Z. Zhuang

Electrical properties of garnet-like lithium ionic conductors Li5+xSrxLa3−xBi2O12 fabricated by spark plasma sintering method

A typical approach involving Pechini method and spark plasma sintering (SPS) method was presented for the preparation of high density Li5+xSrxLa3−xBi2O12 (x = 0, 1) ceramics. Phase formation, microstructure, grain size and electrical properties of the specimens were examined using XRD, SEM and alternating current impedance spectroscopy (ACIS). Dense Li5La3Bi2O12 and Li6SrLa2Bi2O12 ceramics with pure garnet-like phase, relative density of 97% and average grain size of about 5 μm were fabricated using this approach. The total conductivities at 298 K of Li5La3Bi2O12 and Li6SrLa2Bi2O12 ceramics prepared by the SPS method are 5.1×10−5 and 6.8×10−5 S/cm, respectively, 2 times higher than that of samples prepared by the conventional sintering method.

Internal friction study of oxygen ion conductors La1.95K0.05Mo2−xTxO9−δ (T=Fe,Mn)

Based on the oxygen ion conductor La 2 Mo 2 O 9 , a series of K, Fe, and Mn doped samples La 1.95 K 0.05 Mo 2 − x T x O 9 − δ ( T = Fe , Mn ; x = 0 , 0.025 , 0.05 , 0.1 ) were prepared with conventional solid-state reaction method. The effects of Fe and Mn doping on the cationic and anionic ionsdiffusion,phase transition, and electrical conductivity were studied. The mechanical spectroscopy (or internal friction) technique was exploited to investigate the microscopic transport mechanism and to deduce the dynamical relaxation parameters. Three internal friction peaks ( P L , P H , and P M ) were observed in the La 1.95 K 0.05 Mo 2 − x T x O 9 − δ samples, among which P L and P M are of relaxational type while P H is associated with a kind of phase transition. The results revealed that peak P L is originated from the short-distance jumps of oxygen vacancies, while peak P M is associated with the short-distance diffusion of Mn (Fe) ions and peak P H with the phase transition from the static disordered state to the dynamic disordered state of oxygen ion distribution. Based on the results of conductivity measurements, it can be concluded that the high-temperature conductivity of La 2 Mo 2 O 9 can be improved obviously by the Fe or Mn doping at Mo site and the low-temperature conductivity can be increased by the simultaneous K doping at La site.

Relaxation Model of Lithium Ions in the Garnet-like Li5La3Bi2O12Lithium-Ion Conductor

The relaxation mechanism of lithium ions in Li5La3Bi2O12 electrolyte was investigated by internal friction (IF) method. A prominent relaxation-type IF peak was observed. From the shift of peak position with frequency, the activation energy of E=1.0-1.1 eV and the pre-exponential factor of relaxation time in the order of τ0 =10-25 ~10-18 s were obtained if one assumes a distributed Debye relaxation process. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Basing on the coupling model, the values of E and τ0 were determined as 0.5-0.6 eV and 10-17 ~ 10-15 s, which still deviated from the typical values for point defect relaxation. A new phenomenological model was used to describe the relaxation process, and the values of E and τ0 were determined as 0.46 eV and 3.6×10-14 s, which is in agreement with the typical values for point defect relaxation.

Phase Transition and Oxygen Ion Diffusion in (La1−xLnx)2Mo2O9 (Ln = Nd, Gd, x = 0.05-0.25) Using Dielectric Relaxation Method

Dielectric relaxation method was employed to study the properties of oxygen ion diffusion and phase transition in the oxide-ion conductors (La1−xLnx)2Mo2O9 (Ln = Nd, Gd, x = 0.05-0.25). Two dielectric loss peaks were observed: peak Pd at about 600 K and peak Ph around 720 K. Peak Pd is a relaxational peak and associated with the short-range diffusion of oxygen ions, while peak Ph hardly changes its position and dramatically decreases in height with increasing frequency, exhibiting non-relaxational nature. With increasing Ln3+ concentration, the heights of peak Ph and Pd increase at first and then decrease after passing a maximum at 15% doping. It is suggested that peak Ph is related to the phase transition of a static disordered state to a dynamic disordered state in oxygen ions/vacancies distribution. It is found that the 15%Gd or 15%Nd doped La2Mo2O9 samples exhibit the highest conductivity in accordance with the highest height of peak Pd at this doping content.

Internal Friction Study of the Rare Earth Ion Substituted Fast Oxide-Ion Conductors (La1-ΧreΧ)2Mo2O9 (Re=Nd, Gd)

The relaxation and phase transition behaviors of rare-earth ion substituted fast oxide-ion conductors (La1-xRex)2Mo2O9 (Re=Nd, Gd) were investigated by internal friction (IF) measurement in the temperature range 300 K - 950 K. Three different IF peaks (labeled as PL, PH, and PG, respectively) were observed in the rare-earth ion doped La2Mo2O9 samples. Peak PL corresponds to short diffusion processes of oxygen ions among different oxygen vacancy sites. Peak PH is associated with the static/dynamic disorder transition in oxygen ion distribution. Peak PG is a newly discovered peak embodying phase transition-like characteristics and is suggested to be related to order-disorder transition associated with the rearrangement of La/ Re sub-lattice.

Enhancement of Ionic Conductivity in La2Mo2O9-Based Nanocrystalline Films Synthesized by a PVP Assisted Sol-Gel Method

High-quality 5% W- and Gd-doped La2Mo2O9 nanocrystalline films were successfully synthesized on poly-alumina substrates via a PVP-assisted sol-gel method with inorganic salts as precursors, combined with the rapid thermal annealing and novel three stages annealing procedures. Dense and uniform La2Mo2O9-based films with 90 nm in grain size and 1 μm in thickness were obtained by only twice spin coatings and sintering at 600 oC. The electrical conductivity of the films at 600 oC reaches 0.028 and 0.025 S/cm for W- and Gd-doped samples, respectively.