Fabrication of transparent MgAl2O4 spinel through

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 13354–13360 www.elsevier.com/locate/ceramint

Fabrication of transparent MgAl2O4 spinel through homogenous green compaction by microfluidization and slip casting Jin-Myung Kim, Ha-Neul Kimn, Young-Jo Park, Jae-Woong Ko, Jae-Wook Lee, Hai-Doo Kim Engineering Ceramics Research Group, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam 641-831, Republic of Korea Received 12 May 2015; received in revised form 17 July 2015; accepted 20 July 2015 Available online 28 July 2015

Abstract In this study, a transparent magnesium aluminate (MgAl2O4) spinel prepared by homogeneous green compaction and sinter-HIP processes was investigated. The uniformity of the particle configuration in a green body could be improved by both a wet shaping via slip casting and a microfluidization process, which effectively disintegrates particle agglomerates in aqueous slurry by passing the material through a narrow channel with a high shear stress. The relation between the green body state and the sintering behavior showed that the densification during presintering was accelerated by reducing the mean pore size in the green body, which resulted in a finer microstructure. After post-HIP processing, the final grain size and the in-line transmittance was found to be significantly affected by the pre-sintered state. A high in-line transmittance (ILT ¼79.3% at λ¼ 550 nm) was achieved in the specimen fabricated via microfluidization and slip casting. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Slip casting; D. Spinel; MgAl2O4; Transparent; Sinter-HIP; Microfluidization

1. Introduction The sintering of transparent polycrystalline ceramics for armor and window applications is a cutting-edge technology requiring both extreme densification and grain growth inhibition to occur simultaneously [1]. Various candidate materials such as Al2O3, AlON and MgAl2O4 have outstanding mechanical and optical characteristics [2], and among them, magnesium aluminate spinel (MgAl2O4), which is developed as translucent bulk material by Westinghouse [3], have been investigated as a promising candidate for transparent window and IR dome applications due to its excellent mechanical properties and relatively high IR cut-off wavelength [4]. For highly-transparent MgAl2O4, its total volume porosity should be lower than tens of ppm, and its nano-size pores (40–70 nm in diameter, which is one-tenth of a visible wavelength) also should be minimized based on the Mie-scattering theory [5]. To achieve these characteristics, various sintering methods have been investigated, such as hot pressing [6], hot pressing n

Corresponding author. Tel.: þ82 55 280 3426; fax: þ 82 55 280 3392. E-mail address: [email protected] (H.-N. Kim).

http://dx.doi.org/10.1016/j.ceramint.2015.07.121 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

followed by hot isostatic pressing (HIP) [7,8], spark plasma sintering (SPS) [9,10] and sinter-HIP [11,12]. It is also important in armor applications for fine-grained bulk material to be present to enhance the hardness of the material, improving ballistic resistance [13]. To date, the fabrication of large MgAl2O4 bulk material with both a high transmittance (4 80%) and a sub-μm microstructure has only been achieved using the sinter-HIP process with a low sinterHIP temperature (i.e., less than 1500 1C at which rapid grain growth occurs) [14]. It has been proposed that the homogeneity of a green compact should be precisely controlled because it determines the final sintering temperature. Krell et al. reported that the sintering temperature used to achieve a dense Al2O3 material is significantly dependent on the pore size distribution of the Al2O3 green compacts, which typically have a narrow pore size distribution and a smaller mean pore size when made at significantly lower sintering temperatures [15]. Lallemant et al. also investigated the fabrication of transparent Al2O3 via SPS using varying green compaction methods where higher in-line transmittances were obtained by enhancing the uniformity of particle packing and by reducing the amount of large agglomerates [16]. For transparent

