EP3887316A1

EP3887316A1 - Method for producing zirconium dioxide nanoparticles in the presence of an amino acid

Info

Publication number: EP3887316A1
Application number: EP19832565.6A
Authority: EP
Inventors: Amandine VENIER; Erik Camposilvan; Miguel COMESANA-HERMO
Original assignee: Mathym
Current assignee: Mathym
Priority date: 2018-11-30
Filing date: 2019-11-28
Publication date: 2021-10-06
Also published as: FR3089136B1; KR20210096611A; WO2020109477A1; FR3089136A1; US20220009790A1; CN113165900A; CN113165900B; SG11202104964UA; US12071355B2; JP2022510347A

Abstract

The present invention relates to a method for producing zirconium dioxide, ZrO2, nanoparticles by hydrothermal treatment of a zirconium IV compound in the presence of water, at a pH below 7, and of at least one amino acid comprising at least 4 carbon atoms, said amino acid being present in an acid function to amine function ratio greater than or equal to 1. The invention also relates to zirconium dioxide nanoparticles which exhibit a transmittance in the visible spectrum of greater than or equal to 20% at 400 nm and greater than or equal to 95% at 800 nm, measured in water at a concentration of 40% by weight.

Description

PROCESS FOR THE PREPARATION OF NANOPARTICLES OF DIOXIDE

ZIRCONIUM IN THE PRESENCE OF AMINO ACID

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of zirconium dioxide nanoparticles by hydrothermal treatment carried out in the presence of an amino acid. The invention also relates to the use of these nanoparticles, in particular in the form of dispersions.

PRIOR STATE OF THE ART

There is currently a strong interest in nanoparticles of zirconium dioxide (zirconia) in various industrial sectors. One of the techniques for preparing nanoparticles of zirconium dioxide, ZrCL, consists in hydrothermally treating a zirconium compound, in particular a zirconium salt, in which the zirconium has an oxidation state of 4. Thus, during the preparation of ZrCL particles by hydrothermal treatment, the zirconium is neither oxidized nor reduced, and its degree of oxidation is unchanged.

Zirconium dioxide can be in crystalline form and have three different crystal structures depending on the temperature, the presence of dopants and the size of the crystals: monoclinic, quadratic (tetragonal) and cubic. For particles of nanometric size, the quadratic and cubic phases are difficult to distinguish by the X-ray diffraction measurement method (XRD), in the description of the prior art and of the invention, they are labeled together under the name of "quadratic / cubic". The monoclinic phase is easily distinguishable from the other two. It is possible to favor one or other of these crystal configurations by adjusting the preparation process.

Nevertheless, the preparation of stable dispersions of zirconium dioxide nanoparticles having improved properties in terms of crystallinity, colloidal stability, narrow polydispersity and transparency remains a real challenge. Indeed, the methods available to date present for the most part a weak control of the size of the particles with a colloidal stability which can be greatly improved. In particular, these dispersions generally have a high degree of aggregation, which has a negative impact on the properties of the desired final product. Nanoparticles of zirconium dioxide can in particular be used in the preparation of nanocomposite and ceramic materials.

In the case of ceramic materials and ceramic composites, the reuse of colloidal dispersions of zirconium dioxide, as an additive, as a main or secondary component can make it possible to obtain improved optical, thermal and mechanical properties, thanks to the possibility of better control the micro structure and porosity. The use of colloidal dispersions makes it possible to decrease the temperature of the heat treatments compared to the processes which do not involve such dispersions, which makes it possible to obtain generally finer microstructures or crystals better dispersed in the composite.

Among the applications of these ceramics, mention may be made of dense or nanoporous coatings, the manufacture of high transmittance nano-ceramics based on zirconia, the manufacture of nano-composites based on zirconia / alumina, the use as an additive sintering, the preparation of ceramic inks for 2D and 3D printing.

In the case of nano-composites, a dispersion of nanoparticles of zirconium dioxide is generally mixed with a monomer, an oligomer, a polymer or a prepolymer resin. A nano-composite material containing large fractions of zirconia nanoparticles has an increased refractive index, abrasion resistance, elastic modulus and radiopacity and reduced shrinkage relative to the matrix. At the same time, if the nanoparticles are not agglomerated in the matrix and have a sufficiently small size, the nano-composite material can maintain a high transmittance in the visible. The total transmittance (T) of a material corresponds to the ratio of the light flux transmitted through a fixed thickness of said material to the incident flux and can be expressed with a value between 0 and 1 or with the corresponding percentage (T%).

Among the applications of these composites, we can cite the dental field (composites, varnishes, adhesives), optics, electronics and energy (coatings with high refractive index, anti-reflective and anti-scratch), lighting (coatings with high refractive index for the extraction of light from devices, for example OLED and HB-LED) and cosmetics. Furthermore, in the context of these applications, it is preferable that the dispersion has a high visible transmittance, so as not to significantly alter the aesthetic appearance of the composite material obtained from a dispersion of nanoparticles.

Although stable dispersions are available to date, they are generally not very concentrated in nanoparticles mainly due to the technical difficulties linked to their preparation. Indeed, at high concentrations, zirconium dioxide nanoparticles tend to agglomerate while dispersions tend to gel. In addition, in general, the transmittance of dispersions of zirconium dioxide nanoparticles decreases as the concentration of zirconium dioxide nanoparticles increases.

Patent EP 2 371 768 B1 describes an aqueous dispersion of zirconium oxide having, for a concentration of 30% by weight, a transmittance up to 44.3% at 400 nm and up to 98.2% at 800 nm and a viscosity of less than 20 mPa.s at 25 ° C.

Document US 2005/123465 describes a process for preparing a dispersion of ZrCh particles and their modification with caproic acid. This process involves taking a colloidal dispersion of ZrC colloids and mixing it with a solution containing aminocaproic acid. It is not a synthesis by hydrothermal treatment in the presence of an amino acid.

The document LR 2 899 906 describes the preparation of metal oxide nanoparticles from zirconium tetraisopropoxide in the presence of acetylacetone, propanol and para-toluenesulfonic acid. Once the ZrCh particles are obtained, they can be mixed with aminocaproic acid. It is not a hydrothermal treatment in the presence of an amino acid.

