Ultrahigh Terahertz Signal Transmittance of PTFE@PPS Nanocomposite Foam Designed for Terahertz Antennas

Zhang L., Chen D., Jin B., Zhang B., Gong P., Zhang B., Park C.B., Li G.

Zhang X., Li P., Gong P., Xie Z., Jin B., Park C.B., Li G.

Microelectronic is developing towards high frequency ( GHz ) and high speed ( Gpbs ), putting forward high requirements for low dielectric materials. The most efficient method for fabricating low dielectric materials is the incorporation of air into matrix via supercritical CO 2 foaming. Herein, a low dielectric thermoplastic perfluorinated polymer is selected to be the matrix and another low dielectric perfluorinated polymer which is capable to form in-situ nanofibrils is selected to regulate the matrix viscoelasticity. Supercritical CO 2 foaming method is then applied to introduce a large amount of low dielectric air into nanofibrill modified perfluorinated polymer. Owing to supercritical CO 2 as a residue-free foaming agent (residue impurities in matrix originated from foaming agent would increase dielectric loss significantly at GHz ) and its strong interaction with perfluorinated polymer (ensure large expansion ratio of the obtained foams to introduce a large amount of low dielectric air in matrix), the lowest dielectric loss of 0.00015 (among the existing polymeric materials) is then obtained by supercritical CO 2 foaming of in-situ nanofibril modified perfluorinated polymer. Furthermore, the hydrophobic and oilphobic properties of the perfluorinated polymer were enhanced by supercritical CO 2 foaming to form a cellular structure; simultaneously, the corrosion resistance to strong alkali and V0 flame retarding properties of the ultra-low dielectric foam were maintained resulted from the perfluorinated cell walls. Therefore, such superior comprehensive performance of this ultra-low dielectric perfluorinated foam made from supercritical CO 2 foaming enables it the best alternative for the next-generation high-frequency ( GHz to THz ) and high-speed (sub Tbps ) signal transmission substrate in electronics. • Supercritical CO 2 foaming for ultra-low dielectric loss. • Lowest dielectric loss of 0.00015 among existing polymeric material. • Ultra-low dielectric material withstands harsh environment frequently encountered for PCB substrate.

Li P., Lan B., Zhang X., Lei S., Yang Q., Gong P., Park C.B., Li G.

An environmentally friendly polyester vitrimer was prepared through an efficient one-pot method. The vitrimer possesses advanced recyclability,  foamability, creep resistance and self-healing performance.

Li P., Lan B., Zhang Q., Yang Q., Gong P., Park C.B., Li G.

Foams with excellent mechanical performance are extremely valuable, as playing a key role in structural materials. However, it is still a significant challenge to simultaneously strengthen and toughen foam materials. In this work, a novel strategy combining nano-fibrillation network and supercritical carbon dioxide (scCO 2 ) solid-state foaming was proposed as follows: 1) In designing micro-scale cellular structure, chain extension and fibrillation technologies were synergistically adopted to improve the matrix viscoelasticity for cell stability, scCO 2 solid-sate foaming was then adopted to improve cell nucleation and cell growth. The combination of the above technologies helps to obtain a microcellular foam with large expansion ratio and small cell size; 2) In designing nano-scale hierarchical structure of crystalline and multiphase, flexible fibrils were dispersed in cell walls to compensate for matrix's brittleness and hence increased the foam impact strength, the crystals in cell walls improved the bending strength of cell walls and hence increased the foam compression strength. Herein, the obtained microcellular foam has simultaneous improvements in compression strength from 1.7 MPa to 2.7 MPa and impact toughness from 0.26 kJ/m 2 to 0.46 kJ/m 2 while maintaining a low foam density of 0.13 g/cm 3 . • A novel strategy is firstly proposed to simultaneously strengthen and toughen foams. • Micro/nano structure is regulated by in-situ fibrillation and supercritical solid-state foaming. • Nano fibrils facilitate crystallization and hence improve foam compression strength. • Nano fibril network hinders crack propagation and hence improves foam toughness. • Micro cellular structure improves foam compression strength and toughness.

Kong L., Luo S., Zhang G., Xu H., Wang T., Huang J., Fan X.

