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.     

Fabrication and optical properties of transparent fine-grained Zn1.1Ga1.8Ge0.1O4 and Ni2+ (or Cr3+)-doped Zn1.1Ga1.8Ge0.1O4 spinel ceramics

For the first time, a Zn_1.1Ga_1.8Ge_0.1O₄ transparent spinel ceramic has been fully densified by spark plasma sintering. XRD measurements show that this ceramic is composed of a pure cubic spinel phase. SEM analysis revealed a homogeneous and dense microstructure with the average grain size being 200 ± 100 nm. The transmittance of these fine-grained ceramics reached 70 % in the visible range and is very close to 80 % at 2 µm, thus close to the T_max value deduced from the measurement of the refractive index. The ceramics exhibit excellent mechanical properties with a Young modulus of 222 GPa, a Vickers hardness of 14.25 GPa and a thermal conductivity of 7.3 W.m⁻¹. K⁻¹. By doping with Cr³⁺ ions, transparent Zn_1.1Ga_1.8Ge_0.1O₄ ceramics present both a red luminescence and a long-lasting afterglow during several minutes. Moreover, a near infrared broadband emission at 1.3 µm is also achieved with Ni²⁺ ions.     

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.     

Fine grained Nd3+:Lu2O3 transparent ceramic with enhanced photoluminescence

Fine-grained Nd³⁺:Lu₂O₃ transparent ceramic was developed by a two-step sintering method in flowing H₂ atmosphere at T₁ = 1720 °C for 15 min and T₂ = 1620 °C for 10 h. The initial nanopowders were synthesized by a wet chemical processing with a uniform particle size of about 40 nm. The average grain size of the obtained 3 at.% Nd³⁺:Lu₂O₃ ceramic was 406 nm, which is ∼150 times smaller than the coarse-grained ceramic by normal H₂ sintering. The emission intensity of the fine-grained transparent ceramic is 3 times of its coarse-grained counterpart, indicating higher Nd concentration without serious quenching in fine-grained transparent ceramic is possible, which agreed well with the prediction of an atomistic modeling work with YAG. EXAFS research demonstrated that with decreasing grain size, higher degree of disorder factor of the local environment of doped Nd atoms was discovered.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Grain-size effect on Cr3+ and F-centres photoluminescence in nanophase MgAl2O4 ceramics

Photoluminescence (PL) spectroscopy of transparent MgAl₂O₄ spinel ceramics with grain size between 100 and 300 nm was studied at 7 K temperature in the near-IR-VUV range of spectrum with synchrotron radiation excitation. The PL spectra were composed of optical transitions from spatially different regions of the ceramics, which analysis evidenced grain size effect on the emission line-shapes and intensities. In particular, emission of impurity Cr³⁺ ions, being structured in the crystalline bulk, became broad-band in the grain boundary regions, which was associated with respectively strong and weak local crystalline fields. It was observed that (i) excitons and F centres transfer energy to Cr³⁺ and (ii) Cr(²E_g)/Cr(⁴T_2g) and F-centres/Cr³⁺ PL intensity ratios underwent a linear dependence on the grain size.     

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 2 h 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 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission).     

Pressure-assisted microwave sintering: A rapid process to sinter submicron sized grained MgAl2O4 transparent ceramics

This work focuses on the development of an original process based on a 2.45 GHz single-mode microwave cavity equipped with a uniaxial press, to sinter transparent spinel MgAl₂O₄ ceramic in air. The samples were conventionally pre-sintered to a density of 90% TD before microwave sintering to the final stage of densification. The influence of thermomechanical cycle on the material properties was investigated. Transmittance, grain size distribution, hardness and fracture toughness of the samples were measured and correlated to the microstructure. This new sintering process has allowed obtaining transparent samples with sub micrometric grain size and high mechanical properties, with relatively short times and low temperature. These first results can be compared to some obtained by SPS or HIP. The technical input of this method is that all the process is here conducted in air atmosphere.     

