Dmitry Isakov

Microwave dielectric characterisation of 3D-printed BaTiO3/ABS polymer composites

3D printing is used extensively in product prototyping and continues to emerge as a viable option for the direct manufacture of final parts. It is known that dielectric materials with relatively high real permittivity—which are required in important technology sectors such as electronics and communications—may be 3D printed using a variety of techniques. Among these, the fused deposition of polymer composites is particularly straightforward but the range of dielectric permittivities available through commercial feedstock materials is limited. Here we report on the fabrication of a series of composites composed of various loadings of BaTiO3 microparticles in the polymer acrylonitrile butadiene styrene (ABS), which may be used with a commercial desktop 3D printer to produce printed parts containing user-defined regions with high permittivity. The microwave dielectric properties of printed parts with BaTiO3 loadings up to 70 wt% were characterised using a 15 GHz split post dielectric resonator and had real relative permittivities in the range 2.6–8.7 and loss tangents in the range 0.005–0.027. Permittivities were reproducible over the entire process, and matched those of bulk unprinted materials, to within ~1%, suggesting that the technique may be employed as a viable manufacturing process for dielectric composites.

Oriented single-crystal-like molecular arrangement of optically nonlinear 2- methyl-4-nitroaniline in electrospun nanofibers

In-plane aligned nanofibers of organic 2-methyl-4-nitroaniline (MNA) were produced by the electrospinning technique using a 1:1 weight ratio with poly(l-lactic acid). The fibers are capable of enormous efficient optical second harmonic generation as strong as pure MNA crystals in powder form. Structural, spectroscopic, and second harmonic generation polarimetry studies show that the MNA crystallizes within the fibers in an orientation in which the aromatic rings of MNA are predominantly orientated edge-on with respect to the plane of the fiber array and with their dipole moments aligned with the fiber axis. The results show that the electrospinning technique is an effective method to fabricate all-organic molecular functional devices based on polymer nanofibers with guest molecules possessing strong nonlinear optical and/or polar properties.

Study of ferroelectric domain switching by domain wall induced light scattering

The 90°-light scattering on domain walls was probed in various strontium barium niobate (SBN) crystals for studies of the ferroelectric switching under pulsed fields. The validity of this optical method is proved by a good agreement of the switching parameters deduced from optical scattering data with those obtained with electric methods. Scanning of the scattering over the crystal bulk revealed local specialities of the switching, particularly, a marked distribution of the domain wall density D along the polar axis with a maximum close to the negative electrode. In compliance with these in situ observations, the electro-optic coefficient rc reveals a position dependence in all SBN crystals poled in the ferroelectric phase, rc decreasing from the positive to negative electrode. This regularity is interpreted in terms of the domain density distribution D(z) and accounted for by an asymmetry of the domain nucleation.

Manufacture of electrical and magnetic graded and anisotropic materials for novel manipulations of microwaves.

Spatial transformations (ST) provide a design framework to generate a required spatial distribution of electrical and magnetic properties of materials to effect manipulations of electromagnetic waves. To obtain the electromagnetic properties required by these designs, the most common materials approach has involved periodic arrays of metal-containing subwavelength elements. While aspects of ST theory have been confirmed using these structures, they are often disadvantaged by narrowband operation, high losses and difficulties in implementation. An all-dielectric approach involves weaker interactions with applied fields, but may offer more flexibility for practical implementation. This paper investigates manufacturing approaches to produce composite materials that may be conveniently arranged spatially, according to ST-based designs. A key aim is to highlight the limitations and possibilities of various manufacturing approaches, to constrain designs to those that may be achievable. The article focuses on polymer-based nano- and microcomposites in which interactions with microwaves are achieved by loading the polymers with high-permittivity and high-permeability particles, and manufacturing approaches based on spray deposition, extrusion, casting and additive manufacture.

Ferroelectric switching of strontium–barium–niobate crystals in pulsed fields

In strontium–barium–niobate (SBN) polarization and switching processes under pulsed field differ from those in model ferroelectrics. In most SBN crystals, the polarization changes smoothly with relaxation times up to seconds even under fields strongly exceeding the coercive field. The kinetics are described with a power law. In some samples of a variety of compositions faster changes have been observed, but their characteristics fundamentally differ from those in model ferroelectrics. The peculiarities are evidently caused by the relaxor properties of SBN.

