Yttrium Iron Garnet Ceramics Research Articles

Two-Stage Sintering of Yttrium Iron Garnet Ceramics for Magnetron Sputtering

Two-Stage Sintering of Yttrium Iron Garnet Ceramics for Magnetron Sputtering

The solubility, microstructure, and chemical durability of Ce‐doped YIG ceramics designed as actinide waste forms

Yttrium iron garnet (YIG) is a potential matrix for immobilizing high-level radioactive waste (HLW) due to its high actinide solubility and chemical flexibility. In this study, Ce was employed as a surrogate of Pu. A series of Y3-xCexFe5O12 (0 ≤ x ≤ 1) samples were prepared by solid-state sintering at 1400°C to investigate the solubility, microstructure, and chemical durability of Ce-doped YIG. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the solubility of the Ce in a single garnet was 32 mol% at synthesis temperature of 1400°C. The SEM analysis indicated the YIG ceramic grain growth and compactness as Ce concentrations increased. The XPS analysis indicated that the amount of Ce4+ out of the total Ce increased as the CeO2 doping increased, and the maximum value was more than 75.9% in the sample synthesis at 1400°C. Static leaching experiments were also conducted on the single Ce-doped garnet synthesized at 1400°C. The normalized LRCe and LRY release rates were approximately 10−4 and 10−5 g m−2 d−1 after 42 days, respectively. The chemical durability was comparable to other ceramic matrices (such as pyrochlore and zircon, among others). These results demonstrated that YIG ceramics are suitable for immobilizing HLW, such as Pu or U. Highlights The solubility of Ce in single-phase YIG was 32 mol% at 1400°C. Grain size and compactness increased Ce content. The concentration of Ce4+ in total Ce of sample could be over 75.9%. The obtained YIG ceramics exhibit excellent chemical stability. Novelty Statement Y3-xCexFe5O12 ceramics were prepared by solid-state sintering and the solubility, microstructure, and chemical durability were investigated. The solubility of Ce was 32 mol% at 1400°C. The obtained YIG ceramics exhibit excellent chemical stability.

Effect of Ca dopant on magnetic and magnetodielectric properties of Y3Fe5O12

A series of Ca-doped yttrium iron garnet ceramics (Y3-xCaxFe5O12 (x = 0, 0.05, 0.1, 0.15, 0.2)) were prepared by the solid state method. The structure, morphology, valence fluctuation, thermal analysis, magnetization, magnetodielectric and Mössbauer spectrum of Y3-xCaxFe5O12 ceramics were systematically studied. All of the samples crystalized as single phase of garnet structure with the grain sizes of 2–3 μm. The temperature dependent percentage weight loss and heat flow were investigated by thermogravimetric (TG) and differential scanning caborimetry (DSC). The structure of Y3-xCaxFe5O12 ceramics was studied by XRD, Raman and Infrared (IR) spectra. The results indicated that Ca2+ substitution leads to lattice expansion and presence of Ca–O octahedron, which causes the concentration of Fe2+ decrease and Fe4+ increase. As a result, the saturation magnetization first increases and then decreases with the increase of Ca concentration. A six lines asymmetry of magnetic components were observed in Mössbauer spectrum, which was fitted by two hyperfine magnetic sextets corresponding to tetrahedral and octahedral sites, respectively. With the increase of Ca concentration, the intrinsic magnetodielectric (MD) effect disappears completely due to the dramatic decrease of Fe2+-Fe3+ dipoles. Fe3+-Fe4+ dipoles do not contribute to the MD properties of Y3-xCaxFe5O12 ceramics due to the lack of the defects such as electrons or holes being used as media.

