Smoke analysis characterization cell

Abstract

The invention relates to a smoke analysis characterization cell employing optical spectroscopy, which comprises: a reaction chamber, an inlet orifice for injecting smoke into the reaction chamber; an outlet orifice for discharging the smoke from the reaction chamber; and an analysis window for the entry of a laser beam intended to form the plasma inside the reaction chamber, which cell is characterized in that the system further includes a blower for blowing an inert gas close to the analysis window; and a shielding gas injector for the shielded injection of the smoke into the reaction chamber, the shielding being provided by a jet of inert gas around the smoke.

Claims

1. A characterisation system comprising: a characterisation cell for smoke analysis by optical spectrometry, comprising: a reaction chamber; an inlet orifice for the inlet of smoke into the reaction chamber; an outlet orifice for the evacuation of smoke from the reaction chamber; an analysis window for entry of a laser beam intended to form the plasma inside the reaction chamber; a fan for ensuring scanning of inert gas in the vicinity of the analysis window; and a shielding injector coaxially aligned with the inlet orifice configured to coaxially inject smoke shielded by a jet of inert gas around the smoke into the reaction chamber; a collector downstream of the outlet orifice of the cell configured to recover the smoke after its analysis; a pressure regulator for keeping the pressure constant in the reaction chamber of the cell, wherein the pressure regulator comprises a regulation valve placed downstream of the collector to compensate for a loss of charge due to clogging of filters of the collector, wherein the regulation valve is connected to a pressure probe placed in the cell for measuring the pressure therein for its servo-control, and wherein the regulation valve is servo-controlled as a function of the pressure measured inside the cell and which is adapted to open progressively as the collector gets clogged; a reactor for the generation of smoke; a second collector positioned downstream of the reactor; a second regulation valve for regulating the pressure inside the reactor; wherein the second regulation valve is placed downstream of the second collector, wherein the second regulation valve is connected to a second pressure probe placed in the reactor for measuring the pressure therein for its servo-control, and wherein the second regulation valve is servo-controlled as a function of the pressure measured inside the reactor and which is adapted to open progressively as the second collector gets clogged.

2. The system of claim 1, wherein the cell further comprises an arm extending between the reaction chamber and the analysis window, the arm being formed in two parts of different cross-sections, the larger cross-section part being arranged nearer the analysis window and the smaller cross-section part being arranged nearer the reaction chamber to form a Venturi and to ensure overpressure nearer the window.

3. The system of claim 1, wherein the flow rate of inert gas generated by the fan is adjustable.

4. The system of claim 1, wherein the flow rate of inert gas generated by the coaxial shielding injector is adjustable.

5. The system of claim 1, wherein the injector is a circular double nozzle having first and second coaxial orifices, the first orifice having a disc-shaped cross-section for the inlet of smoke, and the second orifice having a ring-shaped cross-section which encloses the first orifice for the inlet of inert gas.

6. The system of claim 1, wherein the cell further comprises a viewing window for observation of the plasma produced inside the reaction chamber during its operation.

7. The system of claim 1, wherein the fan also ensures scanning of inert gas in the vicinity of the viewing window.

8. The system of claim 1, wherein the flow rate of inert gas generated by the Venturi is adjustable.

9. The system of claim 1, wherein the cell further comprises an arm extending between the reaction chamber and the inlet orifice.

10. The system of claim 1, wherein the cell further comprises an arm extending between the reaction chamber and the outlet orifice.

11. The system of claim 1, wherein the cell further comprises a first arm extending between the reaction chamber and the inlet orifice, a second arm extending between the reaction chamber and the outlet orifice, and a third arm extending between the reaction chamber and the analysis window.

12. The system of claim 11, wherein the first and second arms extend along a first axis, and the third arm extends along a second axis that is perpendicular to the first axis.

13. The system of claim 11, wherein the third arm is formed in two parts of different cross-sections, the larger cross-section part being arranged nearer the analysis window and the smaller cross-section part being arranged nearer the reaction chamber to form a Venturi and to ensure overpressure nearer the window.

14. The system of claim 1, wherein the smoke is a smoke of nanoparticles.

15. The system of claim 1, wherein the second regulation valve regulates the pressure inside the reactor so as to keep said pressure constant.

