## 3 Results and Analysis
### Thermo-Kinetic Analysis of W Skeleton Sintering  
The interfacial energy equation is  
\[\Delta(\gamma A)=\Delta\gamma A+\gamma\Delta A \tag{3}\]  
where \(\gamma A\) represents the total interfacial energy of the powder compaction, \(\gamma\) represents the specific surface (interfacial) energy, and \(A\) represents the total surface (interfacial) area of the compaction.  
The total interfacial energy can be divided into two components, where the variation in interfacial energy (\(\Delta\gamma\)) is caused by densification and the variation in interfacial area (\(\Delta A\)) is induced by grain coarsening. The reduction in the total interfacial energy leads to the sintering phenomena of densification and grain growth and serves as the thermodynamic driving force for sintering. The addition of a small amount of Ni to the W powder significantly enhanced densification. During sintering, the W skeleton particles grew in size, and the particle boundaries transitioned from sharp corners to more rounded shapes. In this process, the excess surface energy and lattice distortion energy within the W powder compact decreased; thus, the compact transitioned from a high-energy state to a low-energy state following sintering. Within the temperature range of 1100-1500 \({}^{\circ}\)C, grain boundary diffusion was the dominant mechanism for material transport during W sintering. The addition of Ni improves the wetting behaviour of Ag on W surfaces. The driving force of  
Figure 15: Physical properties of the AgW materials.  
sintering is the reduction in the system's surface energy [27]. At 1070 \({}^{\circ}\)C, the surface energy of the system decreased by 5% during the Ag infiltration.  
Other driving forces of sintering include the differences in volume pressure, vacancy concentration, and vapour pressure, which are induced by variations in particle surface curvature [28]. The pressure difference \(\Delta P\) is given as  
\[\Delta P\approx\frac{\gamma_{s}}{r} \tag{4}\]  
The difference in vacancy concentration \(\Delta C_{v}\) is given as  
\[\Delta C_{v}=C_{v,\infty}\frac{V_{m}^{\prime}}{RT}\frac{\gamma_{s}}{r} \tag{5}\]  
The vapour pressure difference \(\Delta P\) is given as  
\[\Delta P=p_{\infty}\frac{V_{m}}{RT}\frac{\gamma_{s}}{r} \tag{6}\]  
Here, \(\gamma_{s}\) represents the specific surface energy of the solid (solid surface energy), \(V_{m}^{\prime}\) represents the molar volume of vacancies, and \(V_{m}\) represents the molar volume of the solid. \(r\) represents the curvature radius of the sintered neck, \(R\) is the gas constant (8.3 J/mol\(\cdot\)K), and T represents the temperature. In Equations (4)-(6), the pressure difference, vacancy concentration difference, and vapour pressure difference are all inversely proportional to the curvature radius of the sintered neck. A smaller curvature radius leads to a larger sintering driving force, which facilitates material transport. Material transport is induced by the differing curvatures of the sintered neck interfaces. The flow of viscous substances/materials or atomic diffusion from grain boundaries reduces the particle spacing, leading to densification. Therefore, grain boundaries induce the densification and shrinkage of the W skeleton through material transport. The non-soluble sintering additives aggregate at the contact area between W particles, providing high diffusion pathways for W atoms, which reduces the activation energy for W transport. The acceleration of mass transport results in the rapid rounding of pores and grain growth. [PERSON] et al. [29; 30] attributed the activated sintering to enhanced grain boundary diffusion. [PERSON] [31] found that the rate of grain boundary diffusion in Ni-doped W was 500-5000 times higher than that in undoped W. The enhanced grain boundary diffusion is the reason for the activated sintering in Ni-doped W.