GB2555608A

GB2555608A - A magnetic material and a method of sythesising the same

Info

Publication number: GB2555608A
Application number: GB1618592.8A
Authority: GB
Inventors: V Ramanujan Raju; Parmar Harshida; Xiao Tan
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2016-11-04
Filing date: 2016-11-04
Publication date: 2018-05-09
Also published as: US20180130579A1; US10629344B2; EP3319093B1; EP3319093A1

Abstract

A process for producing Co, Al alloyed NdFeB magnetic nanoparticles comprising: dissolving boric acid in 4 N nitric acid to form first solution; dissolving iron nitrate nonahydrate, neodymium nitrate hexahydrate, cobalt nitrate hexahydrate, aluminium nitrate, glycine and first solution in deionised water; adding glycine in molar ratio 1:1; microwaving to form NdFeCoAlB oxide powder; mixing powder with CaH2 to form second powder (in 1:1.1, oxides: CaH2); annealing in vacuum furnace; washing powder with EDTA acid; and vacuum drying. Preferably, microwave radiation power is 330W for 10 minutes, and annealing temperature is 800°C for two hours. Preferably, washing comprises further washing with methanol. Preferably, EDTA acid solution is EDTA in methanol and triethanolamine. Nd15Fe59Co15Al3B8 nanoparticles may be obtained, preferably with tetragonal strcutre have P42/mnm space group. Co, Al alloyed NdFeB nanoparticles may be obtained, preferably, having mean crystallite size between 30-50nm.

Description

(54) Title of the Invention: A magnetic material and a method of sythesising the same Abstract Title: Method of producing Co, Al alloyed NdFeB magnetic nanoparticles (57) A process for producing Co, Al alloyed NdFeB magnetic nanoparticles comprising: dissolving boric acid in 4 N nitric acid to form first solution; dissolving iron nitrate nonahydrate, neodymium nitrate hexahydrate, cobalt nitrate hexahydrate, aluminium nitrate, glycine and first solution in deionised water; adding glycine in molar ratio 1:1; microwaving to form NdFeCoAlB oxide powder; mixing powder with CaH₂ to form second powder (in 1:1.1, oxides: CaH₂); annealing in vacuum furnace; washing powder with EDTA acid; and vacuum drying. Preferably, microwave radiation power is 330W for 10 minutes, and annealing temperature is 800°C for two hours. Preferably, washing comprises further washing with methanol. Preferably, EDTA acid solution is EDTA in methanol and triethanolamine. Ndi₅Fe59Coi5AI₃B8 nanoparticles may be obtained, preferably with tetragonal strcutre have P4₂/mnm space group. Co, Al alloyed NdFeB nanoparticles may be obtained, preferably, having mean crystallite size between 30-50nm.

Nd(NO₃)₂

Fe(N0₃)₃

Co(N0₃)₂ AI(N0₃)₃

C₂H₅NO₂ (H₃B0₃/HNO₃ solution)

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Salts solution

Introduced in Microwave

Metal oxides [WyAuil

Oxides + reducing agent (CaH₂)

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Removal of byproduct

Co, Al- alloyed NdFeB

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FIG. 7

-1 A MAGNETIC MATERIAL AND A METHOD OF SYNTHESISING THE SAME

Field of the Disclosure

The present disclosure relates to a process for producing a NdFeB magnetic material and particularly, but not exclusively, to a process for producing a NdFeB magnetic material for use in electrical machines.

Background to the Disclosure

Conventional hard magnetic materials are generally formed from rare earth materials, which are expensive and their supply can be problematic. Hard magnetic materials are io widely used in large variety of electrical systems, machines and devices, such as, for example, electric motors, electrical generators, hard disk drives, electric and hybrid vehicles, etc.

There is therefore a need for a high performance hard magnetic material composition 15 having a low rare earth material content.

One such composition is Nd-Fe-B which is a hard magnetic material already used in many industrial applications. To date, the experimental behaviour of exchange-coupled Nd-Fe-B magnetic materials has not matched the predicted magnetic properties.

For example, the predicted magnetic properties of exchange-coupled Nd-Fe-B magnets are considerably higher than the experimental values obtained so far. The predicted values are based on efficient exchange coupling, which can only be obtained at the nanoscale level through nanostructured materials.

It is known to produce Nd-Fe-B magnetic materials using techniques such as melt spinning, ball milling and HDDR methods. These methods involve a series of processing steps such as, for example, homogenization at high temperature, melting, casting, and milling, followed by annealing to obtain the final product. A known problem with these

-2 techniques is that they need an excess amount of Nd in order to compensate for the evaporation loss.