J.-M. Kim et al. / Ceramics International 41 (2015) 13354–13360

MgAl2O4, it has been reported that the sintering behavior of MgAl2O4 nanopowders was also dependent on the pore size distribution of green compacts [12,14]. Slip casting is one of advanced wet consolidation techniques that can give a uniform green compact through smooth particle movement and rearrangement during casting process [17]. Several conditions of slip-casting regarding the slurry (i.e. solid loading, viscosity, particle size and its distribution) and a porous medium (i.e. porosity and pore size) should be desirably controlled to make a homogeneous green body. In particular, preparation of well-dispersed and stable slurry is a critical factor to achieve more uniform particle configuration since flocs and agglomerates in slurry generate large interparticle pores that worsen the homogeneity of green compaction. There are a few methods used to make a well-dispersed ceramic slurry, including ball milling, planetary milling and intensive ultrasonication, with an aid of proper deflocculants. The purpose of these methods is to disintegrate agglomerated particles in the raw powder into individual particles. If the size of an individual particle becomes smaller (i.e., tens of nanometer), the disintegration of the agglomerated granules becomes more difficult due to the increasing strength of the van der Waals force. Microfluidization is a common processing method in the food, pharmaceutical and cosmetic industries [18,19]. For example, large agglomerates of ingredients in an emulsion (e.g., milk) can be uniformly disintegrated easily to make the absorption of ingredients into the human body more effective. Fig. 1 demonstrates the operating mechanism of microfluidization which uses Bernoulli's principle: pressure up to 200 MPa is applied to slurry passing through a high pressure module which is composed of a narrow 75-μm channel from a relatively wide tube (i.e., several millimeters). However, most studies in the field of ceramic processing have not specifically investigated the microfluidization process. Limited studies of Al2O3 and ZnO using wet-jet milling were conducted by only one research group of AIST in Japan [20–23]. Based on their studies, mono-dispersed Al2O3 slurries with low viscosities could be effectively prepared by wet-jet milling so that both the sintering activity and the mechanical properties of Al2O3 were significantly enhanced [22].

Fig. 1. A schematic diagram of microfluidization process.

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In this study, the microfluidization method is first used to fabricate a transparent MgAl2O4 spinel. The characteristics of the slurry and the green compact are analyzed and compared to the sintering behavior. The post-HIP treatment transmittance of the HIPed MgAl2O4 is then discussed. 2. Experimental procedure A commercial MgAl2O4 spinel powder that is commonly used (S30CR, Baikowski, France) was selected as the starting material. The primary particle size was reported as 55 nm via direct observation [24]. The raw powders were mixed with deionized water (18.5 M), and 4 wt% of ammonium polyacrylic acid (NH4PAA, R.T. Vanderbilt, Norwalk, CT, USA) was used as a dispersant. Solid loading of MgAl2O4 in the slurry was set at 20 vol%. The slurry was ball-milled in a Nalgene bottle with alumina balls at 80 rpm for 24 h. After ball milling, the slurry was repeatedly passed through a narrow channel with a diameter of 75 μm in the microfluidizer (NLM100, Ilshin Autoclave, Korea) under 100 MPa. The slurry was then slip casted into a porous alumina mold whose average pore size was 0.13 μm. The casted bodies were dried at room temperature for 48 h, and the remaining moisture was removed using a convection oven at 50 1C. Another set of green-bodies was formed by cold isostatic pressing, at 200 MPa for five minutes. For these specimens powder resulting from the comminution of slip cast parts constituted the raw-material. After burnout of NH4PAA at 480 1C was complete, the compacts were sintered under an air atmosphere at 1350–1600 1C for 2 h to analyze the sintering behavior and remove open pores prior to beginning the HIP process. The selected pre-sintered samples were hot pressed in an isostatic environment in a graphite furnace under a 180 MPa Ar atmosphere at 1450 1C for 5 h to create transparent MgAl2O4. To determine whether any contamination occurred during microfluidization, impurities in the slurry before and after microfluidization were identified using induced coupled plasma spectroscopy (Optima-4300 DV, Perkin-Elmer Inc., MA, USA). The particle size distribution of the slurry was measured using a laser diffraction particle size analyzer (LS 13 320 MW, Beckman-Coulter, CA, USA). The rheological behavior of the slurry was measured by a viscometer (LVDV-II þ PRO CP, Brookfield, MA, USA) with a CP40 spindle at a constant temperature of 25 1C. The pore size distributions of the slip-casted MgAl2O4 compacts were measured by a mercury porosimeter (Autopore IV 9510, Micromeritics, GA, USA) to check the homogeneity of the resulting green body. The density of the green and sintered bodies was evaluated by Archimedes' principle (ASTM 792), assuming that the theoretical density of MgAl2O4 is 3.578 g/ cm3. For the green samples, heating at 800 1C for 2 h in air was performed before of the porosity and density measurements. The microstructure of the material was determined by a scanning electron microscope (FE-SEM, JSM-6700F, JEOL, Tokyo, Japan) after mirror polishing and thermal etching. The measurement of the average grain size was performed using the linear intercept method and the equation GS ¼ 1:56 L;

