A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy

We deal here with Sb and Bi doping effects of the n-type half-Heusler (HH) Ti_0.3Zr_0.35Hf_0.35NiSn alloy on the measured thermoelectric properties. To date, the thermoelectric effects upon Bi doping on the Sn site of HH alloys have rarely been reported, while Sb has been widely used as a donor dopant. A comparison between the measured transport properties following arc melting and spark plasma sintering of both Bi- and Sb-doped samples indicates a much stronger doping effect upon Sb doping, an effect which was explained thermodynamically. Due to similar lattice thermal conductivity values obtained for the various doped samples, synthesized in a similar experimental route, no practical variations in the thermoelectric figure of merit values were observed between the various investigated samples, an effect which was attributed to compensation between the power factor and electrical thermal conductivity values regardless of the various investigated dopants and doping levels.     

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd_1?x Mg _x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 10¹⁸ cm^?3 for CdTe and 3 × 10¹⁷ cm^?3 for Cd_0.65Mg_0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd_0.65Mg_0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 10¹⁸ cm^?3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 10¹⁶ cm^?3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.     

A 0.003-mm2, 0.35-V, 82-pJ/conversion ultra-low power CMOS all digital temperature sensor for on-die thermal management

In this paper, a 0.35 V, 82 pJ/conversion ring oscillator based ultra-low power CMOS all digital temperature sensor is presented for on-die thermal management. We utilize subthreshold circuit operation to reduce power and adopt an all-digital architecture, consisting of only standard digital gates. Additionally, a linearization technique is proposed to correct the nonlinear characteristics of subthreshold MOSFETs. A bulk-driven 1-bit gated digitally controlled oscillator is designed for the temperature sensing node. Also, a 1-bit time-to-digital converter is employed in order to double the fine effective resolution of the sensor. The proposed digital temperature sensor has been designed in a 90-nm regular V _T CMOS process. After a two-point calibration, the sensor has a maximum error of ?0.68 to +0.61 °C over the operating temperature range from 0 to 100 °C, while the effective resolution reaches 0.069 °C/LSB. Under a supply voltage of 0.35 V, the power dissipation is only 820 nW with the conversion rate of 10K samples/s at room temperature. Also, the sensor occupies a small area of 0.003 mm².     

Enhancement in Performance of the Tubular Thermoelectric Generator (TTEG)

For use in a tubular thermoelectric generator (TTEG), we fabricated tubular Bi_0.5Sb_1.5Te₃/Ni composite using a melt-spinning technique combined with the spark plasma sintering (SPS) process. With this method, powder sintering, joining of two different materials, and tubular shaping can be achieved simultaneously. The tilted laminate structure which is crucial for the transverse thermoelectric effect was successfully achieved in the sample after SPS densification. The sintered samples showed better mechanical stability and thermoelectric properties compared with the previously studied melt-cast sample. We confirmed larger open-circuit voltage of 240 mV and generating power of 2.5 W with a 100-mm-long TTEG under the small temperature difference of 83 K, and the corresponding power density for a unit heat transfer surface area was approximately 800 W m^?2.     

Crystal Structure and Physical Properties of Skutterudites SrPd4Sn x Sb12−x

A Pd-based skutterudite phase SrPd₄Sn _x Sb_12?x in which the 8c sites of the structure are occupied solely by Pd atoms and with a homogeneity range of 4.3(2) ≤ x ≤ 5.8(2) (wavelength dispersive x-ray spectroscopy) has been synthesized by solid-state reaction then spark-plasma sintering (SPS). It crystallizes as the filled-skutterudite structure type, electronically compensated by substitution of Sn for Sb at framework (24g) positions. The unit cell decreases substantially with increasing nominal and detected Sn content. Magnetization, specific heat, Hall effect, electrical resistivity, thermopower, and thermal conductivity were measured for the SPS-treated samples. SrPd₄Sn _x Sb_12?x is a diamagnetic material. Depending on composition it occurs in the metallic or semiconducting states. Hall effect data show that the type and concentration of most of the carriers depend on the Sn/Sb atomic ratio. The thermal conductivity of SrPd₄Sn _x Sb_12?x is approximately 3.0–5.0 W K^?1 m^?1 at 300 K. The Seebeck coefficient is negative throughout the temperature range covered, reaching approximately ?20 μV K^?1 at 300 K.     

