Sintering Of MgO Research Articles

In situ synthesis and phase analysis of low density O′-sialon-based multiphase ceramics

In situ formed low density O′-sialon-based multiphase ceramics were prepared by liquid-phase sintering method at 1400°C with Si3N4, SiO2 and Al2O3 as raw materials. Crystalline phases were identified by X-ray diffraction (XRD). The quantitative phase analysis was finished by matrix-flushing method and the substitution parameter x value of O′-sialon was estimated. The effects of sintering additives on the phase composition of the material were studied. The results show that, when using Y2O3 alone, Al6Si2O13 phase can be formed in the material, but when using Y2O3 and MgO, MgAl2O4 phase can be preferentially formed and the Al6Si2O13 is not observed. The mechanical properties of the material were measured and the relationships between microstructure and mechanical properties were discussed. The sample with Y2O3 and MgO sintering additives, using fused quartz alone as SiO2 source, displays a combination of high bending strength (163 MPa) and good fracture toughness (3.11 MPa·m1/2). Bending strength and fracture toughness of the samples increase with the increase of the content and aspect ratio of elongated grains and decrease with the increase of the porosity.

Effects of Sintering Additive on Mechanical Properties of Alumina Matrix Composites Reinforced with Carbon Nanotubes

Carbon nanotube is nature's smallest fiber and predicted to have a range of unusual mechanical and electrical properties. One possible route to harnessing these properties for applications would be to incorporate nanotubes in a composite material. Here, we report the mechanical properties of multi-walled carbon nanotube (MWCNT) reinforced alumina composites made with a pristine MWCNT and an acid-treated version that have nanoscale defects on their surfaces from an acid treatment. It was demonstrated that surface modification of the MWCNT is effective in improvement of bending strength and fracture toughness of the MWCNT-reinforced alumina composites. On the basis of the results, we also prepared three sets of the acid-treated MWCNT-reinforced alumina composites having different sintering additives, in order to investigate the effects of sintering additives on their microstructures and mechanical properties. Mechanical properties of the composites were dependent mostly on the type of sintering additives and amount of MWCNT. The 0.9 vol.% acid-treated MWCNT-reinforced alumina composites with MgO sintering additive gave the highest bending strength (689.6 ± 29.1 MPa) and fracture toughness (5.90 ± 0.27 MPa·m1/2), respectively.

Preparation of Nanometer MgO by Ion Exchange Resin Method

In this paper, nanometer MgO powder was prepared by using MgCl2•6H2O as the raw material, and environmental benign strong-base anion exchange resin as the induced-precipitation regent. The influence of the concentration of MgCl2 solution, drying method, calcination temperature and time on particle size and physico-chemical features of MgO was studied. The optimal technical conditions were obtained. The XRD and SEM results show that the nanometer MgO prepared under the optimal technical conditions has regular hexagonal lamellar structure, and is composed of nanocrystals with average size between 10 to 25 nm. The existence of dry N2 prevented the sintering of MgO during decomposition processing of Mg(OH)2. Compared with the other nanometer MgO preparation methods, ion exchange resin method has the advantages of low cost, low pollution, high yield and environmental benign; therefore, it appears to be a promising method for the industrial manufacturing of nanometer MgO.

A practical fabrication method for the advanced heterogeneous fuel with magnesia containing minor actinides

Fabrication tests on advanced heterogeneous fuel with MgO were carried out for the purpose of establishing a practical fabrication method. Advanced heterogeneous fuel consists of spheres (diameter greater than 100 μm) of a minor actinide oxide and MgO inert matrix (macro-dispersed type fuel). Macro-dispersed type fuel pellets with a high density above 90% T.D. were successfully fabricated. In addition, the fabricated pellets showed a homogeneous dispersion of near spherical host phase granules. These were attained by optimization of the fabrication process and conditions; i.e. a preliminary heat treatment of raw powders of host phase, an adjustment of pressure at the granulation process, deployment of a spray-dry process for MgO sphere preparation and sintering in a He atmosphere. From these results, a practical fabrication method for MgO-based macro-dispersed type fuel based on a simple powder metallurgical technique was established.

