Method for producing zirconia colloids

Abstract

The present invention pertains to a method for producing a colloidal suspension of zirconia particles, comprising the following successive steps: a) subjecting a mixture of zirconium oxychloride and an alkali metal halide in an aqueous solvent to hydrothermal treatment at a temperature above 150° C., so as to obtain a suspension in the form of a two-phase mixture comprising a slurry and a supernatant, b) without first peptizing it, desalting said suspension so as to form a colloidal suspension of zirconia.

Claims

1. A method for producing a colloidal suspension of zirconia particles, comprising the following successive steps: (a) subjecting a mixture of zirconium oxychloride and an alkali metal halide AX in an aqueous solvent to hydrothermal treatment at a temperature above 150° C., so as to obtain a suspension in the form of a two-phase mixture comprising a slurry and a supernatant, and (b) desalting said suspension, without first peptizing said suspension, so as to form a colloidal suspension of zirconia.

2. The method according to claim 1, wherein the suspension obtained in step (a) is diluted with water before performing step (b).

3. The method according to claim 2, wherein the diluted suspension is desalted in step (b) by performing ultrafiltration.

4. The method according to claim 1, wherein the alkali metal halide is potassium chloride.

5. The method according to claim 1, wherein zirconium oxychloride and the alkali metal halide are mixed in a molar ratio of AX/ZrOCl.sub.2 from 1/10 to 1/1.

6. The method according to claim 1, wherein zirconium oxychloride and the alkali metal halide are mixed in a molar ratio of AX/ZrOCl.sub.2 from 1/4 to 1/2.

7. The method according to claim 1, wherein zirconium oxychloride concentration in said mixture ranges from 0.5 to 4 mol/l.

8. The method according to claim 1, wherein zirconium oxychloride concentration in said mixture ranges from 1 to 2 mol/l.

9. The method according to claim 1, wherein the hydrothermal treatment is conducted at a temperature from 150 to 220° C. during at least one day.

10. The method according to claim 1, wherein the hydrothermal treatment is conducted at a temperature from 160 to 200° C. during at least one day.

11. The method according to claim 1, wherein the hydrothermal treatment is conducted at a temperature from 175 to 190° C. during at least one day.

12. The method according to claim 1, wherein the suspension obtained in step (a) is desalted in step (b) by performing dialysis in water.

13. The method according to claim 1, further comprising a step of solvent-exchange so as to substitute at least one alcoholic solvent for part or all of the aqueous solvent included in the suspension.

14. The method according to claim 13, wherein solvent exchange is performed by dialysis or diafiltration.

15. The method according to claim 1, further comprising an intermediate or subsequent step of improving zirconia dispersion, either by adding a dispersant, or by surface modification of the zirconia.

16. The method according to claim 1, further comprising an intermediate or subsequent step of pH modification by addition of an organic or inorganic base.

Description

DRAWINGS

(1) FIG. 1 illustrates the XRD pattern of zirconia colloid prepared using ZrOCl.sub.2 as a zirconium source without any mineralizer.

(2) FIG. 2 illustrates the XRD patterns of zirconia colloids prepared using ZrOCl.sub.2 as a zirconium source and NaOH as mineralizer (a: ZrOCl.sub.2:NaOH=1:0.1; b: ZrOCl.sub.2:NaOH=1:0.15; c: ZrOCl.sub.2:NaOH=1:0.2; d: ZrOCl.sub.2:NaOH=1:0.25.

(3) FIG. 3 illustrates the XRD patterns of zirconia colloids prepared using ZrOCl.sub.2 as a zirconium source and KCl as mineralizer (e: ZrOCl.sub.2:KCl=1:0.0625; f: ZrOCl.sub.2:KCl=1:0.25; g: ZrOCl.sub.2:KCl=1:0.5; h: ZrOCl.sub.2:KCl=1:0.75).

(4) FIG. 4 illustrates the XRD patterns of zirconia colloids prepared using different alkali metal halides.

(5) FIG. 5 illustrates the XRD patterns of zirconia colloids prepared at different temperatures and using KCl as mineralizer (ZrOCl.sub.2:KCl=1:0.25).

EXAMPLES

(6) This invention will be further illustrated y the following non-limiting examples which are given for illustrative purposes only and should not restrict the scope of the appended claims.

