Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.     

Thermoelectric Properties of the Narrow-Gap Intermetallic Compound Ga<Subscript>2</Subscript>Ru: Effect of Re Substitution for Ru Atoms

In this paper, the effect of hole doping on the thermoelectric properties of the binary narrow-gap semiconducting intermetallic compound Ga₂Ru in the temperature range from 373 K to 973 K was investigated. We synthesized sintered pellets by spark plasma sintering (SPS) after arc-melting and succeeded in preparing crack-free samples. The maximum dimensionless figure of merit ZT _max was 0.50 at 773 K for the sintered Ga₂Ru alloy. The temperature dependence of the electrical resistivity and its magnitude at 373 K dramatically changed from negative (~11,000 μΩcm) to positive (~200 μΩcm) upon hole doping by the substitution of Re for Ru atoms. Also, the Seebeck coefficient at 373 K changed from 300 μV/K to 75 μV/K. These changes were identified by the increase in carrier concentrations observed by Hall- effect measurements. In particular, large power factors (2.0 mW/m K² to 3.0 mW/m K²) were obtained over a wide temperature range from 373 K to 973 K upon Re substitution. The lattice thermal conductivity beneficially decreased with increasing Re concentration as a result of an alloying effect.     

Exfoliation and blistering of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te substrates by ion implantation

As part of a series of wafer bonding experiments, the exfoliation/blistering of ion-implanted Cd_0.96Zn_0.04Te substrates was investigated as a function of postimplantation annealing conditions. (211) Cd_0.96Zn_0.04Te samples were implanted either with hydrogen (5×10¹⁶ cm⁻²; 40–200 keV) or co-implanted with boron (1×10¹⁵ cm⁻²; 147 keV) and hydrogen (1–5×10¹⁶ cm⁻²; 40 keV) at intended implant temperatures of 253 K or 77 K. Silicon reference samples were simultaneously co-implanted. The change in the implant profile after annealing at low temperatures (<300°C) was monitored using high-resolution x-ray diffraction, atomic force microscopy (AFM), and optical microscopy. The samples implanted at the higher temperature did not show any evidence of blistering after annealing, although there was evidence of sample heating above 253 K during the implant. The samples implanted at 77 K blistered at temperatures ranging from 150°C to 300°C, depending on the hydrogen implant dose and the presence of the boron co-implant. The production of blisters under different implant and annealing conditions is consistent with nucleation of subsurface defects at lower temperature, followed by blistering/exfoliation at higher temperature. The surface roughness remained comparable to that of the as-implanted sample after the lower temperature anneal sequence, so this defect nucleation step is consistent with a wafer bond annealing step prior to exfoliation. Higher temperature anneals lead to exfoliation of all samples implanted at 77 K, although the blistering temperature (150–300°C) was a strong function of the implant conditions. The exfoliated layer thickness was 330 nm, in good agreement with the projected range. The “optimum” conditions based on our experimental data showed that implanting CdZnTe with H⁺ at 77 K and a dose of 5×10¹⁶/cm² is compatible with developing high interfacial energy at the bonded interface during a low-temperature (150°C) anneal followed by layer exfoliation at higher (300°C) temperature.     

Microstructure and Thermoelectric Properties of AgSbO<Subscript>3</Subscript> Ceramics Prepared by Ion-Exchange Powder Synthesis and Normal Sintering

Silver antimonate (AgSbO₃) is a potential high-temperature thermoelectric oxide with a low thermal conductivity, but it is difficult to fabricate dense bulk material with high phase purity. Well-dispersed AgSbO₃ nanopowder with an average diameter of 50 nm was synthesized by an ion-exchange process in this study, the sintering density of which was apparently enhanced. Nearly single-phase AgSbO₃ ceramic samples with a relative density close to 90% were obtained when sintered at a relatively low temperature (1273 K). The electrical conductivity of AgSbO₃ increased greatly with increasing sintering temperature, probably because of defects originating from the compositional deviation caused by sintering, in addition to increased density. This study also confirmed that AgSbO₃ had a low thermal conductivity, from 1.1 W m⁻¹ K⁻¹ at room temperature to 0.8 W m⁻¹ K⁻¹ at 673 K.     

