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).     

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.     

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.     

溶胶-凝胶法引入烧结助剂制备SiC-Y3Al5O12复相陶瓷

以碳化硅、六水硝酸钇、九水硝酸铝和六次甲基四胺为主要原料,通过溶胶-凝胶法引入Al2O3和Y2O3复合烧结助剂,液相烧结制备得到SiC-Y3Al5O12(Y3Al5O12简称YAG)复相陶瓷.采用DTA、TEM、XRD等分析测试技术研究了溶胶-凝胶法引入复合烧结助剂过程及复合烧结助剂对SiC-YAG陶瓷的烧结性能、力学性能、物相组成与显微结构的影响.结果表明干凝胶在920℃左右已完全转变成YAG相,最终获得的YAG粒径小,并均匀分散在SiC表面的SiC-YAG复合粉体;复合粉体先干压、再等静压成型后,在1860℃下烧结45 min,所制得复相陶瓷的相对密度达到了96.5%,抗弯强度达到486 MPa,断裂韧性达到5.7 MPa·m1/2.     

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.     

Preparation and characterization of transparent magneto-optical Ho2O3 ceramics

Transparent magneto-optical Ho₂O₃ ceramics were successfully prepared with an in-line transmittance of ~73% at the wavelength of 1000 nm (~90% of the theoretical transmittance of Ho₂O₃ single crystal) and an average grain size of ~28 μm. The ceramics were fabricated using sulfate-exchanged nitrate-type layered rare-earth hydroxide as the precipitation precursor at a relatively low sintering temperature of 1700°C. The layered compound exhibited nanosheet morphology and fully collapsed into a round oxide powder with an average particle size of ~48 nm by pyrolysis. Calcination temperature for Ho₂O₃ powder significantly affected the optical quality of the sintered body and the optimum calcination temperature was found to be 1050°C. The transparent magneto-optical Ho₂O₃ ceramics displayed wavelength-dependent Verdet constants of −180, −46, and −20 rad/Tm at 632, 1064, and 1550 nm, respectively. Thus, the Ho₂O₃ ceramics show good potential for applications in high-power laser systems.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.     

Fabrication and magneto-optical property of yttria stabilized Tb2O3 transparent ceramics

(Tb_0.5Y_0.5)₂O₃ transparent ceramics have been prepared by wet chemical co-precipitation route and flowing H₂ atmosphere sintering. The optical quality, microstructure and magneto-optical properties of the ceramics were investigated. No sintering aids or milling were adopted in the ceramic fabrication processing. The cold isostatic pressed green compact could be sintered to be transparent (Tb_0.5Y_0.5)₂O₃ ceramic at 1800 ℃ in flowing H₂ atmosphere. The mechanical polished ceramic showed the good transmittance from visible to near infrared wavelength, corresponding to a 71.9% transmittance at 1400 nm wavelength. The Verdet constant measured at 632.8 nm of the (Tb_0.5Y_0.5)₂O₃ transparent ceramics was -220.19 rad T⁻¹ m^-1, which was 1.64 times that of Tb₃Ga₅O₁₂ single crystal.     

Towards fabrication of high-performance Al2O3 ceramics by indirect selective laser sintering based on particle packing optimization

A novel method to fabricate high-performance Al₂O₃ Ceramics by indirect Selective Laser Sintering (idSLS) based on particle packing optimization was reported. Al₂O₃ ceramics with the particle size distribution (PSD) being adjusted based on a particle packing model was prepared with the idSLS method, and the effects of PSD on the macro-performance and microstructure of the idSLSed ceramics part was investigated. Results show that an appropriate PSD of raw material is not only essential to the good SLS formability of powder bed but also contribute to the favorable sintering characteristics of the SLSed green body through an ideal particle packing. The improvement of overall properties including mechanic strength, dimensional accuracy, and surface flatness of the SLSed green body and the final parts were achieved with a fine-tuned PSD exponent. When the raw material has a PSD exponent of 0.5–0.6, the final ceramic parts showed relatively balanced comprehensive properties of bulk density >2.0 g cm⁻³, linear change <1.4%, fracture strength >3.97 MPa, compressive strength >5.25 MPa and surface altitude difference <148 μm.     

