High Insulation Research Articles

Hierarchically multifunctional thermally conductive Boron Nitride/Polyurethane fibrous membranes via constructing alternating multi-layer orientation structure

Thermally conductive polymeric materials with versatility are in high demand in electronic devices. Whereas, the limited thermal conductivity (TC) and versatility restrict the development of such materials. Herein, the hierarchical design and assembly technology was applied to prepare multifunctional functionalized boron nitride/polyurethane multi-layer (m-FBN/PU) fibrous membranes with higher quality and higher quantity of thermally conductive network. Filler layers act as thermally conductive pathways and substrate layers provide mechanical support and the obtained hydrophobic and flexible m-FBN/PU fibrous membranes with 10 wt% FBN filling display high TC (5.78 W·m−1·K−1) and electrical insulation (1014 Ω·cm). Meanwhile, in comparison with the FBN/PU single-layer fibrous membranes, the thermal conductive enhancement reaches 61.5 %, which could be ascribed to the increased phonon transfer network built by the alternated aligned hierarchical structure. Our fabrication provides a promising strategy for developing multifunctional polymeric materials and has the potential in advanced electronics and emerging fields like wearable electronics.

Electrically Insulative Film Adhesive with Enhanced Thermal Performance to Assemble High Power Density Electronic Devices

Assembly adhesives play a critical role in microelectronic packaging and contribute significant value for circuit assembly interconnection, protection, and thermal management. Of particular interest with today’s more powerful systems is thermal management of high-power density electronic systems, including power modules, power amplifiers, communication systems, and directed energy systems. As requirements for performance and functionality increase, heat generated from these devices increases exponentially. For example, local heat flux can easily exceed 50 W/cm2 for IGBT power modules while it may reach several hundred Watts/cm2 at active areas for Gallium Nitride (GaN) power amplifiers. Simultaneously, many of these systems demand a higher level of electrical insulation to eliminate or minimize current leakage to ensure high reliability over a long service life. Effectively and safely dissipating the high heat and ensuring excellent electrical insulation has become an increasingly critical and challenging task in the microelectronics packaging technology for aerospace and defense systems. This paper presents integration of thermally conductive adhesives with an electrically insulative polyimide film into a carefully engineered laminate, which enables high thermal transfer and robust electrical insulation. This new film adhesive provides outstanding compliance and augmented tolerance to large topographical features and warpage (5-7 mils). This film adhesive is one of the potential solutions to addresses the evolving needs of the electronics assembly industry in the areas of management of high-power systems

The application of terahertz technology in ultra high voltage large tonnage insulator detection

Among major nondestructive testing technologies (NDT), such as X-ray and ultra-sound, Terahertz (THz) waves present significant transmission rate through most of nonconductive materials without the need of applying couplant while posing zero ionized radiation hazard. In comparison to microwave NDT, terahertz technology boasts a superior spatial resolution and measurement precision due to its shorter wavelength. As such, it is considered to be the most promising technology for industrial NDT applications. To analyze of THz wave transmission characteristics within porcelain insulator and obtain large quantity of testing data with repeatable measurement condition, this paper reports an experimental approach using Kaolin, in powdery form, to emulate the aging behavior of porcelain, and a THz sensor to detect such imperceptible aging process where micrometer-scale cracks slowly develop inside of porcelain insulators. To support the data acquisition from distance, a LORA communication module was integrated in the sensor for using under high-voltage environment. In addition, a dedicated user-interface software has been developed to receive the wireless data as well as conducting data analysis, crack recognition and graphic display. The experiment successfully demonstrated a very feasible solution for online monitoring of the aging process of porcelain insulators by using compact THz sensor buried in the insulator.

