US20200189005A1

US20200189005A1 - Reduction Expansion Synthesis of Sintered Metal

Info

Publication number: US20200189005A1
Application number: US16/712,622
Authority: US
Inventors: Jonathan Phillips; Zachary David Daniels
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 2018-12-12
Filing date: 2019-12-12
Publication date: 2020-06-18

Abstract

The disclosure provides a method for generating a solid metal object. Initially, a reductant material and a metal precursor particle mixture are arranged in a high temperature furnace that is filled with a chemically inert atmosphere. A temperature of the high temperature furnace is held above the decomposition temperature of the reductant but below a melting point of the metal precursor particle mixture for a predetermined duration to generate the solid metal object. At this stage, the generated metal object is cooled in the inert atmosphere.

Description

RELATION TO OTHER APPLICATIONS

This patent application claims priority from provisional patent application 62/778,785 filed Dec. 12, 2018, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

One or more embodiments relates to the use reduction expansion synthesis to generate a solid metal object.

BACKGROUND

The Reduction Expansion Synthesis (RES) concept in broad terms is as follows: chemical radicals, which are released by thermal decomposition in an inert atmosphere, of solid compounds (e.g., urea) can remove oxygen complexes from nearby structures. Traditional RES processes require intimate mixing of the metal precursors with urea or alternative reduction agents. For example, sub-micron iron and nickel metal particles can be generated simply by physically mixing urea with various iron and nickel oxide particles and heating to ca. 800 C for a few hundred seconds under flowing nitrogen gas. Further, it was shown that alloy particles could be formed using RES. Indeed, sub-micron sized Fe-Ni alloy particles were produced from physical mixtures of urea, NiO and FeOH. Several tests conducted in those studies produced results consistent with the postulate that gas phase radicals produced via the thermal decomposition of urea reacted with oxygen in the metal oxide particles to form volatile oxides, leaving metal behind.
The RES process was also employed to create graphene from graphene oxide. Again, the process followed the same basic pattern: i) physically mix urea and the ‘parent oxide’, graphene oxide, and ii) Heat the mixture in an inert atmosphere at a temperature sufficient to thermally decompose the urea. In another example, a mixture of activated carbon (high concentration of surface oxygen groups), urea and tin chloride were heated to about 900 C in an inert atmosphere. The resulting Sn/carbon electrode was shown to be much more stable as a battery anode than any prior Sn/C electrode. That is, anodes made in this fashion lose less than 20% of capacity over about 200 cycles. In contrast, it has been repeatedly shown that Sn anodes produced using other methods rapidly fail. It was postulated the improved stability resulted from the following process: i) the urea decomposition products removed oxygen groups from the activated carbon and ii) the dangling carbon atoms (‘surface radicals’) left by the removal of surface oxide groups (9,10) reacted with Sn atoms released by the decomposition of SnCl. In this example, the RES resulted in a unique, strong, direct bond between carbon and metal. In contrast, it is postulated that other Sn deposition processes create an oxygen linkage between the metal and carbon (i.e., there is no metal-carbon bond). Activity, stability and TEM observation of very small particles that sintered very slowly, were all consistent with this hypothesis. Another example of RES is the production of a metal coating, specifically chrome coating on iron. Yet another example is an RES process to make a very stable, Pt/conductive carrier catalyst for fuel cells.
The novel apparatus and principles of operation are further discussed in the following description.

