Β-alumina Structure Research Articles

Coloring properties of blue inorganic pigments based on cobalt/nickel doped strontium magnesium β-alumina structure in 3YSZ ceramics

Blue inorganic pigments based on cobalt or nickel doped strontium magnesium β-alumina structure (β-SAM) solid solutions were successfully fabricated using the solid state method for the first time. Compared with commercial cobalt blue (CoAl2O4, 33.32 wt% Co, −b∗ = 18.5), Sr0.857Mg0.714Al9.986Co0.3O17-δ with significantly lower Co content (2.71 wt%) achieved a more vibrant blue color (−b∗ = 57.5). The ocean blue color of Sr0.857Mg0.714Al10.286-xCoxO17-δ (0.05 ≤ x ≤ 0.3) originates from the A24(F4)→T14(P4) transition of the tetrahedral Co2+, while the sky blue color of Sr0.857Mg0.714Al10.286-xNixO17-δ (0.05 ≤ x ≤ 0.3) is due to the T13(F3)→T13(P3) transition in tetrahedral Ni2+. Chemical stability tests demonstrated good acid and alkali resistance of as-prepared pigments. Additionally, blue 3YSZ ceramic composites incorporating 5–10 wt% Sr0.857Mg0.714Al9.986Co0.3O17-δ or Sr0.857Mg0.714Al9.986Ni0.3O17-δ pigments as chromophores were obtained after high temperature sintering. Ocean blue ceramics composed of 10 wt% Sr0.857Mg0.714Al9.986Co0.3O17-δ and 90 wt% 3YSZ with 0.27 wt% Co exhibited a larger −b∗ value (−b∗ = 43.3) compared to 10 wt% CoAl2O4-3YSZ blue ceramic composite (−b∗ = 34.0, 3.33 wt% Co). FE-SEM analysis showed that the pigment phase remained stable after high temperature sintering with 3YSZ, with clear grain boundaries observed. The mechanical properties of the fabricated blue ceramics were comparable to those of pure 3YSZ ceramic, which highlights the potential of Co/Ni doped Sr0.857Mg0.714Al10.286O17 pigments in ceramic applications.

Synthesis and properties of blue zirconia ceramic based on Ni/Co doped Ba0.956Mg0.912Al10.088O17 blue pigments

Novel blue pigments based on Ba0.956Mg0.912Al10.088-xNixO17 (0 ≤ x ≤ 0.3) and Ba0.956Mg0.912Al10.088-xCoxO17 (0 ≤ x ≤ 0.3) solid solutions were successfully synthesized by solid state method. The XRD results confirmed the structure of the as-synthesized sample belongs to hexagonal β-alumina structure with the space group of P63/mmc. The d-d electron transitions in Ni2+/Co2+ tetrahedral sites in the visible light range are the reason for the blue colors. Then, the as-prepared blue oxides were sintered with ZrO2 powders at 1400 °C to prepare blue zirconia ceramic materials. Based on XRD and SEM analysis, the pigment phase is stable after high-temperature sintering with ZrO2, and a clear grain boundary is observed. The XCT results indicate the prepared blue ceramics are dense and very small pores are rarely distributed inside the blue zirconia ceramic body. Additionally, the mechanical properties of the fabricated blue ceramics were maintained compared to pure ZrO2 ceramic.

Investigations on luminescence behaviour of Ce-activated BaMgAl10 O17 phosphor.

The present paper describes the synthesis of cerium-doped barium magnesium aluminate phosphor by combustion method. The crystal structure of synthesized phosphor belongs to the P63 /mmc space group and is related to the β-alumina structure. The photoluminescence emission spectra exhibited a broad peak centered at 440nm showing the Ce3+ emission. The thermoluminescence properties of phosphors under ultraviolet irradiation were investigated. The activation energy was calculated by Chen's empirical method. Fracto-mechanoluminescence properties were also investigated. The phosphor showed mechanoluminescence (ML) properties without irradiation and the ML intensity increased linearly with the impact height of the moving piston. Therefore this compound may have a use as a damage sensor. Copyright © 2016 John Wiley & Sons, Ltd.

