# Controlled-Atmosphere Sintering of KNbO\({}_{3}\)
## 3 Results  
The as-calched KNbO\({}_{3}\) powder's XRD pattern is shown in Figure 1. The pattern was indexed with ICDD Card # 71-0946 for orthorhombic KNbO\({}_{3}\) (space group _Anm2_). No secondary phase peaks were visible. The Archimedes density values of the KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1-10 h in different atmospheres are shown in Figure 2. All of the samples had densities between 95 and 98% theoretical density except for samples sintered in H\({}_{2}\), which had densities between 89 and 92% (theoretical density of KNbO\({}_{3}\) = 4.624 g.cm\({}^{-3}\)[48]). Apart from H\({}_{2}\), the sintering atmosphere did not have much effect on density. Figure 1: XRD pattern of calcined KNbO\({}_{3}\) powder. SEM micrographs of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 3 h in different atmospheres are shown in Figure 3. The O\({}_{2}\)-sintered sample contained cube-shaped equiaxed grains 1-2 \(\upmu\)m in diameter. The tiny particles on the surface of the grains are an artefact caused by the Pt coating. The air- and N\({}_{2}\)-sintered samples appeared similar, with slightly smaller grains. The N\({}_{2}\)-5%H\({}_{2}\)-sintered sample showed very different behavior. Many abnormal grains up to 30 \(\upmu\)m in diameter were present. A few micron-sized grains were still visible in spaces between the abnormal grains. The abnormal grains contained many entrapped pores. In the H\({}_{2}\)-sintered sample, the grains were again micron-sized. The samples sintered for other sintering times showed similar behavior. Figure 4 shows SEM micrographs taken from the edges of samples sintered at 1040 \({}^{\circ}\)C for 10 h. Abnormal grains were present at the edges of the O\({}_{2}\)-, air-, and N\({}_{2}\)-sintered samples. The degree of abnormal grain growth at the edges of the samples increased with sintering time. Samples sintered in N\({}_{2}\)-5%H\({}_{2}\) showed the opposite behavior, with less abnormal grain growth taking place at the edges of the sample than in the bulk. The sample sintered in H\({}_{2}\) contained large plate-like grains of a secondary phase at the edges. EDS analysis of these grains (mean and standard deviation of four point analyses)  
Figure 3: SEM micrographs of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 3 h in different atmospheres. Figure 2: Archimedes density of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1–10 h in different atmospheres. is shown in Table 1. The matrix grains appeared to be potassium-deficient. This may be due to potassium evaporation or to the difficulty in measuring alkali elements by EDS [49]. The K/Nb ratio of the secondary phase was much smaller than that of the matrix grains and was quite close to that of K\({}_{4}\)Nb\({}_{6}\)O\({}_{17}\). Apart from this secondary phase, no abnormal grains were present at the edges of the sample sintered in H\({}_{2}\). It is worth noting that the samples sintered in H\({}_{2}\) were more fragile than the other samples. The mean grain radius of KNbO\({}_{3}\) samples sintered at 1040 \({}^{\circ}\)C for 1-10 h in different atmospheres is shown in Figure 5. Error bars represent the standard deviation. At least 200 grains were measured per sample sintered in O\({}_{2}\), air, N\({}_{2}\), and H\({}_{2}\). For samples sintered in N\({}_{2}\)-5% H\({}_{2}\), between 120 and 180 grains were measured per sample. Only grains from the bulk of the samples sintered in O\({}_{2}\), air, and N\({}_{2}\) were measured, i.e., abnormal grains found at the edges of these samples were not included. O\({}_{2}\)-, air-, and N\({}_{2}\)-sintered samples had similar values of mean grain radius, between 0.4 and 0.6 \(\upmu\)m. The samples sintered in N\({}_{2}\)-5%H\({}_{2}\) had a much larger mean grain radius between 6 and 8 \(\upmu\)m, as well as wider error bars. The mean radius of H\({}_{2}\)-sintered samples was smaller than that of the other samples at -0.35 \(\upmu\)m. Sintering time did not have much effect on the mean grain radius of any of the samples.