Magnesia Ceramics Research Articles

Preparation and properties of porous MgO based ceramics from magnesite tailings and fused magnesia

Porous MgO based ceramics have been widely used in some key industries due to their excellent performance. To investigate the feasibility of preparing porous magnesia ceramics using magnesite tailings as a magnesium source. Porous MgO based ceramics were prepared using magnesite tailings, fused magnesia as main raw materials and soluble starch as pore-forming agent. The starting materials were mixed in different ratios and sintered at 1500 °C for 2 h to obtain good mechanical and thermophysical properties. The Mg2SiO4 plays a fixing role, inhibiting the growth of MgO grains and further promoting the discharge of pores. This fixation can prevent the linear expansion of cracks, thus increasing the fracture surface energy of the specimen. In addition, the scattering effect of pores on gas molecules decreases as the magnesite tailings content increase, so that the thermal conductivity increases. The results showed that when the mass ratio of magnesite tailings to fused magnesia was increased from 1:1 to 1:0 with 4 wt% soluble starch as the pore-forming agent, the apparent porosity decreased from 35.59% to 11.8%; the bulk density increased from 2.37 to 2.82 g∙cm−3; the linear shrinkage ratio increased from 9.28 to 24.23%; the cold compressive strength increased from 47.75 to 226.52 MPa; the residual strength retention rate after one thermal shock cycles at 1100 °C was between 64.05 and 87.76%; and the thermal conductivity increased from 7.44 to 12.23 W∙m−1 K−1.

Molecular dynamics investigation of the thermal behaviors of magnesium oxide ceramics at different pressures and temperatures.

Today, magnesia ceramics have attracted considerable attention due to their essential properties. Therefore, this paper investigates the impact of temperature (T) and pressure (P) on the thermal manner of magnesia ceramics using molecular dynamics simulations (MDS). As the T increases, the mobility of the structures increases. Therefore, the heat flux (HF) in the structures increments slightly due to the greater movement and the larger oscillation amplitude of the atomic samples. On the other hand, with increasing P, the oscillation amplitude and displacement of atomic samples are limited. Therefore, the thermal properties of the structure are expected to decrease. Studies show that increasing T from 250 to 350K increases the average HF from 0.73 to 0.89W/m2. Also, the average thermal conductivity (TC) increases from 30.58 to 38.27W/mK. So, increasing the T means a certain amount of energy is fluxed in a shorter time. On the other hand, increasing the P from 0 to 5bar decreases the average HF from 0.82 to 0.65W/m2. Also, this issue leads to a decrease in the average TC from 33.49 to 30.96W/mK.

Dolomite-zirconia reaction sintered bonded coarse magnesia ceramics: effect of the bonding proportion

Refractory ceramic materials were manufactured, mainly composed of magnesia grains (coarse and medium fraction) and CaZrO3/MgO as a binding phase produced from the reaction sintering between CaMg(CO3)2 (dolomite) and m-ZrO2 (monoclinic zirconia), which constituted the finer fraction. Different relationships between the proportion and the size of the different aggregates were studied. The microstructure of the materials was characterized in terms of density, crystalline phases, grain phase distribution, and morphology. A combination of X-ray diffraction analysis and scanning electron microscopy with microanalysis was used. The mechanical behavior was determined in terms of the room temperature dynamic Young’s modulus and the 3-point bending modulus of rupture (MOR) at room temperature. All the materials remained with porosities above 30%. The microstructure developed was similar between the sintered materials, characterized with MgO coarse grains bonded by CaZrO3/c-ZrO2 phases, and dispersed irregularly shaped pores. The MOR was directly related to the finer fraction content in the composition, where a higher quantity of finer fraction resulted in a higher MOR.

