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Oxide Fibers for High-Temperature Reinforcement and Insulation

Posted on: Thursday, 24 February 2005, 03:00 CST

Oxide fibers find many uses as insulation and as reinforcements. The most widely known oxide fiber is glass fiber, which has a composition based on silica (SiO^sub 2^), but other minerals are added to control their characteristics. One such addition is alumina (Al^sub 2^O^sub 3^), which increases mechanical properties and also resistance to high temperature. Al^sub 2^O^sub 3^ occurs in many transitional forms, all of which are converted to α-Al^sub 2^O^sub 3^ if heated above around 1,200C. A range of fibers exists with compositions consisting of both Al^sub 2^O^sub 3^ and SiO^sub 2^, and a number of manufacturing processes are used to produce them.

INTRODUCTION

Oxide materials, which can possess a Young's modulus of up to 400 GPa, are usually brittle and hard. These characteristics seem to be diametrically opposed to the flexible behavior of fibers but, if made with sufficiently small diameters, even the stiffest material can be made into a supple filament. This is because the flexibility of a filament is related to the reciprocal of the diameter to the third power. The fibers discussed in this article possess diameters of the order of 10 m and often are finer, although, in most cases, health considerations impose a lower limit of around 3 m. This means that many of the continuous-oxide fibers that will be considered can be woven.

GLASS FIBERS

In addition to silica (SiO^sub 2^), glass fibers contain CaO, alumina (Al^sub 2^O^sub 3^), and other oxides added to the composition to control mechanical properties and the melting point. The fibers are usually drawn through a spinneret from the melt, which is around 1,300C, and produced as a continuous roving containing several hundred filaments. Table I shows the compositions of a variety of glass fibers used as reinforcements. ?-type glass is the most widely used glass reinforcement. The other two are higher- performance reinforcements with higher moduli and strengths because of their higher Al^sub 2^O^sub 3^ content. This increases the viscosity of the molten glass, however, and so increases the difficulty of manufacture.

Glass fibers are produced with arange of diameters that are controlled by the size of holes in the platinum-rhodium alloy spinneret and the drawing rate used. A typical diameter of a glass fiber is 14 m. The cost of glass-fiber production is sensitive to the purity of the raw materials, for which only very small amounts of iron are desired, for example, and to the use of expensive batch materials, such as materials containing boron oxide and sodium oxide. ' Table II gives some mechanical characteristics of the glass fibers shown in Table I.

Drawing takes place at high speed. As the glass leaves the spinneret, it is cooled by a water spray so that by the time it is wound onto a spool its temperature has dropped to around 200C in between 0.1 s and 0.3 s. An open atomic network results from the rapid cooling, and the structure of the glass fibers is vitreous. There are no appreciable residual stresses within the fiber and the structure is isotropic. The glass fibers have slightly lower densities than the equivalent bulk glass. The difference is approximately 0.04 g/cc.

Many varieties of glass fiber exist. Continuous fibers are produced as described and can then be wound onto bobbins for further processing, often by traditional textile techniques, or used to make random gl as s fi ber mats that can be impregnated with a thermosetting resin to make a structural composite material. Alternatively, they can be cut into short lengths before being combined with a resin to produce a molding compound. They can also be included in a thermoplastic matrix and then cut into short pellets for injection molding. Insulating glass wool is made by pouring the molten glass onto a rotating plate made of a nickel- based alloy that spins the glass to its periphery, where it is ejected through small holes. Glass wool is widely used for roofing insulation.

DISCONTINUOUS OXIDE FIBERS

Melt-Spun Aluminosilicate Fibers

Short melt-spun aluminosilicate fibers are made by the melting, blowing, or spinning of calcinated kaolin clay or a combination of Al^sub 2^O^sub 3^ and SiO^sub 2^. Oxides such as zirconia, ferric oxide, magnesium oxide, calcium oxide, and alkalines may be added.

