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.     

Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering

Transparent MgO·1.5Al₂O₃ spinel ceramics were successfully prepared via reactive sintering of Al₂O₃ and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05?wt% CaO, the sample with high transmittance (82.3% at 400?nm), small grain size (<5?μm) and high strength (228?±?15?MPa) was obtained after HIPing at 1550?°C. However, when the amount of CaO increased to 0.1?wt%, non-cubic and columnar-shaped grains generated at low HIP temperatures (1550–1650?°C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al₂O₃/MgO, CaO amount and sintereing temperature.     

Effects of pre-sintering and annealing on the optical transmittance of Zr-doped Y2O3 transparent ceramics fabricated by vacuum sintering conjugated with post-hot-isostatic pressing

In this work, Zr-doped Y₂O₃ transparent ceramics were fabricated by vacuum pre-sintering at various temperatures ranging from 1650 to 1800 °C combined with a post-hot-isostatic pressing (HIP) treatment. The pre-sintered samples all show black opaqueness mainly due to the formation of oxygen vacancies, which can be removed by post-annealing in air. The HIP treatment can also eliminate the blackness as well as residual pores, giving rise to optical transparency. The in-line transmittance of the samples after HIP was found to depend strongly on the microstructure of the as-sintered samples. The optimal microstructure of these types of samples before the HIP treatment should be fine grained with only intergranular pores, which can easily be removed by HIP. Annealing before HIP was shown to be necessary to enhance the transmittance of the samples. The effects of the pre-sintering heating rate on the optical transmittance of the HIP-treated samples were also studied.     

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%.     

Fabrication,microstructure and spectroscopic properties of Yb:Lu2O3 transparent ceramics from co-precipitated nanopowders

Ytterbium doped lutetium oxide (Yb:Lu₂O₃) transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) of the powders synthesized by the co-precipitation method. The effects of calcination temperature on the composition and morphology of the powders were investigated. Fine and well dispersed 5?at% Yb:Lu₂O₃ powders with the mean particle size of 67?nm were obtained when calcined at 1100?°C for 4?h. Using the synthesized powders as starting material, we fabricated 5?at% Yb:Lu₂O₃ ceramics by pre-sintering at different temperatures combined with HIP post-treatment. The influence of pre-sintering temperature on the densities, microstructures and optical quality of the 5?at% Yb:Lu₂O₃ ceramics was studied. The ceramic sample pre-sintered at 1500?°C for 2?h with HIP post-treating at 1700?°C for 8?h has the highest in-line transmittance of 78.2% at 1100?nm and the average grain size of 2.6?µm. In addition, the absorption and emission cross sections of the 5?at% Yb:Lu₂O₃ ceramics were also calculated.     

Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering

Transparent Al-rich spinel ceramics (MgO·nAl₂O₃, n = 1.05–2.5) were prepared by reactive sintering in air followed by the hot isostatic press (HIP). Commercial MgO and γ-Al₂O₃ powders were used as the raw materials, and the effects of composition and HIP temperature on the transmittance and microstructure of resulting samples were investigated. To obtain the high optical quality, extra alumina (n ≥ 1.1) was used to help eliminate residual pores and suppress abnormal grain growth during the sintering process. The appropriate HIP temperature was also critical to realize the single-phase formation and prevent the generation of second-phase precipitates. The resulting samples with n = 1.1 and 1.3 exhibited excellent optical quality and fine grains below 5 µm after HIPed at 1550 °C.     

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.     

Sintering parameter optimization of Tb3Al5O12 magneto-optical ceramics by vacuum sintering and HIP post-treatment

Transparent terbium aluminum garnet (TAG) ceramics were achieved by the vacuum sintering plus HIP post-treating from the coprecipitated TAG nanoparticles. The influences of vacuum sintering temperature and sintering aid TEOS on the optical quality of the TAG ceramics were studied. The results show that with the increase of sintering temperature, the optical quality of TAG ceramics is improved gradually, and the in-line transmittance of the TAG ceramics treated at 1720°C for 20 hours under vacuum and then HIP post-treated at 1700°C for 3 hours under 200 MPa argon gas is 81.6% at 1064 nm. The sintering additive TEOS can improve the optical quality of TAG ceramics and inhibit the valence state change of Tb³⁺ ions to Tb⁴⁺ during the annealing process. The Verdet constant of the TAG ceramics at 632.8 nm is about −178 rad·T⁻¹·m⁻¹ at room temperature, which is 1.3 times that of the commercial TGG crystals (−134 rad·T⁻¹·m⁻¹).     

