Thermoelectric properties of Ca3Co4−xMnxO9+δ with x = 0.00, 0.05, 0.10, and 0.15

Ca₃Co_4?xMn_xO_9+δ (x?=?0.00, 0.05, 0.10, and 0.15) sample were prepared by conventional solid state synthesis. The thermoelectric properties were measured at 15 K–300 K. The XRD results revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers were holes over the entire temperature range. Both the electric resistivity and the thermopower increased with increasing Mn content. Ca₃Co_3.95Mn_0.05O_9+δ had the highest power factor of 3.40 μW cm^?1 K^?2 at 163 K, representing an improvement of about 187 % compared to undoped Ca₃Co₄O_9+δ. These results suggested that there is scope for improving the thermoelectric characteristics via partial substitution of Mn for Co.     

Thermoelectric properties of Ca3-xEuxCo3.95Ga0.05O9+δ (0 ≤ x ≤ 0.10)

Polycrystalline samples of Ca_3–xEu_xCo_3.95Ga_0.05O_9+δ (x?=?0.00, 0.02 and 0.10) have been prepared by conventional solid-state synthesis and their thermoelectric properties measured at 25 K to 300 K. The XRD results revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower were simultaneously increased with increasing Eu³⁺ content. The total thermal conductivity decreased with increasing Eu³⁺ content. A maximum dimensionless figure of merit of 0.033 at 300 K was reached for Ca_2.9Eu_0.1Co_3.95Ga_0.05O_9+δ, which is about 27 % higher than that of the undoped sample. These results suggest that the Eu is an effective doping element for improving the thermoelectric properties of Ca₃Co_3.95Ga_0.05O_9+δ.     

Thermoelectric properties of rare earth doped Ca3-xRExCo4O9 (RE = Dy,Er, Gd,and Tb; x = 0, 0.01, 0.03, and 0.05)

Ca_3-xRE_xCo₄O₉ thermoelectric ceramics with different rare earth (RE = Dy, Er, Gd, and Tb) substitution for Ca have been successfully produced by the classical solid state method. All these rare earths in small proportions produce the same effects on the thermoelectric phase, reflected on a similar and significant decrease of the electrical resistivity, compared with the undoped samples. The Seebeck coefficient slightly decreases with all the studied substitutions while the power factor values appreciably increase at high temperatures for small amounts (0.01) of doping elements. Further doping produces the decrease of power factor which indicates that the optimal doping level has been determined to be 0.01 for all the tested rare earths which produces, approximately, the same improvement, around 20 % of the power factor obtained in the undoped samples at 800 °C.     

Structural and electrical properties of Sr(Ti, Fe)O3-δ materials for SOFC cathodes

Doped strontium titanates are very versatile materials. Iron doped SrTiO₃ can be used, for example, as a material for resistive gas sensors and fuel cell electrodes. In this paper, two compositions based on Fe doped SrTiO₃ were studied as possible candidates for cathode application in SOFCs. Namely, SrTi_0.65Fe_0.35O₃ and SrTi_0.50Fe_0.50O₃ were examined. A chemical reactivity between electrode and YSZ electrolyte material was investigated, since Sr containing cathode materials in contact with YSZ electrolyte are prone to form insulating phases. Electrical conductivity of bulk samples showed relatively low total conductivities of 0.4 S cm⁻¹ and ~2 S cm⁻¹ for STF35 and STF50 respectively. Suitability for cathode application was studied by Electrochemical Impedance Spectroscopy in a symmetrical electrode configuration. Area specific resistance (ASR) was determined in the temperature range from 600°C to 800°C. At 790°C samples show polarization ASR of approximately 0.1 Ω cm². It can be expected that further reduction of electrode ASR can be obtained by introduction of ceria barrier layer and tailoring of the electrode microstructure.     

