DensitySometimes it is necessary to estimate the density of a sample or coating from its chemical composition alone. This is especially true of quantitative depth profiling with GDOES. Currently the most effective method is to calculate the unknown density r from the densities of pure materials, assuming constant atomic volume. That is^{(1)} where c_{i} is the mass % of element i and r_{i} the density of pure element i. For a proof of this equation, go further down on this page. How good is this equation? Here is a selection of results: Name  Description  Density (kg m^{3})  Calc. (kg m^{3})  Error (%)  ZnAl  Zn50 Al50  3902  3997  0.4  Brass  Cu60 Zn30  8550  8327  2.7  Stainless steel  Fe75 Cr12 Ni12  8010  7864  1.8  Aluminium oxide  Al_{2}O_{3}  3980  2833  28.8  Silicon nitride  Si_{3}N_{4}  3300  2900  12.1  Acetaldehyde  CH_{3}CHO  805  843  4.7 
Generally the approach works well for metal alloys, typically giving results within a few percent of measured values. Nonmetals, in particular oxides, do not fair as well, being in error by up to 40%. The assumption of constant volumes is clearly not working for these materials. The main reason lies with the nature of chemical bonds, the stronger the bond the closer the atoms are drawn together and the smaller the atomic volumes; some of the strongest bonds are in oxides. A secondary reason is in the arrangement of atoms, i.e. the crystal structure, since a different structure may have a different number of atoms per unit volume. So where do we go from here? A more refined model must include additional information about chemical bonds and crystal structure.
Correction for Oxides and NitridesClearly the constant atomic volume equation is not accurate for oxides, it is also not accurate for other compounds with strong covalent bonds, such as nitrides, which reduce the apparent size of the atoms and hence increase the density. To date, there is no general and easy method to include covalent bonds. But we know from the analysis whether O and N and other covalent bond forming elements are present. So we will assume that if they are present then they will form such bonds, and they will form them preferentially with those elements which give the lowest energy. We will assume the order of oxide or nitride formation is determined by the electronegativities of the other elements present, beginning with the lowest. Hence, for example, Mg (1.31) will form an oxide before Al (1.61) and Al before Zn (1.65). Then, as suggested by Analytis,^{(2)} we simply look up a table of densities for known oxides and nitrides. To determine the density we then sum the specific densities (inverse densities) of the metals, oxides and nitrides, into the density equation above. So now we can calculate densities using constant atomic volumes for up to five elements ae, with oxide and nitride correction. If, instead, you would like to measure density there is a simple method described by Jukka IsoPahkala of SP Swedish National Testing and Research Institute. References: (1) R Payling, in R Payling, D G Jones and A Bengtson (Eds), Glow Discharge Optical Emission Spectrometry, John Wiley & Sons, Chichester (1997), pp 287291. (2) M Analytis, Spectruma Analytik GmbH, private communication (1998). Author: Richard Payling First published on the web: 15 May 2000.
Prove of density equation:Let's assume we have a binary alloy of elements a and b. One atom of a has a volume of V_{a} and one of b has a volume V_{b}. The volume occupied by one mole of a is V_{a} x N_{A}, where N_{A} is Avogadro's number (the number of atoms in a mole), and forb is V_{b} x N_{A}. The mass of one mole of a is w_{a} x N_{A}, where w_{a} is the atomic weight of a, and for b is w_{b} x N_{A}. The density r_{a} of a is therefore mass/volume = w_{a} x N_{A} / V_{a} x N_{A} = w_{a} / V_{a}, and for b is w_{a} / V_{a}. Now suppose we mix A% of a with B% of b, and assume the atomic volumes do not change. The total volume of one mole is (A x V_{b} x N_{A} + B x V_{b} x N_{A}.)/ 100. The mass of one mole is (A x w_{a} x N_{A} + B x w_{b} x N_{A}) / 100. But instead of the density we will calculate the inverse density (the reason will become apparent) This process can then be generalised to any number of elements with constant atomic volumes.
First published on the web: 15 February 2000. Author: Richard Payling
