WO2010011311A1

WO2010011311A1 - Nanolabeling of metals

Info

Publication number: WO2010011311A1
Application number: PCT/US2009/004255
Authority: WO
Inventors: James H. Dickerson; David Britton; Margit Harting
Original assignee: Cape Town University; Vanderbilt University
Priority date: 2008-07-22
Filing date: 2009-07-22
Publication date: 2010-01-28

Abstract

Methods are described for labeling, tagging or otherwise marking metallic materials, and artifacts made of such materials, to identify uniquely their origin, manufacturer and supplier.

Description

DESCRIPTION

NANOLABELING OF METALS BACKGROUΉD IMFORMATION

Field of the Invention Embodiments of the invention relate generally to the field of nanolabeling of metals. Discussion of the Related Art

Metals and alloys are commonly identified by an analysis of their composition, with particular attention paid to trace elements^{(l 8)}. Such practice is well known and widely used in many areas ranging from forensics and archeometry, identifying the provenance of jewelry and precious metals, and the identification of bullets and other weapons materials. In the latter case, the composition is often deliberately modified to provide a distinctive signature. The most common detection methods are X-ray fluorescence and neutron activation analysis. A common modification is to detect different isotopes of common elements, in which case mass spectrometry is also used as a detection method. Other, more specialized labeling methods include adding trace elements which provide a specific signature when investigated with a particular technique. One such example is the addition of an atomic species, such as boron, with a high neutron absorption cross-section, as disclosed by G.B. Smith in GB 872154. As a means of labeling or tagging, the above methods are not commonly used for heavily traded metals, which are in wide circulation, such as copper, because the elemental composition can easily be changed by refining or mixing of alloys.

For soft materials and fluids, labeling with microparticles and nanoparticles is a well known extension of the use of dye markers. In common with dyes, the main detection signal is the optical fluorescence. In polymers, a combination of different nanoparticles and dyes can be combined to provide a unique signature, as disclosed by Hubbard et al in US 2005/095715. Another optical signature, which has been used for liquid fuels, is the Raman spectrum of the nanoparticle surface, as disclosed by Natan et al in WO 2008/019161.

Unlike liquids and low melting point soft materials, the incorporation of foreign nanoparticles into a metallic medium is not trivial. The main methods of are electro- deposition or vapor deposition of the metal matrix around the pre-existing nanoparticles. A particular modification, disclosed by Jin in WO 2005/065281, is the co-deposition of the metallic matrix, by electrolysis, and conducting nanoparticles by electrophoresis. However, such methods cannot be used for all metals, and are only suitable for small scale processing and the production of coatings. Furthermore, the production of the composite material is performed at low temperature, less than 500 C in the case of vapor deposition and normally at ambient temperature for electrodeposition. It is therefore not clear whether the nanoparticle tags will survive any high temperature post-processing, such as the re-casting of the artifact into another form.

Another method of incorporating nanoparticles into a metallic body is to use the established techniques of powder metallurgy. In this case, a green form consisting of a mixture of metal powder and nanoparticles is pressed and sintered, without melting. This method has been disclosed as a means of labeling bullets, with luminescent nanoparticles by Lowden et al in WO 2002/086413. Metal bodies produced in this manner are brittle, and may be porous, and so are not suitable to most processes such as hot and cold forming (e.g. wire drawing). Also, because of the method of manufacture, the nanoparticles are situated exclusively at grain boundaries, and not integrated into the material. Hence, they can be more easily separated by mechanically breaking up the body, e.g. by milling, and applying standard separation techniques.

