Self-dispersing nanoparticles

Abstract

The invention relates to a process for manufacturing nanoparticles that are self-dispersing in water. It also relates to the self-dispersing nanoparticles obtained by the process of the invention and also a process for manufacturing a heat-transfer fluid containing the nanoparticles according to the invention or obtained by the process of the invention. The process of the invention comprises the following steps: a) optionally, manufacture of an aqueous dispersion of nanoparticles chosen from the nanoparticles of alumina (Al.sub.2O.sub.3), of zinc oxide (ZnO), of titanium oxide (TiO.sub.2), of silica (SiO.sub.2) and of beryllium oxide (BeO), b) addition to an aqueous dispersion of nanoparticles chosen from nanoparticles of alumina (Al.sub.2O.sub.3), of zinc oxide (ZnO), of titanium oxide (TiO.sub.2), of silica (SiO.sub.2) and of beryllium oxide (BeO), of a water-soluble polymer chosen from polyvinyl alcohols, polyethylene glycols, polyvinylpyrrolidones, polyoxazolines, starches, and mixtures of two or more thereof, and c) thermal quenching of the dispersion obtained in step b), and d) lyophilization of the quenched dispersion obtained in step c). The invention finds an application in the field of coolants in particular.

Claims

1. A process of manufacturing self-dispersing nanoparticles of a metal oxide, comprising: a step of addition of a water-soluble polymer to a dispersion of nanoparticles; a step of thermal quenching of the resulting dispersion in liquid nitrogen, wherein the step of thermal quenching comprises adding the resulting dispersion dropwise to the liquid nitrogen to form beads; and a step of lyophilization of the quenched dispersion obtained, and comprising, optionally, a step of manufacture of the dispersion of said nanoparticles, before the step of addition of the water-soluble polymer, wherein the dispersion of nanoparticles is an aqueous dispersion of nanoparticles selected from the group consisting of nanoparticles of alumina (Al.sub.2O.sub.3), of zinc oxide (ZnO), of titanium oxide (TiO.sub.2), of silica (SiO.sub.2) and of beryllium oxide (BeO), and the water-soluble polymer is selected from the group consisting of polyvinyl alcohols, polyethylene glycols, polyvinylpyrrolidones, polyoxazolines, starches, and mixtures of two or more thereof.

2. The process of claim 1 wherein the water-soluble polymer is selected from the group consisting of polyvinyl alcohols, polyethylene glycols and polyvinylpyrrolidones.

3. The process of claim 1 wherein the water-soluble polymer is a polyvinyl alcohol having a weight-average molecular weight of between 31000 and 50000 g.Math.mol.sup.1 inclusive.

4. The process of claim 1 wherein the water-soluble polymer is poly(2-ethyloxazoline) with a weight-average molecular weight of 50000 g.Math.mol.sup.1.

5. The process of claim 1 wherein the water-soluble polymer is a polyvinylpyrrolidone with a weight-average molecular weight of between 10000 and 40000 g.Math.mol.sup.1 inclusive.

6. The of claim 1 wherein the water-soluble polymer is a polyethylene glycol having a weight-average molecular weight of between 7000 and 35000 g.Math.mol.sup.1 inclusive.

7. The process of claim 1 wherein the water-soluble polymer is a polyvinyl alcohol having a weight-average molecular weight of between 31000 and 50000 g.Math.mol.sup.1 inclusive, the nanoparticles are nanoparticles of alumina or of zinc oxide, and the nanoparticles/water-soluble polymer weight ratio is between 4.5 and 30 inclusive.

8. The process of claim 7 wherein the nanoparticles/water-soluble polymer weight ratio is between 6 and 7 inclusive.

9. The process of claim 1 further comprising a step of milling of the aggregates obtained after lyophilization of the quenched dispersion, when such aggregates are obtained after this step.

10. Self-dispersing nanoparticles obtained by the process as claimed in claim 1, characterized in that said self-dispersing nanoparticles are selected from the group consisting of self-dispersing nanoparticles of alumina, of zinc oxide, of titanium oxide, of silica oxide and of beryllium oxide, in that said self-dispersing nanoparticles comprise a water-soluble polymer which is polyvinyl alcohol, and in that said self-dispersing nanoparticles are instantaneously dispersible in water.