Investigation of mechanical, thermal, and acoustic performance of insulation materials containing microwave and ultrasound pretreated coffee wastes and estimation of thermal parameters using artificial neural network

AbstractAs a result of increasing coffee consumption in the world, the amount of coffee waste, which is an important environmental problem, is increasing. Due to their complex organic properties, they should not be discharged to landfills. At the same time, these wastes with a porous and cellulosic structure, have high acoustic and heat insulation performances in the building material they are used. In this study, commercial coffee wastes were evaluated without pre‐treatment (CW) and with microwave (MCW) and ultrasound (UCW) pre‐treatment in insulation plaster containing boron. First, the morphological, thermal, and chemical properties of coffee wastes were determined by SEM, TGA, and FTIR analyzes. CW, MCW, and UCW wastes were replaced with insulation plaster at rates of 10%, 20%, and 30%. The physical, mechanical, thermal, and acoustic properties of insulation plasters produced from coffee waste were determined by the analysis methods given in Turkish Standards. According to the results, insulation plasters obtained from pre‐treated wastes were more efficient in terms of mechanical and thermal properties. In terms of thermal and acoustic insulation, UCW30 insulation plasters showed the best performance. Thermal conductivity values were estimated by entering dosage, density, water absorption, bond strength, compressive strength, and moisture values as input dataset to artificial neural network (ANN). The developed ANN model and the experimentally obtained data were compared and it was observed that the results were in harmony. As a result, with this study, a new generation green insulation material was produced by recycling coffee wastes. In this way, a different solution has been found for an important environmental problem.

Versatile spider-web-like cross-linked polyimide aerogel with tunable dielectric permittivity for highly sensitive flexible sensors

With the development of intelligent electronic power systems, the performance of flexible electronic sensors with portability and miniaturization is increasingly in demand. It is extremely challenging to achieve high modulus and high elasticity of polymer matrix simultaneously due to the inherent strength-elasticity conflict of elastic polymer. Here, a series of “spider-web-like” cross-linked polyimide aerogels (CPAs) had been successfully prepared. Benefiting from the robust dynamic network structure, the CPAs exhibited excellent compressive resilience, tunable dielectric permittivity, high thermal insulation property (λ = 0.040 W/m∙K) and modulus (E = 1073 KPa). Additionally, due to the excellent resilience of the CPAs and tunable permittivity, a CNTs/CPA-3 composite aerogel-based strain sensor obtained by assembling the CPA-3 with carbon nanotubes (CTNs) possessed synergistically enhanced sensitivity, high selectivity and “multi-touch” stress detection, without deterioration even after 2000 cycles. Furthermore, the established mathematical model revealed the internal mechanism of the synergistic enhancement of sensing performance of sensor with strain-controllable permittivity. Thus, the fabricated CPAs might be an ideal substrate material for flexible electronic sensors in electronic power systems, and the mathematical model established could also provide a reliable reference for improving the sensitivity and detection limits of sensors.

Development and characteristics of multifunctional ultra-lightweight engineered cementitious composites incorporating cenospheres and PE fibre

Ultra-lightweight, high ductility, high strength, and multi-functionality are recent development trends in concrete. This study proposed a design concept of multifunctional ultra-lightweight engineered cementitious composites (ULW-ECCs) without functional fillers using fly ash cenosphere (FAC) and polyethylene (PE) fibre, which has heat insulation, self-sensing, and self-healing functions. The mechanical (tension, compression, flexure) and corresponding self-sensing properties under monotonic and cyclic loading, self-healing, thermal conductivity, and thermal effusivity properties were examined. The ULW-ECCs were developed based on the maximum packing density of the matrix, which showed high pseudo-strain-hardening indices and had an apparent density of 1055–1333 (Oven-dry density: 946–1261) kg/m3, compressive strength of 36–58 MPa, and a tensile strain capacity of 4%–8% under standard curing condition. With the use of FAC with a highly stiff shell, the ULW-ECCs show high strength and high thermal insulation properties. ULW-ECCs incorporating FAC show excellent self-sensing properties without any conductive fillers under compression, bending and tension. The electromechanical behaviour was consistent and repeatable during cyclic and monotonic loading processes, and a microstructure model was used to explain the self-sensing mechanism. The cement paste in the matrix can form a 3D-like honeycomb structure because a large number of FACs evenly divide the cement paste, which shows sensitive self-sensing ability. Additionally, the multiple micro-cracks of ULW-ECCs show excellent self-healing properties. Optical microscope and SEM analyses of ULW-ECC were used to explain the results. The developed multifunctional ULW-ECCs without functional fillers would be a promising material for sustainable development.

