Method of synthesizing a metal foam, metal foam, uses thereof and device comprising such a metal foam

Abstract

A method of synthesizing a metal foam of at least one metal M having a porous micrometric structure, the method including a step of contact glow discharge electrolysis in an electrolytic plasma reduction conducted in an electrolytic solution in which are immersed an anode and a cathode connected to a continuous electrical power supply, the electrolytic solution including at least one first electrolyte in a solvent, the first electrolyte being the at least one metal M in cationic form, the electrolytic solution further including a gelatine, as well as a metal foam obtained by this method, and a device comprising such a foam.

Claims

1. A metal foam, comprising: at least one metal M having a porous structure and being in the form of an entanglement of multiple strands with a length of between 0.01 m and 100 m, wherein said metal foam is formed by contact glow discharge electrolysis (CGDE) in an electrolytic plasma reduction conducted in an electrolytic solution in which are immersed an anode and a cathode connected to a continuous electrical power supply, wherein the electrolytic solution includes at least one first electrolyte in a solvent, wherein the first electrolyte is said at least one metal M in cationic form, and wherein the electrolytic solution further comprises gelatine.

2. An article of jewellery, comprising the metal foam according to claim 1.

3. A microelectrode, comprising the metal foam according to claim 1.

4. A micro-sensor, comprising the metal foam according to claim 1.

5. A storage device, comprising the metal foam according to claim 1.

6. A catalysis system, comprising the metal foam according to claim 1.

7. An absorbent, comprising the metal foam according to claim 1.

8. A battery component, comprising the metal foam according to claim 1.

9. An energy supply system component, comprising the metal foam according to claim 1.

10. An electronics system, comprising the metal foam according to claim 1.

11. The metal foam according to claim 1, wherein the at least one metal M comprises at least one element selected from transition metals and poor metals.

12. The metal foam according to claim 11, wherein the at least one metal M comprises at least one element selected from nickel, copper, silver, tin, platinum, and gold.

13. The metal foam according to claim 1 having a thickness of between 0.1 mm and 10 mm.

14. The metal foam according to claim 13, wherein the thickness is between 0.3 mm and 5 mm.

15. The metal foam according to claim 14, wherein the thickness is between 0.5 mm and 2 mm.

16. The metal foam according to claim 1 having an apparent density less than or equal to 10% of a theoretical density of the at least one metal M.

17. The metal foam according to claim 16, wherein the apparent density is between 1% and 8% of the theoretical density of the at least one metal M.

18. The metal foam according to claim 17, wherein the apparent density is between 1.5% and 5% of the theoretical density of the at least one metal M.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of the experimental device used during the tests.

(2) FIG. 2 represents the curve of the intensity, noted I (in A), measured as a function of the applied electric voltage, noted U (in V), as obtained experimentally.

(3) FIG. 3 corresponds to an image of a deposition of copper on a cathode as obtained by conventional electrolysis during the application of an electric voltage of 10 V.

(4) FIG. 4 corresponds to an image of a deposition of copper on a cathode as obtained by contact glow discharge electrolysis, during the application of an electric voltage of 25 V.

(5) FIG. 5 represents the curve of temperatures noted T (in C.) as a function of the electric voltage noted U (in V) applied as obtained experimentally, these temperatures being measured at the surface of a thermocouple replacing the cathode of the experimental device represented in FIG. 1.

(6) FIGS. 6A to 6D schematically represent the successive steps of the operating protocol followed to synthesize, by contact glow discharge electrolysis, a copper foam.

(7) FIG. 7 corresponds to an image of the copper foam as obtained at the end of the operating protocol illustrated in FIGS. 6A to 6D.

(8) FIGS. 8A and 8B correspond to microscopic images of the copper foam of FIG. 7, FIG. 8A following observation using a binocular magnifier and FIG. 8B following observation using a scanning electron microscope (SEM).

(9) FIGS. 9A and 9B correspond to images taken using a scanning electron microscope (SEM) of this same copper foam of FIG. 7, FIG. 9A corresponding to the inside of the foam whereas FIG. 9B corresponds to the outside of the foam.

(10) FIGS. 10A and 10B correspond to images of copper foams as synthesized at the surface of a cathode after the implementation of an electrolytic plasma electrolysis method.

(11) FIG. 11 represents the curve of the time of maintaining the gaseous envelope, noted t (in s), as a function of the gelatine concentration, noted [gelatine] (in g/l), of the electrolytic solution considered.

(12) FIGS. 12A and 12B correspond to images of copper foams as synthesized at the surface of a cathode after the implementation of an electrolytic plasma electrolysis method of a duration of 45 and 60 seconds, respectively.

(13) FIG. 13 represents the curve of the intensity, noted I (in A), measured as a function of the applied electric voltage, noted U (in V), as obtained with the implementation of the electrolytic solutions A to E in the electrolytic plasma electrolysis method.

(14) FIG. 14 represents the curve of cathode efficiency, noted R (in %), measured as a function of the gelatine concentration, noted [gelatine] (in g/l), of the electrolytic solution considered, during the implementation of an electrolytic plasma carried out under an applied electric voltage of 30 V, for a time of 10 s.

