Ruo Peng Liu

On the Damping Properties of a Polyurea Elastomer

The development of materials with high damping and energy absorption properties in the form of bulk materials or coatings is today a target for designing components and structures with improved durability and survivability to serve in fields such as aeronautical, mechanical, civil and environmental engineering. This paper presents the results of the dynamic mechanical characterization conducted on reinforced polyurea composites. The mechanical properties of two types of composite materials utilizing the polyurea elastomer as a matrix are compared with those of the pure polymer. The composite materials show a substantial increase of the damping ability. Such materials can be produced in the form of coatings and successfully applied for maximizing the damping of mechanical and structural vibrations or for limiting the damage effects of impact events on structures and components.

Artificial Ceramic Metamaterial with Meshed Grid Structure for Radome Application

We investigated a novel artificial metamaterial that includes two plates of quartz glass dielectric material and a Ag microstructure sandwiched between the two plates. The Ag grid layer was designed and subsequently prepared by tape casting and screen printing. The transmission characteristics of this metamaterial were able to be controlled by adjusting the geometry parameters of the Ag grid such as the width of the strip and the size of the unit cell. Our work has demonstrated the possibility that the ceramic metamaterial can be used as a transmission material capable of work at high temperatures below the melting point of the metal.

Fused Silica Ceramics and Composites for Radome Applications

The processing methods, strengthening methods, water-proof coating procedures, and some ways to improve the transmission of microwaves have been reviewed in relation to fused silica ceramics and their composites for radome applications. Fused silica ceramics are characterized by a residual porosity (up to 18%), low dielectric constant (3.06-3.32), low loss tangent (0.00053-0.0065), excellent thermal properties, but low mechanical strength (37-65 MPa). To achieve higher mechanical strength and better transmission efficiency, new randome materials would be those engineered composites consisting, for instance, of a dense layer-porous core structure, a continuous fiber reinforcement, a multilayered structure, and/or frequency selective surface (FSS) layer (s)/metamaterials.

Thermal Characterization of a Reinforced Elastomer

The effect of the presence of fillers on the thermal properties of a high performance elastomer was investigated in this work. The characterization of the specific heat capacity (Cp), the specific heat flow and the glass transition temperature of a polyurea elastomer reinforced with two different classes of fillers, i.e. short glass fibers and alumina nanoparticles, was conducted by using a differential scanning calorimeter (DSC). We present and discuss the results of the experimental characterization carried out on the reinforced material. The results are compared to those obtained by testing the pure polymer.

Electromagnetic Materials Design for the Enhancement of the Microwave Power Transmission through Polymeric Slabs

In order to improve the energy transmission of an electromagnetic wave in microwave frequency regime propagating through a slab made of a polymeric material, we integrated conductive metamaterials particles in the polymeric medium and used an optimization method for determining the overall size and geometrical characteristics of the metamaterial element that can meet the desired energy transmission requirements. The numerical results show that to attain high transparency the effective parameters of the homogenized medium can be manipulated as required by optimizing the geometry of the metamaterial, the unit cells size and their distribution. To validate this design approach, we present the results of the experimental tests conducted on a 300 mm x 300 mm x 4 mm board with optimized metamaterials structures.

Characterization of TC-3 Glass/Fused Silica Composites for LTCC Applications

A glass powder of the SiO2-Al2O3-K2O-Na2O system was added to fused silica particles to form composite green compacts by tape casting. This procedure was able to lower the sintering temperature of the fused silica particles making the composites suitable for being co-fired with the silver-palladium (Ag-Pd) paste commonly used as a conductive circuit in several microwave applications. The resulting new ceramic composite with the composition of 50 wt% fused silica and 50 wt% glass (brand name: TC-3) had a low dielectric constant and a low loss tangent of 2.7 and 2.5-3.7 x10-3 , respectively, and was able to be co-fired with the Ag-Pd conductive paste at the temperature of 895 °C, resulting in a potential low temperature co-fired ceramic (LTCC) system for microwave applications.

Compressive Behavior of a Polyurea Elastomer

The application of elastomeric coatings for improving the ability of already existing structures to dissipate the energy released by impact events has been investigated by many researchers in the past decade and is today an area of considerable interest. In recent years, polyurea has been successfully applied as a coating material for enhancing the impact protection of buildings and it has also demonstrated a considerable improvement of the survivability of metallic and non-metallic structures subjected to severe shock and impact loading conditions. Given its remarkable properties in terms of impact energy mitigation, life endurance and corrosion resistance, this material is currently of interest for its application in many fields of engineering. This paper presents and discusses the results of the mechanical characterization conducted on a polyurea elastomer fabricated following two different procedures and subjected to varying strain rates of compression load. The tests were conducted to verify the sensitivity of the material behavior to the varying loading conditions and to verify how the fabrication of the material in the laboratory can influence the test results.

Brief Introduction of Microwave Ceramics Developed in our Labs for Dielectric Resonators

Microwave filters require dielectric ceramic resonators attached to ceramic supports via joining. Among the BaTiO3, CaTiO3, and Ba2Ti9O20 based-dielectric ceramics prepared in our labs., the CaTiO3 ceramics modified with NdAlO3 and Al2O3 gave satisfactory dielectric properties: dielectric constant DK = 40.69, quality factor Qxf = 18842 GHz (fo = 6.551 GHz), and frequency temperature coefficient tf = 2.542 ppm °C-1. For replacing the hard-to-sinter Al2O3 based-ceramic supports, a glass-ceramic of the SiO2-B2O3-P2O5-ZnO system was developed, leading to a low sintering temperature of 900 °C, and a low dielectric constant (<4). Finally, a Bi2O3-ZnO-SiO2-Al2O3-CaO glass was used as a sealing agent to replace the traditional gluing consisting of an adhesive. The present work hinters that with proper sintering aids, one can produce multilayered and integrated resonators with and without embedded passive elements.

Broadband Microwave Transmission Achieved by Using Engineered Sandwich Materials

In order to improve the transmission of the energy and broaden the frequency band of an electromagnetic wave in the microwave frequency spectrum propagating through a slab of a polymeric material, we designed an optimized material system where a periodic array of metamaterial unit cells are embedded into a polymeric medium in a sandwich-like configuration. The optimization method targeted the overall size and geometrical characteristics of the metamaterial element that can meet the desired energy transmission requirements given a certain materials thickness and materials dielectric properties. The numerical results conducted on this type of metamaterials in a sandwich-like configuration showed that it is possible to attain a high degree of transparency with broadband characteristics by optimizing the performance of the metamaterials in correspondence of the transmission zeros of the substrate.

Characterization of Radome Materials Comprising LTCC-Derived FSS on a Quartz Glass Plate

Frequency selective surfaces (FSS) and recent metamaterials (MTM) have shown unique electromagnetic characteristics and are of potential benefits for radome applications. To make the radomes/windows high-temperature resistant, the substrates of the frequency selective surfaces or metamaterials should be made of ceramic/glass-based materials of a low dielectric constant and a low loss tangent. However, fabricating ceramic/glass-based FSS or MTM is always challenging. In this paper, a constrained low-temperature co-fired ceramic (LTCC) technique was used to produce quartz glass-based radome material consisting of a frequency selective surface (FSs) layer embedded in a surface laminate. Due to the constrained sintering shrinkage, the geometry and the dimensions of the unit cells of the FSs were not subjected to significant variations and thus the measured electromagnetic (EM) wave transmission spectra matched those of the computer simulation results quite well. This preliminary work marks the beginning of our long-term efforts toward the goal of achieving high-temperature resistant, highly electromagnetic wave transparent, as well as carefullydesigned and fabricated radome materials.