Acoustic Waves Studies in Diamonds Could Make Room for Developing Novel Microsensors

Diamond-based microstructures have been mathematically modeled by physicists from the Technological Institute for Superhard and Novel Carbon Materials, the Siberian Federal University and the Moscow Institute of Physics and Technology, to produce compact, highly sensitive sensors.

Acoustic and ultrasonic sensor. Credit: MIPT

The research probes into the problem of identifying a useful acoustic signal considering the excitation of Lamb waves in potential microwave microresonators with substrates of synthetic diamonds. The researchers analyzed acoustic waves within the piezoelectric layered structure, explained their dispersion, suggested numerous ways of reducing the spurious peaks effects, and proposed a mathematical model. Diamond crystal based structures might be used in the future as high sensitivity sensors to sense temperature, pressure, acceleration, the thickness of ultrathin films etc. The results have been reported in Applied Physics Letters.

I think that the results we have obtained from a piezoelectric layered structure based on synthetic diamonds are ahead of world-class research in this field. Our microresonators were used to obtain resonances at record high microwave frequencies in a range of up to 20 GHz, with the quality factor remaining at several thousand. The behavior of diamond as a substrate for the acoustic microresonator was very significant and I hope that using diamonds in acoustics and electronics will lead to more exciting discoveries.

Boris Sorokin, Corresponding Author

A piezoelectric layered structure resembles a “sandwich” of a variety of materials having a piezoelectric effect. Under tension or compression, an electric field appears near the material and during application of electrical voltage, the material alters its shape. Non-scientists would have viewed piezoelectric effect in lighters. By pressing the button, the piezoelectric compresses, and gives sufficient voltage for a spark. Apart from lighters, this effect is utilized in precise micromanipulators, microphones, and different types of temperature, pressure, humidity sensors etc. Piezoelectrics is also found to have application in extremely stable piezoelectric resonators, which permit quartz clocks to present accurate time, or computers to run programs efficiently.

The influence of an electric field on a piezoelectric, like a thin film of aluminum nitride AlN, results in deformation. It forms elastic waves that are transferred to the substrate in a way similar to that of an elastic wave falling on the piezoelectric film causing an electric field. When the wave reaches the substrate’s edge, it is reflected and inside several materials layers, numerous oscillations take place simultaneously. This effect is similar to an echo that could be heard when a person shouts inside a tunnel or into a thick tube.

Diamonds and waves

Piezocrystals are found to be ideal as they have combined properties like a high electromechanical coupling coefficient, low acoustic absorption, and a high speed of sound. Diamonds meet all of these demands except for one - there is no piezoelectric effect. This is the reason why the devices required aluminum nitride film. Engineers are slightly concerned about the cost, however synthetic diamonds are becoming more affordable. The properties of these diamonds are superior to those noted in natural diamonds, especially with regard to their reproducibility and impurity profile. However, huge natural gem-quality diamonds are extremely costly. The researchers suggest that synthetic single crystal diamonds are ideal candidates for the development of new acoustoelectric devices.

Voluminous waves excited within the layered structure can resonate, thereby giving rise to both the basic mode of oscillations, and also the supplementary modes. Apart from the longitudinal-type oscillations, Lamb waves are also formed under certain specified conditions, in the substrate and piezoelectric film. The spectrum of these waves is in different branches and their phase velocity is dependent on the frequency.

Lamb waves are a unique integration of elastic oscillations existing in thin layers of elastic media. These waves were initially explained by the British physicist Horace Lamb. The particles present in these waves take up an elliptical path. Antisymmetric (bending) and symmetric Lamb waves are available. Phase velocity refers to the velocity at which a point shifts from a preset phase e.g. a wave’s crest; the phase velocity of waves in a specific medium often relies on their frequency and this effect is known as dispersion.

Here geometric dispersion of waves occurs in 2D acoustic waveguides. Although Lamb waves excitation is not useful on the basis of acoustic resonator quality factor in the longitudinal mode, these types of waves may be of special interest.

Scientists examined the spectrum of different acoustic modes taking place within the diamond structure in detail, using mathematical modeling to view the areas of acoustic displacement. The researchers gave particular importance to resonances that arise due to a whole spectrum of natural oscillation frequencies within the layered “sandwich”. This frequency correlates to the frequency at which an elastic system can oscillate without external influences. For instance, if an ordinary pendulum is touched and released, it would swing with a natural frequency. Application of force to this frequency is most efficient for its swing. Resonance occurs when the natural frequency and the excitation frequency match. Then the amplitude of oscillation rises sharply.

Natural frequencies rely on the properties of the materials and also on the structure’s geometry. This points out the possibility of developing detectors that are capable of detecting even individual bacteria fixed to the surface-the bacteria slowly increases the mass of the whole system and moves the resonant frequency.

The scientists were successful in selecting and recognizing various types of waves and forming their dispersion laws. These findings would be useful to develop microwave acoustoelectronic devices.

A combination of semiconductors, solid-state physics, and radioelectronics that deals with the study of the principles behind constructing devices to detect, transform, and process signals is a science referred to as acoustoelectronics. Acoustic resonators are commonly applied in science and technology in the form of sensing elements in different types of physical and chemical sensors and medical devices. Cavity resonators are famous due to their small size and large quality factor, even while resonating at high and ultra-high frequencies. The larger the operating frequencies, the lesser the cross-sectional dimensions of resonators are needed (~100 microns for a frequency of ~10 GHz).

MIPT’s Department of Physics and Chemistry of Nanostructures, based at the Technological Institute for Superhard and Novel Carbon Materials develop and analyze the acoustic properties of these sensitive elements. Researchers from various Russian organizations collaborated to develop a technique for creating a material that is harder than diamond at this institute. The secret of the unusual stiffness of polycrystalline diamonds was uncovered in this place and it was discovered that they are stiffer than individual crystals.

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