Key Engineering Materials Vols. 368-372

Abstract: The Ginzburg-Landau theory on ferroelectrics with random field induced by dipole defects is studied using Monte Carlo simulation in order to investigate the possible dipole clustering and freezing behaviors as well as the dielectric relaxation of relaxor ferroelectrics. The dipole clustering above the transition temperature and the cluster-freezing far below this temperature are identified. The intrinsic correspondence between dipole-clustering/freezing and the multi-peaked time-domain distribution of dielectric relaxation is established.

Abstract: Electromagnetic meta-materials are artificial and periodical materials which both the permittivity and permeability are simultaneously negative. Electromagnetic meta-materials represent a new concept where composite materials are designed to display particular properties. The work which we had finished includes theoretical design and numerical simulation of the structure, fabrication of the samples and measurement of the properties. We designed electromagnetic meta-materials which composed by periodic split ring resonators (SRR) based on ceramics and then researched the influence by microwave guide. It is showed that the resonance frequency of meta-materials can be adjusted by dielectric constant and thickness of ceramic substrate.

Abstract: Cofiring behavior of composites consisting of ferroelectric PMZNT and ferromagnetic NiCuZn ferrite was investigated via X-ray diffractometer (XRD), thermomechanical analysis (TMA) and scanning electron microscopy (SEM). Mismatch of sintered camber consisting of two layered structure of ferroelectric PMZNT and NiCuZn ferrite was observed. Mismatched sintering behavior was modified by adopting PMZNT and NiCuZn powders to form composite materials. The temperature of appearance pyrochlore phase in prepared composite materials is lower than that of sintering pure PMZNT material. The grain size of PMZNT and NiCuZn in prepared composites is smaller than that of sintering pure PMZNT and NiCuZn ferrite, respectively.

Abstract: A novel ceramic synthesis technique, microwave-assisted process was investigated for the production of Barium Hexagonal ferrite (BaFe12O19) powders with improved physical properties. Compared to conventional syntheticroute, the new method significantly shortened synthetic steps and reaction time. This technique involves the reaction of stoichiometric amount of metal nitrates and appropriate dosage of citric acid at microwave oven and the whole process took only 15 min. The powders of BaFe12O19 were further investigated by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The results showed that the formation temperature of M-type Ba hexaferrite is significant low, compared to conventional furnace heating. The X-ray diffraction analysis demonstrates that the phase purity of the microwave-processed samples were determined and compared with a conventionally processed material. SEM observations indicate that the size of the synthesized BaFe12O19 powders is small and uniform distribution. Thus, microwave irradiation is proved to be a novel, extremely facile, timesaving and energy-efficient route to the synthesis of BaFe12O19 powders.

Abstract: Sodium-potassium Niobate (K0.4Na0.6NbO3, KNN) nanopowders were prepared by hydrothermal synthesis at the temperature range of 140-180°C for 12-48h using Nb2O5, NaOH and KOH as source materials. By means of XRD and SEM techniques, the effects of composition and hydrothermal treatment process, such as the rate of [R]/[Nb], the concentration of the alkali, the hydrothermal treatment temperature and the hydrothermal treatment time, on the microstructures and the crystallinity of alkali metals niobate were investigated in details. Results show that K0.4Na0.6NbO3 powders could be achieved by hydrothermal synthesis at the temperature range of 140-180°C with the alkalinity concentration of 2-8M. With the increase of hydrothermal reaction temperature and time, the crystallinity of KNN particles was improved. The obtained K0.4Na0.6NbO3 polycrystalline particles have rhombic structure.

Abstract: Yttrium iron garnet (YIG) nanopowders were synthesized by sol-gel method using Fe(NO3)3·9H2O, Y(NO3)3·6H2O as raw materials. The influence of heat treatment, the citric acid to metal nitrates molar ratio (CA / MN) and PH value on the synthesized powders were investigated using scanning electron microscopy (SEM), thermal analyses (DTA/TGA) and X-ray diffraction (XRD). YIG powders with average grain size of 80~90nm were synthesized after calcining at 1000°C for 4h.

Abstract: xBST-(1-x)YIG composite ceramics with x=0~1 were prepared by solid-state reaction method. The variations for phase compositions were determined by XRD, and the magnetic and dielectric properties of the composites were studied in the frequency range of 100Hz~1GHz. The results showed that the composites (x=0.1~0.9) are consisted of BST phase and YIG phase and no significant chemical interaction occurs between these two phases. The composite materials exhibit excellent frequency dependences of dielectric and magnetic properties. The resonance frequency and high frequency characteristics of YIG are improved by additions of BST, and when x=0.3, xBST-(1-x)YIG has the best dielectric and magnetic properties.

Abstract: Single phase YIG powders were synthesized successfully using Fe2O3 and Y2O3 as starting materials by solid state reaction, and YIG ceramics were prepared by pressureless sintering. The influence of synthesizing temperature and Fe2O3 content on the final production were studied The effect of Fe2O3 content on volume density and microstructure of the sintered YIG was also investigated. The results showed that single phase YIG powders were synthesized by solid state reaction at 1400°C for 3h. When Fe2O3 content was excessive 3 wt%, YIG ceramics with a density of 5.294g·cm-3 was fabricated by sintering at 1480°C for 2.5h.

Abstract: The effect of Co-substitution on the complex permeability of LiZn ferrite was studied. The polycrystalline ferrite samples with a composition of ((Li0.5Fe0.5)0.8Zn0.2)1-xCoxFe2.05O4, where x varies from 0 to 0.08, were prepared by solid-reaction method. The sintered samples were annealed at 490oC for 96 h to produce perminvar effect. The complex permeability was measured in the frequency range from 1 MHz to 2 GHz. The Co-substitution can enhance the real part of complex permeability (μ’). The maximum μ’ appears when x=0.02. The Co-substituted LiZn samples, especially for x=0.02 and 0.04, present resonance-type magnetic spectra obviously. The μ’ of the sample for x=0.02 increases because the damping of domain wall movement decreases after the annealing treatment. The resonance character of the annealed sample becomes even more remarkable due to its domain wall stabilization.

Abstract: Nanocrystalline Ni1-xZnxFe2O4 ferrites with 0 ≤ x ≤ 1 were prepared by sprayingcoprecipitation method. The microstructure was investigated by TG-DSC, XRD, SEM, TEM and BET. Magnetic properties were measured with vibrating sample magnetometer at room temperature. The results showed that uniform and fine nanocrystalline Ni1-xZnxFe2O4 ferrites are obtained. The grain size of sample calcined at 600°C for 1.5h is about 30nm. There are a few agglomerates with average sizes below 100nm. The specific saturation magnetization, Ms, of the sample increases with increasing Zn2+ concent x at room temperature, and the maximum Ms is 66.8 A·m2·kg-1 as the Zn2+ content x is around 0.5mol. As calcining temperature increased from 400°C to 1050°C, the Ms of Ni0.5Zn0.5Fe2O4 ferrite increases from 40.2 A·m2·kg-1 to 75.6 A·m2·kg-1. The coercivity maximum is about 5.97 kA·m-1 as its critical grain size is about 62.0nm. The relation between coercivity and grain size for nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite may be explained based on random anisotropy theory.