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.     

Fabrication and microstructural characterizations of lasing grade Nd:Y2O3 ceramics

Transparent 0.6 at% Nd:Y₂O₃ ceramics were fabricated by vacuum sintering at 1550°C and hot isostatic pressing (HIP) at 1540°C. The ceramics sintered at such temperatures had good homogeneity with dense microstructures, without any residual pores and secondary phases. The in-line transmittance reached 81.6% at 1000 nm and remained 81.1% at 650 nm. Continuous wave (CW) laser operation of an uncoated ceramic slab was evaluated. A maximum output power of 3.6 W with slope efficiency of 45.2% at 1.08 μm was obtained.     

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.     

Microstructure and properties characterization of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

5at.% Yb:Lu2O3 transparent ceramics were fabricated successfully by vacuum sintering along with hot isostatic pressing posttreatment from the nanopowders. The influences of calcination temperature on morphology and microstructures of powders and ceramics were studied systematically. The optimal ceramic sample from the nanopowder calcined at 1050°C shows uniform and dense microstructure with the in-line transmittance of 81.5% at 1100 nm. The results of the thermal measurements, that is, thermal conductivity and specific heat, were related to the changes occurring in the microstructure of the ceramics studied. It was shown on this basis that appropriate control of the technological process of sintering ceramics makes it possible to obtain laser ceramics with very good thermal properties as well as maintaining their high optical quality. Concerning the laser performance, the highest-optical quality 5at.% Yb:Lu₂O₃ sample was pumped in quasi-continuous wave conditions measuring a maximum output power of 2.59 W with a corresponding slope efficiency of 32.4%.     

Laser sintering of Al2O3-based green ceramics prepared by different pre-sintering temperatures

Al₂O₃-based green ceramics are prepared by isostatic cold pressing technology. The prepared green ceramics are pre-sintered at the temperature from room temperature to 1100°C, and then Al₂O₃ ceramics are prepared by laser sintering. The effects of pre-sintering temperatures and laser parameters on mechanical properties and the sintering quality are analyzed. The results show that good crystallinity of Al₂O₃ particles is obtained at a higher pre-sintering temperature. The flexural strength and density of green ceramics increase with the temperature of heat treatment. The flexural strength decreases slightly at ∼200°C due to the paraffin binder disintegration. The pre-sintering temperature and laser processing parameters have a significant influence on the sintering quality. With the increase of laser power and laser frequency, dynamic grain growth occurs, and then grains are refined. The majority of plate-like grains are transformed into long cylindrical-like grains in the severe densification process. However, porous flocculation microstructures are generated on the samples pre-sintered at 1100°C after laser sintering, which is due to the material gasification in atmospheric environment during sintering by infrared laser. More uniform microstructure and better sintering quality of samples pre-sintered at 500°C can be achieved after laser sintering with a relatively narrower grain size distribution.     

Fabrication of transparent yttria by HIP and the glass-encapsulation method

This method consists of a combination of vacuum sintering at 1600 °C followed by hot isostatic pressing (HIP) at 1500 °C of a highly agglomerated commercial powder. The use of evacuated glass capsules to perform HIP treatment allowed samples that showed open porosity after vacuum sintering to be sintered to transparency. The sintering response of the investigated powder was studied by careful microstructural observations using scanning electron microscopy and optical microscopy both in reflection and transmission. The successful key of this method was to keep porosity intergranular during pre-sintering, so that it can be removed subsequently by HIP treatment. It was found that agglomerates of closely packed particles are helpful to reach that purpose, since they densify fully and leave only intergranular porosity. However, performing HIP treatment at 1625 °C was found to result in opaque samples. This was attributed to the diffusion of argon inside the capsule. Contamination at different steps of processing was also investigated by inductively coupled plasma mass spectrometry (ICP-MS).     