The effect of sintering on the properties of magnesia-granite sludge ceramics shaped by temperature-induced forming

Recycling waste materials have attracted considerable attention owing to the increased interest in depressing the negative impact on the environment. The present work investigates the effect of increasing the granite sludge-waste materials from granite manufacture-on the physical, mechanical and electrical properties of magnesia ceramics. The temperature-induced forming (TIF) method is used to shape magnesia-granite sludge ceramics. The sintered ceramics are characterized by XRD and SEM, in addition to measuring the bulk density, apparent porosity, linear shrinkage, microhardness, and electrical resistivity. Sintering of magnesia-granite bodies resulted in the formation of periclase-forsterite ceramics. The XRD results show that the forsterite phase increases reaching 78% at 40 wt% granite. The increase in granite sludge from 10 to 40 wt% increases the bulk density from 1.86 to 2.69 g/cm3 and the apparent porosity decreases from 40 to 15%, respectively. Microhardness is improved by increasing the granite sludge wt% reaching 7.9 GPa. The microstructure of the formed periclase-forsterite is versatile with a rhombohedral structure for periclase and a flower structure for forsterite. The forsterite phase is formed by the diffusion of granite sludge into the formed periclase.

Effect of titanium oxide addition on magnesia refractories

This work contains the results of investigations into the influence of titanium oxide (TiO2) addition on the properties of refractory magnesia ceramics. The presented research involved adding titanium oxide in a classic way, i.e. directly to the ceramic mix. The conducted laboratory tests revealed a significant impact of this oxide on the properties of refractory materials. Addition of a small amount of TiO2 favoured the ceramic mix sintering whereas adding a bigger amount—more than 10 wt% resulted in the formation of refractories characterised by considerable porosity and low mechanical strength. Addition of this oxide also slightly improves the corrosive resistance of refractories.

Effects of the surface processing on the tribological performance of C/SiCs under dry friction

A new friction counterpart for carbon fiber-reinforced silicon carbide ceramic-matrix composites (C/SiCs) and zirconia (ZrO2) toughened by magnesia ceramics is proposed. The effects of the C/SiC surface processing parameters friction on the tribological performance are investigated under dry friction and ambient temperature conditions. The wear tests are carried out using the pin-on-disc friction method. Scanning electron microscopy (SEM) on an instrument equipped with an energy dispersive spectroscopy (EDS) is used to observe the surfaces of the pins and discs before and after the application of friction to reveal the wear mechanism. The results show that surface processing influenced the tribological properties of C/SiC significantly. When the pressure is 30 N, the speed is 0.5 m/s, and the C/SiC surface is ground using 1500# sandpaper, the counterpart tribological performance is the best among the samples considered herein. It is found that the retention ability of the counterparts influenced the tribology performance significantly.

Foaming mechanisms of different foaming agents and their effects on the microstructures of porous magnesia ceramics

The foaming and foam stabilization mechanisms of potassium laurate (PL), potassium oleate (PO), sodium dodecyl sulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS) were investigated in this study. Magnesia porous ceramics were fabricated by a foaming method, and the sample microstructures were analyzed by scanning electron microscopy (SEM) following heat treatment. Results showed that the foaming ability of the surfactants decreased in the order PO > SDS > SDBS > PL. The addition of calcium stearate (CS), a foam-stabilizing agent, extended the half-lives of SDS, SDBS, and PO foams in the order SDBS > PO > SDS. However, the addition of CS to PL significantly decreased its foam half-life. Pore sizes of the heat-treated samples decreased in the order PL > SDS > SDBS > PO, while the evenness of the pore distribution decreased in the order PO > SDBS > SDS > PL. Porous ceramics prepared with PO and CS had the smallest and most evenly distributed pores in the range of 5–10 μm.

