Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

Effect of atmosphere on the PTCR characteristics of porous Ba(Ti,Sb)O3 ceramics produced by adding corn-starch

Porous Ba(Ti,Sb)O₃ ceramics were fabricated by adding corn-starch at 20 wt %. The effect of atmosphere on the PTCR characteristics of the porous Ba(Ti,Sb)O₃ ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the Ba(Ti,Sb)O₃ ceramics were investigated. In air, O₂, N₂, and H₂ atmospheres, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics below 150 °C was independent of atmosphere, while it was strongly dependent on atmosphere above 200 °C. The low electrical resistivity in reducing atmospheres was due to a decrease in potential barrier height, which originated from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In a N₂ atmosphere, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics during the cooling cycle was lower than that during the heating cycle, and then the electrical resistivity of the porous Ba(Ti,Sb)O₃ ceramics during subsequent heating and cooling cycles was increased again by exposure to an O₂ atmosphere.     

Influence of sintering and annealing temperature on grain boundary barrier layer capacitors in a modified reduction-reoxidation method

To produce grain boundary barrier layer capacitors with a simpler fabrication process and more-stable dielectric characteristics, a modified reduction-reoxidation method was developed. Nb₂O₅-doped (Ba_0.8 Sr_0.2)(Ti_0.9 Zr_0.1)O₃ (BSTZ) was calcined at 1170 °C for complete formation of ABO₃ phases. After calcination CuO was added to BSTZ as a liquid-phase promoter and insulating boundary layer material. The ceramics were sintered in a reducing atmosphere, and then the fired samples were annealed in air to reoxidize the reduced CuO to form insulating layers. The dielectric constant of the fabricated capacitors was decreased for higher annealing temperature, longer annealing time and smaller grain size. The loss tangent of the fabricated capacitors was increased for BSTZ with more CuO added, and was almost unchanged with annealing temperature, annealing time and grain size because of the existence of an insulating boundary layer material (CuO).     

X-ray photoelectron spectroscopy and electrical properties studies of La2O3-doped strontium titanate ceramics prepared by sol-precipitation method

X-ray photoelectron spectroscopy has been used to investigate the existence of Ti³⁺ on the surface of La₂O₃-doped strontium titanate and to determine its surface characteristics. The surfaces, having Sr/Ti ratios significantly varying from the stoichiometric ratio, revealed the presence of carbon and suggested the presence of hydroxyl groups on the surface, whose concentration largely decreased in the bulk. Ti³⁺ species existed as a function of the sintering conditions and were detected on the surface of (La, Sr)TiO₃ sintered in air or in N₂ by natural cooling. These samples had a lower electrical resistivity, especially when sintered in a N₂ atmosphere. The surfaces of air oxidized SrTiO₃ and quenched from high temperature contained no detectable amount of Ti³⁺, resulting in higher resistivity. However, the N₂-sintered samples were dark blue in color and exhibited lower resistivity, semiconductivity, and lower valence oxidation state Ti existed when sintered above 1350°C.     

Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects on the properties of deposited thin films by direct current magnetron sputtering

To obtain a suitable sputtering target for depositing transparent conducting Al-doped ZnO (AZO) films by using direct current (DC) magnetron sputtering, this study investigates the possibility of using atmosphere controlled sintering of Al₂O₃ mixed ZnO powders to prepare highly conductive ceramic AZO targets. Experimental results show that a gas mixture of Ar and CO could produce a sintered target with resistivity in the range of 2.23 × 10^− 4 Ω cm. The fairly low resistivity was mainly achieved by the formation of both aluminum substitution (Al_Zn) and oxygen vacancy (V_O), thus greatly increasing the carrier concentration. Compared to usual air sintered target, the thin film deposited by the Ar + CO sintered target exhibited lower film resistivity and more uniform spatial distribution of resistivity. A film resistivity as low as 6.8 × 10^− 4 Ω cm was obtained under the sputtering conditions of this study.     

