MgO Grains Research Articles

Tape casting and characterizations of MgO ceramics

ABSTRACTWe report a high density MgO ceramic substrate produced by the tape casting technology. The tape casting formulation and process produced a uniform tape free of cracking. Y2O3 and SiO2 were used as the sintering aid for the pressureless sintering of the green tape. X‐ray diffraction phase identification indicates that MgO is the main phase, while both Y2O3 and SiO2 sintering aids react with MgO to form MgY4Si3O13 as the second phase. Liquid phase sintering occurs in the temperature range from 1030°C to ~1500°C, which is confirmed by the simultaneous Thermal Gravitation Analysis/Differential Scanning Calorimeter (TGA/DSC) and the percent linear shrinkage and densification. A 96.5% theoretical density was achieved by presureless sintering at 1650°C for 2 hours, which was further increased to a fully dense structure using hot‐isostatic‐pressing(HIP) at 1650°C and 207 MPa in argon. Scanning electron microscopy (SEM) and energy dispersive(EDS) spectroscopic analysis on the HIP’ed sample show that MgY4Si3O13is located at the MgO grain boundary and the sample has a fully dense structure. The refractive indices and extinction coefficient were measured on the HIP’ed sample along with thermal properties and dielectric properties. Thermal diffusivity and heat capacity were measured to calculate the thermal conductivity.

Slag corrosion-resistance mechanism of lightweight magnesia-based refractories under a static magnetic field

Slag corrosion resistance of lightweight magnesia-based refractories with micro/nano-sized pores under a static magnetic field is investigated. They exhibited excellent slag resistance under static magnetic field of 8.5 mT with an isolation layer composed of solid phases formed at the refractory/slag interface. The static magnetic field generated a “magnetic brake effect” that suppressed the infiltrating flow of the molten slag and decreased the dissolution rate of MgO grains. Moreover, it also limited the thickness of the isolation layer by reducing the extent of the refractory-slag interaction, which can be applied to further improve the slag resistance of refractories.

Microstructure and electrical conductivity of Ce0.9Gd0.1O1.95-MgO composite electrolyte

Ce0.9Gd0.1O1.95 (GDC)-xMgO (x = 0, 10%, 20%, 30%, 40%, 50 mol%) powders were synthesized by sol-gel method and then the corresponding composite ceramics were sintered in air atmosphere at 1500 °C for 5 h. The sintered samples consist of both GDC and MgO phases on the condition of x ≥ 10%, and the MgO grains segregate at the grain boundaries of GDC. As the MgO content rises from 0 to 20%, the average grain size of GDC decreases from 2.15 to 0.41 μm. The introduction of MgO can improve the specific grain boundary electrical conductivity of GDC-xMgO composite ceramics dramatically. The GDC-20%MgO shows the highest specific grain boundary conductivity, which is 1.726 × 10−5 S/cm at 400 °C, 9.16 times higher than that of pure GDC. The variation in the real total conductivity confirms that the formation of GDC/MgO heterogeneous interfaces is beneficial to the enhancement in the electrical conduction properties of GDC-xMgO composite ceramics.

Corrosion Mechanism of MgO–CMA–C Ladle Brick with High Service Life

Carbon‐bonded magnesia and magnesia alumina carbon (MAC) bricks have been used in the sidewalls and bottom of steel ladles for over 20 years. Due to the increasing ratio of scrap to steel, higher working temperature, and prolonged refining time, steel producers seek refractories with enhanced service lifetime. Therefore, a new generation of MgO–C bricks with addition of calcium magnesium aluminate (CMA) aggregates is tested in a ladle lining of an integrated steel shop for 138 cycles. The corrosion mechanisms are evaluated by digital light microscopy and with scanning electron microscopy (SEM) equipped with energy‐dispersive X‐ray spectroscopy (EDS). The industrial trial tests reveal the formation of a slag coating at the hot face, leading to a much slower corrosion rate compared with a common MgO–C lining. The dissolution of the CMA aggregates and MgO grains results in a significant change of the slag chemistry and viscosity causing a reduced infiltration and corrosion behavior of the ladle slag. Also, the ability of bonding iron plays a major role, leading to the formation of brownmillerite phase in the protective slag coating on top of the bricks.