(15) FIG. 15 groups together images taken at different magnifications using a scanning electron microscope (SEM) of copper foams synthesized with the implementation of the electrolytic solutions A to E in the electrolytic plasma electrolysis method under an applied electric voltage of 25 V, for a time of 10 s.

(16) FIG. 16 corresponds to an energy dispersive analysis spectrum of a metal foam synthesized with the implementation of the electrolytic solution E in the electrolytic plasma electrolysis method under an applied electric voltage of 25 V, for a time of 10 s.

(17) FIGS. 17A and 17B group together images taken by microscope and by scanning electron microscope (SEM) of metal foams synthesized with the implementation of the electrolytic solution E in the electrolytic plasma electrolysis method under two applied electric voltages of 25 V (FIG. 17A) and 35 V (FIG. 17B), for a time of 10 s.

(18) FIGS. 18A, 18B and 18C correspond to images taken using a microscope of three metal foams noted MM1, MM2 and MM3 synthesized with the implementation of the electrolytic solution E in the electrolytic plasma electrolysis method according to the invention, under an applied electric voltage of 25 V, for a time of 15 s.

(19) FIG. 19 corresponds to an image taken by microscope of the metal foam noted MM5 synthesized with the implementation of the electrolytic solution E maintained under stirring in the electrolytic plasma electrolysis method according to the invention, under an applied electric voltage of 25 V, for a time of 15 s, the cathode being moreover rotated.

(20) FIG. 20 corresponds to an image taken by microscope of the metal foam MM5 of FIG. 19, after machining in the form of a cylinder.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(21) Experimental Device

(22) In FIG. 1 is schematically represented the experimental device 10 used during the tests that have been conducted.

(23) This experimental device 10 comprises a beaker 12 of a capacity of 500 ml placed on a magnetic stirrer 14. This beaker 12 comprises an electrolytic solution 16 in which are immersed two electrodes, namely an anode 18 and a cathode 20. The anode 18 and the cathode 20 are respectively connected to the positive and negative poles of a continuous electrical power supply 22, which makes it possible to apply an electric voltage going up to 35 volts. A coulometer (not represented) is also arranged in series between the anode 18 and the cathode 20. Said coulometer makes it possible to measure the amount of electrical charges brought into play during the contact glow discharge electrolysis. A charge-coupled device (CCD) camera (not represented), and which is equipped with an objective arranged inside the beaker 12, makes it possible to visualise in real time the formation of the metal foam on the cathode 20.

(24) The anode 18 is formed of a copper strip of 10 cm length, 5 cm width and 0.2 mm thickness, whereas the cathode 20 is constituted of a tungsten wire, the diameter of which is comprised between 0.4 and 1 mm.

(25) The electrolytic solution 16 comprises at least one first electrolyte in a solvent, the first electrolyte being the metal M in cationic form M.sup.n+. The electrolytic solution 16 further comprises a second electrolyte, constituted of a strong electrolyte.

(26) During the tests carried out, the electrolytic solution 16 is formed of an aqueous solution of copper sulphate at a temperature of 25 C. which comprises: 64 g/l of copper sulphate CuSO.sub.4, 5H.sub.2O as first electrolyte, 80 ml/l of 98% sulphuric acid H.sub.2SO.sub.4 as second electrolyte, and demineralised water as solvent.

(27) The electrolytic solution 16 also comprises gelatine, in this particular case gelatine (CAS 9000708), the concentration of which varies between 0 g/l (this solution then corresponding to a reference electrolytic solution) and 25 g/l.

(28) Copper sulphate is the salt that generates the copper cations Cu.sup.2+ that will be reduced at the cathode 20, as will be seen hereafter. Said copper sulphate is totally ionised in the electrolytic solution 16, thanks in particular to the presence of the second strong electrolyte, in this particular case sulphuric acid. The presence of this sulphuric acid makes it possible to assure good electrical conductivity of the electrolytic solution 16 and moreover favours the corrosion of the copper anode 18.

(29) The stirring of the electrolytic solution 16 may, if need be, be assured by a magnetic bar 26 arranged inside the beaker 12.

(30) In the same way, rotating the cathode 20 may, if need be, be assured using a rotating motor 28 to which the cathode 20 is attached.

(31) Demonstration of the Electrolytic Plasma Electrolysis Method

(32) It will be recalled that the contact glow discharge electrolysis method, also designated electrolytic plasma electrolysis method, is a particular electrolytic method in which a plasma known as electrolytic plasma is localised between a polarised electrode and the electrolytic solution in which the electrodes are immersed.

(33) This electrolytic plasma forms from an electric voltage known as critical electric voltage and noted U.sub.c, following the ionisation of the gas that surrounds the polarised electrode and which is itself formed beforehand during the electrolytic reduction, or oxidation, of the solvent and of some of the compounds ionised in the electrolytic solution.

(34) In this particular case and as will be seen hereafter, the electrolytic plasma, which is localised between the cathode 20 and the electrolytic solution 16, forms following the ionisation of the hydrogen H.sub.2 which surrounds the cathode 20, said hydrogen being itself formed during the electrolytic reduction of the protons H.sub.+ present in the electrolytic solution 16.

