Fabrication of transparent ScSZ ceramics at low temperature

Optical Materials 35 (2013) 782–787

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Fabrication of transparent ScSZ ceramics at low temperature Maxim Ivanov ⇑, Vladimir Khrustov, Anatoliy Medvedev, Sergey Paranin, Oleg Samatov Institute of Electrophysics, Ural Division of Russian Academy of Sciences, 106, Amundsena St., 620016 Ekaterinburg, Russia

a r t i c l e

i n f o

Article history: Available online 10 July 2012 Keywords: Transparent ceramics Stabilized zirconia

a b s t r a c t Laser synthesized nanometer-sized ZrO2 powders stabilized with Sc2O3 (ScSZ) were used to prepare transparent ceramics. ScSZ green bodies with relative density 53% were prepared by magnetic pulse pressing at 1.2 GPa. These green bodies were sintered in the air with heating rate 5 °C/min to 1150 °C and 0 min exposure at the temperature. 15.7 ScSZ (15.7 mol% Sc2O3 and 84.3 mol% ZrO2) ceramics with relative density higher than 99.7% and grain size about 0.41 lm were fabricated. In-line transmittance of the ceramics (diameter 26 mm, thickness 1 mm) reaches 72.4% @1100 nm. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Transparent polycrystalline ceramics with high refractive index are an attractive material for optical application. For example the continuous trend to miniaturization of digital photographic devices like digital still cameras require optical materials with very high refractive indices up to 2.0 or higher whereas industrial devices like optical microscopes require materials with special dispersion characteristics [1]. Transparent ceramics can display extraordinary properties that glass cannot reach reliably [2,3]. Exceedingly high optical quality of YAG ceramics can be achieved by controlled sintering of fine powders [4]. Furthermore, Japanese companies have developed high refracting ceramic lenses made from cubic perovskite of such a quality that principally meets the requirements of digital photographic devices [5]. Stabilized cubic phase zirconia (c-ZrO2) has refractive index 2.2, higher than other’s oxides. Since c-ZrO2 has furthermore high ionic conductivity, the material is used in fuel cells and thoroughly investigated. Several investigations resulted in translucent zirconia ceramics [6–8]. First reports concerning transparent c-ZrO2 were published by Tsukuma (TOSOH CORP.), who studied yttriastabilized zirconia doped with TiO2 [9,10]. Transparent samples (0.76 mm thickness) demonstrated transmittance up to approximately 65%. Late on the transparent 6 mol% Y2O3 stabilized zirconia (6YSZ) ceramics were fabricated with the help of hot isostatic pressing [11]. The sample sintered at 1650 °C had mean crystallite size 15 lm and demonstrated 70% in-line transmittance. Clasen produced c-ZrO2 optoceramic via electrophoretic deposition with in-line transmittance (including reflection losses) of 53% at 600 nm at the sample thickness of around 1 mm [12]. Recent investigation [1] resulted in transparent ceramics made from ⇑ Corresponding author. E-mail address: [email protected] (M. Ivanov). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.06.006

Y2O3-stabilized ZrO2 with TiO2 as sintering aid. The ceramics were obtained after sintering under vacuum (1  103 Pa) at 1650 °C for 3 h followed by hot isostatic pressing at 1750 °C for 1 h at a pressure of 196 MPa. 69% In-line transmission at 600 nm was shown and a variation of refractive index between 2.10 and 2.20 was observed, depending upon addition of TiO2 as sintering aid. It was noted that the addition of TiO2 results in the appearance of a yellowish discoloration and of birefringence in the optoceramic. Therefore the authors drew a conclusion that in order to improve the optical quality ZrO2 transparent ceramics need other additives instead of TiO2. It is generally agreed that the optical material to be widely adopted should have not only high transparency but also high mechanical characteristics. This can be achieved in the case of smaller size of crystallites in the ceramics sintered at lower temperature. For example, YSZ nanocrystalline material (stabilized with 1–1.5 mol% of Y2O3) exhibits a fracture toughness of up to 17 MPa m0.5, which is three times the fracture toughness obtained for submicrometer zirconia [13]. The rate of superplastic deformation obtained for nanocrystalline zirconia is several times in excess of the values obtained for submicrometer ceramics [14,15]. Worth mentioning that in these two investigations the fabricated ceramics were not transparent. Thus, the combination of high transparency and small grain size in material with high refractive index is a matter of current importance. We believe one of the promising materials for the optical applications could be nanocrystalline scandium stabilized zirconia (ScSZ). This material was investigated in detail for fuel cells purpose, but to the best of our knowledge has never been fabricated transparent. A small wonder that in most cases the ScSZ ceramics were not fully dense (it’s not so necessary for solid electrolytes), since to sinter nanocrystalline ceramics nanometer-sized particles should be used, and a fundamental problem in the compaction and sintering of the nanoparticles is their tendency to agglomerate.

