Mullite Lattice Research Articles

Influence of iron on the mullite formation

Influence of iron on the crystallization mechanism of mullite obtained from the hybrid gels and doped with different concentrations of iron was examined. DTA analysis showed that iron addition had a beneficial effect on the mullite formation leading to the temperature decreasing as the amount of iron increased. X-ray diffraction and M?ssbauer spectroscopy confirmed the increased iron content in the mullite lattice, which led to an increase in the unit parameter and was related to the higher iron content in regular octahedra, disordered octahedra, and tetrahedra with oxygen vacancies. Plate-like morphology observed by SEM suggests the two-dimensional crystal growth (n = 2). The most prominent kinetic models were used to estimate n and Ea from the DTA curves. Ea values increases as iron content increase, indicating that mullitization slows down due to iron incorporation. The Ea values for the undoped mullite sample ranged from 820 to 860 kJ/mol, from 820 to 890 kJ/mol for the iron-doped mullite sample containing 6.0 wt% of Fe2O3, and from 960 to 1060 kJ/mol for the iron-doped mullite sample containing 12 wt% of Fe2O3.

Absorption rate and thermal shock resistance of Co2O3-Fe2O3-mullite-based ceramics for solar heat absorbers

Mullite ceramic materials have a number of excellent properties that make them an excellent choice for solar heat absorber materials in Concentrating Solar Power (CSP) systems. In this study, the effects of adding Co2O3 and Fe2O3 on the properties of mullite-based ceramics were studied, aiming to get excellent absorption rate and thermal shock resistance. The results show that the sample C2 (45.05 wt% 75 calcined bauxite, 45.05 wt% Suzhou clay, 9 wt% Fe2O3, 3 wt% Co2O3) sintered at 1400 °C has the best comprehensive performance, with a bending strength of 143.28 MPa and a solar absorption rate of 92.2 % in the wavelength range of 0.3–2.5 μm. The bending strength increased by 31.36 % after thermal shock cycles for 30 times (1000 °C ∼ room temperature, air cooling). Generation of iron cobalt spinel and lattice distortion caused by Fe3+ and Co3+ into the mullite lattice combined to promote the absorption rate. In the meantime, the main crystalline phase columnar mullite showed a tight continuous interlaced network structure. The fracture mode was mainly transgranular fracture, which endowed C2 excellent thermal shock resistance.

Synthesis of mullite by high temperature reaction to realize nontoxic and efficient recycling of vanadium slag chlorination residue

The compositions of the leaching residue obtained by chlorination method to extract valuable elements (Ti, Cr, Fe, Mn and V) from vanadium slag were mainly Al2O3 and SiO2 with a small amount of hazardous elements Cr and V. In order to reduce the pollution of V and Cr to the environment, a novel method for nontoxic and efficient recycling of leaching residue was proposed. Mullite with high compressive strength of 133.345 MPa and bulk density of 3.20 g/cm3 was successfully synthesized by using leaching residue as raw material, adding appropriate SiO2 and roasting at 1600 °C for 5 h. The trace element Ti and hazardous elements V and Cr in leaching residue entered the mullite lattice in the high temperature reaction process to form a solid solution, which were stabilized in the mullite phase. The synthesized samples were tested by toxicity characteristic leaching procedure (TCLP) and GB5085.3—2007. The results showed that mullite met the toxic leaching standard and was a nontoxic product.

Structural, microstructural and mechanical properties of sintered iron-doped mullite

The study of an effect of iron doping on the structural, microstructural and mechanical properties of sintered iron-doped mullite is presented. The results of phase composition, performed in detail by Mössbauer spectroscopy and XRD analysis, revealed that all added iron was inside the mullite lattice forming the single phase up to 12% by weight of Fe2O3 and 1300 °C. Samples, which were processed at 1550 °C, contained secondary phases, hematite or magnetite, regardless of the amount of added iron. Furthermore, the addition of iron decreases the values of relative linear shrinkage comparing to the values of undoped one (~18%) while the densities of the sintered samples rise as well as their values of microhardness. Even though the density values were not too high (90 TD%), the obtained values of microhardness were excellent, 1634 HV0.1 for maximum iron content due to the characteristic mullite microstructure.

