WO2020049019A1

WO2020049019A1 - Process for producing nanoparticles

Info

Publication number: WO2020049019A1
Application number: PCT/EP2019/073527
Authority: WO
Inventors: Pedro LOPEZ DOMINGUEZ; Sebastian Schaefer; Stephan A Schunk; Michael Baecker; Martina Falter; Isabel Van Driessche; Hannes Rijckaert; Katrien DE KEUKELEERE; Jonathan DE ROO; Javier SIERRA; Armin Meyer; Jan BENNEWITZ
Original assignee: Basf Se; Ghent University
Priority date: 2018-09-07
Filing date: 2019-09-04
Publication date: 2020-03-12

Abstract

The present invention is in the field of processes for the production of nanoparticles. It relates to a process for the preparation of nanoparticles comprising exposing a solution to microwave irradiation, wherein the solution contains (a) an alcohol, (b) barium, (c) zirconium or hafnium, and (d) water and/or a hydroxide, wherein the molar ratio of the sum of water and the hydroxide to barium is 2 to 15.

Description

Process for Producing Nanoparticles

Description

The present invention is in the field of processes for the production of nanoparticles.

Nanoparticles have various applications, such as anti-corrosion layer, imaging agent, photolumi- nescent and photocatalytic material, catalyst, or pinning center for oxide superconductors. In most of these applications, it is advantageous to employ small crystalline nanoparticles. BaZrC and BaHfOs are particularly useful as these materials are chemically very robust. Processes for the production of BaZrC and BaHfOs containing nanoparticles are known from prior art.

US 2010 / 0 129 286 discloses a method of preparing BaZrC nanoparticles by heating an alka- line mixture of barium and zirconium precursors. However, average particle sizes below 90 nm cannot be obtained.

It was an object of the present invention to provide a process for the production of crystalline nanoparticles of very small particle size. The nanoparticles should be highly uniform and reliably stabile. Also, it was aimed at a process of production of nanoparticles which are highly effective as pinning centers in superconductors.

These objects were achieved by a process for the preparation of nanoparticles comprising ex- posing a solution to microwave irradiation, wherein the solution contains

(a) an alcohol,

(b) barium,

(c) zirconium or hafnium, and

(d) water and/or a hydroxide, wherein the molar ratio of the sum of water and the hydroxide to barium is 2 to 15.

The present invention further relates to crystalline nanoparticles containing BaZrC or BaHfOs having an average particle size of 2 to 10 nm.

The present invention further relates to a precursor composition for the preparation of a high temperature superconductor containing the crystalline nanoparticles according to the present invention.

Preferred embodiments of the present invention can be found in the description and the claims. Combinations of different embodiments fall within the scope of the present invention.

The term“nanoparticles” in the present context generally refers to particles with a mass average particle diameter of not more than 100 nm, preferably not more than 80 nm, in particular not more than 60 nm, such as not more than 40 nm. The mass average particle diameter is prefera- bly measured by dynamic light scattering according to ISO 22412 (2008), preferably by using the Mie theory.

The term“solution” in the present context generally refers to a composition which is liquid at room temperature or slightly above, i.e. 20 to 100 °C, and normal pressure, i.e. 1013 mbar. All components of the solution are dissolved which means that no solids of 1 nm or larger are dis persed in the solution. This normally means that the solution is an optically clear solution.

According to the present invention, the solution contains an alcohol. Alcohols include linear alkyl alcohols like methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-oc- tanol, n-nonanol, n-decanol, n-dodecanol, n-tetradecanol; branched alkyl alcohols like iso-pro- panol, sec-butanol, iso-butanol, tert-butanol, neo-pentanol, sec-hexanol, 2-ethylhexan-1 -ol, 2- butyloctan-2-ol; alkenyl alcohols like palmitoleic alcohol, oleic alcohol, linoleic alcohol, arachi- donic alcohol, retinol; aromatic alcohols like phenol, benzyl alcohol, p-cresol, 2-phenylethanol; oligoetheralcohols like 2-methoxyethanol, diethyleneglycol, methyl-diethyleneglycol, triethy- leneglycol, methyl-triethyleneglycol, polyethyleneglycol with a molecular weight of 200 to 1000 g/mol, preferably 300 to 800 g/mol, in particular 400 to 600 g/mol, polypropyleneglycol with a molecular weight of 200 to 1000 g/mol, preferably 300 to 800 g/mol, in particular 400 to 600 g/mol. Alkyl alcohols and oligoetheralcohols are preferred, Ci to C12 alkyl alcohols are more pre- ferred, linear C2 to C10 alkyl alcohols are even more preferred, in particular ethanol.

