US20090274571A1

US20090274571A1 - Nd-Fe-B Permanent Magnetic Material

Info

Publication number: US20090274571A1
Application number: US12/422,385
Authority: US
Inventors: Xin Du; Xiaofeng Cheng; Faliang Zhang
Original assignee: BYD Co Ltd
Current assignee: BYD Co Ltd
Priority date: 2008-05-04
Filing date: 2009-04-13
Publication date: 2009-11-05
Also published as: CN101572146A; CN101572146B

Abstract

Permanent magnetic materials comprising an Nd—Fe—B alloy and an additive comprising at least one boride are disclosed. The boride may be a lanthanide boride being about 0.01% to about 5% of the alloy by weight. In some instances, the lanthanide boride may be at least one of DyB₆, GdB₆, TbB₆, SmB₆, or mixtures thereof.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Chinese Patent Application No. 200810094116.0, filed May 4, 2008.

BACKGROUND

Because of its magnetic properties, low cost and ample reserves, Nd—Fe—B permanent magnets are widely used in vehicles, computers, electronics, mechanical and medical devices, to name a few. In addition, because of its performance to price ratio, Nd—Fe—B materials have been favored to produce magnetic devices with high efficiency, small volume and light mass. However, Nd—Fe—B materials are also known to have poor coercivity and mechanical properties thereby limiting its applications to some extent.

SUMMARY

Nd—Fe—B permanent magnetic materials are disclosed. One embodiment discloses a permanent magnetic material comprising an Nd—Fe—B alloy and an additive comprising at least one boride. The amount of boride may range from about 0.01% to about 5% of the alloy by weight. In one embodiment, the boride is a lanthanide boride. In some embodiments, the lanthanide boride includes at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆. In some embodiments, the lanthanide boride includes TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆. In these embodiments, the weight ratio of the TbB₆to the first compound is from about 1:1 to about 50:1.
In one embodiment, the alloy has the following general formula: Nd_aR_bFe_100-a-b-c-dM_cB_d. In some embodiments, R is at least one element selected from the group consisting of Pr, Dy and Tb, M is at least one element selected from the group consisting of Nb, Co, Ga, Zr, Al, Cu and Ti, and 10≦a≦20, 0≦b≦8, 0≦c≦6 and 5≦d≦7.
One embodiment discloses a method of preparing a permanent magnetic material, the method comprising mixing an Nd—Fe—B alloy and an additive to form a mixture, pressing the mixture in a magnetic field to form a composition, sintering the composition to a first temperature, and tempering the composition to a second temperature. In one embodiment, the sintering and tempering steps can occur under vacuum. In one embodiment, the sintering and tempering steps can occur under an inert gas atmosphere.
In one embodiment, the method includes mixing the alloy and the additive with an antioxidant and a lubricant. In one embodiment, the amount of antioxidant is from about 0.01% to about 5% of the alloy by weight. In one embodiment, the amount of lubricant is from about 0% to about 5% of the alloy by weight.
In some embodiments, the average particle diameter of the alloy is from about 2 to about 10 microns, while the average particle diameter of the additive is from about 2 to about 1000 nanometers. In some embodiments, the pressing step has an intensity of from about 1.2 to about 2.0 T, a pressure of from about 10 to about 200 MPa, and a period of from about 10 to about 60 seconds. In some embodiments, the first temperature is from about 1030 to about 1120° C. for a period of from about 2 to about 4 hours, while the second temperature is from about 500 to about 920° C. for a period of from about 2 to about 8 hours.
Other variations, embodiments and features of the presently disclosed permanent magnetic materials will become evident from the following detailed description, drawings and claims.

