Residual porosity and optical properties of spark plasma sintered transparent polycrystalline cerium-doped YAG

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.     

Single CaO accelerated densification and microstructure control of highly transparent YAG ceramic

In this work, a small amount of CaO single dopant was adopted to realize the densification and microstructure control of fine grained YAG ceramic with excellent optical quality, by a simple solid‐state reaction and one‐step vacuum sintering method. Then, highly transparent YAG ceramics (T = 84.4% at 1064 nm) were obtained just after vacuum sintering at 1820°C for 8 hours. The average grain size was only 2.7 μm, when the total amount of CaO was as low as 0.045 wt%. The effect of CaO on the microstructural evolution and optical property of the as‐fabricated YAG ceramics was systematically investigated in detail. It was found that CaO dopant promoted both densification and grain growth of YAG ceramics when the sintering temperature was lower than 1660°C, however, it dramatically inhibited grain growth when the sintering temperature was further increased.     

Agglomeration Control of Nd:YAG Nanoparticles Via Freeze Drying for Transparent Nd:YAG Ceramics

Neodymium-doped yttrium aluminum garnet (Nd:YAG) nanopowders were synthesized by the carbonate coprecipitation method. The effects of freeze drying and conventional oven drying of the precursor on the agglomeration of the Nd:YAG nanopowders were compared. The optical properties of the Nd:YAG nanopowders and the corresponding sintered Nd:YAG transparent ceramics were also investigated. The Nd:YAG nanopowders synthesized from freeze-dried precursor showed better dispersion and narrower particle size distribution compared with the powders synthesized from conventional oven drying. As a result, the Nd:YAG nanopowders synthesized from freeze-dried precursor have good sinterability, and Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750°C for 5 h.     

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.     

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.     

Effect of the spark plasma sintering (SPS) parameters and LiF doping on the mechanical properties and the transparency of polycrystalline Nd-YAG

Transparent 1 at.% Nd:YAG ceramics were fabricated by spark plasma sintering (SPS) from nanometric Nd:YAG powders, both undoped and pre-mixed with 0.25 wt.% LiF additive. The mechanical and optical properties of the consolidated samples were determined as a function of the processing parameters, namely holding time, peak sintering temperature and heating rate. The presence of LiF accelerates densification and grain growth. Hardness and bending strength are decreased in the presence of the LiF additive, in consistence with the increase of the grain size. The optical transmittance in the doped samples sintered at 1400 °C, reaches 97% of the theoretical transmission and is significantly higher than that of the undoped samples. The increased optical transmittance of the doped samples is attributed to pore elimination by enhanced mass transport and cleansing of the carbon contamination by the fluorine component of the LiF additive. The presence of the latter has no effect on the absorption spectrum of the Nd:YAG ceramic.     

Fabrication and Optical Properties of Highly Transparent Er:YAG Polycrystalline Ceramics for Eye‐Safe Solid‐State Lasers“

Highly transparent 1 at.% Er:YAG ceramics was fabricated by a reactive sintering method under vacuum. The optical properties, the microstructure and the laser performance of the Er:YAG ceramics were investigated. The average grain size of the Er:YAG ceramics was 7~8 μm. The in‐line transmittances of the Er:YAG ceramics at the wavelength of 1800 and 400 nm were about 84.6% and 82.4%, respectively. The absorption coefficient of the Er:YAG ceramics was 1.29 cm^?1. The grain boundaries were very clean and no secondary phase was observed. When end‐pumped by an Er,Yb‐fiber laser at 1532 nm, a maximum output power of 13.8 W lasing at the wavelength of 1645 nm was obtained with a slope efficiency of 54.5%.     

Heating parameter optimization and optical properties of Nd:YAG transparent ceramics prepared by microwave sintering

Nd-doped YAG transparent ceramics were prepared by microwave sintering. In this paper, the green bodies from high-purity commercial powders were sintered from 900 °C to 1750 °C for different lengths of time (0.5–2 h) by microwave heating. By optimizing the microwave heating parameters (the heating rate at different stages of microwave sintering, sintering temperature and holding time), the microstructures and optical properties of transparent ceramics can be effectively improved. The phase transformation, densification process and optical properties of Nd:YAG transparent ceramics were discussed. The liquid phases strongly absorb microwave radiation and affect the sintering results of samples during microwave sintering. The highest in-line transmittances of Nd:YAG transparent ceramic fabricated at 1750 °C for 2 h were 76.5% at 400 nm and 80.6% at 1064 nm. The fluorescence emission spectra and lifetime depending on different heating conditions were also discussed.     