Effect of Different Crystal Form Nanometre Zirconia on Thermal Shock Resistance of Magnesia Ceramic

The magnesia ceramic specimens were prepared using high purity magnesia powder, monoclinic nanometre zirconia, tetragonal nanometre zirconia, cubic nanometre zirconia as starting materials in order to improve thermal shock resistance of magnesia ceramic. The specimens were sintered at 1550°C for 2 hours under air atmosphere. The thermal shock resistance and sintering property of magnesia ceramics with different crystal nanometre zirconia additives were investigated. The results show that density of the specimens with zirconia is improved due to the dispersal distribution of zirconia in periclase grain boundary. The sintering property of magnesia with zirconia additives are considerably promoted. The thermal shock resistance of the specimens with tetragonal zirconia or cubic zirconia is improved due to second crystalline phase toughening mechanism of zirconia.Thermal shock resistance of specimens with monoclinic zirconia is improved due to phase transformation toughening mechanism and second crystalline phase toughening mechanism of zirconia.

Power efficiency enhancement of dielectric assist accelerating structure

Dielectric assist accelerating (DAA) structures, which have extremely high Q-factor and shunt impedance at room temperature, are a type of dielectric loaded accelerating structure. Previous work that focused on developing a prototype DAA structure using high-purity magnesia ceramic showed that the unloaded quality factor (Q0) of the structure was 119,314 and the shunt impedance per unit length (Zsh) was 617 MΩ/m at room temperature. In order to realise a DAA structure with a higher acceleration efficiency than that attained with the prototype, we investigated the selection of an optimum dielectric material based on the analytical calculations of a two-dimensional (2D) DAA structure model and the usefulness of a DAA structure operating at a cryogenic temperature. The analytical calculations of the 2D-DAA structure showed that the Ba(Mg1/3Ta2/3)O3-Ba(Mg1/2W1/2)O3 (BMT-BMW) ceramic was the optimum dielectric material for the DAA structure as it had the smallest rf losses. A simulation of a five-cell C-band (5.712 GHz) DAA structure with a BMT-BMW ceramic indicated that Q0 of the accelerating mode was over 120,000 and Zsh exceeded 850 MΩ/m at room temperature. For the DAA structure operating at a cryogenic temperature of 27 K, Q0 and Zsh of the DAA structure, which consisted of high-purity magnesia ceramics and oxygen-free high-conductivity copper, were estimated by simulation to be greater than 765,000 and 3.8 GΩ/m, respectively.

Effect of Y2O3 Additions on Microstructure and Properties of Alumina–Magnesia Ceramics

The purposes of this research are to prepare and investigate the characterization of 0.7MgO-xY2O3-(99.3-x)Al2O3 by varies x at 0.0, 2.0, 4.0, 6.0, 8.0 and 10.0 wt% (AMY). AMY samples were synthesized by solid state reaction under two different sintering methods conventional sintering method (NS) and two stage sintering method (TS). Ceramic samples were sintered by conventional sintering at 1600°C for 5 h and using two stage sintering with the first sintering temperature (T1) at 1600°C for 30 min and cooling down to the second sintering temperature (T2) at 1450°C for 10 h. The phase composition of the samples were characterized using XRD technique. XRD patterns from two method sintering revealed phase combination of Al2O3, MgAl2O4, MgO and Y2O3. It was found that it is not different XRD patterns from two method sintering. SEM micrographs form NM and TS showed that the shape of the ceramic grains were polyhedron and ellipsoid, while the grain sizes form NS were in the range of 0.91-1.00 μm and the grain sizes from TS were in the range of 0.34-0.48 μm. The samples from NS and TS is showed optimum mechanical properties by AMY with Y2O3 additions between 4-6 wt%.