DETAILED DESCRIPTION

It will be appreciated by those of ordinary skill in the art that the permanent magnetic materials can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive.
One embodiment of the present disclosure discloses a permanent magnetic material comprising an Nd—Fe—B (neodymium-iron-boron) alloy and an additive comprising at least one boride. As used herein, “boride” and the like means a chemical compound between boron and a less electronegative element. In one embodiment, a small quantity of boride may advance the coercivity and mechanical properties of the permanent magnetic material. In one embodiment, the amount of boride may be from about 0.01% to about 5% of the alloy by weight.
In one embodiment, the boride may be a lanthanide boride. In some embodiments, the lanthanide boride includes at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆. In some embodiments, the lanthanide boride includes TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆. In these embodiments, the weight ratio of the TbB₆to the first compound may be from about 1:1 to about 50:1. In some embodiments, the boride may be uniformly dispersed within the Nd—Fe—B alloy, the boride having average particle diameters of from about 2 to about 1000 nanometers. In some embodiments, the boride may have average particle diameters of from about 2 to about 100 nanometers.
In one embodiment, the Nd—Fe—B alloy has the following general formula: Nd_aR_bFe_100-a-b-c-dM_cB_d. In this embodiment, the R is at least one element selected from the group consisting of Pr, Dy and Tb, the M is at least one element selected from group consisting of Nb, Co, Ga, Zr, Al, Cu and Ti, and 10≦a≦20, 0≦b≦8, 0≦c≦6 and 5≦d≦7. In some embodiments, the Nd—Fe—B alloy may average particle diameters of from about 2 to about 10 microns.
One embodiment of the present disclosure discloses a method of preparing a permanent magnetic material, the method comprising: mixing an Nd—Fe—B alloy and an additive to form a mixture, pressing the mixture in a magnetic field to form a composition, sintering the composition to a first temperature, and tempering the composition to a second temperature. In one embodiment, the sintering and tempering can occur under vacuum. In one embodiment, the sintering and tempering occur can under an inert gas. In some embodiments, the inert gas includes at least one member selected from the group consisting of nitrogen, helium, argon, neon, krypton and xenon.
In some embodiments, the sintering process for preparing the permanent magnetic material may include without limitation one or more of the following steps: formulating, smelting, crushing, milling, processing ultrafine powders, pressing in a magnetic field, sintering in vacuum and electroplating. Some of the steps are as follows:
(1) In one embodiment, the Nd—Fe—B alloy may be crushed and grounded to form a powder. The crushing may include hydrogen-induced cracking or mechanical crushing. In one embodiment, jet milling may be utilized to produce powders with average particle diameters of from about 2 to about 10 microns.
In some embodiments, the Nd—Fe—B alloy may be an alloy ingot or a strip casting alloy. In one embodiment, the Nd—Fe—B alloy may be acquired from a third party. In some embodiments, the Nd—Fe—B alloy may be produced by casting or strip casting processes. In one embodiment, the Nd—Fe—B alloy may have the following general formula: Nd_aR_bFe_100-a-b-c-dM_cB_dwhere the R may be at least one element selected from the group consisting of Pr, Dy and Tb, the M may be at least one element selected from the group consisting of Nb, Co, Ga, Zr, Al, Cu and Ti, and 10≦a≦20, 0≦b≦8, 0≦c≦6, and 5≦d≦7.
In one embodiment, the casting process comprises casting a smelted alloy molten in a water-cooled, copper mold. The Nd—Fe—B alloy ingot may have a columnar, crystal structure, where the columnar crystals are separated by Nd-rich phase layers. In these instances, the distance between two Nd-rich phase layers may be from about 100 to about 1500 microns.
In one embodiment, the strip casting process comprises pouring a smelted alloy molten on a copper roller surface. In one example, the rotational linear velocity of the copper roller surface may vary from about 1 to about 2 meters per second. The molten alloy may be cooled to form flakes in different breadths with thicknesses ranging from about 0.2 to about 0.5 millimeter. In some embodiments, the columnar crystals in the flakes may have breadths ranging from about 5 to about 25 microns.
In one embodiment, hydrogen-induced cracking comprises placing an Nd—Fe—B alloy in a stainless steel case, filling the case with high purity hydrogen after vacuumizing, and maintaining at an atmospheric pressure for about 20 to about 30 minutes. In one instance, the alloy may blow out because of hydrogen absorption and produce a hydride. In one embodiment, the hydride may be vacuumized for dehydrogenation for from about 2 to about 10 hours at from about 400 to about 600° C.
In one embodiment, mechanical crushing comprises rough crushing in a jaw crusher, followed by mechanical crushing in a fine crusher. In one embodiment, jet milling comprises accelerating powder grains to supersonic speed in air, and allowing the grains to clash with each other and fall to pieces.
(2) The Nd—Fe—B alloy and the additive may be mixed using a mixer to obtain a powder composition. In one embodiment, the additive comprises at least one boride. The amount of boride may be from about 0.01% to about 5% of the alloy by weight. In some embodiments, the boride may be processed in advance by dispersion treatment. In some embodiments, the boride may include at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆. In some embodiments, the alloy may have an average particle diameter of from about 2 to about 10 microns. In some embodiments, the additive or boride may have an average particle diameter of from about 2 to about 1000 nanometers. In some embodiments, the boride may include TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆, where the weight ratio of the TbB₆to the first compound may be from about 1:1 to about 50:1.
In one embodiment, the alloy/additive mixture may further include an antioxidant and a lubricant. In some embodiments, the amount of antioxidant may be about 0.01% to about 5% of the alloy by weight, and the amount of lubricant may be about 0% to about 5% of the alloy by weight. In some embodiments, the antioxidant includes at least one member selected from the group consisting of polyethylene oxide alkyl ether, polyethylene oxide single fatty ester and polyethylene oxide alkenyl ether. In some embodiments, the lubricant includes at least one member selected from group consisting of gasoline, oleic acid, stearic acid, polyhydric alcohol, polyethylene glycol, sorbitan and stearin.
(3) The alloy/additive mixture may be pressed in a magnetic field to form a composition. In some embodiments, the pressing comprises pressing in a closed glove box with a magnetic field. In some embodiments, the magnetic field has an intensity of from about 1.2 to about 2.0 T and a pressure of from about 10 to about 200 MPa. In some embodiments, the pressing step may take anywhere from about 10 to about 60 seconds.
(4) The composition may be sintered to a first temperature, and tempered to a second temperature. In one embodiment, the sintering and tempering may occur under vacuum. In one embodiment, the sintering and tempering may occur under an inert gas. In some embodiments, the inert gas includes at least one member selected from the group consisting of nitrogen, helium, argon, neon, krypton and xenon. In some embodiments, the mixture may be sintered at temperatures ranging from about 1030 to about 1120° C. for a period of from about 2 to about 4 hours. In some embodiments, the mixture may be tempered at temperatures ranging from about 500 to about 920° C. for a period of from about 2 to about 8 hours. In some embodiments, the mixture may be tempered in two steps, the first tempering at temperatures ranging from about 800 to about 920° C. for a period of from about 1 to about 3 hours followed by a second tempering at temperatures ranging from about 500 to about 650° C. for a period of from about 2 to about 4 hours.
The following provides additional details on some embodiments of the present disclosure.

