CERAMIC COMPOSITE OXIDE

Abstract

The invention provides a ceramic composite oxide of formula (I): (1−x)AaBbOy+xCcDdOz (I) wherein A, B, C and D are each independently selected from the group consisting of Li, Na, Mg, Al, P, K, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, In, Sn, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Er, Tm, Yb, Lu, Ta, W, Bi and mixtures thereof; x is 0.05 to 0.95; y and z are balanced by the charge of the cations; 0≤a, b, c, d≤1; and wherein said ceramic composite oxide has an average particle size diameter of 10 to 700 nm.

Claims

1. A ceramic composite oxide of formula (I):
(1−x)A.sub.aB.sub.bO.sub.y+xC.sub.cD.sub.dO.sub.z  (I) wherein A, B, C and D are each independently selected from the group consisting of Li, Na, Mg, Al, P, K, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, In, Sn, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Er, Tm, Yb, Lu, Ta, W, Bi, and mixtures thereof; x is 0.05 to 0.95; y and z are balanced by the charge of the cations; 0≤a, b, c, d≤1; and wherein the ceramic composite oxide has an average particle size diameter of 10 to 700 nm.

2. The ceramic composite oxide of claim 1, wherein C is Ni and d is 0.

3. The ceramic composite oxide of claim 1, wherein A is Zr and B is selected from the group consisting of Ce, Sm, Y, Yb, Zn, Nd, and mixtures thereof.

4. The ceramic composite oxide of claim 1, wherein A is Ce and B is selected from the group consisting of La, Pr, Nd, Sm, Gd, Y, Yb, Zn and mixtures thereof.

5. The ceramic composite oxide of claim 1, wherein A is selected from the group consisting of Ca, Sr, Ba, La, Pr, Nd, Sm, Gd, and mixtures thereof; and B is selected from the group consisting of Mg, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Y, Nb, Mo, Yb, Ta, and mixtures thereof.

6. The ceramic oxide composite of claim 1, wherein A is selected from the group consisting of Li, Na, K, Sr, Ba, Bu, and mixtures thereof; B is selected from the group consisting of Ti, Nb, Ta, Zr and mixtures thereof; and x is 0.2 to 0.8.

7. The ceramic composite oxide of claim 1, wherein the ceramic composite oxide is of formula (Ia):
(1−x)Ba.sub.(1−m)Sr.sub.m(Zr.sub.(1−n)Ce.sub.n).sub.(1−p)E.sub.pO.sub.y+xNiO  (Ia) wherein E is selected from the group consisting of Y, Yb, Zn, Nd, and mixtures thereof; x is 0.2 to 0.8; y is balanced by the charge of the cations; m is 0 to 1; n is 0 to 1; and p is 0 to 0.4.

8. The ceramic composite oxide of claim 1, wherein the ceramic composite oxide is of formula (Ib):
(1−x)A.sub.aB.sub.bO.sub.y+xCe.sub.1−dD.sub.dO  (Ib) wherein A is selected from the group consisting of Ca, Sr, Ba, La, and mixtures thereof; B is selected from the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Ta, W, and mixtures thereof; D is selected from the group consisting of La, Pr, Nd, Sm, Gd, Y, Yb, and Zn; x is 0.2 to 0.8; y is balanced by the charge of the cations; a is 0.95 to 1; b is 1; and d is 0 to 1.

9. The ceramic composite oxide of claim 8, wherein D is Gd.

10. The ceramic composite oxide of claim 8, wherein A is a mixture of Sr and La.

11. The ceramic composite oxide of claim 8, wherein B is a mixture of Fe and Co.

12. The ceramic composite oxide of claim 1, wherein the first component denoted (1−x)A.sub.aB.sub.bO.sub.y is present in an amount of 10 to 90 wt %; and the second component denoted xC.sub.cD.sub.dO.sub.z is present in an amount of 90 to 10 wt %, relative to the total weight of the ceramic composite oxide as a whole.

13. A process for the preparation of the ceramic composite oxide of claim 1, the process comprising: spray pyrolysis of a solution comprising metal ions of one or both of A and B, when present, and one of both of C and D, when present, wherein the spray pyrolysis comprises atomizing the solution into a furnace at a temperature of at least 500° C. using a dual-phase nozzle, and wherein the ceramic composite oxide is produced at a rate of 0.5 to 10 kg/h per nozzle.

14. The process of claim 13, wherein the solution is an aqueous solution.

15. The process of claim 14, wherein the aqueous solution is prepared from water soluble precursors comprising at least one metal nitrate.

16. The process of claim 13, wherein the spray pyrolysis produces a fine powder product and the process further comprises: collecting the fine powder product by cyclone; and calcining the fine powder product at a temperature in the range of 400-1200° C., thereby forming calcined, spray pyrolyzed particles.

