Solid-state reaction method to 6Li,Ce:YAG ceramics for thermal neutron detection

ABSTRACT

The crystal structures and optical properties of 5% ⁶Li:Ce_0.09Y_2.91Al₅O₁₂ transparent ceramics prepared by solid-state reaction with different vacuum sintering temperature were investigated in this paper. The results reveal that with the increasing of sintering temperature, the transmittance of ⁶Li,Ce:YAG ceramics increases from 36% (1680°C) to 82% (1780°C) at 1000?nm, and the intensity of absorption peaks at 340 and 460?nm increases. The emission peak wavelengths of ⁶Li:Ce_0.09Y_2.91Al₅O₁₂ ceramics have been measured as 534.5?nm, and there is no red shift. The high transmittance and emission peak (at 534.5?nm) suggested that this material could be a candidate for neutron detection applications.     

Optical and Microwave Dielectric Properties of Zn‐Doped MgAl2O4 Transparent Ceramics Fabricated by Spark Plasma Sintering

(Mg_1?xZn_x)Al₂O₄ transparent ceramics were fabricated by spark plasma sintering technique at 1325°C for 10 min. A small mount of Zn²⁺ addition to MgAl₂O₄ ceramics was very effective to the performance improvement, while further increase in Zn‐doped content would give rise to the optical transmittance deterioration. The optical and microwave dielectric properties of MgAl₂O₄ transparent ceramics were improved by Zn substitution for Mg. The in‐line transmittance of the (Mg_1?xZn_x)Al₂O₄ (x = 0.02) ceramics can be as high as 70% at λ = 550 nm and 86.5% at λ = 2000 nm, respectively. The dielectric constant ε_r of (Mg_1?xZn_x)Al₂O₄ just varied from 8.32 to 8.54, however, the Q × f value increased significantly up to a maximal value of 66,000 GHz at x = 0.02. Moreover, the τ_f of (Mg_1?xZn_x)Al₂O₄ transparent ceramics changed from ?74 to ?65.5 ppm/°C. With the increasing of Zn‐doped content, the average grain size and the porosity increased, which was the primary reason for the change in optical and microwave dielectric properties.     

Microstructure evolution during reactive sintering of Y3Al5O12:Nd3+ transparent ceramics: Influence of green body annealing

The effect of green bodies’ mesostructure on the porosity, optical properties and laser performance of reactive sintered Y₃Al₅O₁₂:Nd³⁺ transparent ceramics was studied. Only minor changes in microstructure were revealed for green bodies without annealing and those annealed at 600, 800, 1000 °C, while average pore size increases to 140 nm for sample annealed at 1200 °C. Y₃Al₅O₁₂:Nd³⁺ ceramics sintered at 1750 °C for 10 hours possess significant differences in the final porosity, optical and laser characteristics. Despite all green bodies exhibit a similar phase evolution and sintering behavior on heating, the differences appear in the final stage, when the latest percentage of porosity is removed. The green bodies annealed at 600 °C have an optimal mesostructure from the standpoint of uniform densification. Y₃Al₅O₁₂:Nd³⁺ ceramics prepared using these green bodies exhibit porosity ≤0.001 vol% and yield efficient laser emission at 1.06 μm with slope efficiency as high as 67% in quasi-continuous pumping at 807 nm.     

Photochromism-induced light scattering and photoswithing in Er doped (K,Na)NbO3 transparent ceramics

The (K_0.5Na_0.5)_0.95Li_0.05Nb_0.95Bi_0.05O₃-1mol% Er₂O₃ transparent ceramics were prepared by a pressureless sintering method. The fabricated transparent ceramics not only exhibit high optical transmittance (85.3%) due to dense microstructure (nanoscale size grains), but also show the photochromism-induced light scattering and reversible upconversion (UC)-switching properties by visible light irradiation. Upon 407 nm light irradiation, the optical transmittance intensity is significantly decreased, showing a strong light scattering (ΔAbs = 28.3%). The light scatting degree can be quantitatively reflected by Er³⁺ ion UC emission, and could be recovered to its initial optical transmittance based on photochromic reactions. Meanwhile, the transparent ceramic is found to maintain good energy storage properties with higher W (5.87 J/cm³) and W_rec (1.96 J/cm³) under a higher electric field of 260 kV/cm. These results suggest that Er³⁺ doped (K_0.5Na_0.5)NbO₃ transparent ceramics are promising for the modulation of light scattering and the design of photoelectric multifunction devices.     

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co²⁺:MgAl₂O₄ saturable absorbers for laser passive Q-switching. Densification of commercial MgAl₂O₄ powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co₉S₈ inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co²⁺:MgAl₂O₄. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co²⁺, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ_gs) and excited (σ_es) cross-sections of 3.5×10^-19 and 0.8×10^-19 cm², respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co²⁺:MgAl₂O₄ ceramics.     

