The Role of Diamonds Dispersed in Ferronematic Liquid Crystals on Structural Properties

At a Glance

Metadata	Details
Publication Date	2024-02-20
Journal	Crystals
Authors	Peter Bury, Marek Veveričík, František Černobila, Natália Tomašovičová, Veronika Lacková
Institutions	Institute of Soil Biology, Basque Center for Materials, Applications and Nanostructures
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study investigates the structural and magneto-optical properties of 5CB nematic liquid crystal (LC) composites doped with Fe₃O₄ magnetic nanoparticles (MNPs) and magnetically modified diamond nanoparticles (DNPs).

Threshold Field Reduction: The presence of DNPs significantly decreases the magnetic threshold field (H_th) required for molecular reorientation compared to pure 5CB or 5CB doped only with Fe₃O₄, shifting the onset of structural change toward zero magnetic field for high concentrations (1.60 and 3.20 mg/mL).
Enhanced Memory Effect: The DNP-Fe₃O₄ composites exhibit a large, persistent structural “memory effect” (hysteresis) in the nematic phase, observed in both light transmission and Surface Acoustic Wave (SAW) attenuation measurements.
Quantified Hysteresis: The memory effect at zero magnetic field reached up to 70% (light transmission) and 80% (SAW attenuation) of the total signal change for the highest DNP concentration (3.20 mg/mL).
Transition Temperature Increase: The nematic-isotropic transition temperature (T_NI) increased with the volume fraction of both MNPs and DNPs, suggesting that the nanoparticle aggregates enhance the structural stability of the LC matrix.
Mechanism Hypothesis: The observed memory and structural changes are attributed to the creation of stable diamond aggregate structures or networks within the LC matrix, similar to those formed by SiO₂ aerosils.
Core Value Proposition: DNPs serve as effective modifiers in ferronematic LCs, enabling the tuning of magneto-optical properties crucial for developing highly sensitive and non-volatile magneto-optical memory devices.

Technical Specifications

Parameter	Value	Unit	Context
Host Liquid Crystal	5CB (4-cyano-4’-pentylbiphenyl)	N/A	Nematic phase
Pure 5CB Nematic-Isotropic Transition (T_NI)	~35	°C	Baseline temperature
Magnetic Nanoparticle (MNP) Type	Fe₃O₄	N/A	Dopant material
MNP Diameter	11-14	nm	Particle size range
Nanodiamond (DNP) Crystallite Size	~3.1	nm	Determined by X-ray diffraction (XRD)
Nanoparticle Concentration (Tested)	0.32, 1.60, 3.20	mg/mL	Mass concentration in 5CB
Light Transmission Cell Thickness	50	µm	Parallel alignment cells
SAW Attenuation Cell Thickness	~100	µm	Cells on LiNbO₃ substrate
SAW Operating Frequency	10	MHz	Measurement parameter
Maximum Magnetic Field Applied	400	mT	Range for magneto-optical testing
Maximum Memory Effect (SAW)	~80	%	3.20 mg/mL DNP/Fe₃O₄ composite at zero field
Threshold Field (H_th) Shift	Decreased, approaching zero	mT	Observed for 1.60 and 3.20 mg/mL DNP/Fe₃O₄ composites
Temperature Accuracy (SAW)	±0.2	°C	Measurement stability (5-80 °C range)

Key Methodologies

The study utilized a combination of synthesis techniques for nanoparticle preparation and two primary measurement techniques (Light Transmission and SAW Attenuation) to characterize the structural response of the composites to external magnetic fields and temperature.

