Presence Of Grains Research Articles

Higher temperature order–disorder transition, electrical and optical properties of [N(C2H5)4]2Pd2Cl6compound

[N(C2H5)4]2Pd2Cl6hybrid compound with semiconducting character was synthetized and characterized by X‐ray diffraction, the thermal, optical and electrical properties. X‐ray diffraction indicates that the compound crystallize at room temperature in the orthorhombic system and (Immm) space group. Two endothermic peaks at, 343 K and 440 K were observed in the DSC measurements corresponding to the evaporation of water molecules and order‐disorder transition, respectively. The optical band gap (Eg= 4.34 eV) was deduced by the Tauc relation. The electrical properties was studied using the impedance spectroscopy in the frequency range of 0.1 Hz to 1 MHz and over the temperature range of 270 K to 480 K. The analysis of Nyquist plots revealed the presence of grains, grain boundaries and the electrode effects. The equivalent circuit was determined. The Ac conductivity was analyzed by the Jonsher low and the conduction mechanism was deduced based in the Elliot theory and indicate that the correlated barrier hopping model (CBH) was described the two regions II and IV and the overlapping polaron model (OLPT) the right model described the conduction in the regions I and III.

Tailoring the low dielectric constant in glutamic acid doped ammonium dihydrogen phosphate single crystal by virtue of MPa shock waves for microelectronic applications: the complex impedance and modulus formulation studies

Ammonium Dihydrogen Phosphate, ADP, (NH4H2PO4) crystal has a wide range of applications in integrated and nonlinear optics. The amino acid-like l-glutamic acid (C5H9O4N) causes the defect in the framework of ADP crystal. The 0.6wt% l-glutamic acid doped ADP crystal is grown using the slow evaporation technique. The grown crystal is subjected under the shockwave of Mach number 1.7 and shock number 2, 4 and 6. The Powder XRD study shows the lattice strain introduced in the structure of grown crystals due to impose of the shockwave. The Photoluminescence confirmed the presence of D-defect and Self Trapped Excitons. The influence of shockwave on dielectric, impedance and modulus properties of glutamic acid doped ADP crystal is well studied in the frequency range of 100 Hz to 10 MHz and the temperature range of 323 K to 373 K. The shockwave is reduced the dielectric constant, A.C. Conductivity, strength of polarizibility and the grain capacitance. The Correlation Barrier Hopping (CBH) conduction mechanism is well studied for pristine and shocked glutamic acid doped ADP crystals over the temperature range considered. The presence of grain over the frequency range considered is identified and confirmed using the complex impedance and the complex modulus spectroscopy. The pristine and shocked glutamic acid doped ADP crystals have possessed the temperature-independent, Non-Debye type relaxation process. The results are discussed here.

Design and characterization of novel manganite perovskites Ba1-xBixTi1-xMnxO3 (0≤x≤0.2)

Polycrystalline manganite powders of Ba1-xBixTi1-xMnxO3 (x = 0, x = 0.1 and x = 0.2) were synthesized by the conventional solid-state reaction process. Their crystal structure, morphological, optical, dielectric and electrical properties were investigated. X-ray diffraction of the prepared samples was made at room temperature and confirmed the formation of a perovskite phase. Structural refinement, using the Rietveld method, revealed a tetragonal P4mm phase of pure BTO and a tetragonal P4/mmm phase with the presence of vacancies for both doped samples (x = 0.1 and x = 0.2). Scanning electron microscopy indicated that the perovskite samples had a grain size smaller than 1 μm. From UV–vis–NIR spectra, we found that the band gap reduces from 3.29 eV to 1.48 eV with the increase of Bi and Mn amounts, resulting in a shift of the absorption wavelength region toward the visible range. Dielectric analysis was conducted in a wide range of temperatures at different frequencies. Phase transitions were identified from thermal dielectric results, showing that the samples exhibited a non-relaxor behavior. The structural transformation from tetragonal to cubic structure corresponding to the transition from ferroelectric phase to paraelectric phase was observed in the dielectric properties investigation. The complex impedance spectroscopy indicated the presence of grain and grain boundary effects in the conduction mechanism. Electrical analysis showed that doping with Bi and Mn enhanced the DC conductivity. Furthermore, the DC conductivity temperature dependence confirmed that the studied samples present a semiconductor behavior. The activation energies of grain and grain boundaries depended on the amount of incorporated Bi and Mn. The activation energy of grain varied between 0.54 and 0.87 eV suggesting that the DC electrical conductivity is governed by ionized oxygen vacancies. The activation energy of grain boundaries varied between 0.85 and 0.58 eV.

