Sintering Of MgO Research Articles

Enhancing oxidation resistance of silicon nitride using a Ca2+ stabilizer

Silicon nitrides are widely used in ceramic heaters for combustion engines due to their high strength and wear resistance. However, their poor oxidation resistance at elevated temperatures limits their reliability. Oxidation products, such as silica and intermediate compounds from sintering additives, often suffer from microcracks due to phase transformation and volume expansion mismatch during thermo-mechanical cycles. Use of cation dopants to chemically stabilize the transformation of the oxidation front is proposed. This study investigated the effect of Ca2+ treatment on Si3N4 with MgO and Y2O3 sintering aids. The Si3N4 samples were coated with CaO and the phase composition, microstructure, and chemical distribution of the oxidation products were analyzed using XRD, SEM, TEM, STEM, and EDS. The oxidation coupon tests revealed that the CaO coating effectively protected Si3N4 from oxidation at 1300 ºC for 40h. Moreover, the microstructure analysis showed that CaO coating created more favorable microstructures which reduced oxidation reactions.

Reaction-sintered highly transparent MgGa2O4 ceramics with enhanced dielectric properties

Reaction sintering of MgO and Ga2O3 powders was first used to prepare MgGa2O4 transparent ceramics. The microstructural evolution suggested that the negligible volume change resulting from the solid-phase reaction of MgO and Ga2O3, as well as the close and homogeneous arrangement of both fine particles, are the key factors in obtaining pre-sintered ceramics with an ideal microstructure at lower temperature. After a hot isostatic pressing (HIP) treatment, the fully dense ceramic exhibited improved in-transmittance (74.7%@600nm, and 85.4%@4401nm) and quality factor (Q×f=168000GHz). Due to the combination of high transparency, wide transmission range (6.22 μm at the 60% transmittance), ultra-low dielectric loss, and near-zero temperature coefficient of resonance frequency (–3.9 ppm/◦C), transparent MgGa2O4 ceramic is suggested to be an ideal optical-dielectric integration material.

Effects of laser shock peening on silicon nitride ceramic with varying sintering additive ratios

Silicon nitride ceramic (Si3N4) for industrial applications is conventionally manufactured with sintering additives, and the properties of Si3N4 change significantly based on the contents of these additives. In this study, we investigate the effects of laser shock peening (LSP) on Si3N4 sintered with varying ratios of sintering additives.The Si3N4 samples were sintered with a mixture of Y2O3, MgO, and SiO2 sintering additives at 5, 7, 9, and 11 wt%. A Nd:YAG laser (wavelength = 532 nm, maximum pulse energy = 1.4 J, repetition rate = 10 Hz, pulse duration = 8 ns, beam diameter = 11 mm, top-hat profile) was used to irradiate the Si3N4 samples. The samples were coated with a protective layer (100 μm thick aluminum foil) and a water layer to confine plasma. LSP of Si3N4 with 5% sintering additives resulted in a slight change in surface hardness but a 77% decrease in surface compressive residual stress. In contrast, LSP of Si3N4 with 11% sintering additives led to a simultaneous increase in surface hardness (7.3%) and surface compressive residual stress (67%), indicating a significant difference in the effectiveness of LSP depending on the ratio of sintering additives. Additionally, the surface of Si3N4 with 11% sintering additives showed evidence of grain refinement after LSP. It was demonstrated that the bending strength of Si3N4 with 11% sintering additives increased by 15.2%, and the depth of the fracture origin was significantly deepened.

Combined Influence of MgO and TEOS Sintering Additives on the Structure and Optical Properties of the Ceramic Yag:Yb, Er

Combined Influence of MgO and TEOS Sintering Additives on the Structure and Optical Properties of the Ceramic Yag:Yb, Er

Crosslinking and pyrolysis of a methyl-silsesquioxane: Effect of heating rate on fabrication of polymer derived mullite ceramics using thermoplastic shaping

