Non-Rigid Band Structure in Mg2Ge for Improved Thermoelectric Performance.

Magnesium silicide and its solid solutions are among the most attractive materials for thermoelectric generators in the temperature range of 500-800 K. However, while n-type Mg2(Si,Ge,Sn) materials show excellent thermoelectric performance, the corresponding p-type solid solutions are still inferior, mainly due to less favorable properties of the valence bands compared to the conduction bands. Here, Li doped Mg2Ge with a thermoelectric figure of merit zT of 0.5 at 700 K is reported, which is four times higher than that of p-type Mg2Si and double than that of p-type Mg2Sn. The reason for the excellent properties is an unusual temperature dependence of Seebeck coefficient and electrical conductivity compared to a standard highly doped semiconductor. The properties cannot be captured assuming a rigid band structure but well reproduced assuming two parabolic valence bands with a strong temperature dependent interband separation. According to the analysis, the difference in energy between the two bands decrease with temperature, leading to a band convergence at around 650 K and finally to an inversion of the band positions. The finding of a combination of a light and a heavy band that are non-rigid with temperature can pave the way for further optimization of p-type Mg2(Si,Ge,Sn).

Insight on the Interplay between Synthesis Conditions and Thermoelectric Properties of α-MgAgSb.

α-MgAgSb is a very promising thermoelectric material with excellent thermoelectric properties between room temperature and 300 °C, a range where few other thermoelectric materials show good performance. Previous reports rely on a two-step ball-milling process and/or time-consuming annealing. Aiming for a faster and scalable fabrication route, herein, we investigated other potential synthesis routes and their impact on the thermoelectric properties of α-MgAgSb. We started from a gas-atomized MgAg precursor and employed ball-milling only in the final mixing step. Direct comparison of high energy ball-milling and planetary ball-milling revealed that high energy ball milling already induced formation of MgAgSb, while planetary ball milling did not. This had a strong impact on the microstructure and secondary phase fraction, resulting in superior performance of the high energy ball milling route with an attractive average thermoelectric figure of merit of z T avg = 0.9. We also show that the formation of undesired secondary phases cannot be avoided by a modification of the sintering temperature after planetary ball milling, and discuss the influence of commonly observed secondary phases on the carrier mobility and on the thermoelectric properties of α-MgAgSb.

Application of High-Throughput Seebeck Microprobe Measurements on Thermoelectric Half-Heusler Thin Film Combinatorial Material Libraries.

In view of the variety and complexity of thermoelectric (TE) material systems, combinatorial approaches to materials development come to the fore for identifying new promising compounds. The success of this approach is related to the availability and reliability of high-throughput characterization methods for identifying interrelations between materials structures and properties within the composition spread libraries. A meaningful characterization starts with determination of the Seebeck coefficient as a major feature of TE materials. Its measurement, and hence the accuracy and detectability of promising material compositions, may be strongly affected by thermal and electrical measurement conditions. This work illustrates the interrelated effects of the substrate material, the layer thickness, and spatial property distributions of thin film composition spread libraries, which are studied experimentally by local thermopower scans by means of the Potential and Seebeck Microprobe (PSM). The study is complemented by numerical evaluation. Material libraries of the half-Heusler compound system Ti-Ni-Sn were deposited on selected substrates (Si, AlN, Al2O3) by magnetron sputtering. Assuming homogeneous properties of a film, significant decrease of the detected thermopower Sm can be expected on substrates with higher thermal conductivity, yielding an underestimation of materials thermopower between 15% and 50%, according to FEM (finite element methods) simulations. Thermally poor conducting substrates provide a better accuracy with thermopower underestimates lower than 8%, but suffer from a lower spatial resolution. According to FEM simulations, local scanning of sharp thermopower peaks on lowly conductive substrates is linked to an additional deviation of the measured thermopower of up to 70% compared to homogeneous films, which is 66% higher than for corresponding cases on substrates with higher thermal conductivity of this study.

Synthesis, crystal structure, atomic Hirshfeld surfaces, and physical properties of hexagonal CeMnNi4.

