Development of (Bi,Sb)<Subscript>2</Subscript>(Te,Se)<Subscript>3</Subscript>-Based Thermoelectric Modules by a Screen-Printing Process

In major applications, optimal power will be achieved when thermoelectric films are at least 100 μm thick. In this paper we demonstrate that screen-printing is an ideal method to deposit around 100 μm of (Bi,Sb)₂(Te,Se)₃-based films on a rigid or flexible substrate with high Seebeck coefficient value (90 μV K⁻¹ to 160 μV K⁻¹) using a low-temperature process. Conductive films have been obtained after laser annealing and led to acceptable thermoelectric performance with a power factor of 0.06 μW K⁻² cm⁻¹. While these initial material properties are not at the level of bulk materials, the complete manufacturing process is cost-effective, compatible with large surfaces, and affords a mass-production technique.     

Thermoelectric Properties of SnO<Subscript>2</Subscript> Ceramics Doped with Sb and Zn

Polycrystalline SnO₂-based samples (Sn_0.97−x Sb_0.03Zn _x O₂, x = 0, 0.01, 0.03) were prepared by solid-state reactions. The thermoelectric properties of SnO₂ doped with Sb and Zn were investigated from 300 K to 1100 K. X-ray diffraction (XRD) analysis revealed all XRD peaks of all the samples as identical to the rutile structure, except for the x = 0.03 sample, which had a small amount of Zn₂SbO₄ as a secondary phase. We found that the power factor of the x = 0.03 sample was significantly improved due to the simultaneous increase in the electrical conductivity and the Seebeck coefficient. A power factor value of ∼2 × 10⁻⁴ W m⁻¹ K⁻² was obtained for the x = 0.03 sample at 1060 K, 126% higher than that for the undoped sample.     

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 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.     

Dielectric Properties and Microstructure of MgO-TiO<Subscript>2</Subscript>-ZnO-CaO Ceramics

The effects of CaTiO₃ addition on the microstructure, phase formation, and dielectric properties of MgO-TiO₂-ZnO ceramics were investigated. The sintering temperature of CaTiO₃-doped (Mg_0.63Zn_0.37)TiO₃ ceramics can be lowered to 1290°C when the additive is used. The dielectric properties are found to be strongly correlated with the amount of CaTiO₃ addition. At 1290°C, (Mg_0.63Zn_0.37)TiO₃ ceramic with 1.0 mol% CaTiO₃ exhibited a dielectric constant ε _r of 23.3, dielectric loss tan δ of 1 × 10⁻⁵, and temperature coefficient of capacitance (TCC) of 10 ppm/°C.     

Superionic conductivity and switching effect with memory in TlInSe<Subscript>2</Subscript> and TlInTe<Subscript>2</Subscript> crystals

The temperature dependences of the conductivity σ(T) and the switching and memory effects in one-dimensional TlInSe₂ and TlInTe₂ single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic conductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl⁺ ions over vacancies in the thallium sublattice between (In³⁺Te₂²⁻)⁻ and (In³⁺Se₂²⁻)⁻ nanochains (nanorods). S-type switching and memory effects are revealed in TlInSe₂ and TlInTe₂ crystals, as well as voltage oscillations in the range of negative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electric-field-induced transition of TlInSe₂ and TlInTe₂ crystals to the superionic state is found.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

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 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.     

Preparation and Thermoelectric Properties of LaGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> and SmGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript>

Thermoelectric materials are attractive since they can recover waste heat directly in the form of electricity. In this study, the thermoelectric properties of ternary rare-earth sulfides LaGd_1+x S₃ (x = 0.00 to 0.03) and SmGd_1+x S₃ (x = 0.00 to 0.06) were investigated over the temperature range of 300 K to 953 K. These sulfides were prepared by CS₂ sulfurization, and samples were consolidated by pressure-assisted sintering to obtain dense compacts. The sintered compacts of LaGd_1+x S₃ were n-type metal-like conductors with a thermal conductivity of less than 1.7 W K⁻¹ m⁻¹. Their thermoelectric figure of merit ZT was improved by tuning the chemical composition (self-doping). The optimized ZT value of 0.4 was obtained in LaGd_1.02S₃ at 953 K. The sintered compacts of SmGd_1+x S₃ were n-type hopping conductors with a thermal conductivity of less than 0.8 W K⁻¹ m⁻¹. Their ZT value increased significantly with temperature. In SmGd_1+x S₃, the ZT value of 0.3 was attained at 953 K.     

Effects of Ca and Mn Additions on the Microstructure and Dielectric Properties of (Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript> Ceramics

Dielectric ceramics based on the solid solution (1 − x)Bi_0.5Na_0.5TiO₃ (BNT)-xCaTiO₃ (CT) were synthesized by the conventional solid-state route. BNT with various contents of CT formed a complete solid solution and exhibited a rhombohedral structure. CT in this solid solution with BNT was observed to decrease the dielectric constant at higher temperatures and raise the dielectric constant at lower temperatures. On the other hand, decreased ferroelectricity was observed with increasing CT concentration, resulting in a downward shift of the depolarization temperature and a decrease of the dissipation factor. With the addition of Mn²⁺ to 0.86BNT-0.14CT, the temperature characteristics of capacitance were improved (−55°C to 250°C, ΔC/C _25°C ≤ ±15%). By doping with 1.5 wt.% Mn²⁺, the dielectric constant at room temperature reached over 900, with a dielectric loss of less than 1%.     

