Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment

Highly transparent aluminum oxynitride (AlON) ceramics were fabricated by pressureless sintering with hot isostatic pressing (HIP) post-treatment. The experimental results showed that the optical transparency of AlON ceramics was improved markedly over the visible and near-infrared range by HIP at 1825 °C for 3 h in 200 MPa argon gas, which derived from the elimination of residual pores in the prepared ceramics. For AlON ceramics pre-sintered at 1800 °C, the transmittances of the sample increased from 63.6% to 84.8% at 600 nm and from 75.4% to 86.1% at 2000 nm, respectively. The average grain size of the HIPed sample was about 47.9 μm.     

Hot isostatic pressing of transparent Yb3+-doped Lu2O3 ceramics for laser applications

Yb³⁺-doped Lu₂O₃ nanoparticles produced by laser ablation were used to fabricate transparent ceramics by a combination of pressureless sintering in vacuum (PS) followed by a hot isostatic pressing (HIPing). The samples were subjected to various PS and HIPing conditions and the microstructure evolution and its correlation with the transmittance were investigated. Relative densities of over 97% were achieved after PS at the temperatures of 1250–1700 °C. Rapid grain growth occurred within PS and HIPing temperatures above 1500 °C leading to formation of intragranular porosity which is deleterious for optical quality. Higher transmittance (81.7% at λ = 1080 nm) and ultrafine microstructure with an average grain size of 0.35 μm were obtained by PS at 1250 °C followed by HIPing at 1400 °C for 5 h under 207 MPa. Output power of 2.02 W with a slope efficiency of 46.5% was obtained under a quasi-continuous wave end pumping at 929.4 nm.     

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.     

Fabrication and laser operation of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

In this article, 5 at.% Yb:Lu₂O₃ transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) posttreatment using co-precipitated nano-powders. The influence of precipitant molar ratio, ammonium hydrogen carbonate, to metal ions (AHC/M³⁺, R value) on the properties of Yb:Lu₂O₃ precursors and calcined powders was investigated systematically. It was found that the powders with different R value calcined at 1100°C for 4 hours were pure cubic Lu₂O₃ but the morphologies of precursors and powders behaved differently. The opaque samples pre-sintered at 1500°C for 2 hours grew into transparent ceramics after HIP posttreatment at 1750°C for 1 hour. The final ceramic with R = 4.8 showed the best optical quality with the in-line transmittance of 79.7% at 1100 nm. The quasi-CW laser operation was performed at 1034 nm and 1080 nm with a maximum output power up to 8.15 W as well as a corresponding slope efficiency of 58.4%.     

Highly transparent LiAlON ceramic prepared by reaction sintering and post hot isostatic pressing

A highly transparent polycrystalline LiAlON ceramic with the size of Φ57?mm?×?6?mm was successfully fabricated by reaction sintering (1750?°C, 20?h) and post hot isostatic pressing (HIP, 1850?°C, 3?h, 180?MPa) using AlN, Al₂O₃ and LiAl₅O₈ powders. Related mechanism on the reaction sintering and densification were studied via the analysis of phase and microstructural evolution. High transparency was resulted from full elimination of Al₂O₃ secondary phase and residual pores. It has excellent optical transparency from the visible to middle infrared (IR) bands with the maximum transmittance of ～ 85.5%. The flexural strength and Vikers hardness reach ～303?MPa and ～15.0?GPa, respectively.     

Fabrication and laser oscillation of Yb:Sc2O3 transparent ceramics from co-precipitated nano-powders

Ytterbium doped scandium oxide (Yb:Sc₂O₃) transparent ceramics were fabricated by a co-precipitation and vacuum sintering method. The characteristics of the precursor and the calcined powders were investigated by BET, XRD, and SEM. Ultra-fine and low agglomerated 5at%Yb:Sc₂O₃ powders with the average particle size about 65.4 nm were obtained after calcined at 1100 °C for 5 h. Using the synthesized powders as starting materials, 5at%Yb:Sc₂O₃ transparent ceramics with the in-line transmittance of 71.1% at 1100 nm and average grain size of 145 μm were fabricated by vacuum sintering at 1825 °C for 10 h. Quasi-CW laser oscillation of Yb:Sc₂O₃ ceramics was obtained at 1040.6 nm. A maximum output power as high as 2.44 W with a corresponding slope of 35% was achieved. Finally, the tunability of the ceramic was explored measuring a tuning range up to 55 nm.     