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When compacted, agglomerated powders form green bodies with a nonhomogeneous structure containing large interagglomerate pores, which do not tend to disappear during sintering. To overcome such problems we developed two original methods: laser synthesis of nanopowders [16,17], and magnetic pulse pressing [18]. Laser synthesis means materials evaporation with the help of laser radiation and subsequent vapor condensation in the stream of buffer gas. Nanoparticles produced with this method are the weakly agglomerated, spherical and with narrow size distribution [19]. Such nanoparticles can be arranged into perfect grin body with the help of magnetic pulse pressing because of high shift rate and applied pulse pressure. The technology gives a chance to produce green body with density up to 0.7 of the theoretical one and uniform density distribution, which can be sintered into pore-less transparent ceramics [20,21] and single crystals [22]. In the case of ScSZ worth mentioning the material is quite complicated. There are several contradictory phase diagrams [23], apparently because of metastable states formed at the temperature lower 1000 °C depending on powder fabrication and sintering route. Thus, if one wants to deal with ScSZ special attention has to be placed on phase composition of both nanopowder and ceramics sintered. The objective of the present paper was to fabricate ScSZ transparent nanocrystalline ceramics. This work investigates laser synthesized ScSZ nanopowder, sintering behavior of the powder, nanocrystalline structure and transparency of the ceramics produced. The present investigation tries to determine the processing conditions that are necessary to obtain highly transparent ScSZ ceramics. 2. Experimental The ZrO2 nanopowders with different content of Sc2O3 (from 6.5 to 26 mol%) were synthesized by target evaporation with ytterbium fiber laser. The evaporation setup is described in detail in [24]. The targets were made of ether mixed ZrO2 and Sc2O3 commercially available powders or ScSZ powder prepared by chemical method. The ytterbium fiber laser YLR–1000 (IPG Photonics) worked in modulated mode, modulation frequency was 5 kHz, laser pulse duration – 130 ls, average laser power – 500 W. Some batches of the nanopowders were sedimented to remove particles lager than 200 nm. The sedimentation process is described in [25]. Analysis of chemical composition of the produced nanopowders was made from solutions by atomic-emission method with high stable inductively coupled plasma (ICP) (iCAP 6300, Thermo scientific). Phase structure and composition of the nanopowders were characterized with X-ray analysis (D8 DISCOVER GADDS, Bruker AXS). The specific surface area (SBET) of the nanopowders was measured by nitrogen adsorption according to the BET method (TriStar 3000, Micromeritics). Adsorbates content and exo/endothermal reactions that take place during heating up to 1400 °C were ana-

