Effective Medium Theory for Multi-Component Materials Based on Iterative Method

Contribution of physical and system parameters in the determination of optical constants of thin solid phase samples using THz time domain transmission spectroscopy

Accurate estimation of complex refractive index of optically thin sample is an important challenge in terahertz time domain spectroscopy (THz-TDS). While majority of the previous studies on this topic consider Fabry-Perot (FP) effect as the primary cause of erroneous optical parameter extraction, advanced application criteria in industrial domains, such as, quality control of pharmaceutical composites or semiconductor heterostructure characterization etc. demand further assessment beyond FP effect only. Instead of conventional time-of-flight calculation pivoted to the reference data (usually obtained separately without any sample through air), we employed the differential data obtained from primary and secondary THz pulse through thin pellets to obtain accurate thickness mapping of the same. We observed that for pelletized samples with significantly larger porosity (>15%), the competing optical process like multiple internal scattering has greater impact on the accuracy of estimated optical properties. While in thin pelletized samples with smaller porosity (<10 %), the principal contributor of error is the non-uniform distribution of pellet thickness. For extremely thin pellets the optical parameter extraction is highly susceptible to uncertainties due to external factors such as environmental and/or system drift because of significantly small sample-THz interaction volume. Our observations indicate that short THz-sample interaction, porosity and non-uniformity of sample thickness are the principal causes of erroneous estimation of complex refractive index. It must be noted, however, that, this description does not specify an absolute value of material thickness but is highly dependent on the complex refractive index of the material itself. We also conclude that for specific material, there exists an optimized optical thickness beyond which the contribution of physical and system parameters in the determination of optical constants becomes minimal: it was determined to be around 3 mm for HDPE and 3.2 mm for PTFE. For most of the materials (with lower values of real and complex parts of optical constant) suitable for THz-TDS transmission spectroscopy, this critical thickness for accurate determination of the optical constant, therefore, is expected to be substantial, based on this present study.

Numerical Study of a Capacitive Graphene Oxide Humidity Sensor with Etched Configuration

The geometrical dependence of humidity sensors on sensing performance has not been quantitatively outlined. Furthermore, the etching effect on humidity sensors is still elusive due to the difficulty in separating the effects of the geometrical change and etching-induced porosity on the overall performance. Here, we use COMSOL Multiphysics to perform a numerical study of a capacitive graphene oxide (GO) humidity sensor, with emphasis on the dimensions and etching effect on their sensing performance. GO is a useful and promising material in detecting humidity because of its selective superpermeability to water molecules. The mechanism of improved sensing performance of the etched humidity sensors is discussed in terms of the morphological profile and the effective permittivity including the etching-induced porosity effect. Our study shows that as compared to the unetched sensors, isotropic etching achieves the lowest response time of 1.011 s at 15.75% porosity, while vertical etching achieves the highest capacitance sensitivity of 0.106 fF/RH %.

Deposition of a SiO2 Shell of Variable Thickness and Chemical Composition to Carbonyl Iron: Synthesis and Microwave Measurements

Protective SiO2 coating deposited to iron microparticles is highly demanded both for the chemical and magnetic performance of the latter. Hydrolysis of tetraethoxysilane is the crucial method for SiO2 deposition from a solution. The capabilities of this technique have not been thoroughly studied yet. Here, two factors were tested to affect the chemical composition and the thickness of the SiO2 shell. It was found that an increase in the hydrolysis reaction time thickened the SiO2 shell from 100 to 200 nm. Moreover, a decrease in the acidity of the reaction mixture not only thickened the shell but also varied the chemical composition from SiO3.0 to SiO8.6. The thickness and composition of the dielectric layer were studied by scanning electron microscopy and energy-dispersive X-ray analysis. Microwave permeability and permittivity of the SiO2-coated iron particles mixed with a paraffin wax matrix were measured by the coaxial line technique. An increase in thickness of the silica layer decreased the real quasi-static permittivity. The changes observed were shown to agree with the Maxwell Garnett effective medium theory. The new method developed to fine-tune the chemical properties of the protective SiO2 shell may be helpful for new magnetic biosensor designs as it allows for biocompatibility adjustment.

Mathematical Modeling of Layered Nanocomposite of Fractal Structure

A model of a layered hierarchically constructed composite is presented, the structure of which demonstrates the properties of similarity at different scales. For the proposed model of the composite, fractal analysis was carried out, including an assessment of the permissible range of scales, calculation of fractal capacity, Hausdorff and Minkovsky dimensions, calculation of the Hurst exponent. The maximum and minimum sizes at which fractal properties are observed are investigated, and a quantitative assessment of the complexity of the proposed model is carried out. A software package is developed that allows calculating the fractal characteristics of hierarchically constructed composite media. A qualitative analysis of the calculated fractal characteristics is carried out.

Detection of Microplastic in Salts Using Terahertz Time-Domain Spectroscopy

We report on a prototypical study of the detection of microplastic embedded in table salts by using terahertz time-domain spectroscopy. In the experiment, high-density polyethylene (HDPE) of sizes from 150 to 400 μm are used as a representative microplastic and mixed with table salts. Analyzing terahertz transmittance with an effective medium model, we extract various optical properties such as refractive index, absorption coefficient, and real/imaginary parts of the dielectric constant of the mixture. Consequently, the optical properties exhibit volume-ratio-dependence in 0.1-0.5 THz regimes. Especially, the refractive index and the real part of the dielectric constant possess monotonic frequency dependence, meaning that the quantities can be relevant indicators for the detection of the microplastic in terms of practical applications. Our work proves that terahertz time-domain spectroscopy can pave a way to recognize microplastic mixed with salts and be expanded for detecting various micro-sized particles.