Method of Making Aluminum Nitride Foam

Abstract

Porous aluminum nitride (AlN) provides a greater surface area and higher permeability, which is especially desirable for advanced functional application. Porous or bulk aluminum nitride is very difficult to manufacture due mainly to its high melting point (e.g., 2200 degrees Celsius). A new processing method synthesizes porous aluminum nitride through a complete transformation from porous aluminum using a remarkably low nitriding or sintering temperature. The manufactured porous aluminum nitride foam can be used for such applications as filters, separators, heat sinks, ballistic armor, electronic packaging, light- and field-emission devices, and highly wear-resistant composites when infiltrated with metal such as aluminum, titanium, or copper.

Claims

1. A method comprising: forming an aluminum (Al) green body using aluminum powder and a polyvinyl alcohol binder, wherein a porous structure of the aluminum green body comprises regularly distributed pores on the order of nanometers and microns; and nitriding the aluminum green body through direct nitridation in a nitrogen atmosphere at a temperature of less than about 1000 degrees Celsius to obtain an aluminum nitride (AlN) foam comprising a porosity of about 60 percent to 80 percent.

2. The method of claim 1 wherein the forming the aluminum green body comprises synthesis comprising slurry freezing or drying, or a combination, and thermal sintering or nitriding methods, or a combination.

3. The method of claim 1 wherein the forming the aluminum green body comprises freezing or drying of a slurry comprising the aluminum powder and polyvinyl alcohol binder.

4. The method of claim 1 comprising: forming a slurry comprising the aluminum powder, polyvinyl alcohol binder, and water.

5. The method of claim 1 comprising: while nitriding the aluminum green body, sintering the aluminum green body in the nitrogen atmosphere, wherein a porous structure of the resulting aluminum nitride foam is three dimensional and uniformly distributed.

6. The method of claim 5 wherein uniformly distributed pores of the aluminum nitride foam comprise a plural number of microns in diameter and a nonzero number of nanometer pores.

7. The method of claim 5 wherein uniformly distributed pores of the aluminum nitride foam comprise a plural number of tens to any plural number of hundreds of nanometers.

8. The method of claim 5 wherein a temperature used during sintering is lower than 1700 degrees Celsius.

9. The method of claim 1 wherein the nitriding of the aluminum green body does not use an additive or pressure application.

10. The method of claim 1 comprising: while nitriding the aluminum green body, sintering of the aluminum foam green body is at a temperature between about 500 degrees Celsius and about 900 degrees Celsius.

11. The method of claim 10 wherein the nitriding and sintering of the aluminum green body does not use an additive or pressure application.

12. A method comprising: forming a slurry comprising aluminum powder, polyvinyl alcohol binder, and water; forming a porous aluminum (Al) green body from the slurry; and nitriding the aluminum green body through direct nitridation in a nitrogen atmosphere at a temperature of less than about 1000 degrees Celsius to obtain an aluminum nitride (AlN) foam comprising a porosity of about 60 percent to 80 percent.

13. The method of claim 12 wherein a porous structure of the porous aluminum green body comprises distributed pores on the order of nanometers and microns.

14. The method of claim 12 wherein the forming the porous aluminum green body comprises freezing or drying of a slurry comprising the aluminum powder and polyvinyl alcohol binder.

15. The method of claim 12 comprising: while nitriding the aluminum green body, sintering the aluminum green body in the nitrogen atmosphere, wherein a porous structure of the resulting aluminum nitride foam is three dimensional and uniformly distributed.

16. An apparatus comprising at least one of filters, separators, heat sinks, ballistic armor, electronic packaging, light- and field-emission devices, and wear-resistant composites comprising aluminum nitride foam of method 12.

17. The method of claim 12 comprising: infiltrating the aluminum nitride foam with a metal comprising at least one of aluminum, titanium, or copper.

18. The method of claim 12 wherein the porosity of the aluminum nitride foam provides permeability.

19. The method of claim 12 wherein the aluminum nitride foam comprises a load-displacement curve comprises a first point at a force of about 500 micronewtons and a displacement of between about 28 to 32 nanometers, a second point at a force of about 1000 micronewtons and a displacement of between about 48 to 52 nanometers, and a third point at a force of about 2000 micronewtons and a displacement of between about 78 to 82 nanometers.

20. The method of claim 12 wherein the aluminum nitride foam comprises at a load of 8000 micronewtons, the aluminum nitride foam has a hardness of about 6 to 8.8 gigapascals and an elastic modulus of about 35 to 45 gigapascals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 schematically describes a processing flow from the preparation of starting aluminum powder slurry, freezing or drying, and sintering of porous aluminum green body, and finally synthesizing aluminum nitride foam via low-temperature nitridation.

