Effect of magnesia-alumina spinel precursor sol on the sintering property of fused magnesia refractory

The sintering property of fused magnesia refractory was investigated by the fused magnesia powder as matrix and the synthetic magnesia-alumina spinel precursor sol as binder. The spinel precursor sol was prepared by co-precipitation method and was characterized by its particle size, thermal analysis, microstructure, phase development with temperatures, and so on. The effect of spinel precursor sol on the sintering property of fused magnesia refractory were studied after heat treatment at 1450?°C and 1550?°C. The results showed that the bulk density, flexural strength and linear shrinkage of the sintered samples firstly increase and then decrease with increasing spinel precursor sol. The bulk density and flexural strength (cold modulus of rupture) of the sample sintered at 1550?°C, introduced with 1?wt% spinel precursor sol, reached 3.10?g/cm³ and 47.25?MPa, respectively. From the experimental results, magnesia-alumina spinel precursor sol can replace the traditional binders and improve the sintering property of fused magnesia products.     

Slow sintering in garnet-containing Y and Gd zirconate–aluminate mixtures for thermal barrier coatings

Mixtures of rare-earth zirconates and aluminates containing Y or Y + Gd that form a two-phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y-free, Gd-containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y-free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.     

Development of mechanical and visible transparency properties of spark plasma sintered spinel fabricated by powder injection molding of MgAl2O4 nanoparticles

This research aims to investigate the density, hardness, fracture toughness, and infrared and visible transmittance of spark plasma sintered (SPS) spinel discs fabricated through the powder injection molding (PIM) method. These properties were compared with the sample directly SPSed without the PIM process. For this purpose, initially, a feedstock was prepared with 80 wt% spinel nanopowder and 20 wt% binder. The results revealed that the hardness and fracture toughness of the SPSed spinel disc sintered at 1400 °C were greater than those of the spinel sample without PIM treatment. Also, for the PIMed sample and then SPSed sample, the level of infrared and visible transmittance was ~10% greater than for the SPSed spinel nanopowders.     

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Transparent magnesium aluminate spinel: Effect of critical temperature in two-stage spark plasma sintering

The discolouration of magnesium aluminate spinel caused by carbon contamination is a main drawback of fabricating transparent bodies by spark plasma sintering (SPS). In this study, a two-stage heating rate profile was used to produce transparent MgAl₂O₄ without using sintering aids by SPS at 1250°C. The effect of critical temperature (Tc), at which the heating rate is decreased, on transparency and carbon contamination was investigated: higher critical temperature resulted in higher contamination. Non-uniform densification indicated that fast heating results in a hot-zone formation in the centre of sintered pellets; the higher temperature of centre favoured reaction of graphite die with spinel and formation of disordered carbon structures in residual pores.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering

The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.Fully dense materials were obtained at 1400 °C with a heating rate of 100 °C/min, 10 min soaking time and 100 MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400 °C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.     

Ex situ Raman mapping study of mechanism of cordierite formation from stoichiometric oxide precursors

Ex situ Raman spectroscopy and Raman mapping showed that at temperatures below 1300 °C, magnesia reacted with either alumina or silica to form magnesium aluminate (spinel) or magnesium silicate. At 1300 °C or higher, the reaction between magnesium silicate and alumina, or spinel and silica, led to cordierite formation. Moreover, the presence of silica and spinel, and the disappearance of magnesium silicate at 1400 °C, indicated that cordierite formation was more favored by the reaction between alumina and magnesium silicate than by that between silica and spinel.     

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration

The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.     

In-situ synthesis and low-temperature fabrication of B4C/TiB2-based multilayer graded composites with B4C and Ti–Al intermetallics mixtures through transient liquid phase sintering

