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).     

Magneto-optical,thermal, and mechanical properties of polycrystalline Tb3Al5O12 transparent ceramics

Terbium aluminum garnet (Tb₃Al₅O₁₂, TAG) ceramics have become a promising magneto-optical material owing to the outstanding comprehensive performance, including the magneto-optical, thermal, and mechanical properties. Fine-grained TAG ceramics with high optical quality and mechanical properties have attracted much attention. In this study, TAG ceramics with fine grains and high optical quality are fabricated successfully by a two-step sintering method from co-precipitated nano-powders. After pre-sintered at 1525°C in vacuum and hot isostatic pressed at 1600°C, the in-line transmittance of TAG ceramics reaches 81.8% at 1064 nm, and the average grain size is 7.1 μm. The Verdet constant of TAG ceramics is −179.6 ± 4.8 rad T⁻¹ m⁻¹ at 633 nm and −52.1 ± 1.9 rad T⁻¹ m⁻¹ at 1064 nm, higher than that of commercial Tb₃Ga₅O₁₂ crystals. The thermal conductivity of TAG ceramics is determined from 25 to 450°C, and the result is 5.12 W m⁻¹ K⁻¹ at 25°C and 3.61 W m⁻¹ K⁻¹ at 450°C. A comparison of mechanical properties between large- and fine-grained TAG ceramics fabricated under different conditions is conducted. The fine-grained TAG ceramics possess a bending strength of 226.3 ± 16.4 MPa, which is 9.7% higher than that of the large-grained ceramics. These results indicate that reducing the grain size on the premise of high optical quality helps improve the comprehensive performance of TAG ceramics.     

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.     

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 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.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Development of optical grade (TbxY1−x)3Al5O12 ceramics as Faraday rotator material

Super full dense (Tb_xY_1?x)₃Al₅O₁₂ (x=0.5‐1.0) ceramics with optical grade (pore‐free) were successfully produced by solid‐state reaction between Tb₄O₇ and Al₂O₃ raw powders. Transparent sintered bodies were obtained by sintering at 1720°C for 5 hours in vacuum furnace. By additional HIP treatment, optical scattering centers were effectively removed, and finally the optical quality of the sintered bodies was improved to optical grade. Optical loss of the obtained samples at 1064 nm was approximately 0.1%/cm, and optically inhomogeneous parts were not observed inside the materials. Gaussian mode laser beam quality was not deteriorated after passing through the sample. Transmitted wavefront distortion inspected by interferometry was as excellent as λ/12. Verdet constant increased with an increase of Tb content in the garnet composition. When x=1.0, the Verdet constant was 307, 196, and 60 rad T^?1 m^?1 for 532, 633, and 1064 nm, respectively, at each measuring wavelength. These values were about 1.5 times higher than that of the commercially available TGG (Tb₃Ga₃O₁₂) crystal. Insertion loss of the produced (Tb_0.6Y_0.4)₃Al₅O₁₂ and TAG ceramics at 1064 nm was 0.01 and 0.05 dB, respectively, and extinction ratio was 39.5 and 40.3 dB, respectively. These properties were superior to that of the commercial high‐quality TGG single crystal (insertion loss: 0.05 dB, extinction ratio: 35.0 dB).     

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.     

Fabrication and characterizations of Ho3Sc2Al3O12 magneto-optical transparent ceramics

Currently, Faraday isolators and rotators for high-power lasers are popular research topics; therefore, finding magneto-optical materials with excellent Verdet constants and thermal properties is crucial. In this study, novel holmium scandium aluminum garnet (HoSAG) magneto-optical ceramics were fabricated using vacuum sintering. This study improved the traditional ball milling method and pre-synthesized powders with great sinterability after secondary ball milling and sieving. Different pre-synthesis temperatures were found to have significant effects on the particle size and micromorphology of the synthesized powders. HoSAG ceramics with 0.05 wt% MgO sintering aid and held at 1700°C for 30 h reached a transmittance of 76.7% at 1550 nm. Meanwhile, HoSAG transparent ceramics maintain higher transmittance in the infrared region (>1500 nm) than terbium gallium garnet (TGG) crystals, indicating better application prospects. The Verdet constants of HoSAG magneto-optical ceramics at 405, 532, 808, and 1064 nm were −421.5, −181.2, −65, and −32.3 rad T⁻¹ m⁻¹, respectively, which are slightly less than those of TGG magneto-optical ceramics. Moreover, the thermal conductivity of HoSAG ceramics at 300.00 K was 4.89 W m⁻¹ K⁻¹, which is comparable to that of the TGG ceramics.     

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.     

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.     

Optimizing the piezoelectric properties of Ba0·85Ca0·15Zr0·10Ti0·90O3·lead-free ceramics via two-step sintering

The two-step sintering of lead-free Ba₀·₈₅Ca₀·₁₅Zr₀·₁Ti_0·9O₃·(BCZT) ceramics was investigated as a way to enhance its piezoelectric properties. The variations in grain size as a function of the calcination and sintering conditions and its effect on performance is discussed. Results indicate that as the calcination and first-step sintering temperatures increased, grain size became large and was independent of the second sintering step. Large grains were responsible for the enhanced piezoelectric properties by causing lattice distortion, larger domains, and easy motion of domain walls. The BCZT ceramic calcined at 1200 °C and sintered at 1540 °C without holding and then cooled to 1400 °C and held at 1400 °C for 4 h exhibited optimal performance with the highest remnant polarization P_r ∼13.5 μC/cm², the largest piezoelectric constant d₃₃ ∼ 529 pC/N at room temperature, and the highest Curie temperature T_c ∼125 °C. Two-step sintering has been turned out to be an effective method to realize high-performance BCZT ceramics by microstructure optimization.     