Example 1

Preparation of an Acidic Water-based Zirconia Colloid

(7) 9.9 g of KCl and 170.85 g of ZrOCl.sub.2.8H.sub.2O were dissolved in 289.5 g of deionized water under magnetic stirring.

(8) 375 ml of the mixture thus obtained were filled into a 500 ml Teflon® lined autoclave. The autoclave was placed in an oven and heated to a temperature of 180° C. during 72 h.

(9) 2 l of deionized water were added under stirring to the two-phase mixture thus obtained. The diluted suspension was then desalted by ultrafiltration until a pH of 3.6 was reached. The dry content was adjusted between 16% and 16.5% and resulted in 380 ml of colloidal suspension. The yield of conversion of the zirconium precursor into zirconia particles was close to 90%.

Example 2

Preparation of Acidic Methanol-based Colloidal Zirconia

(10) One part of the suspension obtained in Example 1 was solvent exchanged by diafiltration. To achieve this, the colloidal suspension was concentrated by using an ultrafiltration apparatus while absolute methanol was continuously added to the suspension. The process was stopped when the water content in the suspension was lower than 0.1% by weight and the suspension was then concentrated until reaching a dry content of 21.5% by weight. The yield of conversion of the zirconium precursor into zirconia particles was close to 90%. A colloidal zirconia suspension in methanol was thus obtained.

Example 3

Preparation of Alkaline Water-based Colloidal Zirconia

(11) One part of the zirconia suspension obtained in example 1 was further modified to increase its pH. To achieve this, 54.5 g of deionised water were added to 21.1 g of zirconia suspension obtained from example 1. On another side, 0.35 g of trisodium citrate was dissolved in 30 ml of deionised water. The trisodium citrate solution was then added to the zirconia suspension at a rate of 2.5 ml/min under continuous stirring. After the addition was completed, the suspension was stirred continuously for 12 h. The obtained suspension was then acidified to pH=8.5 and concentrated to 20% dry weight content by ultrafiltration. Stable and transparent alkaline water-based zirconia suspension was thus obtained, with a zeta potential of −62 mV.

Example 4

Characterization of the Colloidal Zirconia of Example 1

(12) A series of experiments were performed on the colloidal suspension obtained in Example 1. TEM observation (JEM-200CX® electron microscope from JEOL) allowed checking particle size, shape and aggregation state. Moreover, a powder XRD pattern of the sample was performed on a D/Max-2200 X-ray diffraction meter (RIGAKU CORPORATION) at room temperature, operating at 30 kV and 30 mA, using Cu kα radiation (λ=0.15418 nm). According to the TEM and XRD observations, the colloidal zirconia had a high crystallinity and it was present as single rod-like monoclinic nanocrystallites with short axis range from 2 to 5 nm and long axis range from 3 to 14 nm. TEM observations confirmed also that the particles were well dispersed and not aggregated, as was also indicated by the transparency of the colloidal suspension.

(13) The zeta potential, as measured with a Zetasizer Nano ZS90® (MALVERN INSTRUMENTS Ltd.), was 34.5 mV, indicating the high stability of the suspension. Moreover, the particle size distribution obtained by Zetasizer was narrow, indicating a uniform size distribution.

(14) Further, elemental analysis was performed by inductively-coupled plasma-atomic emission spectroscopy (ICP-AES, Optima 7300DV) and X-ray fluorescence to assess the bulk chemical composition of the product. They showed the high purity of the zirconia colloid produced.

(15) Moreover, X-ray photoelectron spectroscopy (XPS) was conducted to investigate the surface chemical composition and valence state of the zirconia colloid sample. Spectra were recorded by a PHI-5000C ESCA spectrometer using Mg Ka radiation (hv=1253.6 eV). The C 1 s line was taken as a reference to calibrate the shift of binding energy due to electrostatic charging. From this experiment, one could confirm that the oxidation state of Zr was +4 and that there was only one chemical state of oxygen in zirconia.

(16) Finally, after drying the zirconia sample into a powder at room temperature, FTIR spectra were measured on an AVATAR® 370-IR spectrometer (THERMO NICOLET) with a wavenumber range of 4000 to 400 cm.sup.−1. Absorption bands located at 3396 cm.sup.−1, 1629 cm.sup.−1 and 500-1000 cm.sup.−1 were respectively attributed to the O—H (and molecularly adsorbed water), H—O—H and Zr—O stretching or bending vibrations. These results also confirmed that no organic group was detected in the dried powder, showing that the particles surface was not functionalized with organic groups.

