Improvement effects of sinterability due to B-site excess composition in (K,Na)NbO₃system ceramics

Abstract

The sinterability of Bi-and Cu-doped (K,Na)NbO 3 (KNN) ceramics fabricated from B-site excess calcined powder was investigated. Contrary to conventional reports regarding the sinterability of Bi-free KNN ceramics, it was found that the sinterability was improved with the increase in the B-site excess ratio. To elucidate the reason for this observation, X-ray diffraction and X-ray photoelectron spectroscopy measurements were performed. The formation of K 4 CuNb 8 O 23 was observed in some B-site excess samples. In addition, an apparent change in the valence state was detected, which is thought to be due to the substitution of the A-site ions in KNN with the Bi atoms.

Growth of large unitary (K,Na)NbO3 single crystals using a seed-free solid-state crystal growth method by adjusting the B-site excess ratios and additional Bi2O3

“…We have already reported that it is difficult to sinter the calcined powder compacts densely due to the occurrence of remarkable abnormal grain growth when the B-site excess ratio exceeds 0.75%. 40) The above-mentioned tendencies are consistent with this previous report. Since when the B site excess ratio was 1%, the growth of the large single crystals was observed more frequently compared with the case of the excess ratio of 0.8%, the ratio was fixed to 1%.…”

“…Since increasing the B-site excess ratio decreases the sintering temperature, it improves sinterability and promotes grain growth, unlike Bi 2 O 3 addition. 37,40) Therefore, we attempted to control the AGG activity in the SFSSCG process to grow the large KNN-based single crystals stably by adjusting both the B-site excess ratio and the additional amount of Bi 2 O 3 in the calcined powder.…”

Conditions of large unitary (K, Na)NbO3 system single crystals for rapid solid-state crystal growth method

“…Because of the high concentration of oxygen vacancies detected as defect complexes, Δg c tends to become smaller than Δg max during the KNN crystal growth at B-site excess composition, which promotes the occurrence of AGG. Conversely, when the concentration of oxygen vacancies decrease as a result of the substitution of trivalent Bi ions for monovalent A-site vacancies 36,37) to satisfy the charge neutrality condition, Δg c increases with respect to Δg max ; thus, the crystal growth is suppressed. By utilizing the appropriate B-site excess ratio and amount of added Bi 2 O 3 , the growth rate of only a few specific large grains can be rapidly increased when Δg c is adjusted to an optimum value slightly below Δg max .…”