Dependencies of the open-circuit voltage of Au|YSZ|Pt single-chamber fuel cell on the composition of the uniform CH4 + O2 gas mixture with solid carbon deposition

Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor, may generate a considerable potential difference and electrical power, when supplied by a common fuel + oxidant gas mixture. The Au|YSZ|Pt system in the CH₄ + O₂ gas mixture is one of the simplest examples of such a cell. In this article the open-circuit voltage (OCV) of this cell, supplied with the gas mixture xO₂ + aCH₄ + (1 − x − a)Ar (where a = 0.01, 0.1 or 0.5), is investigated. On the basis of the obtained results, as well as those for the xCH₄ + (1 − x)(0.2O₂ + 0.8Ar) (0 ≤ x ≤ 1) gas mixture, reported in our previous work [Electrochim. Acta, 50 (2005) 2771], we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell. After oxidation of the carbon deposit, the system, once treated by the gas mixture enabling the formation of the carbon phase, shows more and more tendency to generate the OCV. The open-circuit potential of the Au electrode depends only on the O₂ concentration in the initial gas mixture, whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O₂ content in the converted gas mixture. Our results reveal that the OCV achieves a reproducible limiting value of ∼650 mV, which is lower by ∼400 mV than the calculated equilibrium value.     

Determination of open-circuit potentials at gas/electrode/YSZ boundary versus molten carbonate reference electrode at medium temperatures: I. Potentials of Au and Pt in O2 and H2 + H2O atmospheres

A double gas concentration cell as combination of the cell with the yttria stabilized zirconia (YSZ) electrolyte and the cell with molten Li₂CO₃ + Na₂CO₃ eutectics is proposed as an alternative cell system with a standard reference electrode for measurements of the open-circuit potential (OCP) values of electrodes in oxygen concentration cell with the yttria stabilized zirconia (YSZ) electrolyte. In this double-cell one electrode is common for the two cells and the reference electrode is the standard molten carbonate half-cell with 0.33O₂ + 0.67CO₂ atmosphere. This reference electrode should enable the monitoring of OCP and overpotential values in polarization studies in the three-electrodes configuration. If the possible reaction between the solid YSZ and liquid molten carbonates electrolyte is very slow, the measured values of the open-circuit-voltage (OCV) of this cell may be considered equal to the respective reversible electromotive forces (EMF). Very good resistance of the smooth YSZ products to the corrosion in highly dehydrated Li/Na molten carbonates has been shown in experiments lasting few 1000 h. Hence, the consistency of OCV values with the respective EMF values have been tested at various partial pressures of CO₂ and O₂ in the gas mixtures above the molten carbonate electrolyte and at various partial pressures of O₂ + Ar or H₂ + H₂O gas mixtures at the Au or Pt electrodes/YSZ interface. The results have shown the reliability of the double-cell in determination of the open-circuit potentials (OCP) of gas electrodes at the YSZ surface as measured versus the reference electrode with molten carbonate electrolyte. The consistency of OCP and EMF values has been shown satisfying and enhances to use the proposed double-cell in further investigations of OCP and overpotential values at TPB of electrode/YSZ/mixture of reacting gases. At high differences of O₂ partial pressures on both sides of the YSZ membrane some permeation of this gas through the YSZ membrane has been observed. Probably, this effect has an electrochemical character.     

Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes

A systematic investigation was conducted on the mechanism and electrocatalytic properties of O₂ and Cl₂ evolution on mixed oxide electrodes of nominal composition: Ti/[Ru_(0.3)Ti_(0.6)Ce_(0.1−x)]O₂[Nb₂O₅]_(x) (0 ≤ x ≤ 0.1). For the oxygen evolution, a 30 mV Tafel slope is obtained in the presence of CeO₂, while in its absence a 40 mV coefficient is observed. The intrinsic electrocatalytic activity is mainly due to electronic factors, as result of the synergism between Ru and Ce oxides. For chlorine evolution, the Tafel slope (30 mV) is independent on oxide composition. The best global electrocatalytic activity for ClER was observed in the absence of Nb₂O₅ additive. Variation of the voltammetric charge throughout the experiments confirms high CeO₂ content compositions are fragile, due mainly to the porosity caused by CeO₂ presence. On the other hand, Nb₂O₅ addition decreases considerably this instability.     