Microstructural evolution and texture analysis of magnesium phosphate cement

AbstractThree‐dimensional quantitative image analysis from synchrotron X‐ray microcomputed tomography indicated a coarsening of the microstructure of magnesium potassium phosphate cements driven by crystallization of K‐struvite from the first amorphous product. Porosity and pore surface area increased because of the progressive build‐up of a network of elongated/tabular crystal domains, with density higher than the amorphous. The known increase in strength with time is thought to occur thanks to the overwhelming contribution of a developing interlocked lath‐shaped microstructure. Combined X‐ray and neutron diffraction texture analysis indicated that at least a fraction of K‐struvite nucleates at the surface of MgO grains, suggesting the intervention of more than one crystallization mechanism. The detected weak texture, compatible with a nearly random orientation of crystallites, and the isotropic pore fabric, are beneficial with respect to crack propagation.

Synthesis of MgO–MgAl2O4 refractory aggregates for application in MgO–C slide plate

This work provides an effective method for synthesis of MgO–MgAl2O4 (MgO-MA) aggregates with good thermal shock resistance (TSR) using lightly calcined magnesia and α-Al2O3 micro-powder as the starting materials. The effect of α-Al2O3 micro-powder amount and calcination temperature on TSR and microstructure of the synthesized MgO-MA aggregates was investigated. The optimum amount of α-Al2O3 micro-powder was 15 wt%, and the obtained aggregates had the highest density after calcination at 1650 °C. The synthesized MgO-MA aggregates had excellent TSR which was attributed to the thermal mismatch of MgO and MA grains as well as the pinning effect of MA grains on the periclase grain boundaries. TSR of MgO–C slide plate materials was improved significantly by introducing appropriate amount of the synthesized MgO-MA aggregates.

Secondary electron emission enhancement of MgO/Au composite film by adopting a gold buffer layer

An electron-induced secondary electron emission (SEE) enhancement strategy of depositing a gold buffer layer for MgO/Au composite film was proposed. The influence of the gold buffer layer thickness on the microstructure, electrical conductivity and SEE properties of MgO/Au film was investigated. To deposit a gold buffer layer between the substrate and MgO/Au composite film markedly promotes the subsequent MgO grain growth, which leads to an increase in the SEE coefficient. Additionally, the improved electrical conductivity of this composite film caused by the gold buffer layer can effectively suppress the surface charging, which makes the MgO/Au film maintain a higher SEE coefficient for a longer time under continuous electron bombardment. An MgO/Au composite film with a 20-nm-thick gold buffer layer has a SEE coefficient of 5.1 at a primary electron energy of 200 eV with an increase of 13.3% compared with the one without a gold buffer layer.

Preparation and thermal shock behavior of nanoscale MgAl2O4 spinel-toughened MgO-based refractory aggregates

Abstract Nanoscale MgAl2O4 (MA) spinel-toughened MgO-based refractory aggregates were prepared by firing lightly calcined magnesia powder and some nanoscale Al2O3 (≤50 nm) particles at high temperature. The effects of various amounts of nanoscale Al2O3 powder on the phase composition, microstructure, physical properties and thermal shock resistance (TSR) of the prepared MgO-based refractory aggregates were investigated. The results showed that the optimum nanoscale Al2O3 powder addition was 10 wt%, and the nanoscale Al2O3 particles reacted with MgO during firing and formed nanoscale MA creating reactive sintering, which contributed to significant increasing in density and cold strength of the prepared aggregates. The thermal mismatch between nano-MA and MgO could produce microcracks created toughening effect, leading to improvement in TSR of the prepared aggregates. And the formed nano MA grains pinned in MgO grain boundaries and on MgO grain surfaces would lead to microcracks deflection and absorbing much more fracture energy, further improving the TSR. The obtained aggregates were trial used in MgO-C slide plate materials, and thermal shock resistance of the materials was improved.

Stability of point defects near MgO grain boundaries in FeCoB/MgO/FeCoB magnetic tunnel junctions

Magnetic tunnel junctions employing FeCoB as the ferromagnet and MgO as a spacer layer exhibit high performance and are attractive for magnetic random access memory applications. Upon postdeposition annealing, B is observed to diffuse out of the FeCoB layers inducing crystallization of FeCo. It is known that a large proportion of B escapes into the adjacent tantalum underlayer. While diffusion of B into bulk MgO is known to be unfavorable, it is possible that B could diffuse into grain boundaries (GBs) in the polycrystalline MgO layer, affecting its electronic properties. In this paper, density functional theory is used to investigate the stability and electronic properties of oxygen vacancy and B interstitial defects at MgO GBs. We show that both types of defects exhibit increased stability at the GBs, and we introduce electronic states in the gap that could negatively impact performance. These predictions are consistent with recent experimental results, and we discuss further means to confirm the results experimentally using techniques such as x-ray or ultraviolet photoelectron spectroscopy.

The interplay of structure and dynamics at grain boundaries.

