Enhancement of electrical properties of anisotropically conductive adhesive joints via low temperature sintering

The electrical properties of anisotropically conductive adhesives (ACAs) joints through low temperature sintering of nano silver (Ag) particles were investigated and compared with that of the submicron‐sized Ag‐filled ACA and lead‐free solder joints. The nano Ag particles used exhibited sintering behavior at significantly lower temperatures (<200°C) than at the bulk Ag melting temperature (960°C). The sintered nano Ag particles significantly reduced the joint resistance and enhanced the current carrying capability of ACA joints. The improved electrical performance of ACA was attributed to the reduced interfaces between the Ag particles and the increased interfacial contact area between nano Ag particles and bond pads by the particle sintering. The reduced joint resistance was comparable to that of the lead‐free (tin/3.5 silver/0.5 copper) metal solder joints. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1665–1673, 2006     

Capillary and van der Waals force between microparticles with different sizes in humid air

It is well known that surface effect forces, such as van der Waals force and capillary force, are the major contributions to adhesion when microsized particles are in contact in humid environment. But it is very complex to calculate the adhesion force between two smooth unequal particles. In conventional approaches, the effective particle radius approximation and the constant half-filling angle assumptions are often used for computing the van der Waals forces between two microparticles. However, the approximation and the assumption are actually difficult to accurately model the forces between unequal particle sizes when the surfaces are with different properties. In this paper, we present a theoretical study of the van der Waals force and capillary force between two microparticles with different radii and the surface properties linked by a liquid bridge. The proposed model provides the adhesion force predictions in good agreement with the previous formula and existing experiment data. Considering the solid particles are partially wetted by the liquid bridge, the van der Waals force is calculated by divided the particle surface into a wetted part and a dry portion in our stimulation. Since the wetted surface portion of the particle is determined by the half-filling angle, the relationship between two half-filling angles of the unequal size particles is developed from the geometrical consideration, which is relate to the size ratio of the particles, the contact angle, and the separation distance. Then, the van der Waals force is determined using the surface element integration. Moreover, the influences of humidity, particles size, contact angle, and separation distance toward the adhesion forces are discussed using the proposed method. Simulations indicate that a higher relative humidity leads to bigger liquid bridges, suggesting a higher capillary force, but at the same time, the van der Waals force decreases due to the decrease in surfaces energy. As for the influence of contact angle, results show that a higher contact angle, that is, a more hydrophobic surface, reduces the capillary force but increases the van der Waals force (absolute value). The simulations also show that the both the capillary force and the van der Waals force (absolute value) increase as the particle size increases. When the particles are separated from each other, the capillary force and van der Waals force decreases gradually. These results are helpful to understand and utilize the adhesion interaction between particles with unequal sizes at the ambient condition.     

Characterisation of the properties of size fractions from ten world coals and their chars produced in a drop-tube furnace

A range of coals from different parts of the world was studied to determine if there were any common relationships that could be determined to gain a clearer understanding of the distribution of coal properties within different particle-sizes. The properties examined were proximate analysis, maceral analysis and %Unreactives from image analysis. Each fraction was also pyrolysed in a drop-tube furnace at 1300°C, 1 vol% oxygen and a residence time of 200 ms and the resulting chars analysed for morphology using image analysis. There were substantial variations between the particle-size distributions of the different coal samples even though they were ground to the same specification for trials on a combustion rig. Ash distributions showed in all cases that the smallest particle size (−38 μm) had either the highest ash level or was very close to it. However, the trends in ash level for increasing particle size showed variations between coals with some coals showing increases in ash towards the larger particles. Fusinite content did not necessarily concentrate in the smallest size fraction, however, liptinite content did increase with particle size. %Unreactives generally increases with particle size and is related to char morphology through an empirical parameter, the ACA [5]. In addition the ACA [5] parameter showed the effect of both particle size and %Unreactives on char morphology and clearly showed the significant influence of particle size on burnout. A parameter such as this could, therefore, be used in burn-out models and further correlated with %Unreactives and particle size.     