Preparation and Thermoelectric Properties of La-Doped SrTiO<Subscript>3</Subscript> Ceramics

Single-phase polycrystalline La _x Sr_1−x TiO₃ (x = 0, 0.04, 0.06, 0.08, and 0.12) ceramics were prepared by the conventional solid-state reaction method using high-activity hydroxides as the raw materials. The electrical conductivity of all the samples increased with increasing x value and decreased with measurement temperature, while the thermal conductivity decreased with increasing x value and measurement temperature. The La_0.12Sr_0.88TiO₃ sample showed the lowest thermal conductivity of 2.45 W m⁻¹ K⁻¹ at 873 K and the largest ZT of 0.28 at 773 K. The present work revealed that hydroxides with high activity as raw materials are beneficial to improve the thermoelectric properties, especially to decrease the thermal conductivity.     

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.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Thermoelectric Properties of Zintl Compound YbZn<Subscript>2</Subscript>Sb<Subscript>2</Subscript> with Mn Substitution in Anionic Framework

The thermoelectric properties of the Zintl compound YbZn₂Sb₂ with isoelectronic substitution of Zn by Mn in the anionic (Zn₂Sb₂)²⁻ framework have been studied. The p-type YbZn_2−x Mn _x Sb₂ (0.0 ≤ x ≤ 0.4) samples were prepared via melting followed by annealing and hot-pressing. Thermoelectric property measurement showed that the Mn substitution effectively lowered the thermal conductivity for all the samples, while it significantly increased the Seebeck coefficient for x < 0.2. As a result, a dimensionless figure of merit ZT of approximately 0.61 to 0.65 was attained at 726 K for x = 0.05 to 0.15, compared with the ZT of ~0.48 in the unsubstituted YbZn₂Sb₂.     

Super-Large Domain Metal-Induced Radially Crystallized Poly-Si Made Using Ni(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>/NH<Subscript>4</Subscript>OH Mixed Solution

By using an aqueous solution of Ni(NO₃)₂/NH₄OH for formation of Ni media on a-Si, disk-like super-large domain metal-induced radially crystallized (S-MIRC) poly-Si was prepared. The process requires no buffer layer deposition on a-Si. The prepared S-MIRC poly-Si has an average domain size of up to 60 μm, highest hole Hall mobility of 27.1 cm² V⁻¹ s⁻¹, and highest electron Hall mobility of 45.6 cm² V⁻¹ s⁻¹. Poly-Si TFT made on super-large-domain S-MIRC poly-Si had high mobility of ~105.8 cm² V⁻¹ s⁻¹, steep sub-threshold slope of ~1.0 V decade⁻¹, high on/off state current ratio of >10⁷ and low threshold voltage of ~ −6.9 V. A simultaneous Ni-collected and induced crystallization model is proposed to explain the growth kinetics of S-MIRC poly-Si.     

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.     

Thermoelectric Properties of the One-Dimensional Cobalt Oxide CaCo<Subscript>2</Subscript>O<Subscript>4</Subscript>

In this work we studied the crystal structure and physical properties of the new one-dimensional cobalt oxide CaCo₂O_4+δ . The CaCo₂O_4+δ phase crystallizes as a calcium-ferrite-type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. The specific-heat Sommerfeld constant γ was found to be 4.48(7) mJ/mol K². This result suggests that the CaCo₂O_4+δ phase has a finite density of states at the Fermi level. Metallic temperature dependence of the Seebeck coefficient S with a large thermoelectric power (S = 151 μV/K at 387 K) was observed. The origin of the large thermoelectric power may be attributed to the quasi one-dimensional character of the energy band near the valence band maximum in CaCo₂O_4+δ .     

Synthesis and Characterization of In<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films Deposited by Chemical Bath Deposition on Polyethylene Naphthalate Substrates

Indium sulfide (In₂S₃) thin films were deposited on polyethylene naphthalate (PEN) by chemical bath deposition (CBD). The materials were characterized by ultraviolet (UV)–visible spectroscopy, x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), and x-ray diffraction (XRD) to investigate the influence of the polymeric substrate on the resulting thin In₂S₃. The films showed polycrystalline (cubic and tetragonal) structure. A reduction of the ordering of the polymeric chains at the surface of the PEN was also observed, demonstrated by the appearance of two infrared bands at 1094 cm⁻¹ and 1266 cm⁻¹. Presence of oxygen during the early stages of In₂S₃ growth was also identified. We propose a reaction mechanism for both the equilibrium and nucleation stages. These results demonstrate that In₂S₃ can be deposited at room temperature on a flexible substrate.     

Synthesis and Electronic Properties of the Misfit Layer Compound [(PbSe)<Subscript>1.00</Subscript>]<Subscript>1</Subscript>[MoSe<Subscript>2</Subscript>]<Subscript>1</Subscript>

An ultralow-thermal-conductivity compound with the ideal formula [(PbSe)_1.00]₁[MoSe₂]₁ has been successfully crystallized across a range of compositions. The lattice parameters varied from 1.246 nm to 1.275 nm, and the quality of the observed 00ℓ diffraction patterns varied through the composition region where the structure crystallized. Measured resistivity values ranged over an order of magnitude, from 0.03 Ω m to 0.65 Ω m, and Seebeck coefficients ranged from −181 μV K⁻¹ to 91 μV K⁻¹ in the samples after the initial annealing to form the basic structure. Annealing of samples under a controlled atmosphere of selenium resulted in low conductivities and large negative Seebeck coefficients, suggesting an n-doped semiconductor. Scanning transmission electron microscopy cross-sections confirmed the interleaving of bilayers of PbSe with Se-Mo-Se trilayers. High-angle annular dark-field images revealed an interesting volume defect, where PbSe grew through a region where a layer of MoSe₂ would be expected in the perfect structure. Further studies are required to correlate the density of these defects with the observed electrical properties.