Poly-silsesquioxanes have been numerously used to produce SiO2 or SixOyCz ceramics through pyrolysis under different atmospheres. In this study, crosslinking and pyrolysis behavior of a commercial methyl-silsesquioxane, was examined using thermogravimetry analysis and a lower SiO2 yield was observed after pyrolysis at low heating rates of 0.3 and 0.6 K/min under air atmosphere, namely 69.1 and 75.0 wt%, respectively. Using ceramic thermoplastic shaping techniques, a low heating rate, below 2 K/min, is usually required to remove processing additive (e.g., binders) gradually and avoid failures like blisters, pores, and cracks. A mixture of SILRES® MK and γ-Al2O3 was prepared to produce a mullite ceramic with Al2O3/ SiO2 ratio of 60/40 wt%. As expected, the stoichiometric ratio of alumina to silica was altered using a heating rate of 0.6 K/min and mullite ceramic with 61.70/37.70 wt% of Al2O3/ SiO2 ratio was obtained. By mixing above the crosslinking temperature of SILRES® MK, a constant ratio of alumina to silica could be achieved successfully, even at low heating rates. In addition, microstructure indicated the growth of needle-like mullite crystals with some amount of SiO2-rich ternary amorphous phase (MgO–Al2O3–SiO2) between the needle grains. Elemental analysis revealed that MgO sintering additive was mainly in the amorphous SiO2-rich phase.

Utilization of investment casting shell waste in the production of cordierite-mullite-zircon composites for radiation shielding: The influence of La2O3 additive

Cordierite-mullite-zircon ceramic composites were produced via reaction sintering of investment casting shell waste, Al2O3, and MgO at 1350 °C. The influence of La2O3 addition on the crystalline phase composition, microstructure, densification behavior, mechanical strength, and radiation shielding capabilities of the cordierite-mullite-zircon composites has been investigated. The X-ray diffraction (XRD) analysis confirmed the successful synthesis of cordierite-mullite-zircon composites with up to 5 wt% La2O3 additive content. However, a higher La2O3 content, exceeding 7 wt% resulted in cordierite decomposition. Furthermore, the addition of La2O3 significantly enhanced the densification behavior and microstructural development, leading to improved flexural strength and enhanced shielding against fast neutron and gamma radiation. This research suggests the potential of the fabricated ceramic composites derived from investment casting shell waste for applications in radiation shielding.

Preparation of high performance dense Al2O3 ceramics by digital light processing 3D printing technology

The incorporation of a significant amount of organic materials in the fabrication process of Al2O3 ceramics using digital light processing (DLP) 3D printing technology leads to a noticeable reduction in mechanical properties compared to those prepared using traditional methods. To enhance its mechanical performance of Al2O3 ceramics, the addition of MgO and MgO-Y2O3 sintering additives, along with a three-step curing method, has resulted in significant improvements. Al2O3 ceramics fabricated with the addition of MgO exhibited a flexural strength of 454.2 MPa, Vickers hardness of 20.24 GPa and apparent porosity of 0.15 %. Similarly, the addition of MgO-Y2O3 yielded Al2O3 ceramics with a flexural strength of 491.6 MPa, Vickers hardness of 20.91 GPa, and apparent porosity of 0.11 %. These values exceed the typical flexural strength of Al2O3 ceramics prepared using traditional methods. This research provides a fundamental basis for the production of high performance, dense, and complex shaped Al2O3 ceramics using DLP 3D printing technology.

Thermal shock resistant 3D printed ceramics reinforced with MgAl2O4 shell structure

The demand for the swirl nozzle with enhanced temperature resistance and lightweight properties is increasing as the thrust-to-weight ratio of aero-engines rises. The Al2O3 ceramic swirl nozzle can maintain high strength in a hostile environment of high temperature and severe corrosion, while also meeting the requirements of aircraft to enhance efficiency and decrease weight. However, Al2O3 ceramics are limited in their application for aerospace components due to their poor thermal shock resistance (TSR) stemming from their inherent brittleness. This work reported an innovative design and fabrication strategy based on photopolymerization 3D printing technology to realize the three-dimensional shell structure through element interdiffusion and nanoscale stacking of the reinforced phase. With this strategy, a novel type of the new dual-structure Al2O3 ceramic composed of MgAl2O4 shell structure and matrix could be constructed in situ. The nano-sized MgAl2O4 caused a crack passivation effect after the thermal shock, which could improve the strength and TSR of 3D-printed Al2O3 ceramic. In addition, the effects of MgO content and sintering temperature on sintering behavior, flexural strength, porosity, and TSR of Al2O3 ceramics manufactured by digital light processing (DLP) processing were systematically studied. The optimum overall performance of Al2O3 ceramics was obtained at the sintering temperature of 1550 °C and the MgO content of 1.0 wt.%, with a maximum flexural strength of 111.929 MPa and a critical temperature difference of 374.24 °C for TSR. Based on the above research, an aero-engine swirl nozzle with high thermal shock resistance has been successfully prepared by ceramic 3D printing technology, which enhances high-temperature resistance and promotes lightweight design in aero-engine.

