11 research outputs found
Investigation of metal-rich half-Heusler thermoelectrics : synthesis, structure and properties
Thermoelectric materials that can directly convert heat into electricity offer a possible
avenue to address the world’s increasing demand for energy. Metal-rich half-Heusler
compounds are of interest due to their favourable electronic transport properties.
Unfortunately, widespread application is limited by comparatively high thermal
conductivity.
The effect of processing and excess metal (Ni and Cu) on thermoelectric properties of
MNiSn, M0.5M’0.5NiSn (M = Ti, Zr, Hf) and Ti0.5Zr0.25Hf0.25NiSn materials was
investigated and is discussed in Chapters 3 and 4. This work revealed that Cu is effective
n-type dopant, which improves electronic properties of half-Heusler materials. Detailed
structure analysis, which included high-resolution synchrotron X-ray diffraction, neutron
powder diffraction and electron microscopy revealed that most of the excess metals are
randomly distributed on the interstitial sites, producing significant point defect scattering
of phonons. In addition, Cu segregation leads to grain-by-grain compositional variations,
with grains tending towards either half-Heusler or full-Heusler composition.
In addition to the microstructural and properties studies, an in-situ neutron powder
diffraction experiment was used to understand the formation of Ni-rich TiNi1+ySn (y = 0,
0.075 and 0.25) and multiphase M0.5M’0.5NiSn compositions during solid-state reaction.
As described in Chapter 5, the half-Heusler formation occurs through a complex,
multistep reaction, which involves many intermediates. ZrNiSn and Ti0.5Zr0.25Hf0.25NiSn
underwent spontaneous self-propagating combustions, which is a new route to prepare
impurity-free half-Heusler alloys.
The last results Chapter describes the analysis of neutron total scattering data using
Reverse Monte Carlo modelling. This study was performed to gain insight into spatial
distribution of excess Ni within the half-Heusler structure. It confirmed random
distribution of excess metal on the interstitial sites
Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn
TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5–3 mW m−1 K−2 for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m−1 K−1 at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn
Mechanistic Insights into the Formation of Thermoelectric TiNiSn from In Situ Neutron Powder Diffraction
Funding: The EPSRC is acknowledged for funding the research on half- Heusler thermoelectrics (EP/N01717X/1) and for studentships for S.A.B. and B.F.K. The STFC is acknowledged for allocation of neutron scattering beamtime at the ISIS facility (RB1610143).Half-Heusler alloys are leading contenders for application in thermoelectric generators. However, reproducible synthesis of these materials remains challenging. Here, we have used in situ neutron powder diffraction to monitor the synthesis of TiNiSn from elemental powders, including the impact of intentional excess Ni. This reveals a complex sequence of reactions with an important role for molten phases. The first reaction occurs upon melting of Sn (232 °C), when Ni3Sn4, Ni3Sn2, and Ni3Sn phases form upon heating. Ti remains inert with formation of Ti2Ni and small amounts of half-Heusler TiNi1+ySn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNi2y’Sn phases. Heusler phase formation is greatly accelerated by a second melting event near 750–800 °C. During annealing at 900 °C, full-Heusler TiNi2y’Sn reacts with TiNi and molten Ti2Sn3 and Sn to form half-Heusler TiNi1+ySn on a timescale of 3–5 h. Increasing the nominal Ni excess leads to increased concentrations of Ni interstitials in the half-Heusler phase and an increased fraction of full-Heusler. The final amount of interstitial Ni is controlled by defect chemistry thermodynamics. In contrast to melt processing, no crystalline Ti–Sn binaries are observed, confirming that the powder route proceeds via a different pathway. This work provides important new fundamental insights in the complex formation mechanism of TiNiSn that can be used for future targeted synthetic design. Analysis of the impact of interstitial Ni on the thermoelectric transport data is also presented.Publisher PDFPeer reviewe
Thermal properties of TiNiSn and VFeSb half-Heusler thermoelectrics from synchrotron x-ray powder diffraction
Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with αav = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced αav = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport
Mechanistic Insights into the Formation of Thermoelectric TiNiSn from In Situ Neutron Powder Diffraction
Half-Heusler alloys
are leading contenders for application in thermoelectric
generators. However, reproducible synthesis of these materials remains
challenging. Here, we have used in situ neutron powder diffraction
to monitor the synthesis of TiNiSn from elemental powders, including
the impact of intentional excess Ni. This reveals a complex sequence
of reactions with an important role for molten phases. The first reaction
occurs upon melting of Sn (232 °C), when Ni3Sn4, Ni3Sn2, and Ni3Sn phases
form upon heating. Ti remains inert with formation of Ti2Ni and small amounts of half-Heusler TiNi1+ySn only occurring
near 600 °C, followed by the emergence of TiNi and full-Heusler
TiNi2y’Sn phases. Heusler phase
formation is greatly accelerated by a second melting event near 750–800
°C. During annealing at 900 °C, full-Heusler TiNi2y’Sn reacts with TiNi and molten Ti2Sn3 and Sn to form half-Heusler TiNi1+ySn on a timescale of 3–5 h. Increasing the nominal Ni
excess leads to increased concentrations of Ni interstitials in the
half-Heusler phase and an increased fraction of full-Heusler. The
final amount of interstitial Ni is controlled by defect chemistry
thermodynamics. In contrast to melt processing, no crystalline Ti–Sn
binaries are observed, confirming that the powder route proceeds via
a different pathway. This work provides important new fundamental
insights in the complex formation mechanism of TiNiSn that can be
used for future targeted synthetic design. Analysis of the impact
of interstitial Ni on the thermoelectric transport data is also presented
Dataset for Mechanistic Insights into the Formation of Thermoelectric TiNiSn from In Situ Neutron Powder Diffraction
Powder diffraction datasets; data contained in figure
Thermal properties of TiNiSn and VFeSb half-Heusler thermoelectrics from synchrotron x-ray powder diffraction
Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with αav = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced αav = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport
Grain-by-Grain Compositional Variations and Interstitial Metals - A New Route toward Achieving High Performance in Half-Heusler Thermoelectrics
Dataset to support the associated paper