Negative Temperature Coefficient Effect Research Articles

Electrothermal Instabilities in Barium-Titanate-Based Ceramics

An electrothermal analysis for barium-titanate-based ceramics is presented, combining the Heywang–Jonker model for the electric resistivity with a heat dissipation mechanism based on natural convection and radiation in a one-dimensional model on the device level with voltage as the control parameter. Both positive-temperature-coefficient (PTC) and negative temperature coefficient (NTC) effects are accounted for through the double Schottky barriers at the grain boundaries of the material. The problem formulated in this way admits uniform and non-uniform multiple-steady-state solutions that do not depend on the external circuit. The numerical bifurcation analysis reveals that the PTC effect gives rise to several multiplicites above the Curie point, whereas the NTC effect is responsible for the thermal runaway (temperature blowup). The thermal runaway phenomenon as a potential thermal shock could be among the possible reasons for the observed thermomechanical failures (delamination fracture). The theoretical results for the NTC regime and the thermal runaway are in agreement with the experimental flash sintering results obtained for barium titanate, and 3% and 8% yttria-stabilized zirconia.

Theoretical Study of the Decomposition Reactions of 2-Vinylfuran.

With the development of new synthetic methods, 2-vinylfuran (V2F) has become a potential renewable biofuel. In this work, the potential energy surfaces for the V2F unimolecular dissociation reaction, the H-addition reaction, and the H-abstraction reaction were constructed at the G4 level. The temperature- and pressure-dependent rate constants for the relevant reactions on the potential energy surfaces were calculated by solving the master equation based on the transition state theory and Rice-Ramsperger-Kassel-Marcus theory. The results show that the rate constant for the intramolecular H-transfer reaction of V2F with H atoms from the C(5) site to the C(4) site to form 2-vinylfuran-3(2H)-carbene, followed by the decomposition to form h145te3o, is the highest. The rate constants for the H-abstraction reaction of V2F with H atoms were the largest at C(6) on the branched chain, followed by C(7), and the rate constants for the H-abstraction reaction at C(3), C(4), and C(5) on the furan ring were not competitive. Negative temperature coefficient effects are observed for the rate constants of the addition reactions of V2F with H atoms at low pressures, with the H-addition rate constant at the C(5) site being the largest. This work not only provides the necessary rate constants for the reaction mechanism of V2F combustion but also provides theoretical guidance for the practical application of furan-based fuels.

Synergistic effect of positive temperature coefficient and thermally conductive micro‐nano fillers on electrical and thermal properties of epoxy resin

AbstractThis study investigates the effect of positive temperature coefficient (PTC) fillers, i.e., barium strontium titanate (BST‐60) and nano hexagonal boron nitride (h‐BN) fillers, on thermal and dielectric insulation properties of epoxy (EP). The electrical resistivity, DC breakdown strength, space charge, thermal conductivity, thermogravimetric analysis, and glass transition temperature (Tg) of EP composites were measured. Results indicate that, below curie temperature, the EP composites containing 0.5 and 1 wt% of BST and 5 wt% of h‐BN have superior electrical resistivity, breakdown strength, and thermal conductivity. However, above curie temperature, the EP composites containing 2 and 2.5 wt% of BST and 5 wt% of h‐BN have superior insulation properties than the pure EP and other composites. While the thermal properties of EP improve with an increase in h‐BN loading to 10 wt%, its dielectric properties are slightly compromised due to the presence of tiny defects in the composite material. It is hypothesized that the BST‐60 suppresses the negative temperature coefficient effect (NTC) of EP by regulating the electrical resistivity above the curie point, whereas nano h‐BN enhances the thermal conductivity of EP by constructing a heat dissipation path. The proposed mechanism for charge dynamics along with heat conduction‐dissipation explains the fillers' behavior at different temperatures.Highlights Analysis of epoxy composites with polydopamine‐modified BST and h‐BN fillers. PTC fillers are used to mitigate the negative temperature effect of epoxy. Optimal fillers wt% improves dielectric and thermal properties. This study shows that optimized epoxy composites can be used for packaging.

