Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering

Transparent Al-rich spinel ceramics (MgO·nAl₂O₃, n = 1.05–2.5) were prepared by reactive sintering in air followed by the hot isostatic press (HIP). Commercial MgO and γ-Al₂O₃ powders were used as the raw materials, and the effects of composition and HIP temperature on the transmittance and microstructure of resulting samples were investigated. To obtain the high optical quality, extra alumina (n ≥ 1.1) was used to help eliminate residual pores and suppress abnormal grain growth during the sintering process. The appropriate HIP temperature was also critical to realize the single-phase formation and prevent the generation of second-phase precipitates. The resulting samples with n = 1.1 and 1.3 exhibited excellent optical quality and fine grains below 5 µm after HIPed at 1550 °C.     

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce³⁺:(Gd,Lu)₃Al₅O₁₂ with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).     

Densification and microstructure evolution of reactively sintered transparent spinel ceramics

Reactive sintering is an effective and simple method to prepare transparent spinel ceramics. In this research, transparent MgO·nAl₂O₃ (0.98?≤ n?≤?2) spinel ceramics were prepared via reactive sintering in air followed by hot isostatic press (HIP), using MgO and γ-Al₂O₃ powders as raw materials. The influence of composition on densification and microstructure evolution was systemically investigated. More importantly, the relationship between microstructure of presintered samples and final properties of transparent ceramics was singled out. Thermodynamically stable large pores were easily generated in magnesia-rich and stoichiometric samples after presintering in air, causing severe abnormal grain growth during the HIP treatment and poor optical quality of the resulting samples. The presintering temperature of alumina-rich samples widely varied with composition. No large pores were observed in the presintered sample, which was beneficial for the elimination of residual pores in the following HIP process. Highly transparent spinel ceramics with n?=?1.1 and 1.3 were successfully fabricated with the transmittance above 84% even at the short wavelength of 400?nm, close to the theoretical value.     

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.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Photoluminescence and unique magnetoluminescence of transparent (Tb1-xYx)3Al5O12 ceramics

Luminescent transparent ceramics (Tb_1-xY_x)₃Al₅O₁₂ (x = 0, 0.2, 0.5, 0.8) are successfully prepared by a solid-state method with additional hot isostatic pressing (HIP) treatment, and the structure and properties are investigated by XRD, SEM, PL, UV–Vis spectrophotometry and ellipsometry. The Y-containing samples are shown to be solid solution phases between TAG and YAG. The PL intensity is 14 times stronger with the incorporation of 80 mol.% Y, and the ⁵D₄→⁷F₅ emission lifetime of Tb³⁺ is prolonged from 0.357 to 3.035 ms at room temperature. A unique magnetoluminescence emerges upon the incorporation of Y, showing an interesting emission decrease to 55% as the Y content reaches 80 mol.%. Remarkably, this magnetoluminesence can occur at room temperature without an intense magnetic field. Based on our work, transparent (Tb_1-xY_x)₃Al₅O₁₂ ceramics exhibit the potential for applications in green emitters, optical instruments and photoelectric devices. In particular, the magnetoluminescence provides a simple, noncontact and nondestructive route for probing magnetic fields.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Microstructure control,competitive growth and precipitation rule in faceted Al2O3/Er3Al5O12 eutectic in situ composite ceramics prepared by laser floating zone melting

Microstructure control and competitive growth of Al₂O₃/Er₃Al₅O₁₂ eutectic/off-eutectics are explored over wide ranges of solidification rates and compositions. Gradual transformation phenomenon of microstructure morphology from complete eutectic to eutectic + coarse Er₃Al₅O₁₂ phase and to eutectic + Er₃Al₅O₁₂ dendrite is observed and the corresponding influence factors are evaluated. Competitive growth between single-phase Al₂O₃ (or Er₃Al₅O₁₂) dendrite and eutectic is analyzed and coupled growth zone is mapped through comparing interface temperatures of different patterns of microstructures. The complete eutectic microstructure could be obtained at Al₂O₃/Er₃Al₅O₁₂ hypoeutectic (Al₂O₃-17.5 mol% Er₂O₃) under fast solidification rate and the onset growth rate (?0.94 × 10⁴ μm/s) estimated from the measured eutectic spacing (?150 nm) fits well with the result calculated on the basis of competitive growth (?1.27 × 10⁴ μm/s). Transformation of microstructure from irregular eutectic to regular eutectic and probable adjustment mechanism of eutectic spacings are discussed when the eutectic spacings refined from micron-scale (<10 μm) to nano-scale (?20 nm).     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

Gd3+ doping induced microstructural evolution and enhanced visible luminescence of Pr3+ activated calcium fluoride transparent ceramics