Temperature-Dependent Studies of Si-Implanted Al<Subscript>0.33</Subscript>Ga<Subscript>0.67</Subscript>N with Different Annealing Temperatures and Times

Electrical activation studies were carried out on Si-implanted Al_0.33Ga_0.67N as a function of ion dose, annealing temperature, and annealing time. The samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently proximity-cap annealed from 1150°C to 1350°C for 20 min to 60 min in a nitrogen environment. One hundred percent electrical activation efficiency was obtained for Al_0.33Ga_0.67N samples implanted with a dose of 1 × 10¹⁵ cm⁻² after annealing at either 1200°C for 40 min or at 1300°C for 20 min. The samples implanted with doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² exhibited significant activations of 74% and 90% after annealing for 20 min at 1300°C and 1350°C, respectively. The mobility increased as the annealing temperature increased from 1150°C to 1350°C, showing peak mobilities of 80 cm²/V s, 64 cm²/V s, and 61 cm²/V s for doses of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², and 1 × 10¹⁵ cm⁻², respectively. Temperature-dependent Hall-effect measurements showed that most of the implanted layers were degenerately doped. Cathodoluminescence measurements for all samples exhibited a sharp neutral donor-bound exciton peak at 4.08 eV, indicating excellent recovery of damage caused by ion implantation.     

Thermal Conductivity and Other Transport Properties of Mg<Subscript>2</Subscript>Sn:Ag Crystals

The temperature dependence of the thermal conductivity κ(T), electrical resistivity ρ(T), and Seebeck coefficient S(T) of Mg₂Sn:Ag crystals with 0 at.% to 1 at.% Ag content were measured at T = 2 K to 400 K. The crystals were cut from ingots that were prepared by the vertical Bridgman method. Undoped samples show a dramatic κ ∝ T ³ rise at low temperatures to a peak value κ _15K = 477 W m⁻¹ K⁻¹. This leads to exceptionally large phonon drag effects causing giant thermopower with S rising sharply to a peak value S _20K = 3000 μV K⁻¹. At higher temperatures S decreases and changes sign to intrinsic values S ≈ −60 μV K⁻¹. The addition of Ag changes the transport properties as follows: (a) κ decreases systematically, the peak shifts to 30 K and falls to 7 W m⁻¹ K⁻¹; (b) ρ changes from high to low values; (c) S(T) changes to a linear dependence with S _300K ≈ 150 μV K⁻¹ to 200 μV K⁻¹.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Enhancing Thermoelectric Properties of Polycrystalline Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> by Optimizing a Ball-Milling Process

Bismuth sulfide (Bi₂S₃) polycrystalline samples were fabricated by mechanical alloying (MA) combined with spark plasma sintering (SPS). The microstructure and electrical transport properties were investigated with special emphasis on the influence of the ball-milling process. Bi₂S₃ compound powders could be readily synthesized directly from elemental powders under all the investigated conditions, and highly dense n-type bulk Bi₂S₃ samples with high density (>95%) were fabricated by the subsequent SPS process. Changing the MA conditions had no apparent influence on the microstructure or phase structure of the MA-derived Bi₂S₃ powders, but the electrical properties and thermopower of the SPS-sintered Bi₂S₃ bulk samples were greatly dependent on the MA speed and time. The power factor of Bi₂S₃ was increased to 233 μW K⁻² m⁻¹ at 573 K by optimizing the ball-milling process. This power factor is higher than values reported to date for Bi-S binary samples without texture.     