Microwave-Assisted Synthesis of Yttrium Iron Garnet Nano Powders for Low Temperature Sintering

Yttrium iron garnet (YIG) nano powders have been successfully synthesized by a chemical co-precipitation plus microwave heating method. The particle sizes of the synthesized YIG powders ranged from 15 nm to 30 nm. Their heating behaviors indicated that small YIG nano crystallites in the synthesized powders started to melt and form liquid phase at a low temperature about 947°C. By using the synthesized powders as the starting materials, the dense YIG ceramics were sintered at the temperature of about 1050–1100°C, which was much lower than the sintering temperature for the powder derived from traditional solid reactions. The liquid phase from melting of small particles promoted the diffusion and re-crystallization, resulting in the low temperature sintering of YIG ceramics.

Structural and magnetic properties of Al-doped yttrium iron garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study

The structural and magnetic properties of Al substituted yttrium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 °C for 6 h and subsequently at 1200 °C for 6 h to obtain phase-pure garnets. X-ray diffraction (XRD) results confirm the formation of garnets with Ia3¯d structure. The occupancy of Y3+ ions in the dodecahedral site and the distribution of Al3+ and Fe3+ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld refinement of XRD patterns, Mössbauer spectroscopy and 57Fe internal field NMR spectroscopy. For low Al content, Al3+ ions have preference to occupy tetrahedral (Td) sites than the octahedral (Oh) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tetrahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al3+ as compared to Fe3+ ion. All the studied samples show coral-network-like surface morphology. The saturation magnetization (MS) values decrease from ∼26.94 emu/g to ∼ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al3+ reduces the saturation magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

Synthesis of Dense, Fine-Grained YIG Ceramics by Two-Step Sintering

A two-step sintering (TSS) process has been used to fabricate yttrium iron garnet (YIG) ceramics with high density and fine grain size. The densification, microstructure, and magnetic properties were investigated. The sample prepared by the TSS process with first-step sintering temperature (T 1) of 1350°C, second-step sintering temperature (T 2) of 1300°C, and holding time of 18 h had density above 99% of theoretical and exhibited uniform microstructure with small average grain size (2.4 μm). The saturation magnetization (M S) of this sample reached 27.4 emu/g. These results indicate that the TSS process can effectively suppress grain-boundary migration while maintaining active grain-boundary diffusion to obtain dense, fine-grained YIG ceramics with appropriate magnetic properties.

The Investigation of the Phenomenological YIG Phase Formation Within 1000°C to 1250°C: A Kinetic Approach

Presence of unwanted phase(s) such as yttrium iron perovskite (YIP) in yttrium iron garnet (YIG) ceramics has limited the utilization of YIG in the wireless communication domain. These unwanted phase(s) have been deemed responsible for the high dielectric losses, thus contributing to poor performance. This paper focuses on understanding the phenomenological phase transformation during the conventional solid state synthesis of YIG. This is done in order to monitor conditions which favor formation of unwanted phase(s), which shall later be reduced. The phase changes during YIG formation as a function of reaction times and temperatures were determined through XRD analysis. The amounts of YIG formed at various reaction times were fitted into various kinetic models in order to mathematically link what occurs experimentally to the available theoretical descriptions of reactions. It is found that the Ginstling‐Brounstein‐Habert (GBH) model exhibited good mathematical correlation to the formation of YIG. Meanwhile the activation energy (Ea) indicated 490 kJ/mol is required for the formation of YIG. At the end, a reaction model and mechanism between Fe2O3 and Y2O3 were established and illustrated to underline the effect of diffusion controlled environment on the formation of phases in YIG ceramics.

Effects of Bi-Substitution on Dielectric and Ferroelectric Properties of Yttrium Iron Garnet Ceramics

Dielectric and ferroelectric properties of Y3−xBixFe5O12 ceramics were extensively investigated. All the samples show a low temperature dielectric relaxation which originates from the charge carrier hopping between Fe2+ and Fe3+ and a high temperature dielectric relaxation which is associated with conduction. The dielectric constant, dielectric loss and the activation energy of the hopping process between Fe2+ and Fe3+ of Y3-xBixFe5O12 ceramics decrease with increasing Bi substitution. The conductivity decreases and the remanent polarization increases as the amount of Bi increases.

