Valence-induced effects on the electrical properties of NiMn2O4 ceramics with different Ni sources

Spinel-structured NiMn₂O₄ ceramics, with different valence Ni sources, were originally prepared using Ni₂O₃ and NiO as raw materials, and the effects of different valence Ni sources on their electrical properties were first investigated. XRD patterns show that both Ni₂O₃-based and NiO-based NiMn₂O₄ ceramics are single cubic spinel structures. SEM/EDS images indicate that the NiMn₂O₄ ceramics exhibited high density at the experiment-determined sintering temperatures. XPS results and Raman drifts prove that the Ni valence-induced changes in Mn ions at B sites played a significant role in the electrical properties and thermal stability of NiMn₂O₄ ceramics. Compared with NiO-based NiMn₂O₄, the resistivity at 25°C (ρ_25°C) of Ni₂O₃-based NiMn₂O₄ increased dramatically from 3109 to 106958 Ω cm, the thermal constant (B_25/50) increased from 3264 to 4473 K, and the resistance shifts after annealing for 1000 h at 150°C decreased from 0.80% to 0.74%. The investigation of the relationship between the material properties and valence of Ni sources has provided a new and effective way for designing the spinel-structured negative temperature coefficient (NTC) materials by modulating the valence of ions at A sites in the raw materials.     

LaCr1-xFexO3 (0≤x≤0.7): A novel NTC ceramic with high stability

Perovskite-phase LaCr_1-xFe_xO₃ (0 ≤ x ≤ 0.7) ceramics were fabricated by two-step sintering of powders synthesized by conventional solid-state reaction. XRD patterns reveals that the ceramics are single-phased LaCr_1-xFe_xO₃ solid solution at lower Fe contents, but form a biphasic mixture of LaCrO₃-LaFeO₃ at higher Fe contents. SEM analysis indicates that Fe doping causes a reduction in porosity, and a corresponding increase in density. The effects of Fe doping on electrical properties have also been investigated. The electrical resistivity ρ_25°C decreases initially and then increases within a wide range of 634.4–83915.9 Ω·cm with increasing Fe content, whereas B varies within a relatively narrow range of 3651−4301 K. Such combination enables it to be a potential candidate for NTC thermistors. An investigation of elevated-temperature aging behavior shows that the resistance shifts at 25 °C were less than 0.65 % after annealing at 125 °C in air for 1000 h, revealing that the material exhibits excellent stability.     

LaMn1-xTixO3-NiMn2O4(0≤x≤0.7): A composite NTC ceramic with controllable electrical property and high stability

A series of NTC thermistor ceramics based on LaMn_1-xTi_xO₃-NiMn₂O₄(0≤x≤0.7) composite system have been fabricated by solid-state method. X-ray diffraction analysis indicates the composite ceramics mainly consisting of a rhombohedral perovskite LaMn_1-xTi_xO₃ phase and a cubic spinel NiMn₂O₄ phase. SEM images show high density of the as-prepared composite ceramics. The effects of the Ti doping and the weight ratio of LaMn_1-xTi_xO₃/NiMn₂O₄ on electrical property have been studied. The electrical resistivity ρ increases significantly with Ti addition increasing and decreases obviously with LaMn_1-xTi_xO₃ concentration increasing. The ρ_25°C and B values are in the range of 3.2–53,200.0Ω·cm and 1300–4008 K, respectively, and could be adjusted to desired values and applied in various fields. The conductive mechanism may be related to the ion migration and the percolation theory. After annealing at 125°C for 1000h, the resistance shifts are less than 0.52%, suggesting good stability of the composite and high potential for NTC thermistor applications.     

