Electric field effects on phase transitions in the 8CB liquid crystal doped with ferroelectric nanoparticles

The influence of a low ac electric field on phase transitions is discussed in the case of a nematic liquid crystal $4-n-\text{octyl}-4{}^{\prime }$-cyanobiphenyl (8CB) doped with ${\text{Sn}}_2{\text{P}}_2{\text{S}}_6$ ferroelectric nanoparticles. The phase-transition temperatures obtained from temperature-dependent dielectric measurements were higher than those determined by the calorimetric method. This difference is explained by the presence of the measuring electric field which induces two effects. The first one is the amplification of the interactions between the nanoparticle polarization and the liquid-crystal order parameter. The second one is the field-induced disaggregation or aggregation process at high nanoparticle concentrations.

Figure 1

Dielectric spectra of the pure 8CB and $8\text{CB}/{\text{Sn}}_2{\text{P}}_2{\text{S}}_6$ nanocolloid with a volume fraction ${\varphi }_{\text{NP}}=0.54\%$ for (a) homeotropic and (b) planar orientations.

Figure 2

Temperature-dependent (a) ${\varepsilon }_{\parallel }^{\prime }$ and (b) ${\varepsilon }_{\perp }^{\prime }$ of the doped liquid crystal (${\varphi }_{\text{NP}}=0.08\%$). Insets: enlarged regions of the smectic-$A$–nematic phase transition.

Figure 3

The derivative of ${\varepsilon }_{\perp }^{\prime }$ obtained from the planar orientation as a function of temperature for the pure liquid crystal and nanocolloids: (a) smectic-$A$–nematic and (b) nematic-isotropic phase transition regions.

Figure 4

(a) Shifts of smectic-$A$–nematic phase-transition temperatures induced by the nanoparticles; (b) shifts of nematic-isotropic phase-transition temperatures induced by the nanoparticles. (c) Differences between the shifts of phase-transition temperatures observed from DSC and temperature-dependent dielectric measurements: $\delta T=\mathrm{\Delta }{T}_{\text{dielectric}}-\mathrm{\Delta }{T}_{\text{DSC}}$. The lines are a guide to the eye. For some plots, the error bars are not visible due to the scales (see Table 1).

Figure 5

Schematic illustration of a sinusoidal electric field effect on the order parameter of doped nematic liquid crystal. Top: examples of single-dipole nanoparticles and of corresponding aggregates ($p_A>p_B>p_C$). Middle: time-dependent sinusoidal electric field and corresponding optimal values of the order parameter. Bottom: orientation and disaggregation process of nanoparticles in the doped liquid crystal.