Low temperature, transient liquid phase sintering of B 2 O 3 -SiO 2 -doped Nd:YAG transparent ceramics

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B2O3-SiO2 is shown to act as a transient liquid phase sintering aid that reduces the sintering temperature of Nd:YAG ceramics to 1600 °C. 1 at.% Nd3xY3-3xAl5O12 (Nd:YAG) ceramics were doped with 0.34–1.35 mol% B2O3-SiO2 and sintered between 1100 and 1700 °C. Dilatometric measurements show that B2O3-SiO2 doping increases the densification rate during intermediate-stage sintering relative to SiO2-doped samples. B3+ content is reduced to ,5 ppm in samples heated to 1500 °C, as determined by mass spectrometry. For B2O3-SiO2-doped samples, final stage densification and grain growth follow a more densifying sintering trajectory than SiO2-doped 1 at.% Nd:YAG ceramics because there is less SiO2 during final-stage densification. The increased densification kinetics during intermediate-stage sintering lead to highly transparent Nd:YAG ceramics when sintered at 1600 °C in either vacuum or oxygen. Thus, transparent Nd:YAG ceramics can be sintered without the need for expensive refractory metal vacuum furnaces or pressure-assisted densification.

I. INTRODUCTION

It is possible to produce highly transparent polycrystalline Nd:YAG by sintering to .99.99% density because Nd3xY3-3xAl5O12 (Nd:YAG) has a cubic crystal structure. Ikesue et al.1 first reported highly efficient laser gain in transparent Nd:YAG ceramics by sintering a powdered mixture of Al2O3, Y2O3, and Nd2O3 at 1750 °C for 8 h. They found that the addition of 0.14 wt% (1.35 mol%) SiO2 was critical to sintering Nd:YAG to transparency and proposed that SiO2 plays a critical role in incorporating Nd3+ into the YAG lattice. Yagi et al.2 produced transparent Nd:YAG ceramics using coprecipitated Nd:YAG powders doped with several hundred parts per million SiO2 and sintered at 1700 °C for 20 h. Thermodynamic modelings of the SiO2-YAG and SiO2-Nd:YAG systems predict that silica-rich liquid phases form at temperatures as low as 1390 °C.3 Boulesteix et al.4 observed a liquid phase at particle necks in SiO2-doped Nd:YAG ceramics sintered at 1527 °C for 15 min. In 0.3 wt% SiO2-doped 1 at.% Nd:YAG, differential thermal analysis showed an endothermic peak at 1390 °C, which was attributed to liquid phase formation.5 A similar peak was not observed in Nd:YAG samples a)

Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/jmr.2011.45 J. Mater. Res., Vol. 26, No. 9, May 14, 2011

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without SiO2 doping. Using a combination of nuclear magnetic resonance and transmission electron microscopy, Stevenson et al.5 showed that Si4+substitutes into the YAG crystal structure on tetrahedrally coordinated Al3+ sites and no liquid phase remains in 0.035–0.28 wt% SiO2doped 1 at.% Nd:YAG at temperatures between 1600 and 1750 °C. It is apparent that SiO2 acts as a transient liquidphase sintering aid during YAG sinterin

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