Rong-Jun Xie

Dielectric and ferroelectric properties of tetragonal tungsten bronze Sr2−xCaxNaNb5O15 (x=0.05–0.35) ceramics

This letter reports the dielectric and ferroelectric properties of tungsten bronze Sr2−xCaxNaNb5O15 (SCNN, x=0.05–0.35) ceramics. Two dielectric anomalies and a diffuse ferroelectric transition behavior were appreciably observed in the compositions of x=0.05–0.25. The incorporation of smaller calcium cations into the crystal structure resulted in an increase in the Curie temperature, from 279 (x=0.05) to 297 (x=0.35), and a decrease in the permittivity, from 1353 to 543, at their respective Curie temperatures. Ferroelectricity was observed in the compositions with x=0.05–0.25, but absent in the compositions with x=0.30 and 0.35 at room temperature. The maximum spontaneous polarization Ps of 9.1 μC/cm2 and remanent polarization Pr of 3.0 μC/cm2 were achieved in the composition of x=0.15.

Lead-free piezoelectric ceramics in the (1 - x)Sr2NaNb5O15-xCa2NaNb5O15 (0.05 ≤ x ≤ 0.35) system

The study investigating the electrical properties of dense lead-free piezoelectric ceramics in the (1 − nx)Sr2NaNb5O15–xCa2NaNb5O15 n(SCNN) system with x ranging from 0.05 to 0.35 is reported here. X-Ray diffraction patterns all showed a single tetragonal tungsten bronze phase, peak broadening being seen on increasing the Ca content. The lattice constant calculation indicated distortion and shrinkage of the crystal structure with Ca substitution. Dielectric spectra of each composition displayed two dielectric anomalies and diffusive phase transitions. The Curie temperature, Tc, shifted upward whereas the relative permittivity at Tc declined monotonically as the Ca content increased. Ferroelectric and piezoelectric properties of SCNN greatly depended on the Ca content. The composition with x n= 0.15 exhibited the greatest polarization with Pr n= 3.0 μC cm−2 and piezoelectric constant of d33 n= 96 pC N−1, whereas the compositions with x n= 0.30 and 0.35 almost lost their ferroelectricity and piezoelectricity. This study apparently indicates that lead-free SCNN piezoelectric ceramics have potential for electromechanical applications.

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Abstract Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si 3 N 4 and 7 wt.% cordierite, at 1600xa0°C under a constant load of 40 MPa and subsequent annealing at 1750xa0°C for 30 min. The resulting composites featured a microstructure of elongated Si 2 N 2 O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si 3 N 4 matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si 2 N 2 O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si 2 N 2 O grains, the texture formation, and the coarsening of β-Si 3 N 4 . Fracture toughness, however, was still low (∼5.2 MPa m 1/2 ) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si 2 N 2 O-matrix interfacial bond and nearly equiaxed β-Si 3 N 4 with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.

Dielectric Properties of Spark-Plasma-Sintered (Na 0.5 K 0.5 )NbO 3 -PbTiO 3 Ceramics

Dielectric properties of spark-plasma-sintered solid solution (1 m x )(Na 0.5 K 0.5 )NbO 3 m x PbTiO 3 were studied. Both the cubic-tetragonal and the tetragonal-orthorhombic phase transitions become more and more diffuse with increasing x . The strong deviation of the cubic-tetragonal phase transition temperature versus composition curve from the Vegards law is observed. It is considered that (1) the introduction of heterovalence ions to both the perovskite A and B sites, and (2) the change of the mass difference between the perovskite B- and A-site, m B - m A, are two main factors responsible for the observations.

New phosphors Eu2+ and Ce3+ doped Sr4-x(Si,Al)19+x(N,O)29+x for white LED applications (Conference Presentation)

Light-emitting diodes (LEDs) have been steadily consolidating their share in the lighting and display market. Phosphor-converted (pc) white LED becomes the preferred way to generate white light especially for general lighting, as it is much cheaper and simpler than RGB system. Phosphors are essential to high color quality and luminous efficacy. However, the number of commercially available phosphors is very limited. Therefore, developing new phosphors suitable for various white LED applications is very important.nRecently, our group developed the single-particle-diagnosis approach [1-2] to discover new phosphors, with which a tiny luminescent microcrystalline particle down to 5-10 μm can be selected from powder mixtures. In this work, we report a new green emitting Sr-sialon:Eu phosphor discovered by this approach. The crystal structure was solved and refined from single crystal X-ray diffraction data. Sr-sialon:Eu crystallizes in the trigonal space group P3m1 (no. 156) with a = b = 12.1054 A, c = 4.8805 A and Z = 1, and consists of a network of corner sharing (Si,Al)(N,O)4 tetrahedra. Upon doping with Eu2+, the emission band can be tuned from 487 nm to 541 nm with fwhm = 96-124 nm. Ce3+ doped Sr-sialon phosphor shows strong blue emission around 435 nm with a fwhm ≈ 90 nm after 355 nm light excitation. The blue luminescence exhibits a small thermal quenching behavior at high temperature. These performances show that the new Eu2+ and Ce3+ doped Sr-sialon phosphors are promising for white LED applications.nnn[1] N. Hirosaki, T. Takeda, S. Funahashi and R.-J. Xie, Chemistry of Materials, 2014, 26, 4280-4288.n[2] T. Takeda, N. Hirosaki, S. Funahashi and R.-J. Xie , Materials Discovery, 2015, 1, 29-37.