Effects of Al/Sb Double Doping on the Thermoelectric Properties of Mg2Si0.75Sn0.25

Al/Sb double-doped Mg₂Si_0.75Sn_0.25 materials were prepared by liquid–solid reaction synthesis and the hot-pressing technique. The effects of Al/Sb double doping on the thermoelectric properties were investigated at temperatures between room temperature and 900 K, and the resistivity and Hall coefficient were investigated at 80 K to 900 K. Al/Sb double-doped samples were found to be n-type semiconductors in the investigated temperature range. The absolute Seebeck coefficient (α), resistivity (ρ), and thermal conductivity (κ) for Al/Sb double-doped samples at room temperature were in the ranges of 152.5 μV K^?1 to 109.2 μV K^?1, 2.92 × 10^?5 Ω m to 1.29 × 10^?5 Ω m, and 2.50 W K^?1 m^?1 to 2.86 W K^?1 m^?1, respectively. The absolute values of α increased with increasing temperature up to a maximum, and decreased thereafter. This could be attributed to mixed carrier conduction in the intrinsic region. κ decreased linearly with increasing temperature to a minimum near the intrinsic region, then increased rapidly because of bipolar components. The highest ZT value measured was 0.94 at 850 K for Mg_1.9975Al_0.0025Si_0.75Sn_0.2425Sb_0.0075. Sb doping was effective for enhancement of ZT, because of a remarkable increase in the carrier concentration. However, Al doping was almost ineffective for enhancing ZT.     

Effect of Cooling Conditions on the Microstructure and Thermoelectric Properties of Zn/Si-Codoped InSb

InSb is a good candidate thermoelectric (TE) material owing to its high carrier mobility and narrow band gap around 0.18 eV. However, a high figure of merit (ZT) value has not been achieved with InSb because of its high lattice thermal conductivity (κ _lat). To reduce the κ _lat of InSb, we prepared a ZnIn₁₈SiSb₂₀ alloy by Zn/Si codoping into the In lattice sites of InSb. Polycrystalline samples of ZnIn₁₈SiSb₂₀ were prepared by a solid-state reaction method combined with hot pressing. To investigate the microstructures and TE properties resulting from different cooling conditions, samples were prepared by water quenching or slow cooling after an annealing process. The different cooling conditions led to different ZnIn₁₈SiSb₂₀ microstructures and TE properties. The electrical transport properties showed that both samples exhibited metal-like behavior and p-type conduction. The thermal conductivity values of the quenched and slow-cooled samples at room temperature were 8.7 W m^?1 K^?1 and 11.7 W m^?1 K^?1, respectively. A maximum ZT value of 0.23 was obtained at 723 K for the quenched ZnIn₁₈SiSb₂₀ sample.     

Electromagnetic Properties of Ni0.9Zn0.1CoO2

Ni_0.9Zn_0.1CoO₂ was prepared by conventional ceramic processing. X-ray diffraction analysis confirmed the spinel cubic structure of the sample. The dielectric constant and electrical resistance were measured at different temperatures and under different values of the applied magnetic field. The dielectric constant increases with the applied magnetic field while the electric resistance decreases. The studied material has a very high dielectric constant of 8.68 × 10⁵ that increases with temperature due to the thermal activation of hole hopping between Co²⁺ ? Co³⁺ and Ni²⁺ ? Ni³⁺. The studied composition has potential for application as a sensor for detecting the intensity of electromagnetic waves.     

