WO1996014268A1

WO1996014268A1 - Production of metal boride powders

Info

Publication number: WO1996014268A1
Application number: PCT/AU1995/000740
Authority: WO
Inventors: Patrice Millet; James Stanislaus Williams; Barry William Ninham
Original assignee: The Australian National University
Priority date: 1994-11-08
Filing date: 1995-11-08
Publication date: 1996-05-17
Also published as: AUPM933094A0

Abstract

Titanium and zirconium borides have traditionally been produced by means of borothermic and carbothermic reduction. The procedure requires exceedingly high temperatures, namely, about 2000 °C for titanium carbide and between 1400 °C and 1700 °C for zirconium boride, resulting in a polycrystalline material, with an average grain size in microns, less useful than sub-micron sized particles for forming dense refractory products. This problem has been overcome by dry, high-energy milling of the starting material, (ilmenite, zirconia) and the reducing agent boron under vacuum or inert atmosphere to sub-micron dimensions, followed by borothermic reduction at 750 °C to 1100 °C in an inert atmosphere to the boride, (TiB2, ZrB2) and a by-product of amorphous B2O2. Boron is a mild reducer, reacting readily with the iron oxide parts of the ore, but not with TiO2/ZrO2, which it converts to the metaborate, thence to TiB2/ZrB2. The boron in the starting material may be crystalline or amorphous. The resultant boride is sub-micron (∫1ν), less costly that its predecessors and may be readily compacted and sintered into a dense body. The final product has application to ballistic armour, coatings for cutting tools, crucibles and dies for molten metals, cathodes for the electrolytic production of aluminium, leading edges and nosecaps for hypersonic atmospheric re-entry, rocket nozzle inserts and high temperature composites. The B2O2 by-product may be separated from the boride, by leaching away with hot water.

Description

PRODUCπON OF METAL BORIDE POWDERS

Technical Field

This invention concerns the production of metal borides, especially (but not exclusively) borides of metals which are in Group IVB of the Periodic Table. The method of this invention involves the high energy milling of metallic oxides with boron in its amorphous or crystalline form, to produce, after annealing at a relatively low temperature, a reactive powder mixture from which boron oxide, B₂0₂, can be removed to leave the metal boride or a mixture of metal borides.

Background to the Invention

Most metal borides exhibit the properties of extreme hardness, high melting point, high electrical conductivity, good thermal shock resistance, chemical inertness and durability. Borides of titanium and zirconium are refractory materials used as ballistic armour, as coatings for cutting tools, and, in the foundry and refractory industries, as crucibles and dies for handling molten metals. They are also used in the electrolytic production of aluminium, where their superior performance particularly their corrosion resistance - in comparison with the carbon cathodes normally used for this purpose is especially advantageous. In addition, as noted by I M Low and R McPherson, in their paper published in the Journal of Materials Science Letters, Volume 8, 1989, pages 1281-1283, zirconium boride is used in hypersonic re-entry vehicles, for leading edges, nose caps, and rocket nozzle inserts. TiB₂ and ZrB₂ are also used as components of high temperature composite materials such as TiB₂/TiC, ZrB₂/ZrC and Al₂0₃/TiB₂.

The production of titanium boride and zirconium boride usually involves the borothermic and carbothermic reduction of titania and zireonia, respectively. The main disadvantages of such techniques are

(i) that the reaction requires the establishment of a high temperature, which is about 2,000°C when titanium boride is being produced, and is in the range of from 1,400°C to 1,700°C in the case of the production of zirconium boride; and (ii) the boride product is a polycrystalline material having an average grain size measured in microns (μ ) , whereas sub-micron particles are more useful when forming a dense refractory product.

Alternative methods for the production of titanium boride and zirconium boride include:

(a) the so-called "polymer-precursor route", which is described by K Su and L G Sneddon in their paper published in Chemical Materials. Volume 3, 1991, pages 10 to 12;

(b) the use of alkoxides or alcoholates, as described by Z Jiang and W E Rhine in their paper published in Chemical Materials, Volume 4, 1992, pages 497 to 500; and

(c) the borothermic reduction of titanium dioxide or derived polymers in the presence of amorphous boron (see the paper published by Z Jiang and W E Rhine in the Journal of the European Ceramic Society. Volume 12, 1993, pages 403 to 411).

