Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

New process for synthesis of ZnO thin films: Microstructural, optical and electrical characterization

Nanocrystalline ZnO thin films were prepared on glass substrates by using spin coating technique. The effect of annealing temperature (400-700 °C) on structural, compositional, microstructural, morphological, electrical and optical properties of ZnO thin films were studied by X-ray diffraction (XRD), Energy dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), High Resolution Transmission Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Electrical conductivity and UV-visible Spectroscopy (UV-vis). XRD measurements show that all the films are nanocrystallized in the hexagonal wurtzite structure and present a random orientation. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The AFM analysis revealed that the surface morphology is smooth. The HRTEM analysis of ZnO thin film annealed at 700 °C confirms nanocrystalline nature of film. The SEM results shows that a uniform surface morphology and the nanoparticles are fine with an average grain size of about 40-60 nm. The dc room temperature electrical conductivity of ZnO thin films were increased from 10⁻⁶ to 10⁻⁵ (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of ZnO films annealed at 400-700 °C were estimated to be of the order of 4.75-7.10 × 10¹⁹ cm⁻³ and 2.98-5.20 × 10⁻⁵ cm² V⁻¹ S⁻¹.The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 3.32 eV to 3.18 eV with increasing annealing temperature between 400 and 700 °C. This means that the optical quality of ZnO films is improved by annealing.It is observed that the ZnO thin film annealing at 700 °C has a smooth and flat texture suited for different optoelectronic applications.     

Dielectric properties and crystal structure of La(Mg1/2Ti1/2)O3 ceramics with Mg substituted by Co

Solid solutions of (1 − x)La(Co_1/2Ti_1/2)O₃-xLa(Mg_1/2Ti_1/2)O₃ were used to prepare La(Mg_1−xCo_x)_1/2Ti_1/2O₃ using solid-state synthesis. X-ray diffraction patterns of the sintered samples revealed single phase formation. A maximum density of 6.01 g/cm³ was obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ (x = 1) ceramics sintered at 1375 °C for 4 h. The maximum values of the dielectric constant (?_r = 29.13) and the quality factor (Q × f = 80,000 GHz) were obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ with 1 wt% ZnO additive sintered at 1375 °C for 4 h. The temperature coefficient of resonant frequency τ_f was −59 ppm/°C for x = 0.3.     

Sinterability and electrical properties of ZnO-doped Ce0.8Y0.2O1.9 electrolytes prepared by an EDTA-citrate complexing method

The effect of ZnO addition on the sinterability and ionic conductivity of Ce_0.8Y_0.2O_1.9 is investigated. Ce_0.8Y_0.2O_1.9 is prepared using an EDTA-citrate complexing method in order to further improve its electrical properties. Using a ZnO content over 1 mol %, the sinterability of Ce_0.8Y_0.2O_1.9 is significantly improved by reducing the sintering temperature from 1500 to 1350 °C and a relative density of above 95% was achieved. The highest ionic conductivity of 0.0516 S cm⁻¹ was obtained at 750 °C for (YDC)_0.99(ZnO)_0.01 sintered at 1350 °C. Pure YDC sintered at 1500 °C, on the other hand, yielded 0.0289 S cm⁻¹.     

Spectroscopic and magnetic investigation of NiCo nanoferrites

Spectroscopic and magnetic characterization of Ni_1−xCo_xFe₂O₄ (0.0 ≤ x ≤ 0.5) nanoparticles is presented. The infrared spectra are measured in the frequency range 700-350 cm⁻¹. Two prominent bands are observed, low frequency band at about 400 cm⁻¹ and high frequency band at about 600 cm⁻¹ assigned to octahedral and tetrahedral sites, respectively. The force constants K_o and K_t corresponding to octahedral and tetrahedral sites, respectively are also calculated from FTIR spectra. The effect of co-concentration on the magnetic properties has been investigated using a vibrating sample magnetometer (VSM). After appropriate treatments, it is found that both magnetic saturation (M_s) and coercivities (H_c) increase with co-concentration.     

Fabrication and properties of Ba(Zr1−xCex)0.9Y0.1O2.95/NaCl composite electrolyte materials

Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95/NaCl (x = 0.1, 0.2 and 0.3) composite electrolyte materials were fabricated with ZnO as sintering aid. The effect of ZnO on the properties of Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 matrix were investigated. The phase composition and microstructure of samples were characterized by XRD and SEM, respectively. The electrochemical performances were studied by three-probe conductivity measurement and AC impedance spectroscopy. XRD results showed that Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO was perovskite structure. The relative density of this sample was above 95% when sintered at 1450 °C for 6 h. By adding 10 mol% of NaCl to Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO that was sintered at 1400 °C for 6 h, the conductivity was increased. The electrical conductivity of 1.26 × 10⁻² S/cm and activation energy of 0.23 eV were obtained when tested at 800 °C in wet hydrogen.     

