A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 35 (2015) 651–661

A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing Laura Esposito a,∗ , Andreana Piancastelli a , Patrizia Miceli b , Stefano Martelli b a

b

CNR ISTEC, via Granarolo 64, 48018 Faenza, RA, Italy Centro Sviluppo Materiali S.p.A., via di Castel Romano 100, 00128 Roma, Italy

Received 3 June 2014; received in revised form 1 September 2014; accepted 2 September 2014 Available online 22 September 2014

Abstract Hot pressing has been investigated as a method for producing highly transparent, cost-effective MgAl2 O4 for optical applications. The hot pressing of stoichiometric Al2 O3 –MgO powder mixtures has been preferred to the direct pressing of spinel powders, given the ready availability of pure powders and the opportunity to exploit the thermodynamic driving force for spinel formation. Ultra-pure LiF has been used as a sintering additive. An accurate thermodynamic study on the reactions involved in spinel formation has shed light on the mechanisms affecting the material’s transparency, particularly as concerns the presence of a carbonaceous environment. Transparencies up to 70% in the visible range (maximum 78%) have been obtained. The present study focused on the pressure applied during sintering as one of the main parameters influencing the material’s final optical properties. © 2014 Elsevier Ltd. All rights reserved. Keywords: Spinel; MgAl2 O4 ; Thermodynamics; Hot-pressing; Transparent ceramic

1. Introduction Since their official appearance in the 1960s, transparent polycrystalline ceramics1,2 have not only attracted technical and scientific interest, but also captivated science fiction novelists, who have seen them as the materials of 24th-century technology.3 Over the last few decades, transparent polycrystalline ceramics have found numerous applications, ranging from laser hosts4 to transparent armor,5 and next-generation spacecraft windows.6 Several reviews have already been published, giving a thorough description of the extensive effort dedicated to this class of materials.7–10 Magnesium aluminate (MgAl2 O4 ) spinel – simply called spinel from now on – has a cubic crystal structure and is therefore optically isotropic. In addition to a high melting point, it has excellent mechanical properties, outstanding hardness and a superb transparency from the near UV down to the mid IR

∗

Corresponding author. Tel.: +39 0546699763. E-mail address: [email protected] (L. Esposito).

http://dx.doi.org/10.1016/j.jeurceramsoc.2014.09.005 0955-2219/© 2014 Elsevier Ltd. All rights reserved.

range, it remains transparent beyond the limit for ␥-AlON, the direct transparent competitor, which is set at 4–4.5 ␮m. Armorline Company has recently announced that they could reach important milestones in their scale-up programs and reports having obtained 25 × 75 cm rectangular plates and disks 36 in. in diameter made of transparent spinel.11 These exciting results implicitly suggest that a solution has been found for the still debated questions of how transparency can be improved,12,13 and cast some doubts on the real utility of pursuing investigations on the subject. On the other hand, the huge investments needed for high-capacity equipment mean that small or medium enterprises and research institutions operating outside the defense sector cannot afford to manufacture large transparent components. The feasibility of producing limited quantities of transparent spinel ceramics using low- or medium-priced apparatus is reviving interest in investigating the reactions and mechanisms behind highly transparent materials, especially in the case of spinel. There is room for a competitive niche production of small- to medium-sized optical windows in demanding optical applications, such as high-energy laser systems,14 where traditional

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mineral salts and amorphous materials are limited and single crystals are not practical. To give an example, transparent polycrystalline ceramic windows can facilitate breakthrough optical spectroscopy conducted at high temperatures and in aggressive environments, as in LIBS spectroscopy on molten steel.15,16 It was with this in mind that a previous study looked into the densification of transparent spinel using conventional hot pressing of commercial powders.17 A fairly transparent spinel could be synthesized with a transmittance of up to 70% in the visible range and a peak value of 78% at 1100 nm. The best results were achieved starting from separate ultrapure MgO and Al2 O3 powders, while the LiF powder used as the sintering aid was of standard commercial purity (98.5%). The previous study could not demonstrate that the ultimate spinel transmittance had been reached. Further improvements might hopefully derive from the use of ultrapure LiF powder and a better understanding of the role of LiF18,19 and the reactions underway during densification, with a view to fine-tuning the HP cycle.18,20 Even below 10−4 , powder impurities can segregate at the grain boundaries, causing scatter and opacity.13 A simplified thermodynamic model describing the main densification phases was developed to elucidate the optimal processing conditions, in terms of LiF content and the most appropriate hot pressing cycle. This model clarifies the interaction between LiF and the other constituents, the impact of the pressure applied and the influence of the graphite tooling and heating elements generally used in hot pressing hardware. The experimental plan followed the thermodynamic indications. The oxide powder mixture was prepared according to the already tested procedure, using MgO and Al2 O3 powders of the same nominal grade, but ultrapure LiF powder instead of the powder previously used, which contained about 1.5 wt.% of impurities. As before, the hot pressing force applied did not exceed the capability of medium-sized commercial equipment. 2. Thermodynamic model Achieving transparent spinel by hot pressing a starting mixture of separate oxides (MgO, Al2 O3 ) doped with the addition of LiF is a rather tortuous procedure that passes through the low-temperature formation of MgAl2 O4 . There is experimental evidence of LiF promoting the spinel-forming reactions21 and sintering the material, facilitating a cleansing of the compact material thanks to volatile impurity outgassing during hot pressing, and protecting the material against contamination by carbon coming from the tooling.20–23 The role of the fugitive LiF additive is still being debated, however, and has yet to be fully understood.13,19,24,25 MgAl2 O4 spinel formation from the initial oxide powders with LiF evaporation is thermodynamically possible at the temperature considered, but many other species can be formed, in quantities that also depend on pressure and temperature. A thermodynamic study was conducted to study the behavior of the system comprising Al2 O3 , MgO and LiF as a function of temperature and pressure, also taking any effects due to carbon contamination into account (carbon activity: aC = 1). The equilibrium conditions of this complex system were calculated

