|
||||||||||||||||||||||||||
|
||||||||||||||||||||||||||
|
Category: News
End of Year message.
In the year 2021, the pandemic continued to dominate our lives, but the vaccines that science provided us in record time are playing their part. Little by little, we are leaving our virtual life, resuming face-to-face activities to incorporate this new reality.
Unfortunately, our event this year still had to be virtual. Despite that, it was possible to feel the presence of each one of you on the screen!! With each work presented, with each question asked by a student, we felt reassured that science is still well represented in our country! This gives us hope for the future – much-needed hope in the face of the enormous challenges that lie ahead.
Carl Sagan said that we have to know the past to understand the present. And past and present show us that education and science are the main basis for a future with decent living conditions and social well-being. We hope that in 2022 we will continue to fight together for these values, resisting the denialism that still tries to remain present in our society. And that we can finally share our experiences – and our science – in Foz de Iguaçu!
We wish you an excellent end of the year to all, observing all the necessary sanitary care.😊
B-MRS Executive Board
(Português) CARTA CIRCULAR DE CONVOCAÇÃO DE ASSEMBLEIA GERAL ORDINÁRIA DA SOCIEDADE BRASILEIRA DE PESQUISA EM MATRIAIS SBPMAT.
Sorry, this entry is only available in Brazilian Portuguese.
B-MRS Newsletter. Year 8, issue 10.
|
|||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||
|
Featured paper: Solving the paradox of supercooled liquids.
Scientific research carried out at the Brazilian Federal University of São Carlos (UFSCar) sheds new light on supercooled liquids and glasses – two states of matter, in the broadest sense of the expression, which still present many fundamental questions to science. In particular, the work provides strong evidence to solve an old paradox involving supercooled liquids, and opens perspectives for the production of new glassy and crystalline materials.
Supercooled liquids are those that, even at temperatures below the melting point, remain in a liquid state. The best known example is water, which freezes at 0 °C but can be kept as a supercooled liquid even after a few hours in the freezer, as shown in this video.
But not just water, any liquid can be in a supercooled state as long as the conditions that prevent the formation of the first crystal, a phenomenon called nucleation, are met. However, if nucleation does occur, the delicate balance of the supercooled liquid will break down and it will crystallize into a much more stable state. To trigger this process, just waiting for a while is sufficient, stirring the supercooled liquid or introducing a catalyst into it.
In addition to arousing the curiosity of scientists and lay people, supercooled liquids have some applications in situations where it is necessary to lower the temperature to very low levels without causing freezing (crystallization), such as, for example, the preservation of organs donated for transplantation.
In this work, the authors sought to understand the interaction between two phenomena that concur during the crystallization process of supercooled liquids: relaxation (a phenomenon that occurs spontaneously in the amorphous structure of super-cooled liquids on their way to a phase of greater stability) and crystal nucleation. For this, they used atomistic computer simulation tools, that allows describing the position of each atom of a compound as a function of time, to simulate these processes in germanium, whose melting temperature is 938 °C. Above that temperature, germanium crystals “melts”. Below it, if the conditions that prevent the nucleation of crystals are maintained, the liquid germanium does not solidify and remains as a supercooled liquid.
It all started with a paradox
The idea of studying the interaction between nucleation and structural relaxation came from Professor Edgar Dutra Zanotto in 1987, when he was a young professor at UFSCar and coordinated the Vitreous Materials Laboratory, which he had created 10 years before.
It was then that Professor Zanotto began to study the Kauzmann paradox. Published in 1948, this theoretical prediction is named after Walter Kauzmann, who was a professor at Princeton University (USA) and made important contributions to the study of supercooled glasses and liquids. The paradox states that, at a given temperature (called the Kauzmann temperature), the entropy of a supercooled liquid must equal the entropy of the crystalline phase of the same compound. In this context, if cooling continued, the supercooled liquid would end up having zero entropy at a temperature above absolute zero. To avoid this situation, which contradicts the third law of Thermodynamics, supercooled liquids should crystallize before relaxing to the vitreous state, which is a non-crystalline state, above the Kauzmann temperature.
