Bulk Iodoapatite Ceramic Densified by Spark Plasma Sintering with Exceptional Thermal Stability Tiankai Yao, ‡ Fengyuan Lu, ‡ Hongtao Sun, ‡ Jianwei Wang, § Rodney C. Ewing, ¶ and Jie Lian ‡,† ‡ Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 § Department of Geology & Geophysics, Center for Computation and Technology, Louisianan State University, Baton Rouge, Louisiana 70803 ¶ Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115 Iodoapatite powder prepared by high-energy ball milling is den- sified by Spark Plasma Sintering to ~96% theoretical density. X-ray Diffraction and First-principle Calculation indicates the sintered phase is iodine-deficient apatite with chemical compo- sition of Pb 9.85 (VO 4 ) 6 I 1.7 and iodine confinement over 8 wt%. Thermogravimetric analysis shows the bulk iodoapatite dis- plays exceptionally stability without iodine release until 670°C. The greatly improved iodine confinement can be attributed to the dense matrix upon rapid consolidation from highly acti- vated powders by mechanical attrition. I. Main Article A S a byproduct of uranium fission, radioactive I-129, has detrimental effect on both environment and human beings due to its involvement in metabolic process and extre- mely long half-life around 15.7 million yr. 1 Iodine is extre- mely mobile and highly volatile, unreactive with many silicate minerals and rocks, and no engineering barriers can be used to confine iodine. Therefore, the capture and immo- bilization of I-129 into a durable waste form that can retain its integrity in deep geological disposal environment is of particular importance for effective nuclear waste manage- ment. Glass waste form is currently adopted for its versatile incorporation of complex nuclear waste streams, 2 and recent composite glass waste forms were developed, based on Bi–Zn-oxide 3 and Bi–Si–Zn-oxide, 4 in confining iodine. However, the vitrification of conventional glass waste forms requires elevated temperature and relatively long processing duration, and thus is less desirable for volatile elements such as I-129, necessitating the development of new waste forms that can be processed and consolidated at relatively low tem- perature without or minimal iodine loss. Apatite is an earth abundant mineral, normally denoted as A 10 (BO 4 ) 6 C 2 , displaying extraordinary structural flexibility and crystal chemistry. A wide range of radionuclides can be incorporated into apatite structure through coupled cation and anion substitutions (e.g., A = Ca, Na, Pb, rare earth, fis- sion product elements, or actinides; B = P or V; C = F, Cl, I). Among these, Pb 10 (VO 4 ) 6 I 2 was proposed as a potential waste form for iodine immobilization due to its high chemi- cal durability evidenced by its mineral analogue, a measured rate of dissolution of 0.0025 g/m 2 d at pH 6 and 90°C, 5 and high iodine loading of around 8 wt% as inferred from its crystal structure. 5–7 However, it is a great challenge to consolidate lead vana- date iodoapatite into dense ceramics due to the highly vola- tile nature of iodine. Pb 10 (VO 4 ) 6 I 2 also experiences a phase decomposition to Pb 3 (VO 4 ) 2 at a low temperature less than 300°C, and a complete iodine loss occurred at ~400°C. 8 Con- ventionally, it was synthesized by confined reactive high-pres- sure sintering in which PbI 2 as the core part is encapsulated by Pb 3 (VO 4 ) 2 . The vanadinite not only acts as reaction reagent but also a barrier to prevent the release of iodine as the sintering temperature (700°C) is significantly higher than the melting point of PbI 2 (400°C). 5,9 Recently, microwave heating was used to fabricate bulk lead vanadate iodoapatite. However, highly dense pellets cannot be achieved and voids at the order of 10 lm dominated the microstructure of sin- tered pellets. 10 Spark plasma sintering (SPS) was also utilized to sinter Pb–P–V iodoapatite by both nonreactive and reac- tive sintering, 11 in which iodoapatite powders as the starting materials were synthesized by solid-state reaction and calci- nation at 720°C for 15 h in a sealed quartz ampoule. Ther- mal stability and iodine confinement were not reported on the SPS-densified pellets. Consolidation of iodoapatite into dense bulk form with minimal iodine loss is critical for the development of durable waste forms for iodine confinement. In this work, iodoapatite powders were fabricated by solid- state reaction at room temperature using high-energy ball milling (HEBM) of powders of the constituent chemicals 12 followed by a low-temperature annealing (200°C), in which the iodine can be well confined in the powders. HEBM greatly improve the sinter ability of the synthesized powder samples, and thus dense iodoapatite pellets with a relative density of 96% can be consolidated by spark plasma at 700°C at very short durations (several minutes). The low tempera- ture synthesis of iodoapatite and rapid consolidation by SPS greatly mitigate the iodine loss associated with synthesis and densification processes. The SPS-densified iodoapatite pellets shows exceptional thermal stability and no iodine release occurs until 670°C. Those features underline that SPS com- bining with HEBM is promising in fabricating advanced waste form materials with greatly improved thermal stability for incorporation of highly volatile element I-129. Lead vanadate iodoapatite powder were firstly prepared by HEBM (Fritsch, Pulverisette 7, Idar-Oberstein, Germany) of the stoichiometric quantity of constituent compounds including PbO (99.9% purity; Alfa Aesar, Ward Hill, MA), V 2 O 5 (98% purity; Sigma-Aldrich, St. Louis, MO), and PbI 2 (99% purity; ACROS, Fair Lawn, NJ) to target a chemical composition of Pb 10 (VO 4 ) 6 I 2 . The HEBM was performed in R. Ballarini—contributing editor Manuscript No. 35047. Received May 23, 2014; approved June 5, 2014. † Author to whom correspondence should be addressed. e-mail: lianj@rpi.edu 1 J. Am. Ceram. Soc., 1–4 (2014) DOI: 10.1111/jace.13101 © 2014 The American Ceramic Society J ournal Rapid Communication