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.     

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.     

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 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.     

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.     

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.     

The influence of air annealing on the microstructure and scintillation properties of Ce,Mg:LuAG ceramics

The microstructures and optical properties of Ce,Mg:Lu₃Al₅O₁₂ scintillator ceramics are investigated with particular focus on the effect of postannealing in air from 1000 to 1450°C. The formation of Al₂O₃ clusters after annealing above 1300°C is evidenced by scanning electron microscopy. The presence of this secondary phase is tentatively explained by the occurrence of Ce and Mg evaporation, proved by inductive coupled plasma optical emission spectrometry measurements, followed by defect diffusion and clustering during high temperature annealing. Meanwhile, optical investigations including absorption, X-ray induced luminescence, light yield, scintillation decay, and thermoluminescence prove the positive role of post-annealing that leads to a brighter and faster scintillation emission. This behavior is associated to the removal of oxygen vacancies occurring during such treatments. In parallel, the partial conversion of Ce³⁺ ions into Ce⁴⁺ is also observed as a consequence of annealings and the role of Ce⁴⁺ ions in the scintillation process is discussed.     

A single-structured LuAG:Ce,Mn phosphor ceramics with high CRI for high-power white LEDs

The realization of high color rendering index (CRI) is still a great challenge for high-power LEDs (hp-LEDs), which is hindered by the phosphor converter. In this work, based on the strategy of Ce³⁺ and Mn²⁺ multi-ion substitution, the single-structured LuAG:Ce,Mn ceramics with high CRI were prepared via regulating the ratio of tri-color (red, green, and blue) components. The effects of Mn²⁺-Si⁴⁺ pairs doping content on the crystal structure, morphologies, and luminescence properties were investigated in detail. The red emission centered at 590  and 750 nm were effectively compensated by regulating Mn²⁺ occupancy sites, resulting in a significant improvement of CRI. Pure white light with general CRI R_a up to 91.0, special CRI R₉ reaching 37.9 and LE as high as 85.07 lm/W was achieved, when the hp-LEDs were constructed from related phosphor ceramic Ce02Mn7. These results suggest that the LuAG:Ce,Mn phosphor ceramics are highly promising color converters for hp-LEDs application.     

Effect of air annealing on the optical properties and luminescence performance of Ce:YAG ceramics for light-emitting diodes at different Ce concentrations

(Y_1-x%Ce_x%)₃Al₅O₁₂ (x = 0.2,0.4,0.6,0.8,1.0) transparent ceramics were fabricated by vacuum sintering technology, followed by air annealing at different temperatures. Transmittance of ceramics, valence of cerium, and luminescent properties with varying annealing temperatures are studied in detail. The negative effect of Ce³⁺ oxidation induced by annealing gets increasingly evident when Ce concentration increases. Collaborating Ce:YAG ceramics with InGaN blue chips, light-emitting diodes (LEDs) with superior performance were constructed. The relationships between Ce concentration, annealing temperature, and luminous flux of LEDs are elucidated, showing that the optimized annealing temperature of Ce:YAG ceramics decreases from 1200 °C to 900 °C as Ce concentration increases from 0.2 at% to 1.0 at%. The luminous fluxes of optimized LEDs increase by ～10 % compared with that of unannealed LEDs.     

The influences of stoichiometry on the sintering behavior,optical and scintillation properties of Pr:LuAG ceramics

Sintering aids may enter the host lattice, create defects, and seriously deteriorate the scintillation properties of ceramic scintillators. In this study, the 0.3at%Pr:LuAG ceramics with different excess of Lu were fabricated by the solid-state reactive sintering without aids. The influences of stoichiometry on the sintering behaviors, optical and scintillation properties of the ceramics are systematically studied. The results show that the stoichiometric ceramics experience an abnormal grain growth during sintering. The Lu excess can restrain the abnormal grain growth of the ceramics because of the impurity drag effect. The excess Al₂O₃ as optical scattering centers, can also pin in the grain boundary to limit the fast migration of the grain boundaries. The light yield value of the as-sintered ceramics decreases with the increase of Lu content. After air-annealing, the ceramics with appropriate excess of Lu can achieve better optical quality and higher light yield compared to the stoichiometric Pr:LuAG ceramics.     

