Absolute Value Of Seebeck Coefficient Research Articles

Electrical transport and thermoelectric properties of PbTe1−xIx synthesized by high pressure and high temperature

N-type thermoelectric materials PbTe doped with iodine (PbTe1−xIx, x = 0.0001, 0.0003, 0.0007 and 0.001) were synthesized by high pressure and high temperature method (HPHT). Thermoelectric properties of PbTe1−xIx are investigated in the range of 300–600 K. The carrier concentration of PbTe prepared by HPHT is more sensitive to I content compared to that of the PbTe1−xIx sample prepared by other methods. The resistivities and the absolute values of Seebeck coefficients for PbTe1−xIx increase while the thermal conductivities decrease with increasing temperature. A large power factor of 39.1 μW/(cmK2) at room temperature is obtained with the carrier concentration of 9.7 × 1018 cm−3, which is much higher than that of Sb and Bi doped PbTe with the same synthesizing conditions. The high power factor is attributed to the high Hall mobility and large effective mass. The maximum figure-of-merit, ∼0.71 @ 600 K, is obtained for the samples with the carrier concentration of 9.7 × 1018 cm−3.

Structural and Thermoelectric Properties of Ternary Full-Heusler Alloys

The thermoelectric properties of ternary full-Heusler alloys, Co2YZ, which are in a ferromagnetic state up to high temperature above 300 K, were measured and are discussed in terms of the crystal structure and electronic states. Among the full-Heusler alloys studied, the Co2MnSi sample exhibited the highest absolute value of Seebeck coefficient and also the highest electrical conductivity in the temperature range from 300 K to 1023 K. The highest power factor of 2.9 × 10−3 W/m-K2 was obtained for the Co2MnSi sample at 550 K, demonstrating the potential of half-metallic full-Heusler alloys as thermoelectric materials.

Magnetic, magnetocaloric and thermoelectric properties of nickel doped manganites

In this study, we investigate structural, magnetic, magnetocaloric and thermoelectric properties of La0.7Sr0.3Mn1-xNixO3 compounds with 0.025 ≤ x ≤ 0.125. X-ray diffraction analysis shows the structure transformation from the R-3c rhombohedral to the Pbnm orthorhombic structure with Ni2+ doping at x ≥ 0.075 composites. The dc thermal magnetization measurements reveal the monotonic decrease in both Curie temperature and magnetization values with Ni2+ addition. The change in magnetic properties of the studied system is correlated to the ferromagnetism suppression and the antiferromagnetism promotion according to some cooperative intrinsic and extrinsic factors. Results show that Ni2+ addition affects the magnetocaloric properties, where it shifts the maximum value of the magnetic entropy change towards lower temperatures with relative cooling power of 88, 105, 47 J/kg for x = 0.025, 0.075 and 0.125 composites, respectively. Moreover, it is observed that Ni2+ doping increases the absolute value of Seebeck coefficient and decreases hole conduction interval.

Thermoelectric properties of Al and Mn double substituted ZnO

The effects of Al and Mn single and double substitution on structure, composition, and thermoelectric properties of ZnO have been investigated in three series of compounds; Zn1−xAlxO, Zn1−xMnxO (x=0,0.02,0.04,0.06,0.08) and Zn1−2xAlxMnxO (x=0,0.01,0.02,0.03,0.04) prepared by thermal decomposition method. While the lattice structure is not affected by the substitutions, properties of the material are. Al and Mn have opposite effects on electrical conductivity and Seebeck coefficient of ZnO. Al substitution leads to an increase in electrical conductivity while Mn substitution increases absolute value of Seebeck coefficient. Double substituted samples seem to exhibit the effects from both ions though the increase in absolute value of Seebeck coefficient is less significant comparing to that observed in Mn single substituted samples. Nevertheless, the change in electrical conductivity is more pronounced and dominant in the power factor calculation. Thus the most conductive sample in this work, Zn0.98Al0.02O, shows the highest power factor of 1.03×10−4WK−2m−1at 800K. The best double substituted sample is Zn0.98Mn0.01Al0.01O which gives a power factor of 4.79×10−5WK−2m−1 at the same temperature.

Thermoelectric and magnetic properties of Yb2MgSi2 prepared by spark plasma sintering method

An intermediate-valence compound, Yb2MgSi2, has been prepared using a spark plasma sintering method. The magnetic susceptibility and thermoelectric properties of Yb2MgSi2 are measured in the temperature range from 5 to 300 K. From the magnetic susceptibility results, Yb valence of the Yb2MgSi2 is evaluated. As compared with YbAl3, which is one of the promising thermoelectric materials that can be used at low temperatures, Yb2MgSi2 exhibits a lower absolute value of Seebeck coefficient, higher electrical resistivity, and lower thermal conductivity over the measured temperature range. A maximum dimensionless figure of merit, ZT, of 0.0018 is achieved at around 200 K.

