Background

The unique properties found in yttria-stabilized zirconia fiber products have extended their usefulness into applications too corrosive and demanding for other varieties of fibrous ceramic materials. Their characteristics, properties and techniques utilized to make them, are discussed. The availability of these highly stable ceramic fiber materials as well as certain corrosive and demanding applications for them are reviewed.
Yttria Stabilized Zirconia Fiber
Characteristics and Properties

Products made of yttria-stabilized zirconia fiber have exceptional resistance to most corrosive environments. They undergo little attack by molten alkali metal chlorides and carbonates at temperatures as high as 700°C (1300°F) and to aqueous solutions of alkali metal hydroxides at temperatures as high as 230°C (450°F). In addition to being able to withstand short term exposure to mineral acids at their boiling point fibrous yttria-stabilized zirconia has outstanding resistance to oxidizing, reducing and vacuum atmospheres at high temperatures. A high density vacuum formed product, ZIRCAR Zirconia Insulation Type FBD can withstand oxidizing and reducing conditions to temperatures as high as 2200°C (3992°F).
Characteristics of ZIRCAR Zirconia Fiber Type ZYBF-2

Property


Value

Color


White

Diameter


4-6µm

Length, mean mm (in)


1.6 (1/16)

Density


92-97% theoretical
5.6-5.9g/cm3

Crystal Phases


Cubic + tetragonal

Yttria stabilizer content


8 wt%

Composition


99+% (ZrO2+HfO2+Y2O3)

Melting Point


2590°C (4700°C)

Volatile content (0-1000°C)


0-1%
Manufacture

ZIRCAR Zirconia Fiber Type ZYBF-2 is manufactured using a unique precursor process wherein an organic host is converted into an inorganic relic of the original material. Woven and non-woven zirconia textiles are made using the same process. Rigid insulating products are made by combining bulk fiber with various binders into a slurry which is then consolidated by vacuum forming. ZIRCAR Zirconia Fiber Type ZYBF-2 provides significant advantages over other ceramic fibers due to its great refractoriness, resistance to chemical attack and low thermal conductivity.

AZoM - Metals, Ceramics, Polymer and Composites Article - View of ends of ZIRCAR Zirconia Fiber Type ZYBF-2

Figure 1. View of ends of ZIRCAR Zirconia Fiber Type ZYBF-2.

Zirconia Bulk fibers are relatively short, suitable for vacuum forming by themselves, for ultrahigh temperature use or blending into lower temperature fiber mixtures to increase their refractoriness.

AZoM - Metals, Ceramics, Polymer and Composites Article - ZIRCAR Zirconia Fiber Type ZYBF-2

Figure 2. ZIRCAR Zirconia Fiber Type ZYBF-2.
Applications

When blended into mixtures of lower temperature ceramic fibers, Zirconia Bulk Fiber Type ZYBF-2 increases the refractoriness and chemical stability of the fiber system.
Zirconia Cloth Battery Separators

ZIRCAR Zirconia Cloth Type ZYK-15H , another unique material, has become the product of choice for use as battery separators in aerospace nickel hydrogen and nickel cadmium batteries. Zirconia fiber cloth products posses numerous unique characteristics and properties which make them ideal for use in batteries. These include their chemistry, thickness, porosity, physical strength, electrolyte retention and resistance to attack by various electrolytes. Type ZYK-15H exhibits virtually no loss in strength after prolonged exposure (1000+ hrs.) to hot(100°C) KOH.

AZoM - Metals, Ceramics, Polymer and Composites Article - ZIRCAR Zirconia Cloth Battery Separators are die cut in many shapes

Figure 3. ZIRCAR Zirconia Cloth Battery Separators are die cut in many shapes.

To date ZIRCAR Zirconia Cloth Battery Separators have logged millions of hours of space time –without any failures.
Molten Carbonate Fuel Cells

Another application where fibrous zirconia’s chemical stability and resistance to attack make it useful where other materials fall short include the use of ZIRCAR Zirconia Felt Type ZYF-100 in Molten Carbonate Fuel Cells. In this gasketing application the mechanically interlocked zirconia fiber is exposed to high concentrations of hot LiCO3 and shows no signs of degradation. ZYF-100’s compressibility and flexibility enable it to conform to irregular surfaces and to accommodate thermal expansion of various fuel cell components.
Burner Mantles

ZIRCAR Zirconia Felt Type ZYF is also used as the burner mantle in diesel and gasoline fueled heaters used in luxury automobiles and heavy trucks. In addition to having the proper uniform porosity for even distribution of the fuel its resistance to attack by lead oxide is what makes this material so useful in this application.
Thermal Insulation in High Temperature Crystal Growth Applications

ZIRCAR Zirconia Felt Type ZYF-100 is also utilized as thermal insulation in numerous high temperature crystal growth applications. In these systems corrosive salt vapors can attack other insulation materials making the desired thermal gradient difficult to achieve.

A number of ZIRCAR fibrous zirconia materials were used as primary hot face and load bearing components of a Crystal Growth Furnace (CGF) which flew on two space shuttle missions. The chemical compatibility and resistance to corrosion, low thermal conductivity, machineability, thermal shock resistance and mechanical strength of rigid ZIRCAR fibrous zirconia products all contribute to the reason NASA specified them for use in the United States Microgravity Laboratory (USML). All of these fibrous zirconia products with stood the vibration and G forces of take off and landing - both very mechanically corrosive conditions.

AZoM - Metals, Ceramics, Polymer and Composites Article - ZIRCAR Zirconia Insulation - furnace components used in CGF on two Space Shuttle Missions include Types FBD, ZYC and ZYZ-3 ranging in density from 30 pcf to 90 pcf.

Figure 4. ZIRCAR Zirconia Insulation - furnace components used in CGF on two Space Shuttle Missions include Types FBD, ZYC and ZYZ-3 ranging in density from 30 pcf to 90 pcf.

USML-1 flew on the Space Shuttle Columbia (mission STS-50) June 25 to July 9, 1992. The CGF successfully performed experiments involving the growth of cadmium zinc telluride, selenium doped gallium arsenide and mercury cadmium telluride crystals. USML-2 flew on the Space Shuttle mission STS-73) from September 28 to October 14, 1995. The CGF was used to conduct several directional solidification and vapor crystal growth experiments.
Thermal Insulation Products

At elevated temperatures fibrous zirconia insulation products offer the lowest thermal conductivity of all currently available ceramic fiber materials. As with most ceramic fibers, the thermal conductivity of fibrous zirconia is strongly dependent on their bulk density. At high temperatures, where radiation is the primary mechanism of heat transfer, high bulk densities of zirconia fiber provides the most effective insulation. At 1650°C to 1930°C minimal thermal conductivity is obtained at 30 pcf density.
Microstructure of a Vacuum-Formed Fibrous Ceramic Insulation.

The tightly bonded structure which results from the uniform distribution of the binder in these rigid products yields strong fibrous insulating bodies. These products are available in flat boards, cylinders and custom configurations. They can also be machined to exacting dimensional tolerances. Tolerances of +/-.005” are obtainable with CNC equipment.
Conclusion

ZIRCAR’s yttria-stabilized fibrous zirconia insulation products are useful in many demanding applications too corrosive for other ceramic fiber products. These zirconia products offer resistance to chemical attack, high refractoriness and lower thermal conductivity at elevated temperatures than other fibrous ceramic products. ZIRCAR’s alumina-bonded alumina fiber products are stable in reducing conditions and are effective thermal insulators in many corrosive environments to temperatures as high as 1800°C.