Thermal conductivity of Freon-12

The construction of a heated filament measurement device is described. The thermal conductivity of Freon-12 in the liquid phase is measured over the temperature range ?50–100°C at pressures up to 600·10⁵ N/m², and conductivity of the gaseous phase of Freon-12 is measured over the range 30–160°C at atmospheric pressure. The results obtained are approximated by equations and compared with the data of other authors.     

Effect of pH Variation on Structural and Dielectric Properties of Co-precipitated Nanostructured Multiferroic BiFeO3

Phase pure multiferroic BiFeO₃ (BFO) was prepared by the co-precipitation technique using diverse precursors at a temperature as low as 400 °C. The dependence of structural and dielectric properties was investigated with the variation of pH (8–13, with steps of 1). Uniaxially pressed powders were sintered in air at 500 °C for 2 h. X-ray diffraction analysis determined the amorphous nature of the as-synthesized samples and the perovskite nature of the sintered samples. The crystal structure obtained from the lattice parameters of the BFO specimens was R3c hexagonal. The fractional porosity of the pellets was determined from the measured and theoretical density of the samples. The crystallite size varies from 48–62 nm with the variation of the pH value. SEM micrographs exhibited a grain morphology and a wide size distribution. The dielectric constant, loss tangent, and ac conductivity of the pellets were measured as a function of frequency (20 Hz–3 MHz). A high dielectric constant and dissipation factor for the sample prepared with pH=10 was observed in the low frequency range.     

Effect of pyrolytic temperature on the properties of nano-structured Cuo optimized for ethanol sensing applications

In the present work, copper oxide thin films have been deposited at different substrate temperatures from 250 to 400 °C by spray pyrolysis technique. The desired properties of phase pure CuO with good crystal quality and conductivity have been optimized with respect to pyrolytic temperature. X-ray diffraction studies and Hall effect measurements indicated that these two properties are achieved at an optimum temperature of 350 °C. The band gap of CuO films was found to decrease from 1.8 to 1.2 eV with increase in substrate temperature, based on the UV-absorption spectrum of the film. The microstructures revealed that the film optimized at 350 °C, showed uniform surface with trapezium shaped particles, which are well compacted. The dynamic sensing behavior of the optimized p-type CuO sensor, prepared at a substrate temperature of 350 °C, was used to sense ethanol for concentrations: 100 and 200 ppm. The response time and the recovery time were within the range of 15–20 s and 15–18 s, respectively. The results revealed good response even at room temperature, with characteristics dependent on the size of the grains and the concentration of ethanol.     

Investigation of density of states and electrical properties of Ba<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>Bi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> nanoceramics prepared by chemical route

Barium-cobalt-bismuth-niobate, Ba_0.5Co_0.5Bi₂Nb₂O₉ (BCoBN) nanocrystalline ferroelectric ceramic was prepared through chemical route. XRD analysis showed single phase layered perovskite structure of BCoBN when calcined at 650 °C, 2 h. The average crystallite size was found to be 18 nm. The microstructure was studied through scanning electron microscopy. The dielectric and ferroelectric properties were investigated in the temperature range 50–500 °C. The dielectric constant and dielectric loss plot with respect to temperature both indicated strong relaxor behavior. Frequency versus complex impedance plot also supported the relaxor properties of the material. The impedance spectroscopy study showed only grain conductivity. Variation of ac conductivity study exhibited Arrhenius type of electrical conductivity where the hopping frequency shifted towards higher frequency region with increasing temperature. The ac conductivity values were used to evaluate the density of state at the Fermi level. The minimum hopping distance was found to be decreased with increasing temperature.     

Complex impedance spectroscopy studies of PLZT (5/52/48) ceramics synthesized by sol–gel route

The polycrystalline perovskite Pb_0.95La_0.05(Zr_0.52Ti_0.48)O₃ ceramic material was prepared by sol–gel route. Structural, phase formation and thermal properties were confirmed by X-ray diffraction, thermogravimetry and differential thermal analysis. The microstructural study was carried out using scanning electron microscope. The electrical properties of the ceramics were investigated within a wide range of both temperature (from 25 to 500 °C) and frequency (from 100 Hz to 1 MHz) using complex impedance spectroscopy. The impedance spectrum results indicate the decrease in dielectric constant with increase in frequency while the dielectric loss increases with frequency. The activation energy of the sample was calculated as 0.124 eV from the Arrhenius plot of dc conductivity vs. inverse of absolute temperature. The remnant polarization (P_r) and coercive electric fields (E_c) are found out as 5.97 μC/cm² and 8.89 kV/cm from the ferroelectric loop measured at room temperature.     