Fabrication of high-efficiency Yb:Y2O3 laser ceramics without photodarkening

High-efficiency Yb:Y₂O₃ laser ceramics were fabricated using the vacuum-sintering plus hot isostatic pressing (HIP) without sintering additives. High-purity well-dispersed nanocrystalline Yb:Y₂O₃ powder was synthesized using a modified co-precipitation method in-house. The green bodies were first vacuum sintered at a temperature as low as 1430°C and then HIPed at 1450°C. Finally, the samples were air annealed at 800°C for 10 h. Although no sintering aids were used, full density of the samples with excellent optical homogeneity and an inline transmission of 80% at 400 nm could be obtained. Moreover, photodarkening phenomenon was not detected in the ceramics. Preliminary laser experiment with the fabricated ceramics in a two-mirror cavity has demonstrated 32 W continuous-wave (CW) output at ∼1077 nm with an optical-to-optical conversion efficiency of 58.2%. To the best of our knowledge, this is so far the highest CW output power and optical-to-optical conversion efficiency achieved with the Yb³⁺-doped sesquioxide ceramics in a simple two-mirror cavity.     

Preparation of (Tb1-xLux)2O3 transparent ceramics by solid solution for magneto-optical application

Novel (Tb_1-xLu_x)₂O₃ magneto-optical transparent ceramics were prepared through a solid-solution method via vacuum pre-sintering followed by hot isostatic pressing (HIP). The Lu-containing samples were solid-solution phases between Tb₂O₃ and Lu₂O₃, which effectively stabilized the phase transition of Tb₂O₃ during the sintering process. A typical component of (Tb_1-xLu_x)₂O₃ ceramic with x = 0.5 was selected to study the densification behavior and microstructure evolution in detail. The polished (Tb_0.5Lu_0.5)₂O₃ ceramics with a thickness of 3 mm vacuum pre-sintered at 1700 °C under 1.0 × 10^?3 Pa combined with HIP post-treatment at 1600 °C showed good magneto-optical property. The Verdet constant measured at 633 nm was -224.33 rad·T^?1 m^?1, 30% higher than that of Tb₃Al₅O₁₂ (TAG). However, the in-line transmittance was 73.6% at 633 nm, lower than that of TAG. The (Tb_1-xLu_x)₂O₃ ceramics will be a promising material for Faraday devices after further improving the optical quality.     

Fabrication and characterizations of Tb3Al5O12-based magneto-optical ceramics

Transparent terbium aluminum garnet (TAG)-based ceramics were fabricated by vacuum pre-sintering and hot isostatic pressing (HIP) posttreatment from the co-precipitated TAG powders with different stoichiometric ratios. After component optimization, the transparent ceramics with TAG single-phase and attractive optical quality were obtained. The in-line transmittance of optimal Tb_(1+x)3(Al_0.996255Si_0.003745)₅O_{12.0093625+3x/2} (x = −.004, −.002) ceramics (1.7-mm thick) pre-sintered at 1700°C for 20 h with HIP posttreatment at 1700°C for 3 h under 176-MPa Ar reaches 82.6% at the wavelength of 1064 nm. With increasing terbium components, the secondary phase TAP appears in ceramics, which significantly degrades the optical quality of TAG-based ceramics. The Verdet constant of the TAG-based ceramics at 632.8 nm is about −181 rad T⁻¹ m⁻¹ at room temperature, which is about 33% higher than that of the TGG single crystals (−134 rad T⁻¹ m⁻¹).     

Investigation of optical,mechanical, and thermal properties of ZrO2-doped Y2O3 transparent ceramics fabricated by HIP

Highly transparent ZrO₂-doped Y₂O₃ ceramics were successfully synthesized using the hot isostatic pressing (HIP) process. The effects of the ZrO₂ content on the sintering behaviors, optical transmission spectra, Vickers hardness, grain size, size distribution, and Raman spectra were determined. The results indicated that decreased ZrO₂ content could promote increased transmittance, red-shifted infrared cutoff wavelength, increased thermal conductivity, decreased Vickers hardness, and increased lattice ordering. According to the optical transmission spectra, the optimized ZrO₂ content was 0.50 at%, at which point the ceramic exhibited a larger pre-sintering temperature range of 1650–1750 °C and the average grain size of 3.35 µm at 1750 °C. The grain size was significantly decreased at lower pre-sintering temperatures. Furthermore, a moderate Vickers hardness of 8.42 GPa and high thermal conductivity of 10.85 W/m K at room temperature were obtained for the optimized ceramic.     

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.