Grain Boundary Contributions Research Articles

Impedance spectroscopy and magneto-dielectric analysis of La3+ substituted Co2Z hexaferrite

Single phase Ba3Co2Fe24O41 (Co2Z) and Ba2.8La0.2Co2Fe24O41 (La-Z) were prepared by solid state ceramic method. The dielectric, impedance and electric modulus were systematically investigated as a function of temperature from 50 °C–250 °C and in the frequency range of 1 Hz–1 MHz. Frequency dependent imaginary impedance and electric modulus implies that La3+ substitution significantly affect hopping and relaxation mechanism of charge carriers. Cole-Cole plots (Z" vs Z') has been fitted with suitable equivalent circuit for both Co2Z & La-Z. The obtained grain and grain boundary resistances confirmed that charge carriers experienced lower resistance in La-Z. Arrhenius plots of relaxation time (τ) vs 1/T showed parallel conduction mechanism in system. Complex electric modulus spectrum distinguished the capacitive contribution of grain, grain boundary and electric effect. A very high magnetodielectric effect at low frequencies has been observed for La-Z.

Spin-casted (Gd–Zn) co-doped BiFeO3 thin films for sustainable oxide-electronics

The emerging paradigm of oxide-based devices are reshaping the frontiers of sustainable electronics and enabling new functionalities like ferroelectric-photovoltaics, photo-catalytic activity, ferroelectric logics, and terahertz-scale actuation. The intriguing advantages of BiFeO3 (BFO) based oxide electronics offers the possibility of combining electric and magnetic degrees of freedom and is of interest in applications ranging from non-volatile random-access memories, multiple state storage media, tunnelling barriers, actuation, and sensors. Here, we investigate structural evolution and device performance of a cost-effective and unexplored BFO doped derivative i.e. (Gd, Zn) co-doped BFO thin films synthesized using commercially viable spin-casting technique. Owing to the competing change in dopant cation sizes w.r.t. Host lattice, ABO3 rhombohedral perovskite structure of the BFO did not transform while the optical band gap sequentially reduced from 2.65 to 2.53 eV with increasing Gd–Zn co-dopant concentration. Further, the effects of co-doping and asymmetric electrodes on the IV-characteristics of capacitors and the underlying conduction mechanism of these devices are investigated. Photoconductivity studies show three orders larger photocurrent arising from grain-boundary contributions in 2% (Gd, Zn) – BFO film exhibiting rapid, stable and repeatable photo-response which makes them useful for photosensitive capacitor applications.

Quantitative analysis of factors increasing coercive field of iron powder cores – Influence of inclusions

The quantitative contribution of inclusions to coercive field in iron powder cores with different amounts of Si oxide inclusions was separated from that of other microstructural factors and analyzed. Coercive field decreased by 2.8 A m−1 with a 0.01 mass% decrease in the amount of Si in oxide. Si oxide inclusions also affected the recovery and recrystallization of the α-Fe phase, and the contribution of crystal grain boundaries and the contribution of dislocations decreased with a decrease in the amount of Si in oxide. The contribution of inclusions was compared with Kersten's calculation model based on changes of the domain wall area and Neel's calculation model based on changes of the number of magnetic poles, and the observed value was closer to the results of Kersten's calculation model. This suggests that the contribution of magnetic poles generated around an inclusion is small and the change of the domain wall area around the inclusion makes a larger contribution to coercive field.

Electrical and Dielectric Characterization of Bismuth Holmium Nickel Titanate (BiHoNiTiO6)

A lead-free multiferroic double perovskite BiHoNiTiO6 (BHNT) was prepared at high temperature following mixed oxide solid state reaction mechanism. Structural evaluation by X-ray diffractogram suggested monoclinic crystal system for the BHNT. SEM images showed homogeneous morphology with almost similar and uniform grain distribution. The compound possessed low value of tanδ even at a high temperature (400oC) at 100 kHz. Evaluation of the dielectric and electrical plots with frequency and temperature obtained from the impedance spectroscopy revealed that the contributions of grains, grain boundaries and electrode has profound effect on the above properties. These plots are very much helpful to understand the conduction mechanism and microstructure characteristics of the electronic material. The experimental real and imaginary impedance data were modelled by designing an equivalent circuit with the capacitance and resistance parameter. The transport characteristics like σac (ac conductivity), electrical modulus and magneto-electric effect of the material were also reported in this paper.