Niobium and divalent‐modified titanium dioxide ceramics: Colossal permittivity and composition design

Colossal permittivity (CP) (ε_r=10⁴~10⁵) is attained in (A_1/3Nb_2/3)_xTi_1‐xO₂ (A=Ba²⁺, Ca²⁺, Zn²⁺, Mg²⁺) ceramics. Here, (Ca_1/3Nb_2/3)_xTi_1‐xO₂ material was studied as a typical example, and effects of Ca and Nb on their microstructure, dielectric properties and stability were studied. Both backscattering and elements mapping strongly confirmed the formation of secondary phases due to the addition of Ca and/or Nb. Secondary phases‐induced by Ca cannot affect dielectric properties of the ceramics when low Ca and Nb contents were doped, while secondary phases formed by Ca and Nb strongly affected their dielectric properties in a high doping level. In particular, their dielectric properties can be well modified by the optimization of sintering temperatures. In addition, the (Ca_1/3Nb_2/3)_xTi_1‐xO₂ ceramics with x=0.01 exhibited the optimum dielectric properties (ε_r=130500 and tan δ=0.19). Electron‐pinned defect‐dipoles may be suitable to explain CP phenomenon of this work. We believed that this profound investigation can benefit the development of new TiO₂ ceramics as a CP material.     

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.     

New vanadium doped calcium titanate ceramic pigment

In this paper a new pink vanadium doped calcium titanate Ca(V_xTi_1−x)O₃ ceramic pigment in conventional ceramic glazes is obtained by ceramic route and characterized. The limit of solid solution is near by x = 0.2, higher amounts of vanadium crystallizes Ca₂V₂O₇ which dilute the real amount of saturated Ca(V_xTi_1−x)O₃ solid solution and diminish the intensity of colour. The unit cell parameter measurements of Ca(V_xTi_1−x)O₃ agrees with the substitution of Ti⁴⁺ by V⁵⁺ that is associated to a V⁵⁺-O charge transfer at 420 nm on UV-vis-NIR spectra of 5% glazed samples that explain the pink colour obtained. In order to avoid the limitation due to the suppressing of oxygen vacancies by high valence cation V⁵⁺ substitution in a Ti⁴⁺ site of CaTiO₃ perovskite for to preserve the charge neutrality of the lattice; Fe³⁺ and V⁵⁺ codoped samples Ca(Fe_xV_xTi_1−2x)O₃x = 0.1, 0.2 and 0.3 were prepared and show a brown colour fired 1000 °C, but 5% glazed do not produce colour indicating that iron codoping inhibits the pigmenting capacity of vanadium doped CaTiO₃ perovskite.     

Spatial ions distribution of the bonding interface in YAG/Nd:LuAG composite laser ceramic

Composite YAG/Nd:LuAG transparent ceramics were fabricated by a thermal bonding process. The spatial distribution of ions around the original bonding interface of the YAG/Nd:LuAG composite laser ceramic was investigated. Around the original bonding interface, Lu³⁺ and Y³⁺ ions were replaced with each other in dodecahedral symmetry sites. Results from X‐ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) quantitative chemical analyses positively show that the distance of Y³⁺ ions diffused in the LuAG part is about 35 μm, while Lu³⁺ ions’ diffused distance in the YAG part is about 5 μm. This corresponds to the diffusion coefficient of Y³⁺ ions and Lu³⁺ ions (D_Y=2.43 ×10^?10 cm²/s and D_Lu=0.56×10^?10 cm²/s at 1750°C). The formation of Y_xLu_(3?x)Al₅O₁₂ polycrystal in the bonding section explains the complete combination of LuAG and YAG without a bonding interface. Moreover, no diffusion phenomenon of Nd³⁺ ions was detected near the original bonding interface.     

