Abstract:
A system for converting energy, comprising a first device comprising a deformable enclosure containing thermo-reactive molecules suitable for deforming the enclosure when their temperature exceeds a threshold temperature, and a second pyroelectric and/or piezoelectric device making contact with the enclosure.

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
       [0001]    This application is the national phase of International Application No. PCT/FR2014/052709, filed on Oct. 23, 2014, which claims the priority benefit of French patent application FR 13/61163, filed on Nov. 15, 2013, which applications are hereby incorporated by reference to the maximum extent allowable by law. 
     
    
     BACKGROUND 
       [0002]    The present application relates to a system enabling to convert thermal and/or mechanical energy into electrical energy and to a method of manufacturing such a system. 
       DISCUSSION OF THE RELATED ART 
       [0003]    Systems for converting thermal/mechanical energy into electrical energy may in particular be used to form pressure sensors, switches, or energy recovery systems. 
         [0004]    It is known to form energy conversion systems by using piezoelectric and/or pyroelectric films. However, the piezoelectric and/or pyroelectric action of films known to date may be insufficient to form a system of conversion of thermal and/or mechanical energy into electrical energy which has a sufficient sensitivity. 
         [0005]    Further, when the energy conversion system is used to form a switch, particularly a switch manually actuated by a user, it may be desirable, when the user actuates the switch, for the switch to exert in return a mechanical force on the user, for example, the application of an overpressure, particularly so that the user can be sure of having properly actuated the switch. It is then necessary to provide additional means for providing this mechanical reaction. 
       SUMMARY 
       [0006]    An embodiment aims at overcoming all or part of the disadvantages of known systems of conversion of thermal/mechanical energy into electrical energy. 
         [0007]    Another embodiment aims at enabling to use a pyroelectric and/or piezoelectric film to form the energy conversion system. 
         [0008]    Another embodiment aims, in the case of a use of the energy conversion system to form a pressure sensor or a switch, at increasing the sensitivity of the energy conversion system. 
         [0009]    Another embodiment aims, in the case of a use of the energy conversion system to form a switch, at providing a mechanical reaction to the user when he/she actuates the switch. 
         [0010]    Thus, an embodiment provides an energy conversion system comprising: 
         [0011]    a first device comprising a deformable enclosure containing heat-sensitive molecules capable of deforming the enclosure when the temperature exceeds a threshold temperature; and 
         [0012]    a second pyroelectric and/or piezoelectric device in contact with the enclosure. 
         [0013]    According to an embodiment, the second device comprises a film comprising polyvinylidene fluoride and/or at least one copolymer of polyvinylidene fluoride. 
         [0014]    According to an embodiment, the film comprises a polymer selected from the group comprising polyvinylidene fluoride, poly(vinylidene fluoride-trifluoroethylene), poly(vinylidene fluoride-tetrafluoroethylene) and a mixture of at least two of these polymers. 
         [0015]    According to an embodiment, the heat-sensitive molecules are molecules having a characteristic transition temperature and which are adapted, when they are submitted to a temperature variation from a first temperature lower than the characteristic transition temperature to a second temperature higher than the characteristic transition temperature, of passing from a first state where the enclosure occupies a first volume to a second state where the enclosure occupies a second volume different from the first volume, and capable, when they are submitted to a temperature variation from the second temperature to the first temperature, of passing from the second state to the first state. 
         [0016]    According to an embodiment, the heat-sensitive molecules are selected from the group comprising poly (N-isopropyl acrylamide), polyvinylcaprolactame, hydroxypropylcellulose, polyoxazoline, polyvinylmethylether, polyethylene glycol, poly-3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate, poly(propyl sulfonate dimethyl ammonium ethyl methacrylate), and the mixture of at least two of these polymers. 
         [0017]    Another embodiment provides a method of manufacturing an energy conversion system, comprising the steps of: 
         [0018]    forming a first device comprising a deformable enclosure containing heat-sensitive molecules capable of deforming the enclosure when the temperature exceeds a threshold temperature; and 
         [0019]    forming a second pyroelectric and/or piezoelectric device, the second device being in contact with the enclosure. 
