Electrical and Thermal Properties of the Ca3?xGdxCo4O9+δ System

The effects of Gd substitution on the thermoelectric (TE) properties of Ca₃Co₄O_9+δ have been systematically investigated from 25 K to 335 K. Partial substitution of Gd in Ca₃Co₄O_9+δ results in an increase of thermopower and resistivity, and a decrease of thermal conductivity. A maximum dimensionless figure of merit (ZT) of 0.028 was achieved at 335 K for Ca_2.4Gd_0.6Co₄O_9+δ , which is about one order of magnitude larger than that for Ca₃Co₄O_9+δ . The investigation demonstrates that the TE performance of the Ca₃Co₄O_9+δ system can be improved through Gd doping.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

Synthesis and Electrical Properties of γ-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> via a Citrate Sol–Gel Method with Polyethylene Glycol 400

In this work, a citrate sol–gel method (Sol–Gel) with polyethylene glycol 400 (Sol-Gel-PEG400) was developed to prepare γ-Na _x Co₂O₄ by using sodium and cobalt nitrates as the raw materials, citric acid as a complexing agent, and PEG400 as a dispersant. At 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained using Sol-Gel-PEG400. With the addition of 1 vol.% PEG400, smaller, flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. Moreover, polycrystalline sintered bulk γ-Na _x Co₂O₄ with more highly oriented crystals and greater compact density was fabricated using the Sol-Gel-PEG400 synthesized powders compared with the powders synthesized by citrate Sol–Gel. The electrical conductivity (σ) values of Sol-Gel-PEG400 samples were higher than those of Sol–Gel samples between 400 K and 900 K. The σ value of Sol-Gel-PEG400 increased to 3.13 × 10⁴ Sm⁻¹ at 400 K and to 1.84 × 10⁴ Sm⁻¹ at 900 K. Between 400 K and 850 K, the Seebeck coefficient (α) values of Sol-Gel-PEG400 samples were slightly lower than those of Sol–Gel samples. Near 900 K, the α values of these two methods were nearly equal, at 164 μV K⁻¹. Between 400 K and 900 K, the power factor (P) of Sol-Gel-PEG400 was evidently larger than that of Sol–Gel.     

Improved Thermoelectric Characteristics of Si-Doped Misfit-Layered Cobaltite

The cobaltite Ca₃Co₄O_9+δ has shown large thermopower and is considered to be a good candidate for use as a thermoelectric material. The composition of Ca₃Co₄O_9+δ is better expressed as [Ca₂CoO₃][CoO₂] _b1/b2 with the misfit-layered structure featuring different periodicities along the b axis, with b ₁ referring to the b-axis length of the NaCl-type [Ca₂CoO₃] sublattice and b ₂ referring to the b-axis length of the [CoO₂] sublattice. The crystal structure of Ca₃Co₄O_9+δ can be viewed as being of two subsystems, i.e., the distorted NaCl-type [Ca₂CoO₃] sublattice and the CdI₂-type [CoO₂] sublattice, alternately stacked along the c-axis. In this paper, we report measurements of the electrical resistivity and Seebeck coefficient for a series of misfit-layered oxides Ca₃Co_4−x Si _x O_9+δ prepared by solid-state reaction. Structural parameters are refined with the superspace group X2/m(0β0)s0 using powder x-ray diffraction data. With partial substitution of Si⁴⁺ for Co³⁺, the resistivity decreases, while the thermopower increases simultaneously. These results indicate that partial substitution of Si⁴⁺ improves the thermoelectric characteristics of Ca₃Co₄O_9+δ .     

