蓝晶石精矿烧结样品中SiO_2成分相的存在形式

通过对蓝晶石精矿烧结样品的X射线衍射分析（XRD）和扫描电境（SEM）研究,发现蓝晶石转化成莫来石过程中分解出的SiO2存在形式与蓝晶石的较大的体积膨胀效应及原料的原始成分密切相关;并且,形成的方石英相含量多少及其结晶程度的高低与蓝晶石的快速分解阶段相对应,是蓝晶石精矿快速分解和膨胀温度（区间）的指示剂。     

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.     

Fluxing action of illite and microcline in a triaxial porcelain body

Comparative studies were made on two types of triaxial porcelain bodies; one used conventional potash feldspar (ST0) and the other used illite (IT0) as the fluxing agents. Scanning electron micrographs showed that densification and mullite formation in the IT0 body had already occurred at 1000 °C suggesting that there was enough liquid to assist densification and mullite crystal growth. In the ST0 body, mullite crystals started to emerge around 1100 °C, while potash feldspar completely melted around 1200 °C. Full vitrification of the ST0 body required the firing temperature of 1300 °C, which was around 50 °C higher than the IT0 body. The IT0 body showed superior strength to the ST0 body at all studied firing temperature ranges and thus was possibly due to (i) a better densification, (ii) a higher amount of interlocking mullite crystals and (iii) the smaller amounts of residual quartz.     

黏土的杂质含量对堇青石-莫来石复合材料相组成及性能的影响

以滑石细粉、高铝矾土细粉和两种不同杂质含量的黏土细粉为原料,经配料、加水混练成型后,分别于1300、1350和1400℃保温3h制备了堇青石-莫来石复合材料的试样,用XRD、SEM等研究了黏土中杂质含量对堇青石-莫来石复合材料相组成及性能的影响。结果表明:1)使用高杂质含量的黏土能有效降低试样的烧结温度,促进堇青石和莫来石晶体的生成发育,提高试样的体积密度与常温抗折强度,但对其抗热震性损害较大。2)使用高杂质含量的黏土时,提高MgO含量虽然可增加堇青石的含量,但玻璃相含量增加且黏度降低,不能有效提高材料的抗热震性;而使用低杂质含量的黏土时,通过控制引入MgO的含量与煅烧温度,可调节试样中晶相的含量与组成,从而可提高材料的抗热震性。     

Comparative study of the corrosion resistance of thermally sprayed ceramic coatings and their bulk ceramic counterparts

A comparative study of the corrosion properties of thermally sprayed ceramic coatings (Al₂O₃, Al₂O₃–TiO₂ with different ratios, mullite, and ZrSiO₄) and their sintered bulk ceramic counterparts was performed. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying processes. The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄ at 85 °C, respectively. The coating microstructures and phase compositions, as well as the corrosive environment were shown to have a strong effect on the corrosion resistance of the coatings. Al₂O₃–coatings were more sensitive to these factors than Al₂O₃–TiO₂ coatings were.The corrosion resistance of the bulk ceramics was superior to that of the thermally sprayed coatings. This is mainly because the coatings exhibited specific microstructure and contained amorphous and/or metastable phases not appearing in the bulk ceramics.     

Glass–ceramic frits for porcelain stoneware bodies: Effects on sintering,phase composition and technological properties

In the present work, the effects of glass–ceramic frits (10 wt.%) added to a porcelain stoneware body in replacement of non-plastic raw materials were evaluated simulating the tile-making process. Each glass–ceramic frit plays its own peculiar effect on the compositional properties and only some precursors behave as real glass–ceramic materials. The positive influence of glass–ceramic precursors in promoting the sintering stands out when temperature onset densification and sintering rate are considered: both of them are improved with respect to the reference body. The presence of glass–ceramic frits allows to preserve good technological properties, complying with the latest requirements of the industrial practice.     

Development of gold-bronze metallic glazes in a clay-based system for stoneware bodies

In this study, a triaxial glaze system consisting of red clay, kaoline, quartz, MnO, CuO and CoO is systematically developed to produce gold-bronze raw metallic glazes for stoneware bodies. At first, all of the glazed samples in the developed system were fired in an electrically-heated kiln at 1160 °C. Then, the selected successful gold-bronze metallic glazes were applied onto 3-D forms of stoneware bodies and fired at the same conditions. Microstructural characterizations of the glazes are done with scanning electron microscopy (SEM) and energy dispersive x-ray analyses (EDS). This study revealed that triaxial blending of the ceramic raw materials is a beneficial method for glaze production and gold-bronze surfaces are obtained in glazes G 9, G 26, and G 34. It is observed that chemical composition of the glazes directly influence the color and the amount of CuO is more significative than MnO for achieving gold-bronze effect.     

