Sintering behaviour and translucency of dense Eu2O3 ceramics

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 1803–1808

Sintering behaviour and translucency of dense Eu2O3 ceramics Adrián Quesada ∗ , Adolfo del Campo, José F. Fernández Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC, Kelsen 5, Madrid 28049, Spain Received 21 November 2013; received in revised form 18 December 2013; accepted 23 December 2013 Available online 15 January 2014

Abstract Eu2 O3 ceramics have been obtained at sintering temperatures of between 1000 ◦ C and 1550 ◦ C. X-ray diffraction and scanning electron microscopy, in combination with dilatometry experiments, allowed understanding the sintering behaviour. Moderate grain growth followed an efficient densification process between 1400 ◦ C and 1550 ◦ C, which yielded high-density ceramics with an average grain size of 4 ␮m. The ceramics had Young modulus of 125 GPa, in agreement with the previously published data. The dense Eu2 O3 ceramics were translucent (35.1% transmittance at 800 nm of 0.8 mm thick discs), showing in addition a slightly pink colour. We propose that the combination of high density and an average grain size of 4 ␮m is responsible for this novel functionality. © 2014 Elsevier Ltd. All rights reserved. Keywords: Sintering; Europium oxide; Translucent; Densification

1. Introduction Initially employed as a neutron absorber in reactors,1 europium sesquioxide has been extensively studied due in great part to the applications deriving from its optical properties. Among its various other applications, Eu2 O3 is found in MOS devices as the dielectric and has gathered attention from catalysis researchers and from the magnetic semiconductors community.2–4 As a lanthanide sesquioxide, its polymorphism has been thoroughly studied as well, for fundamental and applied reasons5,6 : a transition from cubic (C) to monoclinic (B) takes place at temperatures ranging from 1050 ◦ C to 1350 ◦ C.3 Focusing on its attractive photoluminescent characteristics, Eu+3 has its major emission band centred on a primary colour (red at 612 nm), which is ideal as phosphorescence agent in various light emitting devices such as cathode-ray tube displays.7,8 Moreover, Eu2 O3 presents a reversible spectral transition (ultraviolet-laser-light induced) that leads to utilization as wide-band tunable-laser medium.9 Since the excitation processes and optical transitions are relatively unaffected by size and morphology, Eu2 O3 features in a wide variety of devices and shapes, from nanoscale to macroscopic.

Reports on Eu2 O3 nanostructures have proliferated in the last few years,4,8,10,11 and Eu+3 has been thoroughly employed as dopant in a wide range of matrices12,13 ; however, only a handful of publications (motivated by its reactor applications) focus on bulk Eu2 O3 ceramics. Commonly, sintering temperatures above 1600 ◦ C are utilized with large grain sizes of 50 ␮m and above. The basic physical and mechanical properties can be found in the literature1,14 and methods for avoiding massive grain growth have been proposed.15 However, a thorough study of the sintering processes involved in Eu2 O3 is missing. In this work, the sintering behaviour of Eu2 O3 ceramics was investigated. A structural characterization in combination with dilatometry experiments was undertaken in order to elucidate the densification mechanisms. The mechanical performance of the dense ceramics was also characterized. A previously unreported translucency is demonstrated, which constitutes a novel and promising additional functionality in Eu2 O3 , especially in combination with its photoluminescent properties. Current applications of Eu2 O3 -based devices could potentially benefit from the translucent property of the material, as for instance in the field of laser media. 2. Experimental

∗

Corresponding author. Tel.: +34 91 7355840; fax: +34 91 7355843. E-mail address: [email protected] (A. Quesada).

0955-2219/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.034

Eu2 O3 (99.99% purity) powder from Metal Rare Earth Ltd was used as starting material. The starting powder was

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Intensity (arb. units)

3. Results and discussion 1500 °C

1200 °C

1000 °C

26

28

30

32

34

2θ (º) Fig. 1. Diffraction patterns of the Eu2 O3 powder annealed at temperatures between 1000 ◦ C and 1500 ◦ C. Circle labelled Bragg peaks correspond to the cubic phase, while non-indexed maxima correspond to the monoclinic phase.

