Patent Application
20120092758
The present invention relates in general to a nanocomposite comprising one-dimensional nanomaterials and to applications, especially for protecting against electromagnetic waves.
The present invention relates in particular to a particular nanocomposite, which comprises one-dimensional nanomaterials filled and/or covered with at least one nanooxide of at least one transition metal, to the method for preparing it, and to an optical limiting device comprising such a nanocomposite suspended in a medium which is transparent to visible and infrared radiation. Such an optical limiting device is highly advantageous for applications for protecting against electromagnetic waves ranging from the visible to the medium infrared range.
In the description below, the references in parentheses ([]) refer to the list of references presented after the examples.
At the present time, the known protection systems based on nanoparticles in suspension cannot be used in the infrared band III.
Among these protection systems, mention may be made especially of systems formed from a suspension of carbon black. Such devices give good results in terms of protection against luminous fluxes with wavelengths ranging from the visible range to the near infrared, but their use poses many problems, and especially that of keeping the carbon particles in suspension. In addition, such systems are not at all suited to limitation in the infrared range (in particular the medium infrared) since water is a totally absorbent liquid and thus not transparent in the infrared.
Systems formed from filters also exist, which have the drawback of being wavelength-selective.
Systems based on organic pigments also exist, which are expensive materials, synthesized with solvents of substantial toxicity.
Finally, optical limiting devices for the visible or near infrared range also exist, comprising carbon nanotubes suspended in water or chloroform (French Patent FR 2 787 203 [1] or “Single-wall carbon nanotubes for optical limiting” by L. Vivien, E. Anglaret, D. Riehl, F. Bacou, C. Journet, C. Goze, M. Andrieux, M. Brunet, F. Lafonte, P. Bernier and F. Hache, in “Chemicals Physics Letters” 307 (1999), 317-319 [2], or “Optical limiting properties of single-wall carbon nanotubes” by L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonte, C. Journet, C. Goze, M. Brunet, and P. Bernier, from “Optical Communications” 174 (2000) 271-275 [3], or “Optical limiting properties of carbon nanotubes” in L. Vivien, D. Riehl, F. Hache, and E. Anglaret, in “Physical B” B 323 (2002) 233-234 [4]).
The devices described in patent [1] and the scientific publications [2] to [4] have the drawback of lacking stability in suspension, which limits their long-term use under real conditions of protection against an electromagnetic wave, unless they are functionalized by complex and expensive techniques.
There is thus a real need for a material that is stable in suspension in a medium that is transparent to visible and infrared radiation (for example chloroform), for producing an optical limiting device that overcomes the drawbacks of the prior art, and that can especially constitute a device for protecting against electro-magnetic waves ranging from the visible to the medium infrared range.
The aim of the present invention is, precisely, to satisfy this need by providing a nanocomposite, characterized in that it comprises single-wall and/or multi-wall one-dimensional nanomaterials, and at least one nanooxide of at least one transition metal, said nanooxide filling said nanotubes and/or covering their walls.
Generally, a material is termed as one-dimensional when one of the dimensions is markedly larger (at least 10 times) than the other two.
The term “one-dimensional nanomaterial” is used when these two smallest dimensions are on the nanometric scale (dimensions of between one and a few hundred nanometres, i.e. generally on the submicron scale).
The term “single-wall one-dimensional nanomaterials” means, in the present invention, nanomaterials formed from a single leaflet of atoms rolled up on itself.
The term “multi-wall one-dimensional nanomaterials” means, in the present invention, nanomaterials formed from several leaflets of atoms rolled up on themselves to form concentric cylinders (of Russian doll type) or rolled up in a spiral (of parchment type).
Advantageously, the one-dimensional nanomaterials may be formed from a heat-conducting and/or electrically conducting and/or ion-conducting material.
Advantageously, the one-dimensional nanomaterials are formed from carbon or from a transition metal oxide. However, they may also be formed from transition metal carbides, sulfides, nitrides or borides.
