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.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to the field of online material analysis. The material analysed by these means can be in the form of an aerosol or gas charged with particles making up this material forming smoke. 
         [0002]    More particularly, the invention relates to the field of optical analysis systems for the study of particles formed by pyrolysis inside a characterisation cell. 
       TECHNOLOGICAL BACKGROUND 
       [0003]    The cell forming the subject matter of the invention can be associated with various known characterisation means such as:
       laser-induced fluorescence;   fluorescence spectrometry;   absorption spectrometry;   Raman spectrometry;   infrared spectrometry.       
 
         [0009]    By way of non-limiting example, the following description is based on the use of laser-induced breakdown spectrometry, or LIBS analysis. This method consists in focussing a pulsed laser beam into a reactional mixture to be analysed and forming plasma which is analysed by emission spectrometry. This determines the composition of said reactional mixture. This technique is applied in the description hereinbelow to control of smoke coming from the production of nanometric particles by laser pyrolysis. 
         [0010]    A LIBS system for LIBS analysis is illustrated in  FIG. 1  and comprises a nanoparticle synthesis reactor A 5 , a LIBS cell A 1  , a laser A 2  for emitting a laser beam, a lens A 3  for converging the laser beam inside the LIBS cell A 1 , an optical system A 4  for collecting signals coming from the LIBS cell A 1 , and a spectrometer A 7 . 
         [0011]    Production of nanometric particles within the reactor A 5  is based on the interaction of crossed flows between a beam emitted by a laser, for example a carbon dioxide CO 2  power laser, and a reactional mixture. The beam excites vibrational states of the molecules (so-called precursors) of the reactional mixture. The energy transmitted from the beam to the molecules is redistributed to the entire reactional mixture by collision. There is then very rapid elevation of the temperature of the reactional mixture which causes thermal decomposition of the molecules, resulting in a so-called “supersaturated” vapour in radicals and in energy. Nanoparticles then form from the radicals by homogeneous germination. The nanoparticles grow by a phenomenon of collision/coalescence growth. 
         [0012]    Dissociation and formation of nanoparticles take place in a overlapping volume between the beam and the flow of the reactional mixture observable by way of the production of a flame at this point. 
         [0013]    When the nanoparticles exit from this volume, they undergo a quenching effect which stops their growth. 
         [0014]    The nanoparticles are then guided to the LIBS cell A 1  through an entry conduit A 6 . 
         [0015]    The LIBS cell A 1  comprises a reaction chamber and four arms:
       a first arm A 11  forming inlet orifice A 111  for the smoke;   a second arm A 12  facing the first and forming outlet orifice A 121  for evacuating of smoke;   a third arm A 13  closed by a window A 131  through which the laser beam intended to form a plasma enters; and   a fourth arm A 14  closed by a cache A 141  and facing the third arm A 13  is not used.       
 
         [0020]    The LIBS cell A 1  also comprises a viewing window A 15  for observing the plasma with the naked eye. 
         [0021]    In the LIBS cell A 1 , the nanoparticles behave as a gas and therefore expand inside the reaction chamber and occupy all the space available and form, smoke. 
         [0022]    Inside the reaction chamber, the laser beam F laser  generated by the laser A 2  is focussed by the lens A 3 . When the laser beam F laser  is focussed in the mixture to be analysed there is vaporisation of nanoparticles causing ejection of atoms and forming plasma which expands. During expansion of the plasma, atoms de-energise, causing the emission of light. This light is then received by the optical system A 4  which is adapted and placed to the same side as the laser A 2 . This light is then analysed by the spectrometer A 7  connected to the optical system A 4  via fibre optics A 8  adapted to transport the signal. 
         [0023]    On drawback of this LIBS cell results from the fact that the nanoparticles behave as a gas within the reaction chamber. This is why the analysis window A 131  of the third arm A 13  becomes clogged. The clogged analysis window A 131  acts as a filter which blocks part of the laser beam F laser . Not all the energy of the laser beam F laser  is therefore efficient and only a portion thereof can be used to form the plasma. The formed plasma is therefore less energetic and emits a lower signal. This already weakened signal is further attenuated when it passes back through the analysis window A 131  of the third arm towards the optical system A 4 . 
         [0024]    Another drawback, still linked to the gaseous behaviour of the smoke, is the clogging of the viewing window A 15  tending to obstruct observation of the plasma with the naked eye. 
         [0025]    Yet another drawback is that the formed plasma is not limited to the focal point of the laser beam F laser , that is to say where the latter is the most highly concentrated. In fact, as particles are present throughout the reaction chamber, secondary plasmas Pl sec  can form between the focal point of the laser beam, where the main plasma Pl pr  forms, and the analysis window A 131  through which the laser beam F laser  enters the reaction chamber, as illustrated in  FIG. 2 . The secondary plasmas Pl sec  can be located outside the observation zone by the optical system A 4 . 
         [0026]    Another drawback of the LIBS cell A 1  hereinabove is instability of the signals acquired by the optical system A 4  and by the spectrometer A 7 , the origin of which is numerous. 
       Presentation 
       [0027]    The aim of the invention is to overcome at least one of the drawbacks of the prior art presented hereinabove by way of example. 
         [0028]    To this aim, the invention provides to a characterisation cell for smoke analysis by optical spectrometry, comprising:
       a reaction chamber;   an inlet orifice for the inlet of smoke inside the reaction chamber;   an outlet orifice for the evacuation of smoke from the reaction chamber;   an analysis window for the entry of a laser beam intended to form the plasma inside the reaction chamber;       
 
         [0033]    characterised in that the system also comprises:
       a fan for ensuring scanning of inert gas in the vicinity of the analysis window, and   a shielding injector for shielded injection of smoke inside the reaction chamber, the shielding being ensured by a jet of inert gas around the smoke.       
 