The dilemma aroused so much interest in Zanotto that he set out to investigate whether crystallization of supercooled liquids would occur in less time than structural relaxation. However, this was not an easy task (which is why the paradox persists) and required the mastery of specific computational tools. Thus, the work only started three decades later, when two post-doctoral students specializing in molecular dynamics simulation, Azat Tipeev and Leila Separdar, joined Professor Zanotto’s research group. The new members received co-orientation from Professor José Pedro Rino, also a specialist in the technique, who is a colleague of Zanotto at UFSCar and at the Center for Research, Technology and Education in Vitreous Materials (CeRTEV). While Azat was focusing over liquid germanium, Leila was working on the same problem with other substances. Some of the results of Leila’s work are published in this article in the journal Computational Materials Science.
“Molecular dynamics simulations allow the study of crystallization and relaxation at the atomistic level, in a region of states not yet attainable by laboratory experiments, to obtain essential information about the properties of tiny crystal nuclei in an extremely short time scale and, consequently, testing nucleation and relaxation theories,” explains post-doc Azat, of Russian nationality, who met Professor Zanotto in 2012 at an event on crystallization of glass and liquids in Germany and came for the first time to Brazil in 2015 to participate in the Advanced School of Glass and Vitroceramics organized by Zanotto with funding from FAPESP.
Based on the simulations, the authors determined the structural relaxation and stresses times and compared them with the formation times of the first crystal nucleus at different temperatures. “We found that these curves intersected at the so-called kinetic spinodal temperature, establishing a temperature region where the (strong) interference of relaxation in nucleation must be considered by theoretical models to adequately describe the dynamics of experimental nucleation,” summarize the authors.
Furthermore, the work provided solid evidence for the resolution of the Kauzmann paradox. “Our work demonstrated that the supercooled germanium liquid crystallizes before reaching the Kauzmann temperature, avoiding the entropy catastrophe,” says Azat, who is the first author of the article reporting this research in the journal Acta Materialia.
The new articles co-authored by Azat, Leila and Pedro Rino are part of the vast scientific production that Professor Zanotto and his collaborators have in the area of glass materials. “The crossing of relaxation and nucleation times above the Kauzmann temperature is significantly important to clarify the processes and dynamics of vitrification and crystallization and the very nature of the glassy state,” says Zanotto.
The work was carried out with funding from FAPESP.
Scientific paper reference: Unveiling relaxation and crystal nucleation interplay in supercooled germanium liquid. Azat O. Tipeev, José P. Rino, Edgar D. Zanotto. Acta Materialia. Volume 220, November 2021, 117303. https://doi.org/10.1016/j.actamat.2021.117303.
Author contact: Edgar Dutra Zanotto – dedz@ufscar.br
(Português) Carta dos coordenadores da Área de Física e Astronomia da CAPES sobre o pedido de demissão.
Sorry, this entry is only available in Brazilian Portuguese.
B-MRS Newsletter. Year 8, issue 9.
|
||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||
|
Founding member of B-MRS is elected fellow of TWAS.
Professor Daniel Mario Ugarte (UNICAMP), founding member of B-MRS, was sworn in as a fellow of TWAS (The World Academy of Sciences) in a virtual ceremony held on November 4 within the 15th TWAS general conference.
The election of Ugarte as a member, in the field of Physics, took place in 2019, but the inauguration ceremony was postponed due to the Covid-19 pandemic.
TWAS fellows are scientists whose contributions to science meet international standards of excellence and who work in or promote research in developing countries.
More information on TWAS member election: https://twas.org/directory/regulations.
Featured paper: Optimizing nanotubes for green hydrogen production.
A work carried out in institutions from the state of Pernambuco (Brazil) contributes to the development of nanomaterials with the potential to overcome an important energy challenge: the generation of hydrogen through sustainable processes. In fact, the hydrogen molecule is considered a clean fuel because its use, or “burning”, does not emit greenhouse gases. However, the production of this molecule is responsible for emitting hundreds of tons of carbon dioxide per year.
Fortunately, more sustainable ways to produce molecular hydrogen are being explored by scientists around the world. The “greenest” of all are the photoelectrochemical processes, which consist of breaking down the water molecule (H2O) using electricity from photovoltaic conversion (the transformation of photons into electrons). These processes are carried out in photoelectrochemical cells – simple and low-cost systems basically composed of a photoanode, where sunlight is absorbed, thus generating a current of electrons, and a cathode, on whose surface the hydrogen detaches from the water molecule by the action of the electricity generated in the photoanode. In this context, it is essential to develop materials for the photoanode that are efficient and durable, and which can be produced using low-cost and environmentally friendly processes.
