The Poisson’s ratio of a control specimen was shown as about 0.11~0.13 when compressed at 5 to 25 % strain (see Fig. 5). For the experimental specimen, the Poisson’s ratios in accordance with the permanent volumetric compression with heat treatment (PVCT) ratio of 2.4 and 3.1 were shown as −0.07 and −0.05, respectively. The specimen of PVCT ratio of 2.4 showed more negative Poisson’s ratio than that of PVCT ratio of 3.1. Thus, extremely high compression ratio of the PVCT ratio of 3.1 can be considered as a factor to cause the shape of the pore structure to collapse. Therefore, PVCT ratio of 2.4 is considered as more effective ratio to form negative Poisson’s ratio.Fig. 5Poisson’s ratio variations for control and experimental specimens. The volumetric compression ratio was 2.4 and 3.1 (n = 4, *p < 0.05)