Immobilization of uranium in cristobalite ceramic through adsorption on mesoporous SBA-15 and further sintering process

The development of an adsorbent which can be easily transformed into stable ceramic waste forms through a simple route is necessary for the treatment of the radioactive wastewater. Herein, we report on the immobilization of uranium in cristobalite ceramic through adsorption on mesoporous SBA-15 and further sintering process. The mesoporous SBA-15 with short pore length was synthesized and employed to remove uranium from aqueous solution. Subsequently, the SBA-15 with adsorbed U (U/SBA-15) was solidified by sintering. The effects of sintering temperature and U content on the structure, densification and aqueous durability of the obtained cristobalite ceramic waste forms were investigated. The results indicate that the U/SBA-15 can be transformed into stable cristobalite ceramic after sintering at 1100–1400 °C for 6 h. Furthermore, all the obtained cristobalite ceramic waste forms exhibit good aqueous durability (∼10⁻⁴ g m^-2 d^-1). This work demonstrates a potential route and adsorbent to dispose the radioactive wastewater.     

Effect of pyrophyllite incorporation in porcelain composition on mechanical properties and microstructure

China clay (Kaolin) and quartz in the ratio of 1:2 was progressively replaced by pyrophyllite in a conventional porcelain mix. Addition of 15% pyrophyllite as a replacement of a combination of china clay and quartz decreased the linear shrinkage by 5.03% while fired strength was improved by about 31.5% compared to that of the conventional body fired at 1300 °C. Incorporation of pyrophyllite beyond 15% resulted in early vitrification of porcelain composition, although in lower proportion the effect is not so significant. Percentage of mullite was found to increase in the fired specimens even when kaolinite was progressively replaced by pyrophyllite. However, beyond 22.5% pyrophyllite addition, there occurred large volume of glass formation. Presence of large volume of glassy phase as well as formation of large pores of various shapes resulted in deterioration in ceramic properties. The size and shape of mullite crystals is to a large extent controlled by the fluidity of the liquid matrix from which they grow and this is again a function of temperature and composition.     

Synthesis,phase composition and characterization of Co-diopside ceramic pigments

Ceramic pigments in the system CaO – CoO – MgO – 2SiO₂ were synthesized through solid-state high temperature sintering at 1000, 1100 and 1200 °C. The starting compositions were determined from the stoichiometric mineral diopside, following the expression CaO.xCoO.(1-x)MgO·2SiO₂, where x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 0.7 and 0.9. The effect of sintering temperature and initial cobalt concentration on the phase composition and colour were studied by powder X-ray diffraction analysis, scanning electron microscopy (SEM), infrared (FT-IR), ultraviolet–visible - (UV–Vis) and electron paramagnetic resonance (EPR) spectroscopy. Poly-phase ceramics were obtained depending on synthesis parameters, which in addition to Co-diopside, may contain wollastonite, Co-åckermanite – Ca₂(Co, Mg)Si₂O₇, Co-olivine – CoMgSiO₄, Co-monticellite – Ca(Mg, Co) SiO₄, and cristobalite. Quantitative ratios of detected mineral phases and the degree of cobalt isomorphic substitution were determined by applying Rietveld refinement approach. Trends and consistencies were outlined revealing that concentration of cobalt in the initial synthesis batch and its isomorphous incorporation in the crystal structure of diopside affect the structural perfection of the cation polyhedra. This fact obviously plays a key role in controlling the amount of the main constituent phase in the run-product. Ceramics of highest diopside content and lowest measured degree of structural distortion were only formed at 1200 °C and initial cobalt concentration in the range x = 0.2–0.3. These samples are pink in colour and have the highest value of red coordinate in the CIELab system due to Co²⁺ ions in octahedral coordination in diopside structure.     

Preparation and characterization of a nigerian mesoporous clay-based membrane for uranium removal from underground water

This study reports the removal of uranium in underground wastewater using a Nigerian clay-based membrane. The clay and sintered clay were characterized using XRD, XRF, TGA/DTA, FESEM and PSD. The raw clay was mixed with cassava starch (10, 15, 20 and 25 wt%) and sintered at a temperature of 1300 °C. A multi-point BET analysis of the produced clay-based membranes was conducted to determine the surface area, pore volume and average pore size. Sintering characteristics were determined by apparent porosity, bulk density and flexural strength. The radioactivity of the feed and the permeated water was counted using a gamma spectrometer with an HPGe detector. From the XRD, TGA and FESEM, 1300 °C was found to be optimum for the mullite formation from the clay. The average pore sizes of the produced membranes from the BET results were observed to be in the range from 51 to 70 Å and with a steady state flux range of the tested membranes in the range 1.92×10⁻⁵–2.09×10⁻⁴ m³ m⁻² s⁻¹. The permeation flux produced is of high quality with a rejection in the range of 1.78–2.56 Bq/l of the uranium activity by the tested membranes. This low-cost membrane will have an application for the treatment of uranium-containing wastewater from fracking, oil exploration and phosphate mining industries.     

