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Structure-property investigation of ZnSb, ZnAs, and SiB3: - binary semiconductors with electron poor framework structures
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). (Ulrich Häussermann)ORCID iD: 0000-0002-6886-2649
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In today’s society, where energy conservation and green energy are buzz words, new scientific discoveries in green energy harvesting is key. This work focuses on materials capable of recycling low value thermal energy. Low value thermal energy, waste heat, is for free, and can be transformed into valuable electricity via thermoelectric technology. A thermoelectric device cleanly converts heat into electricity through the Seebeck effect. Thermoelectric devices can play an important role in satisfying the future global need for efficient energy management, however, the primary barrier of improving thermoelectric devices is the materials themselves.

The aim of this thesis is to identify new compositions and structures for thermoelectric materials. In particular, the concept of “electron poor framework semiconductors” is explored. Electron Poor Framework Semiconductors (EPFS) are materials at the border between metals and non-metals, which often show intricate and unique structures with complex bonding schemes. Generally, constituting elements should be from group 12(II) (Zn, Cd), 13(III) (B, Al, Ga, In), 14(IV) (Si, Ge, Sn, Pb), 15(V) (Sb, Bi), and 16(VI) (Te), i.e. elements which have a similar electronegativity (between 1.5-2.0). All EPFS materials have in common highly complex crystal structures, which are thought to be a consequence of their electron-poor bonding patterns. EPFS materials have an intrinsically very low – glass like - lattice thermal conductivity. The focus of this thesis is on combinations of group 12(II) (Zn) with 16 (V) (As, Sb), and 13(III) (B) with 14(IV) (Si).

ZnSb possesses a simple structure with 8 formula units in an orthorhombic unit cell, it is considered a stoichiometric compound without noticeable structural disorder. In this thesis ZnSb is used as a model system to establish more broadly structure–property correlations in Sb based EPFS materials.

ZnSb was established to possess an impurity band that determines electrical transport properties up to 300–400 K. Doping of ZnSb with Ag seems to enhance the impurity band by increasing the number of acceptor states and improving charge carrier density by two orders of magnitude. ZT values of Ag doped ZnSb are found to exceed 1 at 350 K. The origin of the low thermal conductivity of ZnSb was traced back to a multitude of localized low energy optic modes, acting as Einstein-like rattling modes.

ZnAs was accessed through high pressure synthesis. The compound is isostructural to ZnSb and possess an indirect band gap of 0.9 eV, which is larger than that for ZnSb (0.5 eV). The larger band gap is attributed to the higher polarity of Zn-As bonds. The electrical resistivity of ZnAs is higher and the Seebeck coefficient is lower compared to ZnSb. However, ZnAs and ZnSb exhibit similarly low lattice thermal conductivity, although As is considerably lighter than Sb. This was explained by their similar bonding properties.

Lastly, the longstanding mystery of SiB3 phases was resolved. The formation of metastable and disordered α-SiB3-x is fast and thus kinetically driven, whereas formation of stable β-SiB3 is slow and not quantitative unless high pressure conditions are applied. This thesis work established reproducible synthesis routes for both materials. The fast kinetics can be exploited for simultaneous synthesis and sintering of α -SiB3-x specimens in a SPS device. It is suggested that α -SiB3-x represents a promising refractory thermoelectric material.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University , 2019. , p. 114
Keywords [en]
Thermoelectric, Waste heat harvesting, Semiconductor, X-ray powder diffraction, SEM, Electron poor framework, EPFS, Green house effect
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-167789ISBN: 978-91-7797-700-1 (print)ISBN: 978-91-7797-701-8 (electronic)OAI: oai:DiVA.org:su-167789DiVA, id: diva2:1302165
Public defence
2019-05-24, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2019-04-29 Created: 2019-04-03 Last updated: 2019-04-17Bibliographically approved
List of papers
1. Transport properties of the ii v semiconductor znsb
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2013 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 1, no 4, p. 1407-1414Article in journal (Refereed) Published
Abstract [en]

The intermetallic compound ZnSb is an electron poor (II-V) semiconductor with interesting thermoelectric properties. Electrical resistivity, thermopower and thermal conductivity were measured on single crystalline and various polycrystalline specimens. The work establishes the presence of impurity band conduction as an intrinsic phenomenon of ZnSb. The impurity band governs electrical transport properties at temperatures up to 300-400 K after which ZnSb becomes an intrinsic conductor. Furthermore this work establishes an inherently low lattice thermal conductivity of ZnSb, which is comparable to the state-of-the- art thermoelectric material PbTe. It is argued that the impurity band relates to the presence of Zn defects and the low thermal conductivity to the electron-poor bonding properties of ZnSb.

National Category
Physical Chemistry Materials Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-88340 (URN)10.1039/c2ta00509c (DOI)000314633500058 ()
Funder
Swedish Research Council, 2010-4827
Note

AuthorCount:6;

Available from: 2013-03-18 Created: 2013-03-12 Last updated: 2019-04-05Bibliographically approved
2. Synthesis, Structure, and Properties of the Electron-Poor II-V Semiconductor ZnAs
Open this publication in new window or tab >>Synthesis, Structure, and Properties of the Electron-Poor II-V Semiconductor ZnAs
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2014 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 53, no 16, p. 8691-8699Article in journal (Refereed) Published
Abstract [en]

ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil highpressure techniques and structurally characterized by single-crystal and powder X-ray 7 diffraction (space group Pbca (No. 61), a = 5.6768(2) angstrom, b = 7.2796(2) angstrom, c = 7.5593(2) angstrom, Z = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn2As2, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn3As2 and ZnAs2. When heating at a rate of 10 K/min decomposition takes place at similar to 700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2-400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are similar to 1 Omega cm and +27 mu V/K, respectively. These values are considerably higher and lower, respectively, compared to Zn Sb. Above 150 K the thermal conductivity attains low values, around 2 W/m.K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of theta(D) = 234 K, theta(E1) = 95 K, and theta(E2) = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties.

National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-107622 (URN)10.1021/ic501308q (DOI)000340576900054 ()
Note

AuthorCount:7;

Available from: 2014-09-25 Created: 2014-09-22 Last updated: 2019-04-05Bibliographically approved
3. Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity
Open this publication in new window or tab >>Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 22, article id 224309Article in journal (Refereed) Published
Abstract [en]

The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9-400 K) and powder x-ray diffraction (300-725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below 60 cm(-1). Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures Theta(D) = 195 K, Theta(E1) = 78 K, and Theta(E2) = 277K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-delta.

National Category
Physical Sciences Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-118942 (URN)10.1103/PhysRevB.91.224309 (DOI)000356580300002 ()
Available from: 2015-07-24 Created: 2015-07-21 Last updated: 2019-04-05Bibliographically approved
4. Mysterious SiB3: Identifying the relation between a- and b-SiB3
Open this publication in new window or tab >>Mysterious SiB3: Identifying the relation between a- and b-SiB3
(English)Manuscript (preprint) (Other academic)
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-167786 (URN)
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-05Bibliographically approved

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