7-Mar-2022: Ultra-low Thermal Conductivity in Crystalline Solid with promising thermoelectric applications trace to their Local Structural Distortion

In a recent study, Indian Scientists have cracked the origin of ultralow thermal conductivity in silver antimony selenide (AgSbSe2), a crystalline solid having promising thermoelectric applications.

They have found that the AgSbSe2 has deformed local structure, which, while keeping the average structure intact, resulted in the ultra-low thermal conductivity. This work brings out the importance of investigating the local structure of a material to understand the origin of heat transport.

Heat transport is one of the fundamental properties of matter, and it has several applications, such as in semiconductor electronics, thermoelectrics, and thermal barrier coating. Heat propagates from hot end to a cold end of a medium until they are in thermodynamic equilibrium. In crystalline materials, where the atoms are arranged closely in a periodic and ordered fashion, heat mainly passes through conduction. They are highly thermal conductive due to the close arrangement of atoms which facilitates smooth heat transport. Heat conduction takes place when the neighboring atoms vibrate and transfer the heat. The rate at which heat can transfer from a hot end to a cold end is called thermal conductivity, and often the absolute value of this thermal conductivity dictates the applicability of a material in different industries. Materials with very high thermal conductivity are widely used as heat sinks in semiconductor electronics or as heat radiators, while ultra-low thermal conductive materials are useful in heat insulation applications or thermoelectrics. Therefore, studying and understanding the heat transport of materials are extremely important for both fundamental and practical points of view.

Prof. Kanishka Biswas and his student Dr. Moinak Dutta from Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore, an autonomous institute of the Department of Science & Technology, Government of India, in their recent paper published in ‘Angewandte Chemie’ have investigated the origin of ultralow thermal conductivity in AgSbSe2.

While, crystalline materials are highly thermal conductive, AgSbSe2 defies the norm and exhibits thermal conductivity like amorphous materials like glass. To investigate such anomaly in heat transport, they probed the arrangement of atoms inside the locally coordinated environment of the crystal using a technique called synchrotron X-ray pair distribution function (PDF) analysis. Through the analysis, they observed that the so-called periodically ordered and crystalline AgSbSe2 is actually not so ordered in local scale. The cation Sb is found to be off-centered from its ideal position, resulting in the distorting the otherwise perfect ordered arrangement. This breaks the symmetry locally, thus resulting in a low thermal conductivity.

Explaining the initiation impression of AgSbSe2 being a periodically ordered crystal, Prof. Kanishka Biswas said that the distortion of Sb is only restricted to a few angstroms, and there is an equal probability of Sb distortion in six different positions, which are often in polar opposite directions. “So, when viewed for a large number of atoms, the six positions average out to look like that the Sb actually sits in its ideal position. Thus, such deformed local structure of AgSbSe2, while keeping the average structure intact, resulted in the ultra-low thermal conductivity,” he pointed out.

The synchrotron X-ray PDF experiment performed under India-DESY (Deutsches Elektronen-Synchrotron) collaboration supported by DST synchrotron access programme establishes that more often than not, most of the fundamental physical properties of materials can be traced to their local structure, which can only be understood if it is looked deeper into the atomic scale.

11-Feb-2022: Woman scientist from Chennai granted patent for green technology producing medicinally important compound

Dr E. Poonguzhali, a single parent and woman scientist at the Department of Chemistry, Indian Institute of Technology Madras, Chennai, has been granted a patent for developing a green methodology for producing a medicinally important compound called Benzo[b]thiophene.

The compound is present in a range of medicines such as raloxifene (used in osteoporosis), zileuton (used in asthma), and sertaconazole (antifungal medication) and the one-step synthesis of the 2-substituted benzo[b]thiophene can replace hazardous industrial production of the compound.

Currently, available synthesis methods of the compound like Friedel–Craft acylation, mercaptoacetate reaction, subsequent addition and oxidation etc. - all give yields ranging from good to excellent, but these are not environmentally friendly. Besides, it involves the use of very high temperatures. The disadvantages include sulphur emission with an unpleasant smell, expensive starting materials and so on. Apart from this, the reactions are carried out in closed vessels exposing the process to the risk of explosion, use of OLED lights required in the reaction increased the cost of the process, and the various steps involved needs close monitoring while the metal catalyst needed for it is toxic in nature.

Dr Poonguzhali has successfully transferred commercially available starting materials to medicinally important 2-acylbenzo[b]thiophenes in the presence of copper acetate and tetrabutylammonium chloride catalytic system in water medium and open-air atmosphere at room temperature. She worked under the WOMEN SCIENTISTS SCHEME (WOS-A) Programme of the Department of Science & Technology (DST), Govt. of India.

The new method involves using water medium, room temperature, odourless xanthate, open-air atmosphere, handling free inexpensive commercially available starting materials and catalysts in a one-pot manner. It furnished a good to an excellent yield of 2-acylbenzo[b]thiophenes.

Dr. E. Poonguzhali and Prof. G. Sekar treated commercially available 2-iodobenzaldehyde, phenacyl bromide, and xanthate sulphur source in the presence of copper acetate and tetrabutylammonium chloride catalyst to furnish 2-acylbenzo[b]thiophene in water medium at room temperature. The thiolate by-product formed usage is being explored. The remaining side products can be recovered using their solubility properties and reused after purification. The new method also reduces the risk of explosion, decreases the cost of the process and obviates the toxic and hazardous steps involved.

Elaborating on the science behind the mechanism, Dr Poonguzhali said that as water is the medium, no need for organic solvent is involved. Besides, there is no air pollution. Room temperature saves energy, and directly transferring the commercially available starting materials to medicinally essential building blocks in a one-pot manner saves workforce, energy and space. Usage of the thiolate by-product is under process. At the same time, the other side products (KI and KBr) and catalyst (copper acetate and tetrabutylammonium chloride) can be recovered and reused for the same reaction and different applications.

“My family had encouraged me to invent green methodology to replace the hazardous methods. When I started my research career with DST support, I designed the green methodology with the help of Prof. G. Sekar and Prof. Ramesh L. Gardas and succeeded in synthesizing the medicinally important compounds,” said Dr Poonguzhali.

Dr Poonguzhali adds, “My life revolves around my son and my research. My son’s support has helped me overcome the difficulties I have encountered so far and spend time on my research.”