Diffuse Phase Transition Research Articles

Influence of Ba-doping on the structural and physical properties of Sr2−xBaxFeVO6 double perovskites

The influence of A-site Ba-doping on the structural, dielectric, magnetic, and optical properties of double perovskite (BaxSr2−x)FeVO6 (BSFV(x): x = 0.0–2.0) ceramics prepared by solid-state reaction method, is reported. X-ray diffraction patterns and Rietveld refinements revealed that the BSFV(x) samples with x = 0.0–1.0 crystallized in a cubic crystal structure with an Fm3̅m symmetry whereas the samples with x = 1.5 and 2.0 crystallized in a hexagonal crystal structure with R3c symmetry. Their unit cell volumes increased progressively with the Ba-doping content due to the steric effect generated by the progressive increase of the average ionic radius at A-site, 〈rA〉. SEM images revealed the BSFV(x) samples were highly dense with ceramic grains of polyhedral morphology. Their average grain sizes increased from 2.0(2) μm (x = 0.0) to 5.0(3) μm (x = 2.0). X-ray energy dispersive spectroscopy data gave out the chemical compositions of these samples, which were close to the nominal stoichiometric values. FTIR spectra verified the vibrating modes of the [FeO6] and [VO6] octahedrons in the present samples. XPS spectra confirmed dual chemical valence states of Fe element (Fe2+ and Fe3+) and a triple oxidation states of V element (V3+, V4+ and V5+) as well as two species of oxygens in the BSFV(x) samples. The BSFV ceramics exhibited a strongly frequency-dependent dielectric behavior and a diffuse phase transition, which were resulted from the dielectric response of oxygen vacancies and the electron hopping between the variable cations at B-site as well as the B-site cationic random occupancy. Magnetic Curie temperature (TC) and effective magnetic moment (μeff) decreased as increasing the Ba-doping content due to the weakening of superexchange magnetic interactions between the Fe and V ions. The optical band gaps (Eg) of the BSFV(x) samples extracted from UV-Visible spectra, were in the range of 1.94 – 2.42 eV, exhibiting a blue shift towards semiconductor range as increasing the Ba-doping content. The overlapping between the O-2p and Fe/V-3d orbitals contributes to the Eg in the BSFV samples.

Fortified relaxor ferroelectricity of rare earth substituted 4-layered BaBi3.9RE0.1Ti4O15 (RE = La, Pr, Nd, and Sm) Aurivillius compounds

In this report, the effect of rare-earth (RE3+) ion substitution on structural, microstructural, and electrical properties in barium bismuth titanate (BaBi4Ti4O15) (BBTO) Aurivillius ceramics has been investigated. The Rietveld refinements on X-ray diffraction (XRD) patterns confirm that all the samples have an orthorhombic crystal system with A21am space group. Meanwhile, temperature dependent synchrotron XRD patterns reveal that the existence of dual phase in higher temperature region. The randomly oriented plate-like grains are experimentally strived to confirm the distinctive feature of bismuth layered Aurivillius ceramics. The broad band dielectric spectroscopic investigation signifies a shifting of ferroelectric phase transition (Tm) towards low temperature region with a decrease of the RE3+-ionic radii in BBTO ceramics. The origin of diffuse ferroelectric phase transitions followed by stabilization of the relaxor ferroelectric nature at high frequency region is explained using suitable standard models. The temperature dependent ac and dc conductivity results indicate the presence of double ionized oxygen vacancies in BBTO ceramics, whereas the dominance of single ionized oxygen vacancies is observed in RE-substituted BBTO ceramics. The room temperature polarization vs. electric field (P–E) hysteresis loops are shown to be well-shaped symmetric for BBTO ceramics, whereas slim asymmetric ferroelectric characteristics developed at RE-substituted BBTO ceramics.