The one-dimensional nanomaterials that may be used to prepare a nanocomposite according to the invention may comprise, for example, carbon nanotubes (CNTs), carbon nanofibres, titanium oxide TiO2 nanofibres and titanium oxide TiO2 (or titanate) nanotubes. The transition metal nanooxides that may be used to prepare the nanocomposites according to the invention may comprise, for example, niobium pentoxide Nb2O5, titanium dioxide TiO2, tungsten trioxide WO3, iron oxide Fe2O3, vanadium oxide VO2, zinc oxide ZnO or magnesium oxide MgO. A preferred nanooxide is niobium pentoxide Nb2O5.
The preferred nanocomposites that are according to the invention are the following:
A subject of the present invention is also an optical limiting device comprising a nanocomposite according to the invention, which is suspended in a medium that is transparent to visible and infrared radiation. Such an optical limiting device constitutes an ideal device for protecting against electromagnetic waves with wavelengths ranging from the visible to the medium infrared range (medium IR: band III, 8-12 μm).
Advantageously, the medium that is transparent to visible and infrared radiation is a liquid medium, such as chloroform (CHCl3) or carbon disulfide (CS2).
Chloroform will preferably be used as dispersant, due to the stability of the suspension of nanocomposite in this medium. Furthermore, the toxicity of chloroform is much lower than that of carbon disulfide.
The optical limiting device according to the invention formed by a nanocomposite according to the invention in suspension in chloroform in fact leads to a system that has noteworthy non-linear properties if it is subjected to stimulation of laser aggression type. The use of such a device leads to optical limiting that is ideal for protecting a sensor.
To increase the stability of the suspension of nanocomposite according to the invention in the liquid medium that is transparent to visible and infrared radiation, said medium may comprise one or more surfactants or dispersants, such as polyvinyl acetate (PVA) or sodium dodecyl sulfate.
Besides the liquid media that are transparent to visible and infrared radiation, it is also possible to use as medium that is transparent to visible and infrared radiation a medium that is not liquid, for instance liquid crystals or solid thin films.
A subject of the present invention is also a process for preparing a nanocomposite according to the invention, characterized in that it comprises the following steps:
a) either a step of formation via the sol-gel route of a gel from a mixture:
b) or a step of impregnating a salt containing at least one transition metal precursor;
c) optionally, a step of milling the gel obtained in step a); and
d) a step of calcination of said optionally milled gel obtained in step c) at a temperature and for a time of crystallization of the transition metal nanooxide inside the nanomaterials and/or on their walls, while conserving the one-dimensional nature of the nano-materials.
In step a) of formation of the amorphous gel, niobium ethoxide Nb(OEt)5 is advantageously used as transition metal alkoxide in the process of the invention, when it is desired to produce nanocomposites comprising niobium pentoxide (in the present case carbon nanofibres or titanium dioxide nanotubes comprising niobium pentoxide) or titanium tetraisopropoxide Ti(OiPr)4 when it is desired to prepare nanocomposites comprising titanium dioxide as nanooxides (in the present case carbon nanotubes comprising titanium dioxide).
In the impregnation step b), ammonium paratung-state (NH4)10W12O41 is advantageously used as salt when it is desired to prepare nanocomposites comprising tungsten trioxide (in the present case TiO2 nanofibres or TiO2 nanotubes or titanates comprising tungsten trioxide).
Advantageously, the process of the invention comprises, prior to the formation (a) of the amorphous gel or the impregnation (b), a step of ultrasonication treatment of the one-dimensional nanomaterials. This additional step makes it possible to depill (or more generally to deaggregate) the one-dimensional nanomaterials, which are generally in the form of bundles, in particular in the case of carbon nanotubes or TiO2 (or titanate) nanotubes.
Advantageously, the process of the invention comprises, subsequent to the step of formation (a) of the amorphous gel or of impregnation (b), a step of heat treatment at a temperature that allows the excess alcohol or water to be removed.
Advantageously, in the process of the invention, the calcination temperature is between 150° C. and 550° C., as a function of the nature of the one-dimensional nanomaterials and nanooxides used. Thus, in the case of carbon nanotubes covered and/or filled with niobium pentoxide, the calcination temperature is between 450° C. and 530° C., and is preferably about 520° C. For TiO2 nanotubes that are also covered and/or filled with niobium pentoxide, the calcination temperature is between 150° C. and 550° C., and preferably about 350° C.
