Effect of Oxygen Concentration and Tantalum Addition on the Formation of High Temperature Bismuth Oxide Phase by Mechanochemical Reaction

High-temperature face-centered cubic bismuth oxide phase is a material of great interest given its unique properties. In the present study, α-Bi2O3 and tantalum powders were used as the starting powders for the formation of high-temperature bismuth oxide phase via mechanochemical synthesis by high energy ball milling. (Bi2O3)80(Ta)20 and (Bi2O3)95(Ta)5 in weight concentrations were milled in either an oxygen-free argon-filled glove box environment or an ambient atmosphere to investigate the effects of oxygen concentration and tantalum addition. The as-milled powders were examined using X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, and differential scanning calorimetry to reveal the structural evolution. The experimental results showed that for (Bi2O3)95(Ta)5 powder mixtures milled within the glove box, tantalum gradually reacted with the α-Bi2O3 phase and formed a β-Bi7.8Ta0.2O12.2 phase. For (Bi2O3)80(Ta)20 milled under the same conditions, Ta and α-Bi2O3 mechanochemically reacted to form δ-Bi3TaO7 and bismuth after 10 min of high energy ball milling, whereas milling (Bi2O3)80(Ta)20 under the ambient atmosphere with a much higher oxygen concentration accelerated the mechanochemical reaction to less than five minutes of milling and resulted in the formation of high-temperature δ-Bi3TaO7 phase.

The δ-phase is one of the best oxide ionic conductors, but it is stable at temperatures ranging from 729 to 825 • C. Its fluorite-like structure induces 25% oxygen vacancies and cannot be stable at Materials 2019, 12, 1947 3 of 14 performing Rietveld fitting of the XRD patterns using XRD analysis software EVA (Bruker-AXS Diffrac EVA, Bruker, WI, USA). The cross-sectional views of as-milled powders were examined using a Nova NanoSEM 230 scanning electron microscope (FEI, Hillsboro, OR, USA) equipped with an EDAX Apollo X silicon drift detector EDS system (EDAX, Mahwah, NJ, USA) The DSC analysis was performed with a NETZSCH DSC 200 F3 differential scanning calorimeter (Gerätebau GmbH, Selb, Germany) where the sample was heated from room temperature to 300 • C in a purified argon atmosphere at a heating rate of 20 K/min.

Results and Discussion
To understand the structural evolution during the ball milling process, we used XRD and cross-sectional observations using SEM. Figure 1 shows the XRD patterns as a function of milling time for the (Bi 2 O 3 ) 80 (Ta) 20 powder produced using high-energy ball milling within an argon-filled glove box environment. At the early stage of milling, up to the first 25 min, crystalline peaks of the α-Bi 2 O 3 starting powder were observed (Figure 1a). The peaks' intensities decreased with increasing milling time. A similar trend was observed for tantalum. The diffraction peaks of tantalum, however, could be observed throughout the whole three-hour milling process. In addition, three new crystalline phases, which were α-Bi 2 O 4 (ICDD PDF card No. 50-0864), bismuth (ICDD PDF card No. 44-1246), and fluorite-like cubic δ-Bi 3 TaO 7 (ICDD PDF card No. 044-0202), formed after only 10 min of ball milling. The peaks' intensities of α-Bi 2 O 4 did not show noticeable differences throughout the milling process (from 10 min to three hours). The relative amount of bismuth increased with increasing milling time until the end of the three hours of milling. The formation of α-Bi 2 O 4 , bismuth, and Bi 3 TaO 7 suggests that tantalum reacts with α-Bi 2 O 3 particles and the mechanochemical reaction occurs after only 10 min of high energy ball milling. This shows a behavior similar to one previously reported in literature, in which milling α-Bi 2 O 3 and iron leads to the formation of Bi and Fe 3 O 4 [29]. However, the relative peaks' intensities of Bi 3 TaO 7 did not exhibit significant differences at the early stage of milling (i.e., the first 10-30 min). Slight decreases in the peaks' intensities and broadening of peaks was observed after long-term milling from 30 min to three hours ( Figure 1b). Figure 2 shows the cross-sectional SEM images of as-milled (Bi 2 O 3 ) 80 (Ta) 20 powder. Irregular grains with a wide particle size distribution were be observed at all milling stages starting from five minutes of milling (Figure 2a) to the end of three hours' milling ( Figure 2h). Typically, the high-energy ball milling process can be divided into three different types of milling mechanisms, depending on the characteristics of starting