On the other hand, metallic matrix composite (MMC) materials produced by casting are well known. The most common materials of this type consist of microscale oxide (alumina, yttria or vanadia) ceramic fiber or beads embedded in aluminum based alloys. In contrast to labeling, the purpose of incorporating the ceramic filler is to change the physical properties of the host metal, and increase its strength in particular. These effects result, in part, from diffusion bonding at the interfaces of a high density of inclusions. At very high processing temperatures, the inclusions tend to dissolve. Consequently, most MMC materials are based on low melting point metals, such as aluminum. Nevertheless, a similar method has been disclosed by Berger et al in US

2005/01 12360, in which they label aluminum metal with oxide nanoparticles doped with rare earth metals. Berger et al successfully showed that yttria nanoparticles particles were stable in molten aluminium, which is typically at a temperature between 600 ⁰C and 700 ⁰C. The actual temperature is not disclosed, however. In this work, the detection signal used is the luminescence of a combination of rare earth atoms, which are used to dope oxide particles. The same signal was also used by Lowden et al in WO 2002/086413, and is commonly applied in polymers. The main disadvantage of this method, when applied to an opaque metal is that only particles at the surface can be used as the label. Hence, a high concentration of nanoparticles, which may affect the physical characteristics of the metal, is required to provide a measurable signal. Also, as the ceramic nanoparticles are considerably harder than the aluminum matrix, the whole is a typical hard-in-soft composite, for which any surface wear, including polishing, will preferentially remove the particles. BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification are included to depict certain embodiments of the invention. A clearer concept of embodiments of the invention, and of components combinable with embodiments of the invention, and operation of systems provided with embodiments of the invention, will be readily apparent by referring to the exemplary, and therefore nonlimiting, embodiments illustrated in the drawings.

Embodiments of the invention may be better understood by reference to one or more of these drawings in combination with the following description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale.

FIGS. 1 A-IB are large area views of the surface of the copper nugget using SEI (right) and QBSD (left) imaging, representing an embodiment of the invention.

FIG. 2 is a CL image of the lighter area seen in the QBSD image, representing an embodiment of the invention.

FIG. 3 is a higher magnification topographical image of the luminescent region, representing an embodiment of the invention. FIG. 4A is an SEI image of cut surface of the copper nugget, representing an embodiment of the invention.

FIG. 4B is a QBSD image of the cut surface of the copper nugget, representing an embodiment of the invention.

FIG 5A is a: QBSD image of a Eu rich inclusion. FIG. 5B is an SEI micrograph of the same area shown in FIG. 5A.

FIG. 6 is an EDX spectrum of the regions indicated in FIG. 5A.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description.

Descriptions of well known starting materials, processing techniques, components and equipment are omitted so as not to unnecessarily obscure the embodiments of the invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, additions and/or rearrangements within the spirit andVor scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.

Within this application several publications are referenced by Arabic numerals, or principal author's name followed by year of publication. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims after the section heading References. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference herein for the purpose of indicating the background of embodiments of the invention and illustrating the state of the art.

The below-referenced U.S. Patent(s) and U.S. Patent Application(s) disclose embodiments that are useful for the purposes for which they are intended. The entire contents of Patent Cooperation Treaty Publication No(s). WO2008/019161 A2 (Natan et al.) and PCT International Pub No.: WO2005/065281 A2 (Jin) are hereby expressly incorporated by reference herein for all purposes. The entire contents of U.S. Pat. Appl. Pub No(s). US2005/01 12360 Al (Berger et al.) and US2005/0095715 Al (Hubbard et al.) are hereby expressly incorporated by reference herein for all purposes.

In general, the context of an embodiment of the invention can include nanolabeling. The context of an embodiment of the invention can include metal matrix composite production. The context of an embodiment of the invention can also include magnetic detection and characterization of composites.

The invention is a method of labeling, tagging or otherwise marking metallic materials, and artifacts made of such materials, to identify uniquely their origin, manufacturer and supplier. The method, as described below, involves the incorporation of stable nanoparticles that provide a distinctive signature into the grains of the metal.

Novelty of our Invention

Our invention differs from the prior art in several important respects. First, the nanoparticles used here are composed of compounds of the rare earth metals. Preferably these are the lanthanide oxides and sulfides, which are stable at high temperatures. The nanoparticles are included into the molten metal at high temperatures, which are typically between 1000 ⁰C and 1500 ⁰C, or even higher. Working at such a high temperature, common metals such as copper and steel, as well as noble metals can be tagged. The preferred equipment for such a casting process is an induction furnace, which also has the benefit of an intrinsic stirring action caused by the eddy currents generated in the melt. With such equipment, the casting can be performed above 2500 ⁰C, making the labeling of platinum group metals feasible.

