Nanofeuillets de BaTiO3

Abstract

BaTiO 3 nanosheets The present invention relates to BaTiO3 nanoparticles in the form of nanosheets and a method for preparing them. The invention also relates to the use of these BaTiO3 nanosheets for the formulation of inks with particular dielectric properties. Figure for abstract: none

Description

Title of the invention: BaTiO3 nanosheets

[0001] The present invention relates to BaTiO3 nanoparticles in the form of nanosheets and a method for preparing them. The invention also relates to the use of these BaTiO3 nanosheets for the formulation of inks with particular dielectric properties.

[0002] Dielectric inks are typically used in printing processes such as inkjet printing or screen printing, to create electronic circuits or components on different substrates. They can find applications in various industries, including printed electronics, the Internet of Things (IoT) and energy storage. Today, the miniaturization and integration of printed electronics requires the development of new dielectric inks with superior insulating properties, high resolution and compatibility with various printing techniques.

[0003] Most existing dielectric inks are composed of pure polymers and / or spherical nanoparticles, and require the use of a large amount of harmful organic solvents. In addition, the polymers used generally have a low dielectric constant. Another type of ink is based on the use of metal oxide nanoparticles. These nanoparticles have higher dielectric performance and can be dispersed in water or in a solvent by a stabilizer to formulate suspensions. However, most of these inks are composed of 0D spherical nanoparticles or 1D nanofibers which have reduced specific surface areas, due to the challenge of synthesizing 2D nanosheets.

[0004] In this context, the present invention aims to provide BaTiO3 nanosheets having a high aspect ratio and dielectric constant.

[0005] In particular, the present invention aims to provide a method for preparing such nanosheets by a soft chemistry method, which can be carried out on a large scale.

[0006] The present invention also aims to provide a method for controlling the size of BaTiO3 nanosheets.

[0007] The present invention aims in particular to provide BaTiO3 nanosheets making it possible to prepare dielectric inks having superior insulating properties and high resolution, and which are compatible with various printing techniques.

[0008] The present invention also aims to provide BaTiO3 nanosheets making it possible to prepare dielectric inks having better adhesion to the printing substrate and better printing quality.

[0009] The present invention also aims to provide BaTiO3 nanosheets making it possible to prepare dielectric inks which have improved film-forming properties when they are in the form of printed films.

[0010] The present invention also aims to provide BaTiO3 nanosheets for preparing dielectric inks which have improved flexibility when in the form of printed films.

[0011] The present invention also aims to provide BaTiO3 nanosheets making it possible to prepare dielectric inks which, in the form of printed films, have a limited quantity of defects and cracking phenomena.

[0012] The subject of the invention is therefore a BaTiO3 nanosheet characterized in that the ratio between the length Ln of said nanosheet and the thickness en of said nanosheet is greater than or equal to 25.

[0013] In particular, said nanosheet is a particle which extends in three dimensions L n, ln and perpendicular to each other, and characterized in that:

[0014] - Ln is greater than or equal to ln, and

[0015] - Ln and ln are much greater than en, typically the ratio between Ln and en is greater or equal to 6, preferably greater than or equal to 8, preferably greater than or equal to 10, and / or the ratio between ln and en is greater than or equal to 6, preferably greater than or equal to 8, preferably greater than or equal to 10.

[0016] According to the invention, Ln, ln and en are respectively the length, the width and the thickness of the BaTiO3 nanosheet.

[0017] In particular, the aspect ratio is called the ratio of a length to a thickness.

[0018] Preferably, the BaTiO3 nanosheet has a Sn surface area greater than or equal to 10,000 nm2 and / or less than or equal to 1 pm2, preferably between 10,000 nm2 and 1 pm2.

[0019] Typically, the surface area Sn of the BaTiO3 nanosheet is defined as the product of its length Ln by its width ln, i.e. SJ? = L„ x ln.

[0020] Preferably, the BaTiO3 nanosheet is characterized in that it has a thickness of less than or equal to 15 nm, preferably less than or equal to 10 nm, preferably less than or equal to 8 nm, preferably less than or equal to 5 nm, preferably less than or equal to 3 nm.

