Low cost foam-gelcasting preparation and characterization of porous magnesium aluminate spinel (MgAl2O4) ceramics

Porous MgAl₂O₄ ceramics were prepared via a low cost foam-gelcasting route using MgAl₂O₄ powders as the main raw material, ammonium polyacrylate as a dispersant, a small amount of modified carboxymethyl cellulose as a gelling agent, and TH-IV polymer as a foaming agent. The effects of additive's content, solid loading and gelling temperature on slurry's rheological behavior were investigated, and microstructures and properties of as-prepared porous MgAl₂O₄ ceramics examined. Based on the results, the roles played by the foaming agent in the cases of porosity, pore structure, pore size, mechanical properties and thermal conductivity were clarified. Porosity and pore sizes of as-prepared porous MgAl₂O₄ ceramics increased with increasing the foaming agent from 0.05 to 0.6 vol%. Porous MgAl₂O₄ ceramics with porosity of 75.1% and average pore size of 266 µm exhibited a compressive strength as high as 12.5±0.8 MPa and thermal conductivity as low as 0.24 W/(m K) (at 473 K).     

Mechanical and dielectric properties of porous magnesium aluminate (MgAl2O4) spinel ceramics fabricated by direct foaming-gelcasting

Porous ceramic materials have been broadly applied in various fields due to their multifunctional properties. Optimization of their microstructural characteristics, such as pore morphology, total porosity, and pore size distribution, which determine various properties of the final products, is crucial to improve their performances and thus extend their applications. In this study, single-phase porous MgAl₂O₄ materials were fabricated by direct foaming–gelcasting. With an increase in the foam volume from 260 to 350 mL, the total porosity and pore size of the porous ceramic increased, and its microstructure varied from mostly closed cells to open cells containing interconnected large pores (40–155 μm) and small circular windows (10–40 μm) in the ceramic skeleton. The total porosity could be tailored from 84.91% to 76.08% by modulating the sintering temperature and foam volume and the corresponding compressive strengths were in the range of 2.8–15.0 MPa. The compressive strength exhibited a power-law relationship with the relative density with indices of approximately 3.409 and 3.439, respectively. Porous MgAl₂O₄ ceramics exhibited low dielectric constants in the range of 1.618–1.910 at room temperature, which are well matched with theoretical calculations on account of a modified Bruggeman model. The porous MgAl₂O₄ ceramics with good mechanical and dielectric properties controlled easily by various sintering temperatures and foam volumes are promising for practical applications.     

Flash‐sintering of magnesium aluminate spinel (MgAl2O4) ceramics

The sintering behavior of commercially available MgAl₂O₄ spinel was investigated under DC electric field in a range of 0 and 1000 V/cm. Flash‐sintering results in densification close to theoretical density at 1410°C under the DC field of 1000 V/cm, in comparison to the higher sintering temperature of 1650°C in case of conventional sintering. It was observed that the fields less than 750 V/cm had no significant effect on the densification behavior. An abrupt increase in power dissipation was observed corresponding to the occurrence of the flash event. A significant enhancement in grain size was observed in case of flash‐sintered dense spinel samples. The gradual increase in the specimen conductivity observed in the electric field‐assisted sintering (FAST) regime led to Joule heating within the specimen. The increased specimen temperature triggered further increment of current and Joule heating, resulting in the immediate densification.     

Preparation of high-porosity Al2O3 ceramic foams via selective laser sintering of Al2O3 poly-hollow microspheres

In this paper, high-porosity Al₂O₃ ceramic foams called Al₂O₃ PHM ceramics were fabricated through selective laser sintering (SLS) via Al₂O₃ poly-hollow microspheres (Al₂O₃ PHMs). SLS parameters were optimized by an orthogonal experiment as to be laser power = 6 W, scanning speed = 1800 mm/s, and scanning space = 0.15 mm. The effect of sintering temperature on microstructure, shrinkage, porosity, phase composition, mechanical properties and pore size distribution of Al₂O₃ PHM ceramics were investigated. When sintering temperature increased, Al₂O₃ PHM ceramics contained only Al₂O₃ phase and were gradually densified. With the raise of sintering temperature, the porosity of Al₂O₃ PHM ceramics decreased gradually from 77.09% to 72.41%, but shrinkage in H direction and compressive strength of Al₂O₃ PHM ceramics increased from 6.63% and 0.18 MPa to 13.10% and 0.72 MPa, respectively. Sintering temperature had little effect on pore size distribution of Al₂O₃ PHM ceramics, which only declined from 24.2 to 21.4 μm with the increase of sintering temperature from 1600 to 1650 °C. This method can not only directly prepare ceramic foams with complex shapes, but also control properties of ceramic foams. It provides a simple preparation method for many kinds of ceramic foams with complex structure and high porosity by using PHMs with different composition.     