CaMn0.05Zr0.95O3–NiMn2O4 composite ceramics with tunable electrical properties for high temperature NTC thermistors

A series of novel high temperature negative temperature coefficient (NTC) composite ceramics based on (1-x)CaMn_0.05Zr_0.95O₃-xNiMn₂O₄ (x = 0, 0.1, 0.2, 0.3) were fabricated by using the solid-state method. The Rietveld refinement method and backscattered electron (BSE) image confirmed the absence of any other phase. The conduction mechanism of the composite ceramics was determined by analyzing the complex impedance and resistance-temperature characteristics, which could be related to the formation of a continuous Mn³⁺-O-Mn⁴⁺ network. The effect of spinel content on the high temperature stability was analyzed using aging tests. The ρ_400°C, ρ_700°C and B_400/700 values of the NTC thermistors were determined to be 4.9 × 10⁶–1.2 × 10⁴, 1.4 × 10⁵–0.8 × 10³ Ω cm and 12739-5712 K, respectively. This indicated that the electrical properties could be tuned by adjusting the weight ratio of CaMn_0.05Zr_0.95O₃ and NiMn₂O₄. The findings obtained in this study reveal that the composite NTC thermistor exhibits a good application potential at high temperatures and under harsh environments.     

Development of La(CrCoFeNi)O3 system perovskites as interconnect and cathode materials for solid oxide fuel cells

The purpose of this research is to develop interconnect and cathode materials for use in solid oxide fuel cells (SOFCs) which demonstrate desired properties of outstanding sintering properties, high electrical conductivity, and excellent chemical stability at high temperatures. Five different perovskite oxides of lanthanum in combination with chromium, iron, cobalt and nickel oxides powders, i.e. LaCr_0.7Co_0.1Fe_0.1Ni_0.1O₃(LCr7CFN), LaCo_0.7Cr_0.1Fe_0.1 Ni_0.1O₃(LCo7CFN), LaFe_0.7Cr_0.1Co_0.1Ni_0.1O₃(LFe7CCN), LaNi_0.7Cr_0.1Co_0.1Fe_0.1O₃(LNi7CCF), and LaCr_0.25Co_0.25Fe_0.25Ni_0.25O₃(LCCFN), were synthesized through the Pechini method. XRD results show that all materials are in single phase, either rhombohedral or orthorhombic crystal structure. The resulting powders were able to be sintered to a high relative density at a temperature of 1400 °C for 2 h in air. The electrical conductivity of the sintered sample was measured and evaluated from 300 °C to 800 °C. The LCCFN sample appears to have the best combination of sintering property (approximate 94% relative density) and electrical conductivity (88.13 Scm⁻¹ at 800 °C).     

Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites

The Graphene/Mn_0.7Zn_0.3Fe₂O₄ composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343–973 K. The effect of graphene on the thermoelectrical properties of Mn_0.7Zn_0.3Fe₂O₄ was investigated. The dispersion of 2 wt% graphene in Mn_0.7Zn_0.3Fe₂O₄ effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973 K was obtained in the composite with 2 wt% graphene.     

High-conducting Bi4V2−xFexO11-δ ceramics containing Fe2O3 nanocrystals: Structure and properties

The topography, structure, thermal, magnetic, and electrical properties of Bi₄V_2?xFe_xO_11-δ ceramics substituted with x = 0.5 and 0.7 Fe were studied. The microscope analysis showed the presence of iron-rich nanocrystals formed on the Bi-Fe-V-O grains. The X-ray diffraction studies confirmed that grains are built mostly of tetragonal Bi₄V_1.5Fe_0.5O_10.5 phase. Thermal properties analysis showed an order-disorder type γ ? γ? phase transition at a temperature of around 916 K, pronounced in samples doped with x = 0.5 Fe. The magnetic anomaly was observed in ceramics doped with x = 0.7 Fe which was assigned to Morin transition of Fe₂O₃. The conductivity was measured over a wide frequency range from 10 mHz to 1 MHz and at a wide temperature range from 373 to 923 K, using impedance spectroscopy. The D.C. conduction process was due to oxygen vacancies hopping while at low temperatures electron holes hopping is also possible.     

