Negative Temperature Coefficient Behavior Research Articles

Recovery of Ni-Co-Mn Oxides from End-of-Life Lithium-Ion Batteries for the Application of a Negative Temperature Coefficient Sensor

This study demonstrates the current advancements in battery management systems (BMSs), emphasizing the need for precise temperature monitoring within battery packs to enhance safety and performance through efficient thermal management. The increased demand for lithium-ion batteries (LIBs) has driven the development of temperature sensors with improved accuracy and stability. In particular, Ni-Co-Mn-based spinel oxides are commonly used due to their stable negative temperature coefficient (NTC) behavior. However, challenges arise in manufacturing due to the high cost and uncertain supply of critical cathode components (e.g., Co, Ni, and Mn) for LIBs. This research focuses on developing spinel-type (Ni0.6Co0.4Mn2)O4 using recycled Ni-Co-Mn oxides obtained from end-of-life (EOL) LIBs, demonstrating temperature resistance behavior suitable for temperature sensing. The oxides are prepared through hydrometallurgy, oxalate synthesis, and post-heat treatment. Successful integration into spinel-type NTC thermistors suggests broader applications in various industrial fields. A systematic investigation into the synthesis and characterization of recovered Ni-Co-Mn oxides from EOL LIB cathode materials (Li(Ni0.33Co0.33Mn0.33)O2) is presented for NTC thermistor application. Thermogravimetric analysis-derivative thermogravimetry (TGA-DTG) identifies the optimal post-heat treatment temperature. The X-ray diffraction (XRD) patterns confirm a cubic spinel structure of the Ni-Co-Mn oxides, supported by scanning electron microscope (SEM) images showing a uniform microstructure. Also, energy dispersive X-ray spectroscopy (EDS) mapping confirms homogeneous element distribution. Recovered oxide pellets from the sintering process exhibit a single spinel structure, with X-ray photoelectron spectroscopy (XPS) analysis revealing changes in the valence states for Ni and Mn. Resistivity measurements demonstrate semiconductive behavior, which shows a B value (3376.92 K) suitable for NTC thermistor applications. This study contributes valuable insights to black powder recycling from EOL LIBs and its potential in temperature-sensitive electronic devices.

Negative Temperature Coefficient Properties of Natural Clinoptilolite

Negative temperature coefficient (NTC) materials are usually based on ceramic semiconductors, and electrons are involved in their transport mechanism. A new type of NTC material, adequate for alternating current (AC) applications, is represented by zeolites. Indeed, zeolites are single charge carrier ionic conductors with a temperature-dependent electrical conductivity. In particular, electrical transport in zeolites is due to the monovalent charge-balancing cations, like K+, capable of hopping between negatively charged sites in the aluminosilicate framework. Owing to the highly non-linear electrical behavior of the traditional electronic NTC materials, the possibility to have alternative types of materials, showing linearity in their electrical behavior, is very desirable. Among different zeolites, natural clinoptilolite has been selected for investigating NTC behavior since it is characterized by high zeolite content, a convenient Si/Al atomic ratio, good mechanical strength due to its compact microstructure, and low toxicity. Clinoptilolite has shown a rapid and quite reversible impedance change under heating, characterized by a linear dependence on temperature. X-ray diffraction (XRD) has been used to identify the natural zeolite, to establish all types of crystalline phases present in the mineral, and to investigate the thermal stability of these phases up to 150 °C. X-ray photoelectron spectroscopy (XPS) analysis was used for the chemical characterization of this natural clinoptilolite sample, providing important information on the cationic content and framework composition. In addition, since electrical transport takes place in the zeolite free-volume, a Brunauer–Emmett–Teller (BET) analysis of the mineral has also been performed.

On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.

Evolution of conduction mechanism in Ca-doped YFe0.5Co0.5O3 compound by complex impedance spectroscopy

The ceramic samples of Y1-xCaxFe0.5Co0·5O3 (x = 0.15, 0.25, 0.35) has been synthesized using solid-state reaction method. These samples exhibited orthorhombic perovskite structure and belonged to the Pbnm space group. X-ray analysis confirmed that Ca2+ ions replaced Y3+ ions at the A-site of perovskite structured YFe0·5Co0·5O3 compound, while XPS studies confirmed the oxidation states of the constituent atoms. SEM micrographs and Williamson-Hall analysis showed that the crystallite size reduces with increasing Ca content, resulting in a strain-free, accommodated and stable microstructure. Complex impedance spectroscopic investigations established that the dispersion behaviors exhibited by the samples are due to Interfacial and Bulk polarizations. These phenomena are attributed to the accumulation of space charges and the hopping of electrons, respectively. Study further explore the semiconducting nature and negative temperature coefficient behavior of the material. Nyquist plots showed that grain interior is the major contributor to the conduction mechanism, while grain boundaries playing a secondary role. Correlated Barrier Hopping (CBH) has emerged as a suitable model to elucidate the conduction mechanism. The conduction process facilitated by CBH could potentially account for the observed negative permittivity in the material.