powders: both ductile, ductile/brittle, and both brittle [20]. During ball-powder-ball collision, powder particles undergo deformation and/or a fracture process. Ductile powders usually form a lamellar structure at the beginning and result in homogeneous equiaxed particles at the end of milling. For a ductile/brittle system, brittle particles may fracture and are trapped within ductile interfaces and finally form a metal matrix composite with a uniform distribution of embedded brittle particles, whereas brittle/brittle particles are expected to fracture continuously into small particles and sometimes mechanical alloying between starting particles may occur. The present milling system consisted of 20 wt % ductile Ta and 80 wt % brittle α-Bi 2 O 3 particles. The densities of Ta and α-Bi 2 O 3 were 16.69 and 8.9 g/cm 3 , respectively. The relative volume of ductile Ta is low and unable to trap all the fractured brittle Bi 2 O 3 particles within the interfaces. This suggests that the powders were fractured and agglomerated repetitively throughout the milling process. No lamellar structure was observed. Bi3TaO7 suggests that tantalum reacts with -Bi2O3 particles and the mechanochemical reaction occurs after only 10 min of high energy ball milling. This shows a behavior similar to one previously reported in literature, in which milling -Bi2O3 and iron leads to the formation of Bi and Fe3O4 [29]. However, the relative peaks' intensities of Bi3TaO7 did not exhibit significant differences at the early stage of milling (i.e., the first 10-30 min). Slight decreases in the peaks' intensities and broadening of peaks was observed after long-term milling from 30 min to three hours (Figure 1b).  Figure 2 shows the cross-sectional SEM images of as-milled (Bi2O3)80(Ta)20 powder. Irregular grains with a wide particle size distribution were be observed at all milling stages starting from five minutes of milling (Figure 2a) to the end of three hours' milling ( Figure 2h). Typically, the highenergy ball milling process can be divided into three different types of milling mechanisms, depending on the characteristics of starting powders: both ductile, ductile/brittle, and both brittle [20]. During ball-powder-ball collision, powder particles undergo deformation and/or a fracture process. Ductile powders usually form a lamellar structure at the beginning and result in homogeneous equiaxed particles at the end of milling. For a ductile/brittle system, brittle particles may fracture and are trapped within ductile interfaces and finally form a metal matrix composite with a uniform distribution of embedded brittle particles, whereas brittle/brittle particles are expected to fracture continuously into small particles and sometimes mechanical alloying between starting particles may occur. The present milling system consisted of 20 wt % ductile Ta and 80 wt % brittle -Bi2O3 particles. The densities of Ta and -Bi2O3 were 16.69 and 8.9 g/cm 3 , respectively. The To better demonstrate the structural evolution, the cross-sectional views with higher magnification of selective as-milled powders are shown in Figure 3. Notably, in the SEM photos obtained using backscattered electron images, the larger the atom, the brighter the area. As marked in Figure 3, the brightest area is bismuth, whereas the darkest area is tantalum, with the brightness of Bi-Ta-O being in between. Only the tantalum particle is close to being spherical in shape, whereas the others (Bi and Bi-Ta-O) are irregular. This element distribution was confirmed using energy dispersive X-ray (EDX) mapping. Figure 4 shows the three-hour as-milled powder with bismuth, tantalum, and oxygen mappings, where corresponding elements are uniformly distributed within the examined area, with the exception of some localized regions. To better demonstrate the structural evolution, the cross-sectional views with higher magnification of selective as-milled powders are shown in Figure 3. Notably, in the SEM photos obtained using backscattered electron images, the larger the atom, the brighter the area. As marked in Figure 3, the brightest area is bismuth, whereas the darkest area is tantalum, with the brightness of Bi-Ta-O being in between. Only the tantalum particle is close to being spherical in shape, whereas the others (Bi and Bi-Ta-O) are irregular. This element distribution was confirmed using energy   Figure 4 shows the three-hour as-milled powder with bismuth, tantalum, and oxygen mappings, where corresponding elements are uniformly distributed within the examined area, with the exception of some localized regions.   