Self-reinforced Ca-hexaluminate/alumina composites with graded microstructures

The physical, thermal, and mechanical characteristics of self-reinforced calcium-hexaluminate/alumina composites with a graded microstructure are described. The presence of CA₆ phase in Al₂O₃ matrix has a significant effect on the physical, thermal, and mechanical properties of graded Al₂O₃–CA₆ composites. The slightly lower shrinkage and density in the graded composite can be attributed to the presence of CA₆ phase. The thermal expansion and densification behaviour of the graded composite showed that the presence of CA₆ phase hinders the processes of sintering and densification of alumina matrix. When compared to the alumina region, the graded CA₆–Al₂O₃ region is softer by virtue of the presence of the softer CA₆ phase. However, the fracture toughness in the graded region is higher than the alumina region which can be attributed to the display of crack deflection and crack-bridging provided by the CA₆ platelets.     

Corrosion mechanism of MgAl2O4–CaAl4O7–CaAl12O19 composite by steel ladle slag: Effect of additives

Corrosion behaviors and corroded microstructures of MgAl₂O₄-CaAl₄O₇-CaAl₁₂O₁₉ composites containing various additives (ZrO₂ and TiO₂) against steel ladle slag (containing CaF₂) were investigated using a reaction test method at 1600 °C. Thermodynamic calculation, based on the Al₂O₃–CaO–MgO phase equilibrium diagram was used to further reveal the corrosion mechanism. The attack of the liquid slag on the composite substrate was found to take place through an interdiffusion mechanism, producing the precipitation of spinel in the slag and a continuous layer of calcium dialuminate at the interface. This composite showed a high total corrosion depth due to the high porosity of the substrate and the high fluidity of the slag. Fortunately, the addition of ZrO₂ and TiO₂ can greatly improve the slag corrosion resistance by increasing the viscosity of the slag at an earlier stage. Besides, the highly dense microstructures also improved the corrosion resistance against the liquid slag, and thus suppressed the slag penetration. It was also found that the CA₆ grains with low aspect ratios are more difficult to be wetted and dissolved by the slag.     

Fabrication of CaAl12O19–CaTiO3 composites and their potential usage for TiAl alloy smelting

CaAl₁₂O₁₉–CaTiO₃ (CA₆–CT) materials are fabricated using a solid sintering method to investigate the effects of TiO₂ content on the phase composition, properties, and microstructure of the materials. With increasing TiO₂ content, the preferential replacement of Al³⁺ by Ti⁴⁺ in the CA₆ crystal increases the concentration of vacancy defects, thereby promoting ion diffusion and mass transfer, which improves the sintering reaction activity. However, as more TiO₂ is added, the CaTiO₃ phase formed increases the thermal conductivity and mean expansion coefficient of the material at high temperatures, which degrades its thermal shock stability. The CA₆–CT refractory showed good corrosion resistance to the Ti₆Al₄V melt. Except for a slight penetration of iron element, no obvious corrosion occurred at the reaction interface.     

Restructuring-diffusion mechanism of calcium alumino-titanate in CaAl12O19–MgAl2O4–Al2O3 castables

Calcium alumino-titanate (CAT), a secondary material obtained from ferrotitanium slag, was used as a hibonite source to prepare CaAl₁₂O₁₉–MgAl₂O₄–Al₂O₃ castables. The restructuring effect of CAT aggregate was compared by replacing tabular alumina aggregates with CAT aggregates of different particle sizes. The effects of CAT particle size on cold mechanical strength and thermal shock resistance of CaAl₁₂O₁₉–MgAl₂O₄–Al₂O₃ castables were studied. The results showed that CAT aggregates with particle size of 5–3 or 3–1 mm led to more internal cracks or pores and reduced the cold mechanical strength of the castable samples fired at 1600 °C for 3 h. The use of CAT aggregates with particle size of 1–0 mm led to the formation of a well-bonded CAT aggregate and matrix, improving the cold mechanical strength and thermal shock resistance of the castable samples fired at 1600 °C for 3 h. The enhancement mechanism of fine CAT aggregates in this process was proposed based on the sintering of the matrix–aggregate interface with the formation of Ca(Al, Mg, Ti)₁₂O₁₉.     