Distribution of K+ Cs+ Ions in the Alkali Layer of (K+, Cs+)-β-Ferrite

Distribution of K+ and Cs+ ions in (K+ Cs+)-β-ferrite with β-alumina structure was determined by the structure refinement. The site occupancies of K+ and Cs+ ions were determined for (K0.74Cs0.43)- and (K0.64Cs0.53)-β-ferrite. It was confirmed that Cs+ ions dominantly occupied Beevers Roth (BR) sites and that K+ ions occupied both BR and mid-Oxygen (mO) sites. On the other hand, K+ ions in (K0.34Cs0.82)-β-ferrite, which revealed the conduction minimum in ionic conduction, occupied only mO sites. Based on the site occupancies, K+ and Cs+ ions were distributed to the alkali layers in unit cells containing BR site and mO site. On the arrangement of unit cells containing K+ ion(s) or Cs+ ion, two possibilities were suggested.

Multi-sites for Mn2+ in barium β-alumina

Barium β-alumina (Ba0.75Al11O17.25) doped with different contents of Mn2+ was prepared by conventional solid state method and analyzed by photoluminescence (PL) spectroscopy. It is found that there are multi-bands that could be attributed to Mn2+ in the PL spectra in addition to the 515nm band ascribed to Mn2+ in Al(2). By analyzing the crystallographic structure of barium β-alumina, it is concluded that the other sites for Mn2+ can be associated with the interstitial Al(5) and Ba sites in the mirror layers.

Thermodynamic Simulation and Experimental Study of Irradiated Reactor Graphite Waste Processing with REE Oxides

AbstractThermochemical processing of reactor graphite waste is based on self-sustaining reaction 4Al + 3TiO2 + 3C = 3TiC + 2Al2O3 which chemically binds 14C from the irradiated graphite in the titanium carbide. Thermochemical processing was investigated to analyse the behaviour of rare earth elements (REE), where REE = Y, La, Ce, Nd, Sm, Eu and Gd. Both thermodynamic simulations and laboratory scale experiments were used. The REEs in the irradiated reactor graphite are formed as activation products of impurities and spread over the graphite bricks surfaces as well as arise from fission of nuclear fuel. REEs can be used also to substitute for waste actinides as well as to increase the durability of carbide-corundum ceramics relative to waste actinides.Thermodynamic calculations and X-ray diffraction analysis of ceramic specimens synthesized revealed that durable REE's aluminates with perovskite, β-alumina and garnet structures are formed by interaction of REE oxides with the Al2O3 melt during the selfpropagating reaction of ceramic formation.The porous carbide-corundum ceramics synthesized have a high hydrolytic durability, e.g. the normalised leaching rates of 137Cs, 90Sr and Nd are of the order of 10–7 – 10–8 g/(cm2·day).

Formation of ultradispersed systems based on sodium polyaluminates from thermal plasma flows

The process of synthesis of sodium β-alumina in a low-temperature nitric plasma is investigated. It is established that the main product of the plasmachemical reaction is sodium polyaluminate Na2O · nAl2O3 of an unknown structure, whose homogeneity range is within the interval of n = 3.5–8.0. The specifics of phase transformations in heating products of plasmachemical synthesis in the temperature interval of 600–1400°C are considered. The formation of the phase with the β-alumina structure (β-and β-Al2O3) proceeds in several stages with the formation of intermediate metastable sodium polyaluminates.

Observation of Eu3+→Eu2+ in barium hexa-aluminates with β′ or β-alumina structures prepared in air

Abstract In 1.30BaO · 6Al2O3:Eu and 0.82BaO · 6Al2O3:Eu with β′- or β-alumina structure respectively, prepared in air, the doped Eu3+ ions were partially reduced to Eu2+ ions. X-ray powder diffraction patterns and photoluminescent spectra were employed to characterize the compound structures and to detect the existence of Eu2+ ions, respectively. The reduction mechanism of Eu3+ → Eu2+ was proposed to be due to the existence of negative defects (e.g. O Na ‴ ) in these compounds. This is the first observation of the reduction of Eu3+ → Eu2+ in non-stoichiometric compounds prepared in an oxidizing atmosphere. And the luminescent properties of Eu2+ in the compounds of xBaO · 6Al2O3:Eu2+ (x = 0.82, 1.00, 1.30) showed similar behavior due to their similarity in crystal structures.

