Origin Of Unusual Properties Research Articles

Lithium aluminosilicate (LAS) glass-ceramics: a review of recent progress

ABSTRACT The lithium aluminosilicate system (LAS) is being explored for almost seven decades due to its anomalous and attractive properties (especially low/negative thermal expansion and fast ion conduction) and its commercial significance in consumer as well as strategic sectors. This review introduces current commercial applications of LAS systems, necessary background science, structural features of different LAS crystal systems, including their polymorphs and solid-solutions, and the origin of unusual properties. Significant emphasis is provided on processing transparent, nanocrystalline, low thermal expansion glass-ceramic (LEGC), the role of chemical constituents and additives, the effect of heat-treatment, and microstructural evolution while processing LEGC. Detailed discussions are provided on the following areas: LAS matrix composites, chemical strengthening of glass and glass-ceramic, low temperature co-fired ceramics (LTCC) and associated joining technologies (hydrolysis catalysis bonding, anodic bonding, brazing, etc.) that would further extend the application portfolio of LAS system.

Nature and origin of unusual properties in chemically exfoliated 2D MoS2

MoS2 in its two-dimensional (2D) form is known to exhibit many fundamentally interesting and technologically important properties. One of the most popular routes to form an extensive amount of such 2D samples is the chemical exfoliation route. However, the nature and origin of the specific polymorph of MoS2 primarily responsible for such spectacular properties have remained controversial with claims of both T and T′ phases and metallic and semiconducting natures. We show that a comprehensive scrutiny of the available literature data of Raman spectra from such samples allows little scope for such ambiguities, providing overwhelming evidence for the formation of the T′ phase as the dominant metastable state in all such samples. We also explain that this small bandgap T′ phase may attain substantial conductivity due to thermal and chemical doping of charge-carriers, explaining the contradictory claims of the metallic and semiconducting nature of such samples, thereby attaining a consistent view of all reports available so far.

Origin of unusual properties in the ferromagnetic Heusler alloy Ni–Mn–Sn: A first-principles investigation

The martensitic phase transformation temperatures in the Ni–Mn–Sn Heusler alloys increase with the number of valence electrons per atom; however, when Mn atoms are substituted by Fe, Co or Cu atoms, the systems show anomalous behaviors. In addition, large magnetization changes are observed in these alloys. To understand the unusual properties, the phase transitions in Ni–Mn(Z)–Sn (Z = 3d transition metals) are studied using first-principles calculations. It is found that the changes in the magnetic configuration are responsible for the unusual properties observed.

Ferroelectric transition in a random field: Possible relation to relaxor ferroelectrics

Ferroelectric phase transition developing in a random field of static defects is studied by the computer simulation method. The chosen free energy provides the stability of the ferroelectric phase in the absence of defects. The simulation model takes into account the transformation-induced strain and the dipole—dipole interaction. It is shown that the ferroelectric transition in a random field produces the two-phase thermoelastic metastable equilibrium rather than the single-phase ferroelectric state. The metastable state is a dispersion of ferroelectric clusters in the paraelectric matrix. The clusters formation mechanism is discussed in terms of the percolation theory. Increase in the transformation driving force transforms the cluster dispersion to the ferroelectric macrodomain state. The theoretical prediction of comparatively stable polar clusters in the paraelectric matrix in relaxor ferroelectrics provides an additional footing for the superparaelectric origin of unusual properties of the relaxor ferroelectrics.