Solid state single layer Ultracapacitor
Solid State Ultracapacitor power density
A recent development in solid state single layer energy storage devices (SSESD) offers a solution. The SSESD incorporates a high permittivity dielectric with a dielectric constant in the order of 16 million.
A dielectric is any electrical insulator capable of being polarised by an electric field. Under the influence of an electric field, the charge distribution in the dielectric changes so that positive charges align with the field. The three primary polarisations mechanisms are:
- Ionic polarisation where positive ions flow with the field and negative ions flow against the field
- Orientational polarisation where the dielectric contains materials with a permanent dipole moment, in other words, molecules have an uneven charge distribution
- Interface polarisation, where free mobile charges within the material migrate to the dielectric/electrode interface; positive charges move to the negative electrode and positive charges to the negative electrode.
In its simplest form, a capacitor consists of a dielectric layer sandwiched between two conductors. Applying a voltage across the conductors creates an electric field. The dielectric constant or permittivity (K) of the dielectric is the ratio between the field without the dielectric (Eo) and the field with the dielectric (E).
K = Eo/E
In the single layer capacitor described, when a voltage (V) is applied across the conductors, a charge (Q) is induced in the capacitor. The ratio between the voltage and the charge is defined as the capacitance (C) of the capacitor.
C = Q/V
The amount of charge stored depends on area (A) of the conductors, the separation (d) between them, and the dielectric constant.
C = K(A/d)
So, for capacitors with identical dimensions, the higher the value of the K, the greater is the amount of charge that can be stored.
The potential electrical energy (E) stored in a charged capacitor is a function of the capacitance (C), the voltage (V) across the electrodes. It is equivalent to the work done by charging it and can be expressed by:
E = ½ CV2
The experimental work over the past two years consisted of three steps.
The first step centred on the development of bulk capacitor ceramics with high capacitance to be used as reference materials.In contrast to Eestor technology, which has a very basic low dielectric constant of 18,000 against our latest milestone of 16 million.
The second step centred on the development of high surface area ceramics with improved charge storage capabilities.
The third step will centre on the development dielectric films on metal substrate with good adherence to the metal substrate and possessing no pitting or discontinuities responsible for decrease in capacitance. We will use a novel ultrasound technology for mixing and grain refinement.
The powder was pressed and fired at various temperatures ranging to produce dense non-porous samples. The samples were then electroded and tested for capacitance (C), dielectric constant (K), dielectric loss (loss tan) and temperature coefficient of capacitance (TCC).