[0022] FIG. 2 shows comparison of XRD patterns of starting aluminum powder, frozen and dried aluminum green body, and the synthesized aluminum nitride foam after nitriding and sintering.

[0023] FIG. 3 shows morphological images of the final aluminum nitride foam: (a) top-view optical image of a mounted and polished aluminum nitride foam, (b) SEM image of the mounted and polished aluminum nitride sample and the corresponding elemental EDS mapping images, (c, d) SEM images of the synthesized aluminum nitride foam after nitriding and sintering, and (e) the morphology of nanoparticles formed on the surface of the aluminum nitride foam.

[0024] FIG. 4 shows surface morphologies: (a) schematic of the morphological formation mechanism on the surface of the synthesized aluminum nitride foam and (b-e) the SEM images of the surface morphology on the aluminum nitride foam with varying magnifications.

[0025] FIG. 5A-5B shows predicted diffusional profiles of nitrogen atoms in aluminum green-body foam during nitriding/sintering: (5A) the nitrogen atom concentration prediction as a function of time in the strut of aluminum foam on the basis of Fick's second law and (5B) the diffusion distance prediction of nitrogen atoms in the strut of aluminum foam as a function of time.

[0026] FIG. 6A-6C show nanoindentation test results of the synthesized aluminum nitride foam: (6A) the load-displacement curve with a peak load of 5000 micronewtons with the in-situ optical inset image showing the corresponding indentation impression, (6B) variations of elastic modulus and hardness values as a function of the indentation load from 4000 to 10,000 micronewtons, and (6C) surface roughness image of the synthesized aluminum nitride foam with a mean roughness value of 550 nanometers.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Manufacturing the porous foam structure includes the steps: (a) preparing aluminum powder slurry mixed with PVA binder and water; (b) freezing the aluminum slurry when placed in a mold in contact with the cold surface of a copper rod; (c) sublimating the frozen slurry under reduced pressure and low temperature, forming a porous Al foam green body; (d) sintering and nitriding the porous aluminum foam green body at a low temperature of 620 degrees Celsius to transform into a 3D connected porous aluminum nitride foam under nitrogen gas.

[0028] A three-dimensionally (3D) connected porous structure of the aluminum nitride foam is created from the combination of the slurry freezing/sintering. Thermal nitriding methods can be used as an advanced material, which can provide higher surface area with unique aluminum nitride nanoparticles on the surface for potential use in various functional applications.

[0029] The temperature of 620 degrees Celsius is considered unusually very low because this nitriding temperature for metallic aluminum into aluminum nitride is noted as the lowest temperature ever achieved. For example, only surface nitridation (not even the fully complete nitridation) was achieved at 1000 degrees Celsius for 10 hours via a direct melt nitridation process for metallic aluminum.

[0030] The aluminum nitride foam is thus considered to be a reaction product resulting from nitridation, which started on the surface of the aluminum particles and then proceeded toward the inside of aluminum particles under the nitrogen atmosphere. The starting aluminum powder possessed the native oxide layer on the surface, which had been created from the contact with water in slurry preparation (2Al+3H.sub.2O.fwdarw.Al.sub.2O.sub.3+H.sub.2). During the heat treatment for nitridation, the aluminum particle was then covered with a reaction layer containing nitrogen gas as the native aluminum oxide layer was combined with nitrogen gas (Al.sub.2O.sub.3+N.sub.2.fwdarw.2AlON+O.sub.2). The metastable aluminium oxynitride (AlON) could then be transformed into aluminum nitride (AlON+O.sub.2.fwdarw.AlN+O.sub.2).

[0031] When the nitridation process normally occurs at a high temperature of over 2000 Kelvin, the wettability between the molten aluminum and aluminum nitride becomes considerable and the formation of aluminum nitride can be facilitated at the gas-melt interface and further away from the interface in the molten aluminum, which could lead to partial volume nitridation. This is because nitrogen bubbles are recombined at the surface of molten aluminum, resulting in the formation of aluminum nitride particles during the rise of the bubbles in the molten aluminum with high nitrogen content, which leads to the multiple nucleation of aluminum nitride crystals on the surface of the aluminum particle. Therefore, the morphology of the aluminum nitride synthesized at a high temperature typically shows a plate formation on the surface of aluminum with a stacking of aluminum nitride plates on the surface of the core aluminum. The appearance of the nanoparticle branches observed is in agreement with the surface morphology of nitrided aluminum powder at the temperature below 1273 Kelvin, suggesting that a limited growth mechanism of aluminum nitride crystals was also dominant on the surface in our porous aluminum nitride formation, as observed in aluminum powder, due mainly to the relatively low temperature.

