PTCR Materials Research Articles

Gdを添加したBaTiO<sub>3</sub>-(Bi<sub>1/2</sub>K<sub>1/2</sub>)TiO<sub>3</sub>系無鉛高温用PTCR材料の作製

Fabrication of high Curie point (Tc) PTCR materials using BaTiO3 (BT)-(Bi1/2K1/2)TiO3 (BKT) ceramics was investigated. Fine powders of BT, which was synthesized by hydrothermal process, were used as starting materials. BT powders, Bi2O3, KHCO3, TiO2 and Gd(NO3)3 (water solution) were mixed with (C2H5O)4Si by wet milling for 24 h. After drying, the powders were pressed into disks, which were diameter of 20 mm and thickness of 2 mm. These disks were sintered at 1130 – 1280 °C for 2 h in air. The samples containing 5 – 7 mol% BKT have the room temperature resistivity lower than 600 Ωcm, PTCR jump higher than 2.4 order of magnitude, and Tc higher than 175 °C. Especially, the sample containing 6 mol% BKT has the room temperature resistivity of 270 Ωcm, PTCR jump of 2.4 order of magnitude, and Tc of 185 °C and it was the excellent high Tc lead-free PTCR materials which was fabricated by firing in air.

The orientational growth of grains in doped BaTiO 3 PTCR materials by microwave sintering

Doped BaTiO 3 (BTO) PTCR materials were sintered by microwave (MW) of 2.45 GHz at 1250 °C for 15 min. For comparison, identical samples ( Ø30 mm×45 mm) were also sintered by conventional (CO) SiMo furnace at 1350 °C for 2 h. The experimental results show that the MW-sintered samples have a much greater axial linear shrinkage rate of 48% vs. the rate for the CO-sintered sample of 26%. This result has not yet been reported in other researcher works. Marked differences were also observed in the microstructure of the two samples by SEM, the grains in the CO-sintered sample showing on ecliptic spherical shape, whereas in MW-sintered samples the grains show an orientational strip-like microstructure. The XRD analysis indicates that the perovskite structure of MW-sintered BTO is tetragonal, whereas the CO-sintered sample is cubic.

Effect of Isovalent Ba-Site Substitutions on the Properties of (Ba1 – x – yMyYx)TiO3(M = Ca, Sr, Pb) PTCR Ceramics

An approach is proposed for fabricating fine-grained, low-resistivity BaTiO3-based PTCR ceramics via partial isovalent substitutions on the Ba site. The grain size of the ceramics thus prepared is shown to decrease as the ratio of ionic radii r(Ba2+)/r(M2+) (M = Ca, Sr, Pb) increases. Isovalent substitutions on the Ba site narrow down the range of donor dopant (yttrium) concentrations in which PTCR materials can be prepared. The experimental results agree well with thermodynamic calculations under the assumption that the materials contain the Y3+Ti3+O3 phase, as suggested by ESR data, which point to the presence of Y3+–Ti3+ associates. Partial calcium, strontium, and lead substitutions on the Ba site reduce the average grain size of PTCR ceramics, which is probably due to the lattice strain arising from the isovalent substitution. Partial replacement of Ba2+ with mixtures of different isovalent elements (e.g., Sr2+ and Pb2+) offers the possibility of obtaining fine-grained, low-resistivity PTCR ceramics, without changing the phase transition temperature.

THE EFFECT OF ISOVALENT SUBSTITUTIONS AND DOPANTS OF 3D-METALS ON THE PROPERTIES OF FERROELECTRICS- SEMICONDUCTORS

Electrophysical properties and microstructure of PTCR ceramics of the system (Ba,Ca,Sr,Y)TiO3 + y%Mn have been investigated. It has been shown that manganese ions increase the potential barrier at grain boundaries and form a high-resistance outer layer in (Ba,Ca,Sr,Y)TiO3 ceramics. The resistance of grains, outer layers and grain boundaries, the values of temperature coefficient of resistance as well as the varistor effect as a function of manganese content of PTCR materials have been investigated.