Influence of the acceptor-fillers on the dielectric properties of polyimide composites

High-performance polymer dielectrics are critical for high-tech fields such as electrical insulation and aerospace. Nevertheless, it is still challenging to achieve simultaneous improvements in both dielectric constant and breakdown strength in polymer dielectrics. In this work, Al3+ substitution Si4+ in SiO2 by high-temperature sintering to create hole traps to obtain the acceptor-doped fillers Si1-0.75xAlxO2 (SA (x mol%)), and polyimide (PI) dielectric containing SA fillers with different contents were synthesized using in-situ polymerization. The hole traps in the fillers modulate the electrical behavior of the polymer dielectric, thereby significantly increasing the dielectric constant and breakdown strength by modulating the polarization behavior and capture carriers. Among them, the dielectric constant of the 1.25 vol% SA (1.5 mol%)/PI composite was increased to 3.60 at 1 Hz, and the breakdown strength Eb reached 581.5 kV/mm. This work provides a new synthesis strategy for simultaneously achieving high insulation and high polarization of polymer dielectric.

Preparation of Thermally Conductive Silicone Rubber-Based Ultra-Thin Sheets with Low Thermal Resistance and High Mechanical Properties

Thermally conductive silicone rubber (TCSR)-based thin sheets with low thermal resistance and high electrical insulation properties have been widely used in thermal management applications in the electronic and energy storage fields. The low thermal resistance is mainly attributed to the sheets’ small thickness. In order to further decrease the sheets’ thermal resistance, it is necessary to decrease their thickness. However, the sheets mostly have a thickness of at least 0.20 mm, and it is still a challenge to decrease the thickness to less than 0.10 mm mainly due to the difficulty of smooth calendering through a narrow roll-to-roll gap on calenders. Here, a low-viscosity calendering method has been developed to prepare TCSR-based ultra-thin sheets. The sheets present unprecedentedly small thickness (~0.08 mm), low thermal resistance (0.87 cm2K/W), high tensile strength (~8 MPa), high flexibility, high electrical resistance (>1014 Ω·cm), and high thermal dissipation (>30 °C decrease in LED working temperature). Comparison studies between this new method and the conventional preparation method have been carried out to understand the mechanism of the improvements.

Study of acoustic transmission losses in particle-reinforced rubber-based membrane-type acoustic metamaterials

In this work, the mechanical properties of particle reinforced ethylene propylene diene monomer (EPDM)/ethylene tetrafluoroethylene (ETFE) copolymer material were determined by molecular dynamics (MD) method, and the copolymer material was used in the structural design of acoustic metamaterials. Secondly, based on the discrete point matching method, the acoustic transmission loss characteristics of the material are predicted, and the accuracy and effectiveness of the theory are verified through the multi physical field coupling finite element model. Finally, seven key parameters that affect the material structure are studied, and their influencing rules and mechanisms are analyzed. The results show that the structure makes up for the sound insulation defect of traditional ETFE film materials at low frequencies. Through reasonable selection of key parameters, high sound insulation design can be carried out for specific frequency bands, which is of great significance to the potential application in the construction field (new building materials).

