Metalation/De-metalation as a Post-Gelation Strategy to Tune the Mechanical Properties of Catenane-Crosslinked Gels

Mechanically interlocked molecules (MIMs) possess unique architectures and non-traditional degrees of freedom that arise from well-defined topologies that are achieved through precise mechanical bonding. Incorporation of MIMs into materials can thus provide an avenue to discover new and emergent macroscale properties. Here, the synthesis of a phenanthroline-based [2]catenane crosslinker and its incorporation into polyacrylate organogels is described. Specifically, Cu(I) metalation and de-metalation was used as a post-gelation strategy to tune the mechanical properties of a gel by controlling the conformational motions of integrated MIMs. The organogels were prepared via thermally initiated free radical polymerization, and Cu(I) metal was added in MeOH to pre-treated, swollen gels. De-metalation of the gels was achieved by adding cyanide salts and washing the gels. Changes in Young’s and shear moduli, as well as tensile strength, were quantified through oscillatory shear rheology and tensile testing. The reported approach provides a general method for post-gelation tuning of mechanical properties using metals and well-defined catenane topologies as part of a network architecture.

Cold Gas Spray of Copper on Aluminum Nitride as Substrate for Power Electronics

Ceramic substrates for electronic packaging of high-power applications are growing in demand due to their robustness as power and thermal requirements increase. Aluminum nitride (AlN) has excellent thermal and electrical properties with copper currently being bonded to AlN via a direct bond copper (DBC) technique. However, substrates fabricated by DBC are subjected to thermo-mechanical fatigue during fabrication processes and power cycling. DBC substrate’s reliability is negatively affected by the large mismatch in coefficient of thermal expansion that hinders the possibility of thicker substrates, therefore limiting its use for applications above 20 kV. This work employed cold gas spraying (CGS) to mechanically bond Cu on AlN. CGS is a low-temperature additive manufacturing method that accelerates powder particles at near-supersonic velocities to impact a surface causing plastic deformation and mechanical bonding. On ceramic-metal systems CGS has not been widely studied owing to ceramics’ inability to deform plastically, therefore, surface functionalization was performed to enhance the mechanical interlocking mechanism. A factorial design of experiments (DOE) was used to assess the effect of factors: temperature, pressure, stand-off distance, angle of deposition, and travel speed on various substrate surfaces in the CGS fabrication process. These experiments resulted in a successful deposition of copper on AlN.

Impact of Al2O3 Particle Size on the Open Porosity of Ni/Al2O3 Composites Prepared by the Thermal Oxidation at Moderate Temperatures

The performance of attractive Ni-based composites can be affected by changing their microstructures, e.g., introducing pores. Here, we report a novel, relatively low-cost process to fabricate Ni/Al2O3 composites with open porosity modified by the size of Al2O3 particles. The mixture of powders was subjected to thermal oxidation twice in air after a maximal temperature of 800 °C was reached in a stepwise manner and maintained for 120 min. The oxidation kinetics were determined thermogravimetrically. The open porosity was evaluated by an Archimedes’ principle-based method. Localization and quantification of NiO, newly formed on the Ni particle surface and acting as a mechanical bonding agent, were explored by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffractometry. Larger ceramic particles prevented merging of NiO layers on adjacent Ni particles more efficiently; therefore, the open porosity increased from 21% to 24.2% when the Al2O3 particle diameter was increased from 5–20 µm to 32–45 µm. Because both Ni/Al2O3 composites exhibited similar flexural strength, the composite with larger Al2O3 particles and the higher open porosity could be a better candidate for infiltration by molten metal, or it can be directly used in a variety of filtration applications.

Design of Microfabricated Mechanically Interlocking Metamaterials for Reworkable Heterogeneous Integration

Next–generation interconnects utilizing mechanically interlocking structures enable permanent and reworkable joints between microelectronic devices. Mechanical metamaterials, specifically dry adhesives, are an active area of research which allows for the joining of objects without traditional fasteners or adhesives, and in the case of chip integration, without solder. This paper focuses on reworkable joints that enable chips to be removed from their substrates to support reusable device prototyping and packaging, creating the possibility for eventual pick-and-place mechanical bonding of chips with no additional bonding steps required. Analytical models are presented and are verified through Finite Element Analysis (FEA) assuming pure elastic behavior. Sliding contact conditions in FEA simplify consideration of several design variations but contribute ~10% uncertainty relative to experiment, analysis, and point-loaded FEA. Two designs are presented; arrays of flat cantilevers have a bond strength of 6.3 kPa, and non-flat cantilevers have a strength of 29 kPa. Interlocking designs present self-aligning in-plane forces that emerge from translational perturbation from perfect alignment. Stresses exceeding the material yield stress during adhesion operations present a greater concern for repeatable operation of compliant interlocking joints and will require further study quantifying and accommodating plastic deformation. Designs joining a rigid array with a complementary compliant cantilever array preserve the condition of reworkability for the surface presenting the rigid array. Eventual realization of interconnect technology based on this study will provide a great improvement of functionality and adaptability in heterogeneous integration and microdevice packaging.