New insights into the contribution of grain boundaries on fire resistance of 690 MPa ultra-heavy plate steel

In this study, novel insights into the contribution of grain boundary (GB) density on fire resistance is elucidated. We systematically analyzed the relationship between grain boundary density and fire resistance of 690 MPa ultra-heavy plate steel from the perspective of crystallography. The yield strength (YS) at elevated-temperature (ET) of 600 ℃ at the surface and quarter of the plate were 542 and 518 MPa, respectively, while at the center it was only 455 MPa. EBSD analysis indicated that the surface exhibited finer Bain group and higher density of high angle grain boundaries (HAGBs) as compared to the center. The HAGBs can reduce the slip of grain boundaries (GBs) and provide obstacles to the movement of dislocations at ET, which thereby significantly improves the fire resistance. The deformation and cooling rate were decreased rapidly from the surface to the center, which led to Bain group coarsening and drastically decreased HAGBs, resulting in a significant reduction of fire resistance of steel in the center of ultra-heavy plate.

A green approach for the preparation of nanostructured zinc oxide: Characterization and promising antibacterial behaviour

In the present study, nanostructured zinc oxide (ZnO) films have been successfully synthesized using fruit extract of Viburnum opulus L. (VO) on glass slides by successive ionic layer adsorption and reaction (SILAR) procedure. The impact of VO concentrations on the structural, morphological, optical, electrical, and antibacterial attributes of ZnO films has been investigated in detail. The samples' XRD patterns present a hexagonal crystal structure with a preferential orientation along the (002) plane. The crystallite size values of ZnO samples were found to be in the ranges from 14.88 to 9.23 nm. The supplementation of VO to the synthesis solution remarkably affected the surface morphological features of the ZnO films. The optical results demonstrated that band gap energy values of the ZnO films at room temperature were decreased from 3.20 to 3.07 eV as a function of VO content in the bath solution. The films' electrical properties were determined by impedance analysis in the frequency range of 20 Hz −1 MHz. Impedance-frequency measurements showed VO insertion to ZnO thin films cause an increase in impedance value at the low frequencies. Cole-Cole plots with a single semi-circle confirmed the contribution of grain and grain boundary for the electrical conduction process. The agar disk diffusion method was used to test the antibacterial properties of ZnO/VO inserted ZnO and inhibition zones were measured. VO inserted ZnO showed a stronger inhibitory effect on gram-positive bacteria Staphylococcus aureus (ATCC 25923) and gram-negative bacteria Escherichia coli (ATCC 35218) than ampicillin antibiotic used as a control group. In line with the promising bactericidal results of a new generation, VO inserted ZnO, the nanostructured product with this study, it can also be applied in multidrug-resistant clinical isolates obtained from patients.

Effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9Al-1Zn alloy processed by hard-plate rolling

The effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9Al-1Zn alloy (AZ91) processed by hard-plate rolling (HPR) was investigated. Increasing rolling reduction from 55 % to 85 % increases the volume fraction and refines average size of fine grains (< 3 μm, FGs), leading to an optimized bimodal-grained structure consisting of coarse grains (CGs) uniformly embedded in FG regions. The sample with 85 % reduction exhibits the highest yield strength of ∼314 MPa, ultimate tensile strength of ∼381 MPa and elongation of ∼11 %. The high strength is primarily due to the contribution of grain boundaries (GBs) strengthening by FGs (accounting for ∼65 % of strength), meanwhile the improved ductility originates from the optimized bimodal-grained structure and weakened basal texture that favor a higher ductility. The present findings successfully overcome the trade-off dilemma that the large-reduction rolling processing on Mg alloys usually enhances strength at expense of ductility. In addition, the intensified heterogeneous deformation and favorable formation of a bimodal-grained microstructure during large-reduction HPR was addressed by tracing microstructure evolution details in grains of interest via quasi-in-situ electron back scattering diffraction (EBSD). The present study can be instructive for further designing novel Mg alloys by tailoring bimodal-grained structures for superior combination of mechanical properties.