Figure 5 shows the DSC curves of (Bi2O3)80(Ta)20 powder after different milling times. An endothermic peak around 271.1 °C (the melting point of bismuth) was observed for as-milled (Bi2O3)80(Ta)20 powders. The five-minute as-milled powder (black line in Figure 5) did not show a distinct endothermic peak, whereas the 10-min as-milled powder (red line in Figure 5 Figure 5 shows the DSC curves of (Bi 2 O 3 ) 80 (Ta) 20 powder after different milling times. An endothermic peak around 271.1 • C (the melting point of bismuth) was observed for as-milled (Bi 2 O 3 ) 80 (Ta) 20 powders. The five-minute as-milled powder (black line in Figure 5) did not show a distinct endothermic peak, whereas the 10-min as-milled powder (red line in Figure 5) exhibited the largest endothermic peak at 272.8 • C, which corresponds to the melting of bismuth. The endothermic peak position did not show significant differences with long-term milling (30-180 min). The peak height, however, decreased with increasing milling time. This suggests that the amount of bismuth formed by mechanochemical reaction quickly reached a maximum during the early stage of milling (10 min) and gradually decreased with prolonged milling. The aforementioned high energy ball milling process is performed in a glove box, in which the total O2 and H2O concentrations are less than 100 ppm. Figure 6 shows the X-ray diffraction patterns of (Bi2O3)80(Ta)20 powder produced using high energy ball milling within an ambient environment in which the oxygen concentration is ~20 vol.%. As shown in the bottom XRD pattern in Figure 6a, formation of -Bi2O4, bismuth and -Bi3TaO7 can be observed after merely 5 minutes of milling treatment (compared to that of 10 mins. within glove box, Figure 1a). This suggests that the mechanochemical reaction can be accelerated with the aid of oxygen. Similar to that of milling (Bi2O3)80(Ta)20 under glove box conditions, no significant differences in peaks' intensities for -Bi2O4 can be observed. Whereas, the diffraction peaks of bismuth reached a maximum after 10 min of milling. Those of the 30-min as-milled powder exhibited a maximum peak height for -Bi3TaO7 phase. The crystalline peaks of -Bi2O3 diminished with milling time and became undistinguishable after 15 min of milling (30 min within glove box). However, those of tantalum persisted for up to 1 h of milling and could not be recognized at the end of 3 h milling. The aforementioned high energy ball milling process is performed in a glove box, in which the total O 2 and H 2 O concentrations are less than 100 ppm. Figure 6 shows the X-ray diffraction patterns of (Bi 2 O 3 ) 80 (Ta) 20 powder produced using high energy ball milling within an ambient environment in which the oxygen concentration is~20 vol %. As shown in the bottom XRD pattern in Figure 6a, formation of α-Bi 2 O 4 , bismuth and δ-Bi 3 TaO 7 can be observed after merely 5 min of milling treatment (compared to that of 10 mins. within glove box, Figure 1a). This suggests that the mechanochemical reaction can be accelerated with the aid of oxygen. Similar to that of milling (Bi 2 O 3 ) 80 (Ta) 20 under glove box conditions, no significant differences in peaks' intensities for α-Bi 2 O 4 can be observed. Whereas, the diffraction peaks of bismuth reached a maximum after 10 min of milling. Those of the 30-min as-milled powder exhibited a maximum peak height for δ-Bi 3 TaO 7 phase. The crystalline peaks of α-Bi 2 O 3 diminished with milling time and became undistinguishable after 15 min of milling (30 min within glove box). However, those of tantalum persisted for up to 1 h of milling and could not be recognized at the end of 3 h milling.  In order to further reveal the phase transition during ball milling process, the XRD results were analyzed by Rietveld fitting method. Table 1 summarizes the phase transitions and the corresponding phase percentages of (Bi2O3)80(Ta)20 powder at various milling stages during high energy ball milling under either glove box or ambient atmosphere conditions. Figure 7 shows the percentage of individual phase as a function of milling time. As shown in Figure 7a for (Bi2O3)80(Ta)20 powder milled under Ar-filled glove box conditions, the starting powders, -Bi2O3 and Ta, transformed into -Bi2O4, bismuth, and -Bi3TaO7 phases. The relative percentage of -Bi2O4 remained ~8 to 9% after 10 min of milling and did not show significant difference thereafter. The percentage of Ta was 15.4% after 5 min of milling, decreased to 2.5% at 10 min