【作者】 余石金；

【导师】 周东祥；

【作者基本信息】 华中科技大学 ， 材料物理与化学， 2009， 博士

【摘要】 溴化铊（TlBr）晶体具有高原子序数(Z_T1=81，Z_Br=35)、宽禁带（2．68eV）、高电阻率(10¹¹Ω·cm)、高密度（7．56g／cm³）等特点，是目前最有可能成为下一代核辐射探测器用的理想材料之一。TlBr核辐射探测器可在室温条件下工作，并且对X、γ射线有较高的探测效率和较好的能量分辨率，因此可广泛应用于天文物理学、高能物理学、核医学、安检、环境监测等领域。目前，国外关于T1Br的报道主要集中在晶体生长和探测器的研制方面，而对T1Br多晶原料的提纯和晶体退火处理尚末见深入细致的研究。杂质和缺陷导致生长态TlBr晶体质量不高，直接严重影响探测器的性能。在国内，尚末见关于TlBr材料研究方面的报道。本文采用水热重结晶法提纯TlBr多晶粉料，并对单晶体进行了热退火、气氛退火、水热溶液退火研究，制备出了高纯度、低缺陷密度的高质量TlBr晶体材料，为制备高性能TlBr探测器提供实验基础。因此，本文的研究具有重要的理论意义和现实的应用价值。本文采用水热重结晶法提纯TlBr原料粉体。根据TlBr溶解度的正温度系数，通过调整釜体上、下部溶液降温速率，确定了最佳的重结晶提纯工艺参数，取得了很好的提纯效果和提纯效率。经一次水热重结晶提纯后，粉体中杂质Ca、Fe、Mg、K、Zn、Cu、Na、Si的浓度大幅度地下降，提纯前后的浓度比值分别为67．26、28．05、26．76、18．78、11．08、8．69、5．86、5．40，提纯TlBr粉体纯度达到99．999％。系统地研究了各种热退火工艺参数（退火温度、升温速率、退火时间）对退火晶片光学性能的影响。退火过程中的升温速率从40℃／h增大到160℃／h，在晶体表面与晶体中心位置产生一个更大的指向晶体表面的温梯，此温梯将驱动晶内的位错和富Tl相向表面迁移，从而T₄₀₀₀、T₁₀₀₀、T_400-4000、T₅₀₀和T₇₅₀等几项重要指标均有明显提升；退火温度由200℃上升到320℃，晶体光学性能改善更明显，说明高温退火有利于晶内的缺陷向表面扩散迁移，同时参与热迁移的Tl沉淀相临界尺寸也会有所减小；退火时间从40h延长到200h，退火后晶片光学性能略有提升。晶片的氩气、氧气气氛退火过程表明，氧气气氛的退火效果较好，退火后晶片的平均红外光透过率T_400-4000为63．5％，紫外-可见光区透过率T₇₅₀为33．89％，均接近于理论值。氧气气氛退火过程中，氧原子可向晶内扩散至Tl沉淀相处，并将其氧化为Tl₂O，而Tl₂O具有较好的挥发性和在TlBr中很好的相溶性，从而减少了晶片内的Tl沉淀相密度，提升晶体的光学性能。晶片的水热溶液退火研究发现，水热溶液退火可有效地消除富Tl相，但效率较低。将热退火过程与水热溶液退火进行了结合，能快速有效地提高晶片的光学性能。退火晶片的透过率T₄₀₀₀、T₁₀₀₀、T_400-4000、T₅₀₀、T₇₅₀分别为58．56％、62．41％、60．89％、24．68％、34．94％，退火后晶片的电阻率在1．023×10¹¹Ωcm，满足制备TlBr探测器所需的高电阻率要求。更多还原

【Abstract】 Thallium bromide （TlBr） is one of the very promising materials used for next generation nuclear radiation detectors due to its high atomic number （Tl:81 and Br:35）, wide bandgap （2.68eV）, high resistivity (10¹¹Ω·cm) and high density（7.56g/cm³） . TlBr radiation detectors can operate at room temperature, and have high detection efficiency and energy resolution for X- and gamma rays. So TlBr detectors are widely used in X-ray astronomy, high-energy physics, nuclear medicine, safety inspection and environment monitoring.Presently, the studies oversea focus mostly on TlBr crystal growth and preparation of detectors. But TlBr powder purification and crystals modification have not been studied by the numbers. Impurities and defects in TlBr crystals result in poor quality which badly affects the performance of detectors. There is not publication on TlBr material research in our country. The aim of this paper is to purify TlBr powder by hydrothermal re-crystallization method, to get high quality TlBr crystals by the annealing at heat treatment, in atmosphere or under hydrothermal conditions, and to establish an experimental foundation for fabrication of high performance TlBr detectors.In this paper, hydrothermal re-crystallization method was used for TlBr powder purification. Based on the positive temperature coefficient of TlBr solubility, optimal parameters in re-crystallization process were ascertained through adjustment of the solution cooling rate in the autoclave, and good purification effect and high purification efficiency were obtained. After one time hydrothermal re-crystallization process, the impurities of Ca, Fe, Mg, K, Zn, Cu, Na and Si obviously decrease. The ratios of impurity concentration in raw material to that in purified powder are 67.26, 28.05, 26.76, 18.78, 11.08, 8.69, 5.86 and 5.40, respectively. The purity of purified TlBr powder is up to 99.999%.The influences of various annealing parameters （annealing temperature, heating rate, annealing time） on the optical properties were investigated. When heating rate changes from 40℃/h to 160℃/h in the annealing processes, there is a larger temperature gradient which point to the surface of wafers between the surface and the wafer center. The temperature gradient drove out the defects and Tl-rich phases from wafer inner to surface. Therefore, some important property indexes are improved evidently, such as T₄₀₀₀, T₁₀₀₀, T_400-4000, T₅₀₀, T₇₅₀. When annealing temperature changes from 200℃to 320℃, wafers’ optical properties can be improved significantly. It can be explained that high temperature is valid for detects diffusing and migrating to wafer surface, and the critical size of which Tl precipitates can migrate may reduce. With the annealing time prolongs from 40h to 200h, optical characteristics enhanced slightly.Wafers were annealed in argon, oxygen atmosphere. The annealing effect of oxygen is better, and the transmissions of T_400-4000 and T₇₅₀ are 63.5% and 33.89%, respectively, which are closed to the theoretical value. In the oxygen annealing process, oxygen atoms can diffuse to the Tl precipitation locality and Tl precipitation phases are oxidized to Tl₂O. Because Tl₂O has good volatility and is very soluble in TlBr, the concentration of Tl precipitation phase in annealed wafer is decreased and optical characteristics are upgraded.Hydrothermal annealing of wafers was investigated. Hydrothermal treatment can effectually eliminate the Tl-rich phase, but the efficiency is low. Thermal annealing process with a combination of hydrothermal annealing can improve wafers’ optical properties quickly and efficiently. Transmissions T₄₀₀₀, T₁₀₀₀, T_400-4000, T₅₀₀ and T₇₅₀ of the annealed wafer are 58.56%, 62.41%, 60.89%, 24.68% and 34.94%, respectively. The resistivity value of annealed wafer is 1.023×l0¹¹Ω·cm, which meet to the high-resistivity requirement for used as TlBr radiation detectors.更多还原