Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

Author’s Accepted Manuscript Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives Lin-Lin Zhu, Young-Jo Park, Lin Gan, Shin-Il Go, Ha-Neul Kim, Jin-Myung Kim, Jae-Woong Ko www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)31418-9 http://dx.doi.org/10.1016/j.ceramint.2017.07.004 CERI15724

To appear in: Ceramics International Received date: 4 June 2017 Revised date: 29 June 2017 Accepted date: 1 July 2017 Cite this article as: Lin-Lin Zhu, Young-Jo Park, Lin Gan, Shin-Il Go, Ha-Neul Kim, Jin-Myung Kim and Jae-Woong Ko, Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

*

Lin-Lin Zhua, Young-Jo Parka , Lin Gana,b, Shin-Il Goa, Ha-Neul Kima, Jin-Myung Kima, and Jae-Woong Koa

a

Engineering Ceramics Department, Korea Institute of Materials Science 797 Changwondaero, Changwon, Gyeongnam 641-831, Republic of Korea

b

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of

Education Key Laboratory for Green Preparation and Application of Functional Materials, School of Material Science and Engineering, Hubei University, 368 Youyi Avenue, Wuhan, Hubei 430062, China

*

Corresponding author.. Tel.: +82 55 280 3356. [email protected] (Y.-J. Park)

Abstract:

In this study, we report highly transparent Er:Y2O3 ceramics (0-10 at.% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO 2 and La2O3. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y2O3 ceramics were evaluated. The in-line transmittance of all of the Er:Y2O3 ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at.%, the

1

average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m·K. A nearly homogeneous doping level of the laser activator Er up to 10 at.% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO2 and La2O3 are demonstrated to be effective for the fabrication of transparent Y2O3 ceramics. These results may provide a guideline for the application of transparent Er:Y2O3 laser ceramics.

Keywords: Y2O3, transparent ceramics, compound sintering additive, thermal properties, mechanical properties

1. Introduction

There has been increasing interest in the development of transparent polycrystalline ceramics based on the cubic oxides Y3Al5O12 and RE2O3 (RE=Y, Sc, Lu or Gd) in recent decades due to the strong demand for applications such as solid-state laser materials, windows, and scintillators [1]. Among them, the transparent Y2O3 ceramic in particular has attracted considerable attention given its broad wavelength range (0.2-8 μm), high thermal conductivity (approximately 15 W/m·K), capacity for doping with trivalent lanthanide (Ln3+) ions, and relatively low cut-off phonon energy (380 cm-1). In general, the transparent Y2O3 ceramic is considered to be a promising alternative to supplant

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single-crystal materials because its melting point is as high as 2430°C, and the phase transition point is approximately 2280°C [2].

Thus far, three rare-earth ions, namely, Nd3+, Yb3+ and Er3+, have been considered as laser dopants for Y2O3 transparent ceramics [3-6]. The dopants being most intensively investigated at present are Nd 3+ and Yb3+ ions. Zhang et al. reported the effects of Nd doping on the microstructural and thermo-mechanical properties of Nd:Y2O3 transparent ceramics [7]. With an increase in the Nd doping concentration, the average grain size increases while the transmittance, thermal conductivity and the microhardness decrease. Hou et al. also reported the effects of the Yb doping concentration on the microstructural, thermal, and mechanical properties of Y2O3 ceramics in detail [8]. Compared to Nd3+ and Yb3+, there have been fewer studies about Er3+ ions. Er3+ ions with metastable levels 4I11/2 and 4I9/2 are recognized as being among the most interesting RE ions. Specifically, the 4I11/2 → 4I13/2 and 4I13/2 → 4

I15/2 transitions correspond to the mid-IR ~3 μm and the eye-safe 1.6 μm spectral region, respectively

[6]. Wang et al. reported an investigation of the room-temperature laser performance of the polycrystalline Er:Y2O3 ceramic at 2.7 μm with respect to various dopant concentrations [9]. Brown et al. reported the near-infrared and up-conversion luminescence of the 1.5 μm emission of Er-doped Y2O3 ceramic at various concentrations [10]. However, systematic investigations of the fabrication and other properties of transparent Er:Y2O3 ceramics are rare to the best of our knowledge.