         [0036]    The advantage is that the signal obtained at output (light emitted by the plasma and passing through the analysis window) is stabilised relative to the prior art. 
         [0037]    Other optional and non-limiting features of the cell are:
       the cell also comprises an arm extending from the reaction chamber and one free end of which is closed by the analysis window, this arm being formed by two parts of different straight cross-sections, the largest cross-section part being arranged to the side of the analysis window and the smallest cross-section part being arranged to the side of the reaction chamber to form a Venturi and ensure overpressure to the side of the window;   the flow rate of inert gas generated by the fan and optionally the Venturi is adjustable;   the flow rate of inert gas generated by the coaxial shielding injector is adjustable;   the injector is a circular double nozzle having two coaxial orifices, a first having a disc-shaped cross-section for the inlet of smoke and a second having a ring-shaped cross-section which encloses the first for the inlet of inert gas; and   a viewing window is provided for observation of the plasma produced inside the reaction chamber during its operation.       
 
         [0043]    The invention also relates to a characterisation system comprising a cell such as that described hereinabove and also a collector downstream of the outlet orifice of the cell recovering the powder after analysis of the latter and a pressure regulator for keeping the pressure constant in the reaction chamber of the cell. 
         [0044]    Other optional and non-limiting features of the system are:
       the pressure regulator comprises a regulation valve placed downstream of the collector to compensate the loss of charge due to clogging of the filters of the latter;   the regulation valve is connected to a pressure probe placed in the cell for its servo-control; and   the fan also ensures scanning of inert gas in the vicinity of the viewing window.       
 
     
    
     
       PRESENTATION OF THE DRAWINGS 
         [0048]    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: 
           [0049]      FIG. 1  schematically illustrates a conventional LIBS cell; 
           [0050]      FIG. 2  schematically illustrates the formation of secondary plasmas in a conventional LIBS cell; 
           [0051]      FIG. 3  schematically illustrates an example of a characterisation cell forming the subject matter of the invention and integrated into in a characterisation system; 
           [0052]      FIG. 4  schematically illustrates a Venturi such as used in the characterisation cell of  FIG. 3 ; 
           [0053]      FIG. 5  schematically illustrates shielding of the smoke such as used in the characterisation cell of  FIG. 3 ; 
           [0054]      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 
           [0055]      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 
       [0056]    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. 
         [0057]    The LIBS cell for smoke analysis by plasma created by laser comprises a LIBS cell  1 . 
         [0058]    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 . 
         [0059]    The LIBS cell  1  further comprises a third arm  13  closed by an analysis window  131  for entry of a laser beam F laser  intended to form the plasma inside the reaction chamber. 
         [0060]    Facing the third arm  13  a fourth arm  14  closed by a cache  141  can be provided. 
         [0061]    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. 
         [0062]    The laser beam F 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 laser  of the cache  141  of the fourth arm  14  to pollute the measurements made of the smoke is decreased. 
         [0063]    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 laser  inside the reaction chamber and/or the arrival flow of the smoke via the inlet orifice  111 . 
         [0064]    The LIBS cell also comprises a fan  16  for ensuring scanning of inert gas near at least the analysis window  131 . 
         [0065]    This reduces the quantity of smoke in the vicinity of the analysis window  131 , therefore decreasing clogging of the analysis window  131 . 
         [0066]    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 . 
         [0067]    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 S 1 , S 2 . The first part  134 , to the side of its free end, has a cross-section S 1  larger than the cross-section S 2  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 . 
         [0068]    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 . 
         [0069]    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 . 
         [0070]    The flow rate of inert gas of the fan  16  can be adjustable. 
         [0071]    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. 
         [0072]    The shielding of the smoke confines the latter inside the reaction chamber. 
         [0073]    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 laser . 
         [0074]    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. 
         [0075]    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. 
         [0076]    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. 
         [0077]    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 S 1  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. 
         [0078]    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. 
         [0079]    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. 
         [0080]    The regulation valve  52  is connected to a pressure probe S 2  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. 
         [0081]    The collectors  18  and  51  collect nanoparticles of the smoke so that they are not rejected into the atmosphere. 
         [0082]    The gas flows leaving the regulation valves  19  and  52  are combined and directed to the pump  9 . 
         [0083]    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. 
       Example of Operation 
       [0084]    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. 
         [0085]    The reactor  5  is parameterised to give production of 400 g/h of nanoparticles. The pump  9  sets a rate of 160 m 3 /h. 
         [0086]    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. 
         [0087]    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. 
         [0088]    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 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 . 
         [0089]    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). 
         [0090]    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 
       [0095]    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. 
         [0096]      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 Umin) for four different elements: silicon Si, hydrogen H, argon Ar and carbon C. 
         [0097]    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. 
         [0098]    The shielding flowrate is selected such that the combined flow rate of the shielding and of the scanning is 30 L/min. 
         [0099]    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. 
         [0100]      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). 
         [0101]    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). 
         [0102]    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. 
         [0103]      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. 
         [0104]    The shielding flow rate is selected such that the combined flow rate of shielding and scanning is 30 L/min. 
         [0105]    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. 
         [0106]    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. 
         [0107]    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.