Magnesium Aluminate Spinel Ceramics Research Articles

Two-Photon Polymerization of Nanocomposites for Additive Manufacturing of Transparent Magnesium Aluminate Spinel Ceramics.

Transparent polycrystalline magnesium aluminate (MAS) spinel ceramics are of great interest for industry and academia due to their excellent optical and mechanical properties. However, shaping of MAS is notoriously challenging especially on the microscale requiring hazardous etching methods. Therefore, a photochemically curable nanocomposite is demonstrated that can be structured using high-resolution two-photon lithography. The printed nanocomposites are converted intro transparent MAS by subsequent debinding, sintering, and hot isostatic pressing. The resulting transparent spinel ceramics exhibit a surface roughness Sq of only 10nm and can be shaped with minimum feature sizes of down to 13µm. This technology will be important for the production of microstructured ceramics used for optics, photonics, or photocatalysis.

Understanding the effect of lithium fluoride additive on the properties of pressureless sintered magnesium aluminate spinel ceramics

This paper looks at the problem of obtaining high-density transparent ceramics. The paper considers a method for producing such material from magnesium aluminate spinel (AMS) using sintering additives, as well as its potential application. The paper gives an overview of possible additives that are most commonly used by researchers for magnesium aluminate spinel ceramics synthesis. Based on the reviewed literature, lithium fluoride (LiF) was selected by the authors as a sintering additive for the purposes of this research. The paper describes the results of a study that looked at obtaining transparent ceramics by pressureless vacuum sintering at 1,750 оC. By means of thermal synthesis, powders with a single magnesium aluminate spinel phase and bimodal particle size distribution were obtained. Different mechanisms were considered of how LiF densifier acts to create a pore-free ceramic structure. Lithium fluoride was found to form a solid solution with magnesium aluminate spinel, which changes the crystal lattice parameter resulting in a high-density material. Ceramic samples with zero open porosity and relative density of 99% were obtained. The average density was 3.51 g/cm3 at the lithium fluoride concentration of 2.6 wt. %, and 3.55 g/cm3 when the lithium fluoride concentration was 2.7 wt. %. The obtained ceramics can transmit light in the visible region of the electromagnetic spectrum. The residual light diffusion in the obtained ceramic samples can be attributed to the residual closed porosity, which is equal to 1%. The paper describes possible solutions to the problem in order to achieve better light transmission.The researches were conducted using the equipment of the Research sharing center named after D. I. Mendeleev within the framework of the project No. 075-15-2021-688.

Transparent magnesium aluminate spinel ceramics obtained by one-step spark plasma reaction sintering of a sol-gel precursor powder

Transparent magnesium aluminate spinel (MAS) was obtained by one-step spark plasma reaction-sintering (SPRS) of a nanosized sol-gel powder. The sol-gel powder was processed to obtain three powders as precursors materials with different characteristics. The first powder (HT0) was the as-synthesized sol-gel powder, comprising poorly crystallized boehmite and residual magnesium nitrate hexahydrate. The second powder (HT450) was obtained by calcination of the sol-gel powder at 450 °C to produce an incipient formation of MAS. The third powder (HT850) was calcined at 850 °C to produce spinel with low crystallinity. The powders were deagglomerated and then consolidated in one step using a spark plasma sintering (SPS) machine at temperatures ranging from 1260 to 1350 °C to suit their different properties. The one-step SPRS was achieved with relative densities up to 99.98 % and submicron-sized grains. However, transparent ceramics were obtained only with the HT450 and HT850 powders. The ceramic produced from the HT450 powder achieved a light transmittance of 71 % at λ = 1100 nm, Vickers hardness (HV) of 11.98 GPa, grain size of 604 nm, and relative density of 99.98 %. The ceramic produced from the HT850 powder consolidated by one-step SPRS at 1275 °C achieved a light transmittance of 71 %, HV of 12.71 GPa, grain size of 396 nm, and relative density (RD) of 99.90 %. The results showed that the HT450 powder had better sinterability, attributed to the higher reactivity of the partially reacted sol-gel particles used as precursors in this sample.

Low-cost preparation and characterization of MgAl2O4 ceramics

Low-cost magnesium aluminate spinel ceramics were successfully synthesized from the minerals of magnesite(M) and bauxite(B) via the one-step sintering method. The effect of the ratio of bauxite to magnesite on the phase evolution, microstructure, as well as the mechanical properties was thoroughly investigated. The phase structural analysis indicated that Mg2+ or Al3+ ions were dissolved in the spinel lattice and formed Mg-rich/Al-rich spinel MgAl2O4 (MA). The microstructure analysis revealed that the liquid phase gradually appeared with the increment of bauxite content. Appropriate but not saturated cation solution had the effects of structural densification, homogenization of crystal grains, and the enhancement of strength. Ceramics with 1.31 bauxite to magnesite stoichimetric ratio exhibited the highest density of 3.43 g/cm3 and flexural strength of 93.5 MPa.