Synthesis of Hexaaluminogallate Catalysts for NOxReduction

BaCo(Al,Ga)11O19with β-alumina structure was found to be an effective catalyst for NOxreduction and successfully synthesized by the coprecipitation method using metal nitrates and ammonium carbonate. Anisotropic BaCo(Al,Ga11O19particles crystallized at 1100 °C for 2 h through the coprecipitation method. BaCo(Al,Ga)11O19supported on a cordierite honeycomb had the NOxremoval activity with methane over 500 °C in the presence of excess oxygen.

Sm 2+ in BAM: fluorescent probe for the number of luminescing sites of Eu 2+ in BAM

In the literature it is usually assumed that the activator ion Eu 2+ enters the BaMgAl 10O 17 (BAM) lattice substitutionally at the Ba site (the Beevers-Ross site). Recent neutron diffraction results indicate that Eu 2+ ions ([Xe]4f 7) do not occupy this Beevers-Ross site, but an interstitial site (anti-Beevers-Ross site) in this host material with β-alumina structure. According to recent 151Eu Mössbauer measurements however, the europium ions appear to be distributed over more than one type of sites. Since spectroscopical analysis of the broad emission band of BAM : Eu 2+ does not allow a unique determination of the location of Eu 2+ ions in this lattice, we have used in this work the sharp spectral lines of divalent samarium 4f–4f transitions to probe the distribution of Eu 2+ ions. High resolution measurements of BAM : Sm 2+ at liquid helium temperature suggest the presence of more than one site in BAM for Sm 2+. Since Eu 2+ and Sm 2+ possess nearly identical ionic radii and chemical properties, the results from the present investigation provide additional support for the “multiple sites” hypothesis for divalent europium ions in this phosphor.

Catalytic combustion of methane on substituted barium hexaaluminates

A sol–gel method using metallic barium, aluminum alkoxides and metal nitrates has been used to synthesize barium hexaaluminate partially substituted by either manganese, iron or both metal ions. The β-alumina structure was obtained by calcination under oxygen at 1200°C. X-ray analysis revealed that formation of a pure single phase BaM x Al 12− x O 19 occurred up to x=4 for Fe, x=3 for Mn and for Fe 1Mn 1 in the case of mixed substituted hexaaluminates. Incorporation of Mn in excess leads to another phase formation (manganese oxide or spinel). As far as the valence state of transition metal ions is concerned, the introduced Fe ions were always trivalent, whereas the Mn ones were either divalent or trivalent. In the latter case, the first Mn ions were introduced in the matrix essentially as Mn 2+ and only for BaMn 3Al 9O 19 does manganese exist exclusively as Mn 3+, the higher the Mn concentration, the higher the proportion of Mn 3+. All solids were aged at 1200°C under water and oxygen and showed a good thermal resistance. Activity for methane combustion has been measured for fresh and aged solids, light-off temperatures were observed in the 560–640°C range. However, the highest activity was obtained for catalysts containing either 3 Mn, 2 Fe or 1 Fe+1 Mn ions per unit cell. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

Thermodynamic activity of Na 2O in Na β-alumina

β-alumina samples prepared by sintering Na 2O–xAl 2O 3 (5.5 < x < 19) at T > 1900 K were subjected to XRD at room temperature. For all compositions from 5.5 < x < 19, both β″-alumina and β-alumina were observed and α-, β″-, and β-alumina could be found at x > 8.5. The proportion of β″-alumina is observed to reach a maximum value at x = 8.5, which is near the β/α phase boundary and decreases to lower values for both x < 8.5 and x > 8.5. The c parameter for β-alumina is observed to increase from 22.45 Å at x = 5.5 to 22.75 Å at x = 8.5 and then is almost constant for x > 8.5. Emf measurements were made at higher temperature (973 K) using the following galvanic cell: (−) Na (reference) | Pt | Na β-alumina | Pt | Na 2O–xAl 2O 3 (5.5 < x < 19) (+). Calculations from emf data show that the activity of Na 2O in the Na 2O–xAl 2O 3 binary system, decreases with increasing value of x for 5.5 < x < 8.5 and is constant for 8.5 < x < 19. Both lattice parameter and emf data are in agreement with the phase diagram proposed by Y. Le Cars and coworkers, which shows only a single phase β-alumina region within the composition range 5.5 < x < 8.5 and a two-phase β- and α-region from x > 8.5. These results suggest that β″-alumina is not a separate phase and provide new edvidence that β″-alumina can be considered as a region of stacking disorder within the β-alumina phase, as reported by D. Gratias and coworkers. Although XRD can identify the presence of both structures, thermodynamic evidence is provided to suggest that the microdomains of β″-alumina and β-alumina structures are part of one single β-alumina phase, formed syntaxially as long-range stacking disorder.