The hexagonal polymorph of CeMnNi(4) has been synthesized using cold crucible, high-frequency induction melting with subsequent Czochralski crystal pulling. Single-crystal X-ray diffraction, multitemperature synchrotron powder X-ray diffraction (90 to 600 K), and neutron powder diffraction data have been measured to establish the detailed crystal structure and in particular the location of the Mn atoms. The neutron diffraction data provide sufficient scattering contrast between Mn and Ni to establish that the 2c site has an occupancy of 13% Mn atoms, while the 3g site has an occupancy of 25% Mn atoms. Thus, the crystal structure is complex with considerable disorder. Rietveld refinement of the multitemperature synchrotron data establishes a near linear thermal expansion coefficient of 13.9(3) × 10(-6) K(-1) and 14.9(3) × 10(-6) K(-1) for the a and c axes, respectively. Atomic Hirshfeld surfaces are introduced as a new tool to investigate the atomic coordination and interactions in intermetallic compounds. The atomic displacement parameters (ADPs) are observed to be much larger for the heavy Ce atom than for the lighter Mn and Ni atoms, and this correlates with the large atomic Hirshfeld volume of Ce relative to Mn and Ni. The fit of a Debye model to the ADPs gives θ(D) = 312(3) K. Magnetic susceptibility data measured between 2 and 350 K indicate ferromagnetic ordering at 122(2) K (Weiss constant) based on a linear fit of the inverse magnetic susceptibility in the paramagnetic region. Transport properties were measured on a polycrystalline sample containing CeO(2) (2.8%) and Ni (7.7%) impurities. The electrical conductivity is observed to be metallic with a distinct kink in the data around 120 K coinciding with the observed Curie temperature. The lattice thermal conductivity (κ(L)) increases from 0.5 W/Km at 2 K to 8 W/Km at 50 K, and the relatively moderate value of κ(L) probably reflects the significant structural disorder.

Prophylactic nasal continuous positive airway pressure following cardiac surgery protects from postoperative pulmonary complications: a prospective, randomized, controlled trial in 500 patients.

BACKGROUND: Continuous positive airway pressure is a noninvasive respiratory support technique that may prevent pulmonary complications following cardiac surgery. This study was conducted to determine the efficacy of prophylactic nasal continuous positive airway pressure (nCPAP) compared with standard treatment. The primary end points were pulmonary adverse effects defined as hypoxemia (Pao(2)/fraction of inspired oxygen [Fio(2)] <100), pneumonia, and reintubation. The secondary end point was the readmission rate to the ICU or intermediate care unit (IMCU). METHODS: We prospectively randomized 500 patients scheduled for elective cardiac surgery. Following extubation either in the operating room (early) or in the ICU (late), patients were allocated to standard treatment (control) including 10 min of intermittent nCPAP at 10 cm H(2)O every 4 h or prophylactic nCPAP (study) at an airway pressure of 10 cm H(2)O for at least 6 h. RESULTS: Prophylactic nCPAP significantly improved arterial oxygenation (Pao(2)/Fio(2)) without altering heart rate and mean arterial BP. Pulmonary complications including hypoxemia (defined as Pao(2)/Fio(2) <100), pneumonia, and reintubation rate were reduced in study patients compared to controls (12 of 232 patients vs 25 of 236 patients, respectively; p = 0.03). The readmission rate to the ICU or IMCU was significantly lower in nCPAP-treated patients (7 of 232 patients vs 14 of 236 patients, respectively; p = 0.03). CONCLUSIONS: The long-term administration of prophylactic nCPAP following cardiac surgery improved arterial oxygenation, reduced the incidence of pulmonary complications including pneumonia and reintubation rate, and reduced readmission rate to the ICU or IMCU. Thus noninvasive respiratory support with nCPAP is a useful tool to reduce pulmonary morbidity following elective cardiac surgery.

Mumps, mumps vaccination, islet cell antibodies and the first manifestation of diabetes mellitus type I.

To connect mumps and diabetes mellitus in children is an old problem in medical literature. The typical occurrence of ICA at the onset of diabetes in children, as well as the incidence of ICA approximately 3 weeks after mumps infection support the hypothesis of a direct relationship between virus infection and diabetes. But the mumps infection alone is not the key factor. Mumps vaccination may not provide protection against diabetes mellitus, it may even provoke it. (Genetic determination, expressed by the HLA-phenotype in all the patients reported, does not allow a differentiation.)