Effect of Temperature on Electrical and Thermal Properties on Carbon Soot Filled Polyester Graded Composites

This article reports the development, DC conductivity behavior of carbon soot filled polyester graded composites. Carbon soot filled polyester composites having 3 wt% of carbon soot powder and polyester resin were prepared. DC conductivity measurements were conducted on the graded composites by using an electrometer in the temperature range of 28°C to 150°C. DC conductivity increased with increase in carbon soot concentration in the composites; DC conductivity increased with the increase in temperature. Activation energy was calculated by using Arrhenius equation for graded samples exhibited electronic conduction. Linear dependence of pre-exponential factors on activation energy for carbon soot filled polyester graded composites reveals semi-conducting behavior of the composites.     

Structural and electrical properties of V-doped ZnO prepared by the solid state reaction

This paper reports the synthesis, crystal structure and electrical conductivity properties of vanadium (V)-doped zinc oxide (ZnO) powders (i.e. Zn_1?2X V _X O binary system, x = 0, 0.0025, 0.005, 0.0075 and in the range 0.01 ≤ x ≤ 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the V-doped ZnO binary system, were determined by X-ray diffraction (XRD). The limit solubility of V in the ZnO lattice at this temperature is 3 mol % at 950 °C. The impurity phase at 950 °C was determined as ZnV₂O₆ when compared with standart XRD data. The research focused on single I-phase ZnO samples which were synthesized at 950 °C because of the limit of the solubility range is widest at this temperature. It was observed that the lattice parameters a and c decreased with V doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-point probe dc method at temperatures between 100 and 950 °C in an air atmosphere. The electrical conductivity values of pure ZnO and 3 mol % V-doped ZnO samples at 100 °C were 2.75 × 10^?6 and 7.94 × 10^?5 Ω^?1 cm^?1, and at 950 °C they were 3.4 and 54.95 Ω^?1 cm^?1, respectively. In other words, the electrical conductivity increased with V doping concentration.     

Dielectric characterization of gadolinium tartrate trihydrate crystals

Single crystals of gadolinium tartrate trihydrate have been grown by gel diffusion technique. Single crystal X-ray diffraction analysis shows that the crystals belong to the tetragonal system with non-centrosymmetric space group. The dielectric constant, dielectric loss and ac conductivity have been measured as a function of frequency in the range 1 kHz–5 MHz and temperature range 20–300 °C. The dielectric constant increases with temperature, attains a peak around 240 °C and then decreases as the temperature exceeds 240 °C. The dielectric anomaly at 240 °C is suggested to be due to phase transition brought about in the material, which is further supported by the thermal studies. The variation of ac conductivity with temperature has been measured and the material is suggested to show protonic conductivity.     

Stability,viscosity, thermal conductivity,and electrical conductivity enhancement of multi-walled carbon nanotube nanofluid using gum arabic

This experimental investigation discussed on the stability and rheological behavior of multi-wall carbon nanotubes (MWCNTs) nanofluids with and without gum arabic (GA). The stability of MWCNT in the presence of GA dispersant in solar glycol is systematically investigated by taking into account the combined effect of different parameters, such as sonication time, temperature, dispersant and particle concentration. The concentrations of MWCNT and GA have been varied from 0.2 to 0.6% volume concentration and from 0.25 to 1.25 wt%, respectively, and the sonication time has been varied in between 30 and 120 min. The effect of sonication time on viscosity was discussed. It was perceived that the shear thinning behavior is exhibited by all the nanofluid samples. The stability of nanofluid is measured in terms of MWCNT concentration as a function of sediment time using UV-Vis spectrophotometer. Rheological behavior of MWCNT nanofluids is measured using Bohlin CVO Rheometer in the temperature range of 30–50°C, with step sizes of 5°C. Optimal GA concentration is obtained for the entire range of MWCNT concentration and 0.25–1.25 wt% of GA is found to be sufficient to steady all MWCNT range in solar glycol. Rapid sedimentation of MWCNTs is observed at higher GA concentration and sonication time. The presence of MWCNT and GA enhanced the thermal conductivity of the nanofluids by 30.59% at 0.6 vol.% particle concentration and 1.25 GA wt% at 50°C. The electrical conductivity is enhanced in a linear manner with respect to the loading of MWCNT and GA. Nevertheless, the electrical conductivity is increased linearly with increasing the temperature of the nanofluid. At particle concentration of 0.6 vol.% of MWCNT and 1.25 wt% of GA, the electrical conductivity of the nanofluid is improved by 190.57% at a temperature of 50°C.     

