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2	Fundamentals of Acoustic Waves, Materials, and Sensors
3	"Acoustic Wave Sensors consist of" Acoustic Wave Sensors consist of: - a piezoelectric device (usually in a resonator or delay line configuration) - a (partially) selective, sensitive and stable coating (optional and depends on application) - a means to collect, quantify and transmit the detected signal • Sensing is the result of mechanical and/or electrical perturbations at the (coating) surface, which affect the characteristics of the propagating (or resonating) wave such as: -Velocity/frequency of operation -Attenuation (energy loss) -Particle displacement profile
4	Piezoelectricity and Acoustic Fields Piezoelectricity-- application of mechanical energy changes the electrical polarization producing an electric potential Ds, DY ßà DV stress, strain Piezoelectric effect: mechanical strain (displacement) produces an electric potential (or field) Inverse piezoelectric effect: electrical potential (or field) produces a mechanical strain. The coupling between electrical and mechanical fields forms the basis for the acoustic fields utilized by acoustic wave sensors
5	General Acoustic Waves Equations Linear, Piezoelectric Constitutive Relations Cijkl=elastic stiffness tensor for a constant electric field, enij=piezoelectric constant tensor, εin=permittivity constant tensor for constant mechanical strain, Skl=strain tensor (dimensionless) , and En=electric field in the x_n direction. Di=electric displacement component in the x_i direction. Tij=acoustic stress tensor Strain and Electric field are related to mechanical displacement and electric potential as Ukºacoustic displacement in the xk direction
6	"Equations of motion in a..." Equations of motion in a piezoelectric medium Combining the above equations yields the basic differential equations governing acoustic wave propagation in a piezoelectric medium For a given set of boundary conditions, the above equations can be solved for the four variables , and , as well as the wave velocity and attenuation, for a given piezoelectric crystal material and orientation.
7	Perturbations in acoustic wave propagation In acoustic wave sensor applications, changes of wave velocity, v, and/or attenuation, a, are detected as a result of the device interactions with the environments. Device interactions with environments result in perturbations of waves - mechanical loading & electrical loading. Perturbations of acoustic wave, caused by changes in boundary conditions, results in changes in wave properties Changes in wave velocity and attenuation can be calculated from the changes in wave energy density and power dissipation, respectively
8	Creating an Acoustic Wave Sensor The continuous propagation of a wave in the x direction is described generally as Where g is a complex propagation factor representing both attenuation and wavenumber: If frequency is constant, then changes in wave propagation due to perturbations can thus be represented by (k0=unperturbed wavenumber) or, in normalized forms, as
9	In general, the response of a sensor can be written as Dm = change in mass accumulation DC = change in viscoelastic constant Mechanical Interaction De = change in dielectric constant Ds = change in conductivity Acoustoelectric Interaction DT = change in temperature DP = change in pressure v = wave velocity f = frequency a =attenuation
10	Principles of Acoustic Wave Sensor Operation Example: On-line Detection of Proteins
11	Various Types of Applications Gas Sensors CO, CO₂, H₂S, O₂, H₂, SO₂, NO, NO₂,…. Volatile organic, organophosphorus and organosulfur, anesthetic gases, CWA,… Electronic Nose Gas mixtures,odours,… Liquid-Phase Sensors Viscosity meter, chemical probe (concentration of given solution in solvents), organic compounds, BTEX, MTBE, CWA, conductivity meter (for dilute electrolyte solution), detector of specific metal ions in solution,… Biosensor/Immunosensor Bacteria, Fungi, viruses, glucose, lactose, hydrocarbons, pesticides, BWA…
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