Fillers metal particles

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

The defects caused by the high contact resistance especially manifest themselves in the metal-filled composites where the value of the percolation threshold may reach 0.5 to 0.6 [30]. This is caused by the oxidation of the metal particles in the process of CPCM manufacture. For this reason, only noble metals Ag and Au, and, to a lesser extent, Ni are suitable for the use as fillers for highly conductive cements used in the production of radioelectronic equipment [32]. 

Electrically conductive adhesives owe their conductivity as well as their high cost to the incorporation of high loadings of metal powders or other special fillers of the types shown in Table 9.8. If enough metal particles are added to form a network within the polymer matrix, electrons can flow across the particle contact points, making the mixture electrically conductive. Virtually all high-performance conductive products today are based on flake or powdered silver. Silver offers an advantage in conductivity stability that cannot... 

Composites with filler concentrations close to the percolation threshold exhibit conductivity which is sensitive to compressive deformation, since this brings the metal particles into contact, thereby forming percolation pathways. This sensitivity has been exploited especially in anisotropic composites. These are made by prealigning the metal particles with either electric or magnetic fields. This alignment is identical with that produced by external fields in electro- and magneto-rheological fluids where at a critical field continuous threads of... 

In another similar example nanocomposite was formed in a polyurethane matrix. Solvent soluble polyurethane had pyridine groups attached which formed complexes with metal salts. Films were then formed and subjected to a reducing agent in order to produce particulate metal filler. In this case the distribution of the filler which was formed was not uniform because the filler had tendency to aggregate (even though it was chemically attached to the matrix prior to the reduction). The following were factors controlling size and shape of these metal particles ... 

Viscosity increase and, therefore, flow decrease depend on filler concentration and the packing factor which is related to filler volume. Polyethylene filled with metal particulates behaves in a similar way. " Flow was decreasing linearly with the concentration of metal particles. 

Typical fillers magnesium oxide, calcium hydroxide, PTFE, graphite, molybdenum disulfide, carbon black, metal particles... 

Filler Adhesive component in a solid, finely dispersed form that specifically modifies the processing properties of the adhesive and the properties of the adhesive layer (e.g., metal particles in electrically conductive adhesives, chalkstone, carbon black to increase viscosity). Fillers are not reactive partners in adhesive curing. 

The result of these arrangements will be homogeneous metal particles/ nanocomposites distribution in acetylacetone ligand shell. In the capacity of conductive filler silver nanoclusters/nanocrystals and copper-nickel/ carbon nanocomposites can be used. It should be lead to decrease volume resistance from 10" to 10" Q-cm and increase of adhesive/paste adhesion. 

BeryUiuin oxide-filled resins gain high conductivity without loss of electrical properties. Metal particles have heen used as fillers for plastics to improve or impart certain properties. Thus, aluminum has heen used for applications ranging from making a decorative finish to improving thermal conductivity. Copper particles are used in plastics to provide electrical conductivity. Lead is used because it dampens vibrations, acts as barrier to gamma-radiation, and has high density. 

To obtain the lowest electrical volume resistivities (in the 10 to 10 ohm-cm range) and the lowest contact resistances, polymer resins must be highly Ailed with 70-80% by weight (25-30% by volume) of metal particles and must be thoroughly cured. Silver and silver alloys are the most widely used fillers but, in specific applications, other metal fillers including gold, copper, nickel, and even carbon are used. Carbon-filled polymers are often used to dissipate static electricity or to protect from electromagnetic interference (EMI). 

In adding conductive filler to an insulating resin, the volume resistivity changes slowly until a critical level of filler is reached, called the percolation point. The percolation point occurs when the resistivity drops abruptly, then continues to drop slowly (Fig. 2.11). " Almost continuous linkage of metal particles occurs at the percolation point where typical filler volumes for silver flakes are 25-30%. According to the percolation theory, there is a minimum critical volume of filler required for electrical conductivity in a polymer at which each filler particle must contact two other particles. A misconception in the use of silver flakes is that increasing the number of contacts lowers volume resistivity. Actually, the converse is true because, once the percolation point has been reached, each additional contact adds resistance. Thus, increasing the particle size can increase conductivity since the total number of contacts for a fixed volume decreases." ... 

Electrical conductivity in anisotropic adhesives occurs by a mechanism different from that of isotropic adhesives. Although metal fillers are also used, they are used in much lower amounts (0.5-5% by volume) so that the adhesive is essentially an insulator in the x-y directions. On inserting the adhesive between the electrodes (for example, the metal bumps of a flip-chip device with metal pads on a flex circuit) of two parts and applying pressure and heat, the metal particles form a z-direction electrical connection between the electrodes while the surrounding material remains insulating. The... 

The most important additive in an adhesive formulation is the filler. Fillers such as metal particles are used to render adhesives both electrically and thermally conductive while electrically insulative fillers such as metal oxides are used to provide thermal conductivity alone. Regardless of whether they are electrically conductive or not, fillers provide numerous simultaneous benefits, including the following ... 

Increasing the concentration of metal particles in an insulating adhesive matrix changes the electrical properties of the composite in a discontinuous way. Assuming a random dispersion of the metal filler, as the concentration increases no significant change occurs until a critical concentration, pc, is reached. This point, where the electrical resistivity decreases dramatically, called the percolation threshold, has been attributed to the formation of a network of chains of conductive particles than span the composite. A two-dimensional cartoon of a conductive adhesive below p and just above pc is shown in Fig. 3. A typical plot showing the relationship between particle concentration and electrical resistivity is shown in Fig. 4. 

Electrically conductive polymeric composites find many applications. They are usually obtained by mechanically mixing a polymer with particles of a conductive filler metal or carbon black. In such a case a high amount of filler is needed. Conductive charge transfer conplexes were also used to prepare conductive polymer composites, but in the early experiments also a high CT complex content - sometimes as high as 80% - was necessary to obtain a conductive material. 

Within the field of materials science, the term composite is appUed to a material that consists of a combination of phases. Typically, there are two phases, a continuous one, known as the matrix, and a discontinuous one, dispersed within the matrix phase [1]. The majority of composites consist of an inorganic filler, either particles or fibres, dispersed in a matrix of organic polymer. However, other technically useful composites exist, including all-ceramic composites [2] and metal matrix composites [3]. 

Isotropic adhesives conduct current equally in all directions and are the most common and widely used in industry. The anisotropic types, also referred to as z-direction adhesives or anisotropic-conductive adhesives (ACA), although filled with metal particles, are filled at much lower levels (0.5%-5% by volume) than isotropic types (filled 25%-30% by volume). The volume Iraction of filler is well below the percolation threshold at which the adhesive becomes highly conductive in all directions. Because of the low volume Iraction of metal particles, there are no continuous electrically conductive paths in the x-y plane. During the connection process, the anisotropic adhesive, either as a film or paste, is positioned between a flip-chip bumped die or a tape-automated bonded (TAB) die and the corresponding pads of an interconnect substrate. Pressure and heat are... 

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