Lin et al. (Materials Chemistry and Physics, 206 (2018), pages 136-143) have described hybrid compounds based on zirconia and silicone to encapsulate LEDs. The film obtained from this hybrid compound has a transmittance greater than 95% at a wavelength between 400 and 800 nanometers. Perreira et al. (Materials Chemistry and Physics, 152 (2015), pages 135-146) described the synthesis of ZrC-based particles by mixing a zirconium IV compound and an arsenic III and / or L-cysteine dopant at basic pH in an alcohol at room temperature. It is not a hydrothermal treatment at acidic pH.

In any case, having a concentrated dispersion of zirconium dioxide nanoparticles makes it easier to handle these nanoparticles since the user does not have to handle a powdery product. Indeed, even if having a powder can be advantageous for packaging and transport, a concentrated dispersion facilitates the handling of nanoparticles, provided that it is stable.

On the other hand, the stability of a dispersion makes it possible to control the dosage of the nanoparticles, these being dispersed in a homogeneous manner. Finally, having a transparent dispersion of nanoparticles is particularly advantageous in certain fields, for example in the dental field.

Similarly, the methods of preparation of this type of material, described in the literature, include experimental conditions (pressure, temperature or hydrothermal treatment time) which make their implementation on an industrial scale difficult. In view of all these limitations, the development of a technical process capable of producing substantial quantities of ZrCE nanoparticles with good colloidal dispersion, good transmittance and good crystallinity while keeping relatively soft synthesis conditions remains a subject for discussion. to explore. Thus, such a development would in itself constitute a very significant advancement in the industrialization of high quality zirconia nanodispersions, materials which are particularly interesting for many technical sectors.

STATEMENT OF THE INVENTION

The Applicant has found, quite unexpectedly, that it is possible to soften the synthesis conditions and to increase the concentration of zirconium dioxide nanoparticles and the quality of the dispersion by preparing them by hydrothermal treatment in the presence an amino acid. More specifically, the present invention relates to a process for the preparation of zirconium dioxide nanoparticles, ZrCL, by hydrothermal treatment of a zirconium compound IV in the presence of water, at a pH below 7, and at least one acid amino comprising at least 4 carbon atoms, said amino acid having an acid function to amine function ratio greater than or equal to 1.

The invention further relates to zirconium dioxide nanoparticles having a visible transmittance greater than or equal to 20% at 400 nm and greater than or equal to 95% at 800 nm measured in water at a concentration of 40% in weight.

The invention also relates to a dispersion of said zirconium dioxide nanoparticles, ZrCL. These nanoparticles can be stabilized by a conventional stabilizing agent or by at least one amino acid comprising at least 4 carbon atoms, said amino acid having an acid function to amine function ratio greater than or equal to 1. More specifically, the dispersion of nanoparticles of ZrCL can comprise up to 80% by weight of ZrCL nanoparticles, and have a transmittance, for a dispersion at 40% by weight, advantageously ranging from 20% to 83% for 400 nm of wavelength and advantageously going from 95% to 99.9% for 800 nm wavelength.

Also said dispersion has a very low viscosity, advantageously greater than or equal to 1 mPa.s and less than or equal to 10 mPa.s at 40% by weight in water. In addition, the dispersion retains a high transmittance for higher concentrations. A dispersion at 65% by weight has a transmittance ranging from 15%, advantageously 45%, up to 75% for 400 nm of wavelength and ranging from 85%, advantageously 95%, up to 99% for 800 nm. Furthermore, said dispersion has a high refractive index, advantageously greater than or equal to 1.40 for a concentration of 40% by weight and greater than or equal to 1.50 for a concentration of 65% by weight.

In addition, the dispersion after purification with water has a very low organic matter content, advantageously less than or equal to 15% by weight, more advantageously less than or equal to 8% by weight, even more advantageously less than or equal to 5% by weight. weight, even more advantageously less than or equal to 3.5% by weight. Also, said dispersion has a very low dispersion index advantageously between 1 and 7, more advantageously between 1 and 4, even more advantageously between 1 and 2, even more advantageously between

I and 1.5.

The invention also relates to nanoparticles in powder form, having an amino acid stabilizing agent. In particular, the powder is obtained by drying the dispersion described above. Hereinafter, the term “particles” can also be used to denote the nanoparticles of ZrC> ₂ .

Composed of zirconium IV

As indicated above, the method according to the invention makes it possible to obtain nanoparticles of zirconium dioxide by hydrothermal treatment of a zirconium IV compound.

Advantageously, the zirconium compound IV involved in the process according to the invention is chosen from the group consisting of zirconium halides. Preferably, the zirconium compound IV is zirconium oxychloride (ZrOCL).

In a preferred embodiment, the zirconium compound IV is in hydrated form such as for example zirconium oxychloride octahydrate (ZrOCL 8H ₂ O). This embodiment is advantageous because the water molecules provided by the zirconium compound IV in hydrated form may be sufficient and it is then not necessary to add water to implement the hydrothermal treatment.

In a particular embodiment, the water from the hydrothermal treatment comes solely from a hydrated form of the zirconium compound IV.

It should be noted that the zirconium compound IV used in the process according to the invention is not zirconium dioxide.

Amino acid In accordance with the invention, the amino acid comprises at least 4 carbon atoms, advantageously between 4 and 12 carbon atoms, more advantageously between 4 and 6 carbon atoms. The amino function (s) of the amino acid can be primary, secondary, or tertiary. However, the primary amine functions are preferred. The possible carbon atoms of the secondary and tertiary amine functions are counted in the number of carbon atoms of the amino acid.

The amino acid function (s) of the amino acid is a carboxylic acid function, C (= 0) OH, the carbon atom of which is counted in the number of carbon atoms in the amino acid.

The amino acid can be linear or branched. However, it is advantageously linear.

The amino acid not only influences the conditions of the hydrothermal treatment (temperature, pressure) but also the polydispersity and the visible transmittance of the dispersion of zirconium dioxide nanoparticles.

