Humidity detection plays an important role in a variety of commercial and industrial applications, like process monitoring in the chemical industry, agricultural production, meteorology, etc.
Due to high sensitivity to the surface mass loading phenomenon, recently a growing number of surface acoustic waves techniques have been employed for various sensor applications. Surface acoustic wave humidity sensors have attracted much attention.
A Love wave is a kind of layered surface acoustic wave, different from the commonly used Rayleigh type humidity sensor, the properties of Love waves depend on the guiding layer.
As one kind of the guiding layers for Love wave sensors, the elastic layer such as SiO2 have the merits of low acoustic loss and excellent abrasion resistance. However, a sensor coated a SiO2 layer cannot achieve a very high sensitivity because of its fast transverse wave.
Polymers have slower transverse waves and lower density, so they are often used as guiding layer for Love wave sensors to achieve a higher sensitivity. Unfortunately, the viscosity of polymers can produce a great propagation loss.
To get a balanced good performance, researchers prefer the sensors with two layers to combine the advantages of both elastic and polymeric layers. Researchers from Institute of Acoustics (IOA) of the Chinese Academy of Sciences have developed a Love wave humidity sensor based on a device consisting of PVA(polyvinyl alcohol)/SiO2 layers on ST-90°X quartz substrate.
The experimental delay line consisted of a quartz substrate and two 200 nm thick Al interdigital transducers. The uncoated device supported an shear horizontal wave with the frequency of ~ 178.6 MHz and the insertion loss of 26 dB.
Besides, SiO2 thin films were deposited by using Plasma Enhanced Chemical Vapor Deposition technique. The radio frequency glow discharge excited the reaction gas SiH4 and N2O into the plasma depositing a SiO2 layer on the substrate surface. And 30 g of PVA powder and 600 ml of deionized water were put into a clean glass baker.
The mixture was heated and stirred by using a thermo magnetic stirrer, until the PVA powder was completely dissolved. About 5 ml diluted PVA solution was spin coated on the quartz wafer surface for 50 s at a speed of 3000 rpm.
The Love wave device was placed in the plastic container, which had four holes for the connection with the network analyzer, the probe of the standard hygrometer, and wet sponge or silicone particles to change the humidity.
Silica particles and wet sponges were used to adjust the humidity in the test box. When RH=20%, the device coated only a PVA film worked at the frequency of 177.57MHz and the insertion loss of 8.79dB. In the mean time, the device coated PVA/SiO2 layers worked at the frequency of 169.95MHz and the insertion loss of 8.59dB.
Fig. 1 and Fig. 2 showed the frequency and insertion loss shifts of the sensors incorporating a PVA layer versus PVA/SiO2 layers, respectively. As shown in the figures, the same change in humidity caused an obvious larger frequency shift for the Love wave sensor incorporating PVA/SiO2 layers than for the sensor incorporating only a PVA layer. The insertion loss caused by increasing humidity was of a little difference.
These results were consistent with the theoretical analysis. The result indicated that a Love wave sensor coated double layers had a more valuable application prospects than a sensor coated single layer.
Funding for the research came from the National Natural Science Foundation of China (No. 11104314).
Fig.1 Frequency shifts versus relative humidity for ST-90°X quartz substrate/PVA and ST-90°X quartz substrate/SiO2/PVA devices (Image by IOA).
Fig. 2 Insertion loss shifts versus relative humidity for ST-90°X quartz substrate/PVA and ST-90°X quartz substrate/SiO2/ PVA devices (Image by IOA).
References:
WANG Lijun, LIU Jiansheng, HE Shitang. The Development of Love Wave-Based Humidity Sensors Incorporating Multiple Layers. Sensors (Vol. 15, No.4: 8615-8623, Apr 14,2015). Doi:10.3390/s150408615
Contact:
LIU Jiansheng
Institute of Acoustics, Chinese Academy of Sciences, 100190 Beijing, China
Email: liujs98@hotmail.com or liujiansheng@mail.ioa.ac.cn