Jittisa Ketkaew

Pulsed Laser Beam Welding of Pd 43 Cu 27 Ni 10 P 20 Bulk Metallic Glass

We used pulsed laser beam welding method to join Pd43Cu27Ni10P20 (at.%) bulk metallic glass and characterized the properties of the joint. Fusion zone and heat-affected zone in the weld joint can be maintained completely amorphous as confirmed by X-ray diffraction and differential scanning calorimetry. No visible defects were observed in the weld joint. Nanoindentation and bend tests were carried out to determine the mechanical properties of the weld joint. Fusion zone and heat-affected zone exhibit very similar elastic moduli and hardness when compared to the base material, and the weld joint shows high ductility in bending which is accomplished through the operation of multiple shear bands. Our results reveal that pulsed laser beam welding under appropriate processing parameters provides a practical viable method to join bulk metallic glasses.

Three-Dimensional Compatible Sacrificial Nanoimprint Lithography for Tuning the Wettability of Thermoplastic Materials

We report the tuning of surface wetting through sacrificial nanoimprint lithography. In this process, grown ZnO nanomaterials are transferred by imprint into a metallic glass and an elastomeric material, and then etched to impart controlled surface roughness. This process increases the hydrophilicity and hydrophobicity of both surfaces, the Pt57.5Cu14.7Ni5.3P22.5 metallic glass and thermoplastic elastomer, respectively. The growth conditions of the ZnO change the characteristic length scale of the roughness, which in turn alters the properties of the patterned surface. The novelty of this approach includes reusability of templates and that it is able to create superhydrophilic and superhydrophobic surfaces in a manner compatible with the fabrication of macroscopic 3D parts. Because the wettability is achieved by only modifying topography, without using any chemical surface modifiers, the prepared surfaces are relatively more durable. *Corresponding Author Jonathan P. Singer jonathan.singer@rutgers.edu Mechanical and Aerospace Engineering 98 Brett Road Piscataway, NJ 08854 1.0 Introduction Wettability is a functional property of solids which is important from the aspect of fundamental physics and practical applications. Controlling wettability triggers a plethora of emerging applications ranging from household self-cleaning surfaces to advanced fields, including smart filters [1], corrosion resistance materials [2], and anti-fogging surfaces [3]. Comprehensive reviews on incongruous wetting behavior (including full wetting and antiwetting) and applications of special wettable materials can be found in Refs [4-7]. To understand the physics of wetting (both statics and dynamics) thoroughly, 2D surfaces are sufficient. However, for real-world engineering applications such as filtration [8,9], anti-icing [10,11], condensation control [12,13], and biomimetic water striding [14,15], it is crucial to induce special wettability in 3D monoliths. Few studies demonstrated superhydrophobic bulk materials [16,17], but the scope of thermoplastic materials as a 3D superhydrophobic monolith is less explored. Although surface chemistry and surface roughness can control the surface wettability individually, dual-effect of chemistry and roughness is essential to render a surface superhydrophobic or superhydrophilic. High-surface energy materials such as metals and semiconductors facilitate extreme wetting. Conversely, polymeric materials are suitable for making superhydrophobic surfaces because of their low surface energy and facile fabrication of single and dual-scale roughness that maintains a heterogenous wetting state [18]. According to Wenzel’s hypothesis, hydrophilic metals and hydrophobic polymers can be turned into superhydrophilic and superhydrophobic, respectively through topographic modification [18-23]. Numerous techniques that are developed for microand nanostructures fabrication, including molding lithography [24-29], etching [30,31], phase separation [32,33], diffusion-limited growth [34,35], and self-assembly [36,37], have been utilized for making superhydrophobic surfaces. JMNM-18-1027, Singer, 2 These methods are less compatible for patterning 3D objects, however, and as a result, it is difficult to render superhydrophobicity in 3D materials. Therefore, scalable surface texturing methods are of great interest to the functionalization of these surfaces. Nanoimprinting has emerged as an effective route to change the wettability of materials because it is simple and easy to control the surface topography [38-40]. In this method, first, nanostructures generated via some other lithographic process on a master template are transferred to desired materials’ surfaces through either a thermoplastic-, etching-, or photoresinbased forming. Second, the surfaces are optionally modified with low-surface energy materials. These low-surface energy materials can enhance the hydrophobicity of a surface; however, their effectiveness is reduced substantially due to low chemical resistance and physical resistance over time [41]. Moreover, these materials are usually toxic and expensive, which also limits the practical applications of superhydrophobic surfaces. For instance, low-surface energy fluorocarbon materials are contaminants because of their decomposition into perfluorooctanoic acid [42], which is potentially toxic to humans [43,44]. Therefore, rendering superhydrophobicity tuning merely the topography of materials has attracted tremendous interests recently [45,46]. We recently introduced sacrificial nanoimprint lithography (SNIL) of hydrothermally-grown ZnO nanostructures as a means to impart nanostructures to thermoplastically-formed surfaces [28]. Hydrothermal synthesis is a chemical process that permits the seeded growth of ZnO nanostructures on various substrates at low temperature (~ 90 C) [47-49]. This method allows growing nanostructures on a large area with various morphologies by controlling the growth conditions, e.g., seed layer, growth solution composition, and temperature. The seed layer precursor for growing ZnO nanostructures is often nanoparticles; however, dense nanostructures JMNM-18-1027, Singer, 3 can also be grown on other metallic substrates without using any additional treatment [50,51]. One attractive feature of this latter property is that brass and aluminum, both common mold materials for thermoplastic forming, can be used to grow ZnO [28,52]. In SNIL, these structures are simultaneously imprinted and transferred to the molded material during thermoplastic forming and separation. As ZnO is amphoteric, it can then be easily removed in mild acid or base. In our previous work, we demonstrated the wide variety of morphologies possible on even 3D microstructures of thermoplastically-formed metallic glass (MG) created by the SNIL process [28]. In this study, we investigate the use of ZnO SNIL to modify the surface wetting of two different types of thermoplastic materials: (1) MG and (2) thermoplastic elastomer (TPE), through SNIL. MGs have emerged as a representative metal to study the topography-wetting correlation of high energy materials [53-59]. Previous studies focus on hydrophilic to hydrophobic transition, but the potential of MGs used as a superhydrophilic materials has not been investigated, and here we show that SNIL can induce superhydrophilicity in MGs. TPEs are widely used for a range of thermoplastically-formed consumer goods, and we also show that this method is also adaptive to tune the wettability of these polymeric materials. Importantly, we demonstrate that this method is scalable to make large 3D superhydrophobic parts. 2.0 Experimental Procedure Nanoimprint lithography is a top-down and high-throughput surface patterning method in which the surface morphology of a template is replicated oppositely into a pliable material through embossing (hot or cold). In this study, we have used ZnO nanostructures as a sacrificial template, enabling the SNIL fabrication of nanostructures in different thermoplastic materials. JMNM-18-1027, Singer, 4 2.1 Making Template with ZnO Nanostructures The Pt-based MG samples (hereafter referred as Pt-MG) employed in this study were the same structures as used in Ref. [60]. Their synthesis is described in that manuscript. For TPE studies, ZnO nanostructures were grown on aluminum templates by hydrothermal method (Fig. 1a). First, an aluminum template was cleaned ultrasonically with acetone, ethanol, and deionized water, in that order. Then, the template was etched by 0.1 M KOH solution for 0-40 minutes depending on the desired roughness of the template surface. Afterward, it was washed by a stream of deionized water and dried in air. Second, the growth medium was prepared by dissolving 0.00625 M zinc acetate dihydrate (Sigma-Aldrich, >98%) and 0.025 M hexamethylenetetramine (HMTA) (Sigma-Aldrich, >98%) in a solution of deionized water. Finally, to grow the ZnO nanostructures, the template was attached to a glass slide right-sidedown using adhesive and submerged in a vial containing the growth solution. Subsequently, the vial was immersed in a water bath held at 90 C for 30 minutes. The glass slide supported the template by leaning against the sidewall of the vial, resulting in no precipitation on the template. Then, the immersed template was removed from the growth solution, rinsed with DI water to get rid of any residual materials, and dried under ambient conditions. Fig. 1b (i, ii) shows scanning electron microscope (SEM) images of the aluminum template before and after the growth of ZnO nanostructures. The growth of ZnO nanostructures in a substrate for a fixed precursor-base (zinc acetateHMTA in this study) pair can be controlled by the surface roughness of template [61]. Therefore, we roughened aluminum templates by dipping those in 0.1M KOH solution for various times and grew the ZnO nanostructures. Etching created pits on aluminum; these pits restricted the free JMNM-18-1027, Singer, 5

