Diffusion Of Gd Research Articles

Synergistic enhancement of coercivity and thermal stability of nanocrystalline multi-main-phase Nd-Ce-Fe-B magnet via Gd60Y10Cu15Al15 addition

Nanocrystalline multi-main-phase (MMP) Nd-Ce-Fe-B magnet can effectively suppress the magnetic dilution effect of Ce. However, its low coercivity and poor thermal stability have not been adequately overcome. In this work, a novel low-melting-point Gd60Y10Cu15Al15 alloy was introduced into MMP Nd-Ce-Fe-B magnet through intergranular addition for simultaneously enhancing its coercivity and thermal stability. The results show that the intrinsic coercivity Hcj is obviously improved, and its maximum increment is ∼12.3 % at 4 wt% Gd60Y10Cu15Al15 alloy. Especially, the increase in Hcj is more significant, and an abnormally increase in the maximum energy product (BH)max occurs at high temperature of 150 °C. Meanwhile, the reversible temperature coefficients of Hcj (β) and Br (α) are improved simultaneously. These findings imply the enhanced thermal stability for the MMP magnet with Gd60Y10Cu15Al15 addition. The microstructural characterizations, compositional analyses and micromagnetic simulations reveal that the competitive effects of the formed non-ferromagnetic grain boundary (GB) phase and Y or Gd diffusion into the main phase lead mainly to a synergistic improvement in the coercivity and thermal stability of the magnet. This work is expected to provide a promising cost-effective approach for developing the high-performance thermally-stable Nd-Ce-Fe-B magnet and explore more possibilities for effective utilization of Y or Gd rare-earth resources.

Creep behavior and precipitation evolution of Mg-13Gd-2Y-0.5Zr alloy

With exceptional aging hardening effect and a heat resistance temperature up to 250 °C, MgGd alloys have great application prospects in heat-resistant components in new energy vehicle power systems, aviation and aircraft systems, atc. In this study, the creep properties of the Mg-13Gd-2Y-0.5Zr (GW132) alloy at 200–270 °C/ 110–150 MPa are investigated, and the constitutive equation of the steady creep rate is constructed, focusing on the evolution of the precipitated phase in the α-Mg matrix at 250 °C/150 MPa. The alloy exhibits good creep resistance at 200–250 °C/ 110–150 MPa and 270 °C/ 110 MPa. During creep at 250 °C/ 150 MPa for 200 h, the evolution sequence of α-Mg matrix precipitates is mainly β’ phase → β’ + β1 phase → β’ + β1 + β + 18R-LPSO phase, β’, β1, β and 18R-LPSO phases can coexist in the same creep state. The β’ phase is maintained at a large volume fraction and forms a string with the β1 phase. The string-like precipitates are connected to each other to form a continuous dense grid module. Moreover, the β’ phase and the 18R-LPSO phase have excellent high-temperature thermal stability and are coherent with the α-Mg matrix, enabling the alloy to have excellent creep resistance in a certain temperature range. The creep mechanism of the GW132 alloy at 250 °C/ 150 MPa is dominated by the diffusion of Gd and Y atoms and the dislocation slip.

Development of Gd2O3 doped yttria stabilized zirconia based thermal barrier coating for improved high temperature oxidation and erosion resistance

Rear-earth doped advanced thermal barrier coatings (TBCs) have been developed by in-situ doping of Gd into YSZ matrix by atmospheric plasma spray coating of YSZ and Gd2O3 (6 to 15 mol %) premixed powders. These modified TBCs are deposited over CoNiCrAlY bond coated Inconel 718 substrate. Diffusion of Gd or Gd3+ into YSZ matrix resulted in further stabilization of its tetragonal phase which is confirmed by detailed X-Ray diffraction analysis. Microstructure and chemical compositions of doped TBCs are examined by scanning electron microscopy and energy dispersive spectroscopy, respectively. Incorporation of Gd2O3 resulted in relatively denser and smoother coating compared to pure YSZ coating for identical process parameters due to finer particle size and complete melting of Gd2O3 powders. However, in-situ doped coatings are sinter resistant at elevated temperature unlike pure YSZ coating which aids in retention of porosities during high temperature exposure and provide better thermal performances. Modified TBCs showed higher hardness and Young's modulus, as revealed by micro and nano hardness measurements. High temperature isothermal oxidation studies carried out between 1173 K and 1273 K allowed evaluation of the efficacy of modified TBCs for oxidation resistance and related kinetics. The results show that modified TBCs provide better oxidation resistance compared to pure YSZ coating. Erosion tests of TBCs both at ambient and elevated temperature (773 K) showed that modified coatings are more resistant to erosion compared to YSZ coating.

