After DC and AC conductivity measurements, percolation parameters, universality, and time‐temperature‐concentration superposition are evaluated in biocomposites based on poly(lactide) loaded with multiwalled carbon nanotubes either carboxylic, PLA/c‐CNT, or pure but with a larger aspect ratio, PLA/LAR‐CNT, in weight concentrations up to 7%. The polymer matrix is either 100% amorphous or cold‐crystallized. Wide angle X‐ray scattering is used to extract the crystallinity at each temperature. The nucleating effect for the cold crystallization of the PLA is greater for the LAR‐CNT than for the carboxylic ones. DC and AC conductivities are measured in broad frequency and temperature ranges, from 1 mHz to 1 MHz and from 133 to 378 K, respectively. The critical percolation value increases for c‐CNTs (from 1.8 to 2.4 vol%) upon cold crystallization of the PLA matrix as the formation of a conductive path is hindered by the existence of a crystalline phase, which is dominant over the reduction of the amorphous regions. No significant changes upon crystallization were found in the critical concentration value (0.6 vol%) for the LAR composites, where the conductive path is 2D. The universal scaling found here for different concentrations is lost when temperature varies. However, time‐temperature superposition is present for each concentration of LAR or carboxylic CNTs. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers

Refractories containing magnesium aluminate spinel (MgAl2O4) are materials for emerging technology in several applications like cement and steelmaking processes. In order to deep the understanding of these castables, this work presents the microstructural characteristics of an alumina-magnesia/self-forming spinel castable. Several variables such as MgO content, firing temperature and spinel formation are analyzed through XRD and SEM analysis. The results showed that the processes of spinel formation and nucleation are not strongly affected by the MgO content, but that the crystal growth is enhanced for samples with 6.0 wt% MgO. Hibonite (CA6) bonding in the castable matrix showed a needlelike structure that could increase hot properties of the material. MgO content in the castable seems to affect the hibonite development. The development of a self-forming spinel castable without any synthetic spinel grains appears to be promissory for optimum refractory linings.

This study was carried out in conjunction with the investigation, reported in Part I, on the microstructural characteristics of an alumina-spinel castable with several percentages of MgO content. Bulk density and cold crushing strength of samples were evaluated dried and at three fired states (1000, 1200, 1400 °C). Results indicate little influence of MgO additions on physico-chemical properties of the alumina-magnesia/self-forming spinel castable. Characteristics compared with those reported for conventional alumina-spinel castables did not show large difference in values. Therefore, the alumina-magnesia/self-forming spinel castable could be a possible material for substitution of the conventional alumina-spinel castable.