Effects of Sintering Process on Microstructure and Properties of Flake Graphite-Diamond/Copper Composites

Flake graphite-diamond/Cu–Cr–Zr composites with good two-dimensional thermophysical properties were prepared by vacuum hot-pressing technology. The influence and working mechanism of the hot-pressing temperature on the relative density and thermal conductivity of the composites were studied to obtain the optimum sintering process. The results showed that with a pressing pressure of 10 ～ 20 MPa, the relative density and thermal conductivity of the composite materials increased as the sintering temperature increased from 950 to 1010°C. When the temperature rose to 1010 ～ 1040°C, a near fully dense composite material was obtained and thermal conductivity reached maxima of 410 and 119 W/m K parallel and perpendicular to the graphite planes, respectively, both of which are close to the theoretical value. However, relative density and thermal conductivity drastically decreased as the temperature continued to increase beyond 1070°C. This is attributed to the combined effect of sintering temperature and wettability between the matrix and the reinforcements.     

Ferroelectric studies and impedance analysis of PLZT (10/52/48) electroceramics

Perovskite Pb_0.90La_0.10(Zr_0.52Ti_0.48)O₃ ceramic material was prepared through sol–gel process. Structural, phase formation and thermal properties were confirmed by X-ray diffraction, thermogravimetry and differential thermal analysis; size and microstructural study was carried out using particle size analyser and scanning electron microscope. The electrical properties of the ceramics were investigated as a function of both temperature (from room temperature to 500 °C) and frequency (from 100 Hz to 1 MHz) using complex impedance spectroscopy (CIS). The impedance spectrum results indicate the decrease in dielectric constant with increase in frequency while the dielectric loss increases with frequency. The activation energy of the sample was calculated from the slope of the Arrhenius plot as 0.129 eV from the Arrhenius’s plot of dc conductivity versus inverse of absolute temperature. The dc conductivity was obtained from CIS measurements and the activation energy. The remnant polarization (P_r) and coercive electric fields (E_c) are found out as 6.52 μC/cm² and 2.55 kV/cm from the ferroelectric loop measured at room temperature.     

Comparative Study of Nano Crystalline Ceria Synthesized by Different Wet-Chemical Methods

Cerium oxide (CeO₂) nanoparticles were synthesized by using sol–gel methods and the Composite Mediated Hydrothermal Method (CMHM). Sol–gel synthesis was done for 0.6 M molarity of the precursor solution of cerium nitrate hexa hydrated [Ce(NO₃)₃?6H₂O] and the hydrothermal synthesis was done at 180 °C for 45 min. X-Ray diffraction (XRD) was used to measure the structural properties of the prepared ceria. Scherrer’s formula was used to calculate the crystallite sizes of average and most intense peak. Temperature dependent (200 °C to 700 °C) dc conductivity was measured and found to be increasing with the increase in measuring temperature. The frequency dependent (20 Hz to 3 MHz) ac conductivity and dielectric properties were measured at different temperatures. The comparison of electric and dielectric properties was studied. Both synthesis techniques were further analyzed by the Raman spectrum at 514 nm excitation laser line for the different bands of cerium oxide and oxygen vacancies.     