Thermoelectric Properties and Thermal Stability of PEDOT:PSS Films on a Polyimide Substrate and Application in Flexible Energy Conversion Devices

In this study, we investigated the flexibility and thermal stability of films consisting of a complex of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS) cast onto a polyimide substrate. We also prepared a PEDOT:PSS-based flexible device for thermoelectric energy conversion. The thermal stability of a PEDOT:PSS film was evaluated by measuring the Seebeck coefficient (S) and electrical conductivity (σ) for 30 heating and cooling cycles at 330 K to 380 K. Furthermore, the durability of the PEDOT:PSS film was examined by heating at 353 K in air for 4000 h. The approximate values of S and σ were 14 μV K^?1 and 600 S cm^?1, respectively. These values were almost the same before and after repeated bending treatments (10,000 times, radius of curvature 10 mm). In addition, the S and σ values for the PEDOT:PSS film were nearly constant during the heating cycle treatments. In the durability test, σ gradually decreased with increasing heating time (7% at 300 h, 17% at 3600 h). Thus, it was found that PEDOT:PSS films have both flexibility and mechanical toughness as well as relatively good thermal stability in air up to 3600 h. The maximum electric power for the PEDOT:PSS-based flexible device was 0.334 μW at ΔT = 100 K. These results for the power-generating properties of the flexible device are consistent with values calculated from the properties of the constituent materials.     

Thermal Characterization of Polycrystalline SiC

A study is made using fabricated thermal resistors in combination with two-dimensional (2D) electrothermal simulations to determine the thermal conductivity of polycrystalline SiC, single-crystalline SiC, and Si. The results show that the poly-SiC substrate has thermal conductivity of κ _poly-SiC = 2.7 W K^?1 cm^?1, which is significantly lower than that of single-crystalline SiC.     

Effect of Sintering Temperature on Microstructure, Electrical Properties, and Thermal Expansion of Perovskite-Type La0.8Ca0.2CrO3 Complex Oxides Synthesized by a Combustion Method

Perovskite-type La_0.8Ca_0.2CrO₃ complex oxides were synthesized by a combustion method. Microstructural evolution, electrical properties, and thermal expansion behavior of the ceramics were investigated in the sintering temperature range of 1250°C to 1450°C. It was found that the electrical conductivity (σ _e) remarkably improved with increasing sintering temperature from 1250°C to 1400°C, ascribed to the development of microstructural densification, whereas it declined slightly above 1400°C due to generation of excessive liquid. The specimen sintered at 1400°C had a maximum conductivity of 31.6 S cm^?1 at 800°C, and lowest activation energy of 0.148 eV. The improvement of the thermal expansion coefficient (TEC) with increasing sintering temperature was monotonic as a result of the microstructural densification of the materials. The TEC of La_0.8Ca_0.2CrO₃ sintered at 1400°C was about 10.5 × 10^?6 K^?1, being consistent with other components as high-temperature conductors. With respect to microstructure, electrical properties, and thermal expansion, the preferable sintering temperature was ascertained to be about 1400°C, which is much lower than for the traditional solid-state reaction method.     

Solvent-Based Synthesis of <Emphasis Type="Italic">Nano</Emphasis>-Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript> for Low-Temperature Thermoelectric Applications

In this study we show a preparation method for nanostructured Bi_0.85Sb_0.15 powders via a chemical reduction route in a polyol medium, yielding material with particle sizes of 20–150 nm in scalable amounts. The powders were consolidated by spark plasma sintering (SPS) in order to maintain the nanostructure. To investigate influence of the sinter process, the powders were characterized by x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and scanning electron microscopy (SEM) measurements before and after SPS. Transport properties, Seebeck effect, and thermal conductivity were determined in the low temperature range below 300 K. The samples showed excellent thermal conductivity of 2.3–2.6 W/m × K at 300 K and Seebeck coefficients from ?97 μV/K to ?107 μV/K at 300 K with a maximum of ?141 μV/K at 110 K, thus leading to ZT values of up to 0.31 at room temperature. The results show that Bi-Sb-alloys are promising materials for low-temperature applications. Our wet chemical approach gives access to scalable amounts of nano-material with increased homogeneity and good thermoelectric properties after SPS.     