Origin of ferroelectricity in multiferroic ErFeO3

ErFeO3 is a centrosymmetric Pnma/Pbnm structure with no ferroelectricity. However, most surprisingly, it shows ferroelectricity and antiferromagnetism at room temperature. Here, the Fe3+-O-Fe3+ bond angles are 143.50 and 159.280 which is deviated from 1800. This Fe3+-O-Fe3+ super-exchange interaction contributes to an AFM ordering with a weak FM component. An antisymmetric DM exchange interaction (Di. Si x Sj) is attributed to the weak FM component between adjacent spins and introduced spin-orbit interaction. As a result an electric polarization P ~ eij x (Si x Sj) is produced due to local crystal anisotropy. To acquire a conception involving the contribution of grain and grain boundary of ErFeO3, we have analyzed complex impedance and modulus spectra and fitted with the equivalent circuit diagram. The imaginary part of electric modulus is fitted by KWW function and the stretched exponent is found to be 0.3<β< 0.48. The conduction mechanism of ErFeO3 is followed CBH model.

A hardening model considering grain size effect for ion-irradiated polycrystals under nanoindentation

In this work, a new hardening model is proposed for the depth-dependent hardness of ion-irradiated polycrystals with obvious grain size effect. Dominant hardening mechanisms are addressed in the model, including the contribution of dislocations, irradiation-induced defects and grain boundaries. Two versions of the hardening model are compared, including the linear and square superposition models. A succinct parameter calibration method is modified to parametrize the models based on experimentally obtained hardness vs. indentation depth curves. It is noticed that both models can well characterize the experimental data of unirradiated polycrystals; whereas, the square superposition model performs better for ion-irradiated materials, therefore, the square superposition model is recommended. In addition, the new model separates the grain size effect from the dislocation hardening contribution, which makes the physical meaning of fitted parameters more rational when compared with existing hardness analysis models.

Investigate on the phase interfaces and performance evaluation of TiB2p/Al–Si–Cu–Zn (T6) composites

In this paper, the microstructures, mechanical properties, and corrosion behavior of xTiB2/Al–9Si–3Cu-0.8Zn composites after T6 heat treatment were invested in detail through the combination of different technologies, including optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile testing, corrosion testing, etc. The research results showed that the average size of the α-Al grains gradually decreased with the increase of TiB2 particles (TiB2p) content. In addition, there was a semi-coherent relationship between TiB2p and the α-Al matrix, and the nanoscale precipitates were identified as θ and θʹ phases by virtue of their crystal orientation relationship. Tensile testing results revealed that an increase in TiB2 content caused the elastic modulus, yield strength and tensile strength of the composites to increase and then decrease. And with a TiB2p content of 4 wt%, they reached 85.7 GPa, 360 MPa and 398 MPa, reflecting increases of 26.2%, 60.0%, and 30.9%, respectively. The contributions of thermal mismatch, grain boundary and Orowan strengthening mechanisms to the overall yield strength of the 4%TiB2/Al–9Si–3Cu–0.8Zn composite were 72.1, 3.8, and 35.6 MPa, respectively. Unfortunately, the corrosion resistance of xTiB2/Al–9Si–3Cu–0.8Zn composites gradually decreased with increasing TiB2p content. The corrosion morphology analysis indicated that the agglomeration of TiB2p and Si weakened the protection afforded by the oxide film, evolving into a point of nucleation for the subsequent corrosion reaction. Moreover, point–pair correlation was observed between the TiB2p and Al matrix, exacerbating the corrosion. The results implied that extra factors be considered in industrial application of TiB2 ceramic particles in a metallic system.