17. The process of claim 16, wherein the calcination is carried out at a temperature of 550 to 800° C.

18. The process of claim 16, further comprising: forming the calcined, spray pyrolyzed particles into a green body; and sintering the green body.

19. The process of claim 16, further comprising: milling of the fine powder product after calcination.

20. (canceled)

21. A solid oxide cell comprising the ceramic composite oxide of claim 1.

22. A method of use of the ceramic composite oxide of claim 1, the method comprising using the ceramic composite oxide as an electrode or electrolyte in a solid oxide cell.

23. A method of use of the ceramic composite oxide of claim 1, the method comprising using the ceramic composite oxide in a dense gas separation membrane.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0067] FIG. 1: Schematic diagram of spray pyrolysis method

[0068] FIG. 2: Schematic diagram of particle structure at various stages of production process

[0069] FIG. 3: X-ray diffractogram of 30 wt % La.sub.0.6Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3−δ+70 wt % Ce.sub.0.9Gd.sub.0.1O.sub.2−δ (CGO-LSCF)

[0070] FIG. 4: X-ray diffractogram of 50 vol % Sr.sub.0.9La.sub.0.1TiO.sub.3−δ+50 vol % Ce.sub.0.9Gd.sub.0.1O.sub.2−δ (SLT-CGO)

[0071] FIG. 5: X-ray diffractogram of BaZr.sub.0.85Y.sub.0.15O.sub.3−δ+NiO; 40 wt % NiO (BZY-NiO-1)

[0072] FIG. 6: X-ray diffractogram of 40 wt % BaCe.sub.0.7Zr.sub.0.2Y.sub.0.1O.sub.3−δ+60 wt % NiO (BCZY-NiO)

[0073] FIG. 7: Particle size distribution of 40 wt % BaCe.sub.0.7Zr.sub.0.2Y.sub.0.1O.sub.3−δ+60 wt % NiO (BCZY-NiO)

[0074] FIG. 8: Area specific resistance of 30 wt % La.sub.0.6Sr.sub.0.4Co.sub.0.2Fe.sub.0.8O.sub.3−δ+70 wt % Ce.sub.0.9Gd.sub.0.1O.sub.2−δ (CGO-LSCF).

[0075] Samples A950 and A1050 are co-sprayed cer-cer composites, while B950 and B1050 are composites of the same phases prepared by mixing the two oxides.

[0076] LSCF950 and LSCF1050 are single phase cathodes of LSCF for comparison. “950” and “1050” mean the sample was heat treated at 950° C. and 1050° C., respectively.

EXAMPLES

Test Methods

[0077] X-Ray Diffraction

[0078] Powder X-ray diffractograms were obtained from 15 to 70° with step size of 0.0275° and 0.5 s counting time with a Bruker D2 Phaser, equipped with a Lynxeye detector and Ni- and Cu-filters using CuKa radiation accelerated at 40 kV and 40 mA.

[0079] Particle Size Distribution

[0080] Powder was dispersed in an isopropanol solution containing 2 wt % oxide powder and 5 wt % ethyl cellulose relative to the solid content. The instrument used was Horiba LA-960 Laser Particle Size Analyzer equipped with both red (655 nm) and blue (405 nm) lasers measuring 10 000 points for each wavelength and refractive index of 2.15/0.4i.

[0081] Area Specific Resistance

[0082] Electrochemical impedance spectroscopy was performed on symmetric cells using a tubular furnace with a ProboStat sample holder setup for circular samples. An Alpha-A High Performance Frequency Analyzer from Novocontrol was used to analyze the impedance response. The measurements were done in dry air atmosphere. The amplitude of the applied AC signal was 50 mV for the first sample. For the following samples, the amplitude was adjusted to 700 mV due to noise in the measurements. The frequency investigated ranged from 1 MHz to 1 mHz. The symmetrical cells were prepared by spraying dispersions of 5 wt % powder in 92 wt % ethanol and 3 wt % dolacol using an air-brush connected to an argon gas outlet with excess pressure of 0.5 bar. The cells were sintered at 950° C. or 1050° C. for 6 hours with heating and cooling rates of 200° C./h

Example 1

Preparation of 30 wt % La.SUB.0.6.Sr.SUB.0.4.Co.SUB.0.2.Fe.SUB.0.8.O.SUB.3−δ.+70 wt % Ce.SUB.0.9.Gd.SUB.0.1.O.SUB.2−δ (CGO-LSCF-1)

[0083] La(NO.sub.3).sub.3x6H.sub.2O (2 M), Co(NO.sub.3).sub.2x6H.sub.2O (2 M) with EDTA, Fe(NO.sub.3).sub.2x6H.sub.2O (2 M) with EDTA, Ce(NO.sub.3).sub.3x6H.sub.2O (4M) and Gd(NO.sub.3).sub.3x6H.sub.2O (2 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one together with Sr(NO.sub.3).sub.2 crystals according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 100 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 650° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 24 hours by wet ball milling using 5 mm yttria-stabilised zirconia and ethanol, followed by drying and sieving.