Luminescent Properties of RE-doped Mullite (Al6Si2O13) Phosphors(RE = Ce3+, Eu2+, Ce3+-Tb3+, and Eu3+)

The luminescent properties of Ce3+, Eu2+, Ce3+-Tb3+, and Eu3+ in the mullite (Al6Si2O13) host were investigated for realization of full colors phosphors with a single type host. Ce3+ and Eu2+ in mullite showed blue photoluminescence (PL), whereas green and red emission were obtained in the Ce3+-Tb3+ and Eu3+ doped mullite. The prepared phosphors had the excitation band around 300 nm. The PL intensity at 200 °C kept 70 % of the initial intensity at room temperature.The X-ray diffraction study indicated the incorporation of the rare-earth (RE) ions in the mullite lattice without forming secondary phases, although it was not probable that the RE ions substituted for Al3+ and Si4+ because of the significant difference in ionic size. The bond-valence-sum and the differential electron density map analysis indicated that the doped RE ions occupied the stoichiometrically-introduced vacant oxygen site, which was the site for oxygen originally. The amphoteric nature of the oxygen site in mullite was proposed.

Modification of ash fusion behavior of coal with high ash fusion temperature by red mud addition

The influences of two red mud (RM) samples (ZZ and ZM) on fusion behaviors of three high ash fusion temperature (AFT) coals (FZ, JC and JZ) were investigated. It was evaluated by determining AFTs of coals, RMs, and the mixtures of coal ash with RM, as well as their compositions of ashes and mineral. The results showed that the contents of CaO, Fe2O3, and Na2O in RMs were higher than those in coal ashes, which caused RMs’ AFTs lower than those of coals. The formation of high melting-point mullite might be the main factor causing coal AFTs higher. With the increase in RM mass ratio, the increasing amount of low-melting eutectic (anorthite (CaAl2Si2O8), fayalite (Fe2SiO4), and anorthite (Na-rich) (Ca,Na)(Si,Al)4O8)) and amorphous matter made AFTs lower. ZZ reduced AFTs obviously than ZM because the content of Na2O in ZZ was higher than that in ZM. Owing to lower ion potential of Na+ than those of Ca2+ and Fe2+, Na+ is easier to enter into the lattice of mullite and results in the reduction of AFTs.

Mullite crystallization using fully hydrolyzed silica sol: the gelation temperature influence

Mullite is an aluminosilicate widely used as a structural material for high temperature applications. This paper studies the effect of the gelation temperature on the synthesis of two mullite precursors: polymeric and colloidal silica, using both in fully-hydrolyzed silica sol, derived from sodium silicate. The gels were synthesized using aqueous silicic acid and aluminum nitrate. Ethylene glycol was added into polymeric gels. Two gelation temperatures were used: 80 and 100 °C. In the polymeric precursor, the increasing of the gelation temperature caused an increase in the silica incorporation inside the mullite crystalline lattice at 1,000 °C, and it also generated an increase in the reaction extent at all calcination temperatures. In the colloidal precursors, these effects were more intense than in the polymeric precursors in terms of yield. Colloidal samples calcined at 1,250 °C crystallized cristobalite and alpha alumina in addition to mullite when they were previously gelled at 80 °C. On the other hand, the same sample gelled at 100 °C led to only crystallized mullite. The reaction extent increased by more than 20 % for colloidal samples gelled at 100 °C compared to colloidal samples gelled at 80 °C (calcined at 1,250 °C). This increase was due to the almost total incorporation of alumina and silica in the crystalline lattice of mullite.

