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

Behaviour of Magnesium Alloy Under Load-Control Cyclic Testing

The characterization of material fatigue behaviour is crucial in the design of mechanical components. Magnesium alloys, particularly wrought magnesium alloys, are one of the most interesting materials for mechanical components due to their superior physical and mechanical properties and have, therefore, been studied extensively. Primarily, evaluations of the fatigue characteristics of wrought magnesium alloys have been conducted under cyclic stress- and strain-control conditions. However, many engineering components are subjected to cyclic bending moments or load-control cyclic conditions. These conditions can be provided by a rotating bending test (RBT), which is usually employed to extract stress versus number of cycles until failure of the sample (S-N curve). On the other hand, the S-N curve is not sufficient to evaluate cyclic deformations of the material, especially for low-cycle fatigue regimes. To estimate the fatigue behaviour of the material, it is necessary to obtain stress and strain responses of the material under different range of bending moments in load-control condition. To date, however, the evolution of hysteresis curves during rotating bending experiments for asymmetric materials such as magnesium alloys has not been reported in the literature. In the present study, for the first time, Fiber Bragg Grating (FBG) sensors were employed to measure the strain on AZ31B extrusion samples during RBT. First, the accuracy of strain measurement by the FBG sensor in the elastic and plastic regimes was investigated by comparing the FBG measured strain with that obtained by a standard extensometer during uniaxial pull-push test. Results confirmed that the FBG sensor was able to measure the strain up to 1.3%, accurately. In the next step, the strains of the specimens were measured under different bending moments of 2.5, 5, 6.5 Nm, during RBT. Based on these experiments, the strain amplitude of the samples was decreased with increasing the number of cycles in plastic regime until the sample was stabilized, however the strain response of the material under bending moment of 2.5 (in elastic regime) remained constant during the test. To analyze applied stresses to the sample, particularly in the plastic deformation range, the proposed Variable Material Property (VMP) method was employed. In this model, the RBT was simulated by a fixed beam problem that was under alternative loading, and the stress-strain responses of the sample were predicted. Using this simulation method, the hysteresis loops of two critical top and bottom elements of the sample cross-section under different bending moments were obtained. Finally, the strain of the sample during rotating bending, which was measured by the embedded FBG sensor, was replaced with the strains obtained from the modeling by fitting a mapping function. The hysteresis obtained from combination of the modeling results and FBG experiments was relatively precise. To verify this claim, the stresses of the hysteresis loops were compared with the stresses obtained from the proposed strain-control pull-push tests in which, the strain on the sample was adjusted to be equal to the strain measured by the FBG sensor. The results of this test demonstrated that the stresses of the combined VMP and FBG hysteresis loops have good compatibility with the stress responses of the experiment. Therefore, the accuracy of the cyclic deformation hysteresis obtained using modeling and experimental results were confirmed under load-control conditions. Moreover, the fatigue life of the sample was calculated using the obtained hysteresis curve based on the energy approach. The predicted fatigue life of the AZ31B extrusion samples was in good agreement with published experimental data

The effect of cold spray coating parameters on the residual stress and fatigue performance of magnesium alloys