[0084] Other ceramic composite oxides in accordance with the invention can be prepared by this process using any of the chelating agents herein described, or any combination of such chelating agents, to solubilise any of the metal ions.

Example 2

Preparation of 50 wt % La.SUB.0.6.Sr.SUB.0.4.Co.SUB.0.2.Fe.SUB.0.8.O.SUB.3−δ.+50 wt % Ce.SUB.0.9.Gd.SUB.0.1.O.SUB.2−δ (CGO-LSCF-2)

[0085] La(NO.sub.3).sub.3x6H.sub.2O (2 M), Co(NO.sub.3).sub.2x6H.sub.2O (2 M), Fe(NO.sub.3).sub.2x6H.sub.2O (2 M), Ce(NO.sub.3).sub.3x6H.sub.2O (4M) and Gd(NO.sub.3).sub.3x6H.sub.2O (2 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one together with Sr(NO.sub.3).sub.2 crystals according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 100 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 800° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 24 hours by wet ball milling using 5 mm yttria-stabilised zirconia and isopropanol, followed by drying and sieving.

Example 3: Preparation of 50 vol % Sr.SUB.0.9.La.SUB.0.1.TiO.SUB.3−δ.+50 vol % Ce.SUB.0.5.Gd.SUB.0.1.O.SUB.2−δ (SLT-CGO)

[0086] La(NO.sub.3).sub.3x6H.sub.2O (2 M), Ce(NO.sub.3).sub.3x6H.sub.2O (4M) and Gd(NO.sub.3).sub.3x6H.sub.2O (2 M) were dissolved in distilled water in molar concentrations ˜2-4 M to prepare cation precursor solutions. Titanium isopropoxide was mixed with water and citric acid and boiled to decompose the isopropoxide forming a clear solution. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one together with Sr(NO.sub.3).sub.2 crystals according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 100 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 600° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 48 hours by wet ball milling using 10 mm yttria-stabilised zirconia and isopropanol, followed by drying and sieving.

[0087] Other ceramic composite oxides in accordance with the invention can be prepared by this process using any of the chelating agents herein described, or any combination of such chelating agents, to solubilise any of the metal ions.

Example 4: Preparation of BaZr.SUB.0.85.Y.SUB.0.15.O.SUB.3−δ.+NiO; 40 wt % NiO (BZY-NiO-1)

[0088] Ba(NO.sub.3).sub.2 (0.5 M) with complexing agents, Y(NO.sub.3).sub.3x6H.sub.2O (2.5 M) and Ni(NO.sub.3).sub.2x6H.sub.2O (1 M) were dissolved in distilled water to prepare cation precursor solutions. ZrO(NO.sub.3).sub.3x6H.sub.2O was mixed with water and citric acid and heated to form a clear solution. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 160 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 950° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 48 hours by wet ball milling using 10 mm yttria-stabilised zirconia and isopropanol, followed by drying and sieving.

[0089] In this example, the complexing agents can be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 5: Preparation of 40 vol % BaZr.SUB.0.85.Y.SUB.0.15.O.SUB.3−δ.+60 vol % NiO (BZY-NiO-2)

[0090] Ba(NO.sub.3).sub.2 (0.5 M) with complexing agents, Y(NO.sub.3).sub.3x6H.sub.2O (2.5 M) and Ni(NO.sub.3).sub.2x6H.sub.2O (1 M) were dissolved in distilled water to prepare cation precursor solutions. ZrO(NO.sub.3).sub.3x6H.sub.2O was mixed with water and citric acid and heated to form a clear solution. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 160 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 900° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 48 hours by wet ball milling using 10 mm yttria-stabilised zirconia and isopropanol, followed by drying and sieving.

[0091] In this example, the complexing agents can be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 6: Preparation of 40 wt % BaCe.SUB.0.7.Zr.SUB.0.2.Y.SUB.0.1.O.SUB.3−δ.+60 wt % NiO (BCZY-NiO)

[0092] Ba(NO.sub.3).sub.2 (0.5 M) with complexing agents, Ce(NO.sub.3).sub.3x6H.sub.2O (4M), Y(NO.sub.3).sub.3x6H.sub.2O (2.5 M) and Ni(NO.sub.3).sub.2x6H.sub.2O (1 M) were dissolved in distilled water to prepare cation precursor solutions. ZrO(NO.sub.3).sub.3x6H.sub.2O was mixed with water and citric acid and heated to form a clear solution. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The solution was fed into a water-cooled lance at a rate of 160 ml/min and atomised in a nozzle with aid of pressurised air at 2 bar. The droplets were transported through the hot zone in a furnace tube, at a set temperature of 1000° C., by an underpressurised air stream where the time of flight inside the furnace tube was less than one second. The air stream was led into a cyclone where the powder was separated and collected. The collected powder consisted of hollow spheres (agglomerates) with, depending on precursor solution concentration, temperature, feeding rate, air pressure, air velocity and organic additives, diameter 1-20 μm, and primary particles around 100 nm. The powder was further heat treated at 1100° C. for 6 hours to achieve phase purity and decompose any organic residue. To break down the hollow spheres, the powder was milled for 48 hours by wet ball milling using 10 mm yttria-stabilised zirconia and isopropanol, followed by drying and sieving.

