Xianran Xing

Semiconductor/relaxor 0–3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics

Commercial lead-based piezoelectric materials raised worldwide environmental concerns in the past decade. Bi₀.₅Na₀.₅TiO₃-based solid solution is among the most promising lead-free piezoelectric candidates; however, depolarization of these solid solutions is a longstanding obstacle for their practical applications. Here we use a strategy to defer the thermal depolarization, even render depolarization-free Bi₀.₅Na₀.₅TiO₃-based 0-3-type composites. This is achieved by introducing semiconducting ZnO particles into the relaxor ferroelectric 0.94Bi₀.₅Na₀.₅TiO₃-0.06BaTiO₃ matrix. The depolarization temperature increases with increasing ZnO concentration until depolarization disappears at 30 mol% ZnO. The semiconducting nature of ZnO provides charges to partially compensate the ferroelectric depolarization field. These results not only pave the way for applications of Bi₀.₅Na₀.₅TiO₃-based piezoceramics, but also have great impact on the understanding of the mechanism of depolarization so as to provide a new design to optimize the performance of lead-free piezoelectrics.

Hierarchical nanoscale multi-shell Au/CeO2 hollow spheres

Multi-shell ceria hollow spheres (MSCHSs) with a uniform size of ∼300 nm and controlled shell number up to quadruple were synthesized using carbonaceous spheres as the template in a process combining hydrothermally enhanced metal cation adsorption and tunable calcination. The featured sizes of these MSCHSs are in the hierarchical nanoscale region, including the diameters of the interior and exterior hollow spheres (80–300 nm), the thickness of the shells (∼30 nm), the distance between shells (<100 nm), and the pore size in the shells (∼4 nm), which meant that the as-synthesized MSCHSs possess not only a large specific surface area (∼90 m2 g−1) and narrow mesopore distribution, but also nanosized interconnected chambers. With these structural characteristics, the MSCHSs show fascinating capacities as good hosts for noble metals. Gold nanoparticles with sizes below 5 nm could be loaded on these MSCHSs with a high content and good dispersity to construct effective catalysts, which demonstrated a much improved catalytic performance in the reduction of p-nitrophenol. The optimal values of the reaction rate constant (k) reached up to 0.96 min−1. Moreover, this approach opens up a new way to form nanosized multi-shell structures, especially for those with large cation radii.

The role of spontaneous polarization in the negative thermal expansion of tetragonal PbTiO3-based compounds

PbTiO(3)-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO(3)-0.3BiFeO(3) and 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3), whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz(Pb/Bi)) is weakened in 0.7PbTiO(3)-0.3BiFeO(3) but well-maintained in 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3). There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.

Solid solution Pb1-xSrxTiO3 and its thermal expansion

Abstract The solid solution limit of Pb 1− x Sr x TiO 3 was determined in the composition range of 0≤ x ≤1.0 at room temperature (RT). The phases were isolated and indexed in a tetragonal system with x x ≥0.5. The cell parameters of Pb 1− x Sr x TiO 3 continuously, but nonlinearly, change with solubility x . The intrinsic thermal expansions of the solid solution compounds Pb 1− x Sr x TiO 3 ( x =0, 0.15, 0.20, 0.50, 0.90, 1.0) were obtained in the temperature range from RT to 1173 K with high-temperature X-ray powder diffraction. Negative thermal expansion coefficients of Pb 1− x Sr x TiO 3 ( x =0, 0.15, 0.20) were found below the Curie points. The thermal expansions of these titanate ceramics were highly correlated with the solubility in the solid solution Pb 1− x Sr x TiO 3 .

Zero Thermal Expansion and Ferromagnetism in Cubic Sc1–xMxF3 (M = Ga, Fe) over a Wide Temperature Range

The rare physical property of zero thermal expansion (ZTE) is intriguing because neither expansion nor contraction occurs with temperature fluctuations. Most ZTE, however, occurs below room temperature. It is a great challenge to achieve isotropic ZTE at high temperatures. Here we report the unconventional isotropic ZTE in the cubic (Sc1-xMx)F3 (M = Ga, Fe) over a wide temperature range (linear coefficient of thermal expansion (CTE), αl = 2.34 × 10(-7) K(-1), 300-900 K). Such a broad temperature range with a considerably negligible CTE has rarely been documented. The present ZTE property has been designed using the introduction of local distortions in the macroscopic cubic lattice by heterogeneous cation substitution for the Sc site. Even though the macroscopic crystallographic structure of (Sc0.85Ga0.05Fe0.1)F3 adheres to the cubic system (Pm3̅m) according to the results of X-ray diffraction, the local structure exhibits a slight rhombohedral distortion. This is confirmed by pair distribution function analysis of synchrotron radiation X-ray total scattering. This local distortion may weaken the contribution from the transverse thermal vibration of fluorine atoms to negative thermal expansion, and thus may presumably be responsible for the ZTE. In addition, the present ZTE compounds of (Sc1-xMx)F3 can be functionalized to exhibit high-Tc ferromagnetism and a narrow-gap semiconductor feature. The present study shows the possibility of obtaining ZTE materials with multifunctionality in future work.