The seven-layer B4C/TiB2-based graded composites was prepared with B4C and Ti–Al intermetallics through stepped laminating processing and transient liquid phase spark plasma sintering. The sintering strategy of the graded composites was proposed based on the sintering products of monolayer materials with different contents of Ti–Al intermetallics from 5 wt% to 60 wt%. The top three layers and bottom three layers were sintered respectively at 1650 °C and 1500 °C, and then the middle layer was used as the binder to joint the as-preserved two sections at 1550 °C. The apparent density of the as-prepared B4C/TiB2-based multilayer graded composites was 2.94 g/cm3, which was lower than that of most advanced ceramics. With the increase in the addition of Ti–Al intermetallics, the hardness of B4C/TiB2-based multilayer graded composites decreased from 31 GPa (B4C-riched) to 25 GPa (TiB2-riched), whereas the fracture toughness increased from 3.8 MPa·m0.5–6.02 MPa·m0.5. The compressive strength was up to 1100 MPa, displaying the jagged stress-strain curve. Crack propagation resistance mechanisms such as deflection and bridging enhanced the fracture toughness. The B4C/TiB2-based multilayer graded composites fabricated at low temperature possess high front hardness, high rear toughness, high overall strength and low density, and has promising applications in impact-resistant fields such as lightweight ceramic armor.     

Facile preparation of thermal insulation materials by microwave sintering of ferronickel slag and fly ash cenosphere

A novel approach for preparing thermal insulation materials by microwave sintering of ferronickel slag (FNS) in the presence of fly ash cenosphere (FAC) was proposed and evaluated. The study showed that during microwave radiation, the contact interface between FNS and FAC would preferentially form magnesium iron chromate spinel and magnesium iron aluminate spinel particles as hot spots by absorbing microwave vigorously, promoting decomposition and transformation of the raw materials into the thermal insulation phases, mainly cordierite and enstatite. After sintering at 900 °C by microwave for only 20 min with the addition of 25 wt% FAC, a thermal insulation material with thermal conductivity of 0.41 W/(m·K), bulk density of 1.46 g/cm³, compressive strength of 30.72 MPa, water absorption of 21.07%, and linear shrinkage of 7.06% was obtained. Compared with the conventional sintering method, the temperature was reduced by 300 °C, with the sintering time shortened by 6 times. This study represents a good example for clean and efficient value-added utilization of FNS, FAC and other relavent solid wastes.     

Drucker-Prager-Cap modelling of boron carbide powder for coupled electrical-thermal-mechanical finite element simulation of spark plasma sintering

The coupled electrical-thermal-mechanical finite element method in the continuum scale has been widely used to investigate the spark plasma sintering process. An accurate constitutive model of powder material is pivotal for precise continuum finite element simulation. In this study, the Drucker-Prager-Cap model, which is highly accurate in describing the densification behaviour of powder material, was adopted to numerically analyse the spark plasma sintering process of boron carbide powder. First, the parameters of the model were defined to be dependent on temperature and density for higher accuracy; they were determined by minimising the discrepancy between the simulated and experimental results. Based on a spark plasma sintering experiment with a cylindrical sample, the parameters of the Drucker-Prager-Cap model were identified at 1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C. A coupled electrical-thermal-mechanical finite element simulation with the model was performed for spark plasma sintering of boron carbide powder at 1750 °C and 1850 °C. The temperature, stress, and relative density were investigated numerically. By comparing the model results with the temperature and relative density measured in the experiment, the continuum finite element method with the Drucker-Prager-Cap model was validated.     

Properties of liquid phase pressureless sintered silicon carbide obtained without sintering bed

High density pressureless sintered silicon carbide bodies with yttria and alumina as sintering aids were obtained without sintering bed (LPSSC-NB). Sintering behavior of this material was studied between 1850 °C and 1950 °C and it was compared to the liquid phase sintered SiC material obtained using sintering bed (LPSSC-B). Sintered density was 97% of the theoretical density (T.D.) at 1875 °C. Mechanical properties like fracture toughness, hardness, flexural strength were determined and compared to other SiC-based materials. In this manner we were able to demonstrate that silicon carbide could successfully be sintered by means of liquid phase mechanism also without sintering bed. This fact opens liquid phase sintered silicon carbide to a wide range of industrial application.     

Characterization and analysis of high-entropy boride ceramics sintered at low temperature

Multicomponent transition metal boride composite–sintered bodies were prepared by spark plasma sintering, and the composite sintered bodies prepared at different sintering temperatures (1500–1900°C) were characterized. The experimental results showed that several other compounds diffused into the TiB_x phase at lower sintering temperatures under the combined effect of temperature and pressure due to the nonstoichiometric ratio of TiB_1.5 vacancies. When the temperature reached 1900°C, only the hexagonal phase remained. With the continuous increase of sintering temperature, the Vickers hardness and fracture toughness of the sintered bodies had a trend of increasing first and then decreasing, due to the continuous reduction of the porosity of the cross section of the sintered bodies and the growth of the grain size. The Vickers hardness and fracture toughness of sintered body obtained at 1800°C are the best, which are 24.4 ± 1.8 GPa and 5.9 ± 0.2 MPa m^1/2. At 1900°C, the sintered body was a single-phase hexagonal high-entropy diboride. Its Vickers hardness and fracture toughness were 21.9 ± 1.5 GPa and 5.4 ± 0.2 MPa m^1/2, respectively; it showed a clear downward trend.     