Grain size engineering enhanced dielectric, ferroelectric and energy storage properties in SnO2 modified BCZT lead-free ceramics

Grain size engineering is considered as an extremely effective method to realize high electric breakdown strength and enhance the recoverable energy density. In this work, the SnO2 additive is proposed to drive the grain size smaller and enhance the energy storage performance of the (Ba0.85Ca0.15)(Zr0.2Ti0.8)O3 lead-free ceramics. The (Ba0.85Ca0.15)(Zr0.2-xSnxTi0.8)O3 with x = 0.05, 0.10, 0.15, and 0.20 for energy storage application are fabricated via the conventional solid phase reaction method. The influence of Sn4+ addition on the microstructure, dielectric, ferroelectric and energy storage properties of (Ba0.85Ca0.15)(Zr0.2-xSnxTi0.8)O3 ceramics are investigated. As expected, the smallest average grain size and highest diffused phase transition are obtained when x = 0.10. Meanwhile, a maximum electric breakdown field strength 92.08 kV/cm and a recoverable energy storage of 422 mJ/cm3 accompanied with the improved energy storage efficiency of 83.3% are noted. The mentioned results suggest that the effective control of Sn4+ amount in BCZT ceramics is helpful to reduce the grain size and then obtain higher energy storage density and efficiency at relatively low electric field strength.

Giant room-temperature electrocaloric effect within wide temperature span in Sn-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 lead-free thin films

The electrocaloric (EC) effect cooling technique of environmentally friendly lead-free thin film materials driven by electric fields has recently gained tremendous attention due to the urgent demand for microelectronic and integrated circuit refrigeration devices. However, the widespread use of lead-free materials in EC devices is seriously hindered due to the small electrocaloric temperature change (ΔT) within a narrow operation temperature span (Tspan) near room temperature. Here, lead-free Ba0.85Ca0.15Zr0.1Ti0.9-xSnxO3 (BCZT-xSn, 0 ≤ x ≤ 0.03) thin films were prepared on substrates (Pt/Ti/SiO2/Si) via a sol-gel route. The BCZT-0.02Sn thin film presents an excellent EC effect (ΔT = 32.74 K, ΔS = 37.18 J kg−1 K−1) and large EC strength (ΔT/ΔE = 0.033 K cm kV−1, ΔS/ΔE = 0.037 J cm K−1 kg−1 kV−1) over a wide Tspan (∼26 K) under 1000 kV cm−1 near room temperature. The giant ΔT is mainly attributed to the emergence of an intermediate O phase and the formation of a multiphase (R, O and T phases) coexistence structure at room temperature, while the diffuse phase transition behavior is responsible for the wide Tspan. Our study provides a new idea for developing environment-friendly EC materials with an excellent room-temperature ΔT over a broad operational temperature region.

Effect of sintering additives on the densification and dielectric properties of Sr0.5Ba0.5Nb2O6 ceramics synthesized by a soft-chemistry method

Nano-crystalline Sr0.5Ba0.5Nb2O6 powders with addition of LiNbO3 or LiF as sintering additives were prepared by a soft-chemistry synthesis using polyethylene glycol. Calcination at 600 ​°C results in nanocrystalline powders (dcryst. ​≈ ​30 ​nm) which were sintered between 1000 and 1300 ​°C to ceramic bodies. By addition of 10 and 20 ​mol% LiNbO3, the sintering temperature was reduced by about 200 ​K and the activation energy of the initial stage of sintering decreases from 386 to 271 ​kJ ​mol−1. The sintering aid improves the grain growth and dense ceramic bodies were obtained after sintering at 1125 ​°C for 1 ​h. A higher LiNbO3 content favors the formation of a pillar-like microstructure. XRD patterns of ceramics sintered above 1000 ​°C show only reflections of the Sr0.5Ba0.5Nb2O6 phase indicating an incorporation of LiNbO3 in the Sr0.5Ba0.5Nb2O6 structure. Dielectric measurements reveal a diffuse phase transition and a relaxor-like behavior. The phase transition temperature (Tm) depends on the sintering conditions and is between 117 and 127 ​°C for 10 ​mol% LiNbO3, which is very close to pure Sr0.5Ba0.5Nb2O6, while addition of 20 ​mol% shifts Tm to around 200 ​°C and the transition becomes slightly more diffuse. The optical band gap of the samples is ​≈ ​3.3 ​eV and depends slightly on the sintering conditions. We also tried LiF as sintering aid, but this leads to the formation of considerable amounts of secondary phases in the ceramics and relative densities of only 82%.