The starting material is melted at around 2,000C by passing an electric current through it. The molten ceramic is poured into a stream of compressed air which carries the ceramic with it, resulting in drawing. The molten ceramic has to be viscous but needs a low surface tension to be drawn into fiber form; even so, a considerable fraction of the ceramic is not drawn and is known as 'shot.' Turbulence breaks the filaments, which are formed into discontinuous lengths with irregular cross sections and a mean diameter of 2.5 m to 3.5 m. The need for a low surface tension restricts the Al^sub 2^O^sub 3^/SiO^sub 2^ratio to an upper limit of 60/40; pure Al^sub 2^O^sub 3^ is not drawn into filament form if produced by this technique.2 Table III gives some characteristics of commercially produced melt-spun aluminosilicate fibers. The usual starting material for production is kaolin, also known as china clay. It is a natural form of hydrated aluminum silicate (Al^sub 2^Si^sub 2^O^sub 5^(O)^sub 4^). An alternative route is to use mixtures of Al^sub 2^O^sub 3^ and SiO^sub 2^. The fibers are known collectively as aluminosilicate refractory ceramic fibers or simply RCFs. The progressive replacement of SiO^sub 2^ by Al^sub 2^O^sub 3^ improves their refractory characteristics but makes manufacture more difficult.

When natural kaolin is the starting material, there is a possibility of contamination by alkaline oxides (Na^sub 2^O and K^sub 2^O), which has a detrimental effect on the thermal insulation of the fibers produced. These alkaline contaminants can combine with the SiO^sub 2^ in the fiber to form a low-melting-point phase, which can cause failure of the fibers. The presence of vanadia (V^sub 2^O^sub 3^) can further exacerbate this problem by reacting with the alkaline contaminants. Increased thermal resistance is achieved by increasing the Al^sub 2^O^sub 3^ content, although the increased difficulty results in a higher shot content as well as a higher density. Both of the former materials can suffer from significant shrinkage upon heating above 1,200C due to temperature-induced phase changes. The addition of chromia (Cr^sub 2^O^sub 3^) retards these phase changes and reduces shrinkage.

All of these fibers are subject to changes in their microstructures at high temperature, which ultimately limits their use. Al^sub 2^O^sub 3^ and SiO^sub 2^ will combine to form mullite at around 970C; crystoballite, which is a crystalline form of SiO^sub 2^, forms at around 1,260C. These changes will occur progressively, causing the mullite grains to grow, which reduces the flexibility of the fibers. Eventually the fibers fuse together and the product becomes brittle. Shrinkage of the fiber structure also occurs when crystallization is initiated. The higher the Al^sub 2^O^sub 3^content, the greater the shrinkage.

An alternate production route feeds the molten ceramic onto a rapidly rotating disk, or series of disks, from which short fibers are thrown by centrifugal force. It produces longer fibers with a slightly larger diameter (3-5 m) than the first process, which, however, is more common. Both techniques produce fibers of great variability in diameter-generally 1-8 m-and lengths-up to several centimeters-andaconsiderable fraction of non-fibrous shot. Shot is undesirable as it does not contribute to the strength and insulation properties of the product. It is of irregular shape and size and is considerably larger than the fibers formed, ranging from tens to several hundred micrometers. Shot content can be reduced to less than 25% by sifting with a 212 m mesh.

The range of compositions of meltspun aluminosilicate fibers is 45-60 wt. % Al^sub 2^O^sub 3^ with Al^sub 2^O^sub 3^SiO^sub 2^ as the other maj or component, with minor amounts of Fe^sub 2^O^sub 3^, TiO^sub 2^, CaO, and other oxides.3 The limit to the composition is the resistance of the material to devitrification of the glass with, for example, the nucleation and growth of mullite (3Al^sub 2^O^sub 3^.2SiO^sub 2^), which reduces strength dramatically. Strength at temperature increases with Al^sub 2^O^sub 3^ content so that some compositions have 52 wt.% Al^sub 2^O^sub 3^, for use as an insulation up to 1,250C. The highest levels of Al^sub 2^O^sub 3^ allow insulation blankets to be produced for use up to 1,400C. Small additions of Cr^sub 2^O^sub 3^ improve temperature resistance.

The Sol-Gel Process

The Al^sub 2^O^sub 3^ content of Al^sub 2^O^sub 3^ silicate fibers can be increased up to 100% by the use of sol-gel processes. Although this is more expensive than the meltdown process, greater control of the final product is possible. Another advantage is that the precursor is spun at low temperatures before being pyrolyzed. A considerable number of companies have made fibers using the sol-gel route. ICI developed a short fiber with a diameter of 3 m called Saffil in 1974.4 The Saffil fiber, now produced by the company \of the same name, contains 4% SiO^sub 2^ and is produced by the blow extrusion of partially hydrolyzed solutions of some aluminum salts with a small amount of SiO^sub 2^. The fiber contains mainly small δ-Al^sub 2^O^sub 3^ grains of around 50 nm but also some α- Al^sub 2^O^sub 3^ grains of 100 nm. The widest use of the Saffil- type fiber in composites is in the form of a mat that can be shaped to the form desired and then infiltrated with molten metal, usually aluminum alloy. It is one of the most successful fiber reinforcements for metal-matrix composites.