Effect of silver doping on the band gap tuning of tungsten oxide thin films for optoelectronic applications

In the cutting-edge world, semiconductor metal oxides usually tend to have a high optical band gap (>3.0 eV), significantly acceptable for potential optoelectronic applications. The present study discusses the synthesize of pristine tungsten trioxide (WO3) and Silver (Ag) doped WO3 (Ag: WO3) thin films onto a glass substrate at 450 °C, with varying concentrations of Ag doping (2, 4, 6, 8 and 10 at.%) using a simple Spray Pyrolysis Technique. Field emission scanning electron microscopy (FESEM) analysis showed the presence of particles in the WO3 and Ag: WO3 materials. The X-ray diffraction (XRD) pattern confirmed that the samples' hexagonal structure remained intact. In addition, Rietveld refinement was used for the samples to study the crystal structure meticulously. Because of the surface plasmon resonance effect, the samples' distinguishing characteristics were visible in their optical nature. For pristine WO3, the experimental band gap was determined to be 3.20 eV, and for varying doping concentrations, it was found to be 3.15 eV–2.90 eV, respectively. Furthermore, the fracture has remained imperceptible at elevated concentrations, resulting in a substantial influence on the optical characteristics of 10% Ag: WO3 thin films. The estimated redox potential for 2% Ag: WO3 shows a considerable influence of the band edge potential of the Conduction Band (CB) and Valance Band (VB). The activation energy was determined using temperature-dependent electrical resistivity and exhibited an ohmic nature. The synthesized material exhibited a negative temperature coefficient (NTC) effect at higher concentrations of doping, suggesting its potential applicability as a thermistor. A comprehensive analysis of this present study indicates that Ag can be a viable candidate for doping on WO3 thin films for use in optoelectronic devices.

Isothermal low-temperature oxidation behavior and kinetics of heavy oil

In this study, the isothermal low-temperature oxidation (LTO) behavior of heavy oil was investigated by thermogravimetry combined with Fourier transform infrared spectroscopy. Furthermore, the kinetics analysis was conducted using general reaction rate models. The findings revealed that the LTO process of heavy oil could be divided into three distinct phases. The first phase, occurring between 50 and 100 °C, exhibited minimal mass loss and low absorbance of gas-phase products. In the second phase (100–250 °C) the produced gas predominantly consisted of saturated hydrocarbons and substances containing C=O groups. Lastly, in the third phase (250–300 °C), the signals of CO2 and CO in the produced gas were significantly higher compared to the 50–250 °C range, indicating a shift in the oxidation mode of heavy oil from oxygenation to bond cleavage for temperatures higher than 250 °C. Kinetics analysis demonstrated an initial increase in reaction order and reaction rate from 50 to 100 °C, followed by a decrease from 100 to 250 °C, and a final increase from 250 to 300 °C. The negative activation energy observed in the second temperature interval was attributed to the negative temperature coefficient effect. This study provided valuable insights into the isothermal LTO reactions of heavy oil.

Preparation and PTC properties of multilayer graphene/CB/HDPE conductive composites with different cross-linking systems

Multi-layer graphene/carbon black (CB)/high density polyethylene (HDPE) conductive composites with different systems were prepared by melt compounding. The composites were subsequently cross-linked and modified using peroxide cross-linking and radiation cross-linking techniques. The effects of these different cross-linking systems on the room temperature resistivity, positive temperature coefficient (PTC) strength, cyclic stability, and crystallinity of the multi-layer graphene composites were investigated. The results demonstrated that cross-linking modification could increase the room temperature resistivity of the composites. Moreover, as the cross-linking degree increased, the PTC strength also increased, and the negative temperature coefficient (NTC) effect disappeared. Within the range of room temperature resistivity less than 1 Ω cm, the maximum recommended usage of peroxide was 0.3 %. Among the composites with the same ratio, the dual cross-linking system exhibited the best electrical properties. The PTC strength increased by one order of magnitude compared to the un-cross-linked composites, and the cyclic stability was optimal. In the PTC strength test, the result yielded 3.4 in three tests.

