Dielectric properties of SrTiO3/POE flexible composites for microwave applications

Composites fabricated with SrTiO₃ ceramic filler dispersed inside a thermoplastic polyolefin elastomer (POE) polymer matrix, with excellent dielectric properties and good flexibility, have been studied in this paper. The relative permittivity of SrTiO₃/POE composites blended with different volume fraction of ceramic filler was investigated as a function of temperature and frequency. The results indicated that with the increase of ceramic filler, both the relative permittivity and the dissipation factor of composites increased. Good frequency stability within a wide range was observed in all the samples. The theoretical dielectric constant obtained from the effective medium theory of Bruggeman's model is in good agreement with the experimental data. For the composites containing 40 vol.% SrTiO_3, the dielectric constant and the loss tangent were 11.0 and 0.01 at 900 MHz, respectively, while the mechanical test results showed good flexibility on the final quality of the composites with the elongation of 90%. This indicates that the ST/POE composites have the promising characteristics for potential applications in the flexible dielectric waveguide and related flexible microwave devices.     

Novel high dielectric constant and low loss PTFE/CNT composites

Spherical Ca_0.55Nd_0.3TiO₃ ceramic filled polytetrafluoroethylene composites (abbreviated as PTFE/CNT) with different filler volume fractions were prepared. The effects of filler volume fraction on microstructure, dielectric properties and thermal property were studied by scanning electron microscope, vector network analyzer and thermal dilatometer, respectively. The SEM results show that spherical particles are advantageous to reduce the porosity in the interphase which would increase the dielectric loss. Moreover, both the dielectric constant and dielectric loss increased with the increasing volume fraction of CNT microspheres. The high dielectric constant and low dielectric loss composite can be prepared when the ceramic volume fraction is 50?V%: ε_r =?12, tan?δ?=?8.5?×?10^?4 (at 10?GHz). Different models were used to predict the dielectric constant of composite, and the effective medium theory shows the least deviation from the experiment. The experimental coefficient of thermal expansions of composites with different volume fractions were less than theoretical data due to the change from loosely bound polymer chain to tightly bound polymer chain which would restrain the coefficient of thermal expansions of composites.     

Microwave dielectric properties of flexible butyl rubber–strontium cerium titanate composites

Butyl rubber–strontium cerium titanate (BS) composites have been prepared by hot pressing. The tensile tests show that the BS composites are flexible. The dielectric properties of the composites have been investigated at 1 MHz and 5 GHz as a function of ceramic contents. The composite with volume fraction 0.43 of ceramic filler has a dielectric constant (ε_r) of 11.9 and dielectric loss (tan δ) 1.8 × 10^?3 at 5 GHz. The measured values of ε_r are compared with the effective values calculated using different theoretical models. The thermal conductivity of the composites is found to increase with ceramic contents and reaches a value of 4.5 Wm^?1 K^?1 for maximum filler loading 0.43 volume fraction. The coefficient of thermal expansion of the composites decreases gradually with filler loading and reaches a minimum value of 30.2 ppm °C^?1 at a volume fraction 0.43. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of Ca[(Li1/3Nb2/3)0.8Ti0.2]O3‐δ filler on the microwave dielectric properties of polyethylene and polystyrene for microelectronic applications

Ceramic reinforced polyethylene and polystyrene composites were prepared by melt mixing and hot molding techniques. Temperature stable low‐loss Ca[(Li_1/3Nb_2/3)_0.8Ti_0.2]O_3‐δ (CLNT) ceramic was used as the filler to improve the dielectric properties of the polymers. The relative permittivity and dielectric loss in the microwave frequency range were increased with increase in the ceramic loading. As the filler content increased from 0 to 0.50 volume fraction, the relative permittivity increased from 2.3 to 9 and dielectric loss tangent from 0.0006 to 0.005 for polyethylene‐CLNT composite. In the case of polystyrene‐CLNT composite, the relative permittivity and dielectric loss tangent increased from 2.1 to 10.5 and 0.0005 to 0.0032 respectively with increase in filler content from 0 to 0.50 volume fractions. The thermal stability of the relative permittivity of polymer ceramic composites was also investigated. The experimentally observed relative permittivity was compared with theoretical models. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Low dielectric loss LNT/PEEK composites with high mechanical strength and dielectric thermal stability

