Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

Enhancement of reaction-sintering of alumina-excess magnesium aluminate spinel in presence of titania

Alumina-excess magnesium aluminate spinel finds use in different high temperature applications including steel ladles. Alumina-excess spinel was prepared by solid oxide reaction using magnesia (MgO=10?wt%) and calcined alumina (Al₂O₃ = 90?wt%), in the sintering temperature range of 1500–1700?°C. The role of titania on the densification, spinelisation, evolution of microstructure and phase assemblage was investigated in this MgO-Al₂O₃ system. Titania addition increased the rate of densification 20x compared to undoped composition at 1500?°C under dynamic heating condition. However, under static firing, the beneficial effect of titania on densification could only be discerned at lower temperatures. The microstructure of titania doped sintered alumina-excess spinel compacts contain magnesium aluminium titanate phase in the grain boundary of corundum and spinel grains. The beneficial effect of titania on densification is attributed to magnesium aluminium titanate phase (Mg_xAl_2(1-x)Ti_(1+x)O₅) development and also by incorporation of Ti⁴⁺ into the spinel structure.     

Effect of Yb2O3 and TiO2 on reaction sintering and properties of magnesium aluminate spinel

Magnesium aluminate spinel with an initial MgO: Al₂O₃ molar ratio of 2:1 was prepared from its constituent oxides through a solid-state sintering process at temperatures ranging from 1550 to 1700 °C in a normal air atmosphere. The effect of varying amount (0.25–1.0 wt%) of TiO₂ and Yb₂O₃ on densification, phase assemblage, mechanical, thermo-mechanical properties and microstructure of magnesia-rich spinel were investigated under static heating condition. The addition of TiO₂ and Yb₂O₃ favours the densification of magnesia-rich spinel, which is discernible up to 1650 °C. This beneficial effect may be attributed to the development of the secondary phase and formation of solid solution due to the dissolution of the additive ions in the spinel structure. A marginal increase in the average grain size of the samples along with a narrower grain size distribution occurred with the incorporation of both the additives. Both the additives improved the mechanical properties of the magnesia-rich spinel; however, better room temperature flexural strength was achieved with Yb₂O₃ as compared to TiO₂ addition. For the samples sintered at 1550 °C, 1.0 wt% Yb₂O₃ addition resulted in 30% increase in flexural strength; however, same amount of TiO₂ addition increased the strength by 20%. In case of thermal shock resistance, 1.0 wt% TiO₂ and 0.25 wt% Yb₂O₃ addition demonstrated promising result among all the samples.     

Role of Ti and Sn ions in spinel formation and reactive sintering of magnesia-rich ceramics

The solid solubility of magnesia in magnesium aluminate spinel and magnesium aluminate spinel in magnesia does not change with temperature thus not creating bonds or precipitation over periclase grains in a single stage sintering process. In comparison, the precipitated spinels in magnesia-chromia refractories form complex spinel due to inversion in the position of bivalent and trivalent cations within the structure, making them more stable at high temperature than either normal or inverse spinel. Additives form low-temperature compounds that diffuse into the spinel structure and create defects that change the properties of spinel solid solution. In the present study, magnesia and alumina powders along with tetravalent oxide additives were analyzed for their role in reactive densification of spinel in a single stage firing process in order to achieve a better binding system for magnesia-based refractories. These tetravalent oxides on reaction with magnesia form spinel solid solution with MgAl₂O₄ as they have similar crystal structure. The spinel solid solution formed using oxide additives is expected to have higher solubility in magnesia than magnesium aluminate spinel, resulting in improvement of the bonding during sintering through increased in solid solubility at elevated temperatures followed by precipitation of secondary spinel phases, similar to the complex spinel in magnesia-chrome refractories. The formation of spinel during firing remains as a second phase that retards the grain growth of periclase. The changes in unit cell dimensions with temperature and amount of additive were analyzed using Reitveld method and correlated with the densification behaviour at different temperatures.     

