Molecular electronic transducers

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Molecular electronic transducers (MET) are a class of inertial sensors (which include accelerometers, gyroscopes, tilt meters, seismometers, and related devices) based on an electrochemical mechanism. METs capture the physical and chemical phenomena that occur at the surface of electrodes in electrochemical cells as the result of hydrodynamic motion. They are a specialized kind of electrolytic cell designed so that motion of the MET, which causes movement (convection) in the liquid electrolyte, can be converted to an electronic signal proportional to acceleration or velocity. MET sensors [1] have inherently low noise and high amplification of signal (on the order of 106).

Electrolytic cell

An electrolytic cell is an electrochemical cell that drives a non-spontaneous redox reaction through the application of electrical energy. They are often used to decompose chemical compounds, in a process called electrolysis—the Greek word lysis means to break up.

Contents

History of molecular electronic transducers

MET technology had its origins in the 1950s, [2] [3] [4] [5] when it was discovered that very sensitive, low-power, low-noise detectors and control devices could be made based on specially designed electrochemical cells (which were referred to as “solions”, derived from the words solution and ions). Up through the 1970s, the US Navy and others supported development of solion devices for sensitive sonar and seismic applications, and a number of patents were filed. [6] However, early solion devices had a number of serious problems such as lack of reproducibility and poor linearity, and practical production of devices was abandoned in the US and progress languished for decades.

However, fundamental physics and mathematical studies of the underlying electrochemical and fluid flow dynamical processes continued, principally in Russia, where the field came to be known as “molecular electronics”. [7] In recent years both mathematical modeling and fabrication capabilities improved dramatically, and a number of high-performance MET devices have been developed. [8]

Principles of operation

At the heart of a MET device are two (or more) inert electrodes at which a reversible redox reaction occurs, which does not involve either plating of a metal or evolution of a gas. Typically, the aqueous iodide-triiodide couple is used:

3 I → I3 + 2 e anode reaction

I3 + 2 e → 3 I cathode reaction

When a voltage in the range of ~ 0.2 to 0.9V is applied across the electrodes, these two reactions occur in a continuous fashion. After a short time, the electrochemical reactions deplete the concentration of triiodide ions [I3] at the cathode and enrich it at the anode, creating a concentration gradient of [I3] between the electrodes. When the cell is motionless, the electrochemical reaction is limited by the diffusion of I3 to the cathode (a slow process), and the current dies down to a low steady-state value.

Motion of the device causes convection (stirring) in the electrolyte. This brings more I3 to the cathode, which in turn causes an increase in the cell current proportional to the motion. This effect is very sensitive, with extremely small motions causing measurable (and low noise) inertial signals.

In practice, the design of the electrodes to create a device with good performance (high linearity, wide dynamic range, low distortion, small settling time) is a complex hydrodynamic problem.

Advantages of MET sensors

The main advantage of MET sensors over competing inertial technologies is their combination of size, performance and cost. MET sensors have performance comparable to fiber optic gyroscopes (FOGs) and ring laser gyros (RLGs) at a size close to that of MEMS sensors, and at potentially low cost (in the tens to hundreds of dollars range, in production). In addition, the fact that they have a liquid inertial mass with no moving parts makes them rugged and shock tolerant (basic survivability has been demonstrated to >20 kG); they are also inherently radiation hard.

Microelectromechanical systems technology of very small devices

Microelectromechanical systems is the technology of microscopic devices, particularly those with moving parts. It merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines in Japan, or micro systems technology (MST) in Europe.

Applications

Depending on the configuration of the MET device, a variety of inertial sensors can be produced including:

An accelerometer is a device that measures proper acceleration. Proper acceleration, being the acceleration of a body in its own instantaneous rest frame, is not the same as coordinate acceleration, being the acceleration in a fixed coordinate system. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards of g ≈ 9.81 m/s2. By contrast, accelerometers in free fall will measure zero.

Seismometer instrument that records seismic waves (seismograms) by measuring ground motions, caused by earthquakes, volcanic eruptions, and explosions

A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph. The output of such a device — formerly recorded on paper or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, and to study the earth's internal structure.

Gyroscope device for measuring or maintaining orientation and direction

A gyroscope is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rotation is free to assume any orientation by itself. When rotating, the orientation of this axis is unaffected by tilting or rotation of the mounting, according to the conservation of angular momentum.

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A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

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Electrolysis technique that uses a direct electric current to drive an otherwise non-spontaneous chemical reaction

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A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.

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Solid oxide fuel cell fuel cell that has a ceramic electrolyte

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Voltameter

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The alkali-metal thermal-to-electric converter (AMTEC), originally called the sodium heat engine (SHE) was invented by Joseph T. Kummer and Neill Weber at Ford in 1966, and is described in US Patents 3404036, 3458356, 3535163 and 4049877. It is a thermally regenerative electrochemical device for the direct conversion of heat to electrical energy. It is characterized by high potential efficiencies and no moving parts except the working fluid, which make it a candidate for space power applications.

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References

  1. http://www.mettechnology.com
  2. R. M. Hurd and R. N. Lane, “Principles of Very Low Power Electrochemical Control Devices”, J. Electrochem. Soc. vol.104, p. 727 – 730 (1957).
  3. I. Fusca, “Navy wants industry to share burden of solion development”, Aviation Week, vol.66, #26, p.37, 1957.
  4. A. F. Wittenborn, “Analysis of a Logarithmic Solion Acoustic Pressure Detector”, J. Acoust. Soc Amer. vol.31, p. 474 (1959).
  5. C. W. Larkam, “Theoretical Analysis of the Solion Polarized Cathode Acoustic Linear Transducer”, J. Acoust. Soc. Amer. vol.37, p. 664-78 (1965).
  6. See for example US Patents 3,157,832; 3,223, 639; 3,295,028; 3,374,403; 3,377,520; 3,377,521; and 3,457,466
  7. N. S. Lidorenko et al., Introduction to Molecular Electronics [in Russian], Énergoatomizdat, Moscow (1985).
  8. see www.mettechnology.com