Nondestructive testing of defects in adhesive joints

3.3. Thermogravimetric analysis (TGA)
In TGA thermograms of as-synthesized nanohybrids (Figure 3), The observed weight loss
in the temperature range from 75 °C to 550 °C under nitrogen atmosphere can be attributed
to dehydration of water molecules present / adsorbed in the crystals, carbon dioxide
evolution from carbonate defects of calcium phosphates and decomposition of
carboxymethyl cellulose. Since the nanohybrids were thoroughly washed with deionized
water for several times while filtration, only the strongly adsorbed / bound CMC molecules
can contribute to the additional weight loss.
3.4. Wide angle X-ray diffraction (WAXD)
Figure 4 shows the WAXD patterns of as-synthesized and calcined nanohybrids. The
characteristic peaks at 2θ ~10.8, 18.8, 25.8, 31.7 (with maximum intensity), 32.9, 34.0,
39.8, 46.83, 49.4, and 53.1° are attributed to hydroxyapatite. The broadening at foot of
peak at 2θ ~31.7° for as-synthesized samples can be correlated to nanocrystal formation.
For the calcined samples, these peaks are narrow and sharp. The effect of CMC content on
the crystallite size D (nm) of HA in both as-synthesized and calcined samples was
calculated using Scherer formula. With increasing content of CMC, the crystallite size
decreases. After calcination, regardless of CMC content the crystallite size is increased
upto 48 ± 4nm, which can be attributed to the sintering and densification phenomena at a
constant and higher temperature [31]. The consistency in crystallite size can be attributed
to the constant calcination temperature and conditions in which the coalescence of finer
particles is expected to be in similar rate [33, 34].
3.5. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDX)
SEM images of the as-synthesized and calcined samples are shown in Figure 5 (HA-05 and
Figure 5 (HA-1). In the as-synthesized samples, the nanoparticles are embedded /
surrounded by CMC molecules, to form agglomerates whose size is about 100 – 300 nm.
These agglomerates show rough and interconnected porous surface. As FTIR spectra
results suggest the presence of CMC in as-synthesized nanohybrids, the agglomeration of
nanoparticles might be due to CMC molecules. After calcination, particles are bigger in
size and smoother on the surface than that of as-synthesized ones. As mentioned above, the
increase in particle size can be attributed to the sintering of HA nanoparticles at higher
temperature. Figure 7 shows the EDX graph of the as synthesized and calcined sample of
HA-05. The Ca/P ratios for as-synthesized and calcined sample were about 1.45 and 1.67,
respectively. This also confirms the formation of hydroxyapatite precipitates.
3.6. Transmission Electron Microscopy (TEM)
The bright field TEM images of as-synthesized and calcined samples are shown in Figure
6. The as-synthesized nanohybrids can be seen as agglomerates of nanoparticles grown on
the CMC matrix. The size of agglomerates can be found about 100 to 300nm. After
calcination, the HA particles are found to be bigger. In the as-synthesized samples, the
agglomeration of HA nanoparticles can be explained by interactions between
carboxymethyl cellulose, HA and its precursors with each other.
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