According to the invention, the hydrothermal treatment is carried out in the presence of an amino acid having an acid function to amine function ratio greater than or equal to 1. This ratio is preferably of the order of 1.

Indeed, the Applicant has noticed that when the amino acid has more amino functions than acid functions, it was not possible to obtain a dispersion of stable zirconium dioxide nanoparticles. Thus, amino acids such as lysine do not make it possible to obtain a dispersion of nanoparticles of zirconium dioxide stable under the conditions of the present invention.

In a particular embodiment, the amino acid can comprise several acid functions and several amine functions.

In another particular embodiment, the amino acid can comprise several acid functions and a single amine function.

In a preferred embodiment, the amino acid comprises a single acid function and a single amine function.

In another particular embodiment, several amino acids can be used in the process. Thus, the amino acid is preferably chosen from the group consisting of aminobutanoic acids, aminopentanoic acids, aminohexanoic acids, aminoheptanoic acids, aminooctanoic acids, aminononanoic acids, aminodecanoic acids, aminoundecanoic acids and amino acids only or as a mixture, and more preferably in the group consisting of aminobutanoic acids, aminopentanoic acids and aminohexanoic acids and even more preferably in the group consisting of 4-aminobutanoic acid, also called 4-aminobutyric acid, 2- aminopentanoic acid, also called norvaline, 5-aminopentanoic acid, also called 5-aminovaleric acid and 6-aminohexanoic acid, also called 6-aminocaproic acid, alone or as a mixture.

In a particular embodiment, the amino acid is formed in situ, by hydrolysis, from an amino acid precursor. By amino acid precursor is meant a molecule which does not have both an amine function and an acid function, which can be converted to amino acid in the presence of water under the conditions of the hydrothermal treatment according to the invention. In other words, in this particular embodiment, no amino acid is introduced into the reaction medium, but a compound forming an amino acid when it is hydrolyzed is introduced.

A preferred family of amino acid precursors is the lactam family. Thus, advantageously, the amino acid precursor is chosen from the group consisting of pyrrolidone and N-methylpyrrolidone (NMP).

In another particular embodiment, the method is implemented in the presence of an amino acid and an amino acid precursor. Thus, in this embodiment, the method is implemented with two sources of amino acid. The amino acid derived from the precursor may be the same amino acid as that introduced as such or be a different amino acid.

It should be noted that the use of an acid alone, an amine alone or a mixture of the two does not provide the same advantages as the use of an amino acid or a mixture of amino acids. It should be noted that the functionalization with an amino acid of zirconia nanoparticles obtained by a process different from the present invention does not provide the same advantages as the use of an amino acid or a mixture of amino acids during the hydrothermal treatment. . In other words, preparing particles of zirconia in the absence of amino acid and then functionalizing these particles with an amino acid does not make it possible to obtain particles having the properties of the ZrC nanoparticles according to the invention.

Operating conditions for hvdrothermal treatment

According to the invention, a zirconium IV compound undergoes hydrothermal treatment to lead to nanoparticles of zirconium dioxide. This treatment can be carried out under an inert atmosphere (for example under argon or under nitrogen) or under air, advantageously under air.

Advantageously, the hydrothermal treatment is carried out at a temperature greater than or equal to 100 ° C., preferably greater than or equal to 150 ° C., more advantageously between 170 and 220 ° C., and even more advantageously between 180 and 200 ° C.

A person skilled in the art knows how to choose the duration of the hydrothermal treatment, in particular according to the temperature and the pressure chosen. Generally, a longer duration of the hydrothermal treatment can induce a reduction in the temperature of the process necessary to ensure the good formation of the nanoparticles in crystalline form. For example, the duration may be less than 48 hours, or even less than 24 hours. According to a preferred embodiment, it can be between 60 minutes and 180 minutes.

However, depending on the experimental conditions, the rise in temperature may be sufficient to ensure the hydrothermal treatment. This is particularly the case when the hydrothermal treatment is carried out at a temperature of at least 180 ° C., for example with a sufficiently slow rise, advantageously of a few degrees per minute, more advantageously of about ten degrees per minute.

Thus, it is possible to implement the method according to the invention continuously or in batches (“batch” method in English). Those skilled in the art will know how to adapt the experimental conditions, in particular the temperature and the duration of the hydrothermal treatment, but also the rise in temperature, according to the methods of implementation of the chosen method and the desired yield in terms of nanoparticle production.

Preferably, the hydrothermal treatment is carried out at a pressure greater than or equal to 0.1 MPa. Advantageously, the hydrothermal treatment is carried out at a pressure less than or equal to 2 MPa, more preferably between 0.1 and 2 MPa, even more preferably between 0.12 and 1.5 MPa, and even more preferably between 0.15 and 0.6 MPa. Thus, it can be produced at a pressure of between 0.1 MPa and 0.6 MPa.

Thus, the use of an amino acid makes it possible to work under advantageous conditions compared to the processes of the prior art, which makes it possible to use less expensive equipment. For example, an autoclave sized to reach a maximum pressure of 0.6 MPa can be used.

In general, the pressure of the hydrothermal treatment results from the temperature and from the compounds used for the reaction. Optionally, the pressure can be increased by an advantageously inert gas, for example nitrogen or argon.

This treatment is carried out at a pH below 7, more advantageously at a pH between 1 and 6, and even more advantageously at a pH between 3 and 5. However, in addition to the amino acid, the process is advantageously carried out without introducing Brônsted acid. Without wishing to be bound by any theory, it is possible that the reaction between the zirconium compound IV and water acidifies the reaction medium by formation of EL ions. Advantageously, no Brônsted acid base is introduced before carrying out the hydrothermal treatment.

When the hydrothermal treatment is carried out in the presence of additional water, the amino acid concentration is advantageously between 1 and 13 mol.l ¹ , more advantageously between 1, 5 and 4 mol.l ¹ . Still in the presence of additional water, the concentration of zirconium IV is advantageously between 0.1 and 8 mol.l ¹ , more advantageously between 0.5 and 2 mol.l ¹ . As already indicated, the process according to the invention can be carried out with a zirconium IV compound used in hydrated form, in this case it is not necessary to add water. In other words, even if it is possible to supply water, the number of water molecules of the zirconium compound IV in hydrated form is sufficient for the hydrothermal treatment and to lead to nanoparticles of dioxide of zirconium. Thus, in a particular embodiment, the water comes only from a hydrated form of the zirconium compound IV. This embodiment is also applicable when an amino acid precursor is used.