Numerical Modeling of Chemical Kinetics, Spray Dynamics, and Turbulent Combustion towards Sustainable Aviation

With growing interest in sustainable civil supersonic and hypersonic aviation, there is a need to model the combustion of alternative, sustainable jet fuels. This work presents numerical simulations of several related phenomena, including laminar flames, ignition, and spray flames. Two conventional jet fuels, Jet A and JP-5, and two alternative jet fuels, C1 and C5, are targeted. The laminar burning velocities of these fuels are predicted using skeletal and detailed reaction mechanisms. The ignition delay times are predicted in the context of dual-mode ramjet engines. Large Eddy Simulations (LES) of spray combustion in an aeroengine are carried out to investigate how the different thermodynamic and chemical properties of alternative fuels lead to different emergent behavior. A novel set of thermodynamic correlations are developed for the spray model. The laminar burning velocity predictions are normalized by heat of combustion to reveal a more distinct fuel trend, with C1 burning slowest and C5 fastest. The ignition results highlight the contributions of the Negative Temperature Coefficient (NTC) effect, equivalence ratio, and hydrogen enrichment in determining ignition time scales in dual-mode ramjet engines. The spray results reveal that the volatile alternative jet fuels have short penetration depths and that the flame of the most chemically divergent fuel (C1) stabilizes relatively close to the spray.

Preparation and performance study of a novel conductive geopolymer

A novel conductive geopolymer (GP) based on fly ash (FA) and modified by carbon fiber (CF) was successfully developed in this study. Detailed investigations were conducted on various factors influencing resistivity, such as the type and content of CF, liquid-solid ratio, modulus of alkali activator, and the material’s sensitivity to temperature, pressure, moisture. The research findings highlight that short-cut CF enhances both the electrical conductivity and mechanical properties of GP, along with its sensitivity. Specifically, at an FA and short-cut CF mass fraction of 80% and 1.0% respectively, the resistivity and compressive strength of GP reached 5.58 Ω·m and 51.20 MPa, representing a 99.95% reduction in resistivity and a 40.66% increase in strength compared to pure GP. Additionally, the introduction of CF1 into the modified GP exhibited significant negative temperature coefficient and piezoresistive effects. Furthermore, FA not only enhanced the conductivity but also improved its water resistance of GP. This study provides a creative opportunity for design and fabrication of novel conductive intelligent GP materials.

Study on the low Curie point positive temperature coefficient (PTC) material based on the conductive model

PTC material is a positive temperature coefficient heat-sensitive material, and when used as a heating element for electrothermal anti-icing, it exhibits adaptive temperature control. This paper aims to establish and experimentally verify a conductive model for the low Curie point PTC material based on the distribution and blending characteristics of HTPDMS, PDMS, n-tetradecane and acetylene black. Additionally, various mass proportions of the low Curie point PTC materials in the conductive region are prepared and analyzed to understand their PTC effects. Finally, the feasibility of employing the low Curie point PTC material for electrothermal anti-icing is verified through anti-icing experiment. The results demonstrate that the resistivity of the materials with different mass proportions in the seepage and conductive regions conforms to the conductive model proposed in this study. Specifically, when the mass proportion of n-tetradecane/HTPDMS is 1:1, and the mass proportion of PDMS/HTPDMS is 1:2, the low Curie point PTC material exhibits a Curie temperature of 1 °C, with a low-temperature resistivity of only 5.37 Ω cm and a PTC intensity of 2.99. Notably, after subjecting the material to 100 thermal cycle experiments, there is no significant change in the Curie temperature point. The low-temperature resistivity slightly increases, while the PTC intensity declines marginally, and no obvious negative temperature coefficient (NTC) effect is observed, confirming the material's excellent stability. The findings of this study contribute to the understanding of the low Curie point PTC anti-icing materials and lay a solid foundation for their future application in anti-icing systems for fans, airplanes, and various surfaces.