Transparent Pr³⁺ doped Ca_1-xGd_xF_2+x (x = 0, 0.01, 0.03, 0.06, 0.10, 0.15) polycrystalline ceramics with fine-grained microstructures were prepared by the hot-pressing method. The dependence of microstructure, optical transmittance, luminescence performances and mechanical properties on the Gd³⁺ concentrations for Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics were investigated. The Gd³⁺ ions show positive effects on the microhardness of Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics as a result of the decrease in the grain sizes. Excited by the Xenon lamp of 444 nm, typical visible emissions located at 484 nm, 598 nm and 642 nm were observed. Furthermore, the incorporation of Gd³⁺ ions can greatly enhance the photoluminescence performance owing to the improvement in the concentration quenching effect. The quenching concentration of Pr³⁺ ions in CaF₂ transparent ceramics increased to 1 at.% as a result of the positive effect of Gd³⁺ codoping. The energy transfer mechanism of Pr³⁺ in the Pr³⁺:Ca_1-xGd_xF_2+x transparent ceramics has been investigated and discussed.     

Highly-doped YAG:Sm3+ transparent ceramics: Effect of Sm3+ ions concentration

YAG:Sm³⁺ (3–15 at.%) transparent ceramics, a promising cladding material for suppressors of parasitic oscillations at 1064 nm of YAG:Nd³⁺ lasers, have been prepared by solid-state reactive sintering at 1725 °C. The effect of samarium ions concentration on the microstructure and optical properties of YAG:Sm³⁺ sintered ceramics was studied for the first time. The solubility limit of samarium ions in the garnet matrix was found to lie within the range of 9–11 at.%. The spectroscopic characterization of YAG:Sm³⁺ (3–15 at.%) ceramic samples showed that the absorption coefficients corresponding to Sm³⁺ ions transitions increased linearly with increasing Sm³⁺ doping. Also, the increase in the concentration of Sm³⁺ ions contributes to the increase in the intensities of the satellites, leading to the broadening of the main spectral lines and implicitly to the increase of the absorption coefficient around 1064 nm. It was shown that YAG:Sm³⁺ ceramics doped with 9 at.% Sm³⁺ ions possess optical losses of 0.07 cm^?1 at 808 nm and an optical absorption coefficient of 4.45 cm^?1 at 1064 nm. The concentration dependence of the ⁴G_5/2 level decay confirmed that the luminescence extinction is due to the energy transfer between the Sm³⁺ ions through cross-relaxation processes. All these results show that highly-doped YAG:Sm³⁺ (9 at.%) ceramics could be the best candidate for parasitic oscillation suppression in high-power YAG:Nd³⁺ lasers at 1064 nm.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Processing thin,dense, transparent Ce:Y3Al5O12 films from flame made nanopowders for white light applications

Flame made metal oxide nanopowders enable processing of dense, transparent thin (< 50 μm) films of Ce³⁺ doped Y₃Al₅O₁₂ for white light applications. The addition of very small amounts of SiO₂ (0.14 wt. %) and the use of a final 95:5 N₂:H₂ atmosphere sintering step permits nearly complete removal of pores from films originally sintered in O₂. Furthermore, the introduction of this final step allows reduction in processing temperatures needed to effect Ce⁴⁺ reduction to Ce³⁺ by several hundred degrees below typical temperatures of >1600 °C. At 20–50 μm, the reported films are also much thinner than previously reported for the same materials normally produced by solid state reactions of micron size powders. Spectrofluorometric measurements of the dense transparent films exhibit excitation spectra centered around 450 nm and broad emission spectra in the 470–750 nm range with two peaks centered at 537 and 570 nm, confirming their applicability as a phosphor for white light emitting diodes.     

Heating parameter optimization and optical properties of Nd:YAG transparent ceramics prepared by microwave sintering

Nd-doped YAG transparent ceramics were prepared by microwave sintering. In this paper, the green bodies from high-purity commercial powders were sintered from 900 °C to 1750 °C for different lengths of time (0.5–2 h) by microwave heating. By optimizing the microwave heating parameters (the heating rate at different stages of microwave sintering, sintering temperature and holding time), the microstructures and optical properties of transparent ceramics can be effectively improved. The phase transformation, densification process and optical properties of Nd:YAG transparent ceramics were discussed. The liquid phases strongly absorb microwave radiation and affect the sintering results of samples during microwave sintering. The highest in-line transmittances of Nd:YAG transparent ceramic fabricated at 1750 °C for 2 h were 76.5% at 400 nm and 80.6% at 1064 nm. The fluorescence emission spectra and lifetime depending on different heating conditions were also discussed.     

Microstructure and optical properties of Nd: YAG transparent ceramics with the addition of Lu3+ ions

The Nd:YAG transparent ceramics with addition of Lu³⁺ ions were fabricated by co-precipitation method and vacuum sintering. Pure YAG phases were obtained when Lu³⁺ ion content was kept under 4.5?at.-%. Lattice constant of polycrystalline ceramic with 0.8?at.-% Lu³⁺ calculated from XRD patterns was similar to that of YAG single crystal, and its fluorescent intensity arising from ⁴F3/2?→?⁴I9/2 transition of Nd³⁺ ions reached the maximum although the ceramic was opaque. The results indicated that Lu³⁺ ions under 1.5?at.-% relieved strains from lattice distortion and enhanced the fluorescent intensity.     

