Properties of transparent Ce:YAG ceramic phosphors for white LED

Transparent Ce:YAG ceramic phosphors were synthesized from the oxide powder which was produced by co-preparation method of the hydroxides. The Ce:YAG ceramics had a broad emission band peaked at 530 nm due to the 5d → 4f transition of Ce³⁺. The transmittances of the samples obtained were 70-87% at 800 nm. The absorption coefficient and emission intensity of Ce³⁺ were increased with increasing thickness. Under 465 nm LED excitation, the color coordinates of the Ce:YAG ceramics shifted from the blue region to yellow region with increasing sample thickness, passing nearby the theoretical white point in the chromaticity diagram. The highest value of luminous efficacy of the ceramic white LED was 73.5 lm/W.     

Efficient energy transfer between Ce/Cr and Nd ions in transparent Nd/Ce/Cr:YAG ceramics

This paper reports the spectroscopic and laser characteristics of transparent ceramics of YAG:Nd co-doped with Ce and Cr. The ceramics were prepared by advanced ceramic techniques and showed more than 85% of transparency. Diode pumped laser oscillation performed at different output coupler transmissions (T) showed that a maximum slope efficiency of 16% was achieved for T = 3%. The cross energy transfer mechanism under violet laser diode pumping has been analyzed and the possible energy transfer mechanisms have been proposed using the various energy levels of the dopant ions. The emission lifetimes of the dopant ions at two different concentrations have also been found.     

多晶铈掺杂硅酸镥闪烁陶瓷的制备和发光性能

本工作对铈离子掺杂多晶硅酸镥(LSO:Ce)闪烁材料的制备方法进行了系统研究。将LSO:Ce前驱体溶胶喷雾干燥后得到了球形LSO:Ce前驱粉体, 该前驱粉体在1000℃和1100℃的温度下煅烧后分别得到了不同晶型的的单相LSO : Ce球形粉体。显微结构观察显示: 粉体颗粒的平均直径约为2 µm, 是由几十纳米大小的LSO:Ce纳米晶粒堆积而成。A型球形LSO:Ce粉体经1200℃/80MPa的放电等离子体烧结(SPS)后获得了平均晶粒尺寸为1.3 µm, 相对密度高达99.7%的LSO:Ce闪烁陶瓷。由A型球形LSO:Ce粉体压制的素坯在1650℃的空气气氛下烧结4 h后可获得相对密度达98.6%, 平均晶粒尺寸为1.6 μm的LSO:Ce陶瓷。该陶瓷经1650℃/150 MPa的热等静压(HIP)处理1 h后, 获得了相对密度为99.9%的半透明LSO:Ce闪烁陶瓷, 其平均晶粒尺寸为1.7 μm, 晶界干净。该LSO:Ce陶瓷的光产额可达28600 photons/MeV, 发光衰减时间为25 ns。     

The effect of MgO and SiO2 codoping on the properties of Nd:YAG transparent ceramic

Nd:YAG transparent ceramics were fabricated by a reactive sintering method under vacuum using SiO₂, MgO and compound additives (SiO₂ and MgO) as sintering aids. The effects of SiO₂ and MgO on the microstructure and sintering process of Nd:YAG ceramics were studied. High quality Nd:YAG ceramics with compound sintering aids obtained by vacuum sintering at 1780 °C are composed of grains of the size ∼10 μm, and their transmittance is 82% at 400 nm. It was found the absorption coefficient of 1.0 mol% Nd:YAG ceramic was 8.6 cm⁻¹ at 808 nm and its absorption cross section was calculated to be 6.26 × 10⁻²⁰ cm².     

Luminescence quenching processes in Gd2O2S:Pr,Ce scintillating ceramics

Temperature dependent radioluminescence under X-ray excitation (XRL) and luminescence decay time measurements following 430 nm laser excitation have been performed in the 10-775 K range on Gd₂O₂S:Pr³⁺,Ce³⁺ scintillating ceramics. From 200 K to both low and high temperature, XRL light yield decreases by 60%. High temperature luminescence quenching has been revisited. Temperature dependent lifetime measurements imply non-radiative de-excitation mechanism at electronic defects spatially correlated to Pr³⁺ emitting ions. At low temperatures, decreasing XRL light yield with irradiation time is linked to very intense thermoluminescence (TL) peak around 120 K ascribed to sulfur vacancies. These traps cause efficient electron trapping which competes with the prompt recombination mechanism.     

