## 4 Discussion and Conclusions  
### Summary of In Situ CSP Results  
Using KDP as a model system, we have employed uniaxial compressive stresses and temperatures intended to bring about densification through CSP, but to do so at a rate sufficiently slowed so to permit aspects of the process to be interrogated during in situ USAXS/SAXS/WAXS studies of CSP. As to be expected, either higher temperature or application of greater compressive stress increases the densification rate, and also the final density achieved. With p = 100 MPa, temperatures of 40 \({}^{\circ}\)C or 50 \({}^{\circ}\)C may not be sufficient to achieve full density, but near full density does appear to be achievable at 60 \({}^{\circ}\)C. However, this may be at the cost of unwanted grain growth at higher temperatures. With p = 125 MPa, CSP at 40 \({}^{\circ}\)C still may not be able to achieve full densification, but this is possible at both 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C, and there is no significant accompanying grain coarsening. Densification by CSP at 60 \({}^{\circ}\)C with p = 150 MPa is more rapid and complete within the time regime studied, also without grain coarsening. Regardless of densification progress, and sometimes grain coarsening, as reflected in measurements of the main macroscale pore population, CSP clearly brings about changes at the nanoscale level. In almost all the in situ CSP runs studied, there is a reduction both in the nanoscale pore population volume fraction, and in the volume fraction of low-density KDP shell phase. (This is not wholly true for p = 100 MPa at 40 \({}^{\circ}\)C.) The ultrafine low-density KDP volume fraction appears to rise to a maximum and then subsides as overall CSP densification progresses. While specific X-ray scattering-length densities have been assumed for the model-fitting, it is reasonable to assume the true situation is more complex, with some densification of the low-density KDP shell phase as water is removed. As nanoscale pores are lost, some of the now dried shell material is isolated from the pore system, thus providing a source of increasing ultrafine low-density KDP phase. This is then densified by continued application of the compressive stress, causing it to merge with the fully dense KDP in the solid grains, and rendering it increasingly invisible to small-angle scattering (decreasing volume fraction after the maximum). This appearance and behavior of low-density, perhaps amorphous, dissolved and re-precipitated KDP would appear to be the underlying phenomenon enabling CSP that has been observed in other systems, sometimes more clearly when there is also a transient compositional variation [12,25]. The in situ WAXS (XRD) data reveals the presence of microstrain within the sample under compression, likely moderated by the distribution of transient low-density KDP phase, with the microstrain dissipating at later times as the amount of such high-defect, low-density, potentially soft, KDP phase is reduced. Finally, it should be pointed out how densification processes in KDP may be limited by the tendency to fracture along crystal facets, as observed in some  
Figure 12: FWHM in 20 of 200 XRD peak 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. Vertical bars represent the computed peak fitting standard uncertainties, and are mainly smaller than the data points, themselves.  
cases here, and probably attributable in part to the anisotropic elastic constants of KDP. As far as we are aware, this tendency is not reproduced in other materials suitable for CSP that have more isotropic mechanical properties.