Effect of Poling on Dielectric Properties of PMN-PT Ceramics

This article presents the phase transition analysis of PbMg1/3Nb2/3O3-PbTiO3 ceramics. The ceramics were prepared by a conventional solid-state reaction method in two steps. Structural analysis confirms the single phase formation without any impurity phases. Temperature dependent dielectric properties were measured on unpoled and poled ceramics. It is observed that poling process helps to identify diffuse phase transitions.

Bimodal-Structured 0.9KNbO3-0.1BaTiO3 Solid Solutions with Highly Enhanced Electrocaloric Effect at Room Temperature

0.9KNbO3-0.1BaTiO3 ceramics, with a bimodal grain size distribution and typical tetragonal perovskite structure at room temperature, were prepared by using an induced abnormal grain growth (IAGG) method at a relatively low sintering temperature. In this bimodal grain size distribution structure, the extra-large grains (~10–50 μm) were evolved from the micron-sized filler powders, and the fine grains (~0.05–0.35 μm) were derived from the sol precursor matrix. The 0.9KNbO3-0.1BaTiO3 ceramics exhibit relaxor-like behavior with a diffused phase transition near room temperature, as confirmed by the presence of the polar nanodomain regions revealed through high resolution transmission electron microscope analyses. A large room-temperature electrocaloric effect (ECE) was observed, with an adiabatic temperature drop (ΔT) of 1.5 K, an isothermal entropy change (ΔS) of 2.48 J·kg−1·K−1, and high ECE strengths of |ΔT/ΔE| = 1.50 × 10−6 K·m·V−1 and ΔS/ΔE = 2.48 × 10−6 J·m·kg−1·K−1·V−1 (directly measured at E = 1.0 MV·m−1). These greatly enhanced ECEs demonstrate that our simple IAGG method is highly appreciated for synthesizing high-performance electrocaloric materials for efficient cooling devices.

Phase transition and relaxor nature of BaTiO3 ceramics induced by Li/Ga co-doping

The structure, dielectric, and ferroelectric properties of Li/Ga-modified BaTiO3 ceramics were investigated and analyzed. As the Li/Ga doping content x ≥ 0.02, an evident structure transformation occurred from the ferroelectric tetragonal to hexagonal phase due to the Li/Ga dopants replacing Ti-sites. Rietveld refinement results proved an increased content of hexagonal phase with the co-doping. The dielectric measurement results showed that an obvious transition from a typical ferroelectric behavior for the undoped samples to a highly diffusive phase transition (DPT) behavior for the Li/Ga-modified BT ceramics. The diffuseness coefficient γ was found to increase with an increase in the Li/Ga content. The relaxor-like phenomenon can be attributed to the disruption of long-range ferroelectric order and the structure evolution of from tetragonal to hexagonal phase. The slim and thin-waisted hysteresis loops were observed for x = 0.01 and x = 0.02, which may originate from the defect dipoles and relaxor-like behavior.

Efficiency of energy harvesting and storage using a multilayer capacitor based on BaTi0.86Sn0.14O3 ferroelectric lead-free ceramics

The study of the parameters of energy harvesting and storage in the lead-free solid solution BaTi0.86Sn0.14O3 (BTSnO) was performed. The permittivity shows the behavior similar to a diffuse phase transition with the critical exponent γ = 1.84, which is close to the value characteristic of relaxors. Large values of the recoverable energy density, Wrec=(5.8–7.0)∙104 J/m3, and the energy storage efficiency coefficient, η=(82–94) %, are implemented in a wide temperature range, 30–87°С, at a low electric field, E = 18.5 kV/cm. For the first time, the analysis of the Olsen cycle was performed using two phase diagrams: polarization – electric field and entropy – temperature. A fairly good agreement was found between the values of the energy conversion density, ND ≈ 0.15 J/cm3, which was determined using two approaches. A universal parameter is proposed for comparing the energy harvesting density for the materials studied in different ranges of temperature and electric fields.

