铌酸钡钠晶体中铁弹相变的实时观察

IN SITU OBSERVATION OF FERRO-ELASTIC TRANSITION IN BNN CRYSTAL

用锥光干涉图实时观察铌酸钡钠(BNN)晶体中的铁弹相变,实验表明从室温至300℃范围内相变是连续变化的,且呈弥散型转变,较快的区域在250±20℃范围内,升、降温曲线存在热滞约20—40℃,在相变过程中实时观察到孪晶(铁弹畴)的消失和再现均与相变同步,还发现孪晶的存在对相变转折点T_s有很大影响,它使T_s降低,热滞变大,缺陷和局域的组份不均匀是造成相变弥散型的主要原因。

In situ observation of the ferro-elastic transition in BNN crystal is carried out with the convergent polarized interference pattern, it reveals that the crystal transforms from bioaxies to uniaxies in heating with an inflection point Ts at 250±20℃. The transition is of the continuous and diffuse type. A large hysteresis about 20-30℃ is show by the fact that heating and cooling curves are distinct above 210℃.The twins (ferro-elastic domain) disappear and reappear in accordance with the time of phase transition. The density of the twins influences the inflection point T8 of the phase transition and the degree of hysteresis leading to decrease in T8 and increase in the hysteresis. The defects and microscopic compositional inhomogeneities are the main cause for the diffuse of the phase transition.