Preparation of macroporous solid foam from multi-walled carbon nanotubes by freeze-drying technique

Macroporous solid foams, made of multi-walled carbon nanotubes (MWCNTs), having a high porosity of 97% were produced using a freeze-drying technique. MWCNTs were dispersed in a carboxymethyl cellulose (CMC) sodium salt (surfactant) solution. Placed on a heat exchanger, the solution was frozen and lyophilized to obtain a dry sample. The prepared solid foam specimen had a macroporous structure, and its resulting pore sizes were mainly controlled by the initial formulation and freezing condition. A faster cooling rate led to a higher degree of homogeneity and a smaller macropore network in the bulk matrix, as did a higher surfactant concentration or a higher MWCNT content. The shapes of the macropores were quite similar even when the cooling rate and/or initial formulation were different. Interestingly, the electric resistivity of the solid foams, which depended on the preparation conditions, could rapidly respond to an ambient pressure change. It was found that the prepared macroporous solid foams displayed hybrid characteristics of the original MWCNTs and the resulting bulk matrix with a controlled macropore system. These results suggest that the prepared solid foams have high potential to be used as a gas sensing material.