Comparative study of filament-fed and blown powder-based laser additive manufacturing for transparent magnesium aluminate spinel ceramics

Magnesium aluminate spinel is of great interest as a transparent ceramic for its excellent mechanical properties and excellent optical transmittance. Additive manufacturing of this desirable material presents several benefits over traditional manufacturing methods, including reduced fabrication time and cost and the potential to fabricate structures with complex geometries and internal cooling networks. Despite the many benefits, the challenges hindering this technology must be overcome. A primary challenge with powder-based laser additive manufacturing of transparent ceramics is a trade-off between densification and cracking. The fabrication of transparent ceramics requires nearly full densification since pores act as light scattering centers. Even relatively small percentages of porosity render ceramics translucent or opaque. Previous studies on powder-based laser direct deposition of spinel ceramics have shown that densification to transparency is possible with high-laser power deposition. While high-laser powers are beneficial for densification, it also produces high thermal gradients that result in significant crack formation. Cracks hinder mechanical properties and transparency, limiting possible applications. Thus, we propose a filament-based deposition strategy to reduce laser power requirements. Filament-fed laser direct deposition, instead of blown powder, dramatically reduced the amount of gas porosity within the melt. Hence, highly densified, transparent, spinel ceramics were fabricated. Through decreased laser power requirements for high densification, cracking was largely reduced. This paper provides a comprehensive comparison between filament- and powder-based laser direct deposition by analyzing important sample characteristics, including porosity, cracking, grain size, and their controlling mechanisms. This paper also presents a laser direct deposition and postprocessing method to manufacture predensified spinel filaments.

Influence of presintering temperature on magnesium aluminate spinel transparent ceramics fabricated by solid‐state reactive sintering

AbstractTransparent magnesium aluminate spinel (MgO·1.1Al2O3) ceramics were successfully prepared by the solid‐state reactive sintering method using commercial γ‐Al2O3 and MgO powders as raw material. The extra alumina compared with stoichiometric spinel was found efficient to improve the optical quality of transparent spinel ceramics. The influence of presintering temperature on the phase formation, densification process, microstructure evolution, and optical quality of MgO·1.1Al2O3 was systematically investigated. The α‐Al2O3 phase was found in the ceramics calcined at 800°C. With the calcination temperature exceeding 1000°C, the magnesium aluminate spinel ceramics were all pure spinel phase. The density increased from 3.05 to 3.53 g/cm3 and the average grain size increased from 0.5 to 1.9 μm with the increase of presintering temperature from 1450°C to 1600°C. The transparent MgO·1.1Al2O3 ceramic sample presintered at 1550°C for 3 h and hot isostatic pressing posttreated at 1600°C for 3 h showed the best in‐line transmittance of 86.5% at 1100 nm and 84.3% at 800 nm.

Direct 3D Printing of Silica Doped Transparent Magnesium Aluminate Spinel Ceramics.

Transparent magnesium aluminate spinel ceramics were additively manufactured via a laser direct deposition method in this study. With a minimum porosity of 0.3% achieved, highly transparent spinel samples with the highest total optical transmittance of 82% at a wavelength of 632.8 nm, were obtained by a 3D printing approach. However, cracking was found to be a major issue affecting printed spinel samples. To control prevalent cracking, the effect of silica dopants was investigated. Increased silica dopants reduced average total crack length by up to 79% and average crack density by up to 71%. However, a high dopant level limited optical transmission, attributed to increased porosity and formation of secondary phase. Further investigation found that with decreased average fracture toughness, from 2.4 to 1.9 , the obvious reduction in crack formation after doping was related to decreased grain size and introduction of softer secondary phase during deposition. The study demonstrated the feasibility of the proposed laser direct deposition method in directly fabricating transparent spinel ceramics while dopants showed potentials in addressing cracking issues.

Investigation of the influence LiF sintering additive on the properties of magnesium aluminate spinel ceramics

AnnotationThe relevance of developing a technology for creating transparent ceramics based on magnesium aluminate spinel is considered. The spinel precursor was obtained by the reverse coprecipitation method. The effect of the phase composition of spinel powder and sintering additive on the properties of ceramics is shown.