Effect of α-Al2O3 content on microstructures,mechanical properties and purification efficiency on molten steel of MgO-based filters

In this study, five sets of MgO-based ceramic filters were prepared using porous MgO powder and α-Al₂O₃ micro-powder as raw materials. The effect of α-Al₂O₃ content on the microstructures, mechanical properties and purification efficiency of MgO-based filters were investigated via SEM, EDS, and immersion test in molten steel, etc. The results indicate that the reactive sintering between α-Al₂O₃ and MgO promoted the formation and growth of neck connections of magnesium aluminate spinel between grains, increased the density of the filter skeleton, and decreased the pore size inside the skeleton. The higher density and smaller pore size of the skeleton, as well as the neck connection of spinel and crack deflection effect caused by magnesium aluminate spinel, comprehensively improved the strength and thermal shock resistance of MgO-based ceramic filters. Furthermore, the results from the immersion test in molten steel revealed that an appropriate amount of high-temperature liquid phase, smaller pore size, and in-situ generated spinel were beneficial for the filters to adsorb dissolved aluminum, dissolved oxygen, alumina inclusions, and secondary spinel inclusions in the steel. The filter with 20 wt% α-Al₂O₃ had the best overall performance with a compressive strength of 2.35 MPa, a compressive strength after thermal shock test of 1.91 MPa and an efficiency of 77.3% in reducing the total oxygen content of the steel.     

Effect of CeO2 addition on the sintering behavior of pre-synthesized magnesium aluminate spinel ceramic powders

In this work, the effects of 1?wt%, 2?wt%, and 3?wt% CeO₂ as an additive on the sintering behavior of alumina-rich spinel and magnesia-rich spinel powders subjected to sintering at temperatures of 1600?°C, 1650?°C, 1700?°C, and 1750?°C were investigated. The sintering behavior of the ceramics was investigated according to dilatometry measurements, linear shrinkage, bulk density, phase analysis, and microstructure. It was demonstrated that CeO₂ hindered the sintering process in alumina-rich spinel by reacting with Al₂O₃ exsolved from the spinel to form platelet-shaped particles of CeAl₁₁O₁₈ interspersed between the spinel grains. Meanwhile, the presence of CeO₂ promotes the sintering process in magnesia-rich spinel by being distributed in an isolated form among the spinel grains.     

TiO2加入量对固相烧结合成镁铝尖晶石致密化行为的影响

以低品位菱镁矿和工业α氧化铝微粉为主要原料固相烧结合成镁铝尖晶石,探讨TiO2添加剂对合成镁铝尖晶石致密化行为的影响.用X射线衍射(XRD),扫描电镜(SEM)及相关分析软件对烧后试样的相组成和显微结构进行研究,以揭示镁铝尖晶石烧结致密化的过程机理.研究结果表明:添加剂TiO2与形成镁铝尖晶石的置换固溶作用是促进固相合成镁铝尖晶石烧结致密化的重要机理,阳离子空位的产生以及镁铝尖晶石的晶格缺陷,高温下有利于镁铝尖晶石的晶体发育和长大,在空间位阻效应的作用下,达到排除气孔使试样致密化的目的;随着TiO2加入量的增多,镁铝尖晶石的致密化程度也逐渐升高,能够有效的改善制品的烧结性能和显微结构.     

Synthesis and densification behaviour of magnesium aluminate spinel: Effect of Dy2O3

Magnesium aluminate spinels have been developed by reaction sintering of calcined alumina and calcined magnesia and its densification behaviour was studied in presence of Dy₂O₃. Green bar made from stoichiometric spinel composition with and without Dy₂O₃ were subjected to dilatometric study, densification study and microstructural evaluation by SEM. It was found that Dy₂O₃ additive does not have significant effect on the spinelisation but favours the densification of the spinel. Microstructure of sintered spinel without any additive is non-uniform with some exaggerated grain growth. Dy₂O₃ prevents the exaggerated grain growth and thereby helps in the densification process.     

Densification and properties of magnesia-rich magnesium-aluminate spinel derived from natural and synthetic raw materials

Magnesia rich magnesium aluminate spinel (MgO: Al₂O₃=2:1) was developed by reaction sintering of Indian natural magnesite of Salem region as well as from synthetic caustic magnesia with calcined alumina. Dilatometric study of the green compacts was carried out to evaluate the spinelisation and sintering behaviour of both the samples. Green samples were heat treated between 1400 to 1600 °C and characterised in terms of densification behaviour, high temperature flexural strength, microstructure and phase development. Spinel and periclase are the major phases in both the samples, where as forsterite is found only in the sample developed from Indian magnesite due to presence of silica as impurities.     

