Spectrum regulation of YAG:Ce transparent ceramics with Pr,Cr doping for white light emitting diodes application

YAG:Ce transparent ceramics with high luminous efficiency and color render index were prepared via a solid state reaction-vacuum sintering method. Cr³⁺and Pr³⁺ were applied to expand the spectrum of YAG:Ce transparent ceramics. As prepared ceramics exhibit luminescence spectrum ranging from 500 nm to 750 nm, which almost covers full range of visible light. After the concentration optimization of Ce³⁺, Pr³⁺ and Cr³⁺, high quality white light was obtained by coupling the YAG:Ce,Pr,Cr ceramics with commercial blue LED chips. Color coordinates of the YAG:Ce,Pr,Cr ceramics under 450 nm LED excitation vary from cold white light to warm white light region. The highest luminous efficiency of WLEDs encapsulated by transparent YAG:Ce,Pr,Cr ceramic was 89.3 lm/W, while its color render index can reach nearly 80. Energy transfers between Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺ were proved in co-doped ceramic system. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power WLEDs application.     

Sintering additives regulated Cr ion charge state in Cr doped YAG transparent ceramics

Cr: YAG and Cr, Nd: YAG transparent ceramics have significant application prospects in solid state lasers, therefore a controllable charge state of Cr ion in Cr doped YAG transparent ceramics is necessary. In this study, a successful regulation of Cr charge state in both Cr, Nd: YAG and Cr: YAG transparent ceramics was achieved, by a simple optimizing the sintering additives. Both ceramics with the Cr doping concentration of 0.3?at% reached to the theoretical transmittance, after the vacuum sintering and the subsequent annealing process. It was found that by adopting silica additive, divalent charged Cr²⁺ ions could be detected from the vacuum sintered samples, and they were transferred into trivalent state after further annealing in air. Meanwhile, by vacuum sintering ceramics with divalent additives (CaO and MgO), a stable trivalent charged Cr ion could be obtained, and the subsequent air annealing process indicated a significant conversion from Cr³⁺ to Cr⁴⁺. Further increasing the Cr concentration was not benefit to the optical quality as well as the conversion of Cr³⁺ ion in Cr, Nd: YAG transparent ceramics.     

Optimizing liquid phase promotion effect by modifying powder specific surface area in Si4+/Mg2+ co-doped transparent Yb:YAG ceramics

Utilizing the Si⁴⁺/Mg²⁺ co-doping has been considered an effective approach to fabricate highly transparent ceramics. However, the optimum doping concentration has been reported with considerable uncertainties. In this work, highly transparent Yb:YAG ceramics were obtained via the solid-state method and the sintering behavior is discovered to be closely related to both the doping concentration of Si⁴⁺/Mg²⁺ and the specific surface area (S_BET) of powders. S_BET is effectively modified by setting the ball-milling time, where the maximum S_BET (30.914 m²/g) is achieved with 24 h ball-milling time. With increasing S_BET, less Mg²⁺ is required for better optical properties. When S_BET equals 30.914 m²/g, the highest in line transmittance @ 1100 nm of 84.85% is obtained with Si⁴⁺/Mg²⁺ doping concentrations of 0.50 wt% and 0.05 wt%, respectively. The relation between S_BET and optimum doping concentration is explained by the different magnitudes of liquid phase promotion required for different contact areas between powder particles.     

Highly transparent cerium-doped yttria ceramics for full-band UV-shielding window applications

Highly transparent cerium-doped yttria ceramics were fabricated via a pressureless sintering method with an addition of 10 at.% La₂O₃ and 1 at.% ZrO₂ as a binary sintering additive. With an increase in the Ce doping concentration from 0 to 3 at.%, the optical absorption edge of yttria ceramics exhibited a significant red shift from 290 to 380 nm as a result of the 4f–5d transition of Ce³⁺ and the charge transfer of Ce⁴⁺ (i.e., Ce⁴⁺+e^–→Ce³⁺). For a 2-mm thick sample doped with 3 at.% Ce, the total ultraviolet radiation (UV) transmittance (220–400 nm) is only 1.7%, indicating a nearly complete UV absorption. Meanwhile, the specimens possess high in-line transmittance levels in both the visible and the infrared regions (i.e., >70% at 450 nm and ∼80% at 1100 nm). Additionally, the present specimens were confirmed to have good enough mechanical strength levels (∼165 MPa) and thermal conductivity (∼5 W/m·K), which are comparable or even better compared to those of previously reported transparent yttria ceramics. The results of this work indicate that cerium-doped transparent yttria ceramics are promising candidate materials for full-band UV-shielding window applications.     