Effect of particles size on the AC electrical behavior of iron/polystyrene composites

The AC electrical characteristics of polystyrene/Iron composites filled with iron particles of average sizes: 5, 40, 110 and 250 μm, have been investigated. The AC electrical properties were studied in frequency range (50 kHz–1 MHz), and temperature range (30–110 °C) using the impedance method. The AC-conductivity and dielectric constants were determined from the measured impedance data. It was found that the applied frequency, temperature, and iron particles size affect the electrical and dielectric properties of the composites. The AC-electrical conductivity is increasing with temperature. The dielectric constant and the dielectric loss of the composites increase with decreasing the iron particles size. The universal power-law of the electrical conductivity gives exponent with 0 < m < 1 characterizing hopping conduction. The small values of the activation energy indicate that the composite of smallest iron particle size, electrons can tunnel or hop more easily from the valence band to conduction energy band due to the reduction of interparticles separation.     

Effect of temperature on dielectric and electrical properties of Co–Zr doped barium hexaferrites prepared by sol–gel method

In the present research, temperature dependence of dielectric properties of cobalt–zirconium substituted barium hexaferrites, fabricated using citric acid sol gel method, has been reported. The dielectric constant, loss tangent and A.C. conductivity were investigated on the circular pellets in temperature range 30–350 °C and frequency range 10 kHz–1 MHz using impedance analyzer. This paper also presents impedance (Z*) and electric modulus (M*) analysis of all the samples. The single semi-circular arcs, observed in impedance Nyquist plots, suggest the dominance of grain boundaries in the conduction process. Dielectric constant and dielectric loss tangent show very small variation up to 200–250 °C temperature and abrupt increase afterwards up to 350 °C. Thus, these ferrites can be successfully implemented in the practical applications like capacitors, microwave devices etc. up to 250 °C, without any significant change in properties.     

Study of electrical conductivity changes and phase transitions in TiO2 doped ZrO2

The electrical conductivity in ZrO₂ doped with various molar ratios of TiO₂ has been measured at different temperatures. Phase transitions depending on the temperature for different molar compositions were investigated by doping of the samples. The conductivity is also found to increase with rise in temperature till 200 °C and thereafter decreases due to collapse of the fluorite framework. A second rise in conductivity in the doped samples around 462 °C is observed due to phase transition of ZrO₂. X-ray powder diffraction recorded at high temperature show the effect of doping and the phase relationships of doped materials. DTA was also carried out for confirming doping effect and phase transition in samples.     

Structure,dielectric property and impedance spectroscopy of La<Subscript>2/3</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics by sol–gel method

La_2/3Cu₃Ti₄O₁₂ (LCTO) precursor powders were synthesized by the sol–gel method. Effect of sol conditions and sintering process on microstructure and dielectric properties of LCTO powders or ceramics were investigated systematically. The optimum sol conditions for the synthesis of precursor powders were as follows: the Ti⁴⁺ concentration of 1.00 mol/L, the molar ratio of water and titanium of 5.6:1 and the sol pH of 1.0, respectively. After sintered at 1105 °C for 15 h, the LCTO ceramics exhibited more homogeneous microstructure, much higher dielectric constant (ca 09–1.6 × 10⁴) and lower dielectric loss (ca 0.057). The higher dielectric constant of the LCTO ceramics might be due to the internal barrier layer capacitor effect. The LCTO ceramics showed two kinds of conductivity activation energy for grain boundary conductivity from complex impedance analysis. The transition temperature of two activation energy values occured between 170 and 210 °C. The temperature range of 170–210 °C was critical pseudocritical region of the dielectric constant, dielectric loss and activation energy. Furthermore, it was concluded that the grain boundary play an important role for electrical properties.     

Analytical and experimental study on thermal conductivity of hardened cement pastes

Thermal conductivity of hardened cement pastes (hcps) in a wide range of water–cement ratio (w/c) is quantitatively investigated using a transient plane source measurement technique. Alkyl alkoxysilane and rapeseed oil were also added to determine the effect of internal hydrophobation on thermal conductivity of solid structure of hcps. The measurements were performed after drying at 50 and 105 °C as well as water submersion. A nonlinear relation was observed between thermal conductivity and w/c which is in alignment with Powers’ model. Samples dried at 50 °C still contained some moisture which increased thermal conductivity up to 11 % compared to samples dried at 105 °C. Furthermore, hydrophobic agents reduced thermal conductivity of dried samples up to 9 % which indicates the reduction in thermal conductivity of solid structure and is in line with observations by scanning electron microscope. A three phase model which can predict thermal conductivity of plain and hydrophobed hcps at different moisture states is presented by exploiting composite models and Hashin–Shtrikman bounds.     

