## 8 Phase-Contrast X-ray Imaging in Various Thermal Environments
The type of liquid is selected on the basis of the density of the sample and the experimental temperature. For example, the type of liquid can be water, normal saline, or another solution. Here, several observation results for icy materials and their phase changes are introduced. The visualization and identification of crystalline materials are useful for evaluating CXI performance since their density and temperature variations can be understood with high precision. These results demonstrate the advantage of CXI for investigating the phase change of materials because of its high sensitivity to density. One icy material that has been investigated is air hydrate, one of the clathrate hydrates that contain air (N\({}_{2}\), O\({}_{2}\), and other small amounts of gas molecules) in a cage structure composed of water [58]. The analysis of air hydrates is essential for reconstructing the climate over the last several hundred thousand years because the air hydrate in deep ice cores is unique in that it keeps paleo-atmospheric gases in water cages [59]. Figure 13a shows an image of an ice core sample taken at a depth of 1975.8 m from underneath Dome Fuji in Antarctica. Air bubbles in an ice core transform into single crystals of air hydrate over thousands of years. The CXI results show that there are many particles in the ice core sample; they correspond to the areas where the densities are higher than those of the surrounding ice (~0.01 g/cm\({}^{3}\)). XRD analysis has shown that the density of air hydrates is slightly higher than that of hexagonal ice [60]. This means that the particles in the ice core sample are air hydrate crystals [61]. Figure 13b shows the density difference between the air hydrate crystals and the surrounding ice crystals, plotted against the size of the air hydrate crystals estimated by spherical approximation using a voxel size of 18 \(\times\) 18 \(\times\) 18 \(\upmu\)m. The density difference is approximately 14 mg/cm\({}^{3}\), and the density of ice under isothermal conditions (0.923 g/cm\({}^{3}\)) gives an air hydrate density of 0.937 g/cm\({}^{3}\) \begin{table}
\begin{tabular}{c c c} \hline  & **CXI** & **Absorption-Contrast CT** \\ \hline Photon energy [keV] & 35 & 10 \\ Volumetric resolution & (0.013)\({}^{3}\) mm\({}^{3}\) & (0.0013)\({}^{3}\) mm\({}^{3}\) \\ Acquisition time & 0.5 h & 0.3 h \\ Identification of water and ice & Possible & Impossible \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison of CXI and conventional absorption-contrast CT for water and ice. Figure 12: Schematic of experimental setup for CXI under temperature-controlled conditions: (**a**) top view of STXI and (**b**) schematics of experimental setup for cryochamber. Figure 13: (**a**) 3D image of cylindrical ice core sample including air hydrates visualized using a CXI system. The dark gray circles in the image correspond to air hydrate crystals [61]. (**b**) Air hydrates in an ice core sample were visualized using an optical microscope for comparison. (**c**) Density difference between air hydrate crystals and surrounding ice crystals plotted against size of air hydrate crystals. The point considerably higher than the others is possibly not an air hydrate but another type of hydrate, such as a sulphate hydrate, CO\({}_{2}\) hydrate, or Ar hydrate. (Reprinted with permission from [57]. Copyright 2006, AIP Publishing). Tetrahydrofuran (THF) hydrates, which contain THF molecules as guests, form below 277 K even under atmospheric pressure. An in situ observation of THF hydrates using CXI clarified the inhomogeneous growth of THF hydrate crystals caused by the diffusion of THF solute [62]. Figure 14 shows temperature-dependent cross-sectional images of a THF hydrate sample in a cylindrical container, which enables in situ observation of THF growth and the water freeze-thaw process in the interparticle pore spaces using CXI [63]. The light gray circles correspond to the cross sections of polyethylene (density: \(\sim\)0.95 g/cm\({}^{3}\)) beads.