Feasibility of heat conversion to electricity by new spin Seebeck based thermoelectrics

Lead Research Organisation: Loughborough University
Department Name: Physics

Abstract

The UK has committed to meet an 80% reduction in greenhouse emissions relative to 1990 by 2050. Currently, it is recognised that this will likely stem from a diverse portfolio of renewable and existing energy sources as well as the development of technologies for energy storage, conversion and usage. As the majority of the UK's total energy consumption can be attributed to heating (48%) and transport (38%) these are clearly significant targets for change.

One possible route for energy storage on a domestic scale is the storage of heat that could later be converted to electricity if required. Energy harvesters designed to recycle or use various forms of energy that would otherwise be wasted (such as kinetic, thermal, acoustic, or solar), could also find applications with regards to reduction of energy demands. A technology that applies to both these applications is the thermoelectric energy generator (TEG).

The TEG is typically based on the Seebeck effect: a physical process that results in the generation of an electric current when a temperature difference exists between two terminals. Advantages of this technology include reliability, flexibility, and relatively small volumes, however due to low efficiencies and high costs it is currently limited to niche markets. One of the bottlenecks for improvement of the TEG efficiency is the co-dependence of the key material properties (i.e., thermal and electric conductivities) according to the Wiedemann-Franz law. Whilst some progress has been made on this by nano-engineering, there is still some way to go before widespread commercialisation becomes viable.

A solution to this bottleneck could be found in a new phenomena that involves the interplay of thermal and electron spin currents: the spin Seebeck effect. It is similar to the Seebeck effect in that a thermal gradient can be used to generate a current, but with two main differences: the material must be magnetic (whether metallic, insulating or semiconducting), and the electric current generated is spin polarised. This is significant as it has led to the observation of spin dependant conductivity, a feature that could allow us to sidestep the limit imposed by the Wiedemann-Franz Law and thus improve the efficiency of TEGs further.

Harnessing the maximum spin polarised current generated by the spin Seebeck effect typically requires the use of expensive platinum contacts. For such technology to become economically viable therefore would require research into cheaper alternatives. It has been shown that small amounts of platinum, bismuth or tantalum in otherwise 'inactive' copper can result in a similar harvested voltage compared to pure platinum contacts. The aim of this research project therefore, is to explore the possibility of alternative metal contacts with respect to spin Seebeck effect based TEGs and to assess the viability of such an application.

Planned Impact

The potential technology output of this project to reduce energy use as well as provide a vector for energy storage and conversion ultimately aligns it with the UK's 2050 commitment to reduce carbon emissions. As such, it could impact on several areas of society and industry, the most obvious of which is the potential improvement of thermoelectric generators (TEGs). The beneficiaries of this research are outlined below.

1. Society and the public
An increase in efficiency of thermoelectric generators (TEGs) could have a significant impact on society over the next 10-50 years as they become more economically viable. For example, the largest source of immediate energy savings would be the installation of a TEG module in cars where typically 60% of fuel burnt is lost as waste heat (full installation of a TEG with 20% efficiency would result in a UK annual saving of 4 hundred thousand oil equivalent tonnes). Not only would TEGs be useful as energy harvesters, but they could also serve as vectors for energy storage as they allow for conversion of heat to electricity (and vice versa). Any technology that can utilise or direct heat in this way will contribute to the stability of the UK's energy portfolio and thus contribute to stable or reduced energy costs (in the long-term).

2. Emerging industries and the UK economy
Although thermoelectric cells are not a new concept they have been limited by material costs and relatively low efficiencies. Recent improvements in efficiency have required nano-engineering of the basic material that can be difficult to up-scale. Should a new device be developed that could be used to circumvent the problem of low efficiency and fabrication costs then it is highly likely that it will lead to a new (industrial) market. This could, for example, bring in new jobs for production and distribution of the TEGs, or income from patented technology.