Preparation, X-ray phase analysis and dielectric properties of Pb(Fe1-xCox)2/3W1/3O3 and Pb(Co1-yFey)1/2W1/2O3 (0 ≤ x, y ≤ 1) systems

In this work, the Pb(Fe1-xCox)2/3W1/3O3 (PFCW) and Pb(Co1-yFey)1/2W1/2O3 (PCFW) ceramics with 0 ≤ x, y ≤ 1 were successfully fabricated by a solid-state reaction process. X–ray diffraction phase analysis indicate the formation of two different series of solid solutions with a perovskite structure and with the substitution limits of Fe for Co (in PFCW) and Co for Fe (in PCFW) are x = 0.35 and y ≈ 0.05, respectively. Based on the results of dielectric study of the PFCW ceramics, it was shown that a crossover from relaxor ferroelectric to ferroelectric with a diffuse phase transition takes place at x = 0.10. In the case of PCFW ceramics, the observed dielectric maxima correspond to the phase transitions at 320 K and 256 K. The peculiarities of the temperature dependencies of the thermally stimulated depolarization currents of PFCW and PCFW solid solutions were studied and discussed.

Investigation of Dielectric, Ferroelectric and Conduction Behavior of Dy3+ Substituted SrBi2Ta2O9 Bismuth Layer Structured Ceramics

The present manuscript explores the impact of Dy doping in the Tantalum based bismuth layer structured ferroelectrics with chemical composition of Sr(Bi1-xDyx)2Ta2O9 (where x = 0.00, 0.025, 0.05, 0.075 and 0.10) prepared by mixed oxide process. X-ray diffraction study of all the ceramics implement orthorhombic phase without any secondary phase. The polycrystalline nature and grain distribution in the materials is studied from scanning electron microscope study. The temperature dependent dielectric performance of Dy doped SBT ceramics at selected frequencies indicates diffuse order phase transitions with reduction in transition temperature (Tc) and relative permittivity with doping level. The residual polarization and coercive field reduce with doping. The conduction mechanism was analyzed using the frequency and temperature domain impedance spectroscopy for all composition. The electrical contribution from both grains and grain boundary in the doped ceramics in the reported temperatures is confirmed from the Nyquist plots and the non-Debye type of relaxation mechanism is manifested from the depressed semicircles in all of them. The ac conductivities variation with frequencies at the studied temperatures follow Jonscher’s power law and the fitting parameters suggests that the conduction mechanism obey the correlated barrier-hopping model.

Li+ and Sm3+ co-doped AgNbO3-based antiferroelectric ceramics for high-power energy storage

Ag1-x-3yLixSmyNbO3 (x≤0.05, y≤0.05) (ALSN) antiferroelectric ceramics were successfully prepared via conventional solid-state reaction and sinter routes in oxygen atmosphere for improving the energy storage characteristic of pure AgNbO3. The results indicate that all of the studied compositions display a pure orthorhombic antiferroelectric (AFE) perovskite structure, while their key parameters of electric-field-induced antiferroelectric-ferroelectric transition can be affected by Li+ or/and Sm3+ doping contents. The Sm3+ doping can enhance the stability of antiferroelectric state, giving rise to higher antiferroelectric-ferroelectric transition electric-field (EF and EB), while Li+ doping can reduce EF and EB for Sm3+ doped AgNbO3 with low Sm3+ content (y≤0.03). When co-doping the same amounts of Li+ and Sm3+ at x=y≤0.03, both EF and EB almost remain unchanged. At x=y=0.05, the diffuse phase transition (DPT) behavior of antiferroelectric-paraelectric (AFE-PE) phase transition occurred, resulting in a “slim-like” double-polarization hysteresis with significantly enhanced EF. Due to these features, both Wrec and η are improved compared with pure AgNbO3. The Wrec and η with composition at x=y=0.05 is 2.33 J/cm3 and 58% under applying electric field of 240 kV/cm, respectively. The results suggest that building DPT behavior of AFE-PE phase transition could be an alternative strategy to improve the energy storage characteristic of AgNbO3.