Synthesis of spherical (30 nm) and rod-like (200 nm) zirconia co-reinforced mullite nanocomposites

The objective of this research is to study microstructure and phase evolution of zirconia–mullite (ZM) nanocomposites which were synthesized by the direct transformation from amorphous precursor monoliths. The monolithic precursors were heat treated at 950–1250 °C to obtain ZM composites. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were employed to investigate the microstructure of the composites. A unique nano- and submicron-zirconia co-reinforced mullite composite microstructure was obtained after a controlled crystallization. The nano tetragonal zirconia (t-ZrO2) grains (<50 nm) were spherical and embedded in the mullite lattice. The unconfined zirconia grains showed a preferred growth direction along 〈1 0 0〉 direction of t-ZrO2, and formed submicron grains (<650 nm) with bar-like shape. The precursor derived ZM composites also demonstrated enhanced mechanical properties comparing with the conventional powder processed ZM composites.

Main mineral melting behavior and mineral reaction mechanism at molecular level of blended coal ash under gasification condition

The main mineral melting behavior and mineral reaction mechanism at molecular level of Chinese blended coal ash under gasification condition (30% H 2, 66% CO, 4% CO 2) from 1073 K to 1573 K were studied through the ASTM test, X-ray diffraction (XRD), ternary phase diagram system and quantum chemistry calculation with ab-initio calculations. The results show that with increasing blending mass fraction of low ash fusion temperature (AFT) ash (ash B), the location of blended ash in ternary systems is transferred from the mullite region to the anorthite region, as the dominant crystal mineral of blended ash at around DT (XRD analysis) is also transferred from mullite to anorthite. The calcium-bearing minerals, such as anhydrite, calcite etc., can react with mullite and the precursors of mullite (metakaolinite etc.), which is one of the main refractory minerals in high AFT ash (ash A), and is converted into low-melting minerals (anorthite, gehlenite, and fayalite etc.) in the temperature range between 1273 K and 1403 K. The reaction between mullite and CaO to form anorthite plays a significant role in decreasing AFTs of blended coal ash A/B. It is because the chemical activity of the highest occupied molecular orbits (HOMO) in mullite cluster is stronger than that of the lowest unoccupied molecular orbits (LUMO) in mullite cluster, the Ca 2+ as electron acceptor can easily enter into the crystal lattice of mullite mainly through O (7) and O (12) and cause the rupture of bonds Al (1)–O (13) (in the [AlO 6] 9 −-octahedron) and Al (8)–O (13) (in the [AlO 4] 5 −-tetrahedron), which are weaker than any other bonds in crystal lattice of mullite. Finally, the entrance of Ca 2+ can force mullite to transform to anorthite by the effect of Ca 2+, and the entered Ca 2+ is located in the center of [SiO 4] 4 −-tetrahedron ring in the anorthite crystal lattice. Taking the [SiO 4] 4 −-tetrahedron, which is composed of Si (70), O (78), O (48), O (91), O (86) as an example, the Ca 2+ can capture the partial electronics of O (86) and cause the bond length (B.L.) of bond Si (70)−O (86) to become longer and unstable.

Effect of feldspar concentrate on the chemical resistance of acid-resistant materials prepared on the basis of argillaceous materials of different chemical and mineral composition

Research shows that introduction of feldspar concentrate into the composition of ceramic mixes improves mullite crystallization, and this promotes an increase in the acid resistance of refractory materials. An increase in iron oxide content reduces the acid resistance of refractory materials since it does not promote an improvement in mullite lattice crystallization.

Quantum and experimental study on coal ash fusion with borax fluxing agent

Borax fluxing agent was added to four kinds of coal ash in terms of different quality ratios. The ash fusion test (AFT) of the mixtures of coal ash plus borax fluxing agent was carried out and the reactivity of mullite in coal ash was calculated by the density functional theory (DFT). The space group of the mullite is Pbam, belonging to the orthorhombic system. The results suggest that mullite is likely to combine with electron acceptor rather than with electron donor. The activity of Si is higher than that of Al when mullite combines with the electron donor. When borax is added into mullite, Na + as the electron acceptor easily enters into the lattice of mullite, causing a transition from mullite to nepheline. The calculation results agreed well with the AFT results under reducing atmosphere.

