Characteristics of Cu and Mo-doped Ca3Co4O9−δ cathode materials for use in solid oxide fuel cells

In this study, Cu and Mo ions were doped in Ca₃Co₄O_9−δ to improve the electrical conductivity and electrochemical behavior of Ca₃Co₄O_9−δ ceramic and the performance of a solid oxide fuel cell (SOFC) single cell based on NiO-SDC/SDC/doped Ca₃Co₄O_9−δ-SDC were examined. Cu substitution in the monoclinic Ca₃Co₄O_9−δ ceramic effectively enhanced the densification, slightly increased the grain size, and triggered the formation of some Ca₃Co₂O₆; however, no second phase was found in porous Mo-doped Ca₃Co₄O_9−δ ceramics even when the sintering temperature reached 1050 °C. Substitution of Cu ions caused slight increase in the Co³⁺ and Co⁴⁺ contents and decrease in the Co²⁺ content; however, doping with Mo ions showed the opposite trend. Doping the Ca₃Co₄O_9−δ ceramic with a small amount of Cu or Mo increased its electrical conductivity. The maximum electrical conductivity measured was 218.8 S cm⁻¹ for the Ca₃Co_3.9Cu_0.1O_9−δ ceramic at 800 °C. The Ca₃Co_3.9Cu_0.1O_9−δ ceramic with a coefficient of thermal expansion coefficient of 12.1×10⁻⁶ K⁻¹ was chosen as the cathode to build SOFC single cells consisting of a 20 μm SDC electrolyte layer. Without optimizing the microstructure of the cathode or hermetically sealing the cell against the gas, a power density of 0.367 Wcm⁻² at 750 °C was achieved, demonstrating that Cu-doped Ca₃Co₄O_9−δ can be used as a potential cathode material for IT-SOFCs.     

Porous Ca3Co4O9 with enhanced thermoelectric properties derived from Sol–Gel synthesis

Highly porous Ca₃Co₄O₉ thermoelectric oxide ceramics for high-temperature application were fabricated by sol–gel synthesis and subsequent conventional sintering. Growth mechanism of misfit-layered Ca₃Co₄O₉ phase, from sol–gel synthesis educts and upcoming intermediates, was characterized by in-situ X-ray diffraction, scanning electron microscopy and transmission electron microscopy investigations. The Ca₃Co₄O₉ ceramic exhibits a relative density of 67.7%. Thermoelectric properties were measured from 373 K to 1073 K. At 1073 K a power factor of 2.46 μW cm⁻¹ K⁻², a very low heat conductivity of 0.63 W m⁻¹ K⁻¹ and entropy conductivity of 0.61 mW m⁻¹ K⁻² were achieved. The maintained figure of merit ZT of 0.4 from sol–gel synthesized Ca₃Co₄O₉ is the highest obtained from conventional, non-doped Ca₃Co₄O₉. The high porosity and consequently reduced thermal conductivity leads to a high ZT value.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Synthesis of single-phase Ca3Co4O9 ceramics and their processing for a microstructure-enhanced thermoelectric performance

Single-phase Ca₃Co₄O₉ with a high porosity, having a ZT of 0.09 at 627 °C, was successfully prepared by a simple solid-state reaction using fine powders of CaCO₃ and Co₃O₄ after a calcination at 760 °C for 12 hours. It was excellent either for the preparation of the starting powder for the processing of the Ca₃Co₄O₉ ceramics or for direct processing. The influence of already-reported processing methods (classic sintering, hot pressing, free-edge spark-plasma sintering (SPS) of a sintered pellet) and new methods, such as free-edge SPS of a pellet from just-compacted powder, cold pressing of a sintered pellet without post annealing, and free-edge cold pressing of a sintered pellet with post-annealing, on the preparation of Ca₃Co₄O₉ ceramics was studied. The results showed how the density, the grain morphology and the microstructural anisotropy can all influence the thermoelectric characteristics of Ca₃Co₄O₉ ceramics measured in directions parallel and perpendicular to the applied pressure. While the fully dense (99%) and perfectly textured ceramics prepared by the free-edge SPS of a pellet from just-compacted powder had a ZT of 0.17, the highest ZT of 0.31 was obtained for the free-edge cold-pressed and annealed ceramics with modest texturing and low density (65%), having a very low κ of 0.47 W/mK. The results also showed that thin and irregular-shaped plate-like grains with sharp edges are preferred, while their thickening accompanied by rounding of their shape resulted in a reduced thermoelectric performance. The study revealed both the possibilities and the limitations for enhancing the TE characteristics of Ca₃Co₄O₉ ceramics just through microstructure optimisation.     

