The Experts below are selected from a list of 963 Experts worldwide ranked by ideXlab platform
Alan W. Weimer - One of the best experts on this subject based on the ideXlab platform.
-
oxidation kinetics of Hercynite spinels for solar thermochemical fuel production
Chemical Engineering Journal, 2020Co-Authors: Samantha L Millican, Alicia Bayon, Ryan M Trottier, Charles B Musgrave, Iryna Androshchuk, Justin T Tran, Yahya Al Salik, Hicham Idriss, Alan W. WeimerAbstract:Abstract The development of an economically viable solar thermochemical fuel production process relies largely on identifying redox active materials with optimized thermodynamic and kinetic properties. Iron aluminate (FeAl2O4, Hercynite) and cobalt-iron aluminate (CoxFe1-xAl2O4) have both been demonstrated as viable redox-active materials for this process. However, doping with cobalt creates a tradeoff between the thermodynamics and kinetics of H2 production mediated by Hercynite in which the kinetics are improved at the expense of lowering the yield. In this work, we evaluate four spinel aluminate materials with varying cobalt contents (FeAl2O4, Co0.05Fe0.95Al2O4, Co0.25Fe0.75Al2O4, and Co0.40Fe0.60Al2O4) to better understand the role of cobalt in the redox mediating properties of these materials and to quantify its effect on the thermodynamic and kinetic properties for CO2 reduction. A solid-state kinetic analysis was performed on each sample to model its CO2 reduction kinetics at temperatures ranging from 1200 °C to 1350 °C. An F1 model representative of first-order reaction kinetics was found to most accurately represent the experimental data for all materials evaluated. The computed rate constants, activation energies, and pre-exponential factors all increase with increasing cobalt content. High temperature in-situ XPS was utilized to characterize the spinel surfaces and indicated the presence of metallic states of the reduced cobalt-iron spinel, which are not present in un-doped Hercynite. These species provide a new site for the CO2 reduction reaction and enhance its rate through an increased pre-exponential factor.
-
Reduction Kinetics of Hercynite redox Materials for Solar Thermochemical Water Splitting
Chemical Engineering Journal, 2020Co-Authors: Ibraheam Al-shankiti, Alicia Bayon, Alan W. WeimerAbstract:Abstract Solar thermochemical water splitting (STWS) using a Hercynite redox cycle is a promising technology for producing renewable H2. Here, the reduction kinetics of Hercynite (FeAl2O4) is evaluated using thermogravimetric and XRD analyses. The results indicate that as-prepared Hercynite materials undergo reduction via two different reaction mechanisms. The reaction first proceeds by a nucleation and growth mechanism (AE1), followed by a third-order kinetic model (F3). XRD analyses show the occurrence of superstoichiometric oxygen in the spinel structure of FeAl2O4+δ in the second reaction mechanism, which indicates the formation of cationic vacancies. TGA and XRD analyses confirm that Hercynite maintains its spinel structure when the materials are thermally reduced and oxidized with steam. Differential scanning calorimetry (DSC) performed on FeAl2O4 particles shows two heats of reaction in agreement with the two observed mechanisms. This is the first kinetics study to report on the thermal reduction of Hercynite.
-
predicting the solar thermochemical water splitting ability and reaction mechanism of metal oxides a case study of the Hercynite family of water splitting cycles
Energy and Environmental Science, 2015Co-Authors: Christopher L Muhich, Charles B Musgrave, Brian Ehrhart, Vanessa A Witte, Samantha L Miller, Eric N Coker, Alan W. WeimerAbstract:A screening method is developed to determine the viability of candidate redox materials to drive solar thermal water splitting (STWS) and the mechanism by which they operate using only the reduction enthalpy of the material. This method is applied to the doped-Hercynite water splitting cycle, as well as FeAl2O4 and CoAl2O4, materials which have not been previously experimentally demonstrated for STWS. Density functional theory (DFT) calculations of reduction energies coupled with our screening method predict H2 production capacities for iron and cobalt aluminate spinels to be in the order FeAl2O4 > Co0.5Fe0.5Al2O4 > CoAl2O4 with relative H2 production capacity ratios of approximately 1.0 to 0.7 to 2 × 10−4, respectively. Experimental measurements for 1500/1350 °C redox temperatures validate the H2 production capacity predicted by the screening method by demonstrating H2 production ratios of 1.0 to 0.6 to 0. Un-doped Hercynite (FeAl2O4) is shown to be a viable STWS material for the first time with a higher H2 production capacity than traditional doped-Hercynite materials. Theory and experiments show that redox of the aluminate family of spinel materials operates via an O-vacancy mechanism rather than a stoichiometric one, which is more typical for ferrites. The screening approach is generally useful for predicting the ability of new complex materials to drive STWS and the mechanism by which they operate, thus, providing a method to identify promising new candidate STWS materials.
