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Synthesis of highly-infrared transparent Y2O3–MgO nanocomposites by colloidal technique and SPS
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Synthesis of highly-infrared transparent Y2O3–MgO nanocomposites by colloidal technique and SPS Lihong Liua, Koji Moritaa,∗, Tohru S. Suzukib, Byung-Nam Kima a b
Field-Assisted Sintering Group, Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan Ceramics Processing Group, Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Y2O3–MgO composite Colloidal technique Spark plasma sintering Transmittance efficiency Microstructure
Infrared (IR) transparent Y2O3–MgO nanocomposites with a volume ratio of 50:50 were synthesized by combining colloidal and spark-plasma-sintering (SPS) techniques. In order to attain well-dispersed and homogeneous starting Y2O3–MgO nanopowder mixture, the effects of the pH value and the amount of polyetherimide (PEI) dispersant on the suspension stability were studied. Rheological measurement reveals that highly-dispersed and stable suspension was obtained at 7 wt% of PEI dispersant under pH = 10.6. The obtained nanopowders with particle size of 20–30 nm were densified using SPS at several sintering temperatures. The sintered composites show fine grains, narrow grain size distribution and uniform microstructure. The nanocomposite sintered at 1250 °C showed the maximum IR transmittance of 84% at a wavelength range of 2.5–6 μm. The Vickers hardness of the nanocomposite was about 11.9 ± 0.3 GPa, which is significantly higher than those of single phase MgO or Y2O3. Successful fabrication of the high-performance Y2O3–MgO nanocomposite indicates that i) the colloidal technique is an effect method to obtain highly dispersed and homogeneous nanopowders and ii) the SPS technique is a powerful tool to fabricate fine-grained dense transparent ceramics, which are suitable for fabricating IR transparent Y2O3–MgO composite ceramics from commercial starting powders.
1. Introduction Infrared (IR) transparent ceramics, such as Al2O3, MgAl2O4, AlON, and Y2O3, have recently attracted much attention for the application in aerospace field, owing to their excellent optical properties [1–4]. Among them, Y2O3 is recognized as one of the candidate materials for IR windows owing to its wide band transmission, low scattering efficiency and low infrared emissivity at high temperatures. For the nextgeneration IR transparent windows working in harsh environments, however, the further enhancements of durability, mechanical and thermal shock resistance properties are required. For the polycrystalline transparent Y2O3 fabricated by conventional techniques, its limited mechanical properties do not fit with the above requirements due to its large grain size caused by high temperature and longer time sintering processes. In order to improve the mechanical properties, reducing the grain size is an effective method. One technique for reducing the grain size is to disperse second phase particles [5]; dispersed second phases naturally impedes the grain boundary migration of the adjacent phases, and hence, can inhibit grain growth during the densification process [6–9]. This phenomenon is well known as the pining effect for obtaining fine grains in composite
∗
ceramics. For the Y2O3, the dispersion of MgO particle must be a valid candidate. This is because both Y2O3 and MgO are excellent IR transparent materials, and have a longer wavelength cutoff and lower emissivity than other IR transparent materials. Furthermore, according to the phase diagram, these two materials have a eutectic point at 2110 °C, below which both of them are stable and do not react with each other [10]. This characteristic enables to limit the grain growth during the sintering through the pinning effect between Y2O3 and MgO grains. Recently, the Y2O3–MgO nanocomposite ceramics have been reported to show IR transmittance over 3–7 μm wavelength and mechanical properties higher than those of monolithic Y2O3 or MgO polycrystals [11–15]. Realizing the IR transparent Y2O3–MgO composite ceramics, however, is still a big challenge. This is because the large difference in the refractive index between Y2O3 and MgO phases will cause the significant scattering losses at the Y2O3/MgO interfaces, and hence, deteriorate light transmission. Nevertheless, if the grain size is far less than the wavelength of incident lights, the light scattering at the interface boundaries can be negligible and the optical transparency can be improved. In addition, by reducing the gran size, the enhancement of the strength and erosion resistance can also be achieved [10,16,17].
Corresponding author. 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan. E-mail address: [email protected] (K. Morita).
https://doi.org/10.1016/j.ceramint.2020.02.153 Received 8 January 2020; Received in revised form 12 February 2020; Accepted 16 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lihong Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.153
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2.3. Fabrication of Y2O3–MgO nanocomposites
Therefore, the reducing the grain size can satisfy the requirements for IR windows used in the harsher environments. For achieving the fine grain sizes and homogenous microstructures, it is important to control both the nanopowder preparation and sintering conditions. To date, various synthesis methods, such as flame pyrolysis technique, coprecipitation method, hydrothermal method, chemical vapor deposition and the sol-gel method, have been developed for preparing high quality nanopowder mixtures for fabricating composite ceramics [18–22]. Those synthesis methods, however, are often complicated chemical reaction process and need to carefully control the chemical composition, ratio of reactants and reaction speed of two phases to obtain fine and uniform nanopowders. On contrary, since colloidal technique can use commercially supplied raw powders, it is a more simple and cost-effective technique than those of the wet chemical methods for the powder preparation. Furthermore, the colloidal technique offers significant advantages to avoid heterogeneous agglomerates of fine particles by using electrostatic repulsive forces and/ or steric stabilization [23–27]. During the colloidal process, dispersant, solvent, pH adjustment and viscosity should be controlled. Among them, the dispersant will help to induce electrostatic charge to enhance the stability of suspensions through the particle repulsion. The viscosity of the suspension, on the other hand, will help to keep kinetic stability by slowing down the particle aggregation and sedimentation, and finally, a homogeneous mixture can be obtained. In this work, the colloidal technique was used to prepare homogenous Y2O3–MgO nanopowder mixture from the commercially supplied Y2O3 and MgO raw powders. In order to obtain a well-dispersed powder suspension, addition amount of dispersant, pH value of solvent and rheological properties of the suspension were investigated during the processing. The fabricated Y2O3–MgO nanopowder mixture were sintered by using spark-plasma-sintering (SPS) technique, which has been recognized as a powerful sintering tool for attaining dense and fine-grained materials at relatively low temperatures. The microstructure dependent optical and mechanical properties of the fabricated composite ceramics were systemically investigated.
