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Theoretical Modeling of Relative Humidity Sensor based on Cross-Talk Fibers and Micro-emulsion templated Porous Silica Films Soumya Sen1 and V.K.Chaubey2 1
Dept. of Electronics & Instrumentation, BITS, Pilani, Rajasthan-333031, India. 2 Dept. of Electrical & Electronics, BITS, Pilani, Rajasthan-333031, India.
ABSTRACT The article deals with the theoretical modeling of Relative Humidity (RH) sensors based on cross-talk fibers. It also discusses the variation of refractive index of porous silica film with water content that can be used to develop a humidity sensor. The method for fabricating the microemulsion templated porous silica as a sensing medium has been illustrated based on Bruggeman’s Effective Medium theory, utilizing appropriate concentration of Poly Propyl Oxide (PPO) as the swelling agent. It thus proposes the model for a new humidity sensor using cross-talk mediums.
1. INTRODUCTION Porous silicon can be used as a smart transducer material in sensing application by modulating the electrical or optical property of the sensing instruments. An extensive experimental work has been reported employing the porous silicon as the sensing element for optical sensing, chemical sensing or biological sensing [1-3]. Intensity modulated multimode sensors are also widely utilized as a transuding mechanism for acoustic [4], pressure [5] and temperature [6] sensors with promising results. It is a proven fact that moisture absorbs light energy at specific wavelength, which through proper computer control and mathematical treatment may yield the moisture percentage. The water content in solid or granular material is an important parameter for resource management in agro-based industries [7]. Various types of moister sensors employing resistive network, capacitive cells in electrical domain and near infrared (NIR) technology in optical domain have been developed [7-9]. A portable and efficient humidity sensor, employing optical energy as sensing signal with proper transduction medium, may provide a compact, simple and faster transducer. The concept involved in the development of a humidity sensor is based on the principle of change in refractive index of porous silicon with humidity, which in turn leads to change in the signal output from the sensor. The design of the model requires the fabrication of a suitable porous silica film to be sandwiched between the fiber coupling arrangement, so that the intensity cross coupling may become a function of the moisture content in the film. The paper deals with the theoretical modeling of Fiber Optic Relative Humidity sensors based on the cross-talk effects through the porous silica film. The effective dielectric constant of a porous silica (PS) layer undergoes a large change when the moisture diffuses to the inner regions of the pore structure and adsorbs on the oxidized silicon skeleton. Since the effective dielectric constant, εd varies as the square of Refractive Index, n, and related by the equation: n2 =εd , we know that the effective refractive index of the medium will vary with the change in the moisture absorbed by the porous silicon layer. Hence in this model, two multimode fibers should be placed side by side over a small exposed length and this gap should be filled up with porous silica film, as shown in the figure 1. Care must be taken during this fabrication to avoid surface irregularities so that light can pass from the fibers to the PS. We know that the Bruggeman Effective Medium Approximation (EMA) predicts that the effective permittivity of a dielectric PS with a void volume of 60% and εd =2 will increase about 400-500%-which corresponds to experimental data [10]. For the present model, we should fix the porosity of the PS layer such that the layer’s dielectric constant increases from 1.96 (i.e. n=1.4) to say 400% of this value, which is around 8, for which the corresponding refractive index is approximately 2.8. So we realize that the refractive index of the PS layer varies from 1.4 to 2.8, as it adsorbs more and more moisture depending on the ambient humidity conditions. The light thus entering the PS layer from the input fiber will either suffer total internal reflection, or cross over to the second fiber through the sensing interface. The detectors at the ends of the second fiber are placed to measure the intensity of the outcoming light. The refractive index of the PS layer increases with the moisture, and thus reflecting more power leaving a lesser intensity at the detector. The method for fabricating the Microemulsion Templated porous
silica as a sensing medium has been illustrated based on Bruggeman’s Effective Medium Theory and appropriate concentration of PPO (Poly-Propyl Oxide) as the swelling agent for desired porosity of PS. We have developed some experimental module to validate the theoretical predictions. The experimental details have been emphasized in the section 6 of the text. Hence this proposed model for a new humidity sensor based on change in refractive index of crosstalk medium with moisture content, leading to variation in output intensity, could be explored further for improved applications.
Figure 1: Cross-sectional view of the sensor structure.
2. Manufacture of Low Dielectric Porous Silica The International Union of Pure and Applied Chemistry has recommended a classification for porous materials where pores of less than 2 nm in diameter are termed "micropores", those with diameters between 2 and 50 nm are termed "mesopores", and those greater than 50 nm in diameter are termed "macropores". Silica aerogels possess pores of all three sizes. However, the majority of the pores fall in the mesopore regime, with relatively few micropores [11]. The desire for very low dielectric constant materials (