-
near isothermal ferrite alumina Hercynite cycle two step red ox cycle for solar thermal water splitting
Light Energy and the Environment (2014) paper RW3B.3, 2014Co-Authors: Christopher L Muhich, Brian Ehrhart, Ibraheam Alshankiti, Alan W. WeimerAbstract:Hydrogen productivity exceeding 350 micromoles H2/g total redox material has been demonstrated for near-isothermal processing using the “Hercynite cycle” for oxidation with steam carried out at 1350oC following 1500oC reduction.
-
efficient generation of h2 by splitting water with an isothermal redox cycle
Science, 2013Co-Authors: Christopher L Muhich, Charles B Musgrave, Brian Evanko, Kayla C Weston, Paul Lichty, Xinhua Liang, Janna Martinek, Alan W. WeimerAbstract:Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H 2 ) from water and sunlight. Two-step, metal oxide–based STWS cycles generate H 2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS’s overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the “Hercynite cycle” exhibits H 2 production capacity >3 and >12 times that of Hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.
Junhong Chen - One of the best experts on this subject based on the ideXlab platform.
-
improvement of thermal shock performance by residual stress field toughening in periclase Hercynite refractories
Ceramics International, 2018Co-Authors: Haiyang Chen, Junhong Chen, Mingwei Yan, Xinmei HouAbstract:Abstract In this paper, the periclase-Hercynite bricks suffered ten cycles of thermal shocks were characterized, and the micro-mechanism of enhancement thermal shock performance for periclase-Hercynite refractories was analyzed by using residual stress field theory. Due to the diffusion of Fe 2+ (or Fe 3+ ) and Al 3+ , the composite spinel particles formed inside of magnesia grains in contract with Hercynite. When temperature fluctuates, the caused residual stress field and composite spinel particles prevent the extension and propagation of microcracks, enhance fracture toughness, improving of thermal shock stability of magnesia grains. The estimated value of residual stress field and increment of fracture toughness for magnesia grains with composite spinel particles inside by residual stress field toughening model are −1339.19 MPa and 0.49 MPa m 1/2 , respectively. The excellent thermal shock resistance of periclase-Hercynite refractories is derived from the improvement of the internal microstructure of large quantities of magnesia grains around the Hercynite.
-
influence of microstructure on formation of deterioration layer in periclase Hercynite bricks
Refractories and Industrial Ceramics, 2016Co-Authors: Junhong Chen, Peng Jiang, Mingwei Yan, Dongfang Liu, Jialin SunAbstract:The microstructure of the original layer and the cement melt-penetrated layer of a used periclase-Hercynite brick from a cement rotary kiln with a daily output of 5000 tons for 12 months was studied by XRD, SEM, EDS, and a mercury porosimeter. The results show that the cation diffusion between Hercynite and periclase particles in the brick at high temperatures decreases the pore size of the brick. The pore size in the original layer is located mainly in the range of 4 – 20 μm; the decreased pore size increases the penetration resistance of the cement melt to the inside of the brick and makes the cement melt react with the pore walls better. The components of the matrix pore walls such as MgO and Al2O3 dissolve in the cement melt, enhancing the hot properties of the penetrated melt, decreasing the penetration depth, and slowing the formation of the deterioration layer. The pore structure and the element distribution endow the brick with good thermal shock resistance.
-
morphology characterization of periclase Hercynite refractories by reaction sintering
International Journal of Minerals Metallurgy and Materials, 2015Co-Authors: Peng Jiang, Junhong Chen, Mingwei Yan, Xinmei HouAbstract:A periclase-Hercynite brick was prepared via reaction sintering at 1600°C for 6 h in air using magnesia and reaction-sintered Hercynite as raw materials. The microstructure development of the periclase-Hercynite brick during sintering was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The results show that during sintering, Fe2+, Fe3+ and Al3+ ions in Hercynite crystals migrate and react with periclase to form (Mg1-xFex)(Fe2-yAly)O4 spinel with a high Fe/Al ratio. Meanwhile, Mg2+ in periclase crystals migrates into Hercynite crystals and occupies the oxygen tetrahedron vacancies. This Mg2+ migration leads to the formation of (Mg1-uFeu)(Fe2-vAlv)O4 spinel with a lower Fe/Al ratio and results in Al3+ remaining in Hercynite crystals. Cation diffusion between periclase and Hercynite crystals promotes the sintering process and results in the formation of a microporous structure.