Sintering mechanism and properties of corundum-mullite duplex ceramic with MnO2 addition

Corundum-mullite duplex ceramic (CMC) is excellent engineering material that can be strengthened by the addition of various compounds. In this study, the sintering mechanisms of CMC with two nAl₂O₃: mSiO₂ stoichiometric ratios (n = 1, m = 0; n = 3, m = 2) were investigated with different amounts of MnO₂ addition. The phase transition, microstructure, distribution of elements and ionic valence of the ceramics were identified via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The results show that Al³⁺ ions in the corundum lattice are replaced by Mn²⁺ ions. The distorted crystal structure could promote the sintering reaction and leads to a lower sintering temperature. Hence, this study provides beneficial guidance for the industrial production of CMC at lower temperatures and, therefore, with lower energy consumption.     

Effect of mullite phase formed in situ on pore structure and properties of high-purity mullite fibrous ceramics

Porous mullite ceramics are potential advanced thermal insulating materials. Pore structure and purity are the main factors that affect properties of these ceramics. In this study, high performance porous mullite ceramics were prepared via aqueous gel-casting using mullite fibers and kaolin as the raw materials and ρ-Al₂O₃ as the gelling agent. Effects of addition of mullite fibers on the pore structure and properties were examined. The results indicated that mullite phase in situ formed by kaolin, and ρ-Al₂O₃ ensured the purity of mullite samples and mullite fibers bonded together to form a nest-like structure, greatly improving the properties of ceramic samples. In particular, the apparent porosity of mullite samples reached 73.6%. In the presence of 75% of mullite fibers, the thermal conductivity was only 0.289 W/m K at room temperature. Moreover, the mullite samples possessed relatively high cold compressive strength in the range of 4.9–9.6 MPa. Therefore, porous mullite ceramics prepared via aqueous gel-casting could be used for wide applications in thermal insulation materials, attributing to the excellent properties such as high cold compressive strength and low thermal conductivity.     

Corrosion of selected ceramic materials in hot gas environment

The temperature dependence of the hot gas corrosion behaviour of various ceramic materials (Al₂O₃, ZrO₂ (Y-TZP), mullite, ZrSiO₄ and YAG) was investigated. The tests were performed in a high temperature burner rig at temperatures between 1200 °C and 1500 °C, a total pressure of 1 atm with a water vapour partial pressure of 0.24 atm, a gas flow velocity of 100 m/s and test times of about 130–300 h.

ZrO₂ (Y-TZP) showed absolutely no corrosion, however, a very high susceptibility to thermal shock and phase transformation was observed.

The other materials suffered degradation above 1300 °C. This was the consequence of the formation and evaporation of volatile hydroxides (e.g. Si(OH)₄ and Al(OH)₃). YAG showed a low corrosion rate and the formation of a protective surface layer. The corrosion susceptibility of these materials was found to be higher with increasing temperature.

Thermochemical calculations of the partial pressure of volatile species formed in reaction with water vapour, affirm the observed differences in corrosion behaviour.     

Effect of heat treatment on phase composition and properties of modified alumina ceramic

Translated from Steklo i Keramika, No. 11, pp. 17–19, November, 1989.     

Mechanical and kinetic studies on the refractory fused silica of integrally cored ceramic mold fabricated by additive manufacturing

A refractory fused silica based integrally cored ceramic mold, the ceramic core with a ceramic mold shell in a single patternless construction, is fabricated by ceramic stereolithography of additive manufacturing. Refractory ceramic molds should satisfy the following restrictions such as similar strength to that of cast metal during solidification, thermal stability for dimensional accuracy, and easy removal of core after casting. Here, we report mechanical and transformation kinetic studies on the refractory fused silica of integrally cored ceramic mold. The flexural strength of sintered silica continually increases with higher density of better densification up to 11.4 MPa at 1300 °C, while it decreases from 11.3 MPa at 1350 °C to 4.6 MPa at 1500 °C. The degradation of the flexural strength is related to the larger amount of the cristobalite and microcracks generated by the abrupt contraction induced during the transformation of beta to alpha cristobalite. Given the quantitative x-ray diffraction study on transformation kinetics, an apparent activation energy Q is 674 ± 53 kJ/mol and the average time exponent 1.85, suggesting that the transformation kinetic is controlled by 1-dimensional interfacial growth.