## 3 Results
### Pore/Grain Morphology Evolution during CSP  
#### 3.1.1 Coarse (Macroscale) Pore Population and Densification  
Figure 4 presents the variation with cold sintering time of the combined volume fractions of the lognormal pore components representing the macroscopic coarse pores separating powder grains and the separate population of nanoscale pores near triple-point contacts, etc. (populations (i) and (ii) defined above). As indicated below, this total porosity is dominated by the macroscopic pores, which represent more than 98% of the pore volume. The rate and degree of reduction in total porosity (densification) is clearly a function of both temperature and compressive pressure, p. At 40 \({}^{\circ}\)C, loss of porosity and densification by CSP is only partially effective with either p = 100 MPa or p = 125 MPa. At 50 \({}^{\circ}\)C, this is still the case for p = 100 MPa, but densification is significantly more effective with p = 125 MPa applied compression. At 60 \({}^{\circ}\)C, densification appears to be effective at all 3 compressive stresses: p = 100 MPa, 125 MPa, and 150 MPa. (Additionally, see Figure 1) Indeed, densification at 60 \({}^{\circ}\)C for p = 150 MPa seems too fast to be captured properly with the real-time resolution of these experiments.  
Figure 5 presents the variation in coarse pore (macropore) median diameter versus cold sintering time. It should be emphasized that these median diameters have been derived from the lognormal size distributions for the coarse pore component fitted to the scattering data. Diameters greater than \(\approx\)10 \(\upmu\)m imply some significant extrapolation beyond the maximum size that can be measured directly from USAXS data but do emerge from the model fitting. Nevertheless, these variations of the derived coarse pore median diameter with cold sintering time can be used as a measure of how the representative pore diameter changes during CSP. (The median diameter is used because it is at least smaller than the mean diameter.) Note that coarsening of the median or mean pore diameter does not necessarily mean that individual pores become coarser, but more likely implies the loss of finer pores from the distribution due to sintering (and of course the porosities do decrease during sintering). Nevertheless, it is reasonable to associate the mean size of the remaining pores with the mean grain size. In this connection, it is noted that CSP with p = 125 MPa or 150 MPa produces no significant coarsening of the pore size distribution. With p = 100 MPa, significant coarsening of the median pore diameter is indicated at 50 \({}^{\circ}\)C and even more so at 60 \({}^{\circ}\)C, but there is no coarsening at 40 \({}^{\circ}\)C. Comparison of Figure 5 with Figure 4 indicates that significant loss of porosity and densification occurs in all cases (even if not always fully effective as discussed above). While globular pores have been assumed here for convenience, CSP, like other sintering processes [13; 24], seems to cause large pores to flatten and become more oblate with their larger dimensions remaining representative  
Figure 4: Total 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. Logarithmic line fits are guides to the eye. Vertical bars in this and subsequent figures (unless stated) are estimated standard uncertainties based on USAXS intensity calibration and residual stochastics in CSP densification still present using the custom-built CSP stage.  
of neighboring grain sizes that may or may not coarsen during sintering. The progressive flattening of the pore shape implies a reduction in pore volume, and hence densification of the sample. With KDP, there appears to be grain coarsening at higher temperatures (50 \({}^{\circ}\)C, 60 \({}^{\circ}\)C) for a lower compression (p = 100 MPa).