Impact of microwave processing on porcelain microstructure

Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by ~ 75 °C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 °C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (~ 32 ± 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having ~ 63 mol% alumina content compared to ~ 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al⁺³ diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (~ 900–1000 °C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.     

Mullite whiskers derived from kaolin

Short mullite whiskers prepared by firing compacts of kaolin and NH₄Al(SO₄)₂·12H₂O powders, with a small addition (0.8, 1.5 wt%) of NaH₂PO₄·2H₂O, in air 1300 and 1400 °C for 15 h have been characterized in terms of whisker morphology, composition and structure. Relatively uniform whisker shaped crystals grew within the silicate glass matrix. After chemically leaching the glass matrix with HF solution using a microwave heating source, the resulting whiskers were exposed as isolated crystals and exhibited an aspect ratio of >17 (～0.5 μm in diameter). The mullite whiskers had a composition of 51.06 mol% Al₂O₃ and 48.94 mol% SiO₂, with an orthorhombic crystallographic structure.     

Kinetic of mullite formation from a porcelain stoneware body for tiles production

The growth of mullite (3Al₂O₃·2SiO₂) in a porcelain stoneware body for tiles production has been investigated using differential thermal analysis (DTA). The activation energy calculated by both isothermal and non-isothermal treatments is 599 and 622 kJ mol⁻¹, respectively. The growth morphology parameters n and m are both about 1.5 indicating that bulk nucleation is dominant in mullite crystallisation followed by three-dimensional growth of mullite crystals with polyhedron-like morphology controlled by diffusion from a constant number of nuclei. The frequency factor calculated by the isothermal treatment is equal to 8.21 × 10²² s⁻¹.     

Effect of microstructure on mechanical properties of porcelain stoneware

This work examines the effect of microstructure (aspect ratio of mullite crystals and proportion of crystalline and amorphous phases) as well as different physical features (bulk density, closed and open porosity and absolute density) on the mechanical properties of a standard porcelain stoneware composition (50% kaolinitic clay, 40% feldspar and 10% quartz) fired in the 1200–1300 °C temperature interval using a fast firing schedule. The mechanical behaviour was evaluated in terms of bending strength, Vickers microhardness, fracture toughness and Young's modulus. After viewing the results, it can be concluded that increased σ_f, H_v and E values were mainly due to open porosity, percentage of mullite phase and morphology of secondary mullite needles, whereas closed porosity and quartz particles have no influence on these properties.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

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.     

High pressure densification of nanocrystalline mullite powder

Investigations of the high-pressure sintered nanocrystalline mullite powder are presented. The synthesized mullite powder with crystallite size of 51 nm was densified by using high-pressure “anvil-type with hollows” apparatus at 4 GPa over the temperature range of 1100–1500 °C in 100 °C steps. The phase composition and structural parameters of the densified samples were studied as a function of densification temperature. The XRD analysis revealed the appearance of new phases, such as kyanite and corundum, whose development affected the densities of the sintered samples. High relative densities of the sintered samples were obtained because of the application of high pressure. The needle-like microstructure was developed owing to the anisotropic grain growth of mullite. The elongated mullite grains reached the length of approximately 5 µm at 1400 °C, whereas the grains treated at 1500 °C became thicker preserving the same needle length. The Vickers microhardness of the developed microstructures increased with the increase of temperature up to 1400 °C, while at 1500 °C it was slightly reduced due to the grain coarsening.     

Influences of quartz and muscovite on the formation of mullite from kaolinite

A kaolin containing muscovite and quartz (K-SZ) and a pure kaolin (K-SX) with the addition of potassium feldspar, K₂SO₄ and quartz, respectively, were used to investigate the influences of muscovite and quartz on the formation of mullite from kaolinite in the temperature range 1000–1500 °C. In K-SZ formation of mullite began at 1100 °C, and in K-SX at 1000 °C. In K-SZ quartz accelerated the formation of cristobalite and restrained the reaction of mullite and silica. Muscovite in K-SZ acted as a fluxing agent for silica and mullite before 1400 °C and accelerated the formation of cristobalite. The FTIR band at 896.8 cm^− 1 was used to monitor the formation of orthorhombic mullite.     

Glass-ceramic glazes for future generation floor tiles

Glaze in the CaO–MgO–Al₂O₃–SiO₂ system was heated at 950–1190 °C for 2 h and characterized. X-ray diffraction showed that only trace amount of mullite was formed in the glass-ceramic glaze heated at 950 °C. Both mullite and α-cordierite were formed in the glass-ceramic glaze heated at 1050 °C as primary and secondary phases. Glass-ceramic glazes heated at 1120 °C and 1190 °C contained α-cordierite and mullite as major and minor phases. Rietveld analysis revealed that the amount of α-cordierite increased and mullite decreased with increasing heating temperature. Field emission scanning electron microscopy showed presence of mullite crystals dispersed within residual glassy phase in the glass-ceramic glazes heated at 950 °C and 1050 °C. In the microstructures of glass-ceramic glazes heated at 1120 °C and 1190 °C α-cordierite crystals were mainly appeared. Energy Dispersive X-ray analysis corroborated X-ray diffraction results. Vickers microhardness measurement demonstrated highest hardness (8.38 ± 0.07 GPa) of the glass-ceramic glaze heated at 1190 °C.     