Structural,spectroscopic and thermal properties of hot-pressed Nd:(Ca0.94Gd0.06)F2.06 transparent ceramics

0.5–5.0?at.% Nd³⁺ doped (Ca_0.94Gd_0.06)F_2.06 transparent ceramics were fabricated by vacuum hot-pressing sintering. The structural, spectroscopic and thermal properties of Nd:(Ca_0.94Gd_0.06)F_2.06 transparent ceramics, as well as the influence of Nd³⁺ content on these properties were investigated. The as-fabricated ceramic samples exhibited high transparency and nearly pore-free microstructure. The absorption peaks located at 538?nm, 576?nm, 736?nm, 792?nm and 865?nm were attributed to the transitions from ground state to the excited states of Nd³⁺ ions, and the absorption coefficients increased linearly with Nd³⁺ content increasing. The emission band of the sample doped with 1?at.% Nd³⁺ concentration exhibited the highest emission intensity, while the lifetime decreased sharply with the increase of Nd³⁺ concentration. In addition, with Nd³⁺ content increasing from 0.5 to 5.0?at.%, the thermal expansivity coefficients increased slightly, while the thermal conductivity decreased from 4.21 to 2.36?W/m?K at room temperature.     

Highly transparent polycrystalline Gd2O3 ceramic attained via ZrO2 stabilization effect

The transition from the cubic to monoclinic phase of Gd₂O₃ at high temperatures poses a significant challenge to the preparation of transparent Gd₂O₃ materials. In this work, we presented a straightforward yet effective method for fabricating transparent Gd₂O₃ ceramics. Via ZrO₂ stabilization effect for phase structure, highly transparent Gd₂O₃ ceramics were successfully fabricated by vacuum sintering at 1850 °C for 8 h. The effect of different Zr (0 ∼ 13 at%) concentrations on phase transition, grain growth, fracture mode and optical properties of Gd₂O₃ transparent ceramics was investigated. As the Zr content increases, the transition from the cubic (C) to monoclinic (M) phase is effectively suppressed, which is crucial for achieving Gd₂O₃ transparent ceramics. Moreover, the results indicate that the addition of ZrO₂ has a significant effect on grain growth by not only impeding the migration of grain boundaries but also affecting the phase composition. In addition, the 11 at% Zr-doped Gd₂O₃ ceramic exhibits the best optical properties, of which transmittance is about 76% at 850 nm and about 80% in the 2.5 µm ∼ 6 µm mid-infrared range. This work provides an illustrative example for the development of other ceramics with phase transition. The obtained Zr-doped Gd₂O₃ transparent ceramics with high optical quality are potential candidates for optical window, scintillator host and mid-infrared transmission materials.     

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.     

Influence of Fe2O3 addition on the densification and oxygen ion conductivity of the GdSmZr2O7 ceramic

The influence of Fe₂O₃ addition as a sintering aid on the microstructure and electrical properties of the GdSmZr₂O₇ ceramic has been studied. The GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is composed of a pyrochlore-type phase and a small amount of Gd_0.5Sm_0.5FeO₃. Fe₂O₃ is found to be an effective sintering aid for the GdSmZr₂O₇ ceramic, and a reduction in the sintering temperature of about 200 K is achieved. The total conductivity of the GdSmZr₂O₇ ceramic incorporated with or without 1 wt.% Fe₂O₃ obeys the Arrhenius relation. At 1173 K, the highest total conductivity of the GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is about 20% higher than that of the GdSmZr₂O₇ ceramic. The GdSmZr₂O₇ ceramic with or without 1 wt.% Fe₂O₃ is an oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels.     

Mn2+ activated MgAlON transparent ceramic: A new green-emitting transparent ceramic phosphor for high-power white LED

A novel Mn²⁺ activated green-emitting MgAlON transparent ceramic phosphor was synthesized from Mg_0.21Al_2.57O_3.80N_0.20:0.03Mn²⁺ (MgAlON:Mn) phosphor powder by pressureless sintering combining with hot isostatic pressing. By crystalline structure refinement and cathodoluminescence (CL) characterization, it is demonstrated that Mn²⁺ was dissolved in the spinel lattice and occupied the tetrahedral site. The ceramic, retaining high transmittance in UV–vis region (up to 82% at 800 nm) and excellent thermal-mechanical properties of MgAlON transparent ceramic-matrix, shows a strong green emission at 513 nm under 445 nm light excitation. Compared with its powder counterpart, the ceramic phosphor exhibits higher green color purity, higher internal quantum efficiency (47%) and lower thermal quenching. It is suggested that this novel green solid phosphor could be applied in high color rendering and high-power white light-emitting diodes when combined with a red solid phosphor and a blue LED chip.     

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.     