3. The computing industry
For the last 40 years the miniaturisation of transistors in computers has closely followed Moore's law. As transistors continue to be scaled down, however, the increasing power density results in instability of the device and this issue needs to be addressed for continued progress. Spintronics is one research field that aims to move beyond Moore with the manipulation of spin polarised currents. It is not unreasonable, therefore, to argue that the production of a spin polarised current coupled with the control of heat flow in a system could lead to the development of several new spintronic devices. This will have both economic and technological impact on society as it will significantly reduce the heat barrier faced by downscaling components that is common to charge transport based devices. As a result this could lead to new technology and more efficient computing (technology exports and reduced energy costs). For example, magnetic heat switches could be one result of this research area and would lead to intelligent thermal management in electronic devices as they would have the potential to collect waste heat and divert it away from important components. Due to high consumer demand it is realistic to expect commercialisation of this technology within the next 5-20 years.

Publications

10 25 50
publication icon
Caruana A (2016) Demonstration of polycrystalline thin film coatings on glass for spin Seebeck energy harvesting in physica status solidi (RRL) - Rapid Research Letters

publication icon
Cox C (2019) Anomalous Nernst effect in Co 2 MnSi thin films in Journal of Physics D: Applied Physics

 
Description 1) Optimisation of the experimental conditions required to make uniform, reproducable Fe3O4:Pt bilayers.

2) Validation of a thermal model against experimental results as a first step in standardising measurements.
This is essential to developing our understanding (and design) of a potential waste heat harvesting (thermoelectric) device based on this effect.

3) Observation of comparable voltage generation in Fe3O4 deposited on glass.
This is the first step towards low cost, large area fabrication of such devices.

4) Observation of spin conversion in doped alloys.
This demonstrates proof of concept of the grant - that alloys (rather than Pt) can be used as the spin convertor in such devices.
Exploitation Route 1) The deposition parameters required for the thin films (Fe3O4, Co2MnSi) could be applied to any other application using these materials (e.g. Co2MnSi is common in spintronics applications).

2) Standardisation of measurements will be necessary for the research community to compare different materials. This work provided reference material for this.

3 and 4) These findings have provided the backbone of a patent submitted in November 2015. This could be taken forward to develop new technology.
Sectors Electronics,Energy

 
Description The concepts of magnetism, energy harvesting and thermoelectrics were disseminated at various science fairs and workshops. For example, Big Bang Fairs in London and Derby and Loughborough's Community day during British Science week. This had societal impact by increasing public engagement.
Sector Education
Impact Types Societal

 
Description EPSRC Fellowship
Amount £843,483 (GBP)
Funding ID EP/P006221/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2017 
End 06/2021
 
Description Loughborough University Enterprise Fund 2
Amount £16,500 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 09/2016 
End 07/2017
 
Description School of Science Strategic and Major Operational Fund
Amount £18,198 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 04/2015 
End 07/2016
 
Description Small Research Grant
Amount £15,000 (GBP)
Funding ID RG160550 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2017 
End 03/2018
 
Description University Enterprise Fund
Amount £5,000 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 08/2015 
End 09/2016
 
Description University Strategic Investment Fund
Amount £45,000 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 04/2014 
End 07/2014
 
Title Demonstration of polycrystalline thin film coatings on glass for spin Seebeck energy harvesting - dataset 
Description Zip file with all raw XRD, XRR, transport data.Origin project(s) containing raw and processed data for related publication.
Figure 1 was schematic only and not included here.Figure 2 and Figure S2 are in the same origin project (simple and extended TEM data).
Figure captions:Figure 2 TEM analysis of SSE5a. a) & b) STEM/BF and HAADF images of the thin film, respectively. c) Conventional HREM of the PM Pt layer. d) EDX line-scan performed perpendicular to the interfaces of the layers.Figure 3 Summary of the magnetic, electric and thermal properties. a) Spin Seebeck voltage, VISHE (symbols), as a function of applied magnetic field plotted alongside magnetic data (line). b) Resistivity of the devices as a function of tPM. c) Normalised spin Seebeck voltage, SSSE, as a function of tPM, plotted alongside simulated SSSE ( ?SH = 0.1, ?SD = 2 nm, Ms = 90 Am 2/kg, D = 71x10 41 Jm 2[19], gr = 1,3 & 5x10 18 m -2[20]). d) Definition of the parameters used to describe heat flow, (e) & (f) Change in ?T2, and SSSE with substrate's thermal conductivity, ?3.Figure S1 Characterisation of the Fe 3O 4 film. a) SQUID magnetometry above and below the Verwey transition, TV. b) Resistivity as a function of temperature. c) XRD of a set of 4 separately prepared Fe 3O 4 films. The inset shows a close-up of the (311), (222) peaks. d) Example XRR data (symbols) and fit (solid line), indicating thickness = 79 nm, roughness = 1.5 nm.Figure S2 TEM analysis of SSE5a. a) & b) STEM/BF and HAADF images of the thin film, respectively. c) Conventional HREM of the PM Pt layer. d) & e) STEM/BF image of the thin film stack and corresponding EDX line-scan performed perpendicular to the interfaces of the layers, respectively, and f) schematic of the grain growth described in the text.Figure S3 Characteristics of the bilayer film. a) XRD of SSE5a (2.5 nm Pt) and SSE20a (7.3 nm Pt). Inset shows a close-up of the Pt peak. b) XRR fit of SSE5a; Pt thickness = 2.5 nm, roughness = 2 nm.Figure S4 Example spin Seebeck measurement for SSE7a ( tPM = 3.2 nm) measured in fixed field as a function of temperature difference. Note that the sign convention for measurements, defined in Fig 1(a) of the main manuscript follows from Uchida et al.[6]. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Demonstration_of_polycrystalline_thin_film_coatings_...
 