The obtained Y2O3–MgO nanopowder mixtures were consolidated by the SPS machine (LABOX, Sinterland Co., Ltd., Japan). The nanopowders were loaded into the graphite mold with an inner diameter of 10 mm. The interior of the graphite mold was covered with graphite papers. The outside of the mold was covered with a thermal insulator carbon felt to suppress any heat losses from the surface. The SPS process was carried out at a uniaxial pressure of 70 MPa, at a temperature range of 1200–1300 °C, with a fast heating rate of 50 °C/min and a dwelling time of 10 min. During the sintering, the surface temperature of the graphite mold was measured using an optical pyrometer through a hole made in the carbon felt. The sintered Y2O3–MgO composites were postannealed at 1000 °C for 10 h in air to eliminate oxygen vacancies, residual carbon and stress. For IR transmittance measurements, both surfaces of the annealed composites were polished with diamond pastes to ~1.0 mm in thickness. 2.4. Characterization techniques Suspension viscosities and rheological behavior were measured as a function of the shear rate of the suspension by using a cone-plate viscometer (Re-215 Model, Toki Sangyo Co., Ltd., Tokyo, Japan). Zetapotential measurements of the suspensions were performed at 7 wt% PEI as a function of pH using zetasizer nano essentials (Malvern Instruments Ltd., United Kingdom). Thermogravimetric analysis was used to monitor the decomposition of the PEI dispersant by using thermogravimetric and differential thermal analyses (TG/DTA, 2000SR, Bruker, Japan). X-ray diffraction (XRD) analysis of the Y2O3–MgO nanopowder mixtures and sintered bodies was performed by RINT-2500 diffractometer (Rigaku Co., Ltd, Tokyo, Japan, 40 kV 300 mA) using Cu Kα radiation. Microstructures of the nanopowders/nanocomposites were examined by a transmission electronic microscopy (TEM, JEM-2010F, JEOL Co., Ltd, Tokyo, Japan, 200 KV) and field emission scanning electron microscope (FE-SEM, model SU-8000, Hitachi Ltd., Tokyo, Japan). Fourier transform infrared spectroscopy (FT-IR, model 4200, JASCO, Tokyo, Japan) was used to measure the transmittance of the mirror polished samples at a range of 2.5–10 μm. Vickers hardness tester (MVK-E, Akashi Seisakusho, Ltd., Toda, Japan) was used for the hardness measurement at a load of 2 N.
2. Experimental procedure 2.1. Starting materials
3. Results and discussion
Commercially available Y2O3 (BB-type, Shin-Etsu Chemical Co., Ltd., Japan, purity: 99.9%) and MgO (500A-1, Ube Material Industries Ltd., Japan, Purity: 99.98%) nanopowders were used as starting materials. The specific surface area of Y2O3 and MgO powders are 35 m2/g and 28–38 m2/g, respectively. Polyethylenimine (PEI, FUJIFILM Wako Chemical Co., Japan) with 10,000 molecular weight was used as a cationic dispersant.
3.1. Identification of optimal conditions for powder mixture preparation To determine the optimum processing conditions, the effects of PEI amount and pH were investigated in both Y2O3 and MgO suspensions. Fig. 1(a) shows the viscosities of the Y2O3 and MgO suspensions with various PEI amounts under a pH value of 10.6. Both of the suspensions are too sticky to measure the viscosity without PEI addition. The viscosity decreases accordingly with an increase of the PEI amount and shows the lowest value at 7 wt% PEI amounts. With further increasing the PEI amount, the viscosities of both suspensions increase again, indicating that the optimal PEI adding amount for both Y2O3 and MgO suspensions is 7 wt%. The effect of pH on the zeta potential of the Y2O3 and MgO suspensions was also considered at the optimum PEI amount of 7 wt%. As shown in Fig. 1(b), isoelectric points (pHIEP) are 12.2 and 12.4 for the Y2O3 and MgO suspensions, respectively. With decreasing the pH value of each powder (pH < pHIEP), the positive zeta potential increases accordingly. At the pH range of 7.4–11.3, the zeta potential value of each suspension takes over 40 mV, which is enough value to prevent the particles from aggregation by generating the repulsive force between the particles [28]. This indicates that the suspensions with pH at this range (7.4–11.3) can provide much better colloidal stability. The rheological behaviors, which evaluate powder dispersity in the
2.2. Preparation of Y2O3–MgO nanopowder mixture The Y2O3 and MgO nanopowders were separately dispersed into anhydrous alcohol with PEI. The pH value of the Y2O3 and MgO suspensions was adjusted to 10.6. The adding amount of PEI was referred to the total mass amount of Y2O3 and MgO in each suspension. Then the suspensions were deagglomerated by using homogenizer for 10 min. Subsequently, the suspensions were continuously dispersed by stirring for 30 min under ultrasonic dispersion. After that, the prepared MgO suspension was added into the Y2O3 suspension as to be a composition of 50/50 vol% and followed by further ultrasonic stirring for 30 min. The mixed suspension was centrifuged and dried at 60 °C in an oven for 8 h. The obtained nanopowder mixtures were calcined at 650 °C for 2 h in air to remove the dispersant and residual organics, and then were deagglomerated by milling in non-aqueous solvent with a ball milling technique using ZrO2 container and balls. 2
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Fig. 2. TG-DTA results obtained from the Y2O3–MgO nanopowders with 7 wt% PEI.
Fig. 1. (a) Viscosities of the Y2O3 and MgO suspensions as a function of PEI amount, (b) the effect of pH on the zeta potential of the Y2O3 and MgO suspensions with 7 wt% PEI and (c) the shear rate dependence of the viscosities of the Y2O3, MgO and Y2O3–MgO suspensions. Fig. 3. XRD patterns of (a) Y2O3–MgO nanopowders after ball milling and the nanocomposite ceramic after SPS processing at 1250 °C. (b) Enlarged XRD pattern in the range of 2θ = 25–53°. 3
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Fig. 4. (a) Bright-field STEM images and the elemental mapping images of (b) Y, (c) Mg, (d) O, (e) Zr, and (f) EDS spectrum obtained from the Y2O3–MgO nanopowders.
Fig. 5. SEM images of the Y2O3–MgO composite ceramics sintered at 1250 °C using the powder mixtures prepared with (a) and without (b) colloidal process. The Y2O3 and MgO grains are bright and dark contrasts, respectively.
600 °C. The mass change related mainly to the decomposition of PEI completes at around 600 °C, suggesting that the calcination temperature of above 600 °C is enough to remove the remained PEI from the powder mixtures. In this work, therefore, the Y2O3–MgO powder mixtures were calcined at a temperature of 650 °C for 2 h. Fig. 3 shows the XRD patterns of the Y2O3–MgO nanopowder mixtures after ball milling and the nanocomposite ceramic after SPS sintering at 1250 °C. The XRD pattern of the nanopowder can be indexed only by cubic Y2O3 and MgO phases (Fig. 3(a)) and no any ZrO2 impurity phase is observed. More detailed microstructures were characterized by TEM as shown in Fig. 4. It can be seen that the nanopowder mixtures show nearly spherical morphology and the particle size is below 50 nm (Fig. 4(a)). Element distribution and EDS spectra show that the Y2O3 and MgO particles were uniformly distributed in the mixtures (Fig. 4(b–d)). In addition to the Mg, Y and O elements, however, EDS analysis detected a small amount of Zr element (Fig. 4 (e, f)), suggesting that minor Zr element is introduced into the Y2O3–MgO nanopowder mixture during the ball milling process. Nevertheless, it can conclude from Fig. 4 that homogeneous Y2O3–MgO nanopowder mixtures can be obtained by the colloid processing technique.
suspensions, were also checked at the above optimum conditions (7 wt % PEI and pH = 10.6). The rheological behaviors were considered by measuring the effect of the shear rate on the viscosities of the suspensions. As shown in Fig. 1(c), both of the Y2O3 and MgO suspensions exhibited relatively constant viscosities at the wide shear rate of 37.5–380 s−1, indicating a Newtonian response corresponding to a well-dispersed powder suspension. After mixing the well-dispersed Y2O3 and MgO suspensions, although the mixtured suspension shows slightly higher viscosity than those of the Y2O3 and MgO suspensions, it also shows a Newtonian response at the wide shear rate similar to those of the individual Y2O3 and MgO suspension (Fig. 1(c)). The rheological measurements suggest that the well-dispersed and stable Y2O3–MgO suspension can be obtained under the conditions of 7 wt% PEI and pH = 10.6. TG-DTA results obtained from the Y2O3–MgO powder mixtures are shown in Fig. 2. The endothermic peak at 210 °C is assigned to the evaporation of residual water on the particle surface and the exothermic peak at around 315 °C is ascribed to the decomposition of PEI. The thermogravimetric analysis shows that the mass decrease drastically takes place up to 400 °C and becomes almost constant at above 4
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Fig. 6. SEM images of the Y2O3–MgO composite ceramics fabricated from the colloidal processed powder mixture at the sintering temperatures of (a) 1200 °C, (b) 1250 °C, and (c) 1300 °C. Table 1 Comparison of the transparency and microstructure of the Y2O3–MgO nanocomposites fabricated using various powder synthesis and sintering methods [6–7, 12, 27]. Method for nanopowder mixtures
Fabrication technique for composite
Fabrication condition for composite
Thickness (mm)
Transmittance efficiency (%)
Grain size (nm)
Ref.