using the Thermo-Calc26 software package for temperatures in the range of 1100◦ to 1500 ◦ C. The equilibrium calculation was obtained by minimizing the Gibbs free energy. The total Gibbs free energy (Gtot ) of a system is given by: Gtot =

n 

ni μi

(1)

i=1

where ni is the number of moles of species i, and μi is the chemical potential of species i, which can be expressed by μi = G0i + RT ln

fi fi0

(2)

where R is the universal constant, T is the absolute temperature, and fi is the fugacity of species i. G0i and fi0 are the standard Gibbs free energy and standard fugacity of species i, respectively. For gaseous species fi = Φ pi , where Φ is the fugacity coefficient that describes the deviation from the ideal behavior. The composition of the gas phase at equilibrium with the solid MgAl2 O4 spinel and LiF is expressed in terms of the partial pressure, pi , of the i-produced gaseous species: pi = χi × Ptot

(3)

where χi is the mole fraction or (volume fraction) of the gaseous species i, and Ptot is the hydrostatic pressure coming to bear on the gas phase. To construct a thermodynamic model, it is mandatory to first define the environment, i.e. the reacting species, the available reaction volume, the external hydrostatic pressure, and the temperature. The hot pressing cycle is performed under low pressure (∼10–100 Pa). It consists of two main steps: the first up to 1220 ◦ C with no pressure being applied; the second up to 1600 ◦ C under 40 MPa. During the holding time at the end of the first step, the actual available volume can be assumed to be infinite, and the external pressure lower than 100 Pa. As soon as the final pressure is applied on the sample, however, the small volume containing the powder inside the mold becomes a closed system, tightly sealed by the pressed powder. The available volume is reduced to a few cubic centimeters and the external hydrostatic pressure can be assumed to be as high as the pressure acting on the pressing punches (40 MPa). Table 1 and Fig. 1 show the equilibrium partial pressure of the gas phases that, according to Thermo-Calc, occur when MgO–Al2 O3 –LiF are brought together in the presence of carbon contamination. At 1220 ◦ C the partial pressure of the gaseous species predicted remains below 40 Pa, except for the LiF partial pressure, which exceeds 1200 Pa. During the holding time at 1220 ◦ C, before pressure is applied, LiF evaporation is the overwhelming reaction according to the present thermodynamic model. It is worth noting that the equilibrium calculation foresees the presence of CO gas, albeit at a low partial pressure of 22 Pa. The formation of CO gas goes to show that the separate oxides and the spinel phase are both reduced by carbon pollution from the graphite tooling. As for the role of LiF as a carbon cleanser, it has

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Table 1 Partial pressure (Pa) of the gaseous species formed as a function of temperature. * LiF fluorides include LiF, Li2 F2 and Li3 F3 compounds; Mg fluorides include MgF, Mg2 F2 and Mg3 F3 ; Al fluorides include AlF, AlF2, AlF3 . Partial pressure (Pa)

LiF fluorides* Mg fluorides* Al LiF4 Al fluorides* Mg CO

1100 ◦ C

1200 ◦ C

1300 ◦ C

1400 ◦ C

1500 ◦ C

3.55E + 02 1.51E − 02 7.96E − 01 8.86E − 01 8.26E − 01 3.36E + 00

1.26E + 03 1.20E − 01 3.42E + 00 7.64E + 00 5.41E + 00 2.22E + 01

3.74E + 03 7.22E − 01 1.09E + 01 4.69E + 01 2.81E + 01 1.16E + 02

9.64E + 03 3.43E + 00 2.72E + 01 2.21E + 02 1.21E + 02 4.96E + 02

2.22E + 04 1.32E + 01 5.53E + 01 8.39E + 02 4.36E + 02 1.79E + 03

been suggested that LiF acts as a scavenger, capturing the carbon contaminant and forming CFx volatile compounds (x stands for the F stoichiometry), which eventually leaves the sample as a vapor.22 According to the equilibrium calculation (see Fig. 2), the partial pressure of the CFx compounds is pCF < 1 × 10−5 Pa and, under low pressure (∼10–100 Pa), the volume fraction is further reduced to 1 × 10−7 . The experimentally demonstrated carbon cleansing effect is therefore unlikely to be attributable to the formation and volatilization of CFx compounds. Instead, it may be that molten LiF forms a film that is effective in shielding against carbon contamination. In fact, the highly-wetting molten LiF can easily infiltrate the porous powder pre-forms by means of capillary forces.22