Thermal Expansion, Ferroelectric and Magnetic Properties in (1 − x)PbTiO3−xBi(Ni1/2Ti1/2)O3

A zero thermal expansion and multiferroic compound 0.8PbTiO(3)-0.2Bi(Ni(1/2)Ti(1/2))O(3) was developed by a chemical modification route. The structure studies showed that the tetragonality of (1 - x)PbTiO(3)-xBi(Ni(1/2)Ti(1/2))O(3) was gradually weakened to cubic by introducing the dopant Bi(Ni(1/2)Ti)(1/2)O(3), and the thermal expansion coefficient changed from -8.81 x 10(-6)/degrees C to 8.46 x 10(-6)/degrees C in 0.1 < or = x < or = 0.3 around a wide temperature range (from RT to about 500 degrees C). Weak ferromagnetic behavior was observed in the solid solutions, and the superexchange interaction was incorporated to explain its nonmonotonous evolution. Meanwhile, the good piezoelectricity and ferroelectricity were well retained. Further investigations demonstrated that the (1 - x)PbTiO(3)-xBi(Ni(1/2)Ti(1/2))O(3) ceramics possessed good mechanical properties, such as high density and excellent fracture toughness. The improved behaviors make the (1 - x)PbTiO(3)-xBi(Ni(1/2)Ti(1/2))O(3) promising piezoceramics with high thermal stability and mechanical performance. The present work provides a way to design and explore high-performance multiferroic compounds in the synthesis route.

Precursor-induced fabrication of β-Bi2O3 microspheres and their performance as visible-light-driven photocatalysts

Flower-like β-Bi2O3 microspheres with high specific surface area and excellent visible-light-driven photocatalytic activity (for degradation of Rhodamine B) were successfully synthesized via a facile hydrothermal process and subsequent calcination. By precisely adjusting the hydrothermal conditions, the composition and morphology of the microspherical precursors could be well controlled, so that upon further optimized calcination of the precursors, the selective formation of the monoclinic α-Bi2O3 and tetragonal β-Bi2O3 with three dimensional (3D) hierarchical architectures could be achieved. These tetragonal β-Bi2O3 microspheres with an average diameter of 3 μm were constructed by nanoflakes with an average thickness of 50 nm, which, as far as we know, is the first reported result on the 3D hierarchical architectures of tetragonal β-Bi2O3. Its flower-like microspherical architecture made the tetragonal β-Bi2O3 possess not only much improved specific surface area but also a narrower band gap, which significantly enhanced its visible-light-driven photocatalytic activity for the degradation of Rhodamine B (RhB). To further optimize the synthetic conditions and realize the controllable synthesis, the formation mechanism for the morphologies and polymorphs of the Bi2O3 microspheres was discussed in detail.

Phase evolution in low-dimensional niobium oxide synthesized by a topochemical method.

In this paper, we report a catalyst-free topochemical method, combined with molten salt synthesis (MSS), to synthesize, on a large scale, rodlike and platelet single crystals of Nb(2)O(5). Rodlike KNb(3)O(8) and platelet K(4)Nb(6)O(17), which were fabricated as the precursors by the molten salt method, were treated by proton exchange and heat treatment to synthesize the rodlike H-Nb(2)O(5) and platelet T-Nb(2)O(5) single crystal, respectively. The synthesized niobium pentaoxides retained the rodlike and platelet shapes of their precursors. The structural changes involved in the process were investigated by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. A possible topochemical reaction mechanism is proposed. Furthermore, rodlike and platelet KNbO(3) powders were derived from stable H-Nb(2)O(5) and T-Nb(2)O(5), respectively.

Enhanced piezoelectric and ferroelectric properties in the BaZrO3 substituted BiFeO3-PbTiO3

BiFeO3-PbTiO3 (BF-PT) compounds possess very high Curie temperature and tetragonality compared to other PbTiO3-based piezoceramics. The BaZrO3 (BZ), with weakly ferroelectric active cations, was introduced into the BiFeO3-PbTiO3 to reduce the tetragonality (c/a) and improve the piezoelectric property. For the (0.8-x)BiFeO3-0.2BaZrO3-xPbTiO3, the BaZrO3 substitution can effectively decrease the tetragonality (c/a) from 1.18 to 1.02 for those compositions near the morphotropic phase boundary. The piezoelectric property of BiFeO3-PbTiO3 can be much enhanced with an optimal piezoelectric constant ∼270 pC/N with a reduced TC of 270 °C. Both the temperature dependent dielectric properties and polarization loops verified the existence of antiferroelectric relaxor, which was not observed in previous reported BiFeO3-PbTiO3 based materials.

Effectively control negative thermal expansion of single-phase ferroelectrics of PbTiO3-(Bi,La)FeO3 over a giant range.

Control of negative thermal expansion is a fundamentally interesting topic in the negative thermal expansion materials in order for the future applications. However, it is a challenge to control the negative thermal expansion in individual pure materials over a large scale. Here, we report an effective way to control the coefficient of thermal expansion from a giant negative to a near zero thermal expansion by means of adjusting the spontaneous volume ferroelectrostriction (SVFS) in the system of PbTiO3-(Bi,La)FeO3 ferroelectrics. The adjustable range of thermal expansion contains most negative thermal expansion materials. The abnormal property of negative or zero thermal expansion previously observed in ferroelectrics is well understood according to the present new concept of spontaneous volume ferroelectrostriction. The present studies could be useful to control of thermal expansion of ferroelectrics, and could be extended to multiferroic materials whose properties of both ferroelectricity and magnetism are coupled with thermal expansion.