Insignificant impact of designed oxygen release from high capacity Li[(Ni1/2Mn1/2)xCoy(Li1/3Mn2/3)1/3]O2 (x + y = 2/3) positive electrodes during the cycling of Li-ion cells

The properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) Li-ion cells are reported. There is an extended plateau near 4.5 V during the first charging of the cells that corresponds to the simultaneous removal of Li and oxygen from the Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) electrodes. The release of this oxygen directly within a Li-ion cell has been a cause for concern. However, it was found that subsequent to O₂ release, Li-ion cells delivered a high reversible positive electrode specific capacity near 250 mAh/g at C/30 between 2.5 and 4.8 V, the cells did not display increased irreversible capacity relative to counterparts having Li metal negative electrodes and the cells retained 85% of their initial capacity after 70 cycles at C/6 between 2.5 and 4.6 V. Therefore, the O₂ released during the first charge does not significantly impact the electrochemical properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3) lithium-ion cells.     

Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Double channel electrode flow cell application to the study of HO2 production on MnxCo3−xO4 (0 ≤ x ≤ 1) spinel films

We conducted a study on the electroreduction of O₂ in alkaline solution at room temperature on pure thin oxide electrodes of composition Mn_xCo_3−xO₄ (0 ≤ x ≤ 1) using the double channel electrode flow cell (DCEFC). The oxides were prepared at 150 °C and deposited by spray pyrolysis onto titanium substrates. The oxygen reduction reaction (orr) occurs through “interactive” and “parallel” pathways, and the ratio of O₂ molecules reduced to OH⁻ ions with respect to those reduced to HO₂⁻ ions depends on the oxide stoichiometry and on the applied overpotential. The formation of HO₂⁻ increases when the manganese concentration increases. The results obtained for the orr show that the number of electrons transferred per O₂ molecule decreases from 3 to 2 and the ratio k₁/k₂ (the rate constants for direct reduction to OH⁻ and indirect reduction to HO₂⁻) increases, respectively, in the overpotential studied range (−0.05 to −0.6 V). The Mn³⁺ ions placed in the B-sites of the spinel structure seem to be the active centres, where hydrogen peroxide is formed.     

Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

Effects of NO2 ions on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 mol L⁻¹ NaHCO₃ + 0.02 mol L⁻¹ NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with NO₂⁻ concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [NO₂⁻] = 0 up to 400 ± 50 mV/SCE for [NO₂⁻] = 0.1 mol L⁻¹. During anodic polarisation at potentials comprised between E_b([NO₂⁻] = 0) and E_b([NO₂⁻] ≠ 0), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via scanning vibrating electrode technique (SVET). Addition of a NaNO₂ solution after the beginning of the polarisation led to a rapid repassivation of pre-existing well-grown pits. In situ micro-Raman spectroscopy was then used to identify the corrosion products forming inside the pits. The first species to be detected in the presence of NO₂⁻ were mainly dissolved Fe(III) species, more likely [Fe^III(H₂O)₆]³⁺ complexes. Iron(II) carbonate FeCO₃, siderite, and carbonated green rust GR(CO₃²⁻) were also detected in the active pits, as in the absence of nitrite. But they were accompanied by maghemite γ-Fe₂O₃, a phase structurally similar to the passive film, that forms from the Fe(III) complexes. The Raman analyses then correlate with the SVET observations and confirm that the main effect of nitrite ions is to oxidize iron(II) into iron(III). The passive film would then form from the Fe(III) species still bound to the steel surface.     

Characterization of MgxM2 − xP2O7 (M = Cu and Ni) solid solutions

In this study, Mg_xM_2 − xP₂O₇ (M = Cu, Ni; 0 ≤ x ≤ 2) and Mg_3 − yNi_y(PO₄)₂ (0 ≤ y ≤ 3) compositions were synthesized by the chemical coprecipitation method and characterized by X-ray diffraction, UV-vis-NIR spectroscopy and CIE L* a* b* (Commission Internationale de l’Eclairage L* a* b*) parameters measurements.Solid solutions with α-Cu₂P₂O₇ and α-Ni₂P₂O₇ structures and solid solutions with Ni₃(PO₄)₂ structure were obtained from diphosphate and orthophosphate compositions respectively. Isostructurality of α-Ni₂P₂O₇ and α-Mg₂P₂O₇ structures enlarges the compositional range of solid solution formation respect to the Mg_xCu_2 − xP₂O₇ solid solutions one.The CIE L* a* b* parameters in Mg_xNi_2 − xP₂O₇ samples were obtained comparable with these parameters in others yellow materials suitable for ceramic pigments. Mg_0.5Ni_1.5P₂O₇ composition fired at 800 °C or 1000 °C is the optimal composition to obtain yellow materials with α-diphosphate structure in conditions of this study.     

