M. Anis-ur-Rehman

A modified transient method for an easy and fast determination of thermal conductivities of conductors and insulators

The transient plane source (TPS) technique is a well-known technique for the measurement of thermal transport properties. It is applicable to a wide range of temperatures and materials. This paper describes in brief, the modified theory and an improved experimental arrangement for the measurements of thermal transport properties, particularly thermal conductivities. Fused quartz, low carbon steel and silver chloride crystals were studied with the new setup referred to as advantageous transient plane source (ATPS) technique. The measured values of thermal conductivity of these specimens are in excellent agreement with the values reported earlier. The advantage of this modified technique is the further simplicity of the equipment, more automation in data acquiring, better accuracy and ease in calculations. Also, thermal properties data are extended down to liquid nitrogen temperature for silver chloride crystals for the first time with ATPS method.

Structural, electrical and magnetic studies of nickel–zinc nanoferrites prepared by simplified sol–gel and co-precipitation methods

Ferrite nanoparticles, particularly nickel–zinc ferrite nanoparticles, are novel materials for high-frequency applications. Nanoparticles with a composition of Ni0.5Zn0.5Fe2O4 were prepared by two different processes, namely the co-precipitation and simplified sol-gel methods. Powder x-ray diffraction (XRD) patterns confirmed the single-phase spinel structure for the as-prepared samples. Samples were sintered at 555 and 755 °C, after which the structural, electrical and magnetic properties were studied. The crystallite sizes, as determined from XRD data, increased with sintering temperature. The dc electrical resistivity measurements were performed as a function of temperature, with the two-probe method in the temperature range from room temperature to 450 °C. The activation energy and drift mobility were calculated from the temperature-dependent dc electrical resistivity measurements. The dielectric constant and dielectric loss tangent for all the samples were determined as a function of frequency, and the frequency range used was from 20 Hz to 3 MHz at room temperature. The samples prepared using the simplified sol–gel method have lower dielectric constant values compared to those of the samples prepared using the co-precipitation method, and those prepared by the former method are more suitable for high-frequency applications. For the magnetic properties, a vibrating sample magnetometer was used. Saturation magnetization and coercivity increased with an increase in sintering temperature.

Structural, Electrical and Magnetic Properties of Nanocrystalline Mg-Co Ferrites Prepared by Co-Precipitation

Mg-Co nano crystalline ferrites having general formula Mg1-xCoxFe2O4 (x=0, 0.05, 0.1, 0.15, 0.2, 0.25) were prepared by co-precipitation method. X-ray powder diffraction (XRD) patterns of the prepared samples show the formation single spinel phase. The crystallite size, lattice parameters and porosity of samples were calculated by XRD data analysis as function of cobalt concentration. The crystallite size for each sample was calculated using the Scherrer formula considering the most intense (3 1 1) peak lies in the range 27-35 nm. The lattice parameters increased with increase of cobalt concentration. It is because of the fact that cobalt has greater ionic radius then magnesium. The dielectric constant, dielectric loss tangent and ac electrical conductivity of the prepared samples is also measure. The observed variation in electrical and dielectric properties is explained on the basis of cations distribution among tetrahedral (A) and octahedral (B) sites. The variance in saturation magnetization, remanence magnetization and magnetic moment was also measured from BH curve of samples.

Structural and dielectric properties of Cr-doped Ni–Zn nanoferrites

Cr-doped Ni?Zn ferrite nanoparticles having the general formula Ni0.5Zn0.5CrxFe2?xO4 (x=0.1, 0.3, 0.5) were prepared by the simplified sol?gel method. The structural and dielectric properties of the samples sintered at 750?5??C were studied. X-ray diffraction (XRD) patterns confirm the single-phase spinel structure of the prepared samples. The crystallite size calculated from the most intense peak (3 1 1) using the Debye?Scherrer formula was 29?34?nm. Scanning electron microscope images showed that the particle size of the samples lies in the nanometer regime. The dielectric constant (?r), dielectric loss tangent (tan??) and ac electrical conductivity (?ac) of nanocrystalline Cr?Ni?Zn ferrites were investigated as a function of frequency and Cr concentration. The dependence of ?r, tan?? and ?ac on the frequency of alternating applied electric field is in accordance with the Maxwell?Wagner model. The effect of Cr doping on the dielectric and electric properties was explained on the basis of cations distribution in the crystal structure.

Measurement of thermal transport properties with an improved transient plane source technique

The transient plane source (TPS) technique has been revised with the aim of developing a simple and fast system to measure the thermal transport properties of materials at low temperatures, especially high-Tc superconductors. To ensure reliable results, any new system should be tested with known samples. Fused silica, 0.9% carbon steel (215/3), and halide crystals (silver chloride) were studied with the new setup to check its performance. Data were taken from room temperature down to liquid nitrogen temperature. The assembly was designed for cryogenic (79 to 300 K) measurements in an atmosphere free of humidity. Dry nitrogen gas was used as a heat transfer medium around the sample holder assembly. The measured values for thermal conductivity and thermal diffusivity of these samples are in excellent agreement with values reported earlier. The thermal conductivity and thermal diffusivity for silver chloride crystals are extended down to 80 K although recommended data were available only down to 220 K. A Ba-doped, Bi-based, high-Tc superconductor was prepared by a solid-state reaction method. The nominal composition used was Bi1.6Pb0.4Sr1.6Ba0.4Ca2Cu3Oy. Large-sized samples (diameter ∼28mm and length ∼11mm) are investigated for thermal transport properties.

