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Development of novel Diesel Particulate Filters (DPFs) containing fine ceramic fibres

 

Introduction

Carbon particulate matter from Diesel exhausts is a health hazard, since the particles (10-100 nm) can easily reach the lungs.  They are removed by passing the gas stream through a Diesel Particulate Filter (DPF) with an inter-connected pore network.  However, since the flow rate is high, a fine filter generates a large pressure drop across it, impairing operation of the engine. Additionally, the DPF material must remain thermo-mechanically stable during regeneration (a periodic injection of fuel into the exhaust designed to burn off accumulated carbon deposits). Current DPFs are made by the sintering of ceramic particles, giving a relatively coarse, uniform pore architecture, which does not satisfy very effectively these twin requirements of fine scale filtration and minimal inhibition of gas flow. Novel materials will be produced by mixing alumina/silica fibres (with a range of diameters) with currently-used powders and then extruding and sintering the mixture.  The fibres are expected to create a multi-scale pore architecture exhibiting a good combination of filtration efficiency and permeability, whilst also reducing stiffness and raising toughness, hence enhancing the thermal shock resistance. X-ray tomography will be used to explore the pore architecture in the resulting materials and the COMSOL model will be used to simulate flow of particle-containing gas through it.  Permeabilities will be measured and the resistance to thermal shock will also be tested.  Promising materials will be manufactured into DPFs, and tested under engine performance conditions, by industrial partners.

 

Sample Preparation

Miniature DPF samples are manufactured by mixing together particles and fibres in different ratios, with 3 wt.% PVA added as a 10 wt.% solution used as a binder. Often water is also added in order to form a thick paste; the higher fibre content samples, the more water is required. This paste is put into cylindrical rubber silicone moulds, of 30 mm diameter, and height approximately 5 mm. Paper is used to seal the ends of the cylinder. The samples are then sealed in a plastic bag (to prevent oil intrusion) and hydrostatically pressed at 1500 bar to form a green compact. These green compacts are then sintered in air at varying temperatures and times. Once sintered, the filters are ground such that the two circular faces of the cylinder are flat and parallel. The following variables have been systematically altered: fibre/powder ratio, sintering time, sintering temperature, isostatic press time and pressure.

 

Effect of Fibre Content

Both porosity and permeability showed a strong dependence on the volume fraction of fibres in the sample. Figure 1 shows that the variation is approximately sigmoidal. In a DPF situation, a high porosity and permeability are desirable such that a low backpressure is maintained (although of course going too far would begin to reduce the filtration efficiency). Even using fibres of a diameter comparable to that of the mixing powder, it is clear that it is beneficial to add fibres, with the highest porosity and permeability observed in the case where only fibres were present. This is expected as more fibres will provide a more open structure by opening up gaps between particles.

 

Fibres_PorsityPerm 

Figure 1:  Variation of porosity and permeability with respect to the fibre volume fraction.

 

The fibre content also had a noticeable effect on the stiffness of the filters, as shown in Figure 14. The filters made with high alumina powder concentrations (more than a 1:2 fibre:powder ratio) were very friable and impossible to make a four point bent test sample without breaking the filter. Such materials did not have sufficient handling strength to be considered as a viable DPF material. Likewise, the sample made purely of fibres also lacked sufficient handling strength for a test sample to be produced, and so whilst this composition was optimal from the point of view of porosity and permeability, it is also not a candidate material for a DPF.

Stiffness peaked with a fibre fraction of around 67 % (i.e. a 2:1 fibre:powder ratio). Whilst it is desirable to keep our stiffness as low as possible, so as to improve the thermal shock resistance of the material, the observed values were all low (the maximum observed stiffness being only 2.6 GPa). Compared to the stiffness of bulk alumina (400 GPa), this is certainly low enough, even at its peak. The reason for the peak in stiffness is not easy to predict, but will be a consequence of two opposing effects. At high fibre contents the higher porosity will favour a low stiffness, but at low fibre contents the stiffness drops as there is a smaller contribution from the fibres which have a high axial stiffness.

 

 Fibres_Stiffness

Figure 2:  Variation of the filter stiffness with respect to the fibre volume fraction.

 

As the filter with 67% fibres had the largest relative stiffness and best handling strength, whilst also having a high porosity and permeability, it would appear that having a composition of around 2:1 fibres:powder is the optimum ratio for an efficient filter.

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