[Show abstract][Hide abstract] ABSTRACT: Systems and methods are described for producing liquid crystal devices microwave that use metal alignment layers. The liquid crystal microwave devices include a liquid crystal layer sandwiched between metal layers, which are further sandwiched between substrate layers. At least one of the metal layers is rubbed to form micro-grooves that tend to align the liquid crystal molecules adjacent to those layers without using additional rubbed polymeric layers for the alignment. The thickness of the metal layers can be selected to yield a desired electromagnetic effect, such as constraint of propagation of microwave radiation.
[Show abstract][Hide abstract] ABSTRACT: The biomedical applications of ferroelectric nanoparticles rely on the production of stable aqueous colloids. We report an implementation of the high energy ball milling method to produce and disperse ultrafine BaTiO3 nanoparticles in an aqueous media in a single step. This technique is low-cost, environmentally friendly and has the capability to control nanoparticle size and functionality with milling parameters. As a result, ultrafine nanoparticles with sizes as small as 6 nm can be produced. These nanoparticles maintain ferroelectricity and can be used as second harmonic generating nanoprobes for biomedical imaging. This technique can be generalized to produce aqueous nanoparticle colloids of other imaging materials.
[Show abstract][Hide abstract] ABSTRACT: A new module for the measurement of magneto-electric properties was developed as an add-on for a magnetic AC susceptibility option of a Physical Properties Measurement System (PPMS). The module is capable of recording direct dynamic and static converse magneto-electric effect, i.e., the change in electric polarization due to the application of a small AC magnetic field with a DC magnetic field bias, or the change in the magnetic moment induced by an applied electric field. The versatile module setup supports both measurements in a sequential order without the need of removing or repositioning the sample. Furthermore, AC and DC magnetic susceptibilities can be recorded while performing direct and inverse magneto-electric measurements, respectively, which adds outstanding capabilities to the existing instrument while saving time and resources. Measurements are fully automated and integrated in the PPMS Multivu software platform. Magneto-electric behavior of a BaTiO3/CoFe2O4 and BaTiO3/NiFe2O4 magneto-electric composites, and a Pb(Fe0.5Nb0.5)O3 single phase compound were recorded as test measurements.
No preview · Article · Aug 2014 · Review of Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: Fabrication of a thermotropic ferronematic colloid is reported, consisting of a 1% concentration of magnetite nanorods dispersed in a commercially obtained liquid crystal (LC). To produce the nanorods, solvothermal synthesis was utilized by adding Fe(CO)5 to a reaction solution of hexadecylamine and oleic acid in n-octanol. The procedure allows for tailoring of the size and shape of the nanorods. The magnetic nanorods were functionalized with oleic acid. Magneto-optic and electro-optic properties of the colloid were characterized. Specifically, static measurements were performed to determine the birefringence and magnetic and electric Freedericksz thresholds. From the dynamic measurements, the switching time was determined. The nanorods increased the effective magnetic anisotropy of the LC. Depending on nanorod dimensions, the colloid demonstrated faster switching speeds, especially in the presence of crossed electric and magnetic fields.
[Show abstract][Hide abstract] ABSTRACT: To prepare cholesteric liquid crystalline nonlinear optical materials with ability to be vitrified on cooling and form long time stability cholesteric glasses at room temperature, a series of platinum acetylide complexes modified with cholesterol has been synthesized. The materials synthesized have the formula trans-Pt(PR 3)(cholesterol (3 or 4)-ethynyl benzoate)(1-ethynyl-4-X-benzene), where R = Et, Bu or Oct and X = H, F, OCH 3 and CN. A cholesteric liquid crystal phase was observed in the complexes R = Et, and X = F, OCH 3 and CN but not in any of the other complexes. When X = CN, a cholesteric glass was observed at room temperature which remained stable up to 130 °C, then converted to a mixed crystalline/cholesteric phase and completely melted to an isotropic phase at 230 °C. When X = F or OCH 3 the complexes were crystalline at room temperature with conversion to the cholesteric phase upon heating to 190 and 230 °C, respectively. In the series X = CN, OCH 3 and F, the cholesteric pitch was determined to be 1.7, 3.4 and 9.0 µ, respectively.
[Show abstract][Hide abstract] ABSTRACT: Cobalt ferrite (CoFe2O4) and manganese substituted cobalt ferrite (Co0,25Mn0,75Fe2O4) were synthesized by coprecipitation and sol-gel methods. X-ray diffraction revealed that manganese ions slightly enlarged the crystal lattice with respect to pure cobalt ferrite. Magnetization, linear magnetostriction, and piezomagnetic coefficient values tended to decrease with the ion substitution. Linear magnetostriction dropped from a value of −111.5 ppm for cobalt ferrite to values of −22.6 and −30.6 ppm for the substituted samples obtained from coprecipitation and sol-gel, respectively. The sample prepared by sol-gel technique resulted in a higher magnetization, magnetostriction, and piezomagnetic coefficient than that synthesized by coprecipitation method.