[0032] When aluminum particles came into contact with the nitrogen gas during the first stage of nitridation, the native oxide layer of the aluminum particle initially changed to a very thin aluminium oxynitride layer, and further nitridation proceeded through the diffusion of nitrogen through this layer (FIG. 4a). Therefore, the aluminium oxynitride shell acted as a nucleation site by supplying nitrogen atoms for the further growth of aluminum nitride. In the transition state from solid to liquid with reduced viscosity, the aluminum particles may have experienced stresses due to considerable thermal expansion. Some part of the aluminum nitride or aluminium oxynitride layer was then fractured and liquid-like aluminum flowed out of the fractured shell, forming aluminum droplets. The new aluminum nitride created in the droplets formed on top of the aluminium oxynitride layer and grew into the droplet, suggesting that the aluminium oxynitride shell played the role of a seed in the nitridation of the aluminum droplet. If a considerable number of droplets are formed from an active breakage of the aluminum nitride shell, the fresh aluminum can directly react with nitrogen gas; thus, the nitridation rate rapidly increases. Magnified morphological features are seen in FIG. 4b-4e) for the stacking of aluminum nitride particles and rods on the surface of aluminum nitride foam, which supports the aforementioned aluminum nitride formation mechanism.

[0033] The detailed process for manufacturing of aluminum nitride foam can include the following steps: [0034] (1) Placing a mold on a copper rod, or similar material with high thermal conductivity, immersed in liquid nitrogen (or a similar device that can lower the temperature significantly) and pouring the prepared aluminum slurry, containing water and binder (e.g., polyvinyl alcohol or PVA) in the mold. Herein, the presence of binder containing carbon is essential because it exerts a considerable impact on the nitridation of aluminum foam green body through a carbothermal process. [0035] (2) Freezing the prepared aluminum powder slurry in the mold where the aluminum particles are piled up and physically attached between the growing ice crystals. [0036] (3) Forming a aluminum foam green body by drying the ice crystals from the frozen slurry at sufficiently low temperature and reduced pressure (in a freeze dryer), leaving microscale pores in the aluminum foam green-body structure and retaining the physical attachments with the help of polyvinyl alcohol binder. [0037] (4) Finally, constructing aluminum nitride foam with firmly connected pores by debinding (at relatively lower temperature) and sintering or nitriding the aluminum foam green body at sufficiently high temperature (620 degrees Celsius in this invention) under nitrogen atmosphere. Both the presence of carbon in the polyvinyl alcohol binder and the unique three dimensional (3D) porous structure of the Al foam are attributed to the unprecedentedly successful and complete transformation to aluminum nitride foam at such low temperature.

Exemplary Embodiment 1: Synthesizing Aluminum Nitride Foam

[0038] Aluminum powder (with an average particle size of less than 10 microns, 30 volume percent of a water-based solution) was suspended in a solution of deionized water containing polyvinyl alcohol (PVA) binder. The slurry was then dispersed by mechanical stirring for 1 hour and subsequently cooled to a few degrees above the freezing point of water and poured into a mold consisting of insulated polytetrafluoroethylene (PTFE) walls on a copper rod.

[0039] The copper rod was cooled using liquid nitrogen and controlled using a thermocouple and temperature controller. Once the freezing process was complete, the frozen green-body sample was removed from the mold and sublimated at 185 Kelvin (88 degrees Celsius) for 48 hours in a freeze-dryer under a 0.005-torr residual atmosphere.

[0040] The resultant green-body sample was transferred to a tube furnace, first heated to 573 Kelvin (300 degrees Celsius) for 3 hours to remove the polyvinyl alcohol binder, and then sintered and nitrided at 893 Kelvin (620 degrees Celsius) for 5 hours under an nitrogen gas atmosphere with heating and cooling rates of 5 degrees Celsius per minute and 3 degrees Celsius per minute, respectively. It is emphasized that the exemplary embodiment in this invention applied simultaneous sintering and nitriding process at such relatively low temperature enabled the complete transformation of the aluminum foam green body to aluminum nitride foam (FIG. 1).

