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Sodium zirconium phosphate (labeled as NZP)-monazite-type (1-x)Sr0.5Zr2(PO4)3–xCePO4 (x = 0–1.0) composite ceramics, which were designed to simultaneously immobilize simulated fission nuclide Sr and variable valence actinide nuclide Ce, were in situ prepared by one-step microwave sintering technique. The feasibility of Sr/Ce co-immobilization was evaluated via an investigation on the phase evolution, microstructure, density, Vickers hardness, and chemical stability of the composite ceramics. The Ce valence state in the composite ceramics was further ascertained by X-ray photoelectron spectroscopy. It was shown that the Sr/Ce co-immobilized composite ceramics only consisted of Sr0.5Zr2(PO4)3 and CePO4 crystalline phases that were compatible well to each other. Sr and Ce were independently incorporated into Sr0.5Zr2(PO4)3 phase and CePO4 phase, respectively. The valence state of Ce in composite ceramics existed in trivalent state. And the existence of CePO4 phase caused the grain refinement and facilitated the densification of the composite ceramics. The composite samples all showed a highly uniform and dense microstructure, whose relative density was higher than 95% and Vickers hardness could attain 774 HV1. Importantly, the series of Sr0.5Zr2(PO4)3–CePO4 composite ceramics exhibited higher chemical stability than that of the monophase Sr0.5Zr2(PO4)3 or CePO4 ceramics, in which the normalized leaching rates of Sr and Ce were below 10−4 g·m−2·day−1 and 10−7 g·m−2·day−1 order of magnitude, respectively. The NZP-monazite-type composite ceramics has the potential to be a host for the disposal of high-level nuclear wastes containing multiple radionuclides. [ABSTRACT FROM AUTHOR]

Sodium Zirconium Phosphate (NaZr 2 (PO 4) 3 , hereinafter NZP) and monazite are both potential materials for immobilization of nuclear waste. In this work, novel (1- x)Sr 0.5 Zr 2 (PO 4) 3 - x NdPO 4 composite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) for simultaneously immobilizing the simulated fission product (FP) Sr and trivalent minor actinide (MA) Nd were prepared by one-step microwave sintering technique, in which Sr and Nd were immobilized into NZP and monazite type structures, respectively. The phase evolution and microstructure of the samples were investigated by X-ray diffraction (XRD), Raman, and backscattering scanning electron microscopy (BSE). The results showed that the expected composite ceramics were successfully obtained by one-step microwave sintering at 1050 °C for 2 h. The as-prepared samples consisted of Sr 0.5 Zr 2 (PO 4) 3 and NdPO 4 phases, and the content of the two phases varied regularly as x changed, generally conforming to the designed nominal chemical composition. Importantly, the composite ceramics presented the homogenous and dense microstructure. The relative density of the composite ceramics was more than 95%, meanwhile, the Vickers-hardness of the samples was higher than 600 MPa. It was indicated that NZP-monazite type composite ceramics could be a potential matrix for the simultaneous immobilization of actinide and fission product. [ABSTRACT FROM AUTHOR]

In this work, 0.6Sr 0.5 Zr 2 (PO 4) 3 -0.4Ce 1- x Nd x PO 4 (x = 0–1) multiphase ceramics were fabricated using a one-step microwave sintering technique, where Sr and Ce/Nd were utilized as the substitutes for the fission products 90Sr and actinide nuclides in high-level radionuclide wastes, respectively. The influences of Sr/Ce/Nd co-incorporation on the phase evolution, crystal structure, micromorphology, densification, and chemical stability of the as-prepared multiphase ceramic waste forms were systematically investigated. It was detected that the multiphase ceramics were composed of Sr 0.5 Zr 2 (PO 4) 3 (SrZP) phase and monazite phase regardless of x value, and (Ce, Nd)PO 4 monazite solid solution was formed with Ce/Nd substitution. Rietveld refinement results further verified the cell parameters of (Ce, Nd)PO 4 monazite solid solution regularly decreased with increasing Nd replacement, and the structural change of crystalline monazite did not affect the structure of SrZP phase. All samples presented a well-compacted and homogeneous microstructure, and their micromorphology did not change significantly with the replacement of Ce by Nd. Moreover, the relative density of the as-prepared multiphase ceramics all reached more than approximately 94.5 %, demonstrating excellent densification. Importantly, the good compatibility of SrZP and (Ce, Nd)PO 4 monazite phases brought out superior chemical stability. The normalized elemental leaching rate showed no significant change for different Ce/Nd substituted samples, whose normalized leaching rates of Sr, Ce, and Nd were about 10−4 g/(m2·d), 10−7 g/(m2·d), and 10−6 g/(m2·d) order of magnitude, respectively. It is demonstrated that SrZP-(Ce, Nd)PO 4 multiphase ceramics can be used as candidate materials for simultaneously immobilizing fission products and actinide nuclides of high-level nuclear wastes. [ABSTRACT FROM AUTHOR]