No preview · Article · Jan 2014 · IEEE Transactions on Magnetics
[Show abstract][Hide abstract] ABSTRACT: In this paper, we will explore how optical and ferroelectric properties of the stressed ferroelectric nanoparticles prepared through ball milling set a limit on the performance of optical and electro-optical devices based on such materials. It was found that suspensions of BaTiO<sub>3</sub> nanoparticles exhibit a blue shift in the optical band gap with a decrease in particle size. The optical band gap of PbTiO<sub>3</sub> nanoparticles is not affected by the milling time. Polarization switching is composed of slow and fast components. A slow component is threshold-less and is caused by the particle reorientation while a fast component has a threshold, and its rise time is inversely proportional to the electric field. The absorption edge of these suspensions accounts for the applications in the near UV range, while kinetics of the polarization switching governs the speed of electro-optical devices.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we will explore how optical and ferroelectric properties of the stressed ferroelectric nanoparticles prepared through ball milling set a limit on the performance of optical and electro-optical devices based on such materials. It was found that suspensions of BaTiO3 nanoparticles exhibit a blue shift in the optical band gap with a decrease in particle size. The optical band gap of PbTiO3 nanoparticles is not affected by the milling time. Polarization switching is composed of slow and fast components. A slow component is threshold-less and is caused by the particle reorientation while a fast component has a threshold, and its rise time is inversely proportional to the electric field. The absorption edge of these suspensions accounts for the applications in the near UV range, while kinetics of the polarization switching governs the speed of electro-optical devices.
[Show abstract][Hide abstract] ABSTRACT: There are two switching processes where observe in polymer-dispersed
liquid crystals (PDLC) when pulse electric field applied: - Slow
switching process with rise time hundreds microseconds; - Fast switching
process with nanoseconds rise time. The result of research, design and
testing ultra-fast PDLC optical gate is presented. The feasibility of
100 nsec rise time optical gate with 1 square inch crystal clear
transmission (better than 1.54 dB) and attenuation in OFF state more
than 26 dB (30.4 dB for two serial layers) for non-polarized light has
No preview · Article · Mar 2013 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: Liquid crystal (LC)/magnetic nanorods colloids were fabricated and tested using a magneto-optical setup. These thermotropic ferronematics do not show any signs of macroscopic aggregation, exhibit enhanced magnetic sensitivity, and faster time response in the simultaneous presence of crossed electric and magnetic fields. Magnetic nanorods increase an effective magnetic anisotropy of the colloid and decrease magnetic Freedericksz threshold. Applying a magnetic field along the direction perpendicular to the applied electric field leads to a decrease of the time OFF by a factor of 6 for pure liquid crystals, and by a factor of 9—for ferronematics.
No preview · Article · Nov 2012 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We demonstrate an on-wafer liquid crystal phase shifter which has a tunable 0-300 degrees/cm phase shift at 110 GHz. The results show no dispersion over the entire frequency range indicating a tunable "true time delay" of up to 2.5 ps/cm at all frequencies. The inherent losses in the liquid crystal are small, less than 1 dB/cm over the range of 1-110 GHz. The full tunability is achieved using small voltages, close to 10 V. We anticipate that one could achieve a phase shift of 600 degrees/cm at 220 GHz. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691202]
No preview · Article · Mar 2012 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Stressed ferroelectric nanoparticles, less than 10 nm in diameter, are investigated in a nonpolar fluid. Measuring ferroelectric properties of the smallest achieved BaTiO3 ferroelectric nanoparticles allows for the determination of dipole moment and spontaneous polarization as a function of size and concentration; the results are significantly greater than in bulk BaTiO3. This is achieved by using a direct measurement of the displacement current density and either integrating over half a period or fitting the experimental results using the derivative of the Langevin function.
[Show abstract][Hide abstract] ABSTRACT: Nearly all liquid crystal devices use a rubbed organic layer as a method of orienting the liquid crystals. This letter studies the alignment of nematic liquid crystals by rubbed and nonrubbed metallic surfaces. For rubbed metallic films, a homogeneous planar alignment of liquid crystals is found. Nonrubbed metallic surfaces align liquid crystals nonuniformly and randomly. The alignment produced by a single rubbed metallic surface extends from 10 to 50 μ m and is stable in time. These results are important because they show that the organic layer may be eliminated for some applications, including tunable microwave and infrared signal processing elements.