Multifunctionality of rare earth doped 0.925Na0.5Bi0.5TiO3-0.075K0.5Na0.5NbO3 ferroelectric ceramics

Undoped and RE3+ (RE: Pr; Nd; Dy; Ho; Tm) doped 0.925Na0.5Bi0.5TiO3-0.075K0.5Na0.5NbO3 (NBTKN0.075/NBTKN0.075-RE) ceramics were synthesized using the solid-state synthesis method. X-ray diffraction analysis revealed a pure perovskite structure without secondary phases. The Ho, Dy and Tm ions revealed a dielectric curve with a more diffused phase transition associated with a high value of the dielectric constant, while the lowest diffusivities were found for the Nd and Pr ions doped NBTKN0.075. The ferroelectric hysteresis loops show the significant increase of polarization and electric breakdown strengths (BDS) in all doped samples, accompanied by P–E shape changes under the influence of the different RE3+ ions. In the optical studies, the conversion of near- infrared light to visible light was investigated for Ho3+ and Dy3+ doped NBTKN0.075. The sample with Ho3+ produced a strong green-yellow up-conversion (UC) emission. At an excitation of λex = 800 nm, the blue-green UC emission of the Dy ion showed that the UC is due to the energy transfer interaction. CIE diagram with the corresponding (x, y) coordinates were used to clearly identify the overall color of the multichromatic spectral emissions in each spectrum. The additional functionality of up-conversion emission in the ferroelectric NBTKN0.075-RE material, and its positive effects on the electrical proprieties, open the possibility to realize multifunctionality in a wide range of applications.

Correlations between electrocaloric effect and dielectric diffuseness of lead-free Ba(Ti1-Sn )O3 ceramics

This paper reported the correlations between electrocaloric effect (ECE) and dielectric diffuseness of Ba(Ti1-xSnx)O3 (abbreviated as BT-100xSn) ceramics. The BT-100xSn with pure perovskite structure has been synthesized by conventional solid-state reaction method. The temperature dependence of dielectric permittivity indicates that all the BT-100xSn ceramics are normal ferroelectric with diffused phase transition, and the dielectric diffuseness is strengthened as the Sn contents increase. The maximum ECE takes place in the vicinity of phase transition point, which is the characteristic of normal ferroelectrics. The BT-8Sn indicates the largest ∆T = 0.46 K and ∆T/∆E = 0.30 × 10−6 K m/V. The substitutions of Sn contents adjust the maximum ECE to different temperatures, and the good temperature-modulation of ECE is beneficial for the realization of ECE application. Considering the correlations between ECE and dielectric diffuseness, the experimental results demonstrate that the stronger dielectric diffuseness leads to the strengthened temperature stabilities of ECE but accompanied with decreased maximum ECE.

Dielectric properties of composites based on triglycine sulfate and nanocrystalline cellulose

Dielectric properties of mixed composites based on TGS and nanocrystalline cellulose (1-x)TGS-xnCell composites with x = 0.2; 0.7, and 0.9 (x is the mass fraction of the cellulose) were studied within temperature range of 20 ÷ 120 °C. A smearing of the ferroelectric phase transition and an increase in its temperature in TGS crystallites included in the composite were found. An analysis of the experimental results for the 0.3TGS-0.7nCell composite showed that the diffuse ferroelectric phase transition ended abruptly in the vicinity of the temperature T m ≈ 100 °C due to a structural rearrangement in TGS particles.