Effect of Y2O3 content on densification,microstructure and mechanical properties of reaction sintered magnesium aluminate spinel

Dense magnesium aluminate (MgAl₂O₄) spinels were developed via single-stage solid-state reaction sintering method at 1550–1650^oC using combinations of varied commercial grade reactants-three different sources of alumina and two different sources of magnesia. The effect of Y₂O₃ doping in the concentration range of 1–4 wt % on different spinel batches was studied. Y₂O₃ addition was found to favour the densification of all the spinels at all dopant concentrations and maximum densification was found for the 2 wt % Y₂O₃ containing spinel batches. Phase analysis of the Y₂O₃ containing batches revealed the presence of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) at all the sintering temperatures. Owing to similar crystal structure isotropic configuration of YAG (cubic) as that of spinel (cubic), Y₂O₃ doping was found to favour densification of spinel. Microstructural investigation revealed that Y₂O₃ containing batches have a controlled grain structure as compared to the without additive batches. Also, 2 wt % Y₂O₃ containing spinel batches sintered at 1650^oC revealed better mechanical properties such as cold modulus of rupture and strength retainment after thermal shock than that of the undoped spinel batches.     

Sintering paths and mechanisms of pure MgAl2O4 conventionally and microwave sintered

In this paper, a comparative study between conventional and microwave sintering of pure spinel MgAl₂O₄ is presented. The goal is to clarify and identify the possible microwave effects on densification and microstructure of the pure spinel. Sintering trajectories obtained for microwave and conventional sintering are similar and converge into a unique trajectory. Therefore, microwave processing does not refine the grain size of pure spinel. The dominant mechanism of initial and intermediate stages of sintering was determined from the shrinkage curves and sintering trajectory. It was shown that densification is mostly controlled by grain boundary diffusion for both processes. Porosity of microwave and conventionally sintered samples was also characterized by mercury porosimetry and BET analysis. The evolution of the open porosity and pore size distribution is the same whatsoever the process used. This work shows that microwave sintering does not differ that much from conventional sintering on pure MgAl₂O₄ material.     

The effects of in situ formation of Y3Al5O12 on property improvement of magnesium aluminate spinel refractories

Magnesium aluminate spinel is widely used in cement rotary kilns, in the iron and steel industries, as well as in glass melting furnaces due to its excellent performance and chemical stability at both room temperature and elevated temperatures. In spite of these advantages, there are some practical problems during production of magnesium aluminate spinel refractories due to their poor sinterability: poor mechanical properties and poor creep resistance. These issues can cause problems during service. This study improved the sinterability of spinel refractories and in turn improved mechanical properties while decreasing the creep rate. This was done by forming a second low creep rate phase of yttrium aluminum garnet in the matrix structure. The addition of Y₂O₃ and reactive Al₂O₃ accelerated the densification process and increased the cold strength. There was a significant increase in the hot modulus of rupture due to the formation of YAG or the solid solution with spinel.     

Improvement on sintering property of the face coat of yttria mould shells for investment casting

In order to explore methods for improving the sintering property of yttria face coat to prevent the thermal spalling phenomenon during investment casting, without significantly increasing the cost，the effects of electrode types in the vacuum furnace and the doping of oxides on the sintering character of yttria face coat at high temperature have been investigated. By contrast with the results of traditionally sintered pure yttria face coat at 1700?℃, the promotion in densification of the yttria face coat of the mould shells by doping La₂O₃ +?ZrO₂ is the most efficacious, whereas doping CeO₂ +?ZrO₂ can impede the densification of the yttria face coat. After sintering at 2000?℃ in a vacuum furnace with graphite electrodes, the yttria face coat can get complete densification, while the yttria face coat shows incomplete densification in a furnace with tungsten electrodes, whatever doped or undoped. But yttria grains in the face coats doped with oxides are larger than those in the un-doped one. Both CaO?+?ZrO₂ doping and MgO + ZrO₂ doping can make greater acceleration of the yttria grain growth than La₂O₃ +?ZrO₂ doping and CeO₂ +?ZrO₂ doping.     

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.     

Mining tailings as a raw material for glass-bonded thermally sprayed ceramic coatings: Microstructure and properties

Magnesium aluminate, MgAl₂O₄, spinel powders for thermal spraying, were synthesized from secondary raw materials by spray drying and subsequent reaction sintering. Talc ore mining tailings and aluminium hydroxide precipitate from aluminium anodizing process were studied. A stoichiometric MgAl₂O₄ spinel coating was prepared as a reference using pure raw materials. Atmospheric plasma spraying resulted in the formation of ceramic coatings. Microstructural investigations revealed that the reference coatings exhibited crystalline lamellar microstructure of MgAl₂O₄ but secondary coatings contained amorphous areas between the crystalline MgAl₂O₄ clusters. Abrasive wear test results revealed considerably lower wear rate for secondary coatings. It is suggested that the different structure of coatings, particularly the high degree of amorphous phase between the isolated crystalline MgAl₂O₄ clusters caused the higher abrasive wear resistance by changing the wear mechanism. The dielectric breakdown strength of the secondary coatings were at the same level, 24 V/μm, as compared to reference coating, 23 V/μm.     