Electrical conduction mechanism of barium doped strontium hydroxyapatite synthesized by co – precipitation method

The crystalline samples of Barium (Ba) doped Sr10-xBax(PO4)6(OH)2 with x = 0, 0.5 and 1 mol% were prepared by co – precipitation method. The microstructural (SEM) image shows the distribution of densely packed grains over the surface of prepared sample. The nature of cole – cole plots confirm that the conduction mechanism is governed only by the grains with no contribution of grain boundaries for all prepared samples. The resistance of grains decreases with increase in temperature and molar (mol%) concentration, which shows a negative temperature coefficient resistance (NTCR) behaviour for all prepared samples. The a.c. conductivity increases with increase in frequency which shows non – debye nature of the prepared samples.

Synthesis and study of the structural and dielectric properties of La0.67Ca0.2Ba0.13Fe1−xMnxO3 ferrites (x = 0, 0.03 and 0.06)

The La0.67Ca0.2Ba0.13Fe1−xMnxO3 perovskite compounds with different manganese concentrations (x = 0.0, 0.03, 0.06) were synthesized by the autocombustion method and annealed at 700 °C. The X-ray diffraction (XRD) data indicated that all obtained compounds crystallize in the cubic structure with the Pm $$\overline{3}$$ m space group. The increase in the substitution rate has been found to lead to the decrease in the crystallite size value characterized by X-ray diffraction. The dielectric properties of these samples, using a complex impedance spectroscopy technique, were also performed as a function of frequency and temperature. A suitable equivalent electrical circuit was used to assess the contributions of electrode, grains, and grain boundaries in the complex impedance results. Activation energies were determined from different methods. Nyquist plots were consistent with three circuits in series, each one with a capacitance in parallel with a resistance. The conduction mechanism follows the NSPT model for the three samples at T < 280 K, in which the rise in substitution rate increases the binding energy of carriers. Above 280 K, the appropriate model is the Correlated Barrier Hopping (CBH) for x = 0, while the overlapping large polaron tunneling (OLPT) process has been confirmed for both samples x = 0.03 and x = 0.06. Our compounds have high electrical properties, and good thermal and chemical stability. They are susceptible to many technological applications, such as gas sensors, capacitors, filters, resonators, new read heads, and solid oxide fuel cells.

Combined ab-initio and empirical model for irradiated metal alloys with a focus on uranium alloy fuel thermal conductivity

In this work, we developed a model framework of thermal conductivity of irradiated metal and metal alloys based on density functional theory calculations and experimental data. The model incorporates the contributions of point defects (vacancies and transmutation products), grain boundaries, and noble gas bubbles (inter- and intra-granular) resulting from irradiation into the overall thermal conductivity. The model is demonstrated on U-Mo alloys and displays semi-quantitative agreement with experimental data, with a root-mean-square error of about 1 W/m-K (≈9% error versus experiment) over the typical operating temperature and burn-up range for U-Mo alloys. This work provides insight into the relationships of the thermal conductivity changes resulting from the different irradiation-induced changes in microstructure over a wide range of temperatures, and shows that scattering of electrons from point defects produces the majority of the thermal conductivity reduction of U-Mo alloys under irradiation. Specifically, the effect of point defects decreases the thermal conductivity by about 20% when burn-up >20% (e.g., 50% burn-up), while grain boundary and gas bubble formation contribute only about another 10% reduction at 50% burn-up. The ability of our model to individually quantify the effect of different irradiation-related defects on the thermal conductivity as a function of temperature, composition, and irradiation conditions offers both a powerful tool for quantitative semi-empirical modeling of thermal conductivity in irradiated metals and materials design insight for the control of thermal conductivity in metal alloys in applications experiencing irradiation, such as next-generation nuclear reactors.