Sol-gel synthesis,microstructure, and thermoelectric properties of Ca2.8-xBixDy0.2Co4O9+δ

Small-sized Ca_2.8-xBi_xDy_0.2Co₄O_9+δ (0 ≤ x ≤ 0.1) powders with a plate-like morphology were synthesized via the citric acid-assisted sol-gel method. The structural and thermoelectric properties of Ca_2.8-xBi_xDy_0.2Co₄O₉ samples were studied with an emphasis placed on the Bi content and the fabrication process. The as-sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples exhibited a single Ca₃Co₄O_9+δ phase and a plate-like morphology. With increased Bi content, the grain size of the sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples decreased, whereas the density of the sintered Ca_2.8-xBi_xDy_0.2Co₄O₉ samples increased. The incorporation of Bi up to x = 0.075 yielded high electrical conductivity. Meanwhile, the Seebeck coefficient decreased with increases in Bi content. The largest power factor (2.18 × 10⁻⁴ W m⁻¹ K⁻² at 800 °C) was obtained for the twice-sintered Ca_2.725Bi_0.075Dy_0.2Co₄O₉. The partial substitution of Bi for Ca and the twice sintering were a highly effective route for improving the thermoelectric properties of Ca_2.8Dy_0.2Co₄O₉.     

Low temperature sintering of iron deficient Z type hexagonal ferrites

Low temperature sintering of iron deficient hexagonal Co₂Z type ferrites is attempted to prepare a chip inductor for ultra high frequency uses. It should be attained below the melting point of silver (960 °C) used as the electrode material of the chip inductor. The best result is obtained for the sample with the composition 2.15Co_0.6Cu_0.4O·3Ba_0.79Sr_0.16Bi_0.05O·9.725Fe₂O₃ sintered at 925 °C containing 0.5 wt.% of lithium bismuth oxide as a sintering additive. Its density and permeability reaches 5.0 g/cm³ and 7, respectively. As a result, the impedance of the chip inductor also reaches 390 ohm at 2 GHz using this material.     

The sintering behavior,microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets

The properties of sputtering targets have recently been found to affect the performances of sputtered films and the sputtering process. To develop high-quality GZO ceramic targets, the influences of Ga₂O₃ content and sintering temperature on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets were studied.The results showed that the increase in Ga₂O₃ content from 3 wt% (GZO-3Ga) and 5 wt% (GZO-5Ga) not only inhibited the densification but retarded grain growth. During sintering, ZnGa₂O₄ phase formed before 800 °C, and Zn₉Ga₂O₁₂ phase was found after sintering at 1000 °C. Moreover, after sintering at 1200 °C, the number of Zn₉Ga₂O₁₂ precipitates increased at the expense of ZnGa₂O₄ and ZnGa₂O₄ disappearing completely. The relative density, grain size, and resistivity of GZO-3Ga sintered at 1400 °C in air were 99.3%, 3.3 μm, and 2.8 × 10⁻³ Ω cm, respectively. These properties of GZO ceramics are comparable to properties reported in the literature for AZO sintered in air.     

Sintering behavior,microstructure and thermoelectric properties of calcium cobaltite thick films for transversal thermoelectric multilayer generators

The sintering behavior and the thermoelectric performance of Ca₃Co₄O₉ multilayer laminates were studied, and a multilayer thermoelectric generator was fabricated. Compacts and multilayer samples with anisotropic microstructure and residual porosity were obtained after conventional sintering at 920 °C, whereas dense and isotropic multilayer samples were prepared by firing at 1200 °C and reoxidation at 900 °C. A hot-pressed sample has a dense and anisotropic microstructure. Samples sintered at 920 °C exhibit low electrical conductivity due to the low density, whereas the Seebeck coefficient is not sensitive to preparation conditions. However, thermal conductivity of multilayers is very low, and, hence acceptable ZT values are obtained. A transversal multilayer thermoelectric generator (TMLTEG) was fabricated by stacking layers of Ca₃Co₄O₉ green tapes, AgPd conductor printing, and co-firing at 920 °C. The TMLTEG has a power output of 3 mW at ΔT = 200 K in the temperature interval of 25 °C to 300 °C.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