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where L is the mean intercept (ASTM E-112-88). The optical transmittance was recorded by a UV–vis–NIR spectrophotometer (Cary 5000, Agilent Technologies, Santa Clara, CA, USA) in the range of 0.3–3.3 μm. To investigate the in-line transmittance, a 4-mm slit was located between the sample and the detector. 3. Results and discussion The particle size distribution of the MgAl2O4 slurry after ball milling and microfluidization is shown in Fig. 2; for comparison, the particle size distribution of the as-received powder is also analyzed. As shown in Fig. 2, the as-received raw powder primarily consisted of several micron-sized agglomerates due to its high specific surface area and large van der Waals attraction. Pulverized by ball milling, the particles in the slurry showed a significant decrease in the mean particle size from 2.73 μm to 0.195 μm; however, the slurry still contained micron-sized agglomerates, which was exhibited in the particle size distribution curve as three peaks at 0.140 μm, 0.238 μm and 1.87 μm. Conversely, the microfluidized slurry showed a unimodal particle size distribution with a lower mean particle size (0.150 μm) without any micron-sized agglomerates. Additionally, Fig. 2 clearly shows that only 1 pass of the slurry through the microfluidizer is sufficient to fully disintegrate the agglomerates, which is accomplished within a few minutes. In this regard, microfluidization is shown to be a powerful process for the disintegration of large agglomerates in a nano-powder. The forces introduced for disintegration during microfluidization include a high shear stress applied through the narrow channel and also the cavitation phenomenon that can occur when the slurry passes out of the narrow channel into the connected wider tube. To et al. reported that the rapid expansion of a high-pressure suspension, such as cavitation, is effective at deagglomerating nanoparticle aggregates [25]. Therefore, it is reasonable to conclude that both the high shear stress and the strong cavitation in the proposed process improved the resulting deagglomeration of the MgAl2O4 nano-powder.

Volume Fraction (%)

14

Raw powder Ball-milling Microfluidizer (1 pass) Microfluidizer (10 pass)

12 10 8 6

ICP analysis was conducted on the slurry that passed through the channel 10 times to check for any contamination created during microfluidization; these results are summarized in Table 1. It was found that no impurity inflow or contamination from the fluidizing line had occurred comparing the ballmilled slurry to the microfluidized slurry. When deagglomeration is promoted by high-energy milling, impurity contamination due to the abrasion of the milling media and the container is typically unavoidable; however, microfluidization is operated without a milling media and only requires passing the slurry through a narrow channel made by single crystal diamond, allowing a high purity and the resulting composition of MgAl2O4 to be maintained. Fig. 3 shows the rheological properties of the ball-milled and microfluidized slurries. The viscosity versus the shear rate profiles in Fig. 3(a) show that both slurries exhibited a nonNewtonian shear-thinning behavior, which decreases the viscosity as the shear rate increases; however, after reaching a steady viscosity at a sufficiently high shear rate, the viscosity of the microfluidized slurry showed was nearly half of that of the ball-milled slurry. In Fig. 3(b), the time-dependent viscosity profile of the slurries at a constant shear rate (30 s 1) shows that the viscosity increased with time for both slurries, but the viscosity of the ball-milled slurry increased more rapidly with a 58% steeper slope compared to the slope of the microfluidized slurry. While a lower viscosity indicates a weaker interparticle network and less flocculation in the asmilled or as-passed slurry, which agrees with the results of Fig. 2, the slower aging rate can be interpreted as a higher resistivity against re-flocculation and re-coagulation. Recently, Kadosh et al. reported the microstructure of a suspension directly observed by cryo-SEM and showed that a high level of flocculation and inhomogeneous void formations were promoted by aging for 30 min; this phenomenon was shown to be caused by an increase of attractive force between particles with time [26]. It is thus inferred that the homogeneous, mono-sized particle distribution in the microfluidized slurry contributed to the higher stability of the slurry by maximizing the average interparticle distance and weakening the resistance of the flow of the suspension against the external shear force. In addition, Hotta et al. reported that microfluidization enhances the electrostatic repulsion between the particles without degrading the dispersant or the particle surface and enhances the steric repulsion by increasing the average polymer molecular weight and length, which increases the interparticle distance and slurry stability [20]. Fig. 4 shows the pore size distribution in the green compacts that were fabricated with varying slurry conditions and