Development of thermal liner for extreme heat protective clothing using aerogel technology

In this research, an attempt was made to develop and explore the thermal protective performance of the thermal liner and the effect of various preparation parameters of aerogel technology on it. Efforts have also been made to explore the scope of nanotechnology in finding the solution to problems coming in the way of high thermal stability and insulation of the thermal liner. Silica aerogel is a colloidal suspension of silica-based oxides in the air. Hence, it is a highly insulating material and shows one of the lowest densities for a similar performance. Here we took an approach to deploy silica aerogel composite as a thermal liner layer in order to ensure the aforesaid functionalities. To realize the objective, we tried to find the suitable fibre as well as suitable fabric structure for developing an aerogel on it. Further, we studied the effect of processing parameters such as ageing time, the effect of base catalyst and silylation time during aerogel synthesis on the protective performance of the developed thermal liner. Electrospun Nomex® fibre-aerogel composite and nomex® fibre-aerogel composite was also made, and it was compared to the thermal liner made of Nomex® IIIA nonwoven to find out how it worked. Aerogel treated sample with ammonium fluoride as catalyst performed best compared to sample with ammonium hydroxide as a base catalyst. An increase in ageing and silylation time led to significant increase in the thermal protective performance of the developed aerogel-Nomex® composite due to a better aerogel formation inside the composite.

A flexible silica aerogel paper with temperature-switch opacifier for thermal insulation

Aerogels have an impressive thermal insulation performance at room temperature. However, with high infrared transmittance, pure aerogels have no advantages in thermal insulation at high temperature. Opacifiers can reduce infrared transmittance to make a low thermal conductivity with sacrifice of that at room temperature. Herein, VO2 powders undergoing reversible insulator to metal transition at 67.8 °C as opacifier with temperature-switch were introduced into silica aerogel composites to prepare aerogel reinforced by VO2@SiO2 fibers paper (AVSP). At 25 °C, insulator VO2 made little contribution to thermal conductivity (0.025 W/(m·K)) so that it was close to aerogel reinforced by pure SiO2 fibers papers without opacifier (0.023 W/(m·K)). When temperature rose to 500 °C, metal VO2 made thermal conductivity of AVSP (0.051 W/(m·K)) much lower than that of aerogel reinforced by pure SiO2 fibers papers without opacifier (0.076 W/(m·K)). Meanwhile, SiO2 fibers papers with three-dimensional network structure improved flexibility of aerogel. The tensile strength of AVSP was 2.1 MPa with density of 0.186 g/cm3. With 152° contact angle and flame resistance, AVSP showed a great potential in high temperature thermal insulation in a nonplanar condition. Thereby, this work provides a novel strategy to improve thermal insulation property at high temperature with little contribution to that at room temperature.

Experimental modelling and optimisation of WECSC of polymer nanocomposite

ABSTRACT Slicing of material using a thin wire reduces the loss of material and provides better cutting quality compared to any other machining techniques. Moving wire electrochemical spark machining (WECSM) is a hybrid, non-conventional wire slicing process, specially for non-conducting materials such as ceramics, composites and glasses. In this research article, alumina epoxy nanocomposite (AENC) has been taken as a workpiece. AENC is a polymer-based nanocomposite with a property of mixed nature such as ceramics and polymer. Due to its high electrical insulation performance, AENC is in demand in various applications. However, its mixed properties limit the machining capabilities for various applications. In the present work, modelling of the moving WECSM process has been performed using response surface methodology, and GRA with entropy method was applied for finding the optimum set of parameters. The results show that the optimal machining performances for material removal rate, surface roughness and kerf taper were obtained at 55 V applied voltage, 2.5 m/min wire velocity, 200 g/l electrolyte concentration and 200 µs pulse-on time.

Paper-based building envelopes – Thermal and environmental properties of original envelope designs

Paper-based materials, due to their availability, environmental benefits and high thermal properties, have already been implemented in several building envelope designs. However, most of them either did not optimise the use of the material or did not provide sufficient thermal insulation for use in permanent buildings. The presented research analyses six original paper-based building envelope core proposals, suitable for use in permanent buildings in temperate climate of Europe, in terms of thermal and environmental efficiency. Proposals include cores with embedded paper tubes structural elements, timber-cardboard studs and sandwich designs. The heat transfer coefficient of the envelopes was obtained for repeatable modules via 2D computer simulations (ThermCAD software), and the environmental impact was assessed for a representative square metre of the envelope via LCA analysis, based in ecoinvent 3.8 database. All analysed designs showed high thermal insulation properties and met the requirements for Umax = 0.20 W/m2K, did not cause the risk of surface condensation, nor have excessive thermal bridges. The analysed building envelope designs based on embedded frame wall structure showed environmental superiority over conventional envelopes (timber frame and SIP panels). The research may serve as a proof of concept, for using paper-based building envelopes in sustainable architecture.