Because we are using liquid phase mixing, the resulting composite material has a dense microstructure and can be further processed by hot and cold working, or by recasting or annealing. In addition, the nanoparticles are integrated into the grains of the metal and cannot be easily separated. The materials used have more distinctive signals than the rare earth doped transition metal oxides, and the mixing of different particles in known combinations allows the generation of distinctive codes. Such coding further allows the identification of materials from different sources, even when diluted or mixed with other materials, because a measurement of the absolute concentration is not required. In contrast to all prior art, we propose to use a combination of detection signals to give a unique signature. Such a combination includes X-ray fluorescence, which is a nondestructive bulk measurement technique, whose signal depends on the elemental composition of the nanoparticle. Preliminary measurements can therefore be performed in the field, without any sample preparation, and polishing the surface will not remove the label. This measurement can be further combined with optical or magnetic methods measurements, which depend on the size, morphology and environment of the nanoparticles, to give a unique signature. Such techniques can be any type of luminescence, including photoluminescence and cathodeluminescence, and optical spectroscopy, such as Raman spectroscopy, for which chemical etching of the depleted surface layer will be necessary. The higher concentration of rare earth elements in these nanoparticles, compared to the doped transition metal oxide particles also gives a stronger signal. This allows very low concentrations of nanoparticles to be used, to prevent influencing the properties of the metal. Preferably a concentration lower than 1% by weight, and more preferably lower than 500 ppm, should be used. We also propose that when the above primary measurements have been used to identify specific material, then secondary methods may be used to confirm these findings. The specific technique to be used should be electron microscopy to identify the size, morphology and composition of the particles, and their comparison with a catalogue of known combinations of nanoparticle labels.

Current State of the Project

The overall aim of this particular project is to provide metals with a distinctive signature by the inclusion of nanomaterials with distinctive properties, without significantly influencing the bulk properties of the host metal. However, before this can be achieved, the first step is to be able to incorporate nanomaterials into the metallic matrix. These preliminary experiments were therefore designed to firstly investigate the stability of EuS nanopowder during casting in pure copper, and secondly to study its distribution in the cast copper ingot.

Alloy Casting The materials used were 0.99999 pure copper powder, from Goodfellow, and EuS nanopowders produced at Vanderbilt University. The materials were used in the as-received state, but larger aggregates of the nanopowder were broken down with a spatula so that the resulting mixture had a relatively uniform sub-millimeter particle size.

A mixture of 1 % by weight of EuS in Cu was mixed by hand and heated to 1200⁰C for 3 minutes. The heating was performed in air using an induction furnace, at Hot Platinum (pty) Ltd, with the powder in an alumina crucible with graphite liner. During heating, the melt collected as a large drop in the center of the graphite liner, resulting in a nugget, rather than an ingot.

SEM Investigation of the Surface of the Nugget After casting, the surface of the copper nugget had a thick coating (approx 300 μm estimated by eye) of slag. This was removed by etching for approximately 1 minute in a commercial non-ferrous "bright dip". The cleaned nugget was then mounted on an 8mm diameter aluminum SEM stub using double-sided carbon adhesive tape.

Scanning electron microscopy was performed at UCT using the LEO Stereoscan 440, equipped with secondary electron imaging (SEI), full quadrant backscatter detection (QBSD), cathodeluminescence (CL) and energy dispersive X-ray spectroscopy (EDX). For all studies the beam energy was 20 keV, but the probe current was varied according to the requirements of each detection system. In this microscope the SEI detector is positioned to the side of the sample, thus giving a good sensitivity to topography. The QBSD detector is at normal incidence to the sample and therefore gives good contrast between high and low atomic number elements. CL imaging was used to investigate the presence of luminescent materials at the surface, and EDX was used to check for the presence of Eu and S, as well as the presence of other impurities. Figure 1 shows a low magnification image of the same area of the sample surface, taken using both secondary and backscattered electrons. There is a large area, approximately lmm by 0.5 mm, containing heavy elements, which is only faintly visible in the topographic image. A short EDX measurement confirmed the presence of the S Ka line and Eu L series in this region, but in the surrounding material only Cu was detected.

