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.     

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.     

Effect of pH on Structural and Electrical Properties of Electrodeposited Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

Bi₂Te₃ thin films were electrodeposited at various pH values of a bismuth nitrate and tellurium oxide plating solution. Enhancement in pH results in a decrease in grain size. Transmission electron microscopy reveals the transformation of the film morphology from dispersed nanoparticles to connected chain-like nanostructures of Bi₂Te₃ as pH is increased. Electrical characterization for samples deposited in the temperature range of 300 K to 425 K shows a fourfold increase in Seebeck coefficient, S, between its maximum and minimum value as the solution pH changes from 1 to 3.5. Such enhancement of S is attributed to the increased connectivity of the nanostructures at higher pH.     

Thermoelectric Properties of NaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

NaCo₂O₄ has one of the highest figures of merit among all ceramic thermoelectric materials. Because of its large thermopower and low resistivity, the ceramic oxide NaCo₂O₄ is a promising candidate for potential thermoelectric applications. NaCo₂O₄ is, moreover, a ceramic compound with high decomposition temperature and chemical stability in air and it does not contain any toxic elements. Like all 3-d transition ions, Co ions have multiple spin and oxidation states. In this investigation, thermopower and electrical conductivity of NaCo₂O₄ as a function of substitution of Co by Fe ions were measured. Fe substitution for Co causes resistivity to increase, whereas the Seebeck coefficient remained nearly invariant, especially above 330 K. An erratum to this article can be found at     

Microstructures and Thermoelectric Properties of an Annealed Ti<Subscript>0.5</Subscript>(Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)<Subscript>0.5</Subscript>NiSn<Subscript>0.998</Subscript>Sb<Subscript>0.002</Subscript> Ribbon

The thermoelectric half-Heusler compound Ti_0.5(Hf_0.5Zr_0.5)_0.5NiSn_0.998Sb_0.002 was fabricated by spin-casting and subsequent annealing. ZT at room temperature increased with annealing time through an increase in absolute Seebeck coefficients despite a decrease in electrical conductivity. ZT reached 0.10 after annealing at 1050 K for 48 h. In powder x-ray diffraction analysis, each half-Heusler peak was accompanied by a bump at the high-angle side, corresponding to a minor Ti-rich half-Heusler phase. The quantity and Ti composition of the minor phase increased with annealing time, although those of the major half-Heusler phase were almost constant. In transmission electron microscopic analysis, granular domains, several nanometers in size, with atomic ordering or disordering were observed. Thermoelectric properties were␣improved by annealing through the growth of heterogeneous microstructures of the Ti-rich and Ti-poor half-Heusler grains and of the granular domains.     

Thermal Conductivity of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Affected by Grain Size and Pores

A fine measurement system for measuring thermal conductivity was constructed. An accuracy of 1% was determined for the reference quartz with a value of 1.411 W/m K. Bi_0.5Sb_1.5Te₃ samples were prepared by mechanical alloying followed by hot-pressing. Grain sizes were varied in the range from 1 μm to 10 μm by controlling the sintering temperature in the temperature range from 623 K to 773 K. The thermal conductivity was 0.89 W/m K for the sample sintered at 623 K, while a grain size of 1.75 μm was measured by optical microscopy and scanning electron microscopy. The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 K to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.     

Synthesis and Thermoelectric Properties of the Double-Filled Skutterudite Yb<Subscript>0.2</Subscript>In<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript>

Filled skutterudites have long been singled out as one of the prime examples of phonon glass electron crystal materials. Recently the double-filling approach in these materials has been attracting increased attention. In this study, Yb_0.2In _y Co₄Sb₁₂ (y = 0.0 to 0.2) samples have been prepared by a simple melting method and their thermoelectric properties have been investigated. The power factor is increased dramatically when increasing the In content, while the lattice thermal conductivity is lowered considerably, leading to a large increase of the ZT value. A state-of-the-art ZT value of 1.0 is attained in Yb_0.2In_0.2Co₄Sb₁₂ at 750 K.     

Comparative studies of p-type InP layers formed by Zn<Subscript>3</Subscript>As<Subscript>2</Subscript> and Zn<Subscript>3</Subscript>P<Subscript>2</Subscript> diffusion

The Zn₃As₂ and Zn₃P₂ were used as Zn-diffusion sources to form a p-region in undoped-InP wafers. The p-type InP formed by Zn diffusion from a Zn₃P₂ source has higher transmittance over the testing-spectrum range 1,000–1,700 nm versus Zn diffusion from a Zn₃As₂ source. In the case of a p-type region formed from a Zn₃As₂ source, x-ray photoelectron spectroscopy (XPS) showed As atoms were reduced from the oxide state and formed an InAs composition, which introduces more absorption loss.     

Thermoelectric Properties of TlCu3Te2 and TlCu2Te2

We performed thermoelectric characterizations on TlCu₃Te₂: (Tl¹⁺)(Cu¹⁺)₃ (Te²⁻)₂ and TlCu₂Te₂: (Tl¹⁺)(Tl³⁺)(Cu¹⁺)₄(Te²⁻)₄, in order to understand the relationship between the thermoelectric properties (especially the lattice thermal conductivity κ _lat) and the valence states of Tl. The thermal conductivity of TlCu₂Te₂ is high (about 8 W m⁻¹ K⁻¹), while that of TlCu₃Te₂ is extremely low (around 0.5 W m⁻¹ K⁻¹) like other thallium tellurides. This high κ of TlCu₂Te₂ was caused not only by its large electronic contribution but also by its intrinsically high κ _lat. The present study implies that the valence states of Tl would play some important roles in determining the magnitude of κ _lat.     

