Macroscopic Behavior

The macroscopic behavior of refractory oxides is controlled by both the bonding and crystal structure. In particular, the mechanical response and electrical behavior of materials are interpreted in terms of the symmetry of the constituent crystals using matrix or tensor algebra [27]. Other characteristics such as melting temperature and

Table 9 FCC-based crystal structures

Structure

Stoichiometry

Cation

coordination

Oxygen

coordination

Examples

Common

characteristics

Rock salt

MO

6

6

MgO, CaO, NiO, FeO

Fluorite

MO2

8

4

ZrO2, ThO2, CeO2

Oxygen ion conduction

Anti-fluorite

M2O

4

8

Na2O, Li2O, ^2°

Fluxes, prone to hydration

Perovskite

ABO3

A = 12, B = 6

6

PbTiO3, BaTiO3

High dielectric constant

Spinel

AB2O4

A = 4, B = 6

4

MgAl2O4,

MnFe2O4

High solid solubility

Table 10

HCP-based crystal structures

Structure

Stoichiometry

Cation

coordination

Oxygen

coordination

Examples

Common

characteristics

Wurtzite

MO

4

4

ZnO, BeO

Rutile

MO2

6

3

TiO2, MnO2,

Multiple cation oxidation states

Corundum

MA

6

4

Al2O3, Cr2O3

Highly refractory

Table 11 Melting temperature [9], thermal expansion coefficient (0-1,000°C) [1], thermal conductivity (25°C) [17], elastic modulus [60], and heat capacity for some common refractory oxides

Oxide

T

m

(°C)

CTE

(ppm per °C)

k

(W m-1 K-1)

E

(GPa)

Cp

(J mol-1 K-1)

Fused SiO2

1,460

0.5

2

72

42.2

Quartz

1,460

10.7

13

83

56.2

TiO2

1,850

7.3

8.4

290

36.9

3Al2O,-2SiO2

1,850

5.3

6.5

220

77.1

A^3

2,020

QO

QO

36.2

390

78.7

MgA^O4

2,135

7.6

17

239

324

ZrO2

2,700

10

2.3

253

55.1

MgO

2,800

13.5

48.5

300

115.8

stability at elevated temperature are not directional and, therefore, cannot be manipu­lated in the same manner. However, nondirectional properties are still affected by structure in that some crystal structures are inherently more resistant to change than others. For example, structures in which some crystallographic sites are unoccupied, such as spinel, have a much higher solubility for other cations than more close-packed structures like corundum.

Phase diagrams are perhaps the most powerful tool of the materials engineer who needs to choose oxide ceramics for use at high temperature. Phase diagrams are graphical representations of the phases that are stable as a function of temperature, pressure, and composition [28, 29]. Phase diagrams can be used to determine whether a particular compound melts at a specific temperature (congruent melting), decomposes to other compounds while partially melting (incongruent melting), or reacts with another component in the system. A wide variety of phase diagrams for oxide systems are available in various compilations [9-11]. When phase diagrams are not available, behavior can be predicted with at least moderate success, using commercial programs such as FACT-SAGE [30] or using the CALPHAD methodology [31].

Considering potential applications for refractory oxides, phase diagrams also provide useful information on interactions among materials at high temperatures that might limit performance in certain gaseous atmospheres or in contact with specific liquid or solid materials. Interactions can range from the formation of low melting eutectics to reactions that form new compounds. As an example of the former, consider the effect of impurities in SiO2. Pure SiO2 has an equilibrium melting temperature of 1713°C [1]. All SiO2, whether it is naturally occurring or prepared by other means, contains some impurities. If the presence of trace quantities of Na2O are considered, a liquid phase would form at ~800°C, the SiO2-Na2O2SiO2 eutectic [32]. For small impurity levels, the amount of liquid increases as the amount of the second phase increases. If sufficient liquid forms to cause deformation of the component, the use temperature of silica will be reduced drastically. Eutectic liquids form when the Gibbs’ energy released by mixing of the liquid components (entropic) overcomes the energy barrier (enthalpic) to melting of the unmixed solids.

Phase diagrams can also be used as an aid for material selection of oxide com­pounds that can be used at high temperature. Examination of diagrams (summarized in Tables 2-5) reveals that oxide compounds with melting temperatures above 1800°C are predominantly single metal oxides (e. g., Al2O3 or TiO2) or binary oxides (e. g., MgOAl2O3 or BaOZrO2). Very few ternary oxides have high melting temperatures. The complex site occupancies and arrangements necessary to accommodate three or more cations in a single crystal structure reduce the melting temperature of ternary compounds. Upon examining ternary phase diagrams, it becomes apparent that ternary eutectic temperatures are always lower than the three binary eutectic temperatures in the corre­sponding binary systems. As with the binary eutectic, addition of a third component drives the eutectic temperature lower since mixing of the liquid phase components becomes more energetically favorable as the number of components increases.

It is important to distinguish between melting temperature and melting range, as the former is a fundamental property of an oxide, while the latter is a macroscopic behavior that dictates use conditions and tolerable impurity limits. Melting temperature is fairly easily understood requiring little more than observing melting of an ice cube (solid H2O). However, only very pure substances exhibit a true melting temperature. Practical materials, except for the most pure versions (devoid of significant levels of impurity), exhibit a melting range that is defined by the macroscopic environment in which the materials exist.

In a binary combination of two oxides (e. g., alumina and silica), small additions of the second oxide result in the onset of melting at a eutectic temperature that is below the melting temperature for the pure components. For the alumina-silica system, two eutectic compositions exist depending on the overall chemistry of the mixture. For the silica-rich eutectic, all compositions between ~1 wt% and ~70 wt% alumina have an identical temperature for the onset of melting; only the amount of liquid formed will vary with composition. This temperature defines the low end of the melting range, while the temperature at which all of the material is molten (i. e., the liquidus tempera­ture) defines the high end.

Melting range can have a profound impact on performance as liquid formation can lead to shrinkage of the refractory, reaction with the contained product, high tempera­ture softening and flow (especially under pressure), etc. The viscosity behavior of the liquid itself is also important as highly viscous fluids behave very similarly to solids so considerably more can be present before problems occur.