Mechanosynthesis of Nanocrystalline Fully Stabilized bcc γ-phase of Bi2O3 without Any Additive: Manifestation of Ferroelasticity in Microstructure, Optical, and Transport Properties (original) (raw)
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Journal of the Serbian Chemical Society, 2009
A powder mixture of α-Bi 2 O 3 and HfO 2 , in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill under air, using zirconium oxide vials and balls as the milling medium. After 50 h of milling, the mechanochemical reaction led to the formation of a nanocrystalline δ-Bi 2 O 3 phase (fluorite-type solid solution Bi 0.78 Hf 0.59 Zr 0.63 O 3.61 ), with a crystallite size of 20 nm. The mechanochemical reaction started at a very beginning of milling accompanied by an accumulation of ZrO 2 arising from the milling tools. The samples prepared after various milling times were characterized by X-ray powder diffraction and DSC analysis. The electrical properties of the as-milled and pressed Bi 0.78 Hf 0.59 Zr 0.63 O 3.61 powder were studied using impedance spectroscopy in the temperature range from 100 to 700 °C under air. The electrical conductivity was determined to be 9.43×10 -6 and 0.080 S cm -1 for the temperatures of 300 and 700 °C, respectively.
Chemically stabilized δ-Bi2O3 phase: Raman scattering and X-ray diffraction studies
Oriental Journal of Chemistry, 2016
Thanks to its peculiar structural properties, the high temperature ä-phase of Bi 2 O 3 is considered as the best oxide ion conductor. Many efforts to stabilize this structure at room temperature have been deployed. In the present study, we have successfully stabilized the ä-phase by chemically introducing tetra-Te 4+ and pentavalent Ta 5+ cations into the structure. A series of compounds with different percentage of Te 4+ / Ta 5+ were obtained. Their structural and vibrational properties were investigated. From the Rietveld refinement of X Ray diffraction pattern we show that the composition x = 0.2 crystallizes in the cubic symmetry, space group Fm 3m (ITA No. 225) with a lattice parameter a =5.49 Å. The reliability factors are: R F =2.151 % and R Bragg =2.545 % confirm the goodness of the refinement. From the evolution of Raman bands, we confirm the existence of the solid solution features. Furthermore, comparing the spectra of ä-Bi 2 O 3 with the alpha phase, we comfortably suggest that the decrease of the number of Raman bands is a consequence of an increase in the lattice symmetry. Similarly to other fluorite compounds, we show that the structure presents oxygen defects clearly identified in the Raman spectra.
Theoretical investigation of the pure and Zn-doped a and d phases of Bi2O3
Phys Rev B, 2002
We have studied the atomic and electronic structure of pure and Zn-doped ␣ and ␦ phases of Bi 2 O 3 by first-principles calculations. For the pure ␣ phase which is monoclinic, good agreement was obtained between the experimental and calculated structural parameters and, in addition, the calculated density of states in the valence band and the optical band gap correlated well with photoemission spectra. For the pure ␦ phase, which has a defective fluorite structure, the calculations suggest that of three possible oxygen vacancy structures, ͗100͘-vacancy ordering is preferred. This phase, however, must be considered as a supercooled phase at T ϭ0 K since we found that a single displaced vacancy ͑i.e., one that deviates from ͗100͘ ordering͒ can trigger a ␦-␣ phase transition. Similarly, a Zn substitutional impurity in the ␦ phase can also trigger this phase transition. The formation energy of a Zn impurity in the ␣ phase was found to be 1.34 eV, resulting in a maximum impurity concentration of 7.1ϫ10 Ϫ6 at. % Zn at Tϭ1000 K. The low solubility of Zn in the ␣ phase of Bi 2 O 3 is consistent with the observed phase separation between ZnO and Bi 2 O 3 .
Solid State Ionics, 2011
Bi 2 O 3-Dy 2 O 3 Nano-crystalline ceramic SPS Pressureless sintering Oxide ion conductivity Using (Bi 2 O 3) 0.75 (Dy 2 O 3) 0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi 2 O 3) 0.75 (Dy 2 O 3) 0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500°C. During the pressureless sintering process, the grain growth behavior of (Bi 2 O 3) 0.75 (Dy 2 O 3) 0.25 followed a parabolic trend, expressed as D 2 − D 0 2 = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol − 1. Dense (Bi 2 O 3) 0.75 (Dy 2 O 3) 0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi 2 O 3 .
