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.

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Additive manufacturing of complex structures with spatially varying electromagnetic properties can enable new applications in high-technology sectors such as communications and sensors. This work presents the fabrication method as well as microstructural and dielectric characterization of bespoke composite filaments for fused deposition modeling (FDM) 3D printing of microwave devices with a high relative dielectric permittivity ϵ=11 in the GHz frequency range. The filament is composed of 32 vol % of ferroelectric barium titanate (BaTiO3) micro-particles in a polymeric acrylonitrile butadiene styrene (ABS) matrix. An ionic organic ester surfactant was added during formulation to enhance the compatibility between the polymer and the BaTiO3. To promote reproducible and robust printability of the fabricated filament, and to promote plasticity, dibutyl phthalate was additionally used. The combined effect of 1 wt % surfactant and 5 wt % plasticizer resulted in a uniform, many hundreds of meters, continuous filament of commercial quality capable of many hours of uninterrupted 3D printing. We demonstrate the feasibility of using the high dielectric constant filament for 3D printing through the fabrication of a range of optical devices. The approach herein may be used as a guide for the successful fabrication of many types of composite filament with varying functions for a broad range of applications.

Electrospinning of an aqueous poly(vinyl alcohol) and glycine solution has been used to produce nanofibers with an embedded crystalline glycine exclusively in the form of the β polymorph. The β-glycine nanocrystals are highly oriented within the polymer fibers and present good polar properties. Piezoelectric and nonlinear optical responses have been quantitatively examined showing piezoelectric coefficient d eff = 12.5 pm/V and an effective nonlinear optical susceptibility two times greater than that of potassium dihydrogen phosphate (KDP). Additionally, although bulk β-glycine is metastable at room temperature, when confined inside the polymeric nanofibers, it is shown to be remarkably stable.

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.

1 of 6) 1600072 wileyonlinelibrary.com ring resonators have been used as subwavelength unit cells to build a metamaterial-based GRIN medium. [ 5b,10 ] However, these types of metamaterial lenses with wide spatial variation in the refractive index have limited application because of their narrow bandwidth, high dissipation, and the relative diffi culty and high cost of fabrication.Alternatively, spatial variation in refractive index may also be achieved using an "all-dielectric" approach in which the GRIN structure is realized by, for example, patterning air voids within the volume of a polymer lens. [ 11 ] While this approach has some benefi ts in ease of fabrication, especially when using 3D printing technology, [ 12 ] the range of achievable refractive indices is limited to the range between the dielectric properties of the polymer matrix (with relative dielectric permittivity ε typically in the range of 2-2.7) and air ( ε = 1). To expand this range, appropriately designed voids in different 3D-shaped GRIN lenses were fi lled with liquid acetonitrile/benzene mixture with a relative permittivity of up to ε = 37. [ 13 ] However, the performance of the device was constrained by the relatively high intrinsic dielectric loss of acetonitrile. [ 14 ] Alternatively, vacuum casting of titanate powders dispersed in a liquid polymer has been used to fabricate a planar hyperbolic lens. [ 15 ] The process involved the manufacture in sequence of concentric layers of the polymer/titanate composite with hemispherical geometry, each with a different permittivity tailored by a change in the local volume fraction of the titanate powder.Despite these advances in the fabrication of objects with relatively complex geometry and broader range of spatially varying refractive index, these processes are comparatively expensive and infl exible. The ability to fabricate new feedstock materials with much higher dielectric permittivity suitable for mass-market fused deposition 3D printing technology has been demonstrated. [ 16 ] 3D printing potentially offers a more fl exible and scalable capability for the fabrication objects with complex geometries as well as signifi cant contrast in the spatial variation of refractive index. Using a simple process to mix homogeneously polymer pellets and high permittivity and low-loss powder ceramic titanates into long lengths of composite feedstock fi lament with a relative permittivity up to 10, 1D, 2D, and 3D periodic and graded structures [ 16b ] and anisotropic structures with metamaterial features at 12-20 GHz frequency region have been successfully 3D printed. [ 16c ] In this paper, we report the design, rapid fabrication using high-dielectric material, and performance of a 3D-printed Gradient refractive index (GRIN) materials are of interest for various applications where transformation optic principles can be applied to the design of improved photonic and microwave devices. GRIN materials comprise spatially varying electric and/or magnetic properties that challenge conventional manufacturi...

Glycine is a model crystal exhibiting three polymorphic phases and important functional properties such as piezoelectricity and ferroelectricity. We report here in situ observation of the irreversible transformation of the solutiongrown glycine crystals from a β phase into a γ phase. The slow transformation process was monitored by piezoresponse force microscopy at room temperature. The process of β to γ conversion was entirely controlled by the variation of relative humidity in the sample chamber. The results show that the rate of phase transformation in glycine is humidity dependent with a threshold of about 25% RH. It is demonstrated that the phase boundary is highly rugged and the transformation front propagates inhomogeneously along the polar axis of the β phase. The mechanism of the phase transformation is discussed.

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.