21 research outputs found
Sb and Se Substitution in CsBi<sub>4</sub>Te<sub>6</sub>: The Semiconductors CsM<sub>4</sub>Q<sub>6</sub> (M = Bi, Sb; Q = Te, Se), Cs<sub>2</sub>Bi<sub>10</sub>Q<sub>15</sub>, and CsBi<sub>5</sub>Q<sub>8</sub>
The solid solutions of CsBi<sub>4</sub>Te<sub>6</sub>, a high ZT material at a low temperature region, with Sb and Se
were synthesized with general formulas CsBi<sub>4‑<i>x</i></sub>Sb<sub><i>x</i></sub>Te<sub>6</sub> and CsBi<sub>4</sub>Te<sub>6‑<i>y</i></sub>Se<sub><i>y</i></sub>. The introduction of Sb and Se in the lattice of CsBi<sub>4</sub>Te<sub>6</sub> is possible but only to a limited extent. The
Sb and Se atoms substituted are not uniformly distributed over all
crystallographic sites but display particular site preferences. The
structure of new Sb/Bi solid solutions retains the original framework
of CsBi<sub>4</sub>Te<sub>6</sub> composed of NaCl-type Bi/Te slabs
interconnected by characteristic Bi–Bi bonds and Cs atoms located
in the interlayer space. A structurally modified phase in Se/Te solid
solutions was found from the reactions targeted for 0.2 < <i>y</i> < 2.4 with the formula of CsBi<sub>5</sub>Te<sub>7.5‑<i>y</i></sub>Se<sub><i>y</i></sub> (or Cs<sub>2</sub>Bi<sub>10</sub>Q<sub>15</sub>, (Q = Se, Te)). The new structure is
constructed by the same structural motif with an extended Bi/Te slab
(29 Å) compared to that in CsBi<sub>4</sub>Te<sub>6</sub> (23
Å). The CsBi<sub>5</sub>Te<sub>7.5‑<i>y</i></sub>Se<sub><i>y</i></sub> possesses Bi/Te slabs that extend
by an additional “Bi<sub>2</sub>Te<sub>3</sub>” unit
compared to the structure of CsBi<sub>4</sub>Te<sub>6</sub>, which
implies the existence of a phase homology of compounds with the adjustable
parameter being the width of the Bi/Q slab. In the reactions targeted
for the compounds with higher <i>y</i>, a new phase CsBi<sub>5</sub>Te<sub>3.6</sub>Se<sub>4.4</sub> with a different type of
framework was found. The electrical conductivity and thermopower for
the selected samples show p-type conduction with metallic behavior.
The room temperature values measured are in the range of 300–1100
S/cm and 100–150 μV/K for Sb-substituted samples and
20–500 S/cm and 70–140 μV/K for Se-substituted
samples, respectively. Thermal conductivities of these samples are
in the range of 0.9–1.2 W/m·K at room temperature. Tailoring
the transport behavior of these materials for thermoelectric applications
may be achieved by doping, as is possible for the parent compound
CsBi<sub>4</sub>Te<sub>6</sub>
Carrier Mobility Modulation in Cu<sub>2</sub>Se Composites Using Coherent Cu<sub>4</sub>TiSe<sub>4</sub> Inclusions Leads to Enhanced Thermoelectric Performance
Carrier
transport engineering in bulk semiconductors
using inclusion
phases often results in the deterioration of carrier mobility (μ)
owing to enhanced carrier scattering at phase boundaries. Here, we
show by leveraging the temperature-induced structural transition between
the α-Cu2Se and β-Cu2Se polymorphs
that the incorporation of Cu4TiSe4 inclusions
within the Cu2Se matrix results in a gradual large drop
in the carrier mobility at temperatures below 400 K (α-Cu2Se), whereas the carrier mobility remains unchanged at higher
temperatures, where the β-Cu2Se polymorph dominates.
