Sinterability,Mechanical, Microstructural,and Electrical Properties of Gadolinium-Doped Ceria Electrolyte for Low-Temperature Solid Oxide Fuel Cells

In this study, we report key functional properties of gadolinium-doped ceria (Gd_0.1Ce_0.9O_1.95, GDC) sintered at low temperatures as well as single-cell electrochemical performance of a single-cell prepared there of. GDC solid solutions were sintered at various temperatures ranging 1100–1400^∘C and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), density measurements, mechanical strength tests and electrical conductivity measurements. The dry-pressed GDC disc sample sintered at 1100^∘C was found to have 96% of the theoretical density and higher sintering temperatures led to higher densities. SEM micrographs of the fracture and plan surfaces of the sintered discs established the absence of any open pores. The sample sintered at 1100^∘C exhibited high electrical conductivity of 0.027 S/cm at 650^∘C. The mechanical strength of the sintered samples was determined to be in the range of 150–175 MPa. Greater than 96% of theoretical density, good mechanical strength, and high electrical conductivity of GDC disc samples sintered at 1100^∘C established the viability of low-temperature processing of GDC for its use as an SOFC electrolyte. Accordingly, a single-cell was prepared by co-sintering of GDC electrolyte and LSCF-GDC cathode at 1100^∘C and subsequent firing of CuO-GDC anode at 900^∘C. The electrochemical performance of the cell was evaluated in H₂ fuel at 650^∘C.     

Effect of Cr additions on the microstructural stability of Ni electrodes in ultra-thin BaTiO3 multilayer capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCC) is one of the present day challenges to maintain an increase in capacitive volumetric efficiency. This present paper opens a series of investigations aimed to engineer the stability of ultra-thin Ni electrodes in BaTiO₃-based multilayer capacitors using refractory metal additions to Ni. Here, pure Ni and Ni–1 wt.% Cr alloy powders are used to produce 0805-type BME MLCCs with 300 active layers and with dielectric and electrode layer thickness around 1 μm. To investigate the continuity of Ni electrodes, both MLCC chips with pure and doped electrodes were sintered at different temperatures for 5 h. It is found that the continuity of Ni electrodes is improved most likely due to the effect of Cr on the low-melting point (Ni,Ba,Ti) interfacial alloy layer formation. The interfacial alloy layer is not observed when Cr is segregated at Ni-BaTiO₃ interface in the Cr-doped samples, while it is found in all undoped samples. The interfacial alloy layer is believed to increase mass-transfer along the Ni-BaTiO₃ interfaces facilitating an acceleration of Ni electrodes discontinuities.     

Electrical properties of conventional and microwave sintered lead free magnetoelectric composites

Abstract

Here we report comparison of dielectric properties of composition synthesized by microwave and conventional sintering. Microwave sintering requires less time and temperature to achieve the same quality of materials as sintered by conventional route. The material sample was prepared by conventional solid state method and sintered in conventional & microwave furnace. Sintered samples were then subjected to XRD analysis. X-ray diffraction revealed the formation of single phase material. The dielectric and ferroelectric properties were recorded for both the samples and properties were found to improve in microwave sintered samples. There is also a significant improvement in density by microwave processing.     

Base metal Co-fired (Na,K)NbO3 structures with enhanced piezoelectric performance

A NaF-Nb₂O₅ flux doped (Na,K)NbO₃ (NKN) based lead-free ceramic was successfully co-fired with nickel inner electrodes in reduced atmospheres. No chemical reactions and/or inter-diffusion were detected at the interface between the nickel (Ni) electrodes and the NKN-based piezoelectrics. Dielectric, resistivity, and electromechanical performance were measured with processing under different firing conditions and flux additions to obtain high densities. Ceramics are obtained with submicron grain structures with the NaF-Nb₂O₅ sintering aids (2 and 4 wt%) fluxes, and high densities when firing at low pO₂ (10^?10 atms) atmospheres at sintering temperatures ~1150 °C for 2 hours. High resistivities and low losses can be obtained through a second annealing condition at 850 °C and 10^?7 atms at 8 hours. High d ₃₃ values (over 350 pm/V) determined under unipolar converse electromechanical measurements were obtained in the simple prototyped co-fired structures to show feasibility towards base metal electrodes in multilayer actuators.     

