Editorial pubs.acs.org/cm
Thermoelectrics Are Hot (and Cold): Insights from Division of Inorganic Chemistry’s Young Investigator Awardee, Sponsored by Chemistry of Materials properties: electrical conductivity, thermal conductivity, and Seebeck coefficient. It is not impossible, though, and in the field of thermoelectrics a diverse array of materials that can partially overcome the obstacle of interrelated properties have been already discovered.3−5 One such family of materials is the family of cage structures known as clathrates. Derived from the Latin word for “lattice”, inorganic clathrates can have covalent frameworks with ionic guest atoms, bringing about properties which are to some extent decoupled.3 Though conventional clathrates are built up of frameworks of Si, Ge, or Sn, my graduate work is motivated by an unconventional take on traditional clathrates. I work to develop clathrates with frameworks solely composed of transition metals and pnicogens (P, As, or Sb). Surprisingly, even today only a handful of transition metal−pnicogen clathrates are known to exist.3,6−9 My journey began when I became a graduate student at The University of California, Davis, about five years ago. It started with a new student welcome presentation involving the wellknown The Illustrated Guide to a Ph.D.10 I did not have much research experience, and I honestly felt a bit out of place around the other students in my class who really seemed to know what they wanted and where they fit in. Eventually, I realized I was intrigued by solid-state chemistrywith everyone else in my class vying for solution-based chemistry laboratories, there was something exciting and unique about it. I joined Kirill Kovnir’s new solid-state inorganic chemistry lab, and as the first graduate student of my group, I had the benefit of learning techniques by working directly with my advisor in the lab. Consequently, his eagerness and passion for discovery were passed on to me, and I discovered my first new materials in just a few months’ time. As I dug deeper into my research, I began to realize how elaborate and complex solid-state chemistry truly was. In my quest to design new clathrates, I discovered dozens of new materials, some of which included a unique modification of a known phase, binary phosphides, and a whole family of new compounds with P73− clusters.11−14 Just a few months in, I was fortunate to discover my first new clathrate, SrNi2P4 (Figure 2). To my disappointment, it turned out to be a metal with poor thermoelectric properties. However, it proved to be distinctive in other ways. It represented one of only two known systems with a unique clathrate structure made up of a single repeating “twisted Kelvin cell” cage.6 For my research, SrNi2P4 presented proof that new clathrates could still be found in lesser-studied elemental systems, and it further motivated me and my research by conveying the possibility of these new systems to exhibit exciting, uncommon structures. My next goal was to see how easily tailored these unconventional clathrate systems could be, so I set out to
Note f rom Editor: Juli-Anna Dolyniuk Johnson (Figure 1) is a graduate student at the University of California, Davis, working under the supervision of Prof. Kirill Kovnir. At the 252nd ACS National Meeting in Philadelphia, Juli-Anna was awarded the 2016 Division of Inorganic Chemistry’s Young Investigator Award, sponsored by Chemistry of Materials. In this Editorial, Juli-Anna discusses her views on what makes her work exciting.
Figure 1. Juli-Anna Dolyniuk Johnson.
A quick look at Thomson Reuters’ Research Fronts 2013 and 2014 will attest to the fact that renewable energy is one of the most widely studied avenues of modern research.1,2 As people around the world look to appease constantly growing energy demands with alternative sources, we are certainly presented with a vast array of opportunities. Personally, I am enticed by the potential of thermoelectric materials, a topic I have spent my graduate career studying. Their attractiveness stems from an intrinsic ability to use a temperature gradient to generate electricity. The keyword intrinsic suggests that the development of an efficient thermoelectric is not dependent on a wellengineered machine. Thus, their success on the job is not related to external moving components that have the possibility of failing. Similar to how solid-state lighting has changed the way we think about generated light, so could thermoelectrics change the way we think about the generation of electrical energy. Their best feature is their applicability to everyday life. Theoretically, anything with a temperature gradient could be used to generate electricity through the application of a thermoelectric component. From seat cushions to combustion engines to geothermal energy and blazing hot summer days, an enormous amount of otherwise lost heat energy is at our disposal. However, intrinsic developmental challenges as of yet have led to lower-than-ideal efficiencies, preventing the widespread use of thermoelectrics. The great thermoelectric challenge is to simultaneously balance traditionally opposing © 2016 American Chemical Society
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success had I not pushed myself and consciously chosen to work as hard as I did. My path would have also been much rockier had I not had the benefit of a caring and supportive advisor to help guide me along the way. When I think back about the welcome presentation and the Guide, I now realize that I understood what it was saying at the time, but I did not truly grasp the meaning behind it until recently. Perhaps that is how I know I’m ready to move on to the next chapter of my life. It is quite an honor to have been chosen as a 2016 Division of Inorganic Chemistry Young Investigator, and I am very grateful to Chemistry of Materials for their sponsorship and for giving me the opportunity to showcase my work. I would also like to thank the Department of Energy-Basic Energy Sciences and the Department of Energy Office of Science’s Graduate Student Researcher (DOE-SCGSR) Program for funding my projects. I’m excited to see what the next decade will bring for the field of materials science, and I hope I can continue to further our understanding of materials.