Example 1

(1) An Nd—Fe—B alloy is made by strip casting with a rotational linear velocity of a copper roller surface at about 1.5 meters per second. The strip casting alloy has a thickness of about 0.3 mm with the formula Nd_10.2(Dy_2.8Tb_1.3)Fe_75.5(CO_2.3Al_0.7Nb_0.3Ga_0.4)B_6.5.
(2) The alloy is crushed by hydrogen-induced cracking. First, by absorbing hydrogen to saturation at room temperature. Next, by dehydrogenation at 550° C. for about 6 hours. The alloy is milled to produce a powder with an average particle diameter of about microns by jet milling under a nitrogen atmosphere.
(3) An TbB₆additive and an antioxidant are added to the powder. The additive is about 3% of the alloy by weight and has an average particle diameter of about 20 nanometers. The composition is mixed by adding gasoline, which is about 3% of the alloy by weight.
(4) The composition may be pressed by a forming press in a closed glove box under nitrogen gas in a magnetic field. The intensity of the magnetic field is about 1.6 T, the pressure is at about 100 MPa, and the isostatic pressing time is about 30 seconds.
(5) The composition is sintered in a vacuum sintering furnace under an atmospheric pressure of 2×10⁻²Pa, the sintering temperature at about 1080° C. for about 3 hours. The composition is subsequently tempered at about 850° C. for about 2 hours followed by tempering at about 550° C. for about 3 hours.
The Nd—Fe—B permanent magnetic material can be labeled as T1.
Reference 1
The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception of the TbB₆additive.
The Nd—Fe—B permanent magnetic material can be labeled as TC1.

