Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Changes in cation composition and M²⁺/M³⁺ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations.Heating Mg–Al–CO₃ hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations.The presence of Mg²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺ salts or of low soluble hydromagnesite increased the M²⁺/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn²⁺, Ni²⁺, Co²⁺ and Cu²⁺ cations in the newly formed hydrotalcite was detected, while Mg²⁺ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)₃, the M²⁺/Al ratio approximated the minimal possible value of 2.The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value.The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.     

Durable Cr-substituted (Ba,Cs)1.33(Cr,Ti)8O16 hollandite waste forms with high Cs loading

A series of Cr-substituted hollandite solid solution Ba_xCs_yCr_2x+yTi_8−2x−yO₁₆ over a broad range of Cs content (x + y = 1.33, 0 ≤ x and y ≤ 1.33) were systematically investigated. A monoclinic-to-tetragonal phase transition was induced by increasing Cs content in the tunnel sites of the hollandite structure, and all members of the series show structure modulations related to the ordering of the Ba/Cs and vacancies along the tunnels. The thermodynamic stability of the Cr-substituted hollandite samples was measured via high-temperature oxide melt solution calorimetry, which included making the first measurements of the enthalpies of drop solution for Cs₂O and BaO in sodium molybdate solvent at 800°C. Thermodynamic stability increased with increasing Cs content for the series of Cr-substituted hollandite, which also exhibited a greater thermodynamic stability compared to other substituted hollandite analogs including Zn, Ga, Fe, and Al variants. The leaching performance, also known as aqueous durability, demonstrated that the fractional Cs release in the Cr hollandite samples is much lower than in other hollandite systems. After 7 days of leaching at 90°C, the lowest Cs release was observed in the sample with the highest Cs content, approximately 22 wt.% Cs. The Cs release could be further suppressed, by approximately 3× if the sample was further densified and sintered. The Cs release results correlated inversely to the thermodynamic stability, suggesting that the thermodynamic stability may be used in future materials design for nuclear waste immobilization.     

The effect of cesium content on the thermodynamic stability and chemical durability of (Ba,Cs)1.33(Al,Ti)8O16 hollandite

Titanate-based hollandite ceramics are promising nuclear waste forms for Cs immobilization. In this work, a series of Al-substituted hollandite (Ba_,Cs)_1.33(Al,Ti)₈O₁₆ was investigated across a broad compositional range with varying Cs content. Powder X-ray diffraction showed that all samples exhibited a tetragonal hollandite phase. Enthalpies of formation determined by high-temperature melt solution calorimetry indicated enhanced thermodynamic stability with increased Cs content, which generally agreed with sublattice-based thermodynamic calculations. Moreover, enthalpies of formation of the samples were primarily affected by three factors: (a) relative sizes of cations on the A-sites and B-sites, (b) tolerance factor, and (c) optical basicity. Fractional element release revealed that Cs retention was significantly improved for the high Cs-containing hollandite compositions, which were supported by the evolution of microstructure of the pre and postleach particles. Elution studies of Al-substituted hollandite spiked with radioactive ¹³⁷Cs indicated that transmutation of Cs to Ba in the hollandite was accompanied by an increase in the retention of the Cs decay product, suggesting long-term stability of Al-substituted hollandite phase.     

Compositional control of radionuclide retention in hollandite-based ceramic waste forms for Cs-immobilization