X-ray nano-spectroscopy study of two solid oxide cells operated for long-term in steam electrolysis

This study addresses the degradation mechanisms of porous fuel electrode from an electrode (Ni/YSZ) and an electrolyte (Ni/CGO)-supported cell operated for long-term in steam electrolysis mode. Using a nanobeam, spatially resolved X-ray fluorescence and X-ray absorption spectroscopy are combined to acquire high-resolution compositional maps and to probe intra-granular changes with the evolution of the Ni chemical state. This experimental work highlights oxidized states of nickel (i.e. NiO/Ni(OH)2 species) in the case of Ni/CGO while for Ni/YSZ the chemical state is metallic. In both types of fuel electrodes, the Ni depletion occurs and the depletion length is reduced to around 1 μm in the case of Ni/CGO after 23,000 h in contrast to around 4 μm in the case of Ni/YSZ after 6100 h. In the case of Ni/CGO, the coating of Ni particles by the diffusion of Gd and Ce from the CGO contact layer prevents the Ni migration while in the case of Ni/YSZ the Ni migration is not suppressed. These results are in accordance with in-situ impedance data where the main difference observed between Ni/YSZ and Ni/CGO fuel electrodes is a deactivation of the fuel electrode as non-ohmic losses for Ni/YSZ in contrast to Ni/CGO.

Negligible magnetic losses at low temperatures in liquid phase epitaxy grown Y3Fe5O12 films

Yttrium iron garnet (${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$; YIG) has a unique combination of low magnetic damping, high spin-wave conductivity, and insulating properties that make it a highly attractive material for a variety of applications in the fields of magnetics and spintronics. While the room-temperature magnetization dynamics of YIG have been extensively studied, there are limited reports correlating the low-temperature magnetization dynamics to the material structure or growth method. Here we investigate liquid phase epitaxy grown YIG films and their magnetization dynamics at temperatures down to 10 K. We show there is a negligible increase in the ferromagnetic resonance linewidth down to 10 K, which is unique when compared with YIG films grown by other deposition methods. From the broadband ferromagnetic resonance measurements, polarized neutron reflectivity, and scanning transmission electron microscopy, we conclude that these liquid phase epitaxy grown films have negligible rare-earth impurities present, specifically the suppression of Gd diffusion from the ${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ (GGG) substrate into the ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film, and therefore negligible magnetic losses attributed to the slow-relaxation mechanism. Overall, liquid phase epitaxy YIG films have a YIG/GGG interface that is five times sharper and have ten times lower ferromagnetic resonance linewidths below 50 K than comparable YIG films by other deposition methods. Thus, liquid phase epitaxy grown YIG films are ideal for low-temperature experiments/applications that require low magnetic losses, such as quantum transduction and manipulation via magnon coupling.

Solution precursor thermal spraying of gadolinium zirconate for thermal barrier coating

Gadolinium zirconate (GZ) is an attractive material for thermal barrier coatings (TBCs). However, a single layer GZ coating has poor thermal cycling life compared to Yttria Stabilized Zirconia (YSZ). In this study, Solution Precursor High Velocity Oxy-Fuel (SP-HVOF) thermal spray was used to produce a double layer GZ/YSZ TBC and compared the thermal cycling performance with the single layer YSZ TBC. The temperature behaviour of the solution precursor GZ was studied, and single splat tests were carried out to obtain an optimised spray parameter. In thermal cycling tests, the single-layer YSZ reached 20 % failure at 85 ± 5 cycles, whereas the double-layer GZ/YSZ was at 70 ± 15 cycles. The single-layer failed at the topcoat/TGO interface, whereas the double-layer failed at GZ/YSZ interface and topcoat/TGO interface. Moreover, Gd diffusion occurred near the GZ/YSZ interface, resulting in porosities in the GZ layer.