Corrosion testing of zirconia, beryllia and magnesia ceramics in molten alkali metal carbonates at 900 °C

An electrochemical cell containing molten Li2CO3–Li2O at 900 °C has been proposed for the conversion of the greenhouse gas CO2 to CO for chemical energy storage. In the current work, we have examined the corrosion resistance of zirconia, beryllia and magnesia ceramics at 900 °C in the Li2CO3–Li2O and Li–Na–K carbonate eutectic mixtures to identify suitable electrically insulating materials. Conclusions regarding material stability were based on elemental analysis of the melt, primarily via X-ray photoelectron spectroscopy, a particularly sensitive technique. It was found that magnesia is completely stable for at least 33 h in a Li2CO3–Li2O melt, while a combined lithium titanate/lithium zirconate layer forms on the zirconia ceramic as detected by XRD. Under the same melt conditions, beryllia shows considerable leaching into solution. In a Li–Na–K carbonate eutectic mixture containing 10.2 mol% oxide at 900 °C under standard atmospheric conditions, magnesia showed no signs of degradation. Stabilization of the zirconia content of the eutectic mixture at 0.01–0.02 at% after 2 h is explained by the formation of a lithium zirconate coating on the ceramic. On the basis of these results, we conclude that only magnesia can be satisfactorily used as an insulating material in electrolysis cells containing Li2CO3–Li2O melts.

Densification of Nanocrystalline Ceramics by Combustion Reaction and Quick Pressing

The technique of combustion reaction and quick pressing was adopted to prepare dense nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 100 nm was investigated, under the applied pressure of up to 170 MPa, and the temperature of 1740–2080 K with ultra-high heating rate of above 1700 K/min. As a result, pure magnesia ceramics with a relative density of 98.8% and an average grain size of 120 nm was obtained at 1740 K and 170 MPa, while the ones with decreased relative density and increased grain size were produced under the increasing temperature and the identical pressure conditions. The results indicated that grain growth of the nanocrystalline magnesia was effectively restrained by the combined effect of the ultra-high heating rate and the high pressure. Moreover, under the particular sintering conditions, there existed an appropriate temperature range for the preparation of dense nanocrystalline magnesia, and the excessive temperature would not only exaggerate grain growth but also impede densification.

Microstructural evolution and mechanism of grain growth in magnesia ceramics prepared by high pressure and temperature with ultra-high heating rate

The fast densification method of combustion reaction plus quick pressing was adopted to prepare nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 100 nm was investigated, under the applied pressure of up to 170 MPa, and the temperature range of 1740–2080 K with ultra-high heating rate (above 1700 K/min). High-purity magnesia ceramics with a relative density of 98.8% and an average grain size of 120 nm was obtained at 1740 K, and the grain growth during the densification process was effectively restrained. The characteristic morphology of evaporation-condensation was observed in the compact prepared at 2080 K, which revealed the actual process of mass transfer by gas diffusion. Moreover, the investigation on the microstructure evolution and mechanism of grain growth was carried out, on the basis of as-preserved nanocrystalline ceramics. The result indicated that the grain growth of the nanocrystalline MgO was controlled by the mechanism of evaporation-condensation rather than surface diffusion. Furthermore, the pressure had an influence of restraining the grain growth based on solid diffusion and strengthening the effect of gas diffusion with the increasing temperature. Under the particular conditions, there existed an appropriate temperature for the densification of nanocrystalline magnesia, while the excessive temperature would exaggerate grain growth and impede densification.

Dissolution behavior of plutonium containing zirconia–magnesia ceramics

This study explores the dissolution properties of zirconia–magnesia ceramics containing plutonium as the basis of an inert matrix nuclear fuel. The magnesium oxide phase remains pure MgO, while the zirconia incorporates a small amount of magnesium oxide along with all of the plutonium oxide and erbium oxide. The performance of the material under reactor and repository environments was examined. Reactor conditions are examined using a pressure vessel to expose the material to 300°C water. To assess the performance of the material as a waste form it was submerged in 90°C water for 1000h. In both aqueous dissolution studies there was minimal release of less than 0.8wt.% of plutonium from the material. To examine the potential for recycling, the dissolution behavior of the fuel matrix was examined in acidic solutions: pure nitric acid and a nitric acid–hydrofluoric acid–peroxide solution. Both acidic media exhibit potential for dissolving plutonium from the zirconia matrix. The experiments performed in this study are meant to lay a foundation for the chemical performance of zirconia–magnesia inert matrix fuel containing fissile material and burnable poison.