Using CL the Eu rich area was shown to be faintly luminescent (Fig 2). In the CL image, there are several small bright spots, which can be seen as darker regions in the QBSD image. In these regions the Al Ka line could be detected using EDX, suggesting that these areas are the result of contamination with alumina. Under higher magnification, the morphology of the Eu rich region is not like polycrystalline copper. It is very porous, with sintered grains (FIG 3). At first sight, it almost certainly consists of unmixed pure EuS powder which had collected at the surface of the molten copper.

At this stage the conclusion was that EuS survives the casting process, but whether it actually mixes into the copper to form a composite remained an open question.

Microscopy of the Internal Structure of the Copper Nugget

To investigate the dispersion of the EuS powder in the copper, the nugget was sectioned at low speed using a Buehler diamond saw at low speed (approx 0.3mm/min). Three slices were mounted on 8mm diameter SEM stubs and investigated using SEI, QBSD and EDX in the same microscope as used previously. The results were, initially, disappointing. Neither QBSD nor EDX showed any indication of the presence of Eu at any magnification.

However, close comparison of SEl and QBSD images of the same area (Fig. 4) shows micron-sized shadows in the topographical image. These could be pits resulting from inclusions being pulled out during the cutting. The etched slices were therefore etched, using non-ferrous bright dip, and reinvestigated.

After etching, inclusions of various sizes were visible in the QBSD images. Larger inclusions were intergranular, and also occasionally present at the surface of large cavities. However, the inclusion shown in figure 5 is about 1 micron, and has same morphology as other copper grains, indicating that EuS particles are incorporated into the grain.

The areas marked in figure 5a were used for EDX, with the spectra shown in Figure 6. The spectrum for the inclusion clearly shows the Eu L series and the S Ka, both of which are effectively absent in the surrounding matrix. Nevertheless, the Cu Ka and Kβ lines are also present in the inclusion, further confirming that the EuS is contained within the Cu grain.

Discussion and Conclusions

These preliminary experiments have shown that EuS is chemically stable in the molten copper. However, it is clear that we need to work on the mixing techniques, including finding some way of breaking the aggregated powder back down to sub-micron size. In the first instance, gentle grinding with a pestle and mortar should be tried. Other techniques which will be tried to improve the mixing are wet or dry sonication.

In the field, we probably cannot use a surface detection sensitive technique, because of the loss of aggregated powder from the surface during machining. This is a general problem of a hard-in-soft composite material.

However, all the lanthanides have a distinctive L series in the normal X-ray fluorescence range, which gives a convenient bulk detection method using a hand-held XRF system. By combining different nanopowders a unique signature could be developed for any supplier. Practical Applications

A practical application of an embodiment of the invention that has value within the technological arts is address copper and precious metal theft. Copper and precious metal theft is a tremendous problem worldwide, which not only leads to massively increased costs, but often to a dangerous loss of infrastructure in power transmission and communications. Using nanotechnology, objects made from metals such as copper can be invisibly tagged with a unique signature to identify the source of the metal. The nanotags do not influence the properties or appearance of the material, and cannot be removed or destroyed by reworking or melting it. There are virtually innumerable uses for embodiments of the invention, all of which need not be detailed here. Advantages

Embodiments of the invention can be cost effective and advantageous for at least the following reasons. Embodiments of the invention improve quality and/or reduce costs compared to previous approaches.

Conclusion The described embodiments and examples are illustrative only and not intended to be limiting. Although embodiments of the invention can be implemented separately, embodiments of the invention may be integrated into the system(s) with which they are associated. All the embodiments of the invention disclosed herein can be made and used without undue experimentation in light of the disclosure. Although the best mode of the invention contemplated by the inventor(s) is disclosed, embodiments of the invention are not limited thereto. Embodiments of the invention are not limited by theoretical statements (if any) recited herein. The individual steps of embodiments of the invention need not be performed in the disclosed manner, or combined in the disclosed sequences, but may be performed in any and all manner and/or combined in any and all sequences. The individual components of embodiments of the invention need not be formed in the disclosed shapes, or combined in the disclosed configurations, but could be provided in any and all shapes, and/or combined in any and all configurations. The individual components need not be fabricated from the disclosed materials, but could be fabricated from any and all suitable materials. Homologous replacements may be substituted for the substances described herein.