These alternative techniques yield boride powders which have particles in the sub-micron range, thus reducing the temperature to which a pressed mixture of the boride and a binder (and, optionally, a sintering aid such as iron, nickel, cobalt, carbon or tungsten carbide) has to be raised to form a refractory product. However, those techniques all require the establishment of a temperature of at least 1,300°C for the production of the boride. Since the establishment and maintenance of temperatures significantly in excess of 1,100°C requires special heating techniques and consumes significant energy, the alternative techniques for producing borides of Group IVB metals, detailed above, are still costly techniques.

Disclosure of the Present Invention

It is an object of the present invention to provide a method for the production of metal borides - particularly borides of Group IVB metals - which does not require the establishment of a temperature which is significantly in excess of 1, 100°C.

This objective is achieved by high energy ball milling, preferably under vacuum conditions, of a dry powder mixture of a metal oxide (or a mixture of metal oxides) and boron. Heating the fine (nanostructural) powder so produced to a temperature in the range of from about 750°C to 1,100°C enables a thermochemical reaction (the borothermic reduction of the metal oxide or oxides) to take place, with the average particle size of the boride(s) produced by this reaction remaining in the sub-micron range. The oxide of boron that is also produced by this reaction, usually the dioxide, B₂0₂, in the amorphous state, can be removed from the product powder, preferably by the known technique of leaching with hot water - unless the (or a) required product boride is also soluble in hot water.

Thus, according to the present invention, there is provided a method of producing a metal boride which comprises the sequential steps of (a) forming a powder mixture of the metal oxide and boron;

(b) subjecting the powder mixture to high energy dry ball milling under vacuum conditions or in an inert atmosphere for a period sufficient to produce a powder mixture in which the average particle size is a sub-micron value;

(c) heating the milled powder to a temperature in the range of from about 750°C to 1,100°C in an inert atmosphere, whereby borothermic reduction of the metal oxide occurs, to produce the metal boride and an oxide of boron; and

(d) subsequently removing the oxide of boron from the product of the borothermic reduction reaction.

As noted above, the metal oxide is preferably an oxide of a Group IVB metal, titania and zireonia being examples of Group IVB metal oxides.

As also noted above, more than one metal oxide may be mixed with boron in step (a) , in which case the product of this method will be a mixture of metal borides, which can be formed into a composite boride material. A mixture of borides is also produced if the metal oxide of step (a) is a mixed oxide (such as ilmenite, FeTi0₃).

The boron used to form the mixture of step (a) may be amorphous boron or crystalline boron. If crystalline boron is used, a longer milling of the powder mixture will be required to produce the fine powder product of step (b), which is preferably a fine powder in which all the grains have a diameter of less than 1 micrometre (although, in practice, a number of crystallite grains may agglomerate to form a polycrystalline "particle" which has a diameter in excess of 1 micron).

The boride produced by the method of the present invention may be mixed with a fugitive binder and/or a sintering aid, then moulded under pressure and sintered (using either hot pressing or pressureless sintering) to produce a dense body. Alternatively, the boride product may be mixed with another boride, a carbide, an oxide or other suitable material to produce a composite material having desirable properties.

Preferably, the high energy milling step is performed under a primary vacuum of about 10^"2 torr, in a planar ball mill of the type described in the specification of Australian patent No 639,945. That ball mill is also described in the specification of International patent application No PCT/AU90/00471 (WIPO Publication No 91/04810). However, other suitable ball milling devices may be used to perform the present invention. A better understanding of the present invention will be obtained from the following description and discussion of three examples of experimental work in connection with the production of borides, which has been undertaken by the present inventors. In the following description, reference will be made to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a series of X-ray diffraction patterns, in graph form, with the intensity of the x-radiation of each pattern being shown in arbitrary units.

Figure 2 shows, also in graph form, differential thermal analysis data obtained from samples of titanium dioxide milled with amorphous boron.

Figure 3 comprises two further x-ray diffraction patterns, obtained from a sample of zireonia milled with amorphous boron.

Figure 4 presents differential thermal analysis data for the product of a sample of zireonia milled with amorphous boron.

Figure 5 comprises further x-ray diffraction patterns, obtained from a sample of ilmenite that has been milled with amorphous boron.

