Assessing ferroelectric materials for application in τV_MINmode devices

The Development of Materials, Mixtures and Gels for Ferroelectric Displays

“…A great many such mixtures have been formulated through collaboration between the University of Hull and DERA (Malvern), and figure 2 shows a basic example; more recent mixtures include a greater range of host components to give a much lower melting point, and only 1 or 2% of the c h i d dopant to give a higher smectic C phase stability and much better overall performance. [ 15,16,191 The ferroelectric mixture shown in figure 2 contains a component (SO) that does not itself exhibit a smectic C phase, nevertheless in mixtures this compound upholds the smectic C phase stability much better than expected. The ability to generate good mesomorphic behaviour from materials that have relatively short terminal chains, and a compound (SO) that has the fluoro substituents in the centre ring minimises the viscosity of the mixture and hence generates a very short switching time.…”

“…The reinforcing effect of the two fluoro substituents also generates a high lateral dipole which confers a high dielectric biaxiality, an important feature in ferroelectric mixtures operating in TVmin mode. [15,16] The compounds with the two fluoro substituents in the outer ring and one alkoxy terminal chain have the highest smectic C phase stability, but these also have the highest melting points and the highest viscosity. One of the major advantages of the orthodifluoroterphenyls is that the smectic C phase is exhibited over a wide range to high temperatures through the use of two akyl chains (e.g., 11 and 131, which facilitates the formulation of ferroelectric host mixtures For the difluoroterphenyls with the two fluoro substituents in the alkyl end ring (29), the difluorophenyllithium was quenched with an aldehyde to yield the benzylic alcohol (26) which was dehydrated and hydrogenated to generate the necessary alkyl chain (compound 27).…”

The synthesis and properties of host materials with fluoro substituents in the core and in a terminal chain for high dielectric biaxiality FLC mixtures

“…As can be seen from the above equations, the best compromise of 8&, spontaneous polarisation (Ps) and viscosity (q) needs to be used so that the minimum in the response time-voltage relationship occurs at the lowest possible voltage and the shortest switching time. [4,5] The important physical parameters in the above equations are controlled by the structural nature of the compounds in the ferroelectric mixture. Dielectric biaxiality (6E) is proportional to the perpendicular dielectric permittivity (EL), and hence a high lateral dipole confers a high a&.…”

“…Such achiral host mixtures are then doped with a small quantity of a chiral material (e.g., 5 and 6), which need not be mesogenic, to provide the necessary chirality, which on removal of the macroscopic helical structure generates the ferroelectric properties. As can be seen from the above equations, the best compromise of 8&, spontaneous polarisation (Ps) and viscosity (q) needs to be used so that the minimum in the response time-voltage relationship occurs at the lowest possible voltage and the shortest switching time.…”

On the biaxiality of smectic C and ferroelectric liquid crystals

“…These results helped guide the design of superior mixtures for τV min FLC applications Figure 24 shows the composition and results for the mixtures FDD12, [50]: latching with 5µs, 40V pulses is achieved in a 1.3µm device. This translates to sub-40V addressing times below the 12µs target for HDTV.…”