Statements of Disclosure

According to a first aspect of the present disclosure there is provided a process for producing Co, Al alloyed NdFeB nanoparticles, by a microwave assisted combustion process, followed by a reduction diffusion process, the process comprising the steps of:

preparing a first solution of boric acid dissolved in 4 N Nitric Acid (HNO₃); dissolving iron nitrate nonahydrate, neodymium nitrate hexahydrate, cobalt io nitrate hexahydrate, aluminium nitrate, glycine, and the first solution in deionized water to form a second solution;

adding glycine to the second solution in a molar ratio of 1:1 to form a third solution;

subjecting the third solution to microwave radiation, thereby forming an first powder of NdFeCoAlB oxides;

mixing the first powder with calcium hydride in a mass ratio of 1:1.1 (NdFeCoAlB oxides:CaH₂) to form a second powder;

annealing the second powder in a vacuum furnace;

washing the annealed second powder with a solution of ethylenediaminetetraacetic acid; and vacuum drying the second powder.

The process of the disclosure has an advantage that the quantity of amorphous boron required for the reduction diffusion process is reduced over the prior art synthesis techniques.

The magnetic properties of the NdFeB material produced by the process of the disclosure are improved over those of the prior art synthesis techniques.

-3 In the initial step of the process, boric acid is used as source of boron. The use of boric acid will produce boron oxide and will react with CaH₂, to form the desired Nd-Fe-Co-AIB hard magnetic phase.

The boric acid is oxidised during the microwave heating step and is converted to boron oxide. This boron oxide is subsequently reduced as boron during the reduction diffusion steps and subsequently forms the NdFeCoAlB hard phase material.

An advantage of the process of the present disclosure is that the use of boric acid avoids io the problem of boron hydride evaporation that is present in the prior art synthesis techniques. The use of boric acid also reduces the possibility of the formation of boron deficient phases.

An advantage of the process of the present disclosure is that the use of a solution of ethylenediaminetetraacetic acid in methanol and triethanolamine, acts to remove the nonmagnetic calcium oxide (CaO) by-product, and so reduces the absorption of hydrogen in the magnetic phase. This in turn improves the coercivity of the final magnetic material over that produced by prior art synthesis techniques.

Optionally, the step of subjecting the third solution to microwave radiation comprises the step of:

subjecting the third solution to microwave radiation of approximately 330W for a duration of approximately 10 minutes.

The step of microwave heating of the third solution results in the evaporation of water and other volatile species. This evaporation enables an exothermic reaction between the nitrate salts and the glycine results in the third solution being converted to an ultrafine NdFeCoAlB oxide powder.

This in turn reduces the absorption of hydrogen by the third solution, which in turn results in an improvement in the magnetic properties of the end product.

-4Optionally, the step of annealing the second powder in a vacuum furnace, comprises the step of:

annealing the second powder in a vacuum furnace at a temperature of 800°C for 2 hours.

The treatment of the second powder in a vacuum furnace causes reduction of the second powder.

Optionally, the step of washing the annealed second powder with a solution of ethylenediaminetetraacetic acid, comprises the further step of:

further washing the annealed second powder with methanol.

The use of methanol to provide a secondary wash of the annealed second powder assists in removing the non-magnetic calcium oxide by-product.

Optionally, the solution of ethylenediaminetetraacetic acid, is a solution of ethylenediaminetetraacetic acid in methanol and triethanolamine.

Figure 1 shows the chemical formula for ethylenediaminetetraacetic acid. Ethylenediaminetetraacetic acid (EDTA) is a chelating agent with a high affinity for Ca²⁺. Ca²⁺ tends to be bounded with EDTA to form complexants (illustrated in Figure2), which can be utilized to remove CaO. As EDTA was able to dissolve in basic solution, triethanolamine was used to dissolve EDTA. Methanol was added to reduce the viscosity for easier stirring of the liquid and separation of powder from the liquid.

According to a second aspect of the present disclosure there is provided a compound of Ndi₅Fe₅9Co₁₅AI₃B₈ in nanoparticle form obtainable by the method of the first aspect.

Optionally, the compound has a tetragonal structure having a P42/mnm space group.

-5 The Ndi₅Fe59Coi₅AI₃B₈ hard magnetic phase material has a tetragonal structure. The calculated lattice parameters derived from a Rietveld analysis of X-ray diffraction analysis data is a(A) = 8.7826 ±12 and c (A) = 12.2101±ll.