Anomalous grain growth during hot isostatic pressing of magnesia ceramics made from starting powders with different coarse/fine mixing ratios

The influence of coarse powder on grain growth during hot isostatic pressing (HIP) was examined. Powder mixtures containing 0%, 0.5%, 5% and 20% coarse powder were prepared. The magnesia sintered without pressure was subjected to HIP at 1600 °C for 0.5 h at different pressures. The samples produced from fine powder had the largest grains after HIP treatment, although they had the smallest grains before it. The optical transparency of the HIP-treated magnesia ceramics increased with grain size.

Effect of ambient pressure on the grain growth in magnesia ceramics

Effect of ambient pressure on the grain growth in magnesia ceramics

Characterization of the Pore Structure of Ceramics via Propagation of Light and Infrared Radiation

The pore size distributions of alumina and magnesia ceramics were determined by measuring the directional-hemispherical transmittance and reflectance. These values are highly sensitive to changes of the pore structure. The partially sintered alumina samples were measured at room temperature in a wavelength range from 0.5 to 6 μm. The equation of radiative transfer can be solved for absorbing and scattering media by a three-flux solution. With this three-flux solution the scattering coefficients were derived from the measured directional-hemispherical transmittance and reflectance. The scattering coefficients can also be calculated theoretically by the Mie theory, if the pore size distribution is known. Finally, the quantitative pore size distribution was determined by fitting the theoretical scattering coefficients to the experimental scattering coefficients. To check the correctness of the derived pore-size distribution, scanning electron microscopy (SEM) and atomic force microscopy (AFM) pictures of the alumina samples were taken. The pore-size distribution was then derived by counting the pores and determining the diameters D of the spherical pores. Both results agree well and show that the new procedure is a valuable tool to extract structural information during the final sintering state.

A new technique for preparing ceramics for catalyst support exhibiting high porosity and high heat resistance

A new technique for preparing magnesia ceramics of high porosity and high temperature resistance has been developed. Spray freeze drying of magnesium sulfate aqueous solution produced fine salt particles having open pores due to sublimation of ice crystals. The particles were calcined to porous magnesium oxide and formed a green body. Highly porous magnesia was produced by firing the green body. The porous magnesia exhibited a bimodal pore size distribution of macro-pores of micron order and meso-pores smaller than 100nm. Porosity was 87–90%. After addition of an aluminum additive with an amount 3–5mol%, the magnesia exhibited high heat resistance; surface area was greater than 20m2g−1 after 20h exposure in a 1573K oven. Thus, the porous magnesia is expected to be very suitable for combustion catalyst support used in a high temperature environment.

Effect of Silica and Boron Oxide on Transparency of Magnesia Ceramics.

A simple process was developed to produce transparent MgO ceramics. In this process, magnesium hydroxide was initially precipitated by means of dropping an ammonium solution to a magnesium chloride solution in a glass (pyrex) beaker at about 100°C. The resulting precipitate was kept in the aqueous solution for 3h at the same temperature. Sintering was carried out at 1600°C for 2h in vacuum. Aging the precipitate at about 100°C is a key step of the present method, in which silica and boron oxide were dissolved from the used beaker, and then major part of them were adsorbed or absorbed by the precipitate concerned. Coexistence of silica and boron oxide was one of the necessary conditions to produce high transparent MgO ceramics.

Microwave Joining of Magnesia

Two magnesia ceramics were joined by the microwave heating method. The joining was easily accomplished in a short time by covering the joining part with a mullite insulating fire brick during microwave irradiation. The bending strength of the joint increased with increasing joining temperature and pressure. The hardness of the joint was higher than that of the matrix. The maximum bending strength of the sample joined at 2150K and 0.5MPa for 4min was 105MPa and was about 70% that of the matrix.