According to the present invention, the solution contains barium and zirconium or hafnium. It is possible that a compound containing barium and a compound containing zirconium or hafnium is dissolved in the solution or that one compound containing both barium and zirconium or haf- nium is dissolved in the solution, preferably one compound containing both barium and zirco- nium or hafnium is dissolved in the solution.

Various compounds containing barium can be dissolved in the solution such as anhydrous bar- ium halides or barium alcoholates, preferably barium alcoholates, wherein the alcohol is one de- scribed above. Various compounds containing zirconium can be dissolved in the solution such as anhydrous zirconium halides or zirconium alcoholates, preferably zirconium alcoholates, wherein the alcohol is one described above. Various compounds containing hafnium can be dis solved in the solution such as anhydrous hafnium halides or hafnium alcoholates, preferably hafnium alcoholates, wherein the alcohol is one described above.

Preferably the solution contains dissolved BaZr(OR)₆ or BaHf(OR)₆, wherein R is an alkyl, an alkenyl, an aryl or an oligoether group as described above for the residue of the alcohols.

The molar ratio of barium to zirconium or hafnium in the solution is preferably 0.5 to 2, more preferably 0.7 to 1 .4, in particular 0.8 to 1 .2, such as 1 or essentially 1. The concentration of barium in the solution is preferably 0.05 to 0.5 mol/l, more preferably 0.1 to 0.3 mol/l. The con- centration of zirconium or hafnium in the solution is preferably 0.05 to 0.5 mol/l, more preferably 0.1 to 0.3 mol/l.

According to the present invention, the solution contains water and/or a hydroxide. The hydrox- ide can be any compound which dissolves in the solution and contains hydroxide ions. Pre- ferred hydroxides are alkali metal hydroxides or ammonium hydroxide, in particular potassium hydroxide. It is possible that water and/or the hydroxide are added as such to the solution. Pref- erably, water and a strong base, for example an alcoholate, are added to the solution. Prefera- bly, the alcoholate is an alkali alcoholate, i.e. LiOR, NaOR, KOR, RbOR or CsOR, wherein R is an alkyl, an alkenyl, an aryl or an oligoether group as described above for the residue of the al- cohols. Preferred alkali alcoholate is potassium tert-butanolate. By adding a strong base and water, the water gets fully or partially deprotonated and hence forms a hydroxide. The molar ra- tio of the sum of water and the hydroxide to barium is 2 to 15, more preferably 4 to 10, in partic- ular 5 to 8, for example 6 to 7.

The solution can contain a stabilizer or it is free of a stabilizer, preferably the solution is free of a stabilizer. A stabilizer is a compound which has a high tendency to adsorb to the surface of na- noparticles such that it prevents aggregation of the nanoparticles which are formed in the pro- cess according to the present invention. A broad variety of stabilizers can be used, for example alcohols, thiols, carboxylic acids, amines, trialkylphosphine oxides. Preferably, an alcohol, a car- boxylic acid, or a trialkyl phosphine oxide is employed as stabilizer. Preferred trialkyl phosphine oxides are trialkyl phosphine oxides with the same or different C₄ to C20 alkyl groups, for exam- pie trioctyl phosphine oxide. Examples for carboxylic acids are stearic acid, palmitic acid, erucic acid, oleic acid, linoleic acid, linolenic acid, or lauric acid. C₆ to C22 carboxylic acids are pre- ferred, in particular oleic acid or lauric acid. Examples for alcohols include octanol, nonanol, decanol, dodecanol, tetradecanol, benzyl alcohol, phenoxyethanol, hydroxyethylbenzene. C₆ to C22 alcohols are preferred, in particular benzyl alcohol.