-
thermogravimetric analysis of Hercynite synthesized by reaction sintering
2015 International Power Electronics and Materials Engineering Conference, 2015Co-Authors: Junhong Chen, Mingwei Yan, Jialin SunAbstract:The Hercynite synthesized at 1600C in protection atmosphere was thermogravimetrically analyzed. The results show that the change of oxygen partial pressure during sintering transformssome Fe in the Hercynitecrystal structure into Fe but the Hercynitestructure remained. Based on the modal for Hercynite with Fegiven by Dehe, et al., the structural formula was calculated as 2 3 3 2 3 3 4 0.497 0.355 0.148 0.503 0.355 1.142 [ ]O Fe Fe Al Fe Fe Al .
-
two methods of synthesizing Hercynite
Applied Mechanics and Materials, 2014Co-Authors: Junhong Chen, Mingwei Yan, Shu Lei Yang, Wen Na LiuAbstract:Hercynite (FeAl2O4) with octahedral structure rarely exists in nature and can only be synthesized artificially. However, it is hard to synthesize high purity Hercynite. To date, it is known that two main synthetic methods, sintering and electro-fusion processes, are available to synthesize Hercynite. Specimens synthesized by the two processes are different in Microstructure and phase compositions. The variations are researched by SEM, XRD and EDS.
Giovanni B Andreozzi - One of the best experts on this subject based on the ideXlab platform.
-
equation of state of Hercynite feal2o4 and high pressure systematics of mg fe cr al spinels
Mineralogical Magazine, 2015Co-Authors: Fabrizio Nestola, Giovanni B Andreozzi, Benedetta Periotto, Chiara Anzolini, A B Woodland, Davide Lenaz, Matteo Alvaro, Francesco PrincivalleAbstract:In this work a single crystal of synthetic Hercynite, FeAl 2 O 4 , was investigated by X-ray diffraction up to 7.5 GPa and at room temperature, in order to determine its pressure–volume equation of state. The unit-cell volume decreases non-linearly with a reduction of 3.4% (i.e. 18.43 A 3 ). The pressure–volume data were fitted to a third-order Birch-Murnaghan equation of state providing the following coefficients: V 0 = 542.58(3) A 3 , K T0 = 193.9(1.7) GPa, K ′ = 6.0(5). These results are consistent with previous investigations of Cr and Al spinels measured with the same experimental approach but the K T0 differs significantly from the experimental determination carried out more than 40 years ago by Wang and Simmons (1972) by the pulse echo overlap method. Our new results were used to redetermine the FeAl 2 O 4(Hercynite) = FeO (wustite) + Al 2 O 3(corundum) equilibrium in P–T space and obtain geobarometric information for Cr-Al spinels found as inclusions in diamond.
-
hf2epr spectroscopy of fe iii impurities in a blue Hercynite based pigment
Journal of The European Ceramic Society, 2006Co-Authors: F Di Benedetto, Giovanni B Andreozzi, Giovanni Baldi, A Barzanti, Gian Piero Bernardini, Valentina Faso, L A Pardi, Maurizio RomanelliAbstract:Abstract The nature and the behavior of a paramagnetic Fe(III) impurity, occurring in a new charge-transfer Hercynite-based blue pigment and determined through conventional electron paramagnetic resonance (EPR) and Mossbauer spectroscopy, was investigated and ascertained through high-field high-frequency EPR (HF2EPR) spectroscopy. The blue Hercynite-based pigment is biphasic, containing both Hercynite and corundum. The HF2EPR spectral features, together with their temperature dependence allowed to assess the intensity and the symmetry of the Fe(III) crystal field interactions. Numerical simulations were used to determine the relevant hamiltonian parameters. The final attribution of Fe(III) to the octahedral sites in corundum was achieved. The presence of corundum was found to remove ferric iron, eventually formed during the synthesis, from Hercynite. The dilution of Fe(III) in the Al2O3 phase results in a quenching of its coloring effects. As a consequence, corundum acts as a buffer during the synthesis, stabilizing the pigment chromatic yield.
