Refractory Oxides

Jeffrey D. Smith and William G. Fahrenholtz

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influ­ence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials.

1 Introduction

The term refractory refers to materials that are resistant to the effects of heat. Refractory oxides, therefore, are ceramic materials that can be used at elevated temperatures. These nondescript restrictions allow nearly any oxide to be classified as refractory. For this article, refractory oxides will refer, somewhat arbitrarily, to common crystalline compounds with melting temperatures of at least 1,800°C. These compounds can contain one or more metal or metalloid cations bonded to oxygen. As an introduc­tion to the topic, this section provides a brief historic overview of materials commonly used in the refractories industry, including some lower melting temperature materials. The section also reviews some current trends in the industries that produce and use refractory oxides. The other sections of this chapter focus on phase-pure oxide ceramics that can be used at elevated temperatures.

Historically, most of the oxides that were used in refractory applications were traditional ceramics prepared from clays or other readily available mineral-based raw materials. The major categories of traditional refractories are fire clays, high aluminas, and silica [1]. The choice of material for traditional refractory applica­tions, as with advanced material applications, was and is based on balancing cost and performance/lifetime. The ultimate use temperatures and applications for some common refractories are summarized in Table 1 [2, 3]. The production, properties, and uses of some of these materials are discussed in more detail in the other chapters

J. F. Shackelford and R. H. Doremus (eds.), Ceramic and Glass Materials: 87

Structure, Properties and Processing.

© Springer 2008

Class

Material

Phases

Use Temp (°C)

Applications

Fire clay

Low heat duty High heat duty

Mullite, glass, quartz Mullite, glass

Up to 1500

Kiln linings Crucibles

High alumina

Kyanite

a-Al2O3, mullite, glass

Up to 1800

Metal handling Lab ware

Silica

Silica

Tridymite,

cristobalite

1650

Glass tanks crowns

Table 1 Compositions, ultimate use temperatures, and applications for some common tradi­tional refractory materials

Refractory Oxides

Fig. 1 Historic production numbers for fireclay and high alumina (labeled kyanite) brick of this volume. A brief overview of fireclays, high aluminas, and silica is provided here followed by a description of the evolution of the refractories industry.

Although no strict geologic definition exists [4], fireclays can be defined as clay minerals that have pyrometric cone equivalent (PCE) values of 19 or greater following ASTM specification C24-01 (Standard Test Method for Pyrometric Cone Equivalent (PCE) of Fireclay and High Alumina Refractory Materials) [5]. Most refractory products are fabricated from what are considered high heat duty fire clays, which have a PCE value of 27 or higher (~1600°C). Fireclays have Al2O3 contents that range from 20 to 45 wt%, with silica being the other major constituent [6]. Because of their ease of fabrication, resistance to chemical attack, and low cost, fireclay bricks are still widely used as refractory materials. Applications for fireclay refractory brick include insulation behind hot-face materials, furnace linings, and specialty applications such as laboratory crucibles and setters. Historic consumption of fireclay was significantly greater when fireclay refractory brick demand from the U. S. steel industry was at its peak of ~10,000,000 metric tons in the early 1950s (Fig. 1). The decline in demand from the steel industry was due to changes that included higher use temperatures and a shift to
basic practices to improve steel cleanliness. The changing process requirements spurred the development of advanced refractory ceramics such as high alumina casta­bles and basic brick, both of which are prepared from highly beneficiated oxides rather than unrefined minerals. In the past quarter century, fireclay refractories have evolved from a state-of-the-art engineered material to a commodity item that often originates from countries having low labor costs.

Most high alumina refractories are clay-based ceramics to which an alumina-rich mineral is added to chemically react with a majority of the silica present to promote mullite formation [7]. High alumina refractories contain a minimum of 60 wt% Al2O3, although the Al2O3 content can be > 99% for specialty products. High alumina refrac­tories can be produced from fire clays used in combination with alumina-rich minerals such as diaspore or bauxite [8]. Reduction of the amount of free silica (consumed in the formation of mullite) results in increased use temperature for high alumina refrac­tories compared with fire clay refractories, up to 1800°C for some materials. The greater mullite content of high aluminas gives them improved creep resistance and better corrosion behavior. High alumina refractories were developed for steel industry applications that were beyond the performance limits of fireclay refractories. High alumina bricks continue to find use in a wide range of industrial applications including aluminum melting and incineration. Today, use of high alumina materials is approxi­mately equivalent to fireclays (Fig. 1).

Silica refractories can be crystalline or amorphous (fused). Most silica refractories are produced from silica-rich minerals such as quartz and flint and have SiO2 contents of 98 wt% or higher. For crystalline refractories, a mineralizer-like CaO is added to promote crystallization to cristobalite and/or tridymite thereby eliminating the displacive – phase transformation associated with the a to в quartz transition at 573°C. Displacive transformations are typically associated with substantial volume changes that can be quite destructive. Because of the relatively low theoretical density of silica (~2.3 g cm-3 for cristobalite and tridymite), silica bricks are often used to construct arched furnace crowns [8]. Unlike most ceramic materials, silica bricks are resistant to creep at elevated temperature allowing them to be used for extended durations at temperatures approaching the melting temperature. Thus, even though silica melts below the 1800°C limit considered in this article, it has been included because of its high use temperature. The recent trend in the glass industry to convert to oxy-fuel firing has decreased the usage of silica brick because higher temperatures and water vapor concentration in oxy-fuel fired glass hearths promotes alkali-induced corrosion of silica.