Fe₃O₄ Nanoparticle Synthesis:
- Fe₃O₄ precipitates were obtained via coprecipitation of Fe²⁺ and Fe³⁺ ions (from FeSO₄·7H₂O and FeCl₃·6H₂O) dissolved in deionized water.
- The process involved the addition of NH₄OH and subsequent heating to 60 °C.
Nanodiamond (DNP) Preparation:
- DNPs were produced by detonating a mixture of trinitrotoluene and hexogen at a 60/40 ratio, followed by purification.
Magnetic Modification of DNPs:
- 100 mg of DNP powder was mixed with 500 µL of magnetic fluid (stabilized with perchloric acid, 30.4 mg/mL concentration).
- The mixture was dried at room temperature and repeatedly washed with methanol to isolate the magnetically modified DNPs.
Composite Preparation:
- Powdered particles (Fe₃O₄ or DNP-Fe₃O₄) were added to 5CB LC.
- The mixture was sonicated for one hour. Lower concentrations (0.32 and 1.60 mg/mL) were prepared by gradual dilution and re-sonication.
Light Transmission Measurement (Magneto-Optical Effect):
- LC cells (50 µm thickness) with parallel alignment layers were used.
- A linearly polarized green laser beam (532 nm, 5 mW) illuminated the cell.
- Light transmission intensity (I/I₀) was recorded as the external magnetic field was linearly increased and decreased (up to 400 mT).
Surface Acoustic Wave (SAW) Attenuation Measurement:
- LC cells (≈100 µm thickness) were prepared directly on a LiNbO₃ piezoelectric line equipped with interdigital transducers.
- SAW pulses (10 MHz frequency) were generated and received.
- SAW attenuation response was monitored as a function of magnetic field (oriented vertically) and temperature (for T_NI determination).

Commercial Applications

The unique combination of low magnetic threshold and high structural memory induced by the DNPs in ferronematic LCs opens pathways for several advanced engineering applications:

Magneto-Optical Memory Devices: The large, non-volatile memory effect (hysteresis) at zero magnetic field is highly desirable for developing next-generation, low-power, passive magneto-optical storage and switching components.
High-Sensitivity Magnetic Field Sensors: The significant decrease in the magnetic threshold field (H_th) allows the composites to respond structurally to very weak external magnetic fields, enabling the creation of highly sensitive magnetic field detectors and transducers.
Tunable Optical Filters and Modulators: The ability to rapidly and reversibly change the light transmission characteristics using a low magnetic field allows for the development of fast, magnetically tunable optical components.
Advanced Liquid Crystal Displays (LCDs): Modification of the nematic-isotropic transition temperature (T_NI) and the threshold voltage (analogous to H_th in electro-optical systems) can lead to LCs with wider operating temperature ranges and lower power consumption requirements.
Smart Materials and Actuators: The structural changes induced by the magnetic field, particularly the freezing of these changes (memory effect), could be utilized in micro-actuators or smart surfaces where a persistent, magnetically set state is required.

View Original Abstract

A study of the role of diamond nanoparticles on 5CB liquid crystal composites with Fe3O4 nanoparticles is presented. Composite ferronematic systems based on the nematic liquid crystal 5CB doped with Fe3O4 magnetic nanoparticles and additionally bound to diamond nanoparticles (DNPs), of a volume concentration of 3.2 mg/mL, 1.6 mg/mL and 0.32 mg/mL, were investigated using both magneto-optical effect and surface acoustic waves (SAWs) to study the role of diamond nanoparticles on the structural properties of ferronematic liquid crystals. The responses of light transmission and SAW attenuation to an external magnetic field were investigated experimentally under a linearly increasing and decreasing magnetic field, respectively. Investigations of the phase transition temperature shift of individual composites were also performed. The experimental results highlighted a decrease in the threshold field in the ferronematic LC composites compared to the pure 5CB as well as its further decrease after mixing Fe3O4 with diamond powder. Concerning the transition temperature, its increase with an increase in the volume fraction of both kinds of nanoparticles was registered. The role of diamond nanoparticles in the structural changes and the large residual light transition and/or attenuation (memory effect) were also observed. The presented results confirmed the potential of diamond nanoparticles in nematic composites to modify their properties which could lead to final applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst14030202

References

2005 - New non-synthetic method to modify properties of liquid crystals using micro- and nanoparticles [Crossref]
2003 - Dielectric relaxation spectroscopy of a nematic liquid crystal doped with ferroelectric Sn2P2S6 nanoparticles [Crossref]
2008 - Structural changes in 6CHBT liquid crystal doped with spherical, rodlike, and chainlike magnetic particles [Crossref]
2012 - Dielectric properties of nematic liquid crystal modified with diamond nanoparticles [Crossref]
1970 - Theory of magnetic suspensions in liquid crystals [Crossref]
1995 - Macroscopic properties oh ferronematics caused by orientational interactions on the paticle surfaces I, II [Crossref]
2009 - Surface anchoring energy and the Freedericksz transitions in ferronematics [Crossref]
2011 - Magnetic sensitivity of dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystal [Crossref]