MgAl2O4 Ceramics Research Articles

Luminescent properties of MgAl2O4 ceramics doped with rare earth ions fabricated by spark plasma sintering technique

MgAl2O4 ceramics doped with rare earth ions (Eu2+ and Ce3+ ions) were fabricated by spark plasma sintering technique. A complex characterization of the crystalline and defect structure of the ceramic by XRD was carried out. Absorption, excitation, photo- and cathodoluminescence spectra were studied. The photoluminescence spectrum shifts to the blue region with a maximum at λem = 475 nm for the MAS:0.1Ce ceramics. The nature of this luminescence can be caused by the radiative transitions in the cerium ion 5d–4f. The emission spectrum of MAS:0.1Eu has a “green” band emission in range of 400–700 nm centered around 500 nm, which can be ascribed to the allowed 4f65d1→4f7 (5d–4f) transition of Eu2+. In the millisecond time range, simultaneously with the emission of the complex host centers, the impurity luminescence bands of the chromium ion are recorded. It was shown that cathodoluminescence spectra in nanosecond time range can be decomposed into several emission bands at 2.72, 3.01, 3.37, 3.63–3.82 eV caused by F-type centers. It was demonstrated that the Eu2+ and Ce3+ ions lead to change the intensity ratio of the luminescence bands. The luminescence decay kinetics of synthesized spinel ceramics in nano- and millisecond time range were investigated in detail.

Mechanical and Thermal Properties of MgAl<sub>2</sub>O<sub>4</sub>-Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> Ceramic Composites

MgAl2O4-Y3Al5O12 ceramic composites were prepared using fused spinel and a Y2O3 micropowder as the raw materials. The microstructure and thermal properties of the composites were characterized by X-ray diffraction, scanning electron microscopy, laser flash diffusivity measurements. The mechanical properties were also determined. MgAl2O4-Y3Al5O12 ceramic composites are composed of spinel and garnet structures. The thermal expansion coefficients of MgAl2O4 and MgAl2O4-Y3Al5O12 ceramics are similar. The measured thermal diffusivity decreases gradually with increasing temperature. Thermal conductivity of the composites is in the range of 3.3-5.8 W∙m-1∙K-1 from 400°C to 900°C.

Atomic-deficient nanostructurization in water-sorption alumomagnesium spinel ceramics MgAl2O4

Atomic-deficient nanostructurization in alumomagnesium MgAl2O4 ceramics sintered at 1100–1400 °C caused by water sorption are studied employing positron annihilation lifetime spectroscopy. Detected PAL spectra are reconstructed from unconstrained x4-term decomposition, and further transformed to x3-term form to be applicable for analysis with x3–x2-CDA (coupling decomposition algorithm). It is proved that water-immersion processes reduce positronium (Ps) decaying in large-size holes of ceramics (1.70–1.84 nm in radius) at the expense of enhanced trapping in tiny (~ 0.2 nm in radius) Ps-traps. The water sorption is shown to be more pronounced in structurally imperfect ceramics sintered at Ts = 1100–1200 °C due to irreversible transformations between constituting phases, while reversible physical-sorption processes are dominated in structurally uniform ceramics composed of main spinel phase.

Fabrication and Characterization of Porous MgAl₂O₄ Ceramics via a Novel Aqueous Gel-Casting Process.

A novel and aqueous gel-casting process has been successfully developed to fabricate porous MgAl2O4 ceramics by using hydratable alumina and MgO powders as raw materials and deionized water as hydration agent. The effects of different amounts of deionized water on the hydration properties, apparent porosity, bulk density, microstructure, pore size distribution and compressive strength of the samples were investigated. The results indicated that the porosity and the microstructure of porous MgAl2O4 ceramics were governed by the amounts of deionized water added. The porous structure was formed by the liberation of physisorbed water and the decomposition of hydration products such as bayerite, brucite and boehmite. After determining the addition amounts of deionized water, the fabricated porous MgAl2O4 ceramics had a high apparent porosity (52.5–65.8%), a small average pore size structure (around 1–3 μm) and a relatively high compressive strength (12–28 MPa). The novel aqueous gel-casting process with easy access is expected to be a promising candidate for the preparation of Al2O3-based porous ceramics.

