Densification and microstructure evolution of Cr,Nd:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.% Cr, 1.0 at.% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering using commercial Y₂O₃, α-Al₂O₃, Cr₂O₃ and Nd₂O₃ as raw materials. CaO and tetraethoxysilane (TEOS) were used as charge compensator and sintering aid, respectively. The powders were mixed in ethanol and doped with TEOS, dried and pressed. Pressed samples were sintered from 1450 to 1800 °C for 10 h. The relative density increased from 68.8% to 99.4% at the sintering temperature from 1450 to 1700 °C. Grain size increased with increase of sintering temperature and obvious grain growth occurred between 1650 and1700 °C. For the Cr,Nd:YAG ceramics sintered at 1750 and 1800 °C for 10 h, nearly pore-free microstructures with average particle size of ∼10 μm were obtained. The optical transmittance of the 1800 °C sintered sample was ∼70% in the infrared wavelength.     

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.     

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.     

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.     

Direct thermal diffusion bonding of Nd:YAG/YAG composite transparent ceramics by vacuum sintering

Nd:YAG/YAG transparent ceramics were prepared by vacuum sintering at 1780 °C for 40 h and annealed at 1450 °C for 20 h in air. Two separately polished Nd:YAG/YAG samples were bonded into monolithic and uniform composite-material followed by vacuum sintering at 1790 °C for 50 h under uniaxial pressure of 60 MPa, and then annealed at 1450 °C for 100 h in air. The fracture strength of bonded samples at the bonding interface is higher than that of as-prepared Nd:YAG/YAG samples. Meanwhile, the extinction coefficient of bonded samples is 0.0305 cm⁻¹ which is an improvement over as-prepared samples. The microstructure of the contact interface reveals the disappearance of the contact layer at the bond due to the grain growth and coalescence mainly based on grain boundary diffusion at higher temperatures and longer heat-treated time, which indicates that the bonding technology is beneficial to the fabrication of the thick composite materials.     

Influence of powder physicochemical characteristics on microstructural and optical aspects of YAG and Er:YAG ceramics obtained by SPS

The present work aims to establish a correlation between the characteristics of YAG and Er:YAG commercial powders produced by two different synthesis routes and sintered ceramic microstructures and their optical aspect by taking into account the influence of pressure applied during the Spark Plasma Sintering (SPS) process. Physical and chemical characteristics of the powders were compared using various techniques such as SEM, XRD, laser diffraction and chemical analyses. Their behaviours were evaluated through a rheological study, compressibility tests and dilatometry cycles using SPS. This paper pinpoints the most important powder features which influence the optical quality of YAG and Er:YAG ceramics. The optical quality is mainly affected by the porosity, related to powder characteristics that affect particle rearrangement, densification and grain growth. The applied pressure induces microstructural heterogeneities depending on the starting material used and resulting in core-shell aspects of sintered ceramics.     

Preparation of spray-dried powders leading to Nd:YAG ceramics: The effect of PVB adhesive

The mixed powders were obtained with Al₂O₃, Y₂O₃ and Nd₂O₃ powders as starting materials using spray drying technology. The main purpose of introducing polyvinyl butyral (PVB) is to contribute to spray granulation of the powders and inhibition of the compositions segregation. The effect of the addition of PVB (0, 1, 2, and 3 wt%) on the morphologies and compositions of the spray-dried powders is discussed. When calcined at 1000 °C for 2 h, the powders with PVB as an adhesive show sphericity and better dispersion. No compositions segregation can be detected. It is found that the powders with 2 wt% PVB after calcinations are suitable for the fabrication of Nd:YAG transparent ceramics. The corresponding ceramics consists of a well-defined microstructure, and no pores or other defects are observed.     

High efficiency synthesis of Nd:YAG powder by a spray co-precipitation method for transparent ceramics

A spray co-precipitation method was developed to efficiently synthesize Nd:YAG nano-powders. The effects of spray speeds and solution concentrations on the crystallization processes of calcined precursors have been studied. The results indicated that the pure phase of YAG could be obtained by three different crystallization processes owing to different homogeneity levels of Y and Al mixing. Pure YAG powder was obtained at 850?°C and the phase purity persisted to 1600?°C. Using the obtained powders, transparent ceramics with the in-line transmittance up to 80.2%@400 nm and 83.1%@1064?nm were fabricated by gel-casting method and hot isostatic pressing sintering. Furthermore, the microstructure and laser properties of the transparent ceramics have been measured. The maximum laser output of 7.015?W has been obtained with an oscillation threshold and a slope efficiency of 0.235?W and 59.4%, respectively.     

