Study Of Humidity Sensor Based On Surface Acoustic Wave And Micro/Nano Technology

Humidity must be strictly monitored and controlled in many fields, such as industry, medical treatment and environment monitor. Humidity detection is playing quite an important role in storage management, cereal stocking, greenhouse conditioning, electronic devices, transportation, meteorology and military affairs. However, among all the environmental parameters, humidity is still hard to be accurately measured until now. So there's great meaning to research and develop new humidity sensors with more excellent performance, such as high sensitivity and quick response time.Surface acoustic wave (SAW) device was extremely sensitive to the perturbation on its surface, so it has been widely used as sensor components. Its great advantages such as high sensitivity, excellent stability, small size, low cost and ease-to manufacturer made it getting more and more attention. For SAW sensors, their response to the perturbation was caused by multiple factors and mechanisms, which is the basic reason that SAW sensors have so large a sensitivity. The Perturbation Model for the SAW sensors and its mechanisms such as mass loading effect and acoustoelectric effect were deeply analyzed, based on which SAW sensing component and humidity sensitivity material were designed. MEMS photoetching technique was used to fabricate a single port SAW resonator (SAWR) as the final transducer.Humidity sensitive material is a key factor that could determine the humidity sensors'performance. Only when the characteristic of the material was coordinated with the sensing mechanism of the SAW sensors, the humidity sensor could achieve the best performance. The characteristic including adsorption efficiency, conductivity variance and viscoelasticity must be taken into consideration. Polyelectrolyte material, nano material and composite material is the main developing direction and research focus. Designing method of the humidity sensitive material used in SAW humidity sensor was carefully discussed. Nafion/MWCNTs composite material and novel graphene oxide were designed as the alternative material.Coating method of the humidity sensitive film is another critical technique for humidity sensor design. For the SAW device with high frequency, there's only a little area could be coated with the sensing film. Furthermore, there are many restrict requirements for the film, such as reasonable thickness and well-proportioned topography. In another aspect, the sensing film with nano-structure could greatly improve the sensor's performance. Electrospinning is an emerging technique for nano material design. The sensing film made by this method has the advantages of high specific surface, large poriness and uniform morphology. The coating methods based on electrospinning technique were introduced into the design process of the SAW humidity sensor. Nafion/MWCNTs composite nano-fiber films were prepared by the new techniques.SAW sensors are usually working in the RF band. Thus its signal was prone to be interfered with the electromagnetic radiation and other environmental factors. So there are great difficulties for designing SAW sensor detection system. A high frequency resonator circuit and assorted detection system with stable performance were designed. At the same time, a novel RF-triggered semi-passive SAW humidity sensor tag was researched. Furthermore, a new serial topology was studied to propose a new idea for SAW sensor design, but not limited to humidity sensors.Experiment was set up to examine the key indicators for humidity sensors, including sensitivity, stability, repeatability, response-time and temperature coefficient. The results showed that the two types of humidity sensitivity material designed in this paper are all effective in humidity detection. Performance of the SAW humidity sensor has made a great breakthrough compared the commercial devices. The positive effect of electrospinning coating method was also tested. It was really helpful to improve the sensor's sensitivity and dynamic characteristic. The SAW humidity sensor with novel serial topology was also tested in the experiment. The result proved that it was a promising method for high frequency SAW sensor design.Low humidity (<10%RH) or dewpoint detection is always a difficult point in humidity detection. Common resistor type or capacity type humidity sensors are both difficult to meet the accuracy requirements. So there's great meaning to develop a low humidity sensor or dewpoint sensor with low cost, small size and ease-to-use. The present SAW humidity sensor was tried to be used for low humidity and dewpoint detection. The excellent result provided a good prospect for its future.