         [0020]    According to an embodiment, the second pyroelectric and/or piezoelectric device comprises a film comprising polyvinylidene fluoride and/or at least one copolymer of polyvinylidene fluoride, the method comprising the steps of: 
         [0021]    forming a portion of a layer of a solution comprising a solvent and a compound comprising polyvinylidene fluoride and/or said at least one copolymer of polyvinylidene fluoride; and 
         [0022]    irradiating, at least partially, the portion with pulses of at least one ultraviolet radiation. 
         [0023]    According to an embodiment, the duration of each pulse is in the range from 500 μs to 2 ms. 
         [0024]    According to an embodiment, the fluence of the ultraviolet radiation is in the range from 10 J/cm 2  to 25 J/cm 2 . 
         [0025]    According to an embodiment, the solvent has an evaporation temperature in the range from 110° C. to 140° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0027]      FIG. 1  is a partial simplified cross-section view of an embodiment of a system for converting mechanical and/or thermal energy into electrical energy; 
           [0028]      FIG. 2  is a cross-section view similar to  FIG. 1 , in the case of a use of the embodiment of the energy conversion system shown in  FIG. 1  as a switch; and 
           [0029]      FIGS. 3A to 3H  are partial simplified cross-section views of the structures obtained at successive steps of another embodiment of a method of manufacturing the energy conversion system of  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, as usual in the representation of electronic circuits, the various drawings are not to scale. Further, only those elements which are useful to the understanding of the present description have been shown and will be described. In particular, the circuit for processing the electric signals supplied by the energy conversion system is well known by those skilled in the art according to the envisaged application and is not described in detail hereafter. In the following description, unless otherwise indicated, terms “substantially”, “approximately”, and “in the order of” mean “to within 10%”. In the following description, expression element “based on poly-vinylidene fluoride (PVDF)” means an element comprising at least 70% wt. of the PVDF polymer and/or of at least one copolymer of PVDF. 
         [0031]      FIG. 1  shows an embodiment of an energy conversion system  10 . 
         [0032]    System  10  comprises a substrate  12  having an upper surface  14 . Substrate  12  may be made of an insulating or semiconductor material. As an example, substrate  12  is made of glass, of silicon, or of a plastic material. Substrate  12  may be made of a polymer, for example, polyimide, polyethylene naphthalate (PEN), or polyethylene terephthalate (PET). As an example, the thickness of substrate  12  is in the range from 25 μm to 200 μm. Substrate  12  may be flexible. 
         [0033]    System  10  comprises a device  16  which may be actuated with temperature, called heat-actuated device hereafter, and a piezoelectric and/or pyroelectric device  18 . In the present embodiment, heat-actuated device  16  is interposed between substrate  12  and piezoelectric and/or pyroelectric device  18 . However, as a variation, piezoelectric and/or pyroelectric device  18  may be interposed between heat-actuated device  16  and substrate  12 . 
         [0034]    Heat-actuated device  16  comprises a bonding layer  20  laid on surface  14  and having molecules  22  changing state according to temperature, called heat-sensitive molecules hereafter, bonded thereto. The nature of bonding layer  20  depends on the nature of heat-sensitive molecules  22 . The thickness of bonding layer  20  may be in the range from 10 nm to 100 nm, for example, approximately 30 nm. As a variation, layer  20  may be a metal layer or a non-metallic layer, for example, made of fullerene or of polystyrene. 
         [0035]    Term heat-sensitive molecule means a polymer molecule which exhibits a significant and discontinuous change in at least one physical property according to temperature. According to an embodiment, heat-sensitive molecules  22  have a characteristic transition temperature and are in a first state, that is, with a physical property at a first level, when the temperature is lower than the characteristic transition temperature and are in a second state, that is, with a physical property at a second level, when the temperature is higher than the characteristic transition temperature. This change is preferably reversible so that the molecules pass from the first state to the second state when the temperature rises above the characteristic transition temperature and passes from the second state to the first state when the temperature decreases below the characteristic transition temperature. 
         [0036]    According to an embodiment, the considered property is the three-dimensional conformation of the molecule. According to another embodiment, the considered property is the solubility of the molecule in a solvent. According to an embodiment, the considered property is the hydrophobicity of the molecule. 
         [0037]    According to an embodiment, in the first state, heat-sensitive molecules  22  may have a given affinity for water, while in the second state, heat-sensitive molecules  22  may have a reverse affinity for water. For example, in the first state, heat-sensitive molecules  22  may be hydrophobic (conversely, hydrophilic) while in the second state, heat-sensitive molecules  22  may be hydrophilic (conversely, hydrophobic). More generally, heat-sensitive molecules  22  may be such that they are capable of passing from a solvophobic character (conversely, solvophilic) to a solvophilic (conversely, solvophobic) character due to a temperature variation. 