Effects of MgO addition on the microwave dielectric properties of high thermal-conductive silicon nitride ceramics sintered with ytterbia as sintering additives

1 mol% of MgO was added together with 7 mol% of Yb₂O₃ as sintering additives to silicon nitride powder to fabricate advanced silicon nitride ceramics with both high thermal conductivity and low dielectric loss at 2 GHz. The mixed powder was CIPed at a pressure of 120 MPa and was gas-pressure sintered at 1900 °C to >98% of theoretical density. The sintered Si₃N₄ sample exhibited a high thermal conductivity of ～100 W m^?1 K^?1 and a loss tangent (tan δ) of ～4 × 10^?4, concurrently. The tan δ was further reduced by half after the heat treatment at 1300 °C for 24 h. The improvement in tan δ due to the annealing was explained from the point of crystallization of the intergranular glassy phase.     

Preparation of high-performance Ca3Co4O9 thermoelectric ceramics produced by a new two-step method

High-performance Ca₃Co₄O₉ thermoelectric ceramic has been prepared from a Ca_1?xCo_xO/Ca_yCo_1?yO divorced eutectic structure produced by a directional melt-grown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca₃Co₄O₉ thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca₃Co₄O₉ formation with time and related with their thermoelectric performances. The optimum annealing time, 72 h, has been determined by the maximum power factor value (about 0.42 mW K^?2m^?1), which is around the best values reported in textured materials (～0.40 mW K^?2m^?1). This high power factor outcome from the high Ca₃Co₄O₉ phase content, apparent density and Co³⁺/Co⁴⁺ relationship determinations performed in the present work.     

Hybrid microwave processing of Ca3Co4O9 thermoelectrics

In this study, hybrid microwave sintering of Ca₃Co₄O₉ thermoelectric materials was implemented for the first time. Thermoelectric properties of samples sintered in different conditions and in conventional electrical furnace, using the same temperatures and dwell times, were assessed and compared. Microwave processing was found to promote densification and grain texturing in Ca₃Co₄O₉ bulk ceramics, leading to a significant increase of the electrical conductivity. Seebeck coefficient and thermal conductivity were essentially unaffected by the microstructural changes. Prolonged exposure to microwave radiation at 800 °C led to partial phase decomposition and consequent formation of Ca₃Co₂O₆ and Co₃O₄ impurities, with minor effect on the charge transport. Still, the significant presence of residual porosity suggests the need for further optimization of powder and microwave processing conditions.     

The low-temperature sintering of M-type hexaferrites

The sintering of the M-type hexaferrites, AFe₁₂O₁₉ (A = Ba, Sr), was studied. The effects of the A cation type, the partial CuO substitution for Fe₂O₃ and the Fe deficiency on the sintering were examined. Samples with nominal compositions of Ba_1?zSr_zCu_xFe_y?xO_19?δ (z = 0 or 0.5, x = 0–1.0 and y = 11–12) were prepared with a solid-state reaction at 1000 °C. The CuO addition enhanced significantly the sintering rate of the hexaferrites at temperatures below 1100 °C, while a moderate effect on the sintering was caused by a partial substitution of Sr for Ba. The diffusion studies showed that the CuO addition enabled the reactive liquid-phase sintering of hexaferrites at 1000 °C. The relative sintered densities of the Cu-containing hexaferrite ceramics sintered at 1000 °C for 3 h were around 90%. The addition of CuO had a strong effect on the coercivity, but only a moderate effect on the magnetization of the studied ceramics.     

