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iMechanica Journal Club (November 2013): Nano-architected Nanolattice Structures

      In a set of recently published papers, the first in Nature Materials and the second in Advanced Engineering Materials , we investigate methods for designing, fabricating, and testing a new class of nano-architected lattice structures, which we refer to as nanolattices. These nanolattices are comprised of hollow tubes with lengths on the order of microns, diameters on the order of hundreds of nanometers to microns, and wall thicknesses on the order of tens to hundreds of nanometers. During the fabrication, we have a high degree of control over the geometry of the structure, the dimensions of the tubes (both the diameter and the wall thickness), and the overall size of the structure. Depending on the constituent material being used, there is the potential to tune the thickness of the tubes in a way such that a material size effect can be exploited.  There are a number of different geometries that have been designed and are currently being tested, and they can be seen here. The successful fabrication and testing of architected nanolattice structures on this scale, and with this degree of fidelity, constitutes a breakthrough in materials engineering. Using advanced two-photon lithography techniques to make materials with high structural strength in combination with materials size-effects, we have begun to reach into an entirely new material parameter space.


Motivation
      In recent years, there has been a great deal of interest in creating new classes of strong and lightweight materials for advanced engineering applications. It has long been understood that the internal structural landscape of a material, i.e. the order or the disorder, is the major contributing factor to the materials’ overall strength and stiffness. In a collaboration between the Greer group at Caltech, HRL Laboratories, and the Valdevit group at UC Irvine, a paper was published in Science in 2011 on the study of a new class of architected microscale metallic lattice structures (microlattices).  These microlattices were shown to have many novel mechanical properties like ultra-light weight and remarkable recoverability. This work constituted a major development in materials engineering, as no structural materials at that point were able to replicate the same degree of control and precision in the architecture. The commercial manufacturing of microlattices and investigations into their applications is currently continuing to be pursued by HRL Laboratories, researchers at UC Irvine, and a new company, Architected Materials, LLC. The microlattice work was also an inspiration for our current work on nanolattices.


      In addition to creating materials with novel architectures, over the past few decades there has been a major push by materials scientists to understand size-induced material properties. Specifically, it has been observed that as sample dimensions are reduced, a number of continuum mechanical properties of materials begin to break down. Materials such as single crystalline metals and ceramics demonstrate large increases in strength at small scales, and brittle materials like metallic glasses and silicon exhibit ductility at small scales. For materials with a microstructural landscape, like polycrystalline metals, there is often a weakening effect seen as the sample sizes begin to approach the grain size of a material. Many of these phenomena are well known and their mechanisms are still being investigated, but few have been truly utilized in real engineering applications due to fabrication limitations.


      The novelty and promise of the nanolattice work in the Greer group at Caltech comes from our ability to take advantage of both structural and materials size effects in a single structure. For the first time, we are able to control material architecture on incredibly small length scales and use that in combination with our knowledge of size-affected material properties to create truly remarkable new classes of materials. In this first round of work with nanolattices, we have been able to create structures that have constituent materials with strengths on the order of 2 GPa, close to their theoretical strength limit, and the overall strength of the structures has been similar to the highest performance foams to date. These results were obtained without any structural or material optimization. As we continue to explore the new parameter spaces that we can reach with our nanolattice structures, we are continually discovering new and promising results, with some notable examples including ultra-high strength lightweight materials and ductile ceramic structures.


Fabrication
The nanolattice structures that we have published in our recent work are made in a multi-step fabrication process, described in detail here:

  • First we create a polymer scaffold for our structures using a technique known as two-photon lithography (TPL) direct laser writing (DLW). Using a commercial machine made by Nanoscribe GmbH , we use an infrared laser to write three-dimensional structures into a photopolymer in a process similar to 3D printing. The resolution of our structures using TPL DLW is down to 150nm, which is roughly 3 orders of magnitude smaller than even the best commercial 3D printers. Additionally, the writing technique allows us to create 3D structures without the 2D rastering that is required for most 3D printers, so we can create much higher quality 3D structures.
  • Second, we use thin film deposition techniques, like sputtering, atomic layer deposition (ALD), chemical vapor deposition (CVD), etc., to deposit a thin layer of a material onto our polymer scaffold. Here, we have control over the thickness of the coating down to the nanometer level. At this point, the solid structures already have many interesting applications (which will be discussed in the next section), but the composite polymer/material solid structures are less interesting for structural applications because they are not yet ultra-lightweight and they are not able to make proper use of material size-effects.
  • Finally, we use a focused ion beam (FIB) to selectively expose the polymer and then use an O2 plasma etch to remove the polymer. This leaves behind a lattice structure with unit cells composed of hollow tubes. Because we use a 3D printing process, we can create essentially any geometry that we are interested in studying. Additionally, there is the possibility of using double inversion techniques to make negative structures or multilayering the materials to create bulk thin-film composites.