Effect of transition-metal-ion doping on high temperature thermal expansion of 3:2 mullite—An in situ, high temperature, synchrotron diffraction study

The effect of Ti 4+, Cr 3+ or Ga 3+ doping on thermal expansion of 3:2 mullite was examined from 20 °C to 1400 °C by high temperature XRD using synchrotron radiation. The room temperature mullite lattice expanded with metal-ion incorporation and the change in the b-axis was larger than that along the a-axis (Δ b > Δ a) for all ion types. Upon heating, the largest elongation was along the b-axis (Δ b/ b RT) and was similar to that in the pure mullite (∼0.5%). Least expansion was observed along the a-axis (0.28% ≤ Δ a/ a RT ≤ 0.41%). The thermal expansion of mullite was not constant in the temperature range from 20 °C to 1400 °C, and had a linear dependence on temperature ( α( T) = A 0 + A 1Δ T). The α mean values increased with temperature (20 °C ≤ T ≤ 1000 °C), but were always within 3.0–6.7. Cr 3+ and Ga 3+ doping decreased the anisotropy in mullite lattice expansion, however, Ti 4+ doping showed a sharp increase. Trends associated with type and the quantity of dopant are discussed.

CoAl 2O 4–mullite composites prepared by sol–gel processes

The formation of CoAl 2O 4–mullite composites from diphasic sol–gel precursors with 3:2 mullite composition doped with 1, 2 and 3 at.% Co 2+ was studied by differential scanning calorimetry (DSC), X-ray diffraction and Rietveld structure refinement. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different composition and activation energies. The former phase with smaller activation energy (822 kJ mol −1) is attributed to cobalt-containing spinel structure and the latter with larger activation energy (about 1200 kJ mol −1) to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl 2O 4, while Al–Si spinel forms mullite above 1100 °C. Mullite lattice parameters, Rietveld refinement data and the CoAl 2O 4/Co 2+ ratio in annealed samples points out that the majority of cobalt is incorporated in CoAl 2O 4 and only about 0.6 at.% enters mullite structure or the glassy phase, or both.

High-temperature behaviour of mullites: Study by means of X-Ray Diffraction

Results on thermal behaviour of several sol-gel chromium doped mullites (3Al 2 O 3 .2SiO 2 with Cr contents of 1, 3, 6 and 9 wt.% Cr 2 O 3 ) by means of high-temperature X-ray diffraction are exposed in this work. Chromium additions make possible to obtain glass-free materials with very homogeneous microstructures, which is highly convenient as the presence of glass in these materials provides a certain degradation on their mechanical properties, especially at high temperature operations. This study reveals that the chromium introduction in the mullite lattice promotes an effective reduction of its linear thermal expansion coefficient, although also an increase in the anisotropy with which this expansion takes place. Only with the 6 wt.% Cr 2 O 3 content the anisotropy for the lattice expansion is similar to the non-doped mullite.

Microchemical Analysis and Microstructural Development of Cr-Doped Mullites

A characterization study of chromium doped sol–gel mullites (with Cr2O3 contents varying between 1 and 9 wt.%) by means of electron beam techniques is presented. Scanning and transmission electron microscopy studies gave information about general microstructural features, like grain size and shape, presence of glassy phase in triple grain junctions and segregation of secondary crystalline phases. EDX-TEM measurements reveal an upper solubility of chromium in the mullite lattice of approx. 8 wt.% Cr2O3. The effect of Cr doping in the microstructural development of these mullites is discussed.

The Effect of Sesquioxides of 3d-Transition Elements on the Strength of Synthetic Mullite and Mullite-Based Materials

Mullite synthesized from alumina and silica with the sesquioxide of a 3d-transition metal added (Me = Sc, Ti, V, Cr, Mn, and Fe) is capable of forming solid solutions of composition Al6–xMexSi2O13. It is shown, using data obtained by methods of x-ray phase analysis, electron paramagnetic resonance, infrared spectroscopy and magnetic susceptibility, that the aluminum atoms located at octahedral positions of the mullite lattice can be isomorphously replaced by Me metals. The effect of this replacement on the strength of mullite and mullite-based ceramics is considered.