Title Demonstration of polycrystalline thin film coatings on glass for spin Seebeck energy harvesting - dataset 
Description Zip file with all raw XRD, XRR, transport data.Origin project(s) containing raw and processed data for related publication.
Figure 1 was schematic only and not included here.Figure 2 and Figure S2 are in the same origin project (simple and extended TEM data).
Figure captions:Figure 2 TEM analysis of SSE5a. a) & b) STEM/BF and HAADF images of the thin film, respectively. c) Conventional HREM of the PM Pt layer. d) EDX line-scan performed perpendicular to the interfaces of the layers.Figure 3 Summary of the magnetic, electric and thermal properties. a) Spin Seebeck voltage, VISHE (symbols), as a function of applied magnetic field plotted alongside magnetic data (line). b) Resistivity of the devices as a function of tPM. c) Normalised spin Seebeck voltage, SSSE, as a function of tPM, plotted alongside simulated SSSE ( ?SH = 0.1, ?SD = 2 nm, Ms = 90 Am 2/kg, D = 71x10 41 Jm 2[19], gr = 1,3 & 5x10 18 m -2[20]). d) Definition of the parameters used to describe heat flow, (e) & (f) Change in ?T2, and SSSE with substrate's thermal conductivity, ?3.Figure S1 Characterisation of the Fe 3O 4 film. a) SQUID magnetometry above and below the Verwey transition, TV. b) Resistivity as a function of temperature. c) XRD of a set of 4 separately prepared Fe 3O 4 films. The inset shows a close-up of the (311), (222) peaks. d) Example XRR data (symbols) and fit (solid line), indicating thickness = 79 nm, roughness = 1.5 nm.Figure S2 TEM analysis of SSE5a. a) & b) STEM/BF and HAADF images of the thin film, respectively. c) Conventional HREM of the PM Pt layer. d) & e) STEM/BF image of the thin film stack and corresponding EDX line-scan performed perpendicular to the interfaces of the layers, respectively, and f) schematic of the grain growth described in the text.Figure S3 Characteristics of the bilayer film. a) XRD of SSE5a (2.5 nm Pt) and SSE20a (7.3 nm Pt). Inset shows a close-up of the Pt peak. b) XRR fit of SSE5a; Pt thickness = 2.5 nm, roughness = 2 nm.Figure S4 Example spin Seebeck measurement for SSE7a ( tPM = 3.2 nm) measured in fixed field as a function of temperature difference. Note that the sign convention for measurements, defined in Fig 1(a) of the main manuscript follows from Uchida et al.[6]. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
URL https://figshare.com/articles/Demonstration_of_polycrystalline_thin_film_coatings_on_glass_for_spin_...
 