For the non-magnetic materials, polarization and conductivity loss are the keys to affect the electromagnetic wave absorption properties. The heterogeneous interfacial polarization is an important scientific issue in research of electromagnetic wave absorbing materials. Carbon nanotubes/pyrolytic carbon hollow microspheres (CNTs/PyC HMs) are prepared by in-situ growth of CNTs growing on the surface of zero-dimensional PyC HMs carrier. A large number of heterogeneous interface between CNTs and PyC HMs are constructed. The crystallization degree of CNTs can be significantly regulated in a small temperature range. When the reaction temperature increases from 600 to 700 °C, CNTs gradually change from amorphous to crystalline state. The higher crystallization degree of CNTs leads to a greater conductivity differences in heterogeneous interfaces, which results in an enhancement of interfacial polarization intensity. CNTs/PyC HMs with special hollow structure and a large number of heterogeneous interfaces exhibit excellent dielectric loss capability. When the thickness is 2.3 mm and reaction temperature is 650 °C, the minimum reflection coefficient is −56 dB and the effective absorption bandwidth reaches 4 GHz. The polarization loss induced by heterogeneous interfaces is an important avenue to enhance electromagnetic wave absorption properties, and a design strategy of interfacial polarization dominant electromagnetic wave absorbing material is explored.

Liu Y., Li Y., Liu Y., Gong P., Niu Y., Park C.B., Li G.

Zhang Q., Ma H., Gong P., Huang Y., Park C.B., Li G.

Supercritical CO2 foaming is widely used to prepare microcellular foams and particles are frequently added to regulate cellular structure. Nevertheless, microcellular foams present a complicated structure (three-dimension cells and two-dimension cell walls embedded with particles), which is a huge challenge to enlighten scCO2 foaming behavior and corresponding properties. Therefore, a strategy was proposed to take fluorescence’s advantage of real-time and field visualization in scCO2 foaming: (1) cell wall rupture process was studied via in-situ visualization; (2) dual fluorescence design was used to construct particle dispersive state in a cell; (3) polystyrene particles were chemical fluorescence modified to resist strong extraction of scCO2 fluid. This work reveals that effect of particles on bubble nucleation is two folds: interface promoted bubble nucleation, but elastic strain energy inhibited bubble nucleation. By in-situ visualization, irregular particle dominates in initiating cracks in cell walls due to the much larger stress concentration compared with spherical particles.

Li Y., Zhang Z., Wang W., Gong P., Yang Q., Park C.B., Li G.

Due to abundant disposable insulation materials causing serious environmental problem, degradable poly (butylene adipate-co-terephthalate) (PBAT) foam is a solution. To enhance the degradation rate and improve supercritical foaming for PBAT, poly (butylene succinate) (PBS) was selected in this work to blend with PBAT. PBAT/PBS foam with foam density lower than 0.1 g/cm3 and cell size smaller than 40 µm was successfully obtained, whose thermal conductivity was only 34 mW/m K and specific compressive strength was over 3000 N m/kg with an extremely fast degradation rate (97% material degraded within 10 days). The designation strategy is as follows: PBS’s fully aliphatic hydrocarbon structure ensures faster degradation rate than PBAT; PBS’s better molecular chain regularity accelerates crystallization for PBAT/PBS blend which ensures larger strength for blend foams; crosslinking structure and micro-phase separation structure in PBAT/PBS blend ensures better cell growth and hence larger expansion ratio, that is better thermal insulation performance.

Shi S., Liao X., Tang W., Song P., Zou F., Fan Z., Guo F., Li G.

Dielectric materials with ultralow dielectric constant, excellent mechanical properties, and good thermal stability have a broader application prospect in microelectronic devices with the rapid growth of 5G communication systems. On this basis, a series of lightweight polyarylene ether nitrile (PEN)/PSS‐octamethyl substituted (POSS) foams have been prepared by supercritical carbon dioxide foaming technique combining the strategy of low dielectric constant POSS doping and the introduction of bubbles. The effects of the introduction of POSS and foaming temperature on the foaming behavior of PEN/POSS composites are studied. The density of PEN/POSS foam with 5 wt% POSS is as low as 0.208 g cm−3 and the homologous porosity is up to 82.1%, which realizes the lightweight preparation of engineering plastics. Due to the introduction of POSS and pores, the PEN/POSS foam exhibits an ultralow dielectric constant of 1.83 at 1 kHz, while the dielectric loss is as low as 0.0036 and high thermal stability (Td5 > 453 °C). In addition, the specific modulus of PEN/POSS foam is up to 1.07 GPa cm−3 g−1 due to the high performance of the PEN. These excellent properties make PEN/POSS foams have great potential in the application of microelectronic devices.

Sarieddeen H., Alouini M., Al-Naffouri T.Y.

Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advances in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress establishes a foundation for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques, emphasizing ultramassive multiple-input–multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, vital for overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

Li Y., Gong P., Liu Y., Niu Y., Park C.B., Li G.

Mohammadi R.S., Zolali A.M., Kim J., Jalali A., Park C.B.

