Professonial

Energy conversion: Speeding up material discovery

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In even the most fuel-efficient cars, about 60 percent of the total energy of gasoline is lost through heat in the exhaust pipe and radiator. To combat this, researchers are developing new thermoelectic materials that can convert heat into electricity. These semiconducting materials could recirculate electricity back into the vehicle and improve fuel efficiency by up to 5 percent.

The challenge is, current thermoelectric materials for waste heat recovery are very expensive and time consuming to develop. One of the state of the art materials, made from a combination of hafnium and zirconium (elements most commonly used in nuclear reactors), took 15 years from its initial discovery to optimized performance.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an algorithm that can discover and optimize these materials in a matter of months, relying on solving quantum mechanical equations, without any experimental input.

“Semiconducting materials need to have very specific properties to work in this system, including high electrical conductivity, high thermopower, and low thermal conductivity, so that all that heat gets converted into electricity. Our goal was to find a new material that satisfies all the important properties for thermoelectric conversion while at the same time being stable and cheap.”

Kozinsky co-authored the research with Georgy Samsonidze, a research engineer at the Robert Bosch Research and Technology Center in Cambridge, MA, where both authors conducted most of the research.

In order to find such a material, the team developed an algorithm that can predict electronic transport properties of a material based only on the chemical elements used in the crystalline crystal. The key was to simplify the computational approach for electron-phonon scattering and to speed it up by about 10,000 times, compared to existing algorithms.

 

Using the improved algorithm, the researchers screened many possible crystal structures, including structures that had never been synthesized before. From those, Kozinsky and Samsonidze whittled the list down to several interesting candidates. Of those candidates, the researchers did further computational optimization and sent the top performers to the experimental team.

In an earlier effort experimentalists synthesized the top candidates suggested by these computations and found a material that was as efficient and as stable as previous thermoelectric materials but 10 times cheaper. The total time from initial screening to working devices: 15 months.

“We did in 15 months of computation and experimentation what took 15 years for previous materials to be optimized,” said Kozinsky. “What’s really exciting is that we’re probably not fully understanding the extent of the simplification yet. We could potentially make this method even faster and cheaper.”

Kozinsky said he hopes to improve the new methodology and use it to explore electronic transport in a wider class of new exotic materials such as topological insulators.

 

Harmful effects of being overweight underestimated

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The harmful effects of being overweight have been underestimated, according to a new study that analysed body mass index (BMI), health and mortality data in around 60,000 parents and their children, to establish how obesity actually influences risk of death.

Previous studies have suggested that the optimum BMI, at which the risk of death is minimised, appears to be above the range normally recommended by doctors, leading to claims it is good for health to be mildly overweight. However, scientists suspect these studies do not reflect the true effect of BMI on health, because early stages of illness, health-damaging behaviours, such as cigarette smoking, and other factors can lead to both lower BMI and increased risk of death. This makes it difficult to estimate how BMI actually influences risk of death (the causal effect), as opposed to the observed association between BMI and risk of death. This aim of this study was to assess the causal link between BMI and risk of death.

Using HUNT, a Norwegian population-based health cohort study based in a rural county with 130,000 residents, the Bristol Medical School team, with co-workers from the Norwegian University of Science and Technology, were able to see how mortality in the parents related to both their own BMI (the conventional approach) and to the BMI of their adult children. Because BMI of parents and their offspring is related, due to genetic factors, offspring BMI is an indicator of the BMI of the parents. The BMI of adult children is not influenced by illness among the parents, therefore using offspring BMI avoids the problems inherent in simply relating the BMI of the parents to their risk of death.

The health records of around 30,000 mother and child pairs and 30,000 father and child pairs were assessed to examine the extent to which BMI may influence mortality risk in a situation that is not biased by “reverse causation”illness leading to low BMI rather than BMI influencing illness.

The team found that when offspring BMI was used instead of the parent’s own BMI, the apparent harmful effects of low BMI were reduced and the harmful effects of high BMI were greater than those found in the conventional analyses. Importantly, the results suggest that previous studies have underestimated the harmful effects of being overweight.

The current advice from doctors to maintain a BMI of between 18.5 and 25 is supported by this study, and the widely reported suggestion that being overweight may be healthy is shown to be incorrect.

Dr David Carslake, the study’s lead author and Senior Research Associate from the MRC Integrative Epidemiology Unit (IEU) at the University of Bristol, said: “An alarming increase in obesity levels across the world which have risen from 105 million in 1975 to 641 million in 2014, according to a recent Lancet study, create concern about the implications for public health.

“This study demonstrates that correlation is not causation and that when it comes to public health recommendations we need to be cautious interpreting data based on associations alone. We found that previous studies have underestimated the impact of being overweight on mortality and our findings support current advice to maintain a BMI of between 18.5 and 25.”

Professor George Davey Smith, Director of the MRC IEU and Professor of Clinical Epidemiology at the University of Bristol, added: “We are used to seeing conflicting studies purporting to show that something is either good or bad for our health. These generally come from na├»ve observational studies, which can produce seriously misleading findings. More robust approaches for identifying the causal effects of factors influencing health, such as the methods applied in this study, are required if we are to make recommendations for public health based on reliable evidence.”