Temperature Coefficient Of Resistivity Ceramics Research Articles

Spatially Resolved High Voltage Kelvin Probe Force Microscopy: A Novel Avenue for Examining Electrical Phenomena at Nanoscale

AbstractKelvin probe force microscopy (KPFM) is a well‐established scanning probe technique, used to measure surface potential accurately; it has found extensive use in the study of a range of materials phenomena. In its conventional form, KPFM frustratingly precludes imaging samples or scenarios where large surface potential or surface potential gradients exist outside the typical ±10 V window. If the potential regime measurable via KPFM can be expanded, to enable precise and reliable metrology, through a high voltage KPFM (HV‐KPFM) adaptation, it can open up pathways toward a range of novel experiments, where the detection limit of regular KPFM has so far prevented the use of the technique. In this work, HV‐KPFM is realized and shown to be capable of measuring large surface potential and potential gradients with accuracy and precision. The technique is employed to study a range of materials (positive temperature coefficient of resistivity ceramics, charge storage fluoropolymers, and pyroelectrics) where accurate, spatially resolved mapping of surface potential within high voltage regime facilitates novel physical insight. The results demonstrate that HV‐KPFM can be used as an effective tool to fill in existing gaps in surface potential measurements while also opening routes for novel studies in materials physics.

Barium titanate-based thermistors: Past achievements, state of the art, and future perspectives

Barium titanate materials displaying a positive temperature coefficient of resistivity are ubiquitously employed as thermistors in electrical current and voltage control systems, as well as in gas and thermal sensing applications. The positive temperature coefficient of resistivity effect is widely accepted to be a grain boundary-based phenomenon, although detailed studies on grain boundary structure and chemistry, and their influence on the resulting electrical properties, are seriously lacking. Tailoring of the positive temperature coefficient of resistivity electrical characteristics, for specific high-value applications, will require improved understanding and control over grain boundary phenomenon. A comprehensive overview of the development of barium titanate-based positive temperature coefficient of resistivity ceramics is initially presented. We then advance to a discussion on emerging grain boundary characterization techniques, specifically, a stereographic analysis of electron backscatter diffraction data that could assist in enhancing control over BaTiO3 defect chemistry and microstructure, through characterization and subsequent manipulation of the population of grain boundary types. These techniques have great potential for increasing the understanding of the delicate interplay between processing conditions, chemistry, microstructure, and functional electrical properties, and are relevant to the development of advanced, high-performance ceramics and electroceramics in general. Contemporary advancements in the field, such as lead-free positive temperature coefficient of resistivity effect materials and multilayer miniaturized systems based on hypostoichiometric barium compositions, are reviewed. Finally, perspectives on future lines of thermistor research, with a focus on the energy sector, are presented including applications in gas separation and chemical sensing.

Preparation and positive temperature coefficient of resistivity behavior of BaTiO3–BaBiO3–Bi0.5Na0.5TiO3 ceramics

In this study, as a promising material for lead-free positive temperature coefficient of resistivity ceramics, (1 − x − y)BaTiO3 − xBaBiO3 − y(Bi0.5Na0.5)TiO3 (x = 0.001, 0.002, 0.003; y = 0, 0.002, 0.004, 0.006, 0.01) samples were prepared by the conventional mixed oxide method. All the samples were sintered in air atmosphere for 1 h. X-ray diffraction, scanning electron microscope and resistivity-temperature measurement were used to study the crystal structure, microstructure and resistivity-temperature dependence. The results revealed that proper amount of BaBiO3 doping not only decreased the room temperature resistivity, but also increased the Curie temperature. Its optimal doping content was finally chosen as 0.2 mol%. And with an increase of BNT content, both the Curie temperature and the room temperature resistivity increased. When BaBiO3 = 0.2 mol%, BNT = 0.6 mol%, the Curie temperature was 153 °C and the room temperature resistivity was 1512 Ω cm. When the BNT content increased to 1 mol%, the Curie temperature increased to 157 °C but the room temperature resistivity increased sharply to 4.0 × 105 Ω cm.

Fabrication of high Tc BaTiO3–Bi0.5Na0.5TiO3 positive temperature coefficient of resistivity ceramics using the sol–gelmethod

BaTiO3–Bi0.5Na0.5TiO3 (BT–BNT) positive temperature coefficients of resistivity ceramics were successfully prepared by the sol–gel method. The synthesized fine and chemical homogeneous powders could decrease the volatilization of Bi2O3 and Na2O by lowering the sintering temperature. The effect of BNT content on Lg(ρmax/ρmin), the room temperature resistivity, the Curie temperature and the microstructure of the lead-free PTCR ceramics was studied. When BNT ≤4mol%, the PTCR ceramics sintered at 1250°C in air atmosphere displayed excellent PTC effect with a resistivity jump of 3–4 orders of magnitude. The room temperature resistivity and the Curie temperature of the sample with 5mol% BNT were 4.46×106Ωcm and 148°C, respectively. However, when the BNT content exceeded 5mol%, the ceramics displayed dielectric behavior.

