Synthesis and characterisation of gel-derived mullite precursors from rice husk silica

The sol–gel synthesis and characterization of mullite precursor derived from rice husk silica and aluminum nitrate hydrate [(Al(NO₃)₃·9H₂O] has been investigated. The samples were characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) coupled with Rietveld analysis, and scanning electron microscopy (SEM). FTIR results showed the presence of Si–O–Si, Al–O–Al, and Si–O–Al functional groups, which were associated with mullite, corundum, quartz, and cristobalite, as verified by XRD analysis. It is concluded that mullite formation started at 1150 °C, and its abundance increased rapidly with an increase in temperature from 1150 to 1350 °C, resulting in increased phase content from 30.9 to 67.7 wt%. Although mullite was formed at a low temperature, the complete reaction between corundum and silica to form mullite was not achieved. This finding demonstrated that rice husk silica is a potential alternative raw material for the production of mullite ceramic.     

Characterisation of monophasic and diphasic mullite precursors by solid state reaction study

Abstract

The characterisation by various spectroscopic techniques of amorphous mullite precursors formed from monophasic (SH) and diphasic (DG) gels is reviewed. In a new approach, both types of precursor have been heated with CaCO₃ and the crystalline phases formed during solid state reaction monitored using an X-ray powder diffraction technique. The formation of a calcium aluminosilicate phase (gehlenite) rather than calcium aluminate suggests that both precursors consist of an amorphous aluminosilicate phase of composition close to that of 3:2 mullite.     

Structural and morphological characterization of iron-doped sol-gel derived mullite powders

The structural and morphological properties of iron-doped mullite powders are the subject of the present study. The powders of undoped and iron-doped mullite in the composition range of 3–15 wt% Fe₂O₃ were synthesized by a combination of sol-gel and combustion methods. The excess of water and urea were introduced in reaction solutions to enhance the copolymerization of aluminum and silicon species. The results of structural characterization revealed that the synthesized mullite powders were amorphous of a hybrid type. The specific surface area of the undoped mullite powder was 262 m² g^-1 with a maximum pore radius (d_p) of 2 nm classifying it into mesoporous materials. The addition of iron has reduced the specific surface area, while the pore size value remained the same except for the sample with 3 wt% Fe₂O₃ (S_BET = 278 m² g^-1; d_p = 3 nm). The presence of iron caused lowering the temperature of liquid phase formation, while present urea combusted providing the increase of the temperature locally that caused the sintering and formation of agglomerates of smaller particles. However, the results of the particle size analysis are not straightforward. The values of mean volume diameter (D[3,4]) indicated that the particle size increased to 6 wt% Fe₂O₃ (123.6 μm), and then decreased and for the sample with 12 wt% Fe₂O₃, it was equal to 96.6 μm. Thus, the added iron contributed to the more uniform particle size distribution. The SEM analysis has also shown the coarse powder particles consisted of the coalesced smaller particles.     

Synthesis of mullite precursors in molten salts. Influence of the molten alkali nitrate and additives

Silica gel and aluminium salt, in the stoichiometric ratio for mullite (Al/Si=3), have been used to prepare the mullite precursor by reaction in molten alkali nitrates at 450°C. The influence of the nitrate (NaNO₃, KNO₃ or mixture NaNO₃–KNO₃) and the additives (Ca²⁺, Ba²⁺, Bi³⁺) on the phase transformations has been followed between 1000 and 1400°C. It appears that molten NaNO₃ favours the crystallization of cristobalite and α-Al₂O₃ whereas KNO₃ inhibits the formation of silica and mullite. The mixture NaNO₃–KNO₃ seems to be the best choice. Addition of foreign cations improves the mullite formation and favours the synthesis of θ-Al₂O₃, especially in the case of Bi³⁺. But the crystallization temperature of mullite is not changed.     

Synthetic mullite and mullite refractories

Refractories and Industrial Ceramics -     

Microstructural evolution of mullite nanofibrous aerogels with different ice crystal growth inhibitors

Mullite nanofibrous aerogels with low density, low thermal conductivity and high service temperature (as high as 1400 °C) was fabricated by the freeze-casting method, and two different kinds of ice crystal growth inhibitors, glycerol as the cryoprotector and agarose as the gel-former, were adopted respectively to influence the growth of ice crystal, thereby adjusting the sample microstructure (including the pore size and the pore distribution). The addition of glycerol could influence the growth of the ice crystal, leading to the pore structure changing from the cellular pores to the spherical pores and the decrease of the large pore size, consequently resulting in the increase of the sample density and compressive strength. The addition of agarose could inhibit the growth of the ice crystal due to the formation of gel network, resulting in the disappearance of the large pores (cellular pores). The sample with high agarose content exhibited a relatively high compressive strength of 252.41 kPa due to high density and uniform pore structure. The research of this work provides an effective method to regulate the properties of nanofibrous aerogel to meet the demand of different applications.     