Figure 2. (Left) SrNi2P4 clathrate’s thermal conductivity and crystal structure. (Right) New Ba−Cu−Zn−P clathrate structure.
charge balance the clathrate-I Ba8Cu16P30 via a simple elemental substitution with Zn. As many “easy” projects do, this venture ended up taking me about three years and various advanced neutron and synchrotron characterization techniques.15,16 Based on the concentration of Zn added in, multiple structural transitions would occur, leading to a wide array of thermoelectric properties. Ultimately, Zn substitution presented exciting enhancements in the overall thermoelectric properties of Ba8Cu16P30, and it proved transition metal−pnicogen frameworks may be just as tunable as conventional clathrate frameworks. Additionally, my study of the Ba−Cu−Zn−P system led to the exciting discovery of a novel clathrate structure type composed in part of a completely new cage type with 4-, 5-, and 6-coordinated framework atoms (Figure 2).17 Finally, near the end of my graduate career, I had the opportunity to collaborate on a broad survey of the thermoelectric properties of clathrates. Our examination revealed something quite exciting: the vast majority of clathratesincluding the new oneshave consistently low thermal conductivities approaching or lower than those of amorphous silica glass. Overall, the known families of clathrates have properties that are promising but not yet ideal for broadspectrum thermoelectric applications, but with one leg of the thermoelectric challenge nearly solved, only electrical conductivity and Seebeck coefficients have to really be tuned in these systems.3 The range of known clathrates covers approximately half of the periodic system,3 but as recent discoveries have shown, other types of clathrates may still be discovered, crystallizing in new and traditional structure types. Thus, they have great potential to provide different, perhaps better properties than what we’ve seen so far. Additionally, while clathrates may or may not be a solution to our energy problems, they present us with a broad potential for future applications. As we continue to build up our library of clahthrate compounds, we will better detail the interplay between these structures and their properties, and perhaps, someday we will know exactly what we need to make and how we can make it. When I first started graduate school, I had no idea what I was in for. As a barrage of stressful situations, high expectations, and strong personalities, it can be completely overwhelming at times. It can also be very rewarding, though, and you really get the chance to learn about yourself and about your capabilities. There’s no doubt that initiative, hard work, and dedication pay off. In scientific discovery, luck can play a big role, but it can only get you so far, and at some point you have to put in the work. I never would have made it this far and seen so much
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Juli-Anna Dolyniuk Johnson
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) King, C.; Pendleton, D. A. Research Fronts 2013: 100 Top-Ranked Specialties in the Sciences and Social Sciences; Thomson Reuters: 2013. (2) Research Fronts 2014: 100 Top-Ranked Specialties in the Sciences and Social Sciences; The National Science Library, Chinese Academy of Sciences; Thomson Reuters IP & Science; The Joint Research Center of Emerging Technology Analysis: 2014. (3) Dolyniuk, J.-A.; Owens-Baird, B.; Wang, J.; Zaikina, J. V.; Kovnir, K. Clathrate thermoelectrics. Mater. Sci. Eng., R 2016, 108, 1−46. (4) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413 (6856), 597−602. (5) Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Nature 2014, 508 (7496), 373−377. (6) Dolyniuk, J.; Wang, J.; Lee, K.; Kovnir, K. Chem. Mater. 2015, 27, 4476−4484. (7) Fulmer, J.; Lebedev, O. I.; Roddatis, V. V.; Kaseman, D. C.; Sen, S.; Dolyniuk, J.-A.; Lee, K.; Olenev, A. V.; Kovnir, K. J. J. Am. Chem. Soc. 2013, 135, 12313−12323. (8) He, H.; Zevalkink, A.; Gibbs, Z. M.; Snyder, G. J.; Bobev, S. Chem. Mater. 2012, 24, 3596−3603. (9) Liu, Y.; Wu, L.-M.; Li, L.-H.; Du, S.-W.; Corbett, J. D.; Chen, L. Angew. Chem., Int. Ed. 2009, 48, 5305−5308. (10) Might, M. The Illustrated Guide to a Ph.D. Available at http:// matt.might.net/articles/phd-school-in-pictures/. (11) Dolyniuk, J.; Kaseman, D. C.; Sen, S.; Zhao, J.; Kovnir, K.; Osterloh, F. E. mP-BaP3: A New Phase from an Old Binary System. Chem. - Eur. J. 2014, 20, 10829−10837. (12) Dolyniuk, J.; He, H.; Ivanov, A.; Boldyrev, A.; Bobev, S.; Kovnir, K. Ba and Sr Binary Phosphides: Synthesis, Crystal Structures, and Bonding Analysis. Inorg. Chem. 2015, 54 (17), 8608−8616. (13) Dolyniuk, J.; Kovnir, K. Zintl Salts Ba2P7X (X = Cl, Br, and I): Synthesis, Crystal, and Electronic Structures. Crystals 2013, 3, 431− 442. (14) Dolyniuk, J.; Tran, N.; Lee, K.; Kovnir, K. Sr2P7X (X = Cl, Br, and I): Synthesis, Crystal and Electronic Structures of Double Zintl Salts Containing Heptaphosphanortricyclane, P73−. Z. Anorg. Allg. Chem. 2015, 641 (8−9), 1422−1427. (15) Dolyniuk, J.; Kovnir, K. High Pressure Properties of a Ba-CuZn-P Clathrate-I. Materials 2016, 9, 692. 7568
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(16) Dolyniuk, J.; Whitfield, P.; Lee, K.; Lebedev, O.; Kovnir, K. Unpublished. (17) Dolyniuk, J.; Zaikina, Y.; Kaseman, D.; Sen, S.; Kovnir, K. Unpublished.
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DOI: 10.1021/acs.chemmater.6b04459 Chem. Mater. 2016, 28, 7567−7569
