Fabrication and characterization of a multilayer PTCR thermistor

A method has been developed to decrease the room temperature resistance of a PTCR barium titanate ceramic by introducing internal electrodes in a configuration analogous to multilayer capacitors. A multilayer thermistor containing four (n=4) tape cast layers of PTCR barium titanate was fabricated with internal electrodes. A reference sample of the same PTCR barium titanate without internal electrodes was fabricated with the same external dimensions as those of the multilayer thermistor. As predicted by theory, it was found that the resistance of the multilayer thermistor is approximately 16 (n²=16) times smaller than the test specimen with little change in the PTCR effect of the multilayer thermistor.     

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.     

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.     

Effect of Microstructure on the PTCR Effect in Semiconducting Barium Titanate Ceramics

The microstructural influence on the PTCR effect in semiconducting barium titanate ceramics was studied and a method for preparing the ceramic bodies exhibiting a PTCR effect of more than seven orders of magnitude was established. Commercial barium titanyl oxalate was used as a starting material and Sb₂O₃ was added as a doping substance. The average grain sizes of the ceramic bodies prepared were 2 to 5 μm over a sintering range of 60 to 92%, to examine in detail the microstructural influence on the PTCR effect. No extra element, such as Mn or Cr, was added to develop the PTCR effect in the present PTCR materials.     

PTCR Characteristics in Barium Titanate Ceramics with Curie Points Between 60° and 360°C

PTCR characteristics in porous semiconducting barium titanate ceramics with Curie points from 60° to 360°C were investigated. The magnitude of the PTCR effect in these cerumics decreases self-onsistently with increasing Curie point within this temperature range. A PTCR efSect of more than 4 orders of magnitude was ahserved, for a Ba_0.44Pb_0.6TiO₃ ceramic with a Curie point of 360°C .     

Piezoresistivity in PTCR Barium Titanate Ceramics: II, Mathematical Modeling

A modified grain boundary potential barrier model for positive temperature coefficient of resistance (PTCR) barium titanate is developed. It is based on Heywang's double Schottky barrier model, together with Devonshire's thermodynamic phenomenology of polarization behavior. Modifications are made to address the nonlinearity of the dielectric response with respect to electric field in the depletion region adjacent to the grain boundary, and the circular dependence of these quantities with the height of the barrier. Piezoresistivities are calculated for various PTCR compositions and sintering conditions, and these are compared to experimental results from the preceding paper.     

Processing of Laminated Barium Titanate Structures for Stress-Sensing Applications

High-purity, chemically prepared barium titanate was doped with 0.3 at.% lanthanum and, in some cases, counterdoped with 0.04 at.% manganese by precipitation. Using standard techniques, tape-cast sheets of the positive temperature coefficient of resistance (PTCR) compositions as well as the base insulating powder were made. A multilayer sensor was laminated and sintered, with upper and lower PTCR regions separated by an insulating BaTiO₃ region. The sensor was loaded using a four-point bend test configuration. The change in resistivity with increased load was observed to be linear, with positive and negative piezo-resistive coefficients for the PTCR layers under compression and tension, respectively. These trends were in general agreement with a stress-dependent permittivity and the Heywang model of grain-boundary potential barriers. Piezoresistivity values were derived from the resistivity-stress slope; these were examined as a function of temperature and sensor geometry.     

Additive manufacturing of barium titanate based ceramic heaters with positive temperature coefficient of resistance (PTCR)

Lanthanum/manganese doped barium titanate (BT) based PTCR functional heater elements/structures were fabricated with desirable electrical properties for the first time using Additive Manufacturing (AM). 3D printed components of varying size and shape and prototype honeycomb lattices with high density were achieved through AM. Aqueous, less organic containing (2.5 wt% additives versus 10–30 wt% added typically), eco-friendly ink formulations were developed with suitable rheological properties for 3D printing. For BT prints, the sintered densities of the 3D ceramic parts were found to be >99% TD, highest reported value so far. The microstructure, electrical properties and heating characteristics of the printed PTCR components were studied in detail and their thermal stability evaluated using infrared imaging and benchmarked against commercial PTCR heating element. The heating behaviour of the solid and porous 3D printed components was demonstrated to be similar, paving the way for light weight (?47% reduction in weight) heaters suitable for automotive/aerospace applications and less materials wastage during device fabrication.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Spontaneous Polarization Screening Effect and Trap-State Density at Grain Boundaries of Semiconducting Barium Titanate Ceramics

The grain-boundary trap-state density and the polarization screening effect were studied for a series of semiconducting PTCR barium titanate ceramic samples with different manganese (Mn) additives and different thermal treatments. The grain-boundary resistance and capacitance data were measured by the ac complex impedance method. The grain-boundary data obtained were analyzed using a simple double-depletion-layer model and an absolute-zero-temperature approach for the Fermi distribution function for the boundary trap states. The energy density distributions of the boundary trap states were found to be V-shaped for the energy range studied, from 0.35 to 1.35 eV, as measured from the conduction band downward. The "neutral" Fermi level at the grain boundary is taken as 1.35 eV and the bulk Fermi level is taken as 0.15 eV from the conduction band. For the samples without the Mn additive, the PTCR effect is controlled by the trap densities located near 0.35 eV. The trap centers are believed to be barium and oxygen vacancies, or chemisorped oxygens. For the samples with Mn additives, the trap densities increase dramatically near 1.35 eV and play a dominant role in the PTCR effect. These trap centers are believed to be Mn⁴⁺ ions at the titanium sites. The charge-compensation effect of spontaneous polarizations on the trap charges was found to be linearly proportional to the total amount of trap charges at that temperature.     