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl2O4 spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500°C during 2h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl2O4 ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800°C for 10h with an in-line transmission of 81% at 400nm and 86% from 950 to 3000nm for a thickness of 2mm (98.8% of the theoretical transmission).

Effects of cation distribution on microwave dielectric properties of Mg1−xZnxAl2O4 ceramics

The effects of duration of ball-milling on cation distribution of calcined-MgAl2O4 and -ZnAl2O4 powders were investigated by 27Al solid-state nuclear magnetic resonance (NMR) measurement; the degree of inversion (λC), which corresponds to the fraction of Al3+ in tetrahedral site, of the MgAl2O4 powder increased from 0.33 to 0.42 when the duration of ball-milling varied from 0 to 24 h, though that of the ZnAl2O4 powders ranged from 0.04 to 0.07. Thus, the preferential occupation of Al3+ cation in tetrahedral site was obtained for the calcined-MgAl2O4 powders prepared by ball-milling for 24 h. As a result, the Q·f value of the MgAl2O4 ceramics extremely increased from 85,100 to 187,200 GHz with increasing the duration of ball-milling of the calcined powders, while that of the ZnAl2O4 ceramics was almost constant with a Q·f value of approximately 100,000 GHz. Moreover, the effects of Zn substitution for Mg on the microwave dielectric properties and cation distribution of the Mg1−xZnxAl2O4 ceramics were also investigated. 27Al NMR measurement revealed that the high λ0 values, implying the degree of the redistribution of the Al3+ cation in the octahedral site during firing, were observed in the composition range of 0–0.75 and the highest Q·f value of 222,600 GHz was obtained for the Mg1−xZnxAl2O4 ceramics at x = 0.75. Thus, it is considered that the preferential occupation of Al3+ cation at tetrahedral sites of the calcined-powders caused by the ball-milling is effective to enhance the Q·f value of the Mg1−xZnxAl2O4 ceramics.

Fabrication and spectral properties of hot-pressed Co:MgAl2O4 transparent ceramics for saturable absorber

Cobalt-doped magnesium aluminate spinel (Co:MgAl2O4) could be the most effective saturable absorber (SA) for Q-switching solid-state lasers operating at 1.5 μm. Using LiF as the sintering aid, highly transparent Co:MgAl2O4 ceramics were fabricated by hot pressing (HP) of the commercial spinel and the co-precipitated Co:MgAl2O4 powder mixture. The optical properties of Co:MgAl2O4 ceramics were investigated, and the ground state absorption cross section (σgas) of Co2+ was calculated from the curve of the absorption coefficient. For the Co:MgAl2O4 ceramics hot-pressed at 1600 °C for 2 h under 30 MPa with 1 wt.% LiF addition, the in-line transmittance reaches about 77% at 1000 nm and 1700 nm, and the ground state absorption cross section of Co2+ is 2.55 × 10−19 cm2 at the wavelength of 1540 nm, which is promising as a saturable absorber for solid-state pulse laser operating at 1.5 μm.

Fabrication and spectroscopic properties of Co:MgAl2O4 transparent ceramics by the HIP post-treatment

Cobalt-doped magnesium aluminate spinel (Co:MgAl2O4) is one of the most important saturable absorbers for the passive Q-switching of solid-state lasers operating at eye-safe wavelength of 1.5 μm. In this work, highly transparent Co:MgAl2O4 ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) post-treatment, using the mixture of the commercial spinel and the lab-made Co:MgAl2O4 powder as the raw materials. The densification mechanism of Co:MgAl2O4 transparent ceramics was discussed. The microstructure and optical properties of the samples were investigated. The ground state absorption cross section (σGSA) was calculated from the fitted curve of the absorption coefficient spectrum. The results show that Co:MgAl2O4 ceramics fabricated by vacuum sintering at 1500 °C for 5 h and then HIP post-treatment at 1650 °C for 3 h perform good transparency, whose in-line transmittance exceeds 80% at 2500 nm. Moreover, the ground state absorption cross section of 0.02 at.% Co:MgAl2O4 ceramics is calculated to be 3.35 × 10−19 cm2 at the wavelength of 1540 nm, which is promising for the application to the passive Q-switching of solid-state laser operating in the near infrared region (NIR).