(35) Such a contact glow discharge or electrolytic plasma electrolysis method may be demonstrated by establishing a measurement of the intensity I as a function of the applied electric voltage U, I=f(U).

(36) Thus, FIG. 2 illustrates the curve of the intensity I in amperes (A) measured as a function of the electric voltage U in volts (V) applied at the cathode 20, as obtained experimentally with the device 10 described above with the electrolytic solution 16 formed by the aqueous solution of copper sulphate above, at a temperature of 25 C., and in the absence of gelatine, this solution corresponding to the electrolytic solution A, as will be seen later.

(37) In this FIG. 2, it is observed that this curve may be broken down into three parts, respectively designated I, II and III, each of these parts corresponding to a separate process.

(38) The first part of the curve, designated I, corresponds to a conventional electrolysis method (in this particular case, a cathodic reduction), in other words to a purely electrolytic method responding to Ohm's law, in which the intensity increases as a function of the applied electric voltage and does so up to an electric voltage value, known as critical electric voltage and noted U.sub.c, of 25 V. In this range of electric voltages from 0 to 25 V, the surface of the immersed cathode 20 remains constantly wetted by the electrolytic solution 16 and is the seat of the reaction of reduction of copper (4). This reaction of reduction of copper (4) is accompanied, for the highest electric voltages, by the reaction of reduction of water (5) with release of hydrogen, in the form of bubbles that coalesce in the neighborhood of the critical electric voltage U.sub.c.

(39) The corresponding electrolytic reduction reactions are the following:
Cu.sup.2++2e.sup..fwdarw.Cu(4)
2H.sup.++2e.sup..fwdarw.(5).

(40) During this conventional electrolysis method, at the surface of the cathode 20, a deposition of copper is formed which has more and more nodules and an aspect more and more powdery as the applied electric voltage increases. An illustration of such a nodular and powdery deposition is given by FIG. 3, which corresponds to an image of the deposition of copper obtained at the surface of the cathode 20 during the application, to this cathode 20, of an electric voltage of 10 V.

(41) From the critical electric voltage U.sub.c, the intensity decreases sharply as a function of the applied electric voltage and does so up to an electric voltage value of 30 V, value from which the intensity stabilises.

(42) This range of electric voltages, which extends from 25 V to 30 V, corresponds to the second part of the curve, designated II. This drop in the intensity as a function of the applied electric voltage is the consequence of the formation then of the growth, around the cathode 20, of the gaseous envelope, also designated electrolytic plasma. In this part II, the contact glow discharge electrolysis method starts to be established. Since the electrical conductivity of this gaseous envelope is lower than that of the electrolytic solution 16, the intensity drops.

(43) From an electric voltage value of 30 V, which corresponds to the third part designated III of the curve, the gaseous envelope is totally formed and isolates the cathode 20 from the electrolytic solution 16. In this part III, the intensity and the contact glow discharge electrolysis method are stabilised. The intensity, which is then measured, corresponds to the electrical discharges that are created, such electrical discharges being also known as micro-arcs or micro-sparks.

(44) During this process of contact glow discharge electrolysis, at the surface of the cathode 20, a deposition of copper is formed which is in the form of an entanglement of multiple strands. Such an arrangement of metal strands is similar to a foam known as metal foam. An illustration of such a metal foam formed by the deposition of multiple strands of copper is given in FIG. 4, which corresponds to an image of the deposition of copper obtained on the cathode 20 during the application, to this cathode 20, of an electric voltage of 25 V.

(45) This demonstration of the contact glow discharge or electrolytic plasma electrolysis method that has been described may moreover be confirmed by the measurement of the temperature of the surface of the cathode 20 as a function of the electric voltage applied to this cathode 20.

(46) To do so, the experimental device 10 described above has been modified. The cathode 20 has been replaced by a metal thermocouple at the tip of which the electrolytic plasma has been formed. The temperature measurements noted by the thermocouple, during the application of an electric voltage varying from 10 V to 35 V, are reported on the curve represented in FIG. 5. It is pointed out that the measurements are carried out with an uncertainty of +/10 C.

(47) As this curve represented in FIG. 5 shows, the temperature measured by the thermocouple, which thus corresponds to the temperature of the surface of the cathode 20, sharply increases from an applied electric voltage of 30 V. This temperature corresponds to the temperature at which the gaseous envelope, or electrolytic plasma, is totally formed. For an electric voltage of 35 V, the measured temperature reaches 180 C. In the literature, this temperature of 180 C. is designated normal temperature for this type of electrolytic plasma, furthermore also designated cold plasma.

(48) Synthesis of Different Copper Foams

(49) Electrolytic Solutions

(50) Different electrolytic solutions, referenced A to E, have been prepared. The composition of these electrolytic solutions is detailed in table 1 below.

(51) These solutions A to E are aqueous solutions prepared with demineralised water as solvent. The gelatine used, which bears the registration number CAS 9000708, is introduced in the form of powder into the mixture. Said mixture is heated to a temperature of 60 C. so as to enable the complete dissolution of the gelatine in each of the electrolytic solutions B to E.

