## 3 Results
### Pore/Grain Morphology Evolution during CSP
#### 3.1.2 Fine (Nanoscale) Pore Population  
Figure 6 presents the variation with cold sintering time of the volume fraction of the lognormal pore component representing just the population of nanoscale pores. The uncertainties (vertical bars) represent estimated standard uncertainties again resulting from some stochastics in the CSP stage, but also somewhat more prominently here reflecting the uncertainties in extracting the nanoscale component volume fraction from the fits. This is also reflected in the data scatter of the points plotted in Figure 6. Note that the nanoscale pore volume fraction is typically 1% to 2% of the total pore volume fractions presented in Figure 4. As with the coarse macroscopic, hence total, pore volume fractions, the nanoscale volume fraction is assumed to include the shell layer of low-density KDP surrounding each pore at the pore/solid interface. Although showing more data-point scatter than seen in Figure 4, the general trends are similar. At 40 \({}^{\circ}\)C, the loss of nanoscale pores is negligible with p = 100 MPa, and only slight with p = 125 MPa. At 50 \({}^{\circ}\)C, there is some loss of nanoscale porosity with p = 100 MPa, but more with p = 125 MPa. At 60 \({}^{\circ}\)C, nanoscale porosity is removed at all 3 compressive stresses used. Once more, the loss of nanoscale pore volume at 60 \({}^{\circ}\)C with p = 150 MPa occurs too rapidly to be captured properly with the real-time resolution of these experiments.  
Figure 5: Coarse pore median diameter 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. Straight-line fits are simply guides to the eye. The 60 \({}^{\circ}\)C data for all 3 stresses are plotted out to just 50 min to show more clearly the different variations with time.  
Figure 6: Nanoscale pore 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. Straight-line fits are guides to the eye.  
Figure 7 presents the variation in nanoscale pore mean diameter versus cold sintering time. A globular pore shape is again assumed for simplicity but, in this case, the mean diameter refers to the core diameter of the pore, excluding the shell thickness of low density KDP material that is discussed further below. These trends also echo to some extent those observed for the macropore diameters presented in Figure 5. Note that CSP with p = 125 MPa or 150 MPa produces little or no significant coarsening of the nanopore pore population. With p = 100 MPa, significant coarsening of the nanoscale pore mean diameter is indicated at 50 \({}^{\circ}\)C and even more so at 60 \({}^{\circ}\)C, but only slight coarsening is observed at 40 \({}^{\circ}\)C. These results suggest that the nanoscale pore population, although identified as a distinct population from the main macropore population, is nevertheless influenced by grain coarsening effects in a similar way. One important distinction between the macropores and the nanopores is that the shell thickness of low-density KDP phase is negligible compared to the macropore dimensions but, for much of the CSP duration, it is significant compared to the core dimensions of the nanopores (as indicated below).