Jessica’s study on mechanical properties of nanocrystalline MgAl2O4 is published at Acta Materialia! Many congratulations! This works used nanoindentation, microvillar compression, and micro-cantilever bending extensively to investigate the grain size effect on hardenss, yield strength, and fracture toughness. The significant improvement in fracture toughness when the grain size becomes smaller than 10.5nm provides a great promise to develop a high toughness ceramic! Wonderful works!

Jessica M. Maita, Sarshad Rommel, Jacob R. Davis, James A. Wollmershauser, Edward P. Gorzkowski, Boris N. Feigelson, Mark Aindow, Seok-Woo Lee, “Grain size dependence of mechanical properties of nanocrystalline magnesium aluminate MgAl2O4,” Acta Materialia, in press (2023) [PDF][web]

 

 

Shuyang and Zhongyuan gave a presentation at the TMS 2023 at San Diego, CA!

• Zhongyuan Li: Micro-mechanical characterization on amorphous carbon and its nanoporous structures (Poster Presentation)
• Shuyang Xiao: Multi-stage superelasticity in SrNi2P2 intermetallic compound via lattice collapse and expansion and the influence of cryogenic temperature (Oral presentation)

 

 

Zhongyuan’s paper was just published at Materials & Design! Many Congratulations! This work demonstrates how to develop a nano-composite material that can absorb and release the large elastic strain energy.

Zhongyuan Li, Jinlong He, Nikhil Tiwale, Keith J. Dusoe, Chang-Yong Nam, Ying Li, Seok-Woo Lee, “Unraveling the ultrahigh modulus of resilience of sequential-infiltrated core-shell polymer nanocomposite nanopillars,” Materials & Design, 227, 111770 (2023) [PDF][web]

Abstract: Modulus of resilience, the maximum strain energy density that can be stored in an elastically deformed solid, is an important mechanical property for developing artificial muscles in robotics, soft electronics panels, and micro-/nano-electromechanical actuators. In this study, core–shell SU-8 nanocomposites were fabricated via vapor-phase infiltration of nanoscale amorphous aluminum oxides into SU-8 nanopillars and performed transmission electron microscopy, nanomechanical testing, analytical modeling, and atomistic simulations to gain a fundamental insight into the ultrahigh modulus of resilience much higher than that of most high-strength materials. This study shows that the ultrahigh modulus of resilience results from: the low aspect ratio of amorphous aluminum oxide nano-particulates; the particulate size thicker than the free volume size; and the thin aluminum oxide interconnecting links within nano-particulates. These unique microstructural features produce the unusual combination of low specific Young’s modulus, 4 MPa/(kg/m³), and high specific yield strength, 0.2 MPa/(kg/m³), leading to the specific modulus of resilience, 5.21 ± 0.39 kJ/kg about ten times higher than materials with the similar yield strength. This study demonstrates that vapor-phase infiltration is an excellent fabrication method to produce a polymer nanocomposite that can absorb and release a large amount of elastic strain energy.

Zack passed the qualifying exam! Many Congratulations! Yeah!

Zack is going to study the deformation and fracture mechanisms of nanocrystalline ceramics.

Zhongyuan and Shuyang attended the MRS Fall 2022!

 

Multi-Stage Superelasticity in SrNi₂P₂ Intermetallic Compound via Lattice Collapse and Expansion and the Influence of Cryogenic Temperature

Shuyang Xiao, Vladislav Borisov, Adrian Valadani, Guilherme Gorgen-Lesseux, Roser Valenti, Paul Canfield, Seok-Woo Lee

Abstract: Superelastic crystalline solids are able to recover their original shape even after the large amount of deformation is applied. The shape recovery of most superelastic crystalline solids usually occurs through a reversible phase transition between martensitic and austenitic phase. Although this shear process provides up to ~10% of elastic strain, there are several issues that limit their elastic performance. Usually, this shear process requires high activation temperature to reverse the phase transition. So, it is difficult to exhibit superelasticity at cryogenic temperature. Also, the shear process often drives dislocations to move and causes large accumulation of dislocations, which eventually prevent the reversal of phase transition. Therefore, it is desirable to seek out a new class of superelastic material that can overcome these issues through a completely different superelasticity mechanism. In this presentation, we report our first discovery of the giant superlelasticity in a novel intermetallic compound SrNi₂P₂ via a unique phase transition process, lattice collapse-expansion and the influence of cryogenic temperature.