Characterization of Barium Titanate: BaTiO<Subscript>3</Subscript> (BT) Ceramics Prepared from Sol-Gel Derived BT Powders

Sol-gel synthesis was attempted at room temperature by adding drop wise a clear solution (made by reacting BaCO₃ with glacial acetic acid) to an equi-molar solution of titanium tetrabutoxide in isopropanol (IPA) and acetic acid. The gelation occurred within 30 minutes. The as-dried gel was calcined at 750^∘C/6 h in air to obtain carbon free BaTiO₃ powders. The resulting BT powders were further pressed into pellets and sintered at 1280^∘C/4 h in air to get dense ceramics (density ∼ 94%). XRD and SEM techniques were used for phase and microstructure analysis respectively. The room temperature relative permittivity (ε) of 1280 increased to 7200 (at Curie temperature of 127^∘C) at 1 kHz frequency. The dissipation factor (tanδ) ∼1.6% was observed in these samples. Also, the ferroelectric properties such as spontaneous polarization (Ps ∼ 13 μC/cm²), coercive field (E_c∼4.0 kV/cm) and dielectric strength (∼34 kV/cm) are estimated for these samples. These performance parameters are compared with the available standard data from the literature.     

The Effect of Cobalt Oxide Addition on the Conductivity of Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>1.95</Subscript>

The conductivity of cobalt oxide doped Ce_0.9Gd_0.1O_1.95 (CGO10) of various doping concentrations, sintering temperatures, dwell times, and cooling rates was investigated by 4-point DC conductivity measurements. In cobalt oxide doped CGO10, an enhanced total conductivity occuring with a low activation energy of 0.54 eV was detected below 250^∘C in quenched samples. If the same samples were cooled down slowly, only the ionic conductivity of undoped CGO with an activation energy of 0.8 eV was found. The increased conductivity is attributed to a percolating network of an electronically conducting grain boundary phase rich in CoO, which can be retained by quenching from temperatures between 900 and 1000^∘C.     

The effect of sintering atmosphere on V2O5 substituted BiNbO4 microwave ceramics

In multiplayer passive devices, low sintering temperature dielectric materials were needed to co-fire with low melting point inner electrode such as copper or silver, a major problem of base metal electrode (BME) was that the devices must be fired under low oxygen partial pressure atmosphere to protect Cu from oxidation. In this paper, dielectric properties of Bi(V_xNb_1?x)O₄ (x?=?0.001, 0.004, 0.008, 0.016, 0.048) microwave ceramics sintered under air and N₂ atmosphere have been investigated. The densification temperature sintered in different atmosphere decreased from 1010 to 830°C with the amount of V₂O₅ increasing from 0.001 to 0.048. Due to the increasing vacancy defects, the density of ceramics sintered in N₂ was smaller than that sintered in air. The ceramics sintered under N₂ have similar dielectric constant, and its Qf values are higher while x?<?0.016.     

Phase compositions and microwave dielectric properties of MgTiO<Subscript>3</Subscript>-based ceramics obtained by reaction-sintering method

MgTiO₃-based microwave dielectric ceramics were prepared successfully by reaction sintering method. The X-ray diffraction patterns of the sintered samples revealed a major phase of MgTiO₃-based and CaTiO₃ phases, accompanied with Mg₂TiO₄ or MgTi₂O₅ determined by the sintering temperature and time. The microwave dielectric properties had a strong dependence of sintering condition due to the different phase compositions and the microstructure characteristics. The ceramics sintered at 1360 °C for 4 h exhibited good microwave dielectric properties: a dielectric constant of 20.3, a high quality factor of 48,723 GHz (at 9GHz), and a temperature coefficient of resonant frequency of ?1.8 ppm/^oC. The obtained results demonstrated that the reaction-sintering process is a simple and effective method to prepare the MgTiO₃-based ceramics for microwave applications.     