Microstructures and optical properties of 6Li,Ce:YAG ceramics for thermal neutron detection

The microstructures and optical properties of 5%⁶Li: Ce_3xY_3(1-x)Al₅O₁₂ (x = 0.001, 0.003, 0.05, 0.01, 0.02) transparent ceramics prepared by solid-state reaction and vacuum sintering were investigated in this paper. The results revealed that the grain size of ⁶Li,Ce:YAG ceramics at this ration conditions is 4 μm–20 μm. With the doping of Ce³⁺, the transmittance of ⁶Li,Ce:YAG ceramics decreases from 82% (x = 0.001) to 67% (x = 0.02) at 800 nm, and the intensity of transmittance peak at 340 nm and 460 nm increases. The emission peaks show red shift at around 530 nm with the increasing of Ce³⁺ concentration.     

Assessment of conversion efficiency of Cr4+ ions by aliovalent cation additives in Cr:YAG ceramic for edge cladding

0.25at.% Cr:YAG ceramics were successfully fabricated as the edge cladding of Yb:YAG transparent ceramic slabs through vacuum sintering of co‐precipitated powders, using oxide additives to introduce different cations. The effects of various cation additives (Si⁴⁺, Ca²⁺, and Si⁴⁺ + Ca²⁺) on the conversion efficiency of Cr⁴⁺ ions and optical characteristics of the Cr:YAG edge cladding were investigated. Measurements of the absorption spectra of the Cr:YAG ceramics without any additives revealed 2 absorption bands centered at 430 nm and 600 nm, which imparted the sample with a green color. The introduction of only Si⁴⁺‐bearing additive did not promote the transition of Cr ions from the 3+ to 4+ state. Theoretical analysis and experimentation revealed that the addition of CaO not only enhanced the microstructure and improved the transmittance of the Cr:YAG ceramic, but also introduced vacancies that assisted in the formation of Cr⁴⁺ ions. It was determined that CaO has the same effect on the conversion efficiency of Cr⁴⁺ ions whether it is added as a single additive or in combination with SiO₂. The underlying mechanisms by which these aliovalent cation additives influence the formation of Cr⁴⁺ ions and affect optical properties are discussed in detail. High quality composite ceramics with Yb:YAG transparent ceramic slabs and dark brown‐colored Cr⁴⁺: YAG ceramic edge cladding were achieved through the addition of 0.05 wt.% CaO to the edge cladding, with no interfacial effects between the 2 regions being observed.     

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.     

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y₂O₃, α-Al₂O₃ and Tb₄O₇ powders as raw materials. Samples sintered at 1750 °C for 20 h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29 µm. For the 5 at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544 nm originated from the energy levels transition of ⁵D₄→⁷F₅ of Tb³⁺ ions, and the intensity of the emission peak reaches a maximum value when the Tb³⁺ concentration is 5 at%. The in-line transmittance of the 5 at% Tb:YAG ceramics is 73.4% at the wavelength of 544 nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.     

Effect of Ca2+ and Mg2+ ions on the sintering and spectroscopic properties of cr-doped yttrium aluminum garnet ceramics

In this work, we investigated the effects of Ca²⁺ and Mg²⁺ ions and annealing temperature on the spectroscopic parameters of chromium-doped yttrium aluminum garnet ceramics (Cr:YAG). Samples were obtained with either a separate or a simultaneous addition of calcium and magnesium oxides. To achieve this, aqueous suspensions were prepared using Y₂O₃, Al₂O₃, Cr₂O₃, MgO, and CaO high-purity powders as raw materials. The obtained suspensions were freeze-granulated, pressed into pellets, debinded, and subjected to reactive sintering in vacuum at 1715°C for 6 h. Each material was annealed in air with temperatures between 1300 and 1700°C. Samples were also compared to Cr:YAG ceramics with the addition of silica as a sintering aid. All the materials obtained were then exposed to 445 nm excitation, and emission spectra in the visible and infrared wavelengths were recorded. The results showed that the emission spectra of Cr:YAG ceramics varied according to the annealing conditions: as-sintered samples exhibited strong emissions of around 680 nm and, after air annealing, of around 1400 nm. This phenomenon is attributed to the Cr³⁺→Cr⁴⁺ transition. Samples doped solely with MgO exhibited the highest emission intensity in the infrared region. Thus, Mg²⁺ ions provided the best conversion efficiency of chromium ions.     