The LiF equilibrium partial pressure indicates that, when the pressing force is applied, LiF evaporation stops and the mole fraction of LiF gas in the surrounding volume is reduced to 6 × 10−3 (Eq. (3)). If not all of the LiF additive is able to evaporate during the pressureless holding time at ∼1200 ◦ C, the remaining LiF is likely to be trapped inside the structure, damaging the material’s transparency. The high-temperature carbothermal reduction of MgO, Al2 O3 and MgAl2 O4 is well known.25,27 The reaction between MgAl2 O4 and C can be written as: MgAl2 O4 + 3C → Mg(g) + Al2 O(g) + 3CO(g)

(4)

The partial pressure of gaseous Mg(g) rises rapidly to pMg = 450 Pa at 1500 ◦ C (see Fig. 3), i.e. Mg(g) evaporation is not negligible when sintering under low pressure (10–100 Pa), or at atmospheric pressure (∼105 Pa). The generation of Mg vapor gives rise to the formation of pores inside the bulk, making the spinel opalescent. During hot pressing, and assuming a hydrostatic pressure corresponding to the pressing force (40 MPa), the Mg(g) mole fraction χMg(g) at equilibrium drops to ∼1 × 10−5 (from ∼4 × 10−3 at atmospheric pressure), and there should be significantly fewer light scattering pores. The same rules apply to the formation of Al2 O vapor, but its partial pressure at 1500 ◦ C is pAl2 O = 45 Pa, i.e. about one order of magnitude lower, so it is less harmful to the material’s transparency. Fig. 1. Partial pressure of the formed gaseous species as a function of the temperature (see Table 1).

Fig. 2. Partial pressure of LiF, Li, F, and CFx (x, F stoichiometry) as a function of reaction temperature.

Fig. 3. Upper curves: partial pressure of gaseous Mg(g), CO(g), Al2 O at the equilibrium with solid MgAl2 O4 as a function of the temperature and in presence of carbonaceous atmosphere (aC = 1); lower curve Mg(g) partial pressure for clean atmosphere (aC = 0).

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Hot pressing is useful not only to obtain a fully dense material, but also to improve the spinel’s stability, containing harmful material evaporation phenomena. This dual positive effect can be reinforced by upgrading to hot isostatic pressing (HIP), which can easily reach pressures up to 200–300 MPa. Under such a high pressure, there is a stronger mechanical pore closing action, and the Mg(g) mole fraction at equilibrium χMg(g) further drops to 1.5 × 10−6 , thus reducing microvoid formation through MgAl2 O4 decomposition. On the other hand, Liu et al.28 showed that sintering in a clean carbon-free atmosphere (aC = 0) enables the pressureless densification of novel Mg-containing transparent ceramics. In the absence of carbon, the partial pressure of gaseous Mg is reduced by about six orders of magnitude, the equilibrium mole fraction at ambient (∼105 Pa) pressure is 7.1 × 10−9 , and Mg evaporation can be disregarded with confidence (see Fig. 3). Finally, the reaction between the spinel and LiF that leads to the formation of liquid MgF2 and solid LiAlO2 deserves further comment. This reaction may adversely affect the transparency of the hot pressed samples because the LiAlO2 solid phase will more than likely remain trapped inside the structure, contributing to the spinel’s opacity.29 Densification and sintering could be facilitated, on the other hand, by means of a mechanism of dissolution-precipitation.25 The LiAlO2 phase is solid, and the equilibrium partial pressure approach is not applicable to the present case. The interaction between spinel and LiF can be described more appropriately by the variation in the free energy associated with the reaction, and by considering the effect of the presence of carbon. A possible reaction between spinel and LiF can be written as: 3LiF (l) + MgAl2 O4 → LiF : MgF2 (l) + 2LiAlO2 (s)

(5)

Previous thermodynamic studies on the LiF–Al2 O3 –MgO system showed that, while MgO does not react appreciably with LiF, Al2 O3 can form various compounds containing lithium and/or aluminum.30 Reaction (5) can thus be simplified, taking only the interaction between LiF and Al2 O3 into account. For the carbon-free case, Reaction (5) becomes: 3LiF (l) + 2Al2 O3 (s) ↔ AlF3 (g) + 3LiAlO2 (s)

(6)

The associated free energy variation G is: G = G0 + RT ln

a(LiAlO2 )3 × p (AlF3 ) a(Al2 O3 )2 · a(LiF)3

(7)

where: Go = −1.06 × 102 T + 3.05 × 105 (J)

(8)

R is the gas constant (R = 8.314 J/K/mol), and T is the absolute temperature. The activity of the LiAlO2 phase a(LiAlO2 ), and of the LiF phase a(LiF) is set as equating to 1; the activity of the alumina a(Al2 O3 ) corresponds to the activity of Al2 O3 in the MgAl2 O4 spinel phase; and the partial pressure of the AlF3 gas phase p(AlF3 ) is the equilibrium partial pressure in the MgAl2 O4 –LiF system calculated at a given temperature.