Fabrication,microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders

Ytterbium doped lutetium oxide (Yb:Lu₂O₃) transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) of the powders synthesized by the co-precipitation method. The effects of calcination temperature on the composition and morphology of the powders were investigated. Fine and well dispersed 5?at% Yb:Lu₂O₃ powders with the mean particle size of 67?nm were obtained when calcined at 1100?°C for 4?h. Using the synthesized powders as starting material, we fabricated 5?at% Yb:Lu₂O₃ ceramics by pre-sintering at different temperatures combined with HIP post-treatment. The influence of pre-sintering temperature on the densities, microstructures and optical quality of the 5?at% Yb:Lu₂O₃ ceramics was studied. The ceramic sample pre-sintered at 1500?°C for 2?h with HIP post-treating at 1700?°C for 8?h has the highest in-line transmittance of 78.2% at 1100?nm and the average grain size of 2.6?µm. In addition, the absorption and emission cross sections of the 5?at% Yb:Lu₂O₃ ceramics were also calculated.     

Binary transparent (Ho1-xDyx)2O3 ceramics: Compositional influences on particle properties,sintering kinetics and Faraday magneto-optical effects

Binary transparent magneto-optical (Ho_1-xDy_x)₂O₃ (x = 0.01–1) ceramics derived from layered rare-earth hydroxide (LRH) compounds were fabricated by vacuum sintering. They have in-line transmittances of ～67?77 % at the visible wavelength of 700 nm and ～77?84 % at the mid-infrared wavelength of 5 μm with similar maximal infrared cut-off at ～9.5 μm. The impacts of Dy³⁺ doping on particle properties, sintering kinetics and Faraday magneto-optical effects were systematically investigated. The results show that (1) The LRH precursors exhibit the nanosheet morphology with the thickness of ～6?10 nm. Dy³⁺ incorporation not only induces an expansion for the hydroxide host layer but also a contracted interlayer distance; (2) Upon calcination at 1100 °C, the LRH nanosheets collapse into sphere-like oxide particles. The addition of Dy³⁺ leads to increasing lattice constants and decreasing theoretical densities for the (Ho,Dy)₂O₃ solid solutions; (3) A smaller bandgap energy for Dy₂O₃ (～4.85 eV) was obtained relative to those of (Ho_0.9Dy_0.1)₂O₃ (～5.24 eV) and Ho₂O₃ (～5.31 eV); (4) Dy³⁺ dopant promotes grain growth and the pure Dy₂O₃ bulk has a rather smaller grain-boundary-diffusion controlled activation energy (～457 kJ/mol) than the (Ho_0.9Dy_0.1)₂O₃ counterpart (～626 kJ/mol); (5) The Verdet constants of magneto-optical (Ho_1-xDy_x)₂O₃ ceramics generally linearly increase with the rise of Dy³⁺ concentration.     

Photoluminescence and optical properties of Eu3+/Eu2+-doped transparent Al2O3 ceramics

Transparent Eu³⁺-doped (0.05–0.15 at. %) alumina ceramics with fine-grained microstructure were prepared and studied in terms of optical properties and photoluminescence (PL). The light transmission through ceramics up to dopant concentrations 0.125 at. % is dominated by birefringence scattering at grain boundaries. As confirmed by HRTEM/EDS element mapping, high photoluminescence intensity was achieved as the result of the dopant segregation at grain boundaries. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited red light emissions with the highest intensity (394 nm excitation) for material containing 0.125 at. % of Eu³⁺. The luminescence decay was single-exponential with a lifetime ～1.5 ms. The post-sintering reduction of Eu³⁺→Eu²⁺ under an H₂ atmosphere (at 1300 °C) was difficult. Two simultaneously coexisting Eu²⁺ emitting PL centers were identified, one emitting blue light with average decay constant of 150 ns, and the other green light (more intense) with average decay constant of 1.3 μs.     

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.     

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.     

Effect of complex Si4++Mg2+ additive on sintering and properties of undoped YAG ceramics

The paper is devoted to studying of Si⁴⁺+Mg²⁺ complex additive for obtaining transparent YAG ceramics for laser applications. Ceramics were fabricated by reactive vacuum sintering of commercial Y₂O₃, Al₂O₃ powders taken in a stoichiometric mixture with TEOS and MgO as sintering aids. Microstructure and optical properties of YAG:Si⁴⁺,Mg²⁺ ceramics were investigated as a function of the Si⁴⁺/Mg²⁺ ratio. It was found that the influence of complex additive does not correspond to the direct superposition of known Si⁴⁺- and Mg²⁺-induced sintering mechanisms and involves interaction between Si⁴⁺ and Mg²⁺ ions during sintering. It was shown that C_Si/C_Mg> 1 provides more effective pore elimination and uniform microstructure when C_Si/C_Mg< 1 gives more intense inhibition of grain grown which may be important for scaling the size of ceramics.