Thermoelectric properties of nanostructured Al-substituted ZnO thin films

ZnO:Al nano-polycrystalline thin films were deposited by radio-frequency magnetron sputtering on glass substrates. The analysis of the morphology reveals well-connected whiskers with a preferred c-axis orientation perpendicular to the substrate and a dense columnar grain structure. The as-deposited films exhibited a low electrical resistivity of 1 × 10^− 3 Ω cm. Annealing in air produces an increase of the resistivity by more than three orders of magnitude and an increase in the absolute value of the Seebeck coefficient proportional to the resistivity. Annealing of the as-deposited sample in reducing Ar/H₂ atmosphere leads to a decrease in both the resistivity and the absolute value of the Seebeck coefficient. The change in the electrical transport properties is caused by the absorption and desorption of oxygen. Both resistivity and Seebeck coefficient recover to their initial values during annealing of the air-treated sample in reducing Ar/H₂ atmosphere, indicating a reversible process. The analysis by transmission electron microscopy after annealing reveals a stable columnar grain structure with an increase of the grain size. The increase in grain size is larger when the sample is annealed in reducing rather than in oxidising atmosphere. In summary, the reducing Ar/H₂ atmosphere was found to be advantageous for the thermoelectric properties resulting in a maximum power factor of 0.3 mW/K²m at 800 K.     

Effects of reducing and oxidizing atmospheres on the PTCR characteristics of porous n-BaTiO3 ceramics by adding polyethylene glycol

Donor doped BaTiO₃ (n-BaTiO₃) ceramics were fabricated by adding polyethylene glycol (PEG) at 20 wt %. The effects of reducing and oxidizing atmospheres on the PTCR characteristics of the porous n-BaTiO₃ ceramics were investigated. The PTCR characteristics of the porous n-BaTiO₃ ceramics is strongly affected by chemisorbed oxygen at the grain boundaries and are recovered as the atmosphere is changed from the reducing gas to oxidizing gas. The low room-temperature resistivity of the porous n-BaTiO₃ ceramics in reducing atmospheres may be caused by the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In addition, the high room-temperature resistivity of the porous n-BaTiO₃ ceramics in oxidizing atmospheres may be caused by the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries.     

Effect of Cu doping and oxygen-annealing on the magnetic properties of Nd0.5Sr0.5Mn1−xCuxO3−δ (x = 0.0, 0.01, 0.03, 0.05 and 0.10)

The polycrystalline samples with nominal compositions Nd_0.5Sr_0.5Mn_1−xCu_xO₃ (x = 0.0, 0.01, 0.03, 0.05 and 0.10) have been synthesized with the aim to study the change in the structural, magnetic and electrical properties due to substitution of Cu for Mn and oxygen-annealing. The temperature dependence magnetization measurement shows a charge ordering transition (T_CO) at 240 K in Nd_0.5Sr_0.5Mn0₃ sample. The Neel temperature (T_N) increases with increasing the Cu doping content but changes induced by even 0.1 Cu doping are quite small. As compared to the air sintered samples, higher values of T_N are observed for the oxygen annealed samples. Furthermore, oxygen annealing enhances the magnetization of the samples. It is also found that T_C increases and T_N decreases with increase in the applied magnetic-?eld. The value of resistivity decreases with the increasing Cu content from x = 0.0 to x = 0.03 and afterwards it increases with increasing value of x up to 0.10 for both air and oxygen sintered samples. It is also found that oxygen annealed samples exhibit higher resistivity than the air sintered samples. In the present paper the results are discussed according to the change of magnetic exchange interaction caused by Cu-doping. It is also found that the amount of Mn⁴⁺ appears to be the main variable which in?uences the physical properties.     