Study on structure,microstructure and microwave dielectric characteristics of CaV2O6 and (Ca0.95M0.05)V2O6 (M=Zn,Ba) ceramics

In this study, the (Ca_0.95M_0.05)V₂O₆ (M = Zn, Ba) and the CaV₂O₆ ceramics were synthesized through a solid-state reaction method, and the effects of Zn²⁺ and Ba²⁺ substitution on the structure, sintering temperature, densification, microstructure and microwave dielectric properties of CaV₂O₆ ceramic were analysed. The XRD patterns of the sintered samples indicated a single-phase of CaV₂O₆ in all temperatures. Substitution of Zn²⁺ caused a lower sintering temperature and improved the densification of the CaV₂O₆ ceramic. While the dielectric properties of the (Ca_0.95Ba0_.05)V₂O₆ compound were not desirable, the (Ca_0.95Zn_0.05)V₂O₆ sample sintered at 650°C for 4 hours showed significant dielectric properties, with ε_r = 10.29, Q × f ~  53 000 GHz (at 15.5 GHz) and τ_f = −72.37 ppm/°C. Moreover, the chemical compatibility of the CaV₂O₆ ceramic with Al electrode was examined.     

The role of Ca2+ ions in the formation of high optical quality Cr4+,Ca:YAG ceramics

The distribution of Ca²⁺ ions in high optical quality Cr⁴⁺,Ca:YAG ceramics after vacuum sintering followed by air annealing was successfully investigated by HRTEM, STEM, EDX, XPS and optical absorption spectroscopy. The HRTEM microscopy reveals the formation of clear grain boundaries without any impurity phase. A highly-doped thin Ca-rich layer was detected at the grain boundary with Ca²⁺ concentration up to 4.9% RTM, while the concentration of Ca²⁺ ions in the grain volume is less than 0.25%. The layer suppressed grain growth allowing the production of high optical quality ceramics with the average grain size of 1.95 ± 0.27 μm, which is five times smaller than in calcium-free ceramics.The air annealing of Cr⁴⁺,Ca:YAG ceramics results in a 10-fold decrease in Ca²⁺ ion concentration at the grain boundaries, practically removing the Ca-rich layer, moreover, the procedure generates Cr⁴⁺ ions within the grains. Most of the calcium originated from the Ca-rich layer diffuses outside the ceramics or dissolves into Al₂O₃ impurities without interfering with the generation of Cr⁴⁺.     

A realization of excellent piezoelectricity and good thermal stability in CaBi2Nb2O9: Pseudo phase boundary

Phase boundaries (PBs) are known to contribute to the outstanding performances of lead-based and lead-free materials. However, a lack of PBs restricts the promotion of piezoelectric performance in bismuth layer-structured ferroelectrics (BLSFs). In this work, a pseudo PB, ie, pseudotetragonal distortion (regulated by Ce), is proposed to promote the piezoelectric properties of CaBi₂Nb₂O₉-based ceramics, and an excellent piezoelectric constant (d₃₃) of 20.2 pC/N with a high Curie temperature of 923°C is obtained. Verified Ce incorporation into the (Bi₂O₂)²⁺ layer alters the environment of the (Bi₂O₂)²⁺ layer, thereby influencing the atomic displacement in the Nb-O octahedron and modulating the theoretical spontaneous polarization (P_s). Strengthening of the pseudotetragonal distortion is favorable to the polarization switching, and maintains the theoretical P_s of ceramics at a high level, thus realizing the promotion of d₃₃. Furthermore, pseudotetragonal distortion guarantees good thermal depoling performance of the ceramic, which remains at 89.6% (18.1 pC/N) of its initial d₃₃ after depoling at 875°C. This work provides clear guidance on obtaining high d₃₃ and good thermal stability in BLSFs.     

