Joining of Silver Nanoparticles: Computer Simulations and Experimental Observations

Abstract This dissertation introduced a new type of high aspect ratio silver nanoparticle, synthesized by assembly and joining of hexagonal and triangular nanoplates. The synthesis procedure is a simple mixing at room temperature of silver nitrate solution with ascorbic acid and PMAA solution. After mixing, the synthesis starts from reduction of silver ions, leading to nucleation of silver crystals. Growth of the nucleated crystals in presence of capping agent produces thin hexagonal and triangular nanoplates. These nanoplates assemble and join to make one-dimensional segments. This process continue through assembly of the segments to form one- or two-dimensional supercrystals, the configuration of which is controlled by experimental condition such as silver content of the reactor, nitrate ion content, and pH. The introduced synthesis method answers one of the major questions that motivated this research: Finding a simple, feasible, high-yield, low-cost, and mass producible high aspect ratio silver nanoparticles for using as nanofiller in hybrid electrical conductive adhesives and for various other industrial applications. SEM observation reveals that final thickness of the synthesized nanoparticles are around 25 nm, while the other sides are bigger, even up to 100 µm length nanobelts. TEM and XRD demonstrate that the synthesized nanostructures exhibit a (111) crystal texture on their broad surface, inherited from their nanoplatelet structural blocks. One- and two-dimensional silver nanoparticles with (111) surface plane on this scale is a unique material that has not reported before this research. Molecular dynamics simulation reveals that the nanoplates join together to produce a perfect crystal at the formed joint. The simulation results are confirmed by high-resolution TEM observation. This is another important feature of the synthesized nanocrystals that provide unique properties such as good electrical conductivity. Another important property of the synthesized material is high thermal stability, which originates from the (111) crystal texture surfaces being composed of low-energy and high-stability atoms, compared to the surfaces of other silver nanoparticles. This stability is confirmed by molecular dynamics simulation and experimentation, with comparison to pentagonal silver nanowires with (100) surface crystals. This high thermodynamic stability of the synthesized silver nanoparticles raises the working temperature limit of silver nanoparticles by several hundred degrees over that of the competing nanoparticles. In addition to application of the synthesized 1-D high aspect ratio silver nano particles as nanofiller for enhancing electrical properties and reducing the cost of the hybrid electrical conductive adhesive, the synthesized 2-D silver nanosheets were employed as the primary functional material to fabricate an airflow sensor. The fabricated sensor was examined for detecting low airflows (below 5 ml/min). The sensor shows a linear and repeatable response to airflow in the range of experiment. Simple sensor fabrication method, high electrical stability and excellent atmospheric corrosion resistance of silver nanosheets at working temperature of the sensor (120˚C) are other outstanding features of the synthesized silver nanostructures that make them good candidate for future research. Looking at the thermal behaviour of silver nanoparticles a traditional question is raised about the hypothesized existence of a liquid layer on the surface of silver nanoparticles, called “surface premelting” in the literatures. Molecular dynamic simulation was employed to investigate the phenomenon. This thermodynamic investigation uncovered that by increasing temperature, thermal phonons become potent enough to overcome the cohesive energy of the surface atoms, throwing atoms from their lattice positions to create stable surface defects. Accumulation of defects, which is temperature dependent, is able to create a solid disordered-phase layer covering the nanoparticle surface. Existence of this disordered layer was confirmed in the literature, although it is interpreted as liquid layer in some reports. Investigation into the physical state of this disordered surface layer uncovered that it exists as an amorphous solid, which remains solid until melting temperature, and that the melting of the nanoparticle happens at a constant temperature: a usual melting based on classical thermodynamics. This finding contradicts the predicted behavior of the literature’s “surface pre-melting” phenomena, providing an alternative answer to a traditional question that has been asked in this field of science since the 1940s

A Numerical Study on the Powder Flowability, Spreadability, Packing Fraction in Powder Bed Additive Manufacturing

