Thermoelectric Properties of Layer-Antiferromagnet CuCrS<Subscript>2</Subscript>

We have performed a detailed study of the electrical and thermal conductivities and thermoelectric power behavior of an antiferromagnetic-layer compound of chromium, CuCrS₂, from 15 K to 300 K. Unlike previous studies, we find noninsulating properties and sensitive dependence on the preparation method, the microstructure, and the flaky texture formed in polycrystalline samples after extended sintering at high temperatures. Flakes are found to be metallic, with strong localization effects in the conductivity on cooling to low temperatures. The antiferromagnetic transition temperature T _N (=40 K) remains essentially unaffected. The Seebeck coefficient is found to be in the range of 150 μV/K to 450 μV/K, which is exceptionally large, and becomes temperature independent at high temperatures, even for specimens with low resistivity values of 5 mΩ cm to 200 mΩ cm. We find the thermal conductivity κ to be low, viz. 5 mW/K cm to 30 mW/K cm. This can be attributed mostly to the dominance of lattice conduction over electronic conduction. The value of κ is further reduced by disorder in Cu occupancy in the quenched phase. We also observe an unusually strong dip in κ at T _N, which is probably due to strong magnetocrystalline coupling in these compounds. Finally we discuss the properties of CuCrS₂ as a heavily doped Kondo-like insulator in its paramagnetic phase. The combination of the electronic properties observed in CuCrS₂ makes it a potential candidate for various thermoelectric applications.     

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.     

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.     

Off-Center Guest Vibrations and Their Effect on Lattice Thermal Conductivity in <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Type β-Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript>

We report on thermoelectric and Raman scattering studies of single crystalline samples of a type I clathrate Ba₈Ga₁₆Sn₃₀. The n- and p-type samples have a thermopower of −300 μV/K and +270 μV/K, and electrical resistivity of 20 mΩ cm and 40 mΩ cm at room temperature, respectively. Regardless of the charge carrier type, the lattice thermal conductivity κ _L shows typical glass-like behavior. The low-energy vibration of Ba(2) in the tetrakaidecahedron is manifested in the Raman scattering spectrum as a peak at 15 cm⁻¹. This energy agrees with the characteristic vibrational temperature of 20 K derived from the analysis of the specific heat data with a soft-potential model. The fact that both the vibrational energy and the magnitude of κ _L for Ba₈Ga₁₆Sn₃₀ are lowest among the type I clathrates is evidence that off-center vibrations are responsible for the strong reduction of κ _L. Analysis of κ _L(T) revealed that the positional disorder associated with the off-center vibrations acts as the Rayleigh scattering center, and the coupling between guest vibrational modes and acoustic phonons is stronger than that in Sr₈Ga₁₆Ge₃₀ and Eu₈Ga₁₆Ge₃₀.     

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.     

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 <Emphasis Type="Italic">p</Emphasis>-Type Mg<Subscript>2.00</Subscript>Si<Subscript>0.25</Subscript>Sn<Subscript>0.75</Subscript> with Li and Ag Double Doping

Mg₂Si_1−x Sn _x -system solid solutions are ecofriendly semiconductors that are promising materials for thermoelectric generators in the middle temperature range. To produce a thermoelectric device, high-performance p- and n-type materials must be balanced. In this paper, p-type Mg_2.00Si_0.25Sn_0.75 with Li and Ag double doping was prepared by the liquid–solid reaction method and hot-pressing. Effects of Li and Ag double doping on thermoelectric properties were investigated in the temperature range from room temperature to 850 K. All sintered compacts were identified as single-phase solid solutions with anti-fluorite structure. The carrier concentration increased with the double doping. The temperature dependence of resistivity of the double-doped samples was similar to that of a metal. The seebeck coefficient increased with temperature to a maximum value and then decreased in the intrinsic region. Thermal conductivity decreased linearly with increasing temperature, reaching a minimum near the intrinsic region, and then increased rapidly because of the contribution of the bipolar component. The dimensionless figure of merit reached 0.32 at 610 K for Mg_2.00Si_0.25Sn_0.75 double-doped with Li-5000 ppm and Ag-20000 ppm.     

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.     

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.     

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.     

Characterization of Single Barrier Microrefrigerators at Cryogenic Temperatures

The experimental characterization of single barrier heterostructure thermionic cooling devices at cryogenic temperatures is reported. The device studied was a cylindrical InGaAs microrefrigerator, in which the active layer was a 1 μm thick In_0.527Al_0.218Ga_0.255As heterostructure barrier with n-type doping concentration of 6.68 × 10¹⁶ cm⁻³ and an In_0.53Ga_0.47As emitter/collector of 5 × 10¹⁸ cm⁻³ n-doping. A full field thermoreflectance imaging technique was used to measure the distribution of temperature change on the device’s top surface when different current excitation values were applied. By reversing the current direction, we studied the device’s behavior in both cooling and heating regimes. At an ambient temperature of 100 K, a maximum cooling of 0.6 K was measured. This value was approximately one-third of the measured maximum cooling value at room temperature (1.8 K). The paper describes the device’s structure and the first reported thermal imaging at cryogenic temperatures using the thermoreflectance technique.     

