Biomedical Engineering Reference
In-Depth Information
FIGURE 18.6
Internal structure of a PLA (poly
d
,
l
-lactide) microsphere: (A) intact microsphere, scale bar is 30
μ
m and
(B) after milling top half using FIB milling, scale bar is 10
μ
m
[44]
.
surfaces, the microspheres were found to become increasingly porous toward the center of the parti-
cle. FIB was also used to extract a sample from the center of a microsphere, which was then used for
TEM analysis.
Figure 18.7
shows the initial stages of the FIB sample extraction.
18.2.3
X-Ray Diffraction
X-ray diffraction (XRD) is a powerful technique used to uniquely identify the crystalline phases
present in materials and to measure the structural properties (strain state, grain size, epitaxy, phase
composition, preferred orientation, and defect structure) of these phases. XRD is noncontact and
nondestructive. XRD is most sensitive to high-
Z
elements; as a consequence, the sensitivity of XRD
depends on the material of interest
[45]
. The regular array of atoms in a crystalline material forms
a three-dimensional diffraction grating for waves with a wavelength around that of the distance
between the atoms. When waves enter a crystal, they are scattered in all directions by the atoms. In
certain directions, these waves can interfere destructively. In other directions, constructive interfer-
ence will occur resulting in peaks in X-ray intensity. The diffraction pattern that results is a map of
the reciprocal lattice of the crystal and can be used to determine the structure of the crystal
[46]
.
Bragg's law is the basis for crystal diffraction:
(18.1)
n
λ
2 sin
d
θ
where
n
is an integer known as the order of diffraction,
λ
the X-ray wavelength,
d
the spacing
between two consecutive scattering planes, and
θ
the angle between the atomic planes and the inci-
dent (and diffracted) X-ray beam
[47]
.