The Influence of Alumina Bubbles on the Properties of Lightweight Corundum-Spinel Refractory.

The use of a lightweight corundum-spinel refractory in working lining could reduce the thermal conductivity of industrial furnaces. In this study, bubble alumina was introduced to realize a lightweight Al2O3-MgAl2O4 refractory assisted by the reactive sintering of Al2O3 and MgO. The effects of alumina bubble content and sintering temperature on the phase compositions, microstructure and properties of the lightweight refractory were investigated. The results indicated that the overall performance of the lightweight Al2O3-MgAl2O4 refractory was mainly dominated by the content of alumina bubbles. The bulk density, compressive strength and thermal conductivity all decreased when the alumina bubble content increased from 10 to 30 wt%. Meanwhile, the sintering temperature also significantly affected the properties of the obtained refractory. It is worth noting that specimens fired at 1650 °C achieved a high refractoriness under load (RUL) of more than 1700 °C when alumina bubble content was less than 30 wt%, which was comparable to that of the dense Al2O3-MgAl2O4 refractory. The thermal conductivity of the obtained samples was remarkably decreased to no more than 2.13 W/(m·K). In order to overcome the trade-off between the light weight of the refractory and overall performance, it is feasible to adjust the content of alumina bubbles and raise the sintering temperature appropriately.

The effect of size and type of alumina nanopowder phase on the transparency and bending strength of bodies sintered with MgO and La2O3 sintering aid

The effect of size and type of alumina nanopowder phase on the transparency and bending strength of bodies sintered with MgO and La2O3 sintering aid

Effect of Li2CO3–Bi2O3 doping on the low-temperature sintering, structure, and dielectric and mechanical properties of MgO–TiO2(w) ceramics

Dense MgO–12% TiO2(w) ceramics containing 12 wt% TiO2, which were doped with Li2CO3–Bi2O3 composite sintering aids, were prepared at a low sintering temperature of 950 °C in this study. The effects of sintering additives on the sintering characteristics, phase composition, microstructure, and dielectric and mechanical properties of the ceramic samples were systematically investigated, and the influences of their phase composition and microstructure on the dielectric and mechanical properties were examined. The introduction of sintering aids produced a new Bi4Ti3O12 phase in the sample structure, while the residual Bi2O3 mixed with the newly formed Mg2TiO4 and Bi4Ti3O12 phases distributed at MgO grain boundaries formed a structure surrounding MgO grains. This structure filled the pores in the ceramic sample, which increased its density and enhanced the mechanical properties. At a Li2CO3–Bi2O3 content of 15 wt%, the density, flexural strength, and Vickers hardness of the ceramic samples reached their maximum values of 3.4 g/cm3, 218.9 MPa, and 778.7 HV, respectively. However, the further increase in the Li2CO3–Bi2O3 content deteriorated their dielectric properties although the dielectric constant and dielectric loss remained below 13.4 and 2.1 × 10−3, respectively. The findings of this work indicate that Li2CO3–Bi2O3 sintering aids can significantly lower the sintering temperature of MgO–12% TiO2(w) ceramics and control their dielectric and mechanical properties through microstructural changes.