Potential Impact and Future Directions
      The work that we have published in our first two papers focuses on using these materials for structural applications (high strength, lightweight materials). If this work is able to be commercialized, it holds huge implications for the area of lightweight structural materials. Imagine a lightweight insulating foam-like material, for example, in a spacecraft, which could also serve a function as a load bearing material. For the first time, a small-scale size effect in combination with controllable material architecture could be properly utilized in a high-tech engineering application, creating new classes of high-strength lightweight materials


      Given the incredible degree of control that we have on the material architecture and properties, it is limiting to simply think of the nanolattices only in terms of structural materials applications. We have begun to explore a number of alternate avenues in which we can use these nanolattice structures for advanced engineering applications.

  • Strain tunable photonic crystals: The Nanoscribe machine that we are using to create these nanolattices was originally designed for applications in photonics. That is because structures can be made on length scales on the same order of magnitude as the wavelength of light, thereby allowing for photonic bandgaps to be created. These materials have interesting applications as solar cells because it would be possible to trap more incident light on a solar cell, thereby improving efficiency. Because we can create lattice structures with high strength and controllable mechanical properties, it is theoretically possible to create a photonic material whose bandgap properties could be changed with the amount of applied strain, thereby allowing for tunable photonic crystals.
  • Low volume expansion batteries: One of the major limitations of silicon lithium ion batteries is the crack formation that happens as a result of the drastic volume increase when silicon is lithiated.  If the mechanical properties of these silicon/lithium systems could be tuned so that catastrophic failure did not occur upon lithiation, these systems would have a great potential for outperforming current batteries. By creating an architected structure that makes use of the size enhanced ductility of silicon and has a novel structural architecture that can bend and twist, allowing for much lower volume expansion and induced stress, it should be possible to use silicon as a viable material in battery applications.
  • Low thermal conductivity materials: Because we are able to create structures with low densities that are made from a wide range of materials, it is possible to create a low thermal conductivity material with high mechanical strength. Currently, the ultralow thermal conductivity materials that exist, like aerogels, have very poor mechanical strength due to the stochastic nature of the unit cells and the deformation mechanism of the structure, so there is an interesting potential application for low thermal conductivity high strength materials.
  • Thermoelectric devices: There are currently a number of methods for creating materials with very high thermoelectric performance, for example thin film superlattices and quantum wires, but they involve processes that are either incredibly time consuming or ones that cannot create materials large enough for real engineering applications. By using nanolattices as a scaffold on which to pattern thermoelectric materials, it should be possible to create a bulk thermoelectric material that makes use of the novel small-scale thermoelectric properties that have been discovered and incorporates them into a practical engineering material.
  • Biological cell scaffolding: The small scale at which we create nanolattice structures is on the same order of magnitude as biological cells. By coating the nanolattices with biocompatible materials, it is possible to create a structure on which we can grow cells. Because the mechanical properties of the lattice structure can be well characterized and understood, it is possible to then use them as a means to study cell mechanics and better understand the way cells move and interact with their surroundings.


Note for Mass Production
      Currently, the process we use to make nanolattices is with a rapid-prototyping technique, which is never conducive to large scale fabrication. There are more advanced techniques, like single step TPL using photomasks, that could be used to create similar structures with a high degree of fidelity, and these will hopefully eventually provide a route to mass production of these advanced materials. In the scope of our research, it is desired to retain the flexibility of a rapid-prototyping process so that we can explore a wider range of potential applications. As a result, we have not yet looked into mass production of these materials, but it is an interesting avenue for future research.

Acknowledgements
      This work on nanolattice structures was done in the Greer group at Caltech. The main contributors to the work shown here are Lucas Meza, Lauren Montemayor, Dongchan Jang, and Julia Greer (PI/Professor). There are a number of other members of the group, Wendy Gu, Satyajit Das, Victoria Chernow, Chen Xu, Ottman Tertuliano, and Alessandro Maggi, that are also working on projects involving nanolattices and have contributed greatly to the process. We are currently collaborating with a number of other groups on the Caltech campus on projects involving the nanotrusses, including the Minnich group , the Kochmann group , and the Ortiz group . We are also working with the Dionne group at Stanford, along with a few groups at UCSF.

Additional Information
The papers that have been referenced in this post can be found here:

D. Jang, L.R. Meza, H.F. Greer, and J.R. Greer, "Fabrication and Deformation of Hollow Ceramic Nanolattice Structures", Nature Materials, 12, 893-898 (2013)

 

L.C. Montemayor, L.R. Meza, and J.R. Greer, "Design and Fabrication of Hollow Rigid Nanolattices via Two-photon Lithography", Advanced Engineering Materials (2013)

T.A. Schaedler, A.J. Jacobsen, A. Torrents, A.E. Sorenson, J. Lian, J.R. Greer, L. Valdevit, W.B. Carter "Ultralight Mettalic Microlattices", Science 334, 962-965 (2011)

Additional information about nanolattices and other projects we are working on in the Greer group at Caltech can be found at our website here .

