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Theoretical and Experimental Studies on Early Transition Metal Nitrides for Thermoelectrics
Linköping University, Faculty of Science & Engineering. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. 202100-3096.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermoelectricity transforms temperature gradients across thermoelectric material into an external voltage through a phenomenon known as the Seebeck effect. This property has resulted in niche applications such as solid-state cooling for electronic and optoelectronic devices which exclude the need for a coolant or any moving parts and long-lasting, maintenance-free radioisotope thermoelectric generators used for deep-space exploration. However, the high price and low efficiency of thermoelectric generators have prompted scientists to search for new materials and/or methods to improve the efficiency of the already existing ones. Thermoelectric efficiency is governed by the dimensionless figure of merit 𝑧𝑇, which depends on the electrical conductivity, thermal conductivity and Seebeck coefficient value of the material and has rarely surpassed unity.

In order to address these issues, research conducted on early transition metal nitrides spearheaded by cubic scandium nitride (ScN) thin films showed promising results with high power factors close to 3000 μWm−1K−2 at 500 °C. These results are the main motivation behind my thesis where the conducted research is separated into two different routes:

• the synthesis and characterization of chromium nitride thin films and its alloys

• the study of hypothetical ternary nitrides equivalent to scandium nitride

Rock-salt cubic chromium nitride (CrN) deposited in the form of thin films by reactive magnetron sputtering was chosen for its large Seebeck coefficient of approximately -200 μV/K and low thermal conductivity between 2 and 4 Wm−1K−1. The results show that CrN in single crystal form has a low electrical resistivity below 1 mΩcm, a Seebeck coefficient value of -230 μV/K and a power factor close to 5000 μWm−1K−2 at room temperature. These promising results could lead to CrN based thermoelectric modules which are cheaper and more stable compared to traditional thermoelectric material such as bismuth telluride (Bi2Te3) and lead telluride (PbTe).

Although cubic CrN has been shown to be a promising material for research with a large power factor, the electrical resistivity limits applications in pure form as the 𝑧𝑇 is estimated to be slightly below 0.5. To overcome this issue, I enhanced the thermoelectric power-factor of CrN by alloying it with a conductor, Rock-salt cubic vanadium nitride (VN). VN is a suitable choice as both materials share the same crystal structure and have almost equal lattice constants. Through deposition at 720 °C, where a small amount of VN (less than 5%) and Cr2N is introduced into the film, a reduced electrical resistivity averaged around 0.8 × 10-3 Ωcm, Seebeck coefficient value of 270 μV/K and a power-factor of 9.1 × 10-3 W/mK2 is measured at room temperature, which surpasses the thermoelectric properties of Bi2Te3. Hexagonal dichromium nitride (Cr2N) nano-inclusions increase the charge carrier concentration and act as phonon scattering sites. Single crystal Cr2N was also studied separately, as it shows interesting elastic-plastic mechanical properties and high resistance to oxidation at high temperatures for long periods of time.

In the second part of this thesis, hypothetical ternary nitrides equivalent to ScN are investigated for their prospective thermoelectric properties. Scandium nitride has a relatively high thermal conductivity value (close to 10 Wm−1K−1), resulting in a low 𝑧𝑇. A hypothetical ternary equivalent to ScN may have a similar electronic band structure and large power factor, but with a lower thermal conductivity value leading to better thermoelectric properties. Thus, the elements magnesium, titanium, zirconium, and hafnium were chosen for this purpose. DFT calculations were used to simulate TiMgN2, ZrMgN2 and HfMgN2. The results show the MeMgN2 stoichiometry to be stable, with two rivaling crystal structures: trigonal NaCrS2 and monoclinic LiUN2. The calculated electronic band structure of these compounds shows a direct band-gap for the monoclinic and an indirect band-gap for the trigonal crystal structures. These findings, coupled with predicted Seebeck coefficient values, encourages actual synthesis of such materials. DFT calculations were also used to study (Zr, Mg)N and (Hf, Mg)N alloys based on the SQS model. The transition temperature between the ordered monoclinic structure of ZrMgN2 and HfMgN2 and the disordered (Zr, Mg)N and (Hf, Mg)N alloys is calculated to be approximately 800 K and 1050 K respectively. Density of State (DoS) calculations show that similar to (Ti, Mg)N, (Zr, Mg)N and (Hf, Mg)N are also semiconducting. The thermoelectric properties of both compounds are also predicted, and that in the range of a moderate change in the Fermi level, high Seebeck coefficient values at room temperature can be achieved.