Effects of the Content of MgO Additive and Sintering Temperature on the Densification of Alumina Insulator

Effects of the Content of MgO Additive and Sintering Temperature on the Densification of Alumina Insulator

Fabrication and characterizations of Ho3Sc2Al3O12 magneto‐optical transparent ceramics

AbstractCurrently, Faraday isolators and rotators for high‐power lasers are popular research topics; therefore, finding magneto‐optical materials with excellent Verdet constants and thermal properties is crucial. In this study, novel holmium scandium aluminum garnet (HoSAG) magneto‐optical ceramics were fabricated using vacuum sintering. This study improved the traditional ball milling method and pre‐synthesized powders with great sinterability after secondary ball milling and sieving. Different pre‐synthesis temperatures were found to have significant effects on the particle size and micromorphology of the synthesized powders. HoSAG ceramics with 0.05 wt% MgO sintering aid and held at 1700°C for 30 h reached a transmittance of 76.7% at 1550 nm. Meanwhile, HoSAG transparent ceramics maintain higher transmittance in the infrared region (>1500 nm) than terbium gallium garnet (TGG) crystals, indicating better application prospects. The Verdet constants of HoSAG magneto‐optical ceramics at 405, 532, 808, and 1064 nm were −421.5, −181.2, −65, and −32.3 rad T−1 m−1, respectively, which are slightly less than those of TGG magneto‐optical ceramics. Moreover, the thermal conductivity of HoSAG ceramics at 300.00 K was 4.89 W m−1 K−1, which is comparable to that of the TGG ceramics.

The effect of alumina-based sintering aid on the microstructure, selected mechanical properties, and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

Cf-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional Cf/C discs. The Cf-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of Cf-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al2O3–MgO, MgAl2O4, Al2O3–Y2O3, Al2O3–SiO2–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al2O3–SiO2–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al2O3–SiO2–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al8Si4O20 phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al2O3–SiO2–MgO sintering aid were higher than those of other sintering aids. The samples with the Al2O3–SiO2–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.

Effect of extra added Mg2+ and Si4+ on the microstructure and luminescence properties of Ce:YAG ceramic phosphors for high power LED/LD lighting

The study deals with the use of different amounts of MgO or tetraethyl orthosilicate (C8H20O4Si, TEOS), from 0.04 wt %, 0.06 wt %, 0.08 wt %, 0.4 wt %, 0.6 wt % to 0.8 wt %, respectively, as additional sintering aids during the vacuum sintering of transparent Ce3+ doped Y3Al5O12 (Ce:YAG) ceramics to investigate their effect on microstructure and luminescence properties. A comparative study was carried out by using X - ray diffraction, scanning electron microscopy/energy dispersive X - ray spectroscopy, optical and photo-luminesce spectra, as well as electroluminescent spectra. The dominant garnet phase was obtained even when adding 0.8 wt % of MgO or TEOS, while adding MgO led to finer average grain size (∼ 5 - 10 μm) and more homogenous distribution compared to TEOS (∼ 20 - 25 μm) in Ce:YAG ceramics. Adding a large amount of MgO was found to result in the segregation of MgO in Ce:YAG ceramics, while an excess of TEOS led to the segregation of Al2O3. A strong absorption peak at 308 nm was observed in Ce:YAG ceramics with MgO sintering aids, which was ascribed to the existence of Ce4+ induced by a charge compensation effect of Mg2+. The optimum transmittance reached 80% @ 800 nm in Ce:YAG ceramics when adding 0.6 wt % TEOS, which also exhibited a maximum luminous efficacy of 106 lm/W. Si4+ was experimentally proved to have a better optimization effect on luminous efficacy compared to Mg2+.

Highly Transparent Polycrystalline MgO via Spark Plasma Sintering.

Optically transparent ceramics and MgO in particular are promising materials for a wide range of optical applications. This study introduces exceptionally highly transparent MgO ceramics produced via spark plasma sintering (SPS) at relatively low temperature and pressure by optimal incorporation of LiF as a sintering additive. The effect of LiF content on the microstructural and optical properties is presented with emphasis on its function as a densification aid and an agent for minimizing residual carbon contamination. Fully dense MgO discs, 20 mm in diameter and 2 mm thick, with ∼80% in-line transmission at 800 nm and >85% transmission in the infrared range (2-6 μm), are attained. These results demonstrate outstanding transparency in SPS polycrystalline MgO in the 800 nm range, only 7% below the theoretical value. In addition, this work strengthens our understanding of the LiF action mechanism during MgO sintering and its influence on texture development in the SPS-pressing direction. These findings pave the way for fabrication of large, fully dense samples with nearly theoretical transparency.