According to a third aspect of the present disclosure there is provided Co, Al alloyed NdFeB nanoparticies obtainable by the method of the first aspect.

Optionally, the Co, Al alloyed NdFeB nanoparticies have a mean crystallite size of between 30nm and 50nm.

Other aspects of the disclosure provide devices, methods and systems which include and/or implement some or all of the actions described herein. The illustrative aspects of the disclosure are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.

Brief Description of the Drawings

There now follows a description of an embodiment of the disclosure, by way of nonlimiting example, with reference being made to the accompanying drawings in which:

Figure 1 shows the chemical compound of ethylenediaminetetraacetic acid (EDTA);

Figure 2 shows a schematic representation of the complexants after the reactions of CaO, EDTA and triethanolamine;

Figure 3 shows a schematic flowchart for a process for producing Co, Al alloyed NdFeB nanoparticies according to an embodiment of the disclosure;

Figure 4 shows a typical X-ray diffraction pattern for the NdFeCoAlB powder produced by the process of Figure 1;

Figure 5 shows a typical X-ray diffraction pattern for the NdFeCoAlB powder of Figure 4 after removal of the CaO by-product;

Figure 6 shows typical hysteresis loops for NdFeCoAlB powder produced by the process of Figure 3;

-6 Figure 7 shows a Transmission Electron Microcopy micrograph of NdFeCoAlB powder produced by the process of Figure 3; and

Figure 8 shows a Rietveld refinement of NdFeCoAlB powder produced by the process of Figure 3.

It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings.

io

Detailed Description

Figure 3 illustrates schematically a process for the production of Co, Al alloyed NdFeB nanoparticles according to an embodiment of the disclosure.

A first solution is prepared by dissolving boric acid in 4N Nitric Acid (HNO₃).

This first solution is then combined with calculated amounts of iron nitrate nonahydrate (Fe (NO₃)₃), neodymium nitrate hexahydrate (Nd(NO₃)₃), cobalt nitrate hexahydrate (Co(NO₃)₂), aluminium nitrate (AI(NO₃)₃), glycine (C₂H₅NO₂), and dissolved in deionized water to form a second solution.

Glycine is added to the second solution in a molar ratio of 1 : 1 (second solution : glycine) to obtain a stable third solution.

The third solution is then subjected to microwave irradiation at a low microwave power of 330 W for 10 minutes. In one example of the process, a Sharp Model R-899R household microwave oven was used to generate the microwave irradiation.

Microwave heating of the third solution results in evaporation of water and other volatiles from the third solution. Due to the exothermic reaction of nitrate salts and

-7 glycine the third solution is spontaneously converted to a first powder, being an ultrafine Nd-Fe-Co-AI-B oxide powder.

The desired NdisFesgCoisA^Bs nanoparticles are then synthesized by mixing the first 5 powder (the Nd-Fe-Co-AI-B oxide powder) with calcium hydride (CaH₂) in a mass ratio of 1: 1.1 (Nd-Fe-Co-AI-B oxides: CaH₂) to form a second powder. The second powder is then annealed in a vacuum furnace.

Reduction is then carried out at 800 °C for 2 hours to form a powder containing the io desired hard magnetic phase NdisFesgCoisA^Bs together with a soft magnetic phase aFe, with a non-magnetic calcium oxide (CaO) by product, as shown in the x-ray diffraction pattern of Figure 4.

The annealed second powder is then washed to remove the calcium oxide (CaO) by15 product. The annealed second powder is washed with an ethylenediaminetetraacetic acid (EDTA) solution (a solution of ethylenediaminetetraacetic acid in methanol and triethanolamine) to remove the non-magnetic calcium oxide by-product.

The washed annealed second powder is then further washed in methanol. This second 20 washing step is followed by vacuum drying to obtain the dried second powder. Figure 5 illustrates the x-ray diffraction pattern of the washed second powder after the removal of the CaO by-product.

The magnetic properties at room temperature of the second powder are represented in 25 Figure 6 for both the as-synthesised material and for the material after the further removal of the CaO by-product.

As illustrated in Figure 6, after the removal of the calcium oxide by-product, the resultant magnetic properties have been increased by 25% over those of the prior art.

The magnetization (Ms) remanence magnetization (Mr) and coercivity (He) before and

-8 after calcium oxide removal are Ms=37emu/gm, Mr=23emu/gm, Hc=12kOe and Ms=105emu/gm, Mr=71emu/gm, Hc=9.2kOe respectively.

