RU2822375C1 - Multipoint charger for unipolar charging of aerosol nanoparticles - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: invention relates to nanotechnology, particularly to devices for unipolar charging of aerosol nanoparticles in a corona discharge field. Multipoint charger for unipolar charging of aerosol nanoparticles comprises a dielectric housing, which comprises two channels communicated with external gas source for passage of shielding gas flow and channel communicated with external source of aerosol particles. Coaxially to the housing there is a high-voltage anode in the form of a needle having at least three points extending beyond the boundaries of the housing, and a grounded cathode in the form of a plate with a conical hole which expands as it moves away from the housing. Cathode is tightly adjacent to the housing, and the outlets of the channels for the flow of the enclosing gas are located in the immediate vicinity of the anode point.

EFFECT: high efficiency of charging aerosol nanoparticles and reduced losses thereof inside the device.

1 cl, 3 dwg

Description

The present invention relates to the field of nanotechnology, in particular to devices for unipolar charging of aerosol nanoparticles in the field of a corona discharge. Unipolar chargers provide higher charging efficiency than bipolar chargers because charged nanoparticles are prevented from recombining with ions of opposite polarity. This invention is needed in additive manufacturing and other processes where charged aerosol nanoparticles act as functional materials. Effective charging and elimination of nanoparticle loss is an important commercial task when using expensive materials, and also significantly speeds up the technological process and increases the duration of continuous operation of the device without cleaning.

In such devices, as a rule, two electrodes made of conductive material are used, between which a corona discharge occurs, leading to the appearance of a high concentration of ions, which can be estimated by the formula:

where N _i is the concentration of ions in the corona discharge area, I is the electric current of the corona discharge, Z _i is the electrical mobility of ions, E is the electric field strength, A is the effective surface area of the anode, e is the elementary electric charge.

When aerosol nanoparticles pass through the charging area in the device, they are bombarded with a flow of charged ions, as a result of which the nanoparticles acquire a charge. As can be seen from formula (1), an increase in the electric current of the corona discharge I or a decrease in the electric field strength E , with other parameters unchanged, directly leads to an increase in the concentration of _ions Ni . As the ion concentration increases, the average number of collisions between nanoparticles and ions increases, which leads to more efficient charging of nanoparticles. As research has shown [1], for sufficient charging, the ion concentration must be 2-3 orders of magnitude greater than the concentration of nanoparticles. The main problem of similar devices is the loss of nanoparticles on the plate under the influence of an electric field, which increases with increasing voltage on the needle.

An analogue is a device for charging nanoparticles, described in patent US10177541B2. This product includes a housing having a longitudinal axis passing between the inlet and outlet of the housing, and the flow of aerosol nanoparticles flows at a certain angle of the longitudinal axis. An electric field is created inside the housing to direct a flow of unipolar ions to the outlet to charge the flow of aerosol nanoparticles. In addition, an enveloping air flow is supplied between the flow of aerosol nanoparticles and the housing to reduce the loss of charged nanoparticles inside the housing. However, this device has a number of disadvantages: supplying too much enveloping gas flow greatly dilutes the concentration of nanoparticles, which limits the scope of application of the device; a large length of the outlet hole leads to an increase in losses of charged nanoparticles; the selected shape of the grounded electrode also provides additional losses of nanoparticles.

The prototype of the invention is a device for charging nanoparticles, described in patents SU 927318 A2 and US 20110090611 A1. These devices also consist of a housing having a longitudinal axis passing between the entrance and exit of the housing, with the flow of aerosol nanoparticles parallel to the longitudinal axis. The disadvantages of the devices, as in the previous case, are the large length of the outlet, which increases losses, the use of a single-pointed needle forces an increase in voltage to achieve the required concentration, and the shape of the grounded electrode also contributes to an increase in losses of nanoparticles.

The technical problem solved in the present invention is to create a device for obtaining high charging efficiency of nanoparticles.

The technical result is to ensure minimal loss of particles inside the device by choosing the optimal shape of the electrodes.

To achieve this goal, the present nanoparticle charger with high-velocity air flow in a shell to further reduce losses contains a body of dielectric material with inlet and outlet openings for aerosol and enveloping flows, a cathode in the form of a grounded plate, and an anode in the form of a multi-pointed needle. Increasing the efficiency of the charger is achieved by changing the shape of the electrodes, namely, simultaneously increasing the number of points on the needle and choosing a conical shape of the cathode hole with the location of the wide part towards the exit of the device, which leads to a reduction in the loss of nanoparticles inside the device. The first reason for reducing losses is to reduce the field strength required to achieve the required ion concentration. This is achieved by increasing the number of points on the needle, which means increasing the number of ion sources, which allows the voltage on the needle to be lower. The number of anode needle points is at least three. The second reason is to change the electric field to a more non-uniform one by using a conical plate shape. Due to this, charged nanoparticles, due to their low electrical mobility compared to ions, leave the charger without having time to turn in the direction of the field and reach the plate.