Effect of MgO and Fe2O3 dual sintering aids on the microstructure and electrochemical performance of the solid state Gd0.2Ce0.8O2-δ electrolyte in intermediate-temperature solid oxide fuel cells.

Solid state electrolytes have been intensively studied in the solid oxide fuel cells (SOFCs). The aim of this work is to investigate the effects of MgO and Fe2O3 dual sintering aids on the microstructure and electrochemical properties of solid state Gd0.2Ce0.8O2-δ (GDC) electrolytes, which are prepared by a sol-gel method with MgO and Fe2O3 addition to the GDC system. It is found that the addition of MgO and Fe2O3 can reduce the sintering temperature, increase densification and decrease the grain boundary resistance of the electrolyte. The 2 mol% MgO and 2 mol% Fe2O3 co-doped GDC (GDC-MF) exhibits the highest grain boundary conductivity. At 400°C, the grain boundary conductivity and total conductivity of GDC-MF are 15.89 times and 5.56 times higher than those of GDC. The oxygen reduction reaction (ORR) rate at the electrolyte/cathode interface of GDC-MF is 47 % higher than that of GDC. Furthermore, the peak power density of a single cell supported by GDC-MF is 0.45 W cm−2 at 700°C, 36.7% higher than that of GDC. Therefore, the GDC-MF should be a promising electrolyte material for intermediate-temperature solid oxide fuel cells (IT-SOFCs).

Fabrication of periclase and magnesium aluminate spinel refractory from washed residue of secondary aluminum dross

A novel method for fabricating the periclase and magnesium aluminate spinel refractory from the secondary aluminum dross was proposed in the present work by adding magnesium oxide. The fabrication mechanism of the refractory was analyzed by thermogravimetric and differential thermal analysis, scanning electron microscopy, and X-ray Diffraction. The effects of MgO addition and sintering temperature on the mechanical properties and density of refractories were studied. The results showed that with the increase of sintering temperature, the purity, crystallinity, and densification of the refractory were significantly improved, and the porosity of the refractory was decreased. As an obvious second phase in the refractory, periclase can strengthen the grain–grain bonding and inhibit the grain boundary movement. With the increase of MgO addition, due to the significant reduction of porosity, the improvement of grain size uniformity and the absence of microcracks, the flexural strength and the impact toughness were significantly improved. When the MgO addition was 50 wt% at the sintering temperature of 1600 °C, the density and porosity of the refractory were 2.92 g/cm3 and 18.2%, while the flexural strength and impact toughness can reach 270 MPa and 3.7 MPa m1/2, respectively.

Magnesite-derived MgO promoted with molten salts and limestone as highly-efficient CO2 sorbent

This study evaluated the CO2 capture performance of MgO obtained from mineral magnesite and doped with limestone and molten Li, Na and K nitrates under consecutive sorption/desorption cycles via thermogravimetric analysis. Increasing the molten promoter loading resulted in a higher CO2 sorption rate, while elevated CaCO3 amounts impeded CO2 diffusion. When exposed to a 30 % CO2 flow at 300 °C for 30 min, the sorbent with alkali salts and CaCO3 to MgO molar ratios of 0.20 and 0.05 respectively attained a capture of 7.2 mol CO2/kg of sorbent and a negligible activity loss (∼6%) after 50 cycles. This performance was promising, considering the mineral nature of precursors. In-situ X-ray diffraction revealed the growth of the MgO crystal after each desorption, proving the gradual MgO sintering. However, the morphological transformations occurring during cyclic operation and especially if high conversions of MgO are reached, trigger an alkali salt redistribution that grants high stability.

Surface hydrate-assisted low- and medium-temperature sintering of MgO

MgO was readily densified by cold sintering at 180 °C with the help of its surface hydrate, but the product contained a considerable amount of Mg(OH)2. By increasing the sintering temperature to 450 °C to remove Mg(OH)2, pure MgO with a relative density of 93% was achieved. The effects of processing parameters such as Mg(OH)2 amount, initial liquid phase content, uniaxial pressure and dwelling time on densification behavior were investigated. Densification was achieved through a series of processes, including hydration of MgO, decomposition of Mg(OH)2, particle rearrangement, grain boundary diffusion and recrystallization of newly formed MgO. This work provides a novel approach to solve the challenge of densification in most oxides by means of a cold sintering process.