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Effect of point defects on the electronic density of states of ScN studied by first-principles calculations and implications for thermoelectric properties
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0003-1785-0864
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 19Article in journal (Refereed) Published
Abstract [en]

We have investigated the effect of defects and impurities on the electronic density of states of scandium nitride using first-principles calculations with the generalized gradient approximation and hybrid functionals for the exchange correlation energy. Our results show that Sc and N vacancies can introduce asymmetric peaks in the density of states close to the Fermi level. We also find that the N vacancy states are sensitive to total electron concentration of the system due to their possibility for spin polarization. Substitutional point defects shift the Fermi level in the electronic band according to their valence but do not introduce sharp features. The energetics and electronic structure of defect pairs are also studied. By using hybrid functional calculations, a correct description of the band gap of scandium nitride is obtained. Our results envisage ways for improving the thermoelectric figure of merit of ScN by electronic structure engineering through stoichiometry tuning and doping.

Place, publisher, year, edition, pages
American Physical Society , 2012. Vol. 86, no 19
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-87213DOI: 10.1103/PhysRevB.86.195140ISI: 000311694200001OAI: oai:DiVA.org:liu-87213DiVA: diva2:587432
Available from: 2013-01-14 Created: 2013-01-14 Last updated: 2017-12-06
In thesis
1. Scandium Nitride Thin Films for Thermoelectrics
Open this publication in new window or tab >>Scandium Nitride Thin Films for Thermoelectrics
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Thermoelectric devices are one of the promising energy harvesting technologies, since they can convert heat (i.e. a temperature gradient) to electricity. This result leads us to use them to harvest waste heat from heat engines or in power plants to generate usable electricity. Moreover, thermoelectric devices can also perform cooling. The conversion process is clean, with no emission of greenhouse gases during the process. However, the converting efficiency of thermoelectrics is very low because of the materials limitations of the thermoelectric figure of merit (ZTm). Thus, there is high demand to maximize the ZTm.

I have discovered that ScN has high power factor 2.5 mW/(mK2) at 800 K, due to low metalliclike electrical resistivity (∼3.0 μΩm) with retained relatively large Seebeck coefficient of -86 μV/K. The ScN thin films were grown by reactive dc magnetron sputtering from Sc targets. For ScN, X-ray diffraction, supported by transmission electron microscopy, show that we can obtain epitaxial ScN(111) on Al2O3(0001). We also reported effects on thermoelectric properties of ScN with small changes in the composition with the power factor changing one order of magnitude depending on e.g. oxygen, carbon and fluorine content which were determined by elastic recoil detection analysis. The presence of impurities may influence the electronic density of states or Fermi level (EF) which could yield enhancement of power factor.

Therefore, the effects of defects and impurities on the electronic density of states of scandium nitride were investigated using first-principles calculations with general gradient approximation and hybrid functionals for the exchange correlation energy. Our results show that for Sc and N vacancies can introduce asymmetric peaks in the density of states close to the Fermi level. We also find that the N vacancy states are sensitive to total electron concentration of the system due to their possibility for spin polarization. Substitutional point defects shift the Fermi level in the electronic band according to their valence but do not introduce sharp features. The energetics and electronic structure of defect pairs are also studied. By using hybrid functionals, a correct description of the open band gap of scandium nitride is obtained, in contrast to regular general gradient approximation. Our results envisage ways for improving the thermoelectric figure of merit of ScN by electronic structure engineering through stoichiometry tuning and doping.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 72 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1559
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-85917 (URN)LIU-TEK-LIC-2012:44 (Local ID)978-91-7519-733-3 (ISBN)LIU-TEK-LIC-2012:44 (Archive number)LIU-TEK-LIC-2012:44 (OAI)
Presentation
2012-12-11, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2016-08-31Bibliographically approved
2. Design of Transition-Metal Nitride Thin Films for Thermoelectrics
Open this publication in new window or tab >>Design of Transition-Metal Nitride Thin Films for Thermoelectrics
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermoelectric devices are one of the promising energy harvesting technologies, because of their ability to convert heat (temperature gradient) to electricity by the Seebeck effect. Furthermore, thermoelectric devices can be used for cooling or heating by the inverse effect (Peltier effect). Since this conversion process is clean, with no emission of greenhouse gases during the process, this technology is attractive for recovering waste heat in automobiles or industries into usable electricity. However, the conversion efficiency of such devices is rather low due to fundamental materials limitations manifested through the thermoelectric figure of merit (ZT). Thus, there is high demand on finding materials with high ZT or strategies to improve ZT of materials.