Molecular simulations are carried out to address the structure and atomic diffusion at grain boundaries. We use an inherent structure approach, which maps each configuration in a molecular dynamics trajectory to the potential energy minimum ("inherent structure") it would reach by a steepest descent quench. Dynamics are then decomposed into a combination of displacements within an inherent structure and transitions between inherent structures. The inherent structure approach reveals a simple structural picture of the grain boundary that is normally obscured by the thermal motion. We apply our methodology to polycrystalline MgO. Grain boundary atoms are identified as atoms that are undercoordinated in the inherent structure, relative to those in the perfect crystal. Our method enables the calculation of grain boundary diffusion coefficients without arbitrary assumptions about which atoms or spatial regions belong to the grain boundary, and the results are shown to be consistent with estimates from experiments. The inherent structure approach also enables the elementary steps in the diffusion process to be elucidated. We show that the process in MgO grain boundaries primarily involves vacancy hops, but that there is also significant motion of other nearby atoms during such a hop.

Au Doping Effect on the Secondary Electron Emission Performance of MgO Films.

Au-doped MgO films were prepared by reactive sputtering of individual Mg and Au targets, and the Au doping effect on the electron-induced secondary electron emission (SEE) performance was explored by means of surface analysis, first-principle calculation, and electrical characteristic measurement. The results show that the size enlargement of MgO grains and the reduction of surface work functions induced by Au doping are the main reasons for the increase of the SEE coefficient (δ). Additionally, the superior SEE degradation property of the Au-doped MgO film under continuous electron bombardment results from the improvement of electrical conductivity. Through the optimization of Au doping concentration (x), Au-doped MgO film with an x value of 3.0% was found to have the best SEE performance due to its highest SEE coefficient and longest duration of maintaining a relatively high SEE coefficient; its maximum δ value reached 11.5—an increase of 32.2% in comparison with the undoped one.

Impact of boron diffusion at MgO grain boundaries on magneto-transport properties of MgO/CoFeB/W magnetic tunnel junctions

Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.

Light Burning Condition of Preparing Dolomite Clinker Using Natural Dolomite

The influence of the light burning temperature on the sintering property of nature dolomite has been investigated by two-step sintering process in the temperature range 1500 °C to 1600 °C. The resulting bulk densities and apparent porosities of the sintered dolomite samples were examined, and analyzing the sintered dolomite by scanning electron microscopy and X-ray diffraction were performed. The results showed light burned at 850 °C for 3 h, the main phases of the dolomite with 3-5 grain size were MgO, CaO and little CaCO3, and then fired at 1600 °C,the density of sintering dolomite reached to 3.38 g/cm3, the apparent property was 1.2 %, the size of MgO grain up to 3.75 μm . However when dolomite light burned at 1050 °C for 3 h, the main phases were MgO and CaO, and then fired at 1600 °C,the density of sintering dolomite only was 3.30 g/cm3, the apparent property was 2.3 %, the size of MgO only was 3.05 μm .

The Influences of Addition of MgCO<sub>3</sub> on the Properties of the Magnesia Prepared Using Magnesite

Micropowder MgCO3 was added into magnesite as raw materials to prepare magnesia using a two-step calcination method. The sample magnesite was characterized use X-ray diffraction (XRD) and scanning electron microscopy (SEM). Experimental results showed that the sample insulated at 1600° C for 3 hours before and after sintering presented a linear change rate of 15.6 % in the case of without adding micropowder MgCO3, the prepared magnesia had a bulk density of 2.31 g/cm3 and apparent porosity of 32.8 %, while MgO grain size was 3.11 μm. In the case of adding 8 % micropowder MgCO3, the sample magnesite before and after sintering showed a linear change rate of 17.9 %. The bulk density, apparent porosity of prepared magnesia were 2.46 g/cm3 and 28.1 % respectively, while the grain size of MgO was 5.15 mm.

Electron-induced secondary electron emission of Zn-doped MgO/Au composite film

An MgO/Au composite film with a 1.8 at% Zn-doped MgO surface layer prepared by reactive magnetron sputtering exhibited a superior electron-induced secondary electron emission (SEE) performance, and it had a SEE coefficient of 5.4 with an increase of 9.3% and a similar SEE degradation rate in comparison with an undoped composite film under primary electron bombardment of 200 eV. The mechanism for this SEE coefficient increase is tightly related to the reduced work function of Zn-doped MgO crystal analyzed by first-principles calculations and the enlarged MgO grains as well as the decreased surface roughness of Zn-doped MgO/Au film observed by morphological characterizations. In addition, for the Zn-doped MgO/Au film its improved electrical conductivity induced by Zn doping results in a similar SEE degradation rate even though it emits secondary electrons with the higher coefficient all the time under continuous electron bombardment compared with the undoped film.