Adhesion and Removal of Particles from Surfaces Under Humidity Controlled Air Stream

The adhesion and the removal of individual micrometer-sized particles on a plane substrate are studied using an air shear flow cell. Laminar isothermal compressible flow characterization enables us to analyze the effect of various parameters such as particle size, air humidity, surface nature and surface charge on the aerodynamic forces required to remove the particles from the substrate. The results show that the increase of humidity (up to a critical value) favors particle removal when particles adhere under strong electrostatic forces on a non-conductive charged substrate. On the contrary, the existence of a capillary force disfavors particle removal beyond this critical humidity. The increase of the humidity disfavors the removal of particles in contact with an uncharged substrate. The results are interpreted in terms of a global adhesion force using a force and torque balance on a single particle in contact with a plane substrate. Moreover, the use of a high-speed video recording system enables us to determine the particle removal mechanisms as a function of the particle Reynolds number.     

Adhesion and Removal of Particles from Surfaces Under Humidity Controlled Air Stream

The adhesion and the removal of individual micrometer-sized particles on a plane substrate are studied using an air shear flow cell. Laminar isothermal compressible flow characterization enables us to analyze the effect of various parameters such as particle size, air humidity, surface nature and surface charge on the aerodynamic forces required to remove the particles from the substrate. The results show that the increase of humidity (up to a critical value) favors particle removal when particles adhere under strong electrostatic forces on a non-conductive charged substrate. On the contrary, the existence of a capillary force disfavors particle removal beyond this critical humidity. The increase of the humidity disfavors the removal of particles in contact with an uncharged substrate. The results are interpreted in terms of a global adhesion force using a force and torque balance on a single particle in contact with a plane substrate. Moreover, the use of a high-speed video recording system enables us to determine the particle removal mechanisms as a function of the particle Reynolds number.     

Effects of different anisotropically conductive adhesives on the reliability of UHF RFID tags

Radio frequency identification (RFID) system consists of a reader and tags which can be attached to the object to be identified. Such systems are used to identify multiple objects individually and reliably using radio waves without visual connection to the reader. An RFID tag has a simple structure with an antenna to which a chip is attached. Typically anisotropic conductive adhesives (ACAs) are used as attachment materials between the antenna and the chip. ACAs may have a significant impact on the reliability of a tag. In this paper the behaviour of RFID tag interconnections made with four different commercial ACAs was studied. For purposes of comparison the ACAs were characterized using several techniques. The reliability of the interconnections was studied using temperature cycling and constant humidity tests. Clear differences in the failure times between the ACAs were observed. Furthermore, different environments were found to have different effects on the reliability of the ACA interconnections. The results showed that the properties of the ACA should be carefully considered and reliability aspects taken into consideration when ACA for RFID applications is chosen.     

Facile fabrication of elongated polymer micro/nano discs and their surface adhesiveness

The key factor of polymer particles using as a drug carrier is the ability to adhere to the surface of the target sites in order to optimize the drug delivery efficiency. To this end, shape of the particles is one of the essential parameters since nonspherical particles such as discs theoretically have more contact surface area than spherical particles. We herein propose a facile method to fabricate elongated polymer micro/nano discs by combining phase separation and a roll-to-roll coating process with a stretching method. By roll-to-roll coating, biodegradable poly(lactide-co-glycolide) (PLGA) discs formed on polyvinyl alcohol (PVA) film can be instantly stretched by uniaxial stretcher. The effect of stretching temperature and percentage of film elongation on the particle morphology was studied. The results showed that the PLGA discs can be elongated and the aspect ratio of 2.3 can be achieved with 600% elongation at 80°C, reflecting the increase of estimated contact surface area by 1.8 times. The adhesion efficiency of these elongated discs was compared with discs before stretching, spheres, and elongated spheres by using the water-dropping test. The elongated discs showed the best result among these particles and their adhesion efficiency are strongly related to estimated contact surface area.     