In the middle part of the twentieth century, the ceramics industry began a general shift from traditional ceramics toward more advanced (highly engineered) materials. Traditional ceramics are derived from minerals and can have significant variations in composition and performance depending upon the source of the raw material. Traditional ceramics also tend to contain significant amounts of glassy phases or impurities. In contrast, advanced ceramics are usually phase pure oxides that are derived from high-purity industrial chemicals. Advanced ceramics can be single phase or multiphase, but they are essentially phase pure meaning that they contain no significant (0.5 wt% or less) glassy phase or impurities. The cost of advanced ceramics compared with traditional materials created the need for application-specific composi­tions. Thus, advanced materials are implemented specifically where they are needed to optimize system performance. The selection of advanced materials is still driven by the performance-cost balance. Understanding materials performance and selecting the proper material for a particular application requires knowledge of material properties such as those discussed later in this chapter.

Even though many refractory oxides are engineered to optimize performance in a single application, any number of ceramics can be selected for a particular applica­tion. Examples of some of the oxides that can be used at high temperatures, along with their melting temperatures, are listed in Tables 2-5 for oxides containing one, two, or more cations [9-11]. It should be noted that consensus on the melting temperature of specific oxides is tenuous, so values should be considered as approximations; this is especially true in the case of oxides having melting temperatures well above 2000°C. These lists are not intended to be comprehensive (although Tables 2 and 5 contain all of the unary and ternary refractory oxides that the authors could identify), but the lists are long enough to emphasize that a large number of candidates exist for any applica­tion. Tables 3 and 4 are samplings from the hundreds of two component refractory oxides that are available.

From the larger list of binary refractory oxides, aluminate compounds are listed in Table 3 to emphasize that a family of materials that contain one compound with a high melting temperature will tend to form other compounds with high melting tempera­tures. Within the aluminate family, a number of compounds are formed that might not

Table 2 Melting temperatures of refrac­tory oxides containing a single cation

Oxide

Tm (°C)

A2O3

2020

BaO

1925

BeO

2570

CaO

2600

CeO2

2600

2400

CuO

1800

Eu2O3

2240

Gd2O3

2350

HfO2

2780

In2O3

1910

4O3

2315

MgO

2800

MnO

1815

NbO2

1915

^3

2275

NiO

1960

Sc2O3

2450

Sm2O3

2310

SrO

2450

Ta2O5

1875

ThO2

3250

TiO2

1850

Ti2O3

2130

UO2

2750

U2O3

1975

Y2O3

2400

Yb2O3

2375

ZnO

1975

ZrO2

2700

Table 3 Melting temperatures of selected aluminates

Oxide

Tm (°C)

BaO-Al2O3

2,000

BeO-Al2O3

1,910

CaO-6Al2O3

1,850

CeO-Al2O3

2,070

CoO-Al2O3

1,955

FeO-Al2O3

1,820

K2O-Al2O3

2,260

La2O3-Al2O3

2,100

Li2O-5Al2O3

1,975

MgO-Al2O3

2,135

Na2O-11Al2O3

2,000

NiO-Al2O3

2,020

SrO-Al2O3

1,960

m-A^

1,940

ZnO-Al2O3

1,950

Table 4 Melting temperatures of barium-containing binary refractory oxides

Oxide

Tm (°C)

BaO-Al2O3

2,000

3BaO-2Dy2O3

2,050

2BaO-GeO2

1,835

6BaO-Nb2O5

1,925

BaO-Sc2O3

2,100

2BaO-SiO2

1,820

BaO-ThO2

2,300

2BaO-TiO2

1,860

BaO-UO2

2,450

3BaO-2Y2O3

2,160

BaOZrO2

2,700

Table 5 Melting temperatures of ternary refractory oxides

Oxide

Tm (°C)

2CaO-Y2O3-Al2O3

1,810

Na2O-9Y2O3-12SiO2

1,850

2CaO-Gd2O3-Al2O3

1,830

3Ga2O3-2Sc2O3-3Al2O3

1,850

ZnO-ZrO2-SiO2

2,080

normally be expected to be refractory such as those containing potassium oxide, sodium oxide, and even lithium oxide. Individually, oxides such as Li2O, Na2O, and K2O would never be considered refractory, but combined with aluminum oxide they form refractory compounds.

The binary oxides listed in Table 4 were intended as a compilation that is similar to what was presented in Table 3. However, in this case barium oxide was chosen as one component of the binary system. Barium oxide is refractory (Table 2) and forms binary refractory compounds in a number of different families, include aluminates, silicates, titanates, and zirconates. Although not absolute, it is common for an oxide that is refractory in one family of oxides to be refractory in others as well.

Looking toward the future, it is likely that the current trends in production and use of high temperature materials will continue. The users of high temperature structural materials continually push for higher use temperatures and improved component life­time. As use temperatures increase, it is likely that alternate materials that are now considered exotic will have to be developed; this development will be application specific and will occur at a rate that often lags the rate of process development. Consider the thoughts of a steel mill operator from the early 1900s if he had been told that in 50 years his plant would use basic refractories costing orders of magnitude more than fireclay brick. Other developments that are likely in the refractory materials field are the increased use of multiphase materials and coatings. Both technologies offer the promise of unique combinations of physical and mechanical properties that are not available in single-phase materials. For example, a multiphase engineered material could be constructed to have the wear resistance of a hard ceramic with the thermal conductivity and thermal shock resistance of a metal. The possible combina­tions of properties are nearly endless, but development of these materials requires knowledge of interactions at bimaterial interfaces, tailoring of thermal expansion coefficients, and development of cost-effective processing routes.

The purpose of this chapter is to describe the properties and applications for refractory oxides. The sections that follow describe applications, review fundamental chemical and physical aspects, introduce processing methods, list important physical properties, and discuss materials selection criteria for refractory oxides. The organization of this chapter reflects that the performance of ceramic materials depend on interrelation­ships among structure, processing, and inherent properties.