The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite

The influence of nano-silica and zircon additions on the sintering and mechanical properties of in situ formed forsterite fired at 1550 °C for 2 h was investigated. The results indicated that, nano-silica improved in situ formed forsterite at the firing temperature, while zircon additions enhanced the sintering of the investigated samples. XRD analysis and SEM examination observed a good crystallinity of in situ formed forsterite with nano-silica and/or zircon additions. Densification parameter (BD ∼3.22 g/cm³ and AP ∼5.82%), cold crushing strength (CCS ∼285 MPa) and micro-hardness (Hv ∼660) were enhanced with zircon additions.     

Development of fireclay aluminosilicate refractory from lithomargic clay deposits

Lithomargic clay until now has not been utilised to produce refractory bodies due to its low plasticity. In this work, the development and evaluation of fireclay refractory material produced from lithomargic clay deposit has been studied by addition of clay binder. Three formulations were prepared by mixing, semi-dry moulding, drying and firing at temperatures ranging from 1200 to 1400 °C. The fired samples were investigated to determine their physical properties such as bulk density, apparent porosity, linear firing shrinkage, and cold crushing strength. The chemical and mineralogical compositions were also determined. The results show that the linear firing shrinkage values were within limits acceptable for refractory clays. The cold crushing strength increases as temperature increased to 1400 °C. Cold crushing strength increased with increasing binder content. The increase of the highly refractory phases (cristobalite and mullite) and the densification of the bricks due to the presence of fluxing agents were responsible for the high cold crushing strength values. The investigated properties indicate that lithomargic clay underlying bauxite deposits could be used to produce fire clay aluminosilicate refractories.     

Chromium-promoted preparation of forsterite refractory materials from ferronickel slag by microwave sintering

The chromium-promoted preparation of forsterite refractory materials from ferronickel slag was investigated by microwave sintering of the slag with the additions of sintered magnesia and 0–10 wt% chromium oxide (Cr₂O₃). The thermodynamic calculations revealed that the addition of Cr₂O₃ can promote the formations of spinel and liquid phase and maintain high content of forsterite below 1500 °C. The experimental results showed that there existed a stronger promoting effect of Cr₂O₃ additive on the properties of refractory materials in the microwave field than that in conventional sintering. It was attributed to the preferential formation and growth of spinel with stronger microwave absorption than other phases (e.g., enstatite), the existence of more forsterite, and the enhanced densification in association with the presence of more liquid phase at the same temperature. By microwave sintering of the mixture of ferronickel slag, 25 wt% sintered magnesia, and 4 wt% Cr₂O₃ at 1350 °C for 20 min, a superior refractory material with refractoriness of 1801 °C, thermal shock resistance of 6 times, bulk density of 2.97 g/cm³, apparent porosity of 1.4%, and compressive strength of 197 MPa was obtained. Compared with that prepared by conventional sintering at 1350 °C for 2 h, the refractoriness and thermal shock resistance were increased by 175 °C and 100%, respectively. The present study provided a novel method for preparing high-quality refractory materials from ferronickel slag and relevant industrial wastes.     

Preparation and properties of porous ceramic aggregates using electrical insulators waste

In this paper, porous ceramic aggregates were prepared by electrical insulators waste (EIW). Effects of sintering temperature and content of EIW on the aggregates’ properties such as bulk density, and apparent porosity, total porosity, and cold crushing strength were investigated. With increasing sintering temperature and content of EIW, bulk density and cold crushing strength of the aggregates increased, apparent porosity and total porosity decreased. Based on these results, total porosity of specimens in group B sintered at 1200 °C is 62.0%, cold crushing strength is 35.3 N, and thermal conductivity is 0.165 W/(m K) at 300 °C. Comprehensive properties of specimens can be optimized by adjusting sintering temperature. Meanwhile, strength variation resulted from the combined effects of phase transformation and matrix densification under different sintering temperatures.     

High alumina castables reinforced with SiC

Abstract

To improve the sintering, mechanical and refractory properties of high alumina castables, different contents of SiC (up to 8 wt.-%) were added at the expense of high alumina cement. Cold crushing strength of the green samples was measured and hydration behaviour was studied using differential thermal analysis (DTA). After firing at 1550°C for 3 h, the compositions of the fired samples were investigated using X-ray diffraction. Sintering parameters (bulk density and apparent porosity) and mechanical (cold crushing strength) as well as refractory (thermal shock resistance, permanent linear change, load bearing capacity) properties were tested according to standard specifications. It was concluded that increasing content of added SiC results in an improvement in sintering, mechanical and refractory properties but adversely affects green strength. However, castables containing 6 wt.-% SiC show a reasonable compromise between acceptable green strength and improved sintering, mechanical and refractory properties. The improved properties of the fired samples are related to the formation of SiC-mullite system in the matrix.     