EFFECTS OF HYDROCARBON LIQUID FEED IN POLYETHYLENE POLYMERIZATION PROCESS ON PARTICLE SURFACE TEMPERATURE

In the fluidized bed gas phase polymerization of polyethylene (PE), the heat generated by the exothermic polymerization process is dissipated into the gas mixture flowing past the polymer particles. The polymer particle temperature is determined by the extent of convective heat transfer and other mechanisms of heat removal. In addition to the heat removal by convective heat transfer, liquid hydrocarbon (HC) is often injected into the reactor to further remove heat by evaporation but without partaking in the reaction. The effects of adding this liquid HC on the particle surface temperature have been investigated numerically by means of a one-dimensional polar model. Results indicate that the primary mechanism for removal of the heat of polymerization from the particles is by means of convective heat transfer to the bulk gas, which amounts to 99.5% removal of total heat of polymerization. The PE particle temperature rises only by 1–2°C above the surrounding bed gas mixture. The addition of liquid HC to the feed, however, has a pronounced effect on controlling the reactor gas temperature as most of this liquid is evaporated to the gaseous phase before it reaches the polymer particles. To state it clearly, heat of polymerization is transferred from the particles to the reactor bulk gas predominantly by convection, and part of this heat is subsequently absorbed by evaporation of the fresh liquid HC in the feed. Comparison with a detailed computational fluid dynamic (CFD) model of polymerization in a generic gas phase reactor has also been conducted. The results confirm that the particle temperature rise above the reactor gas temperature is consistent with the one-dimensional model. However, local gas temperature variations are present in the reactor due to the unsteady gas-solid hydrodynamics. Hence, there are some zones that are a few degrees hotter/colder than the bulk reactor temperature with corresponding increase/decrease in particle temperature in these zones.     

EFFECTS OF HYDROCARBON LIQUID FEED IN POLYETHYLENE POLYMERIZATION PROCESS ON PARTICLE SURFACE TEMPERATURE

In the fluidized bed gas phase polymerization of polyethylene (PE), the heat generated by the exothermic polymerization process is dissipated into the gas mixture flowing past the polymer particles. The polymer particle temperature is determined by the extent of convective heat transfer and other mechanisms of heat removal. In addition to the heat removal by convective heat transfer, liquid hydrocarbon (HC) is often injected into the reactor to further remove heat by evaporation but without partaking in the reaction. The effects of adding this liquid HC on the particle surface temperature have been investigated numerically by means of a one-dimensional polar model. Results indicate that the primary mechanism for removal of the heat of polymerization from the particles is by means of convective heat transfer to the bulk gas, which amounts to 99.5% removal of total heat of polymerization. The PE particle temperature rises only by 1-2°C above the surrounding bed gas mixture. The addition of liquid HC to the feed, however, has a pronounced effect on controlling the reactor gas temperature as most of this liquid is evaporated to the gaseous phase before it reaches the polymer particles. To state it clearly, heat of polymerization is transferred from the particles to the reactor bulk gas predominantly by convection, and part of this heat is subsequently absorbed by evaporation of the fresh liquid HC in the feed. Comparison with a detailed computational fluid dynamic (CFD) model of polymerization in a generic gas phase reactor has also been conducted. The results confirm that the particle temperature rise above the reactor gas temperature is consistent with the one-dimensional model. However, local gas temperature variations are present in the reactor due to the unsteady gas-solid hydrodynamics. Hence, there are some zones that are a few degrees hotter/colder than the bulk reactor temperature with corresponding increase/decrease in particle temperature in these zones.     

