Method for coating particles

Abstract

The present invention relates to a method for coating primary particles with secondary particles using dual asymmetric centrifugal forces wherein, the primary particles comprise (a) at least one metal, or (b) at least one ceramic; the secondary particles comprise at least one metal or salt thereof; and wherein the secondary particles are more malleable than the primary particles.

Claims

1. A method for forming coated particles comprising coating primary particles with secondary particles using dual asymmetric centrifugal forces, in which two centrifugal forces at an oblique angle to each other are simultaneously applied to the primary particles and secondary particles, wherein: the primary particles comprise (a) at least one metal, or (b) at least one ceramic; the secondary particles comprise at least one metal or salt thereof; and wherein the secondary particles are more malleable than the primary particles.

2. The method according to claim 1, wherein the primary particles comprise the at least one metal and the at least one metal is selected from the group consisting of a single metal, an admix of metals, an alloy and a combination thereof.

3. The method according to claim 1, wherein the primary particles comprise the at least one metal and the at least one metal is selected from the group consisting of Group IVB, Group VB, Group VIB, Group VIIB and Group VIII of the Periodic Table.

4. The method according to claim 1, wherein the primary particles comprise the at least one metal and the at least one metal is selected from the group consisting of titanium, molybdenum, tungsten, nickel and iron.

5. The method according to claim 1, wherein the primary particles comprise the at least one ceramic and the at least one ceramic is selected from at least one of the group consisting of silicon, zirconium, aluminium, yttrium, cerium and titanium.

6. The method according to claim 1, wherein the primary particles are substantially spherical, irregular or a combination thereof.

7. The method according to claim 6, wherein the primary particles are substantially spherical.

8. The method according to claim 6, wherein the primary particles are substantially spherical and have an average diameter of about 2000 m.

9. The method according to claim 8, wherein the primary particles are substantially spherical and have an average diameter of about 1 m to about 45 m.

10. The method according to claim 9, wherein the primary particles comprise titanium.

11. The method according to claim 1, wherein the shape of the primary particles remains substantially unchanged during coating.

12. The method according to claim 1, wherein the secondary particles are selected from the group consisting of a single metal, an admix of metals, an alloy, a metal salt and a combination thereof.

13. The method according to claim 12, wherein the secondary particles comprise the at least one metal salt and wherein the coated particles are further processed by thermal means, chemical means or both.

14. The method according to claim 1, wherein the secondary particles are selected from the group consisting of Group VIII, Group IB and Group IIIA of the Periodic Table, and salts thereof.

15. The method according to claim 1, wherein the secondary particles are at least one of the group consisting of platinum, palladium, rhodium, ruthenium, iridium, osmium, silver, gold, cobalt, copper, nickel, iron and aluminium, and salts thereof.

16. The method according to claim 1, wherein the coating of the secondary particles on the primary particles is in the form of a film or in the form of discrete particles.

17. The method according to claim 1, wherein milling media assists the coating of the primary particles with the secondary particles.

18. The method according to claim 1, wherein the dual asymmetric centrifugal forces are applied for a continuous or aggregate period of about 1 second to about 10 minutes.

19. The method according to claim 1, wherein the speed of the dual asymmetric centrifugal forces is about 200 rpm to about 3000 rpm.

20. The method according to claim 1, further comprising the steps of: (a)compacting the coated particles; and (b)forming an alloy or cermet therefrom.

21. The method according to claim 1, wherein the secondary particles are single crystallites or an agglomerate of many smaller crystallites.

22. The method according to claim 21, wherein the secondary particles are platinum group metal blacks.

23. The method according to claim 1, wherein the method is carried out under an inert atmosphere for at least a proportion of the time.

24. The method according to claim 23, wherein the method is carried out under an inert atmosphere for substantially the whole time.

25. The method according to claim 23, wherein the inert atmosphere comprises argon, nitrogen or a mixture thereof.

26. The method according to claim 24, wherein the inert atmosphere comprises argon, nitrogen or a mixture thereof.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will now be described with reference to the following drawings in which:

(2) FIGS. 1A-C illustrate how the centrifugal forces are applied to the particles in the Speedmixer. FIG. 1A is a view from above showing the base plate and basket.

