RU173873U1 - Device for measuring the specific charge of micron-sized particles - Google Patents

Abstract

Usage: for measuring the charge of micron-sized particles. The essence of the utility model is that a device for measuring the specific charge of micron-sized particles includes a linear quadrupole electrodynamic trap, rod electrodes that are supplied with an alternating electric potential located vertically, an end electrode with a constant potential is located in the lower part of the trap, the lower end electrode is made in the form of a sphere, which is mounted on a thin metal holder, the radius of which is much smaller than the radius of the sphere. Effect: providing the possibility of increasing the accuracy of measuring the charge of particles. 1 s.p. f-ly, 2 ill.

Description

The utility model relates to the field of measurement technology, specifically to devices for measuring the charge of micron-sized particles and can be used both in laboratory research and in various industries, such as microelectronics, mechanical engineering, energy and chemical production, for the diagnosis and control of dispersed solid phases.

Known devices for measuring particle charges based on electrodynamic traps of cylindrical and linear types, often called Paul traps, in which charged particles are captured and oscillated in a combination of alternating and constant electric fields. A known method of measuring the specific charge of particles in air at atmospheric pressure in a cylindrical electrodynamic trap Paul, based on the registration of the displacement of the position of the oscillating particles caused by exposure to a particle of a constant electric field on the lower electrode [N. Winter and H.W. Ortjohann, Simple demonstration of storing macroscopic particles in a "Paul trap", Am. J. Phys., 1991, v. 59, p. 807]. The trap consists of a central grounded electrode, made in the form of a ring of wire with a diameter of 3 mm, and two end ball electrodes located above and below the wire ring along the Z axis, which coincides with the direction of gravity. An alternating voltage is applied to the ball electrodes. In this case, the particle oscillates along the Z axis from the upper ball electrode to the lower one. Applying an additional constant potential to the upper ball electrode shifts the position of the point relative to which the particle oscillates along the vertical Z axis. Registering the average vertical displacement of the oscillating particle makes it possible to determine its charge.

The disadvantage of this solution is that in order to carry out sufficiently accurate measurements in the presence of an oscillatory process superimposed on a constant bias, the observation time should be much longer than the oscillation period, and, in the case when it is necessary to determine the particle charge from one or several snapshots, this method gives a big error. For optical registration of particle displacement, the authors are forced to use a middle electrode in the form of a ring of wire, as a result of which the electric fields become very inhomogeneous. The measured particle can move along elliptical trajectories at small distances between the spherical electrodes, since the fields inside the trap are very inhomogeneous at the ends of both spherical and ring electrodes, which additionally leads to a deterioration in the measurement accuracy. In general, the accuracy of determining the specific charge when the particle performs oscillations is low. The measurement error also increases due to the fact that for micron particles the air resistance force differs from the Stokes law and depends on the size and shape of the particles, which is not always known.

Closest to the claimed utility model is a device based on a linear quadrupole electrodynamic trap. A five-electrode linear quadrupole electrodynamic trap is known with an additional fifth electrode at the end of the trap, adopted as a prototype, in which a method for measuring the specific charge of droplets of a few hundred microns in air at atmospheric pressure based on recording the position shift of an oscillating particle along linear electrodes can be implemented when exposed to a constant electric field [D. Hu and B. Makin, Study of a five-electrode quadrupole levitation System, IEE Proceedings-A, 1991, V. 138, No. 6, p. 320]. The trap consists of four linear coaxial electrodes arranged vertically. An alternating voltage is applied to the linear electrodes, with the help of which an electric field is created, forming an electrodynamic trap for holding charged particles in a horizontal plane. The distance between the linear electrodes is much less than their length, which creates an alternating electric field inside the trap that does not have a longitudinal component. In this case, the charged particle will oscillate only in a horizontal plane perpendicular to the linear electrodes. In the lower part of the trap, at the intersection of the diagonals from the opposite linear electrodes, a fifth electrode with a hemispherical end face, at which a constant potential is applied, is installed. By changing the value of the constant voltage at the hemispherical electrode, it is possible to compensate for the weight and fix the specified distance of the particle from the electrode. The measurement of the position of the particles in the trap, according to which the value of the specific charge is calculated, is carried out by the optical method.

The disadvantage of this method is the significant error in determining the specific charge, which reached 30% compared with the calculated one, which can be explained by the influence of edge effects and structural elements on the measurement results. In this case, the particle may be in the zone of action of the distorted electric field, which leads to a discrepancy between experimental and theoretical results.

The technical result of the utility model is to increase the accuracy of measuring the charge of micron-sized particles by reducing the influence of the edge effects of distortion of the electric field at the ends of the linear electrodynamic trap and eliminating the influence of the dielectric base on the results of fixing the position of charged particles in the trap.