The sharp discrepancy in the electronic transport within the α-Cu2Se and β-Cu2Se matrices is associated with
the formation of incoherent α-Cu2Se/Cu4TiSe4 interfaces, owing to the difference in their atomic
structures and lattice parameters, which results in enhanced carrier
scattering. In contrast, the similarity of the Se sublattices between
β-Cu2Se and Cu4TiSe4 gives
rise to coherent phase boundaries and good band alignment, which promote
carrier transport across the interfaces. Interestingly, the different
cation arrangements in Cu4TiSe4 and β-Cu2Se contribute to enhanced phonon scattering at the interfaces,
which leads to a reduction in the lattice thermal conductivity. The
large reduction in the total thermal conductivity while preserving
the high power factor of β-Cu2Se in the (1–x)Cu2Se/(x)Cu4TiSe4 composites results in an improved ZT of
1.2 at 850 K, with an average ZT of 0.84 (500–850
K) for the composite with x = 0.01. This work highlights
the importance of structural similarity between the matrix and inclusions
when designing thermoelectric materials with improved energy conversion
efficiency
Fabrication and Thermoelectric Properties of n‑Type CoSb<sub>2.85</sub>Te<sub>0.15</sub> Using Selective Laser Melting
We
report a nonequilibrium fabrication method of n-type CoSb<sub>2.85</sub>Te<sub>0.15</sub> skutterudites using selective laser melting (SLM)
technology. A powder of CoSb<sub>2.85</sub>Te<sub>0.15</sub> was prepared
by self-propagating high-temperature synthesis (SHS) and served as
the raw material for the SLM process. The effect of SLM processing
parameters such as the laser power and scanning speed on the quality
of the forming CoSb<sub>2.85</sub>Te<sub>0.15</sub> thin layers was
systematically analyzed, and the optimal processing window for SLM
was determined. A brief postannealing at 450 °C for 4 h, following
the SLM process, has resulted in a phase-pure CoSb<sub>2.85</sub>Te<sub>0.15</sub> bulk material deposited on a Ti substrate. The Seebeck
coefficient of the annealed SLM prepared bulk material is close to
that of the sample prepared by the traditional sintering method, and
its maximum <i>ZT</i> value reached 0.56 at 823 K. Moreover,
a Ti–Co–Sb ternary compound transition layer of about
70 μm in thickness was found at a dense interface between CoSb<sub>2.85</sub>Te<sub>0.15</sub> and the Ti substrate. The contact resistivity
was measured as 37.1 μΩcm<sup>2</sup>. The results demonstrate
that SLM, coupled with postannealing, can be used for fabrication
of incongruently melting skutterudite compounds on heterogeneous substrates.
This lays an important foundation for the follow-up research utilizing
energy efficient SHS and SLM processes in rapid printing of thermoelectric
modules
High‑<i>T</i><sub>c</sub> Ferromagnetism and Electron Transport in p‑Type Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> Semiconductors
Single-phase
polycrystalline powders of Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> (<i>x</i> = 0 and 0.13) were synthesized by a solid-state reaction
of the elements at 773 K. X-ray diffraction on Fe<sub>0.87</sub>Sn<sub>0.13</sub>Sb<sub>2</sub>Se<sub>4</sub> single-crystal and powder
samples indicates that the compound is isostructural to FeSb<sub>2</sub>Se<sub>4</sub> in the temperature range from 80 to 500 K, crystallizing
in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12). Electron-transport data reveal a marginal alteration in
the resistivity, whereas the thermopower drops by ∼60%. This
suggests a decrease in the activation energy upon isoelectronic substitution
of 13% Fe by Sn. Magnetic susceptibility and magnetization measurements
from 2 to 500 K reveal that the Fe<sub>1–<i>x</i></sub>Sb<sub>2</sub>Sn<sub><i>x</i></sub>Se<sub>4</sub> phases exhibit ferromagnetic behavior up to ∼450 K (<i>x</i> = 0) and 325 K (<i>x</i> = 0.13). Magnetotransport
data for FeSb<sub>2</sub>Se<sub>4</sub> reveal large negative magnetoresistance,
suggesting spin polarization of free carriers in the sample. The high-<i>T</i><sub>c</sub> ferromagnetism in Fe<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>Sb<sub>2</sub>Se<sub>4</sub> phases and the decrease in <i>T</i><sub>c</sub> of the Fe<sub>0.87</sub>Sn<sub>0.13</sub>Sb<sub>2</sub>Se<sub>4</sub> sample are rationalized by taking into account (1) the separation
between neighboring magnetic centers in the crystal structures and
(2) the formation of bound magnetic polarons, which overlap to induce
long-range ferromagnetic ordering
Band Ordering and Dynamics of Cu<sub>2–<i>x</i></sub>Te and Cu<sub>1.98</sub>Ag<sub>0.2</sub>Te
<sup>63</sup>Cu, <sup>65</sup>Cu, and <sup>125</sup>Te NMR measurements
are reported for Cu<sub>2–<i>x</i></sub>Te and Cu<sub>1.98</sub>Ag<sub>0.2</sub>Te. The results demonstrate an onset of
Cu-ion hopping below room temperature, including an activation behavior
consistent with high-temperature transport measurements but with a
significant enhancement of the hopping barriers with Ag substitution.