Redox stability and electrical conductivity of Fe2.3Mg0.7O4±δ spinel prepared by mechanochemical activation

This paper addresses the potential of mechanochemical activation of MgO and α-Fe₂O₃ precursor powders to obtain Fe_2.3Mg_0.7O₄ ceramics with enhanced redox stability and electrical conductivity. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest the initial formation of the spinel phase after 5 h of high-energy milling in inert gas, but after 10 h of mechanoactivation, the precursor still comprised hematite as a major phase with minor amounts of magnesiowustite as by-product. The activated mixtures can be nearly completely converted to spinel solid solution by heating to 1173 K, whereas single-phase, dense spinel ceramics can be prepared by sintering at 1773 K in inert atmosphere. These ceramics demonstrated redox stability under mildly reducing conditions (p(O₂) ～ 10 Pa), as confirmed by XRD, thermogravimetry and electrical measurements. The electrical conductivity of Fe_2.3Mg_0.7O₄ at this oxygen partial pressure is lower compared to magnetite, but it is still as high as 60 S/cm at 1073 K and 15 S/cm at room temperature. Cooling below 1473 K in air results in a drop of conductivity due to segregation of hematite phase at the grain boundaries. However, the phase separation is kinetically stagnated at 1073 K, and, after slight initial degradation, the retained electrical conductivity is more than 3 orders of magnitude higher compared to hematite and MgFe₂O₄ spinel.     

High piezoelectric properties in 0.7BiFeO3–0.3BaTiO3 ceramics with MnO and MnO2 addition

BiFeO₃–BaTiO₃–based solid solutions are promising candidates for high–temperature piezoelectric devices because of their high Curie temperature (T_C) and considerable electrical properties. Here, we reported an optimum composition of 0.7Bi(Fe_0.999Mn_0.001)O₃–0.3BaTiO₃ ceramic with a large piezoelectric constant (d₃₃) of 230 pC/N and a high T_C of 505 °C, which was attributed to the intentional introducing of the ceramic with MnO and MnO₂ mixture. Furthermore, an in situ d₃₃ measurement was carried out, demonstrating excellent thermal stability for the 0.7Bi(Fe_0.999Mn_0.001)O₃–0.3BaTiO₃ specimen. The d₃₃ remained above 200 pC/N in the temperature range of 25 °C–400 °C and its fluctuation was less than ± 15 %. It was determined that the high d₃₃ in the 0.7BiFe_0.999Mn_0.001)O₃–0.3BaTiO₃ ceramic originated from a synergistic effect of rhombohedral distortion, intrinsic response, and ferroelectric order. The findings establish a solid correlation between electrical properties and phase/domain structure, and provide a novel approach to improve the piezoelectric properties for BiFeO₃–BaTiO₃–based ceramics.     

Fe3O4-intercalated reduced graphene oxide nanocomposites with enhanced microwave absorption properties

Fe₃O₄-intercalated reduced graphene oxide (Fe₃O₄-rGO) nanocomposites were synthesized by an in situ reduction process. The results of XRD and XPS analyses suggested the successful formation of a Fe₃O₄ crystal phase within the rGO sheets. The SEM and TEM images demonstrated that Fe₃O₄ was flaky and was inserted stably within the rGO layers to form a typical sandwich-like structure. The hysteresis loops revealed the superparamagnetic behavior of the Fe₃O₄-rGO nanocomposites at room temperature. The electromagnetic parameters revealed that Fe₃O₄-rGO nanocomposites exhibited multiple dielectric relaxation and magnetic resonance. The reflection loss revealed that the maximum loss was −49.53 dB at 6.32 GHz for a thickness of 3.4 mm while the highest effective absorption bandwidth was 2.96 GHz.     

