Metin Atmaca 030080007 8th week definitions

1. Conductive Adhesives (Material)

Previous Definition:

A polymer adhesive may contain a filler material such as silver, copper, or aluminum flakes or powders to provide electrical and thermal conductivity. In some cases, thermal conductivity is desired but electrical conductivity is not wanted; alumina, boron nitride, and silica may be used as fillers to provide this combination of properties.

(The Science And Engineering Of Materials, Donald R. Askeland, P.P. Phulé, page: 599 )

New Definition (Better):

A.    Electrically Conductive Adhesives

A most versatile option to make an electrical interconnection between two or more contact points is to use electrically conductive adhesive films that contain graphite fiber nonwovens or metal/metallized particles. These not only allow the use of lighter materials and flat design, while improving the reliability of the finished productsbut also help to reduce costs with regards to materials and processes. Depending on the current flow, a distinction must be made between isotropic and anisotropic electrical conductivity.

Depending on the desired application, anisotropic adhesive films can contain either hard or soft particles. Hard particles are suited for the interconnection of flexible to flexible or flexible to rigid plates, and are embedded into the conductive path by means of pressure; this increases the contact area and obtains good electrical properties. Soft particles are used for flex-to-glass interconnections – that is, the bonding of flexible circuit modules to glass substrates (LCDs). When they come into contact with hard surfaces, they deform and improve the electrical properties.

Selection of the correct system requires a consideration to be made of the type of adhesive and the size and concentration of the particles, as these parameters have an influence on the electrical and mechanical characteristics, the bonding process, and the fields of application. Put simply – the more particles in the matrix, the better the conductivity; the less particles, the better the adhesion. Thermoplastic anisotropic adhesive films have special features allowing smart cards to be converted into multifunction cards. Besides conventional chip bonding, they allow the integration of radiofrequency identification (RFID) antennas, finger print sensors and displays into smart cards. As the processing time of the adhesive films is very short (maximum 3 s), roll-to-roll lamination is possible. In addition, unlike high-temperature curable adhesive films, these adhesives do not require any cooling during storage and transportation.

B.    Thermally Conductive Adhesives

Thermally conductive products are available as adhesive films, elastomer pads and adhesives. They frequently contain thermally conductive, ceramics-based fillers that substantially improve the heat dissipation from electronic components and, at the same time, have an electrically insulating effect. In addition, they level out any possible irregularities of the surface and have a good initial tack and final bond strength to provide for a correct fastening of the components.

Today, thermally conductive adhesive films (Figure 8.58) have partly replaced thermal conductivity pastes, eliminating the need for mechanical fasteners such as clamps or screws. They are capable of leveling out any possible irregularities of up to 20% of their own thickness. A typical application is the bonding of heat sinks to computer central processing units and to flexible or rigid circuit boards. Thick adhesive films are also suited for heat dissipation in modern plasma panels.Weakly adhering, thermally conductive elastomer pads have special gap-bridging properties capable of leveling out differences of up to 2.5mm, although additionalmechanical fasteners are required. Thermally conductive adhesives combine low thermal impedance with high mechanical strength, and also have excellent wetting and gap-filling properties.

(Brockman, W et al, Adhesive Bonding, p. 310)

2. Ferromagnetism (Physical Phenomenon)

Previous Definition:

Ferromagnetism refers to solids that are magnetized without an applied magnetic field. These solids are said to be spontaneously magnetized.  Ferromagnetism occurs when paramagnetic ions in a solid “lock” together in such a way that their magnetic moments all point (on the average) in the same direction. At high enought temperatures, this “locking” breaks down and ferromagnetic materials become paramagnetic. The temperature at which this transition occurs is called the Curie Temperature.

There are two aspects of Ferromagnetism. One of these is the description of what goes on inside a single magnetized domain( where the magnetic moments are all aligned). The other is the description of how domains interact to produce the observed magnetic effects such as hysteresis.

(James D. Patterson, Bernard C. Bailey, Solid-State Physics: Introduction to the Theory, p. 360)

New Definition (Better):

Ferromagnetism signifies in its wide sense the strong magnetism of attracting pieces of iron and has long been used for motors, generators, transformers, permanent magnets, magnetic tapes and disks. On the other hand, the physics of ferromagnetism is deeply concerned with quantum-mechanical aspects of materials, such as the exchange interaction and band structure of metals. Between these extreme limits, there is an intermediate field treating magnetic anisotropy, magnetostriction, domain structures and technical magnetization.

The term ferromagnetism is used to characterize strongly magnetic behavior, such as the strong attraction of a material to a permanent magnet. The origin of this strong magnetism is the presence of a spontaneous magnetization produced by a parallel alignment of spins. Instead of a parallel alignment of all the spins, there can be an anti-parallel alignment of unequal spins.

(Chikazumi, S., Physics of Ferromagnetism, p.118)