lyzed by simultaneous thermo analysis (TG-DSC) with NETZSCHSTA409PC. The particle size and morphology were investigated by transmission electron microscopy (TEM) (JEM-2100, JEOL Ltd.). The nanopowders were compacted into disk-shaped green bodies (diameter 8, 15 and 30 mm, thickness 2–2.5 mm) with the uniaxial magnetic pulsed press [18]. The amplitude of the applied pressure was up to 1.2 GPa, duration of the pulse 300–500 ls. The shrinkage of green bodies was investigated with the help of a hightemperature dilatometer (NETZSCH DIL 402 C) both in vacuum (102 Pa) and in air (gas flow – 100 l/h) at the temperature up to 1500 °C in two heating modes: constant heating rate (CHR) and constant sintering rate also known as rate control sintering mode (RCS). Green bodies were densified by pressureless sintering at different heating rate (0.5–5 °C/min) and temperatures in camber furnace LHT 02/18 (Nabertherm) with MoSi2 heaters and Al2O3 insulation. After sintering, the samples were characterized with respect to their final density and grain size. The Archimedes technique was used to determine the final densities. The green body density was calculated from the size of the samples. Average crystalline size of the samples was determined by atomic-force microscopy (AFM) of the sintered samples with ‘‘Solver 47p’’ (NT-MDT) and scanning electron microscopy (SEM) (LEO982). Disk specimens to be used for measuring optical transmittance were grinded (1 mm thick) and optically-polished on both surfaces with Phoenix Beta Grinder/Polisher (BUEHLER, Germany). The transmittance of polished ceramics was measured over the wavelength region from 200 nm to 20 lm with the help of spectrophotometer (UV-1700, Shimadzu Corp.) and Fourier transform infrared spectrometer with expanded spectral range (Nicolet 6700, Thermo Scientific). 3. Results and discussion 3.1. Characterization of Nanoparticles For all the compositions studied, the output rate of the laser synthesized nanoparticles was 15–20 g/h. Content of adsorbats in all powders was 1.5–2.4 wt.%. The characteristics of the powders: mole content of Sc2O3, specific surface area (SBET), lattice parameters and average crystalline size (dx, measured by XRD) are given in Table 1. All nanopowders produced were enriched slightly with Sc2O3, since scandium, which has higher pressure of saturated vapors, being evaporated in mixture with zirconia evaporates to a higher extent. But the amount of the enrichment was small and close to detection limit of ICP method. The particle sizes dBET were calculated from the specific surface area of powders (assuming spherical and monosized particles). For laser synthesized ScSZ nanoparticles such an assumption was proved to be quite accurate by measuring from TEM microphotographs (Fig. 1). TEM investigation showed that the nanoparticles

Table 1 The main characteristics of laser synthesized ScSZ nanopowders. Sc2O3 content (mol%)

SBET (m2/g)

dBET (nm)

Lattice type

Lattice parameters (nm)

dx (nm)

6.5 7.3 7.9 9.9 10.7 10.9 11.1 11.6 13.6 15.7 20.3 25.9

58 50 54 58 51 52 51 71 75 59 60 61

17.7 20.6 19.2 18.3 20.8 20.4 20.5 14.9 14.1 – – –

Tetragonal 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic 100 wt.% Cubic > 80 wt.% Cubic 46 wt.% Cubic 30 wt%

a = 0.35993 c = 0.51204 a = 0.50973 a = 0.50953 a = 0.50912 a = 0.50882 a = 0.50882 a = 0.50892 a = 0.50884 a = 0.50793 a = 0.50785 a = 0.50687 a = 0.50544

29 31 32 31 32 31 29 24 24 28 47 68

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Fig. 1. TEM micrograph of laser synthesized ScSZ nanoparticles.

are weakly agglomerated, their shape varied between spherical and polyhedron. The study of phase composition showed that in the range 7.3– 13.6 mol% of Sc2O3 content the ScSZ nanopowders are single-phase and their average lattice parameter can be well described by a straight line (Fig. 2). Approximation of the dependence to zero scandium content is close to data for pure ZrO2 [26] obtained by plasmachemical method and containing 90 wt.% of tetragonal phase. But in our case, smaller concentration of scandium leads to formation of entirely tetragonal powder. 6.5ScSZ (number means mole content of Sc2O3 in the material) nanopowder was 100 wt.% tetragonal. Scandium content higher than 14 mol% causes rhombohedral Sc4Zr3O12 (S.G: R-3) phase formation. This phase has lower content of oxygen in comparison with both c-Sc2Zr5O13 (S.G: R-3H) and b-Sc2Zr7O17 (S.G: R-3R). Since the laser synthesized nanopowder always has minor oxygen deficiency, there is a small wonder this very phase is stabilized in the nanopowder. Amount of this phase increases with increase of scandium concentration (Table 1). 3.2. Sintering Behavior of ScSZ Compacts made of the Nanopowders To investigate shrinkage of ScSZ, the green bodies with relative density 53–55% were prepared with the help of uniaxial magnetic