Thermoelectric properties of ytterbium interstitial doped Sr0.7Ba0.3YbxNb2O6-δ ceramics

The thermoelectric properties of interstitial doped Sr_0.7Ba_0.3Yb_xNb₂O_6-δ ceramics were investigated in the temperature range from 323 K to 1073 K. The ytterbium interstitial doping contributes to increasing the power factor (PF) by improving the electrical conductivity. The PF values of Sr_0.7Ba_0.3Yb_0.1Nb₂O_6-δ are comparable to those of the substitution doped sample with twice the doping content. This can be ascribed to the large electronic charge of the dopants relative to the occupied site in the interstitial case. The lattice thermal conductivity of Sr_0.7Ba_0.3Yb_xNb₂O_6-δshows a glass-like behavior due to lattice disorder. The disorder degree deepens as the doping content increases, resulting in plateau values of the lattice thermal conductivity at high temperatures in the Sr_0.7Ba_0.3Yb_0.05Nb₂O_6-δ and Sr_0.7Ba_0.3Yb_0.1Nb₂O_6-δ samples.     

Sintering and conductivity of Sc-doped CaZrO3 with Fe2O3 as a sintering aid

This paper reports the effect of 0.1–0.5 wt% Fe₂O₃ addition on sintering and electrical properties of CaZr_0.95Sc_0.05O_3-δ ceramics synthesized by combustion method. Addition of the sintering aid was shown to enhance ceramic densification and grain coarsening at a reduced sintering temperature and a shorter holding time (1430 °C, 2 h). Effect of the sintering aid on electrical conductivity of the ceramics was investigated using impedance spectroscopy. The highest total conductivity was achieved for the composition with 0.5 wt% Fe₂O₃; it was about an order of magnitude higher than that of the composition without Fe₂O₃. The effect of Fe₂O₃ addition on the conductivity of the grain interior and grain boundaries has been discussed. It was concluded that ceramic densification, grain coarsening and formation of small amounts of calcium ferrite at the grain boundaries upon Fe₂O₃ addition were responsible for the conductivity enhancement.     

Microwave dielectric properties of ultra-low loss Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics sintered at low temperature by LiF addition

In this work, ultra-low loss Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄ ceramics were successfully prepared via the conventional solid-state method. X-ray photoelectron spectroscopy (XPS), thermally stimulated depolarization current (TSDC) and bond energy were used to determine the distinction between intrinsic and extrinsic dielectric loss in (Mg_1/3Nb_2/3)⁴⁺ ions substituted ceramics. The addition of (Mg_1/3Nb_2/3)⁴⁺ ions enhances the bond energy in unit cell without changing the crystal structure of Li₂MgTiO₄, which results in high Q·f value as an intrinsic factor. The extrinsic factors such as porosity and grain size influence the dielectric loss at lower sintering temperature, while the oxygen vacancies play dominant role when the ceramics densified at 1400?°C. The Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄ ceramics sintered at 1400?°C can achieve an excellent combination of microwave dielectric properties: ε_r =?16.19, Q·f?=?160,000?GHz and τ_f =??3.14?ppm/°C. In addition, a certain amount of LiF can effectively lower the sintering temperature of the matrix, and the Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄-3?wt% LiF ceramics sintered at 1100?°C possess balanced properties with ε_r?=?16.32, Q·f?=?145,384?GHz and τ_f =??16.33?ppm/°C.     

Large electric field induced strain of Bi(Mg1/2Ti1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics textured by Template Grain Growth

<001> oriented xBi(Mg_1/2Ti_1/2)O₃-(0.7-x)Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (BMT-PMN-PT) textured ceramics are successfully fabricated by the template grain growth method using BaTiO₃ platelets as template. BMT-PMN-PT textured ceramics with different BMT contents are studied in terms of crystal structure, microstructures, dielectric and ferroelectric properties, and electric field induced strain. The as-fabricated BMT-PMN-PT textured ceramics were found to have a strong orientation along <001> direction. The frequency dispersion of dielectric constant of BMT-PMN-PT textured ceramics increases gradually and its relaxability becomes stronger with increasing BMT content. A large electric-field induced strain (0.42 % at 4 kV/mm) is obtained in 0.25BMT-0.45PMN-0.3PT textured ceramics with Lotgering factor 0.94, which is about 83 % enhancement than that of the randomly oriented ceramics (0.23 % at 4 kV/mm). The strain of 0.25BMT-0.45PMN-0.3PT textured ceramics have a relatively high thermal stability, with a slight decrease from 0.42 % to 0.28 % in the temperature range of 20−100 °C. Our research suggests that 0.25BMT-0.45PMN-0.3PT textured ceramics have a greatly potential for actuator devices applications owing to its advantages of large electric field induced strain response.     

