MRS Spring 2019 Conference: Highlights of Day 1
Tuning Thermal Conductivity with Isotope Engineering
After a bustling registration desk, I kicked off my Spring 2019 MRS Conference experience by attending a talk by Dr. Kai Xiao titled "The Effect of Doping, Vacancies and Isotopes on the Thermal Conductivity of 2D Materials". This talk was really interesting for me personally, because as someone working on thermoelectrics, it was important for me to attend a talk on enhancing (or reducing) the thermal conductivity of materials - which can lead to better thermoelectric performance.
Dr. Xiao's focus on 2D materials is particularly interesting, because (as he mentioned in his presentation) 2D materials have new degrees of freedoms which makes it easy to tune their various electronic and thermal properties. Hence they are able to show insulator, metal, semiconductor, and superconductive properties. Additionally, stacking various single layers of 2D materials can result in new heterogeneous structures and properties. Materials like Bi2Te3 and SnSe hence have great thermoelectric properties due to their high electrical conductivities coupled with low thermal conductivities.
Dr. Xiao continued his talk, mentioning that current techniques to measure thermal conductivity of such materials can pose various challenges. Current methods include steady-state and transient measurement techniques. However, Dr. Xiao presented a new method done in their research of measuring thermal conductivity, by using optothermal Raman. He went on to explain how their research group has experimentally explored how heterogeneity can effect the thermal conductivity of 2D materials. Specifically, they used isotopic doping to synthesize isotopically pure MoS2 monolayers - 100MoS2 using chemical vapor deposition. This isotopically pure MoS2 monolayer had an enhanced thermal conductivity compared to natural MoS2 due to the reduced isotope disorder in the former compared to the latter. This is attributed to the reduced isotope disorder and hence reduced phonon scattering in 100MoS2 compared to natural MoS2.
It was a really interesting talk, and illustrated the future of how 2D materials such as isotopically pure MoS2 can even replace other, popular 2D materials such as graphene as very efficient thermoelectric materials. #s19mrs
Useful Papers to Read:
- Milad Yarali et al. Effect of Metal Doping and Vacancies on the Thermal Conductivity of Monolayer Molybdenum Diselenide. ACS Applied Materials & Interfaces 2018 10(5), 4921-4928, DOI: 10.1021/acsami.7b14310
- Shanshan Chen et al. Thermal conductivity of isotopically modified graphene. Nature Materials 11, 203-207 (2012).
Trash Talking about Sustainability
I felt really lucky to be able to attend the Professional Development Workshop on Designing Sustainability into Materials Research at the MRS Spring 2019 Conference. As someone who works with various hazardous chemicals in the laboratory, I am always interested to know more about the safety and even alternatives for these chemicals. Additionally, as a consumer, it is sometimes difficult to know what you are putting in your body, and the things in your recycling bin are actually being recycled.
The session was divided into two - the first session was on Trash and what we are throwing away, and its implications on the environment. Delivered by Dr. Alan Rae, it was a very engaging and informative talk. We touched upon the various hazardous and recyclable materials we generate in our trash, and how we can replace these materials. Importantly, the talk also discussed the need for life cycle assessment, the need for sustainability, and who the various stakeholders are when we start thinking about sustainability. I learnt a very surprising thing that 50% of all plastics produced are used for packaging and importantly food packaging, with most of it not being recycled. The presentation also helped me understand the complexities that come with sustainability. For example, by reducing the amount of plastics used in food packaging we may be reducing a carbon footprint associated with it, but we also contribute to an increase in food wastage due to the lack of packaging.
We also participated in an interactive activity where we were supposed to think of what alternatives we can provide to a plastic drinking straw at an amusement park. The choices included plastics recyclable, compostable, stainless steel straws, and silicone straws. Dividing into groups and talking to other students was an interesting way to know about everyone's though processes about sustainability and recyclability. Finally, we concluded with The Granta Design Approach to design sustainability into materials research, taking into account the major contributors to a material's sustainability: feed stock, energy, CO2 footprint, and transport.
The second part of the session was a talk by Dr. Julie M. Schoenung on Designing Out Toxic Substances – Useful Data Sources to Promote Better Choices. I really enjoyed her talk as well, especially since she listed so many helpful databases on how to know more about the various chemicals used, toxicology information, and the differences between hazard reduction and exposure reduction.
She started the talk by illustrating an example about how even though the recycling rates of lead acid batteries are around 96% in the US, these are actually just an example of hazard reduction. Due to regulations within the US, these batteries are sent to other countries around the world where people are personally exposed to hazardous materials from lead acid batteries while trying to recycle them. This was a chilling example how we may sometimes be unaware of how recycling is not the end of a product's ability to harm the environment.
Moreover, Dr. Schoenung also highlighted important aspects about databases for accessing toxicology information. Some classification techniques such as Toxicity Characteristics Leaching Procedure (TCLP) used at the federal level and Total Threshold Limit Concentrations (TTLC) used in the state of California are woefully inadequate and only cover in total about 80 of the millions of chemicals present in the world. She also illustrated the difficulty of how it is not possible to completely eliminate hazardous materials use, especially in materials science research, because we want to make products that work and consumers can rely on. However, she also stressed on the importance of at least knowing about the toxicity and hazard of various materials that we encounter.
These two sessions really opened my eyes and educated me about the various sources I can use to know more about what materials I am encountering not just in a laboratory but even at home.