SrNi₂P₂ single crystals were grown using solution-growth technique, and in-situ uniaxial compression and tension tests on [001]-oriented SrNi₂P₂ micropillars, which were fabricated using focused-ion-beam milling, were conducted at room temperature, 200K, and 100K. Room temperature mechanical testing revealed the giant superelasiticity with a remarkably huge elastic strain limit near 20% if both compression and tension are considered. Up to our knowledge, this giant elastic strain corresponds to one of the highest elastic strain limits ever reported for crystalline solids. This excellent superelastic performance is achieved through multiple-stage lattice collapse and expansion under compression and tension by forming and breaking P-P bonds in two co-existing different crystal structures in SrNi₂P₂. Based on Density Function Theory (DFT) calculation and High-Resolution Transmission Electron Microscope (HRTEM), P-P bonds are formed or broken partially depending on the stress state, leading to the multi-stage lattice collapse and expansion. Fraction of P-P bonds seems to be controlled primarily by entropy under a given stress, so the total number of P-P bonds is determined by thermodynamics under a give stress and temperature. Unlike shear process of martensite-austenite transition, no dislocation is involved in the superelasticity process of SrNi₂P₂. Simple making and breaking bond process seems to guarantee ultrahigh fatigue life with presumably nearly infinite number of cycles. Our state-of-the-art in-situ cryogenic nanomechanical testing revealed that forming and breaking P-P bonds is strongly sensitive to cryogenic temperature. At lower temperature, phase transition becomes easier under compression but more difficult under tension. This difference can be explained by thermal contraction. Due to the thermal contraction at lower temperature, which reduces P-P distance, making P-P bonds becomes easier, but breaking P-P bonds becomes more difficult. One noticeable result is that even under cryogenic environment (~100K), SrNi₂P₂ still exhibit superelasticity, implying that SrNi₂P₂ is capable of shape recovery at very low temperature unlike conventional superelastic crystalline solids such as Ni-Ti intermetallic compounds. Our experimental and computational results provide a fundamental insight into understanding the superelasticity mechanisms of SrNi₂P_2. Furthermore, our work suggests its strong potential as a cryogenic superelastic material that could be useful to develop an impact resistance material for aerospace engineering under cryogenic environment.

 

Micro-Mechanical Characterization on Amorphous Carbon and Its Nanoporous Structures

Zhongyuan Li, Ayush Bhardwaj, James Watkins, Seok-Woo Lee

Abstract: Carbon-related nanostructured materials have a strong potential as a mechanical reinforcement material due to their strong ionic/covalent C-C bond, which restricts the motion of dislocation, the carrier of plasticity. However, their brittleness and relatively high density limits their structural application. In this study, therefore, we have developed amorphous carbon with almost no short-range ordering and its nanoporous structures, where the ligand thickness is only around 10nm, by rapid thermal annealing of Brush block copolymer and Phenol formaldehyde. To evaluate the mechanical response, we perform the nanoindentation tests and in-situ compression tests on a micropillar, which was fabricated by utilizing focused-ion-beam milling. Both fully dense and nanoporous structures exhibit a large compressive fracture strain up to ~40% with a significantly high work hardening rate. Raman spectroscopy before and after compression test revealed that this unusual plasticity results from dynamic change in distribution of sp2 and sp3 bonds. During plastic deformation, the number of sp2 bonds decreases but the number of sp3 bonds increases. Carbon atoms, which have sp2 bonds, seem to be reconfigured to form sp3 bond, which is thermodynamically preferable under compression. In addition, we found that both compressive fracture strength and compressive maximum strength are nearly independent of porosity unlike the hardness that scales with the porosity. We think that the porosity-independent strength could be related to the size-affected strength of nanoscale ligand. Larger pores reduce the density, but the size reduction of ligand enhances its strength. These two contributions compensate each other, leading to the negligible change in strength of the entire nanoporous structure. Interestingly, all structures, both fully dense and nanoporous structures, in our study demonstrate the modulus of resilience much higher than most engineering materials due to their low Young’s modulus, implying that amorphous carbon materials have an exceptional capability to absorb and release the mechanical energy per a given volume. However, yield strength of our fully dense amorphous carbon was found to be slightly lower than that of amorphous carbon published in other works. Complete diffuse diffraction pattern indicates that our amorphous carbon possesses the almost complete random arrangement of carbon. This liquid-like structure could be beneficial to lower mass density but does not seem to be desirable to obtain high strength. It would be necessary to control the distribution of a short-range ordering to obtain the improved yield strength without sacrificing the mass density and the ductility much by optimizaing the synthesis condition or performing the post thermal-mechanical treatment. All these efforts will pave a new pathway to create light, strong, and tough carbon nanostructures for their future structural applications.

 

Zhongyuan passed his PhD proposal defense! Many Congratulations!

Presentation title: In-situ mechanical characterization of hybrid organic-inorganic nanocomposites

Abstract: Engineering the modulus of resilience, the measure of a material’s ability to store and release elastic strain energy, is difficult because it requires asymmetrically increasing yield strength and Young’s modulus against their mutual scaling behavior. This task becomes further challenging if it needs to be carried out at the nanoscale such as for micro-/nano-electro-mechanical-systems (MEMS). Polymer based nanocomposite material is promising but it is difficult to introduce a large content filler without aggregation. To overcome these issues, we synthesized AlOx-reinforced SU-8 nanocomposite nanopillars by the vapor phase infiltration (VPI) method. According to the TEM analysis, the AlOx could infiltrate about 50 nm depth beneath the surface with uniform distribution and therefore, our nanocomposite stays as a core-shell structure. To analyze the mechanical performance of our nanocomposite, micropillar compression testing at different strain rates was implemented. Our preliminary results show that AlOx/SU-8 nanocomposite exhibit an ultra-high modulus of resilience (~15 MJ/m3) and the strain rate has weak effects on the resilience property of our composites. To quantitively understand the high performance of our nanocomposite, we utilize analytical modeling and molecular dynamics simulation method to explore the mechanism. The results show that the ultrahigh modulus of resilience originates from the low aspect ratio shape of amorphous aluminum oxide nano-particulates, the full occupation of these nano-particulates in the free volume of the infiltrated layer, and the extremely thin aluminum oxide link emanating from nano-particulates. The strain rate affects Young’s modulus more than the yield strength, which results from the relatively low cross-link degree of the SU-8 matrix, leading to a weak effect on the modulus of resilience.