Effect of Ag-doping on crystal structure and high temperature thermoelectric properties of c-axis oriented Ca3Co4O9 thin films by pulsed laser deposition

Ag-doped Ca₃Co₄O₉ thin films with nominal composition of Ca_3−xAg_xCo₄O₉ (x = 0∼0.4) have been prepared on sapphire (0 0 0 1) substrates by pulsed laser deposition (PLD). Structural characterizations and surface chemical states analysis have shown that Ag substitution for Ca in the thin films can be achieved with doping amount of x ≤ 0.15; while x > 0.15, excessive Ag was found as isolated and metallic species, resulting in composite structure. Based on the perfect c-axis orientation of the thin films, Ag-doping has been found to facilitate a remarkable decrease in the in-plane electrical resistivity. However, if doped beyond the substitution limit, excessive Ag was observed to severely reduce the Seebeck coefficient. Through carrier concentration adjustment by Ag-substitution, power factor of the Ag-Ca₃Co₄O₉ thin films could reach 0.73 mW m⁻¹ K⁻² at around 700 K, which was about 16% higher than that of the pure Ca₃Co₄O₉ thin film.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Electrical characterization of Au/n-GaN metal-semiconductor and Au/SiO2/n-GaN metal-insulator-semiconductor structures

In the present work, we have investigated the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Au/SiO₂/n-GaN metal-insulator-semiconductor (MIS) Schottky diode and compared with Au/n-GaN metal-semiconductor (MS) Schottky diode. Calculations showed that the Schottky barrier height and ideality factor of the MS Schottky diode is 0.79 eV (I-V), 0.87 eV (C-V) and 1.45, respectively. It is observed that the Schottky barrier height increases to 0.86 eV (I-V), 0.99 eV (C-V) and ideality factor deceases to 1.3 for MIS diode. For the MS diode, the calculated doping concentration is 4.17 × 10¹⁷ cm⁻³. However, in the case of the MIS Schottky diode, the decrease in doping concentration is observed and the respective value is 2.08 × 10¹⁷ cm⁻³. The obtained carrier concentration of the MIS diode is reduced about 50% when compared to the MS diode. The interface state density as determined by Terman's method is found to be 3.79 × 10¹² and 3.41 × 10¹⁰ cm⁻² eV⁻¹ for the MS and MIS Schottky diodes, respectively. The calculated interface densities are 2.47 × 10¹¹ cm⁻² eV⁻¹, 3.35 × 10¹¹ cm⁻² eV⁻¹ and 3.5 × 10¹¹ cm⁻² eV⁻¹ for the sweep rates of 300, 450 and 600 mV/s from MOS C-V measurements for the MIS Schottky diode. The interface state density calculated from Terman's method is found to be increased with sweep rate. From the C-V measurement, it is noted that the decrease in the carrier concentration in MIS diodes as compared to MS diode may be due to the presence of oxide interfacial layer. DLTS measurements have also been performed on MIS Schottky diode and discussed.     

SmBaCoCuO5+x as cathode material based on GDC electrolyte for intermediate-temperature solid oxide fuel cells

The performance of SmBaCoCuO_5+x (SBCCO) cathode has been investigated for their potential utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction (XRD), thermal expansion and electrochemical performance on Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte are evaluated. XRD results show that there is no chemical reaction between SBCCO cathode and GDC electrolyte when the temperature is below 950 °C. The thermal expansion coefficient (TEC) value of SBCCO is 15.53 × 10⁻⁶ K⁻¹, which is ∼23% lower than the TEC of the SmBaCo₂O_5+x (SBCO) sample. The electrochemical impedance spectra reveals that SBCCO symmetrical half-cells by sintering at 950 °C has the best electrochemical performance and the area specific resistance (ASR) of SBCCO cathode is as low as 0.086 Ω cm² at 800 °C. An electrolyte-supported fuel cell generates good performance with the maximum power density of 517 mW cm⁻² at 800 °C in H₂. Preliminary results indicate that SBCCO is promising as a cathode for IT-SOFCs.     

Structural, morphological and electrical properties of spray deposited nano-crystalline CeO2 thin films

Nanocrystalline, uniform, dense, and adherent cerium oxide (CeO₂) thin films have been successfully deposited by a simple and cost effective spray pyrolysis technique. CeO₂ films were deposited at low substrate and annealing temperatures of 350 °C and 500 °C, respectively. Films were characterized by differential thermal analysis, X-ray diffraction, scanning electron microscopy, atomic force microscopy; two probe resistivity method and impedance spectroscopy. X-ray diffraction analysis revealed the formation of single phase, well crystalline thin films with cubic fluorite structure. Crystallite size was found to be in the range of 10-15 nm. AFM showed formation of smooth films with morphological grain size 27 nm. Films were found to be highly resistive with room temperature resistivity of the order of 10⁷ Ω cm. Activation energy was calculated and found to be 0.78 eV. The deposited film showed high oxygen ion conductivity of 5.94 × 10⁻³ S cm⁻¹ at 350 °C. Thus, the deposited material shows a potential application in intermediate temperature solid oxide fuel cells (IT-SOFC) and might be useful for μ-SOFC and industrial catalyst applications.     

