Critical Roles of Impurities and Imperfections in Various Phases of Materials

Abstract

In many materials, impurities and imperfections play a critical role on the physical and chemical properties. In the present review, some examples of such materials are discussed. A bulk nanobubble (an ultrafine bubble) is stabilized against dissolution by hydrophobic impurities attached to the bubble surface. An acoustic cavitation threshold in various liquids decreases significantly by the presence of impurities such as solid particles, etc. The strength of brittle ceramics is determined by the size and number of pre-existing microcracks (imperfections) in the specimen. The size effect of a BaTiO3 nanocrystal is influenced by the amount and species of adsorbates (impurities) on its surface as adsorbate-induced charge-screening changes the free energy. The dielectric constant of an assembly of BaTiO3 nanocubes is influenced by a small tilt angle (imperfection) between two attached nanocubes, which induces strain inside a nanocube, and is also influenced by the spatial strain–relaxation due to defects and dislocations (imperfections), resulting in flexoelectric polarization.

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

“…In conclusion, if the number of microcracks in the specimen is very small, it may be possible to achieve the crystallographic limit of dislocation density regardless of the typical size (d 0 ) of pre-existing microcracks. It has already been reported by a statistical theory of fracture that strength of specimen is determined by the number of pre-existing microcracks in the specimen [42,92]. It is the cause of the size effect reported experimentally that smaller specimen exhibits higher strength because the number of pre-existing microcracks is less [42,93,94].…”

“…For ductile metals, on the other hand, the relationship between applied stress and strain deviates strongly from linearity above the yield point due to plastic deformation. For brittle single-crystal ceramics, the condition of fracture is relatively well described by the Griffith criterion which is determined by the size of pre-existing microcracks as discussed in the next section because microcrack formation by pileup of dislocations at grain boundaries under applied stress is absent in single crystals [39][40][41][42]. In the present paper, slightly-ductile single-crystal ceramics are studied such that the condition of fracture is nearly described by the Griffith criterion and that the Bailey-Hirsch type relationship between applied stress and the dislocation density are nearly valid simultaneously.…”

“…In the present paper, slightly-ductile single-crystal ceramics are studied such that the condition of fracture is nearly described by the Griffith criterion and that the Bailey-Hirsch type relationship between applied stress and the dislocation density are nearly valid simultaneously. In case of more ductile singlecrystal ceramics, microcrack formation by pileup of dislocations occurs more frequently at the intersections of dislocation bands (slip bands) and the deviation from the Griffith criterion becomes considerable [42][43][44][45][46][47][48]. Single-crystal ceramics are studied because single crystals have been used in many experiments of dislocation engineering of ceramics [2-4, 6, 7, 9-12, 14, 15, 17, 18, 20, 21, 23].…”

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

“…On the other hand, the dynamic equilibrium model assumes that a part of the bubble surface is covered with hydrophobic impurities . According to the numerical calculations based on the model, more than 50% of the bubble surface is covered with hydrophobic impurities, and such a bulk nanobubble does not necessarily disappear at the liquid’s surface if the covered surface is directed upward. ,, Furthermore, there are TEM (transmission electron microscopy) observations of bulk nanobubbles partly covered with hydrophobic materials which support the model. ,,, The model is based on the fact that gas is concentrated at the surface of a hydrophobic material in liquid water because there is a liquid density depletion layer at the surface of a hydrophobic material in water. ,− The gas pressure in the depletion layer is as high as about 70 atm in liquid water at room temperature . As a result, gas diffuses into a bubble near the periphery of the hydrophobic material on the bubble’s surface, which could be balanced with the gas outflux from the other uncovered surface of the bubble .…”

“…18,30,31 Furthermore, there are TEM (transmission electron microscopy) observations of bulk nanobubbles partly covered with hydrophobic materials which support the model. 1,7,32,33 The model is based on the fact that gas is concentrated at the surface of a hydrophobic material in liquid water because there is a liquid density depletion layer at the surface of a hydrophobic material in water. 30,34−36 The gas pressure in the depletion layer is as high as about 70 atm in liquid water at room temperature.…”