Hollandite materials, as a class of crystalline nuclear waste forms, are promising candidates for the immobilization of radioactive elements, such as Cs, Ba, as well as a variety of lanthanide and transition-metal fission products. In this study, three Ga-doped titanate hollandite-type phases, Ba_1.33Ga_2.67Ti_5.33O₁₆, Ba_0.667Cs_0.667Ga₂Ti₆O₁₆, and Cs_1.33Ga_1.33Ti_6.67O₁₆, were synthesized using a solid-state reaction route. All synthesized phases adopted a single phase tetragonal structure, as determined by powder X-ray diffraction (XRD), and elemental analysis confirmed the measured stoichiometries were close to targeted compositions. Extended X-ray absorption fine structure spectroscopy (EXAFS) was used to determine the local structural features for the framework of octahedrally coordinated cations. EXAFS data indicated that Cs_1.33Ga_1.33Ti_6.67O₁₆ possessed the most disordered local structure centered around the Ga dopant. The enthalpies of formation of all three hollandite phases measured using high-temperature oxide melt solution calorimetry were found to be negative, indicating enthalpies of formation of these hollandites from oxides are thermodynamically stable with respect to their constituent oxides. Furthermore, the formation enthalpies were more negative and hence more favorable with increased Cs content. Finally, aqueous leaching tests revealed that high Cs content hollandite phases exhibited greater Cs retention as compared to low Cs content hollandite. While preliminary in nature, this work draws attention to the links between the capacity for radionuclide retention, atomistic level structural features and bulk thermodynamic properties of materials.     

Effect of B‐site cation on the catalytic activity of La1−xCaxBO3 (B = Fe,Ni) perovskite‐type oxides for toluene combustion

Background The effect of the B cation on the surface properties and catalytic activity in the total combustion of toluene over La_1?xCa_xBO₃ (B = Fe,Ni) perovskite‐type oxides was studied. Result For the La_1?xCa_xFeO₃ series, the perovskite structure was maintained in the range of substitution studied. A completely different behaviour was observed for the La_1?xCa_xNiO₃ series. A Brownmillerite‐type structure (La₂Ni₂O₅) with a large degree of phase segregation as well dispersed mixed oxides was observed upon the substitution of La for Ca. In the Fe series, the catalytic activity in the total combustion of toluene showed that the insertion of calcium ions into the perovskite lattice resulted in higher activity relative to the unsubstituted LaFeO₃ perovskite. In contrast, for the Ni series, substitution results in solids with lower activity than the pure LaNiO₃ perovskite. Conclusion For the Fe series, higher activity and stability are attributed to a synergy between Fe⁴⁺/Fe³⁺ and the oxygen vacancies generated by the calcium substitution. For the Ni counterpart, the structural modification leads to a lower activity of substituted solids compared with the pure LaNiO₃ perovskite, indicating that Ni³⁺ ions are the active sites for toluene oxidation. Copyright © 2011 Society of Chemical Industry     

Effects of BiMO3 on dielectric,ferroelectric, and piezoelectric properties of perovskite lead‐free piezoelectric BaTiO3–(Bi0.5Na0.5)TiO3 ceramics

New lead‐free piezoelectric ceramics of 0.9BaTiO₃–(0.1?x)(Bi_0.5Na_0.5)TiO₃–xBiMO₃, M=Al and Ga, where x=0.00‐0.10, were fabricated by the solid‐state reaction technique. The effect of BiMO₃ contents on the perovskite structure, phase transition, and dielectric, ferroelectric, and piezoelectric properties was investigated. X‐ray diffraction patterns showed that the ceramics exhibit a monophasic perovskite phase up to x=0.06, suggesting stabilized perovskite structures with B‐site aliovalent substitutions. Compositional‐dependent phase transitions were observed from tetragonal to pseudo‐cubic phase with increasing BiMO₃ amounts. Al³⁺ ions were found to stabilize the transition temperature of the ceramics, while significantly decreasing transition temperature, and a change in the dielectric peak were found with an increasing amount of Ga³⁺. Regarding Al³⁺ substitution, the remanent polarization (P_r) values were found to decrease slightly with the Al³⁺ amount. With regard to Ga³⁺ substitution, P_r values decreased with the Ga³⁺ amount up to 0.06 and then increased slightly. The ceramics became softer with a higher degree of substitution according to the lower coercive field (E_c), when compared with 0.9BaTiO₃–0.1(Bi_0.5Na_0.5)TiO₃ ceramics. Ceramics with a lower degree of substitution and tetragonal phase showed butterfly strain loops that correlated with normal ferroelectric behavior.     