Thermic transformation of sillimanite single crystals to 3:2 mullite plus melt: investigations by polarized IR-reflection micro spectroscopy

Oriented and polished high quality sillimanite single crystals were thermically treated for 4, 24 and 96 h at 1600°C. The products were quenched to room temperature and investigated using polarized IR microspectroscopy in reflection mode (600–4000 cm −1 ). The spectra of the 24 and 96 h thermically treated sample indicates the transformation of sillimanite to 3:2 mullite plus melt of eutectic SiO 2 /Al 2 O 3 composition. The anisotropy of the spectra show a high degree of orientation of crystals according to a topotaxic reaction. The high wavenumber mullite lattice bands between 1100 and 1200 cm −1 are assigned to modes of [SiO 4 ] units interconnected within the mullite specific (Al ∗ SiAl–O c ∗ ] 3-clusters, and Si–O c –Si- and Si–O c –Al-bonds. It is shown that the sillimanite to mullite transformation occurs by an increased damping of all modes of the sillimanite lattice due to an irreversible increase of anharmonicity preserved as a typical property of mullite.

Effect of MgO, Y 2O 3, and Fe 2O 3 on silicothermal synthesis and sintering of X-sialon. An XRD, multinuclear MAS NMR and 57Fe Mössbauer study

The separate effects of 10 wt.% MgO, Y 2O 3 and Fe 2O 3 on the silicothermal formation and sintering of X-sialon were investigated by XRD, multinuclear MAS NMR and Mössbauer spectroscopy. The effects of the three oxides on densification follow a similar trend to samples containing 1 wt.% oxide (Y>Mg>Fe), but at the higher additive concentration, the maximum densities are higher and are achieved at lower temperature (1400°C). 10 wt.% MgO and Y 2O 3 facilitate low-temperature mullite formation and its conversion to X-sialon by forming increased amounts of liquid phases. The 25Mg NMR spectra of the MgO-containing samples suggest that their liquid phase is a N-containing Mg silicate glass. These samples also form MgAl 2O 4, depleting the amount of Al available to form X-sialon. The Y 2O 3-containing samples form liquid of a similar composition to N-melilite, depleting the available Si, with the excess alumina appearing as corundum. Under silicothermal conditions, Fe 2O 3 reacts with the Si to form a mixture of magnetic silicide and aluminide, suppressing X-sialon formation by depleting the Si but enhancing the formation and stability of mullite by Fe-for Al substitution in the mullite lattice. Some of the magnetic phases may occur as very fine superparamagnetic particles possibly located in an X-sialon matrix.

Preparation by pyrolysis of aerosols and structural characterization of Fe-doped mullite powders

Amorphous Fe-doped 2SiO 2·3Al 2O 3 powders were prepared by pyrolysis of aerosols generated from solutions of TEOS, Al(III) nitrate, and Fe(III) nitrate in methanol. The study of the thermal evolution of these powders revealed that the presence of Fe(III) promotes mullite formation and that these cations enter into the mullite lattice at the first stages of crystallization at ∼900°C. It was also found that the amount of Fe(III) in solid solution with mullite increased as increasing temperature from 900 to 1400°C. The limit of Fe(III) incorporation into mullite in the as-prepared powders was in the range previously reported for samples prepared by the ceramic procedure (∼11 Fe 2O 3 wt%). The additional iron phase appearing in samples with an iron content above this limit consisted of an iron oxide spinel, which probably contained some Al(III) cations in solid solution. Finally, the analysis of the EXAFS spectra of the heated samples showed that the iron cations incorporated into the mullite lattice are mainly located at octahedral positions.

Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mossbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn2+ and Mn3+ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe3+ ion was detected and conclusive evidence was obtained for the entry of Fe3+ in the octahedral lattice position of mullite from the analysis of Mossbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr3+ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr3+ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti4+ (3d0) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al2TiO5 structures stood as evidence for incorporation of Ti4+ ion in the octahedral site of mullite lattice by replacing Al3+ ion.