Effects of LiF on the microstructure and optical properties of hot-pressed MgAl2O4 ceramics

Transparent magnesium aluminate spinel (MgAl2O4) ceramics were fabricated by hot-pressing of the MgO and α-Al2O3 powder mixture using LiF as a sintering aid. Effects of the LiF additive on densification, microstructure and optical properties of MgAl2O4 ceramics were systematically investigated. It has been found that the addition of LiF can effectively remove the porosity and increase the optical transparency of MgAl2O4 ceramics. For the spinel ceramics HP-ed at 1550°C for 3h with 1wt% LiF addition, the average grain size is about 36µm and the in-line transmittance exceeds 60% at the wavelength of 800nm.

Millimeter-Wavelength Radiation Used to Sinter Radiotransparent MgAl2O4 Ceramics

We have developed a method for production of high-density MgAl2O4 ceramics characterized by low dielectric losses. The method is based on sintering of ultrafine, high-purity powder materials in the electromagnetic field of intense microwave radiation. The initial nanodimensional magnesium-aluminate powder was synthesized by the sol-gel method and sintered in a gyrotron complex operated at a frequency of 24 GHz. The results of studying the conditions and regimes of microwave sintering of MgAl2O4 ceramics are presented, along with its phase features, microstructural peculiarities, and mechanical and dielectric characteristics. The possibility of controlling the microstructure of the sintered material by varying the regime of RF heating is demonstrated.

Microstructure of the microwave fast-sintered MgAl2O4 ceramics

Microstructure of the microwave fast-sintered MgAl₂O₄ ceramics

Fabrication and measurement of optical and spectral properties of the transparent Yb:MgAl2O4 ceramics

In this paper results concerning the fabrication of transparent (with 0.1% mol Yb3+), translucent (with 0.5% mol Yb3+) and opaque (up to 5% mol Yb3+) Yb3+: MgAl2O4 ceramics by hot pressing are presented. The alkoxotechnology of starting magnesium aluminate spinel nanopowder synthesis with homogeneous distribution of Yb ions is proposed. The structural properties of the prepared powders and ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET method. The optical transmittance, absorption and emission cross sections, and the upper level lifetime of the Yb3+:MgAl2O4 ceramics were measured.

Microwave dielectric properties and crystal structures of spinel-structured MgAl2O4 ceramics synthesized by a molten-salt method

The microwave dielectric properties and crystal structures of MgAl2O4 ceramics synthesized using either molten-salt (MA-M) or solid-state reaction (MA-S) methods were characterized in this study. Raman and 27Al solid-state nuclear magnetic resonance spectra indicated that Al3+ cations primarily occupied in tetrahedral sites of the MA-M ceramic. The degree of inversion x, i.e., degree of cation disorder in tetrahedral and octahedral sites, of the MA-M was higher than that of MA-S; such the preferential site occupation of Al3+ cations enhanced the covalency of the MO bonds in the MO4 tetrahedra (M=Mg and Al), leading to a decrease in the lattice parameters. The Q·f of the MA-M fired at 1600°C was 201,690GHz, while the MA-S synthesized at 1600°C exhibited a Q·f of just 85,100GHz. Based on these results, an intermediate spinel structure with a greater x evidently has a higher Q·f, and therefore the cation distribution is closely related to the Q·f of the ceramic.