Fabrication of microinductor using Nanocrystalline NiCuZn ferrites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe₂O₄ was prepared using microwave hydrothermal (M-H) method at a low temperature of 160 °C/30 min. As synthesized powders were characterized using XRD and TEM. The powders were sintered using microwave sintering methods at different sintering temperatures i.e. 750 °C/30 min, 800 °C/30 min, 850 °C/30 min 900 °C/30 min and 950 °C/30 min respectively. The sintered samples were characterized using XRD and SEM. The complex permittivity (ε*) and permeability (μ*) have been measured in the frequency range of 100 kHz to 1.8 GHz. The micro inductors were fabricated with the help of microwave sintered sample by using screen-printing method and co-firing. The electrical properties such as inductance and Quality factor of the prepared inductors were measured over a wide frequency range.     

Magnetic properties of Ni-Zn ferrites prepared by microwave sintering method

The effect of zinc ion substitution for nickel on structural and magnetic properties of NiZn ferrites is reported. The spinel ferrite system Ni_1-xZn_xFe₂O₄ with x = 0.2, 0.3, 0.4 and 0.5 was prepared by microwave sintering method. The uniaxially pressed samples were sintered at various temperatures such as 900°C, 1000°C and 1100°C for 30 min. X-ray diffraction patterns of the samples indicate the formation of single-phase cubic spinel structure. SEM micrographs show that grain size increases with increasing zinc content and sintering temperature. The elemental composition of these ferrites was analyzed by EDS. Lattice constant increases with increase in zinc content, obeying Vegard’s law. The effect of composition and sintering temperature on initial permeability as the function of frequency and temperature was studied. The initial permeability of NiZn ferrite increases greatly with increasing Zn content and sintering temperature. The dependence of initial permeability with respect to temperature shows the decrease in the Curie point with increase in zinc content, is the normal behavior of ferrites. The relative loss factor (tand

Development of Translucent Aluminum Nitride (AIN) Using Microwave Sintering Process

Pure aluminum nitride (AIN) has been successfully sintered to highly translucent form by microwave sintering at 1850°C with a dwelling time of 30–60 minutes. The results showed that the sintering temperature should be at least 1850°C or higher to get reasonable translucency in the AIN sample by the microwave sintering process. On the other hand, the conventional sintering method requires much longer sintering time to obtain a translucent AIN ceramics.     

Piezoelectric Properties of 0.2[Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)]-0.8[PbTiO<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript>] Ceramics Sintered at a Low Temperature with the Aid of Li<Subscript>2</Subscript>O

The dielectric and piezoelectric properties of 0.2Pb(Mg_1/3Nb_2/3)O₃-0.8Pb(Zr_0.475Ti_0.525)O₃ (abbr. as PMNZT) ceramics were measured. Extremely low sintering temperatures of 950^∘C using liquid-phase sintering aid of Li₂O is achieved which was very useful for multi-layered applications. X-ray study shows the splitting of rhombohedral (200) in pure PMNZT to (002) and (200) peaks in Li₂O doped samples. 10 times higher dielectric constant was achieved in Li₂O doped samples to compare to pure ones although the Curie temperature (T_c = 322^∘C) of Li₂O doped PMNZT ceramics was not changed. The value of k_p and k₃₃ increased up to 0.1 wt% of Li₂O and saturating thereafter.     