A solid state NMR and in-situ infrared spectroscopy study on the setting reaction of magnesium sodium phosphate cement

In the present work, the setting reaction of magnesium sodium phosphate cement was followed in time with in-situ attenuated total reflectance Fourier-transform infrared spectroscopy and solid state nuclear magnetic resonance spectroscopy, in order to gain insights into the products and phase transformations. The results indicated that, during the progress of the reaction, amorphous phases containing the H2PO4− structural group converted into phases containing HPO42− and finally PO43−. The increase in pH triggered by the MgO dissolution was considered as the driving force for the transformations. This is supposed to promote the establishment of a high degree of supersaturation close to the surface of MgO grains, resulting in kinetically driven transformations and favouring the amorphous nature of the products. It is suggested that in the later stages of the reaction, two orthophosphate amorphous phases, hosting bound water molecules, coexisted, with the one showing a relatively more ordered 31P local environment, converting into a second with a more disordered phosphorous environment. The densification of the ceramic can be considered as a structural reorganization encompassing bonding of water molecules into a pervasive amorphous phase, containing magnesium-phosphate structural units and involving limited structural rearrangements of the local environment of P and, especially, of Na.

Thin water films and particle morphology evolution in nanocrystalline MgO.

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X‐ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing‐induced morphology changes on powders of MgO nanocubes in different gaseous H2O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H2O) = 10−5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two‐dimensional solvent for Mg2+ ion hydration. In addition, we obtained direct evidence for hydroxylation‐induced stabilization of (110) faces and step edges of the grain surfaces.

Fast and easy preparation of few-layered-graphene/magnesia powders for strong, hard and electrically conducting composites

Composite powders were prepared by the chemical vapor deposition (CH4/Ar atmosphere) of carbon in the form of 2–8 layers few-layered-graphene (FLG) covering the MgO powder grains, without any mixing step. The composites were consolidated to nearly full (99%) density by spark plasma sintering with no or little damage to the FLG. The FLG is located along the MgO grain boundaries, as opposed to be dispersed as discrete particles or flakes. This causes a dramatic hindrance of the MgO grain growth, the average grain size being considerably lower for the sample with 2.08 vol% carbon (200 nm) than for pure MgO (3.7 μm). The samples are investigated by Raman spectroscopy, scanning and transmission electron microscopy. The composites are electrically conducting with a percolation threshold below 0.56 vol%. Compared to pure MgO, the composites are simultaneously stronger (345 vs 200 MPa) and harder (9.8 vs 3.8 GPa). This could arise from reinforcement mechanisms such as crack-deflection and crack-bridging by FLG, but also from MgO grain refinement.

Influence of CaF2 in calcium aluminate-based slag on the degradation of magnesia refractory

High temperature experiments were carried out using an induction furnace to simulate the refractory-slag-metal reactions in conjunction with thermodynamic computations using FactSage7.0 software. The Al-killed steel and the CaO-Al2O3–8SiO2–5MgO-xCaF2 (CaO/Al2O3 = 1.9, x = 0–15 wt%) slag were brought to equilibrium in a magnesia refractory at 1823 K. Moreover, the solubility of MgO in the slag was quantitatively measured using a resistance furnace as a function of CaF2 content and temperature to confirm the thermodynamic driving force of the dissolution of MgO. The penetration depth of a slag into the magnesia refractory increased with increasing CaF2 content, which originated from a decrease in slag viscosity. Also, it was confirmed that MgO grains were dynamically detached from the slag/refractory interface by increasing the content of CaF2.

Influence of the substrate temperature on the microstructure and electron-induced secondary electron emission properties of MgO/Au composite film

The influence of the substrate temperature (Ts) on the microstructure and electron-induced secondary electron emission (SEE) properties of MgO/Au composite film prepared by reactive magnetron sputtering was investigated in order to improve the SEE performance of this composite film. During the deposition of MgO/Au film, a relatively high Ts is conducive to the growth of MgO grains and causes the formation of some large aggregated gold particles on which MgO grains cover in the composite film. In terms of the SEE properties of MgO/Au film, the increase of Ts brings about a significant improvement of SEE coefficient (δ), while it also leads to a slightly accelerated SEE degradation under the continuous electron bombardment due to a faster surface charging. The MgO/Au film prepared at Ts of 773 K has a δ value of 4.9 at a primary electron energy of 200 eV and the maximum δ value of 11.5.