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.     

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.     

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.     

Flash Spark Plasma Sintering of 3YSZ: Modified sintering pathway and impact on grain boundary formation

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS, while allowing high heating rates up to 200 °C/s, which led to the extremely fast densification within a few seconds. The high heating rates had strong impact on sintering mechanisms, in terms of densification and grain growth. While the specimens ended with 5–15 vol% porosity and limited grain growth (< 350 nm), their hardness is higher than fully dense counterpart SPSed ceramics. Using the sintering trajectories, microstructural observations, and impedance spectroscopy, we highlight altered sintering mechanism which resulted in very thin grain boundaries compared to SPS. It appears that densification is largely advanced at grain boundary interfaces, with no residual nano-pores at the grain junctions, where some pores with size comparable to grain size were present. This opens up opportunities for the fabrication of porous lightweight ceramics with good mechanical properties.     

Influence of heating rate on optical properties of Nd:YAG laser ceramic

Using commercial α-Al₂O₃, Y₂O₃ and Nd₂O₃ as raw materials, 0.8 at% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering technology, with tetraethoxysilane (TEOS) as sintering aid. The Nd:YAG ceramics were obtained by sintering at 1750 °C for 20 h under vacuum. The sintering process with different heating rate of the Nd:YAG ceramics have been studied during the present work. The grain sizes, pores and secondary phase amounts increased versus increasing the heating rate. The optical properties of the Nd:YAG ceramics were closely related to the microstructures of the specimens. The lasing performance of the Nd:YAG ceramics changed drastically with change in pores and secondary phase amounts.     

Synthesis and Performance of Advanced Ceramic Lasers

This paper reports recent progress in the production of polycrystalline Nd:YAG (Y₃Al₅O₁₂), Nd:YSAG (Y₃Sc_1.0Al_4.0O₁₂), Yb:YSAG ceramics, and a Nd-doped YAG single crystal with an almost perfect pore-free structure by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd- and Yb-doped ceramics is extremely high, and their optical qualities are comparable with that of commercial high-quality Nd:YAG single crystals. We have also succeeded in the fabrication of a Nd:YAG single crystal, which can be used for laser oscillation, by the solid-state reaction method. Laser oscillation efficiency was very low when the pores remained inside the single crystal; however, the laser oscillation efficiency of the pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having high optical quality. From this fact, it was recognized that optical scattering occurs mainly in the residual pores inside the Nd:YAG ceramics and the scattering at the grain boundary is very less. In addition, we confirmed that a heavily doped Nd:YAG single crystal can be fabricated by the sintering method. Moreover, we have demonstrated the fabrication of a composite ceramic with complicated structures without the need for precise polishing and diffusion bonding. Advanced ceramic processing, which enables design flexibility of the laser element, presented in this work is important in the development of a high-performance laser (high efficiency, high beam quality, and high output energy, etc.)     

Optical Scattering Centers in Polycrystalline Nd:YAG Laser

For the present study, 1.1-at.%-Nd-doped YAG ceramics with controlled amounts of grain-boundary phase were fabricated by a solid-state reaction method using high-purity powders. The optical scattering loss of the Nd:YAG ceramics, obtained from Fresnels equation, increased simply with increased amounts of grain-boundary phase. The continuouswave laser output power of the Nd:YAG ceramics clearly was related to the scattering loss coefficients of the specimens that, in turn, were affected by the amount of grain-boundary phase. Although the scattering loss coefficients of Nd:YAG ceramics with grain-boundary-free structure and a lower pore volume (}150 vol ppm) were almost equivalent to those of a 0.9-at.%-Nd-doped YAG single crystal grown by the Czochralski method, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal with increased exciting power under excitation with an 808 nm diode laser because of the large amount of neodymium additives. Lasing performance was not affected by the existence of grain boundaries in the polycrystalline specimen.     