The physical and mechanical properties of alumina-based ultralow cement castable refractories

In this study, the effects of the type of alumina on the physical, chemical and mechanical properties of the ultralow cement castable (ULCC) refractories were investigated. Brown fused alumina, tabular alumina and rotary bauxite-based ULCC refractories were prepared by mixing each type of alumina with silicon carbide, carbon, cement, metallic silicon and microsilica. The density, porosity and cold crushing strength (CCS) of the refractory castables were measured after drying at 110 °C for 24 h and firing at 1450 °C for 5 h. The slag penetration resistance of the refractory castables was determined using slag corrosion tests. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the castables. It was found that all three refractory castables had strong slag penetration resistance and that the tabular alumina-based refractory castable had the largest specific cold crushing strength with an acceptable percent of porosity among the refractory castables.     

Fabrication of ultralight heat-insulating hollow-fiber mullite fiberboards

Mullite fiberboards have been extensively used in heat-insulating refractory materials. To further improve the thermal insulation properties and reduce the density, we fabricated mullite fiberboards by vacuum filtration using hollow mullite fibers based on a ceiba fiber template. The effects of sintering temperature and type and content of high-temperature adhesives on the density, compressive strength, thermal conductivity, microstructure, and volume stability of mullite fiberboards were investigated. The results showed that the obtained mullite fiberboards have ultralow density of 0.1–0.2 g/cm³, low thermal conductivity of 0.0988–0.1230 W/(m·K) (at 500 °C), and compressive strength of 0.08–0.12 MPa, and they exhibit good volume stability at 1300 °C.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Sinterability of Forsterite Prepared via Solid‐State Reaction

In this work, the sinterability of forsterite powder synthesized via solid‐state reaction was investigated. X‐ray diffraction (XRD) analyses indicate that the synthesized powder possessed peaks that correspond to stoichiometric forsterite. Scanning electron micrographs revealed that the powders were formed agglomerates, which were made up of loosely packed fine particles. Subsequently, the forsterite powders were cold isostatically pressed into a disk shape under 200 MPa and sintered in air at temperature ranging from 1200°C to 1500°C (interval of 50°C) with ramp rate of 10°C/min and dwelling time of 2 h. The sinterability of each sintered samples was examined in terms of phase stability, relative density, Vickers hardness, fracture toughness, and microstructural examination. XRD examination on all the sintered samples exhibited pure forsterite, in which the generated peaks were found to be in a good agreement with JCPDS card no. 34‐0189. The densification of forsterite progressed to reach a maximum relative density of ~91% at 1500°C. This study also revealed that high‐strength forsterite ceramic can be synthesized via solid‐state reaction as forsterite attained favorable mechanical properties, having fracture toughness of 4.88 MPam^1/2 and hardness of 7.11 GPa at 1400°C.     

The use of bagasse in the preparation of fireclay insulating bricks

Acidic insulating refractory bricks were prepared by mixing kaolin and grog with polystyrene (PS) beads up to 1.5% and bagasse up to 5% (by weight). This latter is a fibrous waste residue from the sugar cane industry. The bricks were dried and fired at 1250°C in a cycle lasting for 16 hours. The bricks were tested for water absorption, porosity, bulk density, cold crushing strength and thermal conductivity at three different temperatures (400, 600, and 800°C). It was concluded that bricks containing 1% PS and 3% bagasse abided by ASTM C155-97 for C-30 type insulating refractories with a bulk density of 1.01 g/cm³, a cold crushing strength of 4.08 MPa, and a thermal conductivity of 0.37 W/K/m at 800°C.     

