Sep 12 2008
In every industry, from medical to analytical, designers are under increased pressure to improve performance of equipment. A key element is the electrical system. Designers striving to create the most efficient systems that deliver high performance are increasingly turning to ceramic dielectric components and capacitors.
There are various properties of a dielectric ceramic component that can significantly affect the overall performance of a system. This article examines important material characteristics, some of the ways in which ceramic materials have brought benefits to specific industries, and gives some examples of the types of product it is now found within.
Important materials characteristics
There are a number of specific properties a designer might consider when choosing the right ceramic components for electrical systems. These factors include dielectric constant (Er), low electrical loss and good temperature stability in relation to frequency or capacitance, high dielectric breakdown strength and compatibility in very high magnetic fields.
The dielectric constant, or Er, is an inherent property of the material. The higher the dielectric constant of the material, the smaller the size of component needed to achieve the same frequency or capacitance. This can lead to a reduced weight and a smaller, lighter product, with some significant cost savings.
Low electrical loss, (loss tangent) is often represented as Q (the ratio of 'energy stored' to 'energy dissipated' per cycle). Increased system performance can be achieved through very low electrical loss (high Q), which results in improved frequency selectivity and increased electrical pulse dissipation rates. This is an important factor within a laser discharge system or for improved power handling capability.
Electrical loss is generally proportional to the dielectric constant, i.e. increasing the Er of the material generally increases the electrical loss, so it is often a balance between tailoring all aspects of materials performance to ensure that the properties of the overall package are optimised. For instance, a low material loss tangent coupled with the correct electrode system in a multilayer structure will provide a capacitor with a low equivalent series resistance (ESR) suitable for stable power handling for plasma generation in the semiconductor processing industry.
To improve performance in some applications engineers look to fabricate a system that has a very low variation with temperature. This means that the temperature of the surroundings does not affect the ceramic’s performance and the final product can operate just as effectively over a broad temperature range. The benefits that certain ceramic materials can provide include a high permittivity that can be adjusted accurately to how it responds to variations in temperature fluctuations by subtle compositional modifications.
High intrinsic breakdown strength is an important factor that is considered for pulse power applications, such as those in an Excimer laser or in any electro magnetic pulse generation system. Additionally it is equally important in high power multilayer components where the interlayer ceramic thickness may be 25um with voltages of several kV/mm effective. The drivers here are for higher voltage capabilities through controlled microstructure, especially when encapsulated further within for instance a resin overmould.
In some environments it is extremely important that materials exhibit as little interference as possible when placed in high magnetic fields. A ceramic’s non magnetic capability can be altered by adjusting the compositional make-up of ceramic.
The dielectric ceramic materials that Morgan Electro Ceramics offers are specifically formulated to optimise the above parameters where possible by application. They are ideally suited to a range of high frequency radio frequency (RF) high power and medical applications from AM/FM commercial radio transmitters to filters and tuning circuits used in medical instruments systems.
Medical applications
There are many medical applications which have adopted ceramic dielectric components because of its electrical properties, as outlined below.
Corrective vision systems
Increased demand for laser eye surgery has led to an increased demand of longer life of systems and for a higher repetition capability from the capacitors. This can be achieved though the use of ceramic capacitors.
Excimer lasers are widely used in eye surgery and dermological treatments because the light is exceptionally well focussed and capable of very precise and delicate control that is required. It is also well absorbed by biological matter. The very short wavelength UV light does not penetrate beyond the first nanometer of the target surface and the very short pulse duration of about 10 seconds disintegrate surface material through ablation rather than burning. As a result, it causes almost no heating or change to the underlying and surrounding material.
The lasers are required to produce a stream of high-repetition, short-duration light pulses with an optical beam. The high repetition can be achieved through using high voltage capacitors where a high voltage pulsed discharge is required to energize the laser gases. These pulse discharges can be obtained by rapidly discharging the electrical energy that is stored in these ceramic high voltage capacitors. One key aspect of the material used in this application is that it exhibits little or no piezoelectric effect (mechanical distortion with applied electric field or vice versa).
The pulses ‘give’ gas molecules in the laser a high dose of energy which is released in the form of a light particle or phonon. The phonons are a very high-energy light source that can be used for eye surgery, which has been transformed by laser vision correction. Non-thermally damaging and precisely controllable argon fluoride excimer laser beams are an effective method of treating long and short-sightedness and have already freed millions of people from wearing glasses or contact lenses.