Aluminates

Martin C. Wilding

Abstract Aluminates form in binary systems with alkali, alkaline earth or rare-earth oxides and share the high melting point and resistance to chemical attack of the pure Al2O3 end-member. This means that these ceramics have a variety of applications as cements, castable ceramics, bioceramics, and electroceramics. Calcium aluminate cements are used for example in specialist applications as diverse as lining sewers and as dental restoratives.

Ceramics in aluminate systems are usually formed from cubic crystal systems and this includes spinel and garnet. Rare earth aluminate garnets include the phase YAG (yttrium aluminium garnet), which is an important laser host when doped with Nd(III) and more recently Yb(III). Associated applications include applications as scintillators and phosphors.

Aluminate glasses are transparent in the infrared region and these too have specialist applications, although the glass-forming ability is poor. Recently, rare earth aluminate glasses have been developed commercially in optical applications as alternatives to sapphire for use in, for example, infrared windows.

Aluminates are refractory materials and their synthesis often simply involves solid – state growth of mixtures of purified oxides. Alternative synthesis routes are also used in specialist applications, for example in production of materials with controlled porosity and these invariably involve sol-gel methods. For glasses, one notable, commercially important method of production is container-less synthesis, which is necessary because of the non-Arrhenius (fragile) viscosity of aluminate liquids.

1 Introduction

Glasses and ceramics based on the Al2O3-based systems have important applications as ceramic materials, optical materials, and biomedical materials. Aluminate materials include alkaline earth aluminates, such as those in the CaO-Al2O3 system, which are refractory cousins of hydrous Portland cement [1-3]. Calcium aluminates have a role as both traditional ceramic and cement materials and are used for example as refractory cements; however, calcium aluminates are also important for more novel applications

J. F. Shackelford and R. H. Doremus (eds.), Ceramic and Glass Materials: 49

Structure, Properties and Processing.

© Springer 2008

such as bioceramics [4-8]. Rare earth (yttrium and the lanthanides) aluminates are important laser host materials. Yttrium aluminum garnet (YAG) is one of the most common laser hosts; Nd-doped YAG lasers with powers of up to 5 kW are important for welding and cutting applications and have the further advantage of being solid state, the primary laser component being a single crystal of Nd-doped YAG. Associated with the laser properties of YAG are the materials characteristics of rare earth aluminates, which favor applications as refractory ceramics, composite laser hosts, and glass fibers that are important for optical applications, but also can be used in composite materials [9, 10].

Many of the important desirable properties that make aluminates important in materials science are similar to those of the end-member Al2O3. This includes the refractory nature of aluminates, for example Al2O3 melts at 2,054°C and other impor­tant aluminates have similarly high melting points (Table 1). In addition, aluminates have high hardness, high strength, and are resistant to chemical attack. Al2O3 and both calcium and rare earth aluminate systems can have useful properties such as transparency in the infrared region, and this makes aluminate glasses important for use as optical fibers. Because of their optical applications, aluminate glasses have been studied extensively and as a consequence some very unusual and anomalous thermodynamic properties have come to light.

The refractory nature of aluminates means that high temperature synthesis techniques are required. Depending on the application, aluminates can be made by mixing of oxides and subjecting the mixtures to high temperature, as for example in the manufacture of cement. For other applications, such as optical uses, more exotic techniques are used. These include high temperature melting, single crystal growth [11, 12], container-less synthesis of glasses using levitation [13], and low-temperature routes such as sol-gel synthesis [14, 15] and calcining.

There are a variety of important crystal structures in aluminate systems. Among the most important are the spinel [16] and garnet structures [17, 18]. These various structures reflect differences in the coordination polyhedron of both Al(III) and added components such as Mg(II), Ca(II), and the rare earth ions. In addition, studies of glass structure suggest a wealth of different coordination environments for both Al(III) and added components and structures that are not simply disordered forms of crystalline phases.

For the purposes of this review, aluminates can be defined as a binary section of a ternary oxide system with Al2O3 as one component. A large number of different alumi – nates can be made and it is not the purpose of this chapter to provide an exhaustive list of each different aluminate type or each application. Rather, it is the purpose of this chapter to provide a survey of the range of binary Al2O3-systems and to demonstrate the diversity of both their applications to materials science and to elaborate on the unusual

Table 1 The physical properties of selected binary aluminate ceramics

Melting

temperature

(K)

Density (g cm-3)

Hardness (Knoop/100 g) (Kg mm-2)

Compressive

strength

(MPa)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

a-Al2O3

2,327

3.98

2,000-2,050

2,549

255

393

CaAl2O4

2,143

2.98

MgAlA

2,408

3.65

1,175-1,380

1,611

129

271

LiAlO2

1,883

2.55

350

YsA^U

2,243

4.55

1,315-1,385

280

282

structural and thermodynamic properties of crystalline and glassy aluminate materials. Three subsets of aluminates will be highlighted: binary alkaline earth aluminates (CaO, MgO-Al2O3), which includes calcium aluminate cements and magnesium aluminate spinels, alkali (lithium) aluminates, which potentially have very important applications in the development of new types of nuclear reactors [19], and rare earth aluminates, particularly compositions close to that of the yttrium aluminum garnet (Y3Al5O12; YAG). Each section will discuss the applications, structure, and synthesis of each composition, and finally the thermodynamic and structural properties of these aluminates will be com­pared and summarized.