Synthesis and Pressureless Sintering of Zirconium Phosphate Ceramics

Zirconium phosphate (ZrP2O7) powder is synthesized by direct reaction between ZrO2 and H3PO4 and is used to prepare ZrP2O7 ceramics with MgO and Al2O3 as the sintering additives by pressureless sintering technique. The mechanism of reaction between ZrO2 and H3PO4 is analyzed. The results suggest that the reaction between ZrO2 and H3PO4 is completed at 250°C and ZrP2O7 is stable up to ∼1200°C. At temperatures above ∼1200°C, decomposition starts to occur, resulting in the formation of Zr2O(PO4)2. After exposure to temperatures as high as 1500°C, decomposition is still not complete. The pure ZrP2O7 ceramic, however, is difficult to sinter to high density. In contrast, ZrP2O7 ceramics with MgO and Al2O3 sintering additives can be liquid‐phase sintered to relative densities of 90%–96% at sintering temperatures of 1250°–1500°C, without significant decomposition. The microstructure is observed by scanning electron microscopy. The effect of density, phosphate glass phase, and grain growth on bending strength is analyzed. Because of the abnormal grain growth and microcracking of ZrP2O7 ceramics, the bending strength first increases and then decreases as a function of the sintering temperature.

Densification maps for spark plasma sintering of nanocrystalline MgO ceramics: Particle coarsening and grain growth effects

Recently densification maps for spark plasma sintering (SPS) of nanocrystalline MgO were constructed using a HIP model and constant particle size during the process. Here, the effects of particle coarsening during the early stage and grain growth during the later stage of densification were integrated into the model. Densification diagrams were calculated between 500 °C and 900 °C, pressures range 30–500 MPa and initial particle size 10–40 nm. Plastic yield and diffusion process dominated during the densification in SPS. The power law creep exhibited negligible densification rates. Threshold pressure exists for full densification of nanocrystalline compact with minimal grain growth. Particle coarsening and grain growth resulted in densification patterns that follow the experimental SPS results. The calculated densities were lower than the experimental results and fall short in describing the experimental densification rates of nanocrystalline MgO. Therefore additional densification mechanisms with faster kinetics should be incorporated.

Effect of replacement of MgO by CaO on sintering, crystallization and properties of MgO–Al2O3–SiO2 system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al2O3–SiO2 system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D50 = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10−3) at 1 MHz, a low TCE (4.0–4.25 × 10−6 K−1), very close to that of Si (∼3.5 × 10−6 K−1), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.

Synthesis of high surface area nanometer magnesia by solid-state chemical reaction

Nanometer MgO samples with high surface area, small crystal size and mesoporous texture were synthesized by thermal decomposition of MgC2O4 · 2H2O prepared from solid-state chemical reaction between H2C2O4 · 2H2O and Mg (CH3COO)2 · 4H2O. Steam produced during the decomposition process accelerated the sintering of MgO, and MgO with surface area as high as 412 m2 · g−1 was obtained through calcining its precursor in flowing dry nitrogen at 520°C for 4 h. The samples were characterized by X-ray diffraction, N2 adsorption, transmission electron microscopy, thermogravimetry, and differential thermal analysis. The as-prepared MgO was composed of nanocrystals with a size of about 4–5 nm and formed a wormhole-like porous structure. The MgO also had good thermal stability, and its surface areas remained at 357 and 153 m2·g−1 after calcination at 600 and 800°C for 2 h, respectively. Compared with the MgO sample prepared by the precipitation method, MgO prepared by solid-state chemical reaction has uniform pore size distribution, surface area, and crystal size. The solid-state chemical method has the advantages of low cost, low pollution, and high yield, therefore it appears to be a promising method in the industrial manufacture of nanometer MgO.

Effect of MgO addition and sintering parameters on mullite formation through reaction sintering kaolin and alumina

In the present work, the influence of MgO addition and sintering parameters on the formation and densification of mullite was investigated. The morphology of powders and the microstructure of the sintered samples were characterised by means of a scanning electron microscope. X-ray diffraction was used to characterise phases formed in sintered samples. The density of sintered samples was measured using a densimeter and quantified according to the Archimedes principle. MgO was added at 1, 2, 3, 4, 5 and 6 wt-% to kaolin and alumina and the powders were ball milled for 5 h then uniaxially compacted at 75 MPa and finally sintered at 1500, 1550, 1600 and 1650°C for 2, 4, 6 and 8 h. It was found that addition of MgO not only affected mullite formation but also promoted grain growth. For samples containing 0, 1 and 2 wt-%MgO only mullite was formed. While, in addition to mullite, Al2O3 was present in sample containing 3 wt-%MgO. At higher MgO content (4, 5 and 6 wt-%), three phases, i.e. mullite, Al2O3 and spinel, were formed. Addition of 1 wt-%MgO increased the density of all samples for all sintering times and higher densities corresponded to higher sintering temperatures. At higher MgO content, higher temperatures led to lower densities and lower temperatures led to higher densities for almost all sintering times.