According to the present invention, the solution is exposed to microwave irradiation. By expos- ing the solution to microwave irradiation, the solution is preferably heated to a temperature of 100 °C to 250 °C, more preferably 120 °C to 200 °C, in particular 150 °C to 180 °C. Depending on the specific absorption of the solution, the power input of the microwave can be 10 to 200 W/ml of the solution, for example 50 to 100 W/ml of the solution.

The solution can be exposed to microwave irradiation for various amounts of time, such as 10 to 150 minutes, preferably 15 to 120 minutes, for example 30 minutes. It has been observed that the particle size increases with the time of microwave irradiation.

Usually, the nanoparticles precipitate after having been exposed to microwave irradiation. In this case, the nanoparticles are preferably separated from the liquid phase, preferably by centrifuga- tion. Often is it useful to remove any remaining impurities by washing with a solvent, for exam- pie once or twice or three times. The nanoparticles obtained by the process of the present in- vention can easily be suspended in solvents by adding a stabilizer as described above.

The nanoparticles formed by the process according to the present invention are typically crystal- line. Therefore, the present invention further relates to crystalline nanoparticles containing Ba- Zr0₃ or BaHf0₃. Crystalline in the context of the present invention means that the degree of crystallinity of the particles is at least 50 %, preferably at least 70 %, in particular at least 90 %. The degree of crystallinity is defined as the ratio of the mass average radius of the particles vis- ually observed in the HR-TEM and the radius of the particles determined by evaluation of the full width at half maximum (FWHM) of the dominant peak of the X-ray diffraction pattern (XRD) using the Debye-Scherrer equation. A ratio of 1 determines a degree of crystallinity of 100 %.

According to the present invention, the nanoparticles have a mass average particle size of 1 to 10 nm, preferably 2 to 6 nm. The particle size is preferably measured by HR-TEM. The particles have a low BaCC>₃ content, typically less than 15 %, preferably less than 10 %, more preferably less than 5 %, in particular less than 2 %.

The nanoparticles according to the present invention are suitable for various applications, for example functional coatings, electronic applications or catalysis. The nanoparticles are particu- larly suitable as pinning centers in oxide superconductors. The nanoparticles are preferably added to a precursor composition for the preparation of a high temperature superconductor. Therefore, the present invention also relates to a precursor composition for the preparation of a high temperature superconductor. Preferably the superconductor contains REBa₂Cu₃0_7-x, wherein RE stands for rare earth or yttrium and x is 0.01 to 0.3, more preferably the supercon- ductor contains YBa2Cu307-x.

Preferably the precursor composition contains

(a) an yttrium or rare earth-containing compound,

(b) a alkaline earth metal-containing compound,

(c) a transition metal-containing compound, and

(d) the particles according to the invention.

The yttrium- or rare earth metal-containing compound, the alkaline earth metal-containing corn- pound and the transition metal-containing compound include oxides, hydroxides, halogenides, carboxylates, alkoxylates, nitrates or sulfates. Carboxylates are preferred, in particular acetate or propionate. Carboxylates and alkoxylates can be substituted, preferably by fluorine, such as difluoroacetate, trifluoroacetate, or partially or fully fluorinated propionate.

At least one of the rare earth metal or yttrium containing compound, the alkaline earth metal containing compound and the transition metal containing compound contains fluorine. Prefera- bly, the alkaline earth metal containing compound contains fluorine, for example as trifluoroace- tate. Preferably, the yttrium- or rare earth metal is yttrium, dysprosium, or erbium, in particular yt- trium. Preferably, the alkaline earth metal is barium. Preferably, the transition metal is copper.

Preferably, the molar ratio of the transition metal-containing compound and yttrium or rare earth metal-containing compound in the ink is between 3 : 0.7 to 3 : 2, more preferably 3 : 1.2 to 3 :

1.4. Preferably, the molar ratio of the transition metal-containing compound and the earth alka- line metal-containing compound in the ink is between 3 : 1 to 3 : 2, more preferably 3 : 1.7 to 3 : 1.9.

