PTCR effect in donor doped barium titanate: review of compositions,microstructures, processing and properties

Abstract

Abstract

Barium titanate is widely used in the fabrication of thermistors with a positive temperature coefficient of resistivity (PTCR). The resistivity can increase by several orders of magnitude near the phase transition temperature T_c for the ferroelectric tetragonal to the paraelectric cubic phase transformation. There is general agreement that the anomaly in the change of electrical resistivity of donor doped BaTiO₃ around T_c is due to the grain boundary effect. The Heywang–Jonker model and other mechanisms involving the nature of the electrical barrier formed across the grain boundaries of polycrystalline BaTiO₃ are reviewed. The compositional effect on BaTiO₃ based PTCR properties is listed and discussed. The influences of manufacturing methods under different stages including the initial doping methods, sample forming methods and final heat treatments on PTCR properties are compared. The complex interrelationships between compositions, microstructures, processing and PTCR characteristics are well discussed.     

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm⁻².     

Temperature induced evolution of structure/microstructure parameters and their correlations with electric/magnetic properties of nanocrystalline Nickel ferrite

Nickel ferrite nanoparticles were annealed in order to find dependence of electric/magnetic properties on crystallite size. The following correlations of crystallite size with physical parameters were found: (a) lattice parameter decreases with the increase in size and it reaches value for bulk counterpart approximately for crystallites bigger than 7 nm, (b) ac electrical resistivity at room temperature increases with the increase in crystallite size, (c) for crystallites of ~7 nm or smaller electrical resistivity have maximum value at 50 °C, (d) the real part of permittivity at selected frequency generally decreases with the increase in crystallite size and (e) magnetization increases with the increase in crystallite size. Deviation of stoichiometry, cation polyvalence, and cation redistribution with annealing are the main factors that influence physical properties of Nickel ferrite nanoparticles.     

Effect of crosslinking and field strength on the electrical properties of carbon/polyolefin composites with a large positive temperature coefficient of resistivity

Crosslinking is a promising method for stabilizing the electrical properties of polyolefin composites with a positive temperature coefficient of resistivity (PTCR) for application as self-controlled heaters. An interesting phenomenon is that the measured room temperature resistivity of such materials is independent of applied voltage at low DC fields, but crosses over to display an apparent decrease with the accretion of voltage above a critical value. Measured current vs. time of voltage application has been recorded that shows an equilibrium varying with temperature and voltage. Theoretical approaches to these results are discussed.     

Influence of deposition parameters and annealing treatment on the properties of GZO films grown using rf magnetron sputtering

In this study, transparent conductive films of gallium-doped zinc oxide (GZO) are deposited on soda-lime glass substrates, under varied coating conditions (rf power, sputtering pressure, substrate-to-target distance and deposition time), using radio frequency (rf) magnetron sputtering, at room temperature. The effect of the coating parameters on the structural, morphological, electrical and optical properties of GZO films was studied. This study uses a grey-based Taguchi method, to determine the parameters of the coating process for GZO films, by considering multiple performance characteristics. In the confirmation runs, with grey relational analysis, improvements of 14.1% in the deposition rate, 39.81% in electrical resistivity and 1.38% in visible range transmittance were noted. The influence of annealing treatment, in a vacuum, oxygen, and nitrogen gas atmospheres, at temperatures ranging from 130 to 190 °C, for a period of 1 h, was also investigated. GZO films annealed at 190 °C, in a vacuum, showed the lowest electrical resistivity, at 1.07 × 10⁻³ Ω-cm, with about 85% optical transmittance, in the visible region. It is likely that films grown at lower temperatures (190 °C) could be coated onto polymeric substrates, to produce flexible optoelectronic devices.     