Characterization of Ge<Subscript>22</Subscript>Sb<Subscript>22</Subscript>Te<Subscript>56</Subscript> and Sb-Excess Ge<Subscript>15</Subscript>Sb<Subscript>47</Subscript>Te<Subscript>38</Subscript> Chalcogenide Thin Films for Phase-Change Memory Applications

A phase-change memory device that utilizes an antimony (Sb)-excess Ge₁₅Sb₄₇Te₃₈ chalcogenide thin film was fabricated and its electrical properties were measured and compared with a similar device that uses Ge₂₂Sb₂₂Te₅₆. The resulting electrical characteristics exhibited I _reset values of 14 mA for Ge₂₂Sb₂₂Te₅₆ and 10.6 mA for Ge₁₅Sb₄₇Te₃₈. Also, the set operation time (t _set) for the device using Ge₁₅Sb₄₇Te₃₈ films was 140 ns, which was more than twice as fast as the Ge₂₂Sb₂₂Te₅₆ device. The relationship between the microstructure and the improved electrical performance of the device was examined by means of transmission electron microscopy (TEM).     

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.     

Synthesis,Crystal Structure,and Transport Properties of Na<Subscript>22</Subscript>Si<Subscript>136</Subscript>

The type II clathrate Na₂₂Si₁₃₆ is prepared by the thermal decomposition of NaSi. Thermal analysis indicates this phase is metastable yet has a relatively high decomposition temperature. Rietveld analysis indicates that Na in the larger Si₂₈ cage is shifted off-center, analogous to observations in some type I clathrates. Temperature-dependent electrical and thermal transport properties are reported for Na₂₂Si₁₃₆, for which the spark plasma sintering technique was found to be effective in achieving intergrain sintering in the consolidated specimen. The potential that type II clathrate materials possess for thermoelectric applications is discussed.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

I-型硅基笼合物Ba8Ga16ZnxSi30-x的合成及电传输特性

用固相反应法结合熔融法合成Zn掺杂单相n型Ba8Ga16ZnxSi30-x化合物，探索Zn在Si位的取代对其结构及电传输特性的影响规律. 研究结果表明：x=1时化合物的平均键角畸变Δθ最大为4.4. ；当取代分数x＝0, 2, 4时，对应样品的电导率明显高于x＝1, 3时对应样品的电导率，在室温附近，Ba8Ga16Zn2Si28化合物表现出较高的电导率，约为3.0e5S/m，当x=1时，对应化合物的电导率在测试温度范围内最低；当取代分数x＝0, 2, 4时对应样品的Seebeck系数明显高于x＝1, 3时对应样品的Seebeck系数，且随着填充分数的增加，Seebeck系数分别逐渐降低；Ba8Ga16Zn2Si28化合物在测试温度范围内表现出较好的电性能，在1000K处具有最大的功率因子1.03e-3W/ (m·K2) .     

Electrodeposition and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

Spatial Distribution of the Seebeck Coefficient in Zn<Subscript>13</Subscript>Sb<Subscript>10</Subscript> Determined by a Seebeck Microprobe Measurement System

A Seebeck microprobe (SMP) measurement system has been developed and employed to determine the spatial distribution of the Seebeck coefficient of a polycrystalline Zn₁₃Sb₁₀ specimen prepared by a gradient freeze (GF) method. The spatial distribution of the Seebeck coefficient strongly reflects that of the grains observed using an optical polarizing microscope, the magnitude of which ranges from 100 μV/K to 130 μV/K. This fact strongly indicates that the observed spatial distribution of the Seebeck coefficient arises from the anisotropic Seebeck effect of grains with different crystal orientations in the polycrystalline Zn₁₃Sb₁₀.     

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.     

Wetting Properties and Interfacial Reactions of Mechanically Alloyed Cu<Subscript>5</Subscript>Zn<Subscript>8</Subscript>-Bearing Pb-Free Solders on a Copper Substrate

For enhanced wetting properties of Zn-doped solder alloys, this paper proposes Cu₅Zn₈-bearing solders. A mechanical alloying process with controlled milling time and rotational speed was used to successfully fabricate Cu₅Zn₈-bearing powders with a diameter of 50 μm to 70 μm. Their composition was identified by inductively coupled plasma atomic emission spectroscopy. After the powders were made into a paste with a rosin-activated type of flux, the wetting angles of the Cu₅Zn₈-bearing paste solders on a Cu substrate were compared with the wetting angles of bulk solder alloys with the same amount of Cu and Zn alloying elements. The reason for the enhanced wetting properties of Cu₅Zn₈-bearing solders is explained by thermodynamic calculations and differential scanning calorimetry experiments. In addition, the interfacial reactions and the shear strength with Cu substrates are also discussed.     

Thermoelectric Characterization of (Ga,In)<Subscript>2</Subscript>Te<Subscript>3</Subscript> with Self-Assembled Two-Dimensional Vacancy Planes

The key properties for the design of high-efficiency thermoelectric materials are a low thermal conductivity and a large Seebeck coefficient with moderate electrical conductivity. Recent developments in nanotechnology and nanoscience are leading to breakthroughs in the field of thermoelectrics. The goal is to create a situation where phonon pathways are disrupted due to nanostructures in “bulk” materials. Here we introduce promising materials: (Ga,In)₂Te₃ with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5-nm intervals exist in Ga₂Te₃, scattering phonons efficiently and leading to a very low thermal conductivity.