Structural and Spectroscopic Characterization of Tb3+‐Doped MgAl2O4 Spinel Ceramics Fabricated by Spark Plasma Sintering Technique

Spark plasma sintering (SPS) technique is successfully used for the preparation of magnesium aluminate spinel ceramics doped with different concentrations of Tb3+ ions. Structural, morphological, optical, and photoluminescence (PL) characterization are conducted. It is demonstrated that the redshift in the absorption spectra is possibly due to the energy gap between the valence and conduction bands reducing and the formation of a higher concentration of nonbridging oxygen with an increase in the concentrations of Tb3+ ions in MgAl2O4 ceramics. The PL spectrum excited by nitrogen laser consists of a broad emission band with a maximum at λem ≈ 400 nm and a series of emission bands in the 400–700 nm spectral range responsible for Tb3+ ions. In the “blue” spectral region, the complex host centers such as oxygen vacancies are recorded. An increase in the terbium ion concentration does not lead to the effect of concentration quenching. Luminescence decay kinetics analysis for all ceramics samples is performed. The effect of terbium oxide on the optical, PL properties and luminescence decay kinetics of MgAl2O4 ceramics is discussed.

Magnesium aluminate spinel ceramics infiltrated with lanthanum-glass for dental applications

Abstract In this work, porous ceramic substrates based on magnesium aluminate spinel (MAS), Mg2Al2O4, were infiltrated with a lanthanum-rich glass for applications as dental ceramic material. The s...

Effect of B2O3 Concentration on the Properties of Transparent Magnesium Aluminate Spinel Ceramics

We consider the potential of using boron oxide as a sintering aid for the fabrication of transparent magnesium aluminate spinel (MAS) ceramics and analyze the general tendency in the effect of B2O3 concentration on the average density, open porosity, and optical transmission of MAS ceramics. Boron oxide is shown to have an advantageous effect on the sintering behavior of MAS ceramics: raising its concentration leads to a decrease in open porosity and an increase in the average density of the material.

Effects of ZnO addition on the sintering behavior and mechanical properties of magnesium aluminate spinel ceramics

AbstractThis article discusses the successful preparation of high‐strength solid solution spinel ceramics via a solid‐state reaction sintering process employing MgAl2O4. The effects of ZnO content on the sintering behavior, bulk density, apparent porosity, phase composition, microstructure, cold modulus of rupture, and modulus of elasticity of the MgAl2O4‐based ceramics are investigated. The results show that the formed MgAl2O4‐based ceramics represent an MgAl2O4–ZnO solid solution, which markedly improves the sinterability, densification, and mechanical properties of the pure MgAl2O4 ceramic and decreases the sintering activation energy.

Pulse Cathodoluminescence of the Impurity Centers in Ceramics Based on the MgAl2O4 Spinel

The spectral and luminescence decay kinetics of magnesium aluminate spinel ceramics prepared by spark plasma sintering (SPS) and the starting nanopowders were studied. It was demonstrated that two types of luminescence in spinel ceramics can be observed after pulse e-beam excitation. The first type is due to intrinsic defects, and the second type due to Cr3+ and Mn2+ ions. It was shown that the luminescence decay kinetics for Cr3+ and Mn2+ impurity ions can be approximated by the sum of two exponents. The decay times for the Cr3+ ion are 35 and 409 ns, for the Mn2+ ion are 29 and 340 ns. High-temperature exposure during SPS synthesis does not lead to a change in the nearest surroundings of impurity ions. In the luminescence spectra of ceramic and powder samples the intensity ratio changes of intrinsic and impurity centers is associated with strong diffuse scattering of light for spinel nanopowders.

Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85wt%, the additive content of 0.7wt%, and the gelation time of 350s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100nm.

Transparent ceramics prepared from ultrapure magnesium aluminate spinel nanopowders by spark plasma sintering

We have studied the properties of transparent magnesium aluminate spinel ceramics prepared by spark plasma sintering (SPS) of ultrapure nanopowders. The starting powders, of ≃ 99.98% purity, ranging in specific surface area from 30 to 160 m2/g, were prepared through the hydrolysis of alcoholic solutions of magnesium aluminum alkoxide complexes, followed by calcination at temperatures from 900 to 1100°C. SPS was carried out at 1450°C, with the holding time at the highest temperature not longer than 15 min. The transparent ceramic samples thus prepared have a transmission of up to 73% in the visible and IR spectral regions (λ = 2.5–5.0 µm). The crystallite size in the ceramics is 0.2–0.4 µm, and their microhardness is HV0.1 = 14.8–16.2 GPa.