Simultaneous synthesis and densification of transparent,photoluminescent polycrystalline YAG by current activated pressure assisted densification (CAPAD)

We report a method for the synthesis and processing of transparent bulk polycrystalline yttrium aluminum garnet (YAG) and photoluminescent Ce-doped YAG ceramics via solid-state reactive-current activated pressure assisted densification (CAPAD). The process uses commercially available γ-Al₂O₃, Y₂O₃, and CeO₂ nanopowders. The nanopowders were reacted and densified simultaneously at temperatures between 850 °C and 1550 °C and at a maximum pressure of 105 MPa. The solid-state reaction to phase pure YAG occurs in under 4 min at processing temperatures 1100 °C which is significantly faster (on the order of tens of hours) and occurs at much lower temperatures (∼600 °C) compared to conventional reaction sintering. We found that the reaction significantly improves densification – the shrinkage rate of reaction-produced YAG was three times higher than that of YAG using pre-reacted powder. The Ce additions were found to retard the reaction driven shrinkage kinetics by a factor ∼3, but are still faster (by a factor ∼1.6) than those associated with direct densification (no synthesis). Densities >99% were achieved in both pure YAG and Ce doped YAG (Ce:YAG). Results of optical measurements show good transparency in the visible and photoluminescence (PL) in the Ce:YAG. The PL peak is broad and appears white when excited using blue light confirming that the ceramics can be used in solid state lighting to produce white light.     

Synthesis and dielectric characterization of Ba0.6Sr0.4TiO3 ferroelectric ceramics

Ferroelectric ceramics Ba_0.6Sr_0.4TiO₃ (BST 40) were prepared, by solid-state reaction in the temperature range 1210-1450 °C. Maximum values of the ceramic densities were around 94% of their theoretical value. X-ray diffraction techniques (XRD) and scanning electron spectroscopy (SEM) were used to analyze the structure and the surface morphology of ceramics. Rounded, well defined or abnormal granular growth was observed in the SEM images, vs. sintering conditions and purity of the raw materials. In all samples, BST 40 ceramic is the major phase, but there are also present small amounts of secondary phases, as revealed in XRD diffraction patterns. Permittivity and dielectric loss measurements were performed in the temperature range − 150 to + 150 °C, and 150 Hz-5 MHz frequency values. Permittivity values rising from 1200 to 12,500, with increasing sintering temperatures, were recorded. Narrow and well defined transition peaks were noticed at higher sintering temperatures. Curie temperature was around 2 °C, for samples with the mentioned composition. Permittivity and losses vs. frequency show different behavior whether BST ceramics are in polar or non-polar state and with the distance toward phase transition. Microwave measurements performed at room temperature have shown lower values of permittivity, compared with similar data at low frequency, and dielectric losses lower than 1% at 0.7 GHz. The sintering conditions (temperatures, sintering time, etc.) and purity of the raw materials lead to important changes of transition temperatures in the polymorphic diagram, which we have built—for the other Ba1−xSrxTiO3 compositions (x = 0.25-0.90) sintered at 1260 °C for 2 h.     

Effect of metal halide fluxes on the microstructure and luminescence of Y3Al5O12:Ce phosphors

Yellow-emitting Y_2.95Al₅O₁₂:0.05Ce (YAG:Ce) phosphor particles with high luminescence efficiency under 450 nm ultraviolet (UV) excitation were prepared by a heat treatment of submicrometer-sized oxide powder mixture in a reducing atmosphere at 1550 °C. Prior to the heat treatment, the oxide mixture was blended with 5 wt% of metal halides – BaF₂, BaCl₂, NaF, NaCl, and KF – as a flux. It was observed that YAG:Ce particles prepared with fluorine containing flux demonstrated spherical morphology, high photoluminescence properties, and diameters of 5–20 μm, whereas those prepared with chlorine were small-sized particles (≤5 μm) with relatively low intensity. The highest relative photoluminescence (PL) intensity (∼118%) was obtained for the samples prepared with 5% BaF₂. A set of rules was established for the formation of spherical YAG:Ce particles in the presence of this flux.     

