Negative Temperature Coefficient Characteristics Research Articles

Remarkably improved nonlinearity and thermal stability of CaYNbMnO7 thermosensitive ceramics via the cold‐sintering process

AbstractThe cold‐sintering process (CSP) uses a transient low‐temperature solvent, such as water or water with dissolved solutes consistent with the ceramic composition, to achieve functionalization at lower temperatures via the dissolution and precipitation principle. Here, during the CSP, the use of molten NaOH as a flux solvent promoted the rearrangement and precipitation of CaYNbMnO7 particles. CaYNbMnO7 ceramics were successfully obtained at 350°C and under a uniaxial pressure of 700 MPa. The ceramic was not a single‐phase structure but consisted of a cubic‐phase structure and a layered perovskite structure. To verify the high‐temperature thermal stability of CaYNbMnO7 ceramic prepared at 350°C, a comparative study was conducted with conventional sintering (CS). Results indicate that the ceramics obtained by CSP and CS have negative temperature coefficient characteristics. The cold‐sintered ceramic has a wider temperature range and better aging stability. (The minimum value of the aging coefficient is ∼2.49%.) Simultaneously, CSP is beneficial for promoting the linearization of the relationship between the logarithm in the resistivity (lnρ) and the reciprocal for the absolute temperature (1000/T) in the ceramic. However, the CSP has little effect on the material constant B and thermal activation energy Ea of the ceramic, providing a basis for the low‐temperature synthesis of high melting point thermosensitive ceramics.

Effect of introducing KNO3 on the preparation of athermal fluorophosphate glass and investigation on its thermo-optical property

The effects of introducing some amount of KNO3 into the raw materials on preparation of fluorophosphate glass are investigated in terms of UV–vis transmittance, absorption coefficient and change of refractive index. As the content of KNO3 reaches 50%, the UV absorption coefficient of the optimized fluorophosphate glass reaches the minimum due to absence of microcrystals in the produced glass. The Schott's Sellmeier dispersion formula and Ghosh's two-polar wavelength-dependent Sellmeier equation were both used to calculate dispersion constants of the optimized fluorophosphate glass. The optimized fluorophosphate glass maintains special characteristics of negative temperature coefficient of refractive index and near-zero thermo-optical coefficient. The lower absorption both in UV and near-infrared (NIR) helps to improve its laser-induced damage resistance. The athermal fluorophosphate glass shows great potential for compensation of thermal-induced distortion in high-power solid-state lasers and high-resolution optical systems.

Combustion characteristics of waste styrene–butadiene rubber oil derived from thermal pyrolysis process: experiment and numerical simulation

Owing to the similar carbon range in components, the pyrolytic oil derived from waste styrene–butadiene rubber (SBR) is preferred in aviation, for achieving waste-to-energy and fly net zero missions. The present study investigated the combustion and emission characteristics of SBR using a constant volume combustion chamber (CVCC) spray at 673–873 K initial temperature (Tini), 10 and 15 bar initial pressure (Pini), and 0.5 equivalence ratio (φ). SBR combustion properties were benchmarked with conventional jet propellant 5 (JP-5), and JP-5 and SBR were blended at a 50% volume ratio (JP-5-SBR-50). SBR has a shorter ignition delay (ID) and combustion delay (CD) at 15 bar Pini, compared to 10 bar. SBR exhibits negative temperature coefficient (NTC) behavior starting from Tini = 773–823 K, which was more significant at 10 bar. In the NTC region, at Tini = 723 and 823 K, the ID nearly has a 3-fold variation between 10 and 15 bar, and CD has 4- and 1.7-fold variations at Tini = 723 and 823 K, respectively. Finally, τ has a 3- and 1.5-fold variation at Tini = 723 and 823 K. JP-5 has a shorter ID and CD, followed by JP-5-SBR-50 and SBR. At Tini = 673 K, the addition of JP-5 to SBR significantly shortened ID and CD by nearly 24% and 17.5%. JP-5 has higher CO2, thermal NOx, and unburned HC emissions, whereas those were reduced with SBR addition to JP-5. Moreover, CO emissions at the SBR NTC region (773–823 K) gradually increased more than at any other tested Tini. Meanwhile, the measured ID periods and NTC characteristics for SBR were replicated using a reduced mechanism of 106 species and 2,162 reactions. The reduced mechanism also predicted the measured SBR inhibition effect on the ID periods for JP-5.