Simulations of oxidation of metal nanoparticles with a grain boundary inside

The generic 2D lattice Monte Carlo simulations presented herein are focused on the spatio-temporal kinetics of oxidation of metal nanoparticles composed of two grains separated by a single grain boundary. The oxidation is assumed to occur via inward diffusion of interstitial oxygen ions in the oxide. The results of simulations illustrate that the regimes of oxidation can range from one where the presence of grains is negligible and the oxide shell is formed at the periphery of a whole nanoparticle to one where each grain is oxidized almost independently.

COMPOSITION AND SURFACE MORPHOLOGY OF COMPOSITE ELECTROLYTIC COATINGS CO-W-ZRO2

possibility of electrosynthesis and control of the composition and surface morphology of the composite electrolytic coatings of cobalt with refractory metals by varying the pulse current density has been proved. Composites deposited on copper substrate from biligand citrate-pyrophosphate electrolytes at 4 A/dm2 are distinguished by a developed surface and a more uniform distribution of components on the surface, increased tungsten content, almost 5 times higher than coatings deposited at 10 A/dm2, and a half reduction in oxygen percentage (up to 5.5%). This is explained by the inhibition of the hydrogen gas evolution reaction and the participation of hydrogen ad-atoms in the chemical reduction of intermediate tungsten oxides to metal when polarization is interrupted. Coatings obtained using pulsed current can be considered as composites of the composition Co-W-ZrO2, in which the oxide phase is formed directly in the electrode process as an intermediate for the incomplete reduction of tungstates. The topography of the films is characterized by the presence of grains of elliptical and spherical shape with crystallite sizes of 80 – 180 nm. On the main surface there are protrusions (large grains) with a diameter of 1 – 3 microns. The fractal dimension of the surface is 2.77, which indicates the 3D crystal growth mechanism during coating formation. According to the surface roughness parameters Ra and Rq, coatings belong to the 9th roughness class. By phase composition, composites are predominantly amorphous materials, which include nanocrystalline cobalt and intermetallic compounds Co3W and Zr3Co. Due to the quantitative and phase composition, surface morphology and fractality, the microhardness and corrosion resistance of Co-W-ZrO2 systems deposited at a current density of 4 А/dm2 are 20% higher than the coatings obtained at 10 A/dm2, and 3 times higher than the corresponding characteristics of the substrate.

Structural, DFT, vibrational spectroscopic, thermal, electrical and magnetic characterizations of hydrothermally grown CoCO3 microcrystals

The minerals such as the CoCO3 can be utilized for the fabrication of lithium-grafted batteries and for a variety of food applications. In this study, we have successfully synthesized Cobalt carbonate crystals (CoCO3) using hydrothermal synthesis method. The synthesized micro-crystals consist of hexagonal structure symmetry. The XRD shows the single-phase hexagonal (R-3c) crystal with lattice constants a = 4.661 A, b = 4.661 A, and c = 14.96 A. EDAX spectroscopy analysis confirms the presence of all the elements of sample. FE-SEM analysis suggests nearly rod-shaped crystals with ~ 29 µm average crystalline size. The Fourier Transform Infrared spectrum suggested existence of O–H, C–O etc. bonding. The TGA is carried out to study the thermal stability. The dielectric relaxation is well studied over large angular frequency range. Complex impedance spectrum of the synthesized microcrystals consists of single semi-circular arc due to the presence of grain only. The magnetic study has suggested paramagnetic nature of the sample. The theoretical study such as the density function theory has been carried out using the Quantum ESPRESSO. A reasonably good agreement is observed between experimentally obtained data and DFT data for optimized structures.