Structural study of mullite based ceramics derived from a mica-rich kaolin waste

Mullite-based ceramics have been synthesized by reactive sintering of a mixture containing kaolin and a mica-rich kaolin waste. Samples fired in the temperature range from 1300 to 1500 °C were characterized by X-ray diffraction (XRD). The quantitative phase analysis and unit cell parameters of the mullite were determined by Rietveld refinement analysis of the XRD data. Mullite-based ceramics with 1.2 wt% quartz, 56.3 wt% glass (amorphous phase), 2.64 g/cm³ of apparent density, and 35±1.2 MPa of flexural strength were obtained after firing at 1500 °C. A liquid phase sintering mechanism activated by a total mica content of 13.3 wt% allowed to increase the mullite content to 47.6 wt% (2.3 wt% quartz and 50.1 wt% glass phase) and improve the flexural strength (70±3.9 MPa) after firing at 1400 °C.     

Porous mullite templated from hard mullite beads

Porous mullite bodies were developed by spark plasma sintering (SPS) amorphous mullite beads of about ∼30 μm in diameter at two temperatures, 950 and 1300 °C. Materials showed a close random stacking of solid spheres that retained their original packing but slightly flattened at the contacts in some cases. Depending on the thermal history, the beads were partially or fully crystallized. The thermal conductivity of the different porous mullite materials was analyzed as a function of the microstructure. Owing to the particular porous network, high gas permeability and very low thermal conductivities (1-2 W m⁻¹ K⁻¹) were achieved, among the lowest reported for sintered mullite materials.     

Characterization and mechanisms of the phase's formation evolution in sol-gel derived mullite/cordierite composite

In this work, mullite/cordierite precursor powder was prepared through a technology of low-temperature synthesis by using the sol-gel process, tetraethyl orthosilicate (TEOS) as a source of silicon oxide SiO₂, and aluminum nitrate nonahydrate Al (NO₃)₃.9H₂O as a source of aluminum oxide (Al₂O₃) and magnesium nitrate hexahydrate Mg (NO₃)₂.6H₂O as a source of magnesium oxide MgO was used as raw materials to synthesize mullite/cordierite precursor gel with a concentration (sample containing 50 wt% of cordierite and 50 wt% of mullite) and named as (MC50). The objective of this study is to find a suitable kinetic model to study the phases and the mechanisms of their formation in mixtures, with the prediction of the system's behavior under selected thermal conditions, including finding the kinetic and thermodynamic media that describe these interactions. To follow and characterize the crystalline phases and their transformation as a function of temperature utilizing differential thermal analysis (DTA), Dilatometry (DIL), and powder X-ray diffraction (XRD). The results show that the crystallization process occurred in the temperature interval between (900–1350) °C. In the temperature range of (900–1000) °C, spinels between Al–Si and Al–Mg with the chemical formulas (Al₄Si₃O₁₂ and MgAl₂O₄) were formed. When the thermal treatment temperature increases from (1000–1100) °C, mullite is produced. As the temperature increases, the amount of Mg–Al spinel decreases to form amorphous silica, and μ-cordierite has appeared at 1250 °C. With an increase in temperature up to 1350 °C, α-cordierite appeared as a stable phase. The reason for this is the presence of the spinel (Al–Mg) phase that helped it form.To determine the reaction kinetics of these transformations at high temperatures, the mixture 50/50 mullite/cordierite was selected to study its kinetics. The activation energy values (Ea/Tm) (Tm is the maximum temperature of the transformation, i.e., the maximum peak temperature is not related to the crystallization fraction α) calculated by Ozawa, Boswell, and Kissinger methods are in good agreement with the evident activation energy (Eα/Tα) (Tα is the degree of the heat of transformation in terms of crystallization fraction α changes from 0<α < 1) calculated using the KAS and FWO methods.For the purpose of calculating the interaction model and finding the media that determine the interaction model based on the experimental data, Malék's methodology method was used. The best kinetic model is the Šesták - Berggren model to describe the reaction process to form spinel, mullite, and α-cordierite. From the SB model, the equations Kinetics and all kinetic parameters (n, m, ln(k₀)) that describe the kinetics of the reactions and mechanisms of formation of spinel, mullite, and α-cordierite in the mixture are, respectively, (2.14, 0.023, 65.21), (1.62, 0.1232, 81.76), and (1.41, 0.2859, 91.13). While the values of Gibbs free energy ΔG#, enthalpy ΔH#, and entropy ΔS# were as follows: 407.254 kJ mol⁻¹, 976.756 kJ mol⁻¹ and 415.561 J mol⁻¹K⁻¹ for Mullite formation, and 471.64 kJ mol⁻¹, 1255.16 kJ.mol-1 and 491.75 J mol⁻¹K⁻¹ for the formation of α-cordierite.Comparison of simulation curves with experimental data obtained at different temperatures gives good agreement with the thermal analysis data (Experimental), which indicates that the Model of Šestak − Berggren, is the best suitable kinetic model for studying and describing the reaction technique for MC50 prepared by the sol-gel method.     

