## 3 Results
### Core-Shell Features and High-Defect Low-Density KDP Component
#### 3.2.2 Ultrafine High-Defect Low-Density KDP Component during CSP  
In addition to accommodating a core-shell morphology for the water-filled pore populations described above, a 3 rd ultrafine population of scattering features was needed to provide a complete microstructural scattering model for the data (see Figure S2 in the Supplementary Materials). Consistent among the in situ studies, especially at intermediate sintering times during CSP, this need can be associated with a feature in the USAXS/SAXS profile at q \(\approx\) 0.8 nm\({}^{-1}\) visible in the plots of Figures S3-S5 in the Supplementary Materials. The rise and fall of this component can also be correlated with an associated rise and fall in the scattering flat background, suggesting it may be a glassy or amorphous phase, as has been observed near grain boundaries in other systems undergoing CSP such as ZnO [4, 22, 24]. Due to the weak nature of this ultrafine feature above the scattering background, it was not possible to determine unambiguously the morphology of this component between sheet-like or needle-like features, or they may even have a dendritic fractal-like morphology. With some stochastics, a radius of gyration, R\({}_{\rm g}\), generally in the range of 5 nm to 10 nm was found for the fits. A cylindrical shape and a density one half that of fully dense KDP were assumed as a working model to evaluate evolution of this ultrafine component during CSP.  
With these working assumptions, Figure 11 presents the variations in volume fraction of this component with sintering time during the in situ CSP studies. Despite the fit uncertainties, maxima in the volume fraction of this ultrafine low-density phase (as indicated by arrows) are strongly suggested in all cases measured at 60 \({}^{\circ}\)C, and for both p = 100 MPa and p = 125 MPa at 50 \({}^{\circ}\)C (as well as at 60 \({}^{\circ}\)C). At 40 \({}^{\circ}\)C, any maximum volume fraction appears to occur at later times than those measured for p = 100 MPa or 125 MPa, but the maximum does occur within the in situ measurement time for the case of p = 150 MPa at 40 \({}^{\circ}\)C. Generally, the maximum volume fraction occurs at a shorter cold sintering time for higher temperature and/or greater compressive force. Note that the cold sintering time  
Figure 10: Shell volume fraction versus cold sintering time for 40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, 60 \({}^{\circ}\)C with p = 100 MPa and 125 MPa, and for p = 100 MPa, 125 MPa, 150 MPa at 60 \({}^{\circ}\)C. Lines are guide to the eye.  
Figure 9: Pore shell thickness versus cold sintering time for 40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, 60 \({}^{\circ}\)C with p = 100 MPa and 125 MPa, and for p = 100 MPa, 125 MPa, 150 MPa at 60 \({}^{\circ}\)C. Logarithmic line fits are guides to the eye.  
associated with the maximum in the ultrafine low-density component approximates to that at which the main decrease in volume fraction of low-density KDP shell around the pores is completed, suggesting some of the shell material may be isolated from the porosity as it closes off and densification progresses. Interpretation of this component as low-density KDP phase between sintering grains is reasonable in this context.