4

Table 1 Impurity contents in the ball-milled and microfluidized slurries after 10 passes, as analyzed by ICP.

2 0

0.1

1

10

Particle Diameter (µ m) Fig. 2. Particle size distribution of the as-received raw powder and the ballmilled and microfluidized slurries. Microfluidization was repeated one time and 10 times, which produced a marginal difference in the distribution curves.

S30CR spec sheet Ball-milling Microfluidizer

Fe

Ca

Na

Cr

10 4.21 3.65

5 23.3 20.2

20 44.6 42.7

N/A 0 0


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40

30

Ball-milling Microfluidizer

20

10

Apparent Viscosity (cP)

Apparent Viscosity (cP)

20 18 2.85E-4 cP s-1

16 14

Ball-milling Microfluidizer

12 10 8

1.80E-4 cP s-1

6 0 0

20

40

60

80

100

0

60

120

-1

180

240

300

360

Time (min)

Shear Rate (s )

Cumulative Volumetric Distribution

Fig. 3. Rheological properties of the ball-milled and microfluidized slurries (repeated 10 times) measured at 25 1C: (a) apparent viscosity versus shear rate with shear-thinning behavior shown; (b) time-dependent apparent viscosity profile at a constant shear rate of 30 s 1. The slope of each profile was calculated as a unit of cP over seconds. 1.0

Table 2 Conditions for green body fabrication with respect to the slurry preparation and shaping methods and the resultant pore size distribution and green density of each green compact.

BM MF DP(BM) DP(MF)

0.8

0.6

Specimen Shaping notation

Slurry

Pore size distribution

Green density (%TD)

Slip Dry Ball- Micro- Mean FWHM casting pressing milled fluidized pore size (nm) (nm)

0.4

0.2

0.0

0

10

20

30

40

50

BM MF DP(BM) DP(MF)

o o

o o o o

o o

28.8 25.6 30.7 30.6

10.3 9.1 10.9 10.9

49.0 52.6 46.3 46.5

Pore size (nm) Fig. 4. Pore size distribution of various green compacts measured by a mercury porosimeter. The meanings of the notations in the figure are summarized in Table 2, and the cumulative distributions were found to fit the cumulative Gaussian distribution function well.

consolidation methods; the nomenclature and data of these conditions and methods are summarized in Table 2. The slipcasted body from the microfluidized slurry (MF) showed a narrower pore size distribution with the smaller mean pore size of 25.6 nm compared with the green body prepared from the ball-milled slurry (BM) which displayed a wider distribution with larger mean pore size (28.3 nm). In addition, the green density of MF was higher than that of BM, attributed to the smaller mean pore size in the green compact. It is considered that a narrower particle size distribution without micron-sized agglomerate of microfluidized slurry enabled more uniform and homogeneous particle coordination in the MF green body. Moreover, the higher stability of the microfluidized slurry against re-flocculation which was shown in Fig. 3 is thought to influence the narrow pore size formation during slip casting process that requires a prolonged casting time which may allow the particles to aggregate again. On the other hand, the green bodies fabricated by uniaxial dry pressing of freeze-granules prepared from the ball-milled

