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.     

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.     

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.     

Influence of Cr doping on the phase composition of Cr,Ca:YAG ceramics by solid state reaction sintering

In the present work, the influence of Cr and Ca co-additives on the phase formation under conditions emulated the real sintering process of Cr⁴⁺:YAG ceramics is studied. The XRD analysis of the treated samples revealed the difference in formation rates of intermediate phases between the samples with and without the Cr₂O₃ additive. The formation of intermediate phases in the solid-state reaction between Y₂O₃ and Al₂O₃ is observed to shift toward higher temperatures (ie, toward the stage of fast shrinkage) if the mixture of Cr₂O₃ and CaO is added. The reason for such shift is the appearance of new intermediate, which contains Cr⁴⁺ ions in perovskite structure, as has been established by optical absorption and luminescent investigations. It is found that the Cr,Ca:YAG ceramics prepared by vacuum solid state reaction sintering at 1750°C, 10 hours possesses better optical transparency than Ca:YAG ceramics prepared under the same conditions.     

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.     

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.     

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.     

Fabrication of Ce:YAG,Ce,Cr:YAG and Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramics for the application of white LEDs

(Ce_0.001Y_0.999)₃Al₅O₁₂ and (Ce_0.001Y_0.999)₃(Cr_xAl_1−x)₅O₁₂ (x=0.001−0.005) transparent ceramics were synthesized by the solid state reaction and vacuum sintering and their optical properties were measured. High quality white light was obtained when the Ce:YAG/Ce,Cr:YAG dual-layered composite ceramic was directly combined with commercial blue LED chip. A maximum luminous efficacy exceeding 76 lm/W at a low correlated color temperature of 4905 K was obtained. The color temperature can be controlled by variations of Cr³⁺ concentration and the ceramic thickness. Hence, the Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramic may be a promising candidate for 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.     

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⁺.     

Optical properties of Er,Yb co-doped YAG transparent ceramics

The transparent polycrystalline erbium and ytterbium co-doped yttrium aluminum garnet (Er,Yb:YAG) ceramics with various Yb contents from 5% to 25% were prepared by the solid-state reaction and the vacuum-sintering technique. The in-line transmittances of the mirror-polished ceramics exceed 80% from the visible band to the infrared band. The samples are very compact with few pores. The average grain size of the Er,Yb:YAG ceramic is about 15 μm. The upconversion luminescence spectra, infrared luminescence spectra and luminescence decay curves of the ceramics were observed and discussed. For 1%Er doped YAG ceramic, the best ion ratio of Yb³⁺ and Er³⁺ is around 15:1.     

Preparation and characterizations of Yb:YAG-derived silica fibers drawn by on-line feeding molten core approach

Three Yb:YAG transparent ceramics with Yb₂O₃ doping concentrations of 1, 10, and 15 at%, respectively were made into silica-clad hybrid fibers using an on-line feeding molten core approach. The diffusion of silica was mitigated such that the lowest SiO₂ concentration was 36.4 wt%, and consequently, the Yb₂O₃ content could reach 8.93 wt% in the fiber core. The fiber core transformed from a YAG ceramic to an yttrium aluminosilicate glass, and the formation of abundant Q² silicate species implied that the structure of the core glass maintained some environments similar to that of YAG with Q²–AlO₄ tetrahedra. The absorption and emission spectra of the obtained fibers were compared to those of Yb:YAG ceramics, and the self-absorption effect was analyzed in detail. All three fibers could output lasers under 940 or 970 nm pumping. The maximum output power of the Yb:YAG-derived fibers was higher than that of ceramic wafers owing to the cladding pump technology, which offered a new method to improve the application of ceramics.     

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.     

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.     

Optical and luminescent properties of quasi-stoichiometric YAG: Cr3+ ceramics

The impact of minor stoichiometric variations on the microstructure, optical characteristics, and luminescent properties of YAG:Cr ceramics, synthesized from chemically precipitated ceramic powders, was assessed for the first time. Transparent ceramics with over 70% transparency was produced with a nominal yttrium excess ranging from 0.47 to −1 mol.%. The phase composition, microstructure, and luminescent properties of quasi-stoichiometric YAG:Cr ceramics were examined, and the impact of stoichiometric deviations on the crystal lattice parameter and average grain size in ceramics was outlined. An examination of the optical characteristics of the ceramics revealed a specific absorption band in the case of yttrium excess. The effect of stoichiometry deviation on the luminescent properties of YAG:Cr ceramics was investigated. A change in stoichiometry from −1–0.47 mol.% excess yttrium resulted in a broadening of the luminescence R-line and a decrease in the lifetime of the excited state of Cr³⁺ from 1.91 to 1.81 ms.     

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.     

The Cr‐doping effect on white light emitting properties of Ce:YAG phosphor ceramics

The Cr/Ce‐doped YAG transparent ceramic was fabricated by the solid‐state reaction in vacuum. The Cr/Ce‐doped YAG ceramic phosphor effectively complement the red spectral component and improve the color rendering performance when excited by blue light that is due to the effective energy transfer between Cr³⁺ ion and Ce³⁺ ion. However, the energy transfer from Ce³⁺ to Cr³⁺ ion leads to energy loss and therefore the luminous efficacy of the WLED which is composed of blue LED chip and the Cr/Ce‐doped YAG ceramic phosphor decreases. The composite phase structure of ceramic phosphor is designed for improving the extraction efficacy and increasing the luminous efficacy by breaking the total internal reflection (TIR) at the interface between air and ceramic.     

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.     

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.%.     

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.