Critical Roles of Impurities and Imperfections in Various Phases of Materials

Merits and Demerits of Machine Learning of Ferroelectric, Flexoelectric, and Electrolytic Properties of Ceramic Materials

In the present review, the merits and demerits of machine learning (ML) in materials science are discussed, compared with first principles calculations (PDE (partial differential equations) model) and physical or phenomenological ODE (ordinary differential equations) model calculations. ML is basically a fitting procedure of pre-existing (experimental) data as a function of various factors called descriptors. If excellent descriptors can be selected and the training data contain negligible error, the predictive power of a ML model is relatively high. However, it is currently very difficult for a ML model to predict experimental results beyond the parameter space of the training experimental data. For example, it is pointed out that all-dislocation-ceramics, which could be a new type of solid electrolyte filled with appropriate dislocations for high ionic conductivity without dendrite formation, could not be predicted by ML. The merits and demerits of first principles calculations and physical or phenomenological ODE model calculations are also discussed with some examples of the flexoelectric effect, dielectric constant, and ionic conductivity in solid electrolytes.

Possibility of High Ionic Conductivity and High Fracture Toughness in All-Dislocation-Ceramics

Based on the results of numerical calculations as well as those of some related experiments which are reviewed in the present paper, it is suggested that solid electrolytes filled with appropriate dislocations, which is called all-dislocation-ceramics, are expected to have considerably higher ionic conductivity and higher fracture toughness than those of normal solid electrolytes. Higher ionic conductivity is due to the huge ionic conductivity along dislocations where the formation energy of vacancies is considerably lower than that in the bulk solid. Furthermore, in all-dislocation- ceramics, dendrite formation could be avoided. Higher fracture toughness is due to enhanced emissions of dislocations from a crack tip by pre-existing dislocations, which causes shielding of a crack tip, energy dissipation due to plastic deformation and heating, and crack-tip blunting. All-dislocation-ceramics may be useful for all-solid-state batteries.

Mechanism of the Decrease in Surface Tension by Bulk Nanobubbles (Ultrafine Bubbles)

The mechanism of the decrease in the surface tension of water containing bulk nanobubbles (ultrafine bubbles) is studied theoretically by numerical simulations of the adsorption of bulk nanobubbles at the liquid's surface based on the dynamic equilibrium model for the stability of a bulk nanobubble under the conditions of the Tuziuti experiment (Tuziuti, T., et al., Langmuir, 2023, 39, 5771-5778). It is predicted that the concentration of bulk nanobubbles in the bulk liquid decreases considerably with time, as many bulk nanobubbles are gradually adsorbed at the liquid's surface. A part of the decrease in surface tension is due to the Janus-like structure of a bulk nanobubble that could partly break the hydrogen bond network of water molecules at the liquid's surface because more than 50% of the bubble's surface is covered with hydrophobic impurities, according to the dynamic equilibrium model. The theoretically estimated decrease in surface tension due to the Janus-like structure of a bulk nanobubble agrees with the experimental data of the decrease in surface tension solely by bulk nanobubbles obtained by the comparison of before and after the elimination of bulk nanobubbles by the freeze-thaw process. This effect cannot be explained by the electric charge stabilization model widely discussed for the stability of a bulk nanobubble, although the present model is only applicable to the solution containing hydrophobic impurities. Another part of the decrease in surface tension should be due to impurities produced from a nanobubble generator, such as a mechanical seal, which was partly confirmed by the TOC measurements.

Evidence of laser-induced nanobubble formation mechanism in water

Principles of laser-induced nanobubble formation in water are studied and presented. Nanobubbles were generated by laser light at intensities below threshold for laser-induced breakdown and subsequently expanded by a rarefaction wave to facilitate their observation and analysis. Different methods were used to study nanobubble formation and characteristics. Firstly, probability of nanobubble formation as a function of water sample purity was examined. Secondly, relation between laser fluence at different wavelengths and the number of generated nanobubbles was investigated. Thirdly, measurements of nanobubble lifetime were conducted indicating a contradiction to the Epstein-Plesset equation-based prediction of free bubble dissociation. Accumulated evidence suggests that the presence of physical impurities is a prerequisite for nanobubble formation. Consequently, a lack of impurities results in the absence of nanobubbles in contrast to assumptions by existing studies. The findings presented in this paper provide new insights into the fundamental properties of laser-induced nanobubbles in water.

Theoretical upper limit of dislocation density in slightly-ductile single-crystal ceramics

Upper limit of dislocation density without fracture is numerically calculated for slightly- ductile single-crystal ceramics for which the Griffith criterion for fracture and the Bailey-Hirsch type relationship between applied stress and the dislocation density are nearly valid simultaneously in order to obtain useful information to improve functional, electrical, and mechanical properties of ceramics by the introduction of appropriate dislocations. Two models of fracture as a function of dislocation density are constructed; simple model and probability model. If the diameter of pre-existing microcracks is sufficiently small, the dislocation density could be as high as the crystallographic limit (∼1018m-2). Even if the typical diameter of pre-existing microcracks is not so small, there is some probability that the dislocation density could be as high as the crystallographic limit if the number of microcracks in the specimen is very small. Accordingly, the increase in ionic conductivity by several orders of magnitude without dendrite formation by introducing appropriate dislocations into single-crystal solid electrolytes with the dislocation density higher than about 1017m-2theoretically predicted by the authors may be practically possible.

The Reducing Agents in Sonochemical Reactions without Any Additives

It has been experimentally reported that not only oxidation reactions but also reduction reactions occur in aqueous solutions under ultrasound without any additives. According to the numerical simulations of chemical reactions inside an air or argon bubble in water without any additives under ultrasound, reducing agents produced from the bubbles are H, H2, HO2 (which becomes superoxide anion (O2-) in liquid water), NO, and HNO2 (which becomes NO2- in liquid water). In addition, H2O2 sometimes works as a reducing agent. As the reduction potentials of H and H2 (in strongly alkaline solutions for H2) are higher than those of RCHOH radicals, which are usually used to reduce metal ions, H and H2 generated from cavitation bubbles are expected to reduce metal ions to produce metal nanoparticles (in strongly alkaline solutions for H2 to work). It is possible that the superoxide anion (O2-) also plays some role in the sonochemical reduction of some solutes. In strongly alkaline solutions, hydrated electrons (e-aq) formed from H atoms in liquid water may play an important role in the sonochemical reduction of solutes because the reduction potential is extremely high. The influence of ultrasonic frequency on the amount of H atoms produced from a cavitation bubble is also discussed.