The precursor composition is adapted to the method for making the superconductor. Various methods exist, such as chemical solution deposition, chemical vapor deposition, ion-beam as- sisted deposition. For chemical solution deposition, the precursor composition is preferably an ink containing

(a) an yttrium or rare earth-containing compound,

(b) a alkaline earth metal-containing compound,

(c) a transition metal-containing compound,

(d) an alcohol, and

(e) the particles according to the invention.

For the alcohol, the same details and preferred embodiments apply as described for the pro- cess above. Preferably, the alcohol is a mixture of methanol and C₂ to C12 alcohols.

The ink contains the rare earth metal or yttrium containing compound, the alkaline earth metal containing compound and the transition metal containing compound in a molar ratio deemed op- timal for the superconductor growth and/or properties, taking into consideration the molar corn- position of the respective metals in the superconductor to be produced. Their concentration thus depends on the superconductor to be produced. Generally, their concentration in the solution is independent of each other 0.01 to 10 mol/l, preferably 0.1 to 1 mol/l.

Preferably, the ink contains the nanoparticles at a concentration at which the molar ratio of the sum of all metals in the nanoparticles to the yttrium or rare earth-containing compound is 1 to 30 %, more preferably 3 to 20 %, in particular 5 to 15 %. In many cases this corresponds to 0.1 to 5 weight % of nanoparticles with regard to the ink.

Preferably, the nanoparticles are additionally stabilized by an organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups. More preferably the nanoparticles are additionally stabilized by a compound of general formula (I)

wherein a is 0 to 5, and

b and c are independent of each other 1 to 14, and

n is 1 to 5.

Preferably, a is 0. Preferably, b is 2 to 10, more preferably 3 to 8. Preferably, c is 2 to 10, more preferably 3 to 6. Preferably, n is 2 to 4. In one preferred example, a is 0, b is 6, c is 5, n is 3.

Also preferably, the organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups is a compound of general formula (II)

wherein R¹ and R² are independent of each other H, OH, or COOH, and

m is 1 to 12.

If m is larger than one, it is possible that the R¹ and R² are all the same or different to each other. Examples for the compound of general formula (II) include dicarboxylic acids in which R¹ and R² are hydrogen, such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid; dicarboxylic acids with hydroxyl groups such as tartronic acid, malic acid, tartric acid; or tricarboxylic acids such as citric acid or isocitric acid.

Another preferred organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups is a compound of general formula (III)

wherein e and f are independent of each other 0 to 12. Preferably e is 0. Preferably, f is 2 to 6. Another preferred organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups is a compound of general formula (IV)

wherein g is 0 to 5, and

p and q are independent of each other 1 to 14, preferably 2 to 12. The ratio of p to q is prefera- bly from 20 : 80 to 80 : 20, in particular from 40 : 60 to 60 : 40.

The organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups is brought in contact to the nanoparticles either by precipitating the nanoparticles from a suspension by a highly polar solvent such as acetone, separate the precipitate and redisperse the precipitate in an alcohol with the organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups. Alterna- tively, the organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups is added to a suspension of the nanoparticles, a high boiling alcohol is added and the lower-boiling solvent is removed by evaporation.

Preferably the ink further contains stabilizers, wetting agents and/or other additives. The amount of these components may vary in the range of 0 up to 30 weight % relating to the total weight of the dry compounds used. Additives might be needed for adjusting the viscosity. Additives in- clude Lewis bases; amines such as TEA (triethanolamine), DEA (diethanolamine); surfactant; polycarboxylic acids such as PMAA (polymetacrylic acid) and PAA (polyacrylic acid), PVP (poly- vinylpyrolidone), ethylcellulose.

Preferably the ink is heated and/or stirred to homogenize all ingredients, such as to reflux. In addition, the ink can further contain various additives to increase the stability of the solution and facilitate the deposition process. Examples for such additives include wetting agents, gelling agents, and antioxidants.