[0021] In particular, the BaTiO3 nanosheet has a length Ln less than or equal to 1 pm, preferably less than or equal to 800 nm, preferably less than or equal to 600 nm, preferably less than or equal to 400 nm, preferably less than or equal to 300 nm.

[0022] According to one embodiment, the BaTiO3 nanosheets have, at 100 Hz, a dielectric constant er greater than 150, preferably greater than 200, of preferably greater than 300, preferably greater than 400, more preferably greater than 500.

[0023] Typically, the dielectric constant er, also called relative permittivity er, is defined by the following relationship:

[0024] E = eo x

[0025] in which:

[0026] - e is the permittivity of the medium, and

[0027] - e0 is the permittivity of vacuum, defined as a constant e0 = 8.854 187.10 12 F / m.

[0028] Permittivity is defined as the quotient of the "electric induction" vector by the "electric field" vector which produces it, it describes the response of a given medium to an applied electric field. The dielectric constant er can be calculated from the impedance measurements of a material at a given frequency.

[0029] The breakdown electric field, or breakdown voltage, is defined as the maximum voltage that a material can withstand before dielectric failure.

[0030] According to one embodiment, the BaTiO3 nanosheets have a breakdown electric field greater than or equal to 100 MV / m, preferably greater than or equal to 250 MV / m, preferably greater than or equal to 400 MV / m.

[0031] According to one embodiment, the BaTiO3 nanosheets have a mass of from 1.8 x 10 16 g to 6 x 10 12 g.

[0032] By definition, electrical losses quantify the dissipation of electromagnetic energy from a dielectric material. These electrical losses are usually measured by the loss angle, also called loss tangent or tan(ô), defined as the ratio between the imaginary part (e”) and the real part (e') of the dielectric constant of a material:

[0033] tanri-f

[0034] According to one embodiment, the BaTiO3 nanosheets have, at 100 Hz, loss tangents tan(ô) less than or equal to 0.5, preferably less than or equal to 0.1.

[0035] Preferably, the BaTiO3 nanosheets have, at 100 Hz, an electrical conductivity less than or equal to 106 S / m, preferably less than or equal to 107 S / m, more preferably less than or equal to 108 S / m.

[0036] The invention also relates to a process for preparing nanosheets as described above.

[0037] In particular, the process for preparing BaTiO3 nanosheets according to the invention comprises a step of reacting TiO2 nanosheets with barium hydroxide Ba(OH)2, in the presence of at least one surfactant and at least one co-surfactant,

[0038] said surfactant being chosen from poloxamers,

[0039] and said co-surfactant being chosen from glycols.

[0040] Typically, said step of reacting TiO2 nanosheets with barium hydroxide Ba(OH)2 is step (ii).

[0041] Preferably, the method according to the invention further comprises a step of synthesizing said TiO2 nanosheets. Typically, this step corresponds to step (i), and is followed by step (ii).

[0042] Advantageously, the process for preparing BaTiO3 nanosheets according to the invention optionally comprises a step (i) of synthesizing TiO 2 nanosheets, followed by a step (ii) of reacting said TiO2 nanosheets with barium hydroxide Ba(OH)2, in the presence of at least one surfactant and at least one co-surfactant,

[0043] said surfactant being chosen from poloxamers,

[0044] and said co-surfactant being chosen from glycols. Step (i)

[0045] The method according to the invention may comprise a step (i) of synthesis of TiO2 nanosheets.

[0046] Preferably, step (i) of synthesizing TiO2 nanosheets comprises:

[0047] - the preparation of a solution of titanium isopropoxide in a strong acid, of preferably in hydrochloric acid, and / or

[0048] - mixing said titanium isopropoxide solution with a chosen surfactant among poloxamers, preferably Pluronic P123, and optionally a co-surfactant chosen from glycols, preferably chosen from ethylene glycol, diethylene glycol, and propylene glycol, preferably ethylene glycol, and optionally water.

[0049] Preferably, in step (i), the strong acid is present in a concentration of from 0.05 mol.L 1 to 0.5 mol.L '.