Effect of binder type and other parameters in synthesis of magnesite chromite refractories from process waste

Recycling of the process waste in basic refractory production will not only make it possible to put the waste substances to use but also help to solve the problems of environmental pollution and storing. This paper is the first part of a study on using the magnesite and chromite dusts in refractory production, accumulated in −10⁻³ m and finer particle fraction as process waste at the Konya Chrome-Magnesite Plant. In this work, three different MgO and Cr₂O₃ compositions were studied. Magnesite ore is added to the mixture without any thermal process. The type and proportion of the bond to be used, particle size distribution of the magnesite and chromite ores and the influence of compaction pressure on the refractory properties were examined. Consequently the influence of the changes in the mixture composition and sintering temperature on refractory properties was studied. The results of the experiments revealed the optimum type and content of the bond as MgSO₄·7H₂O and 8%, and optimum pressing pressure of the materials containing raw magnesite at 250 MPa. It was observed that when the chromite content of the material composition increased from 10 to 28% and 50%, the cold crushing strength (CCS) of the material has decreased, yet its porosity (P%) increased. This improves when the sintering temperature increased from 1450 to 1550 °C and 1750 °C. The optimum sintering temperature was found at 1750 °C     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Investigation of slag containing refractory materials for gasification processes

In gasification processes the reaction of carbon feedstocks with oxygen and water leads to production of synthetic gas. The development of a new gasification technique reduces the temperature at the liner wall in the range of 1300 °C. Thus, the currently used high chrome oxide based materials can be replaced by new chrome oxide free materials fulfilling economical as well as ecological aspects. In this contribution the performance of alumina castables with different brown coal ash-contents (slag containing refractory materials) and sintered in different atmospheres have been investigated according to their thermo-mechanical properties, phase formation as well as resistance against thermal shock and corrosion attack under gasification-similar conditions. The slag containing refractory material with 11 wt.% brown coal ash sintered in reducing atmosphere has shown the best results with regard to mechanical properties and corrosion resistance and will be a promising liner material for gasification processes up to 1400 °C.     

Characterization of 3Y-TZP/TiO2 hybrid experimental dental ceramics

The addition of titania to zirconia dental implants has been considered a promising choice to improve its bioactivity. This study aimed to evaluate the effect of different sintering conditions on the microstructure, density, optical properties and flexural strength of a 3Y-TZP/TiO₂ dental ceramic based on zirconia with two different titania contents (7.5 mol% and 12.5 mol%). 3Y-TZP/TiO₂ ceramic powders were synthesized by coprecipitation, uniaxially pressed and sintered at six different sintering conditions. Microstructural analysis of the sintered samples was performed by scanning electron microscopy and X-ray diffraction. Optical properties were measured using a spectrophotometer. The density was determined by Archimedes principle. Flexural strength was estimated by the biaxial flexure device. The microstructure and flexural strength of the 3Y-TZP/TiO₂ dental ceramic with 7.5% and 12.5 mol% were affected by the sintering conditions. Sintering the specimens at 1460 °C for 2 h increased the grain size and significantly decreased the flexural strength of 3Y-TZP/TiO₂ dental ceramic. The interaction (titania content x sintering conditions) affected the relative density and optical properties. A relative density greater than 98% was achieved for the T7.5 groups (sintered at 1260 °C/1 h, 1300 °C/1 h and 1300 °C/2 h) and for the T12.5 groups (sintered at 1260 °C/1 h, 1260 °C/4 h, 1300 °C/1 h and 1300 °C/2 h). The highest values of L*, a* and b* were respectively 87.2 (T7.5 group sintered at 1460 °C/2hs), 4.3 (T12.5 group sintered at 1300 °C/2hs) and 15.8 (T12.5 group sintered at 1300 °C/1 h). The material developed with 12.5 mol% of titania and sintered at 1300 °C/2 h showed high densification, flexural strength of 670 MPa and has good potential to be used in dentistry.     

Investigation on properties of Al2O3–SiO2 complex shaped refractory fabricated by layered extrusion forming

As one of the 3D printing methods, layered extrusion forming (LEF) has distinct advantages to form complex configuration ceramics directly. The feasibility of using LEF to make refractory products with complex shapes was explored by this work, using water-based Al₂O₃–SiO₂ ceramic slurry and specially equipped device. By measuring rheological parameters, the effects of binder addition, dispersant addition and volume proportion of the solid portion composed of α-Al₂O₃ ultrafine powder (92 wt%) and silica fume (8 wt%) on rheological behavior of the slurry were investigated. The green body specimens prepared by the LEF were fired at 1400°C–1600 °C for 3h. The influence of firing temperature on phase composition, microstructure, sintering degree and comprehensive properties of the specimens was investigated. At 2.5 wt% addition of aluminum dihydrogen phosphate as binder, 0.2 wt% addition of sodium hexametaphosphate as dispersant and with solid portion between 56 vol% and 58 vol%, required pseudoplastic behavior of the slurry can be achieved, suitable for the LEF. With the increase of heating temperature, mullitization by the reaction between the α-Al₂O₃ ultrafine powder and silica fume becomes stronger and sintering gets enhanced, leading to improved comprehensive properties of the specimens. Fired at 1600 °C, properties in terms of bulk density 3.03g/cm³, cold compressive strength 190.5 MPa and refractoriness under load 1598 °C are achieved. Crucible slag test shows a good resistance to the glass melt corrosion. Good feasibility of fabricating some complex shaped refractory products by LEF as a novel forming approach has been confirmed by the present work.     

