Showing papers on "Mullite published in 2014"

Plasma spray deposition of tri-layer environmental barrier coatings

Abstract: An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb 2 SiO 5 top coat on SiC substrates. Solidified droplets in as-deposited Yb 2 SiO 5 and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb 2 SiO 5 + Yb 2 O 3 two-phase top coat and 2:1 mullite (2Al 2 O 3 *SiO 2 ) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb 2 O 3 in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al 2 O 3 . Mud-cracks were found in the Yb 2 SiO 5 layer and in precipitated Al 2 O 3 due to the thermal expansion mismatch between these coating phases and the substrate.

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Abstract: Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V 2 O 5 and AlF 3 were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V 2 O 5 and 4 wt.% AlF 3 addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V 2 O 5 lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.

Phase transformation in plasma electrolytic oxidation coatings on 6061 aluminum alloy

Abstract: Oxide coatings were produced on a 6061 aluminum alloy using a pulsed unipolar plasma electrolytic oxidation (PEO) process. The effect of electrical parameters including pulse frequency, duty cycle and current density on phase formation in the coatings was revealed using conventional and glancing angle X-ray diffraction. The results show that PEO coatings are mainly composed of γ-Al2O3. Depending on the electrical parameters employed, the coatings can also contain α-Al2O3 and mullite with varying concentrations. Higher current densities and higher duty cycle were found to favor the formation of mullite. Under the experimental conditions used, the ratio of the integrated XRD peaks for α- and γ-Al2O3 varied from 0 to about 0.6, indicating that the relative content of α-Al2O3 in the PEO coatings varied over a wide range. Longer pulse on-times and higher current densities promoted the gamma to alpha-alumina phase transformation. Depth profiling of PEO ceramic coatings using glancing angle XRD with different incident beam angles revealed that mullite was more concentrated in the top surface of the coatings. No significant variation in α-Al2O3 concentration across the coatings could be concluded in this study, unlike the results of some other studies.

A new and economic approach to fabricate resistant porous membrane supports using kaolin and CaCO3

Abstract: This new and economic approach to fabricate resistant porous membrane supports consists of Algerian kaolin and calcite (CaCO 3 ) instead of Al 2 O 3 . The porous mullite (3Al 2 O 3 ·2SiO 2 ) and anorthite (CaO·Al 2 O 3 ·2SiO 2 ) based ceramics were obtained by solid state reaction. Different calcite amounts (10–28 wt%) have been added into kaolin halloysite type (Al 2 O 3 ·2SiO 2 ·4H 2 O) in order to control pores forming with appropriate distribution and sizes. Based on a pore distribution and formed phases, a kaolin + 15 wt% calcite (K15C) mixture was selected for flat and tubular configurations. A porosity of 45–52% was also obtained when K15C compacts were sintered at 1100–1250 °C. For example, porosity, average pore size (APS) and 3 point flexural strength were 49%, 3 μm and 87 MPa (same as Al 2 O 3 value), respectively when K15C compacts were sintered at 1250 °C for 1 h. Finally, a correlation between microstructure and mechanical properties of elaborated supports has been discussed.

Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation

Abstract: Porous mullite ceramics were prepared from an industrial grade mullite powder by foaming and starch consolidation. The viscosities of the original suspensions and the foamed ones with solid loading of 62.5 and 67.5 wt% were measured. After the steps of forming and drying, the green bodies were sintered under different temperatures from 1,200 to 1,600 °C for 2 h. The influence of solid loading of suspension and sintering temperature on the porosity and compressive strength was evaluated. The sintered mullite ceramics, with porosity from 86 to 73 vol% and corresponding compressive strength from 1 to 22 MPa, contained a multi-modal microstructure with large spherical pores and small pores on internal walls. Thermal conductivity measurement carried out by the transient plane source technique at room temperature resulted in values as low as 0.09 W/mK. In addition, the relationship between thermal conductivity and porosity was discussed in detail.

Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding

Abstract: In the present work, mullite-bonded porous SiC ceramics were fabricated using reaction bonding techniques. The morphologies, phase composition, open porosity, pore size distribution and mechanical strength of porous ceramics were examined as a function of alumina sources (calcined nano-sized alumina powder and alumina sol prepared from hydrolysis of aluminum isopropoxide) and contents. It was found that the addition of alumina in powder form effectively enhanced the strength and decreased the porosity. In contrast, when alumina was added in sol form, a reverse effect was observed. Moreover, it was revealed that when a portion of calcined alumina was replaced by alumina sol, the mechanical properties improved significantly (more than 30%) as well as porosity compared to the traditional method. Pore size distribution analysis showed that the dispersion of nanosize alumina powder and SiC micro-particles in alumina sol is strongly improved compared to mixing in ethanol.