[0050] Preferably, in step (i), the titanium isopropoxide is present in a concentration of from 0.02 mol.L 1 to 0.2 mol.L *.

[0051] Preferably, in step (i), the surfactant is present in a concentration of from 1 mg.mL to 10 mg.mL.

[0052] Preferably, step (i) of the process of the invention is carried out by hydrolysis of titanium isopropoxide.

[0053] Preferably, the synthesis of step (i) is carried out at a temperature greater than or equal to 140°C, preferably greater than or equal to 150°C, preferably greater than or equal to 160°C. Preferably, the synthesis of step (i) is carried out at a temperature of from 140°C to 180°C.

[0054] Preferably, the synthesis of step (i) is carried out for a period of time between between 10 a.m. and 30 a.m., preferably between 3 p.m. and 12 p.m., preferably between 6 p.m. and 10 p.m. Preferably, the reaction of step (i) is carried out for a period greater than or equal to 20 hours.

[0055] Advantageously, the synthesis of step (i) is carried out in a closed, preferably hermetic, environment, for example in an autoclave. In particular, the synthesis is carried out at a temperature below the boiling point of the reaction solvent, and in particular at a temperature below the boiling point of ethylene glycol, typically at a temperature below 197.3°C. Typically, the synthesis is carried out at a pressure above ambient pressure, typically at a pressure above atmospheric pressure.

[0056] In particular, step (i) of the process of the invention is a solvothermal reaction, preferably hydrothermal.

[0057] Advantageously, the synthesized TiO2 nanosheets are washed, typically with an alcohol solution, preferably with ethanol.

[0058] In particular, TiO2 nanosheets are particles which extend in three dimensions Ln', ln' and en' perpendicular to each other, and characterized in that:

[0059] - Ln' is greater than or equal to ln', and

[0060] - Ln' and ln' are much greater than en', typically the ratio between Ln' and en' is greater than or equal to 5, preferably greater than or equal to 8, preferably greater than or equal to 10, and / or the ratio between ln' and en' is greater than or equal to 5, preferably greater than or equal to 8, preferably greater than or equal to 10.

[0061] According to the invention, Ln', ln' and en' are respectively the length, the width and the thickness of a TiO2 nanosheet according to the invention.

[0062] Typically, TiO2 nanosheets have a surface area Sn' which is defined as the product of the length Ln' by the width ln', i.e. S„' = Ln' x ln'.

[0063] Preferably, the surface area Sn' is greater than or equal to 100 nm2, preferably less than or equal to 10,000 nm2, preferably between 100 nm2 and 10,000 nm2.

[0064] Preferably, the TiO2 nanosheets obtained during step (i) are characterized in that they have a length Ln' of between 10 and 100 nm.

[0065] In particular, the TiO2 nanosheets obtained during step (i) are characterized in that they have a thickness en' of less than or equal to 2 nm, preferably 1.5 nm, preferably 1 nm. Preferably, the TiO2 nanosheets obtained during step (i) are characterized in that they have a thickness en' of from 0.4 nm to 2 nm, preferably 0.5 nm to 1.5 nm, preferably 0.6 nm to 1 nm.

[0066] In particular, step (i) is followed by step (ii). Step (ii)

[0067] The method according to the invention may further comprise a step (ii) of reaction of TiO2 nanosheets, preferably obtained during step (i), with barium hydroxide Ba(OH)2, in the presence of at least one surfactant and at least one co-surfactant, said surfactant being chosen from poloxamers, and said co-surfactant being chosen from glycols.

[0068] Preferably, the reaction of step (ii) is carried out at a temperature greater than or equal to 140°C, preferably greater than or equal to 150°C, preferably greater than or equal to 160°C, preferably greater than or equal to 170°C, preferably from 140°C to 180°C.

[0069] Preferably, the reaction of step (ii) is carried out for a period of between 30 minutes and 48 hours, preferably between 1 hour and 40 hours, preferably between 2 hours and 35 hours, preferably between 3 hours and 30 hours, preferably between 5 hours and 25 hours, preferably between 10 hours and 20 hours. Preferably, the reaction of step (ii) is carried out for a period of greater than or equal to 3 hours.

