Patent Publication Number: US-8968865-B2

Title: Methods of electrophoretic deposition for functionally graded porous nanostructures and systems thereof

Description:
The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to functionality graded porous nanostructures, and more particularly, to using electrophoretic deposition to form functionality graded aerogels. 
     BACKGROUND 
     Aerogels are a fascinating class of high surface-area, mechanically-robust materials with a broad range of both commercial and fundamental scientific applications. Owing to its highly porous mass-fractal nanostructure, amorphous silica aerogel has been used as a capture agent in NASA&#39;s cometary-dust retrieval missions, to control disorder in  3 He-superfluid phase transitions, in the fabrication of targets for laser inertial confinement fusion, in low-k microelectromechanical (MEMS) devices, and in Cherenkov nucleonic particle detectors. 
     In particular, amorphous carbon aerogel has received a considerable amount of attention in recent years owing to its low cost, electrical conductivity, mechanical strength, and thermal stability. Numerous applications have been explored for this material including water desalination, electrochemical supercapacitors, and thermal insulation. 
     The electrophoretic deposition (EPD) process utilizes electric fields to deposit charged nanoparticles from a solution onto a substrate. Earlier industrial use of the EPD process employed organic solvent solutions and therefore typically generated hazardous waste as a by-product of the process. In addition, the shapes, compositions, densities, and microstructures of materials formed through EPD processes have typically been difficult if not impossible to control, either separately or in combination with one another. Furthermore, templating has been used in EPD processes to control pore positioning and density; however, templating is restricted in that it is limited by the template material. Also, it is extremely difficult to form structures from more than one material. That is to say, typical EPD processes are limited in that they are only capable of forming planar, homogenous structures. 
     SUMMARY 
     In one embodiment, an aerogel includes a layer of shaped particles having a particle packing density gradient in a thickness direction of the layer, wherein the shaped particles are characterized by being formed in an EPD process using an impurity. 
     In another embodiment, a method for forming a functionally graded porous nanostructure includes adding particles of an impurity and a solution to an EPD chamber, wherein the solution comprises a material to be deposited onto a surface of the particles of the impurity, and wherein the EPD chamber comprises two electrodes at opposite ends of the EPD chamber, applying a voltage difference across the two electrodes of the EPD chamber to create an electric field in the EPD chamber, wherein the electric field causes the particles of the impurity to form a particle packing density gradient in a direction consistent with the electric field, and depositing the material onto surfaces of the particles of the impurity to form shaped particles of the material. 
     Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram of an electrophoretic deposition (EPD) device, according to one embodiment. 
         FIGS. 2A-2C  show the formation of a functionally graded porous nanostructure through EPD, according to one embodiment. 
         FIGS. 3A-3D  show functionally graded porous nanostructures formed through EPD, according to various embodiments. 
         FIG. 4  is a collection of SEM images showing higher and lower particle packing densities of a functionally graded porous nanostructure, according to one embodiment. 
         FIG. 5  is a SEM image of hollow spheres in a functionally graded porous nanostructure, according to one embodiment. 
         FIG. 6  is a flow diagram of a method for forming a functionally graded porous nanostructure through EPD, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations. 
     Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc. 
     It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless otherwise specified. 
     Functionally graded materials (FGM) fabricated with gradients in composition, microstructure, and/or density produce enhanced bulk properties, which typically correspond to a combination of the precursor material properties. For example, controlled composite layers of boron carbide and aluminum may produce lightweight ceramic materials that are both hard and ductile for improved armor. Current graded materials are primarily produced by coarse, layered processing techniques or melt-based approaches which are typically limited to abrupt gradients in composition along one axis only. The techniques described herein overcome these limitations using electrophoretic deposition (EPD) technology to fabricate functionally graded materials. 
     Typically, EPD has been used for forming coatings on surfaces using organic solvents. Recent nanomaterial work has demonstrated that EPD is capable of, at small length scales, being performed using aqueous (water-based) solutions. In addition, EPD may be performed using a wide variety of charged nanoparticles, such as oxides, metals, polymers, semiconductors, diamond, etc. 
     In one general embodiment, an aerogel includes a layer of shaped particles having a particle packing density gradient in a thickness direction of the layer, wherein the shaped particles are characterized by being formed in an EPD process using an impurity. 