Example 2

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that the average particle diameter of the TbB₆additive is about 1.2 microns.
The Nd—Fe—B permanent magnetic material can be labeled as T2.

Example 3

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that the amount of TbB₆additive is about 6% of the alloy by weight.
The Nd—Fe—B permanent magnetic material can be labeled as T3.

Example 4

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception of the antioxidant and gasoline.
The Nd—Fe—B permanent magnetic material can be labeled as T4.

Example 5

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that DyB₆is incorporated as the additive instead of TbB₆.
The Nd—Fe—B permanent magnetic material can be labeled as T5.

Example 6

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that GdB₆is incorporated as the additive instead of TbB₆.
The Nd—Fe—B permanent magnetic material can be labeled as T6.

Example 7

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that SmB₆is incorporated as the additive instead of TbB₆.
The Nd—Fe—B permanent magnetic material can be labeled as T7.

Example 8

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that:
(1) The average particle diameter of the TbB₆additive is about 1000 nanometers;
(2) The amount of TbB₆additive is about 6% of the alloy by weight;
(3) The average particle diameter of the powder alloy is about 7 microns;
(4) The amount of antioxidant is about 5% of the alloy by weight;
(5) The intensity of the magnetic field is about 1.2 T, the pressure is about 200 MPa, and the isostatic pressing time is about 10 seconds;
(6) The vacuum sintering temperature is about 1030° C. for about 4 hours; and
(7) The first tempering is at about 920° C. for about 1 hour and the second tempering is at about 650° C. for about 2 hours.
The Nd—Fe—B permanent magnetic material can be labeled as T8.

Example 9

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that:
(1) A mixture of DyB₆and TbB₆is incorporated as the additive instead of TbB₆;
(2) The average particle diameter of the DyB₆and TbB₆mixture additive is about 20 nanometers; and
(3) The amount of the DyB₆is about 0.2% of the alloy by weight and the amount of the TbB₆is about 4% of the alloy by weight.
The Nd—Fe—B permanent magnetic material can be labeled as T9.

Example 10

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that:
(1) A mixture of GdB₆and TbB₆is incorporated as the additive instead of TbB₆;
(2) The average particle diameter of the GdB₆and TbB₆mixture additive is about 20 nanometers; and
(3) The amount of the GdB₆is about 1% of the alloy by weight and the amount of the TbB₆is about 2% of the alloy by weight.
The Nd—Fe—B permanent magnetic material can be labeled as T10.

Example 11

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that:
(1) A mixture of SmB₆and TbB₆is incorporated as the additive instead of TbB₆;
(2) The average particle diameter of the SmB₆and TbB₆mixture additive is about 20 nanometers; and
(3) The amount of the SmB₆is about 0.01% of the alloy by weight and the amount of the TbB₆is about 0.5% of the alloy by weight.
The Nd—Fe—B permanent magnetic material can be labeled as T11.

Example 12

The sample is substantially similar in all respect to that of EXAMPLE 1 with the exception that:
(1) The average particle diameter of the TbB₆additive is about 100 nanometers;
(2) The amount of TbB₆additive is about 0.1% of the alloy by weight;
(3) The average particle diameter of the powder alloy is about 10 microns;
(4) The amount of antioxidant is about 1% of the alloy by weight; and
(5) The intensity of the magnetic field is about 1.8 T, the pressure is about 10 MPa, and the isostatic pressing time is about 60 seconds.
The Nd—Fe—B permanent magnetic material can be labeled as T12.