Core-shell structure strategy and oxygen vacancy engineering of 0.1LaMnO3@0.9(Ba0.5Sr0.5)TiO3 ceramics towards tuning electrical properties

In recent years, core-shell structures have attracted much attention for their potential in tuning the functional properties of materials and have become an effective method for preparing high-performance materials. In this work, core-shell structured 0.1LaMnO3@0.9(Ba0.5Sr0.5)TiO3 nanoparticles were fabricated by sol-gel method and dense ceramics were prepared at different sintering temperatures. The phase composition, morphology, chemical state and electrical properties were systematically investigated. Structural analysis confirmed the formation of 0.1LaMnO3@0.9(Ba0.5Sr0.5)TiO3 core-shell nanostructures. The results show that the prepared samples exhibit typical negative temperature coefficient characteristics in the range of 50–1000 °C, which is derived from the effect of carrier pairs and oxygen vacancies on the electrical properties. This study provides new insights from structure to material into guiding the development and design of advanced thermistor materials for temperature sensing applications.

Ignition and stabilization of laminar premixed n-heptane/air flames under engine-like conditions

The multi-stage ignition process of a laminar premixed n-heptane/air mixture is investigated under engine-relevant conditions. The cold fuel/air mixture is injected into a hot environment through an inflow boundary at different inlet velocities. Results show that a double-flame structure for stoichiometric mixture is observed and it transitions to a single flame at an inlet temperature (TFuel) of 700 K. The cool flame is stabilized near the inlet for the case with TFuel = 900 K before the hot flame merging with it. However, both the cool and hot flames move upstream for TFuel = 800 K due to the negative temperature coefficient (NTC) behavior. In addition, the effect of inlet velocity is also studied. By increasing the inlet velocity, the hot flame becomes lifted off. Comparisons of the ignition Damkӧhler number (Daign) defined by the ratio of the residence time to the cool- or hot-flame IDTs illustrate the different controlling mechanisms. At a low temperature of 700 K, both low-temperature combustion (LTC) and high-temperature combustion (HTC) have a Daign value of unity, indicating that the stabilization process is mainly controlled by autoignition rather than diffusion. The heating effect from the hot environment enhances the NTC characteristics of the premixed flame, thus, shortening the IDT. Whereas at 800 K, the value of Daign for the LTC is of unity, but that of HTC decreases, especially at a low inlet velocity, indicating that diffusion plays a more important role for HTC and the flame is stabilized by propagation, which can also be confirmed from the comparisons of the 0-D and 1-D ignition delay times (IDTs). Diffusion of intermediate radicals and heat suppresses the appearance of the high-temperature ignition at an inlet velocity of 1 m/s. At 900 K, high inlet velocity can increase the Daign to unity, indicating a transition in the combustion regime from propagation to autoignition.

Electrical conductivity and electrical stability of Bi/Mg modified NiO ceramics for NTC thermistors

Thermistors with negative temperature coefficient (NTC) of resistivity are important components for temperature sensors and actuators. High material constant (B value) of NTC thermistor, i.e. high-temperature sensitivity, is one of key focuses. Herein, Bi/Mg modified NiO based ceramics for NTC thermistors were prepared by conventional solid-state reaction method. Introduction of Bi2O3 significantly enhances the sintering ability of ceramics and reduces the sintering temperature from 1380 to 1250?C. Mg-doping (i.e. preparation of Ni1-xMgxO ceramics, where x=0, 0.02, 0.05, 0.07 and 0.1) has significant effect on room temperature resistivity (?25). Phase composition, microstructure, electrical property and electrical stability were investigated. All prepared ceramics have the phase with rock-salt structure and show typical NTC characteristics with B values higher than 5300K. The electrical stability with an optimized resistance-change rate of 1.02%after being aged at 150?C for 500 h is achieved. The electrical properties of the ceramics were analysed by combining X-ray photoelectron spectra with complex impedance spectra.