16. The system of claim 1, wherein the second regulation valve is placed at the outlet of the second collector.

17. The system of claim 1, wherein the second regulation valve is adapted to open progressively as filters of the second collector gets clogged due to nanoparticles.

18. The system of claim 1, wherein an outlet of the reactor is connected to a pump which creates a flow of smoke.

Description

PRESENTATION OF THE DRAWINGS

(1) Other aims, features and advantages will emerge from the following detailed description in reference to the drawings given by way of illustration and non-limiting, in which:

(2) FIG. 1 schematically illustrates a conventional LIBS cell;

(3) FIG. 2 schematically illustrates the formation of secondary plasmas in a conventional LIBS cell;

(4) FIG. 3 schematically illustrates an example of a characterisation cell forming the subject matter of the invention and integrated into in a characterisation system;

(5) FIG. 4 schematically illustrates a Venturi such as used in the characterisation cell of FIG. 3;

(6) FIG. 5 schematically illustrates shielding of the smoke such as used in the characterisation cell of FIG. 3;

(7) FIG. 6 is graph illustrating the intensity of the signal measured as a function of a scanning rate inside the characterisation cell of FIG. 3 and a shielding rate of the smoke; and

(8) FIG. 7 is a graph illustrating the repeatability of the signal measured as a function of scanning inside the characterisation cell of FIG. 3 and a shielding rate of the smoke.

DETAILED DESCRIPTION

(9) In reference to FIGS. 3 and 4, an example embodiment of a proposed characterisation cell is described hereinbelow. In this example, the characterisation cell is a LIBS system.

(10) The LIBS cell for smoke analysis by plasma created by laser comprises a LIBS cell 1.

(11) The LIBS cell 1 comprises a reaction chamber in which the plasma is formed, a first arm 11 with at its free end an inlet orifice 111 for the inlet of the smoke inside the reaction chamber, a second arm 12 with at its free end an outlet orifice 121 for the evacuation of the smoke from the reaction chamber. The inlet 111 and outlet 121 orifices can be opposite and are arranged advantageously respectively on the upper part and the lower part of the LIBS cell 1.

(12) The LIBS cell 1 further comprises a third arm 13 closed by an analysis window 131 for entry of a laser beam F.sub.laser intended to form the plasma inside the reaction chamber.

(13) Facing the third arm 13 a fourth arm 14 closed by a cache 141 can be provided.

(14) The four arms 11, 12, 13 and 14 can be advantageously arranged in to a cross, where the beam entering via the analysis window 131 intersects the smoke entering via the inlet orifice 111 and exiting via the outlet orifice 121 opposite the latter.

(15) The laser beam F.sub.laser can ablate the material forming the LIBS cell 1. The fourth arm 14 is therefore selected longer than the third arm 13. Thus chance for the particles which result from ablation by the laser beam F.sub.laser of the cache 141 of the fourth arm 14 to pollute the measurements made of the smoke is decreased.

(16) The LIBS cell 1 can also comprise a viewing window 15 to allow an operator to observe the interior of the reaction chamber with the naked eye or by means of a viewing device, for example a video camera connected to a monitor. This viewing window 15 can be arranged on the LIBS cell 1 so that the viewing angle through the viewing window 15 is perpendicular to the incident direction of the laser beam F.sub.laser inside the reaction chamber and/or the arrival flow of the smoke via the inlet orifice 111.

(17) The LIBS cell also comprises a fan 16 for ensuring scanning of inert gas near at least the analysis window 131.

(18) This reduces the quantity of smoke in the vicinity of the analysis window 131, therefore decreasing clogging of the analysis window 131.

(19) The fan 16 can be a pump connected by tubes to an inert gas tank, for example argon, on one side, and on the other side to an intake orifice 132 for inert gas located in the third arm 13 in the vicinity of its end closed by the analysis window 131.

(20) To increase the efficiency of the flow of inert gas in the vicinity of the analysis window 131, the third arm 13 can have the form of a Venturi, as illustrated in FIG. 4, i.e. the third arm 13 is divided into two different cross-section parts S1, S2. The first part 134, to the side of its free end, has a cross-section S1 larger than the cross-section S2 of the second part 135 to the side of the reaction chamber. Overpressure ΔP is then generated in the first part 134, further limiting the quantity of smoke in the vicinity of the analysis window 131.