Experimental phase equilibria study and thermodynamic modelling of the PbO-“FeO”-SiO2-ZnO,PbO-“FeO”-SiO2-Al2O3 and PbO-“FeO”-SiO2-MgO systems in equilibrium with metallic Pb and Fe

Phase equilibria of the PbO-“FeO”-SiO₂-ZnO, PbO-“FeO”-SiO₂-Al₂O₃ and PbO-“FeO”-SiO₂-MgO slags with liquid Pb metal, solid or liquid Fe metal and solid oxides (cristobalite and tridymite SiO₂, willemite (Zn,Fe)₂SiO₄, wustite (Fe,Al)O_1+x, spinel (Fe,Al)₃O₄, olivine Fe₂SiO₄, corundum (Al,Fe)₂O₃, mullite Al₆Si₂O₁₃ and pyroxene (Mg,Fe)SiO₃) were investigated at 1125–1670 °C. These conditions correspond to the minimum solubility of PbO in slag in presence of Pb and Fe metals at reducing conditions and represent the limit of lead smelting and slag cleaning process. High-temperature equilibration on silica, corundum or iron foil substrates, followed by quenching and direct measurement of Pb, Fe, Si, Zn, Al and Mg concentrations in the liquid and solid phases with the electron probe X-ray microanalysis (EPMA) was used. Present data can be used to improve the thermodynamic models for all phases in this system.     

Giant permittivity and low dielectric loss of Fe doped BaTiO3 ceramics: Experimental and first-principles calculations

Fe doped BaTiO₃ ceramics with giant permittivity and low dielectric loss were synthesized in N₂/H₂ atmosphere started with BaTiO₃ powders and iron powders. XRD analysis exhibited the tetragonal-pseudocubic phase transition when the Fe content is 3 mol%. XPS spectra confirmed the iron oxides with mixed-valence structure of Fe²⁺/Fe³⁺, while Ti-ions maintain Ti⁴⁺3d⁰ states without any oxidization-reduction. For the case of ceramics with 5 mol% Fe, the dielectric constant was 66,650 at 1000 Hz at room temperature, 19 times higher than that of pure BaTiO₃ ceramics, while the dielectric loss tangent was 0.13. Comparison with other giant-permittivity materials demonstrated the superior potential of present ceramics. First-principles calculations investigated the interfacial interaction of Fe-[TiO₂] interface and Fe-[BaO] interface. Giant dielectric constant was induced by the interfacial polarization between insulating ferroelectrics and semiconducting iron oxides with mixed-valence states, as well as the contribution from the generated electron hopping conduction.     

Synthesis of a magnesium/aluminum/iron layered double hydroxide and its flammability characteristics in halogen‐free,flame‐retardant ethylene/vinyl acetate copolymer composites

Mg–Al–Fe ternary hydrotalcites were synthesized by a coprecipitation method and characterized with powder X‐ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The flame‐retardant effects of Mg/Al–CO₃ layered double hydroxides (LDHs) and Mg/Al/Fe–CO₃ LDHs in an ethylene/vinyl acetate copolymer (EVA) were studied with the limited oxygen index (LOI), the UL‐94 test, and the cone calorimeter test (CCT), and the thermal degradation behavior of the composites was examined by thermogravimetric analysis. The results showed that the LOI values of the EVA/(Mg/Al/Fe–CO₃ LDH) composites were basically higher than those of the EVA/(Mg/Al–CO₃ LDH) composites at the same additive level. In the UL‐94 test, there was no rating for the EVA/(Mg/Al–CO₃ LDH) composite at the 50% additive level, and a dripping phenomenon occurred. However, the EVA/(Mg/Al/Fe–CO₃ LDH) composites at the same loading level of LDHs containing a suitable amount of Fe³⁺ ion reached the V‐0 rating, the dripping phenomenon disappearing. The CCTs indicated that the heat release rate (HRR) of the EVA composites with Mg/Al/Fe–CO₃ LDHs containing a suitable amount of Fe³⁺ decreased greatly in comparison with that of the composites with Mg/Al–CO₃ LDHs. The introduction of a given amount of Fe³⁺ ion into Mg/Al–CO₃ LDHs resulted in an increase in the LOI, a decrease in the HRR, and the achievement of the UL‐94 V‐0 rating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