Fabrication of high Tc lead free (1 − x)BaTiO3–xBi0.5K0.5TiO3 positive temperature coefficient of resistivity ceramics using reoxidation method

(1 – x)BaTiO3−xBi0.5K0.5TiO3 (abbreviated as BT–BKT, where x = 0, 0.1 and 0.2) ceramics were prepared by solid state reaction method. All ceramic samples were sintered in a pure N2 flow atmosphere, subsequently reoxidized at a temperature range of 800–1,100 °C in air for several hours. The influences of BKT content and reoxidation on the positive temperature coefficient of resistivity (PTCR) behavior of ceramic samples were investigated. BT–BKT ceramic samples sintered in N2 possessed relatively low room temperature resistivity (ρRT) and showed weak PTC effect. Through an appropriate reoxidation, the ceramic samples re-obtained PTC effect of almost three orders of magnitude. With the addition of BKT, the Curie temperature (Tc) was enhanced by ~50 °C than the pure BT ceramics.

Effect of Bi 2O 3 doping methods on the positive temperature coefficient property of Ba 0.999(Bi 0.5Na 0.5) 0.001TiO 3 ceramics

Lead-free positive temperature coefficient of resistivity ceramics based on 0.999BaTiO 3–0.001(Bi 0.5Na 0.5)TiO 3 solid solution were fabricated by the conventional ceramic technique, while Bi 2O 3 was doped directly or after pre-calcining, in the molar ratio of Bi 2O 3:Na 2CO 3:TiO 2 ( G) = 1:1:4, 1.005:1:4, 1.01:1:4, and 1.015:1:4, respectively. There are two synthesizing route, i.e. the materials with G = 1.005:1:4 were pre-calcined at 780 °C firstly and then doped into the basic materials and the materials were directly doped by G = 1.01:1:4 into the starting materials with another holding process at 850 °C for 30 min during co-calcining with BaTiO 3, both of which could obtain samples with the maximum Curie temperature about 144 °C. The latter also possessed comparatively lower room temperature resistance about 22.0 Ω. A further study was deserved to carry out in an attempt to obtain ceramics with higher Curie temperature via more Bi 2O 3-doping.

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba0.89Ca0.08Pb0.03)TiO3 + Y2O3 + MnO + SiO2 have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at tfiring ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.

Synthesis of Y‐Doped BaTiO3–(Bi1/2K1/2)TiO3 Lead‐Free Positive Temperature Coefficient of Resistivity Ceramics and Their PTC Effects

As a lead‐free positive temperature coefficient of resistivity (PTCR) material, (1–x mol%) BaTiO3–x mol% (Bi1/2K1/2) TiO3–y mol% Y2O3–0.5 mol% TiO2 (BT–xBKT–2yY–0.5TiO2) systems were prepared by the conventional solid‐state reaction method. All samples containing <2 mol% BKT sintered in air possessed relatively low room‐temperature resistivity (ρ25) and high positive temperature coefficient (PTC) effect. However, when the BKT content exceeded 2 mol%, the sample was not semiconductive after sintering in air. The effects of sintering schedule on the properties of PTCR ceramics were discussed. The results showed that the optimum composition of BT–1BKT–0.2Y–0.5TiO2, sintered at 1330°C for not‐soaking and then fast quenched in air, achieved rather low ρ25 of 28 Ω·cm and a high jump of resistivity (maximum resistivity [ρmax]/minimum resistivity [ρmin]) of 4.0 orders of magnitude with Tc about 155°C. The ρ25 of the as‐sintered sample could be further reduced to about 10 Ω·cm by annealing in N2 at 450°C for 30 min, accompanied decrease on the PTC effect.

Synthesis and Characterization of La-Doped Nano-BaTiO3 -Based Positive Temperature Coefficient of Resistivity Ceramics

Synthesis and Characterization of La-Doped Nano-BaTiO₃ -Based Positive Temperature Coefficient of Resistivity Ceramics

Annealing effects on the characteristics of high Tc lead-free barium titanate-based positive temperature coefficient of resistivity ceramics

Annealing effects on the positive temperature coefficient of resistivity (PTCR) behavior of (Bi1/2Na1/2)TiO3 and (Bi1/2K1/2)TiO3 modified BaTiO3 semiconducting ceramics are investigated. The annealing treatments result in the occurrence of the PTCR effect and the increased resistivity jump. The impedance spectra reveal that the resistivity of grain interior of annealed sample is little influenced by the annealing process, indicating that the increase in the overall resistivity is entirely a grain-boundary effect. A higher annealing temperature leads to an increase in acceptor state density, potential barrier height and barrier layer width, which contributes to the improved PTCR effect in the annealed samples.

Characterization of manganese-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics using impedance spectroscopy

Small amount of manganese (Mn)-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) positive temperature coefficient of resistivity ceramics are investigated by impedance analyses. The impedance/modulus spectroscopic plots reveal that a third resistance-capacitance (RC) response besides grains and grain boundaries is exhibited in the Mn-free BT-BNT ceramic with 4mol% BNT, but is not observed in the Mn-doped samples. The third RC element can be attributed to a barium vacancy-rich layer in the outer grain region. The evidence of impedance spectroscopy indicates that the highly Bi donor is partially compensated by the Mn acceptor and the predominant charge compensation defect shifts from barium vacancies to electrons with doping small amount of Mn dopant.

