Longtu Li

Synthesis of nanocrystalline NiCuZn ferrite powders by sol–gel auto-combustion method

Abstract A nitrate–citrate gel was prepared from metal nitrates and citric acid by sol–gel process, in order to synthesize Ni 0.25 Cu 0.25 Zn 0.50 Fe 2 O 4 ferrite. The thermal decomposition process was investigated by DTA-TG, IR and XRD techniques. The results revealed that the nitrate–citrate gel exhibits self-propagating combustion behavior. After combustion, the gel directly transformed into single-phase, nano-sized NiCuZn ferrite particles with spinel crystal structure. The synthesized powder can be densified at a temperature lower than 900°C. The sintered body possesses fine-grained microstructure, good frequency stability and high-quality factor compared to the sample prepared by conventional ceramic route.

Ferroelectric characteristics of ytterbium-doped barium zirconium titanate ceramics

Abstract Ferroelectric characteristics of ytterbium-doped barium zirconium titanate (BZT in brief) ceramics have been investigated. It is found that ytterbium dopant greatly influences the dielectric properties of BZT ceramics. With the increase of ytterbium content, the Curie temperature is lowered by three apparent stages identified by the falling “rate”, and the relative permittivity maximum is enhanced and broadened enormously. The diffuseness of the ferroelectric phase transition for BZT is increased monotonously as well. The A/B ratio in perovskite structure plays another important role on the ferroelectric phase transition characteristics for ytterbium doped BZT. The alternation of the substitution preference of ytterbium ions for the host cations in perovskite lattice seems to account for these effects. Thanks to the modifications of ytterbium on BZT, we developed a novel Y5V dielectric material with the room temperature dielectric constant over 23,000, and the dielectric strength higher than 4.0 kV/mm under AC field, which can be used in manufacturing high voltage ceramic capacitors.

Phase transition and high dielectric constant of bulk dense nanograin barium titanate ceramics

Barium titanate (BTO) ceramics with an average grain size of 8nm were fabricated and studied. The temperature dependent Raman spectra revealed successive transitions from rhombohedral to orthorhombic, tetragonal, and cubic as the temperature increased from 87to673K. Local piezoresponse force measurements indicated that the ceramics had switchable polarization at room temperature. Dielectric measurements showed a broad ferroelectric-to-paraelectric phase transition with a maximum permittivity of 1800 at 390K. All these results suggest that ferroelectricity could remain in BTO ceramics with grain size as small as 8nm in diameter.

Ferroelectric properties of nanocrystalline barium titanate ceramics

Dense nanocrystalline BaTiO3 ceramics with the average grain size of 20nm obtained by spark plasma sintering were investigated. The dielectric data show a ferroelectric to paraelectric phase transition with a maximum permittivity of ≈930 at 115°C and at 1kHz. The polarization-reversal characteristics and the local ferroelectric switching behavior were measured; the typical piezoelectric hysteresis loops were recorded. The present results provide experimental evidence, indicating that if a critical grain size exists for ferroelectricity it is less than 20nm for polycrystalline BaTiO3 ceramics.

Combined effect of preferential orientation and Zr/Ti atomic ratio on electrical properties of Pb(ZrxTi1−x)O3 thin films

Lead zirconate titanate [Pb(ZrxTi1−x)O3, PZT] thin films with various compositions, whose Zr/Ti ratio were varied as 40/60, 48/52, 47/53, and 60/40, were deposited on Pt(111)/Ti/SiO2/Si substrates by sol-gel method. A seeding layer was introduced between the PZT layer and the bottom electrode to control the texture of overlaid PZT thin films. A single perovskite PZT thin film with absolute (100) texture was obtained, when lead oxide was used as the seeding crystal, whereas titanium dioxide resulted in highly [111]-oriented PZT films. The dielectric and ferroelectric properties of PZT films with different preferential orientations were evaluated systemically as a function of composition. The maximums of relative dielectric constant were obtained in the morphotropic phase boundary region for both (100)- and (111)-textured PZT films. The ferroelectric properties also greatly depend on films’ texture and composition. The intrinsic and extrinsic contributions to dielectric and ferroelectric properties were dis...