(3-Aminopropyl)triethoxysilane treated La_(2−x)/3Na_0.06TiO₃ (x = 0.06) (LNT) microparticles filled polyetheretherketone (PEEK) composites were prepared using hot pressing process. The effects of variation of LNT ceramic filling fraction on dielectric properties, water absorption, thermal stability and mechanical strength were investigated. All composites demonstrate low water absorption (less than 0.4%) when the ceramic filling fraction is lower than 0.6V_f. The obtained composites exhibited dielectric permittivities varying from ~4 to ~22 as the ceramic fillers increased from 0.1 to 0.8V_f and low losses (~10⁻⁴ @1 MHz, 3~5 × 10⁻³ at the frequencies of microwave (10 GHz) and millimeter wave (29-50 GHz), respectively). The mechanical strength, dimensional and dielectric thermal stability of the composite are remarkably improved by the addition of LNT ceramic fillers. A composite with near zero temperature coefficients of dielectric permittivity or resonant frequency and flexural strength of ~140 MPa could be obtained. The out-of-plane coefficient of thermal expansion (CTE) could be reduced to ~20 ppm/°C as the ceramic filler loading reached 0.7V_f.     

Effect of silane coupling agent on the dielectric and thermal properties of DGEBA-forsterite composites

Diglycidyl ether of bisphenol A (DGEBA) -forsterite composites have been prepared through mechanical mixing process and the influence of silane coupling agent on the microstructure, dielectric and thermal properties were studied. Phase pure forsterite (Mg₂SiO₄) powder was prepared through solid state ceramic route. Filling fraction of forsterite in DGEBA matrix was varied from 10 to 40 vol%. The morphology and filler distribution of filled composite were studied by Scanning Electron Microscopy. Waveguide cavity perturbation technique was employed to measure the dielectric properties of composites. It is found that aminosilane treatment increased the dielectric constant and dielectric loss of the composites in both microwave and radio frequency ranges compared to composites prepared using untreated powders. Coefficient of thermal expansion of composites decreased with the forsterite addition and attains a relatively low value of 45 ppm/°C for composite containing 40 vol% surface treated filler.     

Dielectric,Mechanical, and Thermal Properties of Low-Permittivity Polymer–Ceramic Composites for Microelectronic Applications

A new low-permittivity polymer–ceramic composite for packaging applications has been developed. The ceramic-reinforced polyethylene and polystyrene composites were prepared by melt mixing and hot molding techniques. Low-loss, low-permittivity Li₂MgSiO₄ (LMS) ceramics prepared by the solid-state ceramic route were used as the filler to improve the dielectric properties of the composites. The relative permittivity and dielectric loss were increased with the increase in the ceramic loading at radio and microwave frequencies. The mechanical properties and thermal conductivity of the Li₂MgSiO₄-reinforced polymer–ceramic composite were also investigated. The stability of the relative permittivity of polymer–ceramic composites with temperature and frequency was investigated. The experimentally observed relative permittivity, thermal expansion, and thermal conductivity were compared with theoretical models.     

Microwave dielectric properties of PTFE/CaTiO3 polymer ceramic composites

CaTiO₃ ceramic powder filled polytetrafluoroethylene (PTFE) composites with various filler volume fractions up to 60 vol.% were prepared. The effects of volume fraction of the ceramic filler on the microstructure and microwave dielectric properties of the composites were investigated in detail. As the volume fraction of the ceramic filler increases, the dielectric constant (?_r) and the temperature coefficient of resonant frequency (τ_f) of composites increase, while the product of quality factor and frequency (Q × f) decreases. Composites with 40 vol.% CaTiO₃ exhibited good microwave dielectric properties: ?_r = 13 at ∼5 GHz, Q × f = 930 GHz, and τ_f = 260 ppm/°C. Different mixing rules were used to predict the dielectric constant of composites, and it was found that the dielectric constants predicted by Effective Medium Theory (EMT) were in good agreement with experimental data.     