Sol-Gel Combustion Glycine-nitrate Process Esterification sol-gel Flame spray Pyrolysis Flame spray Pyrolysis This work
Hot pressing Hot pressing SPS SPS Hot isostatic pressing SPS
1350 °C × 50 MPa 1300 °C × 40 MPa 1100 °C × 50 MPa 1200 °C Not mentioned 1250 °C × 70 MPa
0.5 Not mentioned 1 1 3 1
83.5% (at 5 μm) 84.9% (at 4.5 μm) Above 50% (at 3.7 μm) 80% (at 4 μm) 80% (at 4–6 μm) 84.3% (at 5 μm)
~130 ~110 ~100 ~90 ~150 ~142
[27] [8] [12] [6] [7]
3.2. Microstructures of Y2O3–MgO composite ceramics sintered by SPS method at various conditions
technique shows uniform phase distribution and phase agglomeration is highly limited: both the Y2O3 and MgO phases distributed homogeneously. However, the composite fabricated without using the colloidal technique does not give homogeneous microstructure and exhibits agglomerated phase regions with dark and white contrasts as shown in the SEM image: particularly, the Y2O3 grains (dark contrast) strongly agglomerates in the composite. This result indicates that the colloidal technique can be considered as an effective method for obtaining the homogeneous Y2O3–MgO nanopowder mixture and composite. The microstructures of the Y2O3–MgO composite ceramics, which are fabricated using the colloidal technique and sintered at different temperatures, are shown in Fig. 6(a–c). It can be seen that irrespective of the sintering temperature, the composites show dense microstructures, and the Y2O3 and MgO grains distribute uniformly in the composites. Although a few nano-sized residual pores are observed to remain in the multiple grain junctions as indicated by the small arrows, those tend to decrease with the sintering temperature. Moreover, it is of importance that even though grain size increases with increasing the sintering temperature as well known, in this work, the composites maintain fine microstructures with average grain sizes of about 127 nm (Fig. 6(a)), 142 nm (Fig. 6(b)) and 260 nm (Fig. 6(c)) at 1200, 1250, and 1300 °C, respectively.
The XRD pattern of the nanocomposite after SPSed at 1250 °C is shown in Fig. 3(a). The sintered composite can also be indexed only by the cubic MgO and Y2O3 phases as well as that of the nanopowders. In contrast to the nanopowders, however, the peaks relating only to the Y2O3 phase shift to large angle after the SPS process though no peak shift is observed in the MgO phase, as shown in Fig. 3(b). According to the phase diagram of ZrO2–Y2O3 reported in literature [29], ZrO2 has a large solubility in the Y2O3 phase at high temperatures. These indicate that lattice shrinkage happened only in the Y2O3 phase owing to the dissolution of Zr element into the Y2O3 phase. Since the radius of Zr4+ (0.72 Å) is smaller than that of Y3+ (0.89 Å), it caused the decrease in the Y2O3 lattice parameter when Y–O bond was substituted by the shorter Zr–O bond in the Y2O3 phase. It is deduced, therefore, that such a small amount of Zr coming from the ball milling process were totally dissolved into the crystal structure of Y2O3 and no ZrO2 phase can be detected in the sintered composites in this work. Fig. 5 shows the SEM images of the composites fabricated from the powder mixtures with and without the colloidal technique. The sintered microstructure of the Y2O3–MgO composite is strongly influenced by the powder processing. The composite fabricated using the colloidal 5
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effectively act to prevent the motion of the grain boundaries by counteracting the grain boundary migration. When the MgO and Y2O3 particles were uniformly located to each other in three dimensions, the grain growth is described by the following equation [33]:
fβ ⎞ dDα 9 Dα ⎛ 3γ = k ⎜ α − ⋅γα⋅ 2 ⋅ dt Dα 8 Pβ 1 − fβ ⎟ ⎝ ⎠
(1)
Here, Dα is the mean matrix grain size, γα is the grain boundary energy of the matrix phase, Pβ is the particle size of the second phase, fβ is the volume fraction of the second phase particles and K is a constant. From the equation above, it is known that when the volume fraction of the second phase is 50 vol% and the second phase was uniformly distributed in the matrix phase, it will efficiently inhibit the grain growth of the matrix phase. In this work, since the volume fractions of MgO and Y2O3 are both 50 vol%, each of them can act as the second phase and the matrix phase of the other phase. In the present composites, therefore, each phase significantly suppresses the grain growth for each other by pinning the grain boundary and hindered the grain boundary migration, finally resulted in the small grained Y2O3–MgO composite ceramics. The small grain size also gives an indirect proof that uniformly-dispersed Y2O3–MgO nanopowders were successfully obtained from the colloidal technique. The composite ceramic sintered at 1200 °C shows the smallest grain size (average grain size of ~127 nm), however, there are some pores were observed (Fig. 6(a)), which may degrade the optical properties. The composite ceramic fabricated at 1250 °C exhibits small grain size (142 nm) and high relative density (99.4%) and therefore is expected to have the highest transmittance efficiency among the samples.
Fig. 7. Transmittance spectra of the composite ceramics fabricated from the colloidal processed powder mixtures at different sintering temperatures of 1200–1300 °C, comparably with that from the powder mixed by ball milling but not by colloidal technique at 1250 °C sintering temperature.
3.3. Optical and mechanical properties of the Y2O3–MgO composite ceramics Transmittance spectra of the composite ceramics fabricated by using the powder mixtures with and without the colloidal process at different sintering temperatures are demonstrated in Fig. 7. Although the IR transmittance efficiency shows almost the same value of about 80%, it slightly changes with the sintering temperature. The transmittance is enhanced with increasing the sintering temperature and the highest transmittance was obtained at 1250 °C. Whereas further increasing the sintering temperature to 1300 °C, the transmittance reversely decreased. As a whole, the transmittance efficiencies at a wavelength of 5.0 μm are 80.9%, 84.3% and 81.7% for the sintering temperature of 1200, 1250 and 1300 °C, respectively. Moreover, these transmittance efficiencies are appreciably higher than that (43.7%) of the composite fabricated from the powders mixed by the ball milling process without the colloidal technique (see Fig. 7), suggesting that the colloidal technique is responsible for the homogeneous microstructure, as shown in Fig. 5. The temperature dependent transmittance is likely to be related to the microstructures of the density and grain size. The lower transmittance at the lower sintering temperature would be relating to the lower density. Since some pores exists inside the composite ceramic (see Fig. 6(a): residual nano-size pores are located at the grain junctions), the light scattering caused by the pores severely degraded the transmittance. For the higher sintering temperature of 1300 °C, on the other hand, the decrease in the transmittance is related to the excessive grain growth (260 nm) because the scattering caused at the Y2O3/MgO interface boundary is known to increase with the grain size [11]. As the results, the Y2O3–MgO nanocomposite ceramic fabricated at the sintering temperature of 1250 °C shows an excellent IR transparency of 84.3% at 5 μm, which is close to the theoretical value of 85%, owing to the higher density of 99.4% and the smaller grain size of 142 nm. The transmittance spectra of all the fabricated Y2O3–MgO nanocomposite ceramics show two absorption peaks. The small absorption peak at 4.28 μm can be associated with CO2 in environment [34]. There
Fig. 8. Vickers hardness of the Y2O3–MgO nanocomposite ceramics fabricated at different sintering temperatures.