[0070] Advantageously, the reaction of step (ii) is carried out in a closed, preferably hermetic, environment, for example in an autoclave. In particular, the reaction is carried out at a temperature below the boiling point of the reaction solvent, and in particular at a temperature below the boiling point of ethylene glycol, typically at a temperature below 197.3°C. Typically, the reaction is carried out at a pressure above ambient pressure, typically at a pressure above atmospheric pressure.

[0071] According to one embodiment, the reaction of step (ii) is a solvothermal reaction, preferably hydrothermal.

[0072] Preferably, the surfactant used in step (ii) is chosen from the poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) three-block copolymers of the following formula (I):

[0074] in which:

[0075] - x is an integer from 5 to 150, and

[0076] - y is an integer from 30 to 70.

[0077] Preferably, x is 20 and / or y is 70.

[0078] Preferably, the surfactant is chosen from poloxamer copolymers, in particular of formula (I), having a molar mass Mn of from 2,000 g.mol1 to 12,600 g.mol *, preferably of from 5,000 g.mol1 to 6,000 g.mol '.

[0079] Advantageously, the surfactant is chosen from the poloxamer copolymers Pluronic P123, Pluronic L61, Pluronic L121, Pluronic F68 and Pluronic F127, preferably the surfactant is the poloxamer copolymer Pluronic P123.

[0080] Preferably, the surfactant is chosen from poloxamer copolymers, in particular of formula (I), having a molar mass of 2,000 g.mol *, 4,400 g.mol *, 5,800 g.mol *, 8,400 g.mol1 or 12,600 g.mol *, preferably having a molar mass of 5,800 g.mol *.

[0081] Typically, a surfactant that can be used is P123 marketed by Sigma-Aldrich under the reference Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) with a molar mass Mn = 5,800 g.mol *, also called Pluronic P-123, PEG-PPG-PEG (CAS number: 9003-11-6).

[0082] Preferably, the co-surfactant used in step (ii) is chosen from ethylene glycol, diethylene glycol, and propylene glycol, preferably ethylene glycol.

[0083] Advantageously, the BaTiO3 nanosheets obtained at the end of step (ii) are washed, typically with water.

[0084] The invention also relates to the use of BaTiO3 nanosheets as described above, for the manufacture of dielectric inks.

[0085] The invention also relates to the formulation of dielectric inks comprising BaTiO3 nanosheets as described above.

[0086] Preferably, said formulation comprises BaTiO3 nanosheets according to the invention, and optionally at least one polymer additive chosen from polyvinyl alcohol (PVA), poly(vinyl butyral) (PVB), cellulose derivatives such as hydroxyethylcellulose (HEC) or methylcellulose (MC), and amphiphilic block copolymers such as polyethylene glycol - polycaprolactone (PEG-PCL), polyethylene glycol - polypropylene glycol (PEG-PPG), polyvinylpyrrolidone - polystyrene (PVP-PS), polyethylene oxide - polypropylene oxide (PEO-PPO), preferably polyvinyl alcohol (PVA).

[0087] According to one embodiment, the dielectric ink formulation according to the invention comprises from 1 to 5% by weight of BaTiO3 nanosheets relative to the total mass of the ink formulation, preferably from 1.5 to 4% by weight, preferably from 2 to 4% by weight, preferably from 2.5 to 3% by weight.

[0088] According to one embodiment, the dielectric ink formulation according to the invention comprises from 1 to 5% by weight of at least one polymer additive relative to the total mass of the ink formulation, preferably from 1.5 to 4% by weight, preferably from 2 to 4% by weight, preferably from 2.5 to 3% by weight.

[0089] Preferably, the dielectric ink formulation according to the invention has a ratio of weight of BaTiO3 nanosheets: weight of polymer additive of between 2:1 and 1:2, preferably 1:1.

[0090] Advantageously, the dielectric ink formulation according to the invention comprises at least 80% by weight of water relative to the total mass of the ink formulation, preferably at least 85% by weight, preferably at least 90% by weight, preferably at least 94% by weight.