Effect of Microstructural Development on Mechanical and Electrical Properties of Inkjet-Printed Ag Films

The microstructural characterization of inkjet-printed Ag films sintered at various conditions was carried out to analyze the effect of microstructure on mechanical and electrical properties. As expected, the films became denser with grain growth with increasing sintering time and temperature, which resulted in improvement in mechanical properties. However, the resistivity of the films reached a minimum value of 3.0 μΩ cm before full densification. In order to improve the mechanical properties, pressure-assisted sintering was introduced. As a result, inkjet-printed Ag films sintered at 250°C under 5 MPa showed a tensile strength of 550 MPa, elongation of 2.4%, Young’s modulus of 55 GPa, and resistivity of ~3.0 μΩ cm.     

Growth and doping of SiC-thin films on low-stress,amorphous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/Si substrates for robust microelectromechanical systems applications

The N₂-doped 3C-SiC thin films have been grown by low-pressure, chemical vapor deposition (LPCVD) on amorphous Si₃N₄/p-Si (111) substrates using the single, organosilane-precursor trimethylsilane [(CH₃)₃SiH]. The effects of N₂ flow rate and growth temperature on the electrical properties of SiC films were investigated by Hall-effect measurements. The electron-carrier concentration is between 10¹⁷–10¹⁸/cm³. The lowest resistivities at 400 K and 300 K are 1.12×10⁻² and 1.18×10⁻¹ cm, respectively. The corresponding sheet resistances are 75.02 Ω/□ and 790.36 Ω/□. The SiC film structure was studied by x-ray diffraction. The 3C-SiC films oriented in the 〈111〉 direction with a 2ϑ peak at 35.5° and line widths between 0.18–0.25° were obtained. The SiC/Si₃N₄ interface is very smooth and free of voids. The fabrication of microelectromechanical (MEMS) structures incorporating the SiC films is discussed.     

Thermoelectric Properties of <Emphasis Type="Italic">In Situ</Emphasis> Formed Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript>/Bi-Rich Particles Composite

A Bi-15 at.%Sb alloy, homogenized by equal channel angular extrusion (ECAE) at T = 523 K, has been treated just above its solidus temperature, causing segregation of a secondary Bi-rich phase at the grain boundaries. This process results in an in situ composite. The thermoelectric properties of the composite have been measured in the range of 5 K < T < 300 K. The results are compared with those of the homogeneous alloy. The presence of a Bi-rich phase improves the Seebeck coefficient at T < 50 K, and enhances the electrical conductivity by a factor of 1.4 at T = 300 K up to a factor of 3.4 at T = 50 K; unfortunately, the thermal conductivity also increases by about 50% in the same temperature range. As a result, the figure of merit, Z, is slightly suppressed above T = 110 K, but increases at lower temperatures, reaching a peak value of 4.2 × 10⁻³ K⁻¹ at T = 90 K. The power factor considerably increases over the whole temperature range, rendering this material suitable as the n-type leg of a cryogenic thermoelectric generator for cold energy recovery in a liquefied natural gas plant.     

Thermoelectric Properties of Ag-doped Mg<Subscript>2</Subscript>Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg₂Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg₂Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 × 10¹⁸ cm⁻³ for undoped films to 8.8 × 10¹⁸ cm⁻³ for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200–650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 μV K⁻¹, while it was 280 μV K⁻¹ for the most heavily doped film at ∼400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of ∼10⁻⁵ W cm⁻¹ K⁻² at 700 K.     

Preparation of Well-Tiled <Emphasis Type="Italic">γ</Emphasis>-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> by a Novel and Simple Sodium Alginate Gel Method and Its Electrical Properties

In this paper, a novel and simple sodium alginate (SA) gel method was developed to prepare γ-Na _x Co₂O₄. This method involved the chemical gelling of SA in the presence of Co²⁺ ions by cross-linking. After calcining at 700°C to 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained. The arrangement of about 1 μm to 4 μm flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. SA not only acted as the control agent for crystal growth, but also provided a Na source for the γ-Na _x Co₂O₄ crystals. The electrical properties of γ-Na _x Co₂O₄ ceramics prepared via ordinary sintering after cold isostatic pressing were investigated. The Seebeck coefficient and power factor of the bulk material were 177 μV K⁻¹ and 4.3 × 10⁻⁴ W m⁻¹ K⁻² at 850 K, respectively.     