Kinetic study of ignition process of ammonia/n‐heptane fuel blends under engine conditions

AbstractPilot‐ignited ammonia‐fueled engines have drawn more and more attention for low carbon emissions compared to traditional diesel engines. The ignition processes of NH3/NC7H16 mixtures under compression ignition engine‐like conditions are numerically investigated. By comparing the ignition delay times (IDTs) calculated by six ammonia mechanisms with experimental data, the Glarborg mechanism is selected. Then, the Glarborg mechanism and the Zhang detailed n‐heptane mechanism are merged into a new mechanism, which is adopted in the present study. Results show that the negative temperature coefficient behavior of the IDTs is only observed as the ammonia mass fraction is 70%. Only temperature has a significant effect on IDTs at all research conditions, and the effect of ammonia mass fraction is significant when the temperature is lower than 1000 K. However, the effects of equivalence ratio and pressure are small, especially at high temperatures, high equivalence ratios, and high pressures. Interestingly, the IDTs are categorized into three regions by temperature and ammonia mass fraction. The sensitivity analysis indicates that the sensitivity coefficients of most reactions associated with ammonia decrease with a decrease in ammonia mass fraction, whereas only R4210 is sensitive to ammonia mass fraction for n‐heptane‐related reactions. Rates of production and consumption (ROP) analyses indicate that the ammonia mass fraction mainly affects the ROPs of NC7H16, NH3, and NNH at low and medium temperatures, whereas the ammonia mass fraction affects the ROP of H2NO before the temperature of 2000 K. The ROPs of NC7H16, NH3, and NNH significantly increase with increasing temperature, whereas the ROP of H2NO slightly increases with increasing temperature. The increase of temperature in the early and middle stages is mainly contributed by the oxidation of n‐heptane, while the increase of temperature in the middle and late stages is mainly contributed by the oxidation of ammonia.

Understanding the Nonlinear Reactivity Promoting Effect of n-Heptane Addition on the Binary Mixture From Low to Intermediate Temperature: A Case of Methane/n-Heptane Mixtures

Abstract To understand the effect of n-heptane (NC7H16) addition on the auto-ignition of methane (CH4) at low to intermediate temperatures, the ignition delay times (IDTs) of stoichiometric CH4/NC7H16 blends with varying NC7H16 concentrations were measured at temperatures from 600 to 1000 K, pressures of 20 and 40 bar. Detailed chemical kinetic mechanisms were validated against the newly measured IDTs. Adding NC7H16 in the binary mixture shows a nonlinear promoting effect on the IDTs: micro-addition of NC7H16 can significantly reduce the IDTs of the binary mixture when the NC7H16 is lower than 20%. However, the decrease of the IDTs becomes much slower when further increasing the NC7H16 addition. Affected by the negative temperature coefficient (NTC) behavior of NC7H16, this nonlinear effect is particularly notable at around 795 K, the low boundary of the NTC region. To reveal the nonlinear reactivity-promoting effect of NC7H16 addition on the binary mixture, reaction flux, ignition sensitivity, rate of production of the key radicals along heat production analyses were conducted. Apart from contributing more O˙H production through the low-temperature chain-branching reaction pathways of NC7H16, adding NC7H16 also promotes the pre-ignition heat release of the binary mixture. The heat release raises the system temperature and further promotes the mixture ignition, enhancing the nonlinear effect at low temperatures.