Synthesis and study of nanocrystalline Ni-Cu-Zn ferrites prepared by oxalate based precursor method

Nanocrystalline ferrites of compositions Ni_0.5+1.5xCu_0.3Zn_0.2Fe_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized by using oxalate based precursor method at very low temperature. The Ni-Cu-Zn ferrite powder particles were obtained at 450 °C and they exhibit a crystallite size of 16-24 nm. The lattice constants were found nearly equal in all these samples due to minute difference in the ionic radius between Ni²⁺ and Fe³⁺ ions. The thermal analysis has showed the ferrite phase formation at very low temperature 377 °C. The two main spectroscopic bands corresponding to lattice vibrations were observed in the wavelength range from 300 to 1000 cm⁻¹. The IR bands at 570 cm⁻¹ (v₁) and 390 cm⁻¹ (v₂) were assigned to tetrahedral (A) and octahedral [B] groups. The spectroscopic bands shift with the increase of doping concentration. The magnetization was found to decrease with increasing doping concentration. The dielectric constant (?′) and dielectric loss tangent (tan δ) decreased with increase of frequency. The dielectric constant and dielectric loss obtained for the nanocrystalline ferrite samples appeared to be lower than that of the ferrites prepared by other synthesis techniques.     

High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9+δ

Misfit-layered oxides Ca_3−xLn_xCo₄O_9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca_3−xLn_xCo₄O_9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln³⁺. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca_2.8Lu_0.2Co₄O_9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart.     

Sintering behaviors, magnetic and electric properties of Bi-Zn co-doped Co2Y ferrites

The Bi and Zn substitution effects on the sintering behaviors, magnetic and electric properties of hexagonal ferrites with a composition of 2(Ba_1−xBi_xO)·2(Zn_yCo_0.8−yCu_0.2O)·6(Fe_2−x/3Zn_x/3O₃) were investigated. The results showed that the addition of Bi and Zn can significantly promote Co₂Y densification. The Y phase may be triggered to decompose into M and spinel phases at high sintering temperatures (above 1050 °C) for samples with excess Bi (x = 0.2) substitution, which resulted in densification and magnetic properties degradation. Co₂Y ferrites with x = 0.1 and y = 0.4 sintered at 1050 °C show a relative density of 94%, a high initial permeability of 4.5, a quality factor (Q) of 50.     

Dense La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte for IT-SOFC's: Sintering study and electrochemical characterization

This paper presents the sintering behaviour of a La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 coral-like microstructure powder. This is prepared by a successive freeze-drying and self-ignition process followed by calcination at 1200 °C during 1 h. This synthesis method gives great uniformity of the powder and allows shaping into compacts without requiring a grinding step. The grain size distribution (between 0.5 and 4 μm) favours a good sintering behaviour: open porosity disappear at 1400 °C and relative densities over 99% can be achieved after 6 h at 1450 °C. The same powder can also be sintered into a thin disc of ∼100 μm thickness. The characterization of the dense material by impedance spectroscopy shows that the activation energies below and above 600 °C are 1.0 eV and 0.7 eV, respectively. The conductivity at 800 °C is ∼0.11 S cm⁻¹. Special attention is devoted to the temperature range between 200 °C and 400 °C, where the intragrain and intergrain contributions can be distinguished. The analysis of the parameters describing the intragrain constant phase element in the equivalent circuit suggests that, above 325 °C, the system evolves from a distribution of relaxation time to only one relaxation time. The analysis of the data by the complexes permittivity show that ionic oxide conduction mechanism would occur in two steps. In the first, an oxygen vacancy would be released and, in the second, the migration of the ionic oxide would take place in the material.     

Thermoelectric properties of Bi2SexTe3−x prepared by Bridgman method

Bi₂Se_xTe_3−x crystals with various x values were grown by Bridgman method. The electrical conductivity, σ, was found to decrease with increasing Se content. The highest σ of 1.6 × 10⁵ S m⁻¹ at room temperature was reached at x = 0.12 with a growth rate of 0.8 mm h⁻¹. The Seebeck coefficient, S, was less dependent on Se content, all with positive values showing p-type characteristics, and the highest S was measured to be 240 μV K⁻¹ at x = 0.24. The lowest thermal conductivity, κ, was 0.7 W m⁻¹ K⁻¹ at x = 0.36. The electronic part of κ, κ_el, showed a decrease with increasing Se content, which implies that the hole concentration as the main carriers was reduced by the addition of Se. The highest dimensionless figure of merit, ZT, at room temperature was 1.2 at x = 0.36, which is attributed to the combination of a rather high electrical conductivity and Seebeck coefficient and low thermal conductivity.     