[0093] In this example, the complexing agents can be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 7: 50 wt % Li.SUB.6.25.Al.SUB.0.25.La.SUB.3.Zr.SUB.2.O.SUB.12.+50 wt % LiCoO.SUB.2 .(LALZ-LCO)

[0094] La(NO.sub.3).sub.3x6H.sub.2O (1 M), LiNO.sub.3 (2.5 M), Co(NO.sub.3).sub.2x6H.sub.2O (1 M), ZrO(NO.sub.3).sub.3x6H.sub.2O and Al(NO.sub.3).sub.3x9H.sub.2O (1 M) were dissolved in distilled water and complexing agents to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The Li solution was added with 0-30% Li excess. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner to produce the ceramic composite oxide. The complexing agents may be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 8: 85 wt % La.SUB.0.67.Ca.SUB.0.33.MnO.SUB.3.+15 wt % Sm.SUB.2.O.SUB.3 .(LCM-SmO)

[0095] La(NO.sub.3).sub.3x6H.sub.2O (0.5 M) with complexing agents, Mn(NO.sub.3).sub.2x4H.sub.2O (2.5 M), Ca(NO.sub.3).sub.2x4H.sub.2O (0.5 M) and Sm(NO.sub.3).sub.3x6H.sub.2O (1 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner to produce the ceramic composite oxide. The complexing agents may be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 9: 80 wt % La.SUB.0.7.Sr.SUB.0.3.MnO.SUB.3.+20 wt % ZnO (LSM-ZnO)

[0096] La(NO.sub.3).sub.3x6H.sub.2O (1 M) with complexing agents, Mn(NO.sub.3).sub.2x4H.sub.2O (2.5 M) and Zn(NO.sub.3).sub.2x6H.sub.2O (1 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one together with Sr(NO.sub.3).sub.2 crystals according to the stoichiometric ratio in the formula. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder treated in a similar manner to produce the ceramic composite oxide. The complexing agents may be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 10: 86 wt % 0.93Ba.SUB.0.5.Na.SUB.0.5.TiO.SUB.3.-0.07BaTiO.SUB.3.+14 wt % ZnO (BNT-BT-ZnO)

[0097] Ba(NO.sub.3).sub.2 (0.25 M) with complexing agents, titanium isopropoxide, C.sub.12H.sub.28O.sub.4Ti (1 M) with complexing agents, bismuth citrate, C.sub.6H.sub.5BiO.sub.7 (0.5 M) with complexing agents, NaNO.sub.3 (2 M) and Zn(NO.sub.3).sub.2x6H.sub.2O (0.5 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one according to the stoichiometric ratio in the formula. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner to produce the ceramic composite oxide. The complexing agents may be selected from any of the chelating agents herein described, or any combination of such chelating agents.

Example 11: 50 wt % Pr.SUB.0.4.Sr.SUB.0.6.(Co.SUB.0.2.Fe.SUB.0.8.).SUB.0.9.Mo.SUB.0.1.O.SUB.3−δ.+50 wt % Ce.SUB.0.9.Gd.SUB.0.1.O.SUB.2−δ (PSCFM-CGO)

[0098] (NH.sub.4).sub.6Mo.sub.7O.sub.24x4H.sub.2O (0.1 M) with complexing agents, Co(NO.sub.3).sub.2x6H.sub.2O (0.5 M) with complexing agents, Fe(NO.sub.3).sub.2x6H.sub.2O (0.5 M) with complexing agents, Ce(NO.sub.3).sub.3x6H.sub.2O (0.5M) with complexing agents, Pr(NO.sub.3).sub.3x6H.sub.2O (0.5 M) with complexing agents and Gd(NO.sub.3).sub.3x6H.sub.2O (0.5 M) were dissolved in distilled water to prepare cation precursor solutions. Each solution was thermogravimetrically analysed to determine the exact concentrations and the solutions mixed into one together with Sr(NO.sub.3).sub.2 crystals according to the stoichiometric ratio in the formula. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder treated in a similar manner to produce the ceramic composite oxide. The complexing agents may be selected from any of the chelating agents herein described, or any combination of such chelating agents.