###### Abstract
In order to explore these issues, in situ ultra-small- and small-angle X-ray scattering (USAXS and SAXS) [15; 16] methods have been applied to characterize the evolving pore morphology during CSP in a model ceramic system: potassium di-phosphate or KH\({}_{2}\)PO\({}_{4}\) (KDP). USAXS and SAXS provide information on the pore/solid microstructure evolution while wide-angle X-ray scattering (WAXS) provides simultaneous structural information. Initial studies published previously [17], used an adapted compression stage. Its performance, with the applied stress and consequent sintering/densification somewhat stochastic in nature, was insufficient to provide results for systematically selected temperatures and stresses, but it was possible to capture various aspects of CSP. These early studies showed evidence of a core-shell void morphology over the early part of the sintering range, followed by a discrete regime in which excess pore fluid (water) was excluded, with a sharp transition from water saturated CSP to effective dry-state sintering. The present studies have been carried out in real-time under controlled conditions of temperature and uniaxial stress, utilizing a purpose-built cold sintering stage. The new stage with its optimized uniaxial stress control, together with a new feedstock particle morphology, has enabled CSP phenomena to be studied under a range of controlled stress and temperature conditions. The new sample morphology and a higher water content to aid the cold sintering have resulted in a significantly higher scattering background that masks the previous direct observation of core-shell pore features in the scattering. Nevertheless, the core-shell features can be modeled, based on residual variation of the scattering profile and an assumption of an equivalent shell thickness for both the nanoscale and macroscale pore populations. Due to the more efficient and smooth operation of the customized CSP stage, as well as the higher water content, the previously observed sharp transition from wet to dry conditions during densification is replaced by progressive removal of fluid as densification proceeds. KDP has been shown to densify readily under CSP conditions [5] even though it cannot be densified at conventional sintering temperatures since it decomposes well below those required for conventional sintering. However, with its relative molecular mass of 136.09 and theoretical density of 2.34 g/cm\({}^{3}\), use of KDP as a model material for CSP allows absorption and multiple scattering effects of the X-ray beam to be minimized for the available beam line X-ray energy while using a sample thickness sufficient to be compatible with the in situ uniaxial stress geometry requirements for CSP. In this paper, we describe in situ CSP studies using the new custom-built instrumented CSP stage. Results are presented after checking the CSP process visually using a lab-based video camera, through microstructure and structure evaluation using synchrotron-based SAXS and WAXS (XRD) measurements. Results are presented in regard to the evolving residual pore morphology as densification progresses for CSP experiments carried out at three different temperatures (40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, 60 \({}^{\circ}\)C) and two applied uniaxial compressive stresses, p (100 MPa and 125 MPa) with one additional experiment conducted at 60 \({}^{\circ}\)C with 150 MPa). The results are discussed both in terms of the temperature- and stress-dependence for CSP of KDP, and in terms of the more fundamental questions underlying CSP phenomena raised above, including the applicability of potential Arrhenius analysis to at least some aspects of CSP. The potential for extending these methods to a broader range of technologically applicable systems is also briefly considered.