Fabrication of Titanium Oxide-Based Composites by Reactive SPS Sintering and Their Thermoelectric Properties

Titanium oxide-based composites containing (1) Nb, (2) Nb and Sr, and (3) Sr and La were fabricated by a combination of wet processing and reactive spark plasma sintering in which the metal oxide components were reduced by reaction with titanium nitride. If only TiO₂ was used as the starting material, several Magneli-type phases of oxygen-deficient titanium oxides were obtained. When mixed with Nb ions with Ti:Nb = 0.9:0.1, microsegregation of Nb ions was observed (case 1). If Sr was added, a perovskite, SrTiO₃ (STO) phase occurred (case 2), which contained La ions in the case of La addition (case 3). The sintered compacts consisted largely of grains of about 1 μm in size. In the case of Ti-Nb combination (case 1), a unique stripe pattern also appeared inside the grains. The electrical conductivity increased monotonically with increasing temperature in the case of the pure Magneli phases and the Nb-containing composite, whereas bow-shaped temperature dependences with a maximum were observed in the case of the composites containing STO phases. The Seebeck coefficients were commonly negative, and the absolute values increased with temperature. The thermal conductivity was between 2 W m^?1 K^?1 and 4 W m^?1 K^?1 in the temperature range from room temperature to 800°C. A maximum ZT of 0.34 was achieved at 800°C (case 2).     

Crystallographic, Thermoelectric, and Mechanical Properties of Polycrystalline Ba8Al x Si46−x Clathrates

The Al content dependence of crystallographic, thermoelectric, and mechanical properties is reported for polycrystalline Ba₈Al _x Si_46?x (nominal x = 15 to 17) clathrates prepared by combining arc melting and spark plasma sintering methods. The elastic constants and the coefficient of thermal expansion (CTE), which are also important properties for designing thermoelectric devices, are presented. Powder x-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy (EDX) indicate that the type I clathrate is the major phase of the samples but impurity phases (mainly BaAl₂Si₂, Si, and Al) are included in the samples with high Al contents. The actual Al content x determined by EDX ranges from approximately 14 to 15. The absolute value of the Seebeck coefficient increases and the electrical conductivity decreases as the Al content increases. The changes in Seebeck coefficient and electrical conductivity are explained in terms of the dependence of the carrier concentration on the Al content. The elastic constants and the CTE of the samples depend weakly on the Al content. Some of the properties are compared with reported data of single crystals of Ba₈Al₁₆Ge₃₀, Ba₈Ga₁₆Ge₃₀, Sr₈Ga₁₆Ge₃₀, silicon, and germanium as standard references. The effective mass, Hall carrier mobility, and lattice thermal conductivity, which govern the transport properties, are determined to be ~ 2.4m ₀, ~ 7 cm² V^?1 s^?1, and ~ 1.3 W m^?1 K^?1, respectively, for actual Al content x of about 14.77. The thermoelectric figure of merit ZT is estimated to be about 0.35 at 900 K for actual Al content x of about 14.77.     

Thermoelectric Properties of Polycrystalline SrZn2Sb2 Prepared by Spark Plasma Sintering

Compact polycrystalline samples of SrZn₂Sb₂ [space group $ P\overline{3} m1 $ , a = 4.503(1) Å, c = 7.721(1) Å] were prepared by spark plasma sintering. Thermoelectric performance, Hall effect, and magnetic properties were investigated in the temperature range from 2 K to 650 K. The thermoelectric figure of merit ZT was found to increase with temperature up to ZT = 0.15 at 650 K. At this temperature the material showed a high Seebeck coefficient of +230 μV K^?1, low thermal conductivity of 1.3 W m^?1 K^?1, but rather low electrical conductivity of 54 S cm^?1, together with a complex temperature behavior. SrZn₂Sb₂ is a diamagnetic p-type conductor with a carrier concentration of 5 × 10¹⁸ cm^?3 at 300 K. The electronic structure was calculated within the density-functional theory (DFT), revealing a low density of states (DOS) of 0.43 states eV^?1 cell^?1 at the Fermi level.     