Tuning Thermoelectric Properties of Type I Clathrate K8–xBaxAl8+xSi38–x through Barium Substitution

The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses a high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K guest atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8–xBaxAl8+xSi38–x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3n), confirmed by X-ray diffraction. The guest atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-containing samples were characterized from 2 to 300 K. The K–Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the tempera...

Mechanical alloying-spark plasma sintering synthesis and thermoelectric properties of n-type NiSe2+x semiconductors: analysis of intrinsic defects and phase structures

A series of bulk samples NiSe2+x (x = 0, 0.01, 0.02, 0.06) were prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The effect of excessive selenium content (x) on the phase structure, microstructure and thermoelectric properties was investigated. The formation energy of intrinsic defects in NiSe2 was calculated using first principle theory. The added selenium initially entered into the Se vacancy ( $$V_{Se}^{ \cdot }$$ ) which is resulted from the Se volatilization during the MA and/or SPS process and subsequently into the interstitial sites ( $$Se_{i}^{\prime }$$ ). An improved absolute value of Seebeck coefficient (|α|) 9.27 μVK−1 at 323 K for the NiSe2.06 was reached along with an inversely varied electrical conductivity (σ) as increasing x. A minimum thermal conductivity (κ) value 3.43 Wm−1 K−1 was achieved at 323 K for the NiSe2.01, along with a high ZT value 0.0049 at 323 K for the NiSe2.02 among NiSe2+x system. Our work provides a preliminary approach towards TE performance enhancement in n-type NiSe2 semiconductors via tailoring Se content in a nonstoichiometry, and will certainly be of value for further exploration of high performance NiSe2 semiconductors by either doping or microstructure controlling.

Enhancement of the thermoelectric figure of merit in n-type Cu0.008Bi2Te2.7Se0.3 by using Nb doping

Doping with foreign atom has been shown to be an effective way to enhance the dimensionless figure of merit ZT of Bi2Te3-based thermoelectric raw materials. Herein, we report that doping with Nb is effective in enhancing the Seebeck coefficient of n-type Cu0.008Bi2Te2.7Se0.3 polycrystalline bulks. Considering compensation of the Seebeck coefficient due to decrease of the electrical conductivity in Nb-doped compositions, the absolute value of Seebeck coefficient rather increased benefiting from an enhancement of the density of states (DOS) effective mass m* from 1.09m 0 (Cu0.008Bi2Te2.7Se0.3) to 1.21m 0 − 1.27m 0 (Cu0.008Bi2−x Nb x Te2.7Se0.3) due to a DOS engineering effect. The values of ZT were 0.84 at 300 K and 0.86 at 320 K for Cu0.008Bi1.99Nb0.01Te2.7Se0.3. This compositional tuning approach highlights the possibility of further enhancement of ZT for n-type Bi2Te3-based compounds by using a combination of nanostructuring technologies to reduce the thermal conductivity.

Thermoelectric properties of β-Indium sulfide with sulphur deficiencies

We prepared a bulk sample of β-In2S3, which was predicted to possess high thermoelectric performance by ab initio calculations, using pulsed current sintering technique and conventional solid reaction method. The composition of prepared sample was In2S2.94, and the deficient in sulfur led to n-type behaviors of thermoelectric properties. The absolute value of Seebeck coefficient was |S| = 220 μV/K at room temperature, and monotonically increased with temperature up to 600 K then moderately decreased. The thermal conductivity was relatively low (0.68 W m−1 K−1) at 700 K presumably due to the phonon scattering with anharmonic lattice vibrations. The dimensionless figure of merit ZT consequently reached 0.38 at 700 K. We also estimated the value of ZT as a function of carrier concentration, and found that p-type β-In2S3 has potential to possess a large value of ZT exceeding 1.4.

Amorphous-crystalline transition in thermoelectric NbO2

Density functional theory was employed to design enhanced amorphous NbO2 thermoelectrics. The covalent-ionic nature of Nb–O bonding is identical in amorphous NbO2 and its crystalline counterpart. However, the Anderson localisation occurs in amorphous NbO2, which may affect the transport properties. We calculate a multifold increase in the absolute Seebeck coefficient for the amorphous state. These predictions were critically appraised by measuring the Seebeck coefficient of sputtered amorphous and crystalline NbO2 thin films with the identical short-range order. The first-order phase transition occurs at approximately 550 °C, but amorphous NbO2 possesses enhanced transport properties at all temperatures. Amorphous NbO2, reaching −173 μV K−1, exhibits up to a 29% larger absolute Seebeck coefficient value, thereby validating the predictions.

