Grain-boundary segregation and grain growth in nanocrystalline substitutional solid solution alloys

A model for describing solute segregation at grain boundaries has been developed for substitutional solid solution alloys,which integrates multiple factors from atomic to microstructural scales.A concept of mo-lar Gibbs free energy of segregation was introduced to evaluate the segregating capability of the solute elements in a closed system,through which the influences of grain boundary structure,grain size,ma-terial composition,and external conditions were described.Based on the evaluation of various energy forms related to solute segregation and grain growth processes,the nature of the thermal stabilization of nanograin structures by solute segregation was disclosed.A criterion for the destabilization of nanostruc-tures,which is determined by the competition of the change rates between the molar Gibbs free energy of segregation and the total energy of grain boundaries with grain size,has been proposed.This study provided guideline to achieve high-temperature stability of nanograin structures of solid solution alloys even for the weakly segregating nanocrystalline systems.

Incubation of PbSe Thin Films in a Tin(Ⅱ) Salt Aqueous Solution:Modification and Ion-Exchange Reactions

Topochemical ion-exchange reactions between solid micro- and nanostructured metal chalcogenides and aqueous salt solutions are generally used for formation of composite structures based on initial metal chalcogenides and products of their ion-exchange transformation. However, ion exchange has promises as a route to obtaining both composites and solid solutions based on the initial and the end chalcogenide phases. With the help of the ion-exchange technique, single-phase films of Phi xSnxSe substitutional solid solutions with a tin content up to -2 at.%, which are promising for mid- and long-wavelength infrared radiation （IR） optoelectronics, have been obtained at the interface between PbSe poly- crystalline thin films and SnCl2 aqueous solutions containing sodium citrate. It has been shown that the pH value and temperature of the reaction system play an important role in the ion-exchange process. Incubation of lead selenide （PbSe） films in a tin（II） salt aqueous solution also leads to their modification with oxygen-containing tin compounds to a depth of-3 nm. Differences in the film structure, such as changes in the coherent scattering region sizes and orientation of crystallites along the [220] direction, which arise during the contact with citrate-containing SnCl2 solutions, have also been revealed. For the first time, an idea of the existence of a relatively wide reaction zone of an intragranular topochemical ion-exchange reaction in an aqueous solution, within which substitutional solid solutions can form in micro- and nanostructured systems, has been set forth.

Synthesis of substitutional hexaboride of lanthanum and cerium La_(1-x)Ce_(x)B_(6) via aluminothermic reduction

Rare-earth hexaborides(REB_(6))are vital raw materials for cathode materials and high temperature structural ceramics that are widely applied as high-frequency electron tubes and ceramics adaptive for extreme environment,respectively.In this work,single phase substitutional solid solution REB_(6)(LaB_(6),La_(0.75)Ce_(0.25)B_(6).La_(0.5)Ce_(0.5)B_(6),La_(0.25)Ce_(0.75)B_(6) and CeB_(6))powders were prepared with the raw materials of La_(2)O_(3),CeO_(2),B_(4)C and Al powders,after calcining at 1773 K for 4 h and the following alkaline leaching.All substitutional solid solution products have homogeneous distributions of La and Ce in particles.Through microscopic morphology analysis,it is discovered that the formation of solid solution is beneficial for reducing the particle size of product,relative to LaB_(6) and CeB_(6).Moreover,Al flux plays an important role in decarbonizing reaction,and C contents of all products are below 0.4 wt%.