热等静压烧结制备细晶粒Ce,Y:SrHfO3闪烁陶瓷

Ce:SrHfO₃陶瓷因具有高密度和高有效原子序数, 对高能射线具有很强的阻止能力。同时, Ce:SrHfO₃陶瓷还具有快衰减和高能量分辨率等优异的闪烁性能, 引起了研究人员的广泛关注。由于传统的烧结方法难以实现非立方结构Ce:SrHfO₃陶瓷的透明化, 本研究采用真空长时烧结和短时真空预烧结合热等静压烧结(Hot Isostatic Pressing, HIP)方法制备Ce,Y:SrHfO₃陶瓷。以金属氧化物和碳酸盐为原料, 1200 ℃下煅烧8 h可以获得平均粒径为152 nm的纯相Ce,Y:SrHfO₃粉体。1800 ℃真空烧结20 h获得平均晶粒尺寸为28.6 μm的不透明的Ce,Y:SrHfO₃陶瓷, 而两步烧结法可以制备光学透过率良好的Ce,Y:SrHfO₃陶瓷。本研究详细分析了陶瓷致密化过程中微结构的演变, 探究了预烧结温度对Ce,Y:SrHfO₃陶瓷密度、显微结构和光学透过率的影响。真空预烧(1500 ℃×2 h)结合HIP后处理(1800 ℃×3 h, 200 MPa Ar)所获得的Ce,Y:SrHfO₃陶瓷在800 nm处的最高直线透过率为21.6%, 平均晶粒尺寸仅为3.4 μm。在X射线激发下, Ce,Y:SrHfO₃陶瓷在400 nm处产生Ce³⁺ 5d-4f发射峰, 其XEL积分强度比商用锗酸铋(BGO)晶体高3.3倍, Ce,Y:SrHfO₃陶瓷在1 μs门宽下的光产额约为3700 ph/MeV。良好的光学和闪烁性能可以拓宽Ce,Y:SrHfO₃陶瓷在闪烁探测领域的应用。     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Two-step sintering of dense, nanostructural forsterite

This paper investigates a method for preparing the nanocrystalline forsterite dense ceramic via sintering the forsterite nanopowder. The two-step sintering (TSS) method has been applied to suppress the accelerated grain growth of forsterite nanopowder compacts. The effects of sintering parameters on the mechanical properties and the microstructure characteristics of the forsterite ceramic were studied. Results verified the applicability of this method to suppress the final stage of grain growth in the system. The grain size of the high density compacts (~ 98.5% TD) of the forsterite was 60-75 nm. The optimal hardness (940 Hv) and fracture toughness (3.61 MPa.m^1/2) of the prepared nanocrystalline forsterite were found to be higher than those of the currently available hydroxyapatite bioceramics. It was concluded that the two-step sintering method can be used to fabricate improved forsterite dense ceramics with desired bioactivity and mechanical properties that might be suitable for hard tissue repair and biomedical applications.     

Scintillation properties of Ce-doped LuLiF4 and LuScBO3

The crystals of 1 mol% Ce-doped LuLiF₄ (Ce:LLF) grown by the micro-pulling down (μ-PD) method and 1 mol% Ce-doped LuScBO₃ (Ce:LSBO) grown by the conventional Czochralski (Cz) method were examined for their scintillation properties. Ce:LLF and Ce:LSBO demonstrated ∼80% transparency at wavelengths longer than 300 and 400 nm, respectively. When excited by ²⁴¹Am α-ray to obtain radioactive luminescence spectra, Ce³⁺ 5d-4f emission peaks were detected at around 320 nm for Ce:LLF and at around 380 nm for Ce:LSBO. In Ce:LSBO, the host luminescence was also observed at 260 nm. By recording pulse height spectra under γ-ray irradiation, the absolute light yield of Ce:LLF and Ce:LSBO was measured to be 3600±400 and 4200±400 ph/MeV, respectively. Decay time kinetics was also investigated using a pulse X-ray equipped streak camera system. The main component of Ce:LLF was ∼320 ns and that of Ce:LSBO was ∼31 ns. In addition, the light yield non-proportionality and energy resolution against the γ-ray energy were evaluated.     