Applicability of two-current model in understanding the electronic transport behavior of inverse Heusler alloy: Fe2CoSi

Here we explore the applicability of the two-current model in understanding the transport behavior of Fe2CoSi compound by using the first principles calculations in combination with the Boltzmann transport theory. The spin-up channel has small DOS at the EF, whereas spin-dn channel shows almost zero DOS at the EF. The absolute value of Seebeck coefficient in the spin-up channel shows linear increment with the temperature and in the spin-dn channel it varies non-linearly. The electrical conductivity also shows non-linear temperature dependence in both the spin channels whereas, the electronic thermal conductivity shows linear temperature dependence. The values of transport coefficients and their temperature dependence obtained by using the two-current model are found to be in fairly good agreement with the experimental data. Present work clearly suggests the importance of two-current model in understanding the transport properties of the compound.

Effects of WO3 Micro/Nano-Inclusions on the Thermoelectric Properties of Co4Sb11.7Te0.3 Skutterudite

In this study, nano-sized WO3 powder was dispersed into Co4Sb1.7Te0.3 thermoelectric matrix by ball milling and hot-press sintering technology. The results of scanning electron microscopy (SEM) showed that the WO3 phase distributed uniformly in the form of inclusions on the matrix. High-resolution transmission electron microscopy (HRTEM) presented that the average size of embedded nano-WO3 was about 50 nm. With the content of WO3 increasing, the electrical conductivity (σ) of Co4Sb11.7Te0.3 decreased and the absolute value of Seebeck coefficient (a) increased. The absolute value of a for 1.5% WO3/Co4Sb11.7Te0.3 composite was 258 µV/K at 650 K. The minimal lattice thermal conductivity (K(L)) of WO3/Co4Sb11.7Te0.3 achieved in the present work was 1.52 W/m K at 650 K. The total thermal conductivity (K) of WO3/Co4Sb11.7Te0.3 composites decreased obviously, which resulted from the phonon scattering by the WO3 inclusions locating on the grain boundaries of Co4Sb11.7Te0.3 matrix. The highest ZTof 0.71 was achieved at 750 K for 1.5%WO3/Co4Sb11.7Te0.3 composite.

Characterization of Lorenz number with Seebeck coefficient measurement

In analyzing zT improvements due to lattice thermal conductivity (κL) reduction, electrical conductivity (σ) and total thermal conductivity (κTotal) are often used to estimate the electronic component of the thermal conductivity (κE) and in turn κL from κL = ∼ κTotal − LσT. The Wiedemann-Franz law, κE = LσT, where L is Lorenz number, is widely used to estimate κE from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10−8 WΩK−2 (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10−8 WΩK−2 for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L=1.5+exp−|S|116 (where L is in 10−8 WΩK−2 and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si0.8Ge0.2 where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.

Influence of rare-earth elements doping on thermoelectric properties of Ca0.98Dy0.02MnO3 at high temperature

Ca0.98Dy0.02MnO3 and Ca0.96Dy0.02Re0.02MnO3 (Re=La, Pr, Sm, Eu, Ho, and Yb) have been synthesized by the solid state reaction method. Samples with relative densities all over 96% have been obtained. Thermoelectric properties are evaluated between 300 and 1000K. The electrical resistivity shows a typical metal-like conductivity behavior, and at high temperature, 973K, decreases from 36.1mΩcm for Ca0.98Dy0.02MnO3 to 8.6mΩcm for Ca0.96Dy0.02Yb0.02MnO3. Both the absolute values of Seebeck coefficient and thermal conductivity are reduced by the introduction of second rare-earth element. The highest power factor of 415μW/(K2m) is obtained for Ca0.96Dy0.02Yb0.02MnO3 sample, resulting in the highest dimensionless figure of merit (ZT) 0.25 at 973K. This value shows an improvement of 144% compared with that of Ca0.98Dy0.02MnO3 ceramics at the same temperature.

Phase Stability and Optoelectronic Properties of the Bixbyite Phase in the Gallium–Indium–Tin–Oxide System

X‐ray diffraction techniques were used to determine the phase boundaries of the In2O3 solid solution phase in the Ga2O3–In2O3–SnO2 ternary system. The effects of Ga and Sn content on the unit cell dimensions of the bixbyite phase were calculated by a linear regression fit, the results of which indicate the two substitutive cations have opposite and independent effects on the lattice parameter. These results suggest that the cations do not strongly interact with each other in the crystal. Measurements of optoelectronic properties were also taken on single‐phase bulk specimens within the solid solution to establish their dependence on composition. As anticipated, Sn doping yields corresponding increases in conductivity, reduction in the absolute value of Seebeck coefficient, and increase in optical band gap. In contrast, these properties are not significantly affected by varying Ga content, confirming that Ga behaves as an isovalent dopant at the low doping levels involved.