High Energy Harvesting Performances Silicone Elastomer via Filling Soft Dielectric with Stretching Deformability.

Dielectric elastomer generators (DEGs) with high generated energy density and high conversion efficiency are of great interest. Among several dielectric elastomers (DEs), silicone elastomer filled with ceramic fillers have been extensively studied for their high elasticity, insulation and permittivity. However, the stretched breakdown strength (Ebs ) of such composites decreases significantly under large strain, thus sharply reduces its energy harvesting performances. In this study, a polar rubber-based dielectric (GNBR) is synthetized and creatively used as "soft filler" for silicone elastomer. Benefiting from the deformability under stretching and its inherent strong interface bonding with silicone elastomer, this soft filler effectively avoids the formation of weak interface under large strain and reduces the local field strength of interface area. As expected, the composite filled with soft filler (GNBR/PMVS) shows enhanced Ebs of 2.8 times that of composite with traditional hard filler (TiO2 /PMVS) under equibiaxial strain of 200%. As a result, GNBR/PMVS composite exhibits maximum energy density of 130.5 mJ/g with up-to-date highest power conversion efficiency of reported DEG (44.5%). The findings will provide new insights in the rational design of DE composites characterized by high stretched breakdown strength for advanced energy harvesting system. This article is protected by copyright. All rights reserved.

Light-weight and high thermal insulation building material; a comparative study between cenosphere & fly ash

The increasing demand for light-weight sustainable materials have led researchers to find the alternatives to the available natural resources used as construction materials. Cenospheres are hollow fly ash particles which are obtained from fly ash waste by burning coal in thermal plants. Cenospheres have been widely used as filler materials in various industries like aircraft, automation etc, due to its light-weight and high thermal resistivity, However, the use of cenospheres in construction industry has been limited, Therefore, in this research an attempt has been made to replace OPC with cenosphere particles. The cement was replaced from 0 to 30% to produce a light-weight and high thermal insulation concrete. This concrete was studied for the properties like fresh, heat of hydration mechanical, durability and thermal conductivity. It was observed that the heat of hydration significantly reduced by the use of cenosphere particles. The density of the concrete was also reduced with the increment in the content of cenosphere in concrete, however with the decrease in the density the mechanical strength of concrete also reduced but was within the design strength of concrete. It was also observed that the cenosphere contribute to the late gain in the strength of the concrete similar to the fly ash concrete. The thermal conductivity of the concrete reduced by more than 50% with the incorporation of the cenosphere in the concrete. From this research it can be concluded that cenosphere can be used as a replacement to cement in the concrete to produce a light-weight and high thermal insulated concrete.

A Trilayer Polymer Composite with Macroscopic Gradient Dielectric Constant Distribution for Advanced Energy Storage Capacitors

With the increased demand for electrical devices, there is an urgent requirement to explore advanced materials for capacitors. Composite is an effective way to improve performance through materials and structure design. Herein, an asymmetric three-layer structure with gradient dielectric constant through the solution casting method is reported. This unique design of construction consists of a buffer layer pristine poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) embedded between high insulation linear materials polyurea (PUA) and high polarization non-linear materials P(VDF-HFP)/BT nanocomposite. The experimental and simulation results demonstrate that the linear layer ensures high insulation of the composite and relatively high efficiency, while the nanocomposite layer acts as a high dielectric layer to enhance polarization. Typically, the buffer layer could effectively avoid overconcentration of the electric field on the PUA layer, which results in high breakdown strength of the trilayer composite. As a result, an ultrahigh breakdown strength of 5970 kV/cm and excellent energy storage density of 8.2 J/cm3 can be obtained, which were 45% and 2.15 times higher than that of pristine P(VDF-HFP). This asymmetric structure strategy provides a successful case in exploring high-performance energy storage materials.

