Piezoresistivity in PTCR Barium Titanate Ceramics: I, Experimental Findings

Positive temperature coefficient of resistance (PTCR) barium titanate is the active material in a ceramic sensor which employs piezoresistivity to detect changes in applied stress. High-purity, chemically prepared barium titanate is donor-doped with 0.30 at.% lanthanum, and <0.10 at.% of a transition-metal counterdopant may be added to enhance the PTCR effect. Tape-cast sheets of undoped and PTCR BaTiO₃ are laminated to produce a three-layer "trilaminate"—a sintered structure which has two semiconducting PTCR layers separated by an insulating layer. The trilaminate is stressed in a four-point bend configuration (placing one semiconducting layer completely in tension, the other in compression), and the resistivities for both stress states are measured concurrently as functions of applied stress and temperature. Results are presented for various semiconducting layer compositions and sintering conditions.     

Positive Temperature Coefficient of Resistivity Effect in Highly Donor–Doped Barium Titanate

BaTiO₃ ceramics doped with different La concentrations (0–12 mol%) were prepared by sintering under the reducing conditions of a nitrogen atmosphere containing 1% hydrogen. The critical donor concentration that causes blocking of the exaggerated grain growth was observed to be ∼10 mol% La. The samples, which were semiconducting after sintering under reducing conditions, were subsequently reoxidized by annealing in air to induce the positive temperature coefficient of resistivity (PTCR) effect. After reoxidation at 1150°C a noticeable PTCR effect was observed in the samples doped with La concentrations as high as 2.5 mol%. The room-temperature resistivity after reoxidation was found to increase with increasing donor concentration due to an increase in the thickness of the insulating layers at the grain boundaries. TEM analysis showed that reoxidation of the samples caused precipitation of the Ti-rich compound Ba₆Ti₁₇O₄₀ inside the doped BaTiO₃-matrix grains.     

Origin of the Positive Temperature Coefficient of Resistivity Anomaly in the ZnO-NiO System

An anomalous positive temperature coefficient of resistivity (PTCR) was investigated in the ZnO-NiO system. It was found that the ZnOSS (Zn_0.97Ni_0.03O) and NiOSS (Ni_0.6Zn_0.4O) constituent phases of that system exhibit negative temperature coefficient of resistivity (NTCR) character, while their combination shows a PTCR effect with a maximum at 400°C, which coincides with a large difference in the coefficient of linear thermal expansion between the ZnO_SS and NiO_SS phases at that temperature. On the basis of the brick wall model microstructure, the PTCR anomaly of this system can be explained. The magnitude of the PTCR effect is governed by the difference in resistivity of the two constituent phases at the temperature where the maximum of the PTCR anomaly occurs. The predicted temperature dependence of the resistance, R(T) , of a model microstructure consisting of constituent phases with different grain sizes agrees well with the experimental R(T) of the prepared composite ceramics.     

Interfacial Segregation in Perovskites: IV, Internal Boundary Layer Devices

A proposed model for interfacial segregation in perovskites, with induced heterogeneous defect distributions, is extended here to account for the formation of internal boundary layer devices, such as positive temperature coefficient of resistance (PTCR) thermistors and internal boundary layer capacitors (IBLC). Boundary layer effects in doped BaTiO₃ are attributed to factors which contribute to the formation of highly resistive boundary layers by a segregation-induced shift in donor incorporation and/or acceptor segregation, and the inhibiting action of segregated donors on boundary mobility and grain growth. The distribution of space charges, formed by electron transfer from conductive grains to resistive boundary layers, leads to the formation of impedance barriers in the grain-boundary vicinity. Depending on the grain size, and on relative size and spatial distribution of the space charge layer and the resistive layer, a transition from semiconducting properties to insulating properties may take place. This model accounts for the observed PTCR and IBLC phenomena.     

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.     

Single-Grain Boundaries in PTC Resistors

Thin semiconducting barium titanate ceramic bars consisting of single grains joined together in series have been prepared, and the positive temperature coefficient of resistivity (PTCR) characteristics of strictly single-grain boundaries in the materials were investigated. The resistivity ( R )-temperature ( T ) characteristics obtained for the present samples can be classified into typically three categories: (1) normal type PTCR characteristics, similar to those observed in usual ceramic samples, (2) saw-tooth type PTCR characteristics, characterized by an abrupt increase in resistivity by more than three orders of magnitude at the Curie point, immediately follwoed by a monotonous decrease in it, and (3) flat type R–T characteristics, with substantially little or no resistivity jump. Of these R–T characteristics, normal type PTCR characteristics were the most frequently observed (about 60%; a total of 65 samples were examined). Flat type R–T characteristics were least frequently (about 10%) observed. Single boundaries with these three types of PTCR characteristics exhibited essentially the same ferroelectric capacitance–temperature characteristic; this demonstrates that the temperature dependence of the dielectric constant above the Curie point was not responsible for the PTCR anomalies. Single boundaries with normal and saw-tooth type PTCR characteristics showed significantly nonlinear current-voltage characteristics above the Curie point, which may be interpreted to be caused by a current strongly affected by traps (or surface acceptor states) present at the grain boundaries.     

