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BULK AND SURFACE ACOUSTIC WAVE SENSORS

The most common sensor platform (платформа датчика) based on piezoelectric devices (пьезоэлектрические устрой-ства). Figure 1 shows the basic structure of the acousto-electronic gas-sensing platform. Bulk and surface acoustic wave resonators have been used extensively in the design of multifunctional physical and chemical sensors, including

microbalances, viscosity sensors, humidity sensors, immuno-detectors, gas sensors, ion-intercalation monitors, and magnetic- and electric-field sensors.

Because frequency may be measured with higher accuracy than any other parameter, mechanical resonators are well suited for the design of high-sensitivity sensors. Often, thin-film coatings sensitive to the measured parameter are deposited on the resonator because changes in the physical or chemical parameters of the film increase the resonance frequency shift. Resonator-based sensors measure resonance-frequency shifts caused by mechanical, chemical, or electrical perturbations of boundary conditions on the interface.

Mechanical boundary conditions provide continuity of particle velocity and stress on the interface boundary. A resonator without any load film in vacuum behaves like an unbounded, stress-free plate. If the resonator is loaded with a thin film, the boundary conditions will be modified. A dielectric film modifies mechanical boundary conditions, while a conducting film modifies both mechanical and electrical boundary conditions. A resonator coated with a film no longer has stress-free boundaries. Mechanical and electrical perturbations cause resonant-frequency shifts. With the assumptions of no internal electrical sources in quasistatic approximation where and , where F is the electric potential, and h_o is the electrical charge density, lossless boundaries and only external perturbations the resonant frequency shift is given by:

where U is the acoustic energy stored in the resonator mode, T is the stress tensor, and * indicates a complex conjugate. The boundary conditions at the interface are:

and

and

where H is the magnetic field related to the electric field through Maxwell equations.

The regime of oscillation of a piezoelectric resonator can be modified by mechanical or electrical perturbations originating from the surrounding medium. Electrical perturbations can occur in metal films with different conductivity values deposited on the resonator or if the resonator is immersed in ion-conducting electrolyte. The influence of mechanical, chemical, and electrical perturbations in solid and fluid media on the sensor performance depends on the interface between the quartz resonator and the surrounding medium. Various loading effects in liquid and solid media damp the oscillations of the resonator and modify the sensor resolution. The resolution of the sensor is determined by the resonance frequency shift response to external perturbations and the capability of the monitoring electronics to measure accurately this frequency shift.

V. Say if the following statements are true or false.

VI. Choose the best answer: a, b or c to complete the sentences.

VII. Divide the text into logical parts and find the topical sentences in each part.

VIII. Give a short summary of the text.

Part C

I. Read the following text and entitle it.

Another family of multifunctional sensor systems is electrochemical gas sensors (Figure 1), which use solid-state galvanic cells to monitor partial pressures of gases such as CO₂, NO_x, SO_x, and hydrocarbons in gas mixtures. The sensor operates as a battery: the electromotive force (EMF) changes a function of chemical modifications of the cathode in the presence of the gas according to the following arrangement:

Pt electrode, gas | gas sensitive film ion conductor reference electrode | Pt electrode.

For example, to measure the EMF and the NO₂ partial pressure, a galvanic cell of the following type can be used:

Pt | Na | Nasicon | NaNO₃ | Pt, NO₂, O₂

The reaction in the cell is

Na + NO₂ + 0.5 O₂ = NaNO₃

The relationship between the EMF and the NO₂ partial pressure is given by:

EMF ≈ -DG[NaNO₃] + DG[NO₂] + kTIn P[NO₂] + (kT/2)InP[O₂]

Where G is the Gibbs energy, P is the partial pressure, k is the Boltzmann's constant, and T is absolute temperature.

An electrochemical gas sensor for measuring partial pressure of CO₂ is fabricated using thin-film technology and silicon micromachining. The sensor operates at 350°C and the whole structure is built on top of micromachined silicon with platinum thin film heater. Thin Nasicon film is the ion-conducting separator. The sensing electrode is a mixture of barium and sodium carbonate deposited through reactive magnetron sputtering. The reference electrode is the isolation film of silicon nitride covering the platinum heating elements. The micromachined silicon substrate allows the heater to operate at 350°C while the surrounding aria is at room temperature and the same silicon substrate could be used as a base of the controlling electronics.

The fabrication technology of the sensor allows integration of various gas sensors on the same substrate. Nasicon is used for all sensors as an ion-conducting separator of the battery. Only the sensing electrodes are different. For example, to detect NO_x a sensing electrode of sodium nitride can be used.

II. Answer the following questions on the contents of the text.

	What are solid-state galvanic cells used for?
	What device operates as a battery?
	At what temperature does an electrochemical gas sensor for measuring partial pressure of CO operate?
	What is built on top of micromachined silicon with platinum thin film heater?
	What is thin Nasison film?

III. Read the text and say what makes it possible to integrate various gas sensors on the same substrate.

IV. List electrochemical sensors, their characteristics and use.

V. Give the main points of the text in 5-6 sentences.

UNIT 7

Part A

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