A new, efficient and easy to scale up approach to enhance the melt strength of linear low density polyethylene (LLDPE) through high aspect ratio polyamide (PA) fibrillar inclusions is presented. For the first time, the effect of the incorporation of compatibilizer in the blends on the efficiency of the in-situ fibrillation approach to improve rheological properties and foamability is discussed. The morphological analysis indicates high aspect ratio fibers were achieved for both compatibilized and non-compatibilized systems. Also, very fine fibers with nano-scale cross-sectional diameter were achieved for compatibilized blends. Comparable rheological responses and substantial improvement of the foaming ability with superior foam morphology and high cell density was obtained, for both fibrillated blends with and without compatibilizer. Investigation of the crystallinity of the blends through DSC and WAXD indicated that bimodal melting and crystallization of LLDPE is remarkably affected by the presence of PA inclusions, particularly for the blends containing fibrillated PA. Further investigation of the crystallinity through 2D-SAXS revealed formation of shish-kebab crystalline structures as a result of drawing and shear induced crystallization. • Melt strength and foaming ability of LLDPE drastically improved by incorporation of a small amount of polyamide nanofibrils. • High aspect ratio PA fibrils renders blends with favorable rheological properties in blends with and without compatibilizer. • Higher cell density foams are achieved for blends with fibrillar morphology compared to blends with droplet morphology. • Shish-kebab crystalline structures are formed as a result of drawing and shear induced crystallization.

Niijima S., Taniguchi H., Matsuda H., Hashimoto N., Murate K., Kawase K.

We recently reported that the terahertz (THz)-wave characteristics of pottery bodies varied significantly with firing temperature. This suggested the feasibility of conducting nondestructive inspec...

Cao X., Wen J., Song L., Liu X., He G.

Nowadays, polyimide (PI) with low dielectric constant is expected to be widely applied in microelectronics. For this reason, hollow glass microspheres (HGM) modified by silane coupling agent KH-550 (K-HGM), a series of HGM/PI and K-HGM/PI composite films with excellent thermal performance, hydrophobic and low dielectric constant were fabricated by in situ polymerization. The effect of HGM/K-HGM content on the properties of composite films was studied. The superior heat resistance of HGM can improve the thermal performance of composite films. Due to silane coupling agent KH-550, K-HGM exhibits a good interfacial compatibility with PI matrix and forms an interfacial adhesion region. With the HGM loading of 6%, comparing with pure PI films, the glass transition temperatures (Tg) of composite films were dramatically increased by 32.3°C. Especially, the low dielectric constant of 2.21 and dielectric loss of 0.0059 at 1 MHz were obtained for the PI/K-HGM composite film with addition of 8 wt%. Thus, PI/K-HGM composite films show more excellent performance. The current work provides a promising solution for fabrication of PI with low dielectric constant and superior thermal performance that may be applied in microelectronics industry.

Xu P., Jin Z., Zhang T., Chen X., Qiu M., Fan Y.

The composition of ship exhaust gas is complex and a great challenge to the long-term stability of a membrane contactor. In this work, a facile method for constructing a functional surface on an al...

Ezika A.C., Kupolati W.K., Sadiku E.R., Idumah C.I.

Li H., Cao Y., Skorobogatiy M., Atakaramians S.

The transmission line is one of the most fundamental components for the implementation of electromagnetic systems, such as electric cables and optical fibers for microwave and optic applications, respectively. The terahertz band, sandwiched between those two well-developed spectra, is not an exception. To meet such essential demand, low-loss, flexible, wideband terahertz fibers and corresponding functional devices have witnessed a blooming interest in the past two decades, being considered as a promising candidate for building compact, robust terahertz systems thus advancing the practicality and commercialization of terahertz science and technology. In this tutorial, we will provide a concise introduction to the fundamental characteristic parameters and prevalent hosting materials of terahertz fibers. Subsequently, we will look backward over the developments of terahertz hollow-core and solid-core fibers, as well as fiber-based terahertz functional devices for communication, sensing, spectroscopy, and imaging applications. Moreover, we will discuss several remaining challenges hampering the practical utilizations of terahertz fiber devices and propose some potential solutions to current major bottlenecks.

Hu D., Yue P., Huang Y., Wang Y., Chen Y., Zhao L.

Yu J., Chen Y., Zhong W., Hu D., Jia X., Sun J., Jiang Y., Jiang X., Wang H., Zhao L.

Zhang Z., Li S., Zhao X., Tang X., Lu Y., Zhu M., Chen H., Wang Z., Hu Z., Long L., Wang Z., Liu T.