Thermoelectric Properties of Co-Doped TiS<Subscript>2</Subscript>

The thermoelectric properties of cobalt-doped compounds Co _x Ti_1−x S₂ (0 ≤ x ≤ 0.3) prepared by solid-state reaction were investigated from 5 K to 310 K. It was found that the electric resistivity ρ and absolute thermopower |S| for all the doped compounds decreased significantly with increasing Co content over the whole temperature range investigated. The increased lattice thermal conductivity of the doped compounds would imply enhancement of the acoustic velocity. Moreover, the ZT value of the doped compounds was improved over the whole temperature range investigated, and specifically reached 0.03 at 310 K for Co_0.3Ti_0.7S₂, being about 66% larger than that of TiS₂.     

Magnetron Deposition of In Situ Thermoelectric Mg2Ge Thin Films

Thin films of the semiconducting compound Mg₂Ge were deposited by magnetron cosputtering from source targets of high-purity Mg and Ge onto glass substrates at temperatures T _s = 300°C to 700°C. X-ray diffraction shows that the Mg₂Ge compound begins to form at a substrate temperature T _s ≈ 300°C. Films deposited at T _s = 400°C to 600°C are single-phase Mg₂Ge and have strong x-ray peaks. At higher T _s the films tend to be dominated by a Ge-rich phase primarily due to the loss of magnesium vapor from the condensing film.␣At optimum deposition temperatures, 550°C to 600°C, films have an electrical conductivity σ _600 K = 20 Ω⁻¹ cm⁻¹ to 40 Ω⁻¹ cm⁻¹ and a Seebeck coefficient α = 300 μV K⁻¹ to 450 μV K⁻¹ over a broad temperature range of 200 K to 600 K.     

Preparation and Thermoelectric Characterization of SiC-B<Subscript>4</Subscript>C Composites

SiC-B₄C composites with various values of SiC-to-B₄C ratio and grain size were fabricated by pressureless sintering. This paper presents the results of current investigations of this composite material. This includes the parameters of manufacture (shrinkage, density, and open porosity), thermoelectric properties (electrical and thermal conductivity, and thermopower), and material characterization (x-ray diffraction, scanning electron microscopy, oxidation resistance, and thermal expansion). The results indicate high potential of this composite as an alternative material for thermoelectric applications at high temperatures. The Seebeck coefficient of the composite was higher than that of the single-component materials B₄C and SiC and reached 400 μV/K at 500°C.     

Magnetoresistance and magnetization studies of the layered magnanite system La<Subscript>1.2</Subscript>Ca<Subscript>1.8</Subscript>Mn<Subscript>2−x</Subscript>Ru<Subscript>x</Subscript>O<Subscript>7</Subscript> (0 ≤ x ≤ 1)

We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the temperature range of 4.2–320 K and in the presence of magnetic fields up to 10 T on the Ru-doped, bilayered manganite system, La_1.2Ca_1.8Mn_2−xRu_xO₇ (0≤x≤1). We find that the Ru doping affects the magnetotransport properties considerably. The ρ versus H data were analyzed by fitting the data to the power-law equation, ρ = ρ₀ − αHⁿ. The isothermal magnetoresistance (MR) versus H curves taken up to ± 10 T are highly symmetrical, and their curvature changes from concave up to concave down as the temperature increases. The MR, defined as [ρ(H) − ρ(0)]/ρ(0), is found to increase with Ru doping from 58% to 64% up to x=0.1 and to decrease to 45% for the x=1 sample at 10 K. Analysis of the ρ-T data below 30 K shows that, at low temperature, the system behaves like a disordered metal.