Use of Succinic Acid to Test the Stability of PTCR Barium Titanate Ceramics under Reducing Conditions

On heating positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramics at 350°C in succinic acid powder, a reducing atmosphere is created as the succinic acid pyrolyzes. Grain boundary resistances, which may give a PTCR effect, are reduced by this treatment, whereas surface layer resistances, which may also give a PTCR effect, are much less affected.     

Characterization of the Firing Schedule for Positive Temperature Coefficient of Resistance BaTiO3

The development of positive temperature coefficient of resistance (PTCR) behavior during the firing procedure of semiconducting BaTiO₃ was characterized. The PTCR properties of BaTiO₃ were shown to be sensitive to the material's microstructure, liquid-phase distribution, and extent of grain-boundary oxidation. The PTCR behavior first became pronounced as the material cooled from the sintering stage at 1350°C to the annealing stage at 1175°C. Within this region, rapid oxidation of the grain boundaries occurred, which resulted in significant formation of charge carrier traps and a potential barrier. The rapid oxidation of the grain boundaries corresponded with the redistribution and solidification of the liquid phases. Once the carrier traps were established, the magnitude and slope of the PTCR jump increased during the annealing and cool-down stages of the firing procedure because of further oxidation of the grain boundaries.     

Influence of Ba/Ti Ratio on the Positive Temperature Coefficient of Resistivity Characteristics of Ca-Doped Semiconducting BaTiO3 Fired in Reducing Atmosphere and Reoxidized in Air

The positive temperature coefficient of resistivity (PTCR) characteristics of donor-doped BaTiO₃ fired in a reducing atmosphere and reoxidized in air are investigated. The result reveals that conventional semiconducting BaTiO₃ ceramics fired in a reducing atmosphere and reoxidized at a low temperature of 800°C in air show minimal PTCR characteristics, as reported earlier; however, Ca-doped BaTiO₃ with compositions in the range of 1.005≤(Ba+Ca+La)/Ti≤1.010 exhibit pronounced PTCR characteristics, even when reoxidized at such a low temperature. The semiconducting BaTiO₃ ceramics with {(Ba+Ca+La)/Ti}=1.005 and Ca-doped to 20 mol% exhibit remarkable PTCR characteristics with a resistivity jump of two orders of magnitude when they have been reoxidized at 800°C after firing in a reducing atmosphere.     

Barrier Layers in Semiconducting Barium Titanate Glass-Ceramics

Methods were developed for the reduction and subsequent oxidation of glass-crystallized barium titanate for obtaining surface and grain-boundary barrier-layer dielectrics, together with the solid state chemistry compatible with glassmelting, crystallization, and the diffusion processes involved. The material obtained can be described as an interconnected dispersion of semiconducting BaTiO₃ crystallites sealed in a mostly glassy silicate matrix. Oxidation of this semiconductor leads to surface layers with very high dielectric strength, up to 10⁶ V/cm, and resistivity of the order of 10¹⁴ω·cm. The properties of compound barriers consisting of a space charge layer and a thin insulating film are also described and conditions leading to glass-ceramic materials having a positive temperature coefficient of resistance (PTCR) by the use of an anionic dopant are discussed.     

Grain Orientation Dependence of the PTCR Effect in Niobium-Doped Barium Titanate

The positive temperature coefficient of resistivity (PTCR) effect is directly measured in single grain boundaries in 0.1-mol%-Nb-doped BaTiO₃ with 1 mm coarse grains. The PTCR effect largely depends on grain boundary structure. Random grain boundaries exhibit the PTCR effect as in polycrystalline samples, but the PTCR effect does not appear in highly coherent boundaries such as small-angle boundaries, twin boundaries, and coincidence site lattice (CSL) boundaries with low Σ values. For Σ= 3 boundaries, the resistance increase above the Curie temperature is a function of deviation angle. A small PTCR effect is observed in Σ= 3 boundaries with a deviation angle of about 9° in contrast with ideal Σ= 3 boundaries and boundaries with a deviation of about 4°.     