Figure 6 shows the differential thermal analysis data obtained from a sample of ilmenite which has been milled with amorphous boron. Best modes of performing the invention Example 1

In each of a first series of experiments, a quantity of titanium dioxide (a material marketed by Fluka, having a purity of better than 99 per cent and an anatase structure) and a quantity of amorphous boron (a material marketed by Johnson Mattey Electronics, having a purity of better than 92 per cent) were added to a ball mill of the type described in WIPO Publication No WO 91/04810 in quantities such that the atomic ratio of titanium to boron was 1:4. This powder mixture was subjected to high energy milling at room temperature under a vacuum of 10"² torr. The evolution of the structure of the powder as the milling time increased was monitored by observing the x-ray diffraction patterns of the milled powder, obtained using cobalt Kα radiation. In addition, the thermal properties of the milled powder were studied using a Shimadzu differential thermal analyser model No DTA-50. The size of the particles in the milled powder was also monitored, using a scanning electron microscope.

The data obtained for powder samples which had been milled for (i) 25 hours and (ii ) 140 hours are shown in Figures 1 and 2.

Trace (a) of Figure 1 is the x-ray diffraction pattern of the sample milled in vacuo for 25 hours. This trace shows that the Ti0₂ component of the milled powder had retained the anatase structure. The broadening of the x-ray diffraction peaks and the low intensity observed are due to the very small grain size of the milled powder; they are characteristic features of milled powders. After milling for 140 hours (trace (b) of Figure 1), there has been a phase transformation of the titanium dioxide, from the anatase structure (which is stable below 700^βC) to the rutile structure. This mechanically induced phase transition is consistent with other reported observations of milled titanium dioxide.

Figure 2 shows the conventional differential thermal analysis (DTA) traces which were obtained for the samples of the powder mixture which had been milled for 25 hours (trace (a)) and 140 hours (trace (b)). It will be noted that the DTA trace for the powder mixture milled for 25 hours includes three exothermic peaks, at 762.62°C, at 857.21^βC and at 897.56^βC. The higher temperature peaks overlap and form a broad second exothermic peak in the temperature range 785°C to 1,000°C.

The DTA trace for the powder mixture milled for 140 hours contains a similar first exothermic peak, but with a small shift to a lower temperature (760.07^βC), and a second broad peak having a more complicated structure. Several overlapping peaks are clearly visible in a temperature range which begins at a lower temperature than that of the beginning of the broad peak of the powder sample milled for 25 hours.

To investigate the thermochemical reaction corresponding to the first DTA trace peak, the powder sample milled for 25 hours was heated (in the Differential Thermal Analyser) in an atmosphere of argon to 785°C, maintained at that temperature for about two minutes, then cooled rapidly. The x-ray diffraction pattern of the resultant material is shown as trace (c) of Figure 1. This XRD trace clearly shows that there has been a reaction of the titanium dioxide with the amorphous boron, leading to the production of a mixture of Ti₂0₃ and TiB0₃.

It was not possible to separately assess the reactions which occur at the two peaks which form the broad exothermic peak in the temperature range 785^βC to l,OOO^βC, but the XRD pattern obtained by heating ( in the differential thermal analyser) the powder milled for 25 hours in an atmosphere of argon to a temperature of 1,050°C, holding it at that temperature for about 2 minutes, then allowing the heated powder to cool, is shown as trace (d) of Figure 1. Trace (d) of Figure 1 shows that there has been a full reaction of the Ti0₂ with the amorphous boron, producing titanium boride, TiB₂, and the amorphous dioxide of boron, B₂0₂.

Removal of the B₂0₂ product could have been achieved by heating to a temperature of about 1,500°C, to vaporise the B₂0₂. However, it was felt preferable to remove the B₂0₂ by the technique described in the aforementioned paper by Z Jiang and W E Rhine in the Journal of the European Ceramic Society, namely washing with hot water for a period of about 45 minutes. The B₂0₂ went into solution and the fine particle size of TiB₂ product was not destroyed.

A scanning electron microscope analysis of the TiB₂ product showed that it comprised particles in the sub-micron range, with particle diameters ranging from O.lμm to 0.5 μm. Example 2

A powder mixture of zirconium dioxide (the material marketed by Hopkin and Williams Ltd, having a baddeleyite structure) and amorphous boron was prepared such that the atomic ratio of zirconium to boron was 1:4. This powder mixture was dry milled in the ball mill used for Example 1, for 20 hours, under the same vacuum conditions. The x-ray diffraction pattern obtained from the milled powder is shown as trace (a) of Figure 3. It will be noted that the peaks in this XRD pattern correspond to the zireonia structure, with broadening due to a decrease in the crystallite size of the milled powder.