Thermoelectric Properties of (Na<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>)<Subscript>1.4</Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> (M = Sr,Li)

Oxide thermoelectric materials (Na_1−y M _y )_1.4Co₂O₄ (M = Sr, Li; y = 0 to 0.4) were prepared by a sol–gel method. The influence of doping on the thermoelectric properties was investigated, and the phase composition was characterized by x-ray diffraction. Experimental results showed that the main crystalline phase of the undoped and Sr/Li-doped samples was γ-Na_1.4Co₂O₄. The thermoelectric properties of Na_1.4Co₂O₄ can be improved slightly by doping with Sr. Doping with Li improves the thermoelectric properties of Na_1.4Co₂O₄. For a doping fraction of y = 0.1, the electrical conductivity of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieves its maximum value of 301.19 (Ω mm)⁻¹. The Seebeck coefficient and power factor of (Na_1−y Li _y )_1.4Co₂O₄ at 288 K achieve their maximum values of 172.28 μV K⁻¹ and 7.44 mW m⁻¹ K⁻² at a doping fraction of y = 0.4.     

Power Factor Anisotropy of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Conductive Thermoelectric Bi-Sb-Te Thin Films

The best films for thermoelectric applications near room temperature are based on the compounds Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃, which as single crystals have distinct anisotropy in their electrical conductivity σ regarding the trigonal c-axis, whereas the Seebeck coefficient S is nearly isotropic. For p- and n-type alloys, P _⊥c > P _||c, and the power factors P _⊥c of single crystals are always higher compared with polycrystalline films, where the power factor is defined as P = S ² σ, ⊥c and ||c are the direction perpendicular and parallel to the c-axis, respectively. For the first time in sputter-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ and n-type Bi₂(Te_0.9Se_0.1)₃ thin films, the anisotropy of the electrical conductivity has been measured directly as it depends on the angle φ between the electrical current and the preferential orientation of the polycrystals (texture) using a standard four-probe method. The graphs of σ(φ) show the expected behavior, which can be described by a weighted mixture of σ _⊥c and σ _||c contributions. Because (σ _⊥c/σ _||c) _p  < (σ _⊥c/σ _||c) _n , the n-type films have stronger anisotropy than the p-type films. For this reason, the angular weighted contributions of P _||c lead to a larger drop in the power factor of polycrystalline n-type films compared with p-type films.     

Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca2?xBix?δMnO4?γ

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca₂MnO_4−γ . The electrical conductivity σ and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity κ of doped Ca₂MnO_4−γ is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = σS ² T/κ, increases with Bi doping. The maximum ZT is 0.023 for Ca_1.6Bi_0.18MnO_4−γ at 877°C, which is ten times higher than that of the end member, Ca₂MnO_4−γ . The increase of ZT mainly results from the considerable increase of σ, which can be explained in terms of structural change. The␣Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO₃ layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.     

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.     

Thermoelectric Properties of Ca3Co4O9/[Ca2(Co0.65Cu0.35)2O4]0.624CoO2 Composites

Ca₃Co₄O₉ is one of the most promising p-type thermoelectric materials because of its high dimensionless figure of merit ZT. However, polycrystalline Ca₃Co₄O₉ ceramics shows lower ZT value than that for single crystal Ca₃ Co₄O₉ due to its higher electrical resistivity ρ. Mikami et al. have reported that the addition of Ag to Ca₃Co₄O₉ ceramics could successfully reduce ρ and enhance the power factor. On the other hand, Ohtaki et al. reported that a composite structure could be highly effective to reduce κ for ZnO dually doped with Al and Ga. In this work, we tried to enhance the power factor and reduce κ by forming Ca₃Co₄O₉/[Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624CoO₂ composite structure. As a result, the ZT value for Ca₃Co₄O₉/[Ca₂(Co_0.65Cu_0.35)₂O₄]_0.624CoO₂ composites reached 0.164 at 700 °C, which was 40 % higher than the value for Ca₃Co₄O₉.     