The mechanical and thermal properties of transparent Er:Y2O3 ceramics are very important factors related to their design and practical industrial use. As is well known, there is a need for structural designs that ensure good mechanical reliability during operation in any application. On the other hand, the light absorption of laser gain media will cause a thermal gradient in the media during the operation

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of a solid-state laser system [11]. This thermal lensing effect greatly deteriorates the laser oscillation efficiency, which may even lead to ceramic cracking. Thus, the fundamental mechanical and thermal properties must be evaluated, as these will determine the overall reliability of a laser system.

ZrO2 and La2O3 are most frequently employed as sintering additives because they are effective, inexpensive, and environmentally friendly [12-14]. However, ZrO2 addition requires a high sintering temperature and a long holding time, and the introduction of La 2O3 usually results in a large grain size [15, 16]. In recent years, the co-addition of ZrO2 and La2O3 has been proven to be very effective to obtain fine-grained transparent ceramics with pore-free and homogenous microstructures [17]. In the present study, highly transparent Er:Y2O3 ceramics with various doping concentrations (0-10 at.% Er) were fabricated by a solid-state reaction method using ZrO2 and La2O3 as sintering additives. The optical transmittance, microstructure, microhardness, and thermal properties of the samples were investigated with respect to the effects of the Er dopant concentration.

2. Experimental procedure Commercially available oxide powders of Y2O3 (99.99%, Rare Metallic Co Ltd., Tokyo, Japan) and Er2O3 (99.99%) were used as raw materials, with lanthanum oxide (La 2O3, 99.99%, Aldrich) and ZrO(CH3COO)2 (98%, High Purity Chemicals, Sakado, Japan) as sintering aids. Based on the formula of (Y0.87-xErxLa0.1Zr0.03)2O3 (x=0, 0.01, 0.03, 0.05, 0.1), the powders were weighed and milled with ZrO2 balls in anhydrous alcohol (99.9%, Samchun, Pyeongtaek, Korea) for 12 h. After the milled slurry was dried by a rotary evaporator at 70°C, the powder mixture was ground and sieved though a 150-mesh screen and then calcined at 1200°C for 4 h to remove organic components. The calcined powders were uniaxially pressed at 5 MPa into Φ18 mm disks in a steel mold and further

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cold-isostatically pressed (CIP) at 200 MPa. The as-obtained green bodies were sintered at 1800°C for 16 h in a tungsten furnace under a vacuum of 1.0×10 -3 Pa. After sintering, the samples were annealed at 1400°C for 4 h in air.

The XRD patterns for phase identification were collected at ambient temperature by X-ray diffraction (XRD; D/Max 2500, Rigaku, Tokyo, Japan) analysis using CuK α radiation (λ=1.5406 Å) at 40 kV and 100 mA. A step size of 0.01° was used with a scan speed of 6 °/min. In-line transmittance was measured by UV-VIS-NIR spectrophotometry (Cary 5000, Varian, USA) over the wavelength region of 200 to 2200 nm. The microstructures of the polished surface of the specimens were observed by scanning electron microscopy (SEM, JSM-6700F, JEOL, Japan). The average grain size was examined by the intercept method using the equation G=1.56L, where G is the average grain size, and L is the average intercept length.

The thermal diffusivity and heat capacity were measured by the laser flash method using a laser flash analyzer (LFA 467 HyperFlash NETZSCH, Germany). The samples were machined to Φ13.0 mm×1.2 mm in size and were coated with platinum and graphite before the measurement. The thermal conductivity was calculated as

k=α·ρ·Cp,

(1)

where k, α, ρ and Cp denote the thermal conductivity, thermal diffusion coefficient, density and heat capacity, respectively. All measurements were conducted at room temperature. The Vickers microhardness of the samples was assessed from nine indentations made using a hardness testing machine (HM, Mitutoyo, Tokyo, Japan). The microhardness was calculated as

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H=k(P/d2),

(2)

where P is the load (10 N) on the indenter, d is the indentation diagonal, and k is the shape factor of the indenter, which was 1.854 in this case. A macroscale composition analysis was performed using an electron probe microanalyzer (EPMA, SX100, CAMECA, France) with wavelength dispersive spectrometry (WDS). A nanoscale composition analysis was carried out using transmission electron microscopy (TEM, JEOL, JSM-2100F) equipped with an energy dispersive X-ray (EDS) detector.