Effect of the filler morphology on the crystallization behavior and dielectric properties of the polyvinylidene fluoride-based composite

Ceramic/polymer composites can be chemically stable, mechanically strong, and flexible, which make them candidates for electric devices, such as pressure or temperature sensors, energy storage or harvesting devices, actuators, and so forth. Depending on the application, various electrical properties are of importance. Polymers usually have low dielectric permittivity, but increased dielectric permittivity can be achieved by the addition of the ceramic fillers with high dielectric constant. With the aim to enhance dielectric properties of the composite without loss of flexibility, 5 wt% of BaTiO₃-Fe₂O₃ powder was added into a polyvinylidene fluoride matrix. The powder was prepared by different synthesis conditions to produce core/shell structures. The effect of the phase composition and morphology of the BaTiO₃-Fe₂O₃ core/shell filler on the structure and lattice dynamics of the polymer composites was investigated. Based on the results of the thermal analysis, various parameters of ceramic/polymer composites were determined. Differences in the phase composition and morphology of the filler have an influence on the formation of various polyvinylidene fluoride allomorphs and the degree of crystallinity. Furthermore, the dielectric performances of pure polyvinylidene fluoride and the polymer/ceramic composites were measured.     

Composition dependence of dielectric properties,elastic modulus,and electroactivity in (carbon black‐BaTiO3)/silicone rubber nanocomposites

The dielectric properties, elastic modulus, and electromechanical responses of dielectric elastomers (DEs) consisting of silicone rubber and carbon black (CB) incorporated with BaTiO₃ (BT) were studied. When compared with single filler/rubber composites, the resulting three‐component nanocomposites yielded very abnormal phenomena. They might be attributed to the interactions between the two kinds of fillers. The increase in concentration of CB (BT) would play a destructive role to the network structure formed by BT (CB) particles. The maximum electromechanical strain of the nanocomposites achieved at mass fraction m_CB = 0.03 and m_BT = 0.06. The resultant electromechanical strain would be attributed to the large dielectric permittivity in the three‐component nanocomposites, in which the BT particles themselves have a high dielectric permittivity and the electrical networks of CB particles have a contribution on the increase in dielectric permittivity of the three‐component nanocomposites. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Butyl rubber–Ba0.7Sr0.3TiO3 composites for flexible microwave electronic applications

Butyl rubber–Ba_0.7Sr_0.3TiO₃ composites (BR–BST) were prepared by sigma mixing followed by hot pressing. The stress–strain studies show the good flexibility of the composite. The dielectric properties of the composites were investigated at both radio and microwave frequencies. The relative permittivity (ε_r) and loss tangent (tan δ) improved with filler loading at both the frequencies. The relative permittivity and loss tangent of the BR–BST composites at a maximum filler loading of 0.39 volume fraction (v_f) are 13.1 and 0.009 respectively at 5 GHz measured by Split Post Dielectric Resonator (SPDR). The effective relative permittivity of the BR–BST composites is compared with theoretical models. The variation of ε_r with temperature was also investigated in the range 22–80 °C at 1 MHz. The microwave dielectric properties of the composites are also studied after repeated bending. The coefficient of thermal expansion (CTE) of the butyl rubber–BST composites decreased with the addition of the BST ceramic.     

Preparation and characterization of high permittivity and low loss PTFE/CaTiO3 microwave laminates

This article discusses the preparation and characterization of CaTiO₃ filled PTFE flexible laminates for microwave substrate applications. Single phase CaTiO₃ filler composition was prepared through solid‐state ceramic route. Phase formation was confirmed by powder X‐ray diffraction studies. Morphology and filler distribution of the composites were studied using scanning electron microscopic technique. Permittivity and loss tangent of the composite substrates were measured at X‐band frequency region (8.2–12.4 GHz) using waveguide cavity perturbation technique. Effective permittivity of the composites was compared with theoretically predicted values. Temperature coefficient of permittivity (τ_εr) of the composites was also measured in the 0–100°C temperature range. PTFE/CaTiO₃ composite has an effective permittivity of 11.8 and a loss tangent of 0.0036 at optimum filler loading. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Enhancement in electrical and thermal performance of high-temperature vulcanized silicone rubber composites for outdoor insulating applications