We also separated the Korringa behavior by combining NMR line shape
and relaxation measurements, thereby identifying large negative chemical
shifts for both nuclei, as well as large Cu and Te s-state contributions
in the valence band. Further comparison was obtained through heat
capacity measurements and chemical shifts computed by density functional
methods. The large diamagnetic chemical shifts coincide with behavior
previously identified for materials with topologically nontrivial
band inversion, and in addition, the large metallic shifts point to
analogous features in the valence band density of states, suggesting
that Cu<sub>2</sub>Te may have similar inverted features
Optimized Thermoelectric Properties of Sb-Doped Mg<sub>2(1+<i>z</i>)</sub>Si<sub>0.5–<i>y</i></sub>Sn<sub>0.5</sub>Sb<sub><i>y</i></sub> through Adjustment of the Mg Content
Mg<sub>2</sub>Si<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub> compounds are low-cost and environmentally
friendly
thermoelectric materials expected to be applied as power generators
in the intermediate temperature range. Optimization of the thermoelectric
properties of Mg<sub>2</sub>Si<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub> compounds can be accomplished by
the precise control and adjustment of the Mg content. A series of
Mg<sub>2(1+<i>z</i>)</sub>Si<sub>0.5–<i>y</i></sub>Sn<sub>0.5</sub>Sb<sub><i>y</i></sub> (0 ≤ <i>y</i> ≤ 0.015 and 0 ≤ <i>z</i> ≤
0.15) compounds with controlled Mg content were synthesized by a two-step
solid-state reaction method, followed by a spark plasma sintering
technique. On the basis of optimized thermoelectric properties via
doping with Sb, the effect of a variable content of Mg spanning from
understoichiometry to overstoichiometry has been systematically explored.
The results indicate that when the actual Mg content exceeds the stoichiometric
amount, the dominant point defects in Mg<sub>2(1+<i>z</i>)</sub>Si<sub>0.49</sub>Sn<sub>0.5</sub>Sb<sub>0.01</sub> compounds
are interstitial Mg and Si/Sn vacancies. At the same time, the electron
concentration is enhanced with increasing content of Mg. However,
when the actual Mg content is substoichiometric, the point defects
consist mainly of Mg vacancies that tend to counteract the doping
effect of Sb. Thus, the electron concentration of the nominal Mg<sub>2</sub>Si<sub>0.49</sub>Sn<sub>0.5</sub>Sb<sub>0.01</sub> compound
(in reality a 2 mol % deficiency of Mg) is markedly lower compared
with the Mg<sub>2.10</sub>Si<sub>0.49</sub>Sn<sub>0.5</sub>Sb<sub>0.01</sub> compound, which actually had a 2 mol % excess of Mg. Furthermore,
a modest overstoichiometry of Mg enhances the power factor and improves
the dimensionless figure of merit. The highest value of <i>ZT</i> = 1.25 at 800 K among the compounds was obtained on Mg<sub>2.20</sub>Si<sub>0.49</sub>Sn<sub>0.5</sub>Sb<sub>0.01</sub>, which had an
actual Mg excess of 5.5 mol %. The study suggests that point defects,
such as interstitial Mg and Si/Sn vacancies, which are created by
an overstoichiometric content of Mg, have a positive effect on the
electron concentration and thermoelectric properties of n-type Mg<sub>2</sub>Si<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub>-based compounds. This research has also established an essential
foundation for further optimization of the thermoelectric properties
of Mg<sub>2</sub>Si<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub> compounds
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Å, <i>b</i> = 4.262(2) Å, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)Se<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite Homologue with Promising Thermoelectric Properties
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Å, <i>b</i> = 4.262(2) Å, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)Se<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples
Optimization of the Electronic Band Structure and the Lattice Thermal Conductivity of Solid Solutions According to Simple Calculations: A Canonical Example of the Mg<sub>2</sub>Si<sub>1–<i>x</i>–<i>y</i></sub>Ge<sub><i>x</i></sub>Sn<sub><i>y</i></sub> Ternary Solid Solution
The
