples pre-sintered at different temperatures were analyzed by X-ray diffraction (XRD, D8, Bruker, Germany) in the range of 2q ¼ 10e70 . The shrinkage behavior of the green bodies was measured by a thermal dilatometer (DIL, 402E, Netzsch, Germany) and relative densities were measured by the Archimedes method. Thermally etched surfaces of the presintered and HIPed samples were observed with Scanning Electron Microscopy (SEM, JSM-6390, JEOL, Japan). The in-line transmittance was measured by a UVeVISeNIR spectrometer (Carry 5000 spectrophotometer, Varian, USA). 3. Results and discussions The XRD patterns of the samples pre-sintered from 800 to 1200  C are shown in Fig. 1. For the reason of peaks of g-Al2O3 and MgAl2O4 overlap in XRD patterns, it’s difficult to estimate the reaction from Fig. 1b alone. XRD patterns of g-Al2O3 calcined between 800 and 1200  C (Fig. 1a) were also measured for comparison. With the rise of temperature, the crystalline of g-Al2O3 increased, leading to the increase of intensity of the diffraction peaks. g-Al2O3 transformed to a-Al2O3 at 1100e1200  C. For the mixture of g-Al2O3/ MgO, g-Al2O3 firstly reacted with MgO directly which started at 1000  C, when the temperature was above 1100  C, the unconsumed g-Al2O3 transformed to a-Al2O3 (Fig. 1b). For the sample from a-Al2O3/MgO, the onset of the reaction was located at 900  C (Fig. 1c). It seems that the reactivity of a-Al2O3/MgO was a little higher than that of g-Al2O3/MgO. This may be caused by the high relative density (50%) and large contact area between Al2O3 and MgO particles in the green body from a-Al2O3/MgO. With further increasing the temperature to 1200  C, all the peaks of the samples from g-Al2O3/MgO and a-Al2O3/MgO were indexed to MgAl2O4 (PDF#82-2424) and no other impurity was found. Fig. 2 shows the shrinkage curves of the green bodies between 20  C and 1650  C. For the sample from g-Al2O3/MgO, its shrinkage started at ~1000  C and ended at ~1400  C without any expansion (Fig. 2a). Generally, 5%e8% volume expansion will happen during the reaction process, because of the different densities of MgO (3.58 g/cm3), Al2O3 (3.98 g/cm3), and MgAl2O4 (3.58 g/cm3). The reason why no expansion happened in the present case may include two aspects: one is that the expansion by reaction was lower than the shrinkage by sintering; the other is the quite low relative density of the green body from g-Al2O3/MgO which can offset the expansion caused by reaction. In contrast, the shrinkage of the sample from a-Al2O3/MgO started at 1400  C and it demonstrated a slight expansion between 1100  C and 1300  C (Fig. 2b) which was caused by the reaction of Al2O3 and MgO. The relative density and grain size of the pre-sintered samples as a function of sintering temperature are shown in Fig. 3. For the sample from g-Al2O3/MgO, the relative density increased rapidly to 98.5% with the temperature rising to 1400  C, then showed little change when the temperature continued to rise (Fig. 3a), it is corresponding to the shrinkage curve (Fig. 2a). In contrast, the grain size of the sample grew slowly before 1400  C, and when the temperature was above 1400  C, the rapid grain growth happened

Fig. 1. X-ray patterns of the samples from (a) g-Al2O3, (b) g-Al2O3/MgO and (c) aAl2O3/MgO pre-sintered at different temperatures. (g: g-Al2O3, a: a-Al2O3, M: MgO, S: MgAl2O4).