Our experimental and numerical results demonstrate that vapor-phase infiltration is an excellent compositization method to produce a polymer nanocomposite that can absorb and release a significantly large amount of elastic strain energy. However, the infiltration depth of AlOx is only ~50 nm, which strongly limits the reinforcement efficiency. And our recent study shows that ZnOx fillers could fully infiltrate inside the SU-8 micropillar.

In this proposal, therefore, we propose to investigate the mechanical properties of ZnOx-reinforced and InOx-reinforced SU-8 nanocomposites and try to identify the infiltration mechanism of inorganic fillers to advance the VPI synthesis method. Since the rapid thermal annealing manufacturing process could crystallize the ZnOx fillers, we also plan to analyze whether filler crystallization could influence the modulus of resilience properties of the nanocomposite. Research outcomes will eventually allow us to develop high-performance polymer nanocomposites with the high modulus of resilience, which will be greatly useful to create high-power artificial muscles in robotics, soft electronics panels, and energy-efficient micro-/nano-electro-mechanical actuators.

Kyle finishes his Master degree in Lee’s group, and he will start his new engineering role in US Army in Kuwait! We hope that he enjoys his new work in a new place and that we see again once he finishes his duty!

Kyle passed his Master (Plan A) defense! Many Congratulations. In his presentation, Kyle has discussed “Micro-mechanical characterization of additive manufactured metals”. His projects (cold spray and wire-arc-additive-manufacture) have been been supported by US Army Research Laboratory.

 

Zhongyuan participated in the GRC ceramics and gave a poster presentation!

Title of poster: Micromechanical characterization on amorphous carbon and its nonporous structure.

(Bryan, Karla, Seok-Woo, Zhongyuan, Luis): Prof. Bryan Huey’s group were there together!

Abstract:

Micro-mechanical characterization on amorphous carbon and its nanoporous structures

Zhongyuan Li^1,*, Ayush Bhardwaj², James Watkins², Seok-Woo Lee¹

1. Department of Materials Science and Engineering & Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, United States
2. Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA

Carbon materials, such as diamond, graphite, and their related structures, have been often classified as a ceramic due to their high melting point, high hardness, and high brittleness. Carbon materials has a strong potential as a structural material due to their strong ionic/covalent C-C bond, but the brittleness of most carbon materials have limited their structural applications. In this study, we have developed amorphous carbon and its nanoporous structures with 40, 50, and 60% of porosity by rapid thermal annealing of Brush block copolymer and have investigated their plasticity and fracture behaviors. To evaluate the mechanical response, we performed nanoindentation and in-situ compressive tests on a micropillar, which was fabricated by utilizing focused-ion-beam milling. Both fully dense and nanoporous structures exhibit a large compressive fracture strain up to ~40% with a high work hardening rate. This exceptional fracture strain could result from dynamic re-distribution of sp2 and sp3 bonds during plastic deformation. In addition, we found that compressive fracture strength is nearly independent of porosity unlike the indentation hardness that scales with the porosity. We think that the porosity-independent fracture strength could be related to the size-affected strength of nanoscale ligands. The size reduction of ligand at the nanoscale could enhance its fracture strength, leading to the negligible decrease in fracture strength of the entire nanoporous structure. Also, we found that all samples demonstrate the excellent modulus of resilience the orders of magnitude higher than most engineering materials due to their low Young’s modulus, implying that amorphous carbon materials have a strong capability to absorb and release the mechanical energy. However, the yield strength of our fully dense amorphous carbon (1.5 GPa) is much lower than that of amorphous carbon that includes tetra-type C-C bonds (20 GPa). Lack of the halo ring pattern in TEM diffraction pattern indicates that our amorphous carbon possesses the random atomic arrangement even for the nearest neighbor carbons. This fully liquid-like atomic arrangement does not seem to be desirable for the yield strength but to be greatly beneficial for the ductility. It would be necessary to control the degree of a short-range C-C ordering or the population of tetra-type C-C bond to enhance the yield strength without sacrificing the ductility through the optimization of synthesis conditions or the post thermal and mechanical treatments.

Kyle gave a poster presentation at CSAT 2022. Kyle’s poster discussed micro-mechanical characterization of Ta cold spray deposits. He used nanoindentation, spherical indentation, and microvillar compression to investigate the spatial variation of mechanical properties of Ta.

Title of poser: “Micromechanical Characterization of Cold Sprayed Tantalum Powders Using Nanoindentation and Ex-situ Microvillar Compression”