Elsevier

Ceramics International

Volume 45, Issue 3, 15 February 2019, Pages 3217-3222
Ceramics International

Highly transparent Yb:Y2O3 ceramics obtained by solid-state reaction and combined sintering procedures

https://doi.org/10.1016/j.ceramint.2018.10.224Get rights and content

Abstract

(Y0.87-xLa0.1Zr0.03Ybx)2O3 (x = 0.02, 0.04, 0.05) transparent ceramics were obtained by solid-state reaction and combined sintering procedures with La2O3 and ZrO2 as sintering additives. A method based on two-step intermediate sintering in air followed by vacuum sintering was applied in order to control the densification and grain growth of the samples during the final sintering process. The results indicate that La2O3 and ZrO2 co-additives can improve the microstructure and optical properties of Yb:Y2O3 ceramics at relatively low sintering temperature. On the other hand, the addition of Zr4+ ions leads to the formation of dispersed scattering volumes in the ceramic bodies. Transmittance of 78.8% was measured for the 2.0 at% Yb:Y2O3 ceramic sample at the wavelength of 1100 nm. The spectroscopic properties of Yb:Y2O3 ceramics were investigated at room temperature. The obtained results show that the absorption cross-section at 978 nm is in the range of 2.08 × 10–20 to 2.36 × 10–20 cm2, whereas the emission cross-section at 1032 nm is ~1.0 × 10–20 cm2.

Introduction

Nowadays, ceramic laser technology has emerged as a promising candidate because of its numerous advantages over single-crystal lasers. First, ceramics laser media can be produced in large volumes, which makes them attractive for high-power laser generation. Second, composite laser ceramics with complicated structures can also be fabricated, whereas it's impossible for single crystals. Besides, ceramics can be heavily and homogeneously doped with trivalent lanthanide laser-active ions [1].

Yb3+-ion doped laser materials have received considerable attention for laser applications due to its simple energy level diagram that leads to low quantum defects, reducing thermal loads and preventing undesired effects such as excited state absorption [2]. Moreover, Yb3+ doped transparent ceramics are suitable for InGaAs laser diodes pumping. Cubic Y2O3 sesquioxide is a promising host material for solid-state lasers, due to its high thermal conductivity ∼14 W/(m·K), wide transparency range (0.2–8 µm), high refractive index (∼1.8) and low cut-off phonon energy (380 cm−1) [3], [4], [5]. The high melting point ∼2430 °C and the phase transformation from cubic phase to a high-temperature hexagonal phase at about 2350 °C make it difficult to grow Y2O3 crystals with high quality and large dimension. Instead, Y2O3 ceramics are easy to obtain for low sintering temperature, which is about 700 °C lower than its melting point [3], [4]. Therefore, polycrystalline transparent ceramics of Y2O3 would be a new candidate for high-power solid-state laser applications, such as thin disk or microchip lasers [5], [6], [7].

In order to fabricate dense Y2O3 ceramics, Wang et al. [8] and Chen et al. [9] have proposed a new sintering method. Thus, starting from nanopowders obtained by wet-chemical methods, nanocrystalline yttria ceramics with full density were prepared using a two-step sintering procedure based on the low-temperature sintering process to suppress grain growth without affecting densification. However, the optical characteristics of the obtained ceramics were not provided. Recently, Huang et al. [10] reported the fabrication of transparent lanthanum-doped yttria ceramics starting from nanosized powders; a two-step sintering in air procedure followed by one-step sintering in vacuum were combined in order to control separately both densification and grain growth processes.