Please cite this article in press as: D. Han, et al., Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.016

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Fig. 2. Shrinkage behavior of the samples from (a) g-Al2O3/MgO and (b) a-Al2O3/MgO.

and the intracrystalline pores may generate, which was difficult to be removed in the following HIP treatment process. For the sample from a-Al2O3/MgO, the relative density and grain size increased continuously with the rise of temperature (Fig. 3b). Based on the relative density results, 1350  C and 1550  C have been respectively selected as the pre-sintering temperatures of the samples using gAl2O3 and a-Al2O3. Fig. 4 shows the in-line transmittance curves of the samples HIPed under 200 MPa at 1650  C for 4 h in argon gases. The transmission of the sample from g-Al2O3/MgO was lower than that from a-Al2O3/MgO especially in the ultraviolet and visible light region. The in-line transmittance of the samples from g-Al2O3/MgO and a-Al2O3/MgO were 81.4%, and 84.3% at 1100 nm, respectively. At the short wavelength range, the transmittance of the samples both dropped rapidly, indicating the residue pores existed in the sintered ceramics. Fig. 5a, b shows the thermally etched surfaces of the presintered samples. The average grain size of the sample from gAl2O3/MgO is about 0.6 mm (Fig. 5a), smaller than that from aAl2O3/MgO which is about 1.5 mm (Fig. 5b). The pores of the presintered sample from g-Al2O3/MgO own the coordination numbers (i.e. the number of grains surrounding each pore) of 4e7 (Fig. 5a). The large pores with coordination numbers 6 are thermodynamically stable and difficult to “pop out” even under the high temperature and pressure [18,19]. For the sample from aAl2O3/MgO, no large pores were observed and a few small pores were situated at the boundary or the triple junctions. The microstructures of the HIPed samples are revealed in Fig. 5c, d. The sample from g-Al2O3/MgO exhibited an apparent bimodal microstructure. There are large grains (50e150 mm) containing intracrystalline pores, and fine grains (~10 mm) with intercrystalline

Fig. 3. Relative density and grain size of the samples from (a) g-Al2O3/MgO and (b) aAl2O3/MgO.

Fig. 4. In-line transmittance curves of the samples from g-Al2O3/MgO and a-Al2O3/ MgO (2.0 mm thick).

pores. The large grains may be caused by the small grain size of the pre-sintered sample. When the grain size is below 1 mm, the size of the grain boundary is the determinant of grain growth [20]. While the grain boundary size is nearly equal to the size of small grains, the grain boundary will quickly swept small grains, which will cause the rapid grain growth. The size of pores embedded in the large grains or located among small grains were several hundred nm, according to the scatter theory [1], the residual pores were the

Please cite this article in press as: D. Han, et al., Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.016

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Fig. 5. Microstructures of the samples from g-Al2O3/MgO and a-Al2O3/MgO pre-sintered at a) 1350  C, b) 1550  C respectively and then HIPed at 1650  C (c, d).

reason of the rapid decrease of the transmission in the UVeVIS region. On the contrary, no apparent pores can be observed in the sample from a-Al2O3/MgO and the grain size is quite homogeneous. At the same HIPed temperature, the grain sizes of sample from aAl2O3/MgO were much smaller than that from g-Al2O3/MgO, this is because the activation energy of grain growth of spinel prepared by a-Al2O3 is higher than that by g-Al2O3 [21]. In a word, g-Al2O3 showed quite higher densification rate than a-Al2O3. From Fig. 3, it can be found that the relative density of the sample from g-Al2O3/MgO reached 93.5% at a pre-sintering temperature as low as 1300  C. However, in order to reach the similar relative density for the sample from a-Al2O3/MgO, the pre-sintering temperature has to be higher than 1525  C. If the pre-sintering conditions are optimized properly for the sample from g-Al2O3/ MgO to avoid the formation of thermodynamics stabilized pores, highly transparent MgAl2O4 ceramics could be obtained at a much low HIPing temperature. 4. Conclusions Transparent MgAl2O4 ceramics were fabricated by reactive sintering of Al2O3 with MgO in air and further sintering by HIP. The densification temperature of the sample from g-Al2O3/MgO was around 1400  C, 200  C lower than that from a-Al2O3/MgO. The pre-sintered sample from g-Al2O3/MgO contained large pores which were thermodynamically stable and the grain size was below 1 mm, which caused the residual pores and the bimodal microstructure in the transparent ceramics. The transparency of the sample from a-Al2O3/MgO was high, and the in-line transmittances at the wavelength of 600 nm and 1100 nm reached 77.7% and 84.3%,

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Please cite this article in press as: D. Han, et al., Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.06.016