Exceptional thermoelectric performance in Mg_3Sb_(0.6)Bi_(1.4) for low-grade waste heat recovery

Bi_2Te_3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg_3Sb_(0.6)Bi_(1.4), with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi_2Te_3. This exceptional performance is achieved by tuning the alloy composition of Mg_3(Sb_(1−x)Bi_x)_2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg_3Bi_2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg_3Sb_(0.6)Bi_(1.4), after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg_3Sb_(0.6)Bi_(1.4) is a significant advancement towards sustainable heat recovery technology

Crystal growth behaviour of clathrate hydrate in flowing liquid water saturated with natural gas

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.This paper reports the visual observation on the growth and formation of clathrate hydrate crystals in the stream of flowing liquid water presaturated with simulated natural gas: methane + ethane + propane gas mixture. The composition of the methane + ethane + propane gas mixtures were 90:7:3 and 98.5:1.4:0.1 in molar ratio. The morphology (the geometric configuration of crystal such as crystal size or shape) of natural gas hydrate crystals grown in liquid water stream varied depending on the system’s subcooling temperature. The subcooling temperature is defined as the difference between equilibrium temperature of the hydrate and experimental temperature. At subcooling temperature larger than 6.0 K, the crystals grew from the porous pipe surface into the bulk of liquid water in both gas mixture systems. At a lower subcooling condition, polygonal flat plate crystals were observed. At ~9.0 K to ~12.0 K, the crystal morphology enters the transition phase, where the polygonal flat plate crystals started to change into dendritic crystals with increase in subcooling. When the subcooling temperature is greater than 12.0 K, polygonal crystals were completely replaced by the dendritic crystals.am201

Exceptional thermoelectric performance in Mg_3Sb_(0.6)Bi_(1.4) for low-grade waste heat recovery

Bi_2Te_3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg_3Sb_(0.6)Bi_(1.4), with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi_2Te_3. This exceptional performance is achieved by tuning the alloy composition of Mg_3(Sb_(1−x)Bi_x)_2. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg_3Bi_2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg_3Sb_(0.6)Bi_(1.4), after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg_3Sb_(0.6)Bi_(1.4) is a significant advancement towards sustainable heat recovery technology

Band engineering in Mg_3Sb_2 by alloying with Mg_3Bi_2 for enhanced thermoelectric performance

Mg_3Sb_2–Mg_3Bi_2 alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg_3Bi_2-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg_3Sb_2–Mg_3Bi_2 and also Mg_3Bi_2-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50% of the improvement. We find that Mg_3Bi_2 alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg_3Bi_2 because the peak zT in unalloyed Mg_3Sb_2 is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg_3Bi_2 alloying, we model the optimum Mg_3Bi_2 content for thermoelectrics to be in the range of 20–30%, consistent with the most commonly reported composition Mg_3Sb_(1.5)Bi_(0.5)

Improving the thermoelectric performance in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) by reducing excess Mg

The thermoelectric performance of Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal xfrom 0.2 to 0.01 in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01), leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01); however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg_(3.2)Sb_(1.5)Bi_(0.49)Te_(0.01)

Zintl compound n-type Mg_3(Sb,Bi)_2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material

Airway management in cardiac arrest -- not a question of choice but of quality?

This study presented an innovative method in order to estimate training required for skilful and successful intubations during ED cardiac arrests. Video reviews were taken from a system that routinely records ED staff during cardiac arrests and as these recordings are already part of everyday clinical practice, it is likely that there is minimal Hawthorne effect. Cardiac arrest research often reiterates the fact that the basics should be done well. It is commendable that intubations by the residents in this observational study resulted in a modest mean delay in chest compressions of only 8.6 seconds for the intubation attempt. However, nearly a third of intubation attempts were unsuccessful at the first attempt, and there were 11 oesophageal intubations (albeit they were all recognised) in the 93 patients that were included

Grain boundary dominated charge transport in Mg_3Sb_2-based compounds

Thermally activated mobility near room temperature is a signature of detrimental scattering that limits the efficiency and figure-of-merit zT in thermoelectric semiconductors. This effect has been observed dramatically in Mg_3Sb_2-based compounds, but also to a lesser extent in other thermoelectric compounds. Processing samples differently or adding impurities such that this effect is less noticeable produces materials with a higher zT. Experiments suggest that the behavior is related to grain boundaries, but impurity scattering has also been proposed. However, conventional models using Matthissen's rule are not able to explain the dramatic change in the temperature dependency of conductivity or drift mobility which is observed in Mg3Sb2-based compounds. We find that it is essential to consider the grain boundary region as an effectively separate phase rather than a scattering center, taking into account the weaker screening in semiconductors compared with classical metals. By modeling a grain boundary phase with a band offset, we successfully reproduce the experimentally observed conductivity versus temperature and thermopower versus conductivity relations, which indicate an improved description of transport. The model shows good agreement with measured grain size dependencies of conductivity, opening up avenues for quantitatively engineering materials with similar behavior. Model estimates predict room for >60% improvement in the room temperature zT of Mg_(3.2)Sb_(1.5)Bi_(0.49)Te_(0.01) if the grain boundary resistance could be eliminated

Mg3(Bi,Sb)2 single crystals towards high thermoelectric performance

The rapid growth of the thermoelectric cooler market makes the development of novel room temperature thermoelectric materials of great importance. Ternary n-type Mg3(Bi,Sb)2 alloys are promising alternatives to the state-of-the-art Bi2(Te,Se)3 alloys but grain boundary resistance is the most important limitation. n-type Mg3(Bi,Sb)2 single crystals with negligible grain boundaries are expected to have particularly high zT but have rarely been realized due to the demanding Mg-rich growth conditions required. Here, we report, for the first time, the thermoelectric properties of n-type Mg3(Bi,Sb)2 alloyed single crystals grown by a one-step Mg-flux method using sealed tantalum tubes. High weighted mobility ∼140 cm2 V−1 s−1 and a high zT of 0.82 at 315 K are achieved in Y-doped Mg3Bi1.25Sb0.75 single crystals. Through both experimental angle-resolved photoemission spectroscopy and theoretical calculations, we denote the origin of the high thermoelectric performance from a point of view of band widening effect and electronegativity, as well as the necessity to form high Bi/Sb ratio ternary Mg3(Bi,Sb)2 alloys. The present work paves the way for further development of Mg3(Bi,Sb)2 for near room temperature thermoelectric applications