Atomic-resolution interfacial structures and diffusion kinetics in Gd/Bi0.5Sb1.5Te3 magnetocaloric/thermoelectric composites

The demand of a full solid-state cooling technology based on magnetocaloric and thermoelectric effects has led to a growing interest in screening candidate materials with high-efficiency cooling performance, which also stimulates the exploration of magnetocaloric/thermoelectric hybrid cooling materials. A series of Gd/Bi0.5Sb1.5Te3 composites was fabricated in order to develop the hybrid cooling technology. The chemical composition, phase structure and diffusion kinetics across the reaction layers in Gd/Bi0.5Sb1.5Te3 composites were analyzed at different reaction temperatures. Micro-area elemental analysis indicates that the formation of interfacial phases is dominated by the diffusion of Gd and Te while the diffusion of Bi and Sb is impeded. The interfacial phases, including GdTe2, GdTe3, and intermediate phases GdTex, are identified by atomic-resolution electron microscopy. The concentration modulation of Gd and Te is adapted by altering the stacking of the Te square-net sheets and the corrugated GdTe sheets. Boltzmann-Matano analysis was applied to reveal the diffusion kinetics of Gd and Te in the interfacial layers. The diffusion coefficients of Te in GdTe2 and GdTe3 are much higher than that of Gd while in GdTe the situation is reversed. This study provides a clear picture to understand the interfacial phase structures down to an atomic scale as well as the interfacial diffusion kinetics in Gd/Bi0.5Sb1.5Te3 hybrid cooling materials.

Unexpected structural and magnetic depth dependence of YIG thin films

We report measurements on yttrium iron garnet (YIG) thin films grown on both gadolinium gallium garnet (GGG) and yttrium aluminium garnet (YAG) substrates, with and without thin Pt top layers. We provide three principal results: the observation of an interfacial region at the Pt/YIG interface, we place a limit on the induced magnetism of the Pt layer and confirm the existence of an interfacial layer at the GGG/YIG interface. Polarised neutron reflectometry (PNR) was used to give depth dependence of both the structure and magnetism of these structures. We find that a thin film of YIG on GGG is best described by three distinct layers: an interfacial layer near the GGG, around 5 nm thick and non-magnetic, a magnetic bulk phase, and a non-magnetic and compositionally distinct thin layer near the surface. We theorise that the bottom layer, which is independent of the film thickness, is caused by Gd diffusion. The top layer is likely to be extremely important in inverse spin Hall effect measurements, and is most likely Y2O3 or very similar. Magnetic sensitivity in the PNR to any induced moment in the Pt is increased by the existence of the Y2O3 layer; any moment is found to be less than 0.02 uB/atom.

Effect of Gd-Doped Ceria Interlayer Microstructure on the Interdiffusion Behavior Between La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes and Yttria-Stabilized Zirconia Electrolytes

Formation of secondary phases at the interface between cathode and electrolyte is one of the issues affecting the long-term stability of solid oxide fuel cells. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite cathode is known to react with yttria-stabilized zirconia (YSZ) forming SrZrO3 at the interface. To reduce secondary phase formation, a barrier layer such as Gd-doped ceria (GDC) is inserted between LSCF and YSZ. However, the stability of the interlayer may be directly affected by the microstructure and cation diffusion through GDC. In this work, the microstructure of the reaction barrier layer is controlled by growing the GDC thin films on polycrystalline and single crystal YSZ (100) electrolytes using pulsed laser deposition technique. Electrochemical impedance spectroscopy and microstructural analyses were performed on LSCF|GDC|YSZ|Pt cells with different GDC interlayers under an open-circuit condition at 800ºC for 300 h in air. It is observed that SrZrO3 formed at the LSCF/GDC interface of cells with polycrystalline GDC interlayers resulting in a rapid increase in area-specific resistance (ASR) with time. In the case of GDC on YSZ(100), the LSCF/GDC interface showed no SrZrO3. Instead, a significant interdiffusion between LSCF cathode and GDC interlayer occurred. The active diffusion of Gd into LSCF and possible formation of perovskite phases at the LSCF/GDC interface may have contributed to slow increase in the ASR with time. Microstructural changes and type of secondary phase formation at the LSCF/GDC interface give rise to a different rate of increase in ASR and is shown to be directly related to the microstructure of GDC interlayer.