Thermal conductivity of ceramic fibres as a function of temperature and press load

Three ceramic fibres VK-60, ABK and Nextel/VK-80 produced by the steam blowing and nozzle dissemination methods have been investigated for the effect of press load per unit area and temperatures using the transient plane source (TPS) method in air. It was noticed that, with the increase of the aluminium content in the composition of the ceramic fibres, the thermal conductivity of the material decreases and the isolation properties improved. The Nextel/VK-80 fibre has the lowest and VK-60 the highest value of thermal conductivity at room temperature. The application of a press load results in an increase in the value of the thermal conductivity for all the fibres analysed. ABK fibres showed the least increase and Nextel/VK-80 registered an increase of about 10% in the value of thermal conductivity within the load increase from 0.6 to 6.6 kN m-2. However, above 6.6 kN m-2 the thermal conductivity of all the samples increased almost linearly. The thermal conductivity measurements as a function of temperature indicated the same trend for an increase in thermal conductivity for all the samples.

Proficient magnesium nanoferrites: synthesis and characterization

Ferrite materials are potential candidates for modern technological applications because of their tunable electrical and magnetic properties. The excellent combination of magnetic and dielectric properties of magnesium ferrites can be used to fulfill the future demand for high-frequency applications such as antennas. The electrical transport properties of these materials depend on the synthesis conditions such as sintering and composition. The aim of this work has been to correlate the synthesis conditions and induced electrical transport properties, so that these materials prepared in optimized conditions can be used for the miniaturization of high-frequency application devices. X-ray diffraction (XRD) patterns of samples prepared by the co-precipitation method confirmed the formation of a single spinel phase. The crystallite size, lattice parameters and porosity of the samples were calculated from XRD data. The scanning electron microscopy results showed the formation of rods in the case of the samples sintered at 950 C. All the electrical and dielectric properties showed strong dependence on structural properties. The dielectric constant, dielectric loss tangent and ac electrical conductivity of nanocrystalline Mg ferrites were investigated as a function of frequency and sintering temperature. Dielectric, ac electrical properties and the effect of sintering temperature are explained in accordance with the Maxwell‐Wagner and the Koops models.

Annealing effects on the microstructural properties of complex sulfosalt (SnS)x–(Bi2S3)1−x gradient thin films

The annealing temperature dependencies of microstructural properties of complex sulfosalts (SnS)x–(Bi2S3)1−x gradient films have been studied. Films were prepared by thermal evaporation with a combinatorial approach on glass substrates. This work aims to investigate the high throughput screening of these sulfosalts as new photovoltaic materials and also to search Cd free photovoltaic materials. The microstructural behaviour and changing of nanocrystalline (SnS)x–(Bi2S3)1−x structures with respect to composition and annealing temperature (200–400 °C) in vacuum was studied. Structural properties were investigated using x-ray diffraction, energy-dispersive x-ray spectroscopy and Raman spectroscopy measurements. At lower annealing temperature only binary phases such as SnS, Bi2S3, SnS2 and Sn2S3 exist while at higher temperature a ternary phase SnBi2S4 was also developed. These crystalline structures originated in orthorhombic and hexagonal phases and depend on annealing temperatures and composition. The phase formation was different at SnS and Bi2S3 rich ends of thin films. Also, crystallite size, dislocation densities and microstrain gradually varied at both ends. Considering the structural point of view, annealing temperature played a key role on enhancement of crystal quality and phase growth in structures from binary to ternary. Raman microscopy showed that films had bands for different phases. In this work we achieved binary, to a combination of binary and ternary phase conversion of (SnS)x–(Bi2S3)1−x system.

Thermal transport properties of synthetic porous solids as a function of applied pressure

The thermal conductivity, thermal diffusivity and specific heat per unit volume of three types of porous insulating solids were measured, simultaneously, by the transient plane source (TPS) technique. The samples, namely, fibreglass, closed-cell foam and foam insulation, were subjected to external pressure, taking into consideration the compressibility of the samples and the requirements of the TPS technique. These materials are used for insulation for temperature control of air-conditioned space and therefore are helpful in reducing energy losses. Although all three samples are good insulators, having very small thermal conductivity values (~0.03-0.05 W m-1 K-1), closed-cell foam insulation shows insignificant changes in thermal conductivity for the selected pressure range (8-15 kPa). Therefore it is a better choice for thermal isolation. For the other materials investigated, the thermal conductivity value increases slightly with external pressure.

Thermal and electrical properties of crystalline silver bromide

Simultaneous measurements of the thermal conductivity, thermal diffusivity and heat capacity per unit volume of monocrystalline silver bromide (AgBr), using the Gustafsson probe, in the temperature range 77–350 K are reported. Both thermal conductivity and thermal diffusivity follow Euckens law in the temperature region studied. The heat capacity at constant pressure (CP), determined from the volumetric heat capacity, agrees with the calculated value at room temperature. The ratio of the thermal conductivity (λ) of AgBr and AgCl calculated from the Leibfried–Schlomann formula is in excellent agreement with the ratio λ(AgBr)/λ(AgCl) measured experimentally. The dielectric constant and loss factor of crystalline AgBr are also reported as a function of frequency in the range 100 Hz–1 MHz at room temperature.