High-temperature vulcanized silicone rubber composites are highly desirable as outdoor insulating materials due to their immense thermal and electrical performance. The aim of this work is to study the role of co-combined fillers (modified fumed silica [MFS], titanium dioxide [TiO₂], with graphene [G]) on electrical and thermal properties of silicone rubber (S) composites. The dielectric response of S/MFS_10 phr and S/TiO₂_20 composites tailored with 2 phr G was characterized by broadband dielectric spectroscopy. The hybrid filler/composites were found to show higher thermal stability when 2 phr G was added. In addition, a low quantity of G filler was found to slightly increase the AC dielectric breakdown strength of the S/MFS_10 and S/TiO₂_20, where an improvement of 3 and 5% was found, respectively. Several steps were observed in the thermal decomposition of the S rubber composites by thermogravimetric analysis-Fourier-transform infrared spectroscopy. Our findings revealed great potentials for fabricating hybrid-filler/silicone rubber composites with enhanced electrical and thermal properties for outdoor insulating applications.     

Preparation,Characterization, and Dielectric Properties of PP/CaTiO3 Composites for Microwave Substrate Applications

Calcium titanate (CaTiO₃) filled polypropylene (PP) composites have been fabricated through compression molding method. The phase purity of the PP/CaTiO₃ composites was studied using X‐ray diffraction studies. Scanning electron microscopy technique has been employed to study the dispersion of the particulate filler in the PP matrix. The dielectric constant and loss tangent of the composites were measured at X‐band frequency region using waveguide cavity perturbation technique. PP/CaTiO₃ composite has an effective dielectric constant of 11.74 and loss tangent 0.007 at optimum filler loading. The experimental dielectric constant of filled composites was compared with theoretically predicted dielectric constant values obtained using different modeling approaches. The linear coefficient of thermal expansion of PP/CaTiO₃ composites was studied using thermomechanical analyzer.     

Flexible and low cost lead free composites with high dielectric constant

Highly flexible lead free composite film having random distribution of ceramic filler was synthesized using Barium Titanate (BT) as a filler and inexpensive Thermoplastic Polyurethane (TPU) as a matrix. The results show that the 30 vol% BT-TPU composite has a dielectric constant of ~31 which is comparable to the expensive and difficult to produce PVDF based composites. With a breakdown field of 150 kV/mm, an energy density value of ~3 J/cm³ was estimated. These lead-free TPU based composites provide an alternative to PVDF based composites for energy storage applications.     

Low dielectric loss BST/PTFE composites for microwave applications

Ba_0.3Sr_0.475Ce_0.03La_0.12Ti_0.997Mn_0.003O₃/Polytetrafluoroethylene (PTFE) composites were prepared using powder processing technique. The effects of the ceramic filler volume fraction and the coupling agent on the phase composition, microstructure, dielectric and thermal properties of the composites were investigated in this paper. The ceramic filler dispersion in the PTFE matrix, thus the dielectric loss, permittivity, and dimensional thermal stability of the composite was considerably improved by the modification of BST filler surface using phenyl trimethoxy silane (PTMS) coupling agent. Variation of the dielectric permittivity of the composite with composition was well fitted by the effective medium theory (EMT) model in the experimental compositional range. The obtained silane-treated composite with 0.5 V_f BST exhibits extremely low dielectric loss: ε_r = 16, tan δ = 5.4 × 10⁻⁴ @1 MHz and 5.16 ± 0.6 × 10⁻³ @ 10 GHz. The CTE of the composites was reduced to 43 ppm/°C.     

Medium dielectric constant and low-loss PTFE composites filled with MgO-LiF co-doped Li2TiO3 particles