(Ba, Y)(Ti, Zr, Sn)O3-based PTCR materials

PTCR materials based on the systems (Ba, Y)(Ti1−yZry)O3 and (Ba, Y)(Ti1−ySny)O3 were investigated. It has been found that in the case of partial isovalent substitution of titanium ions by zirconium or tin ions, the grain size passes through a maximum at a concentration of 15 and 5 mol.% respectively. This may be attributed to the different grain growth mechanisms in the presence of a small and large amount of liquid phase. It has been shown that increasing the amount of isovalent substitution narrows the donor dopant concentration range, within which PTCR effect occurs. These results agree with the thermodynamic calculation of the range of existence of solid solutions (Ba, Y)(Ti1−xMy)O3, which were regarded as a result of interaction between three phases with perovskite structure: BaTiO3, YTiO3 and BaMO3 where M = Zr, Sn, and of the presence of complex centers Y3+-Ti3+, as indicated by ESR data.

Effect of BaO and SiO2 addition on PTCR BaTiO3 ceramics

The influence of Ba-excess and liquid phase sintering with SiO2 on the electrical conduction and microstructure in PTCR BaTiO3 has been investigated. Dense (95–96%), small grain (5–10 μm) PTCR materials were obtained in Ba-excess (Ba/Ti = 1.006) BaTiO3. The materials exhibit low room temperature resistivity ρRT (100–102 Ω·cm) and high PTCR respond (more than 5 orders). Solid state sintering was found to inhabit the semiconducting and PTCR behavior in Ba-excess materials. Liquid phase sintering, using SiO2 in the Ba-excess BaTiO3, resulted in low ρRT and significant PTCR response. Through domain observation, interior “Polaron deficient zones” were found in samples which exhibit limited liquid phase sintering, leading to non-uniform directional domains and low charge carrier mobility. Proper control of the SiO2 concentration was found critical for obtaining uniform directional domain microstructures and low ρRT.

Temperature and voltage effects on microstructure and electrical behavior of donor modified BaTiO3

A high temperature NTC resistivity transition (Tc2) near 600°C has been found in donor-modified BaTiO3. Thermal treatment near Tc2 on developed PTCR materials significantly affects the resistivity (lowering the Prt and increasing the resistivity transition) and stabilizes the response with time and cycling. Thermal treatment plus applied voltage (Tc2-Poling) affects both electrical behavior and ferroelectric domain structure in samples which have not been annealed after sintering. Poling results in alignment of the domain structure in direction of the applied voltage, suppressing the resistivity transition, while typical PTCR behavior exists in the anti-poled direction. These properties are dependent on controlling domain structure and grain boundary coherency. It has been found that atmosphere and time during these treatments have little effect on bulk domain alignment and electrical behavior. Other resultant properties include improved aging and voltage-withstand characteristics.

High temperature poling effects on conducting barium titanate ceramics

A high temperature NTC resistivity transition (T c2) near 600°C has been found in donor modified barium titanate (BaTiO3). Thermal treatment around this temperature regime plus applied voltage (T c2-Poling) on previously developed PTCR materials significantly affects both electrical behavior and ferroelectric domain structure in samples which have not been annealed after sintering. This type poling results in alignment of the domain structure in a coherent nature in direction of the applied voltage. Samples with a highly suppressed resistivity transition have been developed within direction of the applied voltage, while typical PTCR behavior exists in the anti-poled direction. Properties are dependent on domain structure orientation and grain boundary coherency developed during T c2-poling. Annealed samples are not affected in the same fashion and do not show enhanced domain alignment due to inhomogeneous grain boundaries which pin the domain structure. Results indicate that conductivity and PTCR behavior is notably dependent on grain boundary and domain structure coherency. Conversion from typical PTCR behavior to one which exhibits minimal dependence on the phase transition and results in a highly conductive cubic state is described in terms of grain boundary/domain alignment.