Fig. 3. Linear shrinkage of 13.6ScSZ at heating rate 0.5 (green), 2 (blue), 5 (black) and 20 (red) °C/min in air (solid lines) and in vacuum (dashed lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

pulsed press. The shrinkage behavior varies only slightly, when elemental composition lies in the range of 9.9–15.7 mol% Sc2O3 content. Shrinkage curves of 13.6ScSZ green body at different heating rate both in the air and vacuum are shown in Fig. 3. The comparison characteristics are collected in Table 2. In all samples the rapid shrinkage starts at the temperature about 800–900 °C (To). At higher heating rate T0 increased slightly. The densification happened quite fast during 145–175 °C range of heating up and nearly accomplished when the temperature reached 1050–1075 °C. For instance the shrinkage of the green body being heated 5°C/min commenced at 850 °C and completed after 175 °C heating up. In all cases 1174 °C was enough to sinter ceramics with density higher 96% (Table. 2). If to compare shrinkage in the air with one in vacuum (Table. 2) it’s easy to notice the vacuum sintering runs more actively: T0 is lower, shrinkage speed – higher. The difference is distinctly seen at lower heating rate (Fig. 3). On the other hand density of the sample heated with constant rate to 1174 °C in vacuum is lower then one heated in the air. The change of volume densification speed of the samples being sintered in vacuum is shown in Fig. 4. Maximum speed lies in relative density range 0.75–0.81. There is certain asymmetry of the curves relative to maximum. At higher density the densification speed decreases faster. If to compare the shrinkage speed maximum with one for Al2O3 powder [27] and 3YSZ [28] (in both cases close to 0.72), the maximum is significantly shifted to higher density. Since as the maximum of shrinkage speed is achieved the grains grow is believed to become more considerable, this shift makes us think the laser synthesized ScSZ nanopowders have good chances to be sintered to fully dense nanocrystalline ceramics. Fig. 5 shows the comparison of densification speed in air and vacuum. It confirms the air slows down ScSZ sintering. Activation energy calculated in accordance with MSC method [29] is 580 ± 20 kJ/mol. Any noticeable difference in the energy between sintering in air and in vacuum was not revealed. It means the densification mechanism is identical in both cases. 3.3. Transparency of ScSZ ceramics

Fig. 2. ScSZ lattice parameter vs. Sc2O3 content.

All samples made of nanopowders, which elemental composition fall in the range 13.6ScSZ–15.7ScSZ become translucent after heating to 1100 °C in the air. The most transparent ScSZ ceramics with the smallest crystallite size were sintered at temperature 1100–1200 °C. Heating up to 1500 °C made the ceramics less transparent. This seemed to happen because of second phases formed at

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M. Ivanov et al. / Optical Materials 35 (2013) 782–787 Table 2 Characteristics of sintering process of 13.6ScSZ green body. Heating rate, °(C/min)

Medium

To, (°C)

DT, (°C)

Tmax, (°C)

vTx1e3

qmax,%

vqx1e3

qe%1174 (°C)

½

Air Vacuum

850 810

150 140

950 940

1.3 1.4

84.3 81.9

4.44 5.04

97.8 96.0

2

Air Vacuum

860 830

155 150

965 950

1.2 1.3

83.5 –

4.39 –

98.6 96.3

5

Air Vacuum

850 840

175 160

975 945

1.1 1.3

82.9 80.0

4.06 4.44

99.7 96.8

20

Air Vacuum

900 900

175 145

995 975

0.9 1.0

78.0 74.5

3.50 4.09

96.4 D L
Notice: To – temperature when shrinkage starts, Tmax temperature when shrinkage speed is maximum, Te – temperature when shrinkage ceases, V T ¼ d Lo =dT - module of maximum linear shrinkage speed, qmax – relative density of ceramic when shrinkage speed is maximum, V q ddTq  q1 volume shrinkage speed, DT = Te-To –temperature diapason where shrinkage happens, qe – ceramic density at 1174 °C and zero holding time.

Fig. 4. Shrinkage speed of 13.6ScSZ green body in vacuum (105 Pa) at heating rate 0.5 (green triangles), 5 (black squares) and 20 (red circles) °C/min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

To compare transparency of the samples sintered attenuation of He-Ne laser beam ðk ¼ 630 nm) passed through the ceramics were measured. In-line transmission (at wavelength 630 nm) of 15.7ScSZ ceramics heated in the air up to 1174 °C with different heating rate and zero holding time is shown in Fig. 6. The dependence reveals the transparency of the ceramics is higher when green body is heated at lower rate. A reason of this could be prolonged sintering when the heating rate is low, but this explanation conflicts with dependence on Fig. 7, which is described below. Primary cause of relative low transparency of the ceramics, which were sintered in air, is porosity. If the air was not removed from the green body at the stage of open porosity the gas-filled pores could hardly be eliminated during long time sintering at high temperature. To sinter the ScSZ samples in vacuum the dilatometer’s (NETZSCH DIL 402 C) tube furnace made of corundum with isolated inner space was used. Being heated up to 1150 °C in vacuum (102 Pa) the samples were found to become translucent, but cracked. Further heating up to 1500 °C made the ceramics black and opaque. The change of color seemed to be caused by reduction of the metals and the rhombohedral phases formed in ceramics. For instance, in 15.7ScSZ sample sintered in vacuum the amount of the second phases was 3 wt.% more than in one sintered in the air. Annealing in the air at 1500 °C made the samples green with shadow of yellow, but didn’t improve transparency. To reduce both porosity and second phase amount a combined sintering mode was used. The green body was heated up to T0 in the air, then the air was pumped out and the sample was heated

