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.     

Effect of atmosphere on the PTCR properties of BaTiO3 ceramics

The influence of low-temperature annealing, at < 360 °C, in various reducing and oxidizing atmospheres for a series of BaTiO₃ ceramics with a positive temperature coefficient of resistance (PTCR) is discussed. Combined impedance and modulus spectroscopy is used to analyse a.c. impedance data and shows that the total resistance of the sample can be composed of up to three components, dependent on the cooling rate from the sintering temperature. For quickly cooled samples the PTCR response is dominated by an outer shell on individuals grains, whereas for slowly cooled samples the grain boundary resistance dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas the outer-shell grain PTCR effect is relatively insensitive to the reducing atmosphere. It is proposed that the acceptor states responsible for the outer-grain and grain-boundary PTCR effects are predominantly intrinsic metal vacancies, i.e. Ba and/or Ti, and adsorbed oxygen, respectively.     

Effect of partially oxidized Ti powders on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Rutile TiO₂ (a=4.594 å and c=2.958 å) phase was formed on the outer region of Ti powders after oxidation at 600 °C for 1–300 h. Porous (Ba,Sr)TiO₃ ceramics were fabricated by adding partially oxidized Ti powders (4–8 vol %) into (Ba,Sr)TiO₃ powders, and showed excellent positive temperature coefficient of resistivity (PTCR) characteristics after paste-baking treatment at 580 °C in air. The PTCR characteristics of the porous ceramics were mainly attributed to the adsorption of oxygen at the grain boundaries. The microstructure and electrical properties of the porous (Ba,Sr)TiO₃ ceramics containing the partially oxidized Ti powders oxidized at 600 °C for different oxidation times (1–300 h) were investigated.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

TiCl3-doped Ba0.92Ca0.08TiO3 PTCR ceramics with low r.t. resistivity

The influence of TiCl₃ solution on the room temperature (r.t.) resistivity and electrical properties of Ba_0.92Ca_0.08TiO₃ PTCR ceramics was studied. The results indicate that the PTC effect can be improved significantly when an appropriate amount of TiCl₃ in solution is added to the original materials. Some of the doped Ti³⁺ ions segregate at grain boundaries behaving as acceptors by substituting for Ti site or valence varying (from Ti³⁺ to Ti⁴⁺). As a result, the surface charge density N _s) increases and the barrier height at grain boundaries () is enhanced.     

Effect of pore-forming agent on PTCR characteristics of <Emphasis Type="Italic">n</Emphasis>-BaTiO<Subscript>3</Subscript>

Porous n-BaTiO₃ ceramics are synthesized by the addition of pore-forming agent into the (Ba,Sr)TiO₃ powder. From the DTA-TGA analysis for samples containing the PEG and corn-starch, it was found that an exotherm occurred at 262 and 315°C, respectively, weight loss commenced at 165 and 252°C, and was virtually complete by 265 and 472°C, respectively. The porosity of n-BaTiO₃ ceramics increased and the grain size decreased with increasing the pore-forming agent. From the XRD analysis at high angles, all the samples with and without the pore-forming agent at room-temperature exhibit the tetragonal structure. PTCR jump of the samples containing pore-forming agent is 1-2 orders higher than that of sample without the pore-forming agent. It is also found that the development of PTCR characteristic in the porous n-BaTiO₃ ceramics containing pore-forming agent is related to grain boundaries, which basically equals that in ordinary BaTiO₃ without pore-forming agent, from the complex impedance results.     

Electrical properties of polycrystalline PTCR barium titanate

Semiconducting n-type barium titanate with a positive temperature coefficient of resistance (PTCR) has been made by doping BaTiO₃ with 0.4 mol% Ho₂O₃. The d.c. resistivity, a.c. resistivity (1.2 kHz) and relative permittivity (1.2 kHz) at different temperatures between room temperature and 523 K have been measured. The high relative permittivity and the PTCR effect are attributed to the existence of potential barriers at the grain boundaries as proposed by Heywang. The height of the potential barrier has been calculated as a function of temperature on the basis of the Heywang model, using the measured resistivity versus temperature and relative permittivity versus temperature above the Curie temperature. Several different kinds of electrode have been used to study the effect of the contact on measurements of resistivity and relative permittivity.     