For refractory insulation applications, heat treatments of the fiber above 1,000C induce the δ-Al^sub 2^O^sub 3^ to progressively change into α-Al^sub 2^O^sub 3^. After 100 h at 1,200C, or 1 h at 1,400C, acicular α-Al^sub 2^O^sub 3^ grains can be seen on the surface of the fiber and mullite is detected. After 2 h at 1,400C, the transformation is complete and the equilibrium mullite concentration of 13% is established. Shrinkage of the fiber and, hence, dimension of bricks are controlled up to at least 1,500C.3

Table I. The Chemical Composition of Three Types of Glass Fiber Used as Reinforcements

Table II. Properties of Glass Fibers

CONTINUOUS FIBERS FOR REINFORCEMENT

Continuous oxide fibers for hightemperature applications are based on Al^sub 2^O^sub 3^ combined with other minerals, often SiO^sub 2^, but also other minerals such as zirconia and mullite.

Precursors of Al^sub 2^O^sub 3^ can be obtained from viscous aqueous solutions of aluminum salts. The precursor gel fibers are spun and then dried and heat treated. This causes the precipitation of aluminum hydroxides such as boehmite (AlO(OH)) and the outgassing of large volumes of residual compounds. The associated volume change and porosity at this step has to be carefully controlled if useful fibers are to be produced. It is also possible to spin directly aqueous sols based on aluminum hydroxide. Heating the precursor fibers induces the sequential development of transition phases of Al^sub 2^O^sub 3^ which, if heated to a high enough temperature, all convert to α-Al^sub 2^O^sub 3^. Above 400C and up to around 1,000C, transitional phases of Al^sub 2^O^sub 3^ are produced with grain sizes of 10 nm to 100 nm. Above 1,100C, α-Al^sub 2^O^sub 3^ is formed. However, this transformation is followed by a rapid growth of porous α-Al^sub 2^O^sub 3^ grains of micrometer sizes and above. It is essential that this rapid grain growth be controlled or retarded if fibers with useful properties are to be obtained. Applications of Al^sub 2^O^sub 3^ fibers above 1,100C require that the nucleation and growth of the α-Al^sub 2^O^sub 3^ grains be controlled and porosity limited. This is achieved by either adding to the fiber precursors, SiO^sub 2^ precursors, or seeds for α-Al^sub 2^O^sub 3^ formation.

Table III. Examples of Melt-Spun Short Aluminosilicate Fibers Used for Thermal Insulation

Al^sub 2^O^sub 3^ is most commonly combined with SiO^sub 2^, which has the effect of controlling and retarding grain growth. The fiber produced has a reduced Young's modulus, which can be an advantage as the fibers can be more easily handled. The Young's modulus of SiO^sub 2^ is 70 GPa whereas that of α-Al^sub 2^O^sub 3^ is 400 GPa, so it can be seen that the greater the amount of SiO^sub 2^ in the fiber, the lower its stiffness. There is no similar relationship for fiber strength, however, which is much more dependent on grain size. The smaller the grains making up the fiber, the higher its strength. The susceptibility of the fiber to creep at high temperatures, however, is also increased. Al^sub 2^O^sub 3^, which is intimately associated with SiO^sub 2^, is partially converted to mullite when heated to around 1,200C, and this can have a range of compositions from 2Al^sub 2^O^sub 3^. SiO^sub 2^ to 3Al^sub 2^O^sub 3^.2SiO^sub 2^. The interatomic bonds governing creep in Al^sub 2^O^sub 3^, which are ionic and covalent, lead to creep at temperatures above 1,000C. The controlled production of mullite can limit creep as its complex crystalline structure offers few easy slip planes and sliding of the structure is hindered. The need to reduce creep at high temperatures has led to the development of fibers combining both Al^sub 2^O^sub 3^ and another phase, such as mullite or zirconia. Zirconia is added as grains of around 0.1 m which pin the crystalline structure, hindering creep at least up to 1,100C.

The first Al^sub 2^O^sub 3^-based continuous fiber is sold under the name Nextel 312 by 3M. it contains 62% Al^sub 2^O^sub 3^, together with boria and SiO^sub 2^. With an essentially amorphous structure, it is limited to use below 1,000C because of the volatility of boria. However, it remains the foundation of the 3M Nextel range of oxide fibers. In the same decade, DuPont developed a continuous α-Al^sub 2^O^sub 3^fiber, called Fiber FP, by spinning a slurry composed of an aqueous suspension of α- Al^sub 2^O^sub 3^ particles and aluminum salts. The fiber was then dried and fired. The fiber was not developed commercially as it proved to be too brittle.

