dastgir - 21-5-2009 at 18:20
Dears,
How process of sintering in different than fusion? What are the effects of the different process on porosity of the resulting material.
Kind Regards,
Gulam Dastgir
The_Davster - 21-5-2009 at 18:26
Sinter-solid-state diffusion occurs consolidating a powder into a solid mass. It is not melted at any time. Heating is at a temperature below the
melting point.
Fuse-=melt, heating is at a temperature above the melting point.
Sintering usually gives a more porous product, but very long sinter times can give non-porous.
[Edited on 22-5-09 by The_Davster]
daragh8008 - 22-5-2009 at 06:03
I hear calcinate used a bit how does that differ?
not_important - 22-5-2009 at 19:03
To calcine is to heat to the point of some change - dehydration (CaSO4 . nH2O), decomposition (CaCO3), driving off some other volatile component, or
phase change - often used to cause an ore to break up from the fine structure changes.
roasting is calcining with good exposure to air in order to effect oxidation. Often used with sulfide and/or arsenic containing ores to convert them
to oxides.
Neither generally causes fusion/melting, although sintering may occur. Sometimes roasting may also result in the formation of a melt, such as in
cases where partial oxidation results in the remaining sulfide reducing the newly formed oxide to give the possibly liquid metal.
Sintering involves the formation of inter-particle bonding to some degree, from weak ones that crumble at a touch to strong ones giving results
similar to bricks. Sintering may be the result of diffusion at the points of contact, or particularly with mixtures the formation of a liquid phase
through reactions at those points of contact.
Nicodem - 22-5-2009 at 22:52
The term calcination is also commonly used for the solid-solid reactions between two or more compounds, and not only for decompositions of a single
compound. For example, the synthesis of polycrystalline materials (ceramics) starts first with a calcination where two or more metal oxides (also
carbonates or other) are reacted in a solid-solid reaction. This reaction/calcination is conducted at a temperature high enough, where the ion
diffusion between the crystals is substantial, yet much bellow the melting point of the starting materials and product(s). It is also advisable to
keep the temperature bellow the sintering temperature or else one gets an unmanageable ugly piece of ceramics that can not be powdered down by any
simple method. Often a single calcination is not enough to complete the reaction (this is checked by powder XRD), and the reaction mixture must be
brought back to a fine powder and further calcinated (a double or triple calcination reduces the reaction times to much shorter compared to the
reaction times needed if only a single calcination is used - which is obvious due to the solid-solid reaction depending on diffusion).
Needles to say, for small scale experiments of this type one needs a tube furnace (the tube is generally made of ceramic alumina in order to withstand
temperatures up to 1500°C). Or you can use a muffle furnace.