Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

Nonstoichiometric LuAG:Ce Ceramics ([Lu_(1–x)Ce_x]₃Al₅O₁₂, x = 0.005) with different excess of Lu³⁺ were designed on the basis of Lu₂O₃‐Al₂O₃ phase diagram and fabricated by a solid‐state reaction method. Without using any traditional sintering aids, pure phase and good optical performance were obtained in such a Lu‐rich LuAG:Ce ceramics. In addition, scintillation efficiency and light yield of 1% excess of Lu³⁺ ceramic sample were found 16 times and 1.82 times higher than that of commercial Bi₄Ge₃O₁₂ (BGO) single crystals, respectively. Such values are comparable or even better than those in most of LuAG:Ce single crystals. However, antisite defects were also induced by excess of Lu doping, whose luminescence was found at 350–410 nm in Lu‐rich LuAG:Ce ceramics. The relationship of excess content of Lu and the microstructure, optical quality, and scintillation performance were clarified and discussed. Furthermore, by utilizing X‐ray absorption near edge spectroscopy technique, the charge state stability of cerium in Lu‐rich LuAG:Ce ceramics was examined. It appears that the excess of isovalence Lu³⁺ doping has a negligible effect on the cerium valence instability and creation of stable Ce⁴⁺ center.     

Composition and properties tailoring in Mg2+ codoped non-stoichiometric LuAG:Ce,Mg scintillation ceramics

A comprehensive study of the optical, radioluminescence and scintillation properties of both the Lu³⁺ rich and Lu³⁺ deficient non-stoichiometric Lu_3+xAG:Ce,Mg (Lu_3+xAl₅O₁₂:Ce,Mg, x = −4, −1, +1 and +4 at.%) ceramics are performed, completed further by the microstructure and defects characterization. Small deviation from the stoichiometric composition as well as Mg²⁺ codoping plays a crucial role in ceramic transparency, radioluminescence intensity and the timing characteristics of scintillation response. The Lu_Al antisite defects could be suppressed efficiently by controlling Lu³⁺ content below stoichiometry of LuAG host. MgO (Mg²⁺ ions) as effective sintering aids, can improve both the optical quality and scintillation performance (light yield, scintillation decay times and the ratio of fast decay components). We generally discuss the composition dependence of defects and properties tailoring. We also performed the systematic comparative study with the stoichiometric LuAG:Ce,Mg ceramic and the commercial BGO and LuAG:Ce single crystals.     

Mixed precipitants derived nanocrystalline powders and RE doped LuAG transparent ceramics

Lu₃Al₅O₁₂ (LuAG) nanocrystalline powders were synthesized by using ammonium hydroxide (NH₄OH, AH) and ammonium hydrogen carbonate (NH₄HCO₃, AHC) as mixed precipitant. In the absence of sintering aids such as TEOS, MgO or ZrO₂, the obtained LuAG powders showed good sinterability in H₂ atmosphere (PLSH) at low temperature. The in-line transmittance of LuAG ceramic reached 81% in the whole visible light band from 400 nm to 800 nm. The average grain size of obtained transparent ceramics was ranged in 1–6 μm at different sintering temperatures by PLSH. Various kinds of rare earth ions, such as Nd, Yb, Ce, Pr, and Tm doped RE:LuAG transparent ceramics could be prepared by PLSH technology without sintering aids and HIP post-treatment. Through PLSH technology, RE:LuAG transparent ceramics show high optical quality and large aperture size.     

Ce3+ doped Lu3Al5O12 ceramics prepared by spark plasma sintering technology using micrometre powders: Microstructure,luminescence, and scintillation properties

Ce³⁺ doped Lu₃Al₅O₁₂ (Ce:LuAG) ceramics were fabricated by the solid-state reaction method through spark plasma sintering (SPS) from 1350 °C to 1700 °C for 5 min at a pressure of 50 MPa using micro powders. The average grain size of the SPSed ceramics gradually grew from 0.42 µm (1400 °C) to 1.55 µm (1700 °C), which is nearly one order of magnitude lower than that of vacuum sintered (VSed) Ce:LuAG ceramics (~24.6 µm). Characteristic Ce³⁺ emission peaking at around 510 nm appeared and 92% photoluminescence intensity of room temperature can be reserved at 200 °C revealing excellent thermal stability. The maximum radioluminescence intensity reached around 3 times of VSed Ce:LuAG ceramics and 7.8 times of BGO crystals. The maximum scintillation light yield under γ-ray (¹³⁷Cs) excitation reached 9634 pho/MeV @ 2 μs. It is concluded that SPS technology is a feasible way to develop Ce:LuAG ceramics and further optical enhancement can be expected.     