Effect of grain size on the static and dynamic mechanical properties of magnesium aluminate spinel (MgAl2O4)

Samples of transparent polycrystalline spinel with average grain size varying from 0.14 to 170 μm were prepared by different sintering approaches. The effect of grain size on the flexural strength, hardness and Hugoniot elastic limit (impact loading) was investigated. It was found that values of hardness divided by three for samples with grain size in the 0.14–15 μm range were almost equal to the dynamic yield strength values, estimated based on the Hugoniot elastic limit. This led to the assumption that the onset of inelastic deformation at the Hugoniot elastic limit was brittle rather than ductile. The observed departure of the dynamic yield strength from the hardness divided by three value for ceramics with grain size >15 μm was associated with either impact-induced shear banding or twinning. The feasibility of such banding/twinning intervention in initiating inelastic deformation in the spinel is supported by the values of apparent Hall-Petch coefficients in the corresponding grain size domains.     

Sintering behavior of magnesium aluminate spinel MgAl2O4 synthesized by different methods

This paper is focusing basically on the ceramic technology, of which several methods for the synthesis of MgAl₂O₄ have been investigated. The synthesis conditions regarding the powders cleanliness, microstructure, and sintering parameters of MgAl₂O₄ were studied. MgAl₂O₄ powder was synthesized via conventional solid-state route using different milling process: vertical attrition milling, WAB as a high-energy horizontal attrition milling, and Pulverisette as a planetary ball miller, and via solution combustion route using Urea, Glycine, and a mixture of Urea/Glycine. Urea and Glycine was used as fuel. The white powders were obtained for all solid-state routes and for Urea-combustion technique. The black and gray powders were obtained in the case of combustion technique, respectively, using a fuel of Glycine and Glycine/Urea mixture. The obtained powders and pellets were characterized by XRD, SEM, and Dilatometry. The results show that, among all the solid-state route processes, wet attrition milling gives the better and clean spinel phase. The WAB milling and Pulverisette miller introduce a contamination by some yttria-stabilized zirconia balls in the corresponding powder. Furthermore, the flash combustion technique permit to have nanoparticles with a dense spinel phase of MgAl₂O₄ and with lower sintering temperature in less time and with no calcination step.     

Preparation of Si3N4-BCxN-TiN composite ceramic aerogels via foam-gelcasting

Recently, high-performance ceramic aerogels (CAs) have become a research hotspot because of their multi-functional properties. Compared to their oxide counterparts, non-oxide CAs show better thermal shock resistance and other high-temperature properties, making them suitable for applications in harsh environments. In this work, Si₃N₄-BC_xN-TiN ceramic aerogels (SBCNT CAs) with molar ratios of B/C/Si/Ti = 2:1:5:5 were synthesized at 1623 K for 3 h via foam-gelcasting, from Si, Ti, melamine and boron acid raw materials. The prepared SBCNT CAs containing 91.0% porosity exhibited the highest compressive strength up to 1.0 MPa, and thermal conductivity of 0.144 W/(m·K) at 298 K. They also exhibited excellent service performance at 1473 K in Ar, demonstrating their great potential for high-temperature applications.     

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.     

Preparation and characterization of novel nonstoichiometric magnesium aluminate spinels

Magnesium aluminate spinel is of great importance for nuclear industry, and its structure, showing a great impact on properties, is sensitive to the composition. In order to explore the stoichiometric effect on structure and properties of spinels, several different spinel compositions with MgO·nAl₂O₃ (n?=?0.5–2.4) were synthesized via solid state reaction. Synthetic samples were characterized by X-ray diffraction, scanning electron microscope and nanoindentation tests. The results of XRD and SEM indicate that the single-phase magnesia alumina spinels have been prepared successfully for the first time ranging from n?=?0.667 to n?=?1.5, which is beyond the previous reported ranges of n$\ge$ 0.91. The hardness and modulus decrease with increasing n, implying further that the nonstoichiometric spinel crystal structures are likely to exhibit superior mechanical properties.     