Abstract: The combustion reaction mechanism of the Ni-Ti-C-BN system was investigated by a combustion front quenching method. The results showed that the SHS reaction in the Ni-Ti-C-BN system starts with the formation of TiN x and TiB from the solid-state reaction between Ti and BN. Meanwhile, the solid-state reaction between Ni and Ti occurs at some locations to form Ni3Ti. Subsequently, the resultant TiN x reacts with Ni to form the Ni-Ti compounds such as NiTi and Ni3Ti. As the temperature increases further, Ni-Ti liquid appears between NiTi and Ni3Ti, and some B atoms that diffused away from BN could dissolve into the Ni-Ti liquid to form TiB2. The heat generated from these reactions promotes the further dissolution of the C, B and N atoms in the Ni-Ti liquid to form the Ni-Ti-B-N-C liquid. As the B, N and C atoms in the liquid become sufficiently supersaturated, precipitation of plenty of TiB2 and TiC x N y grains occurs. In addition, the reasons for the prior formation of TiB2 over TiC x N y and separated residence of the TiB2 and TiC x N y grains in the microstructure were discussed. [Copyright &y& Elsevier]

Abstract: In this paper, a simple way of fabricating TiC x N y –TiB2 ceramics through the combustion reaction of Ti, C and BN powder mixtures in an argon atmosphere is reported with an emphasis on the effects of the C/(C+N) ratio on the SHS reaction behaviors and mechanism. With the increase in the C/(C+N) ratio, the combustion temperature shows a zigzag variation behavior; the combustion wave velocity displays a similar variation tendency as did in the combustion temperature while the ignition delay time increases progressively. XRD results confirmed that TiC x N y –TiB2 could form in all the samples. Microstructural observations revealed that both TiC x N y and TiB2 grains had fine sizes of less than 1μm in the products when the C/(C+N) ratio was lower than 0.5. Based on the characterization of quenched samples, the formation mechanism of the titanium carbonitride is proposed. Namely, the formations of TiN0.3 and TiN are followed by the incorporation of C in TiN x to form the titanium carbonitride solid solution. [Copyright &y& Elsevier]

The dense NaZr 2 (PO 4) 3 (NZP) ceramics were successfully prepared by a microwave sintering process with x wt.% ZnO as a sintering aid (where x = 0, 0.5, 1.0, 2.0 and 3.0). The effects of ZnO additive on the crystal structure, microstructure and sintering behavior of as-prepared NZP ceramics were systematically investigated. The single NZP phase can be achieved in the wide temperature range of 900–1200 °C holding for 2 h by microwave-sintering technology. The addition of ZnO sintering aid would not noticeably change the crystal structure of NZP. Importantly, ZnO additive significantly promoted the densification and 1.0 wt% ZnO-added sample possessed the maximum relative density of 96.5% after sintering at 1100 °C, considerably higher than that of pure NZP sample. Besides, the Vickers hardness of the above sample could attain near 800 MPa, which is about four times as hard as the pure NZP ceramics without ZnO additive. It was suggested that the combination of microwave sintering with appropriate addition of ZnO sintering aid would provide a convenient and efficient method for rapid fabrication of dense NZP ceramics. [ABSTRACT FROM AUTHOR]