Mass transport via grain boundary in Pr-based ZnO varistors and related electrical effects

Mass transport and the grain boundary formation behaviors in Pr, Co-doped ZnO ceramics are strongly affected by firing atmosphere and temperature. Drastic change of grain boundary morphology and mass transport behavior were observed in the specimens sintered between 1350 and 1500°C which is ascribed to the increase of the vaporization contents of zinc oxide and praseodymium oxide with the formation of liquid phase. The penetration of liquid (ZnO–PrO_x) into the grain boundaries of sintered, Co-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1–3 V range and nonlinearity coefficients of 15–58.     

Fabrication of high Tc lead free (1???x)BaTiO3�CxBi0.5K0.5TiO3 positive temperature coefficient of resistivity ceramics using reoxidation method

(1 ?C x)BaTiO₃?xBi_0.5K_0.5TiO₃ (abbreviated as BT?CBKT, where x = 0, 0.1 and 0.2) ceramics were prepared by solid state reaction method. All ceramic samples were sintered in a pure N₂ flow atmosphere, subsequently reoxidized at a temperature range of 800?C1,100 °C in air for several hours. The influences of BKT content and reoxidation on the positive temperature coefficient of resistivity (PTCR) behavior of ceramic samples were investigated. BT?CBKT ceramic samples sintered in N₂ possessed relatively low room temperature resistivity (??_RT) and showed weak PTC effect. Through an appropriate reoxidation, the ceramic samples re-obtained PTC effect of almost three orders of magnitude. With the addition of BKT, the Curie temperature (Tc) was enhanced by ~50 °C than the pure BT ceramics.     

Structural and Electrical Properties of Bismuth Ferrite Ceramics Sintered in Different Atmospheres

Bismuth ferrite (BiFeO₃) ceramics were synthesized by the solid-state reaction method followed by rapid liquid phase sintering. The effect of sintering atmosphere (N₂, air and O₂) on the structure and electrical properties of BiFeO₃ multiferroic ceramics were investigated. XRD analysis revealed that N₂ sintering was effective in reducing impurity phases and improving the crystallization behavior. XPS analysis showed that fewer Fe²⁺ ions but more oxygen vacancies were involved in the N₂ sintered ceramics. The SEM investigations suggested that the grain size of the BiFeO₃ ceramics sintered in nitrogen are larger than those sintered in air and O₂. Electrical measurements revealed that the ceramics sintered in N₂ showed lower leakage current, superior dielectric and ferroelectric properties.     

Sintering and properties of highly donor-doped barium titanate ceramics

The electrical properties of donor (La³⁺) doped BaTiO₃ samples with a donor concentration in the range from 0.3 to 1.5 mol.% of La were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly, the samples were annealed in air at 1100°C. Results show that with the use of a specific sintering profile PTCR ceramics containing an amount of donor dopant >0.3 mol.%, can be prepared. Heavily doped samples which do not exhibit anomalous grain growth show a core shell structure.     

Structural, dielectric and electrical properties of lithium silicate ceramics: a comparative study

Lithium silicate (LS) ceramics were prepared via a solid-state reaction technique. Two ceramics with different molar ratios of Li₂CO₃/SiO₂ (1:1 and 1:2) were prepared for dielectric studies. X-ray diffraction pattern showed that Li₂CO₃/SiO₂ (1:1) ceramic is obtained as single phase composition whereas Li₂CO₃/SiO₂ (1:2) ceramic exhibit a multiphase structure with Li₂SiO₃ as a major phase. Microstructural analysis shows that the compounds have well defined grains separated by grain boundaries. Dielectric and electrical properties of the samples were studied in a wide temperature (30–400 °C) and frequency ranges (100 Hz–5 MHz). From the dielectric studies, we can prescribe Li₂CO₃/SiO₂ (1:2) ceramic as a good dielectric material since it possesses low dielectric loss. Detailed analysis of the impedance data suggests that the conduction of Li₂CO₃/SiO₂ (1:2) ceramic is mainly due to the bulk effects whereas the complex impedance plots of Li₂CO₃/SiO₂ (1:1) ceramic suggests the presence of both bulk as well as grain boundary contributions. The bulk resistance of the samples has been observed to decrease with rise in temperature showing a typical negative temperature coefficient of resistance behavior. The low activation energies of the samples suggest the presence of singly ionized oxygen vacancies in the conduction process.     