As summarized in Table 1, these grain sizes are comparable to those of some composites reported in some literatures [11,30]. In the previous works, although grain size show slightly smaller than those of the samples prepared in this work [10,12,16], their powder synthesis methods are complicated involving some chemical reaction process and need to precisely control chemical composition, ratio of reactants and reaction speed of two phases to obtain fine and uniform composite nanopowders. In this experiment, on the contrary, the use of commercially-supplied raw powders provide a simple and cost-effective technique for powder preparation. In addition, the grain sizes in the composite prepared in this work are much smaller than those of the singlephase Y2O3 (about 550 nm) or MgO (about 700 nm) sintered by SPS reported in the literatures as well [31,32]. These fine grains would be attained owing to the pinning effect on grain growth between MgO and Y2O3. During the consolidation process, the dispersion of the MgO and Y2O3 fine particles affects to the movements of the grain boundaries in the Y2O3–MgO composite. Especially, small MgO/Y2O3 particles 6
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are also some absorption peaks at around 7 μm. Those would be caused from the asymmetrical and symmetrical stretching vibrations of the carbonate groups (O–C]O), which may be formed from the contamination from the graphite molds and papers during the SPS processing. Further research work will focus on the reduction of O–C]O and O–H impurities by improving the powder preparation conditions and modifying the sintering technology. In this work, although the sintering temperature has an effect on the transmittance, all the composites (thickness: ~1 mm) show the transmittance higher than 80% at the range of 4–6 μm. As shown in Table 1, the attained transmittance are higher than those of the Y2O3–MgO composite ceramics (thickness: ~1 mm) fabricated by SPS method [10,16], and comparable to those of the samples fabricated by the other sintering techniques [11,30]: 83.5% of transmittance at ~0.5 mm thickness and 80% of transmittance at ~3 mm thickness by using hot pressing and hot isostatic pressing sintering methods, respectively. These results indicate that the colloidal technique is benefit for attaining homogeneous starting powders for fabricating Y2O3–MgO nanocomposite ceramics. Fig. 8 gives the relationship between Vickers hardness and the sintering temperature. The hardness of the composite ceramic sintered at 1200 °C is about 11.3 ± 0.5 GPa due probably to the lower density. The sample fabricated at 1300 °C also shows lower hardness (10.8 ± 0.7 GPa) due to its coarse grains (260 nm). In this work, the composite ceramic sintered at 1250 °C, which has higher density (99.4%) and smaller grain size (142 nm), gives the highest hardness (11.9 ± 0.3 GPa) among all the samples. Thus, it is deduced that both the porosity and grain size of the composite ceramic influence significantly on the hardness similar to the transmittance. That is, the effect of porosity is more pronounced when the density is low, but the grain size acts as an important role when the density is high. This indicates from the obtained results that combining the colloidal and SPS techniques is an effective method to obtain highly dispersed and homogeneous nanopowders for fabricating highly IR transparent Y2O3–MgO nanocomposite ceramics.
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Palmero, Structural ceramic nanocomposites: a review of properties and powders synthesis methods, Nanomaterials 5 (2015) 656–696. [15] Z.Y. Deng, J.L. Shi, Y.F. Zhang, D.Y. Jiang, J.K. Guo, Pinning effect of SiC particles on mechanical properties of Al2O3–SiC ceramic matrix composites, J. Eur. Ceram. Soc. 18 (1998) 501–508. [16] S. Xu, J. Li, H. Kou, Y. Shi, Y. Pan, J. Guo, Spark plasma sintering of Y2O3–MgO composite nanopowder synthesized by the esterification sol–gel route, Ceram. Int. 41 (2015) 3312–3317. [17] D. Jiang, A.K. Mukherjee, Synthesis of Y2O3-MgO nanopowder and infrared transmission of the sintered nanocomposite, Nanophotonic Materials V vol. 7030, International Society for Optics and Photonics, 2008, p. 703007. [18] P. Pawinrat, O. Mekasuwandumrong, J. Panpranot, Synthesis of Au–ZnO and Pt–ZnO nanocomposites by one-step flame spray pyrolysis and its application for photocatalytic degradation of dyes, Catal. Commun. 10 (2009) 1380–1385. [19] G. Kafili, B. Movahedi, M. Milani, A comparative approach to synthesis and sintering of alumina/yttria nanocomposite powders using different precipitants, Mater. Chem. Phys. 183 (2016) 136–144. [20] P.K. Sharma, M.H. Jilavi, D. Burgard, R. Nass, H. Schmidt, Hydrothermal synthesis of nanosize alpha-Al2O3 from seeded aluminum hydroxide, J. Am. Ceram. Soc. 81 (1998) 2732–2734. [21] T. Zhang, L. Kumari, G.H. Du, W.Z. Li, Q.W. Wang, K. Balani, A. Agarwal, Mechanical properties of carbon nanotube–alumina nanocomposites synthesized by chemical vapor deposition and spark plasma sintering, Compos. Appl. Sci. Manuf. 40 (2009) 86–93. [22] H. Zhang, G. Chen, Potent antibacterial activities of Ag/TiO2 nanocomposite powders synthesized by a one-pot sol− gel method, Environ. Sci. Technol. 43 (2009) 2905–2910. [23] H. Windlass, P.M. Raj, D. Balaraman, S.K. Bhattacharya, R.R. Tummala, Colloidal processing of polymer ceramic nanocomposite integral capacitors, IEEE Trans. Electron. Packag. Manuf. 26 (2003) 100–105. [24] M. Suárez, A. Fernández, J.L. Menéndez, M. Nygren, R. Torrecillas, Z. Zhao, Hot isostatic pressing of optically active Nd: YAG powders doped by a colloidal processing route, J. Eur. Ceram. Soc. 30 (2010) 1489–1494. [25] J. Sun, L. Gao, W. Li, Colloidal processing of carbon nanotube/alumina composites, Chem. Mater. 14 (2002) 5169–5172. [26] T.S. Suzuki, T. Uchikoshi, Y. Sakka, Control of texture in alumina by colloidal processing in a strong magnetic field, Sci. Technol. Adv. Mater. 7 (2006) 356. [27] F.F. Lange, Colloidal processing of powder for reliable ceramics, Curr. Opin. Solid St. M. 3 (1998) 496–500. [28] J. Duffy, A. Hill, A. Walton, F. Mazzeo, Suspension stability: why particle size, zeta potential and rheology are important, Ann. T. Nord. Rheol. Soc. 20 (2012) 209–214.