Elucidating the low temperature chemistry of o-xylene enhanced by the reaction pool of dimethyl ether

The branched aromatic hydrocarbon with short side chains, such as o-xylene, has weak low temperature oxidation reactivity, while it could proceed low temperature chain recycling reactions by blending a fuel component with higher oxidation reactivity. However, the study of low temperature oxidation kinetics of o-xylene is very scarce. Therefore, this work investigated the low temperature oxidation process of o-xylene in an atmospheric jet stirred reactor by adding a biofuel with higher low temperature oxidation reactivity, i.e. dimethyl ether (DME). A series of characteristic low temperature intermediates, such as phenyl hydroperoxides and ketohydroperoxide from the oxidation of o-xylene, were detected by using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The chemical structures of representative low temperature intermediates were determined by comparing the ionization thresholds from measured photoionization efficiency curves with calculated ionization energies. A kinetic model for o-xylene and DME mixture was developed and verified by literature and present experimental data. Rate of production (ROP) and sensitivity analyses were performed, which indicate that the low temperature oxidation chemistry of o-xylene, such as the negative temperature coefficient behavior, is enhanced by the reaction pool of DME. According to the classical low temperature oxidation mechanism of n-alkanes, the low temperature reactions of o-xylene were proposed. In particular, phenyl hydroperoxides and ketohydroperoxides were revealed to be key indicators for the occurrence of first and second O2 addition reactions during the oxidation of o-xylene, respectively. In addition, the sensitivity analysis uncovers the interaction kinetics between the reaction pools of o-xylene and DME, as well as their effects on fuel consumption and products formation. In summary, present evidence elucidates the low temperature chemistry of a representative branched aromatic, which could contribute to the development of future engines and control of combustion pollutants.

Theoretical study on low-temperature oxidation kinetics of methyl pentanoate

In order to develop reliable combustion models of biodiesel, the low-temperature oxidation kinetics of methyl pentanoate (MP), the smallest methyl ester with the negative temperature coefficient (NTC), is theoretically investigated. We use the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ and CASPT2/CBS(T-Q)//CASSCF(7e,5o)/cc-pVTZ methods to explore the potential energy surfaces (PESs) of the decomposition of different MP peroxy radicals, including dissociation, isomerization, and concerted HO2 elimination. Comparisons among the PESs of MP and other methyl esters reveal that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. Based on the determined PESs, the pressure- and temperature-dependent rate constants for the low-temperature oxidation of MP are computed at 298−1200 K and 0.01−100 atm. The kinetic calculations suggest that the dissociation rate constants of different methyl ester peroxy radicals are similar at the same active C site, and the channels that lead to tri-molecular products are significant during the low-temperature oxidation of large methyl esters. The literature combustion model of MP is revised with our calculations. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.Novelty and Significance statementMethyl pentanoate (MP) is the smallest methyl ester with the negative temperature coefficient (NTC), but the low-temperature oxidation kinetics in the available combustion model of MP is still insufficient to interpret its NTC behavior. To develop a reliable combustion model for MP, the potential energy surfaces (PESs) of the decompositions of different MP peroxy radicals are investigated. Results show that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. With our calculated rate constants, the literature combustion model is revised. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.

Revealing the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene sensitized by dimethyl ether

Low to moderate temperature oxidation has been a vital topic in the combustion chemistry due to its critical role in controlling the combustion characteristics such as autoignition and pollutant emissions. Low temperature oxidation kinetics of alkanes and oxygenated biofuels have been widely explored, while that of branched aromatics is poorly understood. This work aims to investigate the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene (TMB, C9H12), which is a multi-branched alkylbenzene with weak low temperature oxidation reactivity. To enhance the low temperature oxidation reactivity of TMB, dimethyl ether (DME) was added to efficiently provide chain initiators. The oxidation experiment of TMB/DME mixture was performed with an atmospheric jet stirred reactor coupled with synchrotron vacuum ultraviolet radiation photoionization mass spectrometry. The negative temperature coefficient (NTC) behavior, as well as featured aromatic intermediates, especially phenyl hydroperoxides and unexpected highly oxygenated molecules (HOMs), were observed in the experiment. The detection of these fingerprint aromatic intermediates provide crucial evidence for revealing the low to moderate oxidation kinetics of TMB. In addition, a kinetic model was developed to provide in-depth kinetic analysis. The results demonstrate that the active chain initiators produced from DME oxidation trigger the low temperature chain-branching reactions of TMB. The chain recycling process is further sustained due to the regeneration of active intermediates such as phenyl hydroperoxides and •OH radicals. Aromatic intermediates with chemical compositions of C9H12Ox and C9H10Ox were measured and identified to be key indicators for the low temperature chemistry of TMB. In particular, the aromatic HOMs (C9H12O6 and C9H10O5) are fingerprint intermediates produced from the third O2 addition and subsequent reactions of TMB. Besides, the sensitivity analysis indicates that the interaction kinetics between TMB and DME by sharing the highly sensitive reactions of hydroperoxides and •OH radicals. The present work extends the conceptual reaction schemes proposed in alkanes towards branched aromatics, especially sequential O2 addition reactions that contributing to the formation of phenyl hydroperoxides and HOMs.