Fast Ce,Ca:LuAG scintillation ceramics for HEP: Fabrication,characterization, and computation

High-energy physics community is looking for a hard, fast, and low-cost scintillation material, and Ce:Lu₃Al₅O₁₂ (Ce:LuAG) ceramic is one of the competitive candidates. This work presents Ce,Ca:LuAG scintillation ceramics with good optical quality, and the influence of Ce and Ca concentrations on optical and scintillation properties was fully analyzed. At relatively low level of Ce concentration, the less Ca²⁺ content is needed to achieve a significant intensity increase in fast scintillation component while maintaining a relatively high light yield (LY). The introduction of only 0.1 at% Ca²⁺ could increase the LY_{0.5 μs}/LY_{3.0 μs} from 79.9% to 96.1% in Ce,Ca:LuAG ceramics of 0.1 at% Ce. First-principles investigations are further performed to reveal the tuning mechanisms of the scintillation properties of LuAG by Ce and Ca codoping. We show that the Fermi level shifts down with Ca codoping, which increases the Ce⁴⁺ content and decreases the depth of the electron traps (V_O), resulting to a faster decay. Moreover, the formation preference of Ca-V_O complexes over Ce-V_O leads to the suppression of the non-radiative decay of Ce via V_O. In summary, our study demonstrates the realization of the performance tuning of LuAG via Ce and Ca codoping.     

The phase,microstructure evolution and the Nd3+ function in the fabrication process of LuAG transparent ceramics

Nd:LuAG transparent ceramics were fabricated by the solid-state reaction under vacuum sintering using SiO₂ and MgO as sintering aids, commercial Lu₂O₃, Al₂O₃ and Nd₂O₃ as raw materials. The Nd doping concentration was adopted from 0 at. % to 1.3 at. %. The phase transformation and microstructure evolution of 1.3 at. % Nd:LuAG ceramics under different sintering temperature was investigated in detail. Meanwhile, the effects of Nd₂O₃ on the grain growth were surveyed. The results shown that when the samples were sintered at 1780?°C, the 1.3 at. % Nd:LuAG ceramic had clean gain boundary, and the transmittance of it reached 83.8% at 1064?nm.     

Optical and Scintillation Properties of Ce3+‐Doped LuAG and YAG Transparent Ceramics: A Comparative Study

Cerium‐doped lutetium aluminum garnet (LuAG:Ce) and yttrium aluminum garnet (YAG:Ce) transparent ceramics of same dimension were fabricated and their optical and scintillation properties were studied. LuAG:Ce transparent ceramic showed higher light yield under UV and X‐ray excitation with respect to YAG:Ce transparent ceramic. YAG:Ce transparent ceramic showed higher light yield under gamma excitation and better energy resolution, which could be due to the considerable amount of slower emission (38.5%) in LuAG:Ce as well as lower optical transparency with respect to YAG:Ce ceramic.     

Influence of calcium doping concentration on the performance of Ce,Ca:LuAG scintillation ceramics

Ce,Ca:LuAG scintillation ceramics with different Ca²⁺ co-doping concentrations were prepared by the solid-state reaction method. The concentration of Ce³⁺ was fixed at 0.3 at% and the concentration of Ca²⁺ ranged from 0 to 1.2 at%. We systematically studied how the Ca²⁺ concentration affects the optical quality of Ce,Ca:LuAG ceramics by influencing the microstructure in the vacuum sintering and HIP post-treatment. Good optical transmittance could be obtained with Ca²⁺ concentrations between 0.05 and 0.8 at%, which reached 76.0–81.9 % at 520 nm. The PL and scintillation decay times decrease with increasing Ca²⁺ concentration up to 0.6 at% with no clear trend above this value. The light yield (LY) values at different shaping times decrease with increasing Ca²⁺ concentration but the fast scintillation component (LY_0.5 μs/ LY_3.0 μs) increases significantly from 79 % to 97 %. The co-doping of Ca²⁺ also reduces the afterglow level by more than one order of magnitude.     