4. Conclusions Highly dispersed and homogeneous Y2O3–MgO nanopowders were prepared by a colloidal technique with using PEI as dispersant. When PEI adding amount is 7 wt%, highly-dispersed and stable suspension was prepared. Homogeneous Y2O3–MgO nanopowders with an average particle size of 20–30 nm were obtained from the prepared suspension after calcination at 650 °C, and then, these nanopowders were consolidated into almost the full density by using SPS sintering method. The highest transmittance and hardness were achieved for the composite ceramic sintered at 1250 °C. At the wavelength of 5 μm, the transmittance reaches to 84.3%, which is nearly the same as the theoretical value of 85% for Y2O3–MgO composite ceramics and comparable to those of the composites fabricated using the powders prepared from chemical reaction process. The hardness is as high as 11.9 ± 0.3 GPa. These experiment results indicate that combining the colloidal and SPS techniques is an effect way to obtain the highly-dispersed and homogeneous Y2O3–MgO nanopowders for fabricating highly IR transparent Y2O3–MgO composite ceramics. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by Innovative Science and Technology Initiative for Security, ATLA, Japan. The authors also 7
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Int. 42 (2016) 7819–7823. [32] N. Jiang, R.J. Xie, Q. Liu, J. Li, Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering, J. Eur. Ceram. Soc. 37 (2017) 4947–4953. [33] G. Grewal, S. Ankem, Modeling matrix grain growth in the presence of growing second phase particles in two phase alloys, Acta Metall. Mater. 38 (1990) 1607–1617. [34] D.T. Jiang, A.K. Mukherjee, The influence of oxygen vacancy on the optical transmission of an yttria–magnesia nanocomposite, Scripta Mater. 64 (2011) 1095–1097.
[29] American Ceramic Society, National Institute of Standards and Technology, Phase Equilibrium Diagram Database 3.1.0, American Ceramic Society. National Institute of Standards and Technology, Wiley-Blackwell, Malden, MA, 2003. [30] S. Xu, J. Li, C. Li, Y. Pan, J. Guo, Infrared‐transparent Y2O3–MgO nanocomposites fabricated by the glucose sol–gel combustion and hot‐pressing technique, J. Am. Ceram. Soc. 98 (2015) 2796–2802. [31] R.S. Razavi, M. Ahsanzadeh-Vadeqani, M. Barekat, M. Naderi, S.H. Hashemi, A.K. Mishra, Effect of sintering temperature on microstructural and optical properties of transparent yttria ceramics fabricated by spark plasma sintering, Ceram.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis of highly-infrared transparent Y2O3–MgO nanocomposites by colloidal technique and SPS Lihong Liua, Koji Moritaa,∗, Tohru S. Suzukib, Byung-Nam Kima a b
Field-Assisted Sintering Group, Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan Ceramics Processing Group, Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Y2O3–MgO composite Colloidal technique Spark plasma sintering Transmittance efficiency Microstructure
Infrared (IR) transparent Y2O3–MgO nanocomposites with a volume ratio of 50:50 were synthesized by combining colloidal and spark-plasma-sintering (SPS) techniques. In order to attain well-dispersed and homogeneous starting Y2O3–MgO nanopowder mixture, the effects of the pH value and the amount of polyetherimide (PEI) dispersant on the suspension stability were studied. Rheological measurement reveals that highly-dispersed and stable suspension was obtained at 7 wt% of PEI dispersant under pH = 10.6. The obtained nanopowders with particle size of 20–30 nm were densified using SPS at several sintering temperatures. The sintered composites show fine grains, narrow grain size distribution and uniform microstructure. The nanocomposite sintered at 1250 °C showed the maximum IR transmittance of 84% at a wavelength range of 2.5–6 μm. The Vickers hardness of the nanocomposite was about 11.9 ± 0.3 GPa, which is significantly higher than those of single phase MgO or Y2O3. Successful fabrication of the high-performance Y2O3–MgO nanocomposite indicates that i) the colloidal technique is an effect method to obtain highly dispersed and homogeneous nanopowders and ii) the SPS technique is a powerful tool to fabricate fine-grained dense transparent ceramics, which are suitable for fabricating IR transparent Y2O3–MgO composite ceramics from commercial starting powders.
1. Introduction Infrared (IR) transparent ceramics, such as Al2O3, MgAl2O4, AlON, and Y2O3, have recently attracted much attention for the application in aerospace field, owing to their excellent optical properties [1–4]. Among them, Y2O3 is recognized as one of the candidate materials for IR windows owing to its wide band transmission, low scattering efficiency and low infrared emissivity at high temperatures. For the nextgeneration IR transparent windows working in harsh environments, however, the further enhancements of durability, mechanical and thermal shock resistance properties are required. For the polycrystalline transparent Y2O3 fabricated by conventional techniques, its limited mechanical properties do not fit with the above requirements due to its large grain size caused by high temperature and longer time sintering processes. In order to improve the mechanical properties, reducing the grain size is an effective method. One technique for reducing the grain size is to disperse second phase particles [5]; dispersed second phases naturally impedes the grain boundary migration of the adjacent phases, and hence, can inhibit grain growth during the densification process [6–9]. This phenomenon is well known as the pining effect for obtaining fine grains in composite
∗
ceramics. For the Y2O3, the dispersion of MgO particle must be a valid candidate. This is because both Y2O3 and MgO are excellent IR transparent materials, and have a longer wavelength cutoff and lower emissivity than other IR transparent materials. Furthermore, according to the phase diagram, these two materials have a eutectic point at 2110 °C, below which both of them are stable and do not react with each other [10]. This characteristic enables to limit the grain growth during the sintering through the pinning effect between Y2O3 and MgO grains. Recently, the Y2O3–MgO nanocomposite ceramics have been reported to show IR transmittance over 3–7 μm wavelength and mechanical properties higher than those of monolithic Y2O3 or MgO polycrystals [11–15]. Realizing the IR transparent Y2O3–MgO composite ceramics, however, is still a big challenge. This is because the large difference in the refractive index between Y2O3 and MgO phases will cause the significant scattering losses at the Y2O3/MgO interfaces, and hence, deteriorate light transmission. Nevertheless, if the grain size is far less than the wavelength of incident lights, the light scattering at the interface boundaries can be negligible and the optical transparency can be improved. In addition, by reducing the gran size, the enhancement of the strength and erosion resistance can also be achieved [10,16,17].