Improvement of Moisture Susceptibility for Asphalt Mixture with Ceramic Fiber

Moisture damage is one of the most significant troubles that destroy asphaltic pavement and reduces road serviceability. Recently, academics have noticed a trend to utilize fibers to enhance the efficiency of asphalt pavement. This research explores the effect of low-cost ceramic fiber, which has high tensile strength and a very high thermal insulation coefficient, on the asphalt mixture's characteristics by adding three different proportions (0.75%, 1.5%, and 2.25%). The Marshall test and the Tensile Strength Ratio Test (TSR) were utilized to describe the impact of ceramic fiber on the characteristics of Marshall and the moisture susceptibility of the hot mix asphalt mixture. The Field Emission Scanning Electron Microscopy (FE-SEM) analysis was used to investigate ceramic fibers' microscopic structure and clarify the mechanics of their improved behavior and their distribution within the asphalt concrete mixture. The results showed that the incorporation of ceramic fibers improved the Marshall properties and the asphalt mixture's susceptibility to moisture damage with an optimum fiber content equal to 1.5%, where Marshall stability increased by 39.04%, and the TSR increased by 11.06% at this content compared with the control asphalt mixture.

High Voltage Cable Fault Location Technology Based on Traveling Wave

High voltage cable line is an important part of power grid; its stable operation is of great significance to urban power system. This paper proposed a high voltage cable insulation monitoring system using traveling wave. The travelling wave data are collected by high precision satellite timing signal and an on-line fault location system for high voltage cable is designed.

The Lifetime Prediction and Insulation Failure Mechanism of XLPE for High-Voltage Cable

The performance parameters and service life of the insulation layer of high-voltage cables are the key to ensure safe cable operation. Through extracting the mechanical parameters of high voltage cable insulation layer under different thermal aging temperatures, a normal linear regression model is established to predict the service life of high voltage cables. Meanwhile, the physicochemical characteristics such as microscopic morphology and molecular structure changes of the cable insulation layer are analyzed, the dielectric properties and breakdown field strength under ac condition are tested and the macroscopic electrical properties of the insulation layer are studied, and the insulation failure mechanism is further analyzed. The experimental results show that the elongation at break retention rate of cross-linked polyethylene (XLPE) increases slowly with the aging time, and then decreases rapidly when it reaches 50% of the initial elongation at break retention rate. The normal linear regression prediction model is established based on the Arrhenius formula, and it can be obtained the life endpoint of XLPE high voltage cable about 65 years at an operating temperature of 70 °C. Further analysis, under the action of heat and oxygen, the crystal zone structure of cable insulation is seriously damaged, and the carbonyl index is significantly increased. Accordingly, when the aging failure point is reached, the dielectric permittivity and dielectric loss of insulation are significantly increased, and the breakdown performance is significantly decreased. The work has an important guiding significance for insulation condition assessment and life prediction of high voltage cable.

Development of 80-kW High-Voltage Power Supply for X-ray Generator

This paper describes the development of an 80kW high voltage power supply (HVPS) to drive an X-ray tube. Since the developed HVPS is based on an LCC resonant converter with a symmetrical bipolar voltage multiplier (SBVM), it shows many superior features such as soft switching capability and low voltage imbalance among stages. In addition, the proposed circuit has high power density because a series resonant capacitor is removed by using a method converting the SBVM into capacitors and the full-wave rectifier. Furthermore, a suggested transformer achieves high insulation performance and effective winding area utilization by applying half of the output voltage to the core. The detailed design based on the fundamental harmonic approximation (FHA) is presented for the practical use of the LCC resonant converter with the SBVM in high voltage and high power applications. Also, some significant issues in the implementation are addressed. The performance of the developed HVPS is verified by simulations and experimental results.