Charge-Up Scanning Electron Microscopy Images of Intergranular Phases in Semiconducting BaTiO3 Positive Temperature Coefficient of Resistivity Ceramics

A charge-up image was observed in the intergranular phases of semiconducting BaTiO₃ positive temperature coefficient of resistivity (PTCR) ceramics when the ceramic specimens were heated to the Curie temperature in a scanning electron microscope. Experiments showed that this image was caused by intense secondary electron emission localized in the phases. This charge-up state seemed to be closely related to the PTCR mechanism.     

正温度系数电阻陶瓷复阻抗频谱的计算机模拟

通过建立数学模型,编制了具有不同数量等效电阻-电容(resistor capacitor,RC)电路结构单元的正温度系数电阻(positive temperature coefficient resistance,PTCR)陶瓷的复阻抗频谱的计算机程序.对在还原气氛下烧结后再氧化的高施主掺杂BaTiO3陶瓷的阻抗虚部和电模量虚部随测试频率变化的曲线进行了拟合,结果表明:还原性的Ba1-xLa3 xTi4 1-xTi3 xO3再氧化后形成Ba1-xLa3 Ti4 1-x/4(V""Ti)x/4,在晶界处可能形成了两个具有PTCR效应的RC结构单元.     

Direct Observation of the Double Schottky Barrier in Niobium-Doped Barium Titanate by the Charge-Collection Current Method

Charge-collection (CC) current was measured at a single grain boundary, which exhibited positive temperature coefficient of resistivity (PTCR) effects, in 0.1-mol%-Nb-doped BaTiO₃. The CC current systematically reversed across the grain boundary above the Curie point, which indicated the presence of a double Schottky barrier (DSB) at the grain boundary. In contrast, the CC current was constant across the grain boundary below the Curie temperature. The result obtained from the CC current measurement agrees with the classical DSB model in donor-doped BaTiO₃, indicating the PTCR effects.     

Positive Temperature Coefficient of Electrical Resistivity below 150 K in Barium Strontium Titanate

The resistivity of Ba_{(1– y )(1– x )}Sr _y (1– x )La _x TiO₃ceramics with x = 0.0025 and y = 0.25, 0.5, 0.75, and 0.9 was measured between 50 and 400 K. A resistivity anomaly corresponding to the positive temperature coefficient of electrical resistivity (PTCR) effect was observed for all compositions. The onset temperature decreased from 320 K ( y = 0.25) to 70 K ( y = 0.9). The extent of the PTCR effect was significantly enhanced for the strontium-rich composition and reached ∼8 orders of magnitude for y = 0.9. These results strongly suggested the possibility to fabricate PTCR devices based on (Sr,Ba)TiO₃ ceramics for application at cryogenic temperatures.     

Depletion-layer Dielectric Properties of Positive Temperature Coefficient of Resistance Barium Titanate

For pure and impurity-added positive temperature coefficient of resistance (PTCR) barium titanate ceramic samples, a −11°C shift of the Curie point at the grain-boundary/depletion-layer region was observed. This result is obtained by fitting the PTCR grain-boundary resistance and capacitance data to a theory which combines a double-depletion-layer model with the Devonshire thermodynamic theory of barium titanate. The parameters used in the fitting are obtained from independent experiments. The shift of the Curie point is believed to result from the grain-boundary clamp ing effect near the cubic-tetragonal phase transition point.     

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.     

Lead Titanate Ceramics with Positive Temperature Coefficients of Resistivity

Lead titanate ceramics were successfully made into semiconductors exhibiting anomalous positive temperature coefficients of resistivity (PTCR) about 3 orders of magnitude above the Curie point (480° to 490°C). The PTCR characteristics of the materials prepared were found to be unstable and to show a significant degradation in both room-temperature resistivity and magnitude of the PTCR effect with time. The instability of the PTCR characteristics observed in the present materials is considered to be related to the morphologies of their grain structures.     

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.     

Proposed Phenomenological PTCR Model and Accompanying Phenomenological PTCR Chart

The positive temperature coefficient of resistivity (PTCR) behavior of semiconductive BaTiO₃ is well explained by the Heywang model, which predicts the resistivity behavior above the Curie point based on the acceptor state density at the grain boundaries, the charge carrier density, and the energy gap, E _s, between the conduction band and the acceptor levels. However, the relationship between these parameters and the production parameters (sintering time, composition, and cooling rate) is not well understood. Recently, the present authors have found that E _s can be increased by thorough oxidation. This increase is attributed to a change in the oxidation state of the acceptor. Based on this finding and results from the literature, a phenomenological PTCR model and an accompanying PTCR chart for acceptor–donor-codoped BaTiO₃ are proposed to clarify this relationship. The PTCR chart clarifies that acceptor dopant concentrations, oxidation time, and oxygen partial pressure during oxidation or cooling can be optimized simultaneously to obtain optical PTCR properties.