slurry and microfluidized slurry (DP(BM) and DP(MF), respectively) followed by CIP at 200 MPa showed an overall larger and wider pore size distribution compared to that of the slip-casted specimen, resulting in lower green densities in Table 2. The conditions used here for pressing were not optimal so that the results do not represent the best values attainable by pressing. It was shown by Goldstein et al. [11] and Krell et al. [12] that dry pressing of granules with optimal conditions (i.e. solid loading, solvent, binder, granulation method and CIP schedule) can give a uniform green compact comparable to slip-casted bodies in this study. Nevertheless, it is interesting that when dry pressing was used as a consolidation method, the characteristics of the different slurry preparations were shown to marginally affect the properties of the resulting green body; however, using a wet shaping process allowed prominent improvement to the homogeneity of the green compact with respect to different slurry preparation methods. It is inferred that the disintegrating effect of microfluidization was readily exploited in slip-casting in absence of a drying process which might cause flocculation or agglomeration. Thus, the remaining part of the study will only discuss the slip-casted compacts, not dry pressed samples in order to observe the effects of microfluidization and pore size

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distribution of the green body to sintering behavior and optical properties. The difference in the pore size distribution is directly linked to the sintering behavior of MgAl2O4. Fig. 5(a) shows the relative densities of the pressureless-sintered specimens at an elevated sintering temperature. The compacts prepared by slip casting were sintered up to 99% at 1550 1C. MF was found to always produce higher pre-sintered densities than those of BM, indicating that a smaller and narrower pore size distribution in a green compact led to the rapid densification behavior at lower temperatures, which agrees with previous reports [12,14]. It has been proposed that the inter-aggregated particles form large pores with a high pore coordination number that are thermodynamically less shrinkable and also cause internal tensile stresses via local densification of dense agglomerates, resulting in grain boundary cracks or large pores, which are inhibit full densification [27]. Fig. 5(b) shows the profile of the grain size versus the relative density of each specimen after pre-sintering. The sintering path is shown to be comparable to that reported by Benameur et al. [24], who showed that grain growth was inhibited until densification proceeded to the final stage of sintering when the last open porosity was removed. Remarkably, the onset temperature of grain growth differed in the different processing routes investigated; because the densification proceeded rapidly at lower temperatures in Fig. 4(a), the sintering path shifted to right; the grain growth during densification was effectively hindered before reaching the final stage of sintering. This shift of the sintering path could be caused by the following: pores with a higher pore coordination number (R) than the critical coordination number (Rc) are thermodynamically stable and not shrinkable but can convert to be shrinkable because the pore coordination number tends to decrease with grain growth as sintering proceeds [27]. Thus, when the initial proportion of pores with R4Rc decreases, densification can proceed without excess grain growth at high temperatures, inhibiting grain growth during densification. As mentioned above, a finer grain size is preferred after post-HIP treatment to improve certain

mechanical properties, such as hardness and ballistic resistance. To reduce the grain size of the final material, the prerequisite condition is to achieve close porosity with a refined microstructure at the pre-sintering stage. In this regard, slip casting combined with microfluidization can be an effective method for preparing a fine-grained MgAl2O4 spinel. A post-sintering HIP process was performed on selected specimens with a relative density higher than 94%, which were considered to be the last stage of sintering without an open porosity. The variations in grain size after HIP are summarized in Fig. 6. All of the HIPed specimens were fully densified based on Archimedes' method except for BM pre-sintered specimen at 1500 1C which is denoted as BM-1500 (likewise for the other specimens) with a relative density after HIP of 97.5% due to the remaining open pores after pre-sintering. It is noted that the grain growth significantly increased in specimens with densities near 95%; for BM-1500 and MF-1500, the grain sizes were shown to increase to approximately 1.3 μm after HIP, which was comparable to the grain size of the specimens pre-sintered at 1550 1C. Conversely, the specimens with pre-sintered densities higher than 98% maintained a comparable grain size after the HIP treatment; these specimens included BM-1550 and MF-1550. Grain growth and densification under high pressures have been explained by either plastic deformation, such as grain-boundary sliding, or grain/volume diffusion processes. According to Kim et al., grain-boundary sliding is more prevalent in the intermediate stage of sintering, while diffusion becomes more dominant at the last stage of sintering when the average pore size decreases to below three times the grain size [28]. In this experiment, it is expected that densification during HIP was performed by the diffusion process because the selected pre-sintered specimens primarily contained closed pores. For the grain growth controlled by diffusion during HIP, a linear relationship between the logarithms of the porosity and the grain size was found [29], indicating that the higher porosity and the smaller grain size of a pre-sintered specimen promoted more grain growth during HIP, which agrees with the trends found in this study.

