NaNO2 and KH2PO4: KH2PO4 (>99%) and

NaNO2 and KH2PO4: KH2PO4 (>99%) and NaNO2 (98%) were purchased from a commercial source. The raw powders were mixed with 7 wt% deionized water, ground using a pestle and mortar for 1–2 min and then uniaxially pressed under 350 MPa at 120°C for 1 min to 3 h in a steel die to achieve dense pellets (12.7 mm in diameter and 1–2 mm in height). The die was preheated at 120°C for more than 1 h before hot pressing. The as-prepared ceramic pellets were then baked at 120°C for 6–12 h to remove possible water residue. The densities were calculated by measuring the weight/geometry ratio of the ceramic bulks. Theoretical densities of 2.338 and 2.136 g/cm3 were, respectively, adopted for KH2PO4 and NaNO2.