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.%.     

Quantitative relationship between microstructure and mechanical properties in Nd: YAG transparent ceramics

In this work, stereology and fractals were applied to identify the quantitative relation between stereology parameters, fractal dimension, and mechanical properties of Nd: YAG transparent ceramics sintered at 1750 °C for 8–50 h. Mechanical properties and microstructure of the samples were investigated by using universal testing machine, micro-hardness tester, and scanning electron microscopy (SEM), respectively. When the ceramics were sintered at 1750 °C for 50 h, the compressive strength, flexural strength, and Vickers hardness reached 381.6 ± 5.2 MPa, 275.0 ± 5.5 MPa, and 1330.4 ± 18.5 MPa, respectively. Besides, the fracture toughness of ceramic samples was calculated by Vickers hardness. Micrographs of the sample surface and frequency distribution of crystal grains were analyzed by using metallographic image analyzer software. Findings suggest that compressive strength, flexural strength, and Vickers hardness linearly increase upon an increase in equivalent sphere diameter (D_3S). However, compressive strength, flexural strength, and Vickers hardness decrease as a function of specific surface area per unit volume of the grains (S_V) and discrete grains (S_VP) and mean free distance (λ). Perimeter and area of crystal grains were obtained by using Image-Pro Plus image analysis software. The relationship between the fractal dimension of grain boundary and mechanical properties was analyzed based on the area-perimeter (small-island) method. When the grain boundary fractal dimension is close to 1.0, the geometry of ceramic grains tends to be regular, and mechanical properties of ceramic samples increases.     

共沉淀法合成Nd：YAG粉体及pH值的影响

以Y2O3、Al(NO3)3·9H2O和Nd(NO3)3为原料,碳酸氢氨为沉淀剂,均相共沉淀法制备Nd0.03Y2.97Al5O12(Nd:YAG)粉体.采用DSC/TG、XRD和TEM测试手段对粉末进行表征,研究pH值的影响.结果表明:pH值对合成粉体的成分和性能影响显著,当pH值为8.0时,前驱粉末在1 000℃热处理2 h后,全部转化成纯YAG相,粉体产生软团聚,主要为层状结构,并伴有小量纳米颗粒.     

The phase,microstructure evolution and the Nd3+ function in the fabrication process of LuAG transparent ceramics

Nd:LuAG transparent ceramics were fabricated by the solid-state reaction under vacuum sintering using SiO₂ and MgO as sintering aids, commercial Lu₂O₃, Al₂O₃ and Nd₂O₃ as raw materials. The Nd doping concentration was adopted from 0 at. % to 1.3 at. %. The phase transformation and microstructure evolution of 1.3 at. % Nd:LuAG ceramics under different sintering temperature was investigated in detail. Meanwhile, the effects of Nd₂O₃ on the grain growth were surveyed. The results shown that when the samples were sintered at 1780?°C, the 1.3 at. % Nd:LuAG ceramic had clean gain boundary, and the transmittance of it reached 83.8% at 1064?nm.     

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.     

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.     

Influence of the ceramic powder morphology and forming conditions on the optical transmittance of YAG:Yb ceramics

The influence of the dispersity and morphology of ceramic powders on the characteristics of green bodies and the optical transmittance of YAG:Yb (20?at%) ceramics was considered. The effect of the specific surface area on the relative density of compacts was studied. An increase in the specific surface area from 1.45 to 12.38?m²/g led to a decrease in the relative density of green body compacts from 52% to 38% under the fixed uniaxial pressure of 50?MPa. An increase in the uniaxial pressing value up to 150?MPa provided a maximum increase of the optical transmission of ceramics. However, an increase in the specific surface area and uniaxial pressure led to the appearance of macrodefects in ceramic samples. Cold isostatic pressing at 200?MPa after uniaxial pressing at 50?MPa resulted in an increase of optical transparency and the elimination of the macrodefect formation in ceramics. Dispersity and morphology of ceramic powders have a significant effect on the optical transparency of ceramics. Forming conditions had insignificant influence on optical characteristics. Highly transparent YAG:Yb ceramics with 80% transmittance were developed.     