Synthesis of a scheelite barium molybdate: Structural, topological, electrical, and transport properties for negative temperature coefficient thermistor application

Preliminary structural analysis of Scheelite barium molybdate BaMoO4 (BMO) ceramic using NTREOR-09 and McMaille methods has confirmed the tetragonal symmetry with room temperature X-ray diffraction (XRD) spectrum. Further, other structural parameters like crystallite size and compressive lattice strain are found to be of 31 nm and −0.00151 respectively. The topographical study of scanning electron microscope (SEM) image confirms the polycrystalline nature with no other impurities in the sample. Detailed analysis of dielectric permittivity (εr), and loss (tanδ) supports Maxwell-Wagner and Koop's phenomena. The non-overlapping small polaron tunnelling (NSPT) and correlated barrier hopping (CBH) model have emphasis on transport mechanism. The presence of semiconducting behavior of the scheelite material at various temperatures and frequencies examined by complex impedance and modulus spectra. The value of grain resistance decreases as temperature increases demonstrating the existence of negative temperature coefficient (NTCR) behavior of the material. The increase of the value of thermistor constant (β) (67.339, 516.613, and 772.485 in the different high-temperature range) suggests an efficient application of the material for NTC-thermistor.

Estimation and testing of single-step oxidation reactions for hydrogen and methane in low-oxygen, elevated pressure conditions

Advanced combustion concepts, such as moderate or intense low-oxygen dilution (MILD) combustion, offer a reduction in NOx emissions and increased thermal efficiency. MILD is characterised by low-oxygen, high-temperature conditions, where finite-rate chemistry effects are significant. Modelling this regime using reduced or single-step chemistry remains a challenge in part due to the finite-rate chemistry. This work proposes a generalised method for assigning Arrhenius coefficients of a single-step reaction using the outputs of a detailed mechanism. Assigning the typical chemical conservation equation to a progress variable, activation energy and pre-exponential factor for an Arrhenius kinetics global reaction are determined for hydrogen and methane across a number of conditions, with the proposed method extended to n-heptane as well. The temperature exponent in the modified Arrhenius equation is determined by minimising the ignition delay error between detailed and single-step simulations. Functional forms of each coefficient are calculated from a multivariate regression, dependent on initial temperature, pressure, and oxidant mole fraction. The predicted mechanisms are compared against the detailed kinetics in closed homogeneous batch reactors, with comparisons for hydrogen extended to both laminar opposed flow diffusion flames, and computational fluid dynamics (CFD) simulations. Ignition delay and equilibrium temperature are both well predicted for all three fuels in the batch reactors. Notably, the negative temperature coefficient behaviour of n-heptane is successfully recreated with the single-step mechanism. Temperature and heat release of hydrogen flames are well captured in both opposed flow laminar flames, and in turbulent CFD simulations. The computational time was also significantly reduced through the single-step mechanisms, resulting in ∼100 times reduction in compute time for CFD simulations. The function form of the Arrhenius coefficients shows promise for extension outside of the ranges and fuels analysed herein, and presents interesting phenomena for exploring how initial reactant temperature and pressure influence the effective activation energy of an oxidation process.

Experimental and kinetic modeling study on ammonia/polyoxymethylene dimethyl ether 2 oxidation in a jet-stirred reactor