The differential thermal analysis trace obtained for this milled powder is presented in Figure 4. The DTA trace of Figure 4 is clearly different from the DTA traces obtained from the milled Ti0₂/boron powder mixtures. The trace of Figure 4 has a weak exothermic peak at approximately 750°C and a more pronounced exothermic peak at 1,012°C.

The x-ray diffraction pattern of the material obtained when this milled powder mixture was (i) heated, in the differential thermal analyser, in an atmosphere of argon, to a temperature of about 800°C, (ii) held at that temperature for several minutes, then (iii) rapidly cooled, is not shown in the accompanying drawings. However, that XRD pattern indicated, in addition to the starting material, the presence of a Zr0₂ phase having a structure of higher symmetry. The closest standard diffraction pattern to the XRD pattern obtained of the higher symmetry structure material was the pattern of Zr0₂ JCPDS file No 27-997. The x-ray diffraction pattern that was obtained from the same mixture of zireonia and amorphous boron, milled in vacuo for 70 hours, indicates the presence of the same Zr0₂ phases as those shown by the XRD pattern described above for the Zr0₂/boron mixture which was milled for 20 hours, then annealed at about 800°C.

The zireonia/boron mixture, milled for 20 hours, was heated to 1,100° (which is a temperature above the second exothermic peak of the DTA trace) , held at that temperature for several minutes, then cooled. The x-ray diffraction pattern of the resultant powder is shown as trace (b) of Figure 3. Most of the peaks of this trace correspond to zirconium boride, ZrB₂. Some small peaks corresponding to zireonia are also present, together with two small peaks which the present inventors have not yet matched with a chemical compound. It is clear from the XRD pattern, however, that ZrB₂ is the major product of the thermochemical reaction that has occurred during the short annealing at 1,100°C.

Boron dioxide, which was also present in the material annealed at 1,100°C, was removed by washing with hot water.

Incidentally, none of the experiments with the zirconia/boron powder mixtures provided evidence of the formation of zirconium borate, even as in intermediate phase. This observation is in accordance with the experience of other workers, for as far as the present inventors are aware, there is no publication which affirms that zirconium borate can be prepared. Example 3

Ilmenite, supplied as a mineral sand by Westralian Sands Limited, was mixed with amorphous boron and samples of this powder mixture were dry milled in the ball mill used for Examples 1 and 2, under the same vacuum conditions. The ilmenite had the following composition:

FeO 20.8 per cent (by weight) Fe₂0₃ 20.0 per cent (by weight) Ti0₂ 53.6 per cent (by weight) MnO 1.54 per cent (by weight)

Si0₂ 1.27 per cent (by weight) Zr0₂ 0.62 per cent (by weight) A1₂0₃ 0.58 per cent (by weight) MgO 0.25 per cent (by weight)

Preliminary experiments indicated that when the milled powder mixture was heated, both the titanium and iron oxides were reduced by the boron to form titanium boride (TiB₂) and iron boride (FeB). Accordingly, powder samples, having iron, titanium and boron present in the atomic ratio 1:1:6, were prepared, with a view to performing the transformation indicated by the equation

2FeTi0₃ + 12B - 2FeB + 2TiB₂ + 3B₂0₂.

If this reaction should be obtained, removal of the boron dioxide would leave an intimate mixture of titanium and iron borides, from which a composite body comprising a hard material and a magnetic material could be produced.

A sample of this selected ilmenite/boron mixture was dry milled for 20 hours. The product powder had the x-ray diffraction pattern of trace (a) of Figure 5 and the differential thermal analysis trace of Figure 6. The DTA trace has three exothermic peaks, at 637.84°C, 773.51°C and 822.56^βC.

When the milled sample was heated in the differential thermal analyser to about 700"C, held at that temperature for about 2 minutes, then rapidly cooled, the resulting powder had the XRD pattern of trace (b) of Figure 5. This XRD pattern shows the presence of αFe, Ti0₂, amorphous boron, and some TiB0₃. It was felt that the TiB0₃ phase may be due to a reaction that would not normally occur until the powder had been heated to the temperature of the second exothermic peak, at 773°C, for an analysis of the transformation at 773°C showed that the αFe had reacted with amorphous boron to form FeB and all the titanium had been converted to TiB0₃.

