Garnet Ceramics Research Articles

Comparative spectroscopic investigation of rare earth-doped oxide transparent ceramics and single crystals

Abstract The high-resolution optical spectroscopy and emission decay of rare earth-doped garnet and sesquioxide polycrystalline transparent ceramics produced by solid-state synthesis, with grains of tens of microns, indicate that the nature, variety, and distribution of structural centers formed by the doping ions, their quantum states (energy levels, transition probabilities) and the energy transfer processes are similar to those reported for the corresponding single crystals. New systems based on simple- or multiple-doped Sc 2 O 3 ceramics indicate that these materials show potential for new laser materials in conventional or sensitised emission schemes.

Synthesis and Microwave Dielectric Properties of Re3Ga5O12 (Re: Nd, Sm, Eu, Dy, Yb, and Y) Ceramics

Re3Ga5O12 (Re: Nd, Sm, Eu, Dy, Yb, and Y) garnet ceramics were synthesized and their microwave dielectric properties were investigated for advanced substrate materials in microwave integrated circuits. The Re3Ga5O12 ceramics sintered at 1350°–1500°C had a high‐quality factor (Q×f) ranging from 40 000 to 192 173 GHz and a low‐dielectric constant (ɛr) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τf) in the range of −33.7 to −12.4 ppm/°C. In particular, the Sm3Ga5O12 ceramics sintered at 1450°C exhibited good microwave dielectric properties of ɛr=12.4, Q×f=192 173 GHz, and τf=−19.2 ppm/°C.

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.

Cryogenic temperature characteristics of Verdet constant on terbium gallium garnet ceramics

As the first demonstration of Faraday effect in a TGG ceramics, its Verdet constant at 1053 nm is evaluated to be 36.4 rad/Tm at room temperature which is same as that of the single crystal. In addition, the temperature dependence of Verdet constant is obtained experimentally. At liquid helium temperature, it is 87 times greater than that at room temperature.

Microwave dielectric properties of Re 3Ga 5O 12 (Re: Nd, Sm, Eu, Dy and Yb) ceramics and effect of TiO 2 on the microwave dielectric properties of Sm 3Ga 5O 12 ceramics

Re 3Ga 5O 12 (Re: Nd, Sm, Eu, Dy and Yb) garnet ceramics sintered at 1350–1500 °C had a high quality factor ( Q × f) ranging from 40,000 to 192,173 GHz and a low dielectric constant ( ɛ r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency ( τ f ) in the range of −33.7 to −12.4 ppm/°C. In order to tailor the τ f value, TiO 2 was added to the Sm 3Ga 5O 12 ceramics, which exhibited good microwave dielectric properties. The relative density and grain size increased with addition of TiO 2, resulting in the enhancement of Q × f value. The τ f increased with the addition of TiO 2. Excellent microwave dielectric properties of ɛ r = 12.4, Q × f = 240,000 GHz and τ f = −16.1 ppm/°C were obtained from the Sm 3Ga 5O 12 ceramics sintered at 1450 °C for 6 h with 1.0 mol% TiO 2. Therefore, Re 3Ga 5O 12 ceramics, especially TiO 2-added Sm 3Ga 5O 12 ceramics are good candidates for advanced substrate materials in microwave integrated circuits (MICs) applications.

Cavity-Perturbation Measurement of Complex Permittivity and Permeability of Common Ferrimagnetics in Microwave-Frequency Range

This paper investigates complex permittivity and permeability of several commercially available yttrium iron garnet (YIG) and nickel-ferrite ceramics in the demagnetized state. The measurements were realized by using the rectangular cavity resonators in the frequency range 4.5–26.5 GHz.

Fabrication of Transparent Cerium‐Doped Lutetium Aluminum Garnet Ceramics by Co‐Precipitation Routes

Nanosized, highly sinterable Ce‐doped lutetium aluminum garnet (LuAG:Ce) powders were synthesized by the urea homogeneous co‐precipitation routes. LuAG:Ce transparent ceramics with an average grain size of about 6 μm were successfully fabricated at 1800°C for 10 h by vacuum sintering without sintering aids using the powders calcined at 1000°C for 2 h. The transmittance in the visible light region reaches 65%. The radioluminescence spectrum displays a broad band located at 480–650 nm consisting of two emissions due to the transitions from the lowest 5d excited state to the 4f ground state of Ce3+, which is consistent with that of LuAG:Ce single crystal and well coupled with the silicon photodiodes.