X‐ray Absorption Spectroscopy Studies of the Room‐Temperature Ferromagnetic Fe‐Doped 6H–BaTiO3

We investigated the effect of annealing temperature on magnetic properties of 2% and 10% Fe‐doped BaTiO₃. To understand the possible structural differences between samples treated at different annealing temperatures, and to correlate them with the magnetic properties, several characterization techniques, such as X‐ray diffraction and X‐ray absorption spectroscopic methods (XANES and EXAFS) were employed. We found that the 2% Fe‐doped BaTiO₃ pseudocubic perovskite is paramagnetic regardless of the heat‐treatment conditions. Initially paramagnetic 10% Fe‐doped 6H–BaTiO₃, treated at 1250°C, became ferromagnetic after additional annealing at higher temperature. We have crystalographically characterized the cation ordering processes in the 6H–BaTiO₃ that occurred during the high‐temperature annealing. The ferromagnetism that is induced in this stage is most probably associated with the observed diffusion processes but it extrinsic character still cannot be fully disregarded.     

Microstructure and dielectric properties of high entropy Ba(Zr0.2Ti0.2Sn0.2Hf0.2Me0.2)O3 perovskite oxides

A series of high entropy Ba(Zr_0.2Ti_0.2Sn_0.2Hf_0.2Me_0.2)O₃ (Me=Y³⁺,Nb⁵⁺,Ta⁵⁺,V⁵⁺,Mo⁶⁺,W⁶⁺) perovskite oxides were synthesized by using a solid state reaction method. Three multiple-cation solid solutions formed pure phase compounds, and only two compounds were sintered into ceramics. Microstructure analysis showed the influence of configurational entropy on phase stability and grain growth. Dielectric measurements showed that the high entropy ceramics possessed decent temperature stability of permittivity from 25 °C to 200 °C, low dielectric loss (<0.002) from 20 Hz to 2 MHz, high resistance and moderate breakdown strength (290 kV/cm, 370 kV/cm). Evidence strongly confirmed that controlling configurational entropy could be a feasible perspective to set up highly tunable perovskite structures and explore novel species of dielectric materials.     

Phase‐dependent radiation‐resistant behavior of BaTiO3: An in situ X‐ray diffraction study

The radiation‐resistant response of BaTiO₃ in the tetragonal and rhombohedral phases on exposure to 100 MeV Ag⁷⁺ ion irradiation was investigated by in situ X‐ray diffraction (XRD) at room temperature (300 K) and low temperature (25 K), respectively. This study revealed that the BaTiO₃ in rhombohedral phase retained crystallinity up to an ion fluence of 1×10¹⁴ ions/cm², whereas tetragonal phase amorphized at much lower fluence viz. 1×10¹³ ions/cm². The in situ XRD along with Raman spectroscopy studies revealed that BaTiO₃ in rhombohedral phase is more radiation resistant than that of tetragonal phase. The density functional theory (DFT) calculations confirmed higher bond strength of rhombohedral phase as compared to tetragonal phase, which supported the experimental result of higher radiation stability of rhombohedral phase. The theoretical predictions on high‐temperature phase will be of relevance to the nuclear waste applications.     

Cationic substitution in tricalcium aluminate

Cubic and orthorhombic crystals of tricalcium aluminate doped with Na₂O, Fe₂O₃ and SiO₂ were prepared and examined using an electron probe microanalyzer. The cationic ratios based on six oxygen atoms were derived from the oxide compositions. These data, together with those in previous studies for clinker aluminates containing Mg²⁺ and K⁺, provided excellent correlations between Al+Fe and Si (Al+Fe=2.001−1.03Si) and Ca+Mg and Na+K+Si [Ca+Mg=3.006−0.51(Na+K+Si)]. The chemical variation that is constrained by these equations is well accounted for by the general formula (Na,K)_2x(Ca,Mg)_3−x−y[(Al,Fe)_1−ySi_y]₂O₆, where x is the amount of Ca substituted by Na and K (0≤x<0.158), and y is the amount of Al substituted by Si (0≤y<0.136). The crystal system changed from cubic to orthorhombic with increasing x value. The substitution of Si and Fe for Al extended the solid solution range of the orthorhombic phase to lower values of x, while its effect on the solid solution range of the cubic phase was reversed.     