The sintering temperature of Y2O3 ceramics can be lowered by adding different additives, like La2O3, Al2O3, ZrO2, or HfO2, as sintering aid [11], [12]. Sintering additives form solid solutions in the grain boundary regions that decrease the surface energy and mobility of boundaries and inhibit the abnormal grain growth leading to a lower porosity of ceramics. It is well known that La2O3 is a commonly additive used as sintering aid in order to promote the densification of Nd:Y2O3 ceramics [13]. Zhang et al. reported that transparent Yb:Y2O3 ceramics can be fabricated by using up to 10 mol.% La2O3 sintering additive; ceramics with maximum transmittance of 75.0% in the 1–2 µm range were fabricated [14]. It was found that ZrO2 is an effective additive used as sintering aid in concentration of about 3.0–5.0 at%. However, the presence of high content of tetravalent additives has a negative effect on thermal properties of Y2O3 ceramics. Transparent Y2O3 ceramics with 3.0–5.0 at% ZrO2 as additive are unfavourable for high-power laser application, and the unique addition of ZrO2 lower than 3.0 at% cannot produce transparent Y2O3 ceramics [15], [16]. Some authors reported that highly transparent Y2O3 ceramics were obtained by solid-state reaction and vacuum sintering method using La2O3 and ZrO2 as sintering additives. Adding 9.0 at% La2O3 and 3.0 at% ZrO2 the transmittance of 3.0 at% Tm:Y2O3 [17] and 5.0 at% Yb:Y2O3 [18] ceramics was slightly below 80.0% at the wavelength of 2000 nm and 1100 nm, respectively. By using 10 at% La2O3 and 3.0 at% ZrO2 a transmittance of about 81.0% at 1100 nm was reported for Nd:Y2O3 ceramics [19].

In this work, 2.0, 4.0 and 5.0 at% Yb:Y2O3 transparent ceramics were fabricated by solid-state reaction and combined sintering procedures with 10 at% La2O3 and 3.0 at% ZrO2 as sintering additives. The densification and grain growth of the ceramics was controlled with a two-step intermediate sintering in air method followed by vacuum sintering. Our goal was to improve the optical properties of Yb:Y2O3 ceramics at relative low temperature using commercial micropowders (< 10 µm) and solid-state reaction, which may reduce the costs and make the densification process more applicable. The phase structure, microstructure and optical properties of obtained ceramics were systematically investigated and related to the sintering procedure.

Section snippets

Experimental procedure

High purity commercial available micropowders of Y2O3 (99.999%) and Yb2O3 (99.998%) were used as starting raw materials with La2O3 (99.99%) and ZrO2 (99 +%) as sintering additives (Fig. 1). According to the stoichiometric formula (Y0.87-xLa0.1Zr0.03Ybx)2O3 (x = 0.02, 0.04, 0.05), the weighted powders were blended and ball milled in absolute ethyl alcohol for 12 h with agate balls. After mixing, the slurries were dried and sieved through a 200-mesh screen to obtain a soft agglomeration of the

Results and discussion

The XRD patterns of Yb3+ doped Y2O3 ceramics are shown in Fig. 2. All sintered specimens are well crystallized indicating the cubic Y2O3 phase (Ia3 space group) and confirm that the introduction of La2O3 and ZrO2 co-additives do not change significantly the structure of Yb:Y2O3 ceramics. No extra peaks are found, which means that no secondary phase was formed during the sintering procedure.

The SEM micrographs of the polished and etched surface of Yb:Y2O3 ceramics are presented in Fig. 3. The

Conclusions

(Y0.87-xLa0.1Zr0.03Ybx)2O3 (x = 0.02, 0.04, 0.05) transparent ceramics were obtained for the first time by solid-state reaction and combined sintering procedures. The combination of two-step sintering in air followed by vacuum sintering proves to be a feasible approach to control the densification of Yb:Y2O3 ceramics with limited grain growth. The influence of Yb3+ ions concentration, as well as La2O3 and ZrO2 sintering additives on the structure, microstructure and optical properties of

Acknowledgements

This work was financed by the Romanian National Authority for Scientific Research and Innovation through the program NUCLEU, under grant agreement LAPLAS V 3N/2018 and partially supported by Laserlab-Europe EU-H2020, Project 654148. The authors thank Nicolaie Pavel and Traian Dascalu for their help during laser emission experiments and for the discussion of the results.

References (26)

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    In Table 1, the relatively density of the samples slightly decreased with an increase in the Yb concentration, which is in consistence with the variation of the density of the samples. The transmittance levels of Yb:Y2O3 ceramics fabricated by the present hot-pressing method are higher than those of previous reported Yb:Y2O3 transparent ceramics via other methods[6,28–30]. In addition, an absorption band appeared in the range of 821–1061 nm, which is ascribed to the transition of Yb3+ from ground state of 2F7/2 to excited state of 2F5/2.

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