Tunable thermoelectric properties of Ca0.9Yb0.1MnO3 through controlling the particle size via ball mill processing

This paper reports the thermoelectric properties of ytterbium doped calcium manganite with different porosities through controlling the particle size via ball mill processing. It was found that powders without ball milling only yields ~67% of theoretical density after sintering, whereas 10h milling improves the density to 98%. The enhanced bulk density is due to the reduction of particle size via ball milling. The change in morphology and porosity leads to notable influence on the thermal and electrical transport properties of this material system. The absolute value of Seebeck coefficient decreases with increasing the ball milling time which may be associated with modification of Mn3+/Mn4+ ratio in the system, as evidenced by the results of X-ray photoelectron spectrometry and Raman spectroscopy. A maximum figure of merit (ZT=0.11 at 700°C) was achieved from the sample that was processed with ball milling of 6h.

Thermoelectric and optical properties of CuAlO2 synthesized by direct microwave heating

A single phase of delafossite CuAlO2 (CAO) was successfully synthesized by a 600 W microwave radiation for 20 min. The CAO sample was composed of quite distorted single-crystalline plates with 200–350 nm thick. Its atomic vibrations were detected at 760 and 550 cm−1 belonging to Al–O and Cu–O stretching, respectively. The direct and indirect energy gaps were respectively determined to be 3.9 and 2.9 eV. The photoluminescence (PL) at room temperature was at 585 nm (2.12 eV) corresponding to the indirect energy gap and at 760 nm (1.63 eV) corresponding to the p-type native defect. For its thermoelectric (TE) properties, the Seebeck coefficient (S) was positive value, with holes as the majority of charge carriers. By increasing of the test temperature, both the electrical resistivity and absolute value of Seebeck coefficient were decreased, but the power factor was in the opposite manner. The dimensionless figure of merit (ZT) of the crystalline CAO was evaluated to be the maximum of 9 × 10−3 at 1073 K.

THERMOELECTRIC AND MAGNETO-THERMOELECTRIC PROPERTIES OF Ga-DOPED ZnO THIN FILMS BY RF MAGNETRON SPUTTERING

Zn (1-x) Ga x O thin films (x = 0.01, 0.03, 0.05, 0.07 named as GZO1, GZO2, GZO3, GZO4, respectively) were deposited on glass substrates by RF magnetron sputtering. The crystal structure, electrical, thermoelectric and magneto-thermoelectric properties of GZO films were investigated. It is found that all the GZO films are polycrystalline and preferentially oriented in the c-axis. The electrical resistivity of GZO films decreased first with increasing Ga doping content before it reached a minimum at x = 0.05, and then increased with further increasing Ga doping content. The magnetic fields (B) ranging from 0 to 1.5 T are perpendicularly applied to the films to study the magneto-thermoelectric properties. It is observed that the absolute values of Seebeck coefficients (|S|) of GZO1, GZO2, GZO3 show marked variation with magnetic field and obtain the maximum value at B = 0.5 T. Whereas the |S| value of GZO4 fluctuates slightly with magnetic field and reaches its peak at B = 1.0 T. The magneto-thermoelectric properties are analyzed and we propose that this behavior is mainly attributed to the effect of magnetic field on the electron transport.

A study of Yb0.2Co4Sb12–AgSbTe2nanocomposites: simultaneous enhancement of all three thermoelectric properties

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

Structural and electrical transport properties of Mg-doped n-type Sn-based type Ⅷ single crystalline clathrate

Mg-doped Sn-based single crystalline samples Ba8Ga16-XMgXSn30(0 X 1.5) were grown from Sn flux to characterize their structural and electrical transport properties. Research results show that the prepared compounds are well indexed by the type-Ⅷ clathrate structure with a space group I43 m. With the increase in Mg content, the melting point of the corresponding compounds increases. On the contrary, the lattice constant decreases. The actual content of filling atomic Ba in doped samples is below the ideal value of 8.0, the occupancy of Ba in the dodecahedron is about 0.93 for the sample with X=1.5. All the samples show n-type conduction. The Mg atoms have an effect on the band structure of the materials, and the carrier concentrations in the doped samples are reduced. However, the absolute values of Seebeck coefficient and the resistivity of doped compounds increase, respectively. By calculation, the sample of X=1.5 obtains the maximum value of power factor 1.2610-3 Wm-1K-2 near 430 K.