To achieve the technical result, the end electrode is made in the form of a sphere, which is mounted on a thin metal holder, to increase the distance between the spherical electrode and the lower end of the trap, and thereby reduce the influence of end effects. A thin holder is chosen so that it has a slight effect on the distribution of the electric field of the sphere during measurements, as well as to reduce the mutual capacitance between the holder and linear electrodes.

The technical result is achieved by the fact that the specified spherical electrode is installed at a distance of at least four radii of its sphere from the lower end of the linear trap. Removing a spherical electrode at such a distance reduces the influence of the electric fields of the image and the polarization of the dielectric elements that are structurally present in the end of the trap.

Thus, the use of an end spherical electrode mounted at a distance of at least four radii from the end wall of the trap allows one to increase the accuracy of determining the specific charge of particles.

A diagram of a device that implements the proposed method is presented in FIG. 1. The device includes four identical metal electrodes of circular cross section 1 (side electrodes) mounted vertically on a dielectric base 2 at the corners of a square, the side of which is much smaller than the length of the electrodes, but much larger than their diameter. Since the electric field inside the trap is very inhomogeneous at the ends of the electrodes, to compensate for this effect, the length of the trap should be much larger than its width. The electrodes are connected to a source of alternating voltage with a phase shift between the electrodes located on different diagonals. The device also includes a spherical electrode 3 on a thin metal holder 4, mounted on the base 2 in the center of the square formed by the side electrodes. The diameter of the spherical electrode should be limited by the condition that excludes electrical breakdown between the surface of the sphere and the side electrodes of the trap. The spherical electrode is connected to a constant voltage source. The electrode power devices of FIG. 1 are not shown. In FIG. 2 is a photograph of a device with a particle 5, the remaining symbols as in FIG. one.

. Величину удельного заряда частицы определяют непосредственно из условия равенства силы тяжести, действующей на частицу, и электрической силы отталкивания между одноименно заряженными электродом и частицейThe operation of the device by the proposed method is as follows. On the side electrodes of the trap, located on different diagonals, an alternating voltage is applied, the same in amplitude, but phase shifted by 180 °. In an alternating electric field in the horizontal section of the trap, a potential well is formed for charged particles, where the minimum potential falls on the central axis of the trap. To compensate for the weight of the particle, the potential V repelling the particle is supplied to the ball electrode 3. By selecting the trap parameters, the particle position is stabilized at a certain point on the central axis of the trap. Next, the optical height of the particle is measured above the spherical electrode

. The specific charge of a particle is determined directly from the condition of equality of gravity acting on the particle and electric repulsive force between the electrode and particle of the same charge

,

,

- расстояние от частицы до центра сферического электрода, R - радиус сферического электрода. Заряд сферического электрода Q выражается через потенциал V по формулеwhere m is the particle mass, g is the gravitational acceleration, ε ₀ is the dielectric constant of vacuum, ε is the dielectric constant of air, q is the charge of the particle under study, Q is the charge of a spherical electrode,

is the distance from the particle to the center of the spherical electrode, R is the radius of the spherical electrode. The charge of the spherical electrode Q is expressed through the potential V according to the formula

.

.

Substituting the expression of the charge of the spherical electrode in the initial equality of forces, the formula for calculating the specific charge of the particle will look like this:

.

.

, откуда

с точностью не хуже 2%.This device was tested to measure the charge of a particle acquired by it in a corona discharge. In FIG. Figure 2 shows a photograph of a working device with a captured particle 5. As side electrodes 1, rods 65 mm long and 3 mm in diameter were used, as an end electrode 4, a ball of radius R = 4 mm was used on a thin metal holder 3 18 mm long. The distance between the side rods was 14 mm. The negatively charged particle 5, falling into the trap, was captured on its central axis. An alternating voltage with an amplitude of 840 V and a frequency of 50 Hz was applied to the side electrodes, and a negative potential V = 250 V was applied to the end spherical electrode. A particle hanging above the spherical electrode was photographed from the side by a CCD camera (Fig. 2). The measured distance from the particle to the surface of the spherical electrode was

from where

with an accuracy of no worse than 2%.

Thus, the claimed technical solution provides a more accurate determination of the specific charge of charged particles.

Claims (2)

1. A device for measuring the specific charge of micron-sized particles, including a linear quadrupole electrodynamic trap, rod electrodes that are supplied with an alternating electric potential, located vertically, in the lower part of the trap is an end electrode with a constant potential, characterized in that the lower end electrode is made in a sphere, which is mounted on a thin metal holder, the radius of which is much smaller than the radius of the sphere. 2. A device for measuring the specific charge of micron-sized particles according to claim 1, characterized in that the length of the thin metal holder of the end electrode is at least four radii of the sphere.

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