[0091] Preferably, the BaTiO3 nanosheets used for the formulation of dielectric inks and obtained during step (ii) are first dispersed in water, and optionally washed with an alcohol solution, preferably ethanol.

[0092] In particular, the BaTiO3 nanosheets used for the formulation of dielectric inks according to the invention are previously heat-treated following their synthesis described in step (ii). Typically, said BaTiO3 nanosheets are optionally dried, preferably at a temperature greater than or equal to 80°C, preferably greater than or equal to 85°C, more preferably greater than or equal to 90°C, and then may be heated to a temperature of at least 600°C, preferably greater than or equal to 1000°C, preferably for a period of at least 4 hours. After heat treatment, the BaTiO3 nanosheets may be dispersed in water, before adding at least one polymer additive for the dielectric ink formulation as described above. Description of figures

[0093] [Fig.l] [Fig.l] represents a TEM image of TiO2 nanosheets obtained after washing and re-dispersion in water, according to the synthesis described in Example 1.

[0094] [Fig.2] [Fig.2] represents the TEM images of BaTiO3 nanosheets obtained after washing and re-dispersion in water, according to the synthesis described in Example 2. Shown from left to right are the BaTiO3 nanosheets obtained with P123 concentrations of 0.5, 1, 2 and 3 mg.mL ' respectively, and from top to bottom with a volume percentage of ethylene glycol of 3, 6, 9 and 12% respectively.

[0095] [Fig 3] [Fig 3] represents the TEM images of BaTiO3 nanosheets obtained after washing and re-dispersion in water, according to the synthesis described in Example 2, for different surfactants according to formula (I) (with x and y as described in formula (I) and Mn the molar mass of the surfactant):

[0096] - Figure 3a: P123, Mn ~ 5,800 g.mol *; x ~ 20; y ~ 70,

[0097] - Figure 3b: L61, Mn ~ 2,000 g.mol *; x «5; y “30,

[0098] - Figure 3c: L121, Mn ~ 4,400 g.mol *; x ~ 10; y ~ 68,

[0099] - Figure 3d: F127, Mn ~ 12,600 g.mol *; x ~ 100; y ~ 65,

[0100] - Figure 3e: F68, Mn ~ 8,400 g.mol *; x ~ 150; y “30.

[0101] [Fig.4] Figure 4a represents the BaTiO3 formulation without heat treatment + PVA prepared in Example 3, and Figure 4b shows the BaTiO3 formulation with heat treatment + PVA of BaTiO3 nanosheets prepared in Example 3.

[0102] [Fig 5] [Fig 5] shows the results obtained for BaTiO3 nanosheets without heat treatment and with heat treatment, and for composite films prepared from said BaTiO3 nanosheets with and without heat treatment, and PVA (prepared in Examples 2 and 3). Shown in [Fig 5] are photographs of the nanosheets and composite films (Figure 5a), as well as their dielectric properties by the respective measurements of real permittivity (Figure 5b), real conductivity (S / m) (Figure 5c) and losses (Figure 5d), as a function of the frequency in Hertz (Hz). The legend for Figures 5a to 5d is as follows: +: BaTiO3 nanosheets without heat treatment *: BaTiO3 nanosheets with heat treatment, 4h at 1000°C O: BaTiO3 composite film without heat treatment + PVA : BaTiO3 composite film with heat treatment + PVA Examples