The powder bed fusion (PBF) process is widely adopted in many manufacturing industries because of its capability to 3D print complex parts with micro-scale precision. In PBF process, a thermal energy source is used to selectively fuse powder particles layer by layer to build a part. The build quality in the PBF process primarily depends on the thermal energy deposition and properties of the powder bed. Powder flowability, powder spreading, and packing fraction are key factors that determine the properties of a powder bed. Therefore, the study of these process parameters is essential to better understand the PBF process. In our study, we developed a two-dimensional powder bed model using the granular package of the LAMMPS molecular dynamics simulator. Cloud-based deposition was adopted for pouring powder particles on the powder bed. The spreading of particles over the substrate was mimicked like a powder bed system. The powder flowability in the proposed study was analyzed by varying the particle size distribution. The simulation results showed that a greater number of larger particles in a power sample results in an increase in the Angle of Repose (AOR) which ultimately affects the flowability. Two different kinds of recoater geometry were considered in this study: circular and rectangular blades. Simulation results showed that depending on the recoater shape there is a change in the packing fraction in the powder bed. Cross-sectional analysis of the power bed showed a significant presence of voids when a greater number of larger particles existed in the powder batch. The packing fraction of the powder bed was found to be a strong function of particle size distribution. These analyses help understand the influence of particle size and recoater shape on the powder bed properties. Findings from this study help to provide a guideline for choosing particle size distribution if the spherical particles are considered. While the present study focuses on the spherical powder particles, the proposed system can also be adapted to the study of powder bed with aspherical particles

Characterization of single- and multilayer cold-spray coating of Zn on AZ31B

Zinc, a soft material with a low melting point and high corrosion resistance, was coated onto AZ31B Mg alloy using different cold spraying process parameters. The physical and mechanical properties of the resulting Zn/AZ31B samples were then investigated to explore the effect of the process parameters on the microstructural and mechanical characteristics. The results obtained via X-ray diffraction show the formation of an intermetallic material at the interface of Zn/AZ31B even at low process temperatures. In addition, spherical droplets of Zn were observed at the surface, confirming the partial melting of Zn particles during the impact. This partial melting is believed to lead to the formation of intermetallic compounds during solidification. To engineer the residual stress induced in the cold spraying process, a thin layer of dense Zn was then used as an intermediate layer before coating with Al7075, forming a multilayered surface of Al7075/Zn/AZ31B. Because of the higher thermal expansion coefficient of Zn compared with those of Al7075 and AZ31B, beneficial compressive residual stress could be created in all three layers of this novel multilayer deposition. Without the Zn interlayer, Al7075/AZ31B under the same coating parameters exhibited undesirable tensile residual stress in the substrate.Financial support through funds from Natural Sciences and Engineering Research Council of Canada (NSERC) RTI program under EQPEQ458441-2014 grant, and NSERC through APC under APCPJ 459269–13 grant are gratefully acknowledged

Application Of FBG Optical Sensors To In-Situ Monitoring The Thermo-Mechanical Behaviour Of Cold Spray Coated Samples

In this research, a fiber Bragg grating (FBG) sensor is employed for monitoring thermal and mechanical strain induced by severe plastic deformation during high thermal and mechanical strain rate of cold spray technique. The FBG sensors are embedded in magnesium alloy substrates and the strain evolutions of the substrates are recorded during the cold gas spray coating process. In these experiments, the localized transient thermo-mechanical strain induced in the close vicinity of the substrate surface is monitored. Qualitative analysis of the complicated spectra shapes obtained during coating and cooling processes demonstrates the repeatability and sensitivity of the sensors in this condition. In addition, the obtained result from FBG sensors reveals the existence of compressive strain in the substrate near the interface during peening; however, it is released after a few second because of the high impact temperature of cold spray coating

Role of heat balance on the microstructure evolution of cold spray coated AZ31B with AA7075