Development of ceramic electrochemical sensor based on Bi2Ru2O7+x − RuO2 sub-micron oxide sensing electrode for water quality monitoring

Sub-micron Bi₂Ru₂O_7+x + RuO₂ oxide sensing electrodes (SE) for water quality sensors were prepared on platinised ceramic substrate of the sensor. Their morphology was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Sensing properties of the Bi₂Ru₂O_7+x + RuO₂-SE were investigated for potentiometric detection of pH and dissolved oxygen (DO) in water in the temperature range of 4–30 °C. Sensor was capable to measure DO from 0.5 to 8.0 ppm and pH from 2.0 to 13.0, respectively. The obtained results show acceptable linearity of the measuring characteristics. Long-term stability trial for Bi₂Ru₂O_7+x + RuO₂-SE revealed that bio-fouling can be one of the main destructive factors affecting the performance of the sensors in the long run. The screen-printing technology used in the multi-sensory implementation provides fundamental properties of miniaturization, reasonable accuracy and low cost.     

Surface, kinetics and electrocatalytic properties of the Ti/(Ti + Ru + Ce)O2-system for the oxygen evolution reaction in alkaline medium

Ti-supported (Ti + Ru + Ce)O₂ electrodes, prepared at 450 °C, were characterised by XRD, open-circuit potential (E_oc), capacity data (C) and morphology factor (φ) determinations. XRD measurements showed mixed oxides present a low degree of crystallinity. E_oc-data and CV-spectra support surface electrochemistry of mixed oxides is governed by the Ru(III)/Ru(IV) redox couple. In situ surface characterisation revealed the active surface area increases on increasing nominal CeO₂-content. φ-Values remained in the 0.18-0.3 interval supporting the coatings have a low electrochemical porosity. Kinetics was studied recording polarisation and chronopotentiometric curves, which permitted to determine the Tafel slope and reaction order (with respect to OH⁻), in the low and high overpotential domains. Tafel slope data, b, presented a dependence on overpotential and oxide composition indicating the OER electrode mechanism depends on these variables. A unit reaction order with respect to OH⁻ was found for all electrode compositions investigated. The theoretical analysis of the electrode mechanism permitted to analyse the changes in the experimental Tafel slopes taking into account modifications in the apparent electronic transfer coefficient, α_ap. Analysis of the true and apparent electrocatalytic activities revealed the O₂-evolution reaction rate is affected by oxide composition due to morphologic effects.     

Synthesis optimization of Li1 + x[Mn0.45Co0.40Ni0.15]O2 with different spherical sizes via co-precipitation

Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂ spherical cathode materials with different sizes (about 2 and 5 μm) were fabricated by calcining uniform spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃ with lithium hydroxide at high temperature. The precursor of spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃, was obtained via co-precipitation method at room temperature, which was significantly dependent on synthetic conditions, such as the reaction temperature, the concentration of NH₄HCO₃, and stirring speed, etc. The optimized condition resulted in [Mn_0.45Co_0.40Ni_0.15]CO₃, of which the particle size distribution was uniform and the particle shape was spherical. The final products, Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂, had a well-ordered layered structure and uniform homogeneity. Raman spectroscopy analysis showed the Raman-active species E_g and A_1g modes were observed at 488, 473 cm^− 1 and 597, 590 cm^− 1, respectively, for the obtained spherical cathode materials.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Stable spinel type cobalt and copper oxide electrodes for O2 and H2 evolutions in alkaline solution