Fig. 4. Comparison of the free energy change (G) for the formation of the LiAlO2 phase from the reaction of MgAl2 O4 and LiF as the function of the temperature with (carbon activity aC = 1) or without carbon contamination (carbon activity aC = 0).

Taking carbon contamination into account, Reaction (6) is transformed into: LiF (l) + Al2 O3 (s) + C (s) ↔ AlF (g) + LiAlO2 (s) + CO (g) (9) and: G = G0 + RT ln

a (LiAlO2 ) × p (AlF) × p (CO) a (Al2 O3 ) × a (LiF) × a (C)

(10)

where Go = −3.13 × 102 T + 6.7 × 105 (J)

(11)

the carbon activity a(C) is set to 1, and the partial pressure of the AlF gas phase p(AlF) is now calculated in the MgAl2 O4 –LiF–C system. The free energy variations G calculated from Eqs. (7) in the absence of carbon, and (10) in the presence of carbon, are shown in Fig. 4 as a function of the reaction temperature. Judging from the energy change calculated, in the absence of carbon contamination (when using molybdenum furnaces, for instance), LiAlO2 formation is exergonic and occurs spontaneously in the whole range of temperatures.30 LiAlO2 formation becomes endergonic in the presence of carbon, and for temperature below 1300 ◦ C, while the reaction becomes exergonic again if the temperature is raised above 1300 ◦ C. The thermodynamic results give the impression that carbon contamination inhibits the reactions leading to LiAlO2 formation up to temperatures where the fugitive LiF can be assumed to have evaporated completely, in which case the subtle carbon contamination improves the spinel’s transparency by preventing the formation of the opaque LiAlO2 phase. The main indications emerging from the thermodynamic model can be summarized as follows. - The presence of carbon in the surrounding atmosphere (carbon activity aC = 1) strongly affects both the stability of the spinel phase and the interaction between the LiF and the spinel.

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Fig. 5. Commercial ultra-pure LiF powder morphology resulting from SEM analysis.

- The cleansing action of LiF seems to relate more to the formation of a liquid phase surrounding and shielding the grain than to it removing the carbon contaminant by forming volatile CFx compounds. - Carbon contamination positively hampers the reaction between LiF and Al2 O3 that might lead to the formation of a stable LiAlO2 solid phase. - A carbonaceous atmosphere favors spinel phase decomposition, evaporation of the Mg(g) species, and the formation of light scattering pores. - At high temperatures, raising the ambient pressure counteracts spinel decomposition and the formation of Mg(g) species, i.e. the higher the pressure, the greater the stability of the spinel phase. 3. Experimental Three powder mixtures made from commercial Al2 O3 and MgO products were prepared, taking the stoichiometry of spinel into account, and containing 0.0, 0.5 and 1.0 wt.% of LiF powder, respectively. The properties of the selected commercial powders are given in Table 2. Only powders with a purity of 99.99% or higher were used. The Al2 O3 and MgO powders were chosen in the light of the results of a previous work.17 The Al2 O3 powder was free of hard aggregates and characterized by a sharp particle size distribution with a low specific surface area. The MgO powder had a high purity associated with a high specific surface area and a broad particle size, although a few aggregates were detectable. The LiF powder chosen for this study had a purity of 99.995% and consisted of large cubic particles (Table 2, Fig. 5). Gentle wet ball milling was used to mix the powders homogeneously in a laboratory jar ball mixer (Tecnotest, Modena, Italy, Mod. D768/2V) in absolute ethanol, using polyethylene bottles and 96% pure Al2 O3 milling media with a diameter of 1 cm. The solid content of the slurry was 24 wt.%. Ball milling was done at a constant speed of 40 rpm and lasted for 24 h. The solvent was removed with Rotavapor to avoid any selective powder sedimentation. The powders were then dried completely at 60 ◦ C for 24 h and sieved at 130 ␮m.