(52) TABLE-US-00001 TABLE 1 Electrolytic solution A B C D E Copper sulphate 64 64 64 64 64 (g/l) Sulphuric acid 80 80 80 80 80 (ml/l) Gelatine (g/l) 0 1 5 10 25

(53) These electrolytic solutions A to E are going to be successively introduced into the beaker 12 of the experimental device 10 described previously for the implementation of the contact glow discharge electrolysis method.

(54) We will now describe the operating protocol followed for the implementation of said electrolytic plasma electrolysis method.

(55) Operating Protocol

(56) The operating protocol, which is going to be detailed below, is described in relation with the implementation of an electrolytic solution 16, the composition of which corresponds to the composition of the electrolytic solution A as detailed in table 1 above. This electrolytic solution A, which does not comprise gelatine, thus corresponds to a reference electrolytic solution as taught by the publication [1]. It is nevertheless pointed out that this operating protocol is entirely transposable for the implementation of the method of synthesizing metal foam according to the invention, with the electrolytic solutions B to E.

(57) As has been seen previously, the formation and the growth of the gaseous envelope, or electrolytic plasma, occur around the cathode from the application of the critical electric voltage U.sub.c, which is 25 V in the present case.

(58) Reference may be made to FIGS. 6A to 6D, which schematically illustrate the successive steps of the operating protocol that has been followed to synthesize copper metal foams.

(59) As represented in FIG. 6A, the anode 18 and cathode 20 of the experimental device 10 are immersed in the electrolytic solution 16. The anode 18 is connected to the positive pole of the electrical power supply 22 and the cathode 20 is, for its part, connected to the negative pole of this electrical power supply 22. As represented in FIG. 6A, the electrical power supply 22 does not deliver, at this precise moment, any electric voltage (U=0 V).

(60) It is pointed out that, unless expressly indicated to the contrary, the following tests have been carried out in the absence of stirring of the electrolytic solution 16 and without rotating the cathode 20.

(61) The electrical power supply 22 is then adjusted to an electric voltage comprised between 25 V and 35 V, then started to assure the formation and the growth of the electrolytic plasma 24 (FIG. 6B). In the case illustrated in FIGS. 6B and 6C, the electric voltage delivered by the electrical power supply 22 is set at U=25 V.

(62) As soon as an electric voltage of 25 V is applied, the electrolytic plasma 24 forms then grows around the cathode 20. Electric micro-arcs form and evolve from the surface of the cathode 20 towards the interface situated between the electrolytic plasma 24 and the electrolytic solution 16. Since these electric micro-arcs are composed of negative charges, the copper cations Cu.sup.2+ present in the electrolytic solution 16 are reduced at the end of each electric micro-arc, according to the aforementioned electrolytic reduction reaction (4). This reduction of the copper cations Cu.sup.2+ creates, at the surface of the cathode 20, a copper foam 30 constituted of an entanglement of a multitude of copper strands, said strands representing the negative of the micro-arcs formed during the contact glow discharge electrolysis (CGDE) or electrolytic plasma electrolysis method. The formation and the growth of this metal foam 30 are illustrated in FIGS. 6B and 6C. The growth of this metal foam 30 continues as long as the electrolytic plasma 24 is maintained.

(63) After ten or so seconds of functioning of the electrical power supply 22, the cathode 20 is withdrawn from the electrolytic solution 16 (in this particular case the electrolytic solution A), while remaining under the applied electric voltage of 25 V. In fact, to conserve the integrity of the copper metal foam 30 formed on the surface of the cathode 20, it is preferable to cut off the electric voltage delivered by the electrical power supply 22 after the cathode 20 is taken out of the electrolytic solution 16, as represented in FIG. 6D. In such conditions, the metal foam 30 formed at the surface of the cathode 20 is completely dry. Although it is relatively fragile, this copper foam 30 has nevertheless sufficient mechanical strength which makes it possible to detach it from the cathode 20, by pushing it using a small brush.

(64) The image of the copper metal foam 30 thereby obtained with the electrolytic solution A is reported in FIG. 7. In this FIG. 7, it may be observed that said copper foam 30 has an alveolar structure resulting from the entanglement of numerous strands of copper formed during the electrolytic plasma electrolysis method.

(65) To confirm the spatial arrangement of the metal foam 30, microscopic images of said metal foam 30 have been taken. FIG. 8A is an image resulting from observation using a binocular magnifier whereas FIG. 8B is an image resulting from observation using a scanning electron microscope (SEM). It may be observed that the copper strands are very thin, with a dimension of the order of micrometer, and that they have a molten aspect that is unusual for structures that are obtained with the implementation of a conventional electrolysis method, as illustrated in FIG. 3.

(66) Referring to FIGS. 9A and 9B, which correspond to images taken using a scanning electron microscope (SEM), respectively of the inside and of the outside of the metal foam 30, it may be noted that this copper metal foam 30 has a nanornetric structure in its core and at its edges, which is identical in terms of structure and density. The porosity is, consequently, constant throughout the thickness of the deposition of metal foam 30 obtained by the implementation of the electrolytic plasma electrolysis method.

(67) Tests Carried Out and Results Obtained

(68) The operating protocol described previously has been reproduced with the electrolytic solutions B to E of table 1 above.