Global warming, dwindling fossil fuels, and the low efficiency of alternative energy sources such as batteries and solar panels compared to traditional carbon-based energy sources, have shifted the industrial design paradigm towards minimizing structural weight. In this scenario, employing low density grade metals is inevitable. Magnesium (Mg) alloys, such as AZ31B, are one of the lightest structural materials, and have been able to gain strong foothold in the industrial world due to their high “strength to weight ratio”. However, the long-term performance of parts manufactured from Mg alloys are required for reliable structural applications. Hence, research on the methods to enhance the fatigue properties of Mg alloys is crucial. Surface treatment, particularly surface coating of manufactured parts with a thin layer of high fatigue strength material is considered a versatile approach to address fatigue shortcomings of Mg alloys. Cold gas dynamic spray is an emerging technology that deposit metal powder with a supersonic velocity on substrate in a solid-state. The nature of this technology (peening) promotes compressive residual stress and results in a refined grain structure, which reinforces a material’s surface and prolong crack initiation, thus enhancing the fatigue performance of materials like AZ31B alloy. Among all potential candidates for coating Mg parts, aluminum alloys are one of the best, due to their low density and strong fatigue performance. Hence, in this research, cold spraying is employed to deposit a thin layer of Al7075 on AZ31B-H24 Mg substrates in order to improve their fatigue performance. The complexity of the cold spray process requires some in-depth study through in situ measurements. For this, fiber Bragg grating (FBG) sensors is employed for monitoring and assessing the thermo-mechanical behaviour of Mg alloy during cold-spray coating. Furthermore, the hole drilling, X-ray diffraction and in situ FBG sensors residual stress measurement techniques are used to explore the effect of processing parameters and thermal mismatch on the residual stress distribution of coated Mg samples. In addition, SEM, TEM, CT scanning tomography, micro indentation hardness, surface topography and roughness measurements are utilized to identify the physical and microstructural characteristics of the coating, interface and substrate. Based on the findings of this research, the common understanding that cold spray induced residual stresses are compressive is questioned. It is shown that the heat associated with the spraying, although much lower than melting temperature, can wash out the peening effect and result in tensile residual stress in the substrate. Hence, to customize the residual stress and for inducing compressive residual in the Mg substrate, the cold spray coating parameters were tuned to decrease the heat input to the substrate. Moreover, a cooling system was designed that increased heat transfer from the substrate during the coating process. Another major factor affecting the state of the residual stress is the thermal mismatch of the coating and the substrate materials. The detrimental effect of thermal mismatch between the Al7075 coating and AZ31B substrate on the residual stress of substrate and coating was addressed by adding an interlayer of zinc, which has a higher thermal expansion coefficient than the substrate and coating material. Zinc is successfully deposited, and its coating parameters were selected in a way that resulted in inducing compressive residual stress in the substrate, coating and the zinc interlayer. However, detail characterization of the zinc and AZ31B substrate reveals that intermetallic phases have formed at the interface. Therefore, despite the induced compressive residual stress, cracking at the zinc magnesium interface restricts the application of this interlayer. Two extreme coating conditions that lead to induced tensile and compressive residual stress in the Mg substrate have been selected for the rest of this research. The quality of the coating is examined by CT-scan, which demonstrates that the compressive samples have less porosity than the tensile samples, although the densities of both samples are above 99%. The physical and mechanical properties of the compressive samples, including hardness and surface roughness, have also been significantly improved compared to tensile samples. Finally, the fatigue performance of the two types of coated samples (tensile and compressive) is investigated revealing that the compressive samples demonstrate exceptional fatigue life in high cycle regime compared to the bare AZ31B samples with 130% life enhancement

A New Framework to Estimate Breathing Rate from Electrocardiogram, Photoplethysmogram, and Blood Pressure Signals

Breathing Rate (BR) is a key physiological parameter measured in a wide range of clinical settings. However, it is still widely measured manually. In this paper, a novel framework is proposed to estimate the BR from an electrocardiogram (ECG), a photoplethysmogram (PPG), or a blood pressure (BP) signal. The framework uses Empirical Mode Decomposition (EMD) and Discrete Wavelet Transform (DWT) methods to extract respiratory signals, taking advantage of both time and frequency domain information. An Extended Kalman Filter (EKF), incorporating a Signal Quality Index (SQI), enabled our method to achieve acceptable performance even for significantly distorted periods of the signals. Using state vector fusion, the output signals are combined and finally the BR is estimated. The framework was tested on two publicly available clinical databases: the MIT-BIH Polysomnographic and BIDMC databases. Performance was evaluated using the mean absolute percentage error (MAPE). The results indicated high accuracy: MAPEs on the two databases of 3.9% and 3.6% for ECG signals, 6.0% for PPG, and 5.0% for BP signals. The results also indicated high robustness to noise down to 0dB. Therefore, this framework may have utility for BR monitoring in high noise settings