SUMMARY OF THE INVENTION

Embodiments in accordance with the invention relate a method for generating a solid metal object. Initially, a reductant material and a metal precursor particle mixture are arranged in a high temperature furnace that is filled with a chemically inert atmosphere. A temperature of the high temperature furnace is held above the decomposition temperature of the reductant but below a melting point of the metal precursor particle mixture for a predetermined duration to generate the solid metal object. At this stage, the generated metal object is cooled in the inert atmosphere.
In some embodiments, the high temperature furnace bakes the reductant and metal precursor particle mixture in order to generate a chemical radical species by decomposing the reductant material and expose the metal precursor within the inert atmosphere to the chemical radical species needed to generate the solid metal object.
In some embodiments, the inert atmosphere is nitrogen or argon. In some embodiments, the reductant material is urea. In other embodiments, the reductant material is a petroleum gel.
In some embodiments, the metal precursor includes a metal oxide and metal particles. In some cases, a weight ratio of the metal oxide and the metal particles in the metal precursor is approximately 1 to 1. The metal particles in the metal precursor can include more than one type of metal. In some embodiments, the more than one type of metal includes at least two of iron, nickel, and chromium. The metal oxide can include particles of nano-scale and the metal particles are micron-scale. The metal particles can include nano-scale metal particles and micron-scale metal particles.
In some embodiments, the metal precursor further comprises molecular precursors.
In some embodiments, the method further comprises grinding the metal precursor to combine the metal oxide and the metal particles.
In some cases, approximately 99% of air is flushed from the high temperature furnace by the flow of inert atmosphere.
In some embodiments, the metal particle precursor is arranged above, and within two centimeters, of a bed of the reductant material prior to heating.
In some embodiments, the metal precursor particle mixture is compressed or molded.
Embodiments in accordance with the invention are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a workflow for generating a solid metal object.

FIGS. 2A and 2B show an example system for performing the workflow of FIG. 1.

FIG. 3 shows results of x-ray powder diffraction (XRD) of objects created according to embodiments of the invention.

FIG. 4 shows scanning electron microscope (SEM) images of objects created according to embodiments of the invention.