Corresponding author. 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan. E-mail address: [email protected] (K. Morita).
https://doi.org/10.1016/j.ceramint.2020.02.153 Received 8 January 2020; Received in revised form 12 February 2020; Accepted 16 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Lihong Liu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.153
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2.3. Fabrication of Y2O3–MgO nanocomposites
Therefore, the reducing the grain size can satisfy the requirements for IR windows used in the harsher environments. For achieving the fine grain sizes and homogenous microstructures, it is important to control both the nanopowder preparation and sintering conditions. To date, various synthesis methods, such as flame pyrolysis technique, coprecipitation method, hydrothermal method, chemical vapor deposition and the sol-gel method, have been developed for preparing high quality nanopowder mixtures for fabricating composite ceramics [18–22]. Those synthesis methods, however, are often complicated chemical reaction process and need to carefully control the chemical composition, ratio of reactants and reaction speed of two phases to obtain fine and uniform nanopowders. On contrary, since colloidal technique can use commercially supplied raw powders, it is a more simple and cost-effective technique than those of the wet chemical methods for the powder preparation. Furthermore, the colloidal technique offers significant advantages to avoid heterogeneous agglomerates of fine particles by using electrostatic repulsive forces and/ or steric stabilization [23–27]. During the colloidal process, dispersant, solvent, pH adjustment and viscosity should be controlled. Among them, the dispersant will help to induce electrostatic charge to enhance the stability of suspensions through the particle repulsion. The viscosity of the suspension, on the other hand, will help to keep kinetic stability by slowing down the particle aggregation and sedimentation, and finally, a homogeneous mixture can be obtained. In this work, the colloidal technique was used to prepare homogenous Y2O3–MgO nanopowder mixture from the commercially supplied Y2O3 and MgO raw powders. In order to obtain a well-dispersed powder suspension, addition amount of dispersant, pH value of solvent and rheological properties of the suspension were investigated during the processing. The fabricated Y2O3–MgO nanopowder mixture were sintered by using spark-plasma-sintering (SPS) technique, which has been recognized as a powerful sintering tool for attaining dense and fine-grained materials at relatively low temperatures. The microstructure dependent optical and mechanical properties of the fabricated composite ceramics were systemically investigated.
The obtained Y2O3–MgO nanopowder mixtures were consolidated by the SPS machine (LABOX, Sinterland Co., Ltd., Japan). The nanopowders were loaded into the graphite mold with an inner diameter of 10 mm. The interior of the graphite mold was covered with graphite papers. The outside of the mold was covered with a thermal insulator carbon felt to suppress any heat losses from the surface. The SPS process was carried out at a uniaxial pressure of 70 MPa, at a temperature range of 1200–1300 °C, with a fast heating rate of 50 °C/min and a dwelling time of 10 min. During the sintering, the surface temperature of the graphite mold was measured using an optical pyrometer through a hole made in the carbon felt. The sintered Y2O3–MgO composites were postannealed at 1000 °C for 10 h in air to eliminate oxygen vacancies, residual carbon and stress. For IR transmittance measurements, both surfaces of the annealed composites were polished with diamond pastes to ~1.0 mm in thickness. 2.4. Characterization techniques Suspension viscosities and rheological behavior were measured as a function of the shear rate of the suspension by using a cone-plate viscometer (Re-215 Model, Toki Sangyo Co., Ltd., Tokyo, Japan). Zetapotential measurements of the suspensions were performed at 7 wt% PEI as a function of pH using zetasizer nano essentials (Malvern Instruments Ltd., United Kingdom). Thermogravimetric analysis was used to monitor the decomposition of the PEI dispersant by using thermogravimetric and differential thermal analyses (TG/DTA, 2000SR, Bruker, Japan). X-ray diffraction (XRD) analysis of the Y2O3–MgO nanopowder mixtures and sintered bodies was performed by RINT-2500 diffractometer (Rigaku Co., Ltd, Tokyo, Japan, 40 kV 300 mA) using Cu Kα radiation. Microstructures of the nanopowders/nanocomposites were examined by a transmission electronic microscopy (TEM, JEM-2010F, JEOL Co., Ltd, Tokyo, Japan, 200 KV) and field emission scanning electron microscope (FE-SEM, model SU-8000, Hitachi Ltd., Tokyo, Japan). Fourier transform infrared spectroscopy (FT-IR, model 4200, JASCO, Tokyo, Japan) was used to measure the transmittance of the mirror polished samples at a range of 2.5–10 μm. Vickers hardness tester (MVK-E, Akashi Seisakusho, Ltd., Toda, Japan) was used for the hardness measurement at a load of 2 N.
2. Experimental procedure 2.1. Starting materials
3. Results and discussion
Commercially available Y2O3 (BB-type, Shin-Etsu Chemical Co., Ltd., Japan, purity: 99.9%) and MgO (500A-1, Ube Material Industries Ltd., Japan, Purity: 99.98%) nanopowders were used as starting materials. The specific surface area of Y2O3 and MgO powders are 35 m2/g and 28–38 m2/g, respectively. Polyethylenimine (PEI, FUJIFILM Wako Chemical Co., Japan) with 10,000 molecular weight was used as a cationic dispersant.
3.1. Identification of optimal conditions for powder mixture preparation To determine the optimum processing conditions, the effects of PEI amount and pH were investigated in both Y2O3 and MgO suspensions. Fig. 1(a) shows the viscosities of the Y2O3 and MgO suspensions with various PEI amounts under a pH value of 10.6. Both of the suspensions are too sticky to measure the viscosity without PEI addition. The viscosity decreases accordingly with an increase of the PEI amount and shows the lowest value at 7 wt% PEI amounts. With further increasing the PEI amount, the viscosities of both suspensions increase again, indicating that the optimal PEI adding amount for both Y2O3 and MgO suspensions is 7 wt%. The effect of pH on the zeta potential of the Y2O3 and MgO suspensions was also considered at the optimum PEI amount of 7 wt%. As shown in Fig. 1(b), isoelectric points (pHIEP) are 12.2 and 12.4 for the Y2O3 and MgO suspensions, respectively. With decreasing the pH value of each powder (pH < pHIEP), the positive zeta potential increases accordingly. At the pH range of 7.4–11.3, the zeta potential value of each suspension takes over 40 mV, which is enough value to prevent the particles from aggregation by generating the repulsive force between the particles [28]. This indicates that the suspensions with pH at this range (7.4–11.3) can provide much better colloidal stability. The rheological behaviors, which evaluate powder dispersity in the
2.2. Preparation of Y2O3–MgO nanopowder mixture The Y2O3 and MgO nanopowders were separately dispersed into anhydrous alcohol with PEI. The pH value of the Y2O3 and MgO suspensions was adjusted to 10.6. The adding amount of PEI was referred to the total mass amount of Y2O3 and MgO in each suspension. Then the suspensions were deagglomerated by using homogenizer for 10 min. Subsequently, the suspensions were continuously dispersed by stirring for 30 min under ultrasonic dispersion. After that, the prepared MgO suspension was added into the Y2O3 suspension as to be a composition of 50/50 vol% and followed by further ultrasonic stirring for 30 min. The mixed suspension was centrifuged and dried at 60 °C in an oven for 8 h. The obtained nanopowder mixtures were calcined at 650 °C for 2 h in air to remove the dispersant and residual organics, and then were deagglomerated by milling in non-aqueous solvent with a ball milling technique using ZrO2 container and balls. 2
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Fig. 2. TG-DTA results obtained from the Y2O3–MgO nanopowders with 7 wt% PEI.
Fig. 1. (a) Viscosities of the Y2O3 and MgO suspensions as a function of PEI amount, (b) the effect of pH on the zeta potential of the Y2O3 and MgO suspensions with 7 wt% PEI and (c) the shear rate dependence of the viscosities of the Y2O3, MgO and Y2O3–MgO suspensions. Fig. 3. XRD patterns of (a) Y2O3–MgO nanopowders after ball milling and the nanocomposite ceramic after SPS processing at 1250 °C. (b) Enlarged XRD pattern in the range of 2θ = 25–53°. 3
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Fig. 4. (a) Bright-field STEM images and the elemental mapping images of (b) Y, (c) Mg, (d) O, (e) Zr, and (f) EDS spectrum obtained from the Y2O3–MgO nanopowders.