2020
Thanks to its peculiar structural properties, the high temperature ä-phase of Bi 2 O 3 is considered as the best oxide ion conductor. Many efforts to stabilize this structure at room temperature have been deployed. In the present study, we have successfully stabilized the ä-phase by chemically introducing tetra-Te 4+ and pentavalent Ta 5+ cations into the structure. A series of compounds with different percentage of Te 4+ / Ta 5+ were obtained. Their structural and vibrational properties were investigated. From the Rietveld refinement of X Ray diffraction pattern we show that the composition x = 0.2 crystallizes in the cubic symmetry, space group Fm 3m (ITA No. 225) with a lattice parameter a =5.49 Å. The reliability factors are: R F =2.151 % and R Bragg =2.545 % confirm the goodness of the refinement. From the evolution of Raman bands, we confirm the existence of the solid solution features. Furthermore, comparing the spectra of ä-Bi 2 O 3 with the alpha phase, we comfortably sugge...
Holmium (Ho)-doped Bi2O3 nanoceramic powders derived from sol-gel method have been studied in terms of structural, morphological, and electrical properties. The morphology of the nanoceramic materials was analyzed by scanning electron microscopy (SEM) and their structure by Xray powder diffraction (XRD). Temperature dependence of DC conductivity measurements of nanoceramic powders were carried out by using DC four-point probe technique (4PPT) in air at temperatures ranging from 429 °C to 896 °C (702-1169 K). Electrical conductivity results demonstrate that there is a sharp increase at around 700 °C, which indicates an existence of order-disorder transition. This result supported by the Differential Thermal Analyzer (DTA) curve and XRD pattern which show that the sample has stable high oxygen ionic conductivity fluorite type face centered cubic -phase. Electrical characteristics also show that the DC conductivity in the studied materials obeys Arrhenius relation with different activation energies and conduction mechanisms: two temperature regions with activation energies Ea1=1.40 eV (702-993 K) and Ea2=0.66 eV (1006-1169 K). The analysis of experimental data revealed that the translation motion of the charge carrier, oxygen vacancies, and space charge polarization are responsible for the change in activation energy as a function of temperature.
Synthesis of nanocrystalline Bi2Te3 via mechanical alloying
Journal of Materials Processing Technology, 2009
Nanocrystalline Thermoelectric a b s t r a c t Bi 40 Te 60 thermoelectric compound was fabricated via mechanical milling of bismuth and tellurium as starting materials. Effect of the milling time and heat treatment temperatures were investigated. In order to characterize the ball milled powders, the X-ray diffraction (XRD) was used. Thermal behavior of the mechanically alloyed powders was studied by differential thermal analysis (DTA). The morphological evolutions were studied by scanning electron microscopy (SEM). Results showed that the nanocrystalline Bi 2 Te 3 compound was formed after 5 h of milling. Further milling (25 h) and heating to 500 • C showed that the synthesized phase was stable during these conditions. Nanocrystalline Bi 2 Te 3 with 9-10 nm mean grain size and flaky morphology (lamellar structure) was obtained at the end of milling.
Thorium doped and thorium-carbon co doped metastable β-Bi2O3
Solid State Sciences, 2019
β-Bi 2 O 3 was stabilized by doping it with 10 mol % thorium following solution combustion synthesis and co precipitation method. The samples have been characterized extensively. Formation of tetragonal β-Bi 2 O 3 was confirmed for these samples from powder X-ray diffraction and Raman spectroscopy measurements. The presence of oxygen vacancies in these samples was inferred from the band near 615 cm-1 in the Raman spectra as well as emission in the blue region in photoluminescence spectra. While flaky morphology was noticed for β-Bi 2 O 3 sample from combustion synthesis, sample from co precipitation method showed porous morphology. The inclusion of thorium was verified from energy dispersive spectral analysis. The optical band gap of 10 mol % thorium doped β-Bi 2 O 3 sample prepared by combustion synthesis was 2.24 eV which was lower than undoped α-Bi 2 O 3 (2.67 eV). Further reduction in the band gap was observed for 10 mol % thorium doped β-Bi 2 O 3 sample from co precipitation method (2.03 eV). Such a drastic reduction was attributed to the additional doping of carbon (7.87 %) in the sample as verified from X-ray photoelectron spectroscopic analysis. Samples from combustion synthesis and co precipitation methods exhibited BET surface area of 209 and 117 m 2 /g, respectively. Although the samples from both these synthetic methods showed the presence of defects, β-Bi 2 O 3 from co precipitation method showed better efficiency than the sample from combustion method due to band gap narrowing.