Study of electrical conduction mechanisms of CaCu3Ti4O12–MgAl2O4 composite ceramics with NTC effect via in-situ XPS and complex impedance analysis

In this study, a series of xMgAl2O4-(1-x) CaCu3Ti4O12 (0.05 ≤ x ≤ 0.20) composite ceramics was fabricated via solid-state method. The X-ray diffraction (XRD) data revealed the coexistence of cubic perovskite and spinel phases, and the scanning electron microscopy (SEM) images highlighted different grain distributions in the produced ceramics. The decrease in resistivity with increasing temperature indicated the NTC (negative temperature coefficient) effect in all samples. As the MgAl2O4 content rose, the values of ρ25, B25/75 and Ea decreased gradually from 3.25 × 108 Ω•cm, 7911 K and 0.6852 eV to 1.46 × 106 Ω•cm, 5669 K and 0.4549 eV, respectively. In situ XPS analysis showed a conduction mechanism based on electron hopping. According to the complex impedance results, both grains and grain boundaries contributed to the NTC effect, and the relaxation behavior of ceramics was mainly due to the oxygen vacancies.

Electrical performance and conductive mechanism of xLaCoO3-(1-x)Ba0.98Ca0.02Ti0.96Sn0.04O3 ceramics with NTC effect via chemical modification

xLaCoO3-(1-x)Ba0.98Ca0.02Ti0.96Sn0.04O3 (xLCO-(1-x)BCTS) ceramics were manufactured using a solid-phase reaction method. All samples exhibited single cubic perovskite structures and a typical negative temperature coefficient (NTC) effect. The grain sizes increased with increasing LCO doping content. For all samples, the resistivities (ρ25), activation energies for electrical conduction (Ea), thermal constants (B25/75), and activation energies of the relaxation process (Erelax) were in the ranges of 5.93 × 104-9.36 × 108 Ω•cm, 0.3700–0.2951 eV, 3177–3988 K, and 0.3539–0.2890 eV, respectively. Such a combination of ρ and B renders these materials to be potential candidates for NTC thermistors. According to the XPS spectra and due to the difference in ion radius among Co3+, Co4+, Ti3+, and Ti4+, Co3+-O-Co2+ and Ti3+-O-Ti4+ bonds were formed, and the electrons in the continuous bonds generated jumping conduction. The fitted results of the equivalent circuit showed that Rg and Rgb decreased with increasing LCO content, which is consistent with the DC resistivity at 25 °C. The results of the analysis indicate that the conductive mechanism of xLCO-(1-x)BCTS ceramics was electron hopping conduction.

Understanding the reversal dilution effect of the species sampling in the negative temperature coefficient region

Species sampling during fuel autoignition can provide valuable insights in studying the chemical kinetic mechanism. Various species sampling techniques have been developed for species quantification in different reactors. However, unavoidable dilution in the physical sampling brings uncertainty to the measurements. Particularly in the negative temperature coefficient (NTC) region, the effect of dilution remains unclear. To address this issue and improve measurement accuracy, numerical species sampling during the autoignition of n-butane mixture within the NTC region was studied in a rapid compression machine reactor. Results show that greater fuel consumption and intermediate generation were observed in regions with lower temperature instead of the hot core zone. Consequently, the dilution effect on the sampling results is reversed inside the NTC region. Furthermore, results from two typical sampling methods were compared with the target core profiles to determine their uncertainties. The feasibility of applying dilution ratio calibration on the sampling results of these two methods was also tested, which further indicates the necessity of considering the NTC effect, particularly for intermediates with lower concentrations. Alternatively, increasing the sampling duration of the fast-sampling method effectively reduces the uncertainty of species concentration and enhances the robustness of mechanism validation regardless of the NTC behavior.