Microstructure and performances of corundum−mullite composite ceramics for heat transmission pipelines: Effects of Ho2O3 additive content

Ho₂O₃ was employed to improve the microstructural densification and performances of pressureless sintered corundum–mullite ceramic composites. This study investigated the influences of Ho₂O₃ addition on the microstructure, physical properties and thermal shock resistances of the composites. The results indicated that sample AH5 (80 wt% Al₂O₃, 20 wt% coal series kaolin, and 5 wt% additional Ho₂O₃), which was sintered at 1550 °C, showed the best comprehensive properties. In this Al₂O₃-rich and SiO₂-poor system, a reaction between the Ho₂O₃ and Al₂O₃–SiO₂ system produced an Ho₂O₃–Al₂O₃–SiO₂ liquid phase. This liquid phase increased the microstructural densification and resulted in a lower sintering temperature. The generation of mullite and holmium disilicate during thermal shocks improved the thermal shock resistance. The high bending strength and satisfactory thermal shock resistance of the as-prepared corundum–mullite ceramic composites showed their potential for use in heat transmission pipelines.     

Microstructure and mechanical properties of CaAl12O19 reinforced Al2O3-Cr2O3 composites

Al₂O₃-Cr₂O₃ refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. However, Al₂O₃-Cr₂O₃ refractories have poor mechanical properties and sintering properties. In order to improve the mechanical properties of Al₂O₃-Cr₂O₃ materials, the CaAl₁₂O₁₉ reinforced Al₂O₃-Cr₂O₃ composites were prepared by pressureless sintering process, and the influences of CaO content on the sintering properties, mechanical properties, and microstructure evolution of the composites were studied. The results show that a small amount of CaO can significantly improve the compactness of the composites, which is mainly due to the formed sheet-like CA6 fill the gap between the solid solutions, and reduces the porosity of the composites. In addition, the sheet-like CA6 makes the connection between solid solutions closer, and the intergranular fracture gradually transforms into a mixed mode of intergranular and transgranular fracture. The best mechanical propertie is observed at S4 with the CaO content of 2 wt.%. Compared with sample S0 without CaO, the hardness, compressive strength and flexural strength of the S4 were increased by 35.19 %, 49.69 %, and 68.34 %, respectively. The addition of excessive CaO will deteriorate the mechanical properties of the composites, because the formation of a large number of layered CA6 increases the porosity of the composites. Furthermore, a small amount of CaO addition can significantly improve the thermal shock resistance of the composites. After 10 and 20 thermal shock cycles, the strength loss rates of S4 are only 5.83 % and 8.74 %, respectively.     

Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite

Materials designed in the high-alumina region of Al₂O₃–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl₁₂O₁₉) and spinel (MgAl₂O₄). Considering this aspect, the present work investigated the effect of TiO₂ addition on the sintering densification and mechanical properties of MgAl₂O₄–CaAl₄O₇–CaAl₁₂O₁₉ composite by solid state reaction sintering. The results showed that the CA₆ grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti⁴⁺ into its structure, which greatly improved the densification after heating at 1600 °C. The flexural strength was greatly enhanced with increasing addition of TiO₂ due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al₂TiO₅. The increased content of TiO₂ also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation.     

The impact of bonite aggregate on the properties of lightweight cement-bonded bonite–alumina–spinel refractory castables

The lightweight bonite–alumina–spinel (CA₆–Al₂O₃–MA) refractory castables with bonite aggregate and different spinel sources (pre-formed and in situ formation) were prepared in this study. The phase composition, microstructural features, and mechanical and thermo-mechanical properties of CA₆–Al₂O₃–MA castables treated at various temperatures were investigated by techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending method, and thermal shock test. The results indicated that the incorporation of bonite aggregate had a positive influence on the strength, thermal shock resistance and slag corrosion resistance. It especially decreased the thermal conductivity and had a slightly negative influence on the refractory under load and slag penetration resistance of the castables. For the in situ spinel-containing castable, the formation of in situ spinel with finer particle sizes and acicular CA₆ grains led to higher overall volume expansion, resulting in higher thermal expansion (∆L/L₀), linear change and the apparent porosity of castables. Also, the heat insulation, thermal shock and slag penetration resistance of castables with in situ spinel improved, while the strength, displacement, refractory under load and slag corrosion resistance decreased sharply.     