Phase separation in Zr56-xGdxCo28Al16 metallic glasses (0 ⩽ x ⩽ 20)

The influence of Gd addition on the microstructure of Zr56Co28Al16 metallic glasses was investigated for the exchange of Zr by up to 20 at.% Gd. Due to the large positive enthalpy of mixing between Zr and Gd, liquid–liquid phase separation occurs during rapid quenching of the melt. For a low concentration of Gd (x=2 at.%), a homogeneous amorphous structure is obtained for the as-quenched state. Early stages of spinodal decomposition are observed in the as-quenched state of the glasses with x=5 and 10 at.% Gd. Gd-enriched clusters 4–7nm in size are formed, as shown by atom probe tomography (APT). Annealing below the crystallization temperature Tx leads to an increase in the amplitude of compositional fluctuations and the analysis of the spatial atomic distribution by APT provides direct evidence of the spinodal character of the decomposition by uphill diffusion of Gd into the clusters. For higher Gd content (x=15 and 20 at.%), a coarsened microstructure of the phase-separated glass is obtained due to growth and coalescence while quenching the melt. The microstructure formation is essentially determined by the thermodynamic properties of the metastable undercooled liquid.

Electron magnetic resonance investigation of gadolinium diffusion in zircon powders

The electron magnetic resonance (EMR) technique was used to investigate the diffusion of gadolinium in zircon (ZrSiO 4) powders. The EMR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323 and 1373 K. The activation energy for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E A =506±5 kJ mol −1. This value is close to the ones for the diffusion of Gd in UO 2 and CeO 2, but much larger than for the diffusion of gadolinium in a compound with the same crystal structure as zircon, YVO 4. This is attributed to a difference in the relative sizes of the ions involved in the diffusion process.

Effect of <formula formulatype="inline"><tex Notation="TeX">$\hbox{NH}_{3}$</tex></formula> Plasma Nitridation on Hot-Carrier Instability and Low-Frequency Noise in Gd-Doped High-<formula formulatype="inline"><tex Notation="TeX">$ \kappa$</tex></formula> Dielectric nMOSFETs

The effects of post-NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma nitridation on device's hot-carrier instability and low-frequency noise in the Hf-based high-κ/metal-gate n-channel metal-oxide-semiconductor field-effect transistors (nMOSFETs) with gadolinium (Gd) cap layers are investigated. With postnitridation, the direct-current and 1/f noise characteristics can be improved apparently. Moreover, a hot-carrier stressing-induced threshold voltage shift can be also suppressed despite of a similar transconductance degradation when comparing with that in the device without nitridation. With the charge-pumping and low-frequency noise measurements, we find that the bulk- and interfacial-trap densities can be reduced with nitrogen incorporation. The reduction of bulk and interfacial traps can be contributed to the suppression of Gd diffusion into a high-κ layer. In this paper, appropriate post-NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma nitridation can improve the device performance and reliability and low-frequency noise for a gate-first high-κ/metal-gate nMOSFET with a Gd cap layer.

The thermal stability of Pt/epitaxial Gd2O3/Si stacks and its dependence on heat-treatment ambient

The stability of Pt/epitaxial Gd2O3/Si stacks is studied by monitoring the chemical and electrical properties following heat treatments in forming gas and in vacuum at temperatures between 400 and 650 °C. Our results show that stack instability is realized via diffusion of Gd through the Pt grain boundaries, which was observed after forming-gas annealing at 550 °C for 30 min. The Gd diffusion kinetics in forming gas is studied by secondary ion mass spectrometry analysis, showing that the diffusion process occurs according to C-type kinetics with an activation energy of 0.73±0.04 eV. Following vacuum heat treatments at 600 °C for 30 min, Si outdiffusion is observed, in addition to Gd outdiffusion. Si outdiffusion results in the formation of PtSi clusters on the metal surface following vacuum annealing at 650 °C. In contrast, in the case of forming-gas treatments, Si diffusion and silicide formation were detected only after annealing at 700 °C. The better stability of Pt/Gd2O3/Si stacks in forming gas is correlated with the content of oxygen in the Pt layer during the treatment.

Influence on the magnetoresistance of a spin valve due to the insertion of an ultrathin Gd layer in the free layer

The effect of inserting an ultrathin Gd layer (1–2 nm) in the free layer of a Permalloy-based spin valve has been investigated. In the current in-plane configuration, samples with Gd show a reduced magnetoresistance (MR) value, which tends to zero as Gd gets closer to the nonmagnetic spacer, although good MR values can be sustained when Gd is more than 5 nm away from the spacer. We studied also the effect of adding thin Fe layers around Gd in order to avoid Gd diffusion within the Permalloy. Samples with an Fe/Gd/Fe trilayer inserted in the free layer show some improvement at low temperatures over those without Fe, although the Fe introduces some detrimental additional roughness.