Preparation of proton–β/β″-aluminas by the ion-exchange under hydrothermal conditions and their characterisation

Preparation of proton–β/β″-aluminas is realised by an ion-exchange process under hydrothermal conditions using dilute acetic acid or sulphuric acid as the medium, and the corresponding alkali β/β″-aluminas as the starting materials. The optimum conditions involved in this process is around 425–450 K under the autogenerated pressure below 50 MPa, and a duration of 2–10 h. At higher temperatures and pressures or extended duration, proton–β-alumina converts to boehmite, γ-AlOOH, in the same medium. This method yields products with a variable range of composition with [H 2O]:[Al 2O 3]=1:5 to 1:17. The resulting products convert to α-alumina when heated to temperatures above 1273 K under ambient pressures. High resolution transmission electron microscopy (HRTEM) indicates the variable contents of microsyntactic defects and stacking faults with the changing composition. This study also shows the retention of the β-alumina structure during the displacement of alkali by hydroxonium ions under optimum hydrothermal conditions.

Sensing mechanism of high temperature hydrogen sulphide sensor based on sodium β-alumina

A hydrogen sulphide galvanic probe for reducing gas mixtures (e.g., coal gasification) has been developed. Sodium β-alumina was selected as the electrolyte. The reference consists of a sodium ferrite and ferric oxide powder mixture open to the air. The equilibrium thus generated fixes a constant sodium reference activity. Both measuring and reference electrodes consist of Pt layers. Conditioning of the as-fabricated probes was performed by exposure to a H 2S/H 2 gas mixture at the operating temperature. The performance of the probes was evaluated in synthetic gas mixtures containing well defined H 2S/H 2 partial pressures as well as in an actual coal gasifier. In this paper a possible sensor reaction mechanism is discussed in detail using (electro)chemical as well as thermodynamic considerations. The results are in agreement with the hypothesis that oxygen ions on Na 2O sites in the conduction planes at the surface of the electrolyte were in situ catalytically exchanged by sulphur ions. The activity of Na corresponds to Na x S within β-alumina rather than to pure Na x S. At the measuring electrode, sulphur partial pressures are then automatically converted to sodium activities without the need for a macroscopic auxiliary layer. At higher temperatures (e.g., 860 K), it is postulated that Na 2S is incorporated within the β-alumina structure, while at lower temperatures (e.g., 593 K), NaS appears to be the preferred product in the ion-exchange reaction.

Synthesis, structure and catalytic properties of Fe-substituted barium hexaaluminates

A sol–gel method using Ba and Al isopropylates and iron nitrate has been used to synthesise barium hexaaluminate partially substituted with iron. After calcination under oxygen at 1200°C the β-alumina structure was obtained. Formation of the mixed BaFexAl12-xO19 phase occurred for x=1–4. XRD measurements showed a good crystallinity of the structure and expansion of unit cell parameters due to the presence of larger Fe3+ ions substituting Al3+ ones in octahedral sites only. Mossbauer spectroscopy revealed that Fe3+ ions are present in four different octahedral sites slightly distorted. Catalytic activity in methane combustion showed that an optimum was obtained for solid containing 2 Fe ions per unit cell: the increase of the amount of introduced iron was counterbalanced by the decrease of specific surface area. Intrinsic activities have been calculated for the four solids in both the fresh and aged states. It is observed that increasing iron content increases relative activities in the same ratio as the populations of iron located in two sites as deduced from Mossbauer spectroscopy. It is then tentatively assumed that activity is attributed to octahedrally coordinated Fe3+ ions in some specific sites.