In this thesis, I discuss the basics of thermoelectrics and how to improve ZT of materials, including present-day strategies. Based on these ideas, I propose a new class of materials for thermoelectric applications: transition-metal nitrides, mainly ScN, CrN and their solid solutions. Here, I employed both experimental and theoretical methods to synthesize and study their thermoelectric properties. My study envisages ways for improving the thermoelectric figure of merit of ScN and possible new materials for thermoelectric applications.

The results of my studies show that ScN is a promising thermoelectric material since it exhibits high thermoelectric power factor 2.5x10-3 Wm-1K-2 at 800 K, due to low metallic-like electrical resistivity while retained relatively large Seebeck coefficient. My studies on thermal conductivity of ScN also suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN thin films. Furthermore, my theoretical studies on effects of impurities and stoichiometry on the electronic structure of ScN suggest the possibly to improve ScN ZT by stoichiometry tuning and doping. For CrN and Cr1-xScxN solid solution thin films, the results show that the power factor of CrN (8x10-4 Wm-1K-2 at 770 K) can be retained for the solid solution Cr0.92Sc0.08N. Finally, density functional theory was used to enable a systematic predictionbased strategy for optimizing ScN thermoelectric properties via phase stability of solid solutions. Sc1-xGdxN and Sc1-xLuxN are stabilized as disordered solid solutions, while in the Sc-Nb-N and Sc-Ta-N systems, the inherently layered ternary structures ScNbN2 and ScTaN2 are stable.

Abstract [sv]

Sedan den industriella revolutionen har fossila bränslen varit vår huvudkälla till energi i motorer för transport, elproduktion och uppvärmning av byggnader. Eftersom mänskligheten och vår teknik växer för varje år som går, fortsätter efterfrågan på fossila bränslen att öka. Med tanke på att fossila bränslen inte är förnybara, riskerar vi att de tar slut. Dessutom är resultatet av denna ständiga förbränning av fossila bränslen generering av växthusgaser, t.ex. kolmonoxid och koldioxid, som orsakar klimatförändringar, som ett ytterligare problem. Således finns det ett ökande behov av nya former av energikällor som kan ersätta fossila bränslen.

För närvarande finns det olika typer av tekniker för förnybar energi som solceller, vätgasteknik (bränsleceller), vindkraftverk, vattenkraft, etc. Ett annat koncept som har studerats är energiåtervinning, vilket innebär att fånga eller lagra spillenergi och förvandla det till användbar energi. Spillenergi är den energi, oftast värmeförluster, som förloras i generatorer, vibrationer från motorer, och så vidare. Ungefär 60% av den ursprungliga energin avges som spillvärme. Om vi kan återvinna all denna förlust till användbar energi igen, kan vi spara stora mängder bränslen utsläppen av koldioxid kommer att minska.

Med hänsyn till dessa krav, så är termoelektriska komponenter intressanta kandidater. En termoelektriska komponent är tillverkad av material som direkt återvinner värme (en temperaturgradient) till elektrisk energi utan utsläpp av växthusgaser. De kan också kyla genom den omvända processen, när de genererar en temperaturgradient från en pålagd ström. Detta innebär att de kyler utan rörliga delar eller något kylmedel som kan orsaka miljöproblem. Verkningsgraden är emellertid låg, för närvarande 10% -15%, dessutom är de flesta av dagens termoelektriska material giftiga. Jag har därför studerat en ny klass av material, övergångsmetallnitrider, som en kandidat för termoelektriska tillämpningar. Övergångsmetallnitrider är kända för sina utmärkta mekaniska egenskaper, de används till exempel som beläggningar på skärverktyg i syfte att förbättra prestanda och livslängd. De uppvisar ocksåolika elektriska egenskaper (metaller, halvledare och supraledare). Min studie är inriktad på att förstå de termoelektriska egenskaperna hos övergångsmetallnitrider, främst skandiumnitrid och kromnitrid. Resultaten visar att båda materialen kan vara bra kandidater för termoelektriska tillämpningar.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 178 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1667
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117760 (URN)10.3384/diss.diva-117760 (DOI)978-91-7519-067-9 (ISBN)
Public defence
2015-06-01, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-05-08 Created: 2015-05-08 Last updated: 2016-08-31Bibliographically approved

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Kerdsongpanya, SitAlling, BjörnEklund, Per

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