The effect of surface morphology of Co-W-ZrO2 electrolytic composite coatings on the functional properties of alloys

The possibility of electrosynthesis and control of the composition and surface morphology of the composite electrolytic coatings (CEC) of cobalt with refractory metals by varying the pulse current density has been proved. Composites deposited on copper substrate from biligand citrate-pyrophosphate electrolytes at 4 A/dm2 are distinguished by a developed surface and a more uniform distribution of components on the surface, increased tungsten content, almost 5 times higher than coatings deposited at 10 A/dm2, and a half reduction in oxygen percentage (up to 5.5%). This is explained by the inhibition of the hydrogen gas evolution reaction and the participation of hydrogen ad-atoms in the chemical reduction of intermediate tungsten oxides to metal when polarization is interrupted. Coatings obtained using pulsed current can be considered as composites of the composition Co-W-ZrO2, in which the oxide phase is formed directly in the electrode process as an intermediate for the incomplete reduction of tungstates. The topography of the films is characterized by the presence of grains of elliptical and spherical shape with crystallite sizes of 80 – 180 nm. On the main surface there are protrusions (large grains) with a diameter of 1–3 microns. The fractal dimension of the surface is 2.77, which indicates the 3D crystal growth mechanism during coating formation. According to the surface roughness parameters Ra and Rq, coatings belong to the 9th roughness class. Due to the quantitative composition, surface morphology and fractality, the microhardness and corrosion resistance of Co-W-ZrO2 systems deposited at a current density of 4 А/dm2 are 20% higher than the coatings obtained at 10 A/dm2, and 3 times higher than the corresponding characteristics of the substrate. It was shown the catalytic activity of tungsten containing CEC in ethanol oxidizing reaction and corrosion resistance in media of different acidity.

The complex impedance, dielectric relaxation, complex modulus and photoluminescence studies of pure and L-Methionine doped ammonium dihydrogen phosphate

The ammonium dihydrogen phosphate (ADP) is one of the pioneer nonlinear optical materials utilized for frequency doubling experiment of Nd:YAG laser. The amino acids are bifunctional material that contains the weak hydrogen bonded side chain. In the present study the authors have studied influence of L-Methionine on structural, photoluminescence and a.c.electrical properties of ADP. The powder XRD study suggested tetragonal structure symmetry and single phasic nature of all the samples. The photoluminescence is confirmed existence of the Self Trapped Excitons. The Correlation Barrier Hopping and Nonoverllaping Small Polaron Tunnelling kind of conduction mechanism is observed in present system. The Complex Impedance and Complex Modulus spectroscopy have suggested presence of grain and grain boundary within the samples.

Investigation of physicochemical and electrical properties of $$\hbox {TiO}_{{2}}$$ nanotubes/graphene oxide nanocomposite

Graphene oxide (GO)-based nanocomposites have received a great attention due to their emerging applications. Here, we thoroughly examined the structural, electronic and surface properties of \(\hbox {GO}/\hbox {TiO}_{2}\) nanotubes nanocomposite. The nanocomposite is prepared by simple impregnation of \(\hbox {TiO}_{2}\) nanotubes (HNT400) with GO dispersion. GO is elaborated by an improved Hummer’s method, while HNT400 is obtained using alkaline hydrothermal treatment of \(\hbox {TiO}_{2}\) P25, followed by calcination at \(400^{\circ }\hbox {C}\). XRD and Raman analyses show that GO nanosheets do not change the structural properties of \(\hbox {TiO}_{2}\) nanotubes. TEM analysis confirms the formation of GO nanosheets assembled to \(\hbox {TiO}_{2}\) nanotubes. XPS and EPR results confirm the electron transfer between GO and \(\hbox {TiO}_{2}\) nanotubes. PL analysis reveals that GO inhibits the recombination of photogenerated electron–hole pairs in the nanocomposite. The ac conductivity measurements suggest the presence of grain and grain boundary effects in GO/HNT400.

Lead-Free BNT–BT0.08/CoFe2O4 Core–Shell Nanostructures with Potential Multifunctional Applications

Herein we report on novel multiferroic core–shell nanostructures of cobalt ferrite (CoFe2O4)–bismuth, sodium titanate doped with barium titanate (BNT–BT0.08), prepared by a two–step wet chemical procedure, using the sol–gel technique. The fraction of CoFe2O4 was varied from 1:0.5 to 1:1.5 = BNT–BT0.08/CoFe2O4 (molar ratio). X–ray diffraction confirmed the presence of both the spinel CoFe2O4 and the perovskite Bi0.5Na0.5TiO3 phases. Scanning electron microscopy analysis indicated that the diameter of the core–shell nanoparticles was between 15 and 40 nm. Transmission electron microscopy data showed two–phase composite nanostructures consisting of a BNT–BT0.08 core surrounded by a CoFe2O4 shell with an average thickness of 4–7 nm. Cole-Cole plots reveal the presence of grains and grain boundary effects in the BNT–BT0.08/CoFe2O4 composite. Moreover, the values of the dc conductivity were found to increase with the amount of CoFe2O4 semiconductive phase. Both X-ray photoelectron spectroscopy (XPS) and Mössbauer measurements have shown no change in the valence of the Fe3+, Co2+, Bi3+ and Ti4+ cations. This study provides a detailed insight into the magnetoelectric coupling of the multiferroic BNT–BT0.08/CoFe2O4 core–shell composite potentially suitable for magnetoelectric applications.