Title Optimisation of Co2MnSi thin films and ANE 
Description Characterisation of Co2MnSi thin films and corresponding ANE measurements.
Origin dataset associated with article: Anomalous Nernst Effect in Co 2MnSi Thin Films
AbstractSeparation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect becomes non-negligible. Co2MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L21 order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co2MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the anomalous Nernst Effect (ANE) is then investigated to inform future measurements of the spin Seebeck. A maximum ANE coefficient of 0.662µV/K is found for an A2 disordered polycrystalline Co2MnSi film. This value is comparable to ordered Heusler thin films deposited on to single crystal substrates but obtained at a far lower fabrication temperature and material cost.
 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Optimisation_of_Co2MnSi_thin_films_and_ANE/5117569/1
 
Title Optimisation of Co2MnSi thin films and ANE 
Description Characterisation of Co2MnSi thin films and corresponding ANE measurements.
Origin dataset associated with article: Anomalous Nernst Effect in Co 2MnSi Thin Films
AbstractSeparation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect becomes non-negligible. Co2MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L21 order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co2MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the anomalous Nernst Effect (ANE) is then investigated to inform future measurements of the spin Seebeck. A maximum ANE coefficient of 0.662µV/K is found for an A2 disordered polycrystalline Co2MnSi film. This value is comparable to ordered Heusler thin films deposited on to single crystal substrates but obtained at a far lower fabrication temperature and material cost.
 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Optimisation_of_Co2MnSi_thin_films_and_ANE/5117569
 
Description Investigating Effect of Interface on Spin Injection 
Organisation Science and Technologies Facilities Council (STFC)
Department ISIS Neutron and Muon Source
Country United Kingdom 
Sector Academic/University 
PI Contribution Samples
Collaborator Contribution Characterisation: Off specular x-ray reflectivity measurements and grazing incidence x-ray diffraction.
Impact Beamtime awarded. Still early measurements.
Start Year 2016
 
Description Sample exchange 
Organisation National Institute of Meteorological Research (INRIM)
Country Italy 
Sector Academic/University 
PI Contribution Samples provided to INRIM for analysis. Deposition of Pt on bulk YIG for spin Peltier measurements.
Collaborator Contribution Calibration measurements of new measurement system. Expertise.
Impact Initial data obtained. Too early to list other outcomes.
Start Year 2017
 
Description Sample exchange 2 
Organisation University of York
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Samples provided for scanning thermal microscopy.
Collaborator Contribution Measurements.
Impact Still early.
Start Year 2017
 
Description Sample exchange 3 
Organisation Durham University
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Spin Seebeck Measurements
Collaborator Contribution CFB:Pt, W, Ta samples provided for characterisation of SSE. This has been followed up by a series of Co_xFe_yB_z films with and without Pt for further measurement.
Impact Initial measurements indicated very low signal, which is not unexpected due to the thermal conductivity of the magnetic layer. Follow up measurements are to build a comparative study.
Start Year 2017
 
Description TEM analysis 
Organisation Loughborough University
Country United Kingdom 
Sector Academic/University 
PI Contribution Supply of samples.
Collaborator Contribution TEM analysis of samples
Impact In the process of submitting paper.
Start Year 2015
 
Title Apparatus and Method for Harvesting Electrical Energy 
Description Proposed design for a low cost waste heat energy harvesting device based on the spin Seebeck effect. 
IP Reference GB1521016.4 
Protection Patent application published
Year Protection Granted 2016
Licensed No
Impact Have only just initiated discussions with regards to the next step.
 
Description Big Bang Fair, London 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact Several school groups were introduced to the concepts such as magnetic fields (ferrofluids) and peltier cells (solid state cooling) by integrating with Dr K. Morrison or student helpers provided by Loughborough University. The students were also encouraged to think about suitable alternative energy sources and the best locations for them across the globe (building on the UK distribution of solar, wind and tidal sources).

This interactive stand sparked several keen questions from students, parents and teachers.

Engagement of students with the ferrofluid display in particular led to several questions with regards to electricity and magnetism, helping students to 'visualise' electric and magnetic fields.
Year(s) Of Engagement Activity 2013,2014
 
Description Community day 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact During British Science Week the campus is open on the weekend and the local community is invited to interact with various aspects of science and engineering. The primary aim is to inspire children to learn more about STEM, but it also provides the opportunity to communicate the main aims of local research to the community. Typically, ~1000 attendees. I participated by designing and running several tables/events over the years.
Year(s) Of Engagement Activity 2014,2015,2016,2017,2018,2019
URL https://kmphysics.com/2019/03/10/stem-community-day-at-loughborough-16th-march-2019/
 
Description WISE workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Women in Science and Engineering Event at a local school to try and encourage more girls into science related subjects by demonstrated the types of careers they could have.
Year(s) Of Engagement Activity 2015