Impurity diffusion of cerium and gadolinium in single- and polycrystalline yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive material for solid oxide fuel cells or for nuclear applications such as inert matrix for the destruction of excess plutonium and host material for nuclear waste storage. Long-term properties such as phase stability depend on cation diffusion. Therefore, the present work is focused on the diffusion of Ce and Gd in YSZ single crystals and high-density polycrystals. A thin film of Ce or Gd was deposited either by the spin-coating method or by physical vapour deposition on the surface of polished samples. Diffusion experiments were performed between 1173 and 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. Analysis of the penetration profiles led to the determination of bulk diffusion coefficient (D b) in single crystals and the effective diffusion coefficient (D eff), bulk and grain boundary diffusion coefficients (D gb) were determined in polycrystals. The dependence of diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with results in the literature.

Data acquisition in thermodynamic and electrochemical reduction in a Gd(III)/Gd system in LiF–CaF 2 media

The electrochemical reduction of GdF 3 was studied in the LiF–CaF 2 eutectic (77/23 mol%) on a tantalum electrode for various GdF 3 concentrations in the 800–900 °C temperature range. A previous thermodynamic study showed that gadolinium trifluoride is reduced into metal in a one-step process: G d ( III ) + 3 e − = Gd Cyclic voltammetry and square wave voltammetry were used to confirm this mechanism, and the results show that the electrochemical reduction process is limited by the diffusion of Gd(III) in the solution. The Gd(III) diffusion coefficient was calculated at different temperatures and the results obey the Arrhenius law with an activation energy of 83 ± 4 kJ mol −1. The activity coefficient of Gd(III) and the Gd(III)/Gd standard potential E° were determined in the melt at various temperatures and concentrations. The values obtained for the activity coefficients were close to 1 at the highest temperatures and between 1.3 and 1.6 at lower temperatures.

Rare earth element diffusion in natural enstatite

Chemical diffusion coefficients of La, Nd, Eu, Gd, and Yb in natural enstatite have been measured at 850–1250 °C and 1 atm. Anhydrous diffusion experiments were run in Pt capsules in air, or in sealed silica glass capsules under an iron–wüstite (IW) solid buffer. The sources of diffusant were pre-reacted mixtures of synthetic enstatite powder and microcrystalline rare-earth aluminate garnet. Rutherford Backscattering Spectrometry (RBS) was used to measure diffusion profiles. For Gd diffusion in air over the temperature range 1000–1250 °C, the following Arrhenius relation is found for diffusion normal to (210): D Gd,air = 2.55 × 10 - 9 exp ( 321 ± 85 kJ mol - 1 / RT ) m 2 s - 1 . There is no significant difference between Gd diffusion in air and under IW-buffered conditions. Behavior similar to Gd is also noted for Nd. The Arrhenius relationship for Eu diffusion in enstatite, normal to (210) and at 850–1150 °C and IW-buffered conditions, is D Eu,IW = 6.93 × 10 - 6 exp ( - 384 ± 29 kJ mol - 1 / RT ) m 2 s - 1 . For Eu diffusion in air over the temperature range 1000–1200 °C for the same orientation, the following Arrhenius relation is found: D Eu,air = 1.70 × 10 - 8 exp ( - 350 ± 42 kJ mol - 1 / RT ) m 2 s - 1 . For Eu diffusion under IW-buffered conditions and for experiments run in air, diffusivities normal to (001) are similar to those for diffusion normal to (210). Eu diffusion under IW-buffered conditions is more than an order of magnitude faster than Eu diffusion in air. It is likely that majority of Eu is in the divalent state for diffusion under IW-buffered conditions, but Eu is in the trivalent state for diffusion in air. In the case of Nd and Gd, where valence state does not change under the investigated fO 2 conditions, diffusivities measured for experiments run both in air and under IW-buffered conditions are comparable to those obtained for trivalent Eu. Further, measurements of La, Nd, Eu +3, Gd, and Yb diffusion suggest that diffusion of trivalent REE in enstatite is not sensitive to ionic size, in contrast to that observed for REE diffusion in diopside [Van Orman, J.A., Grove, T.L., Shimizu, N., 2001. Rare earth element diffusion in diopside; influence of temperature, pressure, and ionic radius, and an elastic model for diffusion in silicates. Contrib. Mineral. Petrol. 141, 687–703]. These differences in diffusive behavior of REE between diopside and enstatite, as well as Eu 2+ and Eu 3+ in enstatite, can result in significant REE fractionation between coexisting pyroxenes during partial melting, melt migration, and subsolidus reequilibration processes in the Earth’s mantle and that of the Moon.

