Ultralight Nanolattices with Co-Optimized Mechanical, Thermal, and Optical Properties

Existing porous materials have low material density and excellent thermal properties. However, owing to construction from random architecture, they do not possess favorable scalability in mechanical strength and have poor optical clarity. This research aims to develop a novel class of ultralight, multifunctional, crystalline cellular material with co-optimized mechanical, thermal, and optical properties. The proposed material consists of tubular nanolattices constructed systematically from hollow-core elements in a 3- dimensional (3D) periodic array, as depicted in Figure 1. In contrary to existing cellular materials, the lattice parameters (period, feature sizes, tube shell thickness, etc.) are precisely prescribed, allowing for material properties to be directly tailored. This approach enables the ability to simultaneously design properties across multiple physical domains (density, strength, thermal conductivity, optical clarity, etc.). The mechanical, thermal, and optical properties of such material will be modeled, and an algorithm to optimize geometric parameters will be developed. This work is enabled by a novel fabrication process utilizing 3D colloidal phase lithography (CPL) and atomic layer deposition (ALD), which will enable to control structures parameters with nm-level precision. Advances in the proposed research will help address the challenge of “Materials: Lightweight Structure (TA 12.1.1)” in the NASA Space Technology Roadmaps, as it enables low-density material design with multiple functionalities. Preliminary experiments will focus on demonstration of ultralight, impact-absorbing, transparent thermal insulator for multifunctional windows. 
Under this project we have successfully develop a top down and bottom up fabrication process for nanolattice films with a variety of materials, submicron periodicity, extremely thin shells, and multiple functionality. An illustrative structure is shown in Figure 2, where Al2O3 nanolattice material with 6 nm shells, 125 nm shell radius, and 500 nm lattice period has been demonstrated. The process can enable multiple materials, and we have demonstrated Al2O3, ZnO, Pt, and W. Shell thicknesses down to 2 nm have also be demonstrated without systematic structure collapse. Such a thin film exhibit ordered porosity in the 96% range, and have demonstrated unique mechanical and optical properties. The develop large-area process has enable prototype in the cm range, and can be scaled for continuous manufacturing setting. The fabricated nanolattice material has been extensively studied using nanoindentation (Figure 3), and has demonstrated exciting mechanical properties. First the scaling of the material is significantly better than random porous aerogels. In these experiments, homogenous Al2O3 shells with thicknesses of 4 to 40 nm and ZnO shells with thicknesses of 30 to 94 nm were tested, achieving porosity of 71 to 20% and 96 to 74 %, respectively. It can be observed that these preliminary results have illustrated a stiffness scaling n ~ 1.1, which is significantly better than biological cellular structures (n ~ 3-4) and comparable to enhanced properties recently demonstrated in much larger microlattice structures (n = 1.1). The structure also demonstrated better hardness scaling n ~ 1.5 than the lowest reported value in literature (n = 1.76). image with residual indentation imprint, showing the failure mechanism of the nanolattice film. The structure also has enhanced energy dissipation properties. The non-linear nanoindentation response in the later loading-unloading cycles results from elastic-plastic deformation of the nanolattice film. During unloading, the elastic work done is released by partial recovery of the nanolattice, but the plastic work done will be dissipated through localized, permanent deformations of the structure. As expected, the thicker structures dissipate higher energy before undergoing yielding. Specific energy dissipation for the 4 nm Al2O3 nanolattice sample was 36.3 kJ/kg at average effective strain lower than 10%. This specific energy dissipation is much larger than the previously reported polymer foams (0.15 kJ/kg at 21 kg/m3 density) and ordered polymer frames (7.67 kJ/kg at 600 kg/m3 density). In addition, the enhanced energy dissipation by the film was also achieved at lower total strain and density. The energy dissipation capacity ranged from 36.3 – 166.5 kJ/kg and 21.5 – 48.2 kJ/kg for Al2O3 and ZnO nanolattices, respectively. The film also has enhanced hardness. These results indicate that thicker nanolattices can have potential applications as impact absorbing films that can be broadly applied to a variety of surfaces. Beyond mechanical properties, the nanolattice material also exhibits novel optical properties. Using spectroscopic ellipsometry, we have examined the optical refractive index for ZnO and Al2O3 nanolattice material. Here the measured refractive indices for alumina nanolattice materials with 2, 6, 9, 16, and 19 nm thicknesses are recorded. The thinnest 2 nm-thick sample exhibits n ~ 1.025 from the visible to infrared. The index is close to air, and significantly less than those of bulk alumina (n ~ 1.6-1.7). These materials behave similar to air, while being mechanically stable to support structures. The sample is over 97.5% porous, and has the lowest index reported for porous oxide materials to date. The material thickness and lattice geometry can also be tuned to precisely control the index, filling in the air-gap index range between 1 and 1.3 where no natural material exist. Such materials can be integrated into multilayer devices to enhance photonic trapping and low-dielectric material. We have also performed broadband transmission measurements to examine scattering, as shown in Figure 7. The specular and diffused scattering transmissions are characterized using a spectrophotometer with an integrating sphere. The nanolattice materials with 15 nm and 20 nm shells on glass samples show slight thin-film interference effects, which leads to enhanced transmission in certain wavelength ranges. The scattering of the nanolattice material is generally low in the visible and near-infrared range, but peaks to around 6% at around 320 nm in the UV. The scattering is dominated by diffraction effects from the lattice geometry, which can be seen in the scattering angular profiles shown in the inset diagram. Here it can be observed that for 325 nm scattering a peak exists at around 48º, corresponding to the diffraction ring radius, while the 633 nm scattering decreases monotonically. It is important to note that the scattering angular profile differs significantly from Rayleigh scattering seen in random porous materials, where scattering is independent of angle. Such angle-independent scattering leads to hazy appearance, which is suppressed in the fabricated samples. Here a nanolattice sample is placed next to a bare glass slide, and both samples can be observed to have broadband clarity.