Fig. 5. SEM images of the Y2O3–MgO composite ceramics sintered at 1250 °C using the powder mixtures prepared with (a) and without (b) colloidal process. The Y2O3 and MgO grains are bright and dark contrasts, respectively.
600 °C. The mass change related mainly to the decomposition of PEI completes at around 600 °C, suggesting that the calcination temperature of above 600 °C is enough to remove the remained PEI from the powder mixtures. In this work, therefore, the Y2O3–MgO powder mixtures were calcined at a temperature of 650 °C for 2 h. Fig. 3 shows the XRD patterns of the Y2O3–MgO nanopowder mixtures after ball milling and the nanocomposite ceramic after SPS sintering at 1250 °C. The XRD pattern of the nanopowder can be indexed only by cubic Y2O3 and MgO phases (Fig. 3(a)) and no any ZrO2 impurity phase is observed. More detailed microstructures were characterized by TEM as shown in Fig. 4. It can be seen that the nanopowder mixtures show nearly spherical morphology and the particle size is below 50 nm (Fig. 4(a)). Element distribution and EDS spectra show that the Y2O3 and MgO particles were uniformly distributed in the mixtures (Fig. 4(b–d)). In addition to the Mg, Y and O elements, however, EDS analysis detected a small amount of Zr element (Fig. 4 (e, f)), suggesting that minor Zr element is introduced into the Y2O3–MgO nanopowder mixture during the ball milling process. Nevertheless, it can conclude from Fig. 4 that homogeneous Y2O3–MgO nanopowder mixtures can be obtained by the colloid processing technique.
suspensions, were also checked at the above optimum conditions (7 wt % PEI and pH = 10.6). The rheological behaviors were considered by measuring the effect of the shear rate on the viscosities of the suspensions. As shown in Fig. 1(c), both of the Y2O3 and MgO suspensions exhibited relatively constant viscosities at the wide shear rate of 37.5–380 s−1, indicating a Newtonian response corresponding to a well-dispersed powder suspension. After mixing the well-dispersed Y2O3 and MgO suspensions, although the mixtured suspension shows slightly higher viscosity than those of the Y2O3 and MgO suspensions, it also shows a Newtonian response at the wide shear rate similar to those of the individual Y2O3 and MgO suspension (Fig. 1(c)). The rheological measurements suggest that the well-dispersed and stable Y2O3–MgO suspension can be obtained under the conditions of 7 wt% PEI and pH = 10.6. TG-DTA results obtained from the Y2O3–MgO powder mixtures are shown in Fig. 2. The endothermic peak at 210 °C is assigned to the evaporation of residual water on the particle surface and the exothermic peak at around 315 °C is ascribed to the decomposition of PEI. The thermogravimetric analysis shows that the mass decrease drastically takes place up to 400 °C and becomes almost constant at above 4
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Fig. 6. SEM images of the Y2O3–MgO composite ceramics fabricated from the colloidal processed powder mixture at the sintering temperatures of (a) 1200 °C, (b) 1250 °C, and (c) 1300 °C. Table 1 Comparison of the transparency and microstructure of the Y2O3–MgO nanocomposites fabricated using various powder synthesis and sintering methods [6–7, 12, 27]. Method for nanopowder mixtures
Fabrication technique for composite
Fabrication condition for composite
Thickness (mm)
Transmittance efficiency (%)
Grain size (nm)
Ref.
Sol-Gel Combustion Glycine-nitrate Process Esterification sol-gel Flame spray Pyrolysis Flame spray Pyrolysis This work
Hot pressing Hot pressing SPS SPS Hot isostatic pressing SPS
1350 °C × 50 MPa 1300 °C × 40 MPa 1100 °C × 50 MPa 1200 °C Not mentioned 1250 °C × 70 MPa
0.5 Not mentioned 1 1 3 1
83.5% (at 5 μm) 84.9% (at 4.5 μm) Above 50% (at 3.7 μm) 80% (at 4 μm) 80% (at 4–6 μm) 84.3% (at 5 μm)
~130 ~110 ~100 ~90 ~150 ~142
[27] [8] [12] [6] [7]
3.2. Microstructures of Y2O3–MgO composite ceramics sintered by SPS method at various conditions
technique shows uniform phase distribution and phase agglomeration is highly limited: both the Y2O3 and MgO phases distributed homogeneously. However, the composite fabricated without using the colloidal technique does not give homogeneous microstructure and exhibits agglomerated phase regions with dark and white contrasts as shown in the SEM image: particularly, the Y2O3 grains (dark contrast) strongly agglomerates in the composite. This result indicates that the colloidal technique can be considered as an effective method for obtaining the homogeneous Y2O3–MgO nanopowder mixture and composite. The microstructures of the Y2O3–MgO composite ceramics, which are fabricated using the colloidal technique and sintered at different temperatures, are shown in Fig. 6(a–c). It can be seen that irrespective of the sintering temperature, the composites show dense microstructures, and the Y2O3 and MgO grains distribute uniformly in the composites. Although a few nano-sized residual pores are observed to remain in the multiple grain junctions as indicated by the small arrows, those tend to decrease with the sintering temperature. Moreover, it is of importance that even though grain size increases with increasing the sintering temperature as well known, in this work, the composites maintain fine microstructures with average grain sizes of about 127 nm (Fig. 6(a)), 142 nm (Fig. 6(b)) and 260 nm (Fig. 6(c)) at 1200, 1250, and 1300 °C, respectively.
The XRD pattern of the nanocomposite after SPSed at 1250 °C is shown in Fig. 3(a). The sintered composite can also be indexed only by the cubic MgO and Y2O3 phases as well as that of the nanopowders. In contrast to the nanopowders, however, the peaks relating only to the Y2O3 phase shift to large angle after the SPS process though no peak shift is observed in the MgO phase, as shown in Fig. 3(b). According to the phase diagram of ZrO2–Y2O3 reported in literature [29], ZrO2 has a large solubility in the Y2O3 phase at high temperatures. These indicate that lattice shrinkage happened only in the Y2O3 phase owing to the dissolution of Zr element into the Y2O3 phase. Since the radius of Zr4+ (0.72 Å) is smaller than that of Y3+ (0.89 Å), it caused the decrease in the Y2O3 lattice parameter when Y–O bond was substituted by the shorter Zr–O bond in the Y2O3 phase. It is deduced, therefore, that such a small amount of Zr coming from the ball milling process were totally dissolved into the crystal structure of Y2O3 and no ZrO2 phase can be detected in the sintered composites in this work. Fig. 5 shows the SEM images of the composites fabricated from the powder mixtures with and without the colloidal technique. The sintered microstructure of the Y2O3–MgO composite is strongly influenced by the powder processing. The composite fabricated using the colloidal 5
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effectively act to prevent the motion of the grain boundaries by counteracting the grain boundary migration. When the MgO and Y2O3 particles were uniformly located to each other in three dimensions, the grain growth is described by the following equation [33]:
fβ ⎞ dDα 9 Dα ⎛ 3γ = k ⎜ α − ⋅γα⋅ 2 ⋅ dt Dα 8 Pβ 1 − fβ ⎟ ⎝ ⎠
(1)
Here, Dα is the mean matrix grain size, γα is the grain boundary energy of the matrix phase, Pβ is the particle size of the second phase, fβ is the volume fraction of the second phase particles and K is a constant. From the equation above, it is known that when the volume fraction of the second phase is 50 vol% and the second phase was uniformly distributed in the matrix phase, it will efficiently inhibit the grain growth of the matrix phase. In this work, since the volume fractions of MgO and Y2O3 are both 50 vol%, each of them can act as the second phase and the matrix phase of the other phase. In the present composites, therefore, each phase significantly suppresses the grain growth for each other by pinning the grain boundary and hindered the grain boundary migration, finally resulted in the small grained Y2O3–MgO composite ceramics. The small grain size also gives an indirect proof that uniformly-dispersed Y2O3–MgO nanopowders were successfully obtained from the colloidal technique. The composite ceramic sintered at 1200 °C shows the smallest grain size (average grain size of ~127 nm), however, there are some pores were observed (Fig. 6(a)), which may degrade the optical properties. The composite ceramic fabricated at 1250 °C exhibits small grain size (142 nm) and high relative density (99.4%) and therefore is expected to have the highest transmittance efficiency among the samples.
