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Synthesis of ZrTiO4 by ball-milling and sintering of equimolar mixture of TiO2 and ZrO2

The high-energy ball-milling induced the solid state reaction in equimolar mixture of TiO2 and ZrO2. Synthesis of ZrTiO4 was monitored during and after sintering to find out the conditions for the preparation of ZrTiO4 with improved properties. The milling of equimolar mixture of TiO2 and ZrO2 was performed in air using Fritsch planetary ball mill "Pulverisette 6" with vial and balls made of zirconia (94% ZrO2). The rotation speed was 500 rpm. Two different powder-to-ball weight ratios (R) were used ; R1=1:50 and R2=1:10. The milling time varied from 1 min to 10 hours. The effect of milling on the formation and microstructure of ZrTiO4 was investigated by transmission electron microscopy (TEM), selected area electron diffraction (SAED), Raman spectroscopy (RS) and X- ray powder diffraction (XRD). The sintering processes were monitored in situ at high temperature (300 to 1100  C) by RS. The final products of sintering were additionally investigated by scanning electron microscopy (SEM) to determine their grain sizes. The ball-milling induced the structural changes in the starting materials, as well as the solid state reaction. In samples milled for longer time the presence of ZrTiO4 was evidenced by RS and XRD. With prolonged milling the materials were partially amorphised and the sizes of particle decreased to nanovalues (Fig 1), and consequently the Raman bands and XRD lines were broadened. The rate of solid state reaction, amorphisation and the contamination, due to wearing of milling media, depended on the powder-to-ball weight ratio. The process was faster using R1=1:50, but the contamination was greater. The milling of oxides did not complete the solid-state reaction, so the additional sintering was necessary. Raman spectroscopy performed in situ at high temperature showed that the mixture milled for 10 h with R2=1:10 was partially amorphous at the beginning and stayed amorphous during the heating (Fig. 2a). The spectrum recorded upon the cooling contained only bands of orthorhombic ZrTiO4, so we could conclude that heating at 1100  C completed solid state reaction between TiO2 and ZrO2. The solid state reaction was completed faster and at temperatures much lower than expected from the reference literature (1500 - 1600  C). On the other hand, during sintering of the 1 h milled mixture with R1=1:50 the Raman spectrum recorded in situ at 900  C showed the shape and relative band intensity that corresponded to the spectrum of ZrTiO4 at this temperature (Fig 2b). Sintering at higher temperature amorphised material and after cooling to room temperature the bands of impurities dominated in the spectrum. In the mixture milled for 20 min with R2=1:10 containing a micron sized particles, amorphisation was not observed earlier than 1000 °C (Fig 2c) and Raman bands of ZrTiO4 was not observed upon cooling. The sintered oxides were investigated by SEM (Fig. 3). The sintering to 1100  C in argon proceeded through the second stage that is formation of continues phase of pore. The small grain sizes ( 1  m) were evident in long milled mixtures. Samples sintered from micron-sized powders had grains of several  m (Fig 3c). The high-energy ball milling initiated solid state reaction in equimolar mixture of TiO2 and ZrO2. The oxides achieved by ball-milling underwent amorphisation during sintering due to lowering of the diffusion temperature. It accelerated solid state reaction at temperatures much lower than it was necessary, and facilitated the formation of sintered materials with small grain, called nanoceramics.

ZrTiO4 ; solid-state reaction ; in situ sintering

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