Effect of ZrO2 Addition on Densification and Mechanical Properties of MgAl2O4–CaAl4O7–CaAl12O19 Composite

Although the platelet structure of calcium hexaluminate (CaAl₁₂O₁₉, or CA₆) grains can strengthen and toughen the Al₂O₃–MgO–CaO system materials designed in the high‐alumina region, it also results in poor densification and subsequent accelerated slag penetration for refractory application. Considering this aspect, MgAl₂O₄–CaAl₄O₇–CaAl₁₂O₁₉ composite was fabricated by solid‐state reaction sintering in this work, and the effect of ZrO₂ addition on densification and mechanical properties was investigated. The results showed that the CA₆ grains presented a more equiaxed morphology by addition of ZrO₂, contributing to form highly dense microstructures after heating at 1600°C without evident grain coarsening. The compressive strength and flexural strength were greatly enhanced mainly due to the significant decrease in porosity and pore sizes. Besides, the increased content of ZrO₂ plays an active role in toughening this composite attributed to the dense microstructure and strong bonding with higher strength, as well as considerable t‐ZrO₂ transformability.     

Preparation and thermophysical properties of LaMgAl11O19–Yb3Al5O12 ceramic composites

LaMgAl₁₁O₁₉–Yb₃Al₅O₁₂ ceramic composites were prepared by pressureless sintering process at 1700 °C for 10 h in air. The microstructure and thermophysical properties of the composites were characterized by X-ray diffraction, scanning electron microscopy, high-temperature dilatometer and laser flash diffusivity measurements. LaMgAl₁₁O₁₉–Yb₃Al₅O₁₂ ceramic composites are composed of magnetoplumbite and garnet structures. LaMgAl₁₁O₁₉–Yb₃Al₅O₁₂ ceramic composites exhibit typical linear increase in thermal expansion with the increase of temperature. The measured thermal diffusivity gradually decreases with increasing temperature. Thermal conductivity of LaMgAl₁₁O₁₉–Yb₃Al₅O₁₂ ceramic composites is in the range of 2.6–3.9 W·m⁻¹·K⁻¹ from room temperature to 1200 °C.     

Slag resistance mechanism of CaO·6Al2O3 refractory and its effect on inclusions of aluminum deoxidized steel

In order to verify the advantage of CaO·6Al₂O₃ (CA₆)-based refractories on the inclusions of aluminum deoxidized steel, the five refractories, CA₆, alumina, spinel, and CA₆-alumina and CA₆-spinel composition refractories were prepared into crucibles, and then the laboratory smelting experiments were conducted. After experiment, the slag resistance of the crucible and the variation on inclusions in steel were characterized and discussed. A dense CaO·2Al₂O₃ (CA₂) layer, which was produced by CA₆ reacting with the slag, was distributed between the original bricklayer and the slag layer, improving the slag resistance of refractories. Meanwhile, the 12CaO·7Al₂O₃ (C₁₂A₇), generated by the reaction between CA₂ and refining slag, would release much Ca into the molten steel. The Ca would react with inclusions to produce low melting point substance to float up and remove, contributing to the reduction of the proportion of large size inclusions. In addition, typical inclusions in steel smelted with CA₆ crucible were small-sized MgO·Al₂O₃ inclusions, whereas those of other crucibles are MnS–MgO·Al₂O₃ composite inclusions with MgO·Al₂O₃ as the core, implying CA₆ may absorb sulfur during the smelting process.     

Mechanical performance and microstructure of 3Y-TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

In this study, 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP)/Al₂O₃/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y-TZP/Al₂O₃ composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y-TZP/Al₂O₃. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m^1/2) greater fracture toughness than 3Y-TZP/Al₂O₃. The main toughening mechanisms of 3Y-TZP/Al₂O₃/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t-ZrO_1.95. The two-stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y-TZP/Al₂O₃/GNPs composites with a two-stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y-TZP/Al₂O₃/GNPs composite material.     

