Study In Theory And Applications Of Piezoelectric Langasite Crystal Sensor

Studies with the microbalance sensor of high sensitivity became popular after the original work of Sauerbrey in 1959. An increase in mass bound to the quartz surface causes the crystal's oscillation frequency to decrease linearly. Near half a century has passed since Sauerbrey's work, piezoelectric quartz crystal is the predominating piezoelectric material absolutely in microbalance sensors due to its excellent physical and chemical properties. Hence, quartz crystal microbalance (QCM) is in fact the standard term for microbalance mass sensor. Recently, the rapid development in material science offers the opportunity to study the microbalance sensor beyond of quartz crystal. Langasite (La₃Ga₅SiO₁₄) single crystal is an excellent material better than quartz crystal. In this dissertation, piezoelectric sensors made by a Y-cut langasite crystal are reported and their responses are investigated. The main results were listed below.1, The frequency responses of resonators made from Y-cut langasite crystal in a liquid phase were investigated. It was shown that the oscillating frequency of the langasite resonator decreased with increasing the surface mass loading, viscosity(η)and density(p)of the liquid phase. The correlation of the frequency shift and the mass loading was similar to the Sauerbrey equation. A linear dependence of the frequency shift on the surface mass loading was obtained within 3% of the intrinsic frequency of the langasite resonator. The frequency-mass coefficient of the langasite crystal microbalance (LCM) was 71.1% of that of the classical quartz crystal microbalance (QCM). The frequency of the LCM decreased linearly with increasing (ρη)^1/2. The LCM had a low frequency -temperature coefficient in water in the temperature range of 30-50℃. The LCM possessed excellent oscillating ability in the oscillator method, especially in high viscosity liquids. The LCM was applied to monitor the coagulation process of whole blood.2 A new series piezoelectric langasite crystal (SPLC) was developed in which a langasite resonator oscillating at 9MHz and a pair of parallel electrodes were utilized. By analysing the equivalent electrocircuit model of the sensor, the resonance frequency shifting from the change of conductivity was discussed. The sensor system possesses two advantages: One advantage over more conventional oscillator circuits is the high frequency stability, and narrow bandwidth obtainable with the oscillators. Another is that the interferences caused by mass or viscoelastic properties can be avoided. Only the alternation of conductance and permittivity in a test solution can produce the frequency shift of the sensor system. We also use this method to monitor the adsorption process of one room temperature ionic liquid (RTIL)[C₈mim][PF₆] to vapors of acetone, ethanol, cyclonexane, ethyl acetate, tetrahydrofuran and tetrachloride carbon., a thin RTIL film was spread onto the surface of the conductance electrode. The adsorption of organic solvent vapors to RTIL film causes a significant respond in conductivity and permittivity of the RTIL film, which can be monitored by the resonant frequency and motional resistance of the SPLC in an impedance analysis method. On the basis of the responses of the sensor, we got the adsorbed amount of six organic vapors to RTIL film.3 Room temperature ionic liquids (RTILs) have shown potential as unique solvents with wide range of solubility, miscibility, and other physicochemical properties accompanied by an extremely promising non-volatile behavior. One of the advantages arising from the chemical structures of RTILs is that alteration of the cation or anion can cause changes in properties such as viscosity, melting point, water miscibility, and density. It is not surprising that RTILs have shown tremendous applications in a variety of chemical processes. A combination series piezoelectric device employing RTILs ([C₈mim] [BF₄] and [C₈mim] [Br]) as the sensing materials for organic vapors has been developed and evaluated. The sensing mechanism is based on the fact that the conductivity and permittivity of the ionic liquid membrane changes rapidly due to solubilization of analytes in the ionic liquids. This change in conductivity and permittivity, which varies with the chemical species of the vapors and the types of ionic liquids, results in a frequency shift of the corresponding langasite crystal. The sensor demonstrated a rapid response to organic vapors with an excellent reversibility because of the fast diffusion of analytes in ionic liquids. Furthermore, the ionic liquids, with zero vapor pressure and stable chemical properties, ensure a long-term shelf life for the sensor.4 We found an interesting phenomena of langasite crystal sensor. When the property of liquid changed, except of oscillating at the fundamental resonant frequency, an unexpected strong resonant peak was observed in the much higher frequency region. We called it as a big wave. The frequency of the big wave is slso sensitive to the mass change. But the change in solution conductivity results in a much larger frequency shift of the big wave. The experimental phenomena of the big wave was investigated .When we evaluate the origin and discipline of big wave specificly, we will use the new response mode of the sensor to new application in analysis and afford more abundant information of interface chemistry.