The effect of increasing MgO content in dendromass on ash fusibility and corrosion of corundum refractory castable

Low melting temperatures of the biomass ash at the intensive combustion can generate the slag with a portion of unburned fuel. The portion of imperfectly burned fuel reduces the efficiency of its use. An alternative route to solve the problem without changing the combustion condition is a modification of the chemical composition of the ash with the addition of an agent increasing the melting temperature of the ash/slag. In this paper, we report on the impact of adding the magnesite waste sludge to ash on their melting. Three types of ashes with different SiO₂ and K₂O content were selected for the tests. The melting temperatures of ash and ash mixtures with magnesite sludge (mass ratio 0.5–2:1) have been determined. The addition of magnesite sludge to ash increased the temperature of fusibility index of the mixtures by 50–100 °C. The ash fusibility temperature rose with a decrease of the SiO₂ content in the ash. Subsequently, the interaction of the ash/ash mixtures with the refractory corundum castable was monitored at a temperature of 1450 °C using a static crucible corrosion test. The addition of magnesite sludge to ash reduced the aggression of the slags to the refractory corundum materials. The Al₂O₃ concentration in the post mortem slags of the ash mixtures with the magnesite sludge (2:1) was about 20–23 wt% lower in comparison to the slag without magnesite. The evaluation of the effect of magnesite sludge addition to ash and the elements (K, Ca, Mg, Fe, Si) penetration on the extent of corrosion is the subject of further work aimed at analyses of the corrosion interface of the slag – corundum refractory material.     

Mechanical properties and pre-oxidation behavior of spark plasma sintered B4C ceramics using (Ti3SiC2+CeO2/La2O3) as sintering aid

B₄C ceramic with the addition of 5 wt % (Ti₃SiC₂+ CeO₂/La₂O₃) as sintering aids was fabricated by spark plasma sintering at a relatively low temperature of 1650 °C for 5 min at 80 MPa. The phase composition, microstructures, and comprehensive mechanical properties of the ceramics were studied in detail. The existence of reinforced second phase particles, the refinement of the matrix grains, the formation of residual stress along the grain boundaries and the appearance of the mixed fracture mode had a synergetic strengthening effect on the mechanical properties. The flexural strength, fracture toughness and Vickers hardness of B₄C ceramics reached 565.2 ± 21.8/551.0 ± 25.2 MPa, 6.28 ± 0.01/6.41 ± 0.12 MPa·m^0.5, and 28.51 ± 0.86/27.23 ± 1.08 GPa, respectively. In addition, to reduce the crack sensitivity of the ceramic, the ceramics were pre-oxidized at 800 °C for different durations. The flexural strength was increased by approximately 13.4% after the ceramic was oxidized at 800 °C for 45 min due to the crack-healing effect induced by the oxide glass B₂O₃ on the ceramic surface.     

Variations in structure and properties of vitrified bonds and vitrified diamond composites prepared by sol-gel and melting methods at different sintering temperature

Differences in structure and properties of Na₂O–Al₂O₃–B₂O₃–SiO₂ vitrified bonds and vitrified diamond composites prepared by sol-gel and melting methods were methodically discussed. Results showed that the vitrified bond prepared by sol-gel method contained more [AlO₄] tetrahedron and owned higher bending strength, with the maximum value reaching 137 MPa, 31.73% higher than that prepared by melting method (104 MPa). As the sintered temperature rose, coefficient of thermal expansion of the vitrified bond prepared by sol-gel method increased first and then decreased, acquiring a maximum value of 5.75 × 10⁻⁶ °C ⁻¹ at 720 °C, which was still much lower than the minimum value of vitrified bond prepared by melting method (7.02 × 10⁻⁶ °C ⁻¹). The vitrified diamond composite prepared by sol-gel method possessed lower sintering shrinkage than that prepared by melting method, and could be applicable to the production of grinding tools with high dimensional accuracy. What's more, the maximum bending strength of vitrified diamond composites obtained by sol-gel method was 106 MPa, 24.7% higher than that of vitrified diamond composites prepared by melting method (85 MPa).     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Low-temperature joining of SiC ceramics by NITE phase using CaO-Al2O3-MgO-SiO2 glass as an additive combined with surface oxidation

