Papers by Keyword: Order-Disorder Transition

Abstract: Chemical co-precipitation method was used to prepare Gd₂Zr₂O₇ powders. The powders were then heated in air at 1500°C, 1510°C, 1525°C, 1530°C, 1550°C, 1575°C, 1600°C for 5 h, and 1575°C, 1600°C for 10 h, respectively. The samples after heat treatment were characterized by X-ray diffraction and Raman spectroscopy. X-ray diffraction patterns reveal that order-disorder transition of Gd₂Zr₂O₇ occurs between 1550°C and 1575°C. Prior to the transition, the relative intensity of peaks corresponding to the super-lattice of pyrochlores increases with the increasing temperature. On the other hand, Raman spectra give an inconsistent result from the X-ray diffraction data. No appreciable difference can be observed for all the samples though with peaks broadening as temperature increases. The spectra of the samples indicated as fluorite structure in X-ray diffraction patterns appear with six resolvable peaks which is quite different from the spectrum of ideal fluorite structure.

Abstract: Pyrochlore oxides La₂Zr₂O_7, Ce₂Zr₂O₇ and their solid solutions were studied by in situ x-ray diffraction (XRD) measurement at high pressures. Pressure dependence of cationic disordering was derived from Rietveld refinement of the XRD patterns. The results indicated that the mixed occupancy in the cation site enhanced the compressibility obviously. All the pyrochlore structure became unstable at ~20 GPa and an orthorhombic high-pressure phase formed. The cations changed from the ordered state in pyrochlore to a disordered state in the high-pressure phase. The high-pressure phase is about 8% denser than the pyrochlore phase and not stable at ambient conditions and a disordered defect-fluorite structure was quenched for all the samples.

Abstract: Kinetics of order-disorder transition at antiphase domain boundary (APDB) formed between DO₂₂ (Ni₃V) phases during stress aging was investigated using microscopic phase field model. The results demonstrated that whether order-disorder transition happens or not depends on the atomic structure of the APDB. Accompanied with the depletion of V and enrichment of Ni and Al, order-disorder transition happened at the APDB (001)//(002). Whereas at the APDB {100}·1⁄2[100], which remains ordered with temporal evolution, Ni and Al enrich and V depletes. Composition evolution of APDB with order-disorder transition favors the nucleation of the L1₂ and disordered phase. Some of the grains grew bigger while the others disappeared, accompanying the formation of disordered phase layer during order-disorder transition of APDBs, and the order-disorder transition of APDBs can be considered as accompanying process of coarsening of ordered domain phases and growth of disordered phases.

Abstract: The order-disorder transition at antiphase domain boundary (APDB) between DO22 (Ni3V) phases is investigated using the microscopic phase-field model. After the formation of ordered APDB, the order-disorder transition at APDB is happened, and the ordered APDB transforms into a thin disordered phase layer. Accompanied with the enrichment of Ni and Al at the disordered APDB, the second phase L12 nucleates at the order-disorder interface between DO22 phases and grows along the disordered phase layer. The order-disorder transition at the ordered APDB makes the nucleation and growth of the second phase L12 much easier and faster. The disordered phase caused by the order-disorder transition at the APDB can be considered as the transient phase during the precipitation process of L12 phase.

Abstract: Based on the microscopic phase-field model, the precipitation process of Ni75Al4.3V20.7 alloy at 1190K is simulated, and the kinetics of order-disorder transition at antiphase domain boundary (APDB) formed between DO22 (Ni3V) phases is investigated. After the ordered APDB formed by the impingement of growing DO22 (Ni3V) domains, the order-disorder transition at APDB is happened. Accompanied with the enrichment of Ni and Al at the APDB, the ordered APDB transforms into a thin disordered phase layer. The second phase L12 nucleates at the order-disorder interface between DO22 and disordered phases, and grows along the disorder phase layer quickly. The order-disorder transition at the ordered APDB accelerates the nucleation and growth of L12 phase at the APDB. The disordered phase caused by the order-disordered transition can be considered the transient phase during the precipitation process of L12 phase.

Abstract: The phase equilibria at 900 °C and B2/L21 order-disorder transition in the Ni-Mn-Ga ternary system were investigated by analyzing the equilibrated alloys and diffusion couples using a combination of techniques. It was confirmed that a bcc single phase region exists in a wide composition range at 900 °C, and the critical temperatures of B2/L21 order-disorder transition were determined in Ni-50 at.% section, which exhibits a maximal ordering transition temperature of 796 at Mn content of 25 at.%. The obtained results will be helpful for the preparation and annealing of Ni-Mn-Ga alloys in the specific temperature.

Abstract: Origins of high Q are considered on intrinsic as high symmetry, ordering structure and high density crystal structure. It was concluded that the high symmetry brings high Q instead of ordering comparing some cases as follows: As if ordering ratio of Ba(Zn1/3Ta2/3)O3 (BZT) is high of about 80%, Q values are distributed from low to high Q. Disordered BZT ceramics with high density obtained for short sintering time by spark plasma sintering (SPS) showed high Q. Ba(Zn1/3Nb2/3)O3 (BZN) with order-disorder transition showed high Q at disorder form sintered over the transition temperature. And, the disordered BZN with high Q annealed at lower temperature changed to order structure without improvement of Q.

Abstract: We have discovered a high oxide ion conductor within the perovskite-type (Ba1-x-ySrxLay)InO2.5+y/2 solid-solution system. The system was derived from brownmillerite-type Ba2In2O5, which possessed a ordered oxide ion vacancies. When we doped La3+ into the Ba site, the vacancy changed to a disordered state. The oxide ion conductivity increased with the amount of doped La3+, reaching a maximum value of 0.12 (S/cm) at 800 oC in (Ba0.3Sr0.2La0.5)InO2.75, a level exceeding that of yttria-stabilized zirconia. The oxide ion conductivity of this system was strongly dependent on the unit cell free volume, which appears to be the key parameter governing oxide ion mobility.

Abstract: The tracer diffusion coefficients of Fe and Pd have been measured in FePt and FePt3 by standard radiotracer experiments. In FePt3 the diffusion coefficients are lower than extrapolations of those in the high-temperature disordered state, exhibiting curved Arrhenius plots below the transformation temperature. The diffusion of Pd is faster than that of Fe, and noticeable dependence on composition is found for the latter. In the L10 ordered FePt, the diffusion of Fe has been verified to be slower in the direction of the tetragonal axis, but the case of Pd appears more complex. The diffusion coefficients of Fe and Pd are similar in magnitude in the L10 ordered state.