SOURCES AND OCCURRENCES OF POROSITY AND MICROCRACKS
Porosity originates from the voids, which are created within the bulk. These pores are
of two kinds, fig. 1; open and closed pores. The open or otherwise known as apparent
porosity measures the fraction of open void volume to the material volume. The open
pores are usually interconnected so that they provide passages through which gases can
pass. If one maximizes the porosity for a given application, one can obtain a foam fig
2. Such foams are typically used for filtration or as a catalyst because of their high
surface area
Fig. 1 Types of pores
Porosity is often detrimental to important properties such as elastic modulus, tensile
strength, hardness or the thermal and electrical conductivities. The desired final
properties depend on the final application of the ceramic. Porosity is obviously
desirable in light-weight products, thermal insulation, catalyst, a high specific surface
area is usually essential, supports, and filtration Negative aspects include friability, loss
of strength, undesirable fluid absorption etc.
There are several characteristics that will influence the porosity:
1. The connectivity of the pores: there exists open and closed porosity. In the first case,
the pores communicate with the outside of the material, they can theoretically be filled
with a fluid by immersion of the piece. In the case of closed porosity the pores are
isolated inside the material.
2. The pore size: the pore size is important for the final properties as well as during the
sintering as small pores are much more difficult to eliminate than large ones.
3. The shape of pores: the pores can have different shapes which influence their
behavior. Open pores are typically thin, elongated and irregularly shaped. The closed
pores are typically more equiaxed.
4. The distribution of the porosity: the distribution of the porosity can be of importance.
For example during the production of ceramic layers, the porosity is not always uniform
throughout the thickness of the layer.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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