High-Temperature Capacitor Based on Ca-Doped Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> Ceramics

High-temperature capacitors were prepared by the conventional oxide method based on Bi_0.5Na_0.5TiO₃-BaTiO₃-CaTiO₃ (BNT-BT-CT) lead-free piezoelectric ceramics. BNT-BT is one of the promising candidates as a high-temperature relaxor, and has a high Curie temperature and broadened dielectric constant. The addition of CT increases the dielectric constant at lower temperatures and decreases the dielectric constant at higher temperatures, so that the variation of capacitance is decreased. The effect of BT on the temperature characteristic of dielectric constant is contrary to that of CT. A single-phase rhombohedral perovskite and square grains were obtained in this study. With the proper amount of BT and CT additions, the high-temperature specification can be met: from −55°C to 200°C, the variation of capacitance is within ±15% of room-temperature capacitance.     

Dielectric properties of low loss Ba<Subscript>6−3x</Subscript>Nd<Subscript>8+2x</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript> thin films prepared by pulsed laser deposition for microwave applications

Ba_6−3xNd_8+2xTi₁₈O₅₄ with x=0.25 (BNT-0.25, or simply, BNT) dielectric thin films with a thickness of 320 nm have been prepared on Pt-coated silicon substrates by pulsed laser deposition (PLD) at the substrate temperature of 650°C in 20 Pa oxygen ambient. X-ray analysis showed that the as-deposited films are amorphous and the films remain amorphous after a postannealing at 750°C for 30 min. The dielectric constant of the BNT films has been determined to be about 80 with a low loss tan δ of about 0.006 at 1 MHz. The capacitance-voltage (C-V), capacitance-frequency, and capacitance-temperature characteristics of a BNT capacitor with Pt top electrode were measured. A low leakage-current density of 4×10⁻⁶ A/cm² at 6 V was measured, and a preliminary discussion of the leakage-current mechanism is also given. It is proposed that amorphous BNT-0.25 thin films will be a potential dielectric material for microwave applications.     

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).     

High-Temperature Capacitor Materials Based on Modified BaTiO<Subscript>3</Subscript>

High-temperature capacitor materials sintered at 1120°C were prepared in a BaTiO₃ (BT)-Na_0.5Bi_0.5TiO₃ (NBT)-Nb₂O₅-ZnO-CaZrO₃ system. The Curie temperature of BaTiO₃ was increased by NBT doping, and a secondary phase occurred when adding ≥5 mol% NBT. The effects of Nb₂O₅, ZnO, and CaZrO₃ on the dielectric properties and the microstructure of BT ceramics doped with 1 mol% NBT were analyzed. The overall dielectric constant decreased when the Nb₂O₅ content increased, and increased when the ZnO content increased. The dielectric constant peak at the Curie temperature was effectively depressed, and a broad secondary dielectric constant peak appeared at 60°C when the ZnO concentration was ≥4.5 mol%. Significant grain growth was observed by scanning electron microscope (SEM) analysis as the amount of ZnO increased. The high-temperature capacitor specification (−55°C to +175°C, ΔC/C _25°C less than ±15%) is met when 7 mol% to 8 mol% CaZrO₃ is added.     

Mn-Substitution Effect on Thermal Conductivity of Delafossite-Type Oxide CuFeO<Subscript>2</Subscript>

We have prepared the CuFe_1−x Mn _x O₂ solid solution to enhance the thermoelectric performance of CuFeO₂ by reducing its thermal conductivity κ. With increasing x above 0.4, the crystal structure changes from delafossite ( R[`3] m ) \left( {R\overline{3} m} \right) to crednerite (C2/m). CuFe_0.5Mn_0.5O₂ exhibits the lowest κ value of 2.28 W/m K at the theoretical density, being about one-quarter of that of the end members, CuFeO₂ and CuMnO₂. We discuss the temperature dependence of κ in terms of a classical phonon transport model, and conclude that local structural modulation due to the mixture of undistorted FeO₆ octahedra and distorted MnO₆ octahedra in CuFe_1−x Mn _x O₂ leads to the significant reduction of κ.     

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.     

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.     