(21) The fan 16 can also be connected to an intake orifice located in the vicinity of the end of the fourth arm 14 which is closed by a cache. This helps balance the flow of argon gas inside the LIBS cell 1.

(22) The fan 16 can also be connected to an intake orifice located in the vicinity of the viewing window 15. This also reduces the clogging of the viewing window 15. In this case, to balance out the flow of inert gas inside the LIBS cell 1, scanning can also be ensured in the same way to the side opposite the viewing window 15.

(23) The flow rate of inert gas of the fan 16 can be adjustable.

(24) The LIBS cell further comprises injector 17 for the coaxial shielded injection of the smoke also the reaction chamber, the shielding being ensured by a jet of inert gas coaxial a the smoke and enclosing the latter.

(25) The shielding of the smoke confines the latter inside the reaction chamber. Therefore, the nanoparticle smoke will not tend to occupy all the space available inside the LIBS cell 1 and especially towards the analysis window 131 and the viewing window 15. This also prevents the formation of secondary plasmas outside the focal point of the laser beam F.sub.laser.

(26) As illustrated in FIG. 5, the injector 17 can be a double nozzle 17 with truncated cone shape having two coaxial orifices 171 and 172, a first 171 with a disco-shaped cross-section in for the inlet of smoke Fu and a second 172 with a ring-shaped cross-section enclosing the first 171 orifice for the inlet of the inert gas.

(27) In this way the injected inert gas encloses the smoke which is confined inside the cylinder formed by the inert gas. The inert gas is for example argon Ar.

(28) The LIBS cell 1 can form part of a LIBS system also comprising a LIBS collector 18 downstream of the outlet orifice 121 of the LIBS cell 1 and a pressure regulator 19 for keeping the pressure constant in the reaction chamber.

(29) The pressure regulator 19 can be a regulation valve placed downstream of the LIBS collector 18 to compensate the loss of charge due to clogging of the filters of the latter. The regulation valve LIBS 19 is connected to a pressure probe S1 placed inside the LIBS cell 1 for measuring the pressure therein. A servo-control is provided for controlling the regulation valve LIBS 19 as a function of the pressure measured inside the LIBS cell 1. The regulation valve LIBS 19 opens progressively as the LIBS collector 18 is clogged by smoke.

(30) The LIBS system further comprises a reactor 5 for the generation of smoke such as described in the technological background section. The outlet of the reactor 5 is connected to a pump 9 which creates a flow of smoke.

(31) As it leaves the reactor 5, the smoke is led in part to the LIBS cell 1 and in part to a collector 51 of the reactor. Arranged at the outlet of the collector 51 is a regulation valve 52 for regulating the pressure inside the reactor 5 which must be kept constant. The regulation valve 52 is connected to a pressure probe S2 placed inside the reactor 5 for measuring the pressure therein. A servo-control is provided for controlling the regulation valve 52 as a function of the pressure measured inside the reactor 5. The regulation valve 52 opens progressively as the filters of the collector 51 of the reactor 5 become clogged due to nanoparticles.

(32) The collectors 18 and 51 collect nanoparticles of the smoke so that they are not rejected into the atmosphere.

(33) The gas flows leaving the regulation valves 19 and 52 are combined and directed to the pump 9.

(34) The presence of the regulation valve LIBS 19 is needed to conserve a stable observed signal. In fact, in the absence of the regulation valve LIBS 19, the clogging of the collector 51 of the reactor causes opening of the regulation valve 52, which boosts the flow rate in the path outside LIBS cell and decreases the flow rate in the path of the LIBS cell. At the same time, the LIBS collector 18 also clogs up, which varies the pressure in the path of the LIBS cell and therefore inside the LIBS cell 1. The drop in flow rate and the variation in pressure in the path of the LIBS cell make the resulting plasma instable.

(35) Example of Operation

(36) In operation, the pressure inside the reactor 5 is kept below atmospheric pressure to prevent the produced nanoparticles from escaping into the ambient atmosphere, for example, the pressure is servo-controlled at 900 mbar.