Synthesis and piezoelectric properties of (1 − x)(Na0.5K0.5)NbO3–x(Ba0.95Sr0.05)TiO3 ceramics

(1 ? x)(Na_0.5K_0.5)NbO₃–x(Ba_0.95Sr_0.05)TiO₃ [(1 ? x)NKN–xBST] ceramics were synthesized by the conventional solid-state sintering, and their microstructure and piezoelectric properties were investigated. The sintering temperature of the specimens was 1075 °C in air atmosphere and a morphotropic phase boundary (MPB) was observed in the specimens with 0.03 ≤ x ≤ 0.05. Compared with the piezoelectric properties of the NKN ceramics, the enhanced d ₃₃ value of 136 pC/N and ? ₃ ^T /? _o value of 671 were obtained for the (1 ? x)NKN–xBST specimens with x = 0.03.     

Synthesis, electrical and thermal properties of Bi4V2−xZrxO11 (x = 0.0, 0.02, 0.06 and 0.10) ceramics

Polycrystalline ceramic samples of Bi₄V_2?xZr_xO₁₁ (x?=?0.0, 0.02, 0.06 and 0.10) have been synthesized by standard solid state reaction method using high purity oxides. The formation of the compounds have been checked by X-ray diffraction method. The dielectric constant, dielectric loss and ac conductivity as a function of frequency and temperature have been measured. The dielectric studies indicate that the material is highly lossy and hence its ac conductivity increases with the increase of temperature. The dc conductivity of all the materials has been measured as a function of temperature from room temperature to 673 K and its activation energy was calculated using the relation σ?=?σ_oexp (?Ea/kT). The Modulated Differential Scanning Calorimetry (MDSC) has been used to investigate the effect of substitution on the heat capacity and heat flow of the compounds. The results are discussed in detail.     

Microstructure and microwave dielectric properties of (1 − x)Ca(Mg1/3Nb2/3)O3/xCa0.61Nd0.26TiO3 complex perovskite ceramics

Ceramics in the (1?x)Ca(Mg_1/3Nb_2/3)O₃/xCa_0.61Nd_0.26TiO₃ system were prepared and characterized. Single phase solid solutions were obtained up to x?=?0.4, and the crystal structure belonged to monoclinic space group P21(4) for x?=?0.1 and orthorhombic space group Pbnm(62) for x above 0.1. The secondary phase of Ca₂(Ti,Nb)₂O₆ was observed for x?=?0.6 and 0.8. Good combination of microwave dielectric properties was achieved in the present ceramics with x?=?0.4: ? _r?=?46.5, Qf?=?14,136 GHz, τ _f?=??12.5 ppm/°C.     

The microstructures and dielectric properties of xSrZrO3–(1−x)SrTiO3 ceramics

In this paper, the microstructures and the dielectric properties of xSrZrO₃–(1?x)SrTiO₃ (x?=?0.01, 0.03, 0.05, and 0.07) ceramics were investigated. The composition can form a single-phase structure solid solution as x increases from 0.01 to 0.07, and the average grain size decreases from 15 to 2 μm with the x increasing. With the variation of SrZrO₃ content, dielectric constant, dielectric loss, and breakdown strength (BDS) of xSrZrO₃–(1?x)SrTiO₃ samples are changed. When x?=?0.05, the samples exhibit high BDS more than 14 kV/mm and low dielectric loss about 1?×?10^?3 with dielectric constant of 330. It can be a candidate material of energy storage for pulsed power application.     

Tunable dielectric characteristics of (Ba0.95Ca0.05)(Ti1−y Sn y )O3 ferroelectric ceramics

In order to explore potential candidates for field tunable dielectric materials, the present work was focused on investigating the microstructure, dielectric and tunable dielectric properties of A- and B-site substituted barium titanate ceramics. Large tunability with lower dielectric loss was obtained in the present ceramics at a quite weak bias field. The excellent tunable characteristics were achieved for y?=?0.15 at room temperature (25 °C): tunability?=?26%, tanδ?<?0.01 at 10 kHz under a very weak bias DC electric field of 2.1 kV/cm.     