The behavior of high frequency tunable dielectric resonator antenna (DRA) with the addition of excess Fe2O3 in Y3Fe5O12 (YIG) formulation

This manuscript discusses the potential of yttrium iron garnet (YIG) with various amounts of Fe2O3 excess, as a tunable dielectric resonator antenna. The exploration of YIG’s antenna microwave properties is a key to determine stability of the antenna radiation pattern and frequency shifting. YIG with various amounts of excess Fe2O3 is prepared through a mixed oxide powder technique. Through X-ray diffraction analysis, it was found that addition of 5 wt% excess Fe2O3 to YIG stoichiometric formulation has depleted YIP phases in YIG ceramics. However, increasing amount of excess Fe2O3 in excess of 15 wt% led to increase in the Fe2O3 residue in YIG ceramics. The grains observed using high resolution electron microscopy displayed a well-packed structure. The size of the grains became bigger when the percentage of Fe2O3 increased to 20 % but accompanied with significant changes in electrical properties. This change has led to a frequency shift from 14.14 to 17.32 GHz, which cover applications within the Ku-band (12.00–18.00 GHz) together with radiation stability. These results imply that inclusion of excess Fe2O3 can be used for tunable DRA.

Magnetic and magnetodielectric properties of Bi-substituted yttrium iron garnet ceramics

Effects of Bi substitution upon magnetic and magnetodielectric properties of Y3Fe5O12 ceramics were investigated. It was found that the saturation magnetization decreases and the magnetic ordering temperature increases as the amount of Bi increases. The contribution of intrinsic and extrinsic effects to magnetodielectric effects decreases with increase of Bi, which results in the decrease of magnetodielectric coefficient of Y3−xBixFe5O12 ceramics.

Effects of Al Substitution on Dielectric Response and Magnetic Behavior of Yttrium Iron Garnet Ceramics

Structure, dielectric response, and magnetic properties of Y3Fe5−xAlxO12 (x = 0, 0.5, 1, 2, and 3) ceramics were investigated. X‐ray diffraction (XRD) patterns indicate a single phase with garnet structure in space group Ia3d for all sintered samples. Rietveld refinement of XRD data suggest that Al3+ substitutes for Fe3+ at tetrahedral sites. All the samples show a giant dielectric response which originates from the charge carrier hopping between the Fe2+ and Fe3+. The activation energy of the hopping process increases with increasing Al substitution. The a‐d super‐exchange interaction becomes weak as the content of Al increases, which results in the decrease of saturation magnetizations and magnetic ordering temperatures.

The electromagnetic properties of Cu-substituted garnets with low sintering temperature

Yttrium iron garnet ceramics are used as non-reciprocal materials in circulators because they present low dielectric and magnetic losses, and their magnetic properties can be engineered in order to optimise the material for specific applications. The main drawbacks of circulators are their size and cost, due to complex mechanical assembling of different materials. A possible solution would be to adapt the different materials to a common LTCC process in order to produce circulators with a multilayer process. We showed that Cu substitutions enable us to decrease considerably the sintering temperatures of garnets, from about 1450 °C to about 1070 °C, and our more recent results show it will be possible to decrease this temperature to 1000 °C, which would be a suitable temperature for co-firing with gold. We present new microwave characterizations (the dielectric constant and losses, the different magnetic line-widths: Δ H eff, Δ H K ) of Cu-substituted garnets. The important impact of these characteristics on the properties of circulators is explained, and their variations versus composition and microstructure of the materials are shown.

Magnetic composite materials obtained by swift heavy-ion irradiation of yttrium iron garnet ceramics

High-energy heavy-ion irradiation of magnetic insulators provides a unique way to generate reproducible random composites of ferrimagnetic crystals and paramagnetic amorphous latent tracks. Moreover, high-energy heavy-ion irradiation of the garnet ceramics Y3Fe5O12 is able to induce a third domain exhibiting a rotation of the hyperfine magnetic field parallel to the latent track axis. An analysis of the Mössbauer spectra allowed the intensities of the three contributions to be determined. A damage model, based on two shells surrounding an amorphous core, is proposed to estimate the volumes of the three components.