Electrochemical Corrosion Behavior of Pure Mg Processed by Powder Metallurgy

Pure Mg samples were prepared by powder metallurgy using the cold and hot compacting methods. Cold compacted pure Mg (500 MPa/RT) was characterized by 5% porosity and the mechanical bonding of powder particles. Hot compacted samples (100 MPa/400 °C and 500 MPa/400 °C) exhibited porosity below 0.5%, and diffusion bonding combined with mechanical bonding played a role in material compaction. The prepared pure Mg samples and wrought pure Mg were subjected to corrosion tests using electrochemical impedance spectroscopy. Similar material corrosion behavior was observed for the samples compacted at 500 MPa/RT and 100 MPa/400 °C; however, hot compacted samples processed at 500 MPa/400 °C exhibited longer corrosion resistance in 0.9% NaCl solution. The difference in corrosion behavior was mainly related to the different binding mechanisms of the powder particles. Cold compacted samples were characterized by a more pronounced corrosion attack and the creation of a porous layer of corrosion products. Hot compacted samples prepared at 500 MPa/400 °C were characterized by uniform corrosion and the absence of a layer of corrosion products on the specimen surface. Powder-based cold compacted samples exhibited lower corrosion resistance compared to the wrought pure Mg, while the corrosion behavior of the hot compacted samples prepared at 500 MPa/400 °C was similar to that of wrought material.

Geochemistry, Mineralogy and Textural Properties of the Lower Globigerina Limestone Used in the Built Heritage

The Lower Globigerina Limestone Member, the oldest member of the Globigerina Limestone Formation, outcrops over most of the Maltese archipelago, notably Malta. It has provided the islands’ main building material since the Neolithic period. This paper makes available a corpus of findings relating to the geochemistry, mineralogy and textural properties of this limestone—mostly unpublished and undertaken nearly three decades ago—which provide a useful source to understand its behavior. Bulk chemistry and mineralogy showed that non-carbonate and clay content is higher in limestone of inferior quality. Textural analyses gave insight into the fabric of the matrix, including inter- and intra-particle porosity. These analyses were supplemented by an array of petrophysical tests, including color (a parameter which has a correlation with density and Fe2O3 content), ultrasonic pulse velocity and compressive strength. The findings not only give insight into the composition of the limestone, using insoluble residue content of ≥5% as the threshold of inferior quality lithotype, but provided an insight into the physico-mechanical bonding present, a characteristic which has a bearing on the deterioration of this limestone.

Copper-Tantalum Metal Matrix Composites Consolidated from Powder Blends by Severe Plastic Deformation

We investigated the effectiveness of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of metal powders into metal matrix composites. Equal volumes of copper (Cu) and tantalum (Ta) powders were consolidated at ambient temperature via different ECAE routes. Composites processed by ECAE routes 4E and 4Bc were also processed at 300 °C. The resulting materials were characterized by scanning electron microscopy (SEM) and compression testing. Processing by route 4Bc at 300 °C resulted in the highest compressive strength, lowest anisotropy, and least strain rate sensitivity. We conclude that the superior properties achieved by this route arise from mechanical bonding due to interlocking Cu and Ta phases as well as enhanced metallurgical bonds from contact of pristine metal surfaces when the material is sheared along orthogonal planes.

Effect of Friction Pressure on Microstructure and Properties of Friction Welded Joints of Pure Aluminum/304 Stainless Steel

Continuous drive friction welding was used to realize the high quality connection between pure aluminum and 304 stainless steel. The composition of interface micro-zone and mechanical properties of joint were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), tensile test and hardness test. The formation mechanism of intermetallic compound (IMC) during friction welding was discussed. The results show that under the experimental parameters, the joint surface is uneven and two intermetallic compounds, Fe2Al5 and FeAl3, are formed. With the increase of friction pressure, the mechanical bonding degree of the joint decreases, the metallurgical bonding degree increases, the element diffusion distance increases from 1.4 to 1.9 um, the tensile strength of the joint can reach or even higher than that of the base metal on the aluminum side, and the maximum hardness increased from 414 HV to 447 HV.