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The powder mixture’s reactivity as a function of temperature was tested by high-temperature X-ray diffractometry using a BRUKER D8 ADVANCE diffractometer equipped with an MRI BASIC attachment. The powder was placed on the Pt strip, which also serves as thermocouple, heating element and sample holder. Temperature calibration was based on the thermal expansion of a standard sample of pure synthetic periclase (MgO MERK SP65). To assess phase transformation and chart the course of spinel crystallization, patterns were collected for every 100 ◦ C increase in temperature from 900 ◦ C to 1400 ◦ C (raising the temperature at a rate of 10 ◦ C/min), in the range of 20–70 2Θ, with steps of 0.02◦ and 0.5 s real-time steps, using a LynxEYE multichannel detector. For the hot pressing cycles, pellets of the same weight (approximately 26 g) were pre-shaped by linear pressing under a pressure of 15 MPa in metallic dies of the same diameter (30 mm) as the graphite hot pressing dies. The green pellets were placed in a graphite mold lined with graphite paper and hot pressed under a vacuum (10–100 Pa). The final temperature selected was 1600 ◦ C. The thickness of the samples after the HP cycle was approximately 7 mm. A pressureless soaking stage at 1220 ◦ C, lasting between 0.5 and 2 h, was included before the final densification step with a view to promoting the homogeneous distribution of molten LiF (melting point 845 ◦ C) inside the compacted powder, and the complete removal of any residual liquid by evaporation. After completing the soaking stage, a pressure of 40 MPa was brought to bear on the material (at a temperature of 1220 ◦ C). The holding time at the final temperature (1600 ◦ C) was 60 min. Previous investigations had indicated that a maximal sample transparency cannot be achieved using this thermal treatment. A longer holding time (about 180 min) is needed to achieve a high transmittance, but this causes significant grain growth too, cancelling the beneficial effect on the microstructure of preliminary soaking at 1220 ◦ C. After selecting the best balance between LiF content and soaking time at 1220 ◦ C, the best transparency was achieved after holding the material at 1600 ◦ C for 180 min. The heating rate was initially 15 ◦ C/min up to 1220 ◦ C, then it was reduced to 7 ◦ C/min up to sintering temperature. After the high-temperature soaking stage, powder heating was stopped, and the furnace was allowed to cool freely (∼10 ◦ C/min) down to room temperature. The hot pressing cycles performed are listed in Table 3. Powder morphologies and sample microstructures were investigated with a scanning electron microscope (SEM, Cambridge S360, Cambridge, UK and FE-SEM, Carl Zeiss Sigma NTS GmbH, Öberkochen, Germany) coupled with an energy-dispersive x-ray spectrometer (EDS, INCA Energy 300, Oxford Instruments, UK). Mean linear intercepts were used as a measure of the average grain size. Thin slices of 1 to 5 mm were cut from the samples and optical polished on both surfaces up to 1 ␮m using diamond pastes. Optical transmittance was measured with a UV/vis Lambda 35 spectrometer (Perkin Elmer Instruments, Waltham, MA, USA) over the wavelength region from 200 to 1100 nm. The transmittance data were recorded and are reported without correcting for reflection.

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Table 2 Selected ceramic powders and manufacturers’ data. D50: mean grain size; SSA: specific surface area. The D50 of LiF powder was not available from the producer so it was calculated by SEM microstructural analysis. Powder

Producer

Grade

Purity (wt.%)

D50 (␮m)

SSA (m2 /g)

Impurities (ppm)

␣-Al2 O3 MgO LiF

Taimei Baikowski Sigma Aldrich

TM-DAR M30CR –

99.99 99.99 99.995

0.15 1.35 ∼5.00

14.1 28.0

Si 4, Fe 4, Na 2, K 1, Ca 2, Mg 1 Ca 7, Fe 11, K 28, Na 10, Si 13 <60

Table 3 Results obtained with the cycles performed. The theoretical density of MgAl2 O3 is 3.578 g/cm3 . According to the Archimedes method, all samples were fully dense. RIT: real in-line transmittance. Notes: sample 7 was the same as sample 3, but soaked for longer at the highest temperature to improve its transmittance; grain size was measured in the two distinct regions in sample 1 (see Fig. 8). Soaking @1220 ◦ C (min)

Soaking @1600 ◦ C (min)

Sample

LiF (wt.%)

1

0.0

60

60

2 3 4 5 6 7

0.5 1.0 0.5 0.5 1.0 1.0

60 60 30 120 120 60

60 60 60 60 60 180

Grain size (␮m)

RIT @1100 nm (%)

Thickness (mm)

3 ± 0.8 38 ± 13 19 ± 11 14 ± 10 16 ± 9 20 ± 10 18 ± 14 35 ± 23

34

0.98

42 70 16 40 44 83 76 71 58

1.70 1.30 1.62 1.37 1.70 1.00 1.8 2.2 5.0

4. Results 4.1. High-temperature X-ray diffraction analysis The X-ray diffraction patterns at increasing temperatures of the mixtures with no LiF or with the addition of 1 wt.% LiF are shown in Fig. 6. In the mixture with no added LiF, the onset of the reaction between Al2 O3 and MgO to form spinel was located at 1000–1100 ◦ C, confirming previous reports from Esposito et al. and Sutorik et al. [17,24]. In the presence of LiF, the spinel peaks are seen at a considerably lower temperature (800 ◦ C), suggesting that LiF plays an active part by markedly reducing the activation energy of the reactions leading to the formation of spinel phase.21 In both cases, the transformation into spinel is completed at 1400 ◦ C. 4.2. Hot pressing (HP) cycles Table 3 shows the hot pressing cycles performed and the corresponding average grain size and transparencies of the dense samples obtained. 4.2.1. LiF content Samples 1, 2 and 3 contained 0.0, 0.5 and 1.0 wt.% LiF, respectively, and were processed under identical HP cycles, i.e. 60 min of soaking time at both 1220 ◦ C and 1600 ◦ C (Table 3), to highlight the effect of adding LiF. The microstructures obtained are shown in Figs. 7–9. Sample 1 without any added LiF exhibits an apparent bimodal microstructure characterized by distinct crystallite configurations (Fig. 7), with almost pore-free regions with large and approximately equiaxed spinel grains (∼38 ␮m), and other