Sintering behavior, structures and microwave dielectric properties of a rutile solid solution system: (AxNb2x)Ti1–3xO2 (A=Cu, Ni)

The sintering behavior, structures and microwave dielectric properties in a rutile solid solution system—(A_xNb_2x)Ti_1–3xO₂ (A=Cu, Ni)—were investigated and the samples were prepared by conventional solid state reaction method. Single phase of tetragonal rutile structure has been obtained through the entire range of compositions (0.02 ≤ x ≤ 0.20). The sintering temperature was lowered to 900°C by (Cu _{x /3}Nb_2x/3)⁴⁺ substituting for Ti⁴⁺ in the solid solution. Comparing with that of rutile TiO₂ (465 ppm/°C), the temperature coefficient of resonant frequency (TCF) of the rutile solid solution is much lower (about 250 ppm/°C), and the dielectric constant and the quality factor (Qf value) of the solid solution are about 70~80 and 7,000G Hz. The substitution of (Cu _{x /3}Nb_2x/3)⁴⁺or (Ni _{x /3}Nb_2x/3)⁴⁺ for Ti⁴⁺ in the solid solution improved the microwave dielectric properties of the rutile TiO₂ ceramics.     

Low-temperature sintering and dielectric properties of the Bi(Nb1−x Ta x )O4 system

Low-temperature sintering and dielectric properties of the Bi(Nb_1?x Ta _x )O₄ (x?=?0.1, 0.3, and 0.5) system was investigated as a function of the zinc borosilicate (ZBS) glass content with a view to applying this system to LTCC technology. The addition of 7 wt% ZBS glass ensured a successful sintering below 900°C. The complete solid solution of Bi(Nb, Ta)O₄ with an orthorhombic structure was formed and the high temperature form of Bi(Nb, Ta)O₄ with a triclinic structure was not observed. The second phase of Bi₂SiO₅ was observed for all compositions. The non-relative liquid phase sintering (NLPS) occurred and the one-stage sintering was conducted. The Q?×?f values were improved by the addition of Ta. Bi(Nb_0.7Ta_0.3)O₄ with 7 wt% ZBS glass sintered at 900°C demonstrated 35.8 in the dielectric constant (? _r), 2,200 GHz in the quality factor (Q?×?f ₀), and ?48 ppm/°C in the temperature coefficient of resonant frequency (τ _f).     

Structural, dielectric and electrical studies of MgAl2−2xY2xO4 (x = 0.00–0.05) cubic spinel nano aluminate

Investigations were carried out on a series of MgAl_2-2xY_2xO₄ (x?=?0.00–0.05) nanoparticles prepared in steps of 0.01 by chemical co-precipitation method to study the effect of yttrium substitution at aluminum site on the structural, dielectirc and electrical properties. The single phase cubic spinel structure of all the samples was confirmed by X-ray diffraction (XRD). The Fourier transform infrared spectroscopy (FTIR) study shows two strong absorption bands in the frequency range 400–800 cm^?1, on the tetrahedral and octahedral sites respectively. Elemental analysis by Energy dispersive X-ray fluorescence (EDXRF) shows that samples are stoichiometric. The scanning electron microscopy (SEM) study reveals surface morphology of nanoparticles. Transmission electron microscopy (TEM) study shows the individual nanoparticles size and validates the nanocrystalline nature of the samples. The variation of dielectric permittivity at room temperature as a function of frequency (1 KHz to 1 MHz) suggests the dielectric dispersion due to Maxwell-Wagner Interfacial Polarization. AC conductivity study reveals that the conduction is due to small polaron hopping. The electrical modulus analysis shows that nanocrystalline MgAl_2?2xY_2xO₄ system exhibits non Debye type relaxation. The dc resistivity was found to increase with increase in yttrium content.     

Electrical Properties of 6H-BaTi0.95M0.05O3−δ Ceramics where M = Mn, Fe, Co and Ni

Hexagonal BaTiO₃ materials have been stabilised at room temperature according to the formula BaTi_0.95M_0.05 O_3– where M = Mn, Fe, Co and Ni. Dense ceramics (> 96% of the theoretical X-ray density) were sintered at 1450C in flowing O₂ gas from calcined powders prepared by the mixed oxide route at 1300C. All samples were single-phase and the bulk conductivity, _b, measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. _b at 300C was 10^–7, 10^–5.5, 10^–5.5 and 10^–4 Scm^–1 for M = Mn, Fe, Ni and Co, respectively and Q.f at 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between _b and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions.     

