Zhenxing Yue

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.

Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

Abstract A novel nitrate–citrate gel process was used to synthesize NiCuZn ferrite powders. The thermal decomposition and combustion process of nitrate–citrate gels were investigated by using DTA-TG, IR spectra and XRD techniques. The results revealed that the nitrate–citrate gels exhibit self-propagating behavior after ignition in air at room temperature. NiCuZn ferrite powders with a particle size of 20–50 nm were directly formed after combustion. The ratio of metal nitrates to citric acid in the starting solution affects the combustion process, and then determines the particle size of the as-synthesized powder. The magnetic structure of powders was characterized using Mossbauer spectroscopy.

Enhanced microwave absorption in nickel/hexagonal-ferrite/polymer composites

Ni/hexagonal-ferrite (Co2Z or Zn2Y barium ferrites)/polymer composites were prepared for electromagnetic interference (EMI) shielding materials. In comparison with the hexagonal-ferrite/polymer or Ni/polymer composites, such three-phase composites exhibit excellent microwave absorption properties in the X-band frequency range, strongly depending on the combination of these three phases. For the Ni∕Co2Z/polymer composites with volume fraction ratio of Ni/ferrite equal to 0.70∕0.30, the EMI shielding effectiveness (SE) of the composites with a thickness of 2mm can be as high as ∼67dB. The high EMI SE of the composites and much larger contribution to total EMI SE from absorption than from reflection suggest that these composites would be very promising for applications as thinner and lighter EMI absorption materials.

High and Temperature-Insensitive Piezoelectric Strain in Alkali Niobate Lead-free Perovskite

With growing concern over world environmental problems and increasing legislative restriction on using lead and lead-containing materials, a feasible replacement for lead-based piezoceramics is desperately needed. Herein, we report a large piezoelectric strain (d33*) of 470 pm/V and a high Curie temperature (Tc) of 243 °C in (Na0.5K0.5)NbO3-(Bi0.5Li0.5)TiO3-BaZrO3 lead-free ceramics by doping MnO2. Moreover, excellent temperature stability is also observed from room temperature to 170 °C (430 pm/V at 100 °C and 370 pm/V at 170 °C). Thermally stimulated depolarization currents (TSDC) analysis reveals the reduced defects and improved ferroelectricity in MnO2-doped piezoceramics from a macroscopic view. Local poling experiments and local switching spectroscopy piezoresponse force microscopy (SS-PFM) demonstrates the enhanced ferroelectricity and domain mobility from a microscopic view. Distinct grain growth and improvement in phase angle may also account for the enhancement of piezoelectric properties.

Energy-storage performance and electrocaloric effect in (100)-oriented Pb0.97La0.02(Zr0.95Ti0.05)O3 antiferroelectric thick films

(100)-oriented Pb0.97La0.02(Zr0.95Ti0.05)O3 (PLZT) antiferroelectric films with a thickness of about 1.7 μm were deposited on Pt-buffered silicon substrates via a sol-gel process. The room-temperature capacitance density of the films was above 195 nF/cm2 over the frequency range from 1 kHz to 1 MHz. The recoverable energy density was enlarged with increasing measurement field, and a maximum value of 12.4 J/cm3 at 1120 kV/cm was obtained at room temperature. From the temperature-dependent electric-field-induced polarization hysteresis loops, a maximum reversible adiabatic temperature change, ΔT = 8.5 °C, was obtained near the phase-transition temperature.

A simple way to prepare nanosized LaFeO3 powders at room temperature

Abstract Nanosized LaFeO 3 powders of 30 nm particle size were directly prepared by sol-gel auto-combustion at room temperature. The overall process involves three steps: formation of homogeneous sol; formation of dried gel; and combustion of the dried gel. Experiments revealed that LaFeO 3 dried gel derived from citrate and nitrate sol exhibits self-propagating combustion at room temperature once it is ignited in air. After auto-combustion, the gel directly transforms into nanosized LaFeO 3 particles. The auto-combustion was considered as a heat-induced exothermic oxidation–reduction reaction between nitrate ions and carboxyl group. Differential thermal analysis–thermogravimetry (DTA–TG) was used to study the decomposition of the precursor. The structure of the nanosized LaFeO 3 powders was characterized by X-ray diffraction (XRD).

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.

Complex Y-type hexagonal ferrites: an ideal material for high-frequency chip magnetic components

Abstract The magnetic properties of complex Y-type hexaferrites series with Ba2Me2Fe12O22 (Me=Zn, Co, Cu) have been investigated. Y-type polycrystalline hexaferrite powders prepared by the solid-state reaction method were characterized by X-ray diffraction and vibrating sample magnetometer. Permeabilities of the samples were investigated using an impedance analyzer. Experimental results show that the substitution of Co for Zn leads to a decrease of saturation magnetization and an increase of magnetic anisotropy. At room temperature, saturation magnetization does not increase linearly as Zn content increases due to the effect of the thermal agitation. The saturation magnetization and magnetic anisotropy of the sample determine the variation of initial permeability. Cu additions can lower the sintering temperature distinctly as the magnetic properties debase slightly. It is shown that these materials should be promising for next generation of high-frequency chip magnetic components (including chip inductors and chip EMI filters).

Effect of Mn substitution on the magnetic properties of MgCuZn ferrites

Abstract A series of Mn substituted MgCuZn ferrites (Mg 0.2 Cu 0.2 Zn 0.6 O) (Fe 2− x Mn x O 3 ) 0.97 with x =0.00,0.01,0.03,0.05,0.07 were prepared with nanosized precursor powders synthesized by a sol–gel auto-combustion method. All the ceramic samples can be sintered at low temperature (930°C) (below the melt point of Ag (961°C)). The effect of Mn content on microstructures and magnetic properties were investigated. Experiment shows that low temperature sintered MgCuZn ferrites doped with Mn possess higher initial permeability and better grain structure than that of low temperature sintered NiCuZn ferrites prepared by the same method. Therefor, Mn doped MgCuZn ferrites should be ideal materials for high inductance multilayer chip inductor. It is thought that the variation of initial permeability of MgCuZn ferrites with the Mn substitution was attributed to the decrease of magnetostriction constant.