# Photoconductive TiO\({}_{2}\) Dielectrics Prepared by Plasma Spraying
## 3 Results and Discussion  
### Microstructure and Porosity  
The coating was formed mainly by smooth circular splats, Figure 4, which is a near-optimum character of the coating building blocks. However, when those blocks were stacked to make a thick coating, various imperfections accumulated and formed cracks, pores, and non-well-flattened particles or particle clusters. An example of such a structure, typical for plasma-sprayed ceramics, is shown in Figure 5. The as-sprayed porosity, Table 2, can be characterized not only with its area fraction on the cross-section (\"porosity\") but also with its size (\"ED\", equivalent diameter) and count (\"pores per mm\({}^{2}\)\"). High porosity with a low count indicates large pores (confirmable with high ED as well), and oppositely, a high quantity of pores per area unit in combination with low porosity would signal very fine pores. The limit of such quantification, used in Table 2, is around 3 \(\mu\)m, and finer pores are not counted reliably. The shape criterion used here is the minimal circularity (CIR). Globular pores have a cross-section circularity value one, whereas very flat pores approach zero. \"CIR\({}_{\text{min}}\)\" is the minimum CIR value for the field of view, i.e., detection of an extremely flat void. The image analysis values for the as-sprayed TiO\({}_{2}\) (labeled n-TiO\({}_{2}\) as \"nano\") should be shown in relations with similar coatings to provide an idea if, for example, ED 13.55 \(\mu\)m is a large value or not. For this reason, another two coatings sprayed by the same team and evaluated with the same approach were added in Table 2.  
A comparison of the samples (Table 2) showed n-TiO\({}_{2}\) to be rather porous, with a large ED and a low count, whereas the CIR\({}_{\text{min}}\) parameter was medium, rather high in fact, since the gas-stabilized plasma torches produced a typi  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Porosity [\%]** & **ED [\(\mu\)m]** & \
\begin{tabular}{c} **Pores** \\ **Per mm\({}^{2}\)** \\ \end{tabular} &
\begin{tabular}{c} **CIR\({}_{\text{min}}\)** \\ \end{tabular} \\ \hline n-TiO\({}_{2}\)* & 8.53 \(\pm\) 1.93 & 13.55 \(\pm\) 1.17 & 171 \(\pm\) 68 & 0.095 \(\pm\) 0.026 \\ TiO\({}_{2}\)** & 4.60 \(\pm\) 0.90 & 9.00 \(\pm\) 0.50 & 389 \(\pm\) 57 & 0.030 \(\pm\) 0.010 \\ TiO\({}_{2}\)** & 3.33 \(\pm\) 0.55 & 5.45 \(\pm\) 0.49 & 363 \(\pm\) 56 & 0.105 \(\pm\) 0.028 \\ \hline \hline \end{tabular}  
* the actual research; ** TiO\({}_{2}\) sprayed from fused and crushed natural rutile TiO\({}_{2}\)[18]; *** TiO\({}_{2}\) sprayed from Metco 6232B commercial powder with a gas-stabilized plasma torch [21].  
\end{table}
Table 2: Image analysis of pores in the coatings.  
with finer, and also on average flatter, voids. There would be room for microstructure improvements in the n-TiO\({}_{2}\) coding via some post-processing as post-HIP (hot isostatic pressing), laser re-melting of simply long-time high-temperature annealing. Often, the microstructure obtained applying this post-processing approaches the one typical for sintering. However, our annealing was only very mild to restore the coating stoichiometry to the extent to behave dielectrically only (not like a conductor, which an oxygen-deficient titania could [10; 15]) and, on the other hand, not to affect the microstructure with crack healing, pore coagulation, or similar effects.  
The HR-TEM image, Figure 6, shows at least one kind of defect that can often be observed--dislocations. The extra atom plane of the dislocations can be found by viewing along the direction indicated by the arrows seen in the figure. This dislocation should have an edge component, since there is an extra plane of atoms. It has been noted that this type of dislocation is more often observed in the region in which a strong lattice strain exists [23].  
Figure 5: Coating’s polished surface, SEM-SE—500\(\times\) magnification, tilt 75\({}^{\circ}\).  
Figure 6: HR-TEM of the n-TiO\({}_{2}\), scale bar 5 nm.