Low temperature sintering of PLZST-based antiferroelectric ceramics with Al2O3 addition for energy storage applications

Antiferroelectric (AFE) materials have superior energy storage properties in high power multilayer ceramic capacitors (MLCCs). To adapt to the sintering temperature of inner metal electrodes with less palladium content, in this work, Al₂O₃ was added to Pb_0.95La_0.02Sr_0.02(Zr_0.50Sn_0.40Ti_0.10)O₃ (PLSZST) AFE ceramics, in an attempt to reduce the sintering temperature. Results of this study demonstrate that the optimal composition of PLSZST-0.8 wt% Al₂O₃ sintered at a lower temperature 1040 ℃, has a high recoverable energy density (W_re, 3.23 J/cm³) and a high efficiency (η, 90 %) at room temperature. It is also high in pulse discharge energy density (W_dis, 2.45 J/cm³), current density (1369 A/cm²), and has an extremely short period of discharge (less than 500 ns). In addition, both W_re and η demonstrate a good stability in temperature within a wide range of 30 ℃-100 ℃. In sum, this novel AFE composition has great potentials for energy storage applications such as high energy density MLCCs.     

Transparent alumina ceramics fabricated by 3D printing and vacuum sintering

Transparent alumina ceramics were fabricated using an extrusion-based 3D printer and post-processing steps including debinding, vacuum sintering, and polishing. Printable slurry recipes and 3D printing parameters were optimized to fabricate quality green bodies of varying shapes and sizes. Two-step vacuum sintering profiles were found to increase density while reducing grain size and thus improving the transparency of the sintered alumina ceramics over single-step sintering profiles. The 3D printed and two-step vacuum sintered alumina ceramics achieved greater than 99 % relative density and total transmittance values of about 70 % at 800 nm and above, which was comparable to that of conventional CIP processed alumina ceramics. This demonstrates the capability of 3D printing to compete with conventional transparent ceramic forming methods along with the additional benefit of freedom of design and production of complex shapes.     

Fabrication and characteristics of Ce-doped Y3Fe5O12 ceramics by hot-press sintering with oxygen/nitrogen atmosphere

Infrared transparent Ce-doped Y₃Fe₅O₁₂ (Ce_: YIG, Ce_xY_3-xFe₅O_12, x = 0, 0.12, 0.24, 0.36) ceramics were successfully produced by the solid-state reaction using a hot-press sintering process from the Y₂O₃, Fe₂O₃, and CeO₂ powders. The phase structure, microstructure, infrared transmittance, and magnetic and magneto-optical properties of the Ce-doped Y₃Fe₅O₁₂ ceramics were measured and analyzed. The in-line transmittances of the Ce-doped Y₃Fe₅O₁₂ ceramics with the x = 0, 0.12, 0.24 (L = 0.5 mm) at 1550 nm were about 72%, 66.5%, and 57.6%, respectively. In the state of saturation magnetization, the Faraday rotation angle per centimeter (θ_F) of Ce_xY_3-xFe₅O₁₂ (x = 0, 0.12, 0.24) ceramics measured by the light extinction method was 182.5, −410.4, and −958.3 deg./cm, respectively. The change of the θ_F was about −142.5 deg./cm when per 1at.% Ce was substituted in the dodecahedral site of YIG materials. The (Ce_0.24Y_2.76)Fe₅O₁₂ ceramics were determined as the optimized composition for its excellent infrared optical and magneto-optical properties.     

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.     