The oxidation of ammonia/polyoxymethylene dimethyl ether 2 (NH3/PODE2) was investigated in a jet-stirred reactor with ammonia addition of 10 %, 25 %, and 50 % in mole fraction. Atmospheric experiments were conducted at an equivalence ratio of 1.0, within the temperature range of 500–1100 K. The initial fuel mole fraction and residence time were fixed at 0.5 % and 2 s, respectively. PODE2 and NH3, along with other intermediates and products such as C1-C2 hydrocarbons and permanent gases, were analyzed using gas chromatography and Fourier transform infrared spectroscopy. The experimental results demonstrate that the addition of NH3 has little impact on the consumption of PODE2, primarily influencing the mole fraction of hydrocarbon intermediates. All three blends exhibited negative temperature coefficient (NTC) behaviors. However, NTC behavior was weakened as the NH3 blending ratios increased due to the competition for O2 at low temperatures. A detailed kinetic model for PODE2/NH3 blends was constructed, incorporating updated rate coefficients from the recent literature and analogy methods. The model also considers the interaction between PODE2 and NH3, which was investigated through reaction pathway analysis and sensitivity analysis. Simulated results show that the mole fraction profiles are reasonably predicted by this model at the test conditions. The interaction between PODE2 and NH3 is facilitated by CH3, NH2 radicals, and NO-NO2 cycle. The rapid reversible reaction between NO and NO2 alters the reaction pathway of radicals in the blends. At low temperatures, PODE2 in blend follows the same oxidation pathway as pure PODE2, while NH3 oxidation is affected by the introduction of additional HO2 and H from PODE2. New OH formation pathways involving NH2, NO, and NO2 occur, while the decomposition of H2O2 is inhibited, thereby affecting the oxidation of PODE2 at intermediate temperatures. Additionally, a notable decrease in the intermediates with carbon–carbon double bonds is observed, due to the consumption of CH3 through the reaction NO2 + CH3 = NO + CH3O. This important reaction also promotes the oxidation of CH3 and the NO-NO2 cycle, resulting in an improvement in system reactivity.

Efficient two-dimensional simulation of primary reference fuel ignition under engine-relevant thermal stratification

Despite vast research on engine knock, there remains a limited understanding of the interaction between reaction front propagation, pressure oscillations, and fuel chemistry. To explore this through computational fluid dynamics, the adoption of advanced numerical methods is necessary. In this context, the current study introduces ARCFoam, a computational framework that combines dynamic mesh balancing, chemistry balancing, and adaptive mesh refinement with an explicit, density-based solver designed for simulating high-speed flows in OpenFOAM. First, the validity and performance of the solver are assessed by simulating directly initiated detonation in a hydrogen/air mixture. Second, the study explores the one/two-dimensional (1D/2D) hotspot ignition for the primary reference fuel and illuminates the impact of transitioning to 2D simulations on the predicted combustion modes. The 2D hotspot simulations reveal a variety of 2D physical phenomena, including the appearance of converging shock/detonation fronts as a result of negative temperature coefficient (NTC) behavior and shock wave reflection-induced detonation. The main results of the paper are as follows: (1) NTC chemistry is capable of drastically changing the anticipated reaction front propagation mode by manipulating the local/global reactivity distribution inside and outside the hotspot, (2) subsonic hotspot ignition can induce detonation (superknock) through the generation of shock waves and subsequent wall reflections, and (3) while the 1D framework predicts the initial combustion mode within the hotspot, significant differences between 1D and 2D results may emerge in scenarios involving ignition-to-detonation transitions and curvature effect on shock/detonation front propagation.

Development and validation of a comprehensive combustion kinetic model for the oxidation of 3-hexene

Alkene with a double bond present in the middle of the molecule is attractive as a representative of the olefinic group present in the long-chain unsaturated methyl esters, which make up biodiesel typically. Also the study of alkenes has long gained importance owing to its potential as a gasoline surrogate and its formation in significant amounts during alkane oxidation. These considerations motivate the study of kinetics of trans-3-hexene. In the current work, 3-hexene mechanism is developed upon the base chemistry of comprehensive NUIG1.3 mechanism. Several important reactions are revisited and are assigned rate constants from the literature, including theoretical studies. The revised kinetic mechanism has been validated at various temperatures, pressures, and equivalence ratios for different experimental configurations. The mechanism shows good agreement with ignition delay time data in the low to high temperature range. The mechanism also predicts the well known lack of negative temperature coefficient (NTC) behaviour in the case of alkenes with the central double bond. With the proposed kinetic model, it is possible to predict intermediate species profiles produced during 3-hexene oxidation satisfactorily. Path flux analysis and sensitivity analysis are performed in order to understand important reaction pathways and their effect on the combustion characteristics.