Sintering temperature, microstructure and resistivity of polycrystalline Sm0.2Ce0.8O1.9 as SOFC's electrolyte

In this work, near-completely soft-agglomerated Sm_0.2Ce_0.8O_1.9 powders have been prepared. The pellets were formed and sintered at various sintering conditions of temperature and time. It was found that the sintering conditions have significant effects on the pellet resistivity. By the measurements with the DC four-probe method, it was found that the overall resistivity of the polycrystalline Sm_0.2Ce_0.8O_1.9 material sintered at 1500°C increases linearly with the reciprocal of the average grain size. The AC impedance spectroscopy has been used to distinguish the grain resistivity and the apparent grain boundary resistivity. The brick layer microstructural model has been used to provide an estimate of the apparent grain boundary resistivity and to relate the electrical properties to the microstructural parameters. By lowering the sintering temperature to 1100–1200°C, the true grain boundary resistivity was nearly two orders lower than that sintered at 1500°C, and thus the overall resistivity decreases to about 31 ohm-cm at 700°C measurement. This makes the Sm_0.2Ce_0.8O_1.9 material capable of working as SOFC's electrolyte at temperatures lower than 700°C.     

Structure and impedance of ZrO2 doped with Sc2O3 and CeO2

Scandia and ceria doped zirconia samples, with 10 mol% Sc₂O₃ and different content of CeO₂, were synthesized and characterized. The XRD results depict that the sintered samples have a cubic phase structure. However, Raman spectra show that besides the main cubic phase, a secondary phase is also present in the sintered samples. The addition of CeO₂ can raise the content of the cubic phase, but the minor meta-stable tetragonal phase (t′-phase) exists even at the CeO₂ content as high as 10 mol%. The near-UV Raman spectra indicate that the deformed tetragonal structure predominates at the grain boundary. The addition of CeO₂ can reduce the impurity at grain boundary, and no impurity can be found by near-UV Raman spectroscopy at the grain boundary of the samples with high CeO₂ content. The impedance measurements show that with the increase of CeO₂ content, the impedance of grain boundary decreases and the bulk impedance increases. The low impedance of grain boundary can be attributed to the formation of a clean grain boundary upon CeO₂ doping, and the increase of the bulk impedance is due to the blocking effect of the large Ce(IV) ions.     

Effect of sintering temperature on the microstructure and high-frequency magnetic properties of Ni0.467Zn0.07Co0.015Fe0.511O4 ferrite

In this work, an attempt is made to study the effects of sintering temperature on the microstructure and high-frequency (HF) magnetic properties of a nickel zinc ferrite compound of very low ZnO content of Ni_0.467Zn_0.07Co_0.015Fe_0.511O₄ composition. Samples were prepared by a conventional ceramic route and sintered for 2 h at 1150, 1200, 1250, and 1300 °C. It was shown that the higher the sintering temperature the higher the saturation magnetization and the measured initial permeabilities, and the lower was the H _c of the samples. This was related to the increased sintered densities and grain sizes. The magnitudes of the electrical resistivity of the samples sintered at 1300 °C compared to those of the samples sintered at 1150 °C and 1200 °C showed four orders decrease. This is thought to be due to the grain-size increase and possibly the formation of higher Fe²⁺/Fe³⁺ concentration. The lowest measured quality factor (Q-factor) obtained in the range of 60–210 MHz, corresponds to the samples sintered at 1300 °C. The highest Q-value in the frequency range of 125–210 MHz was obtained for the samples sintered at 1150 °C, which has also shown the highest electrical resistivity.     