基于分子运动学的水汽在细颗粒表面异质核化的数值模拟

为研究过饱和水汽在细颗粒表面异质核化特性,准确预测成核参数,基于分子运动学异质核化理论建立了过饱和水汽在燃煤细颗粒表面异质核化的运动学模型。数值分析了液滴晶核长大过程中水汽分子和水分子两种扩散凝结机制对晶核长大的促进作用及其相对重要性,数值预测了水汽过饱和度和宏观接触角对成核速率的影响;数值计算了不同温度和宏观接触角下细颗粒的临界过饱和度。结果表明：当液滴晶核尺寸小于临界晶核半径时,颗粒表面吸附水分子扩散凝结速率与水蒸气分子直接扩散凝结速率的比值大于100,颗粒表面吸附水分子的扩散凝结机制对晶核长大起主导作用。提高水汽过饱和度或减小宏观接触角均可显著提高液滴晶核的成核速率;成核速率随水汽过饱和度增大呈指数型增长。提高气相主体中水汽温度或减小细颗粒物的宏观接触角均可显著降低异质成核的临界过饱和度;对于粒径小于0.1 μm的细颗粒物,随着细颗粒粒径的增大,异质核化的临界过饱和度显著减小。     

Highly filled thermoplastic composites. II: Effects of particle size distribution on some properties

The effect of irregularly shaped glass particle size and size distribution on the packing density and flexural mechanical properties of highly-filled composites with a rubbery thermoplastic matrix was studied. Increasing the particle's median size and size distribution width significantly increases the packing density of the composites. Compression molding causes the glass particles to fracture at a decreasing level with an increasing distribution width. Particle median size, rather than size distribution, affects the mechanical properties; The flexural modulus and strength increase and the ultimate deflection in flexure decreases with a decreasing median size. A “glass network” is formed in the compression molded composites because of the mechanical interlocking of particles. The nature of this continuous glass phase predominates the composites mechanical behavior. The particle's size and shape determine the nature of the glass network and, thus, have a dominating effect on the mechanical properties. The latter are significantly affected by the particle's surface properties. A specific silane treatment of the glass particles acts to reduce the particle/particle friction, resulting in a higher packing density. The treatment also acts as a cohesive liquid to increase the strength of the glass network, and to increase the particle/polymer adhesion, increasing the composites' strength and ductility.     

A new model to determine contact angles on swelling polymer particles by the column wicking method

The contact angle determination on swelling polymer particles by the Washburn equation using column wicking measurements may be problematic because swelling occurs during the wicking process. The objective of this research was to develop a new model to more accurately determine contact angles for polymer particles that undergo solvent swelling during the column wicking process. Two phenomena were observed related to the swelling effect during the wicking process: (1) a temperature rise was detected during the wicking process when the swelling polymer particles interacted with polar liquids, and (2) a smaller average capillary radius (r) was obtained when using methanol (polar liquid) compared to using hexane (non-polar liquid). The particle swelling will induce both particle geometry changes and energy loss which will influence the capillary rise rate. The model developed in this study considered the average pore radius change and the energy loss due to the polymer swelling effect. Contact angle comparisons were conducted on wood with formamide, ethylene glycol, and water as test liquids, determined by both the new model and the Washburn equation. It was shown that the contact angles determined by the new model were about 4-37° lower than those determined by the Washburn equation for water, formamide, and ethylene glycol. Todetermine whether the polymer particles are swelling, two low surface tension liquids, one polar (methanol) and the other non-polar (hexane), can be used to determine the average pore radius (r values) using the Washburn equation. If the same r values are obtained for the two liquids, no swelling occurs, and the Washburn equation can be used for the contact angle calculation. Otherwise, the model established in this study should be used for contact angle determination.     

Particle dimensions in polystyrene/polyethylene blends as a function of their melt viscosity and of the concentration of added graft copolymer

Apparent particle dimensions in blends of low density polyethylene (PE) and polystyrene (PS) made by mixing in the molten state have been measured and have been shown to depend on the viscosity of the mixed system. At a certain shearing rate small particles of the dispersed polymer are obtained if the polymer in excess has a high viscosity. At a higher concentration however this effect is counteracted completely if the dispersed phase has a low viscosity and thus lowers the overall viscosity. Coalescence is affected in the same way and the size of the particles increases greatly with increasing concentrations. If a highly viscous polymer is dispersed in a low viscosity polymer matrix relatively large particle sizes will be found at the preset shearing rate mentioned above. With increasing concentration of the highly viscous dispersed phase the particle size decreases due to the increasing viscosity. This effect is counteracted by coalescence leading to a relatively slow increase of particle size with concentration. Addition of surfactants such as graft copolymers based on PS and PE leads to smaller particle sizes as expected.     