An experimental and modeling study on autoignition of 2-phenylethanol and its blends with n-heptane

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.

High-solid content, low viscosity and long-term stability Mn–Co–Ni–O ceramic slurry for the fabrication of free-standing ultrathin NTC sheets

This study systematically investigated the relationships between Mn–Co–Ni–O ceramic slurry formulations and their volatility, rheology and stability, as well as the electrical properties of free-standing ultrathin sheets made from them. A ceramic slurry with a high solid content of 90%, viscosity <12,000 mPa s, and 720 h long-term stability was successfully obtained by optimizing a spherical clusters powder and organic carrier. The powder was formed by secondary aggregation, which reinforced the wettability of its surfaces. Such spherical clusters ensure that the powders created for ceramic slurry are compact and have good fluidity features microscopically. The optimal formula of the organic carrier system contained three components: 90 wt% organic solvent, 5 wt% ethyl cellulose and 5 wt% Span85. Meanwhile, an organic solvent was made with terpineol, butyldiglycol, 2-(2-butoxyethoxy) ethyl acetate at a ratio of 70:10:20. Mn–Co–Ni–O free-standing ultrathin sheets were prepared by a screen printing method using this slurry, which had a resistivity of 278.25 Ω cm at 25 °C, thermal constant of 3404.3 K, activation energy of 0.29 eV, and temperature coefficient of resistance of −3.83%/K. Furthermore, the response time of a small thermal sensor based on this sheet reached 32 ms. Such Mn–Co–Ni–O free-standing ultrathin sheets with excellent negative temperature coefficient characteristics are promising candidates for fast response temperature measurement.

A comparative study of different oxidation states of raw materials on the properties of a novel medium-entropy Co2.77Mn1.71Fe1.10Zn0.42O8 thermistor materials

In recent years, high entropy oxides (HEOs) have aroused extensive attention due to their potential applications in catalysis, energy storage and thermal protection. However, as an emerging material, the synthesis, properties and applications of HEOs have yet to be further developed. Herein, we reported a novel non-equiatomic medium-entropy Co2.77Mn1.71Fe1.10Zn0.42O8 oxides for temperature sensing applications by using the different oxidation states of cobalt and iron as raw materials. Results indicate that the prepared multi-component samples present a pure spinel phase, highly dense microstructure, and typical negative temperature coefficient characteristics. Our results also suggest that the different oxidization states of cobalt and iron of raw materials induce obvious influences on resistivity and aging stability. In particular, the medium-entropy oxides can exhibit excellent superior aging stability (the resistance drift ≤0.2% after accelerated aging in 125 °C for 600 h) by a suitable selection of the starting oxides. This work will provide new material design strategies for sensors or other electronic devices and guidance on the raw material selection from laboratory research to industrial production processes.

New high temperature NTC thermistors based on BaSm2Ti4O12 ceramics

In our research it is firstly found that BaSm2Ti4O12 (BSTO) ceramics only show negative temperature coefficient (NTC) effect in wide temperature range. The effects of sintering temperature on the microstructure and electrical properties of BSTO have been investigated. The relative density of as-sintered ceramics increases at first and then decreases with increasing sintering temperature. In the temperature of 400-850℃, all samples show a linear relationship between the natural logarithm of resistivity and the reciprocal of temperature, indicating the NTC characteristics. The electrical resistivity decreases with increasing sintering temperatures. However, the activation energy Ea400/850℃ and material constant B400/850℃ decrease first and then increase, which are attributed to the variation of oxygen vacancy. The value of resistivity ρ400℃, Ea400/850℃ and B400/850℃ is in the range of 4.8×107–1.43×108Ω•cm, 1.33–1.41 eV and 15300–16400 K, respectively. This material has broad prospects in the field of high-sensitivity high-temperature thermistors research.