milling, and fluctuated around 1.4% to 3.1% through the In order to further reveal the phase transition during ball milling process, the XRD results were analyzed by Rietveld fitting method. Table 1  5 min of milling, decreased to 2.5% at 10 min milling, and fluctuated around 1.4% to 3.1% through the milling process. A gradual increase of bismuth from 23.4% (10 min of milling) to 69.7% (3 h of milling) can be observed. The relative amount of δ-Bi 3 TaO 7 phase, however, reached a maximum of 52.8% after 10 min of milling and gradually decreased to 18.8% after 3 h of milling treatment. Figure 7b shows the percentage of individual phase for (Bi 2 O 3 ) 80 (Ta) 20 powder milled under ambient atmosphere conditions. Similar behavior can be noticed as those shown in Figure 7a, except for the accelerated mechanochemical reaction after 5 min of milling.  Figure 7b shows the percentage of individual phase for (Bi2O3)80(Ta)20 powder milled under ambient atmosphere conditions. Similar behavior can be noticed as those shown in Figure 7a, except for the accelerated mechanochemical reaction after 5 minutes of milling.   Though not shown here, cross-sectional examination of as-milled (Bi2O3)80(Ta)20 powder under ambient atmosphere showed widely distributed irregular grains at all milling stages and did not reveal significant differences from those milled within the glove box ( Figure 2). Figure 8 shows the DSC curves of as-milled powder at various stages. The 5-min as-milled powder exhibited an endothermic peak at 273.1 °C, which is slightly higher than the 271.1 °C melting point of bismuth. This accounts for the existence of bismuth within the 5-mins as-milled powder. After 10-mins of milling, the endothermic peak exhibited a maximum peak height and indicated the reduced bismuth reached a maximum. This confirmed the XRD results discussed in Figure 6. The endothermic peak height decreased with prolonged milling similar to those milled under an Ar-filled glove box environment ( Figure 5). DSC examination confirms that the mechanochemical reaction began at a very early stage of milling (5 min) and continued thereafter throughout the ball milling process. Though not shown here, cross-sectional examination of as-milled (Bi 2 O 3 ) 80 (Ta) 20 powder under ambient atmosphere showed widely distributed irregular grains at all milling stages and did not reveal significant differences from those milled within the glove box ( Figure 2). Figure 8 shows the DSC curves of as-milled powder at various stages. The 5-min as-milled powder exhibited an endothermic peak at 273.1 • C, which is slightly higher than the 271.1 • C melting point of bismuth. This accounts for the existence of bismuth within the 5-mins as-milled powder. After 10-mins of milling, the endothermic peak exhibited a maximum peak height and indicated the reduced bismuth reached a maximum. This confirmed the XRD results discussed in Figure 6. The endothermic peak height decreased with prolonged milling similar to those milled under an Ar-filled glove box environment ( Figure 5). DSC examination confirms that the mechanochemical reaction began at a very early stage of milling (5 min) and continued thereafter throughout the ball milling process. Though not shown here, cross-sectional examination of as-milled (Bi2O3)80(Ta)20 powder under ambient atmosphere showed widely distributed irregular grains at all milling stages and did not reveal significant differences from those milled within the glove box ( Figure 2). Figure 8 shows the DSC curves of as-milled powder at various stages. The 5-min as-milled powder exhibited an endothermic peak at 273.1 °C, which is slightly higher than the 271.1 °C melting point of bismuth. This accounts for the existence of bismuth within the 5-mins as-milled powder. After 10-mins of milling, the endothermic peak exhibited a maximum peak height and indicated the reduced bismuth reached a maximum. This confirmed the XRD results discussed in Figure 6. The endothermic peak height decreased with prolonged milling similar to those milled under an Ar-filled glove box environment ( Figure 5). DSC examination confirms that the mechanochemical reaction began at a very early stage of milling (5 min) and continued thereafter throughout the ball milling process. (Bi 2 O 3 ) 80 (Ta) 20 powders milled within either the glove box or ambient environments were used to investigate the effects of oxygen concentration. Additionally, to determine the influence of tantalum addition, the amount of tantalum was decreased to 5 wt % and high-energy ball