Structural and electrical features of lead-free ferroelectric: Li1/2Bi1/2TiO3

In this communication, the detailed analysis of structural (including microstructural) and electrical (dielectric, impedance, conductivity, and modulus) features of a perovskite Li1/2Bi1/2TiO3 are reported. The compound is prepared by a high-temperature solid-state reaction technique. From X-ray diffraction, the crystal structure is fully satisfied with the monoclinic system of the material. Both dielectric permittivity and loss tangent of the pellet sample is scanned and analyzed in extensive temperatures (30–500 °C) and frequencies (100 Hz–1 MHz). Dielectric anomaly is detected at a particular temperature of 109 °C that may be associated with the ferroelectric-paraelectric phase transition. Through the Nyquist diagram, the presence of grains and grain boundary effect in the compound is detected. The observed non-Debye type of relaxation feature is confirmed through the complex impedance spectroscopy. Impedance analysis suggests that the grain resistance reduces with the rise of operating temperature, showing the negative temperature coefficient of resistance (NTCR) nature. Also, the non-Debye type of relaxation phenomenon is established from the modulus spectroscopy analysis. The significant result infers that this compound may be utilized for different high-temperature applications.

Temperature and Frequency Dependent Multiferroic Features of Gadolinium Doped BiFeO3-PbTiO3 Electronic System

Multiferroic gadolinium (Gd) doped BiFeO3 (BFO) and PbTiO3 (PT) samples [(Pb1−xBi0.5xGd0.5x)(FexTi1−x)O3] with x = 0.1, 0.3, 0.5 and 0.7 are formulated by adopting the solid state reaction process. The impact of gadolinium substitution in the crystal structure, morphological behavior and electrical properties has been studied. By altering the Gd concentration in the solid solution a change in the structure is observed that is from tetragonal to rhombohedral. The size of the grain in the compounds reduces as a result of enrich in gadolinium content of the furnished sample. The subsidence of lead minimizes the toxic behavior of the material with significant improvements in dielectric response of gadolinium modified BiFeO3–PbTiO3 synthesized system. From the impedance study, the material presents negative temperature coefficient of resistance characteristics and the impedance is due to the presence of grain and grain boundary effect inside the materials. It is realized that the remnant polarization decreases and frequency dependent ac conductivity increases with the concentration of gadolinium in the electronic system. From conductivity study, the equipped electronic materials show non-exponential type of conductivity relaxation.

Annealing-temperature dependence of the electrical properties of Ba0.8Pb0.2TiO3 as a PTC material

In this research, we investigate how the annealing temperature affects the electrical properties of Ba0.8Pb0.2TiO3 as a positive temperature coefficient material. The material was synthesized via solid-state sintering. The grain size was varied by varying the annealing temperature between 850 °C and 1000 °C for 4 h. Characterization via X-ray diffraction showed that a single-phase compound was formed exhibiting a tetragonal phase. The microstructure was observed using scanning electron microscopy. Average grain size was observed to be between 5 and 10μm. The electrical properties as characterized using impedance spectroscopy showed the presence of grain and grain-boundary contributions in the annealed samples. Dielectric constant and dielectric relaxation phenomenon as a function of annealing temperature have been studied.