A comparative study of the properties of terbium aluminum garnet powder sintered in air,vacuum and by laser irradiation

The structural and optical properties of terbium aluminum garnet (TAG) powder sintered (～1100 °C) in air, vacuum (～10^?6 mbar) and with 70 W of unfocussed CW CO₂ laser radiation, have been studied using X-ray powder diffraction (XRD), scanning electron microscopy (SEM/TEM), FTIR, optical absorption and photoluminescence techniques. Structural properties of TAG are found to be independent of the sintering procedure except that the pure TAG crystalline phase (Tb₃Al₅O₁₂) is evolved in about 2 h in the case of laser sintering compared to 8 h needed in air sintering (by furnace) and 4 h needed in the case of vacuum furnace sintering. On the other hand, the absorption/emission intensity (300–600 nm region) of TAG samples sintered in vacuum is higher compared to that of laser/furnace sintering in air.     

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.     

Properties of MgO transparent ceramics prepared at low temperature using high sintering activity MgO powders

Transparent MgO ceramics are successful fabricated via spark plasma sintering at lower temperature using the high sintering activity powders synthesized by precipitated method. The samples were detected by XRD, SEM, TEM, BET, UV-Vis-NIR, microhardness, and so on. The results show that all ceramics prepared at 700°C-900°C are visually transparent and the sample sintered at 860°C for 5 min exhibits the superior transmittance of 60% (800 nm). It is also found that the mechanical and thermal properties of MgO ceramics are all increasing firstly and then decreasing with the increase in the sintering temperature. And the maximum value of hardness, fracture toughness, MSP strength, and Young's modulus of MgO ceramics is 8.25 GPa, 2.01 MPa·m^1/2, 206 MPa, and 286 GPa, respectively. Moreover, the thermal conductivity of MgO ceramics sintered at 860°C can reach 48.4 W/mK at room temperature.     

Photoluminescence and unique magnetoluminescence of transparent (Tb1-xYx)3Al5O12 ceramics

Luminescent transparent ceramics (Tb_1-xY_x)₃Al₅O₁₂ (x = 0, 0.2, 0.5, 0.8) are successfully prepared by a solid-state method with additional hot isostatic pressing (HIP) treatment, and the structure and properties are investigated by XRD, SEM, PL, UV–Vis spectrophotometry and ellipsometry. The Y-containing samples are shown to be solid solution phases between TAG and YAG. The PL intensity is 14 times stronger with the incorporation of 80 mol.% Y, and the ⁵D₄→⁷F₅ emission lifetime of Tb³⁺ is prolonged from 0.357 to 3.035 ms at room temperature. A unique magnetoluminescence emerges upon the incorporation of Y, showing an interesting emission decrease to 55% as the Y content reaches 80 mol.%. Remarkably, this magnetoluminesence can occur at room temperature without an intense magnetic field. Based on our work, transparent (Tb_1-xY_x)₃Al₅O₁₂ ceramics exhibit the potential for applications in green emitters, optical instruments and photoelectric devices. In particular, the magnetoluminescence provides a simple, noncontact and nondestructive route for probing magnetic fields.     

Low-temperature synthesis of K0.5Na0.5NbO3 ceramics in a wide temperature window via cold-sintering assisted sintering method and enhanced electrical properties

High temperatures (≥ 1100 °C) and narrow temperature window (～ 20 °C) for sintering dense K_0.5Na_0.5NbO₃ ceramics always deteriorate their electrical properties. Here, via cold-sintering assisted sintering method, dense K_0.5Na_0.5NbO₃ ceramics were obtained in a wide temperature span between 800 °C and 1000 °C. An aqueous solution of NaOH and KOH mixture was used as transient liquid. Effects of liquid content (LC), molar concentration (MC) of liquid, cold-sintering temperature (T_CS), and post-annealing temperature (T_AN) on densification and electrical properties of the ceramics were investigated in detail. The ceramics prepared using LC = 10 wt%, MC = 10 mol/L, T_CS = 350 °C, and T_AN = 900 °C exhibit excellent electrical properties with d₃₃ = 123 pC/N, ε_r = 609, tanδ = 0.021, P_r = 28.0 μC/cm², P_m = 39.2 μC/cm², and E_c = 20.3 kV/cm. Compared to the ceramics with same or similar compositions via conventional solid-state sintering, the present K_0.5Na_0.5NbO₃ ceramics exhibit excellent electrical properties. The study endows the cold-sintering assisted sintering the successful method to prepare K_0.5Na_0.5NbO₃ ceramics at low temperatures and in a wide temperature window.     

High electrocaloric effect in hot‐pressed Pb0.85La0.1(Zr0.65Ti0.35)O3 ceramics with a wide operating temperature range

The temperature stability of the electrocaloric effect (ECE) in relaxor ferroelectric Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) prepared by the hot‐press sintering method has been investigated. Compared to the PLZTs prepared via the conventional sintering process, the hot‐pressed PLZTs exhibit larger ECE and superior temperature stability. The hot‐pressed sample with an appropriate content of excess PbO presents a high ΔT of 2.4°C and ΔS of 2.3 J kg^?1·K^?1, both of which are 30% greater than those of the conventionally sintered samples measured at 100 kV·cm^?1. More importantly, the hot‐pressed specimens display great stable electrical properties, including the dielectric breakdown strength and electrical resistivity in the temperature range from 0°C to 100°C, whose ECE instability, especially, is only one‐half that of the samples prepared by the conventional solid‐state method. In addition, the ECE and its stability of the hot‐pressed sample can be further enhanced by increasing the operating electric field to a relatively high level of 200 kV·cm^?1. This work demonstrates hot‐press sintering is an effective method to fabricate ferroelectric ceramics with high ECE as well as desirable temperature stability.     

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.     

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%.