(3) The base plate rotates in a clockwise direction.

(4) FIG. 1B is a side view of the base plate and basket.

(5) FIG. 1C is a view from above along line A in FIG. 1B. The basket rotates in an anti-clockwise direction.

(6) FIG. 2 is a backscattered electron image of substantially spherical titanium powder (<45 m) coated with 0.2 wt % palladium.

(7) FIG. 3 is a backscattered electron image of substantially spherical titanium powder (<45 m) coated with 0.2 wt % palladium.

(8) FIG. 4 is a SEM image of palladium dispersed on the surface of irregular titanium particles.

(9) FIG. 5 is a SEM of palladium dispersed on the surface of substantially spherical zirconia beads.

(10) FIG. 6 is a SEM image of ruthenium dispersed on the surface of substantially spherical titanium powder.

(11) FIG. 7 is a graph comparing the corrosion potential (open circuit potential) vs. time for (a) a HIP-ed powder produced according to the present invention and (b) commercially available Grade 7 TiPd alloy.

DETAILED DESCRIPTION OF THE INVENTION

(12) The coating process may be controlled by various parameters including the rotation speed at which the process takes place, the length of processing time, the level to which the mixing container is filled, the use of milling media and/or the control of the temperature of the components within the milling pot.

(13) The dual asymmetric centrifugal forces may be applied for a continuous period of time. By continuous we mean a period of time without interruption. Preferably, the period of time is about 1 second to about 10 minutes, more preferably about 5 seconds to about 5 minutes and most preferably about 10 seconds to about 200 seconds.

(14) Alternatively, the dual asymmetric centrifugal forces may be applied for an aggregate period of time. By aggregate we mean the sum or total of more than one periods of time. The advantage of applying the centrifugal forces in a stepwise manner is that excessive heating of the particles can be avoided. The dual asymmetric centrifugal forces are preferably applied for an aggregate period of about 1 second to about 10 minutes, more preferably about 5 seconds to about 5 minutes and most preferably about 10 seconds to about 150 seconds. The number of times (e.g. 2, 3, 4, 5 or more times) in which the dual asymmetric centrifugal forces are applied will depend upon the nature of the primary and secondary particles. For example, when the primary particles comprise titanium, stepwise application of the centrifugal forces minimises heating of the particles thus minimising the risk of oxidation and/or combustion. In a particularly preferred embodiment, the dual asymmetric centrifugal forces are applied in a stepwise manner with periods of cooling therebetween. In another particularly preferred embodiment, the dual asymmetric centrifugal forces may be applied in a stepwise manner at one or more different speeds.

(15) Preferably, the speed of the dual asymmetric centrifugal forces is from about 200 rpm to about 3000 rpm. In one embodiment, the speed is from about 300 rpm to about 2500 rpm. In another embodiment, the speed is from about 500 rpm to about 2000 rpm.

(16) The level to which the mixing container is filled is determined by various factors which will be apparent to the skilled person. These factors include the apparent density of the primary and secondary particles, the volume of the mixing container and the weight restrictions imposed on the mixer itself.

(17) Certain metals or metal alloys possessing a strong affinity for oxygen suffer from excessive surface oxide growth if the milling is carried out in the presence of oxygen. In particular, if the coated particles are to be used to produce a final, compacted article that should conform to a recognised specification for oxygen content, it may also be required that an oxygen-deficient atmosphere is to be used. Moreover, an oxygen-deficient atmosphere may be suitable when the secondary particles comprise at least one metal salt which is air sensitive. Accordingly, the coating process of the present invention may be carried out under an inert atmosphere for at least a proportion of the process time and, in one preferred embodiment, for substantially the whole process. Within the context of the invention, an inert atmosphere is one which has limited or no ability to react with the primary and/or secondary particles. Preferably, the inert atmosphere comprises argon, nitrogen or a mixture thereof.

(18) Milling media may be used to assist the coating of the primary particles with the secondary particles. The primary particles can themselves act as milling media.