Finally, in order to complete the mentioned study on hypothetical ternaries, I deposited (Ti, Mg)N thin film alloys by reactive magnetron sputtering. These films, which were deposited at 400 °C, are porous and are crystallized in the rocksalt cubic structure. As-deposited films show an electrical resistivity of 150 mΩcm and a Seebeck coefficient of -25 μV/K, which shows semiconducting properties. In order to initiate a phase transformation, these films when annealed at approximately 800 °C, where nano-inclusions of a titanium/magnesium oxynitride are formed in a LiTiO2-type superstructure are identified by XRD and TEM analysis.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 56
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-162020DOI: 10.3384/diss.diva-162020ISBN: 9789179299644 (print)OAI: oai:DiVA.org:liu-162020DiVA, id: diva2:1370775
Public defence
2019-11-28, Hörsal BL Nobel, B-huset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
EU, European Research Council, 335383Linköpings universitet, 200900971Swedish Foundation for Strategic Research Swedish Research Council, 62120124430Swedish Research Council, 201603365Swedish Research Council, 33020146336Swedish Research Council, 201604810Available from: 2019-11-18 Created: 2019-11-18 Last updated: 2019-12-04Bibliographically approved
List of papers
1. Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films
Open this publication in new window or tab >>Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films
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2018 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 35, article id 355302Article in journal (Refereed) Published
Abstract [en]

CrN thin films with an N/Cr ratio of 95% were deposited by reactive magnetron sputtering onto (0001) sapphire substrates. X-ray diffraction and pole figure texture analysis show CrN (111) epitaxial growth in a twin domain fashion. By changing the nitrogen versus argon gas flow mixture and the deposition temperature, thin films with different surface morphologies ranging from grainy rough textures to flat and smooth films were prepared. These parameters can also affect the CrN(x )system, with the film compound changing between semiconducting CrN and metallic Cr2N through the regulation of the nitrogen content of the gas flow and the deposition temperature at a constant deposition pressure. Thermoelectric measurements (electrical resistivity and Seebeck coefficient), scanning electron microscopy, and transmission electron microscopy imaging confirm the changing electrical resistivity between 0.75 and 300 m omega cm, the changing Seebeck coefficient values between 140 and 230 mu VK-1, and the differences in surface morphology and microstructure as higher temperatures result in lower electrical resistivity while gas flow mixtures with higher nitrogen content result in single phase cubic CrN.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
magnetron sputtering; thermoelectrics; chromium nitride; seebeck coefficient; thin films
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:liu:diva-150201 (URN)10.1088/1361-6463/aad2ef (DOI)000440718100002 ()
Note

Funding Agencies|European Research Council under the European Community/ERC [335383]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 5 program; Knut and Alice Wallenberg Foundation under the Wallenberg Academy Fellows program; Swedish Research Council (VR) [2016-03365]; European Union; Danish Innovation Fund for the NanoCaTe project [604647]; Danish Innovation Fund for the CTEC project [1305-00002B]

Available from: 2018-08-22 Created: 2018-08-22 Last updated: 2019-11-18
2. Synthesis and characterization of single-phase epitaxial Cr2N thin films by reactive magnetron sputtering
Open this publication in new window or tab >>Synthesis and characterization of single-phase epitaxial Cr2N thin films by reactive magnetron sputtering
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2019 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 54, no 2, p. 1434-1442Article in journal (Refereed) Published
Abstract [en]

