Piezoresistivity in Semiconducting Positive Temperature Coefficient Ceramics

The experimental and theoretical aspects of piezoresistivity in semiconducting, positive temperature coefficient (PTC) of resistivity ceramics will be reviewed. Emphasis will be placed on their potential application in sensor technology. Future material and modeling challenges are highlighted.     

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.     

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.     

Lanthanum-Magnesium and Lanthanum-Manganese Donor-Acceptor-Codoped Semiconducting Barium Titanate

BaTiO₃ powder doped with La donor and codoped with Mn or Mg acceptor was sintered at 1350°C/1 h in air. For Ladoped BaTiO₃, the room-temperature resistivity decreased to a minimum at [La³⁺] ∼ 0.15 mol%. For La-Mn-codoped BaTiO₃, the minimum resistivity occurred at [La³⁺] - 2[Mn²⁺] ∼ 0.15 mol%. When the ceramic was changed to a fine-grained insulator by high donor doping ([La³⁺] >0.15 mol%), its semiconductivity was restored, and the relatively homogeneous, coarse-grained microstructure recurred by codoping with either Mg or Mn acceptor, with the transition at [La³⁺] - 2[Mg²⁺] = 0.15 mol% or [La³⁺] - 2[Mn²⁺] = 0.15 mol%. The analogy of a compensation effect between La-Mn- and La-Mg-codoped BaTiO₃ suggested that Mn acceptor added to BaTiO₃ exists as Mn²⁺ ion in the bulk grain region; its influence on the positive temperature coefficient of resistivity behavior is then discussed.     

Positive Temperature Coefficient of Resistivity Effect in Undoped, Atmospherically Reduced Barium Titanate

A sharp positive temperature coefficient of resistivity effect for undoped, fluorinated BaTiO_3−x samples was observed. The qualitative agreement between the behavior of BaTiO_3−x and La-doped BaTiO₃, with respect to the trap density at the boundaries, suggests that in the absence of counterdopants such as Mn, the main origin of surface acceptor states is chemisorbed gases. Impedance analysis indicates that, under the conditions of the present work, fluorine seems not to diffuse in significant amounts into the lattice. The treatment of the fluorinated samples in inert/reducing atmospheres did not markedly decrease the resistivity jump, suggesting that fluorine might be chemisorbed more strongly than oxygen. The composition of two different liquid phases, produced by excess titania and silica additions, did not have an important effect on the resulting properties.     

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.     

Preparation of Semiconducting Barium Titanate by Excimer Laser Irradiation

A drastic drop of the electrical resistivity was observed on a ferroelectric insulator BaTiO₃ surface, after KrFlaser irradiation in air at atmospheric pressure. The temperature dependence of the resistivity was employed to demonstrate semiconducting behavior. Such behavior was seen when the resistance was measured in nitrogen. When measured in air, the semiconducting layer was oxidized and the insulating behavior restored.     

Effect of Acceptors on the Segregation of Donors in Niobium-Doped Barium Titanate Positive Temperature Coefficient Resistors

Small amounts of niobium (donor) doping in barium titanate (BaTiO₃) cause semiconductivity in this material. When the amount of niobium is increased slightly, the BaTiO₃ is converted back to an insulator. This phenomenon is attributed to donor segregation. Such an insulating material can recover its semiconductivity when specific amounts of acceptors are introduced. The space-charge segregation theory explains that such acceptors can prevent the segregation of donors. Impedance/modulus analysis of BaTiO₃ with a slightly increased niobium content and with no or a minimal amount of magnesium (acceptor) show a response of a third resistance–capacitance (RC) network, in addition to the two that have been thought to be caused by the grains and grain boundaries. The response of the third RC network is not observed at greater magnesium concentrations that give a recovery of the semiconductivity. This study suggests that acceptors can inhibit the donor segregation.     

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.     

固相法烧结温度对钛酸钡陶瓷介电性能的影响

研究了固相法烧结温度对钛酸钡陶瓷介电性能的影响.采用固相法在不同温度下烧结钛酸钡陶瓷.结果表明,不同烧结温度对钛酸钡陶瓷晶体结构、微观形貌、介电常数、介电损耗、居里温度等都会产生不同的作用效果.钛酸钡陶瓷只有在最佳烧结温度附近才具有最好的结构和性能,烧结温度过低会使烧结过程不充分,引起过多的点缺陷;而过高的烧结温度也会由于过烧现象的存在而使晶粒与晶界间相互作用出现异常,两种情况都会导致钛酸钡陶瓷介电性能的劣化.     

A Novel Method for the Uniform Incorporation of Grain-Boundary Layer Modifiers in Positive Temperature Coefficient of Resistivity Barium Titanate

The presently developed two-stage process involves diping the prefired porous disks of n -BaTiO₃ in nonaqueous solutions containing Al-buty rate, Ti-isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain-boundary layer (GBL) modifiers (Al₂O₃+ TiO₂+ SiO₂) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrations (< 1%).     

