FR2658916A2 - Electrostatic aerosol particle sensor and apparatuses comprising application thereof - Google Patents

Abstract

Application of the electrostatic sensor according to Claim 1 of the main patent for producing an electrical mobility selector for the particles of the aerosol, characterised in that at least one of the two coaxial conducting discs (16, 18) is provided with an annular slot (76) centred on the axis common to the two discs and in connection with means (84) for sucking through this slot the particles which would normally be deposited on the disc at this slot if it did not exist.

Description

ELECTROSTATIC PARTICLE SENSOR
AEROSOL AND APPARATUSES INCLUDING APPLICATION.

First certificate of addition to French patent application No. EN 89 084 00 of June 23, 1989.

 The present addition to the main patent No. EN 89 084 000 of June 23, 1989 relates generally to the field of electrostatic capture of aerosol particles with a view to studying or treating them and more specifically relates to a number of specific applications of the electrostatic sensor described in the main patent application.

 We will begin by recalling that the electrostatic sensor of aerosol particles, the subject of the main application is characterized in that it comprises two coaxial, spaced and parallel discs between which an electric field is established by bringing them to different potentials , the space between the two discs communicating over its entire periphery with the atmosphere to be examined, a central aspiration being provided in this space to circulate therein, from the periphery of the discs, part of this atmosphere in the form of '' a centripetal and stable laminar flow.

 The main patent application describes and claims a certain number of alternative embodiments of the electrostatic sensor, object of the invention, as well as a number of applications of this sensor, in particular in the production of a mobility spectrometer electric particle, and a collector of particles contained in an atmosphere to be examined.

 The Applicant has found that it is still possible to widen the fields of application of the electrostatic sensor which is the subject of the main patent application and it is these fields of complementary applications which are precisely the subject of this certificate. 'addition.

 A first new application of the electrostatic sensor which is the subject of the main patent resides in the production of an electric mobility selector for aerosol particles, characterized in that at least one of the two coaxial conductive discs is provided with an annular slot. centered on the axis common to the two discs, in connection with suction means, through this slot, particles which would normally have been deposited on the disc at the location of this slot if it did not exist.

 According to an alternative embodiment of this mobility selector, the above suction means consist of a cylindrical casing, attached to one of the discs and containing the slot, and provided with suction tubing to allow the selective extraction of particles with electric mobility corresponding to the radius of the slit.

 A second equally new application of the electrostatic sensor according to the main patent application lies in the production of an apparatus for obtaining a standard of uniform surface deposition of particles on a disc-shaped substrate.

formule dans laquelle :: - h est la demi-distance entre les plaques; - Z est la mobilité électrique commune aux
particules; - rA est la distance du point d'injection de
l'aérosol par rapport à l'axe du capteur; - Q est le débit d'extraction de la phase gazeuse
hors du capteur; - N est la concentration des particules dans le
débit d'extraction Q, lorsque V = 0 - S est la densité surfacique des particules que
l'on souhaite obtenir sur le disque de
collection; - t est le temps.In accordance with the present addition, this device is characterized in that the electrostatic sensor, the substrate of which constitutes one of the conducting discs, is supplied with the aid of a stream of aerosol particles all having the same value for two out of three of the magnitudes of electric mobility, charge and dimension, and in that the potential difference V between the discs of the sensor is varied as a function of time during deposition by law

formula in which :: - h is the half distance between the plates; - Z is the electric mobility common to
particles; - rA is the distance from the injection point of
the aerosol with respect to the axis of the sensor; - Q is the extraction rate of the gas phase
outside the sensor; - N is the concentration of particles in the
extraction rate Q, when V = 0 - S is the surface density of the particles that
we want to get on the disc of
collection; - t is the time.

 A third new application of the electrostatic sensor which is the subject of the main patent lies in the production of a total collection electrostatic sensor according to claim 9 of the main patent, capable of selecting all the aerosols having a particle size smaller than a given threshold, characterized in that '' it includes, on the path of introduction of the gas phase containing the aerosols, an annular slit calibrated to achieve, for an appropriate extraction rate, the inertial separation of particles with a particle size below the threshold and the deposition of these particles on a collection disc.

Anyway, the invention will be better understood by referring to the following reading of examples of implementation of the previous new applications of the electrostatic sensor according to the main patent, description which will be given by way of illustration only and without limitation. referring to Figures 1 to 4 in which:
- Figure 1 shows the schematic principle of producing an electric mobility selector of aerosol particles according to the invention;
- Figure 2 is a diagram for explaining the dispersion of particle deposits on the collector plate of an electrostatic sensor;
- Figure 3 is a simplified diagram of an installation for producing an apparatus for obtaining a standard surface deposition of particles on a substrate;
- Figure 4 shows a schematic example of implementation of a total collection electrostatic sensor according to the invention.

