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The Sol-hydrothermal Synthesis of Potassium Bismuth Titanate Nanoparticles



To propose a method of KBT synthesis at a lower temperature to solve the volatility
of the components.



Lead-based perovskite materials have long been employed in electroceramic industries
due to their excellent piezoelectric, ferroelectric, and dielectric properties. The high toxicity
of lead, however, leads to the replacement of the use of lead-based perovskite in devices with more
environmentally friendly materials. KBT powders are traditionally prepared by solid-state reaction
through the calcinations of K2CO3, Bi2O3 and TiO2 at high temperature. The high-temperature calcination
process leads to serious particle agglomeration, grain growth and small surface area,
which decrease the activity of the KBT powder. Instability of the KBT ceramic according to the
high volatility of its component ions at elevated temperatures was the main concern for the application
feasibility.



This work was aimed to present the simplified method called “sol-hydrothermal” for
the synthesis of KBT nanoparticles. Microstructure and phase evolution of the nanoparticles were
investigated in detail.



The sol-hydrothermal method was performed in potassium hydroxide (KOH) solution at
140-200°C for 2-24 h.



The result showed that increasing hydrothermal temperatures from 140°C to 200°C, the
crystal structure was changed from pseudo-cubic to tetragonal. At 200°C, phase separation was
observed. Suitable hydrothermal time was found to be between 6-12 h, above which phase separation
was also observed. Increasing the KOH solution concentration from 10 to 12 or 15 and finally, 20 M
gave rise to greater KBT peak intensity, suggesting a more complete crystallization process when the
concentration was increased. Tetragonal KBT nanoparticles with c/a ratio of 1.0620 were obtained
under the synthesis condition of 180°C for 12 h in 20 M KOH solution. Sinterability of the synthesized
KBT nano-particles was further investigated by varying the sintering temperatures from 1000°C
to 1080°C; the highest relative density of 97% was obtained in the sample sintered at 1050°C. However,
at this sintering temperature and beyond, the sublimation of the K-containing component occurred
as evident by the appearance of Bi2O3 and Bi4Ti3O12 phases.



In summary, KBT nanoparticles have been successfully prepared by the simple solhydrothermal
method in a basic solution at low temperatures. Synthesis temperature, time and
KOH concentration were found to affect the powder characteristics greatly. Increasing synthesis
temperature was found to affect the phase development while increasing synthesis time resulted in
the development of crystallinity of the KBT powder obtained. Increasing KOH concentration from
10 M to 20 M gave rise to different particle growth and agglomeration degrees. The optimum synthesis
conditions were at 180°C for 24 h in 10 M KOH solution. At this condition, KBT powder
with a uniform particle size distribution and tetragonal structure could be obtained. The synthesis
powder showed excellent sinterability. Sintering at only 1020°C for 2 h gave rise to fine grain ceramics
with 95% relative density. However, as potassium was prone to sublime, increasing sintering
temperature to 1050°C and beyond resulted in K-deficient phases. Sintering of the KBT should
be done in K-saturating atmosphere to suppress this sublimation.