     In another general embodiment, a method for forming a functionally graded porous nanostructure includes adding particles of an impurity and a solution to an EPD chamber, wherein the solution comprises a material to be deposited onto a surface of the particles of the impurity, and wherein the EPD chamber comprises two electrodes at opposite ends of the EPD chamber, applying a voltage difference across the two electrodes of the EPD chamber to create an electric field in the EPD chamber, wherein the electric field causes the particles of the impurity to form a particle packing density gradient in a direction consistent with the electric field, and depositing the material onto surfaces of the particles of the impurity to form shaped particles of the material. 
     For the sake of clarity and simplicity, for the remainder of this description, the porous nanostructure will be referred to as an aerogel. However, the descriptions and embodiments presented herein are not meant to be limited to only aerogels, as any suitable porous nanostructure or microstructure may be formed using the methods described herein. 
     By controlling certain characteristics of formation of structures in an EPD process, such as the precursor material composition (e.g., homogenous or heterogeneous nanoparticle solutions) and orientation (e.g., non-spherical nanoparticles), deposition rates (e.g., by controlling an electric field strength, using different solvents, particle concentrations, etc.), material layers and thicknesses (e.g., through use of an automated sample injection system and deposition time), and deposition patterns with each layer (e.g., via use of dynamic electrode patterning), intricate and complex structures may be formed using EPD processes that may include a plurality of densities, microstructures (e.g., ordered vs. random packing), and/or compositions, according to embodiments described herein. 
     Equation 1 sets out the basic system-level model for electrophoretic deposition, where W film  is the mass of the deposition layer, μ is the electrophoretic mobility, E is the electric field, A is the area of the electrode substrate, C is the deposition particle mass concentration, and t is the deposition time.
 
 W   film =∫ t2   t1   μEACdt   Equation 1
 
     Combining these principles with dynamic patterning and sample delivery (which is described in more detail later), EPD may be employed to produce a diverse set of products with unique and/or difficult to obtain shapes, designs, and properties custom-fitted to any of a number of practical applications. 
     In one approach, EPD technology may be combined with organic sol-gelation, and carbonization if desired, to produce complex, functionally graded, porous nanostructures with properties custom fitted to a number of practical applications. 
     Referring now to the figures, as shown in  FIG. 1 , an EPD device  100  may include a first electrode  110  and a second electrode  106  positioned on either side of an EPD chamber  118 , with a voltage difference  116  applied across the two electrodes  106 ,  110  that causes charged particles  102  in a solution  108  to move toward the first electrode  110  as indicated by the arrow. In some embodiments, a substrate  112  may be placed on a solution side of the first electrode  110  such that particles  102  may collect thereon. The EPD device  100 , in some embodiments, may be used to attract particles  102  of an impurity toward the first electrode  110  or toward a conductive or non-conductive substrate  112  positioned on a side of the electrode  110  exposed to a solution  108  including the impurity  104  that will aid in formation of the aerogel. 
     By controlling certain characteristics of formation of structures in an EPD process, such as the precursor material composition (e.g., homogenous or heterogeneous nanoparticle solutions) and orientation (e.g., non-spherical nanoparticles), deposition rates (e.g., by controlling an electric field strength, using different solvents, particle concentration, etc.), material layers and thicknesses (e.g., through use of an automated sample injection system and deposition time), and deposition patterns with each layer (e.g., via use of dynamic electrode patterning), intricate and complex structures may be formed using EPD processes that may include a plurality of densities, microstructures (e.g., ordered vs. random packing), and/or compositions, according to embodiments described herein. 
       FIG. 2A  shows an EPD device  100  setup with a solution  108  therein prior to addition of particles  102  of the impurity. The solution, in some embodiments, may include a material to be deposited onto a surface of the particles  102  of the impurity. The electrodes  106 ,  110  are positioned on opposite ends of the EPD chamber  118 , in one approach. A substrate  112  may be positioned on a solution side of the electrode  110  to which charged particles are attracted, in some approaches. 
     As depicted in  FIG. 2B , the solution  108  may comprise an organic sol-gel, or other monomer solution, and charged particles of an impurity  102  may serve as structural template materials for guiding functional gradation of the solution  108  after gelation. Structural template materials  102  may comprise long-chain polymers such as polystyrene, or any other polymer, which may have carboxylated or other surface functionalization for causing a desired effect. A surface functionalization for causing any desired effect as would be understood by one of skill in the art may be chosen, depending on the desired effect. 