Cr2N is commonly found as a minority phase or inclusion in stainless steel, CrN-based hard coatings, etc. However, studies on phase-pure material for characterization of fundamental properties are limited. Here, Cr2N thin films were deposited by reactive magnetron sputtering onto (0001) sapphire substrates. X-ray diffraction and pole figure texture analysis show Cr2N (0001) epitaxial growth. Scanning electron microscopy imaging shows a smooth surface, while transmission electron microscopy and X-ray reflectivity show a uniform and dense film with a density of 6.6gcm(-3), which is comparable to theoretical bulk values. Annealing the films in air at 400 degrees C for 96h shows little signs of oxidation. Nano-indentation shows an elastic-plastic behavior with H=18.9GPa and E-r=265GPa. The moderate thermal conductivity is 12Wm(-1)K(-1), and the electrical resistivity is 70cm. This combination of properties means that Cr2N may be of interest in applications such as protective coatings, diffusion barriers, capping layers and contact materials.

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-152796 (URN)10.1007/s10853-018-2914-z (DOI)000448833200035 ()
Note

Funding Agencies|European Research Council under the European Communitys Seventh Framework Programme (FP/2007-2013)/ERC Grant [335383]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 5 program; Knut and Alice Wallenberg foundation through the Wallenberg Academy Fellows program; Swedish Research Council (VR) [2016-03365]; Aforsk Foundation [16-359]; Carl Tryggers Foundation [CTS 17:166]

Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2019-11-18
3. Theoretical study of phase stability, crystal and electronic structure of MeMgN2 (Me = Ti, Zr, Hf) compounds
Open this publication in new window or tab >>Theoretical study of phase stability, crystal and electronic structure of MeMgN2 (Me = Ti, Zr, Hf) compounds
2018 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 53, no 6, p. 4294-4305Article in journal (Refereed) Published
Abstract [en]

Scandium nitride has recently gained interest as a prospective compound for thermoelectric applications due to its high Seebeck coefficient. However, ScN also has a relatively high thermal conductivity, which limits its thermoelectric efficiency and figure of merit (zT). These properties motivate a search for other semiconductor materials that share the electronic structure features of ScN, but which have a lower thermal conductivity. Thus, the focus of our study is to predict the existence and stability of such materials among inherently layered equivalent ternaries that incorporate heavier atoms for enhanced phonon scattering and to calculate their thermoelectric properties. Using density functional theory calculations, the phase stability of TiMgN2, ZrMgN2 and HfMgN2 compounds has been calculated. From the computationally predicted phase diagrams for these materials, we conclude that all three compounds are stable in these stoichiometries. The stable compounds may have one of two competing crystal structures: a monoclinic structure (LiUN2 prototype) or a trigonal superstructure (NaCrS2 prototype; RmH). The band structure for the two competing structures for each ternary is also calculated and predicts semiconducting behavior for all three compounds in the NaCrS2 crystal structure with an indirect band gap and semiconducting behavior for ZrMgN2 and HfMgN2 in the monoclinic crystal structure with a direct band gap. Seebeck coefficient and power factors are also predicted, showing that all three compounds in both the NaCrS2 and the LiUN2 structures have large Seebeck coefficients. The predicted stability of these compounds suggests that they can be synthesized by, e.g., physical vapor deposition.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-144130 (URN)10.1007/s10853-017-1849-0 (DOI)000418294200030 ()
Note

Funding Agencies|European Research Council under the European Communitys Seventh Framework Programme (FP)/ERC [335383]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 5 and 6 programs; Knut and Alice Wallenberg foundation through the Academy Fellow program; Swedish Research Council (VR) [621-2012-4430, 2016-03365]; Swedish Research Council (VR) through International Career Grant [330-2014-6336]; Marie Sklodowska Curie Actions, Cofund [INCA 600398]; VR Grant [2016-04810]; Swedish e-Science Research Centre (SeRC)

Available from: 2018-01-10 Created: 2018-01-10 Last updated: 2019-11-18

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