The invention is illustrated by drawings.

In fig. Figure 1 shows a diagram of the developed needle charger with a multi-edged corona needle.

In fig. Figure 2 shows a diagram of the experiment to determine the characteristics of the charger (loss L _E , the fraction of uncharged nanoparticles h and the fraction of charged nanoparticles at the output η _extr ).

In fig. Figure 3 shows the dependence of the characteristics of the charger (loss L _E , the fraction of uncharged nanoparticles h and the fraction of charged nanoparticles at the output η _extr ) when using a needle with one tip (on the left) and a cylindrical hole in the plate and a needle with eight (on the right) tips and a conical hole in plate at different values of the enveloping flow Q _sh equal to 1, 2 and 3 l/min.

Method [1] for determining optimal installation parameters is presented in Fig. 2. Using this method, it is possible to evaluate the efficiency of the charger by measuring aerosol concentrations without charging, with charging, and after removing charged nanoparticles using an electrostatic absorber. By comparing the obtained values, it is possible to calculate the fraction of losses, the fraction of charged nanoparticles, and the fraction of uncharged nanoparticles at the output of the device. Using this technique, studies were carried out on the effectiveness of the proposed charger and a similar device with a single-pointed needle and a cylindrical hole in the plate. The measurements were carried out for silver nanoparticles obtained by spark ablation with a size of 20-180 nm. The enveloping air flow varied from 1 to 3 l/min, the aerosol flow was 1 l/min. The corona discharge current was constant at 20 μA, which ensured a sufficient ion concentration. The voltage on a single-pointed needle was 2.1 kV; on a multi-pointed needle, the specified current was achieved at 1.8 kV. An explanation for the decrease in voltage on the needle when changing the shape of the electrodes was presented earlier. The research results are presented in Fig. 3. show that for all three values of the enveloping flow, our device with an improved form of electrodes is preferable and increases the proportion of charged nanoparticles at the exit from the device from 12% to 24%, the maximum value of which is 59% for an enveloping flow of 3 l/min. The share of losses in this case is 20%.

Application example

To obtain nanoparticles, a generator with a pulse-periodic gas discharge was used, providing high mass productivity for obtaining nanoparticles with a size of 20-180 nm and a concentration of about 10 ⁷ cm ^-3 .

The resulting nanoparticles were sent to the charger in an aerosol air flow of 1 l/min. A corona discharge with a current of 20 μA was created between a tungsten multipoint needle and a grounded copper plate by applying a voltage of 1.8 kV to the needle. The edges of the plate were blown with an enveloping flow of 3 l/min.

As a result, at the exit from the device, about 59% of the initial concentration of nanoparticles had a charge. About 20% of the nanoparticles were lost inside the device, and 21% of the nanoparticles had no charge.

Thus, this device has high nanoparticle charging efficiency and low nanoparticle loss, and is suitable even for high nanoparticle concentrations. It also has customizable corona discharge and air flow parameters.

Information sources

1. Alonso M., Martin M.I., Alguacil F.J. The measurement of charging efficiencies and losses of aerosol nanoparticles in a corona charger. Journal of Electrostatics. 2006;64(3):203-14. 10.1016/j.elstat.2005.05.008.

2. Patent US 20110090611 A1, publ. 04/21/2011, IPC H05H1/48, Particle charger with sheath air flow for enhancing charging efficiency.

3. Patent US 10177541 B2, publ. 01/08/2019, IPC H01T23/00, Concentric electrical discharge aerosol charger.

4. Patent SU 927318 A2, publ. 1982.05.15, IPC B03C 3/38, Device for charging aerosol particles.

5. Patent SU 1786496 A1, publ. 1993.01.07, IPC G08B 17/00, Device for charging aerosol particles of a fire hazard detector.

Claims (1)

A multi-point charger for unipolar charging of aerosol nanoparticles, which is a dielectric housing containing two channels connected to an external gas source for passing a flow of enclosing gas and a channel connected to an external source of aerosol particles for passing an aerosol flow, coaxially with which a high-voltage anode is located in in the form of a needle, having at least three points extending beyond the boundaries of the housing, and a grounded cathode in the form of a plate with a conical hole, which expands as it moves away from the housing, while the cathode is closely adjacent to the housing, and the outlets of the channels for the flow of the enclosing gas are located in direct close to the anode tip.