Pure aluminum: physical properties

The degree of purity of aluminum

The traditional system of describing the purity metals, including, aluminum, It based on the measurement of the total amount of impurity elements in percentage and subtract this amount from 100 %. Typically the result is expressed in number of nines. for instance, "Five nines" indicates the purity 99,999 %. It means, the total content of impurities is 0,001 % or 10 ppm (10 ppm).

Method of measuring metal purity

Resistance to passage of electrons through a sample of high purity metal, especially at low temperatures, strongly depends on the quantity of impurity elements, which are present in it. This fact is based on a very sensitive qualitative purity test method 99,999 % and higher. This method is valuable not only because of its sensitivity, but also because, that the measurement of electrical resistance is relatively simple.

Classification purity alumina

Typically used as follows, conventional, classification of degrees of purity of aluminum [1]:

  • 99,50-99,79 - aluminum of industrial purity (commercial purity)
  • 99,80-99,949 - high purity aluminum
  • 99.950-99,9959 - superpure aluminum (super purity)
  • 99.9960-99,9990 - extra pure aluminum (extreme purity)
  • more 99,9990 - ultra pure aluminum (ultra purity).

Below is an overview aluminum properties with a purity of 99,50 and higher. Effect of alloying additions and impurities on the properties of aluminum and aluminum alloys, see. here.

The mechanical properties of pure aluminum

Data on tensile strength aluminum multiple degrees of purity are presented in Table 1.

The influence of cold plastic deformation - cold working - on the strength characteristics of pure aluminum of the degree "five nines" (99.999 %) is shown in the table 2.

The hardness and strength of aluminum, depending on the degree of purity is shown in Figure 1.

Table 1 - Mechanical properties of pure aluminum
at room temperature

Table 2 - Mechanical properties of aluminum 99,999+ %

Figure 1 - hardness and strength of aluminum
depending on its degree of purity

Other mechanical properties of pure aluminum:

  • Elastic modulus: 62 GPa
  • Shear modulus of elasticity: 25 GPa at 25 oC
  • Sound speed: at 25 oC - 6200 MS; at 660 ºС (liquid state) - 4650 m / s

atomic properties

Aluminum has [1]:

  • atomic number 13
  • atomic weight 26,9815.

crystal structure

Aluminum crystallizes into a face-centered cubic lattice (FCC), which is the stable temperature 4 K and the melting point.

Figure 1.1 – Atomic structure of aluminum [2]

Density

theoretical the density of aluminum at room temperature (20 ºС), which is based on the size of the crystal lattice is 2698,72 kg / m3 [1]. The experimental data are in the range of from 2996,6 by 2698,8 kg / m3.

The dependence of the density of aluminum on the degree of purity and the solid and liquid states is shown in Figure 2.

Figure 2 – aluminum density depending on the degree of purity
in a solid state (20 degrees Celsius) and
liquid state (1000 degrees Celsius)

The density of the liquid aluminum purity 99,996 % is shown in Table 3.

Table 3 - density liquid aluminum purity 99,996 %

thermal properties

Thermal expansion

Coefficient of thermal expansion (CTE) data for purity aluminum 99,99 % at different temperatures indicated in Table 4.

Table 4 - Coefficient of thermal expansion (CTE)
pure aluminum 99,99 %

The melting temperature

The melting point of pure aluminum is very sensitive to the degree of cleanliness - the content of impurities. It was established experimentally, that the melting temperature of aluminum, 99,996 % is 933,4 K or 660,25 oC.

aluminum Pour point 660,323 ° C is one of the control points of the International Practical Temperature Scale 1990 года, which is used to calibrate the thermometer and thermocouple.

Other thermal properties of pure aluminum:

  • Boiling temperature: 2494 oC
  • Specific heat: 0,900 kJ / kg K at 25 ºS; 1,18 kJ / kg K at 660,40 ºС (liquid state)
  • Latent heat of melting: 397 kJ / kg
  • Latent heat of steam production: 10,78 MJ / kg
  • Heat of ignition: 31,05 MJ / kg

Coefficient of thermal conductivity

At temperatures of moderate to high, such as above 100 TO, thermal conductivity coefficient k of annealed high-purity (99.99+ %) is relatively independent of the degree of purity. Below 100 To the aluminum becomes very sensitive to the purity. pure aluminum heat conductivity coefficient in the temperature range from 100 by 933 K (from -173 by 660 ºС) gradually decreases from 3,02 by 2,08 W cm-1·TO-1 [1].

Figure 2.1 – Thermal conductivity of aluminum in comparison with other metals [2]

Specific electrical resistance

The accepted value of the specific electrical resistance of ultrapure aluminum (99.990 %) at 20 ºC is 2,2548 × 10-8 Om · m or 64,94 % from International Annealed Copper Standard (IACS). The electrical resistance of pure aluminum 99,9 % at various temperatures shown in Table 5.

Table 5 - Specific electrical resistance of pure aluminum (99.90 %)

The electrical conductivity of the pure aluminum is isotropic, unless there are present-oriented deployment.

Influence of grain size on the electrical resistivity of aluminum is industrially insignificant. However, cold-worked aluminum has in the direction of the resulting cold plastic deformation slightly higher - by 0,5-1,0 % - electrical conductivity.

magnetized

Since aluminum has an odd number of valence electrons (3), it is paramagnetic. Under normal conditions, shows no magnetized.

optical properties

The spectral reflectivity

The reflectivity of a smooth aluminum surface with respect to light is more than 90 % at wavelengths of from 0,9 by 12,0 m, For wavelengths 0,2 um, this reflectance decreases to 70 %. For wavelengths less than 0,2 um reflectance sharply decreases.

Maximum reflectivity is achieved by deposition of aluminum from the gas phase (vapor), which gives a very smooth surface. For maximum reflectivity of these films require the thickness of the gas phase is not less than 10-5 look.

The reflectivity of the aluminum surface decreases with increasing roughness of its. The aluminum surface after sandblasting can reflect only 15-25 % light compared with the polished surface of the material of the same chemical composition. On the image 3 It illustrates the normal reflectance of pure aluminum surfaces of different.

Figure 3 - Spectral reflectivity of pure aluminum [1]

issuable

Emission - the ability of the surface to absorb heat and to reflect it. Emission polished aluminum at room temperature is only a few percent of the black body emission. roughening the surface can raise the emission to 20-30 %. Emission increases with temperature and reaches 15-20 % in the liquid state.

A source:

  1. Aluminum and Aluminum Alloys, AMS International, 1993.
  2. TALAT 1501