Structural and Optical Properties of Highly Nd‐Doped Yttrium Aluminum Garnet Ceramics from Alkoxide and Glycolate Precursors

Yttrium aluminum garnet (YAG) nanopowders doped with high neodymium (Nd) content (3 at.%) were synthesized by the sol–gel processing of (i) alkoxide precursors and (ii) metal chelates formed by complexing the cations with polyethylene glycol. A stoichiometric YAG composition was obtained following both procedures; however, the agglomeration of particles was significantly higher in glycolate synthesis, which shielded residual organics from oxidation (elemental analyses). Distribution of Nd3+ ions in the YAG matrix, as shown by the absorption of pump energy and photoluminescence spectra of Nd:YAG ceramics, was more homogeneous in alkoxide‐derived powders. The segregation of Nd centers in the glycolate‐derived sample was supported by the precipitation of a crystalline Nd2O3 phase (X‐ray diffraction) during sintering. High‐resolution absorption spectra (4I9/2(1)→4F9/2(1)) of the powders showed that a higher absorption cross‐section of glycolate‐derived powders is due to Nd3+–Nd3+ ion pairing, which leads to the quenching of photoluminescence. Owing to the better dispersion of optically active centers, the photoluminescence signal was found to be substantially enhanced in alkoxide‐derived Nd:YAG ceramics.

Refine Yttria Powder and Fabrication of Transparent Yb,Cr:YAG Ceramics

Commercial Y2O3 powders were subjected to high energy ball milling in a planetary ball mill using high purity alumina balls and vial. The refined Y2O3 power ,with commercial high-purity Al2O3, Cr2O3 and Yb2O3 powders, were used as raw materials. Transparent ytterbium and chromium codoped yttrium aluminum garnet (Yb,Cr:YAG) ceramics were fabricated by a solid-state reaction method. The Yb,Cr:YAG ceramics exhibit a pore free structure and the average grain size is about 10 micron. The strong absorptions at 940 and 968 nm of Yb3+ are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switch laser output at 1030 nm. Transparent Yb,Cr:YAG ceramics may be a potential material for compact, efficient, high-stability diode-laser-pumped passive Q-switched solid-state lasers.

Stark components of lower-lying manifolds and emission cross-sections of intermanifold and inter-stark transitions of Nd 3+(4f 3) in polycrystalline ceramic garnet Y 3Al 5O 12

Stark energy levels of the 4F 3/2, 4I 9/2, and 4I 11/2 manifolds have been characterized using the room temperature fluorescence spectra for the 4F 3/2→ 4I 9/2 and 4F 3/2→ 4I 11/2 transitions of Nd 3+(4f 3) in polycrystalline ceramic garnet Y 3Al 5O 12 (YAG). The emission cross-sections of the intermanifold transitions, 4F 3/2→ 4I 9/2 and 4F 3/2→ 4I 11/2, as well as the principal inter-Stark transitions, R 1→Z 5 (945.3 nm) and R 1→Y 2 (1063.5 nm), have also been determined. These results are finally compared with those of Nd 3+:YAG single crystal.

Fabrication of transparent Yb,Cr:YAG ceramics by a solid-state reaction method

Transparent ytterbium and chromium codoped yttrium aluminum garnet (Yb,Cr:YAG) ceramics, containing Ca as charge counter element and TEOS as sintering aid, were fabricated by a solid-state reaction method using high-purity Y 2O 3, Al 2O 3, Cr 2O 3 and Yb 2O 3 powders as raw materials. The mixed powder compacts were sintered at 1770 °C for 10 h under vacuum and annealed at 1450 °C for 20 h in air. The Yb,Cr:YAG ceramics exhibit a pore free structure with an average grain size of about 40 μm. The optical transmittance of 5 at% Yb, 0.025 at% Cr:YAG reaches 75% at 1100 nm. The strong absorptions of Yb 3+ at 940 and 968 nm are suitable for InGaAs diode laser pumping; an absorption band at 1030 nm is suitable for passive Q-switch laser output at 1030 nm. Transparent Yb,Cr:YAG ceramics may be a potential material for compact, efficient, high-stability diode-laser-pumped passive Q-switched solid-state lasers.

Fabrication of Transparent Cerium‐Doped Lutetium Aluminum Garnet (LuAG:Ce) Ceramics by a Solid‐State Reaction Method

Polycrystalline, transparent LuAG:Ce ceramics were fabricated by a solid‐state reaction method using high‐purity Lu2O3, Al2O3 and CeO2 powders. The mixed powder compacts were sintered at 1770°C for 10 h under vacuum and annealed at 1450°C for 20 h in air. The transmittance in the visible light region reaches 70%. The emission spectra at 470–650 nm wavelengths are consistent with that of LuAG:Ce single crystals, and satisfy the property requirements of a scintillator.