YCr0.5Mn0.5O3 negative temperature coefficient perovskite ceramics sintered under reducing atmosphere: Microstructure and electrical properties

Instead of the common oxidizing atmosphere, three-phase high-temperature negative temperature coefficient (NTC) materials were prepared under reducing atmosphere from raw powders by conventional solid-state method in the Y2O3-Mn2O3-Cr2O3 ternary diagram. Owing to dilatometric analysis, the sintering stage was performed at 1500 °C for 1 h, 12 h, and 30 h under reducing atmosphere (1 % H2 – 99 % N2) whereas sintering is not reached at 1600 °C in air atmosphere. The sintered ceramics had 96 to 98 % of relative density. XRD patterns and SEM observations revealed that the sintered pellets consist of three phases: a phase with a structure similar to that of the orthorhombic perovskite YCrO3 and two other phases with structures close to that of Y2O3 and a solid solution of MnO. The results of electrical measurements as a function of temperature have shown that despite this dissociation of the phases, samples kept an NTC behavior with interesting characteristics for low and high temperatures: the resistivity increased slightly with dwell time, but the order of magnitude remained unchanged (105 Ω*cm) at 25 °C… Depending of the dwell time, activation energy obtained from material constant B varied from 0.276 eV at low temperature to 0.690 eV at high temperature. Such materials could be successfully used as potential candidates for temperature sensors applications in a wide range temperature from ambient temperature to 1000 °C.

Crystallographic dependence of the electrical transport mechanism in La-Mn-O thermosensitive thin films

Bimetallic oxide materials with amorphous state usually exhibit singular peculiarities due to their periodic disorder structure, implying great potential applications as high-performance electronic devices. In this report, La-Mn-O thin films with different crystallographic features (amorphous and polycrystalline) were deposited on Si (100) substrates using magnetron sputtering method. The as-deposited thin films were successfully convert from the amorphous state to the polycrystalline state by tuning the post-annealing temperatures. X-ray diffraction (XRD) confirmed that La-Mn-O was amorphous at annealing temperatures of 500 °C, while the films were polycrystalline at higher annealing temperatures (600 °C and 700 °C). Scanning electron microscopy (SEM) and high-magnification transmission electron microscopy (HRTEM) distinguished the microscopic features of LaMnO3 thin films in polycrystalline and amorphous states. The effects of annealing temperatures and structural disorder on the temperature-dependent electrical transport properties of La-Mn-O thin films are investigated. The resistance-temperature relationship curves of both polycrystalline and amorphous films show negative temperature coefficient behavior, and the amorphous films follow more than one conduction mechanism. Results show that the electrical transport of the crystalline La-Mn-O films is mainly controlled by thermal activation mechanism, while the electron transport behavior of the amorphous film is simultaneously influenced by thermal activation and Mott VRH variable range hopping conduction.

Modulation of optical absorption and electrical properties in Mn-Co-Ni-O-based high-entropy thin films

High-entropy thin films of Mn0.6Co0.6Ni0.6Mg0.6Cu0.6O4, Mn0.6Co0.6Ni0.6Mg0.6 Zn0.6O4, and Mn0.6Co0.6Ni0.6Cu0.6Zn0.6O4 (MCNMC, MCNMZ, and MCNCZ) with equiatomic proportions were synthesized using chemical solution deposition on silicon substrates. Structural analysis confirmed a consistent face-centered cubic spinel structure, while significant differences in surface morphology were observed. Quantification of the valence states of Mn ions revealed an inverse variation in the concentrations of Mn4+ and Mn2+ ions. The heightened infrared light absorption of the MCNMC thin film was assigned to Cu-induced Jahn-Teller distortion and highly polarized Mg-O bonds. All samples exhibited negative temperature coefficient behaviors in their electrical properties. Additionally, the MCNMC thin film demonstrated the lowest resistance due to its denser microstructure, close proximity of Mn3+/Mn4+ ion concentrations, and additional Cu+/Cu2+ ion pairs, enhancing small polaron hopping conductivity. In contrast, the MCNMZ thin film showed moderate resistance but boasted the highest thermal constant (B25/50) of 3768 K, attributed to its distinctive grain chain structure, facilitating carrier transport while introducing migration barriers.