(17) These experiments confirm that pure zirconia colloids with high dispersion, transparency, stability, refractive index and solid content could be prepared by the method if this invention.

Example 5

Influence of the Mineralizer

(18) Zirconia colloids were prepared according to a process similar to that described in Example 1, except that various mineralizers were used instead of KCl, as well as no mineralizer at all.

(19) The colloid obtained without any mineralizer resulted in the XRD pattern shown in FIG. 1. Using KCl as a mineralizer resulted in the XRD patterns shown on FIG. 3. Diffraction peaks without mineralizer are very weak, characteristic of nearly amorphous particles. On the contrary, the peaks are intense and sharper when KCl is used as mineralizer, showing that the high crystallinity can only be obtained by the use of a suitable mineralizer, like KCl.

(20) NaOH was then used as a mineralizer in different molar ratios of ZrOCl.sub.2:NaOH. The XRD patterns of the colloids obtained are shown on FIG. 2. A comparison of FIGS. 2 and 3 shows that, whatever the molar ratio tested, the diffraction peaks are stronger, and thus the crystallinity of the zirconia colloid is always higher, when KCl is used as a mineralizer.

(21) Various other alkali metal halides were then investigated. The XRD patterns obtained are shown on FIG. 4. As illustrated, all samples prepared by using different alkali metal halides as mineralizers exhibited much higher crystallinity when compared with the samples prepared by using NaOH as a mineralizer (see FIG. 2).

(22) Among those alkali metal halides, the diffraction patterns obtained by use of alkali metal fluoride exhibit sharper and more intense peaks than the others (see FIG. 4). This shows that alkali metal fluorides lead to larger crystal size than when other alkali metal halides are used as mineralizers. This fact was confirmed also by particle size measurements from TEM images. The crystallites obtained from KCl had average dimensions of 3.5 nm width for 8 mm length, whereas those prepared from KF had 8 mm width for 13 mm length.

(23) This example demonstrates that the type of mineralizer significantly affects the crystallinity and the particle size of the zirconia colloid obtained, which directly affects the refractive index and the transparency of this product, and its achievable maximum solid content. Alkali metal halides provide for a significantly enhanced crystallinity compared with NaOH, which is thought to be due to their ability to modify the viscosity and the solubility properties in the reactive solution during the hydrothermal treatment. This example demonstrates also that the choice of the alkali metal halide mineralizer can allow tuning the final particle size.

Example 6

Influence of the Reaction Temperature

(24) Two zirconia colloids were prepared according to a process similar to that disclosed in Example 1, in which the reaction temperature was lowered to 120 and 150° C., respectively.

(25) As evident from FIG. 5, which illustrates the XRD spectra of these samples and of that of Example 1, the crystallinity of zirconia was better at 180° C., which was reflected by sharper peaks. Moreover, TEM images of these samples showed the samples prepared at 120° C. and 150° C. had more agglomerated particles. The colloidal suspension obtained by thermal treatment at 180° C. exhibited also higher transparency than those prepared at 120° C. and 150° C. This shows that the colloids prepared at 120° C. and 150° C. were poorly dispersed, compared with that prepared at 180° C. This result was confirmed using a higher molar ratio of KCl to ZrOCl.sub.2 (0.5:1) .

(26) This example shows that the reaction temperature has an obvious effect on the crystallinity and dispersion of the zirconia colloid, and that temperatures above 150° C. give better results in this respect.

Example 7

Influence of the Zirconium Source

(27) A zirconia colloid was prepared according to a process similar to that described in Example 1, in which zirconium nitrate was substituted for zirconium oxychloride as the zirconium source.

(28) It was observed that, compared with the sample using ZrOCl.sub.2 as the zirconium source, the sample prepared using Zr(NO.sub.3).sub.4 possess a higher crystallinity but lower dispersion and transparency, which are however crucial from the standpoint of the stability of the zirconia colloid formed and of the maximum solid content which may be achieved.

(29) This example shows that the zirconium source affects the dispersion of the zirconia colloid obtained and that ZrOCl.sub.2 is a better zirconium source, in this regard, than Zr(NO.sub.3).sub.4.