11. The self-dispersing nanoparticles of claim 10 wherein the polyvinyl alcohol has a weight-average molecular weight of between 31000 and 50000 g.Math.mol.sup.1 inclusive.

12. A process of manufacturing a dispersion of self-dispersing nanoparticles, comprising mixing self-dispersing nanoparticles obtained by the process as claimed in claim 1, in water.

13. The process of claim 12 wherein the self-dispersing nanoparticles/water weight ratio is between 5/100 and 10/100 inclusive.

14. A heat-transfer fluid comprising water and self-dispersing nanoparticles obtained by the process as claimed in claim 1.

15. The process of claim 1 wherein the step of thermal quenching is performed at a temperature below 180 C.

16. A process of manufacturing self-dispersing nanoparticles of a metal oxide, comprising: a step of addition of a water-soluble polymer to a dispersion of nanoparticles; a step of thermal quenching of the resulting dispersion at a temperature below 80 C.; and a step of lyophilization of the quenched dispersion obtained, and comprising, optionally, a step of manufacture of the dispersion of said nanoparticles, before the step of addition of the water-soluble polymer, wherein the dispersion of nanoparticles is an aqueous dispersion of nanoparticles selected from the group consisting of nanoparticles of alumina (Al.sub.2O.sub.3), of zinc oxide (ZnO), of titanium oxide (TiO.sub.2), of silica (SiO.sub.2) and of beryllium oxide (BeO), and the water-soluble polymer is poly(2-ethyloxazoline) with a weight-average molecular weight of 50000 g.Math.mol.sup.1.

17. The process of claim 16 wherein the nanoparticles are nanoparticles of alumina or of zinc oxide, and the nanoparticles/water-soluble polymer weight ratio is between 4.5 and 30 inclusive.

18. The process of claim 16 further comprising a step of milling of the aggregates obtained after lyophilization of the quenched dispersion, when such aggregates are obtained after this step.

Description

EXAMPLE 1

Synthesis of Nanoparticles of Alumina, According to the Invention

(1) 5 g of polyvinyl alcohol (PVA) with a weight-average molecular weight of between 31 000 and 50 000 g.Math.mol.sup.1 (Fluka: PVA 4-88), in powder form, are added to 95 ml of DI water and the reaction medium is stirred for 1 h at 50 C. until complete dissolution of the polymer.

(2) The PVA solution is added at ambient temperature to 45.33 g of an aqueous colloidal sol of Baikowski BA15PS -alumina, of which the alumina titer by weight is 75%.

(3) The reaction medium is milky white, and very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. (in a thermostatic bath) using a K2D Pro apparatus.

(4) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained after quenching in the liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h. After this lyophilization, porous aggregates of self-dispersing nanoparticles are obtained, these aggregates having the shape and the size of the drops introduced into the liquid nitrogen. Of course, the size and the shape of the drops added to the liquid nitrogen, and therefore of the aggregates obtained, can be modified, in particular by using a means for injecting the drops into the liquid nitrogen which modifies this shape, or alternatively according to the quenching technique used.

(5) The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h, and then they are re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads are then packaged under argon and stored at ambient temperature.

(6) The polyvinyl alcohol used here has a molecular weight <100 000.

(7) Above this molecular weight, it is necessary to use a longer mixing time, at ambient temperature, for the polyvinyl alcohol in the colloidal sol of nanoparticles.

(8) The aggregates obtained in this example after the lyophilization have the size of drops of approximately 5 mm in diameter.

(9) After the lyophilization, the dry beads retain the same size.

(10) It should be noted that the polyvinyl alcohol/colloidal suspension order of addition does not in any way modify the result obtained, just like heating or increasing the polyvinyl alcohol+aqueous colloidal sol mixing time.

EXAMPLE 2

(11) The process was carried out as in example 1, except that the amount of polyvinyl alcohol added was 2.5 g, which corresponds to an Al.sub.2O.sub.3/PVA weight ratio of 13.6.

EXAMPLE 3

(12) The process was carried out as in example 1, but adding only 1.5 g by weight of PVA.