The ratio Mr/Ms is termed reduced remunence and is <0.5 for isotropic magnets. In the present example, the reduced magnetization for the final product of the process of the disclosure is 0.67. Since this value is greater than 0.5 it indicates that the magnetic phases are exchange coupled.

A morphological analysis of the powder material shows the particles are nano sized, as io illustrated in the sample micrograph of Figure 7. The nanoparticles are faceted, with their size varying between 7nm to 45 nm. The Rietveld refinement of the X-ray diffraction data for the Nd-Fe-Co-AI-B powder (after removal of the CaO by-product) indicates a composition made up of 94% Nd-Fe-CO-AI-B hard magnetic phase and 6% of alpha-Fe soft magnetic phase, as illustrated in Figure 8.

The average crystallite size calculated from Rietveld refinement of X-ray diffraction pattern was ~40nm for Nd-Fe-Co-AI-B hard magnetic phase and ~30nm for α-Fe soft magnetic phase.

Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

The foregoing description of various aspects of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person of skill in the art are included within the scope of the disclosure as defined by the accompanying claims.

Claims

1 A process for producing Co, Al alloyed NdFeB nanoparticles, by a microwave assisted combustion process, followed by a reduction diffusion process, the process comprising the steps of:

5 preparing a first solution of boric acid dissolved in 4 N Nitric Acid (HNO3);

dissolving iron nitrate nonahydrate, neodymium nitrate hexahydrate, cobalt nitrate hexahydrate, aluminium nitrate, glycine, and the first solution in deionized water to form a second solution;

adding glycine to the second solution in a molar ratio of 1:1 to form a io third solution;

mixing the first powder with calcium hydride in a mass ratio of 1:1.1 (NdFeCoAlB oxides:CaH2) to form a second powder;

15 annealing the second powder in a vacuum furnace;

20

2 The process as claimed in Claim 1, wherein the step of subjecting the third solution to microwave radiation comprises the step of:

25

3 The process as claimed in Claim 1 or Claim 2, wherein the step of annealing the second powder in a vacuum furnace, comprises the step of:

annealing the second powder in a vacuum furnace at a temperature of

800°C for 2 hours.

-10

4 The process as claimed in any one of Claims 1 to 3, wherein the step of washing the annealed second powder with a solution of ethylenediaminetetraacetic acid, comprises the further step of:

further washing the annealed second powder with methanol.

5 The process as claimed in in any one of Claims 1 to 4, wherein the solution of ethylenediaminetetraacetic acid, is a solution of ethylenediaminetetraacetic acid in methanol and triethanolamine.

io

6 A compound of NdisFesgCoisALBg in nanoparticle form obtainable by the method of any one of Claims 1 to 5.

7 The compound as claimed in Claim 6, wherein the compound has a tetragonal structure having a P42/mnm space group.

8 Co, Al alloyed NdFeB nanoparticles obtainable by the method of any one of Claims 1 to 5.

9 Co, Al alloyed NdFeB nanoparticles as claimed in Claim 8, having a mean crystallite size of between 30nm and 50nm.

23 05 17

Intellectual

Property

Office

Application No: GB1618592.8 Examiner: Mr Peter Banks Claims searched: 1-5 Date of search: 5 May 2017

9 Co, Al alloyed NdFeB nanoparticles as claimed in Claim 8, having a mean 20 crystallite size of between 30nm and 50nm.

10 A process substantially as hereinbefore described with reference to Figures 3 to 8.

25 11 A compound substantially as hereinbefore described with reference to Figures 3 to 8.

12 Co, Al alloyed NdFeB nanoparticles substantially as hereinbefore described with reference to Figures 3 to 8.

Amendments to the claims have been filed as follows

23 05 17

5 preparing a first solution of boric acid dissolved in 4 N Nitric Acid (HNO3);

15 annealing the second powder in a vacuum furnace;

20 2 The process as claimed in Claim 1, wherein the step of subjecting the third solution to microwave radiation comprises the step of:

25 3 The process as claimed in Claim 1 or Claim 2, wherein the step of annealing the second powder in a vacuum furnace, comprises the step of:

annealing the second powder in a vacuum furnace at a temperature of

800°C for 2 hours.

further washing the annealed second powder with methanol.

io

6 A compound of NdisFesgCoisAbBs in nanoparticle form obtainable by the method of any one of Claims 1 to 5.