熔盐法合成片状NaNbO3晶体

Molten salt synthesis of platelet NaNbO_3 crystals

片状NaNbO3晶体可作为反应模板晶粒生长技术(RTGG)制备高取向铌酸钾钠无铅压电陶瓷的模板晶体,通常采用两步熔盐法合成.首先合成片状Bi2.5Na3.5Nb5O18(BNN5)晶体,然后以BNN5晶体作为前驱体合成NaNbO3晶体.借助X射线衍射(XRD)仪、扫描电镜(SEM)及高分辨透射电镜(HRTEM),考察合成温度以及保温时间对晶体物相、形貌和尺寸的影响并揭示片状NaNbO3晶体的形成机理.结果表明:以NaCl为熔盐,1150℃保温4h,得到物相单一、形貌规则、尺寸均一的片状NaNbO3晶体,晶体尺寸约为15μm×10μm×1μm.片状NaNbO3晶体的形成机理是高温条件下由于熔盐中Na2CO3作用,BNN5晶体中[Bi2O2]层被破坏,Bi3+、O2-不断从结构中析出,[Bi2O2]层脱离.同时一部分Na+进入晶格中取代[NbO6]八面体空隙中的Bi3+,另一部分Na+进入[Bi2O2]层脱离后形成的空位中与NaNbO3基元进行结构重排,最终形成NaNbO3晶体.

Platelet NaNbO3 crystals were synthesized by a two-step molten salt method and acted as template crystals to prepare highly textured potassium sodium niobate free-lead piezoelectric ceramic by reactive templated grain growth （RTGG） technique. Bi2.5 Na3. 5Nb5O18 （BNN5） crystals were prepared as precursors for the formation of NaNbO3 crystals. Effects of the synthetic temperature and the soaking time on the phases and the morphology of crystals were studied by X-ray diffraction （XRD） , scanning electron microscopy （SEM） and high resolution transmission electron microscopy （HRTEM）. Results indicated that the single-phase platelet NaNbO3 crystals with the regular morphology and the size could be synthesized in NaCl flux at 1 150 ℃ for 4 h, and the mean size was about 15 μm × 10 μm × 1 μm. Meanwhile, the formation mechanism of platelet NaNbO3 crystals was also proposed. Under the high temperature, [ Bi2O2 ] layers in BNN5 crystals were destroyed due to the effects of Na2CO3, thus Bi^3＋ , O^2- dissolved out uninterruptedly. Simultaneously, a part of Na^＋ penetrated into crystal lattice and replaced Bi^3＋ located in the interspaces of [ NbO6 ] octahedron, and the rest part of Na^＋ diffused into the vacancies where [ Bi2 O2 ] layers broken away, finally, the remaining NaNbO3 blocks rearranged to form NaNbO3 crystals.