## 2 Flash Sintering  
### FS Process and Mechanism  
Flash sintering was first documented for zirconia. This process is often accompanied by a flash of light. The initial study involved compacting 3 mol% yttria-stabilized zirconia (3 YSZ) powder into a dog-bone shape, which was subsequently suspended within a heating furnace using two platinum wires attached to the dog-bone sample. A constant DC voltage was applied to the sample while it was heated at a constant rate. Rapid sintering occurred when the electric field intensity exceeded the critical value of 40 V/cm. With the rise in electric field intensity from 60 V/cm, there was a corresponding drop in the onset temperature of flash sintering. At an electric field strength of 120 V/cm, the 3 YSZ sample could be sintered at a furnace temperature of 850 \({}^{\circ}\)C in a few seconds. Since the report, researchers have persistently engaged in discussions and investigations regarding the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sintering** & **Major Mechanism** & **Typical** & **Typical** & **Typical Process** & **Major Limitations** & **References** \\
**Technique** & **Major Limitations** & **(\({}^{\circ}\)C min\({}^{-1}\))** & **Time (min)** & **Advantages** & **Major Limitations** & **References** \\ \hline \multirow{6}{*}{MWS} & The materials & & & Uniform heating, low energy consumption, the ability to sinter complex-shaped samples. Samples exhibit uniform microstructure, high density, and excellent mechanical properties. & The equipment is complex, and the process requires microwave absorption properties of the materials or uses susceptors. & [36; 37; 50] \\  & The electric current & & & Low sintering temperature, fast speed, and high efficiency. Samples exhibit high density and fine grain size. & The equipment is complex and expensive, pressure-assisted, and has limitations in part dimension and geometries. & [31; 51] \\  & The electric current & & & & The sample was simple in shape and limited in size. The process usually requires expensive Pt electrodes and preheating, and depends strongly on the electrical characteristics of the material. & [52; 53] \\  & The electric current & & & & & \\  & generates Joule heating within the sample. Promotion of the ion diffusion depending on the material system. & & & & \\  & The electric current & & & & & \\  & The electric current & & & & & \\  & generates Joule heating within the sample. Promotion of the ion diffusion depending on the material system. & & & & \\  & The ion diffusion & & & & & \\  & The ion diffusion & & & & & \\  & depending on the material system. & & & & \\  & The electric current & & & & & \\  & generates Joule heating within the sample. Promotion of the ion diffusion depending on the material system. & & & & \\  & The ion diffusion & & & & & \\  & The ion diffusion & & & & \\  & The ion diffusion & & & & & \\  & The ion diffusion & &  
mechanism of FS; nevertheless, the exact mechanism remains controversial. The postulated mechanisms for this phenomenon based on experimental data and assumptions include:  
1. Joule heating and the resulting local overheating of grain boundaries: The electrical current flowing through the sample can produce Joule heating, and particle-to-particle contacts at the grain boundaries can lead to high local temperatures [42; 52; 54; 55]. 2. Nucleation of Frenkel pairs: The applied electric field can promote the nucleation of Frenkel pairs (vacancies and interstitials) with opposite charges (one is a hole; the other is an electron). These pairs, when stripped of their charge by the electric field, become electrically neutral. As a result, they can move freely within the grain, migrate into grain boundaries and pores under the influence of sintering pressure, and thus contribute to sintering. Concurrently, the movement of holes and electrons generates an electric current that further promotes sintering [55; 56; 57; 58]. 3.