Formation of Potential Barrier Related to Grain-Boundary Character in Semiconducting Barium Titanate

Resistance–temperature ( R – T ) characteristics were measured directly at single-grain boundaries in 0.1-mol%-niobium-doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain-boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low-Σ boundaries exhibit a similar grain-boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R – T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain-boundary character is likely to affect the diffusivity of O²⁻ ions and, hence, is crucial to the formation of the potential barrier.     

添加剂对BaTiO_3基PTC陶瓷性能的影响

本文研究了Ｎｂ_２Ｏ_５、 Ｙ（ＮＯ_３）_３、 ＴｉＯ_２、及 Ａｌ_２Ｏ_３、 ＳｉＯ_２对 ＢａＴｉＯ_３半导体陶瓷性能的影响。结果表明室温电阻率的波动随不同施主掺杂物量的改变而不同。Ｙ（ＮＯ_３）_３在较宽范围内变化时,试样的室温电阻率能保持较好的稳定性。同时对ＴｉＯ_２及Ａｌ_２Ｏ_３、ＳｉＯ_２都能提高ＰＴＣ效应的机理进行了解释。     

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.     

Effect of Soaking Time on the Temperature Coefficient of Resistivity of Semiconducting Barium Titanate PTCR Ceramics

The influence of soaking time at 1200°C on the temperature coefficient of resistivity (TCR) of semiconducting barium titanate ceramics has been investigated. It is found that soaking duration has influence on the room-temperature resistivity, the maximum resistivity, the temperature ( T _max) at which the maximum resistivity appears, and TCR of the positive temperature coefficient of resistivity (PTCR) ceramics. The increased room-temperature resistivity with the increase of soaking time is contributed mainly by the increase of grain boundary resistivity examined by the complex-plane impedance method. The obtained surface acceptor density ( N _s) by capacitance–voltage measurement is found to increase with the soaking duration. The higher N _s contributes to higher built-in potential and results in lower T _max and higher maximum resistivity. Therefore, the increased surface acceptor density from increasing the soaking time at 1200°C increases the temperature coefficient of resistivity of PTCR ceramics.     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

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.     

纳米钛酸钡的掺杂对陶瓷性能的影响

采用掺杂纳米级钛酸钡和碳酸锰的方法,观察了掺杂不同量纳米级钛酸钡和碳酸锰后的钛酸钡坯片烧结所得陶瓷表面的显微组织形貌的变化,研究了单一掺杂纳米级钛酸钡、单一掺杂碳酸锰、复合掺杂碳酸锰+纳米级钛酸钡对陶瓷制品晶粒尺寸与介电性能的影响。结果表明：掺杂纳米级钛酸钡对钛酸钡陶瓷制品的介电性能有显著的提升,但是随着掺杂量的进一步增加,其介电性能的变化不大;掺杂碳酸锰对钛酸钡陶瓷晶粒的细化效果优于掺杂纳米级钛酸钡的效果;复合掺杂1%（质量分数,下同）碳酸锰+1%纳米级钛酸钡所得陶瓷的致密性高于单一掺杂1%碳酸锰的效果。     

Influence of Stoichiometry on the PTCR Effect in Porous Barium Titanate Ceramics

Porous semiconducting barium titan ate ceramics with small grain sizes (2 to 5 tun) exhibit large PTCR effects (>7 orders of magnitude). The magnitude of PTCR in the ceramic bodies is significantly affected by the grain structure. This study shows the influences of Stoichiometry on both the magnitude of PTCR and resultant grain structures in the sintered bodies .     

A New Phenomenon in Barium-Lead Titanate Ceramics with Positive Temperature Coefficients of Resistivity

A curious resistivity anomaly in high-Curie-point barium-lead titanate materials with positive temperature coefficients of resistivity (PTCR) has been observed just below the Curie point (∼420°C), besides the normal PTCR anomaly. The additional resistivity anomaly was observed in the resistivity-temperature characteristics, especially on cooling, and shewed a significant dependence on the cooling rate.     

空心钛酸钡的制备及其形成机理

以二氧化钛（TiO₂）和八水合氢氧化钡[Ba（OH）₂·8H₂O]为原料,采用水热合成法,成功制备出具有四方相晶型和空心形貌结构的钛酸钡粉体。利用透射式电子显微镜（TEM）和X射线衍射（XRD）对样品的形貌结构和晶型做了表征,通过研究反应过程中钛酸钡粉体的结构和晶型随时间的变化过程,对空心钛酸钡的形成机理做了详细的探讨,并考察了不同反应温度和钡浓度对钛酸钡粉体结晶和生长的影响。研究结果表明：反应温度和钡浓度会影响钛酸钡粉体的晶型和形貌,并以此提出了一种空心钛酸钡的形成机理;当反应温度为180 ℃、钡浓度为3.0 mmol/L时,能获得粒径分布均匀的四方相晶型的空心钛酸钡粉体。