Electrocaloric effect and temperature dependent scaling behaviour of dynamic ferroelectric hysteresis studies on modified BTO

The large electrocaloric effect associated with the ferroelectric-relaxor and ferroelectric-paraelectric phase transition, and various temperature dependent scaling behaviours on A-site modified BaTiO3 (Ba1-xSrxTiO3, x = 0.00, 0.10, 0.20, and 0.25) have been reported in this article. The compositionally induced relaxor behaviour with diffuse phase transition has been reported. The Rietveld refinement of XRD patterns reveals that all the samples crystallize to the P4mm space group. Raman spectra analysis at room temperature supports the XRD patterns analysis. The diffuse phase transition behaviour of the prepared sample has been confirmed by estimating the diffuse phase parameter (γ = 1.444 ± 0.003) from the temperature variation dielectric plot. A large adiabatic temperature change (ΔT) of 0.278 K and isothermal entropy change (ΔS) of 0.494 J/kgK have been observed for the sample x = 0.20 (Ba0.8Sr0.2TiO3) at 338 K. The temperature dependent scaling relation for remnant polarization (Pr), coercive field (Ec), and hysteresis area <A> are found to be Pr∝T−1.009,Ec∝T−1.890 and <A>∝T−1.169, respectively. The present comprehensive studies on A-site modified BaTiO3 illustrate the electrocaloric cooling efficiency of this material.

Enhanced relaxor behavior and high energy storage efficiency in niobium substituted (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics

The microstructure, dielectric properties, relaxor behavior, and energy storage efficiency of un-substituted and niobium (Nb) substituted (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 (for x = 0, 0.02 and 0.05) samples prepared by the solid-state reaction method has been studied in detail. All the samples exhibited perovskite structure with no trace of impurity. Composition-dependent phase transition was also observed on the addition of Niobium. At room temperature, the co-existence of rhombohedral and tetragonal phases is observed in the unsubstituted samples. As the composition changes from x = 0.02 to x = 0.05, a structural change from tetragonal to cubic is observed. A remarkable reduction in grain size, from 90 μm (for x = 0) to 1.21 μm for (x = 0.05), is observed with the addition of Niobium. This result suggests that Niobium acts as a grain growth inhibiter in barium calcium zirconium titanate (BCZT) ceramics. The effect of Niobium on transition temperature is studied from the temperature-dependent dielectric permittivity graph. It was clear that the transition temperature shifted to a lower temperature region, and for x = 0.05, at a very low temperature (−23 °C/250 K) the tetragonal to cubic transition was observed. At x = 0.05, the temperature-dependent dielectric permittivity showed a broadened curve, indicating a diffuse phase transition. The diffuse phase transition in Nb substituted samples is explained by Uchino and Nomura modified Curie Weiss law.Moreover, the observations on temperature-dependent dielectric permittivity measurements at various frequencies suggest that the substitution of Nb5+ induces relaxor behavior. The energy storage efficiency of unsubstituted and Nb substituted samples was calculated from the polarization versus electric field graph. A high storage efficiency of 84% was obtained for the Nb substituted sample (x = 0.05) at 12 kV cm−1. Enhanced relaxor behavior and increased storage efficiency were observed in (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 at x = 0.05. Thus we suggest that these are promising materials for energy storage applications.

Dielectric properties and relaxor behavior of (1-x)Ba(Zr0.15Ti0.85)O3 - xBa(Mg(1⁄3)Nb(2⁄3))O3 ceramics for capacitor applications