Homogeneous Precipitation Synthesis and Sintering of Magnesium Aluminate Spinel Nanoparticles

A wide application of magnesium aluminate spinel powder has attracted a number of studies concerning the preparation of magnesium aluminate spinel powder. In this study, a precursor for magnesium aluminate spinel was synthesized by a homogeneous precipitation method using urea as a precipitant. The precursor and the calcined powders were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. After precipitation, the precursor was magnesium aluminium hydrate carbonate compound. By calcining, the precursor decomposed to MgO and an amorphous phase after calcining at 600°C. The formation of magnesium aluminate spinel via a reaction between MgO and the amorphous phase was observed after calcining over 800°C. The equiaxed magnesium aluminate spinel nanoparticles with particle size of 20-40 nm were obtained after calcining at 1100°C for 2 hours. Sinterability of the obtained magnesium aluminate spinel nanoparticles was also investigated by sintering compacts of magnesium aluminate spinel nanoparticles in the temperature interval of 1300-1650°C. Sintering temperature of 1600°C allowed the fabrication of dense magnesium aluminate spinel ceramics with relative density >95%.

Evolution of porous structure and free-volume entities in magnesium aluminate spinel ceramics

Porous structure evolution in functional humidity-sensitive MgAl2O4 spinel ceramics prepared at different sintering temperatures is studied. It is shown that the increase in sintering temperature from 1100 to 1400°C results in the transformation of the pore size distribution in ceramics. It is established that porous ceramics with tri-modal pore size distribution including open macro-, meso- and micropores up to a few nanometer is humidity sensitive in the 25–95% relative humidity range. It is demonstrated that positron annihilation lifetime spectroscopy supported by four-component fitting procedure is a promising tool for nanostructural characterization of MgAl2O4 spinel-type ceramics. Microstructure of these materials is improved with the increase in sintering temperature; the effect mainly results in the decrease of the additional near grain boundaries phases amount. These phase extractions and free-volume entities in ceramic structure serve as specific trapping centers for positrons penetrating ceramics. It is demonstrated that Tao–Eldrup model can be adequately used for the calculation nanopores size in MgAl2O4 ceramics using o-Ps lifetimes of the third and the fourth components.

Effect of Chemical Composition on the Optical Properties and Fracture Toughness of Transparent Magnesium Aluminate Spinel Ceramics

Polycrystalline transparent magnesium aluminate “spinel” ceramics were fabricated by hot-pressing and hot isostatic pressing (HIPing) using commercially available MgO and Al2O3 powders. Al2O3 content of spinel was systematically changed that can be expressed as MgO·nAl2O3 with n=1.0, 1.5 and 2.0. UV/visible and near-IR wavelength region light reflection and transmission behaviors of the spinel ceramics were quantitatively correlated to their microstructure to account for the optical quality of the fabricated materials. The stoichiometric spinel ceramic with n=1.0 revealed a relatively poor optical transparency due to pronounced light scattering at the microcracked grain boundaries with a specular light transmission of ∼20–40% in the visible wavelength range. On the other hand, Al2O3 rich compositions revealed a specular transmission of ∼40–60% in the same wavelength range with a high degree of transparency. Additionally, effect of chemical composition on the fracture toughness of spinel ceramics was investigated applying indentation and chevron notched specimen fracture toughness measurement techniques. The spinel ceramic with n=2.0 revealed the highest fracture toughness with a mean value of ∼2.02 MPa·m1⁄2. Based on their optical and mechanical properties, potential of Al2O3 rich non-stoichiometric polycrystalline spinel ceramics for engineering applications requiring high optical transparency and improved fracture toughness was addressed.

Recovery of neutron-induced defects in near-stoichiometric spinel ceramics irradiated at around 500°C

Magnesium aluminate spinel ceramics with different stoichiometry were irradiated with fast neutrons up to 2.3 × 10 24 n/m 2 ( E > 0.1 MeV) at around 500°C. Near stoichiometric compositions, average length change and corresponding lattice parameter change by the irradiation were very small, but some specimens contracted relatively large amount. These specimens were isochronally annealed up to 1000°C. During the annealing, macroscopic length of the specimens whose length reduced by the irradiation showed stepwise increase at around 650°C, and returned to the pre-irradiation value. On the other hand, macroscopic length of the specimens whose length increased very slightly due to the irradiation almost unchanged. The changes in macroscopic length matched well with the changes in lattice parameter of all specimens due to the irradiation and post-irradiation annealing. The reason of the stepwise increase in length and lattice parameter during annealing may be related to the order-disorder transition of cation distribution in spinel.