Toward vacuum sintering of YAG transparent ceramic using divalent dopant as sintering aids: Investigation of microstructural evolution and optical property

Transparent YAG ceramics were fabricated by solid state reaction sintering using divalent dopants (CaO and MgO) as sintering additives without TEOS doping, and the effects of divalent dopants on their microstructure evolution and optical properties were investigated. It was found that CaO was more effective with respect to inhibiting grain growth than MgO, but not as effective as MgO in promoting densification. Fully dense, transparent YAG ceramics with excellent optical qualities could be achieved by optimizing the doping concentrations of CaO and MgO; the transmittance at 1064 nm was as high as 84.5% for 3 mm thick sample at the molar ratio of Ca: Mg=1:4, after sintered at 1840 °C for 8 h in vacuum.     

Fabrication, microstructure and optical properties of polycrystalline Er:Y3Al5O12 ceramics

Highly transparent polycrystalline Er³⁺:Y₃Al₅O₁₂ (Er:YAG) ceramics with different Er³⁺ ions content from 1% to 90% were prepared by the solid-state reaction and the vacuum-sintering technique. The grain boundary is clean and narrow with a width of about 1 nm. The best sintering temperature of the ceramics is about 1800 °C. The relationships between fabrication, microstructure and transparency of the ceramics were discussed. Grain size distributions in axial direction of cylinder samples were characterized by electron probe micro-analyzer (EPMA). The luminescence spectra were measured and discussed.     

Densification and microstructure evolution of reactively sintered transparent spinel ceramics

Reactive sintering is an effective and simple method to prepare transparent spinel ceramics. In this research, transparent MgO·nAl₂O₃ (0.98?≤ n?≤?2) spinel ceramics were prepared via reactive sintering in air followed by hot isostatic press (HIP), using MgO and γ-Al₂O₃ powders as raw materials. The influence of composition on densification and microstructure evolution was systemically investigated. More importantly, the relationship between microstructure of presintered samples and final properties of transparent ceramics was singled out. Thermodynamically stable large pores were easily generated in magnesia-rich and stoichiometric samples after presintering in air, causing severe abnormal grain growth during the HIP treatment and poor optical quality of the resulting samples. The presintering temperature of alumina-rich samples widely varied with composition. No large pores were observed in the presintered sample, which was beneficial for the elimination of residual pores in the following HIP process. Highly transparent spinel ceramics with n?=?1.1 and 1.3 were successfully fabricated with the transmittance above 84% even at the short wavelength of 400?nm, close to the theoretical value.     

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.     

Surface etching and silane heating using Er:YAG and Nd:YAG lasers in dental ceramic luted to human dentin

Investigate the tensile bond strength, surface morphology, and wettability of reinforced glass-ceramic etched with high power laser at different protocols and luted to human dentin. Fifty carious-free human molars were used in this study and distributed in five groups according to surface etching: (Control [HF10% + Silane]; Er [Er:YAG + Silane]; Sil + Er [Silane + Er:YAG]; Nd [Nd:YAG + Silane]; and Nd + Sil [Silane + Nd:YAG]). After, the tensile bond strength test was performed. Three specimens per group were used to perform the surface morphologies by using scanning electron microscopy analysis and wettability by using the sessile drop technique. Failure modes were determined. Data were analyzed with two-way analysis of variance and Tukey tests, with α = .05. The bond strength data showed statistically significant differences among tested groups for the laser and silanization technique type (p < .001). The highest calculated bond strength values were obtained with Er (19.25 ± 3.70 MPa) followed by Sil + Er (14.11 ± 4.11 MPa), Control (9.42 ± 2.27 MPa), Nd (9.66 ± 2.02 MPa), and Nd + Sil (6.71 ± 1.88 MPa), respectively. The silane application prior to the laser irradiation showed an inferior bond strength compared to the conventional silanization technique. The surface etching using Er:YAG laser showed promissor results for the lithium disilicate.