# Controlled-Atmosphere Sintering of KNbO\({}_{3}\)
## 3 Results
\begin{table}
\begin{tabular}{c c c} \hline
**Element** & **Matrix Grains** & **Secondary Phase** \\ \hline K & \(22.3\pm 5.1\) & \(18.8\pm 8.1\) \\ Nb & \(24.1\pm 5.9\) & \(29.6\pm 12.3\) \\ O & \(53.6\pm 10.5\) & \(51.6\pm 20.4\) \\ K/Nb ratio & \(0.9\pm 0.1\) & \(0.6\pm 0.0\) \\ \hline \end{tabular}
\end{table} 
Table 1: EDS analysis of matrix grains and secondary phase in a KNbO\({}_{3}\) sample sintered at 1040 \({}^{\circ}\)C for 10 h in H\({}_{2}\). Figure 4: SEM micrographs of the edges of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 10 h in different atmospheres. Grain size distributions of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1 h in different atmospheres are given in Figure 6. Dashed black lines indicate the radius value that was three times the mean radius. Grains larger than this value were classified as abnormal grains [50]. Samples sintered in O\({}_{2}\), air and N\({}_{2}\) showed narrow unimodal distributions. O\({}_{2}\)- and air-sintered samples did not have any abnormal grains. A small fraction of abnormal grains appeared in the sample sintered in N\({}_{2}\). The sample sintered in N\({}_{2}\)-5% H\({}_{2}\) showed a very broad size distribution with many abnormal grains. The grains were also much larger than in the previous samples. For the H\({}_{2}\)-sintered sample, the grain size distribution became narrow and unimodal again with no abnormal grains. The grain size distributions at increased sintering times did not change much. SEM micrographs of KNbO\({}_{3}\) powder samples annealed for 1 h at 1040 \({}^{\circ}\)C in O\({}_{2}\) and H\({}_{2}\) are shown in Figure 7. The powder particles were of a cubic morphology with faceted faces and sharp edges and corners. The H\({}_{2}\)-annealed sample had rounder edges and corners than the O\({}_{2}\)-annealed sample. Figure 5: Mean grain radius of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1–10 h in different atmospheres. Figure 6: Grain size distributions of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1 h in different atmospheres. Figure 8a shows the XRD patterns of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 5 h in different atmospheres. All patterns were indexed using ICDD Card # 71-0946 for orthorhombic KNbO\({}_{3}\) (space group _Amm2_). No secondary phase peaks were visible in O\({}_{2}\)-, air-, N\({}_{2}\)-, and N\({}_{2}\)-5% H\({}_{2}\)-sintered samples. The H\({}_{2}\)-sintered sample contained Nb\({}_{12}\)O\({}_{29}\), K\({}_{4}\)Nb\({}_{6}\)O\({}_{17}\), and NbO\({}_{2}\) secondary phases (Figure 8b). The (011)/(100) peak splitting at 22-23\({}^{\circ}\), the (002)/(020)/(111) peak splitting at 31-32\({}^{\circ}\), and the (022)/(200) peak splitting at 44-46\({}^{\circ}\) progressively diminished in reducing sintering atmospheres. Unit cell parameters were refined via the least-squares method (MDI Jade 6.5, Materials Data Inc., Livermore, CA). Results are shown in Figure 9. In progressively reducing sintering atmospheres, the \(a\) unit cell parameter tended to increase, whereas the \(b\) and \(c\) unit cell parameters decreased. The difference in value between the \(b\) and \(c\) unit cell parameters decreased noticeably for the H\({}_{2}\)-sintered sample. Figure 8: (**a**) XRD patterns of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 5 h in different atmospheres; (**b**) magnified pattern of the H\({}_{2}\)-sintered sample. Figure 7: SEM micrographs of KNbO\({}_{3}\) powder annealed at 1040 \({}^{\circ}\)C for 1 h in (**a**) O\({}_{2}\) and (**b**) H\({}_{2}\). Figure 10 shows DTA traces of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 3 h in different atmospheres. Endothermic peaks corresponding to transitions from orthorhombic to tetragonal phases and from tetragonal to cubic phases can be seen in the ascending traces (Figure 10a) [45; 51]. The peak temperatures initially increased slightly as the sintering atmosphere changed from O\({}_{2}\) to air and then progressively decreased in reducing sintering atmospheres.