Some improvement was obtained by adding 20 wt.% zirconia as a second phase, which reduced the grain size of the Al^sub 2^O^sub 3^ phase and also went some way to stabilizing the fiber above 1,000C. Again, the fiber was not taken to production. During the 1980s and 1990s, a number of companies developed oxide fibers that overcame the difficulties encountered by the fibers produced by DuPont. Sumitomo Chemicals produces the continuous Altex fiber in which the 15% amorphous SiO^sub 2^ stabilized the Al^sub 2^O^sub 3^ grains in theyphase, which means the grain size was only 25 nm.5 The Altex fiber has only half the Young's modulus of a pure, dense α- Al^sub 2^O^sub 3^ fiber and so can be more easily handled and woven. It was developed as a reinforcement for aluminum alloys and retains its properties up to around 900C, but strength falls rapidly at higher temperatures. Mitsui Mining produces the Almax fiber, the composition and grain size of which are very similar to those of the Fiber FP. However, the diameter of Almax, at 10 m, is only half the diameter of the latter.6 The reduction in diameter means an eight- times increase in flexibility, so the fiber can be woven. The fiber contains considerable porosity and residual stresses, which allow grain growth to occur on heating above 1,000C. Grain growth and creep by grain boundary sliding limit this fiber and others to temperatures 1 ower than 1,100C. Later, 3M produced Nextel 610, which has the same diameter as that of the Almax fiber but with grain sizes of around 0.1 m that double the fiber strength.8 The fibers were examined for potential applications at higher temperatures because of their lack of reactivity in air, but all were found to creep rapidly above 1,000C. At 1,200C, Nextel 610, subjected to a stress of 1 GPa, creeps at a rate of about 8 10^sup - 5^S^sup -1^. None of the fibers mentioned so far retain useful mechanical properties above 1,200C. Some improvements to the creep behavior and high-temperature strength retention of α-Al^sub 2^O^sub 3^ fibers have resulted from adding zirconia to their composition but neither the DuPont PRD-166 fiber nor the 3M Nextel 650 fiber were taken to market.7 The properties of many of the oxide continuous fibers that are commercially available are shown in Table IV.8

3M has produced a range of oxide fibers with increasingly high performance properties. The sol-gel process used to produce Nextel 312 was modified to produce Nextel 440. The composition of 3 mol of Al^sub 2^O^sub 3^ for 2 mol of SiO^sub 2^ was maintained but the boria content was reduced to increase its high-temperature stability. Nextel 440 is formed of nanosized γ-Al^sub 2^O^sub 3^ grains in an amorphous SiO^sub 2^ phase. The fiber has been used to reinforce mullite for applications up to 1,200C. The 3M Nextel 720 fiber is made up of aggregates of mullite grains in which are embedded α-Al^sub 2^O^sub 3^ grains.9 This gives the Nextel 720 fiber the lowest creep rate of any oxide fiber at temperatures above 1,000C.10 Under an applied stress of 1 GPa at 1,200C, its creep rate was 7 10^sup -8^S^sup -1^. The fiber is, however, sensitive to alkaline contamination above around 1,100C, which induces large aAl^sub 2^O^sub 3^ grain growth and a subsequent fall in strength.11 The mechanisms involved are the production of a low melting point phase by the combination of the alkaline contamination and the SiO^sub 2^ content in the mullite phase. This allows aluminum ions to diffuse rapidly from the mullite to the Al^sub 2^O^sub 3^ phase, which increases grain growth and reduces strength. This limits the fiber to temperatures below 1,200C.

Table IV. Properties and Compositions of Alumina-Based Fibers

The initial interest in small-diameter polycrystalline oxide fibers for potentially extending the temperature range of ceramic- matrix composites reinforced by small-diameter SiC fibers has been largely unfulfilled. Although the oxide fibers do not suffer from oxidation, as do the SiC fibers, they are inherently less mechanically stable above 1,000C. Whereas the covalent bonds in SiC resist creep, the ionic bonds in oxides allow easier movement of the structure. The complexity of the crystal structures of some oxides, such as mullite, does impart good inherent creep properties. Ultimately, however, grain boundary sliding and also the metastable state of some of the more complex systems means that oxide fibers are primarily limited to uses below 1,200C if they have to carry loads. Such oxide fibers find important roles in filters and as reinforcements for light alloys.