Fig. 5. Shrinkage speed of 13.6ScSZ green body in vacuum (105 Pa, violet circles) and air (orange squares) at heating rate 5 °C/min. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. In-line transmission (at wavelength 630 nm) of 15.7ScSZ ceramics heated to 1174 °C with different heating rate.

the temperature. Though the amount of the second phases in 13.6ScSZ samples was lower than XRD detection limit, in 15.7ScSZ ceramics rhombohedral c-Sc2Zr5O13 (S.G:R-3H) and b-Sc2Zr7O17 (S.G: R-3R) were detected.

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Fig. 7. In-line transmission (at wavelength 630 nm) of 15.7ScSZ ceramics vs. sintering time, T = 1174 °C.

Fig. 8. Picture of 15.7ScSZ transparent ceramic, diameter 26 mm, thickness 1 mm.

Fig. 10. SEM picture of 15.7ScSZ ceramics sintered in the air at 1150 °C during 2 h.

to decrease of transparency. Furthermore prolonged sintering leads to crystallites overgrowth. The most transparent ceramics (Fig. 8) were prepared from sedimented 15.7ScSZ nanopowder, which was compacted to green body with relative density 53%. This green body was sintered in the air with heating rate 5 °C/min to the temperature 1150 °C and 0 min exposure at the temperature. Diameter of the sample is 26 mm, thickness – 1.0 mm, relative density higher than 99.7%. Average grain size in the ceramics is about 0.41 lm (measured with SEM pictures). Dependence of in-line transmission of the ceramics on wavelength is shown in Fig. 9. Transparency of the ceramics is quite high and reaches 72.4% @ 1100 nm. In the long-wave part the transparency of the ceramics approaches the theoretical limit. The slope in the left side of the curve is caused by both dependence of the refractive index on wavelength and scattering on nano-size pores. To find out the pores 15.7ScSZ ceramics were sintered in the air at 1150 °C during 2 h. The SEM picture of the sample is shown in Fig. 10. Undo these conditions the grain size is enlarged to 1100 nm. The picture of the sample reveals the presence of pores with size about 50 nm. 4. Conclusions

Fig. 9. Dependence of in-line transmission of 15.7ScSZ ceramics on wavelength.

up in vacuum to Te, and then once again the furnace chamber was filled with air to sinter the ceramics to final density. In-line transmission (at wavelength 630 nm) of 15.7ScSZ ceramics heated in this combined mode and sintered at 1174 °C during 0, 1 and 2 h is shown in Fig. 7. In this graph the points 4 and 20 h correspond to sintering in air (not in combined mode). Two conclusions are worth mentioning: transparency of the ceramics sintered in combined mode is worse than sintered in air if other conditions are equal (compare Figs. 6 and 7), and increase of sintering time leads

To the best of our knowledge the transparent ScSZ ceramics were sintered for the first time. The technology, which consists of several consecutive steps: (a) laser synthesis of weakly agglomerated ScSZ nanopowder, (b) compacting of green body with magnetic pulsed press, (c) sintering in air at heating rate 5 °C/min up to 1150 °C and 0 min holding time, gives a chance to sinter ScSZ ceramics with density higher than 99.7%, grain size about 0.41 lm and transparency 72.4% @ 1100 nm. The relatively low transparency in short-wave region seems to be caused by scattering on nano-sized pores, therefore substantial development efforts should be made at (b) and (c) stages of the technology in order to reduce porosity and improve the transparency of ScSZ ceramics in visible region needed to meet requirements of optical devices. Acknowledgements The authors would like to thank the following persons for their cooperation during research: Olga Timoshenkova, Alexey Spirin, Sergey Zayats and Anatoliy Moskalenko.

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