Thermoelectric properties of Higher Manganese Silicides

This work aims to cover a variety of aspects relating to the Higher Manganese Silicide (HMS) system, e.g. composites, substitutions, synthesis methods, and structural evolutions. The composites made of HMS-based compounds and nano-inclusions have been prepared via two different procedures, i.e. (i) solid state reaction, manually mixing, and hot pressing, or (ii) soft ball milling and reactive spark plasma sintering. The later approach has proved its effectiveness in preparing the multi-walled carbon nanotube (MWCNT)/HMS-based material composites mainly containing the HMS phases with a homogeneous distribution of MWCNTs. It was demonstrated that a fine distribution of the nano-inclusions played a crucial role in reducing thermal conductivity through enhancing phonon scattering in HMS-based materials, resulting in an improvement by about 20% for the maximum efficiency for the MWCNT/HMS-based material composite with 1.0 wt.-% MWCNTs. The substitution of molybdenum, tungsten, or silver at the Mn sites, and of germanium or aluminium at the Si sites has been studied for the HMS-based materials. The best thermoelectric efficiency among different Ge contents was achieved for the phase mixture of the non-stoichiometric composition MnSi1.75Ge0.02, which was then chosen to be the base material for further substitutions. No crucial modification of the electrical properties of the base material was observed, but large decreases of lattice thermal conductivity were achieved because of enhanced phonon scattering, with the highest reduction up to 25% for molybdenum substitution. The maximum figure of merit, ZT, value was approximately 0.40 for the material with 2 at.-% molybdenum substitution at the Mn sites. The maximum ZT values ranging from 0.31 to 0.42 have been achieved for various compositions prepared by mechanical alloying, mechanical milling and heat treating in conventional furnace, as well as by solid state reaction, which could possibly be improved by completely eliminating the side products. Subsequently, a simple and effective process was used to synthesize undoped HMS, involving ball milling in n-hexane under soft conditions to obtain homogeneous mixtures of constituting elements, and subsequent spark plasma sintering for a direct solid state reaction. The obtained fine particles after the milling process in n-hexane helped to improve the reaction rate later on, resulting in pure HMS materials. As a consequence, the maximum thermoelectric figure of merit obtained was 0.55 at 850 K, a high value for undoped HMS. Moreover, single crystals of HMS have been prepared using chemical vapor transport with very low yield, but their poor qualities resulted in low resolution in single crystal XRD. HMS-based materials including the ones with different Si/Mn atomic ratios and various dopants, e.g. Ge, Al, Cr, and Mo, have been prepared for the investigation of structural evolution upon heating up from room temperature to high temperature. The average structural formula at room temperature and its temperature dependence were strongly impacted by the phase compositions of the starting materials as well as the nature of dopants. Physical property measurements on the MnSi1.75 compound revealed that a correlation between the thermoelectric properties and the average structural formula of bulk HMS-based materials could be expected

Global age-sex-specific mortality, life expectancy, and population estimates in 204 countries and territories and 811 subnational locations, 1950–2021, and the impact of the COVID-19 pandemic: a comprehensive demographic analysis for the Global Burden of Disease Study 2021

Background: Estimates of demographic metrics are crucial to assess levels and trends of population health outcomes. The profound impact of the COVID-19 pandemic on populations worldwide has underscored the need for timely estimates to understand this unprecedented event within the context of long-term population health trends. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 provides new demographic estimates for 204 countries and territories and 811 additional subnational locations from 1950 to 2021, with a particular emphasis on changes in mortality and life expectancy that occurred during the 2020–21 COVID-19 pandemic period. Methods: 22 223 data sources from vital registration, sample registration, surveys, censuses, and other sources were used to estimate mortality, with a subset of these sources used exclusively to estimate excess mortality due to the COVID-19 pandemic. 2026 data sources were used for population estimation. Additional sources were used to estimate migration; the effects of the HIV epidemic; and demographic discontinuities due to conflicts, famines, natural disasters, and pandemics, which are used as inputs for estimating mortality and population. Spatiotemporal Gaussian process regression (ST-GPR) was used to generate under-5 mortality rates, which synthesised 30 763 location-years of vital registration and sample registration data, 1365 surveys and censuses, and 80 other sources. ST-GPR was also used to estimate adult mortality (between ages 15 and 59 years) based on information from 31 642 location-years of vital registration and sample registration data, 355 surveys and censuses, and 24 other sources. Estimates of child and adult mortality rates were then used to generate life tables with a relational model life table system. For countries with large HIV epidemics, life tables were adjusted using independent estimates of HIV-specific mortality generated via an epidemiological analysis of HIV prevalence surveys, antenatal clinic serosurveillance, and other data sources. Excess mortality due to the COVID-19 pandemic in 2020 and 2021 was determined by subtracting observed all-cause mortality (adjusted for late registration and mortality anomalies) from the mortality expected in the absence of the pandemic. Expected mortality was calculated based on historical trends using an ensemble of models. In location-years where all-cause mortality data were unavailable, we estimated excess mortality rates using a regression model with covariates pertaining to the pandemic. Population size was computed using a Bayesian hierarchical cohort component model. Life expectancy was calculated using age-specific mortality rates and standard demographic methods. Uncertainty intervals (UIs) were calculated for every metric using the 25th and 975th ordered values from a 1000-draw posterior distribution. Findings: Global all-cause mortality followed two distinct patterns over the study period: age-standardised mortality rates declined between 1950 and 2019 (a 62·8% [95% UI 60·5–65·1] decline), and increased during the COVID-19 pandemic period (2020–21; 5·1% [0·9–9·6] increase). In contrast with the overall reverse in mortality trends during the pandemic period, child mortality continued to decline, with 4·66 million (3·98–5·50) global deaths in children younger than 5 years in 2021 compared with 5·21 million (4·50–6·01) in 2019. An estimated 131 million (126–137) people died globally from all causes in 2020 and 2021 combined, of which 15·9 million (14·7–17·2) were due to the COVID-19 pandemic (measured by excess mortality, which includes deaths directly due to SARS-CoV-2 infection and those indirectly due to other social, economic, or behavioural changes associated with the pandemic). Excess mortality rates exceeded 150 deaths per 100 000 population during at least one year of the pandemic in 80 countries and territories, whereas 20 nations had a negative excess mortality rate in 2020 or 2021, indicating that all-cause mortality in these countries was lower during the pandemic than expected based on historical trends. Between 1950 and 2021, global life expectancy at birth increased by 22·7 years (20·8–24·8), from 49·0 years (46·7–51·3) to 71·7 years (70·9–72·5). Global life expectancy at birth declined by 1·6 years (1·0–2·2) between 2019 and 2021, reversing historical trends. An increase in life expectancy was only observed in 32 (15·7%) of 204 countries and territories between 2019 and 2021. The global population reached 7·89 billion (7·67–8·13) people in 2021, by which time 56 of 204 countries and territories had peaked and subsequently populations have declined. The largest proportion of population growth between 2020 and 2021 was in sub-Saharan Africa (39·5% [28·4–52·7]) and south Asia (26·3% [9·0–44·7]). From 2000 to 2021, the ratio of the population aged 65 years and older to the population aged younger than 15 years increased in 188 (92·2%) of 204 nations. Interpretation: Global adult mortality rates markedly increased during the COVID-19 pandemic in 2020 and 2021, reversing past decreasing trends, while child mortality rates continued to decline, albeit more slowly than in earlier years. Although COVID-19 had a substantial impact on many demographic indicators during the first 2 years of the pandemic, overall global health progress over the 72 years evaluated has been profound, with considerable improvements in mortality and life expectancy. Additionally, we observed a deceleration of global population growth since 2017, despite steady or increasing growth in lower-income countries, combined with a continued global shift of population age structures towards older ages. These demographic changes will likely present future challenges to health systems, economies, and societies. The comprehensive demographic estimates reported here will enable researchers, policy makers, health practitioners, and other key stakeholders to better understand and address the profound changes that have occurred in the global health landscape following the first 2 years of the COVID-19 pandemic, and longer-term trends beyond the pandemic