Preparation and magnetic properties of titanium-substituted LiZn ferrites via a sol-gel auto-combustion process

Abstract A series of nitrate–citrate gels were prepared from metal nitrates and citric acid via a sol-gel process, in order to synthesize titanium-substituted lithium zinc (LiZn) ferrites with composition of Li 0.5(0.4+ x ) Zn 0.6 Ti x Fe (2.2−1.5 x ) O 4 ( x ranging from 0.05 to 0.20). The thermal decomposition process was investigated by DTA-TGA, IR and XRD techniques. The dried gels can burn in a self-propagating combustion process in air to transform into single-phase, nano-crystalline ferrite particles. The low-temperature sintering was realized using the synthesized powders, and the sintered ferrites have fine-grained microstructures and excellent magnetic properties. Appropriate amounts of titanium substituted for Fe in LiZn ferrites can significantly increase the permeability value. The prepared LiZn ferrites are good materials for multilayer chip inductors.

Effect of copper on the electromagnetic properties of Mg–Zn–Cu ferrites prepared by sol–gel auto-combustion method

Abstract A novel gel auto-combustion method was used to synthesize Mg–Zn–Cu ferrite powders with compositions of (Mg 0.5− x Cu x Zn 0.5 )O(Fe 2 O 3 ) 0.98 (where x =0.20, 0.25, 0.30, 0.35, 0.40). X-ray diffraction results showed that the dried gels, synthesized from metal nitrites and citric acid, transformed directly into nano-sized ferrite particles after an auto-combustion process in air. The synthesized powders exhibited high-sintering activity, and can be sintered at temperature less than 950 °C. The low-temperature sintered Mg–Zn–Cu ferrites prepared possess good electromagnetic properties, as well as fine-grained microstructures, making them become good materials for multilayer chip inductors with high-performance and low-cost. Copper content has significant influence on the electromagnetic properties, such as initial permeability, quality factor, DC resistivity, dielectric constant and dielectric loss tangent as well as their frequency dispersion for Mg–Zn–Cu ferrites. The possible reasons that are responsible for the composition dependence of main electromagnetic properties were discussed.

Effects of MnO2 on the electromagnetic properties of NiCuZn ferrites prepared by sol-gel auto-combustion

Abstract Mn-doped NiCuZn ferrites with compositions of (Ni0.2Cu0.2Zn0.6)O(Fe2−x,MnxO3)0.98 (x=0, 0.02, 0.04, 0.06) were prepared by a novel sol–gel auto-combustion process. The synthesized nano-sized ferrite powders can be sintered at 900°C, and the sintered ferrites are characterized by fine-grained microstructural feature and high permeability. Mn content in formulations largely affects the grain size and main electromagnetic properties of sintered NiCuZn ferrites. With increasing Mn content, the initial permeability is significantly increased, while the electrical resistivity and quality factor are decreased. The dielectric constant and dissipation factor are also affected by the incorporation of MnO2. The possible mechanism for the influence of MnO2 on the electromagnetic properties was discussed.

Electric field induced intermediate phase and polarization rotation path in alkaline niobate based piezoceramics close to the rhombohedral and tetragonal phase boundary

High resolution synchrotron x-ray and dielectric measurements on unpoled and poled (Na,K)(Nb,Sb)O3-LiTaO3-xBaZrO3 lead-free ceramics close to the rhombohedral-tetragonal (R-T) phase boundary have suggested an additional lattice distortion induced by poling field. This intermediate phase (IP) is consistent with the orthorhombic (O) symmetry but lower symmetries cannot be discarded. As a result, a modified polarization rotation path along R-IP-T in poled ceramics would be responsible for their high piezoelectric activity owing to the effect of the IP bridging the R and T phases. Simultaneously, the electric field induced phase transition would probably contribute to the observed large piezoelectric strains.

Epitaxial growth of sol-gel derived BiScO3-PbTiO3 thin film on Nb-doped SrTiO3 single crystal substrate

BiScO3–PbTiO3 (BSPT) thin film was grown on electrical conductive Nb-doped SrTiO3 (100) (Nb-STO) single crystal substrate by a sol-gel method. The x-ray diffraction and high resolution transmission electron microscopy proved the epitaxial growth relation between the BSPT thin film and Nb-STO substrate. The [001]-epitaxial BSPT thin film exhibited considerable high remanent polarization of 74μC∕cm2 and effective piezoelectric coefficient d33* of 130pm∕V, which were much higher than those of the (100)-oriented BSPT thin film grown on traditional silicon substrate.