Electric-field-assisted processing of ceramics: Nonthermal effects and related mechanisms

Abstract

Field-assisted processing methods, such as spark plasma sintering and flash sintering, have considerably expanded the toolbox of ceramic engineering. Depending on the conditions, substantial electric currents may flow through the material resulting in fast heating rates due to Joule heating. Here, we focus on nonthermal effects induced by electric fields during processing of fluorite- and perovskite-based ceramics. The fundamentals of how a field can directly modify defect formation and migration in crystals are discussed. In addition, the interplay of ion transport and electrical conductivity is considered, this interplay being crucial to understanding nonthermal effects caused by electric fields (as in memristive switching). Electrochemical reactions leading to new phases or reduction are also described, as are densification rates and sintering parameters that are significantly affected even though the sample temperature is held constant. Finally, as grain-boundary properties and segregation are changed by ion transport, we describe how both retardation and acceleration of grain growth can be achieved including graded microstructures.

Shell-like structure to limit further densification formed during flash sintering under alternative current electric field for 8 mol %Y₂O₃-doped ZrO₂

“…In contrast, the relative density increases near linearly, in which the shrinkage-rate is well controlled by controlling compact electric current in SCF-sintering as previously reported. 14) The achieved density obtained by Archimedes method is 5.92 g/cm 3 , which is very close to the theoretical density of 8YSZ, 5.99 g/cm. 13) This achieved density is much higher than that obtained from flash sintering.…”

Research Progress on the Flash Sintering Mechanism of Oxide Ceramics and Its Application

“…Al2O3/YAG陶瓷基复合材料(见图7)，共晶片层的 厚度达到了亚微米尺寸，共晶区域的硬度和断裂韧 性可以与定向凝固技术获得的Al2O3/YAG共晶结构 相媲美 [50] 。在此基础之上，我们还通过临界电场的 作用将具有共晶成分的致密多晶Al2O3/YAG陶瓷基 复合材料转化成共晶结构，从而大幅提升材料的力 学性能 [51] 。此外，通过调控工艺参数，我们还成功 实现了Al2O3/YAG共晶结构的细化，使得陶瓷材料 也可以像金属材料一样通过晶粒细化来提升其力学 性能。 这种电场辅助下的共晶生长方法在Al2O3/YSZ 体系中也得到了验证, 通过闪烧Al2O3和8YSZ混合 粉体可以获得具有共晶形貌特征的Al2O3/YSZ陶瓷 [52] 。 图7 闪烧形成Al2O3/YAG共晶结构的SEM照片 [53] ，为陶 瓷塑性变形提供了新途径。我们已经在较低温度和 极低应力下成功实现了3YSZ陶瓷的快速拉伸和弯 曲变形(见图8)，为开展陶瓷塑性成形技术的研究 指出了一个新方向 [54] [57] ，这将对小尺寸(~厘米级)陶瓷样品的 制备产生颠覆性的影响 [55] ，同时也会对放电等离子 烧结技术的工业应用产生促进作用。 闪烧过程中产生的非焦耳热效更值得关注 [58] 。 本团队在自然科学基金重点项目支持下开展的陶瓷 闪焊技术、闪烧合成技术和电塑性成形技术的研究 已经充分证明：电场作用下产生的非焦耳热效应将 对陶瓷制备技术产生积极作用。加强对非焦耳热效 应微观机理的研究，将使我们对材料缺陷的认识达 到新的高度，对材料科学的发展会产生重大影响 [59] 。 利用闪烧过程产生的非焦耳热效应还可以创新陶瓷 材料的制备加工技术，提升陶瓷材料的性能 [51,60] ， 拓宽陶瓷材料的应用范围，对陶瓷工业的发展有重 要意义。 b a…”

Microstructure and conductivity of blacklight‐sintered TiO2, YSZ, and Li0.33La0.57TiO3

“…Alongside the high heating and cooling rates, 13 electric fields within the ceramic and/or reducing atmosphere are key contributors to defect creation in established rapid sintering techniques. 14 Avoiding currents in the sample or a reducing atmosphere will allow more control over the densification process and material properties. Therefore, blacklight sintering has great potential to provide material qualities that match or even exceed large-scale furnace sintering.…”

“…The modified defect chemistry ultimately translates to an impact of the processing technique on the behavior of the final material. Alongside the high heating and cooling rates, 13 electric fields within the ceramic and/or reducing atmosphere are key contributors to defect creation in established rapid sintering techniques 14 . Avoiding currents in the sample or a reducing atmosphere will allow more control over the densification process and material properties.…”