[0103] [Tableauxl] Reagents Reference Titanium isopropoxide (TTIP) Titanium(IV) isopropoxide, Sigma-Aldrich, CAS number: 546-68-9 Concentrated hydrochloric acid (37%wt.) Hydrochloric acid 37%, AnalaR NORMAPUR, VWR Chemicals, CAS number: 7647-01-0 Pluronic P123 (Mn = 5800) Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), Mn = 5800, Sigma-Aldrich, CAS number: 9003-11-6 L61 Pluronic® L 61, Mn = 2000, Fluka, CAS number: 9003-11-6 L121 Pluronic® L121, Mn = 4400, Fluka, CAS number: 9003-11-6 F68 Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), Mn = 8400, Sigma-aldrich, CAS number: 9003-11-6 F127 Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), Mn = 12600, Sigma-aldrich, CAS number: 9003-11-6 Ethanol Absolute ethanol >99.8%, AnalaR NORMAPUR, VWR Chemicals, CAS No.: 64-17-5 Ethylene glycol (EG) Ethylene glycol, ReagentPlus - 99%, Sigma-Aldrich, CAS No.: 107-21-1 Barium hydroxide oc-tahydrate Barium hydroxide solution, Ba(OH)2, 0.3 N (0.15 M), Sigma-Aldrich, CAS No.: 17194-00-2 Polyvinyl alcohol (PVA) Polyvinyl alcohol, Mw 130,000,99+% hy-drolyzed, Sigma-Aldrich, CAS No.: 9002-89-5 .

[0104] Example 1: Synthesis of TiO2 nanosheets (step (i))

[0105] The following two solutions A and B are prepared from the ingredients described in Table 2 below, mixed under magnetic stirring:

[0106] [Tables2] Ingredients Quantity A Titanium Isopropoxide (TTIP) 6.25 mL Concentrated Hydrochloric Acid (37% wt.) 2.50 mL B Pluronic P123 (Mn = 5800) 1.0 g Ethanol 19 mL

[0107] Solution B is then added to solution A with continuous stirring for 30 minutes: solution AB is obtained.

[0108] Once solution AB is homogeneous, solution C described below is then prepared:

[0109] [Tables3] Ingredients Quantity C Solution AB 12.5 mL Ethylene glycol 100 mL Ultrapure water 1 mL

[0110] Solution C is transferred into a 200 mL Teflon-lined stainless steel autoclave. The reaction is carried out under hydrothermal conditions. The autoclave is heated to 150°C for 20 h. After natural cooling to room temperature, a white precipitate is obtained, which is centrifuged (8,000 rpm) and then washed once with ethanol, and once with ultrapure water. The TiO2 nanosheets are finally redispersed in 100 mL of ultrapure water followed by 5 minutes of homogenization in an ultrasonic bath, their final concentration is 80 mmol.L 1 and the zeta potential is measured at +30 ± 3 mV. The TiO2 nanosheets thus obtained are shown in [Fig.l].

[0111] Example 2: Synthesis of BaTiO3 nanosheets (step (ii))

[0112] An aqueous solution of Pluronic P123 (Mn = 5800) at 20 mg.mL-1 is prepared by dissolving Pluronic P123 in ultra-pure water under magnetic stirring.

[0113] A saturated aqueous solution of Ba(OH)2 at 0.27 M is prepared by dissolving barium hydroxide octahydrate in ultrapure water, with magnetic stirring and heating at 80°C for 24 h.

[0114] In a 40 mL Teflon-lined stainless steel autoclave, the following are added in order:

[0115] 1.5 mL of TiO2 dispersion at 80 mmol.L 1 obtained in Example 1;

[0116] 2. 0.475 mL, 0.85 mL, 1.9 mL or 2.85 mL of Pluronic P123 solution at 20 mg.mL 1 ([P123]final: 0.5, 1, 2 or 3 mg.mL 1 respectively);

[0117] 3. 0.57 mL, 1.14 mL, 1.71 mL or 2.28 mL of ethylene glycol (EG) (%v. final: 3, 6, 9 or 12%v. respectively); and

[0118] 4. 10.55 mL of 0.27M Ba(OH)2 solution.

[0119] The synthesis volume is adjusted to 19 mL each time by adding ultrapure water. The synthesis is carried out under hydrothermal conditions. The autoclave is heated to 160°C for 3 h. After natural cooling to room temperature, the white precipitate is centrifuged (3,000 rpm) and washed several times with ultrapure water to remove the ionic species present in the supernatant; this step is repeated until the dispersion reaches neutral pH. The BaTiO3 nanosheets dispersed in water are then washed and centrifuged (3,000 rpm) three times with ethanol to remove the P123 adsorbed on the surface of the nanosheets. The BaTiO3 nanosheets obtained form a white precipitate and are shown in [Fig.2].