attrition-milled with mixture of 1 and 2 mm ZrO2 balls in distilled water for 3 h which was then dried at 80 ◦ C and sieved (63 ␮m). The obtained powders were attrition milled again, resieved and cold-uniaxially pressed at 55 MPa into disks of 15 mm in diameter and 1 mm in thickness. The pellets were finally sintered in air at temperatures of between 1000 ◦ C and 1550 ◦ C for 2 h, at 5 ◦ C/min. Density of the sintered samples was measured by using the Archimedes method, while theoretical density (Dth ) of the monoclinic phase was calculated from the lattice parameter extracted from the X-ray diffraction patterns (8.16 g/cm3 ). An average green density of 48% Dth was measured. Crystalline phases were characterized by using X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) with Cu Kα radiation. Dilatometry study was performed on a Netzch 407/E dilatometer at 5 ◦ C/min, and 8 mm high uniaxially pressed cylinders were employed. Laser diffraction particle size analysis was performed on a Malvern Mastersizer S system. Microstructures were evaluated by using Hitachi Tabletop TM-1000 Electron Microscope and a field emission scanning electron microscope, FE-SEM (Hitachi S-4700, Tokyo, Japan). Young modulus of the samples was determined from the results of Vickers and Knoop hardness tests performed on a LECO Hardness Tester indenter with a load of 0.5 kg/mm2 . UV–vis Spectroscopy experiments were performed on a PerkinElmer Lambda 950 spectrometer, for 0.8 mm thick pellets, and by measuring the diffuse transmittance (using a large aperture before the detector).

3.1. Structural properties and sintering behaviour As-received Eu2 O3 powder is a mixture of cubic and monoclinic phases. After annealing at 1000 ◦ C, the cubic phase remained the predominant one as shown in Fig. 1. After annealing at 1100 ◦ C (not shown) and 1200 ◦ C, the monoclinic phase became increasingly dominant, although the cubic phase was still detected. Annealing at temperatures of ≥1300 ◦ C led to samples with pure monoclinic Eu2 O3 . A SEM micrograph of the milled powder and the particle size distribution of both the as-received and the attrition milled material are presented in Fig. 2. The attrition milling process successfully reduced the average particle size of the starting powder. In addition, the milling process yielded a bimodal particle size distribution, with a maximum centred on 400 nm and a second maximum at about 1.5 ␮m. A third maximum is detected at 30 ␮m, which is attributed to the presence of large agglomerates. The bimodal character is confirmed in SEM micrographs, where particles with an average size >1 ␮m coexist with a significant population of smaller particles, often in agglomerates. Fig. 3a shows density values of the discs as a function of sintering temperature. Density increases with temperature, reaching 97.2% Dth at 1550 ◦ C. Given the density values, the ceramics are in the final stage of sintering. A first steep increase in density takes place between 1200 ◦ C and 1300 ◦ C, which can be explained by the phase transition since the density of the monoclinic phase is higher than that of the cubic phase.16 The second jump can be correlated with the maximum shrinkage rate between 1400 ◦ C and 1500 ◦ C. At higher temperatures, density reaches a plateau. Sintering times for 2–16 h were investigated, however no significant increase in density was observed. This result indicates that 2 h is appropriate for densification. In addition, the shrinkage rate presents two consecutive minimum values at 1350 ◦ C and 1500 ◦ C, respectively. Smaller particle radius yields larger curvature differences. An increase in curvature leads to an increment of two parameters governing the reduction of surface energy that is associated with sintering: chemical potential and partial pressure at the surface. Therefore, the existence of this double feature in the shrinking rate is attributed to the bimodal character of the starting powders: the lower temperature minimum corresponds to the atomic sintering

Fig. 2. (a) SEM micrograph of the Eu2 O3 powder attrition-milled for 3 h. (b) Equivalent particle diameter distribution curves of the as-received and the milled Eu2 O3 .

A. Quesada et al. / Journal of the European Ceramic Society 34 (2014) 1803–1808 0,1

98 96

7,8

94 7,6

92

7,4

90

7,2

88

0

densification (%)

a

86

7,0

84 1000

1100

1200

1300

1400

1500

1600

Shrinkage (dL/L0)

3

Density (g/cm )

8,0

b

0,0

-5 -0,1

-10

-0,2

-15

-0,3

-20 -25

0

T (°C)

200

400

600

800

1000 1200 1400 1600

-0,4

Shrinkage rate (dL/min)

1805

T (ºC)

Fig. 3. (a) Relative density versus sintering temperature and (b) shrinkage curve at 5 ◦ C/min between room-temperature and 1500 ◦ C.