Fabrication of transparent neodymium-doped yttrium aluminum garnet ceramics by high solid loading suspensions

Dense neodymium-doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was obtained by slip casting and solid-state reaction. The colloidal behavior of the aqueous suspensions of neodymia, yttria, and alumina mixed powders using Dispex A as dispersant was investigated. The variation in zeta potential due to pH alteration was studied. The isoelectric point (IEP) was at pH 4.5 and 4 for the specimens with and without Dispex A, respectively. The optimal dispersion conditions were achieved for the suspensions at pH 9.6 with 0.4 wt% Dispex A. The green body prepared by slip casting was vacuum sintered from 1200 °C to 1750 °C. The grain size of the sintered body increased, and the pore size decreased with increasing sintering temperature. Pore-free Nd:YAG transparent ceramic with a grain size of 5–10 μm was obtained by sintering at 1750 °C for 10 h. The in-line transmittance of the annealed specimen reached 80.8% at 1064 nm.     

Porous Ce:YAG ceramics with controllable microstructure for high-brightness laser lighting

In order to meet the increasing demand for high-power laser diode lighting and displays, phosphor converters with high-brightness and high-directionality ought to be constructed to enhance the luminance and luminous efficacy. However, the pores formed during the sintering of phosphor ceramics affect the scattering effect and directionality of light. Therefore, porosity optimization and pore size regulation need to be explored. In this work, a series of Ce:YAG ceramics with various porosities and pore sizes were prepared. The influences of porosity and pore size on the microstructure, light confinement ability, and optical properties of Ce:YAG ceramics were studied. The ceramic phosphor with a porosity of 10 vol.% and a pore size of 3 μm exhibits a good spot confinement ability and shows a high luminous flux value of 3430 lm and a central luminance (1669 592 cd/m²) under blue laser excitation. The 10 vol.% Ce:YAG ceramic phosphor with a pore size of 5 μm has the highest emission intensity and gives a maximum luminous efficacy of 268 lm/W and a luminous flux of 4020 lm under 30 W/mm² blue laser excitation. Thus, the porous Ce:YAG ceramics are expected to be a promising candidate for high-brightness laser lighting and projection applications.     

Novel model for the sintering of ceramics with bimodal pore size distributions: Application to the sintering of lime

A mathematical model for the sintering of ceramics with bimodal pore size distributions at intermediate and final stages is developed. It considers the simultaneous effects of coarsening by surface diffusion, and densification by grain boundary diffusion and lattice diffusion. This model involves population balances for the pores in different zones determined by each porosimetry peak, and is able to predict the evolution of pore size distribution function, surface area, and porosity over time. The model is experimentally validated for the sintering of lime and it is reliable in predicting the so called “initial induction period” in sintering, which is due to a decrease in intra‐aggregate porosity offset by an increase inter‐aggregate porosity. In addition, a novel methodology for determination of mechanisms based on the analysis of the pore size distribution function is proposed, and with this, it was demonstrated that lattice diffusion is the controlling mechanism in the CaO sintering. © 2016 American Institute of Chemical Engineers AIChE J, 63: 893–902, 2017     

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.     

Post-treatment of nanopowders-derived Nd:YAG transparent ceramics by hot isostatic pressing

Neodymium doped yttrium aluminum garnet (Nd:YAG) transparent ceramics were fabricated from Nd:YAG nanopowders synthesized via a reverse precipitation method by vacuum sintering and successive hot isostatic pressing (HIP) post-treatment. The powders obtained by calcining the precursor at 1100 °C for 4 h and then ball milling for 2 h with 0.5 wt% TEOS as sintering aid were used to fabricate Nd:YAG ceramics. The green bodies were vacuum sintered at 1500–1800 °C for 10 h, followed by the HIP at 1600 °C for 3 h in 200 MPa Ar atmosphere. Influence of the calcination temperature on the phase, morphology and particle size evolution of the nanopowders, as well as the optical transparency and microstructure of the obtained Nd:YAG ceramics before and after the HIP post-treatment was investigated in detail. It was found that for the post-treated 1800 °C-vacuum-sintered Nd:YAG ceramic sample, the in-line transmittance increased from 48.0% up to 81.2% at the lasing wavelength of 1064 nm.