Optimization of the technological properties of porcelain tile bodies containing rice straw ash using the design of experiments methodology

This study examines the effects of replacing fluxing and filler materials with rice straw ash (RSA) in manufacturing porcelain stoneware tile, using the design of experiments (DOE) methodology. The results of the characterization were used to obtain statistically significant, valid regression equations, relating the technological properties of the dried and fired test pieces to the raw materials content in the unfired mixtures. The regression models were analysed in relation to the X-ray diffraction and scanning electron microscopy results and used to determine the most appropriate combinations of traditional raw materials and RSA to produce porcelain stoneware tiles with specific technological properties. The studied range of tile body compositions: clay (40 wt%), feldspar (20–50 wt%), feldspathic sand (5–20 wt%), and RSA (0–25 wt%) was shown to be appropriate for porcelain stoneware tile manufacture.     

Crystalline phases and physical properties of modified stoneware body with glaze sludge

Ceramic stoneware body has been modified with ceramic and glass industrial wastes by replacing 25–100% as flux in the formula. The effects of solid wastes added to the bodies were studied after firing in the temperature range 950–1280 °C. The physical properties of linear shrinkage, bulk density, apparent porosity, water absorption and 3-point bending strength were determined. A composition which related to the general stoneware properties was found when using soda-lime cullet and glaze sludge. It had a firing range lowered to 1050–1100 °C. SEM results demonstrated the sintered microstructure increased in density with increase in solid waste in the modified body. XRD results after firing showed the crystalline phases comprised of mullite, albite calcian and quartz. Thermal expansion was measured in the range 6.53–6.67 × 10^?6 K^?1 at 30–500 °C. The modified bodies were capable of forming prototype products by slip casting and jiggering.     

Utilization of recycled paper processing residues and clay of different sources for the production of porous anorthite ceramics

Production of porous anorthite ceramics from mixtures of paper processing residues and three different clays are investigated. Suitability of three different clays such as enriched clay, commercial clay and fireclay for manufacturing of anorthite based lightweight refractory bricks was studied. Porous character to the ceramic was provided by addition of paper processing residues (PPR). Samples with 30–40 wt% PPR fired at 1200–1400 °C contained anorthite (CaO·Al₂O₃·2SiO₂) as major phase and some minor secondary phases such as mullite (3Al₂O₃·2SiO₂) or gehlenite (2CaO·Al₂O₃·SiO₂), depending on the calcite to clay ratio. Anorthite formation for all clay types was quite successful in samples with 30–40 wt% of paper residues fired at 1300 °C. A higher firing temperature of 1400 °C was needed for the fireclay added samples to produce a well sintered product with large pores. Gehlenite phase occurred mostly at lower temperatures and in samples containing higher amount of calcium (50 wt% PPR). Compressive strength of compacted and fired pellets consisting of mainly anorthite ranged from 8 to 43 MPa.     

Influence of iron on the occurrence of primary mullite in kaolin based materials: A semi-quantitative X-ray diffraction study

Iron-enriched reference kaolins (KGa-1b, KGa-2 and KF) were used to study the effect of iron on the development of mullite phases during the sintering of kaolin-based materials. Up to 1050 °C, primary mullite formation occurred at earlier temperature within iron-enriched kaolins than in the case of iron-free kaolins. At 1150 °C, the presence of ferric ions tended to promote the transformation of the spinel (γ-Al₂O₃-like) phase into primary mullite. This action was correlated with an enhancement of the diffusion mechanism of the main constitutive species of the samples (Al, Si). In the range 1300–1400 °C, iron-enriched kaolins exhibited an abnormal grain growth of secondary mullite crystals and a partial reduction of hematite (Fe₂O₃) into magnetite (Fe₃O₄). These two iron compounds reacted with mullite and cristobalite, leading to the occurrence of eutectic liquids as expected from phase equilibrium diagrams.     

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.     