Fig. 7. Transmittance spectra of the composite ceramics fabricated from the colloidal processed powder mixtures at different sintering temperatures of 1200–1300 °C, comparably with that from the powder mixed by ball milling but not by colloidal technique at 1250 °C sintering temperature.
3.3. Optical and mechanical properties of the Y2O3–MgO composite ceramics Transmittance spectra of the composite ceramics fabricated by using the powder mixtures with and without the colloidal process at different sintering temperatures are demonstrated in Fig. 7. Although the IR transmittance efficiency shows almost the same value of about 80%, it slightly changes with the sintering temperature. The transmittance is enhanced with increasing the sintering temperature and the highest transmittance was obtained at 1250 °C. Whereas further increasing the sintering temperature to 1300 °C, the transmittance reversely decreased. As a whole, the transmittance efficiencies at a wavelength of 5.0 μm are 80.9%, 84.3% and 81.7% for the sintering temperature of 1200, 1250 and 1300 °C, respectively. Moreover, these transmittance efficiencies are appreciably higher than that (43.7%) of the composite fabricated from the powders mixed by the ball milling process without the colloidal technique (see Fig. 7), suggesting that the colloidal technique is responsible for the homogeneous microstructure, as shown in Fig. 5. The temperature dependent transmittance is likely to be related to the microstructures of the density and grain size. The lower transmittance at the lower sintering temperature would be relating to the lower density. Since some pores exists inside the composite ceramic (see Fig. 6(a): residual nano-size pores are located at the grain junctions), the light scattering caused by the pores severely degraded the transmittance. For the higher sintering temperature of 1300 °C, on the other hand, the decrease in the transmittance is related to the excessive grain growth (260 nm) because the scattering caused at the Y2O3/MgO interface boundary is known to increase with the grain size [11]. As the results, the Y2O3–MgO nanocomposite ceramic fabricated at the sintering temperature of 1250 °C shows an excellent IR transparency of 84.3% at 5 μm, which is close to the theoretical value of 85%, owing to the higher density of 99.4% and the smaller grain size of 142 nm. The transmittance spectra of all the fabricated Y2O3–MgO nanocomposite ceramics show two absorption peaks. The small absorption peak at 4.28 μm can be associated with CO2 in environment [34]. There
Fig. 8. Vickers hardness of the Y2O3–MgO nanocomposite ceramics fabricated at different sintering temperatures.
As summarized in Table 1, these grain sizes are comparable to those of some composites reported in some literatures [11,30]. In the previous works, although grain size show slightly smaller than those of the samples prepared in this work [10,12,16], their powder synthesis methods are complicated involving some chemical reaction process and need to precisely control chemical composition, ratio of reactants and reaction speed of two phases to obtain fine and uniform composite nanopowders. In this experiment, on the contrary, the use of commercially-supplied raw powders provide a simple and cost-effective technique for powder preparation. In addition, the grain sizes in the composite prepared in this work are much smaller than those of the singlephase Y2O3 (about 550 nm) or MgO (about 700 nm) sintered by SPS reported in the literatures as well [31,32]. These fine grains would be attained owing to the pinning effect on grain growth between MgO and Y2O3. During the consolidation process, the dispersion of the MgO and Y2O3 fine particles affects to the movements of the grain boundaries in the Y2O3–MgO composite. Especially, small MgO/Y2O3 particles 6
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are also some absorption peaks at around 7 μm. Those would be caused from the asymmetrical and symmetrical stretching vibrations of the carbonate groups (O–C]O), which may be formed from the contamination from the graphite molds and papers during the SPS processing. Further research work will focus on the reduction of O–C]O and O–H impurities by improving the powder preparation conditions and modifying the sintering technology. In this work, although the sintering temperature has an effect on the transmittance, all the composites (thickness: ~1 mm) show the transmittance higher than 80% at the range of 4–6 μm. As shown in Table 1, the attained transmittance are higher than those of the Y2O3–MgO composite ceramics (thickness: ~1 mm) fabricated by SPS method [10,16], and comparable to those of the samples fabricated by the other sintering techniques [11,30]: 83.5% of transmittance at ~0.5 mm thickness and 80% of transmittance at ~3 mm thickness by using hot pressing and hot isostatic pressing sintering methods, respectively. These results indicate that the colloidal technique is benefit for attaining homogeneous starting powders for fabricating Y2O3–MgO nanocomposite ceramics. Fig. 8 gives the relationship between Vickers hardness and the sintering temperature. The hardness of the composite ceramic sintered at 1200 °C is about 11.3 ± 0.5 GPa due probably to the lower density. The sample fabricated at 1300 °C also shows lower hardness (10.8 ± 0.7 GPa) due to its coarse grains (260 nm). In this work, the composite ceramic sintered at 1250 °C, which has higher density (99.4%) and smaller grain size (142 nm), gives the highest hardness (11.9 ± 0.3 GPa) among all the samples. Thus, it is deduced that both the porosity and grain size of the composite ceramic influence significantly on the hardness similar to the transmittance. That is, the effect of porosity is more pronounced when the density is low, but the grain size acts as an important role when the density is high. This indicates from the obtained results that combining the colloidal and SPS techniques is an effective method to obtain highly dispersed and homogeneous nanopowders for fabricating highly IR transparent Y2O3–MgO nanocomposite ceramics.
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4. Conclusions Highly dispersed and homogeneous Y2O3–MgO nanopowders were prepared by a colloidal technique with using PEI as dispersant. When PEI adding amount is 7 wt%, highly-dispersed and stable suspension was prepared. Homogeneous Y2O3–MgO nanopowders with an average particle size of 20–30 nm were obtained from the prepared suspension after calcination at 650 °C, and then, these nanopowders were consolidated into almost the full density by using SPS sintering method. The highest transmittance and hardness were achieved for the composite ceramic sintered at 1250 °C. At the wavelength of 5 μm, the transmittance reaches to 84.3%, which is nearly the same as the theoretical value of 85% for Y2O3–MgO composite ceramics and comparable to those of the composites fabricated using the powders prepared from chemical reaction process. The hardness is as high as 11.9 ± 0.3 GPa. These experiment results indicate that combining the colloidal and SPS techniques is an effect way to obtain the highly-dispersed and homogeneous Y2O3–MgO nanopowders for fabricating highly IR transparent Y2O3–MgO composite ceramics. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by Innovative Science and Technology Initiative for Security, ATLA, Japan. The authors also 7
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