Porosity

The porosity is a dimensionless parameter and represents the ratio of void volume to total volume of a substance or mixture of substances Represent serves as a classifying measure of the actually existing cavities. For use, the size is in the range of material and construction equipment in the geosciences. The porosity has a great influence on the density of a material as well as the resistance to the flow of bulk material ( Darcy's law ).

Originally by natural circumstances related and usually especially in the production of sophisticated castings undesirable, there are now also of an artificially created, inasmuch desired porosity, primarily serving the manufacture of lightweight materials. Aluminum foam and lightweight concrete ( poroton ) are examples of porosity, but is not as such the subject of this post.

• 2.1 Structural Engineering
• 2.2 Materials Engineering
• 2.3 Geosciences

Definition

The porosity is defined as 1 minus the quotient of the bulk density ( a solid ) or bulk density ( a bulk material ) and the true density:

As a percentage size it is calculated as follows:

Alternatively, the porosity as the ratio of void volume to total volume with state as a pure volume of the solid:

In soil mechanics is as a measure also used the void ratio (ratio of void volume to solid volume ).

Open and closed porosity

The total porosity of a substance is composed of the sum of the cavities, which communicate with each other and with the environment ( open porosity, Nutzporosität ) and the non- interconnected voids ( abzementierte, closed or even dead-end porosity).

A high open porosity is called open-pore material, or ideally considered a honeycomb structure with pure closed porosity is referred to as foam.

Typical values

The following geometrically determinable total porosities an array of solid spheres of the same size can be considered as typical:

• For an ordered cubic close ( face-centered ) sphere packing as well as for an ordered, hexagonal closest packing of spheres it is Φ = 0.26
• For the body-centered cubic packing of spheres it is Φ = 0.32.

These values ​​are derived directly from the packing density, which gives a density ratio of 74 % for the cubic and hexagonal close packing. Kepler postulated that this is the largest value that a sphere packing can take at all. However, this so-called Kepler conjecture was until today mathematically not yet established, it was recorded by David Hilbert in 1900 as the 18th problem in his list of 23 mathematical problems.

In a body-centered cubic lattice (as in tungsten - bcc) is the value of only 0.68 and a primitive cubic lattice (as in alpha - polonium - sc) only 0.52.

For any sphere packings of a non - porous material inside ( solid spheres) following rough estimate holds:

Occurrence of porosity

Structural Engineering

In structural engineering, the term porosity refers to the void fraction of a bed or a muck. Porosity and bulk density are doing in context. The porosity defined as the ratio of void volume to total volume of the bulk material VHohl Vtot. Commonly used is the letter ε or PW, less common, however, is that Φ has already been introduced.

It is usual following definition:

The total volume Vtot sets itself from the solid volume Vs (corresponding to pure volume VF) and the void volume VH together.

Materials Engineering

In materials science, the classification of porous materials according to the size of the pores takes place:

• Microporous: pores < 2 nm
• Mesoporous: pore size between 2 and 50 nm
• Macroporous: pores > 50 nm

In cast iron parts, made ​​of sand casting, as there is a pin - holes " ( pinholes " ) designated pore shape. They may be visible on the surface or are close to lower. These are reactions of the melt with the moisture of the mold material, but also with nitrogen-containing binders of the same. In this respect, a distinction hydrogen pinholes and hydrogen-nitrogen pinholes.

Earth sciences

In geology, hydrogeology and soil science, the porosity is the ratio of the volume of all cavities of a porous soil or rock to its external volume. It is a measure of how much space the actual soil or rock fills cavities or which he leaves behind in this because of its grain or fracturing within a certain volume. The pores or capillaries are usually filled with air and / or water. The porosity is usually expressed as a percentage or as a fraction ( a fraction of 1 = 100%) and designated by the formula Φ letters.

The porosity of rocks describes the volume of void fractions, which can be used by mobile, migratable media such as water and gases. Occasionally, the same significant Undichtigkeitsgrad terminus is used, the porosity of rocks. In addition, there are the rocks technical values ​​void ratio ( symbols ) and porosity (symbol ).

When considering the weathering properties of natural stones, one starts from the open porosity ( πwi ). They describe only that pore spaces, are involved in the liquids and gases on exchanges.

Sediments and sedimentary rocks have a porosity of about 10 to 40% on, metamorphic and igneous rocks, however, only about 1 to 2%. Typical real measured total porosities are:

• Sandstone: 5 to 40%, typically 30 % (depending on particle size distribution, the type of binder and consolidation)
• Limestone or dolomite: 5 to 25 % (depending on solution processes through groundwater and weathering )
• Mudstone: 20 to 45% ( due to the small diameter of the pores but not the reservoir rock )
• Shale: less than 10%
• Loose sands and gravels: up to 40 %

In the oil and gas industry, the mining and geology in geothermal energy, the effective porosity plays an important role, as can (water, oil or gas) flow only through the under intercommunicating pore fluids. In connection with storage properties of a rock is spoken in hydrogeology also usable porosity.