-
57fe mossbauer and electronic spectroscopy study on a new synthetic Hercynite based pigment
Journal of The European Ceramic Society, 2004Co-Authors: Giovanni B Andreozzi, Giovanni Baldi, Gian Piero Bernardini, Francesco Di Benedetto, Maurizio RomanelliAbstract:AbstractA new blue Hercynite-based pigment,stable at the processing temperatures for the ceramic application,has been synthesized. It isconstituted by corundum and Ti-bearing Hercynite. In order to clarify the complex Fe distribution evidenced by Baldi et al. [Proc.CIMTEC 2002(2003)],a 57 Fe Mo¨ssbauer and diffuse reflectance spectroscopy study has been undertaken. Fe was found to be dis-tributed between corundum,as trivalent,and Hercynite,both as Fe(II) and Fe(III). Moreover,Fe(II) occupies both tetrahedral andoctahedral sites in the latter mineral,according to a temperature dependent disorder. UV–VIS investigation revealed the presence ofa broad band in the blue region,the shape and position of which was found to be due to an intervalence charge transfer mechanismbetween Fe(II) and Ti(IV) in two edge-sharing octahedra of Hercynite.# 2003 Elsevier Ltd. All rights reserved. Keywords:Al 2 O 3 ; Colour; Pigment; Spectroscopy; Spinels 1. IntroductionCeramic pigments are white or coloured crystallinematerials with high thermal stability and high chemicalresistance. These properties are strategic for their use inhigh temperature treatments. In recent years the devel-opment of new ceramic materials has fostered theresearch of pigments stable at application temperaturesover 1200 C. The crystalline coloured substances thatremain unaltered in such conditions belong to a restric-ted number of structures: zirconium silicate ZrSiO
-
intersite distribution of fe2 and mg in the spinel sensu stricto Hercynite series by single crystal x ray diffraction
American Mineralogist, 2002Co-Authors: Giovanni B Andreozzi, Sergio LucchesiAbstract:The influence of composition on Fe2+-Mg intracrystalline distribution was studied in eleven synthetic crystals belonging to the spinel (sensu stricto)–Hercynite series (Mg1−yFey2+)Al2O4, with 0 ≤ y ≤ 1, produced by flux-growth at 800 °C. Samples were analyzed by single-crystal X-ray diffraction and electron microprobe methods, and found to be chemically homogeneous with only minor Fe3+, which substitutes for Al and increases up to 0.09 atoms per formula unit with total Fe. Structural parameters a , u , T-O, and M-O increase with Hercynite content and, among bond distances, T-O shows the maximum change, from 1.920 to 1.968 A. The a variation from 8.0855 to 8.1646 A is essentially caused by the T-O increase that, in turn, is due to the cooperative effects of (1) Fe2+ → Mg substitution and (2) decrease of inversion from 0.23 to 0.15 along the series. Intracrystalline cation distribution was obtained by a minimization procedure that takes into account structural and chemical data. The T site is mainly populated by Mg and Fe2+ but, at a given temperature, Fe2+ shows a marked preference for tetrahedral coordination with respect to Mg. The influence of composition and temperature on Fe-Mg intracrystalline distribution was modeled within the framework of the general thermodynamic model of O’Neill and Navrotsky for spinel binary solid solutions. The inversion values observed in our samples agree very well with those calculated by the model. Both measured and calculated amounts of octahedral Fe2+ (VIFe2+) show a non-linear increase from spinel s.s. to Hercynite. Consequently, the VIFe2+/Fetot.2+ ratio is not constant along the join, but increases from zero to 15% toward the Hercynite end-member. This behavior explains the very limited Fe2+ inversion observed in natural spinels, which usually belong to the Hercynite-poor part of the join.
-
synthetic spinels in the mg fe2 zn al fe3 2o4 system ii preliminary chemical and structural data of Hercynite and magnesioferrite samples
Periodico Di Mineralogia, 2001Co-Authors: Giovanni B Andreozzi, Ferdinando Bosi, Flavia GarramoneAbstract:Flux-grown spinel samp les belonging to spinel s.s. -Hercynite and spinel s.s. - magnesioferrite solid solutions were investigated by electron micro probe and sin gle-crystal X-ray diffraction. Results confirmed the compositional homogeneity of the samples, and str uctural refinements gave final disagreement factors R ranging from 1.33 to 3. 71%. The solid solution
Maurizio Romanelli - One of the best experts on this subject based on the ideXlab platform.
-
hf2epr spectroscopy of fe iii impurities in a blue Hercynite based pigment
Journal of The European Ceramic Society, 2006Co-Authors: F Di Benedetto, Giovanni B Andreozzi, Giovanni Baldi, A Barzanti, Gian Piero Bernardini, Valentina Faso, L A Pardi, Maurizio RomanelliAbstract:Abstract The nature and the behavior of a paramagnetic Fe(III) impurity, occurring in a new charge-transfer Hercynite-based blue pigment and determined through conventional electron paramagnetic resonance (EPR) and Mossbauer spectroscopy, was investigated and ascertained through high-field high-frequency EPR (HF2EPR) spectroscopy. The blue Hercynite-based pigment is biphasic, containing both Hercynite and corundum. The HF2EPR spectral features, together with their temperature dependence allowed to assess the intensity and the symmetry of the Fe(III) crystal field interactions. Numerical simulations were used to determine the relevant hamiltonian parameters. The final attribution of Fe(III) to the octahedral sites in corundum was achieved. The presence of corundum was found to remove ferric iron, eventually formed during the synthesis, from Hercynite. The dilution of Fe(III) in the Al2O3 phase results in a quenching of its coloring effects. As a consequence, corundum acts as a buffer during the synthesis, stabilizing the pigment chromatic yield.