Example 13

(1) An Nd—Fe—B alloy is made by casting, the smelted molten alloy being cooled and solidified in a water-cooling cooper mold. The cast alloy has a general chemical formula Nd_10.25(Pr_3.30Dy_1.15)Fe_78.33(Al_0.75Cu_0.05)B_6.17.
(2) The alloy is rough crushed in a jaw crusher, followed by mechanical crushing in a fine crusher, and milled to powder form with an average particle diameter of about 3.5 microns by jet milling under a nitrogen atmosphere.
(3) An TbB₆additive and an antioxidant are added to the powder. The additive is about 0.1% of the alloy by weight and has an average particle diameter of about 100 nanometers. The amount of antioxidant is about 1% of the alloy by weight.
(4) The composition may be pressed by a forming press in a closed glove box under nitrogen gas in a magnetic field. The intensity of the magnetic field is about 2.0 T, the pressure at about 10 MPa, and the isostatic pressing time is about 60 seconds.
(5) The composition is sintered in a vacuum sintering furnace under an atmospheric pressure of 2×10⁻²Pa, the sintering temperature at about 1120° C. for about 2 hours. The composition is first tempered at about 800° C. for about 3 hours followed by second tempering at about 500° C. for about 4 hours.
The Nd—Fe—B permanent magnetic material can be labeled as T13.
Reference 2
The sample is substantially similar in all respect to that of EXAMPLE 13 with the exception of the TbB₆additive.
The Nd—Fe—B permanent magnetic material can be labeled as TC2.
Testing
1. Magnetic Property
Using a curve measurement system for permanent magnetic materials (NIM200C/China National Institute of Metrology), the magnetic properties of materials T1-T13, TC1 and TC2 were carried out and recorded in Table 1. The magnetic properties tested included remnant magnetism (Br) and maximum magnetic energy product (BHmax).
2. Mechanical Property
Using a universal testing machine (CMT5105/XinSanSi (ShenZhen) Group
Company), the mechanical properties of materials T1-T13, TC1 and TC2 were carried out and recorded in Table 1. The mechanical properties tested included coercive force (Hcj) and bending strength (MPa).

TABLE 1

Magnetic and mechanical properties of materials T1-T13,
TC1 and TC2.

				Bending
		BHmax	Hcj	Strength
No.	Br (KGs)	(MGOe)	(KOe)	(MPa)

T1	11.66	32.80	27.95	188.21
TC1	11.68	33.11	25.46	179.37
T2	11.59	32.75	26.62	186.37
T3	11.43	31.40	25.96	182.78
T4	11.59	32.13	26.10	183.48
T5	11.64	32.72	26.45	187.68
T6	11.57	32.50	26.19	187.36
T7	11.56	32.43	26.08	186.41
T8	11.59	32.79	26.60	185.74
T9	11.64	32.81	27.46	188.03
T10	11.60	32.77	27.09	187.76
T11	11.67	32.87	26.75	187.66
T12	11.66	32.91	26.89	187.80
T13	11.78	34.76	18.92	193.19
TC2	11.75	34.51	17.43	186.42

Based on the results of Table 1, T1 exhibited better coercive force (27.95 KOe v. 25.46 KOe) and improved bending strength (188.21 MPa v. 179.37 MPa) than its counterpart TC1 without the TbB₆additive, while T13 likewise performed the same versus its counterpart TC2 (18.92 KOe v. 17.43 KOe and 193.19 MPa v. 186.42 MPa). At the same time, the permanent magnetic materials T1 and T13 are able to maintain comparable magnetic properties (remnant magnetism of 11.66 KGs v. 11.68 KGs for T1 v. TC1 and 11.78 KGs v. 11.75 KGs for T13 v. TC2; maximum magnetic energy product of 32.80 MGOe v. 33.11 MGOe for T1 v. TC1 and 34.76 MGOe v. 34.51 MGOe for T13 v. TC2). In addition, the permanent magnetic materials according to the presently disclosed embodiments are also able to maintain comparable mechanical properties including coercive force and bending strength against the reference samples while keeping magnetic properties including remnant magnetism and maximum magnetic energy product substantially invariant.
Although the permanent magnetic materials have been described in detail with reference to several embodiments, additional variations and modifications exist within the scope and spirit as described and defined in the following claims.