Phase structure and thermophysical properties of co-doped La2Zr2O7 ceramics for thermal barrier coatings

La₂Zr₂O₇ has high melting point, low thermal conductivity and relatively high thermal expansion which make it suitable for application as high-temperature thermal barrier coatings. Ceramics including La₂Zr₂O₇, (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ were synthesized by solid state reaction. The effects of co-doping on the phase structure and thermophysical properties of La₂Zr₂O₇ were investigated. The phase structures of these ceramics were identified by X-ray diffraction, showing that the La₂Zr₂O₇ ceramic has a pyrochlore structure while the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ exhibit a defect fluorite structure, which is mainly determined by ionic radius ratio r(A_av.³⁺)/r(B_av.⁴⁺). The measurements for thermal expansion coefficient and thermal conductivity of these ceramics from ambient temperature to 1200 °C show that the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ have a larger thermal expansion coefficient and a lower thermal conductivity than La₂Zr₂O₇, and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ shows the more excellent thermophysical properties than (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ due to the increase of Yb₂O₃ content.     

Evolution of microstructure and electrical properties of Aurivillius phase (CaBi4Ti4O15)1-x(Bi4Ti3O12)x ceramics

(CaBi₄Ti₄O₁₅)_1-x(Bi₄Ti₃O₁₂)_x (CBT-xBIT) Aurivillius phase ceramics were synthesized by the conventional solid reaction method. The evolution of the structure and the electrical properties of CBT-xBIT ceramics were systematically investigated. Due to the enhanced spontaneous polarization induced by internal stresses on the Bi₂O₂ layers in the CBT-xBIT structure, the optimal piezoelectric coefficient (d₃₃ ~ 13?pC/N) was obtained in the ceramics with x?=?0.3 while exhibiting a relatively good thermal stability in the temperature range of 20–700?°C. The dc resistivity (ρ_dc) of the CBT-xBIT ceramics exhibited a higher value (≥?10⁹ Ω?cm) at room temperature, and the tan δ value of CBT-xBIT (x= 0, 0.1 and 0.3) within the temperature range of 20–500?°C maintained stability as a result of the domain structure and point defect concentration in the ceramics. In addition, a distinctive double dielectric peak anomaly was observed in the ε_r-T curves of the CBT-xBIT (x= 0.3, 0.5 and 0.7) ceramics, and it plays a remarkable role in the thermal stability of the piezoelectricity of CBT-xBIT ceramics. As a result, such research can benefit high temperature practical piezoelectric devices.     

High‐Temperature Electrical Conductivity of LaCr1−xCoxO3 Ceramics

LaCr_1?xCo_xO₃ solid‐solution ceramics (x = 0.0–0.3) were prepared by pressureless sintering of a submicrometer powder. The powder was synthesized by a modified glycine nitrate process at 800°C. The electrical conductivity of the material sintered at 1600°C was measured by AC four‐wire method from room temperature to 1200°C. While undoped (x = 0) LaCrO₃ revealed semiconductivity dominated by thermally activated mobility of small polarons over a vast temperature range, substitution of Co for Cr gave rise for a pronounced enhancement of conductivity at temperatures >200°C. XPS analysis showed that the concentration of Cr⁴⁺ on the Cr‐site and Co²⁺ at the Co‐site increased with Co substitution suggesting a thermally activated redox reaction Cr³⁺ + Co³⁺→Cr⁴⁺ + Co²⁺ to create additional charge carriers. Thus, Co doping offers a high potential for designing the electrical conductivity making LaCr_1?xCo_xO₃ an interesting resistivity material for high temperature applications.     