Exemplary Embodiment 2: Phase Analysis and Mechanical Properties of the Synthesized Aluminum Nitride Foam

[0041] To confirm the complete transformation of porous aluminum into aluminum nitride foam, an XRD analysis was carried out. FIG. 2 compares the XRD patterns of the prepared Al foam green body and synthesized aluminum nitride foam, before and after the simultaneous nitrification/sintering process in a box furnace under a nitrogen gas atmosphere. The XRD patterns of aluminum powder and aluminum green body are essentially the same or identical except that gradually diminishing peaks are seen at around 20 degrees as magnified in the inset of the XRD pattern of the aluminum foam green body (FIG. 2), which indicates the presence of carbon in the aluminum foam green body due to the use of PVA for the slurry preparation. Most importantly, the starting aluminum powder was completely transformed to the aluminum nitride phase, as seen from the XRD pattern of the final synthesized aluminum nitride foam, which demonstrates that aluminum nitride foam product could be successfully achieved using us a simultaneous nitridation or sintering processing method at such low temperature of 620 degrees Celsius under an nitrogen atmosphere.

[0042] The optical image in FIG. 3a shows the porous microstructure of the final aluminum nitride foam with the inset showing a magnified microstructure; note that the black areas in FIG. 3a correspond to the epoxy resin that was used to fill the pores during the metallographic mounting. The mean pore size and wall width were measured as 74.0 plus or minus 27.7 microns and 29.1 plus or minus 6.3 microns, respectively; additionally, the porosities measured by the image analysis and physical measurement methods were 72.4 percent and 79.2 percent, respectively; the estimated some 70 percent porosity for the sample obtained in this invention is considered sufficiently high for potential use in a variety of functional applications that can utilize its large surface area.

[0043] As confirmed from the SEM and corresponding EDS images in FIG. 3b, the presence of an aluminum nitride phase was clearly identified throughout the entire sample area. SEM micrographs of the final aluminum nitride foam in FIG. 3c-3e also show a uniform microscale pore structure with aluminum nitride nanostructures grown from the surface of aluminum nitride foam struts.

[0044] The porous aluminum nitride could be achieved by reacting the prepared porous aluminum foam green body with the surrounding nitrogen gas molecules that were dissociated into nitrogen atoms, the nuclei of aluminum nitride nanoparticles were created and enriched on the surface of aluminum, and the nuclei then acted as a template for the further volumetric growth of the aluminum nitride particles, as shown by the schematic diagram of the representative aluminum nitride morphology formed in FIG. 4a. The unique three-dimensional porous structure of the prepared aluminum foam green body could further assist the creation of aluminum nitride nanoparticles on the surface of aluminum.

[0045] The reaction of nitrogen atom diffusion is determined by the concentration gradient in the aluminum boundary layer and is affected by the chemical reaction between nitrogen and aluminum atoms to form the aluminum nitride particles at the interface both in the aluminum boundary layer and the aluminum core. The diffusion of nitrogen in aluminum can be estimated by Fick's second law. According to FIG. 5A, the nitrogen atom concentration was saturated at 2.710{circumflex over ()}4 after 200 seconds. Furthermore, FIG. 5B shows that the nitrogen atoms could diffuse up to 100 microns into the aluminum strut after 300 seconds.

[0046] Nano-indentation testing was used to determine the physical and mechanical properties of the struts of the synthesized aluminum nitride foam. Given the dimensions of a few tens of micrometers of the strut, the physical and mechanical properties of aluminum nitride foam were characterized using the nanoindentation test with the micrometer-scale Berkovich diamond tip. FIG. 6A displays a representative force versus displacement curve with calculation results of the Young's modulus and hardness values obtained from the nanoindentation test curve, where the peak load is 5000 micronewtons and the maximum indentation depth is 148 nanometers. The Young's modulus (E) of 122.3 plus or minus 4.4 gigapascals (GPa) and an indentation hardness (H) of 12.1 plus or minus 1.6 gigapascals were obtained. The inset image in FIG. 6A shows an optical image of an indentation impression under a peak load of 5000 microns with a mean diameter of about 1 micron and no pile-up observed near the impression.

[0047] The load dependence of E and H of the aluminum nitride foam is also shown in FIG. 6B, with their values varying in the range of 35.7-122.3 gigapascals and 4.4-12.2 gigapascals, respectively.

[0048] The surface roughness of an aluminum nitride strut was measured quantitatively using the corresponding nanoindentation image (FIG. 6C). The average surface roughness (Ra) value of the aluminum nitride strut surface was estimated to be 550 nanometers.

[0049] The hardness and elastic modulus data obtained from the nanoindentation test were also used to calculate the Gilman-Chen parameter (H/G) for the aluminum nitride foam manufactured using this technique to determine the dominant type of bonding. The H/G value for our aluminum nitride foam material was roughly 0.2, suggesting that its dominant bonding type should be covalent. This result is in good agreement with the strong covalent nature of the generally known bonding for bulk aluminum nitride.

[0050] This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.