Embodiments in accordance with the invention are further described herein with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is provided to enable any person skilled in the art to use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide a method for rapidly generating a sintered metal body of a designed shape from a mixture of metal oxide particles and metal particles at temperatures several hundred degree K below the melting temperature of the metal.
Embodiments of the invention use reduction expansion synthesis to create a sintered metal object (RES-SM). In most cases the sintered brown body quickly produced with RES-SM will not be as dense, hard, ductile, electrically or thermally conductive as a metal piece of the same size produced by casting from liquid metal. However, the sintered body is a ‘brown’ metal parts, that is a metal part that can be densified using standard ‘post processing’ techniques. Indeed the required post-processing techniques are the same as those employed to densify the brown parts created commercially using the well-known technique of particle injection molding (PIM). Moreover, PIM employs two steps to create brown parts: i) debonding and ii) ‘post-processing’. In contrast, RES-SM requires only the post-processing step to make brown parts. In sum, the slow and expensive ‘debonding’ step of PIM, is not required for RES-SM. Thus, RES-SM may be a suitable technology for the same applications presently dominated by PIM, a technology employed to create more than $2 billion/yr in parts for vehicles. Another application that could be developed is a form of metal additive manufacturing (M-AM). A motivation for developing M-AM technology based on a mature version of RES-SM is that the projected cost of the tools/hardware investment is less than 5% that of current generation M-AM tools.
FIG. 1 illustrates a workflow 100 for generating a solid metal object. As is the case with the other processes described herein, various embodiments may not include all of the steps described below, may include additional steps, and may sequence the steps differently. Accordingly, the specific arrangement of steps shown in FIG. 1 should not be construed as limiting the scope of reduction expansion synthesis of sintered metal.
In block 104, a reductant material and metal precursor particle mixture are arranged in a high temperature furnace. The starting material for the precursor mixture is a physical mixture of metal oxide particles and metal particles. The weight ratio of the metal/metal oxide can span a broad variation, from more than 99% metal to more than 99% metal oxide; although, a near even mixture generally creates a superior product. The size of the particles in the precursor mixture can also span a broad range. For example, all the particles, metal and metal oxide, can be nano-scale. Alternatively, the oxide particles can be micron scale and the metal oxide particles can be nano scale. Moreover, it is not required that all particles of any one type be of a similar size. Examples of reductant material include urea and petroleum gel.
The material and mixture arrangement can be ‘organized’ in a variety of fashions. For example, the particle mixture can be held in a ‘mold’ made of a material such as graphite, alumina, or silica that does not (1) volatilize during heating or (2) interact with the metal in the particle mixture. In another example, the mold can be made of a material that volatilizes, partially or completely, during the heating process but retains shape long enough to allow the solid body to match the shape of the original compact body. In this example, the mold can be made of a high temperature plastic that was created using standard additive manufacturing equipment.
Furthermore, the reductant material should not be directly mixed with the particle mixture because the decomposition process releases gases that cause physical spreading of the metal/metal oxide particles, which prevents a solid body from forming.
In block 106, a temperature of the furnace is held above a decomposition temperature of the reductant material but below a melting temperature of the precursor mixture to generate the sintered metal object. Initially, an inert gas (e.g. nitrogen) is flowed through the furnace until at least 99% of the air is flushed out. Once the oxygen is thoroughly flushed, the furnace can be rapidly heated to the target temperature, for example 800 C, and held for a predetermined duration (e.g., few hundred seconds). In some cases, it is preferable to remove the sintered metal object from the heat while continuing to flow inert gas over it. Further, it may be preferable for the furnace to be continuously but slowly flushed with inert gas during the process.
In block 108, the sintered metal object is cooled. After cooling, the sintered metal object can be removed without damage.
Some features to the reduction process described above are: i) removal of oxygen from the furnace and ii) arranging within the furnace the metal/metal oxide physical mixtures (PM) and a reductant material that creates radicals upon thermal decomposition, where the mixtures and materials are in close proximity (ca. 10 cm or less).
A simple chemical model of the process helps explain the method outlined above. The chemistry, part of a new class of chemical processes known as Reduction Expansion Synthesis (RES), is based on the following model: Radical species, created during the thermal decomposition of a reductant material, diffuse to an area containing ‘reducible’ metal oxides. The radicals interact with oxygen in the metal oxides to form volatile CO2, which enters the gas phase leaving non-volatile, but surface mobile, reduced, metal atoms. For example, assume the reductant material is urea. At above ca. 500 C, urea begins to decompose thermally and release volatile radicals. The radicals travel to a nearby compact body composed of a particle mixture, for example, of nickel and nickel oxide, iron and iron oxide, chromium and chrome oxide, etc. The radicals react with the oxygen in, for example, NiO, forming a volatile gas species such as CO2. The nickel oxide is thus ‘reduced’. The reduced Ni metal atoms created by this process diffuse to and bond to other nearby Ni (or other) metal atoms, which is the basis of the sintering. Said another way, the metal atoms formed by the reduction process create ‘necks’ or bridges between adjacent Ni (or other metal) particles.
The proposed mechanism is consistent with the following observations. For example, sintering does not occur if the reductant material is mixed with the particle mixture. Specifically, gases form rapidly when the reductant material decomposes, which creates a localized pressure burst. If the pressure burst occurs within the particle mixture, the precursor particles are forced apart, preventing necks from forming. In addition, if the reductant material is arranged in a deep bed, it is unlikely any reductant material decomposition products will have sufficient lifetime to reach oxidized material at the center, which results in incomplete reduction to metal. Thus, the process works better with shallow particle mixture beds. Also, a flow process will minimize the possible physical impact of pressure bursts on the particle mixture structure. Finally, it is clear the reductant material and particle mixture ratio should be large enough that more than one radical species is generated per oxygen in the particle mixture bed.
The method described above may be conducted in any suitable apparatus.
Particular configurations of the arrangements described in block 104 are illustrated in FIGS. 2A and 2B. FIG. 2A shows a side view of an arrangement 200 that includes a metal precursor particle mixture 202, perforated flexible graphite 204, reductant material 206, and an alumina boat 208. FIG. 2B shows a top view of the same arrangement 200.