In order to make a superconductor with the ink according to the present invention, the ink is usually deposited on a substrate. The deposition of the ink can be carried out in various ways. The ink can be applied for example by dip-coating (dipping of the substrate in the ink), spin- coating (applying the ink to a rotating substrate), spray-coating (spraying or atomizing the ink on the substrate), capillary coating (applying the ink via a capillary), slot die coating (applying the ink through a narrow slit), and ink-jet printing. Slot die coating and ink-jet printing are preferred.

Preferably, the ink is evaporated after deposition to form a film at a temperature below the boil- ing point of the solvent, such as 10 to 100 °C below the boiling point of the solvent, preferably 20 to 50 °C below the boiling point of the solvent. The substrate may be any material capable of supporting buffer and/or superconducting layers. For example suitable substrates are disclosed in EP 830 218, EP 1 208 244, EP 1 198 846, EP 2 137 330. Often, the substrate is a metal and/or alloy strip/tape, whereby the metal and/or alloy may be nickel, silver, copper, zinc, aluminum, iron, chromium, vanadium, palladium, molyb- denum, tungsten and/or their alloys. Preferably the substrate is nickel based. More preferably, the substrate is nickel based and contains 1 to 10 at-%, in particular 3 to 9 at-%, tungsten. Lam- inated metal tapes, tapes coated with a second metal like galvanic coating or any other multi- material tape with a suitable surface can also be used as substrate.

The substrate is preferably textured, i.e. it has a textured surface. The substrates are typically 20 to 200 pm thick, preferably 40 to 100 pm. The length is typically greater than 1 m, the width is typically between 1 cm and 1 m.

Preferably the substrate surface is planarized before the film comprising yttrium or a rare earth metal, an alkaline earth metal and a transition metal is deposited onto, for example by elec- tropolishing. It is often advantageous to subject the thus planarized substrate to a thermal treat- ment. This thermal treatment includes heating the substrate to 600 to 1000 °C for 2 to 15 minutes, wherein the time refers to the time during which the substrate is at the maximum tem- perature. Preferably, the thermal treatment is done under reducing atmosphere such as a hy- drogen-containing atmosphere. The planarization and/or thermal treatment may be repeated.

Preferably, the surface of the substrate has a roughness with rms according to DIN EN ISO 4287 and 4288 of less than 15 nm. The roughness refers to an area of 10 x 10 pm within the boundaries of a crystallite grain of the substrate surface, so that the grain boundaries of the metal substrate do not influence the specified roughness measurement.

Preferably, between the substrate and the film there are one or more buffer layers. The buffer layer can contain any material capable of supporting the superconductor layer. Examples of buffer layer materials include metals and metal oxides, such as silver, nickel, TbO_x, GaO_x,

Ce0₂, yttria-stabilized zirconia (YSZ), Y2O3, LaAIOs, SrTi03, Gd₂03, LaNiOs, LaCuOs, SrRuOs, NdGaOs, NdAI0₃ and/or some nitrides as known to those skilled in the art. Preferred buffer layer materials are yttrium-stabilized zirconium oxide (YSZ); various zirconates, such as gadolinium zirconate, lanthanum zirconate; titanates, such as strontium titanate; and simple oxides, such as cerium oxide, or magnesium oxide. More preferably the buffer layer contains lanthanum zir conate, cerium oxide, yttrium oxide, gadolinium-doped cerium oxide and/or strontium titanate. Even more preferably the buffer layer contains lanthanum zirconate and/or cerium oxide.

To enhance the degree of texture transfer and the efficiency as diffusion barrier, multiple buffer layers each containing a different buffer material are between the substrate and the film. Prefer- ably the substrate includes two or three buffer layers, for example a first buffer layer comprising lanthanum zirconate and a second buffer layer containing cerium oxide. The film is preferable heated to a temperature of 300 to 600 °C, preferably 350 to 450 °C to re- move remaining organic parts of the precursors. The substrate is kept at this temperature for 1 to 30 min, preferably 5 to 15 min.