         [0038]    Advantageously, heat-sensitive molecules  22  may be selected from one or a plurality of the following polymers: poly(N-isopropylacrylamide) (polyNIPAM), polyvinylcaprolactame, hydroxypropylcellulose, polyoxazoline, polyvinylmethylether, polyethyleneglycol, poly-3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (PDMAPS), and poly(propyl sulfonate dimethyl ammonium ethylmethacrylate). 
         [0039]    The characteristic transition temperatures of these materials are the following:
   polyNIPAM: between 30 and 37° C.;   polyvinylcaprolactame: 37° C.;   hydroxypropylcellulose: between 40 and 56° C.;   polyoxazoline: 70° C.;   polyvinylmethylether: 45° C.;   polyethyleneglycol: between 100 and 130° C.;   PDMAPS: between 32 and 35° C.;   poly(propyl sulfonate dimethyl ammonium ethyl methacrylate): 30° C.   
 
         [0048]    In the present embodiment, for an application where system  10  is used as a mechanical switch actuated by a user, the characteristic transition temperature of heat-sensitive molecules  22  is preferably in the range from 30° C. to 37° C. 
         [0049]    For an application as a switch actuated by an operator&#39;s finger, heat-sensitive molecule  22  is preferably PDMAPS having a characteristic transition temperature in the range from 32° C. to 35° C. and which passes from a hydrophobic state to a hydrophilic state when the temperature exceeds the characteristic transition temperature. 
         [0050]    The material comprising the PDMAPS molecules may appear in the form of an aqueous gel which occupies a first volume when the temperature is below the characteristic transition temperature and a second volume, larger than the first volume, when the temperature is above the characteristic transition temperature. 
         [0051]    According to an embodiment, heat-sensitive molecules  22  may be formed of a plurality of types of polymers capable of being activated by temperature, in particular with different respective characteristic transition temperatures. 
         [0052]    It is possible to modify the characteristic transition temperature of the heat-sensitive polymer by adding a salt or by adding an appropriate surface-active agent or solvent to the polymer. Similarly, a modification of the characteristic transition temperature for a family of heat-sensitive polymers may be performed by forming of a copolymer, the copolymer supporting as desired a filler or an amphiphilic group. 
         [0053]    Device  16  comprises a cap  24  covering heat-sensitive molecules  22  and which defines, with substrate  12 , an enclosure  26  containing heat-sensitive molecules  22 . Cap  24  is capable of being deformed on application of external mechanical stress. To achieve this, as an example, the thickness of cap  24  is in the range from 1 μm to 2 μm, to obtain a flexible membrane. 
         [0054]    Preferably, cap  24  is made of a material which enables to have a good moisture input in enclosure  26 . As an example, to confine water or humidity in enclosure  26 , one may provide on the internal walls of enclosure  16  one or a plurality of areas having a good affinity for water such as, for example, polyimide (PI) or polydimethylsiloxane (PDMS). As an example, cap  24  is made of a material selected from the group comprising polyimide, poly(methyl methacrylate) (PMMA), poly(vinylcrotonate), and PET. Cap  24  may comprise openings for giving way to moisture. 
         [0055]    Pyroelectric/piezoelectric device  18  comprises: 
         [0056]    a first electrode  28  which extends over a portion of cap  24  and over a portion of surface  14 ; 
         [0057]    a pyroelectric and/or piezoelectric film  30  covering a portion of electrode  28 ; and 
         [0058]    a second electrode  32  which extends on film  30  and on a portion of surface  14 . 
         [0059]    First electrode  28  is preferably made of a material reflecting ultraviolet radiation, for example, over a wavelength range between 200 nm and 400 nm. It may be a metal layer. As an example, the material forming first electrode  28  is selected from the group comprising silver (Ag), aluminum (Al), gold (Au), or a mixture or an alloy of two or more than two of these metals. 
         [0060]    Film  30  comprises a pyroelectric and/or piezoelectric material. Preferably, pyroelectric and/or piezoelectric film  30  is arranged to have a pyroelectric and/or piezoelectric activity along a direction perpendicular to surface  14 . According to an embodiment, film  30  is made of a polymer material. 