EXAMPLE I
REALIZATION OF A MOBILITY SELECTOR
By mobility selector or differential mobility analyzer is meant an apparatus capable of sorting the aerosol particles contained in a gaseous phase determined as a function of their electric mobility, a size well known and the definition of which has been recalled in the patent application. main. Such devices already exist in the industry and are used either to produce aerosols of known electrical mobility, or even in certain cases, aerosols having a perfectly determined diameter and often designated by specialists under the terminology of monodispersed aerosols (in Anglo-Saxon terminology "mono-dispersed").

 In the embodiments known to date, the electric mobility selectors have a cylindrical symmetry and the two electrodes present consist respectively of two hollow metallic coaxial conductive cylinders of different diameters. The central cylinder is provided with a circular slot into which particles of determined electrical mobility are sucked. This geometry has the disadvantage of not being sufficiently compact for certain applications that are envisaged.

 On the other hand, by making a mobility selector according to the model shown diagrammatically in FIG. 1, in which there is the electrostatic sensor according to the main patent with its two coaxial discs 16 and 18, and its extraction pipe 8, one can solve the problem in a simple and elegant way.

 Indeed, in Figure 1 we see that the side edges of the previous electrostatic sensor have two openings: an upper opening 70 through which the aerosols to be analyzed are sucked and a lower opening 72 provided with a cleanliness filter 74 through which is injected the sweeping air extracted through the tubing 8.

 According to the invention, the lower conductive disc 16 is provided with an annular slot 76 centered on the common axis 78 of the two discs 16 and 18 and, by this annular slot 76, the particles whose average trajectory between the point A and point B is shown diagrammatically in dotted line 80 on the figure; these particles are sucked through the slot 76 due to the fact that their normal place of impact in the absence of this slot 76 would have been the point B located at the distance from the axis 78 equal to the radius of the annular slot 76.

To achieve this aspiration, several means can be used without departing from the scope of the invention; in particular, and as is the case in the embodiment shown in FIG. 1, it is possible to use a cylindrical box 82 attached to the disc 18 and provided with an axial pipe 84 for example, through which a flow q much lower than the primary extraction flow rate Q drawn off through the pipe 8. In this way, all the aerosol particles which have passed through the annular slot 76 and which by definition have the same electrical mobility are collected by the tubing 84.

 To adjust this electric mobility to the desirable value case by case, one can act on the two parameters which are the flow Q on the one hand and the potential difference V applied according to the invention between the two coaxial conductive discs 16 and 18 .

EXAMPLE II
APPARATUS FOR OBTAINING A STANDARD SURFACE DEPOSIT
The example which follows relates to an application of the electrostatic sensor according to claim 1 of the main patent to the production of an apparatus for obtaining a standard of homogeneous surface deposition of particles on a disc-shaped substrate.

 A number of recent industrial developments, and in particular the need to achieve high purity atmospheres in large work volumes against contamination by dust particles (clean rooms), make it increasingly necessary 'the study of aerosols contained in the atmosphere and thus the knowledge as exact as possible of the rate of contamination of a given atmosphere. For this purpose, it is useful to have surface standards, in particular silicon wafers (in English terminology "waters") "contaminated" which are standards in size or concentration of the particles deposited on a substrate.

 Thus, “contaminated” silicon wafers have already been produced artificially using aerosols of polystyrene microspheres in monodispersed latex which allow the calibration in particle size of the surface deposition analysis apparatuses.

 The methods used to make such deposits are currently poorly understood and therefore cannot be easily used or reproduced.

 In other embodiments of the same nature, particle concentration standards have been produced by etching microscopic points on a substrate, the number of these points being perfectly determined and known by photolithographic process. It is obvious that this technique is delicate to implement and that it leads to relatively expensive standards.

 The present invention relates to a process which makes it possible to carry out standard surface contaminations, both in size and in concentration while guaranteeing a particularly homogeneous deposit. It can also be applied to deposit on substrates particles of a physico-chemical nature perfectly controlled for the needs of research, in particular in microelectronics.

 The Applicant has actually found that it was possible to apply the electrostatic sensor of aerosol particles which is the subject of the main patent application to obtain such a deposit by simply constituting one of the two conductive discs of the electrostatic sensor by the substrate on which one wants to deposit the standard contamination.

