Restructuring-diffusion mechanism of calcium alumino-titanate in CaAl12O19–MgAl2O4–Al2O3 castables

Calcium alumino-titanate (CAT), a secondary material obtained from ferrotitanium slag, was used as a hibonite source to prepare CaAl₁₂O₁₉–MgAl₂O₄–Al₂O₃ castables. The restructuring effect of CAT aggregate was compared by replacing tabular alumina aggregates with CAT aggregates of different particle sizes. The effects of CAT particle size on cold mechanical strength and thermal shock resistance of CaAl₁₂O₁₉–MgAl₂O₄–Al₂O₃ castables were studied. The results showed that CAT aggregates with particle size of 5–3 or 3–1 mm led to more internal cracks or pores and reduced the cold mechanical strength of the castable samples fired at 1600 °C for 3 h. The use of CAT aggregates with particle size of 1–0 mm led to the formation of a well-bonded CAT aggregate and matrix, improving the cold mechanical strength and thermal shock resistance of the castable samples fired at 1600 °C for 3 h. The enhancement mechanism of fine CAT aggregates in this process was proposed based on the sintering of the matrix–aggregate interface with the formation of Ca(Al, Mg, Ti)₁₂O₁₉.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).     

Ultra-rapid microwave sintering of pure and Y2O3-doped MgAl2O4

MgAl₂O₄ samples were microwave sintered to near-full density in rapid processes with heating rates on the order of 100°C/min and zero isothermal hold. The experiments were carried out using a gyrotron system for microwave processing of materials operating at a frequency of 24 GHz with a maximum power of 6 kW. In the regimes with a preset heating rate sustained by the automatically regulated microwave power, the maximum achieved density was about 95% of the theoretical value in pristine MgAl₂O₄ samples (maximum sintering temperature 1650°C) and about 97% in 1 wt.% Y₂O₃-doped samples (1700°C). In the regimes with a fixed microwave power (about 3.5 kW), translucent spinel samples with a relative density above 99% were obtained at 1700°C. The duration of the high-temperature stage of sintering was 1.5-10 minutes. The suggested mechanism responsible for the enhanced densification involves development of a thermal instability and formation of transient liquid phases at grain boundaries. The estimated specific absorbed power in the samples during the high-temperature stage of ultra-rapid microwave sintering was 27-80 W/cm³, similar to the values observed in dc field-assisted flash sintering experiments.     

Interfacial evolution mechanism of MgAl2O4/MgAl2O4 joints bonded with lanthanum glass

The La₂O₃-SiO₂-B₂O₃ (LSB) glass filler with high softening temperature was first used to join MgAl₂O₄ ceramic. An interfacial layer composed of Al₂O₃ was formed due to the solubility difference of MgO and Al₂O₃ in the LSB glass filler. As a result, the addition of Al₂O₃ into the LSB glass filler caused the increase of interfacial layer thickness. On the contrary, the addition of MgO into the LSB glass filler led to the decrease of interfacial layer thickness. When the adding content of MgO was 6 wt%, the interfacial layer disappeared and completely amorphous brazing seam was obtained. The in-line transmittance of joints decreased with the increase of the thickness of interfacial layer. The optimal in-line transmittance of joint bonded with La₂O₃-SiO₂-B₂O₃-MgO (LSB6M) glass filler reached 82.9% at 1000 nm. Meanwhile, the average flexural strength of joints was about 196.2 MPa, which was equal to the strength of MgAl₂O₄ substrate.     

Sintering behavior and microstructure of (1-x)MgAl2O4-xMg2TiO4 spinel solid solutions prepared by isostructural heterogeneous nucleation

Magnesium aluminate (MgAl₂O₄) spinel has grasped considerable attention in high-temperature application by right of its excellent properties. However, the poor sintering behavior of MgAl₂O₄ is detrimental to its further development. In the present work, the application of isostructural heterogeneous nucleation method provides a novel idea for optimizing the sintering behavior of refractory materials. A series of (1-x)MgAl₂O₄-xMg₂TiO₄ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) spinel solid solutions with a present ration of components were fabricated from light calcined magnesia, reactive alumina and pre-preparation Mg₂TiO₄. The effect of Mg₂TiO₄ heterogeneous nucleating agent on the crystalline phase, densification, and microstructure evolution of MgAl₂O₄–Mg₂TiO₄ spinel solid solutions was studied. The XRD, XPS, and EDS results showed that Mg₂TiO₄ entered the lattice of MgAl₂O₄ to form a spinel solid solution, and the heterovalent substitution process was identified, where Ti⁴⁺ and Mg²⁺ ions of larger radius in the Mg₂TiO₄ replaced the Al³⁺ of smaller radius in the MgAl₂O₄. For the sample at x = 0.08, the spinel solid solutions exhibited the optimized densification with a relative density of 93.3%, an apparent porosity of 1.2%, and a compressive strength of 84.5 MPa. A significant increase in densification was related to the lattice distortion induced by ion size mismatch during the heterovalent substitution, thus accelerating the diffusion rate of Mg²⁺ and Al³⁺ ions in the spinelisation state. Moreover, the solid solubility content of Ti⁴⁺ in the MgAl₂O₄–Mg₂TiO₄ spinel solid solutions had a significant effect on the grain morphologies. The Mg₂TiO₄ heterogeneous nucleating agent significantly increased the spinelisation rate of MgAl₂O₄ spinel with negligible effect on densification.     

Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Computer modeling of the local structure and mixing properties of a solid solution of MgAl2O4-MgCr2O4

An original method of the atomistic computer modeling of substitutional solid solutions is applied to the noble spinel MgAl₂O₄-magnesia chromite MgCr₂O₄ binary system. The parameters of the interatomic potentials in the partially ionic approximation are optimized using the experimental values of the structural, elastic, and thermodynamic characteristics of these minerals. The point defect formation energy is estimated. The modeling of Mg(Al _x Cr_{1 ? x} )₂O₄ solid solutions is carried out in a supercell of the 4 × 4 × 4 structural type of spinel that contains 3584 atoms; the GULP 3.4.9 program, which is implemented in the SKIF Chebyshev supercomputer (Moscow State University), is used. The mixing properties of the solid solution such as the mixing enthalpy, the interaction parameters, the mixing volumes, the deviations of the compression moduli from additivity, and the oscillatory entropy were determined. Calculations of the Gibbs energy have yielded the assessments of the stability fields of the MgAl₂O₄-MgCr₂O₄ solid solution and the comparisons of these assessments with the experimental data. Histograms of interatomic spacings are plotted and the values of the relaxation parameters of the Cr-O and Al-O bonds are estimated; these values agree well with the experimental values obtained by measuring the optical absorption spectra on a Cr³⁺ ion.     

Metastability in the MgAl2O4–Al2O3 System

Aluminum oxide must take a spinel form (γ‐Al₂O₃) at increased temperatures in order for extensive solid solution to form between MgAl₂O₄ and α‐Al₂O₃. The solvus line between MgAl₂O₄ and Al₂O₃ has been defined at 79.6 wt% Al₂O₃ at 1500°C, 83.0 wt% Al₂O₃ at 1600°C, and 86.5 wt% Al₂O₃ at 1700°C. A metastable region has been defined at temperatures up to 1700°C which could have significant implications for material processing and properties. Additionally, initial processing could have major implications on final chemistry.     

Electrical properties of Co-doped CaCu3Ti4O12

Co-doped CaCu₃Ti₄O₁₂ samples were synthesized by solid-phase reaction. Electrical properties were studied by impedance spectroscopy in wide temperature (25–450 °C) and frequency (10 Hz–10 MHz) intervals. It was shown that the presence of the copper oxide interlayer significantly reduces the value of the dielectric constant. The amount of impurity copper in the CaCu₃Ti_4-хCo_хO_12-δ samples (x = 0.06; 0.12; 0.24) rise with an increase in the cobalt content. The samples are characterized by a granular microstructure, with an average grain size ranging from 2 to 10 μm. The impedance of the samples was simulated at a temperature of 25 °C and in the range of 100–450 °C. It was found that the samples are characterized by low- and high-frequency polarization. The conductivity activation energy varied from 0.94 to 0.87 eV depending on the cobalt content. The CaCu₃Ti_3.94Co_0.06O_12-δ sample are characterized by the best values of the dielectric permittivity and the dielectric loss tangent, ε = 400 and tanδ = 0.2 (at 1 MHz and room temperature), respectively.     

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%.     

Interaction of MgAl2O4 and MgCr2O4 spinels with calcoferruginous melts

Conclusions The interaction was studied between MgAl₂O₄ and MgCr₂O₄ spinels (and their solid solutions and compositions with periclase) with calcoferruginous melts, which are the essential constituents of various slag reagents.On exposure to a highly basic calcoferruginous melt, the magnesium-aluminum spinel forms magnesio-ferrite, brownmillerite, and some calcium aluminate.On interaction of magnesiochromite with calcium ferrite, chiefly high-temperature compounds form.With the action of highly ferruginous melts on magnesiochromite, the resulting solid solution increases in volume, and a distortion is observed at the boundary of the reaction and little-change zones of the samples.The addition of granular MgO to spinels or their solid solutions improves the chemical stability of the compositions against calcoferruginous melts [4].During use in contact with highly basic calcoferruginous melts, the addition of periclase to chromium spinellide additives is recommended; for use in contact with highly ferruginous melts, periclase and aluminum spinellide compositions are recommended.Translated from Ogneupory, No. 7, pp. 44–47, July, 1969.     

Reactions between MgAl2O4 and MgCr2O4 spinels and phosphate bonds

Conclusions A study was made of the reaction between spinels MgAl₂O₄ and MgCr₂O₄ with orthophosphoric acid and sodium chromophosphate bond. The setting of the material is related to the formation of various hydrated phosphates and hydrates. During heating complex phase inversions take place, with the formation of intermediate compounds. At elevated temperatures the stable phases are spinels.Translated from Ogneupory, No. 2, pp. 53–55, February, 1973.     