(69) Effects of Gelatine on the Form and the Volume of the Metal Foams

(70) As has been seen, the metal foam obtained with the electrolytic solution A has a porous micrometric and homogenous structure throughout its thickness. However, as may be seen in the images of FIGS. 4 and 7, the fact remains that the metal foam synthesized with the electrolytic solution A has a general form which is systematically irregular.

(71) On the contrary, the metal foams synthesized from electrolytic solutions B to E have, for their part, not only these same characteristics of porous micrometric and homogenous structure throughout their thickness, but also a particularly regular general form.

(72) To visualise the important difference residing in the general forms of the synthesized metal foams, reference may be made to FIGS. 10A and 10B.

(73) These FIGS. 10A and 10B correspond to images of copper foams as obtained at the surface of a cathode after the implementation of an electrolytic plasma electrolysis method in identical operating conditions, namely under an electric voltage of 25 V maintained for 5 seconds. More precisely, the image of FIG. 10A corresponds to the copper foam obtained with the implementation of the electrolytic solution A whereas the image of FIG. 10B corresponds to the copper foam obtained with the implementation of the electrolytic solution E.

(74) It is clear from a comparison of these two images of FIGS. 10A and 10B that the general form of the metal foam synthesized according to the method of the invention is regular (FIG. 10B) and does not have zones of fragility or fracture, unlike the metal foam of the image of FIG. 10A.

(75) Furthermore, as also indicated above, the metal foam synthesized from the electrolytic solution A is relatively fragile. Although can be detached, by proceeding delicately using a small brush, from the cathode at the surface of which it has formed, it absolutely cannot be envisaged to subject said metal foam to a subsequent step of shaping, for example, by machining. This fragility is in particular linked to its general irregular form, as appears clearly in the image of FIG. 10A.

(76) This irregularity of form, which characterises the metal foam obtained with the implementation of the electrolytic solution A, is brought about by the deformation of the gaseous envelope (or electrolytic plasma) during the contact glow discharge electrolysis method and, more particularly, during the growth of the metal foam. In fact, observation, using a CCD camera, of the phenomena occurring at the level of the cathode 20 during the contact glow discharge electrolysis shows that the formation of the metal foam occurs rapidly, in ten or so seconds, and in a quite violent manner: the gaseous envelope does not seem to have a sufficient robustness to withstand the forces created by the growth of this metal foam and is not able, consequently, to conserve a regular form around the cathode 20. It is even observed that after 15 seconds, this gaseous envelope breaks, thereby leading to the interruption of the electrolytic plasma as well as the total destruction of the metal foam formed.

(77) It is thus noted that the synthesis of metal foam by contact glow discharge electrolysis as taught by the publication [1] does not make it possible to obtain a foam having a regular form but is also limited in terms of volume of metal foam obtained. In fact, in the absence of gelatine, the thickness of metal foam capable of being obtained is at the most of the order of 0.5 mm, on account of the destruction of this metal foam if the duration of the electrolytic plasma exceeds 15 seconds.

(78) On the contrary, the implementation of an electrolytic solution comprising gelatine makes it possible to obtain a gaseous envelope which, as the CCD camera images show, remains uniform around the cathode during the growth of the metal foam and does so for a duration which is clearly longer than 15 seconds.

(79) FIG. 11 clearly illustrates this phenomenon. In this FIG. 11 are reported the times at the end of which occurs the rupture of the gaseous envelope, which is formed during the electrolytic plasma electrolysis method generated under an electric voltage of 25 V, as a function of the gelatine concentration of the electrolytic solutions A to E implemented.

(80) As FIG. 11 shows, the higher the gelatine concentration in the electrolytic solution, the greater the resistance of the gaseous envelope and, consequently, the greater the time of maintaining the electrolytic plasma.

(81) As has been seen previously, if the electrolytic plasma breaks at the end of 15 seconds with the implementation of the electrolytic solution A, this electrolytic plasma may be maintained for a time of two minutes with a concentration of only 1 g/l of gelatine in the electrolytic solution B, or even attain 4 minutes with concentrations of 10 g/l and 25 g/l of gelatine (electrolytic solutions D and E).

(82) It is not nevertheless imperative to maintain the electrolytic plasma over times longer than 1 minute to obtain a sufficient volume of metal foam. In fact, after only 45 seconds of electrolytic plasma implemented with the electrolytic solutions B to E, one obtains, at the surface of the cathode, depositions of metal foams which have a thickness of the order of 3 to 4 mm, thickness which is well above that of depositions of metal foam that it is possible to obtain with gelatine-free electrolytic solutions.

(83) In fact, beyond one minute, the metal foams take a general form that is more irregular, as the image of FIG. 12B shows.

(84) FIGS. 12A and 12B correspond to images of copper foams as obtained at the surface of the cathode after the implementation of the electrolytic solution E in the method according to the invention. The electrolytic plasma, generated under an electric voltage of 25 V, has been maintained for 45 seconds in the case of FIG. 12A, and for 60 seconds in the case of FIG. 12B.