Diffusion Study of Cerium and Gadolinium in Single- and Polycrystalline Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive matrix for nuclear applications, such as inert matrix for the destruction of excess plutonium or good host material for nuclear waste storage. Some actinide elements in high-level radioactive wastes can be simulated by cerium as tetravalent actinide, and gadolinium as trivalent actinide or neutron absorber. The present work is focused on the diffusion study of Ce and Gd in YSZ single crystal and high density polycrystals. A thin film of Ce or Gd was deposited either by spin-coating method or by physical vapour deposition on the surface of polished samples. The diffusion experiments were performed from 1173 to 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. The experiments led to the determination of effective diffusion coefficient, Deff, bulk and grain boundary diffusion coefficients, DB and DGB. The dependence of these diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with literature.

The effect of Gd diffusion-doped on structural and superconducting properties of YBa 2Cu 3O 7− x superconductors

The effect of Gd diffusion on structural evolution and superconducting properties of YBa 2Cu 3O 7− x (Y123) prepared in the bulk polycrystalline form were studied. The gadolinium (Gd) diffusion in superconducting Y123 ceramic has been studied over the temperature range of 600–900 °C by energy dispersive X-ray fluorescence technique (EDXRF). The temperature dependence of Gd diffusion coefficient in grains ( D 1) and over the grain boundaries ( D 2) are described by the relations: D 1 = 8.9 × 10 −6 exp(−1.25/ kT) and D 2 = 2.3 × 10 −5 exp(−1.09/ kT). For the Gd diffused-doped samples, magnetization and resistivity measurements show that the critical transition temperature, T c, increased from 88 to 91 K and the critical current density, J c, which was calculated from M-H loops taken at 77 K, increased from 55 to 122 A cm −2 in comparison with those of undoped Y123. The possible reasons for the observed increases in T c and J c due to Gd diffusion were discussed. In addition, magnetic hysteresis curves of the Gd900, Gd800, Gd700 and undoped Y123 are presented for temperature of 5 K. It was found that the hysteresis is enhanced by the Gd diffusion. Such enhancement, which is considered to represent a characteristic strength of intergrain coupling, is more clearly recognized when critical current densities are compared.

Growth of ternary oxides in the Gd 2O 3–Fe 2O 3 system. A diffusion couple study

The simultaneous, diffusion-controlled growth of GdFeO 3 (perovskite) and Gd 3Fe 5O 12 (garnet) was studied at 1200–1400 °C in Gd 2O 3–Fe 2O 3 diffusion couples. Both compounds were found to grow as parallel layers according to the parabolic rate law. The parabolic rate constants of the second kind for the exclusive growth of each compound ( k 1 II for GdFeO 3, k 2 II for Gd 3Fe 5O 12) were calculated from the experimentally determined rate constants of the first kind assuming coupling between diffusion fluxes and chemical reactions at phase boundaries. In the case of GdFeO 3, the calculated values of k 1 II are in good agreement with the experimental values measured on Gd 3Fe 5O 12–Fe 2O 3 couples, where exclusive growth of GdFeO 3 is observed. Growth of the perovskite phase, probably related to formation of gaseous Fe(OH) 2, is also observed on Gd 2O 3–gas–Fe 2O 3 couples. The values of k 2 II for of Gd 3Fe 5O 12 are very close to those found for the growth of the isostructural compound Y 3Fe 5O 12. The most likely reaction mechanism is the coupled diffusion of Gd 3+ and O 2− for garnet growth and the coupled diffusion of Fe 3+ and O 2− for perovskite growth. The activation energy is ≈550 kJ mol −1 for Gd 3Fe 5O 12 and ≈400 kJ mol −1 for GdFeO 3.