EXAMPLE 4

(13) The process was carried out as in example 1, but adding 7 g of PVA.

(14) The weight of alumina, the weight of PVA, the alumina/PVA weight ratio, the percentage by weight relative to the total dispersion obtained of Al.sub.2O.sub.3 and the percentage by total weight relative to the total weight of the dispersion obtained in the end for examples 1 to 4 are reported in table 2 hereinafter:

(15) TABLE-US-00002 TABLE 2 Weight Al.sub.2O.sub.3/ % Al.sub.2O.sub.3 by % PVA by Weight PVA PVA total weight total weight Sample Al.sub.2O.sub.3 (g) (g) by weight (dispersion) (dispersion) Example 1 34 5 6.8 11.32 1.6 Example 2 34 2.5 13.6 11.32 0.8 Example 3 34 1.25 27.2 11.32 0.4 Example 4 34 7 4.85 11.32 2.3

EXAMPLE 5

(16) In this example, poly(ethyl-2-oxazoline) with a molecular weight of 50 000 was used as water-soluble polymer.

(17) 5 g of poly(ethyl-2-oxazoline); Aldrich: MW 50 000 g.Math.mol.sup.1, in powder form, are added to 95 ml of DI water and the reaction medium is stirred for 1 h at 50 C. until complete dissolution of the polymer.

(18) The poly(ethyl-2-oxazoline) solution is added at ambient temperature to 45.33 g of an aqueous colloidal sol of Baikowski BA15PS -alumina; the alumina titer by weight is 75%.

(19) The reaction medium is milky white, and very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. using a K2D Pro apparatus.

(20) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained (aggregates of self-dispersing nanoparticles) after quenching in liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h.

(21) The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h, and are then re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads are then packaged under argon and stored at ambient temperature.

EXAMPLE 6

(22) Polyvinyl alcohol having a weight-average molecular weight of between 9000 and 10 000 g.Math.mol.sup.1, which was dissolved beforehand in water, was used as water-soluble polymer.

(23) 5 g of polyvinyl alcohol (Aldrich: MW 9000-10 000), in powder form, are added to 95 ml of DI water and the reaction medium is stirred for 1 h at 50 C. until complete dissolution of the polymer.

(24) The PVA solution is added at ambient temperature to 45.33 g of an aqueous colloidal sol of Baikowski BA15PS -alumina; the alumina titer by weight is 75%.

(25) The reaction medium is milky white, and very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. using a K2D Pro apparatus.

(26) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained after quenching in liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h.

(27) The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h, and are then re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads are then packaged under argon and stored at ambient temperature.

EXAMPLE 7 (COMPARATIVE)

(28) In this example, a polyvinyl alcohol having a weight-average molecular weight of 89 000 to 98 000 g.Math.mol.sup.1 was used as water-soluble polymer.

(29) 5 g of polyvinyl alcohol (Aldrich: MW 89 000-98 000 g.Math.mol.sup.1), in powder form, are added to 95 ml of DI water and the reaction medium is stirred for 1 h at 50 C. until complete dissolution of the polymer.

(30) The PVA solution is added at ambient temperature to 45.33 g of an aqueous colloidal sol of Baikowski BA15PS -alumina; the alumina titer by weight is 75%.

(31) The reaction medium is milky white, and is very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. using a K2D Pro apparatus.

(32) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained after quenching in liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h.

(33) The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h, and are then re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads are then packaged under argon and stored at ambient temperature.

EXAMPLE 8 (COMPARATIVE)

(34) In this example, water-soluble polymer was not added to the colloidal suspension of alumina, which was simply quenched and lyophilized.

EXAMPLE 9 (COMPARATIVE)

(35) In this example, water-soluble polymer was not used and the colloidal sol of alumina was simply dried, without quenching treatment or lyophilization.

EXAMPLE 10 (COMPARATIVE)

(36) In this example, a colloidal sol of zinc oxide was used without the addition of water-soluble polymer. The colloidal sol of zinc oxide was then quenched in liquid nitrogen and then lyophilized.