Phase Relations in the BaO–Al2O3–AIN System: Materials with the β-Alumina Structure

It is generally known that aluminates with the β-alumina-type structure exhibit very useful optical and electrical properties. In this paper the synthesis of a new compound, namely barium aluminium-oxynitride with the β-alumina structure (space groupP63/mmc) is reported. To obtain additional information, the BaO–Al2O3–AlN phase diagram was investigated at 1400, 1500, 1650, and 1700°C. The ideal composition of this new material is BaAl11O16N. The unit cell dimensions of the material with the ideal composition (after firing at 1700°C) area=5.6019 (2) Å andc=22.6579 (8) Å, soc/a=4.0447 (2). We determined a solid solution between the already known barium β-alumina (or barium hexaaluminate phase I) with the composition BaAl13.8O21.7and the new barium aluminium-oxynitride. The extent of this solid solution is dependent on the firing temperature. For this solid solution thecaxis is decreasing, while theaaxis is increasing with increasing nitrogen content.

Crystal structure and electric conductivity of K +-β-ferrite with ideal composition KFe 11O 17

Fe 2+ in K 1.22Fe 11O 17 single crystals has been oxidized by iodine to obtain an ideal composition of KFe 11O 17. The crystal structure was analysed based on its β-alumina structure and a final R F value of 0.034 was obtained. The atom positions in the spinel block were unchanged by the oxidation. On the other hand, while the amount of K + in BR sites was comparable to that in K 1.22Fe 11O 17, that in the aBR sites was largely decreased. The ionic conductivity of β-ferrite with the ideal composition was measured at 300 °C and found to be 3.53 · 10 −3 Ω −1 cm −1 in the (001) plane. The activation energy was E a = 0.19 eV. This ionic conductivity was twenty times lower than that of K 1.22Fe 11O 17. The activation energy was also lowered. This is because the amount of K + in aBR sites was relatively small. The electronic conductivity was also measured and found to be 2.05 · 10 −5 Ω −1 cm −1 at 300 °C, which is two orders of magnitude smaller than that of K 1.22Fe 11O 17. However, the activation energy (0.45 eV) was unchanged. The hopping pairs of Fe 2+ and Fe 3+ would be largely decreased by the oxidation.

Crystal Structure of the New Magnetoplumbite-Related Compound in the System SrO–Al2O3–MgO

A new phase in the system SrO–Al2O3–MgO, which was analyzed by EPMA as Sr2MgAl22O36, was recently found. It is located in the middle of the join connecting strontium hexaaluminate (SrAl12O19) of magnetoplumbite structure and strontium magnesium hexaaluminate (SrMgAl10O17) of β-alumina structure. The crystals were grown by the floating zone method and its crystal structure was analyzed by using the single crystal X-ray was analyzed by using the single crystal X-ray diffraction method. The space group wasP6m2 with lattice parametersa= 5.583 Å andc= 22.225 Å. The observedc-axis length is intermediate between those of SrAl12O19(c= 22.00 Å) and SrMgAl10O17(c= 22.399 Å). The structure refinement result has shown that it had the alternate stacking of the conduction layers of magnetoplumbite-type (SrAlO3) and β-alumina-type (SrO) separated by a spinel block ((Al, Mg)11O16). This is the first time that such a “mixed-layer” structure of β-alumina and magnetoplumbite is found to exist in the hexaaluminates containing one species of large cation.

Single-crystal X-ray structure analysis of Mn-substituted barium hexaaluminates as-grown and after reduction

An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Ba0.780Mn0.254Al10.706O17.153) with the β-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 °C. Both as-grown and reduced crystals have a hexagonal structure (P63/mmc, Z= 2) and their unit-cell dimensions are a= 0.5591(1), c= 2.2659(2) nm and a= 0.5587(2), c= 2.2656(3) nm respectively. The structure refinement from the X-ray reflection data collected by using the ω-2θ scan technique was carried out by the least-squares method to give a final R value of 0.03. The resultant structure was identical with that of β-alumina (Ba0.75Al11.0O17.25) reported by Iyi et al.1 Di- and tri-valent manganese partially and preferentially replaced Al(2) sites in a tetrahedral environment in the spinel block. UV–VIS spectra showed that the evacuation treatment brought about the reduction of some of the Mn3+ to Mn2+. The charge compensation for the reduction was found to be effected by O2– defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.

Crystal structure and mixed alkali effect of (K+, Cs+)-β-ferrite

Crystal structure determination has been accomplished for (K+0.3, Cs+0.7)-β-ferrite with β-alumina structure to investigate the mixed alkali effect in ionic conduction. According to the structure of conduction plane, BR sites were occupied by cesium ions and mO sites were occupied by potassium ions. It was concluded that blocking effect of cesium ions in BR sites dominate the mixed alkali effect in this ferrite.