(Press-News.org) The ability of materials to conduct heat is a concept that we are all familiar with from everyday life. The modern story of thermal transport dates back to 1822 when the brilliant French physicist Jean-Baptiste Joseph Fourier published his book "Théorie analytique de la chaleur" (The Analytic Theory of Heat), which became a corner stone of heat transport. He pointed out that the thermal conductivity, i.e., ratio of the heat flux to the temperature gradient is an intrinsic property of the material itself.
The advent of nanotechnology, where the rules of classical physics gradually fail as the dimensions shrink, is challenging Fourier's theory of heat in several ways. A paper published in ACS Nano and led by researchers from the Max Planck Institute for Polymer Research (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) at the campus of the Universitat Autònoma de Barcelona (UAB) (Spain) and the VTT Technical Research Centre of Finland (Finland) describes how the nanometre-scale topology and the chemical composition of the surface control the thermal conductivity of ultrathin silicon membranes. The work was funded by the European Project Membrane-based phonon engineering for energy harvesting (MERGING).
The results show that the thermal conductivity of silicon membranes thinner than 10 nm is 25 times lower than that of bulk crystalline silicon and is controlled to a large extent by the structure and the chemical composition of their surface. Combining state-of-the-art realistic atomistic modelling, sophisticated fabrication techniques, new measurement approaches and state-of-the-art parameter-free modelling, researchers unravelled the role of surface oxidation in determining the scattering of quantized lattice vibrations (phonons), which are the main heat carriers in silicon.
Both experiments and modelling showed that removing the native oxide improves the thermal conductivity of silicon nanostructures by almost a factor of two, while successive partial re-oxidation lowers it again. Large-scale molecular dynamics simulations with up to 1,000,000 atoms allowed the researchers to quantify the relative contributions to the reduction of the thermal conductivity arising from the presence of native SiO2 and from the dimensionality reduction evaluated for a model with perfectly specular surfaces.
Silicon is the material of choice for almost all electronic-related applications, where characteristic dimensions below 10 nm have been reached, e.g. in FinFET transistors, and heat dissipation control becomes essential for their optimum performance. While the lowering of thermal conductivity induced by oxide layers is detrimental to heat spread in nanoelectronic devices, it will turn useful for thermoelectric energy harvesting, where efficiency relies on avoiding heat exchange across the active part of the device.
The chemical nature of surfaces, therefore, emerges as a new key parameter for improving the performance of Si-based electronic and thermoelectric nanodevices, as well as of that of nanomechanical resonators (NEMS). This work opens new possibilities for novel thermal experiments and designs directed to manipulate heat at such scales.
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Scientists at The University of Manchester have made an important finding that could help develop an early test for kidney disease.
Dr Rachel Lennon from the Wellcome Trust Centre for Cell-Matrix Research has been studying why some people are more susceptible to kidney disease because of their race and gender.
She explains: "It's well known that impaired kidney function is more common in Afro-Caribbean individuals compared to those from a Caucasian background, and in men compared to women. However, the reasons for the difference in susceptibility are only just being ...
This news release is available in German.
The size of the neonatal skull is large relative to the dimensions of the birth canal in the female pelvis. This is the reason why childbirth is slower and more difficult in humans than in most other primates. Scientists from the Universities of Oslo and Vienna, identified adaptations in the morphology of the human body, which were unknown so far. The results of this new study appeared in the current edition of PNAS.
Upright walking and difficult birth
In hominids, upright walking evolved 4-5 million years ago. The ...
Is it possible to prevent mental health problems in higher education students? The answer is "yes" according to a team of psychologists from Loyola University Chicago who conducted a careful, systematic review of 103 universal interventions involving over 10,000 students enrolled in 2- and 4-year colleges and universities and graduate programs. The findings appear in the May 2015 issue of Prevention Science, published by Springer.
Researchers indicated that universal prevention interventions - that is, programs targeting general students, not just students who are at ...
Researchers from Plymouth University Peninsula Schools of Medicine and Dentistry are part of a team led by the University of Oxford, who have carried out new research that suggests mindfulness-based cognitive therapy (MBCT) could provide an alternative non-drug treatment for people who do not wish to continue long-term antidepressant treatment.
The results are published in "The Lancet".
The results come from the first ever large study to compare MBCT - structured training for the mind and body which aims to change the way people think and feel about their experiences ...
Fragments of asteroids regularly land on Earth as meteorites. If you examine such a find, you can see that it comprises millimetre-sized round stones, known as chondrules. These small particles are believed to be the original building blocks of the solar system. However, the research community has not previously been able to explain how the chondrules formed asteroids. A new study shows that asteroids were formed by capturing chondrules with the help of gravitational force.
"The chondrules are of exactly the right size to be slowed down by the gas that orbited the young ...
Researchers at the Institut Hospital del Mar d'Investigacions Mèdiques (IMIM) have identified a new way of treating colorectal cancer. In the study published in the journal Science Signaling, the team led by LLuís Espinosa, investigator of IMIM's research group into stem cells and cancer, have shown that inhibition of endosomal activity is a potential therapeutic strategy for the treatment of cancers with the BRAF mutated gene. This discovery is an important step in the personalisation of the treatment of colorectal cancer, as the presence of this mutation is ...
Bethesda, MD (April 22, 2015) -- The May issues of AGA's journals -- Clinical Gastroenterology and Hepatology and Gastroenterology -- highlight important research updates on the most deadly forms of liver disease. Here's what you need to know:
Researchers confirm that NAFLD worsens heart disease.
One specific cardiovascular disease risk factor -- psychological distress -- is linked to death from liver disease in a large, general population sample.
Improvements in cirrhosis care have contributed to a 41 percent decrease in inpatient mortality.
For access to any of ...
Researchers from North Carolina State University have discovered that electron spin brings a previously unknown degree of order to the high entropy alloy nickel iron chromium cobalt (NiFeCrCo) - and may play a role in giving the alloy its desirable properties.
"High entropy alloys have garnered a lot of attention over the past 10 years because they have remarkable properties," says Doug Irving, an associate professor of materials science and engineering at NC State and corresponding author of a paper describing the work. High entropy alloys are materials that consist ...
COLUMBUS, Ohio--An international research team is bringing a new weapon to bear against invasive earthworms.
The ongoing research project at The Ohio State University, the University of Alberta and Simon Fraser University uses statistical analysis to forecast one worm species' spread, in hopes of finding ways to curtail it.
Most recently, they've focused on the boreal forest of northern Alberta. No native worms live in the forest whatsoever; the region had been worm-free since the last ice age 11,000 years ago, until invasive European species began working their way ...
Alternative variations from popular artists' videos may reach an audience of millions, shows the new study from Finland's Aalto University.
Music is the most popular YouTube content by several measures, including video views and search activity. The world's first academic study on YouTube music consumption by Aalto University in Finland shows that one reason for its popularity lies in users' own video. People re-use original music by popular artists to create their own alternative video variations, which may reach an audience of millions and can be found alongside any ...