3. Results and discussion

Fig. 1 displays the XRD patterns of the Er:Y2O3 ceramics with various Er concentrations sintered at

1800°C for 16 h in a vacuum. All of the peaks perfectly match the standard cubic Y 2O3 phase (JCPDS

card, No. 41-1105) with a space group of Ia-3 without impurity phases. Upon careful examination, the

diffraction peaks of Er:Y2O3 were found to gradually shift to higher angles with an increase in the Er

concentration (see the inset of Fig. 1), which is clear evidence of shrinkage of the unit cell of Y 2O3

caused by the partial substitution of smaller Er3+ ions (R=0.89 Å, CN=6) for larger Y3+ ions (R=0.90 Å,

CN=6) [18]. We calculated the lattice parameters and unit cell volumes of the Er-doped Y2O3 samples

based on the XRD patterns, which are presented in Table 1. With an increase in the Er doping

concentration from 0 to 10 at.%, the lattice parameter decreased from 1.06920 to 1.06758 nm and the

unit cell volume decreased from 1.22230 to 1.21675 nm3.

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Fig. 2 exhibits images of samples doped with various Er concentrations. Each pellet had a diameter

of approximately 13 mm with a thickness of 1.2 mm. All of the samples had high transparency, and the

letters under the ceramics could be seen distinctly. The pink color deepened with increasing Er content.

The in-line transmittance levels of samples (1.2 mm thick) doped with various Er concentrations are

presented in Fig. 3. All of the sintered transparent Er:Y2O3 ceramics always showed optical

transmittance over 83% at 1100 nm. It is important to note that, in the visible region, the in-line

transmittance of all samples retained a relatively high value of over 81% at 600 nm, which indicates

that nanometer-range pores and inclusions were mostly absent in the studied specimens. The high

transmittance is attributed to the benefit of the compound sintering additives, ensuring the output of the

laser. With the small amount of ZrO2 (3 at.%) suppressing grain boundary migration and the large

amount of La2O3 (10 at.%) accelerating the mass transfer during the sintering process, the co-existence

of sintering aids is complementary and appropriate for the removal of residual pores, with the

consequent realization of fine microstructures with smaller grain sizes, providing high optical

homogeneity and good mechanical properties. The room-temperature absorption spectrum of 10 at.%

Er:Y2O3, derived from the measured transmittance spectrum, from 340 nm to 1700 nm, is exhibited Fig.

3(b), in which a number of absorption peaks are presented. The absorption bands centered at 654 nm,

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800 nm, 972 nm, and 1535 nm are attributed to the transitions of Er 3+ ions from the ground state of

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I15/2 to the excited states of 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively [9].

SEM micrographs of the polished surfaces of the Er:Y 2O3 ceramics with various Er doping concentrations are presented in Fig. 4. All of the samples featured comparatively fine and homogeneous microstructures without abnormal grain growth. It should be noted that the average grain size of all samples was estimated to be approximately 13 μm from a statistical evaluation regardless of the wide variation in the Er concentration (Fig. 5(f)). This is mainly attributed to the facts that Er 2O3 and Y2O3 have similar crystal structures and the ionic radius of the Er 3+ ion (R=0.89 Å) is close to that of the Y3+ ion (R=0.90 Å). The difference between the self-diffusion coefficients of Er3+ and Y3+ is within one order of magnitude [19]. Consequently, this trivalent cation-doping does not have much of an effect on the sinterability of Y2O3.

The variations in the thermal diffusivity and specific heat of transparent Er:Y 2O3 ceramics as a function of the Er concentration at room temperature are presented in Fig. 5 (a). The thermal diffusivity of the Y2O3 ceramic without Er doping was 2.134×10 -6 m2/s, and it decreased by as much as 14.0% to 1.836 ×10-6 m2/s for the 10 at.% Er:Y2O3 specimen. On the other hand, the heat capacity was reduced by 4.3%, which was not that strongly affected by variation of the doping concentration. Owing to the substitution of heavier Er3+ ions (atomic weight: 167.26) for Y3+ ions (atomic weight: 88.9) and shrinkage of the unit cell volume (see Table 1), the theoretical density showed a monotonous increase from 5.139 to 5.505 g/cm3 (see Fig 5. (b)). As displayed in Fig. 5 (b), the influence of the Er doping concentration on the room-temperature thermal conductivity is apparent. With an increase in the Er doping concentration from 0 to 10 at.%, the thermal conductivity decreased by 12%, from 5.55 to 4.89