Use of mechanical activation in the processing of gold-bearing ores and anthropogenic materials

The results of experimental studies on the use of mechanical activation in the processing of ores and anthropogenic materials of the Far Eastern region are presented. The effectiveness of the use of electrophotoactivated solutions and complex reagents at the stage of mineral preparation has been established. Technological solutions have been proposed to reduce the loss of finely dispersed and chemically bonded gold during the processing of gold-bearing materials based on a directed change in the properties of the surface of minerals. The effectiveness of the use of the mechanochemical activation process for various aspects of the technological chain has been experimentally confirmed – a decrease in the sorption properties of materials, development of microcracks in oxidized ores for the penetration of leaching solutions into mineral aggregates, oxidation of the surface of primary ores, and removal of films from the mineral surface. Indicators of technological refractoriness of the studied ores and anthropogenic materials were revealed – a high proportion of chemically bound gold with atoms of basic and accessory mineral-forming elements, the presence of grains of the valuable component covered with oxide films, enclosed in carbonates, quartz and waste rock. The refractoriness of ores is confirmed by a high sorption capacity (the predominance of carbonaceous and clay matter), the presence of trace impurities in gold (Sb, As, Cu, Fe, Hg), which increase the consumption of cyanide during leaching. The material composition is dominated by a significant amount of chemical elements – cyanosides, actively interacting with the main complexing agent of leaching solutions – cyanide of alkali metals.

Regional study of Europa’s photometry

The surface of Europa is geologically young and shows signs of recent activity. Studying it from a photometric point of view gives us insight on its physical state. We used a collection of 57 images from Voyager’s Imaging Science System and New Horizons’ LOng Range Reconnaissance Imager for which we corrected the geometric metadata and projected every pixel to compute photometric information (reflectance and geometry of observation). We studied 20 areas scattered across the surface of Europa and estimated their photometric behavior using the Hapke radiative transfer model and a Bayesian framework in order to estimate their microphysical state. We have found that most of them were consistent with the bright backscattering behavior of Europa, already observed at a global scale, indicating the presence of grains maturated by space weathering. However, we have identified very bright areas showing a narrow forward scattering possibly indicating the presence of fresh deposits that could be attributed to recent cryovolcanism or jets. Overall, we showed that the photometry of Europa’s surface is more diverse than previously thought and so is its microphysical state.

Reservoir quality controls on Rotliegend fluvio-aeolian wells in Germany and the Netherlands, Southern Permian Basin – Impact of grain coatings and cements

Abstract Reservoir quality in sandstones can be a function of temperature, pressure, and chemical alterations over time. Some intrinsic properties of sandstones can additionally impact the evolution of reservoir quality. The detrital composition, depositional environment, and properties controlled thereby, dictate some of the diagenetic changes. Extrinsic processes, like external fluids, e.g. from hydrothermal events or dewatering of clay minerals, can additionally result in changes of reservoir quality. One of these properties are grain coatings on quartz grains, which can strongly affect pore cementation by quartz overgrowths. To predict and infer the quality of undrilled reservoirs, constraints are needed to predict their occurrence. We analyse two reservoir settings in the Southern Permian Basin and delineate the impact of depositional systems, lithologies, detrital and authigenic composition on reservoir quality and with regard to grain coatings. Results highlight the importance of diagenetic alterations and the source area in regards to burial development in different parts of the Southern Permian Basin. Samples from the Rotliegend Bahnsen Sandstone Member from Germany and the Slochteren Sandstone Formation from the Netherlands have been petrographically and petrophysically analysed and linked to depositional environments. Our data shows that the emplacement and presence of grain coating minerals cannot be simply linked to the depositional environment or processes controlled thereby (e.g. grain size and sorting), neither on a reservoir-, nor on a basin scale. The dissolution of alumosilicates additionally has no consistent impact on the authigenesis of chloritic grain coatings on the reservoir- or basin scale in this study. This implies the necessity of sample specific analyses and importance of the assessment of diagenetic alterations when analysing, modelling, or predicting reservoir quality in similar settings worldwide.