Finally, a subject of the present invention is the use of an optical limiting device according to the invention for protecting an optical and/or optronic device against electromagnetic waves with wavelengths ranging from the visible to the medium infrared range (in particular in the infrared band III), and in particular an optical switch. Such a limiting device then plays the essential role of switch.
In fact an optical system not bearing a switch can be damaged when the optical flux is higher than the threshold for damaging the various components. In contrast, when the optical switch is incorporated into the optical line, it will clip the high fluxes, which will have the effect of protecting the optical components located downstream of the line. A good optical limiter will have rapid “switching” and a low switching threshold, which is reflected physically by high non-linearity.
Placing particles of the nanocomposite according to the invention in suspension in a solvent makes it possible to produce the switching function while limiting the transmission of the incident beam for high fluxes.
Other advantages may also emerge to those skilled in the art on reading the examples below, which are illustrated by the attached figures, given for illustrative purposes.
15
absolute ethanol: FLUKA product sold by the company Sigma-Aldrich,
acetic acid: FLUKA product sold by the company Sigma-Aldrich,
multi-leaflet carbon nanotubes: sold by Pyrograf Products Inc. under the trade name Pyrograf®-III Carbon Nanotube,
niobium ethoxide (Nb(OEt)5) at 99.95%: sold by the company Sigma-Aldrich under the trade name Niobium (V) ethoxide.
To study the non-linear behaviour of the examples of nanocomposites according to the invention, the Z-SCAN method is used, the operating principle of which is illustrated in
The principle of this method is as follows:
If sample 1 has non-linear behaviour, the energy collected by the detector is minimal when the power density received by the sample is maximal, i.e. when the sample is placed at the focal point of the lens.
To study the change in the switching rate of the examples of nanocomposites according to the invention, the pump-probe method is used, the operating principle of which is illustrated in
This method makes it possible simultaneously to measure rapid phenomena such as a drop in transmission, and slow phenomena such as the relaxation of the system and the return to the initial state thereof.
It consists in superposing two laser beams of different wavelengths on the sample which are arranged as follows:
A modification of the properties of the medium (via the formation of a thermal lens, by non-linear scattering) will have an effect on the trajectory of the probe beam.
The probe radiation is collected by an optical fibre connected to a photodiode. The transmission drop is visualized on an oscilloscope. To discern the various phenomena associated with different time regimes, it suffices to change the time basis of the oscilloscope.
A nanocomposite according to the invention based on commercial powders of multi-leaflet carbon nanotubes (CNTs) and niobium pentoxide (Nb2O5/CNT powders) is prepared via the sol-gel route.
The sol-gel method used in the context of the present invention is based on the hydrolysis and condensation of niobium alkoxides or chlorides in an alcoholic solution in acidic medium. This method, known as “mild chemistry” (on account of the absence of use of pollutant solvents and of harsh experimental conditions), makes it possible to obtain solid particles of nanometric size.
Depilling/Deaggregation of the Bundles of CNT tubes
20 ml of absolute ethanol are mixed with 20 ml of acetic acid (these products are used without additional purification). The carbon nanotubes are added to the mixture and ultrasonicated for one hour so as to depell the bundles of carbon nanotubes. The solution obtained is black.
1 ml of niobium ethoxide is added to the preceding solution, and the mixture thus obtained is ultrasonicated for one hour. Next, the whole is diluted with 20 ml of ethanol and the mixture is then left in the open air (for maturation) all day and all night so as to allow the formation of a sol, the evaporation of the solvent and the formation of the gel. A “dry” gel is then obtained, which is placed in an oven for 15 hours at 110° C. to remove the excess solvent.
The gel obtained is milled manually in a mortar using a pestle, and a black powder is obtained.
After milling the gel, the powder obtained is placed in a calcination oven so as to obtain crystallization of the niobium pentoxide. The heat treatment that is performed therein is a calcination in air comprising a temperature increase ramp at 10° C/minute up to a temperature of 520° C. (increase over a period of about 50 minutes), and then maintenance at this temperature for 2 hours 45 minutes. At the end of calcination, a grey powder formed from carbon nanotubes covered and/or filled uniformly with niobium pentoxide nanoparticles is obtained.