Comparative study of LuxY1-xAG (x=0..1) laser ceramics doped with 5% Yb3+

Materials with high thermo mechanical properties are required as laser media for high-powered lasers. One of such materials is lutetium-aluminum garnet (LuAG) doped with ytterbium. In the present work, we discuss for the first time a full comparative study of 5% Yb-doped LuYAG ceramics with variation Lu/Y ratio. Samples were prepared with addition of B₂O₃, MgO and SiO₂ which were used as sintering aids. Grain sizes, shrinkage curves, lattice constants and transmission spectra were measured for all samples. Thermal conductivity was measured in a wide temperature range for all studied samples. Output power of 12 W and 60% slope efficiency were obtained on the 5% Yb:LuAG disk laser.     

On fast LuAG:Ce scintillation ceramics with Ca2+ co-dopants

“Defect engineering” was a valid strategy to modify the performance of LuAG:Ce scintillator, usually realized by Me²⁺/Me⁺ co-doping. To investigate the effects of Ca²⁺ co-doping on the scintillation properties of LuAG:Ce, a set of LuAG:Ce ceramics with Ca²⁺ concentrations ranging from 0 to 0.5 at.% were manufactured. The absorption spectra, radioluminescence spectra (RL spectra), light yield, RL spectra as a function of temperature, decay time, and TSL curves of the ceramic products were carefully measured. With Ca²⁺ co-doping, the scintillation performance of LuAG:Ce ceramics was greatly improved. Especially for the 0.2 at.% Ca²⁺ co-doped one, it has a high light yield value of 24, 400 ph/MeV, a fast scintillation decay time of 48 ns, and a small slow component contamination. And the role of Ca²⁺ in the scintillation mechanism of LuAG:Ce ceramics was also discussed in this paper.     

Fabrication of Transparent Cerium-Doped Lutetium Aluminum Garnet Ceramics by Co-Precipitation Routes

Nanosized, highly sinterable Ce-doped lutetium aluminum garnet (LuAG:Ce) powders were synthesized by the urea homogeneous co-precipitation routes. LuAG:Ce transparent ceramics with an average grain size of about 6 μm were successfully fabricated at 1800°C for 10 h by vacuum sintering without sintering aids using the powders calcined at 1000°C for 2 h. The transmittance in the visible light region reaches 65%. The radioluminescence spectrum displays a broad band located at 480–650 nm consisting of two emissions due to the transitions from the lowest 5 d excited state to the 4 f ground state of Ce³⁺, which is consistent with that of LuAG:Ce single crystal and well coupled with the silicon photodiodes.     

Fabrication,Microstructure, and Luminescent Properties of Ce3+ ‐Doped Lu3Al5O12 (Ce:LuAG) Transparent Ceramics by Low‐Temperature Vacuum Sintering

The fabrication of 0.5 mol% Ce:LuAG transparent ceramics starting from synthetic nanosized Ce:LuAG powders was investigated by low temperature vacuum sintering. It was found that high quality optical Ce:LuAG ceramics could be densified successfully by vacuum sintering (<10^–3 pa) at 1750°C for 10 h. The in‐line optical transmittance of as‐sintered Ce:LuAG ceramics with thickness of 0.7 mm could reach 73.48% at the wavelength of 550 nm. The microstructure observations revealed that transparent Ce:LuAG ceramics were composed of uniform LuAG grains with average size of 9 μm and HRTEM morphology indicated that no impurity segregation existed at grain boundaries or within Ce:LuAG grains. It was also demonstrated that the annealing treatment (at 1450°C for 20 h in air) could greatly enhance the luminescent intensity of as‐sintered Ce:LuAG ceramics under excitation of X‐ray radiation (75 kV, 25 mA), which makes it a potential candidate to be applied in radiation detector.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics

Ce³⁺-doped Gd₃(Al_1-xGa_x)₅O₁₂ (Ce:GAGG) transparent ceramics were successfully prepared via a solid state reaction/oxygen sintering method. The effects of Ga substitution on the structure and optical properties of the ceramics were investigated. The highest quantum yield and relatively high scintillation light yield were achieved in the Gd₃(Al_0.6Ga_0.4)₅O₁₂. The investigated processing technique demonstrated advantages such as increased flexibility and short processing time, thus being very cost effective. The investigated approach provides a much more economical alternative to the conventional melt growth processes used to fabricate single crystals.     

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.     