Effect of Polaron formation in conduction and dielectric behavior in La0.7Sr0.25K0.05MnO3 oxide

The purpose of this paper is to analyze the dielectric properties of La0.7Sr0.25K0.05MnO3 (LSKM) using impedance spectroscopy over the frequency range of 100 Hz to 1 MHz at different temperatures from 260 to 380 K. In fact, the dielectric analyses of the sample showed two relaxation processes at different relaxation times related to the effect of microstructural inhomogeneities and the contribution of grain and gain boundaries. However, Ac conductivity behavior reveals a metal–semiconductor phase transition at a specific temperature TM-S = 260 K and showed an important impact of the electron–lattice interaction in polaron formation. Also, the conductivity curve is analyzed by Johnscher’s universal power law and the variation of the exponent ‘n’ with temperature suggests the domination of small polaron hopping (SPH) model governed by an activation energy Ea = 0.12 eV. Further investigations using modulus formalism proved the presence of electrical relaxation, which is in good agreement with impedance results. The dielectric poly-dispersive behavior is related to the conduction process and the large value of the dielectric constant $$\varepsilon^{\prime\prime}(\omega )$$ in the order of 106 is correlated to the Maxwell–Wagner relaxation.

Synthesis, structural refinement and physical properties of novel perovskite ceramics Ba1-xBixTi1-xMnxO3 (x = 0.3 and 0.4)

The crystal structure, morphological, optical, electrical and dielectric properties of polycrystalline perovskites of Ba1-xBixTi1-xMnxO3 (x = 0.3 and x = 0.4), prepared by the standard solid-state reaction process, were investigated systematically. From the X-ray diffraction results at ambient temperature, both samples showed the pure perovskite phase. Rietveld refinements revealed that the studied compounds adopt the cubic structure symmetry with Pm3‾m as a space group. Scanning electron microscopy demonstrated that the samples show irregularly spherical grains with an average size of 2 μm. From UV–vis–NIR absorption spectra, the synthesized compounds exhibit a maximum absorbance in the whole visible range. Furthermore, the direct band gap energy was found to be 1.07 and 0.85 eV for x = 0.3 and 0.4, respectively. The electrical transport in these materials was studied by using the impedance spectroscopy from the room temperature to 110 °C. The modelling of complex impedance data was performed to separate the grain and grain-boundary contributions in the conduction process. The addition of Bi and Mn ions increased the dc electrical conductivity. The thermal variation of the dc electrical conductivity proved the semiconductor behavior of both studied compounds. The obtained activation energies from the dc conductivity for grain and grain-boundary contributions were found to be 0.39–0.36 eV for x = 0.3 and 0.20–0.32 eV for x = 0.4. The conduction mechanism was explained by hopping process along ionized oxygen vacancies. The dielectric permittivity and dielectric loss of both compositions showed an increase with increasing temperature.

Atmospheric Dependence of Thermoelectric Generation in SnO2 Thin Films with Different Intergranular Potential Barriers Utilized for Self-Powered H2S Sensor Fabrication

The profound contribution of grain boundaries to the Seebeck voltage generation in the polycrystalline semiconductors has recently been stated. Considering the direct link between the chemiresistiv...

Enhancing the electrical and dielectric properties of ZnO nanoparticles through Fe doping for electric storage applications

The present paper provides significant results about the impact of iron doping on the ZnO nanoparticles’ structural and electrical properties. Fe-doped ZnO (ZnO:Fe) nanoparticles with varying doping concentrations from 0 to 5% were effectively synthesized by a simple co-precipitation method. The X-ray diffraction (XRD) studies revealed that all compositions crystallize in the hexagonal wurtzite structure with the P63mc space group. They also proved the presence of a secondary phase accredited to ZnFe2O4 for ZnO:Fe 5% sample. Concerning the transmission electron microscopy, it demonstrated that the formed nanoparticles are spherical. The electrical properties were explored by complex impedance spectroscopy in the 40–107 Hz frequency range and 400–500 K temperature domains. The comparative Nyquist plots at fixed temperature 440 K suggested that the impedance value dropped with the augmentation of Fe doping concentration. Furthermore, the electrical conductivity and dielectric properties were explored as a function of frequency and temperature in the same range. The obtained results demonstrated that iron doping enhanced the AC conductivity at the same selected temperature 440 K. The analysis of the AC conductivity frequency dependence of the ZnO:Fe 1% sample was carried out by Jonscher’s universal power law and the conduction mechanism was interpreted by the overlapping-large polaron tunneling (OLPT) model. Both impedance and modulus analyses were found to display the contribution of grain and grain boundary to the electrical response of the ZnO:Fe 1% sample. Moreover, the dielectric characterization had affirmed that both dielectric constant and dielectric loss decrease with the increase in frequency and increase with the increase in temperature. These two parameters were found to augment with Fe doping. The observed properties had proven that Fe doped ZnO nanoparticles was very functional for the electric storage applications.