Effect of potato-starch on the microstructure and PTCR characteristics of (Ba,Sr)TiO3

Porous semiconducting (Ba, Sr)TiO₃ ceramics were fabricated by the addition of potato-starch to the (Ba, Sr)TiO₃ powders. The effects of potato-starch on the microstructure and PTCR characteristics of the porous (Ba, Sr)TiO₃ ceramics have been investigated. The porosity largely increased and the grain size slightly decreased with increasing potato-starch content. The crystalline structure of (Ba, Sr)TiO₃ ceramics was independent of the potato-starch content. The (Ba, Sr)TiO₃ ceramics containing potato-starch showed higher PTCR (10⁶) characteristics than that of the (Ba, Sr)TiO₃ ceramics without potato-starch. It was found that the development of PTCR characteristic in the porous (Ba, Sr)TiO₃ ceramics containing various amounts of potato-starch, in the same way as normal BaTiO₃ ceramics without potato-starch, is associated with grain boundaries, from the impedance analysis.     

Effect of the Distribution of Manganese Ions on the Properties of Mn-Doped (Ba,Y)TiO3 PTCR Ceramics

The electrical properties and microstructure of (Ba,Y)TiO₃ PTCR ceramics were studied. The results indicate that the Mn ions increase the intergranular barrier height and produce a high-resistance layer on the grain surface. The temperature-dependent resistances of the grain bulk, surface layer, and grain boundaries, the temperature coefficient of resistance, and the magnitude of the varistor effect were assessed as a function of Mn content.     

Influence of Bi3+ ions in enhancing the magnitude of positive temperature coefficients of resistance in n-BaTiO3 ceramics

Bi³⁺ ions substituting at Ba-sites in a limited concentration range with another donor dopant occupying the Ti-sites in polycrystalline BaTiO₃ enhanced the positive temperature coefficient of resistance (PTCR) by over seven orders of magnitude. These ceramics did not require normal post sinter annealing or a change to an oxygen atmosphere during annealing. These ceramics had low porosities coupled with better stabilities to large applied electric fields and chemically reducing atmospheres. Bi³⁺ ions limited the grain growth to less than 8 m in size, they enhanced the concentration of acceptor-type trap centres at the grain-boundary-layer regions and maintained complete tetragonality at low grain sizes in BaTiO₃ ceramics.     

Influence of the sintering process on the PTCR effect of chip-type Ba1.022–x Sm x TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigated the effects of sintering temperature and reoxidation annealing on the positive temperature coefficient of resistance (PTCR) effect of Ba_1.022–x Sm _x TiO₃ ceramics that were sintered at 1,180–1,260 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h. Results indicated that the room-temperature (RT) resistivity and resistance jump of the ceramics decreased with increasing sintering temperature; moreover, the samples exhibited a remarkable PTCR effect with a resistance jump of 3.3 orders of magnitude and achieved a low RT resistivity of 374.4 Ω cm at a lower sintering temperature. Furthermore, the higher grain-boundary resistivity of the ceramics obtained at a high reoxidation temperature after sintering at low temperature was estimated using an impedance analyzer. In addition, the voltage versus current behavior was investigated in present study.     

BaTiO3基PTCR气敏陶瓷的晶界化学研究

陶瓷化学传感器的敏感机制与晶界的化学缺陷结构在周围微环境下的变异密切相关．晶粒表面和晶界是空位源，研究陶瓷材料在环境气氛中晶界化学缺陷的变化，对于探索陶瓷材料的敏感机制和开拓新应用具有重要意义．本文就BaTiO3基PTCR陶瓷中本征缺陷的生成和分布，缺陷在高温冻结、吸附和脱附过程中的变化，缺陷的重分布及其对晶界势垒的贡献进行深入的讨论．     

Formation and electrophysical properties of Y-containing positive temperature coefficient of resistance ceramics doped by calcium, strontium, and manganese

Formation, microstructure, and electrophysical properties of positive temperature coefficient of resistance ceramics of the systems (Ba_0.996Y_0.004)TiO₃ and (Ba_0.746Ca_0.1Sr_0.15Y_0.004)TiO₃ with manganese as acceptor dopant have been investigated. Is has been shown that manganese ions increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers and grain boundaries, and the value of the temperature coefficient of resistance as a function of the manganese content of positive temperature coefficient of resistance materials have been investigated.     

BaTiO3-based positive temperature coefficient of resistivity ceramics with low resistivities prepared by the oxalate method

Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4 cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm^–1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature.     

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.     