Various substitutions, modifications, additions and/or rearrangements of the features of embodiments of the invention may be made without deviating from the spirit and/or scope of the underlying inventive concept. All the disclosed elements and features of each disclosed embodiment can be combined with, or substituted for, the disclosed elements and features of every other disclosed embodiment except where such elements or features are mutually exclusive. The spirit and/or scope of the underlying inventive concept as defined by the appended claims and their equivalents cover all such substitutions, modifications, additions and/or rearrangements.

REFERENCES

1. Pat Appl.: Pub No.: US2005/01 12360A1 (Berger et al.)

2. Mat. Sci. and Techno 1. Feb 2001, VoI 17, PP 187-194 (Geng et al.) 3. IEEE Trans on Appl ied Superconductivity, VoI 10, No. 1 , March 2000 pp 1273-1276 (Renaud)

4. Applied Surf. Sci. 235 (2004) pp 132-138 (Niederkofler and Leisch)

5. UK Patent 872, 154 (Smith)

6. US Pat Appl: Pub. No. US2005/0095715A1 (Hubbard et al.) 7. PCT International Pub No.: WO2008/019161 A2 (Natan et al.)

8. PCT International Pub No.: WO2005/065281 A2 (Jin)

Claims

1. A method comprising incorporation of stable nanoparticles into grains of a metal, characterized in that the incorporation provides a distinctive signature in grains of the metal.

2. The method of claim 1 , characterized in that the stable nanoparticles include compounds of rare earth metals in a concentration lower than 1% by weight.

3. The method of claim 2, characterized in that the stable nanoparticles include at least one lanthanide selected from the group consisting of lanthanide oxides and lanthanide sulfides in a concentration lower than 500 ppm.

4. The method of claim 1, characterized in that the stable nanoparticles are included into a molten metal at a temperature of greater than approximately 1000 °C.

5. The method of claim 4, characterized in that the stable nanoparticles are included into the molten metal at a temperature of greater than approximately 1500 ⁰C.

6. The method of claim 5, characterized in that the stable nanoparticles are included into the molten metal at a temperature of greater than approximately 2500 ⁰C.

7. The method of claim 1, characterized in that the metal includes at least one member selected from the group consisting of copper, steel and noble metals.

8. The method of claim 7, characterized in that the noble metals include platinum group metals.

9. The method of claim 1, characterized in that the incorporation includes a casting process using an induction furnace.

10. The method of claim 1, characterized in that the incorporation includes mixing different stable nanoparticles in a predetermined combination to generate a distinctive code.

1 1. A method comprising incorporation of stable nanoparticles into a material, characterized in that the incorporation provides a distinctive signature in the material.

12. A method of obtaining a signature from a material, characterized by the use of a combination of detection signals to give a unique signature from stable nanoparticles included in the material.

13. The method of claim 12, characterized in that the combination of detection signals includes X-ray fluorescence.

14. The method of claim 13, further characterized by the combination of detection signals including at least one measurement method selected from the group consisting of an optical measurement and a magnetic measurement, which depend on size, morphology and environment of the stable nanoparticles, to give the unique signature.

15. The method of claim 14, characterized in that optical measurement includes at least on member selected from the group consisting of luminescence and optical spectroscopy.

16. The method of claim 12, further characterized by using a secondary method to confirm the unique signature.

17. The method of claim 16, characterized in the secondary method includes electron microscopy to identify the size, morphology and composition of the stable nanoparticles.

18. A composition of mater, comprising stable nanoparticles in a material, characterized in that the stable nanoparticles provides a distinctive signature in the material.

19. The composition of claim 18, characterized in that the material includes grains of a metal and the nanoparticles include compounds of rare earth metals in a concentration lower than 1% by weight.

20. The composition of claim 19, characterized in that the nanoparticles include at least one lanthanide selected from the group consisting of lanthanide oxides and lanthanide sulfides in a concentration lower than 500 ppm.