The microstructure and mechanical properties of Ni/Al2O3 composites by in-situ generated CaAl12O19 and ZrO2 via hot pressing sintering

Ni/Al₂O₃ composites with a varying mass fraction of CaZrO₃ (0–12 wt%) were prepared by vacuum hot pressing sintering at 1650 °C under a pressure of 30 MPa for 30 min to investigate how CaZrO₃ affect the mechanical properties and morphology of the composites. The results show that CaZrO₃ can react with Al₂O₃ and form new strengthening and reinforcing phases of CaAl₁₂O₁₉ and ZrO₂, which can promote complete densification and solve the problem of uneven distribution due to the poor wettability between Al₂O₃ and Ni. Additionally, composites showed satisfactory mechanical properties when 6.0–9.0 wt% CaZrO3 was added and the major toughening mechanism involved the typical fracture of delamination and the transgranular mode.     

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO₃ coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al₂O₃–MgO castables. The effects of Ca²⁺/Cit^3? mole ratio in precursor solution on coating characteristics of CaCO₃ coated corundum aggregates as well as the effects of CaCO₃ coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO₃ coating is increased and the size of CaCO₃ particles in coatings decreases first and then increases as Ca²⁺/Cit^3? mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA₆) layer at aggregate/matrix interface after sintering at 1600 °C. The Al₂O₃–MgO castables exhibit the best thermal shock resistance when Ca²⁺/Cit^3? mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA₆ layer with thickness of 10 μm, which is in-situ formed through reaction between Al₂O₃ and CaO derived from CaCO₃ coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al₂O₃–MgO castables.     

Flash sintering of 3YSZ/Al2O3-platelet composites

Preparation of 3YSZ/Al₂O₃-platelet composites always requires high temperature, long duration, and/or high pressure. Herein, 3YSZ/Al₂O₃-platelet composites are prepared at low temperature of 492°C-645°C in 30 seconds by flash sintering under the electric field of 300-800 V/cm. The influence of electric field and current limit on the densification and grain growth of composites is investigated. The onset temperature for flash sintering is determined by electric field, which is decreased with increasing the electric field. Under the constant electric field, the current limit has a great effect on the density and grain size of composite. The flash-sintered 3YSZ/Al₂O₃-platelet composites exhibit relatively high hardness and elastic modulus. Both Joule heating and defects generation are proposed to be responsible for the rapid densification in flash sintering. This work demonstrates the feasibility of employing the flash sintering to prepare ceramic composites with fine grain size.     

Properties and rapid consolidation of nanostructured Al2O3-Al2SiO5 composites by high frequency induction heated sintering

The rapid sintering of nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites was investigated by a high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were produced with simultaneous application of a 80 MPa pressure and induced output current of a total power capacity (15 kW) within 3 min. The sintering behavior, grain size and mechanical properties of Al₂O₃ and Al₂O₃ to Al₂SiO₅ composites were investigated.     

Synthesis and characterization of reaction-bonded calcium alumino-titanate-bauxite-SiC composite refractories in a reducing atmosphere

To take full advantage of the excellent properties of CA₆ present in calcium alumino-titanate (CAT) and reduce the formation of the low melting point phase (anorthite), CAT-bauxite-SiC composite refractories were fabricated under buried sintering in order to achieve low thermal expansion, superior high-temperature performance, and increased alkali resistance. Furthermore, the corrosion mechanism of K vapor was investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results show that CA₆ present in CAT can be partially retained and the hot strength of CAT-bauxite-SiC composites slowly decreases when the amount of CAT added is less than 21.6?wt%. The cold strength and bulk density decrease with the CAT content, and the residual ratio of MOR firstly decreases and subsequently increases with the CAT content. For the specimens with CAT additions, 43.2?wt% CAT results in the highest volume expansion at high temperatures. It is proposed that the corrosion mechanism of CAT aggregates under buried sintering is as follows: 1) K vapors penetrate into the CAT with high CA₆ content through the lamellar CA₆ gap and deposit on the inner regions of CAT; and 2) K vapors react with corundum and anorthite present in CAT and cause the microstructural destruction of CAT due to a decrease in the amount of the Al₂O₃-CaO-SiO₂ liquid phase in the CAT. The alkali resistance of the CAT-bauxite-SiC composites decreases as the CAT content increases, which is attributed to poor sintering densification and high apparent porosity.     