Besides the fact that in this particular embodiment the quantity of water used can be greatly reduced, it is particularly advantageous because the Applicant has noticed that the pressure could be greatly lowered. In fact, in this particular embodiment it is possible to carry out the hydrothermal treatment at a pressure greater than or equal to 0.1 MPa and less than or equal to 0.6 MPa, more advantageously less than or equal to 0.4 MPa.

The implementation of hydrothermal treatment without adding water generally leads to a viscous paste. Surprisingly, this viscous paste can be easily dispersed in water and a stable dispersion of nanoparticles of zirconium dioxide can be obtained.

Advantageously, the amino acid is present at the rate of a molar ratio relative to the zirconium compound IV of at least 1, more advantageously between 1 and 50, even more advantageously between 2 and 50 and even more advantageously between 3 and 30.

In the case where the molar ratio of amino acid to zirconium compound is less than 1, the amount of amino acid added is not sufficient to interact with the surface of the nanoparticles as a whole. This deficiency gives rise to the formation of polydisperse agglomerates of anisotropic nanoparticles of monoclinic ZrCE similar to those obtained by the hydrothermal treatment of a zirconium precursor in an aqueous medium in the absence of stabilizing agent.

Advantageously, the zirconium dioxide nanoparticles can be doped with different dopants, comprising the transition metals for example of yttrium and / or the lanthanides for example of gadolinium and / or cerium and more particularly the oxides of these metals, preferably Y2O3, CeC> 2 or Gd ₂ C> ₃ . To do this, the hydrothermal treatment can be carried out in the presence of one or more sources of dopant, preferably a source of yttrium and / or a source of cerium and / or a source of gadolinium. Preferably, the source of yttrium is YCb. Preferably, the source of cerium is CeCb. Preferably, the source of gadolinium is GdCb. In general, the source of dopant can be hydrated or not.

Generally, the source of doping element is introduced at a maximum of 20 mol%, advantageously at most 12 mol%, and advantageously greater than or equal to 0.1 mol%, more advantageously greater than or equal to 1 mol%, relative with zirconium IV. Thus, the source of doping element can in particular be between 0.1 and 20 mol%.

In general, the source of doping element is introduced into the reaction medium before the hydrothermal treatment.

Doping improves certain properties. For example in the case of yttrium, cerium and gadolinium, doping in particular makes it possible to modify the size, the morphology and the crystalline phase of the particles. In general, the particles have a smaller size, a higher quadratic phase content and a more spherical morphology with controlled doping of these elements. In addition, during the production of dense ceramic materials from these nanoparticles, the presence of doping elements makes it possible, depending on the doping rate, also to stabilize the quadratic phase at room temperature in a metastable state, making it possible to obtain solid ceramic materials without cracks and with remarkable mechanical properties of toughness and resistance to rupture. The cubic phase can also be stabilized in a fraction or in the whole of the material by increasing the amount of dopant to obtain materials with high ionic conductivity or high transmittance. During the sintering heat treatment, the presence of doping elements makes it possible to limit the growth of the grains in order to obtain a finer microstructure, often associated with increased hardness and with improved mechanical properties.

The method according to the invention can also include a step of purifying ZrCh nanoparticles. This step can be carried out by washing and / or rinsing, for example with water. This step notably makes it possible to eliminate the amino acid molecules which do not interact with the nanoparticles as a functionalizing and / or stabilizing agent. The ZrCh nanoparticles can be isolated, for example by drying, so as to obtain a powder. They can then be dispersed in a fluid, for example water, an alcohol, in particular glycerol or propylene glycol.

ZrO nanovarticles?

As already mentioned, the invention also relates to a dispersion in water of up to 80% by weight of zirconium dioxide nanoparticles stabilized with an amino acid and having a transmittance, for a dispersion of 40% by weight, advantageously going from 20% up to 83% at 400 nm wavelength and advantageously ranging from 95% up to 99.9% at 800 nm wavelength. It also relates to nanoparticles in powder form, advantageously stabilized with an amino acid. The amino acid advantageously comes from hydrothermal treatment.

By transmittance, we mean total transmittance, that is to say the sum of direct transmittances (corresponding to transmittance called "in line transmittance" in English) and indirect (corresponding to transmittance called "diffuse transmittance" in English) .

The total transmittance of the dispersion is generally measured in water at 20 ° C., for example with a double beam spectrophotometer, such as for example the V-670 model from JASCO, at ambient temperature. A quartz cell having an optical path length of 10 mm is used. The ZrCh nanoparticles are dispersed in water, at the desired concentration. The transmittance value measured on the dispersion, described as a percentage relative to the transmittance measured on the tank filled with deionized water, is an increasing function of the incident wavelength in the range 200 nm-1000 nm, including the range visible (380-780 nm). For example, the value measured at 600 nm will be greater than the value measured at 400 nm and less than the value measured at 800 nm. The total transmittance value at a fixed wavelength generally increases as the concentration of nanoparticles decreases. On the other hand, the total transmittance is not proportional to the concentration of ZrCh nanoparticles and there is no correlation between the transmittances at different wavelengths. In other words, it is not possible to predict transmittance at a given concentration or at a given wavelength from a single measurement at a different concentration or at a different wavelength. The level of organic matter (TMO) present in an aqueous dispersion is calculated as a percentage relative to the mass of nanoparticles. This measurement is carried out on the nanoparticles after drying at 120 ° C. using a Kern DBS type desiccator. The dry particles are introduced into a crucible, then the loss of mass as a function of the temperature is recorded by thermogravimetric analysis (ATG) using a thermal analyzer, for example of the TGA4000 type from Perkin Elmer, between 30 and 900 ° C at a speed of 10 ° C / min. The percentage of organic matter corresponds to the ratio between the loss of mass between 160 and 600 ° C and the initial mass of the dry particles.

