Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates

Abstract

This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln2Hf2O7 (Ln=Sm3+, Eu3+, Gd3+, Dy3+, Y3+, Ho3+, Yb3+); the title samples were prepared at room temperature by mechanically milling stoichiometric mixtures of the corresponding elemental oxides. Irrespective of the lanthanide ion involved, milling promotes the formation of highly disordered fluoritelike materials. Postmilling thermal treatments facilitate the formation of the fluorite ordered derivative, the pyrochlore structure, but only for the larger lanthanides (Sm3+, Eu3+, Gd3+). Impedance spectroscopy measurements revealed that these materials show a moderate‐to‐good oxygen ion conductivity at high temperatures; furthermore, those adopting the pyrochlore structure give higher σdc and lower Edc than their fluorite analogues (σdc at 750°C>10−3 S·cm−1 vs <5·10−4 S·cm−1, respectively). The same trend also holds for the thermal resistivity at high temperatures; the highest thermal resistivity and thus, lowest κ was obtained for Eu2Hf2O7 (κ~1.3·W·m−1·K−1 at 800°C). Therefore, Ln2Hf2O7 phases might be attractive component materials for electrochemical devices and thermal insulating coatings.

Environmentally sustainable facile synthesis of nanocrystalline holmium hafnate (Ho2Hf2O7): Promising new oxide-ion conducting solid electrolyte

“…Stable defect fluorite structured nanopowder of Ho 2 Hf 2 O 7 with unit cell lattice parameters a = b = c = 5.1308 Å and α = β = γ = 90° were prepared employing LAP which is in excellent agreement with the corresponding values reported in literature of Ho 2 Hf 2 O 7 obtained mechanochemically [14,15] and thermodynamically [16]. XRD reference pattern (ICDD-00-024-0473) considered as standard for the determination of the lattice parameters and a standard deviation (σ) of 0.0346 was found.…”

“…Pyrochlore oxides have six theoretical Raman active modes [16,[29][30][31][32][33][34][35][36][37] These Raman active modes represent the vibrations of 〈Ln-O〉 and 〈Hf-O〉 bonds [14,[32][33][34][35][36][37][38][39][40]. Raman spectroscopy is more suitable to study anion disorder in pyrochlores as these modes involve motion of oxygen atoms.…”

“…The interplanar distance calculated by Shojan et al [15] is 2.60 Å for (X00) plane is in good agreement for (200) plane as shown in Table 2. Using high purity feed material nanoparticles of Ho 2 Hf 2 O 7 can be successfully prepared by employing novel Leeds Alginate Process (LAP) at significantly low temperature (~ 600 °C) and in an energy efficient manner compared with other processes [14][15][16]. Nanopowders of holmium hafnate were pressed into pellet and sintered at 1500 °C for 2 h. Ac-impedance spectroscopy was carried out at selected three temperatures i.e.…”

“…Ho 2 Hf 2 O 7 has been prepared mechanochemically [14,15] and thermodynamically [16] for metal-oxide semiconductor devices [15], electrochemical devices [15] and nuclear waste storage materials [8]. Mehanochemical synthesis of holmium hafnate involved the extensive ball milling and overnight heating at 900 °C [14] and in other case ball milling of holmium and hafnium salts followed by calcination at 1500 °C for 24 h [15]. Synthesis of holmium hafnate thermodynamically involved the three heating protocol of 1200 °C for 36 h, 1300 °C for 36 h and 1400 °C for 48 h along with intermittent grinding [16].…”

Fabrication and electrical conductivity of Lu2(Ti1-xHfx)2O7 transparent ceramics prepared by spark plasma sintering

“…With further increasing x to 0.5, the conductivity decreased slightly. The highest conductivity was lower than that of Lu 2+x Ti 2-x O 7-x/2 (1.0-2.5 × 10 −1 S m −1 at 1013 K) [5] and RE 2 Hf 2 O 7 (2.2 × 10 −2 -1.9 × 10 −1 S m −1 at 1023 K) [24] and much higher than that of Lu 2 Hf 2 O 7 (10 −4 -10 −3 S m −1 at 1013 K) [6]. The temperature dependence of the conductivity for the Lu 2 (Ti 1-x Hf x ) 2 O 7 (x = 0-0.5) transparent ceramics was analyzed by using Arrhenius formula: σ = (σ 0 /T) exp(-E a /k B T), where σ 0 is the pre-exponential constant, E a is the activation energy, k B is Boltzmann's constant, and T is the absolute temperature, as shown in Figure 10.…”