Electrical conductivity measurements were conducted on porous ceramic bodies formed by uni-axial pressing of the calcined spinel powders and sintering in air at 1400 °C (chapter 2 - section 2.1). Electrical conductivity of MnMxCr2-xO4 series
measured in air showed increase in conductivity for most of substitutions at B site, but for MnFe0.1Cr1.9O4, whose conductivity was lower than for MnCr2O4. As most of
the MnMxCr2-xO4 spinels proved to be unstable to reduction, their conductivity was
determined in air (Figure 3-15 (a)). MnCr2O4 showed chemical stability to reduction,
as described in the previous subsection and the electrical conductivity was determined as a function of reduction time and also partial pressure of oxygen (pO2). The MgMxCr2-xO4 series showed higher conductivities in air than MnCr2O4, but
comparable with the conductivity of other MnMxCr2-xO4 spinels. The conductivity of
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and Cu both in air and reducing conditions, as observed in Figure 3-15 (b). The conductivity decreased in reducing conditions for all substituted cations, but the trend was similar with an increase in conductivity for Li and Cu doped samples. This observation is consistent with considering a formal increase of Cr cation charge induced by the aliovalent substitution (section 1.5).
The conductivity measured in air for MgFexCr2-xO4 (x = 0, 0.1, 0.5, 1) series
decreased with the increase of the iron content. The composition with x = 0.5 had a minimum value of 2x10-3 S∙cm-1 and for x = 1 conductivity increased again to 0.14
S∙cm-1. In reducing conditions, the conductivity increased with the iron content, as
shown in Figure 3-15 (c), where the maximum value corresponded to x = 1 with 0.4 S∙cm-1.
The other cations considered for x = 0.1 series were studied as well for a higher extent of substitution on B site and the results indicated forming of secondary phases for Li and Cu. A higher amount of Ga (x = 0.2) has been included on B site with formation of pure phase spinel. The influence of Ga on the electrical conductivity was negative in air and reducing atmosphere with decreasing the conductivity from MgCr2O4, as presented in Figure 3-15 (d). The plotted points represent conductivity
values after the apparent equilibration of the samples in air or reducing conditions (see Figure 5-1 and Figure 5-14).
93 b
94
d
Figure 3-15 Log(σ) vs. a) cell parameter of MnMxCr2-xO4; b) cell parameter of MgM0.1Cr1.9O4; c) the Fe
content of MgFexCr2-xO4; d) the Ga content of MgGaxCr2-xO4 in air and reducing conditions at 850 °C.
Electrical conductivity measurements showed p-type conductivity, with a decrease of the conductivity in reducing conditions. Each considered substitution had the same p-type electronic conductivity as for MnCr2O4 and MgCr2O4, while MnFeCrO4,
MgFeCrO4 and Mg1.5Ti0.5CrO4 developed n-type conductivity in reducing conditions.
Titanium containing samples showed n-type conductivity for low pO2 and long
reducing times, while Fe containing samples showed n-type conductivity for a higher amount of Fe content (x = 1) and the n-type response was almost instantaneous with switching the atmosphere from static air to 5%H2/Ar. The apparently equilibrated
data as a function of pO2 is presented in Figure 3-16 for MnCr2O4, Mg1.5Ti0.5CrO4 and
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Figure 3-16 Evolution of electrical conductivity of Mg1.5Ti0.5CrO4 and MgFeCrO4 and MnCr2O4 in
reducing conditions at 850 °C, with p-type conductivity of MnCr2O4 and n-type conductivity of
Mg1.5Ti0.5CrO4 and MgFeCrO4.
The conductivity is influenced directly by the cation distribution in spinels. The conduction mechanism proposed for Cr3+ cations at B site (chapter 1 - section 1.5.3)
anticipated that the reduction of these materials results in a decrease of the materials conductivity and this was confirmed by the experimental conductivity measurements. Substitution with Ga3+ on the B site facilitated the reduction of the spinel, with a
more pronounced decrease of the electrical conductivity in fuel testing conditions, while substitutions with Cu2+ or Li+ would induce a higher charge for Cr cations with
positive effects for the conductivity. The two spinel series MgMxCr2-xO4,
MnMxCr2-xO4 had their electrical properties influenced positively by Fe substitution at
the B site, increasing the conductivity when reduced. The phenomena was assumed to happen because of the formation of a conduction couple Fe2+/Fe3+ resulting from
the partial reduction of Fe3+ cations to Fe2+, increasing the total number of charge
carriers.
The variation of activation energy (Ea) values with temperature has been considered
for two temperature domains resulting two Ea values for each spinel (see Figure
96
(450 to 850 °C). These Ea values were plotted vs. the cell parameter for MgMxCr2-xO4
and MnMxCr2-xO4. The plot illustrates that Ea decreased with increasing temperature
for MgMxCr2-xO4, while Ea increased with temperature for MnMxCr2-xO4 and Ea
generally increased with the unit cell. There are some exceptions from the linear trend for each spinel series and both are connected to the substitution of Cr with Fe on the B site. The samples and the corresponding activation energies represented in Figure 3-17 correspond to the unit cell parameters listed in Table 3-3.
Figure 3-17 Activation energy dependence upon unit cell parameter and temperature in air of listed materials; the two marked positions correspond to MgFe0.5Cr1.5O4 (full squares), MgFeCrO4 (full square
red) and MnFe0.1Cr1.9O4 (empty squares).