Admittance spectroscopy study of NiGe contact on GeSn

The direct gap semiconductor germanium tin (GeSn) is an attractive material for next-generation devices in nanoelectronics as well as in photovoltaic applications. However, its detailed electronic properties have not yet been clearly understood. Recently, admittance spectroscopy has become a popular analytical tool in materials research and development because it involves a relatively simple electrical measurement whose results may establish accurate characteristics of materials. The aim of this work is to study the effects of the annealing temperature on the electrical nature (rectifying or non-rectifying) of the metal contact by admittance spectroscopy. A numerical method, which is based on the solution of the basic semiconductor equations, is applied to simulate the material structure. From the calculation of microscopic quantities such as the modulated carrier concentrations and current densites, we can compute the theoretical admittance and impedance curves as function of frequency and external parameters (temperature and steady-state voltage) and then extract information on the electrical properties of the heterostructure. By a detailed investigation of the impact of microscopic parameters such as the dopant concentrations and the metal barrier height on the electrical characteristics, our objective is to understand the mechanisms of charge transport between the two electrodes

High efficiency Mg2(Si,Sn)-based thermoelectric materials: scale-up synthesis, functional homogeneity, and thermal stability

Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed. The synthesis procedure consists of a melting step followed by high energy ball milling. All the materials synthesized via this method demonstrated not only high functional homogeneity but also high electrical conductivity and Seebeck coefficients of around 1000 Ω−1 cm−1 and −200 μV K−1 at 773 K, respectively. The measured values were similar for all the samples extracted from the ∅50 mm and ∅70 mm compacted pellets and were stable upon thermal cycling. Thermal stability experiments from 168 hours to 720 hours at 723 K revealed no significant change in the material properties. The low thermal conductivity of ∼2.5 W m−1 K−1 at 773 K led to a maximum figure of merit, zTmax, of 1.3 at the same temperature and an average value, zTavg, of 0.9 between 300 K and 773 K, which enables high efficiency in future silicide-based thermoelectric generators

Solid solution formation in Mg2(Si,Sn) and shape of the miscibility gap

Investigation of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for further development of thermoelectric modules. There is a miscibility gap reported for the quasibinary Mg2Si–Mg2Sn series, though the exact compositions of its limits are disputed. Gaining a better understanding of intersolubility limits in Mg2(Si,Sn) is important for further optimization of material performance by exploiting the gap-induced phase segregation. For a better understanding on the boundaries of the miscibility gap below 700 °C, two approaches were taken to provide evidence of thermodynamic stable phases and, hereby, monitor the borders of the miscibility gap. The approaches cover the homogenization of Mg2SixSn1-x at 700 °C and diffusion couple experiments at 600 °C, 525 °C, and 450 °C. For 600 °C we find two ranges where Mg2Si and Mg2Sn are not miscible, namely x = 0.35 ± 0.05 and x = 0.75 ± 0.05 for miscibility gap I and 0.85 ± 0.05 < x < 0.95 ± 0.05 for the second gap. The deduced complex temperature dependence of the mixing / demixing behavior helps to understand the previously observed, apparently contradicting experimental results. We also show that there is no demixing for the compositions with the best thermoelectric properties at temperatures above 700 °C