High Tc lead-free BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics with electrically heterogeneous structure

A distinct positive temperature coefficient of resistivity effect has been observed in BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) ceramics sintered in a N2 flow with low O2 concentration. With the addition of BNT, the samples exhibit resistivity jumps of ∼103–105 starting at ∼190–210°C. X-ray diffraction results indicate that the BNT phase and BT phase formed a solid solution during sintering. An electrically heterogeneous structure, consisting of the grain interiors, outer grain shells, and grain boundaries, is revealed by the complex impedance analyses. The observed dc resistivity jump is attributed to the rapid resistivity rise in both grain boundaries and grain shells.

Formation and electrophysical properties of Y-containing positive temperature coefficient of resistance ceramics doped by calcium, strontium, and manganese

Formation, microstructure, and electrophysical properties of positive temperature coefficient of resistance ceramics of the systems (Ba 0.996Y 0.004)TiO 3 and (Ba 0.746Ca 0.1Sr 0.15Y 0.004)TiO 3 with manganese as acceptor dopant have been investigated. Is has been shown that manganese ions increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers and grain boundaries, and the value of the temperature coefficient of resistance as a function of the manganese content of positive temperature coefficient of resistance materials have been investigated.

BaTiO3-based positive temperature coefficient of resistivity ceramics with low resistivities prepared by the oxalate method

Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4Ω cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm−1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature.

Effects of thermal cycle and electric field on the domain structure and electrical properties of positive temperature coefficient of resistance ceramics

Effects of thermal cycle and electric field on the domain structure and electrical properties of positive temperature coefficient of resistance ceramics

Calculations and modeling of grain-boundary acceptor states for (Ba,Pb)TiO3 positive temperature coefficient ceramics

The electrical properties of high-Curie-point positive temperature coefficient of resistance ceramics of composition Ba0.497Pb0.5La0.003Ca0.005Ti0.995O3 prepared from commercial BaTiO3 powder were studied. The acceptor-state energy and acceptor-state density were determined from the calculated ρmax vs Tmax plot, where ρmax and Tmax are the resistivity and temperature corresponding to the maximum in resistivity. These two parameters, combined with the effect of nonvanishing spontaneous polarization below the Curie point, were treated quantitatively to demonstrate the dependence of measured resistivity ρdc on the measured permittivity εm. The obtained values of potential barrier height and acceptor-state density based on the procedure above were found to be in excellent agreement with those extracted from the slope in the Arrhenius plots of resistivity versus 1/(Tεm), where T is the absolute temperature. Also, the deviation of the measured dielectric constant εm from the Curie–Weiss law’s prediction was explained by the narrowing of the grain-boundary barrier layers.

Evaluation of deep levels in manganese-doped positive temperature coefficient of resistivity ceramics by isothermal capacitance transient spectroscopy

Evaluation of deep levels in manganese-doped positive temperature coefficient of resistivity ceramics by isothermal capacitance transient spectroscopy

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.

The influence of silicon on microstructure and electrical properties of La-doped BaTiO3 ceramics

BaTiO3 positive temperature coefficient of resistance ceramics were prepared with the general composition (La0.002Ca x Ba0.998−x ) (Ti1.01−y Mn y )O3 ·zSiO2 and sintered in air. The Ca content as well as the amount of doped Mn were varied ranging from 4–20 mol% and 0.02–0.04 mol%, respectively. Up to 4 mol% Si was added using a new method as well as by the classical means as solid silica. In the new method, the desired quantity of Si(OC2O5)4 (Si(OEt)4), contained in dry tetrahydrofurane (THF), was dropped under a slight stream of inert gas into the vigorously stirred aqueous slurry which was manufactured after the calcination process. The addition of Si by this technique was found to result in a homogenization of the microstructure and an improvement in electrical properties. The effect of Si on electrical properties is explained by the influence of the observed second phase on the equilibrium of Ba vacancies. The results of further variation of Ca and Mn contents are presented.

On the Influence of Silicon on Microstructure and Electrical Properties of La-Doped BaTiO<sub>3</sub>-Ceramics

BaTiO3 positive temperature coefficient of resistance ceramics were prepared with the general composition (La0.002Ca x Ba0.998−x) (Ti1.01−yMn y )O3 ·zSiO2 and sintered in air. The Ca content as well as the amount of doped Mn were varied ranging from 4–20 mol% and 0.02–0.04 mol%, respectively. Up to 4 mol% Si was added using a new method as well as by the classical means as solid silica. In the new method, the desired quantity of Si(OC2O5)4 (Si(OEt)4), contained in dry tetrahydrofurane (THF), was dropped under a slight stream of inert gas into the vigorously stirred aqueous slurry which was manufactured after the calcination process. The addition of Si by this technique was found to result in a homogenization of the microstructure and an improvement in electrical properties. The effect of Si on electrical properties is explained by the influence of the observed second phase on the equilibrium of Ba vacancies. The results of further variation of Ca and Mn contents are presented.