Effects of ZrO2 on the photoluminescence of 0.5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics

Zr-codoped 0.5 at% Eu: (Y_0.9La_0.1)₂O₃ ceramics sintered in H₂-reducing atomsphere, together with the ceramics with annealing treatment, were fabricated by solid-state reactions and the effects of Zr codoping on these materials’ photoluminescence examined. The obtained emission spectra showed that Zr codoping adjust the materials’ photoluminescence with UV excitation and a logical explanation was proposed. The results suggested that an Eu-doped, yttrium-lanthanum oxide transparent ceramic with Zr in low concentration appeared to have promising potential in modern lighting applications.     

Site-selective occupation and luminescence property investigation of Ca18Na3Y(PO4)14:Eu2+ phosphor for full-spectrum LED

Cyan-emitting phosphors have attracted widespread attention as an integral part to realize full-spectrum lighting. Understanding the site occupation of luminescence centers is of great importance to design and clarify the luminescent mechanism for new cyan-emitting phosphors. Here, we report a cyan-emitting phosphor Ca₁₈Na₃Y(PO₄)₁₄:Eu²⁺ synthesized by the high-temperature solid-state method. The crystal structure is characterized by X-ray diffraction and refined by the Rietveld method. The diffuse reflectance spectra, excitation/emission spectra, fluorescence decay curves, thermal stability, and related mechanism are systematically studied. The results show that Ca₁₈Na₃Y(PO₄)₁₄:Eu²⁺ crystallizes in a trigonal crystal system with space group R3c. Under excitation at 350 nm, a broadband cyan emission can be obtained at 500 nm with a half-width of about 120 nm, which is caused by Eu²⁺ occupying five different sites in host, namely, Na2O₁₂ (450 nm), (Ca3/Na1)O₈ (485 nm), Ca2O₈ (515 nm), Ca1O₇ (565 nm), and (Ca4/Y)O₆ (640 nm), respectively. Moreover, crystal structure, room and low temperature spectroscopy, and luminescence decay time are used to skillfully verify the site-selective occupation of Eu²⁺. Finally, a full-spectrum light-emitting diode (LED) lamp is fabricated with an improved color rendering index (∼90.3), CCT (∼5492 K), and CIE coordinates (0.332, 0.318). The results show that Ca₁₈Na₃Y(PO₄)₁₄:Eu²⁺ has the potential to act as a cyan emission phosphor for full-spectrum white LEDs.     

Investigation of grain boundary diffusion and grain growth of lithium zinc ferrites with low activation energy

Activation energy and diffusion kinetics are important factors for grain growth and densification. Here, Bi₂O₃ was introduced into Li_0.43Zn_0.27Ti_0.13Fe_2.17O₄ ferrite ceramics via a presintered process to lower the reaction activation energy and to achieve low temperature sintering. Interestingly, Bi³⁺ ions entered the lattice and substituted for Fe³⁺ in the B‐site (i.e., a pure LiZn spinel ferrite). Also, SEM image results show that Bi₂O₃‐substituted LiZn ferrite ceramics have low critical temperature for grain growth (920°C), which is very advantageous for LTCC technology. This indicates that Bi₂O₃ is an excellent dopant for ceramics. Furthermore, to promote normal grain growth of the ceramics at low temperatures, different volumes of V₂O₅ additive were added at the final sintering stage. Results indicate that an optimal volume of V₂O₅ additive promotes grain growth (with no abnormal grains) and enhances magnetic performances of the ceramics at low sintering temperature. Finally, adding the optimal volume of V₂O₅ additive resulted in a homogeneous and compact LiZnTiBi ferrite ceramic with larger grains (average size of ~8 μm), high 4πM_s (~4100 gauss), and low ΔH (~190 Oe) obtained (at 900°C). Moreover, the doping method reported in this study also provides a reference for other low temperature sintered ceramics.