Ferroelectric characterization of aligned barium titanate nanofibres

We report the synthesis, structural and ferroelectric characterization of continuous well-aligned nanofibres of barium titanate produced by the electrospinning technique. The fibres with average diameter of 150–400 nm consist of connected nanoparticles of BaTiO3 stacked together to form the shape of a long filament. The tetragonal phase in the obtained nanofibres was revealed by the x-ray diffraction and Raman spectroscopy and has been also confirmed by the second harmonic generation (SHG) and piezoresponse force microscopy (PFM). The temperature dependence of the SHG in the vicinity of the paraelectric–ferroelectric phase transition suggests that barium titanate nanofibres are indeed ferroelectric with an apparent glass-like state caused by metastable polar nanoregions. The existence of domain structure and local switching studied by PFM present clear evidence of the polar phase at room temperature.

Piezoresponse force microscopy studies of the triglycine sulfate-based nanofibers

Local ferroelectric and piezoelectric properties of triglycine sulfate (TGS) nanocrystals embedded into poly(ethylene) oxide (PEO) electrospun fibers were examined by piezoresponse force microscopy (PFM). Piezoresponse contrast was found to be strongly dependent on the position being much stronger at the fiber periphery. A model of the distribution of TGS crystals inside the core-shell PEO-TGS fiber structure was proposed. TGS nanocrystals were unevenly distributed along the fiber axis and tend to grow near the surface of the fiber. The volume fraction distribution of the TGS crystals extracted from PFM measurements is in a good agreement with the data obtained from the dielectric constant measurements.

High nonlinear optical anisotropy of urea nanofibers

Nanofibers consisting of the optically nonlinear organic molecule urea embedded in both poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) polymers were produced by the electrospinning technique. The second-harmonic generation produced by aligned fiber mats of these materials displays a strong dependence on the polarization of the incident light. In PVA-urea nanofibers the effectiveness in generating of the second-harmonic light is as high as that of a pure urea powder with an average grain size of 110 μm. The results suggest that single crystalline urea nanofibers were achieved with a long-range crystalline order extending into the range of 2–4 μm with PVA as the host polymer.

Polar and antiferroelectric behaviour of a hybrid crystal – piperazinium perchlorate

Monoprotonated piperazinium perchlorate, [NH2(CH2)4NH][ClO4], appeared to be a novel room temperature polar material (P1). Its acentric symmetry was confirmed by single-crystal X-ray diffraction, second harmonic generation (SHG) and pyroelectric measurements. Differential scanning calorimetry (DSC) measurements revealed a complex sequence of phase transitions above room temperature: I ↔ II at 433/422 K (heating–cooling), II ↔ III at 417/411 K, III ↔ IV at 403/395 K and IV ↔ V at 397 K (the lowest temperature phase transition recorded only upon heating). The characteristic feature of the structure of [NH2(CH2)4NH][ClO4] is the presence of two parallel cationic chains which are connected with each other by strong N–H⋯N hydrogen bonds. In phase V, these strongly polar non-equivalent chains contribute to spontaneous polarization. 1H NMR measurements disclosed the reorientational motions of the piperazinium ([NH2(CH2)NH]+) cations as well as the proton motion in the N–H⋯N hydrogen bonds along the piperazine chain. Over phase I, the overall motions of the ClO4− anions and reorientational motion of cations are postulated. The dielectric response, e′(T), accompanying the PT I ↔ II indicates possible antiferroelectricity in phase II.

Modification of the optical and photorefractive properties of Ce-doped strontium-barium niobate by co-doping with a nonphotorefractive La impurity

We demonstrate a possibility to modify photorefractive properties of a ferroelectric crystal by means of controlling the ferroelectric phase transition. This is shown by an example of co-doping a photorefractive SBN-0.61:Ce crystal with a nonphotorefractive La impurity, which strongly lowers the ferroelectric phase transition and in turn drastically enhances the linear electrooptic coefficients. The observed low coercive fields enable us to build in SBN:(Ce+La) a reproducible fast switching of the energy transfer direction. This is done by an electric field switching which changes the sign of the two-beam coupling gain factor Γ.