This rate varies depending on the embodiment used. Advantageously, the organic matter content is less than or equal to 15% by weight, more advantageously less than or equal to 8% by weight, even more advantageously less than or equal to 5% by weight, and even more advantageously less than or equal to 3.5% by weight.

The dispersion refractive index is generally measured in water at 20 ° C at a wavelength of 589 nm, using a refractometer, such as the Abbemat 200 model from Anton Paar. The ZrCk particles are dispersed in water at the desired concentration. The refractive index value measured on the dispersion generally increases as the particle concentration increases.

In general, the dispersion according to the invention has a high refractive index, advantageously greater than or equal to 1.40 and less than or equal to 1.90 for a concentration of 40% by weight and greater than or equal to 1.50 and less than or equal to 2.00 for a concentration of 65% by weight.

Zirconia (zirconium dioxide) has one of the highest refractive indices in the family of metal oxides. The value of the refractive index varies slightly depending on the crystal structure (monoclinic, quadratic and cubic). In addition, in solid, well crystallized ceramic materials, it is between 2.16 and 2.24. This property of the material makes it possible to envisage its use to increase the refractive index of a medium, for example a solvent or a resin. If the nanoparticles are well dispersed in the medium and have a primary size sufficiently small to avoid light scattering phenomena, the resulting composite nanomaterial may have a high refractive index as well as good transmittance. The closer the refractive index of nanoparticles to the theoretical refractive index of ceramic materials (2.16 to 2.24), the more effective the nanoparticles are in increasing the refractive index of the composite, allowing d '' obtain a higher index for the same charge rate. Generally, well-crystalline nanoparticles have a higher refractive index.

The refractive index of a nanocomposite material with well dispersed charges and of approximately spherical morphology can be calculated from the refractive index of each of the components. Conversely, the refractive index of nanoparticles can be calculated from index measurements carried out on the nanocomposite at different concentrations with a model representative of the system. Different equations obtained from different mathematical models can be used for this calculation. Commonly used models in these systems include the linear approximation as well as the models of: Lorenz-Lorentz, Maxwell-Gamett and Bruggman (J. Humlicek, “Data Analysis for Nanomaterials: Effective Medium Approximation, Its Limits and Implémentations,” in Ellipsometry at the Nanos cale, 2013, pages 145-178).

The model that most underestimates the refractive index of nanoparticles is that of the linear approximation, which is described by the following equation:

RInc = f _m * RI _m + f _{\ p} * RI _{\ p}

In this equation, RI _nc is the refractive index of the nanocomposite, f _m the volume fraction of the medium, RI _m the refractive index of the medium, f _{\ p} the volume fraction of the nanoparticles and RINP the refractive index of the nanoparticles .

Advantageously, the zirconia nanoparticles according to the invention have a refractive index of between 2.0 and 2.2, more advantageously between 2.10 and 2.15. The refractive index of nanoparticles does not correspond to that measured in water for dispersions of nanoparticles. As indicated in the section concerning the examples according to the invention, it is determined from the measurements carried out on the dispersions of nanoparticles.

The viscosity of the dispersion is generally measured in water at 20 ° C, using a rheometer, such as for example the Kinexus Pro + model from Malvem Instruments. The ZrCh nanoparticles are dispersed in water, at the desired concentration. Viscosity is measured at different shear rates between 0.1 s ¹ and 100 s ¹ to verify that it is a Newtonian fluid with a constant viscosity as a function of the shear rate. The value measured at 1 s ¹ is reported as the value of the viscosity of the dispersion. The viscosity, at a fixed shear rate, generally increases as the concentration increases.

In general, the nanoparticles according to the invention have, in water and at a concentration of 40% by weight, a viscosity of between 1 and 10 mPa.s, advantageously between 2 and 8 mPa.s and so even more advantageous between 2 and 6 mPa.s.

The nanoparticles obtained by the process according to the invention have an average primary size, measured by image analysis from TEM (transmission electron microscopy) images of the nanoparticles deposited on a support transparent to the electron beam. Average primary size is defined as the average over at least 300 nanoparticles of the maximum length between two points on the surface of a nanoparticle. Thus, when the nanoparticle is perfectly spherical, the average primary size corresponds to the average diameter, while when the nanoparticle is not spherical the average primary size corresponds to the average height of the minimum cylinder in which the nanoparticle is inscribed. In this way, the average primary size of the nanoparticles obtained by TEM is advantageously between 2 nm and 60 nm, more advantageously between 3 nm and 40 nm, even more advantageously between 4 nm and 20 nm, and even more advantageously between 5 nm and 7 nm. A second indirect method for estimating the primary particle size is the Scherrer method. This method makes it possible to calculate the primary size from the measurement of the width of the main diffraction peaks at mid-height (full width at half maximum - FWHM) after having subtracted the Ka2 component from the diffraction spectrum and having corrected the measurement. FWHM taking into account the widening of the peaks due to the device (instrumental peak broadening). The peaks selected for the measurement are (- 111) and (111) for the monoclinic phase and (111) for the quadratic / cubic phase. The size of the crystallites is then calculated using the Scherrer equation with a form factor of 0.89.

The DLS technique (from the acronym “Dynamic Light Scattering” meaning dynamic light scattering) makes it possible, by spectroscopic analysis, to measure the hydrodynamic size of particles present in a liquid medium, for example an aqueous medium such as water . In general, the hydrodynamic size is different from the primary particle size measured by TEM or by DRX (Scherrer's method). This technique is also sensitive to the presence of agglomerates of particles. The case if necessary, the value measured by DLS is greater than the size of the particle in the absence of agglomerates. In addition, this analysis allows to give as results, the average hydrodynamic size in volume Dv, in intensity Di, in number DN as well as the associated D50 and D90. The value D50 corresponds to the hydrodynamic diameter for which 50% of the particles measured have a diameter less than or equal to D50. The value D90 corresponds to the hydrodynamic diameter for which 90% of the particles measured have a diameter less than or equal to D90. These latter values can be calculated in volume (Dvso / Dv9o), number (DNSO / DN9O) OR intensity (D150 / BOD).