Low-temperature joining of SiC ceramics using raw CaO-Al₂O₃-MgO-SiO₂ mixed powders (CAMS-P) and CaO-Al₂O₃-MgO-SiO₂ glass (CAMS-G) as additives for the liquid-phase sintering of SiC nanopowders (NITE phase) in combination with surface oxidation was studied. Using CAMS-P as an additive for SiC joining required 1650 °C and produced a lower shear strength of 77.2 ± 3.5 MPa, compared to untreated SiC (108.2 ± 15.8 MPa). Using CAMS-G as an additive, the compactness of SiC joints obtained at 1550 °C was higher than that of NITE joints obtained at 1650 °C with CAMS-P as an additive. Nonetheless, the low joining temperature resulted in gaps and delaminations at the interface, and the joints strength was only 48.3 ± 5.6 MPa. Incorporating CAMS-G as an additive and surface oxidation achieved reliable NITE phase joining at 1550 °C, with no observed gaps or delaminations at the interface. The resulting joints strength was as high as 100.0 ± 0.8 MPa. The joining mechanism was discussed and compared, involving microstructural analysis.     

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.     

Microstructure and mechanical properties of TiB2–B4C ceramics fabricated by hot-pressing

In this study, TiB₂–B₄C composite ceramics were prepared using Y₂O₃ and Al₂O₃ as the sintering aids. Different contents of B₄C were added to seek promoted comprehensive mechanical properties of the composites. The mixed powders were sintered at 1850 °C under a uniaxial loading of 30 MPa for 2 h via hot-pressing. Through the measurement of XRD, SEM and related mechanical properties, the influence of B₄C content on the microstructure and mechanical properties of TiB₂–B₄C composites ceramics was discussed. The experimental results show that TiB₂–B₄C composite ceramics exhibit excellent mechanical properties, which can be attributed to the dense microstructure and fine grain size. In addition, TiB₂–B₄C composite ceramic shows a relatively high comprehensive properties when the addition amount of B₄C is 20 wt%. The relative density, Vickers hardness, fracture toughness and flexural strength are measured to be 99.61%, 27.63 ± 1.73 GPa, 4.77 ± 0.06 MPa m^1/2, 612.5 ± 28.78 MPa, respectively.     

Low-temperature joining of SiC ceramics with Y2O3-Al2O3 interlayer by SiO2-based liquid phase extrusion strategy

In order to reduce the joining temperature of SiC ceramics by glass-ceramic joining, some oxides were usually introduced into to Y₂O₃–Al₂O₃ for reducing the eutectic temperature. However, the joints might have poor high-temperature resistance due to the low melting point of the joining layer. In the present work, based on novel SiO₂-based liquid phase extrusion strategy, joining of SiC ceramics with Y₂O₃–Al₂O₃ interlayer was carried out by using Y₂O₃–Al₂O₃–SiO₂ as the filler through spark plasma sintering (SPS). The SiO₂-free interlayer of Y₂O₃–Al₂O₃ was used for comparison. It was found that SiC joints using Y₂O₃–Al₂O₃ could be only joined at a high temperature of 1800 °C, and the thickness of the interlayer was about 20 μm. The shear strength of the joint obtained at 1800 °C was 89.62 ± 4.67 MPa and the failure located in the SiC matrix. By contrast, reliable joining of SiC ceramics could be finished at as low as 1550 °C by extrusion of SiO₂-containing liquid phase when using Y₂O₃–Al₂O₃–SiO₂ as the interlayer, alongside the interlayer thickness of only several microns. The joint strengths after joining at 1550 °C was 84.90 ± 3.48 MPa and the failure located in matrix position. The joining mechanism was discussed by combining the detailed microstructure analysis and phase diagram.     

Properties of TiB2–PSO modified SiHfBCN-based adhesive through polymer-derived-ceramic route

High temperature SiHfBCN-based ceramic adhesives are fabricated by polymer derived ceramic route with SiHfBCN precursors, TiB₂ and polysiloxane (PSO). The phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscopy, respectively and the evolution of pores was analyzed by Micron X-ray 3D Imaging System and VG Studio MAX 3.0.2 software. After heat-treating at 80 °C and curing at 170 °C in air, the adhesion strength detected in air of SiHfBCN adhesives is 3.22 MPa at room temperature (RT) and can rise to 5.47 MPa at 1000 °C after pyrolysis at 1000 °C in air for 2 h with a universal testing machine. By modifying SiHfBCN with TiB₂–PSO, the adhesion strength can be enhanced to 9.49 MPa at RT and 6.37 MPa at 1000 °C. The results indicate that the formation of SiO₂–B₂O₃–TiO₂ ternary glasses play an important role in improving the adhesion strength. The present study broaden the high temperature adhesive family suitable for large-scale complex ceramic components in harsh environments.     

