Phase Diagrams in Advanced Ceramics

Allen M. Alper, Gernot Kostorz, Herbert Herman

• Publisher: Academic Press
• Year: 1995
• Print ISBN: 9780123418340
• eBook ISBN: 9780080538723
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The investigation of multi-component complex systems composed of oxides, nitrides, and carbides has intensified in the last few years. Phase Diagrams in Advanced Ceramics reviews some of the recent advances inthe understanding of these composite systems, providing insight into how phase diagrams can be utilized in the fabrication of whiskers and ceramic-matrix whisker-reinforced ceramics. Phase relations and sintering information is reviewed for transparent polycrystalline oxides. Phase diagrams are discussed to predict alkali oxide corrosion of alumino-silicate references. - Understanding the development, manufacture, and use of complex, multi-component ceramic materials composed of silicon nitride-metal oxides-nitride-carbide systems - Development and use of whisker and whisker-reinforced ceramics composed of materials such as alumina, silicon-nitride, silicon carbide, and directly solidified eutectic ceramics - Application of phase diagrams to the production of advanced composites such as alumina-matrix, zirconium diboride and titanium, hafnium, zirconium, carbides, and borides - Phase chemistry in the development of transparent poly-crystal and oxides, including yttria, alumina, and magnesium aluminate - Improvements concerning the knowledge of complex multi-component materials composed of oxides, nitrides, and carbides, and knowledge of how to fabricate composite materials containing whiskers and ceramic hosts - New developments in making transparent ceramic materials

1

Phase Chemistry in the Development of Transparent Polycrystalline Oxides

W.H. RHODES, OSRAM SYLVANIA, Inc., Danvers, Massachusetts 01923

I. Introduction

II. Yttria

A. Y₂O₃–Lanthanide Additives

B. Y₂O₃–Group-Four Additives

C. Y₂O₃–Al₂O₃

III. Alumina

A. Al₂O₃–MgO

B. Al₂O₃–MgO–Y₂O₃

IV. Magnesium Aluminate

A. Stoichiometry Variations

B. MgAl₂O₄–LiF

V. Aluminum Oxynitride Spinel

VI. Lead–Lanthanum–Zirconium–Titanate

A. Phase Relations

B. Liquid-Phase Sintering

VII. Conclusions

References

I. Introduction

Polycrystalline oxides have been available for optical applications since the early 1960s when Coble [15] invented translucent Al₂O₃. Before this, it was thought that porosity, with its inherent light scattering property, was a necessary consequence of the ceramic fabrication process. Translucent Al₂O₃ is a key element in high-pressure sodium lamps manufactured all over the world. Although it is not clear that Coble made extensive use of phase diagrams in his original development, phase relations are important to the success of translucent Al₂O₃, and subsequent researchers seeking to sinter other oxides to transparency have found phase relations important to their success in achieving their ultimate goal in sintering: elimination of residual porosity. For example, in the La₂O₃–Y₂O₃ system, Rhodes [54] used a two-phase field to control grain growth during the pore-removal period of sintering and then shifted to a single-phase field to anneal to transparency.

A number of oxides have been developed for optical applications in which glass, because of refractoriness, chemical compatibility, or limitations on bandwidth, typically cannot compete. These oxides are Al₂O₃, MgAl₂O₄, ALON, and Y₂O₃. Although other oxides have been sintered or hot pressed to transparency, these four are considered to be the prime candidates to extend optical applications.

Another important category of optical application uses the electro-optic switching character of perovskites. The system (Pb, La)(Zr, Ti)O₃ has been hot pressed and sintered to transparency by Haertling [26] and Snow [65], respectively. Phase relations are critical to successful fabrication where Pb volatility makes it difficult to retain compositional limits, and compositional variations make possible memory, linear, or quadratic applications.

II. Yttria

Yttria is of interest for optical applications principally because of its high melting point (2464°C) and capacity for wideband transmittance (0.23 to 9.5 μm). This combination of properties makes possible a material that will perform at high temperatures with low emittance and minimal migration of the phonon edge into the 3- to 5-μm band of interest for many infrared applications. Early development of transparent Y₂O₃ was directed toward the lamp envelope application in competition with Al₂O₃. Interest in this application fell mostly because of the high cost of powder, but it remains viable for applications requiring the thermodynamic properties of Y₂O₃. The infrared window application has received considerable attention throughout the world in the last 10 years. Also, unique x-ray scintillators based on transparent Y₂O₃–Gd₂O₃ developed by Greskovich et al. [25] are extremely successful commercially.

The early work of Brissette et al. [12] and Lefever and Matsko [39] demonstrated that pure undoped Y₂O₃ could be fabricated to transparency by employing a combination of fine active powders and high-temperature press forging. More recently, Hartnett et al. [34] and Shibatta et al. [63] have taken a similar powder approach with hot isostatic pressing to fabricate transparent complex geometries. Success in these approaches relies not only on powder properties and the application of pressure to enhance densification kinetics, but also on high purity to prevent precipitation of second phases or coloration from transition or rare-earth ions.

Phase relations become critical to success when pressureless sintering is the fabrication mode chosen to attain transparency. A number of sintering mechanism options are available to ceramists in their quest to affect the densification and grain-growth control necessary to eliminate porosity and achieve transparency. These options include liquid-phase sintering, transient second-phase sintering, and doped solid-state sintering. All require accurate knowledge of solid solution limits, eutectics, and other phase relations governing microstructure development.

The phase field diagram of Foex and Traverse [18] is useful in evaluating possible sintering aids for Y₂O₃. Figure 1 shows that the rare-earth oxides are generally found in one of three structures, depending on cation size. The cubic, or C, structure is a distorted fluorite structure having octahedral cation coordination with 32 MO_1.5 groups per unit cell and a large vacancy in the center. Y₂O₃ has this structure since it has the same ionic radius (0.892 Å) as Ho (0.894), atomic number 67. The lower atomic numbers have larger ionic radii, ...

Table of contents

• Cover image
• Title page
• Table of Contents
• A VOLUME OF THE TREATISE ON MATERIALS SCIENCE AND TECHNOLOGY
• Copyright
• Dedication
• Contributors
• Preface
• Chapter 1: Phase Chemistry in the Development of Transparent Polycrystalline Oxides
• Chapter 2: The Use of Phase Diagrams to Predict Alkali Oxide Corrosion of Ceramics
• Chapter 3: Application of Phase Diagrams to the Production of Advanced Composites
• Chapter 4: Use of Phase Diagrams in the Study of Silicon Nitride Ceramics
• Chapter 5: The Use of Phase Studies in the Development of Whiskers and Whisker-Reinforced Ceramics
• Index