Fig. 6. High temperature X-ray analyses of Al2 O3 –MgO powder mixtures without LiF(a) and with 1.0 wt.% LiF(b).

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Fig. 7. SEM microstructure of sample 1 not containing LiF addition, hot pressing at 1600 ◦ C for 60 min, intermediate soaking at 1220 ◦ C for 60 min. Polished surface (a), fracture surface (b).

regions with fine grains (∼3 ␮m) and a large fraction of intergranular pores. Adding LiF makes the microstructure more homogeneous. Sample 2 contained 0.5 LiF and the average grain size was approximately 20 ␮m (Table 3, Fig. 8). Regions containing mainly small grains can still be seen, but they are not accompanied by a larger amount of residual porosity (as in sample 1), and this is due to LiF aiding the sintering process. As known the presence of Li atoms cannot be detected by SEMEDS analysis. On the other hand the absence of any residual F-rich second phases seems to indicate that LiF fully evaporated during soaking at 1220 ◦ C. When the amount of LiF additive was 1.0 wt.% (sample 3), the average grain size was smaller than in sample 2, which contained less LiF), and less than half the size of the grains in sample 1, which contained no LiF (Fig. 9a). Some residual porosity was visible along the grain boundaries in sample 3 (Fig. 9b), which can be attributed to the trapping of some residual fluoride phase.

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Fig. 8. SEM microstructure of sample 2 with 0.5 wt.% LiF addition, hot pressing at 1600 ◦ C for 60 min, intermediate soaking at 1220 ◦ C for 60 min. Polished surface (a), fracture surface (b).

4.2.2. LiF evaporation LiF evaporation can be investigated more thoroughly by looking at the microstructures obtained from cycles performed with a different LiF content or soaking time at 1220 ◦ C: samples 4 and 5 contained 0.5 wt.% LiF, while sample 6 contained 1.0 wt.% (Table 3); and sample 4 was treated at 1220 ◦ C for 30 min, samples 5 and 6 for 120 min. Sample 4 exhibited a considerable residual porosity and residual fluoride-rich, likely LiF- or MgF-rich phases along the grain boundaries, suggesting that the soaking time was too short to promote complete fluoride evaporation (Fig. 10). Evaporation of the trapped residual fluoride at the higher temperature (1600 ◦ C) eventually led to pores forming along the grain boundaries, making the transmittance of sample 4 lower than for the sample 2 treated at 1220 ◦ C for 60 min. Conversely, extending the intermediate soak at 1220 ◦ C to 120 min appeared to negatively affect the material’s structure, since the light transmittance was worse for samples 5 and 6. 4.2.3. Highly transparent material The highest transparency was obtained with sample 7, which contained 1.0 wt.% LiF and was sintered at 1600 ◦ C for 180 min (Table 3, Figs. 11 and 12). The transparency obtained is

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Fig. 10. SEM microstructure of sample 4 with 0.5 wt.% LiF addition, hot pressing at 1600 ◦ C for 60 min, intermediate soaking at 1220 ◦ C for 30 min. Polished surface (a), fracture surface (b).

Fig. 9. SEM microstructure of sample 3 with 1 wt.% LiF addition, hot pressing at 1600 ◦ C for 60 min, intermediate soaking at 1220 ◦ C for 60 min. Polished surface (a), fracture surface (b).

comparable with the values already achieved in a previous work,17 and confirms that the high-temperature soaking needed to obtain a pore-free material from mixed oxides is about 3 h, but the ultra-pure LiF additive unfortunately did not improve the transmittance. The strong dependence of the sample’s transmittance on its thickness means that there are causes of scatter – mainly pores – in the final material. As known, for a ceramic sample of thickness t, the in-line transmittance can be expressed by the Lambert–Beer equation: T = (1 − R) × exp (−γext × t)

(12)

R is the total reflection loss, which depends on the refractive index n of the ceramic. The attenuation coefficient γ ext describes the total light losses: γext = γabs + γsca

the real in-line transmittance as a function of the sample’s thickness (Fig. 11). From the slope of the lines, values of γsca were estimated to be ∼ =0.8–1 cm−1 , at 1100 and 633 nm, respectively. Analogous intrinsic absorption coefficients measured on similar cubit transparent ceramic suggest that the measured γsca corresponds to a residual porosity ≤0.005%, whereas a transparency ≥80% requires the porosity to be ≤0.002%.31 Lastly, when the final soaking at 1600 ◦ C was extended from 60 to 180 min, and the amount of LiF added was 1.0 wt.% (sample 7), the average grain size doubled, and there was no longer any visible residual porosity or detectable remnant LiF (Fig. 12).