Increase in the Thermoelectric Efficiency of the Disordered Phase of Layered Antiferromagnetic CuCrS<Subscript>2</Subscript>

The thermoelectric figure of merit (ZT) of the layered antiferromagnetic compound CuCrS₂ is further improved with increase in the Cr-vacancy disorder on sintering above 900°C. X-ray photoelectron spectroscopy and x-ray diffraction refinement results for different samples show that the chromium atoms are transferred from the filled layers to the vacant sites between the layers. This atomic disorder increases the electrical conductivity (σ) due to self-doping of the charge carriers and reduces thermal conductivity (κ) due to increase in phonon scattering. The Seebeck coefficient (S) is p-type and remains nearly temperature independent with values between 150 μV/K and 450 μV/K due to electronic doping in different samples.     

Transport and Thermoelectric Properties of the Ca1?xSrxRu1?yMnyO3 System

The magnetic, transport, and thermoelectric properties of Ca_1−x Sr _x Ru_1−y Mn _y O₃ have been investigated. Ferromagnetism with relatively high T _C (>200 K) was introduced by Mn doping. In particular, ferromagnetism appeared in the Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system at y > 0.2. The maximum T _C (=270 K) was recorded for a specimen of Ca_0.5Sr_0.5Ru_0.4Mn_0.6O₃. The ferromagnetism seems to be due to the mixed-valence states of Mn³⁺, Mn⁴⁺, Ru⁴⁺, and Ru⁵⁺ ions. The metallic character of Ru-rich specimens was suppressed by Mn substitution, and the system was transformed into a semiconductor at relatively low Mn content near y = 0.1. Specimens with higher Mn content (y > 0.8) had large thermoelectric power (50 μV K⁻¹ to 130 μV K⁻¹ at 280 K) accompanied by relatively low resistivity (0.03 Ω cm to 1 Ω cm). The Ca_0.5Sr_0.5Ru_1−y Mn _y O₃ system seems to have good potential as a thermoelectric material for use above 300 K.     

Improved Thermoelectric Properties of Bi-M-Co-O (M = Sr,Ca) Misfit Compounds by Laser Directional Solidification

In this work, improvement of the thermoelectric properties of bulk samples due to texture developed by a directional laser-assisted solidification process (laser floating zone melting method) is reported for cobaltite materials with compositions Bi₂Sr₂Co_1.8O _y and Bi₂Ca₂Co_1.7O _y . Sample composition and microstructure have been studied using x-ray diffraction and scanning electron microscopy. Thermoelectric properties have been measured between 4 K and 300 K by simultaneous determination of electrical resistivity and thermopower. All textured samples showed remarkable increase of power factor values as compared with conventional sintered ceramics.     

On the Excess Oxygen in Four-Layered Rock-Salt-Type Units of Modulated Thermoelectric Bi-Sr-(Co,Rh)-O Compounds

Excess oxygen exists in four-layered rock-salt (RS)-type units of modulated misfit-layered Bi-Sr-(Co,Rh)-O compounds, which consist of interpenetrating CdI₂-type (Co,Rh)O₂ and distorted four-layered RS-type block subsystems, which have two b-axes, i.e., b ₁ and b ₂, respectively. From carefully determined chemical contents and misfit ratios, p = b ₁/b ₂, two structural characteristics are concluded, namely, intermixing metal ions in the RS-type layers and excess oxygen δ in, or in the vicinity of, them. The chemical formulae are proposed as [(Bi_1−x (Co,Rh) _x )₂(Sr_1−y Bi _y )₂O_4+δ ] _p (Co,Rh)O₂. The valence states of␣cobalt and rhodium ions are close to 3.3+. These valence states are quite reasonable for good thermoelectric oxides, such as γ-Na_0.7CoO₂ and [Ca₂CoO₃]_0.62CoO₂. Excess oxygen would cause the undulated atomic arrangement.     