The reductant material 206 is placed in the bottom of an alumina boat 208, and separately, the particle mixture 202 is shaped on top of the flexible graphite mold 204. The flexible graphite mold 204 is then placed about 5 mm above the reductant material 206. The bottom of the flexible graphite 204 contains numerous pinholes in order to facilitate gas/radical transport between the reductant material decomposition products and the particle mixture above.
Examples of reductant materials 206, that is materials that thermally decompose to create radicals, include i) urea, which thermally decomposes starting at ˜500 C to create radical species such as NHx, H atoms, carbon monoxide, etc., and ii) organic materials, such as petroleum gel, that thermally decompose to yield reducing radicals such as CxHy groups, CO, etc.
In some cases, optimal results are observed if the thickness of the particle mixture 202 is no more than a millimeter or two. Further, the reductant material 206 to particle mixture 202 mass ratio should be one or larger.
The starting material for the particle mixture 202 may be a physical mixture of metal oxide particles and metal particles. In this case, the weight ratio of the metal/metal oxide can span a broad variation from more than 99% metal to more than 99% metal oxide. However, a near even mixture of metal-to-metal oxide generally creates a superior product. The particle mixture 202 can be made by stirring the components together to obtain a uniform mixture. In some cases, brief grinding (ca. <5 minutes) by hand or in a grinder is performed. Further, the mixture 202 can be made of a mix containing only one metal (ca. nickel and nickel oxide) or from a mix composing several metals. Examples of mixtures 202 include: i) nickel metal particles and iron oxide particles and ii) nickel metal particles, nickel oxide particles, iron particles and iron oxide particles. The product generated from a mix containing several miscible metals is generally a true alloy. The mixture 202 can also contain ‘molecular’ precursors. For example, a particle mix 202 could contain nickel metal particles, nickel oxide particles, and nickel nitrate. In another example, a particle mix 202 could contain nickel particles, nickel oxide particles and iron nitrate.
The size of the particles in the particle mixture 202 can also span a broad range. For example, all the particles, metal and metal oxide, can be nano-scale. Alternatively, the oxide particles can be micron scale and the metal oxide particles can be nano-scale. Moreover, it is not required that all particles of any one type be of a similar size. A mix 202 containing a wide range of iron oxide particles from nano to tens of microns and iron metal particles of a range of sizes from nano to hundreds of microns can sinter to form a solid body. Many variations on this theme are possible such as mixtures 202 containing three different sized particles; however, a superior product generally results from a mixture in which one of the components is significantly larger than the other components. Thus, a mixture of nano-scale NiO and micron scale Ni generally form a superior sintered Ni metal product relative to the case in which both Ni and NiO are micron scale.
Five physical and chemical analysis tools were used to characterize sample morphology, composition, and strength of samples created as discussed above. i) X-ray powder diffraction was performed on samples using a Rigaku Mini-flex600 X-ray diffractometer operated at 40 kV and 15 mA with a Cu metal target (1.54 Å Kα lines). Diffraction data was collected in the 2θ range of 10° to 90° at 3°-5°/minute with a step width of 0.02°. Diffraction data analysis and structural refinement were performed using Jade 9. ii) Morphology of Ni specimens was investigated using a Zeiss Neon 40 scanning electron microscope (SEM) with a 30 μm aperture and an accelerating voltage of 20 kV. Images from the SEM were used to qualify level of sintering in samples. iii) Chemical analysis was performed using an EDAX energy dispersive spectrometer (EDS) attached to the SEM. iv) The density of the products was determined using a system that employs a simple buoyancy method, combined with direct weight determination. Two different fluids were used for the buoyancy determination, distilled water and ethanol. For the solids produced, the latter generally indicated a final product density of ˜95%, and the latter a density of 90%. v) The strength of the material was tested using an Instron machine and dogbone samples.
It is important to note that the three control samples, i) pure metal, ii) pure oxide, and iii) metal/oxide/nitrate, all failed to form self-supporting bodies. The product in these cases was a bed of particles, where mechanically little changed from the precursor.
All of the samples containing both nickel oxide and nickel metal particles generated solid, self-supporting bodies. The color change clearly suggest Ni oxide (green) had been reduced to metal (‘silver’), a conclusion consistent with XRD. Also, it is clear from the results that some shrinkage occurred during firing. Several measurements were taken to assess the degree of shrinkage along both long and short axes. In all cases, the final solid product had a dimension of 75 +/−3% of the original particles-only body.
It is also informative to compare the top and bottom sides of the self-supporting body formed during firing. In reviewing the samples, it was found that the top of the samples is smooth, showing some limited layer ‘delamination’, whereas the bottom shows no delamination but does show dimples, which formed directly above the pin holes in the underlying flexible graphite.
Although color change indicates some nickel reduction, it is not quantitative. FIG. 3 shows results of x-ray powder diffraction (XRD) of objects created according to embodiments of the invention. The XRD results show clearly more than 90% reduction after one bake 308 and 100% reduction after two bakes 306. Specifically, as shown in FIG. 3, all NiO is reduced after only one heat treatment with the 25% NiO loaded samples 300. X-ray diffraction of 50% NiO specimens showed after one heat treatment 301. The NiO peaks disappeared after a second heat treatment indicating final removal of the metal oxide. This outcome is consistent with the expectation of the RES process. That is, the radical species generated by the thermal decomposition of the urea interact with oxygen in the nickel oxide to form volatile oxides, such as CO2, leaving reduced metal atoms behind.
FIG. 4 shows scanning electron microscope (SEM) images of objects created according to embodiments of the invention. As shown in the SEM images 402-408, the RES-SM process creates metal ‘necks’ between metal particles present in the original mix. The existence of necks between particles is a classic feature of sintered metal particles. Metal particles are generally joined to at least two other particles and often many more. It is also apparent from the SEM images 402-408 that there is void space in the sintered mass. Void space is an inevitable outcome of the sintering of particles, as contrasted to nucleation and solidification from a liquid.
Analysis of the product using EDX capability was employed to confirm that the particle surfaces were fully reduced. EDX, a surface sensitive technique, compliments the bulk characterization of XRD. The analysis showed that there is no evidence from EDX of any significant impurities in the particles. Specifically, neither oxygen nor carbon was discovered in significant quantities. There is, possibly, a shoulder on one nickel peak belonging to oxygen; however, limited oxidation of metal surfaces is anticipated. Thus, the existence of some oxygen is consistent with complete reduction during the bake and some oxidation occurring naturally during subsequent air handling.
The key observations regarding the formation of a hard self-supporting, virtually 100% nickel metal objects from a mixture of NiO and Ni metal particles undergoing the RES-SM protocol described above are the following:

- A solid body forms if both metal oxide and metal particles are present in the precursor.
- At least 90% of the oxide in the particle compact is reduced to metal, and ‘double baking’ reduces close to 100%.
- The process leads to the formation of metallic necks between the original metallic particles in the precursor.
- Significant shrinkage occurs during the formation of the solid ‘brown body’ from the particle compact. Linear dimensions of the final product were approximately 75% those of the original particle compact.

It is also important to note that the process included some fundamentally novel features. First, a green body is formed from a mix of metal and metal oxide. In all other systems, including PIM and M-AM, the green body is formed entirely from metal particles. Second, no ‘binder’ is required as per PIM. This is particularly important as ‘debonding’, that is removal of the binder, which is generally an organic material, is the slowest, most complex step in the PIM process. Third, unlike M-AM, at no point is there a need to reach the metal melting point. Indeed, using the embodiment described herein, a solid nickel body formed at 950 C, whereas the nickel melting temperature is 1455 C, which is 500 C higher.
Embodiment described herein can also be compared to other traditional RES processes. Among the traditional processes based on RES are the reducing of carbon oxide to form graphene, which is described in U.S. Pat. No. 8,894,886, the reduction of metal oxide particles to create metal particles, the reduction of chrome oxide in a paste to yield a layer of chrome metal, and the formation of reduced Sn particles on a high surface area carbon to form a high density battery anode. There are several general differences between RES-SM and all of these earlier examples. First, in all of these traditional processes, the material to be reduced and the reductant material are intimately mixed. In contrast, in RES-SM, the particle mixture and the reductant material should not be intimately mixed. For example, the paste used to make chrome and other coatings on metals is composed of an intimate mix of metal precursor materials and reductant material such as urea. Second, RES-SM is the only processes that employs a mixture of metal oxide and metal in the precursor physical mix. In all of the other traditional RES processes, the metal precursors are always of the same oxidation state. Thus, for example, to produce metal particles using RES, it is taught that metal oxides, metal nitrates, metal chlorides, etc. are intimately mixed with urea. It is not taught that a mixture of metal and metal oxide should be used. Third, RES-SM is the only version intended to create a ‘stand alone’ metal object. Most of the earlier work was intended to create coatings. For example, one metal coat is created on top of an existing metal object. In another example, a high surface area material is coated with very small catalyst particles produced using an RES process. In yet another example, a fine powder of graphene is produced using an RES process. In a final example, very fine metal particles or metal alloy particles are created using an RES process.
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention and it is not intended to be exhaustive or limit the invention to the precise form disclosed. Numerous modifications and alternative arrangements may be devised by those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the claims appended hereto.
In addition, the previously described versions of the present invention have many advantages, including but not limited to those described above. However, the invention does not require that all advantages and aspects be incorporated into every embodiment of the present invention.
All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.