Co-precipitation synthesis route to yttrium aluminum garnet (YAG) transparent ceramics

Yttrium aluminum garnet (YAG) precursor was synthesized via a coprecipitation method with aluminum nitrate and yttrium nitrate as raw materials, using ammonium hydrogen carbonate (AHC) as the precipitant. Fine and low-agglomerated YAG powder was obtained by calcining the precursor at 1200 °C. The primary crystallites were measured to be ～120 nm in size and weakly agglomerated to a particle size of ～500 nm, indicating a high degree of sinterability. With 0.5 wt% tetraethyl orthosilicate (TEOS) and 0.1 wt% magnesia as sintering aids, transparent YAG ceramics were fabricated by vacuum sintering at 1730–1790 °C for various hours. The influences of sintering temperature and holding time on the microstructure and transmittance of YAG ceramics were discussed.     

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.     

Effect of MgO doping on the structure and optical properties of YAG transparent ceramics

The paper studies the features of Mg²⁺ ions as sintering aid for reactive solid-state sintering of YAG transparent ceramics. Phase composition, microstructure and optical properties of YAG ceramics, doped by 0 ÷ 0.15 wt.% MgO, were investigated. Solubility limit of Mg²⁺ ions in YAG crystal lattice was found to be in the range of 0.06 ÷ 0.1 wt.% of MgO additive. Substitution mechanism of Mg²⁺ in ceramic YAG was identified by comparison of XRD data and ab initio calculation. It was shown that within the solubility limit Mg²⁺ ions most likely substitute Al³⁺ sites. Doping by MgO above solubility limit led to precipitation of spinel secondary phases. It was found that doping by Mg²⁺ ions increases concentration of oxygen vacancies in YAG lattice that effectively promote sintering. The optimal concentration range of MgO sintering aid that allow to achieve YAG transparent ceramics was defined as 0.03 ÷ 0.06 wt.%.     

Fabrication and microstructure development of Yb:YAG transparent ceramics from co-precipitated powders without additives

Sintering additives are generally considered to be important for improving densification in fabrication of transparent ceramics. However, the sintering aids as impurities doped in the laser materials would decrease the laser output power and produce additional heat during laser operation. In this work, Yb:YAG ceramics were vacuum-sintered without additives at different temperatures for various soaking time through using ball-milled powders synthesized by co-precipitation route. The densification behavior and grain growth kinetics of Yb:YAG ceramics were systematically investigated through densification curves and microstructural characterizations. It was determined that the densification in the 1500°C-1600°C temperature range was controlled by a grain-boundary diffusion. It is revealed that the volume diffusion is the main mechanism controlling the grain growth between 1600°C and 1750°C. Although SiO₂ additives can promote densification during low-temperature sintering, the optical transmittance of Yb:YAG ceramic with no additives, sintered at 1800°C for 15 hours, reaches a maximum of 83.4% at 1064 nm, very close to the measured transmittance value of Yb:YAG single crystal. The optical attenuation loss was measured at 1064 nm in Yb:YAG transparent ceramic, to be 0.0035 cm⁻¹, a value close to that observed for single crystals.     

Sintering parameter optimization of Tb3Al5O12 magneto-optical ceramics by vacuum sintering and HIP post-treatment

Transparent terbium aluminum garnet (TAG) ceramics were achieved by the vacuum sintering plus HIP post-treating from the coprecipitated TAG nanoparticles. The influences of vacuum sintering temperature and sintering aid TEOS on the optical quality of the TAG ceramics were studied. The results show that with the increase of sintering temperature, the optical quality of TAG ceramics is improved gradually, and the in-line transmittance of the TAG ceramics treated at 1720°C for 20 hours under vacuum and then HIP post-treated at 1700°C for 3 hours under 200 MPa argon gas is 81.6% at 1064 nm. The sintering additive TEOS can improve the optical quality of TAG ceramics and inhibit the valence state change of Tb³⁺ ions to Tb⁴⁺ during the annealing process. The Verdet constant of the TAG ceramics at 632.8 nm is about −178 rad·T⁻¹·m⁻¹ at room temperature, which is 1.3 times that of the commercial TGG crystals (−134 rad·T⁻¹·m⁻¹).     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Production and optical properties of Ce3+-activated and Lu3+-stabilized transparent gadolinium aluminate garnet ceramics