Claims

1. A permanent magnetic material comprising:

an Nd—Fe—B alloy; and

an additive comprising at least one boride.

2. The material of claim 1, wherein the amount of boride is from about 0.01% to about 5% of the alloy by weight.

3. The material of claim 1, wherein the boride is a lanthanide boride.

4. The material of claim 3, wherein the lanthanide boride includes at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆.

5. The material of claim 3, wherein the lanthanide boride includes TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆, and wherein the weight ratio of the TbB₆to the first compound is from about 1:1 to about 50:1.

6. The material of claim 1, wherein the alloy has the following general formula:

Nd_aR_bFe_100-a-b-c-dM_cB_d, wherein:

R is at least one element selected from the group consisting of Pr, Dy and Tb;

M is at least one element selected from the group consisting of Nb, Co, Ga, Zr, Al, Cu and Ti; and

10≦a≦20, 0≦b≦8, 0≦c≦6 and 5≦d≦7.

7. A permanent magnetic material comprising:

an Nd—Fe—B alloy, wherein the alloy has the following general formula:

Nd_aR_bFe_100-a-b-c-dM_cB_d, wherein:

R is at least one element selected from the group consisting of Pr, Dy and Tb;

10≦a≦20, 0≦b≦8, 0≦c≦6 and 5≦d≦7; and

an additive comprising at least one lanthanide boride.

8. The material of claim 7, wherein the amount of lanthanide boride is from about 0.01% to about 5% of the alloy by weight.

9. The material of claim 7, wherein the lanthanide boride includes at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆.

10. The material of claim 7, wherein the lanthanide boride includes TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆, and wherein the weight ratio of the TbB₆to the first compound is from about 1:1 to about 50:1.

11. A method of preparing a permanent magnetic material, the method comprising:

mixing an Nd—Fe—B alloy and an additive to form a mixture;

pressing the mixture in a magnetic field to form a composition;

sintering the composition to a first temperature; and

tempering the composition to a second temperature, wherein the sintering and tempering steps can occur under a first atmosphere.

12. The method of claim 11, wherein the first atmosphere is at least one of vacuum or inert gas.

13. The method of claim 11, wherein the additive comprises at least one boride, and wherein the amount of boride is from about 0.01% to about 5% of the alloy by weight.

14. The method of claim 13, wherein the boride includes at least one member selected from the group consisting of DyB₆, GdB₆, TbB₆and SmB₆.

15. The material of claim 13, wherein the boride includes TbB₆and a first compound selected from the group consisting of DyB₆, GdB₆and SmB₆, and wherein the weight ratio of the TbB₆to the first compound is from about 1:1 to about 50:1.

16. The method of claim 11, further comprising mixing the alloy and the additive with an antioxidant and a lubricant, wherein the amount of antioxidant is from about 0.01% to about 5% of the alloy by weight, and wherein the amount of lubricant is from about 0% to about 5% of the alloy by weight.

17. The method of claim 11, wherein the alloy has the following general formula:

Nd_aR_bFe_100-a-b-c-dM_cB_d, wherein:

R is at least one element selected from the group consisting of Pr, Dy and Tb;

10≦a≦20, 0≦b≦8, 0≦c≦6 and 5≦d≦7.

18. The method of claim 11, wherein the average particle diameter of the alloy is from about 2 to about 10 microns, and wherein the average particle diameter of the additive is from about 2 to about 1000 nanometers.

19. The method of claim 11, wherein the pressing step has an intensity of from about 1.2 to about 2.0 T, a pressure of from about 10 to about 200 MPa, and a period of from about 10 to about 60 seconds.

20. The method of claim 11, wherein the first temperature is from about 1030 to about 1120° C. for a period of from about 2 to about 4 hours, and wherein the second temperature is from about 500 to about 920° C. for a period of from about 2 to about 8 hours.