Sintering, crystallization and properties of MgO–Al 2O 3–SiO 2 system glass-ceramics containing ZnO

The effect of replacement of Al 2O 3 by ZnO on the sintering and crystallization behavior of MgO–Al 2O 3–SiO 2 system glass-ceramics was investigated. The results show that with increasing ZnO content, the melting temperature and crystallization temperature of the glass-ceramics were lowered, and the glass transition temperature firstly decreased and then increased. With the replacement of 5 and 8 wt.%, the predominant crystalline phase in the glass-ceramics was found to be α-cordierite and the secondary crystalline phase to be gahnite and quartz. When the replacement was increased to 11 wt.%, the predominant crystalline phase was found to be gahnite and quartz and the secondary phase to be α-cordierite. Only the sample containing 8 wt.% ZnO (abbreviated as sample Z8) can be fully sintered before 1000 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and gahnite is achieved by using fine glass powders ( D 50 = 3 μm) fired at 900–925 °C. The as-sintered density approaches 97.0% theoretical density. The flexural strength of sample Z8 increases with sintering temperature, which closely corresponds to its relative density. The thermal expansion coefficient (TCE) of sample Z8 sintered at different temperatures mainly depends on its crystalline phases. The dielectric property of sample Z8 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic Z8 exhibits a fairly low dielectric constant (5.0–5.2), a low dielectric loss (≤10 −3) at 1 MHz, a low thermal expansion coefficient (4.0–4.2 × 10 −6 °C −1), very close to that of Si (∼3.5 × 10 −6 °C −1) and a higher flexural strength ≥125 MPa), suggesting that it would be a promising material in the electronic packaging field.

On the precipitation of coherent spinel nanoparticles in Ti-doped MgO

The Mg 2TiO 4/MgO composites prepared by reactive sintering of MgO and TiO 2 powders (9:1 molar ratio) at 1600 °C and then air-cooled or further aged at 900 °C were studied by X-ray diffraction and analytical electron microscopy in order to characterize the microstructures and formation mechanism of nanosized Mg 2TiO 4 spinel precipitated from Ti-doped MgO. Expulsion of Ti 4+ during cooling caused the formation of (001)-specific Guinier–Preston zone under the influence of thermal/sintering stress and then the spinel precipitates, which were about 30 nm in size and nearly spherical with {111} and {100} facets to minimize coherency strain energy and surface energy. Secondary nanosized spinel was precipitated and became site saturated during aging at 900 °C, leaving a precipitate-free zone at the grain boundaries of Ti-doped MgO. The intergranular spinel became progressively Ti-richer upon aging at 900 °C and showed 〈110〉-specific diffuse scatter intensity likely due to short-range ordering and/or onset decomposition.

Research for sintering process of AlN ceramics with MgO sintering additive

Research for sintering process of AlN ceramics with MgO sintering additive

The sintering and microwave dielectric characteristics of MgO–CaO–Al 2O 3–SiO 2 glass-fluxed Ba(Mg 1/3Ta 2/3)O 3 ceramics

Samples of 6 wt.% MgO–CaO–Al 2O 3–SiO 2 (MCAS) glass-fluxed Ba(Mg 1/3Ta 2/3)O 3 ceramics are investigated, and its sintering and microwave dielectric properties are measured. The densification of the MCAS-fluxed Ba(Mg 1/3Ta 2/3)O 3 ceramics can be performed well at 1350 °C. Although the 1350 °C-sintered Ba(Mg 1/3Ta 2/3)O 3 ceramics own 96.3% theoretical density, the satellite phases of Ba 5Ta 4O 15, Ba 4Ta 2O 9, Ba 7Ta 6O 22 and Mg 4Ta 2O 9 are difficult to eliminate completely. As the sintering temperatures increase from 1250 to 1450 °C, the microwave dielectric properties of MCAS-fluxed Ba(Mg 1/3Ta 2/3)O 3 ceramics change as follows: The ε r values first decrease, saturate at 1350 °C-sintered ceramics and then slightly increase. The Q× f values first increase, saturate at 1350 °C-sintered ceramics and then slightly decrease. The τ f values linearly shift from 95.3 to −39.9 ppm/°C.

Correlation between MgO Doping and Sintering Characteristics in Hydroxyapatite/β-Tricalcium Phosphate Composite

Correlation between MgO Doping and Sintering Characteristics in Hydroxyapatite/β-Tricalcium Phosphate Composite

Sintering Kinetics Of MgO Studied on Magnesite of Provenance from the Chita Deposit

The potential use of a mathematical simulation method for accumulative recrystallization of magnesite of provenance from the Chita deposit is considered. The densification rate in sintered MgO, that is, the rate at which grain boundaries form at the expense of contracting intergranular pores, is shown to be much higher than the rate at which the grain boundaries disappear during the accumulative crystallization. An equation is derived that provides an adequate description of the kinetic features of MgO sintering. The numerical parameters of this are physically meaningful, which allows one to gain deeper insight into the sintering mechanism of MgO and thus go beyond the limits of a purely phenomenological approach.