Dielectric and pyroelectric properties of (1 − x)PST-xPZT ferroelectric ceramics

Pb(Sc_1/2Ta_1/2)O₃ (PST) ceramics were investigated greatly in the world for their unique pyroelectric, ferroelectric and dielectric properties and comprehensive applications on uncooled focal plane arrays, infrared detectors and other electronic devices. However, the PST ceramics doped with other perovskite ferroelectrics showed more excellent electrical and electronic properties. In this paper, (1???x)PST-xPZT(PSTZT) ferroelectric ceramics were prepared by conventional solid state process. The experiment results demonstrated that the PSTZT ceramics had pure perovskite phase. The temperature dependence of permittivity of PSTZT ceramics was investigated in detail, which indicated that PSTZT was not a complete diffusive phase transition ferroelectric ceramics. At room temperature, the pyroelectric coefficient of PSTZT (x?=?0.1) ceramics was about 15*10^?8C/(cm² K).     

Effect of Temperature on the Elastic and Anelastic Behaviour of Magneto-Ferroelectric Composites Ba0.8Pb0.2TiO3 + Ni0.93Co0.02Mn0.05Fe1.95O4-δ in the Ferroelectric Rich Region

Composites with composition xBa_0.8Pb_0.2TiO₃+ (1 –x) Ni_0.93Co_0.02Mn_0.05Fe_1.95O_4- in which x varies as 1.0, 0.9, 0.7 and 0.5 in molar percent have been prepared by the conventional ceramic double sintering process. The presence of the two phases has been confirmed by X-ray diffraction. These composites were prepared for their use as magnetoferrolectric devices. Variation of longitudinal modulus (L) and internal friction loss (Q ^–1) of these samples with temperature at 142 kHz has been studied in the wide temperature range 300 to 630 K. The elastic behaviour (L) showed a break at the ferroelectric Curie temperature (498 K) in the case of pure ferroelectric material (Ba_0.8Pb_0.2TiO₃). This break shifted to lower temperature side as the ferrite component increases in the composite. The temperature variation of internal friction loss (Q ^–1) showed a corresponding stress induced relaxation peak at the ferroelectric-non-ferroelectric phase transition. This behaviour is explained in the light of structural phase transition.     

Structure and microwave dielectric properties of double vanadate Ca9A(VO4)7 (A= La,Pr, Nd and Sm) ceramics for LTCC applications

Journal of Electroceramics - Ca9A(VO4)7 (A?=?La, Pr, Nd and Sm) ceramics have been prepared by conventional solid state ceramic route. The phase purity of the samples was confirmed by...     

Effects of partial substitution of Fe for Co on the low-temperature thermoelectric and magnetic properties of Ca2.7Bi0.3Co4-xFexO9+δ (0 ≤ x ≤ 0.15)

Polycrystalline samples of Ca_2.7Bi_0.3Co_4-xFe_xO_9+δ (x?=?0.00, 0.025, 0.05, 0.10 and 0.15) have been prepared by conventional solid-state synthesis and their thermoelectric and magnetic properties measured. The X-ray diffraction patterns revealed that all the samples are single phase. The electrical resistivity results indicated that all the samples obey the variable range hopping in the low temperature regime. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower were increased with increasing Fe content. Among the samples, Ca_2.95Bi_0.10Co_3.95Fe_0.05O_9+δ had the highest dimensionless figure of merit of 0.102 at 300 K. Magnetic measurements indicated that all the samples exhibit a low-spin state of cobalt ion. The effective magnetic moments were decreased with increasing Fe content.     

Microstructural Design and Properties of LaCoO3/La2(Zr,Y)2O7−δ Composites

LaCoO_3–/La₂(Zr,Y)₂O₇-based composites were designed in view of three main properties: electrical conductivity, thermal expansion and resistance to thermal cycling. Composition and processing conditions were investigated on the basis of an experimental plan according to the Taguchi method. The phase distribution was estimated from image analysis and related to electrical and thermo-mechanical behavior. Results indicate that an homogeneous and fine grained phase distribution is essential in order to obtain materials with the desired thermal expansion, electrical properties and thermomechanical properties.