(85) Effects of Gelatine on the Curve of the Intensity as a Function of the Applied Electric Voltage

(86) To evaluate the effects of gelatine on the curve of the intensity as a function of the applied electric voltage, reference will be made to FIG. 13.

(87) This FIG. 13 represents the curve of the intensity, noted I (in A), measured as a function of the applied electric voltage, noted U (in V), as obtained with the implementation of the electrolytic solutions A to E, at a temperature of 25 C., in the electrolytic plasma electrolysis method.

(88) It is observed that, the higher the gelatine concentration, the lower the intensity measured in the first part (Ohmic part) of the curve, which corresponds to electric voltages below the critical electric voltage U.sub.c and for which one is in the presence of a conventional electrolysis. In fact, the higher the gelatine concentration in the electrolytic solution, the greater the electrical resistance thereof. This increase in the electrical resistance limits electron transfer and, consequently, the amount of electrical charges exchanged during electrochemical reactions.

(89) It may also be observed that the value of critical electric voltage U.sub.c from which the gaseous envelope begins to form around the cathode, which is 25 V with the implementation of the gelatine-free electrolytic solution A, attains a value of 20 V with the implementation of the electrolytic solutions B, D and E that comprise gelatine.

(90) It will be recalled that the contact glow discharge electrolysis stabilises when the gaseous envelope is totally formed around the cathode, such a stabilisation occurring when the applied electric voltage value is situated in a range of electric voltages in which the intensity is substantially constant as a function of said electric voltage.

(91) Yet, it is moreover observed that this contact glow discharge electrolysis stabilises at a lower electric voltage value when the electrolytic solution comprises gelatine and, in particular, from a value of 25 V with the implementation of the electrolytic solutions C, D and E.

(92) Without the following tying down the inventor, the hypothesis retained to explain the aforementioned observations is that the gelatine could make it possible to better contain, at the surface of the cathode, the gaseous envelope formed by the release of hydrogen from the electrolytic reduction of the protons contained in the electrolytic solution, while reducing the solubility of said gaseous envelope in said electrolytic solution. Thus, when the electrolytic solution comprises gelatine, the gaseous envelope is entirely created at a lower electric voltage than when the electrolytic solution does not contain same and thus enables an ionisation of the gas and thus the complete formation of the electrolytic plasma at an electric voltage that is also lower.

(93) It may finally be pointed out that FIG. 13 shows that, whatever the gelatine concentration in the electrolytic solution, the value of the intensity measured in the third part of the curve corresponding to the stabilisation of the contact glow discharge electrolysis method is constant and of the order of 0.5 A, as in the case of an electrolytic solution not comprising gelatine. This reflects the fact that the amount of electrical charges within the electrolytic plasma is constant.

(94) Effects of Gelatine on the Reduction of Copper

(95) To determine the effects of gelatine on the reduction of copper, the cathode efficiency R has been determined, defined as being the ratio between the mass of copper effectively deposited on the cathode (noted m.sub.Cu deposited) and the theoretical mass of copper which should have been deposited on said cathode, if the totality of the electrical charges, brought into play during the electrolytic plasma electrolysis method, had been used for the electrolytic reduction of copper (noted m.sub.Cu theoretical), according to the following formula (6):

(96) $\begin{array}{cc}R=\frac{m_{\mathrm{Cu}\mathrm{deposited}}}{m_{\mathrm{Cu}\mathrm{theorectical}}}& \left(6\right)\end{array}$

(97) This cathode efficiency R has been calculated for each of the metal foams obtained at the end of the implementation of an electrolytic plasma electrolysis method carried out under an applied electric voltage of 30 V, for a time of 10 s, with each of the electrolytic solutions B to E.

(98) To determine the theoretical mass of copper m.sub.Cu theoretical, Faraday's first law is used, which postulates that 96 485 C are required to reduce 1 gram-equivalent of metal, gram-equivalent being the ratio between the mass number of said metal and the number of electrons exchanged to reduce one atom of said metal. The theoretical mass of copper m.sub.Cu theoretical as a function of the amount of electricity that has flowed through the circuit is given by the following formula (7):

(99) $\begin{array}{cc}{m}_{\mathrm{Cu}\mathrm{theoretical}}=\frac{QM}{Fn}& \left(7\right)\end{array}$
with: Q=amount of electricity (in C) as measured F=Faraday constant, i.e. 96 485 C/mol M=mass number of copper (in g/mol), i.e. 63,546 g/mol n=the number of electrons brought into play during the electrolytic reduction reaction (4) of copper as described above, i.e. n=2.

(100) The determination of the total amount of electrical charges brought into play during the electrolytic plasma electrolysis method, which corresponds to the amount of electricity noted Q, has been made by a coulornetric measurement carried out using an EGG PARC type coulometer.

(101) In table 3 below, are reported the values: of the deposited mass of copper m.sub.Cu deposited (in g) measured at the end of the tests, of the amount of electricity Q (in C) measured throughout the duration of the electrolytic plasma generated during the tests, of the theoretical mass of copper m.sub.Cu theoretical (in g) calculated by the application of the aforementioned formula (7), and of the cathode efficiency R (in %) calculated by the application of the aforementioned formula (6).