Dopant incorporation in ceramics prepared by binder coagulation casting

Uniform incorporation of inorganic dopants and sintering aids into ceramic green bodies is necessary for the development of controlled microstructures during sintering. Binder coagulation casting (BCC) is a novel near-net shape fabrication process that is driven by controlled adsorption of polymeric binders. Two methods were used to incorporate small amounts of inorganic additives into ceramic compositions formed using BCC: the conventional mixed powder method and a novel method of using the organic additive system in BCC as a chelating agent for dopant cations. Cobalt dopant in zirconia was used as a model system for this work. MgO sintering aid in alumina was also examined. Sintering behavior and microstructure analysis using AFM are discussed.

Influence of Mechanical Pressure and Temperature on the Chemical Interaction Between Steel and Silicon Nitride Ceramics

The chemical interaction between a Si3N4ceramic, with Al2O3and MgO sintering additives, and DIN 100Cr6 steel was studied by means of static interaction couple experiments between 500 and 1200 °C. At 500 °C, the ceramic was chemically stable in contact with the steel. In the temperature range between 700 and 1100 °C, the silicon nitride dissociated in contact with the steel. The Si dissolved and diffused into the steel, whereas a nitrogen pressure built up in the micropores at the interface, limiting and inhibiting the reaction rate. The strength of the obtained interfacial bond was too low to withstand the residual stresses formed during cooling, and therefore, the interaction couples fell apart during cooling. Above 1100 °C, the nitrogen also dissolved and diffused into the steel, enhancing the overall rate of interaction. A strong interface was formed, resulting in a well-defined interaction layer on the ceramic side of the interaction couple. The mechanical pressures on the interaction couples were adjusted to study the influence of plastic deformation of the steel on the chemical interaction. Higher contact pressures resulted in more homogeneous and uniform interaction layers. The reactivity of plastically and elastically deforming steel, however, was found to be the same.

Effects of Additives on Microstructure and Properties of Alumina‐Silicon Carbide Nanocomposites

Al2O3/5‐vol%‐SiC nanocomposites have been fabricated by using pressureless sintering with MgO and/or Y2O3 sintering aids and post‐hot isostatic pressing (HIPing), which circumvents the limitations of hot pressing. Al2O3/SiC nanocomposites that have been doped with 0.1 wt% MgO and 0.1 wt% MgO + 0.1 wt% Y2O3 show an increased sintering density and a homogeneous microstructure, as well as a high fracture strength (1 GPa) after HIPing. In contrast, using Y2O3 as a dopant has a negative impact on the microstructure and the fracture strength. The results suggest that MgO, as a sintering additive, has a key role in improving the densification and controlling the microstructure of Al2O3/SiC nanocomposites.

Oxidation Behavior of NBD 200 Silicon Nitride Ceramics

The oxidation behavior of NBD 200 Si3N4 containing 1 wt% MgO sintering aid was investigated in oxygen at 900°‐1300°C. The oxide growth followed a parabolic rate law with an apparent activation energy of 260 kJ/mol. The oxide layers were enriched with sodium and magnesium because of outward diffusion of intergranular Na+ and Mg2+ cations in the ceramics. The 2‐4 orders of magnitude higher oxidation rate for NBD 200 Si3N4 than for other Si3N4 ceramics with a similar amount of MgO could be attributed to the presence of sodium. The oxidation process was most likely rate limited by grain‐boundary diffusion of Mg2+.

The influence of B 2O 3 on the sintering of MgO–CaO–Al 2O 3–SiO 2 composite glass powder

B 2O 3 has been found to be a useful flux to densify the MgO–CaO–SiO 2–Al 2O 3 (MCAS) composite glass powders, which are obtained by the sol–gel method. The temperatures needed to fully densify MCAS decrease with increase of B 2O 3 addition. For MCAS with different amounts of B 2O 3 addition, no apparently exothermic peaks are associated to the glass crystallization and the polymorphic transformation of μ-cordierite to the α-form. The softening temperature ( T s) of MCAS glass decreases with the increase of B 2O 3 additive. For MCAS with 6 wt% B 2O 3 added, only one broad endothermic peak associated T s is observed in the differential thermal analysis curve. For the same sintering temperature, as the amount of B 2O 3 addition increases, the crystallization rates of anorthite and cordierite firstly increase, after reaching a maximum (900°C for 3 wt%, 930°C for 1 wt%), and then decrease. Too much B 2O 3 is not necessary because it inhibits the cordierite and anorthite crystallization.