Regulating the temperature‐sensing behavior of poly(lactic acid) by incorporating multiwalled carbon nanotubes and graphene nanoplatelets

AbstractIn this work, the synergistic effect of using multiwalled carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) to regulate the temperature response of electrical conductivity (σ) for poly(lactic acid) (PLA) composites were reported. The addition of CNTs into PLA/GNPs composites markedly enhanced their crystallization capacity and σ, resulting in a reduction of percolation threshold from 2.87 (for PLA/GNPs) to 0.39 wt% (for PLA/GNPs/CNTs), which is attributed to the formation of a three‐dimensional conductive network within PLA consisting of CNTs and GNPs. Furthermore, elevating the heat treatment temperature from 80 to 140°C resulted in a reduced response time of PLA/CNTs/GNPs composites from 57 to 43 s at the Tend of 140°C. Notably, the combined use of CNTs and GNPs considerably amplified the value and stability of max . For example, the maximum increased from 120% for PLACNT0.6 to 240% for PLA/(CNT1/GNP1)0.6. The suppression of volume expansion of PLA led to a negative temperature coefficient (NTC) effect at relatively low temperatures, resulting in a reduction of NTC transition temperature from about 160 to 110°C. Additionally, the PLA/(CNT1/GNP1)0.6 exhibited a swift and sensitive response to temperature fluctuations in various heating scenarios, which demonstrated a potential for use as a thermosensitive material with fast response and temperature‐sensing ability.Highlights PLA‐based composites with NTC effect was prepared by adding CNTs and GNPs. The transition temperature from PTC to NTC effect decreased by adding hybrid fillers. The addition of hybrid CNTs and GNPs favored the formation of conductive network. The mechanism for NTC transition was explained.

Negative temperature coefficient of ZnO microwires for cryogenic temperature sensing

Negative temperature coefficient (NTC) thermistors are expected to be developed at cryogenic temperature sensing. In this paper, a kind of cryogenic thermistor was developed based on NTC response of single crystal ZnO microwires (MWs). The current–voltage (I–V) characterization demonstrated a NTC response, which was temperature dependence of resistance that decreased with increasing temperature. A sensitive NTC response, especially at the cryogenic temperature range, was observed. The defects in ZnO MWs play an important role in NTC response; therefore, we studied the origin of NTC effect associated with defects to improve the recognition of relationship between defects and the NTC effect and further optimize the temperature sensor design for high performance. Annealing at 800 °C in air was undertaken to make a significant influence on the concentration of defects in as-grown sample, and a series of temperature-dependent features were investigated by photoluminescence, Raman, XPS, and EPR measurements. The results indicated the zinc interstitials to be effective donors for sensitive NTC effect at the cryogenic region. This study provides insight into the ZnO NTC effect and sensitive cryogenic sensing technology.

Effects of load level, dimensional effects and temperature conditions on the pressure sensitivity of short carbon fiber cementitious composites

Carbon fiber reinforced cementitious composites (CFRCC) are a kind of sensing materials in structural self-monitoring system. However, their performances still need to be enhanced. In this study, the effects of load level, dimensional effect, temperature and temperature cycles on the pressure sensitivity of CFRCC with hydroxyethyl cellulose (HEC) dispersant were systematically investigated. Simultaneously, the change rate of resistance was adopted to quantify the pressure and temperature sensitivity of CFRCC. It is found that the pressure sensitivity of CFRCC is stable as load level is below 50 per of the ultimate load. With the increasing size of the specimens, the compressive strength of CFRCC gradually declines whereas its change rate of resistance hikes quickly, both of which show obvious size effect. Furthermore, in the range of 5 °C to 80 °C, the resistivity of CFRCC demonstrates the effect of negative temperature coefficient (NTC), and the sensitivity coefficients of CFRCC tend to decrease with the increasing temperature. Compared with the sensitivity coefficient at 70 °C, the sensitivity coefficient of each specimen at 15 °C have increased by four times. In addition, the pressure sensitivity of CFRCC has significantly decreased after temperature cycles, and especially after 20 temperature cycles, CFRCC’s maximum change rate of resistance decreases by about 30% compared to the initial one. Therefore, CFRCC can be directly used in concrete structures to undertake the task of structural health self-monitoring.