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W/m·K, according to equation (1), offset by the increased density. In the matrix of the Y2O3 ceramic, the heat transfer is conducted by lattice vibrations (phonon transport)

[7]. When the Er3+ ions enter

the lattice, some degree of structural distortion and point defects can be introduced, resulting in stronger phonon scattering which causes a reduction in the phonon mean free path. The reduction in the thermal conductivity from the pure Y2O3 ceramics (about 15 W/m·K) is mainly ascribed to the high concentrations of the composite sintering additives of 3 at.% ZrO2 and 10 at.% La2O3. A high thermal conductivity of laser materials is crucial to minimize the thermal lensing effect and improve the laser beam quality. Therefore, it is necessary to enhance the thermal conductivity of ceramics with low concentrations of sintering additives via process optimization or with a post-treatment, such as hot-isostatic pressing (HIP).

The microhardness and fracture toughness of the samples are compared in Table 2. The microhardness and fracture toughness of the samples with the compound sintering additives exhibited narrow variations, consistent with the changes in the average grain sizes. These results suggest that the Er doping concentrations do not have much of an effect on the microhardness or toughness of transparent Er:Y2O3 ceramics. Moreover, the mechanical properties of Er:Y2O3 ceramics are improved by the additives of ZrO2 and La2O3 compared to the 0.25 at.% Er:Y2O3 ceramics prepared without additives but with the identical traditional sintering method [20].

One of the technical issues associated with conventional melt-growth single-crystal technology is the difficulty of achieving heavy and homogeneous doping of laser active ions with a small segregation coefficient in the host materials [1]. To investigate the homogeneity of the ceramic specimen with high Er doping concentration, macro- and nanoscale line scan was adopted. Fig. 6(a) illustrates the

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distribution of the Y, Er, La and Zr elements in the radial direction from the center to the edge of the 10 at.% Er:Y2O3 specimen. It was found that all the metal elements manifested a uniform distribution along the analyzed length of about 6 mm. In nanoscale, Fig. 6(b) displays EDS line profiles of the Y, Er, La and Zr elements across the grain boundary in a range of 300 nm as indicated in the TEM image. There is no obvious enrichment or deficit in all the detected elements profiles. In macro-and nanoscale, the desired homogeneous distribution of Y, Er, La and Zr elements was confirmed despite the high doping concentration.

In order to decrease the optical loss, it is extremely important to fabricate fully dense transparent Er:Y2O3 ceramics with a pore-free structure and clean grain boundaries. The optical transmittance of the Er:Y2O3 samples by the combination of vacuum sintering and the co-addition of 3 at.% ZrO2 and 10 at.% La2O3 reaches the highest level ever reported, which indicates that nanometer-range pores and inclusions are mostly absent in the considered ceramics, facilitating the following thermal and mechanical property measurements. Compared to the two typical lasing Nd 3+ and Yb3+ ions, the wide range of Er doping concentrations did not have much of an effect on the transmittance, grain size, microhardness or fracture toughness of the samples due to the identical structures and the similar lattice parameters of Er2O3 and Y2O3. However, with high doping concentration, the average interionic distance between Er3+-Er3+ is relatively short. There is a larger probability that the Er3+ ions will compete with each other, resulting in nonradiative loss [21]. Therefore, a relatively low doping concentration is suggested for Er-doped materials, which possess higher thermal conductivity and consequently better thermal shock resistance. These results may provide a guideline for the application of transparent Er:Y2O3 laser ceramics.