The number average hydrodynamic size (DN) is advantageously between 3 nm and 50 nm, more advantageously between 4 nm and 30 nm. They have a DNSO size advantageously between 3 nm and 50 nm and more advantageously between 4 nm and 35 nm. They have a size D _N 90 advantageously between 5 nm and 50 nm and more advantageously between 6 nm and 40 nm.

The volume-average hydrodynamic size (Dv), measured by DLS, is advantageously between 3 nm and 50 nm, more advantageously between 4 nm and 30 nm. The nanoparticles have a size Dvso advantageously between 3 nm and 50 nm and more advantageously between 4 nm and 30 nm. They have a size Dv90 advantageously between 5 nm and 70 nm and more advantageously between 6 nm and 40 nm.

The dispersion index (DI) corresponds to the ratio between the hydrodynamic size of the particle in volume measured by DLS and the primary size of the particle measured by TEM.

In general, the dispersion according to the invention has, in water, a very low dispersion index advantageously between 1 and 7, more advantageously between 1 and 4, even more advantageously between 1 and 2, even more advantageously between 1 and 1.5.

Depending on the experimental conditions, the nanoparticles obtained by the process according to the invention can be in quadratic / cubic or monoclinic crystalline form or in a mixture of the two. In a particular embodiment, the dispersion, or the paste, can be dried. In this case, a powder is obtained. Thus, the invention also relates to a powder obtained by drying the dispersion, or the paste, described above.

Obtaining a powder form provides a definite advantage since a powder can be stored and transported in a smaller volume than a dispersion. Furthermore, the powder obtained by drying the dispersion according to the invention can be easily dispersed not only in water but also in an organic solvent such as acetone, alcohols such as ethanol, isopropanol, glycerol or propylene glycol. It can also be dispersed in a resin, in particular for manufacturing composites. In general, good dispersion in organic solvents with different polarities is obtained during the substitution of the amino acid with another organic molecule according to the general knowledge of the skilled person.

Similarly, the dispersion of the powder can be carried out at any pH, advantageously between 3 and 12. It can be carried out from nanoparticles whose amino acid has been replaced by an organic molecule.

Indeed, in a particular embodiment, the amino acid molecules are displaced after the formation of zirconium dioxide nanoparticles.

As already mentioned, a dispersion, or a powder, of zirconium dioxide nanoparticles can be mixed with a resin to form a composite material.

Use of ZrO nanovarticles?

Zirconia is one of the most used materials in the dental sector for the formulation of dental composites. The latter are photopolymerizable materials (near UV light, wavelength of 400 nm +/- 20 nm) consisting of an organic phase and an inorganic phase. The organic phase is essentially composed of a di-methacrylate monomer, a polymerization agent and a photo-initiator. The inorganic phase consists of mineral fillers intended to provide or reinforce certain properties such as, for example, improving the mechanical properties of the composite, reducing the polymerization shrinkage, providing radiopacity to the dental composite. The dispersion of zirconia nanoparticles according to the invention is of particular interest in the dental field thanks to its high transmittance thus promoting deep light-curing of the dental composite and improved aesthetic properties.

Thus, composite materials particularly suitable for the dental field can be obtained from the dispersion described above.

ZrCh nanoparticles can also be used for their high refractive index (areas of surface coatings, composite materials, adhesives), their biocompatibility (areas of dental and orthopedic prostheses), their dielectric permittivity (area of electronics) , their aesthetic quality (jewelry and watchmaking). They are also used for shaping and densification of a dense ceramic material or coating, which may have mechanical properties, such as resistance to abrasion, to bending or to compression and to toughness, and improved aesthetic properties, such as visible transmittance (areas of technical ceramics, biomedical ceramics and 3D-printing materials).

The invention and the advantages which ensue therefrom will emerge more clearly from the following figures and examples given in order to illustrate the invention and not in a limiting manner.

[Fig. 1] FIG. 1 is a TEM image of the zirconium dioxide nanoparticles of Example 16.

[Fig. 2] FIG. 2 is a photograph of different samples of dispersions according to the invention.

EXAMPLES OF EMBODIMENT OF THE INVENTION

Examples 1 to 23 (invention)

Unless otherwise indicated, the percentages of the constituents of a dispersion are expressed by weight.

Doping is expressed in terms of molar percentage relative to the molar amount of zirconium. This is generally the content of yttrium oxide (Y2O3), cerium oxide (CeCh) or gadolinium oxide (Gd ₂ C> ₃ ). Thus, 3 mol% of yttrium corresponds to a doping of 3mol% of Y ₂ 0 ₃ . In an autoclave, the zirconium compound IV, the amino acid, its precursor or a mixture of several amino acids and, if appropriate, water and / or a dopant are introduced. The autoclave is then sealed and the desired temperature and pressure are applied.

The process conditions are listed in Table 1 below. The pH before hydrothermal treatment of examples 1 to 23 is between 1 and 5.

[Table 1]

AAC4 = 4-aminobutyric acid

AAC6 = 6-aminocaproic acid

CEI to CE6 examples (comparative examples)

According to the method indicated for Examples 1 to 23, comparative tests were carried out (Table 2) in the presence of the following compounds or mixtures:

Butanoic acid + butylamine (example CEI).

Amino acid with two amino functions and one acid function (example CE2). - Butanoic acid (example CE3).

Butylamine (example CE4).

Aminocaproic acid + NH ₄ OH added gradually, leading to a pH of 12 before the hydrothermal treatment (example CE5).

Addition of NaOH, formation of a precipitate purified with water (up to a conductivity of less than 250 pS / cm), addition of acetic acid, the final pH being less than 7 (example CE6, produced according to EP 2 371 768). Table 2]

Once the hydrothermal treatment is complete, the temperature and pressure are lowered so that the autoclave can be opened securely. The reaction media are then optionally washed and / or diluted or concentrated in order to obtain dispersions in water, fluids, having a concentration of 40% or 65% by weight of nanoparticles of zirconium dioxide.

