Highly transparent cerium-doped yttria ceramics for full-band UV-shielding window applications

Highly transparent cerium-doped yttria ceramics were fabricated via a pressureless sintering method with an addition of 10 at.% La₂O₃ and 1 at.% ZrO₂ as a binary sintering additive. With an increase in the Ce doping concentration from 0 to 3 at.%, the optical absorption edge of yttria ceramics exhibited a significant red shift from 290 to 380 nm as a result of the 4f–5d transition of Ce³⁺ and the charge transfer of Ce⁴⁺ (i.e., Ce⁴⁺+e^–→Ce³⁺). For a 2-mm thick sample doped with 3 at.% Ce, the total ultraviolet radiation (UV) transmittance (220–400 nm) is only 1.7%, indicating a nearly complete UV absorption. Meanwhile, the specimens possess high in-line transmittance levels in both the visible and the infrared regions (i.e., >70% at 450 nm and ∼80% at 1100 nm). Additionally, the present specimens were confirmed to have good enough mechanical strength levels (∼165 MPa) and thermal conductivity (∼5 W/m·K), which are comparable or even better compared to those of previously reported transparent yttria ceramics. The results of this work indicate that cerium-doped transparent yttria ceramics are promising candidate materials for full-band UV-shielding window applications.     

Pressureless sintering of transparent AlON ceramic with assimilable γ-Al2O3 as sintering promoting additives

Transparent aluminum oxynitride (AlON) ceramic was successfully fabricated without doping sintering additive by the pressureless sintering method. Γ-Al₂O₃ nano-powder that can be assimilated by the AlON matrix was used for promoting the densification of AlON during the sintering. The sintering behavior of AlON and the effects of γ-Al₂O₃ nano-powder on the phase, hardness, and transmittance of AlON have been investigated in detail. The mechanisms of γ-Al₂O₃ nano-powder on the AlON green body modifying and the sintering promoting are revealed. The transmittance of the AlON ceramic is dramatically enhanced by doping γ-Al₂O₃ nano-powder and the 2 mm thick sample doped with 2.5 wt% γ-Al₂O₃ nano-powder shows an inline transmittance above 81% at 1500 nm.     

Preparation of nanocrystalline ultra-high temperature Ta4ZrC5 ceramics by joint processes of solvothermal and carbothermal reaction

An attractive way to prepare nanocrystalline tantalum zirconium carbide ternary ceramics was proposed and confirmed experimentally. The experimental results showed the Ta₄ZrC₅ powders were successfully fabricated by joint processes of solvothermal and carbothermal reaction. The thermodynamic change process in the Ta₂O₅-ZrO₂-C system was studied. The reactions were substantially completed at relatively lower temperatures (∼1873 K/1 h) and the synthesized powders had a small average crystallite size (∼10 nm). The crystalline structure and the nitrogen sorption isotherms patterns of the product were studied. Besides, a monolithic Ta₄ZrC₅ ceramics was densified without sintering additives by pressureless sintering.     

Fabrication of 0.24Pb(In1/2Nb1/2)O3-0.42Pb(Mg1/3Nb2/3)O3-0.34PbTiO3 transparent ceramics by conventional sintering technique

0.24Pb(In_1/2Nb_1/2)O₃-0.42Pb(Mg_1/3Nb_2/3)O₃-0.34PbTiO₃ transparent ceramics were fabricated by a conventional sintering technique. Through optimization of sintering conditions of calcination and sintering temperatures and time, the obtained ceramics showed high optical transmittance of 53% and 71% at light wavelengths of 1300 and 2000 nm, respectively. The ceramics showed a rhombohedral to tetragonal phase transition at ~120°C and a tetragonal to cubic phase transition at 222°C. These transition temperatures were higher than those of 0.67Pb(Mg_1/3Nb_2/3)-0.33PbTiO₃ ceramics. In addition, the ceramics had a ferroelectric hysteresis loop, a large piezoelectric constant d₃₃ of 407 pC/N, and a planar electromechanical coupling factor k_p of 52%. These results suggest that the transparent ceramics may be used as a temperature-stable, linear electro-optic material.     

Fabrication and characterization of highly transparent ZrO2-doped Tm2O3 ceramics

Highly transparent polycrystalline Tm₂O₃ ceramics were successfully fabricated by vacuum sintering at temperatures from 1650 to 1850 °C for 8 h using commercial Tm₂O₃ and ZrO₂ (1 at%) powders as starting materials. It is the first time that ZrO₂ was reported as a sintering additive to prepare Tm₂O₃ transparent ceramics. The effects of sintering temperature on the optical transmittance and microstructure of Tm₂O₃ transparent ceramics were studied. The desired Tm₂O₃ ceramics with relative density of 99.8% and an average grain size of approximately 9.7 μm were obtained at 1800 °C and the in-line transmittance reached 75% at 880 nm and fluctuated around 80% from 2100 to 2400 nm, respectively. This study demonstrated that Tm₂O₃ transparent ceramics with higher in-line transmittance and smaller grain size could be prepared by using ZrO₂ as sintering additive at a relatively lower vacuum sintering temperature compared to those already reported in open literatures.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Study on robust additive of CaCO3 for fast fabrication of highly transparent AlON ceramics by pressureless sintering