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 0.6vol%. Porous MgAl2O4 ceramics with porosity of 75.1% and average pore size of 266µm exhibited a compressive strength as high as 12.5±0.8MPa and thermal conductivity as low as 0.24W/(mK) (at 473K).

Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics

Transparent MgAl2O4 ceramics were fabricated by reactive sintering in air followed by hot isostatic press treatment using commercial Al2O3 powder (γ-Al2O3 or α-Al2O3) and MgO powder as raw materials. The densification rate, microstructure and optical properties of the ceramics were investigated. Densification temperature of the sample from γ-Al2O3/MgO was lower than that from α-Al2O3/MgO. However, in-line transmission (2 mm thick) of the sample from α-Al2O3/MgO at the wavelength of 600 nm and 1100 nm were respectively 77.7% and 84.3%, higher than those (66.7%, 81.4%) of the sample from γ-Al2O3/MgO. SEM observation revealed that the sample from α-Al2O3/MgO exhibited a homogeneous and pore-free microstructure, while, the sample from γ-Al2O3/MgO showed an apparent bimodal microstructure containing pores.

Synthesis and Sintering of Magnesium Aluminate Spinel Nanopowders Prepared by Precipitation Method using Ammonium Hydrogen Carbonate as a Precipitant

Magnesium aluminate spinel (MgAl2O4) is widely used in many engineering applications due to its high melting point (2135°C), high mechanical strength, chemical inertness, and good optical properties. Precipitation method is recognized as a convenient and cost-effective method for the synthesis of nanopowders. In this present work, MgAl2O4 nanopowders were prepared by precipitation method using ammonium hydrogen carbonate as a precipitant. The precipitated precursors were a mixture of ammonium dawsonite (NH4Al (OH)2CO3·H2O) and hydrotalcite (Mg6Al2(CO3)(OH)16·4H2O). After calcining at 1100°C for 2 hours, The MgAl2O4 nanopowders with particle size of 20-170 nm were obtained. The sinterability of the MgAl2O4 nanopowders was evaluated by sintering compacts of the MgAl2O4 nanopowders at temperature of 1300-1650°C for 2 hours. The relative density of the sintered MgAl2O4 ceramics reached about >97% of theoretical density after sintering at 1500°C for 2 hours. The Vicker’s hardness of the sintered ceramics reached a value of 1414 HV (13.9 GPa) after sintering at 1650°C for 2 hours.

Characterization of nanostructured MgAl2O4 ceramics fabricated by spark plasma sintering

The aim of this work is to investigate the effect of sintering temperature on the properties of nanostructured spinel (MgAl2O4)fabricated by Spark Plasma Sintering (SPS). The starting material was a pure spinel (MgAl2O4) nanopowder. The sintering was carried out at T=1300°C, 1350°C and 1400°C under a pressure of 73MPa.The samples sintered at 1300°C exhibit a finer microstructure (mean grain size is about ~250nm) than those sintered at 1350°C and 1400°C. The relative density is about 99.93% at 1300°C, 99.63% at 1350°C and 99.58% at 1400°C due to the presence of some porosity in the fabricated samples. Samples sintered at 1300°C have a good transmittance (70% at 550nm and 78% at 1100nm) compared to those sintered at 1350°C and 1400°C.Due to their high density and fine grain size, samples sintered at 1300°C exhibit a Vickers micro-hardness Hv=18GPa, elastic modulus E=228GPa relatively more important than those measured on samples sintered at 1350°C (Hv=15GPa, E=172GPa,) and at 1400°C (Hv=12GPa, E=136GPa). They present the reverse behavior for fracture toughness kIC (1.05MPa √m for samples sintered at 1300°C, 1.43MPa √m for samples sintered at 1350°C and 2.23MPa √m for that sintered at 1400°C). The tribological behavior of the MgAl2O4ceramic was equally studied: the friction coefficient µ is measured and increases with sintering temperature µ=0.06, µ=0.13, µ=0.18, respectively for sintering temperature of 1300°C, 1350°C and 1400°C. The wear behavior is also examined and the mass loss increases with increasing sintering temperature.