Novel electrical properties of Mn-doped LaCrO3 ceramics as NTC thermistors

A series of Mn-doped LaCrO3 ceramics, LaCr1−xMnxO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, the concentration of doping Mn), were synthesized by traditional solid-state reaction and their novel electrical properties as the negative temperature coefficient (NTC) thermistors were first investigated. X-ray diffraction results indicate that the Mn-doped LaCrO3 ceramics formed a solid solution with an orthorhombic perovskite structure when x ≤ 0.6, with a tiny amount of Mn2O3 remaining when x = 0.7. The SEM/EDS images and calculated relative densities show that all the LaCr1−xMnxO3 ceramics had a uniform grain size distribution, accurate stoichiometric ratio and high density over 98.0%, enabling them to have high stability. After aging for 1000 h at 150 °C in air, the resistance shifts ΔR/R0 were less than 0.57%. The investigation of electrical properties manifests that the LaCr1−xMnxO3 ceramics had excellent NTC characteristics, and that their resistivities and thermal constants decreased with increasing Mn content and exhibited high controllability. The ρ25°C, B25/50 and activation energy Ea varied in the range of 15.0–51,505.3 Ω cm, 3191–4301 K, and 0.28–0.37 eV, respectively. This combination of parameters makes this system an outstanding candidate for NTC thermistors applied to a low temperature range from − 100 to 200 °C.

Investigation of electrical and aging properties of Bi-modified (Zn0.4Ni0.6)1-xNaxO ceramic thermistors

Bi-modified (Zn0.4Ni0.6)1-xNaxO (denoted as xZNN, x < 0.1) ceramics were prepared by conventional solid-state reaction process. The xZNN ceramics have rock-salt type structure. Besides the rock-salt phase, there is a monoclinic Bi2O3 secondary phase coexisting in the Bi-modified xZNN ceramics. These ceramics exhibit typical characteristic of negative temperature coefficient (NTC) of resistivity. The NTC material constants (B values) of all prepared ceramics are higher than 4000 K. The room temperature resistivity and B values can be adjusted in a large range by altering either or both content of Na-ions and Bi-ions. The effect of Bi2O3 addition is remarkable in term of enhancing sinter ability to reduce sintering temperature from 1350 °C to 1150 °C, and improving aging stability to drop resistance drift rate from 237 % to 1.8 %. Aging characteristic was discussed based on models of space charge distribution and interfacial barriers.

Nano-negative temperature coefficient thermistor with unique electrical properties of high B constant and low resistivity

Reports on extraordinary electrical properties of nano-scale (< 200 nm)-negative temperature coefficient (NTC) thermistors are rarely seen. In this study, nano-scale Ni0.54Mn1.26Fe1.2O4 NTC thermistors were prepared using the spark plasma sintering (SPS) approach. After SPS sintered at 750 °C and then heat-treated at 650 °C, highly dense material with an average grain size of about 133 nm was obtained. The sintered samples showed NTC characteristics in their resistance–temperature relationships and exhibited higher material constants (B) and lower room temperature resistivities (ρ25) than their micro-scale counterparts. The B constant and ρ25 of a nano-grained sample were 5630 K and 1.55 MΩ·cm, respectively; these of a micro-meter sample, however, were 4930 k and 2.7 MΩ cm, respectively. The differences can be explained by two factors: smaller grains of the nano-scale sample produce a larger number of grain boundaries, which enhances the energy barriers at grain boundaries, which, in turn, increases the B constant; both the grains and grain boundaries of the nano-scale sample follow the model of hopping conduction, which eases electron migrations and stabilization, thereby leading to lower resistivity.