 Referring to the attached FIG. 2 which shows an electrostatic sensor according to the main patent, we will first explain the physical principle which is the basis of this application.

formule dans laquelle V est la différence de potentiel appliquée entre les électrodes coaxiales circulaires 16 et 18.Dans la formule précédente, si l'on considère que dans un cas particulier donné, les valeurs du débit Q de la tension V sont fixées, on voit donc que c'est uniquement en fonction de leur mobilité électrique Z que les différentes particules constituant l'aérosol pénétrant par la fente annulaire 12 dans le capteur se répartissent selon différents rayons à la surface de l'électrode circulaire 16.In this FIG. 2, the sensor is represented diagrammatically by its two coaxial conductive discs 16 and 18 and its tube 8 for suction of the extraction flow rate Q. The entry of aerosol particles which it is desired to deposit on the disc lower 16 is located on the surface of the disc 18 and takes the form of an annular injection slot 12 through which these particles are sucked. If the suction gas flow through the annular slot 12 is q, it is clear that the flow of driving gas required at the open periphery of the two coaxial discs 16 and 18 is equal to Qq. If Z is called electrical mobility of the particles, 2h the distance separating the electrode discs 16 and 18, rA the radial distance of the annular slot 12 relative to the axis of the system and r the impact radius of a given particle, the theory of l the device shows that inside the sensor on the disc 16, the aerosol particles are deposited on a circumference of radius r defined by the following formula:

formula in which V is the potential difference applied between the circular coaxial electrodes 16 and 18. In the previous formula, if we consider that in a given particular case, the values of the flow rate Q of the voltage V are fixed, we see therefore it is only as a function of their electrical mobility Z that the different particles constituting the aerosol penetrating through the annular slot 12 in the sensor are distributed according to different radii on the surface of the circular electrode 16.

If we therefore want to move the deposit of particles from radius r to radius r + dr there are only two possibilities which are either to decrease the suction rate Q or to increase the tension
V. As experimentally this second solution is by far the easiest and above all the most precise, it is that which one chooses within the framework of the present invention. Consequently, it is by scanning the voltage and passing it continuously through various values that a dispersion of the deposit of aerosol particles on the circular electrode 16 used as substrate of the future standard will be obtained. surface. But of course, this voltage V must not vary in any way as a function of time if one wishes to produce a standard whose surface concentration is homogeneous, that is to say which always has the same number of points at all points. particles per unit area.

formule dans laquelle S est la densité surfacique que l'on veut obtenir sur l'électrode 16 de dépôt, t est le temps et N la concentration des particules dans le débit Q d'extraction lorsque V = 0.To achieve the homogeneity of such a deposit, the voltage
V must vary over time according to a particular law which the plaintiff has determined and which is expressed by the following formula:

formula in which S is the surface density that we want to obtain on the deposition electrode 16, t is the time and N the concentration of the particles in the extraction rate Q when V = 0.

Furthermore, the above result cannot be obtained from any population of aerosol particles. In fact, three quantities characterize the particles of any aerosol, namely electrical mobility, electrical charge and the size of each particle. Without going into the details of the theory, which is moreover not necessary for the understanding and the reproducibility of the means of the invention, it will nevertheless be indicated that these three quantities are linked together by a relation which s 'writes as follows:
Z = peC
3tb d equation in which Z is the electric mobility of a particle, e the elementary charge, p the number of elementary charges of a particle, C is a coefficient and d is the diameter of each particle.

it therefore follows that equality for all the particles of at least two of these three magnitudes is sufficient so that, from the aerosol thus formed, one obtains, by applying the preceding law of variation of the voltage V in function of time, a homogeneous surface deposition.

 Referring to Figure 3, we will now describe the block diagram of the application of the electrostatic sensor of aerosol particles to the manufacture of a surface concentration standard.

 In this Figure 3, there is shown the electrostatic sensor 87 according to the main patent provided with its concentric electrode discs 16 and 18 and its annular slot 12 for access of aerosols to be dispersed inside the sensor 87, all being located in a closed enclosure 86 supplied by line 88 using a gas flow containing the aerosol particles. The main suction flow Q of the electrostatic sensor 87 circulates in the latter and in the enclosure 86 by means of a pipe 90; it is injected through a filter 92 and rejected outside the enclosure 86 after having fulfilled its driving function.

 A pipe 94 makes it possible to extract and evacuate towards the outside through the filter 96, the excess atmosphere possibly contained in the enclosure 86.

 According to the invention, the aerosol particles which penetrate into the enclosure 86 via the line 88 have predetermined characteristics and in particular the same value for all the particles of two out of three of the magnitudes that are the electric mobility, the charge and the dimension. In a particular case, but of frequent use, the aerosol is produced using calibrated latex beads and the skilled person knows then that it is easy, by passing the aerosol thus formed through '' a differential mobility selector, to obtain a population of aerosol particles which all have the same electrical charge, the same dimension and the same electrical mobility. This particularly easy-to-implement case makes it possible in particular to supply the pipe 88 at the entrance to the enclosure 86.