[0120] It is found that the aspect ratio of BaTiO3 nanosheets (ratio of length to thickness) can be controlled by the concentrations of surfactant (P123) and co-surfactant (EG) during synthesis:

[0121] - a high concentration of Pluronic P123 tends to form nanosheets of low thickness,

[0122] - a high concentration of EG tends to form nanosheets of short length.

[0123] Conditions for synthesizing BaTiO3 nanosheets were modified in order to test the robustness and reproducibility of the described method, and to identify the repercussions on the morphology of the synthesized nanosheets:

[0124] - influence of temperature

[0125] Different reaction temperatures were tested: at 140°C, 150°C and 160°C. It was found that the highest temperature (i.e. 160°C) favors the formation of thicker and better crystallized nanosheets.

[0126] - influence of reaction time

[0127] Different reaction times were tested: 1h, 2h, 3h and 4h. It was found that longer reaction times (>3 hours) allow the formation of better crystallized and more monodisperse nanosheets.

[0128] - influence of the nature of the surfactant

[0129] P123 was replaced by other poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer surfactants of different block lengths: L61, L121, F68 and F127. It was found that all these surfactants also allow the expected BaTiO3 nanosheets to be produced. No significant impact on the morphology was identified. The results can be seen in [Fig 3].

[0130] This synthesis of BaTiO3 nanosheets from the TiO2 nanosheets prepared in Example 1 can also be carried out in larger volume (Up-scaling). For example, in the case of a multiplication of the synthesis volume by a factor of 5: all the volumes of the reagents are multiplied by 5, the latter are then introduced into a 200 mL autoclave. The use of a larger autoclave requires extending the synthesis time to 4 hours in order to compensate for the slower temperature rise of the autoclave in order to obtain well-crystallized particles. The BaTiO3 nanosheets are subsequently manipulated in water at neutral pH for the formulation of inks.

[0131] Example 3: Formulation of dielectric inks

[0132] - without heat treatment

[0133] The BaTiO3 nanosheets obtained in Example 2 are re-dispersed and homogenized in 200 mL of distilled water, to which 0.5 wt. % of polyvinyl alcohol (PVA) of the desired molar mass is added. The dispersion is placed under gentle magnetic stirring for 24 h. The BaTiO3 nanosheets stabilized by the PVA are then centrifuged (3,000 rpm). A gel is obtained which is recovered and homogenized under magnetic stirring for 2 h. A desired volume of concentrated aqueous PVA solution is then added to the BaTiO3 gel. The formulation is then homogenized under magnetic stirring for 2 h. This last addition of PVA constitutes the future polymer matrix of the film which improves the film-forming properties of the ink.

[0134] The quantity of PVA introduced makes it possible to modify the BaTiO3:PVA ratio within the formulation. The percentage of residual water makes it possible to modulate the viscosity of the formulation depending on the deposition method carried out during the next step.

[0135] An example of a prototype formulation is presented in Figure 4a, and consists of 2.7 wt. % of BaTiO3 nanosheets, without additional heat treatment, 2.7 wt. % of 131 kDa PVA and 94.6 wt. % of distilled water. The BaTiO3:PVA ratio is therefore 1:1.

[0136] - with heat treatment

[0137] A second formulation is prepared in order to observe the influence of an additional heat treatment on the performance of the ink formulation. The BaTiO3 nanosheets obtained in Example 2 are thus annealed before being incorporated into the formulation.

[0138] For this, the BaTiO3 nanosheets obtained previously are dried in an oven at 90°C under vacuum for 24 hours. The dry powder is placed flat in an alumina crucible and undergoes a conventional heat treatment in air at 1000°C for 4 hours (3.5 hours of temperature rise and 3.5 hours of temperature fall). After returning to room temperature, the BaTiO3 nanosheets are placed in approximately 10 mL of distilled water (quantity which can be adjusted according to the desired ink composition).

