| Summary: | In the present thesis two new ceramic materials of high porosity (>85 % in volume) have been developed for the application in combustion boilers.
Primarily, materials for its use as gas burners based on highly pure silica fibres (>95 % wt. of silica, called “High silica” HS) and linked together by a coating of silicon carbide deposited by chemical vapour deposition (CVD) from methyltrichlorosilane (MTS) as a precursor were developed. These materials are proposed as an alternative for the actual commercial ones, made by the same technology but based on Tyranno® fibres which are much more expensive.
The other type of investigated materials are based on pyrogenic silica which also has a little addition of cited fibres to avoid the formation of cracks during compaction. In these materials, for thermal insulation, a minimal value of hardness is needed for their later manipulation without a substantial modification of the pore structure, which is the key to maintaining good insulation properties. In this sense, the effect of the addition of several borides on the densification kinetic and hardness of sintered materials has been investigated.
In the case of gas burners, it has been confirmed that highly pure silica fibres start to crystallise to cristobalite at temperatures much greater than those of condensing boilers. In this way, damage produced by phase transformations of cristobalite during heating and cooling rates is avoided.
One of the most important parameters for the fabrication of burners is the permeability of gas across the porous structure, which is the key to regulating the combustion power and the emissions of CO and NOx. It has been verified that this property mainly depends on the length and on the diameter of the fibres. In the case of HS fibres, they have lower diameters than Tyranno® ones which cause a decrease of permeability due to the increase of the number of contacts between fibres and the decrease of the distance between them.
The other studied alternative to reduce the cost of the fabrication of burners based on Tyranno® fibres coated by CVD-SiC, is to replace this coating method by a lower complex and costly ones like sol-gel route and dip coating in colloidal silica processes. Both methods have demonstrated the formation of strong bonds between fibres by the creation of silica deposits, not only as coating but also as bridges between fibres. The method of dip coating in colloidal silica, leaves large quantity of these residues which deteriorate in a notorious way the permeability properties. Sol-gel route allows for obtaining closer values to those of the application. However, the combination of rigidity and permeability is greater in the case of the fibres coated by CVD-SiC.
Pyrogenic silica based materials reinforced by fibres have been achieved by the conventional powder consolidation route consisting in mixing, compaction and sintering. The effect of different parameters on the densification and hardness of these materials including the specific surface area of pyrogenic silica, green density, quantity of reinforcement fibres (HS) and their hydroxylation state and the addition of diverse borides: titanium diboride (TiB2) and boron carbide (B4C) has been investigated.
The results of the dilatometric analysis agree with the predictions of the viscous flow sintering model proposed by Frenkel. Above 450 - 500 ºC, the borides oxidise creating local fused points that accelerate the shrinkage, to a greater extent as the boron addition of the system increases. The activation energies estimated from dilatometric curves for temperatures below 1000 ºC for all studied materials agree with reported ones of silica gels, independently of boron content. At temperatures above 1000 ºC, the activation energies of the materials with boride additions increase significantly while the ones of the materials without boride additions do not change.
Finally, it has been confirmed that borides, even in small amounts, cause notably changes in hardness without modifying the density in a substantial way. This result is important because it is possible to increase the consistency of the material without altering the porous structure.
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