Multiscale patterning of a metallic glass using sacrificial imprint lithography

Bulk metallic glasses (BMGs) have been developed as a means to achieve durable multiscale, nanotextured surfaces with desirable properties dictated by topography for a multitude of applications. One barrier to this achievement is the lack of a bridging technique between macroscale thermoplastic forming and nanoimprint lithography, which arises from the difficulty and cost of generating controlled nanostructures on complex geometries using conventional top-down approaches. This difficulty is compounded by the necessary destruction of any resulting reentrant structures during rigid demolding. We have developed a generalized method to overcome this limitation by sacrificial template imprinting using zinc oxide (ZnO) nanostructures. It is established that such structures can be grown inexpensively and quickly with tunable morphologies on a wide variety of substrates out of solution, which we exploit to generate the nanoscale portion of the multiscale pattern through this bottom-up approach. In this way, we achieve metallic structures that simultaneously demonstrate features from the macroscale down to the nanoscale, requiring only the top-down fabrication of macro/microstructured molds. Upon detachment of the formed part from the multiscale molds, the ZnO remains embedded in the surface and can be removed by etching in mild conditions to both regenerate the mold and render the surface of the BMGs nanoporous. The ability to pattern metallic surfaces in a single step on length scales from centimeters down to nanometers is a critical step toward fabricating devices with complex shapes that rely on multiscale topography for their intended functions, such as biomedical and electrochemical applications. Biomedical and optical devices stand to benefit from a multiscale patterning technique developed by researchers in the USA. Bulk metallic glasses (BMGs) are extremely strong and corrosion-resistant alloys that can be thermoplastically molded with features spanning centimeter to nanometer dimensions. The generation of multiscale molds with conventional lithography, however, requires several costly steps. To simplify BMG patterning from the bottom-up, Chinedum Osuji from Yale University and co-workers from Yale and Rutgers grew tunably packed nanowires and nanosheets of zinc oxide (ZnO) directly onto mold surfaces. When a BMG is formed then detached from this modified mold, the nanostructures embed themselves into the metallic surface. A subsequent mild etching procedure removes the ZnO and gives the BMG a nanoporous surface—an economical route to biomimetic textures for applications ranging from biomanipulation to fuel cells.