Some of these sources are mentioned below:
- Hazardous Waste Classifications: TCLP, TTLC
- Toxic Release Inventory (TRI): TRI Explorer
- Occupational Exposure Limits: MSDS, NIOSH Pocket Guide
- Carcinogens: IARC, NTP
- Globally Harmonized System for Classification and Labeling of Chemicals: (GHS)
3D Printing the Next Generation
Hyunwoo Yuk, a doctoral student at Massachusetts Institute of Technology (MIT) with Prof. Xuanhe Zhao in the Department of Mechanical Engineering delivered his presentation on "Preparation, Adhesion and 3D Printing of Highly Conductive PEDOT:PSS Hydrogels" as a part of the Graduate Student Award Finalists' Special Talk, Session #1.
This was a very interesting presentation as Yuk started with what 3D printing (3DP) and how it is crucial to develop three important aspects of 3DP to enable the next generation of soft material additive manufacturing. These are:
- Developing the 3DP process
- Developing novel soft materials with new functionalities for 3DP
- Developing new applications for soft material structures using 3DP
Yuk illustrated the various research work he has carried out in all three of these aspects: he has developed a phase diagram for 3DP soft materials to identify the optimal ratios of nozzle height and nozzle speed during printing to obtain uniformly printed materials, he has developed novel ferromagnetic soft materials and hydrogel based cell-laden inks for 3DP printing actuating circuits and cell-based living sensors, respectively. Finally, he has worked on printing conductive circuits and organic electronics that are usually made via cleanroom techniques, using 3DP of PEDOT:PSS (an organic electrically conductive polymer).
It was a very impressive presentation on what the future of materials research, manufacturing and applications holds. 3DP has the potential to completely transform how we perceive manufacturing around us, and Yuk did an excellent job of highlighting the interesting intersection of materials development, process development, and materials applications. To read Hyunwoo Yuk's research, please click here.
High Thermal Conductivity in Boron Arsenide
Joon Sang Kang, a graduate student at University of California, Los Angeles with Prof. Yongje Hu presented his talk titled "Experimental Observation of Ultrahigh Thermal Conductivity in Boron Arsenide" as a part of the Graduate Student Award Finalists' Special Talk, Session #2.
Kang started by illustrating the need for this research - a technological challenge being faced by the transistor industry in terms of heat dissipation in shrinking transistor sizes. In the US, data centers exhaust more than 50% of their total electricity to remove heat. To combat this, it is important to develop materials with high thermal conductivity (TC). Currently, diamond and graphite have very large TC but have their own drawbacks. Diamonds are too expensive and their mechanical properties make them hard to integrate with existing semiconducting materials used commercially. Graphite has high in-plane thermal conductivity but low out-of-plane thermal conductivity due to low van der Waals forces.
Since 2013, BAs material with high TC have been predicted; however it was hard to grow them as a single crystal without inducing defects. Kang and their research group developed and observed a very high TC of 1300 W/mK in Boron Arsenide (BAs) crystals. The first challenge that they did encounter, however, was selecting the best substrate to grow a single crystal of BAs. After much experimentation, they landed on Boron Phosphide (BP), which was a good candidate to grow BAs, since they both have similar lattice constants, thereby reducing the lattice constant mismatch. This was followed by measurement of the TC of the thus-grown BAs, which was observed to be very high at room temperature (1300 W/mK). This was 3 times higher than industrially used high TC materials such as SiC and Cu. This observation of high TC in BAs crystals was attributed by Kang to the very long phonon mean free paths observed in BAs crystals via measurements made in the quasi-ballistic heat transporter regimes. Since thermal conductivity is directly proportional to the mean free path, this resulted in BAs crystals having significantly higher TCs.
Such materials could mean a reduction in the amount of energy required to cool electronics and various semiconducting transistors and devices, as well as the development of smaller and smaller transistors that can transfer data faster. As the world moves towards a digital paradigm, such materials can significantly impact the way in which we harness technology, without putting undue pressure on our energy sources. To read more about this work by Kang et al., please click here.
Liquid Crystal Polymers as Biological Actuators
Jennifer Boothby, a graduate student at The University of Texas at Dallas with Prof. Taylor Ware presented her talk titled "Engineering Liquid Crystalline Polymers for Biological Applications" as a part of the Graduate Student Award Finalists' Special Talk, Session #2.
Boothby started the talk with the need to use polymers instead of the various rigid devices such as endoscopic probes and micropumps currently used for biological monitoring. These materials are difficult to scale down, especially the interfaces and powering peripherals that are required for them. Hence, the solution would be to use materials which integrate all of these functionality into one. Recently developments in polymer hinges, anisotropic actuation in polymers via swelling, and creating 3D structures from flat polymers indicate that there is a growing interest in miniaturization and polymerization of biological machines.
Boothby's research focuses on developing liquid crystal elastomers (LCEs) that self-assemble to impart molecular order to polymeric materials, thereby enabling anistropic actuation at small scales and in biologically feasible environments. These can be used to create tissue scaffolds, soft robotics, and smart actuators with controlled molecular order. By using a lyotropic LC system which self-assembles in water due to hydrophobic interactions, but can also be suspended in water due to its external polar groups, she was able to create chromonic molecules which showed shape changing, anistropic behavior. Interestingly, these structures showed good anisotropic actuation without needing to create bilayer films - thereby enabling a range of motions without delamination, as is commonly observed in bilayer actuators. Adding a functional comonomer to these chromonic molecules can create chromonic gels that are responsive to other biological stimuli such as pH and heat. Thus, microstructures with meaningful actuation could be created using this technique. To read more about Boothby's research, please click here.