Each of the dispersions obtained is then analyzed. The agglomeration state of the dispersion is determined by DLS. The stability of the dispersion at different concentrations is determined visually by the appearance or not of a precipitate after 10 days of waiting, without disturbance, at 20 ° C. The dispersion index (DI) is determined by the ratio between the hydrodynamic size of the particle in volume measured by DLS and the primary size of the particle measured by TEM, only in the examples having a single crystalline phase with a spherical morphology, which can correspond to the monoclinic phase (M) when the particle size is less than 10 nm or to the quadratic / cubic phase (Q). In the case where the dispersion has different crystalline phases, including a monoclinic phase (M) having anisotropic particles with a primary size greater than 10 nm, the DI is not calculated. The primary size measured by TEM, on the particles of monoclinic phase and of anisotropic shape of more than 10 nm, corresponds to the length of the major axis of the particle. The viscosity is determined with a Malvem Instrument, model Kinexus Pro +, using the cone / plane geometry of 40 mm in diameter and 4 ° inclination at 20 ° C. The refractive index of the dispersion is determined at 20 ° C, at 589 nm, with an Anton Paar device, model Abbemat 200.

X-ray diffractograms were obtained from nanoparticle powders from Examples 1-23 after drying at 120 ° C. As such, a Bruker D8 Advance diffractometer with CuKalpha radiation was used for 2-theta angles between 10-75 ° and the characteristic peaks were identified and attributed to either the quadratic / cubic phase (Q) or to the phase monoclinic (M) by comparison with the X-ray diffraction database of the International Center for Diffraction Data. The relative intensity of peak 111 in phase Q and the sum of the relative intensities of peaks -111 and 111 in phase M were used to determine the majority phase (Table 3).

Table 3]

Dv = hydrodynamic diameter by volume

T% = transmittance in percent

Q = quadratic / cubic phase particles

M = particles of monoclinic phase

In all of examples 1 to 23 according to the invention, a stable dispersion of ZrCh nanoparticles at each of the concentrations was obtained.

[Table 4]

(1) formation of a precipitate at all concentrations

(2) dispersion at a concentration less than or equal to 40% by weight; solid formation at a concentration greater than 40% by weight

In the absence of amino acid, or when the amino acid does not correspond to that used in the invention, or when a mixture of acid and amine is used, or when in the presence of amino acid used in the invention the pH is greater than 7, the dispersions obtained are not stable, in particular due to the presence of agglomerates of particles. Thus, among the counterexamples, only CE6 makes it possible to obtain a stable dispersion when the concentration is less than or equal to 40% by weight. On the other hand, beyond 40% by weight of nanoparticles, the dispersion becomes very viscous and begins to solidify and dry. Thus, it is not possible to obtain a dispersion at a concentration greater than 40% by weight of nanoparticles in the case of the counterexample CE6.

The transmittances at a concentration of 40% by weight of a dispersion of nanoparticles according to examples 6, 12 to 16, 19 and 22 and according to the counter-example CE6 were measured. The transmittance, in the range of 400 to 800 nm, was determined with a Jasco model V-670 device. These are listed in Table 5.

[Table 5]

Thus, at the maximum concentration making it possible to obtain a stable dispersion according to the CE6 counterexample, namely 40% by weight of nanoparticles, the dispersions according to the invention have better total transmittance at 400 nm and at 800 nm.

Furthermore, excellent transmittances, even at 65% by weight can be obtained. The transmittance of dispersions with the nanoparticles of Examples 14, 15 and 16 were measured according to the above method at different concentrations. The results are listed in Table 6 below. Table 6]

FIG. 2 visually illustrates the transmittance of dispersions according to the invention. Three samples 1, 2 and 3 of dispersions according to the invention are positioned in front of an image 4. Sample 1 is a dispersion comprising 50% by weight of nanoparticles from Example 16. Sample 2 is a dispersion comprising 65% by weight of nanoparticles of example 16. Sample 3 is a dispersion comprising 40% by weight of nanoparticles of example 9. The dispersion of sample 3 has a lower transmittance than the dispersions of the two samples 1 and 2. The transmittance of sample 3 is still very satisfactory, since the image 4 positioned behind the samples can be easily distinguished, despite a slight coloration of sample 3. The two samples 1 and 2 have , as for them, a very high transmittance which makes it possible to see image 4 very distinctly without modification of the color. The magnifying effect seen in Figure 2 is due to the containers and not to the dispersions.

Examples 24 and 25:

The particles from Examples 12 and 16 were redispersed in acetone using a precipitation and functionalization procedure with a molecule endowed with a phosphate function. Stable and transparent dispersions of particles in acetone have been obtained. Examples 26 and 27:

The particles from Examples 12 and 16 dispersed in acetone, after replacement of the amino acid by a molecule endowed with a phosphate function, were redispersed in a monomer, such as 1, 10-decanediol dimethacrylate (D3MA) , using a procedure of incorporation and evaporation of the initial solvent. Stable and transparent dispersions of particles in D3MA were obtained.

Example 28:

The particles from Example 16 were redispersed in propylene glycol, without substitution of the amino acid, using a precipitation and redispersion procedure in the final solvent. A stable and transparent dispersion of particles in propylene glycol was obtained.

The total transmittances at different concentrations of the dispersions of Examples 24 to 28 were measured. The total transmittance value is described as a percentage relative to the transmittance measured on the tank filled with the corresponding pure solvent. These are listed in Table 7.

[Table 7]

It is thus possible to redisperse the zirconium nanoparticles obtained according to the invention in a solvent other than water and to maintain excellent transmittance at high concentration, as well as low viscosity and great stability over time.

Example 29 (invention):

Example 29 can be compared to Example 9, only two synthesis parameters are modified. The amounts of zirconium precursors, yttrium, aminobutyric acid and water are the same, 6-aminocaproic acid is added. The procedure for dissolving solids is different. The zirconium precursor is dissolved in water together with 4-aminobutyric acid. The yttrium precursor is dissolved in water in the presence of 6-aminocaproic acid. After complete dissolution, the two solutions are mixed.