Fig. 4. (a) Surface SEM image of the sintered Eu2 O3 ceramics at 1500 ◦ C. Red lines highlight 120◦ dihedral angles and blue circles indicate occasional deviations towards other angles. (b) Average grain size evolution as a function of density of the samples sintered as a function of temperatures. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

processes initiated at the smaller particles, and the higher temperature minimum to the larger ones. Fig. 4a shows SEM micrograph of the sample sintered at 1500 ◦ C. Both thermal and chemical etching of polished Eu2 O3 ceramics led to a recrystallization process at the surface that impeded an accurate study of the grain structure of the dense ceramics. It is thus worth clarifying that the micrograph pertains to an unpolished as-sintered pellet. It can be observed that the bimodal distribution of the starting powders partially remains in the sintered samples: large grains (of 4–5 ␮m) coexist with smaller ones (around 1 ␮m). Most commonly, 120◦ dihedral angles appear at the grain interfaces (highlighted by the red lines in Fig. 4a). However, deviations towards larger angles occasionally occur (two examples are marked by blue circles in the image), especially at the interfaces involving smaller grains that present less than six sides. Abnormal grain growth has been reported before in this system15 but is clearly not taking place in this case. The average grain size was estimated according to the microstructure of all samples, and its evolution with density is plotted in Fig. 4b. As expected, the average grain diameter increases with temperature, from 700 nm at 1300 ◦ C to 4.5 ␮m at 1550 ◦ C. The shape of the curve corresponds to a prototypical sintering process where densification mainly occurs before grain growth is activated. The pore size and distribution was monitored as a function of temperature in polished samples. Selected micrographs are shown in Fig. 5 where the progressive elimination of pores can be followed. From the analysis of the pore size and area

Table 1 Summary of the Vickers hardness and Young modulus values (determined from Vickers and Knoop hardness tests) for different sintering temperatures. T (◦ C)

Hv (GPa)

1300 1400 1500 1550

2.86 3.44 4.01 4.2

± ± ± ±

0.31 0.32 0.41 0.43

E (GPa) 98 110 114 118

± ± ± ±

11 13 10 11

fraction (Fig. 5e), it is deduced that the sintering process effectively reduces porosity down to 1% in area fraction and below 100 nm in size. Even though porosity is reasonably efficiently removed, large defects that could be originated by compaction inhomogeneities still remain in the dense ceramics, as shown in Fig. 5d. Such defects can account for the remaining 1.8% deviation from total densification (considering a porosity of 1%). 3.2. Mechanical and translucent properties The mechanical properties of dense ceramics constitute an important factor that must be taken into account in terms of potential applications. Using Vickers and Knoop hardness tests, these properties were evaluated. The results, including Vickers hardness (Hv ) and Young’ modulus (E), are presented in Table 1. The behaviour observed is in good agreement with previously reported results. The evolution of the Young modulus with

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Fig. 5. Surface SEM images of the polished discs sintered at (a) 1300 ◦ C, (b) 1400 ◦ C and (c) 1500 ◦ C, showing the pore concentration, size and distribution. (d) SEM image of the sample sintered at 1500 ◦ C. Arrow indicates the presence of large compaction defect. (e) Average pore size and porosity plotted as a function of sintering temperature.

temperature, and thus with density/porosity, seems to follow the following equation15 : E = E0 (1 − bP)

(1)

where E0 is the Young modulus at zero porosity, b is an experimentally determined constant (which is known to have a value

of 2) and P is the volume fraction porosity. From this equation we can deduce that in our case E0 = 125 ± 13 GPa. This value is slightly lower than that reported in the literature (147 GPa).1,14,15 The determination of the modulus by the Vickers and Knoop hardness tests method yields a standard deviation of about 10%, which could constitute one of the main causes behind the slight

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Fig. 6. (a) UV–vis transmittance of the 0.8 mm thick Eu2 O3 pellets sintered at different temperatures over 1300–1550 ◦ C. (b) Picture demonstrating the translucency of a 1500 ◦ C sintered ceramic in comparison to a 1300 ◦ C non-translucent pellet. A red LED is used as the illumination source.

