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The Pennsylvania State University The Graduate School Department of Materials Science and Engineering THE EFFECTS OF SINTERING AIDS ON DEFECTS, DENSIFICATION, AND SINGLE CRYSTAL CONVERSION OF TRANSPARENT ND:YAG CERAMICS A Dissertation in Materials Science and Engineering by Adam J. Stevenson 2010 Adam J. Stevenson Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2010 The dissertation of Adam J. Stevenson was reviewed and approved* by the following: Gary L. Messing Distinguished Professor of Ceramic Science and Engineering Head, Department of Materials Science and Engineering Dissertation Advisor Chair of Committee Elizabeth C. Dickey Professor of Materials Science and Engineering Venkatraman Gopalan Professor of Materials Science and Engineering Patrick M. Lenahan Distinguished Professor of Engineering Science and Mechanics *Signatures are on file in the Graduate School ii ABSTRACT 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. To explore the role of SiO2 doping in densification and microstructure development of Nd:YAG transparent ceramics, 1 at% Nd:YAG powders were doped with 0.035-0.28 wt% SiO2 and vacuum sintered between 1484oC and 1750oC. 29Si magic-angle spinning nuclear magnetic resonance showed that Si4+ substitutes onto tetrahedrally coordinated Al3+ sites at sintering temperatures > 1600oC. High resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600-1750oC. 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 Nd3+ concentration near grain boundaries. Increasing SiO2 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 μm – 18 μm) in highly transparent ceramics using a combination of SiO2 content, sintering temperature, and sintering time. B2O3-SiO2 was shown to act as a transient liquid phase sintering aid that reduces the sintering temperature of Nd:YAG ceramics to 1600oC. 1 at% Nd:YAG ceramics were doped o o with 0.34-1.35 mol% B2O3-SiO2 and sintered between 1100 C and 1700 C. Dilatometric measurements showed that B2O3-SiO2 additions increase the densification rate during 3+ intermediate stage sintering relative to SiO2 doped samples. B content is reduced to < 5 ppm in o the samples at temperatures above 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 B2O3-SiO2 doping reduces SiO2 content during final stage densification. The increased densification kinetics during intermediate iii stage sintering lead to highly transparent (84% in-line transmission at 400 nm) Nd:YAG o ceramics when sintered at 1600 C in either vacuum or flowing O2. Optical absorption spectroscopy and electron spin resonance were used to study the effects of SiO2 doping on color center formation in Nd:YAG transparent ceramics. The primary color centers in sintered samples were F and F+-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 1600oC 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% SiO2 doped 1 at% Nd:YAG. This indicates that SiO2 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 Fe2+/3+. 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 F2 and F2 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 - and Oh center formation. SiO2 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% SiO2 doped 1 at% Nd:YAG transparent ceramics. Two different SCC methods have been demonstrated for Nd:YAG ceramics. The solid state method utilizes SiO2 to increase grain boundary mobility at the seed crystal/polycrystal interface leading to crystals up to 5 mm x 5 mm x 460 μm. Exaggerated grain growth in highly SiO2 doped 1 at% Nd:YAG ceramics was found to depend on both SiO2 content and temperature. Surface doping 1 at% Nd:YAG ceramics with 1 wt% SiO2 led to extended crystal growth up to 1.5 mm at 1600oC. iv TABLE OF CONTENTS LIST OF FIGURES………………………………………………………………….viii LIST OF TABLES ....................................................................................................... xvi Chapter 1 Introduction ................................................................................................. 1 1.1 Motivation ....................................................................................................... 1 1.2 Scientific Approach ........................................................................................ 5 1.3 Organization of the Thesis .............................................................................. 7 1.4 References ....................................................................................................... 9 Chapter 2 Background ................................................................................................. 12 2.1 Transparent Ceramics Versus Single Crystals ................................................ 12 2.2 Transparent Ceramics in the Literature ......................................................... 17 2.1 YAG and Nd:YAG ........................................................................................ 19 2.3 YAG Ceramic Processing .............................................................................. 23 2.3.1Powder Source ....................................................................................... 23 2.3.2Green Forming ....................................................................................... 27 2.3.3Densification .......................................................................................... 28 2.3.4The role of SiO2 in YAG and Nd:YAG sintering .................................. 30 2.3.5Pressure Assisted densification ............................................................. 35 2.3.6Summary of YAG and Nd:YAG Processing ......................................... 38 2.4 Nd:YAG Laser Performance ......................................................................... 39 2.4.1Spectral Properties of Nd:YAG Ceramics ............................................. 41 2.4.2Optical Quality and Scattering in Nd:YAG Transparent Ceramics ...... 45 2.5 Summary ........................................................................................................ 47 2.6 References ..................................................................................................... 48 Chapter 3 Effect of SiO2 on Densification and Microstructure Development in Nd:YAG Transparent Ceramics ........................................................................................... 53 3.1 Introduction ..................................................................................................... 53 3.2 Experimental Procedure .................................................................................. 56 3.2.1 Powder Synthesis and Green Forming ................................................. 56 3.2.2 Sintering ............................................................................................... 57 3.2.3 Microstructural and Optical Characterization ...................................... 57 3.2.4 Nuclear Magnetic Resonance (NMR) .................................................. 57 3.2.5 TEM ...................................................................................................... 58 3.3 Results and Discussion ................................................................................... 59 3.3.1 SiO2 in Nd:YAG Ceramics ................................................................... 59 3.3.2 Densification ......................................................................................... 64 v 3.3.3 Grain Growth ........................................................................................ 70 3.3.4 Microstructural Control and Sintering Trajectory ................................ 76 3.4 Conclusions..................................................................................................... 79 3.5 References ......................................................................................................
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