100

95

Grain Size (µm)

Relative Density (%)

2.0

90

BM MF

85

BM MF

1.5

1.0

0.5

0.0

80 1400

1450

1500

1550

Pre-sintering Temperature (oC)

1600

80

85

90

95

100

Relative Density (% TD)

Fig. 5. Sintering behavior during pressureless air sintering: (a) relative density with respect to the pre-sintering temperature; (b) grain size versus relative density (i.e., sintering path).


13359 2.5

Grain Size (µm)

2.5

MF MF-HIP

BM BM-HIP

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5 0.0

0.0 1400

1450

1500

1550

1600

1400

1450

1500

1550

1600

Pre-sintering Temperature (oC) Fig. 6. Variation of the grain size before and after the post-HIP treatment (T ¼1450 1C, t¼ 5 h, P ¼180 MPa of Ar). Selected specimens for HIP had a relative density above 94% after pre-sintering.

Although BM-1550 would increase in transmittance after the HIP treatment at higher temperatures and with a longer soaking time, it must accompany grain growth; in this regard, MF showed the advantage of producing finer microstructures compared to that of BM with a similar in-line transmittance. The improved transmittance and finer microstructure were likely produced by the homogeneous microstructure of the presintered specimen and the low pre-sintering temperature, which was enabled by the fabrication of uniform green compacts.

In-line Transmittance (%)

100

80

60

BM-1550

MF-1500

40

BM1550-HIP MF1500-HIP

20

0 500

1000

1500

2000

2500

Wavelength (nm) Fig. 7. In-line transmittance of the HIPed specimen after mirror polishing and images of the specimens. The selected pre-sintering temperature was that which showed the best transmittance for each specimen conditions. Due to the different sample thicknesses, calibrated plots with a normalized thickness of 1 mm are shown.

The in-line transmittance and the images of the HIPed specimens are shown in Fig. 7. For the BM and MF specimens, pre-sintering at elevated temperature was found to deteriorate the in-line transmittance due to an increase in the intragranular pores trapped during the pre-sintering process [11]. BM-1550 showed a lower transmittance than MF-1500 which showed a 79.3% of the ILT at λ ¼ 550 nm, while ILT of BM-1550 was 47.5% at λ ¼ 550 nm. It has been reported that the degradation of the transmittance in polycrystalline ceramics with cubic crystal structures is caused by scattering factors that produce different refractive indices and include residual pores, secondary phases or absorption factors, such as vacancies or color centers. Among these, in most cases, the residual porosity has a dominant effect on the transmittance, and a pore with a size near 1/10 of the incident light's wavelength severely deteriorates the light transmittance due to Mie scattering [5]. Thus, improvement of the visible light transmittance could be obtained by reducing the amount of nano-size pores less than 100 nm in diameter. In this regard, the enhanced transmittance of MF-1500 was associated with a smaller amount of residual pores, particularly heterogeneous large pores and intragranular pores, which were not easily removed by the HIP treatment.

4. Conclusion A transparent MgAl2O4 spinel was fabricated by homogeneous green compaction via microfluidization, slip casting and the sinter-HIP process. The various factors affecting the homogeneity of the green compacts used and the associated effects on the sintering behavior and transmittance were investigated. (1) Microfluidization was confirmed to be an effective dispersion process for disintegrating particle agglomerates in slurries by passing the slurries through a narrow channel with a high shear stress. The unimodal particle size distribution of the microfluidized slurry resulted in a smaller mean pore size and a narrower pore size distribution in the green compact. (2) The densification behavior during pressureless air sintering was shown to be influenced by the state of the green body; a more homogeneous green compact tended to accelerate the densification process. The sintering path showed that rapid densification at lower temperatures was more favorable for reducing grain size. Additionally, a lower presintering temperature and a uniform pre-sintered microstructure were required to enhance the transmittance by decreasing the number of large defects and intragranular pores present in the material. Acknowledgments This work was supported by the Materials and Components Technology Development (MCTD) Program (PN: 10047010,

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