Structure and dielectric properties of Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>ZnTi<Subscript>7</Subscript>O<Subscript>16</Subscript> hollandite ceramics

Hollandite-type Ba_1−xSr_xZnTi₇O₁₆ (x=0, 0.2, 0.4, 0.6, 0.8, and 1) ceramics have been synthesized by the conventional solid-state ceramic route. The phase purity and microstructure of these compositions have been characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis shows that an increase in strontium concentration in the A-site causes pairing of vacant tunnel sites, and hence the structure becomes unstable due to the collapse of the tunnel walls. The dielectric properties such as dielectric constant (ε_r), loss tangent (tan δ), and temperature variation of dielectric constant (τε_r) have been measured up to the 13 MHz region. The present study shows that zinc hollandites have relatively high dielectric constant and low loss tangent. The temperature variation of dielectric constant studies reveal that Ba-rich compositions have high positive τε_r and Sr-rich compositions have high negative τε_r in the 0–100°C region. Proper tailor making of these compositions has been attempted to arrive at near-zero temperature variation of the dielectric constant.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

Thermoelectric Properties of Mn-Doped Ca<Subscript>5</Subscript>Al<Subscript>2</Subscript>Sb<Subscript>6</Subscript>

Ca₅Al₂Sb₆ is a relatively inexpensive Zintl compound exhibiting promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Motivated by our previous studies of Ca₅Al₂Sb₆ doped with Na and Zn, this study focuses on doping with Mn²⁺ at the Al³⁺ site. While Mn is a successful p-type dopant in Ca₅Al₂Sb₆, we find that incomplete dopant activation yields lower hole concentrations than obtained with either previously investigated dopant. High-temperature Hall effect and Seebeck coefficient measurements show a transition from nondegenerate to degenerate semiconducting behavior in Ca₅Al_2−x Mn _x Sb₆ samples (x = 0.05, 0.1, 0.2, 0.3, 0.4) with increasing Mn content. Ultimately, no improvement in zT is achieved via Mn doping, due in part to the limited carrier concentration range achieved.     

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.     

Dielectric Properties of (Bi<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>)ZrO<Subscript>3</Subscript> Modified (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> Ceramics as High-Temperature Ceramic Capacitors

(1???x)K_0.5Na_0.5NbO₃-x(Bi_0.5K_0.5)ZrO₃ [abbreviated as (1???x)KNN-xBKZ, 0?≤?x?≤?0.08] lead-free ceramics have been fabricated by a solid-state processing route. Based on the x-ray diffraction data and temperature-dependent dielectric characteristics, an orthorhombic phase for x?≤?0.03 and single rhombohedral one for x?≥?0.05 at room temperature were determined. The cell volume firstly increases, then decreases and finally increases with increasing BKZ, depending on ionic size and crystallographic structure. For the sample of x?=?0.05, a temperature-stable high permittivity (~?1736?±?15%) along with low dielectric loss tangent (≤?5%) is recorded from 158°C to 407°C. In addition, the activation energies of dielectric relaxation and dc conductivity at high temperatures were characterized by impedance spectroscopy. A combined effect of lattice distortion and oxygen vacancies on the magnitude of activation energies was discussed.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.     

Dielectric Properties in the Microwave Range of K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript> Ceramics

Dielectric properties of a potassium sodium niobate (KNN) system in the microwave range up to GHz have rarely been studied. Since K_0.5Na_0.5NbO₃ is the most common and typical type of KNN materials, non-doped K_0.5Na_0.5 NbO₃ ceramics were synthesized at different temperatures (1080°C, 1090°C, 1100°C, and 1110°C) by a traditional solid reaction method for further characterization and analysis. The ceramics were in perovskite phase with orthorhombic symmetry. A small quantity of second phase was found in the 1110°C sintered specimen, which resulted from the volatilization of alkali oxides as the temperature increased. The complex permittivity was measured for the first time in the microwave range (8.2–12.4 GHz) and in the temperature range from 100°C to 220°C, and the effects of annealing on the dielectric properties were studied. The results indicate that the complex permittivity of KNN ceramics over the microwave range increases mainly due to high bulk density and the additional dielectric contributions of oxygen vacancies at high temperature.     

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.     

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.     

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.     

Enhancement of ferromagnetism in the colossal-magnetoresistance layered manganite La<Subscript>1.2</Subscript>Ba<Subscript>1.8</Subscript>Mn<Subscript>2−x</Subscript>Ru<Subscript>x</Subscript>O<Subscript>7</Subscript> (x = 0, 0.1, 0.5, and 1)

Electrical transport and magnetic measurements of the Ru-doped, layered manganite system, La_1.2Ba_1.8Mn_2−xRu_xO₇ (x=0, 0.1, 0.5, and 1), have been carried out in the temperature range of 5–310 K and in the presence of magnetic fields up to 10 T. The magnetic transition temperatures are found to be above room temperature, and colossal magnetoresistance (CMR) is present at low temperatures and even close to room temperature. Magnetoresistance (MR) is found to obey power-law behavior as a function of applied field with an exponent close to 0.5 at low temperatures.