First ZnGa2O4 transparent ceramics

Transparent polycrystalline ZnGa₂O₄ ceramics are synthesized, for the first time, by combining high-energy ball milling, solid-state reaction and spark plasma sintering. They appear transparent in both the visible and near infrared (up to 9 μm) ranges after a post-SPS annealing in air converting the raw semiconductor into an electrical insulator. The maximum of transmittance is reached in the near infrared region, at around 2.5 μm, with a value of 78 % (1 mm thick sample) close to the maximum value of transmittance previously measured for single crystals. These transparent ceramics present a classic cubic spinel ZnGa₂O₄ structure and a dense microstructure (> 99 %) attained without sintering aids, with an average grain size of 600 nm and a random orientation of the crystallites. TEM observations performed on thin foils have revealed limited nanometer scale intergranular porosity which does not affect much the transparency. As a proof of interest, red long-lasting luminescence arising from the entire sample volume is observed in these Cr³⁺ doped transparent ceramics. This innovative work is anticipated to further drive the development of transparent ZnGa₂O₄ ceramics towards a wider range of performing optical applications such as laser emission.     

Dy3+-doped Y2Zr2O7 highly transparent ceramics for ultraviolet excitable warm white light-emitting applications

Attaining effective warm white light emitting in functionally advantageous transparent polycrystalline ceramics is vitally important to guarantee the development of both human and botanical systems. In response to this aim, a series of Dy³⁺-doped Y₂Zr₂O₇ (YZO) transparent ceramics were prepared via a solid-state reaction and vacuum sintering approach in this work. These fabricated ceramics show high transparency, where the in-line transmittance at 700 nm is about 76%, which is very close to the theoretical limit (78%). In addition, under the excitation of UV light sources (358 and 384 nm), strong warm white light emissions were observed in these YZO:Dy transparent ceramics. The corresponding photoluminescence characteristics and mechanisms of YZO:Dy ceramics are investigated carefully. The Dy-doped YZO ceramics integrate with high transparency and UV-excitable warm white light emission properties, making them promising light-emitting converter materials for light-emitting source applications.     

Fabrication of Transparent Dysprosium Aluminum Garnet (Dy3Al5O12) Ceramics via a Solid‐State Reaction Method

Polycrystalline, transparent Dy₃Al₅O₁₂ ceramics were firstly fabricated by a solid‐state reaction method using high‐purity Dy₂O₃ and Al₂O₃ powders. The fully dense Dy₃Al₅O₁₂ ceramic with an average grain size of less than 10 μm was obtained by vacuum sintering at 1820°C for 6 h. The in‐line transmittance of the optimized sample reaches 80% in the visible region. Scanning electron microscopy reveals that no secondary phases and almost no pores are observed at grain boundaries or triple junctions, and the fracture mode of the ceramic is mainly transgranular. The Dy₃Al₅O₁₂ ceramic is promising for magneto‐optical applications. Verdet constant of the Dy₃Al₅O₁₂ transparent ceramic is as high as ?0.41 min·(Oe·cm)^?1.     

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.     

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.     

Importance of optical homogeneity for high-quality transparent ceramics

Two types of transparent Y₂O₃ ceramics without including large scattering sources such as residual pores, one with very high optical homogeneity (type A) and another one with slightly insufficient optical homogeneity (type B), are purposely prepared, and their optical properties are investigated and compared qualitatively and quantitatively. Type A ceramic exhibits transmittance characteristics with very low internal loss in the visible to infrared wavelength region, while type B ceramic is inferior in various optical performances especially in the short (visible) wavelength region. In type B ceramic, birefringence occurs due to optical inhomogeneity in the visible region, resulting in a decrease in the extinction ratio. Non-uniform refractive index distribution is also observed in the Schlieren image of type B ceramic, hence the laser beam quality through that material is degraded. This study proved the importance of optical homogeneity of transparent ceramics and clarified the problems in actual applications.     

Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering

Highly transparent 0.5 at.% Tm:Y₂O₃ ceramics were prepared by using solid‐state reaction combined with vacuum sintering method, with ZrO₂ and Al₂O₃ as sintering aids. Doping amount of ZrO₂ was fixed at 1 at.%, while the effect of Al₂O₃ on densification, microstructure evolution, and transmittance of the Y₂O₃ ceramics was carefully studied. It was found that the addition of Al₂O₃ was very effective in improving densification of Y₂O₃, due to the formation of an Al‐rich eutectic phase Y₄Al₂O₉ (YAM) during the sintering process. As the content of Al₂O₃ was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al₂O₃ was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al₂O₃ led to an increase in both amount and size of the secondary phase. At the optimized content of Al₂O₃ with 81.8 wt ppm, the Tm:Y₂O₃ ceramics sintered at 1860°C for 13 h exhibited an in‐line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.     

