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 3 h with 1 wt% LiF addition, the average grain size is about 36 µm and the in-line transmittance exceeds 60% at the wavelength of 800 nm.     

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.     

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⁺.     

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.     

Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

Calcium oxide (CaO) as sintering additive was first used to fabricate La_0.4Gd_1.6Zr₂O₇ transparent ceramics by a simple solid-state reaction and one-step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as-fabricated La_0.4Gd_1.6Zr₂O₇ ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca²⁺ entered the lattice of La_0.4Gd_1.6Zr₂O₇ ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La_0.4Gd_1.6Zr₂O₇ ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).     

Fabrication and properties of Co:MgAl2O4 transparent ceramics for a saturable absorber from coprecipitated nanopowder

The 0.05 at.% Co:MgAl₂O₄ precursor was synthesized by the coprecipitation method from a mixed solution of magnesium, aluminum, and cobalt nitrates using ammonium carbonate as the precipitant. 0.05 at.% Co:MgAl₂O₄ transparent ceramics were successfully obtained via vacuum sintering and hot isostatic pressing (HIP) of 0.05 at.% Co:MgAl₂O₄ nanopowder calcined at 1100°C for 4 hours. The properties of powder and ceramics were comprehensively investigated. X-ray diffraction (XRD) results showed that Co:MgAl₂O₄ nanopowder had a pure spinel phase. Also, the in-line transmittances of the HIP posttreated Co:MgAl₂O₄ ceramics with the thickness of 1.2 mm were 82% at 400 nm and 84.7% at 900 nm. The average grain sizes of 0.05 at.% Co:MgAl₂O₄ ceramics before and after the HIP posttreatment were 11 and 28 μm, respectively. The calculated ground state absorption cross section of 0.05 at.% Co:MgAl₂O₄ ceramics was 2.9 × 10⁻¹⁹ cm², indicating that this ceramics is a promising material applied as a saturable absorber for passive laser Q-switches in the 1.3-1.7 μm domain.     

Role of different rare earth oxides on the reaction sintering of magnesium aluminate spinel

Role of three rare earth oxides, viz., La₂O₃, CeO₂ and Yb₂O₃ on reaction sintering of magnesium aluminate spinel having molar ratio of MgO:Al₂O₃?=?1:2 from its solid oxide precursors was investigated in static and dynamic heating conditions. Effect of these additives (3?wt%) on densification behavior, phase assemblage and microstructure development were studied in the temperatures of 1500–1700?°C. Yb₂O₃ enhanced the sintering of spinel, while La₂O₃ and CeO₂ negatively impacted the sintering of magnesium aluminate spinel which can be discerned from the shrinkage curve of TMA as well as from static firing regime. This is ascribed to the formation of secondary phases in La₂O₃ and CeO₂ containing samples which have different crystalline structures to that of spinel. This anisotropy due to different crystallinity hindered the pore shrinkage and pore removal and thereby retarded the densification. Whereas, the cubic structure of the secondary phase formed in Yb₂O₃ containing sample which is isotropic with the crystalline orientation of the parental spinel phase assisted the densification.     

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.     

Gel-casting of MgAl2O4 transparent ceramics using a common dispersant

The ethanolaminic salt of citric acid (commercial name Dolapix CE 64) has commonly been used as a dispersant for colloidal based ceramic forming process. In this paper, a surprise was presented that MgAl₂O₄ spinel slurries consisting of MgAl₂O₄ spinel nanoparticles and Dolapix CE 64 gelled in air at room temperature spontaneously. The MgAl₂O₄ spinel slurries with high solid loading (54 vol%) were prepared with Dolapix CE 64 and the green body with up to 57% relative density was obtained. MgAl₂O₄ transparent ceramics with small grain size (0.92 μm) and high transmittance (81.7% at 600 nm) were fabricated after pre-sintering at 1500°C and hot-isostatic sintering at 1550°C.     

High-performance B4C-LaB6 composite ceramics fabricated via hot-pressing sintering with La2O3 as sintering additive

The B₄C-LaB₆ composite ceramics were fabricated via hot-pressing sintering at 2050 °C and 20 MPa pressure with the mixture of boron carbide (B₄C) and 2–5 wt% lanthanum oxides (La₂O₃) as raw materials. The effects of additive La₂O₃ content on the microstructures and mechanical properties of composite ceramics were investigated, and reaction mechanisms of La₂O₃ and B₄C at different temperatures were studied in detail. La₂CO₅, La₃BO₆ and LaBO₃ were formed by the reactions of La₂O₃ and B₄C at different temperatures, and finally LaB₆ was formed below 1600 °C. The comprehensive mechanical properties of B₄C-LaB₆ composite ceramics were optimized by adding 4 wt% La₂O₃, the flexural strength, fracture toughness and Vickers hardness reached 350 MPa,4.92 MP am^1/2 and 39.08 GPa, respectively. The high densification and flexural strength of composite ceramics achieved in the present study were attributed to LaB₆ hindering the movement of grain boundary. However, the densification was reduced caused by CO as La₂O₃ content increased to 5 wt%. The fast channel was formed via B₄C reacting with La₂O₃, which accelerated migration of B₄C in the sintering process. The content of La₂O₃ played an important role in the fracture mode of the composite ceramics, and ultimately affected the fracture toughness of the composite ceramics.     

Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite

Materials designed in the high-alumina region of Al₂O₃–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl₁₂O₁₉) and spinel (MgAl₂O₄). Considering this aspect, the present work investigated the effect of TiO₂ addition on the sintering densification and mechanical properties of MgAl₂O₄–CaAl₄O₇–CaAl₁₂O₁₉ composite by solid state reaction sintering. The results showed that the CA₆ grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti⁴⁺ into its structure, which greatly improved the densification after heating at 1600 °C. The flexural strength was greatly enhanced with increasing addition of TiO₂ due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al₂TiO₅. The increased content of TiO₂ also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation.     

Effect of Y2O3 and ZnO co-doping on the densification and properties of magnesium aluminum spinel

The effects of Y₂O₃ and ZnO co-doping on the densification and properties of magnesium aluminum spinel were investigated. The physical phase composition, microstructure, elemental distribution, densification, apparent porosity, particle size distribution and average corrosion depth of the specimens were investigated by XRD, SEM-EDS, Archimedes drainage method, Nano Measurer 1.2 software, and molten salt corrosion tests. The results showed that after co-doping with Y₂O₃ and ZnO, the cations in the sintering aids could dissolve into the spinel structure, forming solid solution ZnAl₂O₄, second phase Y₃Al₅O₁₂ and Al₂Y₄O₉, which inhibited the abnormal grain growth and made the grain distribution more uniform, thus promoting the densification of the samples. The best co-doping amount of Y₂O₃ and ZnO was 1 wt% Y₂O₃-3 wt.% ZnO, the relative density of the sample was 99.3%, the apparent porosity was 0.021%, and the grain size was the most uniform (6.52 μm). After the sample was placed into the aluminum electrolytic molten salt electrolyte for 1 h, and it was found that the sample doped with 1 wt% Y₂O₃-3 wt.% ZnO had the minimum average corrosion depth (131.9 μm) and the best corrosion resistance.     

Direct measurement of interface energies of magnesium aluminate spinel and a brief sintering analysis

Surface and grain boundary energies are key parameters for understanding and controlling microstructural evolution. However, reliable thermodynamic data on interfaces of ceramics are relatively scarce, limiting the realization of their relevance in processes such as sintering and grain growth. In this work, the heat of sintering itself was used to quantify both surface and grain boundary energies in MgAl₂O₄ spinel. Nanoparticles were compacted and heated inside a Differential Scanning Calorimeter (DSC) when densification and grain growth were observed. The evolved heat signal was quantitatively attributed to the respective microstructural evolution in terms of interfacial area change, allowing determination of average surface and grain boundary energies for MgAl₂O₄ as 1.49 J m⁻² and 0.57 J m⁻², respectively. The data was then used to interpret the thermodynamics involved in density and grain growth during isothermal sintering of MgAl₂O₄.     

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.     

Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl₂O₄ powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl₂O₄ powder was modified by introducing Ga³⁺, and stable MgAl_1.9Ga_0.1O₄ aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s^-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl_1.9Ga_0.1O₄ and MgAl₂O₄ transparent ceramics were systematically compared. It is indicated that the transparent MgAl_1.9Ga_0.1O₄ ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm^-1 at 5 µm, which could be ascribable to the fact that Ga³⁺ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al³⁺. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Fabrication and characterization of IR-transparent Fe2+ doped MgAl2O4 ceramics

Initial investigations on the preparation of highly transparent Fe²⁺:MgAl₂O₄ ceramics using nanopowders synthesized in a laser plume were carried out. For the first time, dense Fe²⁺:MgAl₂O₄ ceramics with high transmission in the mid-IR range were fabricated at a temperature as low as 1300°C and with a short sintering time (1 hour). The obtained Fe²⁺:MgAl₂O₄ ceramics contain a secondary (MgO)_0.91(FeO)_0.09 phase with a low wt% content, causing a substantial decrease in transmittance in the visible range. The transmittance increases with an increase in wavelength due to a decrease in Rayleigh scattering and reaches 85.6% at λ = 4 μm, which is close to the theoretical value. The absorption cross section of divalent iron ions was estimated to be σ = (1.66 ± 0.14) × 10⁻²⁰ cm².     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.