A promising solid-state coating mechanism based on the cold spray technique provides highly advantageous conditions on thermal-sensitive magnesium alloys. To study the effect of heat balance in cold spray coating on microstructure, experiments were designed to successfully coat AA7075 on AZ31B with two different heat balance conditions to yield a coated sample with tensile residual stress and a sample with compressive residual stress in both coating and substrate. The effects of coating temperature on the microstructure of magnesium alloy and the interfaces of coated samples were then analyzed by SEM, EBSD, TEM in high- and low-heat input coating conditions. The interface of the AA7075 coating and magnesium alloy substrate under both conditions consists of a narrow-band layer with very fine grains, followed by columnar grains of magnesium that have grown perpendicular to the interface. At higher temperatures, this layer became wider. No intermetallic phase was detected at the interface under either condition. It is shown that the microstructure of the substrate was affected by coating temperature, leading to stress relief, dynamic recrystallization and even dynamic grain growth of magnesium under high temperature. Reducing the heat input and increasing the heat transfer decreased microstructural changes in the substrate.Natural Sciences and Engineering Research Council of Canad

On the measurement of relative powder-bed compaction density in powder-bed additive manufacturing processes

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.matdes.2018.06.030 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Experimental studies in the literature have identified the powder-bed compaction density as an important parameter, governing the quality of additively manufactured parts. For example, in laser powder-bed fusion (LPBF), the powder-bed compaction density directly affects the effective powder thermal conductivity and consequently the temperature distribution in melt pool. In this study, this physical parameter in a LPBF build compartment is measured using a new methodology. A UV curable polymer is used to bind powder-bed particles at various locations on the powder-bed compartment when Hastelloy X was used. The samples are then scanned using a nano-computing tomography (CT) system at high resolution to obtain an estimation of the relative powder-bed compaction density. It is concluded that due to the interaction between the recoater and the variation in the powder volume accumulated ahead of the recoater across the build compartment, the relative powder-bed compaction density decreases along the recoater moving direction (from 66.4% to 52.4%.). This variation in the powder-bed compaction density affects the density and surface roughness of the final printed parts that is also investigated. Results show that the part density and surface quality decrease ~0.25% and ~20%, respectively, along the build bed in direction of the recoater motion.Natural Sciences and Engineering Research Council of Canada (NSERC) Federal Economic Development Agency for Southern Ontario (FedDev Ontario

Nanocasting Synthesis of Ultrafine WO3 Nanoparticles for Gas Sensing Applications

Ultrafine WO3 nanoparticles were synthesized by nanocasting route, using mesoporous SiO2 as a template. BET measurements showed a specific surface area of 700 m 2/gr for synthesized SiO2, while after impregnation and template removal, this area was reduced to 43 m 2/gr for WO3 nanoparticles. HRTEM results showed single crystalline nanoparticles with average particle size of about 5 nm possessing a monoclinic structure, which is the favorite crystal structure for gas sensing applications. Gas sensor was fabricated by deposition of WO3 nanoparticles between electrodes via low frequency AC electrophoretic deposition. Gas sensing measurements showed that this material has a high sensitivity to very low concentrations of NO2 at 250°C and 300°C

Influence of Pore Formation and Its Role on the Tensile Properties of 17-4 PH Stainless Steel Fabricated by Laser Powder Bed Fusion

Additive manufacturing (AM) is a promising technique due to the scope of producing complex objects from a digital model, where materials are deposited in the successive layers as distinct from the conventional manufacturing approaches. In this study, laser powder bed fusion (LPBF), a class of additive manufacturing (AM), is used to make testing samples with gas atomized 17-4 PH stainless steel (SS) powder at different process parameters in argon (Ar) environment. A thorough study on powder characteristics, such as particle size distribution, powder morphology, phase formation at different atmospheres, as well as the microstructure and tensile properties of the printed parts at various energy densities were carried out. The microstructural analysis discovered the presence of columnar dendrites with complete martensite phases regardless of the process parameters. A detailed X-ray computed tomography (CT) scan analysis on printed samples explored the correlation between the pores and energy density. The sample printed with adequate energy density obtained lower porosity (volume of pores: 2 × 104 to 9 × 104 µm3, compared to 2 × 104 to 130 × 104 µm3) resulting in maximum tensile strength and elongation of 770 MPa and 38%, respectively. Therefore, it is obvious that the quantity, size and shape of pores in the printed parts significantly affect the fracture mode