In Situ Mechanical Properties of Chamotte Particulate Reinforced, Potassium Geopolymer

Abstract: Geopolymers are an inorganic polymeric material composed of alumina, silica, and alkali metal oxides. Monolithic geopolymer is brittle and susceptible to dehydration cracking at elevated temperatures. The addition of a reinforcing phase not only improves strength and toughness but also maintains the structural integrity of the material at elevated temperatures. For this study, potassium-based geopolymer (KGP) is reinforced with varying weight percent of chamotte particles. Chamotte is kaolinite grade clay calcined at 1350°C to produce 38% crystalline mullite, as well as metastable cristobalite and quartz. The chemical composition of the chamotte is almost identical to that of the metakaolin used to create the geopolymer, however, its crystalline nature prevents reactivity with the caustic potassium silicate solution and it remains as a particulate reinforcement. Flexural strength is evaluated at room temperature and in situ at elevated temperatures to just below the leucite crystallization temperature. Reinforcement with 25 wt% chamotte has shown a two-fold increase in room-temperature flexural strength. Flexural strength is also evaluated at room temperature after heating above the leucite crystallization temperature to determine if the chamotte aids in maintaining structural integrity during the volumetric contraction and destructive transformation from cubic to tetragonal symmetry upon forming leucite.

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Abstract: Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.

Anisotropic mechanical properties and fracture mechanisms of textured h-BN composite ceramics

Abstract: The hexagonal boron nitride (h-BN) grain shows typical lamellar structures, so textured materials can be achieved by arranging h-BN grains along the same direction. In this work, textured h-BN composite ceramics with the c-axis orientation arranged along pressure direction were manufactured by hot-press sintering using mullite as the sintering additive. The results show that sintering pressure is an important factor that affects not only the density and the textured degrees of composite ceramics, but also the mechanical properties. Based on the textured microstructure features, the composite ceramics show the obviously anisotropically characterized mechanical properties, such as elastic modulus, flexural strength and fracture toughness, together with various fracture mechanisms.

Preparation of Porous Mullite Ceramics Using Fly Ash Cenosphere as a Pore-Forming Agent by Gelcasting Process

Abstract: Porous mullite ceramics were fabricated from an industrial grade mullite powder by gelcasting process using fly ash cenospheres (FAC) as a pore-forming agent. The influence of content of FAC and sintering temperature on the density and strength was evaluated. The microstructure showed that FAC can act as a sintering aid and a pore-forming agent. When the sintering temperature at 1200°C, porous mullite ceramics with a relatively high porosity (48.1–72.2%), low density (0.84–1.64 g/cm3), low thermal conductivity (0.16–0.22 W/m · K), and high compressive strength (6.21–14.70 MPa) have been obtained.

Thermal Conductivity of Very Porous Kaolin‐Based Ceramics

Abstract: A clay-based material exhibiting high pore volume fraction and low thermal conductivity suitable for thermal insulation is described. Starting with a commercial clay containing >75% kaolinite, foams were made by mixing in water and methyl cellulose as a surfactant then beating. After drying at 70°C, the pore volume fraction >94% remains almost constant for treatments up to 1150°C. In contrast, the phases constituting the solid skeleton evolve strongly with removal of surfactant, dehydroxylation of kaolinite, and formation of mullite. The latter leads to greater mechanical strength but also an increase in thermal conductivity. Thermal treatment of the kaolin foam at 1100°C yields a suitable compromise between low thermal conductivity of 0.054 W.(m.K)−1 at room temperature with a compressive yield stress of 0.04 MPa. The radiation component in the effective thermal conductivity is 50% at 500°C.

Thermal activation of coal fly ash by sodium hydrogen sulfate for alumina extraction

Abstract: The extraction of alumina from coal fly ash (CFA) is a good way to enhance its value-added utilization The presence of inert matter with a high degree of polymerization, such as mullite and other aluminosilicates, greatly restricts the reactivity of CFA Thus, the CFA needs to be activated before use Thermal activation by calcination with added sodium hydrogen sulfate (NaHSO4) was carried out in this study The calcination products were characterized by means of X-ray diffraction, thermal gravimetric and differential scanning calorimetric analysis, and scanning electron microscopy to assess the effects of the additive, calcining temperature, and calcining time on the products The results indicated that NaHSO4 facilitated the decomposition of the polymeric phases and significantly improved the alumina extraction Alumina in CFA was transformed into soluble Na3Al(SO4)3 When CFA was mixed with sodium hydrogen sulfate at a NaHSO4/Al2O3 molar ratio of 6, the alumina extraction reached 90 % at 500 °C after 2 h