Views
5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Figure 2.15

Figure 2.15 Microstructure of ceramic foam with a relative density of 17% (125) . At 91% porosity the medium window diameter was 80 µm compared to 20 µm at 78% porosity. The ratio of window size to cell size k could be predicted within less than 10% deviation by using: 3 Π ⎡ 3 5 ⎛ 1 ⎞ ⎤ V ⎢ ⎜ ⎟ po = − − ⎥ ⎢ − ⎜ ⎟ 1 2 Equation 35 2 1 k 3 2 ⎣ ⎝ 1− k ⎠ ⎥ ⎦ where Vpo was the pore volume fraction. According to this, a critical pore volume fraction is reached at Vpo of 0.74, above which a sudden transition from an open to a closed cell foam occurs. It is evident that closed cell foams cannot be infiltrated to form an MMC. 2.4.4. Pore formation with pyrolysable additives The usage of sacrificial pore forming agents (PFA) is a very flexible method to achieve porous ceramic parts. With this method pyrolysable organic agents are added to a slurry of the ceramic powder. After pressing and heat treatment, this organic agent pyrolyses and forms volatile products, forming pores of corresponding shape and size between the ceramic 51

structure. This is shown schematically in Figure 2.16. Due to simplification reasons, here the sintering shrinkage of the ceramic network was neglected. Figure 2.16 Schematic of pore formation using a PFA between the ceramic particles. A large variety of PFAs has been reported in previous work, among them carbon fibres (123) , flakes (126) and starch (127) . The method is very flexible as the porosity can be controlled in the range of 0 to approximately 80% by the amount of PFA added, depending on densification behaviour of the initial particles. By default, the porosity created is not open, especially at low PFA additions. Percolation thresholds are in the range of 15–30%, depending on PFA size, shape and homogeneity of distribution leading to an interpenetrating network. The green compact consolidation can be either dry-pressing or suspension-based processes which allow a further influence on the microstructure. Alignment of non-spherical particles in a suspension flow is possible, as well as the creation of functional porosity gradients by using a special pressure filtration technique (128) . 2.5. Pressure assisted infiltration methods Apart from special systems such as the Lanxide process (16) and metallization of ceramic particles with copper or nickel (129) , the non-wetting nature of aluminium prevents spontaneous infiltration of the metal melt into the preforms below processing temperatures of 1000°C. Thus a differential pressure between the inside of the capillaries and the metal melt 52

PDF File 2 - University of Alabama at Birmingham
no· l'2-'1t1 - Repository - Texas A&M University
eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
MODULE 2 - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
eA - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
1 - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Cymbeline - eTheses Repository - University of Birmingham
Caroline Winstanley - eTheses Repository - University of Birmingham
epinician precepts - eTheses Repository - University of Birmingham
Witch Shoes - eTheses Repository - University of Birmingham
pains of sin - eTheses Repository - University of Birmingham
5% - eTheses Repository - University of Birmingham
here - eTheses Repository - University of Birmingham
View - eTheses Repository - University of Birmingham
J - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
THE GOD–MAN - eTheses Repository - University of Birmingham
Engineering texts - eTheses Repository - University of Birmingham
Life on Wings - eTheses Repository - University of Birmingham
ford madox brown - eTheses Repository - University of Birmingham
Table of Contents - eTheses Repository - University of Birmingham
Rewriting history - eTheses Repository - University of Birmingham
Four Quartets - eTheses Repository - University of Birmingham
part i - eTheses Repository - University of Birmingham
PIERRE BOAISTUAU - eTheses Repository - University of Birmingham