Electromagnetic and microwave absorption properties of ( Co 2 + -Si 4 + ) substituted barium hexaferrites and its polymer composite Microstructural and high-frequency magnetic characteristics of W -type barium ferrites doped with V 2 O 5The microwave permittivity and permeability of Co 2 Z barium ferrite composite samples are measured as functions of frequency and volume fraction of the ferrite. Magnetostatic properties of the bulk ferrite are determined. This allows Snoek's law [J. L. Snoek, Physica 14, 204 (1948)] to be verified by comparing the microwave and magnetostatic Snoek's constants. The modification of Snoek's law for hexagonal ferrites suggested recently by Acher et al. [Phys. Rev. B 62, 11324 (2000)] is also verified. Acher's constant is found from microwave measurements to agree with the value calculated from the magnetostatic properties of bulk ferrite, but microwave and magnetostatic Snoek's constant do not agree. This may be attributed to the effect due to demagnetizing factors of ferrite inclusions that are not considered in the derivation of Snoek's and Acher's laws. The measured frequency-dependent permeability of composites satisfies the Lorentzian dispersion law and is consistent with the Maxwell Garnett approximation [J. C. Maxwell Garnett, Philos. Trans. R. Soc. London 203, 385 (1904)]. According to the theoretical analysis based on the Lorentzian dispersion law and the Maxwell Garnet mixing rule, both Snoek's and Acher's constants must be linear functions of the volume fraction, independent of whether microwave values of the constants are in agreement with the magnetostatic values. In contrast, the experimental measurements reveal a steady decrease of both constants with the volume fraction. The disagreement is discussed in terms of the influence of effective medium in composite on the inherent permeability of ferrite particles.
DC resistivity, dielectric, and magnetic properties of Mg‐ferrite ceramics (Mg1−xCuxFe1.98O4, with x=0.10–0.30, and Mg0.90−xCoxCu0.10Fe1.98O4, with x=0.05–0.20) were investigated. A primary objective is to develop magneto‐dielectric materials with almost equal values of permeability and permittivity, as well as low magnetic and dielectric loss tangent, for miniaturization of antennas. The MgFe1.98O4 ceramic sintered at 1125°C possessed values of permeability and permittivity of ∼6.5, and relatively low magnetic and dielectric loss tangents of <10−2, with a sintered density of only ∼70% of the theoretical density. Incorporation of Cu was found to be able to not only improve the densification and grain growth but also alter the electrical and magnetic properties of MgFe1.98O4. Further modification by Co resulted in promising magneto‐dielectric materials, with an almost equal permeability and permittivity of ∼9.5 over 3–30 MHz (HF band). Together with their low magnetic and dielectric loss tangents and good sinterability, this class of magneto‐dielectric materials could be potential candidates for the design of small antennas in the HF band (3–30 MHz). The DC resistivities and complex relative permittivities of the ferrite ceramics were discussed with respect to their microstructure, grain size, and the formation of Fe2+ ions. The variation of high‐frequency magnetic properties of the ferrite materials with sintering temperature can be quantitatively understood by the magnetic circuit model and the Snoek‐like law.
The development of resistant lines and hybrids is an economical way to control disease and improve yield stability. The objectives of this study were (i) to investigate if differences in resistance to gray leaf spot (GLS, caused by Cercospora zeina) exist among recombinant inbred lines (RILs) with and without the quantitative trait locus encompassing the resistance‐carrying GZ204/IDP5 DNA segment (RDNAS) and to determine its effect on grain yield, and (ii) to determine general combining ability and specific combining ability effects for grain yield and GLS scores (GLSS). Four RILs (three with RDNAS [RL1_1, RL1_2, and RL2_1] and one without RDNAS [RL2_2]) were developed via marker‐assisted selection from a cross between YML32 and Q11—an elite line susceptible to GLS. The four RILs and the susceptible parent (Q11) were crossed as testers with 13 maize (Zea mays L.) lines of known heterotic groups (Suwan1, Reid, and non‐Reid). The three RDNAS‐carrying RILs showed reduced GLSS and improved grain yield stability, but grain yield itself was not significantly increased. These three RILs also showed negative general combining ability effects for GLSS. RL2_1 was the best line for improving GLS resistance. The RILs possessing the RDNAS in crosses with lines from the Suwan1 heterotic group had lower GLSS than those from Reid and non‐Reid heterotic groups, suggesting that resistance genes or quantitative trait loci, in addition to RDNAS, might be present in Suwan1.
The densification, grain growth, and microstructure development of Mg–Cu–Co ferrite ceramics (MgFe1.98O4, Mg1−xCuxFe1.98O4, with x=0.10–0.30 and Mg0.90−xCoxCu0.10Fe1.98O4, with x=0.05–0.20) were studied. The primary objective was to develop magneto‐dielectric materials for miniaturization of high frequency and very‐high frequency antennas. It was found that magnesium ferrite (MgFe1.98O4) is a promising magneto‐dielectric material. However, due to its poor densification, it could not be fully sintered at a temperature below 1200°C. High‐temperature sintering resulted in undesirable electrical and dielectric properties, due to the formation of Fe2+ ions. The poor densification and slow grain growth rate of MgFe1.98O4 can be considerably improved by incorporating Cu, due to the occurrence of liquid‐phase sintering at a high temperature. A critical concentration of Cu was observed for Mg1−xCuxFe1.98O4, above which both densification and grain growth were maximized or saturated. The presence of Co did not have a significant influence on the densification and grain growth of the Mg‐based ferrite ceramics.
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