A series of deuterated potassium dihydrogen phosphate (DKDP) crystals with different degrees of deuteration are grown from aqueous solution by the point-seed technique. The microhardness of (100), (001) and so-called ‘tripler’ faces for these DKDP crystals were measured. Initially the hardness number of (001) face for each crystal increases with the increase of the applied load until it reaches 25 g. With further increase in load, the hardness number decreases gradually. The hardness numbers decline with the increase in deuterium content. These composition dependences are expected since the bond strength is weakened by the substitution of deuterium for hydrogen. The hydrogen bond is considered to play the key role in effecting the crystal’s hardness. The visible hardness anisotropy of the different faces is attributed to the inhomogeneous distribution of the oxygen–hydrogen bond on these faces.

In this paper, we investigated the effect of thermal-treatment under an Ar atmosphere on the structural evolution of silicon oxycarbide-derived carbons (SiOC-DCs) by adjusting the temperature from 1200 to 2100$^{\circ}$C, which will be characterized by means of N$_2$ adsorption, X-ray diffraction, Raman and transmission electron microscopy techniques, and studied their CO$_2$ capture performances. The results show that the structure of SiOC-DCs varied regularly with treatment temperature. The porosity and crystallinity of the as-received sample are almost stable when the thermal-treatment temperatureis <1500$^{\circ}$C. Subsequently, increasing the temperature (especially up to 1800$^{\circ}$C) will lead to an obvious improvement in the carbon crystallinity at the cost of pore structure breakage, which can be characterized by a quick decrease in the surface area and total pore volume of SiOC-DCs. Interestingly, the as-received SiOC-DC sample exhibits good CO$_2$ captureperformance at 0◦C under ambient pressure, up to 3.16 mmol g$^{−1}$. The thermal-treatment process under an Ar atmosphere in the range of 1200–1500$^{\circ}$C could further help in increasing the CO$_2$ adsorption ability by increasing the ultra-micropore ($d$ < 0.6 nm) volume.