Alkyl-π engineering in state control toward versatile optoelectronic soft materials

Organic π-conjugated molecules with extremely rich and tailorable electronic and optical properties are frequently utilized for the fabrication of optoelectronic devices. To achieve high solubility for facile solution processing and desirable softness for flexible device fabrication, the rigid π units were in most cases attached by alkyl chains through chemical modification. Considerable numbers of alkylated-π molecular systems with versatile applications have been reported. However, a profound understanding of the molecular state control through proper alkyl chain substitution is still highly demanded because effective applications of these molecules are closely related to their physical states. To explore the underlying rule, we review a large number of alkylated-π molecules with emphasis on the interplay of van der Waals interactions (vdW) of the alkyl chains and π–π interactions of the π moieties. Based on our comprehensive investigations of the two interactions’ impacts on the physical states of the molecules, a clear guidance for state control by alkyl-π engineering is proposed. Specifically, either with proper alkyl chain substitution or favorable additives, the vdW and π–π interactions can be adjusted, resulting in modulation of the physical states and optoelectronic properties of the molecules. We believe the strategy summarized here will significantly benefit the alkyl-π chemistry toward wide-spread applications in optoelectronic devices.     

Preparation and phase transformation behavior of χ-alumina via solvothermal synthesis

Solvothermal reaction of aluminum isopropoxide (AIP) in mineral oil at 250–300 °C over 2 h duration provides χ-alumina powder, which transforms directly to α-alumina after calcination at high temperature. The mechanism of the crystallization process appears to be the initial formation of a spherical complex which subsequently decomposes further to precipitate a solid. This mechanism is suggested by XRD, IR, TG/DTA, SEM and TEM characterization of the powder formed. χ-Alumina attains a critical crystallite size around 15 nm through accretion on calcination and then transforms directly to α-alumina through nucleation and growth process. Direct α-phase transformation of χ-alumina powders rather than passage through κ-alumina can be explained by the absence of the cation contamination and the higher crystallinity of χ-alumina in the AIP decomposition process.     

Formation of η and σ phase in three polycrystalline superalloys and their impact on tensile properties

The formation and phase transformation mechanism of η and σ phases in three experimental polycrystalline superalloys were studied. It was shown that a high (Ti + Al) content in the alloys would favor the formation of η and σ phases in the interdendritic region. Different as-cast microstructures resulted in different phase transformation processes during heat treatment and thermal exposure. Influence of η and σ phase on tensile properties had been investigated as well. The tensile properties of the alloys were sensitive to γ′ volume fraction of the alloys, as well as morphologies of η and σ phases in the interdendritic area. Formation of plate-like η phase had negative impact on the low and intermediate temperature tensile properties of the polycrystalline superalloy.