High-strength TiB2-B4C composite ceramics sintered by spark plasma sintering

Spark plasma sintering (SPS) is an advanced sintering technique because of its fast sintering speed and short dwelling time. In this study, TiB₂, Y₂O₃, Al₂O₃, and different contents of B₄C were used as the raw materials to synthesize TiB₂-B₄C composites ceramics at 1850°C under a uniaxial loading of 48 MPa for 10 min via SPS in vacuum. The influence of different B₄C content on the microstructure and mechanical properties of TiB₂-B₄C composites ceramics are explored. The experimental results show that TiB₂-B₄C composite ceramic achieves relatively good comprehensive properties and exceptionally excellent flexural strength when the addition amount of B₄C reaches 10 wt.%. Its relative density, Vickers hardness, fracture toughness, and flexural strength reach to 99.20%, 24.65 ± .66 GPa, 3.16 MPa·m^1/2, 730.65 ± 74.11 MPa, respectively.     

Residual stresses in alumina matrix composites reinforced with Ti(C,N)

Ti(C_0.5,N_0.5)-reinforced alumina matrix composites with an addition of 2 wt% ZrO₂ were tested to determine residual stresses of Al₂O₃ and Ti(C_0.5,N_0.5) phases. The advanced sintering technique (spark plasma sintering ―SPS) at various temperatures of 1600°C and 1700°C was used. Vickers hardness HV1, Young’s modulus E, apparent density ρ and indentation fracture toughness K_IC(HV) were evaluated. An indirectly residual stress measurement by the XRD method using the sin² ψ technique was applied. Compressive residual stresses in both phases: α-Al₂O₃ and Ti(C_0.5,N_0.5) were observed. Residual stresses of α-Al₂O₃₍₂₂₆₎ are in the range between ?204 ± 20 MPa and ?120 ± 20 MPa (for 1600 °C and 1700 °C respectively) are lower compared to Ti(C_0.5,N_0.5)_(420), for which the stresses are in the range of between ?292 ± 20 MPa and ?256 ± 20 MPa (for 1600 °C and 1700 °C respectively). The results exhibit the influence of the sintering temperature on the residual stresses of the tested phases. The residual stresses revealed at 1700°C are lower by about 40% for α-Al₂O₃₍₂₂₆₎ and much less for Ti(C_0.5,N_0.5)₍₄₂₀₎, by only about 15%. Microstructure studies using scanning electron microscopy, X-ray and electron diffraction phase analysis were used.     

Glass infiltration of gelcast zirconia-toughened alumina ceramics for dental restoration

This study investigated the effects of process parameters and material characteristics in glass infiltration of gelcast zirconia-toughened alumina (ZTA) ceramics for dental applications. Nine types of lanthanum-based silicate glasses with different concentrations of La₂O₃, B₂O₃, and Li₂O were prepared and characterized on the basis of their melting temperature, crystal structure, and thermal characteristics. These glasses were infiltrated into a ZTA matrix that was semi-sintered at 1200 °C to obtain a ceramic material with low shrinkage and high strength. The performance of the glass-infiltrated ceramic samples obtained with various glasses for various infiltration durations and temperatures was evaluated on the basis of the linear shrinkage rate, three-point bending strength, fracture morphology, and elemental composition. From the results, the infiltration temperature was identified as 1150 °C, and the optimum glass composition was 30%La₂O₃–15%Al₂O₃–15%B₂O₃–15%SiO₂–5%Li₂O by mass. The glass-infiltrated gelcast ZTA ceramic has a strength of 291.24 ± 27.94 MPa with a shrinkage of 1.8548 ± 0.2663%. These findings could provide a basis for further development of gelcast dental ceramics, which will make all-ceramic oral restoration less reliant on special equipment and materials.     

Microstructural refinement and mechanical response of Al2O3–ZrO2 eutectics fabricated by a novel pulse discharge plasma assisted melting method

In the present work, microstructural refinement and mechanical response of Al₂O₃–ZrO₂ eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al₂O₃–ZrO₂ eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al₂O₃–ZrO₂ eutectics is investigated.     