(102) TABLE-US-00002 TABLE 3 Electrolytic m.sub.Cu deposited m.sub.Cu theoretical solution (in g) Q (in C) (in g) R (in %) B 0.00034 7.386 0.00243 13.98 C 0.00048 7.272 0.00239 20.04 D 0.00060 7.590 0.00250 24.01 E 0.00062 7.166 0.00240 26.27

(103) Appended FIG. 14 represents the curve corresponding to the cathode efficiency R thereby calculated as a function of the gelatine concentration, noted [gelatine], of these electrolytic solutions B to E.

(104) It may be noted that this cathode efficiency R increases with the gelatine concentration in the electrolytic solution. Thus, for comparable amounts of electricity Q, the proportion of electrical charges dedicated to the electrolytic reduction of the copper cations Cu.sup.2+ increases with the gelatine concentration of the electrolytic solution until tending towards a levelling off for a gelatine concentration of the order of 10 g/l.

(105) Although it has previously been shown than the increase of the gelatine concentration causes the increase of the electrical resistance of the electrolytic solution, the electrical resistance thus becoming theoretically less favourable to the electrolytic reduction of the cations Cu.sup.2+, the appearance of the curve of FIG. 14 tends to prove that the increase in the cathode efficiency R as a function of said gelatine concentration is directly linked to an increase in the electrical density in the gaseous envelope or electrolytic plasma. Thus, the more the gelatine concentration increases, the less the gaseous envelope generated at the cathode dissolves in the electrolytic solution. In these conditions, the gas pressure inside the gaseous envelope increases, leading to a more energetic electrolytic plasma, which thus has a higher electrical density.

(106) Effects of Gelatine on the Structure of the Metal Foams

(107) Different metal foams have been obtained from the implementation of the electrolytic solutions B to E in the electrolytic plasma electrolysis method under an applied electric voltage of 25 V, for a time of 10 s.

(108) To characterise each of these metal foams, images thereof have been taken at different magnifications (2 000, 10 000 and 70 000) using a scanning electron microscope (SEM), referenced ESM LMTDIMEB002.

(109) As the images of FIG. 15 show, the increase of the gelatine concentration in the electrolytic solution makes it possible to refine the structure of the metal foam.

(110) As an illustration, it is observed that the metal foam obtained with the electrolytic solution B comprising 1 g/l of gelatine is formed of copper strands having a dimension comprised between 500 nm and 1 000 nm whereas the metal foam obtained with the electrolytic solution E comprising 25 g/l of gelatine is formed of copper strands, the dimension of which is only of the order of 100 nm.

(111) Furthermore, the increase of the gelatine concentration in the electrolytic solution brings about the increase of the number of metal strands as well as the reduction in the size of the interstices between the metal strands.

(112) Finally, the images obtained at high magnification of 70 000 show that the metal strands of the foams obtained with the electrolytic solutions B to D comprise small nodules that have a structure similar to that of depositions of copper obtained with the conventional electrolysis method. It is nevertheless important to note that the presence of such nodules decreases as the gelatine concentration in the electrolytic solution increases to totally disappear at a concentration of 25 g/l (electrolytic solution E). This finding also seems to confirm that the higher the gelatine concentration in the electrolytic solution, the more rapid and intense the reaction of electrolytic reduction of the cations Cu.sup.2+, which takes place at the interface situated between the gaseous envelope and the electrolytic solution, in all likelihood due to a more and more energetic electrolytic plasma.

(113) In addition to characterisation using the scanning electron microscope, the metal foam obtained with the implementation of the electrolytic solution E in the electrolytic plasma electrolysis method under an applied electric voltage of 25 V, for a time of 10 s, has been analysed using an energy dispersive X-ray spectrometry (EDX), referenced ESM LMTPCMEB001 in order to determine the chemical composition of said metal foam.

(114) The spectrum obtained, reported in FIG. 16, show that the majority element of the metal foam remains copper, with a concentration of the order of 80% atomic. The only other two elements also present are oxygen and sulphur, which come from the other compounds present in the electrolytic solution (CuSO.sub.4, H.sub.2SO.sub.4 and H.sub.2O), with concentrations of the order of 15% atomic and 5% atomic, respectively.

(115) This finding is the same, whatever the gelatine concentration present in the electrolytic solution B to E considered.

(116) Effects of the Applied Electric Voltage on the Structure of the Metal Foams

(117) If reference is made to FIG. 13, it may be observed that, for a gelatine concentration of 25 g/l in the electrolytic solution E, the curve of the intensity I as a function of the electric voltage U applied shows that it is possible to stabilise an electrolytic plasma from an applied electric voltage of 25 V (see FIG. 13).

(118) To determine the effect that the applied electric voltage may have on the structure of the metal foams, two metal foams have been synthesized from the implementation of this electrolytic solution E in the electrolytic plasma electrolysis method, for a time of 10 s, under two separate electric voltages applied, one of 25 V and the other of 30 V.