 Anyway, respecting the conditions of a particular nature of the aerosol injected and variation of the potential difference V between the two discs 16 and 18 as a function of time, we obtain on the electrode 16, used as deposition substrate, a homogeneous surface contamination having a value S (in number of particles per unit area) chosen in advance.

EXAMPLE III
SENSOR WITH TOTAL SELECTION OF PARTICLES OF
GRANULOMETRY LESS THAN A THRESHOLD
This example relates to the application of the electrostatic sensor according to claim 9 of the main patent to the production of an apparatus which makes it possible to capture all the aerosols having a particle size below a given threshold. Such an apparatus has a large number of possible applications, one of the main ones being the study in an atmosphere comprising aerosols dispersed in particle size, of those which are normally inhaled by the human respiratory tract, since generally admits that the particles inspirable by man and likely to cross the larynx are those which have a particle size of less than 10 micrometers. This device therefore makes it possible to collect in any atmosphere all the dust that a human being, present in this atmosphere, would have inhaled.

 The total selection apparatus, object of the present application combines the principle of particle size selection by inertial separation with an electrostatic sensor of total collection as it has been defined and claimed with reference to FIG. 7 and to claim 9 of the main patent.

 In FIG. 4 which schematically shows the construction of the total selection sensor, the sensor proper can be recognized as it was described in FIG. 7 of the main patent with its electrodes 16 and 18 of which the electrode 16 is an ionizing emitter, its equipotential grid 60 and its central suction pipe 8 through which the flow Q2 is evacuated. In accordance with the present invention, this structure has been combined with a particle size selection structure by inertial separation which essentially consists, in the gas stream penetrating at the periphery 12 of the apparatus under a flow rate Q1 + Q2 of an annular slot 98 with a width of the order of a millimeter through which the flow of atmosphere Q2 is sucked into the space between the coaxial electrodes 16 and 18. The remaining fraction Q1 of the flow entering 12 into the apparatus which does not not enter the electrostatic sensor is discharged directly through another central tube 100.

 In accordance with the invention, the slot 98 in combination with the suction flow rate Q2 which passes through it constitutes an inertial separator for particle size selection and it is by suitable choice and according to methods known to those skilled in the art that the cutting is carried out at a desired particle size of the aerosol particles entering the sensor by choosing the size of the annular slot and the flow rate Q2 which passes through it.

 In a frequent example of implementation of the present application, the dimensions of the annular slot 98 and the value of the flow rate Q2 are precisely chosen to obtain a cut-off of particles with an aerodynamic diameter of 10 micrometers, which achieves, by physical means , the size selection which would have taken place in the larynx of a human being. In this way, all the particles which this human being would have inhaled penetrate through the slot 98 and are then absorbed and fixed on the collection plate 18 of the electrostatic sensor with total selection for the purpose of a subsequent analysis.

Claims (4)

 1. Application of the electrostatic sensor according to claim 1 of the main patent for the production of an electric mobility selector for aerosol particles, characterized in that at least one of the two coaxial conductive discs (16, 18) is provided with an annular slot (76) centered on the axis common to the two discs in connection with suction means (84), through this slot, particles which would normally have deposited on the disc at the location of this slot if it did not exist.  2. Application of the electrostatic sensor according to claim 1, characterized in that the suction means consist of a cylindrical housing (82), attached to the disc (16), containing the slot (76), and provided with a tube suction (84) to allow the selective extraction of particles having electric mobility corresponding to the radius of the slot (76).  3. Application of the electrostatic sensor according to claim 1 of the main patent to the production of an apparatus for obtaining standards of homogeneous surface deposits of particles on a disc-shaped substrate, characterized in that the electrostatic sensor is supplied the substrate of which constitutes one of the conductive disks, using a stream of aerosol of particles all having in common the same value for two out of three of the magnitudes of electrical mobility, charge and dimension, and in that the potential difference V between the sensor disks is varied as a function of time during deposition according to the law:

  formula in which - h is the half distance between the plates; - Z is the electric mobility common to  particles; - rA is the distance from the aerosol injection point  relative to the axis of the sensor; - Q is the extraction rate of the gas phase  outside the sensor; - N is the concentration of particles in the flow  of extraction Q, when V = 0; - S is the surface density of the particles that  we want to get on the disc of  collection; - t is the time.  4. Application of the total collection electrostatic sensor according to claim 9 of the main patent for the production of a sensor with total selection of all aerosols having a particle size less than a given threshold, characterized in that this sensor comprises on the path d introduction of the gaseous phase containing the aerosols a calibrated annular slot (98) to achieve, for an appropriate extraction rate, the inertial separation of the particles with a particle size below the chosen threshold.