Low temperature pressureless sintering of dense silicon nitride using BaO-Al2O3-SiO2 glass as sintering aid

Dense Si₃N₄ ceramic with BaO-Al₂O₃-SiO₂ low temperature glass powders as sintering aids were prepared by pressureless sintering techniques at a relatively low temperature (1550 °C). Four kinds of glass powders of compositions melting at 1120 °C, 1300 °C, 1400 °C and 1500 °C, respectively, have been introduced as sintering aids. XRD results demonstrate that the BaO-Al₂O₃-SiO₂ glass powders reacted with BaAl₂O₄ and converted into hexagonal celsian, which is a high-temperature phase with melting point of 1760 °C, so being beneficial to the high temperature properties of the materials. In addition, a portion of α-Si₃N₄ transformed to rod like β-Si₃N₄ with high aspect ratio as shown by XRD and SEM analysis. The bulk density increased with the rise of the melting temperature of the BaO-Al₂O₃-SiO₂ glass powders, the sample obtained with the BaO-Al₂O₃-SiO₂ glass powder melting at 1500 °C reaching a maximum density of 98.8%, an high flexural strength (373 MPa) and a fracture toughness (4.8 MPa m^1/2).     

Structure and mechanical properties of hot-pressed B4C-NdB6 composites

High-performance B₄C-NdB₆ composites were fabricated by hot-pressing sintering at the temperature of 2050 °C for 20 min holding time and 20 MPa pressure with Nd₂O₃ (1～4 wt%) as the aiditive. The effects of Nd₂O₃ on the sintering process of the B₄C were studied. The reaction mechanisms of B₄C and Nd₂O₃ at different temperature were investigated. Based on the results of TG-DSC and thermodynamic calculation,. NdB₆ was formed via Nd₂O₃ react with B₄C in the sintering process, which greatly enhanced the densification of B₄C and promoted the sintering process. The flexural strength, fracture toughness and hardness of the B₄C-NdB₆ composites rose to 366.42 MPa, 5.27 MPa m^1/2 and 38 GPa by adding 3 wt% Nd₂O₃, respectively. The coexistence of transgranular and intergranular fracture is the major fracture mode. The phenomenon of pull-out contributed to improvement of the fracture toughness.     

High quality microwave dielectric ceramic sintered at extreme-low temperature below 200° and co-firing with base metal

Ultra-low temperature co-fired ceramics technology (ULTCC) requires the microwave dielectric ceramics with lower intrinsic sintering temperature than the melting point of inner electrodes. In the present work, a novel HBO₂ ceramic was found to be densified at extreme-low temperature below 200 °C, with pores, residual H₃BO₃, amorphous B₂O₃ inside, with a relative permittivity ∼2.12 ± 0.02, a Qf value ∼32,700 ± 300 GHz and a temperature coefficient of resonant frequency value ∼ − 43 ± 3 ppm/°C. This material can be easily obtained by dehydration from H₃BO₃ by sintering at low temperature below 200 °C. Its extreme-low sintering temperature and water solubility also provides the possibility to achieve some novel multi-functional inorganic-organic composite in the future.     

Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

High‐temperature time‐of‐flight neutron diffraction experiments were performed on cubic yttria‐stabilized zirconia (YSZ, 10 mol% YO_1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO₂ laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10^?6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10^?6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10^?6/K at 2350°C–2550°C with oxygen displacement parameters (U_iso) reaching 0.1 Å², similar to behavior observed in UO₂. Acquisition of powder‐like high‐temperature neutron diffraction data from solid‐levitated samples is feasible and possible improvements are outlined. This methodology should be applicable to a wide range of materials for high‐temperature applications.     

Strengthening of Gadolinia‐Doped Ceria (Ce0.80Gd0.20O2‐δ) Thick Ceramic Membranes with Co‐Doping of 1 mol% CuO

Nano‐sized Ce_0.79Gd_0.2Cu_0.01O_2‐δ (CGCO) and Ce_0.80Gd_0.2O_2‐δ (CGO) powders were synthesized via a facile PVA‐assisted combustion method. Full densification can be achieved at much lower temperature (950°C) with minor CuO doping (1 mol%, CGCO) as compared with CGO (1400°C); meanwhile, mechanical strength can be enhanced with CuO co‐doping. The CGCO sintered at 1100°C possesses a relative density of 98.80% and biaxial flexural strength of 302 ± 35 MPa, which is higher than those of CGO sintered at 1400°C (96.43%, 250 ± 39 MPa). A tentative microstructural analysis indicates that this strength enhancement can be attributed to a change in fracture mode from partial to complete transgranule dominant.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.