The Ba(Zr0.15Ti0.85)O3 (BZT) ceramics were fabricated by the conventional solid-state method and the impact of Ba(Mg(1/3) Nb(2/3))O3 (BMN) addition into the BZT matrix on their microstructure, dielectric and ferroelectric properties were examined. According to the XRD analysis, the (1-x)Ba(Zr0.15Ti0.85)O3-xBa(Mg(1⁄3)Nb(2⁄3))O3 ceramics retain a pseudo-cubic ABO3 perovskite structure with an apparent decrease in c/a ratio as x increases. The SEM images show that the BZT-BMN ceramics instigate the reduction of the average grain sizes. And as the BMN content increases to 0.01, the Mg2+ and Nb5+ ion substitutions in the B-sites greatly improved the dielectric constant of BZT ceramics at room temperature. The frequency dispersion of the dielectric constant and the diffuseness of phase transition for BZT-BMN ceramics are enhanced by increasing the BMN addition. In the modified BZT ceramics, the dielectric maximum temperature (Tm) is reduced as the BMN content increases, and the temperature stability of the permittivity is modified by BMN addition. The hysteresis loop grows thinner and the remnant polarization, as well as coercive field, is greatly reduced as the BMN addition increases.

Effect of Sintering Temperature and Polarization on the Dielectric and Electrical Properties of La0.9Sr0.1MnO3 Manganite in Alternating Current

The electrical characterization ofa La0.9Sr0.1MnO3 compound sintered at 800, 1000 and 1200 °C was investigated by means of the impedance-spectroscopy technique. As the results, the experimental conductivity spectra were explained in terms of the power law. The AC-conductivity study reveals the contributions of different conduction mechanisms. Indeed, the variation in the frequency exponents (‘s1’ and ‘s2’) as a function of the temperature confirms the thermal activation of the conduction process in the system. It proves, equally, that the transport properties are governed by the non-small-polaron-tunneling and the correlated-barrier-hopping mechanisms. Moreover, the values of the frequency exponents increase under the sintering-temperature (TS) effect. Such an evolution may be explained energetically. The jump relaxation model was used to explain the electrical conductivity in the dispersive region, as well as the frequency-exponent values by ionic conductivity. Under electrical polarization with applied DC biases of Vp = 0.1 and 2 V at room temperature, the results show the significant enhancement of the electrical conductivity. In addition, the dielectric study reveals the evident presence of dielectric relaxation. Under the sintering-temperature effect, the dielectric constant increases enormously. Indeed, the temperature dependence of the dielectric constant is well fitted by the modified Curie–Weiss law. Thus, the deduced values of the parameter (γ) confirm the relaxor character and prove the diffuse phase transition of our material. Of note is the high dielectric-permittivity magnitude, which indicates that the material is promising for microelectronic devices.

Role of redox reactions involving interfacial oxygen migration in ignition of nanolayered Al/a-Al2Ox/CuO energetic composite

Thermite reactions of nanoparticles involve complex interplays between phase transition and heterogeneous atomic diffusion, especially at the early stage where the temperatures are relatively low and the systems remain as condensed phases. Moderate-temperature redox reactions among these condensed-phase reactants are investigated for a nanostructured 2.2 nm thick Al/CuO multilayer using ab initio molecular dynamics simulations. In order to achieve metastability, one amorphous alumina (a-Al2Ox) nanolayer of 0.6 nm is created between the Al and CuO layers. While the simulation is capable of predicting the energetic behaviors of the composite, focus is placed on investigating the species migration and reaction kinetics near these two interfaces, i.e., Al/a-Al2Ox and a-Al2Ox/CuyO. A set of redox reactions, possessing low activation barriers and a high exothermicity, are found to be critical to initiate the overall thermite reaction through generating localized hot spots within the a-Al2Ox layer and at its two interfaces, which has not been reported before. Driven by this exothermic process, migration of reactive species is promoted in condensed phases and a rate-limiting energy barrier (∼208.8 kJ•mol−1) at the Al/a-Al2Ox interface is overcome, which subsequently triggers a massive oxygen diffusion throughout the multilayer. Examination on chemical kinetics further reveals that the redox capacity of the a-Al2Ox layer determines the solid-state species migration during the ignition stage of the thermite reaction through two pathways: (1) the low-barrier and exothermic O migration in the a-Al2Ox layer and at the corresponding interfaces; (2) the migration-induced processes including decomposition of the CuO layer and melting of the Al. Contributions of the new simulation results to the existing reaction mechanisms are clarified.