Space Shuttle Tiles

In spite of the difficulties this technique of multiple deionizations and stabilization with ammonia is the technique used to generate the high purity silica needed for the insulating tiles that protect the space shuttle from burning up on re-entry. We will discuss the use in the space shuttle tiles below. 

The presence of alumma in the surface of colloidal sdica particles not only provides for charge sites independent of pH but also inhibits the conversion to cristobalite. Hence it is possible to add alumina to the surface of colloidal silica by reacting with sodium aluminate. The manufacture of such products is discussd above in the first part of this chapter. After alumina is added to the particle surface, the particles are deionized to remove the sodium. This provides a product that is both stable, low in sodium, and has a pH of 3-5. Adjusting the pH with ammonia is not necessary. The product is stable at the lower pH that results from being deionized because the alumina provides sufficient charge density to avoid coagulation. If mixed bed deionization is used then the absence of salts further enhances the stability of such a product. 

It should be pointed out however, that the green bonds of such a product are not as strong as sodium stabilized products. The sodium catalyzes the formation of bonds between particles and other oxide surfaces and without in bond strength is cut by roughly 40-50%. 

New fibers are being developed which dissolve in the slightly alkaline pH of the lung. These fibers may eventually replace the more common alumino-silicate fibers especially in use where human contact with breathable fibers is likely. Alumino-silicate fibers do not have the same small size as asbestos fibers, but the mere fact that they are fibers, could become airborne, and the body has no rejection mechanism, has prompted some countries to view them with suspicion. Having fibers which provide the same thermal properties but which would dissolved in the lung would preempt the possible problem. However, fibers which dissolve in alkaline conditions cannot be used with alkali stabilized colloidal silicas. They need a binder which is stable but acidic in pH as described above. Such products were first developed at DuPont and are now available through W.R. Grace who acquired the Ludox colloidal silica business from DuPont 

If you ever visited the Kennedy Space Center in Florida you may have seen a surprising demonstration. A small block of the silica material that makes up the tiles used on the shuttle is heated to well above red heat. The demonstrator turns off the flame and almost immediately picks up the block with bare hands to demonstrate how fast the material can dissipate heat. Some of the silica that goes into this remarkable material is multiply deionized colloidal silica re-stabilized with ammonia. The material is aged in between deionizations to allow sodium to leave the particles and enter the water phase where they can be removed. The product is shown to be suitable for use when it can be heated to a designated temperature and avoid significant conversion to cristobalite. 

---

The practice of employing reusable thermal protection systems for reentry is becoming more common. These are essentially ablative materials exposed to environments where veryHtde ablation actually occurs. Examples iuclude the space shuttle tiles and leading edges, exhaust no22le flaps for advanced engines, and the proposed stmctural surface skin for the National Aerospace plane. 

Figure 1.1 Microstructure of space shuttle tile material, a high-porosity fibrous silica secondary electron image using the scanning electron microscope (SEM). (Reproduced courtesy of Plenum Press, New York.)...

Figure 8.81 Impact failure origin in glaze coating from a space shuttle tile scanning electron micrograph. (Reproduced courtesy of The American Ceramic Society, Westerville, OH.)...

The following data were measured for the tensile strength of a space shuttle tile material (assume the specimens to be unit volume). 

Refractories are one example where a high-density ceramic product is not desirable—the space-shuttle tiles being the extreme example. The thermal conductivity, p, of air is only 0.026 W m K , significantly less than that of most crystalline ceramics. The thermal conductivity of a porous ceramic can be calculated using Eq. 9.10. 

Pumice is a natural porous ceramic. It is produced by volcano eruptions and the gas is trapped inside the solid as it rapidly cools. The matrix is mainly glass, but it can contain small crystals. Synthetic ceramic foam is illustrated in Eigure 15.15. Uses for ceramic foam are summarized in Table 15.3. One of the best-known applications for a porous ceramic is the space shuttle tile. An SEM image of such a tile is shown in Eigure 15.16. Notice that in this case, the ceramic consists mainly of fiber (pressed not woven), so the principle is the same as for ceramic (glass) fiber for house insulation. 

Space shuttle tiles are made of silica glass. We need to be concerned about the impact of space debris. Radomes are made of fused silica and silicon nitride. They have to be transparent to infrared (IR) and radio waves and resist the impact of atmospheric particles. Ceramic bearings are used in low-load applications (watch bearings of ruby or sapphire jewels ), but for high-load and high-speed use metals are often preferred because ceramics have low fracture toughness. 

Disasters, e.g., space shuttle tiles to make sure problems do not reoccur... 

Example application SiOi fibers used for space shuttle tiles... 

Ceramic fibers can be coated with ceramic slurries and hot-pressed to make dense fiber reinforced ceramic- or glass-matrix composites, Alternatively, continuous ceramic fibers can be combined with other fibers, whiskers, or powders and formed into porous shapes used for insulation (e.g., space-shuttle tiles) or subsequently infiltrated by CVD techniques to form fiber reinforced ceramic matrix composites without hot-pressing. For example, Nextel-SiC composites made by infiltration are being considered for applications such as heat exchangers and radiant gas burner tubes [210]. 

Aerospace applications have always been extensive and are still growing, due to the outstanding resistance of these sealants to extremes of temperature and various forms of radiation. Sealants designed to emit virtually no volatile components in the high-vacuum environment of deep space are used to fasten solar panels in place and to perform other sealing functions in delicate satellite assemblies where stray condensable contaminants must be avoided near sensitive optical and electronic devices. Other sealants are used to fasten space shuttle tiles in place and for other applications where the maintenance of elastomeric properties is essential over a wide range of temperatures. 

---