The Power Factor of Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Tl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>3</Subscript> Single Crystals

Single crystals of the ternary system Bi_2−x Tl _x Se₃ (nominally x = 0.0 to 0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by measurement of lattice parameters, electrical conductivity σ _⊥c, Hall coefficient R _H(B║c), and Seebeck coefficient S(ΔT⊥c). The measurements indicate that incorporation of Tl into Bi₂Se₃ lowers the concentration of free electrons and enhances their mobility. This effect is explained within the framework of the point defects in the crystal lattice, with formation of substitutional defects of thallium in place of bismuth (Tl_Bi) and a decrease in the concentration of selenium vacancies (V_Se ^{+ 2} V_{\rm{Se}}^{ + 2} ). The temperature dependence of the power factor σS ² of the samples is also discussed. As a consequence of the thallium doping we observe a significant increase of the power factor compared with the parent Bi₂Se₃.     

The Difference Between Thermo- and Pyroelectric Co-Based Rare-Earth (Nd,Y, Gd,Ce) Oxide Composites Measured Using a High Temperature Gradient

The Seebeck voltage U _S of cobalt-based oxide composites containing rare-earth elements such as Nd, Y, Gd, and Ce was measured under high temperature gradients of up to ΔT = 700 K, as well as its time dependence U _S(t). The closed-circuit electric current as a function of Seebeck voltage I _S(U _S) or time I _S(t) was used for further characterization. While Nd₂O₃ + CoO and Y₂O₃ + CoO showed linear n-type U _S(t) behavior, as it is usual for thermoelectrics, Gd₂O₃ + CoO exhibited large hysteresis. For both Gd₂O₃ + CoO and Ce₂O₃ + CoO, a significant time dependence of the electric current I(t) was measured, indicating pyroelectric material behavior with large capacitance. Both anomalies became smaller when Fe₂O₃ was added, but became more apparent in Nd₂O₃ + CoO and Y₂O₃ + CoO when Al₂O₃ was added. This behavior can be explained by electron pull-out into the interfacial space-charge region, which is considered as a material phenomenon with great potential for further development.     

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.     

Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

Performance of a Thermoelectric Module Based on n-Type (La0.12Sr0.88)0.95TiO3−δ and p-Type Ca3Co4−xO9+δ

Here, we present the performance of a thermoelectric (TE) module consisting of n-type (La_0.12Sr_0.88)_0.95TiO₃ and p-type Ca₃Co_4?xO_9+δ materials. The main challenge in this investigation was operating the TE module in different atmospheric conditions, since n-type has optimum TE performance at reducing conditions, while p-type has optimum at oxidizing conditions. The TE module was exposed to two different atmospheres and demonstrated higher stability in N₂ atmosphere than in air. The maximum electrical power output decreased after 40 h when the hot side was exposed to N₂ at 600°C, while only 1 h at 400°C in ambient air was enough to oxidize (La_0.12Sr_0.88)_0.95TiO₃ followed by a reduced electrical power output. The module generated maximum electrical power of 0.9 mW (～?4.7 mW/cm²) at 600°C hot side and δT?～?570 K in N₂, and 0.15 mW (～?0.8 mW/cm²) at 400°C hot side and δT?～?370 K in air. A stability limit of Ca₃Co_3.93O_9+δ at ～?700°C in N₂ was determined by in situ high-temperature x-ray diffraction.

     

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.     

Characteristics of Ga-doped ZnO films deposited by pulsed DC magnetron sputtering at low temperature

Characteristics of Ga-doped ZnO (GZO) transparent conductive oxide films have been investigated based on the absorption behavior and chemical states of dopant Ga in the film. GZO samples were prepared by pulsed DC magnetron sputtering at 423 K by varying the sputtering power from 0.6 to 2.4 kW and the Ga₂O₃ concentration in the targets from 0.6 to 5.7 wt%. Absorption spectra of the GZO films in the visible to ultraviolet range were characterized by long absorption tails and shoulders near the absorption edges indicating the presence of impurity states or bands that overlap with the conduction band. X-ray photoelectron spectroscopy and X-ray diffraction revealed that substantial portion of the dopant exists as finely dispersed or amorphous metallic Ga and oxide of Ga, which would be related to the formation of the impurity bands or states, especially in the samples with lower Ga content. Presence of these species is correlated to the limited doping efficiency observed in the GZO films.