High Tc positive temperature coefficient resistivity (Pb0.6Sr0.3Ba0.1)TiO3 materials prepared by microwave sintering

PTCR materials with very high critical temperature (Tc=420°C) have been prepared by microwave sintering technique with the composition (Pb0.6Sr0.3Ba0.1)TiO3. Incorparation of SiO2 as sintering aids has lowered the sintering temperature required from 1250°C (10 min) to 1140°C (10 min) and stabilized the electrical properties. The resistivity ratio, and minimum resistivity achieved for these samples were ρmax/ρmin ≈ 102.85 ∼ 103.2 ohm-cm and ρmin ≈ 102 ohm-cm, respectively. The electronic parameters such as donor level (ε d∼0.077 eV), PTCR jump and trap level (ε a∼0.70 eV+ εF) were found to be the better parameters to indicate the perfecness of the densification process than the crystal structures and the microstructure examinations

Valence Change of Mn Ions in BaTiO3‐Based PTCR Materials

The effect of Mn on the PTCR characteristics of La‐doped BaTiO3 ceramics was studied. Acceptor state density was measured from a p‐1/T graph as a function of Mn concentration. The behavior of room‐temperature resistivity with Mn concentration was explained by compensation of acceptors at grain boundaries. The valence state of Mn was analyzed by ESR, and it changed near the Curie temperature of BaTiO3 from 3+ in tetragonal phase to 2+ in cubic phase. Therefore, the number of electron traps increased at the Curie temperature, leading to the great improvement in the PTCR effect. The valence change of Mn ions was explained by vanishing of the Jahn‐Teller effect as the phase changes from tetragonal to cubic.

PTCR Properties of Composites Consisted of Metals with Low Melting Point and Conductive Ceramics

Many metals show the steep increase of their specific resistivity on melting. This phenomenon was applied for producing new PTCR materials in the Pb-TiO2 and Sn-TiO2 systems. The xPb-(100-x)Ti0.99Nb0.01O2 samples with x>15 showed the steep increase of resistivity (PTCR properties) around 327°C, and it was noted that the temperature coefficient of resistivity below the PTCR temperature changes from negative to positive with increasing Pb content. The xSn-(100-x)Ti0.99Nb0.01O2 samples showed PTCR properties around the melting point of Sn (232°C), but PTCR properties are less significant than those observed in the Pb-TiO2 system.

Problems associated with imaging resistive barriers in BaTiO 3 PTC ceramics using the SEM conductive mode

The micro-electrical properties of BaTiO 3 have been studied using the remote electron beam induced current technique. Difficulties associated with applying this technique to low resistance, inhomogeneous ceramics are discussed and illustrated with the example of PTCR materials. At temperatures above the Curie temperature, contrast steps corresponding to resistive interfaces are observed with differing sensitivities according to the imaging conditions used. The image noise decreases and the current sensitivity increases with the sample resistance. Remote electron beam induced current contrast in BaTiO 3, showing the high resistivity transition does not occur uniformly as a function of temperature across all grain boundaries.

Preparation and ac Electrical Response Analysis for (Ba,Pb)TiO3 PTCR Ceramics

For those PTCR materials with higher Pb content, the thermodynamic derivation of increasing values for “inversion temperature” and cation vacancy diffusion rate accounts for the increases in the calculated acceptor‐state densities Ns (cm−2). The influence of frequency‐dependent properties for the dielectric constant on the calculation of acceptor‐state densities is also presented. Applying the concept of suitable value for Ns to obtain a strong PTC effect, (Ba,Pb)TiO3 PTCR ceramics with Curie point Tc over 340°C up to 420°C have been successfully prepared by adopting higher cooling rates.

Improved PTCR Materials Based on Ba1-xPbxTiO3 Systems

Improved PCTR materials based on Ba1-xPbxTiO3 systems have been fabricated by the mixed oxide method. Experimental results proved that PTCR materials with a small grain-sized in the range of 2 to 5 µm and an average density around 95% of theoretical can be produced from the solid state reaction of mixed oxides while rapid firing technique involving extremely short hold times at the highest sintering temperature is applied. The kind of specimens with the Curie temperature around 300°C exhibited the resistivity at room temperature less than 103 ohm-cm and the PTCR effect of 4 orders of magnitude.

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.