Silica reactivity during reaction‐sintering of Nd:YAG transparent ceramics

Silica (SiO₂) is widely used as sintering aid during vacuum sintering of YAG (Y₃Al₅O₁₂)‐based transparent ceramics. These ceramics are mainly used for laser applications when they are doped with rare‐earth luminescent elements such as Yb³⁺ or Nd³⁺. By means of microstructural, chemical, dilatometry, and thermogravimetry analyses, this study has evidenced that sufficiently high amount of silica (ie above the solubility limit in YAG) forms intergranular transient liquid phase of mixed composition Y‐Al‐Si‐O that vaporizes rapidly for temperatures higher than 1350°C. As a result, silica content after sintering remains always lower than the solubility limit in YAG ceramics (ie lower than 900 ppm). Finally, vacuum sintering with an external source of gaseous Si was proven to be suitable to manufacture highly transparent Nd:YAG ceramics.     

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.     

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.     

Optical properties of YMASG:Ce fluorescent ceramics prepared by hot-pressure sintering

Cerium-doped yttrium aluminum garnet (YAG:Ce) based transparent ceramics have been widely used in fluorescent lighting as high-quality inorganic fluorescent conversion materials. This paper further explores the Mg²⁺-Si⁴⁺ ions doped YAG:Ce transparent ceramics by combining the solid-phase reaction method with vacuum hot-pressure sintering and implementing protection measures against hot-pressure mold contamination, and also investigates the effect of different Mg²⁺-Si⁴⁺ doping contents on the structure, transmittance and luminescence properties of the ceramics under hot-pressure sintering. In this work, pure-phase YMASG:Ce transparent fluorescent ceramics with a grain size of about 3-6 μm and clear and clean grain boundaries were obtained with an In-line transmittance of 67% at 800 nm. Under the excitation at 460 nm, the emission peak was red-shifted by 26 nm and the full width at half maxima (FWHM) was broadened with the increase of Mg²⁺-Si⁴⁺ content, which shows that the Mg²⁺-Si⁴⁺ ion pair effectively complements the absence of the red light component in the YAG:Ce emission spectrum. The optimized YMASG:Ce ceramics obtained high-quality warm white light with a low correlated color temperature (CCT) and a high color rendering index (CRI) under the excitation of the blue LED chip. This work proved the feasibility of vacuum hot-pressure sintering to prepare YMASG:Ce transparent fluorescent ceramics, and provided a new approach for studying YMASG:Ce-based ceramics, which was significant for the application of high-power visible laser illumination.     

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.     

MgO assisted densification of highly transparent YAG ceramics and their microstructural evolution

In current study, various amounts of MgO single dopant was adopted to fabricated high quality transparent YAG ceramics, by utilizing a simple one-step solid state reaction sintering method in vacuum. At a MgO doping amount of only 0.03 wt.%, YAG transparent ceramics with a transmittance of 84.5% at 1064 nm could be obtained, after sintering at 1820 °C for 8 h. The microstructure evolution and optical property of as-fabricated YAG ceramics as a function of MgO doping concentration were systematically investigated. MgO dopant could effectively promote densification of YAG ceramics when the sintering temperature was lower than 1660 °C, and dramatically accelerate its grain growth between 1540 °C and 1660 °C. Further increase the doping amount of MgO would not benefit to the optical quality of YAG ceramics, and the intragranular pores as well as the Mg-riched secondary phase were observed from the MgO heavily doped ceramics.     

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.     

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.     

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.     

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.     

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 NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Effect of MgO and Y2O3 Doping on the Formation of Core–Shell Structure in BaTiO3 Ceramics

This study investigates the effects of doping BaTiO₃ with MgO and Y₂O₃ on the formation of core–shell structure. The MgO and Y₂O₃ enhanced the shrinkage upon sintering and inhibited the grain growth, respectively. However, increasing the amount of Y₂O₃ to 3.0 mol% suppressed the shrinkage upon sintering. The results of the diffusion experiment revealed that Y³⁺ was dissolved in the BaTiO₃ lattice to a depth of 5–10 nm inside the grains, whereas Mg²⁺ tended to remain close to the surfaces of the grains when sintered at 1150°C for 18 h, suggesting that Y³⁺ may have had a higher diffusion rate than Mg²⁺. The Mg²⁺ prevented the diffusion of Y³⁺ into the core during sintering. Therefore, Mg²⁺ plays an important role as a shell maker in the formation of the core–shell structure in the codoped system. The core–shell structure can be obtained in BaTiO₃ ceramics that are codoped with MgO and Y₂O₃ upon sintering at 1150°C for 3 h.     