Low-Temperature Physical and Thermoelectric Properties of Ba8Ni5Ge41

Resistivity, Hall resistivity, thermopower, thermal conductivity, and magnetization are reported for polycrystalline Ba₈Ni₅Ge₄₁. Ba₈Ni₅Ge₄₁ is diamagnetic with susceptibility χ _dia = (?2.4 to ?2.82) × 10^?7 emu/g. Semiconductor-like behavior was observed for the resistivity. The thermopower shows positive values for a wide temperature range. The Hall resistivity indicates the dominance of electrons, suggesting the existence of multiband conductance. At room temperature, the thermal conductivity is 1.78(5) W/K m. The highest ZT of Ba₈Ni₅Ge₄₁ is 0.0016 at about 278 K.     

Preparation and Thermoelectric Properties of Doped Bi2Te3-Bi2Se3 Solid Solutions

Since Bi₂Te₃ and Bi₂Se₃ have the same crystal structure, they form a homogeneous solid solution. Therefore, the thermal conductivity of the solid solution can be reduced by phonon scattering. The thermoelectric figure of merit can be improved by controlling the carrier concentration through doping. In this study, Bi₂Te_2.85Se_0.15:D _m (D: dopants such as I, Cu, Ag, Ni, Zn) solid solutions were prepared by encapsulated melting and hot pressing. All specimens exhibited n-type conduction in the measured temperature range (323 K to 523 K), and their electrical conductivities decreased slightly with increasing temperature. The undoped solid solution showed a carrier concentration of 7.37 × 10¹⁹ cm^?3, power factor of 2.1 mW m^?1 K^?1, and figure of merit of 0.56 at 323 K. The figure of merit (ZT) was improved due to the increased power factor by I, Cu, and Ag dopings, and maximum ZT values were obtained as 0.76 at 323 K for Bi₂Te_2.85Se_0.15:Cu_0.01 and 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005. However, the thermoelectric properties of Ni- and Zn-doped solid solutions were not enhanced.     

Effects of Second Phase Yb5Sb3 on the Thermoelectric Properties of YbAl3

The compound YbAl₃ exhibits a very high power factor but also rather a large thermal conductivity, leading to a low figure of merit. The second phase Yb₅Sb₃ was introduced in the YbAl₃ matrix to reduce its thermal conductivity. The composites (YbAl₃)_1?x (Yb₅Sb₃) _x with x = 0, 0.01, 0.05, 0.10, and 0.20 were synthesized by high frequency induction melting, annealing treatment, and spark plasma sintering. The thermoelectric properties of the composites were evaluated. The composites are of n-type conduction. The pure YbAl₃ obtained in this work shows a high power factor of 11,500 μW m^?1 K^?2 but also a high thermal conductivity of 19.6 W m^?1 K^?1. However, the existence of Yb₅Sb₃ compound in the YbAl₃ matrix enhances the electrical resistivity and the absolute Seebeck coefficient of the composite, but significantly reduces its thermal conductivity in the temperature range considered, thereby enhancing the figure of merit. The highest ZT value of 0.23 may be obtained in the sample (YbAl₃)_0.95(Yb₅Sb₃)_0.05 at room temperature, which is apparently higher than that of pure YbAl₃.     

Preparation and Thermoelectric Properties of p-Type Yb-Filled Skutterudites

p-Type Yb _z Fe_4?x Co _x Sb₁₂ skutterudites were prepared by encapsulated melting and hot pressing, and the filling and doping (charge compensation) effects on the transport and thermoelectric properties were examined. The electrical conductivity of all specimens decreased slightly with increasing temperature, indicating that they were in a degenerate state due to high carrier concentrations of 10²⁰ cm^?3 to 10²¹ cm^?3. The Hall and Seebeck coefficients exhibited positive signs, indicating that the majority carriers are holes (p-type). The Seebeck coefficient increased with increasing temperature to maximum values of 100 μV/K to 150 μV/K at 823 K. The electrical and thermal conductivities were reduced by substitution of Co for Fe, which was responsible for the decreased carrier concentration. Overall, the Yb-filled Fe-rich skutterudites showed better thermoelectric performance than the Yb-filled Co-rich skutterudites.     

Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.