discrepancy. Regardless, we can conclude from this study that the ceramics obtained in this work possess competitive mechanical properties. UV–vis spectroscopy transmittance was measured for samples sintered at different temperatures; the results are presented in Fig. 6a. It is worth reminding that the transmittance was measured on unpolished samples, with a thickness d = 0.8 mm, and using a large aperture. As can be inferred from the spectra, transmittance increases non-linearly as a function of temperature, reaching a value of 35.1% at a wavelength of 800 nm for a 0.8 mm thickness. Moreover, the characteristic absorption peaks of Eu2 O3 can be observed between 400 and 650 nm. These excitation processes are known to be associated with transitions from 7 F0.1 to excited states.17 The resulting emission presents a major band at 612 nm (red) which is likely to be responsible for the pink colour of the Eu2 O3 ceramics studied here. The picture shown in Fig. 6b demonstrates translucency by lightning with a red LED the discs fired at 1300 ◦ C (not translucent) and 1500 ◦ C (translucent). Transparency in Eu2 O3 thin-films has been evidenced recently,18 and Eu3+ ions have been embedded in a transparent ceramic matrix before,19 but no reports exist on bulk translucent Eu2 O3 ceramics to the best of our knowledge. Moreover, transparency is usually associated with cubic structures, which leads to very scarce literature on monoclinic translucent ceramics.20 Besides the use of a material with the appropriate band structure, high density and purity, absence of pores and secondary phases, specific grain sizes and isotropic lattice structure are generally required conditions for translucency.20 Based on the structural characterization, we suggest that translucency of Eu2 O3 ceramics is a consequence of high-purity, high-density and moderate average grain sizes. In fact, huge grain sizes have been previously reported in dense Eu2 O3 ceramics (100 ␮m average grain size),15 which could explain the lack of reports on its translucent properties. The sintering process of the Eu2 O3 ceramics, where grain growth is activated after densification, favours the requirements for translucency. On the other hand, our findings shed light on the factors that are most likely limiting the translucent performance: (i) the relative non-uniformity of the grain sizes due to the bimodality of the base powder and (ii) the large pores/defects caused by

compaction imperfections. Moreover, for translucency or transparency applications, densities of 99.9% are frequent, which hints at the need for higher density. It is proposed that conditioning the base powder to smaller and more uniform particle sizes and employing isostatic pressing should afford higher densities and thus improved translucent performance. 4. Conclusions In summary, dense Eu2 O3 ceramics with moderate grain sizes (approximately 4 ␮m) and 1% porosity have been obtained from attrition milled commercial powders. The resulting microstructure hosts translucent properties in combination with promising mechanical behaviour of the Eu2 O3 ceramics that opens the door to novel functionalities for this material. Further works needs to be envisioned in order to optimize the optical properties of the ceramics. Acknowledgements The authors would like to thank Dr. Miguel Ángel Rodríguez for the valuable advice during scientific discussions. The authors would like to thank financial support from the Spanish Ministerio de Economía y Competitividad through the Juan de la Cierva programme and through Project MAT 2010-21088-C03-01. References 1. Curtis CE, Tharp AG. Ceramic properties of europium oxide. J Am Ceram Soc 1959;42:151–6. 2. Dakhel AA. Characteristics of deposited Eu2 O3 film as a thick gate dielectric for silicon. Eur Phys J Appl Phys 2004;28:59–65. 3. Antic B, Mitric M, Rodic D. Cation ordering in cubic and monoclinic (Y, Eu)2 O3 : an X-ray powder diffraction and magnetic susceptibility study. J Phys Condens Matter 1997;9:365–9. 4. Zhang P, Zhao Y, Zhai T, Lu X, Liu Z, Xiao F, Liu P, Tong Y. Preparation and magnetic properties of polycrystalline Eu2 O3 microwires. J Electrochem Soc 2012;159:D204–13. 5. Jiang S, Bai LG, Liu J, Xiao WS, Li XD, Li YC, Tang LY, Zhang YF, Zhang DC, Zheng LR. The phase transition of Eu2 O3 under high pressures. Chin Phys Lett 2009;26:076101–76103.

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