Thermoelectric properties of porous zinc oxide ceramics doped with praseodymium

We evaluated praseodymium (Pr) doping effects on thermoelectric properties of porous zinc oxide (ZnO) ceramics. The low density ceramics composed of ZnO and an additive of Pr (0.5 and 0.1 mol%) oxide were prepared by sintering processes in different atmospheres (air and oxygen), where the additives were Pr₆O₁₁ known for phase transformations and the trioxide Pr₂O₃ with low valence state different from Pr₆O₁₁. Thermoelectric properties of the samples were measured between 313 K and 903 K. At high-temperature around 590 K, some samples showed a maximum of electrical resistivity. We discussed the origin of the maximum on the basis of carrier transport model on gas sensor and varistor. From the results, The maximum appearance was attributable to an interchange of carriers through defects complex closely related with enhanced zinc vacancies acceptor-like in the vicinity of grain boundaries (GB), where the defect complex seemed to be caused by Pr with valence state between 3+ and 3.78+ around GB. The doping Pr into ZnO matrix is expected to be advantageous to improve thermoelectric power.     

Ionic conductivity ageing behaviour of 10 mol.% Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>–1 mol.% CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> ceramics

The long-term ionic conductivity behaviour of samples of zirconia co-doped with 10 mol.% of Sc₂O₃ and 1 mol.% CeO₂ is evaluated in oxidizing and reducing atmospheres at 600 °C. After 3,000 h, the sample kept in reducing atmospheres exhibits 20% loss in the ionic conductivity, while the sample kept in air shows 6% degradation. No phase transitions were observed in the samples after both the ageing studies. The main contribution towards the loss in the ionic conductivity of the sample kept in air comes from grain boundaries; however, for the sample aged in reducing conditions, both grain and grain boundary contribute similarly towards the increase in the total resistivity. This is tentatively explained by the reduction of Ce⁴⁺ cations, dissolved in the fluorite lattice of ZrO₂.     

Effect of secondary-phase segregation on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

Modifications in the positive temperature coefficient in resistance (PTCR) of n-BaTiO₃ ceramics are brought about by specific additives such as Al₂O₃, B₂O₃ or SiO₂, leading to the segregation of secondary phases such as BaAl₆TiO₁₂, BaB₆TiO₁₂ or BaTiSi₃O₉ at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas B₂O₃ addition gave rise to steeper jumps and SiO₂ addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the BaTiO₃ grains. Electron paramagnetic resonance investigations indicated barium vacancies, V_Ba, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process V^X _Ba + e V_Ba. The grain size dependence of the intensity of the V_Ba signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl₆TiO₁₂ gave rise to Al-O^– hole centres whereas no paramagnetic centres corresponding to boron could be detected on B₂O₃ addition. Such secondary phases, forming epitaxial layers over the BaTiO₃ grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as R _g, R _gb and R _{secondary phase}. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in n-BaTiO₃ ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in n-BaTiO₃.     

Enhancement of the Grain Boundary Conductivity in Ceramic Li0.34La0.55TiO3 Electrolytes in a Moisture‐Free Processing Environment

The grain boundary resistivity problem of highly conductive bulk Li_0.34La_0.55TiO₃ perovskite has been investigated by means of impedance spectroscopy and solid‐state NMR of samples processed in controlled atmospheres. The samples were sintered in air, synthetic air, and oxygen, in which the level of moisture varied. A dry atmosphere is critical to obtain dense ceramics with a low grain boundary resistivity. The grain boundary conductivity is five times higher for samples sintered in oxygen atmosphere due to the suppression of Li₂CO₃ secondary phase formation, which is responsible for low lithium ion diffusion at the grain boundary.     

The non-ohmic and dielectric behavior evolution of CaCu3Ti4O12 after heat treatments in oxygen-rich atmosphere

CaCu₃Ti₄O₁₂ (CCTO) ceramics pellets were prepared using the solid-state reaction method, and then they were heat-treated at different temperatures in oxygen-rich atmosphere. The effect of heat treatments on the non-ohmic behaviors and dielectric properties were investigated. EDS analysis results indicate that the percent of oxygen at grain boundaries of CCTO ceramics heat-treated in oxygen-rich atmosphere increases markedly with the rise of temperature and approaches saturation state at about 850 °C. The breakdown voltage and nonlinear coefficient also exhibit an increase trend with the rise of temperature. In addition, the calculated results manifest that the height of Schottky potential barrier is closely related to the oxygen content at the grain boundaries. The permittivity and dielectric loss of samples heat-treated present a relatively intense decrease with the rise of temperature. But the permittivity has a behavior just reverse to the non-ohmic characteristics, which can be explained by the Schottky potential barrier theory.