Electrical properties and microstructure of Y-doped BaTiO3 ceramics prepared by high-energy ball-milling

The effects of corn-starch content and high-energy ball-milling time on the microstructure and electrical properties of porous Y-doped (Ba, Sr)TiO₃ samples were investigated. All the (Ba, Sr)TiO₃ samples at room temperature crystallized in the tetragonal structure and the crystal structure was independent of the corn-starch content and ball-milling time. We found that the corn-starch additive and the ball-milling time affected the microstructure and the electrical properties. The (Ba, Sr)TiO₃ samples exhibited a large PTCR jump (>10⁵) due to the high porosity. The PTCR jump of the samples increased with increasing corn-starch content, while it decreased with increasing ball-milling time. In addition, the resistivity increased with increasing corn-starch content, while it decreased with increasing ball-milling time.     

Effect of processing on the positive temperature coefficient of resistivity in lead metaplumbate/polyethylene composites

The positive temperature coefficient of resistivity (PTCR) effect of a barium metaplumbate/polyethylene [(BaPbO₃)/PE] composite with 12 vol% of BaPbO₃ was studied. The composite samples were prepared by hot-pressing a mixture of BaPbO₃ ceramic and high-density polyethylene powders around the melting point of polyethylene. The composites exhibit a pronounced PTCR effect of up to a six-decade increase in resistivity within a narrow range of temperature (∼10°C). The dependences of the room temperature resistivity and the magnitude of the resistivity jump on the pressing and annealing temperature, and the electrical behavior after repeated heating-cooling cycles were investigated. The fracture surfaces of the composite samples were examined in a scanning electron microscope in order to correlate the electrical behavior with the microstructure.     

PTCR Characteristics and Fabrication of Porous, Sb-Doped BaTiO3 Ceramics

Sb-doped BaTiO₃ ceramics containing corn-starch were prepared by sintering at 1350°C for 1 h in air. In this study, the effect of corn-starch on positive temperature coefficient of resistivity (PTCR) characteristics and microstructures of Sb-doped BaTiO₃ ceramics was investigated. It was found that the porosity and pore size increased and the grain size slightly decreased with increasing corn-starch content. XRD results showed the presence of BaTiO₃ peaks only in the Sb-doped BaTiO₃ ceramics with and without corn-starch. The PTCR jump of the Sb-doped BaTiO₃ ceramics with corn-starch was over 10⁶ and 1–2 orders higher than that of samples without corn-starch. The increase in the room-temperature resistivity with increasing corn-starch content was attributed mainly to the increase in the electrical barrier height of grain boundaries and the porosity as well as the partial decrease in the donor concentration of grains and the grain size. It was also noticed that the grain boundary resistivity contributed largely to the total resistivity of the Sb-doped BaTiO₃—corn-starch ceramics.     

Influence of cooling mode on the electrical properties and microstructure of Ba1.022−xSmxTiO3 ceramics sintered in a reducing atmosphere

We investigated the effects of the Sm-dopant content and the cooling rate on the electrical properties and microstructure of Ba_1.022−xSm_xTiO₃ (BST) ceramics, which were sintered at 1200 °C for 30 min in a reducing atmosphere and then reoxidized at 800 °C for 1 h. The results indicated that the cooling rate affected the electrical properties and the microstructure of the BST samples, whose room-temperature resistivity increased with increasing cooling rate. The semiconducting BST ceramics showed a pronounced positive temperature coefficient of resistivity effect, with a resistance jump greater by 3.16 orders of magnitude, along with a low room-temperature resistivity of 157.4 Ω cm at a cooling rate of 4 °C/min. The room-temperature resistivity of the specimen was lower after sintering for 30 min at 1150 °C during cooling.     

Lead free PTCR ceramics and its electrical properties

Lead free positive temperature coefficient of resistivity (PTCR) ceramics based on BaTiO₃–(Bi_1/2Na_1/2)TiO₃ (BT–BNT) solid solution were prepared by a conventional solid state reaction method using high purity reagents. Temperature dependence of the electrical resistivity was measured and a typical PTCR effect was confirmed in the solid solutions up to 30 mol% addition of BNT. Especially 8.8 mol% BNT added sample indicated superior PTCR properties by optimizing the preparation conditions. Thermal electrical properties and crystal structure were investigated in order to elucidate the origin of the PTCR effect in this system. The DSC spectra of this system indicated the transition to cubic from tetragonal crystal structure and the transition temperature increased with BNT addition up to about 175 °C. Also the boundary layer on the grains was observed by Scanning Spreading Resistance Microscope (SSRM) technique. And then the broad signal of electron emission concerning the interface states was observed using Isothermal Capacitance Transient Spectroscopy (ICTS) technique. In our study, it was found that the PTCR ceramics in BT–BNT system has extremely similar properties to the conventional lead contained one.     