dependence of the electronic band structure of Mg<sub>2</sub>Si<sub>0.3–<i>x</i></sub>Ge<sub><i>x</i></sub>Sn<sub>0.7</sub> and Mg<sub>2</sub>Si<sub>0.3</sub>Ge<sub><i>y</i></sub>Sn<sub>0.7–<i>y</i></sub> (0 ≤ <i>x</i>, and <i>y</i> ≤ 0.05) ternary solid solutions
on composition and temperature is explained by a simple linear model,
and the lattice thermal conductivity of solid solutions with different
Si/Ge/Sn ratios is predicted by the Adachi model. The experimental
results show excellent consistency with the calculations, which suggests
that the approach might be suitable for describing the electronic
band structure and the lattice thermal conductivity of other solid
solutions using these simple calculations. Beyond this, it is observed
that the immiscible gap in the Mg<sub>2</sub>Si<sub>1–<i>x</i></sub>Sn<sub><i>x</i></sub> binary system is
narrowed via the introduction of Mg<sub>2</sub>Ge. Moreover, for the
Sb-doped solid solutions Mg<sub>2.16</sub>(Si<sub>0.3</sub>Ge<sub><i>y</i></sub>Sn<sub>0.7–<i>y</i></sub>)<sub>0.98</sub>Sb<sub>0.02</sub> (0 ≤ <i>y</i> ≤
0.05), the energy offset between the light conduction band and the
heavy conduction band at higher temperatures (500–800 K) will
decrease with an increase in Ge content, thus making a contribution
to the conduction band degeneracy and enhancing the power factor in
turn. Meanwhile, mass fluctuation and strain field scattering processes
are enhanced when Ge is substituted for Sn in Mg<sub>2.16</sub>(Si<sub>0.3</sub>Ge<sub><i>y</i></sub>Sn<sub>0.7<i>–y</i></sub>)<sub>0.98</sub>Sb<sub>0.02</sub> (0 ≤ <i>y</i> ≤ 0.05) because of the large discrepancy between the mass
and size of Ge and Sn, and the lattice thermal conductivity is decreased
as a consequence. Thus, the thermoelectric performance is improved,
with the figure of merit ZT being >1.45 at ∼750 K and the
average
ZT value being between 0.9 and 1.0 in the range of 300–800
K, which is one of the best results for Sb-doped Mg<sub>2</sub>Si<sub>1–<i>x</i>–<i>y</i></sub>Ge<sub><i>x</i></sub>Sn<sub><i>y</i></sub> systems with
a single phase
Manipulating the Combustion Wave during Self-Propagating Synthesis for High Thermoelectric Performance of Layered Oxychalcogenide Bi<sub>1–<i>x</i></sub>Pb<sub><i>x</i></sub>CuSeO
Novel
time- and energy-efficient synthesis methods, especially
those adaptable to large-scale industrial processing, are of vital
importance for broader applications of thermoelectrics. We herein
reported a case study of layer-structured oxychalcogenides Bi<sub>1–<i>x</i></sub>Pb<sub><i>x</i></sub>CuSeO
(<i>x</i> = 0–10%) with emphases on the reaction
mechanism of self-propagating high-temperature synthesis (SHS) and
the impact of SHS conditions on the thermoelectric properties. The
combined results of X-ray powder diffraction, differential scanning
calorimetry, and quenching experiments corroborated that the SHS process
of BiCuSeO consisted two fast binary SHS reactions (2 Bi+3 Se →
Bi<sub>2</sub>Se<sub>3</sub> and 2 Cu+Se → Cu<sub>2</sub>Se)
intimately coupled with two relatively slow solid-state diffusion
reactions (2 Bi<sub>2</sub>Se<sub>3</sub>+B<sub>2</sub>O<sub>3</sub> → 3 Bi<sub>2</sub>SeO<sub>2</sub> and then Bi<sub>2</sub>SeO<sub>2</sub>+Cu<sub>2</sub>Se → 2 BiCuSeO). The formation
rate of the reaction intermediate Bi<sub>2</sub>SeO<sub>2</sub> was
the bottleneck in the SHS process of BiCuSeO. Importantly, we found
that adding PbO in the starting materials has (i) facilitated the
formation of Bi<sub>2</sub>SeO<sub>2</sub> and thus significantly
reduced the SHS reaction time; (ii) improved the phase purity and
sample homogeneity; (iii) increased the power factor via increasing
both carrier concentration and effective mass; and (iv) reduced the
lattice thermal conductivity via more point defect phonon scattering.
As a result, a state-of-the-art <i>ZT</i> value ∼1.2
has been attained at 923 K for Bi<sub>0.94</sub>Pb<sub>0.06</sub>CuSeO.
These results not only open a new avenue for mass production of single
phased multinary thermoelectric materials but also inspire more investigation
into the SHS mechanisms of multinary materials in diverse fields of
material science and engineering