Transparent magneto-optical Ho2O3 ceramics: Role of self-reactive resultant oxyfluoride additive and investigation of vacuum sintering kinetics

Polycrystalline transparent magneto-optical Ho₂O₃ ceramics were successfully fabricated by vacuum sintering using the self-reactive resultant HoOF as a sintering aid. They had an in-line transmittance of 72.0% at 1550 nm and the wavelength-dependent Verdet constants of −182 ± 8, −46 ± 3, and −22 ± 2 rad/Tm at 633, 1064 and 1550 nm (approximately 1.3 times those of terbium gallium garnet ceramics), respectively. The HoOF additive was generated by pyrolyzing Ho(OH)₂F from the chemical reaction between the layered Ho₂(OH)₅NO₃ compound and NH₄F in the liquid phase. The role of the HoOF additive and the sintering kinetics of the Ho₂O₃ ceramic were systematically studied. The results show that: (1) the proper amount of F⁻ additive helps to obtain an oxide powder with a narrow particle distribution and significantly improves the optical quality of the sintered body; (2) HoOF induces a transient second phase to effectively suppress the grain growth in the Ho₂O₃ ceramics during sintering; (3) the diffusion mechanism is primarily controlled by grain-boundary diffusion around 1400 °C; (4) the activation energy of the grain-boundary-controlled diffusion was determined to be approximately 545 kJ/mol via the grain growth method.     

热压烧结Al2O3-ZrB2-SiC复相陶瓷的高温力学性能研究

为提高Al2O3陶瓷的高温力学性能,采用热压烧结工艺(烧结温度1 800℃,烧结压力20 MPa,保温1 h)制备了Al2O3-ZrB2-SiC复相陶瓷(简称AZS),并研究了ZrB2含量对Al2O3基陶瓷高温抗折强度和抗热震性的影响。结果表明:1)在Al2O3基陶瓷中加入第二相ZrB2能有效改善材料的高温抗折强度和高温强度保持率,在1 000和1 200℃时,加入20%体积分数ZrB2的AZS陶瓷试样具有最高的高温抗折强度,而加入24%体积分数ZrB2的AZS陶瓷试样具有最高的高温强度保持率。2)AZS陶瓷的抗热震性能优于纯Al2O3陶瓷。经100℃温差急冷后,加入20%体积分数ZrB2的AZS陶瓷具有最高的残余强度,比纯Al2O3陶瓷提高了17.2%;经300和500℃温差急冷后,加入24%体积分数ZrB2的AZS陶瓷都具有最高的残余强度,比Al2O3陶瓷分别提高了35.3%和20.9%。     

Highly transparent yttria ceramics with Nb2O5/Ta2O5 as novel sintering additives by pressureless vacuum sintering

Highly transparent yttria ceramics were fabricated by pressureless sintering with Nb₂O₅ or Ta₂O₅ as a novel sintering additive. The optical transmittance, microstructural evolution, and thermo-mechanical properties of the samples were investigated. The optimal doping concentrations of Nb₂O₅ and Ta₂O₅ are 0.3 and 0.2 at.%, respectively, which are much lower compared to those of previously reported counterparts. The transmittance of the sample with 0.3 at.% Nb₂O₅ reaches 81.6% at 1100 nm and 72.4% at 400 nm (2 mm in thickness), similar transmittance was obtained in the sample with 0.2 at.% Ta₂O₅. The microhardness (∼7 GPa), fracture toughness (∼0.85 MPa·m^1/2), and biaxial strength (∼200 MPa) of the present samples were confirmed to be comparable or even better compared to those of previously reported transparent yttria ceramics fabricated by pressureless sintering. Furthermore, the present samples, by virtue of the low doping concentrations, possessed relatively high thermal conductivity values (>10 W/m·K), which substantially guaranteed high thermal shock resistance.     

Densification behaviors of Al2O3 ceramics during flash sintering

The densification behaviors of MgO-doped-Al₂O₃ ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO-doped Al₂O₃ was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current-induced point defects on the diffusion.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce³⁺:(Gd,Lu)₃Al₅O₁₂ with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).