 

Comments

Xuanhe Zhao's picture

Hi Lucas,

Thank you for posting this interesting entry on nanolattice structures. I was at the 2013 IMECE and attended the Koiter Lecture given by Prof. Norman Fleck. He discussed nanolattices with gyroid topology potentially capable of large-scale fabrication. It seems the fracture toughness of nanolattice may be relatively low due to relatively small yielding zones around crak tip, despite their various merits. Can you comment on the fracture toughness of your metalic and ceramic nanolattice structures? Can you also compare the nanolattices fabricated using your method with others, in terms of manufactring and properties? Thank you.

Best,

Xuanhe

 

Hi Xuanhe,

Thank you for beginning the discussion. I have seen some of the work done by Norman Fleck on gyroid structures, and it is indeed very interesting. I think, in addition to the potential for large scale fabrication, one of the most exciting things they have found is that the constituent materials appear to reach very near theoretical strengths. This would be due to their single crystalline nature in combination with their small scale, which I believe is on the order of nanometers, possibly tens of nanometers. I would be careful in calling them nanolattices though, because they are very different from our structures. 

On that note, to answer your second question, our fabrication method is very different than any traditional nanofabrication technique. Up to now, most three-dimensional nanomaterials have needed to be fabricated with either self-assembly techniques or by multilayering 2D nanofabricated structures. Self-assembly is the most promising of the two because it allows for the potential of large area fabrication. For example, I believe the gyroid structures are self-assembled in a matrix and then the matrix material is dissolved away to allow them to be freestanding.

Our technique uses two-photon lithography (TPL) direct laser writing (DLW) in a technique that is much more akin to a rapid-prototyping than traditional nanofabrication. We write our structures into a photopolymer using TPL DLW to make a nanoscale polymer scaffold. From there, we can coat onto and dissolve out the polymer to create free-standing hollow lattices. We have a much greater degree of control over the size and the geometry of our structures than any existing nanofabrication technique, mainly because we are essentially rapid prototyping our parts. This gives us a much greater flexibility to study the effect of different materials and geometries. The downside of our method is that it is very slow, so it is not practical for large scale applications, only small testing samples. 

With regard to your first question, the fracture toughness of the nanolattices is a very interesting pursuit that we plan to investigate soon. The fracture toughness of cellular solids generally scales with the square root of the unit cell size. This means a smaller unit cell would be weaker, and our nanolattices would not be good model materials for higher fracture strength. However, we do see an increase in the strength of the constituent material as we decrease the wall thickness. The most important question that will come into play is whether or not we can use the benefit of a size effect to outweigh the detriment of shrinking the unit cell size. Because we are still in the early phases of understanding the results of our systems, this is not a question we have been able to fully answer yet.

With regard to the material, metals have much higher fracture strength than ceramics to begin with, so we would assume a metal truss will have a higher fracture strength than a ceramic one. However, metals and ceramics have different scaling laws due to different size-affected deformation mechanisms. Therefore, the potential gain (or loss) for a metal vs a ceramic nanolattice may be very different, and both of them will likely be investigated during the course of our research.

Thank you,

Lucas

MGRashed's picture

Hi,

 

Is there any development on metallic nanolattices? HRL Labs made headlines in 2011 with metallic microlattices, Did your group attempted at metals on nanoscale? 

Yes, we are well underway with our work on metallic nanolattices. The second paper that we published (in Advanced Engineering Materials) was on the fabrication of metallic trusses (link here), which, in this case, were made from gold. We use sputtering to fabricate the trusses, which is different than the method HRL had used (which was electroless plating followed by electroplating) because our structures are on a much smaller length scale. We have already begun mechanical testing, and the preliminary results are very promising. My colleague, Lauren Montemayor, is the main person working on the metallic nanotrusses, and she will be presenting her work so far at the 2013 MRS Fall meeting in Boston next week on Monday.

Rashid K. Abu Al-Rub's picture

Dear Lucas,

Thank you very much for sharing with the community your knowledge about nano-architectured materials. I was wondering if your manufacturing technique can be used to create geometric features that are of arbitrary shapes and not just truss-type elements (e.g., curved structural elements).

Thanks,

Rashid

Hello Rashid,

Yes, using this technique it is possible to create geometric features that are made of arbitrary shapes. Up to this point we have been focusing on studying different truss geometries, so we haven't looked much into different shaped structural elements. There have been groups who have used two-photon lithography to made helical structures for use as photonic crystals (http://onlinelibrary.wiley.com/doi/10.1002/adma.200901601/full), and some that have made cone shaped elements for use as pentamode materials (http://scitation.aip.org/content/aip/journal/apl/100/19/10.1063/1.4709436). Essentially, the writing process is analogous to 3D printing, so any geometry truss structure or structural element you want, you can make. It's simply a matter of designing it properly.

 

-Lucas

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