Stabilization and Memory of the Domain Structures in Barium Titanate Ceramics: Microstructural Observation

From microstructural observations, we have shown that the domain structures of BaTiO₃ ceramics are stabilized by aging. Once stabilized, the original domain structures reappear even when the stress conditions during phase transformation are changed. The pinning of the domain band walls as a result of a space charge is suggested to cause the stabilization of domain structures.     

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.     

Dielectric Properties of Fluxed Barium Titanate Ceramics with Zirconia Additions

BaTiO₃ compacts, when fluxed with <2 vol% of a complex borate glass phase, were sintered to near theoretical density at temperatures <1175°C in 2 h. Microstructural analysis showed a uniform grain size <1.0 μm with 0.75 wt% ZrO₂ added to the flux phase as a grain growth inhibitor. TEM analysis revealed a microcrystalline grain-boundary phase with the ZrO₂ resident along the grain boundaries. These samples displayed an essentially flat dielectric profile, low dissipation factors (<2%) over the range 25° to 125°C, a near linear dependence (≅±15%) between 25° and −55°C, and significantly increased voltage stability. X-ray diffraction analyss of these small-grained materials indicated a suppression of the tetragonal structure toward a more cubic modification.     

Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics

Dielectric properties and structural characteristics of BaTiO₃ ceramics are significantly influenced by small addition (2 wt%) of ZrO₂. SEM and TEM observations revealed enhanced microstructural uniformity and retarded grain growth depending on sintering temperature. Above 1320°C, Zr diffusion into the BaTiO₃ lattice resulted in a chemical modification of the tetragonal structure and the development of core–shell grains. Below 1320°C, TEM analysis showed ZrO₂ at the grain boundaries as discrete particles (∼0.03μm). X-ray diffraction analysis revealed a decrease in the axial (c/a) ratio with decreasing grain size. A corresponding decrease in the spontaneous polarization, and twinned domain structures, were also observed in the fine-grained ceramics. These samples also showed a flattened permittivity response with temperature and significantly lower losses.     

Anomalous Grain Growth in Donor-Doped Barium Titanate with Excess Barium Oxide

Grain growth and semiconductivity of donor-doped BaTiO₃ceramics with an excess of BaO and additions of SiO₂or B₂O₃were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO₂or B₂O₃exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO₃seed grains were embedded in a sample of donor-doped BaTiO₃with an excess of BaO (without SiO₂or B₂O₃), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor-doped BaTiO₃and not NGG.     

Laser-Sintered Barium Titanate

Laser sintering of alkoxy-derived ultrafine BaTiO₃ powders was investigated. The temperature increases of the sample with laser irradiation were measured with a thermocouple. It was found that laser irradiation could generate enough heat to sinter ceramics. A slurry was prepared by mixing an alkoxy-derived BaTiO₃ powder, binder additives, solvent, and plasticizer. The slurry was tape cast and dried to give a green sheet. The green sheet was laser sintered and was then characterized by SEM, XRD, and density measurements. The effect of burnout before laser irradiation and the characteristic microstructure of laser-sintered BaTiO₃ are described.     

Effects of Silver on Barium Titanate as a Function of Stoichiometry

This work examines the effects of Ag on stoichiometric and nonstoichiometric BaTiO₃ in terms of the unit cell dimensions, the polycrystalline microstructure, and the dielectric properties. Stoichiometric BaTiO₃ and compositions with 0.5 mol% TiO₂ excess and 0.5 mol% BaO excess were prepared via solid-state synthesis with varying amounts of Ag up to 0.3 mol%. Experimental results indicate that stoichiometry plays a significant role in the solubility of Ag and its effects on the physical properties. Overall, the solubility of Ag was negligible in stoichiometric BaTiO₃. However, compositions with excess TiO₂ stabilized the solubility of Ag as evidenced from changes in the unit cell dimensions and dielectric properties. Based on these data, it is proposed that Ag occupies the A-site in the perovskite structure with an upper limit of Ag solubility of 0.06 mol% Ag in BaTiO₃ with 0.5 mol% excess TiO₂. Dielectric measurements showed that Ag concentrations approaching 0.3 mol% gave rise to an increase in the space charge effect, especially at temperatures above T _C. In both nonstoichiometric compositions, the presence of a liquid Ag phase during thermal processing was found to affect microstructural development and sintering.     

Effect of Green States on Sintering Behavior and Microstructural Evolution of High-Purity Barium Titanate

Compacts prepared from three differently agglomerated powders were studied. Hg-penetration results and SEM observations were employed to compare the uniformity of powder compacts and to investigate the pore-size evolution and the microstructural development during sintering. It was found that the more nonuniform the powder compact, the higher the degree of pore growth in the initial and at the beginning of intermediate stages of sintering. Moreover, a higher sintering temperature and a nonuniform microstructure with larger grains could not be avoided. Microstresses might develop because of the differential shrinkage, but they would be released thereafter via the change of grain morphology. It was observed that the aggregate and pore-boundary separation might not be the primary reason for the initiation of discontinuous grain growth.     

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.