Absorption intensities and emission cross sections of principal intermanifold and inter-Stark transitions of Er3+(4f11) in polycrystalline ceramic garnet Y3Al5O12

A comparative spectroscopic study is performed on Er3+(4f11) ions doped in polycrystalline ceramic garnet Y3Al5O12 (YAG) and single-crystal laser rod, both containing nominal 50 at. % of Er3+. The standard Judd–Ofelt (JO) model is applied to the room-temperature absorption intensities of Er3+(4f11) transitions in both hosts to obtain the phenomenological intensity parameters. These parameters are subsequently used to determine the radiative decay rates, radiative lifetimes, and branching ratios of the Er3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds LJ2S+1 of Er3+(4f11) in these garnet hosts. The emission cross sections of the intermanifold Er3+I13∕24→I15∕24 (1.5 μm) transition as well as the principal inter-Stark transition Y1→Z4 (1550 nm) within the corresponding multiplet manifolds have been determined. The room-temperature fluorescence lifetimes of the I13∕24→I15∕24 (1.5 μm) transition in both polycrystalline ceramic and single-crystal YAG samples were measured. From the radiative lifetimes determined from the JO model and the measured fluorescence lifetimes, the quantum efficiencies for both samples were determined. The comparative study of Er3+(4f11) ions performed suggests that polycrystalline ceramic YAG is an excellent alternative to single-crystal YAG rod for certain applications.

Effects of SiO2 Addition on Microstructure and Magnetic Properties of Yttrium Iron Garnet

Pore free Yttrium Iron Garnet (YIG) ceramics with high density and low magnetic loss were manufactured by conventional ceramic processing. By small additions of SiO2, pores captured inside grains were effectively eliminated, and the number of pores in grain boundaries was markedly reduced. The density of specimens was 99.6% of the theoretical density. The FMR linewidth of a specimen was 47 Oe, which is comparable to that of other YIG specimens produced by more expensive and elaborate process such as hot pressing. However, adding more than 1 mol% SiO2 deteriorated the sintering characteristics of the YIG specimen. The added SiO2 may segregate at grain boundaries and inhibit the grain growth. As a result, the average grain size and sintered density were reduced in the case of SiO2 addition more than 1 mol%.

Spectral analysis and energy-level structure of Er3+(4f11) in polycrystalline ceramic garnet Y3Al5O12

Absorption spectra obtained between 1550 and 440nm and fluorescence spectra obtained between 1700 and 1500nm are reported in a comparative spectroscopic study of polycrystalline ceramic Y3Al5O12 (YAG) and single-crystal laser rod YAG, both containing 50at.% Er3+ as a dopant in the garnet host. The spectra are observed in both samples at temperatures between 8K and room temperature. The detailed splitting of individual multiplet manifolds, LJ2S+1, Er3+(4f11), by the crystalline electric field is similar in both the ceramic sample and the single-crystal laser rod. With few exceptions, there is little shift in energy (few wave numbers) of individual Stark levels within a manifold, between dilute and concentrated Er3+ samples. This is not too surprising since Y3Al5O12 and Er3Al5O12 form a solid solution with the majority of Er3+ ions occupying cation sites having D2 symmetry in the garnet lattice over the entire solid solution range. As a check on the observed manifold splittings of Er3+ in ceramic YAG, we compared the present results to the calculated splitting that several of us reported earlier for Er3+ in single-crystal YAG. To within the standard deviation reported earlier, the present results agree with that calculation. Thus, one set of atomic and crystal-field parameters describes the splitting of the Er3+ energy levels in either the ceramic or single-crystal host for Er3+ in D2 sites.

Energy-level structure and spectral analysis of Nd3+(4f3) in polycrystalline ceramic garnet Y3Al5O12

A detailed crystal-field splitting analysis is given for the 26 lowest-energy multiplet manifolds, LJ2S+1, of Nd3+(4f3) in polycrystalline ceramic garnet Y3Al5O12 (YAG). The absorption spectra obtained between 8K and room temperature, and between 1750 and 350nm, and the fluorescence spectrum obtained at 8K and observed between 1450 and 875nm are analyzed for transitions between individual energy (Stark) levels that characterize the energy-level structure of Nd3+ ions in D2 symmetry sites, replacing Y3+ ions in the garnet host lattice. A model Hamiltonian including atomic and crystal-field terms is diagonalized within the complete 4f3SLJMJ basis set which includes 364 states. The calculated splitting of the Nd3+ energy levels by the crystal field is compared with the experimental splitting observed in both the ceramic sample and a single-crystal laser rod. Both samples have approximately the same Nd3+ concentration, about 1at.%. By varying the atomic and crystal-field parameters, we obtain a standard deviation of 18cm−1 between 106 calculated-to-observed Stark levels found between the ground state and the D1∕24 at 28 369cm−1. Within this standard deviation the energy-level structure of Nd3+ is found to be similar in both samples. Low temperature visible and near IR spectra of ceramic Nd3+:YAG show it has comparable properties to the crystalline analog.

