THE EFFECTS OF SINTERING AIDS ON DEFECTS, DENSIFICATION, AND SINGLE CRYSTAL CONVERSION OF TRANSPARENT ND:YAG CERAMICS

Open Access
Author:
Stevenson, Adam Joseph
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 29, 2010
Committee Members:
  • Gary Lynn Messing, Dissertation Advisor
  • Gary Lynn Messing, Committee Chair
  • Elizabeth C Dickey, Committee Member
  • Venkatraman Gopalan, Committee Member
  • Patrick M Lenahan, Committee Member
Keywords:
  • transparent ceramics
  • grain growth
  • sintering
  • ceramic processing
Abstract:
<p> Nd:YAG transparent ceramics have the potential to replace Czochralski grown single crystals in high power laser applications. However, after more than 20 years of development, there has been only limited application of these potentially revolutionary materials. In order to improve the processing and properties of Nd:YAG transparent ceramics and facilitate increased adoption, this dissertation explores the effects of sintering aids on defects, densification and single crystal conversion (SCC) of Nd:YAG ceramics.</p> <p>To explore the role of SiO<sub>2</sub> doping in densification and microstructure development of Nd:YAG transparent ceramics, 1 at% Nd:YAG powders were doped with 0.035-0.28 wt% SiO<sub>2</sub> and vacuum sintered between 1484<sup>o</sup>C and 1750<sup>o</sup>C. <sup>29</sup>Si magic-angle spinning nuclear magnetic resonance showed that Si<sup>4+</sup> substitutes onto tetrahedrally coordinated Al<sup>3+</sup> sites at sintering temperatures > 1600<sup>o</sup>C. High resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600-1750<sup>o</sup>C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and TEM energy dispersive x-ray spectroscopy observations of increased Nd<sup>3+</sup> concentration near grain boundaries. Increasing SiO<sub>2</sub> content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. The average grain size could be controlled (2.8 &#956;m – 18 &#956;m) in highly transparent ceramics using a combination of SiO<sub>2</sub> content, sintering temperature, and sintering time. </p> <p>B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> was shown to act as a transient liquid phase sintering aid that reduces the sintering temperature of Nd:YAG ceramics to 1600<sup>o</sup>C. 1 at% Nd:YAG ceramics were doped with 0.34-1.35 mol% B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> and sintered between 1100<sup>o</sup>C and 1700<sup>o</sup>C. Dilatometric measurements showed that B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> additions increase the densification rate during intermediate stage sintering relative to SiO<sub>2</sub> doped samples. B<sup>3+</sup> content is reduced to < 5 ppm in the samples at temperatures above 1500<sup>o</sup>C, as determined by mass spectrometry. For B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> doped samples, final stage densification and grain growth follow a more densifying sintering trajectory than SiO<sub>2</sub> doped 1 at% Nd:YAG ceramics because B<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> doping reduces SiO<sub>2</sub> content during final stage densification. The increased densification kinetics during intermediate stage sintering lead to highly transparent (84% in-line transmission at 400 nm) Nd:YAG ceramics when sintered at 1600<sup>o</sup>C in either vacuum or flowing O<sub>2</sub>. </p> <p> Optical absorption spectroscopy and electron spin resonance were used to study the effects of SiO<sub>2</sub> doping on color center formation in Nd:YAG transparent ceramics. The primary color centers in sintered samples were F and F<sup>+</sup>-centers as evidenced by optical absorption in the 250 nm to 400 nm wavelength range and the presence of an electron spin resonance line at g = 1.9977. Annealing in air at 1600<sup>o</sup>C for 10 h eliminated/reduced the number of color centers in the sample. The color center induced optical absorption was similar in the 280 nm to 400 nm wavelength range between 0.035 wt% and 0.28 wt% SiO<sub>2</sub> doped 1 at% Nd:YAG. This indicates that SiO<sub>2</sub> doping has little or no effect on color center formation during sintering. Instead, color center formation was shown to be controlled by oxidation and reduction of variable valence impurity ions, primarily Fe<sup>2+/3+</sup>. After irradiating samples with ultraviolet light, optical absorption increased in the 250 nm to 800 nm wavelength range. Optical absorption in this range was associated with the formation of aggregate F-centers such as F<sub>2</sub> and F<sub>2</sub><sup>+</sup> centers. After ultraviolet irradiation, two overlapping electron spin resonance lines were observed at g = 1.9987 and g = 2.0232 that are consistent with F<sup>+</sup>- and O<sub>h</sub>- center formation. SiO<sub>2</sub> content did not affect irradiation induced color center formation, as shown by similar optical absorption and electron spin resonance spin counts in 0.035 and 0.28 wt% SiO<sub>2</sub> doped 1 at% Nd:YAG transparent ceramics. </p> <p> Two different single crystal conversion methods have been demonstrated for Nd:YAG ceramics. The solid state method utilizes SiO<sub>2</sub> to increase grain boundary mobility at the seed crystal/polycrystal interface leading to crystals up to 5 mm x 5 mm x 460 &#956;m. Exaggerated grain growth in highly SiO<sub>2</sub> doped 1 at% Nd:YAG ceramics was found to depend on both SiO<sub>2</sub> content and temperature. Surface doping 1 at% Nd:YAG ceramics with 1 wt% SiO<sub>2</sub> led to extended crystal growth up to 1.5 mm at 1600<sup>o</sup>C. </p>