Dielectric and impedance characteristics of Ba(Bi0.5Nb0.5)O3 ceramics

A lead free polycrystalline material Ba(Bi_0.5Nb_0.5)O₃ was prepared using a high-temperature mixed oxide technique using high purity ingredients. The formation of the material in monoclinic crystal structure was confirmed by an X-ray structural analysis at room temperature. The nature and texture of microstructure by scanning electron microscopy show that the compound has well defined grains uniformly distributed throughout the surface of the sample. Detailed studies of dielectric and impedance properties of the material, carried out in the frequency range of (1 kHz–1 MHz) at different temperatures (30 °C to 475 °C), have shown many interesting properties. Dielectric study showed an existence of diffuse phase transition around 317 °C. The temperature dependence of impedance parameters (impedance, modulus etc.) of the material exhibits a strong correlation of its micro-structure (i.e., bulk, grain boundary, etc.) with the electrical parameters. An existence of negative temperature coefficient of resistance (NTCR) type behavior in the material similar to that of semiconductors was also observed. The complex electric modulus analysis indicates the existence of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature.     

Synthesis and characterization of antimony-doped tin oxide (ATO) with nanometer-sized particles and their conductivities

This study focused on finding the optimum conditions for displaying higher conductivity in an ATO-PET film prepared using the solvothermal method. The conductivity of the ATO film with Sb1.5:Sn8.5 increased to 800 °C calcinations. It did not, however, increase further even though the calcinations were at a temperature above 1000 °C. The average grain size measured from the FESEM micrographs was distributed within a 5.0-nm (at 400 °C) to 50.0-nm (at 1000 °C) range. It was determined from the XRD results that the special peaks assigned to the SnO₂ tetragonal type dominated until 1000 °C in the ATO particles with Sb1.5:Sn8.5. It was also confirmed that the hydrophilicity (the T–OH/T–O ratio was larger) of the ATO nano-particle with Sb1.5:Sn8.5 was largest at 600 °C calcinations. Its binding energy remarkably increased at 1000 °C calcination. In various Sb:Sn mole ratios, the conductivity was at its best in ATO films (for Sb1.5:Sn8.5) with 600 °C calcinations, 9.0×10⁵ (Ω/□). When 1,4-butanediol was used as a solvent, the conductivity of the ATO film was enhanced and the ATO film exhibited higher distribution than the other solvents did. The conductivity of the ATO film prepared in basic conditions (pH=10.0) was enhanced compared to those in acidic conditions.     

Low-temperature sintered pollucite ceramic from geopolymer precursor using synthetic metakaolin

In this article, pollucite ceramic with high relative density and low coefficient of thermal expansion (CTE) was prepared from Cs-based geopolymer using synthetic metakaolin. Crystallization and sintering behavior of the Cs-based geopolymer together with thermal expansion behavior of the resulted pollucite ceramic were investigated. On heating at 1200 °C for 2 h, the amorphous Cs-based geopolymer completely crystallized into pollucite based on crystal nucleation and growth mechanism. Selected area diffraction analysis and XRD results confirmed the resulted pollucite ceramic at room temperature was pseudo-cubic phase with superlattice structure. Compared with Cs-based geopolymer using natural metakaolin, geopolymer using synthetic metakaolin in this article showed a much lower viscous sintering temperature range, which started at 800 °C, reached a maximum value of ?7.47 × 10^?4/°C at 1121.9 °C, and ended at 1200 °C. Cesium volatilization appeared only when temperature was above 1250 °C. Therefore, densified pollucite ceramic can be prepared from Cs-based geopolymer using synthetic metakaolin without cesium volatilization. Abnormal thermal shrinkage of pollucite ceramic was observed at temperature range from 25.3 to 54.6 °C because of pseudo-cubic to cubic phase transition, and its average CTE was 2.8 × 10^?6/°C from 25 to 1200 °C.