New negative temperature coefficient ceramics in ACu3Ti4O12 (A = Sr, La2/3, Ca, Y2/3) system

In this work, new ACu3Ti4O12 (A = Sr, La2/3, Ca, Y2/3) negative temperature coefficient (NTC) ceramics have been successfully fabricated by traditional solid-state reaction method. XRD analysis shows that the ACu3Ti4O12 ceramics are of the CaCu3Ti4O12 phase with body-centered cubic structure, suggesting that the change of A-site ion does not change the crystal structure. The scanning electron microscope image shows that the difference of A-site ion in ACu3Ti4O12 results in the different micromorphology. The X-ray photoelectron spectroscopy analysis further confirms the coexistence of Cu+/Cu2+ and/or Ti3+/Ti4+ in the compounds, which are one of the important factors for the electrical conductivity in ACu3Ti4O12 ceramics. Electrical performance analysis proves that the effect of grain size on resistivity is greater than that of the A-site ion species. The resistivity decreases with increasing temperature from 25 to 500 °C, indicative of NTC characteristics.

Graphene Fiber-Based Strain-Insensitive Wearable Temperature Sensor

A strain-insensitive wearable temperature sensor based on graphene fibers is proposed to solve the temperature measurement strain interference of wearable temperature sensors. The stress, strain, and conduction characteristics of the sensor are discussed based on finite element simulations and the principle of graphene conduction. The wearable temperature sensor showed excellent negative temperature coefficient characteristics. The responsivity of the sensor is 0.85% °C−1, and the sensing resolution is 0.1 °C. The sensor shows good antiinterference below a tensile strain of 90% or a curved surface with a curvature of 0.5. When the sensor is attached to the human body, it reliably detects skin temperature changes at different curved surfaces.

Effects of Globally Stratified Temperature Distributions and NTC Characteristics on End-gas Combustion Modes

ABSTRACT This study numerically investigates the effects of stratified temperature distributions on the end-gas combustion mode in a constant volume reactor. The initial temperature in the reactor was globally stratified with linear gradients. The effects of negative temperature coefficient (NTC) characteristics are addressed through a comparison between the results of a non-NTC fuel and an NTC fuel. The compressible Navier–Stokes equations are solved with detailed chemistry. The results show that the addition of large temperature gradients such as −10 or −5 K/cm can prevent the pressure wave development associated with end-gas autoignition, and the knocking intensity is significantly reduced. In contrast, small temperature gradients such as −1 K/cm lead to the generation of the developing detonation mode and thus to large knocking intensities. In the NTC fuel case, the knocking intensity becomes relatively large because the spatial gradient of the ignition delay time decreases even with the addition of large temperature gradients, especially in the NTC regime. Based on all the results, a correlation is presented between the initial temperature and the reaction front speed (the spatial gradient of the ignition delay time) for the prediction of end-gas combustion mode. The correlation suggests that the possibility of preventing large knocking intensities by temperature gradients is reduced under high-pressure conditions because of the increase in the reaction front speed. Consequently, this study suggests that very large temperature gradients would be required under high-pressure conditions to prevent large knocking intensities under the concept of thermal stratification.

Fabrication of high-sensitivity Bi0.2-xCexSr0.5La0.3TiO3 (0

High density negative temperature coefficient (NTC) thermistor ceramics Bi0.2-xCexSr0.5La0.3TiO3 (0 < x < 0.05) (BCSLTO) were synthesized by the Pechini method. This work is the first to characterization analysis the excellent NTC characteristics of BCSLTO ceramics at high temperatures. BCSLTO NTC thermistor ceramics have wide operating temperature is 300 °C-1000 °C, and mild sintering conditions with the conventional sintering at 1250 °C. The important electrical performance parameters of BCSLTO ceramics are as follows: the resistivities at 300 °C, 700 °C, and 1000 °C are 3.26 × 109–8.99 × 107 Ω cm, 2.21 × 105–1.05 × 105 Ω cm, and 1.18 × 103–1.13 × 103 Ω cm, respectively; the thermal constant B300/700 is 13385.9–9408.8 K; the activity energy Ea300/700 is 1.15–0.81 eV. The resistivity and the thermal constant B can be widely control by doping Ce. In addition, the resistivity drift improves as the Ce doping content increases (below 5%). The new high-temperature Bi0.2-xCexSr0.5La0.3TiO3 ceramic materials with mild sintering conditions, wide operating temperature and high-temperature stability can be used as potential candidates for high temperatures and harsh environments.