In this work, we presented a simple strategy to fabricate medium dielectric constant and low-loss composites for microwave substrate applications. MgO-LiF co-doped Li₂TiO₃ (LT) powders were fabricated by the solid-state reaction route and modified by perfluorooctyltriethoxysilane (F8261). The LT/polytetrafluoroethylene (PTFE) composites were fabricated by cold pressing and hot treatment. The XPS and contact angle analysis indicated that the fluorinated group was introduced to LT particle successfully. The effects of modified LT powders content on the dielectric, thermal, and mechanical properties of composites were investigated. As the modified LT content increases, the dielectric constant, dielectric loss, and temperature coefficient of dielectric constant (τ_ε) increase while the bending strength and coefficient of thermal expansion (CTE) decrease, which is attributed to the higher dielectric constant of LT ceramic, more pores, stable τ_ε of LT ceramic, interface defects and low CTE of LT ceramic, respectively. The composites with 60 wt % LT exhibit the best microwave dielectric properties: ε_r = 6.8, tanδ = 0.001, τ_ε = −29.6 ppm °C⁻¹ at 8 GHz and acceptable coefficient of thermal expansion (28.3 ppm °C⁻¹). Therefore, modified LT powders filled PTFE composites are potential materials for high-frequency microwave substrate applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47980.     

Core@double-shell structured fillers for increasing dielectric constant and suppressing dielectric loss of PVDF-based composite films

High dielectric constant polymer dielectrics have attracted a great deal of attention in flexible electronics. However, it appears to be a paradox for polymer dielectrics that the enhancement of their dielectric constant often comes along with the increase of dielectric loss. Hence, we reported core@double shell structured filler/poly(vinylidene fluoride) (PVDF) composites to overcome this paradox. The hybrid filler with BaTiO₃ (BT) as the core, conductive carbon as the inner shell, and insulating polydopamine (PDA) as the outer shell was synthesized. As a result, the BT@C@PDA/PVDF composites at the filler content of 11 vol% exhibit an outstanding dielectric performance with a dielectric constant of 45 and a dielectric loss of 0.053 at 10³ Hz. This phenomenon can be attributed to the increased interfacial polarization induced by the inner carbon shell and the conductive paths blockade caused by the outside PDA shell inside the BT@C@PDA/PVDF composites. This work reveals that rational design of core@double shell structured hybrid fillers maybe a promising way to optimize the overall dielectric performance of the PVDF-based composites.     

Preparation and properties of novel,flexible, lead‐free X‐ray‐shielding materials containing tungsten and bismuth(III) oxide

Novel, flexible, lead‐free X‐ray‐shielding composites were prepared with a high‐functional methyl vinyl silicone rubber (VMQ) matrix with W and Bi₂O₃ as filler materials. To verify the advanced properties of the lead‐free material, composites with the same mass fraction of PbO were compared. With the X‐ray energy ranging from 48 to 185 keV, the W/Bi₂O₃/VMQ composites exhibited higher X‐ray‐shielding properties. As the filler volume fraction decreased, the tensile strength, elongation, tear strength, and flexibility of the W/Bi₂O₃/VMQ composites increased. The Shore hardness of the W/Bi₂O₃/VMQ composites had a maximum value of 46.6 HA and was still very flexible. With decreasing filler volume fraction, the water‐vapor transmission performances of the W/Bi₂O₃/VMQ composites increased, and the W/Bi₂O₃/VMQ composites also showed better water‐vapor permeability. The heat‐transfer properties of the W/Bi₂O₃/VMQ composites increased with increasing W content, and when the W content exceeded 70 wt %, the thermal conductivity of the W/Bi₂O₃/VMQ material was about 70.45% higher than that of the PbO/VMQ composite. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43012.     

The surface modification of BaTiO3 and its effects on the microstructure and electrical properties of BaTiO3/silicone rubber composites

Interfacial interaction and compatibility between the ceramic dielectric and polymer matrix have strong impact on the dielectric constant and dielectric loss of their composites. In this work, the BaTiO₃ (BT) nanoparticles were modified by (1) stearic acid (SA); (2) aluminate coupling agent (ACA); (3) the combination of SA and coupling agent aluminate, and then incorporated them with silicone rubber (SR) matrix to prepare BT/SR nanocomposites. The effects of the surface modification methods of BT on the microstructure and electrical properties of BT/SR nanocomposite films were studied. The results showed that SA and ACA were beneficial to the dispersion of BT and resulted in the strong interfacial interaction. The composite of BT modified by SA not only had high dielectric constant of 10.5, 2 times higher than that of the unmodified BT/SR nanocomposite (4.7), at the same time showed lower dielectric loss. J. VINYL ADDIT. TECHNOL., 24:288–294, 2018. © 2017 Society of Plastics Engineers