Spherical perovskite oxide powders ($ABO_3$) were prepared by hydrothermally treating spherical gel powders, composed of A-site ions, in the aqueous solution of B-site ion sources. The spherical gel powders were prepared by the thermal hydrolysis of alcohol(aqueous salt solutions.
1. Synthesis of $PbTiO_3$ powder
$PbTiO_3$ powders were prepared by hydrothermally treating $TiO_2$ gel powders in the $Pb(Ac)_2$ aqueous solutions. Two kinds of $TiO_2$ gel powders were used to investigate the effect of the size and shape of the gels on the resulting $PbTiO_3$ powders. One, having irregular shape of a few nm, was prepared by adding KOH solution to the 1-PrOH(aqueous $TiCl_4$ solution. This $TiO_2$ gel powder will be called as nano $TiO_2$ gel. The Other, having spherical shape with a diameter of 0.71 μm and composed of a few nm primary particles, was prepared by the thermal hydrolysis of 1-PrOH(aqueous $TiCl_4$ solution. This one will be called as spherical $TiO_2$ gel. By hydrothermally treating at 220℃ and 16 h the nano $TiO_2$ gel in the $Pb(Ac)_2$ aqueous solution, which was a conventional process used by many other researchers, acicular PY phase- and cubic PE phase-$PbTiO_3$ powder were obtained at the solution pH of 10 and 14, respectively. Thus, by the known conventional process, we could not obtain spherical $PbTiO_3$ powder, so called as ideal powder. By hydrothermally treating at 220℃ and 16 h the spherical $TiO_2$ gel in the $Pb(Ac)_2$ aqueous solution, at the solution pH of 10, the spherical gel powder retained its spherical shape to produce spherical PE phase-$PbTiO_3$ powder but at the solution pH of 14, dissolved out to produce cubic PE phase-$PbTiO_3$ powder. This was due to the difference of solubility of $TiO_2$ gel in the reaction solution that the $TiO_2$ gel have a higher solubility at pH = 14 than at pH = 10. Because the spherical $TiO_2$ gel have a low solubility at pH = 10, the spherical $TiO_2$ gel did not dissolve in the reaction solution, retaining its spherical shape and reacted with Pb ion in the solution to produce to spherical $PbTiO_3$ powder. The spherical $PbTiO_3$ particles were composed of primary acicular particles whose diameter was 20 ~ 50 nm depending on the reaction times. Because the acicular particles started to form at several positions of the surface of the spherical gel and grew into the inner part of the gel, resulting spherical $PbTiO_3$ particle was composed of acicular particles aligned with many directions.
2. Synthesis of PZT powder
PZT powders were prepared by hydrothermally treating spherical $ZrTiO_4$ gel powder in the $Pb(Ac)_2$ aqueous solutions. The $ZrTiO_4$ gel powder was prepared by the thermal hydrolysis of a mixture of $TiCl_4$ and $ZrOCl_2$ in the 1-PrOH(aqueous solution. The powder had spherical shape with a diameter of 0.85 μm and was composed of a few nm primary particles. By hydrothermally treating at 220℃ and 50 h the spherical $ZrTiO_4$ gel in the $Pb(Ac)_2$ aqueous solutions at the solution pH of 10, 12, 13, and 14, at the only solution pH of 10, the spherical gel powder retained its spherical shape to produce spherical PZT powder but at all other pHs, dissolved out to produce cubic PZT powder. This was due to the difference of solubility of $ZrTiO_4$ gel in the reaction solution that the $ZrTiO_4$ gel have the lowest solubility at pH = 10 in the pHs. Because the spherical $ZrTiO_4$ gel have a low solubility at pH = 10, the spherical $ZrTiO_4$ gel did not dissolve in the reaction solution, retaining its spherical shape and reacted with Pb ion in the solution to produce to spherical PZT powder. The spherical PZT particles were composed of a few nm primary particles. The primary PZT particles formed from the surface of the spherical gel and to the inner part of the gel. Thus, partially crystallized spherical gel showed core-rim structure where core and rim was composed of unreacted gel and formed PZT primary particles, respectively.
3. Synthesis of spherical perovskite powders
From the above results, a methodology to synthesize spherical perovskite powders could be established: 1. preparation of a spherical gel powder composed of B-site ions which was easily synthesized by the known process (Sol-Gel and precipitation methods). 2. Selection of A-site ion sources and hydrothermal conditions where the spherical gel has the low solublity to retain its spherical shape and B-site ion sources have a high solubility to exist ions in the reaction solutions under processing conditions. Hydrothermally treating aqueous solution of a mixture of spherical $TiO_2$ gel powder and Ba $Ba(OH)_2ㆍ8H_2O$, spherical $BaTiO_3$ powder was obtained. Spherical $SrTiO_3$, $PbZrO_3$, and $Ba(Zr_0.5, Ti_0.5)O_3$ powders were obtained from a mixture of spherical $TiO_2$ gel and $Sr(OH)_2ㆍ8H_2O$, a mixture of spherical $ZrO_2$ gel and $Pb(Ac)_2$, and a mixture of spherical $ZrTiO_4$ gel and Ba $Ba(OH)_2ㆍ8H_2O$, respectively. All the obtained spherical perovskite powders had a structure of spherical secondary particle composed of nano-sized primary particles. It is expected that the methodology presented in this work could provide a simple method for preparing other spherical multi-component powders including dopped powders as well as spherical perovskite powders.