Effect of phase structures and substrate temperatures on NTC characteristics of Cu-modified Ba–Bi–O-based perovskite-type thermistor thin films

Based on the formula of BaBi0·92Cu0·08O3, Cu-modified Ba–Bi–O-based perovskite-like negative temperature coefficient (NTC) thermistor thin films were successfully prepared by radio frequency (RF) magnetron sputtering method with the various deposition temperatures (25 °C, 100 °C, 200 °C and 250 °C). The effects of the substrate temperatures on the phase structures and electrical properties were respectively characterized and investigated by X-ray diffraction (XRD), resistance-temperature measurement, and complex impedance analysis for the Ba–Bi–O-based thin films. Additionally, the cross-sectional microstructures and grain morphologies of the BaBi0·92Cu0·08O3 thin films were analyzed by scanning electron microscope (SEM) and atomic force microscope (AFM). The XRD patterns results showed that the perovskite monoclinic structure with a small amount of BaBiO2.5 s phase could be obtained for the BaBiO3 thin film deposited at 200 °C, and also, the main crystalline phase was transferred from BaBiO2.82 to BaBiO2.5 when the BaBi0·92Cu0·08O3 thin films were deposited at 200 °C and 250 °C, respectively. Temperature dependent complex impedance spectroscopy was carried out to make clear the resistance contribution of grain (Rg), grain boundary (Rgb) and contact (Rs) response for the conduction mechanisms in the Ba–Bi–O-based thin film deposited at a given substrate temperature. Compared with the electrical properties of the BaBiO3 thin film deposited at 200 °C [such as the room-temperature resistivity (ρ25 ~ 1548 Ω cm) and the thermistor constant (B25/85–3245 K)], the better NTC characteristics of a lower ρ25 value of about 956 Ω cm and a higher B value of approximately 3480 K were found in the BaBi0·92Cu0·08O3 thin film deposited at a same temperature (200 °C). It is believed that the phase structures and electrical properties of the BaBiO3 perovskite-type NTC thermistor thin films could be adjusted and optimized by a trace amounts of Cu-ions substitutions and depositing at an appropriate substrate temperature, which also showed that such as these novel BaBiO3-based thin films are feasible with the extensive of actual applications for the NTC thermistors.

Highly dense 0.3CaCeNbWO8‐0.7LaMnO3 composite ceramics fabricated by cold sintering process

AbstractThe cold sintering process (CSP) has been used for fabricating functional ceramics at a low sintering temperature. In this study, highly dense 0.3CaCeNbWO8‐0.7LaMnO3 composite ceramics have been successfully fabricated by CSP. The phase structure, microstructure, and electrical properties of composite ceramics have been investigated. The composite ceramic is mainly composed of a tetragonal CaCeNbWO8 phase with scheelite structure and an orthorhombic LaMnO3 phase with perovskite structure. The relative density of composite ceramic is 94.5%, and is higher than that of single phase ceramic. The resistivity of composite ceramic exhibits negative temperature coefficient characteristics, with an aging coefficient less than 2%. Such a sintering methodology is of great significance, since it provides a feasible idea for preparing composite ceramics.

Aging characteristic of Cu-doped nickel manganite NTC ceramics

The drift in electrical properties (e. g., the resistivity and B-value) is vital to the practical application for negative temperature coefficient (NTC) ceramics. In this work, Cu-doped nickel manganite ceramics were prepared by solid-state method, to investigate the degradation mechanism of both the resistivity and B-value. Results showed that the as-prepared ceramics exhibited typical NTC characteristics with cubic spinel phase, and that Cu doping greatly reduced the resistivity and B-value. However, the increase of Cu content in system aggravated the drift in the resistivity and B-value after the samples were aged at 300 °C for 30 h. XPS was carried out to further clarify the change in electrical properties. It was found that the decline of properties was due to the decrease of Mn4+ content in system after aging, which derived from the increase of chemical valence of Cu.