Xi’an Fan

Colossal Volume Contraction in Strong Polar Perovskites of Pb(Ti,V)O3

The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb(Ti1-xVx)O3 by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (c/a) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti-O and Pb-O2 bonds from first-principles calculations. As a consequence, Pb(Ti0.9V0.1)O3 exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion αV = -3.76 × 10-5/°C (25-550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb(Ti0.7V0.3)O3 during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferroelectrostriction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics.

Intergranular insulated Fe-6.5 wt% Si/SiO2 composite compacts with tunable insulating layer thickness for low core loss applications

Core–shell structured Fe-6.5 wt% Si/SiO2 particles and intergranular insulated Fe-6.5 wt% Si/SiO2 composite compacts with tunable insulating layer thickness were prepared by in situ chemical deposition combined with a subsequent spark plasma sintering process. Most of conductive Fe-6.5 wt% Si alloy particles were coated with a SiO2 insulating layer in the Fe-6.5 wt% Si/SiO2 composite compacts. The SiO2 insulating layer could increase the electrical resistivity and reduce the effective radius of the eddy current in Fe-6.5 wt% Si/SiO2 composite compacts. Therefore, the intergranular insulated Fe-6.5 wt% Si/SiO2 composite compacts presented lower core loss than those of Fe-6.5 wt% Si compacts without SiO2 insulating layer. Based on this, several types of Fe-6.5 wt% Si/SiO2 core–shell particles and intergranular insulated Fe-6.5 wt% Si/SiO2 composite compacts (with various SiO2 insulating layer thicknesses) were selectively prepared by simply varying the TEOS concentration (the volume of TEOS for each gram of Fe-6.5 wt% Si particles, ml g−1). The results indicated that the thickness of the SiO2 layer and electrical resistivity of Fe-6.5 wt% Si/SiO2 composite compacts increased with increasing the TEOS concentration from 0.1 to 0.4 ml g−1, while saturation magnetization, relative permeability and core loss displayed the reverse tendencies. The as-prepared intergranular insulated Fe-6.5 wt% Si/SiO2 composite compacts exhibited excellent magnetic properties, which inspire us to develop new high silicon containing soft magnetic cores which can be widely used in various electromagnetic devices.