Impedance Spectroscopy of Dielectric BaTi<Subscript>5</Subscript>O<Subscript>11</Subscript> Film Prepared by Laser Chemical Vapor Deposition Method

BaTi₅O₁₁ film was prepared on Pt/Ti/SiO₂/Si substrate by the laser chemical vapor deposition method. A single-phase BaTi₅O₁₁ film with ([`3] 22)/([`2] 23) (\overline{3} 22)/(\overline{2} 23) preferred orientation and columnar cross-section was obtained at high deposition rate (154.8 μm h⁻¹). The dielectric constant (ε _r) of the BaTi₅O₁₁ film was 21, measured at 300 K and 1 MHz. The electrical properties of the BaTi₅O₁₁ film were investigated by ac impedance spectroscopy from 300 K to 1073 K at 10² Hz to 10⁷ Hz. Plots of the real and imaginary parts of the impedance (Z′ and Z″) and electrical modulus (M′ and M″) in the above frequency and temperature range suggested the presence of two relaxation regimes, which were attributed to grain and grain boundary responses. The ac conductivity plots as a function of frequency showed three types of conduction process at elevated temperature. The frequency-independent plateau at low frequency was due to dc conductivity. The mid-frequency conductivity was due to grain boundaries, while the high-frequency conductivity was due to grains.     

Heat Capacity Study on Anharmonicity in Ae<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Ge<Subscript>30</Subscript> (Ae = Sr and Ba)

The heat capacity of single-crystalline samples of Sr₈Ga₁₆Ge₃₀ (SGG) and Ba₈Ga₁₆Ge₃₀ (BGG) clathrates was measured to investigate the anharmonicity of the encapsulated atoms. At low temperatures, BGG can be well described by a standard Debye model, and the C _p/T ³ versus T plot can be fitted with two Einstein temperatures: θ _E1 = 42 K and θ _E2 = 74 K. On the other hand, SGG shows deviation from the Debye model. Moreover, neither the Einstein model nor the soft potential model (SPM) alone can fit the peak in the C _p/T ³ versus T plot, and the peak should be fitted by employing both models. Our results indicate that the effective electron mass is enhanced by the anharmonic phonons.     

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.     

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.     

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.     

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.     

A 5.4-GHz high-<Emphasis Type="Italic">Q</Emphasis> tunable active-inductor bandpass filter in standard digital CMOS technology

A transistor-only CMOS active-inductor with an all-NMOS signal path is presented. By tuning the varactor-augmented parasitic capacitance at the only internal node the circuit losses from submicron MOSFETs can be partially or fully compensated to permit realizing unlimited values of Q, with little frequency and no power-consumption penalties. Transistor-only second-order bandpass filters using the active inductor were built in the TSMC 0.18-μm CMOS process, and high filter Q was obtained by tuning the varactor. The highest center frequency measured was f ₀ = 5.7 GHz for 0.2-μm gate lengths and the maximum repeatably measured Q was 665. Lower Qs can be obtained by reducing the capacitive compensation or by adjusting the circuit biasing. f ₀ and Q are tunable via separate varactors. IIP ₃ and input 1-dB compression point were simulated as 0.523 V_PP and 0.128 V_PP (−1.65 and −13.9 dBm from a 50-Ω source) at 5.7 GHz with Q = 100 and midband gain equal 4.7 dB. For the same conditions, the output noise and noise figure (R _S = 50 kΩ) were simulated to be 0.8 μV/Hz^1/2 and 25.6 dB, respectively. The filter core occupies an area of 26.6 μm × 30 μm and dissipates 4.4 mW at 5.4 GHz from a 1.8-V power supply. As the circuits use only MOSFETs they are fully compatible with standard digital CMOS processes. f ₀ statistics were obtained by measuring 40 chips at identical biasing condition.     

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 .     

Electrodeposition of <Emphasis Type="Italic">p</Emphasis>-Type Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films

Thermoelectric Sb _x Te _y films were potentiostatically electrodeposited in aqueous nitric acid electrolyte solutions containing different concentrations of TeO₂. Stoichiometric Sb _x Te _y films were obtained by applying a voltage of −0.15 V versus saturated calomel electrode (SCE) using a solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The nearly stoichiometric Sb₂Te₃ films had a rhombohedral structure, R[`3]m R\bar{3}m , with a preferred orientation along the (015) direction. The films had hole concentration of 5.8 × 10¹⁸/cm³ and exhibited mobility of 54.8 cm²/Vs. A more negative potential resulted in higher Sb content in the deposited Sb _x Te _y films. Furthermore, it was observed that the hole concentration and mobility decreased with increasingly negative deposition potential, and eventually showed insulating properties, possibly due to increased defect formation. The absolute value of the Seebeck coefficient of the as-deposited Sb₂Te₃ thin film at room temperature was 118 μV/K.