The x-ray diffraction pattern which was obtained when the selected ilmenite/boron mixture was milled for 20 hours, then annealed at 1,050°C (above the temperature of the third, broad exothermic peak of the DTA trace of Figure 6), is shown as trace (c) of Figure 5. This XRD pattern contains only peaks which are characteristic of titanium boride and iron boride.

In the light of their experimental observations, the present inventors have proposed the following sequence of reactions for this borothermic transformation:

FeTi0₃ + B → αFe + Ti0₂ + amorphous (Ti0₂, B₂0₂, B) (1) αFe + B → FeB (2)

Ti0₂ + B → TiB0₃ (3)

2TiB0₃ + 8B - 2TiB₂ + 3B₂0₂ (4) It will be noted that, contrary to the experience with the borothermic reduction of Ti0₂ alone, there was no evidence of the production of Ti₂0₃ in an intermediate stage in the borothermic reduction of ilmenite.

In a further investigation of iron phase transformations, a preliminary Mossbauer analysis has indicated the formation of first Fe₂B and then FeB. A complete reaction sequence, therefore, may include the additional reactions

2αFe + B → Fe₂B (2' ) Fe₂B + B - 2FeB, (3' ) with the reaction (3' ) occurring after the reaction (3) .

As in the experiments of Examples 1 and 2, the powder product obtained by annealing the milled ilmenite/boron mixture at 1,100°C contained the amorphous B₂0₂ phase. Hot water leaching of the product powder, however, removed the B₂0₂ and also a significant amount of the FeB. Thus an alternative technique for removing the B₂0₂ would be required if the desired end-product of the borothermic reduction is a mixture of iron boride and titanium boride.

The present invention provides a convenient and economic method for producing borides, which has the following advantageous features:

(a) the raw materials for the process are inexpensive;

(b) a high purity boride product is obtained; (c) only a relatively short period of high energy dry milling is required; (d) the annealing of the milled powder product requires a relatively low energy input as the annealing time is short and the annealing temperature is relatively low; and (e) the powder product comprises fine (sub-micron) grains, which are particularly useful for the production of refractory and other dense bodies, and composites with other materials.

This list is not exhaustive.

It will be appreciated that although examples of the present invention have been provided, those examples are for the purpose of illustration only, and variations to and modifications of the exemplified processes can be made without departing from the present inventive concept.

Claims

1. A method of producing a metal boride which comprises the sequential steps of:

(a) forming a powder mixture of the metal oxide and boron;

2. A method as claimed in claim 1, wherein the metal oxide is an oxide of a Group IVB metal.

3. A method as claimed in claim 2, wherein the metal oxide is one of titania and zireonia.

4. A method as claimed in claim 3, wherein the atomic ratio of zirconium to boron or titanium to boron is about 1 to 4.

5. A method as claimed in claim 1 or claim 2, wherein said power mixture comprises two or more metal oxides and boron.

6. A method as claimed in claim 1 or claim 2, wherein said metal oxide is a mixed oxide.

7. A method as claim in claim 6, wherein the mixed oxide is ilmenite.

8. A method as claim in claim 7, wherein the atomic ratio of iron to titanium to boron in the powder mixture is about 1 to 1 to 6.

9. A method as claimed in any one of claims 1 to 8, wherein the boron is amorphous born.

10. A method as claimed in any one of claims 1 to 9, wherein the boron is crystalline boron.

11. A method as claimed in any one of claims 1 to 10, wherein said fine powder comprises grains having a diameter of less than 1 micrometre.

12. A method as claim in any one of claims 1 to 11 further comprising the steps of mixing a fugitive binder and/or a sintering aid with the boride, moulding the mixture under pressure and sintering the mixture to produce a dense body.

13. A method as claimed in any one of claims 1 to 12 further comprising the step of mixing the boride with one of the group comprising another boride, a carbide, and an oxide to produce a composite material.

14. A method as claimed in any one of claims 1 to 13, wherein the high energy ball milling is performed in a ball mill of the kind described and claimed in the specification of International Patent Application No. PCT/AU90/00471.

15. A method as claimed in any one of claims 1 to 14, wherein said vacuum conditions comprise a vacuum of about 10^~2 torr.

16. A method as claimed in any one of claims 1 to 15, wherein the step of removing the oxide of boron comprises leaching with hot water.

17. A method of producing a metal boride substantially as herein described in any one of examples 1 to 3.