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This paper reports a novel composite-based processing route for improving the electrical performance of Ca3Co4O9 thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca3Co4O9/xCo composites (x = 0%, 3%, 6% and 9% vol.) have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%–61% of ρth) reached maximum PF values of around 210 and 140 μWm−1·K−2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca3Co4O9 matrix. Although 2ST sintering resulted in rather dense samples (80% of ρth), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca3Co4O9 matrix (224 μWm−1·K−2 at 975K).

This work explores how, by redox-sensitive substitutions, the thermoelectric properties of oxides can be tuned and enhanced.

The energetics and phase behavior of the MIL-53­(Al) metal–organic framework upon low-temperature (15–260 K), subatmospheric H2 adsorption are studied experimentally using a volumetric technique and theoretically by grand canonical Monte Carlo simulation. The adsorption equilibrium data are recorded for a fixed amount of H2 in the system at stable increasing temperature steps starting from 15 K while recording the equilibrium pressure attained at each step. The adsorption isotherms are generated by repeating the experiments for different fixed amounts of adsorbate in the system and connecting the equilibrium points obtained at the same temperature. The solid–fluid interactions are modeled using the TraPPE-UA force field and the fluid–fluid interactions using a parametrization consistent with the same force field; quantum effects on H2 adsorption are taken into account via a quartic approximation of the Feynman–Hibbs variational approach. The use of a consistent force field with proven transferability of its parameters provides an accurate description of the experimental adsorption equilibria and isosteric heats of adsorption. Because of the weak solid–fluid interaction, the Henry constant for H2 adsorption in the large-pore (LP) form of MIL-53­(Al) surpasses that for H2 adsorption in the narrow-pore (NP) form at a temperature lower than that at which the dehydrated structure of the material collapses. However, the saturation capacity of the LP form is always higher than that of the NP phase. The phase behavior of MIL-53­(Al) upon temperature-induced H2 desorption is interpreted in terms of the osmotic thermodynamic theory. For the conditions spanned in the experiments MIL-53­(Al) exhibits at most a single structural transition and its phase behavior depends not only on pressure and temperature but also on the thermal history of the bare material.

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