         [0061]    According to an embodiment, film  30  is based on PVDF. It may comprise the PVDF polymer alone, a single copolymer of PVDF, a mixture of two or more than two copolymers of PVDF, a mixture of the PVDF polymer and of at least one copolymer of PVDF. Preferably, the PVDF copolymer is poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFe)) or poly(vinylidene fluoride-tetrafluoroethylene), particularly P(VDFx-TrFe100-x) where x is a real number in the range from 60 to 80. Film  30  may further comprise fillers. The fillers may correspond to ceramic particles, for example, to particles of barium titanate (BaPiO3) or particles of lead zirconate titanate (LZT). The concentration by weight of fillers in film  30  may vary from 5% to 25% wt. The thickness of film  30  is in the range from 200 nm to 4 μm. The PVDF polymer or the PVDF copolymer of film  30  is a semicrystalline polymer comprising, in particular, a β crystalline phase which may have pyroelectric and/or piezoelectric properties. 
         [0062]    Second electrode  32  is, for example, made of a metallic material selected from the group comprising silver, copper, or a mixture or an alloy of at least two of these materials. 
         [0063]    A protection layer  34 , for example, made of an insulating material, covers the entire structure. Openings  36 ,  38  may be provided in protection layer  34  to expose a portion  40  of first electrode  28  and a portion  42  of second electrode  32 . Protection layer  34  is made of a dielectric material. The dielectric material may be selected from the group comprising polytetrafluoroethylene (Teflon), a fluorinated polymer of the type of the polymer commercialized by Bellex under trade name Cytop, a polystyrene, and a polyimide. 
         [0064]      FIG. 2  illustrates an example of illustration of system  10  as a switch actuated by finger  44  of an operator. For such an application, heat-sensitive molecules  22  are preferably made of PDMAPS having a characteristic transition temperature in the range from 32° C. to 35° C. PDMAPS passes from a hydrophobic state to a hydrophilic state when the temperature exceeds the characteristic transition temperature. The material forming bonding layer  20  may be gold. 
         [0065]    The PDMAPS molecules may be arranged in enclosure  26  in the form of an aqueous gel which occupies a first volume when the temperature is below the characteristic transition temperature and which occupies a second volume, larger than the first volume, when the temperature is above the characteristic transition temperature. 
         [0066]    When a user presses finger  44  on the portion of protection layer  34  covering pyroelectric/piezoelectric film  30 , a pressure is exerted on film  30 , which results in the occurrence of a voltage between electrodes  28 ,  32 . 
         [0067]    In the case where film  30  has both piezoelectric and pyroelectric properties, which may be the case for a PVDF-based film, the presence of finger  44  causes a rise in the temperature of film  30 , which increases the voltage between electrodes  28 ,  32 . 
         [0068]    Further, the presence of finger  44  causes a rise in the temperature in enclosure  26  beyond the characteristic transition temperature of heat-sensitive molecules  22 . This causes an increase in the volume occupied by the heat-sensitive molecules  22  and a deformation of cap  24 . As an example in  FIG. 2 , cap  24  has been shown with an outward-bulged shape due to the increase in the volume of enclosure  26 . However, the deformed shape of cap  24  may be different from the shape shown in  FIG. 2 . The thin thickness of cap  24  advantageously provides a significant deformation of cap  24  as the volume of enclosure  26  changes. 
         [0069]    The deformation of cap  24  causes an additional deformation of film  30 , in addition to the pressure exerted by finger  44 . Thereby, the voltage between electrodes  28 ,  32  is greater than that which would be obtained by only applying finger  44 . The switch sensitivity is thus improved. 
         [0070]    Further, when he/she actuates system  10  by touching it with finger  44 , the abrupt increase in the volume of enclosure  26  is sensed by the user. A mechanical return function is thus obtained without using additional means. 
         [0071]    According to another example of use, there is no application of pressure on piezoelectric film  30  by an external member. The deformation of piezoelectric film  30 , and thus the occurrence of a voltage between electrodes  28  and  32 , is only obtained by the change of volume of enclosure  26  when the temperature in enclosure  26  exceeds the characteristic transition temperature of heat-sensitive molecules  22 . As an example, system  10  shown in  FIG. 1  may be used as a thermally-actuated switch. In this case, the characteristic transition temperature of heat-sensitive molecules  22  is selected according to the temperature threshold beyond which an actuation of the switch is desired. Indeed, when the temperature in enclosure  26  exceeds the threshold temperature, the volume of enclosure  26  increases, which causes a deformation of piezoelectric film  30  and thus the occurrence of a voltage between electrodes  28  and  32 . According to another example of use, the temperature modification in enclosure  26  may be obtained by the application of a local heat source at the level of enclosure  26 , for example, with a laser. A system for converting thermal energy into electrical energy is then obtained. 