milling was executed within a glove box. Figure 9 shows the X-ray diffraction patterns of the as-milled (Bi 2 O 3 ) 95 (Ta) 5 powders at various milling stages. With a limited amount of tantalum addition, the superfluous reactant α-Bi 2 O 3 phase persisted for up to three hours of milling. The crystalline peaks of tantalum were observed after one hour of milling. A new crystalline phase of tetragonal β-Bi 7.8 Ta 0.2 O 12.2 (ICDD PDF card No. 043-0451) was observed after 30 min of milling. During the three hours of milling, bismuth was not reduced, and tantalum was continuously merged into the α-Bi 2 O 3 phase, which induced the formation of β-Bi 7.8 Ta 0.2 O 12.2 phase. When the milling time was increased to 10 h, the crystalline peaks of the starting powders α-Bi 2 O 3 and Ta disappeared, whereas the peak heights of β-Bi 7.8 Ta 0.2 O 12.2 phase increased, and diffraction peaks of newly-formed bismuth were observed. Table 2 summarizes the corresponding crystalline phases and relative phase percentages at various milling stages of (Bi 2 O 3 ) 95 (Ta) 5 powders. The relative amounts of individual phases can be observed better in Figure 10, in which α-Bi 2 O 3 was 56.0% after 30 min of milling, and the amount decreased with increasing milling time and became indistinguishable at the end of milling (10 h). The amount of 30-min as-milled β-Bi 7.8 Ta 0.2 O 12.2 phase was 41.6%, which gradually increased with increasing milling time and reached 90.8% after 10 h of milling. In addition, the diffraction peaks of Ta disappeared after three hours of milling and the formation of bismuth was observed after 10 h milling.  Figure 8. Differential scanning calorimetry (DSC) curves of (Bi2O3)80Ta20 for different milling times under air.
(Bi2O3)80(Ta)20 powders milled within either the glove box or ambient environments were used to investigate the effects of oxygen concentration. Additionally, to determine the influence of tantalum addition, the amount of tantalum was decreased to 5 wt % and high-energy ball milling was executed within a glove box. Figure 9 shows the X-ray diffraction patterns of the as-milled (Bi2O3)95(Ta)5 powders at various milling stages. With a limited amount of tantalum addition, the superfluous reactant -Bi2O3 phase persisted for up to three hours of milling. The crystalline peaks of tantalum were observed after one hour of milling. A new crystalline phase of tetragonal -Bi7.8Ta0.2O12.2 (ICDD PDF card No. 043-0451) was observed after 30 min of milling. During the three hours of milling, bismuth was not reduced, and tantalum was continuously merged into the -Bi2O3 phase, which induced the formation of -Bi7.8Ta0.2O12.2 phase. When the milling time was increased to 10 h, the crystalline peaks of the starting powders -Bi2O3 and Ta disappeared, whereas the peak heights of -Bi7.8Ta0.2O12.2 phase increased, and diffraction peaks of newly-formed bismuth were observed. Table 2 summarizes the corresponding crystalline phases and relative phase percentages at various milling stages of (Bi2O3)95(Ta)5 powders. The relative amounts of individual phases can be observed better in Figure 10, in which -Bi2O3 was 56.0% after 30 min of milling, and the amount decreased with increasing milling time and became indistinguishable at the end of milling (10 h). The amount of 30-min as-milled -Bi7.8Ta0.2O12.2 phase was 41.6%, which gradually increased with increasing milling time and reached 90.8% after 10 h of milling. In addition, the diffraction peaks of Ta disappeared after three hours of milling and the formation of bismuth was observed after 10 h milling.     The starting powder, -Bi2O3, exhibits a monoclinic structure that is stable at room temperature. Unlike -Bi2O3, -Bi2O3 is fluorite-like cubic structure that is stable at high temperatures (730-825 o C). During phase transformation, two intermediate metastable phases, which are either a tetragonal  phase or a body-centered cubic  phase, can be observed. In the present study, for the (Bi2O3)95(Ta)5 powder mixture milled under glove box conditions, tantalum slowly but continuously reacted with -Bi2O3 phase and formed tetragonal -Bi7.8Ta0.2O12.2 phase. When (Bi2O3)80(Ta)20 was milled under glove box conditions, the mechanochemical reaction began after 10 minutes of high-energy ball milling and formation of fluorite-like -Bi3TaO7 was observed. By milling under ambient environment conditions, the oxygen concentration was higher, allowing the mechanochemical reaction to accelerate, which triggered the reaction within less than five minutes of milling.

