Precise determination of cation diffusivity in garnet can provide critical information for quantitatively understanding the timescales and thermodynamics of various geological processes,but very few studies have been performed for Fe-Mn interdiffusion.In this study,Fe-Mn interdiffusion rates in natural single crystals of Mn-bearing garnet with 750 ppm H2O are determined at 6 GPa and 1273-1573 K in a Kawai-type multi-anvil apparatus.Diffusion profiles were acquired by electron microprobe and fitted using Boltzmann-Matano equation.The experimental results show that the Fe-Mn interdiffusion coefficient(DFe-Mn)slightly decreases with increasing XFe.The experimentally determined DFe-Mn in Mn-bearing garnet can be fitted by the Arrhenius equation:DFe-Mn(m2/s)=D0XFenexp(-E*/RT),where E*=(1-XFe)E*Mn+XFeE*Fe,D0=8.06-6.04+9.87×10-9 m2/s,E*Mn=248±27 KJ/mol,E*Fe=226±59 KJ/mol,n=-1.36±0.51.The comparing the present results with previous experimental data suggest that water can greatly enhance the DFe-Mn in garnet.Our results indicate that the time required for homogenization of the compositional zoning of a garnet is much shorter than previously thought.
By employing metal oxides as oxygen carriers,chemical looping demonstrates its effectiveness in transferring oxygen between reduction and oxidation environments to partially oxidize fuels into syngas and convert CO_(2)into CO.Generally,NiFe_(2_)O_(4)oxygen carriers have demonstrated remarkable efficiency in chemical looping CO_(2)conversion.Nevertheless,the intricate process of atomic migration and evolution within the internal structure of bimetallic oxygen carriers during continuous high‐temperature redox cycling remains unclear.Consequently,the lack of a fundamental understanding of the complex ionic migration and oxygen transfer associated with energy conversion processes hampers the design of high‐performance oxygen carriers.Thus,in this study,we employed in situ characterization techniques and theoretical calculations to investigate the ion migration behavior and structural evolution in the bulk of NiFe_(2_)O_(4)oxygen carriers during H2 reduction and CO_(2)/lab air oxidation cycles.We discovered that during the H2 reduction step,lattice oxygen rapidly migrates to vacancy layers to replenish consumed active oxygen species,while Ni leaches from the material and migrates to the surface.During the CO_(2)splitting step,Ni migrates toward the core of the bimetallic oxygen carrier,forming Fe–Ni alloys.During the air oxidation step,Fe–Ni migrates outward,creating a hollow structure owing to the Kirkendall effect triggered by the swift transfer of lattice oxygen.The metal atom migration paths depend on the oxygen transfer rates.These discoveries highlight the significance of regulating the release–recovery rate of lattice oxygen to uphold the structures and reactivity of oxygen carriers.This work offers a comprehensive understanding of the oxidation/reduction‐driven atomic interdiffusion behavior of bimetallic oxygen carriers.
Da SongYan LinShiwen FangYang LiKun ZhaoXinfei ChenZhen HuangFang HeZengli ZhaoHongyu HuangFanxing Li
Delafossite CuFeO_(2) is a promising photocathode material for cost-efficiently photoelectrochemical(PEC)water splitting,but the unfavorable conductivity and fast recombination dynamics of photogenerated carriers limit its PEC activity for water reduction.Here,we developed a heterostructure photocathode consisting of the Cu-doped NiO(Cu:NiO)hole selective layer(HSL)and Ni-doped CuFeO_(2)(Ni:CuFeO_(2))active layer by simply annealing a homogeneous Cu-Fe oxalate layer grown on the Ni film deposited on the fluorine doped tin oxide(FTO)substrate.The obtained heterostructure of Cu:NiO/Ni:CuFeO_(2) with enhanced charge carrier transportability and high-quality interface greatly promotes the separation of photogenerated carriers.Accordingly,the Cu:NiO/Ni:CuFeO_(2) photocathode exhibits a high photocurrent density of~0.9 mA·cm^(-2 )at 0.2 V(vs.reversible hydrogen electrode,RHE),outperforming most of the reported bare CuFeO_(2) photocathodes in the literature.And the photocurrent density can be further improved to 1.2 mA·cm^(-2) after decorating NiSx cocatalyst.
Fei HanWei XuChun-Xu JiaXiang-Tao ChenYing-Peng XieChao ZhenGang Liu
The interdiffusion interface microstructures and composition evolution of Ni-Al-Cr/Ni-Al-Cr layered alloys under different temperature gradients were studied using phase-field simulation.A precipitate-free zone forms near the interface of the layered alloys,and the interface does not move with time under temperature gradients.When Cr concentrations in the alloy layers are different,Al diffuses from a low-to high-temperature region,whereas Cr from a high-to low-temperature region.The width of the precipitate-free zone changes from 0.5 to 2.5μm as the temperature gradient changes fromΔT=0.625 K·μm^(−1) to 1.250 K·μm^(−1);the width of the interdiffusion zone is also enlarged.Additionally,the temperature gradient promotes the interdiffusion of elements through the interface.In contrast,when the initial Al concentration is different in the alloy layers,an uphill diffusion of Al occurs,which is driven by the chemical potential of Al.
Yong-Sheng LiHui-Yu WangJing ChenShuai-Ge YangPeng SangHong-Li Long
The oxidation behaviour of a fourth-generation single-crystal superalloy without coating and with two types of MCrAlY coatings at 1140℃was studied.The results showed that both coatings greatly improved the oxidation resistance of the superalloy,and the addition of Hf further improved the oxidation resistance by pinning the oxide layer into the coating.Before and after oxidation,obvious Cr and Al interdiffusion was detected.Inward Cr diffusion induces the precipitation of a topologically close-packed phase,while the diffusion of Al affects the structure of theγ/γ’phase,the solubility of refractory elements,and the formation of an interdiffusion zone.