Porous mullite blocks with compositions containing kaolin and alumina waste

In the production of alumina by the Bayer process, the calcination step generates a waste containing ~90% aluminum oxide (Al₂O₃). Due to the high content of this oxide, this waste can be used as a source of alumina in porcelain formulations, especially those used in the synthesis of mullite. The purpose of this study was to produce porous mullite blocks using compositions containing kaolin and alumina waste. The compositions were formulated based on a mullite stoichiometry of 3:2. Heat treatments were carried out in a conventional furnace at temperatures of 1450 to 1500 °C, applying a heating rate of 5 °C/min and a 1-h hold time at the firing temperature threshold. The powders were characterized by means of X-ray fluorescence (XRF); X-ray diffraction (XRD); thermal analysis (TGA-DTA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physic mechanical properties of the test specimens: water absorption, apparent porosity, linear shrinkage and flexural strength were also evaluated. The XRD results revealed the formation of mullite as the major phase. The morphological analysis by SEM revealed typical mullite needles originating from clay minerals. The size of the mullite needles was calculated based on the TEM analysis, which indicated diameters smaller than 400 nm, confirming the nanometric dimensions of the needles. The flexural strength test of the specimens indicated that this parameter ​​tends to increase as the temperature is raised.     

Mechanical,morphological and dielectric properties of sintered mullite ceramics at two different heating rates prepared from alkaline monophasic salts

The sintering behavior of nanocrystalline orthorhombic mullite powders was investigated. The changes in microstructure, mechanical and dielectric properties with two different heating rates were explained. Microstructural characteristics depending on heating rate were explained at different sintering temperatures. Dielectric properties of prepared mullite nanocomposites were studied to examine the synthesized mullite ceramics as high permittivity materials in the microwave range. It was indicated that a sharp decrease in bulk density was observed at 1600 °C due to the exaggerated growth of mullite grains. Moreover, a maximum hardness value of 4.97 GPa was obtained at 1600 °C with slow heating rate (5 °min^?1). The DC electrical resistivity with a slow heating rate at 1300 °C was approximately three times the value of the mullite sample sintered with a fast heating rate (30 °min^?1). The minimum dielectric loss of about 0.017 at 1.5 GHz was achieved at a sintering temperature of 1500 °C with a slow heating rate.     

Mullitization,microstructure and physical properties of mechanically activated andalusite sintered by microwave

In this work, the effects of mechanical activation and microwave heating of andalusite on mullite formation have been investigated. XRD results revealed that andalusite peaks disappeared after 60 h of milling and the peaks of alumina were observed. The formation of mullite from activated and as-received andalusite occurred at 800° and 1250 °C, respectively, while mullitization was completed at 1100° and 1400 °C in the former and latter, respectively. Mullite samples prepared from activated andalusite showed better densification with an elongated morphology.     

Preparation and characterization of glazes from combinations of different industrial wastes

The work reported here involves the preparation and characterization of ceramic glazes made from combinations of different industrial wastes. The wastes were float glass, granite and lime shale (a raw material waste from the oil shale industry in São Mateus do Sul, state of Paraná, Brazil), which were used to replace natural raw materials in a proportion of up to 50% in weight. The compositions were formulated using the Seger method and prepared by conventional ceramic processing. The stabilized suspensions were applied in commercial wall tile and porcelain stoneware tile, which were sintered at temperatures of 1080 °C and 1150 °C, respectively, using two different heating cycle. Three compositions were developed, two of which yielded opaque glazes and one a transparent glaze. Linear thermal expansion coefficients (α) of 80.10^?7 °C^?1 to 100.10^?7 °C^?1, and glaze softening temperatures of 600–700 °C were characterized by dilatometric analysis. The glaze compositions showed chemical resistance against acid and alkaline attack after 96 h, showing a mass loss of less than 0.1% in weight. The surface hardness of the glazes determined by the Mohs scale, Vickers microhardness and abrasion resistance (PEI indices) were between 6–7, 3–3.7 GPa and 3–4, respectively. These properties are compatible with those of commercial glazes for wall tiles and porcelain stoneware ceramics.     

Effect of boron oxide on the microstructure of mullite-based glass-ceramic glazes for floor-tiles in the CaO–MgO–Al2O3–SiO2 system

The effect of increasing replacement of Al₂O₃ by B₂O₃ in a parent glass on the sintering and further crystallization of mullite was investigated. The composition of the parent glass was chosen in the mullite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system. Glass powder pellets were heated under standard (10 °C/min and 2 h of hold time) and fast heatings (25 °C/min and 5 min of hold time) at different temperatures from 700 to 1190 °C. Sintering of B₂O₃-containing glasses took place in the range between 850 and 1050 °C. X-ray diffraction results showed that mullite formed as unique crystalline phase for glasses containing amounts of B₂O₃ larger than 6 wt%. For lower amounts of boron oxide cordierite was formed as secondary crystalline phase. Quantitative determination of mullite by Rietveld analysis indicated that the higher amount of mullite present in the glass-ceramic fast heated at 1160 °C was 19.5 wt% for the glass containing 9 wt% of B₂O₃. The final microstructure of the glass-ceramic glazes showed the presence of well shaped, long acicular mullite crystals dispersed within the residual glassy phase. Results of glass-ceramic glazes when applied as slurry and under industrial heating conditions pointed out promising mechanical properties.     

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

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.