Formation peculiarities and optical properties of highly-doped (Y0.86La0.09Yb0.05)2O3 transparent ceramics

Formation peculiarities of highly-doped (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been studied by X-ray diffraction and electron microscopy methods. The phase composition evolution of 1.81Y₂O₃∙0.18La₂O₃∙0.01Yb₂O₃ powder mixtures annealed at the temperatures of 1100, 1200, 1300, and 1400 °C has been studied by XRD. It has been shown that Yb₂O₃ phase dissolves in Y₂O₃ matrix in the calcination temperature range of 1300–1400 °C. Complete dissolution of La₂O₃ in Y₂O₃ matrix occurs at temperatures above 1400 °C. La³⁺ ions enter in Y₂O₃ and Yb₂O₃ crystal structures simultaneously in the 1200–1300 °C range, which leads to a remarkable increase in the volume of the corresponding crystal lattices. The possible reasons for suppressing the crystalline growth of Y₂O₃ and Yb₂O₃ cubic phases have been discussed. Finally, (Y_0.86La_0.09Yb_0.05)₂O₃ transparent ceramics have been obtained by solid-state vacuum sintering at 1650–1750 °C. Ceramics synthesized at a temperature of 1750 °C have been characterized by an in-line optical transmittance of 60% and a homogeneous distribution of constituent components within the volume and along the grain boundaries.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Designing high-temperature thermal insulators based on densification-resistant in situ porous spinel

In situ formation of magnesium aluminate spinel (MgAl₂O₄) based on Al₂O₃ and MgO sources is an expansive reaction that hinders its densification. Although its side effects have been extensively investigated for dense materials, its potentially useful role to produce refractory porous structures remains unexplored. In this study, spherical clusters of brucite (Mg(OH)₂) nanoparticles were synthesized and combined with Al₂O₃ in different ratios for producing porous alumina-spinel-MgO structures. The evolution of their microstructure and physical properties was followed in the 300?1500 °C range. Different porogenic mechanisms related to particles’ packing, dehydroxylation of brucite, MgO consumption in the reaction with alumina (Wagner’s mechanism leading to the formation of Kirkendall voids) and expansive spinel formation were identified during initial heating. Variations in Mg(OH)₂-Al₂O₃ ratio and sintering temperature resulted in structures of tailored porosity, compression strength, thermal conductivity and pore sizes in the 1?5 μm range, suitable for materials to be used at high temperatures.     

Properties of magnesium-aluminate spinel derived from bauxite and magnesia

The synthesis of magnesium-aluminate spinel divided from bauxites and magnesias, the starting materials with different molar mass ratios (Al₂O₃: MgO) of 3, 1, and 0.6 were developed using solid-state reaction sintering at 1350-1500°C. The effects of different mass ratios and sintering temperatures on the phase composition, densification behavior, shrinkage, flexural strength, and microstructure of the synthetic materials were studied. It was found that as the relative content of bauxite decreased, the flexural strength first decreased before increasing. When n(Al₂O₃)/n(MgO) was 1, the spinel was the primary phase and the sample was dense. When the temperature became 1450°C, the flexural strength became maximized at 106.48 MPa.     

Preparation and characterization of porous MgAl2O4 spinel ceramic supports from bauxite and magnesite

Porous magnesium aluminate spinel (MgAl₂O₄) ceramic supports were fabricated by reactive sintering from low-cost bauxite and magnesite at different temperatures ranging from 1100 to 1400 °C and their sintering behavior and phase evolution were evaluated. The effects of sintering temperature on the pore structure, size and distribution as well as on the main properties of spinel ceramic supports such as flexural strength, nitrogen permeation flux and chemical resistance were investigated. The supports prepared at 1300 °C showed a homogeneous pore structure with the average pore size of 4.42 μm, and exhibited high flexural strength (35.6 MPa), high gas permeability (with nitrogen gas flux of 3057 m³ m⁻² h⁻¹ under a trans-membrane pressure of 0.1 MPa) and excellent chemical resistance.