Fabrication of transparent Tb3Al5O12 ceramics by hot isostatic pressing sintering

Tb₃Al₅O₁₂ (TAG) transparent ceramics were prepared by a reactive sintering method using presintering in a muffle furnace combined with hot isostatic pressing (HIP) sintering. The dilatometric, differential scanning calorimetry‐thermogravimetric (DSC‐TG) curves and optical quality were investigated. The microstructure evolution of the TAG ceramic samples was clarified. Two successive transformations were found to generate a TAG phase, as observed in the dilatometric and DSC‐TG curves and XRD patterns of TAG ceramics sintered at different temperature. The changes in average grain size and densification suggest that a 1600°C presintering temperature is suitable for HIP. The optical transmittance of the obtained 0.4 wt% TEOS:TAG transparent ceramics, which were fabricated by a new two‐step sintering of presintering at 1600°C in a muffle furnace followed by HIP at 1650°C, can reach above 80% in the visible (vis) and near‐infrared (NIR) regions. Its transmittance was very close to the theoretical limit. To the best of our knowledge, this is the first time that TAG transparent ceramics with ideal optical quality were obtained without vacuum sintering.     

Fabrication and properties of transparent Tb2Ti2O7 magneto-optical ceramics

Transparent Tb₂Ti₂O₇ magneto-optical ceramics were fabricated from co-precipitated nano-powders by vacuum pre-sintering with hot isostatic pressing (HIP) post-treatment. The formation of pyrochlore phase, decomposition of the precursor, and the morphology of powders calcined at different temperatures were investigated. The in-line transmittance of Tb₂Ti₂O₇ ceramics, which were pre-sintered at 1350 ℃ for 2 h with HIP post-treatment at 1450 ℃ for 3 h and subsequently annealed at 800 ℃ for 20 h in NH₃ atmosphere, reaches 65.5 % at 1064 nm. The Verdet constant of Tb₂Ti₂O₇ ceramics is −229.0 ± 0.6 rad·T⁻¹ m⁻¹ at the wavelength of 633 nm, which is 71 % higher than that of the commercial Tb₃Ga₅O₁₂ crystals. Tb₂Ti₂O₇ magneto-optical ceramics show a promising application for Faraday rotators.     

Polycrystalline transparent magnesium aluminate spinel processed by a combination of spark plasma sintering (SPS) and hot isostatic pressing (HIP)

A two-stage processing approach combining spark plasma sintering (SPS) and hot isostatic pressing (HIP) was employed for the fabrication of relatively large (30?mm diameter) and thick (up to 8?mm) samples of transparent polycrystalline magnesium aluminate. The effects of sample thickness, heating rate during SPS, and the temperature and duration of HIP treatments were investigated. It was established that the heating rate during SPS had a major influence on discoloration due to carbon contamination, which increased with sample thickness. HIP treatment allowed for the elimination of cloudiness due to samples porosity, although carbon contamination present after the SPS step could not be reduced by HIP treatment, regardless of the temperature and duration applied. Highly transparent specimens with thicknesses of 4 and 8?mm exhibiting an in-line transmittance of 85.2 and 83.2% at 600?nm, respectively, were fabricated.     

Fabrication,microstructure and optical characterizations of holmium oxide (Ho2O3) transparent ceramics

Ho₂O₃ transparent ceramics were fabricated by vacuum pre-sintering combined with hot isostatic pressing (HIP) post-treatment at relatively low temperature from high-purity Ho₂O₃ powder calcined at 1000 °C for 4 h. The optimal Ho₂O₃ ceramic sample prepared by vacuum pre-sintering at 1250 °C and HIP post-treating at 1450 °C has a dense microstructure with average grain size of 0.77 μm, and the in-line transmittances reach 80.7 % at 1550 nm and 76.7 % at 1064 nm. The effect of air annealing on the optical quality of Ho₂O₃ ceramics was studied, and the existence of compressed pores in the HIP-ed Ho₂O₃ ceramics was confirmed. The Verdet constants of Ho₂O₃ ceramics were measured to be -47.4 rad/(T m at 1064 nm and -15.4 rad/(T m at 1561 nm. High transmittance and large Verdet constant in the wavelength regions 1–1.07 μm, 1.3–1.5 μm make Ho₂O₃ transparent ceramics promising for magneto-optical devices for lasers based on Yb-, Nd-doped materials and telecom lasers.     

Transparent tetragonal-cubic zirconia composite ceramics densified by spark plasma sintering and hot isostatic pressing

Targeting higher toughness transparent ceramics, tetragonal (3 mol % yttria) and cubic (8 mol % yttria) ZrO₂ starting powder mixtures were densified by spark plasma sintering (SPS) in vacuum at 1100 °C and post hot isostatic pressing (HIP) in argon at 1100 °C. The influence of the ultra-fine microstructure and phase composition on the fracture resistance and light transmission in the visible and infra-red range was assessed. Of special interest was the influence of a thermal annealing step in air on the transparency of the SPS and SPS-HIP ceramics.     