Microstructural and electrical properties of sintered tungsten trioxide

Tungsten trioxide sintered wafers were prepared from WO₃ powder obtained when ammonium paratungstate is decomposed in air at moderate temperature. Two wafer series of five samples were sintered under the same conditions in the temperature range 600–1000 °C. One of these wafers series was submitted to a subsequent annealing at 700 °C under a hydrogen atmosphere. All samples were characterized at room temperature by X-ray diffraction and electrical measurements. X-ray spectra show that WO₃ ceramic presents a mixture of the triclinic and monoclinic phases before the reduction process. After the reduction process, WO₂ and four hydrogen tungsten bronze phases are present in wafers. Capacitance measurements showed that the samples submitted only to the sintering process changed the dielectric constant with the frequency according to the Debye model. The reduced WO₃ shows a semiconductor behavior, as determined by electrical resistivity measurements.     

HgO as an internal oxygen source in Y1Ba2Cu3O7-δ

This paper reports an alternative technique for preparing Y₁Ba₂Cu₃O_7-δ with uniform oxygen stoichiometry which eliminates the need for flowing oxygen during sintering. Our studies show that when HgO is added to the YBaCuO system it provides an abundant source of oxygen during sintering. Mercury diffuses out of the system leaving the crystal structure intact. We have thus been able to obtain a T _c (zero resistivity) of 90 K consistently in HgO doped YBaCuO specimens and sintered in air. An optimum value of HgO leads to a strongly coupled grain material. Resistivity, X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility data are reported. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigations of the electrical property, diffuse reflectance and ESR spectra of the La-(Fe,Mn)-codoped PTCR BaTiO3 annealed in reducing atmosphere

Electrical properties of La-(Fe,Mn)-codoped positive temperature coefficient of resistivity (PTCR) BaTiO₃ ceramics were studied by combining their diffuse reflectance measurements and electron spin resonance (ESR) spectroscopy. La-(Fe,Mn)-codoped samples showed high durability to reducing atmosphere. It is assumed that Fe and Mn ions segregated in the grain boundary contribute to the density of surface acceptor states, meanwhile localizing electrons in a form of Ti³⁺ and stabilizing the chemisorbed oxygens through La³⁺-Mn^3+,4+ or La³⁺-Fe³⁺ pairs. In addition, ESR signals of Fe³⁺ in annealed samples was intensified above Curie temperature (T_c), indicating that Fe ions still maintained its high valence states (Fe³⁺) in the grain boundary even after annealing in reducing atmosphere.     

Effect of grain size and Cu-rich phase on the electric properties of CaCu3Ti4O12 ceramics

The CaCu₃Ti₄O₁₂ ceramics were prepared by the traditional solid-state reaction method under different sintering conditions. The XRD patterns show that crystal structures of the samples are basically single-phase pseudo-cubic, except little second phases of CuO and Cu₂O in the samples sintered in air at 1050 and 1100 °C, respectively, for 12 h. The SEM results indicate that the pellet sintered at 1100 °C for 12 h possess larger grain size and more Cu-rich phases at the grain boundaries than the pellet sintered at 1050 °C for 12 h. It is interesting that the pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h shows smaller grain size (~1 μm) and no Cu-rich phases due to the higher pressure during the sintering process. The results show that the grain size has a reverse effect on the values of the permittivity and the values of breakdown electric field (E _b) and nonlinear coefficient. The pellet sintered at 1100 °C for 12 h exhibits a higher permittivity, but with a lower breakdown electric field (E _b) and a lower nonlinear coefficient due to larger grain size. The pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h exhibits a lower permittivity, but with a higher breakdown electric field (E _b) and a higher nonlinear coefficient due to smaller grain size. The Cu-rich phases at grain boundaries can raise the resistance of the grain boundary leading to the lower dielectric loss tangent, which has been supported by the results of impedance spectroscopy analysis.