The stability of one material, Ti/Cu_xCo_3−xO₄, as anode and also cathode was investigated for electrolysis of alkaline aqueous solution. The electrodes were prepared by thermal decomposition method with x varied from 0 to 1.5. The accelerated life test illustrated that the electrodes with x = 0.3 nominally showed the best performance, with a total service life of 1080 h recorded in 1 M NaOH solution under alternating current direction at 1 A cm⁻² and 35 °C. The effects of copper content in electrode coating were examined in terms of electrode stability, surface morphology, coating resistivity and coating compositions. The presence of Cu in the spinel structure of Co₃O₄ could significantly enhance the electrochemical and physicochemical properties. The trends of crystallographic properties and surface morphology have been analyzed systemically before, during and after the electrodes were employed in alkaline electrolysis. The oxygen evolution would lead to the consumption of the coating material and the progressive cracking of the coating. Along with hydrogen evolution, cobalt oxide could be reduced to metal Co and Co(OH)₂ with particle sizes changed to smaller values in crystal and/or amorphous form at the cathode. The formation of Co is the key process for this electrode to serve as both anode and cathode. It is also the main reason leading to the eventual failure of the electrodes.     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

The electrochemistry of Pd-coated MgySc(1 − y) thin film electrodes: A thermodynamic and kinetic study

The thermodynamic and kinetic properties of Pd-coated Mg_ySc_(1 − y) thin film electrodes are investigated. These thin film electrodes can be described as a two-layer structure, in which the Pd and Mg_ySc_(1 − y) layer contribute to the overall electrochemical response. In order to identify the response of the Pd layer in the two-layer system, thin films consisting of solely Pd, with identical thickness and orientation, were measured. Based on the fact that the chemical potentials of the individual layers of the Pd-coated Mg_ySc_(1 − y) thin films are equal at equilibrium, the exact hydrogen concentration in each layer could be determined. It is shown that during the major part of the hydrogen extraction process of the Mg_ySc_(1 − y) thin films, the composition of the Pd topcoat is close to PdH_0.001. The kinetics of the surface reactions was investigated using electrochemical impedance spectroscopy and showed that, when cross-correlating the results of Mg_ySc_(1 − y) thin films (y = 0.65 − 0.85) and pure Pd films, the surface kinetics are completely dominated by the Pd topcoat. Additionally, it was shown that the charge transfer reaction, and not the absorption reaction is the rate-determining step. The impedance response, dominating the overall kinetic impedance at the hydrogen-depleted state, could be linked to the transfer of hydrogen across the Pd/Mg_ySc_(1 − y) interface in the two-layer thin film electrode.     

Synthesis of negative thermal expansion materials ZrW2−xMoxO8 (0 ≤ x ≤ 2) using hydrothermal method

Negative thermal expansion materials ZrW_2−xMo_xO₈ (0 ≤ x ≤ 2) have been successfully synthesized by the reaction of a mixture of ammonium tungstate and ammonium molybdate with zirconium oxynitrate using a hydrothermal method. Effect of substituted ion Mo on the microstructure, α-to-β and cubic to trigonal phase transition in resulting ZrW_2−xMo_xO₈ powders was examined by the XRD experiments. It was found that the structural phase transition temperature decreased slightly with increasing substituted content. The cubic to trigonal phase transition was also influenced by substituted content. The resulting products decomposed to WO₃/MoO₃ and ZrO₂ as temperature increasing when x ≤ 0.5 and while x > 0.5, the cubic phase transited to trigonal phase. The effect of substituted Mo on the morphology of resulting products was also investigated by SEM experiments.     

Microwave dielectric properties of the (1 − x)Mg2TiO4-xCaTiO3-y wt.% ZnNb2O6 ceramics system

Composite ceramics based on (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12-0.16, y = 0-8) were prepared by a conventional mixed-oxide route. Zn²⁺ partially replaced Mg²⁺ in Mg₂TiO₄ and formed the spinel-structured (Mg_1−δZn_δ)₂TiO₄ phase. Nb²⁺, is known to be solid soluble in CaTiO₃, was found to change its shape from cubic to pliable. A bi-phase system (Mg_1−δZn_δ)₂TiO₄ and CaTiO₃ exhibited in all samples, where a small amount of second phase Mg_1−δZn_δTiO₃ was also detected. The microwave dielectric properties of specimens were strongly related to ZnNb₂O₆ and CaTiO₃ content. As y increased, ?_r and τ_f decreased, however, Q × f decreased to a minimum value and started to increase thereafter. It was also found that ?_r and τ_f increased and Q × f decreased with increasing x. The optimized microwave dielectric properties with ?_r = 18.37, Q × f = 31,027 GHz (at 6 GHz), and τ_f = 0.51 ppm/°C were achieved for (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12, y = 4) sintered at 1360 °C for 6 h.