Enhancing mechanism of improved slag resistance of Al2O3-spinel castables added with pre-synthesized (Al,Cr)2O3 micro-powder

In order to enhance the slag resistance of Al₂O₃-spinel castables, (Al,Cr)₂O₃ is added into Al₂O₃-spinel system as a pre-synthesized micro-powder. Firstly, (Al,Cr)₂O₃ micro-powder is synthesized by sintering under reduction conditions to prevent the formation of hexavalent chromium. The Al₂O₃-spinel castables are prepared using tabular alumina, fused spinel, α-Al₂O₃ micro-powder, calcium aluminate cement and (Al,Cr)₂O₃ micro-powder as the raw materials. The bulk density, porosity, mechanical properties, and slag resistance of the samples are tested. Afterward, the effects of (Al,Cr)₂O₃ micro-powder (0–3 wt%) on the slag resistance and microstructures of the Al₂O₃-spinel castables are assessed by X-ray diffraction (XRD) and energy-dispersive (SEM-EDS) analysis. The results show that the addition of (Al,Cr)₂O₃ micro-powder can could inhibit the deteriorating effects of Cr³⁺ on the mechanical properties of the samples. The microstructure results also shows that with the addition of the (Al,Cr)₂O₃ micro-powder, a secondary solid solution of Ca(Al,Cr)₁₂O₁₉ formed, causing the unit cell to become larger. In the slag erosion area, CA₆ crystals formed with network-like interwoven structures, high density, and greater thickness. These characteristics significantly reduce the erosion and permeability indices of the castables, and improve the slag erosion resistance of the material.     

A comparative study of thermal conductivity and thermal emissivity of high temperature solar absorber of ZrO2 /Fe2O3 and Al2O3/CuO ceramics

Oxide ceramics are considered as promising high temperature solar absorber materials. The major aim of this work is the development of a new solar absorber material with promising characteristics, high efficiency and low-cost processing. Hence, this work provides a comparative and inclusive study of densification behavior, microstructure features, thermal emissivity and thermal conductivity values of the two new high temperature solar absorbers of ZrO₂/Fe₂O₃ and Al₂O₃/CuO ceramics. Ceramic composites of ZrO₂/(10–30 wt%) Fe₂O₃ and Al₂O₃/(10–30 wt%) CuO were prepared by pressureless sintering method at a temperature of 1700 °C/2hrs. Identification of the solar to thermal efficiency of the composites was evaluated in terms of their measured thermal emissivity. Thermal efficiency and heat transfer homogeneity were investigated in terms of thermal conductivity and diffusivity measurement. The results showed that both composites exhibited comparable densification behavior, homogenous and harmonious microstructure. However, Al₂O₃/10 wt% CuO composite showed higher thermal and solar to thermal efficiencies than ZrO₂/Fe₂O₃ composites. It gave the lowest and the best thermal emissivity of 0.561 and the highest thermal conductivity of 15.4 W/m. K. These values proved to be the best amongst all those of the most known solar absorber materials made from the expensive SiC and AlN ceramics. Thus, Al₂O₃/CuO composites have succeeded in obtaining outstanding properties at a much lower price than its other competitive materials. These results may strongly identify Al₂O₃/CuO composites as promising high-temperature solar absorber materials instead of ZrO₂ and the other carbide and nitride ceramics.     

Sintering of compositions in the system Al2O3-Cr2O3-SiO2

The sintering behaviour of refractory compositions in the system Al₂O₃-Cr₂O₃-SiO₂ was studied. The conditions for densification, the mechanism of densification and the reactions during sintering were investigated. Powders of Al₂O₃, Cr₂O₃, mullite and SiO₂ were used as a starting material and were sintered in compacted form under different conditions at 1300 to 1550°C. A dense body was obtained by heating in a carbon powder bed at temperatures around 1500°C. The microstructure of the body was skeleton-like and contained rectangular crystals of (Cr, Al)₂O₃ s.s., which appeared to be desirable for an improvement of the thermal shock resistance.