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

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.     

Fabrication of magnesium aluminate (MgAl2O4) spinel foams

This paper reports on a novel-processing route for fabricating magnesium aluminate (MgAl₂O₄) spinel (MAS) foams from aqueous suspensions containing 30–35 vol.% solids loading. A stoichiometric MAS powder formed from alumina (71.8%) and magnesia (28.2%) at 1400 °C was surface passivated against hydrolysis in an ethanol solution of H₃PO₄ and Al(H₂PO₄)₃ at 80 °C for 24 h. Stable aqueous suspensions with 30–35 vol.% solids loading were prepared using the surface passivated MAS powder with the help of tetra-methylammonium hydroxide (TMAH) and an ammonium salt of polyacrylic acid (Duramax D-3005) employed as dispersing agents. An aqueous solution of N-cetyl-N,N,N-trimethylammonium bromide (CTMAB) was utilized to create foam in aqueous MAS suspensions by mechanical frothing. Liquid foam was then consolidated in non-porous moulds by introducing a polymerization initiator and a catalyst under ambient conditions. Dried (at >90 °C for 24 h) MAS foams were then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS bodies out of an un-passivated stoichiometric MAS powder, and, dense as well as foams out of alumina were also prepared in this study. The sintered properties of MAS and alumina ceramics were characterized by various means and thus obtained results are presented and discussed in this paper. The sintered MAS foams exhibited a porosity of about 74–76% and a compressive strength of about 4–7.2 MPa inline to values reported for other ceramic foams in the literature.     

Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.     

Mechanical and optical properties of MgAl2O4 ceramics and ballistic efficiency of spinel based armour

This study was devoted to the understanding of the influence of MgAl₂O₄ ceramic properties on their ballistic performances. By modifying the processing parameters, ceramics with different microstructures were obtained. Among them, a transparent MgAl₂O₄ spinel with an in-line transmission between 77% and 83% in the visible range, an average grain size of 8.6 μm and good mechanical properties (11.3 GPa in Knoop hardness and 2.5 MPa√m in fracture toughness) was produced. A thorough characterisation of the ceramics was accomplished in order to establish a link between microstructure, mechanical properties and ballistic protective performances against an armour piercing projectile of calibre 7.62x51 mm. The ballistic evaluation demonstrated the advantage of using a spinel layer as the strike face to stop a threat, while reducing drastically the thickness and the areal density of the transparent multilayer, compared to a simple glass armour. MgAl₂O₄ spinel with fine grains presented a better combination of mechanical properties compared to coarser microstructures, hence a better potential to damage a projectile at the impact.     

Preparation and characterization of porous mullite ceramics via foam-gelcasting

Porous mullite ceramics were prepared via foam-gelcasting using industrial grade mullite powder as the main raw materials, Isobam-104 as the dispersing and gelling agent, sodium carboxymethyl cellulose as the foam stabilizing agent, and triethanolamine lauryl sulfate as the foaming agent. The effects of processing parameters such as type and amount of additive, solid loading level and gelling temperature on rheological properties and gelling behaviors of the slurries were investigated. The green samples after drying at 100 °C for 24 h were fired at 1600 °C for 2 h, and the microstructures and properties of the resultant porous ceramic samples were characterized. Based on the results, the effects of foaming agent on the porosity level, pore structure and size and mechanical properties of the as-prepared porous mullite ceramics were examined. Porosity levels and pore sizes of the as-prepared samples increased with increasing the foaming agent content up to 1.0%, above which both porosity levels and pore sizes did not change. The compressive strength and flexural strength of the as-prepared sample with porosity of 76% and average pore size of 313 μm remained as high as 15.3±0.3 MPa and 3.7±0.2 MPa, respectively, and permeability increased exponentially with increasing the porosity.     

