BaTa2O6 production by mechanochemical synthesis, Fe removal, luminescence and thermodynamic properties

Mechanochemical synthesis of BaTa2O6 ceramic powder using BaCO3 and Ta2O5 starting powders was made in a planetary ball mill. The effect of milling time and heat treatment temperature on BaTa2O6 formation and microstructure was investigated. BaTa2O6 phase started to form after 1h of milling and single phase was obtained after 10h of milling due to high energy milling. The crystallite size of the powder after 10h of milling was determined as 22 nm and didnot decrease with milling time. While single phase BaTa2O6 was obtained at 1200 °C for 1h milled powder, it decreased to 900 °C when powder was milled for 5h. BaTa2O6 particle size was identical with milled powder below 900 °C but it increased to 180 and 650 nm at 1100 and 1200 °C. Ba6Fe4Ta12O42 phase apeeared at 1100 °C and increased to high amount at 1200 °C due to iron contamination. The elongated and large grains of Ba6Fe4Ta12O42 phase were different from BaTa2O6 grains having fine size and rounded shape. Both milled and heat treated powders had tetragonal tungsten bronze (TTB) type structure. In order to remove Fe coming from the milling environment, mechanochemically produced BaTa2O6 ceramic powder was leached with 6 and 12 M HCl at 95 °C and 105 °C for time periods varying between 1 and 15 hours. Amount of Fe in leached samples was determined by EDS analysis. Fe concentration decreased from 10,36% to 1,97-3,16% after leaching indicating 70-81% decrease in iron. Leaching temperature had a more pronounced effect on iron removal than concentration. Most of the iron impurity in BaTa2O6 ceramic powder was removed after 1h of leaching due to low activation energy and rapid micropore growth. Nevertheless, the iron content cannot be decreased to low levels due to high iron content in the cracks and voids. In order to investigate the effect of heat treatment temperature on the crystallization and phase development, ceramics made from leached milled powder were sintered between 1000-1450 °C for 5h. Iron related phases formed due to iron impurity appeared above 1000 °C but single BaTa2O6 phase was obtained at 1425 °C. Liquid phase was formed above 1300 °C led to an increase in grain size and density. The structure of BaTa2O6 grains were converted from round shape at 1425 °C to elongated shape at 1450 °C. Rare earth oxide (Eu2O3, Dy2O3) doped BaTa2O6 ceramic powder was produced by solid state method and single phase BaTa2O6 was obtained at 1425 °C after 20 hours of heat treatment. It was determined that high amount of addtive can be doped into TTB BaTa2O6 structure. Although single phase structure was observed until 20 mol% of Eu2O3 and Dy2O3 doping, EuTaO4 and Dy3TaO7 second phases were observed above 20 mol%. Both Eu2O3 and Dy2O3 dopings led a decrease in BaTa2O6 grain size. While undoped BaTa2O6 did not indicate any luminescence property, emission peaks were observed for Eu2O3 and Dy2O3 doping of 0,5-40 mol%. The emission peak intensities increased with doping content and after reaching a maximum value at 20 and 10 mol%, respectively, for Eu2O3 and Dy2O3 decreased to lower values due to concentration quenching. Although asymmetry ratio (electric dipole emission/ magnetic dipole emission) increased by the addition of Eu2O3, it did not change with Dy2O3 addition. In order to investigate into the effect of production methods on luminescence properties, materials were produced by mechanohemical synthesis and mixed oxide technique and their luminescence properties were compared. 5 and 20 mol% Eu2O3 doped BaTa2O6 samples that gave higher luminescence intensities were produced by mechanochemical method after 20h of milling. In addition, mechanochemically synthesized powder was leached using optimum leaching conditions at 105 °C with a 12 M HCl concentration for 3 hours. Leached powders were heat treated at 1425 ?C for 5h. Unleached and leached powders were examined by XRD and SEM which indicated that Fe, Cr, Ni related secondary phases were observed before leaching but their content were decreased significantly after leaching. Metallic impurities were due to high energy milling. The spectrofluorometer analysis of unleached and leached powders indicated that impurities coming from milling environment inhibited luminescence property. However, the luminesce property were observed in mechanochemically synthesized powders after leaching due to decrease in impurity phases. While no luminescence property were detected in mechanochelly synthesized powders, luminesce property were observed in leached powders. However, the luminescence intensity of mechanochemically synthesized and leached powders were 7 to 10 times lower than the same powders produced by mixing oxide method. The kinetic properties of barium tantalate compound was examined by DTA analysis between 25 °C to 1200 °C using BaCO3 and Ta2O5 starting chemicals mixed at stoichiometric ratios. The activation energy of the reaction was calculated by two differents methods which gave compatible results. While the activation energy was found as 190,90 kJoule/mol by Ozawa method-McCarty apporach, it was found as 189,85 kJoule/mol by Matusita-Sakka method. SEM analysis were made to determine m and n coefficents in Matusita-Sakka method. Moreover, the kinetic rate constant of the reaction and crystallization peak temperature of BaTa2O6 were found. BaTa2O6 specimens heat treated at 1425 °C for 20 hours were used to determine the thermodynamic properties of BaTa2O6. The thermodynamic functions of BaTa2O6 ceramic were determined by DSC analysis performed between 70 °C to 1190 °C using the heat capacity curve between 298,15 K to 1463 K. While the standart molar heat capacity (C°p298,15) of BaTa2O6 compound was calculated as 179,71 Joule/mol.K, the standart molar entropy (S°298,15) was calculated as 213,12 Joule/mol.K. These results were in agreement with Neumann-Kopp rule. The other thermodynamic functions (enthalpy, entropy, Gibbs energy changes and temperature dependence of Gibbs energy) of BaTa2O6 ceramic were calculated using heat capacity curve between 298,15-1463 K. Key Words: Mechanochemical synthesis, high energy milling, BaTa2O6, leach, iron removal, luminescence properties, Eu2O3, Dy2O3, kinetic properties, thermodynamic properties, SEM, XRD, EDS, DTA, DSC.