[0139] The nanosheets are re-dispersed using an ultrasonic probe (3 sessions of 10 minutes at 70% power in an ice bath). This step is essential for the fragmentation of the aggregates, the exfoliation and the re-dispersion of the nanosheets in water. The desired volume of PVA is then added to the dispersion and the whole is homogenized for 24 hours under magnetic stirring.

[0140] A second example of a prototype formulation is presented in Figure 4b, and the latter comprises the same BaTiO3:PVA ratio of 1:1: 2.7 wt. % of BaTiO3 nanosheets, annealed at 1000°C for 4 h, 2.7 wt. % of 131k PVA and 94.6 wt. % of distilled water.

[0141] The formulations obtained can then be shaped to form composite films, allowing permittivity, conductivity and loss measurements to be carried out:

[0142] The homogenized formulations are deposited on PMMA (Poly(methyl methacrylate)) substrate, spread with a Doctor Blade and air-dried at 60°C for 3 hours. In particular, the deposited volume and the height of the Doctor Blade blade make it possible to control the final thickness of the film according to the composition of the formulation.

[0143] Here, 150 μL of formulation are deposited, and the height of the slide is set at 250 μm. The thickness of the dry films is between 5 and 8 μm.

[0144] Measures:

[0145] After drying, circular Au electrodes are deposited on both surfaces of each film, this step is carried out using an Au metallizer and a mask whose electrode pattern is adapted to the electrical measurement device. After metallization, the films are dried in an oven at 80°C under vacuum for 2 hours before characterization, in order to carry out the dielectric measurements in anhydrous conditions. The impedance measurements (permittivity, conductivity and losses) presented in [Fig 5], are carried out from 0.1 Hz to 1 MHz at room temperature using a Materials Mates 7260 Impedance Analyzer impedance meter.

[0146] It is found that the heat treatment described above significantly increases the dielectric permittivity of the composite and pure BaTiO3 nanosheets. The electrical conductivity and overall losses are low and suitable for various printed capacitor applications.

Claims

Claims ^Claim 1] BaTiO3 nanosheet characterized in that the ratio between the length Ln of said nanosheet and the thickness en of said nanosheet is greater than or equal to 25. ^Claim 2] Nanosheet according to claim 1, characterized in that it has a surface area Sn greater than or equal to 10,000 nm2 and / or less than or equal to 1 pm2, preferably from 10,000 nm2 to 1 pm2. ^Claim 3] Nanosheet according to any one of the preceding claims, characterized in that it has a thickness en less than or equal to 15 nm, preferably less than or equal to 10 nm, preferably less than or equal to 8 nm, preferably less than or equal to 5 nm, preferably less than or equal to 3 nm.^Claim 4] Nanosheet according to any one of the preceding claims, characterized in that it has a length Ln less than or equal to 1 pm, preferably less than or equal to 800 nm, preferably less than or equal to 600 nm, preferably less than or equal to 400 nm, preferably less than or equal to 300 nm. ^Claim 5] Nanosheet according to any one of the preceding claims, characterized in that it has, at 100 Hz, a dielectric constant er greater than 150, preferably greater than 200, preferably greater than 300, preferably greater than 400, more preferably greater than 500.^Claim 6] A process for preparing nanosheets according to any one of claims 1 to 5, comprising a step of reacting TiO2 nanosheets with barium hydroxide Ba(OH)2, in the presence of at least one surfactant and at least one co-surfactant, said surfactant being chosen from poloxamers, and said co-surfactant being chosen from glycols. ^Claim 7] A process according to claim 6, wherein the surfactant is chosen from poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) three-block copolymers of the following formula (I): rn ® h3c x Jy Jx i.

8.

9.

10. in which: - x is an integer from 5 to 150, and - y is an integer from 30 to 70. Method according to one of claims 6 or 7, in which the co-surfactant is ethylene glycol. Use of nanosheets according to any one of claims 1 to 5 for the manufacture of dielectric inks. Formulation of dielectric inks comprising BaTiO3 nanosheets according to any one of claims 1 to 5, and optionally at least one polymer additive chosen from polyvinyl alcohol (PVA), poly(vinyl butyral) (PVB), cellulose derivatives, and amphiphilic block copolymers.

Images