(13)

where γ abs is the intrinsic absorption coefficient, and γ sca is the scattering coefficient due to pores, grain boundaries and second phase particles. Assuming that the residual pores are incoherently scattering particles and that the intrinsic light absorption is negligible, then γext ≈ γsca and it can be ascertained from the logarithmic plot of

5. Discussion The present work aimed to further improve on the transparency of hot pressed spinel ceramic achieved in a previous study.17 Commercial high-purity LiF powder was used to minimize the occurrence of hazardous optical impurities. Although the use of specially-synthesized, ultra-pure spinel powder enable a transparency beyond 85% to be achieved after hot pressing under comparable conditions,32 for the purposes of the present study only readily-available commercial powders were selected and processed.

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Fig. 11. (a) Real in-line transmittance measured by UV–vis spectrophotometer as a function of the sample thickness. The image on the right shows the sample used for preparing the measured samples. (b) Plot of Ln(RIT) vs. thickness for determining the extinction coefficient.

Fig. 12. SEM microstructure of sample 7 with 1 wt.% LiF addition, hot pressing at 1600 ◦ C for 180 min, intermediate soaking at 1220 ◦ C for 60 min. Polished surface (a), fracture surface (b).

5.1. LiF content

carbon and spinel could also only partially reduce the spinel phase:

According to the literature, the LiF additive should be no more than 1 wt.%13,18,22 because excessive amounts of LiF can remain trapped and result in scatter and opacity.29,33 The purpose of adding LiF is to foster powder densification and protect the material against carbon contamination. The liquid LiF film forms already at low temperatures (845 ◦ C) and facilitates the reaction between the separate oxides to form spinel by lowering the activation energy of the reaction. The liquid-phase LiF also promotes powder densification through a liquid phase sintering effect, facilitating particle sliding and helping to release jamming. The thermodynamic estimate suggests that the liquid LiF prevents carbon contamination in the material rather than acting as a cleanser, so the sample containing no LiF is more liable to carbon attack. The bimodal grain growth of LiF-free samples has already been observed during MgAl2 O4 spinel sintering without any sintering aids and in reducing atmospheres, as encountered in hot pressing with graphite tooling.34,35 When no liquid phase is formed, the end result is generally a fine-grained, poorlysintered material. On the other hand, to explain the exaggerated grain growth, Meir [34] suggested that, alongside the Mg(g) evolution (e.g. Eq. (6)), the high-temperature reaction between

MgAl2 O4 + 2C → MgO + Al2 O(g) + 2CO(g)

(14)

causing MgO enrichment of the spinel phase and a consequent formation of structural oxygen vacancies. The formation of free MgO during spinel sintering has been documented experimentally by processing the material under either CO/CO2 reducing atmosphere with oxygen partial pressure PO2 ∼10−10 Pa or simply in air (PO2 ∼2 × 104 Pa).35 Large grains grow at the expense of smaller ones, which eventually form regions where pores tend to segregate and remain trapped. Pore closure is inhibited when the pores are trapped amongst grains no larger than the pores themselves, leading to the apparent bimodal grain distribution.36 5.2. Intermediate soaking In hot pressing, the main purpose of the intermediate soaking is to promote homogeneous powder wetting by the molten LiF, while ensuring the latter’s complete vaporization at the same time, before applying the final pressure. According to the suggested thermodynamic model, all reactions possibly underway are stopped when the final pressure is applied. The formation