EXAMPLE 11 (COMPARATIVE)

(37) In this example, a colloidal sol of zinc was used without the addition of water-soluble polymer. The colloidal sol of zinc was simply dried at ambient temperature.

EXAMPLE 12

(38) In this example, the process of the invention was applied to the colloidal sol of zinc oxide used in examples 10 and 11.

(39) 1.25 g of PVA (Fluka: 4-88) are added to 33.75 g of DI water, and the mixture is stirred at ambient temperature until the PVA has completely dissolved. The PVA solution is added at ambient temperature to 50 g of a Nyacol aqueous colloidal ZnO sol, the ZnO titer by weight of which is 17%. The reaction medium is milky white, and is very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. using a K2D Pro apparatus.

(40) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained after quenching in liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h. The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h and are then re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads (aggregates) obtained are then packaged under argon and stored at ambient temperature.

EXAMPLE 13

(41) In this example, the process was carried out as in example 12, but adding only 0.75 g of water-soluble polymer, which in this case is PVA.

(42) 0.75 g of PVA (Fluka: 4-88) is added to 33.75 g of DI water, and the mixture is stirred at ambient temperature until the PVA has completely dissolved. The PVA solution is added at ambient temperature to 50 g of a Nyacol aqueous colloidal ZnO sol, the ZnO titer by weight of which is 17%. The reaction medium is milky white, and very homogeneous without any formation of precipitate. After 1 h of mixing at ambient temperature, the thermal conductivity is then measured at 20 C. using a K2D Pro apparatus.

(43) The same reaction medium is then added dropwise to liquid nitrogen (5 l Dewar); the diameter of the drops is approximately 5 mm. The beads obtained after quenching in liquid nitrogen are then filtered off on a plastic Buchner funnel. They are weighed and lyophilized for 48 h. The lyophilization generally lasts 48 h. After 36 h, the lyophilization is stopped and the beads are weighed. They are then re-lyophilized for 12 h and are then re-weighed. It is considered that the lyophilization is complete if the variation in weight between t36 h and t48 h does not exceed 0.005%, typically 0.5 g for 100 g of material used. The beads are then packaged under argon and stored at ambient temperature.

EXAMPLE 14

(44) The self-dispersing nanoparticles obtained in the previous examples were then redispersed according to the following protocol:

(45) 5 g of beads are placed on a 125 ml filter funnel with a porosity of 1 (diameter corresponding to the largest pores: 101 to 160 microns), the diameter of the sintered plate being 60 mm, the whole assembly being mounted on a vacuum flask. 100 ml of DI water are then added, said water is then left for 20 seconds without stirring and then filtration is carried out. Since the residue remains on the sintered plate, said residue is then dried and then weighed.

(46) The dispersibility properties of the various nanoparticles obtained in examples 1 to 13 are grouped together in the following table 3:

(47) TABLE-US-00003 TABLE 3 Weight recovered on Observations regarding Samples the sintered plate dispersibility Al.sub.2O.sub.3 Example 1 <<0.1 g Very good dispersibility Example 2 <<0.1 g Very good dispersibility Example 3 0.8-1 g Medium dispersibility Example 4 <<0.1 g Very good dispersibility Example 5 <<0.1 g Very good dispersibility Example 6 0.8-1 g Medium dispersibility Example 7 3 g Poor dispersibility Example 8 4 g Poor dispersibility Example 9 4.5 g-5 g Very poor dispersibility ZnO Example 10 3-4 g Poor dispersibility Example 11 4 g Poor dispersibility Example 12 0.3 g Very good dispersibility Example 13 0.5 g Very good dispersibility

(48) The variations in the thermal conductivity of various fluids as a function of the weight of nanoparticles added to these fluids were then studied.

