Unlike traditional seismic sensors, molecular-electronic ones made for geological exploration by IGEO use a liquid inertial mass. A sensitive element inside consists of a system of electrodes attached to the body. These electrodes convert an alternating flow of a working liquid caused by the inertia force into a variation of an electric current between electrodes.
Meanwhile, a high ratio of mechanical signal conversion into an electric current and a wide frequency and dynamic ranges are achieved. These properties, in particular, provide a possibility to make seismic sensors of a cutoff frequency at low frequencies up to thousandths of a hertz, high sensitivity and low noise level.
The use of deep feedback provides characteristics identical to the level of 0.2%.
No moving components of precision mechanics and a relative simplicity of the structure provide a high reliability to such devices and resistance to adverse external influences inevitable while transporting and operating in the field.
Principles of molecular electronic motion sensors are based on the mechanism of convection-diffusion charge transfer between the transducer electrodes under the conditions of forced convection which occurs with an external mechanical action. The most important difference of molecular-electronic sensors from other typesof inertial meters for motion parameters is a liquid working as an inertial mass which is an electrolyte solution flowing through the transducer due to an external mechanical action.
The basic element of these devices is a molecular-electronic transducer shown as a sketch at Fig. 1. Inside of a tube 1, made of dielectric and chemically resistant material and filled with an electrolyte solution 2, are two pairs of perforated electrodes 3 and 4 enabling free flow of liquid through the electrode assembly in case of electrolyte motion in a transducer tube.
In an operating mode the constant potential difference is applied to each pair of electrodes, in the absence of an external signal it is providing background current flow between the anode and cathode of each electrode pair; this current occurs due to reversible oxidation-reduction reactions in the anode and cathode.
In the case of the work liquid motion, in the transducer tube occur a convective component of the current between the electrodes of the transmitter and the corresponding response in the form of an electrical signal of the electrodes. Standard gain for an external mechanical signal into an electric current has an extremely high value ensuring the highest sensitivity of the sensor.
Fig. 1. Molecular-electronic transducer: 1 — dielectric tube (transducer body); 2, 4 — installation elements; 3 — electrolyte, 5, 6 — electrodes.
To use a transducer as a linear motion sensor, the tube caps 1 are closed with elastic membranes (Fig. 2). Then with an external acceleration, the electrolyte in a transducer channel is set in motion with holding or assigning charge carriers to the electrodes and is making the current change in an external circuit.
Fig. 2. Sensor of linear motion.
The basic element of the developed devices is a molecular-electronic cell which converts a mechanical motion of work liquid into an electric signal. A transforming cell consists of multiple electrodes placed in a liquid electrolyte. In most applications, the electrolyte is a highly concentrated solution of potassium iodide with a relatively small addition of an active component – molecular iodine.
The iodine solution is reacted with iodide ions to form triiodide ions. The constant of the chemical reaction is very high and finally in a solution iodine presents in two ion forms: iodide (background electrolyte) and tri-iodide (active component).
If the electrode potential difference is applied, a current flows in a cell. In this case, the cathode tri-iodide is converted into iodide with two electrons coming from the metal electrode. The anode gets a backlash.
If the applied potential difference is large enough, all of iodide ions entering the cathode react immediately and saturation occurs when a further increase in the applied voltage does not lead to an increased current in the system. In this mode the current value is limited by the rate of delivery of the active component to the cathode. In a fixed electrolyte the delivery is made by diffusion (migration component does not contribute to transport an active component because the electrical fields in the system are screened by a background electrolyte of a very high concentration). In the presence of liquid motion a convective transportation is added to the diffusion and saturation current increases or decreases depending on the direction of liquid flow.
It is noteworthy that dissolution or precipitation of the agent does not occur on electrodes in this system and that allows the system to maintain its properties unchanged at a basic level over many years of operation.
Initially while developing a molecular-electronic technology, a four-electrode cell consisting of two symmetrical pairs of anode-cathode was used as a sensing element. The difference in cathode currents is used as the output signal of motion sensors. The symmetry of the structure is crucial for the dynamic range of the sensor because all even harmonics arising due to nonlinear effects in each transforming each pair disappear with subtracting the currents in the output signal.
In addition to metal electrodes, each pair included a dielectric spacer having a geometry, as well the geometry of the electrodes, optimized for a wide frequency range, high standard gain, minimal non-linear effects and noise level. Numerical methods are widely used for optimization. In particular, depending on the geometry of the cell, the output current may be proportional to the liquid volume flow or liquid volume flowing through the transforming cell. The configuration of the first type is commonly used in accelerometers, the second type is used in seismometers and the angular velocity sensors based on this technology.
Finally the transforming cell is placed in a certain mechanical system that provides liquid flow through the cell under the influence of external measured signals while eliminating the transducer sensitivity to other influences.
Despite these relatively high output parameters, the sensors which had been developed and produced until now had a number of shortcomings that are ultimately limiting the scope of their application. The basic ones are the following:
1. high production cost of transforming elements;
2. fairly wide range of options for transforming elements handcrafted by men – that requires settings of associated electronics for each individual sensor which also increases the cost of the sensor;
3. an early decline in the sensitivity of the sensing element at high frequencies
These difficulties have been overcome in the up-to-date technical environment by using a planar technology to make transducer elements with characteristic dimensions of tens to hundreds of nanometers.
When using a planar technology for making sensors, a key parameter is the distance between the cathode and the anode in each pair of electrodes, and it is the distance that has a significant influence on characteristics of the transducer whereas the parameters such as a sectional shape, an electrode height, a width of deposited electrodes, a distance between the cathodes do not influence significantly on the shape of the characteristics.
A new type of an electrode pack consists of two plates, one of which has electrode strips (two pairs of the anode-cathode in each group of four electrodes) and the other has bars which are used as spacers; the both plates have holes.
Precision photolithography, electron-beam evaporation of nanotapes of refractory metals, chemical etching techniques and other advanced micro- and nanoelectronic technologies were used to produce samples. These methods have never been used before to produce devices based on molecular- electronic transport in solid-liquid micro- and nanostructures.
A distinctive feature of this technology and molecular-electronic sensors based on it are high gain of the transformation of mechanical motion into an electrical signal, wide dynamic and frequency ranges which have never been achieved previously using traditional electromechanical devices. The absence of precision mechanics elements as well as moving mechanical details on the principle level ensures low costs, high reliability, ease of use and resistance to mechanical shock and vibration which are all inherent in the transportation and deployment of sensors in the field. A unique feature of the molecular electronic sensors is the ability to measure both linear motions and signals of rotational nature. With using a planar technology, the output characteristics of devices will be qualitatively improved and it will allow using the sensors for many different applications.
Electrochemistry Journal, 2012. ''Technological Principles of Motion Parameter Transducers Based on Mass and Charge Transport in Electrochemical Microsystems''