In an article recently published in the Journal of Power Sources (impact factor 9.1270), scientists from the Center for Strategic Technologies of the Northeast (CETENE) and the Federal University of Pernambuco (UFPE) report a simple and clean method to produce nanocomposites capable of generating an electrical current from sunlight. The work also presents good results in the application of the material as a photoanode for hydrogen production.
Challenge: increase photoanode sensitivity
Titanium dioxide (TiO2) is the most used material in photoanodes. Unlike other semiconductors, it is non-toxic and does not degrade easily in contact with light and water. However, this material has a limitation that affects its efficiency: it can only absorb ultraviolet light, not taking advantage of other radiations present in sunlight. For this reason, scientific efforts have been made to expand the sensitivity of TiO2. This was exactly the objective of the CETENE and UFPE team at the beginning of the collaborative work. The strategy they adopted was to integrate semiconductor nanocrystals (quantum dots) to TiO2 nanotubes and, in this way, obtain a material that is more sensitive to sunlight thanks to the synergistic action of both semiconductors.
The researchers started by sensitizing the nanotubes with bismuth sulfide (Bi2S3) nanocrystals, says Denilson V. Freitas, who now works as a researcher at CETENE and has participated in this research line since the beginning, when he was doing his doctorate in Chemistry at UFPE. In the experiments, the scientists noticed that the method of preparing the nanocomposite significantly impacted its photoelectrochemical performance, and reported these results in an article published in 2018 in ACS Applied Energy Materials (impact factor 6.024). “We found that the best photoelectrochemical results were for the linker-assisted method when compared to the hydrothermal method,” says Denilson. Thus, the first method was chosen. In
linker-assisted sensitization, TiO2 nanotubes, supported on titanium sheets, are submerged in a solution containing the chosen nanocrystals. Both materials interact and, at the end of the process, the quantum dots are adsorbed on the surface of the nanotubes.
The second phase of the research was carried out within the Master’s Degree project in Materials Science by Danilo A.P. Velásquez, carried out at UFPE. This time, the scientific team used silver, indium and selenium nanocrystals (AgIn5Se8) with the main objective of determining what would be the optimal submersion time of the nanotubes in the solution, as the researchers had noticed that high concentrations of nanocrystals on the surface of the nanotubes affected in a negative way the performance of the nanocomposite. For this, they performed a series of experiments varying the submersion time between 1 hour and 48 hours.
In addition to observing through electron microscopy techniques the concentration of nanocrystals obtained in each case, the researchers checked the performance of each sample. The results showed that the photoletrochemical performance of nanotubes improved with increasing sensitization time up to 24 hours of immersion, when the obtained nanocomposite generated a photocurrent 2.4 times greater than that of pure nanotubes. Furthermore, the optimized nanotubes also improved their performance in hydrogen production, which was 3.1 times greater than that of the material without quantum dots. The experiments also demonstrated that, after 24 hours of immersion, the concentration of nanocrystals became excessive and impaired the functionality of the nanocomposite. “The work showed that the temporal optimization of the sensitization of nanotubes is an important step in the production of more efficient systems,” summarizes Denilson.
The research was carried out by researchers and students linked to postgraduate programs in Chemistry and Materials Science at UFPE, coordinated by professor Marcelo Navarro, and to CETENE, led by researcher and director of the center Giovanna Machado. The acquisition of images of nanotubes sensitized by high resolution transmission electron microscopy were performed at SENAI-MG. The works were funded by Brazilian research funding agencies CNPq, FACEPE, CAPES and Finep.
Scientific article reference: Boosting the performance of TiO2 nanotubes with ecofriendly AgIn5Se8 quantum dots for photoelectrochemical hydrogen generation. Danilo A. P.Velásquez, Felipe L. N.Sousa, Thiago A. S. Soares, Anderson J. Caires, Denilson V. Freitas, Marcelo Navarro, Giovanna Machado. Journal of Power Sources. Volume 506, 15 September 2021, 230165. https://doi.org/10.1016/j.jpowsour.2021.230165.
Contact of the corresponding author: Giovanna Machado – giovanna.machado@cetene.gov.br.
(Português) Manifesto de coordenadores da Área de Materiais da CAPES pela retomada da avaliação.
Sorry, this entry is only available in Brazilian Portuguese.