Improved thermal shock resistance of low-carbon Al2O3–C refractories fabricated with C/MgAl2O4 composite powders

The addition of C/MgAl₂O₄ composite powders can improve the thermal shock resistance of low-carbon Al₂O₃–C refractories attribute to the formation of microcracks in the agglomerated structure, thus consuming more thermal stress and strain energy. Moreover, C/MgAl₂O₄ composite powders additive promote the formation of short fibrous ceramic phases in the refractories, which suggest a bridging role in the interior of the refractories and increase its toughness. Furthermore, the C/MgAl₂O₄ composite powders also result in a remarkable enhancement of the slag corrosion resistance in the refractories.     

Eu3+ -Nd2O3 blue pigmented solid solutions

Abstract

Usually, Nd₂O₃ shows a polymorphism based on the hexagonal Ln₂O₃ A type and cubic Ln₂O₃ C type structures, which have coordination numbers of 7 and 6 respectively. The Nd³⁺ (4f²) ion in an octahedral environment produces a reddish shade. In the present study, Eu_xNd_2-xO₃ compositions were prepared by ceramic and coprecipitation (from chloride solution) methods using several flux agents. Using the ceramic route, Nd₂O₃ C type structure was detected and the samples were colourless, but using coprecipitation, NdOCl crystallised from an amorphous coprecipitate at low temperature, developing the unusual monoclinic EuNd₂O₃ B type doped polymorph which is blue in colour. When enamelled using a conventional industrial tile glaze, these blue coprecipitated samples gave L*a*b* = 75/1/-11 in the CIE colour convention. Samples were characterised by X RD, U V-V-NIR spectroscopy, and SEM-EDX techniques.     

透明MgAl2O4晶须的制备与表征

本文采用MgO粉和金属Al粉为原料,在氮气气氛下制备MgAl2O4晶须,研究了TiO2、SiO2或Cr2O3等供氧添加剂和制备条件对晶须合成率的影响并分析了晶须的物相组成和结构.结果表明:最佳MgO/Al=2/3;添加SiO2或Cr2O3不利于晶须合成;添加TiO2对合成晶须有促进作用,最佳TiO2加入量为40%;采用氧化铝坩埚比采用釉面陶瓷坩埚好,最佳保温时间为6 h.合成的晶须为MgAl2O4,空间群Fd3M,晶格常数a=b=c=0.8083 nm,MgO 28.3%,Al2O3 71.7%;晶须呈透明柱状,最大长度为20～30 mm,长径比20～400.     

Preparation and properties of MgAl2O4 based ceramics reinforced with rod-like microcrystallines by co-doping Sm2O3 and La2O3

A rod-like microcrystalline reinforced MgAl₂O₄ (MA) based ceramic with high strength and good thermal shock resistance has been successfully prepared by co-doping Sm₂O₃ and La₂O₃ via a single-stage solid-state reaction sintering (SRS) process. The effects of addition amounts of Sm₂O₃ and La₂O₃ on the phase compositions, microstructures, shrinkage ratio, apparent porosity, bulk density, high temperature compressive strength and thermal shock resistance of the MA based ceramics have been investigated. The results showed that MA, SmAlO₃, La₁₀Al₄O₂₁, Sm_4.67(SiO₄)₃O and La_4.67(SiO₄)₃O phases could be found in the ceramics after co-doping additives. The new formed rare earth compounds could prevent the growth of MA grains leading to a densification microstructure of the MA based ceramics. The Sm_4.67(SiO₄)₃O and La_4.67(SiO₄)₃O were formed by the reaction between additives and impurities of raw materials, which presented as rod-like microcrystallines to effectively clean the impurities in the grain boundaries of the MA based ceramics. Accordingly, the sintering properties, high temperature compressive strength and thermal shock resistance of the MA based ceramics were improved markedly.     

Self-segregation and solubility in nonstoichiometric MgAl2O4 nanoparticles

Magnesium aluminate spinel (MAS) has a wide range of technological applications owing to its exceptional mechanical properties and good chemical stability. MAS phase diagrams indicate that only stoichiometric MgAl₂O₄ is stable at ∼25°C and Mg²⁺ and Al³⁺ exhibit solubility in nonstoichiometric MAS only at temperatures higher than ∼1200°C. In this study, the synthesis of nonstoichiometric single-phase spinel nanopowders at low temperatures is reported, and the role of the chemical distribution of Al³⁺ and Mg²⁺ excess on the stability of these nanopowders is examined. We performed selective lixiviation to examine the surface segregation of Mg²⁺ and Al³⁺ in nonstoichiometric MAS to investigate its effect on interfacial solubility and consequently the stability of nonstoichiometric MAS. Furthermore, we plotted an experimental phase diagram of nano MAS that predicts the crystallite size limits for the nonstoichiometric single-phase MAS.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.