Afterwards, the film is preferably heated to a temperature of 700 to 900 °C, preferably 750 to 850 °C in an atmosphere containing water and oxygen to crystallize the film. The partial pres- sure of water is 1 to 99.5 % of the total pressure of the atmosphere, and the partial pressure of oxygen is 0.5 to 90 % of the total pressure of the atmosphere, preferably 2 to 90 %. Even more preferably, during the first stage of heating to 700 to 900 °C the partial pressure of water is 1 to 20 % of the total pressure of the atmosphere, preferably 1.5 to 5 %, and during the second stage of this heating the partial pressure of water is 90 to 99.5 % of the total pressure, prefera- bly 95 to 99 %.

Often, the superconductor wire is cut into smaller bands and stabilized by coating with a con- ductive metal such as copper for example by electrodeposition.

Brief Description of the Figures

Figure 1 shows an HR-TEM of the nanoparticles obtained in example 2.7.

Examples

Example 1 : Synthesis for Ba and Zr containing compounds

Ba(0-iPr)₂ and Zr(0-iPr)₄ are dissolved at a molar ratio of 1 in 2-methoxyethanol (OME) under an inert atmosphere. The mixture is heated to 130 °C, whereby the alcohol evaporates to leave pure BaZr(OME)6.

Example 2:

300 mg BaZr(OME)₆ are added to 4 ml of ethanol under inert atmosphere. An amount of dried KOH according to the following table is added. A pressure resistant vial is charged with the so- lution and sealed. The solution is heated with microwaves (CEM Discovery, maximum power 300 W) to 150 °C for 30 min.

A turbid solution is obtained. After addition of acetone, the solution is centrifuged at 5000 rpm for 3 min. The supernatant is removed, fresh acetone added, resuspended, and centrifuged again. This washing process is repeated once more. The remaining powder is dried and subject to X-ray diffraction in order to determine the crystallite size. The results of the following table were obtained.

A sample of example 2.7 was subject to HR-TEM analysis. The image in figure 1 shows a Ba- ZrC>₃ nanoparticle as confirmed by EDX of about 5 nm size. The crystal plane can be seen indi- cating a high degree of crystallinity.

Example 3

Example 2 was followed with the difference that instead of KOH, potassium tert-butanolate (KOtBu) was added in a molar ratio KOtBu/BaZr(OME)₆ = 6 and water according to the following table. The solution was heated to 180 °C for 30 minutes.

Claims

Claims:

1. A process for the preparation of nanoparticles comprising exposing a solution to micro- wave irradiation, wherein the solution contains

(a) an alcohol,

(b) barium,

(c) zirconium or hafnium, and

(d) water and/or a hydroxide, wherein the molar ratio of the sum of water and the hydrox- ide to barium is 2 to 15.

2. The process according to claim 1 , wherein the water and/or the hydroxide are added to the solution by adding an alcoholate and water.

3. The process according to claim 1 or 2, wherein the alcohol is ethanol.

4. The process according to claim 1 or 3, wherein the hydroxide is potassium hydroxide.

5. The process according to any of the claims 1 to 4, wherein the temperature upon expo- sure of the solution to microwave irradiation is 150 to 180 °C.

6. The process according to any of the claims 1 to 5, wherein the solution is exposed to mi- crowave irradiation for 15 to 120 minutes.

7. The process according to any of the claims 1 to 6, wherein the molar ratio of the sum of water and the hydroxide to barium is 4 to 8.

8. The process according to any of the claims 1 to 7, wherein the solution contains dissolved BaZr(OR)₆ or BaHf(OR)₆, wherein R is an alkyl, an alkenyl, an aryl or an oligoether group.

9. The process according to any of the claims 1 to 8, wherein the molar ratio between water and the hydroxide is 0.5 to 2.

10. Crystalline nanoparticles containing BaZrC>₃ or BaHfOs having an average particle size of 1 to 10 nm.

1 1. The crystalline nanoparticles according to claim 10, wherein the average particle size is 2 to 6 nm.

12. A precursor composition for the preparation of a high temperature superconductor con- taining the crystalline particles according to claim 10 or 11.