     In more embodiments, the impurity may comprise metals, oxides, etc., as would be understood by one having ordinary skill in the art. While the solution  108  is characterized by a fluid state, charged impurity particles  102  migrate toward the first electrode  110 , separating charged impurity particles  102  according to electrophoretic mobility and generating a gradient distribution of charged impurity particles  102  throughout the solution  108 . 
     The gradient distribution may further be characterized by a decreasing concentration of impurity particles  102  with increasing distance d from the first electrode  110 . After the gradient in particle packing density is formed, the gelation process takes place to form an aerogel. 
     In some preferred embodiments, and as shown in  FIG. 2C , the gradient distribution may exhibit one or more distribution zones. As shown in  FIG. 2C , four or more zones may be formed, according to one embodiment: a first zone  120  located nearest the first electrode  110  and exhibiting tight, ordered packing of impurity particles  102 , a second zone  122  exhibiting random packing of charged impurity particles  102 , a third zone  124  nearer the second electrode  106  exhibiting charged impurity particles  102  solvated in the solution  108 , and a fourth zone  126  substantially devoid of charged impurity particles  102 . This gradient may be unidirectional or may be formed in multiple directions by applying multiple electric fields, in more embodiments. 
     According to one approach, as gelation takes place, the material in the solution  108  may be deposited upon surfaces of the impurity particles  102 , shown as coating  126  in  FIG. 2C . However, it is not required that the material be deposited in this manner, and may be formed in other ways, as would be understood by one of skill in the art upon reading the present descriptions. 
     In one embodiment, EPD may be used in conjunction with controlled electric field patterns to direct the composition of deposited material in an x-y plane parallel to a plane of deposition, including multilayer deposition of a single pattern as well as dynamically changing patterns as the particles build up in the z-dimension, perpendicular to the x-y plane. 
     According to one embodiment, as shown in  FIG. 3A , an aerogel  300  may comprise a layer  302  of shaped particles  126  having a particle packing density gradient  306  in a thickness direction (as indicated by d) of the layer  302 . The shaped particles  126  are characterized by being formed in an EPD process using an impurity  102 . 
     In one embodiment, the layer  302  may be substantially free of the impurity  102  and the shaped particles  126  may be hollow spheres, as shown in  FIG. 3A . 
     In another embodiment, as shown in  FIG. 3C , the layer  306  may further comprise the impurity  102  having a particle packing density gradient corresponding to the particle packing density gradient of the shaped particles  126 . In this or any other embodiment, the impurity  102  may comprise a metal or an oxide. 
     According to some embodiments, the shaped particles  126  may comprise carbon or any other material capable of forming an aerogel. 
     Now referring to  FIGS. 3B and 3D , the aerogel  310 , may be formed with more than one gradient in particle packing density, such as two gradients in opposite directions, as shown, or any other arrangement as would be understood by one of skill in the art upon reading the present descriptions. 
     As would be understood by one of skill in the art upon reading the present descriptions, one or more additional layers may be arranged above the layer  302 - 308  as shown in  FIGS. 3A-3D , thereby forming a structure that may have complex layering and/or composition, with gradients possible in the thickness direction across all the layers. 
     In one embodiment, EPD may be used in conjunction with controlled electric field patterns to direct the composition of deposited material in a plane parallel to a plane of deposition, including multilayer deposition of a single pattern as well as dynamically changing patterns as the particles build up in the thickness direction, perpendicular to the plane. This technique enables, for example, transparent ceramic optics with a controlled, smooth, concentration of dopant material. 
     Furthermore, along a plane of deposition perpendicular to the plane, the aerogel may exhibit a functional gradient as described above and with reference to FIGS.  3 A- 3 D. Generally, the functional gradient may be characterized according to decreasing concentration of structural template nanoparticles  102  with increasing distance d from the first electrode  110 . 
     In applications where structural template impurity particles  102  are removed during carbonization, or where structural template impurity particles  102  exhibit low density relative to the solution  108 , the resulting final aerogel may therefore exhibit at least a particle packing density gradient characterized by a density inversely proportional to a distance d from the first electrode  110 . 
     Conversely, where structural template impurity particles  102  exhibit higher density than the solution  108 , and are not removed during carbonization, the resulting aerogel exhibits at least a particle packing density gradient characterized by a density directly proportional to distance d from the first electrode  110 . 