The calcination temperature adopted is a compromise between a temperature that limits the degradation of the nanotubes and a temperature that allows excellent crystallization of the niobium pentoxide, which is about 550° C. Calcination in air performed at this temperature of 520° C. makes it possible to produce, from the starting materials such as commercial multi-leaflet carbon nanotubes and niobium ethoxide, a powder formed from carbon nanotubes that are slightly degraded in terms of one-dimensional morphology but which are covered and filled uniformly with niobium pentoxide nanoparticles. The whole is well crystallized.
The nanocomposite thus obtained is then characterized by laser granulometry (measurement of the size of the grains of the powder obtained in suspension) and by high-resolution transmission electron microscopy (morphology and structure of the nanotubes and of the nanooxides). Next, the following are studied via optical measurements:
An image of the structure and morphology of the nanocomposite of Example 1 was obtained by high-resolution transmission electron microscopy (TEM).
This image presented in
The nanocomposite of Example 1 is suspended in chloroform at a rate of 2 g/l. The change as a function of time of the mean size of the particles or aggregates of the nanocomposite of Example 1 suspended in chloroform is then recorded by laser granulometry.
In parallel, the change as a function of time of the mean size of the carbon nanotubes suspended in chloroform (also at a rate of 0.2 g/l) after ultrasonication is also recorded.
The nanoparticle concentration of each suspension is adjusted so as to have identical linear transmissions (of nanocomposite according to the invention, on the one hand, and of carbon nanotubes, on the other hand). It is thus 2 g/l for the suspension containing the nanocomposite, and 0.2 g/l for the suspension containing the carbon nanotubes. This concentration adjustment is necessary for studying the non-linear behaviour of the suspensions.
The changes as a function of time of the mean size of the particles of the nanocomposite of Example 1 and of CNTs are presented in
This stability of the nanocomposite of Example 1 suspended in chloroform is observable with the naked eye, whereas the carbon nanotubes alone are particularly unstable, as shown in
For this study, the non-linear behaviour of the nanocomposite of Example 1 suspended in chloroform, that of chloroform (solvent) and that of CNTs suspended in chloroform were studied via the Z-scan technique. The concentrations are identical to those used in the particle size measurements.
The results are presented in
The appearance (
The switching times of the suspensions are obtained by linear regression of the curves and reflect the extinction rate of the optical limiting system. The measurements were taken just after ultrasonicating the suspensions. They show that the switching rate of the nanocomposite of Example 1 is much higher than that of the carbon nanotubes alone.
The TiO2/carbon nanotube nanocomposites are obtained by dispersing, by sonication, an amount of pulverulent carbon nanotubes (0.2 or 0.4 g) in a solution of ethanol (20 ml) for 1 hour, followed by addition, with slow stirring, of titanium isopropoxide (7 ml), the precursor for the sol-gel route synthesis.
The sol then obtained after addition of 2 M hydrochloric acid (12 ml), with vigorous stirring, is matured with stirring for 72 hours.
To finish, the milky solution thus obtained is then dried at room temperature for 24 hours, and then at 110° C. for 1 hour, before being calcined at 350° C. for 3 hours.
An image of the structure (or morphology) of the nanocomposite of Example 5 was obtained by high-resolution transmission electron microscopy (TEM) on the nanometric scale. This image presented in
A nanocomposite according to the invention based on powders of TiO2 nanotubes and of niobium pentoxide (Nb2O5/TiO2 nanotube powders) is prepared via the sol-gel route, using the same sol-gel method as in Example 1.
20 ml of absolute ethanol are mixed with 20 ml of acetic acid (these products are used without additional purification). The TiO2 nanotubes are added to the mixture and ultrasonicated for one hour so as to separated the nanotubes.
Growth of the TiO2 nanotubes (or titanate nanotubes) is performed via hydrothermal treatment at 130° C. of a TiO2 powder in a concentrated (10 M) sodium hydroxide solution. Typically, 1 g of pulverulent TiO2 (P25, Degussa) is added to 50 ml of an NaOH solution (10 M) in a Teflon autoclave; the whole is stirred for one hour and then left at 130° C. for 48 hours. The white powder obtained is then filtered off under vacuum and washed with HCl (2 M) until neutral, rinsed with distilled water then dried overnight at 110° C. A post-synthesis calcination treatment is performed at 380° C.