Fabrication of transparent AlON ceramics is extra sensitive to both particle size of starting powder and sintering additive due to shuttling transformation between AlON and Al₂O₃ + AlN during heating. One possible solution is to select robust additive to suppress the shuttling transformation. In this work, three AlON powders with different median particle sizes of 0.6, 0.9, and 1.1 μm were prepared. After studying the effect of CaCO₃ on densification process, AlON ceramics with the maximum transmittance of ≥81.1% were successfully fast prepared by pressureless sintering (PS) at 1880°C for only 2.5 h by using three AlON powders doped with different CaCO₃ amount. Specifically, AlON ceramics prepared from 1.1 μm with 0.5–0.8 wt.% CaCO₃ doping consistently showed the maximum transmittance of ≥85.3%, which indicates that CaCO₃ can serve as a robust additive to enable fast fabrication of highly transparent AlON ceramics even by PS.     

Reactive sintered highly transparent AlON ceramics with Y2O3-MgAl2O4-H3BO3 ternary additive

In this study, highly transparent aluminate oxynitride (AlON) ceramics were prepared via the reactive sintering of Al₂O₃ and AlN powders using a Y₂O₃-MgAl₂O₄-H₃BO₃ ternary sintering additive. The ternary doping process resulted in the efficient preparation of transparent AlON ceramics with small grains and high transmittance as compared to the binary doping (Y₂O₃-MgAl₂O₄) process. The addition of 0.1 wt.% Y₂O₃-0.4 wt.% MgAl₂O₄-0.12 wt.% H₃BO₃ resulted in the formation of a 4-mm-thick AlON ceramic with high transmittance (81% at 600 nm) and low haze (3.46%). This is the best performance in terms of the thickness and transmittance reported for AlON transparent ceramics prepared by the reactive sintering method.     

Highly transparent Mg0.27Al2.58O3.73N0.27 ceramic fabricated by aqueous gelcasting,pressureless sintering,and post-HIP

Magnesium aluminum oxynitride (Mg_0.27Al_2.58O_3.73N_0.27, termed as MgAlON) ceramics with high transparency and complicated shape was prepared by aqueous gelcasting, pressureless sintering, and followed by hot isostatic pressing. No obvious hydration was found by the characterizations of X-ray diffraction, pH value, Fourier transform infrared and thermal analysis for the interaction between MgAlON spinel powders and water, leading to the stable MgAlON slurry with high solid loading (52 vol%) and low viscosity. This result may be due to different composition of MgAlON from that of MgAl₂O₄ and AlON. Besides, transparent MgAlON ceramic (1.95 mm in thickness) with a high in-line transmittance ~86.3% at 3.7 μm was fabricated. The refractive index ~1.7499 at 589.3 nm and absorption coefficient ~1.2 cm⁻¹ at 5 μm of MgAlON are between those of AlON and MgAl₂O₄ transparent ceramics, and Abbé number ~73.66 of MgAlON is the highest.     

Direct tape casting of Al2O3/AlN slurry for AlON transparent ceramic wafers via one-step reaction sintering

In this work, transparent aluminate oxynitride (AlON) ceramic wafers were successfully fabricated by the direct non-aqueous tape casting of Al₂O₃/AlN slurry and the one-step reaction sintering for the first time. The reaction sintered AlON ceramic wafer exhibits high transmittance of 73.2 % at the wavelength of 1600 nm. This fabricating route realizes smooth and flexible tape without cracks or pinholes in Al₂O₃/AlN system and efficiently shortens the preparation cycle of transparent AlON wafers, which is a feasible way to prepare high-quality transparent AlON ceramics with large lateral sizes and thin thicknesses by reaction sintering, might also promote the application of transparent AlON ceramic wafers.     

Photochromic effect of transparent lead-free ferroelectric KSr2Nb5O15 ceramics

Transparent KSr₂Nb₅O₁₅ (KSN) lead-free ferroelectric ceramics have been synthesized via modified pressureless sintering method. A significant photochromic effect was observed for the transparent KSN ceramics prepared without rare-earth dopant modification. The piezoelectric properties depend on the grain orientations were investigated. The optical transmittance of the KSN ceramics is greater than 40% in the wavelength range of 530–800 nm. After NUV irradiation, the absorbance was enhanced by more than 40% in a broad visible range (more than 79%). The absorbance returned to the initial value after a thermal bleaching process. The results of the cycling tests and response experiments showed the stability and saturation of the photochromic effect. In addition, the possible photochromic mechanism of the KSN ceramics is discussed and the photochromic centers are identified. This transparent KSN ceramics exhibits an obvious photochromic effect and is a potential candidate materials for optical data storage and information recording applications.     