(19) However, the incorporation of further hard, non-contaminating media can additionally assist in the breakdown of the secondary particles where agglomeration has occurred, for example, as a result of the manufacturing process or during transit. Such breakdown of the agglomerates further enhances the coating of the secondary particles on the primary particles. The use of milling media is well-known within the field of powder processing and materials such as stabilised zirconia and other ceramics are suitable provided they are sufficiently hard.

(20) Preferably, the secondary particles may be single crystallites or an agglomerate of many smaller crystallites, for example, platinum group metal blacks.

(21) The coating of the secondary particles on the primary particles may be in the form of a film or in the form of discrete particles. The degree of coverage will depend on factors that include the malleability of the secondary particles, the length of time allowed for the coating process and/or the quantity of the secondary particles present. The secondary particles may be present in any suitable quantity provided the secondary particles coat the primary particles e.g. palladium may be added to titanium alloys in a proportion of about 0.05% to about 0.25%, which is recognisable as the levels of addition in ASTM/ASME Ti grades 7, 11, 16, 17, 24 and 25. The quantity of secondary particles can also affect one or more properties of a desired alloy or cermet subsequently formed. For example, when the quantity of Pd is increased in a Pd/Ti alloy, the corrosion resistance of the alloy to chloride-containing solutions (such as salt water) improves.

(22) The method of the present invention further comprises the steps of: (a) compacting the coated particles; and (b) forming an alloy or cermet therefrom.

(23) Suitable methods for compacting either the coated metallic particles or coated ceramic particles include Hot Isostatic Pressing (HIP-ing), Cold Isostatic Pressing (CIP-ing) and Metal Injection Moulding (MIM). The coated metallic particles may also be compacted using high energy beam fabrication methods, such as Direct Laser Fabrication (DLF), and Electron Beam Melting. The coated ceramic particles can also be compacted using slip casting.

(24) Despite the fact that articles produced after compaction have an inhomogeneous distribution of the metal from the secondary particles, the inventors have found that the corrosion resistance of an alloy formed by the claimed method is independent of the method used to compact the particles and equal to the corrosion resistance of articles produced using commercial alloys. Therefore, whichever technique best suits the article to be made from the alloy may be used.

(25) The mechanical properties, however, of the alloy or cermet formed by the claimed method do depend on the method used to compact the particles and, as such, the compaction technique must be carefully selected depending on the required mechanical properties of the final article to be made.

(26) In yet another aspect, the present invention provides an alloy or cermet formed by the claimed method, and articles formed from such an alloy or cermet.

(27) In another aspect, the present invention provides coated particles, wherein the primary particles are coated with secondary particles. The primary and secondary particles are as described above.

EXAMPLES

(28) The invention will now be described by way of the following non-limiting examples.

Example 1

(29) 10 g of substantially spherical titanium powder (<45 m, Advanced Powders and Coatings, Raymor Industries) were weighed into a suitable pot for the Speedmixer Model DAC150FVZ. 0.02 g of palladium black (Johnson Matthey) was added, the pot sealed and the contents mixed. The dual asymmetric centrifugal forces were applied for 20 seconds at 1000 rpm and 20 seconds at 2000 rpm.

(30) An image of the coated particles produced by backscattered electron imaging can be seen in FIG. 2. The substantially spherical shape of the coated particles is clearly visible

Example 2

(31) 150 g of substantially spherical titanium powder (<45 m, Advanced Powders and Coatings, Raymor Industries) were weighed into a suitable pot for the Speedmixer Model DAC600. 0.3 g of palladium black (Johnson Matthey) was added, the pot sealed and the contents mixed for 320 seconds at 2000 rpm.

(32) A SEM image of the coated particles produced by backscattered electron imaging can be seen in FIG. 3.

Example 3

(33) 25 g of irregular HDH titanium powder (<45 m, Chemetall Industries) was weighed into a Speedmixer pot suitable for the Speedmixer Model DAC 150FVZ and 0.05 g of palladium black was added. The pot was sealed and mixed using a cycle with a mixing period of 320 seconds. A SEM image of the palladium dispersed on the surface of the irregular primary Ti particles is shown in FIG. 4.

