简介:The use of X-raymethods in the field ofmaterials analysis is
now entering its seventh decade.While the broad definition
of X-ray methods covers many techniques based on the
scatter, emission and absorption properties 简介:The use of X-raymethods in the field ofmaterials analysis is
now entering its seventh decade.While the broad definition
of X-ray methods covers many techniques based on the
scatter, emission and absorption properties of X-radiation,
the two most common are X-ray fluorescence (XRF)
spectrometry and X-ray powder diffractometry (XRD).
When a sample of material is bombarded with energetic
radiation (X-rays, g-rays, electrons, protons, etc.) vacancies
may arise from the removal of inner orbital electrons. One
of the processes by which the atom regains stability is
by transference of electrons from outer to inner electron
shells. Each of these transitions is accompanied by the
emission of an X-ray photon having an energy equal to
the energy difference between the two states. The X-ray
emission wavelengths are characteristic of the atom in
question and there is a simple relationship (Moseley’s law)
between the wavelength of the emission line and the atomic
number of the atom. Thus when a sample is made up of
many different types of atoms, each atom will produce a
series of wavelengths, and all of the contributions add up to
become the total X-ray emission from the sample. XRF is
a technique which utilizes the diffracting power of a single
crystal, or the proportional characteristics of a photon
detector, to separate the polychromatic beam of radiation
from the sample into separate wavelengths, thus allowing
qualitative and quantitative elemental measurements to be
made.
A beam of monochromatic radiation may also be
scattered when X-ray photons collidewith atomic electrons.
Where the scattered wavelengths interfere with one another
diffraction of X-rays occurs. All substances are built
up of individual atoms and nearly all substances have
some degree of order of periodicity in the arrangement
of these atoms. It is the scattering from these periodic
arrays that leads to the diffraction effect, and there is a
simple relationship (Bragg’s law) between the scattering angle, the wavelength of the radiation and the spacings
between the planes of atoms. Since the distances between
the atomic planes are dependent on the size and distribution
of atoms – i.e. the structure of the material, XRD can be
used for qualitative and quantitative phase identification.详细>