SINGLE-CRYSTAL, LARGE-DIAMETER OXIDE FIBERS

Removing grain boundaries by growing single-crystal oxide filaments from the melt either by heating the \ceramic in a crucible or by laser has been explored since the 1960s.12 It has been shown that such α-Al^sub 2^O^sub 3^ fibers with their C-axis aligned parallel to the fiber axis can resist creep up to 1,600C.13 Saphikon produced fibers composed of single-crystal α-Al^sub 2^O^sub 3^ and also yttrium aluminum garnet-Al^sub 2^O^sub 3^. However, due to the large diameters of 100 m and above, coupled with their high cost, so far they have yet to find an application. A cheaper process developed in Russia consists of infiltrating the molten oxide along channels formed by sandwiching molybdenum wires between sheets of molybdenum.14 When the filaments are formed, the molybdenum is etched away.

NANO-METRIC OXIDE FIBERS

Fibers are being developed with nano-metric sized diameters. It has been known since the 1950s that single-crystal filaments, of oxides and other materials, with micrometer-size diameters, can be grown.15 These filaments, which are known as whiskers, possess very high strengths because of the lack of defects that otherwise weaken larger-diameter fibers. Whiskers have diameters in the range of 0.5 m to 1.5 m and lengths that can range from tens of micrometers to centimeters. A technology that is still in the laboratory electrospins sol-gel precursors, which can then be pyrolysed to form even finer, nano-oxide fibers. Little is known about the properties that can be expected of such fibers. However, the reduction in grain size could lead to a remarkable increase in strength. Their development shows that the evolution of oxide fibers is far from over.

References

1. RK. Gupta, "Glass Fibers for Composite Materials," Fibre Reinforcements for Composite Materials, ed. A.R. Bunsell (Amsterdam: Elsevier, 1988), pp.19-71.

2. Eiji Horie, ed., Ceramic Fiber Insulation Theory and Practice (Osaka, Japan: The Energy Conservation Center, 1986), pp. 43-150.

3. J.D. Birchall, Concise Encyclopedia of Advanced Ceramic Materials, ed. RJ. Brook, (Oxford: Pergamon Press 1991 ), pp. 236- 238.

4. MJ. Morton, J.D. Birchall, and J.E. Cassidy, UK Patent Specification 1360197, 17 July 1974.

5. Y. Abe et al., Proc. 4th Int. Con. Composite Mater. (Tokyo, Japan: Japan Society for Composite Materials, 1982), p. 142.

6. Y. Saibow et al., Proc. 37 Int. SAMPE Symp. and Exhib., 35 (Covina, California: Society for the Advancement of Materials and Processing Engineering,1992), pp. 808-819.

7. A. Poulon-Quintin, M-H. Berger, and A.R. Bunsell, J. European Ceramics Society, 24 (2004), pp. 2769-2783.

8. A.R. Bunsell and M-H. Berger, Fine Ceramic Fibers (New York: Marcel Dekker, 1999), pp. 111-164.

9. D.M. Wilson, S.L. Lieder, and D.C. Lueneburg, Ceram. Eng. Sd. Proc. 16 (Westerville, Ohio: The American Ceramic Society, 1995), pp. 1005-1014.

10. F. Deleglise, M-H. Berger, and A.R. Bunsell, High Temperature Ceramic Matrix Composites, ed. W. Krenkel, R. Naslain, and H. Schneider (Weinheim, Germany: Wiley VCH, 2001), pp. 84-89

11. F. Deleglise et al., J. European Ceramic Society, 21 (2001), pp. 569-580.

12. H.E. Labelle, Jr., Method of Growing Crystalline Materials, U.S. patent No 3,591,348 (1971).

13. J.T.A. Pollock, J. Mat. Sci, 7 (1972).

14. S.T. Mileiko et al. Composites Sei. & Tech., 59 (1999), pp. 1763-1772.

15. M. Akiyama "Ceramic Fibres," Fibre Reinforcements for Composite Materials, ed. A.R. Bunsell (Amsterdam: Elsevier, 1988), pp. 427-478.

A.R. Bunsell is a professor at Ecole des Mines de Paris, Centre des Matriaux, Evry Cedex, France.

For more information, contact A.R. Bunsell, Ecole des Mines de Paris, Centre des Matriaux, BP 87, Evry Cedex, France; 33 (0) 160753015; e-mail anthony.bunsell@ensmp.fr.

Copyright Minerals, Metals & Materials Society Feb 2005

Source: JOM

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