Dielectric properties, relaxation and study of conduction process of Ba0.5Sr0.5Ti0.99Sn0.01O3 ceramics

Polycrystalline compound of Ba0.5Sr0.5Ti(1-x)SnxO3 (x = 0.01) prepared via solid-state reaction route was analyzed by XRD, SEM, dielectric and CIS examination of the sample. Study of XRD data confirmed the material to crystallize in cubic structure for the produced sample. The SEM micrograph depicted distributed grains with clear grain boundary. Loss tangent and dielectric constant variations were studied for frequency ranging from 1 kHz to 1 MHz and thermal range of room temperature (RT) to 400 °C. The loss tangent was found to be significantly less than Ba0.5Sr0.5TiO3. The presence of peak in loss tangent spectroscopy plot suggested dielectric relaxation occurring in the material within studied frequency range. Impedance spectroscopy data were analyzed in different formalism for the contribution of grain boundary and grain in conduction/relaxation process and the activation energies of charge carriers involved were estimated. These values suggested that conduction process is primarily dependent on oxygen vacancies. Jonscher’s power law defines the frequency dependant AC conductivity at different temperature values.

Deformation of Aluminum in Situ Sem and Full Field Measurements by Digital Image Correlation: Evidence of Concomitant Crystal Slip and Grain Boundary Sliding

Mechanical testing in situ scanning electron microscopy (SEM) has become a current technique for multiscale micromechanical investigation of polycrystalline materials, because direct observation of deformation microstructures allows identification of strain heterogeneities and related mechanisms. Yet, most of the studies are based on the inherited post mortem microstructures, thus precluding to unravel the local loading history, to understand the development of localization patterns, the potential interactions of concomitant mechanisms and to quantify their respective contributions to the overall strain. We therefore developed a novel experimental setup for thermomechanical testing in situ SEM, especially suited for full strain field measurements based on digital image correlation (DIC) from the sample scale, to the scales of the aggregate and the single grain. We present results obtained during simple compression, at controlled displacement rate and at temperatures up to 400°C, for polycrystalline aluminum presenting randomly oriented coarse grains (ca. 300 m in size). According to the different scales of interest, specific surface marking patterns were realized by electron microlithography. Kinematic analysis by digital image correlation (DIC) allowed to retrieve full surface strain fields. The latter evidenced that the overall viscoplastic response was dominated by crystal slip plasticity. Increasing temperature favored the activation of non-octahedral slip, but also substantial and continuous contribution of grain boundary sliding (GBS). We suggest the latter mechanism as necessary to accommodate local plastic incompatibilities between neighboring grains.

Effect of substitution of La3+ on structural and electrical properties of Sr(Fe0.5Nb0.5)O3 ceramic

In this work, we have mainly reported the effect of lanthanum substitution on structural, dielectric, impedance and transport properties of strontium iron niobate (i.e., Sr1-xLax(Fe0.5Nb0.5)1-x/4O3 (x = 0, 0.05, 0.1, 0.15, 0.2)). The materials were synthesized using standard ceramic technology. The preliminary structural analysis was done by using the room temperature X-ray diffraction data. The samples of higher concentrations (x = 0.15 and x = 0.20) show the development of an additional phase (i.e., LaNbO4 and Sr3La4O9). Studies of frequency and temperature dependence of dielectric parameters exhibit an anomaly and relaxor behavior in the compounds. The electrical impedance and modulus analysis of frequency and temperature-dependent data show the contributions of grains and grain boundaries in the resistive and capacitive properties of the compounds. The study of transport properties of AC conductivity has provided the conduction and relaxation mechanism. The substitution of La3+ has significantly changed the dielectric constant, tangent loss, and transport properties of the material.