36 mmol of zirconium oxychloride, 119 mmol of 4-aminobutyric acid (AAC4) and 36 ml of water are introduced into a beaker. In a second beaker, yttrium chloride (doping with 6 mol% Y2O3), 23.8 mmol of 6-aminocaproic acid (AAC6) and 36 ml of water are introduced. After complete dissolution the 2 solutions are mixed and introduced into a 100 ml autoclave. The autoclave is then sealed and heated to 200 ° C for 3 h, the pressure is between 12 and 15 bar.

In Example 29, a stable dispersion of ZrC nanoparticles was obtained.

Unlike example 9, the diffractogram obtained by X-ray diffraction analysis reveals the unique presence of the quadratic / cubic phase. Image analysis from TEM images of the nanoparticles obtained in Example 29 reveals only the presence of spherical particles with an average primary size of 20 nm.

Example 30:

Nanoparticles obtained according to Examples 12 and 15 were washed with water and concentrated to different concentrations. The level of amino acid present on the surface of the nanoparticles was measured by ATG (thermogravimetric analysis). The refractive index of the resulting dispersions was measured using a refractometer (Anton Paar, Abbemat 200) at a wavelength of 589 nm and a temperature of 20 ° C. The density of nanoparticles having an amino acid on the surface (or functionalized) was calculated by linear approximation. The refractive index of the functionalized nanoparticles was calculated by linear regression from the refractive index values of the dispersions measured as a function of the volume fraction of the functionalized nanoparticles according to the linear approximation model. Taking into account the level of functionalizing present on the surface, the refractive index of nonfunctionalized (or naked) nanoparticles was calculated. The results of these measurements and calculations are presented in Table 8:

[Table 8]

For the nanoparticles from Example 12, after washing, the level of amino acid present on the surface and measured by ATG is 4.3% by mass of the total mass of the nanoparticles after drying, the density of the non-functionalized nanoparticles is 6.14 g / cm ³ . The density of the functionalized nanoparticles is 5.16 g / cm ³ . The density of the medium is 0.998 and its refractive index is 1.3330. The density of the functionalizing agent (AAC6) is 1.13 g / cm ³ and its index is 1.4870. The refractive index of the resulting nonfunctionalized nanoparticles is 2.1434 and the associated coefficient of determination is 0.9990. For the nanoparticles from Example 15, after washing, the level of amino acid present on the surface and measured by ATG is 6.3% by mass of the total mass of the nanoparticles after drying, the density of the non-functionalized nanoparticles is 6.00 g / cm ³ . The density of the functionalized nanoparticles is 4.72 g / cm ³ . The density of the medium is 0.998 and its refractive index is 1.3330. The density of the functionalizing agent (AAC6) is 1.13 g / cm ³ and its index is 1.4870. The refractive index of the resulting nonfunctionalized nanoparticles is 2.1011 and the associated coefficient of determination is 0.9999. Note that the linear approximation model underestimates the refractive index of nanoparticles. Consequently, the non-functionalized nanoparticles have a refractive index greater than or equal to that calculated.

Claims

1. Process for the preparation of zirconium dioxide nanoparticles, ZrCh, by hydrothermal treatment of a zirconium compound IV in the presence of water, at a pH below 7, and at least one amino acid comprising at least 4 atoms carbon, said amino acid having an acid function to amine function ratio greater than or equal to 1.

2. Method according to claim 1, characterized in that the hydrothermal treatment is carried out at a temperature greater than or equal to 100 ° C, and at a pressure less than or equal to 2 MPa.

3. Method according to one of the preceding claims, characterized in that the zirconium compound IV is chosen from the group consisting of zirconium halides, preferably the zirconium compound IV is zirconium oxychloride.

4. Method according to one of the preceding claims, characterized in that the amino acid is chosen from the group consisting of aminobutanoic acids, aminopentanoic acids and aminohexanoic acids, preferably in the group consisting of 4-aminobutyric acid, the norvaline, 5-aminovaleric acid and 6-aminocaproic acid.

5. Method according to one of the preceding claims, characterized in that the amino acid is formed in situ, by hydrolysis, from an amino acid precursor advantageously chosen from the group of lactams, advantageously from the group consisting of pyrrolidone, N-methylpyrrolidone.

6. Method according to one of the preceding claims, characterized in that the water of the hydrothermal treatment comes solely from a hydrated form of the zirconium compound IV.

7. Method according to one of the preceding claims, characterized in that the hydrothermal treatment is carried out at a pressure between 0.1 MPa and 0.6 MPa.

8. Method according to one of the preceding claims, characterized in that the amino acid is present at the rate of a molar ratio relative to the zirconium compound IV of between 1 and 50, advantageously between 3 and 30.

9. Method according to one of the preceding claims, characterized in that the hydrothermal treatment is carried out in the presence of one or more sources of dopant, preferably a source of yttrium and / or a source of cerium and / or a source gadolinium.

10. Zirconium dioxide nanoparticles, having a visible transmittance greater than or equal to 20% at 400 nm and greater than or equal to 95% at 800 nm measured in water at a concentration of 40% by weight, at 20 ° C with an optical path length of 10 mm.

11. Zirconium dioxide nanoparticles according to claim 10, characterized in that they have, in water and at a concentration of 40% by weight, a viscosity of between 1 and 10 mPa.s, advantageously between 2 and 8 mPa.s and even more advantageously between 2 and 6 mPa.s.

12. Zirconium dioxide nanoparticles according to one of claims 10 or

11, characterized in that they have, in dispersion in water, a refractive index greater than or equal to 1.40 and less than or equal to 1.90 for a concentration of 40% by weight and greater than or equal to 1, 50 and less than or equal to 2.00 for a concentration of 65% by weight.

13. Zirconium dioxide nanoparticles according to one of claims 10 to 12, characterized in that they have, in dispersion in water, a dispersion index of between 1 and 7, more advantageously between 1 and 4, even more advantageously between 1 and 2, even more advantageously between 1 and 1.5.

14. Zirconium dioxide nanoparticles according to one of claims 10 to 13, characterized in that they have a refractive index of between 2.0 and 2.2, advantageously between 2.10 and 2.15.

15. Dispersion comprising nanoparticles of zirconium dioxide, ZrC, according to one of claims 10 to 14.