Two-step pulsed electric current sintering of transparent Al2O3 ceramics

Abstract

Fully densified Al₂O₃ ceramics with fine grain size were obtained by pulsed electric current sintering through a two-step heating profile (referred to as TS-PECS). Highly transparent Al₂O₃ polycrystals with fine grain size (400 nm) were successfully fabricated by the TS-PECS process, namely, sintering at 1000°C for 1 h and followed at 1200°C for 20 min under uniaxial pressure of 100 MPa. Effects of the first step temperature and heating rate were discussed for bulk density, grain size and transparency. The temperature in the first step strongly affects densification and grain growth of Al₂O₃. On the other hand, heating rate, even of 100 K min^?1, in TS-PECS does not give significant influences on densification and grain growth of Al₂O₃. Inline transmittance at 640 nm in wavelength normalised to 1 mm in thickness is increased by decreasing heating rate even in TS-PECS.     

溶胶-凝胶法引入烧结助剂制备SiC-Y3Al5O12复相陶瓷

以碳化硅、六水硝酸钇、九水硝酸铝和六次甲基四胺为主要原料,通过溶胶-凝胶法引入Al2O3和Y2O3复合烧结助剂,液相烧结制备得到SiC-Y3Al5O12(Y3Al5O12简称YAG)复相陶瓷.采用DTA、TEM、XRD等分析测试技术研究了溶胶-凝胶法引入复合烧结助剂过程及复合烧结助剂对SiC-YAG陶瓷的烧结性能、力学性能、物相组成与显微结构的影响.结果表明干凝胶在920℃左右已完全转变成YAG相,最终获得的YAG粒径小,并均匀分散在SiC表面的SiC-YAG复合粉体;复合粉体先干压、再等静压成型后,在1860℃下烧结45 min,所制得复相陶瓷的相对密度达到了96.5%,抗弯强度达到486 MPa,断裂韧性达到5.7 MPa·m1/2.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Dielectric resonator antenna with Y3Al5O12 transparent dielectric ceramics for 5G millimeter-wave applications

Microwave dielectric ceramics are considered to be one of the key materials for dielectric resonators (DR) and have very broad application prospects in the fifth generation (5G) mobile communication system. Here we have prepared high-quality factor Y₃Al₅O₁₂ (YAG) transparent dielectric ceramics using high-purity α-Al₂O₃ and Y₂O₃ powders by cold isostatic pressing of the vacuum sintered with tungsten meshes as the heating elements. Optimum relative permittivity () ~10.53, quality factor Q × f (Q = 1/dielectric loss, f = resonant frequency) ~95, 270 GHz (at =7.37 GHz), and temperature coefficient of resonant frequency (TCF) ~ −51.7 ppm °C⁻¹ were obtained at a sintering temperature of 1780°C for 12 h. For the first time, YAG transparent ceramic dielectric resonator antenna (DRA) is designed as a dominant mode and a higher-order mode using the aperture coupling feeding configuration excitation. The proposed transparent dielectric ceramic DRA can provide a broad impedance bandwidth of 4.193 GHz (ranging from 21.90 to 26.09 GHz) for S₁₁ < −10 dB, radiation efficiency of 92.1%, and compact DR unit. The proposed DRA can be used potentially as a 5G millimeter (mm)-wave multiple-input-multiple-output (MIMO) antenna unit.     

Influence of calcium concentration on formation of tetravalent chromium doped Y3Al5O12 ceramics

Yttrium Aluminium Garnet (YAG) ceramics doped with chromium were prepared by solid-state reactive sintering in a vacuum. The influence of the charge compensator Ca²⁺ concentration on microstructure, optical properties and efficacy of Cr³⁺ oxidation to Cr⁴⁺ under air annealing was investigated. A non-monotonic dependence of these features on the amount of CaO as an additive was found. The changes in ceramic transparency and microstructure were explained considering the interaction between CaO and Cr₂O₃ at the ceramic grain boundaries, which leads to a different pore evolution in distinct samples during sintering. The efficacy of the oxidation of Cr³⁺ to Cr⁴⁺ strongly depends on the concentration of Ca dissolved in the YAG. The calcium solubility decreases due to the higher oxygen partial pressure of the extra phases on the grain boundaries that decreases the amount of generated Cr⁴⁺ ions. Such phenomenon explains the lower concentration of Cr⁴⁺ ions in the sample with 0.8% of Ca against the one with 0.5%. The experiment shows that the ceramic with 0.5% of Ca has a better in-line transmission and a higher concentration of Cr⁴⁺ ions in comparison with samples with a different Ca concentration.