(37) The reactor 5 is parameterised to give production of 400 g/h of nanoparticles. The pump 9 sets a rate of 160 m.sup.3/h.

(38) A loss of excessive charge between the path outside LIBS and the path of the LIBS cell should be avoided. Indeed, this is harmful for stability of the plasma to be generated.

(39) The pressure inside the LIBS cell 1 can be servo-controlled at 850 mbar. The overall flow rate of inert gas (argon) used for scanning the windows 131 and 15 and shielding the smoke is 30 L/min, distributed as follows: 20 L/min for scanning the windows 131 and 15 and 10 L/min for shielding the smoke.

(40) The laser 2 used is a nanosecond laser of Nd:YAG type. The energy per pulse of the laser 2 is set at 50 mJ. A converging lens 3 is positioned between the laser 2 and the analysis window 131. The laser 2 and the converging lens 3 are positioned so that the focal point of the laser beam F.sub.laser emitted by the laser 2 is at the junction of the four arms 11, 12, 13 and 14, or under the inlet flow of the smoke, and opposite the viewing window 15 if the latter is provided on the LIBS cell 2.

(41) The signal emitted by the plasma is collected by the optical system 4 placed at outlet, facing the analysis window 131. The optical system 4 sends the collected signal to a spectrometer 7 which analyses the spectrum of the signal emitted (which is the light of the plasma).

(42) The dimensions of the cell are (from the end of the arms to the centre of the cell, that is, where the plasma is created): first arm 11: 53 mm second arm 12: 160 mm third arm 13: 50 mm fourth arm 14: 100 mm
Comparative Test

(43) Comparative tests were conducted on a LIBS cell, the dimensions of which are specified hereinabove for measuring the combined effect of the shielding and of the scanning.

(44) FIG. 6 illustrates a graph illustrating the intensity of the measured signal (in arbitrary unit) as a function of the scanning flow rate used (in L/min) for four different elements: silicon Si, hydrogen H, argon Ar and carbon C.

(45) The intensity of the signal for silicon Si and hydrogen H shows up on the scale of ordinates to the left. The intensity of the signal for argon Ar and carbon C shows up on the scale of ordinates to the right.

(46) The shielding flowrate is selected such that the combined flow rate of the shielding and of the scanning is 30 L/min.

(47) So if the scanning flow rate is 0 L/min, the shielding flow rate is 30 L/min. If the scanning flow rate is 10 L/min, the shielding flow rate is 20 L/min.

(48) FIG. 6 therefore shows that with shielding alone (scanning flow rate is zero), the intensities of the signals for the four elements are much lower than for a shielding flow rate of 10 L/min (or a scanning flow rate of 20 L/min).

(49) This FIG. 6 also shows that with scanning alone (shielding flow rate is zero), the intensities of the signals for the four elements are lower than for a shielding flow rate of 10 L/min (or a scanning flow rate of 20 L/min).

(50) The conditions of shielding flow rate at 10 L/min and scanning flow rate at 20 L/min are close to the optimum and produce signal intensities close to the maximum.

(51) FIG. 7 shows the combined effect of shielding and scanning on the repeatability of the signal. The repeatability is given in ordinates for four elements (same as for FIG. 6) and is expressed in relative standard deviation of the intensity of lines and calculated over fifty spectra, one spectrum resulting from integration of the signal over thirty shots by a laser. The lower the standard deviation the better the repeatability.

(52) The shielding flow rate is selected such that the combined flow rate of shielding and scanning is 30 L/min.

(53) It is noticed that the repeatability of the measured signals is better when the shielding and the scanning are combined relative to the use of the shielding alone or the scanning alone with a low value translating considerable repeatability. When the scanning flow rate is 20 L/min and that of shielding is 10 L/min the repeatability is close to the minimum.

(54) Both FIGS. 6 and 7 therefore show that the effect of shielding alone and of scanning alone are not added together, but much more, signal quality is unexpectedly improved.

(55) Even though the description has been given in reference to a LIBS cell, the invention is not limited to the latter and also relates to other cells and especially those adapted for the following spectrometries: laser-induced fluorescence; fluorescence spectrometry; absorption spectrometry; Raman spectrometry; and infrared spectrometry.