     Referring now to  FIG. 4 , a scanning electron microscope (SEM) image of an aerogel  402 ,  404  after carbonization is shown according to one embodiment. The images exhibit a particle packing density gradient characterized by a density inversely proportional to a distance from the first electrode (located at the bottom of each aerogel shown in  FIG. 4 ). The first blow-up image  406  shows loosely packed particles, the second blow-up image  408  shows slightly more tightly packed particles, the third blow-up image  410  shows even more tightly packed particles, and the fourth blow-up image  412  shows very closely packed particles. A gradual particle packing density gradient such as that shown in  FIG. 4  may be formed through the techniques and methods described herein, according to various embodiments. 
     Referring now to  FIG. 5 , a SEM image of an aerogel  500  after carbonization that exhibits a characteristic of being formed through an EPD process is shown according to one embodiment. As can be seen in  FIG. 5 , hollow spherical particles  502  are shown. In this image, the hollow spherical particles  502  represent a spatial void where, according to one embodiment, charged impurity particles settled during EPD and were subsequently removed during carbonization. As a result, a low-density gap remains in each of the spatial voids distributed within the aerogel, surrounded by deposited material. 
     Now referring to  FIG. 6 , a method  600  for forming a functionally graded porous nanostructure is shown according to one embodiment. The method  600  may be carried out in any desired environment, including those shown in  FIGS. 1-5 , among others. 
     In operation  602 , particles of an impurity and a solution are added to an EPD chamber, wherein the solution comprises a material to be deposited onto a surface of the particles of the impurity, and wherein the EPD chamber comprises two electrodes at opposite ends of the EPD chamber. The solution and the impurity may be mixed prior to being added, mixed when added, added separately, or in any other combination. 
     In operation  604 , a voltage difference is applied across the two electrodes of the EPD chamber to form an electric field in the EPD chamber, wherein the electric field causes the particles of the impurity to form a particle packing density gradient in a direction consistent with the electric field. 
     In operation  606 , the material is deposited onto surfaces of the particles of the impurity to form shaped particles of the material. 
     In some embodiments, the method  600  may further comprise allowing the shaped particles of the material, the impurity, and the solution to gel, drying the shaped particles of the material and the impurity to remove remaining solution, and pyrolizing the shaped particles of the material to form a functionally graded porous nanostructure. 
     In a further embodiment, the shaped particles of the material may be pyrolized to substantially remove the impurity resulting in hollow shaped particles of the material. 
     Additionally, the impurity may comprise a polymer with carboxylated surface functionalization or some other surface functionalization for causing a desired effect and the shaped particles may comprise hollow spheres. In one embodiment, the shaped particles may be spherical and/or hollow spheres. In addition, the material may comprise carbon or any other suitable material. A surface functionalization for causing any desired effect as would be understood by one of skill in the art may be chosen, depending on the desired effect. 
     According to one example, EPD was used, to form a gradient in polystyrene particle concentration in an organic sol before gelation of the organic sol. The gradient is preserved during gelation of the organic sol and subsequent carbonization to form a graded density carbon aerogel. The particle concentration gradient concurrently forms a density gradient in the carbon aerogel as the polystyrene is removed, leaving a gradient in void space as well. 
     In this example, an organic sol was prepared by combining 7.5 g water, 6.125 g resorcinol, 9 g formaldehyde, and 220 μL acetic acid. 5 mL of 10 vol % 920 nm polystyrene particles with carboxylated surface functionalization was added to the solution to form a suspension, which was placed in an EPD chamber. An electric field of 10 V/cm was applied to the suspension for 15 minutes. The entire EPD assembly was sealed and incubated at 85° C. overnight to gel. After gelation, the sample was washed in acetone and dried in air at room temperature. Carbonization of the sample was performed at 1050° C. in nitrogen. The final product was 20 mm×20 mm×3.6 mm (L×W×H) and had an estimated density greater than 243 mg/cc. 
     SEM analysis of this sample shows a gradient density aerogel having hollow carbon spheres with a range of packing densities depending upon film depth in a deposition (film thickness) direction. Transition from low to high density is relatively gradual compared to coarse deposition techniques described earlier. 
     As the embodiments described herein aptly demonstrate, the EPD methods and structures formed through the EPD methods disclosed herein, according to various embodiments, may be used for any number of novel materials and structures. 
     According to some embodiments, the methods and structures described herein may be used for hydrogen or other gas storage (such as for fuel cell technology), targets for capturing and analyzing thrown particles, capacitors, sensors, catalysis, filtering, water purification, and batteries, among other uses. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.