Enhancement of Piezoelectric Performance of Lead‐Free NKN‐Based Ceramics via a High‐Performance Flux—NaF–Nb2O5

x(NaF?0.5Nb₂O₅)?(1 ? x)[(Na_0.5K_0.5)(Nb_0.8Ta_0.2)O₃] (100xNN?NKNT) piezoelectric ceramics were fabricated to high densities above 97% when sintering at temperatures ~1200°C. Compared with pure (Na,K)NbO₃ (NKN), dielectric constants of the NaF–Nb₂O₅ flux‐doped NKNT ceramics were increased whereas dielectric losses remained low. High field polarization switching showed very square hysteresis loops, but the coercive fields were decreased through a “softening” doping effect induced by the flux. A Rayleigh analysis inferred that the extrinsic contribution from the domain wall dynamics was increased with the flux addition. Collectively, all the experimental observations suggested that NaF–Nb₂O₅ was, in part, soluble in the structure and highly suitable for obtaining a stable NKN system with improved piezoelectric performance.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Fabrication of nanocrystalline Sc2O3-Y2O3 solid solution ceramics by spark plasma sintering

Sc₂O₃-Y₂O₃ solid solution (SYSS) ceramics with novel nanocrystalline structures have been successfully fabricated by spark plasma sintering (SPS) method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), FT-IR and indentation techniques. The SYSS ceramics can possess high hardness by keeping their excellent infrared transmission performance. Especially, the transmittance of as-obtained SYSS ceramics can exceed as high as 80% in the infrared region of 2–6 µm. Moreover, the Vickers hardness is increased to 9.18 GPa by increasing the addition ratio of Sc₂O₃ to 20 at%. Here, it is very important to point out that the mechanical strength of yttria ceramics can be evidently enhanced without deteriorating their optical transmission performance by the addition of Sc₂O₃ as well as the application of SPS technique, where the solid solution strengthening and fine grain strengthening are involved during the synthesis process, respectively.     

Effect of Ta2O5 Substituting on Thermal and Optical Properties of High Refractive Index La2O3–Nb2O5 Glass System Prepared by Aerodynamic Levitation Method

High refractive index glasses with nominal composition of 0.35La₂O₃–(0.65?x)Nb₂O₅–xTa₂O₅ (x ≤ 0.35) were prepared by aerodynamic levitation method. The effect of Ta₂O₅ substituting on their thermal and optical properties was investigated. All the glasses obtained were colorless and transparent. Differential thermal analyzer results show that as the content of Ta₂O₅ increased, the thermal stability of the glasses increased but the glass‐forming ability decreased. The transmittance spectra of all the obtained glasses exhibited a wide transmittance window ranging from 380 to 5500 nm. As the content of Ta₂O₅ increased, the refractive index of the glasses was enhanced from 2.15 to 2.21 and the dispersion was reduced with the Abbe number increasing from 20 to 27.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

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.     

Fast pressureless solid-state reaction sintering of highly infrared transparent MgAlON ceramics from MgAl2O4 and AlON powders by two-step heating

A two-step heating strategy was proposed to fabricate transparent MgAlON ceramics by solid-state reaction of MgAl₂O₄ and AlON powders via pressureless sintering. By dwelling 60 min at 1700 ℃ followed by 150 min at 1880 ℃, highly infrared transparent MgAlON ceramics with transmittance up to 80.4 % were successfully fast prepared. The phase transformation and microstructure evolution during heating from 1400 ℃ to 1800 ℃ and dwelling at 1700 ℃ for 0–90 min was thoroughly studied to reveal the solid-state reaction and densification mechanism of MgAlON by two-step heating. Surprisingly, it was found that the grown grains could break during dwelling at 1700 ℃. This secondary massive fragmentation of grown grains resulted in the minimized grain size and improved moveability of grains, which in turn prompted fast and high densification with pore free in the following sintering step. The grain breakage at 1700 ℃ could be attributed to the decomposition of AlON and formation of MgAlON.     

Optical and thermo-mechanical properties of fine-grained transparent yttria ceramics fabricated by hot-press sintering for infrared window applications

In this work, highly transparent yttria ceramics Φ?=?55?mm in size were fabricated by a hot-pressing method with 1 at.% ZrO₂ or 12 at.% La₂O₃ as a sintering additive. For a 4-mm-thick specimen doped with ZrO₂, the in-line transmittance reaches 71.1% at 400?nm and 80.9% at 1100?nm, and the transmittance of the La₂O₃-doped specimen is comparable to that of the ZrO₂-doped specimen. By means of the relatively low sintering temperature of 1600?°C, the present samples exhibited very fine microstructures (<2?μm), giving rise to excellent mechanical strength levels (～200?MPa). With regard to the 1 at.% ZrO₂-doped specimen, the combination of high strength and high thermal conductivity (～10 W/m?K) substantially improved parameters related to the thermal shock resistance. The results of this study indicate that the hot-pressed transparent yttria ceramic doped with 1 at.% ZrO₂ is optically, mechanically, and thermally suitable for high-temperature IR window applications.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.