Thermal evolution and structural study of 2:1 mullite from monophasic gels

Single phase mullite gels with composition 2Al₂O₃·SiO₂ (2:1) were prepared by the slow hydrolysis method using aluminium nitrate nonahydrate and tetraethylorthosilicate as reagents. The evolution to mullite from gels was studied by infrared (IR) spectroscopy and X-ray diffraction (XRD). Gels thermally treated under fast schedules showed mullite formation below 900 °C. Compositional and microstructural changes in 2:1 mullites through the range of temperature from 900 to 1600 °C were determined by the measurement of lattice parameters and field emission scanning electron microscopy. The alumina-rich mullites formed at low temperatures become almost the nominal 2:1 at 1600 °C. This result is consistent with available thermodynamic data for mullite formation from alumina and silica. Microstructural examination indicated an almost constant grain size for mullite from 900 to 1600 °C.     

Electrospun mullite nanofibers derived from diphasic mullite sol

Fine‐grained mullite nanofibers derived from the diphasic mullite sol were successfully fabricated by electrospinning and subsequent pyrolysis at 1500°C. Polymethylsiloxane and aluminum tri‐sec‐butoxide were selected as the silicon and aluminum source to synthesize the diphasic sol. Results show that the weight loss of mullite precursor fibers in our work was about 60 wt.%, which is similar with that of fibers fabricated using the monophasic sol. This low weight loss was mainly attributed to the high ceramic yield of polymethylsiloxane and low introduced polyvinylpyrrolidone content, which ensures the integrity of fiber morphology during the sintering process. Mullite fibers with 216 nm average diameter were fabricated after sintered at 1500°C and the corresponding grain size was only ~100 nm, much smaller than that in mullite fibers derived from monophasic sols. Therefore, it can be predicated that mullite fibers in this work should possess a higher mechanical strength than those derived from monophasic sols when the sintering temperature was higher than 1400°C and therefore was an ideal starting materials for the fabrication of mullite nanofibrous ceramics used as the high‐temperature thermal insulation materials.     

Structural and microstructural comparison of bioactive melt-derived and gel-derived glasses from CaO-SiO2 binary system

Two glasses from CaO-SiO₂ binary system were obtained by sol-gel and melting techniques. The effect of two different glass obtaining methods was investigated using X-ray diffraction, FTIR, Raman and ²⁹Si MAS NMR spectroscopic methods. The measurements revealed significant differences in the glasses structure. Although both glasses were fully amorphous, the gel-derived glass had a more polymerized structure compared to the melt-derived one. The studied glasses were characterized by BET analysis to provide information about specific surface area of the obtained materials. Apart from microstructural evaluation, thermal properties and in vitro bioactivity study of all glasses were conducted to demonstrate differences in performance of the samples. The formation process of hydroxycarbonate apatite (HCA) layer was investigated using inductively coupled plasma mass spectrometry (ICP-MS) and structural studies.     

Structural evolution in ceramic technology and processing

A survey of the development trends in ceramic technology and processing from the standpoint of synergetics and chemistry of imperfect crystals is given. __________ Translated from Novye Ogneupory, No. 1, pp. 56–61, January, 2006.     

Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.     

Structural evolution of recycled tire rubber in asphalt

The performance of recycled‐tire‐rubber‐modified asphalt mainly depends on the structure of rubber in the asphalt. The effects of the curing temperature, mixing time, and shearing on the evolution of the chemical structure and morphological structure of crumb rubber in asphalt were investigated. The crosslink density, compositions, and morphological evolution of the residual crumb rubber were characterized. The results show that the structure evolution of tire rubber in asphalt was greatly affected by the curing temperature. At a low curing temperature (180°C), the crosslinking network of the tire rubber was broken down, and this led to the partial dissolution of natural rubber (NR). However, at high curing temperature (240°C), the dissolution of NR, synthetic rubber, carbon black, and inorganic filler was observed. The released carbon black covered with a thin layer of bound rubber dispersed at a microstructured or nanostructured size in the asphalt. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42954.     

Structural evolution of aging agar-gelatin co-hydrogels

Agar-gelatin co-hydrogel was investigated over a period of ≈30-days by dynamic light and small angle neutron scattering, and rheology to quantify changes occurring inside the hydrogel. Degree of non-ergodicity was extracted as a heterodyne contribution from the measured dynamic structure factor data. From the analysis of the data, we observed two relaxation modes namely fast and slow modes with relaxation times τ_f and τ_s whose dependence with aging time, t_a fall on a scaling behavior given by power-laws, τ_f  (t_a)^−1/5 and τ_s  (t_a)^3/5 respectively. Further the analysis showed the heterogeneity size (ξ_SANS) obtained from SANS also follows power-law behavior, ξ_SANS  (t_a)^−1/5. The data taken together revealed the “speeding up” of fast and “slowing down” of slow mode relaxation processes.