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4. Conclusions

Highly transparent Er:Y2O3 ceramics were fabricated by a solid-state reaction method at 1800°C using 3 at.% ZrO2 and 10 at.% La2O3 as sintering additives. We investigated the effects of the Er doping concentrations on the optical transmittance, microstructure, mechanical properties and thermal conductivity of the transparent Y2O3 ceramics. Due to the identical structures and the similar lattice parameters of Er2O3 and Y2O3, the wide variance in the Er doping concentrations did not have much of an effect on the transmittance, average grain size or mechanical properties, though this was not the case for thermal conductivity. With an increase in the Er doping concentration from 0 to 10 at.%, all compositions demonstrated a high transmittance over 83% at 1100 nm and over 81% at 600 nm, while the thermal conductivity decreased from 5.55 to 4.89 W/m·K. The microhardness and fracture toughness were improved compared to those of other Er:Y 2O3 ceramics fabricated without additives but with the same traditional sintering method. A nearly homogeneous distribution of the laser activator Er up to 10 at.% in macro-and nanoscale was verified in the Er:Y2O3 ceramic, which is the prominent advantage of polycrystalline over single-crystal materials. The co-addition of ZrO2 and La2O3 was confirmed to be effective for the fabrication of transparent Er:Y2O3 ceramics with improved mechanical properties. These results may provide a guideline for applications of transparent Er:Y 2O3 laser ceramics.

Acknowledgements

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This work was supported by the Materials & Components Technology Development (MCTD) Program (PN: 10047010, Development of 80% Light-Transmitting Polycrystalline Ceramics for Transparent Armor·Window Applications) funded by the Ministry of Trade, Industry & Energy of Korea.

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phys. stat. sol. (a) 175 (1999) 705-710.

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Fig. 1.

X-ray diffraction patterns of Er:Y2O3 ceramics doped with various Er concentrations.

Fig. 2.

Photographs of transparent Er:Y2O3 ceramic samples doped with various Er concentrations of

(a) 0 at.%, (b) 1 at.%, (c) 3 at.%, (d) 5 at.%, and (e) 10 at.% (1.2 mm thick).

Fig. 3. (a) In-line transmittance levels of transparent samples doped with various Er concentrations (1.2 mm thick), and (b) the room-temperature absorption spectrum of the 10 at.% Er:Y 2O3 transparent

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ceramic with 3 at.% ZrO2 and 10 at.% La2O3. Labels in (b) represent the final excitation levels from the ground state of 4I15/2.

Fig. 4. SEM micrographs of the Er:Y2O3 ceramics doped with various Er concentrations of (a) 0 at.%,

(b) 1 at.%, (c) 3 at.%, (d) 5 at.% and (e) 10 at.%; (f): average grain size of the Er:Y 2O3 ceramics.

Fig. 5. (a) Thermal diffusivity and specific heat of transparent Er:Y2O3 ceramics with various Er contents, and (b) density and thermal conductivity of transparent Er:Y 2O3 ceramics with various Er contents.

Fig. 6. (a) EPMA line scan profiles of Y, La, Er and Zr in the radial direction from the center to the edge of 10 at.% Er:Y2O3 transparent ceramic. Inset: photo of the 10 at.% Er:Y 2O3 transparent ceramic, (b) EDS line scan profiles of Y, La, Er and Zr across the grain boundary as indicated in the inset. Inset: TEM image of 10 at.% Er:Y2O3 transparent ceramic.

Table 1. The lattice parameter (a=b=c) and unit cell volume of cubic Er:Y 2O3 doped with various Er concentrations.

.

Lattice parameters a=b=c Er concentrations (at.%)

Unit cell volume

(nm)

(nm3)

0

1.06920

1.22230

1

1.06819

1.21884

3

1.06805

1.21836

15

5

1.06769

1.21713

10

1.06758

1.21675

Table 2. Microhardness and fracture toughness of Y 2O3 ceramics with varying Er doping contents.

Er concentration

Microhardness

Fracture toughness

(at.%)

H (GPa)

KIC(MPa·m1/2)

0

8.11 ± 0.09

1.03 ± 0.05

1

8.06 ± 0.06

1.04 ± 0.04

3

8.05 ± 0.06

0.99 ± 0.06

5

8.05 ± 0.07

1.03 ± 0.06

10

8.10 ±0.06

1.24 ± 0.04

0.25 (Ref. [20])

7.23±0.35

0.81±0 .07

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Fig. 1

Fig. 2

17

Fig. 3

Fig. 4 18

Fig. 5

Fig. 6

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