Suppression of Interfacial Diffusion in Mg3Sb2 Thermoelectric Materials through an Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni-Graded Structure.

The Zintl compound, n-type Mg3Sb2, has been extensively investigated as a promising thermoelectric material. However, performance degradation caused by the loss of Mg element during device preparation and service is a main disadvantage in its utilization in thermoelectric devices. To suppress volatilization, diffusion, or reaction of Mg, we designed a graded concentration junction to control the interfacial elemental diffusion and improve the stability of the thermoelectric joint. We utilized the reaction product at the Ni/Mg3.2Sb2Y0.05 interface, the phase Mg4.3Sb3Ni, as a barrier layer material, and prepared Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni junctions. The results show that the interface behavior of the thermoelectric junction is optimized by the gradation of elemental concentration, thermal expansion coefficient, and work function. The Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni single-leg device showed high thermal stability at 673 K for 20 days, the contact resistance was stable at around 10 µΩ cm2, and the shear strength was maintained at about 20 MPa. The conversion efficiency of its single-leg device maintains nearly 90% of the best performance after aging at 673 K for 20 days.

Enhancing the zT Value of Bi-Doped Mg2Si0.6Sn0.4 Materials through Reduction of Bipolar Thermal Conductivity.

Solid solutions of Mg2Si0.6Sn0.4-xBix with 0 ≤ x ≤ 0.03 were prepared by a one-step synthesis and consolidation method, using MgH2 as a starting material. The thermoelectric properties of these samples were evaluated over the temperature range 300-775 K. Figure of merit, zT, values were determined over this range for all compositions and found to increase with temperature reaching a value of 1.36 at 775 K for samples with x = 0.02. Examination of the components of the total thermal conductivity showed that the bipolar thermal conductivity is suppressed by an increase in the band gap, resulting from solid solution formation, and by low minority carrier mobility. The suppression of the bipolar thermal conductivity is believed to be the consequence of charged grain boundaries.

1H-NMR measurements of proton mobility in nano-crystalline YSZ.

We report nuclear magnetic resonance (NMR) results on water saturated, dense, nano-crystalline YSZ samples (9.5 mol% yttria doped zirconia) which exhibit proton conductivity at temperatures as low as room temperature. (1)H-NMR spectra recorded under static and magic angle spinning conditions show two distinct signals. Their temperature-dependent behavior and their linewidths suggest that one can be attributed to (free) water adsorbed on the surface of the sample and the other one to mobile protons within the sample. This interpretation is supported by comparison with measurements on a single-crystalline sample. For the nano-crystalline samples motional narrowing is observed for the signal originating from protons in the sample interior. For these protons, the analysis of temperature and field dependent spin-lattice relaxation time T1 points towards diffusion in a confined two-dimensional geometry. We attribute this quasi two-dimensional motion to protons that are mobile along internal interfaces or nanopores of nano-crystalline YSZ.

On the conduction pathway for protons in nanocrystalline yttria-stabilized zirconia.

In this communication we elucidate a microstructural picture of proton conduction in nano-crystalline yttria-stabilized zirconia at low temperatures (Kim et al. Adv. Mater., 2008, 20, 556). Based on careful analysis of electrical impedance spectra obtained from samples with grain sizes of approximately 13 and approximately 100 nm under both wet and dry atmospheres over a wide range of temperatures (room temperature-500 degrees C), we were able to identify the pathway for proton conduction in this material. It was found that the grain boundaries in nano-crystalline yttria-stabilized zirconia are highly selective for ion transport, being conductive for proton transport but resistive for oxygen-ion transport.

Direct calorimetric measurement of grain boundary and surface enthalpies in yttria-stabilized zirconia.

We measured the change in enthalpy with grain size of a dense nanograined yttria-stabilized zirconia by oxide melt solution calorimetry and derived a grain boundary enthalpy, 0.73 +/- 0.19 J m(-2). Surface enthalpies of nanopowders are 2.21 +/- 0.14 J m(-2) (anhydrous surface) and 1.60 +/- 0.09 J m(-2) (hydrous surface). The grain boundary enthalpy is about a factor of two smaller than the enthalpy of the anhydrous surface, suggesting that densification which maintains nanosized grains is indeed thermodynamically driven. This is the first direct calorimetric measurement of grain boundary enthalpy in a ceramic.

Oxygen diffusion in nanocrystalline yttria-stabilized zirconia: the effect of grain boundaries.

The transport of oxygen in dense samples of yttria-stabilized zirconia (YSZ), of average grain size d approximately 50 nm, has been studied by means of 18O/16O exchange annealing and secondary ion mass spectrometry (SIMS). Oxygen diffusion coefficients (D*) and oxygen surface exchange coefficients (k*) were measured for temperatures 673

Simulation study of wave propagation instabilities for the combustion synthesis of transition metals aluminides.

Interest in the mode of propagation of self-sustaining reactions has been motivated by the influence of the mode on the microstructure and composition of the final product. However, comprehensive studies relating the onset of the various propagation modes to the chemical and phase transformations taking place in the sample are still lacking. In the present work propagation instabilities in self-propagating high-temperature synthesis (SHS) of transition metal aluminides are studied using a computer simulation approach. The results are presented for the SHS of NiAl, CoAl, TiAl, and NbAl(3). Particular emphasis is made with respect to the influence of process variables and system parameters on the onset of propagation instabilities, in relation to the physicochemical processes taking place during the propagation of the combustion front.