Electrical resistivity of porcelain bodies with natural zeolite addition

In this study, the effect of natural zeolite addition on the electrical properties of porcelain bodies was investigated. Clinoptilolite, which is a type of natural zeolite, was added partially or fully in replacement of quartz in selected electro-porcelain compositions. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1200 and 1250 °C with a period of 60 min. The electrical resistance measurements of samples were performed at 50, 200, 400 and 600 °C. It was shown that the resistivity of samples increased at 50 °C temperature after zeolite addition, while it was decreasing after zeolite addition at higher temperatures. At the same time, it was recognized that the resistivity of samples depends on sintering temperature. The activation energy of electrical resistivity of samples was found to be in the range of 0.79–0.87 eV.     

Instability of the Characteristics of the Positive Temperature Coefficient of Resistivity in High-Curie-Point Barium-Lead Titanate Ceramics and Their Grain Structures

A significant isothermal increase in resistivity with time has been observed in high-Curie-point barium-lead titanate ceramics in the temperature region of the Curie points, giving rise to an instability of the characteristics of the positive temperature coefficient of resistivity (PTCR). The isothermal increase in resistivity (ρ) with time (t) observed at various temperatures can be expressed by the equation log ρ=A+B√t, where A and B are both temperature-dependent constants. The temperature dependence of A gave the usual PTCR characteristics because A is log ρ at t=0, while the temperature dependence of B was found to show a characteristic curve with a peak around the Curie point. Observation by SEM revealed a considerable difference between the grain structure of a sample showing a significant resistivity change and that of a sample showing little resistivity change.     

Experimental electrical resistivity of liquid carbon in the temperature range from 4800 to ∼20,000 K

Specimens of high density pyrolytic graphite (2.2 g/cm³) were placed inside thick-walled sapphire tubes and heated over several microseconds by an electric current of 68 kA. The electrical resistivity of the liquid carbon was measured in a constant volume heating process. The transition of liquid carbon from semi-metal properties (resistivity decreasing with increase of input energy) to metal-like behavior (resistivity increasing with increase of input energy) was obtained at a high input energy (25-75 kJ/g) and at a high, but not measured, pressure. The transition temperature, T, was roughly estimated through the C_V value (heat capacity under constant volume). The relationship between the density and the transition temperature is as follows: for 1.88 g/cm³ density, the transition temperature T = 6300 K, for 1.76 g/cm³, T = 10,100 K, and for 1.1 g/cm³, T = 13,500 K. The estimated temperature at the maximum input energy (75 kJ/g) for liquid metal-like carbon (just before the destruction of the sapphire tube) is 23,000 K, with a corresponding measured electrical resistivity of 3000 μΩ cm.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Effect of sintering temperature on magnetic and electrical properties of nano-sized Co2W hexaferrites

Nano-sized and single phase W-type hexaferrite (BaCo₂Fe₁₆O₂₇) powders synthesized by sol-gel autocombustion method have been investigated. The samples were sintered in a temperature range of 1000-1200 °C for 5 h. The thermal decomposition behavior of as-prepared powder was studied by thermogravimetry (TG), differential thermal analysis (DTA) and infrared spectra (IR). The structural and magnetic properties of powders were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The grain size calculated by Scherer equation was found in the range of 36-47 nm which is small enough to obtain a suitable signal-to-noise ratio in high density recording media. Hysteresis loop measurements show that the coercivity values lie in the range of 210.61-1602.6 Oe with increasing sintering temperature. Magnetization studies show a significant increase in the values from 15 to 22 emu/g. The dc electrical resistivity is observed to decrease up to a certain value as the temperature increases and then rises at higher temperature.     