-
57fe mossbauer and electronic spectroscopy study on a new synthetic Hercynite based pigment
Journal of The European Ceramic Society, 2004Co-Authors: Giovanni B Andreozzi, Giovanni Baldi, Gian Piero Bernardini, Francesco Di Benedetto, Maurizio RomanelliAbstract:AbstractA new blue Hercynite-based pigment,stable at the processing temperatures for the ceramic application,has been synthesized. It isconstituted by corundum and Ti-bearing Hercynite. In order to clarify the complex Fe distribution evidenced by Baldi et al. [Proc.CIMTEC 2002(2003)],a 57 Fe Mo¨ssbauer and diffuse reflectance spectroscopy study has been undertaken. Fe was found to be dis-tributed between corundum,as trivalent,and Hercynite,both as Fe(II) and Fe(III). Moreover,Fe(II) occupies both tetrahedral andoctahedral sites in the latter mineral,according to a temperature dependent disorder. UV–VIS investigation revealed the presence ofa broad band in the blue region,the shape and position of which was found to be due to an intervalence charge transfer mechanismbetween Fe(II) and Ti(IV) in two edge-sharing octahedra of Hercynite.# 2003 Elsevier Ltd. All rights reserved. Keywords:Al 2 O 3 ; Colour; Pigment; Spectroscopy; Spinels 1. IntroductionCeramic pigments are white or coloured crystallinematerials with high thermal stability and high chemicalresistance. These properties are strategic for their use inhigh temperature treatments. In recent years the devel-opment of new ceramic materials has fostered theresearch of pigments stable at application temperaturesover 1200 C. The crystalline coloured substances thatremain unaltered in such conditions belong to a restric-ted number of structures: zirconium silicate ZrSiO
Xinmei Hou - One of the best experts on this subject based on the ideXlab platform.
-
improvement of thermal shock performance by residual stress field toughening in periclase Hercynite refractories
Ceramics International, 2018Co-Authors: Haiyang Chen, Junhong Chen, Mingwei Yan, Xinmei HouAbstract:Abstract In this paper, the periclase-Hercynite bricks suffered ten cycles of thermal shocks were characterized, and the micro-mechanism of enhancement thermal shock performance for periclase-Hercynite refractories was analyzed by using residual stress field theory. Due to the diffusion of Fe 2+ (or Fe 3+ ) and Al 3+ , the composite spinel particles formed inside of magnesia grains in contract with Hercynite. When temperature fluctuates, the caused residual stress field and composite spinel particles prevent the extension and propagation of microcracks, enhance fracture toughness, improving of thermal shock stability of magnesia grains. The estimated value of residual stress field and increment of fracture toughness for magnesia grains with composite spinel particles inside by residual stress field toughening model are −1339.19 MPa and 0.49 MPa m 1/2 , respectively. The excellent thermal shock resistance of periclase-Hercynite refractories is derived from the improvement of the internal microstructure of large quantities of magnesia grains around the Hercynite.
-
morphology characterization of periclase Hercynite refractories by reaction sintering
International Journal of Minerals Metallurgy and Materials, 2015Co-Authors: Peng Jiang, Junhong Chen, Mingwei Yan, Xinmei HouAbstract:A periclase-Hercynite brick was prepared via reaction sintering at 1600°C for 6 h in air using magnesia and reaction-sintered Hercynite as raw materials. The microstructure development of the periclase-Hercynite brick during sintering was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The results show that during sintering, Fe2+, Fe3+ and Al3+ ions in Hercynite crystals migrate and react with periclase to form (Mg1-xFex)(Fe2-yAly)O4 spinel with a high Fe/Al ratio. Meanwhile, Mg2+ in periclase crystals migrates into Hercynite crystals and occupies the oxygen tetrahedron vacancies. This Mg2+ migration leads to the formation of (Mg1-uFeu)(Fe2-vAlv)O4 spinel with a lower Fe/Al ratio and results in Al3+ remaining in Hercynite crystals. Cation diffusion between periclase and Hercynite crystals promotes the sintering process and results in the formation of a microporous structure.