Molybdenum (Mo) based ceramic-metallic (cermet) fuel element is a promising fuel form used in nuclear thermal propulsion (NTP) because of its excellent advantages compared with the tungsten (W) based fuel. Mo, as a matrix material, plays a crucial role in containing the radioactive fission products released from the kernel and hindering the inward diffusion of hydrogen (H), thus reducing fuel loss. Therefore, in this study, the phase composition, microstructural evolution, grain growth kinetic, and mechanical properties of nanosized Mo powders fabricated by spark plasma sintering (SPS) are investigated in detail. The results demonstrate that Mo matrix consistently retains the body-centered cubic structure from 1000 to 1700 °C, with no phase transformation occurring. A relative density of 90% can be obtained at the sintering temperature of 1200 °C and >98% of theoretical density is achieved at 1700 °C under the external pressure of 50 MPa. Meanwhile, the average size of Mo grain grows to only about 1 μm from about 60 nm under 1700 °C due to rapid heating and cooling rate of SPS. Kinetic analysis indicates that the grain growth mechanism of nano-Mo powder is controlled by the surface diffusion with an activation energy of 414.16 KJ/mol. Mechanical properties illustrate that the hardness values of the Mo bulk is governed by the porosity fraction under low preparation temperatures (1000 to 1300 °C) and controlled by the Mo grain size at high preparation temperatures (1300 to 1700 °C). At 1300 °C, the corresponding Vickers hardness value is 316.6 Hv and the Rockwell hardness value is HRB 97. Moreover, Mo powders of different size sintered with various techniques are summarized and discussed. This work can enhance our understanding of the crucial role of advanced sintering techniques in combination with fine-grained powders for use in NTP systems.

中文翻译：

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核热推进放电等离子体烧结纳米钼粉的微观结构演变、晶粒生长动力学和机械性能


钼（Mo）基陶瓷金属（金属陶瓷）燃料元件由于与钨（W）基燃料相比具有优异的优势，是一种很有前景的核热推进（NTP）燃料形式。 Mo作为基体材料，在遏制堆芯释放的放射性裂变产物、阻碍氢（H）向内扩散、从而减少燃料损失方面发挥着至关重要的作用。因此，在本研究中，详细研究了通过放电等离子烧结（SPS）制备的纳米级Mo粉末的相组成、微观结构演变、晶粒生长动力学和机械性能。结果表明，Mo基体在1000至1700°C范围内始终保持体心立方结构，没有发生相变。在1200℃的烧结温度下可以获得90％的相对密度，在50MPa的外部压力下在1700℃下可以达到>98％的理论密度。同时，由于SPS的快速加热和冷却速率，Mo晶粒的平均尺寸在1700℃下从约60 nm生长到仅约1 μm。动力学分析表明，纳米Mo粉的晶粒长大机制受表面扩散控制，活化能为414.16 KJ/mol。机械性能表明，Mo 块体的硬度值在低制备温度（1000 至 1300 °C）下由孔隙率决定，在高制备温度（1300 至 1700 °C）下由 Mo 晶粒尺寸控制。在1300℃下，相应的维氏硬度值为316.6 Hv，洛氏硬度值为HRB 97。此外，还对采用各种工艺烧结的不同粒度的钼粉进行了总结和讨论。 这项工作可以增强我们对先进烧结技术与细粒粉末相结合在 NTP 系统中的关键作用的理解。