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of lithium aluminates (e.g. LiAlO2 ) is unlikely in the presence of carbon contamination and any cleansing effect of LiF is due only to the shielding effect of the liquid film. The appropriate intermediate soaking stage is thus a delicate balance between the time it takes to ensure homogeneous wetting and to complete LiF evaporation. This simple wetting-evaporating mechanism can explain our experimental results. A shorter soaking time (30 min) is not long enough to ensure complete LiF release. A longer soaking time (120 min) leaves the sample without protection against carbon attack. In the present study, the best compromise proved to be a soaking time of 60 min. 5.3. Role of pressure The transparency obtained in this study is comparable with previous results and indicates that using high-purity LiF powder and optimizing the intermediate soaking time only have a minor influence on the final transmittance. The main parameter affecting the optical properties seems to be the pressure applied during densification. For instance, high transparencies are reached by hot isostatic pressing under pressures higher than 200 MPa.12,37 It is worth noting that spark plasma sintered samples with quite a high transparency (80%) have been processed under a pressure of 250 MPa, which corresponds to the pressure commonly used during HIP,38 with the added advantage of the plasma-assisted sintering process. The transparency reached in the present study (76.5% at 1100 nm and 70.6% at 633 nm, for samples 1.8 mm thick) is consistent with the values of around 70–75% at 1000–1100 nm typical of HP processes when the pressure is limited to between 40 and 80 MPa.17,20 Alongside the trivial pore-closing action of the pressure applied, the thermodynamic model also shows that high pressures help to stabilize the spinel phase by minimizing the Mg gas formation due to the reaction with carbon. Actually the transmittance is strongly affected by differences of some ppm in the residual porosity, i.e. the reduction of the Mg evaporation by only one order of magnitude is certainly not negligible. In short, the literature data, the present results, and thermodynamic considerations all seem to indicate that hot pressing at moderate pressure is not suitable for high transparency. Hot pressing is therefore not applicable for producing large components, such as flat panels or windows for spacecraft since only moderate pressure can be achieved,6 and the Mg evaporation can be regarded as a concomitant cause. 6. Conclusions This study aimed to produce highly transparent spinel ceramics by hot pressing at moderate pressures (≤40 MPa) in the light of a thermodynamic investigation into the reactions involved and a fine tuning of the experimental conditions. The role of LiF as a sintering aid was thoroughly elucidated by the thermodynamic model, which showed that the hypothesized carbon cleansing action stems more from the formation of a protective film than from the evaporation of volatile carbon fluoride components. At the same time, the thermodynamic picture

pointed to the important role of the carbonaceous environment in hot pressing. On the one hand, the presence of carbon inhibits the reaction between molten LiF and spinel with a consequent formation of LiAlO2 . On the other, carbon readily reacts with and destabilizes spinel at high temperatures, generating Mg and CO gaseous species and ultimately inducing microporosities and spoiling the material’s optical properties. Finally our findings suggest that, also due to spinel destabilization, transparencies close to the theoretical value can be reached only under very high pressures (200 MPa or more), like those achieved in hot isostatic pressing. Acknowledgments This work has been supported by the Flag Project RITMARE, La Ricerca Italiana per il Mare—coordinated by CNR and funded by the Ministry of Education, University and Research (“Flag” Project 2012-2016) within the National Research Program 2011–2013, Italy. The authors wish to thank Daniele Dalle Fabbriche, CNR ISTEC, Italy, for the hot pressing cycles. References 1. Gatti A. General Electric Company Space Division, Development of a Process for Producing Transparent Spinel Bodies. In: Final Report Contract no 00019-69-C-013 3. Naval Air Systems Command, Department of the Navy; 1969. Code—AIR-52039A. 2. C.A. Bruch AD number AD860196, Transparent magnesia-alumina spinel and method, US Patent 3516839, 1970. 3. Bormanis A. Needed: materials for 24th century starships—considering the materials demands of the star trek universe. J Miner Met Mater Soc 1996;48(6):14–6. 4. Wiglusz RJ, Grzyb T, Lukowiak A, Głuchowski P, Lis S, Strek W. Comparative studies on structural and luminescent properties of Eu3+ :MgAl2 O4 and Eu3+ /Na+ :MgAl2 O4 nanopowders and nanoceramics. Opt Mater 2012;35:130–5. 5. Krell A, Klimke J, Hutzler T. Advanced spinel and sub-␮m Al2 O3 for transparent armour applications. J Eur Ceram Soc 2009;29(2):275–81. 6. Salem JA. Transparent armor ceramics as spacecraft windows. J Am Ceram Soc 2012;96(1):281–9. 7. Wang SF, Zhang J, Luo DW, Gu F, Tang DY, Dong ZL, et al. Transparent ceramics: processing, materials and applications. Prog Solid State Chem 2013;41:20–54. 8. Reimanis I, Kleebe H-J. A review on the sintering and microstructure development of transparent spinel (MgAl2 O4 ). J Am Ceram Soc 2009;92(7):1472–80. 9. Bakas M, Chu H. Pressureless reaction sintering of AlON using aluminum orthophosphate as a transient liquid phase. In: Swab JJ, Singh D, Salem J, editors. Advances in Ceramic Armor V. Hoboken, New Jersey: The American Ceramic Society, John Wiley and Sons, Inc.; 2010. 10. McCauley JW, Patel P, Chen M, Gilde G, Strassburger E, Paliwal B, et al. AlON: a brief history of its emergence and evolution. J Eur Ceram Soc 2009;29(2):223–36. 11. Spilman J, Voyles J, Nick J, Shaffer L. Manufacturing process scale-up of optical grade transparent spinel ceramic at ArmorLine corporation. In: Tustison RW, Zelinski BJ, editors. Proc. SPIE 8708, Window and Dome Technologies and Materials XIII. 2013. Conference Volume 8708. 12. Goldstein A. Correlation between MgAl2 O4 -spinel structure, processing factors and functional properties of transparent parts (progress review). J Eur Ceram Soc 2012;32(11):2869–86. 13. Rubat du Merac M, Reimanis IE. Effect of impurities and LiF additive in hotpressed transparent magnesium aluminate spinel. Int J Appl Ceram Technol 2013;10(S1):E33–48.

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