Li₂O‐doped MgAl₂O₄ nanopowders: Energetics of interface segregation

Abstract

Manufacturing nanoceramics is challenging owing to the instability of the grain size resulting from the high driving force toward growth associated with the interfaces. Nanometric ceramics of some oxides have exceptional mechanical and optical properties, eg, magnesium aluminate spinel (MgAl 2 O 4 ). The production of these fully conformed ceramics requires a precursor powder, which generally contains sintering-promoting additives. Li salts are typically used as sintering promoters for MgAl 2 O 4 , but the interface stability associated with the segregation of the additive is poorly understood. In this study, MgAl 2 O 4 samples containing 0-2.86 mol% Li ions were synthesized via a simultaneous-precipitation method in an ethylic medium and subsequently calcined at 800°C in air. The nanopowders exhibited only the MgAl 2 O 4 phase, and the crystallite size was determined by the Li 2 O concentration. The crystallite size was changed via the chemical modification of the interfaces by the segregation of Li ions. The solubility in the bulk material was very low at the fabrication temperature, and small amounts of Li ions saturated the bulk material and segregated to the grain boundaries (GBs), significantly stabilizing the grain-grain interface compared with the surface. The resulting powder was then aggregated further owing to the initial stage of sintering. The surface excess obtained via the selective lixiviation method confirmed that the segregation to the GBs was greater than that to the surface. Energetics calculations confirmed these results, indicating a high enthalpy of segregation at the GBs (ΔH segr GB = −52.0 kJ∕mol) compared with that at the surfaces (ΔH segr s = −31.5 kJ/mol). The enthalpy of segregation together with theinterface excess allowed us to estimate the reduction in the interface energy with Li + segregation of 0.8% to the surface and 11.2% to the GBs. The Li + segregation to the surfaces started by Al 3+ substitution, and for powders with ≥1.8 mol% Li ions, Mg +2 was preferentially substituted at the surfaces.

Solid‐state NMR for the analysis of interface excesses in Li‐doped MgAl2O4 nanocrystals

“…The dissolution in the lattice is the most accepted pathway taken by dopants and is predicted by phase diagrams; however, particularly at the nanoscale, the dopant distribution cannot be solely determined by the bulk solubility but by a complex energy balance among the surface, the GB, and the bulk. This balance of energy is primarily related to the impact of dopants on each local energetics, affecting the overall thermodynamic metastability as previously described in the literature 6–14 . Thus, one can expect a decrease in the surface and GB energies associated with dopant segregation, which decreases the grain growth rate.…”

“…The distribution of Li + across the system certainly plays an important role in governing the sintering pathways 7 . This is because, according to the Gibbs adsorption isotherm, the amount of surface and GB excesses should control the local energies.…”

“…Figures 5 and 6 exhibit the Li + and Mg 2+ surface excesses (Γ Li and Γ Mg ) for 1.00 and 2.50 mol% Li‐doped MgAl 2 O 4 ‐sintered samples, respectively. The results were acquired by chemical analysis of the leaching supernatant obtained by the selective lixiviation method 20,7,21 normalΓLi${\Gamma _{Li}}$ increased significantly from 0 to 15 min of sintering for both samples, which was due to grain growth and the consequent reduction in SSA (Table 3).…”

Interface segregation of iron sintering aid in gadolinium-doped ceria

“…The crystallite sizes decreased as the iron content increased, a feature associated with the effect of segregated dopants on reducing the interfacial energies. 44–47 According to coarsening models, during the isothermal growth of nanoparticles, the average particle size ( d t ) dependence with time ( t ) follows a general law of growth (eqn (2)). d n t − d n 0 = kDσ i ( t − t 0 )where d 0 is the average particle size at t 0 ; D is the diffusion coefficient; σ is the interfacial energy; k is a constant that depends on the approach to develop the kinetic equation; the exponent n = 2, 3, …, generally reflecting the growth mechanism.…”

“…The crystallite sizes decreased as the iron content increased, a feature associated with the effect of segregated dopants on reducing the interfacial energies. [44][45][46][47] According to coarsening models, during the isothermal growth of nanoparticles, the average particle size (d t ) dependence with time (t) follows a general law of growth (eqn (2)).…”