Local Structure Investigation in Multiferroic BiFeO3–BaTiO3 Ceramics by XAS Technique and Their Relevant Properties

The (1?x)BiFeO₃‐xBaTiO₃ (with x = 0.1, 0.2, 0.3, and 0.4) ceramics were fabricated successfully by solid‐state reaction method. Single‐phase perovskite was obtained in all ceramics, as confirmed by XRD technique. It was observed that 0.7BiFeO₃–0.3BaTiO₃ was the morphotropic phase boundary (MPB) between rhombohedral and cubic phases, as also revealed from ferroelectric and magnetic properties. The simulated and experimental X‐Ray Absorption Spectroscopy (XAS) study revealed that BT in 0.75BF‐0.25BT is possibly taken a rhombohedral structure. Furthermore, the rounded ferroelectric hysteresis loops observed for 0.9BiFeO₃–0.1BaTiO₃ and 0.8BiFeO₃–0.2BaTiO₃ compositions could be attributed to their microstructure and surface charge effects and electron transfer between Fe³⁺ and Fe²⁺ ions. It was also found that high dielectric constant of 0.9BiFeO₃–0.1BaTiO₃ composition was a result of grain and grain‐boundary effects, as observed in SEM micrographs. In addition, a strong signature of dielectric relaxation behavior was observed in this ceramic system with the activation energy 0.467 eV obtained from the Arrhenius' law. Finally, the local structure investigation with XAS technique provided additional information to better understand the electric and magnetic properties in the BF‐BT ceramic system.     

Formation of Tetravalent Fe Ions in LaFeO3 Perovskite Through Mechanochemical Modification by Ball Milling

Mechanochemical modification of previously synthesized LaFeO₃ perovskite‐type oxide by a high‐energy ball milling was investigated to introduce Fe⁴⁺ ions or transform some Fe³⁺ into Fe⁴⁺ in LaFeO₃. X‐ray absorption fine structure studies revealed that the formation of Fe⁴⁺ ions into LaFeO₃ perovskite has been achieved at first time by ball milling at room temperature without any additives or replacement of La³⁺ ions by some divalent cations. The structural model of Fe⁴⁺ containing LaFeO₃ could be described as with a modified perovskite having equal amounts of La and Fe vacancies, which is supported by a good correlation between the results of Fe K‐edge XANES spectra and O₂‐TPD. The synthesis of Fe⁴⁺‐containing LaFeO₃ perovskite by ball milling was able to produce the O₂ adsorption capacity of nonsubstituted perovskite‐type oxide.     

Effects of B‐site doped elements in the electronic‐conducting perovskite phase on the property of dual‐phase membranes

A series of dense oxygen permeable dual‐phase membranes with a composition of 60 wt% Ce_0.8Gd_0.2O_2?δ‐40 wt% Ba_0.95La_0.05Fe_0.9M_0.10O_3?δ (CGO‐ BLFM_0.10, M=Fe, Nb, Zr, Zn, Sc, Y) were successfully synthesized and evaluated as potential ceramic membranes for oxy‐fuel combustion. The effects of B‐site doped elements in electronic‐conducting phase (BLFM_0.10) on the crystal structure, microstructure, chemical compatibility, oxygen permeability, as well as chemical stability of CGO‐BLFM_0.10 were systematically investigated. All electronic‐conducting phase BLFM_0.10 oxides exhibited a pure cubic perovskite structure and showed good chemical compatibility with ionic‐conducting phase CGO. CGO‐BLFSc_0.10 showed the best oxygen permeation stability under a pure CO₂ atmosphere. CO₂‐corrosion on the perovskite phase is the main reason for the property deterioration of fluorite‐perovskite‐typed dual‐phase membrane materials. The stability of dual‐phase membrane materials can be effectively enhanced by reducing the basicity of electronic‐conducting phase of perovskite materials.     

