Microwave dielectric characterization of hevea rubber latex at 2.6, 10 and 18 GHz

Dielectric properties of natural rubber Hevea brasiliensis latex were measured at frequencies 0.2 to 20 GHz, at temperatures of 2, 15, 25, 35, and 50°C and around 30-98% moisture content. Measurements were done using open-ended coaxial line sensor and automated network analyzer. As expected, results showed that dielectric constant increased with increasing moisture. From 0.2 to 2.6 GHz, the losses were governed by conductive losses but for frequencies greater than 2.6 GHz, these were mainly due to dipolar losses. The former is due to conducting phases in hevea latex, while the latter is mainly governed by the orientation of water molecules. The results were analyzed at 2.6, 10, and 18 GHz, respectively. These were then compared with the values predicted by the dielectric mixture equations recommended by Weiner, Bruggeman and Kraszewski. All the measured values were found to be within the Weiner's boundaries and close to the upper limit of Weiner's model. It is also close to the predicted values of Bruggeman's model with a/b = 0.1. All the models including Kraszewski are suitable for predicting the dielectric properties of hevea latex for frequencies 2.6 to 18 GHz, moisture content 30 to 98% and temperatures 2 to 50°C

The analysis and design of multi-layer microstrip moisture sensor for rice grain

The analysis and optimal design of a multi-layered microstrip sensor for measuring moisture content of ricegrain are described. The microstrip sensing structure consists of three layers: substrate, protective layer and semi-infinite grain medium. The effective dielectric constant, characteristic impedance and attenuation or insertion loss of this structure are calculated at various moisture contents with respect to the thickness of the protective layer thickness. The sensor is operated at 9 GHz and is suitable for a broad range of moisture contents (MC) ranging from 10% to 30% (wet basis). This study investigated the optimal thickness of the protective layer for suitable sensitivity, the thickness of grain medium which can be consider as semi-infinite thickness, the effect of kernel density and loading method. The experimental results for the attenuation of the signal at various moisture contents agree reasonably with the theoretical prediction however for better accuracy, the compactness of grain medium should be observed. The effect of air inside the medium and compactness of the material under the test has been studied and the errors of ±0.16 dB have been calculated for the measured reflected attenuation of the sensor. The overall accuracy of the sensor is about ±1.5% moisture content as compared to oven drying method

Dielectric properties of strontium titanate filled mullite composites in microwave region

This research was designed to form better dielectric composite material using one steady state dielectric with a good dielectric material. Distinct dielectric composite was successfully produced using locally sourced kaolinite clay. The samples were made using kaolinite as the base matrix and Strontium Titanate (ST) added in varying ratios. Strontium Titanate were synthesized via solid‐state reaction using Strontium Carbonate and rutile Titanium (IV) Oxide with sintering at 1300 °C. Local white kaolinite was used to fuse the barium titanate material in varying weight ratios. The powders were dry‐mixed and made into pellets for calcination at 1000 °C. The dielectric measurements were carried out using the HP 4291B Impedance Analyzer dielectric setup. Three samples were prepared, namely 10%ST, 20%ST and 30%ST. The dielectric measurements were carried out at room temperature. Microwave region measurements showed steady state and linear dielectric relaxation ranging from 7 in the control sample and dropping down to 5 in 30%ST. The responses indicate linear relation between ST addition and microwave region dielectric permittivity

Effect of AgI addition on elastic properties of quaternary tellurite glass systems

A series of quaternary tellurite glasses {[(TeO2)70(B2O3)30] 90[Ag2O]10}100-z{AgI}z with z=5, 8, 10, 13 and 15 mol% were fabricated by rapid quenching technique. Pulse echo technique was employed to detect the longitudinal and shear ultrasonic velocities generated by 5 MHz transducer room temperature. Elastic properties, Poisson’s ratio, micro hardness, softening temperature and Debye temperature calculated from the measured density and ultrasonic velocity were observed to decrease monotonously with the increase of AgI content. This shows that the presence of AgI inside the glass network creates a loose packing structure of the glass network and hence reduces the rigidity and the strength of the glass system

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co0.5Zn0.5Fe2O4 nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g−1. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency

X-ray diffraction studies on crystallite size evolution of CoFe2O4 nanoparticles prepared using mechanical alloying and sintering

Nanosized cobalt ferrite spinel particles have been prepared by using mechanically alloyed nanoparticles. The effects of various preparation parameters on the crystallite size of cobalt ferrite which includes milling time; ball-to powder weight ratio (BPR) and sintering temperature, were studied using X-ray diffractometer (XRD). Scherrer's equation was used to study the crystallite size evolution of the as-prepared materials. The results of the as-milled sample revealed that both milling time and BPR plays a role in determining the crystallite size of the milled powder. However, where sintering is involved, the sintering temperature results in grain growth, and thus plays a dominant role in determining the final crystallite size of the samples sintered at higher temperature (above 900 °C). From the vibrating-sample magnetometer (VSM) measurement it was observed that the coercivity of the as-milled samples without sintering is almost negligible, which is a type characteristic of superparamagnetic material. However, for the sintered samples, the saturation increases while coercivity decreases with increases sintering temperature

Dielectric properties of rubber wood at microwave frequencies measured with an open-ended coaxial line

Dielectric properties of rubber wood were studied at different microwave frequencies, structural directions, and moisture contents using an open-ended coaxial probe. Frequencies used for this study were 1.00, 2.45, 6.0, 8.0, 10.0, 14.0, and 17.0 GHz; and the measurements were carried out at a room temperature of 22-24°C. The dielectric constant and dielectric loss factor were found to increase continuously as the moisture content increased. A sharp rise in the dielectric constant and dielectric loss factor was obtained at high moisture content, and the trends became concave upward. As the frequency increased, the dielectric constant decreased, whereas the dielectric loss factor increased. The dielectric loss factor remained almost constant above the frequency of 6 GHz for all structural directions. Fourth-order polynomial equations were found suitable for the best fit curve. Dielectric constant and dielectric loss factor of oven-dry wood were higher in the longitudinal direction than in the radial and tangential directions. With respect to frequency, the dielectric loss factor exhibited a peak value around 10 GHz. The dielectric anisotropy of wood may be attributed to the microscopic and macroscopic molecular structures as well as to chemical constituents of wood

Effect of moisture content and grain direction on the dielectric properties of rubber wood at low frequencies

Dielectric constant and dielectric loss factor of rubber wood have been studied at different moisture content, grain direction and frequency. Different dielectric dispersion mechanisms are also observed at different ranges of moisture content. The moisture content above the fiber saturation point does not contribute much to the dielectric properties following a single dielectric mechanism. But the moisture content below the fiber saturation point shows considerable effect on the variations of dielectric properties having different dispersion processes at different moisture content ranges. Based on the shape of the curves, five different moisture content ranges have been identified such as 1) 25% and above, 2) 18-25%, 3) 11-17%, 4) 5-10% and 5) below 5%. It may be possible to explain all these dispersion processes by means of dielectric mechanism for dipole, quasi-dc and diffusive processes. Longitudinal direction shows a higher dielectric constant when compared to radial and tangential directions in ovendry condition. This dielectric anisotropy may be attributed to the microscopic, macroscopic and molecular structures of wood