Ceramic Fibers and Cartridges

W. Addy Majewski

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Abstract: Filter cartridges for filtering of diesel particulates can be assembled from high-temperature ceramic fibers. Fiber filters capture particulates through depth filtration mechanisms. A number of cartridge designs have been developed, some of them incorporating electric heaters for regeneration.


Fiber filters utilize depth filtration over high temperature resistant ceramic fiber cartridges. The filters can be regenerated by fuel additives (fuel bone catalysts, FBC), electrically, or using fuel burners. Various types of ceramic fiber filters were commonly used in the 1990s, in the early years of DPF technology, Table 1 [899]. With time, fiber filters lost their market share to wall-flow filters and their use became limited to some niche applications such as retrofit filters for industrial engines.

Table 1
Estimated utilization of filter materials circa 2000
Ceramic wall-flow monolith70%
Ceramic fiber (wound and knitted)25%
Sintered metal4%
All other1%

Performance, design parameters, and other features of fiber filters can be compared to those of wall-flow monoliths, which have become the benchmark in diesel filter materials. The advantages of fiber filters include:

On the negative side, fiber filters have showed a number of disadvantages:

Fiber Materials

Diesel particulate filter cartridges have been developed utilizing high-temperature continuous ceramic fibers. A number of such fibers—typically polycrystalline metal oxide fibers made from alumina and silica—have been used in textiles intended for 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 had been extensively tested for diesel filter applications. Typical properties of Nextel 312 fibers [304], compared with commercial SiO2 fibers [473], are listed in Table 2. The data indicates that Nextel fibers exhibit favorable strength and flexibility.

Table 2
Physical properties of Nextel 312 and silica fibers
Property Nextel 312 Silica
Form Polycrystalline Amorphous
Composition Al2O3 - 62%, SiO2 - 24%, B2O3 - 14%wt. SiO2
Fiber length Continuous Continuous
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
Elongation 1.2 %  
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))  
Linear 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 of fibers together.

The fibers have a virtually round cross-section and their surface is very 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 the fibers. A special texturization process had been developed for the manufacturing of the Nextel 312 fibers used for diesel filtration [476]. In the process, the raw fiber bundles were separated and the individual fibers were spread in a controlled manner. Spreading and separation produced “expanded” material with a number of 10-12 micron continuous fibers. The degree of expansion could be controlled, which allowed manufacture of 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 [473]. 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 was also confirmed by the Nextel fiber manufacturer through experiments with animals [304]. 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.