Cr‐Doped La‐Ni‐O Catalysts Derived from Perovskite Precursors for CH4‐CO2 Reforming under Microwave Irradiation

The nickel catalysts derived from Cr‐doped LaNiO₃ perovskite‐like precursors were characterized by X‐ray diffraction, high‐resolution transmission electron microscopy, temperature‐programmed oxidation, temperature‐programmed reduction, and X‐ray photoelectron spectroscopy. Their catalytic performance in CO₂ reforming of methane under microwave irradiation was investigated. It was found that the structure and morphology of the oxide composites in this research were influenced by the ratio of Ni and Cr, and the mismatch of La³⁺, Ni³⁺, and Cr³⁺ may cause phase segregation. The catalytic performance of the Ni catalysts is dependent on the oxygen mobility of the perovskite oxide matrix, the content of the reduced Ni⁰, and the content of the remaining perovskite structure. The mobile oxygen in the perovskite matrix in the catalyst may enhance the conversion of CO₂ during the reaction.     

Microstructures and strength of microporous MgO-Mg(Al,Fe)2O4 refractory aggregates

Microporous MgO-Mg(Al, Fe)₂O₄ refractory aggregates were prepared using magnesite, Al(OH)₃ and Fe₂O₃ applying an in-situ decomposition synthesis method. At 1400–1600 °C, there was a Mg(Al, Fe)₂O₄ with Fe³⁺, which had two structures. One was a ring structure formed from Al(OH)₃ pseudomorph particle as a template and a low content of Fe³⁺. The other was the dot and strip structures precipitated in magnesite pseudomorph particles with a high content of Fe³⁺. Besides, at 1550–1600 °C, microporous MgO-Mg(Al, Fe)₂O₄ refractory aggregates had an excellent compressive strength (75.8–81.5 MPa) and apparent porosity (26.8%?28.2%).     

Structure–property relationship amphoteric oxide systems via phase stability and ionic structural analysis

The effects of basicity and amphoteric oxides (Al₂O₃ and Fe_tO) on the structure–property relationships of CaO–SiO₂–(Al₂O₃ and Fe_tO) and CaO–SiO₂–Al₂O₃–Fe_tO slags were investigated to determine the constitutional effects on the structure of high-temperature ionic melts. The proportion of Qⁿ species, which is determined by Raman spectroscopy, and the viscosity measured by the rotating cylinder method are both correlated and shown together with the slag structure index (NBO/T) concept. The NBO/T of CaO-SiO₂ binary slags showed a linear relationship with basicity (CaO/SiO₂), including an inflection point at CaO/SiO₂ = 1.0 resulting from the stability and Qⁿ-dominant unit of the melt, which changes close to the wollastonite (CaSiO₃) congruent point. This inflection point changes with the increasing amphoteric oxide content (Al₂O₃ and Fe_tO) because of the change in the dominant polymeric unit (Si⁴⁺–O–Si⁴⁺→M⁴⁺–O–Si⁴⁺; M: Al and Fe), in accordance with the equilibrated primary phases. As the Al₂O₃ content increased, the viscosity and activation energy of slags both drastically increased owing to the change in the flow unit (Si–O–Si, Al–O–Si, and Al–O–Al). In contrast, as Fe_tO increased, the viscosity and activation energy (E_η) of slags decreased because of the change in the flow unit (Si–O–Si, Fe–O–Si, and Fe–O–Fe). Ultimately, the flow unit (T–O–T; T = Si, Al, and Fe) and activation energy of the slags were found to be closely related to the solid primary phase on the phase diagram, and the physical-property–structure relationship was determined from the phase stability.