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Filter cartridges for filtering of diesel particulates can be assembled using high-temperature continuous ceramic fibers. A number of such fibers, typically polycrystalline metal oxide fibers made from alumina and silica, have been developed for textiles used in high temperature operating environments. Typical applications include insulation for thermocouple wires, belts for high temperature furnaces, filter bags for hot gas filtration, components of space shuttle tiles, and flame curtains. Traditional fiber materials include leached silica, fused silica, asbestos or glass. In general, these fibers are characterized by high strength, high rigidity, low coefficients of thermal conductivity and expansion, and large active surface area.
Nextel, a brand of synthetic fiber developed by 3M, is an example high temperature fiber that was also extensively tested for diesel filter applications. Typical properties of Nextel fibers , compared with a commercial SiO2 fibers , are listed in Table 1. The data indicates that Nextel fibers exhibit favorable strength and flexibility.
|Composition||Al2O3 - 62%, SiO2 - 24%, B2O3 - 14%wt.||SiO2|
|Fiber Diameter||10 to 12 µm (0.394 to 0.472 mil)||7 to 13 µm|
|Fiber Density||2.7 g/cm3 (0.097 lb/in3)||3.0 g/cm3|
|Surface Area||<1 m2/g||0.1-0.3 m2/g|
|Filament Tensile Strength||1.72 GPa (250 × 103 psi)||0.80 GPa|
|Filament Tensile Modulus of Elasticity||138 GPa (20 × 106 psi)||66 GPa|
|Continuous Use Temperature *||1204 °C (2200 °F)||1100 °C|
|Short Term Use Temperature *||1371 °C (2500 °F)|
|Melting Point||1800 °C (3272 °F)|
|Specific Heat||1046.7 J/(kg K) (0.25 BTU/ (lb °F))|
|Lineal Shrinkage 1093 °C (2000 °F)||1.25%|
|Thermal Expansion Coefficient
25 to 500 °C (77 to 932 °F)
|3.0 × 10-6 1/°C
(1.7 × 10-8 1/°F)
|* - in diesel filter application, fiber degradation occurs above 900°C due to ash melting and bonding fibers together.|
The fibers have a virtually round cross-section and their surface is extremely smooth and uniform. In filter applications, the adhesion of filtered particles to the fibers can be greatly enhanced through special treatment to increase the surface roughness and the active surface, such as leaching of the fibers. A special texturization process has been developed for the manufacturing of the Nextel 312 fibers used for diesel filtration . In the process, the raw fiber bundles are separated and the individual fibers are spread in a controlled manner. Spreading and separation produces “expanded” material with a number of 10-12 micron continuous fibers. The degree of expansion can be controlled, which allows to manufacture consistent material.
Fiber diameter is one of the most important parameters that determine the filter performance. Generally, small diameter fibers give better filtration efficiencies. Coarse fibers tend to show low efficiency, critical overloading phenomena, and blow-off . However, very small fiber diameters may present a health hazard, as well as increased costs and problems with handling. A typical diameter of diesel filter fibers is in the range of 10 µm.
Figure 1. Nextel 312 and Asbestos Fibers in 1000× Enlargement
Fibers of 10 micron diameter are generally considered not to be in the respirable range and to present a minimal health hazard. This opinion, based upon current theory and knowledge, was also confirmed by the Nextel fiber manufacturer through experiments with animals . Figure 1 shows Nextel fibers compared with asbestos in 1000× magnification. The Nextel fibers are very uniform in diameter. In contrast, the asbestos sample represents an inhomogeneous collection of fibrils with diameters 100 to 1000 times smaller than those of Nextel fibers.