Tribological behaviour of transparent ceramics: A review

Owing to superior properties, i.e. high hardness, high wear resistance, and weight reduction of transparent ceramics (TCs) over glasses, TCs have shown promising tribological potential for applications such as face shields, explosive ordnance visors, windows for aircraft, spacecraft and, re-entry vehicles, electromagnetic windows, laser igniter windows, screens for smartphones and more. Researchers globally have been attracted to explore more about TCs, considering the tremendously increasing demand over different other transparent materials. The optical quality of TCs is mostly characterized by the in-line transmittance, and the effect of various processing parameters on transmittance has already been studied by various researchers. In this review, the current research progress regarding tribological performance of TCs is compiled. TCs with potential in tribological applications include MgAl₂O_4, Al₂O₃, AlON, Lu₂O₃, c-BN, Y₂O₃, Si₃N₄, and SiAlON. The relevant strategies to improve the tribological properties, including microstructures and mechanical properties are comprehensively discussed. In addition, the mechanisms of material removal of different transparent ceramics are also presented. It is well observed that surface fracture comprising three stages is found as one of the dominant wear mechanisms during wear. This review aims to provide some meaningful guidelines for development of transparent ceramics with enhanced wear resistance, while identifying the wear mechanisms in particular wear conditions.     

Conventional HP sintering of asymmetric hexagonal structure Yb3+-doped Sr5(PO4)3F transparent ceramic without additives

The 2 at.% Yb³⁺:Sr₅(PO₄)₃F (S-FAP) polycrystalline transparent ceramic with asymmetric hexagonal structures has been synthesized by vacuum hot-pressing the nanoparticles prepared via coprecipitation method. X-ray diffraction results of powder and ceramic indicate that their phase peaks are well matched to the crystal structure of S-FAP. The average particle size of 35.5 nm has been exhibited by powder scanning electron microscopy images, and subsequent images of the ceramic cross section and surface morphology revealed a homogenous and compact microstructure with an average grain size of around 220 nm. The relationship between the optical loss caused by the scattering of anisotropic ceramic grains and the optical transmittance of ceramics was revealed in the hexagonal S-FAP transparent ceramics with different thicknesses. The in-line transmittance of hot-pressed ceramics with 1.5-mm thickness achieved 79.95% at 1100-nm wavelength, and the room-temperature absorption and emission spectra of Yb³⁺ in S-FAP polycrystalline ceramic matrix were measured using a spectrofluorometer.     

Er3+-doped CaF2 polycrystalline ceramic with perfect transparency for mid-infrared laser

Er³⁺-doped CaF₂ transparent ceramics are promising mid-infrared gain materials because of their utra-low phonon energy as well as excellent physical, chemical, and optical properties. However, existing hot-pressed and hot-formed CaF₂ ceramics are very difficult to be used in practical applications due to residual pores and weak polycrystallization, respectively. Here, we developed the high quality Er³⁺-doped CaF₂ transparent polycrystalline ceramic by single crystal ceramization. The sample exhibits obvious polycrystalline structure, good mechanical properties, perfectly transmittance, and excellent mid-infrared performance, which provides significant and wide-ranging opportunities for advanced mid-infrared gain materials.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Y3Al5O12 translucent nanostructured ceramics—Obtaining and optical properties

Y₃Al₅O₁₂ nanostructured ceramics with total transmittance of 63% at λ = 1064 nm has been obtained by low-temperature high-pressure sintering. According to high-resolution transmission electron microscopy ceramics is near pore-free and consists of close-packed grains of 20–40 nm in size. The transmittance spectrum in the visible and IR wavelength range, stationary X-ray excited luminescence and thermally stimulated luminescence were studied to characterize Y₃Al₅O₁₂ nanostructured ceramics in comparison to single crystals of the same composition. The observed differences in the optical and luminescent properties were interpreted as a consequence of high defectivity level of nanoceramics arising from its non-equilibrium character, extremely large concentration of grain boundaries and surface states.     

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.     

Laser-quality Tm:(Lu0.8Sc0.2)2O3 mixed sesquioxide ceramics shaped by gelcasting of well-dispersed nanopowders

Tm³⁺-doped mixed sesquioxide transparent ceramics are attractive candidates for the generation of robust ~2.1 μm lasers. In this paper, laser-quality Tm:(Lu_0.8Sc_0.2)₂O₃ mixed sesquioxide ceramics were shaped for the first time by gelcasting of well-dispersed nanopowders, which were obtained using a modified coprecipitation method. The dispersibility of starting nanopowders was largely improved using alcohol-water solvent. The rheological properties of slurries were optimized for gelcasting. We also investigated the densification behavior of the gel-casted green compacts. In contrast to the dry-pressing route, it was found that gelcasting could yield more homogeneous and transparent ceramics. The optical in-line transmittance of the ceramic rod 12 mm in length was as high as 80.3% at 2090 nm. Upon pumping the ceramic rod by 796 nm diode laser, a 1.88 W CW laser at 2090 nm was acquired with a slope efficiency of 24.6% (with respect to the input pump power).     

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.