For 5G microwave-transparent materials, traditional polymer-based foams possess a lightweight characteristic, while the poor mechanical and thermal properties limit the further application. In this work, lightweight polymethacrylimide (PMI) foams with infrared stealth and microwave transparent performance are prepared by using acrylonitrile as the monomer to reduce the cost, and α-methylstyrene as the retarder to alleviate the bursting phenomenon. The thermal insulating properties bring an excellent infrared stealth performance due to the construction of porous structure in PMI foam. Meanwhile, the low dielectric properties endow PMI foam with excellent microwave transmission performance. Such excellent comprehensive performance makes it possible for PMI foam to become a promising microwave transparent/infrared stealth material, which may be employed in 5G base stations or radome of warplane.

Ma H., Wu J., Gao C., He S., Gong P., Shi Q., Wang Z., Li G., Park C.B.

In the context of the future sixth-generation (6G) communication system, the development of multi-functional materials with ultra-low dielectric permittivity (Dk) and ultra-low dielectric loss (Df) is of paramount importance for the design of terahertz (THz) frequency electromagnetic wave transmission devices. In this work, polyimide-block-polydimethylsiloxane (PI-b-PDMS copolymer, PIC) material with low Dk, high flexibility and high supercritical fluid foaming ability was fabricated via copolymerization. Additionally, core–shell SiO2@BN nanofillers were synthesized by condensing tetraethyl orthosilicate (TEOS) on hexagonal boron nitride (BN) nanofiller surface. Subsequently, flexible polyimide/SiO2@BN foam was fabricated via supercritical N2 foaming. The resulting PIC/SiO2@BN nanocomposites exhibit exceptional flexibility and impressive thermal resistance (thermal decomposition temperature 472.2 °C). Notably, the incorporation of core–shell SiO2@BN nanofillers leads to the formation of a molecular interface anchoring structure through hydrogen bonding effect. This structure effectively reduced the mobility of the flexible PIC polymer chains on the SiO2@BN nanofiller surfaces. Consequently, the porous PIC/SiO2@BN materials show a low shrinkage ratio, low dielectric property (Dk = 1.21, Df = 0.012 at 1 THz), excellent THz transmission rate (91.4 %) and long distance 6G THz signal coverage (125.99 m). Furthermore, porous PIC/SiO2@BN materials show superior flame-retardant and thermal infrared stealth properties.

Wu C., Li Q., Hong Y., Feng B., Hu W., Sun L.

AbstractCeramic hollow spheres and polytetrafluoroethylene (PTFE) resin are essential for the fabrication of ultra‐low dielectric constant laminates for high‐frequency printed circuit boards. The poor bonding between ceramic hollow spheres and PTFE is the fundamental obstacle to the homogeneity of the composite. In this work, the broken ceramic hollow spheres were first removed by flotation, and then the surface of the floated hollow spheres was modified using different silane coupling agents at various concentrations. By comprehensively considering the results of the orthogonal experiment, the optimal coupling agent is determined to be 3‐(methacryloyloxy) propyltrimethoxysilane (KH560). The effect of KH560 content, ethanol/water volume ratio, and pH value on the dielectric constant, dielectric layer density, water absorption, and Z‐axis thermal expansion coefficient of the parallel samples was investigated in detail. The optimum surface modification parameters are found to be 6 wt % KH560, an ethanol/water volume ratio of 9 : 1 and a pH of 4. Under optimum technological conditions, the fabricated composites exhibit superior properties with dielectric constant less than 2, indicating a potential application in high performance high frequency copper clad laminates.

Hao S., Feng D., Wu F., Xie Y., Xu Z., Zhao W., Xie D.

It is a challenge all the time to prepare transparent flame retardant polymer composites. In this contribution, a biodegradable carbon dioxide derived copolymer polypropylene carbonate (PPC) has been chosen considering its signality for human environmental protection. Nevertheless, the low glass transition temperature (Tg) of PPC limits its practical applications. Herein, polylactic acid (PLA) and flame retardant guanidine phosphate (GP) were selected to obtain PPC composites by the interaction of intermolecular hydrogen bonds (H-bonds). The properties of PPC/PLA/GP composites were systematically studied. The test results showed that GP can be uniformly dispersed and contributed to the composites with enhanced Tg and flame-retardant properties. As a model system, 3 wt.% of GP enabled the PPC/PLA/GP-3 composites with Tg of 60.0 ℃, UL-94 grade of V-0, limiting oxygen index (LOI) of 30.8 %, and a significant reduction of THR (13 %), pHRR (32 %), and pCO (84 %), respectively. Besides that, the introduction of GP displayed slight influences on the intrinsic transparency nature of PPC/PLA. This study provided a facile approach for solving the bottleneck of PPC for advanced applications.