Magnetic and FTIR studies of BixY3−xFe5O12 (x = 0, 1, 2) powders prepared by the metal organic decomposition method

The crystallization process of bismuth substituted yttrium iron garnet (Bi_xY_3−xFe₅O₁₂; x = 0, 1, 2) powder prepared by the metal-organic decomposition method has been studied with various sintering temperatures. The pure garnet phase was observed for the x = 1 powder at 900 °C sintering temperature, whereas the x = 0, 2 powder showed secondary phases. The x = 0 powder showed a similar crystallization process to that of the solid state reaction method. For the x =1, 2 powders, it is proposed that the lowering of the crystallization temperature is due to the lowered stability of the intermediate phase. The infrared spectroscopy and magnetic properties were also investigated. The pure garnet phase showed three absorption bands located at 563, 598, 655 cm⁻¹ that shifted to 555, 588, 639 cm⁻¹ along with an increase of bismuth concentration. The maximum values of saturation and remanence magnetization and the minimum value of coercivity were observed for the x = 1 powder sintered at 900 °C, which were 20.8 emu/g, 2.67 emu/g, and 31.9 Oe, respectively.     

Transparent conducting oxide films of heavily Nb-doped titania by reactive co-sputtering

Niobium-doped titania (TNO) films of various Nb content were deposited on glass and silicon substrates by reactive co-sputtering of Ti and Nb metal targets. Nb content in the TNO films was varied from 0 to ∼13 at.% (atomic percent), corresponding to Ti_1−xNb_xO₂ with x = 0-0.52, by modulating the Nb target power from 0 to 150 W (Watts). The influence of ion bombardment on the TNO films was investigated by applying an RF substrate bias from 0 to 25 W. The as-deposited TNO films were all amorphous and insulating, but after annealing at 600 °C for 1 h in hydrogen, they became crystalline and conductive. The annealed films crystallized into either pure anatase or mixed anatase and rutile structures. The as-deposited and the annealed films were transparent, with an average transmittance above 70%. Anatase TNO film (Ti_1−0.39Nb_0.39O₂) with Nb 9.7 at.% exhibited a dramatically reduced resistivity of 9.2 × 10⁻⁴ Ω cm, a carrier density of 6.6 × 10²¹ cm⁻³ and a carrier mobility around 1.0 cm² V⁻¹ s⁻¹. In contrast, the mixed-phase Ti_1−0.39Nb_0.39O₂ showed a higher resistivity of 1.2 × 10⁻¹ Ω cm. This work demonstrates that the anatase phase, oxygen vacancies, and Nb dopants are all important factors in achieving high conductivities in TNO films.     

Structure and high-temperature oxidation behaviour of Cu-Ni-Fe alloys prepared by high-energy ball milling for application as inert anodes in aluminium electrolysis

Cu_xNi_85−xFe₁₅ (0 ? x ? 85 wt.%) compounds were prepared by mechanical alloying. Monophased face centered cubic (fcc) Cu-Ni-Fe alloys were obtained after 10 h of milling for x varying from 0 to 50, whereas bi-phased compounds fcc Cu-Ni-Fe + body centered cubic (bcc) Fe were formed with richer-Cu compounds. Their oxidation kinetics in air at 750 °C is parabolic for all compositions and increases drastically for x > ∼30. A stable anode for aluminium electrolysis in low-temperature (700 °C) KF-AlF₃ electrolyte was obtained for 65 ? x ? 85. However, a substantial increase of the Cu contamination in produced aluminium was observed for x > 70.     

Nickel-lithium oxide alloy transparent conducting films deposited by spray pyrolysis technique

In this research, nickel oxide (NiO) transparent semiconducting films are prepared by spray pyrolysis technique on glass substrates. The effect of Ni concentration in initial solution and substrate temperature on the structural, electrical, thermoelectrical, optical and photoconductivity properties of NiO thin films are studied. The results of investigations show that optimum Ni concentration and suitable substrate temperature for preparation of basic undoped NiO thin films with p-type conductivity and high optical transparency is 0.1 M and 450 °C, respectively. Then, by using these optimized deposition parameters, nickel-lithium oxide ((Li:Ni)O_x) alloy films are prepared. The XRD structural analysis indicate the formation of the cubic structure of NiO and (Li:Ni)O_x alloy films. Also, in high Li doping levels, Ni₂O₃ and NiCl₂ phases are observed. The electrical measurements show that the resistance of the films decreases with increasing Li level up to 50 at%. For these films, the optical band gap and carrier concentration are obtained to be 3.6 eV and 10¹⁵-10¹⁸ cm⁻³, respectively.