溶液含浸法によるスピネル多孔体の作製におけるアルミナ骨格の微細構造の影響

Porous MgAl2O4 ceramics were fabricated by impregnating porous Al2O3 bodies calcined at 1250℃ with Mg(NO3)2 saturated aqueous solution under reduced pressure, followed by firing at 1500℃. Porous Al2O3 bodies with continuously long-ranged pore structure and cross-linked one were prepared by a unidirectional freeze casting using aqueous Al2O3 slurries containing urethane latex binder (ULB) and water-soluble binder (PVA), respectively. The effect of the micro structure of Al2O3 bodies on MgAl2O4 formation was investigated. In the case of the porous Al2O3 body with continuously long-ranged pore structure, the amount of MgAl2O4 formed on firing at 1500℃ was larger than that for the porous Al2O3 body with cross-linked pore structure and a single-phase MgAl2O4 porous ceramic was obtained by four times impregnation. However, in this case, the surface of the porous Al2O3 body peeled off gradually with repeating impregnation. On the other hand, in the case of the porous Al2O3 body with cross-linked pore structure, about 10% Al2O3 remained after four times impregnation, but the structural disintegration caused by repeating impregnation was not observed. The cross-linked pore wall was effective to increase the strength of the calcined porous Al2O3 bodies but it was difficult to make the solution of Mg(NO3)2・6H2O into the small pore, resulting in a decrease in the rate of formation of MgAl2O4.

Microstructure and properties of transparent MgAl2O4 ceramic fabricated by aqueous gelcasting

A nontoxic, water-soluble copolymer consisting of isobutylene and maleic anhydride (PIBM) was used as both dispersant and gelling agent to mold MgAl2O4 green body by gelcasting at room temperature in air. In this paper, MgAl2O4 slurries with solid loadings from 43 vol% to 50 vol% were prepared by adding PIBM. The rheological properties of the slurries were investigated. As the solid loadings increased from 43 vol% to 50 vol%, the linear shrinkage in the drying and pre-sintering process decreased from 6.9% to 2.8% and from 16.0% to 14.7%, respectively. The bending strength of the green body with 50 vol% solid loading can reach 2.62 MPa. The in-line transmission of the MgAl2O4 ceramics gradually increased from 61% to 86.9% (1100 nm) with the solid loadings increasing.

Enhanced infrared transmittance properties in ultrafine MgAl2O4 nanoparticles synthesised by a single step combustion method, followed by hybrid microwave sintering

Infrared transparent ceramics found to have potential applications as infrared windows and domes in strategic defence and space missions. Synthesis of ultrafine nanostructured MgAl2O4 ceramics by a modified single step auto-igniting combustion technique, followed by sintering of the sample by resistive and resistive-microwave hybrid heating to high density and their excellent infrared transmission characteristics are presented in this paper. Structural characterisations of MgAl2O4 nanoparticles reveal that the as prepared powder is phase pure, with average crystallite size ∼15nm and possess a cubic structure. Optical band gap calculated using the Kubelka–Munk method is 5.75eV. The thermal stability of the nanopowder at elevated temperatures has been studied using thermo gravimetric analysis (TGA) and differential thermal analysis (DTA). Hybrid heating yield a substantial reduction in sintering temperature and soaking time relative to the conventional resistive heating, and the samples achieved >99% density by microwave-resistive hybrid heating. Scanning electron micrograph (SEM) showed that the pellets are well sintered. The pellet sintered by hybrid heating showed a better transmittance of ∼79% in the UV–Visible region and ∼82% in the mid IR region compared to pellet sintered by resistive heating which has ∼68% in the UV–Visible region and ∼66% in the mid IR region. The results confirm the effective use of nanocrystalline powders from modified combustion synthesis as starting material for the development of high quality IR transparent windows and domes. In addition the microwave hybrid sintering technique employed in the present study also contributes to the results of better transmittance characteristics in highly densified MgAl2O4 ceramic pellets.