         [0072]    The present energy conversion system  10  may also be implemented as a thermal or electrical energy recovery system. 
         [0073]      FIGS. 3A to 3H  illustrate an embodiment of a method of manufacturing energy conversion system  10  shown in  FIG. 1 . 
         [0074]      FIG. 3A  shows the structure obtained after having formed bonding layer  20  on substrate  12 . The bonding layer may be deposited by physical vapor deposition (PVD). 
         [0075]      FIG. 3B  shows the structure obtained after having grafted heat-sensitive molecules  22  to bonding layer  20 . The grafting method may be implemented as described in A. Housni and Y. Zhao&#39;s publication entitled “Gold Nanoparticles Functionalized with Block Copolymers Displaying Either LCST ou UCST Thermosensitivity in Aqueous Solution”,  Langmuir,  2010, 26 (15), pp. 12933-12939. Other examples of grafting methods are described in French application FR13/54701 which is herein incorporated by reference. 
         [0076]      FIG. 3C  shows the structure obtained after having formed cap  24 . Cap  24  may be formed by printing techniques, for example, by inkjet printing or by sputtering. An anneal step enabling to evaporate the solvents having the polymers dissolved therein may be provided to form a film. The anneal step may be formed by irradiation by a succession of ultraviolet (UV) radiation pulses, or UV flashes. UV radiation means a radiation having its wavelengths at least partly in the range from 200 nm to 400 nm. According to an embodiment, the duration of a UV pulse is in the range from 500 μs to 2 ms. The duration between two successive UV pulses may be in the range from 1 to 5 seconds. The fluence of the UV radiation may be in the range from 10 J/cm2 to 21 J/cm2. 
         [0077]      FIG. 3D  is a partial simplified cross-section view of the structure obtained after having formed first electrode  28  on cap  24  and on substrate  12 . The deposition of first electrode  28  may be formed by PVD or by printing techniques, particularly by silk screening or by inkjet printing. 
         [0078]      FIG. 3E  shows the structure obtained after having formed a liquid portion  46 , possibly viscous, which extends on the portion of first electrode  28  covering cap  24  and, possibly, directly on a portion of cap  24 . Liquid portion  46  comprises a solvent and a PVDF-based compound dissolved in the solvent. The thickness of portion  46  is in the range from 200 nm to 4 μm. 
         [0079]    The PVDF-based compound may comprise the PVDF polymer alone, a single copolymer of PVDF, a mixture of two or more than two copolymers of PVDF or a mixture of the PVDF polymer and of at least one copolymer of PVDF. Preferably, the PVDF copolymer is poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFe)) or poly(vinylidene fluoride-tetrafluoroethylene), particularly P(VDFx-TrFe100-x) where x is a real number in the range from 60 to 80. 
         [0080]    The PVDF-based compound may further comprise fillers. The fillers may correspond to ceramic particles, for example, to particles of barium titanate (BaPiO3) or particles of lead zirconate titanate (LZT). The concentration by weight of fillers in the PVDF-based compound may vary from 5% to 25% wt. 
         [0081]    Preferably, the solvent is a polar solvent. This advantageously enables to improve the dissolution of the PVDF-based polymer. Preferably, the solvent is capable of absorbing, at least partially, the UV radiation, for example, over a wavelength range between 200 nm and 400 nm. According to an embodiment, the evaporation temperature of the solvent is in the range from 110° C. to 140° C., preferably from 110° C. to 130° C., more preferably from 120° C. to 130° C. The solvent may be selected from the group comprising cyclopentanone, dimethylsulphoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), or N-methyl-E-pyrrolidone (NMP). Preferably, the solvent is cyclopentanone. 