Synthesis of NASICON with New Compositions for Electrochemical Carbon Dioxide Sensors*

Powders and pellets of new NASICON compositions have been synthesized using a mixed inorganic-organic sol-gel synthesis, by the preliminary formation of a pre-hydrolized TEOS xerogel. The investigated compositions can be described by the general formula Na₃Zr_2-(x/4)Si_2-xP_1+xO₁₂, obtained by keeping the Na concentration constant (= 3) at the optimum value reported for ionic conductivity, with x = 0 (the usual NASICON composition), 0.667, and 1.333. The xerogels were calcined at various temperatures in the range 400–1200°C. The powder samples were analyzed by TG/DTA, BET measurements, XRD, and SEM. The powders calcined at 500°C were sintered into pellets at 1100°C for 6 h. The sintering behavior of the pellets was investigated by dilatometric measurements and SEM observations. The sinterability increased with increasing x value. Dense samples of the new compositions were obtained by sintering at only 1100°C. This is attributed to the occurrence of liquid phase sintering. The electrical conductivity of the NASICON sintered bodies was measured by ac impedance spectroscopy. The conductivity decreased with decreasing c₀ lattice parameter of the hexagonal structure or increasing x value. The CO₂ gas sensors, using as electrolyte the dense samples of the new NASICON compositions, showed good EMF response that was very close to the theoretical value, even for the sample with x = 1.333 that showed much lower conductivity.     

Dielectric Degradation and Microstructures of Heterogeneous Interfaces in Cofired Multilayer Ceramic Capacitors

The compatibility of electrodes and dielectrics in cofired MLCCs with both Ni and Ag/Pd electrodes was characterized by transmission electron microscopy (TEM) using tripod polished samples. Tripod polishing procedures can reduce entire devices to a thickness of less than 1 m. After low angle ion milling for a short time, many regions across several dielectric and electrode layers are electron transparent, which makes it possible to characterize the cofired interfacial microstructures. When analyzed by convergent beam electron diffraction (CBED) and energy dispersive X-ray spectrometry (EDS), NiO lamellae and P-rich intermediate layers were found in highly accelerated life tested (HALT) MLCCs with Ni electrodes. CBED confirmed that the P-rich layers had a Ba₄Ti₁₃O₃₀ (B₄T₁₃) structures. Oxidized Ni layers containing Mn were also found in the HALT samples. It is believed that Mn ions were reduced by the Ni electrodes, as P-rich and Mn-rich segregated layers were observed in the virginal non-life tested MLCCs. Grains with stacking faults, containing dopants such as Mn, Si, and Mg, had the BaTi₄O₉ (BT₄) structure. No silver diffusion was found in either the BaTiO₃ based perovskite lattices or the flux phases in air-fired X7R type MLCCs.     

Microwave dielectric properties of B2O3-added Li2MgTiO4 ceramics for LTCC applications

Li₂MgTiO₄ (LMT) ceramics which are synthesized using a conventional solid-state reaction route. The LMT ceramic sintered at 1250°C for 4 h had good microwave dielectric properties. However, this sintering temperature is too high to meet the requirement of low-temperature co-fired ceramics (LTCC). In this study, the effects of B₂O₃ additives and sintering temperature on the microstructure and microwave dielectric properties of LMT ceramics were investigated. The B₂O₃ additive forms a liquid phase during sintering, which decreases the sintering temperature from 1250°C to 925°C. The LMT ceramic with 8 wt% B₂O₃ sintered at 925°C for 4 h was found to exhibit optimum microwave dielectric properties: dielectric constant 15.16, quality factor 64,164 GHz, and temperature coefficient of resonant frequency -28.07 ppm/°C. Moreover, co-firing of the LMT ceramic with 8 wt% B₂O₃ and 20 wt% Ag powder demonstrated good chemical compatibility. Therefore, the LMT ceramics with 8 wt% B₂O₃ sintered at 925°C for 4 h is suitable for LTCC applications.     

Investigation of the Partial Oxidation of Methane at the Interface Metal (Pt,Au)|YSZ

The reactions occurring at the electrodes of Pt|YSZ|Au cell fed with a mixture of CH₄+air were investigated at 600–850∘C. The electrodes of this cell were made from Pt and Au wires, providing a small contact area between metal and YSZ (quasi-point electrodes). The dependences of the cell voltage on the composition of gas mixture and time were studied. A sequence of chemical and electrochemical reactions was proposed to explain the observed abrupt variation of the cell voltage from positive to negative values. Also the impedances of the Pt and Au electrodes were measured and analysed in order to justify the proposed mechanism of reactions.     