Conclusions
By milling (Bi2O3)80(Ta)20 and (Bi2O3)95(Ta)5 compositions, high-temperature fluorite-like phases were prepared using a mechanochemical reaction. By milling (Bi2O3)95(Ta)5 powder mixtures within an Ar-filled glove box, tantalum gradually reacted with -Bi2O3 phase and formed -Bi7.8Ta0.2O12.2 phase after 10 minutes of milling. By increasing tantalum addition to 20 wt %, superfluous Ta can be mechanochemically reacted with -Bi2O3 to form -Bi3TaO7 and bismuth phases after 10 minutes of ball milling. The mechanochemical reaction can be shortened to less than five minutes by increasing oxygen concentration (i.e., milling (Bi2O3)80(Ta)20 under ambient atmosphere conditions), which resulted in the formation of a high temperature -Bi3TaO7 phase. The starting powder, α-Bi 2 O 3 , exhibits a monoclinic structure that is stable at room temperature. Unlike α-Bi 2 O 3 , δ-Bi 2 O 3 is fluorite-like cubic structure that is stable at high temperatures (730-825 • C). During phase transformation, two intermediate metastable phases, which are either a tetragonal β phase or a body-centered cubic γ phase, can be observed. In the present study, for the (Bi 2 O 3 ) 95 (Ta) 5 powder mixture milled under glove box conditions, tantalum slowly but continuously reacted with α-Bi 2 O 3 phase and formed tetragonal β-Bi 7.8 Ta 0.2 O 12.2 phase. When (Bi 2 O 3 ) 80 (Ta) 20 was milled under glove box conditions, the mechanochemical reaction began after 10 min of high-energy ball milling and formation of fluorite-like δ-Bi 3 TaO 7 was observed. By milling under ambient environment conditions, the oxygen concentration was higher, allowing the mechanochemical reaction to accelerate, which triggered the reaction within less than five minutes of milling.

Conclusions
By milling (Bi 2 O 3 ) 80 (Ta) 20 and (Bi 2 O 3 ) 95 (Ta) 5 compositions, high-temperature fluorite-like phases were prepared using a mechanochemical reaction. By milling (Bi 2 O 3 ) 95 (Ta) 5 powder mixtures within an Ar-filled glove box, tantalum gradually reacted with α-Bi 2 O 3 phase and formed β-Bi 7.8 Ta 0.2 O 12.2 phase after 10 min of milling. By increasing tantalum addition to 20 wt %, superfluous Ta can be mechanochemically reacted with α-Bi 2 O 3 to form δ-Bi 3 TaO 7 and bismuth phases after 10 min of ball milling. The mechanochemical reaction can be shortened to less than five minutes by increasing oxygen concentration (i.e., milling (Bi 2 O 3 ) 80 (Ta) 20 under ambient atmosphere conditions), which resulted in the formation of a high temperature δ-Bi 3 TaO 7 phase.