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.     

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 MgO addition on the microstructure and dielectric properties of BaTiO3 ceramics

MgO-doped BaTiO₃ (BaTiO₃/MgO) ceramics were prepared by a solid-state sintering method. The effects of MgO doping on the dielectric properties of BaTiO₃/MgO were investigated in terms of its microstructural development. The BaTiO₃/MgO was characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and x-ray powder diffraction. Results show that grain growth of the BaTiO₃/MgO during sintering was inhibited by adding MgO at least 0.5 mol%. It resulted in a high resistance of the BaTiO₃/MgO sintered at high temperature. The BaTiO₃/MgO possessed a broad temperature stability and met Electronic Industries Association (EIA) ×7R specification. The improved dielectric properties of the BaTiO₃/MgO are attributed to the decreased tetragonality of BaTiO₃ lattice due to Mg²⁺ substitute for Ti⁴⁺.     

Thermodynamic properties of aluminum titanate-mullite composite ceramics containing heat stabilizer

Using Al₂O₃ and TiO₂ as raw materials, adding MgO as heat stabilizer and mullite as enhancer, aluminum titanate-mullite multiphase ceramics were successfully prepared by solid phase synthesis. The effects of MgO and mullite were systematically studied on the phase composition, microstructure, thermal stability, sintering properties, and mechanical properties of aluminum titanate ceramics. The results showed that the introduction of Mg²⁺ can partially replace Al³⁺ to form Mg_xAl_2(1-x)Ti_(1+x)O₅ solid solution, improved the thermal stability of aluminum titanate ceramics, and promoted the formation and growth of grains, which reduced the sintering temperature. The crack deflections caused by mullite particles improved the mechanical properties. The filling effect of mullite particles and the formation of silica in mullite raw materials were conducive to ceramic densification. The statistics of Mg4M10 sample were as follows: the porosity was only 2.9%, the flexural strength was as high as 64.15 MPa, and the thermal expansion coefficient was 1.35 × 10⁻⁶ K⁻¹ (RT-700°C), encouraging the application of ceramics with high thermal mechanical properties.     

Transparent polycrystalline MgO ceramic fabricated by low-temperature vacuum sintering with MgF2 additive

This study introduces transparent MgO ceramics produced via simply vacuum sintering at 1200–1500 °C by optimal incorporation of MgF₂ as a sintering additive. The effect of MgF₂ content and sintering temperature on the densification process, optical, and thermal properties of MgO ceramics is presented with emphasis on its function as a sintering aid and adverse effect of MgF₂ evaporation in the condition of high MgF₂ content or high sintering temperature. MgO ceramic with 1.0 mol% MgF₂ sintered at 1300 °C exhibits the highest relative density of 99.95% with average grain size of 17.46 μm. The in-line transmittance attains 60% at 1000 nm and >80% in the infrared range (3.8–6.8 μm), without absorption bands originated from the carbon contamination. The corresponding room-temperature thermal conductivity reaches 47.25 W/(m∙K). These results demonstrate that MgF₂ is an outstanding sintering additive for the preparation transparent MgO ceramics.     

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.     

Highly enhanced dielectric loss of MgO-doped Ba0.67Sr0.33TiO3 ceramics prepared by a non-contamination DCC-HVCI method

MgO-doped Ba_0.67Sr_0.33TiO₃ (BST) ceramics with uniform microstructure and enhanced dielectric loss were prepared by direct coagulation casting via high valence counter ions (DCC-HVCI) method. MgO was utilized as acceptor dopant as well as coagulating agent to release Mg²⁺ solidifying ceramic suspensions. Effect of sintering temperature and MgO doping content on microstructure and dielectric properties of BST ceramics was investigated. It was found that the loss tangent value (tan δ) decreased remarkably from 0.025 to 0.004 with increasing MgO concentration from 0.5 to 1.5 mol%. The 1.5 mol% MgO-doped BST ceramics processed high density of 99.5% and tan δ of 0.004 which is improved by 150% compared with that of dry-pressed samples. This improvement can be attributed to the ubiquitous distribution of MgO in BST ceramic matrix leading to the enhanced inhibitive effect. This paper provides a novel facial route to prepare high-performance functional ceramics with high reliability and low cost.