Formation of translucent hydroxyapatite ceramics by sintering in carbon dioxide atmospheres

Hydroxyapatite is used in a variety of clinical applications as a result of the apparent adherence to and mild reaction of bone and soft tissue to it owing to its structural similarity with bone mineral. Transparent hydroxyapatite has previously been fabricated by either or both of two methods; namely the application of pressure during sintering and/or the use of fine particle sized apatite prepared by either a sol-gel process or aqueous precipitation. Recently it has been shown that translucent carbonate hydroxyapatite may be formed by sintering nanocrystalline gels of carbonate hydroxyapatite in a wet carbon dioxide atmosphere. In this study we report for the first time that this atmosphere can be used to sinter microcrystalline powder compacts of hydroxyapatite to form translucent ceramics at ambient pressure. The effect of water partial pressure and sintering time at 1300°C on the optical transmission and microstructure of the ceramic was investigated. It was found that translucent ceramics were formed in all carbon dioxide atmospheres and that optical transmission varied with sintering time. Maximum transmission (13%) of 2 mm thick ceramic was obtained in materials sintered for four hours at 1300°C in a mixture of carbon dioxide containing water at a partial pressure of 4.6 kPa.     

Comparison of the optical and scintillation properties of Gd2O2S: Pr,Ce ceramics fabricated by hot pressing and pressureless sintering

Gd₂O₂S: Pr, Ce ceramics were successfully fabricated by hot pressing (HP) and pressureless sintering. Absorption spectra, phase composition and microstructure of Gd₂O₂S: Pr, Ce ceramics are presented. The intensity of X-ray excited luminescence (XEL) and photoluminescence (PL) are compared, respectively. The sample fabricated by pressureless sintering has almost the identical position of photopeak and shorter afterglow compared with the one fabricated by the hot pressing. The different afterglow of samples fabricated by different methods can be attributed to the presence of different quantity of electronic traps which is illustrated by the Thermoluminescence (TL) study.     

Transparent Lu2O3:Eu ceramics by sinter and HIP optimization

Evolution of porosity and microstructure was observed during densification of lutetium oxide ceramics doped with europium (Lu₂O₃:Eu) fabricated via vacuum sintering and hot isostatic pressing (HIP’ing). Nano-scale starting powder was uniaxially pressed and sintered under high vacuum at temperatures between 1575 and 1850 °C to obtain densities ranging between 94% and 99%, respectively. Sintered compacts were then subjected to 200 MPa argon gas at 1850 °C to reach full density. Vacuum sintering above 1650 °C led to rapid grain growth prior to densification, rendering the pores immobile. Sintering between 1600 and 1650 °C resulted in closed porosity yet a fine grain size to allow the pores to remain mobile during the subsequent HIP’ing step, resulting in a fully-dense highly transparent ceramic without the need for subsequent air anneal. Light yield performance was measured and Lu₂O₃:Eu showed ∼4 times higher light yield than commercially used scintillating glass indicating that this material has the potential to improve the performance of high energy radiography devices.     

A low-firing microwave dielectric material in Li2O-ZnO-Nb2O5 system

LiZnNbO₄ ceramic was fabricated by the conventional solid state reaction method and its microwave dielectric properties were reported for the first time. The phase structure, microstructure, and sintering behavior were also investigated. The LiZnNbO₄ ceramic could be well densified at around 950 °C and demonstrated high performance microwave dielectric properties with a low relative permittivity ~ 14.6, a high quality factor (resonant frequency/dielectric loss) ~ 47, 200 GHz (at 8.7 GHz), and a negative temperature coefficient of resonant frequency approzmiately −64.5 ppm/°C. The LiZnNbO₄ ceramic is chemically compatible with Ag electrode material at its sintering temperature. It can be a promising microwave dielectric material for low-temperature co-fired ceramic technology.     