Water-Stable Hybrid Lead-Free Perovskite for Negative Temperature Coefficient Thermistors

Negative temperature coefficient (NTC) thermistors feature higher sensitivities and faster response speeds and thereby have particular applications in many fields. However, current NTC thermistors are mostly based on inorganic ceramic materials, which show obvious drawbacks in material synthesis, property modulation, and flexible film fabrication. Herein, we report, for the first time, the promising application of an inorganic-organic hybrid NTC thermistor. A new lead-free hybrid iodo bismuthate [1,1',1″-(benzene-1,3,5-triyl)tris(3-methyl-1H-imidazol-3-ium)]Bi2I9 [denoted as (Me3TMP)Bi2I9] was synthesized by a "double-free" strategy. (Me3TMP)Bi2I9 features a lead-free binuclear bismuth iodine anion charge compensated by a "classic hydrogen-bond-free" cation. (Me3TMP)Bi2I9 exhibits remarkable stability in water and UV light irradiation and shows the largest temperature sensitivity coefficient among all reported NTC materials. Theoretical calculation and detailed structural analysis disclose that the seriously distorted (BiI6) octahedra are responsible for the intriguing NTC effect for (Me3TMP)Bi2I9.

Novel DAZO Film with a Negative Temperature Coefficient Effect and Its Application Prospects for the Thermal Runaway Warning System in Batteries

A novel dopamine- and aluminum-doped zinc oxide (DAZO) sensing film was prepared by blending DAZO nanoparticles with silicone latex; this film has promising applications for the thermal runaway warning system of new energy batteries. The DAZO nanoparticles were generated by the coprecipitation–calcination method through the structure-orienting modification of eco-friendly polydopamine biomaterials, followed by the doping of aluminum and nitrogen into the zinc oxide lattice. Dopamine was used as a nitrogen source; it easily self-polymerizes to form polydopamine, which is then pyrolyzed and doped into the zinc oxide lattice. The negative temperature coefficient (NTC) effects of the DAZO film and the aluminum-doped zinc oxide (AZO) film below 150 °C were tested and compared. The results showed that the DAZO film had a more pronounced NTC effect than the AZO film because of the doped nitrogen element. DAZO films exhibited good recyclability and thermal stability during the high-low temperature NTC cycle test. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry/mass spectrometry (TG–MS), and energy dispersive X-ray spectroscopy were performed to characterize the fabrication of the synthesis mechanism of the DAZO nanoparticles. The NTC effects of DAZO films with varying aluminum doping amounts, varying dopamine doping amounts, and different pH values were investigated. Nitrogen doping in the conventional AZO nanoparticles enhanced the sensitivity and low-temperature NTC effect; this is because of the increase in the DAZO grain size due to doping and the structure–orientation of polydopamine, which further leads to the increase in the height of the grain boundary potential barrier. On the basis of the test results, an alarm response device at different temperatures was initially established, and its alarm response at different temperatures was simulated and tested, which showed the possibility of the application of the DAZO film to the runaway warning system for the new energy batteries.

High-Temperature Response Polylactic Acid Composites by Tuning Double-Percolated Structures.