Positive Temperature Coefficient of Resistivity in Ba1-xSrxPb1+yO3-8 Ceramics

The positive temperature coefficient of resistivity (PTCR) effect in Ba_1-xSr_xPb_1+yO_3-s ceramics is systematically studied. The influence of the preparation conditions on the PTCR properties is experimentally tested. The PTCR effect in metallic-conducting BaPb_1+yO₃ is confirmed around 700°C. The temperature where the PTCR effect starts can be shifted to a higher temperature range by substituting strontium for the A-site barium. By the enhancement of the sintering, the magnitude of the PTCR effect was increased and the resisitivity was reduced. In addition to Pb(IV) in the perovskite structure, Pb(II) is detected at the grain boundary in the sintered body.     

Influence of Potassium on Preparation and Performance of PTC Resistors

Different amounts of K₂CO₃ were added to (Ba,Sr)TiO₃-based PTCR (positive temperature coefficient of resistance) ceramics to investigate their influence on the microstructural and electrical properties. Experimental results showed that the incorporation of K acted as an A-site acceptor-type dopant. In addition to enhancing discontinuous grain growth, the increase of K₂CO₃ was found to raise the room-temperature resistivity which was dominated by grain-boundary resistance rather than grain resistance. By adjusting to a suitable amount of donor dopant, the inherent contamination of K in raw material can be compensated to achieve a high-quality PTC resistor.     

Synthesis, formation and characterization of ZnTiO3 ceramics

Zinc titanate (ZnTiO₃) powders of perovskite structure were synthesized by conventional solid state reaction using metal oxides. Powders of ZnO and TiO₂ in a molar ratio of 1:1 were mixed in a ball mill and then heated at temperatures from 700 to 1000 °C for various time periods in air. The crystallization temperature of ZnTiO₃ powder was 820 °C, activation energy for crystallization was 327.14 kJ/mol and for grain growth was 48.84 kJ/mol. A transition point was observed when the electrical resistivity was measured versus temperature. Like some ferroelectric materials, a PTCR behavior above the transition temperature was observed with Curie temperature of 5 °C.     

Yttria-stabilized zirconia closed end tubes prepared by electrophoretic deposition

The electrophoretic deposition technique was used for the preparation of ZrO₂:8 mol% Y₂O₃ (yttria-stabilized zirconia, 8YSZ) closed end tubes for application in high temperature oxygen sensing devices. The 8YSZ ceramic suspensions with different average particle sizes were investigated looking for the best conditions for electrophoretic deposition of thin wall closed end ceramic tubes. High deposition rate of the ceramic particles onto graphite were obtained with isopropanol as solvent and 4-hydroxybenzoic acid as dispersant, with good surface quality of the deposited layer. The green tubes were dried and sintered at 1500 °C, and their properties were analyzed by X-ray diffraction for determination of the structural phases, scanning probe microscopy for observation of grain morphology, and impedance spectroscopy for evaluation of the oxide ion electrical resistivity. Pt/YSZ tube/Pt electrochemical cells were assembled for exposure to oxygen in the 60-650 ppm range using an electrochemical YSZ oxygen pump and sensor system. The signal response of the electrophoretic deposited sensor was similar to the response of the sensor of the oxygen pump. Several thin wall 4 mm diameter × 30 mm length closed end tubes may be obtained in a single operation, showing the ability of this technique for processing large quantities of tubular solid electrolytes with electrical properties suitable for use in high temperature devices.     

钡钛比对钛酸钡陶瓷PTCR性能的影响

钡钛比是影响钛酸钡陶瓷PTCR性能的重要因素，采用固相反应法制备了不同钡钛比的样品，研究了钡钛比对钛酸钡陶瓷的显微结构和性能的影响。实验表明，轻微钛过量的系统具有较好的PTCR特性和较低的室温电阻率，与此相反，过量钡的存在将对烧结和半导化产生不利影响。并且钛过量的系统中过量的钛应该在一次配料中加入，否则由于钛在晶界上过量的偏析将导致难于烧结和过高的电阻。在实验的基础上对钛酸钡基PTCR陶瓷表面态的本质进行了讨论，认为受主表面态是VBa”和吸附O^2-共同作用的结果。