Introduction to heat transfer

This review offers new perspectives on the subject and highlights an area in need of further research. It includes an analysis of current scientific literature mainly covering the last decade and examines the trends in the development of electronic, acoustic and optical-fiber humidity sensors over this period. The major findings indicate that a new generation of sensor technology based on optical fibers is emerging. The current trends suggest that electronic humidity sensors could soon be replaced by sensors that are based on photonic structures. Recent scientific advances are expected to allow dedicated systems to avoid the relatively high price of interrogation modules that is currently a major disadvantage of fiber-based sensors.

Toward a New Generation of Photonic Humidity Sensors

Synthesis of TiO2 nanoparticle through hydrolysis method is presented followed by TiO2nanoparticle doped polyvinyl alcohol nanocomposite by solution process. FTIR, XRD, DSC-TGA, FESEM, TEM analysis are used to identify the nature of synthesized nanoparticle and loading uniformity of developed composite material. A simple modified clad based optical fibre sensor is developed to measure relative humidity. Coated modified clad optical fibre exhibits excellent relative humidity sensing performance with improved thermal stability of coating material in wide range of 9-95 % RH with good process repeatability. Sensor response is also observed to be very fast and highly reversible. Advantage of our developed composite material become evident when it exhibits wider range of moisture sensitivity compare to pure PVA or pure TiO2 material found in literature. Performance of PVA-TiO2 nanocomposite thick film is also evaluated by capacitance method and result found to agree with coated modified clad optical fibre.

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Modified clad optical fibre coated with pva/tio2 nano composite for humidity sensing application

In our previous work (Lazarus and Fedder 2011 J. Micromech. Microeng. 21 0650281), we demonstrated a CMOS-MEMS capacitive humidity sensor with a 72% improvement in sensitivity over the highest previously integrated on a CMOS die. This paper explores a series of methods for creating a faster and more manufacturable high-sensitivity capacitive humidity sensor. These techniques include adding oxide pillars to hold the plates apart, spin coating polymer to allow sensors to be fabricated more cheaply, adding a polysilicon heater and etching away excess polymer in the release holes. In most cases a tradeoff was found between sensitivity and other factors such as response time or robustness. A robust high-speed sensor was designed with a sensitivity of 0.21% change in capacitance per per cent relative humidity, while dropping the response time constant from 70 to 4s. Although less sensitive than our design, the sensor remains 17% more sensitive than the most sensitive interdigitated designs successfully integrated with CMOS.

https://iopscience.iop.org/article/10.1088/0960-1317/22/8/085021/pdf

Designing a robust high-speed CMOS-MEMS capacitive humidity sensor

Photonic Crystal Fiber Interferometers (PCFIs) are widely used for sensing applications. This work presents the fabrication and the characterization of a relative humidity sensor based on a polymercoated photonic crystal fiber that operates in a MachZehnder Interferometer (MZI) transmission mode. The fabrication of the sensor involved splicing a short (1 cm) length of Photonic Crystal Fiber (PCF) between two single-mode fibers (SMF). It was then coated with a layer of agarose solution. Experimental results showed that a high humidity sensitivity of 29.37 pm/%RH was achieved within a measurement range of 27–95%RH. The sensor also showed good repeatability, small size, measurement accuracy and wide humidity range. The RH sensitivity of the sensor has a significant dependence on the thickness of the coating and the sensor with the highest sensitivity showed a linear response for RH change in the range of 27-95% RH and a fast response time of 0.8 sec for an RH change from 50% to 90%.

Enhancement the Sensitivity of Humidity Sensor Based on an Agarose Coating Transmission-Type Photonic Crystal Fiber Interferometer

The present project is aimed at developing a capacitive humidity microsensor to measure humidity profiles inside the human respiratory tract. The ultimate goal of the project is to design the sensor with minimum size and with maximum sensitivity providing minimum response time. The sensor will allow determination of humidity variation in different generations of the human lung during a breath. The design process involves an appropriate simulation of the sensor to investigate the influence of several parameters on its sensitivity, and to develop a microheater that would be used to thermally reset the sensor. This sensor consists of interdigitated electrodes formed as a capacitor using aluminum as conductor and polyimide as dielectric built on a silicon substrate. For the thermal resetting, a polysilicon microheater will be built just underneath the sensor. The system should have quick absorption of moisture and ideally have a response time of less than 1 s. This specific requirement would allow for several measurements during one breath period. In order to meet these constraints a capacitive sensor was designed with a dielectric sensitive to humidity. Other parameters such as shapes of air contact surfaces, domain dimensions, properties of materials, etc. are analyzed and results are shown as well.

W. Moussa

Papers

Development of a humidity microsensor with thermal reset