Full-text · Article · Mar 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: In this work we report methods of formation of three-dimensional structures of particles in a liquid crystal host. We found that, under the appropriate conditions, the particles are captured and dragged by the moving isotropic/nematic front during the phase transition process. This movement of the particles can be enhanced significantly or suppressed drastically by application of an electric field and/or by changing the conditions of the phase transition, such as the rate of cooling. As a result, a wide variety of particle structures can be obtained ranging from a fine-grained cellular structure to stripes of varying periods to a course-grained "root" structure. Changing the properties of the materials, such as the size and density of the particles and the surface anchoring of the liquid crystal at the particle surface, can also be used to control the morphology of the three-dimensional particle network and adjust the physical properties of the resulting dispersions. INTRODUCTION Colloidal dispersions of small particles in nematic liquid crystals are a novel, interesting type of soft matter. The difference from ordinary colloids arises from the orientational ordering of the liquid crystal molecules and the resulting structure in the colloid. Topological defects [1, 2] and additional long-range forces between the colloidal particles  are immediate consequences of this ordering. The nematic-induced interparticle interaction brings a new range of effects to the system: supermolecular structures [4,6], cellular structures [7,8], and even a soft solid  can be observed. Colloidal dispersions in liquid crystals also have a wide variety of potential applications . A range of problems also arises in nematic colloidal dispersions. The nematic ordering makes it difficult to suspend small particles in a liquid crystal host . Particles often segregate into agglomerates distributed non-uniformly in the cell. The resulting spatial distribution of the particles is difficult to control. Our research therefore explores the factors that affect the spatial distribution of these particles and thus control morphology of nanoparticles – liquid crystal colloids. In our previous work, we reported the first demonstration of drag on colloidal particles by a moving nematic-isotropic (NI) interface . We calculated a critical radius above which the particles cannot be captured by the moving interface. We showed that this critical radius is sensitive to the viscous properties of the host liquid crystal, the value of the anchoring coefficient of the liquid crystal on the particle surface, and the velocity of the moving interface. In the experimental work reported here we demonstrate the methods used to control the spatial distribution of particles of different sizes in a liquid crystal cell. We show that a uniform or specially designed non-uniform electric field and/or changing the conditions of the nematic-isotropic phase transition, such as the rate of cooling, can be used to control the morphology of the three-dimensional particle network and adjust the physical properties of the resulting dispersions and displays.
[Show abstract][Hide abstract] ABSTRACT: Ordered colloids are of great scientific and practical interests. Liquid crystals offer enhanced ways of producing and stabilizing these complex structures. We therefore studied the Theological and electro-rheological properties of the structured colloids as a means of probing this stabilization. We found that the mechanical properties of the colloids and stability of their 3D structures can be controlled by the particles size and distribution. In addition, when an electric field is applied, we observed an increase in the apparent viscosity with saturation at high electric fields. This effect depends on the shear rate and temperature. The results are also compared with the published data for the viscosity measurements of pure liquid crystals and isotropic colloids. While we are only beginning to understand the details of these complex colloids we expect they will find a wide variety of applications.
No preview · Article · Jan 2011 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: We report on the first observation of a non-monotonic UV exposure dependence of the pretilt angle of a liquid crystal on a photoaligning surface. We found this non-monotonic dependence on fluorinated poly(vinyl cinnamate) (PVCN-F) surface while non-fluorinated PVCN revealed a monotonic decrease of the pretilt angle with exposure. These dependences correlate with light-induced changes of the polarity of PVCN-F and PVCN surfaces. The obtained data point to an important role of the surface polarity and the light-induced changes in mechanisms of pretilt angle generation on photoaligning materials.
[Show abstract][Hide abstract] ABSTRACT: We describe techniques to selectively harvest single ferroelectric domain nanoparticles of BaTiO <sub>3</sub> as small as 9 nm from a plethora of nanoparticles produced by mechanical grinding. High resolution transmission electron microscopy imaging shows the unidomain atomic structure of the nanoparticles and reveals compressive and tensile surface strains which are attributed to the preservation of ferroelectric behavior in these particles. We demonstrate the positive benefits of using harvested nanoparticles in disparate liquid crystal systems.
Full-text · Article · Oct 2010 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Nanoparticle/liquid crystal colloidal dispersions are a new, rapidly developing, and exciting frontier of soft matter science and technological progress. This is both because of the dramatic changes in liquid crystal characteristics caused by adding nanoparticles and because of the limited understanding of the structure and interactions of the components of such composite systems.
In this review, we summarize the basic science and applications of various nanoparticles, 1–100 nm in diameter, dispersed in liquid crystals. The historical development of this area of science is divided into two main periods. The first period, from the early 1970s until the year 2000, covers the structural organization and the interaction of passive nanoparticles (mostly dielectric) dispersed in liquid crystals. This period initiated the development of novel electro-optic effects and devices based on the properties of such composites. The second period covers progress during the first decade of the millennium. It is a Renaissance era in the development of liquid crystalline nanocolloids when the power of nanoscale was demonstrated to the full extent. Active nanoparticles, mostly ferroelectric, ferromagnetic, and ferroic, added to liquid crystals, shared their intrinsic properties with the liquid crystal host, and allowed for the creation of unprecedented materials with astonishing applications. These active particles generate ultrahigh electric and/or magnetic fields within the liquid crystal host, which, combined with their small size, produces a uniquely exciting and largely unexplored system of composite materials which exhibit novel collective particle–host interactions and which promise a variety of exotic electro-magneto-optic and other applications. These internally generated fields, larger than any macroscopic field that can be applied, serve both to spatially organize the particles and to couple the particle system to the liquid crystal thus enabling the liquid crystal ordering to affect the collective particle order.
Although we have made an effort to review liquid crystalline/nanocolloids research as broadly as possible, many great results are omitted from this chapter. We hope this review will be an invitation to further explore this unique and exiting collection of knowledge.
No preview · Article · Jan 2010 · Solid State Physics