Influence of cerium doping concentration on the optical properties of Ce,Mg:LuAG scintillation ceramics

Ce,Mg:LuAG scintillation ceramics with Ce dopant content ranging from 0.025?at.% to 0.3?at.% and constant 0.2?at.% Mg codoping were fabricated by solid-state reaction. The effects of Ce concentration and annealing conditions on the microstructure, optical quality and scintillation properties are studied in great details. Lattice parameters as well as the absorption, photoluminescence, radioluminescence and thermoluminescence characteristics are investigated as a function of Ce content. Both the photoluminescence and scintillation decays are measured as well in order to study re-absorption and concentration quenching processes. In addition, an enhanced positive effect of air annealing on radioluminescence intensity and light yield is put in evidence. Moreover, the role of the charge transfer absorption of Ce⁴⁺ is investigated. Thermoluminescence measurements are performed to investigate the influence of both air annealing and Ce concentration on defects acting as traps. Finally, the correlations among steady state scintillation efficiency, light yield, thermoluminescence and Ce³⁺ concentration are found and discussed.     

Effective calcination pretreatment of Lu2O3 powders for LuAG transparent ceramics

Lutetium aluminium garnet (LuAG) ceramics as host materials has been widely used in lighting, laser, displaying and scintillators after doping different rare earth ions. Right selection of raw powder and controlling its characteristics during the preparation can greatly improve the optical quality of transparent ceramics. In this paper, the influence of different pretreatment temperatures of commercial Lu₂O₃ powders in oxygen atmosphere on solid state sintering of LuAG ceramics was systematically investigated. The pretreatment gradually decreased the specific surface area of Lu₂O₃ powders and greatly removed the absorbed impurities, which seriously deteriorated the optical quality before. The mean particle size increased from 4.53 to 5.66 μm, and the in-line transmittance of samples (Thickness = 2 mm) at 1064 nm was 68.4% for the pretreated Lu₂O₃ powder at 1000 °C without any sintering additives. Further increase of pretreatment temperature would lead to the coarsening of Lu₂O₃ powders and the decrease of sintering activity, which finally resulted in a large number of micro pores in LuAG ceramics. These results revealed that the pretreatment of Lu₂O₃ powders has prodigious impact on the optical quality of LuAG transparent ceramics, and the adsorbed materials should be removed as much as possible for their applications in lasers or lighting.     

Preparation,crystal structure and luminescence properties of red-emitting Lu3Al5O12: Mn4+ ceramic phosphor

A series of red-emitting Mn⁴⁺ doped Lu₃Al₅O₁₂ (LuAG) ceramic phosphors were successfully prepared by a simple solid-state reaction method in a high-temperature muffle. MgO was co-doped as sintering aids and Mg²⁺ ions helped to realize the charge balance. The relations between the luminescence properties, crystal structures and the microstructures were well established. Results indicated that MgO promoted the densification of the ceramics as the specimens’ relative densities were up to 99%. Moreover, the substitution of Al³⁺ with Mg²⁺ have changed crystal structures and further affected the luminescent properties. Overall, the obtained ceramic phosphors showed strong red-light emission under excitation of ultraviolet and blue light. By optimizing the Mg²⁺ and Mn⁴⁺ concentration, a quantum efficiency (QE) as high as 47.8% can be achieved under the excitation of 460 nm light, indicating that the LuAG: Mn⁴⁺ ceramic phosphors are promising candidates for WLEDs applications.     

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.     

Optimized processing of high density ternary hafnium-tantalum carbides via field assisted sintering technology for transition into hypersonic applications

Ultra-high temperature ceramics (UHTCs) present great opportunities for hypersonic applications, but densification via conventional sintering is challenging and often requires sintering aids, limiting UHT applications. Field-assisted sintering technology (FAST) can produce dense, mechanically-robust components without the need for sintering aids. In this work, we have developed an optimized set of FAST processing conditions without sintering aids for various compositions in the (Hf,Ta)C ternary system. The novel processing approach yields high-density ceramics with minimal grain growth. It was found that 50 vol% HfC (～55 mol%) demonstrated record-breaking nanohardness (41.45 ± 1.37 GPa), Vickers microhardness (30.2 ± 3.1 GPa), and elastic (indentation) modulus (590.12 ± 10.64 GPa). These peak mechanical properties arose from the balance of two underling structure-property relationships: solid solution strengthening and the Hall-Petch effect. The interplay of these compositionally-linked phenomena yields an optimal regime of superior mechanical properties. Combining this interplay with optimized FAST parameters, superior ternary HfC-TaC ceramics can be realized for next-generation hypersonic applications.