17. Diffusion in Oxides

Orman, James A. Van; Crispin, K. L.

doi:10.1515/9781501508394-018

Cited by 24 publications

(30 citation statements)

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“…This relation is similar to that of the diffusion of other cations in magnetite with a hematite buffer as reported in literature, 8 as shown in Figure 5. For the grain boundary diffusion coefficient, D gb , the temperature dependence can be described as follows …”

Section: ■ Results and Discussionsupporting

confidence: 91%

Gas–Solid Reaction Kinetics of ZnFe₂O₄Formation from 907 to 1100 °C

et al. 2015

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The reaction kinetics of Zn vapor with Fe3O4 (magnetite) were studied from 907 to 1100 °C using a new experimental setup that only allows contact between the reactants through a gas-solid reaction. Hematite was used to create the reaction pellets. Because of the reducing atmosphere in the setup, a magnetite layer is formed on the outside of the pellet, which in turn reacts with the Zn vapor. After reaction, Zn concentration profiles were measured in the reacted magnetite layer using field-emission gun electron probe microanalysis. The reaction was confirmed to be diffusion-controlled. The effect of both volume and grain-boundary diffusion was observed in each experiment. The temperature dependence of both the volume and grain-boundary diffusion coefficients was obtained along with the activation energies of the diffusion coefficients. This study provides crucial information for the development of technologies that are dependent on the reaction. One example is the in-process separation technology for the separation of Zn vapor from electric arc furnace off-gas.

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Section: ■ Results and Discussionsupporting

confidence: 91%

Gas–Solid Reaction Kinetics of ZnFe₂O₄Formation from 907 to 1100 °C

et al. 2015

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“…. [ 36 ] Hence, the observed cathodic shift of the V ph,onset here is mainly because the dense-layer suppresses the electron back-injection by blocking the FTO surface. [ 7,20,22 ] The fi rst two effects are not substantial here because the morphology of the hematite NRs remains the same after the dense-layer deposition.…”

Section: Combination Of Flame-doping and Dense-layer Depositionmentioning

confidence: 76%

Enhancing Low‐Bias Performance of Hematite Photoanodes for Solar Water Splitting by Simultaneous Reduction of Bulk, Interface, and Surface Recombination Pathways

Cho

Han

Logar

et al. 2015

Advanced Energy Materials

171

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However, the PEC performance of hematite, especially at low bias, is still severely hindered by three main electron/hole recombination pathways that occur in the bulk, interfaces, and surfaces. [ 1,3 ] As schematically illustrated in Figure 1 a, hematite nanorods (NRs), one of the representative nanostructures adopted for photoanodes, exhibit large bulk recombination owing to its poor majority carrier conductivity, via small polaron hopping conduction with a low electron mobility of 10 −2 cm 2 V −1 s −1 , [ 4 ] and short hole collection depth (≈12 nm = hole diffusion length (≈5 nm) + space charge layer width (≈7 nm). [ 5 ] The low intrinsic conductivity and short hole collection depth greatly limit the charge transport/ separation effi ciency of hematite for PEC water oxidation. In addition to bulk recombination, there are interfacial recombinations between the hematite NRs and the conductive substrate, frequently fl uorinedoped SnO 2 (FTO), and recombination losses due to the electron back-injection into the electrolyte on the exposed areas of FTO. [ 6,7 ] Finally, there are signifi cant surface recombination losses due to the presence of surface states and sluggish oxygen evolution reaction (OER) kinetics of hematite. [ 8 ] As a result, these recombination losses lead to low photocurrent density and large overpotential for hematite based PEC water oxidation.Extensive amount of work has been done to reduce those recombination losses, and most of them normally focuses on reducing one or two recombination losses. For the reduction of bulk recombination, great efforts have been devoted to nanostructuring and/or doping of hematite. To date, a number of nanostructures including nanowires, [ 9 ] nanorods, [10][11][12] nanotubes, [ 13 ] nanosheet, [ 14 ] caulifl ower [ 15 ] and porous structures, [ 16,17 ] and diverse metal dopants including Si, Ti, Sn, Zr, Nb, Ag, Pt, Mn, and Al [ 12,[17][18][19] have been studied to shorten the hole transport distance and to increase the electrical conductivity respectively. Separately, it was shown that adding an under-layer of SiO x , TiO 2 , Nb 2 O 5 , or Ga 2 O 3 suppresses the back electron injection and reduces the interface recombination of hematite. [ 7,20,21 ] For the surface recombination, various approaches including For a hematite (α-Fe 2 O 3 ) photoanode, multiple electron/hole recombination pathways occurring in the bulk, interfaces, and surfaces largely limit its low-bias performance (low photocurrent density at low-bias potential) for photoelectrochemical water splitting. Here, a facile and rapid three-step approach is reported to simultaneously reduce these recombinations for hematite nanorods (NRs) array photoanode, leading to a greatly improved photocurrent density at low bias potential. First, fl ame-doping enables high concentration of Ti doping without hampering the morphology and surface properties of the hematite NRs, which reduces both the bulk and surface recombinations effectively. Second, the addition of a dense-layer between the hematite NRs and fl uo...

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“…24,[33][34][35] In the Fe 2 O 3 doped YSZ system, the Fe 3+ (0.55-0.78Å) radius is much smaller than that of Zr 4+ (0.84Å) and Y 3+ (1.019Å). Thus, the Fe 3+ ion doping weakens the bond strength of the Zr O bonds.…”

Section: Discussionmentioning

confidence: 99%

“…29,[33][34][35] The diffusion rate of atomics is proportional to the jump frequency. 34,35 In other words, each position of Fe 3+ substitutent could be a available site for the Zr 4+ ion migration if the interstitial migration of Fe 3+ ions could operate during the sintering process. Therefore, the interstitial defects of Fe 3+ ions partially contribute to an increase in the diffusion coefficient D.…”

Section: Discussionmentioning

confidence: 99%