1 ml of niobium ethoxide is added to the preceding solution, and the mixture thus obtained is ultrasonicated for one hour. Next, the whole is diluted with 20 ml of ethanol and the mixture is then left in the open air (for maturation) all day and all night so as to allow the formation of a sol, the evaporation of the solvent and the formation of the gel. A “dry” gel is then obtained, which is placed in an oven for 15 hours at 110° C. to remove the excess solvent.
The gel obtained is milled manually in a mortar using a pestle, and a cream-coloured powder is obtained.
After milling the gel, some of the powder obtained is placed in a calcination oven in order to obtain crystallization of the niobium pentoxide. The heat treatment performed therein is a calcination in air comprising a temperature increase ramp at 10° C/minute up to a temperature of 450° C., followed by maintenance at this temperature for 2 hours 45 minutes. At the end of calcination, a white powder formed from TiO2 nanotubes that are covered and/or filled uniformly with niobium pentoxide nanoparticles is obtained.
The rest of the powder obtained is subjected to an air calcination treatment comprising a temperature increase ramp at 10° C/minute up to a temperature of 550° C., followed by maintenance at this temperature for 2 hours 45 minutes. At the end of calcination, a white powder formed from TiO2 nanotubes that are uniformly covered and/or filled with niobium pentoxide nanoparticles is also obtained.
Crystallization of the niobium pentoxide takes place above 520° C., and the calcination temperatures were chosen so as to scan a temperature range extending from the degradation of the morphology of the TiO2 nanotubes and the crystallization of the Nb2O5.
The two powders of nanocomposite thus obtained are then characterized by laser granulometry (measurement of the size of the grains of the powder obtained) and by high-resolution transmission electron microscopy (morphology of the nanotubes). Next, a study is made via optical measurements of the non-linear behaviour (via the Z-scan technique) of these powders, and also their switching rate (via the pump-probe method).
An image of the morphology of the nanocomposite of Example 7 (obtained by calcination at 450° C.) was obtained by scanning electron microscopy. This morphology is presented in
The two nanocomposites of Example 7 (powder calcined at 450° C., on the one hand, and powder calcined at 550° C., on the other hand) are suspended in chloroform at a rate of 3 mg of powder per 20 g of solvent. For comparison, titanium dioxide nanotubes that are neither filled nor covered with niobium pentoxide are also suspended in chloroform.
Next, their non-linear behaviour is studied using the Z-SCAN method, the operating principle of which is illustrated by
The results, which are presented in
This shows that the Nb2O5/TiO2 nanotube nanocomposites do indeed show non-linear behaviour that is clearly reflected by a drop in transmission, but which is, however, less than that observed with the Nb2O5/CNTs nanocomposite.
The two nanocomposites of Example 7 (powder calcined at 450° C., on the one hand, and powder calcined at 550° C., on the other hand) are suspended in chloroform in a proportion of 3 mg of powder per 20 g of solvent. For comparison, titanium dioxide nanotubes that are not filled and not covered with niobium pentoxide are also suspended in chloroform. Next, their switching rate is determined (by means of the pump-probe method, the operating principle of which is illustrated by the figure in
The results obtained just after suspending the samples are presented in
The results obtained at t0+42 hours are presented in
These results show that the calcination temperature of a material has an influence on the stability in suspension and also on the switching efficacy: the nanocomposite according to the invention, Nb2O5/TiO2 nanotubes calcined at 450° C., shows higher stability than the nanocomposite according to the invention, Nb2O5/TiO2 nanotubes calcined at 550° C.
However, the nanocomposite according to the invention, Nb2O5/TiO2 nanotubes calcined at 550° C., has a much shorter switching time than the nanocomposite according to the invention, Nb2O5/TiO2 nanotubes calcined at 450° C. Moreover, an XRD measurement performed on this material shows that at 550° C., it is crystalline, whereas at 450° C. it is still amorphous. The suspension stability thus appears to be linked to the degree of crystallization of the nanocomposite.