Claims

What is claimed is:

1. A method of generating a solid metal object comprising:

arranging a reductant material and a metal precursor particle mixture in a high temperature furnace that is filled with a chemically inert atmosphere;

holding a temperature of the high temperature furnace above the decomposition temperature of the reductant but below a melting point of the metal precursor particle mixture for a predetermined duration to generate the solid metal object; and

cooling the generated metal object in the inert atmosphere.

2. The method of claim 1 where the high temperature furnace bakes the reductant and metal precursor particle mixture in order to:

generate a chemical radical species by decomposing the reductant material; and

expose the metal precursor particle mixture within the inert atmosphere to the chemical radical species needed to generate the solid metal object.

3. The method of claim 1 where the inert atmosphere is nitrogen or argon.

4. The method of claim 1 where the reductant material is urea.

5. The method of claim 1 where the reductant material is a petroleum gel.

6. The method of claim 1 where the metal precursor particle mixture comprises a metal oxide and metal particles.

7. The method of claim 6 where a weight ratio of the metal oxide and the metal particles in the metal precursor particle mixture is approximately 1 to 1.

8. The method of claim 6 where the metal particles in the metal precursor particle mixture include more than one type of metal.

9. The method of claim 8 where the more than one type of metal includes at least two metals of group consisting of iron, nickel, and chromium.

10. The method of claim 8 where the metal oxide includes particles of nano-scale and the metal particles are micron-scale.

11. The method of claim 8 where the metal particles includes nano-scale metal particles and micron-scale metal particles.

12. The method of claim 6 where the metal precursor particle mixture further comprises molecular precursors.

13. The method of claim 6 further comprising grinding the metal precursor particle mixture to combine the metal oxide and the metal particles.

14. The method of claim 1 where approximately 99% of air is flushed from the high temperature furnace by the flow of inert atmosphere.

15. The method of claim 1 where prior to heating, the metal particle precursor is arranged above, and within two centimeters, of a bed of the reductant material.

16. The method of claim 1 where the metal precursor particle mixture is compressed or molded.