(119) The images of the metal foams obtained by microscope and by scanning electron microscope (SEM) show that the variation of the applied electric voltage has an incidence on the structure of the metal foam. The copper foam obtained with an applied electric voltage of 35 V (FIG. 17B) has a less regular structure, characterised by the presence of cracks, than the copper foam obtained with an applied electric voltage of 25 V (FIG. 17A). In addition, the metal strands of this copper foam obtained with an applied electric voltage of 35 V are thicker than those of the copper foam obtained with an applied electric voltage of 25 V. It thus appears that the higher the applied electric voltage, the more micro-arcs are generated in the electrolytic plasma.

(120) Evaluation of the Apparent Density of Metal Foams

(121) First Series of Metal Foams MM1 to MM3

(122) Measurements aiming to determine the mass (noted m and expressed in g) and the apparent volume (noted V and expressed in cm.sup.3) have been carried out to make it possible to calculate the apparent density (noted p and expressed in g/cm.sup.3) of three copper foams MM1, MM2 and MM3 synthesized successively from the electrolytic solution E, according to the synthesis method according to the invention conducted under an applied electric voltage of 25 V and for a time of 15 s, in the absence of stirring of the electrolytic solution E and in the absence of rotation of the cathode.

(123) The apparent density is calculated from the following formula (8):

(124) $\begin{array}{cc}=\frac{m}{V}& \left(8\right)\end{array}$

(125) The microscopic images of these three metal foams MM1, MM2 and MM3 are reported in appended FIGS. 18A to 18C.

(126) It is observed that these three metal foams MM1, MM2 and MM3 indeed have a same porous micrometric structure.

(127) The noted values of mass m, apparent volume V and apparent density calculated for the three metal foams evaluated are reported in table 4 below.

(128) In table 4 are also shown the values of apparent density calculated in percentage, relating to the theoretical density of copper, which is 8.92 g/cm.sup.3 at 20 C.

(129) It should however be pointed out that if the metal foams obtained by the implementation of the method according to the invention have a homogenous and regular structure, they are not nevertheless perfectly spherical, as may be observed in the images of these FIG. 18A 18C. Thus, the determination of the apparent volume V and, consequently, the apparent density is tainted by an uncertainty that is mentioned in table 4 below.

(130) TABLE-US-00003 TABLE 4 Metal foam MM1 MM2 MM3 Electrolytic solution E E E Figure 18A 18B 18C m (in g) 862 1240 204 V (in Cm.sup.3) 0.00362 0.00413 0.00045 (in g/cm.sup.3) 0.238 +/ 0.026 0.3 +/ 0.073 0.451 +/ 0.097 (in %) 2.66 3.35 5.06

(131) Whatever the case, the apparent density percentage values as calculated and reported in table 4 give an order of magnitude of the apparent densities of the metal foams that it is possible to obtain with the method according to the invention.

(132) Such apparent density percentage values of the metal foams capable of being obtained and obtained by the method of the invention are at the most of the order of 10%.

(133) Second Series of Metal Foams MM4 to MM7

(134) Similar measurements aiming to determine the mass (noted m and expressed in g) and the apparent volume (noted V and expressed in cm.sup.3) have been carried out to make it possible to calculate the apparent density (noted and expressed in g/cm.sup.3) of four copper foams MM4, MM5, MM6 and MM7 synthesized successively from the electrolytic solution E, according to the synthesis method according to the invention conducted under an applied electric voltage of 25 V and for a time of 15 s.

(135) In this second series of tests, and unlike the preceding, the electrolytic solution E is maintained under stirring at an angular velocity of 220 rpm, whereas the cathode is maintained in rotation at an angular velocity of 300 rpm, throughout the synthesis.

(136) The image of FIG. 19 corresponds to the metal foam MM5.

(137) The values of mass m noted as well as the values of apparent volume V, apparent density in g/cm.sup.3 and in % calculated as above for the four metal foams are reported in table 5 below.

(138) TABLE-US-00004 TABLE 5 Metal foam MM4 MM5 MM6 MM7 Electrlytic E E E E solution Figure 19 m (in g) 870 560 890 760 V (in Cm.sup.3) 0.0245 0.0253 0.0237 0.0189 (in g/cm.sup.3) 0.035 0.022 0.038 0.040 (in %) 0.39 0.25 0.43 0.45

(139) The observation of the image of FIG. 19, which corresponds to the foam MM5, shows that, when the synthesis method according to the invention is implemented with a rotating cathode, the metal foam obtained, in addition to having a homogenous and regular structure, is also of regular shape.

(140) Moreover, and in a completely surprising manner, the rotation of the cathode makes it possible to attain apparent density values practically ten times lower than those obtained and reported in table 4.

(141) Despite this particularly low apparent density value, the metal foam MM5 has been able to be detached easily from the cathode, machined in the form of a cylinder and handled using a micro-sucker in the absence of any degradation thereof, as illustrated in the image of FIG. 20.

BIBLIOGRAPHY

(142) [1] C. Zhou et al., Electrochemistry Communications, 2012, 18, pages 33-36 [2] K. Azurni et al., Electrochimica Acta, 2007, 52, pages 4463-4470 [3] R. Wthrich et al., Electrochimica Acta, 2010, 55, pages 8189-8196 [4] T. A. Kareem et al., Ionics, 2012, 18, pages 315-327 [5] O, Takai, Pure Appl. Chem., 2008, 80(9), pages 2003-2011