The results at t0+66 hours are presented in
The TiO2 (or titanate) nanotubes synthesized are then impregnated with an ethanol/deionized water solution (⅓) containing a tungsten salt, (NH4) 10W12O41·5H2O. After stirring for 1 hour, the mixture is sonicated for 1 hour, followed by evaporation at room temperature for 24 hours with stirring. The powder thus obtained is then dried at 110° C. overnight.
Two images of the structure (morphology) of the nanocomposite of Example 11 were obtained by high-resolution transmission electron microscopy (TEM).
These images, represented respectively by
10 ml of absolute ethanol (CH3CH2OH) are mixed with 10 ml of acetic acid (CH3COOH). The solvents were purchased from Fluka and Aldrich and used without additional purification. 0.036 g of carbon nanofibres, synthesized in the laboratory by the present Applicant company, are added to the mixture and ultrasonicated for one hour. The aim of this step is to deaggregate the carbon nanofibres as much as possible. The solution obtained is black.
The details of the synthesis of carbon nanofibres are given below:
The carbon nanofibres are synthesized by catalytic decomposition of a reaction mixture of ethane and hydrogen with nickel particles supported on a graphite felt serving as macroscopic support:
The support prepared is dried at room temperature in air for 12 hours, and then calcined at 400° C. for 2 hours in a Pyrex tube under a stream of air (20 cm3/min) so as to convert the metal nitrate into oxide. The heating is performed at a rate of 20° C./minute.
The metal oxide supported on carbon felt thus obtained is reduced in situ at 400° C. under a stream of H2 (20 cm3/min) for 1 hour. The temperature is then increased from 400 to 750° C. (20° C./min) and the stream of H2 is replaced with the C2H6/H2 reaction mixture (60/40).
0.5 ml of niobium ethoxide (Nb(OEt)5) purchased from Sigma Aldrich, 99.95%, is added and the mixture is ultrasonicated for one hour. Next, the whole is diluted with 10 ml of ethanol. The mixture is left in the open air (maturation) all day so as to allow the formation of the sol, the evaporation of the solvent and the formation of the gel. The dry gel is placed in an oven for 15 hours at 120° C. to remove the excess solvent. After milling the gel, the powder obtained is placed in a calcination oven so as to obtain the crystallization of Nb2O5.
100 mg of powder are calcined in a calcination oven at 450° C. The ramp is 10° C./minute, the final temperature is 450° C. and the sample is calcined in air for 2 hours 45 minutes. At the outlet, the powder obtained is black.
100 mg of powder are calcined in a calcination oven at 520° C. The ramp is 10° C./minute, the final temperature is 520° C. and the sample is calcined in air for 2 hours 45 minutes. At the outlet, the powder obtained is grey. The powder begins to crystallize at and above 520° C.
The nanocomposite of Example 13 is presented in
[1] FR 2 787 203
[2] “Single-wall carbon nanotubes for optical limiting” by L. Vivien, E. Anglaret, D. Riehl, F. Bacou, C. Journet, C. Goze, M. Andrieux, M. Brunet, F. Lafonte, P. Bernier and F. Hache, in “Chemicals Physics Letters” 307 (1999) 317-319,
[3] “Optical limiting properties of single-wall carbon nanotubes” by L. Vivien, E. Anglaret, D. Riehl, F. Hache, F. Bacou, M. Andrieux, F. Lafonte, C. Journet, C. Goze, M. Brunet, and P. Bernier, in “Optical Communications” 174 (2000) 271-275,
[4] “Optical limiting properties of carbon nanotubes” by L. Vivien, D. Riehl, F. Hache, and E. Anglaret, in “Physical B” B 323 (2002) 233-234,
[5] “Two methods of obtaining sol-gel Nb2O5 thin films for electrochromic devices”, C. O. Avellaneda, A. Pawlicka and M. A. Aegerter, Journal of Material Science 33 (1998) 2181-2185,
[6] “Sol-gel synthesis and characterization of Nb2O5 powders”, M. Ristic, S. Popovic and S. Music, Materials Letters 58 (2004) 2658-2663.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09/00695 | Feb 2009 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2010/000128 | 2/16/2010 | WO | 00 | 12/23/2011 |