Performance of borosilicate glass/Ba3(VO4)2 ceramic composites and chemical stability with Ag electrodes

In the present work, a systematic study on performance of borosilicate glass/Ba₃(VO₄)₂ ceramic composites synthesized by the traditional solid-state reaction method was conducted. Borosilicate glass is beneficial for reducing the sintering temperature of glass/Ba₃(VO₄)₂ ceramic composites below 900 °C due to the formation of a liquid phase. The ceramic composites with 35 %∼40 % Ba₃(VO₄)₂ can effectively adjust the temperature coefficient of resonant frequency (τ_f) to approximately 0. The diffusion activation energy (Q_Ag) increases from 113.92 kJ/mol to 185 kJ/mol, as Ba₃(VO₄)₂ content increases from 0 to 40 % in the ceramic composites. A 0.65 borosilicate glass-0.35Ba₃(VO₄)₂ ceramic composite possesses excellent properties including an ε_r of 8.66, Q × f value of 20,338 GHz, the largest volume resistivity of 5.1 × 10¹³ Ω·cm and a flexural strength of 259 MPa. Ag exists only in the zero-valence state in the ceramic composite, and Ba₃(VO₄)₂ ceramic can block the diffusion channels and increase the barrier to the movement of Ag ions.     

凝胶成型Al2O3-(30vol%)MgO·1.35Al2O3泡沫陶瓷的研制

以板状刚玉和富铝尖晶石为主原料,采用泡沫浸浆凝胶工艺制备了Al2O3-(30 vol%)MgO@1.35Al2O3泡沫陶瓷.研究了分散剂AN-2000与pH值对Al2O3-(30vol%)MgO@1.35Al2O3混合浆料流变特性的影响.在pH值=9.5、AN-2000添加量为0.4 mL@m-2的条件下制备的粘度为O.14 Pa@s,固相体积分数为55%的Al2O3-(30vol%)MgO@1.35Al2O3混合浆料,通过有机泡沫浸浆、引发凝胶反应、湿坯干燥、排胶和烧结,制备出了气孔率为89.3%,强度为5.2 MPa的Al2O3-(30vol%)MgO@1.35Al2O3泡沫陶瓷.     

Experimental phase relations between MgO-saturated magnesium-aluminate spinel (MgAl2O4) and CuOx-rich liquid

Experimental phase equilibrium data for the Cu-O-Al₂O₃-MgO system is required to improve the performance of MgAl₂O₄-containing refractories and slagging in non-ferrous smelting. In this work, the phase relations of MgAl₂O₄ in the Cu-O-Al₂O₃-MgO system were studied experimentally in air within a temperature range of 1100–1400 °C using the equilibration and quenching method. The chemical compositions of the phases in the quenched samples were determined using electron probe microanalysis (EPMA). Less than 1 wt% of Al₂O₃ or MgO were found in the oxide liquid phase, whereas the solid MgAl₂O₄ and MgO phases contained up to 23 wt% and 30 wt% of ‘Cu₂O’, respectively. Discrepancies between these results and the corresponding calculated values generated by the MTDATA 6.0 software and Mtox database Version 8.2 ranged from 3 wt% to 19 wt%. The results of this work indicate that the MgAl₂O₄ spinel is chemically stable in the presence of a CuO_x-rich liquid under the conditions studied.     

Preparation and characterization of ceramic foam supported nanocrystalline zeolite catalysts

The influence of the viscosity of the ceramic slip on the manufacture of ceramic foams by the polymer sponge method was studied. The foams were coated with silicalite (100 nm) and HZSM-5 (650 nm) crystals by dipping them in a zeolite suspension without binder additives. The amount of zeolite loaded can be controlled through the zeolite content of the dipping suspension. Uniform coatings are achieved starting with about 1 g zeolite/m² foam.     

DC conductivity and AC impedance of Mn doped magnesia alumina spinel (MgAl2-2xMn2xO4) over a large temperature range

MgAl_2-2xMn_2xO₄ (MAMO) with x = 0-0.12 was synthesized in a single-phase form by solid-state reaction. XRD analysis showed that the samples had the cubic center structure of the Fd-3 m space group. Electrical properties of the samples were studied over the temperature range of 300 K∼1073 K. The results showed that the DC conductivity (σ_DC) increased from 10⁻¹¹S/cm at 300 K (MAMO, x = 0) to 10^-3S/cm at 1073 K (MAMO, x = 0.12). The equivalent circuit of the complex impedance spectra suggested that the relaxation of charge carriers was of non-Debye type. The conduction was mainly caused by grain boundaries and the capacitance was mainly attributed to polarization. The complex permittivity values (ε’ and ε’’) were increased by two orders of magnitude with the increase in Mn content and temperature over the measured frequency range (1 Hz-1 MHz). Therefore, doping with Mn could be applied to modify the electrical properties of MAMO at high temperature.