Microstructure Control And Performance Optimization Of Gd₂O₂S:Tb Scintillation Ceramics

Scintillation material is a kind of functional material which can convert high-energy particles or rays into visible light.It plays an important role in high-energy physics,medical imaging,nondestructive testing and other fields.Gadolinium oxysulfide（Gd₂O₂S）has attracted much attention due to its excellent physicochemical stability,wide band gap structure,high X-ray stopping ability and high luminous efficiency.The preparation of Gd₂O₂S material is a trade secret.Siemens,Hitachi and Paul Scherrer Institut have prepared Gd₂O₂S ceramics with high application value.The basic performance requirements of Gd₂O₂S scintillators are less internal defects,high chemical purity and good uniformity of light output.In this study,the preparation process of Gd₂O₂S powder was optimized and the reaction mechanism of the precursor synthesized by hot bath method was explored.The factors affecting the performance of the precursor were determined.The effects of the introducing method of Tb³⁺,initial temperature of water bath and mole ratio n of Gd₂O₃and H₂SO₄on the properties of Gd₂O₂S:Tb powders and ceramics were studied.The main research work of this paper is as follows:（1）The precursor was synthesized from gadolinium oxide and sulfuric acid with the molar ratio of 1:1 under the condition of hot water bath.After calcination in air at400～1100℃,the precursor decomposed into Gd₂O₂SO₄.The Gd₂O₂S powder was obtained by reduction in H₂atmosphere.Then a new introducing method of Tb³⁺was proposed to improve the luminescent properties of Gd₂O₂S:Tb powders and ceramics,and the differences of the properties of the powders and ceramics prepared with Tb（NO₃）₃and Tb₄O₇were compared.The results show that the change of introducing method of Tb³⁺has no effect on the morphology of Gd₂O₂S:Tb powder,however,the powders prepared with Tb（NO₃）₃has higher XEL intensity.Then Gd₂O₂S:Tb scintillation ceramics were prepared by vacuum pre-sintering combined with hot isostatic pressing（HIP）post-treatment.The effects of introducing method of Tb³⁺on densification behavior,microstructure and luminescent properties of Gd₂O₂S:Tb ceramics were investigated.The results show that the Gd₂O₂S:Tb ceramics prepared with Tb（NO₃）₃have higher relative density,and the HIP-ed ceramics also have better luminescent properties due to the uniform doping of Tb³⁺in the ceramics.In addition,the XEL spectra and low temperature thermoluminescence spectra ranging from 77 K to 350 K show that the Gd₂O₂S:Tb ceramics prepared with Tb（NO₃）₃have good green luminescence thermal stability,and there are only thermoluminescence peaks centered at 90 K and 120 K in the ceramics,which may be caused by S vacancies.（2）The effects of initial temperatures of hot water bath on the microstructure and scintillation properties of Gd₂O₂S:Tb powders were studied,and the effects of vacuum pre-sintering temperature and initial temperature of water bath on the properties of Gd₂O₂S:Tb ceramics were investigated.The results show that the Gd₂O₂S:Tb powders prepared at different initial temperatures of water bath all present sheet structure.With the increase of initial temperature of water bath,the size and thickness of the sheet increase,and the agglomeration of the powders intensifies.The results show that the powders prepared at 50℃have the strongest XEL intensity.However,the optical quality and luminescent properties of Gd₂O₂S:Tb ceramics decrease with the increase of vacuum pre-sintering temperature.The results show that the powders prepared at the initial temperature of 7℃in water bath show high sintering activity.The Gd₂O₂S:Tb ceramics pre-sintered at 1250℃and HIP-ed at1450℃have the best optical quality and luminescence properties.The total transmittance of 1 mm thickness ceramics at 545 nm reaches 25.5%.However,there is a secondary phase in the ceramics,which is the reason for the poor optical quality of the ceramics.（3）The effects of molar ratio n of Gd₂O₃and H₂SO₄on the morphology and luminescent properties of Gd₂O₂S:Tb powders were studied,and the effects of n value on the densification behavior,microstructure and scintillation properties of the ceramics were also investigated.The results show that pure Gd₂O₂S:Tb powders can be obtained by calcining the precursors with different n values in air and reducing them with H₂,and the average grain size of the powders is about 30 nm.When n is less than 2,the reduced Gd₂O₂S:Tb powders have sheet structure,and the emission intensity is the same under X-ray excitation.When n is greater than or equal to 2,the reduced powder becomes massive,and the XEL intensity of Gd₂O₂S:Tb powder is significantly enhanced.XEL intensity of Gd₂O₂S:Tb powder with n equal to 3 is 88%of that of commercial micron powder.The results show that the powders with flake structure have faster densification rate after vacuum pre-sintering at 1300℃.The powders with bulk morphology have lower sintering activity,and the ceramics are difficult to be compacted after HIP post-treatment.With the increase of the molar ratio of Gd₂O₃to H₂SO₄,the content of Gd₂O₃in the ceramics decreases obviously,and the luminescence intensity of the ceramics under X-ray excitation tends to increase.