         [0082]    Liquid portion  46  comprises from 1% to 30%, preferably from 1% to 20%, by weight of the PVDF-based compound, and from 70% to 99%, preferably from 80% to 99%, by weight of the solvent. Advantageously, the concentration by weight of the solvent is selected to adjust the viscosity of the obtained solution to enable to implement printing techniques. The method of deposition portion  46  may correspond to a so-called additional method, for example, by direct printing of portion  46  at the desired locations, for example, by inkjet printing, photogravure, silk-screening, flexography, spray coating, or drop casting. The method of depositing portion  46  may correspond to a so-called subtractive method, where portion  46  is deposited all over the structure and where the non-used portions are then removed, for example, by photolithography or laser ablation. According to the considered material, the deposition over the entire structure may be performed, for example, by liquid deposition, by cathode sputtering, or by evaporation. Methods such as spin coating, spray coating, heliography, slot-die coating, blade coating, flexography, or silk-screening, may in particular be used. 
         [0083]      FIG. 3F  illustrates a step of irradiating at least a portion of liquid portion  46 , which causes the forming, in the portion, of a PVDF-based film having the desired pyroelectric and/or piezoelectric properties. The UV irradiation is schematically shown in  FIG. 3F  by arrows  48 . The irradiation is carried out by a succession of UV radiation pulses. According to an embodiment, the duration of a UV pulse is in the range from 500 μs to 2 ms. The duration between two successive UV pulses may be in the range from 1 to 5 seconds. The fluence of the (UV) radiation may be in the range from 10 J/cm2 to 25 J/cm2. The number of UV pulses particularly depends on the thickness of portion  46 . As an example, for a 200-nm thickness of portion  46 , the number of UV pulses may be in the range from 1 to 2 with a fluence between 10 J/cm2 and 15 J/cm2 and for a thickness of portion  46  in the order of 4 μm, the number of UV pulses may be in the range from 2 to 6 with a fluence between 17 J/cm2 and 21 J/cm2. 
         [0084]    Advantageously, during the irradiation of portion  46 , first electrode  28  reflects a portion of the UV radiation having crossed portion  46 . This enables to improve the quantity of UV radiation received by portion  46 . The reflection of UV rays is schematically shown in  FIG. 3F  by arrows  50 . 
         [0085]    Advantageously, the solvent of portion  46  at least partly absorbs the UV radiation. This enables to improve the UV-based heating of the compound and favors the forming of the β crystalline phase. The evaporation temperature of the solvent is advantageously higher than 110° C. to avoid too fast an evaporation of the solvent before the forming of the β crystalline phase, which occurs between 120° C. and 130° C. 
         [0086]    Preferably, the irradiation step causes an evaporation of more than 50%, preferably more than 80%, by weight of the solvent of portion  46 . The irradiation step causes the forming of pyroelectric and/or piezoelectric film  30 . 
         [0087]    The inventors have shown that the diffraction diagram of film  30  comprises two peaks representative of two β crystalline phases having different directions. The inventors have further shown that film  30  based on PVDF has a pyroelectric or piezoelectric activity improved over that of a PVDF-based film which would be heated by a heating plate for a duration varying from several minutes to several hours. 
         [0088]      FIG. 3G  shows the structure obtained after having deposited second electrode  32  on film  30  and on a portion of substrate  14 , and second electrode  32  does not come into contact with first electrode  28 . Electrode  32  is for example made of a metallic material selected from the group comprising silver, copper, or a mixture or an alloy of at least two of these materials. According to the considered material, electrode  32  may be deposited by PVD or by printing techniques, for example, by inkjet or by silk screening. In this case, an anneal step may then be provided, for example, by irradiation of the ink deposited by UV pulses having a fluence between 15 J/cm2 and 25 J/cm2. 
         [0089]    A subsequent step of application of an electric field to the structure may be provided. As an example, the electric field may vary between 20 and 80 V/μm and may be applied at a temperature in the range from 70 to 90° C. for from 5 to 10 minutes. 
         [0090]      FIG. 3H  shows the structure obtained after the forming of protection layer  34 . According to the considered material, protection layer  34  may be deposited by chemical vapor deposition (CVD) or by printing techniques, for example, by inkjet printing or by silk screening. In this case, an anneal step may then be provided, for example, by irradiation of the ink deposited by UV pulses having a fluence between 10 J/cm2 and 21 J/cm2. 
         [0091]    The fact of carrying out the steps of heating the materials forming cap  24  and pyroelectric and/or piezoelectric device  18  by UV irradiation advantageously enables to perform a local heating without deteriorating the heat-sensitive molecules. 
         [0092]    Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art.