Example 4

(34) 30 g of fully-dense substantially spherical zirconium oxide beads (YTZ Grinding Media, Tosoh Corp.) and 0.06 g of palladium black were weighed into the Speedmixer pot suitable for Speedmixer Model DAC 150FVZ. The pot was sealed and mixed for 320 seconds.

(35) The SEM image in FIG. 5 shows the dispersion of the palladium upon the surface of the substantially spherical zirconia beads.

Example 5

(36) 30 g of substantially spherical titanium powder (detailed in Example 1) and 0.06 g of ruthenium black powder (Johnson Matthey) were weighed into the Speedmixer pot suitable for Speedmixer Model DAC 150FVZ. The pot was sealed and mixed using a cycle in which the dual asymmetric centrifugal forces were applied for a total of 180 seconds at 3000 rpm.

(37) The SEM image in FIG. 6 shows the dispersion of the ruthenium powder upon the surface of the substantially spherical titanium powders.

Example 6

(38) 25 g of substantially spherical titanium powder (<45 micron, AP&C, Raymor Industries, Quebec) was weighed into a Speedmixer pot and 0.139 g of tetraamminepalladium hydrogencarbonate dry powder (Johnson Matthey) was added. The pot was sealed and mixed on the Model DAC 150FVZ for 320 seconds. A 5 g sample of the resulting material was heated in a 50 ml/min stream of 5% H.sub.2 in N.sub.2 at 300 C. for 30 minutes.

(39) The dispersion of the palladium on the surface of the titanium powder, measured using a standard carbon monoxide adsorption technique, was found to be around 3%.

Example 7

(40) 25 g of substantially spherical titanium powder (<45 micron, AP&C, Raymor Industries, Quebec) were weighed into a Speedmixer pot and 0.106 g of hexakis(aceto)tripalladium(II) Pd-111 dry powder (Johnson Matthey) was added. The pot was sealed and mixed on the Model DAC 150FVZ for a mixing period of 60 seconds. A 5 g sample of the resulting material was heated in a 50 ml/min stream of 5% H.sub.2 in N.sub.2 at 300 C. for 30 minutes.

(41) The dispersion of the palladium on the surface of the titanium powder, measured using a standard carbon monoxide adsorption technique, was found to be around 3.5%.

Example 8

(42) 12 g of 1 mm alumina beads (SASOL, Product Code 1.0/160) were weighed into a pot suitable for the DAC 150FVZ Model Speedmixer. 0.067 g of tetraamminepalladium hydrogencarbonate dry powder (Johnson Matthey) was added, equivalent to 0.2 wt % Pd on the final coated material. The pot was sealed and subjected to a mixing period of 60 seconds. The resulting composition was heated to 500 C. in an air atmosphere for a period of two hours, during which the palladium salt was decomposed.

(43) The dispersion of the palladium upon the alumina beads, measured using a standard carbon monoxide adsorption technique, was found to be around 4%.

Example 9

(44) A solid CPTi (AP&C, <45 micron powder)+0.2 wt % Pd alloy was prepared by hot isostatic pressing of Pd-coated spherical CPTi powder produced using the dual asymmetric centrifugal forces. The hot isostatic pressing was carried out at 930 C. for 4 hours at 100 MPa.

(45) The corrosion behaviour of the above alloy was compared with that of a wrought titanium Grade (ASTM Grade 7 TiPd alloyTimet UK Ltd.). Polarisation curves were measured on surfaces ground to 1200 grit, washed in deionised water, rinsed in ethanol and then dried. Testing was performed in 150 ml of 2M HCl at 37 C. immediately after cleaning of the surface.

(46) Polarisation curves, shown in FIG. 7, were measured after 30 minutes immersion at open circuit potential. Scans were carried out from 200 mV to +700 mV, relative to the open circuit potential, at 1 mV/second. Tests were carried out using a saturated calomel electrode (SCE) as the reference electrode and Pt wire as the counter electrode. As can be seen, the corrosion resistance of the alloy produced according to the present invention is substantially the same to that of the commercially available alloy.

(47) Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.