We first report the novel Ce³⁺-activated and Lu³⁺-stabilized gadolinium aluminate garnet (GAG) transparent ceramics derived from their precipitation precursors via a facile co-precipitation strategy using ammonium hydrogen carbonate (AHC) as the precipitant. The resulting precursors in liquid phase were substantially homogeneous solid solutions and could directly convert into sinterable garnet powders via pyrolysis. Substituting 35 at.% of Lu³⁺ for Gd³⁺ was effective to stabilize the cubic GAG garnet structure and transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics were successfully fabricated by vacuum sintering at 1715°C. The ceramic transparency was improved by optimizing the particle processing conditions and the best sample had an in-line transmittance of ~70% at 580 nm (Ce³⁺ emission center) and over 80% in partial infrared region with a fine average grain size of ~4.5 μm. Transparent (Gd,Lu)₃Al₅O₁₂:Ce ceramics have a short critical wavelength (<200 nm) and a maximal infrared cut-off at ~6.6 μm. Both the (Gd,Lu)₃Al₅O₁₂:Ce phosphor powder and the transparent ceramic exhibited characteristic yellow emission of Ce³⁺ with strong broad emission bands from 490 to 750 nm upon UV excitation into two groups of broad bands around 340 and 470 nm. The photoluminescence and photoluminescence excitation intensities as well as the quantum yield were greatly enhanced via high-temperature densification. Both the phosphor powder and ceramic bulk had short effective fluorescence lifetimes.     

Dy3+/Ce3+ Codoped YAG Transparent Ceramics for Single‐Composition Tunable White‐Light Phosphor

Superior optical, thermal, and mechanical properties of transparent ceramics are very important in the applications of solid lasers, solid‐state lighting, and transparent armors. Herein, a series of (Dy_0.03Ce_xY_0.97?x)₃Al₅O₁₂ transparent ceramics were fabricated using vacuum reactive sintering method. Importantly, these Dy³⁺/Ce³⁺ codoped yttrium aluminum garnet (YAG) transparent ceramics served as single‐composition tunable white‐light phosphors for UV‐LEDs is developed for the first time. By combining with commercially available UV‐LEDs directly, the optimal chromaticity coordinates and correlated color temperature (CCT) are (x = 0.33, y = 0.35) and 5609 K, respectively. Notably, the codoping of Ce³⁺ enhances the luminescent intensity of Dy³⁺ ions while excited at 327 nm. The emission color of YAG transparent ceramics can be tuned from white to yellow through energy transfer between Dy³⁺ and Ce³⁺. These new phosphors, possessing of pure CIE chromaticity and environmentally friendly nature, are promising for applications in white UV‐LEDs.     

Optimization of Ce3+ concentration and Y4MgSi3O13 phase in Mg2+-Si4+ Co-doped Ce: YAG ceramic phosphors

As a promising replacement for nitride red phosphors, Ce: Y₃(Mg_1.8Al_1.4Si_1.8)O₁₂ (Ce: YMASG) ceramic phosphors have attracted significant attention recently for their advantages in inorganic encapsulation and massive red-shifting of Ce³⁺ emission. In this work, Ce: YMASG with different doping concentrations of Ce³⁺ and Al₂O₃, was fabricated by vacuum sintering to investigate its effects on the elimination of the impurity phase and the enhancement of the luminescent properties of white light-emitting diodes (w-LEDs). It was discovered that the emission wavelength redshifts from 592 to 606 nm as the Ce³⁺ concentration increases, while at 450 K, the emission intensity deteriorates from 0.47 to 0.36 of its initial value. The Rietveld analysis revealed the presence of an impurity phase of Y₄MgSi₃O₁₃ with a concentration of 17.021 wt% in Ce: YMASG. With the introduction of Al₂O₃, the impurity phase was eliminated from the matrix completely, the emission peak shifted to a shorter wavelength, and the thermal stability was greatly improved. When the correlated color temperature was controlled at around 3000 K in the packaged w-LEDs, the commission international de l'éclairage (CIE) chromaticity coordinates shifted toward the bottom left corner of the diagram with increasing concentration of Ce³⁺. Conversely, the luminous efficiency (LE) increased from 36 lm/W to 58.6 lm/W as the concentration of Al₂O₃ increased from 0 to 10 wt%, which demonstrated the application prospect of the fabricated phosphor in warm w-LEDs.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.