Due to the properties of a positive temperature coefficient (PTC) effect and a negative temperature coefficient (NTC) effect, electrically conductive polymer composites (CPCs) have been widely used in polymer thermistors. A dual percolated conductive microstructure was prepared by introducing the polybutylene adipate terephthalate phase (PBAT) into graphene nanoplatelets (GNPs)-filled polylactic acid (PLA) composites, intending to develop a favorable and stable PTC material. To achieve this strategy, GNPs were selectively distributed in the PBAT phase by injection molding. In this study, we investigated the crystallization behavior, electrical conductivity, and temperature response of GNP-filled PLA/PBAT composites. The introduction of GNPs into PLA significantly increased PLA crystallization capacity, where the crystallization onset temperature (To) is raised from 116.7 °C to 134.7 °C, and the crystallization half-time (t1/2) decreases from 35.8 min to 27.3 min. The addition of 5 wt% PBAT increases the electrical conductivity of PLA/PBAT/GNPs composites by almost two orders of magnitude when compared to PLA/GNPs counterparts. The temperature of the heat treatment is also found to play a role in affecting the electrical conductivity of PLA-based composites. Increasing crystallinity is favorable for increasing electrical conductivity. PLA/PBAT/GNPs composites also show a significant positive temperature coefficient, which is reflected in the temperature-electrical resistance cycling tests.

Polymer Composites with Self-Regulating Temperature Behavior: Properties and Characterization.

A novel conductive composite material with homogeneous binary polymer matrix of HDPE (HD) and LLDPE (LLD), mixed with conductive filler consisting of carbon black (CB) and graphite (Gr), was tested against a HDPE composite with a similar conductive filler. Even the concentration of the conductive filler was deliberately lower for (CB + Gr)/(LLD + HD), and the properties of this composite are comparable or better to those of (CB + Gr)/HD. The kinetic parameters of the ρ-T curves and from the DSC curves indicate that the resistivity peak is obtained when the polymer matrix is fully melted. When subjected to repeated thermal cycles, the composite (CB + Gr)/(LLD + HD) presented a better electrical behavior than composite CB + Gr)/HD, with an increase in resistivity (ρmax) values with the number of cycles, as well as less intense NTC (Negative Temperature Coefficient) effects, both for the crosslinked and thermoplastic samples. Radiation crosslinking led to increased ρmax values, as well as to inhibition of NTC effects in both cases, thus having a clear beneficial effect. Limitation effects of surface temperature and current intensity through the sample were observed at different voltages, enabling the use of these materials as self-regulating heating elements at various temperatures below the melting temperature. The procedure based on physical mixing of the components appears more efficient in imparting lower resistivity in solid state and high PTC (Positive Temperature Coefficient) effects to the composites. This effect is probably due to the concentration of the conductive particles at the surface of the polymer domains, which would facilitate the formation of the conductive paths. Further work is still necessary to optimize both the procedure of composite preparation and the properties of such materials.

In situ XPS and impedance spectroscopy studies of (1 − x)Ba0.5Bi0.5FeO3 − xGd2O3 ceramics with negative temperature coefficient

Perovskite-phase (1 − x)Ba0.5Bi0.5FeO3 − xGd2O3 ceramics with negative temperature coefficient (NTC) were synthesized by solid-state reaction. X-ray diffraction (XRD) measurements showed that all samples exhibited a tetragonal perovskite structure. In addition, all samples showed a typical NTC effect; their room resistivity ρ25 and thermal constant B25/75 were regulated by the Gd2O3 doping content, and varied in the 4.70 × 105–4.72 × 106 Ω cm and 5573–6697 K ranges, respectively. In situ X-ray photoelectron spectroscopy (XPS) elucidated the conduction mechanism of Ba0.5Bi0.5FeO3-based ceramics was the electron hopping type, and the source of NTC effect was the change in the content of different valence ion pairs and the increase in electronic mobility with increasing temperature. Impedance analysis showed that the samples were electrically inhomogeneous, and their grain boundary resistance was higher than the grain resistance. The corresponding relaxation process might originate from the release of space charge and the ionization of oxygen vacancies.