Thermal stability of Mg_2Si_(0.4)Sn_(0.6) in inert gases and atomic-layer-deposited Al_2O_3 thin film as a protective coating

Mg_2Si_(1−x)Sn_x solid solutions are promising thermoelectric materials to be applied in vehicle waste-heat recovery. Their thermal stability issue, however, needs to be addressed before the materials can be applied in practical thermoelectric devices. In this work, we studied the crystal structure and chemical composition of Mg_2Si_(1−x)Sn_x in inert gas atmosphere up to 823 K. We found that the sample was oxidized even in high-purity inert gases. Although no obvious structural change has been found in the slightly oxidized sample, carrier concentration decreased significantly since oxidation creates Mg vacancies in the lattice. We demonstrated that an atomic-layer deposited Al_2O_3 coating can effectively protect Mg_2Si_(1−x)Sn_x from oxidation in inert gases and even in air. In addition, this Al_2O_3 thin film also provides in situ protection to the Sb-doped Mg_2Si_(1−x)Sn_x samples during the laser-flash measurement and therefore eliminates the measurement error that occurs in uncoated samples as a result of sample oxidation and graphite exfoliation issues

The characteristics of soil salinization effects on nitrogen mineralization and nitrification in upland fields

The influence of soil salinization on nitrogen (N) transformation is largely unknown, which impedes the reasonable management of N in saline fields. A comprehensive meta-analysis was thus conducted to evaluate the effects of salinity and relative soil physicochemical properties on net N mineralization and nitrification in upland soils. Results showed that effects of salinity on the net-N mineralization rate (Min) and nitrification rate (Nit) changed with the salinity level and incubation time. Generally, the inhibitory effect of salt on Min and Nit decreased gradually with incubation time. At 14–16 days of soil incubation, significant stimulatory effects on Min were observed in middle-level (ECe: 12–16 dS m-1) and high-level (ECe >16 dS m-1) saline soils, and on Nit in low-level (ECe: 4–12 dS m-1) saline soils. Regression analysis revealed that the effects of soil organic carbon (SOC), total N (TN), C/N, pH, and clay content on Min and Nit were closely related to salinity levels. Nit at 5–7 days of soil incubation first enhanced and then decreased with C/N increase, and the threshold value was 34.7. The effect of pH on Nit changed with salinity levels, and shifted from stimulation to inhibition with increasing pH. Min at 5–7 days of soil incubation in middle-level group first increased with increasing pH, and decreased when pH was higher than 8.1. Salinization deeply affected soil properties, which further influenced N turnover via alteration of the availability of substrates and microbial biomass and activities. Our findings suggest that the influence of salinity on soil N turnover closely related with salinity level, and salinity level should be considered fully when optimizing N management in saline upland fields

Thermal stability of Mg_2Si_(0.4)Sn_(0.6) in inert gases and atomic-layer-deposited Al_2O_3 thin film as a protective coating

Mg_2Si_(1−x)Sn_x solid solutions are promising thermoelectric materials to be applied in vehicle waste-heat recovery. Their thermal stability issue, however, needs to be addressed before the materials can be applied in practical thermoelectric devices. In this work, we studied the crystal structure and chemical composition of Mg_2Si_(1−x)Sn_x in inert gas atmosphere up to 823 K. We found that the sample was oxidized even in high-purity inert gases. Although no obvious structural change has been found in the slightly oxidized sample, carrier concentration decreased significantly since oxidation creates Mg vacancies in the lattice. We demonstrated that an atomic-layer deposited Al_2O_3 coating can effectively protect Mg_2Si_(1−x)Sn_x from oxidation in inert gases and even in air. In addition, this Al_2O_3 thin film also provides in situ protection to the Sb-doped Mg_2Si_(1−x)Sn_x samples during the laser-flash measurement and therefore eliminates the measurement error that occurs in uncoated samples as a result of sample oxidation and graphite exfoliation issues

Phase diagram of In–Co–Sb system and thermoelectric properties of In-containing skutterudites

In-containing skutterudites have long attracted much attention and debate partly due to the solubility limit issue of indium in CoSb_3. The isothermal section of the equilibrium phase diagram for the In–Co–Sb system at 873 K is proposed using knowledge of the related binary phase diagrams and experimental data, which explains the debated indium solubility that depends on Sb content. In this paper, a series of In-containing skutterudite samples (In_xCo_4Sb_(12−x/3) with x varying from 0.075 to 0.6 and In_(0.3)Co_(4−y)Sb_(11.9+y) with y changing from −0.20 to 0.20) are synthesized and characterized. X-ray analysis and scanning electron microscopy images indicate that, up to x = 0.27, single-phase skutterudites are obtained with lattice constant increasing with In fraction x. A fixed-composition skutterudite In_(0.27±0.01)Co_4Sb_(11.9) was determined for the Co-rich side of In–CoSb_3 which is in coexistence with liquid InSb and CoSb_2. Indium, like Ga, is expected, from DFT calculations, to form compound defects in In-containing skutterudites. However, relatively higher carrier concentrations of In-containing skutterudites compared to Ga-containing skutterudites indicate the existence of not fully charge-compensated compound defects, which can also be explained by DFT calculations. The net n-type carrier concentration that naturally forms from the complex defects is close to the optimum for thermoelectric performance, enabling a maximum zT of 1.2 for the fixed skutterudite composition In_(0.27)Co_4Sb_(11.9) at 750 K