Effects of Bi2O3–Li2CO3 additions on dielectric and pyroelectric properties of Mn doped Pb(Zr0.9Ti0.1)O3 thick films

Pb(Zr_0.9Ti_0.1)O₃ pyroelectric thick films adding various amounts of the sintering aids Bi₂O₃–Li₂CO₃ have been deposited on the substrates Al₂O₃ by the screen-printing process, and the dependence of microstructure, dielectric and pyroelectric properties on the content of sintering aids has been studied. When the amount of Bi₂O₃–Li₂CO₃ increases from 0 wt% to 5.4 wt%, the sintering temperature of the thick films decreases from 1100 °C to 900 °C, and the grain size and the lattice constant decrease either, but the density and the dielectric constant increase. The Pb(Zr_0.9Ti_0.1)O₃ thick film with 5.4 wt% of Bi₂O₃–Li₂CO₃ sintered at 900 °C has the maximum pyroelectric coefficient 10.51×10^?8 C/cm^?2 K^?1 and the highest figure-of-merit 10.58×10^?5 Pa^?0.5.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Spark plasma sintering of silicon nitride/barium aluminum silicate composite

Si₃N₄ based composites were successfully sintered by spark plasma sintering using low cost BaCO₃, SiO₂ and Al₂O₃ as additives. Powder mixtures were sintered at 1600–1800 °C for 5 and 10 min. Displacement-temperature-time (DTT) diagrams were used to evaluate the sintering behavior. Shrinkage curve revealed that densification was performed between 1100 and 1700 °C. The specimen sintered at 1700 °C showed the maximum relative density (99.8±0.1%), flexural strength (352±16 MPa), Vickers harness (11±0.1 GPa) and toughness (5.6±0.05 MPa m^1/2).     

Synthesis and characterization of Ca3-xLaxCo4-yCuyO9+δ cathodes for intermediate temperature solid oxide fuel cells

The calcium cobaltite Ca_3-xLa_xCo_4-yCu_yO_9+δ with x and y = 0 and 0.1 were synthesized and the electrical, thermal, and catalytic behaviors for the oxygen reduction reaction (ORR) for use as air electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) were evaluated. X?ray diffraction confirms the Ca_3-xLa_xCo_4-yCu_yO_9+δ samples were crystallized in a monoclinic structure and scanning electron microscopic image shows lamella-like grain formation. Introduction of dopants decreases slightly the loss of lattice oxygen and thermal expansion co-efficient. The Ca_3-xLa_xCo_4-yCu_yO_9+δ samples exhibit good phase stability for long-term operation, thermal expansion, and chemical compatibility with the Ce_0.8Gd_0.2O_2-δ electrolyte. Among the studied samples, Ca_2.9La_0.1Co₄O_9+δ shows a maximum conductivity of 176 Scm^?1 at 800 °C. Although the doped samples exhibit a higher total electrical conductivity, an improved symmetrical cell performance is displayed by the undoped sample. Comparing the sintering temperatures, the composite cathode Ca₃Co₄O_9+δ + Ce_0.8Gd_0.2O_2-δ sintered at 800 °C exhibit the lowest area specific resistance of 0.154 Ω cm² at 800 °C in air. In the Ca_3-xLa_xCo_4-yCu_yO_9+δ + GDC composite cathodes, the charge-transfer process at high frequencies presents a major rate limiting step for the oxygen reduction reaction.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Single step reactive sintering and chemical compatibility between La9Sr1Si6O26.5 and selected cathode materials

Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells (SOFC). However more studies on the chemical compatibility of these materials with common SOFC electrodes are required. Here, we report the synthesis of single phase La₉Sr₁Si₆O_26.5 composition by reactive sintering at 1650 °C for 10 h. Fully dense pellets showed very high oxide-anion conductivity, 25 mS cm^?1 at 700 °C. Furthermore, the chemical compatibility of La₉Sr₁Si₆O_26.5 with some selected cathode materials has also been investigated. The lowest reaction temperatures were determined to be 1100 °C, 1000 °C and 900 °C for La_0.8Sr_0.2MnO_3?δ, La₂Ni_0.8Cu_0.2O₄ and La_0.6Sr_0.4Co_0.8Fe_0.2O₃, respectively. The segregation of minor amounts of SiO₂ seems to be a key limiting factor that must be overcome. Finally, these cathode materials were deposited over dense oxy-apatite pellets and the area specific resistances in symmetrical cells were determined. These values, at 700 °C, were 14.4 and 2.6 Ω cm² for La_0.8Sr_0.2MnO_3?δ and La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ, respectively. Furthermore, the area specific resistances are notably improved 0.6 Ω cm² when a 50 wt.% composite of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ and Ce_0.8Gd_0.2O_1.9 is used.