Interaction of end-gas autoignition and cold wall in closed chamber

The presence of cold walls in spark-ignition (SI) engines could induce the interaction with end-gas autoignition. This interaction is numerically studied in the current work considering detailed chemistry and transport, with emphasis on near-wall flame structures and dynamics, as well as autoignition-affected wall heat flux. Two alternative fuels, hydrogen, and dimethyl ether (DME), are considered. In hydrogen/air mixtures, autoignition first develops in the end-gas and leaves an unburnt near-wall region. The combustion mode in this region then sequentially behaves as spontaneous ignition, laminar flame, and low-to-intermediate temperature reaction. Correspondingly, the contribution of diffusion and convection in the energy budget respectively increases and decreases as the reaction front moves towards the wall. End-gas autoignition also introduces strong pressure waves propagating back-and-forth, which subsequently lead to multiple local maxima on the temporal evolution of wall heat flux. Consequently, the maximum wall heat flux no longer changes monotonically with both the wall temperature and the quenching distance. Furthermore, the role of low-temperature chemistry (LTC) is studied using DME/air mixtures, which exhibits typical multi-stage heat release and Negative Temperature Coefficient (NTC) behaviors. It is also noted that, the influence of pressure wave is minimal when the unburnt mixtures have relatively low energy density, such as at off-stoichiometric conditions and/or lower initial pressures. Specifically: consistent to distinct three-stage homogeneous ignition at certain fuel-lean conditions (ϕ = 0.25) and intermediate initial temperature (typically around T0 = 600–750 K), in the corresponding 1D case three-stage end-gas autoignition respectively evolves into the near-wall cool, warm and hot flame; in the case at relatively low initial pressures, only the first-stage autoignition occurs in the end-gas, and as such LTC slightly increases the wall heat flux and advances flame quenching. As the reactivity of unburnt mixtures becomes higher, LTC and HTC respectively introduce strong and weak knocking combustion. Multiple weak and strong local maxima are observed in the temporal evolution of wall heat flux, both of which changes non-monotonically with the wall temperature.

Structural, morphological, optical and thermoresistive study of the polyaniline/polylactic acid/ZnO films produced by solution blow spraying for temperature sensors

The present study aimed to obtain polyaniline/polylactic acid/ZnO (PAni/PLA/ZnO) films via Solution Blow Spraying (SBSp) in order to evaluate their structural, morphological, optical and thermoresistive properties in application such as in temperature sensors. The SBSp technique is a variation of Solution Blow Spinning (SBS), allowing film deposition on any surface using pressurized air and a polymeric solution. ZnO was synthesized via combustion reaction, a method known for producing nanosized particles. PLA was incorporated in the proportion of 10 wt%, while ZnO concentrations ranged from 10% to 60 wt %. Structural analyzes were performed by X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) for the polymers; the morphology of the films was analyzed by Scanning Electron Microscopy (SEM); Ultraviolet–visible (UV-VIS) spectroscopy was performed to obtain the bandgap of the material; thermal analysis was carried out by thermogravimetry and the thermosensitive properties were evaluated by a multimeter in a muffle. XRD results confirmed that the PANi/PLA/ZnO film was obtained using the SBSp method, with crystalline phases corresponding to ZnO and PANi. SEM showed an agglomerated and porous appearance of the samples. Decomposition temperatures according to thermogravimetric analysis (TG) were in the range of 200–400 °C, showing that under this regime the films can present their thermoresistive properties without decomposing with temperature. The bandgap was in the range of 2.32–3.92 eV, confirming values of a semiconductor, and that the presence of PLA did not affect this property. Thermoresistive tests demonstrated a Negative Temperature Coefficient (NTC) behavior, indicating that the PAni/PLA/ZnO films obtained by SBSp possess thermoresistive properties suitable for low-temperature sensing applications.

Negative temperature coefficient and positive temperature coefficient behavior of electrically conductive silicone/carbon black composites

As highlighted in several studies, the thermal expansion of the polymer matrix plays an important role with respect to the thermo-electrical behavior of conductive composites. Although it is known that the crosslink density of polymers affects the thermal expansion of polymers, no studies are known on whether different crosslink densities can affect the thermoelectric behavior of electrically conductive composites. In this study, silicone/carbon black composites with different crosslink densities, were investigated with respect to their self-heating properties under applied voltages and their thermo-electrical behaviors like positive and negative temperature coefficients. Specific resistances ρ about 55 +/− 4.5 Ω-cm have been achieved and uniquely high negative temperature as well as low temperature coefficients were demonstrated, which make these materials interesting for sensor-technical applications. Regarding their self-heating behaviors, average surface temperatures of up to 147°C were measured, which further make these materials very interesting as flexible heating elements. Finally, it could be shown that the crosslink density of addition curing silicones is adjustable over a wide range and that it can be further significantly changed by the addition of carbon black and platinum content.