Dielectric and impedance spectroscopic investigation of the Ba0.3Sr0.7Ti0.873Zr0.097Mn0.03O3 ceramic system

Polycrystalline Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics were processed via a mixed-oxide solid state sintering route at 1500 °C in air for 6 h. X-ray diffraction and scanning electron microscopy were used for phase and microstructural analyses of the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The magnitude of relative permittivity (ε_r) was measured as 247 and tan δ as 0.008 at 1 kHz around the room temperature. Complex impedance spectroscopy (CIS) analysis was used to understand the electronic microstructure of the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The magnitude of total resistance (R_T) was observed to decrease with increasing temperature, which confirmed a negative temperature coefficient of resistance (NTCR) for the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ ceramics. The Cole–Cole plot showed only one semicircle for the sintered sample, which confirmed the grain boundary contribution to the conduction mechanism. A non-Debye type of relaxation behavior was observed in the Ba_0.3Sr_0.7Ti_0.873Zr_0.097Mn_0.03O₃ sintered sample.     

Magneto-electric properties of xNi0.7Zn0.3Fe2O4 – (1-x)BaTiO3 multiferroic composites

Di-phase ceramic composites, with general formula xNi_0.7Zn_0.3Fe₂O₄ – (1-x)BaTiO₃(x = 0.9, 0.7, 0.5, 0.3, 0.1), were prepared by a mixing method. X-ray analysis, for powder and ceramics, indicated the formation of ferrite and barium titanate phases without the presence of the impurities. SEM analysis indicated that the composite morphology contained two types of grains, polygonal and rounded. Homogeneous microstructure and the smallest grain size were obtained in ceramics with 70% of barium titanate. The electrical properties of these materials were investigated using impedance spectroscopy, dielectric and ferroelectric measurements. The NZF-BT(30-70) composite has shown better electrical properties in comparison to other investigated ceramics, confirmed by dielectric and ferroelectric data analysis. Saturation magnetization and coercive field decreased with the increase of the content of ferroelectric phase.     

Lead-free 0.7BiFeO3-0.3BaTiO3 high-temperature piezoelectric ceramics: Nano-BaTiO3 raw powder leading to a distinct reaction path and enhanced electrical properties

Herein, nano-BaTiO₃/Bi₂O₃/Fe₂O₃ and BaCO₃/TiO₂/Bi₂O₃/Fe₂O₃ were respectively used to prepare 0.7BiFeO₃-0.3BaTiO₃ (0.7BF-0.3BT) ceramics by conventional solid-state method and clarify the reaction path, phase structure, microstructure, ferroelectric, and piezoelectric properties. 0.7BF-0.3BT ceramic using nano-BaTiO₃ with cubic phase (C_BT) undergoes the formation of rhombohedral α-phase (R_α) and the transition from R_α and C_BT to rhombohedral β-phase (R_β) and pseudo-cubic (PC) phases, while the counterpart using BaCO₃/TiO₂ directly generates R_β and PC. Nano-BaTiO₃ can decrease pores and oxygen vacancies in the resultant ceramics comparing to BaCO₃/TiO₂, which is due to an inhibited decomposition of R_α and a weaker reduction of Fe³⁺ to Fe²⁺, leading to increased density and reduced leakage current density. Benefitting from a proper phase ratio, increased density and reduced leakage current density, the enhanced piezoelectric properties d₃₃?=?210?pC/N, k_p?=?0.34, P_r?=?31.2?μC/cm² and T_C?=?514?°C are obtained in 0.7BF-0.3BT ceramic using nano-BaTiO₃. Our work reveals the importance of raw materials and the potential of BF-BT as a high-temperature lead-free piezoelectric ceramic material.