Measurements of Complex Permittivity and Permeability of Common Ferrimagnets at Millimeter Waves

In this paper, complex permittivity and permeability of several yttrium iron garnet (YIG) and hexagonal ferrite ceramics are investigated in a broad-band spectrum for the first time. The measurements were realized using a broad-band quasi-optical millimeter-wave (QOM) system with a backward-wave oscillator as the source of tunable coherent radiation. The dielectric and magnetic parameters of the ferrimagnets were obtained by fitting theoretical curves to the experimental spectra. Three types of YIG specimen were initially measured and compared to theoretical results, and then hexagonal ferrites were measured in the demagnetized state for the first time. It was shown that this method provides an accurate characterization of soft ferrites as well as hard anisotropic ferrites in the entire millimeter-wave range.

Corrosion Study of Actinide Waste Forms with Garnet-Type Structure

AbstractGarnet-based ceramics doped with Gd, Th, and Ce as waste surrogates and compositions: (Ca1.5GdTh0.5)ZrFeFe3O12 (G3) and Ca2.5Ce0.5Zr2Fe3O12 (G21) were prepared at 1300 °C in air. Corrosion rates in deionized water measured using an MCC-2 procedure were found to be (in g × m-2 × day-1): 10-6 for Ce (G21), 3×10-5 for Gd and 10-6 for Th (G3). Corrosion rates of Ce, Gd, and Th in an acid solution (0.01 M HCl) were 2 × 10-1 (G21), 4 × 10-3 and 10-3 (G3), respectively. Chemical durability of the garnet ceramics is comparable to durability of titanate zirconolite and pyrochlore samples under similar conditions. Scanning electron microscopy has revealed no alteration of the sample surfaces after their contact with water, and the presence of newly formed iron-enriched phases of 1 to 3 micrometers in size, after interaction of the G21 sample with acid solution for 30 days. High leaching of the elements from the Ce-loaded garnet may be due to reduction of Ce in acid solution to Ce(III) or may be explained by lower stability of the structure of garnets (Ca2.5An0.5)Zr2Fe3O12 compared to (Ca1.5GdAn0.5)(ZrFe)Fe3O12, An = Ce4+ and Th4+. It was found that the (Ca2.5Ce0.5)Zr2Fe3O12 and (Ca2.5Th0.5)Zr2Fe3O12 garnets have distorted the elementary unit cell with decreasing lattice symmetry to tetragonal.

Ionic Transport in Gd3Fe5 O 12- and Y 3Fe5 O 12-Based Garnets

The oxygen permeability of dense garnet-type and membranes at 1173-1273 K is limited by the bulk ambipolar conductivity. The ion transference numbers, calculated from results on the oxygen permeation and total conductivity, vary from to increasing with temperature. Ionic conduction in ferrite garnets, primarily determined by the oxygen vacancy concentration, increases with acceptor dopant additions. The activation energies for the oxygen ionic and electronic transport in air are in the ranges 176-224 and 20-81 kJ/mol, respectively. The ceramic microstructure of garnet-based materials has no essential effect on their ionic conductivity, which is low compared to that of perovskite-related ferrites. The low mobility of oxygen ions, probably limited by the ion transfer along the edges of Fe-O tetrahedra in the garnet lattice, is likely to result from a crooked diffusion pathway. Decreasing the A-site cation radius leads to a higher ionic conductivity of garnet phases. The thermal expansion coefficients of ferrite garnet ceramics at 300-1200 K in air are in the range © 2003 The Electrochemical Society. All rights reserved.

Role of Diffusion Phenomena in the Processing of Ceramics

The role of diffusion in ceramic processing is discussed. The utility of secondary ion mass spectroscopy (SIMS) is highlighted for the evaluation of diffusion coefficients in ceramic materials. By comparing diffusion coefficients obtained in this manner with those obtained indirectly from creep measurements, we identified the ion species that govern creep in yttrium aluminum garnet (TAG) ceramics and we showed that diffusion involving metastable defects plays an important role in ceramic processes such as sintering. The oxygen diffusion coefficients of thin films were evaluated, and the defects involved were shown to be metastable, and their origins were clarified. Furthermore, their relationship with reactivity was discussed in thin films.