(49) For this, the variation in thermal conductivity as a function of the weight of nanoparticles and/or of water-soluble polymer added: of deionized water alone, of deionized water containing 1.6% by weight of polyvinyl alcohol water-soluble polymer having a weight-average molecular weight of between 31 000 and 51 000 g.Math.mol.sup.1, inclusive, of a mixture of deionized water+alumina containing 10% by weight of BA15PS -alumina, of a mixture of deionized water and alumina nanoparticles not treated by means of the process of the invention, which is the mixture of example 1 before the quenching treatment, lyophilization and redispersion and of 1.6% by weight, relative to the total weight of the water+colloidal sol+alumina nanoparticles+water-soluble polymer suspension, of polyvinyl alcohol having a weight-average molecular weight of 31 000 to 51 000 g.Math.mol.sup.1, and of a heat-transfer fluid according to the invention containing the self-dispersing nanoparticles according to the invention obtained in example 1, was measured.

(50) The results are represented in the form of curves in FIG. 1.

(51) The curve of variation of thermal conductivity of deionized water alone serves as a reference.

(52) It is observed, from FIG. 1, that the heat-transfer fluid consisting only of polyvinyl alcohol in deionized water exhibits a slight drop in thermal conductivity compared with water. This is consistent, since the thermal conductivity of water-soluble polymers is 0.1 W m.sup.1 K.sup.1.

(53) Any addition, to a system, of a thermal conductor which is not as good leads to a drop in the thermal conductivity of the mixture proportionally to the ratios of the constituents.

(54) It is also noted from FIG. 1 that, when a mixture of water+1.6% by weight of polyvinyl alcohol is quenched and lyophilized and deionized water is added so as to again obtain a concentration by weight of 1.6% of polyvinyl alcohol, the thermal conductivity moves slightly closer to the thermal conductivity of deionized water alone, but remains below it.

(55) It is also noted that, by using self-dispersing nanoparticles according to the invention, or obtained by means of the process according to the invention, the thermal conductivity of the fluid obtained is further improved.

(56) However, entirely unexpectedly, it is especially noted that, at the low concentrations by weight (10%) of self-dispersing nanoparticles according to the invention or obtained by means of the process of the invention, the heat-transfer fluid according to the invention shows a thermal conductivity which is much higher than a nanofluid containing the same amounts of alumina and of polyvinyl alcohol but not treated by means of the process of the invention.

(57) Thus, the process of the invention makes it possible to increase the thermal conductivity of a nanofluid.

(58) With a concentration by weight of 10% of self-dispersing nanoparticles of alumina according to the invention, the nanofluid of the invention behaves like a fluid containing 17% by weight of nanoparticles. There is therefore a considerable increase in the thermal conductivity relative to the weight of material used. Furthermore, being able to work at a lower charge makes it possible to minimize the problems of settling out and/or of abrasiveness which occur when the charge is too great.

(59) This is true both with the self-dispersing nanoparticles according to the invention containing alumina and the water-soluble polymer, and with the self-dispersing nanoparticles according to the invention comprising zinc oxide and the water-soluble polymer.

(60) This is shown by table 4 hereinafter which shows the thermal conductivities of nanoparticles of zinc oxide before treatment according to the invention, after treatment according to the invention and after a storage period of 15 days.

(61) TABLE-US-00004 TABLE 4 Thermal Thermal Thermal conductivity conductivity conductivity before after after Samples 10% by conditioning conditioning conditioning (15 d) weight Wm.sup.1K.sup.1 Wm.sup.1K.sup.1 Wm.sup.1K.sup.1 ZnO Example 12 (10%) 0.572 0.590 0.590 by weight Example 13 0.572 0.590 0.590 Al.sub.2O.sub.3 Example 1 0.595 0.620 0.620 Example 2 0.595 0.624 0.62 Example 3 0.570 0.570 0.570 Example 4 0.550 0.550 0.550 Example 5 0.60 0.630 0.630 Example 6 0.570 0.570 0.570 Example 7 0.57 0.57 0.57

(62) It is also seen, from FIG. 1, that the addition of nanoparticles of aluminum oxides to deionized water containing polyvinyl alcohol clearly increases the thermal conductivity of the resulting fluid and that this thermal conductivity is directly proportional to the amount of oxides added.

(63) Moreover, the size of the self-dispersing nanoparticles according to the invention and the size of the same nanoparticles before treatment according to the process of the invention were measured.

(64) The process for manufacturing the nanoparticles according to the invention affects the particle size distribution and the size of the self-dispersing nanoparticles little or not at all, when said particles are redispersed in water.