In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.

/pdf/microwave-ferrites-part-1-fundamental-properties-124qghgxm4.pdf

Microwave ferrites, part 1: fundamental properties

Researches on flexible thermoelectric materials usually focus on conducting polymers and conducting polymer-based composites; however, it is a great challenge to obtain high thermoelectric properties comparable to inorganic counterparts. Here, we report an n-type Ag2Se film on flexible nylon membrane with an ultrahigh power factor ~987.4 ± 104.1 μWm−1K−2 at 300 K and an excellent flexibility (93% of the original electrical conductivity retention after 1000 bending cycles around a 8-mm diameter rod). The flexibility is attributed to a synergetic effect of the nylon membrane and the Ag2Se film intertwined with numerous high-aspect-ratio Ag2Se grains. A thermoelectric prototype composed of 4-leg of the Ag2Se film generates a voltage and a maximum power of 18 mV and 460 nW, respectively, at a temperature difference of 30 K. This work opens opportunities of searching for high performance thermoelectric film for flexible thermoelectric devices. Although flexible thermoelectric materials based on conducting polymers are attractive for energy harvesting, their performance is inferior to their inorganic counterparts. Here, the authors present a facile method to deliver inorganic nanowire films with high power factor and flexibility.

High performance n-type Ag 2 Se film on nylon membrane for flexible thermoelectric power generator

Silicates are one of the most important classes of compounds on this planet, and more than 1000 silicates have been identified in the mineral kingdom. Additionally, several hundreds of artificial silicates have been synthesized. The substitution of oxygen by nitrogen leads to the structurally diverse and manifold class of nitridosilicates. Silicon nitride, one of the most important non-oxidic ceramic materials, is the binary parent compound of nitridosilicates, and it symbolizes the inherent material properties of these refractory compounds. However, prior to the last decades, a broad systematic investigation of nitridosilicates had not been accomplished. In the meantime, these and related compounds have reached a remarkable level of industrial application. This review illustrates recent progress in synthesis and structure-property relationships and also applications of nitridosilicates, oxonitridosilicates, and related SiAlONs.

Nitridosilicates and oxonitridosilicates: from ceramic materials to structural and functional diversity.

Magnetron sputter deposition: Linking discharge voltage with target properties

The magnetic and electronic structure of strontium hexaferrite SrFe12O19 has been investigated using density functional theory within the local spin density approximation. The calculations show that in this ferrite all the Fe3+ ions are fully spin polarized with S = 5/2. The electronic structure of this ferrite is strongly influenced by the exchange interactions between the iron ions. The most stable form of the ferrite is a ferrimagnet with the Fe3+ ions at the 4f sites having their spin polarization anti-parallel to the rest of the Fe3+ ions, in agreement with Gorter's prediction. The ferrite SrFe12O19 is calculated to be a semiconductor with both the top of the valence band and the bottom of the conduction band being dominated by Fe 3d states. A strong anisotropy was found for the conductive charge carriers. The calculated results are in good agreement with the available experimental data.

/pdf/magnetic-and-electronic-properties-of-strontium-hexaferrite-3graecjalh.pdf

Magnetic and electronic properties of strontium hexaferrite SrFe12O19 from first-principles calculations

Ab initio band structure calculations were performed for MgSiN2 and Mg3N2. Calculations show that both nitrides are semiconductors with direct energy gaps at . The valence bands are composed mainly of N 2p states hybridized with s and p characters of the metals. The bottom of the conduction band consists of the s characters of Mg and N for Mg3N2, as well as for MgSiN2, while the characters of Si are higher in energy. The optical diffuse spectra show an energy gap of about 2.8 eV for Mg3N2 and 4.8 eV for MgSiN2, in agreement with the calculated values.

/pdf/ab-initio-band-structure-calculations-of-mg3n2-and-mgsin2-5dp3rrnnu5.pdf

Ab initio band structure calculations of Mg3N2 and MgSiN2

https://pure.rug.nl/ws/files/3761019/2002JPhysChemSolidsFang.pdf

Ab initio band structure calculations of the low-temperature phases of Ag2Se, Ag2Te and Ag3AuSe2

Results of first-principles band-structure calculations for the ternary alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) are presented. In the structures of M2Si5N8 (M = Ca, Sr and Ba), the N atoms show connections to two (N[2]) and three (N[3]) neighbouring silicon tetrahedral centres. Calculations show that the local electronic structure is strongly dependent on the local chemical bonding. The valence band is dominated by N 2p hybridized with the s, p states of the alkaline-earth-metal and silicon atoms. The upper part of the valence band is dominated by the 2p states of N[2] atoms, while the N[3] 2p states lie about 2 eV below the Fermi level. The bottom of the conduction band consists of the N 3s characters hybridized with s orbitals of the alkaline-earth metals, while the s character of Si atoms is higher in energy. Sr2Si5N8 is a semiconductor with a direct energy gap at Γ, while Ca2Si5N8 is an indirect semiconductor. Optical diffuse reflectance spectra show an energy gap of 4.9 eV for Ca2Si5N8, 4.5 eV for Sr2Si5N8, as well as 4.1 eV for Ba2Si5N8, in fair agreement with the calculated values.

/pdf/electronic-structure-of-the-alkaline-earth-silicon-nitrides-37swppcl2e.pdf

Electronic structure of the alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) obtained from first-principles calculations and optical reflectance spectra

https://pure.rug.nl/ws/files/3626531/2001JAlloysCompFang.pdf

de Ra Groot

Papers

Magnetic and electronic properties of strontium hexaferrite SrFe12O19 from first-principles calculations

Ab initio band structure calculations of Mg3N2 and MgSiN2

Ab initio band structure calculations of the low-temperature phases of Ag2Se, Ag2Te and Ag3AuSe2

Electronic structure of the alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) obtained from first-principles calculations and optical reflectance spectra

First-principles electronic structure calculations of Ba5Si2N6 with anomalous Si2N6 dimeric units