Effect of shape and size of nickel‐coated particles fillers on conductivity of silicone rubber‐based composites

Electrically conductive polymer composites have been widely used in recent years. The resistivity of the composites is influenced by several factors. The conductive silicone rubbers (CSRs) were prepared by adding the nickel‐coated graphite (Ni‐G) and/or nickel‐coated carbon fiber (Ni‐CF) into liquid silicon rubbers which were then subjected to the vulcanization. The effect of particle shape and size on the electrical conductivity of CSR was investigated; the results indicate that Ni‐CF filled CSR have lower percolation threshold than Ni‐G filled CSR. Compared to the filled particles with larger size, a higher amount of smaller particles are needed to form the same conductive pathway in CSR. Thus, there are more contact points in conductive pathway, which increase the total contact resistance. The volume resistivity of CSR can be significantly reduced by the doping of Ni‐CF particles, while the effect begins to wear off under high doping content. The fiber‐like Ni‐CF has a good “bridging” function for the formation of conductive pathway in CSR, which is greatly enhanced by increasing the length to diameter (l/d) ratio of Ni‐CF particles. Therefore, doping of the particles that have low percolation threshold can reduce the resistivity of the composites. POLYM. COMPOS., 36:1371–1377, 2015. © 2014 Society of Plastics Engineers     

The effect of particle size of fly ash (FA) on the interfacial interaction and performance of PVC/FA composites

In this study, the effect of particle size of fly ash (FA) on the interfacial interaction between the filler particles and the polymer matrix is investigated. Structural and physical characterization of FA with different particle sizes show that its chemical composition is highly dependent on the particle size. The mechanical, dynamic‐mechanical, structural, and microstructural properties of the composites are evaluated. Interfacial interaction between FA particles and the polymer matrix is assessed experimentally using a nanoindenter and numerically using two different models developed by Pukanszky and Kubat. The composites reinforced with smaller particles exhibit better mechanical, viscoelastic, and microstructural properties. Structural and interfacial studies show that, although the characterized amount of silicon oxide in the small particles is lower than the large particles, the concentration of –OH group in SiO₂ is particle‐size and surface‐area dependent. Therefore, smaller particle inclusions result in better interfacial interaction and improved properties. This observation is consistent with the numerically estimated interfacial interaction. J. VINYL ADDIT. TECHNOL., 25:134–143, 2019. © 2018 Society of Plastics Engineers     

Composite magnetic particles: 1. Deposition of magnetite by heterocoagulation method

In this paper we report synthesis and characterization of composite polymeric particles bearing magnetite inclusions and reactive β-diketone groups on the surface. Composites were prepared by two-step method in which first step requires preparation of the functionalized polystyrene core and during second step magnetite was deposited onto core particle surface. This procedure gives a possibility to obtain composite particles with core-shell morphology and both the core size and magnetite shell thickness can be varied. Highly monodisperse PS/AAEM microspheres were synthesized by surfactant-free emulsion polymerization. Change of monomer fleet-ratio gives a possibility to change effectively the final particle size of dispersions without strong changes in particle size distribution. PS/AAEM particles were characterized by light scattering techniques (DLS, SLS) and electron microscopy (SEM) with respect to their particle size and morphology of the surface layer. Magnetite was deposited in form of nano-crystals onto PS-AAEM particle surface by heterocoagulation process. It has been established that more uniform magnetite coating was obtained at lower base amounts used for synthesis of magnetite. Amount of the magnetite on the polymeric particle surface can be effectively controlled by changing the initial FeCl₂ and FeCl₃ concentrations and/or variation of the PS/AAEM core dimensions. It has been confirmed by separation centrifugation technique, that stepwise increase of the magnetite content on the particle surface decrease gradually the stability of colloidal system. Magnetization curves for composite particles indicate that deposited magnetite content is high enough to achieve considerable magnetic response to external magnetic field.     