Spatially resolved spectroscopy of the debris disk HD 32297

Context. Spectro-photometry of debris disks in total intensity and polarimetry can provide new insight into the properties of the dust grains therein (size distribution and optical properties). Aims. We aim to constrain the morphology of the highly inclined debris disk HD 32297. We also intend to obtain spectroscopic and polarimetric measurements to retrieve information on the particle size distribution within the disk for certain grain compositions. Methods. We observed HD 32297 with SPHERE in Y, J, and H bands in total intensity and in J band in polarimetry. The observations are compared to synthetic models of debris disks and we developed methods to extract the photometry in total intensity overcoming the data-reduction artifacts, namely the self-subtraction. The spectro-photometric measurements averaged along the disk mid-plane are then compared to model spectra of various grain compositions. Results. These new images reveal the very inner part of the system as close as 0.15″. The disk image is mostly dominated by the forward scattering making one side (half-ellipse) of the disk more visible, but observations in total intensity are deep enough to also detect the back side for the very first time. The images as well as the surface brightness profiles of the disk rule out the presence of a gap as previously proposed. We do not detect any significant asymmetry between the northeast and southwest sides of the disk. The spectral reflectance features a “gray to blue” color which is interpreted as the presence of grains far below the blowout size. Conclusions. The presence of sub-micron grains in the disk is suspected to be the result of gas drag and/or “avalanche mechanisms”. The blue color of the disk could be further investigated with additional total intensity and polarimetric observations in K and H bands respectively to confirm the spectral slope and the fraction of polarization.

Bismuth-substituted hydroxyapatite ceramics synthesis: Morphological, structural, vibrational and dielectric properties

New hydroxyapatite (HAp) ceramics substituted with 10, 30 and 50 at.% of bismuth were synthesized by conventional solid state reaction at high temperature T = 1300 °C, and characterized via X-ray diffraction, scanning electron microscopy, Fourier transform infrared and Raman spectroscopies; dielectric properties were also investigated. The hydroxyl groups observed in infrared spectra confirmed the HAp phase in the studied samples. The crystallite size was estimated by Scherrer's formula and the Williamson–Hall plot, while lattice parameters and the unit cell volume of the samples were measured according to Bi content.Scanning electron microscope images revealed the presence of grains with irregular sizes developed within larger aggregations. Measured relative dielectric permittivity and dielectric loss are slightly affected by Bi content, while the alternating current conductivity increases with a rise frequency, and decreases at increasing of Bi content. The HApBi50% sample presents the optimal dielectric properties.

Growth, structural, third order nonlinear optical properties, dielectric properties, conductivity mechanisms and spectroscopic characterization of a luminescent material trans-diaqua-bis(pyridine-2-carboxylato)-cobalt(ii) dihydrate for optoelectronic applications

Single crystals of trans-diaqua-bis(pyridine-2-carboxylato)-cobalt(ii) dihydrate (PCOH) have been successfully grown by slow evaporation technique. The formation of crystalline structure and cell parameters were confirmed by single crystal and powder X-ray diffraction techniques respectively. The structural and optical characterization of the PCOH were analyzed experimentally by FT-IR, FT-Raman and UV–visible spectroscopic technique and theoretical studies by density functional theory method. The charge transfer interactions leading to nonlinear properties taking place in the molecule can be studied by HOMO-LUMO analysis and natural bond orbital analysis. DFT computations using CAM-B3LYP/GenECP and B3LYP/GenECP were performed to determine the linear as well as NLO properties. Open aperture Z-Scan results confirm the competence of these materials as optical limiters (NLO). Furthermore, the PCOH exhibits a blue luminescence emission, as can be seen from the (CIE) chromaticity diagram, and hence the complex may be a promising blue organic light-emitting diode (OLED) in the development of electroluminescent materials for flat-panel-display applications. The electrical properties such as dielectric constant, dielectric loss and ac conductivity of the sample were carried out at different temperatures. The Cole-Cole plot shows the presence of grain and grain boundary, it is found that single relaxation is observed in the crystal. These attractive luminescence emission and third-order nonlinear results indicated that PCOH appear to be promising candidates for optoelectronic applications.

Vacuum annealing effect on physical properties and electrical circuit model of ZnO:Sn/SnO2:F bilayer structure

Tin doped Zinc oxide/Fluorine doped tin dioxide bilayer films (ZnO:Sn/SnO2:F) were deposited on glass substrates using spray pyrolysis technique. The effect of vacuum annealing at different temperatures was investigated. Both structural and morphological analysis have shown that there is a significant modification in the bilayer film structure and surface following the vacuum annealing process at 450 °C. Electrical properties have been investigated using the Hall Effect measurements as well as the impedance spectroscopy at room temperature. The circuit parameters were determined using an equivalent circuit model fitted from the impedance spectra and suggesting the presence of grain and grain boundary conductions in the bilayer structure. It was found that the film annealed in vacuum for 1 h at 350 °C is optimal in all respects, as it possesses all the desirable characteristics including the lowest resistivity, high porosity and better grain boundary conductivity.