Humidity control porous ceramics prepared from waste and porous materials

This study investigated the feasibility of generating humidity control porous ceramics by sintering a mixture of volcanic ash, weathered volcanic ash, and waste glass. The sintering conditions, including sintering time, sintering temperature, and various mixed portions of glass amendment, were tested. The final ceramics were analyzed for their pore distribution, pore volume, adsorption and desorption characteristics, and bending strength. The results indicate that the pore size of the volcanic ash ceramics tends to distribute in the range from 70 to 100 nm, whereas that of the weathered volcanic ash ceramics is in the range of 7-9 nm. The humidity controlling conditions of the ceramics take advantage of the inherent porosity of the natural materials. The bending strengths in both cases were all satisfying with the fractural intensity specification of ceramics (> 6 Mpa).     

Luminescence and scintillation properties of BaF2Ce transparent ceramic

Cerium doped Barium Fluoride (BaF₂Ce) transparent ceramic was fabricated and its luminescence and scintillation properties were studied. The photoluminescence shows the emission peaks at 310 nm and 323 nm and is related to the 5d-4f transitions in Ce³⁺ ion. Photo peak at 511 keV and 1274 keV were obtained with BaF₂Ce transparent ceramic for Na-22 radioisotopes. Energy resolution of 13.5% at 662 keV is calculated for the BaF₂Ce transparent ceramic. Light yield of 5100 photons/MeV was recorded for BaF₂Ce(0.2%) ceramic and is comparable to its single crystal counterpart. Scintillation decay time measurements shows fast component of 58 ns and a relatively slow component of 434 ns under 662 keV gamma excitation. The slower component in BaF₂Ce(0.2%) ceramic is about 200 ns faster than the STE emission in BaF₂ host and is associated with the dipole-dipole energy transfer from the host matrix to Ce³⁺ luminescence center.     

Eu-doped CsBr phosphor as a new optically-stimulable phosphor material for medical X-ray imaging sensor

CsBr phosphor ceramics doped with different luminescence centres such as In2O3, Eu2O3, EuCl3, SmCl3, TbCl3, GdCl3 or NdCl3 as the candidate for a new optically-stimulable phosphor for medical X-ray imaging sensor were prepared using a conventional ceramic fabrication process. It was found that X-ray-irradiated Eu-doped CsBr (CsBr:Eu) exhibited intense optically stimulated luminescence (OSL). The peak wavelength of the OSL emission and stimulation spectra of CsBr:Eu phosphor ceramic sample were 450 and 690 nm, respectively. The dependence of OSL properties on the conditions of preparation of phosphor ceramic samples, such as Eu concentration, sintering temperature and sintering time, were studied. The optimum preparation conditions were also studied. It was found that the OSL intensity of CsBr:Eu phosphor ceramics fabricated under optimum preparation conditions is higher than that of commercially available imaging plates using BaFBr:Eu.     

Crystal growth and scintillation properties of Ce and Eu doped LiSrAlF6

Ce and Eu doped LiSrAlF₆ (LiSAF) single crystals for the neutron detection with different dopant concentrations were grown by the micro-pulling-down method (μ-PD). In Ce:LiSAF, intense emission peaks due to Ce³⁺ 5d-4f transitions were observed at approximately 315 and 335 nm in photo- and α-ray induced radio-luminescence spectra. In case of Eu:LiSAFs, an intense emission peak at 375 nm due to Eu²⁺ 5d-4f transition was observed in the radio-luminescence spectra. The pulse height spectra and decay time profiles were measured under ²⁵²Cf neutron irradiation to examine the neutron response. The Ce 3% and Eu 2% doped LiSAF showed the highest light yield of 2860 ph/n with 19 ns main decay time component and 24,000 ph/n with 1610 ns.