Time Dependent Adhesion Induced Phenomena: The Flow of a Compliant Silicone-Polyester Copolymer Substrate Over Rigid Micrometer Size Gold and Polystyrene Particles

A polyester-polydimethylsiloxane block copolymer substrate (Young's modulus of approximately 9.2 × 10⁶ N/m2) was observed by SEM to gradually creep over higher modulus gold and polystyrene micrometer sized particles in contact with its surface. The particle/substrate and particle/particle interfaces were examined shortly after particle deposition and again at various intervals up to a month later. SEM micrographs clearly showed an increase in the size of the contact menisci with time (for periods up to approximately two weeks) at both sets of interfaces. After only one week the gold particles appeared to be completely encapsulated by the substrate. These time-dependent observations confirm the plastic deformation mechanism proposed previously to explain similar observations on micrometer- and submicrometersized glass, polystyrene, and poly-vinylidene fluoride beads contacting the same substrate (L. P. DeMejo, D. S. Rimai, J. Chen, and R. C. Bowen, J. Adhesion 39. 61 (1992)).     

Anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion above critical micelle concentration

Batch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion using nonionic and cationic emulsifiers was studied. The concentration of emulsifiers was set above their critical micelle concentration. Effects of emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/initiator (KOH) ratio on the kinetics, average particle size and distribution and on the average molecular weight and distribution were investigated and discussed. At the beginning of the polymerization, empty micelles, active micelles (polymer particles) and monomer droplets co-exist in emulsion. The transport of monomer from monomer droplets toward empty micelles was confirmed by monomer droplets and empty micelles disappearance and by formation of smaller particles. The transport of monomer from monomer droplets toward polymer particles was not confirmed, since the average polymer particle size did not increase during polymerization. It was proposed, that at lower conversions, monomer diffuses from polymer particle interior to particle surface, while at higher conversions, the monomer diffuses from larger to smaller polymer particles. Emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/KOH ratio have an evident effect on the kinetics and on the average molecular weight, thus demonstrating that cationic emulsifier participates to the initiation reaction.     

An investigation of engineering parameters for the use of immobilized biomass particles in biosorption

Immobilized, inactive mycelia of Rhizopus arrhizus are preferential to native biomass for use in the biosorption of metal ions. Refinement of a proprietary immobilization technique previously developed at McMaster University enabled production of particles of immobilized Rhizopus arrhizus biomass having a 12–23% wt of polymer additive. The effects of production stage parameters on the intrinsic uptake capacity of the immobilized biomass were examined. Kinetic experiments showed the following trends: a decrease in the weight percent of the added polymer leads to an increase in the apparent uranium uptake capacity of the immobilized biomass particles far a given contact time. A decrease in the particle size improved the kinetics of metal uptake and led to an increase in the apparent uranium uptake capacity for the same contact time. An increase in the initial concentration of the uranium solution caused equilibrium conditions to be attained faster.     

Adhesion forces of charged particles

Adhesion forces of toner and polymer particles to aluminum substrates were measured by the centrifugal, detachment field and microelectrode detachment field methods, and factors affecting the adhesion forces are discussed. The adhesion forces of toner particles increased with an increase in either particle size or particle charge. The adhesion force of an irregularly shaped toner was larger than that of a spherical toner. The mean adhesion force of polymer particles to aluminum substrates decreased with an increase in surface roughness of the substrates. The CF₄ plasma treatment of the polymer particles shifted their adhesion force distribution in a smaller direction. It was confirmed that the results by the centrifugal and the detachment field methods were in good agreement with each other. The contribution of van der Waals, electrostatic and water bridging forces to the adhesion forces of toner particles are also discussed.