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

Enhanced Sliding Mode Wheel Slip Controller for Heavy Goods Vehicles

This paper introduces an improved version of a sliding mode slip controller for pneumatic brake system ofheavy goods vehicles, HGVs. Using the Fast Actuating Brake Valve, FABV, allows to adopt advance control approaches forwheel-slip controllers which provide features such as fast dynamic response, stability and robustness. In this paper, the slidingmode algorithm which was developed for the speed dependent wheel slip control using the FABV hardware is analysed andimproved. The asymptotic convergence properties of the control algorithm are proven using Lyapunov stability theory and therobustness of the method is investigate

Structural optimization of MacPherson control arm under fatigue loading

U ovom istraživanju izvršena je optimizacija topologije i oblika MacPherson upravljačke poluge u svrhu postizanja manje težina. Postojeće automobilsko tržište traži dijelove niske cijene i male težine, a za potrebe energetski učinkovitog, a jeftinog vozila. To zahtijeva učinkovitiju uporabu materijala za automobilske dijelove koji mogu dovesti do smanjene mase vozila. Budući da su automobilski dijelovi izloženi dinamičkim opterećenjima koja uzrokuju oštećenja zbog zamora, uzimanje u obzir kriterija zamora čini se bitnim u dizajniranju automobilskih dijelova. Kako bi se stvorili što teži uvjeti opterećenja upravljačke poluge, najprije su spektralnom gustoćom snage generirane neravne ceste. Zatim su, dinamičkom analizom karoserije kompletnog modela vozila, dobiveni najkritičniji uvjeti opterećenja. Nakon toga, izvršena je optimizacija topologije pomoću kriterija vijeka trajanja do zamora primjenom HyperMesh softvera, što je rezultiralo smanjenjem mase od 50 %. U sljedećem koraku kreiran je CAD model primjenom CATIA softvera i provedena optimizacija oblika kako bi se dobile točne dimenzije s manje mase.In this research, the topology and shape optimization of a MacPherson control arm has been accomplished to achieve lighter weight. Present automotive market demands low cost and light weight component to meet the need of fuel efficient and cost effective vehicle. This in turn gives the rise to more effective use of materials for automotive parts which can reduce the mass of vehicle. Since automotive components are under dynamic loads which cause fatigue damage, considering fatigue criteria seems to be essential in designing automotive components. At first, in order to create severe loading condition for control arm some rough roads are generated through power spectral density. Then, the most critical loading conditions are obtained through multi body dynamics analysis of a full vehicle model. Then, the topology optimization is performed based on fatigue life criterion using HyperMesh software, which resulted in 50 % mass reduction. In the next step a CAD model is created using CATIA software and shape optimization is performed to achieve accurate dimensions with less mass

Development of fuzzy anti-roll bar controller for improving vehicle stability

The main objective of this paper is to develop active control mechanism based on fuzzy logic controller (FLC) for improving vehicle path following, roll and handling performances simultaneously. At the first stage, 3DOF vehicle model which includes yaw rate, lateral velocity (lateral dynamic) and roll angle (roll dynamic) are developed. The controller produces optimal moment to increase stability and roll margin of vehicle by receiving the steering angle as an input and vehicle variables as a feedback signal. The effectiveness of proposed controller and vehicle model are evaluated during fishhook and single lane-change maneuvers. Simulation results demonstrate that FLC by reducing roll angle, lateral velocity and acceleration, vehicle roll resistance and handling properties are improved. Finally the sensitivity and robustness analysis of developed controller for varying longitudinal speeds are investigated

Application Of Fiber Bragg Grating Sensor For Strain Measurement At The Notch Tip Under Cyclic Loading

Notches are inevitable in many components and structures due to design limitations. In addition, they are the locations for stress concentration and are susceptible to fatigue failure. As a result, the cyclic stress/strain response at a notch is of key importance. Fiber Bragg Grating (FBG) sensors have been successfully utilized for mechanical and thermo-mechanical strain measurement in many cases; nevertheless, their capability of measuring strain at spots with intensive stress/strain has not yet been explored. In this research, FBG sensors are employed for strain measurement at the notch tip. A verification test was designed to substantiate the FBG measurements. The test involves a rectangular magnesium sheet with a center hole, subjected to uniaxial cyclic loading while the strain was measured at the notch tip using three different methods: strain gage, digital image correlation (DIC), and FBG. There were good agreements between the three measurements

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