Influences of starting particulate materials on microstructural evolution and electrochemical activity of LSM-YSZ composite cathode for SOFC

LSM-YSZ composite electrodes are prepared from a mixture of YSZ and LSM particles. Commercial YSZ particles were mixed with polymerizable complex method-driven LSM powders of two different particle sizes, 81 and 210 nm. The correlations between sintering temperature, microstructure and performance of composite cathode have been studied. It was found that optimum sintering temperature differs depending upon the LSM particle size. The composite cathode derived from finer LSM particles displayed lowest polarization resistance ∼0.46 Ωcm² when sintered at 1100^∘C, whereas the same resistance was observed for the electrode involving larger LSM sintered at 1200^∘C. The microstructural changes evolved during the course of sintering and the resulting electrodes were investigated by scanning electron microscopy and impedance spectroscopy.     

Synthesis and characterization of novel Ca2Zn4Ti15O36 microwave ceramics derived from sol-gel powder

A novel microwave dielectric ceramics with composition of Ca₂Zn₄Ti₁₅O₃₆ (CZT) have been synthesized at different sintering temperatures, using citrate sol-gel derived powder. The sintering behavior and the phase identification of the powders were evaluated using differential thermal analysis-thermo gravimetric analysis and X-ray powder diffraction analysis techniques. The phase of CZT can be observed in the powder calcined at 900 °C. The single-phase of CZT, however, can only be obtained at sintering temperature of 1,000 °C or above. The single-phase CZT ceramics can be sintered into dense at 1,100 °C, exhibiting excellent microwave dielectric properties of ? _r?=?48.1, Q?×?f?=?27,000 GHz, and τ _f?=?+53.5 ppm/°C. The effects of sintering temperature on the density, microstructure, and dielectric properties of the sintered ceramics were investigated. The mechanism responsible for the change of dielectric properties with sintering temperature was also discussed.     

Effect of B2O3 and CuO on the sintering temperature and microwave dielectric properties of the BaTi4O9 ceramics

The effect of B₂O₃ and CuO on the sintering temperature and microwave dielectric properties of BaTi₄O₉ ceramics was investigated. The BaTi₄O₉ ceramics were able to be sintered at 975^∘C when B₂O₃ was added. This decrease in the sintering temperature of the BaTi₄O₉ ceramics upon the addition of B₂O₃ is attributed to the formation of BaB₂O₄ second phase whose melting temperature is around 900^∘C. The B₂O₃ added BaTi₄O₉ ceramics alone were not sintered below 975^∘C, but were sintered at 875^∘C when CuO was added. The formation of BaCu(B₂O₅) second phase could be responsible for the decrease in the sintering temperature of the CuO and B₂O₃ added BaTi₄O₉ ceramics. The BaTi₄O₉ ceramics containing 2.0 mol% B₂O₃ and 5.0 mol% CuO sintered at 900^∘C for 2 h have good microwave dielectric properties of ε_r = 36.3, Q× f = 30,500 GHz and τ_f = 28.1 ppm/^∘C     

Dielectric properties of Bi2O3–ZnO–Ta2O5 ceramics sintered by microwave

Here we report dielectric studies carried out on a Bi₂Zn_2/3Ta_4/3O₇ (abbreviated as β-BZT) composition. The material was synthesized by conventional ceramic method and microwave sintering processing. The dielectric properties were studied as a function of frequency and temperature. Dielectric constant of Bi₂Zn_2/3Ta_4/3O₇ ceramics prepared from microwave is slightly smaller than that of the conventional sintered ones. The dissipation factor and temperature coefficient of dielectric constant are low for microwave-sintered samples. Microwave sintering of Bi₂Zn_2/3Ta_4/3O₇ ceramics led to higher densification and the fine microstructure in much shorter time duration compared to conventional procedures, improved microstructure and dielectric properties. To achieve the same densification, it requires 4 h of soaking at the same temperature in conventional sintering process. Microwave sintering method may lead to energy savings because of rapid kinetics of synthesis.