Promoting spinel formation and growth for preparation of refractory materials from ferronickel slag

The study provides a method for improving the quality of the refractory material prepared from ferronickel slag by promoting the spinel formation and growth in the slag which was sintered with sintered magnesia and chromium oxide in a broad sintering temperature range from 1200°C to 1500°C. According to the thermodynamic analysis, except for forsterite due to the addition of sintered magnesia, a number of high-melting point spinel phases can also be formed in the presence of chromium oxide and this trend becomes more apparent with increasing sintering temperature, along with declined presence of low-melting point clinopyroxene, mainly enstatite. This expectation was verified by conversion of a part of the original phase of ferronickel slag, olivine, to two main spinel phases, including magnesium aluminate spinel and donathite which was produced by the replacement of nontoxic Cr³⁺ ions with Fe³⁺ ions in the octahedral vacancies of magnesium chromate spinel. The formation and growth of these spinel phases were promoted by elevating temperature from 1200°C to 1500°C, which accelerated the transition of initially generated enstatite to a glassy phase, in favor of densification. The formation and growth of spinel during sintering contributed to high refractoriness and compressive strength of the resulting refractory materials     

Low temperature synthesis of highly pure cordierite materials by spark plasma sintering nano-oxide powders

The development of cordierite ceramics, using traditional materials and conventional methods, remains a key challenge because of the high and narrow sintering temperature range. In this work, single-phase cordierite ceramics were produced by spark plasma sintering nano-oxide powders, at low temperatures. A starting mixture of Al₂O₃, SiO₂, and MgO nano-powders with the composition of stoichiometric cordierite was first sonicated, then, sintered in the temperature range 900–1200 °C. The raw powders, sonicated powder mixture, phase transformations, and fracture surface of sintered specimens were characterized by using an x-ray diffractometer and a field emission scanning electron microscope (FE-SEM). The hardness and fracture toughness were measured using the indentation technique. The influence of testing conditions, process parameters, characterization technique, and calculation method on hardness and fracture toughness was investigated. The FE-SEM images, x-ray maps, and EDS results revealed the homogeneity and stoichiometry of the sonicated powder and sintered samples. Highly pure α-cordierite was formed at 1150 °C. Samples that were sintered at 1150 and 1200 °C had bulk density of 2.58 g/cm³ (relative density of 100%), and maintained low average grain sizes of 28.68 and 34.95 nm, respectively. With the decrease in the temperature from 1200 to 1150 °C, the hardness of cordierite was slightly increased from 8.25 ± 0.158 to 8.46 ± 0.188 GPa, the fracture toughness was marginally improved from 2.2 ± 0.158 to 2.25 ± 0.238 MPa mm^1/2, and the critical strain energy release rate was raised from 32.46 to 33.95 J/m².     

On the effects of LiF on the synthesis and reactive sintering of gahnite (ZnAl2O4)

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl₂O₄) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al₂O₃ mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.     

Properties of MgO transparent ceramics prepared at low temperature using high sintering activity MgO powders

Transparent MgO ceramics are successful fabricated via spark plasma sintering at lower temperature using the high sintering activity powders synthesized by precipitated method. The samples were detected by XRD, SEM, TEM, BET, UV-Vis-NIR, microhardness, and so on. The results show that all ceramics prepared at 700°C-900°C are visually transparent and the sample sintered at 860°C for 5 min exhibits the superior transmittance of 60% (800 nm). It is also found that the mechanical and thermal properties of MgO ceramics are all increasing firstly and then decreasing with the increase in the sintering temperature. And the maximum value of hardness, fracture toughness, MSP strength, and Young's modulus of MgO ceramics is 8.25 GPa, 2.01 MPa·m^1/2, 206 MPa, and 286 GPa, respectively. Moreover, the thermal conductivity of MgO ceramics sintered at 860°C can reach 48.4 W/mK at room temperature.