The effects of hot isostatic pressing temperature on the structure and properties in GdMnO3 ceramics

In this paper, the effects of hot isostatic pressing (HIP) temperature on the crystal structure, optical, dielectric and magnetic properties of GdMnO₃ ceramics were studied. All samples form a single-phase structure without structural transformation, while HIP temperature induces the changes in lattice parameters. HIP causes the change in Mn ions valence state, oxygen vacancy concentration, Raman vibration modes and microscopic morphology of GdMnO₃. Vacancy concentration of the samples prepared by HIP at 800 °C increases compared with that of the samples without HIP, then remains unchanged when the HIP temperature is from 800 to 900 °C, and finally decreases with the further increase of HIP temperature. Appropriate HIP temperature can increase the dielectric constant and decrease the dielectric loss. The transition temperature of paramagnetism to antiferromagnetism and magnetization can be significantly affected by HIP temperature. Magnetic transition temperature and magnetization are closely related to Mn²⁺ ions concentration and cation vacancy concentration, respectively.     

Fast fabrication of AlON ceramics with ultra-high transmittance by the assistance of hot isostatic pressing treatment

By employing hot isostatic pressing (HIP) technology, AlON ceramics having ultra-high transmittance up to 88.7% were successfully fast fabricated via dwelling 2.5 h pressureless sintering (PS) and subsequent 2 h in HIP treatment of AlON powder with Y₂O₃ as additive. Different dwelling durations of PS were conducted to study the mechanism of ultra-high transmittance induced by HIP treatment. The evolution of element distribution, pore size and distribution, relative density and transparency before and after HIP treatment was thoroughly explored. Y concentration in AlON ceramics was observed for the first time. It was found that it was high relative density, pore free (no large pores and much fewer small pores) and no Y concentration that led to the ultra-high transmittance for the fabricated AlON ceramics. Therefore, a rational design of PS and HIP treatment is an effective approach to fabricate AlON ceramics of high transmittance.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Optical and thermal properties of TiO2-doped Y2O3 transparent ceramics synthesized by hot isostatic pressing

Highly transparent Y₂O₃ ceramics using TiO₂ as an additive were synthesized by presintering and hot isostatic pressing (HIP). The effects of TiO₂ contents and sintering conditions on the optical properties of the final transparent ceramics were investigated. A small amount (0.04-0.16 wt%) may decrease the densification temperature by about 200°C. The Y₂O₃ ceramics doped with 0.16 wt% TiO₂ revealed a transparency of 82% in the wavelength range 1-6 μm. The thermal conductivity of the samples is about 11.8 W/m K at 25°C, which is close to that of the undoped Y₂O₃ ceramics.     

Hot-pressed Fe2+:ZnSe transparent ceramics with different doping concentrations

In this study, Fe²⁺:ZnSe transparent ceramics with different doping concentrations were prepared from annealed FeSe and ZnSe powders by hot pressing. The as-prepared ceramics consisted of a cubic ZnSe phase and compact microstructures. Doping concentrations of Fe²⁺ ions in the range 0.66–3.05 at.% were accurately realised, which could influence the absorption intensity of Fe²⁺:ZnSe transparent ceramics. An absorption peak was observed at ～3 μm, and its intensity could be controlled by the concentration of Fe²⁺ ions. Fe_xZn_1-xSe (0.0066 ≤ x ≤ 0.0305) ceramics with Fe²⁺ ions concentrations in the range 0.66–3.05 at.% exhibited significant absorption cross sections from 0.6676 × 10^?19 to 0.1075 × 10^?18 cm². The specimen doped with 1.55 at.% Fe²⁺ ions displayed the highest transmittance of 67% at a wavelength of 14 μm and a carbonate absorption peak at 9 μm. The proposed transparent ceramic technique appears promising for preparing Fe²⁺:ZnSe laser gain media because of its advantage of allowing control over Fe²⁺ concentration.     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Comparative study of Yb:Lu3Al5O12 and Yb:Lu2O3 laser ceramics produced from laser-ablated nanopowders

We present a comparative study of two Lu-based oxide ceramics doped with Yb³⁺ ions, namely Yb:Lu₃Al₅O₁₂ (garnet) and Yb:Lu₂O₃ (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu₂O₃ and Al₂O₃ produced by laser ablation: Yb:Lu₃Al₅O₁₂ – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu₂O₃ – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb³⁺ multiplets is revealed for Lu₃Al₅O₁₂. A continuous-wave (CW) Yb:Lu₃Al₅O₁₂ ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu₂O₃ ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior.     

Fabrication of Y6MoO12 molybdate ceramics: From synthesis of cubic nano-powder to sintering

The family of cubic yttrium molybdate (Y₆MoO₁₂) has been chosen due to the possibility of substituting Y³⁺ cations by luminescent trivalent rare earth (RE³⁺) ones and to obtain transparent optical ceramics. The goal of the paper is to find a route towards the fabrication of dense Y₆MoO₁₂ ceramics from nano-powders and using well-known ceramic sintering techniques such as Spark Plasma Sintering (SPS) and Hot Isostatic Pressing (HIP). We have investigated in detail process of the powder preparation by a combustion method as well as the sintering processes taking into account all important sintering parameters. Evaporation of MoO₃ was found to be correlated to reductive conditions or heating rate. Sintering was optimized in terms of phase composition, microstructure and porosity.