Processing and properties of cordierite-silica bonded porous SiC ceramics

Cordierite-silica bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC with cordierite sol followed by sintering at 1300–1400 °C in air. The porosity, average pore diameter and flexural strength of the ceramics varied 30–36 vol%, ~ 4–22 µm and ~ 13–38 MPa respectively with variation of sintering temperature and SiC particle sizes. In the final ceramics SiC particles were bonded by the oxidation-derived SiO₂ and sol-gel derived cordierite. The corrosion behaviour of sintered SiC ceramics was studied in acidic and alkaline medium. The porous SiC ceramics were observed to exhibit better corrosion resistance in acid solution.     

Hot pressing of bimodal alumina powders with magnesium aluminosilicate (MAS) addition

High quality alumina ceramics were fabricated by hot-pressed sintering using bimodal alumina with superfine component as raw material and magnesium aluminosilicate (MAS) glass as sintering aid. Densification behavior, microstructure evolution and mechanical properties of alumina were investigated from 1300 °C to 1450 °C. The bimodal alumina powders were sintered to 99.8% of the theoretical value at 1400 °C and a comparative dense microstructure with a few plate-like abnormal grains was observed. With increase of sintering temperature up to 1450 °C, many fine matrix grains were consumed and quite a few abnormal grains impinged upon each other. For the alumina ceramics hot-pressed from bimodal alumina with 30 wt.% superfine component, optimal mechanical properties were obtained at 1400 °C. The bending strength and fracture toughness were 522 MPa and 5.0 MPa m^1/2, respectively.     

Densification and Properties of Fe2O3 Nanoparticles added CaO Refractories

Up to 8 wt. % of Nano-iron oxide was added to CaO refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. The mechanical behavior was studied by a cold crushing strength (CCS) and flexural strength at 1200 °C test. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe₂O₃ (C₂F), CaO.Fe₂O₃ (CF) and 3CaO.Al₂O₃ (C₃A) phase’s formation, which improved the sintering process. Nano-iron oxide also influenced the bonding structure through a direct bonding enhancement. On the Other hand, the presence of Nano-iron oxide resulting in improvement properties of CaO refractory matrix refractories such as bulk density, hydration resistance and cold crushing strength. The maximum flexural strength at 1200 °C is achieved by the samples containing 4 wt. % nano-Fe₂O₃.     

Fabrication of MgO-CaZrO3 refractory composites from Egyptian dolomite as a clinker to rotary cement kiln lining

Dolomite is used as basic lining for rotary cement kiln due its high refractoriness, corrosion resistance against basic environments and high coating performance, but its poor hydration resistance limits its use. In this work, Egyptian dolomite was converted into outstanding refractory magnesia-calcium zirconate composite (M-CZ) by the addition of 37.8–47.8?wt% zirconia. The mixtures were milled, uni-axially formed and fired at a temperature of 1400–1550?°C for 2?h. Thermal analysis, phase composition, microstructure, densification parameters, and other technological properties of the fired specimens were investigated. It was found that sintered M-CZ composite with bulk density (3.95?g/cm³), cold crushing strength (170?MPa), and high coating ability can be obtained by firing Egyptian dolomite with 37.8?wt% zirconia at temperature 1500?°C with the formation of belite as a secondary product. This composite can be nominated to be produced on a large industrial scale.     

Low-temperature synthesis of mullite-based ceramics from Moroccan naturally occurring andalusite and marble waste

The present work aims to prepare mullite-based ceramics at low temperature mainly from andalusite and marble byproduct by a solid-state interaction method. Marble powder byproduct was used, in the prospect of waste management, as a sintering aid. The influence of marble powder byproduct (5 wt.%) on the phase formation, microstructure-temperature evolution, densification, and mechanical strength was evaluated by means of XRD, TGA-DTA, and SEM. The results revealed that the andalusite remained present with mullite up to 1400°C, while the addition of 5 wt.% of marble byproduct involved its complete transformation into mullite and a trace of anorthite. The transformation of andalusite into mullite upon the heating in the range 1300–1400°C involved the densification of the matrix body and a significant increase in mechanical strength from 60 to 170 MPa, which promoted its application for refractory materials.